Articles of Concrete

Techniques for Concrete Quality Measure...

Application of Ultra-Sonic Pulse Velocity Techniques for Concrete Quality Measurements

Concrete Quality Measurements

Uday Bhise, Engineer. NDT; Durocrete Engineering Services Pvt. Ltd.

Introduction

This method of testing was originally developed for use on concrete and the published accounts of its application are concerned predominately with this material. A considerable volume of literature has been published describing the results of research on the use of ultrasonic testing for concrete and for fuller details of this application the reader is referred to the Bibliography in the end of this paper. In Britain, the method was first developed by Jones and Gatfield at the Road Research Laboratory between 1945 and 1949 and also independently in Canada by Leslie and Cheesman at about the same time. The apparatus developed at that time made use of a cathode-ray oscilloscope for the measurement of transit times and modified forms of this equipment have been widely used in many countries. The equipment was particularly useful in the laboratory but was less easy to use under field conditions. Today’s various nondestructive testing equipment have been designed particularly for field testing being light, portable and simple to use. They can be operated independently of the mains power supply when used in the field and directly from the ac mains supply for laboratory use.

Ultrasonic testing is now widely used throughout the world and it is clear that the advantages of this method over traditional methods of testing are likely to increase further its application. In particular, its ability to examine the state of concrete in depth is undoubtedly very good.

Applications for Pulse Velocity Testing

The velocity of ultrasonic pulses traveling in a solid material depends on the density and elastic properties of that material. The quality of some materials is sometimes related to their elastic stiffness so that measurement of ultrasonic pulse velocity in such materials can often be used to indicate their quality as well as to determine their elastic properties. Materials which can be assessed in this way include, in particular, concrete and timber but exclude metals. When ultrasonic testing is applied to metals its object is to detect internal flaws which send echoes back in the direction of the incident beam and these are picked up by a receiving transducer. The measurement of the time taken for the pulse to travel from a surface to a flaw and back again enables the position of the flaw to be located. Such a technique cannot be applied to heterogeneous materials like concrete or timber since echoes are generated at the numerous boundaries of the different phases within these materials resulting in a general scattering of pulse energy in all directions.

The pulse velocity method has been shown to provide a reliable means of estimating the strength of timber and has been used to test various kinds of timber products. It is in use for the detection of rot in telegraph poles and provides a very economic method of inspecting these poles while in service. The same equipment can be used to test rock strata and to provide useful data for geological survey work. The method has also been used for testing graphite and it is likely that it will prove useful for testing other non-metallic materials.

Velocity of Longitudinal Pulses in Elastic Solids

It can be shown that the velocity of a pulse of longitudinal ultrasonic vibrations traveling in an elastic solid is given by:

V = E(1 - ν) / ( ρ(1 + ν)(1 - 2ν) )

Where E is the dynamic elastic modulus
where ρ is the density
where ν is Poisson’s ratio.

Table 1
Sr. No.MaterialFrequencies
1Concrete24-150kHz
2Timber150-220kHz
3Graphite200kHz & upwards
4Cast Iron1MHz
5Ceramics24-220kHz
The measurement of the velocity of ultrasonic pulses as a means of testing materials was originally developed for assessing the quality and condition of concrete and the equipment for the same are undoubtedly be used predominantly for this purpose. This paper is therefore to provide some technical guidance in the field of quality measurements & various defects finding in the concrete by ultra-sonic methods. Generally, the lower frequencies are for large, dense, non-homogeneous samples and the higher frequencies are for smaller, less dense, more homogeneous samples. For some typical examples see in Table 1.

Applications for Concrete

The pulse velocity method of testing may be applied to the testing of plain, reinforced and pre-stressed concrete whether it is pre-cast or cast in situ. The measurement of pulse velocity may be used to determine:
  • The homogeneity of the concrete
  • The presence of voids, cracks or other imperfections
  • Changes in the concrete which may occur with time (i.e. due to the cement hydration) or through the action of fire, frost or chemical attack
  • The quality of the concrete in relation to specified standard requirements, which generally refer to its strength.
The Indian Standard Institute has issued the code for method of test & specifications of velocity of ultrasonic pulses in concrete. (I.S. 13311-1992: part 1) as follows.

Sr. No.Velocity Range Concrete Quality
1Below 3.0 Km./Sec.Poor
23.0 – 3.5 Km./Sec.Medium
33.5 - 4.5 Km./Sec.Good
44.5 Km./Sec.& aboveExcellent

Quality of concrete can be assessed in terms of uniformity, incidence or absence of internal flaws etc., in turn indicative of the level workmanship employed. Internal flows & cracks can also be assessed using the pulse velocity techniques. The values of pulse velocities obtained, depend upon number of factors, any single criteria for assessing the concrete only on the basis of pulse velocity given in above table can be held to a satisfactory to a general extent.

Accuracy

In most of the applications, it is necessary to measure the pulse velocity to high degree of accuracy since relatively small changes in pulse velocity usually reflect relatively large changes in the condition of the concrete. For this reason, it is important that care be taken to obtain the highest possible accuracy of both the transit time and the path length measurements since the pulse velocity measurement depends on both of these. It is desirable to measure pulse velocity to within an accuracy of ±2% which allows a tolerance in the separate measurements of path length and transit time of only a little more than ±1%. When such accuracy of path length measurement is difficult or impossible, an estimate of the limits of accuracy of the actual measurements should be recorded with the results so that the reliability of the pulse velocity measurements can be assessed.

Coupling the Transducers with the Concrete Surface

Accuracy of transit time measurement can only be assured if good acoustic coupling between the transducer face and the concrete surface can be achieved. For a concrete surface formed by casting against steel or smooth timber shuttering, good coupling can readily be obtained if the surface is free from dust and grit and covered with a light or medium grease or other suitable couplant. A wet surface presents no problem. If the surface is moderately rough, stiffer grease should be used but very rough surfaces require more elaborate preparation. In such cases the surface should be ground flat over an area large enough to accommodate the transducer face or this area may be filled to a level smooth surface with a minimum thickness of a suitable material such as plaster of Paris, cement mortar or epoxy resin, a suitable time being allowed to elapse for the filling material to harden. If the value of the transit time displayed remains constant to within ±1% when the transducers are applied and reapplied to the concrete surface, it is a good indication that satisfactory coupling has been achieved.

Concrete Testing Transducer Arrangement

The diagrams above show three alternative arrangements for the transducers when testing concrete. Whenever possible, the direct transmission arrangement should be used. This will give maximum sensitivity and provide a well defined path length. It is, however, sometimes required to examine the concrete by using diagonal paths and semi-direct arrangements are suitable for these.

Concrete Quality Measurements

The indirect arrangement is the least satisfactory because, apart from its relative insensitivity, it gives pulse velocity measurements which are usually influenced by the concrete layer near the surface and this layer may not be representative of the concrete in deeper layers. Further more the length of the path is less well defined and it is not satisfactory to take this as the distance from centre to centre of the transducers. Instead, the method shown below should be adopted to determine the effective path length.

Concrete Quality Measurements

In this method, the transmitting transducer is placed on a suitable point on the surface and the receiving transducer is placed on the surface at successive positions along a line and the centre to centre distance is plotted against the transit time. The reciprocal of slope of the trend line plotted through these points gives the mean pulse velocity at the surface as illustrated below.

Concrete Quality Measurements

In general, it will be found that the pulse velocity determined by the indirect method of testing will be lower than that using the direct method. If it is possible to employ both methods of measurement then a relationship may be established between them and a correction factor derived. When it is not possible to use the direct method an approximate value for VD may be obtained as follows:

VD. ~ 1.05V1

Where VD is the pulse velocity obtained using the direct method.
V1 is the pulse velocity obtained using the indirect method.

If the points do not lie in a straight line, it is an indication either that the concrete near the surface is of variable quality or that a crack exists in the concrete within the line of the test position.

Concrete Quality Measurements
Concrete Quality Measurements

A change of slope (as shown above) in the plot could indicate that the pulse velocity near the surface is much lower than it is deeper down in the concrete. This layer of inferior quality could arise as a result of damage by fire, frost, sulphate attack, etc. For transducer separation distances up to Xo to the pulse travels through the affected surface layer and the slope of the line gives the pulse velocity in this layer. Beyond Xo the pulse has travelled along the surface of the underlying sound concrete and the slope of the line beyond Xo gives the higher velocity in the sound concrete.

The thickness of the affected surface layer may be estimated as follows:

t = ( Xo/2 ) ( (Vs - Vd) / (Vs + Vd) )

Vd is the pulse velocity in the damaged concrete (in km/s)
Vs is the pulse velocity in the underlying sound concrete (in km/s)
t is the thickness of the layer of damaged concrete (in mm)
Xo is the distance at which the change of slope occurs (in mm).

Influence of Test Conditions

The pulse velocity in concrete may be influenced by any or all of the following:
  1. Path length
  2. Lateral dimensions of the specimen tested
  3. Presence of reinforcing steel
  4. Moisture content of the concrete
The influence of path length will be negligible provided it is not less than 100mm when 20mm size aggregate is used or not less than 150mm for 40mm size aggregate.

Pulse velocity will not be influenced by the shape of the specimen provided its least lateral dimension (i.e. its dimension measured at right angles to the pulse path) is not less than the wavelength of the pulse vibrations. For pulses of 50 kHz frequency, this corresponds to a least lateral dimension of about 80mm. Otherwise, the pulse velocity may be reduced and the results of pulse velocity measurements should be used with caution. The temperature of the concrete has been found to have no significant effect on pulse velocity over the range from 5° to 30°C. So that except far abnormally extreme temperatures.

Pulse Velocity in Steel Bar

The velocity of pulses in a steel bar is generally higher than they are in concrete. For this reason, pulse velocity measurements made in the vicinity of reinforcing steel may be high and not representative of the concrete since the ultra sonic equipment indicates the time for the first pulse to reach the receiving transducer. The influence of the reinforcement is generally very small if the bars run in a direction at right angles to the pulse path and the quantity of steel is small in relation to the path length. It is however preferable to avoid such a path arrangement and to choose a path which is not in a direct line with the bar diameters. When the steel bars lie in a direction parallel to the pulse path, the influence of the steel may be more difficult to avoid as can be seen. It is, however, not easy to make reliable corre- ctions for the influence of the steel.

Pulse Velocity in Moist Concrete

The moisture content of concrete can have a small but significant influence on the pulse velocity. In general, the velocity is increased with increased moisture content, the influence being more marked for lower quality concrete. The pulse velocity of saturated concrete may be up to 2% higher than that in dry concrete of the same composition and quality, although this figure is likely to be lower for high strength concrete.

Conclusions

  • The ultrasonic principle is the best principle to study the homogeneity of the concrete.
  • The best method to interpret the concrete quality by ultrasonic technique is the direct method to have better clarity of homogeneity in the heart of concrete.
  • Some of the ultrasonic applications stated above need to ascertain by the actual use in the field by the expert ultrasonic technician under the guidance of an expert civil engineer.
  • The effect on the pulse velocity due to present of steel is not very significant if the bars are running perpendicular to the pulse path. The further effect of steel running parallel to the pulse path need to be studied in detail & accordingly the correction factors are need to be developed.
  • When pulse velocity measurements are made on concrete as a quality check, a contractor may be encouraged to keep the concrete wet for as long as possible in order to achieve an enhanced value of pulse velocity. This is generally an advantage since it provides an incentive for good curing practice.

Bibliography

  1. Properties of concrete By Neville.
  2. I.S.13311(Part-1)-1992 : ultra sonic pulse velocity method of testing the concrete
  3. Durocrete Engineering Services Pvt. Ltd. Various reports & research on NDT.
  4. Research into the correlation between concrete strength and UPV values By P. Turgut Harran University, Engineering Faculty, Civil Engineering Department Osmanbey Campus, 63000, Sanliurfa, Turkey.
  5. Overview of Nondestructive Evaluation Projects and Initiative at NSF By Chong, K. P., Scalzi, J. B., and Dillon, O. W., "Journal of Intelligent Materials, System and Structures, Vol. 1, pp. 422-431, October 1990.

NBMCW June 2011

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Effect of Humidity and Temperature on ...

High performance concrete (HPC) has been used more widely in recent years due to the increasing demand for durable concrete. Structures that are exposed to aggressive environments reveal that high strength concrete alone cannot guarantee long-term performance. Concrete is required to exhibit performance in the given environment. However, there has been no established method whereby the mixture proportions of concrete can be optimized according to the required performance. The methods adopted for design of conventional concrete mixes are not directly applicable to HPCs. Several methods have been proposed over the years for the proportioning of mineral admixture – based HPC mixes. The methods namely ACI, modified ACI, DOE etc are found to be suitable for designing HPC mixes especially in cold countries where temperature hardly goes beyond 25οC.

India, being a tropical country has different environment in its different parts. Tropical countries usually receive significant rainfall during only some part of the year leading to substantial variation in the level of humidity in many parts of the tropics. So the variation in temperature and humidity has profound effect on the properties of HPC such as strength and durability since the mix proportions are usually decided at laboratory conditions. Therefore, HPC mix design in tropical climate need to be given special attention to incorporate the variation in its properties.

In the present study, an attempt has been made to study the effect of humidity and temperature on workability and strength properties of M50 grade HPC by exposing it to varying humidity and temperature conditions in a chamber where controlled conditions for different humidity and temperature are mainted are monitored. The results indicate, there is a significant effect on strength and workability of HPC mixes due to variation and temperature and humidity.

Kumbhar, P.D. Asst. Professor, Dept of Civil Engg,, K.E.Society's, Rajarambapu Institute of Technology, Rajaramnagar,

Murnal, P. Professor and Head, Dept of Applied Mechanics, Govt College of Engg, Satara

Introduction

These days, more concrete is used in infrastructure projects. High cost of such projects coupled with non replacement possibilities put higher emphasis on durability. Normal concrete, though versatile is not very suitable against severe aggressive conditions, chemical conditions and thermal stresses. High strength concretes were introduced few years back to take care of strength requirement for such highly durable structures. However, structures exposed to aggressive environments have revealed that high strength of concrete alone cannot guarantee long-term performance. This fact has led to the development of high performance concrete (HPC)1. In the present scenario, HPC is emerging as a construction material which will serve the basic dual purpose of strength and durability. However, the basic method of mix design of HPC has not yet been established as it includes other admixtures to serve the requirements of fresh and hardened concrete. These admixtures include silica fume, fly ash and plasticizer or superplasticizer2.

India has very aggressive and corrosive climatic conditions. In some areas, it rains heavily for more than four months a year. Concrete is constantly subjected to various external destructive forces like humidity, heat, cold, industrial pollution, rains etc. So, concrete structures are subjected to gradual deterioration, leading to leaching, carbonation, cracks, separation plaster and corrosion of steel reinforcement, causing substantial reduction in the life of the building and with high costs on repair and maintenance. The long stretch of coastal belt causes saline winds and the summer months are very hot. The problem is aggravated by pollution from increasing industrialization, auto emissions and chemical attacks. Over a period of time, concrete loses its ability to protect itself; cracks and leaks develop and steel reinforcement corrodes, becoming vulnerable to structural failures3.

India, being a tropical country has different environments with varying humidity and temperature conditions in its different parts. The weather and environmental conditions at the time of casting of the concrete may require variation in the proportions. Since the water / binder ratio significantly affects the properties of concrete in both fresh and hardened state, the variation in the temperature and humidity of the surroundings could significantly affect the properties if the water/binder ratio as per the mix-design is used. The water / cement ratio and minimum cement content may also have to be varied for durability considerations. Under such conditions high performance concrete has become quite popular in recent years. However, various required performance attributes of HPC, including strength, workability, dimensional stability and durability, often impose contradictory requirements on the mix parameters to be adopted, thereby rendering the concrete mix design a very difficult task. There are many methods of mix design but the British or American methods will not be applicable for our country as the specific relationships constituting figures and tables are based on their materials4. The conventional mix design methods are no longer capable of meeting the stringent multiple requirements of HPC. The mix design of HPC cannot be based on general tables and graphs. The mix has to be developed for the specific application and for the given set of ingredients5.

Temperature is one of the important factors which affect the durability of concrete. Concrete structures deteriorate more rapidly when exposed to hot environment. High temperature associated with other factors like high humidity has significant effect on the durability of concrete6. Hence, it has become necessary to study the effect of such environmental conditions on workability and strength properties of high performance concrete. The present experimental work deals with study of effect of varying humidity and temperature conditions on workability and strength properties of M50 grade high performance concrete by exposing it to varying humidity and temperature conditions in a room controlled for specified humidity and temperature ranges.

Experimental Investigations

Materials

The materials used in making HPC mixes along with their various properties have been given in Table 1.

Mix Design of HPC

The mix design of HPC was done by using the guidelines of IS Code method (IS10262-1982)8. The design stipulations and the data considered for mix design HPC has been presented below.

Characteristic Strength, fck (MPa)    : 50

Max Size of Coarse Aggregate    : 20mm (Crushed)

[Fraction I-60%, 20mm-12.5mm]

[Fraction II-60%, 12.5mm-10mm]

Degree of Quality Control    : Good

Type of Exposure    : Severe

Degree of Workability    : 0.95 (Compaction Factor)

Target Mean Strength (f'ck), MPa    : fck + t x S = 50+1.65x5 = 58.25 Where,

fck = characteristic compressive strength at 28 days,

S = standard deviation, and

t = a statistic, depending upon the accepted proportion of low results and the number of tests; for large number of tests, the value of 't' is given in Table 2 of IS 10262-1982 code.

Mix Proportions

Mix proportion of M50 grade HPC mix was obtained by making certain modifications in the mix proportion arrived at using the guidelines of IS Code method. The target mean strength is determined by considering the standard deviation value as recommended by IS 456-20009 Code. The mix proportion was obtained without considering any addition or replacement of mineral admixture (i.e. micro silica).

Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 1: Room Controlled for Humidity and Temperature conditions

After several trials, a cement content of 450 kg/m3 and w/b ratio of 0.33 were finalized based on 28 days compressive strength gain of HPC mix and desired workability properties (slump/flow). Thus, for making HPC mixes a cement content of 450 kg/m3 and a w/b ratio of 0.33 were used along with optimum content of micro silica as mineral admixture. After carrying out several preliminary mix trials, the optimum contents of micro silica at 10% and superplasticizer at 1.55%, both by weight of cement, were found to give desired workability and strength properties. The w/b ratio was calculated by dividing the weight of mixing water by combined weight of cement and micro silica. The final mix proportion was arrived at by altering the ratio of fine aggregate to coarse aggregate and is expressed as parts of water: cement: fine aggregate: coarse aggregate as given by 0.33: 1:1.75:2.50.

Preparation of HPC Mix

The required quantities of all the ingredients were taken by weigh batching, with appropriate coarse aggregate fractions and mineral admixtures. Mixing of the ingredients was done in a pan mixer as per the standard procedure. A reference mix was prepared under the prevailing humidity and temperature condition in a room controlled for specified humidity and temperature conditions (Fig.1), using a water-binder ratio of 0.33 and suitable superplasticizer content (by weight of cement) in order to get desired workability. The workability of the concrete was studied by conducting slump and flow tests as per the standard procedure10 (Fig.3 and Fig.4). Standard cube specimens of 150mm x 150mm x 150mm size were cast using the procedure described in IS 516 code11 and were immediately covered with plastic sheet and kept there for 24 hours and then released in water tank for 28 days curing.

Effect of Humidity and Temperature on Properties of High Performance Concrete Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 2: Slump Test Figure 3: Flow Test
All the HPC mixes were prepared using the same mix proportion, w/b ratio and superplasticizer dose under different combined humidity and temperature conditions. A humidity range of 20–90% and a temperature range of 30οC, 35οC and 40οC were considered for study of workability and strength properties of HPC mixes.

The humidity was defined by considering a permissible variation of ± 5% whereas the temperature was defined by considering a permissible variation of ±0.5οC. Thus, a humidity of 50% means the humidity variation which has a range of 15%, 20%, 25% and a temperature of 30οC means the temperature variation which has a range of 29.5οC-30οC- 30.5οC. The details of workability (slump and flow) properties of the mixes prepared under various humidity and temperature conditions along with their quality are given in the Table.2.

Testing of Specimens

After 28 days curing period, the specimens were taken outside the curing tank and were tested under a compression testing machine of 2000KN capacity. The crushing loads were noted and the average compressive strength of three specimens is determined. The compressive strength values of specimens subjected to different combined humidity and temperature conditions has been presented in Table 3.

Result and Discussion

Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 5: Strength Variation at different Relative Humidity (Temp 30OC)

To examine the effect of humidity and temperature on properties of HPC, the specimens with reference mix proportion have been exposed to different humidity and temperature conditions during mixing and casting. From Table 2, it is observed that slump and flow significantly increase in humidity for a given temperature. This means that the increase in surrounding humidity is responsible for maintaining the water content in the mix thereby increasing the workability. From Figures 5 to 6, it is observed that the compressive strength of a given mix is significantly affected by variation in temperature and humidity. There is a general tendency of reduction in compressive strength with increasing humidity (Fig.5 to Fig.7). All the specimens have gained the target mean strength of 58.25MPa. The effect of temperature on compressive strength with varying humidity conditions is indicated in Fig.8. The humidity conditions have been broadly classified into low (30%), medium (50%) and high (80%) humidity levels. From Figure 8, it is observed that there is a general tendency of increase in compressive strength with increase in temperature.

Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 6: Strength Variation at different Relative Humidity (Temp 35oC)

Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 7: Strength Variation at different Relative Humidity (Temp 40oC)

Effect of Humidity and Temperature on Properties of High Performance Concrete
Figure 8: Effect of Temperature on Compressive Strength of HPC

The concrete specimens subjected to different humidity conditions at 35OC have shown some deviations in compressive strength results, however a trend of reduction of compressive strength has been observed.

Conclusion

  • The HPC mixes can be designed using existing IS Code method of mix design with some modifications to achieve a specified target strength using locally available materials and appropriate dosages of superplasticizers and by incorporating micro silica.
  • The behavior of the design concrete mix is significantly affected by variation in humidity and temperature both in fresh and hardened state.
  • It is necessary to develop some correction schemes for the designed mix, which would be applied to the mix at site depending on the site environmental conditions.

References

  • Shridhar, R. (2002), "Use of Chemical Admixtures in HPC for Durable Structures," The Indian Concrete Journal, September, pp.579-580.
  • Krishnan,B., Singh, A., and Singhal, D.(2006), "Mix Design of High Performance Concrete and Effects of Different Types of Cement on High Performance Concrete," Proceedings of National Conference on High Rise Buildings: Materials and Practices, New Delhi, October, pp.11-18.
  • Yargal, S.C., Ravikumar, C.M. and Babu Narayan K.S.(2006), "Design of Various Grades of Concrete in Coastal Environment: A Case Study," CE &CR, December, pp74-83.
  • Maiti S.C., Agarwal, R.K. and Kumar R. (2006), "Concrete Mix Proportioning," The Indian Concrete Journal, December, pp.23-26.
  • Rama Rao, G.V. and Seshagiri Rao, M.V.(2005), "High Performance Concret Mix Proportioning with Rice Husk Ash as Mineral Admixture," NBM&CW, January, Vol.10 (7), pp.100-108.
  • Patnaikuni, I. and Roy, S.K. (1997), "High Strength-High Performance Concrete for Tropical Climates," International Conference on Mainte- nance and Durability of Concrete Structures, March, JNT University, Hyderabad (AP), pp.148-150.
  • BIS Code IS: 383-1970. "Specification for Coarse and Fine Aggregates from Natural Sources for Concrete. (Second Revision), September 1993.
  • BIS Code IS: 10262-1982. "Code of Practice for Recommended Guidelines of Concrete Mix Design." March 1998.
  • BIS Code IS: 456-2000. 'Code of Practice for Plain and Reinforced Concrete (fourth revision).' July 2000.
  • BIS Code IS: 1199–1959. "Code of Practice for Methods of Sampling and Analysis of Concrete." November 1991.
  • BIS Code IS: 516–1959. "Code of Practice for Methods of Tests for Strength of Concrete." (Reaffirmed 1999).

NBMCW May 2011

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Precast Ferrocement Roofing Units

P.C.Sharma, Retd, Head Material Sciences S.E.R.C (G), Chairman Indian Concrete Instt. Western U.P. Ghaziabad Centre, Chief Editor, New Building Materials & Construction World, New Delhi.

Ferrocement is one of the most suitable construction materials for roofing systems is Precast or cast in situ form. It offers high strength, resistance against ingress/seepage of water, high crack resistance and can offer solution with reduced weight & cost. Casting with or without mould is possible. Many shapes and types of roofing systems have been developed such as trough shaped, segmental shell elements, domes, bamboo F.C. domes, F.C roofing has been used at large scale for housing, schools, hospital buildings, godowns, sentry posts, toilets etc. Single precast roofing units can be used to cover small structures. Roofing system with in between support elements can span larger roofs.

Precast Ferrocement Roofing Units
Photo 1: Making of mould 2.5m x 2.0m

This paper presents a very simple structurally sound F.C. roof which can be precast and used for smaller, medium span structures in rural areas and in urban slum development projects.

The whole process has been presented through five photo figures. The mould for such units can be prepared using a masonry, steel or wood. Masonry mould is the cheapest and easiest to make. When only few units are to be produced then a soil deposit mould with cement plaster lining at top is good enough. Photo 1 shows making a peripheral brick on edge lining and depositing of soil in shape for mould. Soil is compacted well to form the shape if mould and the top surface is plastered using 1:4 cement sand, mortar.

Precast Ferrocement Roofing Units Precast Ferrocement Roofing Units
Photo 2: Mould surface plastered and covered with plastic sheet, reinforcement being fabricated Photo 3: Wire mesh reinforcement being fixed

Photograph 2&3 show a thin plastic sheet laid over the mould and making a skeletal cage reinforcement for the roof. Two layers of hot dip galvanized woven wire mesh of 20 g x ½" x ½" were provided as mesh reinforcement one at top and 2nd at bottom of 5 cage. The casting matrix used, photo Figure 4, was designed to obtain 250 kg/cm2 strength at 28 days and mix used was one part of ordinary Portland cement and 2.5 parts of good quality graded sand. Water-cement ratio was maintained at 0.40 .Pidiproof L.W, a waterproofing chemical from Pidilite was used for improving water resistance and workability of mortar. The size of roof shown was 2.5 m x 2.0 m and the photographs belong to a trg programme organized by author for National Drinking Water Mission (when he was with SERC) at Agratalla for PHED Engineers.

An orbital vibrator developed at SERC (R&G) was used for compacting the unit. The thickness at top was kept as 2.5 cm and near the edge as 3.0 cm. Produced units were used for assembling roofs of 2.5 x 4.0m, Figure 1.

Precast Ferrocement Roofing Units
Figure 1: Top plan for twin unit

The Large areas can be covered by providing precast support beams in between. Larger units can be produced if erection facilities are available. A single unit for a one room low cost house designed as a composite unit having toilet and kitchen, can also be produced. Such a system can reduce the construction time drastically. The only disadvantage is its use as upper most roof or as single story construction.

The cost of such roofs come to almost at par with G.I sheet roof when life cycle cost is considered. Several other type of roofs have also been developed at SERC (R) and extensively used in field. Photo 5 shows a 2.5 m x 1.6 m extensively used for making army sentry posts.

Precast Ferrocement Roofing Units Precast Ferrocement Roofing Units
Photo 4: Casting of F.C. Roofing unit at Agartala (Tripura) Photo 5: Precast F.C. Roof for sentry post for BEG Roorkee. (1976) these units used at large scale by indian army

A do it yourself book on F.C. roof, written by author for international Ferrocement information centre AIT Bangkok, Thailand is in wide circulation since 1978. This publication provides a total package on Trough Shaped F.C. roofing unit up to 6 m span. We can produce such booklets for indian condition.

Large number of training programmes have been conducted by author for IFIC, Unicef, NDWM (Min of R.D Govt. of India), 35 point programme, CAPART etc in which F.C. roofing has been covered.

References:

  • Ferrocement roofing panels for single and multiple use - technical report 7/76, SERC Roorkee.
  • Rainwater harvesting and ferrocement structures for north eastern states in India - report & Trg manual, National Drinking water mission project structural Engg Research Centre Ghaziabad - 1998.
  • Ferrocement roof do it yourself manual - P.C.Sharma V.S. Gopalaratnam International Ferrocement Information Centre (IFIC) A.I.T. Bangkok (Thailand) 1978.

NBMCW May 2011

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Effect of Blended Fly ash and Superplas...

Effect of Blended Fly ash and Superplasticizer on Pozzolanic Activity and Compressive Strength of Cement Paste

Preeti Sharma, Research Scholar, Devendra Tyagi, Associate Professor, Department of Chemistry, D.A.V College Dehradun, H.N.B Garhwal University, Srinagar. S.K Agarwal, Scientist, C.B.R.I, Roorkee

It is a very well known fact that the use of fly ash in masonry and concrete enhances the durability of structure hence use of fly ash is gaining momentum in the cement/ concrete Industry. But due to lack of awareness, utilization of flyash is still too low in India.

Present study covers the factors affecting the pozzolanic activity of flyash
  1. Effect of blending of flyash from different sources such as Silo1 (Hopper 1 & Hopper 2 mixture), Silo 2 (Hopper 3, Hopper 4 & Hopper 5 mixture).
  2. Effect of superplasticizer.
The main properties that influence the pozzolanic activity of Flyash are Loss on ignition and Fineness.

In this paper, the effect of fly ash percentage of different fields on the compressive strength of cement paste with and without superplasticizer has been studied

Fly ash of Silo-1 is very coarse in nature where as flyash of Silo-2 is fine. Hence flyash from Silo1 and Silo 2 were blended in varying proportion to study the effect on pozzolanic activity. Since Silo 1 flyash account for 80 % of total flyash generated hence efforts have been made to maximize the utilization of Silo 1 flyash with addition of superplasticizer.

The pozzolanic activity increases from field I to V as expected, the noticeable observation is that with the use of superplasticizer, the pozzolanic activity of Silo-1 is comparable to control value of 358 kg/cm2.

Introduction

Fly ash is a byproduct of the combustion of the pulverized coal in thermal power plants. Fly ash collected from each hopper in the ESP system are transported and stored in the silo. It is known that the properties of fly ashes collected from each hopper in an ESP system varies as we move from the boiler (Hemming et al., 1994; Itskos et al., 2009; Monzo et al., 1994; Erdogdu et al., 1998; Lee et al., 1999). Fly ash consists of inorganic matter present in the coal that has been fused during coal combustion. The particle diameter of fly ash ranges from <1 to 150 µm. Specific surface area is extremely variable ranging from <200 m2/kg to 800 m2/kg.

It is a most common artificial pozzolana, which is defined as a chemically inert silicious and aluminous material that possesse little or no cementitious value. But when it is in finely divided form and in the presence of water, it reacts with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties.

Pozzolanic activity is indicative of lime pozzolana reaction, it is mostly related to the reaction between reactive silica and alumina in fly ash with Ca(OH)2 librated during the hydration of portland cement to form CSH and calcium aluminate hydrate. Fly ash is suitable for massive concrete structures because its addition as a partial substitute for cement reduces the heat of hydration, thereby improving the overall durability of the concrete.

Researchers have given various factors for measuring or assessment of pozzolanic activity:
  1. Quantity of reactive silica
  2. Composition of SiO2 + Al2O3 + Fe2O3
  3. Fineness / surface area of the fly ash particles
  4. Measurement of compressive strength/ strength activity index
  5. Lime reactivity test specified in Indian Standard IS 3812
Fineness of fly ash is very important property affecting pozzolanic reactivity. Different techniques used for measurement of fineness have shown different pozzolanic activity. Fineness of fly ashes plays an important role in strength development. Some researchers give more emphasis on % retained on sieve while some have shown correlation between blain’s fineness and compressive strength.

The sum of Silica + Alumina + Iron oxide is stipulated in most of the standards as a major requirement. It is also observed that silica and alumina in amorphous form contribute to pozzolanic reactivity. Small quantity of iron present in glass phase is reported to have deleterious effect on pozzolanic activity.

Measurement of compressive strength is rated as the best technique for measurement of pozzolanic reactivity. There are incidences where low lime reactivity fly ash has shown more compressive strength while fly ash having higher lime reactivity shown less compressive strength.

It is generally accepted that the fly ash collected at various Sillo/ESP exhibit no greater difference in their chemical composition, but the glassy content of the higher fields is greater [5-7]. Due to this reason cement/ concrete industry is hesitant to use it in concrete. First and second field fly ashes is coarser than other fields and the quantity generated by these fields are nearly 70-80% of the total generation, so it is very important to enhance its reactivity.

There are different ways to enhance the reactivity like
  1. Grinding of fly ash
  2. Use of chemical admixtures
  3. Blending of fly ash of different fields
In the present study blending of Silo one fly ash with subsequent fields has been evaluated for pozzolanic activity. Effect of superplasticizer on the pozzolanic activity has also been studied. This paper reports the effect of fly ash collected from different fields with different percentages on the compressive strength of cement paste.

Material Used

Fly Ash: Fly ash of different fields from thermal power stations (Near Delhi) was collected. The flyash was analyzed and their physical and chemical properties are given in Table 1 & 2.

Table 1: Physical characteristics of fly ash sample of different field
S.NoFly ash FieldsLOI Specific GravitySurface Area (m2/Kg)
1.S-11.102.25398
2.S-20.672.37500

Table 2: Chemical Characteristics of Fly ash of Different Fields
S.NoSample Details (Fly ash)SiO2R2O3CaOSO3
1.S-160.2731.483.680.12
2.S-261.8229.543.740.11

Table 3: Physical and Chemical Properties Of Portland Cement
Compressive Strength
3 Day260 kg/cm2≤230
7 Day370 kg/cm2≤330
28 Days450 kg/cm2≤430
Fineness334 m2/kg≤300 m2/kg
Setting Time
Initial185 mins≤ 30 mins
Final230 mins≤ 600 mins
Insoluble Residue1.2
Magnesia3.2
Alkalies0.40
SO32.5
Silica Content20.5
CaO60.5
Specific Gravity3.14

Cement: Ordinary Portland cement 43 grade confirming to BIS 8112/1989 had been used in present study. The cement was analyzed and its physical and chemical properties are given in Table 4.

Table 4: Pozzolanic Activity Index Of Fly Ash Of Different Fields With and Without Superplasticizer
SystemCASE-1
Without Superplasticizer c.s (kg/cm2)
CASE- 2
With Superplasticizer
c.s (kg/cm2)
Control 340.0420.0
S-1307358.0
S-2370421.0

Superplasticizer: Sulphonated Naphthalene Formaldehyde Condensate (SNF) conforming to BIS 9103 (2004) was used in the present study Binder had been used as 1% dose by weight in all the test mixture.

Sand: Standard sand (annore) had been used in the present study.

Experimental Procedure

Pozzolanic Activity

Control Mixture:- The control was prepared with 250 gm of portland cement, 687.5 gm ( 229 fraction I + 229 fraction II + 229 fraction III) of graded sand and 121 ml of water.

Test Mixture: The test mixture was prepared with 225 gm of cement and 25 gm of pozzolana. The flyash was blended in the following proportions :
  • SET 1: Silo 1 (90%) + Silo 2 (10%)
  • SET 2: Silo 1 (70%) + Silo 2 (30%)
  • SET 3: Silo 1 (50%) + Silo 2 (50%)
The above set of experiments was repeated with 1% superplasticizer addition.

Mixing Procedure: The mortar mixture was prepared using ELE (UK) Automatic mixture. 50 mm cubes were casted for the present study.

Storage of Specimens: After 24 hours of initial curing in a moist room (25 ± 2ºC) with relative humidity not less show 95%. The cubes were placed in air tight glass containers and stored at (65 ± 2ºC) for 6 days.

Determination of Compressive Strength: The compressive strength of mortar cubes was determined after 7 days of demoulding of control and test mixture and average of the three samples has been reported in Table 5

Table 5: Pozzolanic Activity Index Of Fly Ash Of Different Fields With and Without Superplasticizer
SystemWithout Superplasticizer c.s (kg/cm2)With Superplasticizer
c.s (kg/cm2)
Control340.0420.0
SET 1: Field I + Field II
(90% + 10%)
325.0360.0
SET 2 : Field I + Field II
(70% + 30%)
350.0393.0
SET 3 : Field I + Field II
(50% + 50%)
410.0440.0

Paste Studies

Cement cubes of 25mm were cast with various percentages (10, 30 and 50%) of fly ash of different Silo with and without superplasticizer at the same consistency level. The compressive strength of these cubes was determined at different time interval of 1,3,7,28,90 and 360 days.

Results and Discussion: In the present study compressive strength method had been used to evaluate pozzolanic activity.

Table 1 clearly indicates that the surface area of fly ash increases as we move from Silo 1 to 2.

The pozzolanic activity of fly ash of Silo 1 & 2 with and without superplasticizer has been reported in Table-5. It was observed in both the cases (Case 1 & Case 2) that the pozzolanic activity increases as we move from Silo-1 to Silo-2.

It can be depicted for Table 5 that with the use of superplasticizer it is possible to enhance the pozzolanic activity of flyash.

Table 5 indicates the effect of blending of flyash from different fields in varying proportions. SET 1 pozzolanic activity was comparable to control value i.e. 340 kg/cm2, whereas in case of SET 3 the pozzolanic activity was approximately 20 % more than control, thus indicating that the pozzolanic activity was enhanced from SET 1 to SET 3. Hence it was observed that the activity increased with the increasing proportion of Silo 2 flyash thus indicating that fly ash particles with larger medium size particle are more reactive. Enhancement of pozzolanic activity of fly ash through blending helps reduce the cost of superplasticizer.

Table 6: Compressive Strength of Cement Paste with Fly Ash (10%) of Different Fields
System
1day3days7days28days90days360days
Control180280390515535560
S-1168265368515540570
S-2210305425580597616

Table 7: Compressive Strength of Cement Paste with Fly Ash (10%) Of Different Fields with Superplasticizer
System1day3days7days28days90days360days
Control225340450523550600
S-1215365450528562590
S-2242420480585604620

The results of the compressive strength of cement paste with different percentages (10, 30 and 50%) of fly ash of Silo-1&2 with and without superplasticizer up to 360 days are given in tables 6-11.

It is clear from the Table 6-11 that the compressive strength of cement paste increases from S-1 to S-2. Since the fineness of fly ash increases from S-1 to S-2, this indicates that the fine fly ash is very reactive and has larger influence on the strength.

Table 8: Compressive Strength of Cement Paste with Fly Ash(30%) of Different Fields
System1day3days7days28days90days360days
Control180280390515535560
S-195170280350425472
S-2117205360465520548

Table 9: Compressive Strength of Cement Paste with Fly Ash (30%) Of Different Fields with Superplasticizer
System1day3days7days28days90days360days
Control225340450523550600
S-1150208298440479545
S-2167280440520590625

Table 6 and 7 gives strength data of cement paste with 10% fly ash of different fields with and without superplasticizer up to 360 days. The high pozzolanic activity of fly ash of Silo-2 the strength is 10-15% more at one day. The trend is similar up to one year. However, for Silo-1 the strength is slightly less compare to control up-to 7days but beyond that it is comparable to control.

With the use of superplasticizer the 1&3 day strength is more than the control and the gain in strength is observed up to one year.

The results of 30% replacement of fly ash of different fields are given in table 8 and 9. Compressive strength of cement paste without superplasticizer exhibits lower values for Silo-1 upto 360 days. However in case of field Silo-2 strength at 90 days are comparable to control. With the use of superplasticizer Silo-1 show comparable strength at 360 days and Silo-2 show comparable strength at 28 days and at 90 and 360 days the strength is approx. 7 % higher than control. This gain in strength is due to reduction in w/b as we move from field Silo 1-2.

Table 10: Compressive Strength of Cement Paste with Fly Ash (50%) of Different Fields
System1day3days7days28days90days360days
Control180280390515535560
S-17062100197234415
S-211580120225275480

Table 11: Compressive Strength of Cement Paste with Fly Ash (50%) Of Different Fields with Superplasticizer
System1day3days7days28days90days360days
Control225340450523550600
S-187101160235390520
S-2140160205400495575

The strength data of 50% replacement of fly ash is given in table 10 and 11. The strength is less compare to control upto 28 days with and without superplasticizer for fly ash of different fields. At 90 days compressive strength for this is comparable to control when superplasticizer has been used. Beyond 90 days the compressive strength is either equivalent or more compare to control for Silo-2. The use of fine fly ash also has a packing effect and the filling of the small voids and this helps in the strength development (Chindaprasirt et al., 2004).

Conclusions

  • Pozzolanic activity of fly ash of Silo-1 is less than control.
  • Pozzolanic activity of Silo 2 is 5-8 % more than control.
  • Pozzolanic activity of Silo-1 fly ash with 1% superplasticizer is comparable to control value.
  • Blending of fly ash Silo-1 (90%, 70% as 50%) with Silo-2 has comparable pozzolanic activity to control. However, use of superplasticizer with blended fly ash shows enhanced pozzolanic activity
  • This study may help in identifying the optimized blend of fly ash from different fields with/without addition of superplasticizer, to be used in cement / concrete industry.
  • With 30% replacement of cement with fly ash the compressive strength of cement paste from Silo-1 to Silo-2 is comparable to control in case of Silo-1, where as for Silo-2; the strength is more at 360 days in the presence of superplasticizer. However compressive strength of cement paste with 50% replacement for Silo-2 in the presence of superplasticizer beyond 90 days is either comparable or more than control.
  • Test results indicate that fly ash of different fields have noticeable effect on the compressive strength due to different fineness. The fine fly ash (Silo-2) with high surface area is more reactive and thus results in increase in strength.

References

  • Hemming, R.T., and Berry, E.E., On the glass in coal fly ashes: recent advances, Mater. Res. Soc. Symp. Proc. 113, p3-38 (1998)
  • Lee, S.H., Sakai, E., Daimon, M., and Bang, W.K. Characterization of fly ash directly collected Research, Vol. 29 p1791-1797 (1999)
  • Lee, S.H., Sakai, E., Watanbe, K., Yanagizawa, T., and Daimon, M., Properties of classified fly ashes by using electrostatic precipitator and the modification of fly ashes by removal of carbon, J.Soc. Mater. Sci. Jpn. 48, p837-842 (1999).
  • Ranganath, R.V., Sharma, R.C., and Krishnamorthy, S., Proceeding of fifth CANMET/ACI international conference on fly Ash, Silica fume, slag and natural pozzolana in concrete, Miwauke, SP-153, vol. I p 355-366, ACI Detroit (1995)
  • Stanica, S., Cement and concrete, Research, p21 p285 (1991)
  • Mora, E., Paya, J., and Monzo, J., Cement and concrete Research Vol. 41, p29 (1991)
  • Dodson, V., Concrete Admixtures, Structural Engineering Series P 162-164. Van Norsland Rein hold, N.Y.1994.
  • Dongxu, Li., Yimin, C., Jinlin, S., jiashna, S., and Xuequan, W., Cem. Concr. Res. Vol. 30, p 881-886(2000)
  • Yueming, F., Suhong, Y., Zhiyum, W., and jingyn, Z., Activation of fly ash and its effect on cement properties, Cement and Concrete Research, Vol. 29 p 407-472 (1999)
  • Antiohos., S., and Tsiman, S., Activation of fly ash cementitious systems in the presence of quicklime. Part I Compressive strength and Pozzolanic reaction rate Cement and Concrete Research, p 769-779 (2004);
  • IS 1727-1967, methods of test of Pozzolanic Materials, BIS Manak Bhavan, New Delhi (1999)
  • American Society for Testing and materials 1990 Standard test methods for sampling and testing fly ash or natural Pozzolanas for use as mineral admixture in Portland cement concrete ASTM, Pliladelphia PA, ASTM-C311
  • Throne, D.J., and Watt, J.D., Composition and Pozzolanic properties of pulverized ashes II Pozzolanic properties of fly ashes as determined by crushing strength tests on lime mortars, J. Appl. Chem. 15(1965) p 595-604.
  • Watt. J.D., and Throne, J.D., The composition and pozzolanic properties of pulverized ashes. III Pozzolanic properties of fly ashes on determined by chemical methods, J. Appl. Chem 16(1996) p 33-39
  • Aggarwal, S.K. Pozzolanic activity of various siliceous material’, Cement and Concrete Research, Vol. 36 p 1735-39 (2006)
  • Aggarwal, S.K.,Pozzolanic activity of Blended Fly ash, Cement and Concrete Research, Vol. 36 p 1735-39 (2006)
  • A.M Pande, L.M Gupta , Properties of fly ash & Pozzolanic activity, Fly ash Utilization Programme (FAUP), TIFAC
  • Erdogdu K, and Turker P (1998) Effects of fly ash particle size on strength of Portland cement fly ash mortars. Cem. Concr. Res. 29:1217
  • Lee SH, Sakai E, Diamond M and Bang WK (1999) Characterization of fly ash directly from the electrostatic precipitators. Cem. Concr. Res. 29: 1791.
  • Agarwal S.K, and Sharma P, (2009) Role of additives in optimization of flyash in cement. NBM &CW March , p-132, 2009
  • Nagataki S, Sakai E, and Takeuchi T (1984) The fluidity of fly ash cement paste with superplasticizer. Cem. Concr. Res. 14: 631.

NBMCW April 2011

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Cohesive Soil and Fly Ash Mixture As Wa...

Study on Cohesive Soil and Fly Ash Mixture As Reinforced Earth Retaining Wall Filling Material

Vashi, Jigisha M, Research Scholar, Desai, A.K. Associate Professor, Solanki, C.H. Associate Professor, AMD, SVNIT, Surat.

The performance of filling material and its interface friction properties with the geosynthetics would directly influence the application properties of the geosynthetics reinforced earth retaining walls. Through Triaxial test experiments performance include studies of shear parameters of cohesive soil and fly ash mixture in this paper. The results indicate that the mixture of cohesive soil and fly ash have higher strength and rigidity & good interface friction. The technical performance of the mixture conform to the requirements of geosynthetics reinforced earth retaining walls, so it can be used as the filling material of geosynthetics reinforced earth retaining walls in region where specified graded sand is not available.

Introduction

Because of the good engineering performance, a large number of reinforced earth retaining walls have been constructed throughout the world. Compared with the traditional gravity earth retaining walls, geosyntheric reinforced earth retaining walls have better engineering characteristics of light deadweight, beautiful shape, construction convenience etc. Especially on the soft ground, the better performance would be embodied in virtue of their light deadweight.

The national planners in India have put infrastructure development on priority. This has resulted in transport planning, widening of National Highways and new roads in the country. Thus work of Retaining Earth (RE) structures/slopes will be designed in very large numbers over different areas. Thus there is a huge possibility of RE wall being constructed for every 2 Km of 6-4 lane road of NH, State Highways where there is a need of large fill/backfill quantity of sand. But in futures sand is not likely to be a source, so there is a need for use of local waste/fill materials as backfill and hence the present study was taken up.

Filling material’s performances and interface friction properties with the geosynthetics directly influenced the application performances of the geosynthetics reinforced earth retaining walls. The standards used filling materials are cohesion less soil having percentage of fines (<0.08mm) is <15%, free-draining backfill & Soil reinforcement friction factor tan d not = 30o)12. As the filling material of geosynthetics reinforced earth retaining walls, it should have the following engineering properties: (1) good mechanical properties which include the strength and rigidity; (2) better interface friction property with the geosynthetics; (3) the material, better be lightweight.

The shear resistance is function of size, type Undisturbed/Remoulded/ critically weathered, state of drainage in shear, degree of saturation has been studied by many researchers. Following parameters for inorganic cohesive CH soils could be adopted Skempton (1948); Nagaraj (1964); Focht & Sullivan (1969); Stauffer & Wright (1984); Green & Wright (1986); Kulhawy & Mayne (1990); Kayyal & Wright (1991); etc. for preliminary analysis, for compacted soil at OMC & 95% of MDD by Proctor test and observed the range of cohesion of about 100 kPa & angle of internal friction about 20o to 23o for un-drain conditions and cohesion of about 50 kPa & angle of internal friction about 18o for drain condition. Desai, M.D, (1967) studied the expansive CH soil (low to medium) of region has average OMC 20 to 25% and MDD 1.48 to 1.52 g/cc, and this soil remoulded to 95% MDD has Cu = 80 kPa to 100 kPa, Φu = 15 to 18o. Ke Zhao, et al. (2001) studied the performances of saponated residue and added-calcium fly ash mixture in highway application. Desai, N.H (2007) performed box shear test for Fly ash and CH soil in different proportions and showing good results of cohesion in range of 2 to 11 kPa and angle of internal friction in range of 20 to 45 degree.

Through experiments, the technical performances of coshessive soil and fly ash mixture, and the shear parameters for mixture of 80% FA + 20% Cohessive soil & 75% FA + 25% Cohessive soil, were studied. The purpose was to investigate the application properties of the mixture used as filling material of geosynthetics reinforced earth retaining walls.

Experimental Work

The purpose of this experimental work is to review the previous research conducted for region in India having CH soils and combined pertinent data with results from the technical literature to develop guidelines for selection of shear strengths for reinforced cohesive-soil structures.

Soil sample of high plastic clay (CH) and fly ash was collected from GIPCL, Nani Naroli, Kim, Dist. Surat. Physical properties of soil sample were determined by standard lab tests and are represented in Table 1. (Grain size distribution as per IS 2720 Part 4, Specific Gravity as per IS 2720 Part 3 Section I, Liquid & Plastic Limit test as per IS 2720 Part 5, Compaction test as per IS 2720 Part 7, and Free Swell Index test as per IS 2720 Part 40). Physical and chemical properties of fly-ash were also tested and results are presented in Table 1.

Table: 1. Properties of Soil & Fly-ash
Test Physical Properties Chemical Properties of Fly Ash (% content)
Soil Fly-ash
Test 1 Test 2 Test 1 Test 2
% Passing
I S Sieve size in mm 4.75mm 100 100 100 100 SiO2 24.30
2.00mm 100 100 100 100 Al2O3 13.11
1.00mm 97 96 100 100 Fe2O3 17.16
0.425mm 95 95 100 100 TiO2 2.51
0.250mm 95 94 100 100 CaO 27.00
0.075mm 76 75 79 78 MgO 0.32
Specific Gravity 2.497 2.488 2.547 2.526 Na2O 1.05
Liquid Limit Immediate 58 61 44 45 K2O 0.16
LiO2 Nil
After 24 Hrs Soaking - 51 52 SO3 9.50
After 48 Hrs soaking 62 61 LOI (Loss on Ignition) 4.78
Plastic Limit 34 38 NP NP -
Plasticity Index 24 23 -
Standard Proctor Test MDD in kN/m3 16.20 16.10 1.29 1.26
OMC in % 23.5 23.5 32.0 33.0
Free Swell Index in % 50 48 -

For the investigation purpose 80% Fly Ash + 20% Soil mix and 75% Fly Ash + 25% Soil mix was decided. Two different tests namely Triaxial test, (IS 2720: Part 11: 1993/2002) and Permeability (IS 2720 Part 17:1986/2002, falling head method) were carried out for this combination. Specimens were cured for 3 days, 7 days, & 28 days and tested for the said test. Modified proctor test was conducted on 80:20 & 75:25 (F:S) mix proportion. Modified Proctor Test result graph is as shown in Fig.1. Moisture v/s Density relation for Fly Ash: Soil (80:20 & 75:25) ratio mention is given in Table 2.

Table: 2 Moisture-Density Relationships with Mix Proportion
Mix Proportion MDD in kN/m3 OMC in %
80:20 13.89 30
75:25 14.35 30

Moisture Density Relationship
Figure 1: Moisture Density Relationship for 80:20 and 75:25 Proportions

Pilot test results of triaxial test of samples, proportion in 80:20 & 75:25 (Flyash: Black soil) with 3 days, 7 days and 28 days currying period at OMC-MDD; at different cell pressure of 0.5 kg/cm2, 1.5 kg/cm2 and 2.5 kg/cm2; and at 1.5 mm/min strain rate. The values of cohesion C and angle of internal friction φ are found out from modified failure envelope of traixial test are finding out for 3 days, 7 day, and 28 days respectively. The summary of the test result are shown in Table 3.

Table: 3. Testing Result
Tests 3 Days 7 Days 28 Days
Triaxial Test - UU Cuu (kPa) Φ(º) Cuu (kPa) Φ(º) Cuu (kPa) Φ(º)
80:20 (F:C) Mix 414 47.02 365 51.13 336 56.36
75:25 (F:C) Mix 388 42 429 39 457 40
Permeability K for
80:20 (F:C) Mix
2.66E-05cm/sec (Falling Head Permeability Test was conducted on 80:20 mix proportion immediately after casting of the sample.)
Permeability K for 75:25(F:C) Mix 1.23E-05 cm/sec (Falling Head Permeability Test was conducted on 80:20 mix proportion immediately after casting of the sample.)

Conclusion

Through systematic experiments, the application performances of cohesive soil and fly ash mixture have been studied and presented in this paper. The main conclusions obtained are summarized below.

(1) The mixture of cohesive soil and fly ash has higher strength, rigidity, and good water stability. (2) Triaxial shear parameters of the mixture of cohesive soil and fly ash are relatively higher & meet normal design parameters of backfill. These indicate better interface friction if it’s used with geosynthetics. (3) The typical test presented, conformed that mix design of (80:20 & 75:25) can be evolved  for a site to provide low cohesion, high  Φ > 30° material for backfill in RE structures.  This requires placement at 2% less than OMC & MDD. (4) The performance of cohesive soil and fly ash mixture conform to the requirement of filling material of geosynthetics reinforced earth retaining walls. So it can be used as a good filling material for geosynthetics reinforced earth retaining walls at site where BS 8006:1995 specified material is not available.

Acknowledgment

Authors wish to express their deepest gratitude and sincere appreciation to Dr. M. D. Desai (Visiting Prof SVNIT) for his constant guidance, dedication, and encouragement. Authors would like to thank Er. H. H. Desai & Er. N. H. Desai (Owner of Unique Research Center) for providing the lab facilities for the experimental work.

References

  • British standard code of practice for “strengthened / reinforced soils and other fills.” BS: 8006-1997.
  • Desai, M.D. (1967), “Experience in Shear Testing for Problems of Earth Dam Foundations & Embankment Materials.” Pre-conference Symposia on Pore Pressure & Shear Resistance of Soils, INS of SMFE, New Delhi.
  • Desai, N.H, (2007), “Experimental Investigation for use of Flyash as a Major Constituent with Clay for Construction of Embankment.” M.Tech Thesis, D.D.University, Nadiad.
  • Focht, J. A., & Sullivan, R. A. (1969), “Two Slides In Over-consolidated Pleistocene Clays.” Proceedings of the Seventh International Conference on Soil Mechanics and Foundation Engineering, Mexico City, 1969, Vol. 2, pp: 571–576.
  • Green, R., & Wright, S. G. (1986), “Factors Affecting the Long Term Strength of Compacted Beaumont Clay.” Research Report 436-1, Center for Transportation Research, The University of Texas at Austin.
  • Kayyal, M. K., & Wright S.G. (1991), “Investigation of Long-Term Strength Properties of Paris and Beaumont Clays in Earth Embankments.” Research Report 1195-2F, Center for Transportation Research, The University of Texas at Austin.
  • Ke Zhao et al. (2001). “A Research on the Performances of Saponated Residue Added-Calcium Fly Ash in Highway Application”. Chinese Journal of Fly Ash Comprehensive Utilization, 14 (1):6-10.
  • Kulhawy, F. H., & Mayne, P. W. (1990), “Manual on Estimating Soil Properties for Foundation Design.” EPRI EL-6800, Research Project 1493-6, Final Report, Cornell University, Ithaca, August.
  • Nagaraj, T.S. (1964), “Soil Structure and Strength Characteristics of Compacted Clay” The International Journal of Soil Mechanics, Geotechnique, No. 2, pp: 103-114.
  • Skempton, A.W. (1964), “Long Term Stability of Clay Slopes” The International Journal of Soil Mechanics, Geotechnique, No. 2. pp: 77-100.
  • Stauffer, P. A., & Wright, S. G. (1984). “An Examination of Earth Slope Failures in Texas.” Research Report 353-3F, Center for Transportation Research, The University of Texas.
  • Vashi, Jigisha M., Desai, A.K., Solanki, C.H., & Desai, M.D. (2010), “Fly Ash as Backfill Material for Reinforced Earth Structures.” National conference on Fly ash/Futuristic Materials in Civil Engineering Construction for Sustainable Development, V.V.Nagar, Anand, India.

NBMCW April 2011

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Potential Benefits of Flyash in Attaini...

Potential Benefits of Flyash in Attaining the Workability of Silica Fume Concrete

Dr. Jaspal Singh, Professor, Civil Engg Department, Er. Anil Kumar Nanda, Civil Engg Department, PAU Ludhiana

Introduction

Building industry, is one of the key areas of infrastructure development & for catering to the requirements of building materials, we are dependent on natural resources. As natural resources are depleting day by day, we have to think of alternate measuries. Use of industrial wastes for this purpose is beneficial as by this, not only natural resources are conserved but solution to safe disposal of industrial waste is obtained. Flyash and silica fume are the promising industrial wastes which can be easily harnessed in construction. With the increase in the number of coal-based thermal power plants in India, generation of fly ash has reached enormous proportions. In India, about 100 million tonnes of flyash is accumulated every year which is generated as waste from thermal plants. This is causing enough concern as its disposal involves design and installation of ash ponds covering large areas at each plant site. In spite of concerted efforts on a national scale, only a very small fraction (around 6%) of the fly ash is put to use in India, compared to its utilization to a greater extent in other countries.

Silica fume is also a waste by-product from the silicon metal and ferrosilicon alloy industries. The chief problems in using this material are associated with its extreme fineness and high water requirement when mixed with Portland cement. However, if used with superplasticizers, we can attain good workability of concrete.

Workability of concrete plays a vital role in all construction works, affecting the speed of construction and placing of concrete, which in turn affect the financial aspect of construction project. The use of silica fume reduces the workability of fresh concrete or mortar due to its very high specific surface area; however, it improves many of the properties of hardened concrete or mortar. Earlier workability of concrete was controlled by amount of water added during mixing and setting characteristics were adjusted with the help of admixtures to modify the properties of concrete. Nowadays, Superplasticizers are added to concrete to get highly workable concrete. They are assuming increasing popularity for use in concrete, because of advantages they offer in handling, placing and compaction of concrete. Though use of superplasticizers is very common in developed countries, the superplasticizers are not so common in developing countries like India. It is in this context that effort has been made to study the effect of addition of superplasticizer in addition to fly ash and silica fume on workability of concrete.

Materials

Portland cement

xOrdinary Portland cement (OPC) of 43 grade (Ultratech) confirming to IS: 8112:1989 was being used for making concrete. The relevant cement properties experimentally obtained are given in Table 1.

Table 1: Properties of OPC 43 grade cement
S. No. Characteristics Value obtained experimentally Value specified by IS:8112:1989
1. Specific gravity 2.975 -
2. Standard consistency 32% -
3. Initial setting time 123 min 30 min (minimum)
4. Final setting time 266 min 600 min (maximum)
5. Compressive strength
3 Days
7 Days
28 Days

29.18 N/mm2
33.78 N/mm2
47.36 N/mm2

23 N/mm2
33 N/mm2
43 N/mm2
The values obtained conform to specifications given in code

Aggregates

i) Coarse Aggregate

The coarse aggregate used were a mixture of two locally available crushed stone of 10 mm and 20 mm size in 50:50 proportion. The aggregates were washed to remove dirt, dust and then dried to surface dry condition. Specific gravity and other properties of coarse aggregate are given in Table 2. Then sieve analysis of coarse aggregate was done. Proportioning of coarse aggregate was done and Fineness Modulus was obtained as given in Table 3.

Table 2: Properties of coarse aggregates
Characteristics Value
Colour Grey
Shape Angular
Maximum size 20 mm
Specific gravity 2.63

Table 3 : Fineness modules of proportioned coarse aggregate
IS Sieve designation Weight retained on sieve in gms (10 mm aggregates) Weight retained on sieve in gms (20 mm aggregates) Average weight retained (gm) Cumulative weight retained (gm) Cumulative @age weight retained (gm) %age passing
80 mm 0.00 0.00 0.00 0.00 0.00 100.00
40 mm 0.00 0.00 0.00 0.00 0.00 100.00
20 mm 0.00 270 135 135 2.7 97.3
10 mm 2190 4710 3450 3585 71.7 28.3
4.75 mm 2780 20 1400 4985 99.7 0.30
2.36 30 0 15 5000 100 0
PAN - - - - - -

Cumulative percentage weight retained = 674.1

Fitness Modules (F.M.)= Say 6.74

It lies within desirable range 5.5-8.0

Table 4 : Fineness Modulus of the fine aggregates
Sieve no. Retained on each sieve weight (gm) Retained on each sieve
(%age)
Cumulative %age retained %age
Passing
%age
passing
for zone-II
as per
IS-383-1970
10mm 0 0 0 0 100
4.75 mm 80 gm 4 4 96 90-100
2.36 mm 200 gm 10 14 86 75-100
1.18 mm 500 gm 25.0 39 61 55-90
600 Micron 510 gm 25.5 64.5 35.5 35-59
300 Micron 240 gm 12.0 76.5 23.5 8-30
150 Micron 360 gm 18.0 94.5 5.5 0-10
Pan 110 gm 5.50
Total wt. of sample 2000 gm 100 292.5
Fineness modulus = 2.925 and it falls in zone II

ii) Fine Aggregates

Fine aggregates were collected from Chaki River Pathankot. It was coarse sand brown in color. Specific gravity of fine aggregates was experimentally determined as 2.62. Then test on sieve analysis of fine aggregates was performed to get Fineness Modulus.

Fly Ash

Fly ash used in present work was obtained from Guru Hargobind Thermal Plant Lehra Mohabbat, Distt. Bathinda. The fly ash, which was used, falls under class F category. The results of physical properties are given in Table 5.

Table 5: Physical properties of fly ash

S. No.
Characteristics Values
1 Colour Light brown
2. Specific gravity 2.09
3. Class F
4. Chemical composition
SiO2
AL2O3
Fe2O3
CaO
MgO
SO3­
LoI
IR
Lime reactivity

57.41
26.92
5.21
2.05
0.71
0.085
1.18
93.12
59.50
5. Sieve analysis
Sieve no. %age of weight retained
100
140
200
270
Pan
0.00
31.35
26.51
24.72
17.42

Silica Fume

The silica fume used was obtained from Orkla India (Pvt) Ltd (Brand name: Elkem Microsilica 920-D), Navi Mumbai. Its chemical composition and other properties are given in Table 6.

Table 6 : Physical properties of silica fume
S. No. Characteristics Values
1 Specific gravity 2.26
2. Color Grey
3. Chemical composition
SiO2
AL2O3
Fe2O3
CaO
MgO
K2O
Na2O

93.800
0.206
0.096
0.426
0.222
0.337
0.107

Super Plasticizer

The super plasticizer used in the study program was Rheobuild SPI obtained by Basf construction chemicals (India) Pvt. Ltd., Navi Mumbai. It was based on Naphthalene formaldehyde polymer. The physical and chemical properties of super plasticizer, which was obtained from the company, conform to IS-9103-1979 and are given in Table 7.

Table 7 : Properties of super plasticizer Rheobuild-SP1
S. No. Parameter Specifications (As per IS 9103) Properties of Rheobuild SPI
1. Physical state Dark brown free flowing liquid Dark brown free flowing liquid
2. Chemical name of active ingredient Naphthalene formaldehyde polymers Naphthalene formaldehyde polymers
3. Relative density at 250C 1.15 ± 0.02 1.151
4. pH Min 6 7.34
5. Chloride ion content (%) Max 0.2 0.0010
6. Dry material content 32 ± 5 (%) 32.04
7. Ash content 8 ± 5 (%) 8.01

Mix Design by Indian Standard Recommendations

Present investigation includes design of concrete mix (non-air entrained) for medium strength concrete. The guidelines given in various codes like SP: 23-1982, IS: 10262-1982 and IS: 456-2000 have been adopted for mix design of concrete.

Table 8 : Quantities per cubic meter for trial mixes with compressive strength
Mix No. Water cement ratio Cement (kg) Sand (Kg) Coarse aggregate (Kg) Average cube strength at 7 days (N/mm2) Average cube strength at 28 days (N/mm2)
1 0.32 579.375 467.58 1108.52 47.21 60.42
2 0.32 540 487.69 1156.27 44.0 58.56
3 0.32 500 508.15 1204.77 48.6 57.31
4 0.32 480 518.00 1229.00 34.85 54.71
5 0.32 450 533.75 1265.40 40.22 44.26

Table 9: Workability with the varying percentage of silica fume & flyash
%
fly ash
4%
silica fume
8%
silica fume
12%
silica fume
Reference mix
0 0.870 0.852 0.845 0.92
10 0.887 0.860 0.857
15 0.895 0.874 0.869
20 0.902 0.886 0.882

Table 10: Analysis of variance for various percentage of fly ash & silica fume for compaction factor
Source/Treatment Mean values of compaction factor  of reference mix Mean values of compaction factor Critical difference
(C.D.)
Silica fume 4% Silica fume 8% Silica fume 12%
Compaction factor with 0 % flyash 0.92 0.870 0.852 0.845 0.0230
Compaction factor with 10 % flyash 0.887 0.860 0.857 0.0224
Compaction factor with 15 % flyash 0.895 0.874 0.869 0.0196
Compaction factor with 20% flyash 0.902 0.886 0.882 0.0221

For the present investigation, it is required to have characteristic compressive strength 40 N /mm2. the mean target strength is 49.24N/mm2 The compaction factor for the design mix is taken as 0.9. The maximum size of aggregate is 20 mm (angular). Type of exposure is taken as moderate and degree of quality control as very good.

Trial Mixes

The quantity of cement obtained after mix design i.e. 579.375 is much more than the maximum range of cement i.e. 450 kg/m3 as specified in IS 456-2000. So five trial mixes were prepared and average cube strength were obtained after 7 days & 28 days as given in Table 8.

Workability of Concrete

The compaction factor test was performed to see the effect of addition of silica fume and flyash on concrete. The workability of reference and all other concrete mixes as detailed in Table 9 was measured in terms of compaction factor test. It is observed that compaction factor lies between 0.845 to 0.92. Workability of concrete slightly improved with the addition of percentage of flyash to all the percentage of silica fume. In the case of 4 % silica fume and at 0% level of flyash, compaction factor was 0.87. With the addition of 10%, 15% and 20% of flyash, compaction factor improved / increased to 0.887, 0.895 and 0.902 respectively. For 8% of silica fume & at 0% level of flyash, compaction factor was 0.852. With the addition of 10%, 15% & 20% of flyash, compaction factor improved to 0.86, 0.874 & 0.886 respectively. Similarly, for 12% of silica fume and at 0% level of flyash, compaction factor was 0.845. With the addition of 10%, 15%, and 20% of flyash, compaction factor improved to 0.857, 0.869 and 0.882 respectively. The improvement in workability with the addition of flyash to the concrete can be explained on the basis of ball bearing effect of spherical particles of flyash as spherical particles needs less water as compared to other shapes. Probably, another factor contributing to the improvement in workability is increased amount of paste in mix which in turn produces a lubricating effect on ingredients of concrete and helps in achieving a free flowing concrete with closer packing of materials. Conversely, the workability decreased with the addition of percentages of silica fume to all the percentages of flyash. At 0% level of flyash, compaction factors were 0.87, 0.852 & 0.845 with the addition of 4%, 8% & 12% of silica fume respectively. At 10% level of flyash, compaction factors were 0.887, 0.860 & 0.857 with addition 4%, 8% & 12% of silica fume. At 15% level of flyash, compaction factors were 0.895, 0.874 & 0.869 with the addition of 4%, 8%, & 12% of silica fume. Similarly, at 20% level of flyash, compaction factors were 0.902, 0.886 & 0.882 with addition 4%, 8% & 12% of silica fume. The optimum value of compaction factor was at the replacement level of 24% i.e. 20% of flyash & 4% of silica fume by weight of cement. After the optimum level of replacement of flyash & silica fume, if we still add silica fume corresponding to 20% of flyash, the compaction factor starts decreasing. It is due to the fact that surface area is increased due to increased fineness and greater amount of water is required to get a closer packing which results in decrease in workability of concrete mixes at higher replacement levels. The variation of workability with different %ages of flyash and silica fume is as shown in Figure 1.

Moisture Density Relationship
Figure 1: Workability with the varying percentage of silica fume and flyash Statistical analysis

Effect of various %ages of silica fume and flyash on Workability.

The effect of various %age of silica fume and fly ash on workability was statistically significant at 5% level of significance. The values of critical difference and mean compaction factor are given in Table 10.

Summary

The workability was determined using compaction factor test. The statistical analysis was applied on values/results of workability of concrete. All the values/results were found statistically significant.

From the experimental investigation, the following main conclusions can be drawn:

  1. Low water cement ratios like 0.32 can be tried for producing a concrete for commercial purposes but appropriate superplasticizer compatible with the materials are required to be used.
  2. Optimum level of replacements of cement by flyash obtained from Guru Hargobind Thermal Plant Lehra Mohabat, Distt. Bhatinda is around 10% for producing medium range of workability concrete.
  3. Optimum level of replacements of cement by silica fume is around 4% for producing medium range of workability concrete.
  4. The combination of flyash and silica fume is capable of producing a medium range of workability of concrete as partial replacement of cement. The optimum replacement levels of flyash and silica fume are 20% and 4%. This optimum level of combination gave maximum value of compaction factor i.e 0.902
  5. As silica fume & superplasticizer are costly materials and it may not be economical to use them. But when these materials are used with flyash (a waste), workability is likely to improve as evident from the investigation carried out by the authors.

References

  • Bhatnagar Anil and Kumar Rajesh (2007) Use of flyash in rooler compacted concrete dams. The Indian Concrete Journal 81:90-100.
  • Cohen Menashi D (1990) A look at silica fume and its actions in Portland cement concrete. The Indian Concrete Journal 64:429-38
  • Gambhir M L (1992) Concrete manual. Pp. 44. Dhanpat Rai & sons, New Delhi.
  • Gambhir M L (1996) Concrete technology. Pp. 23-25. Tata Mcgraw- Hill Publishing company limited, New Delhi.
  • Gopalakrishanan S, Rajamane N P, Nelamegam M, Peter J A and Dattatreya J K (2001) Effect of partial replacement of cement with fly ash on the strength and durability of HPC. The Indian Concrete Journal 75: 335-41.
  • Goyal S, Kumar M and Bhattacharjee B (2008) Potential benefits of incorporating fly ash in silica fume concrete. The Indian Concrete Journal 82: 38-46.
  • IS: 10262-1982 (Reaffirmed 2004): Recommended guidelines for concrete mix design, Bureau of Indian Standard, New Delhi-2004.
  • IS: 8112:1989 (Reaffirmed 2005): Specification for 43 grade Ordinary Portland Cement, Bureau of Indian Standard, New Delhi-2005.
  • IS: 9103:1999 (Reaffirmed 2004): Concrete Admixtures-Specifications, Bureau of Indian Standard, New Delhi-2004.
  • IS: 383-1970: Specification for Coarse and Fine Aggregates from Natural Sources for Concrete, Bureau of Indian Standard, New Delhi-1970.
  • IS: 1199-1959 (Reaffirmed 1999): Methods of Sampling and Analysis of Concrete, Bureau of Indian Standard, New Delhi-1999.
  • IS: 2386 (Part I,III)-1963: Methods of Test for Aggregates for Concrete, Bureau of Indian Standard, New Delhi-1963.
  • IS: 4031 (Part 4,5&6)-1988: Methods of Physical Tests for Hydraulic Cement, Bureau of Indian Standard, New Delhi-1988.
  • Jolicoeur C, Mikanovic N, Simard M A and Sharman J (2002) Chemical admixtures: Essential components of quality concrete. The Indian Concrete Journal 76: 537-47
  • Mittal Amit (1998) Development of high perpormance concrete for containment dome of kaiga atomic power project. The Indian Concrete Journal 72: 193-202
  • Mullick A K (2008) Cement – superplasticizer compatibility and method of evaluation. The Indian Concrete Journal. 82: 8-15.
  • Ojha R N (1996) Use of fly ash and condensed silica fume in making concrete. The Institute of Engineers (I) Journal-CV 77: 170-73.
  • Shetty M S (2009) Concrete technology. Pp. 29-184. S.chand & Company limited, New Delhi.

NBMCW April 2011

.....

Utilization of Fly Ash in producing bri...

Use of Cement Fly Ash and Gypsum as an Alternative Material for Low Cost Housing (Feasibility Study)

N.S.Naik, Asst.Prof., Civil Engg. Dept. SRES C.O.E. Kopargaon, U.P. Naik, Professor, Civil Engg.Dept. PREC Loni, Tal.Rahata, Dist. Ahmed Nagar and M.S.Purkar, Professor., Civil Engg. Dept. SRES C.O.E. Kopargaon.

In Civil Engineering traditionally, we are using different materials in the form of building units. Some of the building units which we are using for construction are bricks and concrete blocks. Especially for bricks we have to use good plastic clay as primary raw material. This clay is often obtained from prime agricultural land, causing degradation as well as economic loss due to diversion of agricultural land. Manufacturing of bricks produces harmful gases which results in significant air pollution.

This feasibility study is an attempt to use cement, flyash and waste gypsum for the manufacturing of building units so as to replace the traditional building material at least partially.

Introduction

Traditionally, we are using burnt clay bricks for construction. It is a clay product which proved its importance since the dawn of civilization. For production of bricks, we are using good plastic clay as primary raw material. This clay is often obtained from prime agricultural land, causing land degradation as well as economic loss due to diversion of agricultural land. Though clay is easily and abundantly available in nature, its resources has a threshold limit and utilization of clay has reached such a point in construction. Excess use of good clay caused erosion of fertile soil and soil degradation and disturbed the ecology.

The burnt clay brick industry in India produces over 60 billion clay bricks annually resulting in strong impact on soil erosion and unprocessed emissions. For production of these bricks about 160 million tones of top soil, making barren 7500 acres of fertile land. (1) Because of all these ill effects, this is proper time to search an alternative of the burnt clay bricks.

Use of Cement, Fly Ash and Phosphogypsum As An alternative for burnt Clay Bricks

This is new technology which works with the strength of fly ash, lime and gypsum chemistry. The slow chemistry of fly ash and lime is maneuvered by tapping ettringite phase to its threshold limits through sufficient limit of gypsum. Therefore, it does not require heavy duty press or autoclave, which is otherwise required in case of only fly ash and lime. The process completely eliminates the thermal treatment (except open air drying) and does not require combustion of any fossil fuel. The ingredients of the units such as bricks and blocks, fly ash, lime (from OPC) and gypsum are well-known minerals that are widely used in the industries. All these minerals are available in the form of wastages and by-products from industrial activities. In certain areas where by product lime is not available in adequate quantity, Ordinary Portland Cement (OPC) can be used as the source of lime producing the good quality of bricks and blocks. This technology is proved to be environmentally safe and sound.

Materials

Fly ash used for the present study is obtained from the Thermal Power Plant, Eklehara, Nashik. As good quality of lime is not available in the vicinity OPC is used as a source of lime and Phosphogy- psum (CaSO4 2H2O) is obtained from Savil Agrovates, a Kopargaon based company producing agricultural product where phosphogypsum is available as a waste.

Mix Proportions

Table 1: Mix Proportion
Sr. No. Mix Designation Constituent materials (Percentage)
Fly Ash Cement P.G.
01 M-1 25 50 25
02 M-2 30 40 30
03 M-3 35 30 35
04 M-4 40 20 40
As our main intention is to search an alternative material for the conventional burnt clay bricks by using the waste materials and to produce a low cost building material, emphasis is given to use the waste products to the maximum extent and hence following mix proportions are used for the present study. The mix proportions are given in the Table No.1 are in terms of dry weights of the ingredients. Shrinkage cracking is a major weakness in gypsum based blocks. Shrinkage cracking can be minimized by keeping the water content of the binder as low as possible.

Methodology

Mixing of Raw Materials

The weighed quantity of Phosphogypsum, Cement and fly ash were thoroughly mixed in dry state in a pan with the help of a trowel. The mixture in dry state is mixed till it attains a uniform color. When the mixture attains uniform color weighed quantity of water is added in the mixture of fly ash, cement and phosphogypsum. After addition of the required quantity of water the mixture is thoroughly mixed with the help of trowel in a pan. After mixing the mix initially with the trowel the mixture is again mixed thoroughly by kneading until the mass attained a uniform consistency.

To calculate the quantity of water to be added Standard normal consistency test was performed and the water content for the normal consistency was determined. The water content used in the mix for strength tests was 90% of that required to produce the standard normal consistency.

Preparation of Mortar Blocks

Standard cement mortar cube moulds of size 70.7mm x 70.7mm x 70.7mm were used for preparation of blocks. The mixed binder was placed in the cube mould and was compacted properly by rod. Excess paste was hand finished. The mould was filled in three layers and each layer was compacted properly.

Method of Curing

The blocks were taken out from the moulds after 24 hours. After removal from the moulds the blocks were kept for air drying for 2 days. After sufficient strength was gained, these blocks were transferred to water filled curing tanks.

Experimental Work

To check the feasibility of Cement, F.A. and P.G. binder as an alternative material for traditional burnt clay bricks following tests are performed on the binder.
  1. Compression Strength Test
  2. Water Absorption Test
Above tests were performed as per Indian Standards (18), (19)

Observations

Mix M-1
Fly ash Cement P.G.
25% 50% 25%

Observation Table
Sr.No. Age Surface Area in mm2 Comp Load in Newton Comp. Strength in MPa
01 07 4998.49 55250 11.05
02 07 4998.49 66250 13.25
03 07 4998.49 72500 14.50
04 14 4998.49 90500 18.1
05 14 4998.49 88500 17.7
06 14 4998.49 88000 17.6
07 28 4998.49 118500 23.7
08 28 4998.49 117500 23.5
09 28 4998.49 117500 23.5

Mix M-2
Fly ash Cement P.G.
30% 40% 30%

Observation Table
Sr.No. Age Surface Area in mm2 Compressive Load in Newton Compressive Strength in MPa
01 07 4998.49 48500.00 9.70
02 07 4998.49 49000.00 9.80
03 07 4998.49 52500.00 10.50
04 14 4998.49 85000.00 17.00
05 14 4998.49 84500.00 16.90
06 14 4998.49 85500.00 17.10
07 28 4998.49 104000.00 20.80
08 28 4998.49 104000.00 20.80
09 28 4998.49 102000.00 20.40

Mix M-3
Fly ash Cement P.G.
35% 30% 35%

Observation Table
Sr.No. Age Surface Area in mm2 Compressive Load in Newton Compressive Strength in MPa
01 07 4998.49 49750.00 9.90
02 07 4998.49 48750.00 9.75
03 07 4998.49 50000.00 10.00
04 14 4998.49 69500.00 13.90
05 14 4998.49 72000.00 14.40
06 14 4998.49 72500.00 14.50
07 28 4998.49 91000.00 18.20
08 28 4998.49 90000.00 18.00
09 28 4998.49 90000.00 18.20

Mix M-4
Fly ash Cement P.G.
40% 20% 40%

Observation Table
Sr.No. Age Surface Area in mm2 Compressive Load in Newton Compressive Strength in MPa
01 07 4998.49 28500.00 5.70
02 07 4998.49 28000.00 5.60
03 07 4998.49 28000.00 5.60
04 14 4998.49 50000.00 10.00
05 14 4998.49 50500.00 10.10
06 14 4998.49 49000.00 9.80
07 28 4998.49 88000.00 17.60
08 28 4998.49 87000.00 17.40
09 28 4998.49 87000.00 17.40

Variation of Compressive Strength Variation of Compressive Strength
Graph No.1: Variation of Compressive Strength with age for M-1 Mix Graph No.2: Variation of Compressive Strength with age for M-2 Mix
Variation of Compressive Strength Variation of Compressive Strength
Graph No.3: Variation of Compressive Strength with age for M-3 Mix Graph No.4: Variation of Compressive Strength with age for M-4 Mix

Water Absorption Test

After casting the cubes of each mix proportion, the cubes were immersed in water and after 28 days of curing the cubes were taken out of the curing tank and their saturated mass was recorded after the cubes were kept in oven at 1050C and dried to a constant mass and dry mass of the cubes was recorded. After that finding the difference between the saturated and dry mass percentage water absorption was calculated.

Density Test

After 28 days of casting, the cubes was dried to a constant mass in an oven at 1050C they were cooled to room temperature and their density was obtained

Observations: (For density and Water absorption)
Sr. No. Age in days Saturated Mass ( Kg) Dry Mass (Kg) % water absorption Dry density Kn/m3
Mix M-1
01 28 0.660 0.510 29.41 14.44
02 28 0.665 0.510 30.39 14.44
03 28 0.665 0.512 29.88 14.50
Average values 29.89 14.46
Mix M-2
01 28 0.650 0.498 30.52 14.10
02 28 0.645 0.495 30.30 14.02
03 28 0.645 0.495 30.30 14.02
Average values 30.37 14.048
Mix M-3
01 28 0.640 0.515 24.27 14.58
02 28 0.645 0.515 25.24 14.58
03 28 0.645 0.515 25.24 14.58
Average values 24.91 14.58
Mix M-4
01 28 0.625 0.520 20.19 14.73
02 28 0.620 0.515 20.38 14.58
03 28 0.625 0.520 20.19 14.73
Average values 20.25 14.68

Summary of Test Results

Table No. 2 shows summary of all tests results.

Table 2: Shows Summary Of All Tests Results.
Mix Compressive Strength in MPa Percentage Water Absorption Dry Density in  KN/m3
7 days 14 days 28 days
M-1 12.93 17.80 23.56 29.89 14.46
M-2 10.00 17.00 20.67 30.37 14.04
M-3 9.88 14.26 18.23 24.91 14.58
M-4 5.63 9.96 17.46 20.25 14.68

Test Results and Discussions

A large number of blocks were made with different proportions of fly ash, cement and phosphogyp- sum. The experimental results are presented in graph No.1 to 4 and Table No. 2. The results of compressive strength are presented in Graph No.1 to 4, Graph No. 1 to 4 show the compressive strength of cubes having different proportions of fly ash and phosphogypsum. The results indicate that the cement, F.A. and P.G. Mix have potential to use as building material as an alternative for traditional burnt clay bricks. This is due to the fact that fly ash acts as a source of reactive Silica and Alumina, to give Calcium Sulphoaluminate and Silica hydrates, which are responsible for strength. It is observed that the strength of these blocks increases with age. The maximum compressive strength of these blocks can be obtained with a specific proportion of the ingredients. It can be observed from the test results that the strength of the cubes decreases as the percentage of fly ash and phosphogypsum increases. From the results of water absorption, For mix M-1 (with 50% fly ash and phosphogypsum) and for M-2 (with 60% fly ash and phosphogypsum) the percentage water absorption is almost same. For 70% and 80% fly ash and phosphogypsum water absorption decreases.

For economy, the fly ash and phosphogypsum content should be kept as high as possible. The minimum average crushing strength prescribed in Indian code for burnt clay bricks is 3.5 MPa (20). As the compressive strength at 7 days for M-4 is 5.63 MPa and that at 14 days is 9.96 MPa, it can be said that the material is having adequate strength to produce bricks from it.

From Table No.2 it can be observed that the water absorption of cubes in the present investigation was obtained to be in between 24.91% to 30.37%. As per the code (20) the water absorption of ordinary burnt clay bricks should not be more than 20% by weight. Clearly the water absorption for some proportions is more compared to that of traditional burnt clay bricks. Nevertheless such bricks can be used for curtain wall and partition walls where the consequences of the high water absorption capacity will be less severe. However, this aspect certainly needs further investigation.

The weights of various designated cubes are found to be less than that of ordinary burnt clay bricks. The reduced weight of Cement, F.A. and P.G. will provide a working comfort and ease of handling, in addition to reduction in dead weight of structures.

Conclusion

Based on the experimental investigation reported in this study, following conclusions are drawn.

Unique possibility exists for the bulk utilization of fly ash in producing bricks from Cement, F.A. and P.G. in the proximity of thermal power plants, phosphoric acid and fertilizer industries.

The test cubes are having sufficient strength and have potential as a replacement for conventional burnt clay bricks.

Cementitious binder with fly ash and phosphogypsum content equal to 80% gives better compressive strength and 20.25% water absorption and thus suitable for use in construction industry.

Being lighter in weight, Cement, F.A. and P.G. mix will reduce the dead weight and material handling cost in multi storied constructions. The utilization of wastes in making cementitious binder will help in solving the disposal and health hazard problems. It is further needed to develop awareness among users, professionals and financial supporters for using these waste materials for techno-economic reasons in addition to balance economy and achieve energy conservation. The use of these wastes for building industries will definitely reduce the environmental pollution which will be there because of use of burnt clay bricks. Such products can be used for low cost construction practices.

References

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  • Stefania Grzeszczyk et al., Effect of Super Plasticizers on the reheological properties of fly ash suspensions containing activators of the pozzolanic reaction, Cement and Concrete Research, June 2000,pp.1263-1266.
  • Amit Mittal et al., Parametric Study on Use of Pozzolanic Materials in Concrete, New Building Materials and Construction World, October 2006,pp94-112.
  • Environmental and Social Review for Fal-G bricks projects, Eco Carbon Private Limited, Jan 2006, pp.6-14.
  • Manjit Singh, Dr. Mridul Garg, Environment Friendly Building Materials from Waste Gypsum, NBM&CW, June 2007, pp.116-126.
  • www.Science direct.com
  • www.fal-G projects.com
  • I.S.4031-1968, Method of physical tests of hydraulic cement ( Part-I), Bureau of Indian Standards, New Delhi.
  • I.S.269-1976, Specification for Ordinary Portland Cement and low heat cement, Bureau of Indian Standards, New Delhi.
  • I.S.1077-1976, Specifications for common burnt clay building bricks, Bureau of Indian Standards, New Delhi.
  • T.T.T.I., Chandigarh, Civil Engineering Materials, Tata McGraw- Hill Publishing Company Limited, New Delhi.
  • M.S.Shetty, Concrete Technology, S. Chand and Company Ltd, New Delhi
  • M.L. Gambhir, Concrete Manual, Dhanpat Rai and Sons, New Delhi

NBMCW March 2011

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Concrete Overlays

Concrete Overlays

The need has never been greater for engineered strategies to preserve and maintain the nation's pavements. With shrinking budgets, ever increasing traffic volumes and loads, and the critical emerging focus on infrastructure sustainability, highway agencies and local road consultants and municipal bodies are being asked to "do more with less" in managing their pavement networks.

Agencies need to be proactive, sustainable pavement mountainous and rehabilitation strategies that last longer and at reasonable costs. In many situations, concrete overlays or similar to "Thin White Toppings" represent such strategies. They offer cost effective, versatile, short-and long-term solutions for the full range of concrete, asphalt, and composite pavements needs. In addition, they contribute to more sustainable construction practices by preserving pavement service for several decades beyond original service life.

Many concrete overlays have been in service for decades, extending the life of the original pavement structures for 30 years or more! Because concrete distributes the traffic loads over a wide area, the underlying pavement does not experience highly concentrated stresses. As long as the original pavement remains stable and uniform, a concrete overlay can be placed, replaced, or recycled as needed for maintenance and or rehabilitation cycles.

In this manner, the concrete overlay systems can be sustainable for 100 years or longer. Rather than removing and reconstructing on year on year basis adding top coats which give away within weeks of facing rains or heavy traffic conditions, a concrete overlay effectively returns its original investment within 2-3 years of the differential cost and does away with yearly maintenance and suffering to the general public and saving a lot for nations spending.

There are several genuine benefits of the Concrete Overlays:

1) Concrete Overlays consistently provide cost effective solutions. Rupee for Rupee they are one of the most effective long-term maintenance and rehabilitation options for the existing pavements with distress.

2) Concrete Overlays can be constructed quickly and continently as

a) The existing pavement does not need to be removed.

b) Simple operations of surface milling or scarifying is enough for "bonded overlays"

c) Few or no pre-overlay repairs are necessary or even desirable in most cases.

d) Concrete Overlays are placed better using TRTP Fixed Form with Gang Mounted Vibrators or such machines with normal construction practices.

e) Many concrete overlays can be opened to traffic within 3-4 days to traffic from date of placement.

f) Accelerated construction practices can be used throughout the normal construction season.

3) Concrete Overlays are easy to repair.

4) Concrete Overlays are durable rehabilitation tool and most cost effective option

5) Concrete Overlays can survive in and of themselves, as complete preventive maintenance or rehab solutions, or can be used in conjunction with spot repairs of isolated distress.

6) Concrete Overlays enhances pavement sustainability by improving surface reflectance, increasing structural longetivity and enhancing surface profile stability.

Concrete Overlays

A recent event has carried this feature at a heavily trafficked junction near Electronic City where with the joint inputs of Ultratech and ACC and the vital study by their team and the approval of the Bruhat Bangalore Mahanagar Palike a 4 lane road stretch of almost 10,000 SQ. Mtrs with equipment from Allen Concrete Pavers (USA) a leader in Fixed Form Paving technology with more than a dozen machines performing at various sites in India was selected for this sensitive task. The project has been successfully executed to the satisfaction of the general public and the inspection agencies too. Once the outcome is quantified and commercial considerations weighed against the age old practice of asphalt top coats and repeated costs etc are tabled, it is expected to save the agency enough to divert funds to more creative infrastructure needs while the Concrete Overlaid pavements meets the best ride quality and almost nil involvement of rehab for years to come is a new chapter herald by the citizens of Bangaluru.

NBMCW March 2011

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Polished Concrete Floors

Polished Concrete Floors
Anant Shekhar Sahay, Vice-president Business Development-South East Asia, Bomanite India.
Although currently there are no industry standards specifying concrete tolerances specific to polished concrete, manufacturers as well as installers are coming up with their own set of standards to obtain a quality polish job. On new concrete installations that will ultimately receive polishing, certain performance criteria need to be met so the polishing company can perform a uniform quality job.

On new concrete installations, the concrete mix design is critical. Using a cement-rich concrete (typically 3,500 psi) provides a dense surface free of voids for the polishing process. We can also use many times water-reducing admixtures to keep a low water cement ratio without jeopardizing the workability during the concrete placement. Wet curing of the new slab is an excellent way of curing but many times it is not practical. If this is the case, dissipating liquid applied curing compounds are a good way to cure the slab. Any remaining curing compound that does not dissipate over time is usually easily removed during the coarse-grinding phase of the polishing process.

Polished Concrete Floors Polished Concrete Floors

Of equal importance to the appropriate mix design is how the concrete is placed and finished. The flatness and levelness of the concrete substrate is crucial not only for a uniform polish, but also how the project is bid. Floors that have excessive highs and lows will require a tremendous amount more grinding, increasing your labor costs as well as the costs of your diamond tooling.

According to industry standards, finished concrete shall have a minimum Floor Flatness rating of at least 40. The industry standard for terrazzo is no more variance than 1/4 in. in a 10-ft. span, which can also be applied to polished concrete. If the slab is being machine troweled with walk-behind machines, it is imperative to not leave indentations or boot marks behind since they are not easily removed during the polishing phase.

The main methods of coloring polished concrete consist of integrally coloring the concrete mix or staining or dyeing the surface after the fact. By far the easiest and most controllable method is to integrally color your concrete. Most integral color is either in the liquid or powder form and added at the ready-mix batch plant and then mixed for a specified time. Color loads for integral color should never be smaller than 3 cu. yds. for color consistency. Colored aggregate or crushed glass can be added to the concrete mix or hand seeded into the top layer of the mix. The polishing process will reveal the underlying aggregate.

Adding colors to polished concrete, either integrally or with stains and dyes, can create a distinctive look for your floors.

Polished Concrete Floors Polished Concrete Floors

Acid staining or dyeing is a great method of achieving color on polished floors. Different applicators have different methods and time frames of applying the stains and dyes. We have had excellent results with applying dyes around the 400-grit phase. Let's review the process:
  • Coarse grind with 40-grit metal bond (if necessary)
  • Grind with 80-grit metal bond
  • Apply densifier
  • Grind with 150 metal bond
  • Remove scratch pattern from 150 grit with 100/200 resin bond
  • Remove scratch pattern from 100/200 grit with 400 resin bond
  • Apply color stains or dyes
  • Polish with 800-grit resin bond
  • Polish with 1,500-grit resin bond
  • Polish with 3,000-grit resin bond
There are certain situations that the consumer may not choose a 3,000-grit polish. Most Home Depots or Lowe's, for example, typically are polished to 800 grit. There are many levels of polished floors that stop short of truly being "polished concrete," but have their own unique and distinctive look.

Polished Concrete Floors
For the professional polisher, it is often debatable if some type of topcoat is needed with some claiming that it defeats the purpose of polishing relative to the ongoing maintenance. Especially when using dyes, we feel that it is necessary to apply two thin coats of reactive penetrating sealer to help lock the color in. If applied properly - over a clean surface in multiple thin coats (1,500 to 2,000 sq. ft. per coat per gallon) - this provides an extremely hard surface and helps preserve the finished floor. Because most of the material penetrates the surface and is applied very thin, scuffing or scratching is usually not a problem.

Dealing with edges up against walls is always an issue. If the polished surface is to terminate directly against the wall, there are walk-behind machines to address this. Most of the time, tooling by hand with grinders and diamond pads is necessary. Another way to address edges is to use different mediums and create a border making the edge an architectural feature. We have had great results saw-cutting a border in and then using cement based skims or colored epoxy.

If you are considering to pursue the polished concrete market, make sure you do your homework. The purchase of equipment can be a large investment. Talk with manufacturers of polishing equipment and ask why their equipment and services are better than others. Make sure you have a market to support the hefty investment. Polishing machines can also be used for other applications, such as adhesive and coatings removal or to prepare surfaces for decorative overlays, to name a few.

NBMCW March 2011

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Flexural Behaviour of RC Beams Strength...

Experimental Investigation on Flexural Behaviour of Reinforced Concrete Beams Strengthened by Ferrocement Laminates

This paper investigates the flexural behaviour of reinforced concrete beams strengthened by ferrocement laminates. Totally five beams of length 3200mm, width 150mm and depth 250mm were cast and tested. A flat ferrocement laminate 125mm wide, 25mm thick and 3000mm long was cast using cement mortar as binder and weld and woven mesh as reinforcement. The experimental study shows that the ferrocement laminate increases the flexural bahaviour of the distressed reinforced concrete beams. The aim of this project is to bond ferrocement laminates to reinforced concrete beams and strengthen it against flexure.

B.Sivagurunathan, Selection Grade Lecturer in Civil Engineering, Thiagarajar College of Engineering, Madurai. Dr.B.Vidivelli, Professor in Civil and Structural Engineering, Annamalai University, Annamalai Nagar.

Introduction

Increase in service loading, environmental degradation, damage of structure due to poor quality of material and inferior design, upgrading of old structures as per new code and need of seismic retrofit leads to the need of repair and rehabilitation. Rehabilitation of deteriorated civil engineering infrastructure such as beams, girders, bridge decks, parking structures, marine structures, roads etc has been the major issue in the last decades. The need to upgrade the deteriorated civil engineering infrastructure greatly enhances with the ever increasing demands. Therefore rehabilitating existing civil engineering infrastructure has been identified as an important issue to be addressed. The strengthening of concrete structures to resist higher design loads, correct deterioration-related damage, or increased ductility has been traditionally accomplished using conventional materials and construction techniques. Externally bonded steel plates, FRP and ferro-cement laminates are some of the traditional techniques available. In the context of strengthening problem, advanced composites have the potential to prove another promising solution.

Introduction to Ferrocement

Ferrocement is a form of reinforced cement mortar (micro concrete) that differs from conventional reinforced or pre-stressed concrete primarily by the manner in which the following elements are dispersed and arranged. It consists of closely placed, evenly distributed multiple layers of mesh or fine rods completely embedded in cement mortar. Widespread use of ferrocement in construction industry has occurred during the last 40 years. The main worldwide application of ferrocement construction to date has been for silos, tanks, roofs, irrigation channels, manholes & covers, and mostly boats. The construction of ferrocement can be divided in to four phases:
  • Fabricating the steel rods to form a skeletal (cage) framing system;
  • Tying or fastening rods and mesh to the skeletal framing;
  • Plastering and
  • Curing.

Review of Previous Work

Ong.K.C.G, Paramasivam.P and Lim.C.T.E. (1992),
In this study the methods of strengthening of reinforced concrete beams using ferrocement laminates with skeletal bar attached onto the soffit of the beams are reviewed. The methods of anchorage of the ferrocement laminates in the strengthened beams have been investigated. The methods of increasing the ultimate load of the original beams using ferrocement laminate and to control the cracking behavior of the beams as well the effect of the damage of original beams prior to repair are examined.

It is concluded that the strengthened beams have performed better in cracking behavior, reduction in mid-span deflection and increased ultimate load. Further, the pre-cracked beams prior to repair did not affect the ultimate loads of the strengthened beams tested.

Mohd Zamin Zumaat and Ashraful Alam (2006),
In this study, methods of strengthening of reinforced concrete beams using ferrocement laminates with skeletal bar attached on to the soffit of the beams are reviewed. The addition of ferrocement laminates with skeletal bar to the soffit (tension face) of the beams substantially delayed the formations of first crack, restrained the crack widths and increased the flexural stiffness and load capacities of the strengthened beams. The effects of curtailing the ferrocement laminate short of the supports were studied. In the strengthening carried out in the present work, the ferrocement laminates were extended over the supports. This however would be difficult to achieve in practice.

Vidivelli.B, et al (2001),
In this study, the damage due to overloading is considered. A total of three beams were cast and tested. One beam (control beam) was tested to ascertain the load deflection behaviour and ultimate load. The remaining two beams were damaged by overloading. After unloading the damaged beam specimens were repaired with ferrocement laminates with single layer at tension face using epoxy. Static test was conducted on all the beams to determine the load deflection behaviour and ultimate load. A comparison was made on crack width, deflection and ultimate strength at different load stages between control, damaged and repaired beams.

Experimental Programme
Material Used

Experimental Investigation on Flexural Behaviour of Reinforced Concrete Beams Strengthened by Ferrocement Laminates

For the beam specimen, the concrete with design compressive strength of M20 was used. The concrete mix proportions are given in Table1. 43 grade Portland Pozzalana Cement (PPC) confirming to IS: 1489 – 1991, having a specific gravity of 2.92 was used. Locally available river sand having a fineness modulus of 2.61, specific gravity of 2.65 and coarse aggregates of 20mm maximum size, having a fineness modulus of 6.97 and specific gravity of 2.9, were used. High yield strength deformed bars of Fe 415 were used as reinforcement in the concrete beam

Experimental Investigation on Flexural Behaviour of Reinforced Concrete Beams Strengthened by Ferrocement Laminates
Figure 1: Cross section and reinforcement details of RC Beams

For casting of the ferrocement laminates Portland cement mortar matrix, woven and welded meshes of size 24 gauge are tied together to act as reinforcement. Ferrocement may require chemical additives to reduce the reaction between the matrix and the reinforcement.

Details of Test Specimen

Experimental Investigation on Flexural Behaviour of Reinforced Concrete Beams Strengthened by Ferrocement Laminates
Figure 2: Preparation of distressed beam surface

Experimental Investigation on Flexural Behaviour of Reinforced Concrete Beams Strengthened by Ferrocement Laminates
Figure 3: Ferrocement laminate

Five beams were cast for flexural test out of which one beam was control beam the second and third were tested for service load and rehabilitated and the fourth and fifth beams are retrofitted. For rehabilitation and retrofitting ferrocement laminates were used. The details of reinforcement used in the beam are as shown in the Fig.1. Wooden moulds satisfying the above specimen size were used. Before casting, machine oil was applied on the inner surfaces of moulds. Concrete was mixed using tilting type laboratory concrete mixer and was poured into the moulds in layers. Each layer of concrete was compacted well. The specimens were removed from mould after 24 hours of casting and then cured using jute bags. All specimens were moist cured for 28 days.

Bonding of Ferrocement laminate to beam

Experimental Investigation on Flexural Behaviour of Reinforced Concrete Beams Strengthened by Ferrocement Laminates
Figure 4: Bonding of laminnates to the beam

Experimental Investigation on Flexural Behaviour of Reinforced Concrete Beams Strengthened by Ferrocement Laminates
Figure 5: Experimental setup

Before bonding the fabricated ferrocement laminates on the soffit (tension side) of the concrete beam, surface was made rough using sand blasting and cleaned with an air blower to remove all dirt and debris. Once the surface was prepared to the required standard, the epoxy resin was mixed in accordance with manufacturer's instructions. Mixing was carried out in a metal container with base to hardener ratio of 1:2 and was continued until the mixture obtained uniform colour. Then it was applied both on laminate and beam surface and bonded properly without any air gap.

Experimental set-up

Two-point loading system was adopted for the tests. At the end of each load increment, deflection and ultimate load were carefully observed and recorded. The ultimate load for the control beam was found out. Then the other two beams (S) were tested for service load and rehabilitated with ferrocement laminates. The other two beams (R) were retrofitted with ferrocement laminates. The rehabilitated and retrofitted beams were again tested by two point loads and at the end of each load increment; the deflection was carefully observed and recorded. The recorded values are graphically represented and are shown in Fig.6.

Experimental Investigation on Flexural Behaviour of Reinforced Concrete Beams Strengthened by Ferrocement Laminates
Figure 6: Load vs Deflection

Results and Discussions

From the test results summarized in Fig.6, it can be concluded that the rehabilitated beam with ferrocement laminates 12% lesser load than the controlled beam. The retrofitted beam carries 18% more load than that of control and 24% of rehabilitated beam. Fig.6 shows the ultimate load carrying capacity of the beams with different configuration.

Acknowledgements

The experimental work was carried out in the Structural Engineering Laboratory of Thiagarajar College of Engineering, Madurai and Tamilnadu. We would like to thank the faculty members of Structural engineering Department, AnnamalaiUniversity, Annamalai nagar.

References

  • Mohammad taghi kazemi and Reza morshed (2005), "Seismic shear strengthening of RC columns with ferrocement jacket", Journal of structural Engineering, Vol.1.
  • Ong, K.C.G, Paramasivam. P and Lim, C.T.E (1992), "Flexural strengthening of Reinforced concrete beams using ferrocement laminates", Journal of Ferrocement, Vol.22, No.4, PP. 331 - 342.
  • Vidivelli.B, Antony Jeyasehar.C, Srividya P.R. (2001),"Repair and Rehabilitation of Reinforced concrete beams by Ferrocement," In proceedings of the Seventh International conference on ferrocement, Singapore, PP. 465 – 470.

NBMCW March 2011

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Effect of Dosing Sequence of Micro Sil...

Effect of Dosing Sequence of Micro Silica on Slump & Compressive Strength of Concrete

Silica fume has been successfully used in production of high performance and high strength concrete for the past couple of years. The benefits of using silica fume for enhancement of durability and improvement in the microstructure of concrete are already well established through various researches and other developments. Few years back, it was a normal practice to introduce silica fume in the concrete mix in slurry form. However, currently silica fume is dosed in dry form at most of the batching plants. In this paper, an effort has been made to study the effect of dosing sequence of silica fume in the concrete mix in dry form, on the slump and strength characteristics of concrete.

S.B.Kulkarni - AVP, Technical Services & Hemendra S, Manager, Technical Services. UltraTech Cement Ltd. Mumbai.

Introduction

Micro Silica is a mineral composed of ultrafine (fineness in the range of 15,000, m2/kg) amorphous, glassy spherical particles of silicon dioxide produced during manufacture of silicon or ferro silicon. The average particle size is less than 0.5µ meaning that each microsphere of silica fume is about 50 times smaller than an average cement grain particle. The ultrafine particles of micro silica fill the gaps between the cement particles thereby helping to fill the micro voids in the fresh concrete. These particles also act like ball bearings and make the concrete more cohesive. Earlier it was a common practice to use micro silica in slurry form in order to ensure uniform distribution of micro silica in the matrix. However, there were many issues faced at site in introducing silica fume in slurry form.

Issues faced earlier while using silica fume (SF) in slurry form:
  • Separate plant has to be set up for converting silica fume into slurry form.
  • Setting of plant involved an additional capital investment.
  • The pot life of SF slurry is in the range of 30 to 40 minute, so it had to be used within short span of time.
  • Traffic Jams delayed the movement of transit mixer and sometimes led to dosing of slurry even after the lapse of pot life period.
  • If slurry is not used within pot life period, the interaction between water and silica fume particles brings down the effectiveness of silica fume.
  • In Metro cities, it might be difficult to allocate an additional land to set up separate unit for silica fume slurry production.
As such use of silica fume in dry form is considered the most suitable and convenient system. However, the effect of sequencing of dosing of micro silica on concrete slump and strength properties (at 7 and 28 days) need to be studied. To study the impact of dosing of dry silica fume, experiments were conducted in the laboratory with two different dosing sequences and the results were compiled.

Sequence of Micro silica dosing in dry form in the concrete mix:

A. General Practice:

Effect of  Dosing Sequence of Micro Silica
Mixing of ingredients in the process
It has been observed that at most of the batching plants, the various ingredients of concrete and micro silica are loaded in mixer in the following sequence:

1) Coarse Aggregate and Fine Aggregate

2) Cement & Micro silica + Additives (Fly ash / Slag)

3) Mixing of (1) & (2) for approx 30 sec

4) Water & Chemical Admixture

5) Mixing for another, approx 30 sec

6) Unloading the mix

B. New Practice explored for experiment.

It was decided to explore possibility of changing the dosing system of micro silica while keeping the mix design proportion same for all the ingredients and to study its effect on slump and compressive strength of concrete.

For this experiment various materials used were:
  • Cement – UltraTech Slag cement conforming to IS 455.
  • Admixture – Reobuild R620 of BASF.
  • Micro Silica – Elkem.
Grade of Concrete – M35.

Trials were conducted in the laboratory in Pan Mixer: (40 litre Capacity)

Trial A- In this first Trial, following sequence was followed for introducing silica fume in the mix
  1. Coarse Aggregates
  2. Fine Aggregates
  3. Cement
  4. Micro Silica
  5. Water
  6. Plasticizer
  7. Mix all together for 2 minutes.
Trial B- The following sequence was followed in second trial, as given below:
  1. Coarse Aggregates + Micro Silica followed by
  2. Dry mixing for 90sec
  3. Fine Aggregate
  4. Cement
  5. Water
  6. Plasticizer
  7. Mix the ingredients for 90 seconds.
Effect of  Dosing Sequence of Micro Silica Effect of  Dosing Sequence of Micro Silica
Slump Test for Trial A Slump Test for Trial B
From the above two trials, it can be seen that in Trial A silica fume is added after all Coarse Aggregates + Fine Aggregates + Cement have been put in the mixer, whereas in Trial B silica fume is added after Coarse Aggregates and mixed together for 90 seconds and thereafter balance ingredients namely Fine Aggregates, Cement etc. are added. In both the cases, initial slump was measured followed by slump measurement at 30 minutes & 60 minutes. Total 12 cubes were casted for each trial for measuring the compressive strength by
  1. Accelerated Curing Test
  2. At 3 days
  3. At 7 days
  4. At 28 days
The results as referred above were compiled as given below:

Location: UltraTech RMC Lab, Mumbai.

Test Results:
Table - 1A, TRIAL A Table - 1B, TRIAL B
Grade M35 Batch Mix Grade M35 Batch Mix
Cement - PSC 19.60 kg Cement - PSC 19.60 kg
Micro silica 1.40 kg Micro silica 1.40 kg
Crushed Sand 30.72 kg Crushed Sand 30.72 kg
C.A. 10 mm 20.16 kg C.A. 10 mm 20.16 kg
C.A. 20 mm 20.16 kg C.A. 20 mm 20.16 kg
Water 8.28 kg Water 8.28 kg
Admixture (1.8%) 0.378 kg Admixture (1.8%) 0.378 kg
Table - 2 , Slump Results
Trial A Trial B % Increase Remarks
Initial 115 mm 170 mm 47.82 Enhanced Slump in case of Trial B
At 30 min 80 mm 140 mm 75.00
At 60 min 70 mm 125 mm 78.57

Discussion on Test Results

i. Slump test results (as per IS 1199 — 1959)
Effect of  Dosing Sequence of Micro Silica
View of Cube Casting under progress
  • Initial Slump — In Trial A slump was 115mm whereas in Trial B initial slump was 170mm indicating that there was increase in slump by 55mm (47.8% increase).
  • Slump at 30 minutes — In Trial A slump was 80mm whereas in Trial B slump was 140mm indicating that there was increase in slump by 60mm (75% increase).
  • Slump at 60 minutes — In Trial A slump was 70mm whereas in Trial B slump was 125mm indicating that there was increase in slump by 55mm (78.5% increase).
It is interesting to see that in Trial A & B the quantity of various ingredients is same, that includes quantity of water & plasticizer used. Normally, the slump increase happens either by increase in the quantity of water or plasticizer or both. But in this case, it is seen that without changing the mix design proportion or adding extra quantity of water / plasticizer the slump for trial B has increased between 47% and 78% just by changing the sequence of dosing of silica fume (as given in Table 2).

ii. Compressive Strength test result:

a) Accelerated Curing Test cube results (Conforming to IS 9013 – 1978 - Reaffirmed 1999)

Table -3, Compressive Strength results
Trial A Trial B % Increase Remarks
ACT (Mpa) 43 45.56 5.95 Enhanced Compressive Strength in case of Trial B
3 days (Mpa) 24.58 27.87 13.38
7 days(Mpa) 31.76 37.29 17.41
28 days (Mpa) 40.16 45.66 13.70

As mentioned above in the article, concrete cubes were tested by using Accelerated curing test method and results were tabulated as mentioned in Table 3. It is seen that for Trial A, the anticipated 28 days strength was 43MPa, whereas for Trial B it was 45.56MPa. In this case also, the change in sequence of micro silica dosing has resulted into increase in accelerated curing test result value for Trial B by 5.95%.

b) Concrete cube results (as per IS 516 – 1959 - Reaffirmed 1999)

Concrete cubes (150x150x150mm) were casted, cured and tested for 3,7 & 28 days compressive strength as per standard procedure. From Table 3, it is seen that for Trial A the 3, 7 & 28 days strength was noted as 24.58MPa, 31.76MPa & 40.16MPa respectively, whereas for Trial B the compressive strength was noted as 27.87MPa, 37.29MPa & 45.66MPa respectively. It is again interesting to note here also that the compressive strength for 3, 7 & 28 days for Trial B have increased by 13.38%, 17.41% & 13.70% respectively as compared to test results of Trial A. Here also except for change in dosing sequence of micro silica no other changes were made in the mix.

Conclusion

  • It can be seen that introduction of micro silica in the mix immediately after adding coarse aggregates and then mixing the same for 90 sec, has changed the test results considerably both in terms of slump and compressive strength.
  • The test results show that without adding additional water or plasticizers the slump values have enhanced by 47% to 78% and the compressive strength values have enhanced by 13% to 17%.
  • The results are very encouraging as increase in slump values can facilitate reduction of plasticizer dose & enhancement in compressive strength can facilitate reduction in the cement content thereby leading to optimisation in the cost of concrete mix.
  • It is felt that at construction site the Quality Managers should conduct similar trials based on the findings of the above referred article and try to get best benefit in terms of additional retention of slump or reduction in the plasticizer dose resulting in ease in placing, compaction & finishing of concrete, reduced honey comb and also optimization of the concrete mix design cost without spending extra penny for the same.
  • It is hoped that the contents of the article would give further guidance to Quality Managers for exploring the benefits of changing the dosing sequence of micro silica, to their full advantage.

Acknowledgment

Authors acknowledge the experience shared by Mr. S.C. Verma Ex HCC, Q/C Head, on the similar trials conducted earlier.

References

  • IS 456 – 2000 – Plain and Reinforced Concrete – Code of Practice.
  • IS 455 —1989 — Portland Blast Furnace Slag Cement.
  • IS 10262—2009 – Concrete Mix Proportioning – Guidelines.
  • Neville on Concrete by Adam Neville.
  • Construction Technology by Prof. M.S. Shetty.
  • Mineral Admixtures in Cement & Concrete Vol 4 by S.N. Gosh & S. Savkar.
  • Concrete Micro structures, Properties & Material by P.K.Mehta & P.J.M. Monteiro.
  • IS 1199 – 1959 (Reaffirmed 1999) - Methods of Sampling & Analysis of Concrete.
  • IS 9013 – 1978 (Reaffirmed 1999) – Method of making, curing and determining compressive strength of Accelerated cured concrete test specimens.
  • IS 516 – 1959 (Reaffirmed 1999) – Methods of Test for Strength of Concrete.

NBMCW March 2011

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Stamped Concrete: A New Concept in Outd...

Anant Shekhar Sahay
Anant Shekhar Sahay, Vice-president Business, Development-South East Asia
Stamped concrete is one of those home remodeling trends that seem to be catching up very fast these days to beautify outdoor floorings of residence or surrounding such as pool decks, driveways, entries, courtyards, patios, and so on.

Commonly referred to as patterned concrete or imprinted concrete, Stamped concrete is concrete that is designed to resemble brick, slate, flagstone, stone, tile, wood and various other patterns and textures. Earlier only expensive commercial applications looked decent, now with the introduction of less expensive stamped concrete exteriors can also look impressive.

Seeing the burgeoning response to this texturizing and pigmenting trends, Bomanite India is bringing for the first time to India the concept of stamped and decorative concrete which is a well established concept in various parts of the world.

Process of creating a designer outdoor floor

  • Step 1. Wait for the bleed water to evaporate from the surface of the concrete. Then check the concrete to see if it’s firm enough by pressing your finger into the surface about 1/2". It’s all about timing here!
  • Bamonite

    Bamonite

    Bamonite

    Bamonite
    Step 2. Broadcast the release agent over the surface, use a big paint brush or carefully throw it on with your hand. Completely cover the area where you are about to stamp, not too thick though. Wear protective clothes and a mask, this stuff goes everywhere!
  • Step 3. Lay your first stamp on the concrete and lightly press it in with your hand for now. Make sure it is exactly where you want it, the first one is very important to have aligned in the proper position.
  • Step 4. Lay your second stamp next to the first one, make sure they are snug or locked together. Continue laying all your stamping mats across the width of the slab. Pressing or tamping them into the surface of the slab with a hand tamper.

    Don’t tamp too hard, just hard enough to leave the impression of the stamp.
  • Step 5. When you’ve laid out and tamped all the concrete stamps, lift the first one straight up. Be careful not to drag a corner or slide it in any way. Continue that process, leapfrogging them one by one until the entire slab has been stamped.
  • Step 6. Early the next morning saw in the expansion joints to help prevent cracking.
  • Step 7. Clean off the powdered release agent. You can broom off the majority of it and pick it up, then use a garden hose and rinse off the rest. Some will stick to the surface, that’s what creates a two tone colored effect. It will look great after you seal it.
  • Step 8. Apply a concrete sealer after the surface is completely dry. It might be a good idea to wait 1 day after you rinse the stamped concrete to seal it. Moisture and concrete sealers don’t mix well together. That’s it you’ve learned how to stamp concrete.
Hiring a professional concrete stamping contractor is recommended. These steps don’t sound to hard but there are a lot of variables like wind, sun, a hot concrete mix, or half the slab is in the shade and the other half is in the hot sun, that can make stamping concrete very challenging.

With concrete you only get one chance to do it right, especially with stamping, you have a small window of opportunity to make something that looks really good vs something that doesn’t look good at all. Concrete stamping offers a number of advantages when compared to other materials like asphalt, natural stone, and precast pavers.

Benefits of Stamped Concrete

Stamped concrete can make a dramatic impression, and there are many reasons why home owners are choosing this authentic material to enhance their landscapes and buildings. Along with the known durability and long-lasting feature of regular concrete, stamping adds a decorative touch making it a desirable and economical product for all.

Some of the benefits are:
  1. Available in many colors and patterns, concrete stamping offers many design possibilities.
  2. Durable and long lasting, will last for decades when properly installed and maintained.
  3. Can be installed about twice as fast as natural stone or precast pavers.
  4. Can be customized with a one of a kind design of logo.
  5. No special maintenance requirements, just wash occasionally and reseal every few years.
  6. Better resistance to moisture and heavy traffic areas. Sustain a pressure of 600 PSI
  7. Good light reflectivity if lightly colored.

How to stamp and texture concrete

We usually color the concrete, pour the patio or driveway and wait until it is almost hard enough to walk on without sinking in more than a 1/4 to 3/8 of an inch.

Then we cover the surface with a powdered or liquid release agent this keeps the stamps from sticking to the surface. It also adds an antique effect to the final finish.

The next step is to press the rubber stamps into the surface of the concrete pull them up and the texture or pattern is left.

After it cures saw in the expansion joints and apply a good quality concrete sealer.

The end result can be a beautiful concrete slab that looks like slate, stone or even wood planks.

Stamped Concrete Sealers

There are two basic types of stamped sealers; one is a penetrating sealer the other is a film forming sealer.

Penetrating Sealers: A penetrating sealer does actually penetrate the surface and react chemically with the concrete to protect against moisture and deicing salts. It fills the micro and macro spaces, solidifying the entire substrate into one solid mass.

These prevent rain and moisture absorbtion, allowing water to bead up on the surface. The sealed surface is very easy to clean and stays cleaner longer. This has the lowest maintenance and reapplication costs of other sealers.

Penetrating sealers are UV resistant and most allow moisture vapor to escape the concrete.

Use a penetrating stamped concrete sealer for outside and inside stamped concrete where a matte finish is desired. If you don’t want a wet, shiny look this is the sealer to choose.

Film forming sealers: Film forming sealers do just what their name implies, they form a protective film over the surface of the concrete.

Film formers are the most popular choice for decorative stamped concrete because they enhance the colors in the concrete and leave a gloss or sheen looks to the concrete.

Acrylic film forming sealers are the easiest to apply. They can be used on interior or exterior stamped concrete.

Acrylic sealers provide good protection against water and chemical damage. They are UV resistant, non yellowing, fast drying, and come in high or low gloss levels. Acrylics offer a softer surface and are usually the least expansive.

Urethane film forming sealers go on about twice as thick as the acrylic sealer and provide excellent protection against chemicals and abrasives.

These are very good for high traffic areas, really enhance the beauty of decorative concrete, and can be used on both interior and exterior stamped concrete.

Epoxies will give you the hardest, longest lasting, chemical and abrasive resistant finish of all the stamped concrete sealers.

They bond very well to concrete and are generally used for interior applications. They are less UV resistant and may yellow if exposed to the sun. Epoxies are great sealers for high traffic areas and very easy to maintain.

Before installing any sealers to stamped concrete makes sure the surface is completely dry and clean from dirt, dust, oil, and grease.

Applying the sealer correctly will make clean up easier and offer very good resistance to harsh weather conditions, mold, and bacteria growth.

Cleaning and sealing concrete should be done on a regular basis. The frequency will depend on how high a traffic area to cars or foot traffic the concrete is exposed to.

NBMCW February 2011

.....

Compressive Strength of Fiber Reinforce...

An Experimental Study on Compressive Strength of Fiber Reinforced Concrete With Fly Ash

Er.Balvir Singh, Structural Engineer Futura Design Consultants Pvt. Ltd. Ludhiana, Dr.Jaspal Singh, Professor, Deptt. Of Civil Engineering Punjab Agricultural University, Ludhiana

This paper deals with an experimental study on the properties of concrete containing fly ash and fibers. Fly ash content used was 0%, 3%, 9% and 12% of mass basis, and fiber volume fraction was 0%, 0.75%, 1%, 1.25% and 1.5% of volume basis. The experiment conducted show, that steel fiber addition, either into Portland cement concrete or Fly ash concrete, improves the tensile strength and drying shrinkage and decreases workability. Although Fly ash replacement reduces strength properties, it improves workability, reduces drying shrinkage and increases freeze–thaw resistance of fiber reinforced concrete. The performed experiments show that the behavior of Fly ash concrete is similar to that of Portland cement concrete when fly ash is added. In the proposed experimental investigation, the behavior of fiber reinforced concrete with Fly ash as a admixture is studied in compression with the objective to determine the:
  • Optimum value of volume friction of steel fiber for constant value of Aspect ratio and Constant percentage of fly ash replacement with cement.
  • Optimum value of percentage of fly ash replacement with cement for the Constant fiber parameter, i.e. Aspect ratio and volume friction
  • Post cracking behavior of fiber reinforced concrete.

Introduction

Concrete is currently the most widely used construction material as it can be cast to any form and shape at site very easily. However, the relative low tensile strength to weight ratio and limited ductility of plain cement concrete poses major difficulty in its direct applications. To overcome these deficiencies, reinforcing of tensile zone with steel bars and prestressing of plain cement concrete has been invariably used. But all these measures do not improve the post cracking behavior of concrete. In fiber reinforced concrete, short discontinuous fibers are added to plain and reinforced concrete to improve the post cracking behavior and tensile strength to weight ratio.

Fly ash is the fine powder produced as a product from the combustion of pulverized coal. The disposal of fly ash is one of the major issues as dumping of fly ash as a waste material may cause severe environmental problems. Therefore, the utilization of fly ash as an admixture in concrete instead of dumping it as a waste material can have great beneficial effects of lowering the water demand of concrete for similar workability, reduced bleeding and lowering evolution of heat. The use of fly ash in concrete is found to affect strength characteristics adversely. One of the ways to compensate for the early-age strength loss associated with the usage of fly ash is by incorporating fibers. Islam M et al reported that the addition of fibers to concrete considerably improves its structural characteristics such as static flexural strength, impact strength, tensile strength, ductility and flexural toughness. Generally, aspect ratios of steel fibers used in concrete mix are varied between 50 and 100. The most suitable volume fraction values for concrete mixes are between 0.5% and 2% by volume of concrete. In general, the character and performance of fiber reinforced concrete changes with varying concrete formulation as well as material geometry distribution, orientation and concentration of fiber.

Materials and Methods

Properties of Constituent Materials: The components of fly ash fiber reinforced composites are the matrix and the fiber. The matrix generally consists of Portland cement, fly ash, coarse aggregates, fine aggregate, water and fibers.

Cement: The most commonly used cement in concrete is Ordinary Portland Cement. Concrete made with this cement is tested for strength after a curing (and hydration) period of 28 days.

Fly Ash: The Fly ash obtained is finer than Portland cement but contains less oxide as compared to Portland cement. The carbon content in fly ash should be as low as possible, whereas the silicon content should be as high as possible. The fly ash may be used in concrete either as an admixture or in part replacement of cement. The pozzolanic activity is due to the presence of finely divided glassy silica and lime, which produce calcium silicate hydrate responsible for strength development.

Aggregates: The aggregates used in plain concrete are suitable for fiber reinforced concrete. The aggregates are normally divided in two categories, namely fine and coarse. Fine aggregate normally consists of natural, crushed, or manufactured sand. In some instances, manufactured lightweight particles are used for lightweight concrete or mortar. Heavy weight particles made of metallic component are sometimes used to produce heavy weight concrete for nuclear shielding purposes. The maximum grain size and size distribution depends on the type of product being manufactured. Coarse aggregates can be normal weight, lightweight, or heavy weight in nature, even though heavy weight aggregate usage is very limited. Normal weight coarse aggregates can be made of natural gravel or crushed stone. Lightweight coarse aggregates are normally made of expanded clay (such as shale or pumice) or blast furnace slag.

Fibers: The most important parameter describing a fiber is its Aspect ratio. "Aspect ratio" is the length of fiber divided by an equivalent diameter of the fiber, where equivalent diameter is the diameter of the circle with an area equal to the cross sectional area of fiber. The properties of fiber reinforced concrete are very much affected by the type of fiber. Different types of fibers which have been tried to reinforce concrete are steel, carbon, asbestos, vegetable matter, polypropylene and glass. In present experimental study, steel fiber of 26 SWG has been used with aspect ratio of 80.

Results and Discussions

Cement: Locally available ordinary Portland Cement (OPC) 43 Grade was used in the present investigation. This Cement satisfied nearly all the requirements of the IS 8112 and IS 1489. The different properties of the Cement tested in the laboratory are listed in Table 1.

Properties of Cement

Fine Aggregates: Locally available river sand was used as Fine Aggregate. Sieve Analysis of the fine aggregate was carried out in the laboratory as per IS 383 and tested as per IS 2386 as shown in Table 2.

Properties of Aggregate
Compressive Strength of Fiber Reinforced Concrete

Coarse Aggregate: Locally available crushed coarse aggregate was used. Sieve Analysis of the coarse aggregate was carried out in the laboratory as per IS 383 and tested as per IS 2386 as shown in Table 3.

Water: According to IS 3025, water to be used for mixing and curing should be free from injurious or deleterious materials. Potable water is generally considered satisfactory. In the present investigation, tap water was used for both mixing and curing purposes.

Fly Ash: Fly ash used in the experiments is taken from Guru Gobind Singh Thermal Plant, Ropar. Physical properties are checked in laboratory and the chemical properties are reported here for ready reference as obtained from GGS Thermal Plant. The physical properties and chemical properties of fly ash are given in Table 4.

Properties of Flyash

Steel Fibers: Black Annealed mild steel fibers of 26 SWG (average diameters 0.46 mm) were used. The aspect ratio of fiber was kept as 80. The physical properties of fibers are given in Table 5.

Properties of Fibers

Concrete Mix Design and Casting of Specimen: M-20 Concrete mix was used with 10 mm coarse aggregate. The Concrete Mix Design was carried out based on Indian Standard Guidelines. The relation between various constituents arrived at was 1: 1.3: 2.3 respectively by weight and water cement ratio of 0.57. Effectiveness of fibers in the concrete is related among other factors to the 'aspect-ratio' and volume percentages of the fibers, size, gradation, quality of the coarse aggregate and water cement ratio. Increase in the aspect ratio, volume percentage of fibers, coarse aggregate size, gradation and quality intensifies balling of fibers. For uniform mixing and to prevent the 'balling' the aspect ratio of round wires was kept as 80. Volume fraction of steel fibers was taken as 1.5% as beyond that these were difficult to mix. The annealed mild steel wires were cut in lengths of 3.68 cm to have aspect ratios of 80. Care was taken for prevention of fiber balls. Required proportion of coarse aggregate, Fine Aggregate and Cement were mixed with trowels and spade until a homogeneous mix was obtained. The fibers were weighted; half of the fibers were sprinkled by hand from the top by one person while the other person kept mixing them in the dry mix. Then water was slowly added and mixture was thoroughly mixed. The remaining fibers were again sprinkled and same process was repeated. The mould was cast in 3 layers. Table vibrator was used for compaction. It was most suitable, as the fibers tend to align themselves in the plane perpendicular to the direction of vibration. This gives random planar orientation. The top surface of the specimen was smoothened with help of wooden float.

Properties of Control Specimen Beams

Experimental Programme: In the present investigation cubes having size 15 cm x15 cm x15 cm were tested to failure. The cubes were designated as A (1–8), B, C, D, E, F, G, H, I, J, K, L, M, N, O, P and Q. These cubes had varying percentages of fiber by volume with constant aspect ratios of 80. The details of the cubes are given in Table 6. The properties of control specimen are described in Table 7 with 0% fly ash and different volume fraction of fibers and 0% fibers and different volume of fly ash. The different percentages of fibers used were 0.75%, 1.0% 1.25% & 1.5%. The aspect ratio of the steel fibers used was 80. The difference percentage fly ash used was 3%, 6%, 9% & 12%.After de-moulding the specimen, the cubes were allowed to dry for one day before placing them in the curing tank for a period of 27 days.

Compressive Strength of Control Specimen

The result of tests on Auxiliary specimens is given in Table 8 and graphical representation is given in Figure 1 & Figure 2. It is observed that the compressive strength of the auxiliary specimen increased with the increase in Fiber Content and Fly ash. The cube 'K 'with a fiber percentage of 1.25% and 6% fly ash showed ultimate strength of 37.67 N/mm2, which is about 37% more than 'A1' and 33% more than 'A2.'The compressive strength of cubes reduced, when volume fraction of the fibers used was 1.5% and fly ash 12%, which may be due to balling of fibers.

Compressive Strength of Auxiliary Specimen

Conclusion

Graphical representation of Compressive Strength
Figure 1: Variation in 28 days compressive strength at different percentage volume of fibres
  • The optimum value of fly-ash replacement and volume fraction of fibers in concrete mix was found to be 6% and 1.25% respectively.
  • Stiffness of nearly all the cubes was increased due to fly-ash and fiber addition in concrete mix.
  • Post-cracking strength of all the cubes increased significantly.

References:

  • Alguire C. (1987) 'Steel Fibers as Shear Reinforcement in reinforced concrete T beams.' Proceedings of International Symposium on Fiber Reinforced Concrete Dec.pp16-19, Madras.
  • Anderson, B.G. (1957) 'Rigid beams failures' ACI Journal, Proc. V. 53 No. 7, pp. 625-636.
  • Kukerja C.B.(1991) 'Structural characteristics of Fiber Reinforced Concrete' Ph. D.Thesis, University of Roorkee,
  • Batra V.S. Kukreja C.B. (1994) 'The Effect of Steel Fibers on enhancing the life of concrete structures', National Seminar on Fiber Reinforced Concrete, Jan., 28-29, pp. 339-345.
  • Snyder, M.J. and Landard, D.R (1972) "Factor affecting the Flexural Strength of Steel Fibrous Concrete," Journal of A.C.I., Proceedings Vol. 69, No. 2, Feb.
    Graphical representation of Compressive Strength
    Figure 2: Variation in 28 days compressive strength at different percentage volume of fly ash
  • Swamy, R.N. and Mangat, P.S (1974)," A theory for the Flexural Strength of steel fiber reinforced concrete," Magazine of cement and concrete research, vol. 4, No.2, March, pp. 313-315.
  • Pakotiprabha, B., R.P. and Lee, S.L (1974) "Mechanical properties of cement mortar with randomly oriented short steel wires," Magazine of Concrete research, Vol. 26, No. 86, March, pp.3-15.
  • Walkuz, B.R., Januszkiewicz, A. and Jeuzal, J.,(1979) "Concrete composite with cut steel Fiber reinforcement subjected to uniaxial tension," Journal of the American Concrete Institute, Vol. 76. October, pp 1079-1092.
  • Kukreja, C.B., Kaushik, S.K., Kanchi, M.B. and Jain, O.P.(1980) "Flexural characteristic of Steel Fiber reinforced concrete," Indian Concrete Journal, July, pp. 184-188.
  • Haque, M.N., et al.,(1984) " High Fly Ash Concrete," ACI Journal, January- February, pp. 56-60.
  • Islam M. A Fiqual and Alam, A.K.M. Khorshed,(1987) "Study of Fiber Reinforced Concrete with natural Fibers. "Proceedings of the International Symposium on Fiber Reinforced Concrete December 16-19, Madras, India.
  • Chitharanjan, N.(1996) "Feasibility study of using Fly Ash concrete for reinforced flexural members," The Indian Concrete Journal September, pp. 503-508.
  • Sabapathi, P. and Achyutha, H.(1989)" Analysis of Steel Fiber Reinforced Concrete Beams," Indian Concrete Journal May, pp. 246-252.
  • Dwarakanath, H.V. and Naraj, T.S.(1992) "Deformational behavior of Reinforced Fiber Concrete beams in Bending," Journal of Structural Engg., ASCE, Vol. 118, No. 10. October pp 2697-2698.
  • Syed Entesham Hussain and asheeduzzafar, (1994)" Corrosion resistance performance of Fly Ash blended cement concrete," ACI Material Journal, May – June, pp 264-272.
  • Malhotra, V.M., Garette, G. G., and Bilodeau, A.(1994) 'Mechanical properties and durability of polypropylene Fibers Reinforced High-Volume Fly Ash Concrete for shortcrete applications' ACI Materials Journal, Sept-Oct 1994, Vol. 91, No. 5, pp 478-486.
  • Z. Bayasi and P. Soroushian (1989) 'Optimum use of pozzolanic materials in steel fiber reinforced concrete' Transportation Research Record, No. 1226, pp 25-30.
  • R.K. Sharma, C. S. Pant, &R.K. Ghosh,(1975) 'Estimation of Fly Ash content in cement–fly ash mortar' ISI-Bulletin- Vol. 27, March, PP –24-28.
  • IS-516, (1959) 'Indian Standards methods of tests for strength of concrete'
  • IS-383, (1870) 'Specification for Coarse and Fine Aggregate from natural source of concrete'
  • IS-2386, (1963) 'Methods for test for Aggregates for concrete'
  • IS-8112, (1989) 'Grade-43 –Ordinary Portland cements Specification'
  • IS-1489, (1991) 'Portland Pozzolana Cement Specification'
  • IS-3025, (1983) 'Method of sampling and test for water and waste water'

NBMCW October 2010

.....

High–Performance Fiber Reinforced Con...

Properties of Crimped Steel Fibre

P. Ramadoss, Research Scholar and K. Nagamani, Professor, Structural Engineering Division CEG, Anna University, Chennai.

Mechanical properties of fiber reinforcement concrete are needed to use the FRC in various structural applications. This paper presents the experimental investigation carried out to study the behavior of high performance fiber reinforced concrete (HPFRC) under compression and flexure with compressive strength ranging from 60 MPa to 86 MPa and flexural strength ranging from 6 MPa to 10 MPa. Steel fiber volume fraction ranges from zero to1.5 percent (39, 78 and 117.5kg) with aspect ratio of 80 were used. The influence of fiber content on the compressive strength and flexural strength with w/cm ratios ranging from 0.40 to 0.25 is presented. Equations are proposed using regression analysis to predict the strength of HPFRC effecting the fiber addition in terms of fiber reinforcing index. Strength comparison analysis was carried out to validate the empirical equation and the maximum absolute variation obtained was 4 percent. Addition of 1.5 % volume of crimped steel fiber resulted in 10% increase in the compressive strength while flexural strength increased by 37% and the RCSR value evaluated is between 0.107 and 0.149.

Introduction

In last 3 decades, numerous research and development studies have been taken up in the field of steel fiber reinforced concrete (SFRC) for understanding the behavior of sound composites. SFRC has been used in several areas of infrastructure and industrial applications including highway and airport pavements, bridge decks, sky scrappers, hydraulic structures, industrial floors, tunnel lining, etc. [1, 2]. As noted by ACI Committee 544, the composite has great potential for many use in civil Engg., applications, especially in the area of structural elements.

High performance concrete (HPC) is defined according to ACI 363-1992 [3] as concrete, which meets special performance and uniformity requirements that can't always be achieved by using only the conventional materials and normal mixing, placing and curing practice. Typical high performance requirements specify high strength (above 41 MPa), enhanced impermeability (permeability less than 10-10 m/s), and high tensile strength (above 4 MPa) and other special requirements. HPC is achieved by using super plasticizen (SP) to reduce water/cm ratio and by using SCM (supplementary cementing material) as silica fume having pozzolanic reaction and filler effect, which usually combines high strength with high durability [4].

High-performance/highstrength concrete leads to the design of smaller sections and reduces the dead weight, allowing longer spans and more useful area of structures. Reduction in mass is also important for economical design of seismic resistant structures. is responsible for the enhancement of strength and durability of the concrete [4]. Strength, ductility and durability are the important factors to be considered in the design of earthquake resistant R.C. structures. Due to the inherent brittleness of HPC/ HSC, it lowers its post-peak portion of the stressstrain diagram almost vanishes or descends steeply [5]. This inverse relation between the strength and ductility is a serious drawback for the use of HPC/HSC and a compromise to this drawback can be obtained by the addition of discontinuous short steel fibers in to the concrete. When concrete cracks, the randomly oriented fibers arrest a micro cracking mechanism and limit the crack propagation, thus improving the strength and ductility thereby enhances the durability of structural elements. Wafa and Ashour [6] have reported that addition of steel fibers in to HSC changes its brittle mode of failure in to a more ductile one and results in a small increase in compressive strength and more increase in tensile strengths compared to plain concrete. Fibers with 1 percent volume fraction and aspect ratio of 75 provide maximum stiffness to concrete and results in maximum increase in compressive strength [7]. Although a number of researchers investigated the effect of inclusion of discrete steel fibers on the compressive strength [8, 9, 10], research on HPC where fibers play a major role is lacking. The main objective of this paper is to study the influence of crimped steel fibers on the compressive strength and flexural strength of high performance- fiber reinforced concrete with varying w/cm ratios and 10% silica fumereplacement.

Research Significance

High performance concrete with and without fibers possesses mechanical properties that are significantly different from normal strength concrete materials. This paper presents an extensive experimental investigation on the mechanical properties of HPFRC with w/cm ratios of 0.40, 0.35, 0.30 and 0.25 and studies the effects of inclusion of fiber contents on improving these properties.

Experimental Program

Four basic mixes for plain concrete designated as FC1-0.0, FC2-0.0, FC3-0.0 and FC4-0.0 according to the w/cm ratios of 0.4, 0.35, 0.30 and 0.25 were selected.

Materials and Mixture Proportions

Properties of Crimped Steel Fibre
Ordinary portland cement-53 grade satisfying the requirements of IS: 12269–1987 [11] and silica fume supplied by Elkem India Ltd. having specific surface area of 23,000 m2 /kg, a specific gravity of 2.35 and contained 88.7% of SiO2 were used in the ratio of 9:1 (1:1 partial replacement of cement) in all the mixes. Fine aggregate of locally available river sand conforming to IS: 383-1970 [12] with fineness modulus of 2.55 and a specific gravity of 2.63 and coarse aggregate of blue granite crushed aggregates conforming to IS: 383- 1970 with 12.5 mm maximum size and fineness modulus of 6.4, a specific gravity of 2.70 were used.

Fibers used are crimped steel fibers of diameter =0.45 mm and length = 36 mm, giving an aspect ratio of 80. The properties of fibers used are given in Table 1. Mixture proportions used in the test programme are summarized in Table 2 [13, 14, 15]. For each water to cemetitious material ratio three fiborus concrete mixes were prepared with fiber volume fractions, Vf of 0.5, 1.0 and 1.5 percent (39, 78 and 117.5kg/m3). Due to the inclusion of the fibers some minor adjustments in terms different ingredients had to be made as shown in Table 2. A naphthalene sulphonated formaldehyde (NSF) as HRWR admixture (super-plasticizer) conforming to ASTM Type F with dosage range of 1.75% to 2.75% by weight of cementitious materials has been used to obtain the adequate workability of plain and fiber reinforced concrete. Dosage of super plasticizer was arrived based on the workability tests conducted on trial mixes.

Strength Analysis

Mixing and Curing

Silica fume was mixed with cement uniformly and thoroughly till homogeneity is attained. Concrete was mixed using a tilting type mixer and specimens were cast using steel moulds, compacted with table vibrator. For each mix at least three 150 mm cubes and three 100 x 100 x 500 mm prisms were produced. Specimens were demolded 24 hours after casting and water cured at 27± 2oC until the age of testing at 28 days. All the specimens were cured in the same water tank to maintain uniform curing.

Results and Discussions

The results of this investigation are applicable to the material and the type of fibers used.

Compressive Strength

Compressive strength tests were carried out according to IS: 516- 1979 [16] standards using 150 mm cubes loaded uniaxially. The tests were done in a servo–controlled compressive testing machine by applying load at the rate of 14 MPa/min. Minimum of three specimens were tested to assess the average strength.

Silica Fume Concrete CubeFiber Concrete Cube
Figure 1(a) Silica fume concrete cube specimens after failure in compression testFigure 1(b) Fiber concrete cube specimens after failure in compression test

Behavior under Compression

Under uniaxial compressive loading, extensive crack was produced in the concrete during pre-peak stage and then failed suddenly at peak load. Figure 1(a) reveals the failure mechanism of Silica fume concrete and indicates that it increases its compressive strength and makes it more brittle, fails violently and suddenly. It is observed from Figure 1(b) that when fibers in discrete form present in the concrete, propagation of crack is restrained which is due to the bonding of fibers in to the concrete and it changes its brittle mode of failure in to a more ductile one and improves the post cracking load and energy absorption capacity.

Compressive & Flexural Strength

Test results show that addition of fibers has a moderate effect on the improvement of compressive strength values. Table 3 and Figure 2 show that the addition of fiber volume fraction from 0 to 1.5% increases the compressive strength by about 10 percent compared to zero 20 percent given in the literature [5, 7, 9, 10, 17] for normal strength concrete. It is observed from the test results that for 1.0 percent fiber content, the increase in strength is about 10 percent but beyond 1.0% fiber content, there is only marginal increase in strength. Based on the test results, using linear regression analysis, the compressive strength of HPFRC may be estimated in terms of compressive strength of plain concrete, fc and Reinforcing index, RI and volume fraction, Vf respectively, as follows:

Properties of Crimped Steel Fibre
Figure 2: Effect of steel-fiber reinforcing index (RI) on compressive strength
fcf = fc + 1.84 (RI) ----------- (1)

fcf = fc = + 4.74 (Vf ) ---------(2)

Where fcf = compressive strength of fiber reinforced concrete, MPa; fc = compressive strength of plain concrete, MPa;

RI= steel fiber reinforcing index; Vf = volume fraction of steel fiber, percent

RI = wf * (l/d); average density of HPFRC = 2415 kg/ m3

Weight fraction (wf) = (density of fiber/ density of fibrous concrete) * Vf

Aspect ratio (l/d) = length of fiber/diameter of fiber.

Silica Fume Concrete CubeFiber Concrete Cube
Figure3: Relationship between Compressive strength of fiber concrete and Plain concreteFigure 4: Bar chart showing the variation of Compressive strength on the effect of steel fiber content for w/cm ratios

The effect of fibers on 28-day compressive strength of concrete may be evaluated form Figure 3. In this figure, the cube compressive strength of all fibrous concrete irrespective of Vf, is plotted against the plain concrete. The least square line obtained using linear regression analysis with correlation coefficient, r =0.98 is given by:

fcf = 1.087 -----------(3)

Silica Fume Concrete CubeFiber Concrete Cube
Figure5: Effect of steel-fiber reinforcing index (RI) on Modulus of ruptureFigure 6: Modulus of rupture, frf as function of Compressive strength, fcf

The predicted values as obtained by equation (1) and the equations proposed by the researchers [5,7,17] were compared with the experimental values, are presented in Table 4. The proposed equation (1) evaluates the compressive strengths with absolute variations less than 4%. Figure 4 (Bar chart) shows the improvement of compressive strength on the effect of addition of fiber content for different w/cm ratios. It may be seen from the Figure 8 that the predicted compressive strength by the equation (1) and different researchers are having a good correlation with experimental values.

Compressive Strength

Flexural Strength (Modulus of Rupture)

The flexural strength (Modulus of rupture) tests were conducted as per the specification of ASTM C 78- 94 [18] using 100 x 100 x 500 mm prisms under third point loading on a simply supported span of 400 mm. The tests were conducted in a 100 kN closed loop hydraulically operated Universal testing machine. The load was applied at the rate of 0.1 mm/ min. Minimum of three specimens were tested to compute the average strength.

Silica Fume Concrete CubeFiber Concrete Cube
Figure 7: Relationship between Modulus of rupture, fr and Compressive strength (Plain concrete)Figure 8: Comparison of Predicted compressive strengths (MPa) with Experimental values (MPa)

Table 3 and Figure 5 present the variation of the modulus of rupture frf, on the effect of fiber content. Figure 6 shows the variation of the modulus of rupture fcf as a function of the compressive strength fcf of the HPFRC. The equation obtained for predicting the modulus of rupture, frf of fiber reinforced concrete using non– linear regression analysis, as follows:

frf = 0.019fcf1.425, MPa ------------(4)

The Cement and Concrete association of Australia adopts the following relationship for FRC, in its publication Industrial pavement-Guidelines for design, construction and specification.

frf = 0.70 √fc,' MPa which yields lower value compared to the proposed equation (4).

Figure 7 shows the variation of the modulus of rupture, fr as a function of compressive strength √fc of the HPC (plain concrete). The following equation is provided using regression analysis for the test results.

fr = 0.861, √fc MPa ----------------(5)

This equation (4) yields the values less than that obtained by Wafa and Ashour [6] of 1.03' √fc for HSC ACI 363-1992 [3] proposed the equation fr = 0.94 (ƒ'c)0.5 for concrete strength range of 21 MPa c &lt; 83 MPa which yields higher values to the predicted equation (4).

Rashid et al. 2002 have proposed the equation which was obtained from the correlation of data by least square regression analysis (correlation coefficient = 0.94) as = 0.42 (ƒ'c) 0.68 for 5 MPa &lt; ƒ'c &lt; 120 MPa.

It is observed from the test results (Table 3) that there is a significant improvement in flexural strength in increasing the steel fiber content from 0.0 to 1.5 percent for all the mixes, varying from 16 to 38 percent of plain concrete. The increase in strength of 38 percent for 1.5% fiber content and 29 percent for 1.0% fiber content reveal that toughness will be much more than that of plain concrete. Using the tested results presented in Table 4, by performing linear regression analysis, the peak values of modulus of rupture of HP-FRC may be expressed as a function of RI and Vf respectively as follows.

frf = fr +0.665(RI) -----------------(6)

frf = fr + 1.695 ( Vf) ---------------(7)

where frf= modulus of rupture of HPFRC, MPa

frf = modulus of rupture of concrete, MPa

RCSR values obtained, varied in the range of 0.107–0.149.

Where, RCSR is the ratio of rupture strength to compressive strength.

Conclusion

Based on the test results of the experimental investigation using crimped steel fiber reinforcement with an aspect ratio of 80, the following observations can be drawn:
  • High performance concrete (silica fume concrete) is a highly brittle material and fails suddenly. The brittle mode of failure is changed by addition of steel fibers in to HPC, in to a more ductile one. It was observed that SFRC improves the concrete ductility, its post-cracking load carrying capacity.
  • Fiber volume fraction up to 1.0 percent (RI= 2.58) is effective in increasing compressive strength which is by about 10% of plain concrete (silica fume concrete).
  • The addition of steel fibers by 1.50 percent volume fraction results in an increase of about 10 percent in the compressive strength, and results in an increase of 38 percent in the flexural strength compared to no fiber matrix. The improvement in flexural strength varying from 16 to 37 percent of plain concrete.
  • Empirical equations that predict the influence of fiber contents in terms of fiber reinforcing index on mechanical properties of HPFRC are presented.
  • The tensile strength as measured by modulus of rupture of HPC (plain concrete) is closely estimated by the equation fr= 0.861, fc, MPa
  • The validity of the proposed expressions is limited to the type of fiber used up to 1.5 % volume fraction (RI= 3.88).
  • The high-performance fiber reinforced concretes obtained have higher RCSR values, which varied in the range of 0.107–0.149.

Notations

HPC= high performance concrete

HPFRC= high performance fiber reinforced concrete

fc = compressive strength of plain concrete, MPa

fcf = compressive strength of HPFRC concrete, MPa

fr = modulus of rupture plain concrete, MPa

frf = modulus of rupture of HPFRC concrete, MPa

Vf = volume fraction of steel fiber, percent

l/d= aspect ratio

RI= fiber reinforcing index.

References

  • Balaguru, N., and Shah, S.P., Fiber reinforced concrete composites, McGraw Hill international edition, 1992, p.179- 214.
  • ACI Committee 544, "State-ofthe- art report on fiber reinforced concrete," ACI 544.1R- 82, American Concrete Institute, Detroit.
  • ACI Committee 363, State-ofthe- art report on High strength concrete, ACI 363R- 92, American Concrete Institute, 1992.
  • Attcin, P.C., High performance concrete, 1st edition, E& FN, Spon, London., 1998.
  • Samer Ezeldin, A,. Balaguru, P.N., "Normal and high strength fiber reinforced concrete under compression," ASCE, Journal of Mate. in Civil Eng., 4(4) (1992), p. 415- 429.
  • Wafa, F.F. and Ashour, S.A. (1992), "Mechanical properties of high strength fiber reinforced concrete," ACI Materials Journal, 89(5) (1992) p. 445- 455.
  • Saluja, S.K., Sarma, M.S., Singh, A.P., and Kumar. S., "Compressive strength of fibrous concrete," Indian Concrete Journal, 66(2) (1992), p. 99–102.
  • Hsu, L.S., and Hsu, C.T.T., "Stress–strain behavior of steel fiber reinforced high strength concrete under compression," ACI Materials Journal, 91(4) (1994) p. 448-457.
  • Mansur, M.A., Chin, M.S., and Wee, Y.H., "Stress–strain relationship of high strength fiber concrete in compression," ASCEJournal of mate. Civil. eng, 13(1) (1999), p. 21- 29.
  • IS: 12269-1987, Specification for 53-grade OPC, Bureau of Indian standards, New Delhi, India. 
  • IS: 383-1970, Specification for coarse and fine aggregates from natural sources for concrete, Bureau of Indian standards, New Delhi, India.
  • ACI 211.4R-93., Guide for selecting proportions for High strength concrete with Portland cement and Flyash, A C I Manual of concrete practice 1999.
  • IS: 10262-1992, Recommended guide lines for concrete mix design, Bureau of Indian standards, New Delhi, India.
  • ACI Committee 544, "Guide for specifying, mixing, placing and finishing steel fiber reinforced concrete," ACI materials Journal, 90(1) (1993), p. 94-101.
  • IS: 516-1979, Indian standard methods of tests for strength of concrete, BIS 2002 Bureau of Indian Standards, New Delhi, India.
  • Nataraja, M.C., Dhang, N., and Gupta, A.P., "Steel fiber reinforced concrete in compression," ICJ, (July 1998), p. 353- 356.
  • Standard test method for flexural strength of fiber reinforced concrete, ASTM C78-1994, Annual book of ASTM standards, American society for testing and materials, USA.
  • Bayasi, Z, and Soroushan, P., "Effect of steel fiber reinforcement on fresh mix proportions of concrete," ACI materials journal, 89(4)(1992), p. 369-374.
  • Bhattacharya, G.K., Johnson, R.A. Statistical Concepts and Methods, Wiley, New York, 1977.

NBMCW May 2008

.....

Nanosilica Improves Recycled Concrete A...

Swapna Kutcharlapati, Technical Assistant, Ramky Infrastructure, Hyderabad; A. K. Sarkar, Professor, Birla Institute of Technology and Science, Pilani; N P Rajamane, Head, Centre for Advanced Concrete Research, SRM University, Katankulathur

Major problems occurring while using Recycled Concrete Aggregates (RCAs) in concretes are: higher porosity and hence higher water absorption, lower mechanical strengths, residual impurities on the surface of the RCAs creating weaker bond between cement paste and aggregate, etc. The data in this paper shows that aqueous dispersions of nanosilica (NS) obtained from nano-technology can be used to treat RCAs so that the properties of RCAs such as aggregate crushing value (ACV), Lo Angeles Abrasion Loss, specific gravity, etc are improved; concretes made with NS treated Recycled Concrete Aggregates show enhanced compressive strengths.

Introduction

Nanotechnology (NT) is neither a new science nor a new technology; it helps scientists to develop composites with improved and desired characteristics. The size of the particles in the composites is crucial as at the length scale of a nanometer, the properties of many materials could be quite different from their bulk state. Applications of NT are in vary divergent fields and are already employed to enhance properties of many construction materials such as concrete, steel, glass, etc.

Concrete, a very widely used material of construction, can be benefited by NT as concrete becomes more durable, stronger, easily placeable and compactable and self-curable. Nano materials modify the molecular structure of hydrated cement paste and thereby enhancing many properties. NT can make steel to become more tougher, stronger, and corrosion resistant. Using NT, Glass can be made to possess special characteristics such as self- cleaning, insulating and effective water repelling.

This paper deals with application of NT to recycled aggregates prepared from crushed concrete. The nano silica, a product from NT, is used to enhance the properties of recycled aggregates so that concretes with higher strengths can be made. The data in this paper is taken from the work carried out at BITS, Pilani.

Nano Materials

Nano materials have atleast one dimension of the order of a nano which is one billionth of a meter. We may note here that a strand of DNA is only a 2 nm wide and a human hair could have a length of 100,000 nm. A nano particle becomes a quantum dot with dimension of the order of 10 nm and this size is so small that jumps in energy levels occur. Quantum dots are semiconductors whose electronic characteristics are closely related to the size and shape of the individual crystal; the smaller the size of the crystal, the larger the band gap, the greater the difference in energy between the highest valence band and the lowest conduction band becomes, therefore more energy is needed to excite the dot, and concurrently, more energy is released when the crystal returns to its resting state.

Nanotechnology can be defined as ability to create materials, devices and systems, through control of matter in nanoscale by exploitation of properties and phenomena occurring at nanoscale. A Nano Composite is a bulk material containing added nanoparticles to improve the properties of the bulk material.

Self-cleaning glass has nanoparticles to make it photocatalytic and hydrophilic. When UV radiation from light hits the glass, nanoparticles become energized and begin to break down and loosen organic molecules (i.e, dirt). Water on contact with the glass spreads across evenly thereby causing washing action.

Polymeric coatings containing aluminum silicate nanoparticles have increased resistance to chipping/scratching and hence used in everything from cars to eyeglass lenses.

Carbon Nano Tubes (CNTs), first announced by Russian scientists in 1952, was found in the sword of Tipu Sultan as well as in Ajanta paintings [URL3]. The CNTs (with very high Aspect Ratio, (length-to-diameter ratio can be up to 132,000,000:1} have extraordinary strength in terms of tensile strength and elastic modulus [Wang, 2009]. It is observed that CNTs are cylindrical in shape with diameter of nano size and length of several millimeters; they have a density of 1.3 g/cc with a thermal conductivity of 3500 Wme1Ke1 (Note : Copper : 385 W·me1·Ke1) exhibiting an elongation of 15 to 23% at an ultimate tensile strength of about 3,600 MPa.

The nacre, known as mother-of-pearl, is 3,000 times more fracture-resistant than aragonite (basically a calcium carbonate) the mineral it is made of. ninety-five percent of the mass of this biomineral, nacre, is self-assembled, while only 5 percent is actively formed by the organism indicating efficiency of working with nano size particles.

Nanoscale materials can be naturally occurring (e.g. volcanic ash) or incidental (byproduct of human activity e.g., diesel exhaust particulates) or intentionally engineered.

Cellulose Nano-fibers (CNFs) are Ligno Celluloses based and can be made from common materials such as wood pulp (about 55% cellulose) and cottonseed fluff (about 94% cellulose). They are extremely strong and tougher than even Cast Iron (A nanocellulose based paper is stronger and tougher than an iron sheet!) Individual CNFs withstand more stress than glass fibres or steel wire. Cement matrix in presence of CNFs can have at least 10% more compressive strength accompanied by increased tensile strength by several times resulting in 40% more impact strength, and this is attributed to crack suppression & stabilization characteristics of CNFs [Swapna, 2008a and 2008b].

Nano Silica (NS) can contribute to efficient 'Particle Packing' in concretes by densifying the micro and nanostructure leading to improved mechanical and durability properties. NS can control degradation (through blocking of water entry on account of pore refinement) of the fundamental binder system of hydrated cement i.e., C-S-H gel caused usually due to calcium leaching out when immersed in water. NS improves behavior of freshly mixed cement concretes by imparting segregation resistance and by enhancing both workability/mouldability and cohesion of the matrix.

Specific Surface Area of Ingredients of Concrete

Several sizes of particles in a concrete mix cause good filler effect so that the net void volume generated is minimised and thereby an optimum mix with desirable properties is obtained. The actual packing in a multi-size particulate mix depends upon the so called 'grading span' – difference between maximum and minimum sizes of particles in the mix; addition of nanoparticles increases the 'grading span' substantially and thereby contributing to rational packing of particles in concrete mix.

Ingredients of Concrete
Figure 1: Specific Surface Area of Ingredients of Concrete

Nanosilica for Recycled Concrete Aggregates

Recycled Concrete Aggregates

Recycled Concrete Aggregates (RCAs) are obtained by crushing of concretes from demolition of concrete structural components in many structures such as: old buildings, concrete pavements, bridges & structures, at the end of their service life & utility, structures deteriorated beyond the possibility of repairs, structures that are turned into debris resulting from natural disasters (such as floods, earthquake, tsunami, manmade disaster/war, etc), structures not serving the needs in present scenario, old structures to be brought down to pave way for new construction for better economic growth.

RCAs fit into present day motto of 'Reducing, Reusing, Recycling and Regenerating'. Central Pollution Control Board reported in 2004 that solid waste generation in India was about 48 million tons/annum and more than 25% of this is from construction industry which consists of about 7-8 million tons of concrete and brick waste. The waste quantities are estimated to reach to level of atleast 65 million by 2010 (Kumar and Gaikwad, 2004).

RCAs is particularly very promising source of aggregates as 75 per cent of any typical concrete is made of aggregates. RCAs present a unique solution to the problems of large scale demolitions occurring now a days in India. This recycling industry for waste concretes helps reducing management/maintainance costs of dumpsites/landfills and transportation costs

RCAs actually results from crushing of waste concrete and this material as a replacement for natural aggregates can be employed in many applications such as: construction of low rise buildings, manufacture of paving blocks & tiles, laying of flooring and approach lanes, in sewerage structures and sub-base course of pavement, besides drainage layer in highways and retaining walls.

Some of the major problems associated with RCAs can be identified as: lower specific gravity, higher water absorption, lower level of strengths and durability in concretes, impurities on the surface of the RCAs, lack of strong bond between cement paste and RCAs in concrete matrix, etc. However, properties of RCAs can be improved by suitable organic or inorganic treatment systems.

Improving Properties of Recycled Concrete Aggregates with Nanosilica

Treatment of RCAs with Nano silica (NS) is one of the options available for beneficiation of RCAs. The NS used is usually in the form of stable dispersions of nanometer size silica particles and this dispersion is generally in water or other liquid medium [Hosseini, 2009]. Particle sizes of NS range from 4 to 100 nano-meters in diameter with very high surface area of up to 750 m2/gram of silica solids (note the surface area of Portland cement ranges from 0.25 to 0.40 m2/gram). NS is available commercially now as an aqueous solution with a colloidal solid percentage of 30% (nano-particles of SiO2 dispersed in water).

Preparation and Properties of Recycled Concrete Aggregates

Crushed Aggregates
Figure 2: Crushed Aggregates of different sizes

nano-silica particles
Figure 3: TEM image of silica nano-silica particles
RCAs in the present study are prepared by crushing (using jaw crusher) of concrete cubes made of M20 grade concrete and then sieving. Quantities of fine aggregates (size less than 4.75 mm) and coarse aggregates (20 mm size) obtained were 18% and 60% respectively.

Raw RCAs (Fig 2) were found to have specific gravity of 2.41, water absorption of 5.7%, Aggregate crus- hing value (ACV) of 30 % and a Los Angeles Abrasion Loss of 30 %.

It is noted here that Los Angeles test is a measure of degradation of mineral aggregates of standard gradings and it combines many actions such as abrasion or attrition, impact, and grinding. A rotating steel drum containing steel spheres is used to test the aggregate samples and following computation is made:

L.A. Abrasion Loss (%) = (Original Weight – Final Weight)*100/(Original Weight))

Treatment of RCAs with aqueous dispersion of NS is done by soaking the specimens in the solution for 10 days. The Nano-silica (shown in Fig 3) treated Recycled Aggregates have a specific gravity of 2.62 with water absorption of 0.92%. These treated RCAs recorded an Aggregate Crushing Value (ACV) of 5 % (Fig 4a). Indian Road Congress specifies ACV to be less than 30% for cement concrete pavement 45% for concrete other than for wearing surfaces. The ACV indicates ability of aggregate to resist crushing and a lower figure indicates stronger aggregate with greater ability to resist crushing.

The Los Angeles Abrasion Loss of Nano-silica treated RCAs is 5 % (Fig 4b). A reduction in Los Angeles Abrasion Loss indicate increase in strength of aggregates [Kahraman and Fener, 2007].

Tests on Coarse Aggregates
Figure 4: Tests on Coarse Aggregates

Preparation of Concrete Specimens

RCAs were used as coarse aggregates to prepare concrete cube specimens (Fig 5a) made from concrete with mix proportions (by weight) of:

Cement : Coarse Aggregates : Sand : Water = 1:1.49:2.83: 0.45.

A 10% NS dispersion was also added to the fresh concrete mix containing RCAs. Curing of concrete specimens after demoulding was done conventionally by storing the specimens under water (Fig 5b).

Concrete Specimens
Figure 5: Preparation of Concrete Specimens

Discussion of Test Results

Cement hydration generates capillary voids of 10 to 1000 nm size and in a well hydrated paste with a low w/c ratio, the pore size would be less than 100 nm. Hence, NS with nano size dimensions can contribute effectively to the pore size refinement of the hydrated cement matrix.

NS solids could fill the voids between cement grains, resulting in immobilization of "free" water ("filler" effect) and thereby increasing the cohesivity of the fresh nix. Use of colloidal nano silica particles in aqueous medium aids better dispersion of nanoparticles in the concrete matrix and decreases agglomeration of nanoparticles which improves nanoparticles performance in concrete. NS enhances cohesiveness of mix besides reducing segregation and bleeding.

Concrete made with untreated RCAs had a slump of about 15 mm with a compressive strength of 16 MPa. But, concrete made with NS treated RCAs had a slump of about 35 mm with a compressive strength of 22 MPa.

It was observed that NS treatment to the RCAs enhances many aggregate characteristic properties such as abrasion, aggregate crushing value and compressive strengths. This enhanced properties of RCAs lead to higher level of compressive strengths in concretes.

NS treatment to RCAs densifies the loose/weak mortar present on surfaces of RCAs. The reactive and filler nature of NS binds the densified surface mortar to the base stone aggregates. Well-dispersed nanoparticles act as centre of crystallization of cement hydrates, thereby accelerating hydration. However, real challenge is always to get an effective dispersion into NS solids into cement matrix.

NS particles favour the formation of both small-sized crystals of Ca (OH)2 and uniform gel clusters of C-S-H gel. The NS particles very efficiently participate in pozzolanic reactions, resulting in consumption of Ca (OH)2 and formation of an 'additional' C-S-H. Nano-size and superior pozzolanic activity of NS improve/refine/densify interfacial transition zone (ITZ) between aggregates' surface and cement paste, resulting in better bond between aggregates and cement paste.

Concluding Remarks

  • Recycling and reuse of building wastes is an appropriate solution to the problems of dumping hundreds of thousands tons of debris accompanied with shortage of natural aggregates.
  • Recycled aggregates can prove to be a valuable building material from many considerations such as technical, environment and economical.
  • Recycled aggregates possess, as compared to natural aggregate
    • relatively lower
      • bulk density,
      • crushing
      • impact values
    • relatively higher
      • water absorption.
  • Compressive strength of untreated recycled aggregate concrete can be lower by about 15% compared to original concrete.
  • Properties of Recycled Concrete Aggregates and concrete made with them are enhanced by addition of appropriate nano-materials such as Nano-silica.
  • Crack-bridging property of silica nanoparticles and interlocking of silica in the pores of aggregates helps to make the crushed concrete aggregates to gain
  • Regarding cost, it may be noted that
    • Crushed aggregate has almost a zero investment
    • Every 250 grams of Nano-silica solution costs about Rs 150
    • Optimized value 10% implies that only a tiny amount of nano-silica solution is utilised.
    • Use of waste materials means reduced management/requirements of dumpsites, landfills and transportation costs.

Acknowledgments

The test data used in this paper is based on the works carried out by Miss Swapna Kutcharlapati at BITS, Pilani. The authors acknowledge the technical interactions occurred after the invited key note speak given on the topic by Shri N P Rajamane, during the 'Third Edition of Workshop on Emerging Materials and its Applications ( WEMA )' held on 2nd Dec, 2010, at Aarupadai Veedu Institute of Technology (AVIT), Chennai, organised in association with Madras Metallurgical Society.

References

  • Francois De Larrard, (1999), "Concrete Mixture Proportioning: A Scientific Approach", ISBN 0419235000, E & FN Spon, p 440
  • Hosseini P., A. Booshehrian, M. Delkash, S. Ghavami, M.K. Zanjani, (2009), "Use of Nano-SiO2 to Improve Microstructure and Compressive Strength of Recycled Aggregate Concretes", Proceed of the NICOM3, Nanotechnology in Construction, Springer ISBN 978-3-642-00979-2, pp 215-222
  • Ji, T. (2005), "Preliminary study on the water permeability and microstructure of concrete incorporating nano-SiO2", Cement and Concrete Research, Vol 35, pp 1943 – 1947.
  • Jonathan S. Belkowitz, and Dr Daniel Armentrout, (2010), "An investigation of nano silica in the cement hydration process', Concrete Sustainability Conference, National Ready Mixed Concrete Association, pp 1-15
  • Kahraman S. and M. Fener, (2007), "Predicting the Los Angeles abrasion loss of rock aggregates from the uniaxial compressive strength", Materials Letters, Volume 61, Issue 26, October, pp 4861-4865
  • Kumar, S. and Gaikwad, S.A., (2004) "Municipal Solid Waste Management in Indian urban centers: An approach for betterment", Urban development debates in the new millennium, edited by K.R.Gupta, Atlantic Publishers, New Delhi, pp.100-10.
  • Lin K.L., W.C. Chang, D.F. Lin, H.L. Luo and M.C. Tsai, (2008), "Effects of nano-SiO2 and different ash particle sizes on sludge ash–cement mortar", Journal of Environmental Management 88, pp 708–714.
  • Swapna Kutcharlapati, (2009), "A Report On Use Of Nano SiO2 In Recycled Aggregate Concrete", Lab Oriented Project Report on Course BITS C314, Guide: Prof. A. K. Sarkar, Birla Institute Of Technology And Science, Pilani, April.
  • Swapna Kutcharlapati, S.B. Singh and N.P. Rajamane. (2008a), "Influence of Nano Cellulose Fibres on Portland Cement Matrix," National Conference on Advanced materials and Characterization", VIT, Vellore, July23-25 pp. 1-10.
  • Swapna Kutcharlapati, S.B. Singh and N.P. Rajamane. (2008b), "Influence of Nano Cellulose Fibres on Portland Cement Matrix,"Metals Materials And Processes, V. 20, No. 3, pp. 307-314
  • URL1, http://science.howstuffwork s.com/nanotechnology3.htm
  • URL2, http://www.nano.org.uk/forum/viewtopic.php
  • URL3, http://www.eanveshan.com/node/134
  • URL4, http://en.wikipedia.org/wiki/Quantum_dot
  • Wang, X., Li, Q., Xie, J., Jin, Z., Wang, J., Li, Y., Jiang, K., Fan, S., (2009), "Fabrication of Ultralong and Electrically Uniform Single-Walled Carbon Nanotubes on Clean Substrates", Nano Letters, Vol 9, pp 3137–3141

NBMCW January 2011

.....

Sustainable Concrete with Scrap Tyre Ag...

Mukul Chandra Bora, Lecturer (Selection Grade), Department of Civil Engineering, Dibrugarh Polytechnic, Lahowal, Assam

Introduction

With the exponential growth in number of automobiles in India during recent years, the demand of tyres as original equipment and as replacement has also increased. The quantity of scrap tyres produced in India is not exactly available but the increasing trend of use of road transportation will definitely create a problem of disposal in very near future. The total number of registered buses, trucks, cars/jeeps/taxis and two wheelers up to 1997 in India were 0.5 million, 2.25 million, 4.7 million and 26 million, respectively. An annual cumulative growth rate of 8% is expected (Automan, 1999, Statistical Yearbook, 2000). Considering the average life of the tyres used in these vehicles as 10 years after rethreading twice, the total number of waste disposable tyres will be in the order of 112 million per year. Some of the current uses for used tyres in India include tyre rethreading applications, tyre derived fuel for making bricks, making belts for running shafts and making gaskets. This consumes only a fraction of the total tyres discarded every year. The previous common practice of use as fuel is now prohibited by the Indian Government as it causes serious environmental degradation.

The major significance of this research work is to ascertain the replacement of the natural stone aggregate as good quality conventional natural resources like sand, gravel, aggregates etc are depleting very fast with the increase in construction activities in the country and a ban on new quarries are inevitable due to environmental problem. As such, there is a growing search for alternative materials. Keeping in view of the aforesaid reason, a comprehensive experimental investigation was carried out to study the properties of fresh rubberised concrete which in turn provide a useful guideline for its use in concrete.

Experimental Programme

Materials

Cement

Ordinary Portland Cement (53 Grade) conforming to IS: 12269 – 1987 was used in this investigation. The specific gravity and specific surface of the cement was found out to be 3.15 and 3350gm2/gm respectively. The normal consistency of the cement as determined by Vicat Apparatus was found to be 29%.

Sand

Fine grained sand of Dihing River near Dibrugarh was used in this investigation. The sand used was medium sand with fineness modulus as determined was 2.35 and specific gravity is equal to 2.62.

Coarse Aggregate

Table 1. Properties of Natural Aggregate
Name of the PropertiesValue
Classification (USCS)SP
Flakiness Index (%)1.5
Elongation Index (%)2.75
Crushing Value (%)5.0
Crushed stone aggregate of sizes fro 16mm – 20mm collected from Namrup was used in this investigation. Its different properties as determined in the laboratory were tabulated in Table 1.

Water

Water used for making the concrete was treated water of potable standard which is available in the concrete testing laboratory of Dibrugarh Polytechnic. The water was further tested in the Environmental Engineering Laboratory and found to conform to potable water standard.

Tyre chips

Tyre chips was made by cutting the scrap truck tyres into sizes of 12mm and 16mm and used by mixing them in proportion of 2:3. The cutting of tyre was done by hand by labour with chisels & cutters. The maximum and minimum size of chips was 16mm and 12mm respectively. The specific gravity and water absorption was as determined in the laboratory was 0.96 and 0.45% respectively.

Preparation of Concrete Mixture and Test Procedure

A design mix of M25 grade was used in this experimental investigation. The proportion of the design mix was 1: 0.98:2.1 with water cement ratio of 0.39. The percentage of replacement natural coarse aggregate with tyre chips starts from 10% and ends at 30% in increments of 10%. The tyre chips was prepared by cutting the whole scrap tyre into the sizes ranging from 12mm - 16mm without the use of any equipment or machine. A normal concrete of natural aggregate without any replacement (0%) is used for the purpose of reference. Indian Standard Methods of Sampling and Analysis of Concrete (IS: 1199 – 1959) was used to determine the workability and compaction factor of the fresh concrete. The concrete mix was prepared manually and then poured into the cube mould and compacted with surface vibration. The details of the test series are given in Table 1.

The standard procedure as outlined in Indian Standard Code of practice for slump test was followed in this experimental investigation.

The compacting factor of the fresh concrete was also determined to ascertain the workability of fresh concrete as per IS: 1199 – 1959. The standard apparatus as outlined in I.S code was used is for the determination of Compacting factor of the fresh concrete.

Results and Discussion

Properties of fresh Rubberised Concrete: Based on the test conducted on fresh concrete the workability of the concrete in terms of slump value is tabulated in Table 2. It was observed that the workability of the rubberised concrete increases with the increase in tyre chips content. But the compaction factor slightly decreases with increase in rubber content which is negligible. The increase in the value of slump may be due to the lower water absorption capacity of the tyre chips and slight decrease in compaction factor may be due to the cushioning effect provided by the tyre chips aggregate.

Table 2: Workability of the Concrete in terms of Slump Value
Sl. NoTest SeriesWorkability (Slump)
1Normal Concrete85 mm
2Rubberised concrete (10%)100 mm
3Rubberised concrete (15%)125 mm
4Rubberised concrete (30%)150 mm

Table 3: Unit weight of rubberised concrete
Type of ConcreteUnit Weight (Kg/m3)%  reduction
Normal (1:1.5:3)25000
Rubberised (10% replacement)23506
Rubberised (20% replacement)225010
Rubberised (30% replacement)220012

Table 4: Compressive strength of rubberised concrete
Type of Mix (1:1.5:3, w/c = 0.45)Compressive Strength (MPa)
7days28days
Normal Concrete3035
Rubberised concrete (10% replacement)2732
Rubberised concrete (15% replacement)2428
Rubberised concrete (30% replacement)1822

The compaction factor as determined was found to be in the range of 0.8-0.9 and satisfactory.

Properties of rubberised concrete in hardened state: The properties of concrete prepared with partial replacement of natural aggregate with tyre chips aggregate was tested for its weight and compressive strength and the results obtained are tabulated in Table 3 and 4. The photographic view of the compression testing machine (2500kN) and the tested cube containing tyre chips are shown in Fig.1 and 2.

Compression testing machineconcrete cubes
Figure 1: Compression testing machine (Capacity 2500kN)Figure 2: Photograph of the concrete cubes containing tyre chips after compression test

Conclusion

Based on the experimental investigations conducted on rubberised concrete and the subsequent results, the following major conclusions can be drawn.
  1. The workability of the rubberised concrete increases with the increase in rubber content. This may be due to the lower water absorption capacity of the tyre chips.
  2. Due to lower water absorption capacity of the tyre chips, the good workability can be achieved with lower water cement ratio and hence may be useful for high performance concrete having low water cement ratio. This property may lead to the reduction in use of plasticizer and super plasticizer in those concrete.
  3. The use of rubberised concrete may be very much beneficial for a country like India where the problem of scrap tyre disposal is at the very initial stage.
  4. The rubberised concrete may be useful for the bases of foundation which in turn reduces the use of natural aggregates and hence mining.
  5. To be used in reinforced cement concrete, it needs further investigations and field tests.
  6. The unit weight of rubberised concrete decreases with the increase in tyre chips content and hence may be suitable for lightweight construction.
  7. The compressive strength of the concrete with tyre chips does not show any remarkable decrease upto 15% replacement of natural aggregate with tyre chips.
  8. It will offer an opportunity for new entrepreneurs to set up industries for production of tyre chips and hence help in saving our environment as well as employment generation.
  9. The cost analysis reveals that rubberised concrete is cheaper than normal concrete as the scrap tyres are available with nominal cost. There is a direct cost reduction of 10% for optimum replacement percentage of tyre chips.
Studies reveal positive results in terms of workability of concrete and hence utilization of these wastes in the construction industry in large quantities seems to be a reasonable solution for environmental and economic problems. Finally, the use of rubber tire waste in composite materials provides an opportunity to recycle these wastes and thus to achieve an environmental goal.

References

  • Eldin, N.N. and A.B. Senouci, 1993. 'Rubber-tire particles as concrete aggregate.' J. Mater. Civil. Eng. ASCE, 5: 478-496.
  • Biel, T.D. and H. Lee, 1994. 'Use of recycled tire rubbers in concrete. Proceedings of ASCE 3rd Material Engineering Conference Infrastructure: New Materials and Methods of Repair,' San Diego, CA pp: 351-358.
  • Schimizze, R., J. Nelson, S. Amirkhanian and J. Murden, 1994. 'Use of waste rubber in light-duty concrete pavements.' Proceedings of ASCE 3rd Material Engineering Conference Infrastructure: New Material and Methods of Repair, San Diego, CA pp: 367-374.
  • Khatib, Z.K. and F.M. Bayomy, 1999. 'Rubberized Portland cement concrete.' J. Mater. Civil. Eng. ASCE, 11: 206-213.
  • Serge, N. and I. Joekes, 2000. 'Use of tire rubber particles as addition to cement paste.' Cem. Concr. Res., 30: 1421-1425.
  • Segre, N., Joekes, I., 2000. 'Use of tire rubber particles as addition to cement paste.' Cement and Concrete Research 30 (9), 1421–1425
  • Hernandez-olivares, F., G. Barluenga, M. Bollati and B. Witoszek, 2002. 'Statics and dynamic behaviuour of recycled tyre rubber-filled concrete.' Cem. Concr. Res., 32: 1587-1596.
  • Hernandez-Olivares, F., Baluenga, G., 2004. 'Fire performance of recycled rubberfilled high-strength concrete.' Cement and Concrete Research 34, 109–117.
  • Siddique, R., Naik, T.R., 2004. 'Properties of concrete containing scrap-tire rubber an overview.' Waste management 24, 563–569.
  • Siddique, R., Khatib, J., Kaur, I., 2008. 'Use of recycled plastic in concrete: A review.' Waste Management 28, 1835–1852.
  • Khaloo, Ali R., Dehestani, M. and Rahmatabadi, P. (2008) “Mechanical properties of concrete containing a high volume of tire–rubber particles” Waste Management 28; 2472–2482
  • IS: 10262 – 1982, Indian Standard Recommended Guidelines for Concrete Mix Design. Bureau of Indian Standards, New Delhi
  • IS: 12269 – 1987, Indian Standard Specification for 53 Grade Ordinary Portland Cement, Bureau of Indian Standards, New Delhi
  • IS: 1199 – 1969 (2004), Indian Standard Methods of Sampling and Analysis of Concrete, Bureau of Indian Standards, New Delhi.
  • IS: 383 – 1970. Indian Standard Specification for Coarse and Fine Aggregate from Natural Sources for Concrete.

NBMCW December 2010

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Quality and Durability of Concrete

Concrete

Dr S C Maiti, Ex-Joint Director, National Council for Cement and Building Materials, New Delhi

Raj K. Agarwal, Managing Director, Marketing and Transit (India) Pvt. Ltd., New Delhi.

This paper discusses the quality of concrete vis-a-vis the life span of the present-day concrete structures in India. The paper's main focus is on durability of concrete. How to produce good quality durable concrete structures, with the available concrete - making materials! The focus is on materials like aggregates, mineral admixtures like flyash, g.g.b.s., silica fume and on mixing, placing, compaction, and curing of concrete. Use of blended cements to produce durable concrete structures, and details on concrete mixes used for M 70 and M 80 grades of concrete using such cements in developed countries have also been highlighted.

Introduction

Concrete
'Concrete' is a mixture of Cement, water, aggregates and admixtures. The chemical admixtures change the initial characteristics of concrete e.g. setting time, workability, cohesiveness, fluidity etc. The mineral admixtures e.g. flyash and ground granulated blast furnace slag (g.g.b.s.) modifies the resisting capacity of concrete in aggressive environments, and they, being fine materials, help in producing a cohesive, non-bleeding concrete mix. Thus, concrete is a versatile construction material. Its characteristics can be made as desired, using suitable concrete-making materials and their right proportions. The basic characteristics of concrete i.e. workability (fluidity and cohesiveness) and 28-day compressive strength can be obtained as desired, using suitable proportions of the constituent materials. Cement is the binding material in concrete. The 28-day compressive or flexural strength of concrete mostly depends on the water-cement ratio or water-binder ratio and the 28 days compressive strength of cement. The water-reducing admixtures reduce the water content of concrete and thereby, the compressive strength of concrete can be increased by reducing the water-binder ratio. Thus, different grades of concrete i.e. M20, M30, M70 etc. can be produced. Without altering the water content or water binder ratio for a particular grade of concrete, the workability of concrete can be increased with the use of superplasticizers. Thus, in heavily reinforced concrete sections, high-workability concrete can be placed without much compacting effort.

Concrete
In mass concrete structures, bigger sizes of coarse aggregate are used. Air-entraining admixtures are used to produce cohesive concrete mix in such cases. The mineral admixtures like flyash or g.g.b.s. reduces the heat inside the concrete. Thus, PPC (containing flyash) or PSC (containing g.g.b.s.) can be used as low heat cement. For producing very high strength concrete (say M 80), efficient superplasticizer reducing 30-35% of the mixing water will be required. Along with high-strength OPC, mineral admixture silica fume (8-10% by weight of cement) will also be required to produce such high-strength concrete. Silica fume concrete is also abrasion-resistant, and therefore, is being used in construction of spillways of concrete dams.

The long-term strength and durability of concrete are important characteristics of concrete. The concrete structures must provide the designed service life. The timely maintenance of structures is essential to obtain the desired service life.

In spite of good quality cement being manufactured these days, the service life of concrete structures is not increasing. Indian construction industry should rise above certain level, and produce concrete structures, which will have long service life.

Use of Mineral Admixtures in Concrete

Flyash, ground granulated blast furnace slag and silica fume have been recommended to be used in concrete as mineral admixtures1. Flyash and silica fume are good pozzolanas, whereas g.g.b.s. is a latent hydraulic material, which also reacts with the lime liberated due to the hydration of OPC in concrete.

Silica fume, is a very fine non-crystalline silicon dioxide, a by-product of ferrosilicon industries. It is presently being imported in India from Australia, Norway and China. Generally 5-10% silica fume by weight of the cement is sufficient to produce high-strength concrete (M60 grade and above). Being a highly reactive pozzolana, it develops also the early strength of concrete. silica fume (about 8% by weight of cement) has been used in the spillway of Tehri dam, for abrasion resistance. In the spillway of Kol dam (Himachal Pradesh), it is proposed to use about 10% silica fume for M 80 grade of concrete2.

Good quality flyash is available from the electro-static precipitators of our super thermal power stations. But the quantity of Grade 1 flyash is not sufficient to be used to produce portland pozzolana cement (PPC) by the cement manufacturers. The requirement of such flyash by our ready mixed concrete (RMC) plants is also very high.

It is observed that, because of insufficient quantity of good quality flyash, the cement manufacturers are grinding the coarse flyash to the required fineness to produce PPC of minimum fineness of 300m2/kg. By this process, the "ball-bearing effect" of the spherical shape of the flyash particles is lost, and consequently, the beneficial properties of flyash e.g. lower water demand and increased workability of concrete are also lost.

Concrete
We use flyash in concrete, because as a pozzolana, it has good effect in concrete and we get a cohesive concrete mix. In mass concrete, heat development inside the concrete is less. Sometimes, it substitutes part of cement and sometimes, part of the fine aggregate also. When the quality of flyash is not good, it still has beneficial effect. The reactive silica of such flyash may be in less quantity, but in the long run, the part of such flyash is going to provide a denser microstructure. In such case, use of chemical admixtures i.e. superplasticizer has a major role to play. It should reduce the water-binder ratio (w/b ratio) as low as practicable. With reduced w/b ratio, the permeability of concrete will get reduced, and will result in increased compressive strength in the long run. Therefore, it is expected that low-quality flyash in conjunction with the use of compatible and efficient superplasticizer will provide long service-life for concrete structures. Proper mixing of concrete, adequate compaction and longer curing period will definitely provide durable concrete structures is such cases, also.

The mineral admixtures i.e. flyash, g.g.b.s. and silica fume being very fine particles, their uniform blending with cement and aggregates in ordinary drum mixers can not be ensured. In most of our construction sites, batching and mixing plants are not available. Therefore, it is suggested that, slurry can be made with the mineral admixture and part of the mixing water, in the drum mixer, before placing cement and aggregates.

IS 456 stipulates that the mixing of concrete in the concrete mixer shall be continued untill there is an uniform distribution of the materials, and the mass is uniform in color and consistency. If there is segregation after unloading from the mixer, the concrete should be remixed. The mixing time shall be at least 2 minutes for drum mixers. For efficient concrete mixers as in RMC plants, the manufacturer's recommendations shall be followed, and trials may be carried out to produce cohesive concrete mix, using proper combined grading of coarse aggregate fractions.

In our construction sites, generally drum mixers are used. Although, the minimum mixing time for concrete has been fixed as 2 minutes, in many cases, it has been observed that the concrete mix is not a cohesive mix, and is not uniform in color and consistency. The cohesive (not segregated) concrete mix can only be placed properly in the formwork and compacted well. This process of placing a cohesive concrete mass around the reinforcement, and fully compacted and finished well, has got considerable influence on the durability of concrete. The adequate cover to reinforcements, dense concrete cover and well-compacted concrete will not get carbonated during its service life.

To get maximum benefit out of mixing of concrete, it is suggested a two stage mixing : firstly dry mixing of materials (with covered mixer drums) and then wet mixing process. The Japanese researcher used "neiling" of concrete mass (with small quantity of water) and then wet mixing process. The laboratory data of such two stage mixing indicate higher compressive strength and lower permeability of concrete.

In the mixing process, part of the mixing water should be placed in the concrete mixer in the beginning and the chemical admixture along with rest of the mixing water should be placed towards the end of mixing. This procedure provides a concrete of uniform colour and consistency and a cohesive concrete mix.

Concreting in Rural India

Our rural roads and housing are suffering a great set back, because of improper mixing and compaction of concrete. How can we expect durable concrete structures when the quality of concrete placed is not 'good'. The normal drum mixers are not available in rural India. The concrete vibrators are not used, because of non-availability of electricity. In such situation, although rural concrete roads are supposed to be of M 30 grade, and the RCC slabs in houses are supposed to be of M 20 grade, the real scinario is terrible. With hand mixing of concrete, and with rodding of concrete in the formwork, no durable concrete can be produced. The result is shorter life - span of concrete structures.

In rural india, diesel-operated concrete mixers and vibrators can be used to produce cohesive concrete mix and placed and compacted efficiently. However, if hand mixing is the only alternative, 10% extra cement is to be added to the concrete mix, and the mixing time is to be increased, in order to obtain a cohesive concrete mix.

Life Span of Concrete Structures

In India, we design buildings for a life span of about 50-60 years. Bridges are designed for about 100 years, while the concrete dams are designed for about 150 years. Delhi Metro structures have been designed for 120 years. The Euro tunnel connecting England and France under the sea has also been designed for 120 years.

Concrete
Although, our buildings are designed for 50-60 years, it has been observed that the buildings have longer life span. Old buildings in Mumbai and Kolkata developed distress at about 100 years age. With the availability of good construction materials in the present days, our structures should provide longer service life, provided we build them with adequate care, using proper method if mixing, placing, compaction and curing of concrete.

In addition to mineral admixtures like flyash, g.g.b.s. and silica fume, the chemical admixtures e.g. superplasticizers provide the required workability and compressive strength of concrete. But because, both cement and the chemical admixtures are chemicals, their compatibility must be ensured before use.

Fortunately, a large number of chemical admixture companies are operating in our country, specially in cities, and the materials like plasticizers, superplasticizers (normal and retarding type), accelerating and retarding admixtures are available in good quality and quantity. Only their proper dosage and compatibility with different types of cement, if ensured, shall provide cohesive, workable concrete mix, which can be placed comfortably in the formwork and compacted well.

Thus, production of good quality concrete, their proper placing, compaction and curing shall ensure longer life span of structure. In normal "mild" exposure condition, the structures shall have required service life. But in aggressive environmental conditions, they require special care and protection. For example, in marine environment or in heavy rainfall areas, or where aggressive chemical environment exists, the concrete structures should be built with the required materials, and protected well. In sea environment, chloride and sulphate will reduce the service life : the sulphate will attack the concrete, and the chloride will corrode the reinforcements. In such situations, codal provisions must be followed. With the use of superplasticizers, water-cement ratio can be reduced, as low as possible, may be in the range of 0.35 to 0.40, so that the permeability of concrete gets reduced. Portland slag cement with more than 50% slag content is suitable in case both sulphate and chloride are encountered in the environment.

For 'Severe' conditions, such as thin sections under hydrostatic pressure on one side only, and sections partly immersed, considerations should be given to use a water-cement ratio or water-binder ratio (in the case of use of mineral admixtures), as low as possible. IS 456 states that portland slag cement conforming to IS 455 with slag content more than 50% exhibits better sulphate-resisting properties. For thin structures, the life can be increased by providing extra cover to steel reinforcements, by chamfering the corners or by using circular cross sections or by using surface coatings which prevent or reduce the ingress of water, carbon dioxide or aggressive chemicals1.

For high sulphate concentrations i.e. more than 2% in soil or more than 5% in ground water, IS 456 stipulates use of protective coating on the well-made (with lower w/c ratio and with proper cement) concrete surfaces. The coating can be based on asphalt, chlorinated rubber, epoxy or polyerethene materials.

In 'severe' exposure conditions i.e. in marine environment, in many places, fusion-bonded epoxy coatings have been used on steel reinforcements. The TMT bars, and the metallurgically changed corrosion-resistant steel bars are definitely going to increase the service life of concrete structures near the sea. Polymer-based coatings to steel reinforcements have also been found useful.

Concrete Aggregates

Use of good quality aggregates will definitely increase the service life of concrete structures. In many places in India, good quality river bed sand is not available. Sometime, only very fine sand is available. Sand and coarse aggregate must be strong and good, in order to produce durable concrete structures. Well-graded aggregates provide dense concrete mix. Crushed rock coarse aggregate from strong rocks like basalt, quartzite, granite etc. are angular and they provide good bond in concrete and develop strong concrete. Flaky and elongated coarse aggregates should be avoided, if possible. The crushing, impact and abrasion values of coarse aggregates shall not exceed the limits specified in IS 3833.

The fine aggregate should generally be of grading zone II or grading zone III. For very high strength concrete, coarse graded fine aggregate (Zone I) is suitable. Sometimes, river bed sand is not available, so we use crushed stone fine aggregate. They can also produce good quality dense concrete, provided their grading is satisfactory as per IS 383. The deleterious materials like clay lump, coal, lignite, shale and soft fragments should be avoided. Use of good quality well-graded coarse and fine aggregate will definitely increase the service-life of concrete structures.

Use of Blended Cements

Our cement manufacturers are producing more blended cements. The portland pozzolana cement (PPC) is about 50% of the total cement production. The PPC is with about 20% flyash. The portland slag cement (PSC) is a good quality cement, with slag content of about 40-50%. But this cement is limited, about 15% of the cement production. This has become a special cement, because of its availability near the steel plants. The slag (blast furnace slag) is a consistent material, and therefore is of low variability. The use of PSC has become important because this cement in concrete can resist very aggressive environments, and hence increases the life span of concrete structures. It has been prescribed to be used in sea environments and also in concrete dams. The PSC in concrete can resist the alkali-silica reaction in concrete. In aggressive environments, e.g. in concrete piles in sea environment in Gujarat, PSC with 70% slag has been used to produce high-workability M40 grade concrete. Typical concrete mix proportions used are as follows:-

OPC (53-grade) - 30% (141 kg.)

g.g.b.s. - 70% (329 kg.)

sand - 41% (720 kg.)

Coarse aggregate (20mm MSA)) - 1044 Kg.

Retarding superplasticizer - 0.8% (by wt. of OPC + ggbs)

In mass concrete, both PPC and PSC are advantageous, because of low heat of hydration, and also because their alkali is not fully effective. Typically, about 1/6 of the alkali of flyash is potentially reactive, and about 50% of the alkali of ggbs is effective4.

For the underground structural elements of cut and cover tunnels and station boxes, 30% flyash has been used for M35 grade of concrete in Delhi Metro projects5. Delhi Metro structures have been designed for 120 years. The water /(cement + flyash) ratio was 0.40, and the slump of concrete was 120mm.

In the Salhus mono tower cable stayed bridge in Norway, PSC was used. The bridge superstructure main span is 163m. the following mix proportions were used for M70 grade of concrete.

PSC - 450 Kg.

Silica fume - 35 Kg.

Light weight aggregate - 470 Kg.

Water - 195 Kg.

At kualalumpur city centre, there are 450m high twin towers, made of RCC core and columns and composite steel /concrete deck floors. Flyash was used in concrete for a grade of M80. Concrete admixture was used to produce 200mm slump of concrete. The elastic modulus for such concrete at 56 days was 35.5 Gpa. The total alkali in concrete was less than 3kg/m3 of concrete. This satisfies the BRE recommendations6 for resisting alkali-silica reaction in concrete.

The construction of Euro-tunnel under the sea (50-250m below sea level) is designed for 120 years. Each tube is composed of rings made of 5 segments. There are total 2,26,000 segments and 5,00,000 m3 of concrete is used. The length of the tunnel is 51km, of which 37 km is under the sea, 4km under land on French side and 10km under land on British side. The tunnel lining concrete (precast) is of M45 grade, and the cast in place concrete is of M 30 grade.

For the tunnel lining concrete, low C3A cement was not recommended. In marine environment, C3A is important to trap the chloride ions entering the concrete. OPC with 5-8% C3A was used.

For the cast - in - place M30 grade, durability of concrete was of more concern than the strength. Three blended cements were used:
  1. One blend containing 51% OPC, 25% slag and 24% flyash.
  2. One blend of 70% PSC (with 82% slag) and 30% flyash for RCC.
  3. One blend of 72% PSC and 28% flyash for unreinforced concrete.

Conclusion

The paper provides the 'definition', 'durability' and 'applications' of concrete of various grades and various constituents. Use of chemical and mineral admixtures along with good quality cement can produce concrete of any desired grade and if properly constructed, the service life of concrete structures can be enhanced. the mixing procedure and mixing time have considerable influence on the quality of concrete in terms of its cohesiveness and workability. Concreting in rural India has to be given special importance as producing and placing good quality concrete have become difficult, in the absence of electricity and proper mixing and placing equipments. The paper further discusses use of blended cements to enhance durability of concrete structures, and provides details on concrete mixes for M70 and M 80 grades of concrete used in developed countries. Finally the paper provides the details on the selection of concrete making materials (specially on blended cements) for the Euro tunnel, which has been designed for 120 years.

References

  • Indian standard code of practice for plain and reinforced concrete. IS 456:2000, Bureau of Indian Standards, New delhi.
  • Nanda R.L., S. Ratanaramig, P and Boonsiri S. Developing 'High performance concrete for hydraulic structure at Koldam.' the Indian Concrete Journal, Vol. 84, No.1, January 2010, pp. 21-33.
  • Indian standard specification for coarse and fine aggregates from natural sources for concrete. IS 383, Bureau of Indian standards, New Delhi.
  • Neville, A.M. 'Properties of concrete, 4th Edition,' Pearson Education Ltd. 1995.
  • Shetty, M.S., Muenz, K and Gall, N. Delhi Metro : 'Quality control of concrete for underground section.' The Indian Concrete Journal, April 2005, Vol. 79, No. 4, pp. 11-21.
  • BRE. Alkali aggregate reactions in concrete. Building Research Establishment Digest 330, March 1988.
  • Moranville-Regourd, M. 'Selection of Concrete materials for the Euro-Tunnel.' Proceedings, P.K. Mehta Symposium on Durability of concrete, Nice, France, May 23, 1994, pp. 147-159.

NBMCW December 2010

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Recron and Steel Fiber Reinforced Engin...

A Comparative Study of Recron and Steel Fiber Reinforced Engineered Cementitious Composites

Dr. S. C. Patodi, Civil Engineering Department, Parul Institute of Engineering &amp; Technology Limda, Vadodara; Dr. J. D. Rathod, Applied Mechanics Department, Faculty of Technology and Engineering, The M. S. University of Baroda, Vadodara

Fibers in the cementitious matrix tend to reinforce the composite under all modes of loading and the interaction between the fiber and matrix affects the performance of cement based fiber composite material. In the present experimental work, performance of a low modulus fiber (Recron 3S fiber) is compared with the performance of a high modulus fiber (Steel fiber) in producing the Engineered Cementitious Composite (ECC) with suitable mix design. Recron fibers are of polyester type which belongs to ester functional group. They exhibit strong interactions towards other polar substrates being polar in nature and because of this a very good bond is possible between fiber and cementitious matrix. Further, cross section of this fiber is substantial triangular due to which it is 2.2 times more effective than the fiber having circular cross section. On the other hand, corrugated steel fibers with flat geometry have optimized interface properties compared to other steel fibers and have modulus of elasticity more than ten times that of a Recron fiber. In the present paper, mechanical properties of Recron and steel fiber reinforced ECC under tension, compression, shear, impact and flexure are evaluated, with detailed parametric study, by testing different types of specimens. Recron fibers are found to give superb deformation performance under different types of loading with moderate strength enhancement. On the other hand, steel fibers are found to enhance strength of ECC under almost all types of loading but fail to demonstrate the required deformability.

Introduction

Strain hardening through multiple cracking in axial tension and tight crack width control are two unique properties that put Engineered Cementitious Composite (ECC) in a category of smart material [1]. Microstructure tailoring based on micromechanics [2] can lead to composite ductility of several percent in tension; a material property not seen before in discontinuous Fiber Reinforced Concrete (FRC). This makes ECC superior to that of ordinary concrete which is having brittleness in both tension and shear as inherent weakness. The lower fiber content combined with processing ease makes ECC economically and technically feasible to translate material performance to structural performance in the field.

The construction industry is highly cost-sensitive industry. The prime requirement of the society for any newly introduced material is that it must be beneficial, feasible and cost effective. Since, fibers increase cost of the composite and processibility is affected due to lack of workability, it is imperative to minimize the amount of fibers in the composite; yet the amount of the fibers must be good enough to just switch the material from a normal tension-softening FRC behaviour to a strain hardening ECC behaviour. Further, ECC mixture essentially consumes higher cement, typically two or three times than that of conventional concrete which is major factor that affects economy. The best possible way to address this issue is to substitute cement by industrial by-product well known as fly ash, provided the properties of resulting green ECC would still meet the performance requirement of ECC.

Details of a comprehensive study carried out for the performance evaluation of Recron and Steel fibers reinforced ECC (named as RECC and SECC specimens respectively) are given in this paper. Effect of size, C: S ratio, fly ash replacement and fiber volume fraction was studied under tension, compression, shear, flexure and impact type of loading. Briquette, coupon and dog bone type specimens were prepared to evaluate the effect of fiber orientation on tensile strength and strain performance. Indirect tension test was also carried out to establish relation with the direct tensile strength. Cubes were prepared for testing under compression. Inverted L-type shear specimens were prepared to facilitate testing under load and displacement control. Simply supported beams of various depths were tested under static four point loading arrangement to characterize size effect and fiber orientation effects on flexural performance. Also, cylindrical specimens were tested under drop weight test to evaluate impact strength.

For the preparation of specimens, Kamal brand 53 grade OPC, 300µ passing silica sand, w/c ratio of 0.35 and length of fiber as 12.5 mm were kept same in all ECC matrices. Cement: sand ratios of 1:0.5, 1:1, 1:1.5 and 1:2, different percentage volume fraction of Recron and steel fibers, processed siliceous pulverized fly ash confirming to IS 3812 (Part 1): 2003 up to 30% replacement of cement, variable dose of Glenium brand high performance concrete super plasticizer were included in the detailed parametric study performed to characterize ECC under different conditions.

Comparison of Performance Under Tension

Uniaxial tensile test though simple in concept is difficult to perform for concrete and cement matrix and requires attention to many test details. Amongst these is specimen alignment and post crack stability. Also, fiber orientation seems to play very crucial role along with the associated size effect. Testing with small cross section size specimens, in which fibers tend to be oriented favorable to the loading direction typically yields higher tensile strength and strain capacity in comparison to the true properties of the material when used in a structural member with larger dimensions. Therefore, in the present work three different types of specimens as shown in Fig. 1 were designed to give respectively 1D, 2D and 3D fiber orientation effect.

Tension Tests
Figure 1: Test Setups for Direct (1D, 2D and 3D) and Indirect Tension Tests

Evaluation of strength in axial tension requires complicated test setup, time consuming testing procedure and expensive experimentation. Split tensile test seems to be the best option to get tensile strength indirectly for the cementitious composite. It is accepted since long for concrete as it is not only easy to cast the cylindrical specimen but also it is simple to perform avoiding all the precautions as discussed above. Therefore, cylindrical specimens were also tested as shown in Fig. 1(d) to know the indirect tensile strength of ECC.

Tensile Test Specimens
Figure 2: Failure Pattern of Tensile Test Specimens with Recron Fibers

Tensile Test Specimens
Figure 3: Failure Pattern of Tensile Test Specimens with Steel Fibers

Failure pattern of various RECC tensile test specimens is depicted in Fig. 2 and that of SECC specimens is depicted in Fig. 3. Direct tensile test specimens were tested on MTS machine under displacement control using Multi-purpose Testware whereas indirect tensile test specimens were tested on compression testing machine under load control.

Load-displacement curves for briquette specimens for different volume fraction of Recron fiber are shown in Fig. 4 whereas Fig. 5 depicts the load-displacement graphs for briquette specimens having different percentage of steel fibers and fly ash.

Load-Displacement Curves
Figure 4: Load-Displacement Curves with Different % of Recron Fibers

Load-Displacement Curves
Figure 5: Load-Displacement Curves with Different % of Steel Fibers

Tensile strength of Recron fiber reinforced ECC is found less than the steel fiber reinforced ECC for the same fiber volume fraction irrespective of fiber orientation or fly ash replacement. Increase in tensile strength of SECC over RECC ranges between 40 to 60% in different types of tension specimens. Twisting mechanism of Recron fibers dominated over well known size effect of the specimen whereas size effect is more predominant in case of steel fiber reinforced ECC as performance is governed by contribution of cementitious material.

Performance of RECC is focussed on 4% fiber volume fraction being optimized [3] whereas performance of SECC is limited to 3% fiber volume fraction [4]. Maximum strain exhibited by 4% Recron fiber reinforced ECC with c: s ratio of 1:0.5 is found to be 1.53% in briquette specimen in contrast to 0.92 in the same cementitious matrix with 3% steel fiber volume fraction. Approximately 40% strain is reduced in this case. It is important to note here that strain performance of SECC is not at all consistant. It is highly unpredictale and insignificant in most cases.

The key issue to be addressed over here is the excellent strain hardening performance exhibited by 4% Recron fiber reinforced ECC with c: s ratio of 1:0.5 comparable to that with metal. Fly ash replacements in ECC could also maintain the true strain hardening performance with some reduction. Thus, it can be said that the main aim of this investigation to develop truly strain hardening material is satisfied by the use of 4% Recron fibers in ECC. None of the specimens could exhibit strain hardening performance by the use of steel fibers even upto 3% fiber content. Therefore, it can be concluded that the steel fiber reinforced cementitious composite should not be designated as ECC but it can be considered as regular FRC. Fiber volume fraction of more than 6% may convert FRC performance into true ECC performance in case of steel fibers.

Another criterion of ECC performance is also satisfied fully in tensile specimens of RECC i.e. multiple cracking. Density of multiple cracking reduced with increase in fly ash replacement. Combined action of pull out and rupture of the fibers governed the failure mechanism. In contrast, multiple cracking is not seen in any of the specimens in case of steel fiber reinforced ECC. All the specimens failed by formation of single crack in which fracture failure and matrix splitting governed the failure mechanism.

Fiber orientation related with size effect of the specimen plays critical role in triggering strain hardening performance at minimum fiber volume fraction. Strain hardening performance in RECC is geared up at minimum fiber volume fraction of 2% for 1D, 3% for 2D and 4% for 3D specimens. However, only 4% Recron fiber reinforced ECC is selected for further study as most of the specimens tested in the subsequent sections have 3D fiber orientation.

Comparison of Response Under Compression

ECC has shown its compatibility with the concrete and reinforcement. This property of ECC leads to use it in many structural applications where it has to interact with the concrete which is designed for compression. Therefore, one can not neglect this fundamental property of the cementitious material as the end users in the construction industry recognize the construction material by its compressive strength.

For finding the compressive strength of Recron and steel fiber reinforced ECCs, cubes of 150 mm size were tested using a compression testing machine; the failure patterns of the same are shown in Fig. 6.

Failure Patterns of Cube
Figure 6: Failure Patterns of Cube under Compression

Maximum compressive strength in Recron fiber reinforced ECC without fly ash is found as 36.77 N/mm2 at the c: s ratio of 1:0.5 with 4% fiber content. This strength could be elevated to 39.84 N/mm2 by addition of 30% fly ash leading to 8.34% increase in strength. Cement: sand ratio of 1:0.5 proved to be better in Recron and steel fiber reinforced ECC. Average density of 2250 Kg/cm2 and 2350 Kg/m3 respectively is obtained in RECC and SECC. The ratio of compressive strength of cube to the cylinder specimen is about 1.35 in RECC whereas it is found to be 2.5 for SECC.

Maximum compressive strength in SECC without fly ash is found as 49.90 N/mm2 at the c: s ratio of 1:0.5 with 3% fiber content. This strength could not be elevated by addition of fly ash; the strength remains almost the same. Use of fly ash renders only economy in case of SECC while it renders economy and strength in RECC. Maximum enhancement of compressive strength in steel fiber reinforced ECC is 25% over Recron fiber reinforced ECC.

Comparison of Performance Under Shear

Shear failure is generally brittle in concrete structures. The beam-column connection and the base of shear wall are likely to be subjected to intensive shear during earthquake loading. Typical diagonal crack patterns observed in the catastrophically failed structures due to shear in Bhuj earthquake suggest that the structural shear load induces local tensile failure of material. Thus, tensile strength plays a significant role in shear mechanism for brittle materials whereas compressive and tensile strength together governs shear mechanism for ductile material. The present study is primarily concerned with translation of pseudo strain hardening property of ECC in axial tension from material level to the structural level under intensive shear load.

Most of the researchers have carried out the testing of conventional reinforced beams to study the effect of shear span to effective depth ratio, fiber type, fiber volume fraction and aspect ratio and the longitudinal steel content on shear capacity. But the experience shows that the two planes failing simultaneously in double shear for beam specimens is rarely observed in reality and hence shear strength calculated in this manner could be erroneous. Also, an attempt to get one failure plane in shear in beam and column specimen directly under compression testing machine invites undue eccentricity since one portion of the specimen needs to be held fixed with respect to the other part. Hence, there exists no standard, reliable and simple method to get direct shear strength. An attempt is, therefore, made in the present study to get direct shear strength of ECC material by preparing inverted L-type specimens and testing under single shear arrangement as shown in Fig. 7. Recron fibers being polymeric tend to pull out with some resistance or rupture in the matrix. Pull out mechanism of these fibers in the shear zone is very complex in nature whereas it has definite mechanism in tension. Therefore, Recron fibers do not enhance the strength performance up to the extent it is observed in case of steel fibers. However, three dimensional effect because of more uniform distribution of fibers makes it very beneficial to undergo large shear displacement representing pure shear failure. The sliding shear mechanism observed in case of Recron fiber reinforced ECC (Fig. 7(a) is unique in nature and very difficult to get in any other cementituious composites.

Inverted L-type Specimen
Figure 7: Testing of Inverted L-type Specimen under Shear

Advantage of Recron fibers could be quantified to a large extent in shear behavior in comparison to the axial tension. Once again, ECC with c: s ratio of 1:0.5 without fly ash and 4% fiber volume fraction proved to be superior in performance as it could exhibit enhanced strength and strain performance.

Steel fibers being very strong in shear make it possible to resist large amount of the load if fibers are available in the resisting plane. It leads to very uncertain behavior in shear specimens due to this reason only. In some of the specimens none of the fibers could be seen in the failure plane. The specimen looked like plain cementitious matrix. Therefore, it is very difficult to get proper failure pattern or predict ultimate load. Some of the specimens could exhibit shear strength 58% more than the maximum strength obtained in RECC. However, this enhancement is not consistent and can not be guaranteed.

Comparison of Performance Under Flexure

The flexural strength of unreinforced cement-based materials and their fiber composites is important for many applications such as buildings, road pavement slabs, airfield runways, roofing tiles, and architectural wall panels. In addition, using a simple flexure test, an important property of cement-based materials, namely their tensile strength, can be deduced from their flexural strength.

Flexural behavior of plain cementitious matrix is affected by well-known size effect. Fiber orientation and distribution is also affected by size effect. Thus, size effect plays dual role in fiber reinforced composite behavior and therefore it should be taken into account seriously. Also, ECC material composition, which satisfies performance criteria of strain hardening through multiple cracking in axial tension, will definitely satisfy in flexure but not vice versa. Therefore, material optimization strategy in flexure is different than that of axial tension. Lower fiber volume fraction in ECC may satisfy performance criteria under flexure.

Beam Specimens
Figure 8: Four Point Loading Arrangement for Testing the Beam Specimens

Load- Displacement Curves
Figure 9: Load- Displacement Curves for RECC and SECC Beams

In the present work, 500 mm long beam samples having 100 mm x 20 mm and 100 mm x 45 mm cross sections were tested under four point loading arrangement as shown in Fig. 8. Load-displacement response of RECC and SECC beams is shown in Fig. 9 (a) and 9 (b) respectively whereas failure pattern of Recron fiber and steel fiber reinforced beam samples is shown in Figs. 10(a) and 10(b) respectively. Comparison between performance of 4% Recron and 3% steel fiber reinforced ECC beams is made with 20 mm and 45 mm thick specimens in Table 1.

Comparison of RECC and SECC Beams

One of the significant differences in RECC and SECC beams is that none of the SECC beams could exhibit strain hardening (Fig. 9(b)) and multiple cracking (Fig. 10 (b)). All the specimens behaved like plain cementitious matrix. Reserved strength and deflection hardening are not obtained, therefore, they are not considered for the comparison. Ultimate strength for all cases is found to be lower in SECC beams in comparison to RECC beams.

Failure Patterns of RECC and SECC Beams
Figure 10: Failure Patterns of RECC and SECC Beams under Flexure

Comparison of Response Under Impact

Industrial flooring subjected to heavy machine vibration, cavitations and erosion damage in dams and other hydraulic facilities, ability of shotcrete to absorb energy, specialized cementitious coating for repair, fiber reinforced castable refractory, military and blast resistent applications etc. are some of the practical examples where impact test results help to access the performance of the material qualitatively.

Impact Test
Figure 11: Different Cylindrical Moulds and Test Setup for Impact Test

Generally, one cylinder of 150 mm diameter and 300 mm height as shown in Fig. 11(a) is casted and three specimens of 64 mm height are cut from it. This method of sampling was adopted for Recron fiber reinforced ECC which could be cut easily. Cylinders prepared from flat steel fibers, however, could not be cut properly by the available concrete cutting machine. Therefore, directly the samples of 64 mm height were prepared by using the mould as shown in Fig. 11 (b) and were tested using the test set up for the impact test as per the ASTM standard as shown in the Fig. 11 (c). The hammer of weight 4.54 kg was dropped through a height of 457 mm on a steel ball held firmly in the center of the specimen and number of blows required to cause the first visible crack on the top surface were recorded.

Impact Strength of Various Matrices

Performance of 4% RECC is compared here with 3% SECC. Results of 30% fly ash are also included for the comparison. With 4% RECC, 1550 percentage enhancement of impact strength over M40 concrete is found as shown in Table 2. 30% fly ash replacement reduced the enhancement to 1016% which indicates negative effect of fly ash in case where impact energy is to be absorbed. In case of SECC, addition of 30% fly ash resulted in number of blows less than the concrete which is a debatable issue.

Failure Patterns of RECC and SECC Specimens
Figure 12: Failure Patterns of RECC and SECC Specimens

Failure patterns of impact specimens prepared from Recron and steel fiber reinforced ECC are shown in Fig. 12. Failure pattern of RECC shown in Fig. 12 (a) clearly reveals ultra high ductility by the extensive deformation (penetration) of the hard steel ball at the centre of the specimen. There is a perfect splitting crack and large deformability without disruption, disintegration or spalling of cementitious material. On the other hand, the failure pattern of SECC as shown in Fig. 12 (b) is very similar to that of concrete for being brittle in nature.

Energy dissipation in the composite is mainly attributed to the compatible and connected fiber action. Fractured surface of impact specimens is shown in Fig. 13 which indicates that more number of fibers are evenly distributed in RECC compared to SECC. Good strength performance could be obtained under static load in SECC due to fiber geometry, surface deformation and stiffness. However, impact performance of the SECC is very poor. It is almost similar to the M40 concrete; 5.5 percent enhancement in the impact strength can be considered as negligible.

Fractured Surfaces of Impact Specimens
Figure 13: Fractured Surfaces of Impact Specimens

Conclusions

  • Workability aspect of steel fiber reinforced ECC is an appreciable issue as satisfactory workability is observed without use of any chemical admixture. Whereas, 2% dose of high performance concrete super plastisizer is a must to have good workability in case of 4% Recron fiber reinforced ECC.
  • Although, tensile strength of SECC is found more than RECC for the same fiber volume fraction irrespective of fiber orientation or fly ash replacement, none of the specimens could exhibit strain hardening performance and multiple cracking pattern by the use of steel fibers even upto 3% fiber content. Fiber volume fraction more than 6% may lead to its characterization as ECC but such a high volume of steel fibers will make it highly uneconomical.
  • Compressive strength of 4% Recron fiber reinforced ECC is comparable to medium strength concrete whereas compressive strength of 3% steel fiber reinforced ECC is 25% more than RECC. However, material strength can not be directly correlated to the structural strength which seems to increase with the higher ductility. Hence good structural performance requires a balanced material strength and ductility which RECC is able to confirm through ductile failure patterns.
  • Maximum shear strength obtained in SECC is approximately 5 times the strength obtained in ordinary concrete. Also, its shear strength is more than the maximum shear strength of RECC. However, results of SECC are not consistent and most of the specimens failed by fracture control after formation of the first crack. Steel fibers being very strong and stiff in shear makes it possible to resist large amount of the load only if fibers are available in the resisting plane.
  • Ductile nature of failure pattern is observed in RECC while fracture controlled and brittle nature of failure similar to the concrete is exhibited by the SECC under flexural loading which is the major difference between the two.
  • Large difference in the impact strength value between the RECC and SECC may be correlated to the performance of both the composites in axial tension. RECC undergoes extensive deformation with significant stain hardening performance indicating super ductile behavior in axial tension whereas, the SECC fails suddenly after the formation of the first crack similar to the concrete indicating brittle nature. Therefore, there is not much difference in the impact strength of the steel fiber reinforced ECC and M40 concrete.
  • Steel fibers enhance strength of ECC under almost all types of loading but fail to demonstrate deformability. On the other hand, Recron fibers demonstrate superb deformation performance under different types of loading with moderate strength enhancement. This situation gives rise to the hybrid concept of combining steel and Recron fibers to obtain better performance in both strength and strain in the composite.

Acknowledgement

The first author would like to acknowledge with thanks the support provided by the Gujarat Council on Science and Technology (GUJCOST), Gandhinagar to carry out a part of the research work reported here.

References

  • Li V. C.: “On Engineered Cementitious Composites (ECC) - A Review of the Material and its Applications”, Journal of Advanced Concrete Technology, Vol. 1, No. 3, pp. 215-230, Nov. 2003.
  • Li V. C.: “Engineered Cementitious Composites (ECC) – Tailored Composites through Micromechanical Modeling”, Proceedings of Fiber Reinforced Concrete: Present and the Future, Canadian Society of Civil Engineers, 1997.
  • Rathod J. D., Patodi S. C., Parikh B. K. and Patel K. H.: “Study of Recron 3S Fiber Reinforced Cementitious Composites”, Proceedings of a National Conference on “Emerging Technology and Developments in Civil Engineering”, Amaravati, pp. I-88 to I-95, March 2007.
  • Rathod J. D. and Patodi S. C.: “A Comprehensive Study of Mechanical Properties of Steel Fiber Reinforced ECC, International Journal of Earth Science and Engineering, Vol. 03, No. 3 SPL, pp. 303-311, May 2010.

NBMCW November 2010

.....

Chloride Diffusion of Concrete on Using...

Chloride Diffusion of Concrete on Using GGBS as a Partial Replacement Material for Cement and Without and With Superplasticiser

V.S.Tamilarasan, Research Scholar and Assistant Professor, Department of Civil Engineering, Dr.Sivanthi Aditanar College of Engineering, Tiruchendur and Dr. P.Perumal, Professor & Head, Department of Civil Engineering, Government College of Engineering, Salem

Increase in environmental awareness over the past decade, resulted in increasing attention to individual pollution and waste management control. The use of recycled waste cementitious materials is becoming of increasing importance in construction practice.

In India, we produce about 7.8 million tonnes of blast furnace slag, which is a by-product of steel. The disposal of GGBS as a landfill is a problem, which leads to serious environmental hazards. GGBS can be incorporated in cementitious materials to modify and improve certain properties for specific uses.

An attempt is made to replace partially GGBS for cement in concrete of M20 & M25 grades and study its Chloride diffusion. GGBS is replaced for cement in the level of 10%, 20%, 30%, 40%, 50% and 60%. The study results showed that, with the increase in percentage of GGBS, the Chloride diffusion of concrete decreases. Also it is found that the Chloride diffusion in the M25 concrete is less than M20 concrete.

The partial replacement of GGBS for cement in concrete has great potential economical benefits in all areas of construction industry. The GGBS will also make a significant contribution to sustainable development.

Introduction

In recent years there is an increasing awareness regarding environmental pollution due to domestic and industrial waste. Now pollution control board is formed to regulate environmental degradation due to industrial waste. The development and use of blended cement is growing in Asia, mainly due to considerations of cost saving, energy saving, environmental protection and conservation of resources.

Ground Granulated Blast furnace Slag is a by-product obtained in the manufacturing of pig iron in the blast furnace. It is a non-metallic product consisting essentially of silicates and aluminates of calcium and other bases. The molten slag is rapidly chilled by quenching in water to form a glassy sand like granulated material. GGBS is recognized as a desirable cementitious ingredient of concrete and as a valuable cement replacement material that imparts some specific qualities to composite cement concrete.

In India, we produce about 7.8 million tonnes of blast furnace slag and it is available separately as GGBS. The disposal of such slag even as a waste fill is a problem and makes serious environmental hazards with the projected economic growth and development in the steel industry, the amount of production is likely to increase many folds and environmental problem will thus pose a larger threat.

It is seen that high volume eco-friendly replacement by such slag leads to the development of concrete, which not only utilizes the industrial wastes but also saves a lot of natural resources of energy. While using the GGBS in concrete, it reduces heat of hydration, refinement of pore structure, permeability and increase the resistance to chemical attack.

Chloride Permeability of concrete is the relative ease with which chloride ion can penetrate into the pores of concrete. The study of chloride permeability in concrete is of importance when concrete is subjected to chlorine atmosphere such as saline nature, chlorine-manufacturing plants etc. The penetration of chlorine ions into concrete may lead to the corrosion of reinforcement and hence weaken the structures and also adversely affect durability of concrete. Therefore a detailed study has been required to find the chloride permeability of concrete.

The factors affecting chloride permeability are as follows
  • Physico-chemical properties of the mass transport system
  • Chloride source concentration
  • Addition of mineral and chemical admixtures
  • Water binder ratio

Materials Used

Cement

Ordinary Portland cement of 53 grade was used, which has the fineness modulus 1.5, Specific gravity 3.08, Consistency 37%, Initial setting time 2hrs 30 min and Final setting time 3hrs 30min.

Coarse aggregate

Angular shape aggregate of size of 20 mm was used and it has the following properties: Specific gravity 2.935, Flakiness index 100%, Abrasion value 20.4%, Crushing value 30.02%, Impact value 23.6%, Bulk density 1.42 x 103 Kg/m3 and Water absorption 1.01%.

Fine aggregate

River sand conforming to zone III of IS: 383 – 1970 was used and its properties are found as follows: Specific gravity 2.68, Moisture content 0.71 and Fineness modulus 2.75.

GGBS

Physical properties of GGBS are: Specific gravity 3.44 and Fineness modulus 3.36, and the chemical composition of GGBS is Carbon (C) 0.23%, Sulphur (S) 0.05%, Phosphorous (P) 0.05%, Manganese (Mn) 0.58%, Free silica 5.27% and Iron (Fe) 93.82%.

Chloride Permeability

For reinforced concrete bridges, one of the major forms of environmental attack is chloride ingress, which leads to corrosion of the reinforcing steel and a subsequent reduction in the strength, serviceability, and aesthetics the structure. This may lead to early repair or premature replacement of the structure. A common method of preventing such deterioration is to prevent chlorides from penetrating the structure to the level of the reinforcing steel bar by using relatively impermeable concrete. The ability of chloride ions to penetrate the concrete must then be known for design as well as quality control purposes. The penetration of the concrete by chloride ions, however, is a slow process. It cannot be determined directly in a time frame that would be useful as a quality control measure. Therefore, in order to assess chloride penetration, a test method that accelerates the process is needed, to allow the determination of diffusion values in a reasonable time.

Principle

This test method consists of measuring the amount of electrical current passed through 50 mm thick slices of 100 mm nominal diameter cores or cylinders during a 6-h period. A potential difference of 60-voltage dc is maintained across the ends of the specimen. One of which is immersed in a sodium chloride solution, the other in a sodium hydroxide solution. The total charge passed, in coulombs, found to be related to the resistance of the specimen to chloride ion penetration.

Significance and use

This test method covers the laboratory evaluation of the electrical conductance of concrete samples to provide a rapid indication of their resistance to chloride ion penetration. The test method is suitable for evaluation of materials and material proportions for design purposes and research development.

Methodology

10%, 20%, 30%, 40%, 50% and 60% of cement was replaced by means of GGBS, which is the by-product of steel. The mix grades used were M20 and M25. For each level of replacement, 3 cylindrical specimens were cast by using thoroughly mixed cement, fine aggregate, coarse aggregate and water in the mixer machine. All the specimens were kept for curing in the water for a period of 28 days and specimens were arranged in RCPT testing machine and test is carried out for 6 hrs. Afterwards, using formulae, total charge passed was found out. The results are tabulated as shown in tables from 1 to 5 and the conclusions are made.

Test specimen

The specimen was cylindrical shape, size of 100mm diameter, 50mm length. Three cylindrical specimens were used for each percentage of replacement of slag for determining chloride ion penetration.

Procedure

The apparatus consists of two cells. The specimen was mounted as shown in Fig 1 and fixed between the cells in such a way that the round edge surface should touch with the solution. After fixing the specimen, the negative side of the cell was filled with 3% NaCl solution. The positive side of the cell was filled with 0.3M NaOH solution till the top surface of the concrete immerses in the solutions. Leakage was checked. Copper rods were used as electrodes. The wires, electrodes, power supply are connected.

AASHTO T277
Figure 1: AASHTO T277 (ASTM C1202) test setup

A D.C supplier was used to give electrical potential of 12v. The –ve terminal of D.C.S was connected with electrode of NaCl solution. The +ve terminal of D.C.S was connected with electrode of NaOH solution.

As per electro - chemistry principle, due to the applied voltage, the negative ion i.e. the chloride ion was attracted towards positive terminal i.e. NaOH reservoir. Therefore the chloride ion moves through the concrete specimen. Also the positive ion passes towards the negative terminal i.e. NaCl reservoir through the concrete specimen.

Due to the movement of positive and negative ions current was produced. This current was shown in D.C supplier. Reading was taken immediately after voltage supplied at every 30 minutes. This procedure was done for 6 hours duration. Decrease in charge passed values indicates that the concrete has more resistance to chloride ion penetration

Formulae

The total charge passed is a measure of the electrical conductance of the concrete during the period of the test. If the current is recorded at 30 min interval, the following formula, based on the trapezoidal rule, can be used with an electronic calculator to perform the integration:

Q=900(I0+2I30+2I60+……………. +2I300+2I330+I360)

Where:
Q= charge passed (Coulombs)
I0 = current (Amperes) immediately after voltage is applied, and
It = Current (Amperes) at t min after voltage is applied.

Correction:
If the specimen diameter is other than 3.75 inch (95 mm) the value for total charge passed must be adjusted. The adjustment is made by multiplying the value by the ratio of the cross-sectional areas of the standard and the actual specimens. That is:

Qs = Qx x (3.75/X) 2

Qs = charge passed (coulombs) through a 3.75-inch (95-mm) diameter specimen.

Qx = charge passed (coulombs) through X in diameter specimen and

X = Diameter (inch) of the nonstandard specimen.

Mix proportions

Mix proportions are calculated for M20 & M25 grade concrete. The mix ratio for M20 grade concrete is 0.5:1:1.6:3.559 & the mix ratio for M25 grade concrete is 0.44:1:1.326:3.11

Test results

The experimental procedure is conducted on various types of mix containing partial replacement of cement by GGBS. The values of charge passed are tabulated as shown in Table 1 to 5.

Diffusion of Concrete

Diffusion of Concrete

Diffusion of Concrete

Diffusion of Concrete

Diffusion of Concrete

Graphs

Graphs (Fig 2 to 9) are plotted by taking % of replacement of GGBS in x-axis and charge passed in Y-axis for M20 & M25 grades.

Chloride Permeability of M20 Grade
Chloride Permeability of M25 Grade
Figure 2: Chloride Permeability of M20 Grade With GGBS Concrete
Figure 3: Chloride Permeability of M25 Grade With GGBS
Chloride Permeability of M20 Grade
Chloride Permeability of M25 Grade
Figure 4: Chloride Permeability of M20 Grade Super Plasticiser Added GGBS Concrete
Figure 5: Chloride Permeability of M25 Grade Super Plasticiser Added GGBS Concrete
Comparision of M20 & M25
Comparision of M20 & M25
Figure 6: Comparision of M20 & M25 GGBS Concrete
Figure 7: Comparision of M20 & M25 Grade Super Plasticiser Added GGBS Concrete
Comparision of Concrete
Comparision of Concrete
Figure 8: Comparison of M20 grade GGBS Concrete & Super Plasticiser Added GGBS Concrete
Figure 9: Comparision of M25 Grade GGBS Concrete & Super Plasticiser Added Ggbs Concrete

Results & Discussion

The Chloride diffusion tests in M20 & M25 grade concrete were conducted using RCPT testing machine. The results are stated as below:

For conventional concrete, the Charge passed for M20 and M25 grade concrete are 407 Coulombs and 318 Coulombs respectively.

For grade M20 with GGBS the Charge passed values varies from 358 Coulombs to 292 Coulombs and for grade M25 with GGBS the Charge passed values varies from 298 Coulombs to 170 Coulombs.

For grade M20 Superplasticiser added GGBS concrete, the Charge passed values varies from 553 Coulombs to 345 Coulombs and for grade M25 Superplasticiser added GGBS concrete, the Charge passed values varies from 378 Coulombs to 185 Coulombs.

Conclusion

For both the grades of GGBS concrete and Superplasticiser added GGBS concrete, as the replacement level increases, the chloride permeability value decreases which improves the chloride penetration resistance of the concrete and durability of concrete.

By using GGBS as a replacement material for cement, the cost of construction will be reduced. Use of GGBS in concrete also prevents the environment from degradation.

M25 grade concrete has less chloride permeability than the M20 grade concrete. So, the permeability value also depends upon the mix grade of the concrete.

Bibliography

  • Adakhar, "Compatibility of super plasticizer slag added concrete in sulphate resistance and chloride penetration," Advances in Civil Engineering Materials and construction technology, vol.33, 2001.
  • Balamurugan, P. and Perumal, P., "Behaviour of High Performance Concrete under elevated temperature and chloride penetration." Proceedings of the National seminar on Futuristic in concrete and construction Engineering, SRM Engineering College, Kattankulathur, pp 8.1-8.11. 2003
  • Chung-Chia Yang, "Relationship between Migration Coefficient of Chloride Ions and Charge Passed in Steady State," ACI Material Journal, pp. 124-129, March – April 2004.
  • IS: 456-2000, Code of practice for Plain and Reinforced Concrete.
  • IS: 10262-2004, Code of Practice for Concrete Mix Design.
  • Rajamane, N.P. and Annie peter, J. et.al, "Improvement in Properties of High Performance Concrete with Partial Replacement of Cement by Ground Granulated Blast Furnace Slag," IE(I) Journal-CV, Vol.84, pp38-41, May 2003.
  • Shetty, M.S. "Concrete Technology." S.Chand & Co, New Delhi 2002.
  • "Standard Test Method for Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration," ASTM, pp. 646-651.
  • Suvimol Sujiavanich et.al., "Chloride Permeability and Corrosion Risk of High-Volume Fly Ash Concrete with Mid-Range Water Reducer," ACI Material Journal, pp. 177-182, May – June 2005.
  • Tiewei Zhang and Odd E.Gjorv., "Effect of Chloride Source Concentration on Chloride Diffusivity in Concrete,' ACI Material Journal, pp. 295-298, Sep – Oct 2005.

NBMCW November 2010

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A Study on Coefficient of Thermal Expan...

N. P. Rajamane, Head, Concrete Composites Lab, Nataraja M C, Professor, SJ College of Engg, Mysore N Lakshmanan, Former Director, and P S Ambily, Scientist, Structural Engg Research Centre, CSIR, Chennai

Conventional Cement Concrete (CCC) consists of Portland Cement (P-C) as binder which binds the inert aggregate system. However, geopolymer composites (GPCs) have geopolymeric (GP) material as binder instead of P-C. GP is obtained by chemical activation of alumino-silicates present in minerals such as fly ash and Ground Granulated Blast furnace Slag (GGBS). Besides strength characteristics, it is necessary to understand the thermal properties of these new composites. In this connection, coefficient of linear thermal expansions (CLTEs) of two typical GPCs (in the form of mortars) were investigated using the computer controlled Dilatometer installed at Structural Engineering Research Centre (SERC), Chennai. The first GPC had GGBS and the second had a combination of fly ash and GGBS as the ‘starting materials’ for geopolymerisation reactions.

Introduction

Geopolymers are inorganic materials of cementitious nature derived from the alkaline activation of readily available natural or industrial by-product aluminosilicate materials. The term geopolymer was introduced by Davidovits to represent the mineral polymers. Geopolymer-based materials are environmentally friendly since they need less energy to produce. Their performance as a construction material has to be comparable with that of Portland cement in order to accept them as in practical applications. The present paper is concerned with the thermal expansion studies of geopolymer composites. The Coefficients of Linear Thermal Expansion (CLTE) of geopolymer mortars derived from Ground Granulated Blast furnace Slag (GGBS) and fly ash (FA) were measured as per ASTM E 228 on beam specimens using a computer-controlled dilatometer.

Scope of the Study

The knowledge of thermal transport properties of construction materials involved in the process of heat transfer is required for predicting the temperature profile and heat flow through the material. It is also needed in many civil engineering applications such as high rise building subjected to variation of temperature, blast furnace, and pressure vessels. Some information has been generated on the mechanical, microstructure and chemical properties of geopolymer materials. However, the test data on the thermal properties of this novel material is not widely available. The present work was therefore taken up to investigate the thermal properties of the GGBS and FA based typical geopolymer mortars (GPMs), particularly ‘linear thermal expansion coefficients’ (CLTEs). The CLTEs were measured at different temperatures, on prismatic specimens (size : 25 mm*25 mm *100 mm).

Details of Experimental Work

Materials

GGBS from steel plants and Class F FA from coal based thermal power station (near Chennai) were used as the ‘source materials’ (SMs) for the geopolymers (GPs). A combination of sodium silicate solution and sodium hydroxide solution was used as the alkaline activator. GPMs were prepared using locally available river sand as inert filler material. The properties of the ingredients of GPMs are described in Table 1.

Material Properties

Preparation of Test Specimens

Two different GPMs were studied; one with GGBS and the other having a combination of GGBS and FA, mixed in the ratio of 1:4 by mass. The fresh mixes (prepared in an electrically operated mortar mixer) were cohesive and there was no sign of segregation. Conventional table vibrator was sufficient to effect the compaction in the steel moulds. The moulds after casting operations were initially covered with plastic sheets for about 24 hours, after which the specimens were demoulded. Thereafter, the specimens were allowed to cure at ambient conditions inside the laboratory. After 28 days of casting, the cube specimens were taken for compression test and the prismatic specimens were subjected to a temperature of 100°C in a hot air oven for a period of about 24hours before using them for expansion studies.

Dilatometer for thermal expansion measurement
Figure 1: Dilatometer for thermal expansion measurement

Measuring Technique

The thermal expansion test was carried out using equipment called Dilatometer (Fig.1). It consists of specimen holder (Fig.2), furnace, transducer, recorder and temperature sensor. After taking out the specimen from the hot air oven, the specimens were allowed to cool to room temperature. The initial specimen length in the direction of the expansion was measured at room temperature. The specimen (Fig.3) was then placed in the specimen holder of the Dilatometer. The furnace portion of the equipment was moved to enclose the specimen holder. After positioning the specimen accurately, the AC power was switched on. An appropriate force was applied to the sensing probe to ensure that it was in contact with one end of the specimen. The initial displacement was set to zero. The specimen was heated at a constant rate of 3°C/min up to a temperature of 300°C and the changes in specimen length at every 100°C were recorded. After the desired maximum temperature was reached, the furnace was put off to allow the specimen (Fig.4) to cool. Three specimens were tested for each GPM.

Specimen inside the dilatometer Specimen
Figure 2: Specimen inside the dilatometer Specimen size: 100*25*25 mm (prism)

Specimens before testing
Specimens after testing
Figure 3: Specimens before testing
Figure 4: Specimens after testing

The CLTE is calculated as follows:

Linear thermal expansion =[Change in length (linear expansion)] / Original length

The coefficient of linear thermal expansion, α, is computed using the equation

α = (Linear Thermal Expansion / 100) / (TMax - TMin)

Tmax = Maximum temperature (ºC), Tmin = Minimum temperature (Ambient Temperature, ºC)

Test Results

Test Results And Discussions

Coefficients of Linear Thermal Expansion

The variation of CLTE of geopolymer mortars with temperature is shown in Fig.6. The GPM made from mixture of GGBS and FA indicated a lower CLTE value compared to that GPM made from GGBS alone. The CLTE of GPM with GGBS alone was about 4.3x10-6/°C at 100°C and this value increases to 12.3 x10-6 /°C at 300°C; however, in case of GPM containing both GGBS and FA, the CLTEs were 3.3 x 10-6 /°C and 12.25 x 10-6 /°C at 100 °C and 300 °C respectively. It may be noted that in estimating the range of thermal movements (e.g., highways, bridges, etc.), the use of lower and upper bound values of 8.5 x10-6/C and 11.7 x10-6/C has been suggested to be more appropriate for conventional concretes, by ACI 209. Thus, GPMs can be considered to possess comparatively lower values of CLTEs in general at near ambient temperature exposure conditions. It is also reported that for Portland cement paste specimens, though the CLTE may increase initially with temperature, but, beyond about 200 °C, the CLTE could actually decrease with temperature and it may even become negative at higher temperatures; this behaviour may be related to the various chemical changes occurring in the cement matrix (Neville, 1996). However, in case of Portland cement based concretes, the CLTE is reported to be increasing with temperature as seen in the present case of GPMs also.

Compressive Strength of geopolymer mortar
Figure 5: Compressive Strength of geopolymer mortar at 28 Days

Comparison of CLTE of geopolymer mortars
Figure 6: Comparison of CLTE of geopolymer mortars

Compressive Strength

The 28 day compressive of about 63 MPa was obtained for GPM with GGBS alone and this value was 50 MPa for GPM containing GGBS and FA. (Fig.5). These strengths indicate that GPMs develop satisfactory strength levels making them suitable for many structural applications. However, before adopting the GPMs in important practical applications, there is a need to study various aspects of durability of these composites including their potential to provide protection to the embedded steel reinforcement.

Density

The density of GPM with GGBS alone was 2277 kg/m3 and it was 2271 kg/m3 for mix with GGBS and FA. Thus the GPMs have marginally lower density than conventional concretes.

Concluding Remarks

  1. Ground Granulated Blast furnace Slag (GGBS) and fly ash (FA) could be used as ‘starting materials’ for initiation of geopolymerisation reactions so that geopolymeric binder system is evolved.
  2. Densities of Geoplymer mortars (GPMs) prepared with GGBS and FA are lower than those of conventional Portland based concretes.
  3. The 28 day strengths of 63 and 50 MPa achieved for GPMs described in this paper are high enough o conclude that the GPMs can be used as structural materials.
  4. The GPMs had higher CLTE values at higher temperatures. The difference in CLTE values between 300°C and 100°C was about 65% for GPM with GGBS; this difference for GPM having GGBS and FA was 73%.
  5. The lower and upper bound values of 8.5 x10-6/C and 11.7 x10-6/C has been suggested by ACI 209 to be more appropriate for estimating the range of thermal movements in transportation structures such as highways, bridges, etc. The geopolymer composites with lower values of CLTEs (at near ambient temperature exposure conditions) can be considered to be good candidate materials for construction of infrastructures .

Abbreviations

  • GP – Geopolymer, GPM – Geopolymer mortar, GPCs- Geopolymer Composites
  • P-C -Portland cement, CCC-Conventional Cement Concrete
  • GGBS – Ground Granulated Blast furnace Slag, FA - Fly Ash
  • CLTE - Coefficient Linear Thermal Expansion
  • SERC-Structural Engineering Research Centre

Acknowledgement

This paper is published with the kind consent of Director, SERC, Chennai. The co-operation and help received from the scientific and technical staff of Concrete Composites Laboratory, SERC, in creating test data and preparation of paper is gratefully acknowledged

References/ Bibliography

  • ACI 122R-02, “Guide to thermal Properties of concrete and Masonry Systems,” Books of standards, American Concrete Institute, June 21, 2002.
  • Arshad A. Khan, William D. Cook and Denis Mitchell, “Thermal properties and transient thermal analysis of structural members during hydration,” ACI materials journal, May-June, 1998, pp 293-303.
  • ASTM E 228-85, “Standard test method for linear expansion of solid materials with a vitreous silica dilatometer,” Books of Standards, American society for Testing Materials Standards, 1985, Vol. 14.02.
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NBMCW November 2010

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