Starting today, the most notorious winter slick spot on the Twin Cities-area freeway system - the Interstate Hwy. 35W Bridge over the Mississippi River - is being fitted with an automatic de-icing system.
Using temperature- and precipitation-activated nozzles embedded in the bridge deck, the system will spray the bridge with liquid potassium acetate in a bid to rid the surface of the treacherous black ice that has caused more than 120 accidents in the past five winters.
The potassium retains its melting power at 20 to 30 degrees below zero. As of 9 a.m. today, two weeks of traffic disruptions are in store for 134,000 daily users of the bridge as the de-icer system is installed.
The right two lanes of southbound I-35W over the bridge will be closed between University and Washington avenues from 9 a.m. to 3 p.m. through Friday.
Two through lanes will remain open.
Next week, the right two lanes of northbound I-35W between Washington and University will be closed from 8 a.m. to 2 p.m. Monday through Friday. Two through lanes will remain open.
The ramps onto and off the freeway from University and Washington avenues also will be closed during the work. Washington and University are used by motorists headed to the University of Minnesota, and even midday traffic is typically heavy.
To avoid delays during construction, the Department of Transportation recommends that motorists find alternate routes.
Black ice, the invisible film that gives roads a shiny black look, forms regularly on the bridge because of the moisture from the river and the bridge's high elevation, the department said. The bridge can be especially treacherous because ice sometimes forms when the pavement is dry and drivers don't expect it.
The bridge typically generates more complaints than other spots on days with bad driving conditions, said Don Mathisen, a metro-division road maintenance supervisor.
Keeping the bridge clear of black ice has been difficult at sub-zero temperatures when salt is no longer effective. The State Patrol occasionally has closed the bridge to protect drivers.
The automatic system will apply the liquid potassium to the bridge from 68 spray heads in the driving surface placed 59 feet apart and from eight additional spray heads on the north end of the bridge in the median barrier and on the sides of the bridge, said Ia Xiong, project engineer.
The system will cost about $537,000. It is manufactured by the Boschung Co. of Switzerland, which has an office in Brainerd, Minn.
Last year the company installed two bridge de-icers in Pennsylvania and has had others in use in Switzerland, Germany, Russia, France and Italy since 1979, said Mike Strobel, manager of the Brainerd office. A German study found a 51 percent reduction in accidents with the use of the de-icer, he said.
Strobel said the sensors in the bridge detect freezing temperatures before ice forms and activate the potassium spray to prevent ice formation.
The liquid will squirt out of nozzles in an arc 4 to 6 inches high, so motorists will see the spray. The deck surface will be wet.
The plastic nozzles are about an inch thick and 11 inches in diameter. The spray will reach most of the bridge and vehicles will spread it across the entire deck, Strobel said.
Xiong said the substance won't promote vehicle rust.