# Application of Thermal Image Data to Detect Rebar Corrosion in Concrete Structures
## 3 Establishing IRT Database
### Establishment of Optimal Infrared Camera Imaging Conditions
#### 3.3.3 Determination of Amount of Current  
To determine the amount of current in the inverter welder, i.e., the heating temperature, for each concrete cover thickness, the heating time was fixed at 5 min, and the amount of current was changed as shown in Table 8 to observe the changes in temperature with respect to the amount of current.  
Table 9 compares the temperature increase at the surface of the concrete and in the rebar within the concrete. The currents listed in the table were used to heat specimens with a concrete cover thickness of 10 mm and corrosion ratios of 0% and 10%, for 300 s. When a current of less than 100 A was applied, the internal rebar could not be heated sufficiently. When the current was increased, the internal rebar temperature increased by a greater amount. However, when the internal rebar temperature was greater than a fixed temperature, the difference in the maximum surface temperature at each corrosion ratio was reduced. Therefore, the rebar temperature was set higher than the fixed temperature, and the amount of heating current applied to the specimen, with a cover thickness of 10 mm, was set at 150 A. At this condition there was a significant difference in the maximum surface temperature. The shaded part in Table 9 shows the amount of current used in the experiments. For the specimens with cover thicknesses of 20 mm and 30 mm, the amounts of current were determined similarly. As can be seen in Tables 10 and 11, the rebar temperature was increased to more than a set amount, and the amount of heating current was set at 200 A for the specimen with a cover thickness of 20 mm and 250 A for the specimen with a cover thickness of 30 mm, at which there was a large difference in the maximum surface temperature. The shaded parts in Tables 10 and 11 show the amounts of current used in the experiments.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Cover Thickness (mm)** & \multicolumn{4}{c}{**Amount of Current (A)**} \\ \hline
10 & 50 & 100 & 150 & 200 & 250 \\
20 & 100 & 150 & 200 & 250 & 300 \\
30 & 150 & 200 & 250 & 300 & 350 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Changes in the amount of current according to cover thickness.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & \multicolumn{5}{c}{**Current (A)**} \\ \cline{3-6}  & & **50** & **100** & **150** & **200** & **250** \\ \hline \multirow{2}{*}{Corrosion ratio 0\%} & Internal rebar temperature increase & 0.3 \({}^{\circ}\)C & 1.4 \({}^{\circ}\)C & 3.6 \({}^{\circ}\)C & 5.8 \({}^{\circ}\)C & 10.8 \({}^{\circ}\)C \\ \multicolumn{2}{c}{Surface temperature increase} & 0.1\({}^{\circ}\)C & 0.4 \({}^{\circ}\)C & 2.4 \({}^{\circ}\)C & 3.9 \({}^{\circ}\)C & 5.2 \({}^{\circ}\)C \\ \hline \multirow{2}{*}{Corrosion ratio 10\%} & Internal rebar temperature increase & 0.8 \({}^{\circ}\)C & 2.8 \({}^{\circ}\)C & 5.2 \({}^{\circ}\)C & 9.8 \({}^{\circ}\)C & 16.7 \({}^{\circ}\)C \\ \multicolumn{2}{c}{Surface temperature increase} & 0.2 \({}^{\circ}\)C & 1.2 \({}^{\circ}\)C & 3.8 \({}^{\circ}\)C & 4.7 \({}^{\circ}\)C & 5.7 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}
Table 9: Comparison of temperature changes at cover thickness of 10 mm.