Material Science Engineers come up with different ways to enhance the properties of a specific material. One of those methods is post tensioning, through which the strength of a concrete slab can be enhanced. This method actually involves the pre-stressing of the concrete slab. A large enough force is applied on the concrete to minimize the tensile stress to an acceptable level; in layman terms, the concrete is much stronger now. These slabs have firm grid steel which holds up the slab, providing a rigid foundation for homes.
Method to create the foundations.
Before pouring of the post tension slab, high strength steel cables (also referred to as tendons) are laid in a tight grid. This step is performed just to support and strengthen the slab after it has been cured. The tendons are covered in plastic sheets to avoid direct contact with the concrete. Now the concrete is poured extra cautiously so that the tendons
remain at their correct depth.
75% of the concrete is cured that way and at that point post-tensioning is initiated. The term post tension is given to this process because after the concrete has mostly cured, the cables are tightened using a hydraulic jack. This is done to a tension of 25000 pounds per square inch, then they are anchored in the concrete and the slab fully cures.
Advantages of using post-tension slab foundations.
- Higher strength – As told earlier, due to prestressing of the concrete strength is induced in the material.
- More efficient – Less steel and concrete required for the same structure.
- Controls cracking – Due to post tensioning, the cracking is reduced and prevents the formation of further cracks.
- Less bending – addition of strength and stiffness reduces the bending of the slabs.
- Faster installation – With less usage of concrete, the installation procedure becomes rapid.
- Reliability – Many engineers site the factors mentioned above as recommendations to its reliability.
- Less cost – reduced quantity of concrete and steel will lead to lower labor costs.
So post tension slabs have many benefits to it, but with everything else there is a downside. For example, if you are wanting to do an addition of some sort, any new piping or wiring that would normally go through the foundation cannot be done. The Slab has so much tension, that if you were to try to drill through it,then it would possible crack the foundation.
Post Tension Slab Foundations are quite common and as an inspector you may come across these often. Hopefully your understanding of what they are has been increased and will allow you to better assess them.