Optimization of Shotcreting Parameters for Coarse Polypropylene Fiber-Reinforced Concrete Using the Entropy Weight Grey Method

Conventional shotcrete is prone to cracking and exhibits poor impermeability. The incorporation of coarse polypropylene fibers into lining shotcrete can enhance its crack resistance and durability. The shotcreting process parameters are critical factors influencing the performance of coarse polypropylene fiber-reinforced concrete. While existing research has primarily focused on the impact of process parameters on rebound rate, this study investigates their effects on mechanical properties. Experiments were conducted with a fixed spraying angle of 90°, examining the 1 d, 3 d, and 28 d compressive strength, as well as the 28 d tensile, flexural, and bond strength under varying spraying air pressures and distances. The sensitivity of each mechanical property indicator was analyzed using the entropy weight method and grey relational analysis to determine the optimal shotcreting process. The results indicate that a spraying air pressure of 0.5 MPa and a distance of 1.5 m produce denser concrete with higher compressive and bond strengths. Increasing the air pressure to 0.6 MPa increases the proportion of fibers aligned parallel to the spraying surface, resulting in improved tensile and flexural strength. Different mechanical properties exhibit varying degrees of sensitivity to process parameter changes. The 28d tensile strength and 28d flexural strength showed the highest indicator weights, accounting for 20.4% and 22.0% respectively, and were the most sensitive to parameter variation. Grey relational analysis identified the combination of 0.5 MPa air pressure and 1.5 m distance as having the highest relative relational degree, representing the optimal comprehensive performance. These findings provide a valuable reference for the application of fiber-reinforced shotcrete in tunnel engineering.