Minnesota drivers are all too familiar with the rhythmic “thump-thump” and jarring sensation as they travel the state’s roadways in winter. The cause of this rough ride isn’t just normal wear-and-tear—it’s the result of pavement tenting.
Pavement tenting causes crosswise cracks in asphalt to heave upward during frigid months as water freezes and expands below the pavement’s surface. If left untreated, tenting can ruin ride quality (increasing the International Roughness Index, or IRI) and accelerate deterioration, leading to potholes and costly repairs.
During a recent CTS webinar, Matthew Hemmila, the deputy public works director of engineering and program delivery for St. Louis County, Minnesota, explained the scale of the challenge.
“We have about 3,000 miles of roadway, and managing cracks is a huge part of our program,” Hemmila said. “We spend a lot of our annual road and bridge revenue on sealing cracks and fixing the damage that comes from tenting and the problems … that ensue afterwards.”
A research project led by CTS scholar Manik Barman, a professor of civil engineering at the University of Minnesota Duluth, is exploring the causes of tenting and creating a practical roadmap for agencies to address it in work funded by the Local Road Research Board (LRRB). During the webinar, Barman shared preliminary results and conclusions so far.
Barman’s research team conducted extensive work in both the lab and the field. In the field, researchers measured pavement tenting at six St. Louis County sites, recording tenting as high as nearly one inch in the winter. Next, researchers analyzed more than 8,000 MnDOT records to track pavement roughness before and after various treatments. Finally, they designed a new lab test method to assess the frost-heave susceptibility of different aggregate base materials.
Preliminary results reveal that not all treatments are created equal when it comes to smoothing out tented roads. By analyzing historical data, the team identified maintenance strategies that offered the best improvement in ride quality. Top performers included micro-surfacing (a cold-applied thin protective sealcoat) and crack repair (a more extensive crack filling that includes cleaning and sealing).
In addition, the study confirmed that the root cause of tenting often lies below the surface. Base layers with high levels of “fines” (dust particles smaller than 75 microns) hold more moisture, leading to more severe frost heaving. The study suggests the need for optimizing base layer gradation so that larger and smaller pores inside the base layer can be balanced. Notably, tenting occurred regardless of traffic levels or pavement thickness; the base material proved to be the most critical variable.
The final study will include a best-practices manual for transportation practitioners with decision trees for selecting the right tenting mitigation strategy and guidelines for selecting materials to help prevent tenting in new construction. The researchers also recommend additional study on this topic. According to Hemmila, these actionable tools will be a major asset for Minnesota’s city and county engineers.
“We’re excited about what will come out of this study and hope we’ll be able to put it into practice pretty fast,” Hemmila said. “It will give us an edge by helping prevent those factors that exacerbate the cracks, which will give us a huge leg up and could save us quite a bit of money per year.”
—Megan Tsai, contributing writer
