Minimum Reinforcement in ICF Walls

[adubdavis]

I am performing a design of an ICF (insulated concrete form) wall home and have a question related to minimum reinforcement. If I am to design the walls in accordance with ACI 318, vertical and horizontal reinforcement will be required at 18" o.c. per 14.3.5. The ICF documentation (I will be using Amvic blocks) allows for vertical reinforcement at 48" o.c.

Am I required to use the 18" spacing or is there a special approval (ICC or other) that allows the increased spacing for ICF walls? If the 18" spacing requirement is to prevent cracking, I can see where the ICF may not need to meet that requirement since the concrete will not be exposed.

 

[HTURKAK]

I have ACI 318 -14

(11.7.2.1 Spacing s of longitudinal bars in cast-in-place
walls shall not exceed the lesser of 3h and 18 in. If shear )

If reinforcement is not required for shear strength, the maximum spacing for vertical bars shall not exceed the lesser of 3h and 18 in.

This is normative of the code. If you do not comply with the requirements of the applicable code , you will loose a very important shield and curious about , what will be your argument in case of a servicability problem ?

 

[XR250]

The 2018 International Residential Code has tables for this.
The ACI minimums typically do not apply to ICF's

 

[phamENG]

XR - I haven't had much of a chance to work with ICFs. I finally got one a couple months ago but the job was cancelled before I could really dig into the ICF design details. Is that because they base it on plain concrete rules or is it just a matter of ignoring cracking limits because it's all inside the foam?

 

[XR250]

I'm guessing that both of those are the case. I read somewhere that there is less shrinkage as the concrete is in a pretty nice curing environment.

 

[dauwerda]

To be clear, ICF's don't have special rules do they?
It's more that they are typically used in the residential world where the IRC is used. Or to put it another way, a typical house basement wall would have the same minimum reinforcement whether it were formed with traditional concrete forms or ICF's, correct? So, on the flip side, if ICF's were used in a project using the IBC, then they would need to conform with normal reinforcing rules, correct?

 

[phamENG]

I read somewhere that there is less shrinkage as the concrete is in a pretty nice curing environment.

Solid point...if they consolidate it sufficiently. Did a forensic review on a house a few years ago with nearly bowling ball sized voids in the ICF wall when the cut out the foam. Any idea if there's an industry standard practice to check for that without having to run GPR on it? I wonder how GPR would work with the foam? Or just ensuring workability/slump are in the right range and using a vibrator on the form as they pour? This brings up so many questions...

 

[dauwerda]

I have been on a few sites where you could stick a wire all the way through the wall below windows.

 

[RWW0002]

Right or wrong, I think the greater spacing of horiz. reinf. comes from IRC for residential basement walls less than 10' tall. See IRC R404.1.2(1. If your wall is greater than 10' or otherwise beyond the prescriptive requirements of IRC, I think you are required to design the wall per IBC and therefore would need to follow ACI minimum reinf. If you fall within the prescriptive requirements you are able to use IRC reinf. requirements if you choose. There are several threads on here from the past about the differences in IRC and IBC requirements in regards to horiz. reinf. in basement walls.

While there may be some slight differences in curing for IFC vs formed concrete walls, I don't think this is why you have less reinf. I think the 48" spacing option is likely in order to compete with formed walls that are being built per IRC requirements.

 

[phamENG]

ACI 318-14, Table 14.3.1.1 - Minimum Thickness of Bearing Walls (Chapter 14 is PLAIN CONCRETE, so no rebar)

General Walls: Greater of 5.5 inches or H/24 (5.5 governs up to 11feet)

So a 6" ICF wall would do fine for most single story applications, and an 8" is questionable (at best) for a squat 2 story as plain concrete. I don't think the IRC is quite as concerned with ductility - concrete wall provisions are limited to seismic categories A, B, and C. So if you're in D you have to design to ACI 318 anyway.

 

[HTURKAK]

The provisions for plain concrete explained at Ch. 14

Commentary R14.1.3 explains the limitations for plain concrete

(a) That are primarily in a state of compression
(b) That can tolerate random cracks without detriment to
their structural integrity
(c) For which ductility is not an essential feature of design

and the code refers for residential construction ' within the scope of ACI 332',

And , as far as i understand, ACI 332 does allow the plain concrete for footings , foundation walls and slabs on grade . Moreover , ACI 332 has provision for minimum horizontal rebar .

IMO, in this case, one can construct the walls of swine shelter with plain concrete but not the walls of a residential bldg except basement walls ..

If i were at permit office, i would simply reject any residential bldg design with plain concrete superstructure walls ..

 

[dhengr]

Adubdavis:
I was involved with a few of those ICF’s years ago, and haven’t touched one since, and my memory is failing me too. But….,
1. I’ve seen the big voids mentioned above. You can’t have/tolerate them, and they say they never happen because you usually can’t see or detect them without some extra effort. I believe we used self-consolidating concrete and a very small pencil vibrator. You can kinda thump the outside of the forms too, but it isn’t just pump the conc. in from above either. In any case the aggregate must be small. They look kinda like a large/thick waffle if you stirp the forming off. There are continuous voids/cells both horiz. and vert. at regular intervals, so you should have some options for rebar other than 48” o/c. That may work sometimes, and they sell that, it makes their system look inexpensive and easy to do, but they aren’t signing the drawings or dealing with the client when things start to go south. My rebar would fall someplace btwn. IRC and IBC, depending upon the actual conditions. We know that a conc. wall is plenty adequate for a two story house w.r.t. the gravity loads, so it is the lateral loads, soil, wind, etc. which really dictate the rebar and it’s basically in the center of the wall thickness, not out near the face.
2. You have to do a real good job of fitting the ICF blocks together. They have to be carefully plumbed and well braced laterally during the conc. pouring operation, maybe even tied through at some cutouts and the like, with extra bracing too. As I recall, we put a 24 key in the top of the footing, and really cleaned that out before setting the ICF form blocks, so we would get good keying.
3. As I recall, at window openings and the like we put in head and jamb bucks, tied into the forms, with the jambs blocked up by short 2x’s, and we left the sill bucks off, so we could see what was going on down there. We poured from both sides until the cell voids space below the opening started filling up and then moved on for awhile. Then we’d come back and fill and finish the rest of that area from the open sill buck at the bot. of the window opening.
4. This all seemed kinda fiddly to the builder, after the bill of goods he’d been sold by the ICF people, and they bitched a lot. But, we had some good pictures of some major blow-outs, large wall area failures, large voids within the wall cells, out-of-alignment walls, etc., and that made believers out of them. Didn’t stop the bitching tough. Isn’t this always the case, the new guy sells his product as the best thing since sliced bread, and twice as cheap and easy, but forgets to mention all the special precautions needed, and then is long gone when the problems start happening.

 

[phamENG]

Just so we're clear, I wasn't suggesting that you should build a house out of plain concrete...just using as a comparison to better understand some of the reduced requirements and how they relate. Thanks for posting ACI 332. I always forget about that. The only concrete in houses around here is either a footing (and there's nothing special in 332 about that, using 318 is sufficient) or pile foundations with grade beams, in which case I'm usually in 318 anyway.

I think most of the ICFs now have stricter rules on the height of lifts, etc.

Another thing to watch out for - I saw several recent publications and installation videos from ICF manufacturers promoting the wet setting of dowels. No good.