### Non-Destructive Methods of Testing Concrete Structures
Moreover, a thick concrete cover makes evaluating rebar parameters impossible in thermography and modal analysis. Among these universal methods, ultrasonic testing has been shown to provide the most reliable results for evaluating reinforcement. Nonetheless, this technique has limitations, and the effects of concrete properties on the test outcomes are significant [3; 4]. Electromagnetic techniques are the only group of methods dedicated explicitly to directly examining rebars. Their primary advantages over other methods include direct interaction with the reinforcement, the low damping of electromagnetic waves by concrete, and a variety of operational principles and potential applications. This diversity arises from the broad spectrum of excitation frequencies available; different physical phenomena are utilized at various wavelengths [5]. This changes parameters such as effective range and spatial resolution and allows for studying different parameters, leading to various advantages, disadvantages, and limitations. Figure 2 shows the fundamental electromagnetic non-destructive testing (NDT) methods [5]. The rebars in the RC structure are situated at a considerable distance from the sensor. Typically, the thickness of the concrete cover ranges from 20 to 50 mm. In such cases, the eddy current (EC) method is the first choice. This method's practical operational range extends from 0 to 60 mm. The EC method can be successfully applied to detect the presence, distance from the sensor (concrete cover thickness), position, diameter, and steel grade of reinforcing bars [44; 45; 46; 47; 48; 49; 50; 51; 52]. In some cases, the EC method can also detect changes caused by corrosion [53; 54; 55]. The parameters of the method can be regulated by excitation frequency. Higher frequency causes higher spatial resolution and a lower effective range. However, the size of the transducer also impacts on the results. Small-size devices have higher spatial resolution and lower ranges (Figure 3) [44]. As illustrated in Figure 3, a single transducer can be highly effective. However, its efficacy is limited to the relatively narrow spectrum of concrete cover thicknesses. In EC transducers, the excitation subsystem may be realized in many different ways, from the coil excited by a single or multi-frequency signal [44] through pulsed excitation [56; 57] to rotating magnets [58; 59]. Nonetheless, in all cases, the excitation subsystem is integrated with the pickup sensor, which may cause problems in densely reinforced structures. The interaction between adjacent reinforcing bars can distort the obtained results and lead to erroneous classifications [44]. Issues with the EC method are significantly reduced in the case of the magnetic method presented in this paper. The technique is applicable across the entire range of typical concrete cover thicknesses and can be used even in the case of dense reinforced mesh within the RC structure. Additionally, magnetic systems are generally easier to implement and provide greater cost-effectiveness compared to EC techniques [5; 6]. In 2023, the market size of magnetic sensors was estimated at USD 3.25 billion and is expected to reach USD 4.76 billion by 2028, growing at 7.91% per year. The development of magnetic technologies causes these changes. Recent advancements have substantially improved the performance parameters of these sensors. As shown in this paper, the correctness of the classification is comparable to or even better than that of the EC test case. The method can also be effectively utilized even in a passive configuration that leverages residual magnetization. Furthermore, measuring the spatial components of magnetic induction proves to be highly beneficial for non-destructive evaluation, enabling new possibilities in identification [8; 60; 61]. Ground-Penetrating Radar (GPR) is another technique that can be used to assess reinforced concrete (RC) structures. GPR emits high-frequency electromagnetic waves into the tested object and analyzes the reflected waves.