Abstract: As an important component of New Austria Method, the shotcrete technique has the characteristics of simple procedure, fast installation, and wide applications. It becomes an essential tool in the tunnel excavations. Adding the adequate amount of fly ash into the shotcrete mixture can reduce the potential shrink cracks, improve the workability and optimize the construction cost. The impact of fly ash content on the mechanical properties (tensile, compressive and bending strengths) is studied in this paper through the laboratory tests. The results indicate that all indicator strengths reduce with the increase of fly ash content in the mixture. A significant strength reduction can be found at fly ash content of 28%. It is suggested that for a regular shotcrete mix design, the optimum fly ash content should be 9%.

Key words: Shotcrete, Fly Ash Contents, Strength Index, Optimum Content