## 3 Experimental Considerations
### Experimental Results and Considerations  
#### 3.2.1 Temperature Characteristics of Single Reinforced Concrete by High-Frequency Induction Heating  
(1) Compression Strength by Preparation  
Test specimens were produced for different concrete compositions because it was hypothesized that the temperature increase and vulnerability characteristics would vary according to the water-cement ratio. The compression strength test for the material aged of 28 days was conducted as shown in Figure 4 per each specimen and since then, an evaluation with respect to the vulnerability characteristics was performed with high frequency induction heating. The compression strength specimens were cured under the air dry condition in order to account for the climatic conditions of the site. Figure 3: Test specimen production and control method. 2. Increasing Temperature Characteristics Figure 5 shows the increase of temperature with time for the concrete around the reinforcing bars using heat transfer pairs. Figure 4: Stress-strain curve for normal concrete. Figure 5: Cont. In the legend, the center is at the middle of 150 mm concrete specimen. A 75 mm portion has been indicated, and a 15 mm portion from the lower end of the specimen has also been indicated. In addition, 10 and 20 mm in the legend represent the distance from the surface of the reinforcing bar in the test body to the heat transfer pair. In the case of heating at 6 kW for 360 s, the temperature change of the concrete due to the thermocouples was measured at 10 and 20 mm from the center of the rein and end of the reinforcement, respectively. As a result of induction heating, the temperature increases in the order of the reinforcement center at 10 mm, end 10 mm, center at 20 mm, and end 20 mm, with the temperature difference between 10 and 20 mm from the reinforcement center being 46 to 90 \({}^{\circ}\)C.  
As shown in Figure 5d, when the distance from the heating coil in the test specimen using D19 was 40 mm, the temperature difference between 10 and 20 mm from the center of the reinforcement was weak; a temperature difference of 74-83 \({}^{\circ}\)C occurred at the end, and a slight difference occurred compared to the center; however, the difference between the maximum and minimum temperatures was within 30 \({}^{\circ}\)C, and it was judged that there was no significant effect on concrete heat conduction. The maximum temperature of reinforcement for D10 was 510 \({}^{\circ}\)C at the center of the reinforcement at a distance of 30 mm for 360 s, but the maximum temperature of concrete 10 mm away from the center of the reinforcement surface was 106 \({}^{\circ}\)C, when the reinforcement inserted into the concrete was heated, as shown in Figure 5a. Furthermore, for D19 and D25, the maximum temperatures of heating only the reinforcing steel were 651 and 613 \({}^{\circ}\)C, respectively, but when the reinforcing steel inserted into the concrete was heated, as shown in Figure 5c,e, the maximum temperatures were 171 and 139 \({}^{\circ}\)C, respectively. The heat conduction characteristics of concrete change according to the internal moisture content. When high-frequency induction heating is used, the free water inside the concrete evaporates because of the rapid heating of the reinforcing steel; therefore, in this experiment, 450 mm of reinforced concrete was tested in air-dried and absolute-dried  
Figure 5: Increase in temperature with time in reinforced concrete during induction heating. conditions. Four types of reinforcing steel, D10, D19, D25, D32, having coating thicknesses of 30, 40, and 50 mm were considered, and the temperature of a 10 mm-thick layer from the surface of the internal reinforcing steel was measured, as shown in Figure 6.  
Figure 6: _Cont_. The difference in temperature between air-dry and absolute-dry conditions was 45 \({}^{\circ}\)C, and the difference in temperature between the dry conditions was measured high. Further, the difference is evident in the D19 and D25 experiments with high heating efficiency.