## 2 Materials and Methods
### PAU Tests  
Ultrasonic testing (UT) operates on the principle that the incident ultrasonic waves generated by ultrasonic transducers (which transform electrical or optical signals into ultrasonic waves and vice versa) will be reflected back when they encounter an interface between two materials with different acoustic impedances. A phased array ultrasonic (PAU) is an advancement over the UT technology that can be achieved when several of these transducers are arranged together into an array and operated at slightly different times (either electrically or physically) so that the individual waves interact both positively and negatively, allowing beam focusing and beam steering, as shown in Figure 6[108]. The advantages of PAU in comparison to conventional UT include a 5 to 10 times faster scanning rate, better images due to multiple angles and frequencies, which require less interpretation, higher resolution, capability of beam focusing, reliability, portability and mobility[109; 110]. Unlike conventional UT, PAU testing allows signal focusing at desired locations and angles, which is advantageous for the testing of composite materials that have an anisotropic structure, creating challenges in signal evaluation[53; 111]. However, PAU is a relatively new technique compared to the traditional NDTs, and hence the limitations of portable units of PAU include the uncertainty in its application as it has yet to be completely proven.  
Line scan using PAU was performed by moving the array of ultrasonic transducers in a sideways direction along the desired line of inspection, as shown in Figure 7. Each scan at individual positions is stitched together to give one continuous line scan, which is the cross-sectional image through a plane normal to the surface along the direction of the scan. Similarly, the area scan using PAU was conducted as a stripe scan. The stripe scan was performed by positioning the array of ultrasonic transducers perpendicular to the desired line of inspection and then taking scans while moving forward with desired spacing  
Figure 5: GPR systems used: (**a**) Conquest 100 Enhanced, (**b**) C-Thrue radar[105], (**c**) Proceq GP8800[106], (**d**) Proceq GP8000[107].  
\begin{table}
\begin{tabular}{c c c c} \hline
**GPR Systems** & **Center Frequency** & **Radar Technology** & **Depth Range** & **Manufacturer** \\ \hline Proceq GP8800 & 400–6000 MHz & Stepped-frequency & 25.6 in. & Screening Eagle \\  & continuous-wave (SFCW) GPR & (65 cm) & Technologies, Switzerland \\ Proceq GP8000 & 200–4000 MHz & Stepped-frequency & 31.5 in. & Screening Eagle \\  & continuous-wave (SFCW) GPR & (80 cm) & Technologies, Switzerland \\ \hline \end{tabular}
\end{table}
Table 3: _Cont._between the consecutive scans, as shown in Figure 7. Each individual scan collected during stripe scan is simply a line scan having a width of measurement equal to that of the PAU device or the array of transducers. These individual scans along the line of inspection are stitched together to give a time slice or depth slice view, which can be further processed into a full 3D iso-surface representation. The depth slice view is simply the cross-section image parallel to the surface scanned.  
Two different PAU systems were used in this experiment to determine their FRP bar detection capability, as shown in Table 4 and Figure 8. The MIRA 3D device had 64 ultrasonic transducers located in a 16 \(\times\) 4 grid at 3 cm spacing (extended 16 rows of 4 transducers each) where the Pundit Live Array Pro had 24 ultrasonic transducers located in an 8 \(\times\) 3 grid at 3 cm spacing (8 rows of 3 transducers each).  
Figure 8: PAU systems used: (**a**) A1040 MIRA 3D (two devices were attached side by side to increase the number of channels to 16 rows of 4 transducers each) [115], (**b**) Pundit live array pro.  
Figure 6: Phased array with angled wavefront [112].  
Figure 7: PAU test setup: (**a**) Line scan, (**b**) Stripe scan.