Scenario: 1930's garage. 18.5 x 20. Very inconsistent unreinforced slab floor, varying from 1" to 4", sort of cement veneer over very poor, crumbling, larger round rock/cement mix, over wet clay. 6" stem on 2 sides. I've removed a section in the middle, 6.5 x 15, with the expectation of making it 10 x 15 and strong enough for a car. Code requirement is "replacement in kind" for This Olde Garage repair. Next, I'll epoxy coat it.
The question: How thick? What kind of rebar? Underlayment and vapor barrier?
Hint: I generally overbuild my own stuff.
Grateful for any and all advice.