Pedestal Concrete Mass

[blavery]

I'm designing a concrete pedestal for a sculpture. The pedestal is mostly architectural in that it is structurally oversized to support the load. It is a 48"wide X 48" long X 90" tall (42" in the ground, 48" above ground) pedestal to support a 750 lb, 7' tall sculpture. Any advise on when to use a low heat concrete mix? This will be placed in a single pour and I was wondering if I need to worry about using a low heat mix design. This will likely be placed in the spring/summer in the Kansas City area.

Thanks.

 

[phamENG]

You'll want ACI 207.1, guide to mass concrete. Do a quick google search and you can find some PDFs of the document.

 

[ATSE]

Two pours would be much better, with 7 days between (in terms of mitigating the negative heat gradients). Especially if the lower section is below grade.
If the pedestal absolutely has to be placed as a single pour, consider a foam interior blockout. Something like 24" x 24" x 48", securely fastened in the interior of the formwork, to minimum total concrete (and total heat).

 

[Gerber_EIT]

blavery:

In my experience, concrete mix design is typically something the contractor and the concrete supplier work through themselves (e.g. means and methods). Not sure how it is in your area, though. We would consider something of this size to be "mass concrete", which would bring into effect two requirements from ACI 301-10 Section 8 to include in any specifications.
1. The maximum temperature in concrete after placement shall not exceed 158°F.
2. The maximum temperature difference between center and surface of placement shall not exceed 35°F.
Not sure how much that helps. If you're looking to determine your own mix design, I'd look at ACI 207.1R.

 

[SlideRuleEra]

blavery - The pedestal size can be considered mass concrete, but just barely. Agree with Gerber EIT, that the details and timing of the concrete placement are typically the domain of the Contractor and concrete supplier. The way we handle this situation is to require (as part of the bid documents) for the Contractor to submit the details to the Engineer (in advance of the placement) for Engineer's review and acceptance.

If the placement is scheduled to start at or near sunrise, and cold water or ice is used in the concrete mix, outcome should be fine. Temperature of concrete in the ground should not be a problem, soil is an effective heat sink and cooling will occur by conduction.

Placing 4.5 yd[sup]3[/sup] of concrete will take place quickly, you may want to have the Contractor submit concrete forming plans for your review also. Form pressure will be significant.

For a summary of general guidelines on mass concrete see "Engineering Mass Concrete Structures".

http://www.SlideRuleEra.net

 

[blavery]

All very helpful, thank you very much. I'll look into the mass pour resources posted. It's a small job for a parks department, so I'd like to be as specific as I can with what I want and leave as little as possible to the contractor. Sometimes the contractors they get for these small jobs don't have the most experience or attention to detail.

 

[JoelTXCive]

Are the clients fixed on the single pour? What if you poured the in-ground portion and then a "concrete core" for the top. You could then come back and pore a ~6-8" thick concrete cap or shell with a ton of small sized rebar in it to control cracking. You could sell it by saying "if it ever does crack, then it will be easier to fix by having a 2 pour setup. You would only have to bust out the 2nd pour."

 

[blavery]

That could be a good option. There is an opportunity to put a joint where we have a reveal in the form, I'll look into that. I'll probably also use ATSE's idea of the foam blockout.