Ultrahigh performance concrete (UHPC) realized distinctly high mechanical, impermeability, and durability characteristics\nby reducing the size and content of capillary pore, refining the microstructure of cement hydrates, and effectively using\nfiber reinforcement. The dense and fine microstructure of UHPC favor its potential to effectively disperse and interact with\nnanomaterials, which could complement the reinforcing action of fibers in UHPC. An optimization experimental program\nwas implemented in order to identify the optimum combination of steel fiber and relatively low-cost carbon nanofiber in\nUHPC. The optimum volume fractions of steel fiber and carbon nanofiber identified for balanced improvement of flexural\nstrength, ductility, energy sorption capacity, impact, and abrasion resistance of UHPC were 1.1% and 0.04%, respectively. Desired\ncomplementary/synergistic actions of nanofibers and steel fibers in UHPC were detected, which were attributed to their reinforcing\neffects at different scales, and the potential benefits of nanofibers to interfacial bonding and pull-out behavior of fibers in UHPC.\nModification techniques which enhanced the hydrophilicity and bonding potential of nanofibers to cement hydrates benefited their\nreinforcement efficiency in UHPC.
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