The St. Croix Crossing Bridge, a $646 million project that connects Oak Park Heights, Minnesota and St. Joseph, Wisconsin in the United States, uses an innovative extradosed superstructure design that combines prestressed concrete box girders and cable-stays. To protect the post-tensioning (PT) tendons during construction, a corrosion inhibitor was used.
Cable stays support the bridge at five pier locations in the river, while ~1,000 precast boxlike segments are connected by PT tendons that were tensioned and grouted in place. PT cables were also used in the crossbeams connecting upstream and downstream towers at each of the five pier sites.
An unseen but important part of construction was protecting the PT tendons from corrosion before grouting. Grouting is commonly delayed several weeks or months on long-term projects or when extremely cold winter temperatures interrupt continuous grouting. State and federal requirements typically call for corrosion inhibitor application if the waiting period is two weeks or longer. To allow segment erection to proceed through the winter months, tendon grouting was able to be postponed in the winter months with the use of a corrosion inhibitor until substrate temperatures were high enough that freezing of freshly placed grout was no longer a concern.
To protect various post-tension strands placed throughout the bridge during construction, the Lunda/Ames Joint Venture, a major partner in the multi-year construction of the St. Croix Crossing, chose to extensively apply a low-toxicity corrosion inhibitor powder manufactured by Cortec Corp. (Saint Paul, Minnesota) for corrosion protection of ferrous metals in recessed areas, interior cavities, and voids. Corrosion inhibitors have been used to protect PT strands in many bridge projects across the country, including the Wakota Bridge in nearby Saint Paul, Minnesota.
The powder corrosion inhibitor was fogged through post-tension ducts using a low-pressure air hose after PT strands were placed in the duct. The powder vaporized and adsorbed on metal surfaces to form a protective molecular layer on the tendons. The layer helped reduce corrosion by inhibiting interaction with corrosive elements such as air, moisture, and chlorides, and discouraging both cathodic and anodic corrosion reactions from taking place on the tendons. Little or no surface preparation was required before application, and the powder did not need to be flushed out before grouting.
More than 40 extradosed bridges have been built around the world in the past 20 years, with the majority of the them built in Japan. The St. Croix Crossing, which opened on August 2, 2017, is only the fourth extradosed bridge to be built in the United States. The 5,000-ft (1,524-m) long structure spans the St. Croix River and carries four lanes of traffic in each direction. It is 100-ft (31-m) wide and varies between 110 and 150 ft (34 and 46 m) above the water, allowing boat traffic to freely move beneath it. Construction began in 2012, and more than 400 men and women from Minnesota and Wisconsin labored on the project. There are more than 42 million lb (19 million kg) of concrete in the bridge, and its 1.1 million ft2 (102,193 m2) of surface area required more than 22,000 gal (83,270 L) of paint to cover it. This bridge replaces the Stillwater Lift Bridge, which was dedicated more than 80 years ago.
Sources: Minnesota Department of Transportation, www.dot.state.mn.us, and Cortec Corp., www.cortecmci.com.