Experimental investigation on sing fiber pullout behaviour on steel fiber-matrix of reactive powder concrete (RPC)

A new single fiber pullout testing method was applied to characterize the interfacial bond performance, which includes the pullout load-slip relationship, pullout energy, bond strength, and fiber utilization ratio, of the steel fiber-matrix in reactive powder concrete (RPC). The bond properties and bond strengthening mechanism were investigated from the perspective of certain parameters, including the fiber type, fiber embedment angle, silica fume content, magnetic nanoparticles content, and curing time. The microstructure of the steel fiber and longitudinal morphology of the tunnel after the pullout test were determined by scanning electron microscope (SEM). In terms of the results, the bond strength of the end-hooked steel fiber is much stronger than that of the straight steel fiber. With the increasing of embedment angle, the ultimate pullout load and pullout energy of straight steel fibers increase, and the interfacial bond strength increases. It is found that the incorporation of silica fume can effectively enhance the matrix strength. In addition, RPC containing 20%∼30% silica fume shows significant improvement in pullout behavior. The appropriate curing age contributes greatly to the hydration reaction and the matrix strength of RPC. Steaming curing for 2 d can effectively improve the interfacial bond performance. A content of 1% magnetic nanoparticles enhances the pullout behavior of straight steel fibers with no brass coating, because the maximum pullout load and pullout energy increase by 33.6% and 79.0%, respectively. The microstructural observations confirm the conclusions regarding the bond mechanism drawn from the pullout tests. In conclusion, the performance of the matrix, the characteristics of the fiber-matrix interface area and the friction of the steel fiber in the tunnel can influence the bond performance of the fiber-matrix interface of RPC.