3/11/13 3:24 PMEFFECT OF FLY ASH ON THE MECHANICAL PROPERTIES OF CONCRETE

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EFFECT OF FLY ASH ON THE MECHANICAL PROPERTIESOF CONCRETE

This investigation was carried out to determine the mechanical properties of hardened fly ash concrete.Briefly, the water cement ratio are kept close to 0.48 for Mix 1 and 0.5 for Mix 2 in order to study itseffect on the hardened fly ash concrete. The proportion of fly ash content was kept constant with 20%replacement of cement. The properties of hardened concrete investigated included compressive strength,modulus of elasticity, and drying shrinkage (change in length).

The analysis of the test results lead to the conclusion that a good performance of concrete can be producedwith the cement and fly ash used in this investigation.

The concrete also give a good compressive strength specially at late ages. The elastic modulus values arewithin the expected range. The drying shrinkage of the fly ash concrete investigated is relatively low andclose to normal concrete.

1. INTRODUCTION

Fly ash is one of the most widely used material admixtures which results from the combustion ofpulverized coal in thermal power generating plants. For many years, engineers have been interested in andconcerned with the possible use of fly ash in various types of concrete instead of being a waste material.In 1989, the total fly ash production in the world was on the order of 400 million tons annually. It isconsidered a major solid waste, about 70 to 80% of the fly ash goes to landfills.

When fly ash is mixed with lime it will slowly combine with the lime to form a material: similar to naturalcement. Therefore, when the fly ash mixed with portland cement it will! slowly combine with the freelime released as the portland cement hardened. This process NORrmally produces a greater strength in theconcrete after it ages approximately 100 to it) days. Extensive worldwide research [1-8] has shown that

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the use of fly ash can produce concrete of better resistance to sulfate attack, reduce concrete permeability,reduce heat of hydration and slightly affects the drying shrinkage of fly ash concrete or mortars ascompared to normal mixes.

In this investigation, the effect of adding fly ash admixture on the mechanical properties of fly ashconcrete was evaluated.

2. MATERIAL AND SPECIMEN PREPARATION

The concrete mixtures were made of a local Type I cement with specific gravity of 3.2, 2.6 for coarseaggregate (of 3/4 in. maximum size of aggregate) and fine aggregate (sand) with 2.47 specific gravity. Thefly ash used in the study was collected from power plants. Its chemical composition contains Si02 of 57.3,AL203 of 20.8, Fe203 of 9.3 CaO of 2 percent in addition to some other chemical components with lowpercentage. Throughout the experimental investigation a 20% of cement replacement by fly ash wasconsidered.

Four concrete mixes (M1C, M1F, M2C, M2F) were made in order to cast 48 specimens of 10!20 cmcylinders. Twenty four of these specimens were used as a control mix and other

twenty four were used from the concrete mix with 20% fly ash replacement with two different watercement ratio of 0.48 and 0.5. The reason for selecting small difference in w/c, is to see how this smallchange will effect the properties of hardened concrete without keeping the slump constant. This isdifferent from the normal case in which concrete mixes containing fly ash will generally require less water(1% to 10%) for a given slump than concrete containing only portland cement. The mix designation isgiven in Table 1. All specimens were cured in water for 7, 14, 28 and 90 days.

Another two concrete mixes (MSC and MSF as defined in Table 1) were made for shrinkage test with w/cof 0.48 and maximum size of aggregate of 3/8 in. Six prisms of 12"x2"x2" were casted from each concretemix. The prism sizes used here were considered for the purpose of comparison with control mix.Therefore by comparing the fly ash concrete specimens with the control specimens, a good indication ofthe difference in drying shrinkage (volume reduction) can he obtained. However, this comparison can bedone for concrete based on the flexibility given by ASTM C490 standards which allow using any desirecross-section having a 10 in. gage length when measuring the drying shrinkage. After six days of watercuring three prisms from each group were kept in a room with 50% humidity at room temperature of 23°C

3/11/13 3:24 PMEFFECT OF FLY ASH ON THE MECHANICAL PROPERTIES OF CONCRETE

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and the other three were kept in room temperature of 50°C with about 10% relative humidity. A summaryof the concrete mix design is given in Table 2.

3. TEST RESULTS

A large number of specimens were tested for the determination of the mechanical properties of concrete.These includ compressive strength at various ages, stress-strain relationship, modulus of elasticity anddrying shrinkage. Forty eight cylinders were tested in compression to measure the compressive strengthand modulus of elasticity at different ages of moist curing (7, 14, 28 and 90 days). The cylinder specimenswere casted from the concrete mix samples designated as M1C, M1F, M2C and M2F which are describedin Table 2. The mix proportions used to cast the first four samples given in Table 2 were 1:1.82:1,53:0.76(cement:sand:3/4 in. max. size aggregate:3/8 in. max. size aggregate). Also six prisms, casted from eachmix, MSC and MSF in Table 1, were subjected to the drying shrinkage measurements after initial storageof six days in water for periods of 7 to 98 days for both 50% humidity and 50°C temperature.

3.1 Compressive Strength

The data on the compressive strength are given in Table 3 and illustrated in Figs. 1 and 2. It appears fromboth figures that the compressive strength increases with the age of curing. The increase rate in the earlyages (7 to 14 days) is lower than the later ages (28 to 90 days) for both

fly ash and control concretes. For both w/c ratios (0.48 and 0.5) the compressive strength become closer at90 days than at early ages. This trend is attributed to the pozzolanic action of fly ash. For the lower w/cratio, of 0.48, the compressive strength was higher than for the case of 0.5 ratio by about (14 to 22%) forboth fly ash and control specimens at the age of 90 days.

3/11/13 3:24 PMEFFECT OF FLY ASH ON THE MECHANICAL PROPERTIES OF CONCRETE

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Modulus of Elasticity of Concrete

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modulus values for all control and fly ash concretes at different age of curing and different w/c ratios. Theelastic modulus values generally increase with age of concrete attaining maximum value at 90 days of age.The difference between initial and secant modulus is slightly higher at 90 days of age than those of lowerage, i.e. 7, 14 or 28 days. It is of interest to note that the difference between initial and secant modulus at90 days is about 6000 MPa for fly ash concrete and 9500 MPa for control Mix 1. For Mix 2, the differenceat 90 days is about 8000 MPa for the fly ash concrete and 5000 MPa for the control mix. This indicate lessdifference in the case of fly ash concrete than the plain concrete for the considered change of w/c ratiofrom 0.48 to 0.5.

3.3 Drying Shrinkage

The drying shrinkage test results after 6 days of initial water-curing are shown in Fig. 3 for control mixand fly ash (MSC and MSF) specimen under 50°C with 10% relative humidity temperature conditions

3/11/13 3:24 PMEFFECT OF FLY ASH ON THE MECHANICAL PROPERTIES OF CONCRETE

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using the average value of 3 tested specimens. It is clearly observed that fly ash samples shows almostsimilar range of shrinkage values to those of control mix under 50°C temperature condition. The values ofshrinkage at high age i.e. 14 to 98 days, become slightly lower for fly ash samples than the control mixsamples for the 50°C temperature condition. In this case, the compressive strength of the plain concrete isexpected to be higher which correspond to a reduction in permeability. The high temperature environmentcauses a reduction in the size of capillary pores both in the paste matrix and in the paste-aggregateinterface, thus increasing capillary pressure. This will cause the rate of drying shrinkage to be larger, andthe amount of shrinkage to be greater.

Figure 4 shows the same comparison between the fly ash and control samples for 50% humidity at 23°Croom temperature condition. It can be noticed from the figure that fly ash concrete specimen show highershrinkage values with age than control mix samples under humid conditions. In this case due to therelative humidity of 50% it might be expected that the water will take more time to find its way out ofplain concrete specimens which could reduce the rate of drying shrinkage to be smaller than thespecimens with fly ash. These observations agree with some of the results available in literature [91.

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4. CONCLUSIONS

The mechanical properties of fly ash concrete with 20% of fly ash replacement with different water centratio are evaluated in this study. All factors considered in this investigation (fly ash type, fly ash content,moist curing period, and age of testing) influenced the compressive strength of concrete. The relativelyhigh alkali content of the fly ash could be adversary influencing the development of strength in concretematerials. Inspite of the change in water cement ratio, considered here the fly ash concrete shows muchcloser values of compressive strength and modulus of elasticity to values of control mix samples at highages than early ages.

The drying shrinkage of concrete investigated are relatively low. The values of shrinkage strain ofconcrete under high temperature conditions (50°C) is found to be slightly smaller for fly ash concrete thancontrol mix specimens indicating the advantage of the fly ash to reduce the drying shrinkage of concrete.The result was opposite for the case of humid conditions (50% relative humidity), in which the dryingshrinkage strain values were higher in fly ash concrete samples than the control mix samples. Generally,the performed tests of the concrete samples with 20% fly ash replacement of cement can lead to aconsiderable technological and economical benefits in construction firms.

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3/11/13 3:24 PMEFFECT OF FLY ASH ON THE MECHANICAL PROPERTIES OF CONCRETE

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