Novel mechanical headed anchors for strengthening RC slabs with openings under cyclic loading

Reinforced concrete (RC) flat slabs are widely used due to their structural efficiency and design simplicity, but they remain highly vulnerable to punching shear, particularly when openings are located near columns. Despite advances in strengthening methods, the role of headed anchors as mechanical anchorage for shear reinforcement has not been sufficiently examined. This study investigates their performance in RC flat slabs with openings, strengthened using high-strength steel threaded rods and GFRP bars. The headed anchorage incorporated threaded steel components at the ends of the post-installed shear reinforcement to provide mechanical end restraint and enable prestressing. Six interior slab-column connections were constructed and tested under constant gravity load and reversed cyclic lateral loading. Two specimens served as controls without shear reinforcement, while four were retrofitted with vertical shear elements. Of these, five specimens contained openings adjacent to the column face, and one control specimen was cast without openings. The results demonstrated that the headed strengthening configurations enhanced load transfer and cyclic performance. Compared with GB-O, GHS-O increased the peak load and ductility by 33.9% and 31.1%, respectively. At a drift ratio of 1.75%, corresponding to the peak-load drift of both GB-O and CS-O, GHS-O dissipated 64.2% more energy per cycle and exhibited a 10.5% lower damage index than GB-O. Similarly, compared with SR-O, SHS-O increased the peak load and ductility by 5.8% and 15.4%, respectively. At a drift ratio of 2.20%, corresponding to the peak-load drift of SR-O, SHS-O dissipated 4.0% more energy per cycle and exhibited a 4.8% lower damage index than SR-O.