Rainbow Bridge, spanning 410 feet across the pristine North Fork of the Payette River in the scenic State Highway 55 corridor, is the largest single-span concrete arch structure in Idaho. Built in 1933, it was a monumental achievement in Idaho bridge construction reflecting leading-edge bridge engineering. The bridge exemplifies a conscious effort to meld a modern structure with a picturesque natural setting.

With thousands of Idahoans driving across it every year and its exposure to the elements, Rainbow Bridge has been slowly but steadily deteriorating. Travelers could see the pockmarked bridge rails and feel the uneven bridge deck. Due to 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. The rehabilitation plans consisted of a delicate balance was between performing the necessary upgrades to improve travelers’ safety and preserving/protecting the cultural significance of the structure for future generations.

ITD began a major rehabilitation project in 2006. 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.

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.

The International Concrete Repair Institute chose the Rainbow Bridge rehabilitation project as its 2007 Project of the Year. Restoration of this bridge project was completed by Mowat Construction Co. of Desert Hills, Arizona. CH2M Hill served as project engineer and designer.