Critical assessment of the international estimates for relaxation losses in prestressing strands

Synopsis

This report exposes significant variations in the techniques of measuring relaxation losses in prestressing strands and in the relaxation values recommended by the Australian and International Standards and Design Codes. A specially designed test frame is reported and its capabilities are demonstrated on a series of twenty-four relaxation tests. It is concluded on the basis of the test data obtained that test equipment should be more rigidly specified and standardised and that modifications are required to the current methods of estimating the magnitude of the strand relaxation. Modifications to the Australian Standards are suggested. Areas for particular emphasis are test methods, relaxation losses immediately after stressing, isothermal relationship between the relaxation values at 1000 hours and at ultimate and the time dependent relaxation relationships when steam curing is employed.