Transportation planners lack a method to directly compare bridge and road conditions. In a new MnDOT-funded study, University of Minnesota researchers have proposed a Percent Remaining Service Interval (PRSI) measure that can uniformly assess the condition of bridges and pavements, enabling planners to make the most efficient use of preservation and improvement funding.
“Both the MnDOT Bridge Office and the Materials and Road Research Office have very good management systems in place,” says Mihai Marasteanu, a professor in the Department of Civil, Environmental, and Geo- Engineering (CEGE) and the study’s principal investigator. “There is a good potential to develop a new common metric that both offices could use.”
What Did We Do?
To begin developing this new measure, researchers conducted a literature review of current methods used in asset management and life-cycle cost analysis. The review of bridge research focused on performance measures and life expectancy assessment methods, while the study of pavement literature concentrated on performance measures as well as on the use of road service life measures.
Next, the research team, which included civil engineering bridge professor Arturo Schultz, surveyed both bridge management staff and pavement management staff from state transportation agencies. Team members then analyzed the asset management practices of MnDOT’s Office of Bridges and Structures and Office of Materials and Road Research to identify methods for assessing service lives and rehabilitation needs and to highlight the similarities and differences in approaches.
Based on the findings from the survey and analysis, researchers suggested the new method of PRSI that would serve both pavement and bridge needs and offered guidelines for the next steps in developing and implementing a unified PRSI procedure.
“Ultimately, funds for guardrail repairs are drawn from the same purse that pays to fill a pothole or repair a deck joint,” Marasteanu says. “With PRSI, planners could target average values across systems to optimize life-cycle costs and pursue an even distribution of PRSI values to make planning consistent from year to year.”
In the next phase of the project, researchers will work with the pavement office to identify relevant data for calculating PRSI for pavements. “In addition, we plan to identify the time and costs required to reach the evenly distributed configuration of PRSIs necessary for planning consistency, assess how preservation activities impact funding efficiency, and calculate recommended metrics for asset sustainability,” Marasteanu says.
In recent years, MnDOT inspection crews have reported loose anchor bolts on many support structures for overhead signs, high-mast light towers, tall traffic signals, and other signs and luminaires. On newly installed structures, many nuts on anchor bolts may loosen in as little as three weeks; on older structures, they may loosen less than two years after retightening.
Federal standards mandate inspections at least once every five years, a requirement that already stretched MnDOT’s resources for managing light poles, traffic signals and 2,000-plus overhead signs. With an estimated 20 percent of loose anchor bolts in MnDOT’s highway system at any given time, crews would have to inspect structures every year to ensure public safety.
This issue is not unique to Minnesota. In a national survey, some states estimate as many as 60 percent of their anchor bolts may be loose. Minnesota, like other states, tightens anchor bolts according to American Association of State Highway and Transportation
Officials (AASHTO) standards. But the standards and procedures for tightening and retightening bolts were insufficient. To develop appropriate specifications, MnDOT needed to know why bolts loosen. The agency also needed improved standards and procedures to ensure that anchor bolts are tightened effectively
What Was Our Goal?
MnDOT decided to undertake a research project to determine why anchor bolts and nuts on sign and luminaire support structures loosen after installation or retightening, and to develop new standards and procedures that ensure proper and lasting tightening of these bolts.
Researchers from Iowa State University examined specifications and procedures for tightening anchor bolts on support structures in Minnesota. They also developed new specifications and instructions to help crews tighten bolts properly and ensure lasting safety of signs and lights in Minnesota’s highway system.
How Did We Do IT?
Researchers conducted a literature search on anchor bolt loosening. Then they surveyed MnDOT maintenance staff on bolt lubrication and tightening practices, and visited sites in Minnesota and Iowa to observe installation and retightening practices.
In the laboratory, investigators studied the relationship of torque, rotation and tension of various bolt diameters and material grades. They found that bolt stiffness, grip length (the distance between the nuts at each end of an anchor bolt in a two-nut bolt system), snug-tight standards, lubrication and verification after 48 hours played a role in effective tightening practices.
To determine the impact of environmental and structural strain on bolt tightness, researchers monitored sign structures in the field and in the lab. They attached strain gages to the bolts and post of an overhead sign near Minneapolis-St. Paul and installed a wind monitor, camera and data logging unit nearby to collect strain and environmental data for four months. In the lab, they instrumented a post and baseplate mounted in concrete to compare current and proposed tightening specifications and practices.
Investigators developed specifications for each bolt size and grade, anchor baseplate dimension and pole size used by MnDOT based on lab and field results. They also created finite element models to analyze future anchor bolt configurations.
What Did We Learn?
Over- and under-tightening contribute to premature loosening of nuts on anchor bolts. While contractors may lack the experience and training to properly use turn-of-nut guidance, AASHTO recommendations poorly serve the bolt sizes and grades used by MnDOT.
AASHTO’s snug-tight guidance neglects certain characteristics of nuts and bolts, and its turn-of-nut direction is provided for only two bolt sizes and two bolt grades. In some cases, these standards may cause the heads of small bolts to break off and may lead to undertightening of large bolts. MnDOT can measure torque in the field but cannot determine tension, making AASHTO’s equation for verifying torque and tension impractical.
“We have revised our specs to follow the recommended procedures for anchor bolt tightening. The new tables of verification torque values will be fine for both two-nut and one-nut anchor bolt systems,” says Jihshya Lin, Bridge Evaluation and Fabrication Methods Engineer, MnDOT Bridge Office.
Researchers revised the specifications to require bolt lubrication, establish torque levels for snug-tight and specify turn-of-nut rotation after snug-tight for a range of MnDOT materials:
• Eight bolt sizes, ranging from ¾-inch diameter to 2.5-inch diameter.
• Five bolt grades.
• Nine baseplate thicknesses.
• 12 single- and double-mast pole types.
The new specifications provide torque levels in tables to verify the tightness for each bolt, plate and pole type, eliminating the need to run equations. To assist crews that are installing or retightening anchor bolts, researchers developed guidelines that include a compliance form with a checklist to direct crews through each step of the tightening process and ensure proper tension.
The new specifications and procedures should improve public safety and reduce the traffic control, manpower and equipment expenditures required to respond to prematurely loosened nuts. Continued monitoring of bolts installed and retightened under these specifications over time would help evaluate the new procedures.
A new implementation project is underway that will demonstrate these findings in the field. Researchers will also produce educational videos for training and demonstration to MnDOT personnel and contractors. Video topics will include:
Basic Concepts of Bolt Tightening
New Specified Procedures
Signals and Lighting
Additionally, researchers will provide one or more training sessions with training materials. Materials and videos will be posted on a website developed by the researchers.
A research implementation project completed by MnDOT’s Bridge Office shows that phased array ultrasonic 3-D scanning more accurately detects and measures corrosion on steel bridges than traditional methods. More accurate data will allow engineers to correctly evaluate bridge conditions, calculate safe load capacity and make better maintenance recommendations.
“The Phased Array Ultrasonic Testing System (PAUT) can acquire thousands more data points than can traditional methods in the same amount of time, which makes PAUT technology very useful,” said William Lee Nelson, a MnDOT bridge engineering specialist.
What Was the Need?
Corrosion on steel bridges results from exposure to environmental elements and deicing chemicals, and can lead to loss of steel thickness, with subsequent functional and structural issues. Regular inspection to detect and monitor fatigue cracking and other structural damage is critical to extending bridge performance and ensuring traveler safety on the approximately 13,000 bridges in Minnesota. While MnDOT is committed to improving its infrastructure, increasing costs of bridge inspections and maintenance have prompted the agency to seek innovative methods for performing inspections.
Bridge inspectors have been using conventional ultrasonic devices and hand measuring techniques to evaluate corrosion for many years. However, it is not always possible to obtain complete and accurate data using those methods. Accurate steel thickness and corrosion mapping data is critical for bridge engineers to correctly evaluate bridge conditions, calculate safe load capacity and make better maintenance decisions. Without quality data, bridge engineers may make recommendations that can lead to unnecessary and expensive repairs.
Newer versions of ultrasonic devices—such as the phased array ultrasonic testing (PAUT) system—use 3-D scanning technology to produce enhanced images and data. One of the advantages of PAUT devices over conventional ultrasonic models is that they provide thousands more data points, allowing engineers to more accurately measure steel thickness and predict maintenance issues and costs. Another benefit of PAUT devices is that they collect corrosion mapping data much more quickly than conventional ultrasonic devices, which improves safety and efficiency by reducing the time bridge inspectors spend on the bridge.
What Was Our Goal?
The goal of this project was to provide bridge inspectors with training and equipment to collect high-quality data by using the 3-D scanning technology of a PAUT system. The enhanced data would enable bridge engineers to make more accurate assessments of bridge condition and more cost-effective maintenance recommendations.
What Did We Implement?
Investigators reviewed the literature on projects evaluating PAUT systems and identified several studies that assessed these devices favorably. They selected an Olympus OmniScan SX PAUT system for use in this project and used the collected information from the literature review as a point of reference for their field observation testing.
How Did We Do It?
After MnDOT bridge inspectors were trained in the OmniScan PAUT system, they used it to obtain corrosion mapping data for four steel structures in Minnesota: the Sorlie Bridge (Polk County), the Baudette Bridge (Baudette), a high mast light (Duluth) and a test specimen from the Silverdale Bridge (Grant). The project team then compared the PAUT system data with data obtained from traditional (single-beam) ultrasonic methods and traditional field measuring methods.
What Was the Impact?
The comparison showed that the PAUT equipment provided more complete and more accurate corrosion mapping data than did the single-beam ultrasonic and traditional field measuring methods. Based on the findings of the literature review, field observations and the data collected, the project team noted other benefits of using PAUT technology for bridge inspection, including:
Accurately determines the thickness and section of structural steel members, allowing engineers to make better recommendations on load capacity.
Establishes baseline measurements to better predict maintenance costs.
Provides high-quality data that allows engineers to make better repair and maintenance recommendations to avoid unnecessary and costly repairs.
Collects inspection data quickly, resulting in time and cost savings for bridge inspectors in the field.
MnDOT will begin deploying the PAUT system to conduct corrosion inspection of steel bridges and ancillary structures throughout Minnesota. MnDOT will also update the nondestructive testing content in MnDOT’s Bridge and Structure Inspection Program Manual.
Additionally, MnDOT plans to develop and write inspection procedures for the PAUT system and to distribute information about PAUT deployment, targeting MnDOT bridge inspection units, bridge engineers and bridge owners.
By load testing part of a bridge that was removed over Nine Mile Creek, researchers have proven that an innovative and cost-effective method for repairing damaged bridge girders restores them to their original strength.
The findings will help MnDOT and other transportation agencies avoid lengthy traffic closures and more costly techniques when repairing other bridges.
“This innovative method works remarkably well. The amount of damage the crew repaired was pretty extensive. In the end, the strength of the repaired damaged girders was slightly more than the strength of the undamaged girders,” said Carol Shield, Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.
The salting of bridge and roadways during Minnesota winters can create highly corrosive conditions that damage bridges. Such was the case with the Highway 169 Nine Mile Creek Bridge near Edina and Minnetonka, where leaking expansion joints caused corrosion to elements responsible for the strength of bridge girders: shear reinforcement, prestressing strands, and the surrounding concrete.
During a 2013 repair, crews encountered two locations of severe beam deterioration. To repair these areas, MnDOT used a novel method developed in Michigan that involved removing deteriorated concrete and cleaning the area, placing steel reinforcement cages around the damaged beam ends and then encasing the beam ends with concrete. The repair concrete was a specific form of concrete placement called “shotcrete”—a mix of sand, aggregate and cement that is applied with a hose that is wetted at the nozzle before the mixture is sprayed at high velocity onto the repair surface. When the desired thickness of the concrete placement is reached, the placement is troweled and shaped to finish to the desired cross section. The beam end repairs were made in October 2013 and allowed the bridge to continue its function to the public.
MnDOT was able to make the repairs without traffic interruption.
Several years later, the bridge was scheduled for replacement. The repaired girder ends appeared to be in good condition, but the repair technique had not been studied for strength. The bridge replacement presented MnDOT with an excellent opportunity to evaluate the repair method for use on other damaged girder ends.
What Was Our Goal?
When the southbound lanes of the bridge were taken out of service in spring 2017, four prestressed girders were removed from the structure and brought to the University of Minnesota’s Theodore V. Galambos Structural Engineering Laboratory for testing.
Researchers examined and tested the beams to evaluate the effectiveness of the reinforced shotcrete repair method.
“Two of the girders have ends that were repaired by MnDOT, and two girders have ends that never needed to be repaired,” Shield said. “We [tested] the four girders and [compared] their strengths to determine if the repair actually returned the girders to the strength they had prior to the corrosion-related damage.”
The fact that researchers tested good girders alongside repaired girders gave MnDOT a high level of confidence, said Paul Pilarski, Metro North Regional Bridge Construction Engineer, MnDOT Bridge Office.
Bridge girder ends can be repaired for only $5,000 to $10,000, using this new method.
What Did We Learn?
All repairs had been done in field conditions that have the potential to adversely affect the results. But when the beams broke in the lab, the shotcrete repair did not separate from the bonding surface. The repaired reinforced concrete beam ends were found to be at least as strong as similar beams that were in good condition and had not needed repair. The initial repair methods and subsequent testing of the prestressed beam ends are demonstrated in a video created by the research team (testing starts at 3:30 min).
Using this method, severely deteriorated beam ends can be repaired with reinforcement cages and shotcrete for $5,000 to $10,000. The alternative to this type of repair involves constructing a new beam, closing traffic, removing the bridge deck over the damaged beam as well as the beam itself, and recasting the bridge deck and barrier—an intrusive replacement that costs hundreds of thousands of dollars and more than a month of bridge lane closures.
Results have been presented internally at MnDOT, at state and Midwest conferences in late 2017, and at the National Bridge Preservation Partnership Conference in April 2018. Presentations have impressed transportation engineers from around the country and have increased confidence in dealing with aging infrastructure. MnDOT will continue to refine repair methods with the shotcrete treatment based on best industry practices, and will continue to use the beam end repair method if similar conditions are encountered in the state.
MnDOT bridge inspectors often have to find out what lies beneath the surface of Minnesota’s rivers. Thanks to new sonar inspection technology, the Bridge Office now has a way to see previously hidden riverbed floors and underwater bridge structures in far better detail than ever before.
Typically, bridge engineers turn to professional divers to provide information about what’s underwater. But diving inspections don’t always deliver precise information about bridge damage, debris and riverbed topography.
Of the 11,183 Minnesota bridges that span waterways, about 585 require underwater inspection.
In recent years, bridge inspectors turned to underwater inspection technologies to identify areas of interest and direct divers who can inspect hands on. In turbid, sediment-heavy conditions with low visibility such as the Mississippi River, non-optical technologies – laser, radar and sonar – offer safe and useful options. Sonar gathers underwater acoustic data into point clouds for imaging two-and three-dimensional models of conditions.
In winter 2014, a vendor demonstrated the use of sonar at the Third Avenue bridge over the Mississippi River in Minneapolis, where a void was previously discovered during a diving inspection. Acoustic investigation of the frozen-over site delivered a three-dimensional image of a scoured cavity of eroded concrete under a pier in conditions unsuitable to diving inspection. Later that year, the Bridge Office purchased its own three-beam sonar unit with funds secured from MnDOT’s research implementation program. MnDOT is the nation’s first state transportation department to use the technology, according to Petra DeWall, waterway engineer.
The new 3D scanning technology provides much better information than divers can. Nicki Bartelt, hydraulic design engineer, said divers can see up to 2 feet in front of them in good rivers, so most of the work is done by feel.
With the new technology, Bartelt and her colleagues receive a 3D image to base decisions on. Diver reports included only narratives and rough sketches.
“It’s like night and day,” said Nicki Bartelt, hydraulic design engineer. “It’s a picture, but it’s more than that. It’s a point cloud. It’s totally scalable. It is real-world elevations and dimensions. It’s like the difference between Google Earth and a paper map.”
Although the information is far better from these scans, federal bridge inspection standards still require hands-on inspection of bridges, including substructures above and below the water’s surface. For now, the scans will be used to augment diver inspections and other purposes.
“We’re looking to augment with scans to make it safer for the divers,” DeWall said.
The research implementation team identified a host of lessons and best practices after purchasing and testing the new equipment.
“This does have a pretty steep learning curve,” Bartelt said. “It’s not something you can just buy and use. You have to learn how to use it.”
Getting different pieces of the equipment set up and able to communicate with each other was the first difficult step. Field testing also identified the need for a dedicated generator to provide a consistent portable power source because of difficulty with batteries.
MnDOT will develop and publish an underwater imaging policy and reach out to districts, counties, cities and other bridge owners to promote its imaging capability. The hydraulics unit will develop data on completed projects, generate a list of bridges that suit underwater imaging and ensure field personnel are trained to use imaging techniques and inspection.
“I think it’s really exciting, because it opens your eyes to what’s going on in the river,” said DeWall. “We always just assumed before, but now we can see it and document it. The fact that you have the ability to rotate the picture and move it around, and zoom through it makes a huge difference.”
These videos explain the sonar inspection technology:
Low-cost, low-maintenance mussel spat rope can help small fish species navigate through culverts by reducing current velocity and providing protected areas for fish to shelter and rest. Recent research in New Zealand demonstrated the effectiveness of mussel spat rope—rope with long, dense fibers used in mussel aquaculture— to assist small species fish passing through steep, perched or high-velocity culverts. The successful results from this research led MnDOT to investigate mussel spat rope as a method to facilitate fish passage in Minnesota’s culverts.
“Minnesota is a headwater state, and we have a responsibility to keep our fish population healthy. Mussel spat rope will be one more effective tool in the toolbox of methods we have to assist fish passage through culverts,” said Petra DeWall, Bridge Waterway Engineer, MnDOT Bridge Office
What Was Our Goal?
The objective of this project was to determine whether mussel spat rope was an appropriate and effective tool in helping small fish species pass through Minnesota culverts.
What Did We Do?
Investigators conducted a literature review to evaluate previous studies. Then researchers from St. Anthony Falls Laboratory conducted experiments in the laboratory and in the field to investigate the use of mussel spat rope as a fish passage aid.
Hydrodynamic performance. Hydrodynamic performance tests were performed in a 20-inch-wide by 30-foot-long flume fed by water diverted from the Mississippi River into the laboratory. Researchers measured velocity, depth and water surface slope, and sediment accumulation around arrays of ropes. They installed single- and multi-rope configurations and examined many variations of flume flow and depth, recording the rope’s effects on water velocity and turbulence.
In a second experiment, researchers released fine sand into the flume containing two- and four-rope configurations to investigate the rope’s effect on sediment transport. Because the ropes slowed local water velocity, deposits were observed on, between and under the ropes in two different depth tests after one and two hours of sediment feed.
Rope durability, performance and use by fish. Researchers installed mussel spat rope in two Minnesota box culverts: one in the northeast serving a fast current trout stream and one in the southwest serving a slow current prairie stream in critical fish habitat. Double strands of mussel spat rope were installed near a wall in each culvert and examined many times for approximately two years. Each observation included photographic and video recordings of the installations.
Small fish species’ interaction with the rope. Laboratory investigations of fish behavior with the rope were conducted in a 5-foot-wide by 32-foot-long flume with a raised section representing a box culvert. Two Plexiglas windows allowed viewing. Researchers installed two sets of double-strand ropes along a wall, similar to those in the field sites. Four video cameras tracked motion, recorded overhead views of the flume and captured fish behaviors at the midpoint and ends. Researchers used three species of small fish common to Minnesota: fathead minnow, white sucker and johnny darter. Five fish were released into the test area at a time and allowed to swim for an hour. Their progress and behavior were filmed and analyzed.
What Did We Learn?
Key observations from these investigations follow:
Mussel spat rope created small corridors (about 6 inches) of reduced velocity and turbulence along its length, which was sufficient to aid the passage of small fish. Sediment collected in, between and beneath the ropes. The presence of culvert floor sedimentation may assist fish passage.
The rope displayed wear over two years in the field, raising a concern about plastic microparticle release into streams. Sediment covered some ropes over time, suggesting a need for maintenance in some culverts. Only a few fish were observed at the field installations.
In the laboratory flume, test fish swam near and between doubled rope lengths, apparently taking advantage of the reduced current near and beneath the ropes. While there was variation among species, most fish that swam upstream through the simulated box culvert ended their passage on the rope side, evidence that the rope provided cover and refuge from the current.
Mussel spat rope will be a low-cost, low-maintenance tool to help small fish pass through culverts. The final report for this study includes guidance for installing the rope. The low-cost method will also be included in an upcoming guide for designing culverts that allow aquatic organism passage.
Darkness box culverts does not present a complete barrier for southwestern Minnesota fish species, according to a new MnDOT study. The findings will reduce the cost and delay of future box culvert replacement projects.
“This research will allow MnDOT to save both time and money when replacing other box culverts in southwestern Minnesota by eliminating the need for a fish passage study for each one,” said Scott Morgan, Principal Hydraulics Engineer, MnDOT District 7.
In this study, researchers developed several objectives in their efforts to assess the effect of low light levels on fish passage through replacement box culverts. As part of this effort, they wanted to determine typical light levels in the replacement culvert and other box culverts in the region. They also sought to determine if the Topeka shiner and other fish move through culverts in the same numbers they pass through control areas in the same stream, and whether light levels affect frequency of movement. Finally, if a barrier is determined, researchers sought to design or recommend a method for mitigating light in the culvert.
What Did We Do?
In the field, researchers characterized light in long box culverts (at least 8 feet by 8 feet) by collecting many light levels with a light meter at the water surface within the three culverts and at control reaches. They also measured light levels within the water column to characterize the light conditions a fish would experience.
To determine whether Topeka shiners passed through culverts in similar numbers as through control reaches of the same stream, and whether light levels affected their passage, researchers employed a mark-and-recapture process. They caught fish upstream and downstream from the culverts or control reaches, marked them with an identifier indicating where they were caught and released, and then resampled to see where fish moved. They also noted other culvert features that could affect passage, such as water depth and velocity.
To control for confounding variables that could affect fish movement, a laboratory study measured Topeka shiner preference for light or dark channels. Researchers used a 25-foot-long double channel box with water diverted from the Mississippi River. Fish could choose to swim along light or shaded lanes as they preferred in this light manipulation experiment.
What Did We Learn?
Although there has been increasing concern over the potential for culverts to create behavioral barriers for fish and other organisms, this was the first study that quantified these behavioral effects for fish passage. Light levels in large box culverts were not identified as a potential barrier to the fish communities present in southwestern Minnesota. Two out of the three culverts monitored showed reduced fish passage compared to the control reaches; however, fish—including Topeka shiners—were able to pass through all three.
The longest and darkest culvert had the greatest difference in movement between the culvert and the control, but this variation could not be attributed solely to light levels. This finding was supported by experiments at the St. Anthony Falls Laboratory, where fish that could select either a shaded or lighted channel showed no avoidance of the shaded channel regardless of the shading level.
The light measurements in three culverts yielded an extensive data set that can be used to model light levels through culvert barrels. Light levels at the water surface depended on the culvert entrance, dimensions, construction material, orientation and elbows, while light levels in the water column were also affected by turbidity.
The conclusions of this study apply only to Topeka shiners and other small warm water fish species, and to large box culverts like those studied. Additional research is required to investigate possible barriers created by smaller, darker culverts to light-sensitive fish species and the interactions between light and other variables such as velocity.
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.
MnDOT has developed a guide that compares traditional and enhanced culvert inspection methods and tools, their limitations and costs. The guide also includes best practices for identifying when conventional inspection methods work best and when enhanced technologies may offer good value.
“We wanted to document how far you can see into the pipe to get a good inspection and when more than an end-of-pipe inspection was needed. We found that there are some cost-effective options for doing more than end-of-pipe inspections,” said Andrea Hendrickson, State Hydraulic Engineer, MnDOT Office of Bridges and Structures.
“Inspection crews need to understand what type of data they want to gather for each situation, and then balance the quality of data required with the cost of the inspection method,” Doug Youngblood, Environmental Engineer, CDM Smith.
What Was the Need?
MnDOT manages more than 100,000 culverts in the state’s highway culvert system. Culverts are inspected routinely to monitor corrosion and other damage that could lead to expensive repairs and highway closures.
New culverts are inspected to confirm that construction measures up to specification. Centerline culverts, which run from one side of the road to the other under pavement, must be inspected every two to six years. MnDOT also inspects culverts in emergencies or when the public notifies the agency of potential damage or blockage.
Inspection typically begins with an end-of-pipe visual investigation, usually aided by flashlight or occasionally by a camera placed in the pipe. If pipes are large enough, inspectors enter the pipe to examine the walls and measure corrosion or other damage, take photos and conduct hands-on examinations.
But not all culverts are large enough for human access, and inspecting damaged or failing culverts can be dangerous. New, enhanced technologies may offer valuable, safer inspection options.
What Was Our Goal?
This project aimed to review common inspection technologies available for culvert and pipe inspection. The results of this review would then be used to develop guidance for choosing a cost-effective inspection strategy that was appropriate for the site and would provide the required data.
What Did We Do?
The research team began by reviewing literature related to culvert inspection best practices. The team then interviewed inspectors from various Minnesota counties and MnDOT districts, and from five other state transportation agencies to gather additional information on best practices.
Next, investigators reviewed 12 videos of MnDOT inspections performed from 2011 through 2016 and then contracted with a robotics inspection firm to conduct end-of-pipe, laser ring and video inspections of 10 MnDOT culverts that represented a range of sizes, pipe materials and on-site conditions. The results from the three inspection methods were compared to identify best practices, which were incorporated along with the best practices from the literature review and interviews in the Enhanced Culvert Inspections— Best Practices Guidebook.
What Did We Learn?
The guide describes traditional and enhanced inspection technologies and methods, their limitations, costs and best uses for specific situations. Each method offers distinct advantages and disadvantages. End-of-pipe inspection costs about 7 cents per foot, and enhanced inspections cost from 23 cents to $6.50 per foot. Before using enhanced methods, inspectors should have a firm grasp on the quality of data and detail required to best optimize their choices and budget limitations.
End-of-pipe inspections are the fastest and least costly of the methods, but provide the least data. Typically, an inspector with a flashlight can investigate from 5 to 30 feet inside the culvert from the end of the pipe. These inspections work well for determining work conditions and data needs.
Measurement-based inspections include traditional and enhanced methods, including person-entry inspections, hammer sound testing and coring, mandrels and multiple- sensor units such as laser and sonar profilometers. Laser ring scanning offers precise measurement and excellent quantitative data on culvert alignment and geometry. Multiple-sensor units are the most expensive inspection method based on cost per foot and time to process the data, which often takes weeks.
Video inspection typically entails the use of closed-circuit television (CCTV) cameras or consumer-level video from a Hydraulic Inspection Vehicle Explorer (HIVE). MnDOT owns several of both units, which incur labor costs of about 23 cents per foot. CCTV is a national standard for inspection. It offers permanent records with familiar technology; however, lighting, image centering, lens clarity, cumbersome data volumes, and opera-tor training and experience present challenges.
The HIVE is a remotely operated crawler equipped with off-the-shelf cameras and accessories. Developed by MnDOT District 6, the HIVE takes lights and a video camera that is capable of panning and tilting inside a culvert and transmits data wirelessly to a tablet computer. While CCTV offers better measurement ability, a HIVE is lighter, easier to transport and easier to operate. Given that contractor-run CCTV typically costs $2 per foot, the cost of using 750 feet of CCTV would pay for a HIVE.
MnDOT will monitor developments among local contractors, as no Minnesota firms currently offer multiple-sensor inspection capability. MnDOT owns a sonar scanner for use on tripods and floatable platforms, and also owns a laser ring inspection unit. Pilot testing and training may make these options cost-effective. Researchers recommend further development of the MnDOT-developed HIVE, including a foam floating platform and a snap-on laser ring scanner for the camera.
“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.