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You should make sure your site is well drained and stays dry. Moist soil will freeze and heave in the winter. You should run your roof downspouts well away from the building too, a minimum of 4' but 6' or more is better. Before you put down any gravel or sand you should cut back the top of the soil a foot or two and run a sheeps foot compactor over it, if it's mainly clay, or a vibratory roller if it's granular. Then fill back to where you want the top of dirt to be, compacting every 6" or so of fill material. Then put down your gravel/rock/granular base. A thickened slab at the entry doors is a very good idea. Heavy loads comming onto the edge of concrete can crack the concrete. Are you going to have a concrete or block foundation wall around the perimeter of the building? If so, then the slab should be poured over the top of the foundation wall at the entry, with it thickened and rebar added. You should use either 3,500 psi concrete or 4,000 psi concrete. Any weaker strengths are more likely to crack or the surface to be easy to scrape. The higher strength also helps to prevent skrinkage cracks, the concrete shrinks as it dries out. Make sure you use a "low slump" concrete mix, this means there is less water in the mix. You want the mix stiff and taking some work to spread. Don't let the truck driver or the concrete workers add any water to the mix, they want water added to make it easier to move the concrete. The added water makes the concrete more likely to shrink and crack and lowers the strenght of it. Cover the concrete with plastic or better is wet burlap. The longer the concrete takes to dry out the less the shrinking.
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