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I'm having a new home built and found a large crack in the newly poured concrete basement floor and wanted to see if it's something I should be concerned about. The floor was poured sometime in the past two weeks; I need to check with the builder exactly when.

I'd really appreciate your opinion: please see the pictures below. Should I have the builder repair or replace the floor? (The basement will be finished, so cosmetic issues are not a concern, I just want to know the floor will function properly.)

Going to try to get an engineer to look at it...still interested in more comments, thanks!

FYI: There is no rebar in the floor; poured concrete on vapor barrier and gravel.

Click any photograph for full size

crack

crack2

crack is under window well area

Crack is under window well area in picture above.

footer corner sitting in mud and water

Footer corner sitting in mud and water in picture above. It's hard to see in this uploaded version, but standing water is in the corner; if the footer is settling here, that would likely explain the crack, right?

footer forms

enter image description here Footers with some gravel (more was added before the floor).

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Does your floor have wire mesh and re-bar inserted in the concrete? If not then get it replaced. –  Michael Karas Sep 20 '13 at 5:35
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Yes the problem is serious! Very good answer indeed by DMoore. In very rare occasions on such new builds its a setting subsidence that won't happen again. But as DMoore suggested - An engineers needs to come out and do tests now. Do not try to speed things up now because this is the foundation. If it keeps setting your house wont last 10~20 years and you will keep getting cracks in walls and all sorts of problems. This really needs looking at. Do you have a picture before the concrete or while it was poured? –  ppumkin Sep 20 '13 at 8:29
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I would be very worried about that corner - even the walls. If I could put money on this I say the corner is sinking a bit. I am guessing that next issue is vertical cracks on the wall. –  DMoore Sep 20 '13 at 13:39
    
Thanks for your comments! I added pictures of the footers...let me know what you think. –  viatheether Sep 21 '13 at 4:27
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1 Answer

up vote 12 down vote accepted

Well this is a pretty big deal because we don't know the cause.

First let's go over common reasons we get cracks on new floors.

  1. Soil wasn't properly compacted. Soil should be compacted with a rock bed on top.
  2. Bad mixing at site. Especially in the summer contractors pump too much water in the mix. The water makes the concrete weaker and it does crack easier.
  3. Bad mix at plant. Just a poor mix of materials coming in.
  4. Bad environment for mix. Too hot, rain, too long to site, whatever.
  5. They didn't cut the concrete or didn't cut it quick enough. The concrete needs to be cut (I suggest every 10x10 foot area) to allow for expansion. It really needs to be cut the first day.
  6. Didn't use rebar. This is a common cost cutter and I have seen rebar pulled before pour.
  7. House was built on unstable land. Could have been a landfill, could have springs underneath, whatever.
  8. Soil type wasn't conducive to their install method. Really too many things to list here but basically different types of soil expand and contract more due to moisture levels. Also it is possible that it was pushed even further by either a lot of rain or a drought.

Those are what I got off the top of my head. I am positive there are more reasons for cracks in your basement. If I were to totally guess I would say they didn't prep right in that corner - It seems like the corner is dropping on you already and maybe it wasn't floor wasn't compacted correctly. The crack starts on the outward "L corner" which is normal but heads right across the corner - most insignificant cracks in concrete tend to be straight. This crack has its shape for a reason.

What do I think you should do? First ask questions. See if the contractor knows why it cracked. "It happens" is not an answer. My reply would be "Me not paying you happens too." If the contractor/builder doesn't want to take blame (it is their fault) then you need to get a psi reading on the floor, measure its thickness, and also prove that the floor is rebarred/reinforced correctly. You will almost certainly need an engineer. Even if the problem is obvious your builder is less likely to move until it is proved from a creditable source.

You cannot sit on this. If the mix was bad - well did they use the same mix for the walls too? If it was bad prep, what happens when you have issues in a couple years? I have seen basements that have had to be repoured and it is a mess. Also will set you back 10K in the states.

Again, no matter what anyone tells you - cracks in a fresh floor are not acceptable. This is a sizable crack. Even if an engineer checks everything out I would demand a longer warranty on the basement floor and to have them seal it. Either that or they start over.

Note (based on additional pictures: I am not there so it is a bit of conjecture. But it is not a good sign when an area is relatively dry and you have a corner that is not only wet but it seems dropped too. It is a guessing game for me too at this point, not seeing what you have going on. My top 2 guess (and they are guesses) -

#1 - They dug and compacted. While digging they left over dug out that corner and then filled it. Then they never compacted it correctly. After it rained a few times, the corner started dropping. Also when I have a slab compacted the rock is part of the equation. I don't see any rock in your pictures.

#2 - You have some sort of natural underground current or spring. I have a stream that is 3 feet under, about a foot from my house - and I can't imagine if it where a foot over and 2 more feet down.

(Do you have any pictures right before they poured?)

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I would google or yellowpage structural engineer in your area - you can try "foundation engineer" too but that isn't a common term. Then from there you would ask if they have experience with concrete/foundation issues. Most structural engineers have expertise in this area. Be better to find one that is more familiar with basement slabs vs. commercial. Also getting one in your area means they probably have a pretty good background with common issues (soil, cement plant, certain builders, certain install methods) that may be regional. –  DMoore Sep 20 '13 at 16:12
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You can also talk to your city inspector. They probably can get you a list of engineers that work on that type of construction. Also, they can put pressure on the contractor to fix it. –  Mike Wills Sep 21 '13 at 3:23
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I'm not at all convinced that this is a problem. You forgot the #1 reason that concrete cracks: it shrinks as it cures. Cracks are inevitable. Often the concrete tradesmen will cut or score joints in the slab to "control" the cracks (i.e. hide them). People commonly see those as the square segments of sidewalk. I don't see any such cuts in this slab. It's possible the corner is sinking but there's almost no weight on it right now so I would be surprised if that's really the cause. –  Henry Jackson Sep 21 '13 at 4:19
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@HenryJackson - A characteristic of concrete isn't the reason it cracks. Saying concrete shrinks so it cracks means that the uncracked concrete never shrank? Also when is there weight on the corner of a basement slab? I have been flipping houses for years and have seen my fair share of poor workmanship by concrete contractors. This build is done. I would be livid if I were this future home buyer. To the point where I wouldn't even want the lot if the basement was redone. That corner is sinking, I would bet good money on it. –  DMoore Sep 21 '13 at 4:36
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@viatheether - I would say that picture is good news somewhat. It looks like the rock was packed and they continued a few feet outside. You need to get an engineer to look at this - check thickness, psi, and possible soil issue in that corner. I have bought a lot of bad homes (that was my business) on purpose. Usually there are no coincidences... meaning you noticed standing water and soil level dip in a section - and then that corner develops a drop stress crack. –  DMoore Sep 21 '13 at 15:59
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