Post-tensioned slabs are simply foundations that were poured with steel cables running through it in a criss-cross pattern. This results in a much stronger foundation that is less likely to crack. It is similar to reinforced concrete, except that they use cables instead of rebar, and the cable are pulled to tension after the concrete is cured.
In some parts of the Valley, the ground had soft, or “shifting” or “expansive” soil. Essentially the ground was not hard enough to support such a large structure and over time these structures’ foundations started to crack as soil moved differently in one area than another. In the early years of Phoenix, engineers tried to build neighborhoods on the best plots of land with good soil and a slight grade for water runoff. But, as the population kept going up, they had to keep expanding into areas that were previously farmland, with softer soils less ideal for building houses.
When the foundations is poured, it has concrete cables running through it. After curing, the cables are pulled to 33,000 lbs of tension, resulting in 8 inches of stretch for each 100 ft of cable. This provides a constant dynamic tension all throughout the foundation, keeping it together as a single unit and resisting cracks and movement of only one portion pf the foundation. For this reason they have been referred to as “floating slabs”, since they are designed to move as a single unit, even if the soil shifts or moves slightly. In traditional slabs, even a tiny amount of movement in the soil could spell disaster for the foundation. Post tensioned slabs, like snowshoes, are designed to spread the weight over a larger area.
As you can see in the picture at the top of the page, by far the largest limitation of post tensioned slabs is the inability to cut into it after curing. This means if you have a plumbing leak, or want to add an addition and need to tap into the existing plumbing, or put an outlet in the middle of your floor or something like that, you basically can’t. It requires special X-ray scanning to locate the cables in the flooring if you absolutely have to cut into it, since they need to avoid being cut. Luckily, they typically only run every 2 ft, but they also run in both directions, meaning you have to work around the cables in 2×2 sections, if you absolutely must cut into your foundation.
Usually there will be a stamp in the garage similar to the picture at the top of the page that says “Post Tension Slab Do Not Cut”. They will be either in the corner of the garage or in the middle of the garage near the garage door. The other place they may be in the chance that the garage is not a part of the house foundation (if the garage is a step down or something like that) is near the door to the house in the garage. You would also notice the absence of expansion joints, since a post-tensioned slab is designed not to expand.