Investigation of basic properties, microscopic characteristics, and optimized ratio prediction model of ultra-high steel fiber reinforced concrete

Ultra-high steel fiber reinforced concrete (UHSFRC) has poor workability and a low cost-performance ratio, and the application of active admixtures can overcome these shortcomings of UHSFRC. This work investigated the feasibility of fly ash (FA) and silica fume (SF) to improve the performance of SFRC and evaluate the effects of FA and SF on the workability, mechanical properties, microstructure, and economic/environmental performance of UHSFRC. The results showed that FA and SF can increase the slump, compressive, and splitting tensile strength of UHSFRC. Compared with control tests, the slump, 28-d compressive, and 28-d splitting tensile strength increased by 43.25%, 29.35%, and 55.72%, respectively. More importantly, microscopic analysis showed that FA and SF have balling, filling, and adsorption effects, and increase peak intensity of C-S-H gel and ettringite, promoting secondary hydration of cement. According to the experimental data and mechanistic analysis, an innovatively optimized ratio prediction model is adopted to establish the relationship between the slump, compressive, and splitting tensile strength of UHSFRC and the content of FA and SF, and the testing error was less than 5%, indicating that the model is feasible. Moreover, the effect of FA and SF content on cost-performance ratio, carbon emission, and energy consumption of UHSFRC was predicted. The economic/environmental analysis showed the mixing of FA and SF can improve the cost-performance ratio of UHSFRC and reduce carbon emission and energy consumption. This study can provide theoretical guidance for the improvement of the workability and mechanical properties of UHSFRC, and have important research implications for the improvement of the life cycle assessment of UHSFRCs. The results of the study also have some limitations, and further validation is needed for normal steel fiber concrete (compressive strength of 30-60MPa, with fibers with a length of 30-60mm and diameter of approx. 1mm).