Pre - Stressed Concrete

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  rajcivil2k6

  [FONT="]Prestressed concrete is a particular form of reinforced concrete. Prestressing involves the application of an initial compressive load on a structure to reduce or eliminate the internal tensile forces and thereby control or eliminate cracking. The initial compressive load is imposed and sustained by highly tensioned steel reinforcement reacting on the concrete. With cracking reduced or eliminated, a prestressed section is considerably stiffer than the equivalent (usually cracked) reinforced section. Prestressing may also impose internal forces which are of opposite sign to the external loads and may therefore significantly reduce or even eliminate deflection.
  
  One such prestressed beam picture (from my old project) is shown below, if you watch closely you can notice the curvature in upward direction. when the actual load is placed the curvature would decrease according to the intensity of load.
  
  
  
  I would post more details in further posts
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  rajcivil2k6

  Prestressing:
  
  Prestress is usually imparted to a concrete member by highly tensioned steel reinforcement (tendons, wire, strand, or bar) reacting on the concrete. The high strength prestressing steel is most often tensioned using hydraulic jacks. The tensioning operation may occur before or after the concrete is cast and, accordingly, prestressed members are classified as either pretensioned or post-tensioned.
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  rajcivil2k6

  Pretensioned concrete
  
  The prestressing tendons are initially tensioned between fixed abutments and anchored. With the formwork in place, the concrete is cast around the highly stressed steel tendons and cured. When the concrete has reached its required strength, the wires are cut or otherwise released from the abutments. As the highly stressed steel attempts to contract, the concrete is compressed. Prestress is imparted via bond between the steel and the concrete. Pretensioned concrete members are often precast in pretensioning beds long enough to accommodate many identical units simultaneously.
  
  
  
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  rajcivil2k6

  Post-tensioned concrete
  
  With the formwork in position, the concrete is cast around hollow ducts which are fixed to any desired profile. The steel tendons are usually in place, unstressed in the ducts during the concrete pour, or alternatively may be threaded through the ducts at some later time. When the concrete has reached its required strength, the tendons are tensioned. Tendons may be stressed from one end with the other end anchored or may be stressed from both ends, as shown. The tendons are then anchored at each stressing end. The concrete is compressed during the stressing operation and the prestress is maintained after the tendons are anchored by bearing of the end anchorage plates onto the concrete. The post-tensioned tendons also impose a transverse force to the member wherever the direction of the cable changes. After the tendons have been anchored and no further stressing is required, the ducts containing the tendons are often filled with grout under pressure. In this way, the tendons are bonded to the concrete and are more efficient in controlling cracks and providing ultimate strength. Bonded tendons are also less likely to corrode or lead to safety problems if a tendon is subsequently lost or damaged. In some situations, however, particularly in North America and Europe, tendons are not grouted for reasons of economy and remain permanently unbonded. Most in situ prestressed concrete is post-tensioned. Relatively light and portable hydraulic jacks make on-site post-tensioning an attractive proposition. Post-tensioning is also used for segmental construction of large-span bridge girders. Prestress may also be imposed on new or existing members using external tendons or such devices as flat jacks. These systems are useful for temporary prestressing operations but may be subject to high time-dependent losses.
  
  
  
• 

  rajcivil2k6

  Advantage:
  
  Reduction of crack or no crack.
  Slender section of members.
  Increased load carrying capacity.
  Maximum usage of material properties.
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  rajcivil2k6

  Application:
  
  Beam(cast-in-situ & precast), Deck/Roof Slab, Girders, segmental construction etc.
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  rajcivil2k6

  I recommend
  
  "DESIGN OF PRESTRESSED CONCRETE"
  by
  R.I. GILBERT & N.C MICKLEBOROUGH
  
  ISBN 0-203-98586-9 e-book
  ISBN 0-419-16160-0 (Print Edition)