  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.
