Precast deck panels help put bridge projects on the fast track
Precast bridge deck panels are being used to help fast track construction on Edmonton's massive interchange project at Anthony Henday Drive and Stony Plain Road.
Although a design/build/procurement process that was specially developed for the ring road project is being credited with helping to speed things up, the precast panels that are being installed on the $170-million project's seven bridge structures are also making a difference.
"If we used cast-in-place, we would have needed more time, as it would have been more dependent on the weather," says Aamer Shakoor, a senior construction engineer with Alberta Transportation. "The timeline could have been an extra year or more. This is a fast-track project and we need to have it completed by October 14."
The project replaces the current intersections, which include traffic lights at (westbound) Stony Plain Road and at (eastbound) 100 Avenue. The traffic lights, which have been in place at these intersections on Anthony Henday Drive since 1990, are a source of major bottlenecks during rush hour.
Construction on the project began in the spring of 2009, and includes the repaving and widening of three lanes in both directions of 5.6 kilometres of Anthony Henday Drive, repaving of about two kilometres of both 100 Avenue and Stony Plain Road, along with the seven new bridge structures-all while continuing to accommodate traffic flows.
Intersections at other locations along the ring road, including Callingwood, Lessard and Cameron Heights, are also expected to be free of traffic lights by the October deadline. These intersections are all being converted to free-flow interchanges, but, on these, all the bridge deck sections used cast-in-place concrete, Shakoor says.
Although precast bridge deck panels have rarely been used in Alberta, they are used often in the United States, especially for bridge repair and rehabilitation projects. "They minimize traffic disruption, which can be minimized even further by doing much of the work at night," Shakoor says.
Precast deck panels were also used for upgrades on the Whistler Highway shortly before the Vancouver Winter Olympics. More recently, they have been used for interchanges around the Port Mann Bridge. "Precast deck panels are often used in B.C.," says Jay Manton, construction manager for the Kiewit Management Co. and PCL Construction Management Inc. joint venture.
There are two main ways to build with precast bridge deck panels.
One, sometimes called total precast, is to use fairly thick panels with plenty of structural heft, typically with asphalt paving on top, and a waterproof membrane between the precast panels and the asphalt.
The other involves using thinner panels in conjunction with a cast-in-place poured concrete overlay. The bridge deck panels used on the Anthony Henday/Stony Plain Road interchange bridges are of the latter type. They measure three metres by 2.5 metres by 90 millimetres, Manton says.
For this project, he says, the cast-in-place concrete is poured to a thickness of 200-250 millimetres onto the precast panels. Next, the waterproof membrane is applied, and the deck is then paved with about 80-millimetres' depth of asphalt.
Both systems have the advantage of reduced dependency on good weather, reduced site time, work hazards and numbers of workers on site, and, perhaps most important of all, no need for extensive bridge formwork structures.
Lafarge manufactured the precast concrete girders for the project.
"The slabs or bridge deck panels can be seen as primarily a stay-in-place forming system," says Don Zakariasen, director of marketing at Lafarge Canada Inc. "Although the geometry and deck slopes for drainage can be complicated, a full precast system could potentially deliver more efficiency and durability."
He says that Lafarge's Ductal, which has a super-high strength-to-weight ratio, could likely be used successfully for re-decking steel bridges. "It would avoid a huge amount of extra dead weight," he says.
The total precast approach was used recently for Dunvegan Bridge, which spans the Peace River in northern Alberta. "It had been a very old cast-in-place with a crown in the middle. When precast was used to refurbish the bridge, the precast was joined in the middle, with a crown in the middle," says Bill LeBlanc, a senior engineer with Armtec Ltd., which provided the precast panels for the Anthony Henday/Stony Plain Road interchange.
"A LOT OF POTENTIAL"
A total precast bridge deck panel system was also used at a Glenmore Trail interchange in Calgary. "The precast panels were a full nine inches in thickness, and spanned the width of the bridge, about 40 or 50 feet. There's a lot of potential in future for total precast," says LeBlanc, who adds, "A total precast approach can get the job done faster."
The method with thin bridge deck panels and a cast-in-place concrete overlay that is being used on the Stony Plain Road interchange does not come entirely without risk. One of the benefits of using the method, it is hoped, is that Alberta Transportation will have a chance to assess how well the system performs under Alberta's climate conditions, Shakoor says.
One concern about using cast-in-place concrete above the precast deck panels is the risk of cracks reflecting through the cast-in-place above joints between precast panels. "It could lead to water and salt getting into the cast-in-place that's been reinforced with steel," LeBlanc says.
Reg Ball, senior project manager for AECOM Canada Ltd., is philosophical about the lack of literature on how well the precast deck panel system used for the ring road interchange at Stony Plain Road will hold up over time in Edmonton's harsh climate. Noting that precast deck panel systems have a relatively short history-only 10-15 years-in the warmer United States, he says: "Whenever you use innovative techniques, you're extending the boundary. You have a good idea how well it will perform, but you don't know for sure."