## 3 Experimental Considerations
### Experimental Results and Considerations
#### 3.2.1 Temperature Characteristics of Single Reinforced Concrete by High-Frequency Induction Heating
Considering 40 mm coating thickness, the temperature difference between the two laboratories with high heating efficiency was evident, but D10 and D32 had relatively low heating efficiency, which was not significant. Further, for coating thickness of 50 mm, the temperature difference was not large. A relatively uniform rise curve in dry air compared to the gradient of temperature rise curve in dry air was observed. Furthermore, for the air-dried conditions, gel water evaporated for 80 \({}^{\circ}\)C or higher, thus the effects of this may cause an irregular temperature rise curve. Figure 6 shows the temperature rise characteristics of the reinforced concrete by induction heating. In the case of a 5 kW heating power with a coating thickness of 50 mm or greater, the efficiency decreased sharply owing to the penetration characteristics of high-frequency induction heating. However, the temperature rise was small in the case of 10 kW heating power. For D10, however, the temperature increased to 60 \({}^{\circ}\)C, showing no significant difference from the 5 kW case, which seems to be due to a decrease in efficiency owing to a decrease in cross-sectional area in the magnetic field. Figure 6 shows the results of the 10 kW experiment. 2.2 Physical and Mechanical Properties of Single Reinforced Concrete by High-Frequency Induction Heating  
2. Cracks in Reinforced Concrete 1. [label=(0)]
3. Experiment Summary  
Reinforced concrete structures deteriorate in performance due to high heat and constant heating, which results in cracks. Concrete deteriorated by high temperature is mostly repaired when hydrated after cooling, but when heated at 500 \({}^{\circ}\)C or higher it is damaged. For reinforcing steel, the coefficient of thermal expansion from room temperature to 100 \({}^{\circ}\)C can be considered as 1 \(\times\) 10\({}^{-5}\)\({}^{\circ}\)C\({}^{-1}\), as shown in Table 4. The coefficient of  
Figure 6: Increase in temperature with time in reinforced concrete caused by induction heating. thermal expansion increases sequentially when the temperature increases beyond that, and is approximately 0.12 \(\times\) 10\({}^{-5}\)\({}^{\circ}\)C\({}^{-2}\) from 100-200 \({}^{\circ}\)C.  
Therefore, cracks are predicted to occur in the reinforced concrete owing to thermal expansion by induction heating, and the purpose of this section is to determine the crack shapes and to use them as input data to predict the effect of cracks on the weakening of reinforced concrete and steel. Experimental Results and Considerations  
In the case of single reinforced concrete, D10, D19, D25, and D32 reinforcing bars were used, and the cover thicknesses were manufactured at 30, 40, and 50 mm. Furthermore, for high-frequency induction heating, a heating coil was positioned in the center of the test specimen with a frequency of 5 kW and heated for 360 s. Figures 7 and 8 shows the crack formed due to the high-frequency induction heating. In the case of the 30 mm and 40 mm laboratories close to each other, the cracks occurred on the surface of the specimen within 30 s after heating, while the 50 mm specimen cracked within a min of heating, however, the D10 specimen did not. If the cover thickness was 30 mm, cracks occurred along the reinforcing bar and vertical directions on all specimens, and cracks occurred in the concrete within the coil diameter; further, for a cover thickness of 40 mm, cracks occurred in all test specimens except the one using D10 reinforcing steel. In contrast, for 50 mm cover thickness, cracks occurred in specimens other than D10 reinforcing bars within one min, but occurred in the form of a single crack from the reinforcing bar to the concrete surface. For D10 reinforcing bars, the size of the thermal expansion pressure is smaller owing to the reduction in cross-sectional area compared to other reinforcing bars, thus the changes in the occurrence of cracks appeared differently. Figure 8: Crack characteristics of single reinforced concrete by induction heating (full heating).