## 3 Methodology
### Ultrasonic Guided Wave Leakage (UGWL) Method
However, in this novel approach, instead of depending on the longitudinal waves that propagate and attenuate through the wave guide, the energy that is leaked into the surrounding medium (concrete) is measured by placing an array of receivers on the surface of the concrete. As such, the method is also different than other UT methods traditionally used for concrete structures that utilizes bulk waves that propagate perpendicular to the system. When the transmitter is on the rebar and receivers are on the surface of concrete; flaws in the steel rebar, steel-concrete interface, and concrete along the path of the leaked waves are observed as changes in the amplitudes of the leaked waves. Another unique aspect of the UGWL method is the use of amplitude readings (frequency domain) instead of the velocity readings (in time domain) that are typical in the standardized applications of ultrasonic testing. While both sets of data can detect flaws in the RC systems, the changesin the amplitude readings were shown to be far more sensitive and easier to interpret. The change of amplitude is calculated by Equation (1) as follows:  
\[\textit{Change of Amplitude}\left(\%\right)=\left(\frac{A^{i}}{A^{0}}-1\right) \times 100 \tag{1}\]  
where \(A^{i}\) is the amplitude of 54 kHz in the frequency domain of \(i\)th increment of corrosion and \(A^{0}\) is the amplitude of 54 kHz in the frequency domain before corrosion. Further details of the development of this method and previous findings can be found in [1; 2; 3]. In a study conducted by [PERSON] et al. [2] at the University of Nebraska-Lincoln, two sets of concrete slabs were prepared to monitor cracks and corrosion activity. The first set of concrete slabs, 45.7 cm \(\times\) 30.5 cm \(\times\) 12.7 cm (18 in \(\times\) 12 in \(\times\) 5 in), were prepared with a 5% NaCl solution to accelerate corrosion, with a No. 5 rebar at the center level, placed at a depth of 6.3 cm (2.5 in) from the top surface of concrete. The specimens were also submerged in water with the water level just below the reinforcing bar. After monitoring the specimens for a period of 40 days, the measurements of leaked waves using receivers on the surface of the concrete detected corrosion activity. A direct correlation was noted between the increase in UGWL amplitudes and an increase in the length of time the concrete slabs were exposed to corrosive conditions. [PERSON] [2] concluded that the UGWL measurements are sensitive to early corrosion activity. One of the limitations of UGWL is attenuation [28]. This limits the coverage area for testing. Attenuation in UT occurs by intrinsic and geometric effects [22], and UGWL is affected by both [28]. [PERSON] [3] successfully demonstrated the ability of UGWL in the detection of multiple flaws that occur at different locations along the rebar with a single set up. A detection range of up to 427 cm (14 ft.) was achieved in this research using a single test setup. To accurately locate the flaws, a grid of receivers located every 15 cm (6 in) along the rebar is recommended by [PERSON] [3]. Finally, to interpret the UGWL measurements, a specific data analysis method is conducted:  
1. The raw data is stored in a \"4K9\" format using the Pulsonic Ultrasonic Pulse Analyzer. 2. The raw data is then converted to CSV files using the Ultrasonic Pulse Analyzer Data Converter. This raw data in the form of velocities in the time domain. 3. The CSV files are converted to Microsoft Excel documents and time domain data is transformed into frequency domain data using Fourier Analysis. Figure 1: Experimental set-up: (**a**) UGWL; (**b**) HCP. The peak amplitude values in the frequency domain are plotted against distance (i.e., the location of the receiver). This allows to determine the attenuation of the UGWL waves along the wave guide and observe any deviations from the expected (theoretical) attenuation curve that can be due to flaws on the rebar or the rebar-concrete interface.