# Application of Thermal Image Data to Detect Rebar Corrosion in Concrete Structures
## 3 Establishing IRT Database
### Building the IRT Database  
#### 3.4.1 IRT Imaging  
In this study, we planned to capture 16 data values for each of the 270 variables. The database was composed of 4320 captured data values. The detailed infrared imaging plan was designed as follows:  
1. Number of specimens: 6 (degrees of corrosion) \(\times\) 3 (cover thickness) \(\times\) 3 (specimens) = 54;
2. Number of infrared imaging rounds: 54 (specimens) \(\times\) 5 (air temperature) = 270 rounds;
3. Number of data values: 270 (rounds of infrared imaging) \(\times\) 16 (30 s intervals, 480 s of imaging) = 4320.  
Figure 7: Determination of rebar heating time. (**a**) 150 A heating; (b) 200 A heating; (c) 250 A heating.  
As shown in Figure 8, an inverter welder was used at both ends of the target structure's rebar, and current was applied. While maintaining a 40 cm distance from the concrete surface, a chamber was used to shoot infrared images. Thus, the infrared imaging system was built. Next, the electric heater was used to apply 150 A of current to the 10 mm cover specimen, 200 A to the 20 mm cover specimen, and 250 A to the 30 mm cover specimen. The rebar of each specimen was heated for 300 s while the thermography data were collected. Even after the current was cut off 300 s after the heating began, the specimens were left at a high temperature for 180 s, and the system continued to collect thermography data. At this time, the infrared camera was used at 30 s intervals to capture the thermographic data. The system was able to obtain 16 thermographic data values for each specimen.  
Because thermography data are significantly affected by the surrounding environment, including the atmosphere, temperature, and humidity, the experiments were performed indoors, without the effect of direct solar radiation and wind, to reduce the number of factors that impacted the experiments. To minimize the impact of changes in the atmospheric conditions, the thermography data were captured while maintaining atmospheric temperatures of 16, 21, 25, and 28 \({}^{\circ}\)C and a humidity of 60% (\(\pm\)10%).  
Table 11 shows the IRT of specimens with cover thickness of 10 mm and corrosion ratios of 0%, 5%, and 10% when the air temperature was 16 \({}^{\circ}\)C. It can be seen that the surface temperature of concrete gradually rose, starting at the part near the heating unit as 120, 240, and 300 s passed. Even when rebar heating was stopped after 300 s, the concrete surface temperature continued to increase. The thermography of specimens with corrosion ratios of 0%, 5%, and 10% were compared, and it was found that as the rebar corrosion ratio increased, the amount of heat that was transferred to the specimen concrete surface also increased.  
There was a difference in the increase in heat with respect to the cover thickness, but like the specimens with cover thickness of 10 mm, there was an increase in the heat transferred to the specimen concrete surface with the increase in the corrosion ratio. As shown in the section in white in Table 12, 3.12-3.13 is the temperature range that is above the level range that is shown in color when the infrared camera is set before imaging. It shows the high temperature of the temperature distribution of the thermography.  
Figure 8: Infrared imaging system.