We're getting ready to start construction on our home. We are doing 10' below grade walls (two walls against dirt, one wall partial dirt, one wall no dirt) and 10' above grade walls - all ICF. The contractor we are working with has been building ICF homes for 20+ years and says that we should be fine with a 6" form below grade with proper reinforcement. This makes me nervous after reading "typical" foundation walls up to 9 feet are 8" thickness. Thoughts?

How long will the below grade walls be? Do your plans call for any ICF shear walls? That is, reinforced walls 90 degrees from the below grade walls. If so, how frequent? If any doubts, check with a local structural engineer.

I would only do this with 8" walls. It can be done with proper reinforcing as per the Prescriptive Method. It depends on soil used for backfill as well as the height of backfill. The real problem is properly consolidating the concrete at the bottom of the wall. 6" with proper reinforcing is going to retard the flow of concrete. If your builder has 20 years experience he should realize this. I've been doing this for 32 years and 8" is the way to go. One question to ask, is what vibration he intends to do.

You also got to look at the length of the wall. Like my house is very similar to yours with 10' height and going from all in ground to nothing in the ground. But my walls were all around 20' long with a lot of corners to stabilize the foundation (60 foot long wouldn't have worked). We went with 6 inch and was approved my a PE that mainly does ICF.

Did your PE pour the walls? Specs are one thing, best practice is another. As a distributor and a contractor, I'll give you what you want. As a contractor I'm only doing this with 8" except for the no backfill wall. As a distributor, if want 6" so be it.

These are the numbers he rattled off. 10' basement and 10' main level ceilings.

Basement - 1/2" rebar (#4) - 6" BuildBlock forms = 24" vertical spacing / 16" horizontal spacing

Main Level - 1/2" rebar (#4) - 4" BuildBlock forms = 48" vertical spacing / 16" horizontal spacing

Just as I thought. Tell this 20 year pro, go online and get a copy of the Prescriptive Method. It came out in 2002. It is the guide for reinforcing in icfs. He flunked the course. The full backfill wall would need a #4 every 8" to 4" vertically, depending on backfill used. You don't need a horizontal course every 16". It's a waste. 4" above grade 10' wall needs a designed rebar schedule unless your house is never going to experience wind pressure above 20 psi which is the minimum on the chart. Can't even image what the plan is for the doors and windows. Hopefully none are wider than say 30".

Oh I forgot. Did you ask him about vibrating the walls?

I have not asked about vibrating yet. According to the HUD Prescriptive Method we seem significantly off, but according to BuildBlock's engineering manual, we just need to adjust the rebar spacing some.

BuildBlock Engineering Docs
Below Grade - 6" BuildBlock - #4 Rebar - 6-12" vertical spacing / 32" horizontal spacing
Above Grade - 4" BuildBlock - #4 Rebar - 30" vertical spacing / 32" horizontal spacing

So it looks like we if we go from 24" to 6" vertical for below grade and go from 48" to 30" for above grade, we'd be on point for BuildBlock's engineering docs (updated Sep 2019), no? Am I missing something here?

The PM is always is the standard unless a engineer signs off on a different plan. Every BI where I am uses the PM as the guide. When I started the standard was, rebar every course and what for verts. Rebar,shmebar. The point is the more crap in the wall as you get taller, the chance of a successful pour diminishes. Your mix has to right on and your technique has to be perfect. You risk segregation of the concrete when you pour with the more rebar you add. Even the amount of webs and their design affects the results. This will happen in the bottom of the wall which is the worst place for it to happen. If you go back in the archives of this sight when it was the icfweb, you'll see pictures of botched jobs.

I really don't understand why someone would push for 6" and lots of rebar vs 8" and relatively lower rebar density. The cost of the extra concrete is minimal...and at least partially offset by rebar cost differential?

A tangential question...what about micro rebar? Would this be an application where it might help...less risk of consolidation problems (assuming the OP still wants to go with 6")?

Tim

Right, makes little sense.

Yes...but still go with 8".

I've used Helix since 2006 and really like it in certain areas. I use it in floor pours only. Some pump companys won't pour it, each for their own reasons and as far as wall pour it actually stiffens the mix so you can end up with void problems if you don't adjust your slump, and not with water. The other problem is how do you know the the ready mix supplier added the right amount. We batch the fiber on site. Stick with rebar for your walls.

Yes, finding a structural engineer who will design with it and a pump truck that will pump it has limited its use. And the structural engineer will still require rebar for the lintels. There have been law suits against concrete companies that have shorted Helix on projects...like Pensmore:

https://www.news-leader.com/story/news/local/ozarks/now/2018/07/03/pensmore-massive-mansion-near-ozark-continues-construction/651392002/

Very interesting info on the Helix. Thanks gents!

Posted By tarbe on 28 Jan 2022 04:48 PM
Very interesting info on the Helix. Thanks gents!

Your welcome...and gents and gals... Gayle