Chad,

You ask, "Does it matter how thick the concrete is in regards to the structure?". The answer in short is yes. Most all residential concrete above grade and below can be done with 6". I live in the most seismically active zone and I have done some 6" basements. Most of the basements I have done are 8" and once in a great while we do 8" above grade but not very often. Usually this happens under extreme site or building conditions. I work with an engineer that has designed 4" foam core concrete tilt up panels that are used in 30 foot high potato storage sheds. Concrete is a great and strong building product. I would say talking to your local engineers and getting some books to educate yourself is a great first step.

ICF Contractor

Thank's for the post. Looking back, that was dumb to ask "does it matter...in regards to structure" What I meant was, these ICFs are so well re-inforced with rebar, is a 6" below grade as unmarketable as my friend would think?

Chad,

I don't know if I have ever had a homebuyer that checked out the size of the homes foundation. If you have a properly engineered (perscriptive or not) wall that has the proper footing size and proper backfill with the surcharges taken into account, and the proper rebar that is properly placed you will have a great wall weather it is 6" of concrete or 8" of concrete.

Something has struck me about your post. I am wondering if are in one of the areas back east that does not use rebar in their 8" basements. Because if you are your questions make more sense to me.

Some things about concrete and ICFs you need to understand. The ICF product has little effect on the concrete, the need for rebar is a function of the forces at play against the wall as well as thickness of concrete. If you are pouring 8" conventional concrete basements with little to no rebar then you should be able to pour ICF basements with little or no rebar. In the current codes as well as the new 2006 IRC, 2006 IBC, and ACI 332 you can build 8" basements with little to no rebar with the right soil conditions and if you are not in a seismically active zone, although I think I am allowed to build a basic basement at 10" with no rebar.

The current codes and governing body in your area as well as the local engineers would be a great place to start with your questions.

ICF Contractor

I run into problems engineering a 6" wall thickness on multi-story bldgs.  The problem is not the shear strength (in-plane) or the bearing strength (vertical) but the out-of-plane strength.  The strength in this direction is based upon the distance from the center of the steel to the compression face of the concrete.   In an 8" wall this is 4"+ and in a 6" wall this is 3"+.  The problem is that the stiffness (deflection out of plane) is based upon this distance cubed and the strength (bending out of plane) is based upon this distance squared.  So, ROUGHLY, the stiffness is 4^3 or 64 vrs 3^3 or 9, so a 8" wall is 7 times stiffer than a 6" wall.  Also, strength in bending is 16 vrs 9, so a 8" wall is about twice as 'strong' as a 6" wall for out of plane loads.  The thickness also affects my ability to provide embedments into the wall to tie it to the mid-floor framing.

Another issue that affects the use of 6" vrs 8" are tall walls with no framing support (Great rooms or stairwells for example)

Unfortunately, I usually require the use of 8" ICF core on multi-story projects.

Patrick Clark, P.E.

Oops, make that 64 to 27 in terms of stiffness or a little more than twice (Thanks Bruce)

Anyways, there are many design scenarios where you will need to go to 8". I am sitting here in my office engineering one right now. And don't even ask me about 4" unless it's for the family dog.

Pat,

Your numbers are good but my question is how much strength do you need for a 3 story building above grade, also you did not get into the different sizing and grade of rebar. Another factor is the strength of the concrete itself. I understand all things being equal a generic 8" wall is stronger than a 6". What about 4"... I live and work in the siesmically active zone of the Pacific NW and we have used 4" ICF as well as foam core with 2"s of concrete on each side to retain 20 feet of potatoes in concrete storage sheds. Although I don't enjoy working with 4" at times it is a perfect fit for the job.

ICF Contractor

The reason I might prefer the 8" ICF over the 6" is the thermal mass.

Many times it is easier to get an 8" ICF below grade wall past the local code enforcement people.  They will require an engineer on a 6" wall and pass on an 8" wall.  It's what they are familiar with.  It's their call, they have the badge and the power. 

This is the truth, Inspectors are seeing more and more new products on the market these days, ICF's still haven't hit every municipality across the continent. When I started with ICF's the OBC did not require any rebar in an 8" wall below grade, therefore if I build a foundation only I could stand block and no inspection needed, just build and pour, however if I chose to do 6" I now needed the manufacturers prescribed rebar and it needed inspection. If we were building with a particular ICF that did not have prescibed tables for rebar and lintels than an engineer would have to do the design and inspect prior to pour (You could not use another block companies schedules (which is not a bad thing)).

Oh and as for the original question is 8" stronger than 6", depends on the application and it's usage, sometimes using 8" maybe just spending money not needed when 6" will do just fine

Chris

Chad,

     Your mason friend needs to learn a lot more about ICF.  I have built dozens of homes including my own using 6" ICF.  It is the most popular size sold by all the ICF companies out there.  Engineers can tell you the difference between 8" and 6" concrete walls.  The ICF is only the form.  I would check with www.rewardwalls.com.  The engineer there is Kelvin Doerr. 

                                                                                                   Mark Schneider

                                                                                                   ICF Construction

 

Now for my two cents worth, (whatever that’s worth).

First, Pat, I believe that your issue deals more with the lateral surface planes radius of gyration effect on the vertical plane of a wall. You may want to consider taking a close look at the old “ring steel” engineering approach that was used extensively in two way slabs, which helped to stiffen and support large floor areas. The principles work the same for walls, however with the added loads on the tops and side, there is an added stability factor. For example, three stories of thin slab concrete floors can easily be carried on 5” thick core walls with very little reinforcing steel, when combined with non-rigid pinned connection design at interior floor and exterior wall intersections with any interior walls. The problem is the total height, as it limits the design to about 9 feet in height, with a 11’8” limit on 6” walls.

Now for the rest:

The issues with thinner walls are primarily construction issues. I have seen a six inch wall with a tremendous amount of reinforcing steel, properly placed concrete, be only as strong as a 1” thick wall…..Yes, a 1” thick wall. Why this happens, and I see it a lot, is garbage, foam cuttings, sand, leaves, you name it, collect at the bottom of the walls. Static will hold these cuttings in place, in the edge between the form and the footing, inside the form, which creates a cove or coping of loose material, at the bottom of a wall.

As for 8” vrs 6”, 8” is much stronger, with a lot less steel, which makes for easier concrete placement, gives a better safety factor and is much easier to work with, often much quicker to construct as well.

Mark Ross
www.powersmartbuilding.com