From distributed microdamage to localized fracture in a full-scale rebar-free SFRC segmental tunnel lining: AE–MS evidence and multi-source interpretation

The transition from distributed microcracking to localized fracture in steel-fiber-reinforced concrete (SFRC) segmental linings is difficult to identify because visible cracking, internal fracture activity, and structural deformation may evolve asynchronously. A full-scale rebar-free SFRC segmental lining was tested under staged loading and differential lateral unloading. Mechanical measurements, machine-vision crack observations, acoustic emission (AE) and microseismic (MS) monitoring, and Gaussian mixture model (GMM) analysis of AE feature populations were synchronized using horizontal convergence. A system-normalized AE-ratio was introduced to track the relative accumulation of AE and MS responses. The ratio remained at approximately 0.85–0.90 during the pre-localization stage and decreased rapidly over a dominance-transfer interval of 77–91 mm horizontal convergence. This interval occurred mainly during late differential lateral unloading, as the generalized load difference increased from 151.4 to 155.1 kN, the opening of the B2–L1 joint increased from approximately 9.5 to 11 mm, and existing cracks continued to widen. Segment B3 exhibited a segment-body-dominated path with persistent mixed RA–AF feature populations, whereas segments B2 and L1 showed discontinuity-controlled paths characterized by late-stage enrichment of low-RA AE features and crack growth near joints. Crack-width growth, AE feature-population reorganization, and spatial concentration of AE/MS events around the final crack zones showed that the response transfer coincided with structural localization. The procedure provides a deformation-synchronized means of relating AE/MS response evolution to progressive localization at ring and segment scales.