What is a “Post Tension” slab?
A post tension slab is a concrete slab that has steel cables running through it that have been placed under 33,000 +/- pounds of tension. This tension makes the concrete slab and foundation much stronger than concrete without reinforcement and helps reduce cracking.
How to tell if you have a post tension slab
Most homes that have post tension slabs have a sign that is stamped into a concrete garage floor, often near the edge of the garage door in plain sight.
Older home’s with post tension slabs 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 a stamped sign in the concrete floor. In this case 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 (1 1/2″) to three inches (3″) wide that have been patched. These circles are usually about 2 to 4 feet apart. They indicate the end of the post tension cables and at times are not visible because of landscaping or the patching of the cone shaped voids blend in.
Another source of information may be the builder or local building department. They may be able to look at the building plans and tell you if the home has a post tension slab, it’s thickness and location of the cables.
Why do builders use post tension slabs?
The two most common reasons for a home builder or contractor to use a post tension slab relate to cost and type of soils in the area.
The cost of using a post tension slab system is more economical when the soil conditions are poor and would require additional measures to reinforce the area below the slab to be poured.
Areas where soils are expansive in nature or have poor load bearing capacity use post tension slabs more than other areas. The reason for this is that a post tension slab will spread the load of the home over a wider area. In a standard non post tension slab home the weight of the structure is carried on the foundation and piers; whereas 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 are 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 injure someone near it. Therefore, the builder puts up the do not cut or core warning signs for safety to the homeowner or potential contractors working on the building later on.
Seldom does a home owner cut or drill into their slab, but when remodeling or especially while fixing sewer problems, 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.
Take a minute and review the list to see if you have any warning signs of potential issues.
- You may only need preventive maintenance
- If very minor, then maybe you just need to monitor it
- If repair is needed, it is much cheaper to correct in the early stages than after it has become a big problem
- Should it be major, then you may need to consult a qualified professional