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Jacking force for prestressing concrete

Jacking force(Pjk) is the force used at the field to stress a tendon. Jacking force represents the prestressing force including prestressing losses. adding effective prestressing force to prestressing loses will result in jacking force. prestressing losses can be grouped into instantaneous losses related to the mechanics of the post-tensioning system and tendon geometry, and time-dependent losses related to the material properties of the concrete and prestressing steel.

 Instantaneous losses such as:
  • friction and wobble lose
  •  wedge set
  • elastic shortening of concrete
Time-dependent losses such as:
  • shrinkage of concrete
  • creep of concrete
  • relaxation of prestressing steel

the effective prestressing force can be calculated as follows

Pe=Pjk-prestressing losses

the jacking prestressing force can be calculated as follows

Pjk=Pe+prestressing losses

Figure 1

before beginning with prestressing all proposed jacks for prestressing operation shall be calibrated, The purpose of calibration is to calculate the correction factor, because the applied force on jack will slightly differ from the actual force.
Actual load=correction factor*jack gauge reading

Mixing of jacks shall not be allowed. calculation sheet should include prestressing jack number. prestressing jack correction factor is different for different jacks, therefore the actual load or jacking force will be different for different prestressing jack.


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