Hello to all!
I don’t get it. How can you insulate a slab!??!
How can the 1-1/2 to 2 inches of foam that is placed under the mesh and/or rebar ever support a car or even a multiple car load without compressing, and therefore not give all kinds of movement issues?
Regards to all in this great place,
Hunts
Replies
The nominal compressive strength of XPS is 30 psi at 10% maximum deformation, which means that nearly any moderately stiff building material will spread the weight enough to support a car. An 8"x8" piece of 3/4" plywood under each wheel would hold a 7,000 pound car with less than 10% deformation of the XPS.
A 4" concrete slab is much stiffer than 3/4" ply. In fact, if I understand my engineering mechanics correctly, concrete doesn't deflect appreciably under load. It stays rigid until it cracks. That would mean that any load is spread evenly over the whole surface. A 10'x20'x4" concrete slab weighing about 11,000 pounds, plus the 7,000 pound car, would impose a load of 0.625 psi on the underlying XPS. If we assume that the deformation under load is linear up to 10% at 30 psi, then 0.625 psi implies a deformation of about 0.004" in a 2" thickness of XPS.
That's if the foam is actually supporting the slab. If the edges of the slab rest on footings, which is the more usual case, then the load on the foam, and the resulting deformation, is approximately zero, until the load gets large enough to crack the slab.
Wow, thats wonderful. I can see the concept much more clearly now, especially since you made the additional refernce to the slabs edge's resting on the foundation.
I appreciate your time.
Hunts.