Category Archives: Materials and Construction

Bridges and Structures, Materials and Construction, Policy and Planning, research

Report examines condition of Minnesota’s roads and bridges to inform future investments

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Reprinted from CTS News October 6, 2026

From freight, commercial, and public transit to personal commuters, delivery drivers, and emergency services, millions of vehicles travel Minnesota roadways each year. As the number of drivers on the roads grows, ensuring safe and reliable transportation infrastructure is crucial. A recent analysis performed by the Transportation Policy and Economic Competitiveness (TPEC) Program examined the performance and condition of Minnesota’s roadways at the county level from 2018–2024.

Using data regularly collected and maintained by the state, the TPEC team — led by Camila Fonseca-Sarmiento, director of fiscal research at the Institute for Urban and Regional Infrastructure Finance at the UMN’s Humphrey School of Public Affairs and a CTS scholar — assessed condition, safety, and performance trends of Minnesota transportation infrastructure and how these measures have changed over time. Bridge and pavement conditions were categorized as being in good condition, fair condition, or poor condition.

Pavement conditions and trends

The data examined focused on two roadway networks in Minnesota — the National Highway System (NHS) and the County State-Aid Highway (CSAH) system. The NHS, which includes interstates and local and national highways, makes up just 10 percent of statewide lane miles but carries a majority of vehicle traffic. The CSAH system carries significantly less traffic but is more than double the length of the NHS. Pavement types across these roadway networks were evaluated by surface distress or visible defects, ride quality or pavement smoothness, the overall condition of pavement, and the estimated time until the end of the pavement’s service life. Among the analysis’s key insights:

  • More than 80 percent of NHS roads and more than 60 percent of CSAH system roads are in good condition.
  • The CSAH system has more than double the number of roads in poor condition than the NHS.
  • Interstates consistently have the highest pavement quality.
  • Counties with an interstate highway tend to have the most vehicle traffic and pavement in good condition.
  • Counties in northwest Minnesota have the most CSAH roads in good condition while the metro area has the fewest.

Bridge conditions and trends

The team’s research found that the bridge network across Minnesota, comprising 13,500 bridges, remains generally well maintained. Some key trends identified:

  • Bridge conditions have been slowly declining since 2019, with the number of bridges in fair condition increasing.
  • As the population grows, the share of bridges in good condition declines.
  • Counties in northwest Minnesota have a higher rate of bridges in poor condition.

These insights indicate that more bridges will require greater levels of maintenance and preservation efforts in the years to come.

Crash trends

The COVID-19 pandemic altered the driving habits of many — as more people stayed home, fewer vehicles traveled the roads. While the total number of crashes across Minnesota has declined since 2020, the severity of crashes has risen slightly. Among the key crash trends from the report:

  • Minnesota counties average 929 crashes per 100,000 people.
  • Counties in southwest Minnesota have higher rates of property damage, injury, and fatal crashes.
  • Counties in northwest Minnesota have lower rates of property damage, injury, and fatal crashes.
  • Between 2019 and 2023, crashes statewide causing property damage decreased by 22 percent and fatal and injury crashes decreased by 13 percent.
  • Severe weather or wildlife encounters may be contributing to increased crashes in some counties.
  • As the population grows, crash rates increase.
Minnesota county map shaded by rates per 100,000, ranging from 0–500 to 1501+, with some areas marked NA

“The findings emphasize the importance of continued monitoring and investment in Minnesota’s transportation infrastructure,” Fonseca-Sarmiento says. “Understanding how roadway and bridge conditions vary throughout the state is essential for prioritizing resources when investing in the transportation network and improving safety. As transportation demands continue to evolve, data-driven decision-making will remain crucial to ensure a reliable and resilient transportation network for Minnesotans.”

— Krysta Rzeszutek, CTS digital editor

Related MnDOT Research from 2026

Materials and Construction, research

New Project: Tools to improve the durability of asphalt pavements

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This research focuses on improving the durability of asphalt pavements by examining emerging technologies, materials, and temperature‑control methods. Recent advancements—such as warm mix asphalt (WMA), polymer modifiers, bio‑binders, and surface treatments that reduce heat absorption—show strong potential for reducing asphalt aging and increasing resistance to thermal cracking. Environmental factors, including pavement shading from trees, have also demonstrated measurable effects on pavement temperature and long‑term performance.

To receive email updates about this project, visit MnDOT’s Office of Research & Innovation to subscribe.

Building on these developments, this project aims to conduct a comprehensive review of existing tools and evaluate two promising approaches in detail: WMA technology and updated asphalt binder specifications. Experimental testing will compare WMA with conventional mixtures, analyzing compaction behavior and low‑temperature cracking resistance using methods such as gyratory compaction and semi‑circular bend testing. Parallel testing of asphalt binders will assess new specification parameters to predict cracking performance more accurately under Minnesota’s climate conditions.

“We are always looking at increasing durability in our pavements,” said Michael Flaagan, county engineer for Pennington County, Minnesota. ”This research gives us additional choices in increasing performance with the potential for cost savings.”

The anticipated outcome is a set of practical guidelines for Minnesota agencies to integrate WMA and improved binder selection into current pavement practices. These strategies are expected to increase pavement lifespan, reduce maintenance costs, lower emissions, and support better decision‑making for agencies with limited budgets. By identifying and validating the most effective tools and materials, the project contributes to long‑term sustainability, cost efficiency, and improved resilience of the state’s asphalt pavement network.

The Objectives:

  1. Conduct a comprehensive literature review of current technologies, materials, and temperature‑control methods that improve asphalt pavement durability.
  2. Evaluate the compaction behavior and low‑temperature cracking resistance of warm mix asphalt (WMA) in comparison with conventional mixtures through laboratory testing.
  3. Assess newly proposed asphalt binder specification parameters using low‑temperature binder tests, including Bending Beam Rheometer (BBR) and Asphalt Binder Cracking Device (ABCD) methods, to better predict cracking performance.
  4. Develop preliminary guidelines for integrating WMA technologies and improved binder selection into Minnesota’s asphalt paving practices.

Project Details:

Details of the research study work plan and timeline are subject to change.

To receive email updates about this project, visit MnDOT’s Office of Research & Innovation to subscribe.

Materials and Construction, research

New Project: Evaluating and implementing ground penetrating radar for monitoring moisture fluctuations in in-service roads

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Moisture fluctuations within pavement foundations—driven by rainfall, freeze–thaw cycles, and groundwater changes—can significantly impact pavement performance, especially on Minnesota’s low volume roads. Traditional moisture measurement methods, such as in place sensors or falling weight deflectometer testing, tend to be costly, intrusive, slow, and offer limited spatial coverage. Ground penetrating radar (GPR), however, provides a faster, non-invasive, and more affordable way to assess subsurface moisture over large areas, making it particularly useful for local agencies with limited resources.

To receive email updates about this project, visit MnDOT’s Office of Research & Innovation to subscribe.

This research aims to advance the validation and practical use of GPR for evaluating pavement moisture in real‑world low‑volume road settings. The study includes conducting GPR surveys on test sections equipped with moisture sensors, comparing datasets, and assessing how effectively GPR can identify areas affected by drainage or moisture issues. To support broader implementation, the project also involves creating an automatic detection algorithm capable of identifying moisture variations in unbound pavement layers.

The algorithm will be refined into a user‑friendly software tool featuring modular code, a graphical interface, and documentation. Ultimately, the project will deliver implementation recommendations designed to strengthen decision‑support tools and technical guidelines, helping Minnesota’s transportation agencies more effectively monitor and manage pavement moisture conditions.

“For this research to reach its full potential, participation from local engineers and practitioners is essential,” said Eyoab Zegeye, research operation engineer at MnDOT’s Office of Materials and Road Research. “Based on our encouraging findings, the next step is to evaluate whether this approach could be implemented on real low‑volume roads and develop testing and reporting protocols that can be quickly adopted by local agencies. County and municipal staff have the closest understanding of their road networks, challenges, and resource limitations. Their involvement ensures that the tools developed are realistic, usable, and aligned with real‑world needs.”

The Objectives:

  1. Validate and implement GPR-based pavement moisture assessment methods on low-volume roads.
  2. Evaluate GPR’s ability to efficiently survey large pavement areas and monitor moisture conditions.
  3. Identify pavement sections potentially affected by drainage issues using GPR data.
  4. Conduct GPR surveys on low-volume road test sections equipped with in-situ moisture sensors and compare the results.
  5. Develop an automatic detection algorithm to evaluate moisture variation within unbound pavement layers.
  6. Validate the algorithm and implement it as software with modularized code, a graphical user interface, and a user manual.
  7. Produce implementation recommendations to enhance decision-support tools and technical guidelines for assessing moisture fluctuations in Minnesota’s low-volume roads.

Project Details:

  • Estimated Start Date: 7/16/2025
  • Estimated Completion Date: 6/30/2027
  • Funding: MnDOT and Local Road Research Board
  • Principal Investigator: Halil Ceylan
  • Co-Principal Investigators: Sunghwan Kim
  • Technical Liaison:  Eyoab Zegeye

Details of the research study work plan and timeline are subject to change.

To receive email updates about this project, visit MnDOT’s Office of Research & Innovation to subscribe.

Materials and Construction

Incorporating Environmental Assessments of Paving Roadways 

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To limit the environmental impacts of transportation projects, agencies need to identify the sources of greenhouse gas emissions throughout a project’s life cycle. By identifying specific sources and activities that generate emissions, the Minnesota Department of Transportation (MnDOT) can take steps to reduce its carbon footprint. This project explored the use of life cycle assessments and Environmental Product Declarations to measure and reduce the emissions associated with MnDOT paving projects, specifically the transportation of asphalt from production plants to job sites and subsequent paving activities.

Continue reading Incorporating Environmental Assessments of Paving Roadways  →
Materials and Construction, research

New Project: Holistic design and selection criteria for unbound geomaterials used in pavement systems

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Unbound geomaterials used in roadway base layers strongly influence pavement durability, yet current practices in Minnesota often focus narrowly on gradation and index properties. This research seeks to fill that gap by examining mechanical behavior alongside hydraulic, water retention, and suction characteristics to better understand how these materials perform within real-world environmental and pavement systems.

To receive email updates about this project, visit MnDOT’s Office of Research & Innovation to subscribe.

Pavement base layers serve two essential purposes: providing mechanical support that prevents fatigue, settlement, and rutting, and enabling drainage that protects the structure from elevated pore pressures and freeze‑thaw damage. Although millions of tons of these materials are used annually, past studies rarely consider the full interaction between the base layer, surface conditions, and environmental factors.

The project’s primary goal is to develop a new holistic guideline for selecting unbound geomaterials and designing pavement foundations. By integrating both mechanistic and hydraulic considerations, the long‑term objective is to support a shift toward design approaches that enhance roadway performance while reducing maintenance needs and costs.

The Objectives:

This project aims to develop a new holistic unbound geomaterial selection and pavement foundation design guideline. This will be achieved through the completion of four primary objectives:

  1. Review of current roadway foundation design practices and guidelines of MnDOT and local counties and cities in Minnesota.
  2. Collection and analysis of existing data from the literature on the topic (including the extensive previous work of the research team).
  3. Developing a holistic design and material selection framework that explicitly takes material drainage characteristics, material mechanical characteristics, unsaturated soil mechanics properties, and prevailing environmental characteristics into account.
  4. Extensive laboratory and field testing to demonstrate and verify the new approach.
  5. Evaluating and modifying the pore suction resistance factors and seasonal multipliers used in MnPAVE.

Project Details:

Details of the research study work plan and timeline are subject to change.

To receive email updates about this project, visit MnDOT’s Office of Research & Innovation to subscribe.

Materials and Construction

Preventive Road Sawing and Sealing to Reduce Maintenance Costs

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Asphalt pavements will inevitably begin cracking and require maintenance to preserve an adequate level of service. However, preemptively sawing and sealing roads at prescribed locations can reduce cracking and improve maintenance predictability. To help the state’s transportation agencies better manage pavement deterioration, the Local Road Research Board sought to compare the performance of a lower-cost unmodified asphalt binder using the saw-and-seal method with that of a higher-cost polymer-modified asphalt binder without using saw and seal.

Continue reading Preventive Road Sawing and Sealing to Reduce Maintenance Costs →
Materials and Construction, research

New Project: Design guidance and best practices for the use of light fill for sustainable road construction

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Lightweight Fill (LWF) materials—including Tire Derived Aggregates (TDAs), expanded shale/clay, foamed glass, and recycled glass aggregates—show strong potential for sustainable road construction, yet important knowledge gaps remain. Their environmental impacts, long‑term performance, and construction requirements are not fully understood, especially under Minnesota’s freeze‑thaw cycles and high groundwater conditions. Concerns such as leachate production, potential trench settlement, and challenges faced by local road departments highlight the need for further investigation.

To receive email updates about this project, visit MnDOT’s Office of Research & Innovation to subscribe.

This research seeks to address these gaps by reviewing existing practices, evaluating both environmental and mechanical performance, and developing data‑driven recommendations for using LWF materials in road construction. A detailed assessment will help clarify how these materials behave in pavement structures and embankments and how they interact with water‑sensitive environments. A decision matrix will be used to compare LWF materials and support future project planning.

To achieve these goals, the study will evaluate environmental impacts such as leachate risks, analyze long‑term structural integrity, and conduct Life Cycle Assessment (LCA) and Life Cycle Cost Analysis (LCCA) to assess sustainability and financial feasibility. The research will also develop recommended construction techniques that improve performance and support Minnesota’s climate goals and broader sustainability initiatives.

“Lightweight fill materials show promise for sustainable road construction, and Minnesota looks to learn from what has been done previously,” said Andrew Witter, Sherburne County Public Works Director and County Engineer. “This project will evaluate how these materials behave in local conditions and develop guidance for local counties and cities. The results will help agencies make informed decisions for the benefit of the transportation infrastructure in Minnesota.”

The Objectives:

  1. Document existing research and practices related to LWF materials in road construction.
  2. Evaluate the environmental impacts of LWF materials, focusing on leachate risks and interactions with water-sensitive environments.
  3. Analyze the long-term performance and structural integrity of LWF materials in both pavement structures and embankments.
  4. Compare LWF materials in a decision matrix to guide future project decisions.
  5. Perform Life Cycle Assessment (LCA) and Life Cycle Cost Analysis (LCCA) to assess the sustainability and financial feasibility of LWF materials.
  6. Provide recommendations to develop construction techniques that optimize the use of LWF materials and align with Minnesota’s climate goals and broader sustainability initiatives.

Project Details:

  • Estimated Start Date: 06/25/2025
  • Estimated Completion Date: 06/30/2027
  • Funding: Local Road Research Board
  • Principal Investigator: Michele Lanotte
  • Co-Principal Investigators: Angela Farina
  • Technical Liaison: Andrew Witter

Details of the research study work plan and timeline are subject to change.

To receive email updates about this project, visit MnDOT’s Office of Research & Innovation to subscribe.

Materials and Construction, research

Evaluating the Use of Stabilizers on Gravel Roads 

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Gravel roads often experience significant deterioration that requires costly maintenance. To mitigate this deterioration, transportation agencies can apply chemical stabilizers to improve aggregate cohesion. This project examined the mechanical, environmental and economic performance of five gravel stabilizers and three application methods to determine the best options for reducing deterioration.

Continue reading Evaluating the Use of Stabilizers on Gravel Roads  →
Materials and Construction, research

New Project: Are Current Rigid Pavement Roundabout Designs Working in Minnesota?

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As roundabouts become more commonplace in Minnesota, the role they play in the improvement of traffic flow and safety increases. Although rigid roundabouts were originally designed with the assumption that they would behave like rigid pavements on motorways, their performance can be affected by the unique traffic patterns and stresses they experience. The irregular shapes of their concrete slabs face distinct braking, stopping, accelerating, and drainage conditions, which increase the likelihood of cracking and other forms of pavement distress. Heavy trucks navigating sharp curves add additional strain. While concrete pavements are generally durable, pavement wear is greatly affected by elements characteristic of roundabout design.

To receive email updates about this project, visit MnDOT’s Office of Research & Innovation to subscribe.

This research seeks to evaluate and improve the performance of concrete roundabouts in Minnesota, with the aim of extending their service life and increasing their overall effectiveness. The project will start with a comprehensive field survey throughout the state, collecting information on different roundabout designs and documenting the types and levels of pavement distress present. The data will be analyzed to identify performance trends and gain a clear understanding of each design’s advantages and shortcomings. In addition to the field investigation, the study will conduct an in-depth structural review of factors such as joint spacing, dowel bar placement, slab geometry, and drainage features.

Using insights from both the field observations and structural analysis, researchers will propose design strategies to improve the durability of Minnesota’s roundabouts. These recommendations have the potential to strengthen roundabout design practices and contribute to a safer, more dependable transportation system.

“With the rapidly growing number of concrete roundabouts being constructed throughout Minnesota, it is important that we monitor their performance and adjust our design methodologies to ensure that the traveling public can utilize them safely, while maximizing the investments spent on their construction and maintenance,” said Thomas Burnham, research operations engineer with MnDOT’s Office of Materials and Road Research. “This research project is set up to accomplish these goals.”

The Objectives:

  1. To measure rigid pavement roundabout performance for various traffic loads and design features.
  2. Using pavement system performance data, identify any design changes that could minimize maintenance.
  3. Create state-of-the-art guidelines and recommendations for improving the design, function and lifespan of concrete roundabouts in Minnesota.

Project Details:

Details of the research study work plan and timeline are subject to change.

To receive email updates about this project, visit MnDOT’s Office of Research & Innovation to subscribe.

Materials and Construction

Evaluating Recycled Asphalt Pavement Mixtures 

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The use of recycled asphalt pavement (RAP) materials in asphalt pavement construction provides numerous benefits, including reduced material costs, fewer environmental impacts and decreased demand for virgin aggregates and asphalt binders. While previous research has examined the performance of RAP materials, less attention has been given to their compaction properties. This project examined RAP mixtures with varying RAP contents to better understand their implications for pavement construction and environmental sustainability.

What Was the Need?

Road projects commonly use RAP in asphalt mixtures to reduce material costs, conserve virgin materials and improve pavement construction sustainability. However, concerns remain that increased RAP content could reduce compactability and crack resistance, negatively impacting pavement performance. 

A better understanding of RAP material characteristics and mixture behavior is therefore critical to developing mixture designs that can reliably provide pavement durability and field performance. This project investigated RAP mixture design, compaction behavior at varying RAP contents, the impact of graphite nanoplatelet (GNP) modification and the low-temperature performance of different RAP mixtures.

What Did We Do?

This study analyzed RAP samples from three projects that used the same pavement mixture. Because RAP materials are not uniform, investigators evaluated the samples using processed black-and-white curves, chunk index, gradation, moisture content and binder content. For example, the chunk index measure material homogeneity, with higher homogeneity being desirable for mixing with virgin materials.

The first phase of testing analyzed RAP mixtures containing 25%, 40% and 50% RAP using a gyratory compactor. A mix design spreadsheet documented the proportions of virgin aggregates and RAP needed to achieve target gradations. 

Next, investigators evaluated the compaction performance of GNP-modified RAP mixtures by comparing a 50% RAP mixture with and without 6% GNP at compacting temperatures of 135°C, 115°C and 95°C. Previous research indicated that adding GNPs to RAP mixtures can significantly improve low-temperature flexural strength and allow contractors to compact asphalt mixtures to higher densities, potentially improving pavement durability and performance.

Lastly, the project examined the low-temperature strength, fracture and creep performance of RAP mixtures by performing semi-circular bend fracture and bending beam rheometer mixture tests on blends containing 0%, 25%, 40% and 50% RAP.

What Did We Learn?

Gyratory compaction testing showed that mixtures with higher RAP contents required fewer gyrations to reach the target air void level. Specifically, the 50% RAP mixture achieved the targeted 5% air voids with fewer gyrations than the 40% and 25% mixtures. Investigators concluded that the improved compaction was likely due to a greater contribution from mobilized binder under the heating and mixing conditions. As RAP content increased, the amount of effective binder also increased, improving lubrication between aggregate particles and reducing resistance to densification.

Adding 6% GNP moderately improved compaction and reduced gyration demand, although the benefits diminished at lower compaction temperatures. Lowering the compaction temperature from 135°C to 115°C had little impact, but reducing the temperature further to 95°C significantly increased gyration requirements. These findings suggest that adding GNP could improve compactability and may allow for some reduction in heating and compaction temperatures.

Low-temperature semi-circular bend and bending beam rheometer testing showed that RAP mixtures had fracture energy comparable to the virgin mixture and generally higher fracture toughness and flexural strength. Within the range of 25% to 50% RAP, increased RAP content did not result in significant differences in low-temperature creep or strength behavior.

“These results provide promising evidence for potentially increasing the use of RAP materials, but more work is needed to implement it for use in the field,” said Eddie Johnson, Researcher, MnDOT Office of Materials and Road Research.

Overall, the findings demonstrated that RAP mixtures prepared under appropriate laboratory conditions can achieve satisfactory compactability and low-temperature performance. However, replicating these preparation conditions in the field may be challenging and costly. 

What’s Next?

While project findings support the continued use of RAP mixtures, additional research could further improve understanding of RAP performance, including:

  • Quantifying binder activation based on RAP source, heating temperature, mixing procedure and mixing duration.
  • Evaluating the effect of the RAP heating process on binder mobilization and compaction performance.
  • Conducting field validation of findings from this project, particularly the improved compactability with higher RAP contents and the benefit of adding GNP.

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