Tensile performance of cement-based composite reinforced with nano-SiO₂ modified steel fiber and its multiscale enhancement mechanism

Modifying the fiber has become a useful method to enhance the interfacial bond properties, but it was rarely extended to macro tensile performance, limiting the practical application. This paper, therefore, evaluated the enhancement effect of nano-SiO₂ modified steel fibers on the tensile performance of steel fiber reinforced cement-based composite (SFRCC) and investigated its multiscale enhancement mechanism. Direct tensile tests were designed for five typical W/C ratios from 0.40 to 0.20, and the fiber pullout behaviors were also taken into consideration. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were employed to characterize the morphology and composition of the longitudinal section of the pullout tunnel, the incompletely destroyed fiber-matrix interface, and the surface of the pulled-out fiber. The results showed that the improvement of macro tensile performances by using nano-SiO₂ modified steel fiber was effective across all the W/C ratios and became more significant when the W/C ratio decreased below 0.30. The enhancement scope increased and the enhancement ratio in the enhancement scope decreased with increasing curing age. The nano-SiO₂ modification showed a densification effect and phase improvement effect on the micro-zone near the fiber, and a strong chemical bond was established between the fiber and matrix. As the W/C decreased, the failure surface of the modified fiber-matrix kept moving closer to the fiber and changed from flat to tortuous owing to the matrix improvement, resulting in the gradually enhanced tensile performance by using nano-SiO₂ modified steel fiber.