I am building a home in the houston area. nearly 3000 sq. ft. 1 story house, so a pretty big slab.

I am noticing now while walking through the home during the electrical wiring stage that the slab has hairline cracks (probably 1/8") in nearly every room of the home.

Seems like they originate from the edges of the slab or at holes where the plumbing is.

I know that shrinkage is normal, but these cracks run the whole length of the slab and ive found them in all areas of the house. Ive walked through a lot of homes and havnt seen this before but maybe i just wasnt looking close enough.

Do you think this is a structural problem, not structural and just sloppy, or normal concrete laying and ive just never noticed it before.

I am having a home inspector come out for phase 2 to check before the insulation and sheetrock is set, but i guess its too late to do anything about the foundation. I should have had a phase 1 inspection...

I really appreciate any advice. Thank you.

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    Ask your contractor whether any embedded control joints were installed (zip strips). Cracks are normal, but they're typically planned for better. They probably aren't a concern except where you're installing ceramic tile. They'll need to be managed well there. P.S. 1/8" is much larger than hairline and larger than I'd expect. It's probably not a structural concern, but could be a cosmetic one. – isherwood Oct 14 '19 at 16:03
  • I should add the builder got back to my about my concern and said the cables in the foundation have not been stressed yet, and once they are the cracks will go away. Cracks go away? – Aaron Oct 14 '19 at 16:09
  • This house will be almost entirely ceramic tile. Wont the grout fill the crack? – Aaron Oct 14 '19 at 16:11
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    You may mean mortar, and filling the crack doesn't solve the problem of seasonal movement, which will propagate through your tile (as more cracks). – isherwood Oct 14 '19 at 16:15
  • On the jobs that we used cable we had to release the tension plates within 36 hour on a 5 k pad, this was in a seismic zone and sand so the requirements may be different but I would think it should be under tension prior to building. – Ed Beal Oct 14 '19 at 18:20

Is this slab really the foundation? I'm aware of 'slab on grade' shallow foundations but I've never lived in a climate where that is practiced. In my region a foundation with footings would be poured first (even when there's no basement), then the slab would be poured inside the foundation walls. In this latter technique the slab is non-structural.

You also mentioned something about cables being tightened -- is this a post-tension concrete construction?

It's probably a good idea to have a conversation with your contractor about saw-cut control joints. Concrete will move and crack and will continue to do so throughout its life. 'Control joints' are so named because they [attempt to] control and conceal these cracks. If you have the slab cut then you get the opportunity to influence where the cracks will happen. For example, you can encourage the cracks to make straight lines and to fall in places where you wanted a grout joint. If a crack is going to telegraph through your tile floor, it'll be much easier to conceal or repair if it comes up in a grout line rather than crossing a tile.

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    We install control joints under walls. That way they are only visible at doorways and easy to control/fix if necessary. – Lee Sam Oct 14 '19 at 16:32
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    It's obviously much too late for cut control joints. They need to be done within 24 hours, and we know cracks have already formed here. – isherwood Oct 14 '19 at 16:43
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    Here in Minnesota we use "frost-protected" shallow foundations--slabs with thickened edges. They're insulated outside and underneath with closed-cell foam sheet. – isherwood Oct 14 '19 at 16:44
  • It's true that control joints under walls would have been the most ideal placement and that plenty of undesirable cracks have already formed. But even at this late stage isn't it reasonable to expect that cutting joints now could help mitigate the growth of existing or new cracks? – Greg Hill Oct 14 '19 at 16:50

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