Inspection Method with Infrared Thermometry for Detect Void in Subway Tunnel Lining

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 using infrared thermometry to detect these non-visible voids was studied on. Recently, performance of infrared thermometry was improved dramatically. Thus, possibility to detect voids or flacks by the method increased. For this reason, we carried out several measurements and tests in our box-type tunnel and shield tunnel to confirm feasibility of applying this method for maintenance activities. For example, we have conducted a void detection test using infrared thermometry in our subway tunnels during its non-operating hours, and compared the results with that of other conventional test methods, such as a hammering test. This report describes details of the study.