# Cavity Detection in Steel-Pipe Culverts Using Infrared Thermography +
###### Abstract  
Finding efficient and less expensive techniques for different aspects of culvert inspection is in great demand. This study assesses the potential of infrared thermography (IRT) to detect the presence of cavities in the soil around a culvert, specifically for cavities adjacent to the pipe of galvanized culverts. To identify cavities, we analyze thermograms, generated via long pulse thermography, using absolute thermal contrast, principal components thermography, and a statistical approach along with a combination of different pre- and post-processing algorithms. Using several experiments, we evaluate the performance of IRT for accomplishing the given task. Empirical results show a promising future for the application of this approach in culvert inspection. The size and location of cavities are among the aspects that can be extracted from analyzing thermograms. The key finding of this research is that the proposed approach can provide useful information about a certain type of problem around a culvert pipe which may indicate the early stage of the cavity formation. Becoming aware of this process in earlier stages will certainly help to prevent any costly incidents later. c 2021 25 April 2021 29 April 2  
and eddy current), and radiographic testing are examples of well-known techniques for this purpose. For a good overview of various aspects of culvert inspection including available technologies, refer to [4; 5]. Most of these techniques are used to detect anomalies and defects such as blockages, cracks, leaks, corrosion, and pitting in the culvert pipe rather than the surrounding soil. One of the widely used methods for identifying the location and depth of buried pipes is ground penetrating radar (GPR) [6]. Effectiveness of GPR has also been reported for other tasks including the localization of buried pipeline leaks [7] as well as the detection of underground cavities [8; 9]. The GPR method has some limitations such as the impossibility of scanning bottom and lateral sides of a culvert, reduced efficacy for wet clay soil (as the depth of the signal penetration is significantly affected by the soil humidity) and negative effect of environmental electromagnetic noise [4]. Another approach to identify the presence of underground cavities is multi-channel analysis of surface waves (MASW) [10; 11]. In MASW, after generating vibration using a 60 kg hammer, signals of a set of sensors (16 sensors placed with a 1 m spacing) are recorded at once. Through analyzing recorded signals in time-space domain, the dispersion curves of the different Rayleigh modes are created, and corresponding shear wave velocity profiles are determined. The stiffness or rigidity of the scanned ground is directly related to the shear wave velocity. A disadvantage of MASW is the requirement of a complicated interpretation process to determine the presence and location of cavities. Following the rapid advancement of thermographic cameras and development of new techniques such as pulse thermography (PT), principal component thermography (PCT), pulsed phase thermography, and lock-in thermography in the past three decades, Infrared thermography (IRT) [12; 13; 14; 15; 16] has become a powerful tool to detect anomalies, and defects in metallic [17], plastic [17], ceramic [18], and composite [19; 20] materials. Despite the successful application of IRT in detecting corrosion, cracks, delaminations, and internal voids [21; 22; 23; 24; 25], we are not aware of any IRT-based approach for the identification of cavities in the soil around a culvert pipe. Very few papers [26; 27] have recommended IRT for void detection but without providing any reference and evidence to any existing works on this topic. We think the potential of IRT for the identification of cavities in the surrounding area of a culvert has not been explored yet. Therefore, this study assesses the potential of IRT for this task for galvanized culverts. Considering the difficulties of accessing culverts (see Section 4.1), experiments are conducted in a laboratory setup.