Axial compressive behavior and failure mechanisms of ECC-strengthened brick masonry columns with different joint materials

Brick masonry columns with weak joint materials are prone to brittle splitting under axial compression, and engineered cementitious composites (ECC), owing to their multiple cracking and fiber bridging capacities, provide a promising material solution for improving their integrity and deformation capacity. However, the strengthening effectiveness of ECC depends strongly on the joint material of the masonry core and the cooperative behavior between the ECC layer and the masonry core. This study investigates the axial compressive behavior and failure mechanisms of ECC-strengthened brick masonry columns with different joint materials. Twelve short-column specimens were designed and tested under monotonic axial compression, including mud-jointed brick masonry columns (MBMCs), ECC-strengthened mud-jointed brick masonry columns (ECC-MBMCs), cement mortar-jointed brick masonry columns (CMBMCs), and ECC-strengthened cement mortar-jointed brick masonry columns (ECC-CMBMCs). The failure process, bearing capacity, deformation behavior, load–displacement response, load–strain behavior, and bearing capacity calculation method were analyzed. The results show that ECC strengthening significantly improved the behavior of MBMCs. Compared with MBMCs, the average bearing capacity and peak deformation capacity of ECC-MBMCs increased by 131.84% and 39.67%, respectively, and the ultimate deformation capacity increased by approximately four times. The failure mechanism revealed that ECC strengthening changed MBMCs from sudden brittle splitting to ductile failure with post-peak load fluctuations caused by progressive bulging-induced interfacial debonding of the ECC layer. For CMBMCs, ECC strengthening increased the average bearing capacity by 35.81%, but the ductility improvement was limited because interfacial shear stress induced premature debonding of the ECC layer after cracking of the masonry core. A modified bearing capacity model considering fiber bridging and bulging-induced interfacial debonding was proposed. The prediction error for ECC-MBMCs decreased from −25.15% to −7.73%, indicating improved prediction accuracy. Based on the limited experimental data in this study, a conservative bulging-induced interfacial debonding coefficient of 0.35 is recommended as a preliminary value for design-oriented calculations.