I’d be a little cautious dropping in several tons of mass over a few inches of concrete unless I knew it was reinforced (depending on age of house), in good sha

I’d be a little cautious dropping in several tons of mass over a few inches of concrete unless I knew it was reinforced (depending on age of house), in good shape, and soil conditions - even for slab on grade. Pour conditions are highly variable too - how do you know the contractor wasn’t drunk that day. I suppose the point is that it doesn’t matter what concrete was used as long as you knew how it was actually built and current conditions of the foundation (and below the foundation for that matter).