Study of Vertical Prestress Loss in Concrete Box Girder Webs with Twice-tension Strand Technique

To investigate the vertical prestress loss in concrete box girder webs with twice-tension strands technique, a rectangular thin plate test under twice-tension prestressed strands was conducted. At the same time, twice-tension vertical prestress loss used in a practical bridge was also tested, and was compared with the traditional vertical prestressed system using fine-rolled twisted steel bars. Test results confirmed that the immediate vertical prestress loss can be reduced to less than 10% by the twice-tension technology and far less than that of fine-rolled twisted steel bars system. Twice tension system can improve efficiency of vertical prestress and shear reliability in concrete box girder webs effectively. At last based on the test results, some factors that affect twice-tension vertical prestress loss including pipe roughness, joint compression, deformation and retraction of anchorage, elastic compression of concrete, relaxation of tendons and creep & shrinkage of concrete, were discussed, and corresponding methods of design taking value were suggested.