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Re: Pouring Concrete Beam Monolithic with Concrete Column

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From ACI's SP-4 Formwork For Concrete, Fifth edition, page 28 of chapter 9:
"If columns are formed, cast and stripped independently ahead of the forming of other structural members - a fairly common practice to permit columns to take initial shrinkage - the adjusted form should be slightly higher than the true height of the column to allow for shrinkage and variations...."
Steven A.
Los Angeles

Mlcse(--nospam--at) wrote:

Have a concrete question related to the UBC.  The contractor on one of our projects, in order to accelerate the schedule, formed all the columns, beams and underside of the flat plate, laying out rebar, prior to placing any concrete.  The contractor was suppose to only pour all of the columns up to the underside of the concrete beams prior to pouring any beams or slabs.  In one area, the concrete sub placed the concrete in the column integral with the beam/slab prior to completing all of the concrete columns and then doing the beams/slab after waiting for a certain time period.

The question is related to UBC section 1906.4.5 which states "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".  In our particular case, since they were poured monolithic, obviously the concrete in the column was still plastic.

What is the concern with pouring the beam / column/slab monolithic that the code does not want to permit this. This code section is under "1906.4 Construction Joints".  Is it shrinkage stress related? Is the concern curing of the concrete since you have a rather thick mass in the beam column slab interface relative to just the beam and slab which will cure more rapidly since there is more surface area relative to the volumn of the concrete. If you pour the column separately, the beam/slab mass over the column in the next separate pour would be similar to the beam/slab mass away from the column (similar cross sectional area) and therefore a more uniform cure rate.

Any insight would be appreciated.

Michael Cochran S.E.