Cracking slab
JTyler
| Posted in GBA Pro Help on
I had a basement slab poured in May, and it recently cracked in several locations. I am upset that the cracking did not follow control joints. The slab is about 5″ thick, and I was aggravated to find that the control joints are only 1/2″ deep. From everything I can find, 25% of slab depth is standard. Do I have a leg to stand on in asking the contractor to recut joints to appropriate depth and repair cracks? Slab is 4k psi with rebar on 2′ centers poured over poly, 2″ XPS, and ~9″ crushed stone, in case it is relevant. Thanks for any input.
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Concrete cracks. If anything, your slab seems to be better constructed than most. Can you post a couple of pictures?
I understand it cracks - my issue is that the control joints are shallow and functionless. In some locations, cracks run directly across them at shallow angles. I'm wondering if it is reasonable to have expected them to be cut deeper than 1/2".
As far as slab quality - much of what is good about it I specified and installed: compacted stone base, insulation and poly (which contractor advised be installed poly first with insulation on top - I did the opposite), rebar. This crew had problems with attention to detail - daylight 6" wider than specified, missing footings I had to catch and install before pour, anchor bolts set too low to get a nut over a 2x6 sill. I am trying to determine whether these shallow joints fall into that category.
I will try to take pics at some point if helpful. With the rebar grid, they remain essentially hairline cracks without elevation changes.
You're right that control joints should be at least 25% of the slab depth, but there are a lot of other things that can go wrong, and even properly done control joints are not always effective. Was too much water used during the pour? Was the base properly compacted? Is the base dry and on non-expansive soil (which expands when wet)? Are the cracks small hairline fractures, or long structural cracks? Do they radiate from inside corners and/or lally columns (locations of higher stress)?
I respectfully disagree with Steve and others who claim "concrete always cracks." (I realize Steve did not say "always.") Concrete on residential projects the vast majority of the time dries (loses water) before the concrete has time to properly hydrate, leaving it weaker than necessary. It often has more water than it should, leaving the concrete porous and weak once it dries. It also rarely has enough rebar, which is necessary to develop tensile strength.
Your slab exceeds code in several respects: 5" thick instead of code-minimum 3 1/2"; design strength of 4000 psi (in compression) instead of code-minimum 2500 psi; and a rebar grid when none is required are all good upgrades. Poly on top of the foam is the better location. You could ask the contractor to fill the cracks, but at this point deeper control joints probably won't do much--the cracks already show where stress is highest.
Thanks for the input, Mike. I am also concluding repairs are a lost cause, and probably more unsightly than current cracking. Poly is over the foam, to be clear. I compacted the 3/4" stone base with a vibratory compactor over solid dry excavation in non expansive soils, and even kept the slab thoroughly hosed down under poly for about a week. I really didn't expect cracking like this.
Jim, it sounds like you did everything right. A photo of the cracks might help. Nothing to get too concerned about, in any case, but it's always frustrating when quality doesn't meet expectations.
Jim: How far apart were the control joints?
Here is a photo for what it's worth. This is above the corner of the chimney footing. Area in photo is about 2' x 1'. The most substantial cracking surrounds this footing and radiates from two of its corners. Control joints on about 12' spacing.
Jim. I'm not trying to minimize your issue--especially since it looks like you stained your concrete to use it as the finished floor.
How long has it been since the slab was poured? In your photo, it appears that there is some curling or difference in elevation. Is that the case or an optical illusion?
Steven - The slab was poured in May. It is not stained. The house is not heated yet and we had an unusually warm day today; some condensation starting may explain the color? I would say 95+% of the cracks show zero elevation chance, but there are short stretches where you can feel a slight lip...1/32" ish.
Thank you guys for the feedback.
Jim. I found this site and wanted to share it with you: https://www.concretenetwork.com/concrete-joints/contraction-joints.html
It seems you really needed a depth of 3/4 of an inch to 1 inch for the control joints. I also wonder if the contractor created isolation joints at the boundary between the slab and footer.
If the slab is not going to be the finished floor, I would seal the cracks and joints with a high-quality polyurethane caulk before covering it up. I also would test for radon if that is a potential issue in your area of the country.
Since our slab would be the finished floor, we were pretty paranoid about cracks. Like Jim, we used 4000 psi concrete. Our slab was 4' thick, with 1' deep joints. In thinking about control joints, we decided to incorporate them into the floor plan in two ways. Some are design features. For example, our kitchen island is 8' wide, so we set control joints to be exactly under the edges of the island.
The other thing we did was locate joints under interior partitions. That way, many are only visible in doorways. Our joints are more closely spaced than typical. The sections ended up mostly 8' x 10' or smaller.
Like Jim, we had the stone under the foam well compacted. I don't know whether it was the skill of the subcontractors, clever layout/design or just luck, but we don't have any cracks after two years.