New room addition over concrete patio which was very solid. Contractor used fiber fill and rebar in the concrete which is about 4 to 5 inches deep. Poured in mid October. No problems seen until early January. Starting to see long hairline cracks. Added heat to the room in early January. Is this just shrinkage? will it get worse? I thought fiber fill eliminated cracks.
DGB. my view not universally shared by all is, concrete cracks, rebar holds it in place after it cracks*. If the ground is stable, the cracks you see now should stay about the same unless water can get in and subsequently freeze.
*does not apply to prestressed concrete.
DGB (Al-Wa too), It matters what caused the cracks (and their future environment as well)to predict their future. If it is just shrinkage, more likely with larger slabs... What are dimensions? What sort of footer. How much rebar of what size. How is rebar suspended above the bottom? And the list goes on and on...
Fibers were touted to "replace rebar" but don't believe it, they just help control cracks. Like Al-Wa said, concrete cracks. Oh by the way the other half of that mantra is "and steel rusts." If you have sufficient rebar and it is installed correctly it will hold the "pieces" together. The best you can hope for is that the cracks are cosmetic and will not open up or weaken the slab.
I have oodles of cracks in my larger flat concrete work and most are of no real concern. (Except were we sawed control joints)
The first few times I looked over each successively poured new slab and saw it as near perfect only to start to crack, I was concerned but consoled by my GC. Concrete flatwork will crack about every 8-12 feet whether you have rebar or not. You can tool or saw "control joints" or put in divider strips to encourage the cracking to follow your "starter" so it stays neat and tidy but you can't stop cracking on simple flatwork (no pre or post tensioning.) Control joints will really help make the slab look crack free by hiding the cracks under your "feature." We sawed control joints in all of my wrap around porch slabs, the ones on grade and the suspended one and that has nearly eliminated "volunteer" cracking.
Pouring over insulation (bubble wrap type or rigid foam) caused our slabs to crack more. In one pour, about 36' X 48', 1/2 was insulated and 1/2 wasn't. Poured at the same time with the same mix, the insulated slab cracked a bunch more than the non-insulated slab but there is no strength-quality problem, just a lot more cosmetic cracks.
[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]
"I'm not from your planet, monkey boy!"
I seem to recall that I asked a contractor once if they guaranteed for cracks. He said, absolutely he guaranteed the slab would crack.
As I understand it, there's cracks and cracks. There's growing cracks and stable crack, and crack that are just because of internal stresses, which are pretty normal, and crack because of external stresses like load, which may be a cause of concern and remediation.
So, if it makes you feel better, you can get the contractor or an expert to have a peak. If it were me, I'd try and figure out why the crack is there, monitor the situation to see if it gets worth.
By the way, my house is mostly made of concrete - I've seen my share of cracks.
While reading about installing ceramic tile over a crack in a concrete floor, the distinction was made between shrinkage cracks and stress cracks. The author felt that shrinkage cracks will end up with both sides of the cracked concrete remaining in the same plane (ie, no step-off between the slabs), while stress cracks will have one side higher than the other.
Not a concrete worker myself, so I don't know how true it is.
Think about tectonic plates and what causes earthquakes, and apply it to a much smaller scale. In stress, the segments push towards each other, and one has to rise, just like the earth's plates. In shrinkage, there's no stress. In other words, your concrete guy was right.
I own a commercial property that is actually a converted house. Years ago, I had the garage floor poured to raise it 3-1/2" to make it the same level as the rest of the slab. Then I gutted the walls to make a showroom. That 3-1/2" flat pour cracked almost immediately from shrinkage, but has remained constant for 15 years since then. I'm remodeling the building to sell it, and I recently pulled up the old carpet and am laying a vinyl composition tile, so I gained a new acquaintance with the crack -- it hasn't changed a bit. I just laid the tile over it.
...Don
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