Why are there cracks in the newly poured concrete?

[canfossi]

I went to my property today to see the work that was done on the foundation, everything looks great, except that there are some cracks in the crawlspace floor, some a bit bigger than hairline. Is this anything to be concerned about and will they get smaller? The floor was poured roughly a week ago. The foundation is in the full sun and the subfloor is going to be put on in about a week. Thanks Chris

[Richard6br]

The cracks probably won't hurt anything. I assume that the contractor figured no big deal, its just a crawl space. Most floors have a vapor barrier under the concrete so water infiltration should not be a problem. One reason for the cracks is that the floor dried too fast. No, they won't get smaller, they will probably get bigger.

[Farmer Willy]

Only two things I would promise people when we laid concrete--it would get hard and it would crack. How hard depended on the mix and how we cured it. How it would crack depended on how we cured it and how we planned for it (expansion joint, saw cuts, tooled joints). I don't think you'll need to worry about your slab.

[Cabin Fever]

Like the others, I wouldn't worry about hairline cracks. In hindsight, one way to reduce the amount of cracking would be to keep the pour moist by watering it for up to a week afterwards.

[cricket]

I would worry only if the footers aren't adequate and if the contractor failed to place a vapor barrier. It's been known to happen here. Concrete expands and contracts...It's NEVER stable. And depending on the base and the moisture in both the air and the ground, and the mix, and how much potash is added, and how it was finished, is all going to determine how much and how often it cracks. It's a little early for it to be cracking for where you are. Ya'll haven't had a bunch of rain over the last several months so...and you aren't in an unstable area geologically speaking...Hmmm...Did anyone hose the slab after it was poured? You might want to if no one has. It takes on average 45 days to cure so if it's curing too fast, the water will retard it some. What floor treatment are you planning? If you're scoring and staining some cracking can be really cool.

[farminghandyman]

concrete will crack aobut ever 10 feet all most regardless of what one does, that is why most put joints in or use a jointer in about every 10 foot as to control the crack in a straight line and so it is not as noticable,

[Shepherd]

This is an interesting read; appreciate everyone's input. My DH & I are thinking of expanding our patio so I'm very interested. We've had some excellent patio slabs in the last two places we've lived, each had what looked to be separate slabs/sections... is that what the "jointer" does?

[Farmer Willy]

Pretty much. A jointer is a tool run in the wet concrete that makes that grove you see in your sidewalks. It controls were the concrete is going to crack (in theory at least). In big slabs it's easier to pour the whole thing and then come back with a saw and cut relief joints. Either way the idea is that it WILL crack, jsut a matter of deciding in advance where you'd like to see the crack.

[Shepherd]

Thanks FarmerWilly.

[perennial]

I would just say that when we had a concrete basement in our old house eventually those cracks would let in water in real bad rain storms - kind of like
a bubler. Then, someone told us about dryloc and after that not a problem. We just got it in a tube and squirted it over the crack kinda like spackling and
then used a putty knife. We lived with puddles now and then for years until we did that.

[RedneckPete]

Concrete shrinks when it cures. That is why engineered specs will call for non-shrink grout to be used in underpinning or other places where this shrinkage could cause problems. All concrete will crack, cutting the concrete and creating weak points controls the cracks. Most foundations will crack from one of the corners of a window, as this is the weakest point.

Pete

[TNHermit]

Generally when you pour a crawl space you use what is called a slurry mix. Its thinner and not as strong as regular concrete. Usually around 2". Like was said before its a crawl space. the concrete makes it easier to get around. I would say its an upgrade from what most builders do.