The plans for a multistory structure called for a 6-foot-thick base mat. A 4-inch-thick concrete slab was to be placed on a vapor retarder resting on the base mat in preparation for installing a glue-down, moisture-sensitive floor covering. In a value-engineering move, the engineer decided to eliminate the 4-inch-thick slab and finish the base mat so the glue-down flooring could be directly applied to the mat surface. A question then arose as to the need for, and location of, a vapor retarder.
ACI 302.2R-06 ("Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials") recommends sandwiching the vapor retarder between the base course and concrete slab when a moisture-sensitive flooring material will be installed. In this case, though, the slab is 6 feet thick. Should the vapor retarder be placed at the bottom of the slab?
Placing a concrete floor slab directly on top of an intact vapor retarder helps to ensure that moisture from the base course or subgrade is isolated from the concrete. Once the moisture content of the concrete stabilizes as a result of drying, it may increase at the top of the covered slab as moisture moves up from the bottom. But if it reaches an equilibrium moisture content the flooring adhesive can tolerate, the flooring should perform well as long as no more water enters at the bottom of the slab as seen in Figure 2a.
FIGURE 2a. A 4-inch-thick floor slab, before drying, has an equilibrium relative humidity (RH) of about 100 percent (red line). With the top surface exposed to 50 percent RH air, an RH gradient develops, with the bottom concrete still at close to 100 percent RH (yellow line). After the floor surface is covered, a new and acceptable equilibrium RH of perhaps 85 percent (green line) is reached. The drying time needed for this is usually a few months.
Locating the vapor retarder in direct contact with the bottom of the slab makes sense when the slab is inches thick. But when it is 72 inches thick, the reasoning falls apart. Free water in most of the slab will take longer to rise toward the top of the slab. But it will rise until an equilibrium condition is reached. Unfortunately, the equilibrium condition is likely to be a higher one than the flooring adhesive can tolerate without re-emulsifying and causing a flooring failure as seen in Figure 2b. It's not water from the subgrade or base that causes the problem but free water contained in the capillary pores of the concrete. Thus the vapor retarder serves no useful purpose because there is too much water in the 6-foot-thick mat and the drying time needed would unreasonably extend the construction schedule.
FIGURE 2b. A 72-inch thick slab, with the top surface exposed to 50 percent relative humidity and dried for the same amount of time as the 4-inch-thick slab would probably still be at an equilibrium relative humidity of 95 percent (green line) or more. This is too high to permit good performance of most flooring adhesives but drying to an acceptable level would take many months.
But leaving out the vapor retarder doesn't solve the problem. Now water must be prevented from contacting the adhesive at the interface between the concrete and the adhesive. That requires a topical sealer on top of the base mat, or some other way to mitigate the potential flooring failure. Such moisture mitigation systems exist but can be expensive. The question now becomes which costs less: The original design with the 4-inch-thick slab placed on a vapor retarder or the base mat coated with a moisture mitigation system? The original design is likely to be the better choice. So be careful in dealing with such value-engineering decisions.
Random floor cracks
Random cracks in slabs-on-ground are a constant source of problems for concrete contractors. Specifications may require that cracks with widths beyond a given value be repaired. They may even require that every floor panel with a random crack be removed and replaced. In some cases, crack-free floors may be mandatory, for instance in a food processing plant. But for many floors the occasional random crack does not affect performance.