A common occurence these days are for new homes to be constructed with Post Tension concrete slabs. A post tension slab is typically a "monolithic" slab, which means that the slab (the concrete floor that rests on the earth) and the foundation (the perimeter of the concrete slab that is several inches thicker than the slab) are poured at the same time. Prior to the pouring of the concrete, cables are strung throughout the slab area and are set on "benches" so that they are above the ground. The idea is that these cables will be in the middle of the slab when the concrete is poured. After the concrete is poured, and allowed to set for a number of days, the cables are then tensioned. Tensioning means the cables are pulled tight to a pressure of about 10,000 pounds per square inch.
The theory behind post tensioned slabs is that the cables make the concrete much more strong and resistant to earth movement deformation, especially deformation related to expansive soil. Some geotechnical engineers dispute the benefit of post tension slabs in relation to expansive soil. One point that all experts can agree on: do not core or cut into the concrete slab for any reason. The concrete contains cables under tremendous pressure, and cutting one of these cable can result in much damage.