Category Archives: Materials and Construction

Materials and Construction

Using Recycled Concrete Aggregate in New Concrete Pavement Mixes

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Using recycled pavement as aggregate in new concrete mixes can save money and promote environmental sustainability. New design methods published in a new research report allow engineers to create more durable mixes from recycled aggregate than in the past, reducing the need for virgin aggregate, a diminishing and expensive resource.

“This report shows that a lot can be done with recycled aggregate,” said Matt Zeller, Executive Director, Concrete Paving Association of Minnesota. “We can get the strength up to that of concrete with virgin aggregate by bumping our mix design and lowering our water-to-cement ratio.”

“Concrete pavement made with RCA can be beneficial both economically and environmentally,” said Farhad Reza, Professor, Minnesota State University, Mankato, Department of Mechanical and Civil Engineering.

Reza served as the project’s principal investigator.

What Was the Need?

When pavements are due for reconstruction, the old pavement is frequently crushed to aggregate-sized particles and used as the base course for new pavement. In the 1980s, MnDOT and other state transportation agencies began using such recycled aggregate in the concrete course as well. But this latter practice was discontinued by the early 1990s due to mid-slab cracking observed in pavements constructed with such concrete. Using recycled concrete aggregate (RCA) in the base course has continued, however.

Newer mechanistic-empirical design methods and performance engineered mixtures have led to improved RCA mixtures. For example, concrete mixtures now have lower water-to-cement ratios. These advances present an opportunity to re-evaluate the use of recycled aggregates in concrete mixes, which aligns with two important trends: the diminishing availability of virgin, high-quality aggregate, and the growing federal emphasis on sustainable design. Using recycled concrete as aggregate fulfills the three basic principles of sustainability: performance, environmental stewardship and cost-effectiveness.

What Was Our Goal?

Researchers sought to evaluate the performance of selected sections of concrete pavement in Minnesota that had been constructed with RCA; examine field samples and lab mixes; and develop guidelines for successful use of recycled aggregate in new concrete pavements.

What Did We Do?

Researcher vibrates RCA mix samples in a box.
Investigators vibrated RCA mixes in sample boxes to prepare the mixes for mechanical analysis.

After a literature search on the use of RCA in new concrete pavements, investigators examined the following issues:

  • Historical Performance. The research team gathered and compared data on performance, ride quality and durability for 212 miles of RCA pavement and for 212 miles of regular concrete pavement in the state. Both pavement samples had been built in the same time period and had had similar traffic levels.
  • Materials and Constructability. Investigators analyzed the ride quality of two-lift (or two-layer) concrete pavement test sections built in 2010 at the MnROAD test facility, using modeling to project long-term performance based on the historical evaluation. They conducted tests on nine cores pulled from the RCA pavements and tested new mixes made with recycled aggregate from Olmsted County, Minnesota. For comparison, they tested virgin aggregates from a Mankato, Minnesota, plant and fines from a Henderson, Minnesota, site.
  • Life-Cycle Cost Analysis. The research team conducted a life-cycle cost analysis of new RCA mixes and traditional concrete mixes, comparing their performance and cost-effectiveness.
  • RCA Guidelines. Based on the historical analysis, laboratory testing and modeling, and life-cycle cost analysis, the researchers developed new guidelines for the design and construction of pavements containing RCA in concrete mixes.

What Did We Learn?

Results showed that using RCA in concrete pavements can save money and is a sustain-able practice that produces durable concrete pavement.

  • Historical Performance. Most of the existing pavement studied had not reached the terminal ride quality index of 2.5—the level that generally indicates a major pavement rehabilitation must be performed. Analysis showed that rehabilitation is required, on average, at about 27 years of service for RCA pavements and at 32 years for standard concrete pavements.
  • Materials and Constructability. Mix design can be adjusted to achieve traditional strength levels that older RCA mixes did not reach. Elimination of fines and stricter adherence to gradation specifications for concrete aggregate can achieve workable and durable mixes that are less likely to suffer excess drying shrinkage. Pavements designed in this way meet the standards of the Federal Highway Administration’s INVEST program for sustain-ability in highway construction.
  • Life-Cycle Cost Analysis. Long-life RCA pavements are more economical in cost-benefit terms than are thinner, shorter-life RCA pavements.
  • RCA Guidelines. Researchers developed specification recommendations and design guidelines for the use of RCA in new pavement construction. Trial mixes are critical, and absorption and compressive strength must be examined before use. Recycled fines are not recommended, but otherwise RCA can be used in the full range of aggregate sizes between minimum and maximum. Recycled concrete pavement may not produce enough aggregate for both pavement and base course, but acquiring extra RCA to make the base course 70 percent recycled and 30 percent virgin makes the new pavement economical and sustainable.

What’s Next?

Keeping detailed records on mix designs used and tracking mix performance over time will help MnDOT to further refine its use of recycled aggregate in concrete mixes and will provide robust data on the performance of more sophisticated RCA mixes. A research team may want to consider using lower-quality recycled concrete as a bottom lift and higher-quality recycled concrete with virgin aggregate in the top lift. Methods for managing water input with recycled aggregate to achieve proper water-to-cement ratios warrant further study.

This Technical Summary pertains to Report 2017-06, “Evaluation of Recycled Aggregates Test Section Performance,” published February 2017.

Bridges and Structures, Materials and Construction, Policy and Planning

New Project: Creating a Tool to Estimate Bridge Construction Time and Costs

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MnDOT recently executed a contract with WSB & Associates Inc. to begin work on a research project titled “Bridge Construction Time and Costs.”

The research project will help the State of Minnesota’s Bridge Office develop a guidance document and a tool for bridge construction time estimation to be used by MnDOT District project managers and construction staff. The tool will provide a range of production rates based on specific design criteria, being more concise based on the level of information available and will aid in evaluating the potential benefit for accelerated bridge construction (ABC) techniques.

“This research will enable District project managers, who may not have bridge knowledge or background, to complete project planning and scoping more effectively,” said Paul Johns of MnDOT’s Office of Construction and Innovative Contracting.

Mike Rief of WSB & Associates will serve as the project’s principal investigator. Johns will serve as technical liaison.

According to the initial work plan, the project is scheduled to be completed by early March 2018, and WSB & Associates will complete the following tasks:

  1. Conduct an existing practices literature review of current departments of transportation processes around the United States for bridge time and cost estimation.
  2. Review and compile actual case study bridge construction production rates and cost data for major bridge components from state-provided diaries, schedules and bridge plans.
  3. Evaluate and select the best software format and style for a bridge construction time estimation tool. Load state case study production rate data into estimation tool and run validation using bridges currently under construction.
  4. Produce a research report summarizing the literature review on best practices. Produce a user guide for bridge time estimation tool and training presentation.
  5. An optional task, if the budget allows, will include the development of a cost estimating tool. Cost estimation data would be gathered from the literature review and case study analysis during the development of bridge construction time estimation tool for efficiency.
Materials and Construction

MnROAD Breaks New Ground

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In June, MnROAD, the only cold-weather accelerated pavement testing facility of its kind in North America, begins construction on its third phase of research since 1994, the first time MnDOT has rebuilt in partnership with other states.

Dozens of new experiments are planned along MnROAD’s test tracks in rural Albertville: the high-volume original Interstate-94 westbound (built in 1973), the mainline I-94 westbound (originally opened in 1994) and an adjacent low-volume road closed track.

Six states and numerous  industry partners recently formed the National Road Research Alliance (NRRA) to co-sponsor  the reconstruction.

NRRA-prioritized research will support state and local needs, including effective use of fiber-reinforced concrete, asphalt overlays of concrete pavements, cold central plant recycling and concrete partial depth repairs to name a few.

MnROAD has two 3.5 mile test segments on Interstate 94 and one closed 2.5-mile low-volume road.
MnROAD has two 3.5 mile test segments on Interstate 94 and one closed 2.5-mile low-volume road.

“The advantage of having these test  sections at MnROAD is we can take  bigger risks and push the envelope in terms of mix designs and layer thicknesses for both asphalt and concrete  layers that could not be done on a public roadway,” MnDOT Research Operations Engineer,   Dave Van Deusen said.

Forensic analysis of failed cells
Many old test cells will be dug up.  Before any reconstruction starts, however, each test section that is being reconstructed will receive a final forensic study. This allows researchers a look at each layer to see the distress that has occurred over the years—and make the final analysis of why it failed. There are always a lot of theories on the causes of what actually failed, but until the forensic is performed, there isn’t proof on what happened. These findings will help build longer-lasting pavements in the future.

The bid letting date for this year’s construction is April 28, but plans were made available for contractors on March 31. This should give the projects more exposure and generate more interest. Construction begins June 5 and continues until November 2017.

Focus Areas

  • HMA overlay and rehab of concrete and methods of enhancing compaction – States are looking for longer lasting HMA overlays of concrete. New mix designs were developed to promote long-term performance, including how reflective cracking effects can be minimized through design or other joint treatment.
  • Cold central plant recycling – Other states have used reclaimed asphalt pavement stockpiles into plant mix base course mixes (layers below the wear surface) to effectively recycle these materials in a controlled mix design. How can these layers best be used and what type of surface mix or chip seal can be placed on top?
  • Fiber reinforced concrete pavements – Nationally, states want to get a better understanding of the beneficial use of fibers in concrete pavement layers. Is it worth the cost? How can it be best used in both thin city streets and higher volume roadways? Can it be used in new construction and in concrete overlays? The research will provide the answers.
  • Long-term effects of diamond grinding – Each state has aggregates that have been used in concrete pavements that are considered reactive aggregates. Questions arise as to whether diamond grinding might accelerate deterioration in these pavements. What types of topical sealers can be used to treat the surface after the diamond grinding will also be tested.
  • Early opening strength to traffic – What effect does heavy traffic loading have on the long-term performance of full-depth concrete pavement, as well as fast–setting repairs? Test sections will be loaded by a pickup truck in one lane early enough to produce shallow ruts in the surface. In the other lane, a loaded 18-wheeler will travel over the new concrete immediately after it sets, and then sequentially every six hours up to 30 hours. The long-term effects of these early loadings will then be evaluated.
  • Optimizing the mix components for contractors – What effect do low-cementitious content mixes have on long-term performance and constructability of concrete pavements?  Two low cementitious content mixes will be studied to give agencies a better understanding of cost savings. Can these savings be achieved without significantly affecting long-term performance?
  • Compacted concrete pavement for local streets – Compacted concrete pavement is a form of roller compacted concrete that has a standard concrete pavement surface texture. The RCC industry has been successful in Michigan and Kansas constructing CCP pavement on local streets.  This research will determine if the texture that is accomplished is durable in harsh freeze-thaw climates.
  • Recycled aggregates in aggregate base and larger sub-base materials – States continue to look for effective ways to recycle materials into unbound bases. This research will add to MnROAD’s understanding of recycled bases and what seasonal strength values can be used for advanced mechanistic designs–and how they are affected by size/gradation.
  • Maintaining poor pavements– Road owners continually have less funding to maintain their roadway systems. What practices should be used for stabilizing both hot mix asphalt and Portland cement concrete roadways when longer-term repairs cannot be done due to funding levels?
  • Partial depth repair of concrete pavements – Agencies continually seek improved materials and methods for the repair of concrete pavements. In this study, up to 15 innovative concrete pavement repair materials will be evaluated on the concrete panels of the westbound I-94 bypass parallel to the MnROAD mainline.
  • Thin overlays-Experimenting with very thin overlays could provide a real benefit for a lot the roads currently out there. The premise is that with thin overlays, the ride can be smoother and the life of older roads can be extended.

“We don’t often get to reconstruct random roads these days, and when we do, we have much better specifications for low temperature cracking. By the same token, we have to maintain all those older roads built before we had performance grade binders,” said Dave Van Deusen, Materials and Road Research Lab principal engineer. “We will be doing this makeover on an original section of MnROAD built back in the 90’s.”

In one experiment, there is a head-to-head comparison of thin overlays on two sections of road. One section has a thick base and subbase under the asphalt. The other has a heavy asphalt top with very little base.

Van Deusen says if they can get an extra five years of life out of road using thin overlays, he would be pleased. Often, he admits, he is surprised by how long these “short-term” fixes actually last.

Stay up-to-date on construction by signing up for email alerts at mndot.gov/mnroad.  

Materials and Construction, Research

Taconite byproduct reduces road wear from studded tires

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In a recent project, the Alaska Department of Transportation (DOT) used a byproduct of Minnesota’s taconite mining industry for a section of the Alaska Glenn Highway.

The taconite byproduct—Mesabi sand—serves as the aggregate of a sand-seal treatment for a 4,600-foot stretch of the highway just north of Anchorage. Sand seals are an application of a sealer, usually an emulsion, immediately followed by a light covering of a fine aggregate (the sand).

“Our goal was to explore pavement preservation measures that extend pavement life and that also resist studded tire wear,” says Newton Bingham, central region materials engineer with the Alaska DOT. “Studded tires are allowed from mid-September until mid-April, and they cause rapid pavement wear.”

For the project, the Alaska DOT obtained sample pavement cores from the test area in 2014. Researchers then applied sand seals with two different hard aggregates—calcined bauxite and the Mesabi sand—to the surface of the cores to evaluate the effectiveness of each treatment.

Larry Zanko, senior research program manager of the Natural Resources Research Institute (NRRI) at the University of Minnesota Duluth, was the on-site representative for the taconite sand analysis. NRRI focuses on strategies to recover and utilize mineral-resource-based byproducts such as taconite and find potential beneficial end-uses for them.

“Taconite is one of the hardest natural aggregates,” he says. “Minnesota’s taconite mining industry generates tens of millions of tons of byproduct materials every year that could be used as pavement aggregate. Friction aggregates could be a higher-value niche for the industry.”

Testing of the sand-seals showed similar wear resistance for both types of aggregates. “We chose taconite sand since it is available from Minnesota as an industrial byproduct, whereas calcined bauxite sand has to be imported from nations on the Pacific Rim and costs more due to shipping,” Bingham says.

The Alaska DOT reports good performance to date on Glenn Highway and is funding ongoing pavement wear measurement.

NRRI researchers are also studying the use of taconite for other pavement applications. Funded by MnDOT, Zanko’s team developed (and later patented) a taconite compound for repairing pavement cracks and patching potholes (see an article the September 2016 Catalyst). The long-lasting patches reduce maintenance costs and traffic disruption. In continuing work funded by the Minnesota Local Road Research Board, researchers will refine the repair compound and develop and field-test a low-cost mechanized system for pavement and pothole repairs.

Materials and Construction, Research

Thicker may not equal stronger when building concrete roadways

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Transportation agencies have long placed high importance on the thickness of their concrete roadways, making it a major focus of control and inspection during construction. While it is commonly believed thicker concrete pavements last longer, there is little data to support this claim.

“One big reason for the lack of data on the relationship between concrete pavement thickness and performance is the destructive nature of these measurements,” says Lev Khazanovich, a former professor in the University of Minnesota’s Department of Civil, Environmental, and Geo- Engineering. “Concrete thickness is typically assessed by coring—a destructive, expensive, and time-consuming test that only offers widely spaced measurements of thickness.”

In a MnDOT-funded study, U of M researchers set out to fill this knowledge void by leveraging recent advances in the nondestructive testing of pavements that allow for large-scale, rapid collection of reliable measurements for pavement thickness and strength. They conducted four evaluations on three roadways in Minnesota using ultrasonic technology to collect more than 8,000 measurements in a dense survey pattern along with a continuous survey of observable distress.

“We found that both pavement thickness and stress measurements are highly variable, with a half-inch of variation in thickness about every 10 feet,” Khazanovich says. “Interestingly, three of the four surveys averaged less than design thickness, which is contrary to typical accounts of contractors building slightly thicker slabs in order to avoid compensation deductions.”

Data analysis showed that exceeding design thickness did not seem to increase or decrease pavement performance. However, a measurement of pavement strength and quality known as “shear wave velocity” did produce valuable findings. “A drop in the shear wave velocity strength measurement corresponded to an increase in observable pavement distresses such as cracking and crumbling,” Khazanovich explains. “This was especially apparent when we were able to easily identify locations of construction changes, where significant changes in shear wave velocity matched up with observable distress.”

The results of this study illustrate the importance of material quality control and uniformity during construction, since alterations in pavement strength and quality may significantly influence pavement performance. In addition, researchers say that despite inconclusive thickness results, it is still important that pavement has significant thickness to carry its intended traffic load over its service life. Finally, the study demonstrates that new methods of ultrasonic shear wave velocity testing are useful for identifying changes in construction and design that could lead to higher rates of pavement distress.

Materials and Construction

Partner States Get First Look at Minnesota Road Experiment

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Walking along a half-mile segment of Co. Rd. 8 near Milaca last month, materials engineers from around the country got a first look at a shared test site for pavement preservation.

Nearly 60 one-tenth mile sections of Co. Rd. 8 and nearby Hwy 169 were recently treated with various combinations of fog seals, chip seals, crack seals, scrub seals and microsurfacing and a number of thin overlays. Data will be collected from these experimental test roads for three years and compared with the results of a similar experiment in Alabama, where the same test sections were also built on a low- and high-volume roadway, to see which techniques are the most effective for preserving road life.

“Evaluating pavement performance in both northern and southern climates will provide cost-effective solutions that can be implemented nationwide,” said Ben Worel, MnROAD operations engineer.

Photo of Barry Paye, Wisconsin DOT chief materials engineer; and Tim Clyne, MnDOT Metro District materials engineer.

From left, Barry Paye, Wisconsin DOT chief materials engineer, and Tim Clyne, MnDOT Metro District materials engineer, participate in a discussion about future road research needs. Photo by Shannon Fiecke

Nineteen states, which are co-funding the study through MnDOT’s road research facility (MnROAD), were in town Oct. 26-27 for a joint meeting with the National Center for Asphalt Technology in Auburn, Ala. In addition to touring test sections built this summer near Milaca and at MnROAD’s permanent test track in Albertville, the group reviewed preliminary research results and discussed ideas for new experiments.

MnROAD began two joint research efforts with NCAT last year to advance pavement engineering issues that affect both warm and cold climates. In addition to determining the life-extending benefits of different pavement preservation techniques, the partnership has also built test cells to evaluate which asphalt cracking prediction tests best predict future pavement performance. This second study will help state DOTs improve the quality of asphalt mixes, so roads hold up better through harsh winters, leading to less thermal cracking and fewer potholes.

Click here to learn more about the MnROAD-NCAT partnership.

Materials and Construction

MnDOT, Alabama center team up for national pavement research

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The nation’s two largest pavement test tracks are planning their first-ever co-experiments.

The Minnesota Department of Transportation’s Road Research Facility (MnROAD) and the National Center for Asphalt Technology (NCAT) began discussing a formal partnership last year and have now asked states to join a pair of three-year research projects that will begin this summer.

Representatives of the test tracks are meeting next week in Minneapolis at the 19th Annual TERRA Pavement Conference. They said the partnership will develop a national hot mix asphalt cracking performance test and expand the scope of existing pavement preservation research at the NCAT facility in Auburn, Alabama, to  include northern test sections in Minnesota.

MnROAD plans to build test sections at its facility and also off-site on a low- and high-volume road, which may include concrete test sections if funding allows. These Minnesota test sections will supplement 25 test sections built by NCAT on an existing low-volume haul route in 2010 and an off-site high-volume test road planned for this summer in Alabama to assess the life-extending benefits of different pavement preservation methods. Both agencies have also been developing performance tests to predict the cracking potential of asphalt mixes, and they will now work together on that research as well.

“We will collect and analyze the data in similar ways, and I think we’ll have a greater appeal nationally, as we cover a range of climate conditions,” said MnROAD Operations Engineer Ben Worel.

Participation in the pavement preservation study is $120,000 per year for the initial research cycle, which will drop to $40,000 after three years; the cracking study will run three years at $210,000 per year.  Alabama will be the lead state for this effort.

State departments of transportation are asked for commitment letters this month if they are interested in joining either study, even if they do not have SP&R (State Planning and Research) dollars available at the time. Participating agencies will get to design the scope of the research and be kept advised of the ongoing findings, so they can benefit early from the project. Initial planning meetings will be done through a series of webinars in March and April of this year with participating agencies.

At a January 8 webinar, speakers said the research will help states determine how long pavement preservation treatments will last.

“Many DOTs have really well-designed and well-thought-out decision trees, where they can take pavement management data and end up with a rational selection of pavement alternatives. But the issue of extending pavement life is the really big unknown, because references provide a broad range of expected performance,”  NCAT Test Track Manager Buzz Powell said.

Another benefit is that states can learn how pavement treatments hold up in both hot and cold climates.

“It’s 14 degrees right now in Mississippi. It rains about every three days, freezes and then thaws,” said Mississippi Chief Engineer Mark McConnell. “So we need to know how pavement preservation is going to work in the north as well.”

For additional information, contact Ben Worel (ben.worel@state.mn.us) at MnROAD or Buzz Powell (buzz@auburn.edu) at NCAT.

mnroad_ncat
Aerial views of the pavement test tracks at MnROAD (left) and NCAT (right).
Materials and Construction

New Tool Measures Impact of Heavy Trucks

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A new tool developed by the Local Road Research Board helps cities and counties assess how much increased heavy vehicle traffic affects local roads.

Researchers created an analysis method and corresponding spreadsheet tool that city and county engineers can use to calculate the impact of heavy vehicles on asphalt roads beyond what was planned in the original pavement design.

The information will help agencies optimize services, such as garbage collection, for the least amount of damage. It will also help agencies better plan roads in new developments, as well as redesign existing roads that are nearing the end of their lives.

Lack of Data

Heavy trucks cause local roads to deteriorate more quickly than passenger vehicles, but it is challenging to quantify the impacts, especially for areas where traffic was not forecast at the time a road was designed.

Many local engineers in Minnesota have requested information about the impact of heavy vehicles in light of new construction, commercial distribution facilities and hauling routes. This information is needed to assist in local road planning and maintenance.

Two Methods

In a newly completed study, investigators developed two methods for calculating heavy vehicle impact:

  • Calculate the additional bituminous material (and associated costs) that would have been required to construct the pavement had the heavy truck traffic been predicted when the pavement was designed.
  • Calculate the portion of a pavement’s design life, measured in equivalent single-axle loads (ESALs), consumed by unanticipated vehicles.

“Before this project, there wasn’t an easy way for an engineer to determine how much a specific truck was going to decrease the life of a road,” said  Deb Heiser, Engineering Director, City of St. Louis Park.

Whereas previous research has calculated the impact of extremely heavy vehicles over the short-term (typically the course of a construction project), this project calculates the impact of long-term increases in traffic from vehicles that are heavy, but still mostly within normal legal weight limits.

The tool can be used for a single street segment or an entire road network. Users can also compare current situations with proposed ones to evaluate the impact of potential changes in heavy traffic levels.

Related Resources
Maintenance Operations, Materials and Construction

The 411 on Sign Management

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A revised handbook offers Minnesota cities and counties the latest tips on how to meet new sign retroreflectivity requirements, as well as the 411 on sign maintenance and management – everything from knowing when it’s time to remove a sign to creating a budget for sign replacement.

The best practices guide – produced in conjunction with a new sign retroreflectivity study – also offers case studies from around the state.

“The life cycle of traffic signs, from installation to replacement, is a pretty complex issue and it can be a challenge to get your arms around,” said Tim Plath, Transportation Operations Engineer for the city of Eagan. “This handbook really boils it down into some basic concepts and also gives you the resources to dig deeper if necessary. It’s a good resource to have at your fingertips.”

2014RIC20-1

This handbook updates a previous version issued in 2010, to include new FHWA  retroreflectivity and maintenance and management requirements and deadlines.

“Maintenance/management of a large number of signs can potentially be an administrative and financial challenge for many local road authorities,” explained Sulmaan Khan, MnDOT Assistant Project Development Engineer.

Here’s a video demonstration of a sign life reflectometer (the Gamma 922), another resource MnDOT has available for local government agencies. Cities, townships or counties may borrow the reflectomer by contacting the Office of Materials and Road Research, (651) 366-5508.

Related Resources

Traffic Sign Maintenance/Management Handbook (PDF)

Traffic Sign Life Expectancy – Technical Summary (PDF) and Final Report (PDF)

Gamma 922 demonstration (video)

Environment, Materials and Construction

How roadside drainage ditches reduce pollution

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Stormwater can pick up chemicals and sediments that pollute rivers and streams. Roadside drainage ditches, also known as swales, lessen this effect by absorbing water. But until recently, MnDOT didn’t know how to quantify this effect and incorporate it into pollution control mitigation measures.

In a recently completed study, researchers evaluated five Minnesota swales, measuring how well water flows through soil at up to 20 locations within each swale.

“There’s a big push in Minnesota, and probably everywhere, to do more infiltration,” Barbara Loida, MS4 Coordinator Engineer, MnDOT Metro District, said. “We know that our ditches are doing some of that, but we wanted to look at how much infiltration these ditches are providing.”

A key finding: grassed swales are significantly better at absorbing water than expected, which may reduce the need for other, more expensive stormwater management practices, such as ponds or infiltration basins.

This could save MnDOT and counties significant right-of-way and construction costs currently expended on more expensive stormwater management techniques. While swales were recognized in the Minnesota Pollution Control Agency’s new Minimal Impact Design Standards, there was a need to quantify the amount of water a swale can absorb so it could receive the appropriate MIDS credits.

Researchers also tested the ability of carbon, iron chips, steel wool and other materials to remove pollutants as ditch check filters—material put into swales to enhance removal of pollutants.

Gradations on a Modified Philip Dunne infiltrometer allow the measurement of stormwater infiltration.
Gradations on a Modified Philip Dunne infiltrometer allow the measurement of stormwater infiltration.
What’s Next?

A follow-up project, which the MPCA is participating in, will seek to clarify the impact of swale roughness on infiltration rates. The goal is a calculator for real-world infiltration rates that MnDOT and local agencies would be able to implement.

MPCA, MnDOT and the city of Roseville are also partnering on a project to install and test the effectiveness of ditch check filters in real-world locations.

Maintenance recommendations should help MnDOT and local agencies ensure that swales operate at maximum efficiency. These recommendations should continue to be revised as knowledge evolves.

Related Resources

*Editor’s note: This article was adapted from our upcoming edition of the Accelerator. Read the newsletter online, or sign up to receive by mail.