Long-term effect of accelerator content on flexural toughness of steel fiber reinforced shotcrete for tunnel construction

Shotcrete is widely used in underground and tunneling applications due to their superior flexural properties and impact resistance. There is a higher need in improving these properties by adding various admixtures and fiber reinforcements to achieve superior performance to accomplish faster turnout in tunneling constructions. In this study, the effect of three accelerators on the long-term flexural performance of steel-fiber reinforced shotcrete were investigated. The types of accelerators used were aluminate, cement mineral, and alkali-free accelerators, whose effects on the flextural properties were investigated over the periods of 1, 3, 6, 12, and 24 months. The flexural toughness was first estimated in accordance with JSCE SF-04, where up to span/150 deflection area under the load-deflection curve was used for calculating the flexural toughness. Whereas ASTM C1018–97 was used to evaluate the first-crack and post-peak load toughness behavior with three specified points for better understanding the flexural behavior of all mixtures. Moreover, it is important to use alkali-free accelerators in terms of both the long-term performance of the tunnels and the safety of the workers. The long-term performances of the specimens showed by the compressive strength and flexural strength were comparable. Finally, according to the long-term effectiveness and performance, the best mix design for the tunnel construction sites is also proposed.