Q.: I've always been told never to pour concrete on frozen subgrade. Why is that a problem?
A.: There are several reasons for not placing concrete on a frozen subgrade. Some soils expand or heave when they freeze. When they thaw later, the ground subsides and the footing or slab loses subgrade support. This can cause the concrete to crack.
If flatwork is placed on frozen ground there's another danger. The cold concrete at the bottom takes longer to set than concrete near the surface. The surface crusts over and looks like it's ready to finish while the rest of the concrete isn't firm enough to support the finishers' weight. Concrete may bulge out underneath the power trowel, creating a wavy surface. Or the surface may crack.
Finally, there's the possibility that the cold subgrade will freeze part of the concrete in contact with it. Freezing damages green concrete.