Building a structure as long and large as the 1,674-foot Claude Allouez Bridge in De Pere poses continual challenges.
The new bridge's owner (The Wisconsin Department of Transportation), its designer (Graef Anhalt Schloemer & Associates, Inc.), and its general contractor (Lunda Construction, Inc.) recently found an innovative solution to one of the project's major challenges when they used twin conveyors to pour more than 900 cubic yards of concrete for three spans of deck in a continuous pour that reached out more than 400 feet from the concrete delivery point.
The six-hour pour formed the deck for the three western-most spans of the 13-span bridge that will cross the Fox River in De Pere.
Said Mitch Bourin, one of Graef Ahnalt's on-site engineers, "Conveyor systems were the ideal solution for this application. The amount of concrete being placed ruled out using crane-supported buckets, and the long distance ruled out pumping systems. In addition, the construction specifications allow very little slump, so the concrete mix was stiffer than average. That also ruled out pumping. The ideal answer was using conveyors."
WisDOT's project manager, Mike Leitzke, added, "Use of conveyors for placing concrete is relatively rare, compared to pumping, these days. And using two conveyors simultaneously is even more unusual. The long reach and the need to place high volumes of concrete quickly dictated using two conveyors for this job. Fortunately, our general contractor, Lunda, has one of the few two-conveyor systems in the Midwest long enough to handle this job."
The section of deck poured in this session measured 404 feet long, 75 feet wide, and 8 inches deep. It included four traffic lanes, two bicycle lanes, and the curbs, gutters and sidewalks on each side. The concrete has a fully cured strength of 4,000 pounds per square inch. The pour started over the river, above the bridge's third pier and worked back to the western shore.
The conveyors come in 30-foot-long sections that are joined to form the total length needed. The on-shore end of each conveyor has a hopper that funnels concrete from delivery trucks onto the electrically driven conveyor belt.
The opposite, or placing, end of each conveyor is movable and controlled by a portable console of remote controls. The conveyor operators swing the placing ends back and forth to deliver a stream of concrete across the width of the bridge. As sections of bridge are completed, the conveyor system can be retracted to fill the next area.
When a 30-foot length of deck has been finished, one section of conveyor is quickly removed and the pouring restarts. The process continues until the last section of deck has been completed.
As the concrete was poured, Lunda used a Bidwell machine to finish the deck. The Bidwell consists of a lattice truss that rides on temporary, adjustable rails mounted on each edge of the bridge, and a movable head with rollers that put a smooth, uniform finish across the entire width of the bridge deck. The Bidwell also puts the specified slope on the deck to assure proper water runoff for drainage.
As the head travels back and forth along the truss, the work crew makes sure there is enough concrete for it to work properly.
After the Bidwell has completed a pass, a laborer with a hand float puts the final smoothing finish on the surface. To avoid having him walk in the finished concrete, Lunda uses a movable work truss that, like the Bidwell, spans the entire width of the bridge and rides on rails.
Explains Bourin, "One very important factor in constructing a bridge deck is the quality of its surface. That is the part that the driving public comes in contact with and notices most. You want to be sure they experience a smooth, comfortable ride."
After the final surface has been put onto the freshly poured concrete, part of the crew sprays it continuously with water until the whole pour has been completed. Keeping the concrete wet as it cures eliminates the cracking that can happen if it dries too quickly.
The crew then places burlap over the concrete and an automatic spraying system keeps the concrete wet for seven days until it is completely cured.
To be sure the concrete has cured properly, WisDOT, Graef Ahnalt and Lunda are using two methods of monitoring the process. One method is a new method, employing embedded electronic chips that report the amount of curing to a receiver. As a double-check, they are also using the time-tested method of creating hundreds of test cylinders filled with the same concrete poured for the deck.
When the first section of spans has cured, the twin conveyors will be set up starting at the far end of the recently completed section to pour the next, 426-foot-long stretch of deck. This time, the concrete trucks will be able to drive out onto the completed deck to feed the conveyors.
The process will be repeated three times after that to complete the bridge deck.
In all, the completed driving surface of the deck will contain 4,900 cubic yards of concrete, virtually all placed using the conveyors and finished to perfection by the combination of the Bidwell finishing machine and human skill.