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The Truth About Post-Tensioned Foundations

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

In previous blogs, we've discussed how a conventional foundation system gets affected by various forces. Now, let's discuss Post Tensioned foundation systems and the potential for foundation problems.


"Uniform Thickness Post Tensioned (PT) Foundation systems are more susceptible to foundation heave."

- Bob Brown, AZFS Owner & Foundation Repair Expert


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 type of foundation has high-strength cables put in both directions (north/south and east/west) and get spaced about every 30” -40” or so along both sides. These cables get tightened up to about 33,000 pounds a few days after the concrete gets poured. This has the advantage of creating a very stiff slab (see illustration below).

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 greatly increase the tensile capacity at the top and bottom of the beams, this resulted in keeping the foundation system from deflecting or bending as easily. 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 get spread across a larger area, making it harder for settlement to happen.

Because the slab is tied into the foundation monolithically and is located closer to the surface, the entire system is much more at risk for heave. On a conventional foundation, the footings are not likely to heave. The foundations 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, Uniform Thickness Post Tensioned (PT) Foundation systems are more susceptible to  foundation heave.  Although we are seeing mostly edge heave at this time.