## 2 Materials and Methods
### GPR Tests  
Ground penetrating radar (GPR) is a non-destructive testing (NDT) method that is used to the analyze internal characteristics of structures or elements in real-time. It works on the principle that electromagnetic waves reflect back when they encounter an interface between two materials with different dielectric constants [31; 39]. GPR functions by emitting electromagnetic waves (in the form of radio waves) through the test material and then detecting the waves that bounce back off any discontinuities within the material. These discontinuities can take various forms, including interfaces between different materials, such as the concrete-bar interface shown in Figure 3 or the concrete-air/water interfaces (subsurface defects like voids, cracks, debonding and delamination) [97]. Due to the limited research on the use of GPR for FRP-reinforced structures, this method has not yet been able to establish itself as a reliable NDT method. This paper will explore the possibility of using GPR for FRP bar detection. In this study, the GPR tests were conducted both as individual line scans along a straight line and as grid scans over a test area. The line scans were conducted as a reconnaissance survey or preliminary inspection prior to the detailed grid/area scan to form a quick idea of what to expect inside the structure, the orientation of reinforcement or other subsurface features, and the depth of exploration. The GPR line scans conducted \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Slab** & **Parameter** & **Reference** & **Bar 1** & **Bar 2** & **Bar 3** & **Bar 4** & **Bar 5** & **Bar 6** & **Bar 7** & **Bar 8** & **Bar 9** & **Bar 10** \\ \cline{1-1}
**ID** & **Distance to edge** & & & & & & & & & & & \\  & reference (L/in.) & & 3 & 9 & 15 & 21 & 27 & 33 & & & & & \\ \cline{1-1}  & Depth to surface (C/in.) & & 0.75 & 0.75 & 1.5 & 1.5 & 3 & 3 & & & & \\ \cline{1-1}  & Diameter of bar (\(F\)/in.) & & \#4 & \#6 & \#4 & \#6 & \#4 & \#6 & & & & \\ \cline{1-1}  & Material of bar (T) & & & & & & & & & & & \\ \cline{1-1}  & Depth of slab (h/in.) & & & & & & & & & & & \\ \hline \hline \end{tabular}
\begin{tabular}{c c c c c c c c} \hline \hline
**Slab** & **Parameter** & **Reference** & **Bar 1** & **Bar 2** & **Bar 3** & **Bar 4** & **Bar 5** & **Bar 6** & **Bar 7** & **Bar 8** & **Bar 9** & **Bar 10** \\ \cline{1-1}  & **Distance to edge** & & & & & & & & & & \\  & reference (L/in.) & & 3.3 & 8.6 & 14.5 & 20.2 & 25.4 & & & & & \\ \cline{1-1}  & Depth to surface (C/in.) & & 3.3 & 3.3 & 3.6 & 3.5 & 3.8 & & & & \\ \cline{1-1}  & Diameter of bar (\(F\)/in.) & 3 & & & & & & & & & (Bottom mesh) \\ \cline{1-1}  & Material of bar (T) & & & & & & & & & & \\ \cline{1-1}  & Depth of slab (h/in.) & & & & 7.0 & & & & & & \\ \cline{1-1} \cline{2-1}  & Distance to edge & & & & & & & & & & \\ \cline{1-1}  & reference (L/in.) & & & & & & & & & & & \\ \cline{1-1} \cline{2-1}  & Depth to surface (C/in.) & & & & & & & & & & & \\ \cline{1-1}  & Diameter of bar (\(F\)/in.) & 1 & & & & & & & & & & \\ \cline{1-1}  & Material of bar (T) & & & & & & & & & & & \\ \cline{1-1}  & Depth of slab (h/in.) & & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Reinforcement/Dimension details of detectability slab specimens. along a straight line on the surface of the test specimen were used to obtain its cross-section image through a plane normal to the surface along the direction of the scan. However, going back and forth between several line scans and interpretating each line scan was time consuming and labor intensive. Thus, in order to simplify the interpretation of results, grid scans were also conducted over the surface of the test specimens. Grid scans were performed simply by taking GPR data along the straight lines of a grid over an area covering the test specimen, as shown in Figure 4. The grid required for a grid scan was arranged using a grid mat secured to the surface so that it did not move while taking the GPR data.