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Spalling is the most serious problem in the maintenance of subway tunnels. Voids forming on or near the concrete surface are often considered as signs of degradation leading to spalling. However, it is difficult to detect these voids by visual inspection alone because no cracks or other signs of irregularities are visible on the surface (Fig.1). Thus, a non-destructive, contact-free inspection method...
Tokyo Metro ensures the safety of underground tunnels as civil engineering structures by conducting inspections and performing appropriate repairs. Inspections are generally conducted visually, and health grades are determined according to the extent of deformations. However, given the difficulty of comparing the health grades of sections within the structures to the same standards throughout entire...
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