### Non-Destructive Methods of Testing Concrete Structures  
Research on reinforced concrete structures encompasses a range of methods, including destructive testing (which results in the complete failure of the specimen), semi-destructive testing (which inflicts damage necessitating subsequent repairs), and non-destructive testing (NDT) methods. NDT techniques are typically more cost-effective and less time-consuming than their destructive counterparts. NDT does not compromise the tested structure's operational integrity or require repairs following the testing completion. Different non-destructive testing (NDT) methods can identify various parameters. Figure 1 presents the parameters that can be tested with this kind of method. A detailed review of these NDT techniques used in structural diagnostics, along with their applications, benefits, and limitations, is presented in [5]. The NDT methods utilized in construction engineering are usually specifically designed to evaluate the condition of the concrete [20; 21; 22]. Some of these methods are utilized indirectly to assess the risk of corrosion in reinforcing bars by examining the carbonation levels in concrete. These techniques can be categorized into three groups:  
Figure 1: Properties of reinforced concrete structures examined with NDT methods [5]. * Electrical Methods: This category encompasses techniques such as the Half-Cell Potential method, which evaluates the electrochemical potential difference between a reference electrode and the embedded reinforcement, indicating the likelihood of corrosion [5; 23; 24]. Additionally, resistivity measurement methods assess the electrical resistivity of concrete, which correlates with its susceptibility to corrosion [5; 25; 26; 27]. * Methods for Testing Concrete Hardness: this group includes tools such as the Schmidt hammer and Windsor probe, which evaluate concrete's hardness as a proxy for its overall condition and durability [3; 4; 5; 28; 29]. * Chemical Methods: these methods involve phenolphthalein and rainbow tests, which chemically assess carbonation depth and provide insights into the potential for corrosion in reinforcing bars [3; 4; 5; 28; 29]. A comprehensive evaluation, using a simultaneous examination of concrete and reinforcement, can be conducted by the following techniques:  
* Ultrasonic Testing (UT)--methods of this kind allow for structural properties to be investigated by analyzing the propagation of mechanical waves (reflections and scattering phenomena). An example of an ultrasonic technique is the Multi-Channel Ultrasonic Pulse Echo method. In this technique, multiple ultrasonic transducers transmit and receive pulses simultaneously, enabling faster data acquisition across a larger area than single-channel systems. The transducers emit short bursts (pulses) of ultrasonic sound waves into the structure. These waves penetrate the material and reflect back (echo) upon encountering heterogeneities such as rebar. By measuring the time it takes for the echo to return, the distance to the reflecting surfaces can be determined [3; 4; 5; 30; 31; 32; 33; 34; 35]. * Acoustic Emission Testing (AE)--wherein mechanical waves generated by the tested structure are recorded and used for analysis [3; 4; 5; 36; 37; 38; 39; 40; 41]. * Low-Frequency Mechanical Wave Testing (vibrational methods) involves assessing the structure's natural vibrations through modal analysis techniques [42; 43]. * Infrared Thermography (IR) allows for thermal properties to be evaluated by monitoring the structure's heating and cooling processes [3; 4; 44; 45; 46; 47]. These four groups of methods allow the investigation of rebars in reinforced concrete structures. However, their effectiveness is inherently limited. The influence of multiple factors on the results causes this limitation. The most important factors are the concrete cover's thickness, the concrete mix's composition, the type and quantity of used aggregate, air voids within the concrete, contamination, cracks, and other irregularities [3; 4].