Evaluation of residual bond behavior of CFRP and steel bars embedded in UHPC after exposure to elevated temperature

Carbon-fiber-reinforced-polymer (CFRP) bar with high tensile strength is expecting when used with ultra-high-strength-concrete (UHPC), but the reduction of mechanical properties of CFRP under thermal effects is an important issue. This study invesitigated the residual bond strength of CFRP bars in UHPC specimens after exposure to elevated temperatures. For comparison, the residual bond strength of steel rebar was also conducted after exposure to elevated temperatures. For this purpose, the pull-out specimens with dimensions of 150 mm × 150 mm × 150 mm were fabricated to investigate the effects of temperature at ambient temperature and after exposure to elevated temperature at 150 °C and 250 °C. As a result, the CFRP bars embedded in UHPC showed a reduction in the bond stress and these reducing effects is remarkable as the increase of the thermal temperature. Contrary to the test result of the CFRP bars, there was no significant difference in bond strength of the steel rebar in UHPC after exposure to elevated temperature, rather, the residual bond strength after exposure to the elevated temperature at 250 °C increased compared to ambient temperature by 9%. In order to quantitatively identify the initial bond slip behavior after exposure to thermal heating, the experimental results were compared with the modified BPE model and the CMR model, and each parameter was compared by the least square methods. The bond-slip model for CMR was more suitable than the modified BPE model in both the CFRP bars and steel bars, and the experimental results were compared with the previous research; the residual bond stress of CFRP bar in UHPC satisfies the criteria presented by ACI 440.6 M after the heating.