Did an inspection on a 2,541 sf, slab, new construction and noticed square cracking in the garage flooring. I’ve seen the random cracking that’s part of the curring process, but never anything like this. What would cause such a definite pattern? Any input would be appreciated.
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Those are very common / typical expansion cracks. Since most modern day/lazy concrete contractors skip putting in expansion joints
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Looks pretty uniform and closer together than what I see around here. Is there in-floor heat? Possible that they did a poor job of installing insulation under the slab?
Hey Jeff, no, no in floor heat. Just a slab.
Contraction joints are formed by saw cutting, by tooling a joint with a grooving tool, or by inserting a plastic strip into the concrete during finishing (zip-strip). Proper timing and depth of cut are essential. If you wait too long, the slab will crack where it wants to rather than where you want it to.
Hey Christopher, I’ve seen the expansion cracks before, just never so symmetrical.
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I’ve seen this happen in new construction along the edge of a thickened slab.
I have, just like yours. Dozens and dozens of times. Almost exactly where expansion joints would have been installed. It happens soon after the concrete dries.
Though the ones you have are kind of wide, at least looking at these photos.
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If You don’t cut it where You want it to crack, it is going to crack anyway…
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Expansion of rusting rebar placed too close to the surface.
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rebar rusts and expands that fast on new construction…new one for me…
I have a metal detector I keep in my truck. I have used it many times and it will detect the rebar every time on a crack pattern like that. After a short time the concrete will debond and you can an hear a hollow sound when you drag a hammer over it.
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There is the possibility of plastic settlement cracking over the rebar. Google it and you may find cracks similar to yours.
Looks like a sloppy installation of zip strips. They are used to create control joints in the slab.
Christopher Currins is right with the first answer!
Is it post tension cable? I see it pretty regularly and it seems to mimic the placement of the cables. Around here everything is post tension slab over expansive clay soil. I have always assumed since it is in the step down of the garage floor and porches as part of a monolithic slab that the cables were just a bit too close to the surface of the garage/ porch floor. Rusting rebar won’t be a factor if it is post tension cable since there is a plastic sheath and grease.
Water spots caused by standing water on the slab shortly after it was finished. Could also be excess bleed water coming up from underneath because it was poured over a vapor barrier or the substrate was overly saturated.
I agree with @rmayo and @jluttrall . Reinforcement within the slab too close to the surface will do that 90% of the time. Rebar could have been placed in the concrete after it was screeded and then a bull float was used to seal the mud over the top of the rebar. I see contractors do this all the time, and usually have the same results as pictured. It’s no different than tooling a control joint then filling it back in with cream when it’s time to put a trowel to it. The end result will be a crack right where the joint was tooled as there is no aggregate in that spot.
Nope…
@aponder was this a single car garage or a 3 car? I see a single car vehicle door in the background which would put those cracks at about 3-4 feet apart (roughly). Do you have a picture that has the entire floor in it? or at least most of the floor? What were the dimensions of the floor?
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I bet that the cracks are following the rebar. Why, could be that the rebar was set too high or too low or the mix was too wet. It happens.
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