Q.: An epoxy-terrazzo coating on a concrete floor we placed is beginning to blister in two locations. The concrete was placed on a 4-inch-thick granular subbase over a 6-mil vapor barrier placed directly on the subgrade. Do you know of anyone who has tried using a vacuum pump to create a vacuum in the granular fill directly beneath the slab? Would this reverse the migration of the moisture in the slab? The largest area we have to deal with is about 1600 square feet, and I have no idea if the vacuum-pump idea is feasible. The terrazzo may be deformed by the moisture already. We drilled a 3/16-inch-diameter hole in the middle of a blister, and a small geyser of water erupted.

A.: We've never heard of using a vacuum pump to reverse moisture migration through a slab. We heard of an attempt to circulate air through the granular base with a blower in order to correct a similar problem, but it didn't work. We think it's unlikely that the vacuum-pump approach will provide a permanent solution to the blistering problem.