Ultrahigh performance concrete (UHPC) realized distinctly high mechanical, impermeability, and durability characteristics by reducing the size and content of capillary pore, refining the microstructure of cement hydrates, and effectively using fiber reinforcement. The dense and fine microstructure of UHPC favor its potential to effectively disperse and interact with nanomaterials, which could complement the reinforcing action of fibers in UHPC. An optimization experimental program was implemented in order to identify the optimum combination of steel fiber and relatively low-cost carbon nanofiber in UHPC. The optimum volume fractions of steel fiber and carbon nanofiber identified for balanced improvement of flexural strength, ductility, energy sorption capacity, impact, and abrasion resistance of UHPC were 1.1% and 0.04%, respectively. Desired complementary/synergistic actions of nanofibers and steel fibers in UHPC were detected, which were attributed to their reinforcing effects at different scales, and the potential benefits of nanofibers to interfacial bonding and pull-out behavior of fibers in UHPC. Modification techniques which enhanced the hydrophilicity and bonding potential of nanofibers to cement hydrates benefited their reinforcement efficiency in UHPC. 1. Introduction Ultrahigh performance concrete (UHPC) is an emerging class of cementitious composites with outstanding material properties [1–4]. UHPC offers very high strength (compressive strength > 150？MPa, tensile strength > 8？MPa) [2, 5, 6], toughness [3], and impact resistance [7]. UHPC exhibits strain-hardening behavior in tension [8] and provides ductile failure modes with desired postcracking tensile resistance. The extremely low permeability of the dense matrix [2, 9, 10] provides UHPC with excellent durability characteristics. The distinct balance of qualities offered by UHPC are made possible by the use of a high content of cementitious binder (more than three times that of conventional concrete) [11, 12] with a very low water/binder ratio (less than 0.25) [5, 13, 14], dense particle packing through proper grading of (relatively fine) aggregates, cement and supplementary cementitious materials [6], effective use of pozzolanic reactions [15] to refine the pore system and enhance the binding qualities of cement hydrates, and use of relatively high fiber contents to overcome the extreme brittleness of the ultrahigh strength cementitious matrix [1, 11, 16, 17]. UHPC has been used on a limited basis in the united Stated since 2000 [13, 18, 19]. Steel fibers are commonly used in UHPC. The
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