Effect of Strain History and Mixture Proportions on Early-Age Cement Paste and Mortar Stress Relaxation

Abstract

The influence of sand particles, aging, and water–cement (w/c) ratio on the viscoelastic relaxation modulus of cement paste and mortars was measured at early ages (1–28 days). Furthermore, the relaxation modulus obtained approximating the strain state as constant or obtained considering the real strain history observed during testing were compared; significant error was encountered when a constant strain was assumed. In contrast, little error was introduced when aging during the relaxation experiments was neglected, which is likely due to the relatively short duration of the experiments and the nonconstant strain history that led to rapid stress decay. In contrast to conventional wisdom, lower w/c samples loaded at very early ages exhibited similar relaxation to higher w/c samples; however, at later ages, lower w/c materials exhibit lower relaxation rate. For samples loaded at later ages, the similarity between the shape of the relaxation curves for samples containing differing sand content and w/c suggests a method to simply quantify the effects by shifting the spring constants in a Kelvin–Voigt or Maxwell-type model.