## 1 Introduction  
High-frequency magnetic fields are mainly used to induce heat, and heating methods can be classified based on the principle of heat generation and application. In general, radio-frequency heating methods can be classified into two types, namely induction and dielectric heating. Induction heating is a method wherein an object is heated by eddy currents that are generated [1, 2, 3] when a high-frequency magnetic field is applied to a conductor, such as a metal. In contrast, dielectric heating is a method wherein an object is heated by dielectric loss caused by a high-frequency current applied to the dielectric [4, 5, 6]. Conducting a nondestructive investigation to assess steel corrosion in a reinforced concrete bridge throughout its life cycle is necessary because internal corrosion is not visible from the outside. Extensive research has been conducted on this matter, which has led to many nondestructive testing methods being proposed [7], namely the analytical method [8], half-cell potential method [9, 10], resistance probe method [11], ray method [12], ultrasonic detection [13], and optical fiber sensing technology [14]. Although these methods have significantly improved the detection of steel corrosion, certain restrictions exist for their widespread use. For example, the analytical method relies on a reasonable and reliable predictive model for assessing steel corrosion [15], while the electrochemical method can calibrate the corrosion rate but cannot detect the corrosion quantity of the steel bar [16, 17]. Thus, a more effective and convenient technology for corrosion detection is urgently required. The eddy current thermography method is a nondestructive testing technology that has been developed in recent years [18] and is based on the principles of electromagnetic induction and infrared radiation. When a conductor is placed in an alternating electromagnetic field, an eddy current is induced, which causes heat dissipation. The surface temperature of the conductor can be recorded using an infrared camera. The heating efficiency is controlled by the material characteristics of the conductor; therefore, the characteristics of the defects in the object can be assessed by analyzing the temperature variations. Many researchers have conducted research on detecting structural defects [19, 20, 21], pipe leakages [22], and cracks [23]. Further, some research groups have investigated corrosion in steel bars based on eddy current thermography, and found that the temperature of a steel bar is dependent on the degree of corrosion of the specimens [24, 25, 26, 27]. However, many other factors can influence the thermal properties of steel bars, such as the depth of the concrete layer, the diameter of the steel bar, and the humidity of concrete. These factors can affect the ability to detect corrosion in reinforcement structures, and the lack of research regarding their influence limits the application of these methods to quantitatively determine the degree of corrosion in steel bars. Heat is primarily produced by applying high-frequency magnetic fields. Detection methods are thus classified according to the principle of heat generation and application. Typically, radio-frequency heating can be classified as induction or dielectric heating. Induction heating uses an eddy current generated [28, 29, 30] when a high-frequency magnetic field is applied to a conductor such as a metal. Conversely, the dielectric heating method uses the dielectric loss caused by a high-frequency current applied to the dielectric medium to generate heat [31, 32]. This weakening mechanism has already been applied in the manufacturing of recycled aggregates as well as for removing deposited mortar on the surface of aggregates. However, heating methods using external heat sources have low energy efficiency, which results in high energy consumption and greenhouse gas generation through regeneration processes [33, 34, 35].