In the greater Houston area, the most common type of foundation for residential home’s is a post-tensioned slab-on-grade foundation. A post-tensioned foundation is a concrete slab foundation that is reinforced with steel cables that are tensioned after the concrete hardens. When the cables are “pre-stressed” the tensile forces are transferred from the concrete to the reinforcing tendons. Because concrete is excellent at handling compressive forces, and steel is excellent at handling tensile forces, combined; an excellent slab it does make.
Post-tensioned slabs are used widely used in the Greater Houston area due to their ability to sustain differential movement in problem soils (Expansive or plastic type soils). A post-tensioned slab absorbs the load path from the structure above and transmits it to the soil within the entire slab area, as opposed to flooring based slabs, which distribute the load at
each pre-defined footing.
The cables are not normally tensioned until at least 7-days after concrete placement, so it is not uncommon to see the tendon tails still in some time after the pour. Since the cables cannot provide any crack control until after the cables are tensioned, every post-tensioned slab experiences anywhere from 7 to 30 days during which there is no crack control at all. This allows for the development of visible curing cracks called restraint-to-shrinkage (RTS) cracks. In spite of this, post-tensioning is generally considered to be a superior method of bending crack control as compared to conventional slab on grade foundations.
Post-Tensioned Foundation Advantages
The post-tensioned slab has been the primary slab-on-grade foundation type used for many years now when building over expansive soils. There are other benefits as well. to name a few:
- More efficient use of building materials
- Lighter overall structure compared to that of all steel reinforcing bar foundations
- Reduction in cracks and better isolation of deflections
- Primarily used because of its ability to resist expansive soil movement and to span over soil pockets.
How to Tell if Your Foundation is Post-Tensioned
Post-tensioned foundations have distinguishing characteristics that make it easy to identify. Instead of traditional rebar, a post-tensioned slab incorporates tendons that consist of anchors, a steel strand (or stranded), a corrosion resistant coating material and high-density polyethylene sheathing. Of course, by the time most home owners take possession of their home, the slab has already been poured. If that’s the case for you, there is a few distinguishing characteristics that can help you identify the type of foundation that you have.
The stressing end pockets are filled with a high strength non-shrink grout, which leaves a visible circular counter on the perimeter of the exposed foundation. If the pocket is not properly filled with an approved grout, the anchor end will be exposed to moisture, which will cause a significant amount of rust to the steel. This condition can cause a number of problems, ranging from catastrophic cable failure to persistent cracking of the concrete slab as the rusting steel expands.
If your home is being constructed on grade in an area with expansive soils, it is a safe bet that your builder will use a post-tensioned foundation.