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Moisture Causes Microtopping Failure

Question:
A new slab was doweled and pinned to an existing concrete foundation slab to make the footprint larger. The existing slab is 50 years old with an intact vapor barrier. A concrete stain contractor came in and put down an acrylic-modified cement microtopping over the entire new slab, then stained and sealed it. There was no noticeable moisture problem during overlay installation. Now the new area is showing signs of moisture emission, which is causing the microtopping to bubble and fail, as you can see in the photo. The owner of the slab is very frustrated. There also seems to be efflorescence going on in his garage on the new stem walls. Obviously there's a moisture problem underneath the slab, but it only wicks up at the dowelled areas. Was the microtopping the correct product for the application? Do you have any suggestions to repair this problem?

Answer:
I agree that moisture vapor transfer (MVT) is the probable cause of the delamination. The dark area surrounding the delaminated area as well as the white efflorescence on the concrete in those areas indicates moisture. Efflorescence can't occur without water, so that's a "smoking gun" that some water is moving through the slab. The combination of the pressure created by the moisture drive, along with the white salt residue (efflorescence), which acts as a bond breaker, is enough to lift or delaminate most coatings and overlays. Moisture will tend to concentrate and migrate through any joint, crack or sawcut, so the fact that the efflorescence and delamination is worse around the joint does not surprise me. The overlay is a high-strength acrylic polymer mixed with cement and sand. It creates a very durable and rigid thin topping that resists scratches and abrasion. However, it does not have good flexural strength, so when it is subject to pressure and force, it will crack and delaminate before it "bends," so to speak.

Repair of the existing floor involves removing the overlay in the areas where moisture drive is present. The base concrete should then be treated with a nonbreathable, waterproof coating with a crack-suppression membrane embedded into it. This will act as a barrier to stop moisture movement and provide a stable substrate for the microtopping. The microtopping system can then be reapplied, stained and sealed.