## 4 Discussion  
The GFRP bars that dominate the embedded concrete reinforcement application are not conductive and have a density similar to concrete. Therefore, the conventional NDT methods that use electromagnetic and stress waves for the detection of steel bars fail to detect GFRP bars with the same clarity as steel. As explained earlier in Sections 2.1 and 2.2, the GPR and PAU devices work on the basis of differences in dielectric constant and acoustic impedance encountered at an interface. The reflection coefficient of the electromagnetic wave (for GPR tests) and ultrasonic waves (for PAU tests) passing through concrete when they encounter air (bottom surface), steel bar and GFRP bars are shown in Table 6. It can be seen that the concrete-GFRP interface reflection coefficient, i.e., the amount of reflected energy, is less than 8% for GPR tests and has a very minimal value of 0.04% for PAU tests, which is the reason why GPR had limited detectability of GFRP bars and PAU was not able to detect them at all. The dielectric constants and acoustic impedances for average concrete, air and steel used in Table 6 were obtained from the literature [116; 117; 118]. From the specifications chart provided by the manufacturer of the GFRP bar, the dielectric constant was determined to be a value of less than 5, and the acoustic impedance was estimated to be 10.7 \(\times\) 10\({}^{6}\) kg/m\({}^{2}\) s (from the equation \(Z=\sqrt{E\cdot\rho}\), where E = 54.5 GPa is the modulus of elasticity and \(\rho\) = 2.1 g/cm\({}^{3}\) is the density of the GFRP bar). However, the depth slices of GPR tests obtained using four different devices with different antenna center frequency demonstrated that the detection of FRP bars becomes better with the increase in frequency, as shown in Table 7. Apart from the frequencies playing the major role in the detection of FRP bars, different detectability levels could also be due to several other factors related to the radar technologies being used in each of the GPR devices. For example, the 2 GHz GPR device also had a dual polarization feature that enhances the detection of multilevel bars. The maximum frequency GPR devices used in this study also had a Stepped Frequency Continuous Wave (SFCW) system. This provides both the benefits of higher resolution in the detection of shallow targets and the increase in penetration depth. Moreover, there were some disparities in the results of the line scans and the grid scans for the lower frequency GPR device as the bars visible in the line scan were not visible in the depth slices. This could be due to the fact that the depth slices are produced through the interpolation of a series of line scans, and it would show the bars with stronger signals, i.e., stronger hyperbola more dominantly in comparison to the bars with relatively weaker hyperbola. Thus, although it is time consuming, it is recommended to go through each line scan of a grid while analyzing the results for GPR tests. On the other hand, it was found that PAU can only detect FRP strands, which could be due to the presence of (air) voids within the twisted wires of FRP or irregularity of the shape of their surface in comparison with smooth bars.