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   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  
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