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Prestressing jacks

The tensioning of the steel strand (or individual wires) is usually by means of hydraulic jacks. In pre-tensioning, single strand jacks may be used. In post-tensioning the strands are often grouped to form tendons. These may be stressed by large, multi-strand jacks. The load induced into the strand is determined from the pressure in the hydraulic oil supplied to the jack or from the extension of the strand.

Wedge grips are used to grip each strand during stressing and to hold the strand permanently in the tendon anchor after stressing. The jack is removed after stressing and anchoring. In post-tensioning the prestressing force is applied directly to the concrete. In the case of pre-tensioning the anchor holds the strands until after the concrete has gained sufficient strength; the strands are then released, transferring the prestressing force to the concrete.


Monostrand prestressing jack
Photograph courtesy of Freyssinet
 
Acknowledgement: The Concrete Society
 

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