Q.: On a recent project, the corners of a 5-inch concrete slab on grade curled upward 3/8 inch. Tapping with a hammer on the corners produced a hollow sound. The slab, which contained welded wire fabric 2 inches below the top surface, had been placed on a 4-inch layer of crushed stone over a vapor barrier. Why did the slab curl? Some engineers tell us to place the vapor barrier directly under the slab and others require stone or sand between the barrier and slab. Which is correct?

A.: A slab on grade deflects upward because of a variation in moisture content throughout the thickness. This is the result of more rapid drying from the top than the bottom. The drier top therefore shrinks faster than the bottom, causing curling.

When there is a vapor barrier directly under the concrete it creates a greater difference in moisture content and a greater difference shrinkage, and hence more curling, than otherwise. If a vapor barrier must be used it is considered better practice to place a gravel layer between the slab and vapor barrier. Since moisture moves from the slab in both directions, curling is minimized.

Curling can still occur even if a gravel layer is placed. On one job constructed with a gravel layer, the corners were reported to have deflected upward 1/2 inch. But there were two problems. First, cold weather required the slab to be in an enclosed area which had to be heated to 75 degrees F for three days. The heaters accelerated the rate of evaporation from the top. Second, the gravel had been saturated by snow before it was placed above the vapor barrier. The combination of drying from the top and saturation below the slab caused an extreme difference in moisture content from top to bottom, a prime condition for curling.

Anything that will help reduce the variation in slab moisture content will reduce curling. ACI 302.1R-80, "Guide for Concrete Floor and Slab Construction," provides a complete listing of measures that can be taken to minimize curling. This useful publication of 46 pages is available from the American Concrete Institute, Box 19150, Detroit, Michigan 48219. The price is $25.25 ($18.50 to members of the Institute).