Submicroscopic Deformation in Cement Paste and Mortar at High Load Rates

Abstract

Submicroscopic cracking and strain-rate response of cement paste and mortar under uniaxial 11 compression were measured and correlated with applied strain. Cement paste specimens with water-cement ratios of 0.3, 0.4, 0.5, and 0.7 and mortar specimens with water-cement ratios of 0.3, 0.4, and 0.5 were subjected to monotonic load at strain rates ranging from 0.3 to 300,000 microstrain per second. Specimens were tested at ages ranging from 27 to 29 days. After I loading, slices of material were removed from selected specimens for study at magnifications • of 1250x and 2500x in a scanning electron microscope. Image analysis instrumentation was used in later stages of the study. Cracks on transverse and longitudinal surfaces were measured, I' and three-dimensional crack distributions were obtained from the crack data. The portion of _the nonlinear material response caused by the cracks was estimated using a self-consistent material model. The strength and stiffness of cement paste and mortar increase with increasing strain rate. The relative effects of strain rate are largely independent of water-cement ration and sand content. Submicrocracking accounts for a significant portion of the nonlinear response at all levels of compressive strain, and the role of submicrocracking in that behavior appears to increase with increasing sand content and strain rate and decreasing water-cement ratio.