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.
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.
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?
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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
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
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.
Researchers examined mixtures of recycled asphalt pavement (RAP) and aggregate for new gravel road surface layers in the lab and in the field. Although test results did not align perfectly, and field results were somewhat uneven, findings suggest that mixtures with 70 percent RAP content can reduce dust generation. After a year of service these roadways can match all-aggregate gravel road performance in terms of strength, but with a smoother ride.
What Was the Need?
Gravel roads offer a cost-effective option for road departments that wish to avoid the expense of asphalt and concrete roads in rural or low traffic areas. However, about an inch of gravel is lost from these roadways each year. Aggregate resources are diminishing, and gravel and crushed rock aggregate is growing increasingly expensive.
Gravel also generates dust that can reduce visibility, affect road performance and result in complaints from nearby homeowners.
Recycled asphalt pavement (RAP) can be an effective component of new asphalt pavement mixtures. Many aggregate producers stockpile RAP that has been broken into the size of aggregate. But not all RAP works well mixed in asphalt, and some aggregate yards are too far from pavement projects to economically use RAP in pavement.
Road agencies frequently use RAP in gravel roads. The asphalt content in RAP can bind with dust from crushed rock or gravel, helping manage fugitive dust. A recent study in Wyoming found that using RAP in new gravel surface applications at less than 50 percent of the aggregate resulted in good road performance and kept dust to a minimum.
What Was Our Goal?
In light of the findings from the Wyoming study, researchers sought to determine the optimal level of RAP in an aggregate mixture for Minnesota gravel road surfaces. These new applications would offer good driving stability while also controlling fugitive dust.
What Did We Do?
Research began with a review of the literature on RAP as an aggregate component of surface, base and subbase layers, as well as a survey of Minnesota counties on their experience with these mixtures.
In the lab, the research team tested three RAP materials and virgin aggregate from two Minnesota locations in various RAP content levels for strength and compression. Investigators then compared the economic feasibility of 100 percent virgin aggregate use to 50 percent virgin and 50 percent RAP aggregate mixtures on a 1-mile aggregate road, including annual grading and eventual regraveling in the estimations.
Research in the field focused primarily on six 1,000-foot gravel road test sections: four sections in Goodhue County using 15, 30, 45 and 60 percent RAP content, and two sections in Carlton County using 30 and 50 percent RAP. The studies entailed all-virgin aggregate control sections, and installations were made over roads with various subgrade soils that presented a variety of properties. Sites were tested for elasticity, bearing strength and fugitive dust generation.
A secondary field study focused on RAP contents of 50, 70 and 80 percent in 3-inch surface courses for three test sections and one control section in Goodhue County. Sites were tested for elasticity, strength, dust generation, ride quality and surface aggregate looseness over time, and some lab tests were conducted.
“The 70 percent RAP mixture seemed to be about the best combination. We put RAP down in fall 2017, and by the next summer, it was working much like a regular gravel road.” —Charles Jahren, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering
What Did We Learn?
Previous research indicated that RAP can help reduce fugitive dust, offers value as surface courses, and can reduce moisture susceptibility of gravel roads in cold or wet locations.
Lab mixtures with 30 percent RAP consistently produced high compressive strength values, and higher RAP levels generally correlated inversely with bearing strength. Improvements in dust reduction were limited until RAP levels exceeded 50 percent.
Economic analysis determined that a 50/50 percent mix of RAP and aggregate would cost 1.5 percent more than an all-virgin aggregate surface course in terms of construction and maintenance, but potential reductions in dust generation, surface aggregate loss and regraveling after three years of service may produce savings from RAP use.
Results from field testing defied clear recommendations on optimal RAP content. Generally, higher RAP content offered greater elasticity and lower levels of loose aggregate initially, but these benefits fell to equal or below non-RAP levels after a year. Higher RAP correlated with reduced dust generation, but again fell over the first year of service. In secondary testing, initial dust generation was lower with the 50 percent mixture than the others, but after a year was lowest with the 70 percent mixture.
Ultimately, researchers found that after a year, during which fugitive dust production was reduced, the performance of a 70 percent RAP content aggregate surface course was most like a virgin aggregate surface course and offered a smoother driving surface.
“These findings provide another tool in the toolbox. They will be most useful to engineers who haven’t used RAP in gravel roads and to county engineers who have a RAP resource.” —Joel Ulring, Pavement Engineer, MnDOT State Aid for Local Transportation
While this research did not develop a definitive recommendation for an optimal RAP content in surface courses for aggregate roads, it did produce useful data on performance. The study did encourage a general sense that 70 percent RAP content for surface courses of approximately 2 inches may be effective and warrants systematic study for a three-year period.
The Minnesota Department of Transportation (MnDOT) had suspended the use of post-installed epoxy-coated rebar for concrete barrier repairs as a precautionary measure because chemical adhesives used in the process are not designed for use with coated bars. But laboratory testing (conducted in a recent MnDOT-sponsored research study) has now shown that using these adhesives with coated rebar for post-installation works well and provides a safety level 200 to 300 times that predicted by manufacturer specifications. MnDOT is considering research recommendations to modify the installation process in order to resume using coated rebar in post-installed concrete crash barriers.
What Was the Need?
As bridges age, local and state agencies are called on to repair and upgrade elements. Local agencies may install new light posts and replace chain-link rails, while state agencies may replace slabs on wing walls, deck and crash barriers, and other non-hanging bridge components. Work like this requires replacing concrete elements and installing new reinforcement bars. Until recently, MnDOT and local agencies used epoxy-coated steel rebar for these post-installed concrete panels and barriers.
Epoxy-coated rebar resists corrosion from salt and water that penetrate concrete members, especially at seams and cracks. Post-installed rebar requires chemical adhesives to secure the bar in place to effectively transfer loads from one concrete slab to another. Manufacturers test these adhesives with standard uncoated steel rebar to provide users with application guidance and to give engineers data on the tensile pullout strength of the rebar in concrete, a key property in the design of concrete bridge components.
As with many bridge materials, specifications for adhesives are conservative; the tensile strength of chemically adhered rebar can be assumed to be much higher than specified. Despite the almost certain safety of the practice, the MnDOT Bridge Office suspended the use of epoxy-coated rebar in post-installation applications because manufacturer specifications are based only on results from testing with uncoated rebar.
What Was Our Goal?
The goal of this project was to determine the effect of the epoxy coating on the tensile strength of rebar that is post-installed with a chemical adhesive. To achieve this goal, researchers surveyed other state transportation agencies to determine how these agencies use epoxy-coated rebar in certain post-installation practices. In addition, the research team conducted laboratory evaluations of common adhesives and epoxy-coated rebar installed in hardened concrete.
“The point of epoxy-coated rebar is to gain a longer life cycle. While MnDOT practices were not unsafe, there are ways the agency could be more accurate.”—Ben Dymond, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering
What Did We Do?
Two post-installation applications were studied: crash barriers between bridge support piers and crash barriers on the edges of bridge decks. Investigators distributed a survey to all 50 state transportation agencies to learn the state of the practice related to postinstalling epoxy-coated rebar with chemical adhesives in hardened concrete bridge elements. The research team also studied MnDOT procedures for using adhesives with epoxy-coated and uncoated rebar during post-installation of rebar in crash barriers.
Investigators then conducted a laboratory study of the four most common adhesives used in Minnesota. They installed six uncoated lengths of rebar for each adhesive in one concrete slab and six epoxy-coated lengths of rebar for each adhesive in another slab. Rebar was pulled from the concrete to determine the tensile strength.
What Did We Learn?
Thirty states responded to the survey. Twelve of these state agencies do not use epoxy-coated rebar post-installed with chemical adhesives in concrete. Eleven of the 18 agencies that do use epoxy-coated rebar in these applications employ manufacturers’ data on bond strength; another six use a standard, national calculation method.
Tensile pullout strength of the bars varied by adhesive type. In all cases, the manufacturer specifications for strength were very conservative and safe, identifying strength values 200 to 300 times lower than shown in laboratory tests. MnDOT’s approach to adhering epoxy-coated and uncoated reinforcement steel in hardened concrete barriers results in safe, reliable bonding and load transfer.
The difference in strength between chemically adhered epoxy-coated steel and uncoated steel bars was on the order of 10 percent; in some cases, coated rebar was slightly stronger with the tested adhesive. Below are the pullout strength ratios of epoxy-coated bars to uncoated bars (reduced by three standard deviations) for the four adhesives:
Powers AC100+ Gold: 0.89.
Red Head A7+:1.02.
ATC Ultrabond 365CC: 0.98.
Hilti HIT-RE 500 V3: 1.11. (In this case, however, bars ruptured, and pullout strength was not definitively established.)
Researchers recommended a modification factor for calculating bond strength to reflect the findings. Investigators also recommended changing a MnDOT bond stress specification or adopting manufacturer values, which would better meet national specifications for bridge design.
“After exploring the epoxy-coated rebar practices of other states, we identified a difference in the installation design with epoxy-coated bars and uncoated bars.”—Joe Black, Senior Engineer, MnDOT Bridge Office
Although MnDOT is not currently considering the use of epoxy-coated rebar in post-installed concrete, this study suggests what steps may be necessary to reauthorize its use in these applications. One solution would be to specify longer epoxy-coated rebar to mitigate the minor potential loss in pullout strength, allowing bridge managers to leverage the corrosion-resistance benefit of epoxy-coated rebar in these applications.
Bridge decks reinforced with one layer of epoxy-coated rebar and a bottom layer of uncoated steel rebar show corrosion damage sooner than decks constructed with all epoxy-coated rebar. Inspection methods should be enhanced to add a rating for cracking density on the underside of bridge decks. Repairs to mixed rebar decks should be conducted once a key deck surface inspection element has received a condition rating of two and held that rating for seven years, which is sooner than the average repair time of 8.5 years.
What Was the Need?
In concrete bridge decks, steel reinforcing bars are necessary to add tensile strength and transfer loads to beams. Additionally, steel reinforcement in concrete bridge decks is designed to control cracking, which will extend the service life of the bridge.
Steel also corrodes in salt environments, even when embedded in concrete. Water and road deicing chemicals can reach the steel and damage its strength and integrity. Between 1973 and 1990, MnDOT built approximately 660 bridges with more expensive, epoxy-coated rebar in the top layer of reinforcing matting and standard black rebar in the bottom layer. The coated top layer, only 3 inches below the deck surface, was expected to resist corrosion, and damage from salt and water would not reach the next layer of rebar, another 3 inches down in a 9-inch deck.
In recent years, MnDOT has used another reinforcing strategy: mixing noncorrosive fibers into concrete mixes to help prevent or minimize cracking and resist corrosion. The older, mixed reinforcement bridges remain in service and few have been redecked. Performance of mixed reinforcement and fiber reinforcement in Minnesota bridge decks has not been compared to the performance of bridge decks constructed with only epoxy-coated rebar.
What Was Our Goal?
MnDOT sought to compare the performance of mixed rebar decks with all epoxy-coated rebar decks, and the performance of fiber-reinforced decks with no-fiber concrete decks. MnDOT also wanted to learn how to plan preventive maintenance efforts for mixed rebar decks.
What Did We Do?
Researchers reviewed reports from inspections, conducted every two years, for bridges with mixed reinforcement decks and decks with 3-inch strips of fiber reinforcement mixed into the concrete. They narrowed their review to bridge inspection data from 506 bridges with epoxy-coated rebar (including 35 control decks with all-epoxy rebar) built between 1973 and 1990, and 22 bridges with fiber-reinforced concrete and epoxy-coated rebar (including four controls with no rebar) built between 2012 and 2017. All of the bridges were inspected through 2017.
Investigators then conducted site evaluations of 75 mixed rebar decks and 25 all-epoxy rebar decks, as well as 11 fiber-reinforced concrete decks with epoxy rebar and four without rebar. Site surveys focused on confirming the accuracy of recent inspection reports and recording signs of cracking, spalling and other deterioration conditions.
What Did We Learn?
All-epoxy rebar decks outperformed mixed rebar decks, showing less cracking on the top and underside of the decks. Mixed rebar decks deteriorated at a quicker rate on bridges with steel beams than on bridges with prestressed concrete beams. Traffic levels and surface cracking did not appear to affect deterioration of decks in any group.
“There’s not really a good visual inspection standard for quantifying cracking under the bridge deck, and that’s especially important for these types of bridges with mixed rebar. Using only epoxy-coated rebar in decks was a good idea.” —Ben Dymond, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering
Data sets were too small to draw any conclusions about possible differences in performance of fiber-reinforced decks compared to bridge decks that were not built with fibers.
Individual bridge elements, such as bridge deck surfaces, have historically been rated from one (best condition) to five (worst condition). Mixed rebar decks earned element ratings of three to four more frequently than all-epoxy rebar decks, and visual surveys identified more deterioration on the underside of mixed rebar decks than all-epoxy rebar decks.
The research team recommended amending bridge inspection procedures to add a new rating element for quantifying crack density on the underside of decks to anticipate and prevent spalling and delamination on the underside of mixed rebar decks. The team also recommended that once the deck condition element in mixed-bar decks holds a rating of two for seven years, more robust cracking sealing techniques should be considered to prevent it from reaching a rating of three. (For most bridges, that repair typically occurs after 8.5 years of service.)
Finally, the team recommended continued evaluation of fiber-reinforced decks as inspection data is collected over time.
“These findings may help us shift some priorities for repairing or replacing mixed rebar bridges. We will continue to advocate for the use of all epoxy-coated rebar wherever we anticipate high levels of chlorides.” —Nick Haltvick, North Region Bridge Construction Engineer, MnDOT Bridge Office
This research confirms that MnDOT’s current practice of using only epoxy-coated rebar in bridge decks remains a durable solution and offers the best long-term value in terms of repair needs. MnDOT will continue to evaluate fiber-reinforced concrete deck behavior and may adopt a rating method for identifying crack density on the underside of concrete decks.