This technology will be implemented in a ceiling drone as demonstrated in Figure 3d. Figures 5a and 5b show two example GPR scan lines from the top side (1 in. concrete cover) and bottom side (1.5 in. concrete cover) of the RC deck, respectively. The locations of the two scan lines in Figures 5a and 5b were aligned well to ensure that the same cross section was scanned and highlighted. Small and large defects (yellow rectangles) and rebar (red dots) loca- tions near the test surface (1 in. from the top side and 1.5 in. from the bottom side) can be clearly seen regardless of the top and bottom sides of the deck. The defects away from the test surface cannot be identified reliably. The GPR is effective in detecting shallow defects or near-surface features and difficult in detecting deep defects due to electromagnetic wave signal loss through thick heterogeneous concrete and shallow rebar. Figure 6 shows the 3D model of GPR results scanned from the top and bottom side of the RC deck. A total of 33 scan lines on each side of the concrete deck were collected. Since the large defects were placed at different elevations (two close to the top and the other two close to the bottom), the alter- nate locations of large defects identified verified that GPR can detect shallow defects. Defects that were buried deep or too small may not be identifiable. Note that a foam strip is not the best representation for delamination associated with corrosion effects it does not include the corrosion objects that makes concrete more conductive and thus GPR signal more atten- uating. The net result is the reduced resolution for detecting small/large defects indicated in Figures 5 and 6 under realistic field conditions. GPR will be implemented on a robot that will move in a two-dimensional plan following predetermined grid- lines. Results will be summarized in a future report. Concluding Remarks The average age of America’s bridges (numbering more than 617 000) is approximately 45 years based on ASCE (2021) and rapidly approaching the end of their 50-year design life. As they continue to deteriorate, the aging bridges require more effective and reliable inspections and more frequent mainte- nance to ensure safety and serviceability. Current practices require a biennial visual inspection of bridges and is only capable of detecting damage when it has advanced to become visually apparent. As a result, there is a rising demand for aNDT and aNDE tools to assess the condition of bridges both qualitatively and quantitively, thus improving bridge asset management decision-making. GPR and infrared imaging as two potential NDE tools in aNDT&E were demonstrated to be successful in detecting subsurface defects in RC bridge decks. The proposed MR interface and robotic platforms will accelerate the use of the proposed app with MR devices, such as HoloLens 2, in the bridge element inspection field to improve the quality of visual inspection (Moore et al. 2001) and condition state assessment for preventative maintenance workflow. The developed MR interface can assist in bridge inspection education, communication, and operative planning in the years to come. The feasibility of a few key robotic plat- forms and NDE techniques to be installed on the robotic plat- forms has been demonstrated for their potential applications in tele-inspection and tele-maintenance in the future. While the potential of the different robotic technologies presented to augment routine inspection is very high, the more involved hands-on in-depth and special inspections still warrant transformative studies. The robotic platforms and associated NDE technologies presented in this paper require further field validations at bridge sites. Figure 5. Line scans of GPR on the top and bottom surfaces of the concrete deck: (a) top surface with 1 in. (2.5 cm) concrete cover (b) bottom surface with 1.5 in. (3.8 cm) concrete cover. Large defect Small defect Rebar Small defect Rebar Figure 6. Isotropic view of GPR results from the concrete deck: (a) top surface with 1 in. (2.5 cm) concrete cover (b) bottom surface with 1.5 in. (3.8 cm) concrete cover. Small defects Large defects Large defects Small defects ME | AERIALNDTFORBRIDGES 72 M AT E R I A L S E V A L U AT I O N • J A N U A R Y 2 0 2 3 2301 ME Jan New.indd 72 12/20/22 8:15 AM

ACKNOWLEDGMENTS Financial support for the projects covered in this paper is provided in part by T-REX Geo-Seed Grant Program and by the US Department of Trans- portation, Office of the Assistant Secretary for Research and Technology (USDOT/OST-R) under Grant No. 69A3551747126 through INSPIRE Univer- sity Transportation Center (http://inspire-utc.mst.edu) led by Missouri University of Science and Technology. The views, findings, and conclu- sions reflected in this publication are solely those of the authors and do not represent the official policy or position of the USDOT/OST-R, or any state or other entity. The four-wheeled structural crawler was designed and manufactured by Dr. Hung La from the University of Nevada – Reno as part of his technology transfer to the INSPIRE Center. Thanks are due to undergraduate students (Derek Edwards, Daniel McDonald, Rueil Manzambi, and Joseph Ressel) for their assistance to execute technical tasks. REFERENCES Alhaj, A., H.L. Qu, H. Zhang, G. 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