Submicroscopic Cracking of Cement Paste and Mortar in Compression

Abstract

Submicroscopic cracking of cement paste and mortar under uniaxial compression is measured and correlated with applied strain and load history. Cement paste specimens with water-cement ratios of 0.7, 0.5 and 0.3 were subjected to monotonic, sustained or cyclic loading, while mortar specimens with a water-cement ratio of 0.5 were subjected to monotonic loading. One hundred and thirty ( 130) specimens were tested at ages ranging from 27 to 29 days, using a closed-loop servo-hydraulic testing machine. After loading, slices of material were removed for study at a magnification of 1 250x in a scanning electron microscope. Cracking on transverse and longitudinal surfaces was measured. Statistical and stereological models are developed to convert the surface crack distributions to three-dimensional distributions. A self-consistent model is developed tc estimate the elastic moduli of transversely isotropic cracked materials. These models are used to correlate submicrocracking with the reduction in stiffness and the shape of the stress-strain curve. The surface crack densities in cement paste and mortar are about ten times the density of bond and mortar microcracks in concrete at the same value of compressive strain. Submicrocracking accounts for a significant portion (20% to 90%) of the nonlinear respons of cement paste and mortar at all levels of applied compressive strain. As compressiVE strain increases, other mechanisms, such as large microcracks, macrocracks, and creep, play an increasingly greater role.