Experimental and numerical investigation of bond behavior at ECC-concrete interfaces: effects of surface roughness and bonding agents

Abstract The bond between engineered cementitious composites (ECC) and existing concrete governs composite action in strengthened members. This study examined the combined effects of two substrate conditions—an untreated surface (R0) and a mechanically chiseled surface (R1)—and three bonding treatments: no agent (J0), a single-component water-based agent (J1), and a two-component structural agent (J2). Slant-shear and pull-off tests were conducted, followed by inverse identification of equivalent interface parameters using a surface-based cohesive-zone model. After specimens with documented non-target failures were excluded, roughening increased the mean slant-shear strength of the agent-free interface from 1.400 to 1.919 MPa. In contrast, the J1 mean decreased from 2.672 to 2.311 MPa and showed a coefficient of variation of 35.9%, whereas the R1-J2 combination reached 3.156 MPa. These contrasting trends indicate a non-additive interaction between surface condition and bonding-agent type within the tested combinations. The fracture-surface observations and contrasting mechanical responses further indicate that bonding-agent performance depends on its compatibility with the prepared surface and the resulting load-transfer condition. Finally, the observed non-additive response motivates an interaction-based extension of the classical shear-friction framework using a dimensionless state coefficient λ, introduced here as a perspective for future calibration.