Thermo–mechanical modelling of fire-exposed reinforced concrete beams considering temperature-dependent bond–slip degradation

Abstract
This study develops a validated sequentially coupled finite element (FE) modelling framework for reinforced concrete (RC) beams subjected to ISO 834 standard fire exposure, with explicit consideration of temperature-dependent steel–concrete bond degradation. The thermal analysis first predicts transient temperature fields in concrete and reinforcement; these fields are then mapped to a mechanical model incorporating Concrete Damage Plasticity (CDP), temperature-dependent reinforcement properties, and nonlinear interface springs for longitudinal bond–slip. The revised model is strengthened by an explicit bond–slip constitutive relationship, a calibration procedure based on elevated-temperature bond data, and a mesh convergence assessment. The thermal simulation reproduced measured cross-sectional temperatures with typical deviations of 5–10%, while the mechanical analysis captured the overall deflection–time response, crack localization pattern, and failure-time range of 170–200 min. The results show that rapid stiffness loss and accelerated deflection are governed by the combined degradation of concrete compressive strength, steel stiffness/yield strength, and interfacial bond resistance, especially after reinforcement temperatures exceed approximately 600–700 °C. Concrete spalling was observed experimentally but was not explicitly simulated; its influence is discussed as a source of late-stage discrepancy and as a target for future hygro–thermal–mechanical modelling. The proposed framework therefore provides a transparent and reproducible tool for performance-based fire assessment of RC beams, particularly where bond deterioration cannot be represented by a perfect-bond assumption.