Pouring concrete over concrete

Q. I am going to pour 2" of concrete over a 4" cracked slab of concrete behind my back door. If I put wire mesh down first will the concrete I pour crack with such a deep base already present?

A. If the crack in the bottom slab is still active, meaning the bottom slab still moves (expansion & contraction), it is highly likely the crack will mirror through the new 2" of concrete.

If the bottom slab is not moving, it shouldn't crack in the same spot, but possible it may crack somewhere else. If the slab is larger than 8' x 10', I would saw some contraction joints to help prevent it from cracking. If the slab is smaller than this you should be ok.

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Jul 24, 2012
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by: Engineer

If you are laying (Pouring) concrete over existing concrete there needs to be a bond.
When i say bond the concrete you are lying on top off needs to be a rough finish like scabbled.

You could drill holes into the existing concrete surface and insert steel pins and then the mesh on top. It is important you leave a cap of at least 30mm so the concrete can flow under the mesh.

When it comes to curing, wet hessian is to be placed on top of the concrete (not too early though) wait till you can get a steel float finish on it.

Comment: When we pour concrete floors over existing concrete floors and the new floor is less than 3 inches thick, we use a product called Weld Crete to bond the two floors together. It rolls or brushes on like paint, very easy to install & creates a bond between the old & new concrete floors.

If the new floor is over 3" thick we install a bond breaker between the two floors (like typar), this lets the two concrete floors move independently of each other resulting in less uncontrolled cracking.

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