upfrombottom":29cok2ku said:
As has been stated, concrete can't withstand the stress of tension, that is the reason for steel in the concrete. But contrary to what most believe, the greatest stress on concrete is at the surface whether bottom or top. The problem with steel is its deterioration from oxidation and it must have a good covering of concrete to protect it. The horizontal center of a slab is the most neutral part of that slab and ideally the steel should be placed toward the edge where the greatest tension occurs i.e. the top and bottom. The best solution for surface tension is fibermesh. Its noncorrosive nature allows its placement where it is needed most.
I agree with everything but the fibermesh. On the right soil it works fine. ( the standard for steel coverage is 1 ") On expansive clays I have seen to many structural cracks to have confidence in it. I also disagree to your recommendation of using sand. On a flat surface with good drainage it is great but in this case he said that he was in a sandy soil on a slope. Sand will flow in water. Sand will blow in wind. When compacted with the right moisture content it will be like rock. I will also disagree with the suggestion of not using plastic. In utility work or most anything else outside moisture is not a problem. In a silo it is a different situation. When concrete is placed it contains water. Therefore when the water evaporates there is shrinkage. Depending on how it is cured noticeable cracks may or may not appear. Even if you cannot see them they are present. This makes concrete somewhat porous. The exact mix will also make a difference. Plastic is just cheap insurance.
As far as thickness is concerned, If you have ever parked in a modern parking garage it was probable built wit precast panels that are hollow and only about 4" thick. The exterior and interior beams are what give the main structural strength. A footing is used on soils with low compressive strength to spread the load.
Soil compaction will be highly dependent on the make up of the soil and the amount of water in it. Sand gravel and red clay sound like very stable, none or low expansive soil.
A gasoline driven plate compactor should do the job. But again if is to dry or to wet it will not compact properly. The soil fill should be compacted in 6' layers. When you pick up a hand full and squeeze it, it should be just moist enough to hold together as a ball.
I am not saying that my suggestions will work for you with your particular structure. I do not know your soil or the load that will be place on it. No matter how good you make the foundation you are still dependent on the soil beneath it. I have seen many highly engineered buildings fail. We live on the Good Lords dirt and He's in charge.
When in doubt the wisest investment may be to pay the expert. A registered professional engineer familiar with the soils in your area.