huangyhg

delay between pouring columns and slabs/beams over.
how much delay is required between pouring columns and pouring the slab and beams over?
we specify 24 hours. what do you specify and what problems have you seen when there is an insufficient break?
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no delay is required if the formwork is designed for the entire loading of both placements.
civilperson/tomfh,
if you are doing a multi storey it is a good idea to allow the columns to shrink a bit prior to placing the floors over, this helps minimise the column shrinkage effect on the height of the floors.
on a short building it wouldnt make any difference.
24 hours seems like a reasonable minimum.
strengthwise civilperson is 100% correct.
shrinkage of column does not occur until the forms are removed. if there is no removal of form, then the delay interval does not help for column shrinkage effect. if the column forms are removed, then the elevation set for the slab will have to be independent of the column to minimize the column shrinkage effect, (thickening the slab at the location of the column to make up the column shrink).
it is common practice to wait as autogenous shrinkage 鈥?self-desication 鈥?is unavoidable. the paste can reduce up to 25% in volume due to autogenous shrinkage only. not sure how this translates to the total concrete volume change. anyway, it is typical to wait for the columns to shrink before placing the slab.
aci 318 section 6.4.6 and commentary r6.4.6 deals with this.
aci-318 6.4.6 鈥?beams, girders, or slabs supported by columns or walls shall not be cast or erected until concrete
in the vertical support members is no longer
plastic.
r6.4.6 鈥?delay in placing concrete in members supported
by columns and walls is necessary to prevent cracking at the
interface of the slab and supporting member caused by
bleeding and settlement of plastic concrete in the supporting
i don't see any reason to wait except for the column to set up. typically columns have substantial reinforcement so the shrinkage will be small.

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