Why is there a WARNING! POST-TENSION SLAB sticker in my house?

Sunday, July 21, 2019

Post-tensioning is a construction technique for reinforcing concrete. It has been used for many years in commercial construction, but we are now seeing it in some new residences. Steel cables inside plastic sleeves are run across the floor slab along with regular reinforcing steel where needed before the concrete is placed. After the concrete has set, the cables are tightened from the ends to create thousands of pounds of tension. 

    The post-tensioning adds structural strength to the slab, especially in areas with expansive clay soils, but a secondary benefit in residential construction is avoiding those pesky cracks that occur as the concrete cures and shrinks slightly. The shrinkage cracks do not affect the strength or durability of a slab on grade, but are the cause of lots of homebuyer complaints when they appear in places where the slab is exposed, like the garage. The cable tension literally squeezes the concrete together and, if a cable is cut, the release of the tension unleashes an explosive force, like cutting a taut elastic rope, that damages the slab and may cause injury to anyone nearby. 

     Here’s an example of the cables in a sleeve and ready for concrete in a commercial construction project. The box at the end of the sleeve creates a pocket in the concrete for tensioning and then cutting off the ends of the steel.


    So the essence of the warning sticker is the builder telling you: DON’T MESS WITH THE CONCRETE SLAB. LEAVE IT ALONE! Sometimes the warning message is stamped into the concrete floor of the garage instead of using a sticker.

    Another alternative for reducing cracks in a concrete slab is explained in our article What is fiber reinforced concrete?

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