Experimental investigation on bond performance of helically ribbed GFRP rebar embedded in Ultra-high performance concrete

•GFRP rebar diameter and cover thickness have interactions on the failure mode.•Helically ribbed GFRP rebar has the bond strength in UHPC close to steel rebar.•UHPC critical cover thickness is 3 ∼ 4 times of GFRP rebar diameter.•Rib height of GFRP rebar shows significant influence on the bond strength. This study presents an experimental study of the bond performance of helically ribbed glass-fiber-reinforced polymer (GFRP) rebars embedded in ultra-high-performance concrete (UHPC). Pull-out tests were carried out on 40 specimens of GFRP rebar with various parameters, including UHPC cover thickness, rebar diameter and embedment length. The bond behavior was characterized in terms of failure mode, bond strength and bond stress-slip response, and the influence of variable parameters on the bond performance was described. The specimens of GFRP rebar behaved in two types of pull-out failures, namely the direct pull-out failure and the splitting pull-out failure. Those specimens characterized with large diameter of GFRP rebar and relatively small thickness of UHPC cover tended to show the splitting pull-out failure. The GFRP rebar had the bond strength of 20.7 ∼ 56.2 MPa in UHPC when the embedment length equals to the rebar diameter, which is close to that of steel rebar in UHPC. The increase of UHPC cover thickness contributed to the improvement of bond strength when it was smaller than the critical cover thickness (3 ∼ 4 times of rebar diameter). The bond strength grows with the GFRP rebar diameter owning to the increase of rebar rib height. In addition, a fitting equation for bond strength of helically ribbed GFRP rebar to UHPC was suggested based on the present test data.