Curling is the distortion of a slab into a curved shape by upward or downward bending of the edges due to a large differential rate of drying between the top and bottom of the slab. This distortion can lift the edges of the slab from the base leaving an unsupported edge or corner.

Common Causes
• It is common when thin slabs are poured on top of older concrete. Sometimes, curling is evident at an early age. In other cases, slabs may curl over an extended period. Shrinkage or contraction of the top relative to the bottom causes upward curling of the edges of a slab. Changes in slab dimensions, which lead to curling, are most often related to moisture and temperature gradients in the slab. The slab edges curl. Causes include:
• Immediate curling of the slab is most likely related to poor curing and rapid surface drying. Some admixtures will tend to increase shrinkage leading to curling.
• Bleeding and poor curing both tend to produce surface concrete with higher drying shrinkage potential than the concrete in the bottom of the slab.
• Bleeding is increased in slabs on vapor barriers.
• Thin slabs and long joint spacing tend to increase curling.
• Slabs poured outside have an increase in surface temperature from the sun, which can cause a downward warping of the edges.

Prevention
The following practices will help to minimize the potential of curling:

• Proper curing is extremely critical-use a burleen water cure for 2 weeks. Allow the surface to dry for ½ day, then apply a quality surface sealer.
• Use low -slump concrete (not over 4 inch.) Avoid adding retempering water, particularly in hot weather.
• Avoid excessive bleeding-use damp, but absorptive, subgrade.
• Avoid using polyethylene vapor barriers unless covered with at least 2 inches of damp compacted sand.
• Avoid higher than necessary cement content. Use admixtures to reduce the heat of hydration.
• Cure heavier on the edges and areas where curling tends to be a problem.
•  Protect the slab by covering with waterproof paper for 7 days after finishing.
• Use joint spacing in feet equal to two times the slab thickness in inches. Maximum size aggregate should be less than ¾ inch.
• Use a thicker slab.
• The use of properly designed and placed slab reinforcement may help reduce curling.
•   Reduce the as-mixed temperature of the concrete in hot weather.
• Use water-reducing admixtures and / or a high range water-reducer admixture. Avoid admixtures that increase drying shrinkage.
• Avoid aggregates known to have high shrinkage potential.
• Avoid calcium chloride.