Size and boundary effects on desiccation cracking in hardened cement paste

The density of cracks or size of fragments formed in hardened cement paste upon first drying is affected by specimen size as measured with a crack-impregnation technique in free shrinking specimens with a thickness of 4 cm. Fragment size on the drying surface increased with distance away from the specimen corner, resulting in smaller average surface crack densities in larger specimens. Size effect on three- dimensional crack density, that was measured from sections through the impregnated specimens, was weaker. The size effect is explained by higher residual thermal stresses in larger specimens due to the cement hydration process. For comparison a desiccation crack pattern in a 5-mm-thick cement paste layer on a marble substrate was studied. Residual thermal stresses in this specimen were probably low and a uniform crack-pattern with a Gaussian-like fragment size distribution formed. © Springer Science+Business Media B.V. 2008.