Optimization of UHPC based on reactive powder concrete: Box-Behnken Design, analysis of variance, components interaction effects and sensitivity analysis

The primary factors in reactive powder concrete mix design include the water-to-cement ratio, sand-to-cement ratio, and the substitution of silica sand with proper materials. Addressing the inherent behavioral sensitivity of reactive powder concrete to variations in these factors is critical. Accordingly, this research utilized the Box-Behnken Design to meticulously plan and execute experiments, aiming to explore the combined effects and interactions of these main factors on its properties. Results show that the sensitivity of reactive powder concrete to changes in the sand-to-cement ratio and silica sand substitution generally has a more pronounced impact on both compressive and flexural strengths compared to the water-to-cement ratio. Especially in specimens containing fine calcium carbonate, the substitution ratio notably influenced the flexural behavior, whereas, in specimens with fine quartz sand, the water-to-cement ratio's impact on flexural strength surpassed that of the substitution after the sand-to-cement ratio. The more critical interactions occurred between the sand-to-cement ratio and the silica sand substitution ratio than the substitution ratio and water-to-cement, which were influenced by the choice of substitute materials. Also, scrutiny studies on the microstructure of specimens further demonstrated that while the use of fine calcium carbonate enhances the formation of CH within the microstructure, potentially weakening this concrete, it considerably benefits the flexural strength. Conversely, using fine quartz sand fosters a more significant formation of C-S-H gel, which beneficially influences the compressive and tensile strengths of the concrete.