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PJE1 (Materials)
29 Sep 05 10:14
My concrete contractor poured a 5", 4,000 psi basement slab for my new house yesterday.  The pour was complete by 12:30 PM, and the finishing was complete around 3:30 PM.

Yesterday was dry, sunny and relatively wind still in the basement.  Overnight temp fell into the 50's.

I hosed the slab down twice, after it had set up: once just prior to covering the whole 2,900 SF with 6 mil of polyethylene.  Before pouring, we had in place a 15 mil Stego Wrap vapor barrier with the seams and edges taped.  The slab was reinforced with 6X6 mesh sitting on cement bricks.

At the close of the job yesterday, the concrete crew foreman and I discussed proper curing.  For the first time, he brought up the idea that we should cut control joints.  In my reading, I had gotten the impression that this is eldom done in residential basement slabs.  Now, hearing that I was concerned not to develop cracks in the slab, the concrete contractor was saying control joints would be critical.

Reading up on them overnight, I saw a number of references as to when they should be cut, assuming they are not formed in at the pour.  The information I have found suggests noi more than 16 hours after the pour, preferrably right after the concrete is hard enough to saw.

QUESTION: It is now 22 hours after the pour.  Is it too late for sawing control joints?  Is it too late if I already find cracks?  Is it unnecessary if there are no cracks?

Thank you for any help.
Helpful Member!  SlideRuleEra (Structural)
29 Sep 05 15:40
This is the type situation where you have to pick the best of some bad options. Your contractor should have planned the the location of the saw joints in advance and left "gaps" in the wire mesh along those lines. As you have read, the cutting (at the planned locations) should have been done today, at the latest.

My advice at this point would be forget about cutting any joints. You most likely will get cracking, but even if you make the contractor's "afterthought" cuts you could still get just as many (or more) cracks.

To compensate, keep wet curing for 10 to 14 days, and I mean REALLY wet cure it - put the hose under the polyethylene covering and soak the concrete at least twice a day - leave the poly on ALL of the time. That should help, but not solve the problem.

Best Wishes

www.SlideRuleEra.net

Helpful Member!  boffintech (Civil/Environmental)
29 Sep 05 16:24
"keep wet curing for 10 to 14 days, and I mean REALLY wet cure it - put the hose under the polyethylene covering and soak the concrete at least twice a day - leave the poly on ALL of the time."

Good advice.
PJE1 (Materials)
29 Sep 05 18:16
Thanks.

As it turned out, the concrete contractor assigned the crew to a morning pour in a different town.  The foreman, having given me the advice about the control joints, felt guilty about showing up late.  He arrived at noon, roughly 24 hours after the pour, and went right to work.

They cut 5 control joints.  Although he was apparently unaware of the 1/4 the depth of the slab rule.

Unfortunately, between yesterday at 6 PM and today noon, one ugly crack developed radiating out from an outside corner.  Two of the control cuts now flank that crack like a bisected 90 degree angle.  But too late for that one.

Since I was able to have them cut the control joints almost entirely under the location of future stud walls, I figured I might as well still cut them.  No harm, might still help.

Of course, to cut the joints, they pulled up all my neat polyethylene covers.  After they finished, we got a little rain, which covered the slab nicely.  Then I re-covered everything and replaced all the weights.

I'll take your advice and run the hose under the plastic as much as possible.

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