Effect of nickel-coated carbon nanotubes on the tensile behaviors of ultra-high performance concrete (UHPC): insights from experiments and molecular dynamic simulations

Nickel-coated multi-walled carbon nanotubes (Ni-MWCNTs), through combining the high tensile strength and Young's modulus of multi-walled carbon nanotubes with the good wetting characteristics of nanoscale nickel particle, substantially increase their dispersion and composite efficiency in composites. Therefore, Ni-MWCNTs are a promising nano-reinforcement to strengthen the tensile performances of ultra-high performance concrete (UHPC). This study investigated the effects of Ni-MWCNTs with different aspect ratios on the uniaxial tensile behaviors of UHPC and explored the underlaying modification mechanisms by molecular dynamics simulation and microstructural analysis. The results indicated that the presence of Ni-MWCNTs notably modifies the tensile performances of UHPC. Ni-MWCNTs with large aspect ratios are preferable for improving the tensile strength and toughness of UHPC. Adding 0.5 wt.% Ni-MWCNTs with an aspect ratio of 1000 can lead to a maximum increase of 31.9 and 46.4% in the tensile strength and toughness, respectively. While Ni-MWCNTs with small aspect ratios of 125/200 are more effective in enhancing the Young's modulus and Poisson's ratio of UHPC, the increase rate of which can reach 31-53 and 11-30%. Such modifications derive from the enhancement of the interfacial bonding ability between MWCNTs and matrix due to the presence of nanoscale nickel particle and the improvement of matrix compactness owing to the introduction of Ni-MWCNTs. Additionally, the Ni-MWCNTs' bridging/pull-out effects, enhanced by the existence of nanoscale nickel particle, can retard crack development and consume energy under tensile loading.