Concrete floors go through a lot of stress in our northern climate.  A standard slab-on-grade floor, also known as a floating slab, is designed to be pushed around by the frost, and hold its integrity when it cracks.  Yes, I did say “when it cracks”.  Frost drives deeper under the edges of the concrete which creates more movement than the center.  Concrete is a very brittle product and cannot withstand all the pressure placed upon it; therefore, it breaks to relieve that pressure.  The fiberglass fibers, steel mesh, and rebar placed within the slab, are products that keep the concrete together after the inevitable cracking occurs.