concrete placement tolerances at column to slab interface

[boffintech]

ACI-117 doesn't appear to give a straight answer on this. What is the reference for the concrete placement tolerance for top of column to bottom of elevated slab/beam. In other words how far into the bottom of the slab can the column below be over-poured or protrude into the slab above when the slab above is formed up and rebar is being placed.

The EOR says zero.

I understand that this is a function of how thick the suspended slab/beam is: the thinner the slab the more important this is. The engineer wants the full thickness of the slab/beam where the most shear is - the face of the support.

 

[KootK]

I allow 1/2" high any reasonable amount low but know of no code guidance on this. Obviously creating a zero tolerance condition is unreasonable. Contractors in my market seem to shoot for a low value that will fall within the thickness of the slab formwork. That way, they're usually within tolerance but don't have to form the top of the column for the slab pour.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

This is why I generally design my slabs as if the column is 1" high into the slab. But my experience with contractors is similar to KootK. They generally shoot for just low of bottom of slab.

 

[hokie66]

Zero above the soffit. Give an inch, they will take a mile. Below, well, that is up to them, but not a lot.

 

[asixth]

It's a difficult job to do with consolidation, pouring and vibrating all/most of the columns you can in 1 day and the finish of the joint if exposed. If I had to put a number on it I would say 5mm (1/4"), cause this seems to be a minimum acceptance for most tolerance where flatwork is involved. I would not want to go 10mm because the ACI shear provisions give rather high capacities compared to other design codes and too high acceptance criteria creates too much variation to the design capacities.

 

[TXStructural]

ACI 117 doesn't develop tolerances, it only reports them. 318 is silent on this topic. ACI 301-10 says: "Locate joints in walls and columns at underside of slabs, beams, or girders and at tops of footings or slabs;..."

In my personal view, slightly high will result in a socket which is unlikely to materially reduce strength or durability. If I had to put a value on it, I would require that the lower pour stop the greater of one bar diameter or the nominal large aggregate size below horizontal reinforcement to assist in consolidation around those bars. A smaller gap may result in partially embedded and improperly bonded reinforcement.
Slightly low should not cause problems in a properly detailed column (bars developed above the joint.) These joints should be rough.

I will bring up this issue as new business in the next 301. (We are wrapping up 301-15 now so I can't get changes started for that edition.)

Generally speaking, we should not expect unreasonable accuracy in construction. Enforcing limits where they do not matter will increase cost (and reduce the respect contractors have for us) without improving he safety and serviceability of structures.

 

[hokie66]

Punching shear is the issue with slabs. Nearly all columns are cast with the floor forms in place, so it should not be too hard to avoid too high placement of columns.