Question: Although I've been lucky so far and have not had to deal with a failure of flooring materials applied over one of my slabs, it scares me to death thinking about it. What can I do to keep this problem from occurring?
Answer: Definitive information on this problem is finally beginning to emerge, so you might be able to avoid getting stuck in a lawsuit over delaminating flooring. The American Concrete Institute (ACI) has released ACI 302.2R-06, Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials. This new guide covers basic moisture issues, moisture testing, pH testing, how to dry out concrete, use of vapor retarders, and the various floor coverings used on top of concrete floors. There are also two sets of recommendations—one from floor covering an adhesive manufactures and one from the concrete industry. The industry recommendations include:
- Testing: Use qualified testing technicians and ASTM standard test methods.
- Vapor barriers: Place slabs that will receive floor coverings directly on top of the vapor barrier and consider using a low permeance barrier (0.01 perm rating).
- Concrete materials: Admixtures are fine and lightweight concrete also is acceptable, although it dries out more slowly.
- Concrete properties: Since drying time is related to w/cm, using a lower ratio (0.40 to 0.45) helps with drying but may increase shrinkage and curling—0.50 may be better.
- Surface finish: Different floor coverings require different finishes, although most seem to do well with a troweled surface and a light broom finish. For flatness, an over-all FF greater than 35 should not be specified.
- Curing: Don't use water to cure and don't use cure-and-seal materials. Cure by covering with waterproof paper or plastic sheets for three to seven days.
- Surface preparation: This must be done with care, since removing the carbonated surface layer can expose highly alkaline concrete.
These are gross simplifications of the recommendations contained in the guide. We encourage you to purchase the guide from ACI (www .concrete.org) or from the American Society of Concrete Contractors (www.ascconline.org).