Yesterday we experienced unusually warm days and nights in southern Indiana. Amazingly, areas on our concrete garage floor - especially those areas directly underneath our vehicles - were wet. In fact, our jeep actually had condensation droplets hanging from the gas tank. The effect of the colder concrete floor reacting to the warmer southern breezes was surprising.
This concrete floor is 16 years old, was not poured with any kind of vapor barrier below, and the surface has never been treated. Each year we consider treating the surface with some sort of sealer/epoxy but then put the job off due to the size of my garage floor coupled with all the stuff that would need to be moved while the sealing was underway.
We can't stress enough the importance of proper moisture protection for anyone in the building phase. Moisture not only [negatively] affects organic materials in the home but can be a real headache in almost all other areas of the home.
Today it was maybe 78 degrees here. The slabs will show condensation even with plastic under them. When that warm air hits the cold concrete, I don't think it matters if there is a vapor barrier. You can keep your garage door closed as much as possible and let it warm up slowly, I suppose.