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Sidewalk replacement contributes to Minneapolis tree loss

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sidewalk.jpgLast June a windstorm toppled about 1,800 trees in Minneapolis. Many of the fallen trees were in boulevards (the area between sidewalks and streets) rather than in yards. This raised concerns that recent sidewalk replacement—and resulting severed tree roots—had been a factor.

To better understand the higher-than-normal losses, the Minneapolis Park and Recreation Board (MPRB) turned to the U’s Urban Forestry Outreach, Research and Extension lab. “The MPRB Forestry Department has partnered with the University of Minnesota for years,” says Ralph Sievert, MPRB forestry director. “When this study presented itself, we did not hesitate to ask the lab to participate.”

Led by forestry department professor Gary Johnson, the lab studied damaged and undamaged trees along the storm’s path. The data set included 3,076 trees, of which 367 were total failures (tipped or partially tipped) due to the storm.

“The major finding is that replacing the sidewalk increased the odds of root failure by 2.24 times,” Johnson says. For example, when no replacement work was done, the average linden had a 10.6 percent chance of root failure; with sidewalk replacement, this increased to 21.0 percent.

When combined with replacement work, tree species was also a significant factor. Linden trees were most likely to fail, followed by ash, maple, and elm. “Essentially, when replacement work was done near any one of these trees, the rate of failures more than doubled,” Johnson says.

“Now we have a great opportunity to make improvements,” Sievert says. “I’m anticipating this leading to safer, healthier trees with fewer instances of infrastructure damage.”

Read the full article in the June issue of Catalyst.


Whether on a roadside, rest area, park, or lawn, turfgrass diseases can significantly damage turf in the Midwest. As governmental agencies continue to regulate inputs on turfgrass, it is important for managers to know how to meet new turfgrass management challenges—especially concerning diseases.

An online course from the University of Minnesota’s College of Continuing Education gives turfgrass managers the expertise necessary for controlling diseases in this challenging environment. The curriculum was created under the guidance of Associate Professor Eric Watkins of the Department of Horticultural Science. He also serves as the course’s content advisor.

The course prepares students to diagnose Upper Midwest turfgrass diseases and then determine control measures using both cultural and chemical methods. It is presented in Moodle, an e-learning platform that uses a number of interactive instructional tools. Cost is $75, and 12 professional development hours (PDHs) may be earned.

New this spring, the course counts as one elective credit in the Minnesota Local Technical Assistance Program (LTAP) Roads Scholar program. (Minnesota LTAP is a program within CTS.) Aimed at maintenance personnel, the Roads Scholar Program combines a range of training options into a structured curriculum. Graduates earn a valuable professional development credential.

Read the full article in the April issue of Catalyst.

Rebuilding stronger, less expensive roads with recycled asphalt

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While the eco-friendly mantra "reduce and reuse" has been around for decades, its role in asphalt pavement rehabilitation has been discovered much more recently. In years past, when an asphalt road began to deteriorate, the answer was either to apply a hot-mix asphalt overlay to the road's surface or perform a complete reconstruction of the pavement.

"The problem is that other options should be considered," says University of Minnesota civil engineering professor Joseph Labuz. "Fully reconstructing a road is expensive and time consuming, and though the overlay method is fast and less expensive, it doesn't always provide a lasting solution because previous distresses and cracks eventually make their way up to the new layer of pavement."


As an alternative to these two methods, in-place asphalt recycling continues to gain popularity. Full-depth reclamation (FDR) is a pavement recycling technique in which the existing pavement and some of the existing base layer are broken up and blended to form an improved base for a new asphalt surface. Sometimes an additive is mixed in with the recycled pavement layer to further increase its stability, which is known as stabilized full-depth reclamation (SFDR).

As FDR and SFDR gain popularity, highway engineers need ways to effectively evaluate their properties and correctly apply pavement design guidelines. Few documented field studies have measured material performance, however, so assigning the proper design values to FDR and SFDR pavements is done conservatively.

To provide engineers with more guidance, the Minnesota Department of Transportation (MnDOT) sponsored a U of M study to estimate the proper design values and assess the effects of seasonal temperature changes on these pavements.

"MnDOT uses granular equivalency (GE) to describe stiffness of asphalt and base materials," says Shongtao Dai, research operations engineer with MnDOT's Office of Materials and Road Research. When this project was initiated in 2009, there was no well-defined method to determine GE, he explains, and MnDOT recommended a GE of 1.0 (equivalent to a "class 5" aggregate).


Over three years, the U of M research team led by Labuz evaluated tests performed by MnDOT to determine the stiffness of seven sections of FDR and SFDR county roads in Minnesota. During the spring thaw of each year, tests were conducted daily during the first week of thawing to document seasonal weakening of the pavement's base layer. After the spring thaw period, tests were conducted monthly at each location to capture the pavement strength changes throughout the season.

The study results demonstrate the benefits of SFDR pavements in particular: SFDR pavements were determined to have a higher "stiffness rating" than had previously been assigned by MnDOT, and GE was estimated at about 1.5—meaning less expensive aggregates could beconsidered. In addition, while both pavement types exhibited the seasonal effects typical for asphalt pavements—lower stiffness in the spring than in the summer and fall—most of the SFDR pavements showed improved seasonal stiffness.

"The research has provided guidance and confidence to MnDOT on determining GE values for SFDR materials," Dai says.

Brian Noetzelman, county engineer of Pope County and a member of the project advisory panel, adds that if the GE for SFDR rose from 1.0 to 1.5, it would give 50 percent more carrying capacity to the existing base GE at a minimal cost. "Current 9-ton designs could now be 10-ton designs with the same design structure. What a tremendous savings for counties!"

Reprinted from CTS Catalyst, June 2013.

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Center for Transportation Studies

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