Temperature-Dependent Bond–Slip Behavior of CFRP Bars Embedded in Ultrahigh-Performance Fiber-Reinforced Concrete

The flexural and durability performance of reinforced ultrahigh-performance fiber-reinforced concrete (UHPFRC) beam structures is expected to improve if the steel bars are replaced with carbon fiber–reinforced polymer (CFRP) bars with high tensile strength and corrosion resistance. Nevertheless, the interfacial bond characteristics between CFRP bars and UHPFRC may deteriorate when subjected to elevated ambient temperatures during service. To investigate the bond–slip behavior of CFRP bars embedded in UHPFRC at elevated ambient temperature, a series of direct pullout tests were conducted under elevated ambient temperatures ranging from 20°C to 80°C. The testing used CFRP bars with two diameters of 10 and 12 mm. Based on the test results, two temperature-dependent bond–slip models (T-MBPE and T-MCMR-SC models) and a temperature-dependent bond strength model were developed. The results indicated that the bond strength decreases dramatically as the ambient temperature increases. The bond strengths of the specimens with 12-mm-diameter CFRP bars are higher than those with 10-mm-diameter CFRP bars, which is mainly due to the greater rib height of the 12-mm-diameter CFRP bars. Compared with the proposed T-MBPE model, the constructed bond–slip relationships of the proposed T-MCMR-SC model were more consistent with the experimental results. The proposed temperature-dependent bond strength model performed well in characterizing the bond degradation under elevated ambient temperatures, with a prediction error of less than 10% compared to the experimental results. The research outcomes can provide an experimental and constitutive basis for using CFRP bars in UHPFRC structures at elevated temperatures.