Load-Bearing Performance of Steel–Concrete Composite Reticulated Shell Segments Based on the Secondary Service Life Concept

Shield tunnels enter the operation and maintenance stage, ensuring the long-term safety of segmental linings has become a critical challenge. To overcome the limitations of traditional passive repair, this study introduces novel steel–concrete composite reticulated shell segments (SCRSs) based on the secondary service life concept. A scaled-model surcharge test was conducted to evaluate the baseline performance and failure mechanisms of unstrengthened segments (S-CN). Subsequently, a secondary concrete layer was cast inside the deformed segments (S-CR) to assess post-damage structural recovery, supported by theoretical ring mechanics analyses. Experimental results indicate that the unstrengthened SCRS exhibits pronounced load-bearing capacity and ductility, characterized by a progressive three-stage response encompassing the elastic stage, yield plateau, and post-yield stage; the test was terminated at a load of 141.95 kN with the curve still ascending, so this value represents a terminal load and a lower bound on the ultimate capacity. This behavior stems from the sequential engagement of outer reinforcement and the inner steel framework, providing clear deformation warnings prior to structural failure. Furthermore, casting the secondary concrete layer enhanced both the stiffness and the load-bearing capacity of the deformed specimen, which sustained a maximum total load of 203.14 kN; this value is a local failure threshold governed by concrete crushing beneath the concentrated loading heads rather than a structural peak load. The as-built specimens were fabricated from commercially processed steel members and hand-mixed concrete, and their measured material properties (a measured ultimate tensile strength of about 240–400 MPa for the steel, varying between two test batches; a cubic compressive strength of 27 MPa for the concrete) were used throughout the analysis. The experimental data support the feasibility of the secondary service life concept as a proof-of-concept study of the segment body, within the limitations of a single scaled specimen and short-term monotonic loading. This composite design offers a potential pathway for shifting shield tunnel linings from reactive repair to proactive, whole-life management.