Comparative bond-slip response of ribbed CFRP bar to UHPC after exposure to high temperature

Ribbed carbon fiber reinforced polymer (CFRP) bar and ultra-high-performance concrete (UHPC) have high potential in terms of utilization of load-carrying capacity and maintenance of the concrete structure. Bonding behavior of ribbed CFRP bars to UHPC after exposure to elevated temperature is of importance due to its susceptibility of thermal effects. This study investigated the residual bond strength of the ribbed CFRP bar embedded in UHPC at ambient temperature and after exposure to elevated temperature. The bond strength of ribbed CFRP bars with different embedded length and cover thickness were also considered and the bond strength was compared with the sand coated CFRP bars. Both at ambient temperature and after exposure to elevated temperature, the bond strength of the ribbed CFRP bar increased with an increased embedment length and cover layer due to the mechanical resistance of the ribs. However, the difference reduced as the elevated temperature increased. Compared with the sand coated CFRP bars, the decreasing rate of bond strength is slightly lower in ribbed CFRP bars and the residual bond strength after exposure to elevated temperature at 400 ℃ was more than 10 MPa, which satisfies bond strength of FRP bar suggested by ACI 440.5 M. The critical temperature of the ribbed and sand coated CFRP bars was suggested as 300 ℃ with consideration of decreasing rate in this study. The modified bond equations were presented to reflect the reduction of bond strength due to the elevated temperature, and the bonding behavior of the ascending branch was compared using the modified BPE and CMR models and presented the coefficient of each parameter.