ConcreteTech’s lightweight girders carry the day in Interstate bridge span replacement

Sources: Washington State Department of Transportation; CP staff

Accelerated delivery of North Carolina-processed expanded slate aggregate enabled Tacoma, Wash.-based Concrete Technology Corp. to fabricate precast/prestressed girders meeting Washington State Department of Transportation specifications for this year’s highest profile Interstate Highway System bridge repair.

ConcreteTech was awarded a girder contract for replacement of the northern span of an Interstate 5 steel through-truss bridge, which collapsed into the Skagit River after a truck hit overhead supports. With injuries limited to a few motorists, the late-May incident resulted in a three-week closure, during which crews from Guy F. Atkinson Co. erected a galvanized-steel truss structure geared to temporary bridge span service.

A WSDOT-approved lightweight aggregate—Carolina Stalite’s namesake—was central to a rapid construction plan in which the agency sought to limit a permanent replacement structure and deck to 915 tons, the original span structure’s design weight. That target enabled lead designer Parsons Brinkerhoff to use existing concrete piers; structural modification would have added cost and schedule challenges on a Burlington, Wash., crossing serving the principal route between Seattle and British Columbia.  

Based on recent lightweight aggregate testing unrelated to the Skagit River project, WSDOT engineers approved ½-in. Stalite for use in eight 162-ft. precast/prestressed girders supporting a new four-lane span. After one truck delivery, Carolina Stalite, Salisbury, N.C., shipped the expanded slate in 2,000-lb. supersacks, 216 in all, by rail to ConcreteTech’s Port of Tacoma operation. Crews fabricated the long girders with extra wide top flanges, eliminating the need for a cast-in-place concrete deck. At 84 tons each, the girders total nearly
75 percent of the weight WSDOT budgeted for the new span—diaphragms, deck wear course, plus edge and median barriers rounding out allowable tonnage.

ConcreteTech served as a supplier to Max J. Kuney Co., which was awarded the $6.87 million Skagit River Bridge span replacement shortly after the collapse, and bested by two weeks an October 1 project deadline. The agency has a mid-November target for a final contract at the Skagit River crossing: harmonizing remaining spans’ overhead steel supports at 18 feet of vertical clearance, eliminating truck impact-exposed 15-ft. 6-in. low points.