## 3 Results  
### GPR Tests Results  
GPR response or line views traversing perpendicular to bars embedded in each slab specimens obtained using a 1 GHz GPR device are shown in Figure 9, where the top of the hyperbolic shape, i.e., inverted U shape, indicates the location of the bar and the shape of the tails gives a measure of velocity and depth [116]. The background subtraction filter was used to enhance the hyperbolas from embedded FRP bars. Without the application of this filter, the top of the hyperbola of the shallow targets (FRP bars close to the top surface) would have been obscured by the direct wave band that appears at the top surface. However, using the filter also removed the horizontal band indicating the bottom surface of the slab, but because the main objective of this research was to detect the FRP bars, the detection of the bottom surface was deemed not important. Only the line scans conducted at the centerline of the slabs using the lowest frequency GPR device has been presented in this paper for the brevity of the results as it would represent the lowest resolution among the GPR devices used in this study. Further, it should be noted that in order to confirm that the hyperbolas detected were indeed the FRP bars, but not other internal features (such as voids), several line scans were performed and checked for repetition of the same pattern of hyperbolas in each line scan (which indicates the presence of a continuous internal target such as a reinforcement bar). For Slab C with GFRP bars in one direction, bars at up to a depth of 1.5 inch were detectable. For Slab J with GFRP bars in two directions (mesh), the hyperbolas for the bars on both the top and bottom mesh were detectable. The bars near the edge were shadowed by the hyperbola due to the edge of the slab, which could have been avoided by leaving an offset from the edge for taking the measurements. The GPR line scan of Slab L was able to detect carbon strand (Bar 5), steel bar (Bar 6) and GFRP strand (Bar 7), along with the larger diameter #8 GFRP bar (Bar 2). However, it was not able to detect other glass (Bars 1, 3, 4) and basalt (Bars 8, 9) bars. The bar detectability of the line scans conducted using lower frequency device is further summarized in Table 5.  
The depth slices obtained for each slab specimen using each GPR device are further illustrated in Figure 10, where it can be seen that the bar detectability improves as the device central frequency increases. Although the line scan in Figure 9a shows four hyperbolas for Bars 1-4, the depth slice in Figure 10a only shows the presence of Bar 2, which is the larger GFRP bar (#6 GFRP bar) at shallower depth in Slab C. This could be because of the limitations in vertical resolution due to a lower frequency of the GPR device. Larger GFRP bars are detectable using lower frequency GPR device at shallower depth, but they return weaker hyperbolas in comparison to metallic objects. Moreover, the relatively higher  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**PAU** & & & & & & & & \\
**Systems** & & & & & & & & \\ \hline A1040 MIRA 3D & \(8\times 4\) (extendable to 16 \(\times\) 4, 24 \(\times\) 4, etc.) & & Multi-channel ultrasonic pulse & Active dry point & & & & \\ ultrasonic pulse & & contact (A-DPC) & 10–100 KHz & & 6.5 ft (2 m) & ACS-Solutions GmbH, Germany \\ \hline Pundit live array & \(8\times 3\) (with upgrade & & & & Dry-contact ultrasonic pulse & & & \\ probeamplitude of shallower GFRP bars may have dominated and masked the lower amplitude returns from the GFRP bar that are deeper and smaller in diameter, which could be the reason only Bar 2 was visible in Figure 10a. However, the depth slices in Figure 10b,c, obtained using higher frequency GPR devices, are capable of detecting a smaller diameter bar, even at deeper depths.