Optimization of Sprayed Ultrahigh-Performance Concrete Proportion Based on Response Surface Methodology

To study a method that can enhance the early strength and working performance of sprayed ultrahigh-performance concrete (SUHPC), a three-factor, three-level test was designed using the Box-Behnken design method within the response surface methodology. This investigation aimed to examine the effects of three admixtures—steel fiber, ultrafine composite powder, and quick-setting agent—on both the expansion degree and the early compressive strength of SUHPC. A nonlinear regression model was established, followed by an analysis of variance (ANOVA) to evaluate the fitting significance of each regression model and the influence of each factor on the response variables. The optimal mix ratio was then determined, including an error analysis of this ratio. Durability tests for water infiltration resistance and early-age self-shrinkage, as well as spraying tests, were conducted. The results indicated that the interaction between the amounts of ultrafine composite powder and quick-setting agent significantly affected both the expansion degree and the 1-day compressive strength of SUHPC. Additionally, the combination of steel fiber and quick-setting agent had a notable impact on the 3-day compressive strength. The error between the experimental values of the optimized mixes and the predicted values from the response surfaces did not exceed 10%, indicating the model’s reliability. The impermeability grade of the optimal SUHPC mixture was found to be P39, and the early-age self-shrinkage at 72 h was measured at 797 ppm. This performance met the durability test standards while also satisfying the requirements for playability and pumpability, with no occurrences of pipe blockage.