Tag Archives: pothole patching

Taking on potholes with new prevention and repair strategies

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

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

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

What Did We Do?

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

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

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

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

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

What Did We Learn?

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

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

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

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

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

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

 

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

What’s Next?

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

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

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

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

Pothole Patching Study Yields Best Practices Guidance

For most road crews, repairing potholes is an essential and highly visible duty.  Choosing the best or most cost-effective pothole repair method, however, has remained a complicated puzzle.

Researchers identified four pothole repair methods suitable for Minnesota: cold mix, hot recycled asphalt, mastic material and mill-and-fill with hot-mix asphalt. They tracked the performance of each method at five sites in northern Minnesota for two years. Using the results from this monitoring period, researchers developed decision trees for selecting an appropriate repair method and best practices for using each method. The decision trees were developed in two formats: as a flowchart that can be used in a maintenance guide and as flash cards that can be laminated and used by maintenance crews for quick reference in the field.

“We wanted to develop a decision tree for choosing the right pothole repair method that could be laminated for use in the field,” said Susan Lodahl, Assistant State Maintenance Engineer, MnDOT Office of Maintenance.

“This project offers help deciding what kind of pothole patch is appropriate for the conditions, including the pothole dimensions, location in the roadway and the season,” said Manik Barman, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering.

What Was the Need?

Repairing potholes is one of the most public of road crews’ duties. Drivers notice cracks and holes, and feel the effects of short-term repairs that kick up gravel as wheels roll over patched holes.

Selecting the appropriate patching method and materials varies depending on several factors, including the size of the pothole and its location on the roadway. Patching methods and materials face seasonal challenges too. In northern Minnesota, asphalt plants shut down for the winter and don’t reopen before March, if then. Potholes that are repaired in cold weather aim for short-term fixes with patches that can be replaced when warm weather returns or when the pavement can be milled and overlaid with hot-mix asphalt (HMA). Road crews have tried applying HMA in winter using various heating systems and in-place recycling methods, but even warm weather patches only offer semipermanent repair.

Whether it’s spring, summer, fall or winter, choosing the best, most cost-effective or durable pothole repair method has remained a complicated puzzle, one that MnDOT would like to help road crews solve.

What Was Our Goal?

MnDOT funded this research to help road crews choose patching methods that match specific repair conditions. Researchers explored patching tools, materials and methods to identify those most appropriate to specific pothole conditions, road locations and time of year. They also evaluated the effectiveness of different methods based on durability, road safety, ride quality, driver satisfaction and other factors.

A researcher conducts a test on a pothole
A researcher conducts an on-site permeability test to determine how well a pothole repair seals and resists water penetration.

What Did We Do?

Research began with a comprehensive literature search of pothole repair methods in Minnesota and other states. From this search, investigators identified four repair methods that best suit Minnesota: cold mix, hot recycled asphalt, mastic material and mill-and-fill with HMA.

With help from the study’s Technical Advisory Panel, researchers identified five sites in MnDOT District 1 near Duluth, Minnesota, where they oversaw 20 pothole repairs. Investigators monitored these repairs for about two years before assessing the methods and their best applications. Researchers then developed decision trees to help road crews choose the most suitable method for each repair and compiled best practices to provide further guidance.

What Did We Learn?

The best practices describe the best uses of each of the four pothole repair methods along with guidelines for preparing the pothole for repair and placing the patching materials.

Cold-mix patches should be placed only in shallow potholes with depths of 2 inches or less. Deeper potholes should be repaired in two lifts, each compacted with a handheld compactor to prevent dishing or denting when the cold mix settles.

Hot mixes using recycled materials should be avoided. The aged binder slows the heating process, and its fines inhibit the bonding of the new binder and aggregate. The new binder also doesn’t seem to rejuvenate the old, and the patches age more rapidly. When hot mix is used for pothole repair, a handheld compactor is required. Recycling mixers such as the Stepp SRM 10-120 should be used to create virgin hot patch material using asphalt oil and sand or small aggregate.

Mastic material provides a smooth driving surface but appears to dish in potholes along longitudinal cracks because the material lacks the strength to support loads. Mastic should only be used on centerline joints and longitudinal joints along shoulders, where it stays in place. It should not be used in wheel paths.

Mill-and-fill with virgin HMA, when constructed with care, can be effective in eliminating dishing and raveling at the patch-pavement interface. Sufficient tack material must be used, and trucks should not be allowed to drive on the tack. The pothole should be filled with the proper amount of HMA, and the patch must be compacted sufficiently. Failure to carefully apply mill-and-fill can lead to significant deterioration at the patch-pavement interface after about 100 days, which can contribute to additional damage in the distressed area.

Using the findings from this study, researchers developed guidelines for patching method selection, placement, compaction practices and moisture control. They also developed decision trees for selecting the appropriate repair method for conditions. The decision trees are available in two formats: as a flowchart for use in maintenance guides and as flash cards for quick reference by maintenance crews in the field. The final report includes best practices and a step-by-step pictorial guide to patching.

DecisionTree

What’s Next?

The decision trees and best practices developed in this study can be easily combined into a patching guide that, with laminated flash cards, can be distributed to MnDOT road crews around the state. This research could be amplified by repeating the process with more pothole repairs in other areas of Minnesota to increase data for performance evaluation and analysis of best practices.


This post pertains to Report 2017-25, “Comprehensive Field Evaluation of Asphalt Patching Methods and Development of Simple Decision Trees and a Best Practices Manual,” published June 2017. 

‘Three Ways to Cook a Pothole’

In April, we posted about an innovative pothole-filling technology being developed by the Minnesota Department of Transportation and the University of Minnesota, Duluth. The technique involves zapping pothole patches and the surrounding pavement with a special truck-mounted, 50,000-watt microwave. Researchers have found that heating the base and the patch material at the same time creates a stronger, longer-lasting bond that provides for a more permanent pothole fix.

Last week, the MnDOT/UMD microwave technology found its way into a new MnDOT video (above) that also explores two other experimental pothole-patching methods. One involves using a large “electric oven”-type heating element instead of a microwave. The other utilizes a new exothermic (i.e. heat-generating) asphalt mixture containing taconite from northern Minnesota mines. The video compares the potential benefits of all three of the new technologies, which the department hopes will someday lead to “more pothole-patching power for the taxpayer dollar.”

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