Carver County Evaluates Pickle Brine for Ice Control

Like many snow- and ice-control professionals, Carver County Public Works maintenance operations staff are searching for new options to reduce the amount of chloride that reaches our waters from road salt operations. Using food production byproducts such as pickle brine are among the alternatives maintenance staff have been exploring.

Carver County regularly uses salt brine as part of their winter maintenance operations, which has become a widely accepted practice for controlling snow and ice. In the right situation, salt brine can be a more effective alternative to traditional road salt. An opportunity to obtain a free supply of sodium-rich pickle juice from a nearby canning facility seemed like a natural candidate worthy of consideration as a source of brine for county anti-icing and de-icing operations. In addition, recycling the pickle brine could reduce the amount of the waste byproduct.

 

The Carver County Public Works Department began testing samples of the pickle juice in 2016 with some encouraging results. But further testing showed the brine from the pickle cannery had variable salinity and pH levels that could damage maintenance equipment. Given the variables involved, staff determined it would be difficult to manually control the manufacture of the brine into a usable liquid. VariTech Industries recommended purchase of the Brine Boss, an automated brine blending system to manufacture the 23.3 percent brine solution needed for effective ice control operations. In addition, staff found adding potassium hydroxide to pickle brine neutralizes the pH level.

Carver County staff received a grant through the Local OPERA Program to help purchase the brine making system and support further research into the viability of using pickle brine in winter maintenance operations. Staff also restored an old VariTech 600 brine maker obtained from MnDOT for the project.

After extensive testing and analysis, VariTech engineers and Carver County staff concluded that pickle brine acquired from the cannery had to be exactly the same (salinity, vinegar content, and sugar content) for each and every batch or the system sensors would fail. But it turned out that the pickle brine supplier could not provide chemically consistent batches, and the VariTech system was unable to produce a consistent blend of 23.3 percent brine solution using pickle brine. As a result, Carver County staff determined they were unable to continue using pickle brine for snow and ice control.

Nevertheless, this project benefits other agencies considering the use of food production byproducts. The Carver County project demonstrates that there can be an alternative anti-icing product. As technology continues to advance, Carver County may revisit the use of pickle brine as a viable snow- and ice-control option.

For additional information about the project, check out these resources:

Nontraditional Fog Seals Offer Value, Limitations Compared to Traditional Seals

MnDOT conducted field and lab analyses of nontraditional fog seals used by local agencies around the state. Results show that agriculture-based bioseals offer value that must be balanced against temporary reductions in retroreflectivity and pavement friction. Bioseals offer greater friction and visibility than traditional fog seals.

“There is some value to the bioseals. They seal the pavement, and they’re clear so they have a minimal effect on striping. These applications are appropriate in certain areas,” said Bruce Hasbargen, County Engineer, Beltrami County.

What Was the Need?

Maintenance crews often spray pavement surfaces with a “fog” of liquid sealant after pavement has been in service for a year or more. These fog seals extend the water resistance of asphalt and protect pavements from oxidation.

Fog seals wear off after a few years, but can be inexpensively reapplied. The seals lengthen maintenance cycles, protecting asphalt between activities such as crack repair and surface treatment. Traditional fog seals, however, are dark, asphaltic mixtures that obscure pavement striping and reduce the reflectivity of materials. Fog seals also reduce friction, and so typically suit pavements with low-speed service conditions.

In recent years, city and county road agencies in Minnesota turned to bioseals—agriculture-based, clear liquids that manufacturers claim seal pavement against oxidation and water damage without concealing pavement markings. Bioseals are currently not less expensive than petroleum industry products, and little independent work had been performed to identify performance benefits.

What Was Our Goal?

To provide local agencies with more information about bioseal performance, the MnDOT Office of Materials and Road Research studied selected bioseal products in the lab and in the field (MnROAD test site pictured above), comparing them to traditional seals to determine product performance, durability and impact on friction and pavement marking visibility.

What Did We Do?

Following a literature review of fog seal treatments, investigators selected four seals for analysis: a traditional asphalt-emulsion sealer; a nontraditional, polymerized maltene emulsion longitudinal joint sealant (Jointbond); and two soy-based bioseals (RePlay and Biorestor). These seals were applied in 2014 to 8-foot shoulder sections built in 2013 on County Highway 75 in Wright County, north of Monticello. Seals were sprayed on shoulders outside painted markings, in shoulder space where investigators applied geotextile patches and strips of highly reflective striping tape commonly used on some roads. Untreated shoulder areas of 500 feet and 1,320 feet served as control sections.

Reflective marking tape on road shoulder
Researchers placed a swatch of geotextile and reflective pavement marking tape on shoulders before the shoulders were sprayed with nontraditional fog seals. Investigators then moved the textile and tape to MnROAD to study application rates and stripe performance.

After spraying, investigators removed the geotextiles to evaluate the quality of application work by bioseal distributors. They also removed some striping tape and reapplied it as shoulder striping to Cell 33 at the MnROAD test facility, where they could reliably monitor traffic passes over the biosealed markings and evaluate retroreflectivity over time. At the Wright County site, researchers examined pavement distress, friction properties and permeability on the shoulders for three years.

Lab studies included testing seal residue and stiffness in field-aged cores taken from the sealed test sections in year three. Finally, in year three researchers surveyed local agencies in Minnesota about their use of nontraditional fog seals.

What Did We Learn?

Geotextile coating levels showed that vendor application of bioseals is consistent and well-executed. Nontraditional seals do not obscure striping, but bioseals leave residue that temporarily reduces the retroreflectivity of sealed markings to below MnDOT-required levels. Acceptable levels of retroreflectivity returned to the Jointbond samples after 800 truck passes at MnROAD, and to Biorestor and RePlay samples after 1,600 truck passes.

Every tested seal reduced pavement friction. Recovery of friction for the three nontraditional products, which reduced friction by 11 to 17 percent, took about 200 days with no traffic. The traditional, asphaltic fog seal reduced friction by 67 percent and took longer to recover, remaining slippery for turning in wet conditions for over two years.

“Bioseals affect pavement friction, so agencies need to use some caution when using them. City streets are probably going to be very good for nontraditional seals,” said Eddie Johnson, Research Project Engineer, MnDOT Office of Materials and Road Research.

Each seal reduced pavement permeability for about two years; after two years, only the traditional seal continued to provide water protection. The permeability benefit of fog seals lasts significantly longer than the retroreflectivity reduction; when reflectivity recovers, the seals still provide water resistance. Field surveys also found that Biorestor and RePlay may help resist cracks.

Laboratory studies showed that high-temperature stiffness for every treatment was greater than control samples in the top layer than in the middle of cores, suggesting that seals may improve rut resistance of treated pavements in hot weather. Low-temperature stiffness was higher in the top sections for every treatment except the traditional fog seal.

Of the 57 agencies that responded to the survey, 32 have used nontraditional fog seals, preferring Biorestor and RePlay to others. Over half of these users recommend the use of such seals; responses suggest that bioseals offer sealing benefit for two years and, in some cases, up to six years.

What’s Next?

Nontraditional fog seals protect pavements from water and may help prevent cracking. Traditional seals offer longer-lasting water resistance, but also longer-lasting and greater friction reduction. Agencies must consider temporary reductions in retroreflectivity and friction for any seal, and may wish to continue using fog seals only in lower-speed environments.

Continued monitoring of applications would be helpful in determining long-term performance. The study observed that overlaying biosealed asphalt with a traditional fog seal should be effective in extending permeability.

This post pertains to the LRRB-produced Report 2018-18, “Nontraditional Fog Seals for Asphalt Pavement: Performance on Shoulder Sections in Minnesota,” published May 2018.  

Upgrading MnPave-Rigid Design Software

MnDOT has upgraded its software for concrete pavement design. MnPAVE-Rigid 2.0 is now easier to use and allows designers to select from more options for aggregate base types and thicknesses, and incorporate specific traffic values.

“In the original software, we only allowed one aggregate base thickness and one aggregate type. MnPAVE-Rigid 2.0 allows two base thicknesses and three base types,” said Tim Andersen, Pavement Design Engineer, MnDOT Office of Materials and Road Research.

What Was the Need?

MnDOT developed its own pavement design software, MnPAVE-Rigid, in 2014 that incorporated the methodology of the American Association of State Highway and Transportation Officials (AASHTO) Mechanistic–Empirical Pavement Design Guide (MEPDG). Minnesota’s pavement designers use MnPAVE to apply AASHTO’s most sophisticated design principles for both rigid and flexible pavement, focusing on mechanical properties of the pavement and prevention of early cracking and other distress.

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MnDOT engineers use sophisticated and complex M–E pavement design methods from MnPAVE-Rigid 2.0 software to design durable, high-performing concrete pavements.

AASHTO’s mechanistic–empirical (M–E) design methods entail hundreds of inputs, each a mechanical parameter, a measure of site-specific characteristics or a design goal. To simplify the input selection process, AASHTO’s M–E design software offers various input levels to reduce the data gathering and input burden. The most basic level uses default values for most of the inputs based on national averages, but still requires dozens of inputs for the number of pavement layers, traffic expectations, climate and other features.

MnPAVE-Rigid for concrete pavement design reduced that number of inputs to nine, operating like a module of AASHTO’s M–E software. MnPAVE-Rigid inputs work with a set of default values for jointed plain concrete selected by the MnDOT Office of Materials and Road Research in 2014, as described in the MnPAVE-Rigid 1.0 report.

“Many states ignored the challenge of adopting AASHTO M–E or they bought an AASHTO
software license. MnDOT used its accumulated knowledge of AASHTO M–E and Minnesota conditions to build MnPAVE-Rigid, and so can account for its M–E design results firsthand,” said Derek Tompkins, Principal Civil Engineer, American Engineering Testing, Inc.

Since implementing MnPAVE-Rigid 1.0, MnDOT has gathered feedback from users about their experience with the software. In the current project, MnDOT wanted to address this feedback, and expand and improve the original software by exploring additional options with some of the default parameters for concrete pavements.

What Was Our Goal?

The goal of this project was to update MnPAVE-Rigid 1.0 by expanding the range of inputs for traffic, subgrade type, base type and thickness, and to make the user interface more accessible.

What Did We Implement?

MnPAVE-Rigid 2.0 allows users to enter 11 inputs, including inputs related to specific traffic levels and aggregate base types; calculate the new design thickness; and print a project report that summarizes the inputs and the recommended thickness. The upgraded software is more user-friendly, and MnDOT can maintain or make future upgrades to the source code.

How Did We Do It?

Researchers met with the Technical Advisory Panel and reviewed the list of software improvements requested by pavement designers and the MnDOT Office of Materials and Road Research.

Because every change to an input affects a large number of default input variables, investigators ran over 21,000 simulations to analyze the impact of changes made to inputs for base type, base thickness, subgrade type and traffic level. The research team also modified the traffic input calculator to allow designers to enter traffic values from MnDOT’s weigh-in-motion and traffic counting data. The calculator runs input traffic data in software simulations and assigns the input an appropriate axle value for design.

MnPAVE-Rigid 1.0 ran designs based on Class 5 aggregate base over a subgrade like clay loam. Other aggregate types were added to simulations to determine how the software responds to these changes. Investigation also explored the addition of subgrade material options in design simulations.

The code developer modified elements of the advanced inputs tab and PDF report generation features to improve performance for software users, and rebuilt the software in JavaScript 2.0 code, including an installer for use with Windows software.

What Was the Impact?

MnPAVE-Rigid 2.0 is more user-friendly. Its tabs better match designer needs, and the software offers a design report PDF file for export. Instead of selecting from limited options for traffic volumes (default, normal and heavy), users can now input traffic data that the software will categorize. Designers can input Class 5 aggregate, Class 5Q (a higher quality aggregate with fewer fines) and open graded aggregate (no fines). Users can also choose 4-inch or 12-inch aggregate base thicknesses. An additional subgrade option was not included, as simulations indicated a sand subgrade input did not discernibly impact structural thickness outputs.

The AASHTO M–E software is expensive, and agencies that use it have to work closely with consultants to receive training and to explore or modify the code. MnDOT owns and manages the source code for MnPAVE-Rigid 2.0, can keep it secure, and can continue to change and upgrade it internally for Windows and Linux platforms.

What’s Next?

MnDOT designers around the state are reviewing the software, but it is essentially already in use. After full review, the Office of Materials and Road Research will post a link to the program on its website. Presentations about the software upgrades will be made at meetings for materials and soils engineers through the fall of 2018.

Still underway is an effort to further incorporate recycled material properties into MnPAVE Flexible, the design software for asphalt pavement.

This Implementation Summary pertains to Report 2018-17, “MnPAVE-Rigid 2.0,” published May 2018.

New research to explore innovative solutions to aging pavement infrastructure

New solutions are urgently needed to address Minnesota’s aging pavement infrastructure since current materials and technologies can’t keep up with the rate of deterioration and limited funding.  MnDOT recently entered into a contract with the University of Minnesota to further explore new materials and technologies – including taconite and Graphite Nanoplatelets (GNP) – that could offer cost-effective solutions for longer-lasting pavement.

Background

The university has investigated the use of taconite aggregates for more than a decade (see ongoing and completed research), and started investigating GNP-reinforced asphalt materials more than three years ago (see recently completed research). Both materials present very unique properties that can be used to better build and maintain asphalt pavements.

This research project will focus on two applications with significant potential in the pavement area:

  • Early detection and repair of cracking by developing a novel asphalt material in which GNP materials, taconite concentrate, and conventional asphalt binders are combined for damage sensing and healing. The material damage will be assessed by measuring the electrical resistance, while the damage healing will be achieved by applying microwave to the material.
  • Thermal enhancement of tack coat bonding between asphalt overlay lifts, using GNP and taconite concentrate and microwave heating. Poor bonding can result in many different pavement distresses that decrease the pavement structural strength and life, ranging from top-down cracking, potholes and fatigue failure.

Improving pavement durability

The latest data shows that 15 percent of roads in Minnesota are in poor condition, at a cost to each motorist $480 per year. Low-temperature cracking is one of the main causes of pavement failure in Minnesota. Studies have shown that early detection of damage and cracking and timely repair is essential for extending the lifespan of the pavements.

Each dollar spent in the early-stage of pavement life could eliminate or delay $6 to $10 in future rehabilitation or reconstruction costs.

A series of recent studies funded by the National Cooperative Highway Research Program (NCHRP) and MnDOT showed that the GNP-modified asphalt binders and mixtures exhibit a significant improvement in both mechanical and compaction properties. The combination of the previous research and the proposed research will fully explore the properties of GNP-taconite modified asphalt binders and mixtures as a multi-functional pavement material, which will address various needs of MNDOT, including high fracture resistance, efficient compaction process, and cost-effective pavement preservation operations. By addressing these needs, the result of this research will lead to an innovative and efficient means to improve the long-term durability and resilience of asphalt pavements in Minnesota.

 Project scope

The two-year research project aims to explore the damage sensing and healing capability of asphalt binders and mixtures modified by GNP and taconite concentrates. The essential idea is to combine GNP and taconite concentrates with asphalt binders to make the final asphalt products electrical conductive. By measuring the change of electrical resistance, researchers will be able to determine the damage extent. When the damage extent reaches a certain level, the University will apply microwave to the pavement to generate heat, which will heal the cracks through viscous flow of warm asphalt binder. In addition, the thermal bonding capabilities of a novel tack coat material also modified with GNP and taconite concentrate will be investigated. The research will consist of four parts:

  1. Electrical conductivity tests on GNP-taconite modified asphalt binders and mixtures
  2. Modeling of relationship between electrical resistance and damage extent
  3. Investigation of self-healing capability through microwave
  4. Investigation of a microwave-based tack coat system to enhance thermal bonding in asphalt paving

Watch for new developments on this project here.  Other Minnesota pavement research can be found at MnDOT.gov/research.

 

Salt Brine Study Shows Truck Traffic Drives Deicer Effectiveness

Truck traffic significantly improves deicer performance, deicers perform poorly below 10 degrees Fahrenheit, and brine is spread more by traffic spray and snowplow throw than by storm runoff.  This is the outcome of multi-year tests performed in the snow-covered parking lots of two Twin Cities entertainment destinations.

“There is notable airborne removal of deicers from road spray by vehicles and under high wind conditions. For the deicing materials MnDOT uses, little melt is observed below 10 degrees Fahrenheit,” said Tom Peters, Maintenance Research and Training Engineer, MnDOT Office of Maintenance.

MnDOT has conducted several research projects to better understand the performance of both deicing and anti-icing materials (applied to the roadway before a storm to prevent or mitigate ice buildup). Two previous studies evaluated solid materials and liquid brine. In Phase I researchers examined over 50 deicer and anti-icing compounds and blends. They determined that ice melt capacity correlates closely with application temperature, which is the principal factor in effectiveness. Rock salt offers greater ice melt capability, but liquid deicers adhere better to roadways and cause less corrosion and environmental damage to road and bridge environments.

In Phase II, researchers studied deicer performance in the field and considered how traffic levels, truck volumes, weather, pavement type and other factors affect performance. Research showed that deicers work better at warmer temperatures, with little effectiveness below about 10 degrees Fahrenheit. Truck traffic significantly improved deicer performance, contributing to wider and quicker melts. Chlorides were swept off bridge decks by snowplowing, and deicing effectiveness diminished as truck speed rose.

However, severe weather hampered research in Phase II; the winter of 2013-2014 was the coldest experienced in Minnesota in over 30 years. The severity of winter conditions impeded the temperature study of deicer performance and snowplow performance, leaving the research team and MnDOT interested in further study.

What Was Our Goal?

This study aimed to continue the work of Phase II in more representative winter conditions. Researchers evaluated deicer effectiveness, plowing effectiveness, anti-icer persistence in traffic and drains, and pavement shedding of deicers.

What Did We Do?

During the winters of 2015-2016 and 2016-2017, both of which were mild with below-average snow accumulations, the research team followed closely the methods used in Phase II.

Deicers were studied at two facilities in Shakopee, Minnesota. One facility included nine 900-foot lanes on which plow trucks spread deicers at highway speeds. The other facility featured four 500-foot lanes, where deicers were spread by hand in 100-foot segments. Investigators monitored weather and evaluated deicer performance with photography and infrared thermography.

Snowplowing by MnDOT Metro District plow operators was conducted at the two Shakopee facilities as well, at operating speeds of up to 30 mph. Researchers documented performance of various plow configurations in various truck combinations with on-site observation, handheld photography and time-lapse photography.

Anti-icing, in which deicer brines are applied to dry pavement to prevent ice formation, was conducted on an elevated section of U.S. Highway 169 near Mankato that experienced actual traffic. Investigators recorded application rate, time, temperature, precipitation and traffic, as well as deicer flow and concentration in storm drainage runoff over time.

A researcher mixes deicers in bags before spreading by hand at the test site.
A researcher mixes deicers in bags before spreading by hand at the test site.

Researchers studied pavement shedding of deicers in a lab in terms of storm runoff flow and anti-icer concentration in drainage from artificially induced precipitation. Deicers were applied in brine form, dried, chilled and held at temperatures below the freezing point of water and within the range of effective ice melt temperatures.

What Did We Learn?

Research confirmed that deicer performance varies with temperature, with little benefit from rock salt at 10 degrees Fahrenheit or colder unless the pavement is exposed to sunlight. Deicer accumulated in drains at substantially lower levels than roadway applications suggest regardless of traffic conditions, confirming observations that the majority of deicer loss occurs from displacement by traffic and snowplows.

Plow results were fairly uniform across all lanes and along lane lengths for a given plow type, suggesting truck, plow and driver combinations performed uniformly at each track. At higher speeds, snow rises higher in the curvature of the plow, and snow casts more broadly off plow ends and distributes to greater distances.

“Truck traffic makes a huge difference on deicer performance. If two or three dump trucks that aren’t spreading but have weight in them follow a salt truck, salt use might be cut by two-thirds,” said Stephen Druschel, Professor, Minnesota State University, Mankato, Department of Civil Engineering.

Deicers and anti-icers showed wider and quicker melting capability with traffic, especially by trucks, than without. Prewetting offered no significant observable benefit under most conditions, contrary to reports from snowplow drivers in field operations, unless snow was dry; then significant benefits were observed.

Asphalt and concrete pavements shed salinity at high levels initially and at declining levels at about 0.3 inch precipitation. The type of pavement involved had no apparent effect on deicer runoff.

What’s Next?

The impact of truck traffic on deicer performance is significant and needs to be widely communicated. Reports that prewetting improves deicer adhesion in windy conditions and speeds the initiation of ice melt may warrant further review. Evaluation of atmospheric and off-roadway drainage may help quantify traffic-induced brine spray and plow throw.

View Dr. Stephen Druscel’s YouTube presentation, Winter Plowing and Deicing: Saving Money, Salt and Labor by Distinguishing Best Practices.

This post pertains to Report 2017-45, “Salt Brine Blending to Optimize Deicing and Anti- cing Performance and Cost Effectiveness: Phase III.” The full report and appendices can be accessed at mndot.gov/research/reports/2017/201745.pdf and mndot.gov/research/reports/2017/201745B.pdf.

3-D Mapping of Bridges and Riverbeds

MnDOT bridge inspectors often have to find out what lies beneath the surface of Minnesota’s rivers. Thanks to new sonar inspection technology, the Bridge Office now has a way to see previously hidden riverbed floors and underwater bridge structures in far better detail than ever before.

Typically, bridge engineers turn to professional divers to provide information about what’s underwater. But diving inspections don’t always deliver precise information about bridge damage, debris and riverbed topography.

Of the 11,183 Minnesota bridges that span waterways, about 585 require underwater inspection.

In recent years, bridge inspectors turned to underwater inspection technologies to identify areas of interest and direct divers who can inspect hands on. In turbid, sediment-heavy conditions with low visibility such as the Mississippi River, non-optical technologies – laser, radar and sonar – offer safe and useful options. Sonar gathers underwater acoustic data into point clouds for imaging two-and three-dimensional models of conditions.

An engineer prepares to lower tripod-mounted sonar unit into the St. Croix River in Stillwater.

 

In winter 2014, a vendor demonstrated the use of sonar at the Third Avenue bridge over the Mississippi River in Minneapolis, where a void was previously discovered during a diving inspection. Acoustic investigation of the frozen-over site delivered a three-dimensional image of a scoured cavity of eroded concrete under a pier in conditions unsuitable to diving inspection. Later that year, the Bridge Office purchased its own three-beam sonar unit with funds secured from MnDOT’s research implementation program. MnDOT is the nation’s first state transportation department to use the technology, according to Petra DeWall, waterway engineer.

The new 3D scanning technology provides much better information than divers can. Nicki Bartelt, hydraulic design engineer, said divers can see up to 2 feet in front of them in good rivers, so most of the work is done by feel.

With the new technology, Bartelt and her colleagues receive a 3D image to base decisions on. Diver reports included only narratives and rough sketches.

“It’s like night and day,” said Nicki Bartelt, hydraulic design engineer. “It’s a picture, but it’s more than that. It’s a point cloud. It’s totally scalable. It is real-world elevations and dimensions. It’s like the difference between Google Earth and a paper map.”

Although the information is far better from these scans, federal bridge inspection standards still require hands-on inspection of bridges, including substructures above and below the water’s surface. For now, the scans will be used to augment diver inspections and other purposes.

“We’re looking to augment with scans to make it safer for the divers,” DeWall said.

The research implementation team identified a host of lessons and best practices after purchasing and testing the new equipment.

“This does have a pretty steep learning curve,” Bartelt said. “It’s not something you can just buy and use. You have to learn how to use it.”

Getting different pieces of the equipment set up and able to communicate with each other was the first difficult step. Field testing also identified the need for a dedicated generator to provide a consistent portable power source because of difficulty with batteries.

What’s Next?

MnDOT will develop and publish an underwater imaging policy and reach out to districts, counties, cities and other bridge owners to promote its imaging capability. The hydraulics unit will develop data on completed projects, generate a list of bridges that suit underwater imaging and ensure field personnel are trained to use imaging techniques and inspection.

“I think it’s really exciting, because it opens your eyes to what’s going on in the river,” said DeWall. “We always just assumed before, but now we can see it and document it. The fact that you have the ability to rotate the picture and move it around, and zoom through it makes a huge difference.”

These videos explain the sonar inspection technology:

This post pertains to Report 2017-40, “Multi-Beam Sonar Infrastructure
Mapping Research.

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.

County GIS Maps Help Road Departments Anticipate Slope Failure

In a recently completed pilot study, researchers developed maps for two Minnesota counties that rank the failure potential of every slope using a geographic information system (GIS)-based model.

“GIS mapping has been applied to very small watersheds. The two counties in this study are huge areas in comparison. We used a physics-based approach that shows engineers where slope failure is likely to occur,” said Omid Mohseni, Senior Water Resources Manager, Barr Engineering Company.

What Was Our Goal?

The goal of this study was to determine if slope failure models could be developed to help counties anticipate where failures may occur. Researchers used publicly available data, research findings and geotechnical theory to develop failure models that could then be mapped with GIS in two topographically dissimilar Minnesota counties. These maps would identify slopes susceptible to failure so that county highway departments could develop preventive strategies for protecting roadways from potential  lope failure or prepare appropriate failure response plans.

What Did We Do?

Researchers began with a literature review of studies about the causes of slope failure, predictive approaches and mapping. They were particularly interested in research related to potential failure mechanisms, algorithms used for predicting failures and slope-failure susceptibility mapping.

Then investigators collected data on known slope failures in Carlton County in eastern Minnesota and Sibley County in south central Minnesota to identify failure-risk factors not found in the literature. Researchers reviewed various statewide data sets, identifying topographic, hydrologic and soils information that could be used in GIS-based modeling. Next, they developed a GIS-based slope-failure model by incorporating the available data with geotechnical theory and probability factors from hydrologic data, and writing computer code to allow the data to be input into mapping software.

Researchers tested the software on known failure sites to refine soil parameter selection and failure models. The refined models and software were then used to identify and map slope failure risks in Carlton and Sibley counties.

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A detailed GIS map of a length of County Highway 210, color-coded to show slope failure susceptibility along the roadway.

What Did We Learn?

After analyzing the literature and the failure and geotechnical data, researchers identified the following key causal factors in slope failure: slope angle, soil type and geology, vegetation, land use and drainage characteristics, soil moisture, and rainfall intensity and duration.

Researchers then developed mapping models for the two counties using three key data sets. The first was data from 3-meter resolution, high-quality lidar, which measures distances with laser range finders and reflected light, available through Minnesota’s Department of Natural Resources website. The team augmented this data with U.S. Department of Agriculture soils survey data, and with National Oceanic and Atmospheric Administration and National Weather Service hydrologic data for precipitation and storm duration information.

Based on research in geotechnical theory, researchers developed algorithms for anticipating failure and built these into the lidar-based topographic mapping model. They also developed input parameters based on the failure factors and established output parameters representing five levels of failure susceptibility: very low, low, moderate, high and very high.

After testing the GIS-based model against a slope along County Highway 210 in Carlton County, researchers confirmed that failure potential correlated well with documented or observed slope failure. The team further validated the model by applying it to several small areas in the adjacent Carver and Sibley counties, finding similarly effective correlation with identifiable failure sites.

Independent geotechnical experts examined the modeling software and further refined geotechnical, soil and hydrologic elements. Finally, the team developed maps of Carlton and Sibley counties that assigned failure susceptibility levels to slopes in the two counties. Viewing maps through the software remains the most useful way to examine slopes, although large-format maps are available.

“If county engineers have higher slopes adjacent to roadways, they can use this basic tool to predict slope failures and then hire a geotechnical consultant to investigate the site.” – Tim Becker, Public Works Director, Sibley County

What’s Next?

With additional funding, mapping could be extended to every county in Minnesota to further refine failure modeling. Maps may also be useful in identifying structures such as roadways, ecological features, transmission lines and pipelines, bridges and culverts that may be threatened by slope failure susceptibility. Potential risks could be used to prioritize slope treatment plans.

This research effort is part of a slope failure risk mitigation strategy that includes the recently released Slope Stabilization Guide for Minnesota Local Government Engineers. Another project, underway at MnDOT, is identifying, mapping and ranking slopes vulnerable to slides that could affect the state highway network. The project

This post pertains to the Local Road Research Board-produced Report 2018-05, “Storm-Induced Slope Failure Susceptibility Mapping,” published January 2018. More information is available on the project page.

Using Mussel Spat Rope to Facilitate Small Fish Passage in Culverts

Low-cost, low-maintenance mussel spat rope can help small fish species navigate through culverts by reducing current velocity and providing protected areas for fish to shelter and rest.  Recent research in New Zealand demonstrated the effectiveness of mussel spat rope—rope with long, dense fibers used in mussel aquaculture— to assist small species fish passing through steep, perched or high-velocity culverts. The successful results from this research led MnDOT to investigate mussel spat rope as a  method to facilitate fish passage in Minnesota’s culverts.

“Minnesota is a headwater state, and we have a responsibility to keep our fish population healthy. Mussel spat rope will be one more effective tool in the toolbox of methods we have to assist fish passage through culverts,” said Petra DeWall, Bridge Waterway Engineer, MnDOT Bridge Office

 What Was Our Goal?

The objective of this project was to determine whether mussel spat rope was an appropriate and effective tool in helping small fish species pass through Minnesota culverts.

What Did We Do?

Investigators conducted a literature review to evaluate previous studies. Then researchers from St. Anthony Falls Laboratory conducted experiments in the laboratory and in the field to investigate the use of mussel spat rope as a fish passage aid.

Hydrodynamic performance. Hydrodynamic performance tests were performed in a 20-inch-wide by 30-foot-long flume fed by water diverted from the Mississippi River into the laboratory. Researchers measured velocity, depth and water surface slope, and sediment accumulation around arrays of ropes. They installed single- and multi-rope configurations and examined many variations of flume flow and depth, recording the rope’s effects on water velocity and turbulence.

In a second experiment, researchers released fine sand into the flume containing two- and four-rope configurations to investigate the rope’s effect on sediment transport. Because the ropes slowed local water velocity, deposits were observed on, between and under the ropes in two different depth tests after one and two hours of sediment feed.

Rope durability, performance and use by fish. Researchers installed mussel spat rope in two Minnesota box culverts: one in the northeast serving a fast current trout stream and one in the southwest serving a slow current prairie stream in critical fish habitat. Double strands of mussel spat rope were installed near a wall in each culvert and examined many times for approximately two years. Each observation included photographic and video recordings of the installations.

2018-13-p1-image
A long interior view to the open end of a large box culvert shows a double strand of mussel spat rope installed in the water along the left wall.

Small fish species’ interaction with the rope. Laboratory investigations of fish behavior with the rope were conducted in a 5-foot-wide by 32-foot-long flume with a raised section representing a box culvert. Two Plexiglas windows allowed viewing. Researchers installed two sets of double-strand ropes along a wall, similar to those in the field sites. Four video cameras tracked motion, recorded overhead views of the flume and captured fish behaviors at the midpoint and ends. Researchers used three species of small fish common to Minnesota: fathead minnow, white sucker and johnny darter. Five fish were released into the test area at a time and allowed to swim for an hour. Their progress and behavior were filmed and analyzed.

What Did We Learn?

Key observations from these investigations follow:

  • Mussel spat rope created small corridors (about 6 inches) of reduced velocity and turbulence along its length, which was sufficient to aid the passage of small fish. Sediment collected in, between and beneath the ropes. The presence of culvert floor sedimentation may assist fish passage.
  • The rope displayed wear over two years in the field, raising a concern about plastic microparticle release into streams. Sediment covered some ropes over time, suggesting a need for maintenance in some culverts. Only a few fish were observed at the field installations.
  • In the laboratory flume, test fish swam near and between doubled rope lengths, apparently taking advantage of the reduced current near and beneath the ropes. While there was variation among species, most fish that swam upstream through the simulated box culvert ended their passage on the rope side, evidence that the rope provided cover and refuge from the current.

What’s Next?

Mussel spat rope will be a low-cost, low-maintenance tool to help small fish pass through culverts. The final report for this study includes guidance for installing the rope. The low-cost method will also be included in an upcoming guide for designing culverts that allow aquatic organism passage.

This post pertains to Report 2018-13, “Use of Mussel Spat Rope for
Fish Passage in Culverts,” published March 2018.

More Saint Paul drivers stopping for pedestrians, thanks to pilot study

A two-year research project underway in the City of St. Paul is already improving pedestrian safety and driver behavior by applying lessons learned from a national award-winning pedestrian traffic study. The city began using the practices last fall with the “Stop for Me” campaign, and driver yield rates have already gone up by 9 percent.

Background

Each year, dozens of Saint Paul pedestrians legally crossing the street are struck by vehicles driven by motorists who fail to stop. In 2015, 40 pedestrians died in Minnesota after being hit by a motor vehicle; 900 were injured. In 2017, there were 192 vehicle-pedestrian crashes in Saint Paul, three of which proved deadly.

Pedestrian fatalities and injuries represent a growing percentage of traffic fatalities and injuries nationwide. For example, pedestrian fatalities comprised 10.9% of all traffic deaths nationwide in 2004, but 14.5% in 2013.

A recent study supported by the National Highway Traffic Safety Administration demonstrated that driver behavior can be changed on a city-wide basis. The introduction of highly-visible pedestrian right-of-way enforcement in Gainesville, Florida increased driver yield rates for pedestrians by 22% to 30%.

Objective

University of Minnesota researchers are charged with reviewing the City of St. Paul’s efforts to improve pedestrian safety and investigate whether a program similar to the one in Gainesville can change driver yielding for pedestrians and speed compliance. The activities in St. Paul are being planned together with city traffic engineers and enforcement officers and will include various educational, engineering and enforcement countermeasures and media campaigns.

Last fall, St. Paul began the “Stop for Me” campaign, which enforces pedestrian laws, increases driver and pedestrian education and works towards enhanced signage and other changes to crosswalks around the city.

A group of people holding signs with traffic safety messages
Stop For Me is a campaign to improve safety for people who use St. Paul’s sidewalks and cross its  streets.

On June 25, the St. Paul Police Department began the second phase of the campaign by ticketing drivers who fail to stop for pedestrians at crosswalks.

Additionally, police officers are ticketing drivers for “endangerment” if they pass a vehicle that is stopped for a pedestrian at a crosswalk. This citation leads to a mandatory court appearance for the driver.

Weekly stopping percentages can be viewed at eight intersections across the city from now until the end of fall.

Watch for new developments on this project (expected end date of August 2019) here.  Another MnDOT study is looking at pedestrian traffic safety in rural and tribal communities. Other Minnesota research on pedestrian travel can be found at MnDOT.gov/research.

 

 

Minnesota transportation research blog