I poured my 40'x50' garage floor in 2006, did relief cuts on every 10'x10'. My floor is 4"-5" and in 2 corners 7" because I had equipment that weighed 28000 and 31000 lbs on those corners. I have driven my 11000 lb forklift with a 5000 lb lathe on it or lifts of drywall, not a single crack anywhere.
I went heavy on rebar, and have always had the garage heated in winter. I really think that is the key, keep it heated.
I spent a lot of time on prep, tamped the hell out of it for 2 days with a 4 wheel vibrating tamper. I used elimination sand for a base.
I had my basement carpet stripped a couple years ago and there is not a single crack there either, but again I tamped it for 2 days and went heavy on rebar which is cheap if you look at the life of it. I poured the house floor in 2001.
Not all concrete cracks
