Understanding Post-Tensioned (PT) Foundation Systems and Foundation Heave in Arizona: What You Need to Know

In this edition of "Rules of Thumb for Diagnosing Foundation Failure," we shift our focus to Post Tensioned (PT) foundation systems, shedding light on their growing popularity and potential for foundation problems. We’d like to highlight an important observation: "Uniform Thickness Post Tensioned (PT) Foundation systems are more susceptible to foundation heave." Let's delve into the world of PT foundations, particularly the Uniform Thickness PT Foundation prevalent in the Southwest, to understand its intricacies and associated risks.

  

The Concept of Post-Tensioned (PT) Foundations:

Over the past three decades, Post-Tensioned (PT) foundations have gained traction in the construction industry. Among the various types, the Uniform Thickness PT Foundation stands out in the Southwest region. This foundation design incorporates high-strength cables positioned in both north/south and east/west directions, spaced approximately every 30" - 40" along both sides. These cables are tightly tensioned to about 33,000 pounds a few days after pouring the concrete, resulting in a rigid slab with several advantages.

Uniform Thickness PT Foundation Explained

The Uniform Thickness PT Foundation is characterized by a uniform thickness system, which is the most prevalent in the Southwest. The original PT systems featured deep ribs with cables at the top and bottom, enhancing tensile capacity and minimizing deflection or bending. In contrast, the uniform thickness system has a single cable in the middle, creating a "neutral zone." The concrete above and below this zone is not held by the cables, making it more susceptible to tension and potential slab bending.

Challenges Faced by Uniform Thickness PT Foundation

Despite its popularity, the Uniform Thickness PT Foundation comes with inherent disadvantages. First, the lack of edge protection can lead to water penetration around the edges, making soils vulnerable to poor drainage conditions. Moreover, the absence of isolated footings spreads downward roof loads across a larger area, making settlement less likely but significantly increasing the risk of foundation heave.

Understanding Foundation Heave

Foundation heave is a concerning issue where the foundation rises due to the expansion of the soil beneath it. In the case of the Uniform Thickness PT Foundation, the entire system is more vulnerable to heave as the slab is tied monolithically to the foundation and positioned closer to the surface.

Edge Heave Predominance

The most common type of foundation failure observed with Uniform Thickness PT Foundation is edge heave. While its precise cause is still under investigation, it might be attributed to moisture migration or design-related factors.

To summarize, Uniform Thickness Post Tensioned (PT) Foundation systems are more susceptible to foundation heave, with edge heave being the predominant issue. Homeowners, builders, and engineers in the Southwest need to be aware of these risks when considering foundation choices. Adequate drainage solutions are necessary to mitigate potential challenges.

At Arizona Foundation Solutions, we specialize in professional foundation repair and moisture management solutions. Our team of experts can help safeguard your foundation and protect your home from the adverse effects of foundation heave. Understanding the intricacies of PT foundation systems empowers homeowners to make informed decisions and ensure the long-term stability of their homes.