Constructability of a 6" Concrete Wall 7

[KootK]

Sometimes I will make the central walls in concrete stair shafts 6", in an effort to be economical. I've used 6" walls in a few other applications as well, for nominally structural elements. Always, there will be a mat of light rebar centered in the wall. So far, no complaints from the field.

A colleague called me out on this recently with the criticism that such a thin wall is actually difficult to pour and/or vibrate because you've only got a space of 65 mm in which to pour the concrete and execute vibration. And I don't really have a counter argument to that.

Does anyone have any experiences to relate, good or bad, pertaining to the constructability of 6" concrete walls with single, centrally placed reinforcing mats? This isn't a trick. I'm not going to parlay this into some triple digit thread on some arcane theoretical issue. I truly just want some practical advice from my practical minded digi-cohorts. Bring it!

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.

 

[SlideRuleEra]

KootK - When I was in a family bridge construction business in the 1970's, my father (in the bridge business since the 1930's) warned me about 6" thick cast-in-place walls. You would come across them for various applications on highway bridges. His warnings mirrored your colleague's concerns. This issue was so important to us that we would routinely make an unsolicited proposal to the DOT to increase wall thickness to a more suitable value... and the extra concrete would be furnished and placed free. Needless to say this proposal was always accepted. We put a lot of though into the new proposed thickness, it was not necessary an arbitrary number like 8", or so. Depending on the bridge's original design, we could often select a wall thickness that would simplify forming enough to offset the cost of the "free" concrete.

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[Brad805]

6" ICF's are very common in our area. When I poured my own 6" ICF foundation wall I personally did some of the concrete vibrating and it was not a problem. The diameter of the more common concrete vibrator I see is about 1", so if you have a single matt, you have space.

 

[MrHershey]

I've done it before with insulating concrete forms. I don't know that I'd do it again with those just because the forms are stay-in-place and it's difficult to tell if there are any issues without an extensive testing/inspection protocol. The ICF guys (or at least the ones I've worked with) are also trying to limit the amount of vibration done because they don't want their forms to blow out. With standard forms that come off after and you can actually see any surface issues I'd probably be more comfortable. Just have to be real careful with consolidation as you've noted. And watch your detailing and shop drawings, especially at items like trim bars. Had a job with 6" walls where we had just left the typical detail with your typical 2 trim bars over opening and our standard lintel detail showed them side by side. Acted like a dam, barely let any concrete through. Also look at specifying smaller maximum aggregate size. We had issues even with a specified 3/8" max coarse aggregate size. Think there were probably vibrating issues as well, though don't know that contractor ever admitted to it.

 

[manstrom]

I avoid tall 6" walls due to concerns with consolidation, but a short 4" or 6" stem wall (say 3' tall or so) has never been an issue.

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

 

[thaidavid40]

In Alabama we use 6" C-I-P walls often for low-rise construction, and below-grade structures (valve pits, junction boxes, etc.). I've not heard of anyone having any particular problems pouring normally-designed walls this thickness. It's all in the details (and workmanship).

Thaidavid

 

[precast78]

Don't people use self-consolidating concrete nowadays?

 

[BUGGAR]

Use form vibrators.