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.

Monitoring of cracks and crack growth rates is a crucial aspect of structural health monitoring for concrete infrastructure, and multiple manual and automatic monitoring techniques have emerged over the years. This study focuses on an in-depth review of concrete crack sensing using distributed fiber optic sensing (DFOS) technology. DFOS provides the option to sample distributed data points through dedicated optical fibers or cables, thereby effectively addressing the spatial limitations associated with conventional discrete point sensors such as foil strain gauges and transducers. The main findings include that (1) smart concrete crack sensing generally involves three objectives: detecting crack initiation, identifying the crack location and determining the crack width and its evolution; (2) for DFOS used for crack sensing, the three main sensing principles are to measure localized strain spikes in optical fibers or cables that span across cracks, to detect signal intensity losses caused by micro-bending of optical fibers in proximity to cracks and to measure precise local temperature variations within the crack areas; (3) strain-based crack sensing has become the predominant method due to its superior sensing performance and application versatility. This dominance is supported by extensive experimental demonstrations and successful implementations in field monitoring practices; (4) the sensitivity of optical fibers or cables to concrete cracks depends on the installation method, while quantitative crack width measurements require the precise determination of crack locations followed by a subsequent integration or exponential fitting of strain along the length at fiber-concrete interface. This study helps to advance the application of the smart DFOS for structural health monitoring and maintenance of concrete infrastructures.

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.