## 1 Introduction
### Non-Destructive Methods for Testing Concrete Structures
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline \multirow{2}{*}{**Category**} & \multirow{2}{*}{**Example Methods**} & \multicolumn{3}{c}{**Evaluation of:**} & \multirow{2}{*}{**Area Testing**} & \multirow{2}{*}{**Commercial Viability**} \\ \cline{3-3} \cline{5-8}  & & & **Rebar** & & & & \\ \hline
**Mechanical** & & & & & & \\ \hline low frequency & MS [1], HIT [2], VM [3] & & & & & & \\ \hline sonic & IE [4], AE [5] & & & & & & \\ \hline ultrasonic & UT [6] & & & & & & \\ \hline
**Magnetic** & & & & & & \\ \hline active & MFL [7], MPI [8] & & & & & & \\ \hline passive & RM [9] & & & & & & \\ \hline
**Electric** & & & & & & \\ \hline capacity & HCP [10] & & & & & & \\ \hline resistance & CR [11] & & & & & & \\ \hline
**Electromagnetic** & & & & & & \\ \hline low frequency & MFL [7] & & & & & \\ \hline medium frequency & EC [12] & & & & & \\ \hline microwave & GPR [13] & & & & & \\ \hline terahertz & - & & & & & \\ \hline infrared & IR [14] & & & & & \\ \hline visible & VI [15] & & & & & \\ \hline visible & FO [16] & & & & & \\ \hline radiography & X-ray & & & & & \\ \hline
**Chemical** & RT [17], PT [18] & & & & & \\ \hline
**Concrete strength** & WP [19], SH [20] & & & & & \\ \hline  & not suitable & & suitable in some cases & moderate suitable & well suitable \\ \hline  & & & & & & \\ \hline \multicolumn{6}{p{113.8 pt}}{\({}^{1}\) M5–Magnetic Force Induced Vibration Evaluation (MFIVE), [2] HIT–Hammer Impact Test, [3] VM—Vibration Monitoring, [4] LE—Impact Echo, [5] AE—Acoustic Emission, [6] UT—Ultrasonic Testing: [7] MFL—Magnetic Flux Leakage, [8] MPI—Magnetic Particle Inspection; [9] RM—Residual Magnetization; [10] HCP—Half-Cell Potential; [11] CR—Concrete Resistivity; [12] EC—Eddy Current; [13] GPR—Ground Penetrating Radar; [14] IR—Infrared Radiation (Thermography); [15] V—Visual Inspection; [16] PO—Visual Inspection; [16] PO—Fiber Optic; [17] RT—Rainbow Test; [18] PT—Phenolphthalein Test; [19] WP—Windsor Probe; [20] SH—Schmidt Hammer. \\ \hline \end{tabular}
\end{table} 
Table 1: The capability of the NDT methods to RC structure evaluation. primarily designed for evaluating concrete, which means that various factors can affect the results and complicate the identification process. Moreover, the equipment needed for UT tends to be relatively expensive, and the method generally provides lower accuracy in rebar identification compared to EC and MFL [27; 28; 29; 30; 31]. Both eddy current (EC) and magnetic flux leakage (MFL) methods are well-suited for evaluating reinforcing bars in reinforced concrete (RC) structures. Concrete is practically transparent to electromagnetic waves (magnetic permeability is similar to air permeability). This makes direct interaction with the rebar possible with minimal damping. Both techniques prove effective within 0 to 60 mm (a range higher than typical concrete cover thickness). The EC method can reliably detect the presence of reinforcing bars, determine their distance from the sensor (indicative of concrete cover thickness), and assess their position, diameter, and steel grade [34; 35; 36; 37; 38; 39]. Similarly, the MFL method has been utilized to measure concrete cover thickness [40; 41; 42; 43], determine the diameter of reinforcing bars [40; 43], and classify the class of rebar steel [40; 41]. However, achieving simultaneous identification of all three elemental parameters (concrete cover thickness (\(h\)), diameter (\(D\)), and steel class) using this method has posed significant challenges so far. Nonetheless, recent investigations show that it is possible [44]. Moreover, both EC [45; 46; 47] and MFL [48; 49; 50; 51; 52] methods can, in some cases, detect corrosion. In contrast to EC, the MFL method can also be utilized for area testing [6] and mapping reinforcement grids [42; 43; 44; 45; 54; 55].