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PicturesI have a new house under constructions and recently found many many cracks have developed on the concrete slab in the basement. The worst run along straight lines where the sections were poured. These don't have shrink seams in place, they are just cracking along a straight line. There are also some jagged/random cracks that extend off of these.

Cracks in slab

More cracks in slab

Here's the final pour for the slab; you can see it's a non reinforced non-structural slab.

Final pour of slab

My builder has asked the concrete contracting firm to take a look at it but said he things these are just shrinkage cracks and they will be sealed up before they are done.

I would appreciate opinions and learned assessment based on the pictures so I am prepared when the concrete company responds.

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  • cannot see pics. i've been a builder for 20 years and you are being feed a line of BS. There should not be any "shrinkage" cracks in a new poured floor. Sounds to me like it cured way too fast. A few spider cracks after a year or so is not uncommon, but large cracks soon after a pour would not be acceptable on any of my jobs. – shirlock homes Jul 19 '16 at 16:25
  • are the walls built on the slab or slab poured around them? i guess walls are on top of slab. what size is the slab? – shirlock homes Jul 19 '16 at 16:27
  • Your picture that shows the lack of wire in the pour is my biggest concern. The cracks are bigger than I would expect. I am leaving this as a comment only since I have never seen cracking this severe in a basement floor. The straight line cracks are cold joints, as you mentioned, they are in between pours. That's what happens when concrete cures at 2 different times. Although those cracks are bigger than I would expect, my concern is still for the irregular cracks. It may be wise to have concrete strength tested. It can be done after it is poured, it is more expensive to do it that way. – Jack Jul 19 '16 at 16:33
  • thanks guys. Those stud walls are on top of the slab, and are interior walls. I believe that's a load bearing wall, and there is a footer underneath the slab. Those cracks aren't close to the exterior walls. – Bdgregory Jul 19 '16 at 21:55
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Those "straight lines" are control joints (as opposed to expansion joints). The contractor knows that concrete slabs invariably crack, and they install those to make the cracks less conspicuous.

That said, you do seem to have more cracks than I'd have expected, and they appear more open than is typical. The one is oddly close to the exterior wall. I'd ask your builder about backfill and compaction practices.

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    why would they not do a monolithic pour on a small interior slab? No rebar??? WTF – shirlock homes Jul 19 '16 at 16:29
  • I'd agree with isherwood that that seems more than typical. Could you tell us if the land adjacent to the wall nearest the cracks slopes downward and was possibly backfilled? I'd bet it was and there are settlement issues. – Jared Jul 19 '16 at 16:39
  • Even on a monolithic pour we add crack lines to control the cracks. I agree with Isherwood, It is way closer to the wall than I would put them. It looks like an interior wall to me. This may be a mid pour line. + – Ed Beal Jul 19 '16 at 19:14
  • thanks guys, the slab is not small, it's nearly 3000 sq ft (75 ft x 50 ft less some notches). I don't believe any of the ground was backfilled as there was no need to the way the area was dug out. – Bdgregory Jul 19 '16 at 22:00

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