Mechanical Behavior and Prestress Loss of Unbonded Posttension Strands in Self-Centering Structures

Unbonded posttension strands are used between anchorages to provide restoring forces to self-centering structures. In this research, monotonic and quasi-static cyclic tests were performed on strand-anchorage assemblies to understand their mechanical properties and prestress loss. The experimental program included 12.7-, 15.2-, 17.8-mm (0.5-, 0.6-, and 0.7-in.)–diameter, 7-wire, 1,860-MPa (270-ksi) low-relaxation strands between anchorages. Strand stress-strain relationship and wedge seating were measured. Anchorages caused stress concentration for all sizes. The stress-strain relationship given by design guidelines was revised to incorporate the degradation of mechanical properties due to anchorages. Wedge seating, which was linearly related to posttension force in the monotonic and cyclic tests, was considered in a proposed strand force-displacement relationship. A method to predict loss caused by wedge seating and inelasticity was proposed. The predictions were evaluated using results of large-scale precast concrete bridge pier tests under quasi-static and dynamic loading. The proposed force-displacement relationship predicted the elastic response better than design guidelines, but underestimated the capacity. The proposed loss method accurately predicted strand response under quasi-static loading, but overestimated the strand response under dynamic loading.