Traditional reinforced concrete (RC) segments used in shield tunnel linings face limitations in crack resistance and durability, while hybrid fiber-reinforced concrete (HFRC) offers a promising solution to enhance its long-term performance. However, significant knowledge gaps remain regarding the application of HFRC in conjunction with steel reinforcement rebars to improve the structural performance of segmental linings. This study examines the mechanical performance of tunnel linings constructed using four types of segments, namely traditional RC, unreinforced HFRC (UR-HFRC), partially reinforced HFRC (PR-HFRC), and fully reinforced HFRC (FR-HFRC). A refined three-dimensional finite element model (3D FEM) was developed, incorporating a constitutive model for HFRC derived from laboratory tests. The accuracy of the 3D FEM was validated against full-scale load test results. Key findings include: the FR-HFRC segment ring demonstrates the highest ultimate bearing capacity and enhances subsequent stiffness during the hardening phase compared to RC segments. In reinforced segments (RC, PR-HFRC, FR-HFRC), rebars effectively mitigate cross-sectional yielding; however, this advantage comes at the cost of successive plastic hinge formation at segment joints during ultimate failure. Both bolts and reinforcement play a crucial role in load distribution, with HFRC enhancing the synergy between bolts and segments, thereby reducing reinforcement stress levels. Nevertheless, the stress in the reinforcement rarely reaches the yield point, suggesting potential underutilization in certain cases. By optimizing material configurations, HFRC can potentially offer an efficient and cost-effective solution for tunnel lining construction.@en

Concrete segments are commonly utilized as linings in shield tunnels to support the load from the surrounding ground, with their mechanical performance playing a crucial role in ensuring tunnel safety. During the construction of shield tunnels, these segments are assembled on-site, and grouting is performed concurrently to promptly fill the gap between the segment and the surrounding ground. However, inadequate grouting can lead to the formation of voids, which are hidden construction defects that compromise the mechanical stability of the tunnel segments. This study explores the impact of grouting voids on the mechanical performance of concrete segmental tunnels during construction using a 3D numerical simulation. A 3D finite-element model of a segmented shield tunnel with grouting voids was developed based on the load-structure method. The analysis focused on the effects of void characteristics, such as their angle, position, and length, on the tunnel's mechanical behavior. The results indicate that voids located at the tunnel crown reduce the vertical convergence of the tunnel cross-section, while voids at the waist exacerbate its horizontal convergence. Additionally, the presence of voids alters the bending moment distribution in the segments. Compared to the case without a void, there is a reversal of the bending moment when the void is located at the crown, and the bending moment increases from −13 kN·m to 24 kN·m, potentially causing tensile damage. Furthermore, voids also induce stress concentration within the segments, and the maximum stress concentration factor (SCF) occurs at the center of the voids as 2.44. However, when a circumferential joint intersects the void, joint opening causes stress redistribution, with the most significant stress concentration appearing at 45° on both sides of the void. These findings contribute to better damage recognition and enhance the safety assurance of concrete shield tunnels.

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This paper develops a novel lattice diffusive model to quantitatively study the chloride diffusion coefficient in unsaturated cementitious materials, in which the pore voxels are redistributed to make a better representation of a real microstructure of hardened cement paste. Considering the hierarchical microstructure and different drying-wetting cycles, water distributions in multiscale pore structures are modelled and the structure characteristics of water-filled pores, including water connectivity, water tortuosity and effective porosity, are computationally extracted based on that. A lattice diffusion network is established to predict relative chloride diffusion coefficient by combining the effect of both water saturation degree and pore structure characteristics. The predicted results are validated against experimental data, and a concise analytical equation is proposed to predict the relative chloride diffusion coefficient. The equation indicated that the relative chloride diffusion coefficient is proportional to water connectivity but inversely proportional to the square of water tortuosity. Besides, the lattice model's quantitative results reveal that the water connectivity and water tortuosity are highly related to pre-water loading processes, and influenced by the gel pore fraction, which in turn will affect the relative chloride diffusion coefficient. Compared with existing equations and non-redistributed models, the present model could improve the prediction accuracy significantly.

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