Conc. Columns poured over fresh elevated slab

[mlevario99]

Hi, I have a question, I design a 5 story concrete building and is in the construction stage. Last week they poured the first part of the first elevated slab, tehy started around 1:00 AM and they finish pouring concrete around 9:30, they had about 560 cu. yd. The contractor wanted to poured about 4 concrete columns on top of the fresh slab by 1:30 PM, one person call us and ask if it was acceptable to pour columns on top of the fresh slab, we tought it was to soon to start pouring concrete on top of the new slab. Is there any ACI information related to this situation, or any of you guys have had the same problem. The strength of the concrete is 5000 psi.
Thaks

 

[PST09]

It depends on what strength the concrete has obtained already from initial curing. This in turn depends on the type of concrete and any additives that may or may not have been included.

For example a 28 day strength of 50 MPa might result in a strength of around 15 MPa after 24 hours, under “air curing”.

This may provide some insight:

http://commons.wikimedia.org/wiki/File:Age-Compressive_strength_(concrete-curing).png

Depending on the design method you used you could go back to the design calcs and change the compressive strength of the concrete to say 30% of the 28 day strength and see if your structure is still valid with the decreased strength of concrete. Sorry i don’t know of any other way, will be interesting to see what is suggested, I’m sure someone else on here has far more experience than me in this sort of area.

 

[ishvaaag]

In four hours if set some standard model indicates say 3% percent of characteristic stregth, provided it has even set. It might be enough to keep afoot the columns, but we have more here a problem of policy on formwork and concrete placement/conditions for the standing formwork etc than calculation. In any case would have to check for wind load to give the worse pressure on the slab concrete, or stabilize with struts.

I read somewhere that sometimes in the URSS formwork for structures for buildings deemed to stay frozen all the winter was left with the concrete placed till spring. I am quite sure this is the kind of resolution the builder is expecting from you, but to avoid some liquid to flow one needs some kind of a vase.

 

[Ron]

How was it formed and shored? You can pour it all at one time if you've accommodated that in the formwork and shoring design....otherwise, way too early to load the slab with anything heavier than ride on trowel.

 

[ron9876]

That is a common practice on multistory buildings where I am. Obviously the concrete has to take it's initial set. May scuff the curing compound but otherwise not a problem. If they are cycling floors in 4-5 days there isn't much time to waste. Will depend somewhat on the concrete mix.

 

[csd72]

I would be concrened more with abrasion of the concrete from workers walking over it prematurely. For this reason I would recommend it waits one or two days first.

Other than that, as long as the formwork can take the additional weight I cannot see any issue with the strength.

1. The additional weight of 3m of concrete is about 8MPa, compared to a minimum of about 25MPa for mature concrete slabs. (sorry I am too lazy today to convert that)
2. The concrete at the slab is confined by the surrounding slab and therefore has a higher crushing resistance because of this (because of this, most codes allow the slab concrete to be lower grade than the columns).

 

[RHTPE]

This is not at all an unusual request. I have seen columns (above the slab) poured on same day as the slab. The slab concrete has to have acquired enough set to allow workers on it. The weight of the column concrete is insignificant (12.5 psi for a 12' tall column), and has no impact on the shoring below the slab IF there is a column directly below the one(s) being poured.


Ralph
Structures Consulting
Northeast USA

 

[ishvaaag]

I obviously was saying "was NOT what the contractor was expecting from you" but I left it a bit, sounded more ironical.

 

[BAretired]

mlevario99 said

we thought it was to soon to start pouring concrete on top of the new slab.

I agree with that. There are just too many variables to take a chance.

BA

 

[ron9876]

What could it hurt unless foot traffic damages the finish. They will also want to start shoring for the next pour as soon as possible. Don't think there is any problem at all.

 

[BAretired]

The finish is not important structurally. What is important is the resistance of the slab to punching shear. If the slab concrete has not achieved initial set, the addition of fifteen feet of concrete may cause micro cracking around the column which could affect its punching capacity.

There is too much at stake and too little to be gained. It is simply a bad idea.



BA

 

[ron9876]

I certainly don't think that you should be on the concrete before it sets. But after it sets I can't see where there is any stress generated at the column except very small compressive stress.

On most multi-story concrete slab buildings the longest cycle for floor pours is a week. Often a 4-5 day cycle is used. As I said above the contractor will also be placing shoring for the next pour as soon as possible. I don'tsee any reason to penalize the project unless there is some type of damage or risk to the structure.

 

[BAretired]

The contractor should have approached the engineer ahead of the pour regarding the possibility of using excess concrete to pour columns on top of the slab. That way, the engineer is not forced into making a quick decision over the phone without having an opportunity to consider all of the potential consequences.

If the concrete has set, and this would be dependent on curing conditions, chances of harm to the structure are probably minimal, but I still believe the OP gave the best answer in the circumstances.





BA

 

[RHTPE]

A 42 story building I was involved with was able to achieve a floor every 3 days. This cycle occurred on the residential levels, which were not designed for a SLL much more than 40 PSF. I don't recall the f'c. This building, and many others constructed in a similar fashion, suffered no ill effects from their construction means & methods.

The only way this can be achieved is with intense "choreography" of the various trades and crews. Essential to this is being able to get the verticals (columns & walls) poured as soon as the floor slab can be walked on without marring it. Very often, the forms for the verticals would be in place above the floor slab being placed to facilitate the floor-to-floor cycle.

Compared to the methods of high-rise CIP RC construction employed in NYC, I far prefer the above methods.


Ralph
Structures Consulting
Northeast USA