I'm looking to build a simple 4 corner (62'x32') house in GA and I'm wanting to use 6" core ICF's. For cost I'm wanting to use a Monolithic Slab for the foundation and just stack the 6" core ICF's right on top of the slab. Code here only calls for a 24"x18" turn down (I think, maybe less. I need to verify) for a mono slab. If I bumped it up to 24"x24" would that be enough to support the weight of the ICF walls or do I even need to worry? I'm looking at doing 9' walls.

The question is, what's the minimum thickness to support ICF walls on a mono slab?

Is 24" x 18" width x height or height x width?

The depth of the "footing" portion of the slab will depend on local code requirements for frost protection (in my area it is only 12") and is independent of the load you put on it.

The width of the footing depends on the applied load and the soil bearing pressure. A 24" wide footing is pretty common around here (mainly because that is the width of most backhoe buckets) and should be able to easily support a 6" ICF wall if you have moderate strength soil. Assume 150 pounds per cubic foot for concrete. For a 6" core ICF, every 2' of height x 1' of length equals one cubic foot. In your case, 9' / 2' = 4.5 x 150 pcf = 675 pounds per linear foot. With a 2' wide footing that linear foot of wall is spread over 2 square feet, so the pressure exerted on the soil would be 675 lbs / 2 = 337.5 pounds per square foot. This loading doesn't account for load the roof applies to the exterior walls. Soft soil should be able to support ~1000 lbs/sq ft (psf) spread over a large area like a footing. Moderate strength soils should be able to support 2000 - 3000 psf.

If you use a frost protected shallow foundation design, then you get slab and footing insulation (no thermal bridging) and you generally don't have to go as deep.

I assume that you are doing a one story, if so consider a 4" icf

Bad advice: 4" ICF should be illegal. You can't get good consolidation because you can't vibrate it properly and it's a pain in the a$$ to work with. You'll save a few pennies on the concrete (and I do mean pennies) but it'll cost you a world of frustration. People who don't mind spending a buck to save a nickle will disagree, but they're usually the guys who think that if it only costs time, that's no matter. I don't know about you, but I place a very high value on my time.

I wonder what it would cost to get a more self-consolidating mix at the same (or more) psi strength. That takes even less time.

Posted By jonr on 12 Apr 2013 12:06 PM
I wonder what it would cost to get a more self-consolidating mix at the same (or more) psi strength.

A few years ago, in areas where I priced Agilia mix, it was quoted about $30 more per cubic yard than a regular 5" slump mix.  I have not checked lately, but I am hoping that the price will drop as it gets used more.

It's a good data point, although I don't think that one needs to go to 0 slump for a 4" ICF. Maybe $15/yd extra for sufficient admixtures?

http://www.youtube.com/watch?v=pEOmrWBL77w

I always pour 4" walls with a 1/4" pea gravel for aggregate and a 6" slump. Never had consolidation problems. I do however, try to talk home owners out of using 4" block when necessary, as the savings in concrete is not too significant (less concrete, but higher cost per meter by going to special order aggregate). There is also slightly more rebar, which is a marginal cost increase. The big loss is in sound rating of 4" ICF vs 6".

There are exceptions, such as in-fill developments. When the site can only accomodate a 20' wide house, there is no room to lose the 2" on each wall of a 6" ICF. Whenever possible, use a 6" block or you will likely regret it in my experience.

Posted By arkie6 on 12 Apr 2013 06:24 AM
Is 24" x 18" width x height or height x width?

The depth of the "footing" portion of the slab will depend on local code requirements for frost protection (in my area it is only 12") and is independent of the load you put on it.

The width of the footing depends on the applied load and the soil bearing pressure. A 24" wide footing is pretty common around here (mainly because that is the width of most backhoe buckets) and should be able to easily support a 6" ICF wall if you have moderate strength soil. Assume 150 pounds per cubic foot for concrete. For a 6" core ICF, every 2' of height x 1' of length equals one cubic foot. In your case, 9' / 2' = 4.5 x 150 pcf = 675 pounds per linear foot. With a 2' wide footing that linear foot of wall is spread over 2 square feet, so the pressure exerted on the soil would be 675 lbs / 2 = 337.5 pounds per square foot. This loading doesn't account for load the roof applies to the exterior walls. Soft soil should be able to support ~1000 lbs/sq ft (psf) spread over a large area like a footing. Moderate strength soils should be able to support 2000 - 3000 psf.

18" high x 24" wide. I'm around Atlanta GA so we don't get to cold. I just remember 18" for some reason but I'm waiting on a call back from the inspector to know for sure.

Thanks for the advice but 6" blocks are the same price if not cheaper than 4" (considering 4" blocks are special order and not made at the local ICF plants) and concrete isn't all that much more for the 6" core. I don't mind losing the extra 2" on the inside of the house.

Talked with my inspector today and he says code is 12" deep x 16" wide for mono slab. He want's 12" under finished grade and 6" above.

Going by arkie6's math a 16" wide footing should easily support 6" walls on even softer soils.

Posted By gbennett on 12 Apr 2013 06:36 PM
Talked with my inspector today and he says code is 12" deep x 16" wide for mono slab. He want's 12" under finished grade and 6" above.

Going by arkie6's math a 16" wide footing should easily support 6" walls on even softer soils.

But does the guy digging the footing have a 16" wide bucket on his backhoe?  24" is most common around here.  I would err on the side of caution and go 24" wide with three (3) 1/2" rebar, because that 16" wide footing is a minimum for supporting a wood framed wall.  Also, what are you using for the exterior?

arkie6 - Thanks for the help.

You're correct. I will defiantly error on the side of caution and go with the larger 24" footing with (3) 1/2" rebar.

What would you do to get the rebar that needs to be out of slab into position? I was thinking of just tack welding it to the 3 footer rebars.

Plan is to just use vinyl siding.

Roof will be 2x4 trussing and metal roofing.

Posted By arkie6 on 12 Apr 2013 08:30 PM

Posted By gbennett on 12 Apr 2013 06:36 PM
Talked with my inspector today and he says code is 12" deep x 16" wide for mono slab. He want's 12" under finished grade and 6" above.

Going by arkie6's math a 16" wide footing should easily support 6" walls on even softer soils.

But does the guy digging the footing have a 16" wide bucket on his backhoe?  24" is most common around here.  I would err on the side of caution and go 24" wide with three (3) 1/2" rebar, because that 16" wide footing is a minimum for supporting a wood framed wall.  Also, what are you using for the exterior?

If the footing is formed it can be any width narrower than than the trench. When I built my house I calculated the loads on my footings using 1500 psf soil bearing, plus all the live and dead load for the floor (I have a crawl space), plus the additional wall height below floor level (as much as 6'), plus the roof and attic dead and live load, plus snow load. A 16" wide footing was sufficient. The load in GA with slab on grade will be a lot less. I did have to put in some 32" square footing blocks to carry the point load where roofs intersected at 90°.

Posted By gbennett on 12 Apr 2013 10:53 PM
What would you do to get the rebar that needs to be out of slab into position? I was thinking of just tack welding it to the 3 footer rebars.

Wire tie it. Rebar is crappy to weld.

Posted By dmaceld on 12 Apr 2013 11:01 PM

Posted By gbennett on 12 Apr 2013 10:53 PM
What would you do to get the rebar that needs to be out of slab into position? I was thinking of just tack welding it to the 3 footer rebars.

Wire tie it. Rebar is crappy to weld.

My only issue with that is the rebar will want to fall over. My thinking is that I need an L shaped piece sticking up out of the slab. The smaller part of the L will be attached to the footer rebar and the larger part will be up out of the slab. One of the ICF supplier guys I talked to said to just stab the rebar in the slab as it's poured. What's everyones thoughts on that idea? I'm probably over thinking this part.

Posted By gbennett on 12 Apr 2013 11:10 PM
My only issue with that is the rebar will want to fall over. My thinking is that I need an L shaped piece sticking up out of the slab. The smaller part of the L will be attached to the footer rebar and the larger part will be up out of the slab. One of the ICF supplier guys I talked to said to just stab the rebar in the slab as it's poured. What's everyones thoughts on that idea? I'm probably over thinking this part.

I believe stabbing is what I did, spaced at 2'. That should work OK for all the verticals sticking out of the concrete except for the one you want to use for your electrical ground. The rebar piece you leave open so the ground wire can be connected to it must be mechanically secured to the horizontal rebar in the footing. In case you're not aware that is called a UFER ground and is the preferred grounding per National Electrical Code. I opted to run the ground wire to the horiz rebar and clamped it before we poured the footing. Inspector would have preferred to see a rebar sticking out but didn't fuss too much.

I wonder how well a UFER ground works when the entire foundation or slab is isolated from the soil by plastic sheet and foam.