PSI - Issue 84

Mohammad Javad Azari Nezhadian et al. / Procedia Structural Integrity 84 (2026) 1222–1230

1223

1. Introduction The mechanical performance of precast tunnel lining segments has long been recognized as a fundamental issue in mechanized tunnelling, arising from the combined effects of soil-structure interaction, concentrated TBM thrust forces, and the flexural demands imposed during production, transportation, installation, and service life. Early full scale investigations, such as that of Molins and Arnau (2011) performed on a 15-ring section of Barcelona Metro Line 9, showed how tangential soil-structure interaction, non-uniform stress distributions, and stress concentrations at segmental joints are of paramount importance in the bending response of segmental linings, providing a clear motivation for deeply investigating the flexural behavior of tunnel segments under complex loading conditions. In respect to the investigation of different reinforcement strategies able to improve flexural resistance and durability of these structural elements, full-scale four-point bending tests performed by Caratelli et al. (2016) provided one of the first direct comparisons between conventionally RC and Glass Fiber Reinforced Polymer (GFRP) bar-reinforced segments. According to their study, while reaching comparable ultimate capacities, GFRP-reinforced segments avoided corrosion problems and showed different cracking features, representing an important step concerning the assessment of the bending behavior of precast tunnel linings made of new reinforcement materials. Advances in the field of fiber-reinforced concrete further helped improve understanding of the post-cracking flexural mechanism. By conducting three-point bending experiments, Jamshidi Avanaki (2019) illustrated how hybrid steel fiber-reinforced concrete significantly enhances the post-cracking tensile behavior, residual flexural capacity, and performance of damage control. All these improvements have very high relevance for tunnel lining applications, sensitive to control of crack width and energy-dissipation capacity. The comprehensive review by Jusoh et al. (2019) emphasized the crucial role of flexural stiffness, joint rotation, and seam contact behavior in defining the global bending response of segmental linings reaffirming that insufficient flexural resistance often coincides with cracking near bolt holes and joint interfaces, highlighting the need for solutions capable of improving crack control. More recent studies have concerned high-performance materials and hybrid reinforcement strategies. The investigation on Ultrahigh-Performance Concrete (UHPC) segments reinforced with hybrid steel fibers by Abbas and Nehdi (2021)showed that fiber content increased from 1% up to 3%, thereby giving significant increases in flexural capacity, pronounced multi-cracking behavior, and an extended hardening response under bending. Such findings position the UHPC-FRC systems as promising alternatives in cases where severe TBM-induced forces impose substantial flexural demands. Numerical studies, such as Rajput and Joshi (2021), also shed more light on the flexural sensitivity of tunnel linings, which indicates important rotation and stress concentration around longitudinal joints and underlines the need for localized strengthening strategies in hybrid reinforcement systems. In a subsequent full-scale experimental program, Trabucchi et al. (2022) tested Macro Synthetic Fiber Reinforced Concrete (MSFRC), hybrid RC–MSFRC, and conventional RC segments under four-point bending. Accordingly, their test results indicated that fiber-enhanced and hybrid configurations yielded better crack distribution, lower crack widths, and enhanced post-peak behavior while exhibiting comparable bending resistance with substantial reductions in the amount of conventional reinforcement. These results further underpinned the structural efficiency and practicality of hybrid reinforcement systems in the design of precast tunnel segments. The latest developments can be seen in the wide-ranging study by De Andrade et al. (2024), which coupled experimental testing and multiscale numerical modelling in order to investigate the flexural behavior of full-scale RC– SFRC tunnel segments. The authors' work was based on material characterization through EN-14651 bending tests and full-scale three-point bending of precast segments designed for the São Paulo Metro Line 5. Their results clearly showed that hybrid RC–SFRC segments have enhanced crack control with reduced crack spacing, higher flexural resistance, and increased post-cracking stiffness compared with conventional RC segments. This work also confirmed that combining localized traditional reinforcement with optimized steel-fiber dosage indeed leads not only to superior serviceability and ultimate-state performance but also to significant design and durability benefits. Collectively, the studies demonstrate a logical progression from conventional RC reinforcement to fiber-reinforced and hybrid RC-SFRC systems, with this progression being driven by an ever-increasing demand for increased flexural capacity, superior crack control, and greater structural efficiency in relation to both TBM-induced and service-level bending actions. In spite of the progress made so far, the literature still reflects a serious lack of appropriate full-scale