Study on the Mechanical Properties and Durability of Tunnel Lining Concrete in Coastal Areas

To address the problems of the lining cracking and spalling in tunnel structures in coastal areas under the influence of special geological conditions, environmental loading, and the coupling effect of chemical erosion, hybrid fibers were introduced to fly ash concrete in this study. The working performance, compressive strength, split tensile strength, and flexural strength of the hybrid fiber fly ash concrete were tested. A chloride diffusion coefficient under steady-state conditions and a durability test for resistance to sulfate corrosion were carried out. Thus, in-depth analyses of the comprehensive performance of the hybrid fiber fly ash concrete used for the tunnel lining were carried out and the damage mechanism was explored. The results showed that the hybrid fiber fly ash concrete exhibited higher strength compared to the concrete in the control group. However, when the fibers exceeded a certain dosage, the reduction in the working properties of the concrete structure led to the creation of larger pores in the matrix structure, which in turn affected the mechanical properties of the concrete. The most significant reduction in the chloride diffusion coefficient was observed when both steel fibers and coconut fibers were added at a 1.0% volumetric parameter, compared to the control group. The apparent state and compressive strength after sulfate corrosion were also minimally affected. This study ensured that the mechanical properties of the concrete were improved and the corrosion resistance of the matrix also substantially improved, providing a scientific basis for improving the performance of tunnel lining concrete, and confirming that steel–coconut hybrid fiber fly ash concrete has a great potential to improve the structural load-bearing capacity and durability, which may provide theoretical support for its continued use in tunneling projects and construction processes.