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The Skyway Precast Operation
Each of those mammoth sections - the largest of their kind in the world - will be fabricated at KFM's 50-acre Stockton Precast Yard . They will be built using the largest concrete forms and some of the biggest cranes on Earth, and then barged all the way down the Delta to the job site on the Bay.
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Here's how they'll do it.
Each completed precast segment will consist of various components. These include precast panels, steel reinforcing bar and the post-tensioning cables that will hold the segments together once they are raised into place on the bridge.
The roadway sections will be match-cast, a technique that ensures consistency of the sections as they are produced in groups. The precast yard will have three main match-casting beds, two for long-line casting and one for short-line casting.
In long-line casting, an initial roadway section is cast and subsequent sections are poured consecutively in a linear fashion until nine sections have been made - enough to make half a span from pier to pier on the new bridge. Long-line casting is used world-wide, but is not a common method in the U.S.
In short-line casting the bed is shorter, and sections are poured one at a time against the initial section, which is repositioned after each section has cured enough to be moved. In this method, fewer sections are cast than in long-line casting. This technique is used to produce segments with a great variation in depth from the top of the deck to the bottom of the precast panel.
Each main bed will produce a roadway segment every three days.
The casting of each segment will require three giant steel forms: A side form, a core form and a bottom, or soffit form. The core and side forms will be hydraulically controlled to allow for fine-tuning of roadway section angles. The soffit form is static but shaped to produce the required arch angle for the Skyway's parabolic bottom.
Each cast creates shear keys and alignment keys on the face of the section. The shear keys hold the segment together; the alignment keys help ensure correct alignment when the segments are assembled.
In both methods, reinforcing steel cages called the rebar assembly are made on-site and lifted into position in the forms by a rolling gantry crane. (The crane, like the core and side forms and the straddle carrier described below, are made in Italy specifically for the precast operation)
The rebar assemblies include the wings - panels precast from lightweight concrete that make up the sloping side walls of the completed segment. Two of these panels will be made every day in a separate casting yard.
To precast the main sections, the concrete will be placed into the forms by a concrete boom truck and consolidated with a vibration device. The deck surface will be smoothed with a roller screed and then broom-finished. After three days, the sections, still sitting in their soffit forms, will be transported to a curing area by giant straddle-carrier crane.
The one-of-a-kind straddle carrier will be the only piece of equipment on site with enough capacity to lift and transport the heavy completed segments. How heavy? Between 480 and 780 tons, depending on their position on the bridge.
With a span of 105 feet and a height of 34 feet, plus the ability to turn 90 degrees by lifting itself on jack stands, the straddle carrier will move each segment out of its casting bed and place it in a curing area. The carrier is also equipped with modems that feed diagnostic information directly to the manufacturer's facility in Italy. The data allows Italian engineers to address operating problems should they arise, and send solutions directly to Stockton.
The straddle carrier positions segments in rows and arranged in the order in which they will be placed on the bridge. The sections will cure for two to six months, depending on their size. Once fully cured, they will be shipped to the Bay one to four at a time on 240-foot-long barges guided by tugs. The sections will rest on pedestals of varying height to ensure that their top surfaces are level during transport.