# Cavity Detection in Steel-Pipe Culverts Using Infrared Thermography +
## 5 Experimental Results
### Experiment 2: Specimen 1, Cavity Configuration 3  
In this experiment, we combined different cavity configurations, including an air cavity, a water cavity, a thin air layer cavity, and a nonadjacent (in-depth) cavity into one setup (Figure 4e). This time the analysis began with PCT to determine if this method can provide us with any clue about the cavities in a more complicated configuration. Figure 9 shows a few selected EOFs for experiment 2. Even though the signs of cavities are not as bold and clear as in experiment 1, they are still informative and all cavities, except the small ball, can be detected. However, this time checking all first 10 EOFs from different setups was required. Contrary to the previous experiment, in which EOF3-4 carried the most information about cavities, in this experiment information spread among a larger number of EOFs, including EOF8-9. Moreover, detecting the in-depth cavity required a longer thermal stimulation. As the usefulness of pre- and post-processing was demonstrated in the previous experiment, they were also employed for experiment 2 (see Figure 10a-e). Figure 10f is related to a test without specimen while maintaining the other parameters of the setup exactly the same as in experiment 2. This is useful for performance comparison and illustrating the accuracy and quality of the cavity detection by the proposed approach. We also examined the effectiveness of other techniques, such as thresholding and gradients. Figure 10h,i shows the power of the image gradients as four different areas representing the four cavities can be easily differentiated from the rest. Figure 8: PCT analysis (EOF3); experiment 1. (**a**) Setup 1 (best result). The cavity is located exactly under the blue square patch at the bottom of image. (**b**) Setup 2. (**c**) Setup 3. (**d**) Setup 4. Input: cooling down frames for Setup 1, 3, 4 and the entire image sequence for Setup 2. Heating up period: 5 s for Setup 1–3 and 25 s for Setup 4. Temporal filtering was applied only in setup 3. For a better understanding of the influence of different types of cavities in the IRT results, the temperature graphs (Figure 11) of the relevant regions were studied. The nondefective regions have the lowest temperature value during the cooling down phase. The second lowest temperature belongs to zone 2, above cavity 2 (thin air); as indicated in Figure 4e, a zone (1-5) is assigned to each cavity (1-5). The highest values were recorded for zone 4 (thick air cavity) and zone 5 (water cavity), respectively. These results could be anticipated in advance by analyzing the thermal effusivity of the cavity materials. According to [67], the thermal effusivity of air, water, soil (avg. properties) are approximately 6, 1588, and 1067 (588 for soil sandy dry) \(\mathrm{W}\sqrt{\mathrm{s}}/(\mathrm{m}^{2}\,\mathrm{K})\). Since the air thermal effusivity is significantly smaller than the soil's, air cavity allows less thermal energy exchange, and thus the specimen temperature in zone 5 cools down slower. Due to the same phenomenon during the heating up, zone 5 warms up faster as is evident from the related graph in Figure 11. The root of difference between the decay curves of zones 2 and 4 is the presence of a thicker air layer for zone 4 which acts as a stronger insulator (between the soil and the specimen), resulting in a higher temperature increase in the heating phase and a smaller decline in the cooling period in zone 4. The behavior of cavity 5 (water) is more complex. As its thermal effusivity is higher than both dry and average property soil, we expect lower temperatures for this zone in comparison to the nondefective regions which are in contact with soil. We think this inconsistency lies on two issues. First, this cavity is not made completely from water, but a soaked sponge. Although we do not know the exact thermal effusivity for a soaked sponge, it must be smaller than water. Second, the water cavity is surrounded by cavity 4 (air) which is considerably larger.