With the aging of tunnels, additional maintenance and assessment procedures are required to monitor their working conditions. The passive thermal infrared method, which is a nondestructive contact-free method, is used to find defects in a box-type and shield tunnels with concrete lining. In this study, this method is numerically applied to detect defects in an unhealthy concrete structure. The thermal field data measurements are reviewed and the governing heat transfer mechanisms for a defected concrete surface are presented. The heat transfer in the concrete segment, tunnel air, and the air inside the void due to the conduction, convection, and radiation mechanisms is explained. Further, the type of cavity, the temperature difference between the healthy and unhealthy concrete surfaces, the temperature difference between the tunnel air and concrete surfaces, and the influence of the void depth on the detection accuracy are investigated. The results show that the temperature difference between the concrete surface and the tunnel air is more than 0.35 °C, which is an ideal condition for detecting the voids with an approximate depth of 30 mm or less.
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