Maintenance, Repair & Rehabilitation of Concrete Bridges—Winter ...

The historic Rainbow Bridge near Smiths Ferry, Idaho, had its deteriorating rails replacedwith precast concrete units that were match-cast in color and texture to provide aconsistent look for the entire structure.A Perfect Matchby John Hinman, CH2M HillThe Rainbow Bridge near Smiths Ferry,Idaho, was built in 1933 as a cast-in-placearched structure. Its design has made it astatewide landmark and led to its listing onthe National Register of Historic Places. Butits condition had become less noteworthy,leading to the need for delicate repairs thatwould not tarnish its historic nature. Toaccomplish this, new precast concrete railswere installed, with all pieces cast usingcolor and texture to match the existingpieces.The bridge required significant work,because its rails and decorative featureswere disintegrating, and corrosion wasoccurring in the reinforcement of thestringer ends, bents, and spandrel columns.Under ideal conditions, the bridge couldhave been closed and cast-in-place concretecould have been used to replicate theoriginal design, giving the contractor fullaccess. Unfortunately, this wasn’t feasible,as the bridge had to remain open at alltimes.Initially, work focused on the substructure,stabilizing corrosion in the arches andcolumns with electrochemical chlorideextraction (ECE), repairing stringers, andperforming other patching incorporatinggalvanic anodes and replacement work.Analysis showed that the concretehad retained its strength, but neededto be upgraded to control the corrosiondeterioration caused by significant chloridecontamination from deicing chemicals.The main focus at the deck level wasreplacing 841 ft of rail using precastconcrete sections. This approach was takento ensure the bridge could remain openavoiding traffic next to an unprotectededge. The key to success was finding theproper concrete mixture to ensure thenew components would exactly match theshape, color, and texture of the original rails.On the assumption that the originalconcrete mix comprised local aggregates,considerable scouting was done to findsuitable sources. Concrete cores weretaken from the existing bridge, and thesewere compared to cores taken from newsamples cast with different aggregates.Tinting wasn’t an option, as it would beginto weather and create a disparity. It wasdetermined that some local aggregates weretotally unsuitable, but ultimately a closematch was created. The new concrete isexpected to weather over the next few yearsto closely match the existing components.Casting and erecting the precast concreterails created additional challenges, aseach piece was unique due to the grades,superelevations, and curves. The complexitywas immense, with a lot of individualcustomization needed for most of thecomponents. Expanded polystyrene pieceswere carved using computer-controlledcutters to create the forms for each piece.The resulting forms were coated with plasticto achieve a smooth and durable surface.The surface quality of each cast componentstill required close attention. The work wasoverseen by general contractor MowatConstruction Co. in Woodinville, Wash.,with the precast concrete componentsproduced by Central Pre-Mix Prestress Co.in Eagle, Idaho.Officials at the Idaho Department ofTransportation also implemented acorrosion-mitigation program to preventfurther deterioration. After consideringvarious options, they decided on the ECEmethod in the arches and main piers aroundthe joints and the embedment of galvanicanodes in the concrete-patch repairs in thenon-ECE-treated areas.The ECE treatment reduces the amount ofchloride ions in the concrete and generateshigher alkalinity around the reinforcingsteel, reinstating the passivity of steelreinforcement. It directly addresses thecause of the corrosion from the concrete,with no permanent system left in place tobe operated, maintained, and monitored.Approximately 8000 ft 2 of concrete surfacewas treated in less than two months. TheECE treatment was designed by CorrosionControl Technologies in Sandy, Utah. VectorCorrosion Technologies in Wesley Chapel,Fla., supplied the galvanic anodes andexecuted the ECE work as a subcontractorto Mowat.The result of this careful attention to detailis a design that perfectly blends new andold. The proof is in the enthusiastic responsefrom drivers, who had to crawl past theconstruction and wait for traffic when onlyone lane was open. While many timessuch situations create ill will against theconstruction crews; in this case, drivers wererolling down their windows to complimentthe contractor on how good the bridgewas looking. And, with a comprehensivecorrosion-mitigation strategy in place, thebridge is expected to perform for years tocome.The bridge’s arches and main piers aroundthe joints underwent an electrochemicalchloride-extraction process, while galvanicanodes were embedded in the concretepatchrepairs in other areas.The precast concrete rails were cast in formscreated with expanded polystyrene piecesthat were carved using computer-controlledcutters. Each railing piece is unique, owing tothe grades, superelevations, and curves.This bridge was named the 2007 Projectof the Year by the International ConcreteRepair Institute; for more details on theproject, see ICRI’s November/December2007 issue of Concrete Repair Bulletin, orvisit http://www.icri.org/AWARDS/2007/rainbowbridge.asp._____________John Hinman is principal bridge engineerwith CH2M Hill in Boise, Idaho.44 | ASPIRE, Winter 2009