## 3 Methodology  
### Ultrasonic Guided Wave Leakage (UGWL) Method  
UGWL has the ability to identify the onset and progression of flaws in RC structures such as concrete-rebar delamination, corrosion, and cracking. The team's previous work \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Standard**} & \multirow{2}{*}{**Reference**} & **Chloride Threshold Level (CTL) (\%, Cement)** \\ \cline{3-3}  & & **Reinforced Concrete** & **Pre-Stressed Concrete** \\ \hline British Standard & [19] & 0.4 & 0.1 \\ ACI 357 (Water-soluble Cl-) & [20] & 0.1 & 0.06 \\ ASTM 1152 & [21] & 0.2 & 0.08 \\ (Acid-soluble Cl-) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chloride threshold values specified by different standards. successfully demonstrated that UGWL is capable of identifying delaminations as small as 0.2 mm (0.008 in) and the onset of rebar corrosion [1; 2; 3]. A study conducted by [PERSON] et al. [2] successfully demonstrated that the amplitude of the leaked energy that radiates from the reinforcing bar waveguide is sensitive to corrosion initiation and cracks. In ultrasonic testing, the ultrasonic waves can propagate as either bulk waves or guided waves [27]. In case of the bulk wave propagation, which is also known as the normal-beam excitation, the ultrasonic waves are leaked into the surrounding material and the insonified area is limited to the contact point between the sensor and the test material [28]. The insonified area in traditional ultrasonic testing is the region that is flooded with carefully controlled sound waves (i.e., ultrasonic waves), whereas in ultrasonic guided wave propagation, the ultrasonic waves propagate through a waveguide, and the ultrasonic waves can travel longer distances compared to bulk wave propagation. In the UGWL method proposed by the authors, the leaked energy in the guided wave system propagates through the surrounding materials (i.e., concrete) [28], and is detected by a receiver on the surface of the concrete. While a range of ultrasonic frequencies (20-100 kHz) have been used in homogeneous concrete systems, 54 kHz was found to be optimal for this application [1]. Most ultrasonic guided wave studies on RC structures have utilized lower ultrasonic frequencies [1; 2; 3; 27; 29; 30], mostly because the lower frequencies have a higher sensitivity to bond conditions and are easier to solve [27]. Since the concrete is a heterogeneous material, many agree that it is better to use lower frequencies in ultrasonic testing, typically 20 to 100 kHz [27]. Though higher frequencies are more sensitive to smaller defects and can be used with specimens with smaller thicknesses, they are also prone to higher attenuation [31]. In the UGWL testing method, the following two types of transducers are involved: (1) transmitter; and (2) receiver [28]. A transmitter is used to transmit the ultrasonic waves through the waveguide (such as the steel bar in this case). These ultrasonic waves naturally leak into the surrounding material (i.e., the concrete) and are monitored using the receiver. In the UGWL method, the transmitter is connected to the rebar, either directly to its end or at an angle by a couplant. The couplant facilitates the sensor-to-material connection and makes it easy to transmit the ultrasonic pulses from the transmitter to the waveguide. While collecting the test data, the transmitter can either be held connected to the rebar by hand while taking measurements, or attached using a transducer wedge, for hands-free operation and reduced human error. The experimental set-up includes two 54 kHz transducers used as transmitter (T), and receiver (R), both having a diameter of 3 cm (1.18 in). The PULSONIC Ultrasonic Pulse Analyzer 58-E4900 from CONTROLS-Group is used as the data acquisition system and to generate the ultrasonic pulses, Figure 1.  
The ultrasonic guided wave approach is typically used for inspecting long homogenous materials, such as steel pipes, with the receiver typically placed at the other end of the linear element.