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Pre stressed concrete

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Pre stressed concrete

  1. 1.  Prestressed concrete is a method for overcoming concrete's natural weakness in tension.  In 1904, Freyssinet attempted to introduce permanent acting forces in conc. to resist elastic forces under loads and was named “Pre Stressing”.  It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete.
  2. 2.  Pre-stressing is a method in which compression force is applied to the reinforced concrete section.  Pre-stressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that balances the tensile stress that the concrete compression member would otherwise experience due to a bending load.
  3. 3.  This classification is based on the method by which the prestressing force is generated.  They are  Hydraulic Prestressing  Mechanical Prestressing  Electrical Prestressing  Chemical Prestressing
  4. 4.  Pre-stressed concrete refers to a procedure whereby tensile rods are put in place first and tightened, followed by concrete pouring.  Compression can be applied after pouring concrete using bonds. They are tightened once the concrete is dry  Main disadvantage is that a cable can burst out of the slab, if the anchoring system fails.
  5. 5.  Even without a load, the ordinary concrete beam must carry its own weight.  An upward force is created which in effect relieves the beam of having to carry its own weight.
  6. 6.  There are two types of prestressed concrete.  They are  Pre- tensioned concrete  Post tensioned concrete
  7. 7.  Pre-tensioned concrete is cast around already tensioned tendons.  This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension.  The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction.
  8. 8.  Post tensioning is a technique for reinforcing concrete.  Post-tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed.  Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete.  They are classified into two types,  Bonded post tensioned concrete  Unbonded post tensioned concrete
  9. 9.  Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ).  The concrete is cast around a plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element.
  10. 10.  Unbonded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete.  To achieve this, each individual tendon is coated with a grease (generally lithium based) and covered by a plastic sheathing formed in an extrusion process.
  11. 11.  Take full advantages of high strength concrete and high strength steel  Need less materials  Smaller and lighter structure  No cracks  Use the entire section to resist the load  Better corrosion resistance  Very effective for deflection control  Better shear resistance
  12. 12.  Bridges  Slabs in buildings  Water tank  Concrete pile  Thin shell structures  Offshore platform  Nuclear power plant  Repair and rehabilitations
  13. 13.  Thus, pre-stressed concrete increases the quality, strength, span of the structure.  Since it is cost effective, it is used widely on recent days.