Prediction of final concrete strength in hot climates

Abstract

The strength of concrete at long curing times, i.e. final strength, is adversely affected in hot climates. The final strength is known to decrease with increasing curing temperature even when the effects of other related factors, such as dessication due to high curing temperature, are precluded. Physical and chemical causes have been suggested for this phenomenon. Physical causes are believed to be due to the large differences in volumetric expansion of concrete constituents with temperature. According to this theory, the large increase in volume of air and water with high temperatures creates internal stresses in the curing concrete. If the tensile strength of concrete is not sufficient to withstand these internal stresses, porosity increases and microcracks are formed, resulting in the loss of final strength. Other researchers have suggested chemical causes for the loss of final strength of concrete cured at high temperatures, i. e. that the chemical composition of the hydrates is affected by the curing temperature, the microstructure of hydrates is partially changed by the effect of temperature, and the degree of hydration at long curing times is affected by curing temperature. This paper suggests that the temperature dependence of final strength, as well as other phenomena in concrete strength gain, can be explained adequately if the hydration of cement is assumed to follow the nucleation and diffusional growth processes. Then, the direction of final strength change with temperature can be successfully explained by the general concepts of chemical equilibrium. For practical purposes it is important to predict quantitatively the dependence of the final strength on temperature. Based on principles of thermodynamics a linear relationship between the natural logarithm of strength and the reciprocal of curing temperature is suggested. The available data for concrete strength and some results obtained for lime–fly ash mixtures have been used to verify the suggested relationship.