Grade Wall + slab monolithic pour

[ToadJones]

A contractor I do some work for wants to pour a 6" thick grade wall + slab in one monolithic pour for a residential foundation.
Basically he wants a turndown slab with thickened edges that will transition into a 6" wall that will go below frost....about 42".
I have some concerns with frost penetrating the wall and possibly causing heaving of the soil/slab on the inside of the grade wall. I was thinking that placing rigid insulation inside the wall and for some distance inside under the slab might mitigate the problem.
Am I off base here?
I tried to talk him into pouring the foundation wall and slab separate but he does not want to do any extra forming.
Any thoughts?

 

[msquared48]

Toad:

With a 42" frost depth, you have to be in a cold environment. I would be very concerned with the cold being transmitted thru the wall and into the slab. I would also be concerned with expansion and contraction of the interior slab due to the seasonal temperature changes. Is there tile on the floor to worry about?

I would break the slab from the wall and use the insulation you suggest, even installing insulation between the slab and the wall with special detailing.

What do the Architectural details for the residence show?

I think this is another circumstance where the contractor is trying to practice engineering to save himself money in a tight economy. He needs to do it your way. You are the SE, not him.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[mmillerpe]

You said that this is for residential construction, I am assuming that it will be a heated structure? If so, you are going to want the insulation on the exterior of the wall to the top of concrete otherwise there will be a huge cold-bridge at the intersection of the wall and slab. The contractor would need to provide some protection where the insulation is exposed above grade.

By insulation the exterior of the wall, the theory is that the wall and soil below will be part of the heated space, and therefore not subject to freeze.

ASCE 32-01 Design and Construction of Frost Protected Shallow Foundation provides some good information and details for this situation.

 

[ToadJones]

Thanks for the replies...seems like you guys have the same concerns I do.
42" for frost is probably overly conservative for my area.

To clarify the space is actually not heated. It is basically a garage space with residence above (which I really don't care for either).

Am I off base to think that if no insulation were provided on the interior or exterior side of the wall that the frost heave of the interior slab is actually possible? If the slab and wall are poured mono then this could be a real problem, no?

 

[paddingtongreen]

I have used the contractor's system with insulation in the NE. I remember the BOCA Code required insulation inside the concrete wall and for a distance under the slab, but sorry, I don't remember how far inside.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ToadJones]

Pad-
I guess I have no problem with the system in a frost-free climate, but I am in the midwest/NE as well. I will have to talk him into using insulation or I'm out!

 

[mmillerpe]

As you know you need several things for heaving to be possible: Freezing temperatures, water, and frost suseptable soil. If you eliminate one of these items, that the concern for heaving is minimized, if not eliminated.

I grew up in a house in upstate NYwhere the detached garage was built on a haunched slab similar to what you described except the haunch was only 12-24" thick. The soil in the area was primarly gravel. There has been not issues with the system to date.

Again, there are guidelines in the ASCE Guide that deal with unheated systems as well, and the recommendations include a layer of rigid insulation below the entire footprint of the building extending a certain distance beyond the outside of foundations, and a layer of "non-frost susceptable" material among other things. The final details are based on climactic conditins where the building is.

To get back to your original question, yes there is a real concern for frost heave in the condition you describe. There are ways around it, but it is a question of the expense of doing these versus the cost of "traditional" foundation design.

 

[ToadJones]

I know that the area this is going to be built has a lot of frost susceptible soils naturally.
The area scares me, and contractors aren't too big on proper drainage for slab foundations because they know there will never be a leaky basement to deal with.
I think if the grade wall and slab were to be poured separately, then slab heaving in the garage area is really not too big of a concern. Another benefit would be that the grade wall could be poured up above grade so that the wood framing could be held a good distance above grade.

Thanks for your input.