A Study of STRUCTURAL PROPERTIES OF CONCRETE WITH FLYASH

Abstract

This investigation deals with the effect of flyash as partial replacement of cement in structural concretes of equal consistency in general and high strength concrete in particular. Four concrete mixes (identified as MI,MII, Mill and MIV) with compressive strength in the range of 182 to U55 kg/cm and having a maximum cement replacement of 25 per cent were studied. Cement was replaced by equal weight of cement sized particles of flyash. The properties studied were compressive strength, elasticity and dynamic properties for all the mixes. In addition, a detailed experimental investigation on uniaxial compressive creep was carried out for high strength plain and flyash concretes of equal strength at stress-strength ratios of 0.20, 0.55 and 0.50 respectively. Strength, elasticity and dynamic properties were studied over a period of one year while creep studies were confined to a maximum obser vation period of 180 days. Creep recovery and the effect of sustained load on compressive strength were also studied. The tests for various properties were conducted on specimens subjected to controlled environment having a temperature of (27+ 2)°C and relative humidity(90+5) per cent. -XIXThe effect of substituting flyash on 7-day compre ssive strength of all concrete mixes in general was a decre ase varying according to the richness of mix. and its flyash content. At the age of 28 days, approximately equal strength was obtained for the plain and flyash concrete mixes with cement replacement upto 15 per cent in mix MI, upto 10 per cent in Mil and upto 5 per cent each in Mill and MIV respectively. In MI and Mil flyash substitution resulted in a decrease in the vrater to cement plus flyash ratio vrhile a small increase vras found at flyash contents greater than optimum in Mill and MIV. At the age of one year several mixes vrith flyash had reached and some had even exceeded the compressive strength attained by corres ponding mixes without flyash. The coefficient of variation in compressive strength tests was generally lower for flyash concretes. Air curing of flyash concrete after initial wet curing of 28 days vras found to result in reduced development of compressive strength vrith age. Sustained load was found to result in an increase in the compressive strength. Elasticity was found to be slightly higher in case of mixes with flyash for the same strength. It was found to be highly correlated linearly with square root of the compressive strength. -XX* Regression analysis of the test data showed that most of the properties studied during the investigation were ' time variant which could be conveniently related in the form of afunction, f.a +bloget. Very high correlation was often observed while fitting these semi-log lines on the observed data. Mathematical functions in the form of hyperbolic and power expressions also fitted well. Creep time curves for the plain high strength concrete mixes and their corresponding mixes with flyash were quite similar. Creep was found to be approximately related linearly to stress-strength ratio. The creep of flyash concrete was observed to be always lower than that of the reference concrete irrespective of the flyash content. About 75 per cent of creep was found to take place during the first 28 days of loading. Creep recovery was significantly smaller at higher stress-strength ratios and it vras found to increase slightly with increasing flyash content . The nature of creep in flyash concrete had been explained in the light of available information on' the structure of flyash concrete. The effect of age of loading, changes in mix. constituents and different rates of strength gain were identified as the chief factors for reduced creep of flyash concrete. -XXIThe dynamic modulus of elasticity was found to be nearly same fon equal strength. The ratio of the static to dynamic moduli was found to he higher for high strength concretes and it also increaaed with age. The logarithmic decrement, .»., was not significantly affected with flyash content upto the optimum. •&. was found to be the highest at the early ages in saturated state and it dropped down to nearly 30 per cent of its 7-day value at the age of one year when differences in damping between the different concretes were small. The longitudinal pulse velocity was higher for flyash concrete than for plain concrete of equal strength and was found to decrease with the increase in path length. Hyperbolic, logarithmic and power type expressions were found to fit well for all the observed creep time curves and the logarithmic type expression by U.S.B.R. was found to give the best correlation in general, a' '• simple model on the lines of Linear Homogeneous Marhov process was found satisfactory for prediction of long-term values of creep parameter. The unit creep parameter in the U.S.B.R. expression was found to be well correlated with the logarithmic decrement in the saturated state. The creep mechanism of concrete with flyash was indicated to be quite similar to that of plain concrete.