TIME DEPENDENT PROPERTIES OF FLY ASH CONCRETE

Abstract

This investigation deals with the influence of percentage cement replacement and that of the age on strength, elasticity and shrinkage of plain and fly ash concrete mixes in general and on creep in particular. Two plain concrete mixes of M150 and M200 grades and the corresponding fly ash concrete mixes with cement replace ments of 10, 15, 20 and 25 percent were included in the study. All the mixes had a slump of (4+1) cm. The speci mens were cured in the moulds under wet gunny bags for the first 24 hours, kept under water for the next 27 days in the controlled environment room with a temperature of (27+2)°C and relative humidity of (90+5) percent and subsequently stored in controlled ambient environment till ohe time of testing. Compressive strength, elastic modulus and dynamic modulus were observed to increase with ago at a decreas ing rate. At the age of 365 days a maximum increase of 94, 49 and 24 percent in strength, elastic modulus and dynamic modulus had been found over and above the values of the corresponding parameters at the age of 28 days in case of some of the mixes studied. For cement replace ments upto 15 percent all these parameters were indivi dually found to be linearly related with the logarithm of the age. At the age of 28 days compressive strength, elas tic modulus and dynamic modulus of fly ash concrete -XImixes was observed to be smaller than the value of the corresponding parameter for their reference mixes at the same age. Also higher the percentage cement replace ment smaller was the observed value of the individual parameter. 15 percent cement replacement was found to be the optimum in respect of both strength and elasticity. The value of these parameters for the reference mixes, at the age of 28 days, were found to be attained by the corresponding fly ash concrete mixes, with 15 percent cement replacement, within 40 to 60 days. Cement replace ments higher than the optimum resulted in fly ash concrete mixes of comparatively smaller strength and elasticity at all the ages, whereas, fly ash concrete mixes with cement replacements upto 15 percent were found to give strength and elasticity within + 5 percent of the corresponding parameter for their reference mixes at the age of 365 days. When plain and fly ash concrete mixes of equal strength were compared higher elastic modulus was indi cated by fly ash concrete mixes. Strength of plain and fly ash concrete mixes was also observed to increase when the specimens were kept under sustained load. Saturated state logarithmic decrement measured at the age of 28 days, was found to be highest for the mixes. In the controlled environment the value of '6' decreased •Xllrapidly, in the early ages, after removal from water.'6' decreased slowly afterwards and finally stabilized to a constant magnitude. Slight increase in '6* with increasing fly ash content was observed in different mixes. Based on the experimental data interclass correlat ions between compressive strength and elastic modulus, between elastic modulus and dynamic modulus and between compressive strength, dynamic modulus and logarithmic decrement had been established. For the mixes, investigated, shrinkage was found to increase with time at a decreasing rate. An increase in shrinkage was also observed with increasing fly ash content for a given reference mix. Also smaller the aggregate to cement plus flyash ratio, higher was the observed creep. Creep strains, in general, showed an increasingtrend with increasing fly ash content for a given refer ence mix under a given stress-strength ratio. Higher creep strains were observed for the same mix under higher stressstrength ratio and under the same stress-strength ratio concrete having higher cement-flyash paste content exhibited higher creep. The nature of the creep-time curves remained similar for all the mixes irrespective of fly ash content. The increase in creep strain was much faster in the early -Xlllages and approximately 72 percent of the 150 day creep was observed to take place within first 28 days after the creep loading. Least squares regression lines for shrinkage and creep against logarithm of the age after creep loading fitted fairly well on the observed data. Possibility of predicting creep from hyperbolic, power and exponential expressions, in light of the observed data, had also been discussed. A non-linear Burger model was found quite satisfactory to describe creeptime behaviour of the various mixes and its coefficients evaluated. Creep has been found to be highly correlated with the saturated state logarithmic decrement, at the age of 28 days, for a given mix. 'No correlation between creep and dynamic modulus, however, could be established. Creep recovery was found to be comparatively smaller than the preceding creep strain at the time of unloading and that it stabilized much earlier as compared to the creep strain. Creep recovery strains recorded were higher for specimens, of a given mix, under higher stressstrength ratio but no definite relationship was observed between the two parameters. Also creep recovery was observed to increase with cement replacement upto 15 percent. Cement replacements higher than 15 percent reduced creep recovery strains. An integrated hypothesis combining various processes -XIVof water diffusion, delayed elastic action and viscous flow of cement gel had been proposed to explain the creep and creep recovery processes occuring in fly ash concrete mixes.