Assessment of Relaxation Loss Estimates for Strands

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 are discussed. A specially designed test frame is reported and its capabilities are demonstrated on a series of twenty-five relaxation tests. Areas of particular emphasis are test method, relaxation losses during first minute after completion of stressing, isothermal relationships between the relaxation values at 1,000 hours and at ultimate, and the time dependent relaxation relationships when steam curing is employed. It is concluded on the basis of the test data obtained that test equipment should be more rigidly specified and standardized and that modifications are required to the current methods of estimating the magnitude of the strand relaxation. Modifications to the Australian Standards are suggested.