Effects of fiber content and concrete cover on the local bond behavior of helically ribbed GFRP bar and UHPC

The bond performance of GFRP bar to UHPC is of great concern in GFRP reinforced UHPC elements and structures. In this study, pullout tests were conducted on 45 specimens to investigate the local bond behavior of helically ribbed GFRP bar to UHPC which was characterized by the failure mode, the bond strength and the bond stress-slip curve. The test parameters included the fiber content in UHPC V-f (0 %-3 %) and the concrete cover C (12-66 mm) and their effects on the local bond behavior were accessed. Specimens with C >= 36 mm and V-f >= 2 % trended to exhibit the direct pullout failure while those specimens with C < 36 mm and Vf < 2 % generally behaved in the splitting pullout failure. GFRP bars with diameter of 18 mm embedded in UHPC had the bond strength of 18.76-54.11 MPa. The bond strength could be significantly enhanced by increasing the fiber content in UHPC or the cover thickness within a certain range. As the fiber content increased from 0 % to 2 %, the bond strength of GFRP bar to UHPC grew by 74.81 %- 92.22 %. However, the bond strength showed almost negligible growth when the fiber content further increased from 2 % to 3 %. Similar phenomenon was observed in the effect of cover thickness on the bond strength. The critical cover thickness was found to be approximate 3 times of rebar diameter D in the case of embedment length Le = D. In addition, some interactions were observed in the effects of these two factors on the bond performance. Based on the test results, an updated equation including the influence of fiber content was proposed to predict the bond strength of ribbed GFRP bars embedded in UHPC. Further, a fitted model was developed to describe bond-slip relationship between GFRP bars and UHPC.