Investigation of the Age-Dependent Constitutive Relations of Mortar

The stress-strain relation of cement binder is age-dependent and so is the behavior of mortar. In this study, we conducted a concurrent experimental and theoretical investigation into the effect of material age on the properties of both cement binder and mortar. First the stress-strain curves of the binder were measured over a wide range of aging time from 7 days to 18 months. Two types of binder-one with the cement paste alone and the other with the fly ash/cement combination-were chosen in the tests. The test results were simulated with a modified, age-dependent Burgers rheological model. Then measured were the stress-strain curves of the mortar that contains the same types of cement at three levels of aggregate volume concentrations:C ₁= 29, 38, and 49%. A micromechanics-based composite model making use of the secant-moduli was then introduced to predict the age-dependent behavior of mortar at various aging times and aggregate concentrations. Direct comparison between the measured data and the composite predictions is found to be in close agreement. It suggests that the proposed micromechanics model could be a viable approach to the estimate of age-dependent constitutive relations of mortar.