Post Tension Foundation Basics
Post Tension Foundation Basics
When building on our expansive soils, a post tension concrete slab on grade is often the foundation of choice. It’s a method of construction that relies on steel cables (known as “tendons”) that are placed in the foundation’s mold form before concrete is poured and then "post-tensioned" to add the necessary tensile forces. This allows the concrete to be poured thinner than conventional slab on grade concrete and reduces construction costs.
However, despite the obvious benefits of this type of foundation, it can come with some problems. Post-tension foundations need to be carefully maintained and monitored for proper function and durability. Failure to do so can lead to expensive structural damage or even complete building collapse.
Compared to the intricate arrangement of steel rebar needed for conventional slab on grade foundations, post-tension cables are much easier to install. They also help to minimize the weight of a foundation and increase its load-carrying capacity.
While they’re an ideal option for homebuilders in our area with expansive soils, post-tension slabs have a few limitations. For example, they can’t be used for structures that are subject to very high loading and they aren’t suitable for every site. It’s important to consult with a qualified foundation engineer when choosing a foundation for your new project.
One of the most common issues with a post tension foundation is the potential for cracks in the slab. Since the cables laying in the concrete are not stretched or elongated, they don’t provide any crack control and may contribute to early cracking. This is especially true when the concrete is poured in hot weather, which can cause the concrete to shrink and crack prematurely.
Another problem with post-tension slabs is the difficulty of changing or moving plumbing and electrical lines. While it is possible to modify a post-tension slab, it’s usually a complex and risky process. The tendons must be carefully rerouted and tensioned to maintain the required level of strength and safety.
It’s critical to regularly inspect the slab for exposed cable ends. If these aren’t covered with a concrete patch, they will rust quickly and fail, exposing the concrete to water and moisture which can lead to other foundation problems.
Lastly, homeowners should be careful not to cut into the concrete slab or core it for any reason. This is because if you accidentally hit a post-tension steel cable, it could SNAP under tremendous pressure and potentially damage or even destroy the foundation of your home. For this reason, a builder will stamp a “do not cut or core” warning on these types of foundations to ensure the safety of homeowners and contractors working on the house later. Fortunately, it’s easy to hire a professional to scan your concrete floors for these metal cords and confirm they are not exposed. This is a service that’s well worth the investment for anyone who owns a post-tension slab home. It’s also an essential service for commercial building owners who own or operate a facility with post-tension slabs.
