Post - Tensioning       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 as active reinforcement (pro-active to the applied loads), as opposed to rebar, which is considered to be passive 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 Uses Post-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.           Here are typical services we have performed for our clients:         • Shop drawings and stressing calculations       • Installation procedures; friction and modulus testing       • Jack calibrations / traceable to the National Bureau of Standards       • Post-tensioning system development, drawings and testing       • Nuclear containment tendon surveillances per NRC requirements       • Design of tendon manufacturing, installation and stressing equipment       • Jack chair designs for multi-strand tendons and cable-stays       • Onsite technical assistance and stressing supervision       • Seminars and teaching presentations       • Prestressed concrete design       • Rock anchor design       • Cable stays     Here is a quick overview of post-tensioning equipment, materials and field work...   Post-tensioning is a specialty trade and should only be performed by staff experienced in this technology. Lack of proper training and attention to detail will result in mistakes and project delays that can run into the 10s of thousands of dollars. Post-tensioning is typically performed on the critical path and requires a culture of safety and respect. Tendons can carry 800 tons of force or more, which represents a very considerable amount of stored (strain) energy. Until grouted, tendons should be viewed as "loaded guns" and it is best not to stand in line with an ungrouted tendon. Tendon failure is rare. When it does occur, it is typically due to a shearing load on the strand or due to wedge problems. The energy release can be explosive and may result in high-velocity projectiles. Safety goggles must be worn to protect against eye injury. Hardened steels as used in post-tensioning are quite brittle and may chip when struck with a hammer or pipe.   Post-Tensioning Gone Wrong   Top : Home