Strain hardening ultra-high performance concrete (SHUHPC) incorporating CNF-coated polyethylene fibers

A novel idea of using carbon nanofibers (CNFs) to strengthen the interface transition zone (ITZ) and to enhance the interface frictional bond strength between polyethylene (PE) fibers and cement-based matrix was proposed and realized by coating CNFs on surface of PE fibers through hydrophobic interactions. A strain hardening ultra-high performance concrete (SHUHPC) incorporating such CNF-coated PE fibers was developed. The resulting CNF-SHUHPC has a compressive strength over 150 MPa and exhibits 15% enhancement in tensile strength, 20% improvement in tensile strain capacity, and reduced cracking spacing. Single fiber pullout tests showed the interface frictional bond strength of the CNF-coated PE fiber was increased by 22%, which is attributed to CNFs strengthening the ITZ by filling nano-pores and bridging nano-cracks resulting in denser microstructure and higher crack resistance against fiber pullout as revealed by the micrographs. The increased interface frictional bond strength leads to higher tensile strength and increased tensile strain capacity as predicted by the micromechanical model.