# Cavity Detection in Steel-Pipe Culverts Using Infrared Thermography +
## 4 Method  
### Experiment Setup and Data Acquisition  
Conducting experiments with real culverts in many regions of Canada involves some challenges. Gaining access to culverts is either difficult or impossible for most part of the year due to the flow of large volumes of water and icing conditions. Moreover, only culverts whose structure contains at least one cavity with known size and location are useful for the experimental phase of the project; this information is essential for both development and performance evaluation. These kinds of records are often infrequent and accessing them requires obtaining authorization from the Ministry of Transportation. Therefore, the performance of the proposed approach is evaluated in a laboratory setup instead. Figure 1 illustrates the experiment setup, in which a wooden box of size \(78\times 33\times 60\) cm\({}^{3}\) is used to represent the soil around the culvert pipe. The box is filled with a mixture of sand and gravel and a specimen is then placed on the top of it. Four 1000 W halogen lamps are used as the source of thermal excitation. The power of lamps is controlled through an eight-channel dimmer pack and an electronic interface circuit. With the help of this controller and adjusting the location of the lamps, we attempt to reduce the nonuniformity of thermal stimuli. Thermal images are recorded during both heating up and cooling down process using two IR cameras and a PC. The real setup is shown in Figure 2.  
Thermograms are produced by a FLIR Phoenix mid-wave infrared (MWIR) camera and a Jenoptik VarioCAM long-wave infrared (LWIR) thermal camera (see Table 1 for technical specifications). Thermal cameras with cooled detectors are superior to those with uncooled detectors in terms of capture rate, spatial resolution, and thermal sensitivity; this superiority is illustrated well in [64]. Because of these advantages and the availability of the FLIR Phoenix camera in our laboratory, the analyses are made based on thermal images produced by this camera. As this is the first time, the potential of IRT for cavity detection is assessed, it is important to identify the smallest temperature differences and the FLIR Phoenix can be very useful for this purpose. A downside of the cooled models is that they are considerably more expensive. Therefore, we also use the uncooled camera to make sure about the possibility of detection with this model, which is more suitable for field work. The experiments are conducted with two specimens (Figure 3). The first one, referred to as specimen 1, is a 2 mm flat steel plate with dimensions 70 \(\times\) 30 cm\({}^{2}\). To solve the low emissivity and reflection issues, it is coated with black flat paint. Since the laboratory test with this specimen cannot represent a real scenario, we carry out further experiments using specimen 2 which is a horizontal cut of a corrugated galvanized steel culvert pipe (CGSP) with a diameter of 65 cm, a thickness of 1.6 mm, and corrugations with a pitch of 68 mm and a depth of 13 mm. To create cavities, a few different objects, as shown in Figure 4a, are used. The first air cavity is created using a 10 \(\times\) 10 \(\times\) 6 cm\({}^{3}\) cube of foam with a carved area of 6 \(\times\) 6 \(\times\) 3 cm\({}^{3}\) so that a 6 \(\times\) 6 cm\({}^{2}\) surface of the plate is in contact with air rather than sand. The second configuration (Figure 4c,d) is almost similar to the previous one except that 1 cm of sand is located between the cavity and the lower surface of the specimen. Cavity configuration 3 (Figure 4e) is made of five different cavities: (i) cavity configuration 2; (ii) a 10 \(\times\) 6 \(\times\) 1.3 cm\({}^{3}\) foam cube with a 8 \(\times\) 4 \(\times\) 0.5 cm\({}^{3}\) hollowed area; (iii) a ping pong ball; (iv) a cylindrical ceramic ashtray with the interior radius 5.5 cm and height 2.5 cm half filled with water; and (v) a 2 \(\times\) 2 \(\times\) 1.5 cm\({}^{3}\) sponge.