Evaluation of the Seismic Performance of Unbonded Post-Tensioned Precast Concrete Walls with Internal and External Dampers. I: Experimental Research

This paper presents the results of an experimental investigation on the performance of half-scale, unbonded, post-tensioning, precast concrete walls subjected to fully reversed cyclic loads. Moreover, this paper discusses the damage progression and failure mechanism of each specimen, associated with their construction details (damper types and confinement details of the boundary elements). Five isolated walls were tested, each consisting of precast concrete panels joined only by unbonded post-tensioning strands. The bottom joint of the walls featured two kinds of dampers: Three specimens had mild steel reinforcement, crossing the bottom joint, and the other two specimens had external replaceable hysteretic dampers, attached to the wall sides. Different types of confinement details at the boundary elements were used among the specimens. Steel fibers were mixed into the concrete of one specimen. The prestressing load ratio was 0.05 in most of the specimens. An additional axial load of 468.5 kN was applied to each specimen (additional axial load ratio of about 0.04), before any lateral loading. Quasi-static, displacement-controlled loads were applied to the specimens until a significant strength reduction was observed. Although only moderate axial loads were applied, most of the specimens sustained drifts above 3% while maintaining their lateral strength, gravity load stability, and self-centering. A better performance was observed in the specimens with external dampers, with drifts above 4%, less residual drifts and less cover concrete spalling, particularly in the specimen with steel fibers. The external dampers were very effective in dissipating energy before fracture, until 3% drifts. On the other hand, the specimens with internal dampers sustained unexpected damage at their bottom joint, after reaching their peak strength. For instance, the gravity load-carrying capacity of one specimen with internal dampers was compromised after 4% drifts. Then, the use of external replaceable dampers provided higher performance and post-earthquake recovery for unbonded post-tensioned precast walls. Additional design recommendations are provided based on the test results. Keywords: Shear walls; Precast concrete; Post-tensioning; Seismic dampers; Self-centering system; Energy dissipation; Hybrid walls.