by Scott Miller
Q. I had an overlay installed inside a house we are doing a remodel on. The overlay is a self leveling product. the concrete floor was cleaned very well, dried and primer put down. The first bucket of overlay was just a bit too thick so they added a bit more water to the mix. I noticed many air bubbles in the overlay. The bubbles started popping pretty soon after the mix was put down. I watched as they continued and some buckets had more bubbles than others. After about an hour I went back to where they started and I saw many, many holes in the overlay where the air bubbles popped after the overlay had self leveled. What can be done to fix this? Will they have to do another overlay? The temperature was about 70 degrees, low humidity, a/c was off and no windows open, just doors and no wind blowing. They had 1 guy who's job was to maintain consistency of the product by mix time and adding a wee bit of water if needed.
A. Air bubbles in concrete overlays are a common problem. The air bubbles can appear for a couple of reasons.
The first is over mixing the product, using the wrong mixing paddle, and moving the mixing paddle in and out of the material while mixing it.
Doing this traps air in the overlay material and once the material is laid down the air escapes through the surface causing tiny bubbles.
The second reason air bubbles form is when the overlay material is laid down on the sub-base, the mix water in the overlay material settles into the tiny voids in the sub-base. The water replaces the air in those voids, pushing it to the surface causing tiny air bubbles.
If the sub-base material is really porous, then one coat of primer may not be sufficient. It may be necessary to saturate the sub-base material with the primer and mop up any that puddles.
If the self-leveling overlay is the finished floor then you may need to install a micro-topping material over it to hide the pin holes left by the air bubbles.
If you are laying another floor over the concrete overlay then it should be good enough for that.
We use a lot of Mapai products for overlays. You can check out their website for more information or call them for technical assistance.
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