# Application of Thermal Image Data to Detect Rebar Corrosion in Concrete Structures
## 3 Establishing IRT Database
### Establishment of Optimal Infrared Camera Imaging Conditions
#### 3.3.2 Method of Heating Rebar in Concrete Specimens  
The internal rebar heating must occur simultaneously with the beginning of the heating, and a fixed heating rate must be maintained. To satisfy the experiment requirements, such as safety, while heating the entire rebar to a high temperature (100 \({}^{\circ}\)C or more), several heating methods were attempted experimentally, as listed in Table 7. The methods were compared, and the most suitable one for the experimental conditions was used. This method employs an inverter welder to apply a fixed current to the rebar within the specimen and heat it.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Optics**} \\ \hline Pixel resolution & 20 \(\mu\)m \\ Focus & Fixed \\ Focal length & 30 mm \\ \hline \multicolumn{2}{c}{**Measurement**} \\ \hline Calibration rage & Low range: \(-\)15 \({}^{\circ}\)C to 45 \({}^{\circ}\)C \\ High rage: \(-\)40 \({}^{\circ}\)C to 70 \({}^{\circ}\)C \\ Sensitivity & Low range: \(<\)50 mK \\ High range: \(<\)100 mK \\ Accuracy & Low range: +/-2 \({}^{\circ}\)C of reading \\ High range: +/-2 \({}^{\circ}\)C or 2\% of reading \\ \hline \hline \end{tabular}
\end{table}
Table 6: Specification of the infrared camera.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Type**} & \multicolumn{2}{c}{**Pinicple**} & \multicolumn{3}{c}{**Aspect**} \\ \cline{3-6}  & & & **Advantage** & & **Disadvantage** \\ \hline \multirow{4}{*}{Inverter welder} & \multirow{4}{*}{–} & Uses digital signal processing for digital control, applies high current to heat via resistance & \multirow{4}{*}{–} & Can apply fixed quantity of electric charge in stable manner (100–500 A) & \multirow{4}{*}{–} & Temperature at the contact between the rebar and the electric heater is high compared to the middle of the rebar \\ \cline{1-1} \cline{5-6}  & & & & **\#A** (10  
In the connection between the specimen's rebar and the inverter welder, it is essential that sufficient current is applied, the configuration is safe at high-voltage current, and uniform contact is achieved. Methods that can provide these benefits were examined, and preliminary experiments were performed using a method that employs a welding holder, a method that employs a crimpled terminal, and a method that employs an earth clamp. The results showed that the welding holder could not withstand a high-voltage current load. It was destroyed and could not be used. The drawback of the crimpled terminal method is that a deformed rebar is used, and therefore the deformed part of the contact point rebar must be removed. Further, there were difficulties in performing the experiments because the contact between the crimpled terminal and the rebar was not fixed, and a fixed contact point temperature could not be maintained. The earth clamp method maintained a relatively fixed rebar temperature compared to the other methods. In addition, it was possible to connect the rebar and the inverter welder safely, and it was the easiest method to use during the experiments. Therefore, the earth clamp method was used to perform the experiments.