Category Archives: Materials and Construction

Affordable Bridge Girder End Repair Method Restores Concrete Beams

By load testing part of a bridge that was removed over Nine Mile Creek, researchers have proven that an innovative and cost-effective method for repairing damaged bridge girders restores them to their original strength.

The findings will help MnDOT and other transportation agencies avoid lengthy traffic closures and more costly techniques when repairing other bridges.

“This innovative method works remarkably well. The amount of damage the crew repaired was pretty extensive. In the end, the strength of the repaired damaged girders was slightly more than the strength of the undamaged girders,” said Carol Shield, Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.

Background

The salting of bridge and roadways during Minnesota winters can create highly corrosive conditions that damage bridges. Such was the case with the Highway 169 Nine Mile Creek Bridge near Edina and Minnetonka, where leaking expansion joints caused corrosion to elements responsible for the strength of bridge girders: shear reinforcement, prestressing strands, and the surrounding concrete.

During a 2013 repair, crews encountered two locations of severe beam deterioration. To repair these areas, MnDOT used a novel method developed in Michigan that involved removing deteriorated concrete and cleaning the area, placing steel reinforcement cages around the damaged beam ends and then encasing the beam ends with concrete. The repair concrete was a specific form of concrete placement called “shotcrete”—a mix of sand, aggregate and cement that is applied with a hose that is wetted at the nozzle before the mixture is sprayed at high velocity onto the repair surface. When the desired thickness of the concrete placement is reached, the placement is troweled and shaped to finish to the desired cross section. The beam end repairs were made in October 2013 and allowed the bridge to continue its function to the public.

MnDOT was able to make the repairs without traffic interruption.

Several years later, the bridge was scheduled for replacement. The repaired girder ends appeared to be in good condition, but the repair technique had not been studied for strength. The bridge replacement presented MnDOT with an excellent opportunity to evaluate the repair method for use on other damaged girder ends.

What Was Our Goal?

When the southbound lanes of the bridge were taken out of service in spring 2017, four prestressed girders were removed from the structure and brought to the University of Minnesota’s Theodore V. Galambos Structural Engineering Laboratory for testing.

Researchers examined and tested the beams to evaluate the effectiveness of the reinforced shotcrete repair method.

“Two of the girders have ends that were repaired by MnDOT, and two girders have ends that never needed to be repaired,” Shield said. “We [tested] the four girders and [compared] their strengths to determine if the repair actually returned the girders to the strength they had prior to the corrosion-related damage.”

The fact that researchers tested good girders alongside repaired girders gave MnDOT a high level of confidence, said Paul Pilarski, Metro North Regional Bridge Construction Engineer, MnDOT Bridge Office.

Bridge girder ends can be repaired for only $5,000 to $10,000, using this new method.

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

What Did We Learn?

All repairs had been done in field conditions that have the potential to adversely affect the results. But when the beams broke in the lab, the shotcrete repair did not separate from the bonding surface. The repaired reinforced concrete beam ends were found to be at least as strong as similar beams that were in good condition and had not needed repair. The initial repair methods and subsequent testing of the prestressed beam ends are demonstrated in a video created by the research team (testing starts at 3:30 min).

Using this method, severely deteriorated beam ends can be repaired with reinforcement cages and shotcrete for $5,000 to $10,000. The alternative to this type of repair involves constructing a new beam, closing traffic, removing the bridge deck over the damaged beam as well as the beam itself, and recasting the bridge deck and barrier—an intrusive replacement that costs hundreds of thousands of dollars and more than a month of bridge lane closures.

What’s Next?

Results have been presented internally at MnDOT, at state and Midwest conferences in late 2017, and at the National Bridge Preservation Partnership Conference in April 2018. Presentations have impressed transportation engineers from around the country and have increased confidence in dealing with aging infrastructure. MnDOT will continue to refine repair methods with the shotcrete treatment based on best industry practices, and will continue to use the beam end repair method if similar conditions are encountered in the state.

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

 

Nanotechnology Reduces Cold-Weather Cracking in Asphalt Pavements

Adding graphite nanoplatelets (GNP) to asphalt binders and applying the methodology developed in a new MnDOT study could provide a cost-effective approach to reducing cold-weather cracking and increasing the durability of Minnesota pavements.

“This project gives MnDOT a low-cost way to incorporate the latest nanotechnologies into our asphalt mixtures, reducing cold-weather cracking and increasing the durability of Minnesota pavements,” said Shongtao Dai, Research Operations Engineer, MnDOT Office of Materials and Road Research.

What Was Our Goal?

The objective of this project was to develop a cost-effective method to determine the optimum mix design of GNP-reinforced asphalt binders and mixtures. This method would predict the fracture behavior of these materials using a combination of simple laboratory testing and computer modeling.

What Did We Do?

Researchers developed a method for determining the quantity of GNP to add to an asphalt binder to achieve optimal asphalt mixture performance. The method used a computer model to predict the low-temperature fracture behavior of mixtures based on bending beam rheometer (BBR) tests on fine aggregate mixtures. This test applies a load to the center of a thin, rectangular specimen that has been cooled to a low temperature while its edges rest on two elevated supports, and then measures how the specimen bends over time. The results of this test determine the stiffness of materials and their ability to relax the stresses of contraction.

The BBR test is simpler, less expensive and less labor-intensive than the more accurate semicircular bend (SCB) test, which measures fracture resistance—the way cracks in a material form—by loading a semicircular sample from its apex. However, the SCB test can determine the properties of all the particles within a mixture; the BBR test can only evaluate the mechanical properties of coarse aggregates. To obtain the accuracy of the SCB test without the labor and expense, the computer model developed by researchers in this study uses BBR results as inputs to simulate SCB tests and infer the properties of fine aggregates.

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Although simpler and less expensive than a SCB test, a BBR test only evaluates the properties of a mixture’s coarse aggregates.

What Did We Learn?

Researchers validated their computer model by comparing its results with those of  actual SCB tests. They found that the model was able to predict the results of SCB tests for both conventional and GNP-modified mixtures. By performing only a BBR test on the fine aggregates mixture and inputting the results into the computer model, researchers obtained a reasonable prediction of the fracture response of the final asphalt mixtures.

In turn, the model showed that using GNP in asphalt binders can significantly improve the strength and fracture resistance of a mixture compared to mixtures with unmodified asphalt binders. The model can be used as a design tool to determine what percentage of GNP is needed to achieve the necessary tensile strength for a target value of fracture energy.

What’s Next?

Using GNP in asphalt binders, in combination with the methodology developed in this project, could potentially provide MnDOT with a cost-effective approach to improving the cold-weather performance of Minnesota pavements, preventing cracking and increasing pavement durability. MnDOT will continue to evaluate the use of GNP in its asphalt mixes.

This post pertains to Report 2018-02, “A Mechanistic Design Approach for Graphite Nanoplatelet (GNP) Reinforced Asphalt Mixtures for Low-Temperature Applications.” Further GNP research is underway. Find related projects at MnDOT.gov/research.

Managing Stormwater Runoff with Recycled Peat and Taconite Tailings

Researchers have found that peat has high potential to replace commercial compost in MnDOT’s standard bioslope and bioswale design for roadside ditches, and that taconite tailings performed comparably to the sand currently specified in MnDOT designs, with the additional benefit of removing phosphates.

Finding alternatives to commercial compost and sand for use in bioswales will help MnDOT meet regulatory requirements for stormwater runoff, while reducing the costs and environmental effects of transporting and storing these materials.

“The results of this project will very much facilitate the development of green infrastructure by reducing its cost to MnDOT and Minnesota local agencies, helping them to do more with less,” said Dwayne Stenlund, Erosion Control Specialist, MnDOT Erosion Control and Stormwater Management.

What Was Our Goal?

The objective of this project was to evaluate peat and muck excavated from construction activities, taconite tailings from area mining operations, and other stormwater quality filter media for use in bioswales and bioslopes along Minnesota highways. Laboratory and field tests of these products would examine their capacity to absorb water, retain pollutants and support plant growth to determine if they are beneficial and practicable in these designs.

What Did We Do?

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For field tests, researchers created small plots using either peat or  compost mixed with native soil.

Researchers began by conducting a comprehensive literature review on the use of bioslopes and bioswales as stormwater treatment best management practices. Then they collected peat and muck near a highway construction project, as well as locally sourced sand, compost, taconite tailings and commercial peat.

These materials, as well as various combinations of materials, were used in laboratory experiments to determine how well they:

  • Absorbed water, using a falling head test to measure saturated hydraulic conductivity, which indicates the rate at which water infiltrates a material.
  • Retained pollutants, using leaching experiments to quantify how well they removed copper, lead, zinc, nitrate and phosphate.
  • Sustained plant growth, using bioassays and greenhouse studies.

Finally, researchers conducted pilot field tests on three plots containing a 50/50 percent peat and sand mixture, and another three plots with a 50/50 percent compost and sand mixture. Between April and August of 2017, they monitored water infiltration, discharge water quality and vegetation establishment for these sites.

What Did We Learn?

“Ultimately, a combination of peat and taconite tailings will compare favorably with current MnDOT specifications for bioslope and bioswale design,” said Kurt Johnson, Research Fellow, University of Minnesota Duluth Natural Resources Research Institute.

Researchers found that peat has a strong potential for replacing commercial compost in MnDOT’s standard bioslope and bioswale designs, and that taconite tailings also performed comparably to the sand currently specified in these designs. However, muck has little potential to replace commercial compost or peat due to its low permeability, poor infiltration and filtration properties, and lack of support for plant growth.
Results for the three properties of interest follow:

  • Infiltration rate: While muck had an unacceptably low hydraulic conductivity, peat performed at least as well as compost, and taconite tailings as well as sand. Pilot tests showed that a 50/50 mix of peat and taconite tailings had a similar water storage capacity to a 50/50 mix of compost and sand.
  • Pollution retention: Muck absorbed only 50 percent of metals; salvaged peat, commercial peat and compost performed well, absorbing more than 80 percent. However, only taconite tailings showed the potential to remove phosphate. None of  the materials removed nitrate.
  • Plant growth: Mixtures of compost or peat with sand or taconite tailings all performed well in providing a viable substrate for plant growth. Mixes containing compost performed the best in plant growth trials. Muck was difficult to mix with any other material, and its value for plant growth was minimal. Greenhouse study results showed no difference between sand and taconite tailings in their effect on plant growth response.

What’s Next?

In a second phase of this project, “Development and Regionalization of In Situ Bioslopes and Bioswales,” MnDOT will conduct further laboratory tests on alternative materials for bioslopes and bioswales, and expand field tests to several sites in Minnesota that have been constructed using these materials. Researchers also recommend the development of specifications and detail drawings for the use of these materials.

This blog pertains to Report 2017-46, “Comparing Properties of Water Absorbing/Filtering Media for Bioslope/Bioswale Design,” published November 2017.

Taking on potholes with new prevention and repair strategies

Potholes are one of the biggest and most costly ongoing maintenance challenges faced by highway agencies. Despite considerable progress in pavement materials and mechanics, pothole repair has remained an area in which little progress has been made.

To make headway in this area, Minnesota transportation researchers studied critical factors in pothole formation and repair in order to identify solutions that would reduce the occurrence of potholes and increase the durability of repairs. They also investigated the potential of newer materials, such as taconite and graphite nanoplatelets (GNP), in repair mixes. Researchers looked at how to make winter patches more durable and also different shapes of patches.

“Our goal was to provide a scientific assessment of pothole repair materials and practices,” said University of Minnesota professor Mihai Marasteanu, the lead researcher. Project sponsors were the Minnesota Department of Transportation (MnDOT) and the Minnesota Local Road Research Board.

What Did We Do?

Researchers began by reviewing national and international literature about pothole causes and repair activities. They also surveyed MnDOT maintenance superintendents and local engineers on current repair practices.

Next, the research team conducted simulations of square, diamond, and round pothole repair shapes to determine if some shapes were more conducive to reducing stress in repair materials. This stress analysis included the use of different common pothole filling mixes and their interface with existing pavement materials.

In the next stage of research, the team evaluated six asphalt mixes for relevant mechanical properties: four winter mixes, a polymer-modified hot-mastic asphalt mix suitable for winter and summer use, and a summer mix in two forms modified with GNP. Mixes were evaluated for compaction and bonding, tensile strength, and water penetration.

Laboratory test of pothole repair sample
Pothole repair samples performed poorly in water penetration tests, which suggested that most mixes will perform poorly under seasonal freeze-thaw stresses.

Finally, researchers studied national and international pavement preservation and pothole prevention practices and the cost-effectiveness of pothole repair.

What Did We Learn?

Through this work, researchers learned that pothole prevention requires repairing pavement cracks as they develop—and sometimes, even timely repairs only slow pothole development.

Laboratory analysis showed that cold mixes compact and bond poorly. To be more effective, these materials require significant curing not possible in the field unless heating is provided. The polymer-modified mastic patching material that was heated was stronger than the winter mixes even at very cold temperatures. Most mastics are used in warm weather, but this material may be effective for winter uses.

Durable winter repairs require expensive patching materials and on-site heating technologies such as truck-mounted microwaves. “To make winter repairs last longer, you need to provide an external source of heat to cure winter patching materials,” Marasteanu says.

Taconite-based materials activated chemically or by heating potholes before and after filling offer promise for more durable repairs. GNP modifiers improved compaction, tensile strength, fracture energy, and fracture resistance in the summer mix.

Pothole repair samples performed poorly in water penetration tests, which suggests that most mixes will perform poorly under seasonal freeze-thaw stresses.

Also of note, the study’s exploration of pothole repair shapes found that circular repairs offer the best filling and compacting performance; repair materials cannot fill corners, even with significant compaction.

 

“We had been squaring off potholes, making sure patches were all at right angles. But in this study, we found that square patches increase stresses at the boundaries. The ideal is a circular patch,” said Todd Howard, Assistant County Engineer, Dakota County.

What’s Next?

The most common pothole repair in Minnesota is throw-and-roll with HMA (using a truck’s tires to compact shoveled-in asphalt). Newer, more durable repairs include taconite-based materials activated chemically or by heating potholes with a truck-
mounted microwave unit before and after filling. While promising and, in the case of the microwave method, potentially effective in extreme cold, these approaches require further research before becoming widely used in winter and spring repairs.

GNP-modified mixes also warrant further study, especially in winter mixes. If MnDOT can encourage cost tracking, analysis of the cost-effectiveness of various pothole repair methods, including the mastic tested in this research, may become possible.

This research is part of a larger effort by MnDOT to improve pothole repair approaches and develop pothole repair guidance for crews throughout the state, including a recently released asphalt patching best practices guide with decision trees.

This post pertains to the Report 2018-14, “Pothole Prevention and Innovative Repair,” published April 2018. Part of this story was adapted from a June 2018 article by the Center for Transportation Studies. Further information is available on the project page and technical summary.

Using a National Database to Develop Performance Metrics for Local Pavement Markings

Pavement marking performance metrics from a new study will help Minnesota local agencies save time and money by choosing longer-lasting pavement marking products.

Researchers developed pavement marking performance metrics for Minnesota local agencies to use as a guide to make better pavement marking product decisions. The metrics were developed based on an analysis of survey data collected from Minnesota local agencies and MnDOT pavement marking data mined from the National Transportation Product Evaluation Program (NTPEP). Findings showed differences in product performance with regard to retroreflectivity and service life, which were impacted by variables such as road surface type, year of application, traffic volume and type of pavement marking.

“There would be great potential savings in using pavement marking products with a longer service life. Mining NTPEP data to analyze product performance has not been done before and should contribute substantially to this goal,” said Omar Smadi, Director, Iowa State University Center for Transportation Research and Education.

What Was Our Goal?

The goal of this research was to develop pavement marking performance metrics for Minnesota local agencies to use as a guide when choosing the most durable and cost-effective products. Researchers developed the pavement marking performance metrics, specifically for retroreflectivity and service life, by analyzing existing MnDOT data mined from NTPEP. They also used the findings to make recommendations for future pavement marking research to support local agency needs.

What Did We Do?

Researchers designed and conducted a survey to assess pavement marking products used by local agencies in the state. Then they extracted 2010 and 2013 MnDOT pavement marking data from NTPEP to analyze the performance of products that survey respondents identified as commonly used.

NTPEP data included products tested at two different sites and applied on different road surfaces. Researchers analyzed performance with regard to retroreflectivity and deterioration or longevity of the materials under various conditions, such as road surface type, year of application, traffic volume and type of pavement marking. Based on results from the analysis, researchers developed performance metrics for Minnesota local agencies to use as a guide for choosing particular pavement marking product types.

What Did We Learn?

From the survey results, researchers learned that the majority of Minnesota local agencies use either latex or epoxy as their primary pavement marking material. However, epoxy and tape outperformed latex at all levels of conditions and provided a service life of three years or more.

A few survey respondents also reported grooving as a method that seemed to extend the service life of latex paint markings. Researchers were unable to investigate the impact of grooving, however, since MnDOT grooving data was not accessible.

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Grooving may extend the service life of pavement marking materials.

From the NTPEP data analysis, researchers concluded the following:

  • White markings had significantly higher initial retroreflectivity and slower deterioration than yellow markings.
  • Road surface type does not significantly impact retroreflectivity throughout its service life.
  • Epoxy has higher retroreflectivity than latex materials.
  • As expected, markings on wheel zones deteriorated faster, reducing retroreflectivity over time.
  • Deterioration values of markings varied among different test sites, which may be attributed to differences in average annual daily traffic (AADT) values (10,000 in 2010 versus 37,000 in 2013) or installation practices.

“The findings from this research will be beneficial for Minnesota local agencies in determining which pavement marking materials are most effective,” said Kate Miner, then-Scott County Traffic Manager.

What’s Next?

Although the product performance metrics data will help Minnesota local agencies make better pavement marking product decisions in less time, researchers recommend developing a guidebook to make the information more usable. Adding grooving data to the guidebook would also be beneficial to investigate the potential impact grooving provides in extending the service life of pavement markings.

Researchers also recommend testing the same products evaluated in this research on low-volume local roads and on challenging surface types. MnDOT NTPEP data only included products that were tested on high-volume freeways.

This blog pertains to the Local Road Research Board-produced Report 2017-43, “Minnesota Local Agency Pavement Marking: Mining Existing Data,” published November 2017. A related project has developed a spreadsheet tool to help local agencies prioritize pavement markings on low-volume roads.

Design Spreadsheet Offers Alternatives to Protect Pavements from Frost Damage

Researchers have developed a simple design tool for determining the amount of frost-free materials needed for a specific site’s subgrade to prevent frost and freeze-thaw damage to pavements.

“This tool will help us optimize construction to provide the best pavement,” said Steve Henrichs, Assistant Pavement Design Engineer, MnDOT Office of Materials and Road Research

Since 1995, MnDOT has required the use of frost-free materials (FFM) in subgrade depths of 30 to 36 inches for asphalt pavements, based on traffic load requirements. It is not clear that such FFM requirements are effective. In some areas, 30 inches may be excessive and, therefore, unnecessarily expensive; in others, 36 inches of FFM may not be enough, leading to costly pavement failure and repair. MnDOT needed a research-based pavement subgrade design procedure for resisting frost damage in pavements.

“Frost protection has not been studied in depth recently. This research used inputs based on soil type, location and expected frost depth, and didn’t require advanced modeling or expensive laboratory testing,” said Matthew Oman, Principal Engineer, Braun Intertec Corp.

What Was Our Goal?

The goal of this project was to develop a procedure for optimizing subsurface materials and thicknesses based on existing subgrade soils and geographic areas in Minnesota in order to resist pavement damage from frost action.

What Did We Do?

Researchers first reviewed existing literature on frost action and frost susceptibility. They synthesized national and international research and looked at practices and standards for mitigating frost action in states and countries with climates similar to Minnesota’s. Then they reviewed MnDOT’s current and historical policies and practices.

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Researchers reviewed charts and other resources on cold temperatures and structural insulation needs, like this map from the National Oceanic and Atmospheric Administration.

The central effort in this research was to examine existing pavements in Minnesota to characterize pavement performance and winter profiles. Researchers and the Technical Advisory Panel selected 72 pavement sites for study based on soil types (such as glacial till, clay, silt, sand and peat); pavement types (including concrete, asphalt and composite); subsurface materials and thickness; and weather conditions. The team evaluated construction logs, project plans, management data and subsurface investigations, and they augmented Minnesota-specific data with performance and soil data from the Federal Highway Administration’s (FHWA’s) Long-Term Pavement Performance (LTPP) program. Researchers created winter pavement profiles of most of the sites and compared them with roughness and ride quality data collected the previous summer.

Finally, the team analyzed performance trends and design and construction details to assess the effect of frost heave on ride quality. Using the findings from this effort, the team built a design tool for determining what pavement structures require of subgrades to resist environmental effects based on project location, projected frost depth and soil type.

What Did We Learn?

The initial evaluation did not produce strong correlations between winter ride quality and factors like FFM depth, grading soil depth and region. Winter ride quality measurements were poorer than summer measurements, but the role of FFMs remained unclear. Insufficient data, outliers and other questionable information were culled from the records, which were then amplified with data from other pertinent historical sources. Results from this effort suggested that FFM depth may improve pavement performance by incrementally reducing ride deterioration, particularly at depths of 25 inches or greater.

Review of relevant LTPP data established that shallower FFM depths and greater silt content in subgrades correlate with poor pavement ride quality. Silty soils, which have low permeability and produce high capillary effects, have long been considered susceptible to frost damage.

Researchers avoided thermodynamic modeling and analysis—and kept the design tool simple—by selecting subgrade silt content as a proxy for frost susceptibility. The spreadsheet tool uses project location (latitude and longitude), predicted frost depth and subgrade soil silt content as the key factors in frost susceptibility of pavements. The tool recommends frost treatment ranges from about 30 percent of predicted frost depth for soils with zero silt to over 80 percent of predicted frost depth for soils with 100 percent silt. The spreadsheet requires limited laboratory testing of subgrade soils, is simple and inexpensive to implement, and produces results similar enough to MnDOT’s practices that they will not require dramatic change in construction needs.

What’s Next?

Researchers produced four spreadsheets, each employing a different combination of frost depth prediction and soil type characterization. Once MnDOT selects its preferred spreadsheet and determines if additional subsurface tests should be included as inputs, pilot implementation will begin. Additional study to enhance the tool could include investigating MnROAD cells further, collecting more winter ride quality data, developing uniform frost depth prediction methods and tracking more information from new construction.

This post pertains to Report 2018-06, “Designing Base and Subbase to Resist Environmental Effects on Pavements,” published February 2018. The full report can be accessed at mndot.gov/research/reports/2018/201806.pdf.

Design Considerations for Embankment Protection During Overtopping Events

Roadways in Minnesota’s Red River watershed are prone to flooding and overtopping, where wide flows of water wash across the surface of the roadway. Repairing the resulting damage to roadway embankments can be costly and time-consuming, requiring lengthy road closures. Protecting roads from destructive scour could significantly reduce the cost and time of repairs after a flood event. Researchers investigated three “soft” design methods using full-scale models and field monitoring, with flexible geogrid mat providing the best erosion protection. Regardless of protection technique, any physical separation from the soil beneath led to failure by creating a pathway for water to follow. Establishing root growth and vegetation would improve the performance of all techniques by anchoring the soil.

“This project developed a fairly complete matrix of useful erosion protection measures that our own staff can implement—techniques that are less elaborate and more cost-effective than hiring contractors,” said J.T. Anderson, Assistant District Engineer, MnDOT District 2.

“This project was a combination of basic and applied science, and is a great example of the university and MnDOT working together successfully to solve problems unique to our geography and climate,” said Jeff Marr, Associate Director, Engineering and Facilities, University of Minnesota St. Anthony Falls Laboratory.

What Was the Need?

Roadways in the Red River watershed are prone to flooding and overtopping, where wide flows of water wash across the surface of the roadway. Downstream scour and erosion of roadway embankments can result in breach or washout of the entire roadway. Repairing the damage caused by flooding and overtopping can be costly and time-consuming, requiring lengthy road closures. Frequent flood events in recent years reinforce the need to protect roadways where flooding is likely to occur.

Raising the roadway to prevent overtopping is not a feasible solution, as flood plain law does not allow moving the problem elsewhere by backing up the water. The most cost-effective option is to allow floodwaters to overtop roadways and to try to protect their embankments from scour. Protecting roads from destructive scour and erosion by developing cost-effective scour prevention measures could greatly reduce the cost of repairs, as well as the time required to reopen the roadway after a flood event.

What Was Our Goal?

The goal of this project was to investigate the effectiveness of slope protection techniques to shield overtopped roadways and their downstream embankments from scour and erosion. A further goal was to use cost-effective methods that could be installed by local agencies instead of contractors. The researchers evaluated several “soft” design methods using an integrated approach of full-scale models and field monitoring.

What Did We Do?

Using the findings from a literature review, the research team developed a field-based program to collect data on the hydraulics associated with full-scale overtopping events. Researchers recorded flood stage at several locations near the Red River during over-topping events and evaluated the failure modes under natural conditions. Annual field monitoring occurred from 2013 through 2016 during overtopping events.

Next, the research team conducted a series of experiments at a full-scale laboratory facility to study the hydraulic and erosional processes associated with overtopping. The facility simulated a transverse section of a roadway and included an upstream water supply, road crest, shoulder and downstream embankment slope.

Photo of sod growing through square mesh plastic geogrid material
Sod is overlaid with geogrid to help stabilize the sod’s root system and soil beneath.

Two slopes were examined in the lab: 4:1 (horizontal:vertical) and 6:1. With bare soil used as a control, three erosion protection techniques were investigated: armored sod hydraulic soil stabilization, turf reinforcement mat (Enkamat) and flexible concrete geogrid mat (Flexamat). All three are alternatives to riprap and other hardscapes, and encourage vegetation to grow through a mat, helping to stabilize the soil and protect the embankment from scour and erosion.

What Did We Learn?

The researchers were able to draw some definitive conclusions from the laboratory experiments:

  • Bare soil with no vegetative cover (the control) is highly susceptible to erosion and is the worst-case scenario. New installations should have established vegetation before the first overtopping event is expected.
  • All three mitigation techniques reduced erosion, but the flexible concrete geogrid mat provided the best protection. Researchers noted that these results describe overtopping that occurred immediately after the protection treatments were installed. Established vegetation and root growth would likely improve the performance of all techniques.
  • Initiation of erosion appears to be linked to small-scale inconsistencies in the soil, erosion control material and placement of the protection technique. Small failures can quickly develop into mass failure of the embankment.
  • Failure occurred in areas where the protection technique physically separated from the surface of the soil and exposed a direct pathway for the water to flow. Inflexible protection techniques were the poorest performers.
  • Common locations for failure were the toe of the slope and the upstream transition from the shoulder to the soil slope, with steeper slopes failing most often.

What’s Next?

No mature vegetation existed on the embankment slope in the laboratory flume, which mimics the post-construction period in the field. Full vegetation is more typical for much of an embankment’s life cycle. Since one of the most important functions of vegetation on a slope is the ability of its roots to anchor soil, further study of these techniques with mature vegetation could provide a better understanding of their effects.

Future studies should include other stabilization techniques as well as the effects of overtopping on frozen and thawing soils, through-embankment seepage or piping, and various soil types on performance of the stabilization technique. Future projects could also evaluate the use of multiple techniques along with the study of anchoring improvements and longevity of the erosion control products.


This post pertains to the LRRB-produced Report 2017-21, “Design Considerations for Embankment Protection During Road Overtopping Events,” published June 2017. NCHRP Synthesis Report 496, “Minimizing Roadway Embankment Damage from Flooding” provides the state of the practice on mitigating damage from overtopping. 

New Procedures Offer Guidance for Using Bonded Whitetopping on Asphalt Pavements

Researchers developed procedures for selecting asphalt pavements for thin whitetopping based on site examination and lab testing. Test results do not offer definitive indications of how overlaid asphalts will perform, but procedures offer recommendations on pre-overlay pavement treatment, testing protocols and design considerations for bonded concrete overlay of asphalt.

“This research established a procedure for testing pavement cores. However, more performance data on whitetopping is needed to correlate pavement performance and asphalt properties,” said Tim Andersen, Pavement Design Engineer, MnDOT Office of Materials and Road Research.

“These procedures address collecting field data and testing pavement core samples in the lab. They also provide useful guidance for pavement repair and design considerations for overlays,” said Dale Harrington, Principal Engineer, Snyder and Associates, Inc.

A badly rutted pavement.
Rutted and otherwise damaged asphalt pavement is a candidate for a bonded concrete overlay that can mitigate damage under the right site conditions.

What Was the Need?

Many counties throughout Minnesota have used bonded concrete overlays to rehabilitate asphalt pavement. Though not widely used by MnDOT, a bonded concrete overlay, or whitetopping, normally involves milling a few inches of asphalt off the damaged surface and placing 4 to 6 inches of concrete over the asphalt pavement. A well-bonded overlay can add 20 years to a pavement’s service life.

Bonded whitetopping performance has not been care-fully tracked, and correlation of its performance with the underlying pavement condition is not well understood. Be-fore MnDOT can expand its use of bonded whitetopping, materials engineers wanted to better understand what asphalt pavement conditions are best suited to this type of overlay, how asphalt behavior influences the concrete top layer and what underlying pavement characteristics affect the expected lifetime and performance of bonded white-topping.

What Was Our Goal?

This project sought to develop an integrated selection procedure for analyzing existing, distressed asphalt pavement to identify good candidates for bonded whitetopping and establish design considerations for a site-specific, effective concrete overlay. By testing pavement core samples in the lab, investigators wanted to identify asphalt pavement properties that correlate with distresses in concrete overlays that are 6 inches or less. They also sought specific recommendations for managing trans-verse cracking in asphalt to avoid reflective cracking into concrete overlays.

What Did We Do?

Researchers began with a literature review of approaches to selecting pavements for bonded whitetopping. The results of this review were used to develop testing procedures to identify the volumetric properties of existing asphalt pavements. Researchers applied these procedures to 22 pavement cores from six concrete overlay sites in Iowa, Michigan, Minnesota and Missouri. Selected projects entailed 4-inch to 6-inch overlays in fair to good condition that were built from 1994 through 2009. Data about mix design, asphalt condition, pavement thickness, overlay thickness, site conditions and other details were available for each site.

The research team compared roadway data with falling weight deflectometer measurements from pavement cores to evaluate field performance and design recommendations suggested by the selection procedure. To refine the procedures, investigators evaluated volumetric asphalt characteristics for their potential influence on premature overlay cracking due to stripping, slab migration and reflective cracking. Finally, the team developed a detailed selection process that includes steps to identify and test asphalt pavements with potential for bonded whitetopping, repair asphalt before overlays and establish design considerations for overlays based on the test results from the selected asphalt pavement.

What Did We Learn?

The selection procedure, which is based on recommended practices from the National Concrete Pavement Technology Center, has six steps:

  • Perform a desk review of available site data, including design, repair and environmental conditions.
  • Obtain pavement core samples.
  • Conduct site visits to examine existing conditions.
  • Obtain additional core samples for testing, when necessary.
  • Prepare preliminary cost and materials estimates, if practical.
  • Provide design recommendations.

Investigators tested pavement cores for air voids, density, stiffness, fatigue, aging, strip-ping potential and other distress parameters. Results were inconclusive in terms of identifying asphalt properties that lead to specific bonded concrete overlay failures or to long-term performance of bonded whitetopping projects. The pavement cores showed wide variation in material properties, but few of these distresses. Researchers framed the recommendations for testing volumetric properties in the format of MnDOT’s Pavement Design Manual, giving the agency an easily adoptable core testing protocol.

The selection procedures include information about the impact of transverse cracking, rutting, longitudinal cracking and other distresses on concrete overlays, and provide recommendations for treating various distresses before whitetopping. Design considerations for whitetopping are also provided based on site conditions and the results of core, ground penetrating radar and falling weight deflectometer testing.

What’s Next?

Tested overlay sections should be evaluated over time to determine if life expectancy is met or if asphalt stripping, slab migration or reflective cracking has decreased overlay life. Because volumetric tests failed to provide conclusive relationships between asphalt properties and overlay distress, further research is needed to identify mechanistic or field tests that could correlate asphalt properties with concrete overlay performance. Once this additional research is completed, the selection procedures identified could be refined and placed in the design guide. A life-cycle cost analysis of overlays would also be useful for decision-makers considering bonded concrete overlays of asphalt.


This Technical Summary pertains to Report 2017-24, “MnDOT Thin Whitetopping Selection Procedures,” published June 2017. 

Research Confirms Low-Binder Asphalt Pavement Mixtures Prone to Cracking

Disk-shaped compact tension test
The disk-shaped compact tension test determines fracture energy of pavement samples, a strong predictor of cracking performance.

Research showed that lower asphalt binder mixtures are susceptible to premature cracking. The current use of coarse-graded mix designs should be adjusted to narrow the gradation difference between larger and smaller aggregates in the mixes. While the research suggests such mixes should be used sparingly in Minnesota, it did not provide definitive data suggesting the practice should be stopped altogether. The practice may continue on a limited basis.

What Was the Need?

Introduced in 1993, Superpave has successfully helped transportation agencies in northern regions design asphalt pavements that are less susceptible to thermal cracking. When tested, Superpave-compliant designs were found to resist both rutting and thermal cracking.

Gradation-based design approaches have also allowed for the use of coarse-graded, low asphalt binder mixtures. These mix designs establish a maximum aggregate size and reduce the range of usable gradations. Such coarse-graded designs meet MnDOT specifications because the maximum aggregate size falls within the acceptable gradation range. However, the reduced fine aggregate content made possible by the use of coarse aggregates leads to a mix that, while still within specifications, offers less surface area to be coated by the asphalt binder and can encourage unwelcome permeability in the field. To win low-bid competitions, contractors have embraced these low-binder, coarse-graded designs to reduce binder and aggregate costs.

Transportation engineers noticed that these pavements seemed to “gray out” or lose their dark color more quickly than previous asphalt designs. These pavements also seemed to grow somewhat more brittle and were less able to rebound from loading. Such asphalts are thought to be prone to quicker failure than mixes with finer aggregate and more binder. Road designers typically attribute thermal cracking and potholing in low-binder asphalt to the increased permeability that leads to water incursion and freeze-thaw damage.

What Was Our Goal?

The goal of this project was to determine how well low-binder asphalt pavements per-form and whether current designs make sense in terms of cost–benefit and durability. Researchers would identify any relationship between reduced bitumen use and potential for cracking, and would suggest changes to specifications for coarse-graded asphalt pavement mixtures to prevent such cracking issues.

What Did We Do?

Researchers worked with MnDOT to identify 10 pavement locations in Minnesota that used 13 coarse-graded, low-binder asphalt mix designs. Investigators extracted data on cracking, roughness and other factors for these sites from MnDOT’s pavement management system. The research team then visited the sites and inspected the pavements.

Researchers developed a coring plan, and field samples were cored for volumetric analysis to determine the binder, aggregate, air void level and other properties of each mixture. They also tested permeability and dynamic modulus, and conducted fracture energy testing to determine cracking resistance.

Investigators used performance modeling to analyze the test results of pavement proper-ties and project pavement durability. Then they compared the projected performance to actual field performance. From this assessment, they drew recommendations for modifying specifications for MnDOT low-binder, coarse-graded asphalt mixtures.

What Did We Learn?

This study suggests MnDOT should reduce its use of coarse-graded asphalt mixtures, but the findings did not provide sufficient data to justify prohibiting the use of coarse- graded, low-binder asphalt designs.

Low-binder mixtures were prone to thermal and transverse cracking. Their high permeability left them vulnerable to premature moisture and freeze-thaw damage. Field and laboratory testing and modeling demonstrated that coarser mixtures produce excessive cracking in a short period of time. Thin overlays of 3 inches or less crack more quickly than thick overlays of 4 to 6 inches. Mechanistic-empirical simulations showed that low-binder asphalt mixtures were significantly inferior to higher-binder mixtures in terms of thermal cracking.

Most of the high-cracking mixtures showed low fracture energy in testing, suggesting the value of fracture energy testing and modeling. Disk-shaped compact tension testing showed that higher permeability mixtures correlate reasonably well with lower fracture energy. Eight of the 13 mixtures were more permeable than recommended, and six significantly so. Typical volumetric properties poorly predicted cracking.

To better project pavement performance, researchers recommend that MnDOT maintain volumetric testing-based specifications, but add performance testing-based specifications and testing designs for fracture energy, fracture resistance, modulus and other parameters. For Superpave designs, investigators suggest using a narrower aggregate gradation range, reducing the gradation gap between minimum and maximum aggregates in mixes.

What’s Next?

Although the research validates MnDOT engineers’ anecdotal concerns, the pavements evaluated were mostly overlays, which are known to be susceptible to transverse cracking because of flaws in underlying pavement layers. MnDOT may weigh the results and adjust specifications, but would likely require further study of coarse-graded mixture performance before ruling out its use or identifying situations in which coarse-graded mixtures may be the best option. Additional research could consider the use of nonuniform lift designs for asphalt pavements, varying mixes for each lift in the structure rather than using a single, uniform mix for every layer in the full depth of the pavement.


This post pertains to Report 2017-27, “Impact of Low Asphalt Binder for Coarse HMA Mixes,” published June 2017. 

Recycled Asphalt Pavement Use is Increasing

MnDOT has long been a leader in the use of recycled asphalt pavement or RAP. Much of the nation’s current use of RAP in hot mix paving asphalt is based on the methods first used in a 1978 project that reconstructed the streets in what is now the 3M campus in Maplewood.

Subsequent MnDOT projects using as much as 80 percent RAP in hot mix paving revealed significant pavement performance problems, according to Curt Turgeon, state pavement engineer.

Currently, MnDOT asphalt paving specifications allow 30 percent RAP in overlay projects and 20 percent RAP when crack resistance asphalt cements are used in new or reclaimed pavements.

For economic and environmental reasons, Turgeon said MnDOT has renewed interest in increasing the use of RAP. Work includes trials of varying percentages in hot mix, trials at MnROAD of cold central plant recycling, and continued use of cold in-place recycling and full depth reclamation.

Increase in hot mix percentages

In District 6, a 13-mile section of the 30-mile Hwy 52 resurfacing project contains 40 percent RAP on the wide outside shoulders. The mixture contains proprietary additives to potentially assist in the rejuvenation of the RAP.

Tom Meath, District 6 materials engineer, said the higher percentage is being used because of the abundance of RAP available.

“This project allows the contractor to use up stockpiles of pavement from this and other projects and reduces the amount of new material needed, while not diminishing the quality of what’s used in the traveling lanes,” he said.

Meath said there are counties and cities in District 6 already using 40 percent RAP, but this is the first time MnDOT is trying it.

“We’re trying to figure out ways to use more RAP,” he said. “That’s a lot of money sitting there when we remove an asphalt pavement.”

Cold central plant recycling

This year’s MnROAD reconstruction, funded by the National Road Research Alliance, contains test sections of cold central plant recycling. This process uses 100 percent RAP mixed in a standard plant at ambient temperatures using an emulsified or foamed asphalt cement. The result is a product that is not resilient enough be used as a top surfacing so the test sections will receive either a standard hot mix overlay or a double chip seal.

Cold in-place recycling

The resurfacing portion of the Hwy 110 project east of I-35E and I-494 in Mendota Heights and Inver Grove Heights will use 100 percent recycled asphalt as the base layer of pavement.

Tim Clyne, Metro pavement and materials engineer, said using 100 percent saves on rock and asphalt costs, trucking costs and time. Since the material is reused with the cold in-place recycling process, the result is a more variable product than the material produced at the plant. Hot mix will be used as the top surface.

“It’s not a new technology, but this is the first time Metro has used the 100 percent RAP in at least 30 years,” he said. “It provides a long-term pavement solution for an extended pavement life.”

See a video of cold in-place recycling, which shows a milling machine, a machine that screens and crushes oversize materials and then mixes in an asphalt emulsion, an asphalt tank and an asphalt paver and roller.

Full depth reclamation

Full depth reclamation uses equipment often described as a rototiller for pavements. The asphalt pavement and some of the existing base is ground together in place. Multiple passes of the reclaimer are often used. The final pass may include the addition of a binder such as asphalt emulsion, foamed asphalt, cement or lime. The result is an aggregate base with the old crack pattern completely erased.

“Hot mix overlays on full depth reclamation base have shown excellent performance compared to a typical mill and overlay project,” said Turgeon.

Economic and performance benefits of these techniques are well understood.  Until recently, the environmental benefits of using materials in place instead of hauling off to a plant haven’t been well documented. MnDOT participates in the Recycled Materials Resource Center pooled fund project now housed at the University of Wisconsin – Madison.

In June 2017, the RMRC completed an analysis of nine paving projects that documented an average of 22 percent overall savings and 20 percent savings in water usage.


This post was written by Sue Roe and was originally published on MnDOT’s Newsline on  Aug. 23, 2017.