Media schooled on River Crossing highway-surface project
About two dozen Twin Cities-area media members got a quick lesson last week on how River Crossing’s concrete driving-surface segments are being made and how they’ll be transported to the construction site soon.
The hour-long Minnesota Department of Transportation “media visit” March 17 was held at the bridge project’s Grey Island casting yard in Cottage Grove, where about 80 workers have already completed 105 of 650 segments.
Those segments –- each weighing 170 tons and measuring 48 feet wide, 18 feet tall and 10 deep — are now lined up in casting yard’s outdoor storage area in the order they’ll be installed, according to a MnDOT PowerPoint project outline.
From there, they’ll be moved by a mobile segment lifter to the facility’s barge-loading area on the Mississippi River a few hundred yards south of where they were cast indoors.
A succession of barges will then transport four to six of the segments at a time to the River Crossing bridge construction site just south of Stillwater off Hwy. 95.
Asked how soon the segments’ 30-mile barge trip along the Mississippi and St. Croix rivers to the site will begin, MnDOT River Crossing Bridge Construction Engineer Paul Kivisto told the group:
“The weather and the ice on the river will determine when the segments go in. … We do expect that to happen pretty soon though.”
Installation of the highway-surface segments will mark a key next phase of the River Crossing project, which is scheduled to be completed in the fall of 2016.
About 250-300 man hours of labor go into each segment as part of a meticulous indoor casting process, MnDot officials explained at the media visit.
Although the basic casting process is the same for all of the segments, each one is “one-of-a-kind, like a puzzle piece,” noted Project Director Michael Beer.
The casting process, too, produces new segments in the order they will be installed as what will become the new bridge’s driving surface throughout the rest of this year and into 2016, he said.
Six to eight steel-reinforced segments are cast every week at the five-station, indoor Cottage Grove facility, where adjustable forms are used to create each segment’s unique size and shape, according to the MnDOT PowerPoint outline.
About 350 additional highway-surface segments are also being cast at an on-site yard at the bridge location.
Here are the basics of how the Grey Island casting process works:
—Near each adjustable-form casting station, a “skeleton” of rebar steel-reinforcement bars is first assembled according to each segment’s design specifications.
—Crews then use a crane to lift and place the rebar skeleton inside the form.
—Next, hollow plastic ducts are installed inside the skeleton to make space for the “post-tensioned” steel strands that will be used to put each segment into place at the bridge site. Concrete is then poured inside the form, with surveyors measuring “to the thousandth of a foot” for correct specifications before and after each pour.
—Afterward, the segment’s concrete is allowed to cure for about 24 hours to meet its specified strength, and the post-tensioning strands are stressed.
—Crews then move the finished segment to an area adjacent to the casting form, providing a template to assure that it will perfectly match the next segment.
—As the process proceeds, the finished segments are then moved in order by a mobile lifter to the outdoor storage area, where they continue to cure and strengthen until ready for the barge trip to the bridge site.
According to a St. Croix Crossing “Bridge By the Numbers” sheet distributed at the media visit, 563.8 million pounds of concrete will be used to construct the new bridge, along with 42.3 million pounds of steel and 1,969 miles of cable strands.