Spalling is the most serious problem in maintenance of subway tunnels on operation. Voids forming on or near the concrete surface are often considered as signs of degradation which induce spalling. However, it is difficult to detect these voids by visual inspection, as the voids have no cracks or other signs of irregularities and are not visible from the surface. Thus, we studied a non-destructive, contact-free method using infrared thermometry to detect these non-visible voids. The subject tunnel is a box type tunnel, constructed by cut and cover tunneling method and made of reinforced concrete. The third rail method is applied for the line. The subject of the study is 2km length of a railroad line in operation. We performed the following next tests. Analyses on change of environmental temperature in tunnel by taking thermal image photographs of all subject sections and measuring the temperature of the inner tunnel and concrete wall surface at 13 selected points. Analyses on environmental temperature that allows the detection of non-visible voids by taking thermal image photographs seasonally, during off-hour time and observation of thermal image photographs for 6 days on fixed points. Confirmation of measurement accuracy to detect non-visible voids with thermal image photographs by comparison with the results of hammering inspections at the selected points. Analyses of the condition of detectable non-visible voids by non-destructive tests and micro-destructive tests at selected points. Investigation on the possibility of the application of thermal image photographs to business maintenance work.
