Bond behavior of FRP bar in ultra-high-performance lightweight concrete (UHPLC)

The combination of ultra-high-performance lightweight concrete (UHPLC) and fiber-reinforced-polymer (FRP) bar offers an optimal solution in terms of lightweight construction, long-term durability, and enhanced structural strength. However, the bond behavior of FRP bars in high-strength UHPLC with a reduced unit weight remains insufficiently understood. In this study, the bond performance between FRP bar and UHPLC with a compressive strength exceeding 80 MPa and a unit weight below 2000 kg/m 3 was investigated using pull-out tests. A total of 150 mm cube specimens were fabricated considering ten variable combinations, including bonded lengths (1 d , 2 d , 4 d , 8 d ), cover thicknesses (1.5 d , 2.5 d , 3.5 d , 5.4 d ), and bar types (CFRP, GFRP, and steel bars). For the ribbed CFRP bar, the bond strength increased by approximately 34.2 % and 39.9 % as the cover thickness increased from 1.5 d to 3.5 d and 5.4 d , respectively. As the bonded length increased to 4 d and 8 d , the bond strength decreased by approximately 5.84 % and 36.11 %, respectively, reaching 12.10 MPa and 8.21 MPa. All specimens except the ribbed CFRP bars with bonded lengths of 1 d and 2 d , as well as the sand-coated CFRP bar specimens, exhibited splitting failure due to incorporation of lightweight aggregate. The GFRP bar exhibited the highest bond strength of 14.5 MPa, followed by the sand-coated CFRP, steel, and ribbed CFRP bars. For the bond-slip relationship, the CMR model showed better agreement with the experimental results than the modified BPE model, while the average R 2 values of both models exceeded 0.90.