Submicrocracking in Cement Paste and Mortar

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Title: Submicrocracking in Cement Paste and Mortar

Author(s): Emmanuel K. Attiogbe and David Darwin

Publication: Materials Journal

Volume: 84

Issue: 6

Appears on pages(s): 491-500

Keywords: cement pastes; compression; concretes; cracking (fracturing); electron microscopes; measurement; microcracking; microstructure; strains; mortars (material); stresses; stress-strain diagram; Materials Research

Date: 11/1/1987

Abstract:
Submicroscopic cracking of cement paste and mortar under uniaxial compression is measured and compared to applied strain. Cement paste specimens with water-cement ratios of 0.7, 0.5, and 0.3 and mortar specimens with a water-cement ratio of 0.5 were tested at ages ranging from 27 to 29 days. After loading, slices of material were removed for study at a magnification of 1250 x in a scanning electron microscope. Cracking on transverse and longitudinal surfaces was measured, and three-dimensional crack distributions were obtained from the surface crack data. The research demonstrates that surface crack density in cement paste and mortar is an order of magnitude greater than the density of bond and mortar microcracks in concrete at the same value of compressive strain. The mean size of submicrocracks increases with increasing compressive strain, while the number of cracks per unit volume decreases. Differences in water-cement ratio have a small but measurable effect on the crack distributions. Crack density is lower initially in mortar than in cement paste but increases more rapidly as compressive strain increases.