Failure mode of URM walls retrofitted with hybrid basalt-polyethylene fiber ECC at critical thickness: A case study

Engineered Cementitious Composites (ECC) overlays have been widely used to retrofit the unreinforced masonry (URM) walls. However, existing studies have focused primarily on overlay thicknesses within 20 mm. The performance of thicker overlays, particularly the critical 30 mm thickness permitted by some design codes, remains largely unexplored. This study investigates the cyclic behavior of URM walls retrofitted with a novel hybrid basalt-polyethylene fiber ECC (HFECC) overlay of 30 mm thickness under representative axial compression of 1.2 MPa. Three specimens were tested: an un-retrofitted URM wall (URM), a wall retrofitted with normal-strength HFECC (SW-1), and a wall retrofitted with high-strength HFECC (SW-2). Failure modes, hysteretic responses, stiffness degradation, ductility, and energy dissipation were systematically analyzed. The results demonstrate that the HFECC overlay effectively transforms the brittle single-cracking mode of URM walls into a ductile multi-cracking mode, significantly enhancing crack control and damage tolerance. High-strength HFECC improves shear strength by 2.8 times and achieves an ultimate drift ratio of 1/75 with cumulative energy dissipation exceeding 200 kJ-approximately 50 times that of the URM wall. However, progressive detachment of the overlay is identified as the governing failure mode under the tested axial stress, indicating that the full tensile capacity of the HFECC layer may not be fully mobilized. An analytical model based on the superposition principle predicts ultimate strength with test-to-estimated ratios of 0.82 (normal-strength) and 1.13 (high-strength), both within 20% deviation. This study provides the first experimental validation of 30 mm thick ECC overlays for seismic retrofitting of URM walls and highlights that bond capacity and detachment prevention are critical for optimizing retrofitting effectiveness.