OP: https://www.reddit.com/r/AskContractors/s/obwgKP2zhn
I pressure washed the whole floor at 35kPSI. A lot came off but at a certain point, around the bottom of the gravel layer—but with a lot of the gravel still hanging on—it stopped penetrating. I decided to take a scarifier to it to see if it can be planed down and if the concrete under the gravel seems tougher. I’m still working my way down past the gravel layer but I’m starting to hit more areas without it.
I didn’t mention this in the OP but the surface layer of the floor at the time I made that post was of such a quality that it was a health hazard to sweep the shed without a respirator because there was just constantly silica dust across the whole surface.
I honestly can’t tell whether the stuff under the gravel is any better quality. Like I said, the pressure washer did eventually stop talking the floor down, but the layer beneath the gravel seems to be that same powdery quality as the top layer.
I did determine that it is a four inch slab by digging up around the outside a little bit. From the outside it’s not obvious whether the concrete is junk all the way down. If I had no reason to think otherwise, looking from the outside, I’d assume it was perfectly good slab.
All the photos above are after pressure washing and very light passes with the surface planer. None of what’s visible is loose stone sitting atop the concrete: that is the concrete and it won’t move if you poke at it or walk on it. It’s just very chewed up.