Generally, there are two basic types of foundations – “post-tension” foundations and “conventionally reinforced” slabs. Generally, post-tension foundations have cables running through the slab which are tensioned with the goal of keeping the foundation together. Conventionally reinforced slabs have much more steel in them than post-tension slabs and often look like a waffle gridiron prior to the pouring of concrete.
Post-tension foundations have become increasingly common. Their popularity may be due to the fact that since they usually have less steel in them they may be less expensive to install. Many volume or production builders routinely install post-tension foundations.
In some situations, post-tension foundations may be prone to significant foundation movement and resulting damage. This may be due to a number of factors:
- The Post-Tension Institute (“PTI”) has published various editions regarding the proper design and installation of post-tension slabs. The second edition of the PTI manual was published in 1996, and the third edition manual in 2004. The third edition significantly stiffened the design of post-tension foundations, making them more resistant to foundation movement. Despite the fact that the third edition was published in 2004, some volume builders have routinely utilized the second edition (published in 1996) for foundations constructed after the publication of the third edition in 2004. Presumably these builders still utilize the second edition after 2004 (although the 1996 second edition was outdated by then) because use of the second edition may result in cheaper construction costs.
- The spacing and tensioning of the tendons, or cables, is important with respect to the integrity of a post-tension slab. The proper placement and tensioning of tendons requires care and expertise, which may not be utilized in some situations, especially when a volume builder is installing multiple foundations in a short period of time.
- Since a post-tension slab has a relatively small amount of steel when compared to a conventionally reinforced slab, they may be prone to significant movement. I have read multiple subdivision-wide geotechnical reports (which rarely are shown to the homebuyer before their purchase) which warn that one should expect cracking, etc. should some foundation design techniques be chosen, including the installation of a post-tension slab.
If you are experiencing foundation movement you should contact an independent professional as soon as possible to have your particular situation evaluated.
