## 7 Results and Discussion  
The radagrams in Figure 7 represent typical profile images of the top and bottom faces of the prestressed concrete beam used in this study. Figure 7a reveals four longitudinal rebars at the top of the beam, represented by the dark red parabolas, and two air voids, represented by the dark red-blue parabolas below the top rebar row. Figure 7b shows a typical longitudinal profile image of the top face of the beam, revealing the total number and location of each stirrup. Figure 7c represents a typical bottom profile image, where the prestressed tendons are located. Figure 7: Typical profile images of the prestressed concrete beam: (**a**) top face transverse profile no. 15, (**b**) top face longitudinal profile no. 2, and (**c**) bottom face transverse profile no. 20. The speed of the electromagnetic wave, a crucial component of the proposed methodology, was calculated to be 5.31 in. per nanosecond. This calculation was made by correlating Equations (2) and (3) with the vertical position of the steel duct inside the concrete box beam, which is located 7.375 inches below the top surface of the structure. The electromagnetic wave velocity is a crucial parameter in the GPR data analysis, since it enables the calculation of rebar depth, spacing, and concrete cover. The rebar data, which were initially calculated manually by analyzing the radargram for each GPR profile image, are now automatically determined using the algorithm developed for this paper. This new approach not only saves time but also ensures accuracy, making the process more reliable. The comparison of rebar location and depth between manual and automatic rebar detection using the methodology explained in this paper shows excellent agreement, as illustrated in Figures 8 and 9 for the main rebars and Figure 10 for the stirrups. Figure 8: Rebar depth comparisons between manual and auto rebar picking for top rebars: (**a**) Rebar #1; (**b**) Rebar #2; (**c**) Rebar #3; and (**d**) Rebar #4. Figure 9: Rebar spacing comparisons between manual and auto rebar picking for top rebars. Figures 8d and 9 show that the algorithm failed to detect the rebar near profile number 40. This was primarily due to significant signal interference from air voids and stirrups in that area, which prevented the algorithm from identifying the positive amplitude curves using the 80% threshold (\(Q\)) set for this analysis. While reducing the acceptable sum of squared differences between the theoretical and actual hyperbolic curves could help improve rebar detection, it likely leads to many false positives, complicating the data analysis further. Therefore, it is necessary to manually analyze locations where the automatic algorithm is unable to detect rebar because of inconsistencies in the GPR reconstructed image. Figure 10 illustrates the locations where the algorithm identified the stirrups, demonstrating high accuracy, with only four false positives. This performance is largely attributed to the minimal electromagnetic signal interference in the longitudinal profiles, where the air voids are situated far from the stirrups, and there is a clear path between the stirrups and the concrete surface. Similar to the manual data collection, the algorithm failed to detect the stirrups at the ends of the beam due to the tight spacing regime and high EM signal overlap. While the longitudinal profiles proved particularly effective for the automatic rebar detection algorithm, it is important to note that transverse profiles also yielded accurate results. Both scan orientations contribute valuable insights, with the longitudinal profiles offering a slight advantage in terms of clarity due to the reduced electromagnetic signal interference. Although the algorithm failed to calculate rebar depth and spacing in some locations, this novel methodology offers a significant advantage over existing research by eliminating the need for template selection and extensive algorithm development based on an infinite dataset of hyperbolic shapes, as proposed by previous studies [40; 41].