Using Debonded Strands to Reduce End Stress in Bridge Beams

A new MnDOT-funded research study has found that most agencies in states with weather similar to Minnesota’s use debonded strands in prestressed concrete bridge beams. MnDOT may begin piloting debonding as an alternative to draping, which manufacturers claim is time-consuming, challenging to worker safety and expensive.

What Was the Need?

Bridge designers often prestress concrete beams with steel strands to improve the performance of the beams. The strands precompress the beams so that when external loads like vehicle traffic are applied, the concrete is less likely to crack under loading.

When the beams are fabricated, the strands are stretched from one end of the concrete form to the other, and then concrete is poured and hardens around the stretched cable. Once the concrete is cured, the cables are released from the precasting bed. When the cables shorten, they shorten or squeeze the concrete they are bonded to in the beam, precompressing it.

Because concrete is effective in compression and poor in tension (it cracks), precompressing the concrete leads to beams that may not crack in service conditions. It also leads to less deflection of the beams under loading. Both outcomes improve the strength, serviceability and durability of the system.

“We were assessing the current state of the practice of debonding strands in prestressed concrete, learning what other agencies have done and how much success they’ve had,” said Catherine French, CSE distinguished professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.

Prestressing causes high stress at beam ends, which is conventionally mitigated with some combination of two common design approaches. Cables can be debonded at the ends, typically by using a sleeve of a limited length that prevents concrete from directly bonding with the strands where covered. Draping strands can also help with end stresses by reducing the eccentricity of the strands. While regular strands run parallel to the length of the beam, draped strands are pulled in a somewhat V-shape, from the top of the beam at each end to the bottom of the beam in the middle.

MnDOT currently uses draping to relieve the end stresses, but it does not allow debonding due to its potential to provide a path for chlorides to enter the concrete along the debonding tubes, which could lead to corrosion. Manufacturers would like to rely less on draping, which requires more time, cost and care to safely fabricate. Local agencies are interested in debonding because draping requires thicker and more expensive concrete beams. National standards offer the choice of draping, debonding or a combination of both.

What Was Our Goal?

Researchers investigated the state of the practice for using debonded strands in prestressed concrete beams. MnDOT and the Local Road Research Board (LRRB) needed recommendations for using debonded strands to position the agency to adopt current and imminent national debonding standards for prestressed beams and to use debonding as an alternative to draping, where appropriate. 

What Did We Do?

Researchers studied current MnDOT prestressing specifications, prestressing and debonding guidelines established by the American Association of State Highway and Transportation Officials (AASHTO), and research on debonding and draping. They also surveyed 11 agencies in 10 states with climates similar to Minnesota’s about their use of debonding and its performance in terms of reducing beam end stresses and resisting corrosion. They followed up with some respondents to gather more detail on respondents’ practices and experience.

Debonding sleeves protect prestressing strands from bonding with concrete, reducing stress and cracking at beam ends. 

Plastic sheathing is wrapped around a portion of bonded prestressing strands.
In addition to conducting the survey, the research team met with two fabricators who produce MnDOT prestressed concrete beams to review prestressing, debonding and draping procedures, and visited the plants to observe the process.

What Did We Learn?

Debonding appears to reduce cracking at beam ends. Currently, AASHTO allows debonding of up to 25 percent of prestressing strands in concrete beams, though this may soon be revised to allow a higher debonding limit. AASHTO’s T-10 Technical Committee proposes allowing up to 45 percent debonding, while NCHRP Research Report 849 recommends allowing up to 60 percent of strands be debonded.

“Researchers did not find that there was any excessive corrosion with debonded strands. The team is recommending we start at debonding 40 percent of prestressed strands,” said Brian Homan, State Aid bridge plans engineer, MnDOT Bridge Office.

Ten of the 11 responding agencies use debonding, typically in coordination with sealing beam ends with silicone or similar material to protect sleeved cables from water and salt intrusion. Five of the 10 reported debonding as their primary method for reducing end stresses, and three indicated draping as their favored approach. Six limit debonding to 25 percent of strands, though others allow a higher percentage, including Michigan DOT, which allows up to 40 percent of the strands to be debonded. Respondents reported few problems with debonded strands.

Researchers recommend that MnDOT begin debonding up to 40 percent of its strands to refine the practice before it considers adopting the 60 percent standard. Two split sheath tubes, one over the other, should be applied to the strands to achieve debonding in the end regions.

Concrete ends should be sealed as MnDOT currently requires, and silicone sealant should be applied to exposed strand ends. The research team recommended a sequence for releasing prestressing cable to minimize cracking at the beam ends—bonded before debonded and shorter debonded lengths before longer lengths.

What’s Next?

Debonding strands costs less than draping and is favored by local agencies; the practice will reduce prestressed beam fabrication costs for MnDOT and the LRRB. Draping procedures present safety concerns that will be relieved by a reduction in draping, and debonding is expected to reduce end cracking.

MnDOT is developing two pilot projects in which up to 45 percent debonding may be used. Future research may be warranted to identify the best percentage of strands that should be debonded and evaluate debonding sleeve materials, designs and performance.

This post pertains to Report 2019-30, “Debonded Strands in Prestressed Concrete Bridge Girders,” published July 2019. For more information, visit the MnDOT project page.

Smartphone App Alerts Drivers Exceeding Speed Limits on Curves

Researchers have developed a proof-of-concept curve speed warning system for use with mobile phones, a technology they hope car manufacturers might adopt for in-vehicle systems. The proof-of-concept system uses data from local road agencies on curve locations, speed limits and signage with geofencing to trigger cloud-based data alerts to road users driving faster than recommended speeds for curves.

What Was the Need?

Over one-quarter of fatal highway crashes occur at horizontal curves. In Minnesota, these areas are a contributing secondary factor in 49 percent of fatal crashes. Each year, accidents on two-lane, two-way highway curves injure over 4,000 people and result in 70 deaths, almost one-fifth of annual roadway fatalities in the state.

Research has shown that dynamic message signs with speed detection components work well in warning drivers to reduce speeds, but these systems require power supplies and cost approximately $14,000 per site. Minnesota’s Otter Tail County alone has over 400 reduced speed curve sites. The Local Road Research Board (LRRB) and MnDOT have been funding research that examines alternative approaches to speed warning systems for drivers approaching curves.

“This smartphone application stitches together existing technologies for GPS, GIS and mapping to provide an inexpensive, cloud-based warning system for drivers,” said Richard West, public works director, Otter Tail County.

In a 2015 study, researchers developed an in-vehicle, vibration-based warning system tested in a driving simulator that relies on data from highway sensors and other sources. Research in 2018 focused on the use of GPS signals to calculate and recalculate a vehicle’s trajectory on roadways to issue warnings. A new phase of this study is refining the approach to draw on vehicle-to-vehicle data.   

What Was Our Goal?

The goal of this research was to develop a dynamic curve speed warning system that would employ cloud-based data sharing. The system would not require significant infrastructure investment and would be applicable to all reduced speed curves in the MnDOT highway system.

What Did We Do?

Following a literature search, researchers focused on developing a proof-of-concept smartphone app that would warn drivers of upcoming curves and speed reduction requirements. They also created a database for county road agency managers to input curve locations within their jurisdictions, speed limits and sign facing direction for use with the smartphone app.

Researchers layered the database into their geographic roadway inventory tool, which draws on GPS and mapping data, and combined data from the sources into a cloud-based curve database. Then they developed a geofence system that triggers alerts as the tracking device crosses virtual geographic boundaries.

A smartphone app uses GPS and GIS to trigger a warning via the cloud to smartphone users traveling above the curve speed limit as they pass through a geofence, or virtual geographic boundary, before the curve.

An illustration of the system overview that includes a curved road with geofencing overlays marking two warning areas on the curve, a cloud-based warning database and a smartphone screen showing a “Reduce speed” message.
A smartphone app uses GPS and GIS to trigger a warning via the cloud to smartphone users traveling above the curve speed limit as they pass through a geofence, or virtual geographic boundary, before the curve.

The curve speed warning system was tested on roadways in Otter Tail and Pope counties. After county agencies input curve location and speed limit data into the system, researchers tested the system by running the app while driving a number of highways selected for a high density of reduced speed curves. They adjusted the system based on these field tests to accommodate GPS signal speed, travel speed and cloud data transfer bandwidth.  

Researchers then evaluated roadside dynamic speed warning system safety impacts to determine the potential safety and cost benefits from the cloud-based warning system.

What Did We Learn?

The curve speed warning system worked in proof of concept. GPS and cloud data can be drawn on fast enough to provide warnings in time for drivers to respond. Researchers refined geofencing parameters to only pull data for curves within 30 miles of the vehicle to keep data volumes to manageable levels within standard parameters for mobile phone data packages.

“Study results show that this system works accurately. If data from county and state roads were input, the application could be made available to everybody,” said Bradley Wentz, program director, Advanced Traffic Analysis Center, Upper Great Plains Transportation Institute, North Dakota State University.

The core of the system is the curve database, which requires accurate data input by county road agencies. Testing resulted in one performance error, which was traced to incorrect data for the facing direction of a warning sign.

Image of the secondary warning on a smartphone screen showing the message “Slow Down,” the recommended speed of 45 mph and the driver’s current speed of 51 mph in a red circle.
The smartphone app sends a second warning with this message and an audible signal to a driver’s phone.

When vehicles are traveling faster than the speed limit for an upcoming curve, the smartphone app issues a silent, on-screen warning of the approaching curve and speed limit. If the vehicle does not slow its speed sufficiently, the app flashes and issues another warning with an audible signal. 

The smartphone app sends a second warning with this message and an audible signal to a driver’s phone.

Safety implications may match crash and speed reductions identified in research on the safety benefits of dynamic sign warning systems. Researchers believe the cost to maintain the software and warning database roughly matches the cost to maintain a traditional dynamic speed warning sign system. But using a single cloud-based system for the entire roadway inventory offers a dramatic cost savings over installing expensive warning sign systems at every curve.

What’s Next?

Researchers have prepared presentations for local audiences and presented findings at the 2018 National Rural ITS Conference. County road agencies can easily update the database, and the system can accommodate not just reduced speed curve locations, but any reduced speed needs, such as seasonal bumps and cracks in pavement, work zones, special events and controlled intersections.

This post pertains to Report 2019-19, “Cloud-Based Dynamic Warning System,” published June 2019. For more information, visit the research project page.

New Project: Use of Innovative Technology to temporarily Deter Bat-Bridge Use Prior to and During Construction

MnDOT has funded a study to evaluate the use of non-lethal ultrasonic acoustic devices to temporarily deter bats from bridges before and during construction projects.

Background

Brown bat on bridge

When protected bats roost or form colonies on bridges, bridge repair and replacement projects have to follow regulatory requirements to minimize impacts on species protected by state and federally regulations – including the Endangered Species Act. These regulations protect bat populations that have already declined due to white-nose syndrome, which is estimated to have killed more the 5.7 million bats in eastern North America since 2006. The northern long-eared bat (Myotis septentrionalis) is listed as a threatened species as it is one of the species most impacted by white-nose syndrome. Other species are anticipated to be protected as the disease spreads.

To minimize disturbance to protected bats, which may use bridges both for day roosting habitat and sites for maternity colonies where bats give birth and raise their young, MnDOT is evaluating the feasibility and efficacy of ultrasonic acoustic devices to temporarily deter bats from work areas on bridges. These devices emit a sound, inaudible to humans, that disrupts the bat’s ability to echolocate and therefore discourages bats from approaching. This new technology has been utilized for wind turbines with positive results.

Ultrasonic device to deter bats

Temporarily deterring bats from a work site saves taxpayers money and increases bat safety. Regulations for protected species can limit activity that is potentially harmful, including bridge work during times when bats may use bridges for roosts or maternal colonies. Without the use of deterrents, work may be delayed until bats vacate the bridge, which may not occur until bats retreat to hibernacula (such as caves and mines) for the winter. Having control over when bats are present will provide more predictable timelines to projects and reduce engineering and administrative costs associated with delays and changes to work plans. Without a control measure, projects must adhere to timing restrictions that increase construction costs and may even reduce bridge life expectancy. And if bats are kept away from construction sites, they will not be directly harmed or disturbed by the activity.

Project Scope

This one-year study will investigate the efficiency and feasibility of ultrasonic bat deterrent technology for temporary exclusion of bat species on bridges by monitoring bat presence before the ultrasonic devices are installed, during a trial period, and after devices are removed. This technology will be tested on two bridges in Minnesota. Findings (expected in May 2020) will determine if ultrasonic bat deterrent technology can be utilized to exclude bats from construction and maintenance work zones, thereby reducing costs and ensuring the safety of protected species.

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

Guidebook helps cities and counties choose tools for managing fleets

Snowplows and other winter maintenance vehicles clearing roads during a nighttime snowstorm.

Managing a fleet of trucks, heavy equipment, and other vehicles challenges road agencies large and small. While large agencies like MnDOT use software and specialized administrators to manage fleet management systems electronically, city and county agencies often do not. For some small agencies, fleet management may fall to a shop mechanic or two.

In a recent project from the Local Road Research Board’s Research Implementation Committee, researchers identified the fleet management needs of city and county agencies and reviewed various cost-effective tools that could help these agencies make fleet management decisions. They then developed a guidebook for local agencies that addresses the tools and methods needed to manage fleets effectively.

“The guidebook provides the benefits of fleet management, a comparison of various program features and attributes, and a contact for more information about each program,” says Guy Kohlnhofer, county engineer, Dodge County, and the project’s technical liaison.

Fleet Management Tools for Local AgenciesFleet Management Tools for Local AgenciesFleet Management Tools for Local AgenciesFleet Management Tools for Local AgenciesFleet Management Tools for Local Agencies
Fleet Management Tools for Local Agencies

The guidebook—Fleet Management Tools for Local Agencies (2017RIC01)—includes a matrix comparing the eight most widely used fleet management software tools among Minnesota agencies. Costs, equipment needs, tracking features, financial analysis applications, and other attributes are reviewed. Case studies of agencies that use spreadsheets, software, and specific fleet replacement strategies are also included.

Three approaches to fleet replacement planning are presented in the guide. “You may have a vehicle that has been driven 300,000 miles and needed little maintenance, while another vehicle has been driven 100,000 miles and has needed a lot of maintenance,” says Renae Kuehl, senior associate, SRF Consulting Group, Inc., one of the co-authors. “We provide three models to determine when you should replace each.”

One of the findings of the project is that spreadsheets are effective and widely available tools for managing fleets. They are easy to tailor to local needs and fleets, are well understood by most computer users, are part of most office software suites, and work well for small data sets. Disadvantages, however, include limitations in reporting features, easy corruptibility of data, and inconsistent data entry among users.

In contrast, fleet management software offers easy report generation; software linkage to fuel, financial, and other software systems or modules; secure and consistent data; and interagency shareability. However, these tools can be expensive. Software costs for managing fleets average almost $36 per vehicle, and annual support costs average about $18 per vehicle. Other disadvantages include the need for training and internet accessibility.

This article originally appeared in the September issue of the LTAP Technology Exchange 

Leveraging Existing Inductive Loops to Classify Highway Vehicles

Researchers evaluated the use of existing inductive loop installations in Minnesota for vehicle classification. Results showed that inductive loops may be effective at identifying and classifying individual vehicles as they pass, but the system will require further refining for Minnesota use.

What Was the Need?

MnDOT periodically counts vehicles on state highways and uses this data to plan for transportation infrastructure needs, apply for federal funding, anticipate traffic demand and potential congestion, and learn how drivers use the highway system.

Automatic traffic recorders (ATRs) and weigh-in-motion stations count and measure the size of commercial vehicles. Engineers also count total traffic, classifying vehicles by size or axle number according to the Federal Highway Administration’s (FHWA’s) system of 13 vehicle classes, which includes Class 2 for passenger cars; Class 3 for pickup trucks, some SUVs and minivans; Class 4 for buses; and Class 5 through 13 for commercial vehicles.

Vehicle classification counting usually entails manual counting or the use of pneumatic tubes stretched across vehicle lanes to record speed and the number of axles passing. Tube counts are conducted for 48 hours at each of 1,200 sites throughout the Minnesota highway system once every two years. This time-consuming, costly practice also places staff in danger. Video imagery can be used, but this also takes a considerable commitment of labor to view, analyze and record vehicles.

A 2013 U.S. DOT study in California evaluated the use of inductive loops in vehicle classification. This technology is commonly used on highways for monitoring congestion by counting vehicles and measuring speed. Inductive loops are embedded just below the pavement surface and linked to a data station nearby that records electronic signals from the metal chassis of each passing vehicle.

What Was Our Goal?

MnDOT sought to evaluate the U.S. DOT approach in a Minnesota setting that would leverage existing technology. Researchers would use the method to record, identify and classify vehicles passing over inductive loops already installed throughout the Twin
Cities’ highway system.

What Did We Do?

A camera and an inductive loop data box at the U.S. Highway 169 and Trunk Highway 282 intersection.
A camera and an inductive loop data box at the U.S. Highway 169 and Trunk Highway 282 intersection.

Following a review of the 2013 U.S. DOT study and other research, the investigative team installed video systems and new loop signature circuit cards at five test sites: two at Interstate highways, one at a major highway and two at signalized intersections. Investigators gathered data at each location for three to four weeks.

Researchers then analyzed 10 to 14 days of loop and video data from each site. For ground truth, the team identified every individual vehicle from video, then analyzed loop data in two ways. First, they compared video and individual electronic signature readings for every vehicle. Then they analyzed loop signature data in 15-minute interval aggregations to evaluate how well the system works without verification on a vehicle-by-vehicle basis.

After evaluating vehicle classes using the FHWA classification system and a second classification system, researchers presented their findings and conclusions in a final report.

What Did We Learn?

The research team reviewed over 400 hours of video and counted over 807,000 vehicles. The match rate for all 13 FHWA classes averaged 75 percent with a standard deviation of 8 percent for individual vehicle matching. The overall matching rate was biased toward Class 2 and 3 vehicles, as sedans, pickups and SUVs share similar vehicle chassis configurations and loop signature patterns.

The 15-minute aggregated method showed a tendency to undercount Class 2 vehicles and overcount Class 3 vehicles by about 13 percent of total traffic. The secondary classification system results matched the FHWA system fairly well for consumer-level vehicles and tended to undercount some commercial vehicles.

A video camera and an inductive loop data box installed on Interstate 94.

Overall, Class 2 vehicles were matched by inductive loop signatures at a rate of 81 percent accuracy, with 17 percent of passenger vehicles misclassified as Class 3 vehicles. All other vehicle classes had matching rates of less than 50 percent. California results showed an average match rate across classes of about 92 percent.

These results were disappointing. Site conditions may have been a factor, particularly at one site where damaged hardware, broken sealants and other physical conditions were suboptimal. The library of vehicle signature signals in California was used as a basis for Minnesota analysis, but the data sets may not match precisely. Agricultural needs, for example, differ between states, and heavy agricultural vehicles feature different configurations, potentially generating different electronic signatures.

“We need a little more research, which will mostly be done by our office. If we get better accuracy, we’ll be able to get data continuously rather than just 48 hours every couple years,”  said Gene Hicks, Director, Traffic Forecasting and Analysis, MnDOT Office of Transportation System Management.

The U.S. DOT study in California also used loops in circular patterns, and Minnesota’s loops are arranged in rectangular patterns. Data signal crossing, diminished signal quality and shadow data repeated on neighboring lanes may have corrupted findings.

What’s Next?

Further research will be needed before loop signature data can be used reliably in traffic analytics. Researchers suggest that the investigation can be re-evaluated by installing four loop signature cards at two permanent ATR locations with loops, pneumatic tubes and video. Circuit cards can also be updated and classification algorithms better calibrated to vehicle signature profiles.

This post pertains to the LRRB-produced Report 2018-31, “Investigating Inductive Loop Signature Technology for Statewide Vehicle Classification Counts,” published October 2018. For more information, visit MnDOT’s Office of Research & Innovation.

Roadside Turf That Tolerates Salt, Heat and Ice

A recently completed research study has identified turfgrass species and cultivars that perform best under the heat and salt on Minnesota roadsides.

Varieties from fifteen turfgrass species were tested in salt, heat and ice stress protocols. Analysis of color and cell membrane stability yielded recommendations for salt- and heat-resistant turfgrasses, but was inconclusive for ice-resistant cultivars. A mixture of cultivars was recommended for field study.

What Was the Need?

Minnesota’s roadside vegetation prevents erosion and keeps contaminants from reaching ground- and surface water. Turfgrass offers aesthetic value and unobstructed sightlines for drivers, but it must withstand harsh conditions.

Yellowing turfgrass in winter alongside Interstate 94.

In addition to year-round contaminants generated from highway traffic, Minnesota roadside grasses face snow, ice and salt from deicing operations in winter, and heat in summer that is even hotter alongside roadways and in urban environments.

Various turfgrass species may offer better resistance to specific stressors. Several current and recent MnDOT studies have evaluated salt tolerance and watering needs for select species of turfgrass. It was unclear, however, which species performed best under the multiple, combined stressors of Minnesota roadway environments, and would suit a mixture tailored to one of three Minnesota climate regions that could optimize turfgrass performance throughout the year.

What Was Our Goal?

This project sought to identify turfgrass species and cultivars that perform well under the range of stressors common to Minnesota roadsides. Successful cultivars may be candidates for turfgrass mixtures of multiple species that would optimize performance under all conditions in the field.

What Did We Do?

Researchers conducted a literature search to identify promising turfgrass species for harsh environments. Then they contacted seed companies for further recommendations before requesting seeds for multiple cultivars from 15 species. Next, researchers tested cultivars in salt, heat and ice stress protocols. Results from this testing are summarized below.

Salt Stress. The research team grew 38 individual cultivars hydroponically in 4-inch pots for 12 weeks. Then they suspended the pots in a salt solution for three weeks. Salt was added at the end of three weeks and again at three-week intervals at four salt concentration levels. Investigators compared digital images of green cover with in-pot color index meter results and tested cell membrane stability by measuring electrolyte leakage.

Heat Stress. Investigators grew eight samples each of 34 cultivars in 4-inch pots in a greenhouse for 12 weeks. Plants were trimmed manually to 2 inches. Half the plants were put through three heat stress cycles of 49 days of 95 degrees Fahrenheit and 70 percent humidity, followed by 28 days of normal conditions. Researchers then conducted digital imaging, in-pot color indexing and electrolyte leakage testing.

Ice Stress. After 10 weeks in a greenhouse, four samples each of 35 cultivars were placed in cold acclimation for 14 days at 35.6 degrees Fahrenheit. Investigators then moved pots to a chamber held at 28.4 degrees for 24 hours to freeze the soil, and then applied 2-inch layers of ice to each pot. A sample of each cultivar was removed at four, eight, 12 and 16 weeks, thawed for 48 hours in a 35.6 degree chamber and then moved to a greenhouse. Digital images were taken at 31 days in a greenhouse. A second batch was run as a control through the same trial up to the point of ice cover to identify if cold temperature was fatal to any samples.

A grid of 16 photographs shows green cover for four turfgrass cultivars: tall fescue, perennial ryegrass, Kentucky bluegrass and hard fescue. Increasing damage from salt exposure is shown for each cultivar after one, six, nine and 12 weeks.
A grid of 16 photographs shows green cover for four turfgrass cultivars: tall fescue, perennial ryegrass, Kentucky bluegrass and hard fescue. Increasing damage from salt exposure is shown for each cultivar after one, six, nine and 12 weeks.

What Did We Learn?

A mixture of turfgrass varieties and species will likely be the best solution for year-round use in Minnesota, as no one cultivar performed well in every trial.

Salt Stress. Tall fescue and perennial ryegrass sustained the highest percent green cover and lowest electrolyte leakage throughout the salt stress trials. Alkaligrass, considered salt tolerant, did not perform significantly better than other grasses. Only tall fescue emerged as a salt-resistant turfgrass option, though this cultivar is vulnerable to ice cover.

Heat Stress. Performance varied significantly within species, suggesting a potential for breeding improvements. Some species performed poorly under heat but recovered well when returned to normal conditions. Researchers recommended Canada and Kentucky bluegrasses, tall fescue, strong creeping red fescue and slender creeping red fescue as heat-resistant turfgrass cultivars.

Ice Stress. Tall fescue performed best in image and color analysis. Field observations and previous study, however, suggest that tall fescue performs poorly under ice cover. Warm season grasses died during the control cold storage. Researchers concluded that the ice trial did not properly simulate field ice cover conditions.

What’s Next?

The second phase of this study began in 2018 and employs a mixture of six species selected from this study: Kentucky bluegrass, slender creeping red fescue, hard fescue, buffalograss, alkaligrass and tall fescue. Mixtures will be planted in different combinations on roadsides for evaluation. MnDOT will also adjust its seed mixture recommendations for use in the meantime based on the results of this and other studies. Ultimately, MnDOT intends to develop recommendations tailored to three climate regions in Minnesota.

This post pertains to the LRRB-produced Report 2019-01, “Regional Optimization of Roadside Turfgrass Seed Mixtures,” published December 2018. For more information, visit MnDOT’s Office of Research & Innovation project page.

Recycling Asphalt Pavement Offers Strong Alternative to New Aggregate Base

In a newly completed study, researchers found that stabilized full-depth reclamation has produced stronger roads for commercial loads in Minnesota, and the method shows promise for uses in rural agricultural areas. How much greater the strength gained with each stabilizing agent is better understood, though not conclusively.

What Was the Need?

With stabilized full-depth reclamation (SFDR), roadway builders pulverize and mix old (hot-mix or bituminous) pavement and on-site base aggregate with asphalt to create a new, thick layer of partially bound base over the remaining aggregate base of the former roadbed. The process eliminates the cost of hauling away old pavement and hauling in new, expensive aggregate, which is in limited supply.

Cracking and other damage in older pavements usually reflect through new asphalt and concrete overlays. SFDR roads, on the other hand, tend to avoid reflective cracking while meeting the increasing load demands of an aging roadway system in reduced funding environments.

To make a road stronger and more resistant to damage from heavy loads, most rehabilitation approaches require a thicker and wider roadway. SFDR may offer a way to build stronger roads without widening the road and without transporting old material from the road site and hauling new aggregate to the location.

a tanker truck connected to a reclaimer
A train of equipment runs on an SFDR site: a tanker of new asphaltic material connected to a reclaimer that pulverizes the old pavement and mixes in part of the road base and possibly stabilizing agents.

In 2016, performance requirements of SFDR edged MnDOT and the Local Road Research Board (LRRB) closer to design standards for the technique by establishing testing, modeling and analytical methods for evaluating SFDR mixtures. Minnesota designers lack a method for giving SFDR designs structural design ratings to quantify how well the mixture will meet the needs of a new roadway. How much strength is gained by mixing  in a stabilizer and laying the reclaimed road as a thick asphalt pavement base before adding the overlay remains unquantified.

What Was Our Goal?

Most replacement roadways need to be capable of bearing heavier commercial and agricultural loads than the original roads. Researchers sought to determine the structural value of SFDR in mixtures employing various stabilizing agents to help designers better accommodate rehabilitation and increased loading needs with SFDR.

“We’re really big on recycling, and we’ve been using SFDR and FDR for quite some time. We have increased confidence in SFDR. We just don’t know how high that confidence should be,” said Guy Kohlnhofer, County Engineer, Dodge County.

What Did We Do?

Researchers visited 19 Minnesota road sites to look at 24 pavement sections and surveyed pavement conditions, cracking and potholing for each segment. The team conducted stability testing with a dynamic cone penetrometer (DCP) at each section and removed three pavement cores from each for laboratory testing.

Two researchers test the base structural strength of a rural pavement using a dynamic cone penetrometer.
Two researchers test the base structural strength of a rural pavement using a dynamic cone penetrometer.

SFDR pavement can be difficult to properly core, and most specimens failed before laboratory testing. Researchers conducted tests of dynamic modulus in a way that simulated high and low vehicle speeds in the lab on the surviving 14 samples. The tests simulated the movement of wheels over pavement surface and examined the resiliency of the pavements in springing back from these rolling loads.

Based on these results, researchers plotted the laboratory test results in mathematical curves. They then analyzed their findings while referencing flexible pavement design procedures using the concept of granular equivalents (GEs) that is familiar to many  avement designers in Minnesota. Finally, they estimated the structural difference between stabilized and unstabilized reclaimed materials and identified how the structural value varies with selected stabilization agents.

What Did We Learn?

Field surveys found roads performing well. Few of the pavement surfaces showed noticeable distress, and more recent surface coating treatments showed almost no distress over pavements in which distresses would quickly present themselves. DCP testing suggested that asphaltic stabilizers—asphalt, asphalt plus cement and modified asphalt—offered greater stiffness than fly ash and cement stabilization.

“We confirmed that what local engineers are doing has value, even if we weren’t able to generate more optimistic numbers,” said Charles Jahren, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering.

Lab testing suggested that while SFDR mixtures offer less stiffness compared to regular hot-mix asphalt (HMA) layers, their stiffness diminishes less in comparison to HMA for slow-moving heavy loads like seasonal agricultural equipment. SFDR is worthy of additional consideration as a base layer, in such loading environments.

The most critical goal for this study was to quantify the granular equivalency of SFDR mixtures with various additives to standard aggregate bases. Foamed asphalt and engineered emulsion proved the most structurally beneficial stabilizers; SFDR mixtures with these materials offered GE values of 1.46 to 1.55, confirming the general MnDOT approach that SFDR can be used for a GE of 1.5. If road builders pulverize 4 inches of asphalt roadway with 4 inches of base aggregate and add foamed asphalt or emulsion stabilizer, the 8-inch asphalt base offers the strength of a 12-inch aggregate base. A pavement of HMA or portland cement concrete can follow to create a roadway section with greater strength than a roadway section with the same thickness of nonstabilized base.

What’s Next?

SFDR performs well in the field and shows particular promise for use on rural roadways subject to seasonal, heavy agricultural loads. Researchers confirmed current GE inputs for SFDR and documented the performance of specific stabilizer options employed in Minnesota. Continued monitoring of SFDR road performance and additional testing and analysis would add more detail to design procedures and provide designers with greater confidence.

This post pertains to LRRB-produced Report 2018-33, “Field Investigation of Stabilized Full-Depth Reclamation (SFDR),” published November 2018. For more information, visit MnDOT’s Office of Research & Innovation project page.

Testing Methods for Crack Resistance in Asphalt Materials

The Minnesota Department of Transportation is working with other state agencies in a pooled fund study to improve methods for testing crack resistance of asphalt mixtures. To expand options further, MnDOT asked researchers to evaluate alternative tests with standard lab equipment. The new tests produced repeatable results. Methods include the semicircular bend (SCB) test in a nontypical configuration, a dynamic modulus test of smaller asphalt mixture samples, a bending beam rheometer (BBR) test of mixtures, and a BBR of asphalt material for binder selection.

What Was the Need?

A number of factors lead to cracking and other damage in asphalt. Cold temperatures cause pavements to contract, triggering internal tensions that lead to low-temperature cracking. Aging asphalt binder grows brittle and under loading pressure generates bottom-up, or fatigue, cracking. A variety of causes may contribute to top-down cracking, such as mixture properties, construction practices, tire design and loading.

A road crew works at night to place a layer of asphalt pavement.

MnDOT, in partnership with the National Center for Asphalt Technology, and four other state transportation agencies are part of a pooled fund study to develop mixture performance testing focused on cracking. This group, termed the Cracking Group, installed eight different pavement cells at MnROAD in the summer of 2016 to examine pavement performance and testing approaches for low-temperature, top-down and fatigue cracking.

The group’s approach does not embrace every potential test, including some examinations other agencies and research organizations have found potentially valuable in predicting cracking behavior of asphalt pavement materials.

What Was Our Goal?

MnDOT sought to investigate the viability of testing methods not included in Cracking Group studies. These tests would be conducted on asphalt mixtures sampled during construction of the test sections at MnROAD to help in material selection, quality control and forensic investigation of paving materials.

“This was a knowledge-building, data-gathering study that will help fill out our materials library database to correlate test results of asphalt materials to field performance.”
—David Van Deusen, Research Operations Engineer, MnDOT Office of Materials and Road Research

What Did We Do?

Preliminary testing focused on the eight MnROAD cells, pulling cores from the existing pavement before reconstructing new sections. Researchers tested these cores to refine methods for proposed tests. The team then gathered details on the binders and mixtures used in the 2016 reconstruction to use in its planned tests.

Researchers ran three tests on the eight asphalt mixtures and one test on the five asphalt binders used in the pavement mixtures at MnROAD. The asphalt mixture tests were:

  • Bending beam rheometer (BBR) test of mixtures to obtain creep stiffness and strength of asphalt mixtures. This approach uses small beam specimens useful in forensic investigations.
  • Low-temperature semicircular bend (SCB) test to measure fracture energy in mixtures. Currently there is no national standard test for fracture energy, but based on previous pooled fund work, MnDOT implemented the disk-shaped compact tension (DCT) test. The SCB results will be used to tie in the previous work and compare to the DCT.
  • Dynamic modulus test of mixture resilience that uses smaller cylindrical specimens, a benefit in forensic studies.

To obtain asphalt binder strength, researchers used a variation of the BBR test for mixtures.

A sample disk of asphalt stands vertically in testing equipment to be compressed from one edge to the other.
The SCB test applies pressure diametrically on an asphalt pavement puck along the axis of a 6-inch pavement cylinder to measure susceptibility to cracking at low temperatures.

What Did We Learn?

The four tests proved to be viable options for materials selection testing, quality control and forensic examination of samples from existing asphalt pavements. The SCB and dynamic modulus can be run with research equipment. These tests yielded repeatable results and identified differences in the eight mixtures that are expected to impact performance. In particular, the BBR test of mixture has potential for being a practical field screening test.

The BBR test of mixtures measures strength and creep of ½-inch-thick asphalt mixture specimens compared to an indirect tensile test of strength on 2-inch asphalt pucks, and the test produces similar results. The dynamic modulus test uses the same configuration as the indirect tensile test, but instead of applying vertical compression to a 6-inch asphalt core, it applies pressure on a 1.5-inch puck diametrically, yielding similar results on an asphalt mixture’s resistance to loading.

The SCB test, an alternative to the DCT test, provides similar results in measuring the fracture energy of asphalt pavement mixtures. Either of these two newer tests is viable for MnDOT use. The binder BBR strength test represents a viable alternative to the direct tension test that, due to complex sample preparation and expensive equipment, is not frequently used.

“These test methods produce repeatable, consistent results, are simple to perform and differentiate between mixtures. They could provide critical information on the evolution of pavement performance since they can be used for forensic analyses.”
—Mihai Marasteanu, Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering

All tests found sample performance highly dependent on temperature. Fracture resistance does not correlate directly with other tested values; two mixtures that share similar creep stiffness, for example, may not have similar fracture resistance. Results indicate the eight mixtures tested may perform similarly, although one with high recycled asphalt content and another with a highly modified asphalt binder may be outliers. Based on the laboratory test results, mixtures with performance-graded binders do not differ markedly when one is mixed with recycled asphalt materials. As is the case with all pavement field studies, time is required for the mixes to begin to distinguish themselves from one another in terms of field performance.

What’s Next?

MnDOT will share test results from this study with the Cracking Group team and include them in the overall examination of the MnROAD test cells. Researchers recommend comparing results to observed distresses and core tests periodically from these pavement cells to correlate field conditions and tested mixture performance over time. MnDOT will consider some of these testing methods and findings in its continuing effort to develop a performance-based balanced mix design approach for asphalt pavement.

This post pertains to Report 2019-03, “Investigation of Cracking Resistance of Asphalt Mixtures and Binders,” published January 2019. For more information, visit MnDOT’s Office of Research & Innovation project page.

Impact of Arterial Bus Rapid Transit on Traffic and Users

Video and statistical analyses showed that arterial bus rapid transit (ABRT) along Snelling Avenue in Minneapolis-St. Paul had no significant impact on traffic volume and wait times at intersections. Survey results demonstrated that users prefer the A Line over local bus service and consider it roughly equivalent to express bus, light rail and commuter rail service. Though ABRT has not converted automobile drivers to transit riders, users enjoy its easy payment format, cleanliness, route service and convenience. This study also provided recommendations for future ABRT line design considerations.

“Arterial bus rapid transit is perceived positively by users. It’s much like light rail and commuter rail—people think of it as equally useful as light rail.” —Alireza Khani, Assistant Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering

What Was the Need?

Bus rapid transit (BRT) entails dedicated lanes for buses and off-board payment for users who purchase fares before boarding the bus. In recent years, arterial BRT (ABRT) has developed as an alternative for metropolitan areas that lack roadway width for dedicated lanes. ABRT uses off-board payment but not dedicated lanes; instead, it uses existing roadway arterials and limited stops, offering a fast and efficient commute for users.

In 2016, the A Line opened on the Snelling Avenue corridor in Minneapolis-St. Paul, the area’s first ABRT line. It quickly gained popularity among transit customers as an alternative to local bus service, complementing the Twin Cities’ light rail system and commuter rail service from the suburbs.

Because the A Line operates within existing lanes of traffic and does not feature pullouts at its stops, it could slow corridor traffic when buses stop to load and unload. A preimplementation study of the corridor and A Line service suggested that traffic impacts would be minimal. The A Line’s actual impact on traffic, however, had not been determined, and user perceptions had not been assessed.

The A Line, Minnesota’s first ABRT line, has a positive reputation among riders and area residents,
and has had no negative impact on automotive traffic along Snelling Avenue, according to a recent
MnDOT study.
Passengers disembark from an A Line bus on Snelling Avenue.

What Was Our Goal?

MnDOT sought to examine the traffic impacts of the A Line in its first year of deployment, and to identify and quantify the A Line’s appeal to riders, including the service’s strengths and weaknesses, and how the transit experience of the A Line compares to local service. In addition, MnDOT needed to assess the characteristics of the service that could be used in new ABRT lines in the Twin Cities.

What Did We Do?

Researchers employed two strategies to evaluate A Line performance. First, the team conducted a traffic and transit capacity study. Investigators analyzed bus system data for ABRT and regular bus service capacity. In August 2017, researchers deployed four cameras each at two intersections: Snelling and University north of Interstate 94 (I-94), and Snelling and Dayton, south of the Interstate. Cameras collected video data for weeks before the 12-day Minnesota State Fair, which is held at the fairgrounds on Snelling Avenue, and additional video during the fair through its conclusion in September. Researchers analyzed recordings of four signal cycles before and after bus arrival at the intersections for traffic queues and volume.

Next, investigators studied the results of a 2016 Metro Transit survey of passengers on the A Line and four parallel standard bus lines. The study compared transit usage data from 2016 and 2017, before and after the A Line opened. The research team surveyed A Line passengers, station area residents, business workers and owners, automobile users, bicyclists and pedestrians. Team members also reviewed a recent study of Minneapolis-area real estate developers on transit facilities and options.

Traffic moves on northbound Snelling Avenue at Dayton in this image taken from the video analysis.
Researchers used video cameras at two key intersections along Snelling Avenue to evaluate the A Line’s impact on traffic.

What Did We Learn?

Video and data analyses revealed that the A Line increased overall transit capacity, and the time its buses spent not moving while passengers were loading and unloading during a green traffic signal had no significant impact on intersection queue length or traffic flow at the two intersections—during and outside State Fair dates. The A Line carries more riders than the local bus along the same route, and the greatest rider turnover occurs at the Snelling and University station, which connects with light rail service.

Surveys identified the five attributes most important to satisfactory transit service: easy fare payment format, hours of operation, complaint resolution, personal safety while riding and courteous transit drivers. A Line users were more satisfied with ABRT than with local bus service, and showed no significant difference in satisfaction with the A Line compared to express buses, light rail and commuter rail. For most individual service attributes such as payment procedures, travel time, shelter cleanliness, and route and bus signage, the A Line performed better than local buses, the same as light rail but not as well as commuter rail. Nonuser surveys indicated a positive perception of the ABRT, but mixed impact on pedestrian and bicycling activities and little impact on reducing preferences for using automobiles instead of transit.

What’s Next?

To improve A Line service, transit managers should focus on operating hours, the on-board safety of riders, reliability and total travel time. Researchers noted that rider satisfaction does not consider costs associated with improved service and recommended that future ABRT plans weigh improvements in the five key attributes of transit service against costs in planning new lines. The study findings and recommendations will be used in planning future ABRT lines.

“We will use this study to show MnDOT staff that arterial bus rapid transit should have minimal to no impact on existing traffic and signal operations.” —Carl Jensen, Transit Advantages Engineer, MnDOT Metro District

This Technical Summary pertains to Report 2018-35, “After Study of The Bus Rapid Transit A Line Impacts,” published December 2018. For more information, visit MnDOT’s Office of Research & Innovation project page.

New Tools to Optimize Truck Station Locations

The Minnesota Department of Transportation (MnDOT) has 137 truck stations across the state. These stations house and allow maintenance of MnDOT highway equipment as well as provide office and work space for highway maintenance staff. Within 20 years, 80 of these stations will need to be replaced as they reach the end of their effective life spans. Researchers developed a geographic information system based modeling tool to determine the most effective locations for truck stations in the state. Using data from many sources, a new research study has determined that MnDOT could rebuild 123 stations, relocate 24 on land available to MnDOT and combine two. MnDOT would save millions of dollars using the location optimization alternatives over the 50-year life cycle of a typical truck station.

What Was the Need?

MnDOT operates 137 truck stations, 18 headquarter sites for maintenance operations and over 50 areas for materials delivery. Truck stations are used to house and maintain large highway equipment, and to provide office and work space for highway maintenance staff. Some stations also store materials. 

The average life span of a truck station is 50 years. Within the next 20 years, 80 of MnDOT’s truck stations will need to be replaced. With costly capital replacement imminent, MnDOT has considered measures to optimize truck station locations within its eight state districts, including possibilities of reducing the size of some, increasing others, or combining the facilities of some state and local agencies into new partnerships. Determining the best effective locations for new truck stations could reduce costs for both state and local partners.

MnDOT needed a means of selecting and collecting the most appropriate data for an investigation into optimizing truck station locations. The agency also needed tools such as a computer model to analyze the data. These resources would allow MnDOT to determine the most time- and cost-effective locations for future truck stations. 

What Was Our Goal?

The initial objective of this research project was to collect data about truck service areas, including the quantity of highway equipment and materials capacity, and the materials storage capacity of facilities. This information combined with service route data would allow MnDOT to optimize truck station locations by determining whether facilities should be closed, resized, combined or relocated, and whether other materials storage locations would be necessary. An economic benefit–cost analysis would compare alternatives. 

A map of Minnesota indicates the location of each of MnDOT’s 137 truck stations with a blue square and of major highway routes connecting the stations, also shown in blue.
This project will determine the future of more than half of MnDOT’s 137 truck stations in the next two decades.

What Did We Do?

To determine how other departments of transportation (DOTs) and related agencies have addressed choosing the best locations for facilities, researchers conducted a literature review that included reports from six state DOTs and Australia, Transportation Research Board publications and other research papers. In addition, they consulted the standards developed by MnDOT’s Truck Station Standards Committee. 

Researchers also conducted surveys and interviews of both MnDOT and outside agency stakeholders. 

With many data sets collected for each truck station site, researchers used a geographic information system (GIS) platform to solve a location-allocation problem and a multivehicle routing problem for the truck stations. The problems incorporated such factors as amount of equipment, equipment capacity, storage capacity, material demand for road segments and other information. Estimated costs of operation for each location alternative were compared to present costs of each truck station. 

“Using real-world data, we built GIS models of maintenance operations to determine optimal truck station locations. With expected life spans of around 50 years, truck stations that are optimally located will reduce operating costs and save money for MnDOT and Minnesota taxpayers.” —William Holik, Assistant Research Engineer, Texas Transportation Institute

MnDOT’s Maple Grove Truck Station and Maintenance Center is a new 108,000-square-foot facility.
MnDOT’s truck stations range in size from Class 1 buildings of at least 25,000 square feet to smaller
Class 3 facilities with four or fewer overhead doors.
MnDOT’s Maple Grove Truck Station and Maintenance Center is a new 108,000-square-foot facility. MnDOT’s truck stations range in size from Class 1 buildings of at least 25,000 square feet to smaller Class 3 facilities with four or fewer overhead doors.

What Did We Learn?

The literature review showed that optimizations of facility locations may require a second level of sites, such as strategically placed materials storage depots. Some research also showed that both transportation and facility costs must be considered and that after a certain point, consolidation of stations could cost more as vehicles and staff were required to drive farther to reach them. 

Reports of state DOT location optimization efforts were instructive. Iowa DOT noted the need to consider the slow highway speeds of snowplows. This was a critical element for researchers to include in their optimization models as it determines route travel times. Vermont Agency of Transportation highlighted the use of satellite materials depots. Generally, state DOT efforts were confined to small regional issues, unlike MnDOT’s statewide scope.

In interviews with MnDOT and local agency stakeholders, researchers learned about partnerships that already existed between MnDOT and city and county agencies. These partnerships primarily included the sharing of truck stations and sometimes of materials. These partnerships were included in the optimization development.

Researchers optimized the truck station location using a GIS optimization model and separate cost analyses. They developed alternatives for each truck station individually. Each alternative was then analyzed to determine costs and savings over a 50-year life cycle. 

Finally, researchers determined which alternatives could be most effectively executed and their optimum order. They also developed an implementation plan for station relocation and replacement. This modeling was an iterative process: Each optimal location replaced the existing location and became the baseline against which the next station alternative was compared. The result was a comprehensive set of location possibilities for each MnDOT district with multiple alternatives for every truck station, including benefit–cost analyses. Researchers’ optimization solutions determined that 123 truck stations could be rebuilt on-site, 24 could be relocated on land available to MnDOT, and two could be combined. 

“We successfully analyzed all of our truck station and loading locations, determined which were good candidates for potential relocation or consolidation, and developed a data-driven plan of action to save millions of dollars.” —Christopher Moates, Planning Director, MnDOT Building Services

What’s Next?

MnDOT now has the information it needs to effectively implement cost-saving changes in future truck station planning and construction. The agency could use the researchers’ initial recommendations or further employ the GIS modeling tool to examine variations on the results of the project. 

This post pertains to Report 2019-10, “Optimizing Truck Station Locations for Maintenance Operations,” published February 2019. For more information, visit MnDOT’s Office of Research & Innovation project page.

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