Losses in Prestressed concrete

The effective prestress in concrete undergoes a gradual reduction with time from the stage of transfer due to various causes. This is called losses in prestressing.

Types of Losses

  1. Pre tensioning
    • Elastic deformation in concrete
    • Relaxation of stress in steel
    • Shrinkage of concrete
    • Creep of concrete
  2. Post-tensioning
    • No loss due to elastic deformation if all the wires are simultaneously tensioned but if the wires are successively tensioned then there will be elastic deformation.
    • Relaxation of stress in steel
    • Shrinkage of concrete
    • Creep of concrete
    • Friction
    • Anchorage slip

There will be losses due to sudden changes in temperature.

Elastic deformation of concrete: An elastic shortening of the concrete takes place because of the application of pre-stress in concrete.

Loss = m.fc

m – modular ratio and fc – prestress in concrete at the level of steel

Loss due to shrinkage: The change of volume of concrete members. It depends on the humidity and does not depend on the application of load. Type of cement, aggregate, and method of curing affects the shrinkage of concrete.

Loss due to creep: The continuous deformation with time under sustained loading is what we call a creep. This in elastic deformation increases at a decreasing rate during the time of loading and its total magnitude maybe several times as large as the short term elastic deformation.

Relaxation of stress in steel: Relaxation is assumed to mean the loss of stress in steel under nearly constant strain at a constant temperature. It is similar to the creep of concrete. This loss is generally of the order of 2 to 8% of the initial stress.

Friction loss in post-tensioned members: This loss occurs only in the post-tensioned members. These are because of the frictional losses in the jacking equipment. In straight lengths, it occurs due to the wobble effect, and in curved lengths, it occurs due to curvature and wobble effect.

Loss due to anchorage slip: This loss is due to the friction between the cable and the jack. The friction wedges employed to grip the wires, slip over a small distance before the wires are finally housed between the wedges.

Check this post to know more about prestress concrete.

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