Experimental and theoretical study on bond behavior of helically wound FRP bars with different rib geometry embedded in ultra-high-performance concrete

Fiber reinforced polymer (FRP) bar reinforced ultra-high-performance concrete (UHPC) members have attracted wide interest due to their excellent durability and mechanical properties. Determining bond strength is of great importance to the design of FRP bar reinforced UHPC members. This study investigates the effects of bar type, concrete type and rib geometry on the bond behavior of helically wound FRP bars embedded within UHPC. Pullout tests on a total of 48 specimens with three types of concrete (0%, 1% and 2% steel fiber content), three types of bar (steel, basalt FRP and glass FRP) and three types of surface condition (smooth, shallow ribbed and deep ribbed) have been conducted. The results show that the bond strength of FRP bars embedded within UHPC is greatly affected by rib height. Indeed, FRP bars with deep ribs can reach a bond strength comparable to that of ribbed steel bars. A novel bond strength model for FRP bars embedded within UHPC, considering the effect of rib geometry, has been proposed and validated by the experimental results and existing data in the literature.