I built a 20x30 building on a 6" slab with fibermesh and footers. 14 cubic yards of conrete for that slab! Some spots were more than 6". Then again I have to meet a 120mph wind load code which required some large footers.
minimum would be 4 inches but if it were mine I would go at least 6inches, use the fibermesh and put down poly sheeting before pouring, helps keep the moisture in the mix, slows the cure rate and the concrete will cure harder. (The slower the cure the better. Most concrete gets about 90% strength within 27 days. Anything you can do to slow the process is good ie keep cool, keep watered, don't let dry out etc)
4" is standard for a garage. Make sure you are pouring on a well prepared base and use a vapor barrier. The poly will slow drying of the concrete for greater strength and will also prevent moisture from wicking through.
Go with a 6" slab, trowel finish, with the 6mil poly under it. You can level the area out with a sand base, compact it, then place the poly sheeting over it. I would suggest digging a perimeter trench around the slab to keep any soil from washing out around the edges and to support the exterior walls. When I did this, the trench was about 8" deeper than the slab by 6" wide at the bottom and I also put an extra piece of rebar in the lower section, tied back to the main slab steel. If your slab is going to support the exterior walls, then I would make it at least a foot deeper than the slab, going into undisturbed soil. Don't forget to add anchor bolts for the exterior wall sill plate. The slab I poured was inside of an already constructed 40 x 50 pole barn, so it did not have to support any of the exterior walls.
For the slab, I used #4 rebar at 16" on center, placed on 3" chairs - much better than mesh which usually just gets stepped on and winds up at the bottom of the slab. I also added area floor drains and restroom plumbing.
I think "anything" is a bit of a stretch. I doubt it'd hold up too well if I were using it for my semi tractors over an extended period. [img]/forums/images/icons/smile.gif[/img]
A friend of mine built a 50' x 100' steel building himself on a slab and poured his slab THIRTEEN INCHES thick. While I'd never recommend that or probably ever do it, I have to say there's not been the first crack in it in almost a decade now and it gets all sorts of heavy equipment run over it on conventional rubber tires, steel wheels and steel tracks.
To quote him, "Everybody laughed at me when I built this place... 'too much concrete'... 'shoulda just built a pole building'... 'waste of money'..." Well, by gawd, they sure ain't laughin' now when this place still looks new and their slabs are cracked and their buildings look like sh*t! "
I watched briefly while they poured the slabs for an addition at the Anheuser Busch plant in Syracuse NY. They used poly and it was a 12 inch pour. Finish is like glass, then epoxy coated. No cracks there that I've ever seen [img]/forums/images/icons/cool.gif[/img]
Steve, we have a 24" frost line. Floating slabs are not the best things to use. A better choice would be 8" block footers capped with shoe block and 4" of concrete.
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