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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To learn more about exterior walls and structures, see these other blog posts:

• What is the average lifespan of a house foundation?

 • What causes raised white lines of residue on a block wall that are crusty and crumbling?  

• How much ventilation is required for the under-floor crawl space of a home? 

• What causes vertical cracks in fiber cement siding planks?

• What causes stair-step cracks in a block or brick wall?

• What causes a horizontal crack in a block or brick wall? 

• How can I tell if a diagonal crack in drywall at the corner of a window or door indicates a structural problem?

• What causes the surface of old bricks to erode away into sandy powder? 

• What are the pros and cons of concrete block versus wood frame construction?

• Should I buy a house with a crawl space? 

• Why is my stucco cracking?

• There's cracks running along the home's concrete tie beam. What's wrong? 

• What would cause long horizontal lines of brick mortar to fall out?

• How do I recognize structural problems in a retaining wall? 

• What is engineered wood siding?

• Should I buy a house that has had foundation repair? 

• What is a "continuous load path”?

• Should I buy a house with asbestos siding?   

• How can I tell if cracks in the garage floor are a problem or not? 

• What do you look for when inspecting vinyl siding?

• Why is housewrap installed on exterior walls under the siding? 

• How do I recognize serious structural problems in a house?

• Why did so many concrete block homes collapse in Mexico Beach during Hurricane Michael? 

• How can I tell if the concrete block walls of my house have vertical steel and concrete reinforcement?

• Should I buy a house with structural problems? 

• What are those powdery white areas on my brick walls?

• What causes cracks in the walls and floors of a house?

• How can I tell if the exterior walls of a house are concrete block (CBS) or wood or brick?

• What are the common problems of different types of house foundations? 

• What are the warning signs of a dangerous deck?

   Visit our STRUCTURE AND ROOMS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.