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Contraction gradient induced microcracking in hardened cement paste

Drying induced cracking of concrete surfaces and repair layers is a common problem. A principal cause for this type of cracking is the moisture and resulting contraction gradient that develops in the cement paste matrix upon drying. This phenomenon has been experimentally quantified in unconfined hardened cement paste samples using a fluorescent resin impregnation technique. The effects of sample thickness and drying method on surface crack density and crack penetration depth are reported and explained. Finite element modelling of moisture gradients indicate the important role of the film coefficient in desiccation cracking of unconfined samples. The critical thickness for samples to remain crack-free upon drying was in the range of 2-5 mm depending on drying method. In thicker samples a crack spacing doubling process was observed that is in agreement with theoretical predictions.