## 3 Results and Investigation
### Temperature Characteristics of a Cross-Bar during Induction Heating
#### 3.2.2 Experimental Results and Investigation  
(1) Frequency--5 kW output (center heating)  
The high-frequency output of 5 kW (shown in Figure 13) showed the same variation tendency as that of a single reinforcement, and the initial temperature-rise curve showed a steep gradient; however, compared to a single reinforcement, the thermal equilibrium was reached quickly with a temperature reduction of up to 100 \({}^{\circ}\)C. The temperature difference between the upper and lower reinforcing bars was not significant when the heating distance was large; however, the temperature difference between the upper and lower reinforcing bars was 80-180 \({}^{\circ}\)C at a heating distance of 40 mm or more. If the distance increases, a magnetic field is formed around the upper reinforcing bar before reaching the lower reinforcing bar, and the lower reinforcing bar is heated only via conduction. Figure 12: Thermal experiment method for crossed reinforcing bar. Figure 13: _Cont._  
As mentioned above, if the heating distance is short, a magnetic field is formed on the lower reinforcing bar, and the surface area of the reinforcing bar is removed from the magnetic field area, and most of the reinforcing bar is heated via conduction. The temperature distribution was similar for single reinforcing bars; however, for the upper reinforcing bars, the temperature difference between the reinforcing bars inside the diameter and those outside the rectilinear longitude varied from 50 to 200 \({}^{\circ}\)C for a heating distance of 10-30 mm. This is a result of a clear difference in temperature between the heat-generating part and the conducting part in the magnetic field area in the upper part. Figure 13: Temperature characteristics of crossed rebar (5 kW). In the case of the lower part, it seems that the range of temperature rise due to conduction was larger than the phenomenon of heat generation itself. (2) Frequency--5 kW output (side heating)  
In the case of conductive resistors, it is important to select a heating position because the heat transfer rate shows a sharp difference depending on the position and distance of the reinforcing bar applied to the heating coil as shown in Figure 14. In this experiment, it was not an intersection of the reinforcing bars. Reinforcing bars were placed in the center of the well-shaped reinforcing bars, and the temperature rise and temperature distribution characteristics based on the diameter and distance of the reinforcing bars used were examined. Figure 15 shows the temperature characteristics. In the case of temperature-rise characteristics, the center of the well-shaped reinforcement showed a difference of up to 100 \({}^{\circ}\)C when a heating coil was placed at the intersection. When the heating coil was positioned and heated, only one of the three upper and lower reinforcing bars was heated; as the reinforcement bar moved farther away from the heating coil, only the upper reinforcing bars were heated, and only the lower reinforcing bars were heated by heat conduction. When the heating coil was located in the center of the well-shaped reinforcing bar, the temperature increase rate showed the same tendency; however, the heating efficiency showed a slight decrease. In contrast, in the area to be heated, the upper two reinforcing bars and the strain two reinforcing bars applied to the coil were simultaneously heated to increase the width of the heating range. This may have been due to the coupling efficiencies of the reinforcing bars and heating coils. As expressed in Equations (8) and (9) the heating coil is basically a resistor (inductance); it generates heat based on the surface area (current flowing through the coil \(\times\) diameter). Figure 14: Relationship between work (reinforcing bar) and heating coil combination efficiency. Figure 15: _Cont_. The magnetic force is proportional to half of the square of the distance, while the strength of the magnetic force is proportional to the current generated in the coil and the coil differential (relative change between the two coils).