64 Damage to concrete structures
The most typical case in which shrinkage cracks occur is concrete slabs
prior to final finishing and setting as illustrated in Figure 3.16. Due to moisture exchange with the environment, heavily accelerated by strong wind conditions, the exposed top surface of the concrete slab loses water. This drying
process leads to a volume reduction. In totally free conditions, the plastic
concrete could show a shortening even up to a few millimetres, which is an
order of magnitude higher than the drying shrinkage occurring later after
setting. This free plastic shrinkage cannot occur because it is restrained by
the underlying concrete, which is not drying or at least at a much slower rate.
As a result, plastic-shrinkage cracking can occur in the top zone of the concrete slab with crack openings which can go up to a few millimetres in severe
cases. Although quite often plastic-shrinkage cracks are rather superficial
Plastic-shrinkage cracks
Evaporation of water
Figure 3.16 Plastic-shrinkage cracking in concrete slabs.
Figure 3.15 Damage by animals walking in freshly cast concrete.