Post Tension Slab
In 1867, a French florist named Joseph Monier was simply trying to make a stronger flower pot, but his use of reinforcing bars (rebar) in cement ushered in a whole new era of construction possibilities.
Today the building industry is taking the next step in concrete technologies by using post-tension cables in slab construction. Post-tensioning is a method of strengthening concrete using high-strength steel strands or cables, typically referred to as tendons. Slabs using the post-tension method can be built thinner, which can cut down on construction costs and curing time.
The post-tensioning method is the best practice for building stronger, more reliable foundation slabs.
Concrete is very strong in compression, usually rated up to 2,500 PSI, but it's relatively weak in tension. This means that a concrete slab can be prone to cracking due to deflection, or bending, when the earth under the slab sinks or becomes unstable. This is common in areas with expansive soil such as Denver and Houston. Expansive soil is a type of soil that expands and contracts based on its moisture level, and it can damage the structural integrity of homes.
Post-tension concrete construction creates a tight grid of steel cables that actively help support the slab. Unlike conventional "inactive" rebar, which only helps keep the slab intact after cracking, post-tension tendons continually contribute to the structural integrity of home. Plastic sheathing is used to cover the cables to protect them from corrosion.
Once you've performed engineering research on what types of post-tensioning specifications are required for your region, here's how to build a post-tension slab:
- Lay plastic-sheathed steel cables in a grid where the slab will be poured.
- Pour the concrete, making sure that the cables remain at the right depth (these are usually held in place with positioning tools called "chairs").
- After the concrete has cured to 75% of its designed strength, pull the cable taut using a hydraulic jack. This is the 'post-tension' put on the cables. Cables should be pulled to 25,000 PSI.
- Anchor the ends of the cables and let the concrete cure completely.
In addition to being used in residential construction, post-tension concrete construction is widely used in commercial applications today. The combination of structural integrity and thinner pads allows for many practical applications. Office buildings can have thinner floors but retain comfortable ceiling heights to reduce the total height and weight load. More interesting and creative structures can be used in event stadiums and ball parks. Even parking garages can be made stronger, more functional, and less expensive (to construct). For more information on post-tension slabs, visit the Post-Tensioning Institute's web site.
By employing post-tension concrete construction methods, home builders can deliver a superior product at reduced costs. Just like for Monsieur Monier, everything could be coming up roses.