Column-slab junction where column has higher concrete grade

[haideeb]

Referring to a flat slab construction.. With the column designed to higher concrete grade than the slab, current detail shows pouring the portion of the slab around the column with concrete of grade same as that of the column. This is quite difficult to implement on site. Can anyone lead me to referrences on how this can be avoided, i.e. pouring whole slab to one concrete grade?

 

[JAE]

Agree that it is difficult to implement.

We've always tried to avoid that by limiting the column strength to 1.4 times the strength of the slab/floor system concrete.
Per ACI (10.12) that avoids having to puddle the higher strength concrete around the slab.

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[BridgeSmith]

You have the only good solution I know of, if you need the higher concrete strength for the slab under the columns. My question would be, what's the reason for requiring the higher concrete strength there? Can the goals be accomplished another way, such as thickening the slab at those locations?

The first thing I would check is what the cost difference is to get concrete in the higher strength. It may be little or nothing. On many of our projects, the difference in specifying 3500psi or 4000psi concrete is only a matter of a few extra days to reach those strengths. Locally, the standard mix is 4500psi at 28 days. It reaches 3500psi in about 4 days and 4000psi in 8 or 10.

 

[jayrod12]

The issue isn't necessarily the cost of puddling, but the coordination required to ensure it is done properly. So like JAE, we try to avoid it at all costs. Perhaps check if your column works at an fc'= slab strength*1.4 and if it does then you're all good.

 

[MIStructE_IRE]

Perhaps I’m missing the point here - but can you compensate with a couple of extra vertical bars at the junction?

Or is the “couple” of bars I mention above in reality likely to be a couple of hundred bars to make up the difference??! Sorry, I’m too tired to even do a quick calc..

 

[KootK]

Perhaps I’m missing the point here - but can you compensate with a couple of extra vertical bars at the junction?

This is my default. Although I always kinda wonder if the axial really diverts into the bars the way we/I claim that it does.

 

[BridgeSmith]

"... I always kinda wonder if the axial really diverts into the bars the way we/I claim that it does."

Assuming a concrete joint that has no gaps, strain compatibility dictates that the stress in the steel will be Es/Ec, approximately 8 times that of the surrounding concrete. If you have steel equal to 1% of Ag, then the steel will carry about 8% of the compression force, right? Maybe a little more if there's some gaps and local concrete crushing at the joint, but that's still not a whole lot. If your longitudinal column steel ratio is 4% or 5%, then it's a different ball game.

 

[KootK]

Yup, that's the story we tell. My point was really that I don't consider strain compatibility to be entirely valid over the area where newly introduced bars are being rapidly developed. Another thing that comes into play is that, in a situation where axial loads are high like this, creep will move a bunch more load into the rebar than our elastic strain compatibility calculations would indicate. Which plays in our favor in this instance.

 

[BridgeSmith]

Yeah, creep is your friend in this instance (unlike your friend being a creep, which is not helpful). We use a factor of 3 to transform the concrete for composite structures under permanent load, indicating that in the long term, the will carry 3 times what it did initially, so that 8% becomes 24% of the dead load carried by the steel with 1% reinforcement.

 

[haideeb]

Thank you all for your input. My column grade is unfortunately more than 1.4 x the slab grade so have to increase slab grade.

 

[jayrod12]

Do you need all of the concrete strength? If not, that might be your way out. In my experience you can generally get away with less concrete strength at most levels, it's likely not until the bottom level that you actually need the specified strength. Perhaps this is a better solution than increasing concrete strength at each level, keep it to only the areas where the required column strength is 1.4*slab strength, at all other levels where you can use an fc' = 1.4*slab strength don't change anything.

 

[Yao1989]

Column to slab, column to beam, column to pile cap, column above footings, it’s all the same issue; don’t exceed the ~1.4 ratio. Don’t forget that ratio also decreases for edge and corner columns due to lack of confinement.

 

[KootK]

For most applications, I would think that you could compare required column concrete strength to the slab 56 day strength. That, as opposed to comparing provided column concrete strength to slab 28 day strength.