I'm in central MN. When they built my shop the contractor strongly recommended doing the dirt work in the fall and then let it set till late spring to make sure it settles properly. It's been 15 years and not a single crack in the concrete.
As much as it might piss you off, it's actually a good thing that the area sets. Just my .02- even if you don't use it, put tubing in the slab, you cant do it afterward. I use radiant to heat my 2000 foot shop and it is the best shop heat ever. Working on a warm slab is so much nicer than any other way of heating, and it's far more economical if you insulate properly.
The whole time they were excavating at our house —- I kept thinking how much I was on the hook for and I hadn’t even pounded a 2 X 4 together yet...... LOL