Effects of structural fibers on bonding mechanism changes in interface between GFRP bar and concrete

In this study a total of 63 cubic specimens were prepared to investigate the pullout behaviors of the sand-coated and helically-wrapped GFRP bars as well as steel bars in concretes reinforced with structural fibers (steel, PP and PVA fibers). The results of direct pull-out testing were presented and analyzed with the aim of elucidating the effect of surface treatment of bar, fiber type, and fiber volume fraction in interface and suggesting the effective evaluation method for the improved ductility. The structural fibers in the interface changed the interfacial bond behaviors before and after the maximum stress and resulted in significant improvement of the relative bond strength, but bond failure modes largely depended on the interfacial property with the rebar. The fiber's crack closing resistance, determined by evaluating both the residual bond strength and toughness indices, was used to determine optimum amount and type of fibers in the composites. The closing pressures due to PVA and hook end steel fibers had a good effect on resisting and controlling the interfacial crack initiation, growth, and propagation.