Typically, upward curling of the edges of a slab is caused by shrinkage or concentration of the top relative to the bottom. When one surface of the slab changes size more than the other, the slab will warp at its edges in the direction of relative shortening. This curling is most noticeable at the sides and corners.
Change in slab dimensions which lead to curling are most often related to moisture and temperature gradients in the slab. One primary characteristic of concrete which affects curling is drying shrinkage. The most common occurrence of curling is when the top part of the slab dries and shrinks with respect to the bottom. The slab edges curl upward. Immediate curling of a slab is most likely related to poor curling and rapid surface drying; and anything that increases drying shrinkage will tend to increase curling.
In slabs, bleeding and poor curing both tend to produce potential than the concrete in the bottom of the slab. Bleeding is accentuated in slabs on polyethylene or topping mixtures
placed on concrete slabs; and shrinkage differences from top to bottom in these cases are larger than for slabs on an absorptive subgrade.
Thin slabs and long joint spacing tend to increase curling. For this reason, thin unbonded toppings need to have a fairly close joint spacing.
In industrial floors, close joint spacings may be undesirable because of the increased number of joints and increased joint maintenance problems. However, this must be balanced against he probability of intermediate random cracks and increased curling at the joints.
The other factor that can cause curling is temperature differences between the top and bottom of the slab. The top part of the slab exposed to the sun will expand relative to the cooler bottom causing a downward curling of the edges (Figure 1B). Alternatively, during a cold night when the top cools and contracts with respect to a warmer subgrade, the curling due to this temperature differential will add to the upward curling caused by moisture differentials.