Mechanical properties and micro-mechanisms of polyvinyl alcohol fiber-reinforced shotcrete for tunnel support

Conventional shotcrete support technology has inherent limitations, including a high material rebound rate and inadequate crack resistance. With the continuously increasing mining depth of coal-mine roadways, higher requirements are imposed on the workability of shotcrete. To optimise the mechanical performance and supporting effectiveness of shotcrete, and to address construction difficulties caused by poor crack resistance and high rebound loss, this study systematically investigated the effects of polyvinyl alcohol fibre (PVAF) at different dosages and curing ages on the compressive strength, tensile strength, shear strength, and flexural strength of concrete. The test results show that, with increasing PVAF dosage, both the slump and spread of shotcrete decrease markedly, and the rebound rate is significantly reduced from above 21% to approximately 15%. PVAF can significantly improve the crack resistance and shear performance of shotcrete and exhibits excellent toughening effects. The optimum enhancement is achieved at a dosage of 0.1%; at 28 days, the split tensile strength, shear strength, and flexural strength increase by 36.2%, 24.3%, and 21.1%, respectively. Furthermore, quantitative analysis of the crack morphology and its geometric parameters of the failed specimens indicates that PVAF effectively reduces crack area and crack width by altering the failure mode of concrete; Meanwhile, four-point bending tests show that PVAF improves the post-peak load-carrying capacity of concrete under bending and increases the flexural toughness index. Finally, by constructing a mesoscale numerical model of concrete, the micro-mechanism by which PVAF enhances concrete strength and improves toughness is elucidated in depth.