Bond-slip and bond strength models for FRP bars embedded in ultra-high-performance concrete: A critical review

Ultra-high performance concrete (UHPC) composite structures reinforced with fiber-reinforced polymer (FRP) have gained increasing attention due to their superior mechanical strength and durability. The bond properties are prerequisites for applying FRP-UHPC structures in civil engineering infrastructures. Firstly, a database of 232 test specimens was established based on recently published literature to understand the bond behavior of FRP bars in UHPC. Next, the primary factors affecting bond performance were reviewed, including embedment length, surface condition, concrete compressive strength, FRP bar diameter, fibers in the UHPC, and concrete cover thickness. The failure modes of FRP bars in UHPC were discussed in this study. The bond-slip and bond strength models available in the existing codes and literature were compared and discussed based on numerous experimental results. Finally, modified bond strength models were proposed based on traditional regression and gene expression programming (GEP) analyses. The bond strength models proposed based on the GEP method yielded the most accurate estimates for the bond strength of FRP bars to UHPC, with a coefficient of determination (R2) of 0.84 and 0.91 for the pull-out and beam specimens, respectively.