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 , and impact resistance . UHPC exhibits strain-hardening behavior in tension  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 , effective use of pozzolanic reactions  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
V. Corinaldesi and G. Moriconi, “Mechanical and thermal evaluation of Ultra High Performance Fiber Reinforced Concretes for engineering applications,” Construction and Building Materials, vol. 26, no. 1, pp. 289–294, 2012.
K. Habel, M. Viviani, E. Denarié, and E. Brühwiler, “Development of the mechanical properties of an Ultra-High Performance Fiber Reinforced Concrete (UHPFRC),” Cement and Concrete Research, vol. 36, no. 7, pp. 1362–1370, 2006.
N. Yi, J. J. Kim, T. Han, Y. Cho, and J. H. Lee, “Blast-resistant characteristics of ultra-high strength concrete and reactive powder concrete,” Construction and Building Materials, vol. 28, no. 1, pp. 694–707, 2012.
N. van Tuan, G. Ye, K. van Breugel, and O. Copuroglu, “Hydration and microstructure of ultra high performance concrete incorporating rice husk ash,” Cement and Concrete Research, vol. 41, no. 11, pp. 1104–1111, 2011.
K. Wille, A. E. Naaman, S. El-Tawil, and G. J. Parra-Montesinos, “Ultra-high performance concrete and fiber reinforced concrete: achieving strength and ductility without heat curing,” Materials and Structures, vol. 45, no. 3, pp. 309–324, 2012.
. Lai J, W. Sun, S. Xu, and C. Yang, “Dynamic properties and damage model of ultra-high performance fiber reinforced cement composites subjected to repeated impacts,” in High Performance Fiber Reinforced Cement Composites 6, vol. 2 of RILEM State of the Art Reports, pp. 389–396, Springer Science+Business Media B.V., Amsterdam, The Netherlands, 2012.
K. Kobayashi, T. Iizuka, H. Kurachi, and K. Rokugo, “Corrosion protection performance of High Performance Fiber Reinforced Cement Composites as a repair material,” Cement and Concrete Composites, vol. 32, no. 6, pp. 411–420, 2010.
S. Michael and F. Ekkehard, “Ultra-high-performance concrete: research, development and application in Europe,” in Proceedings of the 7th internatinal symposium on the utilization of (UHS/HPC '05), pp. 51–77, 2005.
E. Vejmelková, M. Pavlíková, Z. Ker？ner et al., “High performance concrete containing lower slag amount: a complex view of mechanical and durability properties,” Construction and Building Materials, vol. 23, no. 6, pp. 2237–2245, 2009.
S. J. Barnett, J. Lataste, T. Parry, S. G. Millard, and M. N. Soutsos, “Assessment of fibre orientation in ultra high performance fibre reinforced concrete and its effect on flexural strength,” Materials and Structures, vol. 43, no. 7, pp. 1009–1023, 2010.
C. Schr？fl, M. Gruber, and J. Plank, “Preferential adsorption of polycarboxylate superplasticizers on cement and silica fume in ultra-high performance concrete (UHPC),” Cement and Concrete Research, vol. 42, no. 11, pp. 1401–1408, 2012.
R. Deeb, A. Ghanbari, and B. L. Karihaloo, “Development of self-compacting high and ultra high performance concretes with and without steel fibres,” Cement and Concrete Composites, vol. 34, no. 2, pp. 185–190, 2012.
S. Kang and J. Kim, “The relation between fiber orientation and tensile behavior in an ultra high performance fiber reinforced cementitious composites (UHPFRCC),” Cement and Concrete Research, vol. 41, no. 10, pp. 1001–1014, 2011.
A. Peyvandi, P. Soroushian, A. M. Balachandra, and K. Sobolev, “Enhancement of the durability characteristics of concrete nanocomposite pipes with modified graphite nanoplatelets,” Construction and Building Materials, vol. 47, pp. 111–117, 2013.
M. S. Konsta-Gdoutos, Z. S. Metaxa, and S. P. Shah, “Multi-scale mechanical and fracture characteristics and early-age strain capacity of high performance carbon nanotube/cement nanocomposites,” Cement and Concrete Composites, vol. 32, no. 2, pp. 110–115, 2010.
B. M. Tyson, R. K. Abu Al-Rub, A. Yazdanbakhsh, and Z. Grasley, “Carbon nanotubes and carbon nanofibers for enhancing the mechanical properties of nanocomposite cementitious materials,” Journal of Materials in Civil Engineering, vol. 23, no. 7, pp. 1028–1035, 2011.
G. Z. Voyiadjis and N. Mozaffari, “Nonlocal damage model using the phase field method: theory and applications,” International Journal of Solids and Structures, vol. 50, no. 20-21, pp. 3136–3151, 2013.
L. Y. Chan and B. Andrawes, “Characterization of the uncertainties in the constitutive behavior of carbon nanotube/cement composites,” Science and Technology of Advanced Materials, vol. 10, no. 4, Article ID 045007, 2009.
Z. S. Metaxa, M. S. Konsta-Gdoutos, and S. P. Shah, “Carbon nanotubes reinforced concrete,” in Proceedings of the ACI Fall Convention, pp. 11–20, American Concrete Institute, New Orleans, La, USA, 2009.
S. Musso, J. Tulliani, G. Ferro, and A. Tagliaferro, “Influence of carbon nanotubes structure on the mechanical behavior of cement composites,” Composites Science and Technology, vol. 69, no. 11-12, pp. 1985–1990, 2009.
R. K. Abu Al-Rub, B. M. Tyson, A. Yazdanbakhsh, and Z. Grasley, “Mechanical properties of nanocomposite cement incorporating surface-treated and untreated carbon nanotubes and carbon nanofibers,” Journal of Nanomechanics and Micromechanics, vol. 2, no. 1, pp. 1–6, 2012.
Z. S. Metaxa, J. T. Seo, M. S. Konsta-Gdoutos, M. C. Hersam, and S. P. Shah, “Highly concentrated carbon nanotube admixture for nano-fiber reinforced cementitious materials,” Cement and Concrete Composites, vol. 34, no. 5, pp. 612–617, 2012.
G. Y. Li, P. M. Wang, and X. Zhao, “Pressure-sensitive properties and microstructure of carbon nanotube reinforced cement composites,” Cement and Concrete Composites, vol. 29, no. 5, pp. 377–382, 2007.
O. Galao, E. Zornoza, F. J. Baeza, A. Bernabeu, and P. Garcés, “Effect of carbon nanofiber addition in the mechanical properties and durability of cementitious materials,” Materiales de Construcción, vol. 62, no. 307, pp. 343–357, 2012.
A. Cwirzen, K. Habermehl-Cwirzen, and V. Penttala, “Surface decoration of carbon nanotubes and mechanical properties of cement/carbon nanotube composites,” Advances in Cement Research, vol. 20, no. 2, pp. 65–73, 2008.