Basements

Published on August 18th, 2016 | by Henri

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Parts of concrete slab have sunk

Q. My home has a walkout basement with a concrete slab. A fairly large section of the concrete (10′ x18′) has sunk, but not evenly. That is, part of the concrete has hardly sunk at all whereas part has sunk as much as 1-1/2 to 2 inches.

I thought about having the concrete “mud jacked” but it’s already cracked. Besides, part of the section abuts a vertical concrete half wall on which some of the posts that are used to support the deck are resting. It would seem the best thing to do would be to top off the slab. But what kind of cement should I use? What preparation is needed before application? How do I get such a large area level? How thick or thin can the cement be? The home is 20 years old.

A. I am not clear about whether the slab is the basement slab or an outside slab for a patio. My confusion stems from the mention of a half wall supporting some of the deck posts.

Your problem is evidence that there has been settlement of the substrate under the part of the concrete slab that has settled because the substrate was not solid enough to support the weight of the concrete.

It is likely that, after 20 years, the settling has stopped. The best solution would be mud-jacking if you can find someone doing it and willing to tackle small jobs in your area.

Or have a concrete contractor or mason pour new cement over the sunken part (concrete would not work as it contains aggregate that can’t be used for such a thin coating.)

The existing crack can be patched at the same time.

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