Curling occurs when the top surface of a slab dries faster than the bottom surface. This creates a stress differential within the slab as the top surface reduces in size faster than the bottom. The resulting effect is that the slab curls upwards, this is most pronounced at the edges and especially the corners.
Curl can lead to problems at interfaces with other elements of or around the slab, any internal/external doorways which do not have any load transfer capability are likely to experience curl which will then expose the joint arris to traffic and impact damage. It is therefore vital in design that this phenomenon is considered to reduce possible future maintenance issues.