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Question DetailsAsked on 1/12/2014

Concrete cracking in a basement is that normal?

We are looking at buying a house that was recently built meaning in the last year or two.
Not sure of build date. There are long cracks in the basement floor. The builder says this is normal
And does not affect the quality of the house. Is this true or should we look elsewhere for a new house?
We also want to acid wash the basement floor but afraid th he cracks 's would not look good with the acid
Wash. The builder says it would make it look even better.

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1 Answer

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Depending on the size of the concrete slab many will crack do to shrinking. The concrete mix has alot to do with it as well as just the nature of the product. If the mason added too much water or the choice of mix can add to the chances of it. If they used a lower PSI (compressive strength) concrete it might also add to the likelyhood of cracking. If it is a tract home (one of many the builder built on same street) they may have opted for a cheaper cement. I would use nothing lower than 3000 PSI concrete myself but usually 3500 PSI. I do not know why you want to acid wash the floor but it should not affect the strength of the floor but I wonder if it is due to excessive chaulking. If it to get rid of the chaulking I would wonder about the concrete. Your basement floor is not structural so it will not effect the house as far as that goes but I would look carefully at the foundation walls for cracks. Residential construcion is not checked for things like the proper mix of concrete like commercial work is. On many commercial jobs the concrete delivery slips are saved as proof and samples are sent to labs for testing.


Don

Answered 5 years ago by ContractorDon

1
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Slab cracks can be epoxy or patching grout patched (and let cure) before the acid washing, so that in itself is not a problem, though I have a bit of a problem believing cracks in a floor will look even better ? If this is being bought from the buyildere, I would make him patch the cracks before the sale closes.

Now - as to the cracks - if basically one crack each way near the center, running fairly straight (with minor 1 inch or so deviations as it works around hard aggregate), not a big issue as long as: the floor does not show signs of settling or becoming swaybacked, not displacing vertically across the crack, no water or sand or mud coming up through the crack, not being open more than about 1/16" (ballpoint pen tip roughly), or no multiple cracks in same direction or wandering all over. In a normal slab the cracks will parallel the walls - typically one each way near the center, or may go down or across center about half length of slab, then diagonally off at about 45 degrees to each corner - depends on what is settling. IF there is a floor drain, then usually will run diagonally from drain to near (but not necessarily directly to) the four corners - with or without a short straight section parallel to the walls near the drain.

If there are scribed control joints like this -

http://inspectapedia.com/structure/Co...

(usually not done in basement or garage slab), then cracks meeting same criteria as above that stay within the groove of the expansion joint are not an issue.

Any slab not meeting the above "as long as none of these" list would be suspect - not only because it is cracking badly or in a way that indicate problems, but also because if the basement slab has problems, how about the rest of the foundation concrete ?

Also, as Don said, if chalky on the surface, "crazed" with lots of microcracks, does not feel hard if scratched with steel or a rock, then could be a weak mix, which could then break up more readily with age. Ditto if you can see a rectangular (usually 4, 6 or 8 inch spacing) pattern in the concrete, indicating either too shallow a rebar grid or mesh, or a weak concrete cracking at the reinforcing.

Answered 5 years ago by LCD




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