Incidentally, here's a page from the FoxBlocks published "Structural Engineering Design Guide US" that shows their recommendation of tying the basement slab to the footing rather than the walls. Same effect I suppose.

I showed this to the engineer but he didn't use it. He was pretty adamant about tying directly to the walls.

The FoxBlocks method of tying the slab to a massive footing is commonly done and makes sense to me when this lateral support is desired/required (although the base of the "L" should go to the left of the footing rebar). Tying the slab to a relatively weak ICF core doesn’t make much sense to me, but I am not a structural engineer. Removing all the EPS insulation from area where the slab/wall meet so as to thermally couple the slab to the ICF core also seems suboptimal. I doubt that this design approach would pass code or inspection here in our seismic area as flexibility is generally considered goodness and rigidity is generally considered badness. Smartwall's advice to brace from the exterior side and Arkie's advice to minimize the EPS insulation removal makes good sense to me. But I would still confirm with structural engineer that having any EPS between the slab and ICF core is ok because that is NOT what is shown in the drawing and would reduce the shear strength. Anyhow, good luck with your project.

Posted By arkie6 on 26 Apr 2020 05:57 PM

The red in my diagram below shows the rebar dowel and the green highlight shows the 4"x4" cutout in the foam that I mentioned above where you insert the rebar dowel into the wall, then screw a plywood scab over the foam to hold the rebar dowel in place and retain the concrete during the wall pour.

Thanks, I think this is what I'm going to do. It also seems to be what they did in the second video I sent. Now the only question is those two #5s that run horizontally in the wall at the diaphragm (slab). I'm leaning toward using a half block (full block cut lengthwise) on the first course so that the first rebar chair is upright and at the right level. Someone else told me just stick the two rebars in the top of the first chair and "it'll be fine" even though it's about 6" higher than the slab. I'm not sure I believe that.

Bad practice. Should have come out of the wall with a least a #6, not a #4. I've done suspended slabs for porches and you need stronger rebar. What you should really do is pour the footing, wall and slab in one continuous pour. I've heard of people doing it that way.

I'm going to be serious for a minute. You've given us some of the information, but not all. Block width being used, height of the wall, back fill height and type of soil being used to back fill. There is the book that came out in 2002. It's call the Prescriptive Method For Insulating Concrete Forms. It is the bible for all things icf. Every BI in NY has it and they follow it to a T. Download a copy . Another question: what is the rebar schedule both vertical and horizontal? I take it that you using all #5. Also is this a DIY build and pour ? Footer width?

I did not originally see the 2-#5 horizontals in the wall at the slab elevation...
The more I think about it, the more I am convinced that the 6'x8" dowels are there for for in plane shear... Because of the 6ft length, because of the 2#5's - because horizontal wall rebar is either for temperature (read more or less useless in ICF walls) or for shear walls. Since this is below grade, there must something else at play to require these dowels to be like this...
Also - bear in mind - your rebar is not that strange - there are jobs out there that are really pushing the envelope - count your blessings - poke the dowels through the foam block (sprayfoam the large gap you make this way or use plywood) and calmly carry on. They are 4 feet o/c - they wont bother you as much...
Congrats on choosing ICF for your home - you will not regret it!

Here's some answers to smartwall's questions:

Block Width: 11-1/4" / 6" core

Height: 10'

Backfill height: 8', sandy gravel soils (typical of southern Arizona). It's a walkout basement, so the grade gradually goes to FFE toward the front

Rebar schedule: #5 @16"OC horiz and vertical. 2#5 at roof/floor lines.

Footer width: 36" in the back of the structure, wall centered on footer. 18" in the front (walkout part of the basement).

DIY: Yes, with professional help on pour day.

Hope that helps. Incidentally, we don't get a lot of basements here in Tucson. In fact, I can't think of a single other residence with one. The engineer had never dealt with one either, so he's probably doing belt/suspenders/etc on this. Most other engineers wanted to design the back wall as a cantilevered retaining wall (which IS something the engineers are used to designing here). That would have required a much wider and thicker footer, much more rebar, and a thicker ICF block in the back. This engineer chose a restrained wall design (which is typical in every part of the country with basements), but it was his first one. The detail on the top is driving me nuts. He's got the mud plate on the wall right under the floor sheathing, with the 2nd story frame on that. It means there's no passage in the walls between the basement and 2nd floors. Something about wanting to pass that shear directly into the floor sheating without going through a typical rim joist setup. That said, I've got that part figured out with some special framed out passages/chases between the basement and 2nd floor for mech/plumbing/electric.

With that rebar schedule I would not pour anything less than an 8" wall. With 30 years of pouring these things I could not guarantee a completely consolidated wall, even with my being anal about vibrating. What happens when you pour especially with a double row of rebar at the top, is the concrete will, I repeat, will segregate. Simply put the mix will hit the rebar and separate. You will see the cement on the inside of the form and the aggregate will drop to the bottom. It's called graveling. The kicker will be the 2 bottom bars, where your most concerned, will be a major problem and exacerbate this situation. As far as the floor the engineer is right. Without the floor acting as a direct support for the top of the icf wall, you would be depending on toe nailed floor joist to keep the top of the wall from failing. As far a mix. I would go with at least a 4000 lb mix. Hopefully you can get pea stone. It works better the crushed stone as far as flow. The first pour I would use a water reducer. Make sure you have a vibrator that can reach all the way to the bottom. Should be a least a 1.5" head, not the worthless pencil vibrators. A 2'" would be best but if you stick with the 6" form you'll have problems. All the horizontal rebar should be in the fingers closet to the interior wall not in the center of the block. One trick that I've used is to cut the unused rebar fingers off the lower blocks. It allows the concrete to flow better. I now use a form with no fingers. There like have 4 cup holders in a 2 seat car. Get a whole bunch of electrical ties. I use these instead of wire to tie the rebar together. If you have a steel supplier handy, order all you rebar from them. It's what I do and you'll have less waste, plus it tuns out cheaper. Good luck.

I agree that the rebar seems a little skimpy. Up the verts and size down the horizontals.
I would change it to #5 verts @ 12" o/c and #4 horizontal at 24" o/c. Or maybe even 32". You do not need a lot of it.
Its the verts that do all the work.

Not really skimpy amount of rebar. Just the opposite. The horizontal is over the top. I could tell it was someone who never wrote a spec for an icf. When I started, the spec. was rebar every course, but since the PM came out in 2002 we have guidelines for rebar. I forgot, that the other video that you posted shows what I was talking about. At the end when the guy cuts the foam in the wrong place. Then they show a shot from the top of the wall down inside. That grey stuff on the inside of the wall is the Portland cement. Doesn't really do any good in regards to the strength of the bottom of the wall sitting, attached to the wall above. That's why I said the horizontal rebar has to go closest to the interior wall. Give yourself a chance at a good outcome by keeping the inside of the form clear. That's why I switched back to a form from 25 years ago. It's called Premer Icf. Doesn't have rebar fingers, just ties that hold to rails together, so there is nothing to block the flow of the concrete.

Thought I'd give you guys an update on what's transpired. After a long call with the engineer, he agreed to remove the two bottom rebar at the slab level. Said that's more of a masonry detail anyway. We still have 2 #5s at the top though. In speaking with another contractor that does a ton of ICF, they told me that when they get this detail on a 6" wall, they always move the two rebar ontop of one another until they're about to pour the very topmost lift, then they run around and move the 2nd rebar to it's proper position. That way the 2nd bar isn't there segregating the concrete right at the top (and you don't have concrete on that bar setting over the course of the day).

So that's where we are. Wife and I have stacked up the first 5 courses already (this ICF stuff goes FAST....and it's a lot of fun...)

Waited to see if someone would come up with the right answer. Actually remove both top bars before the pour. You do 4 lifts in order to vibrate the concrete. When you get to the last lift replace the rebar. I never pour with the top rebar in place. It takes no time to reinstall it since it's already been cut.

Smartwall - may I disagree with you? I checked the guide that you recommended before writing my post above.

Yes, his verts are a little skimpy. See table 3.4 in the guide that you recommended.
https://www.huduser.gov/Publications/PDF/icf_2ed.pdf

The guide is asking for #5 vertical rebar spaced as follows:
30pcf soil backfill = 14" o/c.
45pcf soil backfill = 10" o/c
60pcf soil backfill = 6" o/c.
The above is for 9' wall height x 8' backfill.

His detail wants #5 at 16" o/c - which is less than the least dense backfill - in other words skimpy.

When I am out your way - you owe me a lunch!

Point taken. I didn't read the PM because we never get into that situation. It is always better to increase the block size than to cram in more rebar The more rebar, the more voids, which negates the effectiveness of the concrete. With the system were using now, we can go from a 6" to 10" for under $600, so it makes no sense to skimp. Send your address and I'll send you a certificate for a Happy Meal. It will save you a trip.

Baldwin, three things I noticed about that table 3.4. First, it's for 40 bar, but we're using 60 bar. The chart says multiply all spacing by 1.5 for 60 bar. Second, it assumes the rebar is in the middle of the wall, but moving it to the tension side of the wall (inside) is much stronger. Finally, it assumes a 5.5" wall, but these are 6".

Good you have a copy of the PM. You will also note that the compressive strength of the concrete is set at 2500 psi minimum. You would have to be a moron to use that strength. Two tips I forgot. Increase the amount of fly ash in your mix. Normal here is 20% without asking. With what your pouring I would go up to 35%. It actually strengthens the concrete and makes it really snotty. If your pouring in hot weather, pour the earliest the batch plant can deliver. Nothing like hot rebar to set up the concrete on contact. Also use the smallest pump truck you can get that will do the job. Measure the farthest point from the pump set up to the last corner of your foundation. The reason for the smallest truck is that you don't want some Bozo with a pot of high test coffee in his system looking to see what his 50 meter pump can do. I have 1 company that I use. 28 meter with a 15 ' 5" to 3" Ruff Neck reducer, controlled by a power cutoff at the reducer.

Forgot one other very important tip. You will not be able to see the bottom of the forms when you are filling. It's important that you see how your vibrating is going, if you need to adjust anything. I use an 500 watt work light that I shine thru the wall of the icf it shows you exactly what is going on at the bottom of your wall.

Thanks for the tip about using the 500 watt work light to see what is going on inside the IC form. I think that will be very reassuring to the builder and client. Even with vibrating I have been concerned about voids in the wall.