While concrete overlays offer a fabulous design solution, they commonly experience moisture problems. This occurs when water vapor emissions form between the underlying concrete slab and the impermeable layer that forms on top of the slab. When such vapor becomes trapped, it can cause a greater amount of pressure than the bond strength of the topping and slab, resulting in the delamination of the topping. A combination of white salt residue along with the pressure produced by moisture is enough to cause most overlays and coatings to lift or delaminate. Such concrete moisture problems will almost always develop when a concrete slab is lacking an under-slab vapor barrier. Even with vapor barriers, new slabs will need time to dry.
Testing for the Presence of Concrete Moisture
There can be a number of reasons why concrete might experience moisture problems, including it had not fully cured, there is a high water table, or the concrete was produced with a high water-to-cement ratio. When concrete moisture problems are suspected, regardless of the cause, it is important to perform a test to be certain. One option is to perform a tape-down test in which a small area is taped down and sealed overnight. If beads of condensation or sweat appear the next day, you know there is excess moisture. When this is the case, it is vital to reduce the moisture content prior to applying a concrete overlay.
One of the best ways to prevent concrete moisture problems in overlays is to treat the base concrete with a non-breathable waterproof coating. The coating should have a crack-suppression membrane embedded inside it to act as a barrier and prevent moisture movement while providing a stable substrate on which to apply the microtopping for the overlay.
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