Influence of nano-SiO₂ on mechanical properties and microstructure of steel fiber reinforced concrete after heating at high temperatures

The high temperature mechanical properties of the nano-SiO₂ steel fiber reinforced concrete(NSFC), the steel fiber reinforced concrete(SFRC) and the normal concrete(NC) were investigated, such as the compressive strength, splitting tensile strength, flexural strength, and so on. SEM and XRD were used to analyze micro-structure and interfacial transition zone of steel fiber at different temperatures. The results show that the compressive strength, splitting tensile strength and flexural strength of NSFC are better than that of SFRC and NC in the test temperatures, and reach maximum at 400 ℃. Compared with NC, the compressive strength, splitting tensile strength and flexural strength of NSFC increase by 27.01%, 63.28% and 54.12%, respectively, at room temperature, and by 35.09%, 84.62% and 87.23%, respectively, at 400 ℃. SEM and XRD analysis shows that there is a permeable diffusion layer on the steel fiber surface because of solid state reaction in the interfacial transition zone of steel fiber and concrete. This permeable diffusion layer is white, bright, serrated, and mainly consists of FeSi₂ and the complex hydrated calcium silicate. The compounds of this layer change the interfacial transition zone structure, enhance bonding capacity of the steel fiber and matrix, and increase the high temperature mechanical properties of concrete.