Betwixt all my buildings, I started with everything from marginal concrete to dirt. Marginal, meaning 4" thick if LUCKY, and NO reinforcement. Over time, I've fought this same battle, and there's really only one machine that I can count on to get in and out of all the buildings and carry a pallet... that's my farm tractor with a light 3-point fork attachment.
My pole barn has a foot of crushed rock, starting with a base of 4", then 3-and-down, topped with fines. I put it down in 3" lifts, using front end loader to spread, and a plate compactor to hammer it in tight. I did this because the bozo caretaker of the property chose to dig out a foot of dirt from the floor to provide enough overhead clearance to rent the space for RV storage... effectively, it became a swimming pool, and in middle of winter, that water would freeze, crushing RV wheels and tires where they sat...
Suffice to say, that no matter how hard one packs the floor, there's only ONE time of year that I can rexpect my Hyster H50H to be able to get in and out of the building with any servitude, is during the middle of damned-cold December through February... and the reason why, is because Mother Nature provides pavement... ice... to hold it all together. Without it being frozen, the rock simply cannot compact tight enough to maintain cohesion under concentrated loads of the Hyster's small tires... it peels up just a little rock, immediately causing the forklift to lean and rock back and forth, digging divots that it cannot climb out of. Yes, small steer tires, with no load on the mast, is the worst combination.
My Clark IT-60 has Super Singles all around, it does better, but it's too big and tall to maneuver well inside the building.
On-grade, Four inch concrete... even reinforced... lasts about one winter, before it starts cracking up under the weight of machines. There's simply not enough cross section to allow the concrete to support concentrated weight over soil that has moisture under it... frost heaving causes the edges to lift the center, leaving it unsupported... so until the soil beneath can be isolated from moisture ingress, and prevented from freezing, it will NOT support machinery weight worth a hoot. I wish I could do my floors for $5/sqft... but no chance- they'll be twice that... plus a border foundation and drainage, foam insulation for stopping frost.