I am a civil engineer and design slabs, retaining walls, and pier type foundations everyday.
For the average farm tractor weight, a 6" slab will be more than sufficient. You could get away with using 4x4 wire mesh, but I would use #4 rebar on 12" centers to create a grid pattern. The bars should be 2" below the surface. Rebar gives the concrete strength in the upper portion of the slab. Any lower than 1/2 way and it is not helping you with the tension the slab sees. Concrete works great in compression, not tension. Control joints should be cut at no more than 15' spacings. Remember, it is a given all concrete will crack somewhere. You just want to control where. Compacted soil is a must to keep it from settling. If you are filling the interior with subgrade, compact the stone or soil in 6" lifts. Rent a compactor. Driving a tractor over it is too risky and allows uncompacted voids. Use a standard 4000 psi concrete mix with 5%-7% air entrainment. Also use a vapor barrier under the concrete. Concrete should not be placed on dirt. It needs a minimum of 4" of crushed stone. I suggest to thicken the edges to go below frost line depths. This will help in eliminating heaving during freezing and thawing times if water runoff is a problem. If water isn't an issue, the slab will weigh enough that any water that freezes under it will not be able to push the slab upward.
Soil conditions, unless mush really won't play a large part with your type of slab. Thickness of concrete has very little to do with soil conditions. It is mostly the square footage (footprint) that matters. The thickness is determined by what the punch through loading is. 6" of concrete will hold a lot of weight if the footprint is large enough.
If you are pouring it in separate sections on different days, put a 1/2" foam strip expansion board between the slabs. Think of the earth's plates during an earthquake.
Check your local and state building codes. They have them for a reason.
Keep heavy equipment off the floor for at least 7 days. 4000 PSI concrete will cure over 28 days, but after 7 days it has about 75% of its strength.
This post was edited by johndeerefan at 12:38:53 09/23/11 2 times.
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