The OP failed to say very much specifically about what he wanted and nothing about where he is, which matters.
Most concrete sold in the south, which has no seismic requirements for high strength concrete, is 2,500psi. Some mix plants sell 3k psi as the low rated mixes, because of special construction in their areas, which require higher strength mixes, so it makes more sense for them to plan those recipes. Doing so saves them time in planning and QC, which saves them money in the long run. And they simply pass the upcharge onto their customers, who they care very little for anyway.
3-4k psi mix cost is twice that for 2.5k , although not much different between the two higher rated mixes. You get no extra value for a higher rated mix, just higher cost, unless your specific load requirements necessitate it.
The west coast has unique seismic codes due to geological instability west of the mountains. That's why they also have registered SEs, which don't exist east of the mountains, excepting Chicago (which thinks it's all of Illinois, in that regard). That includes Alaska, as well.
Yes, you should use higher strength concrete in foundation piers, but placing a thick floor with high strength concrete rather than constructing foundation piers isn't engineering, it's guesswork. And wrong. Yes, GCs probably do it often, and it's still wrong, because it's wasteful and doesn't take into consideration the particular load bearing requirements of the floor loading. Placing an outdoor pad can be done on cut soil, if it's sound, and the pad would not need to be thicker than the average load on it required it to be. And, if it was in the south, where we don't have to worry about frost heave, it'd be fine.
Yes, common practice is to remove soil and replace it with compacted fill and baselayer, but that's just practice, not gospel. It's done because engineers are lazy, when they're not pressed for time, and lawyers are everywhere. If I wanted to make absolutely certain any site, not a particular site, but a generic site (which doesn't exist except on paper) would support a generic floor design, I'd specify exactly what I just described. If I had a good client who had a good owner's engineer representing his interests, I wouldn't be allowed to get away with it. I'd have to provide calculations based on actual soil samples from his site and tailor the design to match those characteristics and I'd have to back up my design recommendations with a cost analysis. I know all of this: I used to do it for a living.
I had no intention on writing a diatribe when I sat down, but we've been over this before and it's late and I've too many things on my mind at present and my hands were on autopilot.
