Q. We just had our first experience with slab curling. What causes it and what can be done to avoid it?

A. When the edges or corners of a slab curl up, often at construction joints, it's caused by concrete at the top of the slab shrinking or contracting faster than concrete at the bottom. Drying shrinkage is the usual cause. Water leaves the top surface of the slab faster than it leaves the bottom, creating shrinkage forces that can raise slab edges or corners.

All concrete shrinks to some extent, so you can't eliminate drying shrinkage. But you can take the following steps to reduce it and thus avoid excessive curling:

  • Order concrete with the lowest possible water content and the highest possible coarse-aggregate content. This usually requires you to use low-slump concrete and the appropriate methods to place it.
  • Avoid adding excess water at the jobsite.
  • Avoid using concrete containing aggregates that increase shrinkage. In some areas of the country, large differences in drying shrinkage have been traced to the local aggregate used.
  • Order concrete with the largest possible maximum-size coarse aggregate.
  • Avoid ordering concrete made with unwashed aggregate because dust from the aggregate increases water demand.
  • Avoid ordering concrete containing admixtures that may increase shrinkage, such as calcium chloride and some water reducers.

Another method for reducing curling caused by drying shrinkage is placing the slab on a granular layer that absorbs water from the bottom of the slab, thus equalizing drying of the slab top and bottom. This usually isn't recommended if the slab will receive a moisture-sensitive floor covering because water in the granular layer may later exit the slab and cause the flooring adhesive to fail.

Reinforcing steel placed near the top surface of the slab can help restrain curling. Placing #4 bars on 18-inch centers both ways and chairing the bars to keep them within an inch of the surface has been suggested (see "Controlling Curling and Cracking in Floors to Receive Coverings," Concrete Construction, July 1998, pp. 603-605). Load-transfer devices at construction or contraction joints also help restrain curling.

Finally, you can use large-bay construction methods that minimize construction joints. If strip construction is unavoidable in warehouses, you may be able to locate construction joints where racks will cover most of them, thus reducing problems even if the joints do curl.