A post-tensioned slab-on-ground foundation is simply a ground-supported concrete slab foundation that is reinforced with flexible cables that are tensioned after the concrete hardens. When the cables are tensioned, the concrete is placed in compression. The cables are not normally tensioned until at least 7-days (and sometimes not until 30-days) after concrete placement. Since the cables cannot provide any crack control until after the cables are tensioned, every post-tensioned slab experiences anywhere from 7 to 30-says during which there is no crack control at all. This allows for the development of large, visible curing cracks called restraint-to-shrinkage (RTS) cracks.
In spite of this, post-tensioning is generally considered a superior method of crack, especially for bending cracks, compared to conventional reinforcing.