Numerical Modelling of concrete beams strengthened with co-cured CFRP plates with a mussel shell modified epoxy (MME): An XFEM-CZM traction–separation framework with experimental works validation

Concrete beam strengthening using externally bonded CFRP remains a key method for extending service life and capacity. A reliable, mechanism-aware prediction of load-bearing capacity is still required for design. A finite-element framework was therefore established and validated to predict the flexural response of beams strengthened with CFRP plates co-cured with a mussel-shell-modified epoxy (MME) adhesive. The model was implemented in ABAQUS/CAE using a traction separation law (TSL) framework. XFEM simulated concrete cracking, while CZM captured the debonding behaviour at the CFRP and adhesive with concrete interface using the same TSL parameters. Material and interface properties were obtained independently and utilised. The influences of CFRP bonded length, MME volume fraction, and pre-loading before strengthening were examined. Validation employed load–displacement curves, failure modes, and dedicated stress-distribution studies that mapped peel and shear along the plate ends and across the interface. It was found to transition with bonded length from tip-driven shear–tension to flexural debonding; pre-loading introduced carried crack damage that elevated local interface stresses and advanced the onset of debonding. The inclusion of MME enhanced composite action and delayed failure, with moderate filler content performing best; however, excessive content (≈10%) degraded adhesive properties. The predictions of ultimate load showed close agreement with tests: most debonding-controlled cases lay within ≤ 10%, the remainder within 10–20%, and a single boundary-dominated short-plate case exceeded 20%. The framework provides mechanistically transparent stress insight and numerically reliable capacity prediction for CFRP/MME strengthening, supporting sustainable and optimised strengthening configurations.