# Cavity Detection in Steel-Pipe Culverts Using Infrared Thermography +
## 6 Discussion  
In our tests, we observed that the thermal behavior of the real specimen was very different from that of specimen 1 because of the dissimilarity in the material and the surface appearance. Moreover, the thermal behavior of the whole system depends on several other factors such the soil components, size and type of the pipe, the depth in which the pipe has been buried, the pipe distance from the ground surface, and ambient temperature. Preparing an experimental setup taking into account all these factors, even if possible, would be too costly and time consuming. Therefore, to verify the performance of the proposed approach for cavity detection, it is necessary to carry out further tests in real setups. Since the passive thermography approach could not provide enough information about cavities in our experiment, the active approach was used. However, in real scenarios the thermal behavior of the whole system will be largely different and therefore we think that the passive thermography might be able to provide sufficient information for cavity detection. Since the passive approach is easier and less expensive to implement, this possibility should be investigated in a real setup. Figure 15: Probability map. (**a-d**) Experiment 3. (**e-h**) Experiment 2. (**a,e**) Computed using all frames. (**b–d,f-h**) Computed using frames of the cooling down phase. All results, except the one shown in (**h**), were obtained with the same \(c_{1}\) and \(c_{2}\); see the text for the details. _pen_: the penalty factor. In the experiments, some unexpected thermal patterns were observed in thermograms. Following further investigation, we realized that the presence of a thin layer of air between the specimen and the soil was the main reason behind these unexpected behaviors. It occurred because in our experimental setups, the specimens were placed on the surface of the soil and in practice it was extremely difficult to level the soil surface. Even having a leveled soil was not enough since the specimen surface was not perfectly leveled. As a result, we could not completely eliminate the thin air layer. It is worth noting that thin air layers adjacent to the specimen surface can cause a noticeable variation in the thermal radiation from the specimen surface. Nevertheless, we later realized that this feature of thermography can be very helpful in detecting other problems in a culvert. Formation of thin air layers adjacent to a culvert pipe can indicate a sign of cavity creation in earlier stages, and our experiments unexpectedly show that this stage can be identified through IRT. Before achieving the reported results, a set of parameters had to be determined. For example, the duration of thermal stimuli has a crucial impact on the quality of thermal images and may dictate the level of information that can be extracted from thermograms. Very short and weak thermal excitation may not be enough for the detection of the required information while long and strong thermal stimuli may yield more fruitful thermograms, but they extend the test period and usually are more costly. Moreover, the optimal parameters for PCT and ACT were not the same necessarily. For PCT, we obtained the best result when the length of the heating up period was about 5-10 s whereas for ACT the better results were achieved with longer thermal stimulation. Also, we noticed that the detection of deeper cavities requires longer stimulation as the heat propagation is slow, thereby taking more time to reach deeper layers. Consequently, conducting experiments with different heating periods is necessary for a successful cavity detection. In addition to the thermal stimuli, it is necessary to adjust a few other parameters such as the distance between the camera and specimen, the distance between the thermal sources and specimen, locations and directions of thermal sources (to minimize nonuniform excitation on the specimen surface). We think the results, reported in this paper, can be improved by further tuning of these parameters.