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Re: Rigid Foam and Concrete Overlay

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For their products  (Plazamate, Perimate,etc.) Dow lists the compressive strength determined by ASTM D 1621.  Theoretically,  you can use that value to calculate a "foundation modulus" which some (nobody I know)  suggest can be used as a k value.

However, the compressive strength listed for Plazamate is 60 psi;  this is measured at a deformation of 5%.  Per D 1621, the "apparent modulus" is calculated as (load * initial height) / (area * deformation).  Since the load is measured at a deformation of 5%,  the foundation modulus ends up as 60*20 = 1200 psi/in.

There was a recent article in Concrete International that discussed this issue and pointed out that a k value that high was meaningless.

Although Dow doesn't list the k value,  presumably they've done the testing to determine a k value (ASTM 1196).  You could try calling them and asking them.

 

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