Coupling effects of nano-silica and steel fibers on concrete performance and microstructural characterization

The present investigation examines the coupled effects of steel fibre reinforcement and nano-silica (NS) addition on the permeability and mechanical properties of concrete. End-hooked steel fibres (designated as D) and sheared crimped steel fibres (designated as B) of varying lengths were first incorporated into concrete mixtures to evaluate their influence on water permeability, with the objective of identifying the fibre geometry and size that yield optimal impermeability. Experimental results demonstrated that the inclusion of either fibre type effectively enhanced the resistance to water penetration; among all tested variants, D50 (end-hooked steel fibres of 50 mm length) and B40 (sheared crimped steel fibres of 40 mm length) exhibited the most pronounced improvements, corresponding to a 33.33% and 25% reduction in permeability, respectively, relative to the plain concrete(PC) without fibres. Subsequently, D50 and B40 fibres were combined with NS at various dosages to investigate the coupling effect of fibre and NS addition on the mechanical behaviour of concrete. An orthogonal experimental design was adopted, with fibre contents of 1%, 1.5% and 2% by volume, and NS contents of 0.5%, 1% and 1.5% by mass of cementitious materials. The results indicated that the most significant enhancement in mechanical performance was achieved at 2% fibre content and 1% NS content. Under these optimal proportions, end-hooked steel fibre-reinforced concrete (E-S) exhibited a compressive strength improvement comparable to that of sheared crimped steel fibre-reinforced concrete (S-W); however, the splitting tensile strength and bending strength of E-S were considerably superior to those of S-W. The underlying mechanism governing the coupled effect of steel fibres and NS on concrete performance was further elucidated through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses from a microstructural perspective. The findings provide a solid theoretical basis for understanding the combined influence of fibre reinforcement and nanoparticle incorporation on both the impermeability and mechanical response of concrete.