Reinforced concrete bridges are built to handle heavy loads and routine traffic for 75 years or more. But bridges in climates like Minnesota’s are exposed to moisture and chlorides from road salts that may penetrate these structures and corrode the steel.
In a recently completed research project, funded by MnDOT and the Local Road Research Board, researchers studied a rural bridge built in 2017 near Elgin, MN, that used glass fiber–reinforced polymer (GFRP) rebar in the bridge deck. They found that GFRP performed well, proving sufficiently strong for use as an alternative to corrosion-susceptible steel rebar.
Well-documented efforts undertaken two decades ago to mitigate corrosion of a Highway 394 reinforced concrete bridge have given researchers the perfect scenario for evaluating the treatments’ long-term effectiveness. The test results are mixed: State-of-the-art methods for electrochemical chloride extraction and fiber-reinforced polymer wrapping of bridge elements performed well in combination, but poorly in isolation.
A new spreadsheet tool developed by the Minnesota Department of Transportation draws on historical data to help project engineers better estimate bridge construction time. The method allows users to project time-frames based on bridge design elements, work scheduling and other inputs, utilizing estimates from comparable projects in a 10-year database of bridge-building data.
An evaluation of five maintenance paint coating systems on Minnesota steel bridges with localized corrosion found that each maintenance coating performed well, and, if corrosion is identified early and maintenance painting occurs, the service life of the paint coating system can be extended five years before repainting is required. Based on the test data, researchers recommended an update to MnDOT’s Bridge Maintenance Painting Manual that includes guidance on when to apply maintenance paint coatings and when to remove paint and recoat bridges.
A new MnDOT-funded research study has found that most agencies in states with weather similar to Minnesota’s use debonded strands in prestressed concrete bridge beams. MnDOT may begin piloting debonding as an alternative to draping, which manufacturers claim is time-consuming, challenging to worker safety and expensive.
The Minnesota Department of Transportation (MnDOT) had suspended the use of post-installed epoxy-coated rebar for concrete barrier repairs as a precautionary measure because chemical adhesives used in the process are not designed for use with coated bars. But laboratory testing (conducted in a recent MnDOT-sponsored research study) has now shown that using these adhesives with coated rebar for post-installation works well and provides a safety level 200 to 300 times that predicted by manufacturer specifications. MnDOT is considering research recommendations to modify the installation process in order to resume using coated rebar in post-installed concrete crash barriers.
Researchers provided recommendations and general guidance to assist MnDOT in developing cost-effective strategies for future investigation and repair contracts of post-tensioned bridges built in Minnesota before 2003. To develop these recommendations, researchers identified grout voids in post-tensioning ducts on two representative bridges, documented strand corrosion and repaired voids by filling them with grout.
“With the guidelines developed in this project, we have a good basis for cost-effectively and efficiently inspecting post-tensioned bridges built before 2003,” said Dustin Thomas, South Region Bridge Construction Engineer, MnDOT Bridge Office.
“From a fiscal perspective, it makes sense to do limited inspections on these bridges before committing additional resources to more comprehensive inspection and repair,” said Mark Chauvin, Associate Principal and Unit Manager at Wiss, Janney, Elstner Associates, Inc.
What Was the Need?
Some concrete bridges in the United States are strengthened using post-tensioning—a method of reinforcing concrete by running steel strands through a hollow plastic or metal duct placed within the concrete element. Tension is then applied to these strands with a hydraulic jack, com-pressing the concrete and creating internal stresses that resist external traffic loads. Post-tensioning improves the durability of concrete and virtually eliminates cracking.
Post-tensioning ducts are filled with grout—a mixture of cement, sand and water that hardens around the steel strands. This practice prevents the strands from corroding if they are exposed to air, water and deicing chemicals.
Grouting materials used in bridges built before 2003 frequently produced voids where grout did not fully fill the post-tensioning ducts or cover the strands. These post-tensioning strands were vulnerable to corrosion, which can lead to deterioration in bridge elements over time. Once transportation agencies and the industry became aware of these issues, they improved their construction practices and began using prepackaged grout materials with additives so that the post-tensioning ducts would be completely filled.
About 40 post-tensioned bridges were built in Minnesota before 2003 that might still require repair. MnDOT commissioned a two-phase project to develop techniques for evaluating these structures. In the first phase of the project, completed in 2012, researchers inspected a representative sample of these bridges and developed a general inspection protocol to guide future investigations. In the second phase, described here, researchers developed additional guidance about grout repairs, as well as the most cost-effective contracting methods for such repairs.
What Was Our Goal?
The goal of the second phase of this project was to provide recommendations and general guidance that MnDOT could use to develop cost-effective strategies for future investigation and repair of post-tensioned bridges built in Minnesota before 2003. As part of this project, researchers identified grout voids in post-tensioning ducts on two representative bridges, documented strand corrosion and repaired voids by filling them with grout.
What Did We Do?
In 2013, researchers inspected three spans on two Minnesota bridges for voids around post-tensioning strands. They began the project by using ground penetrating radar to map the location of the ducts. Once the ducts were located and mapped, researchers used a borescope to visually inspect the duct interiors at locations where voids were likely to be present. When they found voids, they documented the percentage of the duct filled by grout and the extent of corrosion in the post-tensioning strands within the ducts, if any. Following inspection, researchers filled the voids with grout and installed sensors within the voids at two locations to monitor the long-term corrosion of post-tensioning strands.
Using their experience with these repairs, researchers then created guidelines that would help MnDOT develop cost-effective strategies that can be implemented in future post-tensioning duct investigation and repair contracts.
What Did We Learn?
Researchers found voids in 32 percent of inspected ducts. These voids were typically at least 10 feet long and about one-half the diameter of the duct. Although prestressing steel strands were exposed at approximately half of the grout voids, no significant corrosion of the strands was observed at any location. Light to moderate corrosion was usually observed on the inside surfaces of the galvanized metal ducts at grout voids.
The guidelines developed by researchers address:
Typical work plans for investigation and repair, including considerations for bridge access and traffic maintenance during inspection and repair.
Document review, including bridge design drawings and post-tensioning shop drawings.
Visual surveys to identify signs of distress near post-tensioning ducts.
Procedures for borescope inspection and remedial grouting repair.
Various contract and project approaches for developing specialized inspection and re-medial repair contracts, with a discussion of the advantages and disadvantages of each approach. Using multiple contracts with graduated levels of inspection and repair will most likely provide MnDOT with the best value.
Planning-level cost information for seven representative pre-2003 post-tensioning bridges identified by MnDOT to assist in future budget calculations.
The guidelines developed in this project will provide MnDOT with a framework to solicit and procure similar engineering and construction services contracts for post-tensioning bridges in Minnesota. Researchers recommend exploring additional techniques to more rapidly assess and inspect post-tensioned bridges, including noninvasive investigative methods that do not require drilling holes.
“Phase 3 will allow us to utilize a new drone specific to confined space inspections,” Wells said. “This new drone is meant to reach places the prior drones could not, which will supplement our efforts nicely. Also, Phase 3 will include more bridge inspections in order to get a more comprehensive feel for cost and time savings.”
The increasing costs of bridge inspections are a concern for MnDOT. The use of unmanned aircraft systems (UAS) has been shown to reduce costs, improve the quality of bridge inspections, and increase safety. The UAS can deploy a wide range of imaging technologies including high definition still, video, and infrared sensors, and data can be analyzed using 3D imaging software.
MnDOT completed a small research project in 2015 to study the effectiveness of UAS technology applied to bridge safety inspections. The project team inspected four bridges at various locations throughout Minnesota and evaluated UAS’ effectiveness in improving inspection quality and inspector safety based on field results.
A second research effort demonstrated UAS imaging on the Blatnik Bridge and investigated UAS use for infrared deck surveys. Additionally, a best practices document was created to identify bridges that are best suited for UAS inspection.
It is the goal, based on this next phase of research, to implement a statewide UAS bridge inspection plan, which will identify overall cost effectiveness, improvements in quality and safety, and future funding sources for both state and local bridges.
Collins Engineering will also investigate a collision tolerant drone — the Flyability Elios — for confined space inspections.
As part of the Phase 3 project, Collins Engineering will:
Review current Federal Aviation (FAA) rules, technical literature, owners and industry experiences, and ongoing UAS research.
Develop bridge inspection list based on Phase II research regarding best practices. Approximately 20-25 bridges will be inspected under this contract depending on location and size.
Develop a field work plan for the bridge inspection list. If approvals for these bridges cannot be obtained, suitable alternatives will be chosen. This field work plan will address safety concerns, FAA, and other agency requirements.
Establish a work schedule and deliverable submission schedule.
Establish methods of access and schedule equipment.
Receive training on the Flyability collision tolerant drone for use in the study.
Perform field work at the selected bridges to collect imagery and evaluate the technology to accomplish the project goals.
Inspect known deficiencies identified during previous inspections with the use of the UAS to evaluate the ability to identify deficiencies using photos and video.
Enter bridge inspection data in Minnesota’s Structure Information Management System (SIMS) providing element condition ratings, photos, videos, etc. based on UAS imagery and information.
Prepare a draft report to document project activities, findings and recommendations.
The Phase 3 project is scheduled to be complete by July 2018.
MnDOT’s efforts to study whether drones can help bridge inspectors are progressing, and the second phase project has been completed. (Meanwhile, a third project has just begun.)
Phase 1 of this research project demonstrated that drones can reduce safety risks and inconvenience to bridge inspectors and the traveling public. Phase 2 shows that new drones, designed with vertical and horizontal camera and sensor capabilities for structure inspections, give bridge inspectors safe access to under-deck areas that were previously difficult or impossible to reach. The new drones cost even less than the unit tested in Phase 1.
“Using a drone rather than snoopers for bridge inspection can save significant time and cost. The FHWA approves of this use as well. It’s another tool for inspectors to employ,” said Jennifer Wells, Principal Engineer on Mobility, MnDOT Office of Bridges and Structures.
“We were one of the first transportation agencies and contractors to test and use this new technology for bridge inspections. Drones let bridge inspectors collect more data and collect it more safely and efficiently,” said Barritt Lovelace, Regional Manager, Collins Engineers, Inc.
What Was the Need?
MnDOT and local bridge owners have 600 bridge inspectors who monitor more than 20,000 bridges in Minnesota. Each bridge must be inspected once every 24 months. Bridges in poor condition and those considered fracture-critical (where failure of a single component could cause collapse) must be inspected every 12 months. Large bridges can take weeks to fully inspect and often require inspectors to dangle from ropes or stand in buckets on the end of “snoopers,” cranes that reach from the bridge deck to below-deck level to put inspectors within sight of under-deck elements.
Snoopers are expensive and require traffic lane closures, presenting safety risks to the traveling public and inspectors. MnDOT established in a Phase 1 study that unmanned aircraft systems (UAS) significantly augment inspection findings with infrared and imaging data while reducing safety risks to inspectors and the public. The project earned a 2016 Minnesota State Government Innovation Award as well as awards and recognition from such groups as the American Public Works Association.
UAS designed specifically for structure inspections were unavailable during Phase 1. The UAS used in that phase had key operational limitations, including the inability to proceed when concrete and steel bridge components blocked Global Positioning System (GPS) signals. When that happened, the drone simply returned to base automatically.
What Was Our Goal?
In Phase 2, MnDOT wanted to test the use of an upgraded UAS to examine larger and more challenging bridges. The new UAS, which was specially designed for structure inspections, featured more robust imaging and infrared data-gathering capabilities, and was more flexible to control. Its operational capabilities also were not diminished by the loss of GPS signals. Results from UAS inspections and traditional bridge inspection methods would be compared for quality and cost-effectiveness.
What Did We Do?
Investigators selected a prototype senseFly albris UAS to inspect four bridges:
The Blatnik Bridge over the St. Louis River between Duluth, Minnesota, and Superior, Wisconsin, a 7,980-foot-long steel through-arch bridge with steel deck trusses.
A 362-foot-long two-span steel high truss bridge over the Red River in Nielsville, Minnesota.
A 263-foot-long corrugated steel culvert in St. Paul.
The Stillwater Lift Bridge, a 10-span structure over the St. Croix River with six steel through-truss spans and one movable span.
For each bridge or structure, researchers prepared detailed safety and inspection plans to identify and mitigate potential hazards, inspection needs and Federal Aviation Administration (FAA) requirements. Researchers conducted and evaluated UAS and standard inspection methods for each inspection site, analyzing results in terms of access technique, data collection and usefulness for interim and special inspections.
What Did We Learn?
The senseFly albris UAS offered a clear operational upgrade over the Phase 1 unit. It can operate without GPS; the camera lens can turn up and down at 90-degree angles; and protective shrouds and ultrasonic sensors prevent the propellers from striking bridge elements.
For some inspection functions, lane closures can be curtailed or eliminated altogether. The drone worked well in the high, confined spaces of the Blatnik Bridge and should provide under-deck inspection details otherwise unavailable or too costly for any tall bridge in the MnDOT system. This UAS identifies and measures clearances, rope access anchor points and other pre-inspection conditions for planning large-scale or emergency inspections. Photogrammetry software can be used with the UAS to develop three-dimensional models of bridges and bridge sites. Using infrared thermal sensors, the UAS can detect delamination of concrete while flying adjacent to lanes of traffic. For smaller, confined spaces on bridges and culverts, the senseFly albris may not be ideal. Despite its protective shrouds, it is not as collision-tolerant as needed for very tight spaces.
Currently no UAS replicates hands-on inspection functions like cleaning, sounding, measuring and tactile testing. But the UAS is an additional tool that provides conventional and improved data safely. The FAA and the MnDOT Office of Aeronautics no longer require private pilot certification for drone operators. A new, streamlined certification and licensing procedure makes drone use more practical.
Costs were significantly lower with UAS inspections than with conventional approaches. Conventional inspection of the Blatnik Bridge would have required four snoopers, an 80-foot lift and eight days of inspection, at a cost of about $59,000 (without the cost of mobilizing equipment and traveling). The UAS Blatnik Bridge inspection would contract as a five-day, $20,000 project.
Phase 3, which began in the summer of 2017, uses the senseFly albris and the Flyability Elios, a collision-tolerant drone more suited to confined spaces such as box girders or culverts. During this phase, researchers will identify which situations are best suited for drone use, what parameters should govern drone use in bridge inspections, and how UAS can be integrated into standard inspection operations at a county and district level.