There are much divergent criteria for the design of slabs on grade and entire books devoted to it. The structural design can be made as if a slab on springs of some modulus of subgrade reaction or an elastic half space, bot options more or less in the reach of many computer programs as of now. However other factors influence the design, such curling on drag and differential drying and reinforcement, cracks due to shrinkage etc. My experience in this field has been mainly in parking for condominiums, with no access for trucks. We have used typically 15 to 25 cm thickness where the slab on the ground is not a structural mat, systematically reinforced with 2 wire meshes top and bottom faces and cut at least partial joints after a finishing conrete topping has been added typically from abot 4 to 6 meters or a bit more. We have compacted previously and sometimes added a special cohesionless subgrade. With this approach we have had in the slabs on the ground less problems that in the cases we have used mat foundations all over the plot, where the concentration of rebar and bad curing under extremely hot and windy weather has caused in a scarce number of occassions drying and sinking shrinkage (on the rebar) be noticeable.
If you are interested in concrete slabs-on-grade subjected to heavy loading, I have found the U.S. Army Corps of Engineers publication on the subject useful. It can be found at:
