On strain change of prestressing strand during detensioning procedures

The strain change of a seven-wire strand is experimentally investigated for two different detensioning procedures of a pretensioned concrete bar: flame-cutting and hydraulic-jacking. Based on the results of strain changes, it is confirmed that the prestressing force was stepwise transferred to concrete for several seconds according to the individual cut of different wires in the flame-cutting procedure. In the hydraulic-jacking procedure, the detensioning was smoothly accomplished in a time of about one second shorter than that of flame-cutting. It seems that the pattern of strain changes during the detensioning procedure is more important than the elapsed time for detensioning. In addition, the drop in the strain of each wire at the moment of flame-cutting was reduced by the installation of the debonded region. Moreover, the test results showed that the placing of stirrups has an insignificant effect on the strain change. In particular, the initial prestressing force was estimated simply from the elastic recovering strain of the strand after detensioning. The prestressing force obtained from the procedure was very close to that obtained from a load cell.