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PRECAST: A HIGHWAY STAR

In an era where tighter budgets force state departments of transportation to stretch construction dollars, precast concrete interests are making their value propositions heard by improving established offerings, like prestressed girders and mechanically stabilized earth (MSE) panels, and perpetuating newer ones like bridge deck and pavement panels. Some producers are showing agency engineers solutions that use not only less cast-in-place concrete, but also less precast than called for on working project plans.

In December 2011 and July 2012 cover stories, we tracked precast/prestressed producers’ impressive strides in the super girder class, typically bulb tee bridge members in the 150-foot-plus range. From 2010–2012, at least six state DOTs had set new length records for single bulb tees delivered by trailer. Producers noted that among key drivers in super girder orders were DOTs’ increasing confidence in a) efficient concentrations of widely adopted 0.6 in. diameter strand in bottom and top flanges; b) consistent attainment of higher strength beam mixes, especially up to 10,000 psi; and, c) trailers engineered for safe delivery of inordinately long loads.

This month we revisit super girder practice with two producers leading the charge: Concrete Technology Corp. of Tacoma, Wash., and its neighbor down Interstate 5, Knife River Corp. Northwest, Harrisburg, Ore. In separate articles (Concrete Tech, pages 20-22; Knife River NW, cover story, pages 27-31), we see how each tackled jobs, one emergency, one planned, involving long bulb tee girders to replace spans carried by obsolete steel through-truss members.

The Knife River NW project involved replacement of a dated U.S. 30 crossing at the Burnt River and Union Pacific rail line, and overall design seemingly maxed out with precast. The bridge girders are the Oregon Department of Transportation Bulb Tee 90-in. profile, and have proved their mettle since Knife River NW proposed them on another Union Pacific rail crossing in 2005.

Roadway alignment and clearance requirements for the Union Pacific line originally spelled a two-span, U.S. 30/Burnt River bridge replacement. Availability of the longer spanning BT90, coupled with the expansion of an existing abutment—supported by Reinforced Earth Company’s signature precast MSE panel—enabled ODOT to eliminate the new pier and specify four 162-ft. versus eight 120-ft. bulb tees. Capping off the precast concrete schedule are 15 deck panels, which were part of a design that landed ODOT a grant under the innovations and life cycle-geared Federal Highway Administration Highways for LIFE program. The economy and performance ODOT officials observed in their inaugural precast bridge deck compelled a similar specification on a four-bridge project on Oregon S.R. 58, near Mt. Hood. In addition to a BT girder order, Knife River NW is shipping 74 precast deck panels over the job’s 2013 and 2014 phases.

ODOT entered the super girder class after Knife River NW proposed a suitable profile and a series of successes in Washington State, where Concrete Tech is synonymous with precast/prestressed members in the 180-ft. length league. In the Concrete Tech project visited this month, girder length—162 ft., the same as the Knife River NW order—is less of an issue than production speed and engineering finesse.

Using lightweight aggregate shipped nearly 3,000 miles by rail, Concrete Tech fabricated nine, weight-sensitive bulb tees to replace a span of the Interstate 5 Skagit River crossing that collapsed the week before Memorial Day. Much like the Knife River NW order, the girders Concrete Tech dispatched up Interstate 5 from its Tacoma plant replaced a span bearing on a structure built of material increasingly displaced by precast.