## 2 Materials and Methods  
In order to determine whether GPR and PAU can detect internal FRP reinforcement, small-scale concrete slabs were fabricated. Two slab specimens (i.e., labeled as J and L) were 30 in. wide by 30 in. long and 7 in. deep, and the third specimen (viz. Slab C) was 36 in. wide by 36 in. long and 5 in. deep. The concrete mix used to cast the slab specimens was the 'Class II 4500 Bridgedeck' concrete, as per the Florida Department of Transportation (FDOT). This class specified a guaranteed compressive strength of 4600 psi (31 MPa). Type II Cement was used with a water to cementitious material ratio (w/m) of 0.44, and #57 stone and silica sand was used as coarse and fine aggregate, respectively. To obtain the actual strength value; concrete cylinders were tested at 28 days according to ASTM C39 [94; 95]. An average compressive strength of 31.70 MPa was obtained with a standard deviation of 0.69 MPa (coefficient of variation of 2.2%).  
The slab specimens were fabricated, targeting different parameters such as FRP bars/strands type (GFRP, CFRP, BFRP), bar diameter, bar direction and bar depths. Table 1 shows the identification of the slab specimens by group, highlighting their main parameters. Because GFRP bars are the most commonly used FRP reinforcement in concrete elements, the first two slabs constructed (Slabs C and J) were only reinforced with GFRP bars of different sizes, at different depths and different configuration (bars in one direction and bars in two orthogonal directions, i.e., mesh). The concrete cover specified by ACI CODE-440.11-22 [96] for the concrete members reinforced with GFPR bars ranges from 0.75 in. to 3 in., hence the depth variations in the slab specimens were included to represent the layers of FRP reinforcement, which could be anywhere within the concrete cover range specified. Further, the third slab (Slab L) has different types of internal reinforcement (bars and strands) embedded into it, including one steel bar whose detectability acts as a control for this research. Having GFRP, CFRP, BFRP and steel bars/strands on the same slab specimen allows comparison of the detectability of different FRP bar/stands with the steel bar under the same test conditions.  
All specimens were fabricated and labeled following the same layout as shown in Figure 1. Every slab is identified with a letter (C, J and L), and every side has an identification number (from 1 to 4). The direction of the measurement will be determined by the number where the measurement is started to the end of the measurement.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimen Group** & **Slab ID** & **Bar Diameter** & **No. of Bars** \\ \hline \begin{tabular}{c} Slabs with GFRP bars in one \\ direction \\ \end{tabular} & Slab C & \#4 and \#6 & 6 \\ \hline \begin{tabular}{c} Slabs with GFRP bars in two \\ directions (mesh) \\ \end{tabular} & Slab J & \#6 & 10 \\ \hline \
\begin{tabular}{c} Slab with different FRP bars in \\ one direction \\ \end{tabular} & Slab L &
\begin{tabular}{c} \#3, \#5, \#8 GFRP bars, \#5 CFRP strand, \#3 \\ steel bar, \#6 GFRP strand, \#3, \#5 BFRP bars \\ \end{tabular} & 9 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Identification of small-scale concrete slab specimens.  
Figure 1: Labeling of slab specimens.  
Table 2 shows the details and dimensions of the specimens. For each specimen, the distance to edge, depth to surface, bar diameter, bar material and slab depth are presented based on the convention shown in Figure 1. The formwork for each slab specimen constructed is shown in Figure 2.