Defect Visualization in the Bridge Underpass Arch Structure: A Photogrammetry Assessment Using UAV-Captured Imagery

Concrete structures develop several defects as the structure ages. One of the common concerns in structural integrity is the formation of cracks, which demands regular inspection with precision. In this study, a bridge underpass arch structure was inspected with the help of an Unmanned Aerial Vehicle (UAV) in a coastal region of the Mediterranean Sea, where 2D captured images were transferred into a 3D model for better visualisation from a Structural Health Monitoring (SHM) perspective. The images with cracks were manually annotated, using the VGG tool, by an expert. Using the 3DF Zephyr software, from sparse to dense point clouds, and 3D mesh to orthophoto, all 3D models were constructed from the annotated and unannotated images of the structure. The 3D model achieved a Ground Sampling Distance of 0.0046 m/pixel, with an image alignment of 60%. The Bundle Adjustment Mean Reprojection Error confirmed satisfactory internal model accuracy. The final assessment through the orthophoto, where a resolution of 4531 × 2433 pixels was achieved, revealed that the images were of sufficient quality to capture the details and the defects present, and better visualisation could be made. This output demonstrates that UAV-based photogrammetry is time- and cost-efficient and surpasses the traditional visual inspection of confined structures.