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I live in a lot colder location than most (40 miles south of the Quebec border). My shop is 28x36 and 2 stories. I built it on a "floating pad" and it has not moved or cracked in 15 years. Do a Google on "concrete floating pad". The "howstuffworks" site has a good cross section. I only went down 12 inches at the perimeter but the key was compacting the stone, 4 mil plastic and then 2" pink Styrofoam with all of the seams duct taped together before pouring the slab. The Styrofoam stops frost from creeping under and heaving the slab. The Scandanavians have been doing this forever and it works well! Hire the finishing crew and make sure they put heavy mesh in the concrete. Rebar is over kill and not necessary. Don't forget to put in all of the services you will ever want before you pour. You might think you will never want a bathroom or whatever but once it's poured, you won't have the chance to change your mind. I did mine in 3 days which included the compacting, digging and pour.

Good luck

Tim