## 3 Results and Investigation
### Temperature Characteristics of Single Reinforcement by Induction Heating
#### 3.1.3 Experimental Results and Investigation  
Temperature-rise characteristics of a single reinforcement by high-frequency induction heating  
(1) Rising temperature characteristics  
Figure 7 depicts the results of the temperature-rise characteristics (150 mm length) of the reinforcing steel by induction heating in the order of 5 kW and 6 kW. As depicted in the figure, the results for 5 and 6 kW showed no significant difference. However, temperatures above 800 \({}^{\circ}\)C could not be measured and were considered 800 \({}^{\circ}\)C.  
With respect to time and temperature, if the distance from the heating coil to the surface of the reinforcing steel was 10, 20, or 30 mm, the target temperature reached 300 \({}^{\circ}\)C within 60 s. The smaller this distance, the faster the increase in temperature, and the faster the temperature increased to over 600 \({}^{\circ}\)C. In the case of distances of 10 and 20 mm, a state of thermal equilibrium was attained between 600 and 800 \({}^{\circ}\)C, and in the case of a distance of 30 mm, a state of thermal equilibrium was attained between 500 and 700 \({}^{\circ}\)C. However, in the case of a 50 mm distance, it took more than 300 s to reach 300 \({}^{\circ}\)C. Consequently, the closer the coil was to the reinforcement, the more rapidly the temperature rose in a short time immediately after heating, after which the temperature remained almost constant. In contrast, when the coil was far away from the reinforcement, the temperature increased gradually. Therefore, when heating bars with a maximum output of 6 kW, considering the dismantling performance and energy effect, a distance of 30 mm or less with a steep gradient was considered appropriate. Figure 5: Output of high-frequency induction heating unit. Figure 6: Heating experiment for single reinforcing bar. Figure 7: \(Cont.\)  
Figure 8: Temperature rise characteristics of rebar during induction heating (430 mm—center). (D00-0-0, D reinforcement type; output, kW; distance, mm). (2) Temperature-distribution characteristics  
Excessive heating, insufficient heating, and unevenness in the temperature rise during the heating process of reinforcing steel using an induction heater can increase concrete fragility. Using the infrared radiation temperature measurement method, the temperature distribution at the time of temperature rise in the reinforcement bar was analyzed under induction heating conditions of 5 kW and 6 kW. The results, including the highest temperature increase rate, are shown in Figure 9. The horizontal axis of the figure is 150 mm, which is the length of the specimen; however, there may be some errors in the region due to radiation heat. The \(x\)-axis of Figure 9 is 160 \(\pm\) 5 mm on average, and an error of 10 \(\pm\) 5 mm due to radiant heat is considered to have no significant effect on the measurement of the central and end heating phenomena. In most of the test specimens, the central part was heated more intensively than the end. Reinforcing bars at parts outside the heating coil with a diameter of 120 mm exhibited a temperature of 150-450 \({}^{\circ}\)C, and it was necessary to consider the temperature deviation in subsequent heating conditions. Figure 9: \(Cont\). Figure 9: Cont. In the case of a 5 kW output for the 430 mm specimen reinforcement heating experiment, the diameter of the heating coil was 120 mm. Therefore, the points were measured at a distance of 30 mm in the reinforcement length direction based on the direct light center of the coil, and the results are shown in Figure 10. The 430 mm specimen at 10 kW output was tested in the same way, and the results are shown in Figure 11. The temperature difference between the inside and outside of the coil diameter was not significant; however, the temperature difference between the outside 30 mm and 100 \({}^{\circ}\)C was removed from the magnetic field area from the outside 60 mm. Figure 9: Temperature distribution of reinforcing bar by induction heating (5 kW–150 mm).