huangyhg

construction joint wait time
hi everyone, i have a question regarding construction joints in concrete. i am working on a parking garage (256'x120') that has a 3.5" concrete topping slab supported by precast double t's. the contractor will pour the middle 1/3 of the topping slab first, then pour the two ends after that. my question is, how long should he wait to pour the ends after pouring the middle section? are there any recommended standards by aci for this issue? any help would be appreciated.
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analyzing your question, it seems you are trying to divide the garage into 3 segments?is it possible to pour them at one time then put sawcut joints per 8ft on both direction?
is there a really good reason for pouring the middle third first? my preference would be to pour the full length and establish the pour area on a dt joint. is this possible?
dik
i think the answer is that it probably doesn't matter. if you are casting a topping slab on top of the dt's (field topped), the underlying double tee's will restrain the topping slab from shrinking as it cures.
the contractor would like to pour the entire floor at one time. the engineer that originally started this job said that wouldn't be a good idea because the concrete will be restrained on all 4 sides during shrinkage and would likely crack quite a bit. so he required them to pour the middle section first, allowing it to undergo some shrinkage before pouring the ends. now the contractor is asking how long to wait before pouring the ends. we have saw cut joints detailed at all dt joints (8'-0" oc).
aci committee 224 has a list of resources for reducing cracking.
on crack sensitive structures (normally water holding), i try to wait 14 days between adjacent pours, but often times change that to 7 days at the contractors request. the least amount of time i wait is 3 days.
if you're really concerned about shrinkage, use low w/c ratio concrete and have the batch plant provide documentation of shrinkage rates for the proposed mix prior to approval.
typically continous bridge deckds are poured in this manner. waiting time specifed is 7 to 14 days, this always seems rather conservative to me. if this is a concrete slab, perhaps maturity meters could be used to moniter curing and reduce wait.
if you check aci, that procedure specified by the structural engineer is antiquated and not recommended. pour the whole slab at once and sawcut the joints where needed in combination with pre-set joints.
i agree with lkjh345 and ron. the topping slab will bond with the tee's and there won't be any global shrinkage due to the restraint. pour it all at one time and cut in your joints.
i've worked on projects where we successflly poured topping slabs on dt's in one reasonably sized large pour using concrete with a low w/c ratio and sawed joints aligned with joints between the dt's. bonding between the topping slab and dt's causes restraint everywhere, so pouring in sections doesn't really help control cracking as much as a low w/c ratio.
thanks all for the help...i appreciate it. i can definitely see how the dt's will restrain the concrete as people have noted.

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