ASPIRE Summer 08 - Aspire - The Concrete Bridge Magazine

WSDOT Moving TowardsPerformance-BasedConcrete SpecificationsAlthough the concrete has performedwell overall, future projects similarto this will use performance-basedconcrete specifications. This willallow WSDOT to include additionalperformance requirements such asshrinkage and scaling resistance. Somepotential improvements that couldbe made to this mix include reducingcementitious materials content, addingshrinkage-reducing or corrosioninhibitingadmixtures, or perhapsmoving to self-consolidating concrete.Shifting to performance specificationswill allow contractors to develop mixesthat meet performance requirementsyet are tailored to the forming,placement, consolidation, and the formremoval methods they prefer.The WSDOT is currently consideringuse of the performance-basedspecifications for the next floatingbridge project, which will likely bethe State Route 520 floating bridgereplacement across Lake Washington.This project is currently in the designphase, with pontoon constructionexpected to start late 2009. Pontoonconstruction on this project sharesmany similarities with the Hood Canalproject. The WSDOT is incorporatingvaluable experiences from Hood Canalinto the design of the State Route 520Bridge to further improve performanceand constructability, and reduceconstruction costs.materials and tight constructiontolerances are required. Wideningand improving the bridge’s west halfwas completed in 2005, Since then,the Washington State Departmentof Transportation (WSDOT) has beenconstructing a new east half, which willbe moved into place in May-June 2009,and further upgrading the west half.The work is underway at multiplec o n s t r u c t i o n s i t e s i n w e s t e r nWashington. Since early 2006,construction pro g ress includessuccessfully building and floating 12 ofthe 14 new east-half pontoons, joiningthe draw span pontoons together, andrehabilitating three 1980 pontoons.The original elevated roadways wereremoved from these pontoons, andwider roadways with a new profile gradewere constructed in their place. Also, 20gravity anchors have been constructedand were placed on the canal floor insummer 2007. Other large operationsin various stages of construction includefabrication of the transition spans andspecialized supports, lift spans, largespherical and cylindrical bearings,major mechanical components, andoutfitting of the draw span assemblywith buildings, access ramps, and thehydraulic, mechanical, and electricalsystems.All of these elements of work set thestage for the closure of the bridge inMay-June 2009, when the east half ofthe bridge and the west transition spanwill be replaced. The bridge will be opento traffic while new anchor cables areinstalled and west half mechanical andelectrical upgrades are made.Pontoon ConstructionThe prestressed high-performanceconcrete (HPC) pontoons are beingconstructed in four separate cycles atConcrete Technology Corporation’sgraving dock in Tacoma, Wash. Thelargest of the cellular box structures is60 ft wide, 18 ft tall and 360 ft long.The pontoons are heavily reinforcedwith both conventional epoxy-coatedreinforcing steel and longitudinal,transverse, and vertical post-tensioningtendons.The HPC used for the pontoons wasoriginally developed in the 1990s forthe I-90 Lacey V. Murrow (LVM) floatingbridge across Lake Washington andincludes the following components:Type I/II cement: 625 lb/yd 3Class F fly ash: 100 lb/yd 3Silica fume: 50 lb/yd 3Aggregates: 1/2-in. maximum sizeThe approximately 31,000 yd 3 ofpontoon concrete have a minimumspecified 28-day compressive strengthof 6500 psi, and a maximum 56-daychloride permeability of 1000 coulombs.The actual 28-day compressive strengthshave been approximately 11,000 psiand the 56-day permeability less than800 coulombs. Early in the project, thecontractor realized that the LVM concreteplacement in the pontoon walls wouldbe challenging because the walls are upto 21 ft tall; 6 in., 8 in., and 10 in. thick;and heavily congested with reinforcingsteel and post-tensioning ducts. Toimprove concrete placement andconsolidation, the contractor requestedapproval to exceed the maximum 9-in.slump that was allowed by the contract.This was achieved by using additionalhigh-range water-reducing admixturewithin manufacturer’s allowances. AfterThe elevated roadway was constructed atTerminal 91 at the Port of Seattle on threeexisting pontoons. These three pontoonswere used temporarily in the west drawspan until 1982 to open the Hood CanalBridge quickly after the 1979 storm sunkthe west half. These pontoons weresuccessfully rehabilitated in 2007 after25 years of storage in the Puget Soundand are used in the new east half.