#MAPEITechTip: Why do concrete slabs curl?

Although there are many variables that cause a slab to curl and warp, it typically boils down to changes in moisture content and temperature differential above and below the slab. When the top of the concrete slab dries faster than the underside of the slab—add to the mix, hot, dry, windy exterior conditions, inability to properly cure and high-water to cement ratio—you have the potential for the slab to curl. What are some factors that can help limit or control curling? 
1.) Limit air movement across the surface of the concrete 
2.) Ensure the sub-grade is wet/moistened—reducing shrinkage. 
3.) Slab thickness—the thicker the slab the less potential to curl 
4.) Slab reinforcement, typically 1%, helps to restrain movement. 
5.) Reduce joint spacing—ie: not exceeding 24 times the slab thickness. 
For a more in depth discussion of slab placement, see ACI 302.1R-06 “Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials”

Why it is important to consider ‘slab curl’ when installing a Resilient floor covering?  Since during the ‘curing’ process the concrete slab is experiencing both ‘tensile’ and ‘compressive’ forces causing the slab to ‘lift’ and then ‘relax’.   During this curing cycle where the slab is shrinking due to the ‘water of convenience’ evaporating off of the slab surface—as it lifts and relaxes the result is surface cracking and creating elevated or uneven areas.  In order to meet slab flatness the contractor grinds the slab, once a moisture barrier (see ASTM F3010-13 & ASTM F710-17) is applied, the concrete slab equilibrates (when the pressure, temperature and moisture are in balance between the top and bottom of the slab), the slab relaxes further leaving the areas that were ground down lower then the rest of the slab.  The installer will be the first person notified if the slab has not reached equilibrium (no more slab movement = relaxed) and to make any necessary repairs to ensure an acceptable substrate for installation of the Resilient floor covering.