Thousands of Idahoans experience it every year, but few realized the historic gem was losing its luster, one chunk or chip at a time. Rainbow Bridge, a traditional arched concrete bridge was showing its age, belying its coveted status on the National Register of Historic Places.
Recognizing the slow but steady assault that nature was exacting on the structure, ITD began a major rehabilitation project in 2006. Although work on the arch piers, floor beams, deck joints and columns took place outside the view of the motoring public, someone was paying close attention.
The International Concrete Repair Institute chose the Rainbow Bridge facelift as its 2007 Project of the year. The bridge emerged ahead of some notable competitors:
In all, the Concrete Repair Institute named nine awards of excellence and 15 awards of merit for projects in the U.S. and abroad.
The organization “conducts an awards program each year to honor and recognize outstanding projects in the concrete repair industry,” according to its Web site. Entries are received from around the world, and the winning projects are honored each year at the annual ICRI Awards Dinner and Reception at each ICRI Fall Convention.
“Because of its historic significance (listed on the National Register of Historic Places) and strong support from the local community, the Rainbow Bridge in Idaho was designated for rehabilitation, rather than replacement,” according to the award announcement.
Independent consultants for the ITD project began with a comprehensive corrosion and structural survey. The chloride testing, corrosion potential testing, and a visual and tactile survey indicated that the most severe corrosion in the substructure was near the deck drains and the joints.
The corrosion mitigation strategy included two systems to protect different sections of the structure: electrochemical chloride extraction was used for the concrete arch substructure and galvanic anodes were used around the perimeter of concrete patch repairs in the areas that did not receive the electrochemical treatment, according to the institute.
The specified corrosion mitigation systems offered the best combination of service life, cost and maintenance without major impacts on the appearance of the historic structure.
Rainbow Bridge, spanning the pristine North Fork of the Payette River in a scenic byway corridor, was built in 1933. It was a monumental achievement in Idaho bridge construction for its day, reflecting leading-edge engineering. Rainbow Bridge exemplifies a conscious effort to meld a modern structure with a picturesque natural setting.
The open-spandrel concrete arches stretch 410 feet across the Payette and is the largest single-span concrete arch structure in Idaho.
It was designed by Charles A. Kyle, the first chief bridge engineer for the Bureau of Highways, which operated as part of the Department of Public Works . Construction began in July 1933 and was completed in December – just six months later. In contrast, repairs took place over two construction seasons without significant traffic interruptions
C.C. Dinsmore & Co. of Ogden Utah, built the bridge.
Restoration was completed by Mowat Construction Co. of Desert Hills, Ariz. CH2M Hill served as project engineer and designer.