The 35W _9340 Bridge Anti-Icing System Fact Sheet by qingyunliuliu


									          The 35W #9340 Bridge Anti-Icing System Summary for the NTSB
           compiled by C. Beckwith, Metro Maintenance Operations on 8/5/07

Background: In the fall of 1999, The I-35W #9340 Bridge was fitted with a computerized
anti-icing system that automatically sprayed the anti-icing chemical potassium acetate on the
bridge deck when sensor data indicated that hazardous winter driving conditions were
imminent. The Boschung Company Incorporated ( ), a Switzerland
based company, was hired to design, furnish & install, and support the anti-icing system. The
final cost for the system was $618,450. The Mn/DOT Project Manager for the installation
was Ia Xiong, P.E., formerly of Mn/DOT and now with Hennepin County: Bill Gorse is the current Boschung America contact:, 724-658-3300.

The I-35W Bridge #9340 was a candidate for this high-tech treatment due to the high
incidence of winter traffic crashes on the bridge. The bridge was more susceptible to “black
ice” and slippery conditions because of moisture from the Mississippi River’s St. Anthony
Falls, nearby power plants and industrial facilities, and because of the high volume of traffic
on the bridge. The formation of “black ice” was due to the combination of extreme cold and
heavy vehicle exhaust from congestion on the bridge. In addition to traffic safety, the anti-
icing system also contributed to sustainability, because the chemical used is environmentally
less toxic and corrosive than sodium chloride, which traditionally had been used.

System Operation: The bridge anti-icing system worked with a combination of sensors,
RWIS weather stations, a computerized control system, and a series of 38 valve units and 76
spray nozzles that apply potassium acetate. The system was automatically activated based on
the temperature and atmospheric conditions occurring on and above the bridge deck. A
small pump house, north of the river and west of I-35W adjacent to the southbound lanes,
houses a 3,100-gallon tank for potassium acetate storage, a 100-gallon tank for water storage,
pumps and valves, and software for selecting spray programs. The anti-icing system was
typically activated automatically but could be activated manually from the pump house and
via remote control.

The Boschung anti-icing systems utilize active and passive pavement sensors, under the
brand names “BOSO” and “ARCTIS”, which can determine the actual freeze point and
spray the bridge/roadway before icing occurs (which is why it is called "anti-"icing not
deicing). On the I-35W bridge, there were 76 10-inch diameter spray disks that were
flush-mounted in the pavement approximately every 55 feet (the heads do not pop up like
sprinkler heads which is often assumed). The sensors in the bridge deck were also
flush-mounted. One RWIS station was attached to the bridge rail on the north side of the
span carrying south bound traffic about half way across the span. The other RWIS station
was located on the north end of the bridge. On the 35W bridge, accumulator tanks that were
part of a valve unit assembly were mounted on the outside of the concrete bridge rails
between the spans. These 1-gallon accumulator tanks were pressurized locally to deliver
chemical via solenoid valves to two spray disks each. The pumps at the pump house
delivered chemical/water to accumulator tanks at the valve units.

Data Collection and Archiving: The I-35W system had two sets of BOSO and ARCTIS
sensors, however, at least one set of sensors had already been removed for salvaging
purposes by Mn/DOT on June 18, 2007. The other set of sensors had its cable cut by a PCI
sub-contractor the same day, though it wasn’t going to be salvaged. The last recorded
readings from the sensors were on June 18, 2007 around 8:30 AM. Data is available
prior to June 18, 2007 for air temperature, pavement temperature, humidity, freeze point,
wind speed and direction, dew point, spray program used, tank level and precipitation on the
roadway. A computer, responsible for automatically collecting and recording data that was
received from bridge sensors, is located in the Metro Maintenance dispatch center at the
RTMC, located at the Water’s Edge Metro District Headquarters in Roseville. A server was
also installed early in 2007 to store the data for Metro anti-icing systems and is located in the
Metro Maintenance Operations Engineering Service work area in Water’s Edge. Historical
data is available intermittently throughout the life of the system, but some issues over the
past 8 years with communications, servers and a recent sensor failure in 2006 resulted in
some data being lost. There should fairly complete archived data over the past couple of
seasons, however. Data can be retrieved through the contacts designated at the end of this
summary document.

Anti-Icing Chemical: The Anti-Icing System utilized potassium acetate, brand name
“CF7”, from Cryotech Deicing Technologies ( Tony Myhra is the
contact from Cyrotech:, 218-963-1515. A light coating of this
chemical was applied as needed based on the freeze point calculated by the pavement
sensors at an estimated rate of 12-17 gallons per lane mile. The chemical was sprayed
through the spray disks and was tracked by traffic between disks. It was tracked
approximately 500-feet off of the bridge. The potassium acetate tanks were filled as needed
by Mn/DOT maintenance operations personnel.

Though the installation of the anti-icing system had greatly reduced the amount of chlorides
applied on the bridge, it did not eliminate the application of chlorides. Chlorides were still
sometimes applied on the bridge in the snow and ice seasons for a variety of reasons
including, but not limited to, vehicle tracking of chlorides onto the bridge, snow plow
drivers forgetting to turn off their spreaders, and by Metro Maintenance personnel making a
conscious choice to apply chlorides when conditions or incidents warranted. It should also
be noted that chlorides had been applied exclusively throughout the life of the bridge prior
to the anti-icing system being installed in 1999.

Potassium acetate (CF7) is not corrosive to steel or concrete, but it can be corrosive to
galvanized materials as evidenced in some corrosion that occurred on galvanized floor
grating within the pump house. Cryotech Deicing Technologies claims that galvanizing
corrosion does not occur on bridge or roadway elements such as expansion joints and
guardrail. Our analysis at Mn/DOT was consistent with this claim. In October of 2005,
Todd Nieman and Tim Williams of the bridge office evaluated the galvanized products on
the deck of the bridge. Their inspection was summarized in an October 13, 2005 email from
Todd Nieman:

    “The products we looked at were Expansion Joints (Finger and strip seal types), light poles, and barrier
    expansion plates. We conducted visual inspection and performed coating thickness measurements to
    determine remaining galvanizing thickness. It is unknown what the original galvanizing thickness was on
    these items, so impossible to provide quantitative analysis. However, in all products the present
    galvanizing thickness seemed to measure adequate and "as-would-be-reasonably" expected thickness for
    the age and type of components.
   The one exception to adequate galvanizing thickness was on the top road way surface of the expansion
   devices. These top (traffic) surfaces had virtually no remaining galvanizing. However, turning the probe
   immediately around the corner from the top traffic surface showed good thickness. We believe that the
   lack of galvanizing on the top surface of the expansion devices is attributed to wear and not chemical
   attack from the de-icing solution.

   Our determination was that the anti-icing solution was not contributing to advanced galvanizing losses.”

Potassium acetate was pressurized throughout the entire system during the snow and ice
season months each year (approximately October-April). During the non-snow and ice
season months (April-October) water was pumped through the system and was manually
applied to the pavement 1-2 times per week to keep system components functional and
cleared of debris. At the time of bridge collapse, the system had been switched over to
summer operations (as of April 2007) and therefore was using water. Due to the deck
rehab contract, water had been drained from the system as conduit underneath each
disk had been disconnected in June and July. The only chemical in the mostly empty
system at this time was residual water.

Location of anti-icing system components on Br. 9340: The spray disks, sensors and
RWIS locations were described above. There was also a variety of conduit (PVC, stainless
steel and galvanized) attached to the underside of the bridge. This conduit was attached to
the bridge superstructure in some locations via clamps and ties. Conduit ran up through the
concrete bridge deck from the underside of the deck. Valve units and the RWIS stations
were attached to the bridge rail with brackets that were bolted to the bridge rail.

A set of manuals showing details of system components is available through Metro
Maintenance. An as-built of the anti-icing system on Br. 9340 is on file in the Metro
Maintenance Operations Engineering Services work area, and a copy of the as-built will be
delivered to the bridge office on 8/6/07.

Contact information:
The representative for Metro District anti-icing systems in Maintenance Operations is Chris
Beckwith, P.E.,, 651-234-7921 (work), 651-775-0304

The field expert for Metro District anti-icing systems in Maintenance Operations is Cal
Lucas,, 651-234-7925 (work), 651-775-0303 (cell). (Cal had
recent surgery and may be unavailable for the next 1-2 weeks)

An operational report of the system, written in 2001 by former anti-icing system engineer,
Cory Johnson, can be located at
icing evaluation.pdf This report contains photos that may be helpful as well. Cory Johnson
still works at Mn/DOT in the Central Office, if needed.

There are a number of photos of the system located on our server in Metro Maintenance.
Contact Chris Beckwith for access to the photos or any other files. Chris is also available to
answer any questions as needed.
I-35W System Facts:

Location                 I- 35W Mississippi River Crossing, Minneapolis
Bridge Length Treated    1950 feet, 8 lanes
Roadway Treated          0 feet
Unique Site
                         Major River Crossing, Industrial Plants in Proximity of Bridge
Manufacturer             Boschung
Installer                Boschung
Original Contract Cost   $578,000, extra work brought total to $677,000
Completed                December 1999 (first system of its kind installed in U.S.)
                         RWIS (dew point, wind speed, air and ground temp, wind dir),
                         BOSO (temp and dampness), ARCTIS (active, freeze point)
Accumulator tanks        38
Spray Disks              76 (all bridge)
Supply line loops in
Control System           Fully Automated
                         Potassium Acetate (CF7), PROS: environmentally friendly, less
Chemical Used            corrosive to steel and conc., low freeze point (-76 deg), effective
                         to -15 deg. CONS: corrosive to galv., more expensive
System Pressure          about 170 psi
Storage Tank Capacity    1 tank - 3,100 gal total
Advanced Warning
                         "Anti-Icing in Progress When Flashing"
Pump House Building

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