Slab edges that curl upward and slab centers that are slightly depressed from their original elevation are common in suspended slabs and slabs on grade. This curling is caused by differential drying shrinkage and by moisture or temperature differences across the thickness of the slab.

Curling increases when:

  • The slab is made from high-shrinkage concrete.
  • The slab surface is exposed to low humidity or the subgrade under the slab has a high moisture content or both.
  • The concrete has a high modulus of elasticity (that is, the slab is relatively flexible)

Reducing curling of a slab starts with reducing shrinkage. The problem is often overemphasis of compressive strength and neglect of shrinkage.

Cracking and curling of slabs may be inevitable, but the severity can be controlled with a few simple measures.