A post tension slab is reinforced with stranded steel cables that are tensioned (tightened) after the concrete hardens. The cables are slid inside a plastic sheathing, like a wrapping. This prevents the cables from touching the concrete. When the concrete has sufficiently cured, the plastic sheathing is stretched. Stretching the post-tensioned cables applies significant force to the concrete system, lifting the slab into a compressed state, which reduces shrinkage cracks and cracks caused by difficult soil conditions.
Why Do Builders Use Post Tension Slabs?
There are two reasons why builders used post tensions slabs during the construction of a home or building.
- Due to poor soil conditions found in the Gulf Coast region, builders choose post tension slabs. One soil issue is that the freezing depth in southern climates is shallow.
- Post tension slabs are cheaper, which means more profit for the builder. Unfortunately, the results of this change are devastating for homeowners. Conventional concrete & rebar foundations last about 20-30 years before failure begins to occur. Post-tension slabs are lasting about 15-20 years and are failing at a much higher rate.
Problems With Post Tensions Slabs
One of the biggest problems with a post tension slab is cracks. The cables laying in the concrete are generally not tightened until at least 7-10 days after the concrete is poured. Since the cables are not stretched or elongated, they cannot provide any crack control in those first few days.
Another reason for cracks is when a vehicle or another large object is placed upon the structure, and its concrete slab undergoes tension. To solve this problem, post tensioned steel tendons are placed when the concrete is poured and tensioned after with conventional reinforced bars.
A second issue with post tension slabs is that the footings anchor the foundation in place. The tension required to bind the additional concrete in the footings plus the dirt between the footings can’t be met. The result is that the cable tension is not compressed enough to resist bending movements.
Defects such as slab cracks can be costly to repair after construction is finished. Picture ripping up flooring or carpet to repair a small crack you may never notice. Trying to fix cracks can be time-consuming and expensive. Depending on the extent of damage, a family may have to leave their home while the work is being done. Talk about prices adding up.
Advantages Of Post Tensions Slabs
Below are some advantages that come with using post tension slabs.
- Since the thickness in the post tension slabs is reduced, the amount of concrete and reinforcement is also reduced by 20% – 30% compared to the conventional concrete slab.
- Compared to conventional rebar foundation, the installation of post tension slabs is relatively simple.
- It’s easier for a crew to pour concrete and walk due to the extra space in the foundation.
- Lighter overall structure compared to that of all steel reinforcing bar foundations and can be more economical.
- The slab is easier to inspect and repair.
When these post tensioned steel tendons are stressed, the concrete is squeezed. In other terms, the concrete is compacted, which increases the compressive strength of the concrete, and at the same time, the steel tendons that are pulled increase the tensile strength. As a result, the overall strength of the concrete increases.
When construction teams properly understand and follow correct procedures for post tensioning a slab, they’re adding to a foundation’s life-use.
Who To Call For Post Tension Slab Problems
If you have post tension slab problems, who should you call for help? Check out Foundation Professionals of Florida, an award-winning and top-performing company. We specialize in all your foundation and slab repair needs and can provide any of the services above. Click here for more information and contact us today.