What is a “Post Tension” slab ?
It is a concrete slab that has steel cables running through it that have been placed under 33,000 +/_ pounds of tension. This makes the concrete slab and foundation much stronger than concrete without reinforcement and helps reduce cracking.
How to tell if you have one
Most homes that have post tension slabs have a sign that is stamped in the concrete floor. Often near the edge of the garage door in plain site.
Plastic or paper sign on the wall. Other homes may have a plastic or paper sign fastened to the wall. These are often placed next to a door that goes from the garage into the house.
There may be times when there are no visible signs on the wall or stamped into the concrete floor. Then you may be able to look for evidence of a post tension slab by walking around the perimeter of the home and looking for small circle type areas about one and one half to three inches wide that have been patched. Usually about 2 to 4 feet apart. This would be the end of the post tension cables which is often not visible.
Another source of information may be from the builder or local building department.
Why do builders use post tension slabs ?
The two most common reasons relate to cost and type of soils.
The cost of using a post tension system is more economical under certain circumstances, especially when the soils of the area are poor.
Areas where soils are expansive in nature or have poor load bearing capacity use post tension slabs more than other areas. The reason is that a post tension slabs spreads the load of the home over a wider area. In a standard non post tension home the weight of the structure is carried on the foundation and piers; where as on the post tension slab, the weight is partially carried by the slab as well. This results in smaller and less expensive footings being required.
Other benefits is that less control joints may be required to help control shrinkage cracks. Less concrete or thinner slabs may be possible for certain expansive soils.
Why does the sign say “Do not cut or core” ?
When cutting or drilling into the concrete slab a post tension steel cable may get cut or damaged. This may weaken the slab structurally or the cable (which is under tremendous pressure) may occasionally rip right out of the slab and injury some one near it. Therefore, the builder puts up the warning signs for your own safety.
Seldom does a home owner cut or drill into their slab, but when remodeling or fixing a sewer problem, then there may be a need to cut or drill the slab. Don’t worry, professionals will know how to do it without damage or anyone getting hurt.