Q.: Last year I had a job converting a garage into a living space. The garage is below grade on one side, and the concrete block walls were carefully engineered, water-proofed, and French-drained in an area with rocky, well-drained soil. The slab was designed to be 4 inches thick, but the contractor poured it approximately 18 inches thick to overcome a surveying error. The slab was poured over sand and waterproofing membrane in November 2000. The following July, at the end of construction, the contractor had the garage slab sealed with an epoxy coating to cover the construction blemishes and to make the floor look finished. One month later, long before any rains fell here, the epoxy finish started to bubble with pockets of water. By December, there were about 30 water-filled bubbles, each 2 to 3 inches in diameter, with a tablespoon or more of water in each. When the bubbles are cut open, there don't appear to be cracks in the concrete or any movement of water under the epoxy coating. Most of the bubbles appeared under rubber floor mats that were placed on a portion of the garage floor. There is no moisture apparent on the walls, even after some heavy fall rains. I am guessing that the slab had not fully cured when the epoxy was installed and that moisture is still leaving the concrete. Does that sound right?
A.: Obviously there is water coming out of the concrete. Assuming there are no leaks in your underslab vapor barrier, the water is probably excess water that was in the original mix to make it workable. An 18-inch-thick slab will take a considerable amount of time to completely dry out. The rule of thumb is 1 month of drying time for each inch of slab thickness to get to a water vapor emission rate that's acceptable for most floor coverings. Also, because of the different concentrations of calcium salts in a slab from one spot to the next, it's possible for semipermeable membrane effects to generate high water pressure in localized areas. One man told us that he stuck a knife into a water blister on an epoxy-coated surface similar to what you describe, and water shot 8 feet into the air. Our guess is that this action is causing the blistering to occur in this situation.
Unfortunately an epoxy sealer is not very vapor-permeable and probably trapped the water vapor, resulting in the bubbles. This was made worse where there were rubber mats on the surface. A siloxane or acrylic sealer would have allowed the concrete to continue to breathe. On the other hand, an epoxy sealer is a good idea when a slab is intended to have a moisture-sensitive adhesive applied, as for carpet or linoleum.
In terms of a "fix", time will have to take its course. Wait until no new bubbles have appeared for some time; then cut out the delaminated sections and reapply a sealer. The concrete should then be dry enough for the coating to bond and stay bonded, but you might consider an acrylic or silane sealer, which has good water vapor transmission characteristics. You should be able to get a good smooth floor for covering, but it won't be pretty the repaired spots will always be visible.