In previous blogs, we've discussed how a conventional foundation system is affected by the various forces. Now, let's discuss Post Tensioned foundation systems and the potential for foundation problems.
Post tensioned foundation (or PT foundation) has gained in popularity over the last 30 years. In parts of the Southwest, a particular type has gained enormous popularity: The Uniform Thickness Post Tensioned (PT) Foundation. This is type of foundation has high strength cables put in both directions (north/south and east/west) and are spaced about every 30” or so along both sides. These cables are tightened up to about 28,000 pounds a few days after the concrete is poured. This has the advantage of creating a very stiff slab (see illustration below).
The other disadvantage of this particular system is that it has very little edge protection. Since it projects down very little, water can penetrate around the edge and go underneath very easily. As a result, soils are more easily affected by poor drainage.
You will notice that there are no isolated footings. The downward forces of the roof loads are spread across a larger area, making it harder for settlement to happen.
The overwhelming type of foundation failure we see with a uniform thickness PT is edge heave. It is not clear at this time if that is a function of the time required for moisture to migrate to the center, or if it is a function of design.
To summarize, the Uniform Thickness Post Tensioned (PT) Foundation systems are more susceptible to foundation heave. Although we are seeing mostly edge heave at this time.
Next: Rules of Thumb VI: https://www.foundationaz.com/blog/rules-of-thumb-diagnosing-foundation-problems-vi
Previous: Rules of Thumb IV: https://www.foundationaz.com/blog/rules-of-thumb-for-diagnosing-foundation-problems-part-iv-foundation-types