Roundabouts have been repeatedly shown to reduce fatal and serious crashes, in part by slowing vehicles as they move through intersections. Less was known, however, about how far those speed-reducing effects extend beyond the intersection itself or how they compare with other intersection types. To better understand these upstream and downstream impacts, MnDOT and local transportation agencies worked with investigators to measure vehicle speeds at roundabouts, signalized intersections and all-way stop-controlled intersections. The results showed that roundabouts provide traffic calming benefits beyond the intersection, supporting their continued use as a safe and effective design alternative.
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Incorporating Environmental Assessments of Paving Roadways
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.
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Evaluating a New Low Slump Concrete for Bridge Decks
For over 30 years, MnDOT has used the same low slump concrete overlay mixture to extend the service life of bridge decks. However, MnDOT suspected that changes in the materials used in the mixture over time may have contributed to increased cracking. This project evaluated a new mixture design, with and without glass fibers, to reduce cracking, improve sustainability and lower costs.
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Preventive Road Sawing and Sealing to Reduce Maintenance Costs
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.
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Evaluating the Use of Stabilizers on Gravel Roads
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.
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Mitigating Surface Tenting to Improve Road Quality
Transverse cracking and tenting are significant problems for asphalt pavements, especially in colder climates where the asphalt layer contracts during winter conditions. While the impacts of frost on roads are generally known, more data is needed about treatments to mitigate tenting and the influence of base layers on tenting behavior. This project examined the effectiveness of pavement treatment options and identified primary causes of tenting to help state and local engineers limit future tenting.
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Reducing Phosphorus Pollution from Ponds
Ponds and historic wetlands are essential for preventing excessive phosphorus from reaching downstream waters. These systems remove solids, nutrients, metals and hydrocarbons from stormwater runoff as particles settle to the bottom. However, low oxygen levels in the water can cause phosphorous to be released from bottom sediments. This project examined the effectiveness of strategies to limit that release and reduce negative downstream impacts.
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Evaluating Recycled Asphalt Pavement Mixtures
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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Evaluating the Use of a Common Alternative Deicer
When temperatures fall below 15 degrees Fahrenheit (F), salt loses its effectiveness at melting snow on roads. To accommodate lower temperatures, transportation agencies often combine an alternative deicer with salt brine to treat roads and return them to an appropriate level of service. This project investigated the most common alternative deicer used in Minnesota to provide guidance to state and county winter maintenance managers about its application at different concentrations and temperatures.
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Successfully Integrating Electric Vehicles into Fleets
While electric vehicles (EVs) provide benefits over traditional gas-powered vehicles, they also come with challenges. Investigators surveyed and interviewed staff, analyzed costs and developed an optimization model to address these challenges. Project findings will help agencies across Minnesota achieve a successful and cost-effective transition to the increased use of EVs.
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Crossroads 