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The Truth About Post Tensioned Foundations

Rules of Thumb for Diagnosing Foundation Failure part V: The Truth About PT

In my last blog, I discussed how a conventional foundation system is affected by the various forces. In this blog I will discuss Post Tensioned foundation systems and the potential for foundation problems.

Post tensioned 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).

RoT V 1

The Uniform Thickness Post Tensioned (PT) Foundation has a uniform thickness system. It is the most common in the Southwest. Originally PT systems had deep ribs with cables both at the top and at the bottom. Since the cables could not stretch, this kept the foundation system from deflecting or bending. The uniform thickness system is less robust in that it has one cable in the middle (engineers call this the neutral zone… for Star Trek fans, this is not where the Romulans are). The concrete above and below is not held by the cables and can crack in tension (and thus bend the slab) more easily.

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.

Rot V 1.1Because the slab is tied into the foundation monolithically, the entire system is much more at risk for heave. On a conventional foundation, the footings are not likely to heave. The footings are more likely to heave with The Uniform Thickness Post Tensioned (PT) Foundation.

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 heave.  Although we are seeing mostly edge heave at this time.