OK guys, here are some comments from a geotechnical civil engineer, licensed Professional Engineer now 87 years old. My specialty is foundations.
Some of what is given above makes sense, but you also should know that a concrete slab can bridge a lot of crap and nothing happens. I've built several garages for myself and have buried stumps and roots. Just be aware that any organic matter will eventually rot and take up less space. Under a well built slab, no sweat. I'd not worry about occasional roots 1" or smaller.
For site preparation it is not necessary to remove dark topsoil, since it already had rotted as far as it will. However, sod and brush and other highly organic stuff should go.
Compaction of any fill is necessary, and in layers. Even stripped ground needs compaction. However, the uniform grained "sugar sand" won't show much change if you try to compact it since all the grain sizes are the same. That's like trying to compact a bunch of marbles in a bucket. Try was you might, they won't take less space.
Be sure any compaction and other work extends out from the slab edges at least as far as the thickness of fill under the edges. You need edge support.
Compaction can be done with any vehicle, such as a car, a tractor with LOADED bucket, etc.. Tracked vehicles don[t work well. Use at least 5 tire passes on every square foot. Layers should be thin, certainly not over 10 inches for these light "compactors". If bone dry, a small amount of water, not flooding, helps get best density. If muddy or obviously wet, that water takes up space that eventually will be air, so settlement can occur. The sugar sand might benefit with a little water to keep the vehicle from becoming stuck. ideally if you can grab a handfull and it forms a nice "ball", the moisture content is good.
The recommendation for a vapor barrier underneath is not needed for a garage type building. It only is needed for a building with special flooring that can be affected by moisture coming up thru the slab, such as a store with vinyl flooring or a gymnasium. I will add that having a vapor barrier underneath holds water in the wet mix. That keeps the moisture percentage from top to bottom the same. Since concrete strength and eventual shrinkage varies dependent on the initial water/cement ratio at any part, these, properties then will vary later if the W/C ratio is different, top to bottom. Where no vapor barrier is present and the top W/C ratio is high, (excess water at top to start), the top will eventually shrink and the final slab will develop a curl. You can see this on highways after a rain, with no water standing at slab edges, but ponded inside.
You can avoid the vapor barrier by wetting down the ground just before the slab is placed.
For the slab, I'd be sure to use some form of reinforcement, placed at mid height, for sure where vehicles will sit. Ideally 1/2" bars )(No. 4 size) spaced about 18 inches each way would be good. there are grids of wire fabric that will work. You can order the concrete with small metal fibers in it. Slab thickness at least 4 inches, preferably 5. Some folks will thicken the edge where wall and loads sit. The mix should be stiff as you can handle and call for 5 bags (cement) per cubic yard when ordering it.
Add a bunch of water to make it flow is weakening the resulting final strength. The higher the water /cement ratio of the delivered mix, the weaker it finally will be.
Don't over work the surface. Initially only a quick floating with wood is needed for initial leveling. THAT'S ALL. Working it to make it smooth early on brings up water near the top and surface spalling and flaking will happen later. Wait until the smoothing will ring from the trowel movement and it is damn hard to get the stuff to move. It may be a few hours after placing. Early working with a steel trowel will result in surface that falls apart later. Excessive water content there at the top.
Cut or create joints no farther apart than 12 feet. Cut joints must be cut while the slab is still very early in hours, not the next day.
Then comes the curing. Concrete takes a long time to finally reach final strength; that is years. However, the most important thing in the early stage is not letting it dry. Water is needed for hardening to continue. Ideally covering with plastic for a month would be great, but at least a week covering and keeping moist is mandatory. Yes, add some water if you see it getting dry at any time. It may seem strange that adding a lot of the water in the mix causes weakened final result, but after hardening is underway, you want to retain water so increases strength can develop.
) with the concrete work for a pole building, Morton, and the specs they provided in the contract were "Site preparation consists of the removal of organic matter, loose top soil, vegetation, unsuitable soil types, and cutting high areas or filling low areas with well-graded fill that is free of rocks (4" or larger except in column area where maximum size must be 1" or less) free of debris and frost when placed, and can be compacted to 95% of its maximum density. Earth removed from high areas can only be used if it meets these specifications. If the building will have a concrete floor, a minimum of 4" of well graded fill must be provided to level the rough grade. Well-graded fill is material consisting of particle sizes from its coarsest to finest particles, is trimable, compactable, and granular. A washed sand is not compactable and must not be used."
