THE CONCRETE BRIDGE MAGAZINE

WINTER 2009

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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The historic Rainbow Bridge near Smiths Ferry, Idaho, had its deteriorating rails replaced with precast concrete units that were match-cast in color and texture to provide a consistent look for the entire structure. The precast concrete rails were cast in forms created with expanded polystyrene pieces that were carved using computer-controlled cutters. Each railing piece is unique, owing to the grades, superelevations, and curves. The bridge's arches and main piers around the joints underwent an electrochemical chloride-extraction process, while galvanic anodes were embedded in the concrete- patch repairs in other areas. 44 | ASPIRE , Winter 2009 a peRfect matcH by John Hinman, CH2M Hill The Rainbow Bridge near Smiths Ferry, Idaho, was built in 1933 as a cast-in-place arched structure. Its design has made it a statewide landmark and led to its listing on the National Register of Historic Places. But its condition had become less noteworthy, leading to the need for delicate repairs that would not tarnish its historic nature. To accomplish this, new precast concrete rails were installed, with all pieces cast using color and texture to match the existing pieces. The bridge required significant work, because its rails and decorative features were disintegrating, and corrosion was occurring in the reinforcement of the stringer ends, bents, and spandrel columns. Under ideal conditions, the bridge could have been closed and cast-in-place concrete could have been used to replicate the original design, giving the contractor full access. Unfortunately, this wasn't feasible, as the bridge had to remain open at all times. Initially, work focused on the substructure, stabilizing corrosion in the arches and columns with electrochemical chloride extraction (ECE), repairing stringers, and performing other patching incorporating galvanic anodes and replacement work. Analysis showed that the concrete had retained its strength, but needed to be upgraded to control the corrosion deterioration caused by significant chloride contamination from deicing chemicals. The main focus at the deck level was replacing 841 ft of rail using precast concrete sections. This approach was taken to ensure the bridge could remain open avoiding traffic next to an unprotected edge. The key to success was finding the proper concrete mixture to ensure the new components would exactly match the shape, color, and texture of the original rails. On the assumption that the original concrete mix comprised local aggregates, considerable scouting was done to find suitable sources. Concrete cores were taken from the existing bridge, and these were compared to cores taken from new samples cast with different aggregates. Tinting wasn't an option, as it would begin to weather and create a disparity. It was determined that some local aggregates were totally unsuitable, but ultimately a close match was created. The new concrete is expected to weather over the next few years to closely match the existing components. Casting and erecting the precast concrete rails created additional challenges, as each piece was unique due to the grades, superelevations, and curves. The complexity was immense, with a lot of individual customization needed for most of the components. Expanded polystyrene pieces were carved using computer-controlled cutters to create the forms for each piece. The resulting forms were coated with plastic to achieve a smooth and durable surface. The surface quality of each cast component still required close attention. The work was overseen by general contractor Mowat Construction Co. in Woodinville, Wash., with the precast concrete components produced by Central Pre-Mix Prestress Co. in Eagle, Idaho. Officials at the Idaho Department of Transportation also implemented a corrosion-mitigation program to prevent further deterioration. After considering various options, they decided on the ECE method in the arches and main piers around the joints and the embedment of galvanic anodes in the concrete-patch repairs in the non-ECE-treated areas. The ECE treatment reduces the amount of chloride ions in the concrete and generates higher alkalinity around the reinforcing steel, reinstating the passivity of steel reinforcement. It directly addresses the cause of the corrosion from the concrete, with no permanent system left in place to be operated, maintained, and monitored. Approximately 8000 ft 2 of concrete surface was treated in less than two months. The ECE treatment was designed by Corrosion Control Technologies in Sandy, Utah. Vector Corrosion Technologies in Wesley Chapel, Fla., supplied the galvanic anodes and executed the ECE work as a subcontractor to Mowat. The result of this careful attention to detail is a design that perfectly blends new and old. The proof is in the enthusiastic response from drivers, who had to crawl past the construction and wait for traffic when only one lane was open. While many times such situations create ill will against the construction crews; in this case, drivers were rolling down their windows to compliment the contractor on how good the bridge was looking. And, with a comprehensive corrosion-mitigation strategy in place, the bridge is expected to perform for years to come. This bridge was named the 2007 Project of the Year by the International Concrete Repair Institute; for more details on the project, see ICRI's November/December 2007 issue of Concrete Repair Bulletin, or visit http://www.icri.org/AWARDS/2007/ rainbowbridge.asp. _____________ John Hinman is principal bridge engineer with CH2M Hill in Boise, Idaho.

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