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We are patching an old concrete slab and we are having a problem with the glue or concrete seeping through small cracks onto the floor below. Is there something I can seal it with before we pour?

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So this is a suspended slab? Pictures would be helpful. With concrete, a stiffer mix may be the answer, or at least part of the answer. Depending on the cracks you may also need to form the underside of the slab (ie, the ceiling of where it drips on the floor.) –  Ecnerwal Jun 12 at 21:13
    
How about foam, or foam backer rods? –  getterdun Jun 13 at 3:33
    
I think (from my own house building experience) @ecnerwal's suggestion is correct. By using a stiffer mix (lower water amount in the mix) there would be less possibility for the concrete to escape. I have seen many people using, instead of polyurethane foam which may be expensive, patches made of tin which were blocking the gaps. The problem was that the tin was then a part of the construction. To hide it you had to apply plaster. –  Konstantinos Chertouras Jun 13 at 13:52

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