Architectural concrete

[JStructsteel]

I have a project with a lot of architectural concrete, concrete planters, benches, etc. Some of these are solid concrete, and thus not too much stress for the loads, but I am worried about crack control.

Would you supply the min area, 18%, divide between the two faces and call it good, or would you supply more, than the 18%?

Thanks

 

[BAretired]

18% sounds a little on the high side.

BA

 

[JStructsteel]

Sorry, 0.18%

 

[dik]

Maybe 0.5%? 0.18% is, like, minimum temperature steel.

I don't know for concrete, but steel has a 'finish schedule' AESS. Catch the link:

https://www.aisc.org/globalassets/aisc/aess/all_about_aess_reprint.pdf

Dik

 

[BAretired]

I would use more than minimum temperature steel, but the amount would be dependent on exposure and thickness. For thin sections and exterior exposure, I would reinforce with just one layer in the middle of the section using 0.003Ag but not less than 10M @ 300 each way. For interior exposure, spacing could be increased to 400mm or 500mm.

To minimize shrinkage cracks, small bars are better than large bars.

BA

 

[dik]

BART: I'd use 0.5% as a minimum. Definitely not TS. And, yes, I agree that just 'throwing steel' in it is not a crack solution.

Dik

 

[kipfoot]

not a direct answer to your question, but:

You may want to look at an additive like ultrafiber 500, intended to reduce cracking during curing among other benefits claimed. The items you mentioned, benches and planters, ought to be cast and properly cured under controlled conditions, which I think is really the best thing you can do to reduce the likelihood of cracking.

 

[BAretired]

dik,
Okay, I can't argue with your choice. Another variable is the length of element or distance between control joints. A 6m long concrete element is more likely to crack than one which is only 2m long.

BA

 

[dik]

BART: The whole thing has to be planned... transitions have to be looked at, location and types of joints, etc. I would avoid fibres, either polypropylene or steel, Our Winnipeg Library has Architectural exposed concrete... and, some of the worst concreting I've seen done... like no one cares.

Dik

 

[dhengr]

Jrisebo:
You may have to pay some attention to the conc. furniture dimensions, particularly thickness and corner shapes and the like, to be able to get the rebar in place with sufficient conc. cover. Also, look at the overall dimensions and plan to prefab. the rebar cage/cages to fit into the forms or molds, to be fairly finely/carefully assembled and placed. You might also want to pay attention to lifting, handling and installation stresses for these heavy items. Any fibers in the conc. sound like a bad idea to me too, particularly if people are going to come in contact with the conc. furniture or planters, or whatever. They’ll be suing you for rashes, scratches, cuts and skin burns, etc. That’ll be like sitting on a very stiff wire brush, or a rubdown with same. You want a clean, smooth, easily cleanable surface finish.

 

[kipfoot]

what I mentioned are cellulose fibers. Steel fibers could rust at the surface and burn your {behind} and polypropylene would probably have to be burned off with a torch. I don't want to sound like a fiber salesman, so I'll leave it at that.

But I'll also mention that mock-ups are important in architectural concrete. It's helpful to have an approved result that further work can be compared to and to use the process to learn the best methods for your situation.

 

[dik]

Kipfoot... I'd even avoid cellulose fibres, too.

Dik