The aging of America's 590,000 bridges has led to increased maintenance and rehabilitation funding in recent years, with even higher levels of future spending anticipated to sustain their health and safety. "While 50 years ago the nation faced a historic period of bridge construction, today it faces a historic period of bridge repair and reconstruction," according to the American Association of State Highway and Transportation Officials (AASHTO) in its report titled "Bridging the Gap."
The report credited state departments of transportation with keeping the nation's network of bridges safe through ongoing inspections, the use of improved materials, and "ingenious repairs." At the same time, the report acknowledged, "A significant new investment and national commitment is necessary to protect these invaluable assets."
Special attention is being given to structures that are more than 50 years old and heavily traveled, which AASHTO has labeled, "Baby Boomer Bridges." Falling into this category is the Maurice J. Tobin Memorial Bridge, which stretches across the Mystic River to connect the Charlestown section of Boston with Chelsea. Currently operated by the Massachusetts Department of Transportation, the three-span, cantilevered truss bridge first opened to traffic in 1950.
At approximately 2 1/4-miles long, the bridge is the largest in New England and the first in Massachusetts to use a new, high-tech structural monitoring system that uses wireless sensors attached to various areas of the bridge. A continuous flow of data from these sensors will provide engineers with real-time information on loads, stresses, environmental conditions and corrosion on the bridge. When fully operational, this "smart bridge technology" will enable engineers to address any issues immediately.
Engineers also follow a proactive schedule of bridge inspections and a carefully phased recoating and re-decking program to keep the bridge in good condition. Bridge maintenance has been an ongoing priority since the late 1970s, when the first lead abatement project in the country was initiated by the Massachusetts Port Authority (Massport) engineering department, which operated the bridge at that time. "I started my career on that bridge as a paint inspector," recalled Larry Mitkus, who is currently a coating consultant with Tnemec Company. "I was on that bridge for three straight years, which is why I know it so well."
Over the years, Mitkus has continued his involvement with the Tobin Bridge, specifying high-performance coating systems for major renovation projects, including an on-ramp for a toll plaza that was recoated in 2000. "They can't recoat the entire bridge at once because of its size, so projects are divided under different contracts," Mitkus explained. "The specified color is a federal standard green, which is a government specification."
The three-coat system used on the on-ramp and other sections of the bridge consists of Series 90-97 Tneme-Zinc, a zinc-rich urethane primer, followed by an intermediate coat of Series 27 F.C. Typoxy, a polyamide epoxy. The finish coat, Series 73 Endura-Shield, an aliphatic acrylic polyurethane, provided protection from exterior weathering, abrasion and corrosive fumes. Overall, nearly 16,000 gallons of coatings have been used, representing 15% of the entire bridge. "The coating system provides the sacrificial corrosion performance of the zinc primer, the barrier protection of the epoxy intermediate coat and the UV resistance of the Series 73," Mitkus said. "We have done a number of overpasses and bridge structures over the past 20 years that use a very similar coating system, including work for the Massachusetts Turnpike Authority."
Each coating project was complicated by the need for full containment to prevent abrasives and paint from damaging automotive and truck traffic. "This is a major artery from the north coming into Boston, so there's no way to shut down the bridge for recoating," Mitkus noted. "In addition to protecting traffic, there's the lead issue. A dense urban population is in proximity to the bridge, so there cannot be any abrasives or dust containing lead paint drifting onto surrounding properties. That was a major concern for Massport."
Mitkus expects an increase in major bridge maintenance and repair projects during the next five to 10 years. "You're going to see a definite increase in bridge coating because of the growing need," he predicted. "There are bridges all over the country that are in disrepair, so we're seeing more federal stimulus funding being allocated toward fixing the infrastructure." (Source: Tnemec)
The Tobin Bridge Project
Because of its rapid curing characteristics, Series 90-97 Tneme-Zinc was used as the primer on the Tobin Bridge in Chelsea, MA, allowing the intermediate coat to be applied the same-day.
With more than 80,000 motorists using the Tobin Bridge daily, all recoating work is conducted under full containment. "In addition to protecting the traffic from the coatings, these are lead abatement projects, so you can't allow any abrasives containing lead dust to drift onto surrounding properties," Mitkus explained. "This is a major artery from the north coming south into Boston, so there's no way to shut down the bridge during maintenance."
For each project, the structural steel was prepared in accordance with SSPC-SP10/NACE No. 2 Near-White Metal Blast Cleaning and primed with Series 90-97 Tneme-Zinc, a moisture-cured, zinc-rich urethane primer that was spray-applied. An intermediate coat of Series 27 F.C. Typoxy, a versatile polyamide epoxy, was spray-applied, followed by a finish coat of Series 73 Endura-Shield, an aliphatic acrylic polyurethane that is highly resistant to abrasion, ultraviolet (UV) light, and exterior weathering. "We worked closely with the engineers in designing the coating system, which ended up being used as the standard for quality," Mitkus noted.
Since 2000, nearly 16,000 gallons of the three coatings have been used on Tobin Bridge. "The coating contractor was happy with the products and had no delivery or application issues," Mitkus added. "The coating system has been holding up well and we haven't had any callbacks."
Tobin Bridge has developed a phased maintenance program that includes recoating and redecking to ensure its upkeep. The $1 million structural monitoring system installed during 2009-2010 is designed to provide bridge engineers with real-time information about loads, stresses, environmental conditions, and corrosion. An estimated 26 million vehicle trips are taken over the bridge annually.