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Concrete – Drying time

Joe_Adams | Posted in General Questions on

Hi everyone!

I am about to install some tiles over a concrete slab on grade. I will be using a polymer modified thin set to apply the tiles directly to the concrete.

Because I am using a polymer modified, I want to make sure the concrete is dry enough for the thin set to work.  Otherwise, the polymer modified thin set will not achieve a good adhesion because it needs to dry (and will not dry upwards because of the large ceramic tiles). (I am concern about this issue because of this reading https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-113-stoned).

So, my question is, how much time must I wait after poring until I install the tiles? I will be using a wet concrete cure.

Thank you in advance!

Joe

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Replies

  1. DCContrarian | | #1

    I'm not a tile expert but I'll offer an opinion, although it's probably best to ask in a tile forum.

    I read the Lstiburek article. He talks about very specific conditions, where modified thinset is installed between two non-permeable surfaces and then subjected to repeated wetting, typically from rainwater. In that scenario the polymers in the modified thinset never solidify, because they need to be wetted and dried to solidify. In liquid form they are water-soluble, and with the repeated wetting they wash away, which weakens the mortar to the point of failure.

    My opinion is you have nothing to worry about unless a) you have an impermeable layer beneath the mortar like RedGard or other isolating membrane; and b) the tile is in a wet spot. Concrete doesn't dry as it hardens, it cures. The water that was added to the mix doesn't evaporate, it physically reacts with the cement and is chemically bound in the finished concrete. As Joe notes in the article, concrete will cure underwater, it needs no exposure to air to harden. Cured concrete is typically pretty thirsty. The amount of water that needs to come out of the thinset is pretty minimal. A 50-lb bag of thinset covers about 50 square feet depending on the tile, and it takes 5 quarts of water to mix a bag. So that's 3.2 oz of water per square foot. But most of that is going into the cement in the mix, only a fraction needs to come out of the polymers. Unless the concrete is obviously wet it's going to have no problem absorbing that.

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