Fire performance and post-fire residual behavior of insulated CFRP shear-strengthened RC beams: An experimental and numerical study

This paper presents an experimental and numerical study on the fire performance and post-fire residual behavior of reinforced concrete (RC) beams shear-strengthened with carbon fiber-reinforced polymer (CFRP) U-wraps. Seven full-scale beams were tested, including three specimens under ambient conditions and four subjected to ISO 834 standard fire exposure. The key experimental variables included fire exposure duration, number of CFRP U-wrap layers, shear span-to-depth ratio and the presence of a 20 mm fire-resistant mortar insulation layer. Experimental results showed that the CFRP U-wraps increased the shear capacity of the specimens by approximately 30 %–35 % under ambient conditions. In fire tests, the uninsulated RC beam failed due to diagonal shear cracking, whereas the insulated CFRP shear-strengthened RC beams maintained their structural integrity for a fire resistance rating of 2.5 h, exhibiting only small midspan deflections. Subsequent post-fire bending tests showed that the insulation layer effectively preserved the residual shear capacity of the strengthened beams, maintaining values comparable to their pre-fire capacities. A coupled thermal-mechanical numerical model was proposed by integrating transient heat transfer analysis with a fiber-based sectional method to predict the temperature distribution within the beam cross-section and to quantify the degradation of shear capacity during and after fire exposure. The model explicitly accounted for the temperature-dependent deterioration of concrete, steel stirrups and CFRP U-wraps as well as the bond behavior between the CFRP and the concrete substrate. Numerical predictions showed close agreement with the experimental results, which validated the accuracy and reliability of the proposed numerical model. The validated model offers a robust analytical framework for evaluating the in-fire and post-fire shear performance of CFRP shear-strengthened RC beams, thereby supporting the development of performance-based fire-resistance design provisions for such members.