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Old 06-22-2009, 08:31 AM
blacknoir's Avatar
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Default Foundation Drying question

I poured a 6" Foundation slab and I'm wondering how long I should wait to take off the forms and how long before I can start stacking my cement block for the stand?

It's been a couple days and seems pretty firm but I worry I'll put too much on too quickly and strain anything still too wet lower down.

Thanks,
Shay
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Old 06-22-2009, 09:48 AM
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Default Re: Foundation Drying question

Shay,
Concrete cures by a chemical reaction not by drying. Drying however, can retard the curing and lower the ultimate strength of the concrete. Concrete likes/needs to be kept wet/damp for several days after being poured. When I poured the slab for my house I nailed thin strips on top of the forms and "ponded" the whole slab (28x40ft) for a week which was way overkill, but it was summer and hot. On my slab/foundation for the WFO I covered the surface with old towels and wet them down with a hose several times a day for the first few days.

Concrete will continue to cure and get stonger for decades, however, for all practical purposes concrete will have over 90 % of it's final strength in 28 days. It's not uncommon to pull the forms in three days and have the concrete strong enough to have workmen start to install sill plates within a week. The surface will be somewhat delicate and should you have a fancy ultra smooth finish you want to be careful as it can be fairly easily damaged for the first week.
Hope this is some help,
Wiley
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Old 06-22-2009, 10:22 AM
blacknoir's Avatar
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Default Re: Foundation Drying question

Kinda. You guys and your specifics. I have been keeping it wet but I do not have a ultra smooth finish because I didn't care that much for this part. I'll keep wetting it down and probably start stacking this evening. I poured Saturday morning so that's almost 3 days.
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Old 06-22-2009, 04:30 PM
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Default Re: Foundation Drying question

Keep it damp for at least 6 days. Longer if possible.

Concrete generally reaches 50% of its ultimate strength after 6 days and as Whiley notes, 90% after 28 days. (but only if it has moisture available continually for the entire period.)

You should be OK to start build the block walls after 3 days of curing.

Last edited by Neil2; 06-22-2009 at 04:37 PM.
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Old 06-22-2009, 07:51 PM
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Default Re: Foundation Drying question

I poured my 6-8" foundation on saturday evening and pulled the forms today which is 2 full days. I'll let you know if iI have any surprises.
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Old 06-23-2009, 04:49 AM
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Default Re: Foundation Drying question

I laid the first course of blocks while the slab was still wet (3-4 hours after the pour), and built the rest of the stand the next morning. Also, it is probably best to brush the slab to make it rough. You don't really want the slab too smooth, otherwise you won't get good bonding of the stand to the slab. If you are worried about that, they sell concrete bonding agent at home depot which should do the trick.

Your mix was probably 3000 or 4000 psi, so I wouldn't worry... Just keep off the edges.

--J
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