Enhancing the toughness of bonding interface in steel-UHPC composite structure through fiber bridging

Using Ultra-high performance concrete (UHPC) to replace normal strength concrete (NSC) can greatly reduce the weight of steel-concrete composite structures. However, the bonding interfaces between steel and UHPC in composite structures are prone to brittle failure, and the introduction of ductile fibers can effectively enhance the interfacial toughness. In this study, helical fibers and steel wire mesh fibers were stably embedded in UHPC and epoxy matrix to prepare the steel-UHPC bonding specimen. The interfacial shear properties were studied based on a modified direct shear test method, and the effects of fiber size, fiber number, and fiber direction were considered. Meanwhile, the interfacial properties of bonding specimens with two types of ductile fibers were compared with those of epoxy direct bonding interface and quartz sand particle embedded interface. The results indicated that the toughness enhancement of steel wire mesh fiber on the bonding interface of steel-UHPC composite structure is the best among all interfaces, and its maximum fracture energy Gτ is 25.5 times that of epoxy direct bonding interface. Meanwhile, the toughening mechanism of steel wire mesh fiber on steel-UHPC interface is highly related to its arrangement direction. The interface failure began with the shear fracture of steel wire mesh fiber and ended in the failure of epoxy matrix, when the steel wire mesh was transversely arranged. Only tensile fracture of steel wire mesh fiber occurred, and the failure of the epoxy matrix was not observed when the steel wire mesh was longitudinally arranged. Finally, the shear capacity calculated by the proposed model based on two toughening mechanisms showed good agreements with the experimental results. The feasibility of fiber bridging to enhance the toughness of bonding interface in the UHPC-OSD system is confirmed via the finite element analysis.