Innovative Technologies for Bridge Inspection

For the past several years, unmanned aircraft systems (UAS), more commonly referred to as drones, have been a topic of discussion for a wide range of inspection applications, including building facades and roofs, transmission towers, dams, ancillary highway structures, and bridges. Bridge inspection is an application that has received a lot of attention in recent years, with universities, departments of transportation (DOTs), and consulting firms researching and working to determine the best implementation approaches for these technologies.

DJI Matrice 210 with XT2 sensor

Several of the potential benefits for implementing these drone technologies into bridge inspections include increasing safety by reducing inspector time in hazardous environments, reducing costs for access equipment (i.e. snooper trucks, aerial lifts, scaffolding, rigging, etc.), reducing durations of traffic closures, and providing high quality imagery of difficult to access areas. While it will likely be impossible to eliminate the need for hands-on access, even a small reduction in the amount of time required will provide a benefit for improving safety and reducing inconvenience for the traveling public. 

Snooper truck with traffic closure

It is important to understand that drones cannot be a direct replacement for all types of traditional inspection by a trained inspector. Drones can provide a significant amount of useful data but limitations may result in missing crucial deficiencies in hard to reach locations. Drones should be implemented in situations that make sense and can match or exceed the existing level of details from inspections. For example, truss bridges are comprised of multiple built-up members that are connected by gusset plates. These gusset plate locations are very tight and would not provide adequate accessibility for a drone to view all locations of concern. The interior surfaces of the individual truss members would also be difficult to adequately inspect for similar reasoning. However, the use of a drone for inspection of the truss bridge piers may be an effective application depending on the height of the bridge.

Truss gusset plate connection/bearing

There are numerous factors that should be considered while determining whether drones could make positive contributions to inspections, including the capabilities of the drone itself, federal regulations (14 CFR Part 107), site features, airspace restrictions, identified hazards and mitigation measures, feature(s) beneath the bridge, bridge structure type, bridge materials, vertical clearance, age and condition of bridge, type of inspection, comfort of overseeing agency, and comfort of the inspection team leader. Unless a member of the inspection team is a FAA Part 107 licensed commercial UAS pilot, additional costs for a UAS pilot must also be considered. In general, it is challenging to provide detailed guidelines for implementation because all these different factors must be considered for each bridge.

Every drone has limitations with different strengths and weaknesses depending on the type of work being performed. Whether these limitations are based on the drone software and hardware or site environment; it is important to know the operational limitations. Different types of drones will be needed for different applications. Managing expectations about drone capabilities is important to ensure client expectations are met and the best possible value is provided. 

The most basic application of drones for bridge inspection includes using the drone to collect inventory information and document site conditions. This application can be performed by most types of drones, ranging from prosumer to commercial. While it is not a flashy application, it can help improve safety by eliminating the need for inspectors to climb down steep embankments and uneven surfaces. In this application, the drone can also capture beneficial aerial views to document the site for assisting with future inspection planning.

Bridge elevation taken by drone

The use of drones as a means of access for inspection will vary in effectiveness depending on the type of drone, bridge structure type, and bridge material. The use of drones for the inspection of bridge undersides requires drones with cameras that are capable of looking upwards. While some drone manufacturers, such as Skydio, DJI, and Parrot, have designed drones capable of looking upwards, the number of drones that are capable of an adequate upward view are still limited. Bridges that provide the best opportunity for drones include steel/concrete box girders, butted box beams, prestressed voided slabs, concrete slabs, and concrete arches since there are minimal outstanding elements to obstruct the drone view. Several challenges still exist for these types of bridges, such as gaps between box beam which create a difficult environment for viewing the vertical faces of the box beams. Steel multi-girder bridges may not provide an effective opportunity as the top face of the bottom flange and bottom of the web would not be visible unless the drone was able to fly between the girders. Some drones can safely fly between girders; however, it will depend on the drone being used and the girder spacing.

Steel box girder web from Skydio 2

Skydio 2 beneath box beam bridge

The use of infrared thermal imaging can be used to identify areas of leakage and concrete delamination. This can be implemented with either handheld thermal cameras or drone mounted thermal cameras. Some drones have integrated infrared thermal cameras or are able to be outfitted with them through swappable payloads. The use of thermal imaging for detecting leakage is effective because water has a high thermal capacity – which means that it can absorb a lot of heat before it starts to warm up. Generally, water will be cooler than surrounding objects which is easy to identify using thermal imaging.

Leakage along longitudinal joint

The use of thermal imaging for determining concrete delamination requires ambient air changes or direct sunlight heating the concrete. The air within a delamination acts as insulation separating the delaminated concrete from the remaining concrete. This results in the delamination heating and cooling at a different rate than the remainder of the deck. The use of thermal imaging for these purposes requires having specific atmospheric conditions and temperature changes for the delaminations to be visible. 

Delamination along underside of bridge deck

Delamination along topside of bridge deck

It is important to note that temperature readings are affected by many factors including the type of material, thickness of material, surface texture, dirt/debris that may be present, moisture, and reflectivity of the surrounding environment. These temperature differences could lead to false-positive indications of delamination. Thermal data should be cross checked against visual imagery whenever possible to ensure that false-positives are eliminated from the final data. 

Beyond the use of drones as a means of inspection access, photogrammetric modeling of structures to create a digital twin can be a valuable tool for inspection tracking and asset management. Photogrammetric modeling can be performed with several different commercially available software packages, including Bentley Context Capture, and cloud-based platforms such as Pix4D and Drone Deploy. Photogrammetric modeling relies on distinct tie points in order to stitch photos together. If there are not enough distinct tie points in the images, the software can struggle to accurately stitch images together and can leave gaps in the final product. The modeling process requires a large number of photographs in order to maintain sufficient overlap (approximately 75% to 80%) for the software to sufficiently process tie points. This requires multiple perspectives and angles in order to accurately model the subject geometry. 

The bridge model provides a visualization of the overall bridge condition. The model can be used to extract orthomosaic images of different faces of the model. These orthomosaic images help provide context for the overall condition, which can be lost since inspection reports focus on the deficiencies. This helps to provide a “big picture” view. It also provides an exact image of the structural conditions which can be useful for report documentation in lieu of traditional deficiency sketches, where hand drawn field sketches are translated into final report sketches using AutoCAD. Traditional field sketches can be time consuming to develop and may not be entirely accurate due to the subjective nature of documenting cracking, spalling, and delaminations by different bridge inspectors. 

Orthomosaic of box beam underside

The final photogrammetric model can also be used as a useful tool for quantification of defects, as both length and area can be determined from the model. Additional data process is possible through other commercial software, such as Matlab. Feature extraction processes can be used to highlight visible deficiencies such as cracking. If multiple data sets are available, digital image subtraction processes can be applied to identify the change (typically worsening) of existing defects and deterioration over the time between the data sets. 

Distance measurement of pier delamination

Area measurement of wearing surface patch

The use of innovative technologies and methodologies such as drones and photogrammetric models will be useful tools for bridge inspection work going forward. It is important to make the distinction that at this time these technologies are not a latchkey solution that will fit every scenario but rather should be implemented as another tool-in-the-toolbox in situations that make sense and provide the most benefit.