orf,
First, what part of the country are you in ?
xxxx psi is the strength the concrete should have after setting up 28 days. Your concrete supplier will varie the amount of the basic concrete ingredients of sand, coarse aggregrate (stone), water and cement to get to the strength you want. Typically they have cut sheets/"recipes" of mixes that have already been lab tested for strength as well as field tested for ease of placement. You could probably call some of your local concrete suppliers and get a copy of their recommended mixes for your geographical area and area of use (garage floor).
For a something as simple as a garage floor the following should be looked at :
- your area of the country will have an affect on the recommend strength and "mix" due to such things as possible freeze-thaw cycles, road salt exposure, spalling, etc.
-you will gain strength in your slab quicker by thickening rather than increasing the concrete strength. Don't know the trade-off in cost of the thickness versus increased concrete mix strength though.
-And probably most important, in the long run the slab will only be as good as the base below it which is where many people skimp. Need well compacted good material under the slab.
For my area of Buffalo/Niagara I would be looking at a minimum 4000 psi "air-entrained" (for freeze-thaw,salt) concrete at probably a minimum of 5 inches thick for anything more than normal cars. Don't know what effect the presence of "pex" would have on the required thickness.
As far as heating the slab you might look for websites that show a cross section of the recommend construction/insulation. I bet your state building department has construction recommendations or contact your local building officials to see if they can help.
More than you wanted to know probably.
