Water leakage through soil has been considered the most serious problem and the main cause of concrete degradation in subway facilities. Several deterioration mechanisms are derived from water intrusion, among others are concrete cracking, spalling, and water voids. These mechanisms can compromise the structural integrity and jeopardize public safety. The detection and evaluation of concrete structures are predominantly conducted on the basis of visual inspection (VI) techniques, which are known to be time-consuming, subjective, and qualitative in nature. Although, these technologies may be consistent in finding surface defects, e.g., cracks, and spalling, they fall short in detecting subsurface distresses such as air voids, and water voids. Ground penetrating radar (GPR) has been widely used for the inspection and evaluation of concrete infrastructure. Nevertheless, few research endeavors were conducted for the detection and mapping of air/water voids. This paper presents a GPR-based assessment model for subway networks. The model performs damage identification and localization of air voids and water voids in the concrete subsurface. It provides a systematic approach for the detection and mapping through the incorporation of image-based analysis (IBA) and processing techniques. First, a defect detection scheme is designed to establish a consistent inspection pattern. Second, subsurface data are collected in a subway network facility. Third, the position and dimension of the detected distresses are mapped to estimate the severity of deterioration. The proposed method was implemented on assessing a segment in Montréal subway network. Validation of the results was conducted through visual inspection, digital images, thermal images and concrete coring samples which demonstrated high correlation and compatibility with the constructed GPR-based maps. The proposed system is expected to improve the quality of decision making as it can assist transportation agencies in identifying critical deficiencies and by focusing constrained funding on most deserving assets.