Articles of Concrete

A Study on Engineering Properties of C...

Dr. Samson Mathew, Asst. Professor, P. Selvi, PhD Student, and K.B.Velliangiri, Former P.G.Student, Department of Civil Engineering, National Institute of Technology, Trichirappalli

Soil stabilization is a technique for improvement of weak soils. The engineering properties of soil can be improved by soil stabilization. Two road stretches located in seashore areas have been selected for this study and three samples were collected from each road stretch at various locations. This study deal about the improvement of weak soil particularly in seashore area. Oneis Aranthangi-Kattumavadi road in Pudukkottai District. Another is Mannarkudi–Adhiramapattinam road in Thanjavur District.

The Engineering properties like Specific gravity, Optimum Moisture Content, Maximum Dry Density, Liquid limit, Plastic limit and Grain size analysis of the collected samples were identified by suitable laboratory tests. The strength tests like unconfined compressive strength and California bearing ratio tests were conducted in NIT laboratory.

Thanjavur and Pudhukottai samples along the above road stretches are having silt and clay particles from 30 to 45%. Based on the soil properties, it was considered that the cement is a suitable stabilizer. Cement used as a soil stabilizer in different percentages (2%, 4%, and 6%) is tested in all soil samples. The test results are compared with the initial engineering properties and strength properties.

Introduction

Development of an adequate network of roads, especially in remote areas is of vital importance in the socio-economic development of villages in a country. The transportation facilities have to be continuously upgraded and improved so as to keep pace with the traffic demand, which is being generated, by the development plans and resultant expanding economy. However, development of a large network of roads by traditional practices and techniques require heavy financial investments. Soil stabilization methods using locally available cheaper materials have considerable scope in reducing the initial construction cost of the pavements. The soil deposits along the coast may be of silty sand, silty clay, soft clay or any other soil type. Soil at a particular location may be unsuitable, wholly or partially, to the requirements of the construction engineer. But the various developmental activities necessitate making use of these lands, which are not having the desirable properties as an engineering material.

The following are the ways of dealing with unsatisfactory soils:
  • By pass the bad soil
  • Remove bad soil and replace with good material
  • Redesign the structure
  • Treat the soil to improve its properties. Such treatment is called soil stabilization.
Soil stabilization in the broadest sense is the alteration of any property of a soil to improve its engineering performance.

Research Significance

India has about 6000km long coastline stretching along nine states i.e. Gujarat, Maharastra, Goa, Karnataka, Kerala, Tamilnadu, Andhra Pradesh, Orissa, and West Bengal. There was a tremendous construction activity along the coast. In south India, the Tamilnadu state boundary has a long coastal line in eastern side along Puthukkottaitai, Thanjavur, Thiruvarur, Nagappa- ttianam, Ramanathapuram, Cuddalore, Chennai, Negercoil and Kanyakumari districts.Most of the road network near seashore area is poor in condition due to the settlement of subgrade soil. This settlement is purely occurred by the volume changes of subgrade soil. In its natural state the engineering properties of the soil is considered as very poor, if it is used as a subgrade layer for pavement construction. So, it is necessary to control the variability of the geotechnical properties of the soil using different additives. The main purpose of this research is to improve the load carrying capacity of the natural soil. The soil samples along the road stretches from Puthukkottai and Thanjavur districts are predominantly with silt and clays. Thesilt and clay contents are ranging from 30–50%. It also may affect the strength of the subgrade. In this work, an attempt was made to analyse the engineering properties of the natural soils and their improvement with suitable stabilizer.

Literature Review

Engineers are often faced with the problem of constructing roadbeds on or with soils, which do not possess sufficient strength to support wheel loads imposed upon them either in construction or during the service life of the pavement. It is, at times, necessary to treat these soils to provide a stable subgrade or a working platform for the construction of the pavement. These treatments result in less time and energy required for the production, handling, and placement of road and bridge fills and subgrades and therefore, less time to complete the construction process thus reducing the disruption and delays to traffic. These treatments are generally classified into two processes, soil modification or soil stabilization. The purpose of subgrade modification is to create a working platform for construction equipment. This modification in the pavement has no role in the design process. The purpose of subgrade stabilization is to enhance the strength of the subgrade and this increased strength is taken into account in the pavement design process. Manikant Mandal and Dr. Mayajit Mazumdar (1995), a study was made on the effect of additives on lateritic soil stabilization with cement and lime. Particularly, the strength and fatigue behavior, under repeated flexture, of stabilized latertic soil treated with additives, have not been studied in our country till now. Sodium carbonate analytical reagent grade was used as an additive. Static and dynamic tests were carried out on specimens of soil-cement and soil-lime mixtures prepared under standard as well as modified compaction. It has been found that sodium carbonate used as an additive in trace amounts, improves the strength of soil-cement and soil-lime. Also, the additive increases the value of modulus of rupture and durability of the stabilized soil. A. Arumugam and K. Muralidharan (1997), stabilizing the locally available soils and using them as subgrade materials generally reduce the cost of pavement construction. It was concluded that the mechanical stabilization saving in the construction cost of pavement upto 43% has been effected. Lime and cement stabilization saves the amount by 46.2% and 27.56% respectively. T.Lopez-Lara, J.A. Zepeda-Garrido and V.M. Castario (1999) this paper includes the evaluation of the main index properties of the soil, along with a characterization of the materials through X-ray diffraction. It has concluded that the polyurethane was the one material gives good performance to the soil, with 6% addition. Emhammed. A. Basha, Roslan Hashim and Agus S.Muntohar (1999), This paper presents the chemical stabilisation of soils using cement and rice husk ash. Three types of soils, residual soils, kaolinite and bentonite, were used in the study. Cement and rice husk ash reduced the plasticity of residual soil, kaolin, and bentonite. A considerable reduction was attained by cement-stabilised soils. In general, 6–8% of cement and 10–15% RHA shows optimum amount to reduce plasticity of soils. Reduction in plasticity index is an indicator of improvement. The maximum dry density of cement–stabilised residual soil and kaolin slightly decreased with increases in cement content, in contrast with cement-stabilised bentonite. Adding rice husk ash and cement increase the optimum moisture content of all soils. Azm S. Al-Homoud, Taisir Khedaywi and Abdullah M. Al. Ajlouni (1999), This research was undertaken to compare the effectiveness and economic feasibility of bitumen, lime, and cement as stabilizing agents for reduction of swell potential of a swelling soil from Northern Jordan. The results of this study showed that for a soil containing high percent fines, cutback bitumen treatment causes more reduction in swell potential than cement and less reduction than lime. For this type of soil the bitumen is the economical agent compared to lime and cement. Virender Kumar (2002), A study on the effect of lime as stabilizing agent and Na2 CO3 as an additive, when added to the soil-flyash combination has also been investigated. A combination of 70% soil, 28% flyash , 1% lime and 1% Na2 CO3 has been found to give best results against the optimum use of flyash in soil. In the soil flyash mix, flyash content beyond 15% does not reduce the permeability nor improve the dry density. Anil Misra, Debabrata Biswas and Sushant Upadhyaya (2004), This paper focuses upon the laboratory evaluation of the (1) stabilization characteristics of clay soils blended with self-cementing class C flyash, and (2) residual self-cementation capabilities of ponded class C flyash. In the analyses, it was found that the stabilization characteristics are the functions of curing time, curing condition, and clay mineralogy. Results obtained from the analyses showed that the OMC changes due to the addition of flyash, also the samples rapidly gained the compressive strength and stiffness within 7 days curing period, and the greatest increase occurred in 1 day due to the rapid hydration reaction of class C flyash. Typically the strength tends to increase up to 14 days curing period, and beyond 14 days, the strength retarded. After 28 days the samples became brittle. Costas A.Anagno– stopoulos (2004), In this study, a laboratory test programme was carried out to find out the effect of inclusion of cement and acrylic resin on physical and engineering behavior of a soft clay. A series of tests are conducted with the addition of 5% to 30% of cement contents and acrylic resin of 5% . It is concluded that the development of strength and stiffness for a short curing time (7 days) is delayed significantly because of A.R addition while for long curing time (28 days) the engineering parameters are increased considerably. Also addition of A.R showed a small increase in Cc for all percentages of cement because A.R is having the tendency to keep water in its molecule. S.A.Aiban, H.M.Al-Ahmadi, I.M. Asi, Z.U.Siddique, and O.S.B. Al-Amoudi (2005) The main objective of this study was to upgrade the load carrying capacity of pavements constructed on sabkha soils, using geo textiles, and to assess the effect of geotextile grade, base thickness, loading type (static and dynamic) and moisture condition (as-molded and soaked) on the soil fabric-aggregate (SFA) systems. The effect of geotextile in improving the load-carrying capacity of soil becomes negligible at higher deviator stress (i.e. 200 kPa) level. Also the inclusion of the A-400 geotextile was almost similar to the improvement achieved when adding 6.5% Portland cement. Suksun Horpibulsuk et al. (2006) The characteristics of strength development in cement stabilized low plasticity and coarse-grained soils. A phenomenological model to predict the laboratory and field strength development based on a single trial test is presented in this work.

Experimental Programme

Materials and their Properties

The soil samples used for this study were collected from the following road stretches and their corresponding labels are given against the location. The engineering properties of these samples were found out and suitable stabilization methods have been identified based on these properties. The basic properties are presented in Table 1, 2.

Engineering Properties of Natural Soil

Samples from Pudukkottai district

  • Road stretch from Aranthangi to Kattumavadi at km 8/4 (sample P1)
  • Road stretch from Aranthangi to Kattumavadi at km 14/8 (sample P2)
  • Road stretch from Aranthangi to Kattumavadi at km 24/8 (sample P3)
  • Samples from Thanjavur district
  • Road stretch from Mannargudi to Athirampattinam at km 60/0 (sample T1)
  • Road stretch from Mannargudi to Athirampattinam at km 66/4 (sample T2)
  • Road stretch from Mannargudi to Athirampattinam at km 73/0 (sample T3)
The specific gravity of Pudukkottai and Thanjavur soils are ranging from 2.70-2.75. This range is following the typical specific gravities of Inorganic clay according to IS 2720. Considering the particle size of the natural soil samples in both the locations presence of sand content is high compared to fine contents. Presences of fines in the Thanjavur samples are higher than the Pudukkottai soil. The IS soil classification guidelines were followed to classify the soils. Based on the Consistency limits, both the soils were classified as SC (clayey sand) soil group.

The Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) values for all samples were depicted in Tables 1 and 2. It was observed that the OMC values for the Thanjavur samples are having greater value than Puthukkottai samples. The specific surface area for the fine-grained soil is maximum. So, it is necessary to add more water to get the maximum MDD in the case of Thanjavur soil samples. The Unconfined Compressive strength (UCC) of the natural soils may be expressed in terms of their consistency. Considering the UCC values from Table 1, the P1 and P3 samples were categorized as "very stiff" and P2, T1, T2, T3 are having "stiff" consistency.

Except P2 sample remaining are giving less than 3% California Bearing Ratio values. So, the natural soil strength may be improved by adding Cement.

Properties of Cement

Experimental Programme

The soil samples from Pudukkottai district are mixed with different percentages of cement as 2, 4, and 6, similarly the samples from Thanjavur district are mixed with 1,2,3 and 4% of cement have been subjected to different laboratory tests. According to the Indian standards the consistency limits, compaction characteristics, unconfined compressive strength, California Bearing Ratio tests were conducted on various mixes. The test results are presented in Table 3, 4 respectively.

Engineering Properties of Natural Soil

The UCC test was conducted on cylindrical specimens are prepared at Optimum Moisture Content. In natural and stabilized form the specimens were tested in compression testing machine immediately after moulding. The CBR tests were conducted for four days soaking period only.

Results and Discussions

Consistency limits

From Table 3, 4, it was observed that the Plasticity index decreased from 7 to NP with 6% cement addition in Puthukkottai district samples. The samples P1 and P2 are showing NP state at 6% cement addition. But sample P3 shows the reduction at 4% cement itself. The PI reduction is occurring at 4% cement in T2 and T3 samples and at 3% cement in T1 sample. The addition of cement increases the plastic limit and decreases the liquid limit. The PI is a measure of a soil's cohesive properties and is indicative of the amount and nature of clay mineral. The substantial reduction of Plasticity Index values indicates not only an improvement in the volume change characteristics but also modification of the soils into more stable and workable material. The effects of cement on consistency limits of all the soil samples are graphically represented in Fig 1-6.

Engineering Properties of Natural Soil
Engineering Properties of Natural Soil
Figure 1: Plasticity Index Vs various Cement Content of sample P1
Figure 2: Plasticity Index Vs various Cement Content of sample P2
Engineering Properties of Natural Soil
Engineering Properties of Natural Soil
Figure 3: Plasticity Index Vs various Cement Content of sample P3
Figure 4: Plasticity Index Vs various Cement Content of sample T1
Engineering Properties of Natural Soil
Engineering Properties of Natural Soil
Figure 5: Plasticity Index Vs various Cement Content of sample T2
Figure 6: Plasticity Index Vs various Cement Content of sample T3

Proctor Compaction Test

The results of standard proctor test on soil treated with different percentage of cement are shown in Table 3, 4 for Puthukkottai and Thanjavur district samples respectively. The Pudukkottai soil samples are having the OMC values in the range of 7.2-8.8%. But the stabilized samples are showing a range of 7.6-9.9%. The Optimum Moisture Content increases with increase of cement content in all soil samples. It is observed that the dry density decreases in all soil samples with increase in cement content.This is due to the basic fact that the soil-cement mix may have difference in specific gravity than the original one. Also the addition of water causes the bulking phenomenon in the stabilized soil. During this time the capillary forces resisting the rearrangement of particles against the external compactive energy. The fine cement particles influence the compatibility of soil-cement material. This soil-cement interaction resulting in the cementitious products also it gains strength.

California Bearing Ratio

The effect of cement on California bearing Ratio of the stabilized soils are depicted in Table 3,4 and represented in Figure 7-12. It is observed that the addition of 6% cement increases the CBR value of the Puthukkottai natural soil (P3) from 2.90% to 135%. Similarly, addition of 4% cement increases the CBR value of the Thanjavur natural soil (T3) from 2.50% to 68%. The reason for this strength improvement is the pozzolanic action in soil-cement material. The cementitious reaction between cement and clay takes place as primary and secondary processes. Hydration of the cement is regarded as the primary reaction and forms the normal hydration products that bind particles together. In the secondary process, the fresh calcium hydroxide formed in the primary phase reacts with the silica and alumina in the clay to form additional cementitious material.

Engineering Properties of Natural Soil
Engineering Properties of Natural Soil
Figure 7: CBR value comparison chart for various Cement Content for sample P1
Figure 8: CBR value comparison chart for various Cement Content for sample P2
Engineering Properties of Natural Soil
Engineering Properties of Natural Soil
Figure 9: CBR value comparison chart for various Cement Content for sample P3
Figure 10: CBR value comparison chart for various Cement Content for sample T1
Engineering Properties of Natural Soil
Engineering Properties of Natural Soil
Figure 11: CBR value comparison chart for various Cement Content for sample T2
Figure 12: CBR value comparison chart for various Cement Content for sample T3

Unconfined Compressive Strength

The Unconfined Compressive (UCC) strength values are increased from 2.34 to 5.76 kg/cm2 at 6% cement on Puthukkottai district sample (P3) are given in Table 3 and Figure 13-15. Addition of 4% cement shows a significant improvement in UCC values on Thanjavur samples (T2) from 2.45 to 5.65 kg/cm2 are given in Table 4 and Fig 16-18.

The cement treated specimens exhibit notably higher strength than do natural soil specimens. It was also observed that the cement treated soils achieved higher strength at lower strain when compared to the natural specimens. The higher strength is attributed to the presence of cementation bonds in cement treated specimens.

Engineering Properties of Natural Soil
Engineering Properties of Natural Soil
Figure 13: UCC comparison chart for various Cement Content for sample P1
Figure 14: UCC comparison chart for various Cement Content for sample P2
Engineering Properties of Natural Soil
Engineering Properties of Natural Soil
Figure 15: UCC comparison chart for various Cement Content for sample P3
Figure 16: UCC comparison chart for various Cement Content for sample T1
Engineering Properties of Natural Soil
Engineering Properties of Natural Soil
Figure 17: UCC comparison chart for various Cement Content for sample T2
Figure 18: UCC comparison chart for various Cement Content for sample T3

Conclusions

The following conclusions are drawn from this research work
  • As per Indian Standard based on the consistency limits of the natural soil samples from Puthukkottai and Thanjavur are classified as SC (clayey sand).It was identified that Cement is the suitable stabilizer for this SC soil group.
  • The PI values are ranging from 8 to 9 only. This shows that the amount of clay content is lower than the silt content. Considering the state of plastic of the soils they are falling in 'low' plastic category.
  • Plasticity Index of the all samples from Puthukkottai samples are reduced with the addition of 2-6% cement. With the addition of 4% cement the plasticity index of all the samples from Thanjavur comes down to approximately 0. At this plasticity index soils are quite friable and workable.
  • The maximum dry density decreases and optimum moisture content increases with the addition of Cement. The Pudukkottai soil samples are having the OMC values in the range of 7.2-8.8%. But the stabilized samples are showing a range of 7.6-9.9%.
  • The cement required to get 10% CBR is 1%. Addition of 6% cement gives a maximum CBR values in the range of 126-135% on Puthukkottai samples. Similarly, 4% cement gives a maximum value in the range of 66-68% on Thanjavur samples. As the cement is a costlier one, using a small quantity will economize the road project.

References

  • Manikant Mandal and Dr. Mayajit Mazumdar (1995), "A Study on the effect of sodium carbonate as an additive to stabilized soil," Indian Highways, December 1995, pp. 31–36.
  • Dr. A. Arumugam and K. Muralidharan (1997), "Optimi- sation of Pavement construction cost on stabilized soil subgrade," Indian Highways, March 1997, pp. 33–42.
  • T.Lopez-Lara, J.A. Zepeda-Garrido and V.M. Castario (1999), "A comparative study of the effectiveness of different additives on the expansion behavior of clays," www.ejge.com
  • Emhammed. A. Basha , Roslan Hashim and Agus S.Muntohar (1999), "Effect of the cement–Rice husk ash on the Plasticity and compaction of soil," www.ejge.com
  • Azm S. Al-Homoud, Taisir Khedaywi and Abdullah M. Al. Ajlouni (1999), "Comparison of effectiveness and economic feasibility of bitumen, lime and cement as stabilizing agents for reduction of swell potential of a clayey soil," Indian Highways, January 1999, pp.51–58.
  • Prof (Dr) Virender Kumar, (2002), "Compaction and permeability study of a soil stabilised with Flyash, Lime and Na2CO3 ", Journal of The institution of Engineers" Volume 82, Febraury 2002, pp. 173–176.
  • Anil Misra, Debabrata Biswas and Sushant Upadhyaya (13 Decemeber 2004), "Physio- mechanical behavior of self cementing class C flyash-clay mixtures," www.sciencedirect.com
  • Costas A.Anagnostopoulos (2004), "Physical and Engineering Properties of a cement stabilized soft soil treated with Acrylic Resin additive," www.ejge.com
  • S.A.Aiban, H.M.Al-Ahmadi, I.M. Asi, Z.U.Siddique, and O.S.B. Al- Amoudi (8 March 2005), "Effect of geotextile and Cement on the performance of sabkha subgrad," www.sciencedirect.com.
  • Suksun Horpibulsuk et al. (2006), "Strength Development in Cement stabilized low plasticity and Coarse grained soils: Laboratory and Field Study," Soils and Foundation, vol.46,No.3, pp.351–366.
  • IS 2720 (part 4)-1984 Test For Grain size Analysis
  • IRC 37–2001, Guide lines for the Design of flexible pavements (second revision)
  • IS 2720 (part XXI)–1976 (Test for soils)
  • IS 2720 (part XXII)–1972 (Test for soils)
  • IS 2720 (part XXVII)–1977 (Test for soils)
  • IS 2720 (part XXIII)–1976(Test for soils)

NBMCW January 2009

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Splitting Tensile Strength of HSSFRC wi...

Investigations on the Splitting Tensile Strength of High-Strength Steel Fiber Reinforced Silica Fume Concrete

P. Ramadoss, V. Prabakaran, Faculty, Department of Civil Engineering, Pondicherry Engineering College, Puducherry, and K. Nagamani, Professor, Structural Engineering Division, Anna University, Chennai

This paper presents the investigations on the contribution of steel fibers on the splitting tensile strength of high-strength steel fiber reinforced concrete (HSSFRC) with silica fume replacement. For 32 series of concrete mixes, splitting tensile strengths were determined at 28 days. The variables investigated were steel fiber volume fractions (0 to 1.5%), silica fume replacement level (SF = 5% and 10%) and matrix composition (w/cm ratios ranging from 0.25 to 0.40). The influence of fiber content in terms of fiber reinforcing index on the splitting tensile strengths of HSFRC is presented. Based on the experiments data, using linear regression analysis, empirical expression was developed to predict 28-day tensile strength of HSSFRC in terms of fiber reinforcing index and the absolute variation obtained was within 5%. Relationship between splitting tensile strength and compressive strength has been developed using regression analysis. The experimental values of previous researchers were compared with the values obtained experimentally.

Mix proportions for HSSFRC

Introduction

The use of fibers in concrete has gained momentum in the infrastructural and industrial applications. Studies have shown increasing evidence that brittle behavior of HSC/ HPC can be overcome by the addition of short steel fibers of small diameter in the concrete mix (Hsu and Hsu, 1994). Balaguru and Shah (1992) and ACI 544. 1R- 1996 have reported that the addition of steel fibers in concrete matrix improves all mechanical properties of concrete especially tensile strength, impact strength, and toughness and ductility. An understanding of the tensile strength properties of steel fiber reinforced concrete (SFRC) and its variation with fiber content is an important aspect of successful design. The combined use of superplasticizer, and supplementary cementing materials (SCM) such as silica fume having pozzolanic reaction and filler effect which will in turn improve the interface of the materials, thereby enhancing the strength of the concrete, can lead to economical high-performance concrete (HPC) with enhanced durability. Banja and Sengupta (2005) have observed that silica fume incorporation in concrete results in significant improvement of the tensile strength along with compressive strength.

The relationship between splitting tensile strength and fiber reinforcing index for predicting 28-day strength at any water-to-cementitious materials ratio and fiber content in terms of fiber reinforcing index is quite limited. Wafa and Ashour (1992) have developed equations for predicting the influence of fiber contents on strength properties (modulus of rupture, splitting tensile strength and compressive strength) of HSFRC with w/c = 0.25. Nataraja et al. (2001) have developed expressions for predicting 28-day tensile strength using regression analysis as a function of fiber reinforcing index (RI). In all the empirical equations only a particular w/cm ratio with varying fiber content was used. However, rather absolute strength values have been dealt with in all the models and thus are valid for a particular w/cm or w/c ratio and specimen parameter considered. Ramdadoss and Nagamani (2007) have considered non-dimensional parameter for developing mathematical model for predicting 28-day compressive strength of HPFRC with wide range of w/cm ratios.

The aim of the present investigation was to develop the expression to predict 28-day splitting tensile strengths of HSSFRC for a wide range of w/cm ratios, which may serve as the useful tool to quantify the effect of fiber addition on strength in terms of fiber reinforcing index. This paper presents an experimental investigation on the splitting tensile and compressive strength properties of HSC with w/cm ratios ranging from 0.25 to 0.4 and studies the effect of inclusion of steel fibers (fiber volume fractions, Vf = 0.5%, 1% and 1.5%) on improving these properties.

Materials and Methods

Materials, Mixture proportions, and Preparation of specimens

Ordinary Portland cement-53 grade having 28-day compressive strength of 54.5MPa, and satisfying the requirements of IS: 12269–1987. Condensed silica fume (Grade 920-D) contained 88.7% of SiO2, having specific surface area of 23000 m2/ kg, specific gravity of 2.25, and fineness by residue on 45 micron of 2% conforming to ASTM C1240-1999.

Locally available river sand as fine aggregate conforming to grading zone-II of IS: 383-1978 was used. It has fineness modulus of 2.65, specific gravity of 2.63, and water absorption of 0.98 % @ 24 hrs. Coarse aggregate of crushed granite stone with 12.5 mm maximum size, conforming to IS: 383-1978 was used. The characteristics of coarse aggregate are:

Specific gravity = 2.70; Fineness modulus = 6.0; Water absorption= 0.65 % @24hrs.

Superplasticizer of sulphonated naphthalene formaldehyde (SNF) condensate as HRWR admixture conforming to ASTM Type F (ASTM C494) and IS: 9103-1999 was used. Crimped steel fibers (conforming to ASTM A820-2001) used, having diameter =0.45 mm and length = 36 mm, giving an aspect ratio of 80, and Ultimate tensile strength (fu) = 910 MPa and elastic modulus (Ef) = 200 GPa.

Mixtures were proportioned using guidelines and specifications given in ACI 211.4R–1993, recommended guidelines of ACI 544.3R- 1993. Mixture proportions used in the test program are summarized in Table 2. This aspect of work has been carried out elsewhere (Ramadoss and Nagamani, 2008). For each water-cementitious materials ratio, 6 fibrous concrete mixes were prepared with three fiber volume fractions, Vf= 0.5 and 1.0 % by volume of concrete (39, 78 and 117.5 kg/m3). Slump value obtained was 75 ± 25 mm for silica fume concrete mixes and VeBe value was 12 ± 3 sec for fibrous concrete mixes.

Concrete was mixed using a tilting type mixer and specimens were cast using steel moulds, compacted by using table vibrator. For each mix at least six 150 x 300 mm cylinders were prepared. Specimens were demoulded 24 hours after casting and water cured at 27±2 oC until the age of testing at 28 days. All the specimens were water cured in the same curing tank to maintain uniform curing of specimens.

Splitting Tensile Strength of High-Strength Steel Fiber Reinforced Silica Fume Concrete

Testing for Strength

Splitting tensile strength

Splitting tensile strength tests were conducted according to the specification of ASTM C496-1990 using 150 x 300 mm cylindrical specimens. The tests were conducted in a 1000 kN closed loop hydraulically operated Universal testing machine. Plywood pieces (as packing material) of size 25 x 3 mm were placed on both loading and reaction faces of specimens before testing. Three samples were used for computing the average strength.

Compressive strength

Compressive strength tests were performed according to ASTM C39-1992 standards using 150 mm diameter cylinders without capping, loaded uniaxially. Each strength value was an average of there specimens. Compressive strength ranging from 46.9 to 80.4 MPa as obtained for various hardened concrete mixes is given in Table 2.

Results and Statistical Modeling

Splitting tensile strength

Table 2 presents the variation of the splitting tensile strength, fspf on the effect of fiber content in terms of fiber reinforcing index (RI). It is observed from the test results given in Table 2 that there is a significant improvement in splitting tensile strength due to increase in fiber content from 0 to 1.5% (RI = 0 to 3.88) for all the mixes and the variation is 24-56 percent over reference concrete. An increase in strength of 55.9% for 1.5% fiber content (RI = 3.88) and 45.4% for 1% fiber content (RI = 2.58) of concrete was obtained, revealing that ductility of fiber reinforced concrete has improved considerably. Percentage increase in splitting tensile strength for different fiber volume fractions of concrete mixes with w/cm ratios ranging from 0.25 to 0.40 is shown in Figure 1.

Mix proportions for HSSFRC
Compressive strength of SFRC
Figure 1: Percentage increase in splitting tensile strength for different fiber volume fractions (RI = 0 to 3.88)
Figure 2: Splitting Tensile Strength, fspf as a function of compressive strength, fcf

The relation between split tensile strength and compressive strength of steel fiber reinforced concrete (SFRC) is shown in Figure 2. The empirical equation obtained for SFRC with correlation coefficient, r = 0.86 is given as:

fspf= 0.105fcf0.97 (1)

Strength Prediction Model

Table 2 presents the splitting tensile strength ratios of fiber reinforced and plain concrete (silica fume concrete). Figure 3 shows splitting tensile strength ratios, (fspf / fsp) as a function of the fiber reinforcing index, RI of the concrete. These ratios can be utilized for the development of the generalized expressions, which are free from the influence of varying w/cm ratios and specimen parameters, for predicting the tensile strength. The validity of the model was investigated by examining relevant statistical coefficients (Bhattacharya, 1977).

Mix proportions for HSSFRC
Figure 3: Relationship between splitting tensile strength ratio and fiber reinforcing index
Based on the test results, using the linear regression analysis, the splitting tensile strength ratio (fspf / fsp) of HSFRC and SF concrete may be predicted in terms of fiber reinforcing index, RI as follows:

fspf / fsp= 1+0.150 (RI) (2)

where

fspf = Splitting tensile strength of HSFRC, MPa;

fsp = Spliting tensile strength of SF concrete, MPa

The values of the correlation coefficient (R) and the standard error of estimate (s) have been obtained as 0.96 and ­­0.274, respectively. The percent variation in absolute has been obtained as 3.092.

Comparison of Results of Previous Researchers

Yao et al. (2003) have obtained splitting tensile strength improvement for FRC using steel-hooked end, carbon-straight and polypropylene (PP)-straight fibers at volume fraction, vf= 0.5% for each fiber are 10.1%, 17% and 5% respectively, compared to control concrete. Song et al. (2005) have obtained tensile strength improvement for FRC using nylon-straight fiber of vf= 0.5% and polypropylene - straight fiber of vf= 0.66% are 17.05% and 9.68% respectively, to that of no fiber concrete. Chen and Liu (2005) have obtained tensile strength improvement for FRC with 9.1% silica fume replacement using steel-crimped and carbon-straight fibers at vf= 1% for each fiber are 23.9%, 15.9% respectively, and for concrete with polypropylene-straight fiber at vf= 1%, no improvement (i.e., -2.3%) in tensile strength was obtained compared to reference concrete. Sivakumar and Santhanam (2007) have obtained splitting tensile strength improvement for fiber reinforced concrete with 7% SF replacement using steel-hooked, PP-straight, polyester and glass fibers at vf= 0.5% for each fiber are 26.8%, 7.3%, 14.6% and 4.9% respectively, compared to control concrete. Choi and Yhan (2005) have obtained splitting tensile strength improvement for glass FRC and polypropylene FRC with vf= 1% and 1.5% for each fiber are 34.1% and 37.2%, and 43.9% and 43.9% respectively, compared to plain concrete. The values obtained experimentally are comparable with the reported results of the researchers discussed above.

Mode of Failure under Tension

Mix proportions for HSSFRC
Figure 4: Failure pattern of cylinder specimens under split tension
As a result of the inclusion of fibers into the plain concrete matrix, the brittle mode of failure is changed into a ductile mode as fibers increase the energy absorption capacity and enhance the lateral stiffness to the section. After the occurrence of the cracks, the specimen did not fail suddenly. Instead, the crack propagates due to the bonding of the fibers into the concrete matrix and offers pulling force in the lateral direction to restrain cracking, thereby prolonging the failure state of the specimens. Figure 4 shows failure pattern of cylinder specimens under splitting tension.

Conclusion

Based on the experimental investigations on HSSFRC incorporating silica fume with w/cm ratios ranging from 0.25 to 0.40, the following conclusions can be drawn.
  • The brittle mode of failure is changed by addition of steel fibers into high-strength concrete, into a comparatively ductile one and it was observed to improve its energy absorption capacity.
  • The addition of steel fibers by 1.50 percent volume fraction (RI=3.88) results in an increase of 55.9 percent in the splitting tensile strength compared with un-reinforced matrix.
  • Moderate improvement in compressive strength was obtained for HSFRC.
  • Statistical model developed found to give good correlation with experimental values.

References

  • Hsu, L.S. and Hsu, C.T.T., "Stress-strain behavior of steel fiber reinforced high-strength concrete under compression," ACI Materials Journal, 91[4] [1994], p. 448-457.
  • Balaguru, N. and Sha, S.P., Fiber reinforced concrete composites, McGraw Hill international edition, New York, 1992, p.179- 214.
  • ACI 544.1R-96, "State-of-the-art report on fiber reinforced concrete," ACI Manual of Concrete Practice, 1996.
  • Bhanja, S. and Sengupta, B., "Influence of silica fume on the tensile strength of concrete," Cement and Concrete Research, 35 [2005], p. 743-747.
  • ACI 363R-92, State-of-the-art report on high strength concrete, ACI Manual of Concrete Practice, 1992.
  • Wafa, F.F. and Ashour, S.A., "Mechanical properties of high-strength fiber reinforced concrete," ACI Materials Journal, 89[5] [1992], p. 445- 455.
  • Nataraja, M.C. Dhang, N. and Gupta, A.P., "Splitting tensile strength of steel fiber reinforced concrete," Indian Concrete Journal, 75[4] [2001], p. 287- 290.
  • Ramadoss, P. and Nagamani, K., "Investigations on the compressive of high-performance fiber reinforced concrete," Proceedings of international conference on recent developments in structural engineering, MIT, Manipal, India, 2007.
  • IS: 383-1970, Specification for coarse and fine aggregates from natural sources for concrete, Bureau of Indian standards, New Delhi, India.
  • ACI 211.4R-93, Guide for selecting proportions for High strength concrete with Portland cement and Fly ash, American Concrete Institute, Detroit, 1999.
  • ACI 544.3R-93, "Guide for specifying, mixing, placing and finishing steel fiber reinforced concrete," American Concrete Institute, Detroit, 1999.
  • Ramadoss, P. and Nagamani, K., "Tensile strength and durability characteristics of high-performance fiber reinforced concrete," The Arabian Journal for Science and Engineering, 33[2B], [2008], p. 307- 319.
  • ASTM C496-1990, Standard test method for split tensile strength of cylindrical concrete specimens, Annual book of ASTM standards, American Society for Testing and Materials, USA.
  • ASTM C 39-1992, Standard test method for compressive strength of fiber reinforced concrete, Annual book of ASTM standards, American Society for Testing and Materials, USA.
  • Bhattacharya, G.K. and Johnson, R.A., Statistical Concepts and Methods, Wiley, New York, 1977.
  • Yoa, W., Li, J. and Wu, K., "Mechanical properties of hybrid fiber-reinforced concrete at low fiber volume fraction," Cement and Concrete Research, 33[2003], p. 27-30.
  • Song, P.S. Hwang, S. and Sheu, B.C., "Strength properties of nylon- and polypropylene-fiber-reinforced concrete," Cement and Concrete Research, 35[2005], p. 1545-1550.
  • Sivakumar, A. and Santhanam, M., "Mechanical properties of HSC reinforced with metallic and non- metallic fibers," Cement and Concrete Composites, 29[2007], p. 603-608.
  • Chen, B. and Liu, J., "Contribution of hybrid fibers on the properties of high-strength lightweight concrete having good workability," Cement and Concrete Research, 35[2005], p. 913-917.
  • Choi, Y. and Yuan, R.L., 'Experimental relationship between splitting tensile strength and compressive strength of GFRC and PFRC,' Cement and Concrete Research, 35[2005], p. 1587-1591.

NBMCW February 2009

.....

Role of Additives in Optimization of Fl...

Flyash in Cement

S.K Agarwal and Preeti Sharma, Central Building Research Institute, Roorkee
This paper deals with the effect of various additives in optimizing the flyash content in portland cement. The study showed that with the use of various additives like naphthalene–based superplasticizer, calcium nitrite, calcium formate or blend of these additives. It is possible to load up to 35% of flyash in ordinary Portland cement compared to around 25% of flyash present in Indian portland pozzolana cement. The 35% limit is the upper limit fixed by Indian standard. The 28-day compressive strength of cement with 35% flyash incorporating various additives is comparable to control PPC. Effect of flyash replacement by sand has also been studied.

Introduction

India at present produces about 120 million tons of cement, out of which 30-40 million tons of cement is blended. The major blended cements are predominantly Portland pozzolana PPC (flyash), and Portland slag (PSC). About 30% of flyash is being utilized by various agencies. Thus, there is tremendous pressure for its utilization from environmental angle. The portland pozzolana cement is being sold in the market as 53MPa, composite cement etc. This PPC contains 20-25% flyash. The cement manufacturers are envisaging that by 2010 the share of the blended cements will be about 60% with major share of PPC. It is only possible if more fly ash is loaded in OPC. Keeping this in view, there is scope of loading more flyash in OPC. In India, up to 35% flyash is permitted under BIS 1491-1991.

Many research workers have used additives either as grinding aids for clinker or improving the properties of cement. Effect of additives on individual components of cements have been studied (1-6) extensively to know the hydration behavior.

This paper presents the results of an investigation on the effect of additives in optimizing flyash content in OPC. The compressive strength of cement mortars incorporating two grades (I&II, BIS 3812-2003) of flyash has been chosen for the present study.

Experimental

Flyash in Cement
Materials
53 grade OPC conforming to BIS 12269 and PPC (29% flyash) was used in present study. The physical and chemical properties of OPC and PPC cements are given in Table 1.

Two flyashes one from Dadri and other from Ropar were taken for the present study. The physical and chemical properties of flyash (Dadri and Ropar) are also given in Table 1.

Powdered naphthalene sulfonated formaldehyde condensate, calcium nitrite and calcium formate were procured from the local market.

Method

Stock of Ordinary Portland cement with various percentage of flyash was blended thoroughly using powder mixer.

Casting of 12.5mm cement cubes (OPC) with various percentages of flyash (Ropar and Dadri) were cast with and without additive (SNF) at water content required for the consistency of control cement. The cubes were demolded after 24 hrs. and kept in water at 27 ± 2oC. The compressive strength at 1, 3, 7 and 28 days is given in Table2-3.

The effect on compressive strength by replacing flyash with sand has also been studied and results are given in Table 4.

The casting of 50mm cubes of mortar (1:3) with and without additives (SNF, calcium formate and calcium Nitrate) were cast and cured at 27±2ºC. The compressive strength at 1, 3, 7 and 28 days were obtained. The results are given in Table 5.

Scanning electron micrograph of Dadri and Ropar flyash was determined and shown in figure 1 and 2.

The X ray of both the flyashes was also taken and shown in figure 3 and 4.

The pozzolanic reactivity index of Dadri flyash is 120 and Ropar flyash is 90. The dose of SNF was optimized and found 0.5% by weight of cement in the present study.

Results and Discussion

Effect of additive (SNF) on compressive strength of cement incorporating Ropar flyash:
Table 2 presents the compressive strength data of various percentages of flyash cement with and without additive. The control cement contains 29% flyash, which was supplied by the manufacturer. It is clear from the table that with addition of 0.5% additive (sulfonated formaldehyde condensate powder), up to 35% flyash blended cement, the compressive strength at various ages of 1, 3, 7 and 28 days are comparable to the control cement. However, when fly ash content is increased to 40%, the 3 and 7 day compressive strength is about 25-30% lowers than the control, however at 28 days, it is only 5% lower when compared to control.

Effect of SNF on Compressive Strength of Cement Incorporating Dadri Flyash:

Flyash in Cement
Table 3 presents data of compressive strength with and without additive of cement with various percentages of Dadri flyash. It is evident from table 3 that with the use of 0.5% of SNF, it is possible to increase the flyash content up to 35% in OPC. The compressive strength of cement with Dadri flyash at various ages has been found to be more than cement with Ropar flyash. This is because of higher surface area and pozzolanic activity of Dadri flyash than the Ropar flyash.

Effect of Sand Replacement

In order to find out the contribution of flyash in the strength development of cement, flyash was replaced by sand in the 35% mix. In case of Ropar flyash at 28 days, the compressive strength of cement without additive, there is drop of 54% and 49% with additive. In case of Dadri flyash also the drop in compressive strength is similar. It can be concluded from the observation that the contribution of flyash in strength development is 54 and 49%.

Effect of SNF on Mortar

Table 5 represents the data of 1:3 mortars. The compressive strength of control system with 29% flyash has been compared with mortar with 35% flyash cement and with 0.5% additive. It is clear from the table that compressive strength at 1, 3, 7 and at 28 days it is about 5% more up to 33% flyash cement and it is at par with control up to 35%. When calcium formate and calcium nitrite is added along with SNF the 1-day strength has been found to be 50 and 30% more than control. However, at 28 days, it is similar to control system.

The addition of superplasticizer in cement disperses the cement particles and allows the glassy content of the fly ash to react effectively with the lime liberated by the hydration of the cement to form CSH.

The increase in flyash content from 29% to 35% in a cement plant of 5000 tons capacity per day, extra 462 tons flyash can be loaded. It has been estimated that including the cost of superplasticizer (0.5%), it is possible to save around $2100 per day. It is not only the saving in terms of cost but from environmental point of view.

Conclusion

  1. It is clear from the present studies that with the addition of powdered SNF. It is possible to increase the flyash content up to 35% in OPC.
  2. The addition of flyash has no negative effect on the compressive strength of cement up to 35%.
  3. The study will help to utilize more flyash in OPC, thereby helping to save limestone used in the manufacture of clinker and hence environmental pollution caused due to CO2in the clinker production.
Flyash in Cement

Acknowledgment

The authors are thankful to Director, Central Building Research Institute, Roorkee for pursuing the work in the lab. The paper is being published with the permission of Director, CBRI, Roorkee.

NBMCW March 2009

.....

Influence of Replacement of Cement by M...

Influence of Replacement of Cement by Micro Silica on Flexural Strength of Fibrous Ferrocement made from GI Fibers

N. K. Patil, Assistant Professor, Dept of Civil Engineering; D.Y. Patil, College of Engineering and Technology,Kolhapur, and Dr. K B.Prakash, Professor and Head of Civil Engineering Department, K.L.E. Society's, College of Engineering and Technology, Belgaum
Applications of ferrocement are vast increasing. Ferrocement is lightweight cement composite. It does not require any special construction technique and skilled labor. It has its cracking resistance, ductility and fatigue resistance higher than those of concrete. In addition, impermeability of ferrocement elements is far superior to that of ordinary concrete. It is used for variety of structures such as prefabricated residential units, marine structures and industrial structures. Similarly, fiber reinforced concrete (FRC) has also wide applications. FRC possesses higher compressive strength and toughness, increased resistance to wear and tear, and higher post cracking strength. But these two materials have some limitations also. These two materials cannot be employed where high vibrations, high tensile forces and high impact are to be resisted.

Fibrous ferrocement, which is a combination of ferrocement and fiber reinforced concrete, shows better improvement in some of mechanical properties, such as toughness, impact resistance. This new composite also shows higher compressive strength, higher tensile and impact strength.

Besides strength properties, the performance against secondary effects like temperature and shrinkage also play an important role in the durability of concrete. Use of pozzolanic materials like micro silica can improve the strength properties, serviceability and durability of concrete. The replacement of cement to some extent by such pozzolanic material can also achieve economy.

In this paper, an attempt has been made to study the effect of replacement of cement by micro silica on the flexural strength of fibrous ferrocement using GI fibers. The percentages of GI fibers were varied as 0%, 0.5%, 1.0%, 1.5%, and 2.0%. The cement was replaced by micro silica in different percentages like 5%, 10%, 15%, 20%, 25%, and 30%.

Introduction

Along with many advantages of concrete, it has deficiencies like low tensile strength, prone to cracking, low post cracking capacity, brittleness, low ductility, not capable to accommodate large deformations and low impact strength. These deficiencies lead concrete to be brittle material with low tensile strength and limited ductility. The contribution of conventional steel reinforcement in RCC structural elements to take care of tensile stresses is limited only in its own plane.1

Ferrocement is a particular type of reinforced concrete although the extent of investment is relatively less in case of ferrocement construction. Because of similarity in the nature of the ingredients of conventional concrete and ferrocement, the durability characteristics are expected to be similar, even though the mechanical characteristics may be quite different.2

In terms of structural behavior, ferrocement exhibits very high tensile strength-to-weight ratio and superior cracking performance. The distribution of small diameter wire mesh reinforcement over the entire matrix surface provides a very high resistance against cracking. Moreover engineering properties such as toughness, fatigue resistance, impermeability, etc are considerably improved. Sometimes, conventional reinforcing bars in the skeleton form are added to thin wire meshes in order to achieve a stiff reinforcing cage.3

FRC is a composite material consisting of cement, sand, metal, water and fibers. In this composite material, short discrete fibers are randomly distributed trough out the concrete mass. The behavioral efficiency of this composite material is far superior to that of plain concrete and many other construction materials of same cost. Due to this benefit, the use of FRC has steadily increased during last two decades and its current field of application includes airport and highway pavements, earthquake resistant and explosive resistant structures, mines and tunnel linigs, bridge deck overlays, hydraulic structures, rock slope stabilization. Extensive research work on FRC has established that the addition of various types of fibers such as steel, glass, synthetic and carbon, in plain concrete improves strength, toughness, ductility, and post cracking resistance, etc.4

When the loads imposed on concrete approach that for failure, cracks will propagate, sometimes rapidly, fibers in concrete provide means of arresting the crack growth. Cosequently, the real advantage of using of fibers in concrete can be seen after matrix cracking. These types of materials are useful if a large amount of energy absorption capacity is required to prevent the failure. Reinforcing steel bars in concrete have the same beneficial effect because they act as long continuous fibers. However short discontinuous fibers have the advantage of being uniformly mixed and dispersed throughout the concrete. Fibers are added to a concrete mix, which normally contains cement, water and fine and coarse aggregate.4

The major advantages of fiber reinforced concrete are resistance to microcraking, impact resistance, resistance to fatigue, reduced permeability, improved strength in shear, tension, flexure and compression.5 Fibers influence the mechanical properties of concrete or mortar in all modes of failure, especially those that induce fatigue and tensile stress. The strengthening mechanism of fibers involves transfer of stress from matrix to the fiber by interfacial shear or by interlock between the fibers and the matrix. 6 Addition of steel fibers in concrete mix significantly improves the cracking behavior and ultimate strength of deep beams. Addition of steel fibers in concrete results in an increase of beam stiffness.7 In addition to increase in flexural strength, a considerable increase in toughness is also imparted by the fibers.8

The fiber reinforced concrete is also gaining more importance these days especially in the earthquake resistant structures, where ductility plays an important role.9

In spite of many advantages of FRC, it has some limitations also. It cannot be employed where high impact, high vibration and high wear and tear is expected. Many problems have to be faced during construction of FRC, especially when quantity of fiber used is more. The fibers if put in bulk along with other ingredients do not disperse but nest together. This phenomenon is called balling effect. The balling effect may be reduced to some extent by mixing the fibers and other ingredients in dry form and then adding water. The fibers placed in concrete, may block the discharge port. Since the flow of FRC is low, the FRC has to be placed near to the place where it is to be used finally.10

The fibrous ferrocement, which is a combination of fiber reinforced concrete and ferrocement, can overcome most of the limitations of the FRC and ferrocement. And it can be used with assurance where high impact, high vibration and high wear and tear are expected. In this new material advantages of both FRC and ferrocement are combined.10

Research Significance

Eventhough, performance of fiber reinforced concrete in airfield, pavements, industrial floors and machine foundations is satisfactory, it suffers from some limitations. It cannot be employed where high impact, high vibrations and high wear and tear are expected. Similar to fiber reinforced concrete the ferrocement also have many advantages and its applications are rapidly increasing. The major limitation in ferrocement is that the percentage of reinforcement cannot be increased beyond certain limit. This limitation affects the strength of ferrocement and it cannot be employed where high impact or high load are expected.

The fibrous ferrocement, which is combination of ferrocement and fiber reinforced concrete, can overcome to some extent the limitations offered by ferrocement and fiber reinforced concrete and even improve some of the mechanical and strength properties of ferrocement.

Replacing cement by pozzolanic material like micro silica in fibrous ferrocement, not only its strength gets enhanced but serviceability and durability characteristics can also be improved. The replacement of cement by micro silica can achieve economy too. Thus fibrous ferrocement can be effectively used as an economical construction material.

Experimental Programme

The main aim of this experimentation is to study the effect of replacement of cement by micro silica on the flexural strength of fibrous ferrocement produced with GI fibers. The percentages of GI fibers were varied as 0%, 0.5%, 1.0%, 1.5%and 2.0% by volume fraction of total mix. The cement was replaced by micro silica in different percentages like 5%, 10%, 15%, 20%, 25% and 30%.

Replacement of Cement by Micro Silica

Ordinary Portland cement of 53 grade and locally available sand with specific gravity 2.64 and fineness modulus 2.91 was used in experimentation. To impart additional workability a superplasicizer (Algisuperplast – N), 0.5 % by weight of cement was used. The GI fibers were made available by cutting GI binding wires of diameter one mm. The aspect ratio adopted for fibers was 40. The different percentages of GI fibers used in the experimentation were 0%, 0.5%, 1.0%, 1.5% and 2.0% by volume fraction. The welded mesh (WM) used in the experimentation was having square opening of 25 mm with 2.5 mm diameter. The chicken mesh (CM) used was having a hexagonal opening with 0.5 mm diameter. The cement mortar with a proportion of 1:1 was used with a water cement ratio of 0.4.

To study the effect of replacement of cement by micro silica on flexural strength of fibrous ferrocement, the flexural test specimens of dimension 100x100x500 mm were cast and tested under two point loading on UTM of 1000 kN capacity. The required size of welded mesh and chicken mesh were first cut according to the mould size for flexural test test. The chicken mesh was tied to the welded mesh using GI binding wires at regular intervals. This formed cage of (1WM+1CM) with specific area 49345 mm2. Similarly cages of (1WM+2CM) and (2WM+2WM) were prepared with specific surface areas 60914 mm2 and 98690 mm2 respectively.

Now the fibrous mortar was prepared by mixing the required percentage of GI fibers into the cement mortar prepared by replacing cement by micro silica with varied percentage as mentioned above. This cement mortar was placed inside the mould in which cages were kept. All moulds were kept on the table vibrator and sufficient vibration was given to compact the mortar. The specimens were finished smooth after vibration.

Replacement of Cement by Micro Silica

The specimens were demoulded after twenty-four hours of casting and specimens were transferred to curing tank where in they were allowed to cure for 28 days. After 28 days of curing, they were taken out of water, dried and were tested for flexural strength.

Test Result

Tables 1, 2 and 3 give the flexural strength test results for fibrous ferrocement made from GI fibers with and without replacement of cement by micro silica. The tables also indicate the percentage decrease in the flexural strength of fibrous ferrocement with replacement of cement by micro silica. In tables WM represents welded mesh and CM represents chicken mesh. Table 4 gives multiplying factors for different percentages of GI fibers. Table 5 gives variables C & D for fibrous ferrocement with different specific surface areas. Table 6 gives predicted (from equations 8) and observed value of flexural strength for fibrous ferrocement without replacement of cement by micro silica. Table 7 shows final predicted (from equations 8) and observed value of flexural strength for fibrous ferrocement with 25% replacement of cement by micro silica. The Figures 1, 2 and 3 give the variation of flexural strength for fibrous ferrocement made from GI fibers with and without replacement of cement by micro silica. Figure 4 shows predicted (from equations 8) and observed value of flexural strength for fibrous ferrocement with 25% replacement of cement by micro silica. Figures 5 to 8 show some photographs during experimentation.

Replacement of Cement by Micro Silica

Mathematical Equation

Flexural strength of fibrous ferrocement can be expressed as

S =σ x F ———————— (1)

where σ is flexural strength of ferrocement (1WM+1CM), and F is multiplying factor which depends on volume fraction of fibers and specific surface of welded mesh (WM) and chicken mesh (CM).

Equation (1) can be rewritten as

( S / A1) = σ F1

[S / (A1 x σ )] = F1 ————— (2-a)

Similarly

[S / (A2 x σ )] = F2 ————— (2-b)

[S / (A3 x σ )] = F3 ————— (2-c)

Where A1, A2 and A3 are specific surface areas in m2 for fibrous ferrocement with (1WM+1CM), (1WM+ 2CM) and (2WM+2CM) respectively. F1, F2 and F3 are multiplying factors which depend on volume fraction of fibers and specific surface of welded mesh (WM) and chicken mesh (CM) for fibrous ferrocement with (1WM+1CM), (1WM+ 2CM) and (2WM+2CM) respectively.

Replacement of Cement by Micro Silica

Using experimental values and statistical approach following best fit exponential equations are obtained for multiplying factors F1, F2 and F3, where vf is volume fraction of fibers.

F1= 20923 e0.1687vf, with R2= 0.9298 ————— (3-a)

F2= 19.655 e0.1582vf, with R2= 0.9003 ————— (3-b)

F3= 14.719 e0.1724vf, with R2= 0.9021 —-———— (3-c)

Replacement of Cement by Micro Silica
Factors F1, F2 and F3 can be used to find flexural strength of fibrous ferrocement for specific surface area of A1 = 0.049345 m2 , A2 = 0.060914 m2 and A3 = 0.098690 m2 respectively.

Now equation (2) can be rewritten in generalized form as follows,

σcuff = σcu x A x C x eD vf ———— (4)

Where C and D are variables, A is specific surface area of ferrocement composite and vf is volume fraction of fibers. From equations (3) and (4) it is observed that variable C and D depend on specific surface area of ferrocement composite.

Using equation (3) and statistical approach following best fit polynomial equations are obtained for variables C and D keeping them as dependent on specific surface area (A).

C = -426.83 A2– 62.452 A + 25.048 with R2= 1 -———————— (5)

D = 417.47 A2-46.937 A + 1.4683 with R2= 1 ———————— (6)

Thus equation (4) can be used to predict flexural strength of fibrous ferrocement in which variables C and D can be predicted from equations (5) and (6). Predicted values of flexural strength of fibrous ferrocement are shown in following table.

Using statistical approach 'Average' which is average of the absolute deviations of predicted values from observed values.

Average =Σ(X1+X2+…..+Xn) / n, where n is number of readings, X1, X2, X3…..Xn are readings under consideration.

Average ratio for = 1.001, means average percentage deviation of predicted values from experimental values is (1.001-1) x 100 = 0.1%, which is within acceptable limits. Since predicted values on lower side, suitable multiplying factors which can be used in Equation (4) is = 1 + 0.0037 = 1.001= 1.00.

Hence final equations to predict flexural strength of fibrous ferrocement without 25 % replacement of cement by micro silica.

S = σ x A x C x eD vf ———— (7)

Where S = Flexural strength of fibrous ferrocement without replacement of cement by micro silica.

σ = Flexural strength of ferrocement (1WM+1CM).

A= Specific surface area of ferrocement in m2.

C & D = variables to be obtained from equations (5) & (6).

vf= Percentage volume fraction of fibers.

The average percentage increase in flexural strength as observed from table 1, 2 and 3 is 18.07% with 25 % replacement of cement by micro silica. The multiplying factor = 1+ 0.1807 = 1.18 can be used in equation 7 to predict flexural strength of fibrous ferrocement with 25 % replacement of cement by micro silica.

S = 1.18 σ x A x C x eD vf —— (8)

Where S = Flexural strength of fibrous ferrocement with 25 % replacement of cement by micro silica

Replacement of Cement by Micro Silica
σ = Flexural strength of ferrocement (1WM+1CM).

A= Specific surface area of ferrocement in m2.

C & D = variables to be obtained from equations (5) & (6).

vf= Percentage volume fraction of fibers.

Equation 8 can be used to predict flexural strength of fibrous ferrocement with 25 % replacement of cement by micro silica.

Average ratio = 0.999, means average percentage deviation of predicted values from experimental values is

(1-0.00999) x 100 = 0.1%, which is within acceptable limits. Since predicted values on lower side, suitable multiplying factors which can be used in Equation (8) is = 1 + 0.001 = 1.001 = 1.00.

Hence equations 8 can be used to predict flexural strength of fibrous ferrocement with 25 % replacement of cement by micro silica.

Discussions on Test Results

  1. It has been observed that the flexural strength of fibrous ferrocement increases in the range of 13 to 95% as compared with ferrocement with increase in percentage of GI fibers in it. This is true for all the fibrous ferrocement specimens produced from (1WM+1CM), (1WM+2CM) and (2WM+2CM). It has been also observed that flexural strength of fibrous ferrocement increases in the range of 7to 20% with increase in amount of the welded mesh and chicken mesh in it goes on increasing.

    Replacement of Cement by Micro Silica

    Fibers act as crack arrester and distribute stresses over large area contributing to flexural strength. Therefore more percentage of fibers or more percentage of welded mesh and chicken mesh enhance flexural strength of fibrous ferrocement. It is observed that 1 to 1.5 % fiber content seems to be optimum.
  2. It is observed that there is increase in flexural strength of fibrous ferrocement up to 25 percent replacement of cement by micro silica. After 25 percent replacement of cement by micro silica, strength goes on decreasing. Same trend is observed for fibrous ferrocement produced with (1WM+1CM), (1WM+2CM) and (2WM+2CM).

    This may be due to the fact that 25% replacement of cement by micro silica may give rise to higher pozzolanic reaction and it may fill up all the pores making fibrous ferrocement a dense mass.
    Replacement of Cement by Micro Silica


  3. It is observed that percentage increase in the flexural strength of fibrous ferrocement made from (1WM+1CM), is 15%, 16%, 18%, 19%, 20% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of GI fibers in it respectively, with 25 percent cement replacement by micro silica. It is observed that percentage increase in the flexural strength of fibrous ferrocement made from (1WM+2CM), is 15%, 16%, 18%, 19%, 21% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of GI fibers in it respectively, with 25 percent cement replacement by micro silica. It is observed that percentage increase in the flexural strength of fibrous ferrocement made from (2WM+2CM), is 16%, 18%, 19%, 20%, 21% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of GI fibers in it respectively, with 25 percent cement replacement by micro silica.

Conclusions

  • Flexural strength of fibrous ferrocement goes on increasing as the percentage of GI fibers in it goes on increasing. 1 to 1.5 % fiber content observed to be optimum percentage.
  • Flexural strength of fibrous ferrocement increases in the range of 13 to 95% with increase in percentage of GI fibers.
  • Flexural strength of fibrous ferrocement increases in the range of 7 to 20 % with increase in amount of welded mesh and chicken mesh.
  • Fibrous ferrocement with GI fibers shows better performance as compared to ferrocement.
  • Suggested mathematical equations can be used to predict flexural strength properties of fibrous ferrocement with 25% replacement by micro silica. Predicted values of flexural strength obtained from mathematical equations agree with experimental values.
  • Thus fibrous ferrocement with 25% replacement of cement by micro silica can be used as economical construction material where flexural strength requirement is predominent.

Acknowledgment

The authors would like to thank Dr. S. C. Pilli and Dr. B. D. Dalvi, Principals of KLE's College of Engineering and Technology, Belgaum and D.Y.Patil College of Engineering and Technology, Kolhapur for their encouragement throughout the work. Authors are also indebted to management authorities of both the colleges for their wholehearted support, which boosted the moral of the authors. Thanks are also due to HODs of the civil engineering department and other staff for their kind cooperation.

References

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  • Madan S.K,Rajesh Kumar, "Strength of reinforced steel fibers concrete deep beams in shear," Civil Engineering and Construction Review, December 2005, (pp47 - 51).
  • Johnston, C.D. "Steel fiber reinforced mortar and concrete – A review of mechanical properties" International Symposium on fiber reinforced concrete, ACI, SP-44, Detroit,May 1982, (pp 127 - 136)
  • Kulkarni, D. K, Prakash,K.B, "Effect of addition of combination of adminixtures on the properties of flyash fiber reinforced concrete," Civil Engineering and Construction Review, September 2006, (pp 56 - 64)
  • Prakash, K. B. and Krishnaswami, K. T, "Fibrous Ferrocement – An ideal material for Bridge Overlays," FIP symposium on The Concrete Way To Development (vol. II) held at, Johannesburg, South Africa, 9 – 12 March 1997, (pp 683 – 690)
  • I.S. 516-1959, "Methods of tests for strength of concrete," Bureau of Indian Standard.
  • Balsubramanain, K. et al, 'Impact resistance of steel fibre reinforced concrete,' The Indian Concrete Journal, May 1996, (pp 257 - 262)

NBMCW April 2009

.....

Economic Design of CUSUM Control Charts...

CUSUM Control Charts

Debasis Sarkar, Assistant Professor, Construction and Project Management Dept, CEPT University, Ahmedabad and Prof. Goutam Dutta, Productions and Quantitative Methods Area, Indian Institute of Management, Ahmedabad
The design of a control chart has economic consequences, in which the costs of sampling and testing, costs associated with investigating out of control signals and possibly correcting assignable causes, are all affected by the choice of the control chart parameters. Henceforth, it is logical to consider the design of a control chart from an economic viewpoint. In this paper, we made an attempt to develop an optimum economic model for CUSUM control charts applicable to Indian RMC industry.

Introduction

History reveals that Ready Mixed Concrete (RMC) was patented in Germany in 1903, but the means of transporting it had not developed sufficiently well to enable the concept to be exploited. In India, RMC was launched about two decades ago. Lack of proper manuals, higher initial costs than conventional site mixed concrete, high initial investments for installation of automatic batching plants and also lack of awareness were the major causes that led to an initial setback to the RMC industry. Thus in course of time awareness of the advantages of using RMC and realization of the fact that the conventional concrete may result in higher lifecycle cost due to higher maintenance costs made the construction industry to adopt RMC as a better option economically and qualitatively. But still in India only about 2% of the total cement produced is utilized in RMC production against 70% of the cement produced being utilized in RMC in UK and US.

Control charts can effectively improve the efficiency of the production process of RMC. CUSUM (Cumulative Sum) control charts along with V-mask can be applied to RMC industry as a daily/weekly quality monitoring tool. The design parameters of CUSUM control chart (sample size, sampling frequency and control limits) can be chosen from an economic view point, and an optimum economic model can be developed which can predict the expected production cost per c‎um. of RMC.

Economic Design Philosophy

To formulate an economic model for the design of a control chart, it is necessary to make certain assumptions about the behavior of the process. These assumptions are as follows:
  1. The process is assumed to be characterized by a single in- control state.
  2. The process transitions between states are instantaneous.
  3. The process is not self- correcting, which indicated that once a transition to an out of control state has occurred, the process can be returned to the in control condition only by management intervention following an appropriate out of control signal on the control chart. In some cases, however transitions between out of control states are allowed, provided the transitions are always consistent with further quality deterioration.

Cost Parameters

Generally, three categories of costs are considered in the economic design of control charts. These categories are as follows:
  1. Costs of sampling and testing
  2. Costs associated with the investigation of an out of control signal and with repair or correction of any assignable cause found.
  3. Costs associated with production of nonconforming items
The production, monitoring and adjustment process may be thought of as a series of independent cycles over time. Each cycle begins with the production process in the in-control state, and continues until process monitoring via the control chart, results in an out-of control signal. Following an adjustment in which the process is returned to the in-control state, a new cycle begins.

We assume:
  1. E (T) = Expected length (long term average length / mean length) of a cycle
  2. E (C) = Expected total cost incurred during the cycle
  3. E (A) = Expected cost per unit time
The basic equation for obtaining economic design is expressed as follows:
  1. E (A) = E (C) / E (T) ......... (1)
Applying optimizing techniques to equation (1) economic optimal control chart design can be obtained.

Literature Survey

Control charts are widely used to establish and maintain statistical control of a process. They are also effective devices for estimating process parameters, particularly in process capability studies. The use of control chart requires selection of a sample size, a sampling frequency or interval between the samples and the control limits for the chart. Selection of these three parameters is usually called the design of the control chart. The design of a control chart has economic consequences, in that the costs of sampling and testing, costs associated with investigating out of control signals and possibly correcting assignable causes, and costs of allowing nonconforming units to reach the consumer are all affected by the choice of the control chart parameters. Therefore, it is logical to consider the design of a control chart from an economic viewpoint. In recent years, considerable research has been devoted to this problem.

Duncan (1956, 1971), Taylor (1967, 1968), Goel (1968), Lorenzen and Vance (1986), were the principal researchers who developed optimal economic models of control charts. Duncan (1956) proposed an economic model for the optimum economic design of the X-bar control chart. His work was the first to deal with a fully economic model of a Shewhart type of control chart, and to incorporate formal optimization methodology into the control chart parameters. Duncan's paper was the stimulus for much of the subsequent research in this area. Taylor (1968) was the first to investigate the economic design of CUSUM control charts. His model expresses the expected cost per unit of time as a function of the sample size, sampling interval and the V-mask design parameters. He assumes that a single assignable cause of specified magnitude occurs according to a Poisson process. To solve the model, however he assumes that the sample size and sampling interval are specified.

Goel (1968) has compared economically optimal X-bar and CUSUM charts. He reports that, in general, there is little difference between the two types of charts, with respect to optimum system cost. However, if a smaller than optimum sample size is used, the resulting economic penalties are more severe for the X-bar chart. Futhermore, the CUSUM chart is not quiet as sensitive to errors in specifying the magnitude of the shift as is the X-bar chart.

Ohta et al. (2002) proposed an economic model for the selection of time-varying control chart parameters for monitoring on-line the mean and variance of a normally distributed quality characteristic. The process is subject to two independent assignable causes. One cause changes the process mean and the other changes the variance. The occurrence times of these assignable causes are described by Weibull distributions having increasing failure rates. Yu and Wu (2004), Yu and Chen (2005) are of the view that most of the studies on the economic design of charts focuses on a fixed sampling interval (FSI), however it has been discovered that variable sampling interval (VSI) control charts are substantially quicker in detecting shifts in the process than FSI control charts due to higher frequency in the sampling rate when a sampling statistic shows some indication of a process change. The results of a numerical example adopted from an actual case indicate that the loss cost of VSI moving average control charts is consistently lower than that of the FSI scheme.

Reviewing the available literature of about past two decades we observed that, though substantial work has been carried out to develop economic design models in process and automation industry, but very minimal attempt has been made to develop economic design models applicable to construction industry. Furthermore, we believe that no attempt has been made to develop an optimum economic model for CUSUM control charts applicable to ready mixed concrete industry. In this paper we made an attempt to develop an optimum economic model for CUSUM control charts applicable to Indian RMC industry.

Optimum Economic Model of Cusum Control Chart

Lorenzen and Vance Economic Control Chart Model (Montgomery, 1985) popularly known as the LV model is one of the most widely accepted models for economic and optimum design of CUSUM control charts. This LV model provides the practitioner the most flexibility amongst any of the single assignable cause models available. By using average run lengths instead of Type I and Type II errors, LV allows the analyst to choose from any type of variable or attribute control chart. The LV model incorporates three types of cost ratios into its formulation. These are as follows:
  1. The cost of producing non conforming items
  2. The cost of false alarms and of search and repair of the true assignable cause
  3. The cost of sampling
The control chart design parameters for the LV model are as follows:
  1. Sample size (n)
  2. Sampling interval (h)
  3. Control interval width
These parameters should be selected in such a manner that the expected cost per hour function is minimized.

The variables used in the model are defined as follows:
  1. γ0= Number of assignable causes per hour
  2. T = Expected time of occurrence of the assignable cause (a function of γ0)
  3. S = Expected number of samples taken while process is in-control (a function of γ0)
  4. T0= Expected search time when false alarm
  5. T1= Expected time to discover the assignable cause
  6. T2= Expected time to repair the process
  7. E = Time to sample and chart one item
  8. C = Production cost per hour
  9. C0 = Quality cost per hour while producing in- control
  10. C1= Quality Cost per hour while producing out of control
  11. W = Cost for search / repairs
  12. a = Fixed cost per sample
  13. b = Variable cost per unit
  14. Y = Cost per false alarm
  15. = Mean shift (number of standard deviation slip when out of control)
  16. n = Sample size
  17. h = Hours between samples or sampling interval
  18. K = CUSUM reference value
  19. H = CUSUM decision interval
  20. δ1= Flag for whether production continues during searches (1- yes, 0 – no)
  21. δ2= Flag for whether production continues during repairs (1-yes, 0- no)
  22. ARL1 = In-control average run length
  23. ARL2 = Out of control average run length
  24. ECT = Expected Cycle Time (time between successive in control periods)
The generalized model to obtain optimum economic design for CUSUM control charts can be expressed as:
  1. C = { C0/ γ0+ C1( - T + n E + h (ARL2) + δ1T1+ δ2T2) } / ECT + {( S Y / ARL1) +W } / ECT + { [ (a + bn) / h] [ 1 / γ0– T + n E + h (ARL2) + δ1T1 + δ2T2] } / ECT …… (2)
Where
  1. ECT = 1 / γ0+ [(1 - δ1)S T0] / ARL 1 – T + nE + h (ARL2) + T1+T2

Proposed Optimum Economic Design of Cusum Control Chart Model for Indian RMC Industry

Considering the conditions existing in India an Optimum Economic Design CUSUM Control Chart Model applicable to Indian RMC industry is proposed. As per the basic operations of a typical Indian commercial RMC plant it is convenient to calculate the operation and production cost per m3 of concrete produced. This cost depends on the grade of concrete produced and may vary within a certain range from plant to plant depending upon the location of the plant and the source of raw materials. The parameters of this model are chosen to minimize the expected cost of production / m3 of concrete.

The variables modified as per the proposed model are defined as follows:
  1. C = Expected cost of production / m3 of concrete
  2. C0= Quality cost / m3 while producing in-control situation
  3. C1= Quality cost / m3 while producing in out of control situation
  4. a = Fixed cost / m3
  5. b = Variable cost / m3
  6. h = Sampling interval (typically 1 sample of three cubes per batch of concrete)
  7. δ1= Flag for whether production continues during searches (1- yes, 0 – no)
  8. δ2= Flag for whether production continues during repairs (1-yes, 0- no)
  9. E = Time to sample and chart one item (minutes)
  10. T1= Expected time to discover the assignable cause (minutes)
  11. T2= Expected time to repair the process (minutes)
  12. ARL2 = Out of control average run length
  13. ECT = Expected Cycle Time (minutes)
Proposed Model for Optimum Economic Design of CUSUM Control Chart for Indian RMC industry is given by the following empirical relationship:
  1. C = {C0+ C1 (n E + h (ARL2) + δ1T1+ δ2T2} / ECT + (a + b n) / ECT …….. (3)

Case Study and Testing of the Proposed Model

This proposed model as per equation was tested with data of is a fully computerized and commercial batching plant from Delhi. The production capacity of this plant is 60 m3 / hr and each batch is of capacity 2 m3. The cycle time for each batch is about 2 minutes where time taken to load the raw materials is about 1.5 minutes and mixing time is 0.5 minute.

Thus for M20 grade concrete with 53 grade cement the fixed cost and variable cost per m3 of concrete as per data of year 2007 is calculated as follows:

(a) Fixed Cost

Total Fixed Cost = Rs. 2688 m3

(b) Variable Cost

This includes cost of repairs and replacement of parts during major or minor machinery breakdown which can be taken as lump sum amount of Rs. 150 / m3 of concrete.

(c) Quality Costs

Costs of quality primarily consist of the costs which are involved during production of concrete in, in-control and out of control situation. These are presented in table

Model Input data

CUSUM Control Charts
  1. a = Rs. 2688 / m3
  2. b = Rs. 150 / m3
  3. C0= Rs. 90 / m3
  4. C1= Rs. 130 /m3
  5. h = 2 minutes
  6. n = 1
  7. δ1= 1
  8. δ2= 0
  9. E = 1 minute
  10. T1 = 1 minute
  11. T2= 1 minute
  12. ARL 2 = 10
  13. ECT = 2 minutes
Expected Production Cost / m3
  1. C = {C0+ C1 (n E + h (ARL 2) + δ1T1 + δ2T2} / ECT + (a + b n) / ECT
  2. = {90 + 130[(1 (1) + 2 (10) + 1(1) + 0(1)]} / 2 + {2688 + 150(1)}/2
  3. = 2894
  4. = Rs. 2894/m3
Thus applying the proposed Optimum Economic Design Model the total production cost per m3 of concrete which comprises of the fixed costs, variable costs and quality costs is about Rs. 2894 / m3 of concrete.

Limitations and Concluding Remarks

As per the results obtained from tests conducted on the Optimum Economic Design Model, it appears that for M 20 grade concrete in Delhi (Gurgaon) region as per the rates of materials and labor collected in year 2007, the expected total production cost (including overheads & profit) will be about Rs. 2894 / m3 of concrete. The RMC producers can fix the grade prices accordingly. The present grade prices charged by most of the RMC producers are much higher than Rs. 2894 / m3 for M20 grade. The increased production cost appears to be due to use of over conservative mix design and lack of systematic and judicious quality controlling and monitoring system. Also the production cost is increased due to unnecessary usage of more cement, than that specified, to reduce the producers' risk. Similar methodology can be applied for calculating the production costs of other grades of concrete like M25, M30, M35, M40, M50 and the like.

CUSUM Control Charts
Though the proposed Optimum Design Model seems to give good results, but the complication of this model will make it difficult to be user friendly for Indian RMC industry. The theoretical production capacity of the commercial plant taken as case study is 60 m3/ hr. This production at maximum capacity is rarely achieved and in one hour batches of various grades of concrete depending upon the requirement may be produced. Basically it is very difficult to speculate the production cost of a RMC plant per hour. Thus it is seems to be more logical to develop a model by which the expected cost of production per m3of a specified grade of concrete can be speculated. In the proposed model the expected cycle time is taken as the time for producing 1 m3 of concrete (2 minutes) which primarily comprises of the loading time (1.5 minutes) and mixing time (0.5minutes). Also it is difficult to predict the mean shift, number of assignable causes per cycle and the expected search time when there is a false alarm. However, if proper care is taken during the designing of the mix, and a systematic quality control and monitoring system (using SQC tools like CUSUM technique & V-mask and Exponentially Weighted Moving Average control charts) is followed, application of this model by the RMC plants may not be necessary.

References

  • Barnard, G.A. (1959) "Control Charts and Stochastic Process" Journal of Royal Statistical Society, Vol.21, pp.239 – 271
  • Box, G. (1994) "Role of Statistics in Quality and Productivity Improvement" Journal of the Royal Statistical Society, Series A, Part 2, pp. 209-229
  • Brown, B.V. (1984) "Monitoring Concrete by the CUSUM System" Concrete Society Digest No.6, Concrete Society, London.
  • Chiu, W.K. (1974) "The Economic Design of Cusum Charts for Controlling Normal Means" Applications in Statistics, Vol. 23, pp. 420 – 433
  • Deming, W.E. (1986) Out of Crisis, Massachusetts Institute of Technology, Center for Advanced Engineering Study, Cambridge, Massachusetts
  • Dewar, J.D. and Anderson, R. (1988) Manual of Ready Mixed Concrete Blackie and Son Ltd., Glasgow and London.
  • Duncan, A.J. (1956) "The Economic Design of X-bar Charts Used to Maintain Current Control of a Process" Journal of the American Statistical Association, Vol 51, pp 56-62
  • Duncan, A.J. (1971) "The Economic Design of X-bar Charts When there is a Multiplicity of Assignable Causes" Journal of the American Statistical Association, Vol 66, pp 85- 91
  • Ewan, W.D. (1963) "When and How to Use Cu-Sum Charts" Technometrics, Vol. 5, pp. 1 – 22
  • Gan, F.F. (1991) "An optimal design of CUSUM quality control charts." Journal of Quality Technology, Vol. 23, pp. 279-286
  • Goel, A.L. (1968) "A Comparative and Economic Investigation of X-bar and Cumultive Sum Control Charts" Ph.D Dissertation, University of Wisconsin-Madison, Wisconsin
  • Goel, A.L. and Wu, S.M. (1973) "Economically Optimum Design of CUSUM Charts" Management Science, Vol. 19, pp. 1271 – 1282
  • Grant, E.L. and Leavenworth, S. (2000) Statistical Quality Control, Seventh Edition, Tata McGraw Hill Publishing Co. Ltd., Delhi
  • Johnson N.L. and Leone, F.C. (1962 a) "Cumulative Sum Control Charts–Mathematical Principles Applied to their Construction and Use" Industrial Quality Control, Vol. 18, pp.15 – 21
  • Keats, J.B. and Montgomery, D.C. [Editors](1996) Statistical Applications in Process Control Marcel Dekker, Inc., New York.
  • Kemp, K.W. (1961) "The Average Run Length of the Cumulative Sum Control Chart When a V-Mask is Used" Journal of the Royal Society, Series B, Vol. 23, pp.149 – 153
  • Lorenzen, T.J. and Vance, L.C.(1986) "The Economic Design of Control Charts : A Unified Approach" Technometrics, Vol.28, pp.3 – 10
  • Lucas, J.M. (1976) "The Design and Use of V-Mask Control Schemes" Journal of Quality Technology, Vol. 8, pp 1 – 12
  • Montgomery, D.C. (1985), Introduction to Statistical Quality Control John Wiley & Sons, Inc, New York.
  • Montgomery, D.C. (1980) "The Economic Design of Control Charts: A Review and Literature Survey" Journal of Quality Technology, Vol. 12, pp.75 – 87
  • Montgomery D.C. and Woodall, W.H. (1997)" A Discussion on Statistically Based Process Monitoring and Control." Journal of Quality Technology Vol. 29, pp. 121-162
  • Ohta, H., Kimura, A. and Rahim, A. (2002) An Economic Model for X bar and R charts with Time Varying Parameters. Quality and Reliability Engineering International, Vol. 18 (2), pp. 131-139
  • Page, E.S. (1954) "Continuous Inspection Schemes" Biometric, Vol. 41, pp.100 – 115
  • Page, E.S. (1961) "Cumulative Sum Control Charts" Technometrics Vol. 3, pp. 1 – 9
  • Sarkar, D. and Bhattacharjee, B. (2003) "Quality Monitoring of Ready Mixed Concrete Using Cusum System" Indian Concrete Journal, Vol 7, pp. 1060-1065
  • Taylor, H.M.(1968) "The Economic Design of Cumulative Sum Control Charts" Technometrics, Vol. 10, pp. 479 – 488
  • Woodall, W.H. (1986) "The design of CUSUM quality control charts". Journal of Quality Technology, Vol. 18, pp. 99-102
  • Woodall, W.H.(1985) "The Statistical Design of Quality Control Charts" The Statistician, Vol. 34, pp.155 – 160
  • Yu , F.J. and Chen , Y.S. (2005) "An Economic Design for Variable Sampling Interval X bar Control Chart for a Continuous Flow Process." The International Journal of Advanced Manufacturing Technology, Vol. 25(3 - 4), pp.370 - 376
  • Yu , F.J. and Wu , H.H. (2004) "An Economic Design for Variable Sampling Interval MA Control Charts". The International Journal of Advanced Manufacturing Technology, Vol. 24(1-2), pp.41-47

NBMCW June 2009

.....

Yearnings of a Reinforced Concrete Stru...

Reinforced Concrete

Anil K Kar, Engineering Services International, Kolkata. Satish K Vij, Member Engineering,Railway Board, New Delhi

Introduction

Portland cement based concrete is today the most widely used material of construction.

Ceiling of buildings
Ceiling of buildings
Concrete has attained this pre-eminent position because of many useful properties it is endowed with. Among these properties are the relative ease of formability, rigidity on setting and curing, and the great ability to resist compressive forces (generally in the range of 20-80 MPa and in special cases up to 280 MPa).

Concrete is, however, weak in its resistance to tensile forces, flexural tension, shear and torque. It tends to be brittle in nature.

These weaknesses are overcome with the addition of reinforcing bars (rebars) of steel in the case of reinforced concrete and high strength steel wires or cables in the case of prestressed concrete.

With its weaknesses overcome and its positive properties retained, beautiful concrete structures have given expression to man’s imagination and sense of elegance.

The utility and elegance as well as the durability of concrete structures, built during the first half of the last century with ordinary portland cement (OPC) and plain round bars of mild steel, the easy availability of the constituent materials (whatever may be their qualities) of concrete and the knowledge that virtually any combination of the constituents leads to a mass of concrete have bred contempt. The same thing happened in the case of rebars as bars of higher and higher strength of a radically different type were introduced. Strength was emphasized without a thought on the durability of structures.

As a consequence of the liberties taken, the durability of concrete and concrete structures is on a southward journey; a journey that seems to have gained momentum on its path to self– destruction (Figures 1–7). This is particularly true of concrete structures which were constructed since 1970 or thereabout by which time (a) the use of high strength rebars with surface deformations (HSD) (Figure. 8) started becoming common, (b) significant changes in the constituents and properties of cement were initiated, and (c) engineers started using supplementary cementitious materials and admixtures in concrete, often without adequate consideration.

Common features among the examples in Figures. 1-7 are: (a) all the concrete structures, whether roofs or not, are exposed to the atmosphere, and (b) all the structures contain reinforcing bars (rebars). These bars are in the form of high strength bars with surface deformations (Figure. 8).

Today, it is not uncommon to find instances of distressed concrete elements inside rooms, tunnels, etc., away from any known sources of water, other than atmospheric air, suffering early distress due to corrosion in rebars.

Figure. 9. Self–destruction of concrete through volume expansion due to swelling of gel which is formed on the surface of aggregates (containing reactive silica or silicate) by reaction of reactive silica or silicate with alkali in cement in a process known as alkali-silica reaction.

Figure 9 shows another form of distress in concrete that can occur more often in constructions with modern cement.

High Strength rebars
The setback in the health of newly constructed concrete structures prompted Swamy1, to write: “The most direct and unquestionable evidence of the last two/three decades on the service life performance of our constructions and the resulting challenge that confronts us is the alarming and unacceptable rate at which our infrastructure systems all over the world are suffering from deterioration when exposed to real environments.”

The observation of Swamy1is an echo of the observation which was made in a 1999 document of Central Public Works Department, Government of India2when CPWD reported in its Technical Circular 1/99 that while work as old as 50 years provided adequate service, the recent constructions in concrete were showing signs of distress within a couple of years of their completion.

The plight of reinforced concrete structures of recent vintage has made such structures yearn for the healthy life of the past when concrete structures could expect to have long life spans.

Though it is an international phenomenon, the problem of early distress in concrete structures is most acute in India, where cold twisted deformed (CTD) rebars were most extensively used during the last three decades of the last century.

Being widespread and universal, the tailspin in the life span or durability of concrete structures, constructed in recent decades, came as a surprise to many, specially when concrete was going through the stages of gaining in compressive strength from 10-30 MPa of earlier periods to a more common 20-80 MPa of these days and in special cases up to 280 MPa, with a newly found label of high performance concrete. This surprise finds expression in the words of Swamy1who, being surprised at the alarming and unacceptable rate of deterioration in the condition of infrastructure systems, writes: “What is most surprising is that this massive and horrendous infrastructure crisis has occurred in spite of the tremendous advances that have been made in our understanding of the science engineering and mechanics of materials and structures.”

The surprises and disappointment emanate from a failure to recognize or admit the real causes of the problem. It is pertinent to note that the unhappiness with the performance of concrete structures followed many thoughtless changes in rebar, cement and construction practices. In all these changes, the emphasis in the case of rebars was on higher strength. In the case of cement, the emphasis was on higher strength, high early strength and greater profit margin, whereas in the case of construction practices, the emphasis has been on doing away with or shortening the period of moist curing.

The changes in the principal materials of concrete construction are perpetuated through various provisions in the basic Indian Standard for the design and construction of concrete structures, IS:456-20003, when the standard or code makes no distinction among reinforcing elements of steel of different categories and grades, of shapes as different as plain bars and bars with surface deformations, as disparate as round bars and structural shapes, or among OPC, Portland slag cement (PSC), Portland pozzolana cement (PPC) and seven other types of cement, except may be in the required duration of wet curing of concrete, made with different types of cement.

It will be shown later that, as in the case of reinforcing elements, the Indian codes also provide too much leeway in the requirements for individual types of cement.

In addition, what could be most disturbing is a general lack of discipline in India in the matter of adherence to the codal provisions on the properties of the manufactured materials. A few examples, related to the construction of a single family dwelling in Kolkata in the year 1993, will illustrate the point. It will make one recognize the sufferings and anguish of a reinforced concrete structure and its yearnings for rebars with an unblemished character and cements with an unquestionable performance.

Only with the right rebar and the right cement, where one will complement the other, can the structure promise a trouble-free life of long duration, provided that the other constituents of concrete and the workmanship will be up to the mark.

Case 1: CTD rebars of all sizes from 8 mm to 32 mm dia were purchased from the largest steel maker in the private sector in the country. Bars of all sizes were found to lack utterly in ductility. All bend tests led to brittle failure. The selling company initially claimed to have the requisite test certificates. A site visit by their representatives was convincing enough for them to recognize and admit the hollowness of their earlier claim of conformance of their product with the provisions in BIS standards. All bars, except the 32 mm dia bars, were replaced over a period of about two months. So difficult it was for the largest manufacturer of steel products to find rebars conforming to the standards that money was refunded for their failure to locate and supply 32 mm dia bars which would meet the requirements of BIS standards. It will be seen later in this paper that compliance with the requirements of the relevant standards of BIS would not necessarily make the CTD bars suitable for the construction of durable concrete structures.

Case 2: For the construction of the same building, for which rebars were procured in Case 1, PSC was procured from a leading manufacturer of cements. The cement was procured fresh from the rail wagons. The cement was used in construction on the following day. A day later, as one approached the site, streaks of slurry, which had escaped through the joints of formwork the day before, were found to glisten like fresh snow in sunlight. Obviously, something was wrong and yet the manufacturer, as in the case of the CTD bars, would routinely issue a test certificate claiming conformance with the relevant code of the BIS.

Case 3: though not cement or rebar, machine made modular bricks of burnt clay, considered to be the best in Kolkata, were procured from the Brick Directorate of the Government of West Bengal for the construction of the building in Cases 1 and 2. the bricks, in place, showed serious signs of distress as well as efflorescence due to high contents of aluminium sulphate. The Director of the government organisation would not initially believe it to be possible. He visited the site, saw distressed bricks in place and conducted some tests on unused bricks whereupon he had no hesitation in exclaiming that the bricks were not suitable for building construction. Bricks, which were yet to be used in construction, were withdrawn and money was refunded. Obviously, the best bricks in town would not satisfy the requirements of the BIS standards.

Such manufactured materials would not make durable structures, and yet such can be the quality of manufactured construction materials in India. The problem is magnified when, as it will be shown later in this paper, the code in Ref. 3 and similar codes in India and abroad recommend for use materials which are incapable of giving durable concrete structures.

Though it can be easily recognized that the satisfactory performance of concrete structures, built with plain round bars of mild steel and OPC in the past, played the key role in making concrete structures durable and popular, it cannot be overlooked in this context that relatively new concrete structures, with signs of distress, started becoming a common sight, only after the start of the use of rebars and cement of modern era, which are distinctly different from similar materials of the past.

Principal Causes of Early Distress

The decay and distress in concrete structures have in most cases been in the form of corrosion in reinforcing bars and consequent cracking and spalling of surrounding concrete.

There must be reasons for the early corrosion in rebars4,5.

A change in the environment over the last four decades cannot be that reason, as co-existing concrete structures of different periods show remarkably different performances; older structures, constructed about forty years ago or earlier, performing better than structures which were constructed during the last thirty years or so6,7.

As the differences in internal forces at service load conditions, as would be determined by different methods of design, are not very significant, the switch-over from the working stress method of design of earlier periods to the ultimate strength or limit state method of design is not one of the prime reasons for the very significant downfall in the durability of concrete structures of recent decades.

That brings into focus the use of inappropriate materials and poor workmanship as likely causes of early distress in concrete structures of recent decades. Poor workmanship was there earlier too as it is today. But the all-pervading nature of concrete structures of recent era1,2 reaching states of distress at an early age cannot be explained by poor workmanship alone.

From the three case histories in the preceding section, specially when the eminent positions of the manufacturing firms and their reputation are taken into account, it can be easily visualized that substandard materials are very extensively used in construction in India, and the use of manufactured materials of substandard qualities can have a significant role in the early decay and distress in concrete structures in India. But this cannot still explain the all-encompassing nature of concrete structures of recent vintage, whether in India2 or abroad1,8, reaching states of distress early.

It is explained in the following that there are certain inherent features in today’s rebars and cement, which contribute to the shortening of the life span of concrete structures in India and abroad.

A survey6,7 of several randomly selected buildings and bridges in the public domain in Kolkata revealed that structures with CTD rebars (a special form of high strength deformed or HSD rebars) would become distressed early, no matter what the type of cement was. Also, since corrosion in rebars must have something to do with the properties and characteristics of rebars and since such corrosion does lead to distress in concrete structures this should confirm that the principal cause of early corrosion in rebars, and therewith a significant cause of early distress in today’s concrete structures, must lie with the rebars.

Besides the time dependent distress in concrete structures, initiated by corrosion in rebars, there are many instances of concrete elements (both reinforced and prestressed) with almost instant signs of concern, like cracks in new concrete structures. As the orientation and almost instant occurrence of the cracks cannot be related to corrosion in rebars and as time dependent damages, of the type depicted in Figure. 9, cannot be related to corrosion in rebars and in prestressing elements, such distresses should be identifiable with shortcomings in cement or concrete. It will be seen later that many of these distresses can indeed be related to the significant changes which were brought upon in the physical properties and chemical compositions of cement of modern era.

The roles of rebars and cement in causing early decay and distress in concrete structures of recent periods thus form the focus of study in this paper.

Surface Deformations in Rebars Largely to Blame

Since it was observed in a survey6,7 that concrete structures with CTD bars performed less satisfactorily than concrete structures with plain round bars of mild steel and since it was also observed that early signs of distress in concrete structures in different countries started showing up with the use of rebars with surface deformations (whether CTD or other HSD bars), it should be obvious that this early decay and distress has much, if not all, to do with HSD rebars.

In the matter of corrosion of steel, engineers tend to think of the possible susceptibility of steel only in terms of its metallurgy. It is generally overlooked that the contour or surface characteristics too can have a significant influence on the corrosion of a piece of steel.

Kar4-7, 9-12 had explained how the presence of surface deformations made HSD rebars inherently prone to early corrosion. It was also explained as to why CTD bars (as in Torsteel, TISCON, etc.), with built-in (manufacturing stage) stresses beyond yield, were the worst of all HSD rebars in the context of early corrosion.

For an understanding of the basic causes of early distress in HSD, HYSD or CTD rebars, it will help to recall the phenomena of stress concentration and stress corrosion. According to the phenomenon of stress concentration, unless there will be an yield, the stress at a discontinuity or deformity can be significantly (even several times) higher than the average or nominal stress across the section. Thus, unlike in the case of a round bar with plain surfaces, the stresses in the surface region of a CTD or HYSD or HSD rebar can be much higher than the average or nominal stress across the section. The stresses in HSD bars may reach yield or higher levels under service load conditions or even before use in construction. The phenomenon of stress corrosion, in the context of the nature of the problem under consideration, implies that metals corrode faster with an increase in the stress level, particularly if stresses will reach yield levels or higher. Initially described in Ref. 5, this phenomenon of stress corrosion is further explained here.

The worst performance in the case of concrete structures with CTD reinforcing bars has its source largely in the manufacturing process of CTD bars. The cold twisting beyond yield, as a part of making CTD bars, causes permanent slippages at the interfaces of metallic grains of steel. High stresses at or above yield stress levels are permanently locked in the bars.

CTD Bars
The permanent slippages at the interface of metallic grains, caused during the cold twisting of CTD bars, and the higher strain, under greater stresses from the imposed loads lead to a break down of the protective oxide film on the surfaces of rebars, thereby exposing the intergrain faces of CTD bars to corrosive elements, (e.g., moisture and oxygen) and leading to the formation of electro-galvanic micro cells and generation of damaging electrical currents thereby making the CTD reinforcing bars much more vulnerable to corrosion than would have been the case with plain round bars of mild steel or even with conventional HYSD bars. This can be seen in a comparison of the conditions of rebars in Figures. 10-13.

The top four bars in Figure. 10 with corrosion on entire surfaces show the effect of cold twisting beyond yield as a part of the process of making CTD bars. In Figure. 10, the four untwisted bars at the bottom, which are from the same rod as the top four bars are, show isolated signs of corrosion at the bases of protrusions due to the magnification (effects of stress concentration) of manufacturing stresses even without cold twisting.

It should be evident that CTD bars are a deterrent to the construction of durable concrete structures.

Besides the effects of stress concentration and stress corrosion, there are other causes which make rebars with surface deformations greater prone to early corrosion than plain round bars are7,14.

The recognition of unsuitability for use, though not of the basic reasons for this unsuitability in the case of CTD bars, led to the stoppage of use of such bars fairly early in most countries and a belated introduction of thermo-mechanically treated (TMT) bars in India.

In the absence of the cold twisting, i.e., in the absence of yield (or higher) stress all over the surface, these TMT and other HSD rebars may not be as bad as CTD bars (Figures. 10 and 11) are, but these bars too can be highly susceptible to early corrosion (Figures. 12 and 13), as HYSD or other HSD rebars suffer from the effects of stress concentration due to the presence of surface deformations and the consequent effects of stress corrosion and other factors7,14.

Corrosion at the bends and at bar ends
Figure 13 depicts HYSD rebars of good quality at a construction site in the USA. Stress related corrosion can be seen at bends and at cut ends of rebars.

At bends, stresses in plain round bars of mild steel too reach yield. However, in the absence of adequate ductility, the adverse effects of stresses beyond yield are more pronounced in the case of HSD rebars than in the case of plain round bars of mild steel. Also, unlike in the case of HSD rebars in concrete with blended cements, the localized corrosion at bends of plain round bars of mild steel inside OPC concrete is not transmitted.

A unique feature of HSD bars, inclusive of HYSD and CTD bars, is that the thin edges of the lugs, which have no specific design requirements in India, are often damaged during the making, transportation and handling. The profusion of nicks in the lugs, arising from manufacturing defects and handling operations, can be easily seen on most products of CTD or other HSD bars. The damaged regions lead to the creation of sites with potential differences. Gradually the whole lengths of bars are covered with rust.

As explained in the preceding, topping the list of factors, which make reinforced concrete structures with HSD rebars so strongly predisposed to early decay and distress, are the phenomena of stress concentration and stress corrosion4-7, 9-14. Extensive tests and observations in Russia have revealed that, compared to the durability of plain round rebars, the durability of rebars with surface deformations is one order of magnitude less13.

Tests13 in Russia further revealed that, as the state of stresses at the level of yield limit and beyond was associated with considerable impairment of the steel surface and a breakdown of the natural protective oxide film of Fe2O3 or Fe3O4, the HSD rebar, stressed at yield or beyond, became electrochemically more active than an unstressed reinforcing bar and the passivation of such a highly stressed rebar became very difficult even in a saturated solution of Ca(OH)2.

In other words, it becomes well nigh impossible to mask the electro-potential of a rebar inside concrete if surface stresses in the steel elements would cross the yield level. Deformed rebars are beyond redemption.

Furthermore, inside concrete structures, the process of rebar corrosion in HSD rebars is at a very active state, as besides heightened stresses in keeping with the phenomenon of stress concentration and stress corrosion, particularly under post yield states, the presence of surface deformations on rebars of recent eras leads to gaps between rebars and concrete7,14 and greater microcell and macrocell formations15 than in the case of plain round bars in concrete.

Confirmation of the poor performance of HYSD and other rebars with surface deformations is also recorded in the findings of Mohammed, et al15, who concluded on the basis of tests for specific performance that “Due to the formation of gaps, the bottom part of horizontal steels shows significant macrocell and microcell corrosion …………..Deformed bars corroded more than plain bars.”

In the context of durability of concrete structures, it is evident from the preceding that it was most inappropriate when plain round bars of ductile mild steel were discarded in favour of CTD or other forms of HSD bars with limited ductility which are highly prone to early corrosion.

Since rebar corrosion is in many cases at the root of early decay and distress in reinforced concrete structures and since physical characteristics of rebars of recent periods are largely to blame for this poor performance, concrete structures yearn for a switch back to the use of plain round bars of mild steel as rebars which will have high ductility and where stresses at service load conditions will not cross or reach the yield.

Rebars with Surface Deformations Are beyond Redemption

Epoxy coated rebars
The vulnerability of HSD rebars cannot be camouflaged. Rust shows through epoxy coating in the advertised sample in Figure. 14.

Kar16 explained that at added cost, fusion bonded epoxy coating did not provide any assurance of added life span. In addition, the coating prevented any intimate bond between rebars and concrete as a result of which there could be disastrous consequences under vibratory loads.

Figure 15, which is from another advertisement on pretreated HSD rebars, is an admission (by suppliers of galvanized and epoxy coated rebars) of the fact that on its own neither galvanizing nor FBEC can really mitigate the problem of early corrosion in HSD bars. The advertised HSD rebar in Figure. 15 has both galvanizing and FBEC treatment. At much added cost, the twin protection (Figure. 15) will still make the structure vulnerable due to the loss of an intimate bond between the rebar and its surrounding concrete.

Cement

With the use of high strength rebars with surface deformations, the situation is bad in the realm of today’s concrete structures. It has the potential of going from bad to worse with
  1. the use of cement which is sold before tests are completed whereas subsequent tests at independent laboratories occasionally show the failure of the cement to meet the requirements set in codes
  2. the use of OPC with high C3S/C2S ratios
  3. the use of cement with excessive contents of water soluble alkalis
  4. the use of cement with high contents of gypsum
  5. the use of PPC with the addition of fly ash without any verification (in India) of its properties for its suitability for use in cement
  6. the use of PSC that may be made with ground granulated blast furnace slag of unverified qualities
  7. the use of PPC and PSC which will be made with OPC clinker of the type described in (b)–(d)
  8. inadequate curing of concrete with the availability of OPC, PSC and PPC for general purpose use in India, cement is very often used on consideration of cost and brand name, rather than suitability for specific purpose.

Cement and Impediments to Durability

In the light of (a) past experience of good performance of concrete structures built with OPC, (b) the non-availability of OPC in many parts of India, and (c) aggressive marketing by manufacturers of blended cements in favour of blended cements, the debate goes on as to whether OPC or blended cement should be preferred for the construction of reinforced concrete structures. It has, however, been suggested in the preceding that properties of modern cement may have a role in the early distress in concrete structures of recent periods. Thus, before a judicious consideration can be given to this issue of superiority of OPC or blended cement, it will help to know more about the changes which have taken place in the case of OPC over the years and the likely consequences of such changes in the context of durability of concrete construction.

The significant changes, which have been brought upon OPC over the years, are
  1. higher specific surface of cement that gives higher strength of concrete as well as higher heat of hydration; with specific surface which today varies between 300-450 sqm/kg, as compared to 150-200 sqm/kg of earlier periods; today’s OPC can truly be termed as high early strength cement
  2. large C3S/C2S ratios in OPC that gives high early strength with little reserve for any gain in strength at later ages; these days C3S/C2S ratios in the case of OPC is in the range of 3.0 to 5.0; in comparison, the C3S/C2S ratio was about 0.21 in 1920; about 0.47 in 1970; 0.54 in the late 60’s and about 0.56 in 1990 in the USA; besides the lack of any gain in strength with time beyond the initial few days, high C3S/C2S ratios give high heat of hydration; porosity of concrete is higher and concrete lacks durability as it is deprived of autogenous healing that helps to seal cracks and pores under moist or humid conditions.
  3. high contents of water soluble alkalis in cement, is shown in Table 1; high alkali contents can lead to destructive alkali-silica reaction if the %Na2Oeffective, i.e., % Na2O + 0.658(%K2O) will be greater than about 0.6; aggregates with reactive silica or silicates, which react with high alkali (pH > 13.5), are widespread in India and OPC or even blended cement in India today have high alkali contents (Table 1).
Contents of CaO and Soluble Alkalis in Typical cements
The data on contents of water soluble alkalis, as shown in Table 1, are for some cements of well known brands, which were available in Kolkata in the year 2006.

Compared to the safe limit of about 0.60, the % Na2Oeffective for OPC, PSC and PPC in Table 1 are respectively 2.37, 1.76 and 1.64 which are way too high, compared to the safe limit of about 0.60%.

Besides the possibility of destructive alkali-silica reaction, high contents of water soluble alkalis in cement
Plastic shrinkage cracks in a roadways slabs

Setting Times of Portland Slag Cement
  1. decrease workability of concrete.
  2. lead to the development of high heat in green concrete, thereby leading to plastic shrinkage cracks (Figure. 16) and thermal cracks (Figure. 17)
  3. make concrete highly absorbent, thereby aiding the process of corrosion in rebars and prestressing elements.
The consequences of high specific surface, large C3S/C2S ratios and high contents of water soluble alkalis can be seen in Figure. 18 where, unlike the familiar rising curve, there is no gain in the compressive strength of concrete with continued curing beyond 14 days.

Figures 18-22 show the behavior of some cements (of well known brands) which were available in Kolkata in the year 2008.

Greater details on hitherto unknown short term performance of some of today’s cement can be found in Ref. 19.

Table 2 shows that today’s codes on cement, as in the case of reinforcing elements, may be too permissive to accommodate all that is manufactured.

The high value of the initial setting time, the low figure for the final setting time and the closeness of the initial and final setting times of the cement in Table 2 are indicative of the fact that today’s cement may be significantly different from cement of earlier decades. There are no records of durable concrete structures built with cement of the present period.

It will help to recognize in this context that small scale short term tests on laboratory models do not adequately represent the real structures out in the open environment, which are not built with any special attention given to conducting tests.

The significant delay in the initial setting time is due to high contents of gypsum. the effects of such high contents of gypsum on the durability of concrete is yet to be studied.

The above explains why the durability of concrete structures of recent vintage has many impediments.

Variation in compressive strength of OPC concrete

Yearnings of today’s Concrete Structures

In the light of the pitiable performance in terms of durability, today’s concrete structures yearn that serious thinking, divorced from manufacturers’ commercial interests, be given to rebars and cement if the objective will be the construction of durable concrete structures.

Concrete structures yearn that, instead of the hollow promises of the past four decades, they be given back their normal life of the past where rebars and cement/concrete of unquestionable integrity were compatible and complemented each other, thereby leading to a long life.

Concluding Remarks

28-day compressive strength of ppc concrete
Compared to durable concrete structures of the past, concrete structures, built during recent decades, have been characterized by early decay and distress. Many people have expressed their concern at the unacceptable and alarming rate of decay.

The paper identifies the use of high strength rebars with surface deformations, in lieu of plain round bars of mild steel, as a key factor contributing to the early decay and distress in the case of reinforced concrete structures.

The paper has further identified large specific surface of cement particles, large C3S/C2S ratios for compounds in OPC and excessive contents of water soluble alkalis Na2O and K2O in cement rendering modern cement unsuitable for the construction of durable concrete structures. The paper has also cautioned against the use of fly ash and granulated blast furnace slag in the making of blended cements without verification of the properties of such materials. Questions have been raised also on high gypsum contents in cement.

As it is generally desired or required that concrete structures be durable, the above factors, which have robbed concrete structures of their durability in recent decades, require re-consideration and improvement.

It will be equally important that codes recommend materials which will give durable concrete structures and not what manufacturers make. It should be ensured simultaneously that manufacturers make rebars and cement to more exacting standards of the relevant codes.

References

  • Swamy, R. N., “Infrastructure regeneration: the challenge of climate change and sustainability — Design for strength or durability ?” The Indian Concrete Journal. Vol. 81, No. 7, July 2007.
  • Central Public Works Department, Government of India, Technical Circular 1/99, Memo No. CDO/DE(D)/G-291/57 dated 18/02/1999, issued by Chief Engineer (Designs), Nirman Bhawan, New Delhi - 110 011.
  • IS:456-2000, Indian Standard, Plain and Reinforced Concrete Code of Practice (Fourth Revision), July 2000.
  • Kar, A. K., “Concrete jungle ¾ Calamity may be waiting to happen,” The Statesman, Kolkata, 4 August, 2000.
  • Kar, A. K., “Concrete structures ¾ the pH potential of cement and deformed reinforcing bars,” Journal of The Institution of Engineers (India), Civil Engineering Division, Volume 82, Kolkata, June 2001, pp. 1-13.
  • Kar, A. K., “Deformed reinforcing bars and early distress in concrete structures,” Highway Research Bulletin, Highway Research Board, Indian Roads Congress, Number 65, December 2001, pp. 103-114.
  • Kar, A. K., “Deformed rebars in concrete construction,” New Building Materials & Construction World, Vol. 12, Issue 6, December 2006, pp. 82-101.
  • Papadakis, V. G., Vayenas, C. G., and Fardis, M. N., “Physical and chemical characteristics affecting the durability of concrete,” ACI Materials Journal, American Concrete Institute, March - April, 1991.
  • Kar, A. K., “Concrete structures we make today,” New Building Materials & Construction World, Vol. 12, Issue 8, February 2007.
  • Kar, A. K., “The ills of today’s cement and concrete structures,” Journal of the Indian Roads Congress, Vol. 68, Part 2, July-September 2007.
  • Kar, A. K., “Woe betide today’s concrete structures,” New Building Materials & Construction World, Vol. 13, Issue-8, February, 2008, also Vol. 13, Issue-9, March, 2008.
  • Kar, A. K., “Concrete structures: a tale of reverse technology,” RITES Journal, RITES Ltd., Vol. 10, Issue 2, July 2008.
  • Alekseev, S. N., Ivanov, F. M., Modry, S., and Shiessel P., Durability of reinforced concrete in aggressive media, Oxford & IBH Publishing Co. Pvt. Ltd., New Delhi, 1993. (Translation of Dolgovechnosti zhelexonetona v agressivnikh sredakh, Stroiixdat, Moscow 1990.)
  • Kar, A. K., “Reinforcing bars in the context of durability of concrete structures,” All India Seminar on Utilisation of skyways and subways in the cities in India, American Society of Civil Engineers ¾ India Section and Institution of Engineers, UK, Eastern Region, India, 9th and 10th December, 2005.
  • Mohammed, T. U., Otssuki, N., and Hisada, M., “Corrosion of steel bars with respect to orientation,” ACI Materials Journal, American Concrete Institute, March - April, 1999.
  • Kar, A. K., “FBEC rebars must not be used,” The Indian Concrete Journal, Vol. 78, No. 1, January 2004.
  • IS:456-1978, Indian Standard Code of Practice for Plain and Reinforced Concrete, Third Edition, Bureau of Indian Standards, New Delhi.
  • Sengupta, J. B., et al, “Critical review points on the design and construction of concrete roads in India,” New Building Materials & Construction World, Vol. 14, Issue-6, December 2008, New Delhi.
  • Kar, A. K., et al, “Strange are the ways of cement and concrete,” New Building Materials & Construction World, Vol. 14, Issue-1, July 2008.
  • Roper, H., Kirby, G., and Baweja, D., “Long-term durability of blended cement concretes in structures,” ACI SP 91-22, pp. 463-481.
  • Bier, Th. A., “Influence of type of cement and curing on carbonation progress and pore structure of hydrated cement paste,” Materials Research Society Symposium, 85, pp. 123-134, 1987.
  • Kar, A. K., “IS 456:2000 on durable concrete structures,” New Building Materials & Construction World, New Delhi, Vol. 9, Issue-6, December 2003.
  • Kar, A. K., “Waterproofing of structures: Challenges and solutions,” New Building Materials & Construction World, New Delhi, Vol. 11, Issue 10, April, 2006.
  • Kar, A. K., “Zero-leakage concrete tunnels for durability,” New Building Materials & Construction World, Vol. 14, Issue-6, December 2008, pp. 184-208.
  • Railway Board, Ministry of Railways, Government of India, “Waterproofing of Structures,” Circular No. 2008/LMB/1/7 dated 28.12.2008 and 15.12.2008, issued by Director (L & A), Rail Bhawan, New Delhi.
  • P. W. (Roads) Directorate, Government of West Bengal, “Typical Specifications on Maintenance & Repair of Bridge for the Year 2001-2002,” 01.01.2002, Kolkata.

NBMCW June 2009

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RMC Means Concrete Fit for Posterity

Ready Mix Concrete

Brajendra Singh, Chief Consultant, Cement Manufacturers’ Association, New Delhi.

Background

Concrete is one of the oldest construction materials known to man. Platforms made of lime concrete, found in Israel, have been dated to as far back as 7000 B.C. In India, a sunken city discovered off the coast of Gujarat a few years ago, and estimated to be 7000 years old, has many examples of concrete being used for construction purposes. In ancient Rome, instead of using marble like the Greeks were doing, the Romans used volcanic aggregates (formed out of hardened lava) to produce lightweight concrete, with which they plastered almost all their buildings.

Modern concrete construction only took off after the production of Ordinary Portland Cement (OPC) started in the 19th century. Then in 1903, Ready Mixed Concrete (RMC) was patented in Germany. Unfortunately, RMC could not be commercially exploited for another 10 years, since suitable transport for conveying it to construction sites within the required time frame, did not exist till then. It should be borne in mind that till 1910, several countries had a law that made it mandatory for a man with a red flag to walk in front of any motorized vehicle–mainly to warn horse–drawn transport to move aside. Finally, in 1913, the first dumper type of vehicle for conveying concrete made its appearance in USA, and RMC became a reality.

The next year, 1914, was a fairly significant one, both nationally and internationally as far as concrete was concerned. In India, the first cement plant (capacity 1,000 tonnes per annum) came up in the Mahatma’s birthplace Porbandar, and the country’s first concrete road was built in Madras. In Europe, in the same year, a study found that by mixing flyash with concrete one could obtain significant advantages, such as reduction in the heat of hydration resulting in fewer cracks, and improvement in durability.

What is RMC?

Ready Mix Concrete
RMC has been defined in various ways by different authorities, including the Bureau of Indian Standards (IS 4926: 1990, which is under revision) and the American Concrete Institute (116R-90). A comprehensive definition which covers essential characteristics can be an “RMC is concrete that is manufactured in a stationary mixer, in a central batching and mixing plant, for delivery or supply to a purchaser in a plastic and unhardened state, requiring no further treatment before being placed in the position where it is to set and to harden.”

Advantages of RMC Uniform and Assured Quality of Concrete

Ready Mix Concrete
Various problems arising out of the labor-intensive nature of the site-mixed concrete, including those related to the quality of the concrete, are almost totally eliminated by the use of RMC. This is mainly because of the large degree of mechanization and automation in the production of RMC. Different raw materials like cement, coarse aggregates, sand, water and admixtures are accurately weighed, correctly proportioned and mixed thoroughly in RMC plants, which are almost 100% computer controlled now-a-days. As a result, the batch-to-batch variation in the quality of concrete is negligible. Further, there is generally a better control on the quality of the ingredients used, in view of the care exercised by the RMC manufacturer in selecting appropriate sources of raw materials, as well as their testing which is carried out regularly at the plant laboratory. Thus, the customers get consistently uniform and high quality concrete.

Faster Construction Speed

In view of the automated operations, construction activities can be sped up by continuous supply of concrete and mechanized placing. For big construction works like dams, raft foundations, bridge piers and superstructures, slabs of buildings and commercial complexes, and so on, large volumes of concrete are required on a continuous basis. With the use of ordinary site mixers and their labor-intensive techniques of producing concrete, it is just not possible to accomplish such jobs economically and in time. Therefore, in such cases it becomes essential to use RMC and adopt mechanized methods of handling and placing. Improved speed of construction thus achieved, invariably leads to lowering of the overall project cost.

Storage Needs at Construction Sites Eliminated

With the use of RMC, there is no need to store cement, sand, aggregates, water and admixtures at the workplace thus drastically reducing the space requirements at the construction site. In a metropolitan area, where there is a premium on space, this would lead to large cost savings. Further, the contractor/builder need not invest in temporary/permanent structures required for storing different concrete ingredients, nor worry about pilferage and re-supply problems.

Drastic Savings in Labor Requirements

If concrete is produced at site, a large work force is required for handling the different concrete ingredients, as well as for batching, mixing, transporting and placing operations. By using RMC, this labor requirement is drastically reduced. Further, as the RMC manufacturer looks after the quality control needs, technical manpower of the contractor/builder required for quality assurance and quality control purposes, also gets minimized, leading overall to considerable saving in salaries and other allied costs.

Addition of Admixtures is Easier

Now-a-days, the use of both mineral admixtures (flyash, blast-furnace slag, silica fume etc.) and chemical admixtures (plasticisers, retarders, superplasticisers, etc.) is growing. An RMC plant is well–equipped to handle these materials safely and efficiently. While a batching plant can easily be fitted with separate containers to store and use mineral admixtures, a well-designed dispensing arrangement can also be incorporated in it for adding chemical admixtures to the concrete. Further, in view of the high-efficiency of mixing in RMC plants, the fine powders of mineral admixtures are uniformly blended with other ingredients in the concrete; which is an essential requirement for them to impart the required characteristics to the mix.

Documentation of the Mix Design

With the latest generation of RMC plants having micro-processor-based computer controls, it is possible to automatically document and keep a record of the mixes produced and delivered, as well as the quantities of the ingredients used. These records cannot only be helpful in keeping a control on cost, but also useful for pinpointing the problem area in case of any dispute or trouble at a later date.

Reduction in Wastage of Materials

In a labor-intensive job, wastage normally occurs in the handling of all materials, particularly cement. The losses of cement are generally of the order of 2 to 3 kg per 50–kg bag of cement. This wastage is drastically reduced in an RMC facility. Firstly, because RMC plants generally use bulk cement, which saves handling of individual bags. And secondly, because automatic handling is much more efficient than use of manual labor for the same purpose. Cement is a costly input in RMC and around 5% savings on this account could lead to substantial benefits overall.

RMC is Eco-friendly

At a typical construction site in an urban area using site-mixed concrete, the stored ingredients of concrete, more often than not, spill on the footpaths and roads, obstructing pedestrian and vehicular traffic, besides clogging up manholes and drains. Such hazards do not exist on sites using RMC. A construction site using RMC would generally look quite neat and clean.

Further, as mentioned earlier, the use of RMC enables in reducing the wastage of materials, thus minimizing the uneconomical use of non-renewable raw materials.

At the RMC plant, relevant technology can be used to prevent or minimize dust emissions in accordance with local and national regulations. Plant pollution can be minimized through appropriate plant design, location and technology. Suitable action can be taken to improve effluent quality and reduce volumes of discharge.

RMC can thus be an eco-friendly product, an advantage that is certainly appreciated in the growing environmentally conscious world.

Situation in other Countries

Delivery of RMC, although started in 1913, invariably posed numerous problems, which were only sorted out by 1926, with the advent of the revolving-drum type transit mixer. RMC then came into its own and by the late 1920s and early 1930s, and was introduced in several European countries.

Some of the early RMC plants were of a very small capacity. In 1931, a Ready-Mixed Concrete plant set up in London, had a 1.52 m3 capacity central mixer, supplying six 1.33 m3 capacity agitators with an output of 31 m3 per hour. The cement was handled manually in bags. Till around 1940, there were only six firms producing RMC in the U.K. After the Second World War ended in 1945, there was a boost to the RMC industry in the whole of Europe, including the U.K., due to the large scale reconstruction required in cities which had been extensively damaged due to bombing, shelling or street fighting.

In Europe, the European Ready Mixed Concrete Organisation (ERMCO) was formed in 1967, which is a federation of the National Associations of the respective countries. As of 2004, there were over 6,000 companies represented by it, having a turnover in excess of 20 billion Euros and producing a total of 320 million m3 of RMC. Cement consumption for RMC ranged from 33–66% of total cement sales in different countries, and RMC consumption was of the order of 0.3 to 1.5 m3 per capita, per annum. By early 2004, there were as many as 1,100 RMC plants in the U.K. consuming about 45% of the cement produced in that country.

Coming to the USA, it may be noted that till 1933, only 5% of the cement produced in the country was utilized for making RMC. ASTM published the first specification for Ready-Mixed Concrete, in 1934. However, the industry in USA has progressed steadily since then. Between 1950 and 1975, the RMC industry’s consumption of cement increased from 33% to 67% of the total cement produced in the country; and by 1990, this consumption increased to 72.4 percent of the total cement used in that country. There were as many as 5,000 RMC companies in the country in 1978. However, due to consolidation in the industry, this number dropped to 3,700 in 1994, with only 6 to 7 percent of the companies controlling nearly 50% of the RMC market share. Since then the number of RMC companies in USA has more or less remained the same.

In Japan, the first RMC plant was set up in 1949. Initially, dump trucks were used to haul concrete of low consistency for road construction. In the early 1950s, rotating type truck mixers were introduced and since then there has been a phenomenal growth of the industry in that country. By 1973, there were 3,413 RMC plants in Japan and this number rose to 4,462 by the end of the 1970s. By 1992 Japan was the then largest producer of RMC, producing 181.96 million tonnes of such concrete per year. Even today, Japan is among the largest producers of RMC, though China, from where reliable figures are difficult to obtain, is fast catching up and may even have become No. 1.

In many other countries of the world, including some of the developing countries like Taiwan, Malaysia and Indonesia, as well as certain countries in the Gulf Region, the RMC industry is not only well–developed today, but is also expanding at a rapid rate.

Development in India

Ready Mix Concrete
Ready-Mixed Concrete plants arrived in India in the early 1950s, but their use was restricted to major construction projects such as large dams, where they were used as captive units dedicated to a single consumer. Bhakra, Nagarjunasagar and Koyna dams were some of the early projects where RMC was used. Later on, RMC was also used for other large projects such as construction of long-span bridges, industrial complexes etc. Here too RMC was from captive plants, which formed an integral part of the construction project. RMC in a true commercial sense had yet to arrive in the country.

In 1974, a techno-economic feasibility study for setting up of RMC plants in India was conducted by the Central Building Research Institute (CBRI), Roorkee. This study recommended setting up of RMC plants in major metropolitan towns of the country. It also suggested the use of flyash as a partial replacement of cement to effect savings in cost.

In the late 1970s, the then Cement Research Institute of India (CRI) now the National Council for Cement and Building Materials (NCBM) carried out a techno-economic viability study of RMC, transported without agitation. In this study, it was observed that conventional RMC would be uneconomical under the then prevailing conditions, since only small volumes of concrete (1m3 or less) could be handled by the available transport at a time, thereby making the transportation cost per unit higher. To reduce the total cost, the study suggested that only a part of the mixing water (about 60%) be added at the central plant, and such a “semi-dry” mix be transported in non-agitating trucks to the construction site, where the mix could be discharged from the truck to the mixer and remixed with the addition of the balance required water. This study further recommended that once the demand for RMC goes up, conventional agitator trucks could be introduced, without any change in the central infrastructure. Based on this study, a feasibility report for setting up of an RMC plant at Delhi was jointly prepared by the NCB and the Central Public Works Department in 1988. The report, like many others gathered dust for a long time, before being implemented in the mid-1990s, only to have the plant close down after only a few years of operation.

It was during the 1970s, when the Indian construction industry went overseas, particularly to West Asia, that an awareness of Ready-Mixed Concrete was created among Indian engineers, contractors, builders etc. Indian contractors in their works abroad began to use RMC plants of 15-m3 /hr to 60-m3 /hr capacity, and some of these plants were brought back to India during the mid-1980s. In the meantime, Indian equipment manufacturers had also started manufacturing small RMC plants.

Ready Mix Concrete

However, it was only after cement was fully decontrolled, and particularly since the early 1990s, that RMC has been talked about in our country on a commercial basis. The first plant belonging of Ready-Mixed Concrete Industries, Pune was set up in 1993. However, this plant was not a financial success due to several reasons. These included high taxes; a dispute as to whether RMC was a service (as claimed by the producer) or a manufactured product (as decided by the local authorities), the latter attracting much higher duties; hesitancy by builders to use the new product; and higher cost of RMC vis-à-vis site mixed concrete. Later in 1994, the Associated Cement Companies Ltd. (ACC) set up a commercial plant at Bandra in Mumbai. Since then, a large number of other players have set up RMC plants in the country, mainly in metropolitan areas. Currently, based on the information obtained from various RMC manufacturers, there are 198 RMC plants in existence in the country with a combined capacity of 9887 cubic mts per hour. More RMC plants are being planned to be set-up in the near future.

RMC for Road Works

Concrete roads, though initially somewhat costlier, have a large number of advantages over bituminous ones. It is due to this that a sizeable number of concrete roads are now coming up in our country in all sectors–rural, urban, inter-city, national highway and expressways. For highways, where slip-form pavers are used, single-lane pavers and 60 to 90 cubic metres per hour captive batching plants are often matched together for the work. Although these two machines suit each other, they cause delays on the job, thereby driving up costs. The reason for this is that most modern highways in our country, now have or will have, two or three lanes widths on each side. This road width forces single-lane pavers to go back and forth in order to concrete the entire breadth of the pavement. Extra-wide slip-form pavers that can concrete the entire road width in one go are available, or can be procured, but they would then need 150 to 250 cubic metres of concrete per hour, in order to operate at their optimum capacity. Batching plants that can supply such large quantities of concrete are generally too costly for use by construction contractors. However, these are well within the reach of RMC suppliers. Hence RMC is extremely suitable for concrete road construction, where it can save both time and money.

The Urban Construction Boom

Ready Mix Concrete
Although India is supposed to live in its villages, our country is rapidly developing an almost unmanageable rate of urbanization. At the time of Independence, we had a population of 33 crores, out of which only 14% lived in cities. At the turn of the century, while our population had jumped up to over 100 crores, its urban component had risen to almost 33 percent i.e. today we have more people living in towns and cities, than lived in the entire country in 1947.

With 6 mega cities, 23 metropolitan cities and almost 4,000 large and small towns, India is facing a construction boom. Cement consumption has crossed 120 million tonnes per annum. Unfortunately, most of this cement is used to make concrete in very small, primitive, on-site mixers; this results in large-scale pollution, wastage of cement and low quality output. The resultant construction is weak, sensitive to weather and has a short life. This is a criminal waste of our country’s limited resources. It is therefore essential that on-site mixing be banned and RMC made mandatory for all construction work. The reduction in pollution and improvement in quality that will take place as a result, will more than offset any problems such orders may cause.

Performance Parameters

Till fairly recently, the strength of concrete was almost the sole criterion for its formulation. The other properties required for satisfactory performance of the completed construction, were generally assumed to be based on the strength factor. However, experiments, investigations and experience gradually proved that this was often a fallacious assumption. Today, by and large, performance criteria for concrete end products are formulated independently from strength. It is an accepted fact, by modern-day engineers; architects and builders, that properties for concrete can be predefined; and then obtained by suitably adapting the concrete mix design and the admixtures to be added to it. Concretes can thus be “tailor-made” to suit specifications and requirements laid down by customers but this can only be done in RMC plants.

Modern computer-controlled RMC plants can be used to literally design concrete mixes, thereby producing an almost unrestricted requirement of desirable end products; sleek, sophisticated, long-lasting, superior creations; whether they be buildings, roads dams, bridges or what-have-you. RMC means Concrete fit for posterity.

NBMCW June 2009

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Flexural Behavior of Self–compacting ...

Self compacting Concrete

Prof.K.Vijai, Associate Professor and Dr. R.Kumutha, Professor and Head Department of Civil Engineering, Sethu Institute of Technology, Kariapatti, (TN)

The use of Self–compacting Concrete (SCC) is spreading world wide because of its very attractive properties in the fresh state as well as after hardening. Several attempts have been made in the recent years to study about the strength and behavior of SCC. However, only few studies have been conducted on the strength and behaviour of structural elements made using Fibre Reinforced Self–compacting Concrete (FRSCC). Therefore, an attempt has been made in the present investigation to study the effect of recron polyester fibres on the strength and behavior of FRSCC structural elements subjected to flexure. A mix proportion of SCC was arrived by using trial and error method. Totally, nine trial mixes were investigated and w/c ratio, cement content, water content, was maintained constant for all the mixes. The variable in this study is percentage of volume fraction (0.25, 0.5 and 0.75) of recron fibres of 12mm length. Finally, six beams were cast, out of which two were FRSCC beams, two were conventional concrete beams with fibres and two control beams without fibres one made with conventional concrete (CC) and the other one made with SCC. The effect of addition of recron fibres in the compressive strength and the flexural behavior of conventional and SCC beams has been studied.

Introduction

SCC is now an emerging technique in the field of concrete technology. SCC is an innovative idea to tackle the problem of concreting through dense reinforcement. SCC is unique, because of its properties, like fill ability, flowability, pumpability, and make production of concrete more industrialized. The use of cementitious fines like fly ash makes the concrete economical. It becomes necessary to develop a compaction free production system thereby reducing the overall cost of the project, improve the quality of the work, and providing safety in the work environment. SCC possesses high flowability, resistance to segregation, passing ability which enables the concrete to fill in through the dense reinforcement. In addition to the properties of fresh concrete, SCC should also possess the properties of hardened concrete, in order to ensure the hardened concrete properties. The use of SCC will lead to a more industrialized production, reduce the technical costs of in situ cast concrete constructions, improve quality, durability, pump ability and reliability of concrete structures and eliminate some of the potential for human error. It will replace manual compaction of fresh concrete with a modern semi-automatic placing technology and in that way improve health and safety in and around the construction site. SCC must have a great filling ability, a high segregation resistance during and after placing the concrete and a great filling ability through dense reinforcement and around other obstacles such as recesses and embedded items.

The use of fiber-reinforced concrete or cement composites to enhance the performance of structural elements has been the subject of many research projects during the past few decades. Numerous types of fiber-reinforced concrete or cement composites reinforced with steel, polymeric, glass, and carbon fibers have been evaluated for structural applications. As one might suspect, not all fiber-reinforced concrete or cement composites behave in a similar manner, and thus proper material selection is critical to achieve the desired structural performance.

Steel fibre reinforced concrete (SFRC) is a composite material in which short discrete steel fibres are randomly distributed throughout the concrete mass. Extensive research work on SFRC has established that the addition of steel fibres to plain cement concrete (PCC) improves its strength, durability, toughness, ductility, post-cracking load resistance, etc [1-2].Owing to the favourable characteristics of SFRC, its use has steadily increased during the last two decades all over the world and its current fields of application includes airport and highway pavements, earthquake-resistant and explosion resistant structures, mine and tunnel linings, bridge deck over lays, hydraulic structures, rock-slope stabilisation, etc[3].

Behavior of Steel fibre Reinforced Self–compacting Concrete Flexural Elements has been studied by Ganesan N, Indira P.V and Santhosh Kumar P.T and it was found that the addition of steel fibres improved the crack load and ultimate strength of the concrete [4]. Steel fibres are added to improve tensile strength and fracture properties of concrete. Such an addition results in imparting ductility to an otherwise brittle material. The addition increases the strain capacity and imparts improvement in ductility also known as pseudo-ductility. The ductility factor increased up to a volume fraction of steel fibres of 0.5% for all the aspect ratios. A literature review indicated that while several researchers studied the effect of addition of steel fibres on the strength characteristics of conventional concrete and self compacting concrete, no study has been conducted so far to investigate the effect of addition polyester fibres on the flexural strength of conventional and self compacting concrete. Therefore, an attempt has been made to study the effect of addition of recron 3S polyester fibres on the flexural strength of normal and self compacting concrete. Test results obtained in this investigation are presented and discussed.

Experimental Programme

Materials used

Ordinary Portland cement having a specific gravity of 3.09 was used in the casting of the specimens. Crushed Granite aggregates with a maximum nominal size of 12mm and a specific gravity 0f 2.66 were used as coarse aggregates. Locally available natural river sand with a specific gravity of 2.48 was used as the fine aggregate. Fly ash obtained from the nearest thermal power station was used as a filler material in SCC. Conplast SP430 was used as an admixture to improve the workability of concrete.Recron-3S polyester fibres of 12mm length having a tensile strength of 400 N/mm2to 600 N/mm2were used in fibre reinforced SCC.

Test Methods for SCC

Properties of Fresh Concrete

In order to flow and fill through the dense reinforcement the SCC must possess certain properties like flow ability, fill ability, resistance for segregation. The major fresh concrete properties of SCC are Flow ability, Stability and Fillability. Table 1 lists the test methods to study the properties of SCC in fresh state and their acceptance criteria as per the guidelines provided by EFNARC [5]. The L-box test gives an indication of the resistance of the mixture to flow round observations in the L-box mould as shown in Figure 1. This test also detects the tendency of the coarse aggregate particles to stay back or settle down, when the mixture flows through closely spaced reinforcements. The slump flow test judges the ability of concrete to deform under its own weight. Viscosity of the mortar phase is obtained by a V-funnel apparatus as shown in Figure 2.

L-Box Test

Sequential procedure for arriving mix proportion of SCC

Initially, a normal mix with 100 mm slump is targeted without the use of superplasticizer simply by adjusting the water-cement ratio. In order to arrive at the normal mix of conventional concrete, the ACI method of mix design was used. The concrete mix was designed for a compressive strength of 30 MPa. A vertical slump of 160mm to 180mm is then aimed by adding superplasticizer to the normal mix. If any segregation, especially bleeding takes place at this stage, as judged visually, a part of coarse aggregate was replaced with fine aggregate. The percentage of replacement was chosen to be small, say 5 percent, by weight. To proceed towards achieving SCC, coarse aggregate is then replaced with a filler material namely fly ash, by weight starting from a value of 5 percent, 10 percent, 15 percent etc, until a slump flow of 500mm – 800mm was achieved by slump flow test. Mixes which were arrived, based on the sequential procedure are given in the Table 2.

Mix proportions for various trials

The mix ratios conforming to the slump flow specifications were tested for the V funnel and L Box tests. For the mix to get qualified as SCC it should satisfy the specifications of V funnel and L Box also. Then the fresh properties of concrete were also tested by the addition of recron fibres in the arrived SCC mix. Test results of fresh concrete are given in Table 3. Control SCC mix was obtained by replacing 25% of coarse aggregates by fly ash and by adding superplastcizers of 1% by weight of cementitious materials. When 0.25% and 0.5% of recron fibres were added to the control SCC mix, it satisfies the fresh concrete properties suggested by EFNARC guidelines and it was taken for the investigation of flexural behavior of beam. When 0.75% of recron fibres were added to the control SCC mix, fresh concrete properties given by EFNARC guidelines were not fulfilled and hence it was not taken for further investigation.

Test result of fresh Concrete

Flexural Behavior of Conventional and Self– compacting concrete

Self compacting Concrete
To study the flexural behavior, totally six reinforced concrete beams of size 1200mmx100mmx150mm were cast, out of which two were FRSCC beams, two were conventional concrete beams with fibres and two control beams without fibres one made with conventional concrete and the other one made with SCC. Two numbers of 10mm diameter Fe 415 grade steel bars were provided as longitudinal reinforcement and two legged 6mm diameter stirrups were provided at 150mm spacing in the mid span. The spacing of the stirrups was kept as 80mm in the shear zone. The beams were simply supported and subjected to two point loading as shown in Figure 3. A hydraulic jack of 15T capacity was used to apply the load. The dial gauges were fixed at the centre of the beam and under the load points to record the deflection of the beam during test. The deflection of the beams at mid span and under the load points were measured at every 0.2T intervals of loading. At every loading stage, cracks appearing on the surfaces were marked and crack widths were measured using a crack width measuring microscope The beam was loaded up to failure. The test results for conventional concrete and SCC beams are given in Tables 4 and 5. From the test results, load-deflection characteristics were studied and given in Figures 4 and 5. Figures 6 and 7 show the specimens after failure.

Test result for conventional Concrete
Self compacting Concrete

Self compacting Concrete

Discussion of test results

From the load-deflection curves, it has been noted that when recron polyester fibres are added, energy absorption capacity increases for both conventional and self compacting concrete. The compressive strength of conventional concrete and Self– compacting Concrete was found to be improved by the addition of Recron 3S fibres. Self–compacting Concrete shows higher compressive strength when compared to conventional concrete and this trend was applicable in fibre reinforced concrete also. In case of conventional concrete the compressive strength enhanced by about 9.8% and 4.4% for 0.25% and 0.5% of volume fraction of fibres respectively. In self compacting concrete compressive strength of concrete increased by about 12% and 6.9% for 0.25% and 0.5% of volume fraction of fibres respectively.

The first crack load in conventional concrete beam is 9 kN. When 0.25% and 0.5% fibres are added in conventional concrete beam the first crack load is enhanced to 10 kN and 12 kN respectively. Similarly in SCC beams the first crack load is 8 kN and it is increased to 12 kN and 14 kN due to addition of 0.25% and 0.5% fibres respectively. Because of addition of 0.25% and 0.5% of fibres the load carrying capacity of conventional concrete beam is enhanced by 4.4% and 8.4% whereas the load carrying capacity of SCC beams is increased by 2.8% and 5.3% respectively. The stiffness was also found to be increasing gradually both in Conventional as well as in SCC Beams.

Concrete Beam

Conclusions

Based on the results of this experimental investigation, the following conclusions are drawn:

The compressive strength of SCC was found to be improved by the addition of recron fibres to the mix.Also the SCC shows higher compressive strength when compared to Conventional concrete. The first crack load was also found to be increased in SCC than the CC beams. The ultimate load carrying capacity of the beam has also increased due to the addition of recron fibres in the mix. The stiffness has also increased gradually both in conventional as well as in SCC due to the addition of fibres. The SCC shows good properties than the conventional concrete properties. The SCC does not require compaction or it can be compacted with a very little effort. SCC is very useful in concreting through the congested reinforcement. The use of SCC increases the pump ability of the concrete; this reduces the labour cost and increases the safety at the work site. It improves the economy of the concreting work. Therefore, In general practice also the use of SCC proves to be advantageous.

References

  • B Singh, P Kumar and S K Kaushik. .High Performance Composites for the New Millennium. Journal of Structural Engineering, Vol 28, no 1, April-June 2001, pp 17-26.
  • N Ganesan and K P Shivananda. Strength and Ductility of Latex Modified Steel Fibre Reinforced Concrete Flexural Members. Journal of Structural Engineering, Vol 27, no 2, July 2000, pp 111-116.
  • V Ramakrishnan. .Materials and Properties of Fibre Reinforced Concrete.Proceedings of the International Symposium on Fibre Reinforced Concrete, Vol 1,December 16-19, 1987, Madras, pp 2.3-2.23.
  • Ganesan N, Indira P.V And Santhosh Kumar P.T Ultimate Strength Of Steel Fibre Reinforced Self Compacting Concreete Flexural Elements, The Indian Concrete Journal, December 2006.
  • Poulson, B EFNARC (2002) “Specification And Guidelines For Self-Compacting Concrete”

NBMCW May 2009

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Parametric Study on Use of Pozzolanic M...

Amit Mittal, M. B. Kaisare, Rajendrakumar Shetti, Tarapur Atomic Power Project 3 & 4, Nuclear Power Corporation of India Limited, Maharashtra.

The utilization of pozzolanic materials in concrete as partial replacement of cement is gaining immense importance today, mainly on account of the improvements in the long-term durability of concrete combined with ecological benefits. Fly ash, Ground Granulated Blast Furnace Slag (GGBS) and High Reactive Metakaolin (HRM) are the pozzolanic materials, which conform to these requirements and largely available in India. To study the effect of partial replacement of cement by these pozzolanic materials, studies have been conducted on concrete mixes with 350 to 500 kg/cu‎m cementitious material at 30%, 40%, and 50% replacement levels of fly ash; 50% and 60% replacement levels of GGBS and 7.5% and 10% replacement levels of HRM. In this paper, the effect of these pozzolanic materials on workability, setting time, density, air content, compressive strength, modulus of elasticity and permeability by Rapid Chloride Permeability Test (RCPT) are studied. Based on this study, compressive strength vs W/Cm curves have been plotted so that concrete mixes of grade M15 to M45 with different percentage of any of these pozzolanic materials can directly be designed.

Introduction

The Ordinary Portland Cement (OPC) is one of the main ingredients used for the production of concrete and has no alternative in the civil construction industry. Unfortunately, production of cement involves emission of large amounts of carbon-dioxide gas into the atmosphere, a major contributor for green house effect and the global warming, hence it is inevitable either to search for another material or partly replace it by some other material. The search for any such material, which can be used as an alternative or as a supplementary for cement should lead to global sustainable development and lowest possible environmental impact.

Fly ash, Ground Granulated Blastfurnace Slag, High Reactive Metakaolin, Micro silica, and so on are some of the pozzolanic materials which can be used in concrete as partial replacement of cement. A number of studies are going on in India as well as abroad to study the impact of use of these pozzolanic materials as cement replacements and the results are encouraging.

Fly ash is finely divided residue that results from the combustion of coal and transported by flue gas. India is a resourceful country for fly ash generation with an annual output of over 110 million tonnes, but utilization is still below 20% in spite of quantum jump in last three to four years. Availability of consistent quality fly ash across the country and awareness of positive effects of using fly ash in concrete are pre- requisite for change of perception of fly ash from 'A waste material' to 'A resource material'. Now a days due to strict control on quality of coal and adopting electrostatic precipitators, fly ash of consistent quality is separated and stocked, and it is gaining popularity as a good pozzolanic material for partial replacement of cement in concrete.

Ground Granulated Blastfurnace slag (GGBS) is a byproduct for manufacture of pig iron and obtained through rapid cooling by water or quenching molten slag. If slag is properly processed then it develops hydraulic property and it can effectively be used as a pozzolanic material. However, if slag is slowly air cooled then it is hydraulically inert and such crystallized slag cannot be used as pozzolanic material. Though the use of GGBS in the form of Portland slag cement is not uncommon in India, experience of using GGBS as partial replacement of cement in concrete in India is scanty.

High Reactive Metakaolin (HRM) is a quality enhancing pozzolana. HRM is manufactured from natural kaolin, which is available in abundance in the country. It is produced by calcination of natural kaolin at a temperature of 650oC to 700oC through either dry process or wet process. In India, extensive deposits of kaolins are found in almost all the states. The best quality of kaolin deposits is found in Kundara in Kerala and Singhbum in Jharkhand. The data on studies of use of HRM as a pozzolanic material is very limited. At present refractories, paints, and paper industries etc., are the major users of HRM in India. However, use of HRM in concrete is hardly observed.

Reaction Mechanism of Pozzolanic Materials

Pozzolanic materials contain little or no cementitious properties of their own but they react with calcium-hydroxide produced during the hydration of cement. Fly ash and HRM have almost negligible cementitious properties, however GGBS is a hydraulically latent material and reacts with water directly as well as participates in the secondary reaction with calcium hydroxide. The reaction mechanism of fly ash, GGBS and HRM in concrete is briefly explained below.

Reaction Mechanism of Fly Ash

Majority of the fly ash available in India are silicious type (ASTM C 618 Type-F) which contains reactive calcium oxide less than 10% and posses no hydraulic properties. It does not react with water directly. The silica present in the fly ash reacts with calcium hydroxide (CH), produced during the hydration of cement, and the principal product of reaction is calcium silicate hydrate (C-S-H). The reaction of fly ash depends largely upon breakdown and dissolution of the glossy structure by the hydroxide ions and the heat generated during early hydration of Portland cement fraction. Reaction of fly ash and water in the presence of water is described as below:

Product of hydration of OPC

OPC (C3S/C2S) + H2O————————> C-S-H + CH

Reaction of pozzolanic material

CH + S ———————————————> C-S-H

The reaction of fly ash continues to consume calcium hydroxide to form additional C-S-H as long as calcium hydroxide is present in the pore fluid of cement paste.

Reaction Mechanism of Ground Granulated Blast furnace Slag

Although GGBS is a hydraulically latent material, in presence of lime contributed from cement, a secondary reaction involving glass (Calcium Alumino Silicates) components sets in. As a consequence of this, cementitious compounds are formed. They are categorized as secondary C-S-H gel. The interaction of GGBS and Cement in presence of water is described as below:

Product of hydration of OPC

OPC(C3S/C2S) + H2O ————————> C-S-H + CH

Product of hydration of GGBS

GGBS(C2AS/C2MS) + H2O ——————> C-S-H + SiO2

Reaction of pozzolanic material

SiO2+ CH + H2O ——————————————>C-S-H

The generation of secondary gel results in formation of additional C-S-H, a principal binding material. This is the main attribute of GGBS, which contributes to the strength and durability of the structure. The diagrammatic representation of secondary gel formation is shown below.

Formation of Secondary Gel

Reaction Mechanism of High Reactive Metakaolin

Metakaolin is a lime-hungry pozzolana that reacts with free calcium hydroxide to form stable, insoluble, strength-adding, cementitious compounds.

When Metakaolin- HRM (AS2) reacts with calcium hydroxide (CH), a cement hydration byproduct, a pozzolanic reaction takes place whereby new cementitious compounds, C2ASH8 and CSH are formed. These newly formed compounds contribute to cementitious strength and enhance durability properties to the system in place of the otherwise weak and soluble calcium hydroxide.

Product of hydration of OPC

OPC(C3S/C2S) + H2O ————————> CSH + CH

Reaction of pozzolanic material

AS2 + CH + H2O ———————————> C2ASH8 + CSH

Unlike other commercially available pozzolanic materials, Metakaolin is a quality-controlled, manufactured material. It is not a byproduct of an unrelated industrial process. Metakaolin has been engineered and optimized to contain a minimum of impurities and to react efficiently with cement's hydration byproduct, the calcium hydroxide.

Ingredients

Test Result of Cement

Test Results of Admixtures
Cement

The Ordinary Portland Cement of 43 grade conforming to IS: 8112 was used. The 28 days compressive strength and the specific surface of cement used in this study was 60 N/mm2 and 289 m2/kg respectively. The test results of cement used for this study are given in Table-1.

Coarse Aggregates

The Coarse Aggregates from crushed Basalt rock, conforming to IS: 383 were used. The Flakiness and Elongation Index were well-maintained below 15%.

Fine Aggregates

The river sand and crushed sand was used in a combination as fine aggregate conforming to the requirements of IS:383. The river sand was washed and screened to eliminate deleterious materials and over size particles.

Admixture

The high range water reducing and retarding superplasticizer conforming to ASTM C-494 Type G was used. The base of admixture used in this study was sulphonated naphthalene formaldehyde and water reduction of admixture was around 24%. The test results of admixture used for this study are given in Table-2.

Fly Ash

Fly ash is a finely divided residue that results from the combustion of ground or powdered coal and is transported from the combustion chamber by exhaust gases. The fly ash is extracted from the flu gases by electrostatic precipitators and collected separately in different fields depending upon their specific surface area.

Technological efforts have been made to improve the quality of fly ash. At present most of the power plants are using Electro Static Precipitators (ESP) through which fly ash is collected in different chambers according to its particle size. Hence a uniform good quality of fly ash can be collected from these power plants. Some of the power plants have gone a step further by developing a collection system, in which the fly ash collected from different fields is combined and the final product is taken to an air classifying plant where coarse particles are removed. The final beneficial product is then stored in a silo to be used in cement and concrete industry.

Test Result of Flyash
Fly ash for this study was taken from Dahanu Thermal Power Station (DTPS) at Dahanu. DTPS has installed ESP for segregation and collection of fly ash into 6 different fields. As the field number increases the fineness of fly ash increases but the quantity decreases. Field-1 fly ash has coarse particles and is not suitable for concrete applications. Fly ash from Field-2 onwards is segregated, packed and used for concrete applications. Since maximum availability of fly ash is from Field 2, same was used for our study. This fly ash conforms to the requirements of IS: 3812 Part 1 and also ASTM C-618 Type F.

Generally, fly ash quality is assessed on the basis of some of key parameters like pozzolanic activity, material retained on 45 micron sieve, loss on ignition and other chemical parameters. It is advisable that to qualify a source of fly ash all the test as specified in IS/ASTM will be conducted initially and only key parameters can be tested for each batch to ensure a consistent quality of fly ash. Test results of fly ash used in the experimental study are given in Table-3.

Ground Granulated Blastfurnace Slag

In the process of manufacturing of pig iron, the molten slag is produced which is instantaneously tapped and quenched by water. This rapid quenching of molten slag facilitate formation of "Granulated slag." Ground Granulated Blast furnace Slag (GGBS) is processed from Granulated slag.

Test Result of GGBS
GGBS essentially consists of silicates and alumino silicates of calcium and other bases that is developed in a molten condition simultaneously with iron in a blast furnace. The chemical composition of oxides in GGBS is similar to that of Portland cement but the proportions varies.

The four major factors, which influence the hydraulic activity of slag, are glass content, chemical composition, mineralogical composition and fineness. The glass content of GGBS affects the hydraulic property, chemical composition determines the alkalinity of the slag and the structure of glass. The compressive strength of concrete varies with the fineness of GGBS.

GGBS used in the present study is taken from M/s. Indorama Cement Ltd., Mumbai and it conforms to requirements of BS 6699. Test results of GGBS used in this study are given in Table-4.

High Reactive Metakaolin

HRM, white in color, is a manufactured and process controlled, reactive aluminosilicate pozzolana. It is formed by calcining purified kaolinite at a specified temperature range, generally between 650-7000C. There are various methods of processing kaolin, which could be principally grouped into two types—wet process and dry process. Of these, the wet processing route is reported to be better because it helps to remove most of the impurities.

Test Result of HRM
Unlike other pozzolana such as micro silica and fly ash which are industrial byproducts, HRM is specially manufactured and the process is controlled in a cost effective manner. Moreover, it is water processed to remove non-reactive impurities, producing an almost 100% reactive pozzolana. Its particle size is significantly smaller than cement particles, of the order of 1.5 micron, with a specific gravity 2.5.

As of today no separate ASTM, EN or BIS specification for HRM exists, but it is classified as a natural pozzolana conforming to ASTM C 618 Type N. Indian code of practice for Reinforced concrete, IS 456-2000, recommends the use of HRM in concrete. HRM used in the present study is taken from M/s. 20 Microns, Mumbai, and it conforms to requirements of ASTM C 618. Test results of GGBS used in this study are given in Table-5.

Experimental Set Up

Concrete mixes taken up for this study were proportioned with total cementitious content starting from 350 kg/cu‎m to 500 kg/cu‎m in the increments of 50 kg/cu‎m. For each cementitious content a control mix (without any pozzolanic material) was proportioned as a reference mix. For fly ash, three different replacement levels i.e., 30%, 40%, and 50% were used for each cementitious content. For GGBS two different replacement levels i.e., 50% and 60% were used for 350, 400, and 450 cementitious content. For HRM two different replacement levels i.e., 7.5% and 10% were used for each cementitious content. Each mix was identified by a unique no. 'x''y''z' where 'x' indicates the total cementitious content, 'y' indicates the type of pozzolanic material (F-fly ash, G-GGBS and H-HRM) and 'z' indicates the percentage replacement by pozzolanic material. Control mixes are indicated as 'x' Control. All the concrete mixes were proportioned using absolute volume method.

High range superplasticizer was used in all the concrete mixes to achieve good workability. A slump of 175 mm + 25 mm was maintained in all the mixes to ensure that these mixes could be pumped and placed even in the most congested areas. Unit water content was kept constant for a particular series of mixes of same cementitious content. To achieve the uniform workability, the admixture dosage was adjusted without changing the unit water content. This ensured the identical W/Cm ratio for a particular cementitious content and the effect of pozzolanic material replacement can directly be studied on the various properties of concrete. Mix proportions of all the mixes is given in Table-6.

Concrete Mix Proportions

All the concrete mixes were produced in the concrete technology laboratory of Tarapur Atomic Power Project 3 & 4 (TAPP 3&4) using pan type laboratory concrete mixer. Mixing sequence and time was also standardized in all the mixes to minimize the variations. The 3000 KN automatic compression testing machine was used to determine the strength properties of concrete mixes.

Results and Discussions

Test results of properties of fresh and hardened concrete are given in Table–7. The main observations are as follows.

Properties of Flyash and Hardened Concrete

Workability

Admixture Dosage vs Fly ash content of Concrete Mixes

Admixture Dosage vs GGBS and HRM content
The spherical shaped particles of fly ash act as miniature ball bearing within the concrete mix and which leads to the improvement of workability of concrete or reduction of unit water content. In the present study we have kept the same unit water content for a particular series of mixes of same cementitious content, hence to maintain the same workability of concrete the admixture dosage are reduced as the fly ash content is increased from 0% to 50%. Figure-1 shows the details of admixture dosage versus fly ash content of concrete mixes with different cementitious content. From figure-1, it can be observed that for all ranges of cementitious content, the reduction in admixture dosage with increase in fly ash percentage takes place to maintain the same workability.

The improvement of workability or reduction of admixture dosage is not observed when GGBS and HRM are used as a replacement of cement. This could be due to the fact that the particle shapes of GGBS and HRM are non spherical. The Figure-2 shows the details of admixture dosage versus control, GGBS and HRM mixes with different cementitious content. From Figure-2, it can be observed that to obtain similar workability for control, GGBS and HRM mixes, no specific trend of admixture dosage could be established.

Density and Air Content

It is observed that air content of the concrete mix is unaffected by the replacement of cement by fly ash, GGBS and HRM. As fly ash and GGBS have lower specific gravity as compared to cement, there is a slight reduction in the density of concrete at a higher level of cement replacement.

Initial and Final Setting Time

Setting characteristics of concrete depends upon various parameters like ambient temperature, concrete temperature, cement type, pozzolanic type, admixture type, and their relative quantities. To study the effect of replacement of cement by different pozzolanic material on setting characteristics of concrete in 350 kg/cu‎m and 450 kg/cu‎m cementitious content concrete mixes, the initial and final setting time are determined. For fly ash mixes normally it is expected that use of fly ash will retard the setting time of concrete. In this study high range water reducing and retarding superplasticizer (ASTM C- 494 Type G) has been used and the quantity of superplasticizer is getting reduced as the percentage of fly ash is increasing to maintain the same workability. Therefore, the retardation due to increase in fly ash percentage is getting compensated by the reduction of retarding type super plasticizer in concrete mix.

The initial and final setting time of 350 kg/cu‎m and 450 kg/cu‎m cementitious content concrete mixes with different pozzolanic materials are determined and results are presented in Table-7. No significant change is observed on the setting characteristic of concrete with different pozzolanic materials.

Compressive Strength at Various Ages

Compressive strength of all the mixes is determined at 3, 7, 28, 56, and 90 days. For each set of cementitious content the development of compressive strength at various ages with respect to different percentages of fly ash, GGBS and HRM are shown in Figure 3 to 6, Figure 7 to 9, and Figure 10 to 13 respectively. The observations are as follows:

Compressive Strength
  1. From Figure 3 to 6, it is observed that as the fly ash content increases, there is reduction in the strength of concrete. This reduction is more at earlier ages as compared to later ages.
  2. For a particular fly ash replacement level, say 40%, the strength development with reference to control mix is much lower at early ages than that of later ages. For concrete mixes of 350 kg/cu‎m and 400 kg/cu‎m cementitious content, 3 days compressive strength is only around 45% of control mix, while it improves gradually to 84% of control mix at 90 days. For concrete mixes of 450 kg/cu‎m and 500 kg/cu‎m cementitious content, 3 days compressive strength is around 60% of control mix, while it improves gradually to 90% of control mix at 90 days. It is also observed that the compressive strength development with respect to control mix improves as the total cementitious content increases.
  3. From Figure 7 to 9, it is observed that for 50% replacement of cement by GGBS, compressive strength reduces at all ages for all the cementitious content with respect to control mix. Concrete mixes with 60% replacement shows higher reduction in compressive strength as compared to that of 50% replacement.

    Compressive Strength
  4. The reduction in compressive strength with replacement of cement by GGBS is more at early ages as compared to later ages.
    Compressive Strength
  5. From Figure 10 to 13, it is observed that the replacement of cement by 7.5% and 10% HRM indicates a mixed trend. In majority of cases, there is a slight increase in the compressive strength as compared to control mixes. Compressive strength of control mix and 7.5% HRM mix are almost in the same range. However, 10% HRM mixes indicate higher compressive strength as compared to control mix especially in the 400, 450, and 500 kg/cu‎m cementitious content.
  6. To compare the effect of different pozzolanic materials, 450 kg/cu‎m cementitious content mix is analyzed for compressive strength as a percentage of control mix at 7, 28, and 56 days for all replacement levels of pozzolanic materials.
Compressive Strength
Formation of Secondary Gel
Formation of Secondary Gel
Formation of Secondary Gel
Formation of Secondary Gel
Figure 14 shows the effect of different pozzolanic materials on 7, 28, and 56 days compressive strength of 450 kg/cu‎m cementitious content mix with respect to control mix. It is observed that for fly ash and GGBS mixes; there is an improvement in strength development with increase in ages at all levels of replacements. This confirms that secondary reaction of these pozzolanic materials with calcium hydroxides, which results in C-S-H, improves the strength of concrete at later ages.

For concrete mixes with HRM, it is observed that even at initial ages there is an improvement of strength as compared to control mix. However, the percentage increase with respect to control mix at 28 days and 56 days is relatively less. This is probably due to the fact that higher finesses of HRM leads to early pozzolanic reaction and improved strength at early ages.

Modulus of Elasticity

The modulus of elasticity was determined using 150 mm diameter and 300 mm high cylinder specimen as per IS: 516 at 28 and 56 days. The mix of 450 kg/m3 cementitious content was selected for determination of modulus of elasticity. Figure-15 gives the details of E-value of concrete mixes with various percentage of fly ash, GGBS and HRM. From this figure, it can be observed that at 28 days for all percentage of Fly ash and GGBS, there is reduction in E-value and this reduction increases with the increase in replacement levels. HRM mixes shows E-value very close to control mix and at 10% replacement level E-value is slightly higher than that of control mix. At 56 days E-value of all the mixes increases as compared to 28 days. As the fly ash and GGBS percentage increases the E-value reduces, however this reduction in E-value is much lower as compared to reduction in compressive strength at same age. This indicates that cement and aggregate characteristics have a greater effect on modulus of elasticity than the use of fly ash and GGBS.

Durability of Concrete

One of the major advantages of use of pozzolanic materials in concrete mixes is to improve the durability of concrete. The existence of large pores and large crystalline products in the transition zone in OPC concrete are greatly reduced by the introduction of fine particles of pozzolanic materials. The decrease in the pore interconnectivity of concrete thus decreases the permeability of blended concrete. The reduced permeability results in improved long term durability and resistance to various forms of deterioration of concrete structures.

The permeability of concrete was determined by means of Rapid Chloride Penetration Test (RCPT) as per ASTM C-1202 on 100 mm diameter and 51 mm thick concrete cores extracted from the various samples of concrete mixes. RCPT was conducted at 28, 56, and 90 days of age for various combinations of mixes. Figures-16 to 19 show the details of RCPT value. From these figures, it can be observed that generally there is reduction in permeability of concrete as the cementitious content increases (or w/cm decreases) at all ages. Reduction in permeability of concrete with the incorporation of fly ash is marginal at 28 days of age, but at 56 and 90 days age, the permeability of fly ash concrete is considerably lower as compared to concrete without fly ash. Similar trend is observed for GGBS and HRM concrete mixes also. However, reduction in RCPT value of HRM mixes at 56 and 90 days is lower as compared to the fly ash mixes and GGBS mixes. At 90 days of the age the concrete mixes with fly ash show lower permeability as compared to GGBS and HRM mixes. This is probably due to the extended pozzolanic reaction of fly ash concrete mixes. Therefore, this study proves conclusively that the concrete containing pozzolanic materials is less permeable as compared to OPC concrete for all ranges of cementitious contents.

Development of W/Cm Curves for Mix Design

Based on the various research carried out on fly ash, GGBS and HRM concrete, published data and the results of above experimental study; it is proved that pozzolanic materials can gainfully be used in making concrete of desired grades of improved characteristics at fresh and hardened stage, durable, eco-friendly and economical without any reservation.

Concrete mix proportioning with pozzolanic materials as a cementitious ingredient is slightly tricky as compared to the OPC concrete. It depends on various parameters like type of pozzolanic materials, their characteristics, percentage of replacement, age at which desired strength is required, reheological characteristics of concrete mix and durability criteria. From this study, it is clear that simple replacement of cement by pozzolanic materials reduces/improves the strength of concrete at early ages and the development of strength at various ages is related to total cementitious content or W/Cm and the percentage replacement of cement by pozzolanic materials. To simplify the mix proportioning process, based on the above study, strength versus W/Cm graphs are plotted for different percentages of fly ash, GGBS and HRM. Figures-20 and 21 give the strength versus W/Cm graphs at 28 days and 56 days age respectively. For a given type of concrete ingredients these graphs can be used to quickly design the concrete mix proportions of the desired grade. Based on the 28 days target mean strength the required W/Cm can be selected from the graphs depending on the type and percentage of pozzolanic materials in the mix. Unit water content can be decided by the workability requirements and type and dosage of admixture. Total cementitious contents, quantity of cement and pozzolanic materials is then calculated. Quantities of coarse and fine aggregates shall be worked out based on absolute volume method.

It is also observed from the study that there is a considerable increase in the compressive strength of concrete with fly ash and GGBS beyond 28 days. Therefore, structures like raft, footing and column etc. where design load is not expected to come on 28th day, and the acceptance criteria for fly ash and GGBS concrete can be based on 56 days compressive strength. Strength versus W/Cm graphs at 56 days can be used to design concrete mix proportions. This practice of accepting the concrete mix at 56 days will help in utilizing the development of strength of fly ash and GGBS concrete beyond 28 days, reduction in total cementitious content and overall economy of the concrete mix without compromising the quality of concrete.

To generalize the strength versus W/Cm curves for different pozzolanic material percentages, it is desired that similar type of exercises be conducted by various organizations using different types of concrete ingredients. All such data can be analyzed carefully giving due consideration to input materials and generalized graphs shall be developed. This will encourage many users to adopt concrete with these pozzolanic materials extensively.

Conclusions

India has a vast resource of fly ash generation all across the country. This material if segregated, collected and used properly can solve the major problems of fly ash disposal and reduce the use of cement, which consumes lot of energy and natural resources. Similarly, the vast reserves of Kaolin can also be utilized for manufacturing HRM, which can work as a quality pozzolanic material. In India, the use of GGBS in the manufacturing of Portland slag cement is gaining popularity however for the effective utilization of large quantity of GGBS, it is essential to use it as a partial replacement of cement in concrete. In India, many organizations are putting their efforts to promote the awareness of fly ash concrete and its advantages. However very limited work has been carried out on GGBS and HRM as pozzolanic materials in concrete.

The experimental exercise has helped to study the various properties of concrete with different replacement levels of fly ash, GGBS, and HRM and to develop the mix design curves for concrete mix proportioning with various percentages of these pozzolanic materials. Based on the studies conducted by authors following conclusions are drawn:
  1. Use of fly ash improves the workability of concrete. This phenomenon can be used either to reduce the unit water content of mix or to reduce the admixture dosage. GGBS and HRM mixes show no significant change in workability characteristic of concrete.
  2. Density and air content of concrete mixes are generally unaffected with the use of pozzolanic materials.
  3. Due to adjustment of admixture dosage to obtain similar workability in all concrete mixes, no significant change in setting characteristics are observed for a particular cementitious content.
  4. For fly ash and GGBS mixes as the percentage of replacement increases concrete strength reduces. This reduction is more at earlier ages as compared to later ages. This is expected, as the secondary hydration due to pozzolanic action is slower at initial stage.
  5. The strength of HRM mixes from 7 days onwards is observed to be higher than that of control mixes. As HRM percentage replacement increases further improvement in strength is observed.
  6. Modulus of elasticity of fly ash and GGBS concrete also reduces with the increase in replacement levels for a given W/Cm. Reduction in E-value is much lower as compared to compressive strength.
  7. Concrete with pozzolanic materials is more durable as compared to OPC concrete. Significant reduction in RCPT values at 56 and 90 days is observed for all the three pozzolanic materials. At 90 days, fly ash mixes have lower RCPT values as compared to GGBS and HRM mixes indicating extended pozzolanic reaction.
  8. Compressive strength versus W/Cm curves developed for different percentages of pozzolanic materials can be used as a quick guide for concrete mix proportioning.

Acknowledgments

Authors are grateful to Sh. O. P. Goyal, Site Director and Sh. H.D.Singh, Chief Construction Engineer for their guidance and encouragement. Authors are also thankful to the staff of Concrete Technology Laboratory TAPP 3&4 who have performed the trial mix studies and helped in testing and analyzing the test results.

The opinions expressed in this article, are those of the authors, and do not necessarily reflect the official views of TAPP-3&4.

References

  • ACI Committee, "Use of fly ash in concrete" ACI 232.2R-95.
  • ACI Committee, "Ground granulated blast furnace slag as a cementitious constituent in Concrete" ACI 233 R-95.
  • American Standard specification for Coal fly ash and raw of calcined natural pozzolana for use as a mineral admixture in concrete, ASTM C – 618.
  • Indian Standard, Pulverized fuel ash – Specification, Part I, for use as pozzolana in cement mortar and concrete (Second Revision), IS 3812 (Part 1): 2003.
  • Specification for ground granulated blast furnace slag for use with Portland cement, BS 6699: 1992.
  • Malhotra V. M., Ramezanianpour A. A., "Fly ash in concrete," Second edition, September 1994.
  • Mittal Amit, Kaisare M.B., Shetti R.G., "Experimental study on use of fly ash in concrete," International Congress on fly ash utilization, 4th–7th December 2005, New Delhi.
  • Mittal Amit "Chemical Admixtures—An Experience in using 4000 MT in Nuclear Industry," National Seminar on RMC & Chemical Admixture for Concrete Technology, March, 14–15 2003, Mumbai.
  • Mittal Amit, Kaisare M.B., Shetti R.G., "Use of SCC in a pump house at TAPP 3&4, Tarapur," the Indian Concrete journal Vol 78, June 2004 No 6 pp 30–34.
  • Mittal Amit, Lahari A. K, Bapat S. G, "Use of Fly ash in concrete and Quality Aspects," DAE Concrete Day Celebration, September 22, 2003, Mumbai.
  • Basu P. C., Saraswati Subhajit, "High volume fly ash concrete with Indian ingredients," the Indian Concrete Journal Vol 80, March 2006, No 3 pp 37–48.
  • Basu P C, Saraswati Subhajit, "Concrete composites with ground granulated blast furnace slag," the Indian Concrete Journal Vol 80, June 2006 No 6 pp 29– 40.
  • Mavinkurve S.S, Basu P C and Kulkarni V.R, "High Performance Concrete having High Reactivity Metokaolin," the Indian Concrete Journal Vol 77, May 2003 No 5 pp 1077–1085.
  • Mittal Amit "Experience of using Micro silica in Indian Nuclear Power Plants," New Building Materials and Construction World, Vol.10, Issue-12, June 2005, pp 18–33.
  • Technical literature "Ground Granulated Blastfurnace Slag— A new generation engineering product," M/s Indorama Cement Ltd., Mumbai.
  • Technical literature on High Reactive Metkaolin, M/s 20 Microns, Mumbai.

NBMCW October 2006

.....

Experimental Investigation on Influence...

Column Joints

experimental investigation on influence of development length in the retrofitted concrete beam - column joints

Robert Ravi.S, Senior Lecturer., Prince Arulraj.G, Director, School of Civil Engineering., Karunya University, Coimbatore

In the last few decades, moderate and severe earthquakes have struck different places in the world, causing severe damage to reinforced concrete (RC) structures. Retrofitting of existing structures is one of the major challenges that modern civil engineers have to face. Recent evaluation of civil engineering structures has demonstrated that most of them will need major repairs in the near future. Upgradation to higher seismic zones of several cities and towns in the country has also necessitated in evolving new retrofitting strategies.

One of the techniques of strengthening of the RC structural members is through confinement with a composite enclosure. This external confinement of concrete by high strength fiber reinforced polymer (FRP) composites can significantly enhance the strength and ductility and will result in large energy absorption capacity of structural members. FRP materials, which are available in the form of sheets, are being used to strengthen a variety of RC elements to enhance the flexural, shear, and axial load carrying capacity of these elements.

Beam-column joints, being the lateral and vertical load resisting members in RC structures are particularly vulnerable to failures during earthquakes and hence strengthening of the joints is often the key to successful seismic retrofit strategy. In this paper an attempt has been made to study the influence of development length in reinforced concrete beam-column joints retrofitted with Carbon fiber reinforced polymer wrap and Glass fiber reinforced polymer wrap.

Nine RC beam-column joint specimens (control) were cast and tested to failure during the present investigation. In six specimens, the development length of the beam rods were provided as per IS 456. In remaining three specimens, the development length of the beam rods were provided as per IS 13920. The failed beam-column joint specimens were retrofitted by removing the concrete in the joint portion and recasting with concrete of the same grade and subsequently wrapping of Carbon fiber reinforced polymer (CFRP) sheets. These sheets were used to wrap three specimens and Glass fiber reinforced polymer (GFRP) sheets were used to strengthen the other three specimens. The performance of the retrofitted beam-column joints was compared with the control beam-column joint specimens and the results are presented in this paper.

Introduction

Recent earthquakes have exposed the vulnerability of existing reinforced concrete (RC) beam-column joints to seismic loading. Until early 1990s, concrete jacketing and steel jacketing were the two common methods adopted for strengthening the deficient RC beam-column joints. Concrete jacketing results in substantial increase in the cross sectional area and self-weight of the structure. Steel jackets are poor in resisting weather attacks. Both methods are however labour intensive and sometimes difficult to implement at the site. A new technique has emerged recently which uses fiber reinforced polymer sheets to strengthen the beam-column joints. FRP materials have a number of favorable characteristics such as ease to install, immunity to corrosion, high strength, availability in sheets etc., The simplest way to strengthen such joints is to attach FRP sheets in the joint region in two orthogonal directions.

The initial developments of the FRP strengthening technique took place in Germany and Switzerland. Strengthening of reinforced concrete members with externally wrapped FRP laminates by Carbon and Glass FRP sheets has been studied in detail by researchers at Swiss Federal Laboratories for Materials Testing and Research, German Institute of Structural Materials and Institute for Building Construction & Fire Protection. The results obtained proved that the FRP strengthening technique is highly efficient and effective.

Literature Review

Geng et al (1998) used composite overlays to strengthen simple models of interior beam-column joints, which resulted in increase the strength of joints including stiffness and ductility of the specimens.

Ze-Jun Geng et al (1998) investigated the effect of strengthening of beam-column connection with CFRP sheets wrapped around the joint with a set of steel angles and rods. They reported that retrofitting has resulted in significant improvement in ductility. The ultimate load carrying capacity was also found to increase by 35 %.

Prota et al (2000) have focused on a new technique for the seismic upgradation of RC beam-column connections in gravity load-design frames by the application of FRP rods and laminates. The FRP rods provided flexural strengthening, whereas the lay-up laminates provided confinement and shear strengthening. Along with modeling of such upgraded connections to assess the increase of strength and ductility provided by the composite reinforcement, an experimental program was also carried out by them.

Jianchun Li et al (2002) developed a model for a concrete column–beam connection with fiber reinforced polymer (FRP) reinforcement. Results of the analysis indicated that designed FRP reinforcement greatly improved the stiffness and load carrying capacity of its concrete counterpart. It also delayed the crack initiation at the joint.

D’Ayala et al (2003) have reported the results of cyc1ic tests carried out on beam-column joint specimens strengthened by externally bonded FRP fabric. The specimens tested by them were designed to comply with gravity load design codes and seismic design was not considered.

Kabir et al (2003) investigated the upgradation of a RC joint against reversal loading, which represents seismic excitation. It was reported that the use of composite fabrics enhanced the performance of the joint. The load-deflection curves for strengthened beam-column connections were studied in detail. They also reported that the behavior of retrofitted joints such as moment bearing capacity, ductility, axial load and drift was significantly improved.

Based on the review of literature it is found that only few experimental investigations have been carried out on beam-column joints. Hence an attempt has been made to carry out an investigation on beam-column joint specimens retrofitted with glass and carbon FRP wrap.

experimental investigation on influence of development length in the retrofitted concrete beam - column joints

Experimental Investigations

The experimental program consisted of testing nine reinforced concrete beam-column joint specimens. The details of a typical test specimens are given in Fig.1(a) & Fig.1(b). The column had a cross section of 200 mm x 200 mm with an overall length of 1500 mm and the beam had a cross section of 200 mm x 200 mm with a cantilevered portion of length 600 mm. For six specimens, the development length of the tension and compression rods in beam was provided as per clause 26.2.1 of IS 456. For the remaining three specimens, the development length of the beam rods was provided as per clause 6.2.5 of IS 13920. The concrete mix was designed for a target strength of 20 MPa at the age of 28 days. The load carrying capacity of the column was found to be 440 kN as per the code IS 456-2000.

experimental investigation on influence of development length in the retrofitted concrete beam - column joints

Static tests were conducted on the control and retrofitted reinforced concrete beam-column joint specimens. Generally, when the axial load on the column exceeds 50 to 60% of its capacity, the effect of axial load will be more predominant on the joint. But in the case of the seismic forces, the effect of lateral load will be more predominant. Hence in order to truly reflect the performance of the joint under seismic load conditions, it was decided to restrict the axial loads of column to a maximum of 200 kN which is less than 50 % of load carrying capacity of the column. The experimental investigation consisted of applying three axial loads of 65 kN, 130 kN and 200 kN on the columns and applying a point load at the free end of the cantilever beam portion till the failure of the specimen. The loading was continued till the joint failed by crushing of concrete in the case of control specimens and rupture of wrap in the case of retrofitted specimens. The details of the experiments are given in Table 1

Garbon fiber reinforced polymer wrap (GFRP) was used to strengthen the three failed beam-column joint specimens C1,C2 & C3 and they are redesignated as retrofitted specimens R1, R2 & R3. Carbon fiber reinforced polymer wrap (CFRP) was used to strengthen the other three failed beam-column joint specimens C4,C5 & C6 and they are redesignated as retrofitted specimens R4, R5 & R6.They were again tested to failure. The performance of the retrofitted beam-column joint specimens was compared with that of the control beam-column joint specimens.

Preparation of Test Specimens

The RC beam-column joint specimens were cast using fabricated steel moulds. Reinforcement was prepared and placed inside the mould. The grade of concrete used was M20. Concrete was mixed in a tilting type mixer machine. Care was taken to see that concrete was properly placed and compacted. The sides of the mould were removed 24 hours after casting and the test specimens were cured in water for 28 days.

Preparation of the Retrofitted Specimens

The failed specimens C1, C2, C3 & C4,C5 ,C6 were retrofitted and redesignated as specimens R1, R2, R3 & R4, R5, R6 . The concrete near the area of failure was removed completely. After applying cement paste in this area, the portion was filled and compacted with the same grade of concrete. The specimens were cured for 28 days. Before wrapping the GFRP, CFRP sheets, the faces of the specimens were ground mechanically to remove any laitance. All the voids were filled with putty. Then a two component primer system was applied on the concrete surface and allowed to cure for 24 hours. A two component epoxy coating was then applied on the primer coated surface and the GFRP or CFRP sheet was immediately wrapped over the entire surface of the reinforced concrete beam-column joint.

A hand roller was then applied gently over the wrap so that good adhesion was achieved between the concrete surface and the GFRP or CFRP wrap, as suggested by the manufacturers and allowed to cure for seven days. Another coat of the two component epoxy was applied over the fiber sheet. Then the second wrap was applied by following the same procedure and allowed to cure for a further period of seven days. Both the wrapped layers were orthogonal to each other.

Description of the Test Programme

The specimen C1,C4 & C7 were tested in a loading frame in the horizontal plane. Both the ends of the column were hinged using roller plates. The axial load of 65 kN was applied at one end the column using a hydraulic jack of 250kN capacity and the load was measured using an electrical load cell. The other end of the column was supported by the steel bulkhead attached to the loading frame. A transverse load was applied at the free end of the beam through a hydraulic jack of capacity 100 kN at a distance of 600 mm from the column face to develop a bending moment at the joint. The load on the beam was also measured using an electrical load cell. The deflection at the free end of the beam was recorded at regular load intervals. The specimens C2,C5,C8 and C3,C6,C9 were tested in the same way and the axial loads applied on these specimens were 130 kN and 200 kN respectively. The retrofitted specimens R1&R4, R2&R5 and R3&R6 were also tested for the axial loads of 65kN, 130kN and 200kN

Analysis of the Results

In the case of the specimen C1, first crack was formed in the beam portion approximately at a distance of 45 mm from face of the column at a load of 4.5 kN. At a load of 11 kN, cracks propagated to the compression zone of the beam. Spalling of concrete occurred in the beam at a load of 14 kN. Cracks propagated to the beam-column joint portion at a load of 18 kN. The application of the load was stopped when the deflection at the free end of the beam reached 50 mm. The load corresponding to this deflection was 19 kN.

In the case of the specimen C2, first crack was formed in the beam portion approximately at a distance of 50 mm from face of the column at a load of 5 kN. At a load of 10 kN, another crack was formed in the beam-column joint of the test specimen. The cracks in the beam started to widen at a load of 15 kN. Spalling of concrete occurred in the tension zone of the beam at a load of 18.5 kN. The application of the load was stopped when the deflection at the free end of the beam reached 50 mm. The load corresponding to this deflection was 20 kN.

experimental investigation on influence of development length in the retrofitted concrete beam - column joints

In the case of the specimen C3, the first crack was formed in the beam portion approximately at a distance of 45 mm from face of the column at a load of 4.75kN. At a load of 10.5 kN, cracks propagated to the compression zone of the beam. Spalling of concrete occurred in the beam at a load of 14.5 kN. Cracks propagated to the beam-column joint portion at a load of 18 kN. The application of the load was stopped when the deflection at the free end of the beam reached 50 mm. The load corresponding to this deflection was 18 kN.

In the case of the specimen C4, first crack was formed in the beam portion approximately at a distance of 40 mm from face of the column at a load of 4.25 kN. At a load of 10.5 kN, cracks propagated to the compression zone of the beam. Spalling of concrete occurred in the beam at a load of 13.5 kN. Cracks propagated to the beam-column joint portion at a load of 17.5 kN. The application of the load was stopped when the deflection at the free end of the beam reached 45 mm. The load corresponding to this deflection was 19.5 kN.

In the case of the specimen C5 first crack was formed in the beam portion approximately at a distance of 55 mm from face of the column at a load of 5.5 kN. At a load of 10.25 kN, another crack was formed in the beam-column joint of the test specimen. The cracks in the beam started to widen at a load of 15.25 kN. Spalling of concrete occurred in the tension zone of the beam at a load of 18 kN. The application of the load was stopped when the deflection at the free end of the beam reached 50 mm. The load corresponding to this deflection was 19.75 kN.

In the case of the specimen C6 the first crack was formed in the beam portion approximately at a distance of 42 mm from face of the column at a load of 4.5 kN. At a load of 10 kN, cracks propagated to the compression zone of the beam. Spalling of concrete occurred in the beam at a load of 14 kN. Cracks propagated to the beam-column joint portion at a load of 17.5 kN. The application of the load was stopped when the deflection at the free end of the beam reached 45 mm. The load corresponding to this deflection was 18.5 kN. The load deflection curves for these specimens are shown in Figure 2 .The load carrying capacity of the all six controlled specimens are given in Table 2.

Effect of Axial Load on Column

It is seen from Table 2. that the effect of axial load on column is negligible in altering the load carrying capacity of the beam. There is not much difference in the load deformation characteristics of the beam-column joint specimens with an increase in the axial load. In the case of control specimens, cracks emanated from the beam portion and with an increase in the load, the cracks propagated into the joint portion of the specimens and spalling of concrete was noticed.

Retrofitted Specimens

experimental investigation on influence of development length in the retrofitted concrete beam - column joints
In the case of the retrofitted specimen R1 (GFRP), first crack was formed in the beam portion very close to the column at a load of 10 kN. At a load of 13 kN, a crack propagated to the compression zone of the beam. The cracks in the beam started to widen at a load of 19 kN and bond failure of the wrap was noticed on the tension side of the beam at a distance of 50 mm from the face of the column. The application of the load was stopped when the deflection at the free end of the beam reached 50 mm. The load corresponding to this deflection was 25.5 kN.

In the case of the specimen R2 (GFRP), first crack was formed in the beam portion at a distance of 30 mm from the face of the column at a load of 11 kN kN. At a load of 14.5 kN, the crack propagated to the compression zone of the beam. The cracks propagated into the column portion at a load of 19 kN. Bond failure of the wrap was noticed on the tension side of the beam at a load of 21 kN and the compression side of the beam at a load of 25 kN. The application of the load was stopped when the deflection at the free end of the beam reached 50 mm. The load corresponding to this deflection was 27 kN.

experimental investigation on influence of development length in the retrofitted concrete beam - column joints
In the case of the specimen R3 (GFRP), the first crack formed in the beam portion at a load of 11 kN. Bond failure of the wrap was noticed on the tension side of the beam at a load of 15 kN and on the tension side of the compression side of the beam at a load of 20 kN. The application of the load was stopped when the deflection at the free end of the beam reached 50 mm. The load corresponding to this deflection was 26.5 kN.

In the case of the retrofitted specimen R4 (CFRP) first crack was formed in the beam portion very close to the column at a load of 12kN. At a load of 15 kN, a crack propagated to the compression zone of the beam. The cracks in the beam started to widen at a load of 21kN and bond failure of the wrap was noticed on the tension side of the beam at a distance of 50 mm from the face of the column. The application of the load was stopped when the deflection at the free end of the beam reached 50 mm. The load corresponding to this deflection was 26.8 kN.

In the case of the specimen R5 (CFRP), first crack was formed in the beam portion at a distance of 30 mm from the face of the column at a load of 12.5 kN. At a load of 16 kN, the crack propagated to the compression zone of the beam. The cracks propagated into the column portion at a load of 20 kN. Bond failure of the wrap was noticed on the tension side of the beam at a load of 22 kN and the compression side of the beam at a load of 26 kN. The application of the load was stopped when the deflection at the free end of the beam reached 50 mm. The load corresponding to this deflection was 27.5 kN.

In the case of the specimen R6 (CFRP), the first crack formed in the beam portion at a load of 12 kN. Bond failure of the wrap was noticed on the tension side of the beam at a load of 15.5 kN and on the tension side of the compression side of the beam at a load of 21 kN. The application of the load was stopped when the deflection at the free end of the beam reached 50 mm. The load corresponding to this deflection was 27 kN.

Effect of Retrofitting

The load deflection plots of GFRP or CFRP wrapped and control RC beam-column joint specimens for various column axial loads are shown in Fig.3(a).Fig.3(b) and Fig.3(c). The load carrying capacity of the various reinforced concrete beam-column joint specimens (both control and retrofitted) are given in Table 3. It is seen from the Table 3. that the load carrying capacity of specimen retrofitted with GFRP has increased in the range of 17 % to 27 % and the area of the load deflection curve up to a deflection of 50 mm for specimens retrofitted with GFRP was 13 % to 20 % more compared to the control specimens. It is also seen that the load carrying capacity of specimen retrofitted with CFRP has increased in the range of 28 % to 34 % and the load deflection curve up to a deflection of 50 mm for specimens retrofitted with CFRP 18 % to 30 % more compared to the control specimens. The load deformation characteristics also improved to a larger extent in the case of wrapped specimens over the control specimens. This resulted in a substantial increase in the energy absorption characteristics of the specimens that were wrapped with GFRP or CFRP.

experimental investigation on influence of development length in the retrofitted concrete beam - column joints

Conclusions

Based on the experimental investigations carried out on the control and GFRP or CFRP wrapped beam-column joint specimens, the following conclusions are drawn:
  • The effect of axial load on the performance of beam-column joints is found to be negligible.
  • There is 14.5 % increase in the load capacity and 10 % increase in energy absorption capacity of the RC beam-column joint specimen, as the development length is increased based on IS 13920.
  • The percentage increase in the load carrying capacity of the retrofitted specimens was found to be in the range of 17 %-34 %.
  • The load deformation characteristics also improved to a larger extent in the case of the retrofitted specimens over the control specimens. This resulted in a substantial increase in the energy absorption characteristics of the specimens that were retrofitted with GFRP or CFRP.
  • The enhancement in the energy absorption capacity of the wrapped specimens was in the range 18%-30% over the control beam-column joint specimens.
  • The failure was in the column portion of the joint for the control specimen which is to be avoided. In the case of the wrapped specimens, the failure was noticed in the beam portion only and the column was intact and this is the most preferred type of failure under seismic loads which will prevent progressive collapse of the structure.

References

  • Geng et al (1998) and Mosallam (1999), “Seismic repair and upgrade of structural capacity of reinforced concrete connections: Another opportunity for polymer composites” Proc., int. composites expo ’99, Cincinati, ppl-8.
  • Ze-Jun Geng, Michael J Chajes, Tsu-Wei Chouc & David Yen-Cheng Pand. (1998), “The retrofitting of reinforced concrete column to beam connections.” Composites science and technology, 58, pp 1297-1305.
  • Prota,A., Nanni,A., Manfredi,G. Nd Cosenza,E.,(2000), “Models of concrete confined by fiber composites,” Proceedings of the fifth international conference of fiber reinforced concrete structures (FRPRCS 5), PP 865-874.
  • Jianchun li, A., Bijan Samali, Lin Ye,Steve Bakoss.,(2002), “Behavior of concrete beam-column connections reinforced with hybrid FRP sheet,” Composite structures,57,pp 357-365.
  • D’Ayala, D., Penford,A., and Valentini,S., (2003), “Use of FRP fabric for strengthening of reinforced concrete beam column joint,” Journal of composite for construction,2(4),pp 165-174.
  • Kabir,M.Z., Ashrafi,H.R and Varzaneh,M.N., (2003), “Upgrading ductility of RC beam-column connection with high performance FRP laminates,” Damage and fracture mechanics VII, CA Brebbia & SI Nishdia (Editors) ISBN 1-85312-926-7.

NBMCW April 2009

.....

Modeling of Stress–Strain Response of...

Fiber Reinforced Concrete

Prabhat Kumar, Retd. Scientist, Central Building Research Institute and P.C. Sharma, Retd. Head Material Sciences, Structural Engineering Research Centre, Ghaziabad

Addition of fiber reinforcement in mortar and concrete provides a large number of benefits. The mathematical modeling of this new material is needed to exploit its benefits to fullest extent. This paper brings past and present trends to achieve this. A compact mathematical model is presented and the process of calibration of model is demonstrated. Results are encouraging.

Introduction

Safety and economy are major concerns in building construction. The existing structures support safety aspects of the past and current design practices. However, economy and rationality aspects require continuous research in view of the advancements in material technology and more ambitious functional requirements of new structures. For example, high strength concrete (HSC) offers economic advantage in concrete filled tube columns, core walls, lower storey columns and pre–cast piles. Addition of silica fumes for strength enhancement makes HSC brittle. The economic advantage of HSC is partly off set on account of increased susceptibility to structural damage under abnormal operational conditions, which arise during natural disasters like earthquake and cyclone. Fibers may be added to HSC to compensate for the negative effect of silica fumes [1]. This product is fiber reinforced concrete (FRC). The addition of fibers marginally affects pre–peak stress–strain response of concrete, however, the post–peak response changes substantially (See Figure 1).
Typical effects of fiber addition to concrete
Figure 1: Typical effects of fiber addition to concrete
  • Compressive strength increases
  • Peak strain increases
  • The rate of drop of post–peak descending branch moderates
  • Stress at large strain enhances substantially
Improved structural and strength properties of FRC, like impact and fatigue resistance, are fruitfully exploited in Fiber Reinforced Shotcrete [2], and polymer concrete, which is used in structural repair [3]. Addition of fibers arrests formation and propagation of micro–cracks and improves tensile strength of concrete to delay its fracture.

Recent revolution in the digital computer hardware and software technology places tremendous computation power at the disposal of an analyst. It must be gainfully utilized in the design of structures such as pavement and runways in particular. At present, the material technology appears to be moving faster than the corresponding analytical developments [4–7]. This article presents recent developments in the analytical modeling of FRC. The possibilities which still remain are described and future extensions are proposed. Availability of a satisfactory mathematical model can promote greater utilization of benefits obtained by adding fiber to concrete and also enhance areas of application.

Mechanics of Fiber Reinforcement of Concrete

There are a large number of variables in the problem of FRC.
  1. Strength of the matrix to be reinforced,
  2. Material of fibers–It may be steel, glass or polypropylene,
  3. Aspect ratio of fibers l/f where l is length and f is diameter,
  4. End profile of fibers–hooked, bent or straight,
  5. Body profile of fibers–It may be twisted to improve bonding.
  6. Amount of fiber. Volume fraction Vf and weight fraction wf define the amount of fibers. These are related as in Equation 1. The ratio of unit weights is approximately 3.25 for steel fiber.

    wf = (Unit weight of fiber material/Unit weight of matrix) Vf (1)
Published experimental studies on FRC provide empirical expressions for prediction of stress–strain behavior. These expressions are derived by using boundary conditions of slope at the origin, peak point, inflexion point and at any other point. Some of the expressions consist of a single expression for entire curve whereas others may be in the form of two or more segments, applicable to pre– and post–peak, respectively.

The area under the stress–strain curve is taken as a measure of deformation energy absorbing capacity of a material. The toughness ratio (TR) is defined as the ratio of area under the actual stress–strain curve to the area under an ideal rigid plastic material of similar strength. A toughness index (TI) is also defined as the ratio of toughness of fiber reinforced matrix to that of the un–reinforced matrix. It may be seen that TR is always less than unity while TI is always greater than unity. An integration of Equation of stress–strain curve is required to calculate toughness index. Ductility, on the other hand, is the ability of the material to deform without failure. The shape and rate of decay of descending branch of stress–strain curve characterize ductility property of a material. Toughness and ductility properties are important when loading involves impact, repetition and reversal.

It has been found [6] that everything being equal, the improvement due to fiber addition was relatively more pronounced in lower matrix compressive strengths. To facilitate further mathematical development, reinforcing index (RI) has been introduced. Experimental observations revealed [6] that higher RI resulted in smaller the slope of the post–peak branch of stress–strain curve. Therefore, the area under the stress–strain curve increased. RI can be defined with respect to volume fraction Vf as (VRI = Vf l/Φ) as well as with respect to weight fraction as (WRI = wf l/Φ). These two shall be related as in Equation 1. It was found [6] that VRI in excess of 2.0 resulted in deterioration rather than enhancement of properties. It indicates that there is an optimum benefit that can be derived from fiber addition. This may occur due to difficulty in ensuring uniform distribution of fibers in the body of concrete element.

Mathematical Modeling of FRC

In the derivation of analytical expression for stress–strain curve of FRC, following expressions are most commonly used.

Fiber Reinforced Concrete

A, B, C and D in Equation 2 are parameters which are determined from the boundary conditions at origin, peak point, inflexion point and at any other suitable location. The number of boundary conditions must be equal to the number of parameters. This mathematical fact limits the number of parameters which can occur in a general expression. A mathematical model is meaningless unless all its parameters can be characterized from the measurable quantities. In this presentation stress and strain are denoted by f and ε, respectively. Further, Y = fc/fP, X = εcP. Subscript P applies at peak point while c is a general point. Following studies highlight the difficulties in arriving at a suitable expression for stress–strain curve of FRC.

Fiber Reinforced Concrete

Fiber Reinforced Concrete

Fanella and Naaman [6] described properties of FRC through Equations [3–5]. Starting with the first expression of Equation 2 values for parameters A to D are as given in Tables 1 and 2.

fcf'= 1.9 RI + fc' --------------------------------(3)

Ecf= 162 fcf'+ 8270 ----------------------------(4)

εP= 0.00079RI + 0.0041 (fcf'/ fc') --------------(5)

Mebarkia and Vipulanandan [3] used second expression of Equation 2 as Equation 3.

Fiber Reinforced Concrete

This expression satisfies the boundary conditions that it passes through origin and peak point. The following additional boundary conditions were used to determine parameters p and q (1) At origin initial tangent modulus is Eo/Ep(2) Slope is zero at peak point. Parameters p and q must meet the followings additional conditions p + q > 0; p + q 0. Through regression analysis with the experimental observations, the following model was produced. Xf and XP are fiber and polymer contents by weight, respectively. An integration of Equation 3 is required to calculate toughness index.

q = 1 – p – Ep/ Eo and

p = A + BXf+ CXf2 -------------(4)

Where,

A = – 0.36 + 0.076XP– 0.0029XP2

B = 0.46 – 0.068XP+ 0.0023XP2

C= 0.0055 + 0.0013XP– 0.000070XP2

Ezeldine and Balaguru [1] also used second expression of Equation 2 as Equation 5. One should note that Equations 3 and 5 are identical [â = (1–q)/p]. This model was calibrated for hooked as well as straight fibers with reinforcing index by weigh as RI and ri, respectively.

Fiber Reinforced Concrete

Hooked fibers:

β = 1.093 + 0.074*10-4Eo and

Eo= 9.610*104[RI]-0.926

Straight fibers:

β = 1.093 + 7.4818 [ri]-1.387

Mansur et al [8] used second expression of Equation 2 in his study. Equation 6 was used for the descending branch of stress–strain curve. The ascending branch used k1= 1 and k2= 1. This experimental program used hooked steel fibers.

Fiber Reinforced Concrete

For confined non–fiber concrete, k1= 2.77(ρsfy/fo) and k2= 2.19 (ρsfy/fo ) + 0.17

For confined fiber concrete, k1= 3.33 (ρsfy/fo ) + 0.12 and k2= 1.62 (ρsfy/fo ) + 0.35

Here ρs is the volumetric ratio of lateral ties, fy is proof stress of lateral ties and fo is unconfined strength of concrete.

Complete details of above mathematical models of FRC can be found in the original papers. The presentation of this section shows that analytical modeling of FRC is important and that use of empirical techniques is rampant in this endeavor. Also, the available analytical models are complicated and suffer on account of limited validity. More research effort is obviously required.

Recent Developments

Barr and Lee [8] deviated from the prevailing trend of using either of the expressions given in Equation 2 to propose use of a double exponential model. In this approach, two exponential functions are subtracted to obtain proper shape of the stress–strain curve. Smith and Young [9] first used exponential function for this purpose (Equation 7). The double–exponential model uses Equation 8. This double–exponential model can be further developed by assigning values to parameters C1, C2 and C3 from the boundary conditions of FRC problem.

Y = X EXP(1 – X) ----------------------------(7)

Y = C1[EXP(–C2X) – EXP(–C3X)] -----------(8)

Fiber Reinforced Concrete
Figure 2: Results of parametric study (continued)


Fiber Reinforced Concrete
Figure 2: Results of parametric study (concluded)

The first exponential function dominates the tail end where as the second expression influences the shape of stress–strain curve in the initial part. A parametric study of Equation 8 (Figure 2) shows that different combination of parameters yields a family of stress–strain curves. It should be possible to use this formulation for mathematical modeling of stress–strain behavior of FRC. This model is applied to the published experimental data on steel fibers [6]. The experimental and analytical results are compared in Figures 3 and 4.

Fiber Reinforced Concrete
Fiber Reinforced Concrete
Figure 3: Modeling of FRC with constant aspect ratioFigure 4: Modeling of FRC with constant volume fraction

Compact model [10] is also available in which the analytical line is made to pass through a point located on the tail of stress–strain curve. Equations 9 to 12 describe this model.

Fiber Reinforced Concrete

Equation 9 is the stress–strain relation in which solution of Equation 10 provides the value of exponent n. Equation 9 is obtained from Equation 12 in which value of m is as given in Equation 11. This model is also tried and the tail end point is taken at strain of 0.015. This model is applied to the published experimental data on steel fibers [6]. The experimental and analytical results are compared in Figures 5 and 6.

Fiber Reinforced Concrete
Fiber Reinforced Concrete
Figure 5: Modeling of FRC with constant aspect ratioFigure 6: Modeling of FRC with constant volume fraction

X and Y are non-dimensional strain and stress, respectively; n = Model Parameter; m is a derived quantity (equation 11); U is a derived quantity (Xg/Yg). Subscript g denotes general point; O defines quantity at origin; and P applies to quantity at peak point. The application of compact model is more satisfactory than the double exponential model.

Conclusion

Stress–strain behavior of Fiber Reinforced Concrete is analytically modeled in this paper. Two published analytical models, which were never applied in modeling of FRC, have been used in the present study. Some difficulties were faced in the process. First of all, it was difficult to read the values of experimental observations from the published documents. Secondly, the exact conditions of experimental investigations were not known.Practically all experimental investigators propose an analytical model which applies very well to their data. It is not surprising because, the analytical models are based on the regression analysis of their own data.

This paper considers data of other investigators, so this effort is unique. The effort of analytical modeling reported in this paper is very encouraging. This procedure shall be refined further and shall be reported in subsequent publications on this topic. More experimental data must be generated. For example, all varieties of fibers must be considered. The analytical procedure should then be applied to this more extensive experimental data to develop general expressions for toughness index etc. The proposed models are more suitable for analytical treatment like integration etc.

References

  • Ezeldine, A. S. and Balaguru, P. N. (1992) "Normal and high strength fiber reinforced concrete under compression," Journal of Materials in Civil Engineering (ASCE), V. 4(4), pp. 415–429.
  • Barrett, S. and McGreath, D. R. (1995) "Shotcrete support design in blocky ground," Tunneling and Underground Space Technology, V. 10(1), pp. 79–89.
  • Mabarkia, S. and Vipulanandan, C. (1992) "Compressive behavior of glass fiber reinforced mortar," J of Materials in Civil Engineering (ASCE), V. 4(1), pp. 91–105.
  • Shah, S. P. and Naaman, A. E. (1976) "Mechanical properties of glass and steel fiber reinforced mortar," ACI Journal, V. 73(1), pp. 50 – 53.
  • Hughes, B. P. and Fattuhi, N. I. (1977) "Stress–strain curves for fiber reinforced concrete in compression," Cement and Concrete research, V. 7(2), pp. 173–183.
  • Fanella, D. A. and Naaman, A. E. (1985) "Stress–strain properties of fiber reinforced mortar in compression," ACI Journal, V. 82(4), pp. 475–483.
  • Mansur, M. A., Chin, M. S. and Wee, T. H. (1997) "Stress–strain relationship of confined high strength plain and fiber concrete," Journal of Materials in Civil Engineering (ASCE), 9(4), pp. 171–178.
  • Barr, B. and Lee, M. K., (2003) "Modeling the strain softening behavior of plain concrete using a double–exponential model," Magazine of Concrete research, 55 (4), pp. 343–353.
  • Smith, G. M. and Young, L. E. (1956) "Ultimate Flexural Analysis Based on Stress–Strain Curve of Cylinder." ACI Journal, Proc. V. 53 (6), pp. 597–609.
  • Kumar, P. (2004) A compact analytical model for unconfined concrete under uni axial compression. Materials and Structures (RILEM), V. 37(9), pp. 585–590.

NBMCW April 2009

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Early High Strength Concrete Advantages...

High Strength Concrete

Shivram Bagade, Concrete Technologist – BASF Construction Chemicals India Pvt. Ltd, Bangalore Nagesh Puttaswamy Manager (TASC) UltraTech Cement Ltd., Hyderabad

Fast cars, fast travel schedules, fast track construction has become the order of the day. In many ways, these technological advancements have been an economic boon for the mankind but at what cost? Speeding cars are thrill on the race tracks but the risks are also a part of the system. The concept of ‘time saved is money saved’ induced the fast track work in construction industry, since then engineers and administrators are making a holistic approach by making every part of construction to contribute into the system in making the construction faster. High Strength concrete is also one of them. The history of high strength concrete is about 35 years old, in late 1960s the invention of water reducing admixtures lead to the high strength precast products and structural elements in beam were cast in situ using high strength concrete. Since then the technology has come of age and concrete of the order of M60 to M120 are commonly used. Concrete of the order of M200 and above are a possibility in the laboratory conditions.

With this as a confidence level the industry today has some very challenging demands for the cement manufacturers, admixture manufacturers. The demand for high strength in a very short duration has come up. There have been demands like,
  • 40 MPa of M60 concrete in 3 days,
  • 50% of target strength in 24 hours,
  • 12 MPa in 12 hours,
  • 12 MPa in 10 hours.
The reasons for these demands are many, but as engineers, we need to think about the durability aspects of the structures using these materials. The consumers in these cases have been catered for their requirement through their insitu batching plants. With long term durability aspects kept aside we have been able to fulfill the needs. The concrete of these properties will have a peculiar Rheological behavior. Some observations made during the trials for Early High Strength Concrete (EHSC) are discussed in this paper along with some of the durability issues. The need to understand the rheological parameters in connection with the durability aspects need to be given a careful attention. The technology of EHSC is being used more in the infrastructural projects and pre-cast industry in some cases, it is being used indiscriminately and in few cases it is being adopted without proper technical backup. Cement and admixtures are selected and rejected based on economic criterion, the very sensitive issues like the cement admixture compatibility and micro plastic-cracks are not addressed in the right way. The concept of EHSC is a boon for the pre-cast industry but needs to be nurtured in a better way so that it will not get in to a trap like what happened to the use of flyash in concrete, which even today has not got 100% confidence with the technical fraternity.

Introduction

Concrete based on the 28 day strength, is classified as high strength concrete so and so forth. Till about 1970s the concretes that could achieve strength of above 40Mpa were classified as high strength concrete. When concrete mixtures of about 60Mpa and above were commercially produced the bench mark for the high strength concrete was raised to 55Mpa or more.

The history of high strength concrete is about 35 years now, since the development of super plasticiser admixtures in the late sixties the Japanese with their high strength pre-cast products using ‘naphthalene sulfonate’ and the Germans with under water concrete using ‘melanine sulfonate’ were the pioneers of the technology.

Three to four decades ago despite the availability concrete as versatile construction material. Most of the highrise buildings all over the world have used steel elements as structural frame. The famous Twin Towers at Manhattan (World Trade Centre) had steel frames. The reason was that with the strength of concrete available in those days the members made of concrete would have been bulky and ugly.

With advent of high strength the bulkiness in the concrete members are gone and we are able to make slender sections in concrete too. Since then high strength concrete has come a long way and is running a race to reach the strength of steel. The concrete of the order of 200 Mpa has become a reality at least at the laboratory conditions and concrete of the order of M60 to M120 are commonly used at sites. The properties of the high strength concrete are well studied and understood by the engineers today, the use of very high strength concrete no longer raises eye brows. The drawbacks of the high strength concrete have been countered by the user.

How High Strength in Concrete Achieved

The higher strength in concrete could be achieved by using one of the following methods or a combination some or many of the following:
  • Higher cement content
  • Reducing water cement ratio
  • Better workability and hence better compaction
Some of the Codal requirement of the high strength concrete
Compressive strength60 Mpa or more
DurabilityPermeability < 5 mm as per DIN 1048
Workabilityto be placed in areas of high congestion.

The requirement of high strength concrete requires a higher cementitious material in the concrete mixture, which could be in the range of 400kg plus per M3. The cementitious content of higher value would lead to higher thermal shrinkage and drying shrinkage, and there would be a stage when any further addition of cementitious material will not affect the strength. For the durability aspects, the minimum and maximum cement content in concrete is governed by the codal provisions, reduction of water cement ratio has also its own limitations especially in the site conditions. The hunger for the higher strength leads to other materials to achieve the desired results, thus emerged the contribution of cementitious material for the strength of concrete.
  • Addition of pozzulanic admixture like the Pozzulanic Flyash (PFA) or granulated blast furnace slag (GGBS) which helps in formation of secondary C-S-H gel there by improvement of strength.

    The addition of pozzulanic admixture like the flyash used as admixture will reduce the strength gain for the first 3 to 7 days of concrete and will show gain beyond 7 days and give a higher strength on long term.
  • Addition of mineral admixtures like the silica fumes or metakaolin or rice husk ash.

    The highly reactive pozzulanic admixtures like the silica fume or metakaolin and Rice Husk Ash (RHS) will start contributing in about 3 days. The RHS has an advantage over the PFA because the RHS is more reactive.
  • Use of chemical admixtures like the Super-plasticiser or Hyper-Plasticiser, set controlling admixtures will help in attaining the higher strength in concrete.

    The research and the experience indicate that the admixtures based on the Poly Carboxylic Ethers (PCE) called the hyper plasticisers are the best suited for the job as they have a water reducing capacity of 18% to 40% in reference to the control or reference concrete.
  • Combination of all of the above or some of the above to achieve the desired strength.

    The combination of at least a few of these methods has now become invariable as the HSC came along with some complexities like higher shrinkage, higher heat of hydration etc., all these complexities need to be neutralised or controlled. Most of the problems were handled by a combination of PFA or GGBS and PCE admixture.

    In order to accelerate the cement hydration steam curing methods are also adopted however this may not result in higher strength. The strength gain at early age can be achieved by replacing a part of the fine aggregate by flyash or blast furnace slag, without increase in the water requirement of the concrete mixture.
Properties of the ingredients used in the HSC are:

Properties of Cement Required

Compressive strength> 60 Mpa
C3A Content< 6
Fineness300 + 20 Sq. M per Kg
Total alkali contentMax. 6% expressed as Na2O
C3S> 50%
C2S> 24%
C4AF> 15%
LOI< 2%

Properties of Flyash

PFA required enhancing the strength, impermeability and durability of concrete. Class F PFA has to be used.

It reduces segregation and bleeding in fresh concrete, creep in hardened concrete, it also lowers heat of hydration.

Chemical Requirements

SiO2> 60%
SiO2 + AI2O3 + Fe2O3= 85%
LOI2% Maximum
FinenessMax. 10% (retained on 45 Micron)

Aggregates

Fine Aggregates: Should fall in Zone II

Chemical Admixtures

The High Range Water Reducing Admixture (HRWRA) have to be used, normally the PCE admixtures are formulated for the specific need. High Range Water Reducing Admixture (HRWRA): It is a known fact that for the durability of the structure permeability in concrete plays a major role, one of the important factors that govern the issue water–cement ratio during the manufacture of concrete, lower the water-cement ratio lower will be the capillary pores and hence lower permeability and enhanced durability.

Poly Carboxylic Ethers (PCE) based admixtures called hyper plasticizers invented in the 1990’s have fulfilled the conditions in an excellent way, the advantages of which are being exploited in the production of high strength and high performance concrete. The water reducing capacity of these admixtures is between 18–40% of the control or reference concrete. These admixtures assist in achieving higher slumps more than 180 mm at much lesser w/c ratios (less than 0.30). They impart better control over the rheology of the concrete and that’s one of the reason such admixtures are always used for producing for selfcompacting Concrete. The only disadvantage of these admixtures is that they do not have a longer retention beyond 45 minutes and are always used along with retarding agents, adding to the complexities of the mix. Situations of these complexities need to be handled very carefully, however the construction chemical industry has come out with combination of chemicals or let us say cocktail of chemicals to meet the demand.

Industry’s Demand

The construction industry turning towards pre-cast elements and requirement of post-tensioning has made the requirement of the high strength of concrete invariable and the engineers had to overcome these drawbacks, which to a great extent we have been able to do. The construction of modern days have become fast track where the economics on the investment on the form work are considered the use of high strength concrete is invariable and has become a must. The construction today is crucial when it comes to economics very fine aspects are considered to achieve savings in construction work. The speed of construction and its technology is measured in terms of the number of cycles of the use of formwork. This has now turned into one of the basic requirement of concreting process, the demands of the industry has become very complicated.

We had an opportunity to observe and work with some of these cases. Some of the examples are as follows:

CASE 1

Infrastructure Project

This concrete requirement was for precast segmental girders to be post tensioned for a fast track infrastructural project. The time is the main objective of the contract between the consumer and contractors. The segments need to be hoisted into place within a record low time, and needs to be post-tensioned. The continuity in work, the construction area being very congested for work demands that the infrastructure for the casting moves further at the earliest.

Grade of concrete requiredM50
Special requirement specified
01Minimum strength required in 24 hrs25 Mpa.
02Workability required: Initial collapse slump
03Workability required: Pumpable concrete after 90 minutes
04To get high early strength without stickyness

These large segmental girders for a elevated highway project
Trial 1Trial 2Trial 3
Mix GradeM-50M-50M-50
Cement OPC 53 grade5.45.45.4
W/C, A/C, W/B
CA - I 20mm6.2285.2925.472
CA - II 12.5mm5.7486.3725.472
CA - III 6mm2.42.7122.76
Crushed Sand8.4488.4489.12
Total Water (Incl. Abs)1.9741.981.99
Crushed Sand8.4488.4489.12
Total Water (Incl. Abs)1.9741.981.99
AdmixtureM-715M-715M-715
Dosage (in %)@0.6@0.6@0.7
Slump / Flow (in mm)
0 min210200200
30 min150150165
Wet Density (kg/Cu. M)246224052482
Comp. Strength (N/Sq. mm)
01 Day28.0430.0431.32
02 Days40.1242.4446.20
07 Days47.8048.2050.00
28 Days
Batch Size / Cu. m0.0120.0120.012

CASE 2

Infrastructure project
Grade of concrete requiredM60
Special requirement specified
01Minimum strength required in 24 hrs40 Mpa.
02Workability required as in SCC
03Quantity of cementitious material Pre-fixed - fly ash from a fixed source
04Fine aggregate100% crusher dust

The concrete had to achieve more than 50% of its final strength in 24 hours as we have already discussed earlier the whole process to happen in about 14 hours of the final setting time of cement!

The project requirement was for the shell-plate roof elements to cast on ground and needs to be hoisted to place and was post-tensioned. The requirement of early high strength of concrete was for the fact that the members need to be shuttered early and moved to the curing yard and the number of cycles of formwork was critical economical criterion. The segments had special shapes and folds hence there were a requirement of the self-compacting property. The retention of workability in combination with the requirement of early high strength pulled the system in opposite directions.

The concrete in this case was made at the captive batching plant at site and the necessary conditions were achieved with a couple of alternative cement brands. The concrete was placed into the mould using transit mixers within the site had retain the workability criterion for nearly 45 min to 1 hour. The details of the mix design could not be provided here as all trials were done by the consumer and we could not get the permission to publish here. The admixture there had to be specially fine tuned for the specific requirement and it had to under further fine tuning with respect to change in the ingredient sources. Several thousand cubic meters of concrete has been consumed. Is it going to be the trend setting benchmark for other projects?

CASE 3

For a Segmental Construction of Towers
Grade of concrete requiredM60
Special requirement specified
01Minimum strength required in 12 hrs12 Mpa.
02Quantity of CementPre-fixed
03Quantity of cementitious materialPre-fixed - fly ash from a fixed source
04High reactive mineral admixtures like Silica FumeNot permitted for economic reasons
05The minimum strength requirement was modified to 12 Mpa in 10hrs

Initial slump : Collapse.

Slump after 30 min : Collapse and flow min 450 mm.

The requirement of this concrete was for the making pre-fabricated segments of towers cast at logistically suitable place. The circular segments with variable diameter from one end to other have to be fabricated in a yard and hoisted to the curing yard. The cost of the formwork and the restriction on the space available for the casting area demands the segments to be hoisted from the casting yard to the curing yard, the very early strength requirement is to cater for the hoisting load. The consumer is applying a protective coating on the precast segments for ensuring the durability. The segments are transported on trucks to destinations as far as 600-700 kilometers for actual use. The economic criterion for the consumer demanded that there should be at-least 2 cycles of casting for every 30 hours.

The following mix design was arrived at for getting the desired conditions the average of nearly 10 to 15 trials were made with different some selected brands of cements, the critical results between different brands of cement are listed in the Tables.

Workability
Time in minutesSlump in mmFlow in mmRemarks
0Collapse575 to 625These parameters were kept constant for all brands of cement for a definite dosage of admixtures
30Collapse495 to 525
60110Nil
9050Nil
12020Nil

It was observed that the loss of slump and flow was rapid after the 45-50 minute mark; the values indicated here are the average of several tests that were conducted. After 60 minutes the concrete exhibited unusual stiffness and elastic/spongy feel.

Mix proportioning details
Mix GradeM-60M-60M-60
Cement 53 grade OPCBrand XBrand YBrand Z
Cement470470470
PFA130130130
W/C0.260.260.26
20 mm454454454
12.5 mm454454454
Natural Sand750750750
Free Water154154154
Total water---
AdmixtureG30(S3)G30(S3)G30(S3)
Dosage(%)
Comp. Str. (Mpa) average of all cube results
12 Hours14.155.38*4.5*
14 Hours14.3313.33*12.85*

The results from the Brands Y and Z are very important to be noted the gain of strength between the 12thhour and the 14thhour are almost 3 times.

The Drawbacks of the Technology

The implication of such rapid strength gain is alarming and needs a serious attention. Considering the fact that the final setting time of most brands of cement in India is around 120 minutes, there will be a lot of questions answered regarding the durability aspect.

Irrespective of these question marks, technology has been able to satisfy the requirement of the customer and the work is in progress. As technologists, we need genaralise the requirement for the use as many more requirements may come and all may have to have captive batching plants to produce these special concrete.

The high strength concrete has great advantage in the modern construction scenario as many statistics show that it has not only delivered in the strength aspects but also in terms of economy. Studies show that an approximate increase of 5 times in the strength of concrete will have only about 3 to 3.25 folds increase in concrete. If the designer is able to exploit the conditions the over all costing of the project will definitely come down. Hence lots of projects today are adopting the high strength concrete of the order of M60 or above regularly.

The advancement of the formwork technology is making the erecting and removing of formwork system has become easier and simpler. The cycle time for formwork is reducing drastically one-week one-slab concepts are widely being adopted, in the developed countries this cycle period is about 4 days. In order to achieve this, we need the concrete to attain a minimum amount of strength in that time.

That means Early High Strength Concrete will become the order of the day.

The precast concrete element, manufacturers are already adopting the technology with or without proper technical know how. Why not when we have been able to incorporate the self-compacting property or the self-leveling property with a slight modification of cost. The consumer has been able to make the concrete element with a slightly cheaper cost. Now if the consumer can increase the production of the elements in terms more number of cycles per form work the project cost shall be benefited.

Type of ConcreteCostNo. of Cycles per monthIncrease in costIncrease in no. of cycle
Conventional290030--
Self -compacting3100366.70%20.00%
SCC / EHSC32004510.35%50.00%

With all the parameters kept constant this is a very lucrative offer for any business proposition. The construction industry would be turning towards this technology called the EHSC in near future.

The technologists should deliberate about this concept/technology in a very critical way or else this concept could be abused by indiscriminate use. The deliberations are more important now because this technology is being used in the infrastructural project.

Conclusion

The EHSC is here to stay but we need to be careful that we will not forgo the durability aspects of the material. There are few products that are available to ensure durability in these concretes as coatings and secondary admixtures within the concrete mixture. A careful effort should be taken in to impart knowledge on the protective systems along with the technology of early high strength concrete.

With many of the construction sites and some of the precast units being manned by non-technical or untrained personal the technology should be allowed to have sustained growth not allowed to have premature problems.

Reference

  • Concrete Microstructure properties and Materials P. Kumar Mehta, Paulo J.M. Monterio
  • Workability of self-compacting concrete by Chiara F Ferraris, Lynn Brower, Joseph Daczko
  • Concrete Admixture Handbook V.S. Ramachandran
  • Properties of Concrete A.M. Neville
  • Properties of Ultra High strength Cement Konstantin Sobolev, Svetlana Soboleva
  • The article depicts the views of the authors based on the studies and observations made by them while working on specific cases and observations made there off. The inference of the information given in this article is left to the readers, it is only an effort made by the authors to open deliberations on the topic rather than conclude anything.

NBMCW March 2009

.....

Re-alkalization of Carbonated Reinforce...

Carbonated Reinforced Cement Concrete

M. Raja Shaker, Research Scholar; Prof. Ramesh R Reddy, Dean and Professor, Department of Civil Engineering, University College of Engineering, Osmania University, Hyderabad.

The process by which Carbon dioxide/Carbon monoxide from the atmosphere penetrates into concrete and reacts with Calcium Hydroxide to form Calcium Carbonates is known as Carbonation. Carbonation results in to shrinkage of concrete. Carbon dioxide in the presence of moisture changes into dilute Carbonic Acid and attacks concrete to reduce alkalinity of concrete. The pH of hardened concrete is alkaline and is generally in the range of 12.6 to 13.5. Carbonation will results in reduction of pH of concrete resulting in rupture of passivating layer around the embedded reinforcement which otherwise protects it from corrosion. Carbonation is one of the main reasons for the corrosion of reinforcement, which is almost uniform unlike chloride-induced corrosion. Reinforcement steel corrosion is faster in the presence of chloride ions than that of carbonation corrosion. Several non-destructive tests were carried out for condition assessment of RCC framed structure, which is situated in an industrial environment. Core samples were extracted for compressive strength, chloride content, and carbonation depth. Ultra sonic pulse velocity (UPV) test, Rebound hammer test, half-cell potential measurements, and resistivity measurements were taken. Test results confirmed corrosion of reinforcement was initiated due to carbonation. The corrective action are initiated to restore the building by re-alkalization of the RCC members using migration corrosion inhibitors, and grouting with re-alkalizing cementitious injection grouts to fill up voids, and giving protective breathable coats to prevent the ingression of carbon dioxide, moisture and other acidic fumes. Re-alkalization had improved the pH of concrete considerably.

Introduction

Generally, concrete is expected to give troublefree service throughout its intended life with or without little maintenance. But these expectations could not sustain in many constructions due to structural deficiency, improper workmanship, unanticipated overloading, premature material deterioration, exposure to different aggressive environments etc., Structures built in highly polluted urban industrial areas, aggressive marine environments, and harmful sub soil waters at costal belts are subjected to rapid deterioration. Degree of harshness of environmental condition to which the reinforced concrete structures are exposed became an important consideration for design, in addition to compressive strength. Carbon dioxide/carbon monoxide present in the atmosphere penetrates into concrete and reacts with calcium hydroxide to form calcium carbonate is called carbonation. The alkalinity of hardened concrete reduces due to carbonation effecting rupture of the passivating layer around the reinforcement leading to initiation of corrosion and shrinkage of concrete. Carbonation is one of the main reasons for the reinforcement corrosion, which will be uniform unlike chloride-induced corrosion. The pH of concrete will reduces from 13 to 9 when all the calcium hydroxide present in concrete is carbonated.

Carbonation of Concrete

The concrete comes into contact with carbonic acid resulting from carbon dioxide in the atmosphere, the ensuing carbonation of the calcium hydroxide in the hydrated cement paste leads to reduction of the alkalinity, to pH as low as 8.5, thereby permitting corrosion of the embedded steel. When all Ca(OH)2 in concrete has become carbonated the pH value of concrete will be as low as 8.3 and at that stage the protective layer gets destroyed and steel is exposed to corrosion.[1] The rate of carbonation in concrete directly depends on the water cement ratio of the concrete, i.e., the higher the ratio the greater is the depth of carbonation in the concrete. In consolidate concrete with reasonable quality without any cracks rate of carbonation is expected to be very low. Cement paste contains 25-50 percentages by weight calcium hydroxide Ca(OH)2), with pH of the fresh cement paste is at least 12.5. The pH of a fully carbonated paste is about 7.The concrete will carbonate if CO2from air or from water enters the concrete according to

CO2+ H2O '! H2CO3

Ca (OH)2+H2CO3'! CaCO3+ 2H2O

Alkali + Acid '! Salt + water

It is clear from the above reaction that the alkalinity of concrete gets reduced. When Ca(OH)2is removed from the paste hydrated Calcium silicate hydrate will liberate CaO, which will also carbonate. The carbonation starts from the surface of concrete and proceeds inwards. The rate of carbonation depends on porosity and moisture content of the concrete. The carbonation process requires the presence of water because CO2dissolves in water forming H2CO3. If the concrete is too dry with relative humidity less than 40%, carbon dioxide cannot dissolve and no carbonation occurs. On the other hand it is too wet with relative humidity more than 90%, carbon dioxide cannot enter the concrete and the concrete will not carbonate. Optimal conditions for carbonation occur at a relative humidity of 50% (range 40-90%). Normal carbonation results in a decrease of the porosity making the carbonated paste stronger. Carbonation is therefore an advantage for non-reinforced concrete but it is a disadvantage in reinforced concrete, as pH of carbonated concrete drops to about 7, a value below the passivation threshold of steel.

Carbonated Reinforced Cement Concrete
The structure is located in an industrial area where reinforced members are continuously exposed to aggressive environment of carbon dioxide, carbon monoxide etc., The presence of carbon dioxide is about 0.03% in rural areas and in improperly ventilated or industrial atmospheres the percentages goes upto 0.3%. Carbonated concrete will exhibit slightly increase in strength due to hardness with less permeability. The reduction of permeability is attributed to loss of water released due to decomposition of calcium hydroxide. Carbonation is a slow process and takes years to reach the cover depth. In concrete, the presence of abundant amount of calcium hydroxide and relatively small amounts of alkali elements, such as sodium and potassium, gives concrete a very high alkalinity-with pH of 12 to 13. It is well known that at the early age of the concrete, this high alkalinity results in the transformation of a surface layer of the embedded steel to a tightly adhering film, that is comprised of an inner dense spinel phase in epitaxial orientation to the steel substrate and an outer layer of ferric hydroxide. As long as this film is not disturbed, it will keep the steel passive and protected from corrosion.

Once corrosion sets in on the reinforcing steel bars, it proceeds in electrochemical cells formed on the surface of the metal and the electrolyte or solution surrounding the metal. Each cell consists of a pair of electrodes the anode and the cathode on the surface of the metal and an electrolyte. Basically, on a relatively anodic spot on the metal, the metal undergoes oxidation (ionization), which is accompanied by production of electrons, and dissolution. These electrons move through a return circuit, which is a path in the metal itself to reach a relatively cathodic spot on the metal, are consumed through reactions involving substances found in the electrolyte. In a reinforced concrete, the anode and the cathode are located on the steel bars, which also serve as the return circuits, with the surrounding concrete acting as the electrolyte.

Condition Assessment and Non-Destructive Evaluation

Carbonated Reinforced Cement Concrete
The structure study is located at an industrial atmosphere surrounded by chemical plants. Visual observation was carried out and documented for future reference. The grade of concrete used for the structure is M15. The structure was investigated by conducting non-destructive tests like rebound hammer, core sampling for estimation of carbonation depth, density of concrete, water absorption, and permeability. Rebound hammer test measures surface hardness of the hardened concrete. Low strength concrete having low stiffness will absorb more energy & will give less rebound values. [Figure 1] Powder samples were collected for pH and chloride content. [Table.2] Half-cell measurements and corrosion rate measurements were taken for asserting the presence of corrosion. Ultra sonic pulse velocity test is conducted on selective members to understand the homogeneity, integrity and identification of cracks in the members etc., The average values of UPV are found ranging from 2.4 to 4.1km/see indicating the presence of voids at some locations and also weak bond of the plaster. Core samples were taken with portable core cutting machine. Water absorption, compressive strength and density were determined. The compressive strength of core samples extracted varied from 22 to 26 MPa indicating good strength of core. [Table.1]. The depth of carbonation was checked with 1% phenolphthalein solution. Carbonation depth was found varying from 35 to 60 mm. The change color to pink indicates the portion of unaffected by carbonation. [Figure 2] The carbonation had reached reinforcement at some locations. The pH value of cover zone at many locations were between 9.0 to 10.0 confirming that cover concrete is affected by carbonation, leading to reinforcement corrosion. [3] But the chloride present in concrete is found to be less than 0.4 percentage of weight of cement. This infers that structure is not deteriorated due to presence of chlorides. Measurement of electrochemical parameters was done for assessing and identifying the presence and severity of corrosion. Half-cell potentials with copper-copper sulphate reference electrodes were measured. These test results have indicated the initiation of corrosion process and it is in initial stage. Electrical resistivity measurements were taken. The electrical resistivity is an indication of amount of moisture in the pores, pore system and size of pore. All the non-destructive results conducted confirm that core concrete is having good strength and the cover concrete is carbonated. The presence of chlorides is insignificant. The deterioration of concrete is mainly due to atmospheric pollutants like carbon dioxide and carbon monoxide and also due to aging. Hence structure needs to be protected from CO2,, CO, and chloride attack if any.

Realkalisation of Carbonated Concrete

Carbonated Reinforced Cement Concrete
Realkalisation is an important non-destructive technique used to restore the alkalinity of carbonated concrete for preventing from further corrosion of embedded reinforcement. Removal of carbonated concrete mechanically and replacing the same with new repair mortar involves a lot of work and also consumes time as well as money. Realkalisation helps in increasing the alkalinity of concrete i.e. pH of concrete will be increased to more than 12 initially and keeps the reinforcement in passivating condition for long time. Migration corrosion inhibitors are generally used for realkalisation works. [5]

According to another method of realkalizing concrete, the layers of concrete become carbonated and acidified due to exposure of surface of the concrete. This method assumes and presupposes that the concrete still contains adjacent layers that have not yet become carbonated. A substantially water-tight adherent coating is applied to the surface of the concrete that is exposed to air. Thereafter, the concrete is caused to become saturated with water from a source external to the concrete structure, and this condition of saturation is maintained for a period of time sufficient to effect a diffusion of alkaline materials from the relatively less carbonated layers of the concrete into the relatively more carbonated layers thereof. The carbonated layers thus become realkalized, so that further deterioration of the concrete structure is significantly arrested. [US patented 5049412].

Chloride extraction and realkalisation can be done by electro chemical treatment to halt and prevent corrosion in chloride contaminated and carbonated concrete. [2]. Principle of ion migration is used for removal of chloride ions from the contaminated concrete and simultaneaously pH of the carbonated concrete will be increased through electro-osmosis. Once surface is prepared, a steel or titanium mesh electrode will be attached to surface, and an electrode will be embeded in nontoxic biodegradeble electrolitic media. Once electric field is applied, chloride ions migrate from reinforcement bars inside concrete towards the externally attached electrode. Simultaneously the alkali ions migrate from electrolyte to concrete, thus raising the pH of concrete to original levels.

Repair Methodology Adopted

Concrete penetrating corrosion inhibitors are based on bipolar inhabitation mechanism. These can penetrate even in dense concrete by virtue of its low vapor pressure and natural affinity to metallic surfaces and inhabit corrosion at both anode and cathode surfaces. Conventional inhibitors are based on Calcium nitrate that works only on anodic inhibitors mechanism. Based on the non-destructive tests results it was decided to adopt the realkalisation of concrete for continued use. The existing surface coating of oil bound distemper was removed by scrubbing completely and washed with water and cleaned with compressed air. Holes were drilled at an interval of 500mm center to center along the length of the column on all the exposed sides in such a way that the staggering will be effective to cover maximum area of columns and beams. PVC nozzles are grouted with epoxy grout and air cured. Restoration and increasing the alkalinity concrete, an alkaline inhibitor solution of Calcium hydroxide 2gm/ litre and sodium nitrite of 10 gm/litre solution were grouted in all holes.

After 24 hours, cementious grout of non ferrous fluidiser, non gaseous expanding grout consisting of dry premixed blend of ultra fine special cement, set regulating and reactive chemicals with chlorine free, is mixed with water and injected into concrete at alternate grouting holes. This grouting is required to fill voids and honeycombed locations with cementious grout. It was noted that at some locations considerable quantity of grout was consumed confirming presence of voids. After 24 hours of air cure, single pack ready mix low viscosity epoxy grout was injected. These grouts were injected to take care hairline cracks present in existing concrete surfaces. After the grout, all the PVC nozzles were removed and sealed with epoxy grout.

Protective Coatings For Exposed RCC Members

Several methods are available in the market for protecting the exposed surface of concrete. Silanes and siloxanes are used as protective coats in the form of surface treatments. These react with cement matrix to form hydrophobic layer on the walls of the pores within the concrete. Thus protects the concrete from the ingress of water and water born salts. Moisture is required in optimum content for reaction of Silanes and siloxanes with hydrated cement matrix.

Coatings or impregnation on concrete reduce or impede chloride ion penetration, gas penetration to decrease carbonation or water penetration. Coatings consist of continuous film applied on the concrete surface with a thickness in the range of 100 to 300 microns. The coatings in the liquid form are sprayed or brushed on the concrete surface. Different coatings materials are Acrylic, Butadiene, copolymer, chlorinated rubber, epoxy resins, polluter resins, polyurethane, vinyl etc. The other types of materials used for treating the concrete surface are pore liners, which impregnate in the concrete pores and line them with water repellent materials. Materials used in this category as hydrophobic liner are silicons, siloxanes and Silanes. The third category of is pore blockers, which penetrate into the pores and then react with calcium hydroxide and block the pores. The materials available in this category are liquid silicates and silicofluoride.

All exposed RC members were coated with two-component high build epoxy polyamide composition with micaceous iron oxide pigment primer as base. Over this a coat of two-component high build polyamide cured epoxy coating was applied. Inspite of its unchallenged anti corrosion properties, epoxy resins suffer from its inherent limitation like chalking sensitivity in outdoor due to its poor UV stability. It is for this reason all epoxy based high performance coatings are generally over coated with a suitable polyurethane topcoat for colour and gloss retention in exterior exposure. For external surfaces exposed to UV rays, one coat of two-component air drying acrylic polyurethane coating was provided.

Qualification of Repair Methodology

Carbonated Reinforced Cement Concrete
The confirmatory tests were conducted on the repaired structural members. PH test for old concreting before and after injection re-alkalization chemical, etc., were conducted. Powder samples were collected for evaluation of pH of the realkalisation concrete. There is remarkable improvement of pH as given in Table no 3 and shown in Chart 1.

Discussion of Results

Rate of carbonation depends on the level of pore water, concrete grade, permeability of concrete and cover depth. The carbon dioxide diffusion will be very very slow when the concrete pores are filled with water. The already diffused carbon dioxide will result in formation of carbonic acid, which ultimately results in alkalinity reduction. Periodical maintenance like renewal is required for all concrete members of structures exposed to aggressive environment. The design of structures exposed to such aggressive industrial polluted atmosphere condition, appropriate protective coating for reinforcing steel, all the concrete and masonry surfaces shall be specified for execution right in construction stage.

Conclusions

  • Alkalinity of concrete can be increased and corrosion of reinforcement due to carbonation can be delayed by using certain corrosion inhibiting chemicals such as nitrites, phosphates, benzonites etc. on hardened concrete.
  • Aggressive surrounding environment influence carbonation and corrosion in concrete structures. If the surrounding environment is properly investigated, it is possible to adopt various remedial and precautionary measures so as to delay or minimize carbonation and corrosion initiation in concrete.
  • Corrosion inhibitor selected should be capable of penetrating upto steel reinforcement.
  • The Corrosion inhibitors used for realkalisation are acting as protective coats for the exposed surfaces of concrete due to property of hydrophobic impregnation
  • In the absence of proper investigation on the aggressive effect of the surrounding, the concrete structures have to be built with proper design, construction quality control so as to achieve highly impermeable concrete using mineral admixtures.

References

  • V.G.Papadakis et.al Effect of composition and environmental factors and cement–lime mortar coating on Concrete Carbonation _Material and structure 25 no 149 (1992)
  • LZA Technol., Sten k Henriksen, David Whitemore, "chloride extraction and realkalisation of Reinforced concrete stop steel corrosion, journal of performance of constructed facilities, volume 12, issue 2, pp 77-84 (May 1998)
  • M.Raja Shaker, V.M.Bhat, S.G.Bapat 'Refurbishing of chlorine affected factory building using polymers - a case study" 5thAsian Symposium on Polymers in concrete at Chennai. Organized by Structural Engineering Research Center Chennai Sept 2006.
  • M.Raja Shaker, Ramesh. R. Reddy "Sustainability of repair methods adopted for chloride induced distressed rehabilitated work" International Conference on Advances in Concrete and Construction ICACC-2008 organised by Vasavi College of Engineering, Hyderabad in Association with Osmania University, RMIT University, Melbourne, Australia, & South Dakota School of Mines and Technology, USA, 7-9 February 2008
  • Hand book on repairs and rehabilitation of RCC buildings published by Director General (Works) CPWD, New Delhi.
  • M.Raja Shaker, "Rehabilitation of corroded concrete structures," 14thNational congress on Corrosion Control organized by National corrosion control of India, Karikudi, 18-20, September 2008.

NBMCW February 2009

.....

Quality Control Model for Indian Ready ...

A Framework for Development of Quality Control Model for Indian Ready Mixed Concrete Industry

Ready Mixed Concrete Industry

Debasis Sarkar, Assistant Professor, Constructionand Project Management, CEPT University, Ahmedabad

Quality Control of Ready Mixed Concrete can be divided into three convenient areas like forward control, immediate control and retrospective control. SQC application proves to be a vital tool which can be used effectively for quality and productivity improvement for infrastructure projects. Statistical Quality Control can be effectively applied to RMC industry for online (during production) and also offline (before and after production) quality monitoring and control. SQC and particularly SPC techniques have potentiality to improve efficiency and profitability of the organization. However, in absence of proper and effective quality monitoring systems in most of the batching plants in India, they are lagging behind from their western counterparts in terms of operational procedures, product quality and economy in manufacture of concrete. Hence there is a need to analyze the applicability of scientific quality monitoring techniques to RMC in India. The proposed Quality Control Model for Indian RMC industry is a SQC based, simplified user–friendly model, which if adopted, we think, would make Indian RMC industry much more organized and efficient in terms of production and operation.

Introduction

As per Indian Standard code of practice (IS 4926) Ready Mixed Concrete (RMC) is defined as the concrete delivered in plastic condition and requiring no further treatment before being placed in position in which it is to set and harden. Instead of being batched and mixed on site, concrete is delivered for placing from central batching plant. History reveals that RMC was patented in Germany in 1903, but the means of transporting it had not developed sufficiently well to enable the concept to be exploited. There were significant developments in USA in the first quarter of 20thcentury. The first delivery of RMC was made in Baltimore in 1913, and the transitmixer was born in 1926. In 1931 K.O. Ammentorp, erected a plant at Bedfont, west of London and launched a company named as Ready Mixed Concrete Ltd. At the same time companies like British Steel Piling Company, Scientific Concrete Co. Ltd, Jaeger System Ltd, became interested and entered into RMC business.

The most interesting aspect of RMC industry is its remarkable growth. In United States, in 1925, there were about twenty RMC operators, by 1929 this number had grown to hundred and today there are more than ten thousand operators. In United Kingdom between 1950 and 1974, about thirty one million cubic meters of RMC was produced at its peak. Today this production have exceeded ninety one million cubic meters. Since its inception, major developmental research work pertaining to production and operational procedures has been carried out in UK.

From this growth pattern, it is obvious that the RMC plants had something to offer. The consumer wanted his concrete delivered to the job in a ready-to-place condition. This growth has been accompanied by improvements in the equipment and methods used by the ready mixed operator. Volume batching has completely been replaced by weigh batching and presently computerized weigh batchers are used in most of the batching plants. Aggregates are stored in properly installed bins and cement and flyash are stored in silos. Conveyors are used to transport the aggregates. Cement and flyash is pumped into the central mixer with pneumatic pumps. Electronic moisture meters, digital admixture dispensers are used in fully automatic batching plants. This introduction of improved batching equipment has made possible the exact and scientific formulation of concrete mix. Concrete designed to meet the specific requirements can now be ordered directly from the ready mixed operators.

Quality Control of RMC can be divided into three convenient areas like forward control, immediate control and retrospective control (Dewar and Anderson, 1988). Forward control basically deals with procedures of quality control to be followed before the production process. This covers (i) materials storage, (ii) monitoring of quality of materials, (iii) modification of mix design, (iv) plant maintenance, (v) calibration of equipment and (vi) plant and transit mixer condition. Immediate control is concerned with instant action to control the quality of concrete during production or that of deliveries closely following production. This covers (i) weighing – correct reading of batch data and accurate weighing, (ii) visual observation and testing of concrete during production and delivery (assessment of uniformity, cohesion, workability, adjustment of water content) and (iii) making corresponding adjustments at the plant automatically or manually to batched quantities to allow for observed, measured or reported changes in materials or concrete qualities. Retrospective control primarily deals with the quality control procedures after production. This covers (i) sampling of concrete, testing and monitoring of results, (ii) weighbridge checks of laden and unladen vehicle weights, (iii) Stock control of materials and (iv) diagnosis and correction of identified faults.

In India RMC was launched about two decades ago. Lack of proper manuals, higher initial costs than conventional site mixed concrete, high initial investments for installation of automatic batching plants and also lack of awareness were the major causes that led to an initial setback to the RMC industry. In the present era rapid urbanization has resulted in increase in demand for multistoried housing complexes, commercial complexes like shopping malls, retail units, multiplexes, multistoried office buildings and other real estate projects have largely increased the demand of good quality concrete to make the structures adequately safe and durable. Use of good quality concrete is also one of the basic requirements to make the structure earthquake resistant. Problems in availability of land in urban cities particularly the metro and mega cities like Delhi, Mumbai, Chennai, Kolkata, Ahmedabad, Bangalore, Pune etc. have increased the need for vertical expansion due to restrictions and constraints in horizontal expansion. Thereby construction of tall structures has become an essential requirement of urban cities. Thus in course of time awareness of the advantages of using RMC and realization of the fact that the conventional concrete may result in higher lifecycle cost due to higher maintenance costs made the construction industry to adopt RMC as a better option economically and qualitatively. But still in India about 2% of the total cement produced is utilized in RMC production against 70% of the cement produced being utilized in RMC in UK and US.

In this paper, an attempt has been made to develop a framework for Statistical Quality Control (SQC) based quality control and monitoring model for Indian RMC industry.

Case Study

Mode of Data Collection

To develop the SQC based model for quality control and monitoring for Indian RMC industry data have been collected from four operational RMC plants, two of which are from Ahmedabad and two from Delhi. One of the plant from Ahmedabad is fully automatic and has a production capacity of 30 cu‎m / hr. This is referred as Case 1. The silo for this plant for storing cement is of 100 MT capacity and the silo for storing flyash is of 80 MT capacity. Cement and flyash is pumped with pneumatic pumps. The typical grades of concrete produced are M15 to M50 with both 43 and 53 grade cements. The other plant from Ahmedabad is semi automatic with a production capacity of 15 cu‎m/hr. This plant has been set up by a contracting organization for supplying concrete for their in-house projects. This is referred as CASE 2. Here silos are not available for storage of cement / flyash and raw materials are transported into the central mixer through screw conveyors. The grades of concrete produced are M20 and M25 with 43 and 53 grade of cement. The plants of Delhi are fully automatic and have production capacity of 60 cu‎m / hr. Cement and flyash are stored in silos and conveyors are available for transporting raw materials. The concrete produced are M15 to M50 with 43 and 53 grade cement. High strength concrete upto 80 N/mm2(Mpa) can also be produced. These two plants are referred as Case 3 and Case 4 respectively.

Data pertaining to total quality management system for production and operation of RMC like cube compressive strength of 7 days and 28 days concrete grades of M15, M20, M25, M30, M40, tests of raw materials like cement, coarse aggregates, fine aggregates, admixtures, water, details of present equipment used, cycle time for production, production capacity of the respective plants under study, methods for transportation of raw materials, transportation methods for produced concrete, details of concrete mix designs have been collected. Also observations pertaining to the lacunae in the present production, monitoring and operational procedures and the present day to day problems faced by RMC industry have been noted.

Classification of Mixes and Comparative Analysis of Case Studies

The various concrete mixes collected from Case 1, 2, 3 and 4 have been given a specific designation based on the grade of concrete, slot / batch and specific case from where the batch has been collected. These designations prove to be very useful in referring the concerned mixes during the analysis. For Mix “1 M20 C1,” “1”refers to the slot for which this mix designs is followed (20.05.06 to 04.06.06). “M20” is the specified grade of concrete where “M” is Mix and 20 is the characteristic compressive strength at 28 days which for this case is 20 N/mm2 or 20 Mpa. “C1” represents Case 1. Details of this designation are presented in Table1.

Classification of Mix Based

A comparative presentation of all the samples collected from Cases 1, 2, 3, and 4 along with the calculated mean, plant standard deviation, target mean strength, target mean range and coefficient of variation are presented in table 2. As per Dewar and Anderson, (1988), Target Mean Strength (TMS) = fck + 2 σ…. (1) where fck is the characteristic cube compressive strength at 28 days, σ = plant standard deviation. Target Mean Range (TMR) = 1.128 x assumed standard deviation (plant standard deviation)….. (2).

Comparision of Plant Standard Deviation, Mean, TMS, TMR, and Coefficient of Variation

Table 2 has been formulated using Excel wherein the mean strength of the data of each slot for a specified concrete grade is calculated and presented in column (6). Column (5) represents the values of plant standard deviation as obtained from the calculation of the values of the compressive strengths of the collected samples of different batches of the same slot and mix design. It is more logical to use the values of the plant standard deviations for analysis for target mean strengths greater than 27 N/mm2 (Dewar and Anderson, 1988). From the calculated results it is observed that for four case studies under consideration the value of standard deviation obtained in the plant varies from values as low as 1.92 to as high as 6.57. This reflects the fact that a different degree of quality control is maintained by each plant. The value of standard deviation of 1.92 is for a plant which is semi-automatic. Since this plant is not a commercial RMC plant and is set up by a contracting organization to supply concrete for construction of a management institute, location and working conditions of the plant are excellent and most of operating conditions are under control and the quantity of concrete produced, is of range of 600 to 900 cu‎m per month. Due to this limited production they might be able to maintain good degree of quality control, thus having lower standard deviation values. On contrary for a plant as in case 3 located in Delhi which is fully automatic and has latest imported equipment, but is not able to maintain a good degree of quality control, which is reflected from a very high value of standard deviation like 6.57 may be due to the pressure of producing and delivering very high quantity of concrete which may be of the range of 500 to 800 cu‎m per day during peak demands. Thus the present work aims in developing an effective quality monitoring and control model which will definitely benefit the commercial batching plants located in metros and major cities of India which have a target of producing superior quality concrete of quantities as high as 1000 to 1200 cu‎m per day.

Target Mean Strength (TMS) for each mix is calculated as per equation (1) and the Target Mean Range (TMR) as per the relationship TMR = 1.128 σ (Dewar and Anderson, 1988). The CUSUM for range aims in monitoring the difference between two consecutive values of compressive strength obtained and also monitors the difference between highest and lowest value of the strength obtained in a specified grade of concrete of different batches of the same slot produced from same mix design. The coefficient of variation presented in column (9) is a good indicator of the degree of quality control maintained by the plant. As per the results obtained the coefficient of variation varies from 0.06 (Case 2) to 0.164 (Case 1). The average coefficient of variation of Case 1 is 0.118, Case 2 is 0.068, Case 3 is 0.131 and of CASE 4 is 0.116.

These results reflect the fact that amongst the four cases under analysis best quality control is maintained by the plant of Case 2, followed by Case 4, Case 1 and Case 3.

Proposed Quality Control Model for Indian RMC Industry

Based on the analysis carried out in this present research, a Quality Control Model which can be widely accepted by the Indian RMC industry has been proposed. The schematic representation of this model is presented in Figrue. 1

Proposed Quality Control Model for Indian RMC Industry

Quality Assurance (QA)/ Quality Control (QC) Team

The QA /QC team for the proposed model should be headed by a QA/QC Manager. He is the key person involved in the decision making process. He should be assisted by QA/QC Incharge whose job is to co-ordinate and implements the QA /QC guidelines and test procedures for the entire batching plant. Minimum two QA/QC Engineers should assist the incharge. The engineers should be involved in physically conducting the tests of the incomming raw materials and the final product ie. RMC. They should be physically present to check that all the equipment are calibrated properly. Also they should carry out the daily or weekly quality monitoring as per the proposed model. The QA/QC Incharge will take decisions about the assignable causes noticed during the monitoring phase. At least four lab technicians should be employed per plant to carry out the sampling and testing.

Establishment of Standard QA / QC Lab

A standard QA /QC lab with latest testing equipment should be established at the plant.

Testing of Raw Materials, Fresh Concrete and Hardened Concrete

The incoming materials particularly raw materials like cement, coarse aggregate 10mm, coarse aggregate 20mm, fine aggregate, admixtures should be subjected to testing as per adopted acceptance sampling plan. A proposed scheme for the basic tests to be conducted for raw materials, fresh concrete and hardened concrete along with their frequency of testing is given in Table 3.

Proposed Quality Scheme for Testing of Raw Materials for RMC

Ready Mixed Concrete Industry

Operation as per Proposed Model

The design of the mix should be carried out by competent authority and should be approved by authorities like Indian Institute of Technology (IIT), National Council of Cement and Building Materials (NCCBM), Council of Scientific and Industrial Research laboratories, Concrete Technology laboratories of National Institute of Technologies etc. The approved mix design for each grade of concrete should be tested in the plant as per plant conditions by casting trial cubes. Any major deviations of strength should be immediately communicated to the concerned authority and necessary rectifications should be carried out immediately.

Admixtures

A standard QA /QC laboratory equipped with latest testing equipments should be established at site. Minimum two lab technicians should support two QA /QC engineers. The QA / QC engineers should report to QA / QC Incharge who in turn should report to QA/QC Manager. The incoming raw materials (cement, fine aggregates, coarse aggregates 10mm, coarse aggregates 20mm, flyash, admixtures etc.) should be tested with visual checks as per the testing procedures laid down by the respective QA/QC department of the plant. The raw materials found unsatisfactory in the visual checks should be rejected and as per requirement confirmatory tests should be conducted before rejection. The supplier should be immediately informed about the rejection of his material and should be warned so that if he repeats the same, his contract would be terminated. Standard tests for raw materials, fresh concrete and hardened concrete have to be performed as per Indian Standard (IS) testing procedures. The details of these tests along with the respective IS codes and frequency of testing is given in table

The materials qualifying in the respective tests are accepted and aggregates are stored in respective aggregate storage bins. Cement and flyash should be stored in cement silo and flyash silo respectively. It is desirable to transport the cement in bulkers and directly pump it into the silo through pneumatic pumps. The cement if required to be stored in godowns should be well protected from rains. The godown should be covered on all sides and should be provided with adequate size of lockable entry or exit gates. Cement should always be stored on a plain cement concrete base which is about 200 mm thick or brickwork base which is about 300 mm above the ground. The oldest lot should be used first. It is always better to cover the cement bags with polythene sheets particularly in pre-monsoon and monsoon seasons. The workforce involved in handling cement bags or bulk cement should compulsorily be provided with personal protective equipments like safety helmets, safety shoes hand gloves and safety goggles. During pumping of cement into the silos the workmen should take utmost care so that the cement due to back hammering effect does not create major accidents like severe eye injuries. The workmen should compulsorily wear safety goggles and helmet while working with the pneumatic pumps.

The stored raw materials should be weighed (as per the weights of design mix) in the respective weigh hoppers fitted with load cells. These weighed materials through screw conveyors or loaders should be transported to the central mixer. Measured quantity of water through water meter and admixtures through admixture dispensers should be added into the central mixer after dry mixing of cement, flyash, coarse aggregates and fine aggregates. After addition of water and admixtures the final mixing should be carried out for about 30 seconds. The entire loading process takes about 90 seconds. Thus the ideal cycle time for each of concrete is about 120 seconds. The capacity of each batch may be of 0.5 m3, 1 m3 and 2 m3 depending upon the capacity of the plant.

The RMC thus produced have to undergo tests of fresh concrete like slump tests and corresponding temperatures of the concrete should be noted. If the concrete is produced in summers when the outside temperature is about 40 to 45 degree Celsius the drum of the transit mixer should be covered with wet jute cloth and desirably some ice cubes should be added to the fresh concrete. The slump of the mixes to be placed by concrete pumps should be at least 180mm to 200 mm at the time of pumping. Thus use of flyash in the mix is always desirable for pumpable mixes as flyash due to ball bearing action increases the pumpability of a mix. The slump of concrete to be placed by crane and concrete bucket or by elevator hoists should be about 100 to 120mm at site. Minimum 6 cubes (150mm x 150mm x 150mm) should be prepared for each batch of concrete produced as per standard procedures. The cubes should always be casted in a cool shady place and in no case should the casting procedure be carried out under hot sun. The cubes should be air cooled for about 24 hours and then placed into the curing tank for curing. The curing tank should be completely filled with water and desirably should contain different compartments for 3 days, 7 days and 28 days testing. The fresh cubes should be marked with the specified grade of concrete, date of casting and should be placed in a jute gunny bag and after tying the mouth of the bag and putting a plastic tag of the grade and date of casting of the concrete, the bag should be placed in the desired compartment of the curing tank. Generally if the requirement is that the concrete needs to be tested for 7 days and 28 days compressive strength, then 3 cubes should be packed in the jute gunny bag and placed into the compartment for 7 days curing and other 3 cubes should be placed in the compartment for 28 days curing. As per client requirement if 3 days strength also needs to be tested then total 9 cubes should be casted. Following a systematic procedure of casting and curing the cubes improves the degree of quality control of the plant. The cubes should be tested in hydraulically operated compressive strength testing machine and desirably the manually lever operated strength testing equipment should be avoided. Care should be taken to ensure that the cube is uniformly seated before application of the load and the rate of loading should be uniform and desirably 140 kg /cm2/min. The failure load of the three cubes should be noted separately and the corresponding compressive strength should be calculated. The average of three cube strengths can be considered as the observed strength of a sample.

The fresh concrete thus prepared is loaded in transit mixers and transported to respective sites. The time from batching to placing of the concrete at the site should not exceed 2 hours. If the concrete is transported in summers the drum of the transit mixer should be covered with wet jute cloth and during extremely high temperatures ice cubes should added in the concrete. No additional water should be added in the concrete enroute. Before placing, the concrete should be tested for desirable slump, temperature, uniformity and cohesiveness and if found ok should be placed at the respective site.

The 7 days and 28 days strength obtained should be monitored using CUSUM and V-mask. This technique should be used as a daily monitoring tool and when the results of about four to five samples are obtained the data should be plotted using Excel / SPSS software where the x- axis of the graphical representation of the CUSUM plot (mean strength, range and correlation) represents sample number and the corresponding y- axis represents the CUSUM value in N / mm2 or Mpa. The V-mask plotted in a transparent paper should be superimposed on the obtained CUSUM plot by placing the lead point of the mask on each and every obtained CUSUM value starting from the first sample. If the plot remains within the boundary of the mask, the process is in control. If the plot crosses the boundary of the mask, a significant change has occurred in the process mean and an action is required. The root cause of the problem need to be investigated and accordingly action should be taken. A change in cement content can be calculated by the empirical relationship proposed by Dewar and Anderson (1988), and trial cubes can be casted and tested with the modified mix design value. The proposed Quality Control Model for Indian RMC Industry is presented in Figure 1.

Conclusion

RMC emerges to be an advantageous material in congested sites where setting up of a mixing plant is difficult. Presently construction industry including real estate developers are opting for RMC of grades M20, M25, M30, M35, M40, M50 and even higher grades. However, in absence of proper and effective quality monitoring systems in most of the batching plants in India, they are lagging behind from their western counterparts in terms of operational procedures, product quality and economy in manufacture of concrete. Hence there is a need to analyze the applicability of scientific quality monitoring techniques to RMC in India. The proposed Quality Control model for Indian RMC industry is a SQC based, simplified user friendly model, which if adopted, we think, would make Indian RMC industry much more organized and efficient in terms of production and operation.

Acknowledgment

Author highly acknowledges the cooperation and help of the RMC batching plants of Ahmedabad and Delhi under study, in providing necessary information for this research work.

References

  • Dewar, J.D. and Anderson, R. (1988) Manual of Ready Mixed Concrete Blackie and Son Ltd., Glasgow and London.
  • Box, G. (1994) “Role of Statistics in Quality and Productivity Improvement” Journal of the Royal Statistical Society, Series A, Part 2, pp. 209-229
  • Keats, J.B. and Montgomery, D.C. [Editors] (1996) Statistical Applications in Process Control Marcel Dekker, Inc., New York.
  • Montgomery, D.C. (1985), Introduction to Statistical Quality Control John Wiley & Sons, Inc, New York.
  • Montgomery D.C. and Woodall, W.H. (1997) “A Discussion on Statistically Based Process Monitoring and Control.” Journal of Quality Technology Vol. 29, pp. 121-162.
  • Sarkar, D. and Bhattacharjee, B. (2003) “Quality Monitoring of Ready Mixed Concrete Using Cusum System” Indian Concrete Journal, Vol 7, pp. 1060-1065.
  • Woodall, W.H. (1986) “The design of CUSUM quality control charts.” Journal of Quality Technology, Vol. 18, pp. 99-102.

NBMCW December 2008

.....

Ground Granulated Blast Furnace Slag Bl...

Blended Concrete

D. K. Jain, Research Scholar, J. Prasad, Associate Professor, and A. K. Ahuja, Associate Professor, Department of Civil Engineering, I. I. T. Roorkee.

Cement concrete with Ordinary Portland Cement (OPC) continues to be the pre-eminent construction materials due to its commendable performance in terms of strength aspects, but durability of this is not satisfactory particularly when it is exposed to aggressive environment. Same time, present global environmental requirements suggest the civil engineers for reducing the consumption of OPC. Use of mineral admixtures like Blast Furnace Slag, Fly Ash and Silica Fume etc. in concrete may be a suitable solution in such situation.

Based on literature available, the present paper discusses the effect of GGBFS blending in concrete over its properties in fresh and hardened states. Factors affecting the hydration and strength development of blended concrete are presented. The present paper discusses the resistance of GGBFS blended concrete to Chloride attack, Sulphate attack, Carbonation, Aggregate-Silica reaction and Frost attack with factors influencing its performance. Pore refinement and secondary gel formation in the GGBFS blended concrete are the main reasons.

Introduction

OPC based concrete continues to be the pre-eminent construction materials for use in any type of civil engineering structures because of its easiness in construction, its satisfying performance in strength requirements, better durability in normal environment, in comparison to other construction materials like steel, timber etc but at the same time some problems are also associated with this. First is environmental pollution and large energy requirement in the production of OPC. Production of one tonne OPC required approximate 4.0 G Joule energy and produced approximate one tonne CO2 gas in the environment. At present the cement industries produced approximate 7% of total CO2 produced in the world, which is very alarming to our protective Ozone layer. Second problem is the lower durability in aggressive environment. Concrete with OPC, which performed, very well over a period of about 100 years in the normal environment showed substantial damage within a few years of construction in the aggressive environment. Use of mineral admixtures like Ground Granulated Blast Furnace Slag (GGBFS), Silica Fume (SF), Fly Ash (FA) etc. in concrete may be the better solution in above conditions.These admixtures also offer benefits with respect to the cost of concrete.

Based on the information available in literature the performance of GGBFS in blended concrete, with respect to properties in fresh state, in strength development and in durability aspects are discussed in the present paper. Durability aspects with reference to Chloride resistance, Sulphate resistance, Aggregate-silica reaction and Frost resistance are described. Hydration process of GGBFS and its reactivity with OPC is also discussed. Effects of important factors on properties of blended concrete in fresh and hardened state are highlighted.

Ground Granulated Blast Furnace Slag

Blast Furnace Slag is a by product obtained in the manufacturing of Pig iron in the Blast furnace and is formed by the combination of earthy constituents of iron ore with lime stone flux. Quenching process of molten slag by water is converting it into a fine, granulated slag of whitish color. This granulated slag when finely ground and combined with OPC has been found to exhibit excellent cementitious properties. Glass particles of GGBFS are the active part and consist of Mono-silicate (Q0-type), like those in OPC clinker, which dissolve on activation by any medium. Glass content in GGBFS is normally more than 85% of total volume. Specific gravity of GGBFS is approximately 2.7-2.90, which is lower than of OPC. Bulk density of GGBFS is varying from 1200-1300 kg/m3. Normal chemical composition of Indian GGBFS is shown in Table 1. GGBFS is more closure to OPC in chemical composition in compare to other mineral admixtures.

Hydration of GGBFS

Hydration products of GGBFS are poorly crystalline Calcium Silicate Hydrate broadly similar to that formed from hydration of OPC, but with lower Ca/Si ratio (Jimenez et al., 2003). Due to lower Ca/ Si ratio, these hydrates have more alkali retention capacity. Hydration products of GGBFS effectively fill up the pores and increase the strength and durability of concrete. GGBFS requires activation to initiate hydration and the availability of a medium for continuing the hydration process. Slag hydration can be activated by using alkalies, lime, sulphate etc (Chemically activation), or by fine grinding (Mechanically activation) or by increasing temperature of concrete (Thermal activation). Various alkalies activators like Sodium hydroxide, Sodium carbonate, Sodium sulphate, Sodium silicate (Water glass) etc. can be used for slag. Water glass activated slag produced most cross-linked structures that results in increased mechanical strength of hydration products, while Sodium hydroxide make hydration process of slag more intensive (Garcia et. al., 2003). Due to higher activation energy of blast furnace slag relative to OPC, it has advantage of thermal activation on its hydration (Roy and Idorn, 1982).

Table 1: Chemical Composition of GGBFS
OxidesQuantityRemark
SiO230 - 35%-
Al2O38 - 22%Higher in Indian Slag
CaO27 - 32%Lower in Indian Slag
MgO7 - 9%-
Fe2O38 - 10%Higher in Indian Slag

Reactivity of GGBFS with OPC

Hydration products like alkailies, lime and heat of OPC are activate the hydration of GGBFS particles in blended cement concrete. Initially and during early hydration of concrete containing GGBFS, the predominant reaction is with Alkalies hydroxide, but a subsequent reaction is with Calcium hydroxide (Roy and Idorn, 1982). Reactivity of GGBFS with OPC in blended concrete is depends on chemical & mineralogical composition, glass content, fineness etc. of the GGBFS. Type of portland cement employed and curing conditions also has a significant effect on the rate of formation of hydration products in blended concrete. On the basis of chemical composition of GGBFS various indices have been proposed in the literature to evaluate its reactivity Table 2. These formulas are not considering the effect of composition of OPC on hydration of blast furnace slag. Mantel (1994) reported the more reliability on first three formulas. Reactivity of high glass content slag is normally found more and greater fineness of slag also increases its reactivity due to increase in surface area for reaction with activators. Use of rapid hardening cement in place of OPC increases the reactivity of slag in blended concrete, due to more activators available at early age.

Table 2: Indices for Reactivity of GGBFS
Indices as per Chemical CompositionRecommended values
CaO / SiO21.3 - 1.4
(CaO + MgO) / SiO2> 1.4
(CaO + MgO) / (SiO2 + Al2O3)1.0 - 1.3
(CaO + 0.56Al2O3 + 1.4MgO) / SiO2> 1.65
(CaO + MgO + Al2O3) / SiO2> 1.0

ACI (ACI 233R-95) recommends the use of Slag Activity Index (SAI) to evaluate its reactivity. SAI is the percentage ratio of the average compressive strength of slag blended cement mortar cubes (at 50% slag content), to the average compressive strength of reference cement mortar cubes at a designated age. Based on SAI the GGBFS is classified into three grades namely, Grade 80, 100 and 120. Blended concrete with grade 120 normally achieved strength of OPC concrete at 3rd day and after, while concrete with grade 100 achieved at 7th day and afterward. However, concrete made with grade 80 GGBFS will have a lower strength at all ages and not recommended by ACI for use in structural concrete.

Mix Proportioning

No specific mix proportioning method is available for GGBFS blended concrete. Simplest and most common method of incorporating slag in concrete is a straight forward replacement of OPC by equal weight, but due to the difference in specific gravity of these two materials slight adjustment in aggregate content is needed for correct yield. Swami (1990) suggests that the total cementitious material has to be increased by 10% for 50% replacement and by 20% for 65% replacement level in blended concrete to attain a strength comparable to normal OPC concrete. Babu and Kumar (2000) used the efficiency concept for developing equal 28 days compressive strength for blended and normal concretes. They suggested the value of overall efficiency from 1.29 to 0.7 for 10 to 80% replacement level. Normally 30 to 50% replacement of cement by GGBFS is adopted in the field.

Properties of Fresh Concrete

Effect on Hydration Tempreature
Use of GGBFS in concrete usually improves workability and decreases the water demand due to higher smoothness of GGBFS particles and increase in paste volume of concrete. At higher replacement level (> 50%) the water demand may increased for same workability (Sivasundaram and Malhotra, 1992). The possible reason for this is the greater fineness of GGBFS particle, which increases the surface area of binder in concrete at larger replacement level. Segregation and bleeding chance in GGBFS blended concrete is lower. GGBFS blending increases the setting time of concrete but gap between initial and final setting is reduced. Setting time of blended concrete is reduced with increase in the fineness of GGBFS. Dose of air entraining agent require is higher for GGBFS blended concrete in comparison to OPC concrete, to produce same air entrainment. Use of GGBFS in concrete reduces the hydration temperature and also prolonged the time for peak temperature of concrete as shown in Figure 1 (Brooks and Al-Kaisi, 1990). Results related to creep and shrinkage shows the more detrimental effect of drying environment, or need of an early water curing for better performance of GGBFS blended concrete.

Strength Development

Strength development of GGBFS blended concrete is quite different from OPC concrete. Strength of blended concrete at early ages is lesser in compare to that of OPC concrete. Due to the prolonged hydration process of slag the later age strength of blended concrete is higher than OPC concrete. Results found by Geiseler et. al (1995) for the strength development of two equal grades mortars with and without GGBFS are shown in Figure 2. They also observed the 100% increase in strength of GGBFS blended mortar, after a period of 25 years, in compared to 28 day Strength.

Strength development of blended concrete depends on properties of GGBFS, type of OPC, proportion of ingredients, and curing conditions. Olorunsogo and Wainwright (1998), studies the effects of particle size distribution of GGBFS on compressive strength of blended mortar. They use position parameter (x0) to serves as an indicator of the degree of fineness of slag and the slope of curve (n) to represents the size range of particles of different GGBFS. They found that the strength of mortar is reducing when value of position parameters (x0) is increasing for constant slope value (n), but effect of change in slope value (n) for constant position parameter (x0) is very little (Figure 3 a & b).

Effect of Particle Size Distribution of GGBFS

Lim and Wee (2000) and Jain and Pal (1998) present the effect of replacement percentage and fineness of GGBFS on strength development of blended concrete. They found that after the age of 28 days of moist curing, strength of 50, 65,and 80% GGBFS blended concrete are higher than the OPC concrete, while at 91 days blended concretes (with all replacement level) shows higher strength. Results of both studies show the optimum content of GGBFS equal to 50% for maximum strength. Blended concrete with higher fineness of GGBFS shows higher strength up to the age of 28 days, but at the age of 91 days strength of concrete with GGBFS of different fineness is very similar and significantly higher than the OPC concrete (fig 4 a & b). Therefore, toincrease the strength development at early ages finer grinding of GGBFS is useful.

Curing condition has much more effects on strength development of GGBFS blended concrete due to its slow and prolonged hydration. It required longer and continuous moist curing for complete hydration of slag particles. Sanjayan and Sioulas (2000), Miura and Iwaki (2000) and Brooks and Al-Kaisi (1990) studies the effects of curing conditions on strength of GGBFS blendedconcretes. They found that early age strength development (up to 28 days) of GGBFS blended concrete is increasing with curing temperature, but the effect on later strength is very little. Miura and Iwaki (2000) found that the effects of curing method on strength development are found only after age of 7 days. Blended concrete cured under water shows higher strength than sealed cured concrete. The detrimental effect of isolating specimens from a continuous supply of moisture where more on later age strength of GGBFS blended concrete. Temperature matched curing is also increases early age strength of concrete, particularly in the case of high GGBFS content concrete.

Chloride Resistance

Chloride resistance of concrete is the most important aspects for durability of RC structures. When chloride content in concrete reaches more than the threshold value, the protective layer of alkalinity get broken and steel reinforcement will corrode in the presence of oxygen and humidity. Hydration products of cementitious materials are also reacts with chloride and form Freidel's salt that does not have any harmful effects on concrete durability.

Chloride Resistance
GGBFS blending in concrete increases its resistance to chloride penetration. Effect of GGBFS is not limited to the initiation of corrosion by chlorides, it also increases the critical chloride concentration, beyond which unduly high corrosion progress occurs. Lower corrosion rate of steel in blended concrete is attributed to the conjoint effect of lower permeability and a decrease in the diffusion rate of chloride ions in these concrete. The reduction in the diffusion rate of chloride is attributed to the reduction in the capacity of blended concrete to exchange anions, which is due to the lower concentration of hydroxyl ions in the pore solution.

Kumar et. al. (2002) presents the results of depth of chloride penetration in concrete after 6 months exposure in 3.5 % Sodium chloride solution and found that penetration decreases with increase in GGBFS content. Corrosion studies of Jain and Pal (1998) show that corrosion resistance of blended concrete with 50% or above GGBFS gives real advantages. Swamy (1986) observed that in the first 10 mm layer, there was little difference in the amount of soluble chloride in OPC and GGBFS blended concrete, but at larger depth chloride content in GGBFS concrete were significantly lesser. Author reported that for a particular mix proportion the diffusion of chloride is reduced with decrease in the w/cm ratio or with increase in GGBFS content (Figure 5). Wee et. al. (2000) reported that the higher replacement level, higher fineness, and longer moist curing increase the chloride resistance of GGBFS blended concrete (Figure 6). Hope and Ip (1987), studies the corrosion of steel in blended concrete, after various cycles of wetting and drying in 3.5% sodium chloride solution and found that the corrosion of the steel is decreases with increase in GGBFS content. Al-Amoudi et. al. (1993) studied the long term (7 years) corrosi on resistance of concretes in 5% Sodium chloride solution and found the corrosion rate of 6.5 μm/year of steel in GGBFS blended concrete while it is the 38 μm/year in OPC concrete. Gu et. al. (2000) reported the supremacy of 55% GGBFS blended concrete with 28 days compressive strength of 45.6 MPa over OPC concrete having strength of 61.1 MPa by RCPT test. The accumulated charge passed through GGBFS blended concrete is found 670 coulombs while in normal OPC concrete, it is 1730 coulombs. Sivasundaram and Malhotra (1992) observed electrical charge ranged from 174 to 383 coulombs for various GGBFS blended concrete. Smith et. al. (2004), found that GGBFS blending benefit the initial concrete resistivity and also tend to increase the resistivity with maturity.

Zhang et. al. (1999) found that the charge passed during RCPT is 515, 775, & 675 for GGBFS blended concrete cured under moist room, 7 days burlap & then laboratory air, and with curing compounds, up to the age of testing while in OPC concrete with same curing conditions the values are 1105, 1500, & 1135 coulombs, hence they reported that the very little effect of curing conditions on chloride resistance concrete with low w/c ratio, although for all conditions the charge passed for blended concrete is lower than for OPC concrete. Detwiler et al. (1994) reported that the use of 30% slag has a far more effect on chloride resistance than lowering the w/c ratio from 0.5 to 0.4 in OPC concrete and they also found the increases in chloride diffusion with increase in curing temperature.

Sulphate Resistance

Solid Sulphate does not attack the concrete severely but when chemicals are in solution, they find the entry into porous concrete and react with the hydrated cement products. Sulphate attack of concrete takes place by the reaction of sulphate ions with Calcium hydroxide and Calcium aluminate hydrate and produced Gypsum & Ettringite with larger volume and lesser strength. Concrete with GGBFS shows improved resistance to Sulphate attack because it has lesser concentration of Calcium hydroxide and Calcium Aluminate hydrate in pore solution. Pore size and volume refinement of paste in blended concrete also improve its Sulphate resistance, but it is not a primary cause. Sulphate resistance of blended concrete depends on chemical composition of GGBFS and cement, GGBFS content in concrete, curing condition, and type of ion associated with Sulphate etc.

C3A content of OPC and Alumina content of GGBFS are the primary factors related to chemical composition of binders. A replacement level of 70% or more of OPC by high Alumina slag (13- 15 %) is required for improved Sulphate resistance, where as 50 % replacement level provides good Sulphate resistance when low Alumina slag is used (Hooton and Emery, 1990). In the case of high C3A cement the 70 –80% replacement of OPC is required for better Sulphate resistance. Increase in GGBFS content in a mix increases its Sulphate resistance. Jain and Pal (1998) reported that replacement of 50% OPC by GGBFS improved the Sulphate resistance of concrete, however 70% replacement reduces the expansion of concrete significantly in Sulphate solution at the age of 6 months (Figure 7). Sulphate resistance of concrete also depends on C3S/C2S ratio (Rasheeduzzafar et. al. 1990). Low C3S/C2S ratio at a particular value of C3A increases the Sulphate resistance. Wee et. al. (2000), reported that the reduction in w/cm ratio, or increase in GGBFS content, increases the Sulphate resistance of blended concrete while change in fineness of GGBFS has no effect.

GGBFS blended concrete shows good resistance to Sodium Sulphate, but shows quick deterioration in Magnesium Sulphate attack (Figure 8). Rasheeduzzafar et. al. (1994) attributed this behavior to the depletion of Calcium hydroxide in GGBFS blended concrete. In the absence of Calcium hydroxide, Magnesium ions react more directly and extensively with C-S-H to generate noncementitious M-S-H hydrate, results in aggravated deterioration. Osborne (1999) presents the results of studies carried out at BRE (UK) and found the early curing of concrete is the most significant factor for Sulphate resistance. A beneficial effect of initial short air curing on the long-term Sulphate resistance is also reported by many researchers. This may be due to the formation of a carbonated outer layer in concrete leading to blocking of the pores and refinement of pore structures.

Aggregate-silica Reaction

Aggregate-Silica reaction (ASR) in concrete is a chemical reaction between the active silica constituents of the aggregate and the alkalies (Na+ and K+ ions) of the cement, which formed silicate gel with increase in the volume. The result is the map cracking, and disruption in concrete. Use of GGBFS in concrete reduces the risk of ASR, because of lowCa/Si ratio in the hydration products. Low Ca/Si ratio increases its alkalies binding capacity. Hence, increase in GGBFS content reduces the availability of alkalies for ASR in the concrete (Duchensne and Berude 1994) as shown in fig 9.

Aggregate-silica Reaction

Level of GGBFS required to mitigate damaging ASR is influences by the type of reactive aggregate and the quantity of alkalies available in the mix. Expansion due to ASR is decreases with increase in GGBFS content in concrete, as shown in fig. 10 (Swamy, 1986). The 50% replacement of OPC by GGBFS is found normally satisfactory for preventing ASR with any type of aggregates and cement with less than 0.6% alkalies. As per ACI 233 report a minimum level of 40% GGBFS cement replacement is needed to mitigation of ASR.

Carbonation

In the carbonation of concrete the Carbon Dioxide (CO2) of environment reacts with hydrated compounds of cement paste and forms Calcium Carbonate. Carbonation of concrete lowers the alkalinity of pore solution and destroys the protective passive layer of reinforcement, hence increases the chances of corrosion of steel reinforcement.

Carbonation resistance of GGBFS blended concrete is comparable to OPC concrete. It depends on the strength, GGBFS content and curing conditions of concrete with the environmental condition in which the concrete were situated. Concrete with 50% GGBFS achieved similar resistance to carbonation to that of OPC concrete mixture proportion, in moist environment. But higher GGBFS content (70-80%) concrete has lower resistance, especially if associated wit h a sheltered or drying climate. In dry environment concrete loss its moisture rapidly and hydration is ceases, particularly in cover concrete portion. Under these dry conditions the GGBFS blending concrete reaches lower strength and carbonated faster in earlier ages. Results of a typical field study under industrial area on the carbonation depth is shown in fig 11, suggest a marginal increase in depth of carbonation for GGBFS blended concrete.

Frost Resistance

Freezing and thawing of moisture inside the body of concrete is defined as frost action. Water gains volume (By approximately 9%) when it freezes and looses when it melts. This volume changes induce internal stresses, which do cracking, spalling, and scalling in concrete.

As in OPC concrete the air content has the greatest influence on the frost resistance of GGBFS blended concrete. Frost resistance of both types concrete of similar strength and air content is essentially the same. GGBFS blended concrete without Air entraining admixture (AEA) shows significantly lower air content than in a typical OPC concrete. Dose of AEA required to entrain a given volume of air is more for GGBFS blended concrete, this may be due to the increased workability exhibited by the GGBFS blending and therefore easing the expulsion of entrapped air (Sanjayan and Sioulas, 2000). Gifford and Gillot (1996) found that air voids in GGBFS concrete is more finely divided and closely spaced than in OPC concrete with same air content. They reported that freeze thaw durability of GGBFS blended concrete is at least as good as OPC concrete given adequate air voids parameter. Due to the slower strength development at early ages in GGBFS blended concrete, its frost resistance in low temperature environment can be increase by the use of chemical activators. Deja (2003) reported that the air entraining of the concrete mix up to the level of 5-6% gives good resistance to de-icing salt in frost condition.

Conclusion

Based on the discussions presented in this paper the following conclusion have been found out:
  • GGBFS can be used as a high volume mineral admixture in concrete due to technological, economical, and environmental benefits, without any compromise with its performance.
  • GGBFS has the cementing property, but it required activation by any medium. Hydration product of GGBFS is the same C-S-H gel with more dense and low Ca/Si ratio.
  • Reactivity of GGBFS with OPC depends on the chemical and mineralogical composition of both materials. Higher fineness and glass content increases the reactivity of GGBFS. Slag Activity Index is the best way to predict the reactivity of GGBFS with OPC.
  • GGBFS blending in concrete is increases the workability, consistency and reduces the water demand, bleeding & segregation chances. GGBFS blending in concrete reduces its hydration temperature and also prolonged the time for peak temperature of concrete.
  • Due to the prolonged hydration of GGBFS the later age strength of GGBFS blended concrete is higher than OPC concrete under adequate reactive material and moist curing conditions.
  • Chloride resistance of GGBFS blended concrete is higher than the OPC concrete due to its lower permeability and different chemistry of pore solution. Chloride resistance increases with increase in level of GGBFS in concrete.
  • Sulphate resistance of concrete is increases with blending of GGBFS. Lower concentration of C3A in OPC and Al2O3 in GGBFS are favorable for better Sulphate resistance.
  • Carbonation of GGBFS blended concrete is comparable to normal OPC concrete. Blended concrete exposed in long moist condition shows very little carbonation.
  • GGBFS is very effective in reducing the expansion due to Aggregate-silica reaction in concrete due to higher alkalies binding capacity of hydration products of GGBFS.
  • Frost resistance of GGBFS blended concrete is as good as OPC concrete, subjected to the availability of air voids. Dose of AEA required to produce same air content is higher in blended concrete.
  • To get complete advantages of GGBFS blending in concrete minimum 7 day moist curing is must. Replacement level of 40 to 50% is the optimum from strength and durability aspects.

References

  1. ACI 233R-95 Committee Report, (1997), GGBFS as a Cementitious Constituents in Concrete, ACI Manual of Concrete Practice, Part I
  2. Al-Amoudi, O. S. B., Rasheeduzzafar, Maslehuddin, M., and Al-Mana, A. I., (1993), Prediction of Long-term Corrosion Resistance of Plain and Blended Cement Concrete, ACI Materials Journal, Vol. 90, No. 6, pp. 564-570
  3. Babu, K. G., and Kumar, V. S. R., (2000), Efficiency of GGBFS in Concrete, Cement and Concrete Research, Vol. 30, pp. 1031- 1036
  4. Bijen, J., (1996), Benefits of Slag and Fly Ash, Construction and Building Materials, Vol. 10, No. 5, pp. 309-314
  5. Brooks, J.J., and Al-Kaisi, A. F., (1990), Early Strength Development of Portland and Slag Cement Concrete Cured at Elevated Temperatures, ACI Materials Journal, Vol. 87, No. 5, pp. 503-507
  6. Deja, J., (2003), Freezing and De-Icing Salt Resistance of Blast Furnace Slag Concretes, Cement and Concrete Composites, Vol. 25, pp. 357-361
  7. Detwiler, R. J., Fapohunda, C. A., and Natale, J., (1994), Use of Supplementary Cementing Materials to increase the Resistance to Chloride ion Penetration of Concrete Cured at Elevated temperatures, ACI Materials Journal, Vol. 91, No. 1, pp. 63-66
  8. Duchesne, J., and Berube, M. A., (1994), Available Alkalies from Supplementary Cementing Materials, ACI Materials Journal, Vol. 91, No. 3, pp. 289-299
  9. Garcia, J. I. E., Fuentes, A. F., Gorokhovsky, A., Fraire-Luna, P. E., and Mendoza-Suarez, G., (2003), Hydration Products and Reactivity of Blast-Furnace Slag Activated by various Alkalis, Journal of American Ceramic Society, Vol. 86, No. 12, pp. 2148-2153
  10. Geiseler, J., Kollo, H., and Lang, E., (1995), Influence of blast furnace cements on durability of concrete structures, ACI Materials Journal, Vol. 92, No. 3, pp. 252- 257
  11. Gifford, P. M., and Gillott, J.E., (1996), Freeze-Thaw Durability of Activated Blast Furnace Slag Cement Concrete, ACI Materials Journal, Vol. 93, No. 3, pp. 242- 245
  12. Gu, P., Beaudoin, J. J., Zhang, M. H., and Malhotra, V. M., (2000), Performance of Reinforcing Steel in Concrete Containing Silica Fume and Blast- Furnace Slag Ponded with Sodium Chloride Solution, ACI Materials Journal, Vol. 97, No. 3, pp. 254-262
  13. Hooton, R. D., and Emery, J. J., (1990), Sulphate Resistance of a Canadian Slag Cement, ACI Materials Journal, Vol. 87, No. 6, pp. 547-555
  14. Hope, B. B., and Lp, A. K. C., (1987), Corrosion of Steel in Concrete made with Slag Cement, ACI Materials Journal, Nov.- Dec., 525-531
  15. Jain, M. K., and Pal, S. C., (1998), Utilisation of industrial slag in making high performance concrete composites, Indian Concrete Journal, June, pp. 307- 315
  16. Jimenez, A. F., and Puertas, F., (2003), Structure of Calcium Silicate Hydrates Formed in Alkaline-Activated Slag: Influence of the Type of Alkaline Activator, Journal of American Ceramic Society, Vol. 86, No. 8, pp. 1389-1394
  17. Kumar, S., Rao, B. K., and Mishra, S., (2002), Chloride Penetration Resistance of Concrete Containing Blast Furnace Slag, The Indian Concrete Journal, December, pp. 745-751
  18. Lim, S. N., and Wee, T. H., (2000), Autogeneous Shrinkage of Ground- Granulated Blast-Furnace Slag Concrete, ACI Materials Journal, Vol. 97, No. 5, pp. 587-593
  19. Mantel, D. G., (1994), Investigation into the Hydraulic Activity of Five Granulated Blast Furnace Slag with Eight Different Portland Cements, ACI Materials Journal, Vol. 91, No. 5, pp. 471-477
  20. Miura, T., and Iwaki, I., (2000), Strength Development of Concrete Incorporating High level of ground Granulated Blast- Furnace Slag at Low Temperatures, ACI Materials Journal, Vol. 97, No. 1, pp. 66- 70
  21. Olorunsogo, F. T., and Wainwright, P. J., (1998), Effect of GGBFS Particle-Size Distribution on Mortar Compressive Strength, A.S.C.E. Journal of Materials in Civil Engineering, August, pp. 180-187
  22. Osborne, G. J., (1999), Durability of Portland Blast-Furnace Slag Cement Concrete, Cement and Concrete Composites, Vol. 21, pp. 11-21
  23. Rasheeduzzafar, Al-Amoudi, O. S. B., Abduljauwad, S. N., and Maslehuddin, M., (1994), Magnesium-Sodium Sulphate attack in Plain and Blended Cements, A.S.C.E. Journal of Materials in Civil Engineering, Vol. 6, No. 2, pp. 201-221
  24. Rasheeduzzafar, Dakhil, F. H., Al-Gahtani, A. S., Al-Saadoun, S. S., and Bader, M. A., (1990), Influence of Cement Composition on the Corrosion of Reinforcement and Sulphate Resistance of Concrete, ACI Materials Journal, Vol. 87, No. 2, pp. 114- 122
  25. Roy, D. M., and Idorn, G. M., (1982), Hydration, Structure, and Properties of Blast Furnace Slag Cements, Mortar, and Concrete, ACI Materials Journal, Nov.– Dec., pp. 444-457
  26. Sanjayan, J. G., and Sioulas, B., (2000), Strength of Slag-Cement Concrete Cured in place and in other conditions, ACI Materials Journal, Vol. 97, No. 5, pp. 603- 611
  27. Sivasundaram, V., and Malhotra, V. M., (1992), Properties of Concrete Incorporating Low Quantity of Cement and High Volumes of Ground Granulated Slag, ACI Materials Journal, Vol. 89, No. 6, pp. 554-563
  28. Smith, K. M., Schokker, A. J., and Tikalsky, P. J., (2004), Performance of Supplementary Cementitious Materials in Concrete Resistivity and Corrosion Monitoring Evaluations, ACI Materials Journal, Vol. 101, No. 5, pp. 385-390
  29. Swamy, R. N., (1986), Cement replacement Materials, Surrey University Press, London
  30. Swamy, R. N., and Bouikini, A., (1990), Some Engineering Properties of Slag Concrete as Influenced by Mix Proportioning and Curing, ACI Materials Journal, Vol. 87, No. 3, pp. 210-220
  31. Wee, T. H., Suryavanshi, A. K., and Tin, S. S., (2000), Evaluation of Rapid Chloride Permeability Test, Results for Concrete Containing Mineral Admixtures, ACI Materials Journal, Vol. 97, No. 2, pp. 221- 23

NBMCW November 2007

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High Performance Concrete Using Fumed S...

High Performance Concrete

Dr. P. Jeyabalan, Assistant Professor of Civil Engineering, and B. N. Krishnaswami Faculty in Civil Engineering, National Institute of Technology, Tiruchirappalli, Tamil Nadu.

Concrete is the second largest material consumed by the human being in the world next to water. Environmental attack can severely reduce the strength and life of the concrete. In the present study a series of 5 batches of concrete were cast containing fumed silica and the material properties were determined. These properties include compressive strength and permeability. The test results are compared with the values for high performance concrete without fumed silica as a reference mix.

Introduction

Application of high performance concrete (HPC) has got momentum in various fields of construction globally in the near past. HPC is being practised in the fields like construction of nuclear reactors, runways at airport, railway sleepers, cooling towers, silos, chimneys and all kinds of bridges. Considerable amount of development has been made in the field of HPC using silica fumeas a mineral admixture which is produced from condensing the gases escaping from furnace of a ferro silicon (or) silicon metal manufacturing process.

Properties of Slica Fume

High fineness, uniformity, high pozzolanic activity and compatibility with other ingredients are of primary importance in selection of mineral admixture as per ACI 363R – 5. As per the Table-1 Silica fume has the maximum fineness of 28 m2 g-1 whereas the fumed Silica has the fineness of 190 m2 g-1 which is 6 to 7 times finer than Silica fume. Finer the particle of pozzolano, higher will be the modulus of elasticity, which enhances the durability characteristics of the HPC.

The high performance concrete is essentially a concrete having a low water/binder ratio (W/B). A value of about 0.4 is suggested 1 as the boundary between usual concretes and high performance concrete. Conventional concrete and high performance concrete have not only quite different compressive strength but also significantly different microstructures (quite different shrinkage behavior) and significantly different overall performances. Hence any grade of concrete with water/binder ratio less than 0.4 will be called as high performance concrete and with water/binder ratio greater than 0.4 will be called as an ordinary concrete.

Causes of deterioration of concrete

Durability of Concrete

Concrete should be capable of withstanding the conditions for which it has been designed throughout the life of a structure. Lack of durability can be caused by external agents such as environmental changes (or) due to internal constituents of concrete shown in Table-2. Causes can be categorized as physical, mechanical, and chemical. Physical cause arises from the action of frost. Mechanical cause arises due to abrasion. Chemical causes are due to attack by sulphates, chlorides, acids and seawater etc. These causes can be managed by reducing the permeability of concrete (or) by using impermeable concrete. From the durability point of view, it is important to achieve low permeability as quick as possible in the fresh concrete. Factors Controlling Permeability:

There are 3 major factors, which determine the permeability in concrete.
  1. Water/cement ratio
  2. Compaction
  3. Curing
Each factor is equally important. If one of these factors is not controlled, the result will be increased in permeability.

ACI 301-89 recommends being water tight structural concrete can have a maximum permeability of 1.5 x 10-11 m s-1.

Fumed Silica

It is a high reactive pozzolona made for some purpose. Fumed silica is produced by the vapour phase hydrolysis of silicon Tetra chloride in a hydrogen oxygen flame. The reactions are,

High Performance Concrete

The combustion process creates silicon dioxide molecules, which condense to form particles. Fumed silica is being used in various fields such as pharmaceuticals, paints, adhesives and sealants, plastics, rubber and ink except for concrete.

Properties of the fumed silica and silica fume have been shown in Table – 1. Since fumed silica is much finer than silica fume its influence in the paste aggregate interface was found to be the dominant factor in the development of increased strength.

Mechanism of HPC

Under compressive loads, failure in normal Concrete occurs either with in the hydrated cement paste or along the interface between the cement paste and aggregate particles. This interface called the "Transition zone" is a weak area in normal concrete.

Properties of Portland Cement
To improve the strength and other properties, it is necessary to strengthen the weak areas. Reducing the water-cement ratio and using supplementary cementitious materials like fumed silica tends to strengthen the transition zone. When Portland cement hydrates, a considerable quantity of calcium hydroxide is produced. Fumed silica which has a very high percentage about 99.97 percentage of amorphous silicon dioxide reacts with calcium hydroxide to form calcium silicates hydrates (C-S-H). This improves impermeability as well as strength. There is a distinct change in the refinement of the pore structure in a fumed silica concrete giving less of the capillary pores and more of the finer gel pores, thus by improving the impermeability and strength.

The water demand of concrete containing fumed silica increases dramatically, because of its finerparticle size, but this can be overcome by the use of super plasticizers. Bleeding of concrete mixtures containing fumed silica is almost non–existent. The super plasticizer deflocculates (5) the cement particles and thus fluidifies the mixture so that very low water content is sufficient for an adequate workability. Typically, 5 to 15 litres per of super plasticizer can effectively replace 45 to 70 litres per of water(2).

In the present study, a series of 5 batches of concrete were cast containing fumed silica and the material properties were determined. These properties included compressive strength; permeability. The test results are compared with the values of HPC without fumed silica as a base mix.

Experimental Details

Materials: The cement used was ordinary Portland cement. The Physical and chemical composition as provided by the manufacturer is given in Table 3. Naturally available fine and coarse aggregates were used and their properties were shown in Table 4. Portable drinking water was used. High range water reducing admixtures (HRWRA) were used which conforms to ASTM C494 Type A and F.

Properties of aggregate

Maximum size of coarse aggregate–12.5 mm

Minimum size of coarse aggregate–6.0 m

PH value of water - 6.5

Mix proportions were arrived based on the guidelines given by ACI 211.1 Slump which were maintained as 25 to 50 mm

Required Average Compressive Strength

According to ACI – 318/318 R – 46, Table 5.3.2.2 the required average Compressive strength (f'cr), when data are not available to establish a Standard deviation.

High Performance Concrete

The required average compressive strength f' cr = f' c + 9.6561 = 60 + 9.6561 f' cr = 70 Mpa

Design Mix

Target strength : 70MPa

Ratio of Cement, Fine & Coarse

aggregate : 1:1.35:1.68

W/(C+P) Ratio : 0.31

Slump : 25 – 50 mm

A basic trial mix was prepared with above proportioned materials and without fumed silica. Subsequently, four more companion mix was prepared by replacing 5%, 7.5%, 10%, and 15% of cement that required for basic mix with fumed silica, keeping the same slump and W/C+P ratio.

Compressive Strength

100 mm x 100 mm x 100 mm mould were used to prepare concrete cubes. There are about nine cubes were cast in each trial mix. At the age of 3rd day, 7th day, and 28th day the average compressive strength of all the trial mix are found out and are plotted in chart 1-4.

High Performance Concrete

Results and Discussion

The 3rd day average compressive strength of M60 grade basic mix proportion was only 24 Mpa without any admixture except HRWRA, which was used to maintain the required slump. The 1st, 2nd , 3rd and 4th companion trial mixes in which cement was replaced by fumed silica @ 5%, 7.5%, 10% and 15% respectively of the total cement quantity required for basic mix shows that there were considerable increase in its compressive strength on the same age. This result shows that the use of fumed silica as an admixture gives 'High Early Strength,' so as to attend the emergency repair works immediately.

Similarly, the 28th day average compressive strength of M60 grade basic mix proportion was only 63 Mpa without any admixture except HRWRA, which was used to maintain the required slump. The 1st, 2nd , 3rd and 4th companion trial mixes in which cement replaced by fumed silica with the addition of fumed silica @ 5%, 7.5%, 10% and 15% respectively of the total cement quantity required for basic mix shows that there were considerable increase in its compressive strength on the same age. This result shows that the use of fumed silica as a mineral admixture gives 'High average Compressive strength.'

Permeability Assessment

Five batches of 450 x 450 x 100 mm plain concrete blocks were made to study the permeability of each of trial mix; using Germann water permeability Instruments (GWT – 4000) and the results are shown in Table–5.

Properties of Fumed Slica Mixes

For a fully water saturated concrete without surface porosities and cracking a measure of the water permeability can be obtained without disturbance from capillary absorption. With the GWT – 4000 a sealed pressure chamber is attached to the concrete surface, boiled water is filled into the chamber and a required water pressure is applied to the surface.

High Performance Concrete

High Performance Concrete

High Performance Concrete
High Performance Concrete
Chart 1: Basic Mix Test Results with no Fumed SilicaChart 2: First Companion Mix Test Results using 5% Fumed Silica
High Performance Concrete
High Performance Concrete
Chart 3: Second Companion Mix Test Results using 7.5% Fumed SilicaChart 4: Third Companion Mix Test Results using 10% Fumed Silica
High Performance Concrete
Chart 5: Fourth Companion Mix Test Results using 15% Fumed Silica


High Performance Concrete

High Performance Concrete

High Performance Concrete

The pressure is kept constant using micrometer gauge with attached pin that substitutes the water leaving the chamber, to Chart 2: First Companion Mix Test Reults Using 5% Fumed Silica measure the amount of water penetrating the substrate. The difference in the gauge position over a given time (e.g., 5 minutes) is taken as a measure of the water penetrability for a given water pressure. The pressure applied here is 5 BAR.

Test has been carried out as shown in figures 1 and 2

High Performance Concrete
High Performance Concrete

A flux of water penetrating the surface for a given water pressure may be calculated as simply the difference in micrometer readings in mm for a given testing time (e.g. 5 minutes).

Alternatively, the flux q may be calculated for a given water pressure as

q = B (g1-g2) A-1 .t-1 = 78.6 (g1-g2) 3018-1 t-1= 0.026(g1-g2) t-1 (mm sec-1)

'B' is the area of the micrometer pin being pressed into the chamber, 78.6mm2 for the 10mm pin diameter.

g1 and g2 the micro–meter gauge readings in mm at the start of the test and after the test has been finished.

'A' is the water pressure surface area 3018 mm2 (Gasket inner diameter 62mm), 't' the time the test is performed over in seconds.

Conclusion

Mineral admixture such as fumed silica is an ideal constituent for high performance concrete as it has the inherent ability to contribute to continued strength development through their pozzolanic/ cementations reactivity and to enhance durability and chemical resistance through their pore refinement and reduced sorptivity characteristics.

References

  • P. C. AITCIN "High performance concrete" E&FN Spon –1998 Page- 5.
  • Pierre – Clande Aitcin and Adam Neville – High Performance concrete Demystified " Concrete Int ernational" features January 1993, Volume 15, number I, Page No: 21-26.
  • R.Shridhar and Y. P. Kapoor – "Roll of construction chemicals in ensuring durability of concrete structure " compilation from Indian concrete Journal – Repair and Rehabilitation First edition 2001, Page No: 126 – 130.
  • A.M.NEVILLE and J.J.Brooks – Concrete Technology First ISE serpent, 1999, Publishers M/s. Addison Wesler Longman, Inc. 1999 PP – 262 – 266.
  • V.M. Malhotra–FlyAsh, Silica Fume, and Rice–Husk ash in concrete A Review concrete International features April 1993,Volume 15, number 4,PP 23-28.
  • Aitcin. P.C. and Adam Neville 'High Performance Concrete demnystified,' Concrete International pp 21-26 V-15 (January 1993).
  • S. Rajeev and J.Rajesh – An expert system for diagnoising causes and repairs of defects in R.C. structures. Repairs and Rehabilitation compilation from "The Indian Concrete Journal."
  • Alexander M. Vaysburd, Peter H. Emmons and Gajanan M.Sabnies Repair & Rehabilitation compilation from "The Indian Concrete Journal."
  • J.K. Patel, N.B.Desai and J.C.Rana, Properties and application of steel fibre reinforced concrete.
  • Ambuja Technical Literature on Fibre reinforced concrete.

American Concrete Institute Code

  1. Cement Concrete Terminology–ACI 116R-00.
  2. Standard Practice for selecting proportions for normal, heavy weight and mask concrete–ACI 211.1- 91 (Reapproved 1997).
  3. Guide for selecting proportions for High Strength concrete with portland Cement and Flyash–ACI 211-4R-93 (Reapproved 1998).
  4. Chemical admixtures for concrete–ACI 212 3R-91 (Re,approved 1999)
  5. Guide for the use of High– Range water reducing admixtures ( S u p e r plasticizers) in concrete ACI 212.4.4R-93 (Reapproved 1998).
  6. Guide for silica Fume in concrete ACI 234R-96 (Reapproved 2000).
  7. State-of-the-Art Report on High–strength concrete ACI 363R–92 (Reapproved 1997).
  8. Guide to Quality control and Testing of High strength concrete ACI 363.2R-98.

Bureau of Indian Standard

  • IS: 456-2000 Plain and reinforced concrete–code of practice (Fourth Revision)
  • IS: 12269-1987 Specification for 53 grade Ordinary Portland Cement
  • IS: 383-1970 Specification for coarse and fine aggregates from natural sources for concrete (Second Revision)
  • IS: 3025-1964 Methods of sampling and Test (Physical and Chemical) for water used in Industry IS: 9103-1979 Specification for admixture for concrete.

NBMCW May 2008

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Effect of Fineness of Sand on the Cost ...

Effect of Fineness of Sand on the Cost and Properties of Concrete

Prashant Agrawal, QC Manager, HCC Ltd. Dr. Y.P. Gupta, Materials Consultant, BCEOM-LASA JV, Suryakanta Bal, QC Engineer, HCC Ltd. Allahabad Bypass Project, Allahabad, UP.

The grading and maximum size of aggregates is important parameters in any concrete mix. They affect relative proportions in mix, workability, economy, porosity and shrinkage of concrete etc. Experience has shown that very fine sands or very coarse sands are objectionable – the former is uneconomical, the latter gives harsh unworkable mixes. Thus the object in this paper is to find the best fineness modulus of sand to get the optimum grading of combined aggregate (all-in-aggregate), which is most suitable, and for economy. In general, the grading of aggregates, which do not have a deficiency or excess of any size of aggregate and give a smooth grading curve, produce the most suitable concrete mix. Further a cohesive mix is also desired for the pumped concrete produced by RMC Plant. In the present investigations, effect of the grading of river sand particles has been investigated for a good Concrete mix. Sand has been sorted in three categories i.e. Fine, Medium, and Coarse. These were mixed with coarse aggregate in different proportions so as to keep the combined Fineness Modulus (all-inaggregate) more or less the same. Various proportions of such aggregate are mixed in preparing M 30 grade of Concrete mix. Effect is studied on concrete workability, cube strength, flexural strength and permeability. The results indicate that with the change in fineness of sand, workability gets affected. The details of findings and its effect on compressive and flexural strength and permeability, influencing durability are reported in this paper.

Introduction

Fineness Modulus is a term used as an index to the fineness or coarseness of aggregate. This is the summation of cumulative percentage of materials retained on the standard sieves divided by 100. It is well–known that aggregate plays an important role in achieving the desired properties of concrete. Though, aggregate constitute 80 to 90% of the total volume of concrete, yet very littleattention is given in controlling the grading and surface texture of aggregate to optimize the properties of concrete. Improper blend of aggregate influences the cement and water demand for a given concrete mix and affects workability, compactibility, and cohesion characteristics of pumpable concrete mix. It also influences the compressive strength, flexural strength and other properties like permeability & durability of concrete.

Review of Provisions in Different Specifications

IS 383: "Specifications for Coarse and Fine Aggregates from Natural Sources for Concrete." This publication deals with specifications for Coarse and Fine aggregates from natural sources for Concrete. These specifications do not specify any limit for fineness modulus to be used in concrete. It divides the sand in four zones i.e. from Zone I to Zone IV. Zone I–Sand being very coarse and Zone 4 sand is very fine. It is generally recommended by code to use sands of zones I to Zone III for Structural concrete works.

AASTHO Designation: M6-93- "Standard Specification for Fine Aggregate For Portland Cement Concrete"- It indicates that the fineness modulus of sand will not be less than 2.3 and nor more than 3.1. Further, fine aggregate failing to meet the fineness modulus requirement as above may be accepted, provided concrete made with similar fine aggregate from the same source has an acceptable performance record in similar concrete construction; or in absence of a demonstrable service record, provided, it is demonstrated that concrete of the class specified, made with the fine aggregate under consideration, will have relevant properties at least equal to those of concrete made with the same ingredients, with the exception that a reference fine aggregate be used which is selected from a source having an acceptable performance record in similar concrete construction.

ASTM Designation: C33-93- standard specification for concrete aggregates"–The fine aggregate shall have not more than 45% passing any sieve and retained on the next consecutive sieve and its fineness modulus will not be less than 2.3 and not more than 3.1. Rest is the same as for AASTHO M6-93.

U.S.B.R: The code has specified that the fineness modulus of sand shall not be less than 2.50 and not more 3.0.

Experimental Investigation

Effect of Fineness of Sand on the Cost and Properties of Concrete
In the present investigations, the effect of Fineness Modulus of sand has been investigated. The Fine aggregate (Sand) taken is Yamuna river sand and coarse aggregate taken is Dolomite limestone in crushed form. It has been sorted in several categories starting the Fineness Modulus (FM) of sand from 2.0 to 3.0. These were mixed in different proportions to get a consistent combined FM. The combined FM is determined like All-in-aggregate FM. In the present study we have selected M30 Grade of concrete mix. To find out the effect of fineness modulus (FM) of sand on concrete, sand of different FM from 2.0 to 3.0 is chosen. Two sizes of coarse aggregate particles: i.e. 20 & 10 mm, which are generally used in standard concrete mix, were chosen for the investigations.

Concrete Mix Selected:

Concrete Grade : M30

Water-Cement Ratio : 0.45

Cement: OPC 53 grade (350 Kg)

Aggregate to Cement Ratio : 5.52

Admixture: Super plasticizer (as required)

Fine Aggregate : Yamuna River Sand (Average 777Kg)

Coarse Aggregate: Dolomite;

Crushed Stone (Average 1155Kg)

The material properties are given in Table 1.

When we choose very fine sand (i.e. FM 2.0), and very coarse sand (i.e. of FM 3.0), and if the proportion of sand is fixed in the mix then due to poor all-inaggregate grading, the mix may become very harsh or not give correct results. So in present study proportions of coarse aggregate and fine aggregate, are slightly adjusted in the mixes to keep allin- aggregate grading within envelope of desired all-inaggregate grading given in IS: 383. The FM of combined mix is kept in range of 4.94 to 4.97 as seen from Table 2.

In this study water-cement ratio (W/C) of mix is kept constant for all the trial mixes with sand of different fineness modulus. Workability of mix is also fixed in range of 45 to 55 mm slump. Since mix with so different fineness modulus of sand, will result in different water demands, so watercement ratio is kept constant and to adjust workability slight adjustments in admixture dosages has been made. Various proportions of such ingredients are mixed in laboratory mixer of 0.1 capacity for preparing M30 grade of Concrete mix. Cubes (150 x 150 x 150 mm size), cylinders (150 Ö x 150 mm height) and beams (150 x 150 x 700 mm length) are cast. Effect of varying FM of sand is studied on concrete density, workability, compressive strength, flexural strength and permeability.

Observations & Discussion of Results

Table 3 gives the total observations recorded during the experimental investigations. Effect on Workability, Density, Strength, and Permeability due to variations in FM of sand is discussed here.

A. Workability of Concrete Mix: The workability of concrete mix was measured with the help of 300 mm standard size slump cone. A little amount of admixture dose was added to concrete mix. Each time concrete mix was examined for the behavior in slump, segregation and Bleeding etc. The slump observed was about 50 mm in all cases. No segregation or bleeding was observed in the mix.

Figure 1, shows the type of slump observed. The results indicate that with the increase in fineness modulus of sand, water demand in the mix got affected consequently workability gets affected. Since water-cement ratio is kept constant, so to keep workability in the same range of 50 mm, admixture dosages were varied. The admixture dosages reduced considerably as fineness of sand increases as shown in Figure 2. The Figure 2 shows that:
  • Admixture Dosage reduced from 1.0 percent to 0.2 percent as sand fineness modulus increases from 2.0 to 3.0.
  • For Every 5% increase in FM of sand, admixture dosage reduced by 0.1%.

Effect of Fineness of Sand on Density of Concrete

After measuring the slump, several 150 mm cubes were filled. These were cured in water tank for 28 days. After curing, each cube was weighed using electronic balance and density of concrete was calculated. The variation of density with FM of sand is shown in Figure 3 for different cases. From this figure, it is evident that there is slight increase in density i.e. 0.80 to 1.20 percent, when fineness modulus increases from 2.0 to 3.0.

Effect of Fineness of Sand on Compressive Strength of Concrete

Effect of Fineness of Sand on the Cost and Properties of Concrete
Cubes of 150 mm were tested for compressive strength at 7 and 28 days. This compressive strength is given in table 3 for varying FM of sand. The variation is shown in Figure 4. The figure indicates that:
  • As fineness modulus of sand changes from 2.0 to 2.5 there is an increase in compressive strength from 43.07 to 49.00 MPa. i.e. strength increases by 14%. On the other hand by increasing Fineness Modulus from 2.5 to 3, Compressive strength increases from 49.00 to 56.83 MPa resulting in 16% increase in strength.
  • For Every 0.1 increase in FM of sand from 2.0 to 3.0, 28 days Compressive Strength increases by 2.5 to 3.0%.
  • 7 days compressive strength also increases in the similar proportion.
  • There is faster increase in strength towards coarser side of sand.

Effect of Fineness of Sand on Flexural Strength of Concrete

Flexural strength is calculated from 28 days testing of beam of size 150x150x700 mm by using following formula.

Flex. Strength = P x 1000 x L / [b x d x d], for a > 200 mm but less than 200 mm

= P x (3000 x a) / (b x d x d), for a > 170 mm but less than 200 mm

= Result is discarded when a > 170 mm

Where,

b = width of sample beam (150 mm).

d = depth of sample at the point of failure (1500 mm).

a = distance between the line of fracture and the nearest support (recorded for each sample after test).

P = failure Load.

L = total support length of specimen (600 mm).

The variation of flexural strength with respect to different parameters is also given in Figure 4. This figure indicates the following:
  • As fineness modulusincreases from 2.0 to 2.5 there is an increase in 28 days flexural strength from 3.82 to 4.25 MPa i.e. strength increases by 11.25%. On the other hand by increasing Fineness Modulus from 2.5 to 3, the strength increases from 4.25 to 4.81 MPa resulting in 13.1% increase in strength.
  • For Every 0.1 increase in FM of sand from 2.0 to 3.0, Flexural Strength increases by 2.1 to .5%
  • The increase in strength is more towards coarser side of sand.

Effect of Fineness of Sand on Permeability of Concrete

Effect of Fineness of Sand on the Cost and Properties of Concrete
Permeability of concrete is determined by using cylinder specimen having 150 mm diameter and 160 mm height. They were applied water pressure of 7 Kg/ cm2 for 96 hours in the Permeability Apparatus shown in Figure 5.

Immediately after 96 hours cylinders were split under line load test. The depth of penetration of water in cylinder was measured as well as volume of water lost is recorded.

The results are interpreted as:
  1. Average depth of water penetration in cylinder is measured
  2. Coefficient of permeability is calculated as volume of water lost divided by volume of concrete penetrated with water i.e.
Permeability coefficient = vol. of water lost / (Area of cylinder x Average depth of concrete having effect of penetration of water).

The Permeability Coefficient of concrete Vs FM of sand is plotted in figures 6. It is seen from figure that permeability coefficient is more or less constant with respect to fineness of sand. Thus FM of sand has very little impact on Permeability Coefficient of Concrete and the value remains more or less constant.

Failure Pattern of Beams & Cubes

  1. It is generally seen that the failure occurs at the interface of aggregate and mortar.
  2. In flaky aggregate, some voids are observed at the interface of concrete and mortar. Elongated aggregate pieces are broken.
  3. Mortar matrix isgenerally crushed.

Cost Benefit Ratio

Effect of Fineness of Sand on the Cost and Properties of Concrete
Cost of Concrete mix per is calculated on the basis of unit cost of each ingredient material in the mix. The following market rates have been taken for Cement, Sand, Coarse aggregate, Admixture and a nominal cost for water. No labor cost has been added in the calculation as it will remain constant.

Cement: Rs. 4.25 per Kg

Sand* : Rs. 0.30 to Rs. 0.32 per Kg (depending upon Fineness of Sand)

Coarse Aggregate: Rs. 0.75 per Kg

Admixture: Rs. 40.00 per Kg

Water: Rs. 0.10 per Kg.

* Rate of Sand for FM 2.0 to 2.3 is Rs. 0.30 per Kg, for FM 2.4 to 2.7 is Rs. 0.31 per Kg, and for F. M. 2.8 to 3.0 Rs. 0.32 per Kg. The variation of rate of sand depends on market which can have much more difference.

The quantities of ingredients for one of concrete are given in table 4 (a). Cost of concrete is calculated by taking above rates and quantities given in table 4 (a). On the basis of cost calculated for concrete and the corresponding 28 day compressive strength, the cost benefit ratio is calculated as follows. This is given in Table 4 (b)
  1. Cost of concrete is calculated in terms of quantity of material used & market rates as given above.
  2. Cost Benefit Ratio is calculated as:
C/B ratio = Total cost of Concrete/28 days Compressive Strength

A curve has been plotted between FM of sand Vs C/B Ratio as shown in Figure 6. From this graph, it is seen that C/B ratio reduces considerably as the FM of sand increases. From FM varying from 2.0 to 3.0 the C/B ratio reduces by 71%. Thus, it is advisable to use coarser sand in Concrete.

Conclusion

Fineness Modulus of Sand affects Compressive and flexural strength of Concrete. Sand, with higher FM, results in higher strength of concrete. It is evident by cost benefit ratio that overall concrete mix is becoming economical if we use sand with higher FM. The results indicate that with the increase in FM, workability gets affected considerably. The cement demand also gets modified. Some of the observations are given below:
  • Fineness Modulus has larger impact on 28 days Compressive & Flexural Strength.
  • Fineness Modulus has very little impact on Permeability of Concrete. Permeability coefficientis changed by about 2% for FM from 2.0 to 3.0.
  • Fineness Modulus also affects the density of concrete. It increases by about 2.3% as the FM increases from 2.0 to 3.0. Optimum value of density and other parameters are obtained when FM is 2.8.
  • The optimum value of strength can be taken when workability of concrete is also good. It is obtained when Fineness Modulus is about 2.7.
  • The net cost of Concrete reduces when FM of sand increases. It reduces by about 6.5% for an increase of FM from 2.0 to 3.0.
  • As the fineness modulus of Sand increases, the Cost/Benefit Ratio reduce by a very large factor. This is 29% when FM changes from 2.0 to 3.0. That means we can get large advantage by using concrete having Coarse Sand.
  • A well adjusted grading (all-inaggregate) of concrete mix is also suitable for pumped concrete produced through RMC Plant. This is achieved by using a sand having FM of around 2.5.

References

  • NEVILLE, A.M, 'Properties of concrete', IV edition, Pearson Education Pvt. Ltd. 2005.
  • MEHTA, P.K, PAULO J.M. MONTEIVO, ‘Concrete microstructure, properties and materials’, ICI, 1999.
  • IS: 383-1970; 'Specifications for coarse and fine aggregate from natural sources for concrete', BIS, New Delhi.
  • SP: 23-2001; 'Handbook on concrete mixes based on Indian standards,' BIS, New Delhi.
  • IRC 2001: 'Specifications for Road and Bridge Works,' Indian Road Congress, New Delhi.
  • AASHTO Designation: M6 – 93, 'Standard Specification for Fine Aggregate for Portland Cement Concrete.
  • ASTM Designation: C33 – 93, Standard Specification for Concrete Aggregates.
  • BS: 812, 'British Standard for size and shape of aggregates'

Acknowledgment

The work has been carried out in M/ S HCC Ltd. Site Laboratory at Allahabad. The authors are thankful to them and QC staff of M/S BCEOM and HCC for their help.

NBMCW October 2007

.....

Cementitious Composites with Steel Rein...

Response of Engineered Cementitious Composites with Steel Reinforcement and Concrete in Moment Resisting Frames

Dr. S. C. Patodi, Professor, J. D. Rathod, Applied Mechanics Department, Faculty of Technology & Engineering, M.S.University Baroda.

With the advent of new materials, there is a constant need for designers to find innovative ways to incorporate these materials into new applications. The field of civil engineering is currently at a cross roads of equal significance with development of new materials termed as High Performance Fiber Reinforced Cementitious Composites (HPFRCC). These materials with tensile performance magnitudes higher than Reinforced Concrete (R/ C), allow designers to create structures previously impossible due to limitations of minimum reinforcement, minimum clear cover or excessive cracking in R/C. The replacement of brittle concrete with an Engineered Cementitious Composite (ECC), which represents a class of HPFRCC, micro structurally tailored with strain hardening and multiple cracking properties, has shown to provide improved load-deformation characteristics in terms of reinforced composite tensile strength, deformation mode and energy absorption. This paper reports, investigation of response mechanism of composite moment resisting frame system with large energy dissipation capabilities. Plain cementitious matrix is used in frame specimens to estimate deformation behavior and formation of plastic hinges. Expected plastic hinge regions are properly detailed by steel reinforcement. Deformation mechanism of plain cementitious matrix suggested economic use of ECC by replacement with concrete in some areas. Load-displacement curves are plotted and compared for damage tolerance evaluation. Crack width is measured as a function of load for damage reduction evaluation and toughness index is found out for post peak performance evaluation. Compatibility of ECC with reinforcement and concrete in terms of deformation and strength is discussed.

Introduction

In earthquake resistant design, the structural system performance requirements can be specified in terms of minimum ductility ratio, number of load cycles, sequence of application of load cycles and permissible reduction in strength at the end of loading. At the beam column connection level, the following performances are desirable:
  1. Ductile plastic hinge behavior under high shear stress,
  2. No congestion of transverse reinforcement for confinement and for shear,
  3. Concrete integrity under load reversals and
  4. Concrete damage contained within a relatively short hinging zone. These performances are difficult to achieve with ordinary concrete, although some encouraging results have been obtained with Fiber Reinforced Concrete (FRC)[1].
Desirable performance of the plastic hinge is not easy to translate directly into numerical quantities of materials property requirement. In general, however, it may be expected that the following properties of the concrete material in the plastic hinge should be advantageous:
  1. High compression strain capacity to avoid loss of integrity by crushing,
  2. Low tensile first cracking strength to initiate damage within the plastic hinge,
  3. High shear and spall resistance to avoid integrity loss by diagonal fractures and
  4. Enhanced mechanism that increases inelastic energy dissipation. ECC is a class of ultra ductile fibre reinforced cementitious composite used to achieve above objectives without introducing ductile detailing in a structure. ECC can undergo upto 5% strain in tension, yet at the lower fibre volume of 2% with flexible processing. ECC can be used in some fused zones so that with the above performance, overall performance of the structure can be enhanced[2].
ECC when used with ordinary reinforcement detailing replacing the concrete at some key places, interact with reinforcement and concrete. Both, reinforcing steel and ECC can be considered as elastic-plastic material capable of sustaining deformation up to several percent strains. As a result, the two materials remain compatible in deformation even as steel yields. Compatible deformation implies that there is no shear lag between the steel and the ECC, resulting in a very low level of shear stress at their interface. As a result of low interfacial stress between steel and the ECC, the bond between ECC and reinforcement is not as critical as in normal R/C, since stress can be transmitted directly through the ECC via bridging fibers even after microcraking. In contrast, in R/C members the stress must be transferred via interface to the concrete away from the crack site. After concrete cracks in an R/C element, the concrete unloads elastically near the crack site, while the steel takes over the additional load shed by the concrete. This leads to incompatible deformation and high interface shear stress responsible for the commonly observed failure modes such as bond splitting and/or spalling of the concrete cover. ECC has excellent shear capacity. Under shear ECC develops multiple cracking with cracks aligned normal to the principal tensile direction. Because the tensile behavior of ECC is ductile, the shear response is correspondingly ductile. As a result, R/ ECC elements may need less or no conventional steel shear reinforcement. With tensile strain hardening and ultra high tensile strain capacity, ECC can sustain very large deformation without damage localization. When ECC structural element is loaded in flexure or shear beyond the elastic range, the inelastic deformation is associated with micro cracking with continued load carrying capacity across these cracks[3]. The tight crack width in ECC has advantageous implications on structural durability and on the minimization of repair needs subsequent to severe loading of an ECC member. ECC can eliminate premature delamination or surface spalling in an ECC/concrete combination.

In the present work, the effects of cementitious composite ductility on the steel reinforced behavior are experimentally investigated and contrasted to the unreinforced composite. Interaction between ECC and concrete is observed and possibility of replacing ECC with concrete is explored. Tight crack width control in ECC is examined. L-type plane frame and portal frame specimens are used for the experimental investigation.

Material Composition

Recron 3S brand synthetic fibers of triangular cross section produced by Reliance Industries were used with cementitious matrix. Fiber volume fraction of 4% was used which was found as optimum fiber volume fraction by pilot tests. Kamal brand 53 grade OPC, 300 m passing silica sand, 2% dose of concrete super– plasticizer of conplast SP430 brand with w/c ratio as 0.35 and sand/cement ratio as 0.5 were used for the preparation of samples of ECC in the present experimental investigation. In addition, Kamal brand 53 grade OPC, silica sand confirming to zone III, 12.5 mm size coarse aggregates with w/c ratio of 0.35 and 0.5% dose of superplasticizer were used to produce concrete for use in combination with ECC in C-ECC specimens. Mix proportion for concrete used was 1:1.295:2.407. Mild steel reinforcement having yield strength of 250 N/mm2 was used in R-ECC specimen.

Specimen Configuration

Cementitious Composites with Steel Reinforcement and Concrete
L-type plane frame specimens, 3 specimens each, were cast with plain cementitious matrix and ECC with 4% fiber. Portal frame specimens, 3 specimens each, were cast with plain cementitious matrix (PCC), ECC with 4% fiber, steel reinforced ECC (R-ECC), and combination of ECC and concrete (C-ECC). Specimen configuration of LFigure type frame and portal frame specimens is shown in Figure 1.

Experimental Programme

For the preparation of specimens, the ingredients in required proportion were mixed in Hobart type mixer machine. Flow table test was performed to satisfy workability criteria in fresh state. After filling the mould with the matrix, it was compacted and demoulded after 24 hours. All the specimens were kept in curing tank for 28 days at room temperature. After putting proper identification mark, specimens were fixed into prefabricated experimental set up on MTS machine. Basic Testware available on computer supervised controller was used to conduct the test. All the specimens were tested in flexure at a displacement control rate of 0.005 mm/sec. Load and displacement at the first crack and at ultimate load were recorded during the test. Loaddisplacement curves were plotted and data were automatically recorded using basic Testware data acquisition facility. Crack width was measured for the first initiated crack during the test with the help of travelling microscope having least count of 0.01 mm. Test setups for plane frame and portal frame specimens are shown in Figure 2.

Discussion of Test Results

Cementitious Composites with Steel Reinforcement and Concrete
  • L-type frame specimens were tested under flexure. First crack load and ultimate load results are reported in Table 1 for plain matrix (L-0%) and ECC with 4% (L-4%) fiber matrix. Reserved strength refers to increase in strength of the member upto ultimate load over the first crack strength. This criterion is used to represent residue strength of the material. Deflection hardening refers to increase in deflection of the member upto ultimate load over the first crack deflection. This value shows inelastic deformation capability of the member which represents ductility of the material. Plain matrix failed suddenly with no reserved strength and deflection hardening. ECC-4% matrix has load and displacement values higher than plain matrix. First crack displacement is almost 2 times and ultimate displacement is almost 3.5 times than plain cementitious matrix. There is marginal increase in reserved strength but considerable improved performance in deflection hardening over plain cementitious matrix is observed. Maximum deflection hardening achieved is more than 100%. Ultimate flexural strength of ECC-4% is found as 3.63 N/mm2 against 2.69 N/mm2 that of plain matrix.
  • Load-displacement curves are plotted for all the three specimens of ECC-4% as shown in Figure 3. Strain hardening is observed in all the specimens with very little linear portion in the beginning.Toughness index I5 is calculated as area under the load displacement curve for 3 times first crack displacement divided by the area under load displacement curve for first crack displacement. Post peak performance of the material can be represented by this value, which is also indicative of energy absorption capacity of the material. Toughness index I5 for ECC- 4% specimen is found as 4.21 which for a plain matrix could not be represented as it failed suddenly after the formation of first crack.
  • One can utilize design strength up to ultimate strength of ECC matrix in strain hardening zone. Development of cracks and crack width are therefore important in strain hardening zone. ECC matrix is well known for its tight crack width control which is utmost important for the durability of a member. One should make sure that migration of aggressive substances into matrix should be eliminated so that corrosion of reinforcement and subsequently spalling of matrix and delamination can be prevented. According to ACI committee 224, ultimate crack width should be limited to 150 μm when member is exposed to an environment of seawater and seawater spray in wetting and drying [4]. Rate of increase of crack width as a function of load gives information about consideration of design load for particular crack width criteria. Crack width was measured of the first visual crack and then crack width development with increase in load in the column of L- frame was recorded and was found within 150 μm at ultimate load.
  • Crack generation history in the column of L frame is tabulated in Table 2 in which crack number along with its location from bottom of the beam is presented. Failure of L-type specimens took place due to rotation of column in the middle at crack number 4. First crack generated right below the bottom of the beam and subsequent cracks appeared below first crack with spacing of about 2 cm up to middle of the column as the load increased. Spacing of the cracks was more below the crack number 4. Number of crack formation with increase in load in the column of L–type specimen is shown in Figure 4.
  • Portal frame specimens were tested under flexure to evaluate ECC performance along with combination of reinforcement and concrete. Strain hardening was not observed in plain cementitious matrix. First crack load and ultimate load of PCC, ECC, RECC and C-ECC frames are given in Table 3.
    Cementitious Composites with Steel Reinforcement and Concrete
  • Lower first crack strength and then large amount of plastic hinge formation is desirable for seismic response so as to have large energy dissipation. This behavior is reflected in ECC sample number 1 and 3. Sample number 3 of ECC-4% performed well in both and showed reserved strength as 365.44% and deflection hardening as 331.38% which is the maximum among ECC, R-ECC and C-ECC. In R-ECC samples nominal mild steel reinforcement of diameter 4 mm and 6 mm were used as shown in Figure 1(D). Shear reinforcement was not used looking to the enhanced shear capacity of ECC material. R-ECC specimens showed consistent enhanced performance with percentage reserved strength and percentage deflection hardening. Also, the deformation compatibility between ECC and reinforcement was observed.
  • Concrete of compressive strength 58.89 N/mm2 [5] was used alongwith ECC matrix as per the plastic hinge formation and compression zone requirement in plain cementitious matrix. Replac–ement of ECC by concrete is indicated by dark portion in Figure 1(C). C-ECC specimens render economy in strength perfor– mance as is clear from the higher first crack and ultimate strength compare to ECC. Deformation compatibility between ECC and concrete and enhancement of strength perfor– mance after first crack is, however, questionable which can be observed from the poor results of percentage reserved strength and percentage deflection harde– ning. Bending moment and shear force at the base of column (AB), top ofcolumn (BA), and end of the beam (BC), along with bending moment at the center of the beam are calculated and tabulated in Table 4.
  • Ultimate flexural strength and shear strength in PCC, ECC, RECC and C-ECC are calculated and tabulated in Table 5. Contribution of mild steel in flexure and associated consistent compatible deformation is highlighted in the result of R-ECC. Shear reinforcement is not provided in R-ECC specimen. Shear resistance is contributed by ECC material only. Ultimate shear strength of ECC material for ECC-4% is 7.51 N/mm2 [5] which is approximately double than M20 concrete. Calculated shear strength in sample number 1 is 12.02 N/mm2 which is higher than the ultimate shear strength of ECC. Therefore, shear failure of beam in R-ECC specimen is observed as shown in Figure 5.
    Cementitious Composites with Steel Reinforcement and Concrete
  • Crack width as a function of load was measured on the column of RECC sample number 2 and results are given in Table 6. Crack width remained 100 mm at a load of 19,494 N. Approximately, 20,000 N load is found to act for the threshold crack width of 150 μm. Structural element should be loaded corresponding to maximum permissible crack width of 150 μm from durability point of view. Development of the crack width upto 20,000 N load is slow but then it becomes fast.
  • Development of crack width was also measured in beam of RECC sample number 3. There was a slow crack width development upto 20,833 N load but then suddenly it became fast. Approximately, 21,000 N causes crack width within limit of 150 μm.
  • Crack development along with its location in the column from bottom of a beam for R-ECC was studied and is represented here in Figure 6 and Table 7. First crack initiated right at the bottom of the beam and new cracks generated below the first crack at approximately constant spacing with increase in load unlike ECC specimen.
  • Load displacement curves are plotted in Figure 7 for PCC, ECC-4, R-ECC and C-ECC specimens. PCC and CECC could not show strain hardening. ECC-4 specimen showed well defined strain hardening and post peak performance with less first crack load. R-ECC specimen performed the best with respect to strength, strain hardening and post peak behavior. Toughness indices are found out for ECC, R-ECC and C-ECC and tabulated in Table 8. As load displacement curve of ECC indicates the best post peak performance, the toughness index of 12.67 could be obtained for ECC.

    Cementitious Composites with Steel Reinforcement and Concrete

    Cementitious Composites with Steel Reinforcement and Concrete
  • Crack patterns for PCC, ECC-4, R-ECC and C-ECC are shown in Fig. 8. Single crack formation at the center of the beam and top of the columns were responsible for failure of the PCC specimen. This crack pattern gave information about reinforcement detailing and concrete substitution. Rotation of the beam in the center and at the top of the column was seen in ECC, R-ECC and CECC specimens. The crack pattern of ECC, R-ECC and C-ECC were distinctly different from that of PCC. The first crack started at the midspan of the beam on the tensile face, and multiple cracks developed from the first cracking point and spreaded to the outside of the midspan. The multiple cracks at the outside of the midspan were inclined similar to the shear cracks in the R-ECC beams. As the ultimate load approached, one of the cracks from the midspan started to open up after the development of large damage zone. Horizontal parallel cracks starting from the top of the column at the constant spacing of 2 to 5 cm developed upto the center of the column as shown in Figure 6. The first crack at the top of the column widened and rotation took place from this crack.
  • R-ECC specimens having larger resistance to rotation due to reinforcement did not fail due to rotation. Cracks were not seen along the reinforcement even after such large inelastic deformation which indicates good compatibility between reinforcement and ECC. Shear strength of the beam at support became more than ultimate shear strength of ECC material. Shear reinforcement was not provided in the beam. Eventually, beam of RECC failed due to shear from one of the ends as shown in Figure 5. Fractured surface of combination of ECC material with concrete revealed that there is good bond between two materials, without any delamination and spalling.

Conclusion

  • Plastic hinges were formed at beam column junction in L and Portal frames. ECC plays significant role in rotation of such plastic hinges in ductile manner. Therefore, energy absorption capacity of plastic hinges in such cases is greatly enhanced. Total collapse of structure can be much delayed or damage can be minimized with the help of such fused zones made with ECC and thus the overall performance of the structure can be improved.
  • ECC has compatible deformation and good bond strength with steel reinforcement. Debonding of ECC with steel reinforcement due to shear, spalling, punching was not observed. R-ECC renders maximum improvement in structural performance. Shear resistance of ECC is also quite large. Shear reinforcement can thus be minimized or eliminated, but it requires careful design.
  • C-ECC has no problem with flexural strength compatibility. However, it has poor deformation compatibility. It requires further investigation for proper interface behavior.
  • In R-ECC, ECC and C-ECC, vertical and inclined multiple cracks with close spacing are observed in beam portion while horizontal cracks with 2 to 3 cm spacing are observed in columns of portal frame specimens. Damage zone is large in column compared to beam. This strong column-weak beam concept can be used for specimen configuration and thus hinge formation in the column can be avoided.
  • Tight crack width control is the key property of ECC for durability performance. Ultimate crack width of ECC matrix remains within 150 mm upto quite large load considered to be sound for concrete durability. Thus, ECC can be effectively used in cover with less thickness.
  • The additional cost of ECC over normal concrete is mostly because of the use of fibres, higher cement content and use of high performance super– plasticizer. This is the reason why optimization of the composite to minimize the fibre content is so important. Finally, economy of ECC should be based on cost/benefit analysis. The life cycle cost of structure includes not only the initial material cost but also the construction and maintenance cost.

Acknowledgment

The authors would like to thank Reliance Industries Ltd., Grasim Industries Ltd., and Fosroc Chemicals Ltd. for supporting this research work by providing Recron 3s fibers, Kamal brand 53 Grade OPC cement and Conplast Super Plasticizer respectively. Thanks are also due to the funding agency DST, New Delhi for providing a grant of Rs. 25.6 Lakhs, under FIST Project, to Prof. S. C. Patodi for upgrading the testing facilities used in this investigation.

References

  • Fischer, G. and Li, V. C. "Intrinsic Response Control of Moment-Resisting Frames Utilizing Advanced Composite Materials and Structural Elements," ACI Structural Journal, Title No. 100-S18, March-April 2003.
  • Li, V. C. "Large Volume, High-Performance Applications of Fibers in Civil Engineering," ACE-MRL, Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan, DOI 10.1002/ app. 2263, 2000.
  • Fischer, G. and Li, V. C. "Effect of Matrix Ductility on Deformation Behavior of Steel Reinforced ECC Flexural Members under Reversed Cyclic Loading Conditions," ACI Structural Journal, No. 99-s, pp. 79, 2002.
  • Li, V. C. "On Engineered Cementitious Composites (ECC)- A Review of the Material and its Applications," Journal of Advanced Concrete Technology, Vol. 1, No. 3, pp. 215- 230, Nov. 2003. Rathod, J. D., Patodi, S. C., Parikh, B. K. and Patel, K. H. "Study of Recron 3S Fibers Reinforced Cementitious Composites," National Conference on Emerging Technology and Developments in Civil Engineering, Amravati, pp. I-88 to I-95, March 2007.

NBMCW June 2008

.....

Cements and Concrete Mixtures for Susta...

Cements and Concrete Mixtures

Mehta, P. Kumar, University of California, Berkeley, U.S.A.

The climate changes, due to man-made global warming triggered by steeply rising volume of greenhouse gases, composed mostly of carbon-dioxide, is a very serious issue that is being addressed worldwide by every major sector of economy. There is a general acceptance of the view that firm measures must be taken without delay to bring down the global carbon emissions to the 1990 level or less during the next 15 years.

The focus of this paper is on portland-cement concrete, which is the most widely used manufactured product in the world today. Cement production is not only energy-intensive but also responsible for direct release of nearly 0.9 tonne carbon-dioxide for each tonne of portland clinker, which is the principal component of modern cements. Fifteen years ago, in 1990, the world production of cement was slightly more than 1 billion tonnes. In 2005, it already crossed 2 billion tonnes which means that direct CO2 emissions from the portland clinker production have nearly doubled. Fifteen years from now, with businessas- usual, the estimated cement requirement would be 3.5 bilion tonnes, and direct CO2 emissions from cement kilns would triple the 1990 level. Thus, the challenge before the global construction industry is how to meet the buildings and infrastructure needs of rapidly growing economies of the world, and at the same time, cutting down the CO2 emissions attributable to cement consumption to the 1990 level, in conformity with other sectors of economy.

Different options for consideration of the construction industry are presented in this paper.

The production and use of blended Portland cements containing large proportion of complementary cementing materials, such as coal fly ash and granulated blast-furnace slag provide an excellent strategy for immediate and substantial reduction of direct CO2 emissions associated with the manufacture of portland-cement clinker. Both EU and North American cement standards now permit more than 50 % clinker replacement in composite cements. Furthermore, the use of composite cements and concrete mixtures containing large addition of complementary cementing materials would yield crackresisting structural elements of radically enhanced durability. High-volume fly ash concrete applications for recently built structures in North America are cited as typical examples of possible CO2 reduction.

Sustainability–an Introduction

During the 1990s, it became abundantly clear that industrialization of the world is happening at an unsustainable speed. Among the major sustainability issues of public concern are high rates of consumption of energy and materials, short service life of manufactured products, and lack of space for safe disposal of huge volumes of solid, liquid, and gaseous wastes generated by human activities. Global warming, the cumulative effect of these problems, has emerged today as the most serious sustainability issue of the 21st century.

Cements and Concrete Mixtures
Figure 1: Historical and Future Atmospheric CO2, Based on IPCC Reports (1)
The term, global warming, refers to the greenhouse-gas effect leading to a steady increase in the earth's surface temperature since 1950s. According to a World Watch Institute report, twenty-four of the last 27 years have been the warmest on record. Weather scientists around the world have concluded that a linear relationship exists between the earth's surface temperature and the atmospheric concentration of CO2, which makes up 85 % of the greenhouse gases. The current CO2 concentration, about 380 ppm (mg/L) in 2005, is the highest in recorded history (Figure 1). With business as usual, it is projected to increase at an exponential rate. In 2006, the annual global CO2 output reached a staggering 30 billion tonnes.

Evidence of global warming is not confined to temperature measurements. The following list includes some of the observable effects of the phenomenon:
  • A sharp increase in the melting rates of glaciers, polar caps, and ice sheets.
  • Rising ocean levels–a potential threat to coastal populations.
  • Unusual increase in frequency and intensity of rainstorms, flash floods, cyclones, hurricanes, heat waves, droughts, and wild fires.
  • Adverse impact on current sources of agriculture and water.
  • Disruption of the earth's carbon cycle due to changes in the botanical species on land and oceans.
In a series of reports, issued earlier this year by the United Nations Intergovernmental Panel on Climate Change, leading weather scientists of the world have unequivocally stated that global warming is occurring, and that it has been triggered by human activities. They have warned about devastating consequences of global warming if immediate action is not taken by national and industry leaders to reduce the carbon dioxide emissions to the 1990 level or less.

Although climate change is a global phenomenon, it has to be tackled in every country individually by each of the major CO2 emitting sectors of economy, such as power generation, transportation, and energy consumption associated with the use of buildings, and manufacture of structural materials like concrete and steel. According to Kyoto Protocol, proposed in 1990 and signed in 2005 by 141 countries, the signatories agreed to stabilize the greenhouse gas emissions by 2012 to 6 % below the 1990 level. The two largest polluting countries, the U.S. and China, which are responsible for nearly half of the global CO2 emissions, have yet to show a willingness to commit to any specific goals. However, in 2005, many multinational corporations, State governments in the U.S., and over 400 mayors representing 60 million Americans have signed on to programs that intend to meet or beat the Kyoto targets by 2020. In September 2006, the State of California approved the Global Warming Solutions Act according to which, by 2020, California's CO2 emissions would be reduced to the 1990 level.

Concrete Industry's Environmental Impact

The subject of environmental impact of the concrete industry is covered by numerous publications across the world including those listed in References (1-6). The embodied energy content, i.e., the sum total of energy required to extract raw materials, manufacture, transport, and install building elements is only 1.3 MJ/kg for 30 MPa concrete, compared to 9 MJ/kg for recycled steel and 32 MJ/kg for new steel. However, being the largest manufactured product consumed in the world, quantitatively concrete represents considerable embodied energy.

Worldwide today, approx. 17,000 million tonnes of concrete is being produced annually. Besides natural resources, such as aggregates and water, the concrete industry is a large consumer of cement– a manufactured product directly responsible for high CO2 emissions. In 2005, according to Cembureau, the global cement consumption was 2,270 million tonnes. Therefore, carbon footprints of the global cement industry are very significant considering the amount of fossil fuels and electrical power consumed for crushing, grinding and transport of materials, and for the 1400 to 1500°C burning operation to make portland clinker –the principal ingredient of hydraulic cements. The scope of this paper is limited to direct CO2 emissions, of which approx. 6.3%2 of the global emissions are attributable to portland clinker manufacture.

CO2 Emissions from Cement Kilns

Typically, ordinary portland cement is composed of 95 % clinker and 5 % gypsum, which is a complementary cementing material (CCM) because it enhances the cement performance by improving the setting and hardening characteristics of the product. Depending on the carbon content of fossil fuels used for clinkering, 0.9 to 1.0 tonnes of CO2 is directly released from cement kilns during the manufacture of clinker. In addition to gypsum, sometimes other mineral additives, commonly known as supplementary cementing materials (e.g., coal fly ash, granulated blast-furnace slag, natural and calcined pozzolans, pulverized limestone, and silica fume) can either be interground with clinker and gypsum or added directly during the concrete mixing operation. Large quantities of these materials are available as industrial by-products. As discussed in this paper, when properly used, the mineral additives have the ability to enhance considerably the workability and durability of concrete. Therefore, these additives too are treated as complementary cementing materials (CCM) in this paper.

Global statistics for 1990 and 2005 on cement production, CCM consumption, and direct CO2 emission attributable to Portland clinker manufacture, are presented in Table 1. According to the U.S. Geological Survey records, the world consumption of cement in 1990 was 1,044 million tonnes. From the fragmentary information available it is estimated that, globally, the average clinker factor of cement (units of clinker per unit of cement) in 1990 was 0.9, which means that 940 million tonnes of clinker and 104 million tonnes of CCM were used. Assuming the average CO emission rate as 1.0 tonne CO2/ tonne clinker, in 1990 the direct CO2 emission from clinker production were 940 million tonnes.

In 2005, due to a gradual increase in the use of CCM, it is estimated that 370 million tones of CCM were incorporated into 2,270 million tonnes of cement. This gives a clinker factor of 0.84. Also, in 2005, due to increase in the use of alternate, low-carbon, fuels for burning clinker, the average CO2 emission rate dropped to 0.9 tonne per tonne of clinker. This means that, in 2005, 1,900 million tonnes of clinker was produced, with 1,700 million tonnes of direct CO2 release to the environment. In conclusion, the global cement industry has almost doubled its annual rate of direct CO2 emissions during the last 15 years.

Reducing the CO2 Emissions

Comparing the 1990 and 2005 global CO2 emissions directly attributable to clinker production (Table 1), the magnitude of the problem becomes at once clear. Not only the annual rate of cement consumption in the world has nearly doubled during the last 15 years but also, at the current rate of economic growth in many developing countries, by the end of the next 15 years the cement requirement is expected to go up to about 3,500 million tonnes a year. Assuming that during the same period the use of CCM increases from 15 to 20 % of the total cement, the global clinker production and CO2 emission in 2020 would amount to 2,800 million tonnes, and 2,520 million tonnes, respectively. To bring down the CO2 emission from 2,520 to 940 million tonnes (the 1990 level) involves nearly a two-third reduction in clinker requirement, which is unlikely barring a global catastrophe.

In the portland clinker manufacturing process, direct release of CO2 occurs from two sources, namely the decomposition of calcium carbonate (the principal raw material) and the combustion of fossil fuels. The former accounts for about 0.6 kg CO2/kg clinker and the latter 0.25-0.35 kg CO2/kg clinker (depending on the carbon content of the fossil fuel); the global average being 0.9 kg CO2/kg clinker. Alternate sources of energy other than fossil fuels are being sought but, at present, they are too expensive. Also, there are some cements that do not require calcium carbonate as a raw material (e.g., magnesium phosphate cements) but they are neither economical nor technically feasible for large-scale production. Obviously, it will not be possible to achieve any drastic cuts in CO2 emission as long as technical and economic reasons favor the use of portland clinker as the major component of hydraulic cements.

The golden rule or mantra for successful resolution of all sustainability issues is, "Consume less, and think more." Based on this mantra, the author proposes the following three tools, the simultaneous use of which would enable the cement industry to reduce greatly the direct CO2 emission attributable to clinker production:
  1. Reduce the consumption of concrete: Architects and structural designers must develop innovative designs that minimize the consumption of concrete. Service life of repairable structures should be extended as far as possible by the use of proper materials and methods of repair. Low-priority projects should be postponed or even canceled when possible. Foundations, massive columns and beams of concrete, and pre-cast building components that can be assembled or disassembled as needed, should be made with highly durable concrete mixtures described in this paper.
  2. Reduce the cementing materials in concrete mixtures: Mix design procedures that involve prescriptive codes (e.g., minimum cement content, maximum w/cm, and much higher than needed strength) lead to considerable waste of cement, besides adversely affecting the durability of concrete. Such prescriptive codes have outlived their usefulness and must be replaced with performance-based specifications that promote durability and sustainability. For example, to achieve durability, it is not the w/c but the cement paste content which should be minimized through optimum aggregate grading, use of plasticizing admixtures, and specifying 56 or 91-day strength for the structural components that do not have to meet a minimum 28-day strength requirement.
  3. Reduce the clinker factor of cement: Every tonne of clinker saved would reduce the direct CO2 release from cement kilns by an equivalent amount. Furthermore, as explained below, concrete products made with cements of low clinker factor are expected to be much more durable when compared to ordinary portland cement products.
Imagine if it were possible to enhance the durability of most cement-based products by factor 10 or more, without using any expensive technology and materials! Unquestionably, in the long term, this would serve as an excellent strategy for minimizing the wasteful consumption of cement and other concrete ingredients for general construction.

Published literature contains numerous reports showing that high-early strength concrete mixtures used in modern, high-speed, construction often suffer from lack of durability because they are usually made with high content of a cementing material and a high clinker factor of cement. The hardened product contains a heterogeneous cement paste, with weak interfacial bonding, and is vulnerable to cracking from excessive thermal shrinkage and drying shrinkage. According to Reinhardt (7), to minimize the shrinkage, volume of the paste (cement plus mixing water) in concrete should not exceed 290 L/ m3. High-volume fly ash concrete mixtures, described in this paper are made with cements of low clinker factor (0.4 – 0.5), and less than 290 L/m3 cement paste content. Therefore, they can be used for making relatively crack-free products of excellent durability without any added cost.

CO2 Emission from Cement Kilns

Options

As shown in Table 1, compared to the base year 1990, global carbon emissions direct from Portland clinker production have already doubled in the past 15 years. If no serious measures are put into place quickly by the world's construction industry, i.e. with business-as-usual it is estimated that the rate of direct carbon emissions from cement kilns will almost triple in the next 15 years (Table 2, Option 1). Table 2 also includes data on two other options, an easy option (Option 2) and a challenging but preferable option (Option 3). Note that Option 1 (business-as-usual) data will be used as a reference point for both Options 2 and 3, that are discussed next.

Cement Consumption in 2020

According to Option 2, by 2020, if the global concrete construction industry is able to reduce the concrete consumption by 20 % (compared to Option 1) and at the same time increase the CCM utilization to 30 % of the total cement, these steps will have the effect of reducing the direct CO2 emissions from cement kilns to 1,760 million tonnes. This is nearly twice as much as the 1990 emissions rate of 940 million tonnes.

According to Option 3, in 2020, the total cementing material (2,100 tonnes) would comprise 1050 million tonnes of portland clinker and the same amount of complementary cementing materials. In Table 3, estimates of different types and amounts of complementary cementing materials that would be available for use in 2020 are given. Note that coal fly ash is expected to make up 760 million tonnes or nearly threefourths of the total CCM. Would such a large quantity of fly ash be available in 2020? It is difficult to provide a definite answer, but let us examine the assumptions under which this is possible.

Cements and Concrete Mixtures

In the foreseeable future, fossil fuels will continue to remain the primary source of power generation, and due to the low cost of coal, expansion of the coal-fired power industry will continue in major coalproducing countries such as China, India, and the United States. According to one estimate, approximately 1200 million tones of fly ash would be available in 2020. It would indeed be a formidable job to ensure that nearly two-thirds of the fly ash produced by coal-fired power plants is suitable for use as a complementary cementing material. This goal can be accomplished, provided the key players, i.e., the producers of fly ash, the consumers of cement and concrete, and individuals or organizations responsible for specifications work together to overcome the problems, discussed below.

The power sector of the global economy is the largest single source of carbon emissions in the world. It is estimated that about 7 billion tonnes a year of CO2 is being released today from the combustion of all fossil fuels, and that the coalfired power plants alone generate 2 billion tonnes of CO2. Besides carbon emissions, according to Malhotra (5), coal combustion in 2005 generated approximately 900 million tonnes of solid by-products including 600 million tonnes of fly ash. Due to rapidly changing rates of fly ash production and use in the two large economies of the world, China and India, which meet threequarters of their electrical power requirement from coal-fired furnaces, accurate data on today's global rates of flyash production and utilization are not available. However, a rough estimate shows that the current rate of fly ash production is approximately 750 million tonnes/ year, and that nearly 140 million tonnes/year is being consumed as an ingredient of blended cements and concrete mixtures. The remaining fly ash either ends up in low-value applications, such as road sub-bases and embankments, or is disposed to landfills and ponds.

When used as a complementary cementing material, each tonne of fly ash can replace a tonne of portland clinker. Diverting fly ash from the waste stream and using it to reduce direct carbon emissions from the cement industry is like killing two birds with one stone. Therefore, increasing the utilization of most of the available fly ash as a complementary cementing material is, unquestionably, the most powerful tool for reducing the environmental impact of two major sectors of our industrial economy, namely the cement industry and the coal-fired power industry.

In spite of proven technical, economic, and ecological benefits from the incorporation of high volumes of fly ash in cements and concrete mixtures, why does the fly ash utilization rate as a complementary cementing material remain so low? Obsolete prescriptive codes, lack of state-ofthe- art information to architects and structural designers, and lax quality control in power plants are among some of the reasons. Also, all of the currently produced fly ash is not suitable for use as a complementary cementing material, however cost-effective methods are available to beneficiate the material that does not to meet the minimum fineness and maximum carbon content requirements–the two important parameters by which the flyash suitability is judged by the cement and concrete industries (5).

Sustainable Cements

Sustainable, portland-clinker based cements can be made with 0.5 or even lower clinker factor using a high volume of granulated blast furnace slag (gbfs), or coal fly ash (ASTM Class F or C), or a combination of both. Natural or calcined pozzolans, in combination with fly ash and/or gbfs, may also be used. Compared to portland cement, the high-volume flyash and slag cements are somewhat slower in setting and hardening, but they are more suitable for producing highly durable concrete products. Unfortunately, worldwide, the conventional concrete construction practice is dominated by prescriptive specifications that do not permit the use of high volume of mineral additives.

Cement containing a high volume of complementary cementing materials can now be manufactured in accordance with ASTM C 1157–a new standard specification for hydraulic cements, which is performance-based. However, in North America significant amount of blended portland cements are not produced, because it is customary to add mineral admixtures at the readymixed concrete plants. According to American Coal Ash Association, at present about 14 million of the available 70 million tonnes/year fly ash is being used as a complementary cementing material in concrete mixtures. Reliable estimates are not available from China and India, however, it is reported that significant quantities of blended cements containing 20- 30 % flyash, are being manufactured in these countries.

The European Cement Specification EN 197/1, issued in 2002, contains 26 types of blended portland cements including three cement types that have clinker factors ranging between 0.35 and 0.64. Type III-A Cement covers slag cements with 36-65 % gbfs; Type IV-B Cement covers pozzolan cements with 36-55 % pozzolans including fly ash, natural or calcined pozzolanic minerals, and silica fume; Type V-A Cement covers composite cements containing 18-30 % gbfs plus 18-30% pozzolans. According to Cembureau statistics for 2005, the consumption of ordinary portland cement in the European Union countries has dropped to 30 % of the total cement produced, whereas blended portland cements containing up to 25% CCM have captured 57% of the market share, and blended cements with more than 25% CCM are approaching 10% of the total cement consumption.

Sustainable Concrete Mixtures

For reducing direct carbon emissions attributable to Portland clinker production, the emerging technology of high-volume flyash (HVFA) concrete is an excellent example showing how highly durable and sustainable concrete mixtures, with clinker factor of 0.5 or less, can be produced by using ordinary coal fly ash (ASTM Class F or Class C), which are available in most parts of the world in large amounts. The composition and characteristics of HVFA concrete are discussed in many publications and are briefly described below. Note that concrete mixtures with similar properties can be produced by using a high volume of granulated blast-furnace slag or a combination of flyash and slag, with or without other mineral admixtures.

The cementing material in HVFA concrete is composed of ordinary portland cement together with at least 50% flyash by mass of the total cementing material. The mix has a low water content (100- 130 kg/m3), and a low content of cementing materials (e.g. 300 kg/ m 3 for ordinary strength and max. 400 kg/m3 for high-strength). The plasticizing action of the high volume of flyash imparts excellent workability even at w/cem of the order of 0.4. However, chemical plasticizers are often used, when lower w/cem are required.

Occasionally, an air-entraining admixture is also included in the mix when protection against frost action is sought.

Compared to portland-cement concrete, the HVFA concrete mixtures designed to achieve the same 28-d strength exhibit superior workability without segregating even at slump values of 200-250 mm. Typically, the concrete is slow in setting and hardening, i.e. develop slightly lower strength at 3 and 7-d, similar strength at 28-d, and much higher strength at 90-d and 1-year. The pozzolanic reaction leading to complete removal of calcium hydroxide from cement hydration products enables the HVFA concrete to become highly resistant to alkaliaggregate reaction, sulfate and other chemical attacks, and reinforcement corrosion (due to very low electric conductivity). Furthermore, the HVFA concrete mixtures are much less vulnerable to cracking from both the thermal shrinkage (less heat of hydration), and the drying shrinkage (less volume of cement paste). Therefore, in addition to very low clinker factor, the ability of HVFA concrete to enhance the durability by factor 5 to10 makes it a highly suitable material for construction of sustainable structures in the future. The author has been involved with many field applications of HVFA concrete that are described in earlier publications (8-11). Three recently built structures in the U.S., with large reduction in CO2-emissions resulting from the use of HVFA concrete, are described below.

Cements and Concrete Mixtures
Figure 2: The BAPS Hindu Temple, Chicago, 2004 High-Volume Flyash was used for Unreinforced Monolith Foundations and Drilled Piers
A Hindu Temple, built with concrete members designed to endure for 1,000 years or more, was constructed in Chicago in 2003 (Figure 2). The superstructure of the temple is composed of some 40,000 individual segments of intricately carved white marble (Figure 2). Unreinforced monolith slabs are a part of the foundation, supported by 250 drilled piers, 9 m high and 1 m diameter. All structural elements were made with, cast-inplace, HVFA concrete containing 105 kg/m3 ASTM Type I portland cement and 195 kg/m3 Class C flyash, 2 L/ m3 polycar– boxylate superplasticizer, and 100 kg/m3 water. Note that the total cementing material was 300 kg/m3, the clinker factor was only 0.33, and the w/cem was also 0.33. The fresh mix had 150-200 mm slump and showed excellent pumpability, which made it possible to place and finish 400 m 3 Concrete for the main prayer-hall slab (22 by 18 by 1 m), in less than 5 hours. Typical compressive strength values at 3-d, 7-d, 56-d, and 1-y were 10 MPa, 27 MPa, 48 MPa, and 60 MPa, respectively. No structural cracks in any concrete member were reported. Also, the chloride penetration permeability, which is an excellent index of long term durability of concrete, was surprisingly low (< 200 coulombs) in 1-year old core samples. A conventional concrete mix would have required 400 kg/m3 portland cement to achieve similar3 highstrength. The use of 3,000 m 3 HVFA concrete mix resulted in 900 tonnes of portland cement saving, which corresponds to about 800 tonnes of CO2 emissions reduction.

Cements and Concrete Mixtures
Cements and Concrete Mixtures
Figure 3a: Utah State Capitol Building after Seismic Rehabilation, 2006 high-volume Flyash was used for reinforced foundation, beams, and Shear WallsFigure 3b: Utah State Capitol Building after Seismic Rehabilation, 2006 Excellent Pumpability and Workability of Nearly Self-consolidating Concrete Mixtures

The Utah State Capitol Building, Salt Lake City, underwent seismic rehabilitation in 2006 (Figure 3a). Due to heavily congested reinforcement in the foundations, floor beams, and shear walls, a nearly self-consolidating mix containing 160 kg/m3 ordinary portland cement, 200 kg/m3 ASTM Class F flyash, 138 kg/m3 water, and 1 L/m3 superplasticizer was used. The clinker factor of this mix was 0.44, and the w/cem was 0.38. The specified slump and 28-d compressive strength were 150 mm and 27 MPa, respectively. The field concrete showed an average of 225 mm slump and 34 MPa strength. It is estimated that this 4,500m3 HVFA concrete job, enabled 900 tonnes of reduction in CO2 emissions attributable to clinker saving.

The CITRIS Building at the University of California at Berkeley contains 10,700m3 HVFA concrete – the largest volume ever used for construction of a single building. For foundations and mats, a concrete mix containing 160 kgm3 of ASTM Type II portland cement, 160 kg of Class F fly ash, and 123 kg/m3 water (0.37 w/cem) was used. For heavily reinforced columns, walls, beams, girders and slabs, a concrete mix containing 200 kg/m3 ASTM Type II portland cement, 200 kg/m3 Class F flyash, and 140 kg/ m3 water (0.35 w/cem) was used. In both cases the clinker factor is 0.50. The specified compressive strength was 27 MPa @ 28-d for all structural members except the foundations and mats which were designed for a specified strength of minimum 27 MPa @ 56-d. Note that the concrete used for reinforced columns achieved 20 MPa strength @ 7-d, and nearly 40 MPa @ 56-d. It is estimated that the choice of HVFA concrete as a structural material for the CITRIS Building resulted in a reduction of 1950 tonnes of direct CO2 Emissions attributable to the low clinker factor of the cementing material.

Cements and Concrete Mixtures
Cements and Concrete Mixtures
Figure 4a: CITRIS Bldg., Univ. of California, 2007 Mat Foundation Under ConstructionFigure 4b: CITRIS Bldg., Univ. of California, 2007 Heavily Reinforced Columns under Construction

Economic and Technical Barriers

For utilization of high proportions of complementary cementing materials in general construction, human perception appears to be a far more formidable barrier than actual economic and technical barrier. According to Meryman and Silman (12):

Sometimes, there is a perception that a "green" material or practice is more costly, but on further examination, it proves no to be so; often it is just a matter of getting on the other side of the learning curve. We must clarify the difference between life cycle cost and first cost, since many sustainable products have better life cycle performance. We need to define the term 'economic' and include the collateral cost of using non-sustainable practices.

The use of sustainable cements and concrete mixtures, described in this paper, would undoubtedly produce structural members of high durability. However, a statistical life-cycle analysis is not possible because there are no reliable laboratory tests for quantitative assessment of longterm durability of field structures. Other major barriers are lack of codes of recommended practice and unwillingness of structural designers and engineers to be among the first to champion the use of new materials. Again, according to Meryman and Silman (12):

How can an underused material or method become tried, trusted and ultimately the standard? These materials and methods need advocates. As technical professionals, structural engineers can use specifications to communicate a commitment to and confidence in more sustainable choices. By taking responsibility for those practices, we become their advocates.

From my own personal experience, I confirm the observations of Meryman and Silman. I have come to the conclusion that it is the hand that writes the specifications which holds the power of leading the concrete construction industry to an era of sustainability. Codes of recommended practice advocated by organizations, such as American Concrete Institute and U.S. Green Building Council, can play an important part in accelerating the sustainability of the concrete industry. For instance, the USGBC point-rating system for new construction has already become a powerful driving force for sustainable building designs. The rating system awards sufficient points for buildings that would consume less energy in their use. A similar emphasis is needed in favor of sustainable materials that produce less CO2 during their manufacture. By suitably amending the rating system so that some points based on CO2 emissions reduction are directly assigned for the use of sustainable materials in new construction, the USGBC can help sustainability of the cement and concrete industries.

Concluding Remarks

The high carbon dioxide emission rate of today's industrialized society has triggered climate change that is potentially devastating to life on the planet earth. To meet the global concrete demand, which was 17 billion tonnes in 2005, two billion tonnes of CO2 were directly released to the atmosphere from the manufacturing process of portlandcement clinker, which is the major component of modern hydraulic cements. With business-as-usual, the direct CO2 emissions from portland clinker production, in the year 2020, would triple the 1990 level unless immediate steps are taken to bring down the emissions by making significant reductions in the: (a) global concrete consumption, (b) volume of cement paste in concrete, and (c) proportion of portland clinker in cement.

Examples of recently built structures prove that by using high volume of coal flyash and other industrial wastes as complementary cementing materials with portland clinker, we can produce low cost, highly durable, and sustainable cements and concrete mixtures that would significantly reduce both the carbon footprints of the cement industry and the environmental impact of the coal-fired power generation industry.

It seems that the game of unrestricted growth, in a finite planet, by reckless use of energy and materials, is over. Most sectors of the global economy have already initiated action plans to bring down their share of carbon emission to the 1990 level or less, by the year 2020. The construction industry is already pursuing the goal of designing and constructing sustainable buildings that consume less energy and resources to maintain. Now, all segments of the construction industry–owners, designers, contractors, and cement and concrete manufacturers–will have to join the new game of building sustainable structures using only sustainable materials.

We have the tools to win this game. What is needed now is the will and the individual initiative. To paraphrase John F. Kennedy, "Ask not what others can do. Ask what you can do to promote the use of sustainable construction materials."

Acknowledgement

The author would like to thank Mason Walters of Forell Elsesser Engineers, San Francisco, for the photographs in Figures 3 and 4.

References

  • P.K. Mehta, and P.J.M. Monteiro, "Concrete: Microstructure, Properties, and Materials," McGraw-Hill, New York, 2006
  • ACI Board Advisory Committee on Sustainable Development, "White Paper on Sustainable Development," Concrete International, American Concrete Institute, Vol. 27 No. 2, 2005, pp. 19-21
  • The Concrete Center of U.K., "Sustainable Concrete," www.concretecenter.com, 2007, 18 pages
  • World Business Council for Sustainable Development, "The Cement Sustainability Initiative,"www.wbcsdcement.org, Geneva, Switzerland, 2007
  • V.M. Malhotra, "Reducing CO2 Emissions," Concrete International, American Concrete Institute, Vol. 28 No. 9, 2006, pp. 42-45
  • P.K. Mehta, "Greening of the Concrete Industry for Sustainable Development," ibid., Vol. 24 No.7, 2002, pp. 23-28
  • H.W. Reinhardt, "New German Guideline for Design of Concrete Structures for Containment of Hazardous Materials," Otto Graf Journal, FMPA, Univ. of Stuttgard, Germany, Vol. 17, 2006, pp. 9-17
  • P.K. Mehta and W.S. Langley, "Monolith Foundation Built to Last a 1,000 Years," Concrete International, American Concrete Institute, Vol. 22 No. 7, July 2000, pp. 27-32
  • D. Manmohan and P.K. Mehta, "Heavily Reinforced Shear Walls and Reinforced Foundations Built with Green Concrete," ibid., Vol. 24 No. 8, 2002, pp. 64-70
  • P.K. Mehta and D. Manmohan, "Sustainable, High-Performance Concrete Structures," ibid., Vol. 28 No. 7, 2006, pp. 37-42
  • V.M. Malhotra and P.K. Mehta, "High-Performance, High-Volume Flyash Concrete," Supplementary Cementing Materials for Sustainable Development, Ottawa, Canada, 2002
  • H. Meryman and R. Silman, "Sustainable Engineering–Using Specifications to Make it Happen," Structural Engineering International, Vol. 14 No. 3, Aug. 2004, pp 216-219.

Acknowledgement

The article has been reproduced from the SEWC'07 proceeding with the kind permission from the SEWC organisers.

NBMCW July 2008

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Effects of Adding Chemical Admixtures i...

Effects of Addition of More than two Chemical Admixtures on the properties of Retempered Concrete

Retempered Concrete

D.K. Kulkarni, Assistant Professor, Civil Engineering Department, Rajarambapu Institute of Technology Rajaramnagar Islampur, Maharastra. Dr. K.B. Prakash, Professor Civil Engineering Department, K.L.E Society's College of Engineering and Technology, Belgaum.

In situations like delivery of concrete from central mixing plant, in road construction, in constructing lengthy tunnels, in transportation of concrete by manual labor, in hilly terrain long hauling of concrete is required. Loss of workability and undue stiffening of concrete may take place at the time of placing on actual work site. In such situations engineers at site, many a time reject the concrete partially set and unduly stiffened due to the time elapsed between mixing and placing. Mixed concrete is a costly material and it cannot be wasted without any regard to cost. It is required to see whether such a stiffened concrete could be used on work without undue harm with use of combinations of admixtures. The process of remixing of concrete, if necessary, with addition of just the required quantity of water is known as 'retempering of concrete'. Sometimes, a small quantity of extra cement is also added while retempering.

In the sites sometimes the concrete has to wait for some time to enter in the formwork after it is mixed. This may be due to some break down in the conveyance or quarrel between the labors. In such situations the concrete looses its plasticity. But since the quantity is enormous, such concrete cannot be wasted. In such situations addition of small quantity of cement and water along with combinations of admixtures can bring back the plasticity to concrete. Thus retempering becomes important in such odd situations.

In this paper an attempt is made to study the strength characteristics of concrete containing combination of admixtures at retempering time of 15 min upto 90 min. The combinations of admixture studied in this experimentation is

Superplasticiser + Air Entraining Agent + Water proofing compound (S+AEA+W).

The tests were conducted to evaluate the strength characteristics of concrete like compressive strength, tensile strength, flexural strength and impact strength for different retempering times.

Introduction

One of the adverse effects of hot weather concreting is loss of slump. Delay in the delivery of ready mixed concrete has the same result and leads many people in the concrete industry to regain the original slump by adding water, a process known as 'retempering'1.

Ready-mixed (RMC) concrete, which is mixed at the plant, using a normal, well-designed concrete mix, should arrive at its destination with sufficient workability to enable it to be properly placed and fully compacted. In such circumstances, where there is a significant period of time between mixing and placing the concrete, there will be a noticeable reduction in the workability of the fresh concrete. If for any reason, the placement of the concrete is unduly delayed, then it may stiffen to an unacceptable degree and site staff would normally insist on the rejection of a batch or otherwise good concrete on the grounds of insufficient workability. If not rejected, excessive vibration would be needed to attempt to fully compact the concrete, with the risk of incomplete compaction, expensive repair, or, at worst, removal of the hardened concrete.

If abnormal slump loss in anticipated or if transport times are significant, then the intelligent use of admixtures can alleviate the potential workability difficulties, although at additional cost, and this practice is common place 2, 3, 4. However, in cases where unforeseen delay or some other cause has lead unexpectedly to poor workability, retempering of the concrete by water, while normally considered to be bad practice, may, in reality, be contemplated as a possible course of action. The increase in the water content of the concrete immediately prior to discharge will improve the consistency, but it is widely held that there must be a subsequent increase in the water/ cement (w/c) ratio which will be detrimental to the hardened concrete5, 6.

Adding water to a plastic mix to increase slump is an extremely common practice, even though it is not recommended because it increases the porosity of concrete. Concrete often arrives on site more than half an hour after initial mixing. Placement operations can take anywhere from 10 to 60 minutes, depending on the field conditions and the size of the load. When the slump decreases to an unacceptable level during the operations, water is added to the mix and, very often, experienced field inspectors will tolerate what can be termed 'reasonable' retempering, i.e., enough to increase slump by 50 or 60 mm 7.

Research Significance

In the circumstances like breakdown of any concreting equipment or quarrels between the labors or suddenly erupted strikes on the site may put the green concrete into difficult situation. In such above situations the concrete which is already mixed may have to wait for a longer time before entering into the formwork. This causes the loss of plasticity and if such concrete is used, the strength and other characteristics of concrete are affected. Such concrete has to be either discarded or used with little addition of extra water and cement so that a part of plasticity is regained, and such concrete is called retempered concrete. Probably use of some admixtures may induce some good qualities to such retempered concrete. Therefore, it is essential to study the characteristic properties of retempered concrete containing combination of admixtures.

Experimental Programme

Retempered Concrete
The main aim of this experimentation work is to find the effect of addition of more than two admixtures on the properties of retempered concrete. The combination of admixtures selected for the study on concrete is Superplasticiser + Air Entraining Agent + Water proofing compound (S+AEA+W)

Ordinary Portland Cement and locally available sand and aggregates were used in the experimentation. The specific gravity of fine and coarse aggregate was 2.66 and 2.51 respectively. The experiments were conducted on a mix proportion of 1: 1.26:2.1 with w/c = 0.41 which corresponds to M20 grade of concrete. The admixtures and their dosages used in the experimentation are shown in Table 1.

After thoroughly mixing all the ingredients in dry state, the required quantity of water was added in the mix and thoroughly mixed. At this stage the different admixtures like superplasticiser, air entraining agent and water proofing compounds were added and a homogeneous concrete mix was obtained. This concrete mix was covered with gunny bags for 15 minutes. The time was reckoned, the moment the water was added to the concrete mix. After 15 minutes the mix was poured into the moulds and the specimens were cast with sufficient compaction through vibration. This forms retempered concrete for 15 minutes. Similarly, the specimens were prepared with retempered concrete with a retempering time of 30 minutes, 45 minutes, 60 minutes, 75 minutes and 90 minutes.

Another set of retempered concrete specimens were cast by adding 5% extra cement and the required extra amount of water to balance a w/c ratio of 0.41. All the specimens were demoulded after 12 hours of their casting and were transferred to curing tank to cure them for 28 days. After 28 days of curing the specimens were tested for their compressive strength, tensile strength, flexural strength and impact strength as per IS specifications.

For compressive strength test, the cubes of dimensions 150 X 150 X 150 mm were cast and were tested under compression testing machine as per I S 516-1959.8 For tensile strength test, the cylinders of diameter 100 mm and length 200 mm were cast and were tested under compressive testing machine as per I S 5816- 1999.9 For flexural strength test the beams of dimensions 100 X 100 X 500 mm were cast and were tested on an effective span of 400 mm with two point loading as per I S 516-1959.8 For impact test four different test methods are referred in the literature.10 Drop weight method being the simple method, was adopted to find the impact energy. Impact strength specimens were of dimensions 250 X 250 X 30 mm. A steel ball weighing 12.6 N was dropped from a height of 1 m on the centre point, which was kept on the floor. Number of blows required to cause first crack and final failure were noted down. From these number of blows, the impact energy was calculated as under.

Impact energy = w h N (N-m)

Where w = Weight of steel ball = 12.6 N

h = Height of drop = 1 m

N = Number of blows required for first crack or final failure as the case may be.

Retempered Concrete

Test Results

Table 2 gives the compressive strength test results of retempered concrete. It also gives percentage increase or decrease of compressive strength w.r.t. reference mix. Table 3 gives the tensile strength test results of retempered concrete. It also gives percentage increase or decrease of tensile strength w.r.t. reference mix. Table 4 gives the flexural strength test results of retempered concrete. It also gives percentage increase or decrease of flexural strength w.r.t. reference mix. Table 5 gives the impact strength test results of retempered concrete. It also gives percentage increase or decrease of impact strength w.r.t. reference mix.

The variation of these strengths are depicted in the form of graphs as shown in Figure.1, 2, 3 and 4.

Retempered Concrete

Retempered Concrete

Discussion of Test Results

  1. It has been observed that the concrete without any admixture shows maximum compressive strength, tensile strength, flexural strength and impact strength at a retempering time of 60 minutes. It is true for both concretes which are produced by adding 5% extra cement and water and concrete without adding 5% extra cement and water.

    This may be due to the fact that the evaporated water up to Figure 3: Variation of Flexural Strength w.r.t. Retempring Times Figure 4: Variation of Impact Strength w.r.t. Different Retempring Times 60 minute may bring down the w/ c ratio resulting in an enhanced strength.

    Thus it can be concluded that the concrete without any admixture show maximum strengths at a retempering time of 60 minutes.
  2. It has been observed that the concrete produced with addition of 5% extra cement and water show higher compressive strength, tensile strength, flexural strength and impact strength as compared to concrete produced without 5% extra cement and water. This is true for all the retempering times from 15minutes to 90 minutes.

    Obviously this may be due to the fact of presence of 5% extra cement.

    Thus it can be concluded that the concrete produced with addition of 5% extra cement and water yields more strength, for all the retempering times up to 90 minutes.
  3. It has been observed that the concrete with the combination of admixture (S+AEA+W) shows maximum compressive strength, tensile strength, flexural strength and impact strength at a retempering time of 45 minutes. It is true for both the concretes which are produced by adding 5% extra cement and water and concrete without adding 5% extra cement and water.

    This may be due to the fact that the evaporated water up to 45 minute may bring down the w/c ratio resulting in an enhanced strength.

    Thus it can be concluded that the concrete with the combination of admixture (S+AEA+W) shows maximum strengths at a retempering time of 45 minute.
  4. It has been observed that the concrete produced with addition of 5% extra cement and water show higher compressive strength, tensile strength, flexural strength and impact strength as compared to concrete produced without 5% extra cement and water, when the combination of admixture (S+AEA+W) is used. This is true for the retempering times from 15minutes to 90 minutes.

    Obviously this may be due to the fact of presence of 5% extra cement.

    Thus it can be concluded that the concrete produced with addition of 5% extra cement and water and with combination of admixture (S+AEA+W) yields more strengths for all the retempering times up to 90 minutes.
  5. It has been observed that the compressive strength, tensile strength, flexural strength and impact strength of concrete produced with the combination of admixture (S+AEA+W) is higher than that without any admixture. This is true for all the retempering times and also it is true for the concrete produced by addition of 5% extra cement and water and concrete without 5% extra cement and water.

    This may be due to the fact that the addition of combination of admixture (S+AEA+W) induce more workability which will facilitate for full compaction and in turn this results in higher strengths.

    Thus it can be concluded that the concrete produced with the combination of admixture (S+AEA+W) show higher strengths than that of without admixtures for all the retempering times.

Conclusions

  • The concrete without any admixture show maximum strengths at a retempering time of 60 minutes.
  • The concrete produced with addition of 5% extra cement and water yields more strength, for all the retempering times up to 90 minutes.
  • The concrete with the combination of admixture (S+AEA+W) shows maximum strengths at a retempering time of 45 minute.
  • The concrete produced with addition of 5% extra cement and water and with combination of admixture (S+AEA+W) yields more strengths for all the retempering times up to 90 minutes.
  • The concrete produced with the combination of admixture (S+AEA+W) show higher strengths than that of without admixtures for all the retempering times.
  • Thus instead of wasting the bulk concrete, the retempering can be recommended either with the use of combination of admixture (S+AEA+W) or without admixture.

Acknowledgments

The authors would like to thank Dr. (Mrs) S. S. Kulkarni, Principal, RIT, Sakharale and Dr. S.C.Pilli, Principal, KLESCET, Belgaum for giving all the encouragement needed which kept our enthusiasm alive. Thanks are also due to the management authorities and others who constantly boosted our morale by giving us all the help required. Thanks are also due to authorities of MBT Pvt.Ltd(Degussa) Mumbai for supplying the required admixtures.

Reference

  • M A A l Kubaisy and A S K Palanjian, "Retempering studies of concrete in hot weather," Proceedings of colloquium organized on behalf of the coordinating committee for concrete technology of RILEM, Oct 3-5, 1990, pp.83-91.
  • Previte R W, "Concrete slump loss," ACI Journal, Aug-1977, pp. 361-367.
  • Mayer L M and Perenchio W F, "Theory of concrete slump lossas related to the use of chemical admixtures," Concrete International, Jan-1979, pp. 36- 43.
  • Erlin B and Hime W G, "Concrete slump loss and field example of placement problems," Concrete international, Jan1979, pp. 48- 51.
  • Gonnerman H F and Woodworth P M, "Tests on retempered concrete, ACI Journal, 1929, pp. 25.
  • R P West, "Concrete Retempering without strength loss," Proceedings of colloquium organized on behalf of the coordinating committee for concrete technology of RILEM, Oct 3-5, 1990, pp.134-141.
  • Michel Pigeon, Francois Saucier, and Patrick Plante, "Air-void stability, part IV: Retempering,"
  • ACI Materials Journal, May-June 1990, pp.252-259.
  • I S : 516-1959 "Methods of tests for strength of concrete," Bureau of Indian Standards, New-Delhi.
  • I S : 5816-1999 "Splitting tensile strength of concrete method of test," Bureau of Indian Standards, New-Delhi.
  • Balsubramanain, K. et al, "Impact resistance of steel fiber reinforced concrete," The Indian concrete Journal, May 1996, (pp 257-262).

NBMCW August 2007

.....

Online Strength Monitoring of Ready Mix...

An Application of Operating Characteristic Curves in Online Strength Monitoring of Ready Mixed Concrete

Debasis Sarkar, Lecturer CPM, CEPT University Navrangpura, Ahmedabad, Dr. Manish Thaker, Lecturer in Statistics, Statistics Department M.G. Science Institute, Navrangpura, Ahmadabad.

The operating characteristic (OC) curves measure the performance of a sampling plan. This can be used to find out the producer’s risk and consumer’s risk. In context to commercial ready mixed concrete plants, the producer’s risk is associated with the risk of a good quality concrete being rejected by the client and the consumer’s risk is associated with the risk of accepting a poor quality concrete. Online quality monitoring deals with the monitoring techniques applied during the production of the ready mixed concrete in the RMC plants. 28 days cube compressive strength data of concrete grades have been collected from RMC plants in and around Ahmedabad and attempts have been made to investigate the producer’s risk and consumer’s riskwhich would enable RMC producers and consumers to assure quality levels.

Quality monitoring of ready mixed concrete (RMC) has to be carried out throughout its production process. There are various techniques for online monitoring of the concrete , namely (i) Control charts–Cusum Control charts, Schewart Control charts, EWMA Control charts (ii) Acceptance Sampling (iii) British Ready Mixed Concrete Association (BRMCA) concrete control system. However, if the RMC producers in our country adopts any of the above monitoring techniques, it would enable them to produce and sell quality product at reasonable prices. The operating characteristic curves which measure the performance of a sampling plan can be utilized for finding out the producer’s risk (associated with the risk of a good quality concrete being rejected by the client) and consumer’s risk (associated with the risk of accepting a poor quality concrete). In this paper, an attempt has been made to plot the OC curves for various grades of concrete collected from the RMC plants in and around Ahmedabad. The collected 28 days cube strength data for the various concrete grades produced by these RMC plants have been utilized in finding out the producer’s risk, average quality level, consumer’s risk and limiting quality level from the OC curves plotted. These parameters would provide adequate information about assuring the quality levels for RMC producers and consumers.

Acceptance Sampling Plan

Acceptance sampling is the type of inspection procedure employed when inspection is for the purpose of acceptance or rejection of a product based on adherence to a standard.

In acceptance sampling plans by attribute, a product item is classified as conforming or not, but the degree of conformance is not specified. In acceptance sampling plans by variable the quality characteristic is expressed as a numerical value.

Acceptance sampling can be used as a form of product inspection between companies and their customers, or between departments or divisions within the same company. It does not control or improve the quality level of the process. It is merely a method for determining the disposition of the lot. Acceptance sampling procedures will accept some lots and reject others, even though they are of same quality. There is a risk of rejecting “good” lots or accepting “poor“lots, identified as producer’s risk and consumer’s risk respectively.

Operating Characteristic Curve

Operating characteristic (OC) curve measures the performance of an accepting sampling plan. This curve plots the probability of accepting the lot versus proportion non–conforming. Thus, the OC curve displays the discriminatory power of the sampling plan and it shows the probability that a lot submitted with a certain fraction defective will be either accepted or rejected.

If the producer designs for 97.5% of concrete to be above the specified strength when the theoretical basis of the compliance rules is 95%, the risk of failing on any normal size of contract is acceptably low. By this approach the ready mixed concrete industry runs less risk problems with each individual small contract and also safeguards larger contracts and its overall production Thus the conventional way by which probabilities may be judged by both producer and consumer is through the use of the operating characteristic curve (Figure. 1.0). For any compliance clause, the producer may assess the risk of having complying concrete rejected (producer’s risk) and the consumer can assess the risk of accepting non-complying concrete (consumer’s risk). In the theoretical diagram (Figure. 1.0a), the producer’s risk and consumer’s risk are both nil. In practice, diagrams are usually of the form of Figure 1.0b where both run some risk.

An Application of Operating Characteristic Curves in Online Strength Monitoring of Ready Mixed Concrete
Fig: 1.0 Operating characteristic curves. (a) Ideal (b) Practical solution

Key elements of an OC curve are described as follows.

An Application of Operating Characteristic Curves in Online Strength Monitoring of Ready Mixed Concrete
Fig 2.0: OC curve showing
(AQL , 1 - α) and (LQL , β)
Producer’s risk (α) : This is a risk associated with rejecting a lot of “good” quality. It is generally denoted by "α" Since á is expressed in terms of the probability of non-acceptance, it cannot be located on an OC curve unless it is specified in terms of probability of acceptance. This conversion is given below:

Probability of acceptance (Pa) = 1 - α

Acceptable quality level (AQL): This is a numerical definition of a good lot, associated with the producer’s risk a. Thus AQL is a percent defective that is the base line requirement for the quality of the producer’s product. The producer would prefer the sampling plan to have a high probability of accepting a lot that has a defect level less than or equal to the AQL.

Consumer’s risk (β): This is a risk associated with accepting a lot of "poor" quality. It is generally denoted by "β)"

Limiting quality level (LQL): This is a numerical definition of a poor lot associated with consumer’s risk. Thus the LQL is a designated high defect level that would be unacceptable to the consumer. The consumer would prefer the sampling plan to have a low probability of accepting a lot with a defect level as high as the LQL.

To construct an OC curve, we assume that the process produces a stream of lots and the lot size is large and the probability of non– conforming item is small. Poisson distribution can be used to find out the probability of "x" non - conforming items in the sample (n). This probability is given by:

P(x)= e λx / x!

where x

= 0,1,2, ….. ∞

λ = Average number of non - conforming items in the sample.

= np (where n = sample size and p = proportion of non - conforming items)

The probability of lot acceptance can be found out by using:

Pa = P (x ≤ c) , here 'c' denotes the acceptance number.
An Application of Operating Characteristic Curves in Online Strength Monitoring of Ready Mixed Concrete
OC curve has the following properties:
  1. OC curve in general is continuous in nature
  2. When P = 0 ⇒ Pa = 1 ie. lots with no defectives must always be accepted
  3. When P = 1, ⇒ Pa = 0, ie. lots with all defectives must always be rejected. Probability of acceptance of lots with few defectives can be identified from the OC curve plotted for that particular case.

    All OC curves passes through points (0,1) and (1,0)
  4. As P increases Pa decreases and vice- versa
  5. The points on the OC curve are ( LQL, 1 - α) , ( AQL, β) where a denotes producer’s risk. and b denotes consumer’s risk.
An Application of Operating Characteristic Curves in Online Strength Monitoring of Ready Mixed Concrete
Fig. 3.0 Decision Table defining producers risk and consumers risk of a lot.
The above parameters can also be represented by the decision table Figure 3.0.

Conformity criteria for compressive strength (EN 206- 1:2000)

Conformity assessment shall be made on test results taken during a n assessment period that shall not exceed the last twelve months . Conformity is confirmed if both the criteria given in Table 1.0 for either initial or continuous production are satisfied.

Table : 1.0 : Conformity criteria for compressive strength
ProductionNumber n of test results for compressive strength in the groupCriterion 1Criterion 2
Mean of n results (fcm) N / mm2Any individual test result (fci) N / mm2
Initial3≥ fck  + 4≥ fck - 4
Continuous

Not less than 15

≥ fck + 1.48 σ≥ fck - 4

Current compliance rules of BS 5328

The current strength compliance rules of BS 5328 are:
  1. No result for a batch shall be less than the specified strength less 3 Mpa.
  2. No mean of four consecutive results shall be less than the specified strength plus 3 Mpa.
Where a result is the mean for a pair of cubes from a single batch and tested at 28 days.

Data Collection and Case Study

Data in form of 28 days cube compressive strength results available for a number of grades of concrete, the corresponding mix proportions followed by the RMC plant operating around Ahmedabad were collected. About twenty samples for grade of concrete M20, and about thirty samples for grades M25, M30, M40 have been considered for the analysis of OC curves.

Table 2.0 : Compressive strength data for various concrete grades
Concrete gradeAverage 28 days compressive strength ( Mpa)
M2035.50, 27.80, 35.80, 30.10, 27.60, 32.45, 30.20, 26.85, 31.10, 19.20, 25.86, 31.20, 25.60, 31.15, 35.80 27.50, 28.73, 23.20, 18.95, 24.50, 22.45, 29.80, 35.65, 30.80, 24.01, 25.25, 27.55, 30.15, 24.50, 22.60
M2531.20, 35.86, 31.00, 39.01, 35.60, 38.00, 29.68, 27.26, 30.88, 35.50, 28.88, 38.50, 27.60, 26.00, 37.10, 30.80, 34.45, 38.00, 33.51,35.80, 31.20, 36.52, 29.82, 37.80, 35.01, 36.60, 32.25, 31.50, 28.65, 27.55
M3040.00, 38.30, 45.32, 39.01, 43.50, 44.80, 36.25, 42.20, 30.00,39.70, 36.65, 37.25, 34.00, 29.15, 30.50, 35.30, 29.80, 34.50, 14.20, 45.50, 40.10, 38.20, 37.30, 39.50, 40.10, 42.20, 40.50, 39.80, 38.65, 40.20
M4048.55, 44.50, 56.10, 47.45, 52.50, 51.45, 46.75, 48.85, 53.30, 51.40, 55.65, 52.43, 46.82, 41.03, 41.05, 45.50, 39.90, 39.45, 47.84, 50.15, 52.23, 49.10, 46.63, 48.73, 55.63, 53.70, 45.63, 44.85, 48.80, 47.31

In this paper, an attempt has been made to apply clause BS 5328(ii) to the 28 days compressive strength results for M20 grade concrete, which has resulted in identification of the batches in which “good” quality concrete is being rejected by the client and the batches in which “poor” quality concrete is being accepted by the consumer. The details of this case is demonstrated in Table 3.0. A similar attempt will be made in future papers to understand the application of conformity criteria as per Table 1.0, Criterion 1 (EN- 206-1:2000).

Table 3.0 : Application of BS 5328 clause (ii) "mean of four" to successive results for grade M20 concrete
Cube no.Av. 28 days compressive strength (Mpa)Mean of fourAccept concrete represented byReject concrete represented by
135.50---
227.80---
335.80---
430.1032.301 - 4-
527.6030.332 - 5-
632.4531.493 - 6-
730.2030.014 - 7-
826.8529.285 - 8-
931.1030.156 - 9-
1019.2026.847 - 10-
1125.8625.758 -11-
1231.2026.849 -12-
1325.6025.4710 -13-
1431.1528.4511 - 14-
1535.8030.9312 - 15-
1627.5030.0113 - 16-
1728.7330.7914 - 17-
1823.2028.8115 - 18-
1918.9524.5916 - 19-
2024.5023.8417 - 20-
2122.4522.28-18 - 21
2229.8023.9319 - 22-
2335.6528.1020 - 23-
2430.8029.6821 - 24-
2524.0130.0722 - 25-
2625.2528.9323 - 26-
2727.5526.9024 - 27-
2830.1526.7425 - 28-
2924.5026.8626 - 29-
3022.6026.2027 - 30-

Note: Criterion of acceptance is 20+3 = 23 Mpa for mean of four

For the batch 18-21, result no. 18 (23.20 Mpa), result no. 20 (24.50 Mpa) and result no. 21 (22.45 Mpa) inspite of being above the specified strength (20 Mpa) are being rejected during sampling as the other faulty results of the batch like result no. 19 (18.95 Mpa) together with the good results fail to satisfy the acceptance criteria. Inspite of these strengths being more than the specified strength, but during sampling these three results are being rejected. Thereby the producer has a risk of about three good samples out of a lot of thirty being rejected by the client along with poor quality samples. Thus the producer’s risk can be quantified as 10%. A similar logic can be applied for quantifying consumer’s risk. In the above case one poor sample i.e. result no. 19 (18.95 Mpa) is being accepted by the consumer along with other good samples for a lot of thirty. Thus the consumer’s risk can be quantified as 3.3%.

Similar analysis has been carried out for concrete grades M25, M30, and M40. A comparative statement showing the results of the analysis is represented here.

Table 4.0 : Finding out producer’s risk and consumer’s risk for various concrete grades.
Grade of
Concrete
Number of results more than specified strength in rejected lotPercentage
(%)
Producers risk (α) (%)Number of results less than specified strength in accepted lotConsumers Risk (β) (%)
NumberNumberPercentage (%)
M20031010013.33.3
M25000000000000
M300516.6616.66013.33.3
M40031010000000

From the above Table, it has been observed that the producers risk for the RMC plant under study ranges between to 16.66% and the consumers risk is about 3.3%. An Operating characteristic curve (OC) has been plotted from the data available pertaining to the probability of acceptance (Pa) and proportion nonconforming (p). Poisson Distribution is applied for plotting the OC curve for a reasonably large sample size (n) and small proportion nonconforming (p). The acc eptance number (c) is assumed to be 0 for this analysis. Lot acceptance probabilities for different values of proportion nonconforming for sampling plan n = 30, and c = 0 is analyzed in the Table 5.

Table 5.0 : Lot acceptance probabilities for different values of proportion nonconforming for sampling plan n = 30 , c = 0.
Proportion nonconforming (p)np = λProbability of lot acceptance (Pa) (From Cumulative Poisson Distribution Table as per Appendix 1)
0.000.01.0
0.0050.150.861
0.010.300.740
0.020.600.548
0.030.900.406
0.041.200.301
0.051.500.223
0.061.800.165
0.072.100.122
0.082.400.090
0.092.700.067
0.103.000.049

An Application of Operating Characteristic Curves in Online Strength Monitoring of Ready Mixed Concrete
Fig 4.0: OC curve for M20 Grade Concrete
A plot of Probability of acceptance (Pa) versus Proportion nonconforming (p) for the values obtained from Table 5.0 for concrete grade M20 produces the graph as represented in Figure. 4.0. The values of the producers risk and consumers risk obtained from the analysis of Table 3.0 are represented as comparative statement in Table 4.0. These values of the producers risk and consumers risk can be superimposed on the OC curve to find out the Acceptable quality level (AQL) and Limiting quality level (LQL) for the concrete grade under analysis (M20). Similar analysis can be carried out for concrete grades M25, M30, and M40.

As per Figure. 4.0 and Table 4.0 for M20 grade concrete lot with 10% defectives will be accepted only 3.3 % of times by the consumer (with probability of 0.033). With á = 0.1 the value of AQL obtained from the Figure. 4.0 is 0.0041. Thus with AQL = 0.76% and 1- α= 0.90 lot with 0.76% defectives should then be accepted only 90% of times. Similar analysis for concrete grades M25, M30, and M40 is represented in the Table 6.0.

Table 6.0 : Comparative analysis of producer’s risk, consumer’s risk, acceptable quality level, limiting quality level for various concrete grades.
Grade of concreteProducer’s risk (α)Consumer’s risk (β)Acceptable Quality level(AQL)Limiting Quality level (LQL)
M200.100.0330.00410.106
M250.000.000.001.00
M300.1660.0330.00760.106
M400.100.000.00411.00

As per Table 6.0 analysis of grade of concrete M25 resulted in an ideal situation where the producers risk and consumers risk is zero. Thus the probability of acceptance is 100%. For M30 grade lot with 16.6% defectives will be accepted only 3.3% times by the consumer. For α = 0.166 (16.6%) the AQL obtained from graph (Figure. 4.0) is 0.0076. Thus 1 - α = 0.834, which indicates that lot with 0.76% defectives should then be accepted only 83.4% of times. Analysis of M40 concrete projects a case where a lot with 0.41% defectives had 90% probability of acceptance. The consumers risk α) being nil, the LQL tends to 1.00. Thus a lot with 100% defectives has 0% probability of acceptance. The above analysis of various grades of concrete indicate that the RMC plant operators have a general tendency to reduce the producers risk by following a mix design where the Target Mean Strength (TMS) is obtained from the equation TMS = fck + 2o where fck is 28 days characteristic compressive strength and = Plant Standard Deviation. The design margin is kept as 2o for a confidence level of 97.5%. However, too m u c h overconservative mix design will lead to an uneconomical mix thereby resulting in higher price per cum. of concrete for various grades. A representation of the OC curve by considering the parameters Probability of acceptance Vs Percent within limits (%) is shown in Figure. 5.0.

An Application of Operating Characteristic Curves in Online Strength Monitoring of Ready Mixed Concrete
Fig. 5.0: Ideal limits of α , β , AQL and LQL for concrete grades M20, M25, M30, M40

As per Figure. 5.0 and as per analysis (Table: 7.0) it is suggested that the RMC producers can fix up the AQL = 97.5% which will keep the producers risk (β) within 5% and LQL = 0% which will also restrict the consumers risk (®) within 5%. Thus fixing up the AQL and LQL to the above mentioned limits will develop a practical situation where the chances of a good concrete being rejected by the client and chances of the consumer to accept bad concrete are both restricted to 5%. This criteria can be taken care of during the mix design of the concrete grades.

Table 7.0 : Generalized values of α, β, AQL and LQL for concrete grades M20, M25, M30 , M40.
Producers risk (α)Acceptable quality level (AQL) %Consumers risk(β)Limiting quality level (LQL) %
0.0597.50.050
0.10950.1017.5
0.1591.50.1535
0.20900.2047

Limitations

The major disadvantage of this type of OC curve is that there are no provisions for the AQL and LQL in the input parameters of the Poisson Distribution. In this case, the only input parameters are the sample size (n), acceptance number (c), and proportion nonconforming (p). Also it is not reasonable to assume that the distribution will be the same for any given RMC plant. But when AQL and LQL are specified, it is reasonable to assume that the distribution will vary based on these two limits. In addition to this weakness, it is not reasonable to assume a proportion of nonconforming units of any lot for given RMC producer is unique throughout its production process. In real practice each lot might have different proportion of nonconforming items, which is timated from each lot data.

Conclusions

The OC curve is a widely accepted tool to quantify the producers risk and consumers risk. Sampling plans with large sample sizes are better able to discriminate between acceptable and unacceptable quality. Therefore, fewer lots of unacceptable quality are accepted and fewer lots of acceptable quality are rejected. As per the analysis done in this paper, it is suggested that the RMC producers ideally maintains the Acceptable quality level (AQL) = 97.5% and Limiting quality level (LQL) = 0%, which will restrict the producers risk and consumers risk to 5%. This will also ensure that unnecessarily there is no requirement of over conservative mix design for the concrete grades which will definitely make the grade prices more economical. Thus a good compliance scheme should ensure that the producers risk and consumers risk are at an acceptable level and are properly distributed between the producer and the consumer. Here lies the importance of OC curves which can be used to evaluate the desired compliance scheme.

References

  1. Neville, A.M. and Brooks, J.J. Concrete Technology ELBS with Longman, 1994.
  2. Dewar, J.D. and Anderson, R. Manual of Ready Mixed Concrete Blakie and Son Ltd, Glasgow and London, 1988.
  3. Miller, I and Freund, J.E. Probability and Statistics for Engineers Pearson Education (Singapore) Pte. Ltd., 2001.
  4. Mitra, A. Fundamentals of quality control and improvement Pearson Education (Singapore) Pte. Ltd.,2004.
  5. Montgomery, D.C. Introduction to Statistical Quality Control John Wiley & Sons, Inc. 1985. Indian standard code of practice for plain and reinforced concrete, IS 456: 2000, Bureau of Indian Standard, New Delhi.
APPENDIX 1 : Cumulative Poisson Distribution
λ
x0.010.050.100.200.300.400.500.60
00.9900.9510.9040.8180.7040.6700.6060.548
10.9990.9980.9950.9820.9630.9380.9090.878
20.9990.9990.9980.9960.9920.9850.976
λ
x0.700.800.901.001.101.201.301.40
00.4960.4490.4060.3670.3320.3010.2720.246
10.8440.8080.7720.7350.6990.6620.6260.591
20.9650.9520.9370.9190.9000.8790.8570.833
λ
x1.501.601.701.801.902.002.102.20
00.2230.2010.1820.1650.1490.1350.1220.110
10.5570.5240.4930.4620.4330.4060.3790.354
20.8080.7830.7570.7300.7030.6760.6490.622
λ
x2.302.402.502.602.702.802.903.00
00.1000.0900.0820.0740.0670.0600.0550.049
10.3300.3080.2870.2670.2480.2310.2140.199
20.5960.5690.5430.5180.4930.4690.4450.423
λ
x3.504.004.505.005.506.006.507.00
00.0300.0180.0110.0060.0040.0020.0010.000
10.1350.0910.0610.0400.0260.0170.0110.007
20.3200.2380.1730.1240.0880.0610.0430.029
( Adapted from Introduction to Statistical Quality Control , Douglas C. Montgomery, Appendix I pp. 498 – 499 )

NBMCW June 2007

.....

Use of RECYCLED AGGREGATES In CONCRETE-...

S. K. Singh, Scientist, Structural Engineering Division, Central Building Research Institute, Roorkee and P. C. Sharma, Head ( Retd.), Material Sciences, SERC,(G) and Editor New Building Materials & Construction World, New Delhi, Chairman, Indian Concrete Instt. UP Gaziabad Centre.

RECYCLED AGGREGATES In CONCRETE
One of the major challenges of our present society is the protection of environment. Some of the important elements in this respect are the reduction of the consumption of energy and natural raw materials and consumption of waste materials. These topics are getting considerable attention under sustainable development nowadays. The use of recycled aggregates from construction and demolition wastes is showing prospective application in construction as alternative to primary (natural) aggregates. It conserves natural resources and reduces the space required for the landfill disposal.

This paper presents the experimental results of recycled coarse aggregate concrete and results are compared with the natural crushed aggregate concrete. The fine aggregate used in the concrete, i.e. recycled and conventional is 100 percent natural. The recycled aggregate are collected from four sources all demolished structures. For both types of concrete i.e. M-20 and M-25, w/c ratio, maximum size of aggregate and mix proportion are kept constant.

The development of compressive strength of recycled aggregate concrete at the age of 1,3,7,14,28, 56, and 90 days; the development of tensile & flexural strength at the age of 1,3,7,14 and static modulus of elasticity at the age of 28 days are investigated. The results shows the compressive, tensile and flexural strengths of recycled aggregate are on average 85% to 95% of the natural aggregate concrete. The durability parameters are also investigated for recycled aggregate concrete and are found to be in good agreement with BIS specifications.

Introduction

Any construction activity requires several materials such as concrete, steel, brick, stone, glass, clay, mud, wood, and so on. However, the cement concrete remains the main construction material used in construction industries. For its suitability and adaptability with respect to the changing environment, the concrete must be such that it can conserve resources, protect the environment, economize and lead to proper utilization of energy. To achieve this, major emphasis must be laid on the use of wastes and byproducts in cement and concrete used for new constructions. The utilization of recycled aggregate is particularly very promising as 75 per cent of concrete is made of aggregates. In that case, the aggregates considered are slag, power plant wastes, recycled concrete, mining and quarrying wastes, waste glass, incinerator residue, red mud, burnt clay, sawdust, combustor ash and foundry sand. The enormous quantities of demolished concrete are available at various construction sites, which are now posing a serious problem of disposal in urban areas. This can easily be recycled as aggregate and used in concrete. Research & Development activities have been taken up all over the world for proving its feasibility, economic viability and cost effectiveness.

RECYCLED AGGREGATES In CONCRETE
An investigation conducted by the environmental resources ltd. (1979) for European Environmental commission (EEC) envisages that there will be enormous increase in the available quantities of construction and demolition concrete waste from 55 million tons in 1980 to 302 million tons by the year 2020 in the EEC member countries. As a whole, the safety and environment regulations are becoming stringent, demand for improvement in techniques & efficiency of the past demolition methods is getting pronounced. Special rules and regulations concerning the demolition have already been introduced in several countries like U.K., Holland and Japan.

The main reasons for increase of volume of demolition concrete / masonry waste are as follows:-
  1. Many old buildings, concrete pavements, bridges and other structures have overcome their age and limit of use due to structural deterioration beyond repairs and need to be demolished;
  2. The structures, even adequate to use are under demolition because they are not serving the needs in present scenario;
  3. New construction for better economic growth;
  4. Structures are turned into debris resulting from natural disasters like earthquake, cyclone and floods etc.
  5. Creation of building waste resulting from manmade disaster/war.
In study conducted by authors for RCC buildings, the approximate percentage of various construction materials in demolition waste is presented in Fig. 1. This may vary depending upon the type of structure.

In many densely populated countries of Europe, where disposal of debris problem is becoming more and more difficult, the recycling of demolition waste has already been started. As per the survey conducted by European Demolition Association (EDA) in 1992, the several recycling plants were operational in European countries such as 60 in Belgium, 50 in France, 70 in the Netherlands, 120 in United Kingdom, 220 in Germany, 20 in Denmark and 43 in Italy. The recycling of construction & demolition waste becomes easy & economical, wherever combined project involving demolition and new construction are taken up simultaneously. The possible uses of construction and demolition wastes are given in Table 1.

Recycling and Reuse of Construction & Demolition Wastes in Concrete

RECYCLED AGGREGATES In CONCRETE
The recycling and reuse of construction & demolition wastes seems feasible solution in rehabilitation and new constructions after the natural disaster or demolition of old structures. This becomes very important especially for those countries where national and local policies are stringent for disposal of construction and demolition wastes with guidance, penalties, levies etc. A typical lay out plan of recycling plant for construction waste has been shown in Figure. 2. The properties of recycled aggregate concrete obtained by various authors are given in Table2.

International Status

RECYCLED AGGREGATES In CONCRETE
The extensive research on recycled concrete aggregate and recycled aggregate concrete (RAC) as started from year 1945 in various part of the world after second world war, but in a fragmented manner. First effort has been made by Nixon in 1977 who complied all the work on recycled aggregate carried out between 1945-1977 and prepared a state-of-the-art report on it for RILEM technical committee 37-DRC. Nixon concluded that a number of researchers have examined the basic properties of concrete in which the aggregate is the product of crushing another concrete, where other concentrated on old laboratory specimens. However, a comprehensive state-of-the-artdocument on the recycled aggregate concrete has been presented by Hansen & others in 1992 in which detailed analysis of data has been made, leading towards preparation of guidelines for production and utilization of recycled aggregate concrete.

It has been estimated that approximately 180 million tones of construction & demolition waste are produced each year in European Union. In general, in EU, 500 Kg of construction rubble and demolition waste correspond annually to each citizen. Indicatively 10% of used aggregates in UK are RCA, whereas 78,000 tons of RCA were used in Holland in 1994. The Netherland produces about 14million tons of buildings and demolition wastes per annum in which about 8 million tons are recycled mainly for unbound road base courses.

RECYCLED AGGREGATES In CONCRETE
The 285 million tons of per annum construction waste produced in Germany, out of which 77 million tons are demolition waste. Approximately 70% of it is recycled and reused in new construction work. It has been estimated that approximately 13 million tons of concrete is demolished in France every year whereas in Japan total quantity of concrete debris is in the tune of 10-15 million tons each year. The Hong Kong generates about 20 million tons demolition debris per year and facing serious problem for its disposal.

USA is utilizing approximately 2.7 billion tons of aggregate annually out of which 30-40% are used in road works and balance in structural concrete work. A recent report of Federal Highways Administration, USA refers to the relative experience from European data on the subject of concrete and asphalt pavement recycling as given in Table 3.The rapid development in research on the use of RCA for the production of new concrete has also led to the production of concrete of high strength/performance.

Indian Status

There is severe shortage of infrastructural facilities like houses, hospitals, roads etc. in India and large quantities of construction materials for creating these facilities are needed. The planning Commission allocated approximately 50% of capital outlay for infrastructure development in successive 10th & 11th five year plans. Rapid infrastructural development such highways, airports etc. and growing demand for housing has led to scarcity & rise in cost of construction materials. Most of waste materials produced by demolished structures disposed off by dumping them as land fill. Dumping of wastes on land is causing shortage of dumping place in urban areas. Therefore, it is necessary to start recycling and re-use of demolition concrete waste to save environment, cost and energy.

Central Pollution Control Board has estimated current quantum of solid waste generation in India to the tune of 48 million tons per annum out of which, waste from construction industry only accounts for more than 25%. Management of such high quantum of waste puts enormous pressure on solid waste management system.

In view of significant role of recycled construction material and technology in the development of urban infrastructure, TIFAC has conducted a techno-market survey on 'Utilization of Waste from Construction Industry' targeting housing /building and road segment. The total quantum of waste from construction industry is estimated to be 12 to 14.7 million tons per annum out of which 7-8 million tons are concrete and brick waste. According to findings of survey, 70% of the respondent have given the reason for not adopting recycling of waste from Construction Industry is "Not aware of the recycling techniques" while remaining 30% have indicated that they are not even aware of recycling possibilities. Further, the user agencies/ industries pointed out that presently, the BIS and other codal provisions do not provide the specifications for use of recycled product in the construction activities.

In view of above, there is urgent need to take following measures:-
  • Sensitization/ dissemination/ capacity building towards utilization of construction & demolition waste.
  • Preparation and implementation of techno-legal regime including legislations, guidance, penalties etc. for disposal of building & construction waste.
  • Delineation of dumping areas for pre-selection, treatment, transport of RCA.
  • National level support on research studies on RCA.
  • Preparation of techno-financial regime, financial support for introducing RCA in construction including assistance in transportation, establishing recycling plant etc.
  • Preparation of data base on utilization of RCA.
  • Formulation of guidelines, specifications and codal provisions.
  • Preparation of list of experts available in this field who can provide knowhow and technology on totality basis.
  • Incentives on using recycled aggregate concrete-subsidy or tax exemptions.
Realising the future & national importance of recycled aggregate concrete in construction, SERC, Ghaziabad had taken up a pilot R&D project on Recycling and Reuse of Demolition and Construction Wastes in Concrete for Low Rise and Low Cost Buildings in mid nineties with the aim of developing techniques/ methodologies for use recycled aggregate concrete in construction. The experimental investigations were carried out in Mat Science laboratory and Institutes around Delhi/GBD to evaluate the mechanical properties and durability parameters of recycled aggregate concrete made with recycled coarse aggregate collected from different sources. Also, the suitability in construction of buildings has been studied.

The properties of RAC has been established and demonstrated through several experimental and field projects successfully. It has been concluded that RCA can be readily used in construction of low rise buildings, concrete paving blocks & tiles, flooring, retaining walls, approach lanes, sewerage structures, subbase course of pavement, drainage layer in highways, dry lean concrete(DLC) etc. in Indian scenario. Use of RCA will further ensure the sustainable development of society with savings in natural resources, materials and energy.

Experimental Investigations

In the present paper, an endeavor is made so as to compare some of the mechanical properties of recycled aggregate concrete (RAC) with the natural aggregate concrete (NAC). Since the enormous quantity of concrete is available for recycling from demolished concrete structures, field demolished concrete is used in the present study to produce the recycled aggregates. The concrete debris were collected from different (four) sources with the age ranging from 2 to 40 years old and broken into the pieces of approximately 80 mm size with the help of hammer & drilling machine. The foreign matters were sorted out from the pieces. Further, those pieces were crushed in a lab jaw crusher and mechanically sieved through sieve of 4.75 mm to remove the finer particles. The recycled coarse aggregates were washed to remove dirt, dust etc. and collected for use in concrete mix. The fine aggregate were separated out, and used for masonry mortar & lean concrete mixes, which is not part this reported study. But these were found to suit for normal brick masonary mortar and had normal setting and enough strength for masonary work.

Concrete Mixes

The two different mix proportions of characteristic strength of 20 N/ mm2 (M 20) and 25 N/mm2 (M 25) commonly used in construction of low rise buildings are obtained as per IS 10262 – 1982 or both recycled aggregate concrete and natural aggregate concrete. Due to the higher water absorption capacity of RCA as compared to natural aggregate, both the aggregates are maintained at saturated surface dry (SSD) conditions before mixing operations. The proportions of the ingredients constituting the concrete mixes are 1:1.5:2.9 and 1:1.2:2.4 with water cement ratio 0.50 & 0.45 respectively for M-20 & M-25 grade concrete. The ordinary Portland cement of 43 grade and natural fine aggregates (Haldwani sand) are used throughout the casting work. The maximum size of coarse aggregate used was 20 mm in both recycled and natural aggregate concrete.

The total two mixes were cast using natural aggregate and eight mixes were cast using four type of recycled aggregate concrete for M-20 & M-25. The development of compressive strength is monitored by testing the 150-mm cubes at 1, 3, 7, 14, 28, 56 and 90 days. In one set 39 cubes were cast for each mix. The cylinder strength and corresponding strain & modulus of elasticity were measured in standard cylinder of 150x300 mm size at the age of 28 days. The prism of size 150x150x700 mm and cylinder of size 150x300mm were cast from the same batches to measure Flexural strength and splitting tensile strength respectively. This paper reports the results of experimental investigations on recycled aggregate concrete.

Properties of Recycled Concrete Aggregate

Particle Size Distribution

The result of sieve analysis carried out as per IS 2386 for different types of crushed recycled concrete aggregate and natural aggregates. It is found that recycled coarse aggregate are reduced to various sizes during the process of crushing and sieving (by a sieve of 4.75mm), which gives best particle size distribution. The amount of fine particles (<4.75mm) after recycling of demolished were in the order of 5-20% depending upon the original grade of demolished concrete. The best quality natural aggregate can obtained by primary, secondary & tertiary crushing whereas the same can be obtained after primary & secondary crushing incase of recycled aggregate. The single crushing process is also effective in the case of recycled aggregate.

The particle shape analysis of recycled aggregate indicates similar particle shape of natural aggregate obtained from crushed rock. The recycled aggregate generally meets all the standard requirements of aggregate used in concrete.

Specific Gravity and Water Absorption

The specific gravity (saturated surface dry condition) of recycled concrete aggregate was found from 2.35 to 2.58 which are lower as compared to natural aggregates. Since the RCA from demolished concrete consist of crushed stone aggregate with old mortar adhering to it, the water absorption ranges from 3.05% to 7.40%, which is relatively higher than that of the natural aggregates. The Table 4 gives the details of properties of RCA & natural aggregates. In general, as the water absorption characteristics of recycled aggregates are higher, it is advisable to maintain saturated surface dry (SSD) conditions of aggregate before start of the mixing operations.

Bulk Density

The rodded & loose bulk density of recycled aggregate is lower than that of natural aggregate except recycled aggregate-RCA4, which is obtained from demolished newly constructed culvert. Recycled aggregate had passed through the sieve of 4.75mm due to which voids increased in rodded condition. The lower value of loose bulk density of recycled aggregate may be attributed to its higher porosity than that of natural aggregate.

Crushing and Impact Values

The recycled aggregate is relatively weaker than the natural aggregate against mechanical actions. As per IS 2386, the crushing and impact values for concrete wearing surfaces should not exceed 45% and 50% respectively. The crushing & impact values of recycled aggregate satisfy the BIS specifications except RCA2 type of recycled aggregate for impact value as originally it is low grade rubbles.

Compressive Strength

The average compressive strengths cubes cast are determined as per IS 516 using RCA and natural aggregate at the age 1, 3, 7, 14, 28, 56 and 90 days and reported in Table 5. The table 4 shows that the target cube strength was achieved at 28 days for all types of concrete. As expected, the compressive strength of RAC is lower than the conventional concrete made from similar mix proportions. The reduction in strength of RAC as compare to NAC is in order of 2- 14% and 7.5 to 16% for M-20 & M-25 concretes respectively. The amount of reduction in strength depends on parameters such as grade of demolished concrete, replacement ratio, w/c ratio, processing of recycled aggregate etc.

Splitting Tensile & Flexural Strength

The average splitting tensile and flexural of recycled aggregate are determined at the age 1, 3, 7, 14, & 28 days varies from 0.30 -3.1 MPa and 0.95- 7.2 MPa respectively. The reduction in splitting and flexural strength of RAC as compared to NAC is in order of 5-12% and 4 -15% respectively.

Modulus of Elasticity

The static modulus of elasticity of RAC has been reported in Table 4 and found lower than the AC. The reduction is up to 15% .The reason for the lower static modulus of elasticity of RCA is higher proportion of hardened cement paste. It is well establish that Ec depends on Ec value of coarse aggregate, w/c ratio & cement paste etc. The modulus of elasticity is critical parameter for designing the structures, hence more studies are needed.

Durability

The following parameters were studied to assess the influence of recycled aggregates on durability of concrete:

Carbonation

Freeze-Thaw Resistance

Carbonation

RECYCLED AGGREGATES In CONCRETE
CO2 from the air penetrates into the concrete by diffusion process. The pores (pore size>100nm) in the concrete in which this transport process can take place are therefore particularly crucial for the rate of carbonation. The carbonation tests were carried out for 90 days on the specimens (150x150x150mm) of recycled aggregate concrete and natural aggregate concrete in carbonation chamber with relative humidity of 70% and 20% CO2 concentration. The carbonation depths of recycled aggregate concretes for different grade were found from 11.5 to 14mm as compared to 11mm depth for natural aggregate concrete. This increase in the carbonation depth of RAC as compared to NAC, attributed to porous recycled aggregate due to presence of old mortar attached to the crushed stone aggregate.

Freeze-Thaw Resistance

In the freeze-thaw resistance test (cube method), loss of mass of the concrete made with recycled aggregate was found sometimes above and below than that of concrete made with natural aggregate. The results were so close that no difference in freeze thaw resistance (after 100 cycles) could be found. The literature also found that the effect of cement mortar adhering to the original aggregate in RAC may not adversely affect the properties of RAC.

Obstacles in Use of RCA & RAC

The acceptability of recycled aggregate is impeded for structural applications due to the technical problems associated with it such as weak interfacial transition zones between cement paste and aggregate, porosity and transverse cracks within demolished concrete, high level of sulphate and chloride contents, impurity, cement remains, poor grading, and large variation in quality.

Although, it is environmentally & economically beneficial to use RCA in construction, however the current legislation and experience are not adequate to support and encourage recycling of construction & demolished waste in India. Lack of awareness, guidelines, specifications, standards, data base of utilization of RCA in concrete and lack of confidence in engineers, researchers and user agencies is major cause for poor utilization of RCA in construction. If the Govt wishes these obstacles can easily be removed.

Conclusion

RECYCLED AGGREGATES In CONCRETE
Recycling and reuse of building wastes have been found to be an appropriate solution to the problems of dumping hundred of thousands tons of debris accompanied with shortage of natural aggregates. The use of recycled aggregates in concrete prove to be a valuable building materials in technical, environment and economical respect

Recycled aggregate posses relatively lower bulk density, crushing and impact values and higher water absorption as compared to natural aggregate. The compressive strength of recycled aggregate concrete is relatively lower up to 15% than natural aggregate concrete. The variation also depends on the original concrete from which the aggregates have been obtained. The durability parameters studied at SERC(G) confirms suitability of RCA & RAC in making durable concrete structures of selected types.

There are several reliable applications for using recycled coarse aggregate in construction. However, more research and initiation of pilot project for application of RCA is needed for modifying our design codes, specifications and procedure for use of recycled aggregate concrete. The subject of use of RCA in construction works in India should be given impetus, because of big infrastructural projects are being commissioned including Common Wealth Games in 2010.

References

  1. Hansen, T.C. (1992), "Recycling of Demolished Concrete Masonry, Rilem Report No. 6, E&FN Spon, London, Great Britain, pp. 316.
  2. Oikonomou,N.D.(2005)"Recycled Concrete Aggregates," Cement & Concrete Composites, Vol. 27, pp315-318.
  3. Thielen,G.(2004)"Concrete Technology Reports 2001- 2003,"German Cement Works Association.
  4. US Deptt. of Transportation (2000) "Recycled Materials in European Highways Environment-Uses, Technologies and Policies," Int. Technology Exchange Programme.
  5. Biojen,J. (1996) "Waste Materials and Alternative Products "Pro's and Con's" Concrete for Environmental enhanced and Protection, E & FN Spon, pp. 587-598.
  6. Buchner, S. and Scholten, L.J. (1992). "Demolition and Construction Debris Recycling in Europe," European Demolition Association (EDA).
  7. Ferguson, J.; Kermode, O.N.; Nash, C.L.; Sketch, W.A.J. and Huxford, R.P. (1995), "Managing and Minimising Construction Waste," Institution of Civil Engineers, Thomas, Telford Publications, U.K., pp. 1-60.
  8. Gottfredsen, F.R. and Thogerson,F. (1994), "Recycling of Concrete in Aggressive Environment," Demolition and Reuse of Concrete and Masonry; Rilem Proceeding 23, E & FN Spon, pp. 309-317.
  9. Hansen, T.C. (1986) "Recycled Aggregate and Recycled Aggregate Concrete, Seocnd state of Art Report, Development 1945–1985," Rilem TC-DRC, Material & Structure, Vol. 19, No. III. pp. 201- 248.
  10. Hendricks, Ch.F. (1996), "Recycling and Reuse as a Basis of Sustainable Development in Construction Industry," Concrete for Environment, Enhancement and Protection, E&FN Spon, pp. 43-54.
  11. Kikuchi, M. and Yasunaga, A. (1994), "The Total Evaluation of Recycled Aggregate and Recycled Concrete" Demolition and Reuse of Concrete and Masonry, Rilem Proceedings 23, E&FN Spon, pp. 367-377.
  12. Lauritzen, E.K. (1994), "Introduction," Disaster Planning, Structural Assessment, Demolition and Recycling, Rilem Report No. 9, E&FN Spon pp.1 –10.
  13. Mc Laughliu, J. (1993), "A Review of the Prospect for Greater Use of Recycled and Secondary Aggregate in Concrete," Concrete, The Concrete Society Journal, Vol. 27, NO. 6,pp. 16-18.
  14. Merlet, J.D. and Pimienta, P. (1994), "Mechanical and Physico- Chemical Properties of Concrete Produced with Coarse and Fine Recycled Concrete Aggregates," Demolition and Reuse of Concrete and Masonry, Rilem Proceeding 23, E&FN Spon, pp. 343-353.
  15. Nikon, P.J. (1986), "Recycled Concrete an Aggregate for Concrete–a Review," Rilem TC-37, DRC, Materials Structures, Vol. 19, No. 111.
  16. Pauw, C.D. (1994), "Reuse of Building Materials and Disposal of Structural Waste Material," Disaster Planning, Structural Assessment, Demolition and Recycling, Rilem Report 9, E&FN Spon, pp. 133-159.
  17. RILEM TC 121 DRG Recommendation (1994), "Specification for Concrete with Recycled Aggregates," Materials and Structure, Vo. 27, No. 173, pp. 557- 559.
  18. Singh, S.K., Sharma, P.C., and Nagraj, N. (1997), "State-of-Art Report on Recycled Aggregate Concrete," SERC Report, Ghaziabad.
  19. Sharma, P.C., Singh, S.K. and Nagraj, N. (1998), "Future of Recycled Aggregate Concrete in India," National Seminar on New Materials and Technology in Building Industry, July 24-25, Vigyan Bhawan,New Delhi, pp. IV-197-IV- 205.
  20. Singh, S. K. and P. C. Sharma (1998)"Recycling and Reuse of Building Waste in Constructions- A Review," All India Seminar on Concrete for Infrastructural Development, Roorkee, pp 317-329.
  21. Tavakoli, M. and Soroushian, P. (1996), "Strength of Recycled Aggregate Concrete made using Field Demolished Concrete as Aggregate," ACI Materials Journal, Vol. 93, No.2, pp.182-190.
  22. Tavakoli, M. and Soroushian, P.(1996), "Drying Shrinkage Behavior of Recycled Aggregate Concrete," Concrete International, Vol. 18, No. 11, pp. 58-61.
  23. Vyncke, J. Rousseau, E. (1994), "Recycling and Construction and Demolition Waste in Belgium : Actual Situation and Future Evaluation," Demolition and Reuse of Concrete & Masonry, Rilem Proceeding 23, E&FN Spon, pp. 57- 69.
  24. Yogishita, F. et al. (1994), "Behavior of Reinforced Concrete Beams containing Recycled Coarse Aggregate" Demolition and Reuse of Concrete & Masonry Rilem Proceeding 23, E&FN Spon, pp. 331-342.
  25. Yangani, K., Hisaka, M. and Kasai, Y. (1994), "Physical Properties of Recycled Concrete using Recycled Coarse Aggregate made of Construction with Finishing Mater4ials," Demolition and Reuse of Concrete & Masonry, Rilem Proceeding 23, E&FN Spon, pp. 379-390.
  26. Sharma, P.C., Nagraj, N.(1999), "Recycled Aggregate Concrete and Its Importance in Indian Conditions"– All India Seminar on Indian Cement Industries : Challenges and Prospects of Cement" Chandrapur (Maharashtra)
  27. Ramammurthy, K. & Gumaste, K.S.(1998), "Properties of Recycled Aggregate Concrete," Indian Concrete Journal, pp. 49-53.
  28. Rahal, K. (2007) "Mechnical Properties of Concrete with Recycled Coarse Aggregate," Building & Environment,Vol. 42, pp 407-415.

NBMCW October 2007

.....

The Influence of Flyash Addition on Fre...

Shweta Goyal, Lecturer, Thapar University, Maneek Kumar, Head, Civil Engineering Department, Thapar University, Patiala, Professor Bishwajit Bhattacharjee, Head, Civil Engineering Department IIT Delhi.
This paper deals with the effect of granular characteristics of mineral admixtures like silica fume and flyash added in binary or ternary combinations on the water requirement of resultant concrete. The role of superplasticizers in modifying the rheology has been investigated. Superplasticizers are the admixtures that are added to concrete in very small dosages and modify the water requirement of resultant mix and improve fresh properties of concrete.

Measurement of workability is made by slump test and Vee-bee time test in order to have the correlation between the two and amount of compaction achieved is studied by measuring fresh density of concrete. It is found that superplasticizers become necessary with the reduction of water binder ratio and flyash and silica fume affect the fresh concrete in opposite ways. Also, the relation that exists between slump and Vee–bee time for normal concrete without superplasticizers does not remain valid for concrete having mineral admixtures and superplasticizers.

Introduction

The use of high range water reducers (superplasticizers), condensed silica fume and other fine mineral admixtures have lead to the production of high-strength concrete [1]. Mineral admixtures are used in order to increase strength and improve durability of concrete. Blast furnace slag, flyash and silica fume are some of the mineral admixtures used in varying proportions to achieve the desired results. The mineral admixtures also affect the properties in fresh state, which are directly related to development of strength and durability of hardened concrete. Economics (not always) and environmental considerations have also had a role in the growth of mineral admixture usage.

Much research has been conducted for improving both fresh and hardened properties by using various mineral admixtures. It is reported that fly ash contributes to increase flowability in the fresh state, a dense microstructure and develop higher mechanical properties at the later stage due to the pozzolanic reaction [2,3]. Silica fume, on the other hand, has very fine particles–average particle size is less than 1Fm, which decreases the flowability in fresh state of concrete although, provides a dense microstructure and improved mechanical properties at early stages due to fast pozzolanic reaction [4, 5]. Silica fume is considered to be most efficient in contributing towards both early and later age properties of concrete. However, in India, silica fume comes under the category of costly materials, whereas flyash is abundant in our country and its production is increasing day by day. In the study undertaken, silica fume and flyash are used in combination to see the effect on improvement in fresh properties.

It is widely known that better fluidity is achieved by addition of superplasticizer. The increase of superplasticizer in concrete began in 1960s and has proved to be a milestone in concrete technology and in the field of construction [6]. There is no doubt that the use of admixtures had a profound impact on the concrete practices in India during the last few years [7]. The superplasticizer is adsorbed on the cement particles, which deflocculates and separate, releasing trapped water from cement flocks [8]. Currently available superplasticizers are micro molecular organic agents which are often divided into four groups according to their chemical contents as sulphonate melamine formaldehyde, sulphonate napthalene formaldehyde, modified lignosulphonates and copolymers containing sulphonic and carboxyl groups [9]. The family of superplasticizers based on polycarboxylic products is more recent (1980s). These materials are of higher reactivity; they do not contain the sulphonic group and are totally ionized in alkaline environment. These do not have the side effect of delaying the curing of concrete [10]. In the present study, poly-carboxylic group based superplasticizer is used as a chemical admixture.

It is believed that admixtures mainly affect the flow behavior of cement paste and do not alter the behavior of aggregates. Therefore, in most of the studies on concrete rheology and selection of chemical admixtures, tests on cement pastes have been conducted [1, 3, 8]. The results are then related to concrete workability. Unfortunately, the relation between cement paste rheology and concrete rheology has never been completely established [11]. The main reason behind it is that cement rheology is typically measured under conditions that are never experienced by cement paste in concrete. The values that are usually reported in literature do not take into account the contribution of aggregates [12]. The aggregates act as heat sink and shear the cement paste during mixing process. Therefore, in order to predict concrete rheology accurately, the tests are directly conducted on concrete. For this, one of the most commonly used methods for measuring concrete workability, i.e. slump cone test, is used.
Influence of Flyash Addition on Fresh Properties of Silica Fume Concrete
Slump cone test is the typically quantified field test for measuring concrete workability. However, in a survey conducted by National Ready Mix Concrete Association (NRMCA) and the National Institute of Standards and Technology (NIST) [13], it is determined that slump cone is not representative of the ease of handling high performance concrete in field, because in slump cone test, concrete does not undergo the same treatment as is met in the field. Therefore, along with the slump cone test, Vee–bee time is also noted, because in this test, concrete experience almost same vibrations as experienced in field.

The objective of the study is to look at the rheological characteristics of concrete which has silica fume and fly ash present either as binary or ternary combination with ordinary Portland cement. Secondly, the validity of existing relation between slump and Vee–bee time is checked for the mineral admixture concrete containing superplasticizers.

Materials

Influence of Flyash Addition on Fresh Properties of Silica Fume Concrete
Influence of Flyash Addition on Fresh Properties of Silica Fume Concrete
Cementitious material
ASTM Type I Portland cement is used in this study. Its chemical composition is given in Table 1. The chemical and physical characteristics of two mineral admixtures silica fume and flyash can be seen in this table.

Aggregates

Crushed granite with a maximum nominal size of 10 mm was used as coarse aggregate and natural riverbed sand confirming to Zone II with a fineness modulus of 2.52 was used as fine aggregate. The properties of aggregates are listed in Table 2.

Superplasticizer

Poly-carboxylic group based superplasticizer, Structro 100 (a product of Fosroc chemicals), is used throughout the investigation. This group maintains the electrostatic charge on the cement particles and prevents flocculation by adsorption on the surface of cement particles [14]. It is a light yellow colored liquid complying with requirements of IS 9103 – 79, BS 5075 Part III and ASTM – C494 Type F. The specific gravity of superplasticizer is 1.2 and solid content is 40 percent by mass.

Mixture Details and Preparations

To explore the effect of superplasticizer, the rheological properties are studied for three water binder ratios: 0.25, 0.35 and 0.45. The three series obtained from three water binder ratios are designated as M1, M2 and M3 respectively for water binder ratios of 0.25, 0.35 and 0.45. The quantity of mineral admixtures is varied from 0 to 30 percent and is used either in a binary or a ternary combination. The mix designs used in the study are shown in Figure 1 and the mix details of specimens are listed in Table 3 and Table 4.

Influence of Flyash Addition on Fresh Properties of Silica Fume Concrete
The mix preparation is very important because it influences its rheological behavior. The following procedure was adopted for mixing.

The cementitious materials (Portland cement, silica fume and flyash) were mixed together separately in a container. Coarse aggregates and the fine aggregates were mixed in a mixer rotated at slow speed of about 140 rev./min. for 1 minute. The cementitious material was then put in the mixing drum and the resultant mixture was dry mixed for one minute followed by addition of half of the total water content during the next one-minute mixing. The remaining water along with superplasticizer was then added and mixed at high speed of about 285 revolutions per minute for 1.5 minutes or till the uniform and homogeneous mix is achieved. (Superplasticizer was taken as percentage by mass of binder which included cement, silica fume and flyash if any present. Water content of superplasticizer was taken into account when calculating the total water content of the mix [15].)

The prepared mix is used for obtaining slump and Vee–bee time. In all 24 mixes are prepared and three determinations of slump and Vee–bee time are made for each sample and the mean value is taken. It is worth mentioning at this stage that for the selected dose of superplasticizer, no segregation was observed at any stage.

Results and Discussions

Influence of Flyash Addition on Fresh Properties of Silica Fume Concrete
For each of the mix, the superplasticizer dose is given step increments and the corresponding Vee–bee time and slump is noted. The saturation point is obtained from the slump verses superplasticizer dosage curves; and is taken as that value of superplasticizer beyond which it will not increase the slump with any further increase in dosage. (In other words, superplasticizer has no further plasticising effect). The results of these tests are presented in Figure 2 to 4 where slump is plotted against superplasticizer dosage and in Figure 5 where optimum superplasticizer dosage is plotted against water binder ratio. The nomenclature of mixes used is already presented in Table 4. The results are discussed as below.

Effect on Mineral Admixtures on Rheological Properties

The effect of the addition of a mineral admixture is detected by an increase in the slump or a reduction of water content or a reduction of superplasticizer dosage needed to obtain the same slump. The results are represented in Figure 2 to 4 in which the variation of slump is plotted as a function of superplasticizer dosage for three series of water binder ratios studied.

(a) OPC – SF System

Influence of Flyash Addition on Fresh Properties of Silica Fume Concrete
For the same water binder ratio, with increase in silica fume content in concrete, the value of lump decreases and hence the optimum superplasticizer dosage increases, which can be attributed to high specific surface of silica fume with an average particle size of 0.1Fm. However, this is not the sole factor affecting the increase in superplasticizer demand for silica fume mixture. Long with the high specific surface area, the particles of silica fume are chemically highly reactive and have affinity for multilayer adsorption of superplasticizer molecules, which is also supported by other researchers [16, 17]. As a result, with increase in silica fume percentage, the quantity of superplasticizers in the concrete system decreases leading to steep increase in the superplasticizer dosage. The same type of behavior is observed for entire water binder range with an exception for water binder ratio of 0.25. At this ratio, with the addition of 5% silica fume, the optimum dosage of superplasticizer decreased by a small amount from 4% (for control mix) to 3.75%. This reverse trend can be explained by considering the dispersion action of flocculated cement particles by silica fume particles in combination with superplasticizer. Actually, the effectiveness of superplasticizer is enhanced in the presence of silica fume [18]. Similar observation is also made in some previous studies also [19, 20].

(b) OPC – FA System

The addition of flyash has just the opposite effect on the mix properties in terms of workability and optimum dosage of superplasticizer as compared to silica fume. With incorporation of flyash, the water demand and hence optimum percentage of superplasticizer required reduce as compared to the control mix without mineral admixtures for all water binder ratios studied. The reduction in water demand of concrete caused by the presence of flyash is ascribed to its spherical shape, which reduces the frictional forces among the angular particles of OPC, called ball – bearing effect [21]. These spherical particles easily roll over one another, reducing inter-particle friction. The spherical shape also minimizes the particle’s surface to volume ratio, resulting in low fluid demands. Also, due to the electrical charges, the fine flyash particles become adsorbed on the surface of cement particles, which thus become deflocculated, reducing the water demand [22]. In other way, the effect of flyash can be considered similar to the action of superplasticizer

(c) PC–SF–FA System

From the above discussion, it can be stated that flyash act improves flowability and silica fume has a reverse effect, when added individually. Thus, it is thought that when used in combination, the beneficial effect of flyash on fluidity is used to compensate the loss of slump with silica fume addition. As expected, when the different combinations of silica fume and flyash are used, the slump values were higher and optimum superplasticizer dosage was lower in comparison with the corresponding mixes having only silica fume. The slump obtained increased with increase in flyash content in the mix and decreased with increase in silica fume content. For all the three water binder ratios, TC2 gave least superplasticizer dosage while MC3 gave maximum superplasticizer dosage. Thus, it can be said that the addition of flyash led to the production of economical mixes with greater workability.

Effect of Water Binder Ratio on Optimum Superplasticizer Dosage

Figure 5 shows the results of optimum superplasticizer dosage obtained for all mixes at various water binder ratios. From the figure, it is observed that as the water binder ratio decreases, the optimum dosage of superplasticizer increases. With the decrease in water binder ratio, more number of superplasticizer molecules are required for adsorption on the surface of cement and mineral admixture particles to increase the fluidity of the mix. The optimum dosage increases sharply as the water binder ratio is decreased from 0.35 to 0.25 as compared to the shift from 0.45 to 0.35. For example, in the control mix, the optimum superplasticizer dosage increased from 1.25% to 4% as the water binder ratio is decreased from 0.45 to 0.35. This is because at very low water – binder ratio, cement particles are very close and to overcome inter particle friction and inter particle forces of attraction, higher optimum dose of superplasticizer is required.

Relation Between Slump and Vee-bee Time

Influence of Flyash Addition on Fresh Properties of Silica Fume Concrete
In order to formulate a relation between slump and Vee–bee time for mineral admixture concrete, Vee–bee time test is also conducted simultaneously to slump test. Figure 6 shows the graph for slump with Vee– bee time. In the graph, the doted line shows the approximate relationship between slump and Vee–bee time for the normal ordinary Portland cement concrete without using superplasticizers and the solid line is the best fit obtained for the test results in the present study. The marked shift of the present curve for concrete containing mineral admixtures and superplasticizers from the existing curve for normal concrete can be observed from the graph. For higher values of Vee–bee time, the amount of slump required is almost same from both the curves. However, when the Vee–bee time is lesser than 5 seconds, the difference in the values of slumps obtained from the two curves differ in the range of 20 to 50 mm. Since Vee–bee time is the representative of actual compaction in the field, it can be said that for equal compaction, the mixes with admixtures require 20 to 50 mm higher slump than the mix containing Portland cement only. This shift in the curve can be due to the effect of cohesive nature of the mix with silica fume, flyash and superplasticizers.

Effect of Mineral Admixtures on Fresh Density

Influence of Flyash Addition on Fresh Properties of Silica Fume Concrete
In order to study the effect of mineral admixtures of superplasticizer on the degree of compaction achieved, fresh density of final mixes were also determined and the same is presented in Table 5. The fresh density of all the mixes lies in the similar range, although the mixes with flyash have a density somewhat higher than the other mixes which can again be due to ball bearing effect of flyash.

Conclusion

On the basis of the studies carried out, it can be concluded that in the binary system, silica fume increases the superplasticizer demand at a constant workability due to its high surface area and its strong affinity for multi— layer adsorption of superplasticizer molecules. Flyash addition, on the other hand, decreases the water demand and hence optimum percentage of superplasticiser for constant workability due to its ball – bearing effect that reduces frictional forces among binder particles. Also, due to the electrical charges, the fine flyash particles become adsorbed on the surface of cement particles, which thus become deflocculated, reducing the water demand. Three-component system is much preferred for high performance concrete because in it, silica fume act as a filler and flyash controls rheology.

The existing relationship between slump and Vee –bee time changes with the addition of mineral admixtures and superplasticizer. For equal compaction, the mixes with admixtures require 20 to 50 mm higher slump than the mix containing Portland cement only.

Acknowledgments

This research is supported by the Department of Science and Technology Grant. The authors would like to acknowledge the authorities concerned for its assistance in carrying out the research.
References
  1. Gallias J L, Kara-Ali R, Bigas J P, ‘The effect of fine mineral admixtures on water requirement of cement pastes,’ Cement and concrete research, 30, (2000). 1543 – 1549.
  2. Neville A M, ‘Properties of Concrete,' Pearson Education, (2004).
  3. Park C K, Noh M H, Park T H, ‘Rheological properties of cementitious materials containing mineral admixtures,’ Cement and Concrete Research, 35, (2005), 842–849.
  4. Zhang X, Han J, ‘The effect of ultra-fine admixture on the rheological property of cement paste,’ cement and concrete Research, 30, 5, (2000), 827 – 830.
  5. Bagel, L. (1998): ‘Strength and pore structure of Ternary Blended Cement Mortars Containing Blast Furnace Slag and Silica Fume,’ Cement and Concrete Research, Vol. 28, No. 7, pp. 1011–1020.
  6. Papayianni I, Tsohos G, Oikonomou N, Mavria P., ‘Influence of superplasticizer type and mix design parameters on the performance of them in concrete mixtures’, Cement and Concrete Composites, 27, (2005), 217–222.
  7. Agarwal S K, Masood I, Malhotra S K, ‘Compatibility of superplasticizers with different cements’, Construction and Building materials, 14, (2000), 253–259.
  8. Chandra S, Bjornstrom J, ‘Influence of cement and superplasticizers type and dosage on the fluidity of cement mortars – Part I, cement and Concrete Research, 32, (2002), 1605 – 1611.
  9. ACI COMMITTEE 212, ‘Chemical admixtures for concrete’ ACI Materials Journal, (1989), 297.
  10. Langley W S, Carette G G, Malhotra V M, ‘Structural concrete incorporating high volume ASTM Class F flyash,’ ACI Materials Journal, 86, (1989), 507–514.
  11. Ferraris C F, Obla K H, Hill R, ‘The influence of mineral admixtures on the rheology of cement paste and concrete,’ Cement and Concrete Research, 31, (2001), 245–255.
  12. Bartos P, ‘Fresh Concrete: Properties and tests,’ Elsevier, New York, (1992).
  13. Ferraris C F, Lobo C, ‘Processing of high performance concrete’, concrete International, 20, 4, (1998), 61–64.
  14. Mitsui, K. et al. (1989): ‘Properties of High strength concrete with silica fume using high–range water reducer slump retaining type’ in Superplasticisers and other Chemical Admixtures in Concrete, Ed. V.M. Malhotra, ACI SP – 119, pp. 79 – 97.
  15. Duval, R. and Kadri, F.H. (1998): ‘Influence of silica fume on the workability and compressive strength of high performance concrete; Cement and Concrete Research, Vol. 28, pp. 533 – 547.
  16. Nehdi M., Mindess S. and Aitcin P.C. (1998): ‘Rheology of High performance Concrete: Effects of fine particles,’ Cement and Concrete Research, Vol. 28, pp 687 – 697.
  17. Park C.K., Noh M.H. and Park T.H. (2005): Rheological properties of cementatious materials containing mineral admixtures,’ Cement and Concrete Research, Vol. 35, pp. 842 – 849.
  18. Olliver J.P., Carles-Gibergues A. and Hanna B. (1988); Cement and Concrete Research, Vol. 18, No.3, pp. 438 – 448. (*)
  19. Duval R. and Kadri E.H. (1998): ‘Influnce of silica fume on the workability and compressive strength of high performance concrete,’ Cement and Concrete Research, Vol. 28, No. 4, pp. 533 – 547.
  20. Yogendran V., Langan B.W., Haque M.N. and Ward M.A. (1987), ACI Materials Journal. (*)
  21. Termkhajornkit P., Nawa T. and Ohnuma T. (2001): ‘Effect of properties of fly ash on fluidity of the paste’, Cement Science and Concrete Technology, Vol. 55, pp. 163–169.
  22. Helmuth R. (1987); ‘Fly ash in cement and concrete,’ PCA, Skokie, Ill. pp 203.

NBMCW October 2007

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Trends in Concrete Construction

Trends in Concrete Construction

Brajendra Singh, Chief Consultant, Cement Manufacturers’ Association, New Delhi.

Concrete is one of the oldest and most widely used building materials in the world. In one form or another various types of concrete have been used for construction purpose for around 9,000 years by now. Concrete platforms dating back to 7,000 B.C. have been unearthed in West Asia and concrete structures have been found in a 7,000 years old sunken city, discovered off the coast of Gujarat. These are just two examples, taken at random, from hundreds of concrete structures built throughout known human history.

One of the enduring mysteries of all times, is the answer to the question as to how did the ancient Egyptians, who had no machines worth the name, haul up huge limestone blocks weighing over fifteen tonnes, to construct their massive pyramids. This question has for centuries been very widely debated by archeologists, historians and engineers, and several possible answers arrived at. Leaving aside improbable conjectures like the one that the pyramids were constructed by alien beings who visited our planet from outer space, most other theories focus on methods used to quarry the gigantic blocks, transport them to the building sites, shape and polish them so finely—even though no mortar has been used to join the blocks together, they fit so nicely that even a knife blade cannot be slipped between adjacent stones–and finally haul them into position. Most aver that the blocks were chiselled out of hillside rock formations, floated down the Nile on boats or rafts, moved across land using wooden rollers placed below them, and positioned using long sloping ramps. Human slaves, along with elephants, formed the motive power. Although, eminently feasible, this method would have been painfully laborious and slow. Some years back a new theory of pyramid construction was put forth. According to it, there was no question of quarrying, transporting, shaping and polishing of blocks; nor of hauling them into position. This is because, according to this revolutionary theory, there were no limestone blocks to start with. They were, in actual fact, poured in-situ lime concrete blocks. This process, it is argued, would have saved the almost impossible effort required to construct the 4,500 years old pyramids, especially as the Egyptians of that time apparently had no iron tools, now were aware of the invention of the wheel.

As concrete evolved over the ages, it has become quite clear from recent discoveries, that several ‘modern’ varieties of concrete, may not be so modern after all. Take for instance, lightweight concrete. As far back as 83 B.C. Roman architects used lightweight aggregates formed by the cooling of lava, from volcanoes like Etna, Stromboli and Vesuvius, to build the Temple of Fortune in Palestrina, Italy, whose ruins were discovered some time back. Excavations in Italy have also revealed the remanants of large number of other residential and official buildings, made with lightweight concrete, dating back to between the 1st Century B.C. to the 2nd Century A.D.

Another example of a supposedly modern form of concrete which in actual fact was fairly widely used by the ancients, is fireproof concrete. Sometime during the 3rd Century B.C., the buildings of almost the entire city of Rome, were re-built with fireproof concrete. Then, to give them an aesthetic look, they were given a facing of bricks. When Rome’s first Emperor, Augustus Caesar (after whom the month of August is named), nephew of Julius Caesar, took independent charge of the Roman Empire in 32 B.C., he decided that his capital did not look grand enough. So he had marble facades put on every building, so that Rome literally glittered in the sunshine. Four hundred years later, when the Goths under Alaric looted Rome, they set the entire city on fire. The marble facades and brick burned off, but the basic concrete structures of Rome almost all survived

Innovations

Another innovation that originated over 2000 years ago in Rome, was the blending of reddish volcanic earth with lime. This resulted in a fairly unique product–concrete that set under water. Undersea structures built at that time, are still existing today, though most of them are damaged or broken.

Coming to more modern times, concrete was used for making boats, yes you read that right, it is boats, since 1850s, in France. These boats were made by plastering concrete over an iron mesh boatshaped framework. This composite was named as Fericement in early days and Ferrocement later on. It is still being used for making boats, water tanks, house components, irrigation & sanitation item etc. Such boats had many advantages since they were waterproof and leak proof, did not rot and were also almost damage proof. Right till the 1920s, concrete boats and ships, some as big as 132 metres long and 17 metres wide, weighing over 7,500 tonnes, were plying on ocean going routes. Even today, many colleges in USA, organize regular concrete boat races, which are extremely popular with students.

Next on our list, is fibre reinforced concrete. This product too, is not all that modern as, according to available records, the first fibre-reinforced concrete products were bricks, reinforced with straw fibres, which were in wide use some 3000 years ago. And concrete roofs, reinforced with horse-hair, were all the rage around the 3rd Century A.D. Steelfibre reinforced concrete is a more recent product, since the know-how for the manufacture of steel fibres was not available earlier on. Hence steel fibre reinforced concrete only made its appearance in 1874, a mere 132 years ago.

And do you know when Ready Mixed Concrete (RMC) i.e. concrete mixed in a central batching plant and transported to different work sites, first made its appearance. It was in 1903. Unfortunately, suitable motorized transport for its conveyance, from batching plant to work site, was not available in those days, since the automobile industry was still in its infancy. So, when centrally batched concrete was carried by horse-drawn container vehicle, it often set on the way, as there was almost no knowledge regarding retarding chemicals available at that time. The manufacture and use of RMC was therefore somewhat slow, till about 1914, when the first petrol engine driven RMC trucks made an appearance. Incidentally, 1914 was also the year when the first concrete road was constructed in our country.

Despite all that is mentioned above, modern-day concrete technologists need not feel that “there is nothing new under the sun.” Today, we have a number of innovative usage for concrete, most of which–as far as current knowledge goes–were unknown or even undreamt of, till just a few decades ago. These include flexible concrete, spun concrete, whisper concrete, ultra-thin concrete and even cementless concrete. Some details of these products and processes are given in the succeeding paragraphs.

Flexible Concrete

Trends in Concrete Construction
The term ‘flexible concrete’ seems to be an anomaly, since concrete is generally considered to be inflexible, as in a rigid road pavement. The requirement for a flexible form of concrete has been felt for many years, due to failure of concrete roads and bridge decks, when subject to severe stress by overloaded trucks going across them. In the mid-1990’s, scientists in USA’s University of Michigan, decided to try and design a flexible form of concrete, which would be ductile and elastic. They gave their new product the name of Engineered Cement Composite (ECC) and started carrying out experiments with different trial mixes.

Eventually, after dozens of hits and misses, they produced a fairly satisfactory mix which, after setting, resulted in a concrete that, when overloaded, bent/sagged but did not crack. The mix was similar to a normal concrete mix, except that there were no coarse aggregates in it. Also it contained around two percent fibres, compared to the normal half percent contained in ordinary fibre-reinforced concrete. Additionally, the fibres incorporated in ECC were specially coated ones; this coating allowed the fibres to slide within the concrete, thus imparting flexibility to it.

Concrete produced by ECC techniques, has already been used in projects in several countries, including Australia, Japan, Korea, Switzerland and USA. The latest formula has given an end product that is 40 percent lighter in weight and 500 times more resistant to cracking, than normal concrete. This latest composite concrete has been used for a 5cm ultra-thin deck on a bridge in Japan. The 40 percent saving in weight has led to significant economies in construction cost especially in the understructure on which the dead load came. An additional bonus that the deck’s flexibility gave, was that there was no requirement for expansion joints–the entire deck slab was a continuous one. This not only provided a smoother ride for motorists using the bridge, but also saved on the bother and cost of joint filler maintenance/ replacement.

Spun Concrete

Columns are vital part of most buildings. Load bearing columns, unfortunately, tend to be large in size. Though large columns can be fashioned and designed artistically, thus giving a pleasing appearance, they often take up vital space and obstruct free movement as well as vital viewability. Pre-stressing columns imparts additional load bearing capacity to them, thus allowing them to be made slimmer in size and permitting larger spacing between them but even then, their size can create problems.

Trends in Concrete Construction

To provide columns with even more load bearing strength, so that their diameter could be further reduced, a new technique has been conceived, which is basically German in origin. This technique results in the production of what is known as spun concrete. The procedure for making columns of spun concrete is roughly as follows. A steel mould in the shape of the column is made, in two halves. The reinforcement cage for the column is also made in two parts. One part is placed in each half of the mould, anchored to fixing devices, which are a part of the mould, and pretensioned. High strength concrete, up to M-100, is then poured into the mould halves. After that the halves are bolted together and placed in a centrifuge.

The mould, with the concrete in it, is then spun for approximately 10 minutes, at 600 rpm. After that, the concrete is left to set, for between 12 to 16 hours, depending on various factors, such as strength required, column size, ambient conditions and so on. The mould is then removed and the column cured, then transported to the construction site. This process produces a very dense, high strength concrete structure. Heavy reinforcement ratios up to 15 percent, have enabled production of 28 metre high columns, having a diameter of only 70 cm, capable of taking loads up to 360 tonnes, by the use of spun concrete.

Whisper Concrete

Trends in Concrete Construction
One major disadvantage of concrete roads is that they are noisy; vehicles traveling on them produce a ‘swishing’ sound, due to the friction between their tyres and the hard road surface. In European countries, where long stretches of concrete highways exists, this irritating ‘swish-swish’ was, and is, the cause of much annoyance for road users, and for those whose houses are situated in the vicinity of concrete roads. So much so that many countries have made it mandatory to construct sound deflecting fences along concrete roads, wherever they pass through residential areas. In fact in UK, construction of concrete road pavements was actually banned for a few years due to noise pollution.

And that is how ‘Whisper Concrete’ came into being, although it was partly by accident.

In late 70’s and early 80’s, despite predictions that the then quantum jump in oil prices would drastically reduce individual usage of vehicles, traffic on concreted European roads increased by leaps and bounds. Simultaneously, there was an increase in vehicular speed, particularly on inter-city highways. This caused a greater wearing action on road surfaces and also an almost unbearable increase in the level of sound being produced. Smoothened pavements, worn down due to excessive wear and tear, led to skidding of vehicles, causing accidents; and noise pollution gave rise to headaches and other soundrelated psychological problems. These troubles were particularly noticeable near and on autobahns, motorways and freeways, where speeds generally exceeded 120 kmph, and between 75,000 to 1,00,000 vehicles traversed the facility every day.

Among the first countries to take cognizance of motorists complaints was Belgium. Since accidents due to skidding (which led to a large number of deaths and serious injuries) caused much more damage than mere noise pollution, priority was given for mitigating causative factors for the former.

Investigations into the causes of skidding, showed that when concrete pavements were initially laid, they were invariably given a non-skid surface by brooming; a method in which the surface of the road had grooves etched into it, by dragging steel-wire brooms across the top of the concrete pavement, before it had hardened fully. These grooves, which were generally made two millimeters deep, imparted excellent anti-skid properties to the road. However, continuous heavy traffic over on extended period of time, caused the ridges between the grooves to get worn down, thus flattening the surface of the pavement. In those days, re-grooving of the road surface, though eminently feasible, was a somewhat costly and laborious process (new techniques have made it easier to re-groove the top of a concrete pavement today). Hence road maintenance authorities tended to delay the operations, or even give it the complete go-by. So when it next rained, interaction between vehicle tyres and the wet, smooth road surface, produced a phenomenon known as ‘hydroplaning.’ Hydroplaning is a particularly nasty form of skidding, and normally leads to total loss of control of vehicles by drivers.

An accident rates in the country went up and criticism of official apathy mounted, the Belgian authorities started to take action. They began to look for ways and means to restore the anti-skid surface of concrete roads economically and speedily.

Initially, trial lengths of smoothened road surface, were overlaid with 40-50 mm of concrete having a maximum aggregate size of 6-8 mm. The surface of the new concrete, while still wet, was sprayed with a retarder consisting of glucose, water and alcohol; it was then immediately covered securely with polythene sheeting, to prevent evaporation. This particular retarder, as tests had shown, affected only the top 2 mm of the concrete.

Once partial curing of the remaining concrete had taken place (anything between 8 to 36 hours later, depending on the ambient conditions), the polythene sheeting was removed, and the surface of the road was swept with a machine, which had stiff, rotating wire bristle brushes. These rotating brushes removed the cement mortar from the top 1.5 mm of the pavement, thus exposing the aggregate and making the surface rough enough for safe high-speed driving in wet weather.

Trends in Concrete Construction
When vehicles were driven at expressway speeds over these newly made antiskid surfaces, it was found to every ones surprise that, besides being safer to travel on, such exposed-aggregate pavements were much quieter than normal concrete surfaces. In fact, they eventually proved to be even quieter than blacktopped roads.

Further trials were then carried out, with the emphasis now on reduction of the amount of noise pollution being created. These only served to confirm the earlier findings, that the new type of surface was much quieter than any of the other pavements in service. The delighted public works authorities–who had got two benefits for the price of one–soon labeled the exposed-aggregate pavement as ‘whisper’ concrete, and decided to go in for it in a big way.

However, the Belgians soon discovered that along with its advantages, whisper concrete had one fairly serious drawback. Where overlaying of old smoothened concrete pavements was involved, the cost of using whisper concrete was more or less the same as regrooving, but involved much less effort; and where it was laid on an existing worn-out bitumen pavement, whisper concrete costs matched those of white-topping (re-surfacing of an old blacktopped pavement with thin concrete slabs). But where new roads had to be built, it was found that the pavement had to be constructed in two layers. A lower layer of 200 mm of ‘normal’ concrete, having a maximum aggregate size of 30 mm; and an upper layer of 40-50 mm of whisper concrete, having a maximum aggregate size of 6-8 mm. This double operation increased both time and cost of construction. Nevertheless, Belgian authorities decided that the advantages of whisper concrete far out-weighted its disadvantages. They therefore went on constructing fresh roads and topping existing ones, with the new material. Between 1981 and 1994 eight million cubic metres of whisper concrete was laid down on the country’s roads. Today, CRCP (Continuously Reinforced Concrete Pavement) with an exposed aggregate surface, is the standard form of road construction in Belgium, for all inter–city highways.

After Belgium, whisper concrete was taken up in a big way by neighboring Netherlands. Extremely happy with its performance, but not too pleased in having to build it in two layers, the Dutch carried out some trials of their own. They soon discovered that if the maximum aggregate size in the entire concrete mass was reduced to 20 mm, and a good percentage of small stone chippings added to the mix, whisper concrete pavements could be laid in a single pass. Though driving on such pavements was not as ‘comfortable’ as on two-layer whisper concrete, the noise produced was somewhat less, apart from the considerable saving in time and money since only a single laying operation was involved.

The next European nation to take up the new road building method was Austria. Austria is by and large a mountainous country, with many of its roads running along the lower portions of valleys. Increasing traffic at greater speeds along these arteries, caused noise to roll up the hillsides in waves. This phenomenon started causing ‘adverse political fall-out.’ Fearing loss of votes, worried Government officials, scanned literature, organized conferences and toured Europe, looking for solutions to the problem. It was not long before they discovered whisper concrete. After due trials and deliberations, the Austrians decided to adopt the Belgian two-layer technique of construction, rather than the Dutch single-layer method. This is because in Austria, despite the country’s middle–of–the–Alps location, suitably tough and hard aggregates are extremely costly. Such aggregates are essentially required for that country’s roads because the heavy snowfalls it experiences, means that most vehicles use studded tyres; and such tyres wear out soft aggregates very fast. Hence the Austrians used soft aggregates for the thicker lower layer of their concrete roads, and hard tough aggregates for the thinner, upper whisper concrete layer. Austria’s selection of the twolayer method of construction, proved to be a wise one, because even five years after the initial whisper concrete roads were built, their surfaces showed no signs of wear and tear, despite their regular use by studded tyre traffic.

The British, traditionalists as usual, waited to see the experience of others and then took up the construction of whisper concrete pavements only in 1995. The guidelines provisionally enunciated by them, are probably the most suitable ones for use by those building whisper concrete roads for the first time. These include:
  • Under standard highway conditions, a concrete road should consist of a cement-bound sub-base, between 150-200 mm thick. On top of this, there should be 200 mm of CRCP, followed by 50 mm of whisper concrete surfacing.
  • Existing concrete paving trains should be modified to lay the lower CRCP and the upper whisper concrete surface in the same pass.
  • Full pavement width (even for double-lane roads) on each side of dual carriageway roads, should be laid in a single operation.
  • Normally, 8 mm size coarse aggregate should be used in the surface layer. Not more than 3 percent of these should be oversized and 10 percent undersized.
  • These aggregates should posses a polished stone value greater than 60; this will ensure sufficient hardness to combat wear and tear. The aggregates should also have a ‘flakiness’ index less than 25% which will ensure that they have a fairly uniform shape.
  • Coarse aggregate should form around 60% of the whisper concrete, which should be airentrained. Sand used should be very fine. The cement used should be OPC (Ordinary Portland Cement).
  • The whisper concrete layer should be initially levelled by a conventional mechanical float with oscillating beams. This should be followed by further levelling by a ‘super smooth’ float, set longitudinally down the carriageway, at right angles to the first float, which should remove any remaining imperfections or ridges.
  • Spray the smooth finished surface immediately with a retarder consisting of glucose, water and alcohol. Then cover the surface with a polyethylene ‘cling’ film.
  • Between 8 and 36 hours later (depending on ambient conditions), remove the polyethylene film and brush the surface with mechanically rotating, stiff bristles; to remove cement mortar from the top 1.5 mm.
  • Properly planned operations should enable construction of about 3000 linear metres of whisper concrete per day.

Ultra-Thin White- Topping

Trends in Concrete Construction
Until 1991, most white topping projects did not purposely seek a bond between the interface of the concrete and the underlying flexible surface. Rather, the existing bitumen served as base for the new concrete overlay. Today, we refer to this technique as “Conventional” or “Classical” white topping, defined as: “A concrete overlay, usually of thickness of 100 mm or more, placed directly on top of an existing bitumen pavement.

However, a new technology emerged in the early 1990’s, which has dramatically expanded white topping technology and its use. This rehabilitation technique purposely seeks to bond the concrete overlay to the existing bitumen. As a result, the concrete overlay and the underlying bitumen act as a composite section rather than two independent layers. This composite action significantly reduces the load-induced stresses in the concrete overlay. Therefore, the concrete overlay can be considerably thinner for the same loading as compared to a white topping section with no bond to the underlying bitumen.

When describing pavement thickness, terms such as “thick” and “thin” are relative and depend on the viewpoint and experience of the user. For Ultra-Thin White (UTW) topping, a more definitive description is needed. Based on the international experience, ultrathin white topping can be defined as: “A concrete overlay 50 mm to 100 mm thick with closely spaced joints bonded to an existing bitumen pavement.”

There are three basic requirements for UTW overlays to perform properly. These are:
  • Availability of an appropriately thick existing bitumen layer.
  • Achievement of a bond between the existing bitumen pavement and the UTW.
  • Provision of short joint spacing.
Bonding allows the concrete and bitumen layers to perform as a composite section. This causes the two layers to act monolithically and share the load. With bonding, the neutral axis in the concrete shifts from the middle of the concrete down toward the bottom of the concrete. This shifting lowers the stresses at the bottom of the concrete and brings the stresses into a range that the thin concrete layer can withstand.

The composite section has opposing effects on corner stresses. There is a decrease in the concrete stresses because the whole pavement section is thicker. However, if the neutral axis shifts low enough in the concrete, the critical load location may move from the edge to the corner depending on the materials and layer characteristics. Essentially, the corner stresses decrease because the bonding action creates a thicker section, but increase because the neutral axis shifts down and away from the top surface.

To combat this effect, close joint spacing is critical. All pavement types must absorb the energy of the applied load by either bending or deflecting. Traditional concrete pavements are designed to absorb energy by bending and thus are made thick enough to resist stresses induced by bending. With UTW, short joint spacings are used so that energy is absorbed by deflection instead of bending. The short joint spacing also minimizes stresses due to curling and warping by decreasing the amount of slab that can curl or warp.

For the UTW overlays, the short joint spacing in effect forms a minipaver block system, which transfers loads to the flexible pavement through deflection rather than bending. Typical joint spacings that have performed well on UTW projects are somewhere between 0.6 and 1.5 m. It is recommended that the maximum joint spacing for UTW be between 12-15 times the slab thickness in each direction.

Trends in Concrete Construction
When performing a UTW project, there must be enough bitumen to protect the concrete (minimize stresses), and enough concrete must be placed to protect the bitumen (minimize strains). A thicker bitumen pavement section improves the load-carrying capacity of the system because it creates a thicker final UTW pavement structure, and also carries more of the load. This shifts the neutral axis down in the concrete, which decreases the concrete stresses.

The construction of a UTW consists of three basic steps:
  • Prepare the existing surface by milling and cleaning, or blasting with water or abrasive material.
  • Place, finish, and cure the concrete overlay using conventional techniques and materials.
  • Cut saw joints early at prescribed spacings.
A clean surface is required for proper bond. Milling the surface followed by cleaning improves bond because it opens the pore surface of the bitumen pavement. The milling creates a rough surface that “grabs” the concrete and creates the mechanical bond between the two layers. Once a surface is cleaned it is extremely important to keep it clean until paving commences.

Paving a UTW is no different from paving any other concrete pavement. Conventional slip-form and fixed-form pavers, as well as hand-held equipment–such as vibrating screeds–have all been used successfully without major modifications. The only real change is that the concrete layer is thinner than normal. Normal finishing and texturing procedures are applied to the surface.

Proper curing is critical to avoid shrinkage cracking and debonding between the bitumen and concrete pavements. Curing compound should be applied at twice the normal rate, because the overlay being a thin concrete slab, has high surface area to volume ratio, and can thus lose water rapidly due to evaporation. Care must also be used during application, to avoid spraying curing compound on adjacent uncovered prepared bitumen surfaces, since that would decrease bonding.

Joint sawing should be carried out with lightweight saws, as early as possible, to control cracking. Saw depth should be approximately one–fourth to one third of the total depth of the overlay. Typically, UTW joints are not sealed. Test studies have shown that UTW pavements perform well without sealants because the compactness of the slabs minimizes joint movement.

The concrete mix selected for particular project is matched to the traffic conditions and opened-fortraffic requirements. Synthetic fibers are often added to increase the post-crack integrity of the panels.

Ultra-thin White-topping projects have been carried out in several countries including USA, Brazil and Canada. However, the technique is still regarded to be in its infancy and requires considerable research to streamline and standardize it.

The American Concrete Institute issued ‘Supplement Specification 852’ on 11th July 2000, which laid down specifications for ‘Ultra-thin White-topping Overlay with Steel Fiber Reinforced Concrete.’ As far as is known this is the only existing specification on the subject.

Cementless Concrete

In the late 1980s, Austria was facing a shortage of cement, due to several factors. Shortage of suitable quality limestone was one of them. Another was the extremely stringent emission standards for cement manufacturing plants set by the country’s Government, due to concern about the steadily deteriorating environment.

Both the cement and construction industries were worried, and decided to do something to sort out the problem. Discussions, experiments, laboratory and field trials became the order of the day. Eventually, an absolutely new, novel and unique product was developed, after 15 years of intense effort.

The scientists, technologists and others involved in the project, started off by thinking ‘outside the box.’ They decided that they did not want to produce a modified cement, or even an improved version of OPC. They resolved to create an alternative to cement. This was a tall order indeed, but the experiment team was determined to succeed; and succeed they did. Their basic premise was, that although they did not want cement, their alternative binding material, had to have cementitious properties, if they wanted it to take over cement’s role.

By trial and error, they narrowed down their choice of the base material to slag. Austria, located right in the heart of Europe’s biggest steel producing zone, was ideally situated to procure massive quantities of slag, easily and economically. And blast furnace steel slag is a highly cementitious material.

Once the base element had been identified further experiments and trials were carried out to find ways and means to convert it into a suitable, easy-to-use and economical binding agent. Finally it was determined that by blending gypsum, certain alkaline products and a few other additives with slag, they could obtain a substance that had all the binding properties of cement, yet was superior to it in many ways.

The advantages that this new slag-based binder had included:
  1. No burning process was involved in its production. Hence emission of carbon-dioxide and nitrous oxides was reduced to almost zero, making it extremely friendly to the environment.
  2. It has a very low heat of hydration. Hence it is ideal for mass concrete applications such as dams and foundations. Also, low heat of hydration means almost no cracks in the finished product, hence eminently suitable for water-retaining structures.
  3. High resistance of concrete products made from it, to sulphate and acid attack, as well as damage by alkali-reactive aggregates. Thus can be used with great advantage in aggressive environment.
  4. Energy saving of up to 80 percent in its manufacture, since this involves only grinding.
The above-mentioned binder is still not in general production, as its composition was finalized only around five years ago. Trials on concrete items and structures manufactured using this binder, are still being carried out.

The author is grateful to the International Cement Review, BFT International and the Indian Cement Review for some of the information contained in the above article.

NBMCW June 2007

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Experimental Investigation on the Stren...

Dr. S.M.Gupta, Department of Civil Engineering National Institute of Technology, Kurukshetra.
The use of silica fume as a mineral admixture for the production of high strength concrete and high durable concrete is gaining importance in recent years. The objective of the present experimentation is to study the effect of silica fume as additive on the strength and durability characteristics of concrete obtained using locally available material. Concrete mix for M20 gradeis designed which serves as basic control mix. Silica fume concrete mixes are obtained by adding silica fume to basic control mix in percentages varying from 0 to 16% at an increment of 2% by weight of cement. The compressive strength development and durability against acidic and alkaline attack is studied.

Introduction

The present trend in concrete technology is to increase the strength and durability of concrete to meet the demands of the modern world. These factors can be achieved in concrete by adding various blending materials with cement or separately to concrete. The materials suitable for blending are flyash, blast furnace slag, silica fume, etc. Silica fume concrete (SFC) is emerging as one of the new generation construction material. It can be considered as high strength concrete or high performance concrete

The use of pozzolanic admixtures like condensed silica fume, because of its finely divided state and very high percentage of amorphous silica, proved to be the most useful if not essential for the development of very high strength concretes and/or concretes of very high durability. It is recommended that for applications in concrete silica fume should conform to certain minimum specifications such as silicon dioxide content of not less than 85%, spherical shape with a number of primary agglomerates with particles of size ranging from 0.01 to 0.3 microns (average of 0.1 to 0.2 microns), amorphous structure and a very low content of unburnt carbon.

Silica fume is known to improve both mechanical characteristics and durability characteristics of concrete since, both the chemical and physical effects are significant. Physical effect of silica fume in concrete is that of a filler, which, because of its fineness, can fit into spaces between cement grains in the same way that sand fills the spaces between particles of coarse aggregate and cement grains fill the spaces between sand grains. As for chemical reactions of silica fume, because of high surface area and high content of amorphous silica, this highly active pozzolan reacts more quickly than ordinary pozzolans.

Experiments have revealed that silica fume in concrete essentially eliminates pores between 500 to 0.5 micron sizes and reduces the size of pores in the 50 to 500 micron range. Physical and chemical mechanisms made the silica fume more effective in reducing pore size.

Experimental Programme

An experimental program was carried out to find out the strength and durability characteristic of concrete containing silica fume as an additive. Concrete mix for M20 grade was designed, which served as basic control mix. Silica fume concrete mixes were obtained by adding silica fume to basic control mix in percentages varying from 0 to 16% at an increment of 2% by weight of cement.

Materials Used

Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing
Ordinary Portland cement was used throughout the Experimentation. Silica fume used in the experimentation was obtained from FOSROC Chemicals (India) Limited. The physical and chemical properties of OPC and silica fume (SF) are given in Table 1. Locally available aggregates were used. Coarse aggregates crushed from igneous basalt rock of 20mm and down size having specific gravity of 2.74 and conforming to IS 383-1970 were used. For Fine aggregate local sand having specific gravity of 2.56 and conforming to grading zone I of IS: 383-1970 was used. Superplasticizer based on sulphonated naphthalene formaldehyde was used to impart additional desired properties to the silica fume concrete. The dosage of super plasticizer was 0.7% by weight of cement. Ordinary potable water was used for mixing of the ingredients.

Concrete Mixes

Mix design for M20 grade of concrete was carried out using the guidelines prescribed by IS: 10262- 1982. The designed concrete mix for M20 served as basic control mix (CM). Silica fume concrete mixes were obtained by adding silica fume to basic control mix in percentages varying from 0 to 16% at an increment of 2% by weight of cement. (viz SFC2, SFC4, SFC6, SFC8, SFC10, SFC12, SFC14, SFC16). The Basic control Concrete mix proportion obtained was 1 part cement: 1.62 parts of fine aggregate: 3.28 parts of coarse aggregate with water–cement ratio of 0.5 and 0.7% of Superplasticizer.

Batching, Mixing, and Curing

The concrete ingredients viz. cement, sand and coarse aggregate were weighed according to proportion 1:1.62:3.28 and are dry mixed on a platform. To this the calculated quantity of silica fume was added and dry mixed thoroughly. The required quantity of water was added to the dry mix and homogenously mixed. The calculated amount of superplasticizer was now added to the mix and then mixed thoroughly. The homogeneous concrete mix was placed layer by layer in moulds kept on the vibrating table. The specimens are given the required compaction both manually and through table vibrator. After through compaction the specimens were finished smooth. After 24 hours of casting, the specimen were demoulded and transferred to curing tank where in they were immersed in water for the desired period of curing

Tests Conducted

The tests were conducted both on Fresh and Hardened concrete. The tests on fresh concrete was the workability test conducted through Slump test, Compaction factor test; Table 2 and Vee-bee consistometer test. The strength and durability tests conducted on hardened concrete are briefed here:

Compressive Strength Test

The compressive strength test was carried out on cube specimens of dimensions 150 ´ 150 ´ 150 mm. The compressive strength test specimens were cured and tested for 3-days, 7-days, 28-days, and 60-days in compressive testing machine. Three specimens were used for each test.

Durability Test Resistance Against Acid Attack

For acid attack test concrete cube of size 150 ´ 150 ´ 150 mm are prepared for various percentages of silica fume addition. The specimen are cast and cured in mould for 24 hours, after 24 hours, all the specimen are demoulded and kept in curing tank for 7-days. After 7-days all specimens are kept in atmosphere for 2-days for constant weight, subsequently, the specimens are weighed and immersed in 5% sulphuric acid (H2SO4) solution for 60-days. The pH value of the acidic media was at 0.3. The pH value was periodically checked and maintained at 0.3. After 60-days of immersing in acid solution, the specimens are taken out and were washed in running water and kept in atmosphere for 2-day for constant weight. Subsequently the specimens are weighed and loss in weight and hence the percentage loss of weight was calculated.

Resistance Against Alkaline Attack

For alkaline attack test concrete cube of size 150 ´ 150 ´ 150 mm are prepared for various percentages of silica fume addition. The specimen are cast and cured in mould for 24 hours, after 24 hours, all the specimen are demoulded and kept in curing tank for 7-days. After 7-days all specimens are kept in atmosphere for 2-days for constant weight, subsequently, the specimens are weighed and immersed in 5% sodium sulphate (Na2SO4) solution for 60-days. The pH value of the alkaline media was at 12.0. The pH value was periodically checked and maintained at 12.0. After 60- days of immersing in alkaline solution, the specimens are taken out and are washed in running water and kept in atmosphere for 2-day for constant weight. Subsequently, the specimens are weighed and loss in weight and hence the percentage loss of weight was calculated.

Results and Discussions

Workability Test Results
Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing
The result of workability of concrete as measured from slump, compaction factor and, Vee-bee degree are shown in Table 2. According to these results, workability of concrete decreases as the silica fume content in concrete increases from to 16%. No wide variations in the slump and compaction factor values for the mixes containing increased amount of silica fume were observed. The silica fume concrete did not show tendencies for seggregation and bleeding. This is due to the fact that as the percentage of silica fume increases the water available in the system decreases thus affecting the workability. As compared to control mix (CM), the mix containing 16% silica fume (SFC16) has a slump reduction of 28% and compaction factor reduction of 5.26%. The effect of silica fume content on the workability with regard to slump of concrete is shown in Figure 1.

Compressive Strength Test Results

The compressive strength of concrete containing silica fume given in Table 3 shows an increasing trend as the percentage of silica fume increases, from 0 to 16%. This is true for 3-days, 7- days, 28-days, and 60-days compressive strength. The strength activity index for 3-days, 7-days, 28-days, and 60-days for 16% of silica fume is 1.65, 1.33, 1.49 and 1.41 respectively. The effect of silica fume content on the Compressive strength of concretes is shown in Figure 2.

Resistance Against Acid Attack

Table 4 shows the change in weight of control mix and silica fume mix when immersed in 5% sodium Sulphric acid (H2SO4) solution. Under a very low pH (0.3 pH) of 5% - H2SO4 Solution, all hydrated products, hydrated silicate and aluminate phases and calcium hydroxide, can easily be decomposed. The control mix was markedly affected by 5% - H2SO4 solution with a significant weight loss. On the other hand, the progress of deterioration in silica fume concrete immersed in 5% - H2SO4 solution varied widely depending on the percentage of silica fume. SFCl6 mix was found to be most effective in preventing the Sulphuric acid attack. It appears that in the Sulphuric acid attack, the early decomposition of calcium hydroxide and subsequent formation of layer amount of gypsum are attributed to the progressive deterioration accompanied by the scaling and softening of the matrix. The percentage weight loss, decreases as the percentage of silica fume in concrete increases. The weight loss index for SFC16 is 0.65

The effects of silica fume content on the acidic media durability shown in Figure 3.

Resistance Against Alkaline Attack

Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing

Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing

Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing

Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing
Table 4 shows the change in weight of control mix and silica fume concrete when immersed in % sodium sulphate (Na2SO4) solution. The pH value of 5% sodium sulphate (Na2SO4) solution was found to be 12. The percentage weight loss, which is an indication of durability, decreases as the percentage of silica fume in concrete increases.

The weight loss index for SFC16 is 0.00 while for control mix it is 1.00. This may be due to the fact that the silica fume, which also acts as a filler material, increases the density of concrete by filling the voids. The voids, which are very compactly filled up by the silica fume, do not allow the alkaline media to penetrate into concrete mass and also reduced content of calcium hydroxide in die silica fume concrete due to pozzolanic reaction. Thus the percentage weight loss will be less as the percentage of silica fume in concrete increases. The effect of silica fume content on alkaline media of concretes is shown in Figure 4.

Compressive strength of Silica Fume Concrete after 60-days Immersion in Acidic Media and Alkaline Media

Table 5 shows test result of 60-days compressive strength of silica fume concrete, when exposed to two different media viz Acidic and Alkaline media the strength activity index shows an increasing trend as the silica fume increases from to 0 16%.

The strength activity index for SFC16 is 1.92 for acidic and 1.42 for alkaline as compared to the control mix in the respective media. The effects of silica fume content on the compressive strength after 60-days immersion in Acidic media and Alkaline media of concretes is shown in Figure 5.

Conclusions

These studies have lead to the following conclusions:
  1. The workability of concrete as measured from slump, compaction factor and Vee-bee degree decreases as percentage of silica fume in concrete increases. As compared to the control mix, SFC16 has a slump reduction of 28% and compaction factor reduction of 5.26%. Thus the workability of concrete decreases as the percentages of silica fume in concrete increases.
  2. The compressive strength of concrete shows an increasing trend as the silica fume content increases, from 0 to 16%. This increasing trend is evident for 3-days, 7-days, 28-days, and 60- days compressive strength. The strength activity index for SFC16 is 1.65, 1.33, 1.49, and 1.41 at 3- days, 7-days, 28-days, and 60- days respectively. Thus silica fume acts as a pozzolanic material, hence the compressive strength of concrete increases as the percentage of silica fume increases.
  3. Resistance against acidic attack of silica fume concrete increases as the silica fume content increases from 0 to 16%. The percentage weight loss, which is an indication of durability in acidic media, decreases as the percentage of silica fume in concrete increases. The weight loss index for SFC16 is 0.65. Thus since silica fume acts as a filler material and fills up the voids of concrete, the durability of concrete in acidic media increases as the percentage of silica fume in concrete increases.
  4. Resistance against alkaline attack of silica fume concrete increases as the silica fume content increases from 0 to 16%. The percentage weight loss, which is an indication of durability in alkaline media, decreases as the percentage of silica fume in concrete increases. The weight loss index for SFC16 is 0.00 while for control mix it is 1.00.
  5. The 60-days compressive strength of silica fume concrete, when exposed to two different media viz. Acidic and Alkaline media shows an increasing trend as the silica fume increases from 0 to 16%. The strength activity index for SFC16 is 1.92 for acidic and 1.42 for alkaline as compared to the control mix in the respective media.
References
  1. ACI Committee 226R (March- Apr. 1981), “Silica Fume in Concrete,” ACI Material Journal, 84, 3, pp. 158–166.
  2. Conferences/Seminars/ Workshops.
  3. Cook, J.E., “Research and Application of High Strength Concrete, 10.000 psi Concrete,” Concrete International, Oct., 1989, pp. 67-75.
  4. Duval R. and E.H. Kadri (1998), “Influence of Silica Fume on the Workability and the Compressive Strength of High Performance Concrete,” Cement and Concrete Research, 28, 4, pp.533-547.
  5. Gupta, S.M., “Experimental Studies on the Behavior of High Strength Concrete,” Ph.D. Thesis, 2001, K.U.Kurukshetra.
  6. I.S. : 10262–1962, “Indian Standard Recommended Guidelines for concrete mix design,” BIS, New Delhi.
  7. I.S.: 383-1970 (1990), “Specification for coarse and Fine Aggregate from Natural source for concrete,” Bureau of Indian Standards, New Delhi.
  8. Leming M.L., “Properties of High Strength Concrete: An Investigation of Characteristics High Strength Concrete Using Materials in North Caroling Research Report FHWA/ NC/88-006,” Department of Civil Engineering, North Carolina State University, Raleigh, N.C., July, 1988.
  9. Mehta, P.K., and Gjorv, O.E. “Properties of Portland Cement Concrete Containing Silica Fume,” Cement and Concrete Research, V. 12, No. 5, Sept.1982, pp. 587-595.
  10. Moreno, J., “The State–of–the–Art of High Strength Concrete in Chicago, 225W. Wacka Drive. Concrete International, Jan., 1990, pp. 35-39.
  11. Neville A.M. (2000), “Properties of Concrete,” Fourth and Final Edition - Pearson Education Asia Ltd.
  12. Ojho, R. N., “Use of Flyash and Condensed Silica in Making Concrete,” IE (I), Journal V. 77, November, 1996, pp. 170-173.
  13. Rachel J. Detwiler and P. Kumar Mehta (Nov.-Dec. 1989), “Chemical and Physical Effect of Silica Fume on the Mechanical Behavior of Concrete,” ACI Materials Journal, 86, 6, pp. 609-614.
  14. Sellevold, E.J. and Nilsen, T. Supp1ementary Cementing Materials for Concrete, Ed. By V.M. Malhotra. CANMET, SP 86-8E, pp. 167-246/1987.

NBMCW June 2007

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New Building Material- Freshly Ground L...

New Building Material- Freshly Ground Lime Instead of Cement

Prof M. D. Apte, Pune

By the end of the Nineteenth century, the British rulers had imported ‘Cement’ to India and commenced discouraging the method of using freshly ground lime for masonry construction that was in vogue in India since ages. If properly used, the cement construction could be made fairly waterproof. The construction could last as well appreciably long and gave hardly any trouble of maintenance, similar to slaked lime construction to the users. The Portland cement age was dawning in India! Local industrialists as well went ahead and established cement factories here. Being a factory manufactured material, it was touted to be always of uniform (and good) quality. By the end of Second World War, the fresh lime grinding as a process of preparing masonry material had been fully relegated into an historical construction activity!

In late fifties when we civil engineering students of Government College of Engineering Pune were taught this subject of cement concrete, it was emphasized that Concrete structures like bridges will last for over 60 years whereas residential accommodation can give satisfactory service for over 100 years! The cement concrete was quite strong and durable, even better than “finely and freshly ground lime” under use then. Even RCC was started being used by engineers with success.

We students were awestricken with the new found material and the technique of its use. The mix design for 1:2:4 (volume batch concrete) RCC, we used to need 15-16 one CWT (112 Lbs) bags of cement to make 100 cft of finished concrete. Sometime the cement consumption could go up to even 17 bags.

The mix design was introduced with a hollow box (3’ x 3’ x 3’) packed fully with coarse aggregate additionally packed with fine aggregate and in turn this interfiled with finer powder of cement. The body of concrete being aggregate the cement was only the binding agent as we could understand.

Any more small voids in this box were supposed to be filled with the expanding cement gel after it reacts with the mixing water in the concrete. The resulting concrete was supposed to be even waterproof.

New Building Material- Freshly Ground Lime Instead of Cement
We the engineering students were really overwhelmed by the good qualities of the cement and began enthusiastically looking forward to design cement concrete structures. By that time British and other foreigners had progressed into pre-stressed and/ or post-tensioned concretes. Indians accepted that as well as a technical gift from West. Concrete designing had become a science and wordy wars about volume batching versus weigh batching were fought in technical journals with gusto. Whatever concrete construction was executed before Second World War in India was by the British Engineers done through the Indian ‘Mestries’. They had no restriction of time or money for the projects and accordingly these constructions are standing even today as good examples. (Of course, now the British or other Western engineers have relaxed their vigil and got confused due to the large varieties of cements in the market, their output is dropped to ‘average’). Indian engineers used concrete since independence without teaching their masons and mestries the correct techniques needed to use this new material properly (since they themselves were unaware of that aspect). Cement was being used as readymade ground lime only. The engineers took it to increase the strength or durability of concrete, one needs to add cement in excess to the mixture.

Once a boxed structural member was concreted, the sample of the concrete used therein was to be cast in cubes and after proper curing; the cube was to be crushed to determine the quality (compressive strength) of the concrete used in the member. Bureau of Indian Standards brought a standard for this cube test and use of concrete to give impetus to good concrete construction. After they published IS: 456 of 1978 regarding use of plain & reinforced cement concrete giving the direction to use more cement (up to 540 Kgs per CM3 of concrete) for durability consideration, use of more cement for strength as well as durability purposes became rule rather than exception. Most of the ‘experts’ included in the BIS committee on Cement Concrete Section are either representing large construction companies or cement manufacturing companies who were only interested in increasing the use of cement to earn more money. They were not necessarily interested in propagating use of some other material in construction. Isn’t it? Not only cities but even small towns became concrete jungles and no wonder the mother Earth reacted by increasing environmental temperatures everywhere.

Cement factories grew in number as well as size and they manufactured special cements not only for refractory or sulphate resistance purposes but also to give higher strength in compression (instead of 33 N/mm2) of 43, 53 or even higher (at the end of 28 days curing). When the manufacturers professed that the stronger concrete is more economical, gullible people, even engineers, enthusiastically started using it in their designs. This was the time when dangers of cement concrete even with reinforcement were started appearing on horizon. Defects like, rusting of steel and carbonation of concrete, cracking of “stronger” cement concretes after a few days use, deterioration of the concrete members after a few seasons of intensive (though within designed loads) use, destruction of concrete because of alkali-aggregate reaction in certain circumstances, cracking of concrete with excessive cement quantity leading to destruction of the monolith, members failing because of inadequate concrete strength development etc. made frequent appearances. Deterioration of Vashi Bridge near Mumbai and failure of many bridges like Mandovi River Bridge in Goa as well as overhead tanks, multistoried concrete residential buildings etc compelled Indian civil engineers to wake up and study the technology and its application in India thoroughly. According to their thinking, the defects might be due to various reasons like the pour was incorrect, segregation might have taken place while pouring, placement of reinforcement might not be exactly as per design or might have been shifted to wrong places during concreting or the compaction might not have been done effectively or the curing might not have been done correctly or defect might be in erection and/or removal of form work. Moreover, even when the test cubes were cast along with the member being concreted, further progress in concreting was never held up (as obstacle to maintain progress of work) till the 28 days crushing strength of the cubes certifying the strength of the concrete became available to the site engineer. In the name of progress, the constructors gave more importance to the test cubes being cast and tested successfully than the concreting of the members themselves (to avoid any future problem arising in case the test results were not found satisfactory). This led to having individuals other than site engineers specializing in casting of test cubes. This was found to result in the cubes not really as representative a sample of the concreting of the member as desired. In addition to the use of excessive cement in concrete, this and other shortcomings in use of correct technology and procedures led to the defects in concrete that have surfaced in India over the years.

From the basic principals of concrete technology one can list following essentials of strong and durable concrete by using Ordinary Portland Cement:-
  1. Cement is only binding agent and has hardly any inherent strength as a material. It can adhere to surfaces of strong pieces as gum and give strength to the monolith body.
  2. For convenience, pieces of stones as aggregates of various sizes are considered suitable to give a body to concrete. To have cost within limits, quantity of cement should be small and sizes of coarse aggregate pieces be as large as possible in the mixture.
  3. The cement only binds the various aggregate pieces together to make the concrete monolith. Since cement as binder has no inherent strength, the binding layer should be as thin as possible. Moreover, cement being in very fine form has a high coefficient of thermal expansion/contraction compared to that of aggregates used and hence thinner the layer, safer it is. So cement must be used as least as practicable.
  4. Strength of concrete has very little bearing on the quantity of cement in the concrete in the long run. Larger the (than necessary) quantity of cement, the concrete is likely to deteriorate over time faster due to temperature variations in the environment.
  5. New Building Material- Freshly Ground Lime Instead of Cement
    To make concrete stronger, less voids or gaps should be permitted in the concrete monolith. This is possible by using all the intermediate sizes of aggregate (to reduce the size of gaps) and adequate compaction of the concrete in-situ after pouring. Water should be just sufficient to make the rich chemical gel with cement. Extra water remaining if any is likely to create voids after evaporation.
  6. In the concrete mixture only cement is a manufactured substance and hence is more susceptible to environmental damage and deterioration and hence least durable.

    Therefore, thinnest possible gel around the aggregate pieces is all that is needed to make a durable concrete.
  7. In short, to make strong and durable concrete what we need is well graded aggregate to fill the volume of concreting (box?), added with minimum required cement to cover the interstices with strong gel formed with little more than essential, say within 40% water as compared with cement quantity. To make it durable, prevent any voids within the monolith by adequate compaction. You can provide compaction such that the strength of the concrete is as designed.

    The mixture must be uniform and before setting time of the cement is reached, compaction must be completed. Once concrete is cast and set, it should be cured with water at least for 7 days and then damp curing may be satisfactory.
  8. If possible and convenient, it is suggested that concreting can be done by first filling coarse aggregate in the centering boxes and colloidal mass of sand and (water added) cement is poured to fill the voids before compaction. This will ensure that the semi-elastic gel that is produced by the colloidal mixture will be able to coat the aggregate pieces effectively with less cement at the same time giving better strength.
  9. As far as reinforcement is concerned, the quantity of steel as designed must be placed at correct locations to resist tensile stress development in concrete. It must be ensured that the steel reinforcement bars do not shift during pouring and compaction of concrete. Adequate concrete cover must be around the reinforcement to prevent environmental carbon-di-oxide, chlorine or moisture from reaching the bars and corroding them.
In short, it can be seen that once the ingredients of concrete are properly selected, ensuring W/C ratio around 0.4 and adequate compaction to prevent any voids in hydrating (cement in) concrete are the only ways to ensure strong and durable concrete. If these conditions are adhered to, then adding any extra quantity of cement (per cubic meter of concrete) has no positive effect on the strength or durability of concrete. Rather more cement is likely to make the concrete less durable since thicker cement layer will have more shrinkage/ expansion than other ingredients of concrete as environmental temperature wane and wax giving rise to destruction of the monolith.

The cement is required only to surround the aggregate pieces for binding neighboring pieces. The maximum size of aggregate (MSA) will determine the quantity of cement required per CM of concrete. As the MSA decreases, the required cement quantity will increase since the surface area of the smaller aggregate pieces (to be bound together) will increase. Normally, for RCC we use 20 mm MSA. When this size increases (like for road or foundation purposes) to, say 40 mm, then naturally cement quantity will reduce by around 10%. For soil stabilization, cement is mixed in soil (comparatively coarser, even sandy) at not more than 10% by volume. As soil becomes clayey and finer, the cement content may go even up to 25%. This is natural, since surface area of particles to be covered by cement increases appreciably. The unrestrained compressive strength of this stabilized soil becomes about 6 Kgs per mm2. If properly restrained and compacted the resulting compressive strength, it can be comparable with concrete. Thus compressive strength will depend on compaction and W/C ratio only. For a cubic meter of concrete (MSA 20 mm) about 1300 litres of aggregates are required. Accordingly, for cementing purposes, 160 Kgs of cement should be sufficient. Some small additional quantity of cement may be required to cater for inadequate and/or non-uniform mixing of the concrete and to cater for the rough surfaces of the aggregate pieces being bound by the cement paste. Addition of any extra cement cannot make the concrete more durable. In case the mix is found to be non-workable for want of sufficient fines, an odd bag of pozzolanic powder may be added and/ or some plasticizer used. Since the concrete strength will be limited by that of the aggregate used, any lesser strength of the concrete can be achieved by adjusting the compaction suitably. Addition of extra quantity of cement will not do the trick of giving more strength and/or durability to the concrete, in case adequate control on W/C ratio and/or compaction of concrete could not be maintained.

Amongst the constituents of concrete only cement is a factory manufactured item and therefore susceptible to environmental attacks. Natural materials will always be superior and economical when compared with ‘manufactured’ replacements. Since cement has no intrinsic body and therefore strength, any quantity in excess of binding needs, is likely to make the layers between aggregate pieces thicker. Any exposed cement at the surface may get damaged due to environmental factors in addition. Amongst the constituents of the concrete cement is having the largest coefficient of shrinkage. This will ensure that the thicker cement layers will crack and loosen the aggregate pieces while facing changes of environmental temperature. This finally will result in deterioration of the monolith. Therefore, cement used in the concrete must not be in excess. Cement is factory produced, but its raw materials like lime-stone, clay etc are Natural minerals and therefore cement cannot be (and also is not) a product of identical chemical composition (even from adjacent batches). In short, every bag that one opens, needs field checks for characteristics of the cement before using the same. This makes it further costly.

Thus many defects in concrete may be developed after the structures are in use. This has led to development of construction chemical industry. They have developed chemicals to treat these defects. While treating the intended defects the reactive chemicals create some other side effects (defects?) in the concrete. As a result of all this the cement as a replacement of finely ground lime has become enormously costly and beyond the affordability of common man. Doubts are also cropping up if Cement is really an effective and acceptable replacement for freshly ground lime.

It will be apparent therefore that the New Material (cement) that was initiated (with much fanfare) to replace freshly ground lime is neither advantageous nor economical to anyone (at least in Indian environment) but to the manufacturers of Cement and construction chemicals. The structures constructed in Cement Concrete are non-durable, and cannot be made waterproof by human intervention Even Cement is not an environment-friendly material and its production as well as use add pollutants as well as heat to the atmosphere. The position in foreign countries is also not very good. The costly cement concrete structures need varieties of construction chemicals to add for getting desired results. Lot of technical consideration is needed to determine the type and quantity of the chemical to be mixed. The results are not of required durability or long lasting. The chemicals to be added are not inert and therefore dangerous to life and nature as well. Since however, the Westerners did not have any better method or construction material before cement, they may continue to insist on cement as ‘Best’ building material in use; let them. It is suggested that at this stage of development India should carry out checks on the utility of freshly ground lime against cement. Selection from the large variety of cements and additives in the market and the appropriate practices of complicated processes of designing, mixing, pouring, compacting as well as curing of concrete are confusing even to engineers and the construction is not economical to the consumer i.e. common man. Structures constructed with lime over 60 years ago appear to be still in serviceable state without undue maintenance expenditure.

Cement is manufactured from mixture of lime stone and clay (both ground/crushed) in water (or dry if could be uniformly mixed). The slurry is blended to correct composition. This corrected slurry fed to rotary kiln heated by powdered coal is converted into clinkers. These clinkers ground in ball mill with addition of 2 to 3 % gypsum (for preventing flash setting). This cement is stored in cement silos for loading in bags or vehicles. The coal requirement of the rotary kiln is 350 Kgs per ton for wet process and 100 Kgs for dry process. Many factories produce cement by dry process but still some use wet process. Let us assume that on the average a ton of cement needs 150 Kgs of coal. 200 MT of cement (quantity produced and used in whole year of 2006) has used 30 million tons of coal. This would have added about 75 million tons of CO2 (Carbon-Dioxide) to the environment to increase the atmospheric temperature. Other countries in the world would have added lot more and thus earth temperature would have been raised to a very high extent. It is possible that this has been taken into account in Industry’s contribution to environmental pollution. It therefore can only be noted here as cement factory’s pollution portion.

Once cement concrete is poured in formwork and starts hydrating, it evolves heat. Total heat that cement can generate during hydration is around 125 calories per gram of cement during its complete activity of hydration. This activity is a long drawn process and for our purpose we take 28 days hydration as full hydration for design purpose. A heat of hydration of 90 calories is given out by every gram of cement during that period. Let us consider that on the average 3 calories of heat is given out per day by one gram of cement. As per reports, in whole year India has consumed 200 million tons of cement during 2006 (or say, 17 million tons per month) for construction/repair of structures. Thus during the year 2006 concrete structures (only under construction/repairs) have given out 34 X 10*12 calories (or 34 trillion calories of heat) to atmosphere EVERY DAY!. More heat at a lower rate is being given out in balance period of the year in addition. This is during construction. Once the structures are in use, their exposed concrete bodies absorb heat from the sun during day and reject to the atmosphere during the evening is another aspect of heat evolution by concrete structures. India is considered to be (still) developing indicating that the use of cement is going to increase continuously as the ‘development’ progresses. Environment is getting heated to a large extent by the use of cement. Thus quite an appreciable quantity of heat is generated (for the environment) by cement consumed by all the nations; more by the developed Nations. Thus one can imagine how much heat is daily given out by the cement use to the environment so that earth temperature goes on increasing. It surely cannot be dismissed as minor aspect while considering the ‘Green- House’ effect on the earth due to human activities.

It will be clear from above discussion that cement as building material has technical problems right from start and could not yet been completely rectified. Rather they appear to be increasing continuously. People using it have perforce to add some (more expenditure?) chemicals to overcome the defects. Similar to modern system, an attempt to rectify some defect creates another one in concrete structures as well. In addition, this material requires lot of energy during production process and adds heat and pollution to the environment while in use. It is thus creating havoc all over the world. Actually, Kyoto round of WTO talks during Nineties could have done better by adding ‘scrapping of this material from production (as well as use) slowly’ to its suggestions for corrective measures to reduce rising temperature of the earth.

While discussing this aspect with my friends most of them agreed with the view that cement is surely a building material quite inferior to burnt and ground lime. However, they did insist on telling that the lime is incapable of constructing highrise buildings for which cement concrete is only available. Use of cement therefore can immediately be stopped only for buildings lower than 3 stories high. Quite a large amount of pollution can be reduced by use of this rule since only a fraction of buildings are presently highrise structures. This should be immediately implemented.

If we consider the condition of man on this planet since he came, it will be apparent that getting way from Nature’s contact and desire to abjure physical labor are the two tendencies of man which are detrimental to him. Lack of physical labor has made him fall sick frequently for lack of exercising his body adequately and properly. Distance from Nature has kept him away from Nature’s ways to prevent/recover from various mental as well as physiological illnesses. When man stays in highrise buildings he is necessarily away from earth i. e. Nature and he misses all the advantages of it. Therefore, as a rule man need not operate from high rise structures at all. Therefore, he could have done nicely with burnt & ground lime as construction material and avoided manufacture and use of cement at all. All the pollution of environment as discussed above could have been avoided. Now as well disusing this material he can reduce environmental pollution. He will have to use lime as a new building material instead. In old world at least, he will have few persons familiar with it to help him in this aspect.

Mr Joseph Aspdin, a Leeds constructor took a patent for Portland cement {a fine powder of certain earth crust found in nature which was similar to the rock at Portland (a place in England) in color in 1824. Its use in Europe started in right earnest since they had no other (suitable) construction material prior to that time. As good traders they propagated with zeal and force this material in their colonies. The slave population had no choice to refuse it (though they had better material in ‘finely ground lime’) for masonry. During their rule British always discouraged the use of any indigenous materials or systems like Dhaka Mulmul, Handloom weaving, Ayurved and Gurukul System of learning in India and offered their imported versions instead, specifically to kill the indigenous systems and fleece the riches of the enslaved country and people. Same thing happened concerning use of lime in construction during their rule. This led to disuse of lime slowly till WWII, after which it nearly reached its extinction as a building material. As a young boy, I remember to have seen the use of ‘Ghaani’ being used for grinding lime by bullock when our house at Satara was being extended in around fifties (about 55 years ago) as an only instant.

It is quite likely therefore that some oldies aware about use of this material may still be around and can assist us in redeveloping it into a building material superior to Cement in all aspects. The organization of IITians that is coming up in India to develop technical education (and other aspects) Nationwide can do well to serve the World if they can revive the ‘finely ground lime’ as a building material to replace the ‘dirty’ cement. The world will be saving not only money and energy but it will be saving the environment and Nature as well for our future generations. This will really be going back to the (progressive) future!
Reference
  1. Minimum Cement Content for Strength and Durability of Concrete– Technical Rationale’ by Prof M D Apte published in NBM & CW Jan 2002.
  2. Cement concrete text book ‘Concrete Technology’ by Prof M. S. Shetty.
  3. Indian Standards concerning Concrete Construction as brought out by BIS
  4. Cement Manufacturers Literature and publications
  5. Experiences of the author during his professional career
  6. British rulers’ efforts in imposing Western culture

Addition from Editor

Gangaccanal aqueduct known as Solani Bridge near Roorkee in Uttarakhand is a fine example of lime construction (Masonary line mortar) which is more than 150 years old by now with no problems. Large number of Arch Bridges on Ganga canal (constructed along Solani bridge starting at Hardwar are constructed using Bricks masonary in lime mortar.

NBMCW November 2007

.....

Strange are the Ways of Cement and Conc...

Strange are the Ways of Cement and Concrete

Dr. Anil K Kar, Engineering Services International, Kolkata; Arun Kumar Chakraborty, Asst. Professor,Civil Engineering and A K Sarkar, Civil Engineering Bengal. Engineering, and Science University, Shibpur

Introduction

It is commonly recognized that the compressive strength and other useful properties of concrete increase with increasing duration of curing, more particularly moist curing (lower curve in Figure 1). This knowledge of increasing compressive strength with increasing periods of moist curing has been gained from tests over the years where standard cubes or cylinders of concrete are tested on the last day or a day after a specified period of moist curing.

It is commonly known that the rate of gain in strength in the initial period is faster in the case of ordinary portland cement (OPC) than in the cases of portland slag cement (PSC) and Portland pozzolana or flyash cement (PPC). It is generally... observed that after the first few weeks of moist curing, further gain in strength in the case of OPC concrete is insignificant (lower curve in Figure 1), whereas concrete with blended cement (PSC and PPC) can gain considerable strength beyond the first week or two of concreting (Figure 2).

Strange are the Ways of Cement and Concrete
Much of the knowledge, more particularly impression, about concrete and concrete structures is based on the performance of well cured concrete and concrete structures which were built decades ago with OPC of that time. The impressions, many a people have about concrete, is that concrete is impervious and naturally waterproof. Another impression, people carry from the past, is that concrete structures are durable.

Much has changed over the years with cement and construction practices, hastening the decay and distress in modern concrete structures.

This paper is an attempt to study the changed ways of cement and concrete. This is limited to studying the influence of the duration of moist curing on the compressive strength of concrete with OPC, PSC, and PPC.

Though blended cements find a very considerable share of the construction market in India today, the earlier practice was to use mostly OPC.

It was an old practice in the era of OPC to provide 28 days' moist curing to concrete.

Over the years, OPC went through many modifications in its chemical compositions and physical characteristics, resulting in higher ultimate strength and the development of most of this ultimate strength within a week or two of concreting (lower curve in Figure 1).

This early attainment of much of the ultimate strength and a greater emphasis on early completion of projects made the codes/standards lower the required period of moist curing. The Indian Standard Code of Practice for Plain and Reinforced Concrete, IS:456:20001, as well as its earlier version2 lowered the requirement of the minimum period of moist curing of OPC concrete from 28 days in earlier decades to 7 days.

Increasingly, cement manufacturers in India started marketing blended cements aggressively. Because of slower rates of hydration, it became necessary to set standards at longer periods of moist curing of concrete with blended cements.

In order that concrete of comparable mix proportions with blended cements may yield comparable or higher (than 28-day OPC concrete strength) strengths, researchers generally recommend 56 to 90 days of moist curing of concrete in the case of blended cements with mineral admixtures. The Indian code1, however, considers it sufficient to cure such concrete with blended cements for only 10 days. The code recommends that this minimum period of 10 days may be extended to 14 days. The earlier code required 7 days' moist curing for concrete with blended cements too.

Though the code1 considers moist curing for periods ranging between 7 to 10 days to be adequate for concrete construction with different types of cement and though the duration of effective curing of concrete during construction may be even less thanthe periods specified in the code 1 , the design and construction of concrete structures are based on compressive strength of well compacted concrete samples after 28 days of moist curing.

Besides the shortcomings, which may arise as a result of the gap between the required (considered desirable by researchers on the basis of attainment of strength) and mandated (by the code) periods of moist curing, and besides the obvious gap between the design (tested after 28 days' moist curing) and the actual periods of curing, Kar3-7 has pointed out that today's cement is in many ways different from cements which were used till a few decades ago and which had given durable concrete structures. Furthermore, Kar3-7 has shown that today's cement in India may contain harmful alkalis to make concrete less durable or even self-destructive (curve for PPC in Figure 2).

In this scenario, it is considered appropriate: (a) to study the influence of curing on the development of strength in concrete and (b) to examine the reasonableness of the codal provisions on the duration of moist curing of concrete. This is done for concrete, made with the three basic types of cement (viz., OPC, PSC and PPC).

Codal Provisions on Curing

Among the different provisions on curing of concrete, IS:456-20001 suggests that "Curing is the process of preventing the loss of moisture from the concrete whilst maintaining a satisfactory temperature regime. The prevention of moisture loss from the concrete is particularly important if the watercement ratio is low, if the cement has a high rate of strength development, if the concrete contains granulated blast furnace slag or pulverised fuel ash. The curing regime should also prevent the development of high temperature gradients within the concrete.

" The code1 also requires that "Exposed surfaces of concrete shall be kept continuously in a damp or moist condition by ponding or by covering with a layer of sacking, canvas, hessian or similar materials and kept constantly moist for at least seven days from the date of placing concrete in case of ordinary Portland Cement and at least 10 days where mineral admixtures or blended cements are used. The period of curing shall not be less than 10 days for concrete exposed to dry and hot weather conditions. In the case of concrete where mineral admixtures or blended cements are used, it is recommended that above minimum periods may by extended to 14 days."

It appears from the language of the code that the extension of the curing period from 10 days to 14 days is not mandatory.

It is of interest to note here that the provisions in IS :456-20001 were considered reasonable eight years ago, whereas the provisions in IS :456-19782, which permitted 7 days' moist curing for concrete with OPC, PPC as well as PSC, were considered reasonable at least thirty years ago.

During the intervening 22 years between the two codes, many structures with PSC concrete must have been cured moist for 7 days or less.

During the intervening period between the two codes1.2 and during the period following the more recent code, cement has undergone very significant changes in chemical compositions and physical characteristics3-7.

The resulting effects of such changes include the generation of considerable heat inside concrete at early ages. There is also the exothermic reaction from the high contents of water soluble alkalis in Indian cement of today3-7, further hastening the rate of hydration of cement, thereby leading to still faster gain in strength. All of these can make it possible to lower the required period of moist curing if the attainment of strength in concrete will become the only criterion in the determination of the adequacy of any particular period of moist curing of concrete. This would suggest that even if it may be found that today's high early strength cements may yield strengths within acceptable ranges on satisfaction of the requirements of the mandated moist curing for short periods, such short duration curing in the past might not have yielded concrete strengths close to the design strengths at 28 days.

As stated earlier, it is studied here whether the stipulated periods of moist curing, set in IS:456-2000 1 for concrete with OPC and blended cements, are reasonable or not. This is done with cement that is available in the Kolkata region today. The tests for compressive strength were conducted using 150 mm cubes with cements of several nationally and internationally known brands.

The evaluation of the adequacy of the period of curing is made from the consideration of attainment of strength as a percentage of strength at 28 days of moist curing.

It is well–known that moist curing, particularly at the initial periods, reduces the permeability of concrete. It is further known that greater the impermeability, better is likely to be the durability of concrete structures. In clause 8, the code 1 has, however, given four options for lengthening the life of concrete structures. Kar8 has explained that among the four options, given in the code, only the option of providing surface coatings/protection systems to concrete structures is practical in lengthening the life of concrete structures.

Since the provision of surface protection systems will effectively make concrete surfaces or structures impervious to the external agents of decay, any shortcomings in the form of greater permeability of concrete due to any inadequacy in curing loses some or much of any significance. Accordingly, no serious attempt is made to study the effects of curing on the permeability of concrete at 7 or 10 days vis-a-vis permeability of concrete cured moist for 28 days.

Codal Provisions and Effects of Curing

It is a common practice, in the acceptance of cement and concrete, to ensure that concrete has the required compressive strength.

There may be additional tests for setting times for workability and expansion as an yardstick for durability.

Five sets of curves are presented here as a part of the study on the influence of the period of moist curing on concrete strength.

Figure 1 shows the compressive strength of OPC concrete. The lower curve in Figure 1 represents the strength of OPC on the completion of moist curing for 1, 3, 7, 10, 14, 21 and 28 days as is the conventional practice. In this particular case, the 7-day strength of 26.22 MPa is 77.6% of the 28-day strength of 33.78 MPa.

It would appear that the mandated curing period (minimum) of 7 days may or may not lead to an endangerment of the safety of an OPC concrete structure if it would have been designed and constructed in accordance with the 28-day strength but cured moist for 7 days and loaded immediately thereafter. The real situation is better if the structure will not be loaded immediately after 7 days of moist curing, as explained below.

An interesting observation can be made here with the help of the upper curve in Figure 1 which represents the compressive strength of the concrete as that in the lower curve, except that the upper curve represents the compressive strengths at 28 days for cube samples which were cured moist for different periods from 0 to 28 days. It is observed that the peak concrete strength of 37.90 MPa at 28 days is higher than the concrete strength of 33.78 MPa when it is cured moist for a period less than 28 days. In this particular case, it so happens that the peak strength at 28 days is available when concrete is cured moist for 7 days, and then cured in air a further period of 21 days. In the same token, it is observed that in the event the concrete in Figure 1 would not be cured moist at all, but kept covered or in shade, a minimum strength of 30.46 MPa would develop at 28 days, which is 90% of the strength of the concrete, if it would be cured moist for 28 days. This is not too bad a situation where concrete may not be moist cured at all and the attainment of strength of concrete will be the only consideration.

Figure 2 shows the gain in compressive strength of PPC concrete from one batch and PSC concrete from two batches. The 150 mm cubes were cured moist for different durations. The cubes were tested for compressive strength at the end of each period of curing. The samples were cast in the month of April 2008. It is noticed that the compressive strength of concrete at the end of stipulated 1 periods (10 days) of moist curing is 76 to 80 percent of the 28-day strength in the case of PSC concrete whereas it is 91 percent in the case of PPC concrete.

Since structural elements e.g., floor slabs and floor beams are frequently loaded close to their design loads during construction stages, it is recognized here that it may not be unreasonable if the moist curing will be terminated at the end of 10 days in the case of PPC concrete, but the same cannot be said in the case of PSC concrete. It will be seen later that if loading of the structure will be delayed, moist curing of the structure or structural element for 10 days may not be too unreasonable.

It is recalled here that the code1 permits 7 days' moist curing in the case of OPC concrete and 10 days' moist curing in the case of concrete with blended cements, whereas (a) the design is based on compressive strength after moist curing for 28 days, and (b) real structures are seldom cured moist for the stipulated (in the code) periods.

In consideration of the above, it would appear that if adequate care will not be taken to limit construction or service loads, immediately upon the termination of moist curing, to within 70% of the design loads, and that too with appropriate margins for various uncertainties, the stipulated periods of curing will prove to be unreasonable. It is seen in the following that the picture is not necessarily unreasonable, if
  1. concrete will be cured moist for at least 3 days, and
  2. the structure will not be loaded until 28 days after concreting or some such days after concreting as may be determined from tests for particular batches of cement.
This suggestion is made only in the context of strength. Prolonged curing may increase the durability of concrete by minimizing permeability and shrinkage.

In contention of the above, three cases are studied in the following. These include cases:
Strange are the Ways of Cement and Concrete

Strange are the Ways of Cement and Concrete
  1. When the design is based on compressive strength of concrete after 28 days' moist curing but the concrete is cured for the stipulated period of 7 or 10 days and the structural element is not loaded until 28 days.
  2. The concrete is cured for 3 days and the structural element is not loaded until 28 days.
  3. Where the design is based on compressive strengths of concrete after 28 days' moist curing but the concrete is not provided with any moist curing and the structural element is not loaded until 28 days.
Figures 3, 4 and 5 show the variations in compressive strength of concrete when concrete is tested at 28 days but cured moist for 0, 3, 7, 10, 14, 21 and 28 days, followed by air curing for 28, 25, 21, 18, 14, 7 and 0 days, respectively. Test results on concrete samples from three batches of concrete are represented in each of the three figures.

It is seen in each case in Figures 3 to 5 that the peak compressive strength is recorded when concrete is tested at 28 days but the moist curing is between 3 to 14 days (Tables 1 – 3).

Among all the three batches of OPC concrete (Table 1), it is seen that strengths equal to or higher than the strengths, at 28 days' moist curing, can be obtained if the moist curing will be discontinued after 3 days.

It is further noticed in Figure 3 and Table 1 that in two cases the peak strengths in OPC concrete were obtained when concrete was cured moist for 7 days, followed by air curing for 21 days before the test. In fact, the 28-day (moist curing) compressive strength was lower than the peak strength (at 7- day moist curing followed by 21 days' air curing) by as much as 17.4 percent in one case.

In the case of PSC concrete (Figure 4 and Table 2), the peak strengths were gained when concrete was cured moist for 7 to 14 days, followed by air curing for the remaining days. In the three cases of PSC too, it is seen that the 28-day strength could be reached by discontinuing moist curing after 3 days. It's noticed that with continued moist curing, there is a drop of strength (from the peak) by as much as 11.3 percent.

In the case of PPC concrete (Figure 5, Table 3), the peak strengths were obtained after 7 to 14 days' moist curing, followed by air curing till 28 days from concreting. In two of the cases, the strengths of concrete were higher than the 28-day strengths when the cubes were cured moist for 3 days, followed by curing in air for 25 days. In the remaining case (Case III), the 28-day strength (moist cured) could be obtained with 9 days' moist curing, followed by 19 days of air curing. In case I (Table 3), there is a drop of 25.1 percent in the peak strength with continued moist curing beyond 10 days.

It is seen that the stipulated1 period of 7 days' moist curing in the case of OPC and 10 days in the case of blended cements is justified as far as matching the 28- day (moist cured) strength is concerned provided that the structures will not be loaded till 28 days after concreting.

It is seen that the stipulated2 period of 7 days' moist curing for concrete with blended cements, as in Ref. 2, was not reasonable, particularly when cement of the earlier periods did not gain strength as early as it does today.

Concluding Remarks

There is an increasing trend to shorten the period of moist curing of concrete. The Indian code IS:456- 20001 has lowered the required period of moist curing of concrete to 7 days in the case of OPC and 10 days in the case of blended cements with mineral admixtures. The period of 10 days is, however, higher than what was specified in Ref. 2. In some countries, the minimum period of moist curing has been lowered to 3 days and in real cases in India many concrete structures are not provided any moist curing.

From the performance of four batches of OPC concrete (1 in Figure 1 and 3 in Figure 3), it appears that curing concrete, with today's OPC, for 3 days in moist condition will be sufficient if matching the 28-day design strength will be the only criterion and the structures will not be loaded until 28 days after concreting.

From the performance of nine batches of PPC and PSC concrete (3 each in Figures. 2,4 and 5), it appears that except in one case of PPC concrete, curing concrete, with today's PPC and PSC, for 3 days in moist condition will be sufficient if matching the 28-day design strength will be the only criterion to be fulfilled and the structures will not be loaded until 28 days after concreting. In case III of PPC in Figure 5, moist curing of 9 days, followed by air curing of 19 days, leads to a matching of compressive strength of concrete if such concrete will be cured moist for 28 days before loading.

A closer study of the various curves in Figures 1 to 5 show a decreasing trend for the strength of concrete with continued moist curing after the peak strength will have been reached much earlier than 28 days. This challenges the widely held concept about concrete that concrete increases in strength with continued curing in moist condition.

Studies are underway to determine if the declining strength with increasing moist curing has more to do with impurities in today's cement3-7 or with any lingering damp/ moist condition of the test samples after the initial period of moist curing.

References

  • IS:456-2000, Indian Standard, Plain and Reinforced Concrete Code of Practice (Fourth Revision), Bureau of Indian Standards, New Delhi, July 2000.
  • IS:456-1978, Indian Standard Code of Practice for Plain and Reinforced Concrete, Third Edition, Bureau of Indian Standards, New Delhi, September 1978.
  • Kar, A. K., "Concrete Structures We Make Today," New Building Materials & Construction World, New Delhi Vol. 12, Issue 8,February 2007.
  • Kar, A. K., "The Ills of Today's Cement and Concrete Structures," Journal of the Indian Roads Congress, Vol. 68, Part 2, July- September 2007.
  • Kar, A. K., "Durability of Concrete Bridges and Roadways," New Building Materials & Construction World, New Delhi, Vol. 13, Issue 3, September 2007.
  • Kar, A. K., "Woe Betide Today's Concrete Structures," Part I New Building Materials & Construction World, New Delhi, Vol. 13, Issue- 8, February, 2008.
  • Kar, A. K., "Woe Betide Today's Concrete Structures," Part II New Building Materials & Construction World, New Delhi, Vol.13, Issue- 9, March, 2008.
  • Kar, A. K., "IS 456:2000 On Durable Concrete Structures," New Building Materials & Construction World, New Delhi, Vol. 9, Issue-6, December, 2003.s

NBMCW July 2008

.....

An Overview of Some Development in CONC...

Dr. S.S. Rehsi, Consultant (Building Materials' Former United Nations Expert on Building Materials, Chandigarh

Introduction

There has been rapid advances in concrete technology during the past three decades or so. The improvement in strength and other structural properties achieved earlier through the use of steel reinforcement are now accepted as routine and the reinforced cement concrete and pre–stressed concrete have become conventional materials. Later work led to the development of a variety of concretes in the form of, among others, fibre reinforced concrete, polymer concrete, Ferrocement, sulphur concrete, lightweight aggregate concrete, autoclaved cellular concrete, high-density concrete, ready-mixed concrete, self-compacting concrete, rollercompacted concrete, high strength concrete, super high-strength concrete, high performance concrete, high-volume fly ash concrete, self-curing concrete, floating concrete and smart concrete (1-27). Some of these concretes are briefly discussed here.

Fibre Reinforced Concrete

An Overview of Some Development in CONCRETE TECHNOLOGY
Different types of mineral, organic and metallic fibres have been used. Among the mineral fibres, use of asbestos in the production of asbestos cement products is well known. Since, water absorption of the asbestos fibre is high, its use in concrete increases water requirement. Consequently, there is reduction in strength of the concrete. Organic fibres such as, coir, jute, rayon and polyester are attacked by the highly alkaline condition in concrete. As a result, concrete containing these fibres loses strength with time. Other organic fibres namely, nylon, polypropylene and polyethylene are alkali-resistant. But, due to their lower modulus of elasticity, the incorporation of these fibres do not increase strength. Concrete containing nylon or polypropylene fibres, however is reported to develop higher impact resistance. Virgin Poly-Propylene fibers of structural grades, such as Forta Ferro Fibres, having high strength and moduls of elasticity are now available from FORTA Corpn USA. These are being extensively used all over world for Pavement/highways/Runway construction, Fibreshotcreting of tunnels, Repair and Rehab jobs and Bridge deck construction incl India. Among all fibres the use of steel fibre in concrete has received far greater attention, in the past but because of the corrosion problem structural grade poly propylene and other synth fibers are taking over now.

The compressive strength, tensile strength, fatique strength, modulus of elasticity, abrasion resistance, skid resistance and thermal conductivity of steel fibre reinforced concrete has been found to be slightly higher than the corresponding plain concrete. While creep and shrinkage are more or less unaffected, there is over 100 percent increase in the flexural strength and impact toughness of plain concrete when reinforced with steel fibre, 2 percent by volume. At the same fibre content, use of a blend of fibres having different aspect ratio, in place of single aspect ratio fibre, gives greater structural benefits. It has also been found more beneficial as well as economical to use steel fibres only in the tensile zone of the flexural member. Unlike plain concrete, steel, fibre reinforced concrete is not brittle and offers far greater resistance to cracking. The fibres act as crack arrestors and restrict the growth of flaws in concrete from enlarging under stress into visible cracks. The ultimate failure is reached only when some of the fibres get pulled out of the matrix. As compared to plain concrete, the resistance of steel fibre reinforced concrete to thermal shock and heat spalling is also far superior.

The major applications of steel fibre reinforced concrete are in pavements (both for new construction and overlays), precast concrete units, concrete reactor pressure vessels, blast resistant structures, machine foundations, tunnel linings and structures requiring resistance to thermal shocks, such as refractory linings.

Polymer Concrete

Depending upon the method of monomer incorporation into the concrete, the polymer concrete is termed as
  1. polymer impregnated concrete, when dried precast concrete is impregnated with monomer and polymerized in-situ,
  2. polymer cement concrete, when cement, aggregate, water and monomer are mixed together and polymerized after laying and,
  3. polymer concrete, when aggregate and monomer are mixed together and polymerized after laying.
A number of factors such as distance to be penetrated, degree of drying, total porosity and pore size in concrete, monomer viscosity, whether or not vacuum and/or pressure is applied, influence the extent of monomer filling in polymer impregnated concrete. The widely used monomers are methly methacrylate, styrene, acrylonitrile and chlorostyrene. The monomer polymerization is done either by thermal catalytic process or by radiation.

As compared to plain concrete, the strength and other properties of polymer concrete are considerably higher. At 6 per cent polymer loading, the mechanical properties of polymer impregnated concrete vis-à-vis corresponding plain concrete were found to be as follows:
  • Compressive strength, 2 to 4 times higher
  • Tensile strength, about 4 times higher
  • Modulus or Elasticity, About 4 times higher
  • Creep and Permeability, Almost Nil
With almost nil permeability, the polymer impregnated concrete has much greater resistance to the attack of acidic and/or sulphate containing waters

Economics permitting, applications of polymer concrete having good scope are: concrete pipe manufacture, concrete piles, concrete tiles, tunnel supports and linings, precast concrete decks, precast concrete building units for use in aggressive conditions, desalting structures, lightweight concrete constructions and providing surface protection to cast in-Situ concrete.

Ferrocement

An Overview of Some Development in CONCRETE TECHNOLOGY
Ferrocement is a kind of reinforced concrete in which the matrix is cement mortar, microconcrete and the reinforcement is in the form of layers of wire mesh or similar small diameter steel mesh closely bound together to produce a stiff structural form. The mix proportions of the cement mortar usually are: cement 1 part, sand 1.5 to 2.5 parts and water 0.35 to 0.5 part, by weight. Admixtures are added in the mix for improving to properties. The maximum size of sand grains depends upon the mesh opening and reinforcing system to ensure proper penetration. Different types of wire mesh such as, hexagonal wire mesh (commonly known as chicken wire mesh), welded wire mesh, woven mesh, expanded metal mesh, are used. Use of Hexagonal Mesh is not preferred due to its poor resistance to loads. The mechanical behavior of Ferrocement is greatly influenced by the type, quantity, orientation and strength properties of the mesh. The thickness of ferrocement elements range from 2 to 3 cm with 2 to 3 mm external cover. When additional strength is required, one or more layers of steel bars are inserted between the inner layers of the mesh. Use of short random fibres in Ferrocement elements at the same steel content has been found to greatly increase the modulus of elasticity and strength. Polymer impregnation of the Ferrocement elements, with and without short random fibres is reported to considerably improve upon these properties.

Ferrocement has a variety of applications. The important among these are: construction of fishing and cargo boats, grain storage bins, water storage tanks, biogas holders and digesters, fermentation tanks, precast roofing and walling units, cooling towers, sewage troughs, septic tanks, irrigation channels, drying pans for agricultural products, shutters and formwork for use in concrete constructions, lining for tunnels and mines, and providing waterproofing treatment over RCC or RB roofs, lining of surface of tanks or swimming pools. Ferrocement has been successfully used in india by SERC (G)'s Material Science Group for construction of domes, large tanks, manhole covers, Drainage units and for repair and rehab of structures. the new techniques and applications developed by this group are being used on large scale on commercial basis.

High Strength Concrete

IS: 456-2000 designates concrete having 28-day compressive strength of 60 to 80 N/mm2 corresponding to grades M60 to M80 as high strength concrete (HSC).

The production of HSC requires stringent control on the quality of materials used. The Portland cement should, preferrably, be of 53 grade conforming to IS: 12269- 1987. Crushed stone coarse aggregates produced from trap, quartzite or granite give higher strength and are more suitable than rounded gravel for use in making HSC, particularly when the desired concrete strength is 70 N/ mm2 or more. Studies on the effect of the size of coarse aggregate on the strength of concrete showed that smaller size produced higher strength. A maximum coarse aggregate size of 10 mm is considered suitable for use in HSC. The use of mineral admixtures such as fly ash, silica fume, metakaoline in combination with suparplasticizers in HSC matrix greatly enhances impermeability, durability and strength.

The use of HSC in construction offers the advantages of
  1. reduction in the size of concrete members with resultant reduction in self-weight,
  2. greater stiffness,
  3. early stripping of formwork and
  4. lowering of construction cost due to reduction in the concrete member size and self-weight.
It has, therefore, been widely used in the construction of highrise buildings and bridges in many countries including India. A super high-strength concrete, called reactive powder concrete (RPC) is produced by eliminating the use of coarse aggregate. The concrete matrix consists of cement, finely ground sand with particle size close to that of cement, silica fume and short steel fibres. The water/cement ratio is kept very low, around 0.15. The desired workability is obtained by using higher amounts of super plasticizers. The RPC does not require reinforcement bars. It is suitable for use in building very thin structures meeting different architectural needs.

High Performance Concrete

High performance concrete is defined as concrete that meets special Performance and durability requirements in terms of mechanical properties, volume stability and longer life in severe environmental conditions to which the concrete is exposed during its service life. High performance of concrete is generally linked to strength of the concrete; higher the strength, better the performance. Therefore, in the first place high performance concrete has to be a high strength concrete. Besides high strength, low permeability of concrete is an essential requirement to prevent ingress of corrosive waters containing chlorides, sulphates and /or other deleterious salts. Low permeability is achieved by using higher cement content, mineral admixtures such as fly ash, Silica fumes, metakaoline or granulated blast furnace slag, and keeping water/cement ratio low at 0.35 or less. Higher amounts of super plasticizers are used to obtain the desired workability in the concrete matrix. Workmanship has to be excellent to ensure full compaction and proper concrete cover over embedded steel reinforcement. All these and subsequent adequate curing of concrete after laying and regular maintenance of concrete construction ensure high performance.

High-volume Flyash Concrete

An Overview of Some Development in CONCRETE TECHNOLOGY
High-volume fly ash concrete technology was developed at the Canada Centre for Mineral and Energy Technology (CANMET), Ottawa, Canada in 1980’s. It enables minimizing the amount of cement required to produce high quality concrete for different types of applications by incorporating upto 50 to 60 percent fly ash in the concrete mix. The concrete is prepared using a low water/cement ratio of 0.30, and the desired workability is obtained by using super plasticizers. While highvolume fly ash concrete was initially developed for mass concrete construction where low heat of hydration and just enough early strength were required, later work showed that this concrete developed excellent long-term structural properties, namely compressive strength, flexural strength, splitting tensile strength, and Young’s modulus of elasticity. Its durability measured in terms of its low water permeability, resistance to carbonation, alkaliaggregate reactions and penetration of chlorides and sulphates, was also found to be excellent. In view of this, highvolume fly ash concrete is eminently suitable for structural applications, in addition to its use for building of roads and pavements. It is being used for such constructions in Canada, U.S.A. and other countries. In India, Ambuja Cements Ltd, has made a beginning by building two fly ash concrete roads, one at Ropar (Punjab) and another at Ambujanagar (Gujarat) in 2002, using 50 per cent fly ash in the concrete mix. Both these roads are reported to be performing very well.

Self-compacting Concrete

A concrete that gets compacted by itself totally covering reinforcement in the formwork is called self-compacting concrete (SCC). It is highly flowable, selflevelling, self-defoaming and coahesive and can be handled without segregation. Like any other super plasticized concrete, the ingredients in SCC mix consists of cement, coarse and fine aggregates, mineral and chemical admixtures. A limiting value of coarse aggregate as 50 per cent of the solid volume of the concrete, and of fine aggregate as 40 per cent of the solid volume of the mortar fraction in the SCC mix proportion is suggested for achieving good self-compact ability. Commonly used mineral admixtures are fly ash, silica fume, ground blast furnace slag Chemical admixtures consists of a super plasticizer and a viscosity modifying admixture. The use of one or more mineral admixtures having different morphology and particle-size distribution improves deformability, self-compact ability and stability of the SCC. While the super plasticizer helps achieving high degree of flow ability at low water/ Cementing material ratio, the viscosity modifying admixture increases viscosity of the fresh concrete matrix and reduces bleeding.

The SCC has the advantages of easy placement in thin - walled elements densely reinforced concrete structure, quality, durability and reliability of concrete structures, faster construction and reduced construction cost.

Self-curing Concrete

An Overview of Some Development in CONCRETE TECHNOLOGY
Curing of concrete by which the concrete, after laying, is kept moist for some days, is essential for the development of proper strength and durability. IS 456-2000 recommends a curing period of 7 days for ordinary Portland cement concrete, and 10 to 14 days for concrete prepared using mineral admixtures or blended cements. But, being the last act in the concreting operations, it is often neglected or not fully done. Consequently, the quality of hardened concrete suffers, more so, if the freshly laid concrete gets exposed to the environmental conditions of low humidity, high Wind velocity and high ambient temperature.

To avoid the adverse effects of neglected or insufficient curing, which is considered a universal phenomenon, concrete technologist and research scientists in various countries including India, are working on the development of self-curing concrete. Different lines of action are being pursued. These include
  1. use of water-soaked, surface dry lightweight aggregates which release water when the concrete starts getting dry and losing water,
  2. develop high early strength concrete, which attains a strength of 20N/mm2 in 30 minutes and so may not require further curing, and
  3. develop a system by which some “enteric” coated particles or capsules containing membrane-forming curing compound (or a substance that reacts with water to do so) is distributed over the surface of the concrete slab in the final stages of finishing. The particles will open if the surface becomes dry and a membrane will form while the concrete is still water-saturated upto its top but has no free water on the surface.

Smart Concrete

Smart concrete is a concrete that can take care of its own shortcomings or that can act as a senser to help detecting internal flaws in it. It is produced by incorporating some changes in the ingredients of the concrete mix. For instance,
  1. due to its high density, the high strength concrete does not permit water vapours to go out during fire, leading to spalling off concrete cover and damage to concrete members. Addition of 2kg polypropylene fibres per m3 of high strength concrete mix increases fire resistance. At high temperature during fire, these fibres melt and leave pores for water vapours to escape from the concrete surface, thus preventing spalling and damage,
  2. incorporation of 0.5 per cent specially treated carbon fibre in the concrete mix increases the electrical conductivity of the concrete. Under load, the conductivity decreases but returns to original on removal of the load. The concrete could thus act as a senser to
    1. measure the number, speed and weight of the vehicles moving on concrete highways, and
    2. detect tiny flaws regarding internal condition of concrete construction after an earthquake, and
  3. Use of porous carbon aggregate, available in the form of coke at the steel plant, in the concrete mix imparts good electrical conductivity which can help in room heating, melting of ice on concrete highway and runways by passing low voltage current.

Conclusion

As other areas of research and development (R&D) in concrete technology has been a continuing process, Different types of concretes, as described above, have been developed from time to time, to meet the needs of the construction industry. Technologies for self-curing and smart concrete are still in the development stage, but are expected to be fully developed soon and available for use in constructions.

References
An Overview of Some Development in CONCRETE TECHNOLOGY
  • State–of–the–art Report on Fibre Reinforced Concrete, ACI Committee 544, ACI journal, November 1973, pp-729-743
  • Neville, Adam, End, Proceeding: RILEM Symposium on Fibre Reinforced Cement Ltd. Lancaster(U.K.)
  • Parameswaran, V.S., and Krishnamoorthy, T.S., Eds., Proceedings Fibre Reinforced Concrete, Madras, December 16- 19-1987, Vol.11, Oxford TBH Publishing Co. Pvt. Ltd., New Delhi(India)
  • Shah, S.P., Ed., Proceedings: Conference on new Materials in Concrete construction, University of iiiinoisat Chicago Circle, Chicage, Iiiinois (U.S.A.), 15-17 December 1972
  • Proceedings International Symposium on Fibre Reinforced Concrete, ACI Special Publication, SP-44,American concrete Institute, Detroit (U.S.A.). 1974.
  • Swamy, R.N. Concrete Technology & Design, Vol 1 :New Concrete Materials, Vol.2: New Reinforced Concretes, Vol. 3: Cement Replacement Materials, Vol.4: Ferro cement Current and potential Applications, Blackie and Sons Ltd. (Publishers). London (U.K.), 1988
  • Expansive Cement Concrete, ACI Special Publication, SP- 38,American Concrete Institute, Detroit, 1973
  • Ferrocement: Materials & Applications, ACI Special Publication, SP-61, American Concrete Institute, Detroit (U.S.A.), 1979
  • Sharma, P.C., Ferro cement Segmental Shell – Multipurpose Unit, Proceedings: Asia-Pacific Symposium on Ferrocement Applications for Rural Development, University of Roorkee, Roorkee (India), 23-25 April, 1984,PP. 113- 124
  • Sharma,P.C., A Mechanized Process for producing Ferro cement Roof and wall elements Journal of Ferro cement, January 1983, PP.13-18
  • P.C. Sharma & V S Gopalaratnam - 'Ferrocement Water Tank' - Published by International Ferrocement Information Center bangkok (Thailand)
  • P.C. sharma, K. Shashi Kumar & P. nimityongskul 'ferrocement roofing Elements' published by International ferrocement information Center Bangkok (Thailand)
  • P.C. sharma- 'ferrocement Lining for Waterproofing, Rehabilation and Retrofilling of RCC and masonary Structures' key note address–International workshop on 'Repair Rehabilitation and Retrofitting of concrete and masonary structures Oct 2004, Gedu Bhutan.
  • Malhotra, V.M., Super plasticizers: Their effect on Fresh and Hardened Concrete, Concrete International, May 98 1, pp.66-81
  • State-of-the-art Report on High Strength Concrete, ACI Committee 363, ACI Journal, July-August, 1984, pp. 364-411
  • Kishore, Kaushal, High Strength Concrete, ICI Bulletin No. 51, April-June, 1995,pp.29-31
  • Sen,B., High performance Concrete: Development and Prospects, ICI Bulletin No. 55,April- June,1996,pp.14-19
  • Basu, Prabir C., High Performance Concrete: Mechanism and Application, ICI Journal, April- June,2001,pp.15-26
  • Malhotra, V.M., and Mehta, P.K., High-Performance, High-Volume Fly Ash Concrete, Supplementary Cementing Materials for Sustainable Development Inc., Ottawa, Canada, Marquardt Printing Ltd. Ottawa, Canada, August, 2002
  • High-Volume Flyash Concrete, Green Business Opportunities, Confederation of India Industry, Quarterly, Oct.-Dec.,2003
  • Handbook on High-Volume Flyash Concrete, CII (India) and CIDA (Canada) Publication.
  • Apte, M.D., Innovative Concretes and Fibres, ICI Journal, July-September, 2003, pp.9-10
  • Surlaker, Samir, Self-Compacting Concrete, ICI Journal, January-March, 2002, pp.5-9
  • Kumar, Rakesh and Rao, M.V.B., Self-Compacting Concrete: An Emerging Technology in Construction Industry, ICI Journal, July-September, 2002,pp.9-12
  • Srinivasan, D.,Will there be A Self-Curing Concrete? Concrete International, September 2000
  • Bryant, Mather, Self-Curing Concrete-why Not? Concrete International, January, 2001, Reproduced in ICI Journal,April- June,2003,pp.7-8
  • Subramanian, N. Curing-The Last and The least Considered Aspect in Concrete Making, ICI Journal, April-June,2002, pp.13-25
  • Srinivasan, D., Research Needs in Concrete, ICI Journal, July-September, 2006, pp.5-6
  • Walraven, J.,The Evolution of Concrete, ICI Bulletin No. 70, Jan-March, 2000, pp. 11-19
  • Chen,Pu-Weei,and Chung, D.D.L., Carbon Fibre Reinforced Concrete As An Intrinsically Smart Concrete For Damage Assessment During Static And Dynamic Loading, ACI Materials Journal, April, 1996, pp.341-350

NBMCW October 2007

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Self Curing Concrete An Introduction

Self Curing Concrete An Introduction

Ambily P.S, Scientist, and Rajamane N P, Deputy Director and Head, Concrete Composites Lab Structural Engineering Research Centre, CSIR, Chennai
Excessive evaporation of water (internal or external) from fresh concrete should be avoided; otherwise, the degree of cement hydration would get lowered and thereby concrete may develop unsatisfactory properties. Curing operations should ensure that adequate amount of water is available for cement hydration to occur. This paper discusses different aspects of achieving optimum cure of concrete without the need for applying external curing methods.

Definition of Internal Curing (IC)

The ACI-308 Code states that “internal curing refers to the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing Water.” Conventionally, curing concrete means creating conditions such that water is not lost from the surface i.e., curing is taken to happen ‘from the outside to inside’. In contrast, ‘internal curing’ is allowing for curing ‘from the inside to outside’ through the internal reservoirs (in the form of saturated lightweight fine aggregates, superabsorbent polymers, or saturated wood fibers) Created. ‘Internal curing’ is often also referred as ‘Self–curing.’

Need for Self–curing

When the mineral admixtures react completely in a blended cement system, their demand for curing water (external or internal) can be much greater than that in a conventional ordinary Portland cement concrete. When this water is not readily available, due to depercolation of the capillary porosity, for example, significant autogenous deformation and (early-age) cracking may result.

Due to the chemical shrinkage occurring during cement hydration, empty pores are created within the cement paste, leading to a reduction in its internal relative humidity and also to shrinkage which may cause early-age cracking. This situation is intensified in HPC (compared to conventional concrete) due to its generally higher cement content, reduced water/cement (w/ c) ratio and the pozzolanic mineral admixtures (fly ash, silica fume). The empty pores created during self-desiccation induce shrinkage stresses and also influence the kinetics of cement hydration process, limiting the final degree of hydration. The strength achieved by IC could be more than that possible under saturated curing conditions.

Often specially in HPC, it is not easily possible to provide curing water from the top surface at the rate required to satisfy the ongoing chemical shrinkage, due to the extremely low permeabilities often achieved.

Potential Materials for IC

The following materials can provide internal water reservoirs:
  • Lightweight Aggregate (natural and synthetic, expanded shale),
  • LWS Sand (Water absorption =17 %)
  • LWA 19mm Coarse (Water absorption = 20%)
  • Super-absorbent Polymers (SAP) (60-300 mm size)
  • SRA (Shrinkage Reducing Admixture) (propylene glycol type i.e. polyethylene-glycol)
  • Wood powder

Chemicals to Achieve Self–curing

Some specific water-soluble chemicals added during the mixing can reduce water evaporation from and within the set concrete, making it ‘self-curing.’ The chemicals should have abilities to reduce evaporation from solution and to improve water retention in ordinary Portland cement matrix.

Super-absorbent Polymer (SAP) for IC

The common SAPs are added at rate of 0–0.6 wt % of cement. The SAPs are covalently cross-linked. They are Acrylamide/acrylic acid copolymers. One type of SAPs are suspension polymerized, spherical particles with an average particle size of approximately 200 mm; another type of SAP is solutionpolymerized and then crushed and sieved to particle sizes in the range of 125–250 mm. The size of the swollen SAP particles in the cement pastes and mortars is about three times larger due to pore fluid absorption. The swelling time depends especially on the particle size distribution of the SAP. It is seen that more than 50% swelling occurs within the first 5 min after water addition. The water content in SAP at reduced RH is indicated by the sorption isotherm.

SAPs are a group of polymeric materials that have the ability to absorb a significant amount of liquid from the surroundings and to retain the liquid within their structure without dissolving. SAPs are principally used for absorbing water and aqueous solutions; about 95% of the SAP world production is used as a urine absorber in disposable diapers. SAPs can be produced with water absorption of up to 5000 times their own weight. However, in dilute salt solutions, the absorbency of commercially produced SAPs is around 50 g/g. They can be produced by either solution or suspension polymerization, and the particles may be prepared in different sizes and shapes including spherical particles. The commercially important SAPs are covalently cross-linked polyacrylates and copolymerized polyacrylamides/ polyacrylates. Because of their ionic nature and interconnected structure, they can absorb large quantities of water without dissolving. From a chemical point of view, all the water inside a SAP can essentially be considered as bulk water. SAPs exist in two distinct phase states, collapsed and swollen. The phase transition is a result of a competitive balance between repulsive forces that act to expand the polymer network and attractive forces that act to shrink the network. The macromolecular matrix of a SAP is a polyelectrolyte, i.e., a polymer with ionisable groups that can dissociate in solution, leaving ions of one sign bound to the chain and counter-ions in solution. For this reason, a high concentration of ions exists inside the SAP leading to a water flow into the SAP due to osmosis. Another factor contributing to increase the swelling is water solvation of hydrophilic groups present along the polymer chain. Elastic free energy opposes swelling of the SAP by a retractive force.

SAPs exist in two distinct phase states, collapsed and swollen. The phase transition is a result of a competitive balance between repulsive forces that act to expand the polymer network and attractive forces that act to shrink the network.

Means of Providing Water for Self–curing Using LWA

Water/moisture required for internal curing can be supplied by incorporation of saturated-surfacedry (SSD) lightweight fine aggregates (LWA).

Water Available from LWA for Self–curing

It is estimated by measuring desorption of the LWA in SSD condition after exposed to a salt solution of potassium nitrate (equilibrium RH of 93%). The total absorption capacity of the LWA can be measured by drying a Saturated Surface Dry (SSD) sample in a dessicator.

Water in LWA for Internal Curing

About 67% of the water absorbed in the LWA can get transported to self-desiccating paste. Some water remains always in the LWA in the high RH range and it becomes useful when the overall RH humidity in concrete is significantly reduced. The water retained in LWA in air-dry condition may not be enough to prevent autogenous shrinkage whose magnitude, however, may be reduced significantly. The fine lightweight aggregate, in saturated condition, produce a more uniform distribution of the water needed for curing throughout the microstructure.

The grain size of the LWA used as curing agent should be less in order to minimise the paste– aggregate proximity, i.e. the distance to which the internal curing water could diffuse. The grain size of down to 2–4 mm are found to be beneficial.

Utility of LWA Near Surface of Concrete

At the surface of the concrete, as the water evaporates from the concrete surface, a humidity gradient develops. This accelerates the appearance of the localized humidity gradients. The water from the LWA near the surface is then used up faster than in the interior of the concrete thus causing the near-surface layer of the concrete to become denser in a shorter time. This helps reduce the amount of water that would normally evaporate and contributes to improve internal curing of the concrete. It also leads to reduced or no stresses due to drying helping in eliminating the surface cracking.

Potential of LWA for Reducing Autogenous Shrinkage

As the cement hydrates, the water will be drawn from the relatively “large” pores in the LWA into the much smaller ones in the cement paste. This will minimise the development of autogenous shrinkage as the shrinkage stress is controlled by the size of the empty pores, via the Kelvin- Laplace equation.

The radii of capillary pores formed during hydration in the cement paste are smaller than the pores of the LWA. When the RH decreases (due to hydration and drying), a humidity gradient develops; with the LWA acting as a water reservoir, the pores of the cement paste absorb water from the LWA by capillary suction. The unhydrated cement particles from the cement paste now have more free-water available for hydration and new hydration products grow in the pores of the cement paste thus causing them to become smaller. The capillary suction, which is the inverse to the square of the pore radius, increases as the radius becomes smaller and thus enabling the pores to continue to absorb water from the LWA. This continues until most of the water from the LWA has been transported to the cement paste.

Crushed LWA for Internal Curing

Crushed LWA could provide a better surface for binder interaction as the pelletising process often produces LWAs with sealed surface. The vesicular surface resulting from the crushing operation allows paste penetration and provides more surface area for reaction between the aggregate and paste. The transition zone associated with a crushed aggregate has advantages over a more smooth and sealed surface.

Water Required for Self–curing

It depends upon chemical and autogenous shrinkages expected during hydration reactions.

Types of Shrinkage Drying

Shrinkages may occur at earlyages or at later ages over a longer period; different types of shrinkages may be identified as :

Drying shrinkage, autogenous shrinkage, thermal shrinkage, and carbonation shrinkage.

Reason for Chemical Shrinkage

Chemical shrinkage is an internal volume reduction due to the absolute volume of the hydration

Products being less than that of the reactants (cement and water). For example: Hydration of tricalcium silicate:

C3S + 5.3 H -> C1.7SH4 + 1.3 CH

Molar volumes

71.1 + 95.8 -> 107.8 + 43 i.e, 166.9 -> 150.8

Therefore,

Chemical shrinkage = (150.8 –166.9) / 166.9 = -0.096 mL/mL = -0.0704 mL/g cement

For complete reaction of each gram of tricalcium silicate, there is a need to supply 0.07 gram of extra curing water to maintain saturated conditions. (A value of 0.053 for 75% hydration at 28 day was experimentally observed by Powers in 1935).

Quantity of Chemical Shrinkage

Portland cement hydration is typically accompanied by a chemical shrinkage on the order of 0.07 mass of water per mass of cement for complete hydration: for silica fume, slag, and fly ash, these coefficients are about 0.22, 0.18, and 0.10 to 0.16, respectively. It can be measured by ASTM standard test method, C1608

Autogenous Shrinkage

It is as a volume change in concrete occurring without moisture transfer from the environment intoconcrete. It is due to the internal chemical and structural reactions of the concrete. Autogenous shrinkage is prominent in HPCs due to the reduced amount of water and increased amount of various binders used.

At early ages (the first few hours), before the concrete has formed a hardened skeleton, autogenous shrinkage is often due to only chemical shrinkage. At later ages (>1+days), the autogenous shrinkage can also result from self-desiccation since the hardened skeleton resists the chemical shrinkage.

The external (macroscopic) dimensional reduction of the cementitious system under isothermal sealed curing conditions; can be 100 to 1000 micro strains.

Self-desiccation

It is the localized drying resulting from a decreasing relative humidity (RH) which could be the result of the cement requiring extra water for hydration. It is the reduction in the internal relative humidity of a sealed system when empty pores are generated.

Potential of Selfdesiccation Prominent in HPC/ HSC

The finer porosity of HSC/HPC (with a low w/c), causes the water meniscus to have a greater radius of curvature, causing large compressive stress on the pore walls, leading to greater autogenous shrinkage as the paste is pulled inwards. Self–desiccation is only a risk when there is not enough localized water in the paste for the cement to hydrate and it occurs the water is drawn out of the capillary pore spaces between the solid particles. At later ages, a strong correlation exists between internal relative humidity and free autogenous shrinkage.

Mineral admixtures, such as fly ash and silica fume, in concrete tend to refine the pore structure towards a finer microstructure thereby water consumption will be increased and the autogenous shrinkage due to self-desiccation will be increased.

Inter-dependance of Autogenous & Chemical Shrinkages

Chemical shrinkage creates empty pores within hydrating paste and stress generated is stimated by equation:

σcap = 2 *γ / r = - In (RH) * R * T / Vm

where γ,Vm = Surface tension and molar volume of the pore solution,

r = the radius of the largest water-filled pore (or the smallest empty pore),

R = the universal gas constant, and T is the absolute temperature

The sizes of empty pores regulate both internal RH and capillary stresses. These stresses cause a physical autogenous deformation (shrinkage strain) given by:

ε = ( S * σcap/ 3 ) * [ (1/K) – (1/Ks)]

where ε = shrinkage (negative strain), S = degree of saturation (0 to 1) or volume fraction of waterfilled pores, K = bulk modulus of elasticity of the porous material, and Ks = bulk modulus of the solid framework within the porous material.

The above equation is only approximate for a partiallysaturated visco-elastic material such as hydrating cement paste, but still provides insight into the physical mechanism of autogenous shrinkage and the importance of various physical parameters The internal drying is analogous to external drying shrinkage.

Early External Water Curing and Cracks in HPC

Reduction of autogenous shrinkage due to external curing in HPCs is possible for first one or two days when the capillary pores are yet interconnected. Early water curing can lead to higher strain gradients when the skin of the concrete becomes well cured (no shrinkage) whereas, autogenous shrinkage, which is generally difficult to control, begins at the interior of the concrete. These problems can be mitigated by use of a pre-soaked LWA.

Monitoring of Self – curing

This can be done by:
  1. Measuring weight-loss
  2. X-Ray powder diffraction
  3. X-Ray microchromatography
  4. Thermogravimetry (TGA) measurements
  5. Initial surface absorption tests (ISAT)
  6. Compressive strength
  7. Scanning electron microscope (SEM)
  8. Change internal RH with time
  9. Water permeability
  10. NMR spectroscopy
Advantages of Internal Curing
  1. Internal curing (IC) is a method to provide the water to hydrate all the cement, accomplishing what the mixing water alone cannot do. In low w/c ratio mixes (under 0.43 and increasingly those below 0.40) absorptive lightweight aggregate, replacing some of the sand, provides water that is desorbed into the mortar fraction (paste) to be used as additional curing water. The cement, not hydrated by low amount of mixing water, will have more water available to it.
  2. IC provides water to keep the relative humidity (RH) high, keeping self-desiccation from occurring.
  3. IC eliminates largely autogenous shrinkage.
  4. IC maintains the strengths of mortar/concrete at the early age (12 to 72 hrs.) above the level where internally & externally induced strains can cause cracking.
  5. IC can make up for some of the deficiencies of external curing, both human related (critical period when curing is required is the first 12 to 72 hours) and hydration related (because hydration products clog the passageways needed for the fluid curing water to travel to the cement particles thirsting for water). Following factors establish the dynamics of water movement to the unhydrated cement particles:
    1. Thirst for water by the hydrating cement particles is very intense,
    2. Capillary action of the pores in the concrete is very strong, and
    3. Water in the properly distributed particles of LWA (fine) is very fluid.

Concrete Deficiencies that IC can Address

The benefit from IC can be expected when
  • Cracking of concrete provides passageways resulting in deterioration of reinforcing steel,
  • low early-age strength is a problem,
  • permeability or durability must be improved,
  • rheology of concrete mixture, modulus of elasticity of the finished product or durability of high fly-ash concretes are considerations.
  • Need for: reduced construction time, quicker turnaround time in precast plants, lower maintenance cost, greater performance and predictability.
Improvements to Concrete due to Internal Curing
  • Reduces autogenous cracking,
  • largely eliminates autogenous shrinkage,
  • Reduces permeability,
  • Protects reinforcing steel,
  • Increases mortar strength,
  • Increases early age strength sufficient to withstand strain,
  • Provides greater durability,
  • Higher early age (say 3 day) flexural strength
  • Higher early age (say 3 day) compressive strength,
  • Lower turnaround time,
  • Improved rheology
  • Greater utilization of cement,
  • Lower maintenance,
  • use of higher levels of fly ash,
  • higher modulus of elasticity, or
  • through mixture designs, lower modulus
  • sharper edges,
  • greater curing predictability,
  • higher performance,
  • improves contact zone,
  • does not adversely affect finishability,
  • does not adversely affect pumpability,
  • reduces effect of insufficient external curing.

Effect of Particle Size and Content of LWA

Internal curing by saturated lightweight aggregate can eliminate autogenous shrinkage with the smallest possible amount of lightweight aggregate. The grain size of the LWA used as curing agent needs to be reduced in order to minimize the paste– aggregate proximity, i.e. the distance to which the internal curing water should diffuse. The reduction of the grain size (down to 2–4 mm), is shown to be beneficial. However, the further reduction of grain size could result in a decrease of curing efficiency.

The effectiveness of internal curing depends not only on whether there is sufficient water in the LWA, but also on whether it is readily available to the surrounding cement paste as well. Hence, if the distance from some location in the cement paste to the nearest LWA surface is too great, water cannot permeate fully within an acceptable time interval. This distance can be called the paste– aggregate proximity. Alternatively, aggregate distribution can be described by means of aggregate– aggregate proximity, which is the distance between two nearest LWA surfaces, often called spacing. For a given amount of aggregate, the paste–aggregate proximity can be adjusted by the size of the aggregate. The finer the aggregate size, the closer will be the paste– aggregate proximity.

The LWA can be used for internal curing without considerable detrimental effects on strength when added in the amounts just required to eliminate self-desiccation.

“Protected Paste Volume” Concept in Self-curing

For self-curing, besides providing necessary quantity of water inside the matrix, it is essential to ensure the proximity of the cement paste to the surfaces of the source of water so that required high RH is generated around the cement grains for hydration reaction. In this regard, the “protected paste volume” concept is useful to recognise the effective volume of cement paste. For this, the aggregates are represented by impenetrable spherical or ellipsoidal particles and each aggregate particle is surrounded by a soft penetrable shell representing the interfacial transition zone. Instead of the interfacial transition zones, the saturated LWA (fine aggregate) particles surrounded by a shell of variable thickness can be assumed for evaluation. Then, by systematic point sampling, one can determine the volume fraction of paste contained within these shells and hence the relative proximity of the cement paste to the additional water.

Distribution of Internal Water Reservoirs for Curing

The transport distance of water within the concrete is limited by depercolation of the capillary pores in low w/c ratio pastes. With water-reservoirs well distributed within the matrix, shorter distances have to be covered by the curing water and the efficiency of the internal-curing process is consequently improved. The concept of internal curing was established, based on dispersion of very small, saturated LWA throughout the concrete, which serve as tiny reservoirs with sufficient water to compensate for self-desiccation. The spacing between the LWA particles is conveniently small so that the water travels smaller distances to counteract self-desiccation. The amount of water in the LWA can therefore be minimized, thus economising on the content of the LWA.

Travel of Water from Surfaces of LWA

Estimates of travel of internal water from the surface of water reservoir in the concrete matrix are:
  • early hydration — 20 mm
  • middle hydration — 5 mm
  • late hydration — 1 mm or less
  • “worst case” — 0.25 mm (250 ìm)
(Early and middle hydration estimates in agreement with x-ray absorption-based observations on mortars during curing).

Size of pores for Internal Water Storage

Water is held in pores primarily by capillary forces. Only pore sizes above approximately 100 nm are useful for storage of internal curing water. In smaller pores the water is held so tightly that it is not available for the cementitious reactions. Since some of the water absorbed by the LWA in the smaller pores will not be released to the hardening cement paste, an amount of water more than sufficient to counteract selfdesiccation should be absorbed in the LWA. A great quantity of water is in fact entrapped in the internal porosity of the larger particles; one should consider that only about half of it is available for internal curing. In case of smaller fraction, the opposite seems to hold: the absorption is lower, but almost 80 % of the water is lost by 85% RH.

Usefulness of IC in Pavements

The major problem of cracking in pavements may be alleviated by internal curing, besides imparting many potential benefits.

Usefulness of IC for Early-Age Cracking

The IC can influence the ‘Early- Age Cracking Contributors’ which are mainly thermal effects and autogenous shrinkage. During initial ages of concrete, hydration heat can raise concrete temperature significantly (causing expansion), subsequent thermal contraction during cooling can lead to early-age (global or local) cracking if restrained (globally or locally). Another prominent effect would be autogenous shrinkage, especially in concretes with lower water-binder ratios where sufficient curing water cannot be supplied externally, the chemical shrinkage accompanying the hydration reactions will lead to self-desiccation and significant autogenous shrinkage (and possibly cracking).

Pore Sizes in Internal Reservoirs & Capillary Pores

IC distributes the extra curing water throughout the 3-D concrete microstructure so that it is more

readily available to maintain saturation of the cement paste during hydration, avoiding selfdesiccation (in the paste) and reducing autogenous shrinkage. Because the autogenous stresses are inversely proportional to the diameter of the pores being emptied, for IC to do its job, the individual pores in the internal reservoirs should be much larger than the typical sizes of the capillary pores (micrometers) in hydrating cement paste.

Quantifying Effectiveness of IC

IC can be experimentally measured by:
  • Internal RH
  • Autogenous deformation
  • Compressive strength development
  • Degree of hydration
  • Restrained shrinkage or ring tests
  • 3-D X-ray microtomography (Direct observation of e 3-D microstructure of cement-based materials).

Conclusion

The internal curing (IC) by the addition of saturated lightweight fine aggregates is an effective means of drastically reducing autogenous shrinkage. Since autogenous shrinkage is a main contributor to early-age cracking, it is expected that IC would also reduce such cracking. An additional benefit of IC beyond autogenous shrinkage reduction is increase in compressive strength. As internal curing maintains saturated conditions within the hydrating cement paste, the magnitude of internal self-desiccation stresses are reduced and long term hydration is increased. IC is particularly effective for the highperformance concretes containing silica fume and GGBS. In cement mortar containing a Type F fly ash, the fly ash functions mainly as a dilutent at early ages, and higher and coarser porosity at early ages result in less autogenous shrinkage.

The self-desiccation is the reduction in internal relative humidity of a sealed hydrating cement system when empty pores are generated. This occurs when chemical shrinkage takes place at the stage where the paste matrix has developed a self-supportive skeleton, and the chemical shrinkage is larger than the autogenous shrinkage. Effects of self-desiccation depend on the sizes of the generated empty pores. These pore sizes in turn are dependent on the initial waterto- binder ratio (w/b), the particle size distributions of the binder components, and their achieved degree of hydration. The continuing trends towards finer cements and much lower w/b have significantly reduced the capillary pore “diameters” (spacing) in the paste component of the fresh concrete, and have often resulted in materials and structures where the effects of self-desiccation are all to visible as early-age cracking. Many strategies for minimizing the detrimental effects of selfdesiccation (mainly the high internal stresses and strains that may lead to early-age cracking), such as internal curing, rely on providing a “sacrificial” set of larger water-filled pores within the concrete microstructure that will empty first while the smaller pores in the hydrating binder paste will remain saturated. It may be noted that the effects of self-desiccation are not always detrimental, as exemplified by the benefits offered by self-desiccation in terms of an earlier RH reduction for flooring applications and an increased resistance to frost damage.

IC is useful when ‘performance specifications’ are important than ‘prescriptive specifications’ for concrete. Prime applications of IC could be: concrete pavements. precast concrete operations, parking structures, bridges, HPC projects, and architectural concretes. Concrete, in the 21st century, needs to be more controlled by the choice of ingredients rather than by the uncertainties of construction practices and the weather. Instead of curing through external applications of water, concrete quality will be engineered through the incorporation of water absorbed within the internal curing agent.

Acknowledgment

The authors thank Dr. N. Lakshmanan, Director, SERC, Chennai, for permitting to publish this paper.
Bibliography on Selfcuring (Internal Curing)
  1. Bentz, D.P., “Capillary Porosity Depercolation/Repercolation in Hydrating Cement Pastes via Low Temperature Calorimetry Measurements and CEMHYD3D Modeling,” Journal of the American Ceramic Society, 89 (8), 2606-2611, 2006.
  2. Bentz, D.P., “Influence of Curing Conditions on Water Loss and Hydration in Cement Pastes with and without Fly Ash Substitution,” NISTIR 6886, U.S. Dept. Commerce, July 2002.
  3. Bentz, D.P., and Snyder, K.A., “Protected Paste Volume in Concrete: Extension to Internal Curing Using Saturated Lightweight Fine Aggregates,” Cement and Concrete Research. 29, 1863-1867, 1999.
  4. Bentz, D.P., and Stutzman, P.E., “Curing, Hydration, and Microstructure of Cement Paste,” ACI Materials Journal, 103 (5), 348-356, 2006.
  5. Bentz, D.P., Garboczi, E.J., and Snyder, K.A., “A Hard Core/Soft Shell Microstructural Model for Studying Percolation and Transport in Three–Dimensional Composite Media,” NISTIR 6265, U.S. Department of Commerce, 1999.
  6. Bentz, D.P., Halleck, P.M., Grader, A.S., and Roberts, J.W., “Direct Observation of Water Movement during Internal Curing Using X-ray Microtomography,” Concrete International, 28 (10), 39-45, 2006.
  7. Bentz, D.P., Lura, P., and Roberts, J.W., “Mixture Proportioning for Internal Curing,” Concrete International, 27 (2), 35-40, 2005.
  8. Bilek, B et al, “The possibility of self-curing concrete Proc Name Innovations and developments in concrete materials and construction.” Proc. Intl Conf. University of Dundee, UK. 9-11 September 2002.
  9. Cusson, D., and Hoogeveen, T., “Internally-Cured High- Performance Concrete under Restrained Shrinkage and Creep,” CONCREEP 7 Workshop on Creep, Shrinkage and Durability of Concrete and Concrete Structures, Nantes, France, Sept. 12-14, 2005, pp. 579-584.
  10. De Jesus Cano Barrita, F.; Bremner, T.W.; Balcom, B.J., “Use of magnetic resonance imaging to study internal moist curing in concrete containing saturated lightweight aggregate,” High-performance structural lightweight concrete. ACI fall convention, Arizona, October 30, 2002. ACI SP 218.
  11. Dhir, R.K. Hewlett, P.C. Dyer, T.D., “Mechanisms of water retention in cement pastes containing a self-curing agent,” Magazine of Concrete Research, Vol No 50, Issue No 1, 1998, pp. 85-90.
  12. Geiker, M.R., Bentz, D.P., and Jensen, O.M., “Mitigating Autogenous Shrinkage by Internal Curing,” High Performance Structural Lightweight Concrete, SP-218, J.P. Ries and T.A. Holm, eds., American Concrete Institute, Farmington Hills, MI, 2004, pp. 143-154.
  13. Geiker, M.R.; Bentz, D.P.; Jensen, O.M., “Mitigating autogenous shrinkage by internal curing, High-performance structural lightweight concrete.” ACI fall convention, Arizona, October 30, 2002. ACI SP 218.
  14. Hammer, T.A.; Bjontegaard, O.; Sellevold, E.J., “Internal curingrole of absorbed water in aggregates, High-performance structural lightweight concrete.” ACI fall convention, Arizona, October 30, 2002. ACI SP 218.
  15. Hoff, G. C., “The Use of Lightweight Fines for the Internal Curing of Concrete,” Northeast Solite Corporation, Richmond, Va., USA, August 20, 2002, 37 pp.
  16. Hoff, G.C., “Internal Curing of Concrete Using Lightweight Aggregates,” Theodore Bremner Symposium, Sixth CANMET/ACI, International Conference on Durability, Thessaloniki, Greece, June 1-7 (2003).
  17. Kewalramani, M.A.; Gupta, R, “Experimental study of concrete strength through an eco-friendly curing technique,” Advances in concrete technology and concrete structures for the future. Dec 18-19, 2003. Annamalainagar.
  18. Kovler, K.; et.al., “Pre-soaked lightweight aggregates as additives for internal curing of high-strength concrete”s, Cement, Concrete and Aggregates, No 2, Dec. 2004, pp 131-138.
  19. Lura, P., “Autogenous Deformation and Internal Curing of Concrete,” Ph.D. Thesis, Technical University Delft, Delft, The Netherlands, 2003.
  20. Mangaiarkarasi, V.; Damodarasamy, S.R., “Self curing concrete today’s and tomorrow’s need of construction world,” INCRAC & CT 2005–Proc Intl Conf on recent advances in concrete and construction technology. 7-9 December 2005, Chennai. Vol.2.
  21. Mather, B., Hime, W.G., “Amount of Water Required for Complete Hydration of Portland Cement,” Concrete International, Vol. 24, No. 6, June, 56-58 (2002).
  22. Powers, T.C., Brownyard, T.L., “Studies of the Physical Properties of Hardened Portland Cement Paste,” Bulletin 22, Portland Cement Association, Skokie, Illinois, 992 pp. (1948).
  23. Troli, R. et al. “Self compacting /curing/compressing concrete, Global Constr. : Ultimate concrete opportunities : Admixtures–enhancing concrete performance.” 6th Intl.congress. Univ of Dundee, UK. 5 July 2005.
  24. Zhutovsky, S.; Kovler, K. Bentur, A., “Efficiency of lightweight aggregates for internal curing of high strength concrete to eliminate autogenous shrinkage,” Materials and Structures, 35(246)40, 2002, Page 097-101.

NBMCW July 2007

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GREEN CONCRETE and GLOBAL WARMING

Dr. S.P. Bhatnagar, Tech Dry (India) Pvt. Ltd. Bangalore

There are several causes for global warming including carbon dioxide emission from burning of fossil fuels for the purpose of electricity generation. Coal accounts for 93 percent of the emissions from the electric utility industry. Coal emits around 1.7 times as much carbon per unit of energy when burnt as does natural gas and 1.25 times as much as oil. Natural gas gives off 50% of the carbon dioxide. Carbon dioxide emitted from cars is about 20%. Carbon dioxide emitted from airplanes causes 3.6% of global warming and that the figure could rise to 15% by 2050. Building structures account for about 12% of carbon dioxide emissions.1

It is wellknown that the ecological balance is getting disturbed but we will keep our discussions restricted to the construction industry and utility of the green concrete.

Green Concrete & Global Warming

Green building is all about science-physics, chemistry, and biology. It’s really about ecology because ecology is about physics, chemistry, and because it is all about systems and integration of physics, chemistry, and biology.

Building is the shelter creating boundaries between people and the environment. Green building is about creating optimized boundaries between people and the environment.

The green building programme has identified a set of parameters that should be kept into consideration when the building is constructed and materials are chosen for it. It is vast subject to even define the material, which constitutes as environmental friendly or green material.

Worldwide, the construction industry contributes about 9% to the global GDP, and is one of the most important elements of every economy. Today’s demands on buildings, roads, bridges, tunnels, and dams could not be met without construction chemicals. The strength of concrete has risen dramatically as a result of the development of construction chemicals.

Green Concrete & Global Warming
The global construction chemical industry is a $20 billion business. The United States and Western Europe are the two largest markets, together accounting for 56% of the total market. Japan, China and India come next and together have a market share of about 21%.

The raw materials needed for the production of construction chemicals are manufactured by the big chemical producers. Polymers are the most important group of raw materials and are found in virtually every construction chemical formulation ranging from adhesives to waterproofing treatments. The development of new construction chemicals in many cases requires interaction by the chemical producer, construction chemical manufacturer and end user. The construction chemical industry spends about 3% of its sales on R&D of new products and applications.2

We often hear that India is going to become world power. It sounds musical to our ears but when you see the state of our Infrastructures, it is disappointing. In this paper, we will deal with one important aspect and that is the construction industry.

The production of bricks required the burning of fuels, either fossil fuels or agricultural wastes. The firing of bricks is to increase the strength and durability of the brick and to decrease water absorption. Concrete requires the manufacture of cement. To produce cement, limestone and clay are heated at 1450°C consuming fossil fuels, and cement is formed. The limestone is converted to calcium oxide and carbon dioxide.

CaCO3->CaO + CO2

For every 1000kg of calcium carbonate used, 440 kg of carbon dioxide is produced.

This production of carbon dioxide raises the question for the world of the desirability and economics of emulating western building practices in these countries, given the huge population requiring housing. For the production of bricks and concrete energy intensive activities are undertaken. In addition, the energy use results in carbon dioxide production. In the case of cement production, the demand for cement worldwide is 800 million tonnes per year. Assuming 500 million tonnes of limestone is used for this purpose each year then more than 220 million tonnes of carbon dioxide is emitted to the atmosphere from cement works alone each year. This is the equivalent of 44 kg of carbon dioxide for every inhabitant of the Earth each year.

Cement

Green Concrete & Global Warming
Let us concentrate on some of the major factors contributing to this state of affairs related to construction industry. We believe that every responsible citizen would continue in adopting environmental objectives. The given table will show the energy demand and emissions generated in production of 1kg of cement.

Corrosion

The corrosion of steel reinforcement is by far the single most common cause of structural damage.

The key environmental factors that reduce the passivation of steel are carbonation and chloride. Other factors which may influence either the initiation or rate of reinforcement corrosion include cracks in concrete, temperature, moisture, oxygen and in adequate concrete quality or cover.

There are two major situations in which corrosion of reinforcing steel can occur:
  • Carbonation
  • Chloride contamination

Carbonation

Green Concrete & Global Warming
Carbonation is the process in which the Carbon dioxide (CO2) enters in the concrete as carbonic cid in the presence of moisture and reacts with calcium hydroxide and following reaction takes place

Ca(OH)2 + CO2 -> CaCO3 + H2O

Chloride Ions

Chloride ions can enter concrete in two ways:
  • They may be added during mixing either deliberately as an admixture or as a contaminant in the original constituents.
  • They may enter the set concrete from environment pollutants dissolved in rain water/humidity.
Both Carbonation and Chloride ions damage the protective, highly alkaline passive shield around reinforcement. This leads to the corrosion of reinforcement/ malignancy of reinforcement as we call, which makes the building less durable and vulnerable to natural calamities which leads to human tragedy and loss of property.

Toxicity of Admixture

While using admixture, it is very important to be careful and make judicious decision so that the ingredient of cement does not react with admixture and produce undesirable side products.

Plasticizers tend to liberate cancer causing toxic product like formaldehyde.

Tons and tons of admixtures particularly plasticizers and superplasticizers are used in construction. The study shows that approximately 15-25% of sulphonated naphthalene polymers (SNP), lignosulphonate and polycarboxylates and 30-60% of sulphonated melamine polymers (SMP) were leached. Some additional test showed that this is the only part of leached organic substance that comes from superplasticizers and rest of them come from coating and adhesives.

Togero4 shows in his studies that some small fraction of formaldehyde in both SNF and SMF is liberated, which is not only hazardous but also carcinogenic.

lmpregnants

Green Concrete & Global Warming
Water vapour permeability is an essential requirement for building materials. A satisfactory water repellent leaves the treated substrate permeable to water vapour while restricting the passage of liquid through the capillaries.
Green Concrete & Global Warming
A natural external finish of masonry buildings may be required for aesthetic purposes. Treatment by impregnation does not change the finish appearance and no yellowing is normally developed during use.

Permanent bonding between concrete capillaries and impregnants results in longterm durability.

Solvent Based Impregnants

Impregnants dissolved in a suitable solvent can be used to create a waterproof hydrophobic surface which does not allow the ingress of water. However, solvent being toxic and hazardous has been banned in most of the Western countries. Moreover, solvents are very expensive.

Water Based

Water based impregnants form a zone within the pore of structure after penetration, resulting in a molecular size three to four times the dissolved size, and some impregnants is bound chemically to the silicates in the cement matrix.

Coatings

Coatings have not been successful because they tend to block pores and capillaries with the trapped water underneath it. This trapped water hits the weaker part of the surface and create ingress points in the form of cracks, blisters, honeycombs etc.

The effect of some of the coatings is injurious like:

Asbestos

We are sure that everybody knows that any type of asbestos causes cancer and it is not confined to a specific blue variety.

Asbestos as we all know is the name given to group of minerals that occur naturally as masses of strong, flexible fibers that can be separated into thin threads and woven. These fibers are not affected by heat or chemicals and do not conduct electricity. For these reasons, asbestos has been widely used in many industries.

Membranes

It has become a fashion to use membranes, the word is a misnomer since the elastomeric coating should have the following properties which these membranes do not have.

Tensile strength, Elongation, Crack bridging, Abrasion resistance, Temperature flexibility, Weatherability, Bonding, Flashing attachment, Materials compatibility, Wind uplift resistance.

The conventional membranes or thick toppings are normally bitumen, asphalt, polyurethane, and epoxy based. Besides several disadvantages like blister formation, debonding and other factors which allow water to enter.

Bitumen

The main constituents in bitumen are polyaromatic compounds which undergo photochemical oxidation particularly at high temperature (since they are black, the temperature is much higher).

These photo-oxidation generates gases and strong carcinogenic compounds like Benzo[a]pyrene.

Elastomeric Coatings

Elastomeric coatings should not be misunderstood as the above mentioned membrane since these are normally non-cementitious and are produced by using special polymerization techniques and unlike other membranes they are flexible, breathable with high elongation and weatherability and crack bridging membranes rather than coating which have several problems.

Elastomeric coatings are the latest type of coating which came into roof protection systems and tanking systems in basements.

To understand this concept we must address ourselves to basic questions to why latest membranes in this field are different than the conventional coating membranes.

Let us understand the term elastomers. Elastomers are a class of materials which differ quite obviously from all other solid materials in that they can be stretched easily and almost completely reversibly, to high extensions and before reaching its ultimate breaking elongation – it can be released and will rapidly recover to almost exactly the original length it had before stretching. The material is said to be elastic.

Most synthetic elastomers are not as elastic as natural rubber, but all can be stretched (or otherwise deformed) in a reversible manner to an extent, which easily distinguishes them from all other solid materials.

Elastomers are a special case of the wider group of materials known as polymers. Polymers are not made up of discrete compact molecules like most materials, but are made of long, flexible, chainlike or string-like, molecules. At this scale the inside of a piece of rubber can be thought of as resembling a pile of cooked spaghetti. In spaghetti, however, the chains, though intertwined, are all separate. But in most practical elastomers each chain will be joined together occasionally along its length to one or more nearby chains with just a very few chemical bridges, known as crosslinks. So the whole structure forms a coherent network which stops the chains from sliding past one another indefinitely – although leaving the long sections of chain between crosslinks free to move. The process by which crosslinks are added is known as vulcanization.

Polymers on the other hand are giant molecules of different chemicals. A polymer or a macromolecule is made up of many (poly) molecules (‘mers’) or monomers linked together like wagons in a train, for example poly(vinyl chloride), poly(ethylene), etc. The polymerization of vinyl chloride (VC), which represents some 500 to 2000 molecules of VC linked together to make a giant molecule of commercial PVC. Monomers may have the same or different chemical compositions

Water in the form of vapour, liquid presents below-grade construction with many unique problems. Water causes damage by vapour transmission through porous surfaces, by direct leakage in a liquid state. Water presence in below grade makes interior spaces uninhabitable not only byleakage but also by damage to structural components as exhibited by reinforcing steel corrosion, concrete spalling, settlement cracks, and structural cracking.

Therefore, all elastomeric membranes are not alike and different parameters like nature of monomer cross-linking agent, polymerization technique, initiators, accelerators and fillers can have an influence on the physical and chemical stability of the final elastomeric membrane.

Grouting

Grouting is the injection of a fluidized material into the soil to enhance its strength, density, or to reduce its permeability. Grouting can be more feasible than the cut and cover method, for example, excavating a trench to put in a tunnel lining and filling in the gap with soil. In the city, traffic may have to be rerouted around the cut and cover project site.

In planning a grouting programme for particular conditions, we need knowledge of various types of grouts and their properties. The basic types of grouts now in use and their properties are discussed. Types of admixtures and fillers used and their effects on the grout are also discussed. The most common types of grout are Portland cement, clay, chemical, and asphaltic grouts. No one grout is suitable for every situation.

Now-a-days excellent chemical grouting products have been developed, which can strengthen the voids whether in basements or otherwise. For example there are 2-component system where the damage is not only treated on the surface of the structure but that the complete centre of damage and the whole section of the building structure are completely treated.

These kinds of products do not effect the environment nor pollute the ground water.

Thermal Insulation

This has been a very misunderstood subject and there has been an understanding that Brick bat coba, surkhi or thermal insulation are preferred as Insulation products while thermal insulation does provide insulation but is not very durable. Surkhi which is now-a-days used as burnt bricks but definitely does not provide any thermal insulation on the concrete.

Heat naturally flows from warm areas to cooler areas, regardless of direction. This flow of heat can never be stopped completely, but the rate at which it flows can be reduced by using materials which have a high resistance to heat flow.

The general guideline for thermal insulation is to understand that thermal resistance of insulating material is directly proportional to the type of material and its thickness measured in terms of thermal conductivity.

Thermal insulation for buildings has been known since long and is one of the serious requirements more because of the climatic conditions in India. Moreover, we in India need any new system, which can contribute in saving energy.

For the last few years, lightweight micaceous minerals like Vermiculite have been used. The problem with this product is that it is very porous in nature and absorbs water and therefore has to be waterproofed. Moreover, it is soft, and laying of tiles over it is often required.

The choice of the insulating material depends on the cost, area to be covered and the cost of heating or cooling. There are large numbers of insulation materials available in the market.

Recently, ceramic microspheres and some natural clay along with redispersable spray dried polymers have played a key role. For example, lightweight waterproofing concrete not only replaces brick bat coba and reduced the weight on the surface of the roof, gives a very good insulation.

It is important that the key persons in the field of real estate and construction industry should appreciate the advantages of green building and its benefits, but unfortunately they mix the cost benefit of these green buildings.

In one of the reports conducted by the World Business Council for Sustainable Development (WBCSD). Respondents to a 1400 person global survey estimated the additional cost of building green at 17% above conventional construction, more than triple the true cost difference of about 5%. At the same time, survey respondents put greenhouse gas emissions by buildings at 19% of world total, while the actual number of 40% is double this.3

Existing technologies combined with common sense design can increase energy efficiency by 35 percent and reduce heating costs by 80 percent for the average building in industrialized markets.

Life cycle analysis shows that 80 to 85% of the total energy consumption and CO2 emissions of a building comes from occupancy through heating, cooling, ventilation, and hot water use. Buildings already represent approximately 40% of primary energy use globally and energy consumption in buildings is projected to rise substantially in the world’s most populous and fast growing countries such as China and India.

It would also be interesting to note that we can perhaps use environmental friendly green material, some them are:
  • By-product: Unused or waste material from one manufacturing or energy producing process that can be used in another manufacturing or energy producing process.
  • Diversion: Avoidance of landfill disposal of a material or product through reuse or recycling.
  • Embodied Energy: All of the energy required in the raw material extraction, manufacturing, distribution, and transport of a material product up to its point of use.
  • Global Warming potential: Possible Climate warming effect caused by the manufacture and/ or use of a material or product compared to that of carbon dioxide which has a GWP of 1.0.
  • Indoor Air quality: Condition of air inside buildings with respect to harmful concentrations of contaminants, volatile organic compounds and particulates.
  • Life Cycle: All stages of production, including raw materials extraction, manufacturing, distribution, use, maintenance, reuse or recycling, disposal, and all transportation.
  • Off-Gassing: Releasing of gases or vapours into the air
  • Rapidly renewable: Materials that are replenished relatively quickly, usually in less than 10 years.
  • Recyclable: Having the potential for being recycled by possessing such traits as highly recoverable, easily separated from other materials, not contaminated by toxic coating etc.
  • Recycled content: Portion of material or product that is made from recovered material.
  • Reused or salvaged materials: Materials or products from building deconstruction or demolition that are reused ‘as –is’ with little or no processing or modification
  • Solid waste: Material or product, typically long lasting and not biodegradable, disposed of in landfills or incinerators.
  • Source separation: Separation of waste materials by material type at the point of use to facilitate recycling.
  • Third party certified: Materials or products that are monitored by independent organizations for compliance with recognized environmental standards.
Quite often, it is extremely difficult to accurately assess the environmental performance of a building material or product over its entire life cycle. In many cases, the GBP relies on third party certification organization to accomplish this task.

Green Terrace/Green Roofs

Green Concrete & Global Warming
We have been emphasizing on green concrete. We would once again say that ecological engineering is an emerging field, it permits us to develop design of sustainable ecosystem with integrate human society.

We can avoid sound pollution by using lightweight minerals. Many home owners and the designers prefer to add bright lights because it gives a better feeling of architecture, it lights up garden and tress but we forget that the by-product of all these is light pollution.

What is light pollution? When the light is shining into your neighbor’s house it creates a sky glow effect, it can cause glare and so many other problems. Light pollution is also harmful to wild life and equally to human beings.

Infact several European countries and the US have very aggressively pursued the project of green roofs or terrace gardens. This would help mitigate the urban heat insland effect, reduce storm water runoff, improve building insulation and increase green space and biodiversity in urban centers.

Progress in horticultural engineering, including improvements in drought-resistant plants, and advances in waterproofing systems aided the gradual development of a viable green roof industry. Germany has been on the forefront in this field and has subsidized green roof costs.

Green roofs are the result of a complete underlying roof build-up system, providing continuous, uninterrupted layers of protection and drainage. Recent advances in technology have made them lighter, more durable and better able to withstand the extreme conditions of the rooftop.

Waterproofing

If waterproofing is not done and is not effective, it can encourage the growth of algae, fungus, mosses which are the natural sources of bacteria and in-house pollution, radon gases which causes disease like asthma and diabetes mainly in children.

Conclusion

The benefits of green buildings are many: greater energy efficiency, reduced water consumption, longer useful life, better health conditions for occupants, and much more. All of these factors can improve the value of a building over the long term and reduce operational costs. However, the mistaken perception exists that green building “costs too much” without a commensurate return on investment. Therefore, we conclude that waterproofing is a critical step but should be based on environmental friendly, non-toxic and energy saving techniques.

References:

  1. US Emission Inventory 2004 Executive summary p.10
  2. SRI Consulting SCUP Report
  3. World Business Council for Sustainable Development (WBCSD)
  4. Togero, 2004

NBMCW October 2007

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