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Re: Concrete floor overlay using rigid foam to save weight

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        Back in the 1970s I was Lead Engineer on a project at a beach on the Arctic Ocean.  After about two years of planning and design work the project was shelved for a variety of political/environmental reasons.  One of the features of the project was the site development, the major component of which was the permafrost protection system.  We were planning to install about 25 acres of Dow HI 60 (we checked out several products and considered this one best for our application) insulation 4" thick over a 6" lift of pit run gravel and covered by an oil resistant membrane and another two feet of pit run gravel.  Based on literally hundreds of hours of study we believed that this system would work well (but, of course, it was never proven in the field).

        What you will have if you do this is essentially a "slab on grade" design (with an exceptionally good sub base) for your new floor.  It should work well but there are a couple of negatives you should watch out for.  There may be some "creep" that could affect your slab elevations over the longer term; you should check this out with Dow.  Some chemicals like oil and gasoline seriously deteriorate styrofoam; if there's any chance of a spill (say from cars in a parkade) you may want to provide a membrane or other protection.


H. Daryl Richardson

Eric Green wrote:

I have an existing elevated concrete floor slab that needs elevated 9 inches. Because the floor is only rated to 100 psf LL, I cannot add 9 inches of concrete. Therefore, I am going to use 6 inches of rigid foam insulation with a 3 inch reinforced concrete slab poured over the insulation (The architect & owner understand this will drop the allowable LL and are ok with this). I have see this done several times before, although I have never been involved in the design of such a system.

My question is this. Dow has rigid insulation rated for 25 psi compressive strength, where compressive strength is defined at 10% deflection under load. However, they also sell a material called high load, which has a compressive strength of 40/60/100 psi (there are 3 varieties) at 5% deflection under load. Dow technical services cannot provide me with any information regarding what the allowable load should be when used under a concrete floor slab. Has anyone here ever dealt with this issue?

It seems the 25 psi material should work as this provides a FS of about 18 under DL+LL (100 psf), even using the 5% deflection criteria (i.e. compressive stress of 12.5 psi rather than 25 psi).

Eric Green, PE