## 1 Introduction
### Non-Destructive Methods for Testing Concrete Structures  
Nondestructive testing (NDT) methods fulfill two roles: they can function as preliminary techniques before semi-destructive or destructive testing or as primary diagnostic tools. Various NDT methods can assess reinforced concrete (RC) structures. Some methods are designed explicitly for evaluating concrete [7; 8; 9], while others focus solely on testing the reinforcement mesh. Additionally, some techniques assess the entire structure comprehensively. The RC structure parameters that can be evaluated using NDT methods include:  
1. Visual condition of concrete--the category is represented by the oldest NDT method: visual inspection (VI). 2. Evaluation of electromagnetic properties--this approach offers several advantages, including direct interaction with the reinforcement, minimal attenuation of electromagnetic waves by concrete, cost-effectiveness, and a broad range of operational principles and applications. This category can be further divided into three subcategories: 1. Magnetic properties--this category encompasses passive methods, such as the residual magnetization technique (RMT), and active methods where the object being tested is continuously magnetized, as seen in magnetic flux leakage (MFL), or magnetized before testing, as in magnetic particle inspection (MPI) [4; 5; 6]; 2. AC electromagnetic induction--this category is represented by the eddy current (EC) method [4; 5; 6]: 3. Reflection of electromagnetic waves--this subcategory is represented by ground-penetrating radar (GPR) [10; 11; 12; 13]; 4. Density of components in RC structure--this category is represented by radiography (X-ray) [4; 5; 14]. 4. Carbonation level of concrete--providing insights into the risk of corrosion of the carbonation mechanism (the connection between corrosion and carbonation is described in [12]); this parameter can be evaluated using three different properties: 1. Electrical properties--this category includes the half-cell potential method (HCP) and concrete resistivity method (CR). The HCP technique assesses the capacity (electrochemical potential) between a reference electrode placed on the structure surface and reinforcement placed inside the structure [15; 16]. The CR is used to measure the electrical resistivity of concrete [17; 18; 19]. Both electrical properties change while the carbonation process progresses; 2. pH (chemical tests)--the rainbow test (RT) and phenolphthalein test (PT) fall within this category, allowing for the assessment of carbonation depth through the measurement of concrete pH levels [20; 21]; 3. Strength of the concrete--this category of methods includes the Schmidt hammer (SH) and Windsor probe (WP). Both methods test concrete by hitting the surface and measuring the outcome of the impact [20; 21]. 5. Thermal properties of the whole structure--infrared thermography (IR) enables exploration of the difference between the thermal properties of concrete and steel by monitoring the heating and cooling processes of the RC structure [22; 23; 24]. 6. Mechanical properties of the whole structure--the methods test propagation of mechanical waves (sonic and ultrasonic frequencies)--waves may be generated by the tested object (passive method), like in the case of acoustic emission (AE) [25; 26], by induced electromagnetic waves, like in the case of electromagnetic acoustic transduction (EMAT), or by mechanical wave--impact echo (IE) and most forms of ultrasonic testing (UT) [27; 28; 29; 30; 31]. 7. Natural frequencies of the whole structure (low frequency)--the methods in this category investigate the resonance frequencies of the object. Vibrations can be induced by electromagnetic waves (as seen in the M5 system), physical impact (such as in the hammer impact test (HIT)), or passively through vibration monitoring (VM). Modal analysis is frequently employed in this type of testing [6; 32; 33]. Another approach to categorizing NDT methods employed in the diagnostics of reinforced concrete (RC) structures is based on their operational principles.