A BRIEF OVERVIEW
Prestressing is a technology by which beneficial loads are applied to structural members, mostly made of concrete. To a lesser degree, prestressing is also applied to steel, masonry and wood members. The seven-wire steel strands used for prestressing typically have an ultimate strength of 270 ksi (1860 MPa).
Prestressing can also be viewed as a form of reinforcement, namely asactive reinforcement (pro-active to the applied loads), as opposed to rebar, which is considered to bepassive reinforcement (re-active to the applied loads).
Prestressing can take one of two forms: • pre-tensioning • post-tensioning
Pre-tensioning applies only to concrete members. It requires the strands to be stressed before the concrete is poured. Once the concrete has gained strength, the strands are released from external frames and their tensile loads are transferred to the concrete, which responds by going into compression.
Post-tensioning can be applied to all types of materials but is mostly used in conjunction with concrete. The strands are stressed and locked off after the concrete has gained strength. The member itself provides the reaction for stressing the strands. Again, the concrete responds by going into compression.
Post-tensioning forces are provided by tendons which are made up of one or more strands inside a duct and connected to anchoring devices. The strands are stressed with high-pressure hydraulic jacks and locked off with hardened, serrated steel wedges.
Post-tensioning can be further subdivided into four categories • unbonded - the strands are free to slide inside the duct/concrete • bonded - the strands are an integral part of the member • internal - the tendon runs inside the member • external - the tendon runs outside the member
In bonded tendons, the open space between strands and duct is grouted. In unbonded tendons, the strand is covered in grease and wrapped in a plastic sheath by a continuous extrusion process.
Other UsesPost-tensioning technology can be readily adapted to serve in other construction applications.Some good examples are:
• Rock Anchors - for anchoring to rock or to stabilize rock faces and tunnel walls • Stay Cables - for supporting bridge superstructures • Heavy Lifting - for lifting and moving heavy loads ( 100tons - 10,000tons + )
CTE has been involved in all aspects of post-tensioning since 1986 and offers related services to suppliers, contractors and engineers. We maintain membership with the Post-Tensioning Institute in Phoenix, Arizona, the leading organization dedicated to post-tensioning in the world.