The bridge consists of two travel lanes and a breakdown lane, and required a specially designed parapet wall on both sides. The Constructions Materials Group of Lafarge Canada Inc., a subsidiary of worldwide Lafarge Corp., was responsible for slipforming all of the parapet wall on the project. Under a separate contract, Lafarge also supplied all of the ready mix concrete to the bridge project.
In order to complete the job by its scheduled date, Lafarge bought a new Commander III slipform paver for the project, and a second machine was leased to Autoform Contracting London Ltd., who provided an operator for the machine. Manufactured by Gomaco Corp., Ida Grove, Iowa, the machine features additional vibrator circuits, custom neoprene rubber tracks and special wall forms, with hydraulic sideplates.
The bridge's parapet wall was designed to be 9 in. wide at the top, 20 in. wide at the bottom and 47 in. high off the bridge deck. It overhangs the deck by 7 in. on the bottom. The purpose of the overhang is to conceal the anchor heads for the transverse post tensioning in the concrete deck.
The overhang was poured over wooden soffit panels that were stripped three days later and reused again ahead of the machine. Hydraulic sideplates on the mold prevented the crew from having to make adjustments manually from the outside of the wall.
This time-consuming project required several steps toward completion. The wall was poured over reinforcing steel, which was installed by a crew working for Strait Crossing Joint Venture, Charlottetown, Prince Edward Island, Canada, the prime contractor on the bridge project.
Jeff Kerr, area manager, was asked last fall to explain the slipforming process. "It's a revolving process. The work by one crew directly affects the work of another crew. It's a cycle. We have crews working ahead of us, getting the reinforcing steel ready and installing the soffit panels. So we can pour, depending on what is happening ahead of us."
Concrete light bases were placed every 272 ft along the bridge. Lafarge is pouring the parapet wall right over the bulk heads for the lights, rather than stopping, then digging out the concrete afterward.
Expansion joints were tooled in every 20 ft. A finishing platform traveled behind the machine to enable finishers to work from the back side of the wall.
Approximately 17,003 cu yd of concrete was used to pour the 85,302 ft of parapet wall. A specified mix design of 5,000-psi silica fume concrete mix was used with a slump of 0.79 to 1.18 in. Concrete trucks, varying from three to five trucks per machine, ran to-and-from two plants on Prince Edward Island and one plant in New Brunswick to supply the work.
The number of trucks and speed of service depended on how far out the machines were on the bridge. The plants serviced the entire project so, according to Kerr, there were days when they had trouble keeping up. Fortunately for the crews, during the construction the bridge was only open to construction traffic.
Weather Rears Its Head
Weather also was a factor as the crews worked under the demanding schedule. Winds were very strong while working atop the bridge and the winters were extremely cold.
"Working approximately 43 to 61 m (140 to 200 ft) in the air over the Northumberland Strait, it can be extremely windy. We had to shut down if the winds got too high. It's basically a safety element. It doesn't take long for your hard hat to go flying off in the 50 km per hour (31 mph) winds," said Kerr.
The Construction Materials Group of Lafarge Canada had little experience with slipforming a wall before the project. Fortunately, their parent company provided access to other Lafarge operations with more experience in the area. The British Columbia Lafarge crew came to their aid, lending their experience in helping to get started on the project. With the help of the more experienced group, along with support from Gomaco, the paver manufacturer, and the dealer, the group got off to a good start and had successfully slipformed 32,808 ft of wall by mid-October 1996.