Unbonded Tendons as an Alternative for Bonded Tendons in Post-Tensioned Bridges: Constructability, Structural Performance, and Monitoring

Bonded post-tensioning tendons are used extensively in segmental bridge construction and have generally provided satisfactory service with respect to durability. However, multiple tendon failures in U.S. bridges with grouted tendons have led to the exploration of other forms of corrosion protection. One potential option is to use flexible fillers inside the tendon duct instead of cement grout that do not bond the prestressing strands to the concrete section. The use of these unbonded tendons not only affects the ultimate strength, fatigue, and serviceability behavior of structural systems, but also encourages efficient monitoring methods and facilitates tendon replacement during the structure’s service life. The current bridge design code, however, does not explicitly address flexible fillers, and little guidance is given regarding unbonded tendons in general. To evaluate the structural performance of unbonded tendons, full-scale experiments were conducted with both external and internal I-girder specimens having deviated tendon profile. These investigations provide important information on structural behavior of unbonded systems as well as tendon constructability, and can assist in the preparation of material and construction specifications. Multiple laboratory experiments were conducted with commercially available flexible fillers to examine the constructability of unbonded tendons. These experiments focused on tendon and duct detailing, filler material preparation, injection process and equipment, and venting procedures. An analytical investigation was also performed to compute the rate of cooling of flexible fillers during injection, which is a critical parameter to ensure that flow clogging does not occur. In addition, a promising tendon monitoring algorithm has been developed that uses the strain variations in the wedge plates caused by load relief due to corrosion-induced wire breaks. Extensive analytical and experimental investigations demonstrate the method’s feasibility in detecting, locating, and quantifying tendon damage.