Location makes a big difference in a shop floor. In my area we have a good base of clay to build on and it's very stable. A 6" floor with rebar will work fine. I could see other areas needing more. My floor is 6" thick and made form a 6 bag mix. My heaviest machine is the Haas VF3 at almost 16,000 lbs. No issues at all. The base you build on makes a big difference. If you pour a floor on unstable ground you're asking for problems.
