Q.: Will a vapor barrier accentuate the curling-cracking of a slab on grade? What corrective measures can be taken for a slab that has already curled?
A.: The use of vapor barriers under slabs is one of the conditions that is thought to contribute to curling. If a curling problem is anticipated the vapor barrier should be omitted and the slab placed directly on a granular subbase of crushed rock or gravel so that moisture can leave the bottom of the slab as well as the top. The subbase should be 5 to 12 inches or greater in thickness and designed not to retain water This is only one of a number of measures that can be taken to minimize curling.
If curling has already occurred, it may be possible to flatten the floor by ponding water on its surface until the surface has expanded sufficiently. After the slab has flattened it can be sawed into smaller panels to minimize future trouble with curling.
It may also be possible to prevent curling from reoccurring by applying an impermeable coating. An epoxy or polyurethane should probably be chosen because they not only have low permeability but they are wear-resistant.