Traditionally basement slabs are 3 1/2" thick but?  The makers of "Helix" have a parking lot that is only a 2" slab on EPS foam over grade and it has held up under several years of use.  It would seem that  a 2 1/2" slab of 4000 PSI concrete with 15#/yd of Helix over say 3" of type 1 EPS would be a fine basement floor.  Has anyone gotten an inspector's approval?  The saving in $ for less concrete more than pay for the Helix and I suspect the result has lower embodied energy so it's probably "greener".    Type 1 foam seems to be all that's needed as it has a 10 PSI compressive strength which when derated by 1/3 to prevent "creep" still comes out to 480 PSF while even 3 1/2" concrete @150 #/yd adds a dead load of 31PSF to the residential design floor loading of 40 PSF. giving a "safety factor of 6+.

I think that durability would depend on the soil underneath it - one example doesn't prove much for other cases (like expansive clay). Other techniques for concrete reduction are non-uniform thickness (use foam strips to create beams) and post tensioning (sometimes used for tennis courts). Or high strength concrete (up to 19,000 psi).

Posted By jonr on 20 Jun 2014 12:54 PM
I think that durability would depend on the soil underneath it - one example doesn't prove much for other cases (like expansive clay). Other techniques for concrete reduction are non-uniform thickness (use foam strips to create beams) and post tensioning (sometimes used for tennis courts).

In exactly the same way as plain concrete or otherwise reinforced concrete could suffer over poorly drained expansive soil.  The 2 1/2" slab I've described is in fact stronger than a 5" slab reinforced with minimal 6 x 6 mesh.

You're losing some good quality mass in exchange for that lower embodied energy.

I wonder how much of the parking lot performance comes from the EPS preventing frost from penetrating (a frost protected design). And from a very well prepared base and/or a higher psi mix. "Stronger" is such a vague term when it comes to reinforced concrete performance.

Opps wrong thread sorry! :)

I pour 3" with Helix