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Prestressing losses

In prestressed concrete, we precompress concrete in order to offset undesirable tensile stresses, this will enhance concrete strength and durability. there are two methods for producing prestress concrete, post-tensioned and pre-tensioned. in post-tensioned we cast concrete then tendons are inserted inside ducts placed before concreting and then we tension tendons. in pre-tensioned concrete, tendons are tensioned first then concrete cast. the prestressing force applied at field termed jacking force, this force will not remain constant, this force will encounter instantaneous and time-dependent losses, the designer will consider these losses and the effective prestressing force will be equal to jacking force minus the losses.

Figure 1

instantaneous losses 

  • Friction losses: these losses occur due to friction between tendons and post-tensioning ducts, these losses will increase by increasing the length of the tendon
  • Anchor set: the setting of wedges in anchor head will cause tendons to lose some of prestressing force as shown in figure 2
Figure 2

  • Elastic shortening of concrete: compressing of concrete as a result of applied prestressing force will reduce and shrink the concrete on other words elastic shortening of the concrete, the shortening of concrete will lead to losses of prestressing force, elastic shortening is an accumlative process, the shortening of concrete will increase by increasing the number of tensioned tendons.
Time-dependent losses

  • shrinkage: shrinkage can be defined as the contraction of concrete due to loss of moisture, this type of loss is time-dependent and it doesn't occur instantly 
  • creep: creep can be defined as the deformation under a permanent load, creep also a time-dependent, creep of structure will increase with time until reaching the highest value.
  • relaxation: steel relaxation is a time dependant-losses, the magnitude of relaxation will depend on the properties of prestressing steel, prestressing steel can be treated to reduce the relaxation effects. 
next articles will dicuss the losses of prestressing fore in details....


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