Q: What would cause a well-applied, good-quality clear sealer to peel off a concrete slab in thin sheets? Our crew applied the sealer after thoroughly cleaning the slab, following all of the manufacturer's recommendations. The slab is part of an interior loading dock with a hard-troweled finish. Concrete was placed directly on a vapor barrier with no granular layer on top of the vapor barrier. There is a sand fill beneath the vapor barrier. The slab was about a month old when the sealer was applied. This is all the more puzzling because we applied the same sealer, in the same way, to a similar slab about 30 miles away and had no problems with it. Both slabs were placed by the same subcontractor, with the same mix specified; however, the concrete came from two different suppliers. The same crew (not the slab subcontractor's) applied the sealer to both slabs at the same concrete age about one month and neither slab had any curing compound applied. The site for the problem slab was poorly drained, and there were numerous delays during construction due to moisture conditions. The second slab, where the sealer is successful, is outdoors and has a rougher finish. This site was well-drained, and the slab was placed a little later in the season when rain was not as much of a problem.

We've heard of a similar problem occurring when nonbreathing polymer coatings are applied to concrete slabs. Blisters appear, caused by movement of water vapor to the concrete-slab surface (see Concrete Construction, June 1996

, pp. 480-486). However, we've never heard of this happening with a sealer. Have any of our readers experienced this problem?

Reader Response:

The contractor does not state whether the sealer he applied was water- or solvent-based, but I assume that it was solvent-based. Although conditions such as surface preparation, air temperature, age of the concrete, and surface finish are critical to the successful application of a concrete sealer, a condition often overlooked is the dew point at the time of application. The dew point is the temperature at which condensation of water vapor occurs. On a typical humid summer day in the Midwest, it is not uncommon for the dew point to be only a few degrees lower than the air temperature. Concrete substrates enclosed by new buildings are usually shaded and receive little air movement. As a result, substrate temperatures often are near or below the dew point. Usually, the first coat of sealer will go down with no delamination problems, since it can adhere to a relatively rough or porous substrate, even if the substrate is moderately moist. As the first coat dries, solvent from the sealer leaves the film and cools the concrete substrate a few degrees, probably just enough to lower the substrate temperature below the dew point. You may have experienced this cooling effect if you accidentally spilled on your skin acetone or any solvent with a high speed of evaporation. After the solvent has evaporated from the sealer, water vapor in the air will begin to condense on the surface of the first coat. Also, the first coat of sealer has penetrated the surface pores of the concrete, dramatically reducing the substrate's porosity. This decrease in porosity and the presence of condensed water on the surface interfere with the adhesion of subsequent coats of sealer. As we all know, water and oil (or solvent-based sealers) don't mix. The second coat of sealer will form on top of the first coat, with the condensed water sandwiched, or trapped, between the two. Since the second coat did not adhere to the first coat, it will delaminate in thin sheets, as the contractor described. Cyler F. Hayes

Dayton Superior Corp., Chemical Operations

Oregon, Ill. I have found this to be a common problem, usually caused by troweling the slab surface too hard, thereby bringing excessive laitance to the surface. The sealer adheres only to the laitance and soon flakes off. I have also encountered this problem in a slab containing excess moisture. The slab was subjected to freezing conditions, and moisture that rose to the top of the slab condensed and froze, lifting off the sealer. However, the problem wasn't noticed until long after the sealer was applied.Michael E. Shanok

Forensic Engineering, PC

Hamden, Conn.