From the time I was a young kid traveling from Chisholm to Hibbing to see my grandmother or cousins who lived on Third Avenue, I can recall white-knuckling in that old rickety car as we went over this bridge.”
—Congressman James Oberstar (Minn.)
Congressman Oberstar isn’t alone. The Highway 169 Mitchell Bridge near Hibbing, Minn., has long held a nasty reputation for its tendency to ice up in winter, sending motorists crashing into its sides. Fourteen weather-related traffic accidents occurred on the bridge in the past four winters alone, according to the Minnesota Department of Transportation (MnDOT).
Hibbing lies at the heart of northern Minnesota’s Iron Range—where weather patterns are dictated by close proximity to Lake Superior, the largest freshwater lake in the world.
Winters are long and snow-filled, and keeping roads and bridges free of snow and ice poses a real challenge. So when MnDOT decided to test a new technology to make roads and bridges safer in winter, the Hibbing site seemed a natural candidate.
The technology chosen was a surface overlay made up of a patented combination of epoxy and aggregate rock. Installed in summer, transportation departments “charge” the surface with their standard liquid anti-icing chemicals before frost or ice storms are expected. The material acts like a rigid sponge, storing the chemicals inside and then automatically releasing them as conditions develop for the formation of ice or snow.
The result is safer roads with better mobility and less maintenance, because the overlay helps prevent frost or ice from forming on road or bridge surfaces.
“This is potentially the biggest technological breakthrough for improving highway safety in snow-belt regions since the invention of the snowplow,” said John Bray, District 1 of MnDOT. Minnesota is one of 10 states to expand its highway-safety arsenal by adding the surface overlay test sites this summer.
Equally attractive is the fact that the surface overlay keeps releasing the anti-icing chemicals over multiple events. In 2003, the technology was licensed to Minnesota-based Cargill Inc. and has been met with an enthusiastic response.
Measures that work
The Hibbing installation in July 2006 marked the 15th installation (six this season, nine previously) of the surface overlay at sites in nine states. But it is the first test site within the state of Minnesota, the first installation to be kicked off by a U.S. Congressman and the first time MnDOT crews were called upon to install the new technology.
“We keep learning and improving at each installation,” said Anthony Hensley of Cargill SafeLane, who oversees each installation.
Prior to discovering the surface overlay, the Mitchell Bridge had become so notorious for accidents among local residents that in 1995, MnDOT installed an automated spray system for deicing it. The system included dial-in technology that was supposed to allow MnDOT to call in instructions for the automatic spraying of anti-icing chemicals as weather conditions dictated. The problem, according to Duane Hill, MnDOT assistant district engineer for maintenance operations, was “it never really worked.” MnDOT eventually removed the system after years of struggling with high maintenance and poor reliability.
Then, last summer, transportation officials in neighboring Wisconsin installed the surface overlay on the Blatnik Bridge between Superior, Wis., and Duluth, Minn. The Blatnik Bridge, too, has a long history of accidents. Anecdotal reports reached MnDOT that on snowy days, maintenance crews arriving at the Blatnik test site found the surface wet rather than snow-compacted.
“That install really caught the attention of our district engineer, who places a strong emphasis on safety,” said Hill. “He pushed those of us in District 1 to develop a research project for Minnesota.”
The Mitchell Bridge topped the list of prospective test sites in the district because, Hill noted, “Everyone in Hibbing knows someone who has had a problem there.”
Having selected the site, MnDOT’s next decision was whether to hire a contractor to install the overlay or do the work themselves. MnDOT chose to use their crews, in part, to keep the project within budget.
Overview of the overlay
Although commonly referred to as “the Mitchell Bridge,” the test site is actually two bridges over railroad tracks connected by a roadway. In all, the surface overlay was to be applied to 16,000 sq ft of bridge deck and roadway. Cargill provided technical support for the installation, and as such, Hensley’s work with MnDOT actually began weeks before the project’s scheduled start date. Hensley first discussed with project management installation details such as how to distribute the aggregate, how large a work crew would be needed and job sequencing. Video clips and photos of past installations supplemented the meetings to help managers envision how the project would look.
Next, Hensley provided a “very helpful” list of supplies that would be needed, “right down to duct tape,” according to one supervisor. All epoxy and aggregate was delivered a week prior to the project’s scheduled start date.
Meanwhile, Hensley and project managers nailed down critical details, including traffic control. At the Hibbing site, “traffic control played a huge role,” Hensley noted. The project plan called for the surface overlay to be applied to the southbound lanes of the bridges and roadway only, with northbound lanes serving as the control site. The original traffic plan called for working on one southbound lane at a time, keeping one open for traffic.
“The more we talked,” said Hill, “the clearer it became that work would progress faster, and it would be safer for our employees, if we closed both southbound lanes.”
It was a tough call, Hill noted, because MnDOT’s traffic office worried about the potential impact on motorists of having all traffic re-routed onto northbound lanes.
“As a result, we really went the extra mile with additional flagging to ensure motorists slowed down and paid attention during the installation,” Hill noted.
The Friday before the project’s scheduled start proved to be a day for keeping a close eye on the weather forecast. Predictions of rain would force postponement. However, by midafternoon, with just a 20 to 30% chance of rain in the forecast, Hensley confirmed the installation would move forward.
Monday, July 17, dawned bright and hot in Hibbing. MnDOT project managers and Hensley met with the crew to tell each person what to expect based on their job. With traffic control in place, they moved quickly to step two: shot blasting the bridge surface to clean it, while crews mixed and prepared the epoxy.
Although the surface overlay represents revolutionary new technology for road and bridge safety, it is applied in a decidedly low-tech and traditional manner. Crews must hand-spread the sticky, black epoxy across the road surface. The initial coat is applied at a thin rate. After the epoxy is hand-spread on the surface, the aggregate is immediately shoveled across the surface. Next, a sweeper truck passes by to take out loose aggregate, followed by two leaf blowers to remove any excess stone.
“The pavement was hot, the epoxy was hot and we had the right number of people,” Hensley recalled, “which made for a quick cure time.”
With daytime temperatures in the 80s, the overlay was left to harden for about an hour, then a second coat was applied following the same protocol, but at a thicker rate. The final profile is about 3?8 in. thick.
The installation method follows the recommended installation method outlined in AASHTO Task force 34. The surface overlay is expected to provide a robust surface for more than 15 years of service, plus the much-needed pavement seal to limit chemical and moisture penetration into the concrete bridge deck.
Just 34 hours after the project began, MnDOT was able to reopen one southbound lane of the site to traffic at 5 p.m. Tuesday, with the second lane reopening on Wednesday morning.
“This project moved like clockwork,” said Hill. “The training [that was] provided, the planning and advance preparation all contributed to a job well done. An added unanticipated benefit was that it proved to be an excellent team-building exercise for our employees.”
MnDOT also agreed to evaluate the overlay’s performance at the Hibbing test site over the next three years as part of a partnership agreement with Cargill. Chemistry professor John Evans from the University of Minnesota-Duluth will analyze safety and durability, including skid testing, wear characteristics and chloride permeability as part of a research effort at the Northland Advanced Transportation Systems Research Laboratory.
Dylan the singer, Mitchell the bridge
Meanwhile, an analysis of the surface overlay’s performance during the 2005-06 winter season has already been conducted by Wilfred Nixon, a leading snow- and ice-control authority who is president of Asset Insight Technologies and professor of engineering at the University of Iowa. The report found no weather-related accidents at all nine test sites studied.
In many cases, this contrasted with multiple accidents on nearby untreated stretches of road or bridge deck. And almost all of those treated sites had a history of winter-weather accidents.
The report concluded, “On the basis of the observations made during the 2005-06 winter, [the] overlay provides benefits in both safety and mobility under winter storm conditions, and those benefits may be attained with less chemical than would be needed for highway segments without the overlay.” A complete copy of the report, which was commissioned by Cargill, is available at www.cargillsafelane.com.
MnDOT officials are hoping for similar results at the Hibbing test site, as is Congressman Oberstar. As ranking member of the U.S. House of Representatives’ Transportation & Infrastructure Committee, Oberstar is well aware that adverse weather conditions contribute to an average of 1.4 million car accidents in the U.S. alone each year, resulting in 7,000 deaths, more than 600,000 injuries and $42 billion in economic loss, according to the National Oceanic and Atmospheric Administration.
“This could be the most significant highway advancement in 40 years,” said Oberstar of the surface overlay technology. “One, it will save lives. And two, it will extend the useable life of a bridge or roadway.”
That’s the promise of the surface overlay. Should it fulfill that promise, as first results indicate it will, Hibbing may no longer be best known around the nation and world as the hometown of singer Bob Dylan. Instead, it may become recognized as one of the spots that helped prove a new technology to give motorists better mobility and greater safety, with impacts as far reaching as Dylan’s music.
