If its that hard why bother with concrete? I'd prolly not consider less than 3" with some sort of mesh in it....steel. I'd put plastic down first, and bagcrete is prolly not a good idea either.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.