Durable repair of concrete structures relies critically on the shear bond between new and existing concrete, yet reliable prediction of this bond remains challenging due to highly localized interfacial damage mechanisms. Conventional numerical interaction strategies, such as tie constraints, are unable to capture progressive debonding, often leading to unconservative estimates of load transfer and structural capacity. This study presents a three-dimensional Finite Element approach that explicitly represents the repair interface through thin sacrificial layers governed by Concrete Damaged Plasticity and element deletion. The approach is validated against a dedicated shear push-out experimental campaign in which the cement content of the repair layer was systematically varied from 300 to 550 kg/m³ while all other parameters are held constant. The numerical model accurately reproduces the experimentally observed zipper-type interfacial debonding and captures both the onset and propagation of localized shear damage, with satisfactory quantitative agreement. Building on this validation, continuous calibration curves are derived with high statistical correlation (R2 ≈ 0.95) and low predictive error (NRMSE < 9%), directly relating repair cement content to shear bond strength and interface compressive strength. Specifically, the framework captures the nonlinear increase in shear bond strength from 0.21 to 1.85 MPa. The proposed method provides a physically grounded and design-oriented bridge between mix proportioning and structural simulation, enabling consistent definition of interface parameters without iterative numerical tuning.