Prestress Loss of a New Vertical Prestressing Anchorage System on Concrete Box-Girder Webs

Abstract

A new, double-tensioned prestressing steel strands anchorage system (DTPSSAS), characterized by low prestress loss caused by prestressing tendon retraction, is described. When it is used for short prestressing tendons in concrete box-girder webs, this anchorage system significantly reduces prestress loss. To investigate its efficiency in reducing prestress loss, a rectangular thin plate using the DTPSSAS was devised and tested; the results demonstrated that the average rate of instantaneous prestress loss decreased from 32.35% after the first tension to 2.68% after the second tension. Further, long-term observations of prestress loss with the DTPSSAS were conducted in the laboratory and in a field test. Based on the field test of a continuous concrete box-girder bridge, the average loss rate of instantaneous prestress loss to the control tensile stress of two strands in the web is 7.61%, and the average rate of prestress loss at 150 days is 10.53%. Concurrently, the time-dependent prestress loss was calculated with Models GL2000 and CEB-FIP90 for different concrete shrinkage and creep prediction by the step-by-step calculation method. In comparison with the laboratory test results, the calculated results with the two models were greater than the test values. In addition, the effect of stress redistribution caused by the constraint of reinforcing bars to concrete shrinkage and creep was evaluated. The results revealed that the stress redistribution effect increased the vertical compressive prestress loss of concrete, indicating that the time-dependent compressive prestress loss rate of concrete was greater than the tensile prestress loss rate of prestressed tendons. In this regard, bridge engineers may need to pay attention to the stress redistribution phenomenon in vertical prestressing design.