Tag Archives: pavement maintenance

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.  

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.

2018-02-p1-image
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.

Taconite byproduct reduces road wear from studded tires

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.

New Video: “Why Aren’t They Working on My Road?”

A new video produced by the Local Road Research Board helps the public understand why some bad roads aren’t always fixed first.

The seven-minute video explains what causes road pavements to deteriorate and why, like the saying, “throwing good money after bad,”  it may be more cost-effective to put maintenance dollars into roads that still have life left in them versus roads that are in the worst condition.

In it, city and county engineers discuss how they use a pavement management program to decide which roads to fix when, in order to stretch limited resources in the most effective way possible.

“We’ve learned that if we wait for things to break and fall apart, they’re much more costly to replace than if we put a little bit into it during its life cycle,” says Mark Maloney, City of Shoreview public works director.