Articles of Concrete

Influence of Curing Types on Strength o...

Prof. K. Vijai, Associate Professor, Dr. R. Kumutha, Professor & Head Department of Civil Engineering Sethu Institute of Technology Pulloor, TamilNadu, and Dr B. G. Vishnuram, Principal Easa College of Engineering & Technology, Coimbatore.

In order to address environmental effects associated with Portland cement, there is need to develop alternative binders to make concrete. An effort in this regard is the development of geopolymer concrete, synthesized from the materials of geological origin or by product materials such as fly ash, which are rich in silicon and aluminum. This paper presents results of an experimental study on the density and compressive strength of geopolymer concrete. The experiments were conducted on fly ash based geopolymer concrete by varying the types of curing namely ambient curing and hot curing. The ratio of alkaline liquid to fly ash is fixed as 0.4. For all the samples the rest period was kept as 5 days. For hot curing, the temperature was maintained at 60oC for 24 hrs in hot air oven. The compressive strength test was conducted on each of the sample and the results show that there was an increase in compressive strength with the increase in age for ambient cured specimens. For hot cured samples the increase in compressive strength with age was very less as compared to that of specimens subjected to ambient curing. The density of geopolymer concrete was around 2400kg/m3 which is equivalent to that of conventional concrete. Hence, geopolymer concrete has a great potential for utilization in construction industry as it is environmental-friendly and also facilitates the use of fly ash, which is a waste product from coal burning industries.

Introduction

Demand for concrete as a construction material is on the increase so as the production of cement. The production of cement is increasing about 3% annually. The production of one ton of cement liberates about one ton of CO2 to atmosphere. Among the green house gases, CO2 contributes about 65% of global warming. Furthermore, it has been reported that the durability of ordinary Portland cement concrete is under examination, as many concrete structures especially those built in corrosive environments start to deteriorate after 20 to 30 years, even though they have been designed for more than 50 years of service life.

Although the use of Portland cement is unavoidable in the foreseeable future, many efforts are being made to reduce the use of Portland cement in concrete. It is time to deploy new technology materials like geopolymers that offer waste utilisation and emissions reduction. The term geopolymer describes a family of mineral binders with chemical composition same as zeolite. Hardened geopolymer concrete has an amorphous microstructure which is quite similar to that of ancient structures such as Egyptian pyramids and Roman amphitheaters. Geopolymer pioneered by Joseph Davidovits is an inorganic alumino-silicate polymer synthesized from predominantly silicon (Si) and aluminium (Al) materials of geological origin or byproduct materials like fly ash, metakaolin, Granulated Blast furnace slag etc. The polymerisation process involves a substantially fast chemical reaction under alkaline condition on Si-Al minerals that result in a three dimensional polymeric chain and ring structure consisting of Si-O-Al-O bonds.

The chemical reaction comprises the following steps:
  1. Dissolution of Si and Al atoms from the source material through the action of hydroxide ions.
  2. Orientation or condensation of precursor ions into monomers.
  3. Setting or polycondensation or polymerization of monomers into polymeric structures.
Compared with ordinary Portland cement concrete, Geopolymers show many advantages. Low-calcium fly ash-based geopolymer concrete has excellent compressive strength, suffers very little drying shrinkage and low creep, excellent resistance to sulfate attack, and good acid resistance. It can be used in many infrastructure applications. One ton of low-calcium fly ash can be utilized to produce about 2.5 cubic meter of high quality geopolymer concrete, and the bulk cost of chemicals needed to manufacture this concrete is cheaper than the bulk cost of one ton of Portland cement. Given the fact that fly ash is considered as a waste material, the low calcium fly ash-based geopolymer concrete is, therefore, cheaper than the Portland cement concrete. The special properties of geopolymer concrete can further enhance the economic benefits. Moreover, reduction of one ton of carbon dioxide yields one carbon credit and this carbon credit significantly adds to the economy offered by the geopolymer concrete. In terms of reducing global warming, geopolymer technology could reduce approximately 80% of CO2 emission to the atmosphere caused by cement and aggregate industry.

In this paper, an attempt has been made to study the properties of geopolymer concrete such as density and compressive strength for two different types of curing.

Experimental Programme

Materials

Low calcium fly ash (ASTM class F) collected from Mettur thermal power station was used for casting the specimens. Fine Aggregate (sand) used is clean dry river sand. The sand is sieved using 4.75 mm sieve to remove all the pebbles. Fine aggregate having a specific gravity of 2.62, bulk density of 1701.84 kg/m3 and fineness modulus of 2.42 was used. Coarse aggregates of 20 mm maximum size having a fineness modulus of 6.94, bulk density of 1679.7 kg/m3 and specific gravity of 2.87 were used. Water conforming to the requirements of water for concreting and curing was used through out. Alkaline liquids are used in geopolymerisation. The most common alkaline liquid used in geopolymerisation is a combination of sodium hydroxide (NaOH) or potassium hydroxide (KOH) and sodium silicate or potassium silicate. In the present investigation, a combination of Sodium hydroxide solution and sodium silicate solution was used as alkaline solution. Sodium hydroxide is available commercially in flakes or pellets form. For the present study, sodium hydroxide flakes with 98% purity were used for the preparation of alkaline solution. Sodium silicate is available commercially in solution form and hence it can be used as such. The chemical composition of sodium silicate is: Na2O-14.7%, SiO2-29.4% and Water -55.9% by mass.

Mix Design of Geopolymer Concrete

In the design of geopolymer concrete mix, coarse and fine aggregates were taken as 77% of entire mixture by mass. This value is similar to that used in OPC concrete in which it will be in the range of 75% to 80% of the entire mixture by mass. Fine aggregate was taken as 30% of the total aggregates. From the past literature, it is clear that the average density of fly ash-based geopolymer concrete is similar to that of OPC concrete (2400kg/m3).Knowing the density of concrete, the combined mass of alkaline liquid and fly ash can be arrived. By assuming the ratios of alkaline liquid to fly ash as 0.4, mass of fly ash and mass of alkaline liquid was found out. To obtain mass of sodium hydroxide and sodium silicate solutions, the ratio of sodium silicate solution to sodium hydroxide solution was fixed as 2.5. For the present investigation, concentration of NaOH solution is taken as 8M. Extra water (other than the water used for the preparation of alkaline solutions) and dosage of super plasticizer was added to the mix according to the workability desired. Using the above procedure the mix was designed and the mix proportions are given in Table 1.

Detail of Mix Proportions

Preparation of Geopolymer Concrete

Specimens under ambient curing
Figure 1: Specimens under ambient curing

Hot curing of specimens
Figure 2: Hot curing of specimens
To prepare sodium hydroxide solution of 8 molarity (8M), 320 grams (8x40 i.e, molarity x molecular weight) of sodium hydroxide flakes was dissolved in one litre of water. The mass of NaOH solids in a solution will vary depending on the concentration of the solution expressed in terms of molar, M. The mass of NaOH solids was measured as 248 grams per kg of NaOH solution of 8M concentration. The sodium hydroxide solution thus prepared is mixed with sodium silicate solution one day before mixing the concrete to get the desired alkaline solution. The solids constituents of the fly ash-based geopolymer concrete, i.e. the aggregates and the fly ash, were dry mixed in the pan mixer for about three minutes. After dry mixing, alkaline solution was added to the dry mix and wet mixing was done for 4 minutes. Twelve cubes of size 150mm x 150mm x 150mm were cast and compaction was done by mechanical vibration using a Table vibrator.

Curing of Geopolymer Concrete

After casting the specimens, they were kept in rest period for five days and then they were demoulded. The term ‘Rest Period’ was coined to indicate the time taken from the completion of casting of test specimens to the start of curing at an elevated temperature. This may be important in certain practical applications. For instance, when fly ash-based geopolymer concrete is used in precast concrete industry, there must be sufficient time available between casting of products and sending them to the curing chamber. At the end of the Rest Period, six test specimens were kept under ambient conditions for curing at room temperature as shown in Figure 1. Remaining six specimens were kept at 60oC in hot oven for 24 hrs and is shown in Figure 2.

Results and Discussion

Density of Geopolymer Concrete

Table 2 presents the density values in kg/m3 at 7 days and 28 days of curing. Density values ranges from 2251 to 2400 kg/m3. The density of geopolymer concrete was found approximately equivalent to that of conventional concrete. As the age of concrete increases, there is a slight increase in density as shown in Figure 3.Variation of density is not much significant with respect to age of concrete and type of curing.

Density of Geopolymer Concrete

Variation of density with age of concrete
Figure 3: Variation of density with age of concrete

Compressive Strength

The compressive strength at 7 and 28 days of curing is presented in Table 3. Figure 4 shows a graphical representation of variation of compressive strength for 7 days and 28 days of curing. Compressive strength of hot cured specimens are more than that of ambient cured specimens both in 7 and 28 days. 28 days compressive strength of hot cured specimens was 2 times more than that of ambient cured specimens.7 days compressive strength of hot cured specimens was 7 times more than that of ambient cured specimens. In ambient Curing, the 28 days compressive strength is about 4.5 times 7 days compressive strength. In hot curing, the 28 days compressive strength is about 1.2 times 7 days compressive strength.

Compressive Strength of Geopolymer Concrete

compressive strength with age of concrete
Figure 4: Variation of compressive strength with age of concrete

Conclusion

The compressive strength of hot cured concrete is much higher than that of ambient cured concrete. In ambient curing, the compressive strength increases as the age of concrete increases from 7 days to 28 days. The compressive strength of hot cured fly ash based geopolymer concrete has not increased substantially after 7 days. The average density of fly ash based geopolymer concrete is similar to that of OPC concrete. Geopolymer concrete is more environ-friendly and has the potential to replace ordinary portland cement concrete in many appliances such as precast units.

Acknowledgment

The authors would like to thank Mr. K.Anand Babu, Mr.Gem Tshering, Mr.Sonam Jamtsho and Mr.Upendra Das, final year Civil Engineering students of Sona College of Technology, Salem for their involvement in casting and testing.

References

  • B. Vijaya Rangan, Dody Sumajouw, Steenie Wallah, and Djwantoro Hardjito, “Studies On Reinforced Low – Calcium Fly Ash –Based Geopolymer Concrete Beams And Columns” International Conference on Pozzolan, Concrete and Geopolymer, Khon Kaen, Thailand, May 24-25, 2006
  • Sarawut Yodmunee and Wanchai Yodsudjai, “Study On Corrosion Of Steel Bar In Fly Ash-Based Geopolymer Concrete” International Conference on Pozzolan, Concrete and Geopolymer, Khon Kaen, Thailand, May 24-25, 2006
  • X. J. Song, M. Marosszeky, M. Brungs, R. Munn, “Durability Of Fly Ash Based Geopolymer Concrete Against Sulphuric Acid Attack” International Conference on Durability of Building Materials And Components, Lyon [France] 17-20 April 2005
  • B. Vijaya Rangan, “Fly Ash-Based Geopolymer Concrete” Research Report GC 4 Engineering Faculty in Curtin University of Technology Perth, Australia, 2008
  • Anurag Mishra, Deepika Choudhary, Namrata Jain, Manish Kumar, Nidhi Sharda and Durga Dutt,” Effect of Concentration of Alkaline Liquid And Curing Time on Strength And Water Absorption of Geopolymer Concrete “ARPN Journal of Engineering and Applied Sciences Vol. 3, No. 1, February 2008
  • S. E. Wallah And B. V. Rangan,”Low-Calcium Fly Ash-Based Geopolymer Concrete: Long-Term Properties,” Research Report GC 2 Engineering Faculty in Curtin University of Technology Perth, Australia, 2006.

NBMCW November 2010

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Mechanical Water Dozer for Durable Conc...

Anil. K. Sharma, Chief Engineer, CPWD

Concrete Deterioration

Concrete is a porous material. This is the intrinsic property of concrete. Once the pores within structure of concrete are interconnected, this leads to creation of pathways (i.e. capillary porosity) for migration of aggressive chemicals from surface of concrete to deep inside concrete. The aggressive chemicals react with chemical constituents of cement paste and alter its character from highly alkaline towards acidic. This chemical deterioration of reinforced concrete, which is age related includes:
  • Carbonation of cover concrete
  • Corrosion of reinforcement
  • Effect of aggressive chemicals present

    • Inside concrete
    • In external Surroundings of concrete
Capillary Porosity

Mechanical Water Dozer for Durable Concrete Making
Mechanical Water Dozer for Durable Concrete Making
Porous Concrete Without Inter Connectivity Durable
Porous Concrete (Pores Inter Connected) Leading to Surface Non -Durable

It is this capillary porosity, which can control durability of concrete. Capillary porosity is the major route by which gases and liquids can permeate in the concrete. All these factors lead to deterioration of chemical characteristics of concrete leading to corrosion of reinforcement and cracking of concrete. Higher porosity leads to interconnectivity of pores and makes it lesser durable reinforced concrete. Lower is the capillary porosity, likelihood of interconnected pores are reduced and leads to more durable reinforced concrete.

Water Cement Ratio

The primary factor responsible for porosity in concrete is excessive quantity of water used in its manufacture than its actual need for hydration & hardening. The quantity of water needed for hydration is only 0.23 by weight of cement, whereas water/cement (w/c) ratio in excess of 0.23 is needed from workability consideration for placement and compaction of concrete. Higher is the w/c ratio, higher would be the resultant porosity of concrete. Higher is the concrete porosity, lesser durable is the concrete. Thus, key for getting durable concrete is to effectively control w/c ratio during concrete making.

There are also other secondary factors like improper compaction and/or micro-cracks due to flexure, weathering, heat of hydration etc, which also add to enhancement in capillary porosity and early deterioration of concrete.

Fixed w/c Ratio Assures Proportioning of Constituents

For desired workability, the quantity of water needed is almost fixed and it is in the range of 175 to 180 litres per cubic metre of concrete. The effective control on w/c ratio results in fixed quantity of cement per unit volume of concrete. Thus, with workability of concrete also being fixed, fixed w/c ratio assures automatic control on the proportioning of its constituents [i.e. cement: fine aggregate(sand): coarse aggregate(stone)]. With fixed w/c ratio, even by mistake, any error in proportion of constituents would lead to un-workable concrete and its rejection by the technicians themselves who are actually responsible for placing and compacting concrete in position.

Thus, if we are able to develop an effective control on w/c ratio during concrete making, cement/aggregate ratio gets controlled automatically for a desired workability by the local work force of technicians actually manufacturing and placing concrete. This results in uniform quality of concrete in each batch.

Achieving Fixed w/c Ratio

Mechanical Water Dozer for Durable Concrete Making

Control on w/c ratio in traditional manufacture of concrete had always been a major challenge. It is most difficult to have a check on actual quantity of water added in each batch of concrete manufactured using traditional single bag concrete mixer. It solely depends on the judgment of concrete mixer operator regarding water need to achieve desired workability by technicians actually placing and compacting concrete. If labor force commits any error in ratio of sand and stone aggregate, quantity of water could be varied by the mixer operator to achieve the desired workability. Most engineers have failed to tame the mixer operator responsible for adding water during concrete making.

Mechanical Water Dozer (fabrication drawing given below) has been developed to overcome the difficulty of taming concrete mixer operator and ensure a fixed quantity of water in each batch of concrete made with one bag of 50 Kg cement.

Mechanical Water Dozer

Mechanical Water Dozer for Durable Concrete Making
The calibrated container (10) with controlled & calibrated water storage capacity varying from 17 Litres to 34 litres can yield concrete with w/c ratio from 0.34 to 0.68 using 50 Kg cement bag in each batch. This container capacity can be adjusted up to 4 ml accuracy using flexible overflow pipe (7) by adjusting level with water leveling (9) device using clamp (8) sliding over the Idle Overflow pipe (12). Water is continuously pumped from storage tank (1) using 1/8 HP pump (4) through three-way valve (6) with valve direction from pump towards container. As soon as water level in container reaches the predetermined level, the excess pumped water is overflown through overflow pipe and is received back in water tank. The overflow of water is an indication that the Water Dozer is ready for delivery of fixed quantity of water to concrete mixer.

For adding the aforesaid fixed quantity of water to concrete mixer, the Three Way Valve (6) is to be operated with its direction changed for water flow from calibrated container to concrete mixer (15). During this period of water flowing from calibrated container to concrete mixer, the pumped water entry into calibrated container is blocked and pumped water is diverted to Idle Overflow Pipe (12) with its out fall at highest level.

Conclusion

By the use of Mechanical Water Dozer in concrete manufactured using traditional concrete mixer, following advantages are accrued to concrete and its durability is assured:
  1. Consistently uniform quality of concrete in every batch as per design mix parameters by effectively controlling on w/c ratio and other constituents.
  2. Any error in cement-aggregate ratio results in unworkable concrete and its rejection by workforce themselves.
  3. Quality of concrete is assured with a low cost appliance with fabrication cost less than '7,500-, which can be fabricated locally at site. All raw materials used in its fabrication are locally available.
  4. The Doser has been tested and used at several sites of construction and has been found to be very effective in ratio control which is an essential component of quality control for concrete.

NBMCW November 2010

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Design of Buildings of Steel and Concre...

Design of Buildings of Steel and Concrete: Comparative Assessment of Limited Continuity

Approach and Composite Construction

Dr. P. Suryanarayana, Professor, Maulana Azad National Institute of Technology, Bhopal

Public buildings requiring large floor area can be economically constructed adopting steel framework of columns, girders and flooring of reinforced concrete. The recently published steel code (IS:800-2007) includes Load and Resistance Factor methods of design adopted from the latest foreign codes. The outdated clauses for design of composite beams and columns are now removed from IS:800-2007. It is expected that new versions of composite construction, based on European codes will be published soon by the Bureau of Indian Standards.

This paper presents various alternatives for design of Steel-Concrete Framed Buildings and examines the economy of each arrangement. Comparison is made between two types of steel construction (traditional and limited continuity type) as well as economy and constructional advantage achieved by composite construction. The new Indian steel code as well as the latest European codes are used.

A four storey industrial frame of 18mx36m plan area is designed using five alternate approaches. Each of the five alternative designs is done by Allowable Stress Design (ASD) as well as Load and Resistance Factor Design (LRFD) and results compared.

Introduction

Public buildings requiring large floor area can be economically constructed by adopting steel frame work of columns, girders and flooring of reinforced concrete slabs. The traditional design of steel-concrete buildings has the following features. (Figure 1) The floor system consists of a slab supported by a grid work of beams. The beams frame into columns in such a way that the centre lines of beams in longitudinal and transverse directions intersect at the column centre. The beam column joints are assumed to be pinned. Hence the beams are designed as single span beams with hinge supports.

An alternate arrangement wherein the secondary beams are continuous over the primary beams and the primary beams are continuous over column brackets is suggested by British designers. In this arrangement, the centre lines of primary beam, secondary beam and column do not intersect at a common point. [5*]

A comparison of traditional and alternate designs is made by designing a building panel of 18m x 36m for a four storey building by both the approaches. Calculations for each design are done both by Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD). The designs are based on elastic and plastic analysis, lateral stability calculations and deflection checks [1,2,3,4]. The building frame consists of 6.0m x 9.0m slab panels. Columns are 4.0m high per storey.

The five design alternatives are as follows:
  1. Traditional design (Fig.1): Primary beams (9.0m) and secondary beams (6.0m) designed as single span beams hinged at ends. It is assumed that the slab provides full lateral restraint to beams.
  2. Traditional design (Fig.2): Secondary beams at 3.0m spacing. All beams designed as single span beams hinged at ends.
    * References are listed alphabetically at the end of the paper
  3. Limited continuity design (Fig.3): Secondary beams (at 4.5m spacing) designed as 3 span continuous beams, transmitting concentrated loads to primary beams. Primary beams (9.0m) designed as single span beams hinged at ends.
  4. Same as design 3, except that secondary beams are spaced at 2.25m (Fig.4)
  5. Steel-concrete composite construction (Fig.1): Secondary beams are designed as 3 span continuous beams and primary beams as 4 span continuous (Propped Construction with two props at one third points for all spans).
Design of Buildings of Steel and Concrete


In all designs dead load of 4.0 kN/sqm. (slab, floor, finish, partitions) and live load of 4.0 kN/sqm is assumed. Slab 140mm thick, M20 concrete, steel sections of Fe 250 grade adopted.

Codes and Design Methods

Design of steel beams: The beams are designed both by allowable stress design (ASD) and load and resistance factor design (LRFD). ASD Methods developed by Brett, Galambos, Kirby, Nethercot and Trahair [5] were used and the lightest section obtained from these designs are adopted. The designs are repeated by LRFD methods given in [2.4].

Design of steel columns is done by the latest Indian Code. [2]

Design of composite beams and columns: The Indian code for composite beams (3) does not include design of continuous beams. There are no code provisions regarding the design of beams with slender sections. In this paper, the design procedures given in [4] were followed.

The outdated and irrational design procedure given in IS 800-1984 [1] for the design of composite columns is now removed in the 2007 version [2,7]. At present there are no guidelines available in Indian Codes for composite columns. Therefore the procedure given in [4] is adopted.

Design of Buildings of Steel and Concrete

Summary of Designs

The cross sections of beams, girders and columns obtained by LRFD methods are shown in figs. 5 to 10 and tables 1 and 2. A comparison with ASD results is given in tables 3 and 4.

In the steel column (2 channels ISMC 400) (Fig.6) the channels arranged to form a box section 350mm x 400mm so that the column is equally strong for major axis and minor axis buckling. The steel requirement of the composite column is much less (Fig.5) The same column section can be adopted at all locations with appropriate orientation as shown in Fig.1

Span Beams

Span Beams

From Tables 1 and 2 it can be seen that all the beams are designed for optimum performance. In Table 1, (Design 3) the section is slightly over designed. However the section cannot be reduced, if the deflection criterion is to be fulfilled.

It is seen from Tables 3 and 4 that designs obtained by LRFD methods result is lighter sections as compared to ASD methods. Of the five alternative designs considered, it is seen that composite construction is the best option, resulting in great savings of steel.

Comparison of ASD and LFRD/LSD
Comparison of ASD and LFRD/LSD

There are many structural and constructional advantages of composite construction vis-a-vis steel frame work. These can be summarised as follows:
  1. Due to the composite action of concrete flange and steel rib, the steel section required in a composite beam is smaller that the section needed for a steel beam, both for single span beams and continuous beams.
  2. Due to increased stiffness of the composite section, beam deflections are reduced.
  3. The concrete slab provides adequate lateral restraint for the steel rib.
  4. By comparing Fig.1 with Figs. 2,3 and 4 it can be seen that a frame comprising composite beams and columns is the most stable among the alternatives.
  5. Speedy construction is possible by proper sequencing of operations. Providing continuity and a rigid frame work is easier in composite construction (as compared to steel work) due to monolithic action.

Conclusion

This paper presents design alternatives for a four storey, 18m x 36m steel-concrete framed building. The frame consists of 20 columns and 16 primary beams of 9.0m span. The number of secondary beams (6.0m) ranges from 15 to 42 in the five alternative arrangements. The beams and columns are designed for strength and checked for serviceability both by ASD and LRFD methods. Design methods from Indian and foreign codes, as well as methods developed by different designers are used. The most economical designs are adopted.

The following conclu- sions are drawn from the study.
  1. LRFD methods are more comprehensive than ASD methods and result in economical design. Savings of order 11% for secondary beams and 16% for primary beams are achieved.
  2. Steel-concrete composite construction requires less number of secondary beams compared to limited continuity designs.
  3. Due to composite action, the size of steel sections can be reduced.Savings of order 22% for secondary beams and 15% for primary beams can be achieved. These savings are partially offset by the cost of shear connectors.
It is concluded that safe and economic designs can be obtained by the LRFD method given in IS:800-2007 for steel frames.

Steel-concrete composite frames are preferable to steel frames due to structural and constructional advantages. A comprehensive code of practice comprising the LRFD methods mentioned in this paper is expected to be available soon.

References

  • IS 800-1984 IS Code of Practice for General Construction in Steel (Second Revision), BIS, New-Delhi.
  • IS 800 : 2007 IS Code of Practice for General Construction in Steel. (Third Revision), BIS, New-Delhi.
  • IS 11384-1985 IS Code of Practice for Composite Construction in Structural Steel and Concrete, BIS, New-Delhi.
  • Narayanan, R. et al: Teaching Resource for Structural Steel Design Ch. 10, 11, 12, 21, 22. Indian Institute of Technology, Chennai, INSDAG, Kolkata July 2001.
  • Richariya, A.K. "Design of Buildings of Steel and Concrete: Comparative Assessment of Limited Continuity Approach and Composite Constru- ction" M.Tech Thesis, Maulana Azad College of Technology, Bhopal 1993.
  • Sangamnerkar, P. "Design Alternatives for Steel-Concrete Framed Buildings". M.Tech Thesis. Maulana Azad National Institute of Technology, Bhopal 2009.
  • Suryanarayana, P. "A Proposed Amendment to IS : 800-1984 (Encased Column Design). Civil Engineering & Construction Review, Vol. 2, No.5, May 1989 pp. 44-46.

NBMCW October 2010

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Production of Self–compacting Concret...

Production of Self–compacting Concrete Using Crusher Rock and Marble Sludge Dusts

M. Shahul Hameed, Faculty & Research Scholar, Dept. of Civil Engg., Sethu Institute of Technology, Kariapatti; P. Kathirvel, Faculty & Research Scholar, Dept. of Civil Engg., KLN College of Engg., Madurai, and Dr. A.S.S. Sekar, Asst. Professor, Dept. of Civil Engg., Alagappa Chettiyar College of Engg. & Tech., Karaikudi

For many decades, concrete has largely been used as a construction material, whether in moderate aggressive environment, or in strong aggressive environment. By volume alone, concrete is the world's most important construction material. Self–compacting Concrete (SCC) as the name implies that the concrete requiring a very little or no vibration to fill the form homogeneously. SCC is defined by two primary properties: Ability to flow or deform under its own weight (with or without obstructions) and the ability to remain homogeneous while doing so. The study explores the use of marble sludge dust (MSD) and crusher rock dusts (CRD) to increase the amount of fines and hence achieve self-compactibility in an economical way, suitable for Indian construction industry. The study focuses on comparison of fresh properties of SCC containing varying amounts of MSD and CRD with that containing commercially available viscosity modifying admixture. The comparison is done at different dosages of superplasticizer keeping cement, water, coarse aggregate, and fine aggregate contents constant. Test results substantiate the feasibility to develop low cost SCC using MSD and CRD.

Introduction

Owing to all its properties, use of SCC is constantly increasing all over the world. But the adoption has not been as fast as it should have been due to its higher cost of production. In India, SCC is used in limited owing to lack of awareness and the higher costs associated with its production. SCC is defined by two primary properties: Deformability and Segregation resistance. Deformability or flowability is the ability of SCC to flow or deform under its own weight (with or without obstructions). Segregation resistance or stability is the ability to remain homogeneous while doing so. High range water reducing admixtures are utilized to develop sufficient deformability. At the same time, segregation resistance is ensured, which is accomplished either by introducing a chemical VMA or by increasing the amount of fines in the concrete. These viscosity modifying admixtures are very expensive and the main cause of increase in the cost of SCC. Self-compacting Concrete is considered to be the most promising building material for the expected revolutionary changes at the job sites as well as on the desk of designers and civil engineers. However, the basic principles of this material are substantially based on those of flowing, cohesive, and superplasticized concretes developed in the mid of 1970's. The necessary ingredients for manufacturing SCC are superplasticizers and powder materials (including cement, fly ash, ground fillers or other mineral additions even in the form of fine recycled aggregate) at an adequate content (> 400 kg/m3 of cement and filler), with some limits in the maximum size of the coarse aggregate (< 25 mm).

The self-compacting concrete differs from conventional concrete in the following three characteristic features, namely, (i) appropriate flowability, (ii) non-segregation, and (iii) no blocking tendency. An increase in the flowability of concrete is known to increase the risk of segregation. Therefore, it is essential to have a proper mix design. This paper reports the results of an investigation into the development of low-bleeding self-compacting concrete. V-Funnel test is used to assess the flowability and segregation resistance of the self-compacted concrete.

Self-compacting High Performance Concrete

The prototype of SCC was first completed in 1988 using materials already on the market. The prototype performed satisfactorily with regard to drying and hardening shrinkage, heat of hydration, denseness after hardening, and other properties and was named "High Performance Concrete." Since then, the term high performance concrete has been used around the world to refer to high durability concrete. Therefore, Okamura [1] has changed the term for the proposed concrete to "Self-compacting High Performance Concrete" (SCHPC) and this was defined as follows at the three stages of concrete:
  1. Fresh : Self–compactable.
  2. Early age : avoidance of initial defects.
  3. After hardening : Protection against external factors.
SCHPC can be described as a high performance material which flows under its own weight without requiring vibrators to achieve consolidation by complete filling of formworks even when access is hindered by narrow gaps between reinforcement bars [2]. With the advancement of concrete technology, high performance concrete is getting popular in prestressed applications. The attributes of high performance concrete are as follows:
  • High strength
  • Minimum shrinkage and creep
  • High durability
  • Easy to cast
  • Cost effective.

The Role of Superplasticizers and Powder Materials

With the advent of superplasticizers, flowing concretes with slump level up to 250 mm were manufactured with no or negligible bleeding, provided that an adequate cement factor was used [3]. The slump was increased by increasing the amount of mixing water. When the slump is over 175 mm the bleeding increases too much and this was the reason why ACI in 1973 did not recommend slump higher than 175 mm [4]. The most important basic principle for flowing and cohesive concretes including SCCs is the use of superplasticizer combined with a relatively high content of powder materials in terms of portland cement, mineral additions, ground filler and/or very fine sand.

Characterisation of Wastes Marble Sludge Powder and Quarry Rock Dust

The physical characteristics of the waste are furnished in Table 1. The fineness modulus of marble sludge powder and quarry rock dust is comparable to that of fine sand of F.M. 2.2 to 2.6. The coefficient of uniformity for fine sand generally should be less than 6. Similarly the coefficient of gradation should be between 1 and 3 for fine sand. The chemical characteristics of the River sand, Marble sludge powder and Quarry rock dust are furnished in Table 2. The chemical characteristics are compared with the oxide composition of ordinary Portland cement.

Physical Characteristics of the Waste

Chemical Characteristics of the Waste

Heavy Metal Leachability of the Waste

The waste is of inorganic type with heavy metal concentration in the TCLP extract of the marble sludge powder and quarry rock dust are as shown in Table 3. The heavy metals concentration of As, Ba, Cd, Cr, Hg, Pb and Ag are within the acceptable limits as specified by USEPA. The results indicate that there won't be any impact on the leachability of waste.

Extract of the Marble Sludge Powder and Quarry Rock Dust

Raw Materials and Test Methods

Cement

Ordinary Portland Cement (43 Grade) with 28% normal consistency with specific surface 3300 cm2/g Conforming to IS: 8112-1989 was used.

Marble Sludge Powder

Marble sludge powder was obtained in wet form directly from deposits of Marble factories. Wet Marble sludge powder must be dried before the sample preparation. Marble dust was sieved from 1mm sieve. The high content of CaO confirmed that the original stones were Marble and limestone. The dust was also tested (NP 85) to identify the absence of organic matter, thus confirming that it could be used in concrete mixtures.

Quarry Rock Dust

The quarry rock dust was obtained from a local crusher at Kariapatti, in District Virudunagar. The specific gravity of the quarry rock dust used was 2.677. Moisture content and bulk density of waste are less than the sand properties.

Fine Aggregate

Medium size sand from Madurai Vaigai river with a modulus of fineness = 2.20; Specific gravity 2.677, normal grading with the silt content 0.8%.

Coarse Aggregate

Crushed stone from Madurai district with a size of 5-20 mm and normal continuous grading was used. The content of flaky and elongated particles is < 3%, the crushing index d"6% and the specific gravity 2.738.

Water

Tap water available in the concrete laboratory was used in manufacturing the concrete. The qualities of water samples are uniform and potable.

Super Plasticizer

A superplasticizer based on refined lingo Sulphonates, 'Roff Superplast 320' was used to get and preserve the designed workability.

Mix Proportion of Concrete

For Durability Studies the Indian Standard mix Proportion (by weight) used in the mixes of Conventional Concrete and green concrete were fixed as 1:1.81:2.04, 1:1.73:2.04 after several trials. Based on properties of raw materials, two different mix proportions are taken and given in Table 4. Mix A is the controlled concrete using river sand and Mix B is the green concrete using Industrial waste (50% quarry rock dust and 50% marble sludge powder) as fine aggregate. The water/cement ratio for both the mixes was 0.55% by weight. Water reducing admixture was used to improve the workability and its dose was fixed as 250 m1/50kg of cement.

Mix Proportions

Testing of Specimens

For each mix, slump flow, L-box and V-funnel test were carried out. Brief explanation and illustration of Slump flow, L-Box and V-funnel test is given below.

Slump Flow Test

It is the most commonly used test and gives a good assessment of filling ability. At first, the inside of slump cone and the smooth leveled surface of floor on which the slump cone is to be placed are moistened. The slump cone is held down firmly. The cone is then filled with concrete. No tamping is done. Any surplus concrete is removed from around the base of the concrete. After this, the cone is raised vertically and the concrete is allowed to flow out freely. The diameter of the concrete in two perpendicular directions is measured. The average of the two measured diameters is calculated. This is the slump flow in mm. The higher the slump flow value, the greater its ability to fill formwork under its own weight. As per EFNARC guide, the range is from 650 mm to 800 mm [5].

Slump Flow Test
Slump Flow Test

L Box Test Method

It assesses filling and passing ability of SCC. The vertical section is filled with concrete, and then gate lifted to let the concrete flow into the horizontal section. When the flow has stopped, the heights 'H1' and 'H2' are measured. Closer to unity value of ratio 'H2/H1' indicates better flow of concrete [5].

L Box Test
L Box Test

V-funnel Test And V-funnel At T=5 Min

The test measures flowability and segregation resistance of concrete. At first, the test assembly is set firmly on the ground and the inside surfaces are moistened. The trap door is closed and a bucket is placed underneath. Then the apparatus is completely filled with concrete without compacting. After filling the concrete, the trap door is opened and the time for the discharge is recorded. This is taken to be when light is seen from above through the funnel. To measure the flow time at T–5minutes, the trap door is closed and V-funnel is refilled immediately. The trap door is opened after 5 minutes and the time for the discharge is recorded. This is the flow time at T–5minutes. Shorter flow time indicate greater flowability [5].

V-funnel Test
V-funnel Test

Compressive Strength Test

Compressive strength test usually gives an overall picture of the quality of concrete because strength is directly related to the structure of the hydrated cement paste. The compression test is an important concrete test to determine the strength development of the concrete specimens. Compressive strength tests (BS 1881: Part 103: 1983) were performed on the cube specimens at the ages of 7, 28 and 90 days.

Compressive Strength Test
Compressive Strength Test

Split Tensile Strength Test

The indirect method of applying tension in the form of splitting was conducted to evaluate the effect of MSP and CRD on tensile properties of concrete. The split tensile strength is a more reliable technique to evaluate tensile strength of concrete (lower coefficient of variation) compared to other methods. The split tensile strength of 150 mm diameter and 300 mm high concrete cylindrical specimens was determined to assess the effect of CRD and MSP on the tensile properties of the concrete.

Split Tensile Strength Test
Split Tensile Strength Test

Test Results and Discussions

Fresh Flow Property

Properties of freshly mixed concrete were tested for qualifying within the specified EFNARC range of SCC [3] and given in Table 5. The slump flow of Mix B was 657 mm within the EFNARC range of SCC. The results of slump flow show that the flow increased with the increase in the quantity of fine particle. As far as filling ability of the Mix B, results of V - funnel tests remained more towards the minimum range or even lesser. This showed more filling ability but less viscous mix. Due to the very low w/b-ratio and the very high fineness of Marble sludge dust and crusher rock dust the viscosity of the SCC is obviously higher than that of conventional concrete. The air content is therefore somewhat higher than in conventional concrete. While testing the concrete for passing ability, majority of the mixes passed through the bars very easily and without any blockage. The results of L-box test show that the ratio of L-box increased with the increase of marble sludge dust. V - Funnel at T5 minutes test shows the potential to segregation resistance. V - funnel at T5minutes test results for Mix A and Mix B are 13 sec and 10 sec respectively. The results of this test remained very encouraging and within the EFNARC range.

Freshly Mixed Concrete

Hardened Mixture Properties

Compressive Strength Test

The 7 days and 28 days compressive strength of SCC is increasing up to 21.52 N/mm2 and 42.25 N/mm2 respectively. Whereas, the conventional concrete, the 7 days and 28 days compressive strength is 17.58 N/mm2 and 35.86 N/mm2 respectively. The test results of 7 days and 28 days compressive strength is given in Table 6. CRD and MSP reduce the pore structure of concrete results improve the strength of concrete. The crusher rock particles have greater rough surface than river sand. The rough surface improves the bond strength. Hence, a SCC concrete specimen made with crusher rock dust gives greater compressive strength.

Compressive Strength of Concrete

Split Tensile Strength Test

The 7 days and 28 days split tensile strength is increasing up to 2.68 N/mm2 and 4.85 N/mm2 respectively for SCC. The split tensile strength for conventional concrete is 2.57 N/mm2 and 4.14 N/mm2. The test results of 7 days and 28 days compressive strength and split tensile strength is given in table 6.When comparing the SCC with normal concrete there is much improvement in split tensile strength value. MSP improves the flow property value. But, when MSP percentage increases beyond 15%, the strength reduced.

Cost Analysis

Cost analysis of the materials used, has been analyzed as per the purchased price from the market (as on June 2009). The mixes selected for calculation and analysis were those which could pass maximum properties of freshly mixed concrete. The detailed calculations are summarized in Table 7. It is clear that the cost of ingredients of specific SCC containing MSD and CRD is Rs.3849.10 only, which is 9.91% less than the control concrete.

Cost analysis of the materials

Conclusion

Throughout the world, the waste disposal costs have escalated greatly. At the same time, the concrete construction industry has realized that Marble Sludge Dust and Crusher Rock Dust are relatively inexpensive and widely available by-product that can be used for sand replacement to achieve excellent workability in fresh concrete mixtures. These materials can be used in the manufacturing of economical SCC in different ways. When the crusher rock dust and marble sludge dust were used as replacement of sand, the requirements of expensive chemicals such as HRWRA and viscosity modifying agent (VMA) decreased. Based on field experience and laboratory tests, the properties of SCC, when compared to conventional portland cement concrete, can be summarized as follows:
  • The possibility of developing low cost SCC using crusher rock and marble sludge dusts is feasible. Low cost SCC can be made, by incorporating these industrial waste by replacing the river sand.
  • The utilization of MSD and CRD in SCC solves the problem of its disposal thus keeping the environment free from pollution and enhance the resource productivity of the concrete construction industry.
  • The chemical compositions of Crusher dust and Marble sludge such as Fe2O3, MnO, Na2O, MgO, K2O, Al2O3, CaO, and SiO2 are comparable with that of cement.
  • The replacement of fine aggregate with 30% marble sludge and 70% Crusher dust (Mix B) gives an excellent result in strength aspect and quality aspect. It induced higher compressive strength, higher splitting tensile strength
  • The results showed that the substitution of 70% of the crusher and 30% of marble sludge induced easier flowability, pumpability, and compactability.
  • Early-strength up to 7 days, which can be accelerated with suitable changes in the mix design when earlier removal of formwork or early structural loading is desired.
  • In fresh state, some of the mix results values were out of the EFNARC range and therefore before casting the concrete, the properties of freshly mixed concrete must be checked for SCC.

References

  • Okamura, H. and Ouchi, M., (1999). "Self-compacting concrete- development, present and future", Proceedings of the First International RILEM symposium on Self-Compacting Concrete, pp.3-14.
  • Zhu W., Gibbs C J., and Bartos P J M., (2001). "Uniformity of in situ properties of self compacting concrete in full-scale structural elements," Cement & Concrete Composites, Vol. 23, pp. 57-64.
  • M. Collepardi, "Assessment of the "Rheoplasticity" of Concretes," Cement and Concrete Research, (1976) pp. 401-408.
  • ACI Manual of Concrete Practice, «Recommended Practice for Selecting Proportions for Normal Weight Concrete», 1ACI 211.1-70, Part 1, (1973) pp.211-214, ACI Publications.
  • EFNARC. (2002). "Specifications and Guidelines for Self Compacting Concrete." Available online at: www.efnarc.org

NBMCW October 2010

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Effect of Rice Husk Ash on Cement Morta...

Sudisht Mishra, Faculty Civil Engineer. Deptt NERIST, Itanagar, Prof (Dr.) S. V. Deodhar, Principal, SSVPS BSD College of Engineering, Dhule.

Introduction

Effect of Rice Husk Ash on Cement Mortar and Concrete
Workability, strength, and durability are three basics properties of concrete. Amount of useful internal work necessary to overcome the internal friction to produce full compaction is termed as Workability. Size, shape, surface texture and grading of aggregates, water-cement ratio, use of admixtures and mix proportion are important factors affecting workability. Strength is to bear the desired stresses within the permissible factor of safety in expected exposure condition. The factor influencing the strength are: quality of cement, water-cement ratio, grading of aggregates, degree of compaction, efficiency of curing, curing temperature, age at the time of testing, impact and fatigue. Durability is sustenance of shape, size and strength; resistance to exposure conditions, disintegration and wearing under adverse conditions. Variation in concrete production, loading conditions in service life and subsequent attack by the environment factors are main deteriorating factor of concrete. Properly compacted and cured concrete used in RCC continues to be substantially water tight and durable till capillary pores and micro-cracks in the interior are interconnected to form pathways up to surface.

Durability is mainly influenced by environmental exposure condition, freezing - thawing, contact to aggressive chemicals, type and quality of constituent materials, water-cement ratio, workability, shape and size of the member, degree of compaction, efficiency of curing, effectiveness of cover concrete, porosity and permeability. During service life of structures, penetration of water and aggressive chemicals, carbonation, chloride ingress, leaching, sulphate attack, alkali-silica reaction and freezing-thawing are resulting deterioration. Loading and weathering inter link voids and micro-cracks present in transition zone and network of same micro cracks gets connected to cracks on concrete surface which provides primary mechanism of the fluid transport to interior of concrete. Subsequent increase of penetrability leads to easy ingress of water, oxygen, carbon dioxide and acidic ions etc into concrete resulting cracking, spalling, loss at mass, strength and stiffness.

Low permeability is key to durability and it is controlled by factors like water-cement ratio, degree of hydration, curing, entrapped air voids, micro cracks due to loading and cyclic exposure to thermal variations. Admixture improves workability, compactibility, strength, impermeability, resistance to chemical attack, corrosion of reinforcement and freezing - thawing etc. and in turn to durability. For this study durability is interpreted in terms of porosity, moisture movement, surface strength, ultra sound pulse velocity and elasticity modulus of concrete. Optimum use of Rice Husk Ash (RHA), obtained by open field burning method, is decided for improving workability, strength and durability of concrete.

Rice Husk Ash

RHA, produced after burning of Rice husks (RH) has high reactivity and pozzolanic property. Indian Standard code of practice for plain and reinforced concrete, IS 456- 2000, recommends use of RHA in concrete but does not specify quantities. Chemical compositions of RHA are affected due to burning process and temperature. Silica content in the ash increases with higher the burning temperature. As per study by Houston, D. F. (1972) RHA produced by burning rice husk between 600 and 700°C temperatures for 2 hours, contains 90-95% SiO2, 1-3% K2O and < 5% unburnt carbon. Under controlled burning condition in industrial furnace, conducted by Mehta, P. K. (1992), RHA contains silica in amorphous and highly cellular form, with 50-1000 m2/g surface area. So use of RHA with cement improves workability and stability, reduces heat evolution, thermal cracking and plastic shrinkage. This increases strength development, impermeability and durability by strengthening transition zone, modifying the pore-structure, blocking the large voids in the hydrated cement paste through pozzolanic reaction. RHA minimizes alkali-aggregate reaction, reduces expansion, refines pore structure and hinders diffusion of alkali ions to the surface of aggregate by micro porous structure.

Portland cement contains 60 to 65% CaO and, upon hydration, a considerable portion of lime is released as free Ca(OH)2, which is primarily responsible for the poor performance of Portland cement concretes in acidic environments. Silica present in the RHA combines with the calcium hydroxide and results excellent resistance of the material to acidic environments. RHA replacing 10% Portland cement resists chloride penetration, improves capillary suction and accelerated chloride diffusivity.

Pozzolanic reaction of RHA consumes Ca(OH)2 present in a hydrated Portland cement paste, reduces susceptible to acid attack and improves resistance to chloride penetration. This reduces large pores and porosity resulting very low permeability. The pozzolanic and cementitious reaction associated with RHA reduces the free lime present in the cement paste, decreases the permeability of the system, improves overall resistance to CO2 attack and enhances resistance to corrosion of steel in concrete. Highly micro porous structure RHA mixed concrete provides escape paths for the freezing water inside the concrete, relieving internal stresses, reducing micro cracking and improving freeze-thaw resistance.

Non Destructive Tests (NDT)

NDT, systems required for assessing strength in service feature, is defined as a test which does not impair the intended performance of the element or member under investigation, carried out onsite, with ability to determine the strength and durability of critical constructions without damaging.

For conducting ND Tests, Rebound hammer, Protimeter-moisture measurement system, Porositester and pulse Ultrasonic Non-Destructive Indicating Tester (PUNDIT) equipment are used.

Rebound hammer test is conducted to asses the relative strength of concrete based on the hardness at or near its exposed surface. Concrete Rebound Test Hammer is a traditional instrument used for the non-destructive testing of hardened concrete. This provides a quick and simple test method for obtaining an immediate indication of concrete strength in various parts of a structure. Knob of this instrument is kept perpendicular to the surface (i.e. 90 degree to the surface) for measurement and is push pressed from the bottom towards the surface of the concrete, hammer like sound is produced. The button near the bottom of the instrument is pressed to lock the indicator and reading is taken.

The surface moisture is generally not seen for cleaning and restoring the structure after damage from storms, floods or fires. For exposed or unseen structural damage, the undetected moisture damages strength, durability and reliability. The Protimeter Moisture Measurement System is a powerful and versatile instrument for measuring and diagnosing surface moisture in buildings. This instrument directly displays moisture content (%) along with three conditions of material like DRY, WET and RISK condition. For using Protimeter operational modes are selected and information is presented on a large, back lit liquid crystal display. The radio frequency sensor is positioned so that large number of moisture readings is taken quickly and easily.

Porositester contains of 3 glass tube used to measure the water penetration in concrete at atmospheric pressure. Tubes are fixed with vacuum plate and vacuum pump without any subsequent cleaning. Rubber seal inserted in the tube provides a defined contact surface of dia 25 mm vertical or horizontal. Compressible seals on suction plate and tube permit secure fixing also to uneven surfaces. Current supply is provided from a 12 V rechargeable battery. Vacuum plate is pressed against the façade with a wet sponge, motor is placed on adhesion of the plate is checked. Test tubes are placed under the clip and secured firmly with screws. Test tubes are filled with water up to the zero mark and again refilled after the descent of the water level by 1 or 2 ml and readings of quantity of penetrated water are noted down for 15 minute test time. As per the manual of equipment, water absorption coefficient A is calculated by using following formula:

Effect of Rice Husk Ash on Cement Mortar and Concrete

Where,

X=Amount (level) of water penetrated in ml.

d= Dia. of test tube in mm.

t= Time of penetration in minutes.

Effect of Rice Husk Ash on Cement Mortar and Concrete
Figure 1: Porositester
Figure 1 shows the Porositester mentioned above Pundit is a highly trustworthy equipment for ultrasonic pulse velocity teshon concrete. Pundit plus is used to determine pulse velocity, (UPV) modulus of elasticity, cavities and cracks present in the concrete. Equipment is ruggedly built for on-site reliability for simple, speedy operation with integral RS232 interface, auto memory store for readings, large LCD display with external or battery power supply. The setup parameters are defined for the preferred mode of operation and respective values are displayed.

Workplan

Objective of this work is to study the effects of Rice Husk Ash as an admixture on workability, strength, durability of cement concrete and cement mortar. Based on the above, optimum dose of RHA is determined to enhance the desired properties of concrete without causing any adverse result on other properties.

Rice Husk from local Rice Mills was burnt completely in open field condition and sieved with 150 micron IS sieve. Rice Husk Ash percentage was gradual increased from 7.5%, 10.0%, 12.5%, 15.0% and 17.5%. M20 grade nominal mix concrete (1 : 1.5 : 3) and cement mortar of proportion (1 : 4). Coarse aggregate of 20 mm graded nominal size, river sand zone III type and 53 grades Pozzolana Portland Cement (PPC) were used for this work. For slump values 15 mm to 35 mm and compaction factor 0.85 to 0.90, water cement ratio for plain and RHA mixed concrete was 0.50 and 0.575 respectively. For casting concrete and mortar cubes, 150 mm steel cube moulds and 70.7 mm steel cube moulds were used. Each set Contained Six samples of plain concrete and six of RHA mixed. After 24 hours of casting, samples were opened and kept under tap water curing for 28 days. Later destructive and non-destructive tests were carried out on set of three separate cubes and average values taken for this study.

Test Results
Compressive Strength

Compressive strength for RHA mixed concrete samples increased upto 12.5% of RHA and decreased for higher % of RHA. Highest strength was found 30.3 N/mm2 followed by 30.07 N/mm2 for RHA composition 10.00% and 12.5% respectively. In comparison to normal M20 mix samples, compressive strength decreased by 12.94% and 19.17% for 15.00% and 17.5% of RHA mixed concrete samples. Between RHA compositions 10% and 12.5%, compressive strength increased very marginally (0.80%) whereas same value was highest (3.08%) between RHA compositions 7.50% and 10.00%.

Compressive strength of normal mortar cubes was 10.39 N/mm2 and same increased to 16.43 N/mm2 and 17.44 N/mm2 for RHA composition 7.5% and 10.00% respectively. For higher proportion of RHA, 12.50%, 15.00% and 17.50%, compressive strength decreased to 12.74 N/mm2, 10.73 N/mm2 and 7.71 N/mm2 respectively. Maximum increase in strength was 67.85% followed by 58.13% for 10.00% and 7.50% RHA composition respectively.

Effect of Rice Husk Ash on Cement Mortar and Concrete

Rebound Hammer Test

Surface strength of M20 grade concrete cube was 20.05 N/mm2. For RHA mixed samples, same values increased to a maximum of 28.00 N/mm2 and minimum 23.00 N/mm2 (Table-1) for 10% and 17.5 % rice husk respectively. For RHA mixed mortar samples, surface strength increased marginally from 16.67 N/mm2 to 17.33 N/mm2 (3.96%) which further showed a decreased value of 16.67 N/mm2 for mortar cubes with 17.5% RHA. For concrete samples, strength increased upto 10% RHA and later it started decreasing but for the mortar samples same trend continued upto 15% RHA. Interpreted from the graph that the strength is increased rapidly from 7.5% to 15% and it starts decreasing with increase in percentage of admixture. The reference mortar cube is having strength of N/mm2.

Effect of Rice Husk Ash on Cement Mortar and Concrete

Surface Moisture Test Results

Surface moisture for concrete cubes was 17.31% which increased to 17.97% for 7.5% RHA content. For higher % of RHA, it showed a decreased trend with minimum value 17.2% for 17.5% RHA. For RHA mixed mortar cubes, maximum value was 17.32% with 10% RHA which further decreased to 15.89% for 17.5% RHA.

Effect of Rice Husk Ash on Cement Mortar and Concrete
Figure 2 (a): Variation of NDT Properties of Mortar Cubes

Pulse Velocity Test

Pulse velocity was observed 3258 m/sec. in normal concrete cubes which increased to maximum 3736 m/sec. in 15.00% RHA mixed cubes. Increase in pulse velocity was 6.78%, 10.25%, 12.68%,14.67% and 13.51% for corresponding RHA 7.5%, 10%, 12.5%, 15%, and 17.5% respectively.

Elastic modulus increased from 3.83 GN/m2 to a maximum 5.97 GN/m2 (55.87%) for 12.5% RHA mixed samples.

Effect of Rice Husk Ash on Cement Mortar and Concrete
Figure 2 (b): Variation in Compressive Strength, Rebound Hammer
Strength and Surface Moisture

Porosity

For nominal mix M20 grade concrete cubes, water absorption coefficient was found 1.19 Kg/m2 /” min. In RHA mixed concrete samples, water absorption coefficients exhibited decreasing trend to a minimum of 1.34 (29.84%) for 12.5% of RHA but the same increased with higher percentage of RHA. For maximum proportion of RHA (17.5%) it was found 1.63 Kg/m2 /” min, 15.18% higher than the minimum value.

Effect of Rice Husk Ash on Cement Mortar and Concrete
Figure 2 (c): Variation in Compressive Strength, Rebound Hammer
Strength and Surface Moisture

Conclusion

In nominal mix M20 grade concrete and 1:4 cement mortar RHA was added as an admixture from 7.50% to 17.50% with an uniform variation of 2.5%. During destructive test, compressive strength of mortar cubes and rebound hammer strength of concrete samples found increased with maximum variation of 67.85% and 39.65% for 10% RHA. Maximum variations of elastic modulus were 55.87% followed by 27.94% for 12.50% and 10% RHA mixed samples. Compressive strength of concrete samples showed maximum increase 3.08% between RHA 7.50% to 10.00% which decreased further for higher percentage of RHA.

Reduction in water absorption, from results obtained from 6 tests concrete and 3 tests on mortar samples, it is observed that up to 10% RHA with concrete and mortar enhances all properties (Figures 2a to c) and it is observed that 12.5% of Rice Husk Ash by mass of cement as the optimum doses to be added in concrete production of M20 particularly when the husk is burnt under field condition to utilize the easily available and low cost resources for betterment of concrete structure with respect to economy, durability and strength. So best applicable percentage of rice husk ash as per field condition 10.00% for optimal strength and durability.

References

  • Bronzeoak Ltd, Rice Husk Ash Market Study, ETSU U/00/00061/REP DTI/Pub URN 03/668, 2003.
  • Satish Chandra, Waste materials used in concrete manufacturing, William Andrew Inc. Norwich, NY 13815, 2002.
  • Hwang, C. L., and Wu, D. S., Properties of Cement Paste Containing Rice Husk Ash, ACI SP-114, 1989.
  • Anderson,L.L. and Tillman D.A., Fuels from waste. Academic Press Inc., New York, U.S.A, 1978.
  • E. B. Oyetola and M. Abdullahi, The Use of Rice Husk Ash in Low–Cost Sandcrete Block Production, Department of Civil Engineering, Federal University of Technology, P.M.B. 65, Minna, Nigeria, June 2006.

NBMCW October 2010

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Popular Non Destructive Testing of Conc...

Popular Non Destructive Testing of Concrete Structure-review of Std. Methods

Testing of Concrete Structure

R. B. Singh, Chief Coordinator, ANULAB, Agra

Introduction

In a short span of ten years after Bhuj earthquake, nondestructive testing has achieved an important place in the Quality Assurance of hardened concrete and the evaluation of existing concrete structure with regard to their strength & durability.

This paper deals with the popular, economical and widely used NDT tests in the field in general & national highways in particular. The paper also has discussion on combined methods, when more than one nondestructive test method is used and condition assessment is based on the data obtained from Rebound Hammer, UPV & Core tests.

The aim of the paper is to address the field engineers engaged in evaluation of quality of hardened concrete. An attempt has been made to keep the theoretical part of the subject to an absolute minimum, where necessary tables of std. values, photographs & comparisons have been included. Research oriented engineers who would want through treatment of the material and a more basic approach are referred to the original std. specification, NDT handbooks & original papers and literature on the subject given as reference. Although nondestructive tests are relatively simple to perform & instrument based, the analysis and interpretation of the test data are not easy, because concrete is a complex material, hence the engineers are cautioned that interpretation of the test data must always be carried out by trained specialists in NDT rather than by technicians performing the tests.

If used properly, nondestructive tests can form a vital link in the chain of testing and evaluation of concrete and concrete structures, which starts with crushing of 150 mm cubes and may end with load testing of finished structure.

At present, standard method of determination strength of hardened concrete consist of testing of concrete cubes (03 Nos. of 150X150X150 mm) in compression testing machine following the std. test method IS: 516. In case of PQC for rigid pavement, beam specimens are broken for flexural strength. The strength tests, regardless of the type, are excellent for determining the criteria of quality & quality control during construction, but they leave a lot to be desired. The main disadvantage of such tests are delay in obtaining test results, the fact that the test specimen may not be truly representative of the concrete in a structure, the necessity of stressing the test specimen to failure, the lack of reproducibility in the test results and the relativity high cost of testing & wastage of concrete in form of cubes.

For the NDT tests to monitor the service behavior of concrete structures over a long period, it was imperative that these tests be nondestructive. This approach, though new for the testing of concrete, had long been used in the testing of metals in petroleum exploration & refining projects. The direct determination of strength implies that concrete specimen must be loaded to failure; it becomes clear that nondestructive methods of testing cannot be expected to yield absolute value of strength.

These methods, therefore, attempt to measure some other property of concrete from which an estimate on its strength, its durability and its elastic parameters is obtained. Such properties of concrete are its hardness, rebound number and its ability to allow ultrasonic pulse velocity to propagate through it. The electrical properties of the concrete, allow us to estimate its moisture content, density, thickness and its cement content. Based on above, various nondestructive methods of testing concrete have been developed.

Popular NDT Tests for Concrete Used in field are:
  1. Rebound Hammer Test- RH Test
  2. Ultrasonic Pulse Velocity- UPV Test
  3. Combined Method UPV & RH Test
  4. Core Extraction for Compressive Strength Test
  5. Ingredient Analysis of Concrete Core
  6. Concrete Cover Measurement by Laser Based Instt.
This paper, describes in detail only Rebound Hammer (RH) test, Ultrasonic Pulse Velocity (UPV) test & Core Test which are widely used & accepted by engineers at site and also referred in IS: 456-2000, under Inspection & Testing of Structures. These are followed by a description of the combined methods approach in which more than one nondestructive method is used to estimate strength of concrete. The Ingredient Analysis, Cover Measurement, Permeability, and Density methods are of limited application and are briefly described the concluding part of the paper.

1. Rebound Hammer–RH (Schmidt) Test

In 1948, a Swiss Engineer, Ernst Schmidt from Zurich developed a test hammer for measuring the hardness of concrete by the rebound principle. Since then the Rebound Hammer (RH) test has gained recognition at construction site & precast Industry.

Principle

The Schmidt Rebound Hammer is principally a surface hardness tester with little apparent theoretical relationship between the strength of concrete and the Rebound number of the hammer. However, within limits, empirical correlations have been established between strength properties & rebound number. This correlation between the concrete strength and rebound number is required to be established at site/field laboratories before it is used for strength estimation of concrete. Sometimes it is referred as fieldcalibration of rebound hammer. Lab calibration are based on Brinell Hardness & Rebound Nos. are checked on std. calibrated Anvil for the purpose. Proper site calibrations eliminate the lab calibration, which is for the checking of hammer performance.

Rebound Number and Compressive Strength

There is a general correlation between compressive strength of concrete and the hammer rebound number. Coefficients of variation for compressive strength for a wide variety of specimens averaged 25%. The large deviations in strength can be narrowed down considerably by proper calibration of the hammer, which allows for various variables discussed earlier. By consensus, the accuracy of estimation of compressive strength of test specimens cast, cured, and tested under laboratory conditions by a properly calibrated hammer lies between ±15 and ±20%. However, the probable accuracy of prediction of concrete strength in a structure is ±25%.

Limitations and Usefulness

The limitations of the Schmidt hammer are many; these should be recognized and allowances be made when using the hammer. It cannot be overstressed that this instrument must not be regarded as a substitute for standard compression tests but as a method for determining the uniformity of concrete in the structures and comparing one concrete by the Schmidt hammer within an accuracy of ±15 to ±20% may be possible only for specimens cast, cured, and tested under identical conditions as those from which the calibration curves are established. The prediction of strength of structural concrete by using calibration charts based on the laboratory test is not recommended.

2. Ultrasonic Pulse Velocity-UPV Test

The test instrument consists of a means of producing and introducing a wave pulse into the concrete and a means of sensing the arrival of the pulse and accurately measuring the time taken by the pulse to travel through the concrete.

Portable ultrasonic testing equipment are available. The equipment is portable, simple to operate, and includes rechargeable battery and charging unit. Typically, pulse times of up to 6500 os can be measured with 0.1-os resolution. The measured travel time is prominently displayed. The instrument comes with a set of two transducers, one each for transmitting and receiving the ultrasonic pulse. Transducers with frequencies of 25 to 100 KHz are usually used for testing concrete. These transducers primarily generate compressional waves at predominantly one frequency, with most of the wave energy directed along the axis normal to the transducer face.

Factors Affecting UPV Test

Although it is relatively easy to conduct a pulse velocity test, it is important that the test be conducted such that the pulse velocity readings are reproducible and that they are affected only by the properties of the concrete under test rather than by other factors. The factors affecting the pulse velocity can be divided into two categories: (1) factors resulting directly from concrete properties; and (2) other factors. These influencing factors are discussed below:

Effects of Concrete Properties

  1. Aggregate Size, Grading, Type, and Content
  2. Cement Type
  3. Water-Cement Ratio
  4. Admixtures
  5. Age of Concrete

Other Effects

  1. Transducer Contact
  2. Temperature of Concrete
  3. Moisture and Curing Condition of Concrete
  4. Path Length
  5. Size and Shape of a Specimen
  6. Level of Stress
  7. Presence of Reinforcing Steel

Applications of UPV Tests

UPV Testing
UPV Test being perform on Deck Slab of Flyover on NH-2 at Firozabad (U.P.), India
The pulse velocity method has been applied successfully in the laboratory as well as in the field. It can be used for quality control, as well as for the analysis of deterioration. The applications of the pulse velocity method on a concrete structure are:
  1. Estimation of Strength of Concrete
  2. Establishing Homogeneity of Concrete
  3. Studies on the Hydration of Cement
  4. Studies on Durability of Concrete
  5. Measurement of Surface Crack Depth
  6. Determination of Dynamic Modulus of Elasticity

Combined Method–UPV & RH Test

UPV Testing
UPV Test being perform on Minor Bridge Pier on NH-11 at Dausa (Raj.), India
Hardness scales are arbitrarily defined measures of the resistance of a material to indentation under static or dynamic load or resistance to scratch, abrasion, wear, cutting or drilling. Concrete test hammers evaluate surface hardness as a function of resiliency, i.e the ability of hammer to rebound or spring back.

The interpretation of the pulse velocity measurements in concrete is complicated by the heterogeneous nature of this material. The wave velocity is not determined directly, but is calculated from the time taken by a pulse to travel a measured distance. A piezoelectric transducer emitting vibration at its fundamental frequency is placed in contact with the concrete surface so that the vibrations travel through the concrete and are received by another transducer, which is in contact with the opposite face of the test object.

Conclusion

Portable Concrete Coring Machine
Portable Concrete Coring Machine (BOSCH) in Horizontal Operation on RCC Column
Combined nondestructive methods refer to techniques in which one test is used to improve the reliability of the in site concrete strength estimated by means of another test alone.

The validity of a combined technique can be evaluated from the degree of improvement this additional test provides to the accuracy and reproducibility of predictions, vs. the additional cost and complexity of the combined method and the extent to which it is practicable to perform the additional test in site.

Of the various combinations proposed by different researchers and from the reported data it seems that only the combined techniques based on the Ultrasonic Pulse Velocity and surface hardness measurement have been adopted for practical evaluation of the in site compressive strength of concrete.

Concrete Core from RCC Column
Concrete Core from RCC Column being Extracted after Diamond Bit Core Drilling
The limitations of a combined method are usually those pertinent to the limitations of each component test, except when a variation in the properties of concrete affects the component test, except when a variation in the properties of concrete affects the component test results in opposite directions. In this case, the errors can be self-correcting. Development of a prior correlation relationship is essential if the estimated from the combined test are to be meanigful. The more information that can be obtained about the concrete ingredients, proportions, age, curing conditions, etc. the more reliable the estimate is likely to be.

When testing suspect quality concrete of unknown composition, it is highly desirable to develop a prior correlation relationship in which factor such as aggregate type and approximate age of concrete are introduced as constants. For most in site concrete an approximate age and petrological type of aggregate can be determined, thus reducing the number of uncontrollable variables.

Core Drilling
Core Drilling in Progress on the Inside Wall (After Epoxy Grouting) of Box Culvert on NH-26 at Sagar, (M.P.)
The most important influences on the accuracy and reliability of strength estimates seem to be the coarse aggregate type in the concrete.

When a reliable prior correlation relationship exists for a particular concrete type, the use of combined nondestructive techniques provides a realistic alternative to destructive testing. It often possible to perform a large and thus a representative number of tests at a reduced cost compared with coring, and without an adverse effects on the integrity of structural element.

Core Extraction for Compressive Strength Test

Test Specimens

Core Drilling
Core Dressing-Cutting in Lab using Diamond Wheel Cutter in the Lab before Capping and Curing for Compressive Strength Test on CTM
Core Specimens- A core specimen for the determination of compressive strength shall have a diameter at least three times the maximum nominal size of the coarse aggregate used in the concrete, and in no case shall the diameter of the specimen be less than twice the maximum nominal size of the coarse aggregate. The length of the specimen, when capped, shall be as nearly as practicable twice its diameter.

Procedure

Core Drilling- A core specimen taken perpendicular to a horizontal surface shall be located, when possible, with its axis perpendicular to the bed of the concrete as originally placed.

Measurement of Drilled Core Specimens

Mean Diameter- The mean diameter shall be determined to the nearest millimeter from three pairs of measurements. The two measurements in each pair shall be taken at right angles to each other, one pair being taken at the middle of the core and the other pairs at the quarter points of the depth. The mean of the six readings shall be taken as the diameter.

Position of Reinforcement- The positions of any reinforcement shall be determined by measuring to the nearest millimetre from the centre of the exposed bars to the top of the core. The diameter and, if possible, the spacing of the bars shall be recorded, and also the minimum top and bottom cover.

RCC Structure-Box
Extracted Three Numbers of Cores (Making One Sample) from RCC Structure-Box Culvert on NH-26 at Sagar (M.P.)
Capping- The ends of the specimen shall be capped before testing. The material used for the capping shall be such that its compressive strength is greater than that of the concrete in the core. Caps shall be made as thin as practicable and shall not flow or fracture before the concrete fails when the specimen is tested. The capped surfaces shall be at right angles to the axis of the specimen and shall not depart from a plane by more than 0.05 mm.

Apparatus

Number of Specimens- At least three specimens, preferably from different batches, shall be made for testing at each selected age.

Procedure- Specimens stored in water shall be tested immediately on removal from the water and while they are still in the wet condition. Surface water and grit shall be wiped off the specimens and any projecting fins removed. Specimens when received dry shall be kept in water for 24 hours before they are taken for testing. The dimensions of the specimens to the nearest 0.2 mm and their weight shall be noted before testing.

Concrete Core Specimen
Capped and Cured Concrete Core Specimen under Compressive Strength Test in CTM
Calculation- The measured compressive strength of the specimen shall be calculated by dividing the maximum load applied to the specimen during the test by the cross-sectional area, calculated from the mean dimensions of the section and shall be expressed to the nearest kg per sq cm. Average of three values shall be taken as the representative of the batch provided the individual variation is not more than ± 15% of the average. Otherwise repeat tests shall be made.

A correction factor according to the height/diameter ration of specimen after capping shall be obtained from the hardened curve. The product of this correction factor and the measured compressive strength shall be known as the corrected compressive strength, this being the equivalent strength of a cylinder having a height/diameter ratio of two. The equivalent cube strength of the concrete shall be determined by multiplying the corrected cylinder strength by 5/4.

Report- The following information shall be included in the report on each test specimen/core:
Concrete Core Specimen
Capped and Cured Concrete Core Specimen under Compressive Strength Test in CTM
  1. Identification Mark,
  2. Date of test,
  3. Age of Specimen,
  4. Curing Conditions, Including date of Manufacture of Specimen in the Field,
  5. Weight of Specimen
  6. Dimensions of Specimen,
  7. Cross-Sectional Area,
  8. Maximum Load,
  9. Compressive Strength, and
  10. Appearance of Fractured Faces of Concrete and Type of Fracture, if these are Unusual.

Reference

  • IS: 13311 (Part 1): 1992 Non-Destructive Testing of Concrete-Method of Test; Part 1-Ultrasonic Pulse Velocity.
  • IS: 13311 (Part 2): 1992 Non-Destructive Testing of Concrete-Method of Test; Part 2-Rebound Hammer.
  • BS: 1881: Part 203: 1986 British Standard-Testing Concrete Part 203. Recommendations for Measurement of Velocity of Ultrasonic Pulses in Concrete.
  • ASTM C 597-02 Standard Test Method for Pulse Velocity through Concrete.
  • Handbook on Nondestructive Testing of Concrete, Second Edition, Edited by V.M. Malhotra and N.J. Carino, ASTM & CRC Press, 2006.
  • Testing Hardened Concrete: Nondestructive Methods by V.M. Malhotra, Published Jointly by the IOWA State University Press & American Concrete Institute (ACI) 1976.
  • Near-Surface Testing for Strength and Durability of Concrete, Editor P.A.M. Basheer, Fifth CANMET/ACI International Conference on Durability of Concrete.
  • Advanced Testing Methods and Damage Assessment of Distressed Concrete Structures by H.G. Sreenath Proceedings of the Advanced Course on Structural Health Monitoring, Repair and Rehabilatation of Concrete Structures Feb. 4-6, 2006, SERC, CSIR, Chennai.
  • Advanced Course on Non-Destructive Testing and Evaluation of Concrete Sructures, Organised by SERC, CSIR.Chennai, 2006.
  • Properties of Concrete, Fourth Edition 1996, by A.M. Neville, Published by ELBS-Longman.

Acknowledgement

The article has been reproduced from the proceeding of "National Seminar on Green Structures for Sustainability" with the kind permission from the event organisers.

NBMCW October 2010

.....

Rationalised Codes for Concrete Structu...

The intent of this presentation is to highlight the urgency and necessity of developing the ‘New Generation Codes’ for structural concrete and to inform readers about the efforts being made internationally and in India in this respect. The discussion necessarily traces the history of code making, development of design philosophy, ideal contents of a code, etc. The need to revamp the codes arises from factors like unprecedented growth of knowledge, developments in design philosophy, and rapid advances in construction technology, need to frame codes which do not hinder development, transparency and others. The international and Indian response to this challenge is described. In the Indian context the required nature of the code and its contents are discussed. They are illustrated by example of the contents of the proposed ‘Limit State Concrete code for bridges’.

S. G. Joglekar, Sr. Advisor, STUP Consultants P Ltd., Navi Mumbai

Introduction

Archaeologists distinguish different eons of human history by the main material developed and used by them – such as Stone Age, Bronze Age, Iron Age, Age of Painted pottery etc. Future archaeologist will surely call our period as ‘Concrete Age’. Concrete structures are an essential part of our life and a mainstay of our infrastructure in any developed and developing country. This infrastructure is built spending large sums of money and the structures are expected to remain in service for long period – 50 to 100 years or even more. The dams and aqueducts, ports and harbours, and the monumental structures have even longer life expectancy. No wonder then that these constructions are closely controlled in any country. This control is achieved by creating and enforcing guidelines in form of codes of practices. This control by codes, however, should not restrict the development of new improved technology and better ways of designing and constructing structures. Many new types of concrete and structural solutions are being evolved all over the world. In this sense development of concrete technology and structures is truly an international activity, which has spreads across countries, sharing knowledge and technology. It should be obvious that the practices in any country should remain more or less in line with the international developments, without any country lagging too much behind, or following a totally different path. Need for ‘Internationalisation’ is obvious.

Not so obvious is the need for ‘Rationalisation’ of design practices or the need for revamping existing codes and developing ‘New Generation’ codes! (Note ‘codes’ in plural)

The intent of this presentation is to highlight the urgency and necessity of developing the ‘New Generation Codes’ and to inform readers about the efforts being made internationally and in India in this respect. The discussion necessarily digresses into topics like history of code making, development of design philosophy, ideal contents of a code, etc., which makes the exercise more interesting. In order to keep the presentation short, - like proverbial ideal length of a skirt, - only the concrete related structural codes are discussed. The concepts remain valid for the full range of other structural codes.

International Scenario

In the last 5 to 10 years a number of countries have made major revisions to their codes relating to structural concrete. Bridge codes have also followed the general trend. The revisions are also taking place at closure intervals.
  • Canadians have adopted the Limit State Philosophy and Published new Bridge Code in 1995
  • In USA new LRFD Bridge Design Code is published by AASHTO in 2005(4th edition 2007).
  • ACI also has revised basic Concrete Code (318) in 2005 and 2008; also brought out metric version and a Spanish version (An example of attempt at internationalization).
  • Australians have revised Concrete Code AS: 3600 in 2001. Draft of revision 2005 released.
  • ‘Eurocodes’ is a comprehensive system of codes being prepared from 1989 and by now most of the important codes have been published. All common market countries are expected to adopt the same with minor variations based on the national practices, which have also been restricted to limited number of parameters.
What had been the reasons behind this flurry of activities? Why such short spans of life for new revisions of codes as compared to relatively peaceful longer reign for the codes of olden days?

Unprecedented Growth of Knowledge

In the later half of 20th century developmental efforts picked up pace in the countries which had become newly independent. In the war affected developed countries, reconstruction of damaged economy, taken together with growth of industry, put new demands on engineering infrastructure. “Necessity is mother of invention”’ says an old saying, This increases demand led to search for better methods of construction, efficient use of existing materials, achievement of durability, economy and speed of construction and other such aspects. Rapid developments were made in theoretical and experimental research in universities, and new technologies developed - like prestressed concrete, precast concrete, high strength steels and high performance concretes. New structural forms like cable stayed bridges developed. Design and construction of seismic resistant structures demanded research in plastic behaviour of materials and structures prior to failure. Higher levels of analytical methods were needed. New powerful methods of computer based analysis came to the fore. Pressure of development of infrastructure and durability crisis of concrete structures, motivated the researchers and practicing engineers to understand their materials, and invent new technologies. The construction techniques, like cantilever construction, segmental construction, precasting of large segments of bridge and assembling them by use of heavy lifting equipment developed in this period.

This process has been accelerated in last 15 years with China and India, - words largest and the second largest construction industries, - having tremendously increased investment in infrastructure.

Long and short of it, the explosion of knowledge, advent of new technologies and demand from the market had changed everything in the construction industry so much that revamping and frequent updating of engineering codes has become unavoidable.

Development of Design Philosophy

It is useful to trace briefly the evolution of design philosophies in the order to put the whole issue of new generation of codes in its proper perspective.

The Beginning:
  • The industrial revolution in Europe led to development of cast iron, and then steel as structural materials. The economy of construction and the problem of variability of strength of materials had opposite pulls in achieving optimal solutions. A series of problems faced in practical applications and consequential theoretical studies led to development of concepts of safety, - indicated by safety factors - measured by the ratio of minimum expected strength to maximum working load. This is an overall factor, - combining material factor and load factor in one, - as is presently understood.
  • In case of brittle fracture, or in case of sudden failure like buckling, the acceptable ‘safe’ margins were proposed and were improved from time to time on the basis of full scale tests. (Mathematical solutions did not work well enough !)
  • The theory of bending developed, which combined with the knowledge about ductility (or yielding) of materials led to another approach based on keeping enough margin between the yield stress and permissible stress at all sections of a member (allowable stress approach) Safety factor then is then measured by the ratio of Yield stress to Allowable stress.
  • Serviceability was checked by putting controls on deformations (i.e. deflection)
  • Methods for ensuring safety against dynamic effects of loads, were developed. Impact was handled by increasing the static load to an equivalent dynamic load by a suitable factor. Fatigue was controlled either by limiting stresses bellow endurance limit ( of stress) of the material, or by limiting the number load cycles well bellow the cycles causing failure. These approaches worked well for steel.
  • When reinforced concrete and prestressed concrete developed, both allowable (working) stress methods and overall safety factors on members (i.e. strength factor) were followed by engineers, and as a result a mixed approach has entered in the design of concrete structures.
  • The understanding of the behaviour of concrete structures and its many applications in practice had developed side by side. This led to frequent changes in not only the detailed design rules, but also in the basic approach, or design philosophy depending on the approach of the developer of the concept. These basic philosophies also differed from country to country.
  • The American Practice followed semi-empirical approach. In this approach theories are developed based on the understanding of behaviour to the extent possible, but for the practical use experimentally establishing parameters for prediction of strength are defined. This approach has practical limitations of applicability and built in disincentive for promoting growth of knowledge.
  • The British and European approach was theoretically more ‘pure’, but it had to accept artificial compromises to accommodate observed (or experimental) behavior in its theoretical methods. The example of design of rectangular footings is well known. Another example is the design of reinforced concrete column by working stress method in which the real decision making is based on the observed sharing of load in steel and concrete at failure (after creep and shrinkage takes place), but the strength formulae in working-load codes are made to look live ‘allowable stress’ method.
  • The Russians developed the Ultimate Load Factor methods.

Semi-Probabilistic, Limit-State Philosophy

  • In last few decades, borrowing the philosophical approach from the field of aircraft design, concepts of deciding the acceptable risk of failure and targeting the design strength (just) bellow that was taken as the aim to be achieved.
  • The reliability of design-choices or the risk of failure is ideally assessed by using probabilistic methods. The risk management based on the ‘reliability’ methods was made feasible by greater understanding of the statistical nature of material properties and nature of loads. The earlier established safety approaches are re-formatted in light of the new understanding.
  • In place of theoretically pure probabilistic estimation of risk, semi-probabilistic methods of partial factors was adopted. This was combined with the concept of ‘States of Structure’ described by various ‘Limit States’. This philosophy is a practical compromise between the old methods and the fully probabilistic risk management methods.
  • This semi-probabilistic, limit-state philosophy, at present, forms the basis of many of the national codes.

International Response

Awareness of the rapid developments in for past 40 years had led CEB and FIP in Europe to frame Model Code -1978 (revised in 1990) as a guide for code makers. This is presently under updation for its 2010 version. The formation of European Common Market made it necessary to unify and standardise codes of practices across Europe. A political directive was given for the same, under which, in 1989, the programme for synthesising new Eurocodes from codes of advanced member countries like U.K., France, Germany etc. was taken up. CEB-FIP model codes provided the basic scientific background for incorporating the most up-to-date research.

Other advanced countries like Japan, U.S.A., Canada etc. followed similar approaches as suited their own established organisations.

Thew commercial advantages of having common, - or at least similar – codes became obvious to many countries from the example of Eurocodes. Now some organisations from the Asian countries are attempting to evolve ‘Asian Model Code’ suitable for their region, in spite of many practical difficulties in implementing a common code.

The international organisations like IABSE, CEB & FIP (now merged a FIB), ACI/PCI and others provided common international platforms for exchange of views at regular interval. This has greatly helped development of structural Concrete across the world.

Indian Scenario

Indian codes cannot lag far behind. How they can catch-up with the Progress needs to be discussed.

Code committees of Bureau of Indian Standards (BIS), Indian Roads Congress (IRC), and Indian Railways (IR) are active, but are their efforts need to be speeded up if they have to catch up with the international codes. For this to happen they need to be centrally coordinated and become futuristic in their approach

Presently Indian Standards are written mostly by comparing and updating their knowledge contents borrowing from foreign codes – viz. British, American, Australian, etc. Some of the practical difficulties are:
  • Basic theoretical work or practical research for confirming the design data (e.g. Load and material factors) for Indian conditions is not done in a pro-active way by codal committees.
  • The committee members are voluntary experts supported by their own organisations, who have practical limitations in putting large amount of man-months in these efforts.
In spite of these difficulties the BIS, IR and IRC committees are revising the knowledge contents of the codes, but their efforts even within their own organisations lack central policy directive, and as such different structural codes have discrepancies both between and within the codes.

The IRC has made significant progress in revamping and updating the Concrete Bridge Code based on the limit state concepts following a format of presentation different from the normal code formats. It is interesting to understand it in details. For this one has to go into the nature of code, its status, its value in achieving continued education of users and so on. This is briefly presented in section 9. The remaining article illustrates the general approach that needs to be adopted using the example of the IRC Bridge Code for illustration.

New Strategy is Needed

It should be realised that the ‘New Indian Bridge Codes’, will be used – and extended for newer applications - in early two or three decades of the 21st century. To develop the same, an appropriate common design philosophy and a standardised and purposeful format is essential. For this purpose it is necessary to understand the nature of codes and their place in practice. The remaining parts of this discussion suggests a suitable philosophy and the format, which is suitable for combining established theory, basic common principles and experience based rules’ The code should stipulate the acceptable modelling of the problems and their solutions ( for structural, hydraulic, and geotechnical issues). It should incorporate up-to-date knowledge of materials, and come out with a set of requirements and recommendations which are achievable based on the available technologies, accounting for their strengths and limitations. By being transparent and stating the limits of applicability the format of codes will become helpful to accommodate new developments and be suitable for incorporating changes as the art and science of engineering bridges develops.

Design Philosophy - Limit State, Semi-Probabilistic Approach

It is most appropriate to adopt what is commonly known as “Limit State Philosophy” as the basic philosophy of all bridge codes, with possible exception (at this stage) for the geotechnical aspects of design. The up-to-date information and suitable format of the same is available (for concrete structures) in CEB/FIP Model Code of 1978 and 1990. (The format adopted by ‘Eurocodes’ is based on it, but is a practical compromise between the model code, existing national codes and divergent approaches of member states. Thus, it deviates in details from the model code formats.)

It is useful to know the development of the philosophical basis of “Limit State Design” in relation to the historical developments in structural engineering. Many excellent publications are available on this topic.

The advantages of adopting limit state philosophy are briefly listed below without going in detailed discussion:
  • The basic aims of safety, serviceability, durability and economy are recognised. Other aspects such as fire resistance and environmental impacts can be easily added where applicable.
  • With better understanding of material behaviour in its elastic range, plastic range and failure mechanisms, appropriate mathematical models for the analysis are chosen. The behaviour (i.e. performance) of the structure being designed can be pre-determined as a design choice.
  • Appropriate values of material factors depending upon the variability in properties of different materials subjected to different types of loads can be incorporated.
  • Variability of loads arising from the natural events (e.g. wind, earthquake) and also of those loads from intended use (man-made loads) can be incorporated knowing their statistical distribution functions in time and spatial domains. The probability of combination of normal and extreme values can be estimated and allowed for in the design in an consistent way. This permits proper evaluation of “risk” in probabilistic terms and provides more rational approach to risk management.
  • Where knowledge is inadequate and full rationalisation is not possible, the existing practices and experiences can be incorporated by techniques of retrofitting. This is what was done when the first limit state codes were written and tried in practice concurrently with existing codes for a few years. This approach permits immediate change over to the new formats, and then changing the same in details when further studies / experiences justify the change, (i.e. values of partial factors)

Nature of Code

What ‘Code-is-Not’?

In an engineering environment, it is easier to understand what the code is and what should be its coverage or contents, if one understands what code is not:
  • Code is not a Text Book: A textbook contains fundamental principles and basic knowledge of the subject matter, which is well established and widely accepted. A code refers to this knowledge base, but does not quote it in a comprehensive manner. The codal text is not a ‘stand-alone’ write-up covering theory.
  • Code is not a Hand-Book: A hand-book on any subject is a collection of theory, data-base, statement of practices, rules and information needed by users, and is basically a comprehensive reference document. The code may refer to some commonly known and acceptable data base, acceptable practices, approximations and thumb rules – but it is not a comprehensive collection of the same.
  • Code is not a Statutory Document: In most of the countries code contains recommendations which are not legally mandatory by themselves, - (they may become so by virtue of provisions of contract document between the purchaser and supplier), - nor does the use of code relieves user of his statutory, professional, or moral responsibilities in the social context.
  • Code is not a ‘Standard’: A ‘standard’ tries to formally define as accurately as practicable the properties, methods of measurements, tests and procedures which lend precise meaning to general concepts, and form basis of acceptance/ rejection tests of a product. This is the case of ASTM (American Standards of Testing Materials). In many a case the structural ‘code of practice’ has been raised to a status of national standard, but in reality it continue to be a ‘code’ in its essence. This happens when a statutorily established national body like BIS formulates codes of practices.

Contents of Code

These can be grouped in the following parts:
  • Principles: These are general statements or definitions for which there are no alternatives within the time-frame of validity of code. Principles by this definition also include fundamental knowledge (theory or text-book knowledge).
  • Application Rules: These are the statements of methods acceptable to code, which follow the principles and are consistent the principles in their requirements. Alternative methods may be permitted explicitly provided they comply with the spirit of the principles and satisfy the general requirements, such as strength, durability etc. Often, the use of alternative rules is left unstated (or uncommented in commentary also, where one is provided) which leads to various interpretations and debate about validity and/or acceptability of the new and/or unstated propositions.
  • Commentary: This supplies clarifications, background information, limits of applicability, further guidance and any such help that code writers care to bring to the attention of users.
This division of contents is explicitly used by some codes like French codes in force before Eurocodes, (viz. BAEL and BPEL Codes dealing with RCC and Prestressed Concrete Bridges). This is also a preferred format of Model Code of CEB/FIP-1978 and 1990. Some other code makers publish separate Explanatory Guide. The rest leave it to the enterprising authors, or to the mercy of the owner- authority and users.

Coverage of Code

Whether explicitly stated or implied, the subject matter covered by codes covers the following:
  1. Statement of Philosophy, and Basis of Design: The code should clearly state its aims and its approach adopted to achieve the aims.
  2. Established Knowledge/Theory: Established knowledge/theory is reiterated in code to a minimum necessary extent in order to indicate the basis for application clauses.
  3. Analytical Models and simplifications: Modern codes explicitly cover this topic and distinguish between well established general models and simple, normally used methods (established by long use). More advanced methods of analysis and use of more exact accurate properties of materials are also given. Guidance for the use of latter in preference to ‘simple’ methods in special cases should be clearly indicated.
  4. Stated (or Unstated) Aims: Safety, serviceability, durability, and economy – Most often these aims being of qualitative nature, and not easily measurable, may or may not be stated explicitly, but they form a strong background for contents of the code. New aims are getting added to this list. Eco-friendliness, aesthetics, energy conservation, operation and maintenance requirements are being now added by modern codes directly or indirectly.
  5. Expected Minimum Requirements of Structures: The ‘requirements’ are the description of the characteristics that a structure (and/or its components) should possess in order to meet the “aims” stated in (b). This is achieved by setting up corresponding ‘design criteria’. The code presumes that by satisfying design criteria the requirements will be met.
  6. Rules and Practices: A set of rules, derived from theory or from practice, and acceptable analytical models used in predicting the characteristics/properties of the designed structure (element), which meets the ‘requirements’ stated in (c).
  7. Materials: Acceptable materials and their properties. The most commonly used materials are included covering the essential - but not exhaustive - properties.
  8. Workmanship: Recommended workmanship practices, which essentially are technical specifications, but are not complete by themselves and are not exhaustive like tender specifications are covered. Aspects of Quality Assurance and Reliability, Operation and Maintenance are being addressed to varying degree.
The extent to which the above aspects are detailed decides the extent to which the document stands on its own, (or has to draw support from other similar codes). This choice lies with the code writing bodies.

Proposed Contents of IRC Limit State Concrete Code

The IRC Limit State Code for Concrete Bridges is a unified code being drafted under the following section headings. The headings and sub-headings are self explanatory.

Section
1.0 Contents
2.0 Foreword and Introduction
3.0 Definitions and Notations
4.0 General
5.0 Basis of Design
6.0 Material Properties and their Design Values
7.0 Analysis
8.0 Ultimate Limit State of Linear Elements for Bending and Axial Forces
9.0 Ultimate Limit State for two Dimensional Elements for out of Plane and in Plane Loading Effects
10.0 Design for Shear, Punching Shear and Torsion.
11.0 Ultimate Limit State of Induced Deformation
12.0 Serviceability Limit State
13.0 Prestressing Systems
14.0 Durability
15.0 Detailing - General Requirements
16.0 Detailing Requirements of Structural Members
17.0 Ductile Detailing for Seismic Resistance
18.0 Material Specifications
19.0 Workmanship and Quality Controls
Normative Annexures
A-1: Load Combinations
A-2: Additional Information and Data about Properties of Concrete
A-3: Testing of Ducts for Prestressing tendons
Informative Annexures
B-1: Analysis and Design for Impact
B-2: Analysis and Design for Fatigue
B-3: Recommended Practice for Prestressing Operations and Grouting.

Recapitulation and Conclusions

  1. The topic of Developing New Generation Rationalised Codes of Practices for Bridges in India, has been presented, bringing about the necessity and urgency of the same. Every word of the topic has a deeper meaning attached to it,
  2. The international scenario has been described and the reasons behind the frequently carried out revisions have been explained. Briefly, it is the rapid growth in the scientific knowledge, triggered by the pressure of development of infrastructure and durability crisis of concrete structures. It motivated the researchers and practicing engineers to understand their materials, and invent new technologies.
  3. For proper spread of the knowledge and to assure proper application of the same, codes of practices have to remain up to date.
  4. New rational design philosophy which is based on concepts of reliability, safety, serviceability, durability, economy and is aware of the new issues of eco-friendliness, energy conservation etc. will have to be adopted in the new codes of practices.
  5. This can be done in India by initially borrowing from the work done in developed countries, but it is recognised that it is necessary to carry out research to establish needs and environment which are peculiar to India.
  6. The code makers in India are active and have made a good, although somewhat delayed, .start, Hard work, and centrally planned activity is needed to catch up with the up-to-date international codes.

NBMCW October 2010

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Use of HDPE Flat Duct for Prestressed S...

Shriram Bapat, Retd.Chief Engineer Civil, NPCIL, Mumbai

Prestressed concrete slab (PT slab) construction has gained wide importance as a state of art technology, due to a number of engineering solutions and economy that it provides. Conventional usage of bright metal sheets as duct material have explicit problems of corrosion, duct opening leading to slurry intrusion and higher short-term losses due to the large values of friction and wobble coefficients. Further, one-end stressing operation may lead in to inefficiency of the prestressing system. The use of HDPE material, as prestressing duct, can eliminate the inherent problems associated with the metal ducts and reduce the prestress losses. Both end stressing using HDPE sheathing is the best combination for PT slabs.

Introduction

Prestressed concrete slabs (PT slabs) are the state of art construction technology and are provided these days for almost all commercial complexes. This concept is also being adopted for some large scale residential complexes. As the concrete quantities as well as reinforcement steel quantities get reduced when this concept is adopted, it is getting popular. Additionally, one gets clear headroom, as the floor beams are eliminated. This paper highlights the cost effectiveness of use of HDPE sheathing compared to bright metal sheathing which is conventionally used in the industry.

Sheathing Material

Three different types of sheathing materials are available in the market and are in general being used for housing the prestressing tendons.
  • Bright Metal Sheathing
  • Lead Coated Metal Sheathing
  • Galvanized Metal Sheathing
All the above sheathings are manufactured as round sheathing and then pressed to get a flat sheath, suitable for PT Slab. In the process of converting round sheath to flat sheath, there are chances of getting non-uniform profile of the flat duct as well as there are chances of opening of joints, which further leads to chances entry of slurry into the sheath while concreting. The flat HDPE duct is ideal in this situation, as the shape will be uniform and absolutely no possibility of leakage of slurry into the duct, while concreting.

Table-1: Comparison of Wobble and Friction Coefficient Values
Type of sheathWobble coefficientFriction coefficient
Bright Metal0.00460.25
Galvanized  Metal0.00300.20
Lead coated Metal0.00300.18
H.D.P.E.0.00200.17

In addition to the advantages of the HDPE duct as mentioned above, HDPE ducts have lower values for co-efficient of friction and wobble co-efficient, leading to reduced short-term prestress losses during pre-stressing operation. Table-1 gives values of friction and wobble co-efficients for different types of sheathing ducts.

HDPE flat duct was not being manufactured in India till recently. Now, one manufacturer has successfully developed and came forward to manufacture and supply of HDPE flat duct.

Present Study

In view of lower value of friction and wobble coefficients for HDPE duct, when compared with those values for Bright Metal duct, which is commonly used in PT industry, a study was undertaken to work out economy in using HDPE duct. Different column grids, corresponding column sizes, slab thickness and drop slab thickness were considered. Table-2 gives the considered combinations.

Table-2: Combination Considered for the Case Study
Sr No.Grid Size(m)Column Size(mm)Slab Thickness(mm)Drop Size(mm)
18 x 8700 X 7002003000 X 3000 X 375
29 x 9700 X 7002003000 X 3000 X 375
310 x 10800 X 8002253500 X 3500 X 400
411 X 11900 X 9002504000 X 4000 X 450
512 X 12900 X 9002654000 X 4000 X 475

Five panels have been considered in X-direction and three panels have been considered in Y-direction. Stressing has been done from alternate end in both the direction as one case and in other case, stressing is considered from alternate end in Y-direction and from both ends in X-direction. ADAPT FLOOR PRO 3D (Builder) software of Dr. ALAMI has been used for analytical work.

Table-3: Comparison of H.T. strand requirements using Bright Metal Duct and HDPE Duct
ColumnSlabDrop Panel COLUMN Both End StressingAlternate End Stressing
BMHDPEBMHDPE
HTSHTSHTSHTS
GridThk.SizeSize(Kg/sq.m.)(Kg/sq.m.)(Kg/sq.m.)(Kg/sq.m.)
8X82003X3X0.375700X7003.433.43.1
92003X3X0.375700X70043.6443.81
102253.5X3.5X0.400800X8004.43.94.54
112504X4X0.450900X9004.64.244.74.3
122504X4X.500900X9005.45.05.65.35

Results

On the basis of analytical results, the requirement of H.T. strand per square meter of slab has been worked out. Table-3 gives comparison of H.T. strand requirement using Bright Metal and HDPE flat duct and Table-4 gives percentage saving of H.T. strand when HDPE flat duct is used, in place of Bright Metal Duct.

Table-4: Percentage Saving in H.T. Strand
Column Grid% SAVING
SizeBoth End StressingAlternate End Stressing
8X811.708.0
9X909.004.8
10X1011.411.0
11X1107.808.5
12X1207.4004.50

Conclusion

It can be seen from the Table-4 that there is substantial saving in requirement of H.T. strands when HDPE duct is used, in place of conventionally used Bright Metal duct. The range of saving is 4.50% to 8.5% when conventional alternate end stressing is resorted to. The saving goes up in the range of 7.4% to 11.7%, when both end stressing is resorted to. Additionally, there is marginal saving in requirement of duct length and number of anchorages.

When conventionally used Bright Metal Ducts are provided in PT Slab construction, they get corroded during construction stage itself (particularly in and around Mumbai coastal region). This increases friction and thus the frictional losses also go up. Use of HDPE flat duct will eliminate this problem.

On the basis of study following is recommended:
  • HDPE flat duct shall be used in PT Slab construction.
  • Both end stressing shall be resorted to, in place of conventional method of alternate end stressing.
  • 8 m x 8 m column grid seems to be most economical choice in selecting column grid at planning stage.

Acknowledgment

The author acknowledges the analytical help provided by Shri Umesh Bhujbalrao of M/s.VSIL, Bhopal. Preparation of this paper was possible due to constant pursuance by Shri C.B.Dandekar of M/s.REX Polyextrusion Ltd, Sangli, manufacturers of HDPE flat duct.

NBMCW October 2010

.....

Flexural Strength of Beams

Flexural Strength of Beams Incorporating Copper Slag as Partial Replacement of Fine Aggregate in Concrete

Mrs. D. Brindha, Senior Lecturer, Dr. S. Nagan, Assitant Professor, Dept. of Civil Engineering, Thiagarajar College of Engineering, Madurai

Copper slag is obtained as waste product from the sterlite industries, which produces copper in the form of cathode. Investigations were carried out to explore the possibility of using copper slag as a replacement of sand in concrete mixtures. This paper presents the results of study undertaken to investigate the feasibility of using copper slag as fine aggregate in concrete. The effects of replacing fine aggregates by copper slag on the compressive strength of cubes, split tensile strength of cylinders and flexural strength of beams are evaluated in this study. Five test groups were constituted with the replacement percentages of 0%, 20%, 30%, 40%, and 50% .The results showed the effect of copper slag on RCC concrete elements has a considerable amount of increase in the compressive, split tensile and flexural strength characteristics. Leaching studies revealed that copper slag does not leach heavy metals like Pb, Zn, Cr, Ni, Mo etc and also indicates that the leaching of heavy metals was well below the toxicity limits even under aggressive conditions.

Introduction

Large amounts of industrial waste or by-products accumulate every year in the developing countries. Sustainability and resource efficiency are becoming increasing by most important issues in today’s construction industry. Therefore, nowadays utilization of secondary materials is being encouraged in construction field. For the production of cement and concrete, very high amount of energy is needed. Around 7% of CO2 released to the atmosphere is generated during cement production [1]. Harmful effects of concrete on environment can be reduced by producing good and durable concrete by using industrial byproducts.

The Government of India has targeted the year 2010 for providing housing for all [2]. such large scale housing construction activities require huge amount of money. Out of the total cost of construction, building materials contribute to about 70% of cost in developing countries like India. Therefore, the need of the hour is replacement of costly and scarce conventional building materials by innovative, cost effective and environment– friendly alternate building materials. The new material should be environment–friendly and preferably utilize industrial wastes generated as a result of rapid industrialization.

Copper slag is one of the materials that is considered as a waste material which could have a promising future in construction industry as partial or full substitute of either cement or aggregates. It is a byproduct obtained during the matte smelting and refining of copper [3]. Since it has a higher composition of Fe2O3 the density of copper slag is relatively higher when compared to other materials.

Sterlite industries (India) ltd, Tuticorin, India produces 4 lakh tons of copper per year [4]. For every ton of copper production, about 2.2 tons of copper slag is generated. Therefore, in India 8 lakh tons of copper slag is generated every year. It was obtained as a result of manufacturing of copper electrodes in sterlite industry.

Copper slag have been widely used for abrasive tools, roofing granules, cutting tools, abrasive, tiles, glass, road base construction, rail road ballast and cement and concrete industries[5]. Several researchers have investigated the possible use of copper slag as fine and coarse aggregates in normal concrete and its effect on different mechanical properties of mortar and concrete. Some studies were carried using fine powder copper slag for partial replacement of cement [6]. There are no studies available about the copper slag effect on structural members like RCC beams and RCC columns.

The pozzolanic activity of copper slag has been investigated by O.Pavez etdl. The effect of copper slag on the hydration of cement was investigated by Mobasher et al. [8] and Tixier et al. [9]. Up to 50% by weight of copper slag was used as a Portland cement replacement together with up to 1.5% of hydrated lime as an activator to pozzolanic reaction. Result indicated a significant increase in the compressive strength. Khalifa,S.al/zabri investigated the performance of high strength concrete made with copper slag as a fine aggregate[11]. The results indicated that water demand reduced by 22% at 100% copper slag replacement compared to the control mixture. The strength and durability of HSC were generally improved with the increase of copper slag content in the concrete mixture.

Although there are many studies that have been reported by investigators from other countries on the use of copper slag in cement concrete, not much research has been carried out in India concerning the incorporation of copper slag in concrete. Therefore, this paper presents the study of the effect of copper slag as a supplementary fine aggregate in concrete member like RCC beams. Flexural strength of RCC beams was investigated for the various proportion of copper slag as a sand substitute in concrete.

Physical and Chemical Properties

The copper slag is glassy in nature and has a similar particle size range to sand, indicating that it could be used as a replacement for the sand present in cementitious mixture. The Table-1 shows the physical properties of copper slag replaced with sand. Copper slag has a black color and glassy appearance. The specific gravity of copper slag varies from 3.5 to 4.0. The Unit weight of copper slag concrete is somewhat higher than that of conventional concrete. The weight of copper slag concrete is increasing in the range of 10 to 15% from the conventional concrete. Water absorption of copper slag is typically very low (0.361) when compared to sand. Bulk density of copper slag varies from 1.6 to 2.20 g/cc. It has high friction angle due to sharp angular shape. Since copper slag contains proper angular particles, mostly between 4.75mm to 75 micron in size, it confirms to zone II as per IS-383 shown in Table-2.

Flexural Strength of Beams

Flexural Strength of Beams

Chemical Composition of Copper Slag

Flexural Strength of Beams

Flexural Strength of Beams
Copper slag samples were analysed for constituent oxides including minor oxides and heavy elements besides mineral phases. The results of chemical analysis are shown in Table-3.

Leaching of heavy elements in copper slag

Copper slag samples were dipped in distilled water and studied for leaching of heavy metals from them up to a period of 15 days using ICP technique. No leaching of heavy metals such as Pb, Zn, Cr, Ni, Mo etc was observed. Leaching of very small quantities of Ba (0.008 ppm), Cu (0.087 ppm), Mn (0.008 ppm) and Sr (0.002 ppm) was however observed at 15 days.

The leaching of heavy metals in copper slag samples was tested by National Council for Cement and Building Materials, New Delhi as per the method given in ASTM D-5233-1995d which involves sample treatment under aggressive conditions. The results presented in Table-4 indicate that the leaching of heavy metals was well below the toxicity limits even under aggressive conditions.

Materials
Cement

The cement used in this study was ordinary Portland cement (OPC) of grade 43. The specific gravity of cement is 3.15.

Fine aggregates and coarse aggregates

Coarse aggregates are purchased from a nearby crusher in Madurai. Coarse aggregate with a nominal maximum size of 20mm were used in this study. The fine aggregate was river sand taken from Vaigai River, Madurai which has the specific gravity of 2.55 and bulk density of 1.61.

Copper slag

Copper slag is a by-product obtained during the production of copper in copper industries. Copper slag used in this work was brought from Sterlite industries (India) Ltd, Tuticorin which produces annual average of 8 lakh tones of copper.

Laboratory Testing Program and Mix Proportions

The mix proportion chosen for this study is given Table-4. The following mix proportion is adopted as per IS-10262.

Flexural Strength of Beams

Water : Cement : Fine Aggregate : Coarse Aggregate

0.50 : 1 : 1.38 : 3.23

Five concrete mixtures with different proportions of copper slag ranging from 0% to 50% were considered as shown in Table-5 the materials were mixed in a laboratory concrete mixture. The overall mixing time was about 3 minutes. The mixtures were compacted using vibrating table. The slump of the fresh concrete was determined to ensure that it would be within the designing value and to study the effect of copper slag replacement on the workability of concrete. The specimens were demoulded after 24 hours, cured in water and than tested at room temperature at the required age.

Cube compressive strength

To determine the compressive strength, cube moulds of size 150x150x150 mm were used. 15 cubes were caste with different proportions of copper slag. They were cleaned thoroughly using a waste cloth and then properly oiled along its faces. Concrete was then filled in mould and then compacted using a standard tamping rod of 60 cm length having a cross sectional area of 225mm2.

Reinforcement Detailing

Flexural Strength of Beams

Experimental test set up

Flexural Strength of Beams

Split tensile strength

To determine the split tensile strength, cylinder moulds of diameter 150mm and length 300mm were casted. Totally, 15 cylindrical specimens were casted with different proportions of copper slag. The crude oil was applied as seen earlier along the inner surfaces of the mould for the easy removal of casted cylinder from the mould. Concrete was poured throughout its length and compacted well.

The split tensile strength of concrete is calculated by using the following formula

fst = 2P / π ld

where

P- maximum load at failure

l-length of cylindrical specimen in mm

d-diameter of cylindrical specimen in mm.

Flexural strength of beams

The size of beam specimens is 1000 x150 x150mm. The beam specimens were cast and tested with and without copper slag for normal conditions. 15 Nos of beam specimens were cast using the same reinforcement shown in Figure. the beams were divided into five series A, B, C, D and E.

Series A consisted of three beams designated as A1, A2 and A3. These beams were treated as control specimens and were not replaced with copper slag.

Series B consisted of three beams designated as B1, B2 and B3. 20% of copper slag was replaced for fine aggregate in concrete. Similarly, series C, D and E consisted of C1, C2, C3, D1, D2, D3 and E1, E2, E3 beams replaced with 30%, 40% and 50% of copper slag. To conduct the test the beam specimens were placed on the loading frame. Plumb bop was used to correct the center of the beam. Two distance pieces were kept at the one third distances from both the ends. Loading frame was placed upon the distance pieces and its centre was corrected using plumb bob. Load cell was placed on the beam top and corrected to its centre by using plumb bop. Load cell was connected by the load indicator; three deflectometers were placed on the bottom side of the beam. Two point loading system was adopted for the test.

Test Results and Discussion
Effect of copper slag on compressive strength of concrete

The average 28 days compressive strength for different proportions of concrete mixes shown in fig. The results show that the compressive strength of concrete is increased as copper slag quantity increases up to 40% addition, beyond that the was reduced due to comp. strength significant increase in free water content in the mixes. The excessive free water content in the mixes with copper slag content causes the bleeding and segregation in concrete. Therefore, it leads reduction in the concrete strength.

Table-5 gives Results of the compressive strength test conducted on cube specimens at 28 days.

Flexural Strength of Beams

The highest compressive strength was achieved with 40% replacement of copper slag, which was found about 43N/mm2. This means that there is an increase of compressive strength of more than 30% compared to the control mix. However mix with 50% replacement of copper slag gave the low compressive strength when compare to 40% but still more than 20% compared to the control mix. The results showed that the uses of copper slag as a replacement of sand in concrete mixes resulted high compressive strength of about 30%.

Effect of Copper Slag on Split Tensile Strength of Concrete

Totally 15 cylindrical specimens were tested for finding split tensile strength in accordance with ASTM C 496-96. the splitting tensile strength was determined by using the following formulae Ft = 2P / π LD. The results from the splitting tensile test at 28 days are presented in Table-6.

Flexural Strength of Beams

The tensile strength of concrete showed similar behavior to the compressive strength. The results show that the split tensile strength is increased as copper slag quantity increases up to 40% addition, beyond that the split tensile strength value slightly reduces but still more than 45% compared with control mix. The results showed that the use of copper slag in concrete increases the tensile strength of about 90% with that of control mixture.

Effect of copper slag on flexural strength of RCC beams

Fifteen beams were also tested for flexural strength under two point loading conditions. Out of the 15 specimens A1,A2,A3, were treated as control specimens, remaining set of specimens incorporating copper slag at a percentage of 20%, 30%, 40% and 50% with that of sand.

The average modulus of rupture (flexural strength) was determined using the following expression

Fcr = PL / BD2

Where Fcr = modulus of rupture

P = ultimate load in KN

L = length of beam in m

B = Average width of specimen in m

D = Average depth of specimen in m

All of the beams tested failed in flexure with crushing of concrete in the compression zone at the failure stage after the development of flexural cracks. The first visible cracks formed between the locations of the two point loads in the region of maximum bending moment.

Thereafter, as the load was increased more cracks started to form over the shear span on both sides of the beam. Flexural beams replaced with copper slag gives more flexural strength compared to the control specimens. The results showed the significant increase of flexural strength of beams when copper slag was added for the replacement of sand.

Flexural Strength of Beams

The load deflection curve indicates that the copper slag replaced concrete specimens are withstanding for higher loads. The energy absorption of these beams was calculated as the area under the load Vs deflection curves for flexure failure. From the values, it was observed that the copper slag replaced beams showed an increase in energy absorption values. This enhancement in energy absorption of beams could be attributed to the ductile nature of the copper slag beams.

Conclusion

The following conclusions may be drawn from the study of the effect of copper slag replacement on the concrete properties
  • The addition of copper slag has improved the compressive strength, split tensile strength and flexural strength of concrete.
  • The results also revealed that addition of the slag in concrete increases the density of about 10 to 20% thereby the self–weight of the concrete.
  • The slump value of copper slag concrete lies between 75 to 100 mm.
  • The flexural strength test on beams results show that the ultimate load carrying capacity of the beam increases by 30% for 40% replacement of copper slag.
  • The uses of copper slag as a partial replacement for sand impart strength up to 50% replacement level. Higher level replacement leads to segregation and bleeding.
  • Water absorption value of copper slag concrete is reducing upto 40%. After that the surface water absorption is increased rapidly.
  • The leachant studies revealed that the addition of slag does not pave way for leaching of harmful elements like Copper (Cu) and Iron (Fe) present in slag in concrete. Thus, it does not pose any environmental problem.
  • It was observed that the copper slag replaced beams showed an increase in energy absorption values

Acknowledgment

The authors wish to acknowledge Thiagarajar College of Engineering, Madurai and Sterlite Industries (India) Ltd., Tuticorin, India for providing all the facilities for carrying out this work. The statistical data of leaching studies of copper slag, collected from the report submitted by National council for cement and building materials, New Delhi, July 2009.

References

  • Mehta P.K. Greening of the concrete industry for sustainable development, concr.Int.2002: 24(7);23-7).
  • Fly ash: A resource material for innovative building material- Indian perspective C. N. Jha & J. K. Prasad.
  • Gorai P, Jana RK, Premchand, characteristics and utilization of copper slag, a review. Resource conserve.recy 2003;39;299-313
  • Global slag magazine, may2007
  • Mostafa khanzadi, ali behnood.mechanical properties of high strength concrete incorporating copper slag as coarse aggregate.
  • Copper slag waste as supplementary cementing material to concrete. W.A.Moura, J.P.Goncalves, M.B. Leirelima, 23 jan 2007
  • O.Pavez, F.Rojas, J.Palacios, A.Nazer. pozzolanic activity of copper slag. Conference on clean technologies for the mining industry, 2003
  • Mobasher B,devaguptapu R, arino AM. Effect of copper slag on the hydration of blended cementitious mixtures. Proceedings of the ASCE materials engineering conference. Materials for the new millennium :1996, P/1677/86.
  • Tixier R, devagupta R, Mobasher B, the effect of copper slag on the hydration and mechanical properties of cementitious mixture. Cement concrete Res, 1997;27 (10):1569-80.
  • Boncukcuoglu R, Kocakerim MM, Tosunoglu V. Utilization of industrial boron wastes cement production for the stabilization. Energy Education Science and Technology 1999;3 (1):48–54.
  • Demirboga R, Sahin R, Bingol F, Gul R. The Usability of blast furnace slag in the production of high strength concrete. Fifth international symposium on utilization of high strength/high performance concrete. Norway: Sandefjord; 1999.p. 1083–91.
  • Demirboga R, Orung I, Gul R. Effects of expanded perlite aggregate and mineral admixtures on the compressive strength of low density concretes. Cement and Concrete Research 2001;31 (11):1627–32.
  • Ghafoori N, Bulholc J. Investigation of lignite-based bottom ash for structural concrete. Journal of Materials in Civil Engineering 1996;8 (3):128–37
  • Haque MN, Kayyali OA, Joynes BM. Blast furnace slag aggregate in the production of high performance concrete. American Concrete Institute 1995;SP 153:911–30.
  • Kayali O, Haque MN, Zhu B. Drying shrinkage of fiber rienforced lightweight aggregate concrete containing fly ash. Cement and Concrete Research 1999;29:1835–40.
  • Mehta PK. Durability critical issues for the future. Concrete International 1997;19(7):69–76.
  • Alp.I, Devaki.H, Sungan. H, Utilization of floatation wastes of copper slag as a raw material in Cement production, journal of Hazardous materials, volume 159,issues 2-3,2008.
  • Washington Almeida Moura, Jardel Pereira Gonçalvemonica  Batista  Leite  Lima, April, (2007) Copper slag waste as a supplementary cementing material to concrete, Journal of Material Science, Volume 42, Number 7.
  • Himaru Keisuke, Mizuguchi Hiroyuki, Hashimoto Chikanori, Ueda Takao, Fujita Kazuhiro, Oumi Masaak, (2005) Properties of Concrete Using Copper Slag and Second Class Fly Ash as a Part of Fine Aggregate, Journal of the Society of Materials Science, Vol.54; No.8; page.828-833.
  • Report on ‘Technical suitability of Copper slag for manufacture of cement for Sterlite Industries (India) Ltd., Tuticorin’, submitted by National council for cement and building materials, New Delhi, July 2009.

NBMCW July 2010

.....

Improvement of Shear Strength using Tri...

Prof. Priti A. Patel, Research Scholar in SVNIT and Asst. Prof. in CKPCET, Dr. Atul K. Desai, Associate Prof. and Head AMD, SVNIT, Dr. Jatin A. Desai, Retd. Prof. AMD SVNIT, Surat.

It is now well established that one of the important properties of fibre reinforced concrete is its superior resistance to cracking and crack propagation. As a result of this ability to arrest cracks, fibre composites possess increased extensibility and tensile strength, both at first crack and at ultimate, particular under flexural loading; and the fibres are able to hold the matrix together even after extensive cracking. The net result of all these is to impart to the fibre composite pronounced post – cracking ductility which is unheard of in ordinary concrete. The transformation from a brittle to a ductile type of material would increase substantially & the energy absorption characteristics of the fibre composite and its ability to withstand repeatedly applied, shock or impact loading. In this paper, the mechanical properties of triangular shape polyester fibre, and applications of FRC are discussed.

Introduction

Fibre reinforced concrete (FRC) may be defined as a composite material made with Portland cement, aggregate and incorporating discrete discontinuous fibres. Why would we add such fibres to concrete? Plain, unreinforced concrete is a brittle material with a low tensile strength and a low strain capacity. The role of randomly distributed discontinuous fibres is to bridge across the cracks that develop & provide some post- cracking "ductility". If the fibres are sufficiently strong, sufficiently bonded to material, and permit the FRC to carry significant stresses over a relatively large strain capacity in the post-cracking stage.

There are of course, other ways of increasing the strength of concrete. The real contribution of the fibres is to increase the toughness of the concrete (defined as some function of the area under the load vs. deflection curve), under any type of loading. That is, the fibres tend to increase the strain at peak load, and provide a great deal of energy absorption in post-peak portion of the load vs. deflection curve [4].

When the fibre reinforcement is in the form of short discrete fibres, they act effectively as rigid inclusions in the concrete matrix. Physically, they have thus the same order of magnitude as aggregate inclusions; fibre reinforcement cannot therefore be regarded as a direct replacement of longitudinal reinforcement in reinforced and prestressed structural members. However, because of the inherent material properties of fibre concrete, the presence of fibres in the body of the concrete or the provision of a tensile skin of fibre concrete can be expected to improve the resistance of conventionally reinforced structural members to cracking, deflection and other serviceability conditions.

The fibre reinforcement may be used in the form of three – dimensionally randomly distributed fibres throughout the structural member when the added advantages of the fibre to shear resistance and crack control can be further utilised. On the other hand, the fibre concrete may also be used as a tensile skin to cover the steel reinforcement when a more efficient two – dimensional orientation of the fibres could be obtained.

Improvement of Shear Strength using Triangular Shape Fibre in Concrete
Figure 1: Tensile Load versus Deformation for Plain and Fibre Reinforced Concrete

The behavior of FRC under loading can be understood from Figure 1. The plain concrete structure cracks into two pieces when the structure is subjected to the peak tensile load and cannot withstand further load or deformation. The fibre reinforced concrete structure cracks at the same peak tensile load, but does not separate and can maintain a load to very large deformations. The area under the curve shows the energy absorbed by the FRCs when subjected to tensile load. This can be termed as the post cracking response of the FRCs. The real advantage of adding fibers is when fibers bridge these cracks and undergo pullout process, such that the deformation can continue only with the further input of energy from the loading source.

Importance of Fibre Reinforced Concrete in Shear

Shear failure in concrete is known to be brittle and catastrophic [8]. In usual structural design, shear is accounted for by providing shear reinforcement such as stirrups in beams. Sometimes the shear reinforcement may be less than sufficient if the loading configuration were different from that predicted during design, such as during earthquake or at critical section reinforcement conjunctions. Fibres are effective shear reinforcement, its increased shear strength and ultimately results in ductile flexure failures [7].

If RC structure designed according to IS: 13290 (Code for ductile detailing), heavy reinforcement detailing will be in confinement zone of beam and column. If shear stress enhance by the addition of "Triangle Polyester Fibre" in the concrete, then stirrups spacing in structural component will be increased. Polyester fibre reinforces concrete satisfying ductility, durability and energy absorption criteria of RC structures. PFRC (Polyester Fibre Reinforced Concrete) also give high resistance to CO2, water and atmospheric gases in "cover zone" and protect reinforcement from corrosion. Polyester fibres in cover zone provide mechanism of "crack arrester" under earthquake, reversible cyclic, dynamic, and wind cyclonic load. Recently in IS: 456 in august 2007, some amendment was made for addition of "fibre" in concrete.

Shear strength of fibre reinforced concrete plays very important role in construction of P.Q.C. (Pavement Quality Concrete) for road and at present in Surat in construction of road no steel reinforcements are used only "Triangle Polyester Fibre" are added for flexural shear of road. Steel doweling bars are used only at joints of road panel in longitudinal and lateral directions. Due to triangle shape of fibres in concrete gives more damping property. Ordinary concrete gives 5% damping, while TPFRC gives 6.5% damping and ultimately reduces dynamic loading on structure [6].

In the most advanced and most recent design method, the contribution of fibre reinforcement to shear is completely ignored. Also no Indian design codes allow a reduction or removal of stirrups or dowels from beams, column and beam-column joint. Now a days, these techniques slowly entering into field, provide properties that are easily obtainable through fibre reinforcement.

Research Significance

FRC as a material is emerging from laboratory testing to field applications like in new construction, repair, retrofitting, etc. Structural characteristics should be understood properly for this ductile composite to be used in wide spectrum of applications. An attempt is, therefore, made in the present experimental investigation to analyses these composites, and to predict their performance in various loading conditions.

Test Programme
Material and Mixtures

The mixture proportions and properties of concrete used in the test program are given in Table 1.

Improvement of Shear Strength using Triangular Shape Fibre in Concrete

OPC 53 grade cement, aggregate – maximum size of 9.5 mm and 19 mm, river sand and potable water were used. Super plasticizer – 140 ml/bag was used to achieve adequate fibre dispersion and workability. Polyester fibre (Recron 3S) of length 12mm were used as they exhibit good bond with cement being substantial triangular in cross section and offer better dispersion in the matrix being silicon coated. Variable: Fibre volume fraction – 0%, 0.5%, 1%, 1.5% and 2 %. From each mixture, the following specimens were cast: three cubes (150x150x150mm), cylinders (150mm dia. and 300 mm height), beams (100x100x500mm), L- shape specimen for shear (Refer fig 2.) Specimens will be test at 7 and 28 days.

JSCE-SF6 suggested direct shear method by using beam of size (150x 150x 500 mm) and load applied through male–female arrangement and failure under double shear. Another method suggested by Dr. C.D. Modhera and Dr. N. K. Bairagi,[1] arrangement for which is shown in fig. 2.

Improvement of Shear Strength using Triangular Shape Fibre in Concrete
All Dimensions are in mm.
Figure 2: Arrangement of Specimen for Shear Strength

Results and Discussion
Properties of Fresh Concrete

Workability reduces at higher dosage of fibres compared to initial dosage used. Due to more addition of fibres, there is increase in amount of entrapped air voids due to presence of fibres and therefore increase in air content attributes in reducing workability and difficulty is observed in compaction of mixes.

Failure Shapes

A load resisting mechanism is better due to triangular cross section of fibres undergo twisting kind of the deformation in 3D fibre orientation than circular cross section. A sufficient bond with matrix is observed during the studies. The fibre pull-out is prevented and fibres mainly failed by breaking. Compared to normal concrete, budging effect is observed for FRC cubes. Also no balling off of concrete indicated the gain of residual strength in concrete.

Compressive Strength

The addition of polyester fibres to the mix increased the28 day’s compressive strength of the mix with the dosage of 1.5% by 16% due the confinement provided by fibres. The compressive strength at 1.5% dosage is slightly higher than strength at 2% dosage. Compressive strength increases for all dosage of fibres than normal concrete. Reason is that due to confinement provided by fibre bonding characteristics of concrete increases and hence compressive strength increases with the increases in the fibre content.

Improvement of Shear Strength using Triangular Shape Fibre in Concrete
Figure 3: Graph for Compressive Strength at Different Fibre Content

Split Tensile Strength

Split tensile test does not give perfect estimation about direct tensile strength due to mixed stress field and fibre orientation but its failure pattern gives fairly good idea about ductility of the material. Failure patterns of splitting tensile test indicate that specimens after first cracking do not separate unlike the concrete failure. Large damage zone is produced due to closely spaced micro cracks surrounding a splitting plane. Fibre bridging action and due to that stress transfer mechanism is responsible for such enhanced ductile failure pattern.

Improvement of Shear Strength using Triangular Shape Fibre in Concrete
Figure 4: Graph for Split Tensile Strength at Different Fibre Content

Flexural Strength

The flexural strength of the mix with the dosage of 1.5% increases by 21%. Nominal increases remains for all dosage of fibres compared to normal mixes. Minor gain is due to influence of top compression block in specimen, as when specimen is subjected to two-point loading, half depth of uncracked portion is under compression and bottom half is under tension.

Improvement of Shear Strength using Triangular Shape Fibre in Concrete
Figure 5: Graph for Flexural Strength at Different Fibre Content

Shear Strength

The addition of polyester fibres to the mix increased the shear strength of the mix with the dosage of 0.5% and 2% by 20% and 25% respectively. Addition of fibre reduces the crack spacing, thus indicating a more redistribution of stresses. Plain concrete failed in a very brittle manner, but FRC provided a gradual softening in shear. As the first crack forms, the fibres bridge it, transmitting stresses across the crack surface. In order to enforce further crack opening the applied load has to be increased, which leads to the formation of another crack. This mechanism then repeats until failure.

Improvement of Shear Strength using Triangular Shape Fibre in Concrete
Figure 5: Graph for Shear Strength at Different Fibre Content

Conclusion

  • Inclusion of different fibres content adversely affect flow properties of concrete. For wider application of FRC, careful mix proportioning is highly recommended.
  • Polyester fibres did not disperse properly in the mixing water. Addition of fibres to dry mix was found to be more practical.
  • The presence of fibres in concrete alters the failure mode of material. It is found that the failure mode of plain concrete is mainly due to spalling, while the failure mode of fibre concrete is bulging in transverse directions.
  • Compressive strength of material increases with increasing fibre content. Strength enhancement ranges from 9 % to 16 %. For the material used in this study fibre content of 1.5% by volume of concrete has beneficial effect on strength enhancement.
  • Strength enhancement in splitting tensile strength due to fibre addition varies from 6.5 % to 34%. Its failure pattern gives fairly good idea about ductility of the material. Split tensile strength at 28’days is approximately 50% higher than 7 day’s strength.
  • The addition of fibre has significantly enhanced the performance of beam. During the test it was visually observed that the FRC specimen has grater crack control as demonstrated by reduction in crack widths and crack spacing. The flexural strength increases with increasing fibre content. The maximum increase in flexural strength at 1.5 % fibre content is 27%.
  • The shear strength increases with increasing volume percentage of fibres. This is because of fibres enhances the load carrying capacity of mix. The percentage increase in shear strength of the mix varies from 27 % to 32 %.
  • Different test result indicates that for polyester fibre can effectively be used up to 1.5% fibre content.

References

  • Bairagi N.K. and Modhera C.D., "Shear Strength reinforced concrete," ICI, Jan-March 2001, pp. 47-52.
  • Balaguru P., and Najm H., "Properties of Fiber Reinforced Structural Lightweight Concrete", ACI Material Journal, V. 101, July-Aug 2004, pp. 281-286.
  • Bentur A. and Mindess S., "Fibre Reinforced Cementitious Composites," 2007, Taylor & Francis, London.
  • Chanh N., "Steel Fiber Reinforced Concrete."
  • Chen Pu - Woei, Chung D. D .L. "A Comparative Study of Concretes Reinforced with Carbon, Polyethylene, and Steel Fibers and Their Improvement by Latex Addition", ACI Material Journal, V. 93, March-April 1996, pp. 129-133.
  • Dave U. V. and Desai Y. M. "Effect of Polypropylene, Polyester and Glass fibres on various strength of ordinary and standard concrete" the first international conference on recent advance in concrete technology, Sep. 2007,Washington D.C. U.S.A.
  • Desai A.K., "Earthquake Resistant Ductile Concrete," NBM & CW, Oct 2007, pp.144-159.
  • Desai A.K., "Study of Beam-Column Joint Using HPC with Polyester Fibre under Cyclic Loading," NBM&CW, Jan.2006, pp.140-144.
  • French C., Kreger M.E. , "High Strength Concrete (HSC) in Seismic Regions," 1998, Framington Hills, ACI, vii, 471
  • Karbhari Vistasp M, "Use of Composites for 21st Century Civil Infrastructure," Computer Methods in Applied Mechanics and Engineering, V.185, 2000, pp.433-454.
  • Li V., Ward R. J. and Hamza A., "Steel and synthetic Fibers as Shear Reinforcement," ACI Material Journal, V.89, Sep-Oct 1992, pp. 499-508.
  • Mirsayah A. and Banthia N., "Shear Strength of Steel Fiber-Reinforced Concrete," ACI Material Journal, V.99, Sep-Oct 2002, pp. 473-479.
  • Nagabhushanam M., Ramakrishnan V. and Vondran G. "Fatigue Strength of Fibrillated Polypropylene Fiber Reinforced Concretes" Transportation Research Record, 1989, No. 1226, pp. 36-47.
  • Parameswaran V.S., "High - Performance Fiber Reinforced Concretes," Indian Concrete Journal, Nov. 1996, pp. 621-627.
  • ACI Committee 544, "Measurement of Properties of Fiber Reinforced concrete."

NBMCW August 2010

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Fatigue in Concrete Concerns Security a...

Concern about fatigue in concrete, is on increase in important concrete structures such as flyovers. In flyovers and allied structures when loading, in the form of repeated cycles, occurs for a large number, concrete undergoes phenomena of fatigue. Busy flyovers, in congested urban areas when display cracks in riding surface, or pedestrians; witness falling of chunks of concrete the phenomena creates panic in users. Basically, Fatigue and failure of concrete is a rare phenomenon. It is not a usual mode of failure of concrete structures. This Paper deals with important parameters of fatigue, strength of component materials under cyclic, loading and Codal provisions illustration of a bridge and or flyover affected by fatigue are suitably dealt with.

Notations – fc = compressive strength of concrete in MPa; Flim = fatigue limit in MPa

Dr C.S.Suryawanshi, Former Chief Engineer & Joint Secretary (P.W.D) Senior Consultant Mumbai.

Introduction

Security and Stability of Flyovers
In flyovers, under occasional, over loadings where full design load gets repeated, for a very large number of cycles, concrete undergoes stress concentration, exhibits excessive cracking and may eventually lead to failure after a sufficient number of load repetitions, even if the maximum stress is less than the static strength of a similar specimen. Such members under– go a process of progressive, permanent internal structural (micro-cracking) change in a material subjected to a fluctuating stresses/ strains, conventionally termed as "fatigue." Substantial research and laboratory studies indicate, most existing concrete structures, designed under static load criteria offer a much less resistance to many concentrated loads than, to repeated fixed point loads.

Many countries have recognized the importance of this effect and included design provisions for fatigue in their codes. A study of such phenomena in concrete in a bridge in arid zone and flyover, in coastal region constructed in recent past, on available data is illustrated here, pinpointing some of inadequacies and probable rehabilitation measures.

Fatigue & Material Properties

Unlike other materials, concrete is subjected to the effects of fatigue. Fatigue in a structural member occurs when permanent internal changes in the material viz internal micro cracking gets symptomized on surface initially in the form of visible cracks and later concrete turns brittle leading to failure of concrete. Generally, it is recognized as a cracking developed under repetitive loads that are less than the static load capacity. Fortunately, research has shown that the static load criteria under which most existing concrete structures have been designed have virtually precluded the possibility of fatigue failure in the primary load-carrying elements of the main members. Reports of fatigue failures in these primary elements are, apparently, nonexistent. Also, a recent laboratory study in Japan has indicated that the fatigue resistance of bridge deck slabs to moving concentrated loads is much less than to repeated fixed-point loads. This suggests that the progressive failure of slabs may, in part, be attributed to fatigue where repeated shear and torsional forces are applied. Fatigue is also regarded and experienced as a cause of cracking and failure in concrete pavements.

Research on fatigue dates back to early 1900s. Most of it dealt with fatigue of plain concrete. In the 1960s, very extensive programmes on the fatigue of reinforcing elements were carried out in Europe, Japan, and the United States. Committee 215 of the American Concrete Institute published a report in 1974 that included an extensive list of references. More recent work has been directed to specialized applications. The proceedings of a colloquium held in 1982 by the International Association for Bridge and Structural Engineering contain a substantial number of current and relevant papers.

Phenomena of Fatigue

In general, mechanical properties of structural materials under dynamic loading, get improved with increasing rate of load application. In respect of concrete's dynamic ultimate strength in compression may be much greater than the static strength.

A fatigue loading is a sequence of load repetitions. A distinction is made between high-cycle, low amplitude and low cycle, high-amplitude fatigue loading. The distinction depends on whether the repeated loading causes a failure at less than or more than an arbitrary number of cycles, usually considered in the range of 100-1000 cycles.

Behavior of Concrete Under Loads

In a simple explanation for fatigue, it is interesting to recall basis of stress-strain relation. Figure-1 shows the typical stress-strain curve of a test piece under compressive stress increased monotonically to rupture. The curve may be designated with following zones.

Stress-Strain Curve for concrete in compression
Figure 1: Typical stress-strain curve for concrete in compression
Zone A: stable micro-cracking; stress 0+0.55fcu. The approximate proportion 0.55 is not sensitive to the value of fcu

Strain is partly elastic and partly caused by non-reversible rupture of crystalline bonds, known as micro-cracking, hence the bend of the curve. The rupture is mostly by sliding between un-hydrated cement particles and is accompanied by reduction in volume. On condition that adequate moisture is present, the crystalline bonds will be re-established when the load is released or maintained steadily.

The release of the load will be as shown in Fig. 4-2, with a hysteresis loop and a residual deflection. Successive load cycles tend to a stable residual deflection with a hysteresis loop, the energy represented by the area of the loop being dissipated into heat.

The propagation of micro cracks under loads within this range is stable and the concrete can support an unlimited number of applications of such loads.

Zone B: unstable micro cracking; stress 0.55 fcu – 0.80 fcu. Micro-cracking is stable under a single application of load but increases with repeated applications until the micro-cracks link together to form macro-cracks which in turn progress in extent until rupture ensues. Strains increase with repetition of load. Final rupture is similar to rupture caused by a load monotonically increased and is accompanied by large strains; it is not brittle, As in zone A, after the release of the load or under a steady load, cracks both micro and macro will tend to seal up by the reestablishment of crystalline bonds accompanied by some increase in strength.

Stress-Strain Curve for repeated loadings in compression
Figure 2: Typical stress-strain curves for repeated loadings in compression
Zone C: unstable Under macro-cracking : stress 0.8 fcu – 1.0 fcu – Under a single application of load in this zone, micro-cracks get formed as the load increases, which then link together to form macro-cracks. These macro-cracks extend and lead to rupture with either the repetition or the maintenance of the load.

The material is disaggregated and its apparent volume increases. After the release of the load (but not under steady load), cracks will seal up as in zones A and B.

Zone D: post-rupture stress - with a load possessing sufficient energy-potential (piled-up weights), strain (an extensible test rig) or dynamic (an impact)-as soon as the concrete reaches the maximum stress which it can support, it collapses.

concrete in compression
Where, however, the strain is controlled, as by material elsewhere in a hyperstatic structure or by a testing machine applying a constant rate of strain, or where the load is more properly seen as an imposed strain or again where the load is responsive to strain, it is not unknown for the concrete to continue to support a reducing value of stress with very large strains - as much as three or four times the strain at fcu. This behavior is not easily accounted for and, with present knowledge, is not to be relied upon but its existence in special circumstances is well accredited.

Under steady, long-maintained loads but less good under repeated and high- energy load is perhaps as good a summary of what we know as any. Sufficient repetition of load in the upper half of this zone will cause cracking.

In respect of reinforced concrete members the behavior of concrete is similar upto zone A, however, thereafter it worth noticing.

Zone B: quasi-elastic behavior of the cracked section - The steel behaves elastically and the concrete is generally within zone A of material behavior (stable micro-cracking) but overlapping into zone B (unstable micro-cracking) at the upper end. The non-linearity of the concrete behavior causes the neutral axis to rise, resulting in a larger departure from linearity in the behavior of the beam.

concrete in compression

Repeated loading causes increased deflection and crack widths, but the beam is probably able to support large number of repetitions. Release of the 'load leaves an appreciable residual deflection.

Effect of initial steel tension on load deflection curve
Figure 3: Effect of initial steel tension on load deflection curve of a prestressed concrete beam A Cracking Load B Rupture Load



load deflection curve
Figure 4: Typical load-deflection curve of a prestressed concrete beam
Zone C: non-elastic behavior of tile cracked section terminating in rupture The stress in the steel exceeds the elastic limit and the stress in the concrete enters zone C, the zone of unstable micro-cracking. Deflections are large; the repetition or maintenance of the load will lead to collapse. Release of the load leaves a large residual deflection.

Zone D: Post-rupture load behavior - The ability of the beam to support large deflections but with decreasing load is primarily caused by the large non-elastic strain of the steel prior to rupture but the concrete makes a contribution. If the compressive flange is transversely reinforced with closed stirrups or links, its ability to support large compressive strains is greatly increased. The importance of this zone lies in the large area beneath the deflection curve and, in consequence, in the large energy at rupture, energy of which a small fraction only is resilient but nearly all of which is dissipated. This renders reinforced concrete ideal for high-energy loads (earthquake, explosion) of rare occurrence.

Pre-stressed Concrete

Given a concrete section and a cross-sectional area of high-tensile steel at a defined location in the section, the ultimate strength of a pre-stressed concrete beam has been determined subject only to such variations as are described below. Brittle failures similar to those in reinforced concrete can occur with extreme proportions of steel, but it will be assumed that such proportions have been avoided. There then exists an extra variable at the control of the designer - the initial tension in the steel; the behavior of the beam will differ profoundly according to the value chosen for this initial tension. Figure -3 shows the range of behavior.

At one extreme, where the steel has not been tensioned, behavior is similar to that of reinforced concrete. The difference is that, since the steel is of high tensile strength, the steel strains are large and the neutral axis rises rapidly to near the upper surface and the full strength of the steel cannot be utilized. The cracking load is small and well defined.

At the other extreme, the steel is stressed to near its elastic limit. The cracking point is little lower than the rupture load. Since the steel has been given an initial extension, less strain is needed for it to reach its ultimate tensile strength but here the steel is at its maximum.

With intermediate values of initial tension in the steel (that shown represents a typical design according to current practice whereby the tension in the steel after relaxation is approximately 0.6 fcu the cracking load will be much lower, the ultimate load slightly lower than that with maximum initial tension.

What is the significance of these differences of behavior?

Note first the significance of the cracking load in pre-stressed concrete. From zero load to cracking load, there is little difference in the value of the force in the steel, the increased bending moment being supported by a displacement of the centre' of thrust in the concrete, thus increasing the moment arm. In concrete, stresses vary between limits fixed by the designer as being capable of being supported indefinitely. As a result, until loads appreciably exceed the cracking load, there is no danger of failure under repeated loads.

In curve I (maximum initial tension in steel), the cracking load is a high proportion of the static rupture load; so too is the fatigue limit-indeed, it is doubtful whether any known structural material can withstand indefinite repetitions of so large a proportion of its static rupture load. On the other hand, deflection at rupture is small as is the energy to cause rupture; behavior tends towards the brittle. This is the extreme of resistance to repeated loads.

In zone A, residual deflections on release of the load are small: as we have seen repetition of the load in this zone will not produce failure. Fig-4.

In zone B, residual deflections on release of the load are larger but still small. The area under the curve after loading is large but this energy is not dissipated but for the most part stored resiliently and the beam is still quasi-elastic. The inflection in the curve of deflection is of value in redistributing the load in redundant structures, especially since this redistribution is reversible and residual deflections on release of the load remain small. The beam in this zone can support steady loads indefinitely but repeated loads can cause failure.

In zone C, residual deflections on release of the load are larger but still much smaller than those in curve II. Dissipated energy is significant but a large part of the energy absorbed is restored resiliently until rupture is near. Steady loads in this zone will cause failure as will repeat loads.

From the foregoing fatigue can be simply defined as

The weakness or breakdown of material subjected to stress especially a repeated series of stresses

Fatigue Limit – Tensile Strength of Material

Fatigue in Concrete Concerns Security and Stability of Flyovers

Premature Failure Syndrome (PFS)

Under continuous repetitions of loading, a crack forms at a point of high stress concentration, on repletion of stress the crack slowly spreads under, the member raptures without measurable yielding. Although the concrete is ductile, the fracture looks brittle.

Endurance Limit (EL)

Concrete, has a well defined yield point and has a known endurance limit, which is the maximum unit stress that can be repeated, through a definite range, on an indefinite number of times without causing structural damage. Generally, when no range is specified, the EL is intended for a cycle in which the stress is varied between tension and compression stresses of equal value. For a different range if 'f' is EL, 'fy' the yield point and 'r' ratio of minimum stress to the maximum than

Fatigue in Concrete Concerns Security and Stability of Flyovers

The range of stress may be resolved in two components, a steady or mean stress and an alternating stress. The EL, sometime is defined as the maximum value of alternating stress that can be superimposed on the steady stress on indefinitely large number of times without causing fracture.

If f is EL for completely reversed stresses 's' the steady unit stress and fs the ultimate tensile stress, than alternating stress =

Fatigue in Concrete Concerns Security and Stability of Flyovers

Where 'n' lies between 1 – 2 depending upon mechanical properties of material.

Fatigue Strength of Component Materials

When the phenomena of fatigue alone, is critically examined it will indicate - Test data on the fatigue strength of concrete or reinforcing steels are usually presented in the form of S-N, or (Wohler, diagrams). Where S is a characteristic stress of the loading cycle, (often either the stress range or a function of the maximum and minimum stress), and N is the number or cycles to failure. Fatigue' data on concrete or reinforcing steels have commonly been shown in semi-log plots, where S is plotted linearly as a dependent variable and life is plotted as an independent variable on a log scale. In this form, the mean of the data can often be represented by a straight regression line.

Fatigue life is considered to occur in three stages: initiation of cracking, propagation of cracking, and fracture. In the propagation stage, micro-cracking occurs in the concrete or cracking is growing in a steel element. During most of this stage, the cracking grows slowly, followed by a short period of more rapid growth leading to fracture.

In designing for fatigue, recognition must be given to the variability that is inherent in the phenomenon. The mean regression line of a set or fatigue test data is shown in Fig- 5. The assumption of a normal distribution of the fatigue lives about the mean is usually acceptable. With this assumption, for example, it may be stated that the probability of failure, p at the lower dashed line is 5%. This dashed line is located 1.96 times the standard deviation, s, of the data below the mean. Fatigue data on concrete have been represented in this manner.

It is more common and more useful for design, particularly with fatigue data on steel, to utilize a lower tolerance limit on the data. A tolerance limit associates a confidence level with a probability of survival. This confidence level is dependent on the number of test results in the sample. For the data shown in Fig-5, 95% of the test results are expected to exceed, at a 95% confidence level, the lower tolerance limit. This limit is about twice the standard error of estimate or N below the mean.

Confidence intervals are also commonly shown on fatigue test data. These intervals, for example at the 95% level, indicate the range in which 95% of the means or the population represented by the' data are expected to be included. However, it is never known if the S-N curve of the population actually lies within the confidence interval.

Plain Concrete

When concrete is subjected to increasing compressive load, it is observed that the volume of the concrete ceases to decrease, and instead begins to increase at a stress level as low as approximately 50% of the compressive strength. The initial deviation from linearity of the relationship between stress and decrease in volume is noticed to be related to a significant increase in micro-cracking of the aggregate paste interface, while the stress at which the volume began to increase was related to a noticeable increase in micro-cracking through the matrix. Repetitive loading appeared to have a significant effect on the growth of micro-cracking. As a result, the fatigue of plain concrete may be considered to be a process of progressive, permanent changes occurring within the concrete matrix under repetitive loading.

Fatigue in Concrete Concerns Security and Stability of Flyovers
Figure 5: variability in fatigue life of concrete in composition

Under cyclic compressive loading of concrete specimens, a number of investigators have observed a decrease in the measured value of pulse velocity and an increase in acoustic emissions. These changes are related to the growth of the micro-cracking. More important to the designer is the increase in deformation that occurs during repeated loading, as illustrated by the data obtained by Holmen and shown in Fig - 6.

A rapid increase in strain occurs between and about 10% of the total life, Nr. The increase in strain between 10 and about 80% is slow and uniform, after which the strain increases at an increasing rate until failure occurs. Furthermore, it appears that the strain consists of two components, one related to the micro-cracking and the other which is time development and related to creep deformation. Expressions for estimating the strain have been developed.

Cyclic Compression Fatigue

Numerous investigators have found that the fatigue life of a common group of concrete specimens tested in compression, each under a constant amplitude loading with the stress, f, varying from the same minimum to different maximum, may generally be presented by a straight line, as illustrated in Fig-5. Further more, there is no apparent endurance limit below which the concrete will sustain an unlimited number of repetitions, although it should be noted that very little data are available for loading greater than 107 cycles.

An expression for the fatigue of concrete in compression based on linearity of the relationship between fmax fmin and log N, can be expressed:

Fatigue in Concrete Concerns Security and Stability of Flyovers

in which a value of b based on a review of available data, equal to 0.064 was given.
Fatigue in Concrete Concerns Security and Stability of Flyovers
Figure 6: Variation in measured strain: €0 is the maximum strain in the first cycle; the frequency is 5Hz

In design equation for the fatigue of concrete in compression has been included in the Tentative recommendations for the limit state design of concrete structures by the Japan society of Civil Engineers, as follows:

Fatigue in Concrete Concerns Security and Stability of Flyovers

Where k is the coefficient taken equal to 0.85 to consider the difference in concrete strength measured using standard cylinders and the in-place strength. This equation was reported to have been based on the fatigue life to a probability of failure of about 5%.

Cyclic Tension

A number of investigations have shown that the fatigue strength of concrete under loadings producing axial, splitting or flexural tension is about the same as for compression. Cyclic loading from compression to tension has been reported to cause more damage than zero-to-tension loadings.

Effect of Material Properties

Numerous investigator have studied the effect of such factors as cement content, water/cement ratio, curing conditions, age at loading, amount of entrained air and type of aggregate. Except for the latter factor, there is general agreement that these factors affect fatigue strength in a proportionate manner to the static strength of the concrete, as directly given by the previous expressions for fatigue strength. Furthermore, the fatigue strength of mortar is also comparable to concrete when expressed as a function of compressive strength.

In practice, structural concrete members are normally subjected to randomly varying loads with periods of rest. In laboratory tests on concrete specimens subjected to varying flexural stresses, rest periods were beneficial, increasing the fatigue strength at 107 cycles, expressed as fmax/fc by about 10%. Low amplitude cyclic loading interspersed in higher amplitude loading also has a beneficial effect.

Commonly used Miner's hypothesis to determine the accumulation of fatigue damage under varying stresses can be expressed as:

Fatigue in Concrete Concerns Security and Stability of Flyovers

Where Ni equals the number of constant amplitude cycles at stress level i, Nfi equals the number of cycles that will cause failure at that stress level i, and equals the number of stress levels.

Fatigue in Concrete Concerns Security and Stability of Flyovers

Cyclic Flexural Fatigue

In tests conducted on beams with non-pre-stressed reinforcement, a single reinforcing element has commonly been used. Fatigue failures of these beams occur suddenly, with little Sign of distress. Very few tests have been conducted on beams with multiple non-pre-stressed reinforcing elements. However, fatigue fractures of reinforcing bars were induced in bridges in the AASHTO road test during special testing after the completion of the vehicular traffic tests.

These beams may not exhibit significant distress before failure occurs, because of the high bond between the steel and concrete, unless there is sufficient redundancy in the overall structural system to permit redistribution of the loading.

Multiple reinforcing elements have generally been used in the tests on beams containing pre-stressed steel. Fatigue failure of these beams have occurred only after significant signs of distress in the form of the development of a very wide crack and increasing deflections. As on the load-deflection curve for a pre-tensioned pre-stressed concrete I-beam in which a flexural fatigue failure occurred is shown in Fig- 7, along with a cross-section through the beam showing the location of the six 7/16 in (11 mm) diameter seven-wire 270 000 psi (1860 MPa) grade strand. Compressive strength of the concrete was 7120 psi (49 MPa) at the time of test. This beam was subjected to an initial loading of approximately 80% of the ultimate flexural capacity of the specimen, which was sufficient to fully develop flexural cracking and also cause significant inclined cracking in both shear spans. Next, the beam was subjected to 2 million cycles of a normal design loading with induced moments in the center of the span ranging between 19 and 45% of the flexural capacity of the specimens. Under this loading, the stress in the bottom fibers of the beam ranged between 890 psi (6.14MPa) compression and 440 psi (3.0 MPa) tension, computed on the basic of an un-cracked section. The tensile stress was approximately equal to 5.2 "f'c psi (0.44 √f'c MPa). No damage was observed. The range of the cyclic loading was subsequently increased to between 19 and 50% of the flexural capacity. At the maximum load, the nominal tensile stress in the bottom fibers was 660 psi (4.5 MPa) or 7.8 √f'c psi (0.65 √f'c MPa. The beam sustained 570,000 cycles of this increased above design loading before the test was stopped because of extensive fatigue damage. The first indication of increased deflection corresponding to fatigue damage was evident at 455,000 cycles of the above design loading. After the test was conducted, the pre-stressing strand was exposed and a total of 21 wire fractures were found in the three lower level strands. Four of these fractures were at locations other than at the major cracks.

Fatigue in Concrete Concerns Security and Stability of Flyovers
Figure 7: Deflection of a prestressed beam under static and cyclic loading (Reproduced from Hanson, J.M., Halsbos, C.L. and Vanhorn, D.A., 'Fatigue tests of prestressed concrete I-beams', J.Struct. Div. ASCE. Vol. 96 pp 2443-2464, 1970)

Cyclic Shear Torsion and Bond Fatigue

The effect of shear, torsion and bond are in many a cases quite closely related. For example, the development of inclined cracking will, in the absence of adequate shear reinforcement lead to increased stress in the flexural steel, which may affect the bond strength. On the other hand slip of reinforcement in an adequately reinforced member may induce a shear failure particularly in prestressed beams.

A number of investigations of the shear fatigue behavior of both non-prestressed and pre-stressed beams have been reported. It has been observed that the shear fatigue strength of non-prestressed beams without web reinforcement was approximately 60% of their static shear strength. Fractures of stirrup reinforcement have been reported in fatigue tests on both non-prestressed and prestressed beams.

Designers should recognise that inclined cracking will occur at lower stress under cyclic loading than under static loading. This, of course, is generally regarded as the limit state in a beam without web reinforcement. The number of cycles will be related to the tensile fatigue strength of the concrete. Non-prestressed beams without web reinforcement will therefore have lower shear fatigue strength than prestressed beams. For design, the shear fatigue strength should probably not be taken greater than one-half of the static design shear strength. Pre-stressing contributes to the shear strength of a beam without web reinforcement by imposing a compressive stress in regions where inclined cracks may originate. The shear fatigue strength of these members may be estimated by reducing portion of the contribution which is dependent on the tension in the concrete to one-half of the usual value.

Torsion

Information on the torsional fatigue resistance of concrete members is limited. A recent investigation confirmed that the fatigue properties of plain and prestressed concrete under torsion were about the same as those of concrete under compression or flexure.

Unintended or unrecognized structural interactions may induce torsion in slabs and beams. These torsional forces may be surprisingly large in uncracked members, where the resistance is dependent on the tensile strength of the concrete. Under cyclic loading the torsion may cause cracking that will subsequently contribute to the deterioration of the member.

In torsion, as in shear, it was found that the primary stresses are redistributed to the bars after cracking, and that the torsional stiffness is reduced. However, failures occurred due to fatigue of the concrete rather than the steel.

Bond

Research review on bond fatigue, leads to the conclusion that in the absence of cracks in the anchorage zone, the bond strength for 1 million cycles will be about 60% of the static strength. When cracking occurs, the bond fatigue strength will be strongly dependent on shear effects.

Two areas are of particular importance for bond fatigue - railroad ties and pre-stressed beams with 'blanketed' strands.

Design Parameters and Codal Provisions

Structural concrete members are normally proportioned to carry factored service loads. Fatigue is a serviceability condition that must be checked in the design process if the member is subjected to cyclic loading. This check is made by computing minimum and maximum stress levels under the anticipated cyclic loading at any potentially critical location and comparing the stress variation to the fatigue strength of the material. If there is variability in the cyclic loading, some method of accumulating the effect of the different load levels must be included. Miner's hypothesis, as expressed in Eqn-2-5, is frequently used for both the concrete and steel components of the member.

A satisfactory estimate of the stress levels in both the steel and the concrete can usually be made using the ordinary principles of flexural mechanics. However, these procedures become quite complicated for pre-stressed beams subjected to cyclic loading that induces cracking. These members are often referred to as partially pre-stressed. Aids have been developed for the analysis of these members.

While simple in concept, a check of a member for fatigue can become quite complicated when, for example, different load patterns are required to obtain maximum or minimum stress levels at a selected location. However, in many cases, the question will be whether or not the concrete member is on the threshold of fatigue distress. Hence it will be mainly important to project the numbers of cycles of maximum repeated loading which the member may conceivably resist during its design life.

Most building codes governing the design of concrete structures subjected to in shear reinforcement can be calculated as follows:

Fatigue in Concrete Concerns Security and Stability of Flyovers

σwrp = σwrk (Vpd / Vrd)

Where Vmd is the design maximum shear force, Vcd is the design ultimate shear force resisted by concrete, Vpd is the applied design permanent shear force, Vrd is the applied design variable shear force, Aw is the area of shear reinforcement within a distance s, d is the effective depth and θ is the angle between shear reinforcement and the longitudinal axis of the member.

Fatigue limit states for reinforced concrete beams may generally be examined only for longitudinal tensile reinforcement and shear reinforcement. Fatigue limit states for reinforced concrete slabs may generally be examined only for tensile reinforcement. The examination of fatigue limit state for reinforced concrete columns may generally be omitted.

Codal Provisions

  1. Indian Standards
    1. Plain and Reinforced concrete code of practice IS-456.
    2. Indian Roads Congress Publication 21

      A standard specification and code of practice for Road bridge Section-III deals with along aspects does not contain any provision for evaluation of fatigue, but contain provision. Provision of exposure conditions for concretes from durability point of view.
    3. Indian Roads Congress Special Publication 37

      Provision about rating of bridges are outlined along with procedure to be adopted in evaluating the strength of existing bridges. Indian Standards – Code of Practice – Plain and Reinforced Concrete is 456:2000 contains design requirements of R C Members. Amongst various requirements includes elastic deformation and creep of concrete (6.2.3 & 6.2.5) and limits the concrete to follow law of elasticity as long as the stress in concrete does not exceed 1/3 of its characteristic strength.
  2. Foreign Codes
    European Codes
    Several European countries have codes containing fatigue design provisions. The following provisions are included in Appendix f of the 1978 CEB-FIP Model code for concrete structures.

CEB-FIP Model Code For Concrete Structures 1978

General

Fatigue failure of a material is failure due to frequent repetition of stresses lower than its strength under static loading.

The stresses that are comparable with the fatigue strength should be determined by means of elastic methods, taking into account the dynamic effects, the effects of creep, the losses of pre-stress etc. These stresses are defined as follows:

σmax corresponding to a frequent action repeated 2-10th times at its maximum value;

σmin corresponding either to a quasi-permanent action or to a frequent action repeated 2-10th times at its minimum value, as the case may be.

The condition to be checked is

Δ6 = 6max - 5 min ≤ Δfrep / δfar

Δfrep denoting the strength under repeated load effects.

Fatigue Strength of the Concrete

The fatigue strength is defined as the 50% fractile deducted from test results.

Fatigue Strength of the Steel Reinforcement

For the steel the characteristic strength is the 10% fractile and for the anchorage devices the 50% fractile, deducted from tests in which σmax is repeated 2-10th times and where:

σmax = 0.7 fyk (or f0.2k) for reinforcing steel

σmin = 0.85 f0.2 for pre-stressing tendons.

The reduction of the fatigue strength owing to curvature, welding, mechanical connections, end anchorages etc. should be taken into account in the calculations, preferably on the basis of test results.

Special Considerations

The task is to take into account all the stress concentrations which are liable to affect fatigue behavior but which are beyond the usual objectives of stress checks.

According to the notes (which accompany Appendix f), the provisions represent a simplified approach of practical character which is appropriate for most conventional structures. Only in special cases, it is necessary to check the cumulative effect of the repetitions at different stress levels, using the Palmgren-Miner rule for example, with appropriate limitations and a defined load spectrum, related to the expected life term of the structure. The random nature of the load repetitions is not taken into consideration.

Normally, the following numerical values are introduced:

For steel, Yfat is applied to the characteristic value and is taken at 1.15 i.e. frepfat = f&thet

For concrete, and for the anchorage devices Yfat is taken equal to 1.25 and applied to the mean value i.e. frep/Yfat = Vfcm/1.25 or Δfsm/1.25

In the absence of test results or practical experience, the following lower limits are accepted (σmin = 0) for fcm2

0.6 for the stresses in the concrete and for bond stresses in high bond bars.

0.4 for the bond stresses in smooth found bars.

In the absence of test results, the following values can be adopted for Δfcmmin= 0)

Smooth bars 250 MPa

Prestressing tendons (without bond due to deformed shape) 200 MPa

Prestressing tendons (with bond due to deformed shape) 150 MPa

High bonds bars 150 MPa

In the absence of test results, fsk may be reduced by the following coefficients:

Curvature: (1-1.5Ør), where r denotes the radius of curvature

Spot welding: 0.4

Continuous scam welding: 0.4

Butt welding: 0.7

Bars of small diameter are to be preferred

The spacing between bars should not exceed

10Ø for the longitudinal reinforcement

5Ø for the transverse reinforcement

In general, checking tendons for fatigue in fully pre-stressed elements is not necessary. It should be noted however that the cracking moment can be reduced as a result of fatigue in tension of the concrete.

Ontario Highway Bridge Design Code

This is a code of practice for Design of Highway Bridges. Deals with various aspects of design, states limit states and at cl. 14.6.2 prescribes ultimate limit state to be used for evaluation. Recommends serviceability limit state for evaluation of fatigue- At cl. 2.6.2 presents serviceability limit states where in it requires (Cl. 2.6.2.2). Concern for superstructure vibration in the form of deflection limit, thus a stringent and comprehensive requirement to elaborate fatigue.

In order to minimize over stressing of the box girders under global action, a thin overlay is required to be designed by in-elastic method. Such empirical design is covered in Section 7.4 of Ontario Highway Bridge Design Code.

All these overlay designs though taken up referring to Ontario Code, speaking those designs are beyond the limitations imposed in the Ontario Code. From the satisfactory performance of these repaired and rehabilitated deck slabs one may infer that in such rehabilitation job, design following methodology of Ontario Code, but outside the boundaries or the limits indicated in the code can also perform. This, however, need to be confirmed by checking, the performance of the similar strengthening carried out earlier. In the unlikely event, if the thin RCC overlay provided is seen to be inadequate to give protection to the existing deck slab, additional steel girders can be placed below the deck.

Improvement of Fatigue Strength

Design of members for a repeated loading cannot be executed with the certainty with which member can be designed to resist static loading. Stress concentration may be present for a wide variety of reasons and it is not practicable to calculate their intensities. But sometimes it is possible to improve the fatigue strength of a material or to reduce the magnitude of a stress concentration below the minimum value that will cease fatigue failure.

Fatigue strength of a material can be improved by cold working the material in the region of stress concentration by thermal process or by pre-stressing it in such a way as to introduce favourable internal stresses, there the fatigue stresses are unusually severe special materials may have to be selected with high energy absorption and notch toughness.

In general, avoid design details that cause severe stress concentrations or poor stress distribution. Provide gradual changes in sections. Eliminate sharp corners and notches, do not use details that create high localized constraints. Locate unavoidable stress raisers at points where stress is low and fatigue conditions are not severe. Provide structures with multiple load paths or redundant members so that a fatigue crack in any area of the several primary members is not likely to cause collapse of entire structure.

Case Study

A major bridge constructed around 1964-65 on Aurangabad-Nagar State Highway about 18km from Aurangabad carrying single carriage- way's layout plan and cross sections are shown in Fig-1,2 and 3.

The deck slab of the bridge was provided with asphaltic wearing coarse. In 1974, the deck slab suffered a distress as outlined in para—above.

Background

The Turbhe Flyover at Navi Mumbai, passes over Thane-Belapur railway line with viaduct arms of Mumbai-Panvel, Panvel-Mumbai, Mumbai-Thane and Thane-Mumbai was constructed through a "Design and Construct" contract under design review and supervision by PWD, Govt. of Maharashtra. The layout plan of the flyover is given below.

Fatigue in Concrete Concerns Security and Stability of Flyovers
Figure: 1 Turbhe Flyover at Navi Mumbai

In addition, there are 4 Nos. simply supported spans (spans varying between 38.50 m to 45.50 m) of cast-in-situ, PSC single cell box girder type superstructure; across the railway tracks.

The flyover has the following 4 arms in the via-duct portions.

Mumbai–Thane Arm —

8 Nos spans of 7 m carriageway widths on Thane side

Mumbai–Panvel Arm—

10 Nos. spans of 11 m on Panvel side and 8 (4 + 4) Nos spans of total 14 m carriageway width on Mumbai side.

Thane–Mumbai Arm—

10 Nos. spans of 7 m carriageway width on

Thane side

Panvel–Mumbai Arm—

11 Nos. spans of 11 m carriageway width on Panvel side and 8 (4 + 4) Nos spans of total 14 m carriageway width on Mumbai side.

From the sections of box girders, it is apparent that two types of RCC box girders of 7 m and 11 m carriageway widths have been used.

Fatigue in Concrete Concerns Security and Stability of Flyovers
Figure 2: Section of box girder

The inclined webs of box girders are supporting the top deck slab with outer faces of webs as 6.10 m (with minimum web thickness as 300 mm and minimum deck slab thickness at centre as 230 mm) for 11.00 m wide carriageway and 4.80 m (with minimum web thickness as 250 mm and minimum deck slab thickness at centre as 200 mm) for 7.00 m wide carriageway.

It has been reported that the design was carried out as per relevant IRC and as per loading condition of bridges on National Highways and as per stipulations of contract agreement.

After construction the flyover was opened to traffic in November / December, 1994.

The traffic on all the arms of the flyover is very high, both in terms of volume and heavy vehicles with high axle loads, i.e. of the order of – PCU (Passenger Car Unit). In axle load survey on Mumbai - Nashik highway, it is seen that axle loads exceeding 20 t i.e., gross vehicle weight much higher than permitted by RTO regulation (two axle truck upto 26.0 t and six axle vehicle upto 116.0 t) are plying.

The flyover, in the viaduct portion, has 55 Nos. of simply supported spans (spans in the range of 30 m each) of cast-in-situ RCC, single cell box girder type superstructure, while the obligatory railway spans are with spans of cast-in-situ, prestressed single cell box girders.

Fatigue in Concrete Concerns Security and Stability of Flyovers
Figure 3: Sections of box girders

In July, 1997, a through-hole (pot hole) in deck slab at an isolated location was noticed. Though the pot hole was repaired, but every year one more pot hole started appearing in the deck slab of RCC box girders. Those pot holes were repaired, but lately more and more pot holes started appearing in different isolated locations of the deck slab. Repairs so far carried out have been listed in Table 1.

Fatigue in Concrete Concerns Security and Stability of Flyovers

Field Observations

Concerned PWD officials showed the typical distressed / damaged locations and provided the past repair rehabilitation history of the flyover.

The condition of the flyover with naked eye from a distance of 20' or so revealed, most of the deck slab's soffit contain isolated short length cracks of varying widths (approximately 0.1 mm to 0.2 mm), barring fewer ones exceeding 0.3 mm wide. Similar type of network spread of cracks is visible in the intermediate cross girders and in end diaphragms too.

Fatigue in Concrete Concerns Security and Stability of Flyovers
Photo 4
In Panvel-Mumbai arm (between P27 - P28 & P28 - P29) a large portion (3.0 m x 2.0 m) of surface deck slab is severely distressed with formation of a pot hole of about 1.0 m diameter (Photo 4 & 5), and badly damaged, cracked and depressed (top portion) bituminous wearing course (Photo 6) Similarly in Mumbai - Thane arm, in number of spans, the wearing course displays early symptoms of pot hole formation (Photo 4).

The major damage in the deck slab is noticeable in the suspended portion between two webs. In cantilever portion, as of today at least when observed with naked eye, there is no severe damage, although there are signs of minor distress.

In Mumbai - Thane arm, in span P19 - P20, (where RCC overlay has been carried out in May, 2002) it is noticed top of deck slab, contains a few isolated short length cracks and in a local pocket of about 2.0 m x 2.0 m number of closely spaced cracks (Photo 5). It is informed the span with RCC overlay is functioning satisfactorily without any visual signs of increase in length or width of cracks.

In the complete length of flyover, there is no sign of corrosion of reinforcement.

Fatigue in Concrete Concerns Security and Stability of Flyovers
Photo 5
In the webs of box girder as seen from distance (from outside) and inside some vertical and inclined cracks mostly of small width are noticeable. Similarly, the box girder bottom slab soffit at places is having transverse cracks of small width. It couldn't be established as to whether these cracks in webs and soffit of box girders are progressive or dormant

In addition, the following discrepancies were noticed.
  1. The slab-seal type expansion joints installed on the flyover reveal early signs of damage / deterioration and anchor bolts exposed at many locations and adjacent concrete band damaged.
  2. Across the obligatory railway spans, the gap between superstructures of two arms may cause a concern in near future.

Behavior of Repaired Surfaces

Fatigue in Concrete Concerns Security and Stability of Flyovers
Photo 6
During course of discussion and inspection of the stretches of the deck surface repaired in the past, it is noticed-
  1. The 150mm thick RCC overlay provided over entire deck of span between P19-P20 on Mumbai-Thane Arm taken up in May 2002 has behaved satisfactorily so far. A few short length isolated cracks on top of overlay apparently have not progressed in length or width. With similar progressive deterioration of deck slab in a balanced cantilever box girder repaired in the past, a thin RCC overlay (an overlay of 125mm thick M40 grade concrete with 0.6% isotropic reinforcement and with 15mm integral concrete wearing course in a clear gap of 2.38m between webs of box girders (where existing deck slab thickness of deck slab was 180mm) was installed in first quarter of 1989 in some other bridge. No further problem has been reported so far and the repaired portion has behaved satisfactorily. Similarly, in span between P19-P20 in Mumbai-Thane arm a RCC overlay of 150 thicknesses with 0.5% isotropic reinforcements was installed in May 2002. So far this is also performing satisfactorily without creation of any new pot hole in the span or without any major problem as on the overlay
  2. A few pot holes repaired by replacing the disintegrated concrete with new concrete of same grade performed satisfactorily under existing high volume of traffic mixed with regular heavier wheel loads.
  3. Where as a patch repair with epoxy injection to closely spaced cracks did not stop recurrence of new pot holes, Original deck below the area when inspected showed the process of formation of pot hole this shared with epoxy injection, the strength and integrity of concrete could not be restored.
Fatigue in Concrete Concerns Security and Stability of Flyovers
Photo 7
Recurrence of cracks in deck soffit will establish the effectiveness of the repair already carried out and effectiveness of RCC overlay in such deck rehabilitation work. Any appearance of new cracks or progress of existing cracks needs to be monitored vigilantly.

New cracks below the closely spaced cracks at top are indicative of inadequate repair of the deck slab before laying of overlay concrete.

Even in cracked area, the overlay concrete is performing its intended function.

Discussion on Field Observation

Fatigue in Concrete Concerns Security and Stability of Flyovers
Photo 8
In the absence of detailed reports of strength of concrete, design parameters, and close (hand-shake distance) observations of the cracks and data on quality of concrete, from construction records collation of the field observations (with naked eye) of damage / distress with the academic theory for behavior of structure can be summarised as.
  1. When the first pot hole in the deck slab appeared in deck slab of span (between pier P11a to P12a) in Thane - Mumbai arm (on 29.07.1997), process of progressive permanent internal structural(micro cracking)change in the concrete subjected to fluctuating changes in stresses and strains was already set in
  2. As a result of progressive internal cracking, macro cracks grew below the bituminous wearing coat which went unnoticed and subsequently every year one more pot hole started appearing in isolated locations of other spans.
  3. Local patch repair by replacement of disintegrated concrete with new concrete of similar modulus of elasticity, performed well, but appearance of similar pot holes in other portions of deck slab did not stop.
  4. Lately, the rate of appearing pot holes has increased significantly, especially in the suspended span portion of the deck slab between the webs of the box girder and away from the haunched portion of the deck slab. In the common parlance, the recurrence of pot holes is seen preceded by widening and extension of the cracks in criss-cross direction, forming through cracks and allowing leakage of water, near disintegration of concrete in closely cracked areas, with growing appearance of cracks in asphalt wearing course with local depression in wearing surface and finally forming through hole (pot hole) in the deck slab and wearing course.
  5. Presently, there are many such locations at various stages of formation of pot holes.
  6. The results of inspection of January to March, 2003 revealed that cracks at deck soffit are generally of small lengths and width mostly between 0.1 mm to 0.2 mm while a few ones reached to 0.3 mm and a very small number reached to 0.4 mm. Cracks of non progressive nature upto width 0.3 mm may be acceptable in RCC structures subject to static load unless those are in aggressive environment exposure.
  7. The cracks noticed on intermediate cross-girders are of short length of width between 0.1 mm to 0.2 mm while a small number of cracks reached upto 0.3 mm width and a very few reached to 0.4 mm width. Many of these cracks have been repaired by sealing and grouting with epoxy material. It would be important to note that the cracks in intermediate cross-girders and end diaphragms did not progress with time or additional cracks did not appear after the earlier cracks were repaired.
  8. There are diagonal and vertical cracks on both inclined faces of web of box girders. It is reported cracks from inside are repaired by sealing and grouting with epoxy. Though the outside cracks were not repaired, there is no sign of pressure injected epoxy flown along coming out from outside, which shows that the cracks are not through. No cracks are noticed after inside cracks were sealed. There are transverse cracks at soffit of bottom slab of box girders. Since fully filled with dust and dirt inside the cracks from inside cracks in the bottom of slab are not visible.
  9. The outer faces of box girders are provided with protective epoxy coating. There is no corrosion stain marks visible in any area.
  10. In the reinforcements exposed in pot holes, there is no sign of rusting of reinforcements. From these, it can be concluded that the concrete is without any effect of aggressive salt or chemicals which can start corrosion of reinforcements or deteriorate concrete.
  11. At many locations where depression and cracks in wearing course are noticed, there are reports of water leakage through deck slab which indicate formation/development of cracks below wearing course.
  12. There is no sign of deterioration of concrete in any other parts of the flyover.
  13. 87 mm thick wearing course, consisting of 25mm thick Asphaltic Concrete, 50mm thick Bituminous Macadam and 12mm thick Mastic Asphalt is provided over the deck slab. Except where, the wearing course is damaged due to formation of pot holes and disintegration of deck concrete has taken place, the wearing surface is fairly smooth without much of undulations.
  14. Change in the longitudinal profile of the box girders, could not be observed since no data from strain gauges or deflectometer was available and as such the deck slabs cannot be considered to be performing satisfactorily as per design under global condition, but have been under progressive deterioration, and these slabs are proving to be inadequate under the action of local wheel load with short fatigue life.
The discussion, in absence of detailed information/data cited at the beginning of this para would not lead to any exact recommendation as to whether any major remedial measure is necessary at present or cosmetic repair would suffice.

Inference

Fatigue in Concrete Concerns Security and Stability of Flyovers
Photo 9
In absence of rigorous monitoring of the behavior of Flyover with suitable instruments, the probable sequence of chain of actions that occurred in getting set up and causes of distress taken place in the deck slab and girder can be listed as under:
  1. The concrete in thin deck slab is non-uniform both in design terms of quality and cover to reinforcements.
  2. Heavy wheel loads (In excess of the design permitted), generate severe concentrated local stressing particularly in the longitudinal distribution steel (provided on percentage basis of transverse steel) & this can develop excessive stresses under individual wheel loads. The situation can be aggravated by bigger concrete cover to reinforcements. In such a situation widening of transverse cracks both at top and soffit of deck slab cannot be ruled out.

    The plying of loads heavier than design permitted over thin slab develop higher stresses and strains at soffit of deck slab which attribute to the widening of longitudinal cracks.
  3. The fine cracks in the soffit of deck slab have occurred, as a result of not that of drying shrinkage or variation in temperature gradient but definitely due to increase in fatigue load.
  4. Repeated number of one way vehicles create higher stresses and strains when compared to the normal two way bridges and flyovers with similar type of severe load intensity (as could be).
  5. Once the crack has appeared, there are instantaneous widening and closing of the crack with every passage of heavier wheel loads. This tends to increase the crack in length and width. In this process, the transverse and longitudinal cracks join and develop a mesh pattern of cracks at the soffit of the deck. Further the transverse cracks at deck top developed due to high longitudinal moment due to heavy concentrated wheel load join the soffit crack. Once the crack passage is through rainwater starts trickling through crack passage and wash away, dislodged fine particles from the cracks cement paste aiding widening and propagation of crack in the process. Needless to mention that this process continues with the repeated passage of heavy wheel loads. With spider net (criss-cross) cracks and a few visible through cracks, signifying that the integrity of concrete in flexural behavior is almost lost and the concrete gets fully disintegrated.

    In extreme cases, it is found that portion of the deck slab with such criss-cross wide cracks deflect visibly under wheel load and in some cases spalling of concrete at the top of deck slab is seen to have commenced. Such weak spots and spalling of top concrete are reflected in the form of closely spaced cracks and small depressions in the wearing course surface. With further passage of vehicles the disintegrated concrete starts getting dislodged from the reinforcements and a through hole (pot like hole) gets formed in the deck slab. This progressive deterioration of concrete under the repeated action of heavy wheel loads is comparable to fatigue failure of concrete. The fatigue behavior in such reinforced concrete bridge deck structure is more pronounced as the dead load stress in such deck structure is very small while the fluctuations in stress level due to heavier moving wheel loads are very high and with repetition with every passage of vehicle. This fatigue failure of concrete is further accelerated by passage of rain water through the cracks.
  6. Absence of cracks or any other forms of distress in the cantilever portion and haunched portion of deck slab is indicative that those are not affected by heavier wheel loads or fatigue phenomenon.
  7. The cracks on the webs of box girders are not apparently through type and do not show any signs of increase/decrease in length and width. Apparently these cracks are not influenced by heavier and repeated vehicle loads.
  8. The transverse cracks visible at the soffit of bottom slab of box girders also do not show any signs of increase/decrease in length and width. Apparently these cracks are also not affected by heavier and repeated vehicle loads.
  9. Similar type of fatigue failures have been reported in bridges in Japan and investigated by taking deck panels from distressed bridges and tested in the laboratory by simulating repeated wheel load condition, including the effect of wetting/dying (due to rainfall). The investigation showed the fatigue failure of concrete under the action of repeated wheel and accelerated failure due to rain water passing through the deteriorated deck.
Effective rehabilitation measures, therefore, have to aim at safety to vehicle and retaining the integrity of the superstructure in general and deck slab in particular under the global action by strengthening of the deck slab against fatigue failure for improving the performance against the effects of action of local wheel load.

From the foregoing, it is evident that the problem is with the suspended span portion of deck slab where, in localised pockets progressive and extension of cracks lead to disintegration of concrete. The phenomenon is progressing with time. The minor cracks existing at the soffit of the deck slab are acceptable in RCC structure, but for those which do not give any prior indication of formation of pot holes in future.

From this, one may conclude that in this particular flyover with loads very heavy than permitted, the effect of fatigue is having a significant effect in progressive deterioration of concrete.

Rehabilitation Measures

Since distress in the flyover superstructure in the form of micro cracking of matrix of concrete is the progressive one in isolated pockets rehabilitation of the deck slab is essential. The following steps need to be implemented:
  1. Potential repair of the existing deck slab surface to restore the riding surface and integrity to its design level
  2. Local repair for strengthening of the deck slab to lengthen the fatigue life and resist the local action of repetitive wheel loads.
The decision on the method of repair as well as programming of the entire work has to be weighed on the basis of level of improvement desired and the traffic restriction that would be possible during repair work. The following steps can be considered for the deck repair.

a) Local Repair of Deck Surface

Steps to be taken:
  1. Mapping of cracks at soffit of deck slab from inside the box girder. Mark all visible cracks (cracks upto 0.1mm width). Closely spaced cracks and cracks in criss-cross pattern of width more than 0.3mm shall be marked as potential area of start of the process of disintegration of concrete which might form pot hole in future. By sounding with a hammer the soft patches and the concrete in the process of disintegration can be identified.

    While mapping of deck soffit, other cracks (0.3mm or of more width) in webs of girders, intermediate cross girders and end diaphragms shall be noted.
  2. Total removal of Asphaltic wearing course and air blowing to remove loose particles. Tar or Asphaltic material, if any, remaining after removal of Asphaltic wearing course by chipping off, shall be carefully scrapped off and cleaned.
  3. Marking deficient areas of deck slab along with cracks at top of deck. Dismantling deteriorated concrete of identified pockets in a planned manner without disturbing the existing reinforcements. Re concreting of the removed portion with M-40 grade concrete with suitable anti-shrinkage admixture. Sealing of cracks at both soffit and top of deck with polymer modified cement mortar. Injection of epoxy based compound through cracks from underside of deck slab. Rendering of bad patches of concrete and honeycombs, if any, with cement mortar (1:3) and finishing with polymer modified cement mortar. The bonding between old and new concrete/mortar shall be with polymer cement slurry. All such repair work may be possible with partial closure of traffic. The traffic, however, has to be keep at low speed and if possible closed to trucks and heavy vehicles.

b) Strengthening or shielding the deck slab

Strengthening of deck slab is considered in many ways however the following methods may be tried:
  1. Steel I-beams are placed longitudinally below the deck slab and supported on intermediate cross-girders and end diaphragms. The longitudinal I-girders will reduce the span to 1/3 of the present span in the transverse direction i.e. 1.425m (for box girder with 7m wide carriageway) and 1.825m (for box girder with 11m wide carriageway). This can reduce the transverse bending moment considerably. The negative transverse bending moment created over the new supports, are expected to be small and within the capacity of the existing section. The gap between existing concrete faces and new steel parts, have to be appropriately packed for proper functioning of the additional support.

    All steel sections required for the purpose shall have to be brought inside the box girders through the manhole in the soffit slab of the box girder and further moved forward through the openings in the intermediate cross-girders and end diaphragms. Due to limited depth inside the box the length of steel sections which can be taken inside the box would be limited. Appropriate splice jointing of sections to make the full length has to be taken up inside the box girders. All these will not only put some constraints but also appropriate strict control would be necessary for ensuring quality.

    The efficiency of the system will depend to what extent an appropriately designed and fabricated steel structure can be installed and integrated into the system and to what extent this will reduce the fatigue in concrete under such repeated heavy wheel loads. In case of doubt, this can be further enhanced by additional shielding with RCC overlay at top.
  2. Stitching of Steel strips

    Steel strips are placed below the deck slab parallel to the main steel of the deck slab to reduce the stresses in steel by considerable amount. These steel strips will be of mild steel and will be fixed with expansion anchor bolts to the deck slab in addition to epoxy bonding. The idea of providing these steel strips is that as they are effective only for the live loads, the stresses in concrete and steel due to live load are expected to reduce significantly. Consequently, the strains in concrete would be reduced and the probability of crack formation would be eliminated.

    These steel strips, however, will not increase the strength of the deck slab in the longitudinal direction. Therefore, even after providing steel strips transverse cracks can still appear in between the steel strips. With such possibility in view, the deck slabs may have to be further strengthened by additional shielding with RCC overlay at top.
  3. Shielding the deck slab in the form of RCC over lay

    The purpose of this overlay is to relieve the existing deck slab from the local action of the wheel load, while the repaired deck will perform primarily the global function of the box section. Integrating the overlay with the existing deck slab with shear connectors or relaying on the bond at the interface by providing epoxy bonding compound is not considered appropriate due to the deteriorated condition of the existing deck slab. Effective bonding can only be relied upon the cantilever portion of the deck slab, over the web portion of the box girder and over the haunched portion of the deck slab which are not cracked and have not suffered any apparent distress. In order to minimize over stressing of the box girders under global action, a thin overlay is required to be designed by in-elastic method. Such empirical design is covered in Section 7.4 of Ontario Highway Bridge Design Code. This, however, has to fulfill a few design conditions. With the basic presumption that edge stiffening provided by integrating the overlay along the edges (carriageway end curbs/crash barriers and effective epoxy bonding between the deck and overlay in cantilever portion of deck slab, top portion of webs of box and haunched portion of deck slab) is effective, a thin overlay can be designed to carry the local action of wheel load independent of the existing deck slab. As this overlay would be supported by the existing deck slab, for any non-uniformity in construction of the overlay slab, the existing deck slab will provide the local support. The load transmitted to the repaired deck slab would be much less firstly due to independent behavior of the overlay slab and partly due to dispersal of the load in a much bigger area creating very small moments and stresses in the existing deck slab. Keeping these in view, the thin RCC overlays have to be designed meeting the design conditions of Ontario Code as much as possible and by extrapolation where necessary. The existing clear gap between inner faces of the webs of the boxes with 7.0 m and 11.0 m wide carriageways are 4.3m and 5.5m respectively. Considering the span of the slab as between centre of haunches, the spans can be assumed as 3.70 m and 4.30 m for 7.00 m and 11.00 m wide carriage-ways respectively. The Clause 7.4 of Ontario Code generally limits the span of slab as 3.70 m while the charts are upto 4.50m. Accordingly, 150mm and 200mm thick M40 concrete overlays with 0.6% orthotropic reinforcements are proposed for box girders of 7.00m and 11.00m wide carriageways respectively. Additional 10mm thickness of concrete at top is proposed to be provided as integral wearing course. Alternately, a new wearing course of 40-50mm thickness with ACC Marg can be provided over the overlay. This additional load of RCC overlays with additional wearing course will marginally increase (About 11%) the stresses in concrete and steel of box girders. Considering the increase in allowable stresses indicated in IRC SP-37 this marginal increase in allowable stresses may be considered as not significant and can be allowed.
ii) In case the provisions of Ontario Code has to be complied with to a greater extent, then the effective spans have to be reduced significantly by providing support along the centre as indicated in Figure 2 in sub-item (i) above. Such steel supports have to be provided from inside the box section and thus it can be taken up without disturbing the traffic. Therefore, in this rehabilitation method both RCC overlay and steel frame support from inside the box have to be included.

Considering, the field constraints along with relative ease and economy of construction between different alternatives, the following method of strengthening/shielding of the existing deck slab is recommended.
  1. After completion of steps (i) through (iii) in (a) Local Repair of Deck Slab, the work of RCC overlay has to be taken up. The thickness of RCC overlay would be 160mm and 210mm for 7.00m and 11.00m wide carriageways respectively. The grade of concrete of overlay shall be M40 and reinforcements shall be 0.6% isotropic type (reinforcement of 0.6% of cross-sectional area shall be in both direction in both top and bottom).
  2. Before placement of new concrete over the existing deck slab, the deck top surface shall be thoroughly cleaned of all loose particles, sticking bitumen, oil, grease, paints, etc. The surface may be roughened by minor chipping for ensuring better interface friction between deck and overlay concrete. The reinforcements shall be placed continuous with staggered lapping of reinforcements with appropriate cover to reinforcements. Immediately before placing of fresh concrete of overlay, the existing deck top shall be coated with appropriate epoxy bonding compound. The concrete shall be laid while the epoxy bonding compound is still tacky. The concrete shall be properly vibrated and compacted to the specified thickness and slope and with rough finish at top. The concrete shall be cured by covering with wet hessian cloth and regular sprinkling of water and depending on type of cement used curing shall start early to avoid plastic shrinkage cracks.
Over the concrete overlay with integral wearing course the light vehicular traffic can be allowed 14 days after concreting and heavy commercial vehicles only after 28 days of concreting. In case separate wearing course is placed over the overlay concrete, then the same may be started 14 days after concreting of the overlay. Needless to mention that before any start of vehicles over the overlay concrete, the expansion joints have to be re fixed or new expansion joints installed in line and level of the new wearing surface and traffic allowed only after the jointing concrete gains adequate strength.
  1. After the traffic is allowed over the repaired spans of the flyover, the performance of the repair work shall be monitored on regular basis. The top of overlay shall be checked for appearance of cracks and local depression, if any. The survey from the top may be taken up on half yearly basis. The soffit of the deck shall also be inspected on a yearly basis for appearance of cracks, seepage of water or any other forms of distress. Cracks not widening or extending in time can be sealed with polymer modified mortar. If the deck soffit is showing any sign of extending and widening of cracks, those cracks, in addition to sealing, will need further grouting with epoxy based material. Inspite of this sealing and grouting, in the unlikely event of new cracks still start appearing then the deck slab has to be considered for further strengthening by introduction of steel girder supports. This strengthening, however, can be taken up from inside the box and without disturbing the traffic.
  2. The expansion joints have to be installed at the new deck level (after laying of overlay and wearing course). The existing slab seal type expansion joints are partly damaged and might further get damaged in removal and reinstallation. Considering the limited life of the existing expansion joints, it may be prudent to install new expansion joints of strip seal type. These new joints shall be placed after re-concreting as per requirement of the supplier of new expansion joints.
  3. Miscellaneous repair of longitudinal joints between two adjacent superstructures shall be re-concreted along with overlay concrete and wearing course at top. Such joints shall have angle iron sections anchored along edges and appropriate drip course below. The gap between edges shall be maintained as 20mm without any continuity between adjacent spans. Other minor repairs of damaged or defective components (Say non functioning drainage spouts etc) noticed, if any, shall be repaired and reinstalled.
Based on the above proposed rehabilitation measures a schedule of items of rehabilitation is prepared.

Conclusion

The deck slabs of RCC box girders in the suspended portion are seen to be affected by progressive deterioration due to repeated passage of heavy wheel loads. This phenomenon which can be compared with fatigue in concrete deck structures in bridges of RCC superstructures are being noticed in many bridges in India and abroad. This progressive deterioration of concrete with final visible manifestation in the form of formation of through holes (pot holes) are more common in bridges where the traffic is high in terms of volume and heavier wheel loads. As per conventional methods of analysis (statical analysis), these bridge decks are capable of carrying those heavier wheel loads, but the present IRC codes and other International codes are not appropriately giving guidance on how to take care the effects of repeated wheel loads. Research works throughout the world are in progress to establish the exact source of the problem and for possible solution to the problem. The repair and rehabilitation already carried out in the country, in similar distressed slab with additional RCC overlay designed on the basis of empirical method of Ontario Code has shown some initial promise of trouble free performance in future. Accordingly, for rehabilitation of this flyover, in addition to repair of the distressed deck slabs, an RCC overlay is proposed over the existing deck slab. The increase in stress level due to this additional concrete overlay is minor to affect the overall performance of the superstructure. Once executed with the requisite quality control, this will eliminate progressive deterioration of concrete even with repeated passage of heavy wheel loads. In such a distressed and repaired bridge, irrespective of the method of repair, it would be essential that for assessment of durability and long time performance of the flyover, the monitoring and maintenance inspection is taken up on regular basis. The inspection can identify progressive deterioration, if any, and can set the course of all future maintenance repair work necessary for ensuring durability and trouble free service life of the flyover.

Acknowledgment

Author is grateful to the officers of PWD and Consultants who provided the details of site information.

References

  • Okada, K. Okamura H. and Sonoda K., Fatigue failure mechanism of reinforced concrete bridge deck slabs, Transp. Res. Rec. No.664, 1978, pp. 136-144.
  • ACI Committee 215, Considerations for design of concrete structure subjected to fatigue loading, J. Am. Conc. Inst., Proc., Vol. 71, No.3, March 1974, pp. 97-121.
  • Lenschow R., Fatigue of concrete structures, Proc. Int. Ass. Bridge Struct. Eng. colloq, Lausanne, 1982, IABSE Rep., Vol. 37, pp 15-24.
  • Shah, S. P. and Chandra, S., Critical stress, volume change and micro-cracking of concrete, J. Am. Conccr. Inst... Proc., Vol 65 No.9 Sept 1968 pp. 770-781.
  • Shah, S. P. and Chandra. S., Fracture of concrete subjected to cyclic and sustained loading, J. Am. Concr. Inst., Proc., VoI. 67, No.10, Oct. 1970, pp. 816-824.
  • Holmen, J. 0., Fatigue of concrete by constant and variable amplitude loading, Recent research in fatigue of concrete structures, American Concrete Institute, SP-75, 1982, pp. 72-110.
  • Oplc, F. S. and Hulsobs, C. L., Probable fatigue life of plain concrete with stress gradient, J. Am. Concr. Inst., Proc., Vol. 63. No.1, Jan. 1966, pp. 59-81.
  • Nordby, G. M., A review of research-fatigue of concrete, J. Am. Concr. Inst., Proc., Vol. 55. No. 2, Aug. 1958, pp. 191-220
  • Hilsdorf, H. K., and Kesler, C. E., Fatigue strength of concrete under varying flexural stresses, J. Am. Concr. Inst., Proc. VoI 63. No.10, Oct. 1966. pp. 1069-1076.
  • Miner, M. A., Cumulative damage in fatigue Trans. Am. Soc. Mech. Eng., Vol. 67, 1945, pp A159-164
  • American Association of State Highway and Transportation Officials, Standard specifications for highway bridges, 12th Edn. 1977, Washington. DC. 496 pp.
  • Highway Research Board, AASHTO road test, Report 4, bridge research, Special Report 61D, Publication No.953, National Academy of Sciences. National Research Council, Washington. DC, 1962, 217 pp.
  • Suryawanshi C S (Dr) 1995: Rehabilitation of Structures in Bombay: Ph.D. Thesis submitted to University of Amravati (Maharashtra), India.

NBMCW October 2010

.....

Mix Design for Pumped Concrete with PPC...

A simple method of concrete mix design for pumpable concrete based on an estimated weight of the concrete per unit volume is presented in the article. The Tables and Figures included are worked out by the author from a wide range of materials available in the country. The method is suitable for normal weight concrete and with admixtures.

Kaushal Kishore, Materials Engineer, Roorkee, Uttaranchal

Pumped Concrete

Pumped concrete may be defined as concrete that is conveyed under pressure through either rigid pipe or flexible hose and discharged directly into the desired location. Pumping may be used for most all concrete construction, but is especially useful where space or access for construction equipment is limited.

Pumping equipment consists of pumps which are of three types:
  1. Piston type concrete pump
  2. Pneumatic type concrete pump
  3. Squeeze pressure type concrete pump
Other accessories are rigid pipelines, flexible hose and couplings etc.

A pumpable concrete, like conventional concrete mixes, requires good quality control, i.e. properly graded uniform aggregates, materials uniformly and consistently batched and mixed thoroughly. Depending on the equipment, pumping rates may vary from 8 to 130 m3 of concrete per hour. Effective pumping range varies from 90 to 400 meters horizontally, or 30 to 100 meters vertically. Cases have been documented in which concrete has successfully been pumped horizontally upto 1400 meters and 430 meters vertically upward. New record values continued to be reported.

Pumping

For the successful pumping of concrete through a pipeline, it is essential that the pressure in the pipeline is transmitted through the concrete via the water in the mix and not via the aggregate; in effect, this ensures the pipeline is lubricated. If pressure is applied via the aggregate, it is highly likely that the aggregate particles will compact together and against the inside surface of the pipe to form a blockage; the force required to move concrete under these conditions is several hundred times that required for a lubricated mix.

If, however, pressure is to be applied via the water, then it is important that the water is not blown through the solid constituents of the mix; experience shows that water is relatively easily pushed through particles larger than about 600 microns in diameter and is substantially held by particles smaller than this.

In the same way, the mixture of cement, water and very fine aggregate particles should not be blown through the voids in the coarse aggregate. This can be achieved by ensuring that the aggregate grading does not have a complete absence of material in two consecutive sieve sizes – for example, between 10 mm and 2.36 mm. In effect any size of particle must act as a filter to prevent excessive movement of the next smaller size of material.

Basic Considerations

Cement Content

Concretes without admixtures and of high cement content, (over about 460 kg/m3) are liable to prove difficult to pump, because of high friction between the concrete and the pipeline. Cement contents below 270 to 320 kg/m3 depending upon the proportion of the aggregate may also prove difficult to pump because of segregation within the pipe line.

Mix Design for Pumped Concrete

Workability

The workability of pumped concrete in general has an average slump of between 50 mm and 100 mm. A concrete of less than 50 mm slump is impractical for pumping, and slump above 125 mm should be avoided. In mixtures with high slump, the aggregate will segregate from the mortar and paste and may cause blocking in the pump lines.

The mixing water requirements vary for different maximum sizes and type of aggregates. The approximate quantity of water for a slump of 85 mm and 100 mm is given in Table 1. In high strength concrete due to lower water/cement ratio and high cement content, workability is reduced with the given quantity of water per cu.m. of concrete. In such cases, water reducing admixtures are useful. In the addition to this type of admixtures at normal dosage levels, to obtain a higher workability for a given concrete mix, there is no necessity to make any alteration to the mix design from that produced for the concrete of the initial lower slump. There is generally no loss of cohesion or excess bleeding even when the hydroxycarboxylic acid baset materials are used.

If this class of product is used to decrease the water/cement ratio, again no change in mix design will be required, although small alterations in plastic and hardened density will be apparent and should be used in any yield calculations.

A loss of slump during pumping is normal and should be taken into consideration when proportioning the concrete mixes. A slump loss of 25 mm per 300 meters of conduit length is not unusual, the amount depending upon ambient temperature, length of line, pressure used to move the concrete, moisture content of aggregates at the time of mixing, truck-haulage distance, whether mix is kept agitated during haulage etc. The loss is greater for hose than for pipe, and is sometimes as high as 20 mm per 30 meter.

Aggregates

The maximum size of crushed aggregate is limited to one-third of the smallest inside diameter of the hose or pipe based on simple geometry of cubical shape aggregates. For uncrushed (rounded) aggregates, the maximum size should be limited to 40% of the pipe or hose diameter.

The shape of the coarse aggregate, whether crushed or uncrushed has an influence on the mix proportions, although both shapes can be pumped satisfactorily. The crushed pieces have a larger surface area per unit volume as compared to uncrushed pieces and thus require relatively more mortar to coat the surface. Coarse aggregate of a very bad particle shape should be avoided.

Mix Design for Pumped Concrete
Difficulties with pump have often been experienced when too large a proportion of coarse aggregate is used in an attempt to achieve economy by reducing the amount of cement; such mixes are also more difficult and costly to finish. The grading of coarse aggregate should be as per IS : 383-1970. If they are nominal single sized then 10 mm and 20 mm shall be combined in the ratio of 1:2 to get a 20 mm graded coarse aggregate. In the same way, 10 mm, 20 mm and 40 mm aggregates shall be combined in the ratio of 1:1.5:3 to get a 40 mm graded coarse aggregate.

Fine aggregate of Zone II as per IS: 383-1970 is generally suitable for pumped concrete provided 15 to 30% sand should pass the 300 micron sieve and 5 to 10 percent should pass the 150 micron sieve. Fine aggregate of grading as given in Table 2 is best for pumped concrete. The proportion of fine aggregate (sand) to be taken in the mix design is given in Table 3. However, the lowest practical sand content should be established by actual trial mixes and performance runs.

In practice, it is difficult to get fine and coarse aggregates of a particular grading. In the absence of fine aggregate of required grading they should be blended with selected sands to produce desired grading, and then combined with coarse aggregates to get a typed grading as per Table 4.

Uncrushed Aggregate (River Gravel)

It has become a custom that almost in all the construction sites crushed aggregates are being used. To save environmental pollution as far as possible in ordinary construction works uncrushed aggregates (River Gravel) including river sand should be used.

Production of crushed aggregates from crushers poses air and noise pollution problems.

Crusher & Air Pollution Problem

When the rocks and river bed boulders are crushed, dry surfaces are exposed and air borne dust can be created. An inventory of sources of dust emissions usually begins with the first crusher and continues with the conveyor transfer points to and including the succeeding crushers. Here the aggregate is more finely grounded, and dust emission become greater. As the process continues, dust emission are again prevalent from sources at conveyor transfer points and the final screens.

In the modern screening and washing plant in the production of uncrushed (gravel/shingle) aggregate from the river bed they are not crushed, thus no dust is formed. Further aggregates are washed to remove silt and clay like materials. Therefore, uncrushed (gravel/shingle) aggregates produces in these plant arrive at site in a moist condition, hence do not present a dust problem. Whereas the crushed aggregate leave crushing plant very dry and create considerable dust when handled. To prevent dust in handling it is not possible to wet each load of crushed aggregate thoroughly before it is dumped from the delivery truck. Attempts to spray the crushed aggregate as it is being dumped have had very limited effectiveness.

Mix Design for Pumped Concrete

Mix Design for Pumped Concrete

During crushing of aggregate particles less than 100 micron remains suspended in the air. The suspension of a particle in the air follows a certain trajectory depending on its size, density, shape and other physical properties. In air turbulence the dry crusher dust has long trajectories or suspension time and settling distance to the ground. If a crushing plant is not properly designed and operate without any efficient prevention system this “fugitive” dust may generate air pollution.

Mix Design for Pumped Concrete
Air quality due to pollution should be monitored monthly. The ambient air quality standards as recommended by Central Pollution Control Board (CPCB) India are given in the following table.

Note:

  1. SPm : Suspended particle matters
    SO2 : Sulpher dioxide
    CO : Carbon Mono oxide
    NOx : Nitrogen Oxide
  2. The concentrations for the above pollutants shall be 95% of the time within the limits prescribed.
  3. Category ‘C’ sensitive areas are: Hill Stations, Tourist Resorts, Sanctuaries, National Part, National Manuments etc.
  4. The air quality levels should be determined by sampling and brought down within specified ambient air quality standards through use of various mitigation measures, modern pollution free equipment and advance construction technology, such as giving preference in using uncrushed (gravel/shingle) natural aggregate in the general civil engineering construction work in place of crushed aggregate obtained from crusher.

Pumping

Before the pumping of concrete is started, the conduit should be primed by pumping a batch of mortar through the line to lubricate it. A rule of thumb is to pump 25 litres of mortar for each 15 meter length of 100 mm diameter hose, using smaller amounts for smaller sizes of hose or pipe. Dump concrete into the pump-loading chamber, pump at slow speed until concrete comes out at the end of the discharge hose, and then speed up to normal pumping speed. Once pumping has started, it should not be interrupted (if at all possible) as concrete standing idle in the line is liable to cause a plug. Of greater importance is to always ensure some concrete in the pump receiving hopper at all times during operation, which makes necessary the careful dispatching and spacing of ready-mix truck.

Testing for Pumpability: There is no recognized laboratory apparatus or precise piece of equipment available to test the pumpability of a mix in the laboratory. The pumpability of the mix therefore, should be checked at site under field conditions.

Field Practices: The pump should be as near the placing area as practicable and the entire surrounding area must have adequate bearing strength to support the concrete delivery trucks, thus assuring a continuous supply of concrete. Lines from the pump to the placing area should be laid out with a minimum of bends. For large placing areas, alternate lines should be installed for rapid connection when required.

When pumping downward 15 m or more, it is desirable to provide an air release valve at the middle of the top bend to prevent vacuum or air buildup. When pumping upward, it is desirable to have a valve near the pump to prevent the reverse flow of concrete during the fitting of clean up equipment, or when working on the pump.

Illustration example on Concrete Mix Design.

Mix Design for Pumped Concrete

Test Data for Materials

  1. The grading of fine aggregate, 10 & 20 mm aggregates are as given in Table. 5. Fine aggregate is of zone-I of IS:383-1970. 10 and 20 mm crushed aggregate grading are single sized as per IS: 383-1970.
  2. Properties of aggregates

    Mix Design for Pumped Concrete
  3. Target strength for all A, B and C mixes
    fck = fck + 1.65 x S40 + 1.65 x 5
    = 48.3 n/mm2 at 28 days age
  4. For Mix A and B free W/C ratio with crushed aggregate and given strength for target strength of 48.3 n/mm2 at 28 days from Fig. 1 Curve D found to be 0.40. This is lower than specified maximum W/C ration value of 0.45

    Note: In absence of cement strength, but cement conforming to IS Codes, assume from Fig. 1 and Fig. 2.

    Curve A and B for OPC 33 Grade
    Curve C and D for OPC 43 Grade
    Curve E and F for OPC 53 Grade
    Take curves C and D for PPC, as PPC is being manufactured in minimum of 43 Grade of strength.
  5. Other data: The Mixes are to be designed on the basis of saturated and surface dry aggregates. At the time of concreting, moisture content of site aggregates are to be determined. If it carries surface moisture this is to be deducted from the mixing water and if it is dry add in mixing water the quantity of water required for absorption. The weight of aggregates are also adjusted accordingly.
Mix Design for Pumped Concrete

Design of Mix-a with PPC

  1. Free W/C ratio for the target strength of 48.3 n/mm2 from Fig. 1 curve D found to be 0.4
  2. Free water for 100 mm slump from Table 1 for 20 mm maximum size of aggregate.

    Mix Design for Pumped Concrete

    From trials, it is found that Retarder Superplasticizer at a dosages of 18 gm/kg of cement water may be reduced 25% without loss of workability
    Then water = 200 – (200 x 0.25) = 150 kg/m3
  3. PPC = 150/0.4 = 375 kg/m3
  4. Formula for calculation of fresh concrete weight in kg/m3
    UM 10 x Ga (100 – A) + CM(1 – Ga/Gc) – WM (Ga – 1)

    Where,
    Um = Wight of fresh concrete kg/m3
    Ga = Weighted average specific gravity of combined fine and coarse aggregate bulk, SSD
    Gc = Specific gravity of cement. Determine actual value, in absence assume 3.15 for OPC and 3.00 for PPC (Fly ash based)
    A = Air content, percent. Assume entrapped air 1% for 40 mm maximum size of aggregate, 1.5% for 20 mm maximum size of aggregate and 2.5% for 10mm maximum size of aggregate. There are always entrapped air in concrete. Therefore, ignoring entrapped air value as NIL will lead the calculation of higher value of density.
    Wm = Mixing water required in kg/m3
    Cm = Cement required, kg/m3

    Note:- The exact density may be obtained by filling and fully compacting constant volume suitable metal container from the trial batches of calculated design mixes. The mix be altered with the actual obtained density of the mix.

    Um = 10 x Ga (100 – A) + Cm (1 – Ga/Gc) – Wm (Ga – 1)

    = 10 x 2.75 (100 – 1.5) + 375(1- 2.75/3.00) – 150 (2.75 -1)

    Density = 2477 kg/m3
  5. aggregates = 2477 – 375 – 150 = 1952 kg/m3
  6. Fine aggregate = From Table 3 for zone-I Fine aggregate and 20 mm maximum size of aggregate, W/C ratio = 0.4 found to be 44 – 53%. For consideration of grading of Table 4 let it be 45% Fine aggregate = 1952 x 0.45 = 878 kg/m3 Coarse aggregate = 1952 – 878 = 1074 kg/m3
    10 and 20 mm aggregate are single sized as per IS: 383-1970. Let they be combined in the ratio of 1:2
    10 mm aggregate = 358 kg/m3
    20 mm aggregate = 716 kg/m3

    With the consideration of grading of Table 5 let Fine aggregate, 10 and 20 mm aggregate combine in the ratio of 45%, 19% and 36% and check with the required grading of Table. 5.

    Thus,
    Fine aggregate = 878 kg/m3
    10 mm = 1952 x 0.19 = 371 kg/m3
    20 mm = 1952 x 0.36 = 703 kg/m3
  7. Thus for M-40 Grade of concrete quantity of materials per cu.m. of concrete on the basis of saturated and surface dry aggregates:
    Water = 150 kg/m3
    PPC = 375 kg/m3
    Fine Aggregate (sand) = 878 kg/m3
    10 mm Aggregate = 371 kg/m3
    20 mm Aggregate = 703 kg/m3
    Retarder Super Plasticizer = 6.75 kg/m3
Mix Design for Pumped Concrete

Mix-B with OPC

  1. Water = 200 – (200 x 0.25) = 150 kg/m3
  2. OPC = 150/0.4 = 375 kg/m3
  3. Density:
    10 x 2.75 (100 – 1.5) + 375 (1 – 2.75/3.15) – 150 (2.75 – 1)
    = 2495 kg/m3
  4. Total Aggregates = 2495 – 150 – 375 = 1970 kg/m3
    Fine Aggregate = 1970 x 0.45 = 887 kg/m3
    10 mm Aggregate = 1970 x 0.19 = 374 kg/m3
    20 mm Aggregate = 1970 x 0.36 = 709 kg/m3
  5. Thus for M-40 Grade of concrete with OPC per cu.m of concrete on the basis of saturated and surface dry aggregates.
    Water = 150 kg/m3
    OPC = 375 kg/m3
    Fine Aggregate (sand) = 887 kg/m3
    10 mm Aggregate = 374 kg/m3
    20 mm Aggregate = 709 kg/m3
    Retarder Super Plasticizer = 5.625 kg/m3

Mix. C with OPC + Flyash

Mix Design for Pumped Concrete
With the given set of materials increase in cementitious materials = 13%
Total cementitious materials = 375 x 1.13 = 424 kg

Note:-
  1. Specific gravity of Retarder Superplasticizer = 1.15
  2. Addition of Flyash reduces 5% of water demand.
Note:-
  1. For exact W/C ratio the water in admixture should also be taken into account.
  2. The W/C ratio of PPC and OPC is taken the same assuming that the strength properties of both are the same. If it is found that the PPC is giving the low strength then W/C ratio of PPC have to be reduced, which will increase the cement content. For getting early strength and in cold climate the W/C ratio of PPC shall also be required to be reduced.
  3. PPC reduces 5% water demand. If this is found by trial then take reduce water for calculation.
Mix Design for Pumped Concrete

Conclusion

Pumped concrete may be used for most/all concrete construction, but is especially useful where space or access for construction equipment is limited.

Although the ingredients of mixes placed by pump are the same as those placed by other methods, quality control, batching, mixing, equipment and the services of personnel with knowledge and experience are essential for successful pumped concrete.

The properties of the fine normal weight aggregates (sand) play a more prominent role in the proportioning of pumpable mixes than do those of the coarse aggregates. Sands having a fineness modulus between 2.4 and 3.0 are generally satisfactory provided that the percentage passing the 300 and 150 micron sieves meet the previously stated requirements. Zone-II sand as per IS: 383-1970 meets these requirements, and is suitable for pumped concrete.

For pumped concrete, there should be no compromise in quality. A high level of quality control for assurance of uniformity must be maintained.

A simple method of concrete mix design with normal weight aggregates and with admixtures for pumped concrete is described in the article. The author has worked out the Tables and Figures for materials available in the country by numerous trials. Therefore, the proportions worked out with the help of these Tables and Figures will have quite near approach to the mix design problems in the field.

Note:- When coarse and fine aggregate of different types are used, the free-water content is estimated by the expression,

Mix Design for Pumped Concrete

Where, Wf = Free-water content appropriate to type of fine aggregate.
and, Wc= Free-water content appropriate to type of coarse aggregate.

Note:- The above combined obtained grading is for PPC and OPC mixes. For OPC + Flyash Mix fine aggregate is about 43%, 10 mm 20% and 20 mm is 37%. This is also within the permissible limits of recommended grading for pumped concrete.

References:

  • IS : 383-1970 Specifications for coarse and fine aggregates from natural sources for concrete (second revision) BIS, New Delhi
  • IS: 456-2000 Code of practice for plain and reinforced concrete (fourth revision), BIS, New Delhi
  • IS: 9103-1999 Specification for admixtures for concrete (first revision) BIS, New Delhi
  • IS: 8112-1989 Specifications for 43 Grade ordinary portland cement (first revision) BIS, New Delhi
  • IS: 2386 (Part-III) 1963 method of test for aggregate for concrete. Specific gravity, density, voids, absorption and bulking, BIS, New Delhi
  • IS: 3812 (Part-I) 2003 Specification for pulverized fuel ash: Part-I for use as pozzolana in cement, cement mortar and concrete (second revision) BIS, New Delhi
  • IS: 1489-Part-I 1991 Specifications for portland pozzolana cement (Part-I) Flyash based. (Third revision), BIS, New Delhi
  • Kishore Kaushal, “Design of Concrete Mixes with High-Strength Ordinary Portland Cement.” The Indian Concrete Journal, April, 1978, PP. 103-104
  • Kishore Kaushal, “Concrete Mix Design.” A manual published for Structural Engineering Studies, Civil Engineering Department, University of Roorkee, 1986.
  • Kishore Kaushal, “Concrete Mix Design Based on Flexural Strength for Air-Entrained Concrete,” Proceeding of 13th Conference on our World in Concrete and Structures, 25-26, August, 1988, Singapore.
  • Kishore Kaushal, “Concrete Mix Design,” Indian Concrete Institute Bulletin September, 1988, pp. 27-40 and ICI Bulletin December, 1988, pp. 21-31.
  • Kishore Kaushal, “Method of Concrete Mix Design Based on Flexural Strength,” Proceeding of the International Conference on Road and Road Transport Problems ICORT, 12-15 December, 1988, New Delhi, pp. 296-305.
  • Kishore Kaushal, “Mix Design Based on Flexural Strength of Air-Entrained Concrete.” The Indian Concrete Journal, February, 1989, pp. 93-97.
  • Kishore Kaushal, “Concrete Mix Design,” VIII All India Builders Convention 29-31, January, 1989, Hyderabad, organized by Builders Association of India, Proceeding Volume pp. 213-260.
  • Kishore Kaushal, “Concrete Mix Design Containing Chemical Admixtures,” Journal of the National Building Organization, April, 1990, pp. 1-12.
  • Kishore Kaushal, “Concrete Mix Design for Road Bridges,” INDIAN HIGHWAYS, Vol. 19, No. 11, November, 1991, pp. 31-37
  • Kishore Kaushal, “A Concrete Design,” Indian Architect and Builder, August, 1991, pp. 54-56
  • Kishore Kaushal, “Mix Design for Pumped Concrete,” Journal of Central Board of Irrigation and Power, Vol. 49, No.2, April, 1992, pp. 81-92
  • Kishore Kaushal, “Concrete Mix Design with Fly Ash,” Indian Construction, January, 1995, pp. 16-17
  • Kishore Kaushal, “High-Strength Concrete,” Civil Engineering and Construction Review, March, 1995, pp. 57-61.
  • Kishore Kaushal, “High-Strength Concrete,” Bulletin of Indian Concrete Institute No. 51, April-June, 1995, pp. 29-31
  • Kishore Kaushal, “Mix Design of Polymer-Modified Mortars and Concrete,” New Building Materials & Construction, January, 1996, pp. 19-23.
  • Kishore Kaushal, “Concrete Mix Design Simplified,” Indian Concrete Institute Bulletin No. 56, July-September, 1996, pp. 25-30.
  • Kishore Kaushal, “Concrete Mix Design”, A Manual Published by M/S Roffe Construction Chemicals Pvt. Ltd., Mumbai, pp. 1-36
  • Kishore Kaushal, “Concrete Mix Design with Fly Ash & Superplasticizer,” ICI Bulletin No. 59, April-June 1997, pp. 29-30
  • Kishore Kaushal. “Mix Design for Pumped Concrete,” CE & CR October, 2006, pp. 44-50.

NBMCW September 2010

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Transparent Concrete Litracon™

Transparent Concrete Litracon™

Transparency is the new buzz word that attracts the attention of others whether it's the case of humanities/social context that implies openness in communication/statement or in the case of objects like glass building that is also transparent and can be seen through. Seeing the increasing trend of transparency in every sphere especially in building material sector Hungarian Architect Aron Losonczi thinks, is it possible to make concrete structure transparent? When imaginations run wild they invent ways from their own. Architect Aron Losonczi also got a result for his quest and in 2001 he had invented concrete (Litracon™) that is transparent.

Transparent Concrete Litracon™

We all know that concrete can be concocted to look like many things, but who would have thought that the rock-solid substance could be a substitute for a window or as a partition wall.

Transparent Concrete Litracon™
Patent protected internationally, Litracon™ presents the phenomenon of light transmitting concrete in the form of a widely applicable new building material. It is a combination of optical fibers and fine concrete and can be produced as prefabricated building blocks and panels. Due to the small size of the fibers, they blend into concrete becoming a component of the material like small pieces of aggregate. In this manner, the result is not only two materials - glass in concrete - mixed, but a third, new material, which is homogeneous in its inner structure and on its main surfaces as well.

The fibers lead light by points between the two sides of the blocks. Because of their parallel position, the light-information on the brighter side of such a wall appears unchanged on the darker side. The most interesting form of this phenomenon is probably the sharp display of shadows on the opposing side of the wall. Moreover, the color of the light also remains the same.

Transparent Concrete Litracon™
Thousands of optical fibers are organized into thin layers and run parallel to each other between the two main surfaces of each block. The proportion of the fibers is very small (4%) compared to the total volume of the blocks. Moreover, these fibers mingle in the concrete because of their small size, and they become a structural component as a kind of modest aggregate. Therefore, the surface of the blocks remains homogeneous concrete. In theory, a wall structure built from light-transmitting concrete can be several meters thick, because the fibers work without almost any loss in light up until 20 meters. Load-bearing structures can also be built of these blocks, since the fibers do not have a negative effect on the high compressive strength value of concrete. The blocks can be produced in various colors and sizes.

The technology of concrete (Litracon™) has received several awards like the 'Red Dot: Best of the Best 2005' Award, LEAF Award 2006, iF Material Award 2008 and was also nominated for DESIGNPREIS 2006™ award.

Recently Litracon introduced a new product Litracon Pxl, a novel and widely applicable, transparent building material. It was developed by architect Aron Losonczi. Contrary to Litracon, in Litracon Pxl there is no optical fiber for light transmission but a specially formed plastic unit. The panels can even be storey high that makes installation easier. This new building material can be applied in several fields of design and architecture e.g., illumination façade, window shade, inner partition walls or as a decoration element in interiors architecture.

MGS Architecture- March April 2010

.....

Concrete Construction and Sustainabilit...

The paper provides a detailed discussion on the design and implementation of concrete construction projects to minimize environmental impact of buildings and public works projects during the conception, construction and operation including maintenance. It will help engineers and architects appreciate and utilize concrete building systems to design high performance buildings that conserve energy and maximize occupant's comfort, thus highlighting the contributions of our profession to the sustainability. This paper discusses many aspects of sustainability and concrete industry emanating from the US experience with various professional organizations, personal experience and the prospect of using it to the sustainability in India.

In the US, there have been efforts in education in construction industry management (CIM) which offers sustainability in education, a very important task of four profession. The attributes that concrete has to offer the green building movement and the effect that emerging information on life cycle analysis has on concrete's role in this important revolution to the building industry, which is applicable and useful information in Indian scenario. This paper outlines aspects of this important and holistic research work currently being undertaken by the authors to develop handbook to fulfill the need of the industry as well as professionals and in the education for Concrete Industry Management Program, a four-year degree program currently offered at five universities in the US and can be expanded globally either through direct cooperation or long-distance learning, which is the upcoming feature and needs its application in civil engineering.

Gajanan M. Sabnis, Emeritus Professor, Howard University, Washington, DC; Consultant, Mumbai

Kenneth Derucher, Professor of Civil Engineering, California State University, Chico, USA

Kristin Cooper Carter, Director of Sustainability, Calera Corporation, Los Gatos, USA

Introduction

This paper is a special privilege to present in 2009 as a Lecture at the ICI Annual General Meeting. With the theme of the meeting being Green Structures for Sustainability, I could not resist a little diversion and take the opportunity to review "sustainability" in construction industry on a broader basis. In doing so, I want to bring the attention of the technological advances during the last decade that have affected our lives at all stages.

While sustainability has become the buzzword in our daily life, one should be glad that we are all made aware of this phenomenon for the mankind to realize its importance in our life. It is widely recognized that definitions of sustainability vary considerably based on the reference basis. These definitions and perceptions as they relate to the built-environment are important to understand the scope of the industry's commitment. On a broad basis, sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs. In essence, sustainable development is a process of change in which exploitation of resources, the direction of investments, the orientation of technological developments and institutional changes are all in harmony and enhance current and future potential to meet human needs and aspirations."[1]

There are many viewpoints on sustainability; the most perspectives emphasize socio-economic concepts associated with sustainability, along with an emphasis on holistic long-term life-cycle considerations.

Within a broad societal context sits the built-environment, the infrastructure that concrete helps to shape. According to Brandon, P.S., Lombardi, P. (2005) in their text, Evaluating Sustainable Development in the Built-environment, this environment is generally defined as "…the built-environment…and is concerned with humankind's activity in creating shelter and accommodation for itself, an act which inevitably changes the environment in some way. In particular the development of cities, and the underlying social cohesion and culture which is created through cities, has a big impact on the use of resources, the way people behave, their interaction with nature and the waste products that ensue from this type of living."

The main actors in the North American concrete industry as it has historically been thought of are heavily concentrated in the "production" category above. The place of the construction industry (and the concrete industry that is a component of it) in this big picture of sustainability has already been recognized.

There are many initiatives that concern the built-environment and infrastructure, which could be beneficially influenced by the optimum use of concrete. From this analysis it is recognized that there is an environmental abatement versus growth paradox for the industry. The new and retrofit construction proposed is likely to increase demand for concrete, compounded potentially by a greater use of concrete per structure. This reinforces the need for the industry to drive down the environmental footprint of its product, but also to work urgently to drive down the future environmental costs of maintaining and operating our infrastructure over cradle-to-cradle lifecycles.

This paper has several aspects to cover the large spectrum of sustainability, which all lead to the topic on hand. At the end I hope to draw some conclusions as to how we in ICI and as Concrete Interested Individuals achieve our goals.

Sustainability Globally Speaking

We have come a long way on sustainability around the globe. As our world has become smaller and technology has grown leaps and bound, we cannot ignore what is happening in the world and think only of one country as in the past. A little search on Internet brings out much more information than we ever did. Thus, when we consider sustainability in a broad sense globally, we must see what is taking place in various parts of the world.

The World Business Council for Sustainable Development (WBCSD) seeks to illustrate how companies work independently, or with different stakeholders, to integrate the challenge of sustainable development into their business activities. This is vary important since it brings out sustainability not just in the context of our field but in an overall responsibility of economic improvement. There are more than 100 case studies are currently available on-line. The couple of relevant studies are illustrated here to make the point.

Case I: Reducing raw material and fossil fuel use in cement production by Castle Cement, UK (part of Heidelberg Cement Group). They meet about a quarter of the demand for cement in the United Kingdom, selling more than three million tons of the products. Castle has substantially reduced its use of virgin raw materials and fossil fuels in recent years. In 2003-4, Castle used over 195,000 tons of alternate fuels to replace approximately 160,000 tons of coal in its kilns. Energy has been recovered from scrap tires, Cemfuel (processed from the residues of recycled waste solvents), and Profuel (paper and plastic wastes not viably recycled in other ways). Now, uniquely in the UK, Castle has introduced (Footnotes) a biomass fuel Agricultural Waste Derived Fuel (AWDF), which is meat and bone meal, produced by sterilizing and grinding abattoir waste. Castle continues to use alternative raw materials prolonging the life of the company's quarries. Pulverized fuel ash has been widely used, while other recycled materials include waste plaster moulds from the ceramic industry.

Case II: Sustainable products for the construction industry by SCOTASH, a joint venture (JV) between energy company Scottish Power and Lafarge Cement UK.

The company takes the ash output from Scottish Power's coal-fired power stations and re-engineers it into sustainable products for the construction industry. Environmentally friendly products from ScotAsh are used in the construction of roads, buildings, windfarms, harbors and other major projects throughout the UK, and demand for specialized materials, such as environmental binders and concrete enhancers, continues to grow. (Scottish Power's coal-fired power stations produce around 600,000 tons of ash each year. Until recently, much of this was disposed of to ash lagoons. Now, the majority of Scottish Power's ash output is recycled in Scot Ash products. Using pulverized fuel as (PFA) in cement and concrete enhances their long-term strength, durability and resistance to chemical attack. PFA is also used as a simple fill material, as a grout to repair or stabilize buildings or structures, and as a lightweight aggregate. Eventually, ScotAsh aims to recycle Scottish Power's entire ash output. With over 80% achieved in the last year, that target is now within reach.

Expertise in Civil Engineering is indispensable for realizing a sustainable society in which a comfortable living environment for humans is compatible with a high-quality natural environment. Environmental is one of the basic field, we use for planning and constructing the infrastructure for water supply, sewerage and waste treatment systems.

In Japan at Hokkaido University (HU), they established a new program "The Sustainable Metabolic System of Water and Waste for Area-Based Societies," which is commended as the world's leading research project of its kind. This program is based on the recognition that to achieve a sustainable society, an area-based society, including river basins and coastal waters, needs to serve as the basic unit of human activities. Aiming to keep living environments in an area-based society safe and comfortable and minimize the burdens that urban areas place on the natural environment, the project seeks to establish the sustainable metabolic systems of water and waste in the area-based society. To this end, we are tackling the various technological developments: Leading-edge water and wastewater treatment systems using membrane filtration, Resource recovery technology from waste material, Appropriate disposal system for solid waste, Construction material with long life, Repairing method of infrastructure to keep its long life, New management procedures of infrastructure considering reduction of the environmental/health risk, Reduced consumption of resources/energy, and demand of the people.

On the basis of the research outcomes, the program aims to produce researchers and engineers who can build up the sustainable area-based society with new infrastructure based on the concept of the holistic path and creates the whole system by integrating autonomous sub-systems. The program will establish an international center for education and research of New Socio-Environmental Engineering.

This brief global sustainability perspective takes us to civil engineering as a countrywide phenomenon and its progress and future.

Sustainability and Civil Engineering

In the US scenario, sustainability has been there all the time as everywhere else. In the last two decades though, it has caught up the attention of various professions, including politics. And, this made even more impact on our life. The writer became aware as he was in the process of constructing his own house5 from scratch and had never liked the wood or so called "stick-house," made of 2x4 in wood sections or even plywood at times. So, the "Green House" that he and his wife built in Silver Spring, MD which became them a little recognition and award(s), but it did not catch fire in the industry. It still pretty much is the same after ten years' pitch to many other folks. When he was the officer in the ASCE in 1995-98, ASCE approved the Policy No. 419 related to sustainability and has been accepted with many changes to become a very good direction for our profession. Known as Policy 419, ASCE believes that sustainable development is the challenge of meeting human needs for natural resources, industrial products, energy, food, transportation, shelter and effective waste management while conserving and protecting environmental quality and the natural resource base essential for future development. Sustainable development requires strengthening and broadening the education of engineers and finding innovative ways to achieve needed development while conserving and preserving natural resources.

Sustainability and Concrete Industry Developments

Concrete and its components and composites have been studied by various individuals and by institutes in many forms. At this point it is worth taking a broad look at the inputs and missions of various institutes, such ACI, PCI, PCA, NRMCA, PCI, etc. Sustainable development related to concrete is not just the carbon dioxide and potential for global warming, but should include the social, economic, and environmental aspects of how we use our resources.

ACI has been involved in sustainability efforts since 2000 when the Task Group on Sustainable Development was formed and ACI became an early member of the U.S. Green Building Council. ACI now has committees on sustainability both at Board level and at technical activities level. ACI hosted a workshop in the fall 2008 and spring 2009 conventions at the request of the Chair of ISO/TC 71 Subcommittee 8, Environmental Management for Concrete and Concrete (Structures, to further explore the issues relating to concrete, sustainability, and standardization and will hold another one in Fall 2009 convention.

Engineers understand the numerous social, environmental, and economic benefits of concrete. They know concrete is readily available and regionally produced; it is versatile and cost effective with a lower ecological cost; and, when designed and constructed properly, extremely durable and long lasting. So it is disappointing to see that there is constant challenge about concrete's carbon footprint and the intense energy needs to produce cement. Every material has its benefits and detriments. Every profession must be involved in sound practices of sustainable development using concrete including owners, architects, developers of rating systems and legislators. And the industry must also be proactive in mitigating the environmental shortcomings of concrete. ACI efforts are centered on providing sound and credible technical information on concrete and how it can be used to create a sustainable built environment.

ACI new strategic plan for the first time includes sustainability as one of its primary goals. The second of five goals states, "ACI will lead efforts that position concrete as sustainable and environmentally friendly." The plan calls for expanding understanding of the sustainability issue among membership, expanding resources to support sustainability issues, increasing the content on sustainability in ACI documents and products, and improving the perception of concrete relative to sustainability. ACI role in this effort is to identify issues, problems, and opportunities; to involve ACI members and the concrete industry in the development of technical information, and to inform concrete benefits to non-members from the construction industry and the public. ACI recognizes that construction with concrete is essential to developing a sustainable built environment.

Precast/Prestressed Concrete Institute (PCI) believes that precast concrete has inherent sustainable qualities and therefore the precast concrete manufacturers have a unique opportunity and obligation to participate in the sustainability movement by supporting green building practices and by continually improving their plant practices to reduce their environmental impact. As the voice of the precast concrete industry, PCI has established a sustainability committee to provide leadership through education, sharing of green building practices using precast concrete technology, and best practices in the plants. PCI oversees a certification program that ensures quality and standardizes practices for precast plants and erectors. A key initiative of PCI and its sustainability committee is the development of similar guidelines for sustainable practices, including water recycling and minimizing water use, dust and emissions control, and energy reduction.

Precast concrete contributes to Sustainability or green building practices in significant ways. The low water-cement ratios possible with precast concrete at 0.36 to 0.38 mean that it can be extremely durable. The thermal mass of concrete allows shifting of heating and cooling loads in a structure to help reduce mechanical-system requirements. Precast concrete is generally factory-made, there is little waste created in the plant (most plants employ exact-batching technologies) and it reduces construction waste and debris on site, reducing construction IAQ concerns. The load-carrying capacities, optimized cross sections, and long spans possible with precast concrete members help eliminate redundant members, and concrete readily accommodates recycled content.

Sustainability and Concrete Industry

The National Ready Mixed Concrete Association (NRMCA) strives to transform the built environment by improving the way concrete is manufactured and used in order to achieve an optimum balance among environmental, social and economic conditions. The ready mixed concrete industry is dedicated to upholding the principles of sustainable development that meets the needs of the present without compromising the ability of future generations to meet their own needs by attempting to balance social, economic and environmental impacts. Sustainability has become part of the fabric of society. Corporations in every industry are shaped by their customers' desire to be more environmentally responsible. Companies that adopt sustainable practices will become preferred suppliers. While environmental performance, including greenhouse gas emissions, will be increasingly monitored and regulated, voluntary initiatives such as the one presented here will help achieve ambitious sustainability goals.

Construction industry stakeholders including project owners, designers, contractors and product manufacturers are especially affected by the challenges of sustainable development since the built environment has significant environmental, social and economic impact on our lives and planet. On one hand, our built environment provides us with places to live and work and contributes to a robust economy and societal needs. On the other, operating our buildings, houses and infrastructure consumes enormous amounts of energy and valuable resources. Building products require natural resources and energy to produce and transport. New construction projects can burden natural habitats.

The concrete industry is uniquely positioned to meet the challenges of sustainable development. Its products help improve the overall environmental footprint of the built environment. For example, high performance concrete wall and floor systems help improve energy performance of buildings. Light colored pavements reduce urban heat islands and minimize lighting requirements. Pervious concrete pavements reduce and treat storm-water runoff. Concrete is extremely durable and provides for long service life. And the industry continues to develop new sustainable products through research and development.

The concrete industry is dedicated to continuous improvement through product and process improvements. The industry continues to increase the use of recycled materials, including industrial byproducts, thus conserving valuable natural resources and reducing process energy required to manufacture concrete. The industry continues to explore new ways to further reduce carbon footprint through the development of innovative cements and concrete mixtures. Concrete companies also strive to improve manufacturing processes, including the use of alternative energy sources, to minimize the energy of production and associated greenhouse gas emissions. Finally, the industry continues to enhance transportation efficiency and delivery methods to reduce the environmental impact of the construction process.

The NRMCA Initiative on Sustainability outlines goals for reducing the overall environmental footprint of concrete construction and provides strategies for achieving these goals. The concrete industry has been a key contributor in building this nation's infrastructure and will continue to enhance the sustainability of our built environment for generations to come.

Sustainability and Infrastructure

Highlights the shortcomings of infrastructure as it affects our lives are presented by Dr. Swamy very well. These are presented in his words: "There have been unparalleled advances during the latter half of the last century, to the scientific, engineering and social face of the world, but in that process, the world has also been plunged into several inter-related crises. In the context of the construction industry, these crises can be broadly classified in terms of environment, durability and sustainability. The crises have risen from a number of factors such as technological industrialization, population growth, world-wide urbanization, and uncontrolled pollution and creation of waste. There is now the real danger that the massive, indiscriminate and wasteful consumption of the world's material and energy resources may result in extensive global warming that is hard to reverse. The price for this environmental abuse is the rapid deterioration and destruction of the world's infrastructure, water shortages, environmental disasters, and material/structural deterioration by the forces of nature.

Every crisis experienced in the world has a direct impact on the construction industry, and since the construction industry is closely interlinked with energy, resources and environment, irredeemable environmental degradation can only be prevented by sustainable development of the industry, which alone can give hope for a better world and better Quality of Life. This paper advocates a Holistic approach to design and construction integrating all aspects from conceptual design to completion and maintenance during service life."

In the US, the ASCE Report continuously grade America's Infrastructure. They estimate a five-year total investment need of US $1.7 trillion (2007), just to put back the roads, bridges, dams, drinking water and other infrastructure systems to good serviceable life. The average state of America's infrastructure was given a Grade D - Poor. As a specific example, some 40% of more than 500,000 highway bridges are rated as structurally deficient or functionally obsolete. Some $100 billion is the estimated requirement to eliminate current backlog of bridge deficiencies, and maintain repair levels." The things have changed not much better in 2008 ASCE report and in some respects has even worsened.

Life cycle is an important aspect of sustainability in concrete industry. When designing a building to minimize environmental impact, it is important to look at the complete life cycle of the building including material acquisition, manufacturing, construction, operation, and reuse/recycling. This is called the "cradle to cradle perspective." All phases of a building or product should be considered. Simply looking at the acquisition and manufacturing phases ignores impacts during the operational phase. Reuse/recycling considers new options once the product or building has reached the end of its service life. Its successful integration with the virgin material in construction enhances the industry but also helps appreciate the material much beyond the value that it demonstrates.

Sustainability and Engineering Education

The concept of Sustainable Development attempts to balance social, economic, and environmental impacts or what is commonly referred to as the "Triple Bottom Line." As such, sustainable development attempts to integrate these perspectives on how we live and how we affect the world around us by taking into account local, regional and global impacts. These attempts led to curriculum development, which recognizes the need for sustainable concrete development that has been discussed so far.

The project of Handbook/Textbook for Sustainability and Concrete Construction has been launched by the author (s) in the US, which is in its final stage of preparation. The book was initiated as an activity to develop a text for a course in Sustainability and Concrete Industry as part of the Construction Industry Management (CIM) initiative by the concrete industry at four universities. It soon became evident that the book could best be developed with several individuals in this field of wide coverage. It should be an interesting book with viewpoints from world experts in their field to make this book as complete and as updated of the activities related to this field. Taylor and Francis have scheduled it for publication early 2010. It is anticipated that it will be a state of the art book with information and delivery system with hard copy as well as a CD Audio version of book with extensive bibliography.

Educational efforts are made by Concrete Centre in UK to offer free technical presentations in the clients' office. The Concrete Centre helps all those involved in the design and use of concrete to become knowledgeable about the products and design options available - with the minimum of effort. It is a receptacle for knowledge about the innovative ideas and products emanating from the concrete sector and is available for consultation as part of integrated supply chain teams where it sets out to help teams deliver the best solutions for clients.  As such, it embraces all of the principles set out in 'Rethinking Construction' and 'Accelerating Change' - reduction of costs, improvements in efficiency of designers and constructors, assistance with innovation and integration of the supply chain. It is funded by 15 major cement and concrete organizations and it works alongside the British Cement Association, The Concrete Society, and the ready mix and precast concrete industries to ensure an integrated approach from the concrete sector to technical support, research, education, training and information services. 

Their teams provide expert assistance, advice on concrete issues and solutions on a multitude of topics. The team consists of qualified engineers and architects, housing experts and contractors with years of experience. All are on-hand to pass on their knowledge, free of charge, provide bespoke project advice, offer support and guidance on concrete and advise on current thinking and new innovations. Their presentations are CPD-certified with approved learning outcomes. All presentations related to the issues about Sustainability are segmented into the following related areas and some details of these courses:
  1. Architecture
  2. Building Structures
  3. Civil engineering
  4. Construction
  5. Sustainability
  6. Specification
  7. Concrete technology

Sustainability

SU1 The sustainability credentials of concrete

Definition of sustainability; concrete's initiative to become industry leaders; introduction to legislation; material credentials and inherent benefits; responsible sourcing; specification options; embodied/in-use energy; life-cycle performance; achieving holistic design; current case studies; resources.

SU2 Utilization of thermal mass

Explanation of Principles and design techniques; future use in rising temperatures; passive solar design; embodied and in-use CO2 emissions; case studies.

Some Examples

There are many examples that can be cited here and were also pointed out earlier. This section focuses on the total concept of sustainability.

The Orchid is located in the heart of rapidly growing suburbs with the domestic airport virtually in its backyard. In a polluted city like Mumbai, having won the accolade of being the most eco-friendly hotel is no easy achievement. It displays a heightened level of environmental sensitivity in its revolutionary architecture, design and interior decoration. Rehabilitation of the site, which originally included an old building, provides an interesting backdrop for this leading environmental business. The Orchid is flanked by drip irrigated greenery on either side providing the much needed break from pollution. The hotel while rather ordinary on the exterior includes one of the largest array of environmental technologies ever assembled in a hotel building. Double glass doors open into an imposing 70-feet tall fiber fountain. Passive solar design including a rooftop pool to reduce heat loads on the structure is coupled with adequate day lighting via a central atrium. Triple glazed windows, an anomaly in most of Asia, provide increased thermal and acoustic insulation. The building is constructed from a variety of low resource construction materials and is finished with low-VOC paint. Guests are treated to a host of environmental products from recycled paper guest programs to herbal amenity products. A revolutionary eco-button allows guests to participate in the environmental activities by reducing their air conditioning uses. There are two special guestrooms for the physically challenged people.

Employees and management are actively engaged in the environmental operation of the hotel. Over 10,000 local students have been educated on the environmental features of the hotel. Employees have developed a CD that illustrates and educates viewers on the environmental design and performance of the hotel. And the director has spoken at numerous seminars about the environmental virtues of the project. The Orchid's commitment to environmental excellence is evident everywhere in the hotel. The property's mantra is "Deluxe need not disturb, Comfort need not compromise and Entertainment need not be insensitive."

Second case study is the township in Masdar City in Abu Dhabi covers not just green building and concrete but considerably imbibes more green concepts. It is the most ambitious sustainable development in the world; when completed it will be the world's first zero carbon, zero waste city powered entirely by renewable energy sources. This important Initiative is backed by the long-term strategic commitment from the government of Abu Dhabi to accelerate the development and deployment of future energy solutions. This allows sustainable development and living to a new level.

The City is a clean-tech cluster, which is already attracting the world's best in all areas of sustainability from renewable energy to biomass. All types of companies from innovators, incubators, research and development, pioneers and solution providers will be part of the journey to create, work and live in Masdar City. It will be built over seven years at an investment in excess of US$20 Billion. Its master plan design meshes the century-old learnings of traditional Arabic urban planning and architecture with leading-edge technologies to create a sustainable, high-quality living environment for all residents. The City will be built in seven carefully designed phases, incorporating the latest technological advances generated in its clean-tech cluster and globally. The first buildings under construction already demonstrate the innovative vision: Masdar headquarters building will receive its power required for construction from a vast PV array on its roof built ahead of the remaining structure – a world-first.

Utility services at the City will include energy, district cooling, wet utilities (water, wastewater, re-use water, and storm water), Tele-communications and waste management. Infrastructure support projects at the City will include landscaping, common areas, leisure areas, access roads, bridges, tunnels and Information and Communication Technology (ICT) services as well as development management. To accomplish its ambitious endeavor, Masdar requires access to leading edge thinkers and companies through mutually beneficial partnerships. Masdar is currently embarking on a global drive to attract industry partners to participate in this historic endeavor.

As the first major hydrocarbon-producing economy to take such a step, Abu Dhabi has established its leadership position by launching the Masdar Initiative.The Masdar Initiative driven by the Abu Dhabi Future Energy Company (Masdar), a wholly owned subsidiary of the Mubadala Development Company (Mubadala) is a global cooperative platform for the open engagement in the search for solutions to some of mankind's most pressing issues: energy security, climate change and the development of human expertise in sustainability.

Abu Dhabi is leveraging its substantial resources and experience in global energy markets into the technologies of the future. One key objective of Masdar is to position Abu Dhabi as a world-class research and development hub for new energy technologies, effectively balancing its strong position in an evolving world energy market. A related objective is to drive the commercialization and adoption of these and other technologies in sustainable energy, carbon management and water conservation. In doing so, Masdar will play a decisive role in Abu Dhabi's transition from technology consumer to technology producer.

The goal is to establish an entirely new economic sector in Abu Dhabi around these new industries, which will assist economic diversification and the development of knowledge-based industries, while enhancing Abu Dhabi's existing record of environmental stewardship and its contribution to the global community.

Conclusion

There is work done in Sustainability presently on all fronts of the concrete industry. On a realistic basis, we are only seeing the tip of the iceberg and lot more needs to be done. This paper obviously sets the tone for some accomplishments, but also indicates much more to come in this important field, which will last for longtime in the history of mankind. In India for organisation like ICI we need to learn from global experiences and do our share to become a sustainable International Concrete Institute (ICI) as a leader and the follower.

References

  • Sustainable engineering practice: an introduction by ASCE Committee on Sustainability, Jorge Vanegas, Published by ASCE Publications, 127 pages, 2004.
  • Various documents from US organizations related to concrete available in the full form from the directly.
  • http://www.aiche.org/IFS/Press/oct06newsletter.aspx
  • http://www.sustain.hokudai.ac.jp/researchers_in_network/norihito_tambo/index.php
  • The Concrete Centre, (Riverside House (4 Meadows Business Park (Station Approach (Blackwater (Camberley (GU17 9AB, Email: enquiries@ concretecentre.com
  • http://www.toodoc.com/orchid-hotel-mumbai-ebook.html
  • Appendices: Policies and Support from Various US Organizations
  • ASCE - Civil Engineering
  • https://www.asce.org/pressroom/news/policy_details.cfm
  • http://content.coprinstitute.org/files/pdf/ASCEPolicyStatementSustainableDevelopment.pdf
  • ACI – Concrete; www.concrete.org
  • NRMCA - Ready Mix Concrete Industry – www.nrmca.org
  • PCA - Cement Industry – www.pca.org
  • PCI - Prestressed and Precast Concrete Industry – www.pci.org

Acknowledgement

The article has been reproduced from the proceeding of "National Seminar on Green Structures for Sustainability" with the kind permission from the event organisers.

NBMCW August 2010

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Steel Reinforcement in GFRP Strengthene...

Corrosion Performance of Steel Reinforcement in GFRP Strengthened Concrete Cylinders

The long-term durability of FRP composites is a crucial factor in their successful application as repair materials or as reinforcement for concrete. Extensive research has been carried out on using these FRP composites for repair and strengthening, however information about their performance in environments simulating hostile service conditions and their long-term durability are only beginning to be studied. Little is known about the long-term performance of FRP composites in corrosion prevention. In the present study, a detailed experimental study was carried out to investigate the corrosion performance of embedded steel reinforcement in Glass Fibre Reinforced Polymer (GFRP) wrapped cylindrical reinforced concrete specimens subjected to an impressed current and a high salinity solution. Test variables include types of resins, Configuration of fibre mat and number of wrap layers. Samples were evaluated for corrosion activity by monitoring impressed current flow levels, and by examining reinforcement bar mass loss and concrete chloride content among samples. Test results indicated that FRP wrapped specimens had prolonged test life, decreased reinforcement mass loss, and reduced concrete chloride content .The performance of wrapped specimens was superior to that of control samples. It was concluded that GFRP wraps were able to confine concrete, slowing deterioration from cracking and spalling and inhibiting the passage of salt water.

Dr. R.Kumutha, Dean & Head, Mr.K.Vijai, Associate Professor, Department of Civil Engineering, Sethu Institute of Technology, Pulloor

Introduction

Concrete structures in an aggressive environment, such as coastal areas, marine environments and regions where deicing salts are used, are specifically prone to premature deterioration. The ingress of chlorides present in seawater, salt spray, and deicing compounds into concrete promotes reinforcement corrosion and subsequent deterioration of the entire concrete member. As reinforcement corrosion intensifies, not only the expansive products of corrosion cause failures in the concrete surrounding the reinforcement frequently evidenced by cracking and spalling of the concrete, but may also lead to a loss in the structural integrity of the reinforcing steel. When bridges and structures are built in coastal areas, corrosion related problems are especially evident.

Overall, developing innovative ways to prevent corrosion from taking place and implementing long-term solutions to repair chloride contaminated concrete are necessary endeavors. A recent solution for repairing damages due to corrosion in reinforced concrete is to use fiber reinforced plastic (FRP) composite wrap. Several works have focused on the use of FRP composites for repairing and strengthening of structures, however, information about the corrosion performance of systems using these advanced materials is still lacking. Little is known about the long-term performance of FRP composites in corrosion prevention. This research is directed towards this endeavor.

Materials Used

Ordinary locally available Portland cement was used for the casting of the specimens. The fine aggregate (sand) used is clean dry river sand and hard granite broken stones were used as coarse aggregate. Aggregates passing through 12.5mm sieve and retaining on 4.75mm sieve were used. Three concrete cubes were cast as control samples and the average standard 28 days characteristic compressive strength of concrete cubes was found out to be 32.51 N/mm2 with a mix ratio of cement: sand: gravel: water 1:1.3:3.29:0.47. Concrete cylinders were confined by wrapping them with Glass Fiber Reinforced Plastics (GFRP). Two types of GFRP sheets namely Chopped Strand Mat (CSM) having a density of 300g/m2 and Woven Roving Mat (WRM) having a density of 610g/m2 were used. Two types of resins namely General Purpose Polyester resin and epoxy resin systems were used.

Experimental Programme

Steel Reinforcement in GFRP Strengthened Concrete Cylinders
Thirty-nine test specimens (lollipop samples) were cast, consisting of 51 mm diameter, 102 mm in height concrete cylinders, in which a single 12 mm diameter steel reinforcing bar protruding 19mm(0.75in) from the top centre of the specimen. A typical wrapped specimen, together with reinforcement is shown in Fig.1. Prior to casting, reinforcing bars were cleaned with a wire brush to remove all rust from the surface. As concrete was placed in the moulds, a bar was used to consolidate the concrete by rodding ten times. The reinforcement was secured such that it protruded from the top of the mold by 19 mm thus providing a uniform concrete cover at the sides and bottom. Test specimens were allowed to cure for 24 hours before the moulds were removed. Thirty- nine concrete cylinders were then moist cured in a saturated calcium hydroxide bath at a room temperature of 24ºC for 28 days.

Procedure to Bond FRP

Cylinders were confined by wrapping them with Glass Fiber Reinforced Plastics (GFRP) with a hand lay-up procedure. The surface was first cleaned to remove any dust particles. Then the surface was applied with the mixed solution of resin, accelerator and catalyst for General purpose polyester
Steel Reinforcement in GFRP Strengthened Concrete Cylinders
and resin and hardener for epoxy resins respectively in the proportions as suggested by the manufacturer. Then the properly cut fibre mat was placed over the surface and another coating of the mix was applied. The procedure was repeated for successive layers of FRP. Then the specimens were left to dry for 10 hours at room temperature. Approximately 25mm (1in) of overlap was maintained per layer to allow confinement to develop. Samples were grouped into 13 categories, or styles, each receiving a different type of surface treatment as shown in Table1.

Test Configuration

Steel Reinforcement in GFRP Strengthened Concrete Cylinders
After test parameters were established, specimens were placed in a tank and immersed 89 mm in a 5% NaCl by weight solution, approximately double that of typical seawater, at a room temperature of approximately 24º C. In this experimental investigation, samples were connected to a single 12-volt DC power supply, which impressed a current such that the reinforcing bars are anodic. Fig.2 shows a schematic of test configuration. Samples were wired using parallel circuitry so that the gradual removal of samples would not influence the corrosion current of individual specimens. Electrodes, serving as cathodes, consisted of 25 mm wide, 3 mm thick steel bars, distributed throughout the tank bottom in such a way to ensure a consistent environment for all samples. The high salinity and the impressed current were both used to create an especially aggressive environment by providing an abundance of chloride ions and by stimulating an increased flow of electrons, respectively.

Current Monitoring

Steel Reinforcement in GFRP Strengthened Concrete Cylinders

In an attempt to characterize the impressed current, measurements were taken with an ammeter in each of the parallel electrical circuits twice a day.
Steel Reinforcement in GFRP Strengthened Concrete Cylinders
A current spike or rapid increase in current flow, indicating the short circuit, occurred after cracks had formed in a concrete that failure was imminent. Due to space limitations, current measurements are not presented in this paper, however the data is consistent with other results. For most of the sample styles, type of failure mechanism commonly encountered was excessive current with concrete failure which is a type of failure whereby the concrete cracked and a current spike was observed. Failure modes of some of the sample styles are shown in Fig. 3. Days to failure for all the sample styles are summarized in the Table 2.

Physical Techniques

Samples were visually inspected for cracks daily and removed from the tank when the concrete cracked, the wrap failed, and/or the current flow in the reinforcement spiked. During sample testing, it was noticed that, corrosion products in the form of dark green to black paste, leached out of the concrete from around the steel reinforcement. When the buildup of corrosion products at this interface zone became excessive, it was removed. After each sample was removed from the tank, the reinforcing bar was extracted from the concrete and placed in a 10% solution of Hydrochloric Acid (HCl) for a week to remove all corrosion products and remaining concrete. The acid etched all contaminants away, leaving only the steel bars. The bars, or pieces of bars, were then weighed. Table 3 presents the mass loss data for all samples that were tested until failure.

Steel Reinforcement in GFRP Strengthened Concrete Cylinders

Chloride Ingress Measurements

Chloride penetration into the concrete was established by the chloride ion concentration as obtained from a chemical analysis of concrete samples. After failed specimens had been removed from the tank, powder samples for chloride analysis were obtained by drilling a 19 mm deep hole in a perpendicular direction from the bottom end surface of each concrete specimens directly below the embedded reinforcing bar. Powder obtained from the first 6.4mm was discarded and a powder sample of at least 5 g was collected for analysis.
Steel Reinforcement in GFRP Strengthened Concrete Cylinders
Because the bottoms of the samples were in continual contact with the bottom of the tank, this location was deemed to be suitable for chloride testing. The ingress of chlorides ions into the concrete surrounding reinforced steel was used as a relative measure to compare and evaluate the effectiveness of various surface treatments. Powder samples were analyzed for their soluble chloride content using Nordtest method5 by dissolving the concrete dust in a solution of nitric acid followed by Volhard titration. Table 4 presents the chloride content for all samples.

Experimental Results and Discussion

Current Measurements
It is presumed that the electrical current applied to the reinforcement attracted negatively charged chloride ions from the solution into the concrete towards the positively charged steel bars. As the chloride ion reached the steel-concrete interface, the steel surface began to corrode. The expansive products of corrosion imposed tensile stresses on the concrete cover, and resulted in cracking when the tensile stresses became too high. Cracking especially large cracks, would allow the conductive chloride solution to come in immediate contact with the steel surface thus providing a direct current path between the reinforcement and the electrodes in solution. Therefore a current spike, or a dramatic increase in current flow, followed cracking in the concrete.

From the data and from observations made during testing, it was found that in general, the chloride ions migrated into the concrete quickly in control samples, as evidenced by the rapid onset of cracks or concrete failure and the subsequent current spikes. It was also found the samples treated with polyester generally had longer test lives and had fewer current spikes than treated with epoxy. Fig.4 shows the number of days to failure for the lollypop specimens. Increasing number of wraps from one to two proved to be effective, likely because of increased confinement; however, three wrap layers was not clearly shown to be more effective than two layers. It implies that due to their sequencing or the chronology of spiking, both polyester and epoxy type and number of wraps, affected performance.

Steel Reinforcement in GFRP Strengthened Concrete Cylinders

Reinforcing bar mass loss

In this study, the reinforcing bar mass loss ratio (percentage per day) was calculated, as it would yield a relative figure that could be used to compare the general performance of samples and treatment options. It was found that, in control samples, chloride ions are generally migrated into the concrete very quickly than the wrapped samples. The unconfined samples and wrapped specimens coated with epoxy experienced the highest mass loss ratios indicating the most severe levels of corrosion. The results from the study of the ratio of the percent mass loss per day shown in Fig.5 reveals that sample CSM-P3 and WRM-P3 showed 3.664% and 3.517% less mass loss per day respectively than control samples. In all the samples epoxy appears to allow free migration of chlorides into the concrete while polyester offers much more impervious protection.

Using Chopped strand mat with polyester resin, wrapped samples lost 0.38%, 0.46% and 0.659% less mass loss per day, for one, two, three layers respectively as compared with those of epoxy resin. This reduction in the rate of mass loss was about 40%, 55%, and 64%. The maximum reduction in the rate of mass loss was observed in CSM-P3 specimens, which was about 91% as compared with that of unconfined specimens. Using Woven roving mat with polyester resin, wrapped samples lost 0.23%, 0.26% and 0.52% less mass loss per day, for one, two, three layers respectively as compared with those of epoxy resin. This reduction in the rate of mass loss was about 24%, 32% and 50%.The maximum reduction in the rate of mass loss was observed in WRM-P3 specimens, which was about 87% as compared with that of unconfined specimens. While comparing the results of ratio of mass loss per day between both types of resins, the wrapped samples coated with general purpose polyester proves to be more effective than the wrap- ped samples coated with epoxy.

Chloride content

Results from chloride ingress measurements are shown in Fig.6. It was found that the unwrapped samples had a higher chloride concentration than the wrapped samples. It was also noticed that, samples treated with polyester performed better than those treated with epoxy. The content of chloride in unconfined sample was found to be 0.141% whereas the sample CSM-P3 performed better with a chloride content of 0.105%.
Steel Reinforcement in GFRP Strengthened Concrete Cylinders
Maximum reduction of chloride content was observed in CSM-P3 and it was about 25% as compared to unconfined samples. The effect of an additional wrap beyond the second layer did not prove to be any more effective in reducing the chloride content or rate of migration of chlorides into the concrete. As evident in the figure, the results of the concrete chloride contents provide strong evidence that the number of wraps and type of resins influence the ingress of chlorides.

Conclusion

Based on the results of this experimental investigation, the following conclusions are drawn:
  • Glass fiber-reinforced polymer wraps increase a reinforced concrete sample’s resistance to accelerated corrosion in a submerged environment as evidenced by prolonged life, decreased overall rate of reinforcement mass loss, and the reduced concrete chloride content.
  • It was also found that the samples treated with polyester generally had longer test lives and had fewer current spikes than treated with epoxy. FRP wrapping of samples increased the life span by about six fold compared with control samples.
  • Examination of the experimental data reveals that the type of resins used as a surface treatment has a significant effect on the corrosion resistance.
  • Increasing number of the wraps from one to two proves to be effective, likely because of increased confinement; however, three wrap layers is not clearly shown to be more effective than two wrap layers. This could be because the confining strength of two wraps was sufficient to restrict the expansion of the corrosion residual.
  • FRP wrapping is potentially effective in reducing corrosion in reinforced concrete structures in marine environments and this improved performance is likely due to the establishment of confining stresses in the concrete and the added resistance to the permeation of moisture and chlorides, both provided by the composite wraps.

References

  • Houssam A.Toutanji., "Durability characteristics of concrete columns confined with advanced composite materials", Composite Structures, 44, 1999, pp. 155-161.
  • Houssam Toutanji., and Yong Teng "Strength and Durability performance of concrte axially loaded members confined with AFRP composite sheets", Composites: Part B, 33, 2002, pp. 255-261.
  • Saadatmanesh, H., Eshani, MR., and Li, MW., "Strength and Ductility of concrete columns externally reinforced with fibre composite straps", ACI Structural Journal, V. 91, No. 4, 1994, pp. 434-447.
  • Isaac A Wootton., Lisa K. Spainhour., and Nur Yazdani., "Corrosion of steel reinforcement in carbon fibre-reinforced polymer wrapped concrete cylinders," Journal of Composites for Construction, November 2003, pp. 339-346.
  • NT BUILD 208: Concrete, Hardened: sampling and treatment of cores for strength tests- Chloride content by volhard titration, edition-3.

NBMCW June, 2010

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Geopolymer Concrete - A New Eco-friendl...

N. P. Rajamane, Head, Concrete Composites Lab; N Lakshmanan, Former Director and Presently Project Advisor Structural Engg Research Centre, Chennai, and Nataraja M C, Professor, SJ College of Engg, Mysore

General

The cement industry is the India’s second highest payer of Central Excise and Major contributor to GDP. With infrastructure development growing and the housing sector booming, the demand for cement is also bound to increase. However, the cement industry is extremely energy intensive. After aluminium and steel, the manufacturing of Portland cement is the most energy intensive process as it consumes 4GJ per tonne of energy. After thermal power plants and the iron and steel sector, the Indian cement industry is the third largest user of coal in the country. In 2003-04, 11,400 million kWh of power was consumed by the Indian cement industry. The cement industry comprises 130 large cement plants and more than 300 mini cement plants. The industry’s capacity at the beginning of the year 2008-09 was about 198 million tones. The cement demand in India is expected to grow at 10% annually in the medium term buoyed by housing, infrastructure and corporate capital expenditures. Considering an expected production and consumption growth of 9 to 10 percent, the demand-supply position of the cement industry is expected to improve from 2008-09 onwards.

Coal-based thermal power installations in India contribute about 65% of the total installed capacity for electricity generation. In order to meet the growing energy demand of the country, coal-based thermal power generation is expected to play a dominant role in the future as well, since coal reserves in India are expected to last for more than 100 years. The ash content of coal used by thermal power plants in India varies between 25 and 45%. However, coal with an ash content of around 40% is predominantly used in India for thermal power generation. As a consequence, a huge amount of fly ash (FA) is generated in thermal power plants, causing several disposal-related problems. In spite of initiatives taken by the government, several non-governmental organizations and research and development organizations, the total utilization of FA is only about 50%. India produces 130 million tonne of FA annually which is expected to reach 175 million tonne by 2012. Disposal of FA is a growing problem as only 15% of FA is currently used for high value addition applications like concrete and building blocks, the remainder being used for land filling. Globally, less than 25% of the total annual FA produced in the world is utilized. In the USA and China, huge quantities of FA are produced (comparable to that in India) and its reported utilization levels were about 32% and 40%, respectively, during 1995. FA has been successfully used as a mineral admixture component of Portland pozzolan blended cement for nearly 60 years. There is effective utilization of FA in making cement concretes as it extends technical advantages as well as controls the environmental pollution.

Ground granulated blast furnace slag (GGBS) is a by-product from the blast-furnaces used to make iron. GGBS is a glassy, granular, non metallic material consisting essentially of silicates and aluminates of calcium and other bases. Slag when ground to less than 45 micron from coarser, popcorn like friable structure, will have a specific surface of about 400 to 600 m2/kg (Blaine). GGBS has almost the same particle size as cement. GGBS, often blended with Portland cement as low cost filler, enhances concrete workability, density, durability and resistance to alkali-silica reaction.

Alternative but promising gainful utility of FA and GGBS in construction industry that has emerged in recent years is in the form of Geopolymer cement concretes’ (GPCCs), which by appropriate process technology utilize all classes and grades of FA and GGBS; therefore there is a great potential for reducing stockpiles of these waste materials.

Importance of Geopolymer Cement Concretes

Producing one tonne of cement requires about 2 tonnes of raw materials (shale and limestone) and releases 0.87 tonne (H” 1 tonne) of CO2, about 3 kg of Nitrogen Oxide (NOx), an air contaminant that contributes to ground level smog and 0.4 kg of PM10 (particulate matter of size 10 µm), an air borne particulate matter that is harmful to the respiratory tract when inhaled. The global release of CO2 from all sources is estimated at 23 billion tonnes a year and the Portland cement production accounts for about 7% of total CO2 emissions. The cement industry has been making significant progress in reducing CO2 emissions through improvements in process technology and enhancements in process efficiency, but further improvements are limited because CO2 production is inherent to the basic process of calcinations of limestone. Mining of limestone has impact on land-use patterns, local water regimes and ambient air quality and thus remains as one of the principal reasons for the high environmental impact of the industry. Dust emissions during cement manufacturing have long been accepted as one of the main issues facing the industry. The industry handles millions of tonnes of dry material. Even if 0.1 percent of this is lost to the atmosphere, it can cause havoc environmentally. Fugitive emissions are therefore a huge problem, compounded by the fact that there is neither an economic incentive nor regulatory pressure to prevent emissions.

The cement industry does not fit the contemporary picture of a sustainable industry because it uses raw materials and energy that are non-renewable; extracts its raw materials by mining and manufactures a product that cannot be recycled. Through waste management, by utilizing the waste by-products from thermal power plants, fertiliser units and steel factories, energy used in the production can be considerably reduced.This cuts energy bills, raw material costs as well as green house gas emissions. In the process, it can turn abundantly available wastes, such as fly ash and slag into valuable products, such as geopolymer concretes.

‘Geopolymer cement concretes’ (GPCC) are Inorganic polymer composites, which are prospective concretes with the potential to form a substantial element of an environmentally sustainable construction by replacing/supplementing the conventional concretes. GPCC have high strength, with good resistance to chloride penetration, acid attack, etc. These are commonly formed by alkali activation of industrial aluminosilicate waste materials such as FA and GGBS, and have a very small Greenhouse footprint when compared to traditional concretes.

Basics of Geopolymers

The term ‘geopolymer’ was first introduced by Davidovits in 1978 to describe a family of mineral binders with chemical composition similar to zeolites but with an amorphous microstructure. Unlike ordinary Portland/pozzolanic cements, geopolymers do not form calcium- silicate-hydrates (CSHs) for matrix formation and strength, but utilise the polycondensation of silica and alumina precursors to attain structural strength. Two main constituents of geopolymers are: source materials and alkaline liquids. The source materials on alumino-silicate should be rich in silicon (Si) and aluminium (Al). They could be by-product materials such as fly ash, silica fume, slag, rice-husk ash, red mud, etc. Geopolymers are also unique in comparison to other aluminosilicate materials (e.g. aluminosilicate gels, glasses, and zeolites). The concentration of solids in geopolymerisation is higher than in aluminosilicate gel or zeolite synthesis.

Composition of Geopolymer Cement Concrete Mixes

Following materials are generally used to produce GPCCs:
  1. Fly ash,
  2. GGBS,
  3. Fine aggregates and
  4. Coarse aggregates
  5. Catalytic liquid system (CLS): It is an alkaline activator solution (AAS) for GPCC. It is a combination of solutions of alkali silicates and hydroxides, besides distilled water. The role of AAS is to activate the geopolymeric source materials (containing Si and Al) such as fly ash and GGBS.

Formulating the GPCC Mixes

Unlike conventional cement concretes, GPCCs are a new class of materials and hence, conventional mix design approaches are applicable. The formulation of the GPCC mixtures requires systematic numerous investigations on the materials available.

Preparation of GPCC Mixes

The mixing of ingredients of GPCCs can be carried out in mixers used for conventional cement concretes – such as pan mixer, drum mixer, etc

Mechanical Properties

Compressive Strength: With proper formulation of mix ingredients, 24 hour compressive strengths of 25 to 35 MPa can be easily achieved without any need for any special curing. Such mixes can be considered as self curing. However, GPCC mixes with 28 day strengths up to about 60-70 MPa have been developed at SERC.

Modulus of Elasticity The Young’s modulus or modulus of elasticity (ME), Ec of GPCC is taken as tangent modulus measured at the stress level equal to 40 percent of the average compressive strength of concrete cylinders. The MEs of GPCCs are marginally lower than that of conventional cement concretes (CCs), at similar strength levels.

Stress Strain Curves The stress-strain relationship depends upon the ingredients of GPCCs and the curing period.

Rate of Development of Strength This is generally faster in GPCCs, as compared to CCs.

Reinforced GPCC Beams

Load carrying capacity of GPCC beams, are up to about 20% more of CC beams at similar concrete strength levels. Cracking of concrete occurs whenever the tensile strength of the concrete is exceeded. The cracking in reinforced concrete is attributable to various causes such as flexural tensile stresses, diagonal tension, lateral tensile strains, etc. The cracking moment increases as the compressive strength increases in both GPCC and CC beams.

Reinforced concrete structures are generally analyzed by the conventional elastic theory (Clause 22.1 of IS 456:2000) which is equivalent to assuming a linear moment-curvature relationship for flexural members. However, in actual behaviour of beams, non-linear moment curvature relationship is considered. The moment-curvature relation can be idealized to consist of three straight lines with different slopes. The slopes of these line changes as the behaviour of the beam is changed due to increasing load. Thus each straight line indicates different phases of beam history. The moment-curvature relations of GPCCs and CCs are essentially similar.

The service load is generally considered as the load corresponding to a deflection of span/350 or maximum crack width of 0.2 mm, whichever is less. The deflections at service loads for the GPCC and CC beams are found to be almost same. Thus, at service loads, the behaviour of the GPCC and CC beams are similar.

Ductility factor of the beams is considered as the ratio of deflection at ultimate moment (äU) to the deflection at yield moment (äY). The ductility factor decreases as the tensile reinforcement increased. The ductility factor of GPCC beams could be marginally less than CC beams indicating higher stiffness of GPCC beams. The crack patterns observed for GPCC beams are similar to the CC beams.

Reinforced GPCC Columns

The concrete compressive strength and longitudinal reinforcement ratio influence the load capacity of columns. The load carrying capacity increases with the increase in concrete compressive strength and longitudinal reinforcement ratio. Crack patterns and failure modes of GPCC columns are similar to those of CC columns.

Bond Strength of GPCC with Rebars

The bond strength of GPCCs with rebars are higher compared to CC. Thus developmental length of steel bars in reinforced GPCC can be kept same, as in the case of reinforced CC. The bond strengths of GPCC and PPCC are significantly more and conservative than the design bond stress recommended in IS: 456-2000. The GPCCs possess satisfactory bond with embedded steel bars so that the conventional design process of reinforced structural components can be applied conservatively to GPCCs also.

Durability Aspects of GPCCS

The GPCC specimens have chloride permeability rating of ‘low’ to ‘very low’ as per ASTM 1202C. GPCCs offer generaly better protection to embedded steel from corrosion as compared to CC. The GPCC are found to possess very high acid resistance when tested under exposure to 2% and 10% sulphuric acids.

Concluding Remarks on GPCCS

From the test data generated at SERC, it can be concluded that GPCCs are good candidates materials of constructions from both strength and durability considerations. Geopolymer concrete shows significant potential to be a material for the future; because it is not only environmentally friendly but also possesses excellent mechanical properties. Practical recommendations on use of geopolymer concrete technology in practical applications such as precast concrete products and waste encapsulation need to be developed in Indian context.

Because of lower internal energy (almost 20% to 30 % less) and lower CO2 emission contents of ingredients of geopolymer based composites compared to those of conventional Portland cement concretes, the new composites can be considered to be more eco-friendly and hence their utility in practical applications needs to be developed and encouraged.

NBMCW December 2009

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Effect of Sulphate Attack on Self compa...

P. Kathirvel, Faculty & Research Scholar, Dept. of Civil Engg., KLN College of Engg., Madurai; M. Kapilarasan, Senior Design Engineer, E&G op, L&T, ECC Div., Chennai; M. Shahul Hameed,Faculty & Research Scholar, Dept. of Civil Engg., Sethu Institute of Technology, Kariapatti; Dr. A. S. S. Sekar, Asst. Professor, Dept. of Civil Engg., Alagappa Chettiyar College of Engg. & Tech., Karaikudi.

In Limestone and Granite Quarries, considerable amounts of limestone and quarry dust powder is being produced as by-products of stone crushers. Large Quantity of both type of powder are being collected and utilization of this by-product is a big problem from the aspects of disposal, due to environmental pollution and health hazards. These fines can efficiently be utilized as viscosity enhancers, particularly in Self–compacting Concrete (SCC) applications. Thus the Successful utilization of these powders in SCC could turn this material in to a valuable resource.

The present experimental investigation aims to study the Durability of SCC with Partial replacement of cement by Quarry and limestone (dust) Powder by (10%, 20%, and 30%) and comparing the properties like Density Variation, Compressive Strength, Water Sorptivity for 28, 60, 90 and 120 Days age with respect to control SCC.

Introduction

To achieve SCC properties, the concrete mix should contain lower volume of coarse aggregate(1). SCC requires higher powder content, lesser quantity of coarse aggregate, high range Superplasticizer and VMA to provide Stability and fluidity to the concrete mixes. The flow characteristic increases with increasing VMA. Compressive strength, flexural strength and Split tensile strength decreases with increased addition of VMA

Substitution of 10% of cement with Quarry limestone powder (QLP) improved the compressive strength of cement pastes(2), which can be accepted as a positive factor in utilization of QLP in Self–compacting paste applications. QLP can successfully be used in production stage of proper SCC mixtures. Incorporation of QLP at the same cement content generally reduced the superplasticizer requirement and improved the 28 days compressive strength of SCCs. Normal strength SCC (~30 Mpa) mix that contain approximately 300-310 kg of cement per metre cube can be successfully prepared by employing high amount of QLP. However, substitution of high amount of cement with QLP reduced the strength values.

The SCC has become widely used standard concrete rather than a special concrete(11). The new structural design and construction systems are making full use of SCC in durable and reliable concrete structures. Different testing methods to test high-flowability, resistance against segregation, and passability(6). It is difficult to develop SCC with CA content higher than 45% or lower than 15% of the total aggregate(5). SCC properties can be achieved even for a very high percentage of sand content (up to 85%). Strength and workability properties of SCC containing different combinations of admixtures improved as a dosage of superplasticizer in them increased(4). No adverse effects observed in the combination of admixtures, mainly because of using the admixtures from the same manufacturer.

Optimum water/cement ratio for producing SCC is in the range of 0.84–1.07 by volume(8). The ratios above and below this range may cause blocking or segregation of the mixture, respectively. Self-compactability test method stipulations are not universally accepted rules. Degree of toleration depends on the engineering judgment, material type and variety. Proper concrete mixtures can be produced by trial and error method. Limestone was the most common addition in most of the cases. Approximately half the cases used a viscosity-modifying agent (VMA) in addition to superplasticizer and could therefore be considered as a combined type of SCC, which are generally more robust than mixes without a VMA(7). It was found that the quarry dust could be used successfully in the production of SCC(12). However, due to its shape and particle size distribution, mixes with quarry dust required a higher dosage of superplasticizer to achieve the flow properties.

The optimum dosage of superplasticizer for cement with fillers was about 4.38% of weight of cement(13). Loss in Mass and Compressive strength of concrete cubes were found to be negligible when under marine and Sulphate attack(3).

Material Properties

Cement

Ordinary Portland cement of 53 Grade was used in this investigation. The specific gravity of cement was 3.13.

Lime Stone Powder and Quarry Dust Powder

In general, powder is referred to as materials with particle sizes less than 0.125mm. Here locally available limestone Powder with particle less than 0.125mm having a specific gravity of 2.53 was used.

Aggregate crushers produce significant amounts of crusher dust during the process of producing 20-10mm nominal size aggregates. Utilization of stone crusher dust, a waste material, has not been made in concrete. The reason for this is the greater specific surface area, excess of material smaller than 150 mm and the associated increase in water demand of concrete. However, with the availability of high range water reducers, the increased water demand of the crusher dust can be accommodated without increasing the water-cement ratio. The crusher dust collection for this entire experimental programme was done from locally available crushers. Quarry dust not only reduces the cost of construction but also the impact on environment by consuming the material generally considered as a waste product with few applications. The specific gravity of crusher dust is 2.30.

Locally available Crushed granite aggregate with a maximum nominal size of 12mm with fineness modulus of 7.48 and specific gravity as 2.78 and bulk density 1486 Kg/m3 have been used. Fine aggregate used for the study should be properly graded to give the minimum voids ratio and shall be free from deleterious materials like clay, silt content and chloride contamination. River sand is normally preferred over crushed sand since in the former particle shape is fully water worn by attrition which helps in reduction of water content of mix and also lesser resistance to pumping. Locally available sand passing through 4.75mm sieve with fineness modulus of 2.85, Bulk Density as 1720 Kg/m3 and specific gravity 2.64 which falls under grading zone II were used for the entire investigation.

Chemical Admixtures

High performance concrete superplasticizer (Conplast SP430), based on aqueous solution of lignosulphonates, organic polymer to reduce water cement ratio for a required workability.

Conplast SP 430 is a ready to use admixture that is added to the concrete at the time of batching. The maximum effect is achieved when it is added after the addition of 50 to 70% of the water and it must not be added to the dry materials. Conplast SP430 is differentiated from conventional superplasticizers in that it is based on aqueous solution of lignosulphonates, organic polymer with long lateral chains. This greatly improves cement dispersion. At the start of the mixing process the electrostatic dispersion occurs but the presence of the lateral chains, linked to the polymer backbone, generate a steric hindrance which stabilizes the cement particles capacity to separate and disperse. This mechanism provides flowable concrete with greatly reduced water demand.

Glenium Stream 2 is a viscosity modifying admixture which is used in combination with the Super Plasticizers in order to guarantee maximum efficiency. Glenium Stream 2 consists of a mixture of water-soluble polymers which is adsorbed onto the surface of the cement granules, thereby decreasing the viscosity of the water and influencing the rheological properties of the mix thereby reducing segregation.

Mix Design

A simple mix design method was proposed by Nan Su et al (2001) for the development of self– compacting concrete & same was used in this study for M30 concrete. Assuming Packing factor of 1.18, the mix proportion arrived is given in Table 1.

Table 1: Mix Design
Material Water Cementitious material Fine Aggregate Coarse Aggregate
Cement Filler
Wt. (Kg) 182.26 214.24 200 953.91 736.46
Ratio 0.44 1 2.303 1.78
Table 2: Workability Test Results
Mix ID Slump Flow (mm) T50 cm (sec) Vfunnel sec V @ 5min (sec) L box(H2/H1) U box (mm)
CS 741 1.08 6.82 2.5 0.986 08
1L 748 1.02 6.78 2.34 0.994 05
2L 716 1.24 7.10 2.66 0.917 12
3L 695 1.43 8.64 3.52 0.849 18
1Q 720 1.41 7.38 3.29 0.884 17
2Q 698 1.67 7.65 3.54 0.852 22.5
3Q 672 1.86 11.41 4.62 0.811 27

Test Methods

The investigation was carried out by varying the quarry dust and limestone content (replacement for cement). Table 1 Give the details of the workability test results for different mixes taken for the investigations.

The observations are given below:
  1. While replacing cement by limestone powder by 10% the flow properties was high when compared to control SCC and also the passing ability and filling ability also increased. For 20% and 30% it was within the permissible limit recommended by EFNARC. But it was slightly decreased when compared to control SCC and 10% lime replacement.
  2. While replacing cement by 10% quarry dust the flow properties was high when compared to control SCC. For 20 and 30% the flow properties decreased when compared to 10% quarry replacement but satisfies the acceptable limits.
Effect of Sulphate Attack on Self compacting Concrete

Workability test results were found out as significant as per the recommendations given by EFNARC. The tests were repeated for every replacement by keeping the superplasticizer, VMA, water-powder ratio constant through out the investigation. Immediately after conducting the fresh concrete tests on SCC (Fig. 1 – 4), the concrete cubes were casted for durability study (Fig 5). The tests conducted were carried out at hardened states of SCC were Compressive strength test, Density variation test and Water Sorptivity test.

Sorptivity is defined as the rate of movement of a wetting front through a porous material. The water sorptivity test involves the uni-directional absorption of water into one face of a pre-conditioned concrete sample (Fig 6). At predetermined time intervals, the sample is weighed to determine the mass of water absorbed, and the sorptivity is determined from the mass of water absorbed of time. The lower the water sorptivity index, the better is the potential durability of the concrete.

Effect of Sulphate Attack on Self compacting Concrete

Results and Discussions

Compressive Strength Vs Duration of Immersion


Figures 7 to 12 show compressive strength of the specimens kept in water and 5% of Sodium sulphate solution. Fig 1 shows the comparison of compressive strength of cubes (QD) placed in water and 5% of Sodium sulphate solution. The figure shows that the compressive strength on specimen decreases by average of 0.5% at 28 days. The decrease in compressive strength continuous as the duration of immersion increases.

Effect of Sulphate Attack on Self compacting ConcreteEffect of Sulphate Attack on Self compacting Concrete

At 90 days the compressive strength of control specimens, 1Q, 2Q, 3Q decreased by 1.7%, 1%, 1.7% and 2.3% respectively. For 120 days test results the decrease in compressive strength of control specimens, 1Q, 2Q, 3Q is by 2.2%, 2%, 2.3% and 2.5% respectively. Here the decrease in compressive strength of SCS and 3Q is high when compared to other specimens. The specimen 1Q shows good resistant to sulphate attack when compared to other specimens.

Effect of Sulphate Attack on Self compacting ConcreteEffect of Sulphate Attack on Self compacting Concrete

Fig 8 shows the comparison of compressive strength of cubes (Lime) placed in water and 5% of Sodium sulphate solution. As for the specimens immersed in Sodium sulphate solution, there is gradual decrease in compressive strength up to 120 days. The compressive strength of the specimens, SCS, 1L, 2L and 3L decreases by an average of 0.4%, 0.7% for 28 and 60 days respectively. The decrease in compressive strength of the specimens SCS, 1L, 2L and 3L at 90 days decreases by 1.7%, 0.6%, 1.3% and 2.7% respectively. By the 120 days results the SCS and 3L specimen decreases by 2.2% and 3.5% respectively. The decrease in compressive strength is high when compared to 1L and 2L specimens. The compressive strength of the specimens 1L shows slight decrease in strength by 1.5% showing good resistance to sulphate attack when compared to other specimens.

Density Variation Vs Duration of Immersion

Figs 13 to 18 show the density variation and duration of immersion in water and 5% of Sodium sulphate solution.

Effect of Sulphate Attack on Self compacting Concrete

The figures show that the density of the specimens increased by an average of 0.5% at 60 days. This might be due to the water absorption of the specimens. After 60 days the density of the specimens, immersed in 5% of Sodium sulphate solution start decreasing. This shows the beginning of the deterioration of the specimens.

Effect of Sulphate Attack on Self compacting Concrete

Fig 13 shows the density variation of the control specimens and specimens replaced with lime (10%, 20% and 30%) kept in water and 5 % Sodium sulphate solution. At 90 days of testing the density of the specimens CS, 1L, 2L and 3L decreased by 0.4%, 0.10%, 0.20% and 1% respectively. At 120 days of testing the density of the specimens CS, 1L, 2L and 3L decreased by 2%, 0.5%, 1.2% and 2.4% respectively. The specimen 1L and 2L shows slight decrease in density when compared to the other specimens.

Effect of Sulphate Attack on Self compacting Concrete

Fig 14 shows the density variation of the control specimens and specimens replaced with Quarry dust (10%, 20% and 30%) kept in water and 5 % Sodium sulphate solution. At 90 days of testing the density of the specimens SCS, 1Q, 2Q and 3Q decreased by 0.5%, 0.2%, 0.3% and 0.5% respectively. At 120 days of testing the density of the specimens SCS, 1Q, 2Q and 3Q decreased by 2%, 0.8%, 1.1% and 1.6% respectively. The specimen 1Q and 2Q shows slight decrease in density when compared to the other specimens.

Water Sorptivity Vs Duration of Immersion

Effect of Sulphate Attack on Self compacting Concrete

Figs 19 to 24 show the water sorptivity of the specimens kept in water and 5% of Sodium sulphate solution.

The figures are drawn for 28, 60, 90 and 120 days sorptivity results. Based on the results obtained it can be seen that there is negligible or no change in water sorptivity from 28 day to 60 days. After that, there is reduction in water sorptivity, which denotes the reduction in permeation of water as time goes on.

Effect of Sulphate Attack on Self compacting Concrete

From the Fig 19, the 120 days test results of the water sorptivity specimen SCS, S1L, S2L, S3L are decreased by 1.6%, 0.5%, 1.2%, and 2% respectively. At 120 days of testing the Sorptivity specimens SCS, S1Q, S2Q and S3Q decreased by 1.6%, 0.8%, 1.1% and 1.2% respectively. Though there is saturation of water, due to the deterioration cement matrix, there is reduction in water sorptivity.

Effect of Sulphate Attack on Self compacting Concrete

Conclusion

In this investigation, the self compacting properties were found to be good for cement replacement by limestone powder by 10%. As the percentage of quarry dust increases, the workability properties of SCC decreased with reduction in strength. The flow properties of all the replacements were satisfying the recommended values given by EFNARC.

Compressive strength of replacement of 10% Lime was 5 percent higher than the control specimens. Addition of limestone powder increases the sulphate resistance up to 10% which is 0.5 percent higher than that of concrete without replacement of cement by limestone. Reduction in density was 1.5 percent lesser for replacement of cement by 10% lime when compared to of concrete without replacement of cement by limestone.

The reduction in density was 1 percent lesser for replacement of cement by 10% quarry dust when compared to of concrete without replacement of cement by quarry dust. In sorptivity the reduction in density was 1 percent lesser for both replacement of cement by 10% lime and quarry dust powder. The result of the study indicated that the replacement of cement with 10% lime improved the durability of Self–compacting concrete. The loss in mass, compressive strength and sorptivity of cubes were found to be negligible under Sulphate attack. It was observed that limestone and quarry dust powder resists Sulphate attack within tolerable limits.

References

  • Shankar H.Sanni, 'Effect of Viscosity Modifying Admixtures On Self Compacting Concrete,' CE&CR JULY 2007, pp 66-71.
  • Burak Felekoglu, 'Utilisation of high volume limestone quarry wastes in concrete industry (self compacting concrete case)', Resources Conservation & Recycling (2007), pp 1-22.
  • N.Ganesan et.al, 'Durability aspects of Steel Fibre-reinforced SCC',The Indian Concrete Journal, May 2006, pp 31-37
  • K.B.Prakash & D.K.Kulkarni, 'Effects of addition of more than two chemical admixtures on concrete properties, Indian Concrete Journal, March 2006, pp 17-21.
  • Debashis Das et.al, 'Effect of maximum size and volume of course aggregate on the properties of self compacting concrete', Indian concrete journal, March 2006, pp 53-56.
  • P Kumar, 'Self Compacting Concrete: methods of testing and design', Journal of the Institution of Engineers (INDIA) vol 86, February 2006, pp 145-150.
  • P.L. Domone,' Self-compacting concrete: An analysis of 11 years of case studies', Cement & Concrete Composites 28 (2006), pp 197–208.
  • Burak Felekoglu et al, 'Effect of water/cement ratio on the fresh and hardened properties of self-compacting concrete', Building and Environment, January 2006.
  • H.J.H. Brouwers, H.J. Radix, 'Self-Compacting Concrete: Theoretical and experimental study', Cement and Concrete Research 35 (2005), pp 2116 – 2136.
  • Jagdish Vengala and R.V.Ranganath, 'Mixture proportioning procedures for SCC, Indian Concrete Journal', august 2004, pp 13-21.
  • Okamura. H, Ouchi.M, 'Self Compacting Concrete', Journal of Advanced Concrete Technology, Vol-1, April 2003, pp 5-15.
  • D.W.S.Ho et.al, 'The use of quarry dust for SCC applications', Cement and Concrete Research 32 (2002), pp 505-511.
  • Nan Su et.al, 'A Simple Mix design method for SCC', Cement and Concrete Research 31 (2001), pp 1799-1807.
  • K.Kannan, 'performance of blended cement in Aggressive environment', project report, July 2007, Department of civil engineering, Mepco schlenk Engineering College.
  • 'The European Guidelines for Self-Compacting Concrete,' May 2005.

NBMCW October 2009

.....

Concrete for Tall Residential and Compo...

Burj Dubai
Joseph P. Colaco, President, CBM Engineers, Inc. TX USA

Concrete for New Age Structures especially for tall buildings, concrete has major advantages – it is economical, fire-proof, has lower floor-to-floor heights, higher damping values and higher stiffness. All these are reasons that all the recent tall office buildings are composite (steel and concrete) structures and all the recent residential buildings are pure concrete structures.

Introduction

There are many tall residential towers planned and under construction mostly in Dubai, Mumbai, and in China. The height of these towers range from 30 to 40 stories at the lower end and at the upper end to the world’s tallest tower, the Burj Dubai, (Fig. 1), currently under construction. These tall residential towers have given rise to the need for new structural systems and the adaptation of existing structural systems from tall office towers.

Many of the tall office buildings in the world such as the Shanghai Financial Centre, Taipei 100 and Petronas Towers are all composite steel/concrete structures.

Structural Systems

So far, most of tall residential towers are being constructed in reinforced concrete. The advantages of concrete such as lower cost, faster speed of construction, ease of finishing, fire-proof characteristics and structural stiffness are well known. Moreover, concrete technology is very advanced. Because of the height of the buildings and the desirability of limiting the column sizes, concrete strengths have also increased. Currently, in Dubai, for lower columns and shear walls, C80 concrete is being used routinely and C90 and C100 concrete strengths have been considered. Normal weight concrete is used for all structural elements. In Mumbai, M70 concrete is being used. The collateral benefit of higher concrete strengths is the increase in the modulus of elasticity and earlier stripping times. Currently a 4 to 5-day cycle per typical floor is being achieved.

In Tall Residential Towers, it is desirable to have minimum reduction of outside views from the interior of the floor. One possibility is the use of interior demising walls and shear walls. Framed tubes give less than 50% (in some cases 30%) exterior glass. It was therefore necessary to evolve different systems for Tall Residential Towers

The systems that are developed are:
  • Full Width Systems – Where the concrete shear walls run through the entire width of the building with minimum openings. While these walls are possible in the apartment floors, they are generally very difficult to accommodate in lobbies and parking areas.
  • Core walls with outriggers – Core shear walls with one or two story deep outrigger beams connecting to exterior columns at discrete levels. These outriggers are most advantageous between one-third and two-third of the height.
  • Spine walls with outriggers – Walls that are placed at interior corridor walls with door openings. Outrigger beams connect to exterior columns and are placed usually at service floors.
One of the design considerations for tall apartment towers is the restriction of the motion perception in the ISO guidelines. In general for a 10-year storm the acceleration is restricted to 1.5 to 1.8% G where G is the gravity acceleration. These values are lower than those for an office tower. One year and 5 year storms are also a consideration.

A brief description of key projects will illustrate the point.

A. Burj Dubai

Fig. 1 shows the typical floor plan of the tower designed by Skidmore, Owings and Merrill. The structural system is a triangular core shear wall “buttressed” by 3 double walls along the corridors of each leg of the plan. The floor slab is a 200mm flat plate mild steel reinforced. An elevation of the tower is shown in Fig.2. Extensive wind tunnel tests were run using two laboratories. Calculations of shrinkage and creep of vertical elements were done to develop a differential axial shortening matrix. Moreover, an extensive prototype test was run to measure the deflection (short-term and long-term) for the deflection of the 200 mm. floor slab.

Burj Dubai

B. Princess Tower

This is a 102-story tall tower in the Dubai Marina. Fig. 3 shows the typical floor plan. The lateral load resisting system is a perimeter “framed tube” system. The exterior columns are closely spaced and the spandrel beams are 1.2m deep. The core wall has very little effect on the overall behavior under lateral loads except at the bottom of the building. The floor system consists of steel floor beams, 60 mm deep metal deck and 90 mm concrete slab. The main reason for the structural steel floor framing was to reduce the weight of the building because of foundation capacity restrictions. An elevation of the building is shown in Fig. 4. A wind tunnel test was done both for cladding pressures and dynamic behavior of the structure. A tuned-sloshing damper is being considered.

Princess Tower

C. Elite Tower

This is a 90-story tall tower in the Dubai Marina. A typical floor plan is shown in Fig. 5. Once again a perimeter framed tube with a core shear wall is used for lateral resistance. The core wall is primarily a vertical load resisting element in the upper floors. A cross-wall system (not shown) is used in one direction up to the 29th floor. The floor framing is a 200 mm thick concrete post-tensioned flat slab with a portion around the core wall thickened to 350mm. This system eliminates all interior columns. The architectural advantages of a column-free space are obvious. Structurally, it increases the gravity loads on the exterior columns and minimizes uplift. The elevation of the tower is show in Fig. 6. A wind tunnel test utilized a 2% damping ratio and no extra damping is required to meet ISO guidelines.

Elite Tower

D. Hircon Tower

This is a 72-story tall octagonal tower in the Dubai Marina designed by Indian Architect, Hafeez Contractor. The structural system is shown in Fig. 7 and the elevation in Fig. 8. The lateral load resisting system consists of a core shear wall in one direction and a “full-width” concrete wall in the other direction. The perimeter columns are widely spaced and a flat “band beam” 450 mm deep is used as a spandrel beam. The floor slab is a 240 mm concrete flat plate. The upper floors have internal swimming pools within the apartments. No external damping is required.

Hircon Tower

E. Palais Royale, Mumbai

Palais Royale - Mumbai

This is a 300 m. tall apartment tower currently under construction in Mumbai. One half of the floor plan is shown in Fig. 9 and the elevation is shown in Fig. 10. The building is octagonal in plan and has a central atrium. There are shear walls around the elevators. The floor system is a beam and slab system. A placement boom and MEVA formwork is also used as shown in Figs. 11 and 12.

Palais Royale - Mumbai

Composite Structural Systems for Tall Office Buildings

The modern era of Composite Systems using both steel and concrete for columns began with the work of the late Dr. Fazlur Khan (1) in 1966. He thought that steel and concrete could be combined in the vertical plane just as efficiently as they had been in composite floor beams several decades earlier. His work led to construction of the first modern composite building, the 20-story Control Data Center in Houston in 1968. From a systems standpoint, the structure was constructed as a conventional steel frame except for the use of very small steel erection columns for the exterior columns. The steel frame was built approximately 8 floors ahead of the concrete placement in exterior columns and spandrel beams. The sequence of steel erection above, and concrete columns and spandrels below, was continued until the structure was topped out. Temporary bracing had to be used in the all-steel column portion of the structure until the concrete frame was completed. The resulting structure had an exterior composite frame that carried gravity loads and all the lateral loads. Several advantages were quickly recognized:
  1. The steel structure could be built at its normal speed.
  2. The concrete encasement of the exterior columns provided structural rigidity and fireproofing.
  3. The composite structure was economical.
For a detailed treatise on Composite Design of High Rise Buildings see Ref. 2.

J. P. Morgan Chase Plaza Houston.

Currently, the tallest composite building in North America is the 75-story, 300 m. tall, J. P. Morgan Chase Plaza Houston, which utilizes a composite column and concrete spandrel on the exterior. Both materials were used to their optimum, thereby producing cost savings and an entire new structural genre’ of building construction.

Plaza Houston

Figure 13 shows the typical floor plan. The exterior structure of the building has columns placed at 10’ (3m.) on centers on four sides but on the fifth side, there is a 85’ (26m.) clear span. The exterior columns are composite using a steel erection column and cast-in-place concrete. There is also a cast-in-place concrete 5’ (1.5m) deep spandrel beam at each level (see Figure 13). The interior columns and floor framing use structural steel. The foundation of the tower is a 9’-9" (3m.) thick concrete mat, 63’ (19.2m.) below grade. The exterior composite system, called a “ruptured-tube”, is the main element that is used to carry the wind loads on the structure. Since the closely spaced tube is ruptured at 85’ (26m.) clear span front face, the healing of the rupture was required. Several alternates were considered viz. providing diagonals across the front face to complete the “tube” or providing stiff truss elements at discrete floors to tie the ends of the front face together.
Plaza Houston
These alternates were discarded for aesthetic reasons. Hence, the only option was to use the building core. A concrete shear wall is placed next to the front row of elevators and the connection between the interior shear wall and the exterior tube is by very stiff steel link beams in the plane of the floor. There is a secondary stiffness elements consisting of a steel girder that spans 85’ (26m.) and ties the two triangular concrete front piers of the building. A photograph of the building under construction is shown in Figure 14.

For this tower a maximum 7,500 psi (52 MPa) 56-day strength normal weight concrete was utilized in exterior columns, spandrel beams, and shear wall from the mat foundation up to the 7th floor, and 6,000 psi (42Mpa) was utilized for these elements from the 8th to the 30th floors.

There are several advantages of high-strength concrete and they can be summarized as follows:
  1. Additional Stiffness: One of the considerations in tall buildings is the restriction of the inter-story drift under lateral loads. This is required in order to keep architectural elements from having any distress. Almost all tall building designs are controlled by stiffness requirements and hence, the use of high-strength concrete with its high modulus of elasticity results in a lower inter-story drift for the same member sizes. On this project, nominal 7,500 (52Mpa) concrete had a modulus of elasticity of approximately 5.7 x 106 psi (39,330Mpa) which is substantially higher than that which is obtainable with 5,000 psi (35Mpa) concrete. The net result of this high E value is that the maximum deflection of the building under hurricane wind loads will not exceed 16" (41cm.) at the top of the building for wind in any direction.
  2. Damping; Extensive discussions were held with the University of Western Ontario that conducted the wind tunnel analysis for the project. Since the basic wind resisting elements are concrete members, the damping ratio was 2% whereas for an all-steel building, a 1% damping factor is generally used. This resulted in a much lower peak acceleration in the wind tunnel test. The building accelerations at the top floor of the building were in the range of 18 milli-g’s for a 10-year recurrence interval which is below the generally accepted criteria for the threshold of discomfort due to building motion.
  3. Axial Shortening: The differential movements of vertical elements in tall buildings are a critical item in the constructability of level floors (Ref. 3). In a composite frame, the problem is exacerbated by the fact that the interior columns of structural steel have only elastic axial shortening while the exterior composite columns are subjected not only to axial shortening due to stress but to shrinkage and creep. The use of high-strength concrete which has a higher modulus of elasticity reduces the axial shortening of the concrete columns. The use of limestone aggregate and the use of fly ash (which lowers cement content) reduce the shrinkage of the concrete columns. Axial shortening compensation tables were developed for the entire structure and interior steel column lengths were adjusted at 10-story increments to try to attain level floors. In the worst case, at the roof level the columns at the end of the 85’ clear span were placed 2.5 inches (6.3cm.) higher than the columns in the middle of the long faces.

References

  • Khan, F. R., “Recent Structural Systems in Steel for High-Rise Buildings, Conference of Steel in Architecture, Bethlehem Steel No. 24 – 26, 1969.
  • Viest, Colaco, Furlong, Griffis, Leon and Wyllie, “Composite Construction Design for Buildings, McGraw-Hill 1997.
  • Fintel, Mark, Ghosh, S. K., and Iyengar, Hal, “Column Shortening in Tall Structures – Prediction and Compensation,” Portland Cement Association, Skokie, Illinois, 1987.
  • Sheikh, T. M., Yura, J. A., and Jirsa, J. O. (1987), “Moment Connections Between Steel Beams and Concrete Columns, “PMFSEL Report NO. 87-4, The University of Texas, Austin, Texas.
  • ACI-ASCE Committee 352, “Recommendations for Design of Beam-Column Joints in Monolithic Reinforced Concrete Structures”, Report ACI 352 R-85, 1985.
  • ACI 318-89, “Building Code Requirements for Reinforced Concrete,” American Concrete Institute, Detroit, Michigan.

NBMCW October 2009

.....

Ferrocement Roofing Elements - Analysis...

Analysis of Trough Shaped Ferrocement Roofing Elements Using ANSYS

Ferrocement Roofing Elements Using Ansys

L.Andal, Assistant Professor, Civil Engg. Department, Velammal College of Engineering and Technology, Madurai, M.S.Palanichamy, Vice Chancellor, Tamil Nadu Open University, M.Sekar, Dean, College of Engineering Guindy, Chennai, V.Vanitha, Lecturer, Civil Engg. Department, Mepco Schlenk Engineering College, Sivakasi

Ferrocement is a composite material made up of cement matrix and reinforcement in the form of multiple layers of mesh. Ferrocement structures are flexible and strong, due to the fact that they are thin and the steel reinforcement is distributed widely throughout the mortar. Though the raw materials required for Ferrocement construction is easily available, it is not widely used in our country due to the non-availability of proper design guidelines/code books. This paper aims at the study on the behavior of Ferrocement beams using ANSYS, finite element software. Finite Element Models have been developed for the trough shaped ferrocement roofing unit. The results obtained using the finite element software is compared with the analytical results computed from the formulae available in the literature and also with the experimental results. Design charts are developed for ferrocement trough unit to serve as an aid for a prospective designer to arrive at a section to suit their requirements.

Introduction

Housing shortage is a well recognized problem in this world. As result of uncontrolled population growth, migration to urban centers and the decay of existing low cost housing units, the cumulative needs of housing of many countries are already beyond easy corrective measures. Development of new materials of construction, low cost housing and accelerated construction methods are some of the different measures that research workers and engineers have been attempting towards meeting this challenge. Studies on building costs show that roofs/floors account for as high as 20-25% of the total building cost and saving in these items will go a long way in reducing the overall cost of a building. Traditionally, either ferrocement or corrugated galvanized iron sheets or asbestos cement sheets are being used for low cost roofing. Ferrocement as a roofing component was conceived in this context. As a composite material made of wire mesh, cement and sand, ferrocement posses unique qualities of strength and serviceability. If a roof/floor system in ferrocement is evolved and adopted on a large scale, it will prove a fitting contribution to housing the millions of homeless in the country.

The following are the analytical and experimental works related to ferrocement. Andal et.al (studied the flexural strength of modified ferrocement with polymer mortar. 45 specimens were tested and the load deflection curves, first crack loads and ultimate loads were analysed to obtain the optimum percentage of polymer to be used in modified ferrocement. Expressions were derived for the plastic moment and First Cracking Moment. Ramesht, M.H.(2) compared various analytical procedures which have been developed to predict the ultimate moment of Ferrocement under flexure. Balaji Rao,(3)proposed two methods to predict the first crack and ultimate moments of Ferrocement elements and concluded that both the methods are found to give satisfactory agreement with test data. Mathews,et.al.(4) studied the analytical and experimental investigations of cracking load, ultimate load, deflection, crack spacing and crack width of hollow ferrocement roofing system. Ramachandra Murthy, D.S(6)studied the applications of Ferrocement as housing units, water tanks, grain silos, roofing components, irrigation channels, boats and marine structures. Damodar Maity, et.al (9)studied the deflection and stress behavior of nine different types of ferrocement roofing elements. From the parametric study, it was found that deflection is less in double T-type of element and more in case of trough shaped element. Principal stresses are less in the segmental shell element and more in case of inverted V–shaped element. On the basis of cost analysis it was found that the shell element is the most economical shape as a roofing element.

Moreover, the theoretical analysis of ferrocement trough shaped elements are very tedious and time consuming. So this paper deals with the analysis of ferrocement elements using ANSYS software.

Objective

The main objective of this work is to develop design charts for the Ferrocement trough units. Design aids are essential and will be of immense help to the majority of the users of ferrocement. Moreover, they will increase consideration of ferrocement as an alternative construction material in various applications. In view of this, design charts are developed using the results obtained from ANSYS for ferrocement trough shaped roof/floor system.

ANSYS (Version 8.0)

ANSYS is a general purpose Finite Element analysis software package. The software implements equations that governs the behavior of these elements and solves them all creating a comprehensive explanation of how the system acts as a whole. This type of analysis is used for solving problems which are too complex to analyze manually.

Details of Trough Element

Trough shaped section with lips as shown in Figure 1 is selected for the roofing element. The cross sectional details are given below.
Ferrocement Roofing Elements Using Ansys
  • Span =3.0m
  • Thickness of section, t =20mm
  • Depth of the section, H = 100mm
  • Angle of web with vertical = 59Ú
  • Angle of web with horizontal = 31Ú
  • Characteristic compressive
  • Strength of mortar = 40 N/mm2
  • Moment of inertia, Ig (mm4) = 5956199.0
  • Neutral axis depth Yb =47.5mm
  • Section modulus Z (mm3) = 125393.66

Analysis Using ANSYS

The following steps are involved in analyzing the trough element using the finite element software package ANSYS
  • Element selection
  • Defining material properties
  • Model creation
  • Meshing
  • Applying boundary conditions and loading
  • Analysis
  • Viewing results
The accuracy of the results depends upon the type of element used and the way in which the model is meshed. Among these steps the element selection and meshing are very important.

Elements Used

Ferrocement Roofing Elements Using Ansys
In modeling the trough unit the elements chosen were:
  • SOLID 65 for modeling the mortar layers and
  • SHELL 181 for modeling the chicken mesh and weld mesh layers.
The trough element modeled using ANSYS is shown in Figure 2

Calculation of Cracking Moment

  • Method I: (Ref 3)
    where,
    fr - modulus of rupture of cement mortar
  • Ig – Gross moment of Inertia
  • Yb – Neutral axis depth

  • Method II: (Ref 3)

    Ferrocement Roofing Elements Using Ansys

    where,
    frm - modulus of rupture ofmesh-mortar combination

    Ig – Gross moment of Inertia

    Yb – Neutral axis depth

  • Method III :( Ref 4)

    Ferrocement Roofing Elements Using Ansys

    where,
    fcr - modulus of rupture of cement mortar

    Ig – Gross moment of Inertia

    Yb – Neutral axis depth

  • Method IV :( Ref 1)

    Ferrocement Roofing Elements Using Ansys

    Mcr = a x fcu xZ

    where,

    α - Cracking Moment constant Characteristic compressive strength of mortar

    Z - Section modulus

    S = Percentage of reinforcement

    F = Percentage of flyash
Ferrocement Roofing Elements Using Ansys

Cracking Moment from ANSYS

The stress value from ANSYS is given below from which the Cracking Moment is calculated.

Stress at first visible crack = 14.740 N/mm2

Cracking Moment Mcr= 14.740 x (5956199.0 / 47.5)

= 1.857 KNm

Experimentally the Cracking Moment obtained at the first visible crack (Ref 8) was 1.075 KNm.

The comparison of Cracking Moment for 20 mm thick Ferrocement trough element by various methods with ANSYS results are given in Tables 1 to 4.

Ferrocement Roofing Elements Using Ansys

Development of Design Charts

Design charts are developed for ferrocement trough unit by varying the following parameters
  1. Thickness of the section,t: 20, 25, 30 &35 mm
  2. Characteristic compressive
    Strength of mortar, fcu : 35, 40, 45&50 N/mm2
  3. Mesh mortar parameter, x : 0.2 (4 layers of chicken mesh &1 layer of weld mesh), 0.25 (6 layers of chicken mesh &1 layer of weld mesh) & 0.30 (8 layers of chicken mesh &1 layer of weld mesh).
The calculation of mesh-mortar parameter "x" for the Ferrocement Trough Element with 4 Layers of Chicken Mesh and One Layer Weld Mesh is given below.

Ferrocement Roofing Elements Using Ansys

= 0.059 + 0.116
= 0.175
H" 0.200.

Design charts developed for ferrocement trough for various thicknesses of the specimen and the mortar strengths are given in Figures 3 to 8.

Ferrocement Roofing Elements Using Ansys
Ferrocement Roofing Elements Using Ansys
Ferrocement Roofing Elements Using Ansys

Conclusion

Based on the detailed analysis of Ferrocement trough units using ANSYS the following conclusions have been drawn:
  • The cracking moment by method 4 is found to be in good agreement with both the ANSYS and Experimental results.
  • The methods 1, 2, and 3 underestimated the cracking moment for the Ferrocement trough units.
  • From the results obtained from ANSYS design charts are developed for the ferrocement trough unit. The design charts developed in this paper will serve as an aid for the designer to arrive at a suitable section.

References

  • Andal, L. Palanichamy, M.s. Ponraj Shankar And Vaidyanathan, R.(2005) 'Behaviour of Ferrocement Flexural Member with Polymer Modified Mortar,' Proceedings of International Conference on Advances in Concrete Composite and Structures (ICACS-2005) Jan 6-8.
  • Ramesht, M.H. And Vickridge, I.g.(1996) 'Faoferrs- A Computer Program for the Analysis of Ferrocement in Flexure,' Journal of Ferrocement Vol.26, No.1, pp.21 -31.
  • Balaji Rao, K.(2002)'Estimation of Cracking and Ultimate moments and load-deflection behavior of Ferrocement Elements', National seminar of Ferrocement 18-20 Feb' 2002 @ Anna University, Chennai. pp.G1– G10.
  • Mathews, M.S. Sheela, S. Seetharaman, P.R. And Sudhakumar, J.(1991) 'Analytical and Experimental Investigations of Hollow Ferrocement Roofing Units'.Journal of ferrocement. , Vol.27; NO.1, pp.1 -14.
  • Perumal, P. And Vidhyanathan, R.(1987) 'Strength and Corrosion Resistance of Ferrocement slabs', Proceedings of National Seminar on Modern trends in building materials, Design & Construction, Institution of Engineers (India) Allahabad, Nov.13-15.
  • Dr. Ramachandra Murthy, D.S. (2002) 'Low cost Housing in Ferrocement Technology,' National seminar of Ferrocement 18-20 Feb' 2002 @ Anna University, Chennai, pp. j1 – j8.
  • Rao, P.K. (1992) 'Stress-strain Behavior of Ferrocement Elements under Compression,' Journal of Ferrocement Vol.22, No. 4, pp.343 – 352.
  • Sujatha, T. (2005) 'Flyash based Ferrocement trough shaped roofing/ flooring element for low cost housing,' M.E. Thesis.
  • Damodar Maity, Kalita, U.C. And Nazrul Imam (2002) 'An Investigation On the Shape of Ferrocement Roofing Elements,' Journal of Ferrocement Vol.32, No. 4, pp. 271-284.
  • Naaman, A.E.'Ferrocement and Laminated Cementitious Composites,' Techno Press 3000, USA, pp.1471, 143 &145.
  • ACI Committee Report No. 549 'State–of–the–Art Report on Ferrocement,' ACI-549 -97, American Concrete Institute, Detroit, Part 5, pp. 549.R-2.
  • ANSYS Help Manual (Version 8.0).

From the Editor

Structural Engineering Research centre at Roorkee/Ghaziabad carried out R&D work on ferrocement under the able guidance of its director Prof. G.S. Ramaswamy from 1968 onwards and a very large number of applications were developed and technology released for commercial use through NRDC. The Centre supported the activities of international ferrocement information centre A.I.T. Bangkok by deputing its experts for conducting Try Programme and preparation of do-it-yourselfManuals on tanks, Bins, Roofs, Boats, Bio Gas Plants etc. Large number of papers and manual were produced and the pre–patents were file. The technologies developed are in use by leading engineering department, pre–cast component Producers.

NBMCW October 2009

.....

Biofouling and Corrosion of Specialty C...

Embedded in Concrete Exposed to Natural Marine Environment

P. Venkatesan, Central Electrochemical Research Institute (CSIR), Karaikudi.

The influence of biofouling on concrete was studied by exposing reinforced concrete specimens in natural marine environment. The facilities at Offshore Platform and Marine Electrochemical Centre (OPMEC), Tuticorin were utilized for the study. The OPMEC unit is located at about two Kilometers away from seashore. The reinforced concrete specimens were exposed at atmospheric level, high tide level, and immersed in seafloor for four years. The Marine fouling on concrete including macro fouling and micro fouling were studied. The concrete specimens immersed in seafloor higher biomass of 40.4 g/dm2 (wet weight) while surface sample shows lower biomass of 40.4 g/dm2. Barnacles, Molluscs, ascidians and bryozoans were the major fouling community at, all the three level studies. Corrosion rate was high for reinforced concrete immersed in seafloor.

Corrosion of reinforcement is a major problem for the reinforced concrete structures constructed in marine as well as in industrial areas. In this research paper corrosion of mild steel plain bar reinforced concrete immersed in seafloor for four years is presented. The reinforcements are coated with three different types of corrosion protection by specialty coatings, namely cement polymer composite, interpenetrating polymer network and epoxy coating. The corrosion behavior is monitored using Open Circuit Potential (OCP) measurements periodically. After completion of four years the reinforced concrete specimens were subjected to the following measurements:- linear polarisation resistance (LPR), a.c. impedance, electrochemical noise (ECN) and weight loss technique. Based on the above measurements using Stern-Geary equation the corrosion rate was calculated. The pH measurement, chloride analyses were also carried out. The results are presented and discussed.

Introduction

Marine fouling is the visible sign of infestation by various sedentary organisms practically on all-engineering materials used for marine and offshore services. Materials submerged in the ocean follow a sequence of physical, chemical and biological changes at their surfaces, following a surface conditioning by organic matter. Microorganisms, apart from affecting corrosion by them selves, aid in the colonization of spores of algae and the larval form of macro fouling organisms (Secheer and Gunderson, 1873; Corp, 1976). The composition, extent and rate of development of a macro-fouling community at any site is determined by the interaction of many variables, notably geographic location, nutrient availability, light levels, water depth and its biota. The settlement of some species, particularly barnacle is not always random but may be encouraged by the presence of other individuals of the same species.

Marine organisms can modify the local environment of the structures, by influencing oxygen concentration cells, changing the pH and also through the production of metabolites, which create a more aggressive electrolyte. Algae are capable of raising the pH to above two units than that of the seawater and lower it to 1.8 when they decompose. The hard fouler such as barnacles are capable of reducing the pH to 1-2 beneath the base of their shell.

Fouling can enhance corrosion by bacterial action of both aerobic and anaerobic nature. The sulphur oxidizing bacteria, Thio bacillus sp. oxidise inorganic sulphate and produce sulfuric acid, which is corrosive to steel and concrete.

The corrosion of the reinforcement embedded in the concrete is the main cause of damage and early failure of reinforced concrete structures especially those located in aggressive marine and industrial environment. The reinforcement in all the structures provides a constructional security. Steel embedded in good quality concrete is protected by the high alkalinity of pore water, which in the presence of oxygen, passivates the steel. The loss of alkalinity due to carbonation of the concrete and the penetration of chloride ions to steel can destroy the passive film. It is believed that reinforced concrete structures are durable and maintenance-free for the whole of its design life, approximately more than 60 years. However, the corrosion of reinforcing steel in concrete exposed to aggressive environment affects the life of the concrete and thus has rapidly become a serious problem throughout the world. Parking structures, bridges, buildings, and other reinforced concrete structures exposed to marine and industrial environments are being severely damaged due to corrosion of reinforcing steel within periods as short as 10 to 20 years.

In marine environment, reinforcing steel corrosion is the natural result of the penetration of chloride up to reinforcement position.

As corrosion products have volume two to four times higher than that of iron, stress is created within the concrete causing irreversible damage to the reinforced concrete structure:

(i) cracking of concrete

(ii) reduction in load carrying capacity and

(iii) failure of the structure.

Understanding of the corrosion mechanism as well as the rates of deterioration demands the long-term studies. Among the nondestructive methods: electrochemical measurements (Open circuit potential measurements, corrosion rate measurement) are reliable. Duffo et al. characterised the corrosion products at the steel/concrete interface for 65 years old specimens exposed to atmospheric environment. Chitty et al. studied archaeological analogues corrosion systems taken from different buildings aged from 80 to 1700 years old. A complete characterization based on macro and microscopic methods has been performed.

The main constituents of rust formed on atmospheric corroded steel are iron oxyhydroxide, ferric oxides and magnetite.

The corrosion process of a steel reinforcement in concrete is different from those of a steel coupon exposed to atmosphere, and more similar to steel immersed in alkaline solution.

Jain et al. in addressing rust composition of steel reinforced concrete, inform that the final corrosion products are Fe3O4 and b–Fe OOH. The presence b–Fe OOH in rust was due to the presence of chloride ions in concrete.

Poupard et al. carried out detailed investigation of chloride-induced corrosion on 40 years old reinforced concrete beam exposed in natural marine environment. Visual observations, electrochemical measurements, carbonation depth, total chloride content were located using half-cell potential measurements. The real corrosion state of steel reinforcements was observed by applying destructive methods. The corrosion products and the steel/concrete interface characterized by optical, XRD, SEM, EDS and ì- Raman techniques.

Poupard et al. based on diagnosis results of the corrosion damage of a prestressed concrete beam after 40 years of exposure in marine environment suggested that Chloride ions are only responsible for corrosion attack and he distinguished two types of corrosion zones.

Organic coatings are a cost-effective way to protect metals. However, at defects or sites of damage in the coating, a local corrosion cell develops leading to coating breakdown. Alkali generated at the cathode tends to inhibit anodic attack in these areas, but reduces the adhesion between coating and metal to zero, results in debonding around the defect or blisters near the defect. The controlling factors in coating delamination have been extensively studied. These experiments show the same reversal of polarity seen previously.

On the basis of the data, Asthana concluded that the IPN-coated reinforcing bars have acceptable bond strength with concrete, and have better corrosion resistance than other commercially available treatment used for similar applications. The economics of the treatment is quite attractive since treatment costs about 15 to 20% of the cost of steel. Hence it may be said that IPN-coated reinforcing steel bars fulfill the minimum requirements laid down in various standard specifications. Asthana K.K. concludes from the studies that the IPN coated steel reinforcement bars would have a more extended life in comparison to uncoated reinforcement.

The pH may get lowered if the concrete contains chlorides, sulphates, and other deleterious chemicals. These chemicals diffuse through the concrete and lower the pH value of the water in the pores of concrete. As a result, protective oxide film is pierced by these chemicals, which will then attack the reinforcement.

Alblasand London reviewed the literature concerning the effect of chloride contamination on the corrosion of coated steel surfaces. Appleman, Helvig, Weldon and Flores showed various correlations between level of chloride and premature coating failures. These investigators applied the contaminant (chloride) in known quantities to the steel surface and applied the coating shortly thereafter. Niel and Whitehurst used chloride contamination that remained in the micropits after sand blasting of steel surface for studying FBE coating performance. They found that in the presence of a pitted surface, chloride contamination could cause serious loss of performance in FBE coatings in hot cathodic disbonding and hot water tests. For underground coatings and other immersion coatings in critical applications, a maximum chloride level of 2 ppm was suggested.

It is generally assumed for steels without surface coatings that chloride-induced corrosion results from the breakdown of the passive film. In the presence of a passive film, it is believed that the corrosion process results from the electrostatic attraction between the positively charged metal surface and the negatively charged chloride ions. It is believed that chloride ions react at areas where the passive film is discontinuous, damaged, or at heterogeneous sites on the steel surface. After initiation, the chloride ions are used as a catalyst for the liberation of iron ions, resulting in further corrosion.

The corrosion performance of steel reinforcement embedded in cementitious materials exposed to chlorides is a function of both the concrete and steel characteristics.

In this paper the corrosion behavior of mild steel reinforced plain rods and with three different types of coatings exposed to natural marine environment over a period of four years is presented.

Materials and Methods

Mild steel plain bar of diameter 16 mm and length 200 mm was used as reinforcement for the concrete. The initial weight of the mild steel rebar specimens was recorded for gravimetric studies. Electrical wires were soldered to the mild steel rods for electrochemical studies. The reinforcement concrete specimens of dimensions 150 X 150 X 300 mm reinforced with mild steel plain bars were cast. The concrete mix proportion was 1: 1.2: 2.4. Ordinary Portland cement with river sand as fine aggregate and 20 mm stones as coarse aggregate were used. The water cement ratio used was 0.45. The concrete cover provided to the reinforcement was 50 mm. Three coatings of cement polymer composite coating (CPC), Interpenetrating polymer network coating (IPN) and epoxy coating (EC) were applied for a an average thickness of 150±25mm for various reinforcements. Identical cubes without reinforcing rods were cast and tested in the universal compression-testing machine for obtaining compressive strength. The compressive strength is 40 Mpa.

Corrosion rate of reinforcements embedded in concrete and immersed in seafloor for about four years was measured by the following electrochemical techniques: a) Linear Polarisation Resistance (LPR), b) a.c. Impedance and c) Electrochemical Noise (ECN).

For gravimetric study, the reinforced concrete specimens were broken open. The rust products were removed by immersing the reinforcement in pickling acid. The weight of the reinforcement steel rods was taken then loss in weight was calculated. The corrosion rate (Eq. 1) was estimated using the relationship,

Corrosion rate (mmpy) = 87.6 W . . .(1)

D A T

Where W is the weight loss in milligrams, D is the density of the material in gm/cm3, A is the surface area of the reinforcement in cm2, and T is the exposure time in hours.

A saturated calomel electrode having a wetted cotton tip was placed over the concrete surface. Open circuit potential between the reinforcement rod and the reference electrode was measured periodically using a multimeter.

The water-soluble chloride was estimated by volumetric method by preparing cement extract. The test solution was neutralized with diluted sulphuric acid (H2SO4) then titrated against standard silver nitrate (AgNO3) using potassium chromate as indicator.

Exposure details

For the study of the reinforced concrete specimens in natural marine environment the unique facility available at the Offshore Platform and Marine Electrochemistry Centre (OPMEC), an unit of Central Electrochemical Research Institute (CSIR) situated at a distance of two km from seashore located at Tuticorin, India was made use of. The reinforced concrete specimens were tied using polypropylene wire and suspended from the platform of OPMEC unit. The specimens were positioned at three different levels identified as atmospheric, high tide and seafloor.

Atmospheric Level (AL)

The reinforcement concrete specimens were positioned at the platform and exposed to typical natural marine atmosphere.

High tide Level (HL)

High tide level refers to the depth at which the reinforcement concrete specimen was subjected to severe

wave action of the sea. Due to this, the specimens were subjected to alternate wet and dry conditions.

Seafloor Level (SFL)

Seafloor level corresponds to the level where the submerged reinforcement concrete specimens were placed on the seafloor under the sea. This refers to a depth of approximately nine meter from the offshore platform.

Results and Discussions

Biofouling on concrete exposed at High Tide Level
The deterioration of concrete cubes exposed to natural marine environment over a period of four years is presented. The results based on visual inspection is presented and discussed.

Visual Inspection

The concrete specimens were taken away from exposure site after completion of four years. Then the biofouling scraped without affecting the concrete surface. The deterioration on the concrete surface was visually examined; the observations are presented and discussed.

Atmospheric level exposure

The significant deterioration of concrete is nil, no pit formation on the concrete for specimens exposed at atmospheric level for four years.

High tide level exposure

Concrete specimens exposure at High tide level shown Figure 1, pit measuring 7.04 mm in depth and 17.06 mm in diameter on concrete specimens exposed at high tide level for four years. The effective concrete cover is reduced from 50 mm to 43 mm, thus 14% reduction in the concrete cover provided to the reinforcement. The hardened cement paste was missing giving way to pit formation. At high tide level immersed reinforced concrete specimens deterioration is caused by aqueous organisms of animal and plant life, including barnacles, mussels, algae, and others. The accumulation of barnacles, mussels, algae, and other organisms causes corrosion. The reinforced concrete specimens exposed at high tide level is subjected to alternate wet and dry condition. When sea is calm or at low tide level the specimens are in dry exposure condition. In dry exposure condition sunlight falls on the specimens resulting in the death of the aqueous organisms. These phenomena reduce the corrosion caused by aqueous organisms.

Seafloor immersion

Concrete specimens immersed in seafloor level as shown in figure 2 & 3, pit measuring 9.72 mm depth and 19.36 mm diameter for concrete specimens immersed in seafloor for four years. The effective concrete cover is reduced from 50 mm to 40 mm, thus 20% reduction in the concrete cover provided to the reinforcement. The hardened cement paste was missing giving way to pit formation.

Biofouling present in the specimens immersed at seafloor promotes the microbial induced corrosion in the specimens. Anerobic conditions increases the corrosion rate. The biogeneric bacteria produces the sulphide which promotes the corrosion in anaerobic condition.

Biological corrosion is the deterioration of a metal by corrosion process that occurs directly or indirectly as a result of the activity of living organisms. These organisms include microforms such as bacteria and macro types such as algae and barnacles.

Microorganisms

Biofouling on concrete exposed at Seafloor
Anaerobic bacteria grow most favorably in environments containing little or no oxygen. Anaerobic bacteria that influence the corrosion behavior of buried steel structures are the Sulfate-reducing types. These reduce sulfate to sulfide according to the following schematic equation:

SO4 2- + 4H2 ---> S2- + 4H2O

The source of hydrogen in the above equation can be that evolved during the corrosion reaction or other organic products present in the soil.

Sulfate-reducing bacteria are most prevalent under anaerobic conditions. The presence of sulfide ion markedly influences both the cathodic and anaodic reactions occurring on iron surfaces. Sulfide tends to retard cathodic reactions, particularly hydrogen evolution, and accelerates anodic dissolution. The acceleration of dissolution causes increased corrosion. The corrosion product in the presence of sulfate-reducing bacteria is iron sulfide, which precipitates when ferrous and sulfide ions are in contact.

concrete specimens exposed at different levels
Iron bacteria are a group of microorganisms that assimilate ferrous iron from solution and precipitate it as ferrous or ferric hydroxide. The growth of iron bacteria frequently results in tubercles on steel surfaces and tends to produce corrosion attack. Concrete is less satisfactory in the presence of sulfur-oxidizing bacteria because it is also rapidly attacked by the sulfuric acid environment.

pH Value

The pH values of the extract prepared from the concrete specimens exposed at different levels are presented in Table 1. The pH value got reduced from initial value of 13.2 to approximately 11 for all the three levels.

Chloride Content

chloride content maximum for seafloor level exposure and minimum for atmospheric level exposure
The water-soluble chloride content of the extract prepared from the concrete specimens exposed at different levels is presented in the table 2. As seen from the Table 4, the chloride content was maximum for seafloor level exposure and minimum for atmospheric level exposure.

Biofouling

In this section biofouling on the 100 mm concrete cubes immersed for six months at high tide, low tide levels and immersed at seafloor is presented.

The Table 3 details regarding biomass on the reinforced concrete specimens after six months immersion in sea at various depths.

The following bacteria enumerated after three months exposure to natural marine environment) heterophic bacteria (HB), iron bacteria (FB), manganese depositing bacteria (MB), acid producing bacteria (APB) and sulphate reducing Bacteria (SRB).

Potential-Time Behavior Of Reinforced Concrete Specimens Exposed At Atmospheric level In Natural Marine Environment

The potential time behavior of reinforcement with specialty coating, embedded in concrete and exposed to atmospheric level in natural marine environment are presented and discussed.

Mild steel plain reinforcement:

On the day of exposure at atmospheric level in natural marine environment the mils steel plain reinforcement showed the OCP values of –129 mV vs. SCE. The mild steel reinforcement was in active condition after completion of 157 days of exposure at atmospheric level as per ASTM C876 since the OCP measurement –275 mV vs. SCE was above –275 mV vs.SCE.

Cement Polymer Composite Coated Reinforcement:

On the day of immersion at seafloor in natural marine environment the mild steel plain reinforcement showed the OCP values of–128 mV vs. SCE. The OCP values never crossed–275 mV vs. SCE indicating that the reinforcement was in passive condition.

Interpenetrating Polymer Network Coated Reinforcement:

On the day of immersion at seafloor in natural marine environment the Interpenetrating Polymer Network Coated reinforcement showed the OCP measurement as–112 mV vs. SCE. The mild steel reinforcement was in active condition after completion of nearly four years of exposure at atmospheric level as per ASTM C876 since the OCP measurement–414 mV vs. SCE was above–275 mV vs. SCE.

Epoxy Coated Reinforcement:

On the day of immersion at seafloor in natural marine environment the Epoxy Coated reinforcement showed the OCP measurement as –214 mV vs. SCE. The mild steel reinforcement was in active condition after completion of 722 days exposure at atmospheric level as per ASTM C876 since the OCP measurement –277 mV vs. SCE was above –275 mV vs. SCE.

Potential-Time Behavior Of Reinforced Concrete Specimens Exposed At High Tide Level In Natural Marine Environment

The potential time behavior of reinforcement coated with specialty coating, embedded in concrete and exposed to high tide level in natural marine environment are presented and discussed.

Mild steel plain reinforcement:

On the day of exposure at high tide level in natural marine environment the mild steel plain reinforcement showed the OCP values of –110 mV vs. SCE. The OCP measurement after completion of 44 days of exposure at high tide level was –373 mV vs. SCE and was above –275 mV vs. SCE as per ASTM C876, the mild steel reinforcement was in active condition.

Cement Polymer Composite Coated Reinforcement:

On the day of immersion at seafloor in natural marine environment the Cement Polymer Composite Coated reinforcement showed the OCP measurement as –98mV vs. SCE. The mild steel reinforcement was in active condition after completion of 44 days of high tide level exposure as per ASTM C876 since the OCP measurement –634 mV vs. SCE was above –275 mV vs. SCE.

Interpenetrating Polymer Network Coated Reinforcement:

On the day of immersion at high tide level in natural marine environment the Interpenetrating Polymer Network Coated reinforcement showed the OCP measurement as–132 mV vs. SCE. The mild steel reinforcement was in active condition after completion of 44 days of high tide level exposure as per ASTM C876 since the OCP measurement –357 mV vs. SCE was above –275 mV vs. SCE.

Epoxy Coated Reinforcement:

On the day of immersion at high tide level in natural marine environment the epoxy coated reinforcement showed the OCP measurement as –210 mV vs. SCE. The mild steel reinforcement was in active condition after completion of 44 days of seafloor immersion as per ASTM C876 since the OCP measurement –459 mV vs. SCE was above –275 mV vs. SCE.

Potential-Time Behavior Of Reinforced Concrete Specimens Immersed At Seafloor Level In Natural Marine Environment

Biofouling and Corrosion of Specialty Coated Reinforcement
The potential time behavior of reinforcement coated with specialty coating, embedded in concrete and immersed in seafloor level in natural marine environment are presented and discussed

Mild steel plain reinforcement:

On the day of immersion at seafloor in natural marine environment the mils steel plain reinforcement showed the OCP values of –118 mV vs. SCE. The mild steel reinforcement was in active condition after completion of 121 days of seafloor immersion as per ASTM C876 since the OCP measurement –373 mV vs. SCE was above –275 mV vs. SCE.

Cement Polymer Composite Coated Reinforcement:

On the day of immersion at seafloor in natural marine environment the Cement Polymer Composite Coated reinforcement showed the OCP measurement as –117 mV vs. SCE. The mild steel reinforcement was in active condition after completion of 157 days of seafloor immersion as per ASTM C876 since the OCP measurement –634 mV vs. SCE was above –275 mV vs. SCE.

Interpenetrating Polymer Network Coated Reinforcement:

On the day of immersion at seafloor in natural marine environment the Interpenetrating Polymer Network Coated reinforcement showed the OCP measurement as –140 mV vs. SCE. The mild steel reinforcement was in active condition after completion of 157 days of seafloor immersion as per ASTM C876 since the OCP measurement –528 mV vs. SCE was above –275 mV vs. SCE.

Epoxy Coated Reinforcement:

On the day of immersion at seafloor in natural marine environment the Epoxy Coated reinforcement showed the OCP measurement as –148 mV vs. SCE. The mild steel reinforcement was in active condition after completion of 121 days of seafloor immersion as per ASTM C876 since the OCP measurement –293 mV vs. SCE was above –275 mV vs. SCE.

Linear polarization measurements:

The concrete specimens with mild steel plain reinforcement were exposed at atmospheric level, high tide Level and seafloor immersion in natural marine environment at offshore platform for four years. The Linear polarization measurements of reinforcement with respect to corrosion are presented and discussed.

Linear polarization measurement for specimens exposed at atmospheric level

The linear polarization measurements of reinforcement coated with specialty coating, embedded in concrete and exposed at atmospheric level in natural marine environment are presented and discussed.

The Table 4 shows the corrosion rate at atmospheric level based on linear polarization resistance measurements for mild steel reinforcements after completion of four years of immersion in seafloor.

Linear polarization measurements for specimens exposed at high tide level

The linear polarization measurements of reinforcement coated with specialty coating, embedded in concrete and exposed at high tide level in natural marine environment are presented and discussed.

The Table 5 shows the corrosion rate at high tide level based on linear polarization resistance measurements for mild steel reinforcements, after completion of four years of immersion in seafloor.

Biofouling and Corrosion

Linear polarization measurements for specimens immersed in seafloor

The linear polarization measurements of reinforcement coated with specialty coating, embedded in concrete and immersed in seafloor in natural marine environment are presented.

The Table 6 shows the corrosion rate based on linear polarization resistance measurements for mild steel reinforcements after completion of four years of immersion in seafloor.

Corrosion rate based on a.c impedance measurements:

The concrete specimens with mild steel plain reinforcement were exposed at atmospheric level, high tide Level and immersion in seafloor in natural marine environment at offshore platform for four years. The a.c impedance measurements of reinforcement with respect to corrosion are presented.

A.C impedance measur- ements for specimens exposed at atmospheric level

Biofouling and Corrosion of Specialty Coated Reinforcement
The a.c impedance measurements of reinforcement coated with specialty coating, embedded in concrete and exposed at atmospheric level, Fig. 4, in natural marine environment are presented.

The Table 7 shows the corrosion rate based on a.c impedance method for mild steel reinforcements after completion of four years atmospheric level exposure.

A.C impedance measur- ements for specimens exposed at high tide level

The a.c impedance measurements of reinforcement coated with specialty coating, embedded in concrete and exposed at high tide level, Figure 5, in natural marine environment are presented and discussed.

The Table 8 shows the corrosion rate based on a.c impedance method for mild steel reinforcements after completion of four years high tide level exposure.

A.C impedance measurements for specimens immersed in seafloor

The a.c impedance measurements of reinforcement coated with specialty coating, embedded in concrete and immersed in seafloor, Figure 6, in natural marine environment are presented.

The Table 9 shows the corrosion rate based on a.c impedance method for mild steel reinforcements after completion of four years immersion in seafloor.

Corrosion rate based on electrochemical noise measurements:

environment at offshore platform for four years. The electrochemical noise measurements of The concrete specimens with mild steel plain reinforcement were exposed at atmospheric level, high tide Level and seafloor immersion in natural marine reinforcement with respect to corrosion are presented.

Electrochemical noise measurements for specimens exposed at atmospheric level

The electrochemical noise measurements of reinforcement coated with specialty coating, embedded in concrete and exposed at atmospheric level, Figure 7, in natural marine environment are presented and discussed.

Table 10 shows the corrosion rate based on electrochemical noise method for mild steel reinforcements after completion of four years immersion in atmospheric level

Electrochemical noise measurements for specimens exposed at high tide level

The electrochemical noise measurements of reinforcement coated with specialty coating, embedded in concrete and exposed at high tide level, Figure 8 &9, in natural marine environment are presented and discussed.

Table 11 shows the corrosion rate based on electrochemical noise method for mild steel reinforcements after completion of four years immersion in high tide level.

Electrochemical noise measurements for specimens immersed in seafloor

The electrochemical noise measurements of reinforcement coated with specialty coating, embedded in concrete and immersed in seafloor, Figure 10, in natural marine environment are presented and discussed.

The Table 12 shows the corrosion rate based on electrochemical noise method for mild steel reinforcements after completion of four years immersion in seafloor

Weight loss (Gravimetric) Method:

Biofouling and Corrosion
The concrete specimens with mild steel plain reinforcement were exposed at atmospheric level, high tide Level and seafloor immersion in natural marine environment at offshore platform for four years. The corrosion rate of reinforcement based on weight loss (Gravimetric) method are presented and discussed. The weight loss measurements were performed after removing the specialty coatings by mechanical means.

Weight loss (Gravimetric) measur- ements for specimens exposed at atmospheric level

The weight loss measurements of reinforcement coated with specialty coating, embedded in concrete and exposed at atmospheric level in natural marine environment are presented and discussed.

The Table 13 shows the corrosion rate based on weight loss method for mild steel reinforcements after completion of four years at atmospheric level.

Weight loss (Gravimetric) measurements for specimens exposed at high tide level

The weight loss measurements of reinforcement coated with specialty coating, embedded in concrete and exposed at high tide level in natural marine environment are presented.

The Table 14 shows the corrosion rate for mild steel reinforcements after completion of four years at high tide level based on weight loss method.

Weight loss (Gravimetric) measurements for specimens immersed in seafloor

The weight loss measurements of reinforcement coated with specialty coating, embedded in concrete and immersed in seafloor in natural marine environment are presented.

The Table 15 shows the corrosion rate based on weight loss method for mild steel reinforcements after completion of four years at seafloor level. The corrosion rate was highest at seafloor level.

Visual Inspection

Biofouling and Corrosion of Specialty Coated Reinforcement
The specialty coated reinforcements broken open from concrete specimens were visually examined; the observations are presented and discussed. The reinforcements are in the following order: CPC, control, epoxy and IP net-coated reinforcements.

Atmospheric level exposure

The reinforcements are in the order: CPC, control, epoxy and IP net based on open circuit potential measurements.

High tide level exposure

The reinforcements are in the order: CPC, control, epoxy and IP net based on open circuit potential measurements.

Seafloor immersion

The reinforcements are in the order: CPC, control, epoxy and IP net based on open circuit potential measurements.

Visual examination of the seafloor immersed reinforcement revealed that the corrosion prominently present in the control bar. Corrosion patch measuring 19mm was observed in control reinforcement.

Adhesion/bonding between coating and the concrete was good in Cement Polymer Coated reinforcement.

Adhesion/bonding between coating and the concrete was very poor in I.P net-coated reinforcements. Peeling off of coating was observed in the reinforcement coated with I.P net.

Pitting corrosion was observed in the epoxy coated reinforcement. Pitting corrosion patch measuring 7mm was observed.

Chloride Content

The water-soluble chloride content of the extract prepared from the concrete specimens exposed at different levels is presented in the table 4. As seen from the Table 16, the chloride content was maximum for seafloor level exposure and minimum for atmospheric level exposure.

pH Value

Biofouling and Corrosion
The pH values of the extract prepared from the concrete specimens exposed at different levels are presented in Table 17. The pH value got reduced from initial value of 13.2 to approximately 11 for all the three levels.

Conclusions

The biofouling deteriorates the concrete.

The deterioration due to biofouling reduces the effective cover of the reinforced concrete

The corrosion rate of reinforcement embedded in the concrete is higher at seafloor.

The electrochemical noise method is well suitable for measuring the corrosion rate of reinforced concrete structures. The corrosion rate obtained by using the electrochemical noise measurement is well comparable with the other electrochemical methods as well as Gravimetric method.

Acknowledgments

The author acknowledges the Director CECRI, Karaikudi who had given me the opportunity to work in the field of corrosion behavior of reinforced concrete in natural marine environment.

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NBMCW October 2009

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Experimental Investigation on Behavior ...

Beam Column Joints Retrofitted with FRP Wrap Subjected to Static Load

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

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 engineer have to face. Recent evaluation of civil engineering structures has demonstrated that most of them will need major repairs in the near future. One of the techniques of strengthening 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 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 behavior of the reinforced concrete beam-column joints retrofitted with Carbon fiber reinforced polymer wrap and Glass fiber reinforced polymer wrap.

Nine exterior RC beam-column joint specimens were cast and tested to failure during the present investigation. In six specimens, the reinforcements in both column and beam were provided as per code IS 456:2000 In remaining three specimens, the reinforcements in both column and beam were provided as per code IS 13920:1993. Various percentage of load carrying capacity of column was given as axial load in the column. Static load was applied as cantilever point load on beam till failure. 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 Carbon fiber reinforced polymer (CFRP) sheet was used to wrap three specimens and Glass fiber reinforced polymer (GFRP) sheet was 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 were presented.

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 labor 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

Robert Ravi et al (2009) conducted an experimental investigation on influence of development length in retrofitted reinforced concrete beam-column joints. Nine controlled reinforced concrete beam-column joints specimens were casted, in which six specimens had design and details as per the code IS 456:2000. Remaining three specimens had design and details as per the code IS 13920:1993. Retrofitting was done on failed specimens which had details as per code IS 456:2000.Three specimens were wrapped with GFRP and remaining three with CFRP. Static load test was conducted on control and retrofitted specimens. They conclude that there was an increase in load carrying capacity by 14.5% and an increase in energy absorption capacity by 10% as the development length was increased based on code IS 13920:1993.

K.R.Bindu et al (2008) conducted a detailed investigation on the performance of exterior beam-column joints with inclined bars at joints under cyclic loading. They investigated the effect of inclined bars at the joint region. Four exterior beam column joints were cast and tested under cyclic loading. The performance of specimens which had joint reinforcement with inclined bars was compared with the specimen without inclined bars. They concluded that specimens with inclined bars show more ductility and energy absorption capacity than the specimen without inclined bars.

Alexander G. Tsonos et al (2008) conducted a detailed investigation on effectiveness of CFRP jacket and RC jacket in post earthquake and pre earthquake beam-column sub assemblages. The feasibility and technical effectiveness of high strength fibre jacket system and reinforced jacket system were discussed. Four exterior beam-column joint sub assemblages were tested under cyclic loading. They concluded that in case of post earthquake, specimens retrofitted with RC jacket shows more effective but in case of pre earthquake both retrofitting technique shows equal effectiveness.

G.A. Lakshmi et al (2008) conducted a detailed investigation by numerical and experimental study on strengthening of beam-column joints under cyclic excitation using FRP composites. In that study three typical modes of failure namely flexural failure of beam, shear failure of beam and shear failure of column were discussed. Comparison was made in the terms of load carrying capacity. Three exterior beam-column joint sub assemblages were caste and tested under cyclic loading. All the three specimens were retrofitted using FRP materials and result were compared with control specimens. Finite element analysis has been carried out using ANSYS to numerically simulate each of these cases. They concluded that the shear failure was very brittle and hence retrofitting should be done in such a manner that the eventual failure occurs in the beam in flexure.

G. Appa roa et al (2008) conducted detailed investigation on performance of RC beam-column joints strengthened by various schemes subjected to seismic loads. In this study different strengthening methods such as steel jacketing, fibre wrapping and providing haunch elements were discussed. The important design parameters such as joint shear strength and energy dissipation capacity for various schemes were discussed. They concluded that to enhance the strength, stiffness and energy dissipation, it lacks proper placements and arrangements of FRP sheets and strips. Hence it could not improve the joint shear strength. The numerical studies revealed that the haunch element had significant reduction of shear force and bending moment in the frame members leading to significant reduction of joint shear force.

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 Investigations

The experimental program consisted the testing of nine reinforced concrete beam-column joint specimens. The column had a cross section of 200mm x 200mm with an overall length of 1500mm and the beam had a cross section of 200mm x 200mm with a cantilevered portion of length 600 mm based on the availability of mould. Six specimens had 4nos. of 12mm diameter bars as longitudinal reinforcement in column as per IS 456:2000, cl 26.5.3.1. The lateral ties in the column were provided at a spacing of 180 mm c/c as per IS 456:2000, cl 26.5.3.2(c). Beam had 2 nos.16 mm diameter bar as tension reinforcement and 2nos.12 diameter as compression reinforcement as per code IS 456: 2000, cl. 26.5.1.1 (a) & cl. 26.5.1.2. Beam had vertical stirrups of 6 mm diameter at 120mm c/c as per code IS 456:2000, cl.26.5.1.6. The development length of the tension and compression rods in beam were also provided as per clause 26.2.1 of IS 456:2000.

For the remaining three specimens, 4nos.12mm diameter bars were provided as longitudinal reinforcement. The lateral ties in the column were provided as 8mm diameter bar at 75 mm c/c for the central distance of 1100mm as per IS 13920:1993,cl 7.4.6 and 6mm diameter bars at 100mm c/c for the remaining length of the column. Beam had 2 nos. each 16mm diameter bar as tension and compression reinforcement. Beam had vertical stirrups of 6mm diameter bar at 40mm c/c. up to 340mm from the face of the column as per code IS 13920:1993,cl 6.3.5 and 6 mm diameter bar at 80mm c/c for remaining length of the beam. The development length of the beam rods were also provided as per code IS 13920:1993,cl 6.2.5. The concrete mix was designed for a target strength of 25 MPa at the age of 28 days. The load carrying capacity of the column was found to be 525 kN. The details of the typical test specimens are given in Fig.1(a) & Fig.1(b).

Experimental Investigation

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 240 kN which is less than 50 % of load carrying capacity of the column. The experimental investigation consisted of applying three axial loads of 80 kN, 160 kN and 240 kN on the column 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

Experimental Investigation

Glass fiber reinforced polymer sheets (GFRP) wrap was used to strengthen the three failed beam-column joint specimens C1,C2 & C3 and they are redesignated as retrofitted specimens R1, R2 & R3.
Experimental Investigation
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. The physical properties of GFRP and CFRP were given in Table 2 and Table 3

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 M25.ACI method of mix proportioning namely, ACI 211.4R-99 was adopted to arrive the initial mix proportions. However, the following proportions were arrived at after several trial mixes.

Experimental Investigation

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. Fig .2 a) & Fig. 2 b) shows the typical test specimen before and after concreting.

Experimental Investigation

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. Fig 3.a) & 3.b) shows the reconcreting process.

Experimental Investigation

The specimens were cured for 28 days. Before wrapping 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 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. Fig 4.a) & 4.b) shows the wrapped specimens by GFRP and CFRP.

Experimental Investigation

Description of the Test Programme

Experimental Investigation
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 80 kN was applied at one end the column using a hydraulic jack of 500kN 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 250 kN 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 up to a control deflection of 75mm. The specimens C2,C5,C8 and C3,C6,C9 were tested in the same way and the axial loads applied on these specimens were 160 kN and 240 kN respectively. The retrofitted specimens R1&R4, R2&R5 and R3&R6 were also tested for the axial loads of 80kN, 160kN and 240kN.Fig 5. shows the typical experimental set up.

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 19.5 kN. At a load of 20.5 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 21 kN. Spalling of concrete occurred in the tension zone of the beam at a load of 22 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 23 kN.

In the case of the specimen C2, the first crack was formed in the beam portion approximately at a distance of 47 mm from face of the column at a load of 19.5 kN. At a load of 20.5 kN, cracks propagated to the compression zone of the beam. Spalling of concrete occurred in the beam at a load of 21 kN. Cracks propagated to the beam-column joint portion at a load of 21.5 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75mm. The load corresponding to this deflection was 22 kN.

In the case of the specimen C3, first crack was formed in the beam portion approximately at a distance of 50 mm from face of the column at a load of 20 kN. At a load of 21 kN, cracks propagated to the compression zone of the beam. Spalling of concrete occurred in the beam at a load of 21.5 kN. Cracks propagated to the beam-column joint portion at a load of 22.5 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 23.5 kN

In the case of the specimen C4, first crack was formed in the beam portion approximately at a distance of 50 mm from face of the column at a load of 20.5 kN. At a load of 21 kN, cracks propagated to the compression zone of the beam. Spalling of concrete occurred in the beam at a load of 21.5 kN. Cracks propagated to the beam-column joint portion at a load of 22 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 22.5 kN.

In the case of the specimen C5 the first crack was formed in the beam portion approximately at a distance of 55 mm from face of the column at a load of 19 kN. At a load of 19.5 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 20 kN. Spalling of concrete occurred in the tension zone of the beam at a load of 20.5 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 21 kN.

In the case of the specimen C6, the first crack was formed in the beam portion approximately at a distance of 50mm from face of the column at a load of 19.5 kN. At a load of 20 kN, cracks propagated to the compression zone of the beam. Spalling of concrete occurred in the beam at a load of 20.5 kN. Cracks propagated to the beam-column joint portion at a load of 21 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 21.5 kN. Fig.6 a) & 6.b) shows the beam-column joint specimen before and after testing.

Experimental Investigation

In the case of the specimen C7 the first crack was formed in the beam portion approximately at a distance of 60 mm from face of the column at a load of 21 kN. At a load of 22.5 kN, cracks propagated to the compression zone of the beam. Spalling of concrete occurred in the beam at a load of 23.5 kN. Cracks propagated to the beam-column joint portion at a load of 25.5kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 26 kN.

In the case of the specimen C8 the first crack was formed in the beam portion approximately at a distance of 55mm from face of the column at a load of 22 kN. At a load of 23 kN, cracks propagated to the compression zone of the beam. Spalling of concrete occurred in the beam at a load of 24 kN. Cracks propagated to the beam-column joint portion at a load of 25 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 26.5 kN.

In the case of the specimen C9 the first crack was formed in the beam portion approximately at a distance of 58 mm from face of the column at a load of 21.5 kN. At a load of 22.5 kN, cracks propagated to the compression zone of the beam. Spalling of concrete occurred in the beam at a load of 24.5 kN. Cracks propagated to the beam-column joint portion at a load of 25 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 26 kN. The best load deflection curve among the specimens was shown in Fig.7.a) & 7.b).

Experimental Investigation

The load carrying capacity of six control specimens were given in Table 4.

Experimental Investigation

Effect of Lateral Ties

It was seen from Table.4, there was not much difference in the load deformation characteristics of the beam-column joint specimens with an increase in the axial load. It was seen that there was only 12% increase in the ultimate load capacity and 13% increase in energy absorption capacity of the RC beam-column joint specimen, as the stirrup spacing is decreased as per code IS 13920:1993. 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

In the case of the retrofitted specimen R1, first crack was formed in the beam portion very close to the column at a load of 22 kN. At a load of 24 kN, a crack propagated to the compression zone of the beam. The cracks in the beam started to widen at a load of 26 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 75 mm. The load corresponding to this deflection was 29 kN.

In the case of the specimen R2, first crack was formed in the beam portion at a distance of 30 mm from the face of the column at a load of 24 kN. At a load of 26 kN, the crack propagated to the compression zone of the beam. The cracks propagated into the column portion at a load of 27 kN. Bond failure of the wrap was noticed on the tension side of the beam at a load of 28 kN and the compression side of the beam at a load of 29 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 30 kN.

In the case of the specimen R3, the first crack formed in the beam portion at a load of 23 kN. Bond failure of the wrap was noticed on the tension side of the beam at a load of 25 kN and on the tension side of the compression side of the beam at a load of 27 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 29 kN.

In the case of the retrofitted specimen R4, first crack was formed in the beam portion very close to the column at a load of 23 kN. At a load of 25 kN, a crack propagated to the compression zone of the beam. The cracks in the beam started to widen at a load of 27 kN and bond failure of the wrap was noticed on the tension side of the beam at a distance of 55 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 75 mm. The load corresponding to this deflection was 31 kN.

In the case of the specimen R5, the first crack was formed in the beam portion at a distance of 40 mm from the face of the column at a load of 25 kN. At a load of 27 kN, the crack propagated to the compression zone of the beam. The cracks propagated into the column portion at a load of 28kN. Bond failure of the wrap was noticed on the tension side of the beam at a load of 29 kN and the compression side of the beam at a load of 30 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 32 kN.

In the case of the specimen R6, the first crack formed in the beam portion at a load of 24 kN. Bond failure of the wrap was noticed on the tension side of the beam at a load of 26kN and on the tension side of the compression side of the beam at a load of 28 kN. The application of the load was stopped when the deflection at the free end of the beam reached 75 mm. The load corresponding to this deflection was 30 kN. Fig 8.a) & 8.b) shows the failed retrofitted specimens.

Experimental Investigation

Effect of Retrofitting

Experimental Investigation
The load deflection curves of control, GFRP and CFRP wrapped RC beam-column joint specimens were shown in Fig.9. The load carrying capacity of the various reinforced concrete beam-column joint specimens (both control and retrofitted) are given in Table.5. It was seen from the table that the increase in the load carrying capacity was 13% to 36% for the retrofitted specimens. It was also seen that the area of the load deflection curve up to a deflection of 75 mm for specimens retrofitted with a double layer of GFRP was 14 % and CFRP was 26 % 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 both GFRP and CFRP.

Experimental Investigation

Conclusion

Based on the experimental investigations carried out on the control and retrofitted beam-column joint specimens using GFRP & CFRP wrapping, the following conclusions were drawn:
  • There was 12% increase in load capacity and 13% increase in energy absorption capacity of the RC beam-column joint specimen, as the stirrup spacing is decreased as per code IS 13920:1993
    There was 13% increase in the load capacity and 14% increase in energy absorption capacity of the RC beam-column joint specimen, retrofitted using GFRP wrapping.
  • There was 36% increase in the load capacity and 26% increase in energy absorption capacity of the RC beam-column joint specimen, retrofitted using CFRP wrapping
  • 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 using both GFRP and CFRP.
  • The enhancement in the energy absorption capacity of the wrapped specimens was in the range 14%-26% 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

Robert Ravi.S, Prince Arulraj.G., “Experimental Investigation on Influence of Development Length in Retrofitting Reinforced Concrete Beam-Column Joints”NBMCW 2009, Vol 4, Pg 148-158.

K.R.Bindu and K.P.Jaya, “Performance of Exterior Beam Column Joints with Cross Inclined Bars Under Seismic Type Loading,” Journal of engineering and applied science, 2008 ,Vol 7, Pg 591-597.

Alexander G. Tsonos, “Effectiveness of CFRP Jackets and RC Jackets In Post–earthquake and Pre– earthquake Retrofitting of Beam Column Sub Assemblages,” Journal of engineering structures 2008, Vol 30, Pg 777-793.

G.A. Lakshmi, Anjan Dutta,and S.K.Deb, “Numerical Study of Strengthening of Beam Column Joints Under Cyclic Excitation Using FRP Composites,” Journal of structural engineering 2008,Vol 35, Pg 59-65.

G. Appa roa, M.Mahajan and M.Gangaram, “Performance of Nonseismically Designed RC Beam Column Joints Strengthen by Various Schemes Subjected to Seismic Loads,” Journal of structural engineering 2008,Vol 35, Pg 52-58.

Yousef A. Al-Salloum and Tarek H.Almusallam,”Seismic Response of Interior RC Beam Column Joints Upgrade with FRP Sheets. I: Experimental Study” Journal of composite for construction 2007, Vol 11, Pg 575-589.

Yousef A. Al-Salloum and Tarek H.Almusallam, “Seismic Response of Interior RC Beam Column Joints Upgrade With FRP Sheets. II: Analysis And Parametric Study “Journal of composite for construction 2007, Vol 11, Pg 590-599.

Devados Menon, Pradip Sarkar and Rajesh Agrawal, “Design of RC Beam Column Joints Under Seismic Loading–A Review.” Journal of structural engineering 2007, Vol 33, Pg 449-457.

M.Jamal Shannag, and Nabeela Abu-Dyya, “Lateral Load Response of High Performance Fibre Reinforced Concrete Beam Column Joints” Journal of construction and building materials 2005 Vol 19, Pg 500-508.

A.M.Said and M.L Nehdi, “Use Of FRP For RC Frame In Seismic Zones, Evaluation of FRP Beam Column Joints Rehabilitation Techniques” Journal of applied composite materials, 2004, Vol 11, Pg 205-226.63

Abhijit Mukherjee and Mangesh Joshi, “FRPC Reinforced Concrete Beam Column Joints Under Cyclic Excitation” Journal of composite structures 2004, Vol 70, Pg 185-199.

Costas P.Antonopoulos and Thanasis C. Triantafillou, “Experimental Investigation of FRP-Strengthened RC Beams Column Joints” Journal of composite for construction 2003, Vol 7, Pg 39-49.

Costas P.Antonopoulos and Thanasis C. Triantafillou, “Analysis of FRP-Strengthened RC Beam Column Joints” Journal of composite for construction 2002, Vol 6, Pg 41-51. Ahmed Ghobaraah and A.Said, “Shear Strength of Beam Column Joints.” Journal of engineering structures 2002, Vol 24, Pg 881-888.

T.El-Amoury and A.Ghobarah, “Seismic Rehabilitation of Beam Column Joints Using GFRP Sheets” Journal of engineering structures 2002, Vol 24, Pg 1397-1407.

Ze-Jun Geng, Micheal J Chajes and Tsu-Wei Chou, “Retrofitting of Reinforced Concrete Column-To-Beam Connections” Journel of composite science and technology, Vol 58, Pg 1297-1305.

NBMCW October 2009

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Role of Fibers for Durable Concrete Con...

Role of Fibers for Durable Concrete Construction

Clinton Pereira, Assistant Manager Technical, Grasim Industries Limited, Mumbai

Introduction

The use of fibers to reinforce concrete materials is a well–known concept. It has been practiced since ancient times, with straw mixed into mud bricks and horsehair included in mortars. However, in our modern day construction practices we have forgotten the ancient practices to control cracks in concrete. Concrete cracking is normal. Portland cement concrete is considered to be a relatively brittle material and is prone to crack in the plastic as well as the hardened stage. Plastic shrinkage occurs when the evaporation of water from the surface of concrete is greater than the rising bleed water. As concrete is very weak in tension in its plastic stage, a volume change causes the surface to crack. As it hardens, the water present in the pores of concrete begins to evaporate. This causes the concrete to shrink due to the volume change, which is restrained by the subgrade and reinforcement. This results in a tensile stress being developed in hardened concrete, again causing the concrete to crack.

Cracks lead to negative perception of quality, durability and serviceability, however in most cases they become only aesthetic problems. Cracks also results in disputes between the owner, Architect, design Engineer and contractor which results in job delays and cost increases due to work stoppages and evaluation which is more severe than the actual consequences of cracking. One of the solutions to this problem is the additions of fibers to concrete. An attempt has been made in this article to provide the advantages and benefits of using fiber reinforced concrete for a variety of applications. The use of fibers help in modifying properties of concrete both in plastic and hardened stage and thus results into a more durable concrete. Incorporating Synthetic fibers help to reduce thermal and shrinkage cracks. Addition of steel fibers enhances the ductility performance, post-crack tensile strength, fatigue strength and impact strength of concrete structures. What is Fiber Reinforced Concrete (FRC)?

FRC is Portland cement concrete reinforced with more or less randomly distributed fibers. In FRC, thousands of small fibers are dispersed and distributed randomly in the concrete during mixing, thus improving concrete properties in all directions. Fibers help to improve the pre- crack tensile strength, post peak ductility performance, fatigue strength, impact strength and minimize thermal and shrinkage cracks.

Which types of fibers are used in FRC?

Role of Fibers for Durable Concrete Construction
A wide variety of fibers have been used in concrete. For each application it needs to be determined which type of fiber is optimal in satisfying the concrete application. The different types of fibers used as concrete reinforcement are synthetic fibers and steel fibers. The different types of synthetic fibers used are Polypropylene, Nylon, Polythene, Polyester and Glass Fibers

For architectural and decorative concrete products and for prevention of early age cracking, synthetic fibers may be used. Steel fibers are used for applications where properties of concrete in the hardened stage have to be modified, namely, post crack flexural strength, abrasion resistance, impact resistance and shatter resistance of concrete.

How do fibers work in plastic stage of concrete?

Early age volume changes in concrete causes weak planes and results in the formation of cracks, because the stresses developed in the body of concrete exceeds its tensile strength at that specific time. The growth of these micro shrinkage cracks is inhibited by the mechanical blocking action of both synthetic and steel fibers. The internal support system of the fibers inhibits the formation of plastic settlement cracks. The uniform distribution of fibers throughout the concrete discourages the development of large capillaries, caused by bleed water migration to the surface. Fibers thus lower the permeability of concrete through the combination of plastic crack reduction and reduced bleeding characteristics.

Results of experimental program for reference

The contribution of steel, synthetic and cellulose fibers to the shrinkage-crack reduction potential of cement composites during the initial and final setting of concrete and its evaluation was reported in the ACI Materials Journal, May-Jun 1994, Vol. 91, No. 3, pp 280-288. "Contribution of fibers to crack reduction of cement composites during the initial and final setting period"

The primary variables of the investigation were: fiber types, matrix composition, and test methods. Fiber type consisted of synthetic fibers with lengths varying from a fraction of 1 to 60 mm at volume content of 0.9 kg/m3 and steel fibers of three lengths viz. 30, 50, and 60mm were investigated at volume contents of 45 and 60 kg/m3. The Synthetic fibers were made of nylon, polyethylene, polypropylene and polyester. The longer polypropylene fibers were fibrillated. Nylon and polyester fibers were made of single filaments with lengths varying from 19 to 50mm. The matrix consisted of cement mortar with various cement-sand ratios, concrete containing coarse aggregates, and lightweight concrete. In the case of test methods, the primary variables were specimen thickness and plane dimensions of the test panels. Results indicated that both steel and synthetic fibers made a definite contribution to shrinkage crack reduction during the initial and final setting periods. The microfibers are more effective in rich cement mortar, whereas the longer fibers, are more effective in lean mortars and concrete.

Discussions & Remarks

From the above experiment, it is clearly observed that the addition of fibers to concrete helps in minimizing plastic shrinkage cracks which eventually helps in enhancing the durability of the structure. Reduction in surface and internal cracks prevents the entry of moisture and other harmful chemicals which can have a devastating effect on concrete. As fibers arrests the formation of micro cracks the permeability of concrete is reduced; this property is of prime importance for the manufacturing of waterproof concrete.

How do fibers work in hardened stage of concrete?

The early age benefits of using synthetic and steel fibers continue to contribute to hardened concrete. Hardened concrete attributes provided by synthetic and steel fibers are lowered permeability and the enhanced resistance to shattering, abrasion, and impact forces.

Role of Fibers for Durable Concrete Construction

The ability to resist shattering forces is greatly enhanced with the introduction of fibers to the concrete. When plain concrete is compressed, it will shatter and fail at the first crack. Fibers manufactured specifically for concrete prevent the effect of shattering forces by tightly holding the concrete together. Abrasion resistance of concrete is enhanced when fibers are used because the water-cement ratio at the surface is not lowered by variable bleed water. This improvement is assisted by the internal settlement support value of the fibers contributing to uniform bleeding. Resistance to impact and other suddenly applied loads is also one of the significant improvements in the properties of fiber reinforced concrete. Fibers help in distributing the impact forces to the entire body of concrete, thus reducing the concentration of the braking forces.

Results of experimental program for reference

The contribution of synthetic and steel fibers to enhance the "Flexural behavior and toughness of fiber reinforced concretes" was reported by the Transportation Research Record, 1989, No. 1226, pp 69-77.

This above referred article presents the results of an extensive investigation to determine the behavior and performance characteristics of the most commonly used fiber reinforced concretes (FRC) for potential airfield pavements and overlay applications. A comparative evaluation of static flexural strength is presented for concretes with and without four different types of fibers: hooked-end steel, straight steel, corrugated steel, and polypropylene. These fibers were tested in four different quantities (0.5, 1.0, 1.5, and 2.0 percent by volume), and the same basic mix proportions were used for all concretes. The test program included (a) fresh concrete properties, including slump, vebe time, inverted cone time, air content, unit weight and concrete temperature, and hardened concrete properties; (b) static flexural strength, including load-deflection curves, first crack strength and toughness, toughness indexes, and post-crack load drop; and (c) pulse velocity. In general, placing and finishing concretes with less than 1 percent by volume for all fibers using laboratory-prepared specimens was not difficult. However, the maximum quantity of hooked-end fibers that could be added without causing balling was limited to 1 percent by volume. Corrugated steel fibers performed the best in fresh concrete; even at higher fiber contents (2 percent by volume), there was no balling, bleeding, or segregation. Higher quantities (2 percent by volume) of straight steel fibers caused balling, and higher quantities of polypropylene fibers (2 percent by volume) entrapped a considerable amount of air. Compared with plain concrete, the addition of fibers increased the first crack strength (15 percent to 90 percent), static flexural strength (15 percent to 129 percent), toughness index, post-crack load-carrying capacity, and energy absorption capacity.

Discussions & Remarks

From the above experiment, it is clearly observed that addition of fibers to concrete helps in enhancing resistance to shattering, abrasion, and impact forces, thus improving the mechanical properties of concrete. The increase in the flexural toughness/residual strength of concrete increases the load bearing capacity of concrete, which can potentially reduce the depth of slabs on grade. The increased impact and abrasion resistance increases the durability and reduces the maintenance cost of the concrete structure in use.

Advantages and benefits of Fibers in concrete:-

  1. Fibers inhibit and controls the formation of intrinsic cracking in concrete caused both in the plastic and hardened stage of concrete, thus ensuring a more durable concrete construction.
  2. Fibers reinforce concrete against impact forces, thereby improving the toughness characteristics of hardened concrete.
  3. Fibers improve the resistance to shattering forces caused due to earthquake loads and vibrations induced in machine foundations, thus making concrete a more versatile material for such critical applications.
  4. Fibers enhance the hardness of the surface of concrete against material loss due to abrading forces caused by frequent movement of wheel loads. This enhances the service life and safety of concrete pavements.
  5. Fibers reduce the permeability and water migration in concrete, which ensures protection of concrete due to the ill effects of moisture.
  6. Fibers reduce plastic shrinkage and settlement cracking when concrete is still green, thus enhancing the overall life of the structure and reducing the maintenance cost.
  7. Fibers can replace the secondary reinforcement or crack control steel used in grade slabs, thereby reducing the overall cost of the structure.
Role of Fibers for Durable Concrete Construction

Typical recommended applications of FRC

Fiber reinforced concrete is recommended in all types of concretes which demonstrate a need for enhanced toughness characteristics, resistance to intrinsic cracking and improved water tightness such as:-industrial floorings, canal linings, driveways, bridge decks, pavements, precast structures, water tanks, overlays/toppings, tilt-up panels, RCC elements, composite decks, mass concretes, sloping slabs, walls, thin sections, shotcrete and terrace slabs.

Conclusion

An important focus of our vision should now be on increasing the durability and longevity of various structures. The life of all bridges, pavements and other concrete structures should double in the next century as our country's major financial resources are invested in the construction sector. Carefully selecting materials to optimize and control their properties and use of more performance based specifications will result in advances in the durability of concrete.

Concrete is an inherently brittle material with a relatively low tensile strength as compared to its compressive strength. Reinforcing with randomly disturbed fibers presents an effective approach in controlling cracks and improving the ductility and flexural strength of concrete. A variety of materials such as polypropylene, nylon, polyester, glass, carbon and steel fibers can be used in fiber reinforced concrete. The applications of fibers are wide; however the appropriate fiber has to be used in order to meet the requirements of the structure and achieve maximum effectiveness.

FRC can be used in almost all types of civil structures where durability of the structure and a lower maintenance cost becomes the important factor in the selection of concrete materials. For corrosion protection of structures in the coastal belt cracks should be practically minimized, which can be achieved by the additions of fibers. As FRC provides a higher shatter resistance to concrete, structures in earthquake prone areas would minimize the risk of human casualties. Addition of fibers to concrete pavements enhance various toughness properties and thus reduce maintenance costs, enhance driving safety, controls surface erosion and increases the overall life and durability of the structure. In lieu of the same, it is felt that Architects & consultants should take strong initiatives in promoting the use of FRC by way of specifying the same in tender documents.

In conclusion, the use of fibers help in modifying properties both in the plastic as well as hardened stage of concrete, thus making concrete a more versatile material to be used for variety of applications.

Acknowledgment

I would like to thank Mr.S.B.Kulkarni-Asst. Vice President-Technical, Grasim Industries Ltd-Cement Business who has motivated me to write my first article.

I would also like to thank my colleague Mr. Hemendra Shribatho, Dy.Manager-Technical, Grasim Industries Ltd-Cement Business for proofreading the final version of the article and offering suggestions for improvement.

References

  • R. Brown, A. Shukla and K.R. Natarajan, "Fiber reinforcement of concrete structures" University of Rhode Island URITC project No. 536101, September 2002.
  • ACI Materials Journal, May-Jun 1994, Vol. 91, No. 3, pp 280-288. "Contribution of fibers to crack reduction of cement composites during the initial and final setting period"
  • Transportation Research Record, 1989, No. 1226, pp 69-77. "Flexural behavior and toughness of fiber reinforced concretes"
  • Specification for Concrete Cracking" by Juan Pablo Covarrubias, Concrete International, September2007.
  • Photographs courtesy: Nina Concrete Systems Pvt Ltd, Mumbai. Distributors for Fibermesh Polypropylene fibers in India.

NBMCW September 2009

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Self–compacting Concrete A Paradigm S...

Self–compacting Concrete

S. K. Singh, Scientist, Central Building Research Institute, Roorkee &amp; Honorary Secretary, Institution of Engineering Roorkee Centre.

In the era of highrise and over congested construction sites, the performance required for concrete structures is becoming more and more complicated and diversified. The concrete is required to have properties like high fluidity, self-compactability, high strength, high durability, better serviceability and long service life. The construction activity should have minimal interference with the environment. Self-compactability is desirable to attain rationalized and labor saving effects in production and placing of concrete. Self-compacting concrete (SCC) is a highly engineered material that addresses these requirements of high fluidity and no segregation. The several advantages reported in using SCC are reduction in the construction time and labor cost, elimination of the need of vibration, reduction of noise pollution, improving compactability even in highly congested structural members and finally a better construction ensuring good structural performance. This paper is the review of development in the field of SCC and its basic principle, different mix proportioning, properties in fresh and hardened state, testing methods to test high-flowability, resistance against segregation and its ability to pass through congested reinforcement. Mix design method and effect of incorporating variety of materials have been discussed in this paper. The applications of SCC, practical acceptance at jobsite and its future prospects have also been discussed.

Introduction

From several years, due to poor quality control and quality assurance at site, the problem of durability of concrete structures has been a major issue to engineers. Sufficient compaction is required to make durable concrete structures. Compaction of conventional concrete is done by vibrating whereas over vibration can easily cause segregation and bleeding. Also under vibration can cause honeycombing in concrete. Further, in conventional concrete, it is very difficult to ensure uniform material quality and good density in heavily reinforced locations of the structures. If steel is not properly surrounded by concrete it leads to durability problems. To overcome these problems, SCC was developed in Japan as a means to create uniformity in the quality of concrete. Self-compacting concrete achieves this by its unique properties in fresh state such as workability, water retention, workable time, green stability and early shrinkage. In the plastic state, it flows under its own weight and maintain homogeneity while completely filling any formwork and passing around congested reinforcement. Furthermore, in hardened state, it possesses all important properties such as compressive strength, flexural and tensile strength, elastic modulus, drying shrinkage, freeze-thaw resistance and carbonation resistance as compared to conventional concrete. The concept of SCC was firstly proposed and applied to prototype structure by Okamura [1] in Japan in 1988. Later studies to develop SCC, including a fundamental study on the workability of concrete, have been carried out by Ozawa and Maekawa [2]. SCC has now been used in construction with enthusiasm across Europe, America and other parts of the world in both cast-in-situ and precast concrete work. Earlier, SCC relied on very high content of cementitious paste and the mixes required specialized and well-controlled placing methods to avoid segregation. But the high contents of cement paste made them prone to shrinkage and generation of very high heat of hydration. The overall costs were also very high and therefore, applications remained very limited. After a continuous research and development which created a series of advancement in the field of SCC and it is now no longer a material consisting of only cement, aggregates, water and admixtures.

The SCC is an engineered material consisting of cement, aggregates, water and admixtures with several new constituents like colloidal silica, pozzolanic materials, ground granulated blast furnace slag (ggbs), microsilica, metakaolin, chemical admixtures etc. to take care of specific requirements such as high-flowability, high compressive strength, good workability, enhanced resistance to chemical and mechanical stresses, lower permeability, enhanced durability, resistance against segregation and ability to pass under dense reinforcement conditions. The fluidity, deformability and high resistance to segregation enables the placement of concrete without vibrations and with reduced labour, noise and less wear and tear of the equipment. It also helps to shorten the construction period.

Basic Principle

The SCC is a concrete which gets compacted due to its self-weight and is de-aerated (no entrapped air) almost completely while flowing in the form work. In densely reinforced structural members, it fills completely all the voids and gaps and maintains nearly horizontal concrete level after it is placed. The above mentioned properties of SCC are achieved by limiting aggregate wherein energy required for flowing is consumed by internal stress (it is increased due to decrease distance between particles that is due to high deformability) resulting in blockage of aggregate particles. Limiting coarse aggregate content whose energy consumption is intense to a level lower than normal is effective in avoiding this type of blockage. The high flowability with high deformability can be achieved only by application of a super-plasticizer keeping w/p ratio to a very low value [3]. Fig.1 explains the process of achieving self compactability with the help of a flow chart.

Self–compacting Concrete

The reasons for better performance of SCC are attributed to better microstructure and homogeneity. Many investigations, carried out by means of efficient microscopes/SEM etc, have shown an improved microstructure of SCC opposite to conventional concrete. The void ratio of SCC in the interfacial transition zone between cement paste and aggregate has been found lower and the pores have been distributed much more evenly [1].

The Rheology of Self- compacting Concrete

Self–compacting Concrete
The cements and powdered inorganic additives as shown in Table 1, which form constituents of the concrete are all counted as ultra fines (particle size diameter d"0.125 mm) [5,6,7]. In aqueous suspension, the individual grains exhibit different surface charges depending on their chemically differing compositions. Opposite charges attract one another, so the mineral constituents of the suspension tend to agglomerate as shown in Fig. 2, and this is regardless of whether, involves a pure cement suspension or cement – fly ash – silica fume suspension, or a pure fly ash suspension is involved. It is observed that the rheological behaviour of all these suspensions is substantially that of a Bingham solid. The characteristic features of this are a pronounced yield value and a shear resistance which increases with shear rate and can be assumed to be approximately linear with decreasing shear rate. The gradient of the straight line is a measure of dynamic viscosity. At high water/solids ratios in the suspension the yield value approaches zero because of the large average distance between the particles, and the dynamic viscosity approaches that of water. At low water/solids ratios the yield value increases sharply because of the high tendency to agglomeration, and so does the dynamic viscosity. Investigations demonstrated in detail that the action of super-plasticizers is essentially to equalize the surface charges (zeta potential) on all solid particles in the dispersion and in this way to disagglomerate the particles [9].

Concrete as a five component system

New developments in the field of super-plasticizers [6] exhibit that steric and tribological effects also plays an important role in the mobility of the individual solid particles. Polycarboxylates, for example, are being developed specifically with a correspondingly extended performance spectrum. Increased addition of super-plasticizer is always associated with a drop in yield value towards zero, but the dynamic viscosity of the suspension is governed largely by the water/solids ratio, i.e. by the average thickness of the water layer between the solid particles.

With addition of super-plasticizer, cement-fine solids with water/solids ratios of (for example, 0.6, 0.5 and 0.4), very soon reach the so-called "saturation point" after which no further reduction of the shear resistance can be measured [8]. Because of the large amount of water-filled free space around the disagglomerated solid particles these tend to settle as soon as the suspension is no longer stirred. The coarser particles sink faster than the finer ones in accordance with the laws of physics. The finer cement grains, which individually have already been substantially hydrated in the suspension, therefore only come into contact with one another after a fairly long period, and are no longer capable of developing the required solid hydrate structure with one another. They appear as a chalk-white surface layer. Concretes with excessively high additions of super-plasticizer and excessively low ultrafines content may then exhibit the typical faults during the production of such concrete.

Mix Proportioning

Before any SCC is produced and used at site the mix has to be designed and tested. In this process, local materials should be tested to achieve new concrete mixes with right mixing sequences and mixing time valid for the plant and also suitable for the element to be cast. Various kinds of fillers can result in different strength, shrinkage and creep but shrinkage and creep will usually not to be higher than for that of conventional concrete. Various mix proportions of SCC as shown in Table 2 have been used all over the world depending upon the locally available materials.

Self–compacting Concrete

Mix Design Principles

The flowability and viscosity of the paste is adjusted and balanced by careful selection and proportioning of the cement and additives by limiting water powder ratio and then by adding a super plasticizers and Viscosity modifying agents. By controlling precisely these components of SCC, their compatibility and interaction, good filling and passing ability, resistance to segregation can be achieved. For controlling temperature rise and shrinkage cracking as well as strength, the powder ratio in the concrete can be increased. The paste is the vehicle for the movement of aggregate therefore the volume of the paste must be greater than void volume in the aggregate [4]. The coarse to fine aggregate ratio in the mix should be low to achieve a good passing ability between congested reinforcement.

So SCC should have:-
  • Low coarse aggregate content
  • Increased paste content
  • Low water powder ratio
  • Increased super plasticizers dosage
  • Sometimes VMA can be used

Super Plasticizers and Cementitious Materials

When we increase the slump of concrete over 175mm by increasing amount of water the bleeding increases too much but with superplasticizers flowing concrete with slump level up to 250mm can be manufactured with no or negligible bleeding. The most important basic principle for flowing and cohesive concrete (SCC) is the use of superplasticizers combined with a relatively high content of powder materials in terms of Portland cement mineral additions ground fillers and very fine sand. A particle replacement of Portland cement by fly ash is also good for theological properties (resistance to segregation, strength level and crack freedom) practically in mass concrete structures exposed to resistance thermal stresses produced by the heat of hydration of cement.

Viscosity Modifying Agents (VMA)

With these admixtures (0.1 to 0.2% by mass of cementitious materials) SCC can be made with a reduced volume of fine materials. There are two types of VMA used:
  • Traditional pumping aids, admixtures used to improve the cohesiveness of lean concrete mixtures to be pumped and chemically based on modified cellulose or hydrolyzed starches
  • Polithielen glycol and biopolymers which appear to be the most effective VMA's for SCC

Constituent Materials of SCC

The constituent materials for SCC are same as in the case of conventional concrete except specific addition and chemical admixtures to achieve flowability and performance [29].

Cement

All cements which confirms any type of standard can be used for the production of SCC. However, correct choice of cement type depends on the specific requirements of application rather than the specific requirement of SCC.

Addition

Due to fresh property requirements of SCC, inert and pozzolanic or hydraulic additions are commonly used to improve and maintain the cohesion and segregation resistance. Additions are also limiting the cement content in order to reduce the heat of hydration and thermal shrinkage.

Mineral fillers: The particle size distribution shape and water absorption of mineral fillers may affect the water demand or sensitivity. CaCO3 based minerals fillers (<0.125mm size) are widely used and can give excellent rheological properties and performance.

Fly ash: It enhances cohesion and reduces sensitivity to changes in water content. However, high levels of fly ash may produce a paste fraction which is so cohesive that it can become resistant to flow.

Silica fume: Silica fume or microsilica (very fine amorphous silica particles <1 micron) is complementary material to manufacture concrete with great cohesion in fresh state. Due to high fineness, spherical shape and amorphous nature of silica fume it gives good cohesion, improved resistance to segregation and high reactivity to SCC. Silica fume is also very effective in reducing or eliminating bleeding.

Ground blast furnace slag: It provides reactive fines with a low heat of hydration. GGBS is already exists in some type of cements. A high proportion of GGBS may affect stability of SCC resulting in reduced robustness with problems of consistence control while slower setting can also increase the risk of segregation.

Marble powder: It enhances segregation resistance to the concrete and when marble powder is used in dry form it absorbs moisture also.

Other additions: Metakaolin, natural pozzolana, ground glass, air cooled slag and other fine fillers have also been used or considered as additions for SCC.

Fine Aggregates

In order to ensure sufficient workability while limiting the risk of segregation or bleeding, SCC contains a large amount of fine particles (around 500 kg/m3). Nevertheless, high volume of paste in SCC mixes helps to reduce the internal friction between the fine aggregate particles but well graded sand is still very important. In order to avoid excessive heat generation the Portland cement is generally replaced by mineral admixtures like lime stone fillers and fly ash. Nature and amount of fillers added are cohesive in order to comply with the strength and durability requirements.

Coarse Aggregates

It is possible to use natural rounded semi crushed or crushed aggregate to produce SCC, with maximum aggregate size varying from 10 to 20mm. Their content is kept sufficiently low so that individual aggregate to be lubricated by a layer of mortar paste, thereby increasing the fluidity and reducing segregation potential. The reinforcement spacing is deciding factor in determining the maximum aggregate size in SCC.

Admixtures

Super plasticizers or high range water reducing admixtures are essential components of SCC. Viscosity Modifying Agent (VMA) may also be used to reduce the segregation and sensitivity of the mix due to variation in other constituents especially to moisture content.

Superplasticizers: The admixture should bring about the required water reduction and fluidity but should also maintain its dispersing effect during the time required for transport and application. Precast concrete is likely to require shorter retention period as compared to cast-in-situ.

VMA: VMA significantly increases cohesive property of SCC without altering its fluidity. These admixtures reduce the effect of variations of moisture content.

Air entraining admixtures: It increases freeze-thaw durability of SCC. They are also used to improve the finishing of flat slabs.

Fibres

Both metallic and synthetic fibres have been used in the production of SCC. Due to the use of fibres flowability and passing ability of SCC can be reduced. Trials are therefore needed to establish the optimum type, length and volume fraction to give all the required properties in the fresh and hardened state.

Mixing Water

Same water may be used as used for simple concrete. Where recycled water recovered from processes in the concrete industry is used the type/content and in particular any variation in content of suspended particles should be taken into account as this may affect the batch uniformity of mix.

Properties of SCC

Structural Properties

The basic ingredients used in SCC mixes are practically the same as those used in the conventional concrete, except they are mixed in different proportions and the addition of special admixtures to meet the project specifications for SCC. The hardened properties are expected to be similar to those obtainable with conventional concrete. Laboratory and field tests have demonstrated that the SCC hardened properties are indeed similar to those of conventional concrete as shown in Table 3.

Structural Properties of SCC

Development of Concrete Strength with Time

The difference between 28 days compressive strength of SCC and conventional concrete is not very pronouncing. Isolated cases however showed that at the same water cement ratios slightly higher compressive strengths of SCC is obtained. Some of the published test results show that an increase of the cement content and a reduction of filler content at the same time increase the initial compressive strength and the ultimate concrete strength [12]. The strength development of SCC is studied for various percentage of replacement of additions (fly ash) and results are shown in Fig. 3.

Percentage replacement of additions vs strength

Splitting Tensile Strength

All parameters which influence the characteristics of the microstructure of the cement matrix and of the interfacial transition zone are of decisive importance in respect of the tensile load bearing behaviour. Most of the results of the measured splitting tensile strength values are in the range of valid regulations for conventional concrete with the same compressive strength[13]. However, in about 30% of all data points a higher splitting tensile strength has been found (Fig. 4).

Splitting tensile strength vs. compressive strength

Hence SCC appears to have higher tensile strength. The reason is better microstructure, especially the smaller total porosity and the more even pore size distribution within the transition zone of SCC. Further, a denser cement matrix is present due to the higher content of ultra fines. Only few examples about SCC show more rapid increase of the tensile strength as compared to the corresponding compressive strength.

Modulus of Elasticity

The modulus of elasticity of concrete depends on the proportion of the Young s modulus of the individual components and their percentages by volume. Thus, the modulus of elasticity of concrete increases for high content of aggregates (of high rigidity), whereas it decreases with increasing cement paste content and increasing porosity. It is seen that the modulus of elasticity of SCC can be up to 20 % lower compared with conventional concrete having the same compressive strength and made of the same aggregates [13].

Modulus of elasticity vs. compressive strength

Shrinkage and Creep

SCC is affected in the same way as conventional concrete by the water cement ratio as well as the kind of specimen curing. The drying shrinkage of SCC has been examined several times. The shrinkage deformations can achieve higher values in SCC than in comparable conventional concrete. A large influence on the shrinkage deformations result from the aggregate combination, the relation of coarse to fine aggregates, fineness and content of ultra-fines. So the shrinkage can be reduced by a higher content of coarse aggregates. However, a minimum paste volume must be present, in order to ensure an optimal self-compaction of SCC without segregation. Furthermore, a denser microstructure of the cement paste can be achieved by addition of fillers with fineness larger than that of cement. It is seen that the drying shrinkage of SCC is 10 to 50% higher than that of conventional concrete. There is a steep rise of the deformations particularly for young concrete aged up to 28 days, which decreases again with an increasing age. As regard to creep behavior of the concrete, results are contradictory. Therefore more results are needed to establish creep behavior of SCC.

Bond Behavior

The bond between reinforcement and concrete is influenced by parameters of reinforcing bar and surrounding matrix. In SCC the main factors are the grading of the aggregates and the ultrafine material content, the consistency and application of superplasticizer and stabilizer. It is observed that a lower maximum bond strength, but a higher bond stiffness in the range of low displacements. After reaching the maximum bond stress the slip increases gradually but the bond stress decreases extremely low. In general the bond strength is dependent on the position of the reinforcing bar in the formwork (top bar effect), and the height of fresh concrete above and beyond the bar during casting. SCC shows a lower settlement in the formwork due to self-deaeration during flowing in comparison to conventional concrete [12].

Coefficient of Oxygen Permeability

The SCC mixes have significantly lower oxygen permeability coefficient than the conventional reference concrete mixes. Particularly, for SCC mixes using PFA and limestone powder the coefficient of permeability is only 30 - 40% of the level for the reference concrete mixes.

Sorptivity - Capillary Water Absorption

The research indicates that sorptivity was considerably lower for all the SCC mixes than for the traditional vibrated reference mixes. Such results would suggest that the near-surface concrete was denser and more resistant to ingress of fluid in the SCC mixes than in the corresponding reference mixes [14].

Chloride Diffusivity

Chloride ingress into concrete is one of the most common causes of durability problems, particularly corrosion of reinforcement in structural concrete. The ingress of chloride into concrete may be by permeation and capillary absorption of chloride-containing solutions or by diffusion of chloride ions through saturated internal network of pores in concrete. SCC mixes show similar chloride diffusivity to those of traditional vibrated mixes. However, the chloride diffusivity was found to be very much dependant on the types of powder used in concrete. Both the reference and SCC mixes containing PFA showed much lower values of coefficient of chloride migration than the other mixes.

Micro-Properties of Interfacial Transition Zone around Steel Reinforcement

The interfacial transition zone (ITZ) between cement paste and reinforcement (i.e. aggregates and steel bars) has been recognized as being the 'critical and weak link' in cement composites and structural concrete, and having a considerable influence on the engineering and durability properties. In order to assess the impact of the use of SCC on bond and durability performance, the micro-properties of the ITZ around steel reinforcement was studied using a novel depth-sensing micro indentation test [10]. The research indicates that the average elastic modulus and micro hardness values of the ITZ were higher in the SCC mix than in the corresponding traditional vibrated reference mix. As expected, the ITZ properties below steel bar were found to be relatively weaker than those above the bar, likely to be caused by internal bleeding and settlement of the fresh concrete during the placing and compaction processes. However, the results seemed to indicate that the difference of ITZ properties between top side and bottom side of the steel bar was less pronounced for the SCC mix than for the reference mix [11].

Test Methods for Flowability of SCC

The Slump Flow Test

This is a test method for evaluating the flowability of SCC, The basic equipment is the same as for the conventional slump test. However, the concrete placed into the mold is not rodded. When the slump cone has been lifted and the diameter of the spread is measured rather than the vertical distance of the collapse, as shown in the Fig. 6.

Slump flow test

Funnel Test

The V-funnel test was developed in Japan and used by Ozawa [28]. In this test method, segregation resistance of SCC is being evaluated using a funnel as shown in Fig. 7. The funnel is filled with concrete and the efflux time of SCC with coarse aggregates having the maximum size of less than 25 mm is measured.This test gives account of the filling capacity (flowability). The inverted cone shape shows any possibility of the concrete to block is reflected in the result.

Self–compacting Concrete

T500 Test

This is also a test method for evaluating the material segregation resistance of SCC, where the 500-mm flow reach time is measured in the slump flow test as shown in Fig. 6. SCC should give 500 = 2 - 5 seconds.

U-Type and Box-Type Tests

In this method, flowability of SCC with coarse aggregates having the maximum size of less than 25 mm is measured by passing it through an obstacle (Fig. 8). Time is measured to pass SCC through the obstacle for self-compactability [15,16].

Box Type Flow Obstacle Test

Applications of Self-compacting Concrete

Since the development of the prototype of self-compacting concrete in 1988, the use of self-compacting concrete in actual structures is gradually increasing. The main reasons for its growing application are summarized as follows:
  • To shorten the construction period
  • To assure better compaction in the structure: especially in confined zones where vibration is very difficult
  • To assure uniform construction with better quality
  • To reduce noise level in the factory as well as on the site
  • To reduce personal injuries from noise and manual handling
  • To reduce electricity usage at site
  • To reduce the overall maintenance of structure
  • To save the costs of vibration equipment.
A typical application of SCC is in the two anchorages of Akashi-Kaikyo (Straits) a suspension bridge opened in April 1998, with the longest span (1991 m) in the world. The SCC provides tangible opportunities to both designer and contractor for its applications. It also has a future in the precast industry providing durable concrete at a lower cost due to lower initial investments of vibrating facilities and lower recurring costs due to faster re-usage of moulds. It improves the working environment at plants and sites by eliminating noise of vibration; it is possible for concrete product plants to be located in the urban area.

Summary and Outview

Self-compacting concrete is an innovative construction material, which offers various advantages in construction process due to its outstanding characteristics. Considering the economy and the durability of conventional concrete structures, it is observed that the quality and the density of the concrete, as well as the compaction of the concrete are main parameters that cause deterioration. For this, SCC offers new possibilities and prospects. It is a boon considering improvement in concrete quality, significant advances towards automation and concrete construction processes, shortened construction time, lower construction cost, much improved working conditions and is rightly called as a 'silent revolution' in the field of concrete technology.

The interpretation of various researches can be summarized as follows:
  • The concrete strength of SCC and normal concrete are similar under comparable conditions, this statement includes also the time development of concrete strength,
  • Tensile splitting strength, modulus of elasticity and shrinkage of SCC and normal concrete differ, but the differences vary within the usual scatter width, known for normal concrete
  • No final tendency can be given for creep of SCC.
Based on these facts, there is just no need of specific design rules for SCC. Further research projects are required to interpret influence on the hardened properties of SCC more precisely. Therefore further investigations have to follow, to get knowledge about the influence of any parameter, as for example the cement type, the type of filler and its portion, the water-binder ratio and so on. To use the advantages of SCC efficiently, all parameters affecting the properties of concrete in respect of the production and the durability of concrete structures should exactly be known. In this way only SCC can be designed optimally, without causing later damages by the usage of a new and modern building material. The use of self-compacting concrete is recommended for all applications, where the mentioned advantages are necessary to assure a good concrete quality. Especially in highly congested reinforced concrete members like bridge decks, abutments, nuclear power structure, tunnel linings, tubing segments or where it is difficult to vibrate the concrete, or even for normal engineering structures, SCC is favourably suitable. In our country, the special type of concrete is used on few jobs on experimental basis. However it will be more frequently used as economics and environmental pollution will become major concerns of 21st century.

Acknowledgment

The author is gratefully acknowledged the help rendered by Mr Abhinav Daharwal and Ms Chanchal Jain of VNIT, Nagpur in preparation of this paper.

References

  • H Okamura and M Ouchi. 'Self-compacting Concrete-Development, Present use and Future' Proceedings of the First International RILEM Symposium on 'Self-Compacting Concrete'. Sweden, Proc 7, 1999, pp 3-14.
  • K Ozawa, M Kunishima, K Maekawa and K Ozawa. 'Development of High Performance Concrete Based on Durability Design of Concrete Structures'. Proceeding of EASEC-2, vol 1, January 1989, pp 445-450.
  • F Dehn, K Holschemacher, K and D Weibe. 'Self-Compacting Concrete -Time Development of the Material Properties and the Bond Behaviour'. LACER No 5, 2000, pp 115-124.
  • Horst Grube and Jörg Rickert, 'Self Compacting Concrete – Another Stage in the Development of the 5-Component System of Concrete'.
  • Okamura, Hajime, Ozawa, and Kazumasa: 'Mix Design for Self-Compacting Concrete' Concrete Library of JSCE No. 25, June 1995.
  • Takada, K.; Pelova, G. I.;Walraven, J. C.: Influence of Mixing Efficiency on the Mixture Proportion of General Purpose Self-Compacting Concrete. Int. Symp. on High Performance Concrete and Reactive Powder Concrete. 16.-20. Aug., 1998, Sherbrooke, Canada
  • Takada, K.; Pelova, G. I.; Walraven, J. C.: Self-Compacting Concrete Produced by Japanese Method with Dutch Materials. Congress of European Ready Mixed Concrete Organization, ERMCO 98, Lisbon, 23.-26. June 1998
  • Tattersall, G. H. : Rheology of Portland Cement Paste. British Journal of AppliedPhysics 6 (1955) Nr. 5, S. 165-167.
  • Spanka,G.;Grube,H.;Thielen,G.: Wirkungsmechanismen verflüssigender Betonzusatzmittel. Beton 45 (1995), H. 11, S. 802-808 und H. 12, S. 876-881 ; ebenso Betontechnische Berichte 1995-1997, S. 45-60
  • Zhu, W. and Bartos, P.J.M., 2000, 'Application of depth-sensing microindentation testing to study of interfacial transition zone in reinforced concrete', Cement and Concrete Research, 30, pp.1299-1304.
  • W Zhu J Quinn & PJM Bartos, aspects of durability of self compacting concrete.
  • König, G.; Holschemacher, K.; Dehn, F.; Weiße, D.: Self-Compacting Concrete - Time Development of Material Properties and Bond Behaviour.
  • Poceedings of the Second International Symposium on Self-Compacting Concrete, Tokyo, (2001), pp. 507 - 516.
  • Klaus Holschemacher & Yvette Klug; A Database for the Evaluation of Hardened Properties of SCC.
  • H Okamura and K Ozawa. 'Mix Design for Self-Compacting Concrete'. Concrete Library of JSCE, no 25, June 1995, pp 107-120.
  • 'Specification and Guidelines for Self-Compacting Concrete'. EFNARC, Association House, 99 West Street, Farnham, Surrey GU9 7EN, UK, February 2002.
  • Ferraris, Browner, Ozyildirim & Daczko; 'workability of self compacting concrete'.
  • Hayakawa M, Matsuoka Y, Shindoh T. "Development and Application of superworkable concrete. Proc of Int RILEM workshop on special concretes – workability and mixing, Paisley, Scotland, June. London: E&FN Spon; 1993.p. 891-902.
  • Furuya N, Itohiya T, Arima I. Development and application of highly flowing concrete for mass concrete anchorages of Akashi-Kaikyo Bridge. Proc of Int Conf on high performance concrete. Detroit, USA: American Concrete Institute; 1994. P. 371-396.
  • de Larrard F, Gillet G, Canitrot B. Preliminary HPC mix design study for the Grand Viaduct de Millau. In: Proc of 4th Int Symp on utilisation of high strength concrete, May Paris France, 1996.p. 1323-331.
  • Nishizaki T, Kamada F, Chikamatsu R, Kawashima H. Application of high strength self compacting concrete to prestressed concrete outer tank for LNG storage. Proc of 1st RILEM Int Symp on self compacting concrete, Stockholm. Paris: RILEM; 1999.p. 629-38.
  • Botte J, Burdin J, Zermatten M. SCC tunnel applications: Proc of Int RILEM workshop on self compacting concrete, Stockholm. Paris: RILEM; 1999.p. 681-93.
  • Inoue H, Takeichi Y, Ohtomo T. Construction of rigid foundation of underground diaphragm wall. In: Proc of 2nd Int Symp on utilisation of SCC, Japan 2001.p. 643-50.
  • Johanson K, Kyltveit BP. ScCC in a rock repository for radioactive waste. : Proc of 2nd Int Symp on utilisation of SCC, Tokyo, Japan Oct 2001.p 681-86.
  • Fornasier G, Giovambattista P, Zitzer L. SCC in Argentina; development program and application. In:Proc of 1st north American conference on the design and use of SCC, Chicago, Nov 2002. P. 439-44
  • Collepardi M, Collepardi S, Ogoumah ologat JJ, troli R.laboratory test and field experiences of high performance SCCs. Proc of 3rd RILEM Int Symp on SCC, Iceland, Aug. France; RILEM publications PRO 33;2003.p. 413-16.
  • Jörg Dietz, Jianxin Ma; "Preliminary Examinations for the Production of Self-Compacting Concrete Using Lignite Fly Ash".LACER No.5 – 2000.p.125-140.
  • Dr. R. Sri Ravindrarajah, D. Siladyi, B. Adamopoulos; "development of high-strength self-compacting concrete with reduced segregation potential" Poceedings of the 3rd International RILEM Symposium , Reykjavik, Iceland, 17-20 August 2003, Edited by O. Wallevik and I. Nielsson , (RILEM Publications), 1 Vol., 1048 pp.
  • K Ozawa, N Sakata and H Okamura. 'Evaluation of Self-Compactibility of Fresh Concrete Using the Funnel Test'. Concrete Library of JSCE, vol 25, June 1995, pp 59-75.
  • "The European Guidelines for Self Compacting Concrete" Specification , Production and Use; EFNARC (2005); pp 63.

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Interlocking Concrete Paver Blocks

Interlocking Concrete Paver Blocks

An Easy Approach for Road Construction

Dr. S.D. Sharma, Scientist "F" Central Road Research Institute, New Delhi

Interlocking Concrete Block Pavement (ICBP) has been extensively used in a number of countries for quite sometime as a specialized problem-solving technique for providing pavement in areas where conventional types of construction are less durable due to many operational and environmental constraints. ICBP technology has been introduced in India in construction, a decade ago, for specific requirement viz. footpaths, parking areas etc. but now being adopted extensively in different uses where the conventional construction of pavement using hot bituminous mix or cement concrete technology is not feasible or desirable. The paper dwells upon material, construction and laying of concrete block pavement as a new approach in construction of pavement using Interlocking Concrete Paver Blocks.

Introduction

Concrete paver blocks were first introduced in Holland in the fifties as replacement of paver bricks which had become scarce due to the post-war building construction boom. These blocks were rectangular in shape and had more or less the same size as the bricks. During the past five decades, the block shape has steadily evolved from non-interlocking to partially interlocking to fully interlocking to multiply interlocking shapes. Consequently, the pavements in which non-interlocking blocks are used are designated as Concrete Block Pavement (CBP) or non-interlocking CBP, and those in which partially, fully or multiply interlocking blocks are used are designated as 'Interlocking Concrete Block Pavement (ICBP).

CBP/ICBP consists of a surface layer of small-element, solid un-reinforced pre-cast concrete paver blocks laid on a thin, compacted bedding material which is constructed over a properly profiled base course and is bounded by edge restraints/kerb stones. The block joints are filled using suitable fine material. A properly designed and constructed CBP/ICBP gives excellent performance when applied at locations where conventional systems have lower service life due to a number of geological, traffic, environmental and operational constraints [1-8]. Many number of such applications for light, medium, heavy and very heavy traffic conditions are currently in practice around the world.

Advantages and Limitations

There are many distinct features of ICBP as compared to the conventional methods of pavement construction and hence make it a suitable option for application in the specified areas [7 & 10]. Some of these are:
  • Mass production under factory conditions ensures availability of blocks having consistent quality and high dimensional accuracy.
  • Good quality of blocks ensures durability of pavements, when constructed to specifications.
  • ICBP tolerates higher deflections without structural failure and will not be affected by thermal expansion or contraction.
  • ICBP does not require curing, and so can be opened for traffic immediately after construction.
  • Construction of ICBP is labor intensive and requires less sophisticated equipment.
  • The system provides ready access to underground utilities without damage to pavement.
  • Maintenance of ICBP is easy and simple and it is not affected by fuel and oil spillage.
  • Use of coloured blocks facilitates permanent traffic markings.
  • ICBP is resistant to punching loads and horizontal shear forces caused by maneuvering of heavy vehicles
  • Low maintenance cost and a high salvage value ensures low life cycle cost.
However, important limitations of the technique are the following:
  • Quality control of blocks at the factory premises is a prerequisite for durable "ICBP"
  • Any deviations of base course profile will be reflected on the "ICBP" surface. Hence extra care needs to be taken to fix the same.
  • High quality and gradation of coarse bedding sand and joint filling material are essential for good performance.
  • "ICBP" over unbound granular base course is susceptible to the adverse effects of poor drainage and will deteriorate faster. "ICBP" is not suited for high speed roads (speed above 60 km/h)

Physical Requirements

Interlocking Concrete Paver Blocks
Since zero slump concrete is used in production of paver blocks, the quality of blocks produced will depend upon various parameters like the capacity of compaction and vibration of machine, grade of cement used, water content, quality of aggregates used, their gradation and mix design adopted, additives used, handling equipment employed, curing method adopted, level of supervision, workmanship and quality control achieved, etc. Recommended grades of paver blocks to be used for construction of pavements having different traffic categories are given in Table 1 [9].

Application of ICBP Technology

Some of the proven areas where ICBP technology is being applied are listed below [9 & 10]:
  1. Non-traffic Areas: Building Premises, Footpaths, Malls, Pedestrian Plaza, Landscapes, Monuments Premises, Premises, Public Gardens/Parks, Shopping Complexes, Bus Terminus Parking areas and Railway Platform, etc.
  2. Light Traffic: Car Parks, Office Driveway, Housing Colony Roads, Office/Commercial Complexes, Rural Roads, Residential Colony Roads, Farm Houses, etc.
  3. Medium Traffic: Boulevard, City Streets, Small Market Roads, Intersections/Rotaries on Low Volume Roads, Utility Cuts on Arteries, Service Stations, etc.
  4. Heavy and Very Heavy Traffic: Container/Bus Terminals, Ports/Dock Yards, Mining Areas, Roads in Industrial Complexes, Heavy-Duty Roads on Expansive Soils, Bulk Cargo Handling Areas, Factory Floors and Pavements, Airport Pavement, etc.

Shapes and Classifications

There are four generic shapes of paver blocks corresponding to the four types of blocks as below [9 & 10]:
  1. Type A: Paver blocks with plain vertical faces, which do not key into each other when paved in any pattern,
  2. Type B: Paver blocks with alternating plain and curved/corrugated vertical faces, which key into each other along the curve/corrugated faces, when paved in any pattern,
  3. Type C: Paver blocks having all faces curved or corrugated, which key into each other along all the vertical faces when paved in any pattern and
  4. Type D: 'L' and 'X' shaped paver blocks which have all faces curved or corrugated and which key into each other along all the vertical faces when paved in any pattern.
Interlocking Concrete Paver Blocks
The generic shapes and groups of paver blocks identified to four types are illustrated in Figures 1 & 2.

Materials

The quality of materials, cement concrete strength, durability and dimensional tolerance of paving blocks, etc. is of great importance for the satisfactory performance of block pavements. These aspects and the block manufacturing process itself, which immensely affect the quality of paving blocks, have been outlined in the Indian Roads Congress Special Publications [9]. The Central Road Research Institute (CRRI) has prepared the specifications for ICBP [10]

Paving Blocks

The quality of materials, strength of cement concrete and durability as well as dimensional tolerances etc. are of great importance for satisfactory performance of block pavement. The recommended thickness of block and grades of concrete for various applications and specification for paving in which materials used for preparation of blocks, physical requirements, physical test methods, sampling and acceptance criteria has already been formulated in BIS Code [10].

Bedding and Joint Filling Sand

Interlocking Concrete Paver Blocks
It is well established that if proper attention is not paid to the quality of bedding sand, and if the thickness of bedding sand layer is not uniform enough, serious irregularities in surface profile can result; excessive differential deformation and rutting can occur early in service life of the block pavement. The gaps in between two adjacent paving blocks (typically about 3 mm wide) need be filled with sand, relatively finer than the bedding sand itself. The desired gradation for the bedding and joint filling sands are given in Table 2 [9].

It is necessary to restrict the fines (silt and/or clay passing 75 micron sieve) to 10 percent, since excessive fines make joint filling very difficult. Similarly, it is not advised to use cement in the joint filling sand, which may not only make it difficult to completely fill the joints, but may also adversely affect the desired flexibility characteristics of the paving block layer. The joint filling sand should be advisably as dry as possible; otherwise complete filling of joints may be difficult.

Base and Sub base Materials

The engineering properties of base materials are the load spreading properties to disperse stresses to the subgrade and the desired drainage characteristics, having an important bearing on the performance of a block pavement. Although, local availability and economics generally dictate the choice of base material at the design stage, yet the commonly used materials considered suitable for base courses are unbound crushed rock, water-bound macadam, wet mix macadam, cement bound crushed rock/granular materials, and lean cement concrete/ dry lean concrete etc [11, 12, 13, 14 & 15]. In broad terms, wherever the subgrade is weak (having a CBR value below 5) use of bound granular materials, like, cement treated crushed rock, requiring a relatively thinner base, should be preferred while for high strength subgrades, unbound crushed rock may be used. The climatic and environmental factors also need be considered during the choice of a base material. Sub-base is essential where commercial traffic is expected. The quality of sub-base materials is inferior to the base materials and includes natural gravels, cement treated gravels and sands and stabilized subgrade materials. The quality of sub-base materials should be in conformance with IRC: 37 [16].

Construction of Interlocking Concrete Block Pavement

Interlocking Concrete Paver Blocks

Sequencing of operations

The sequencing of operations (Fig. 3) for construction of block pavement should be as follows [1 & 4]:
  1. Installation of sub-surface drainage structures
  2. Leveling and compaction of subgrade
  3. Provision and compaction of sub-base course (where needed)
  4. Provision and compaction of base-course and checking for correct profile
  5. Installation of edge restraints
  6. Provision and compaction of coarse bedding sand
  7. Laying of blocks and interlocking
  8. Application of joint sealing sand and compaction
  9. Cleaning of surface
  10. Filling any remaining empty portions in the block layer especially near edge restraint blocks with in situ concrete.

Construction of Sub–grade

This is the foundation layer over which the block pavement is constructed. Like in conventional pavements, the water table level should not be at a level of 600 mm or higher, below the subgrade level. It should be compacted in layers of either150 or 100 mm thickness guidelines [10]. The prepared subgrade should be graded and surface dressed to a tolerance of ± 20 mm of the design levels, and its surface evenness should have a tolerance of within 15 mm under a 3 m straight edge [10].

Construction of Base and Sub-base Layers

Base course and sub-base course are constructed in accordance with standard procedures contained in relevant IRC specifications like IRC:SP:49-1998, IRC:50-1973, IRC:51-1992, IRC:63-1976, IRC:19-1997 & IRC:37, 2001, [11, 12, 13, 14, 15 & 17]. When cement bound base is proposed it may be constructed using rolled lean concrete as IRC:SP-49. The quality control specified in IRC: SP-11 [16] shall apply. Constructing the lower layers to proper level and grade is very essential to maintain the top surface level and surface regularity of the block pavement surface.

Edge Restraint Blocks and Kerbs

Interlocking Concrete Paver Blocks
Concrete blocks on trafficked pavements tend to move sideways and forward due to braking and maneuvering of vehicles. The tendency to move sideways has to be counteracted at the edges by special edge blocks and kerbs. The edge block should be designed and anchored to the base such that the rotation or displacement of blocks is resisted [7]. These are to be made of high strength concrete for withstanding the traffic wheel-load without getting damaged. These members should be manufactured or constructed in-situ to have at least a 28-day characteristic compressive strength of 30 MPa or flexural strength of 3.8 MPa. As far as possible the edge blocks should have vertical face towards the inside blocks. A few typical edge-blocks are also shown in Fig. 4. Where the space is not easily permitting the use of plate vibrators, jhurmut or manual compactor using small size plate rammer may be used. The road kerbs provided on edges of roads also serve the purpose of edge blocks. In case the kerbs are not provided, it has to be replaced by edge strips. In case of heavy traffic 150 mm x 150 mm plain cement concrete (M-25) may also be provided over dry lean concrete to give further confinement of blocks. In-between the edge-restraint blocks cement mortar (1: 6, cement: coarse sand) may be used in place of sand for sealing of blocks [7].

Placing and Screeding of Bedding Sand

The thickness of the sand bed after compaction should be in the range of 20-40 mm [10], whereas, in the loose form it should be 25 to 50 mm. It is preferable to restrict the compacted thickness to 20-25 mm to reduce the risk of any localized over-compaction, which would affect the final block surface level. Bedding sand should not be used to fill-up local depressions on the surface of a base or sub-base. The depressions if any, should be repaired with same base or sub-base material in advance before placing sand. The sand of specified gradation to be used, should be uniformly in loose condition and should have uniform moisture content. Optimum moisture content is that when sand is neither too wet nor too dry and has moisture of 6 to 8 percent. Requirement of sand for a day's work should be prepared and stored in advance and covered with tarpaulin or polythene sheets. The processed sand so obtained is spread with the help of screed boards to the specified thickness. The screed boards are provided with nails at 2-3 m apart which when dragged gives the required thickness. The length of nail should take into account the surcharge to be provided in the uncompacted thickness. Alternatively, the screed can be dragged on edge strips kept on both sides as guides [7].

Laying of Blocks

Blocks can be laid generally by manual labour but mechanical aids like hand-pushed trolleys can expedite the work. Normally, laying should commence from the edge strip and proceed towards central line. When dentated blocks are used, the laying done at two fronts will create problem for matching joints in the middle. Hence, as far as possible, laying should proceed in one direction only, along the entire width of the area to be paved [4, 5, 6, 7, 8 & 10].

While locating the starting line, the following should be considered:
Interlocking Concrete Paver Blocks
  • On a sloping site, start from the lowest point and proceed to up-slop on a continuous basis, to avoid down-slop creep in incomplete areas.
  • In case of irregular shaped edge restraints or strips, it is better to start from straight string line as shown in Fig. 5.
  • Influence of alignment of edge restraints on achieving and maintaining laying bond.
Laying of block pavement at National Highway - 08 Jaipur-Kishangarh section for heavy commercial vehicles for truck-lay-byes on both side of pavement are shown in Figs 6 and 7.
Interlocking Concrete Paver Blocks

Establishing the Laying Pattern

The blocks can be placed in different bonds or patterns depending upon the requirement, some popular bonds commonly adopted for block paving are [4]:

1. Stretcher or running bond 2. Herringbone bond 3. Basket weave or parquet bond The typical layouts of these bonds are given in Fig. 8.
Interlocking Concrete Paver Blocks

Typical Pavement Composition

A typical compositions normally used in ICBP are given in Table 3 and a cross-section is shown in Fig. 9 [10].Block Pavements at Typical Locations: Essentially, there are three important aspects in detailing. These typical locations are [10]:Curves: It is necessary to cut the paving units to fit the edge restraints. Rectangular blocks of a similar or contrasting colour as an edging have been used to minimize the visual effects of small errors in block cutting. To avoid unsightly and potentially weak joints, it is often preferable to change the laying pattern at the curve. The curve itself can be installed in Herringbone bond and yet the pavement can revert to stretcher bond on the approaches [10].Pavement Intrusions: On some pavements, like in city streets, there could be several intrusions, like, manholes, drainage gulleys, etc. where coping with these intrusions with the pavement is desirable. Around intrusions, it is good practice to lay along both sides of the intrusion simultaneously so that closure is made away from the starting workface, rather than carrying the pavement around the intrusion to return to the original laying face to avoid accumulation of closing error [10].

Interlocking Concrete Paver Blocks
Changes in alignment: Changes in alignment of a road pavement can some times be achieved by the use of special blocks. However, it is generally easier to choose a block that can be installed in Herringbone bond through simply cutting the blocks to fit into the edge restraints. Where aesthetic requirements of shape of the paving unit dictate the use of Stretcher bond, then only a 90o shape change in alignment can be achieved without cutting the blocks (Fig. 10). At intersections, if a Herringbone bond laying pattern is adopted, the block laying can proceed without the need for construction joints (Figs 11). An alternative to this is to install a shoulder (support) course of rectangular blocks between the main road and the side streets; this permits different laying patterns to be used in two roadways [10].

Compaction

For compaction of the bedding sand and the blocks laid over it, vibratory plate compactors are used over the laid paving blocks; at least two passes of the vibratory plate compactor are needed. Such vibratory compaction should be continued till the top of each paving block is in level with its adjacent blocks. It is not a good practice to leave compaction till the end of the day, as some blocks may move under construction traffic, resulting in the widening of joints and corner contact of blocks, which may cause spalling or cracking of blocks. There should not be delay in compaction after laying of paving blocks to achieve uniformity of compaction and retention of the pattern of laying. During vibratory compaction of the laid blocks, some amount of bedding sand may get filled up into the joints between them; the extent of sand getting filled up into the joints will depend on the degree of compaction of sand, i.e. the force applied by the compactor. Standard compactors may have a weight of about 90 kg, plate area of about 0.3 m2and apply a centrifugal force of about 15 kN, while heavy duty compactors may weigh 300-600 kg, have a plate area of about 0.5-0.6 m2 and apply a centrifugal force of 30-65 kN. Where the bedding sand is required to be compacted for heavy traffic block pavements, heavy-duty compactors should be used. After compaction by vibratory plate compactors, some 2 to 6 passes of a vibratory roller (with rubber coated drums or those of static weight less than 4 tonnes and nominal amplitude of not more than 0.6 mm) will further help in compaction of bedding sand and joint filling [4 & 10].

Laying and Surface Tolerances

Interlocking Concrete Paver Blocks
While constructing the block pavement, the surface tolerances of individual layers may be observed as shown in Table 4 [4 & 10].

Conclusion

  1. ICBP technology can provide durable and sustainable road infrastructure where construction and maintenance of conventional pavements are not cost effective.
  2. ICBP is much cheaper than rigid (concrete) pavement designed for identical conditions. Compared to bituminous pavement for low traffic volumes and high strength subgrade, the initial construction cost of ICBP is likely to be equal to or marginally higher. For high traffic volumes and low strength subgrade, ICBP will be cheaper than flexible pavement.
  3. Guidelines for use of Interlocking Concrete Block Pavement and Specification on Paver Blocks are published in Codes and available with Indian Roads Congress and Bureau of Indian Standards which are very useful for Indian industries and highway professions for adoption of block pavement technology.

Acknowledgments

Author is thankful to Dr. S. Gangopadhyay, Director, CRRI, for his kind permission to publish this paper and continuous encouragement. Sincere thanks are also due to Shri B.M. Sharma, Head, Pavement Evaluation Division for encouraging support to write this paper. The cooperation and valuable suggestions extended by Shri T. Muraleedharan, Retired Senior Scientist, Pavement Evaluation Division is gratefully also acknowledged.

References

  • Muraleedharan, T., Sharma, S.D., Sridhar, S.K. and Nanda, P.K., "Face Lifting of Old Concrete Pavement Using Interlocking Concrete Block Pavement Technique," Proceeding, International Seminar on New Trends in Highway Construction, New Delhi, pp 161-174, Nov-1997.
  • Muraleedharan, T. and Nanda, P.K., (1992) "Application and Performance of Interlocking Concrete Block Pavement – An Overview," The Indian Concrete Journal, pp 395 -400, July 1992.
  • Muraleedharan, T. and Nanda, P.K., Laboratory and Field Study on Interlocking Concrete Block Pavement foe Special Purpose Paving in India," Proceeding, Fifth International Conference on Concrete Block Paving, Tel Aviv, Israel, pp413-422, June-1996.
  • Sharma, S.D., Prashant Kumar, Nanda, P.K., "Interlocking Concrete Block Pavements: New Trends in Construction," Civil Engineering & Construction Review, (Roads), 2005
  • Sharma, S.D., Sood, V.K. and Sikdar, P.K., "Interlocking Block Pavement For Sustainable Road Infrastructure For Cold Region," International Conference on Sustainable Habitat For Cold Climates, Leh, India, September 16-18, 2004.
  • Sharma, S.D., Sikdar, P.K., Rao, Y.V., (2004) "Interlocking Concrete Block Pavements – Its Prospects," Seminar on Design Construction and Maintenance of Cement Concrete Pavements, IRC, October, 2004.
  • "New and Improved Road Technologies (Tenth Five Year Plan 200207) – Material for Special Road Applications – Development of Paving Surface for High Altitude and Desert Areas, Central Road Research Institute, New Delhi, March 2007.
  • Sharma, S.D., "Interlocking Concrete Blocks Pavement: New Approach in Construction for Rural Roads," All India Seminar on "Highways Development: Design, Construction, Operation and Repairs & Concrete Day Celebration – Lucknow, 2008.
  • IS:15658:2006 on "Precast Concrete Blocks for Paving – Specification," Bureau of Indian Standards,
  • IRC SP: 63-2004 "Guidelines for Use of Interlocking Concrete Block Pavement" Indian Roads Congress.
  • IRC: SP: 49 – 1998: "Guidelines for Use of Dry Lean Concrete as Sub-base for Rigid Pavement," Indian Roads Congress.
  • IRC:50-1973, "Recommended Design Criteria for Use of Cement Modified Soil in Road Construction."
  • IRC:51-1992, "Guidelines for the Use of Soil Lime Mixes in Road Construction."
  • IRC:63-1976, "Tentative Guidelines for the Use of Low Grade Aggregates and Soil Aggregate Mixtures in Road Construction."
  • IRC:19-1977, "Standard Specifications and Code of Practice for Water Bound Macadam."
  • IRC SP-11-1988: "Handbook of Quality Control for Construction of Roads And Runways," Indian Roads Congress, (Second Revision).
  • IRC:37-2001: Guidelines for the Design of Flexible Pavements (Second Edition).

NBMCW September 2009

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SFRC for Industrial Floors

Ganesh P. Chaudhary, Bekaert Industries Pvt Ltd, Mumbai

During the last three decades SFRC was considered a new technology for construction industry. However, this technology has found high acceptance among the construction industry. Currently, steel fibers are used mainly in industrial flooring, tunneling, pavements etc.

Construction time and durability are the main factors among the various advantages which help SFRC to command its superiority over other methods.

In our country, lot has been written or published about SFRC, but we are not using this technology as it is being used in other countries there is a definite and detail approach on how to design fiber concrete and achieve a homogeneous dispersion of steel fibres. Steel fibre geometry and grading of concrete play a vital role in SFRC.

Steel fibre reinforced concrete is defined as a concrete, containing discontinuous discrete steel fibres. Steel fibres are incorporated in concrete to improve its crack resistance, ductility, energy absorption and impact resistance characteristics.

Properly designed and dosed SFRC can reduce or even eliminate cracking a common cause for concern in plain concrete.

The article talks about various aspects of steel fibre reinforced concrete Viz. Design of SFRC floor based on lose berg's yield line model, resistance to corrosion, selection criteria, mix design and other practical considerations as applicable to Shapoorji Palonji project site in Coimbatore.

Steel fibre reinforced floor is defined as a floor made of concrete, containing discontinuous discrete steel fibres. Steel fibres are incorporated in floor concrete to improve its crack resistance, ductility, energy absorption and impact resistance characteristics.

Properly designed and dosed SFRC floor can reduce or even eliminate cracking a common cause for concern in plain concrete.

Scope

Concrete composition, admixtures, placing and curing play another evident role but here focus will be on a sample design of SFRC Industrial floors using Drapro and selection criteria of steel fibre.

Industrial floors are generally subjected to Loads such as Point load, UDL and Wheel Load. In Interest of explaining load effects certain loads and sub base values are assumed to arrive at Flexural Stress and corresponding dosage. Other assumptions such as temperature, Joint distance, loading factor can be made available on request.

Input–Loads

sfrc for industrial floors

Point Loads

(Figure i) illustrates Point loads arising from Rack loads, Stacking Area, Lines etc. We need to design a floor which is efficient of taking these loads at various locations such as joint of panels, center of panels, etc.

Anticipated Location of Load

(Figure ii) illustrates various location of loads as discussed in above paragraph.

Wheel Loads

sfrc for industrial floors
Wheel loads are loads coming from moving equipment like Forklift. The diagram gives details of loads arising out of a 6 ton capacity Forklift having a tire pressure of 1.5 N/mm ^2 (Figure iii).

UDL

(Figure iv) illustrates UDL of 5 Ton / M ^2.

Input- Sub Base

sfrc for industrial floors
Sub base plays an important role in floor. Generally, subbase (Figure v) is seen in industries. To analyze the effect of subbase on floor design, it is necessary to arrive at equivalent E-modulus or CBR value of the subbase.

If there are more than 2 layer of sub-base defined the equivalent E-modulus of the ground is calculated using the formula below
sfrc for industrial floors

Result

As it is not known beforehand which yield will occur first, we have to consider all possible load combinations. After considering various load combinations and locations maximum moments (Table i) are foreseen.
sfrc for industrial floors

Steel Fibres

Selection Criteria

The most important aspects controlling the performance of steel fibres in concrete are as follows:
sfrc for industrial floors
  • Tensile Strength on the wire ( > 1000 Mpa)
  • Aspect ratio
  • Geometrical shape
sfrc for industrial floors
Higher aspect ratio (Figure vii) always gives better performance of the SFRC with respect to flexural strength, impact resistance, toughness, ductility, crack resistance etc.

Unfortunately, the higher the aspect ratio and volume concentration of the fibre, the more difficult the concrete becomes to mix, convey and pour. Thus there are practical limits to the amount of single fibres, which can be added to SFRC, with the amount varying with the different geometrical characteristics of the several fibre types. Loose steel fibres with a high 1/d aspect ratio, which is essential for good reinforcement, are difficult to add to the concrete and to spread evenly in the mixture.

sfrc for industrial floors
BEKAERT has glued (Figure viii) the loose fibres together with water-soluble glue into bundles of 30-50 fibres to facilitate handling of the Dramix steel fibres. The individual Dramix steel fibres have the necessary high 1/d aspect ratio, but as they are glued together in compact bundles, they have approximately the same size as the other aggregates. Glued Dramix steel fibres present no difficulty in mixing. They are added as an extra aggregate and require no special equipment to be added to the mix, whether dry mix or wet mix. The hooked ends improve the bond and anchorage of the Dramix steel fibres in the concrete/shotcrete and increase the reinforcing efficiency and ductility. Hooked ends are proved to be best as compared to any other shape of fibres. Bekaert has done extensive research on same copies of which can be made available on request.

Fibre Dosage

This is one of the most important elements in SFRC. As discussed earlier fibre performance clearly depends upon parameters like tensile strength, aspect ratio, and anchorage.

The dosage of fibres for a certain performance varies as per type of fibre used. This can be established by making a proper design followed by field test.

ollowing table gives comparison of various types of fibres in terms of dosage.

Comparison with Alternatives

A conventional pavement with 200 mm Thk with single Mesh can be replaced by a 120 mm Thk (SFRC) pavement with following combinations.
sfrc for industrial floors

* Results valid only for Dramix Fibres

Although unit cost of lower aspect ratio (45) fibre is less, due to high dosage ( 27.5) Kg) per M ^3 cost of SFRC becomes very high as compared to that of SFRC with lower dosage (15 kg) of High Aspect ratio (80) Fibres.

Practical Considerations

sfrc for industrial floors
Steel fibre reinforced concrete is better concrete as compared to RCC in certain applications. To make this technology practically possible, it is very much necessary to give importance to fibre geometry, concrete consistency, gradation etc. What we want is concrete with right mix and homogeneous dispersion of steel fibres (Figures iX & X).

Fibre Geometry

sfrc for industrial floors
Length of the fibre should be more than sum total two Aggregate sizes (Fig Xi). At the same time fibre length should not exceed 2/3rd of the inner dia of the conveying system (Fig Xii).

Here first factor is related to interlocking of two aggregates whereas second factor is related to workability of concrete through the pumping system.

sfrc for industrial floors
In order to have more networking of fibres, it is suggested to have fibres with highest available L/D ratio or least available diameter which finally gives more fibres per kilo ( (Fig Xiii)

Concrete Consistency and Gradation

In addition to selection of appropriate fibres, it is very much necessary to have consistent concrete with continues gradation. What fibres want is concrete with enough paste around the aggregates.
sfrc for industrial floorssfrc for industrial floors

Project at Coimbatore

Given Facts

1. Mix Design

sfrc for industrial floors

2. Steel Fibres

Type 1

Length: 60 MM

Diameter: 0.9 MM

Formation: Glued

Anchorage : Hooked End (Dramix)

Tensile Strength : > 1000 N/MM ^2

Dosage : 30 KG/ M^3

Type 2

Length: 60 MM

Diameter: 0.75 MM

Formation: Glued

Anchorage : Hooked End (Dramix)

Tensile Strength : > 1000 N/MM ^2

Dosage : 20 KG/ M^3

In order to create more paste in existing formulae of concrete following suggestions were made to job site.
  1. Depending on availability pl. add either of following (30-50 Kg per M ^3, 1. Fine sand <= .125mm, 2. Fly Ash, 3. GGBS)
  2. Start from W/C Ratio of 0.5 and take trials up to 0.46
  3. Increase cement content to 380-400 KG ( Trail and error)
  4. Increase slump to minimum 80 and maximum 120 (Trial and Error)
It was difficult to get fine sand of required fineness so it was decided to increase 20-40 KG of existing fine grade sand (ZONE II).

Six Samples of various combinations were checked for fibre dispersion as follows.
sfrc for industrial floors
  • No balls were observed during the mix.
  • W/C Ratio maintained was 0.48/0.49.
Further improvements at the time of actual project can be as follows.
  1. Make fine sand available and reduce cement content
  2. Reduce water cement ratio to 0.46
  3. Maintain slump in the range of 80-120
  4. If possible increase mixer speed to 18 RPM

Conclusion

Steel fibre reinforced Industrial floors can be designed using Lose berg's Yield line model. At www.bekaert.com/building one can register to get a free design of Steel fibre Industrial floors based on the inputs provided.

Steel fibres being an essential part of this design should be selected very carefully as discussed in the paper. More emphasis should be given on total cost impact than per unit cost.

Although proper design and economics is important for the project, it is very much necessary to engineer the concrete to suit the selected fibre geometry. Concrete consistency and gradation should be different for every mix and should depend on the type of fibre as suggested by manufacturer.

References

  • Gerhard Vitt Design–Presentation at Malenovice approach for Dramix Industrial floors.
  • Beckett D, Humphreys J The Thames Polytechnic, Dart ford: Comparative tests on Plain, Fabric Reinforced and Steel Fibre reinforced Concrete Ground Slabs.
  • Lose berg A.: Design Methods for structurally Reinforced Concrete Pavements, Sweden, 1961.
  • Thooft H: Dramix Steel Fibre Industrial floor Design in accordance with the Concrete Society TR34.
  • Practical guide to the installation of Dramix Steel fibre concrete floors.
  • Ganesh P. Chaudhari, Design of SFRC Industrial floor Indian Concrete Institute, Seminar on Flooring and Foundations.
  • Ganesh P. Chaudhari, Design of Durable SFRC Industrial Floor, International conference of "Sustainable Concrete Construction "ACI, 8-10 February Rantagiri, India.

NBMCW Febuary 2009

.....

Matrix Ductility and Reinforcement on t...

Influence of Matrix Ductility and Reinforcement

Influence of Matrix Ductility and Reinforcement Detailing on Response of Portal Frame Under Cyclic Loading

Detailing on Response of Portal Frame Under Cyclic Loading

J. D. Rathod, Lecturer, and Dr. S. C. Patodi, Professor, Applied Mechanics Department,The M.S. University of Baroda, Vadodara

This paper reports the response mechanism of a composite moment resisting portal frame system with self– centering and energy dissipation capabilities under cyclic loading. The load deformation response is primarily influenced by transition mechanism of the relative flexural stiffness of beam and column members, triggered by the formation of plastic hinges in the beam. The portal frames are designed with normal and ductile reinforcement detailing as per the codal provisions and are prepared from concrete, Engineered Cementitious Composite (ECC) and 30% replacement of cement by fly ash in ECC (FAECC) matrices and their response under compressive cyclic loading is evaluated and compared with respect to strength, ductility, damage and energy absorption capability.Load-deformation and moment-curvature graphs are plotted to evaluate displacement ductility and curvature ductility respectively. The interaction of linear elastic steel reinforcement and damage tolerant deformation with ductile stress-strain behavior in tension of ECC results in non linear elastic flexural response with stable hysteretic behavior and prevents premature member failure. Compatible deformation of reinforcement and matrix lead to low interfacial bond stress and prevent composite disintegration by bond splitting and cover spalling.

Introduction

In most structural applications of steel reinforced concrete, a large flexural stiffness is desirable in order to limit member deflections under service load conditions for which the elastic deflection limit can be small and must not be exceeded. In seismic resistant structures, however, inelastic deformations at particular locations of the structural system are intended to dissipate large energy, thereby reducing the effects of seismic excitation on the structure. In particular, moment resisting frames designed according to the strong column/weak beam concept are expected to undergo inelastic deformations by formation of plastic hinges in the beam members, while the columns remain elastic in order to maintain vertical load carrying capacity and prevent possible collapse. Expected plastic hinge regions must be properly detailed to provide confinement, shear resistance and protection against buckling of longitudinal reinforcement [1]. Particular flexural members in seismic structures such as beams and first story columns may be required to undergo relatively large flexural deformations while maintaining their load carrying capacity. In steel reinforced concrete members, these deformations are likely to exceed the elastic deflection limit, which accommodates the need for energy dissipation; however, it also implies relatively large residual deformations after unloading [2]. Generally, the beam column joints of a RC frame structure subjected to cyclic load experience large internal forces. Consequently, the ductile behavior of RC structures dominantly depends on the reinforcement detailing of the beam column joints. Because of placement difficulties, however, reinforcement detailing of the beam column joints is not easy to handle and sometimes it is difficult to carry out the same according to the design drawings [3].

Engineered Cementitious Composite (ECC) is a class of ultra ductile fiber reinforced cementitious composite developed for such kind of potential applications. Microstructure tailoring based on micromechanics can lead to extreme composite ductility of several percent in tension; a material property not seen before in discontinuous fiber reinforced cementitious composites [4]. Strain hardening property of Engineered Cementitious Composite (ECC) material in axial tension has opened a new dimension to its use in construction industry. At increasing load, the induced tensile strains in reinforcement and matrix are accommodated by further elastic deformation in steel and propagation of multiple cracking in the ECC matrix for being ductile in nature [5,6]. However, cost and sustainability of this composite are two important issues to be addressed for making it more acceptable. Replacement of cement by fly ash is the solution in this regard which has been accepted world wide over the last few decades. But this attempt requires a special attention in ECC as tensile strain is the vital issue to be addressed instead of strength.

Response of portal frames made up of steel reinforced concrete and ECC matrix carrying almost same compressive strength with normal and ductile reinforcement detailing is examined in this paper under compressive cyclic loading with respect to load-deformation response, damage tolerance, energy absorption and failure mode. It is aimed to explore possibility of the use of novel material like ECC by which overall performance of the structure can be drastically enhanced. Curvature ductility and displacement ductility are evaluated. Further, 30 % cement is replaced by fly ash to examine its effect. Finally, this experimental investigation provides useful recommendations regarding reinforcement detailing with respect to use of concrete and smart material like ECC.

Matrix Composition

Recron 3S brand synthetic fibers of substantial triangular cross section produced by Reliance Industries Ltd. are used in 4% volume fraction. Kamal brand 53 grade OPC, 300m passing silica sand, 2% dose of high performance concrete super plasticizer of Glenium 51 brand, w/c ratio of 0.35, sand/cement ratio of 0.5 are used in ECC matrix composition for the preparation of frames in the present experimental investigation. Mix proportion for concrete frame is adopted as 1:1.295:2.407 with Kamal brand 53 grade of OPC, silica sand confirming to zone II, 12.5 mm size coarse aggregate, w/c ratio of 0.35 and 0.5% dose of super plasticizer of SP430 brand. 30% fly ash used in the ECC composition is a processed siliceous pulverized ash confirming to IS 3812 (Part 1): 2003 [7]. Plain mild steel reinforcement of 6 mm diameter for longitudinal and 4 mm diameter for transverse direction having yield strength of 250N/mm2 is used in the preparation of specimens with normal and ductile detailing.

Specimen Details and Test Set Up

Influence of Matrix Ductility and Reinforcement Detailing on Response of Portal Frame Under Cyclic Loading
Reinforcement detailing and dimensions of portal frame specimens are selected such that strong column/weak beam concept is satisfied. Normal reinforcement detailing is provided as per IS: 456-2000 [8] and ductile detailing is provided as per IS: 13920 [9] using 6 mm diameter bar for longitudinal reinforcement and 4 mm diameter bar for transverse reinforcement for each matrix composition as shown in Figs. 1 and 2 respectively. All the specimens are cured for 28 days normal water curing and then tested by applying cyclic compressive loading under displacement control on MTS machine. The two columns of a portal frame are fixed into the grips of a fixture which is fabricated locally. Pellets are pasted at the top and bottom of the beam portion near the beam-column connection 60 mm apart to measure the elongation. Compressive cyclic load is applied at the centre of the beam at the interval of 2000 N. The frame is loaded gradually upto 2000 N, then unloaded and reloaded to the next increment of load. This pattern of loading is continued foe each increment until failure. Curvature of the beam at connection with a column is measured at the peak load for each cycle. Average result of three specimens is presented here for discussion. In the result tables given in next section, letter N in bracket indicates normal detailing and D indicates ductile detailing in the frames prepared from concrete, ECC and FAECC.

Discussion of Test Results

Compressive strength of ECC, FAECC and concrete is reported as 36.77 N/mm2, 39.84 N/mm2 and 40.23 N/mm2 [10] respectively which indicates that concrete is the strongest construction material in compression out of the three. Total number of compressive load cycles resisted by each portal frame upto failure is different but overall observation indicates that repairable integrity of all the three matrices is almost lost after 10 cycles; therefore three matrices are compared with each other for first 10 cycles. Table 1 represents load carrying capacity of frames at first crack and ultimate. Frames prepared from concrete, FAECC and ECC indicatec first crack load of 10,000, 9000 and 8666 N respectively for normal reinforcement detailing which indicate that a material which is stronger in compression has higher modulus of rupture indicating truly a matrix property. But after formation of first crack, interaction of matrix with steel reinforcement comes into picture and as a result of which formation of other cracks takes place in inelastic deformation regime.

Influence of Matrix Ductility and Reinforcement Detailing on Response of Portal Frame Under Cyclic Loading
Ultimate loads resisted by concrete, FAECC and ECC frames with normal reinforcement detailing are 26424, 27400 and 34989 N respectively which indicate reverse behavior as compared to first crack. Therefore, magical role is played by ECC by interacting synergetically with reinforcement in inelastic deformation regime. ECC matrix exhibits compatible deformation with reinforcement by numerous wide spread multiple cracking which does not allow yielding of the reinforcement at one location but redistributes the stresses. Similar failure patterns are observed when portal frames are subjected to static loads [11]. All the three matrices exhibit plastic hinge in the beam prior to formation of crack in the column. First crack load in the column for Concrete, FAECC and ECC material is 16666, 20333 and 27000 N respectively along the line of beam column connection. The highest load in column is contributed by ECC due to large inelastic deformation in a beam itself and stiffness of the ECC column. Also, ECC undergoes extensive multiple cracking in the column along with plastic hinge area at center of beam which is not seen in concrete or FAECC column as depicted by failure patterns in Figs. 3 to 5. Shear failure and spalling of concrete in columns and beam are observed in concrete for both reinforcement detailed portal frames unlike FAECC and ECC. Specimens prepared from ECC exhibit numerous shear cracks in the beam column connections for normal reinforcement detailing as ECC contributes as a matrix instead of stirrups. Therefore, additional stirrups are not required when ECC or FAECC is used in the beam-column connection which minimizes placement difficulties of concrete. ECC itself is very effective shear resisting material due to which it may not require steel reinforcement in shear resistance. Therefore, ECC do not exhibit shear failure in either beam or column whereas concrete fails due to shear in spite of ductile detailing as shown in failure patterns. Pure flexure failure at the center of a beam is exhibited by ECC and FAECC. Concrete, FAECC and ECC carry first crack load of 9333, 7800 and 5250 N respectively for ductile reinforcement detailing which indicate that ductile detailing is very effective for concrete compared to FAECC and ECC. Ultimate loads recorded by concrete, FAECC and ECC with ductile reinforcement detailing are 28000, 27059 and 22333 N respectively which indicate that load carrying capacity of brittle material like concrete can be enhanced with the help of ductile detailing whereas fusion of ductile detailing and ductile matrix like ECC will not result in attractive performance. First crack load in the column is highest for concrete portal frame.

Influence of Matrix Ductility and Reinforcement Detailing on Response of Portal Frame Under Cyclic Loading

Considering an isolated segment from the tensile section of a steel reinforced flexural member, prior to formation of flexural cracking the tensile force is proportionally shared between reinforcement and matrix. At formation of an initial flexural crack, the tensile stress in the concrete matrix can not be directly transferred and is diverted into the reinforcement, resulting in tensile stress concentration and strain discontinuity between concrete and reinforcement. In contrast, initiation of flexural cracking in the steel reinforced ECC member does not result in a stress free matrix crack, but tensile stress is directly transferred across through crack. Subsequently, ECC enters the strain hardening regime and stresses are redistributed proportional to the stiffness of reinforcement and matrix at the deformation stage. Although the inelastic stiffness of ECC is significantly lower than in its uncracked state, tensile load in the matrix prior to cracking is transferred by means of fiber bridging and is not diverted into the steel reinforcement. Due to the uniform stress profile in the cracked matrix, initiation of further flexural cracking is dependent on the tensile deformation characteristics of ECC i.e. multiple cracking spacing and width and is effectively independent of interfacial bond properties. Local stress concentration in the steel reinforcement is prevented by direct tensile load transfer in the ECC matrix as well as compatible deformations between reinfor- cement and matrix, thus effectively decoupling reinforcement stress distribution and flexural crack formation from interfacial bond properties. The lack of relative slip between reinforcement and ECC in multiple cracking stages actively prevents interfacial bond deterioration and radial splitting forces.

Influence of Matrix Ductility and Reinforcement Detailing on Response of Portal Frame Under Cyclic Loading
Table 2 represents energy absorbed by portal frames in each cycle and total energy absorbed in 10 cycles which are evaluated from the load-displacement graphs obtained on MTS as shown in Figs. 6, 7 and 8 with the help of NCSS software. As expected, concrete and FAECC portal frames with ductile reinforcement detailing absorb more energy compared to normal detailed frames whereas, ECC exhibits unlike behavior compared to the other two. However, amount of the energy absorbed by ECC portal frames is highest for both the reinforcement detailed portal frames. ECC frames with normal reinforcement detailing absorb maximum energy which may be beneficial in earthquake resistant structures. Thus, the major problems experienced at site of placement of concrete due to ductile detailing can be minimized and performance of the structure can be improved.

Influence of Matrix Ductility and Reinforcement Detailing on Response of Portal Frame Under Cyclic Loading

Displacement of two points in compression and tension zone of beam near column connection is measured with the help of digital vernier having 0.01 mm accuracy from which curvature is evaluated and moment versus curvature graphs are plotted as shown in Figs. 9 and 10 which indicate large inelastic deformation behavior of ECC after formation of first crack compared to FAECC and concrete with normal reinforcement detailing. Behavior of FAECC and concrete is almost same with linear and parallel lines having same slope. ECC behaves distinctly different being ductile in nature compared to other two which strongly recommends normal reinforcement detailing for much enhanced performance. Ductile detailing doesn't make much difference in deformation capability but slope of the moment-curvature line of ECC is minimum indicating larger ductility. However, moment carrying capacity is higher for concrete portal frames.

Influence of Matrix Ductility and Reinforcement Detailing on Response of Portal Frame Under Cyclic Loading
Strain in ECC, FAECC and concrete in tension is noted as 1.53 %, 0.66 % and 0.01 % respectively. Further, ECC has lower elastic modulus compared to concrete and reaches its compressive strength at a larger strain due to the lack of large aggregates. Ductility of the portal frames is evaluated by displacement and curvature ductility factors. The curvature ductility factor is evaluated as curvature at peak load divided by curvature at yield. Curvature at yield can be evaluated from yield strength and modulus of elasticity of reinforcement. Displacement ductility factor is evaluated in same manner as above with the help of displacement values of beam. ECC represents highest curvature and displacement ductility in normal reinforcement detailing. These values are more than ECC itself with ductile detailing. Displacement ductility is found to be less than curvature ductility in all cases.

Conclusions

Ductile detailing is introduced in the code with the intention of improving the seismic performance of the structure. But this provision itself limits good concrete placement in many cases at site and thus does not fulfill the desired purpose. ECC is one of the best solutions in such cases as it does not contain coarse aggregates which minimize the placing problem and ductile detailing may not be necessary with ECC being ductile in nature. Therefore, ECC can be used in such fuse zones where large inelastic energy is expected to be absorbed. This investigation provides enough evidence that higher compressive strength may not improve overall seismic performance of the structure but rather ductile composite like ECC can result in a better performance.

Replacement of cement by fly ash in ECC increases compressive strength but it reduces ductility and energy absorbing capacity. Normal or ductile reinforcement detailing does not make much difference and hence overall economy of a structure can be achieved by replacing 30 % cement by fly ash in ECC and providing normal reinforcement detailing. Shear failure in column or beam and matrix spalling is not observed in ECC and FAECC frames unlike concrete frames.

References

  • Li V. C. and Fisher G., "Intrinsic Response Control of Moment-Resisting Frames Utilizing Advanced Composite Materials and Structural Elements" ACI Structural Journal, Vol. 100, No. 2, pp. 166-176, March-April 2003.
  • Li V. C. and Fisher G., "Deformation Behavior of Fiber-Reinforced Polymer Reinforced Engineered Cementitiuos Composite (ECC) Flexural Members under Reversed Cyclic Conditions," ACI Structural Journal, Vol. 100, No. 1, pp. 25-35, Jan.-Feb. 2003.
  • Gencoglu M. and Ilhan E., "An Experimental Study on the Effect of Steel Fiber Reinforced Concrete on the Behavior of the Exterior Beam-Column Joints Subjected to Reversal Cyclic Loading," Turkish Journal of Engineering and Environmental Science, pp. 493-501, Feb. 2002.
  • Li V. C., "Engineered Cementitious Composites- Tailored Composites through Micromechanical Modeling", Proceedings of Fiber Reinforced Concrete: Present and the Future, Canadian Society of Civil Engineering, Montreal, Canada, pp. 64-97.
  • Li V. C. and Fisher G., "Influence of Matrix Ductility on the Tension Stiffening Behavior of Steel Reinforced Engineered Cementitious Composites (ECC)," ACI Structural Journal, Vol. 99, No. 1, pp. 104-111, Jan.-Feb. 2002.
  • Li V. C. and Fisher G., "Effect of Matrix Ductility on Deformation Behavior of Steel Reinforced ECC Flexural Members under Reversed Cyclic Loading Conditions," ACI Structural Journal, Vol. 99, pp. 781-790, No. 6, Nov.-Dec. 2002.
  • IS 3813 (Part 1): 2003, "Pulverized Fuel Ash – Specification," Part 1 for use as Pozzolana in Cement, Cement Mortar and Concrete, Bureau of Indian Standard, New Delhi, 2003.
  • IS 456: 2000, "Code of Practice for Plain and Reinforced Concrete," Bureau of Indian Standard, New Delhi, 2000.
  • IS 13920: 1993, "Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces," Bureau of Indian Standard, New Delhi, 1993.
  • Rathod J. D. and Patodi S. C., "Effect of Cement: Sand Ratio on Shear Strength of ECC in 1D and 2D Fiber Orientationm," Civil Engineering and Construction Review Journal, Vol. 21, No. 5, pp. 86-96, May 2008.
  • Rathod J. D. and Patodi S. C., "Response of Engineered Cementitious Composites with Steel Reinforcement and Concrete in Moment Resisting Frames," New Building Materials and Construction World Journal, Vol. 13, No. 12, pp. 232-243, June 2008.

NBMCW June 2009

.....

Effect of Replacement of Cement by Sili...

Effect of Replacement of Cement by Silica Fume on the Strength Properties of SIFCON Produced From Waste Coiled Steel Fibres

A. K. Gurav, Lecturer, Department of Civil Engineering, D. Y. Patil College of Engineering and Technology, Kolhapur, Maharashtra. Dr. K. B. Prakash, Professor Department of Civil Engineering, K.L.E. Society's College of Engineering and Technology Belgaum

Slurry infiltrated fibre reinforced concrete (SIFCON), is a relatively new and advanced material of construction. It is fibre reinforced concrete (FRC) containing high percentage of fibres in which coarse aggregates are absent. SIFCON is a fabrication method in which steel fibres are preplaced in the form or in the mould to its full capacity, rather than being mixed and then cast or sprayed along with concrete. After placement of fibres, fine-grained cement based slurry is poured or pumped into the fibre network, infiltrating the air space between the fibres while conforming to the shape of the form or mould. External vibrations can also be used to aid infiltration of the slurry. SIFCON utilizes the fibres in the range of 6-20% by volume fraction as against usual range of 1-3% for fibre reinforced concrete. Due to such a high percentage of fibres tremendous improvement in strength properties can be expected. In this paper, effect of replacement of cement by silica fume on the properties of SIFCON is reported. SIFCON is made from waste coiled steel fibres obtained from lathe machine shop. The percentage of fibres used is 8%. In this study, fibres having aspect ratios like 80, 90, 100, 110, and 120 are used. Specimens are cast by replacing cement by silica fume at varying percentages like 5%, 10%, 15%, 20%, 25%, and 30% by weight of cement. The strength characteristics like compressive strength, tensile strength, and flexural strength and impact strength are evaluated.

Introduction

High performance concrete usually contains Portland cement. However, partial cement replacement by mineral admixtures can be economically advantageous. These minerals act as filler due to their small particle size that enables their penetration between cement grains. This results in reduction in the water cement ratio to achieve a given workability.1

Silica fume is most used mineral admixture in high strength high performance concrete. As defined by ACI 116R, it is very fine non-crystalline silica, produced in electric arc furnaces as a byproduct of the production of silicon or alloy containing silicon. Silica fume is mainly amorphous silica with high SiO2 content. It has extremely small particle size and large surface area. It is not precipitated silica or gel silica or colloidal silica or silica flour.2

In the discussion of high performance concrete role played by fibre reinforced concrete (FRC) is vital. Fibre reinforced concrete (FRC) is defined as a composite material which consists of conventional concrete reinforced by randomly dispersed short length fibres of specific geometry, made up of steel, synthetic material or natural fibres3. The fibres are distributed evenly throughout mix without balling or clustering4. The randomly oriented fibres help to bridge and arrest the cracks. As such, crack widening is gradual as compared to plain concrete5. This leads to better performance of concrete. Fibres have reported to be superior than wire mesh, for shortcrete. Also they overcome a difficultly in placing the mesh, especially on irregular surfaces6.

The concept of steel fibre reinforcement is very old. Steel fibres have been used since early 1900s7. Presently, steel fibres are considered as structural fibres as they enhance strength of the structure to a great extent3. The addition of steel fibres into concrete mass can dramatically increase the strength properties like compressive strength, tensile strength, and flexural strength and impact strength of concrete8. The strength properties of FRC can be increased by increasing the percentage of fibres in the concrete. But as the percentage of fibres increases, there are certain practical problems which have to be faced. The higher percentage i.e. higher volume content of fibres may cause balling effect in which the fibres cling together to form balls. Thus uniform distribution of fibres cannot be guaranteed, if percentage of fibres is more. Also longer fibres interfere with the aggregates during compaction thus hindering the proper orientation of fibres9.This fact limits the fibre content form 1 to 3 percent by volume.

The limitations of FRC and continuous ongoing demand for high performance material has led to the invention of 'Slurry infiltrated fibre reinforced concrete' (SIFCON) by Lankard in 1979. The 'Slurry infiltrated fibre reinforced concrete' is high strength, high performance material containing relatively high volume percentage of fibres as compared to FRC. SIFCON is also sometimes termed as 'High volume fibrous concrete.' In conventional FRC, the fibre content usually varies from 1 to 3 percent, while in SIFCON it varies from 6 to 20 percent by volume depending on the geometry of fibres and type of application.8 The material SIFCON has no coarse aggregates but has a high cementious content.

Research Significance

SIFCON which is considered as a high performance concrete, can also be produced by using waste coiled steel fibres obtained from the lathe machine shops. Since these fibres are available locally, they can be easily used in the production of SIFCON. Due to their coiled nature they may offer more resistance to loads. The study of effect of replacement of cement by silica fume on SIFCON produced from such waste coiled fibres may result in an economic building material.

Experimental Programme

The main aim of this experimental programme is to find out the effect of replacement of cement by silica fume on the strength properties of SIFCON produced from waste coiled steel fibres. Ordinary Portland cement of 53-grade and locally available sand with a specific gravity 2.65 and fineness modulus of 2.92 was used in the experimentation. To impart additional workability, superplastisizer (1% by weight of cement) was used. The waste coiled steel fibres were procured from local lathe machine shops. The fibres were of chrome steel having density 6.8 gm/cm3. The percentage of fibres used in the experimentation was 8%. The average thickness was 0.5 mm, with average coil diameter of 3 mm. The different aspect ratios adopted in the experimentation were 80, 90, 100, 110 and 120 giving fibre lengths 40mm, 45mm, 50mm, 55mm and 60mm respectively. The average thickness of fibres was taken into consideration in fixing the aspect ratios.

Effect of Replacement of Cement by Silica Fume on the Strength Properties of SIFCON Produced From Waste Coiled Steel Fibres
The cement mortar slurry was prepared with 1:1 proportion using w/c ratio 0.42. A superplastisizer (1% by weight of cement) was added to this slurry which increased infiltration capacity of the slurry. To study the effect of replacement of cement by silica fume on SIFCON, slurry was prepared by replacing cement by silica fume at varying percentages like 5%, 10%, 15%, 20%, 25% and 30% by weight of cement. In this experimentation Elkem microsilica grade 920-D was used. Physical and chemical properties of this silica fume are given in Tables 1 and 2 respectively.

The moulds were filled with 8% fibres, and slurry was poured into the moulds. Vibration was given to the moulds using table vibrator. The slurry was poured until no more bubbles were seen. This ensured a thorough infiltration of slurry into the fibres. The top surface of the specimen was leveled and finished. After 24 hours, the specimens were demoulded and were transferred to curing tank where they were allowed to cure for 28 days.

The effect of replacement of cement by silica fume on SIFCON was studied on compressive strength, tensile strength, flexural strength and impact strength. The cube specimens of dimension 150x150x150mm were cast, from which the compressive strength was calculated. The specimens of dimension 150mm diameter and 300mm length were cast for split tensile strength. The specimens of dimension 100x100x500mm were cast for flexural strength test. Two point loading10 was adopted on these specimens with an effective span of 400mm. The impact strength specimens consisted of plates of dimension 250x250x35 mm. For impact strength test four methods are described in the literature11. Out of these methods drop weight method was used owing to its simplicity. A steel ball weighing 20 N was dropped from the height of 1m over the specimen. The number of blows required to cause complete failure were noted. The impact energy was calculated as follows.

Impact energy = w x h x N (N-m).

Where

w = weight of the ball = 20 N

h = height of fall = 1 m

N = number of blows required to cause complete failure

After 28 days of curing, the specimens were taken out of the water. Then they were tested for their respective strengths.

Test Results

Tables 3 to 6 give the compressive strength, tensile strength, flexural strength and impact strength test results respectively, for SIFCON with and without replacement of cement by silica fume. The tables also indicate the percentage increase in the strength of SIFCON due to replacement of cement by silica fume.

Effect of Replacement of Cement by Silica Fume on the Strength Properties of SIFCON Produced From Waste Coiled Steel Fibres

The Figures 1 to 4 give variation of compressive strength, tensile strength, flexural strength, and impact strength respectively, for SIFCON with and without replacement of cement by silica fume.

Discussions on Test Results

1) It has been observed that the compressive strength, tensile strength, flexural strength and impact strength of SIFCON goes on increasing as the aspect ratio of fibers in it goes on increasing. This is also true for SIFCON with and without replacement of cement by silica fume.

Effect of Replacement of Cement by Silica Fume on the Strength Properties of SIFCON Produced From Waste Coiled Steel Fibres

This may be due to the fact of optimum infiltration of slurry for fibres having more aspect ratio.

Thus it can be concluded that the SIFCON produced with fibres having an aspect ratio of 120 yields the maximum strength.

Effect of Replacement of Cement by Silica Fume on the Strength Properties of SIFCON Produced From Waste Coiled Steel Fibres
2) It has been observed that maximum compressive strength for SIFCON is obtained when 20% of cement is replaced by silica fume. Also it is observed that percentage increase in the compressive strength of SIFCON with 8% fibres, due to 20% replacement of cement by silica fume is 20.07%, 20.19%, 20.49%, 20.63% and 20.91% respectively for different aspect ratios of fibres.

It has been observed that maximum tensile strength for SIFCON is obtained when 20% of cement is replaced by silica fume. Also it is observed that percentage increase in the tensile strength of SIFCON with 8% fibres, due to 20% replacement of cement by silica fume is 20.98%, 21.16%, 21.43%, 21.74%, 22.07% respectively for different aspect ratios of fibres.

It has been observed that maximum flexural strength for SIFCON is obtained when 20% of cement is replaced by silica fume. Also it is observed that percentage increase in the flexural strength of SIFCON with 8% fibres, due to 20% replacement of cement by silica fume is 20.41%, 20.63%, 20.91%, 21.14%, 21.55% respectively for different aspect ratios of fibres.

It has been observed that maximum impact strength for SIFCON is obtained when 20% of cement is replaced by silica fume. Also it is observed that percentage increase in the impact strength of SIFCON with 8% fibres, due to 20% replacement of cement by silica fume is 20.84%, 21.17%, 21.46%, 21.65%, 21.88% respectively for different aspect ratios of fibres.

The increase in various strengths of SIFCON due to replacement of cement by silica fume may be attributed to pozzolanic activity of silica fume. Silica fume is highly reactive pozzolana. Further due to its fineness it fills the small pores of the cement paste giving very dense concrete. Due to these facts an increase in the strength properties is seen. However, beyond 20% replacement the strengths decrease. This may be due to less quantity of cement in the mix. Hence it may be concluded that the optimum percentage of replacement of cement by silica fume for SIFCON is 20%.

Conclusions

  • The SIFCON produced with waste coiled steel fibres having an aspect ratio of 120 yields the maximum strength.
  • Optimum percentage of replacement of cement by silica fume for SIFCON is 20% when waste coiled steel fibres are used.
  • Waste coiled steel fibres can be efficiently used in the production of SIFCON.
Effect of Replacement of Cement by Silica Fume on the Strength Properties of SIFCON Produced From Waste Coiled Steel Fibres

Acknowledgment

The authors would like to thank and Dr. A. N. Chapgaon and Dr.S. C. Pilli, Principals of D. Y. Patil College of Engineering and Technology, Kolhapur and KLE Society's College of Engineering and Technology, Belgaum 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

  • Chaid R. et al, 'Influence of natural pozzolana on the properties of high performance mortar,' The Indian Concrete Journal, August 2004, pp 22-26.
  • Tiwari A. K., 'Advances in high performance concrete–the fifth ingredient,' Civil Engineering & Construction Review, June 2005, pp 28-38
  • Sikdar P. K., Saroj Gupta and Satander kumar, 'Application of fibres as secondary reinforcement in concrete,' Civil Engineering & Construction Review, December 2005, pp 32-35.
  • Kieth Carr, 'Polypropylene and steel fibres combinations,' Concrete, September 2004, pp 60-61.
  • Ziad Bayasi and Henning Kaiser, 'Steel fibres as crack arresters on concrete.' The Indian Concrete Journal, March 2001, pp 215-219.
  • Marc Wandewalle N.V., Bekaert S.A., Choudhari G.P., 'Fibres in concrete-Dramix steel fibres for SFRS & SFRC,' The Indian Concrete Journal, March 2003, pp 939-940.
  • Fibre reinforced concrete, A report published by cement and concrete institute, Midrand, 2001.
  • Prakash K. B. et al, 'Performance evaluation of slurry infiltrated fibrous silica fume concrete', Proceedings of International conference on Fibre Composites, HPC and smart materials', Chennai, India, pp 201-211.
  • Saluja S. K. et al, 'Compressive strength of fibrous concrete', The Indian Concrete Journal, February 1992, pp99-102.
  • I. S. 516-1959, 'Methods of tests for strength of concrete', Bureau of Indian standards, New Delhi.
  • Balsubramanain K. et al, 'Impact resistance of steel fibre reinforced concrete', The Indian Concrete Journal, May 1996, pp257-262.2

NBMCW March 2009

.....

Recent Technologies Using Waste Materia...

By - products for Sustainable Development

Satander Kumar, Scientist (Retd), CRRI New Delhi.

Sustainability and long term performance of concrete structures are the two important criteria with respect to the prevailing environmental conditions. Sustainability cannot be sacrificed to attain high strength. High ultimate strength is generally accompanied by a low W/C ratio. This is not, in itself, adequate to satisfy all sustainability related requirements. All types of concrete used in different applications must provide acceptable frost resistance, repeated heat resistance, sufficient fatigue life and adequate serviceability. Such concretes are made with the use of mineral admixture by partly replacing Portland cement or aggregate. This type of concrete for offshore concrete platforms in the North sea, with compressive strength of 45 to 70 MPa has been used, without any problem with respect to corrosion of embedded steel or other environmental reasons so far, after 15 years of the combined exposure to heavy mechanical loading and a severe marine environment.

Good quality fine particles of waste materials or by–products particularly mineral admixtures and superplasticizer make the cement concrete sustainable with improved long term performance because of least permeability and very slow chemical reaction with harmful compounds present in the concrete. Granulated blast furnace slag, fly ash and silica fume are few major mineral admixtures which are by–products of the steel, power and alloy industries which do not require much further processing (grinding or heating etc.) Before their use in the concrete except in case of granulated blast furnace slag which need only grinding. Even, the energy consumed by these mineral admixtures and other by–products of the Industry could be re-utilized also.

Minimum use of natural resources and recycling of unused materials are key elements in sustainable development. Unprecedented rise in our urban population, traffic population and a rapidly changing construction scenario have put tremendous pressure on available resources and carrying capacity of our eco-system. The paper deals with the studies showing the ability of the concrete containing waste materials as compared to conventional concrete, to protect embedded steel from corrosion and concrete from other environmental effects for a very longer service period.

Introduction

The first cement plant in India came up in Porbander, Gujarat, in 1914 and the first concrete road in India was constructed in Chennai in the same year. Indian cement industry, being the second largest in the world, has the annual production capacity of cement 115 million tones (apprx.).

The waste materials which may commonly be used in road construction are: flyash, steel slag, broken bricks, debris from the dismantled concrete and other building structures, rice husk ash, lime sludge, marble slurry dust, silica fume (a by–product of ferro silicon alloy industries), phospho-gypsum, kimberlite tailings from Panna mines, iron ore wastes, recycled bituminous and cement concrete materials etc. Some of the advantages of waste materials are:
  • Wide spread availability,
  • Less haulage cost as compared to borrow soil or other conventional materials, as there is generally a ban on mining of sand/aggregate or use of top soil in some metro cities,
  • Flyash/slag/silica fume mitigate alkali silica reaction,
  • Low energy requirement and environment friendly,
A detailed research work on the use of phospho-gypsum in making concrete roads, has been done at the department of Civil Engineering and Applied Mechanics, Shri Govind Ram Sakseria Institute of Sciences and Technology, Indore (MP). Research work has also been done at Florida Institute of Phosphate Research Institute, Bartow, Florida in 1988 and at other Institutes in India and abroad. 10 percent phospho-gypsum in cement concrete gives, 5-10 percent increase in compressive strength for M-20 grade concrete.

Nearly, 10 million tones of blast furnace slag become available from steel plants in India. About 5 million tones of blast furnace steel slag annually is being produced as granulated slag, which is mainly used in the manufacture of Portland blast furnace slag cement (IS 455-1989) either by inter-grinding or blending after grinding.

The concept of milling to get milled materials is to use existing roads as quarry from which road stone aggregate can be reclaimed. The material removed from the highly distressed bituminous roads by milling machine (grinding) is called milled materials. Milled materials are stabilized with various additives like cement, emulsions, RBI grade 81, etc. When the stone aggregates are replaced by milled materials partly or fully, the behavior of milled material in the base course is comparable to conventional base courses.

As per IRC:63-1976 "Tentative Guidelines for the Use of Low Grade Aggregate and Soil Aggregate Mixtures in Road Pavement Construction," the debris materials of the concrete structures/buildings, brick bats, mortar and concrete as sub base may be used after evaluating their properties especially grading and crushing strength alone or in combination with other binders /additives (5-10%).

The country is generating around 110 million tones of fly ash per annum from its coal based thermal power plants. Silica fume is a byproduct of ferro-silicon alloy industries. Silica fume particles are ultra fine and siliceous in nature containing more than 85 percent reactive silicon dioxide which is about two times more than the silicon dioxide content present in the fly ash collected by electro-static precipitators.

Flyash normally has lime reactivity of 50-60 kg/ sq. cm (5-6 M Pa) or even more, where as the minimum specified value of lime reactivity is 45 kg/sq. cm as per IS: 3812-2003. A good quality silica fume (as mineral admixture) is now available in India from the private firms including firms dealing with super plasticizers. High performance concrete mixes can be designed very easily by using slag, fly ash and silica fume partly and adding super plasticizers into the fresh wet concrete up to maximum 2.0 percent (preferably less than 0.5 percent) by weight of cement to achieve desired workability. This slag has potential reactivity and need alkaline medium for activation. Globally, a mixture of all the three is also being used as a replacement of Portland cement for making high strength and high performance concrete. The reason being higher cement content than specified will reduce the life of the structure and pavement and with regard to objective of sustainability of the pavement will be far away from our reach once the structure or pavement is built.

Properties of Materials

recent technologies using waste materials
  • Cement: The cements when tested for various physical and chemical properties shall conform to Grade 43 (IS: 8112-1989 and Grade 53 (IS: 12269-1987).
  • Coarse and Fine Aggregates: Various tests on aggregates shall conform to Table 1.
  • Super plasticizer: Various tests carried out as per IS: 9103-1999 shall conform to properties given there.
  • Fly Ash:The properties are given in IS: 3812-2003.
  • Silica Fume
Micro silica is available in the following four forms:
  1. Undensified: Its density is typically 200 to 350 kg/m3. It is collected from the filters as a very light powder, which is used in making mortar and grout.
  2. Densified: It has a density of 500 to 700 kg/m3. It is basically processed to increase the bulk density by loosely agglomerating the fine particles. This form is not as 'dusty' as undensified micro-silica and has a fine granular appearance and is being used in ready-mix concrete and pre-cast products.
  3. Palletised: Its density is around 1,000 kg/m3. This form is treated with a small amount of cement and water to give solid agglomerate. It is normally used only for inter-grinding with cement and cannot disperse beyond the actual grinding.
  4. Slurry: It is an aqueous suspension of undensified micro-silica in water, usually in the ratio of 1:1 (by weight). The material has a specific gravity of around 1.4 and as a liquid, is easier to handle than the powders. Slurry freshly made shall be frequently agitated to avoid setting out micro silica fume particles. The slurry shall be used only on a day-to-day and batch-to-batch basis. The slurry may contain other agents to maintain the suspension and the effect of these on the finished concrete would have to be assessed. Various tests carried out on silica fume shall conform to ASTM: C 1240-2001 or IS Specification.
Properties of silica fume are given in Table 2.

Durability

recent technologies using waste materials
To improve the durability of the concrete containing very fine particles, following admixtures are generally used as per the requirement:
  1. Accelerators
  2. Retarders
  3. Water reducing agents which are termed plasticisers and superlasticisers viz.:

    Sulphonated melamine-formaldehyde condensates (SMF)

    Sulphonated nepthalene-formaldehyde condensates (SNF).

    Modified lignosulphonates (MLS),
  4. Air-entraining agents
Sulphonic acid esters, carbohydrate esters etc.

Durability of concrete may be evaluated by the following tests:
  • Chloride Ion Permeability Test (ASTM-C1202)
  • Modulus of Elasticity
  • Abrasion Resistance as per IS: 9284-1979.
  • Bond Strength
  • Cover to concrete
  • Weathering durability tests
The durability of concrete by Chloride permeability test consists of monitoring the amount of electrical current passed through 51 mm thick slices of 102 mm nominal diameter cores or cylinders during a 6 hour period. A potential difference of 60 V DC is maintained across the ends of the specimen, one of which is immersed in a sodium chloride solution and the other in a sodium hydroxide solution.
recent technologies using waste materials
The total charge passed, in coulombs, has been found to be related to the resistance of the specimen to chloride ion penetration. The results are used to assess qualitatively the chloride ion penetrability of concrete as indicated Table 3:

The specimens may also be cast for weathering cycles to undergo
  1. heating for 8 hours at 60°C and cooling them at 27°C,
  2. keeping in 5% sodium sulphate solution at room temperature and heating them at 60°C.,
  3. keeping the specimens at–15°C and cooling them in air at room temperature. Visual observations may be made on the damages and deterioration caused.
Concrete Cover - Plain and Reinforced Concrete: As per clause 304.3 of IRC: 21-2000 regarding the cover of concrete to steel bar in case of plain and reinforced concrete bridges, the minimum clear cover to any reinforcement bar, closest to the concrete surface, shall be 40 mm. Increased minimum cover thickness of 50 mm shall be provided when concrete members are exposed to severe conditions as mentioned in clause 4.0 on concrete except that for the condition of alternate wetting and drying and in case of foundations where the minimum clear cover shall be 75 mm.

New Technologies

  1. Self compacted Concrete (SCC) has its own limitation - it can not, on its own, flow into nooks and corner of the form work. Through compaction, often using vibration is essential for achieving strength and durability of concrete. It has its own limitation depending on the types of structures, its dimensions, and types of reinforcement, location of structures etc. Self– compacting concrete (SCC) may provide remedies to these problems. Developed by Prof. Okamura and his team in Japan in 1986, SCC has evolved as new innovative technology, capable of achieving status of being an out-standing advancement in the sphere of concrete technology. There are now many countries who are also working on SCC viz Sweden, Thailand, UK, India etc.
    • Ensure through compaction employing unskilled labour
    • Minimize repair of finished surface
    • Ensure good finished surface
    • Reduced manpower for casting and finishing
    • Increase in speed of construction
    • Improvement in the performance
    No vibration is necessary for SCC which can flow around obstructions, encapsulate the reinforcement and fill up the space completely under self weight. The salient advantages are: Development of Self Compacted Concrete (SCC) is now being improved and is being used in the concrete industry. As a safe guard against separation of water, use of a viscosity modifying agent is usually essential to minimize shrinkage due to high powder content in SCC. There are typical mixes of SCC similar to conventional concrete where risk of cracking due to shrinkage and thermal stresses could be reduced. Addition of flyash and other siliceous materials such as silica fume and granulated slag make 'Sustainable SCC'.

    There are many organization/academic institutions/cement companies in India who are working in the laboratory and field for the advancement and use of SCC in structures. There is a need to formulate IRC/BIS specifications/ guidelines for the use of SCC in respective structures based on the experience/data gained in India. However, there are guidelines published by Hampshire, UK (EFNARC-2002)/contract documents on use of SCC in Nuclear Structures.
  2. High volume flyash (35 to 90 per cent) concrete is the other area on which work has been carried for making roller compacted concrete pavement in China in 1998, USA, UK etc. There is early strength decrease, but strength will get sharp development later and high-grade cement benefit the early strength
  3. Soil stabilization is extensively used in European and African Countries to optimize and conserve natural resources. The preliminary estimate revealed that we require 175 million cubic meter of aggregate per annum. Such exploitation of natural resources would create serious environment issue. There is therefore need to develop and popularize alternate methods of road construction and thereby make the optimum use of natural material. This is also necessary to reduce diesel consumption required for transporting and processing these materials. There is a need to improve the engineering properties of traditional construction material to reduce the material consumption.. 10mm stabilized soil is equivalent to 15 mm of unbounded granular layer which mean drastically reduction in aggregate consumption. Lime is used to stabilize clayey soil and cement is used for stabilization of sand soils. Now I the market, stabilizers are available such as RBI-Grade 81 which can be used both for sandy and clayey soil with long term performance.
  4. Ultra Thin White Topping (UTWT- The fourth major technology is Ultra Thin White Topping (UTWT) i.e laying of concrete over bituminous pavement with closer joint spacing, the details of which are given n IRC:SP 76. UTWT involves the use of High Performance concrete (HPC) as per IRC: SP 70 and Ultra High performance Concrete i.e HPC contain fibers and special types of aggregates.

    UTWT has been used in India, USA and U.K. as thin rigid overlay over flexible pavements. In case of ultra thin white topping (UTWT), existing bituminous layer is milled before laying UTW over bituminous layer. The minimum thickness of bituminous layer after milling shall be 75 mm for UTWT and also existing bituminous layer shall be free from major defects and cracking so that reflection cracks or sympathetic cracks are as minimum as possible. These cracks, if left out after milling, shall be repaired with either bituminous or cement concrete layer or any other suitable polymers to avoid reflection cracking. The basic purpose of UTWT is to improve the riding quality along with the load carrying capacity.
    1. The roadway is milled to a uniform depth throughout the surface layer of asphalt,
    2. Edges of the milled area serve as forms and significantly reduce the cost of formwork,
    3. Polypropylene/polyester fibres are optional to add to enhance the toughness of UTWT
    4. Maximum W/C ratio 0.33,
    5. Concrete strength (Compressive) = 276 kg/sq cm at 3 days,
    6. Concrete is placed by slipform/fixed form to the finished grade of the existing roadway as per the facilities available,
    7. Joints spacing is normally kept at 100- 150 cm,
    8. Provides additional durability which is achieved due to the additional use of mineral admixtures, and
    9. Bond between concrete overlay and the underlying asphalt, enable UTWT to act as a composite section and require less thickness of UTWT,

    Salient features of UTW:

  5. White topping (WT) is a conventional concrete overlay of 20-30 cm thick with a leveling layer of bituminous\macadam or dry lean concrete over a crack arresting layer over the existing bituminous layer or in some cases, existing bituminous layer is removed before white topping is laid, or repaired with the suitable materials. Mineral admixtures in the WT also will improve the sustainability of the pavement.
  6. Roller Compacted Concrete, Soil stabilization and others- There are other technologies which includes the use of byproducts which are–roller compacted concrete pavement (RCCP) (IRC:SP-68) i.e concrete pavement made with the use of roller/ soil compactor on low slump concrete, concrete, soil stabilization of weak soils, paver block for footpath, shoulder and even for carriageway at the round-abouts or crossings, flowable fills (containg very low quantity of Portland cement) or cement treated bases. Byproducts in these technologies may be used as soil or aggregate, replacement.
If the pavements which are designed and constructed for very longer period than conventional life, these are normally called perpetual pavements. This may be possible with the use of mineral admixtures in the concrete, soil stabilization or other by products as discussed.

General Discussion And Conclusions

There are many mineral admixtures, which are generally industrial by-products and are recommended as replacement of cement or sand in the concrete to increase the performance and durability of concrete structures and to reduce the high cost. Sometimes it is very difficult to design the concrete mixes up to the expected level of quality and service in coastal or other typical areas without the use of mineral admixtures. Flyash, slag and silica fumes are as health hazard as cement and other reactive siliceous particles. These shall also be used in a similar way with proper care and bags shall be opened when required, but used within a period of six months.

A concrete pavement particularly made with mineral admixtures/ by-products maintains the initial roughness of 2000mm/km only if it is very carefully and meticulously designed and constructed. Such pavements require stricter quality control of both materials, which constitute the concrete, and process of laying the same, which otherwise result in failure. During lying of pavement quality or dry lean concrete, adequate quantity of water shall be added. If it is less than the requirement, some of the cement may remain un-hydrated and if it is more, there may be more shrinkage. By the use of by–products, rigid pavements have some advantages:
  • Shrinkage of concrete is directly proportional to water content per cubic meter of concrete,
  • If compaction of concrete layer is excessive, there would be sometime, more free water on the top surface, there by causing more shrinkage cracks,
  • Concrete pavement may perform well in areas having high rainfall, prone to water-logged and flooding where swelling pressure due to expansive soils, is very high.
  • More service life
  • Annual maintenance is very less,
  • Less quantity of aggregates is required, conserving natural resources
Use of mineral admixtures will not only enable substantial savings in the consumption of cement and energy, but also has many advantages such as: improvement in workability, decreases in permeability, decreases potential alkali aggregate reactivity, increases resistance to sulphate reaction, better finishing etc besides saving in time.

In town, streets particularly in North East and Uttar Pradesh, pavement can be made with bricks laid flat or on-edge over 50-75 mm of rammed ballast, lean cement concrete, and well consolidated. The soil should also be well rammed and brought to camber or proper levels.

References

  • IS: 1077–1992Common Burnt Clay Building Bricks– Specifications
  • S: 3495–1992 Methods of Tests of Burnt Clay Building Bricks – Part–1 Determination of Compressive Strength
  • IS: 3495–1992 Methods of Tests of Burnt Clay Building Bricks – Part–2 Determination of Water Absorption
  • IS: 3495–1992Methods of Tests of Burnt Clay Building Bricks – Part–3 Determination of Efflorescence
  • IS: 2180–1988 Specification for Heavy Duty Burnt Clay building Bricks
  • IS: 2248–1992Glossary of Terms Relating to Structural Clay Products for Building
  • IS: 5454–1978Methods for Sampling of Clay building Bricks
  • IS: 1905–1987Code of Practice for Structural Use of Unreinforced Masonary
  • IS: 3316–1974Specification for Structural Granite
  • IS: 3620–1979Specification for Laterite Stone Block for Masonry
  • IS: 4139–1976Specification for Sand Lime Bricks
  • IS 3812 : 2003Specification for Fly Ash for use as Pozzolana and Admixture
  • IS 269 : 1989Specifications for 33 grade ordinary Portland cement
  • IS 650 : 1966 Standard sand for testing of cement (first revision)
  • IS 1344 : 1981 Specification for calcined clay pozzolana (second revision)
  • IS 1727 : 1967 Methods of tests for pozzolanic materials (first revision)
  • IS 2580 : 1982Jute sacking bags for packing cement (second revision)
  • IS 3535 : 1986Methods of sampling of hydraulic cements (first revision)
  • IS 4032 : 1985Methods of chemical analysis of hydraulic cement
  • IS 4845 : 1968Definitions and terminology relating to hydraulic cement
  • IS 4905 : 1968 Methods of random sampling
  • IS 11652 : 1986High density polyethylene woven sacks for packing cement
  • IS 11653 : 1986Polypropylene (PP) woven sacks for packing of cement
  • IS 12089: 1987Specification for granulated slag for the manufacture of Portland slag cement
  • IS 12154 : 1987 Light weight jute bags for packing of cement
  • IS 12174 : 1987Jute synthetic union bags for packing of cement
  • IS 12423 : 1988Method for colouri- metric analysis of hydraulic cement Technical Report on RBI-Grade 81, Legend Surface Developers Pvt Ltd, Sanik Farm, New Delhi.

NBMCW February 2009

.....

Steel Fibre Concrete Composites for Spe...

Steel Fibre Concrete Composites for Special Applications

Normal and High Volume Steel Fibre Concrete Composites for Special Applications

Dr. V.S. Parameswaran, President and Chief Executive, Design Technology Consultants, Chennai Chief Executive, International Centre for FRC Composites (ICFRC), Chennai, Former Director, SERC & Past President, ICI.

In recent times, the sustained efforts of researchers all over the world to innovate and incorporate unmatched excellence in construction have led to development of several advanced concrete construction materials. Of these, composites containing steel fibres have come to stay and deserve special mention. This paper, besides outlining the properties and applications of normal fibre reinforced concrete (SFRC), also describes the emergence and potentials of high-volume fibre composites such as slurry infiltrated fibrous concrete (SIFCON), slurry infiltrated mat concrete (SIMCON), compact reinforced concrete (CRC) and reactive powder concrete (RPC).

Steel Fibre Reinforced Concrete (SFRC)

Concrete is the most widely used structural material in the world with an annual production of over seven billion tons. For a variety of reasons, much of this concrete is cracked. The reason for concrete to suffer cracking may be attributed to structural, environmental or economic factors, but most of the cracks are formed due to the inherent weakness of the material to resist tensile forces. Again, concrete shrinks and will again crack, when it is restrained. It is now well established that steel fibre reinforcement offers a solution to the problem of cracking by making concrete tougher and more ductile. It has also been proved by extensive research and field trials carried out over the past three decades, that addition of steel fibres to conventional plain or reinforced and prestressed concrete members at the time of mixing/production imparts improvements to several properties of concrete, particularly those related to strength, performance and durability.

The weak matrix in concrete, when reinforced with steel fibres, uniformly distributed across its entire mass, gets strengthened enormously, thereby rendering the matrix to behave as a composite material with properties significantly different from conventional concrete.

The randomly-oriented steel fibres assist in controlling the propagation of micro-cracks present in the matrix, first by improving the overall cracking resistance of matrix itself, and later by bridging across even smaller cracks formed after the application of load on the member, thereby preventing their widening into major cracks (Fig. 1).

Steel Fibre Concrete Composites for Special Applications

The idea that concrete can be strengthened by fibre inclusion was first put forward by Porter in 1910, but little progress was made in its development till 1963, when Roumaldi and Batson carried out extensive laboratory investigations and published their classical paper on the subject. Since then, there has been a great wave of interest in and applications of SFRC in many parts of the world. While steel fibres improve the compressive strength of concrete only marginally by about 10 to 30%, significant improvement is achieved in several other properties of concrete as listed in Table 1. Some popular shapes of fibres are given in Fig.2.

Steel Fibre Concrete Composites for Special Applications

In general, SFRC is very ductile and particularly well suited for structures which are required to exhibit:
Steel Fibre Concrete Composites for Special Applications
  • Resistance to impact, blast and shock loads and high fatigue
  • Shrinkage control of concrete (fissuration)
  • Very high flexural, shear and tensile strength
  • Resistance to splitting/spalling, erosion and abrasion
  • High thermal/ temperature resistance
  • Resistance to seismic hazards.
The behavior of SFRC under fatigue loading regime as compared to conventional concrete is shown in Fig. 3, while Fig. 4 illustrates the improvement in impact resistance of SFRC with the increase in the fibre content. The high ductility exhibited by normal SFRC and polymer-impregnated SFRC over conventional concrete is shown in Fig. 5.

The degree of improvement gained in any specific property exhibited by SFRC is dependent on a number of factors that include:
  • Concrete mix and its age
  • Steel fibre content
  • Fibre shape, its aspect ratio (length to diameter ratio) and bond characteristics.
The efficiency of steel fibres as concrete macro-reinforcement is in proportion to increasing fibre content, fibre strength, aspect ratio and bonding efficiency of the fibres in the concrete matrix. The efficiency is further improved by deforming the fibres and by resorting to advanced production techniques. Any improvement in the mechanical bond ensures that the failure of a SFRC specimen is due mainly to fibres reaching their ultimate strength, and not due to their pull-out.

Mix Design for SFRC

Just as different types of fibres have different characteristics, concrete made with steel fibres will also have different properties.

When developing an SFRC mix design, the fibre type and the application of the concrete must be considered. There must be sufficient quantity of mortar fraction in the concrete to adhere to the fibres and allow them to flow without tangling together, a phenomenon called ‘balling of fibres’ (Fig. 6). Cement content is, therefore, usually higher for SFRC than conventional mixes Aggregate shape and content is critical. Coarse aggregates of sizes ranging from 10 mm to 20 mm are commonly used with SFRC. Larger aggregate sizes usually require less volume of fibres per cubic meter.

Steel Fibre Concrete Composites for Special Applications

SFRC with 10 mm maximum size aggregates typically uses 50 to 75 kg of fibres per cubic meter, while the one with 20 mm size uses 40 to 60 kg.

Smaller sections less than about 100 mm in thickness should be considered as requiring 10 mm aggregate size only.

It has been demonstrated that the coarse aggregate shape has a significant effect on workability and material properties. Crushed coarse aggregates result in higher strength and tensile strain capacity.

Fine aggregates in SFRC mixes typically constitute about 45 to 55 percent of the total aggregate content.

Typical mix proportions for SFRC will be: cement 325 to 560 kg; water-cement ratio 0.4-0.6; ratio of fine aggregate to total aggregate 0.5-1.0; maximum aggregate size 10mm; air content 6-9%; fibre content 0.5-2.5% by volume of concrete. An appropriate pozzolan may be used as a replacement for a portion of the Portland cement to improve workability further, and reduce heat of hydration and production cost. The suggested mix proportions for making SFRC mortars and concretes is given in Table 2.

The use of steel fibres in concrete generally reduces the slump by about 50 mm. To overcome this and to improve workability, it is highly recommended that a super plasticizer be included in the mix. This is especially true for SFRC used for high-performance applications.

Steel Fibre Concrete Composites for Special Applications

Generally, the ACI Committee Report No. ACI 554 ‘Guide for Specifying, Mixing, Placing and Finishing Steel Fibre Reinforced Concrete’ is followed for the design of SFRC mixes appropriate to specific applications.

Fibre Shotcreting

Steel Fibre Concrete Composites for Special Applications
“Shotcreting” using steel fibres is being successfully employed in the construction of domes, ground level storage tanks, tunnel linings, rock slope stabilization and repair and retrofitting of deteriorated surfaces and concrete. Steel fibre reinforced shotcrete is substantially superior in toughness index and impact strength compared to plain concrete or mesh reinforced shotcrete.

In Scandinavian countries, shotcreting is done by the wet process and as much as 60% of ground support structures (tanks and domes) in Norway are constructed using steel fibres. In many countries including India, steel fibre shotcrete has been successfully used in the construction of several railway and penstock tunnels (Fig. 7).

Typical mix proportions for making fibre shotcrete with sand only, and with a combination of sand and coarse aggregate, is given in Table 3.

Applications of SFRC

The applications of SFRC depend on the ingenuity of the designer and builder in taking advantage of its much enhanced and superior static and dynamic tensile strength, ductility, energy-absorbing characteristics, abrasion resistance and fatigue strength.

Growing experience and confidence by engineers, designers and contractors has led to many new areas of use particularly in precast, cast in-situ, and shotcrete applications. Traditional application where SFRC was initially used as pavements, has now gained wide acceptance in the construction of a number of airport runways, heavy-duty and container yard floors in several parts of the world due to savings in cost and superior performance during service.

The advantages of SFRC have now been recognised and utilised in precast application where designers are looking for thinner sections and more complex shapes. Applications include building panels, sea-defence walls and blocks, piles, blast-resistant storage cabins, coffins, pipes, highway kerbs, prefabricated storage tanks, composite panels and ducts. Precast fibre reinforced concrete manhole covers and frames are being widely used in India, Europe and USA.

Cast in-situ application includes bank vaults, bridges, nosing joints and water slides. “Sprayed-in” ground swimming pools is a new and growing area of shotcrete application in Australia. SFRC has become a standard building material in Scandinavia.

Applications of SFRC to bio-logical shielding in atomic reactors and also to waterfront marine structures which have to resist deterioration at the air-water interface and impact loadings have also been successfully made. The latter category includes jetty armor, floating pontoons, and caissons. Easiness with which fibre concrete can be moulded to compound curves makes it attractive for ship hull construction either alone or in conjunction with ferrocement.

Use of SFRC for repair work is also a growing market. Several tunnels and bridges have been repaired with spraying of layers of shotcrete after proper surface preparation. A few most common applications of SFRC are illustrated in Fig. 8.

SFRC shotcrete has recently been used for sealing the recesses at the anchorages of post stressing cables in oil platform concrete structures. Recent developments in fibre types and their geometry and also in concrete technology and equipment for mixing, placing and compaction of SFRC and mechanized methods for shotcreting have placed Scandinavian and German consultants and contractors in a front position in fibre-shotcreting operations world wide.

Steel Fibre Concrete Composites for Special Applications
Laboratory investigations have indicated that steel fibres can be used in lieu of stirrups in RCC frames, beams, and flat slabs and also as supplementary shear reinforcement in precast, thin-webbed beams. Steel fibre reinforcement can also be added to critical end zones of precast prestressed concrete beams and columns and in cast-in-place concrete to eliminate much of the secondary reinforcement. SFRC may also be an improved means of providing ductility to blast-resistant and seismic-resistant structures especially at their joints, owing to the ability of the fibres to resist deformation and undergo large rotations by permitting the development of plastic hinges under over-load conditions.

Slurry Infiltrated Fibrous Concrete (SIFCON)

SIFCON is a high-strength, high-performance material containing a relatively high volume percentage of steel fibres as compared to SFRC. It is also sometimes termed as ‘high-volume fibrous concrete’. The origin of SIFCON dates to 1979, when Prof. Lankard carried out extensive experiments in his laboratory in Columbus, Ohio, USA and proved that, if the percentage of steel fibres in a cement matrix could be increased substantially, then a material of very high strength could be obtained, which he christened as SIFCON.

While in conventional SFRC, the steel fibre content usually varies from 1 to 3 percent by volume, it varies from 4 to 20 percent in SIFCON depending on the geometry of the fibres and the type of application. The process of making SIFCON is also different, because of its high steel fibre content. While in SFRC, the steel fibres are mixed intimately with the wet or dry mix of concrete, prior to the mix being poured into the forms, SIFCON is made by infiltrating a low-viscosity cement slurry into a bed of steel fibres ‘pre-packed’ in forms/moulds (Fig. 9).

The matrix in SIFCON has no coarse aggregates, but a high cementitious content. However, it may contain fine or coarse sand and additives such as fly ash, micro silica and latex emulsions. The matrix fineness must be designed so as to properly penetrate (infiltrate) the fibre network placed in the moulds, since otherwise, large pores may form leading to a substantial reduction in properties.

A controlled quantity of high-range water-reducing admixture (super plasticizer)may be used for improving the flowing characteristics of SIFCON. All types of steel fibres, namely, straight, hooked, or crimped can be used.

Proportions of cement and sand generally used for making SIFCON are 1: 1, 1:1.5, or 1:2. Cement slurry alone can also be used for some applications. Generally, fly ash or silica fume equal to 10 to 15% by weight of cement is used in the mix. The water-cement ratio varies between 0.3 and 0.4, while the percentage of the super plasticizer varies from 2 to 5% by weight of cement. The percentage of fibres by volume can be any where from 4 to 20%, even though the current practical range ranges only from 4 to 12%.

Uniaxial Tensile Strength

Unlike the cracks which form in continuous reinforced cementitious composites such as ferrocement, the cracks in SIFCON generally do not extend through the whole width of the specimen. Instead, they can be short and randomly distributed within the loaded volume, i.e. on the surface and through the depth of the specimen. The ultimate tensile strength of SIFCON typically varies from 20 to 50 MPa, depending on the percentage of steel fibres and the mix proportions used.

Steel Fibre Concrete Composites for Special Applications

Compressive Strength

The cement slurry (without fibres) used in the making of SIFCON generally develops a one-day strength of 25 to 35 MPa, and a 28-day strength of 50 to 70 MPa. The corresponding values for SIFCON composites are 40 to 80 MPa and 90 to 160 MPa, respectively, depending on the percentage of steel fibres incorporated in the matrix. Generally, SIFCON exhibits an extremely ductile behavior under compression.

Flexural Strength

The ultimate flexural strength of SIFCON is found to be very high and is in the order of magnitude higher than that of normal SFRC. The values observed by several researchers range from 25 to 75 MPa with an average of about 40 MPa. SIFCON is found to possess excellent ductility both under monotonic and high-amplitude cyclic loading.

Shear Strength

Investigations carried out in USA, Denmark and India have shown that the ultimate shear strength of SIFCON specimens were 30.5, 28.1, 33.3 and 31.8 MPa, respectively, for fibre lengths of 30, 40, 50 and 60 mm, indicating thereby that the fibre length does not seem to affect the shear strength. The average shear strength of SIFCON can be taken as about 30 MPa as compared to just about 5 MPa for plain concrete.

Resistance to Abrasion, Impact, Fatigue, and Repeated Loading

SIFCON possesses extremely high abrasion and impact resistance, when compared with plain concrete and SFRC specimens. The resistance improves further drastically with the increase in the percentage of fibres. It is several times that of ordinary plain or reinforced concrete.

Design Principles

The design methods for SIFCON members must take into account their application or end-use, the property that needs to be enhanced, mix proportion, strength, as well as its constructability and service life. In general, a high-strength SIFCON mix can easily be designed and obtained with virtually any type of steel fibres available today, if the slurry is also of high strength.

Like conventional concrete, the strength of the slurry is a function of the water-cement ratio; because the slurry mixes used in SIFCON usually contain significant percentages of fly ash or silica fume or both, the term “water-to-cement plus admixtures” is used when designing the slurry mix. In addition, the ratio of the “admixtures to cement” is also an important parameter in the design of SIFCON. It is also to be noted that higher volume percentages of fibres need lower viscosity slurry to infiltrate the fibres thoroughly. In general, the higher the strength of the slurry, the greater is the SIFCON strength.

Applications of SIFCON

SIFCON possesses several desirable properties such as high strength and ductility. It also exhibits a very high degree of ductility as a result of which it has excellent stability under dynamic, fatigue and repeated loading regimes (Figs. 10 a & 10 b). It is also quite expensive. Because of this, SIFCON should be considered as an efficient alternative construction material only for those applications where concrete or conventional SFRC can not perform as may be expected/required by the user or in situations where such unique properties as high strength and ductility are required.

Since properties like ductility, crack resistance and penetration and impact resistance are found to be very high for SIFCON when compared to other materials, it is best suited for application in the following areas:

Steel Fibre Concrete Composites for Special Applications
  • Pavement rehabilitation and precast concrete products
  • Overlays, bridge decks and protective revetments
  • Seismic and explosive-resistant structures
  • Security concrete applications (safety vaults, strong rooms etc)
  • Refractory applications (soak-pit covers, furnace lintels, saddle piers)
  • Military applications such as anti-missile hangers, under-ground shelters
  • Sea-protective works
  • Primary nuclear containment shielding
  • Aerospace launching platforms
  • Repair, rehabilitation and strengthening of structures
  • Rapid air-field repair work
  • Concrete mega-structures like offshore and long-span structures, solar towers etc.

Compact Reinforced Concrete (CRC)

CRC is a new type of composite material. In its cement-based version, CRC is built up of a very strong and brittle cementitious matrix, toughened with a high concentration of fine steel fibres and an equally large concentration of conventional steel reinforcing bars continuously and uniformly placed across the entire cross section (Fig. 11).

CRC was initially developed and tested by Prof.Bache at the laboratories of Aalborg Portland cement factory in Denmark. The pioneering experiments carried out at this laboratory established the vast potential of CRC for applications that warrant high strength, ductility and durability.

CRC has structural similarities with reinforced concrete in the sense that it also incorporates main steel bars, but the main bars in CRC are large in number and are uniformly reinforced. Owing to this and also because of the large percentage of fibres used in its making, it exhibits mechanical behavior more like that of structural steel, having almost the same strength and extremely high ductility.

CRC specimens are produced using 10-20% volume of main reinforcement (in the form of steel bars of diameter from about 5 mm to perhaps 40 or 50 mm) evenly distributed across the cross section) and 5-10% by volume of fine steel fibres. The water-cement ratio is generally very low, about 0.18% and the particle size of sand in the cement slurry is between 2 and 4mm.The flow characteristics while mixing and pouring is aided by the use of micro silica and a dispersant. High-frequency vibration is often resorted to for getting a the mix compacted and to obtain homogeneity. Prolonged processing time for mixing, about 15-20 minutes, ensures effective particle wetting and high degree of micro-homogeneity.

Such highly fibre-reinforced concrete typically has compressive strengths ranging from 150 to 270 MPa, and fracture energy from 5,000 to as much as 30,000 N/m.

CRC beams exhibit load capacities almost equivalent to those of structural steel and remain substantially uncracked right up to the yield limit of the main reinforcement (about 3 mm/m), where as conventional reinforced concrete typically cracks at about 0.1-0.2 mm/m.

Some of the properties of CRC as obtained from extensive experiments carried out on CRC specimens are given in Table 4.

Design of CRC

The development and design of CRC is based on fracture mechanics principles/theories, that takes into account the coherent and ductile phase of the composite, cracked pattern and ultimate failure mode. The theories assume that, as in the case of metals, any single, micro crack developed owing to the presence of a local flaw can not propagate and cause sudden tensile failure because of the interlinked pattern of main steel and fibres, thereby rendering the composite highly elastic, ductile and strong.

Applications of CRC

Steel Fibre Concrete Composites for Special Applications

CRC can probably be used especially in the form of large plates or shells designed, for example, to resist very large local loads with unknown attack position (from explosives, say, or mechanical impact) or to resist uniformly distributed pressure, either as pure compression or pure tension (e.g. large pressure tanks).

Because CRC has very high “strength-density ratios” (often greater than those of commonly used structural steel), it offers particularly interesting possibilities for members, where weight and inertia loads are decisive. It could, for instance, be used for different forms of transport (ships, vehicles, etc.), where low weight is essential, or for rapidly rotating large machine parts, where the performance is limited by the capacity of the materials to resist their own inertia loads.

The high degree of ductility of CRC, even at very low temperatures, will make CRC very interesting for large objects that have to resist large loads at low temperatures, where steel will fail due to brittleness or suffer functional deficiency due to progressive corrosion damage.

Because of the far better possibilities of forming CRC and combining it with several other components than those afforded by steel, CRC finds its principal use in hybrid constructions – for example, load-carrying parts in large machines, or special high-performance joints in conventional steel and concrete structures, where large forces have to be concentrated in small volumes.

Slurry Infiltrated Mat Concrete (SIMCON)

SIMCON can also be considered a pre-placed fibre concrete, similar to SIFCON. However, in the making of SIMCON, the fibres are placed in a “mat form” rather than as discrete fibres. The advantage of using steel fibre mats over a large volume of discrete fibres is that the mat configuration provides inherent strength and utilizes the fibres contained in it with very much higher aspect ratios. The fibre volume can, hence, be substantially less than that required for making of SIFCON, still achieving identical flexural strength and energy absorbing toughness.

SIMCON is made using a non-woven “steel fibre mats” that are infiltrated with a concrete slurry. Steel fibres produced directly from molten metal using a chilled wheel concept are interwoven into a 0.5 to 2 inches thick mat. This mat is then rolled and coiled into weights and sizes convenient to a customer’s application (normally up to 120 cm wide and weighing around 200 kg).

As in conventional SFRC, factors such as aspect ratio and fibre volume have a direct influence on the performance of SIMCON. Higher aspect ratios are desirable to obtain increased flexural strength. Generally, because of the use of mats, SIMCON the aspect ratios of fibres contained in it could well exceed 500. Since the mat is already in a preformed shape, handling problems are significantly minimised resulting in savings in labour cost. Besides this, “balling” of fibres does not become a factor at all in the production of SIMCON.

Investigations using manganese carbon steel mats (having fibres approximately 9.5 in long with an equivalent diameter of about 0.01 to 0.02 in) and stainless steel mats (produced using 9.5 in long fibres with an equivalent diameter of about 0.01 to 0.02 in) have revealed that SIMCON has performed very well compared with SIFCON specimens that had a steel fibre content of 14% by volume as illustrated in Table 5.

It is clear from the table that the energy-absorption capacity of SIMCON is far superior to SIFCON. A reinforcement level in SIMCON of only 25% of that of conventional SIFCON is found to provide as much as 75% of the latter’s ultimate flexural strength.

Applications of SIMCON

SIMCON offers the designer a premium building material to meet the specialised niche applications, such as military structures or industrial applications requiring high strength and ductility.

While the use of SIFCON is presently limited only to specialised applications owing to high material and labour costs involved in the incorporation of a very high volume of discrete fibres that are required for achieving vastly improved performance, SIMCON broadens these market applications by cutting the fibre quantity to less than half and there by substantially reducing the product cost.

Reactive Powder Concrete (RPC)

Another recent development in concrete technology is the production of reactive powder concrete (RPC) containing steel fibres as macro-reinforcement. First developed by Bouygues-SA, Paris, its processing has been patented. A high degree of strength, compactness, refined microstructure and homogeneity is achieved by using dense and powder-like particles smaller than 600 microns, and in some cases 300 microns, and by the addition of 2 to 5% of steel fibres. RPC, therefore, do not contain any aggregates, and traditional sand is replaced totally by finely ground quartz of particle size less than 300 microns.

Steel Fibre Concrete Composites for Special Applications

The compactness of an RPC mix is enhanced further by pressing the mix before and during setting, while still in the moulds/forms and by using a very low water-cement ratio (about 0.2%). By subjecting the material to low or high pressure steam curing and by applying pressures up to 50 MPa, the pozzalolanic reaction of the silica fume is accelerated resulting in further modifying of the structure of the hydrates and in concrete strengths as high as 500MPa.

Even though RPC is very strong, it exhibits a brittle failure when fibres are not present. By confining RPC (with steel fibres) in mild steel /stainless steel tubes and applying pressure-cum-heating techniques during its casting, the compressive strength and ductility can be improved tremendously. It is reported that very high strengths of 200 to 800 MPa can be obtained for RPC with cement contents of 955 to 1000 kg/m3. Typical composition of an RPC mix used in the construction of the very first RPC pedestrian bridge built in 1997 in Sherbrooke, Quebec, Canada is given in Table 6. A view of another bridge built in Japan using RPC filled stainless tube supporting columns is shown in Fig. 12.

In due course of time, RPC is expected to outperform normal high performance concretes (HPC) as illustrated in Table 7.

Steel Fibre Concrete Composites for Special Applications

Indian Scenerio

In India, SIFCON, CRC, SIMCON and RPC are yet to be used in any major construction projects. For that matter, even the well-proven SFRC has not found many applications yet, in spite of the fact that its vast potentials for civil engineering uses are quite well known. The reason for these materials not finding favour with designers as well as user agencies in the country could be attributed to the non-availability of steel fibres on a commercial scale till a few years ago. The situation has now changed. Plain round or flat and corrugated steel fibres are presently available in the country in different lengths and diameters. It is, therefore, possible now to use new-age construction materials like SIFCON and CRC in our country in the construction of several structures that demand high standards of strength coupled with superior performance and durability.

Acknowledgment

Some of the pictures and tables included in the paper have been freely extracted from the Keynote paper presented by the author at the National Conference on Advances in Construction Materials, Methodologies & Management organized by the Chaitanya Bharathi Institute of Technology at Hyderabad during 21-22 January, 2009. The author thanks the organizers of the Conference for giving consent to make use of them in the preparation of this paper.

NBMCW July 2009

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New Construction Materials for Modern ...

Construction Materials for Modern Projects

S.A. Reddi, Deputy Managing Director (Retd), Gammon India Ltd.

India is witnessing construction of very interesting projects in all sectors of Infrastructure. High rise structures, under construction, include residential/commercial blocks up to a height of 320 m and RC chimneys for thermal power stations extending upwards up to 275m. Majority of the structures are in structural concrete. The functional demands of such high rise structures include the use of durable materials. High Strength Concrete, Self–compacting Concrete are gaining widespread acceptance. Apart from the basic structural materials, modern projects require a variety of secondary materials for a variety of purposes such as construction chemicals, waterproofing materials, durability aids etc. The paper highlights some of the recent developments.

Durable Concrete

Concrete Design and Construction Practices today are strength driven. Concrete grades up to M80 are now being used for highrise buildings in India. However, due to escalation in the repair and replacement costs, more attention is now being paid to durability issues. There are compelling reasons why the concrete construction practice during the next decades should be driven by durability in addition to strength.

A large number of flyovers and some elevated roads extending up to 20km in length are being realized in different parts of the country and involve huge outlay of public money. However, the concrete durability is suspect. Many of the structures built during the period from 1970 have suffered premature deterioration. Concrete bridge decks built during the period now require extensive repairs and renovations, costing more than the original cost of the project. Multi-storied buildings in urban areas require major repairs every 20 years, involving guniting, shotcreting etc.

A holistic view needs to be taken about concrete durability. In this context, there are a large number of materials in the market which facilitate durable construction. Apart from the materials, the construction processes have also undergone changes with a view to improving the durability of the finished structure.

High Performance Concrete

Construction Materials for Modern Projects
In the United States, in response to widespread cracking of concrete bridge decks, the construction process moved towards the use of High Performance Concrete (HPC) mixes. Four types of HPC were developed1:
  • Very High Early Strength Concrete – 17.5 mPa in 6 hours
  • High Early Strength Concrete – 42.5 mPa in 24 hours
  • A Very High Strength – 86 mPa in 28 days
  • High Early Strength with Fiber Reinforcement
  • High Performance Concrete was introduced in India initially for the reconstruction of the pre-stressed concrete dome of the Kaiga Atomic Power Project, followed for parts of the Reactors at Tarapur and Rajasthan. Subsequently, a number of bridges and flyovers have introduced HPC up to M75 grade in different parts of India.

Self–compacting Concrete (SCC)

SCC was developed by the Japanese initially as a Quality Assurance measure, but now is being widely used for concrete structures worldwide. In India, one of the earliest uses of SCC was for some components of structures at Kaiga Atomic Power Project. Many components of the structures were very heavily reinforced and the field engineers found it difficult to place and compact normal concrete without honeycombs and weaker concrete. SCC was successfully used.

SCC leaving the batching plant is in a semi-fluid state and is placed into the formwork without the use of vibrators. Due to its fluidity, SCC is able to find its way into the formwork and in between the reinforcement and gets self-compacted in the process. SCC is particularly useful for components of structures which are heavily reinforced. The fluidity is realized by modifying the normal mix components. In addition to cement, coarse and fine aggregates, water, special new generation polymer based admixtures are used to increase the fluidity of the concrete without increasing the water content.

Due to its high fluidity, the traditional method of measuring workability by slump does not work. The fluidity is such that any concrete fed to the slump cone falls flat on raising the slump cone; the diameter of the spread of concrete is measured as an indication of workability of SCC. This is called Slump Flow and is in the range of 600 – 800 mm.

Apart from the use of superior grade chemical admixtures, the physical composition of the concrete for SCC has undergone changes. The concrete is required to have more of fine aggregates and compulsorily any of the mineral admixtures – fly ash, ground granulated blast furnace slag (GGBFS), silica fume, metakaolin, rice husk ash etc. Fly ash is abundantly available as a waste product at all the thermal power stations and the Government has encouraged use of fly ash by offering them practically free at the thermal power stations. GGBFS is again a by-product of the steel mills. During the production of steel, a molten steel is poured from blast furnaces and travels in special channels, leaving the impurities on top of the stream. The waste material, being lighter moves on top and easily diverted away from the usable steel.

The diverted slag is quenched and forms small nodules. These nodules are crushed and granulated into very fine product, with particle size smaller than that of cement. The product is marketed in 50 kg bags and available economically in the regions around steel mills with blast furnaces. In other regions, additional transport cost of this bulk material is involved but its use is justified because of contribution to durability of concrete. For the concrete components of the structure for Bandra and Worli sewage outfalls in Mumbai, the German prime contractor insisted on compulsory use of GGBFS for the M40 concrete in order to improve the durability of concrete. GGBFS had to be transported from Vizag in the eastern part of India, in spite of heavy transportation cost. Since then GGBFS is finding widespread use in different parts of India for ensuring durable concrete.

The Use of Mineral Admixtures

Construction Materials for Modern Projects
After realization of the need for durable concrete structures, the composition of concrete has undergone changes. From being a product made of three or four materials (cement, aggregates, water), today a typical durable concrete consists of six or more materials. The use of low water cement ratio enables a reduction in the volume and size of capillary voids in concrete; this alone is not sufficient to reduce the cement based content of concrete which is the source of micro-cracking from thermal shrinkage and drying shrinkage.

To reduce the cement based content, both the water content and cement content must be reduced as much as possible. Concrete mixes with fewer micro cracks can be produced by blending the cement with mineral admixtures either in the batching plant or in the cement plant. This enhances the service life of concrete structures in a cost-effective manner.

Fly Ash

Thermal power stations are left with an undesirable by-product, fly ash, in large quantities which is not able to effectively utilize or dispose of. Currently, (2009) more than 120 million tonne of fly ash are generated annually and the storage and disposal has been costing the power stations substantial unproductive expenditure. Unfortunately, all the fly ash available at the power stations is not fit for use as mineral admixture directly. Fly ash as a mineral admixture should conform to IS: 3812. Such a material is available in the finer streams of Electro Static Precipitators fitted to the power generation system.

The coarser materials are required to be processed (generally with the help of Cyclones) before being considered for use as mineral admixture for concrete. There are only a few processing units in India, including the one as Nashik Thermal Power Station. As per the Euro Code for Concrete, only processed fly ash can be permitted as mineral admixture in concrete. The code limits the use of fly ash. About 35% of cement may be replaced by fly ash; the actual percentage replacement depending on the outcome of trial mixes.

High Volume Fly Ash Concrete (HVFA)

The high volume fly ash concrete (HVFA) represents an emerging technology for highly durable and resource efficient concrete structures. Laboratory and field experience have shown that fly ash from modern coal-fired thermal power plants, when used in large volume (typically 50 - 60% by mass of the total cementitious materials content, is able to impart excellent workability in fresh concrete at a water content that is 15 – 20% less than without fly ash. To obtain adequate strength at early age, further reductions in the mixing water content can be achieved with better aggregate grading and use of super-plasticizers.

HVFA concrete has now been successfully used in a few sporadic projects in India. All SCC in India use HVFA, to the extent of 50% cement replacement. Some concrete roads being built by NHAI have also used HVFA concrete, including the Four-Laning of Satara – Kolhapur National Highway.

Ground Granulated Blast Furnace Slag (GGBFS)

The problems associated with the quality of fly ash do not exist in the case of Ground Granulated Blast Furnace Slag GGBFS, as the produce is necessarily the outcome of grinding to the required particle size. Thus the use of GGBFS as a mineral admixture should be preferred, despite long leads for end users in certain parts of India far from the steel plants. GGBFS sold in India is of uniform quality and particle size gradation. For many landmark structures such as the Burj Dubai (the tallest building in the world in 2009) GGBFS has been extensively used as a mineral admixture, even though the material is imported from other countries, resulting in the landed cost being more than that of cement. This was a conscious decision with a view to obtaining a more durable concrete structure.

In India the use of GGBFS has been fairly limited, in spite of all the technical advantages. The Indian Concrete Code permits up to 70% of cement replacement where GGBFS is used. Technically, the use of GGBFS is more effective only at replacement levels of 50% or more. For a number of structures in a port in Andhra Pradesh, typically the M40 concrete mix contained 100 kg of cement and 300 kg of GGBFS.

Portland Slag Cement (PSC) is also available and useful for ensuring durability of concrete structures. Due to the proximity to steel mills, PSC is generally produced in locations close to steel plants. Here again due to the bulky nature of the product, the transportation cost predominate. Another issue concerning quality of the PSC is the actual percentage replacement while making PSC; this information is not normally displayed on the bags, leaving the user at a disadvantage. In developed countries, information regarding the percentage of slag utilized in making PSC is generally printed on each bag of cement.

Condensed Silica Fume (CSF)

Construction Materials for Modern Projects
CSF is a by-product of Ferro-Silicon industry and at present an imported product, easily available in the Indian market. The particle size is very small, about 100 times smaller than that of cement. It can occupy the voids in between cement particles in a concrete mix, reduce the water demand and thus contribute to a very dense concrete of high durability. Normally, 5 - 10% of cement can be replaced by CSF in order to produce durable concrete. The product is expensive and is used in developed countries only for very high strength concrete (above 75 mPa). Indiscriminate use of CSF for lower grades, barring exceptions, only increases the project cost without corresponding technical benefits. Even when used, the percentage replacement should be based on trial mixes in each case, which may vary from one to 10%. CSF may also be used for High Performance Concrete of lower grades.

Ternary Blends

Ternary blends of mineral admixtures are now recommended for improving the durability of important concrete structures. An outstanding example is the Reconstruction of the New I-35 W St. Anthony Falls Bridge crossing the Mississippi River in Minneapolis, US. The new bridge has been opened to traffic in September 2008, less than 14 months after the collapse. HPC has been used for reconstruction with a target 100 year life span. High Performance Concrete containing silica fume and fly ash was used for low permeability.

Two gleaming white concrete sculptures tower 9 m high at each end of the bridge. The sculptures were pre-cast using an SCC mix that included photo-catalytic cement with self cleaning and pollution reducing characteristics. The photo-catalytic cement is one of the new developments in the construction materials industry. The SCC concrete resulted in a marble-like, smooth white finish to the concrete surface. With a low water cementitious material ratio (w/cm), air entrainment and a rapid chloride permeability test (RCPT) value of less than 1500 coulombs at 28 days, the monument will also be a durable feature in the severe environment adjacent to the I-35 W Roadway.2

For the drilled shaft foundations of the I-35 Bridge, SCC was used. To control temperature during curing, fly ash and slag were incorporated as the majority of the cementitious material. This reduced the heat of hydration by approximately 50%. The concrete mixes for the footings and piers were proportioned for mass concrete and durability through the use of fly ash and slag. As the components were massive in size, concrete mixes were modified by cementitious materials, chilled water and cooled aggregates, use of form insulation and internal cooling pipes.

Cement Silos

The use of batching plants for producing concrete is gaining increasing acceptance. As large volumes of cement are used in a batching plant, the cement is generally stored in vertical steel silos. When cement is received in bulkers from the factory, the same is directly pneumatically pumped into the silos which have capacities ranging from 50 to 500 tonne depending upon the project requirements. If only bagged cement is available, they are emptied into the silos, usually with the help of screw conveyors. For modern applications, more than one silo will be required depending on the types of cement and mineral admixture used in the concrete mix.

In a recently commissioned batching plant complex in the Middle East, each of the two plants feature nine cement silos for Portland cement, slag cement, micro silica, fly ash and SRC cement.

Durability Enhancing Products

A full line of products are available to prevent or repair corrosion damage. A typical corrosion inhibiting admixture prevents deleterious expansion and cracking caused by the formation of rust during over-induced corrosion. There are also penetrating sealants to protect new and repaired concrete from the corrosive effects of chloride. The silane and siloxane based reacting sealers soak into the surface, creating a barrier against water or chlorides.

A number of concrete waterproofing admixtures eliminate the need for conventional external waterproofing membranes and saves time, money and hassle at the construction site. It transforms concrete into a water-resistant barrier by becoming an integral part of the concrete matrix.

Hydrophobic Concrete Waterproofing System

A typical patented product uses three materials to achieve a water-tight concrete structure, a super-plasticizer which reduces batching water requirements, thus limiting the volume of the capillary pour network in the concrete; a reactive hydrophobic pour blocking concrete admixture and product specific water stop protection at construction dams.

Other accessory products include an operation retardant, curing compound, water stops and polypropylene fiber reinforcement. The patented product is typically added while concrete mix is being prepared to assist waterproofing. One product is applied at the rate of 5 liter per of concrete. Typically the manufacturer provides a warranty period of 10 years. The performance warranty provides for repairing water leakage through industry accepted and approved means for a period of 10 years. The product however has some negative impact on the rate of gain of strength of concrete. As a rough indication, the specified characteristic 28-day strength of concrete will not be achieved at 28 days but at 56 days or more.

The cementitious content of concrete using the integral waterproofing compound shall not be less than 325 k g / c u m with up to 50% fly ash or slag replacement. The water cement ratio shall be adjusted to compensate for the water in the waterproofing compound and super-plasticizer and maintain the required workability. The water cement ratio shall not exceed 0.42. The product is of American origin, represented by an Indian company which provides the necessary technical expertise.

Reinforcement

Construction Materials for Modern Projects
The revised BIS Code 1786 provides for four grades of reinforcement characterized by the yield strength – Fe 415, Fe 500, Fe 550 and Fe 600. Each of the first three grades is also available with superior ductile properties and a nomenclature is Fe 415D, Fe500D and Fe550D. Primarily the ductile grades specify a higher elongation value. Use of higher grades reduces the tonnage of steel in compression members e.g. columns substantially, results in decongested reinforcement and facilitates easy placement and vibration of concrete. Fe 415 and Fe 500 are easily available in the market. Fe 550 is now being offered by some prime producers–Tata Steel, Sail etc. After the revision of the Code, Fe 550 is also offered in selected diameters.

Fe 500 bars are now used for a number of highrise buildings, bridges and flyovers in India. Lapping of bars results in congestion of steel creates difficulties in proper placement and compaction of concrete and of course more expensive for large diameter bars. Couplers are now preferred instead of lapping. With widespread use, the cost of couplers has come down. The coupler design and manufacture permits the joints in the same plane without the need for staggering as in the case of lapping Fig. 1 shows typical use of couplers for columns of a multi-storied building in Mumbai.

Ternary Blended Cements

Construction Materials for Modern Projects
Ternary blended cements containing the combination of fly ash–slag, fly ash–silica fume or slag–silica fume are commonly used for concrete in many parts of the world. The European Standard EN 197 for cement lists 27 different combinations for cement. Usually mineral admixture used may present a complimentary effect on cement hydration. Limestone filler addition produces favorable effects on cement test. In particular, the physical effects caused by limestone filler enhance the strength due to hydration acceleration of Portland clinker gains at very early age and the improvement of particle packing of the cementitious system. However, the rate of hydration is initially lower than that corresponding to Portland cement; shows a reduction of strength at early age and similar or greater strength at later ages. Ternary cements containing a limited proportion of limestone filler (no more than 12%) and 20 – 30% GGBFS provide a good resistance to chloride ingress and good performance in sulphate environment of low C3A Portland cement.4

Photo-catalytic Cement

This is a patented Portland cement developed by Italcementi Group. The photo-catalytic components use the energy from ultra-violet rays to oxidize most organic and some inorganic compounds. Air pollutants that would normally result in discoloration of exposed surfaces are removed from the atmosphere by the components, and the residues are washed off by rain. This cement can be used to produce concrete and plaster products that save on maintenance cost while they ensure a cleaner environment.3

In addition to Portland cement binders, the product contains photo-catalytic titanium dioxide particles. The cement is already being used for sound barriers, concrete paver blocks and façade elements. Other applications include pre-cast and architectural planners, pavements, concrete masonry units, cement tiles etc.

Insulated Concrete Form (ICF)

ICF structural elements allow maximum clear spans. The ICF elements are used for large commercial buildings, residential buildings etc.

Exterior Self–leveling Concrete Topping

This is a Portland cement based product for fast track resurfacing and smoothing of concrete. It produces a smooth flat hard surface and dries quickly without shrinking, cracking or spalling. Pourable or pumpable when mixed with water, it installs 6 to 20 mm thick in one application and up to 50 mm thick with the addition of aggregate. It is pourable or pumpable when mixed with water. It can be used on, above or below grade and it makes spalled or damaged concrete look like new. Once sealed it creates an excellent wearing surface.

Carbon Dioxide (CO2)

As part of a future global atmospheric stabilization strategy, industrialized countries may lead to use large amounts of carbon dioxide. CO2 may be used for curing pre-cast concrete units. Manufacturers of concrete masonry units could use CO2 to reduce energy consumption. Steam curing which is conventionally used is energy intensive. Although CO2 curing provides slower strength development than steam curing, the performance can be improved if the blocks are properly pre-conditioned before CO2 curing. It has also been noted that the water absorption of CO2 cured blocks is lower than that of steam cured blocks.

Corrosion Inhibiters for Reinforced Concrete

Construction Materials for Modern Projects
Calcium nitrate has been proven to inhibit reinforcement corrosion. About 3–4% calcium nitrate of cement by weight is sufficient to protect the reinforcement steel against corrosion. Typically a corrosion inhibiter should
  • raise the level of chlorides necessary to initiate corrosion or
  • decrease the rate of corrosion after it has started or
  • both. Since it does not necessarily prevent corrosion from happening altogether, it is more appropriate to call the product as corrosion retarders.

Coarse Aggregates for Concrete

The BIS Code (IS:383) permits the use of three types of coarse aggregates–natural gravel (shingle), crushed stone or a blend of both. Many outstanding structures built in India in the past had used river gravel as coarse aggregate for concrete including dams (Bhakra), prestressed concrete aqueducts and siphons (Kunu Siphon), large number of prestressed concrete bridges, power stations (Trombay 500 MW Unit V) etc. The results are excellent. Use of rounded aggregates, by virtue of their geometry, reduces the cement and water content requirements of concrete, thus contributing to the economy. Almost 50% of all the concrete produced in the developed world utilizes natural gravel and broken stone is used only when gravel is not available within economic leads.

Recycled Aggregates

With continuous development activity worldwide, the availability of coarse aggregates from natural sources or crushed rock are dwindling; at the same time, due to demolition of old structures, roads etc., a large amount of debris is generated annually and their disposal poses problems for the individuals and the Governments. In many countries including the UK, any demolition agency is not permitted to dispose of the debris except at predetermined locations which may involve very long leads, expensive operations.

Extensive research has now established that the debris can be crushed, processed and recycled as coarse aggregate for fresh concrete. Such recycling solves the above mentioned problems of disposal, and also more economical. Many national codes in the developed world permit the use of recycled aggregates in concrete, subject to safeguards.

Lightweight Aggregates

These are manufactured products and are extensively used in all types of structures involving longer spans where the dead-load forms a major component of the loads involved in the design. Such lightweight aggregates are manufactured products using expanded clay, sintered fly ash etc. Their contribution to strength depends on the type and quality of the lightweight aggregate, the size fraction used and the amount of aggregate used as well as the type and quality of binder in concrete. However, the addition of lightweight aggregate in concrete reduces the modulus of elasticity.

High Performance Lightweight Concrete

Construction Materials for Modern Projects
By using high strength/high performance lightweight concrete in prestressed concrete bridge girders, spans of bridge girders can be extended by up to 20%. The implications of using lightweight aggregate on prestressing losses long-term creep and shrinkage deformation should be considered. Compressive strength of up to 75 mPa has been obtained. They also result in reduction in creep and shrinkage and consequently lower prestressed losses. The overall costs for a given load capacity are reduced. The reduction in the structure dead-load leads to a reduction in the foundation size.

Self–curing, Shrinkage-free concrete

Italian researchers have produced a concrete by the combined use of
  1. a water reducing admixture based on polycarboxylate in order to reduce both the mixing water and cement.
  2. a shrinkage reducing admixture
  3. an expansive agent based on a special calcium oxide.
The combined use of an expansive agent and a PC based water reducing super-plasticizer results in a shrinkage-free concrete even in the absence of any wet curing. Due to the water reduction caused by the PC based super-plasticizer at a given w/c, there is a reduction in the volume of cement paste and a corresponding increase in the amount of aggregates. Both are responsible for significant reduction in the drying shrinkage.

Advanced Composite Reinforcement

In highly corrosive environments, the use of advanced composite fiber reinforced polymers (FRP) is attractive as a replacement for conventional steel reinforcements. While the FRP materials can be resistant to corrosion, there is lack of ductility. At the moment FRP reinforcement in India is quite expensive. The main market for FRP in India is for structural retrofit for increasing the load capacity, to remedy construction defects or repair damages.

Application of Nano Technology

Reducing particle size of a material to nano–scale often imparts new properties or enhances existing ones. This is typical of nano particles of titanium dioxide, which maintains its photocatalytic activity even when mixed with cement. External cement based surfaces become strongly photocatalytic, leading to a much better appearance and a significant reduction in concentration of pollutants in the surrounding air.

The photoactive titanium dioxide was found to be a more powerful photocatalytic agent when its particle size decreased to non size. This makes it a ideal vehicle for application in construction. A cement binder containing about 5% of active titanium dioxide produces concrete with a smooth surface and also converts the pollutants, removes them from the surrounding air. In a typical application on a building in France completed in 2000, the quality of concrete surface have remained unchanged till date. The structure looked as if it were freshly built (Fig 3.)

Cleaner Surfaces and Less Pollution

Mixing active titanium dioxide with cement produces a binder that maintains its entire normal performance characteristic when used to make concrete. The photocatalytic action makes the surfaces not only to a significant self–cleaning; it also improves the quality of surrounding environment. Using titanium dioxide in glass fiber reinforced concrete offers more efficient and economical way to achieve the benefits of photocatalytics. The environmentally active e-GRC offers the most economical way to achieve cleaner, brighter facades.

Applications for the e-GRC include
  • Cladding panels and facades elements
  • Permanent formwork and form liners
  • Roofing tiles
  • Motorway and Railway sound barriers

References

  • Goodspeed, Vanikar & Cook “High Performance Concrete defined for Highway Structures,” Concrete International Vol. 18 No.2, Feb. 1996.
  • Alan R. Phipps, FIGG Bridge Engineers Inc “HPC for 100 Year Life Span,” HPC Bridge Views, FHWA Issue 52, Nov/Dec 2008.
  • Concrete that cleans itself and the air,” Concrete International Feb. 2009 Vol. 31 No. 2, The Magazine of the American Concrete Institute.
  • Irassar et al “Durability of Ternary Blended Cements containing Limestone Filler and GBFS,” ACI Publication SP-234, 2006.
  • Peter J M Bartos, e-GRC, CONCRETE, UK April 2009

NBMCW July 2009

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