Concrete for Condo Balconies 1

[ajk1]

Condo balconies in Ontario are exposed to freeze-thaw and as such must be air entrained, by CSA A23.1. However the remainder of the floor to which the balcony is contiguous, is within the heated building envelope and does not require air entrainment. It does not seem practical to pour the balconies with one mix and the remainder of the same floor with another mix; it would create an impractical scheduling problem for the delivery of the concrete. On the other hand, if the same air entrained concrete is used for the entire floor (this is a 10 storey building) that would be a waste of money, and as well the floor finishers prefer the concrete not be air entrained if it does not have to be. This issue must be very common. What is being done? Anyone dealt with this on an actual job?

 

[Ron]

ajk1....although we don't deal with freeze-thaw considerations in my area, we use air entrained concrete for most applications. The difference in cost is rather insignificant considering the benefits of placement and control of the mix. I would not consider using two separate mixes. Use air entrainment and don't worry about the minor cost increase.

 

[concretemasonry]

ajk1 -

Much depends on the construction schedule and how thin to split the hairs.

AE concrete is a minimal material cost increase and if the schedule does the permit large loads, the short load cost, set up costs, crew movements and lack of continuity can more than cover the minimal cost to keep bean-counters happy. Delivery is probably the largest individual cost in a yard of concrete and one minor material in a mix design will not have an appreciable cost effect.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[manstrom]

It is possible to bring in one truck of AE for the balconies and pour the rest non-AE.

Some concrete guys say "impossible", some guys say "we do it all the time"

If they want to use one mix, use AE. You may be able to drop the AE to the low end of the spectrum.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[JAE]

There is a section in ACI 318 (Chapter 10 I think) where ACI includes a reference to puddling a higher strength mix around columns for floors and then mixing a lower strength in the floor beyond the column. So historically this type of concrete placement with different mixes in a wet placement has been done.

I think with proper coordination the balcony concrete could be placed that way as well.

Another option would be to use a construction joint over the cantilever support. This would be more expensive in my view as you would have to build bulkhead forms to separate the two pours but allow all the rebar to extend through.

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[KootK]

I get a ton of pushback on high rise buildings when I try to puddle high strength concrete. I'd expect the same with this. Considering your climate, Isokorb thermal breaks at the balconies would be appropriate and would nicely facilitate the construction joints that JAE mentioned.

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.

 

[ajk1]

I think puddling a different strength mix is risky practice at any time, although I know it is occasionally done... should tint the concrete so can determine immediately if they have the wrong concrete. But I would not puddle the concrete. I like to sleep at night. I think the simplest and safest thing is to use the air entrained balcony concrete throughout the floor. I will look into the Isokorb thermal breaks and think about it.

Thanks to all.