Flexural bond behavior and development length of ribbed CFRP bars in UHPFRC

Ribbed carbon fiber reinforced polymer (CFRP) bars in ultra-high-performance fiber reinforced concrete (UHPFRC) are a high-efficiency combination for its sustainable substitute and high strength to weight ratio. This paper aims to investigate the bond stress and development length of ribbed CFRP bar embedded in UHPFRC using the hinged beam test. A total of 12 groups of hinged beam specimens were tested with different parameters, such as bar type (ribbed CFRP, sand coated CFRP, ribbed glass fiber reinforced polymer (GFRP), and steel bar), bonded length (4d_b, 10d_b, 20 d_b, and 40 d_b), fibers (with and without fibers in ultra-high-performance concrete (UHPC)), and presence of shear reinforcement for assessing the bonding behavior and development length of each bar. Tensile failure occurred in the steel, ribbed GFRP, and ribbed CFRP bars with bonded lengths of 10d_b, 20d_b, and 40d_b, respectively, and pullout failure occurred in sand coated CFRP bars regardless of bonded length. The ultimate load for the bonded length of 10 d_b was in order of ribbed CFRP, ribbed GFRP, steel, and sand coated CFRP bars. Compared with the bonded length, the bond stress of ribbed CFRP bars is larger than that of the ribbed GFRP and sand coated CFRP bar and these trends became more remarkable as the bonded length increased. There was no difference in the bond stress with presence of the reinforcement cage, and the bond stress increased by 20 % when the fibers were incorporated. The code-based equation for development length of ribbed CFRP bar was found to be underestimated, and ribbed CFRP, sand coated CFRP, ribbed GFRP bar of development length was proposed as 24d_b, 20d_b, and 107d_b.