Q.: We're the general contractor for a commercial building in which the concrete floor will receive a moisture-sensitive covering. The floor-covering manufacturer requires a maximum water-vapor emission rate of 3 pounds/1,000 square feet/24 hours. Before applying the floor covering, we plan to wait until the emission rate reaches the 3-pound limit, but we've heard that the concrete surface can absorb moisture from the flooring adhesive.

We asked the concrete contractor and the flooring installer if they know how much moisture might be absorbed, but neither one was sure. Mow much moisture is likely to be absorbed, and is this value enough to affect flooring performance?

A.: We performed pull-off tests to evaluate the effect of water-vapor emissions on floor-covering adhesive strength (search the Concrete Construction article archive for "Effect of Water-Vapor Emissions of Floor-Covering Adhesives," January 1999). During the testing, we considered the same issue, so we performed some laboratory drying tests to determine the amount of water or solvent lost due to evaporation or absorption by the concrete substrate.

We used a tile trowel to place 80 to 100 square inches of tile adhesive (about a 9-inch-square tile size) on a hard 1/4-inch-thick pre-weighed plastic plate. We weighed the plates right after placing the adhesive, after the manufacturers' stated open time, and at various times after that until a constant weight resulted. By measuring differences between starting weight and subsequent weighings, we could then determine how much moisture or solvent was lost with time. Because the plastic plate was nonabsorptive, all of the weight loss could be attributed to evaporation. However, on an absorptive surface, some of the solvent or water would be expected to enter the substrate.

Of the eight tile adhesives tested, average weight loss at the manufacturers' stated open time was about 10% of the initial weight. This water loss represents about 15 to 20 pounds of moisture per 1,000 square feet of floor area. Ninety percent of the total moisture or solvent loss occurred within 24 hours after initial adhesive placement. The final moisture or solvent loss averaged about 48% for all samples tested, although one sample lost 84% of its initial weight. The average total moisture loss from all adhesives tested was about 120 pounds per 1,000 square feet of floor area.

We can estimate the total amount of moisture or solvent in the adhesive that's lost under ambient conditions. And we can assume that most of the moisture or solvent in the flooring adhesive is available for absorption by the floor. However, we don't know how much the absorption would affect adhesive performance, or if this absorption possibility was considered when the 3-pound limit was set. Perhaps other readers can share information on this subject.