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Study Suggests 70 Percent RAP for Minnesota Gravel Road Surfaces

Researchers examined mixtures of recycled asphalt pavement (RAP) and aggregate for new gravel road surface layers in the lab and in the field. Although test results did not align perfectly, and field results were somewhat uneven, findings suggest that mixtures with 70 percent RAP content can reduce dust generation. After a year of service these roadways can match all-aggregate gravel road performance in terms of strength, but with a smoother ride.

What Was the Need?

Gravel roads offer a cost-effective option for road departments that wish to avoid the expense of asphalt and concrete roads in rural or low traffic areas. However, about an inch of gravel is lost from these roadways each year. Aggregate resources are diminishing, and gravel and crushed rock aggregate is growing increasingly expensive.

Gravel also generates dust that can reduce visibility, affect road performance and result in complaints from nearby homeowners. 

Recycled asphalt pavement (RAP) can be an effective component of new asphalt pavement mixtures. Many aggregate producers stockpile RAP that has been broken into the size of aggregate. But not all RAP works well mixed in asphalt, and some aggregate yards are too far from pavement projects to economically use RAP in pavement. 

Road agencies frequently use RAP in gravel roads. The asphalt content in RAP can bind with dust from crushed rock or gravel, helping manage fugitive dust. A recent study in Wyoming found that using RAP in new gravel surface applications at less than 50 percent of the aggregate resulted in good road performance and kept dust to a minimum. 

What Was Our Goal?

In light of the findings from the Wyoming study, researchers sought to determine the optimal level of RAP in an aggregate mixture for Minnesota gravel road surfaces. These new applications would offer good driving stability while also controlling fugitive dust. 

What Did We Do?

Research began with a review of the literature on RAP as an aggregate component of surface, base and subbase layers, as well as a survey of Minnesota counties on their experience with these mixtures. 

In the lab, the research team tested three RAP materials and virgin aggregate from two Minnesota locations in various RAP content levels for strength and compression. Investigators then compared the economic feasibility of 100 percent virgin aggregate use to 50 percent virgin and 50 percent RAP aggregate mixtures on a 1-mile aggregate road, including annual grading and eventual regraveling in the estimations.  

Research in the field focused primarily on six 1,000-foot gravel road test sections: four sections in Goodhue County using 15, 30, 45 and 60 percent RAP content, and two sections in Carlton County using 30 and 50 percent RAP. The studies entailed all-virgin aggregate control sections, and installations were made over roads with various subgrade soils that presented a variety of properties. Sites were tested for elasticity, bearing strength and fugitive dust generation. 

A secondary field study focused on RAP contents of 50, 70 and 80 percent in 3-inch surface courses for three test sections and one control section in Goodhue County. Sites were tested for elasticity, strength, dust generation, ride quality and surface aggregate looseness over time, and some lab tests were conducted.

“The 70 percent RAP mixture seemed to be about the best combination. We put RAP down in fall 2017, and by the next summer, it was working much like a regular gravel road.” —Charles Jahren, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering 

Mounds of RAP at a gravel pit in Carlton County offer road agencies an alternative to
natural gravel and crushed aggregate for gravel roads. But RAP has to be used in the
right proportion with gravel.
Mounds of RAP at a gravel pit in Carlton County offer road agencies an alternative to natural gravel and crushed aggregate for gravel roads. But RAP has to be used in the right proportion with gravel.

What Did We Learn?

Previous research indicated that RAP can help reduce fugitive dust, offers value as surface courses, and can reduce moisture susceptibility of gravel roads in cold or wet locations. 

Lab mixtures with 30 percent RAP consistently produced high compressive strength values, and higher RAP levels generally correlated inversely with bearing strength. Improvements in dust reduction were limited until RAP levels exceeded 50 percent. 

Economic analysis determined that a 50/50 percent mix of RAP and aggregate would cost 1.5 percent more than an all-virgin aggregate surface course in terms of construction and maintenance, but potential reductions in dust generation, surface aggregate loss and regraveling after three years of service may produce savings from RAP use. 

Results from field testing defied clear recommendations on optimal RAP content. Generally, higher RAP content offered greater elasticity and lower levels of loose aggregate initially, but these benefits fell to equal or below non-RAP levels after a year. Higher RAP correlated with reduced dust generation, but again fell over the first year of service. In secondary testing, initial dust generation was lower with the 50 percent mixture than the others, but after a year was lowest with the 70 percent mixture. 

Ultimately, researchers found that after a year, during which fugitive dust production was reduced, the performance of a 70 percent RAP content aggregate surface course was most like a virgin aggregate surface course and offered a smoother driving surface. 

What’s Next?

“These findings provide another tool in the toolbox. They will be most useful to engineers who haven’t used RAP in gravel roads and to county engineers who have a RAP resource.” —Joel Ulring, Pavement Engineer, MnDOT State Aid for Local Transportation

While this research did not develop a definitive recommendation for an optimal RAP content in surface courses for aggregate roads, it did produce useful data on performance. The study did encourage a general sense that 70 percent RAP content for surface courses of approximately 2 inches may be effective and warrants systematic study for a three-year period. 

A researcher scrapes a gravel road surface with a modified garden hoe to measure loose aggregate levels.
A researcher scrapes a gravel road surface with a modified garden hoe to measure loose aggregate levels.

This post pertains to Report 2019-11, “Optimal RAP Content for Minnesota Gravel Roads,” published March 2019. For more information, visit MnDOT’s Office of Research & Innovation project page.

Rout-and-Seal Offers Slight Cost–Benefit Over Clean-and-Seal Repairs

In a recently completed study, Minnesota researchers compare the performance and cost-benefit of the clean-and-seal versus rout-and-seal techniques for repairing asphalt pavement cracks.

Survey results, construction data and field evaluation of new repairs and their performance over two years gave rout-and-seal repairs a slight cost–benefit edge over clean-and-seal repairs. At an average performance index level, rout-and-seal offered about four years of service before failure; clean-and-seal offered about three years. The study also recommends rout-and-seal for use over clay and silt subgrades in most conditions. Decision trees were developed to help planners and repair crews select an appropriate repair method.

Background

Preserving asphalt pavements so they maintain performance for decades requires a variety of repairs, including sealing cracks. Cracks allow water to seep into pavement structures, leading to damage from freeze-thaw expansion, stripping of the asphalt’s bond from the underlying structure, potholes and crack expansion.  

For most crack repairs, road crews clean the crack and apply an asphaltic filler or sealant. MnDOT uses two approaches to repair cracks and create a smooth ride for passing vehicles: clean-and-seal and rout-and-seal. Both treatments force traffic closures. 

Clean-and-seal asphalt crack repair begins by using compressed air to clean the crack before sealing it.

With clean-and-seal, compressed air is used to remove debris from the crack before a sealant is applied. With rout-and-seal, a saw or router is used to grind a shallow trench or reservoir over the crack. The routed seam is then filled with an asphaltic sealant. 

After routing a shallow channel over the pavement crack, repair workers fill the crack with asphaltic sealant.

Rout-and-seal requires more time and, in many cases, slightly more sealant, making it more expensive than clean-and-seal. Some agencies favor clean-and-seal because it is less expensive, reduces the time crews are on the road and frees more time to maintain other cracks. 

What Was Our Goal?

Researchers sought to determine which of the two repair methods offers the better value over time. If rout-and-seal delivers a longer-lasting repair, it may be more cost-effective than clean-and-seal in terms of life-cycle cost. The research team also needed to develop guidelines for selecting the most suitable repair method for the damaged pavement. 

What Did We Do?

Researchers conducted a literature search to see how agencies around the country approach asphalt crack repair. The research team then surveyed Minnesota road agencies to see which repair method agencies prefer and how long repairs typically last. 

To review performance of crack sealing, researchers evaluated the MnDOT construction logs of old repair sites and visited 11 new repair sites. These locations were revisited two, six, eight, 12 and 18 months following the repair. To calculate a performance index rating, researchers recorded data about site conditions that included sealant age, traffic level, subgrade soil type and crack sealing performance. Two sites were removed from the analysis when local crews applied chip seals to the pavements.

Investigators calculated performance index levels for each repair method at each site. They gathered cost data where available from bid-letting paperwork and determined life-cycle costs. Finally, the research team created decision trees that planners and maintenance crews can use to help select crack repair methods. 

What Did We Learn?

“This study provided very useful information. The rout-and-seal has a better cost–benefit over the life of the pavement than the clean-and-seal, however, they are relatively close. Agencies will need to decide if they have the manpower or resources to perform one over the other.”

—Dan Knapek, Assistant County Engineer, Sherburne County Public Works

Limited research was identified that compared clean-and-seal and rout-and-seal treatments. Most studies of asphalt crack sealing compared unsealed and sealed pavement performance and have established that sealing does extend pavement life. None compared cost–benefits of the two methods.

Of 47 survey respondents, 68 percent use rout-and-seal and 32 percent use clean-and-seal. Responses identified no clear trends in life expectancies for the two methods, with predictions for service until failure falling predominantly in two to 10 years for clean-and-seal and two to 15 years for rout-and-seal. The most common criteria for choosing a method were crack or pavement condition (46 percent of respondents) and predetermined maintenance schedules (24 percent). 

Analysis of MnDOT construction data found no statistically significant difference in life expectancies for the two methods, with service lives of 6.4 years for rout-and-seal and 6 years for clean-and-seal. A similarly slight advantage for service lives of both treatments was identified for low-volume roads over higher-volume roads. 

After one year of service, the new seal sites delivered strong performance index figures. Short-term performance on rural roads was identical for the two methods. After the severe 2018-2019 winter, however, performance dropped significantly; spalling damage was frequently observed at rout-and-seal sites. 

Analysis of old and new seal projects showed that at an average performance index level, rout-and-seal repairs last about four years and clean-and-seal about three. Life-cycle cost analysis found rout-and-seal slightly more effective. Because the difference is slight, factors such as treatment cost, life expectancy, ease of operation, traffic level and crew manager opinion may guide selection of sealing strategies. 

What’s Next?

Researchers developed two decision trees for selecting a repair method: one for pavement management and another for maintenance crews. Rout-and-seal is recommended for pavements over clay and silt subgrades. 

Research that extends monitoring of the new crack seal sites for up to five years would provide useful data on performance and comparison of the effectiveness of the two methods.

“To help select an appropriate crack repair method, we developed two decision trees: a detailed one and a simple one with only three variables—crack size, traffic level and the number of times a crack has been sealed.” 

—Manik Barman, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering

This Technical Summary pertains to Report 2019-26, “Cost/Benefit Analysis of the Effectiveness of Crack Sealing Techniques,” published June 2019. Visit the MnDOT research project page for more information.