¿What causes concrete slabs to curl/warp, and how can this mechanism be minimized?

It is important to begin this discussion by defining the difference between slab curling and slab warping.Concrete slabs curl due to the effects of temperature variation between opposing surfaces of the slab. Concrete slabs may curl when the top surface of the slab is cooler than the bottom surface. The cool surface shortens due to thermal shrinkage, which may deform the perimeter of the slab section upward as the temperature of the bottom surface of the slab stays at a higher temperature. On the other hand, the top surface will expand when it is heated (such as with intense sunlight), which may relax curling or cause reverse curling. Reverse curling is when the concrete slab deforms downward instead of upward.Concrete slabs warp due the effects of moisture variation between opposing surfaces of the slab. As the top surface of the slab dries, the volume change causes the surface to shorten while the bottom of the slab stays moist. Because the bottom surface stays moist, there is little or no shortening due to the effects of drying. Again as the top surface of the slab shortens and the bottom surface does not the edges lift.Slab on grade applications are typically too thin to be significantly affected by temperature variations that cause curling, however variations in moisture commonly change the surface profile of slab-on-grade installations leading to warping. The loss of slab support at the edges of the slab panels affected by warping may lead to random cracking and slab performance issues.Slabs designed with appropriate thickness using quality materials, good placement practices, properly jointed and cured will achieve sufficient strength to minimize slab warping. The concrete mixtures should use the largest practical (locally available) aggregate size, smooth aggregate gradations with a wide variety of particle sizes to maximize aggregate packing and minimize aggregate surface area, and low water contents to minimize the shrinkage potential of the concrete mixture to be used for slab construction.

Avoid the following concrete placement and material variables where possible:

1. High initial concrete temperatures.
2. High slump mixtures achieved by the addition of water—water reducers preferred.
3. Small coarse aggregate sizes.
4. Long hauls and long placement delays.
5. Use of aggregates containing excessive contaminates (dirty).
6. High shrinkage aggregate.
7. Use of chemical admixtures known to increase shrinkage.

For applications where excessive warping will be unacceptable it may be appropriate to design the slab as a post-tensioned element, use shrinkage compensating concrete mixtures, or provide a reinforcing steel mat (approximately 1% of the cross section) 2 in. below the slab surface. While these options will have a considerable impact on the economy of the slab placement, they have a proven record of minimizing slab warping.