Policy and Planning, Traffic and Safety

New performance measures identify truck delays and bottlenecks

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A new freight transportation study takes the next step in lessening traffic bottlenecks by pinpointing location and time of recurrent delays.

Freight transportation provides significant contribution to our nation’s economy. Reliable and accessible freight network enables business in the Twin Cities to be more competitive in the Upper Midwest region. Accurate and reliable freight data on freight activity is essential for freight planning, forecasting and decision making on infrastructure investment.

Researchers used detailed and specific data sets as tools to investigate freight truck mobility, reliability and extent of congestion delays on Twin Cities metropolitan area corridors. Precise locations and times of recurrent delays will help to mitigate future traffic bottlenecks.

“This research provided tools and metrics with new levels of precision concerning truck congestion. The results will allow us to take the next steps toward future investment in addressing freight bottlenecks,” said Andrew Andrusko, Principal Transportation Planner, MnDOT Office of Freight and Commercial Vehicle Operations.

What Was the Need?

The corridors of the Twin Cities metropolitan area (TCMA) provide a freight transportation network that allows regional businesses to be competitive in the Upper Midwest. However, traffic volumes on many of these roadways are facing overcapacity during peak travel periods. Heavy truck traffic is only expected to increase, and delays will continue to disrupt freight schedules.

A 2013 study by MnDOT and the Metropolitan Council suggested the need to identify when and where truck congestion and bottlenecks developed in the TCMA. Previous research funded by MnDOT examined heavy truck movement along 38 Twin Cities freight corridors. Researchers created freight mobility and reliability measures, and worked to identify significant bottlenecks. Further research was needed to extract more precise data to better understand TCMA freight traffic congestion.

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The top five congested AM and PM peak corridors in the TCMA are listed above with the delay hours for each period. The large delay hours arise from heavy truck volume and speeds far below base free-flow speeds.

What Was Our Goal?

The aim of this project was to combine data from the U.S. DOT National Performance Management Research Data Set (NPMRDS) with information from other sources to build on the previous study’s analyses of mobility, reliability and delay along key TCMA freight corridors. New performance measures would more clearly identify the extent of system impediments for freight vehicles during peak periods in selected corridors, allowing researchers to identify causes and recommend mitigation strategies.

What Did We Do?

Researchers worked with stakeholders to prioritize a list of TCMA freight corridors with NPMRDS data coverage. The NPMRDS includes travel time data from probe vehicles at five-minute intervals for all National Highway System facilities. The travel times are reported based upon Traffic Message Channel (TMC) segments with link lengths varying from less than 1 mile to several miles. Researchers worked with 24 months of NPMRDS data from the selected corridors.

Because of varying TMC segment lengths, researchers used geographic information system (GIS)–based data to georeference the NPMRDS data to relevant maps. Combining these with average travel time data from passenger and freight vehicles, researchers used their data analysis framework to generate measures of truck mobility, reliability and delay at the corridor level.

A truck mobility analysis of all the selected corridors was performed using the truck-to- ar travel time ratio (TTR) for each TMC segment of each five-minute interval computed in AM (6-10 a.m.), midday (10 a.m.-4 p.m.) and PM (4-8 p.m.) peak periods using the 24- month NPMRDS data. A TTR of 1 describes a truck and a car traveling a distance in the same amount of time. On average, trucks are known to travel 10 percent slower than cars on freeways: a TTR of 1.1. A truck traveling 20 percent slower would have a TTR of 1.2.

Reliability measures evaluated the truck travel time reliability. Researchers computed truck delay during rush hour on the GIS network by fusing truck volumes, posted speed limit and NPMRDS data.

Researchers computed a truck congestion measure by comparing truck travel time with the target travel time in each TMC segment, which provided a measure of delay (in lost hours) at the segment and corridor level.

What Did We Learn?

The truck mobility analysis revealed that roadways with intersections have a higher TTR. Trucks on U.S. and Interstate highways take about 10 percent longer to travel the same distance as cars: TTR 1.1. On state highways, the TTR reaches 1.2 and 1.4 in the AM and PM peak periods, respectively. On county roads, trucks slow considerably: midday TTR is 1.5 and spikes to 1.7 and 1.9 in the AM and PM peak periods. Intersections in a TMC segment and delays at signalized intersections could have caused the TTR increases.

All reliability measures indicated that truck travel time in the PM peak period is less reliable than in the AM peak period. Similar to the TTR measure, roadways with signalized or unsignalized intersections were less reliable for truck traffic than freeways.

Truck congestion and delay measures revealed that the top five TCMA corridors with significant congestion had an average delay of over 3,000 hours in the AM and PM peak periods, with the PM delays notably greater. Also, in the AM peak period, eight additional interchanges had average delays of over 300 hours per mile. In the PM peak period, nine interchanges and eight segments showed significant congestion.

The top six TMC noninterchange segments exhibiting recurring PM peak period delays on average weekdays had delays ranging from 495 hours to 570 hours per mile.

Insufficient capacity, increasing demand, roadway geometry and density of weaving points (on-and off-ramps) were considered key causes of delay among these six bottlenecks.

What’s Next?

NCHRP Research Report 854, Guide for Identifying, Classifying, Evaluating and Mitigating Truck Freight Bottlenecks, provides guidelines for identifying, classifying, evaluating and mitigating truck bottlenecks. Follow-up research by MnDOT could potentially leverage this project’s effort with the NCHRP guidelines to develop mitigation strategies.

This post pertains to Report 2018-15, “Measure of Truck Delay and Reliability at the Corridor Level, published April 2018.

 

Materials and Construction, Traffic and Safety

Prioritizing Pavement Markings on Low-Volume Roads

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Researchers have developed a tool to help Minnesota local agencies make cost-effective pavement marking decisions in their counties. The spreadsheet-based tool was developed as part of a recently completed research study by the Minnesota Local Road Research Board.

What Was the Need?

Minnesota has many miles of low-volume roads, most marked with yellow centerline and white edge lines. Applying and maintaining these markings is a significant financial investment for local agencies, which typically work within very constrained budgets. These agencies needed more information about the value and the initial and ongoing costs of typical 4-inch and enhanced 6-inch pavement markings on low-volume roadways. They also needed clarification and guidance for prioritizing pavement marking installation and maintenance that could work within their limited budgets.

What Was Our Goal?

The goal of this research was to develop a prioritization approach and a decision-making tool for using pavement markings on low-volume roads based on the benefits and costs of these markings. Local agencies could then use these resources to make cost-effective decisions about installing and maintaining pavement markings.

Rural road with cyclist in bike lane
This segment of Minnesota Highway 38 has yellow centerlines and white edge lines that delineate a 4-foot bicycle lane

What Did We Do?

Researchers took a multistep approach to identifying critical pavement marking information and practices:

• Conducted a literature search of existing research on typical (4-inch) and enhanced (6-inch) pavement markings, focusing on the benefits (such as crash reduction and improved lane-keeping), costs and current maintenance practices.
• Surveyed Minnesota counties to learn about their current practices and management approaches for pavement markings.
• Reviewed existing County Road Safety Plan (CRSP) methodology to learn about research and data used to rank at-risk road segments and identify CRSP improvement strategies, specifically the range of pavement markings that CRSPs recommended.

Researchers were then able to develop a prioritization approach and a decision-making tool that incorporated both past research and local state of the practice. In addition to producing a final report describing task results, they developed a brochure explaining the approach, the tool and implementation steps.

“This innovative tool will help local agencies make pavement marking decisions under tight budget constraints, where the question is always how to best allot funds for competing needs. This tool clarifies the problems and helps prioritize the possible solutions,” said David Veneziano, LTAP Safety Circuit Rider, Iowa State University Institute for Transportation.

What Did We Learn?

The literature search revealed limited research addressing traditional pavement marking use and effectiveness on local roadways. Pavement markings produce safety benefits, including reduced crash rates, but showed no real effects on vehicle speed, indicating that pavement markings may not alter driver behavior. Only limited efforts were identified in the literature aimed at investigating the prioritization and management of pavement markings.

The survey of local Minnesota agencies revealed that most counties use centerline and/or edge lines, which may be the result of MnDOT State-Aid Operation Rules. Some counties mark all their roads; most use 4-inch latex paint or epoxy markings. Repainting schedules depend upon road age, marking condition and county budgets.

A review of Minnesota counties’ CRSPs showed they included pavement marking recommendations. The CRSPs recommended, on average, 109 miles of pavement markings in every county. Applying one linear foot of centerline costs about 5 cents; 100 miles of centerline cost $26,400. Because of the extent of these recommendations, researchers directly incorporated the methods and directives from the CRSPs into their prioritization approach and tool.

The spreadsheet tool produced through this project allows users to enter road site characteristics such as pavement condition, road width, the CRSP rating and traffic volume, as well as the age of extant markings, costs, durability and the potential for crash reduction. Pavement marking options include centerline and/or edge lines, high visibility markings and enhanced durability materials. The tool uses factor weights that assign a relative importance to each criterion for any potential marking approach compared to other alternatives. The result is a performance rating score for each marking alternative. Thus, the tool assists not only in identifying the physical aspects of a road segment, it also incorporates the agency’s preferences, priorities and budget through a priority-weighting feature that generates the cost or cost range for a marking project.

What’s Next?

Recommendations for further research include conducting a follow-up survey of users
of the new spreadsheet tool to facilitate future modifications, creating databases of roadway characteristics to simplify agencies’ use of the tool, and performing additional research on the safety and other effects of pavement markings. Researchers also encouraged agencies to keep in mind a proposed national retroreflectivity rule for the Manual on Uniform Traffic Control Devices that could affect pavement marking practices on low-volume roads. This rule has not yet been finalized or implemented.

This post pertains to LRRB-produced Report 2018-21, “Investigating the Necessity and Prioritizing Pavement Markings on Low-Volume Roads,” published June 2018. The Pavement Marking Prioritization Tool can also be found on the project webpage on the LRRB website.

Environment, Maintenance Operations

Carver County Evaluates Pickle Brine for Ice Control

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Like many snow- and ice-control professionals, Carver County Public Works maintenance operations staff are searching for new options to reduce the amount of chloride that reaches our waters from road salt operations. Using food production byproducts such as pickle brine are among the alternatives maintenance staff have been exploring.

Carver County regularly uses salt brine as part of their winter maintenance operations, which has become a widely accepted practice for controlling snow and ice. In the right situation, salt brine can be a more effective alternative to traditional road salt. An opportunity to obtain a free supply of sodium-rich pickle juice from a nearby canning facility seemed like a natural candidate worthy of consideration as a source of brine for county anti-icing and de-icing operations. In addition, recycling the pickle brine could reduce the amount of the waste byproduct.

The Carver County Public Works Department began testing samples of the pickle juice in 2016 with some encouraging results. But further testing showed the brine from the pickle cannery had variable salinity and pH levels that could damage maintenance equipment. Given the variables involved, staff determined it would be difficult to manually control the manufacture of the brine into a usable liquid. VariTech Industries recommended purchase of the Brine Boss, an automated brine blending system to manufacture the 23.3 percent brine solution needed for effective ice control operations. In addition, staff found adding potassium hydroxide to pickle brine neutralizes the pH level.

Carver County staff received a grant through the Local Road Research Board‘s Local Operational Research Assistance Program (OPERA) to help purchase the brine making system and support further research into the viability of using pickle brine in winter maintenance operations. Staff also restored an old VariTech 600 brine maker obtained from MnDOT for the project.

After extensive testing and analysis, VariTech engineers and Carver County staff concluded that pickle brine acquired from the cannery had to be exactly the same (salinity, vinegar content, and sugar content) for each and every batch or the system sensors would fail. But it turned out that the pickle brine supplier could not provide chemically consistent batches, and the VariTech system was unable to produce a consistent blend of 23.3 percent brine solution using pickle brine. As a result, Carver County staff determined they were unable to continue using pickle brine for snow and ice control.

Nevertheless, this project benefits other agencies considering the use of food production byproducts. The Carver County project demonstrates that there can be an alternative anti-icing product. As technology continues to advance, Carver County may revisit the use of pickle brine as a viable snow- and ice-control option.

For additional information about the project, check out these resources:

Materials and Construction, Traffic and Safety

Nontraditional Fog Seals Offer Value, Limitations Compared to Traditional Seals

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MnDOT conducted field and lab analyses of nontraditional fog seals used by local agencies around the state. Results show that agriculture-based bioseals offer value that must be balanced against temporary reductions in retroreflectivity and pavement friction. Bioseals offer greater friction and visibility than traditional fog seals.

“There is some value to the bioseals. They seal the pavement, and they’re clear so they have a minimal effect on striping. These applications are appropriate in certain areas,” said Bruce Hasbargen, County Engineer, Beltrami County.

What Was the Need?

Maintenance crews often spray pavement surfaces with a “fog” of liquid sealant after pavement has been in service for a year or more. These fog seals extend the water resistance of asphalt and protect pavements from oxidation.

Fog seals wear off after a few years, but can be inexpensively reapplied. The seals lengthen maintenance cycles, protecting asphalt between activities such as crack repair and surface treatment. Traditional fog seals, however, are dark, asphaltic mixtures that obscure pavement striping and reduce the reflectivity of materials. Fog seals also reduce friction, and so typically suit pavements with low-speed service conditions.

In recent years, city and county road agencies in Minnesota turned to bioseals—agriculture-based, clear liquids that manufacturers claim seal pavement against oxidation and water damage without concealing pavement markings. Bioseals are currently not less expensive than petroleum industry products, and little independent work had been performed to identify performance benefits.

What Was Our Goal?

To provide local agencies with more information about bioseal performance, the MnDOT Office of Materials and Road Research studied selected bioseal products in the lab and in the field (MnROAD test site pictured above), comparing them to traditional seals to determine product performance, durability and impact on friction and pavement marking visibility.

What Did We Do?

Following a literature review of fog seal treatments, investigators selected four seals for analysis: a traditional asphalt-emulsion sealer; a nontraditional, polymerized maltene emulsion longitudinal joint sealant (Jointbond); and two soy-based bioseals (RePlay and Biorestor). These seals were applied in 2014 to 8-foot shoulder sections built in 2013 on County Highway 75 in Wright County, north of Monticello. Seals were sprayed on shoulders outside painted markings, in shoulder space where investigators applied geotextile patches and strips of highly reflective striping tape commonly used on some roads. Untreated shoulder areas of 500 feet and 1,320 feet served as control sections.

Reflective marking tape on road shoulder
Researchers placed a swatch of geotextile and reflective pavement marking tape on shoulders before the shoulders were sprayed with nontraditional fog seals. Investigators then moved the textile and tape to MnROAD to study application rates and stripe performance.

After spraying, investigators removed the geotextiles to evaluate the quality of application work by bioseal distributors. They also removed some striping tape and reapplied it as shoulder striping to Cell 33 at the MnROAD test facility, where they could reliably monitor traffic passes over the biosealed markings and evaluate retroreflectivity over time. At the Wright County site, researchers examined pavement distress, friction properties and permeability on the shoulders for three years.

Lab studies included testing seal residue and stiffness in field-aged cores taken from the sealed test sections in year three. Finally, in year three researchers surveyed local agencies in Minnesota about their use of nontraditional fog seals.

What Did We Learn?

Geotextile coating levels showed that vendor application of bioseals is consistent and well-executed. Nontraditional seals do not obscure striping, but bioseals leave residue that temporarily reduces the retroreflectivity of sealed markings to below MnDOT-required levels. Acceptable levels of retroreflectivity returned to the Jointbond samples after 800 truck passes at MnROAD, and to Biorestor and RePlay samples after 1,600 truck passes.

Every tested seal reduced pavement friction. Recovery of friction for the three nontraditional products, which reduced friction by 11 to 17 percent, took about 200 days with no traffic. The traditional, asphaltic fog seal reduced friction by 67 percent and took longer to recover, remaining slippery for turning in wet conditions for over two years.

“Bioseals affect pavement friction, so agencies need to use some caution when using them. City streets are probably going to be very good for nontraditional seals,” said Eddie Johnson, Research Project Engineer, MnDOT Office of Materials and Road Research.

Each seal reduced pavement permeability for about two years; after two years, only the traditional seal continued to provide water protection. The permeability benefit of fog seals lasts significantly longer than the retroreflectivity reduction; when reflectivity recovers, the seals still provide water resistance. Field surveys also found that Biorestor and RePlay may help resist cracks.

Laboratory studies showed that high-temperature stiffness for every treatment was greater than control samples in the top layer than in the middle of cores, suggesting that seals may improve rut resistance of treated pavements in hot weather. Low-temperature stiffness was higher in the top sections for every treatment except the traditional fog seal.

Of the 57 agencies that responded to the survey, 32 have used nontraditional fog seals, preferring Biorestor and RePlay to others. Over half of these users recommend the use of such seals; responses suggest that bioseals offer sealing benefit for two years and, in some cases, up to six years.

What’s Next?

Nontraditional fog seals protect pavements from water and may help prevent cracking. Traditional seals offer longer-lasting water resistance, but also longer-lasting and greater friction reduction. Agencies must consider temporary reductions in retroreflectivity and friction for any seal, and may wish to continue using fog seals only in lower-speed environments.

Continued monitoring of applications would be helpful in determining long-term performance. The study observed that overlaying biosealed asphalt with a traditional fog seal should be effective in extending permeability.

This post pertains to the LRRB-produced Report 2018-18, “Nontraditional Fog Seals for Asphalt Pavement: Performance on Shoulder Sections in Minnesota,” published May 2018.  

Materials and Construction, Traffic and Safety

Concrete Design Software Easier-to-Use, Capabilities Expand

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MnDOT has upgraded its concrete pavement design software, MnPAVE-Rigid, to make it easier to use and allow more design inputs.

“In the original software, we only allowed one aggregate base thickness and one aggregate type. MnPAVE-Rigid 2.0 allows two base thicknesses and three base types,” said Tim Andersen, Pavement Design Engineer, MnDOT Office of Materials and Road Research.

MnDOT hired American Engineering Testing to update the design software as part of a research project advised by Andersen and funded by the state research program.

Background

MnDOT developed its own pavement design software, MnPAVE-Rigid, in 2014 that incorporated the methodology of the American Association of State Highway and Transportation Officials (AASHTO) Mechanistic–Empirical Pavement Design Guide (MEPDG). Minnesota’s pavement designers use MnPAVE to apply AASHTO’s most sophisticated design principles for both rigid and flexible pavement, focusing on mechanical properties of the pavement and prevention of early cracking and other distress.

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AASHTO’s mechanistic–empirical (M–E) design methods entail hundreds of inputs, each a mechanical parameter, a measure of site-specific characteristics or a design goal. To simplify the input selection process, AASHTO’s M–E design software offers various input levels to reduce the data gathering and input burden. The most basic level uses default values for most of the inputs based on national averages, but still requires dozens of inputs for the number of pavement layers, traffic expectations, climate and other features.

MnPAVE-Rigid for concrete pavement design reduced that number of inputs to nine, operating like a module of AASHTO’s M–E software. MnPAVE-Rigid inputs work with a set of default values for jointed plain concrete selected by the MnDOT Office of Materials and Road Research in 2014, as described in the MnPAVE-Rigid 1.0 report.

“Many states ignored the challenge of adopting AASHTO M–E or they bought an AASHTO
software license. MnDOT used its accumulated knowledge of AASHTO M–E and Minnesota conditions to build MnPAVE-Rigid, and so can account for its M–E design results firsthand,” said Derek Tompkins, Principal Civil Engineer, American Engineering Testing, Inc.

Since implementing MnPAVE-Rigid 1.0, MnDOT has gathered feedback from users about their experience with the software. In the current project, MnDOT wanted to address this feedback, and expand and improve the original software by exploring additional options with some of the default parameters for concrete pavements.

What Was Our Goal?

The goal of this project was to update MnPAVE-Rigid 1.0 by expanding the range of inputs for traffic, subgrade type, base type and thickness, and to make the user interface more accessible.

What Did We Implement?

MnPAVE-Rigid 2.0 allows users to enter 11 inputs, including inputs related to specific traffic levels and aggregate base types; calculate the new design thickness; and print a project report that summarizes the inputs and the recommended thickness. The upgraded software is more user-friendly, and MnDOT can maintain or make future upgrades to the source code.

How Did We Do It?

Researchers met with the Technical Advisory Panel and reviewed the list of software improvements requested by pavement designers and the MnDOT Office of Materials and Road Research.

Because every change to an input affects a large number of default input variables, investigators ran over 21,000 simulations to analyze the impact of changes made to inputs for base type, base thickness, subgrade type and traffic level. The research team also modified the traffic input calculator to allow designers to enter traffic values from MnDOT’s weigh-in-motion and traffic counting data. The calculator runs input traffic data in software simulations and assigns the input an appropriate axle value for design.

MnPAVE-Rigid 1.0 ran designs based on Class 5 aggregate base over a subgrade like clay loam. Other aggregate types were added to simulations to determine how the software responds to these changes. Investigation also explored the addition of subgrade material options in design simulations.

The code developer modified elements of the advanced inputs tab and PDF report generation features to improve performance for software users, and rebuilt the software in JavaScript 2.0 code, including an installer for use with Windows software.

What Was the Impact?

MnPAVE-Rigid 2.0 is more user-friendly. Its tabs better match designer needs, and the software offers a design report PDF file for export. Instead of selecting from limited options for traffic volumes (default, normal and heavy), users can now input traffic data that the software will categorize. Designers can input Class 5 aggregate, Class 5Q (a higher quality aggregate with fewer fines) and open graded aggregate (no fines). Users can also choose 4-inch or 12-inch aggregate base thicknesses. An additional subgrade option was not included, as simulations indicated a sand subgrade input did not discernibly impact structural thickness outputs.

The AASHTO M–E software is expensive, and agencies that use it have to work closely with consultants to receive training and to explore or modify the code. MnDOT owns and manages the source code for MnPAVE-Rigid 2.0, can keep it secure, and can continue to change and upgrade it internally for Windows and Linux platforms.

What’s Next?

The updated MnPAVE-Rigid is now available online. Presentations about the software upgrades will be made at meetings for materials and soils engineers through the fall of 2018.

Still underway is an effort to further incorporate recycled material properties into MnPAVE Flexible, the design software for asphalt pavement.

This Implementation Summary pertains to Report 2018-17, “MnPAVE-Rigid 2.0,” published May 2018.

Maintenance Operations, Traffic and Safety

Reporting Driver Intrusions in Work Zones

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Data from a new system for tracking work zone intrusions may be used to change work zone design and policies, reducing the risk of injury and death from intrusion crashes.

MnDOT and the Local Road Research Board engaged researchers to develop a user-friendly system that allows highway crews to quickly record instances of motorists’ intrusion into work zones, using a laptop, tablet or paper.

“This collaboration resulted in a fast, efficient and easy-to-use system because crews and supervisors let us know throughout the process exactly what they needed to consistently report work zone intrusions,” said Nichole Morris, Director, University of Minnesota HumanFIRST Laboratory.

What Was Our Goal?

The goal of this research project was to develop and test an efficient, comprehensive and user-friendly reporting system for intrusions into work zones. It was essential for the system to be accepted by highway workers. The information collected from the system, which was modeled after the existing MNCrash report, would then be used to examine risk factors to reduce intrusions and danger to workers. Safety data would be relayed back to workers and to MnDOT managers, providing an empirical basis for design changes to work zones, as well as future policy recommendations to the state government.

“To reduce work zone intrusions and make work zones safer, we need to track and analyze the intrusions. This reporting system will generate the data we need to make smart changes and possibly to influence legislative policy,” said Todd Haglin, Emergency Management and Safety Manager, MnDOT Office of Administration.

What Did We Do?

To design a usable system for reporting work zone intrusions, research designers had to:

  • Understand the characteristics of the typical system user (in this case, the work zone supervisors and crew).
  • Develop common or typical intrusion scenarios to realistically test the system.
  • Conduct iterative testing with typical users (supervisors and crew members) and incorporate revisions based on test results.

The research team interviewed work zone supervisors from rural and urban truck station locations across the state: in Baxter, St. Cloud and Duluth and at Cedar Avenue near Minneapolis-St. Paul. Researchers sought to learn what crews and supervisors considered an intrusion and what they thought should be reportable elements of the intrusion, such as the work zone layout, weather, location, time, visibility, road conditions and maneuvers of the intruding vehicle.

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In a common type of close call, the dark car shown here fails to merge until it is too close to the work zone, forcing the silver car out of its lane.

Researchers used information gathered from the interviews to develop four typical intrusion scenarios—which were reviewed and revised by MnDOT supervisors—and used these scenarios to test the prototype reporting interface. Then they conducted usability tests with these scenarios and with actual intrusions that crews had experienced. Users suggested changes to the report format throughout the process.

Crews and supervisors collaborated with researchers during three rounds of testing, revising the reporting interface after each round. An online beta version had been supplemented with a paper version. Both versions were revised through this iterative design process.

What Did We Learn?

This design approach allowed the research team to produce a report interface incorporating the very specific needs of the work zone crews and supervisors:

  • The third major revision split the report decision flow into two options—a shorter report and a comprehensive report—based on whether the intrusion presented a risk to the crew. Without this revision, intrusions that workers considered minor were not likely to be reported.
  • Researchers surveyed users of the system with each revision. Supervisors liked the drop-down menus, the comprehensiveness of the system and its ease of use. They rated the final revision as good in terms of usability, ease of use and time to completion (five to six minutes on average).
  • The final design version was tested using a laptop, tablet and paper. Multiple reporting options made it more likely that workers and supervisors would quickly report data about a work zone intrusion before details were forgotten.

What’s Next?

Supervisors and workers involved in the design process gave high marks to the final version of the reporting system. The design is considered complete. Researchers had created the interface as a free-standing program, using the University of Minnesota’s digital resources to build and evaluate their design. For this reporting system to be made  vailable for use by MnDOT and other agency workers, MnDOT must engage MNIT, the state’s information technology professionals, to determine where the system will reside and to integrate it into the state’s existing computer platform.

This post pertains to Report 2018-09, “Work Zone Intrusion Report Interface Design,” published in February 2018. 

MnDOT is developing other initiatives to improve work zone safety,  including a personal warning sensor for construction workers. Search for “work zone” research projects here.

Materials and Construction, Research

New research to explore innovative solutions to aging pavement infrastructure

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New solutions are urgently needed to address Minnesota’s aging pavement infrastructure since current materials and technologies can’t keep up with the rate of deterioration and limited funding.  MnDOT recently entered into a contract with the University of Minnesota to further explore new materials and technologies – including taconite and Graphite Nanoplatelets (GNP) – that could offer cost-effective solutions for longer-lasting pavement.

Background

The university has investigated the use of taconite aggregates for more than a decade (see ongoing and completed research), and started investigating GNP-reinforced asphalt materials more than three years ago (see recently completed research). Both materials present very unique properties that can be used to better build and maintain asphalt pavements.

This research project will focus on two applications with significant potential in the pavement area:

  • Early detection and repair of cracking by developing a novel asphalt material in which GNP materials, taconite concentrate, and conventional asphalt binders are combined for damage sensing and healing. The material damage will be assessed by measuring the electrical resistance, while the damage healing will be achieved by applying microwave to the material.
  • Thermal enhancement of tack coat bonding between asphalt overlay lifts, using GNP and taconite concentrate and microwave heating. Poor bonding can result in many different pavement distresses that decrease the pavement structural strength and life, ranging from top-down cracking, potholes and fatigue failure.

Improving pavement durability

The latest data shows that 15 percent of roads in Minnesota are in poor condition, at a cost to each motorist $480 per year. Low-temperature cracking is one of the main causes of pavement failure in Minnesota. Studies have shown that early detection of damage and cracking and timely repair is essential for extending the lifespan of the pavements.

Each dollar spent in the early-stage of pavement life could eliminate or delay $6 to $10 in future rehabilitation or reconstruction costs.

A series of recent studies funded by the National Cooperative Highway Research Program (NCHRP) and MnDOT showed that the GNP-modified asphalt binders and mixtures exhibit a significant improvement in both mechanical and compaction properties. The combination of the previous research and the proposed research will fully explore the properties of GNP-taconite modified asphalt binders and mixtures as a multi-functional pavement material, which will address various needs of MNDOT, including high fracture resistance, efficient compaction process, and cost-effective pavement preservation operations. By addressing these needs, the result of this research will lead to an innovative and efficient means to improve the long-term durability and resilience of asphalt pavements in Minnesota.

 Project scope

The two-year research project aims to explore the damage sensing and healing capability of asphalt binders and mixtures modified by GNP and taconite concentrates. The essential idea is to combine GNP and taconite concentrates with asphalt binders to make the final asphalt products electrical conductive. By measuring the change of electrical resistance, researchers will be able to determine the damage extent. When the damage extent reaches a certain level, the University will apply microwave to the pavement to generate heat, which will heal the cracks through viscous flow of warm asphalt binder. In addition, the thermal bonding capabilities of a novel tack coat material also modified with GNP and taconite concentrate will be investigated. The research will consist of four parts:

  1. Electrical conductivity tests on GNP-taconite modified asphalt binders and mixtures
  2. Modeling of relationship between electrical resistance and damage extent
  3. Investigation of self-healing capability through microwave
  4. Investigation of a microwave-based tack coat system to enhance thermal bonding in asphalt paving

Watch for new developments on this project here.  Other Minnesota pavement research can be found at MnDOT.gov/research.

 

Bridges and Structures

Ultrasonic Testing Method Improves Corrosion Detection on Steel Bridges

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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.

What’s Next?

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.

This post pertains to Report 2017-33, “Phased Array Ultrasonic Steel Corrosion Mapping for Bridges and Ancillary Structures.”

Maintenance Operations

Salt Brine Study Shows Truck Traffic Drives Deicer Effectiveness

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Truck traffic significantly improves deicer performance, deicers perform poorly below 10 degrees Fahrenheit, and brine is spread more by traffic spray and snowplow throw than by storm runoff.  This is the outcome of multi-year tests performed in the snow-covered parking lots of two Twin Cities entertainment destinations.

“There is notable airborne removal of deicers from road spray by vehicles and under high wind conditions. For the deicing materials MnDOT uses, little melt is observed below 10 degrees Fahrenheit,” said Tom Peters, Maintenance Research and Training Engineer, MnDOT Office of Maintenance.

MnDOT has conducted several research projects to better understand the performance of both deicing and anti-icing materials (applied to the roadway before a storm to prevent or mitigate ice buildup). Two previous studies evaluated solid materials and liquid brine. In Phase I researchers examined over 50 deicer and anti-icing compounds and blends. They determined that ice melt capacity correlates closely with application temperature, which is the principal factor in effectiveness. Rock salt offers greater ice melt capability, but liquid deicers adhere better to roadways and cause less corrosion and environmental damage to road and bridge environments.

In Phase II, researchers studied deicer performance in the field and considered how traffic levels, truck volumes, weather, pavement type and other factors affect performance. Research showed that deicers work better at warmer temperatures, with little effectiveness below about 10 degrees Fahrenheit. Truck traffic significantly improved deicer performance, contributing to wider and quicker melts. Chlorides were swept off bridge decks by snowplowing, and deicing effectiveness diminished as truck speed rose.

However, severe weather hampered research in Phase II; the winter of 2013-2014 was the coldest experienced in Minnesota in over 30 years. The severity of winter conditions impeded the temperature study of deicer performance and snowplow performance, leaving the research team and MnDOT interested in further study.

What Was Our Goal?

This study aimed to continue the work of Phase II in more representative winter conditions. Researchers evaluated deicer effectiveness, plowing effectiveness, anti-icer persistence in traffic and drains, and pavement shedding of deicers.

What Did We Do?

During the winters of 2015-2016 and 2016-2017, both of which were mild with below-average snow accumulations, the research team followed closely the methods used in Phase II.

Deicers were studied at two facilities in Shakopee, Minnesota. One facility included nine 900-foot lanes on which plow trucks spread deicers at highway speeds. The other facility featured four 500-foot lanes, where deicers were spread by hand in 100-foot segments. Investigators monitored weather and evaluated deicer performance with photography and infrared thermography.

Snowplowing by MnDOT Metro District plow operators was conducted at the two Shakopee facilities as well, at operating speeds of up to 30 mph. Researchers documented performance of various plow configurations in various truck combinations with on-site observation, handheld photography and time-lapse photography.

Anti-icing, in which deicer brines are applied to dry pavement to prevent ice formation, was conducted on an elevated section of U.S. Highway 169 near Mankato that experienced actual traffic. Investigators recorded application rate, time, temperature, precipitation and traffic, as well as deicer flow and concentration in storm drainage runoff over time.

A researcher mixes deicers in bags before spreading by hand at the test site.
A researcher mixes deicers in bags before spreading by hand at the test site.

Researchers studied pavement shedding of deicers in a lab in terms of storm runoff flow and anti-icer concentration in drainage from artificially induced precipitation. Deicers were applied in brine form, dried, chilled and held at temperatures below the freezing point of water and within the range of effective ice melt temperatures.

What Did We Learn?

Research confirmed that deicer performance varies with temperature, with little benefit from rock salt at 10 degrees Fahrenheit or colder unless the pavement is exposed to sunlight. Deicer accumulated in drains at substantially lower levels than roadway applications suggest regardless of traffic conditions, confirming observations that the majority of deicer loss occurs from displacement by traffic and snowplows.

Plow results were fairly uniform across all lanes and along lane lengths for a given plow type, suggesting truck, plow and driver combinations performed uniformly at each track. At higher speeds, snow rises higher in the curvature of the plow, and snow casts more broadly off plow ends and distributes to greater distances.

“Truck traffic makes a huge difference on deicer performance. If two or three dump trucks that aren’t spreading but have weight in them follow a salt truck, salt use might be cut by two-thirds,” said Stephen Druschel, Professor, Minnesota State University, Mankato, Department of Civil Engineering.

Deicers and anti-icers showed wider and quicker melting capability with traffic, especially by trucks, than without. Prewetting offered no significant observable benefit under most conditions, contrary to reports from snowplow drivers in field operations, unless snow was dry; then significant benefits were observed.

Asphalt and concrete pavements shed salinity at high levels initially and at declining levels at about 0.3 inch precipitation. The type of pavement involved had no apparent effect on deicer runoff.

What’s Next?

The impact of truck traffic on deicer performance is significant and needs to be widely communicated. Reports that prewetting improves deicer adhesion in windy conditions and speeds the initiation of ice melt may warrant further review. Evaluation of atmospheric and off-roadway drainage may help quantify traffic-induced brine spray and plow throw.

View Dr. Stephen Druscel’s YouTube presentation, Winter Plowing and Deicing: Saving Money, Salt and Labor by Distinguishing Best Practices.

This post pertains to Report 2017-45, “Salt Brine Blending to Optimize Deicing and Anti- cing Performance and Cost Effectiveness: Phase III.” The full report and appendices can be accessed at mndot.gov/research/reports/2017/201745.pdf and mndot.gov/research/reports/2017/201745B.pdf.

Bridges and Structures, Materials and Construction

Affordable Bridge Girder End Repair Method Restores Concrete Beams

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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.

Background

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.

2018-07-p2-image
Repaired and unrepaired girders were tested to failure in a laboratory. This repaired beam end remains firmly connected to the beam, even after the girder was crushed.

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.

What’s Next?

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.

This post pertains to Report 2018-07, “BR27568—Experimental Shear Capacity Comparison Between Repaired and Unrepaired Girder Ends,” published February 2018. More information can be found on the project page. (Part of this article was adapted from an October 2017 article by the Center for Transportation Studies.) 

 

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