I worked on a job in which a bank was being built. I thought they would never get finished pouring the floor. 3 levels of 8" pours, each level had criss-crossed rebar every 4". More rebar attaching the levels of pours.Although I do have to say the guys here working were less than impressed when they realized that 4 foot by 25 foot section they cut out of the pad in the back has rebar thru it every 6 inches. BOTH ways ... They just THOUGHT they were gonna bust it up with a hammer and haul it off.
Them boys installing rebar had their work cut out for them.
The floor where the vault was to be was even thicker, with more rebar. It would take some doing to go in from underneath.