It's not the strength of the concrete that matters. It's what is under it that really makes the difference. A highly compacted sub grade is what determines the viability of concrete. If your sub grade is weak, then the concrete will fail. 2400, 3000, 4000 it does not matter. Compact, compact, then compact some more.That's really not true. It might be true in most cases, but there are places where you can't get the soil compacted and stable, regardless of how much you spend. There are places where the soil is going to give you problems.
In those cases, higher strength concrete and maybe post-tensioning the slab come into play. Houston and SE Arizona are classic examples. Expansive clay soils don't really care how much you compact them or the base course. They do what they please and will tear apart a structure if you don't build right for them.
A post-tensioned slab is extremely unlikely to crack because the concrete is under constant compression from the cables inside. A more typical example is bridges and elevated floor slabs where with post-tensioning, it doesn't matter if anything is under the slab. A raft slab does exactly what the name implies. It sits atop the bad soil like a boat on water.
Since none of us know what the OP's soil is like, we're throwing rules of thumb at the question. The OP might have perfectly stable soils where it doesn't take much engineering to get the slab right. If the OP's soil isn't good, there are ways to overcome the problem. Generally, that means engineering a slab, almost as if there is nothing under it. Sometimes, over excavating the pad area and filling it in with engineered fill is enough.
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