Reinforced Concrete: Post Tensioning
The most common way to reinforce concrete is to put rebar – reinforcement steel bars – into the forms prior to pouring the concrete so that they are incorporated into the concrete to strengthen the material.
However, there are many different types of reinforced concrete. One popular method is post-tensioned concrete. This allows for higher structural strength in the concrete than traditional reinforced concrete by pre-stressing the concrete. It can be used in a variety of different applications, but is mainly used in industrial situations where it is expected that a high amount of weight will need to be supported.
What is post tensioning?
Post tensioning uses special tendons, which are pre-stressing steel cables that are enclosed with plastic ducts, generally a coating and a outer sheathing layer. They are placed along the concrete forms or through the center of the slab prior to the concrete being poured. The concrete is poured completely and left to cure until it has gained strength. Before any weight is placed on the concrete, but after it has cured completely, the cables are pulled tight and tensioned. Anchors are then placed along the outer edges of the concrete to secure the cables and provide greater strength to the structure.
Although it may seem counter-intuitive, the process of post tensioning is a method to pre-stress the concrete. This means that stress is added to the concrete in the form of tension prior to being required to bear weight. Normal reinforced concrete can be called pre-tensioned, in contrast, though it is also pre-stressed since the reinforcement bars are tensioned prior to the concrete being poured. By pulling the tendons and adding tension to the concrete prior to applying the service load, the concrete can bear additional weight in the future.
Once the concrete has been pulled after the concrete has been poured and anchors have been installed, the area surrounding the tendons is filled with grout. This serves to both protect the tendons from corrosion and to further join the tendon the cement slab.
Uses of post tension reinforced concrete
Post tensioning has become much more popular as the technique has been refined and more engineers have learned the practice. It requires quite a bit of skill and education to install post tension reinforced concrete, but the benefits are worth the work required.
Some of the most common and useful applications are large scale concrete slabs, such as roads or sports areas, specifically tennis courts. It is especially useful in areas that are on grade or otherwise likely to move due to loose, unstable soil, or even tectonic activity. Due to its ability to be added after the concrete is poured, it can be used to restore damaged buildings especially due to earthquake damage. Post tensioning is a common technique used in bridges, where the post tensioning can help to alleviate the stress caused by frequent vibration, movement, expansion and contraction due to weather, and heavy loads. Other uses include water tanks, masonry structures or large concrete countertops that need to support a large service load.