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August 31, 2012

Cornwall, Ontario bridge project uses recycled tires

New ground was broke literally and figuratively earlier this summer when recycled tires — or tire-derived aggregates (TDA) — were used for the first time in Ontario by the Ministry of Transportation at the site of the new $26-million Boundary Road Bridge over Highway 401 near Cornwall.

The north and south embankments were partially built with a tapered, three-metre-thick layer of TDA, which is shredded scrap tires generally ranging in length between 12 millimetres (0.5 inches) and 305 millimetres (12 inches). About 3,000 tonnes were used.

Installed at the maximum three-metre-thickness to limit internal heating, the shredded bits were spread out with a D5 dozer, compacted by a vibratory steel drum roller, wrapped with a protective geotextile fabric, and then covered with two metres of soil to limit air and water infiltration. There are also several granular drains to remove any water that does get in. Below the layer are two metres of borrow material.

It only took bridge contractor Aecon Construction about 10 weeks to build the embankments which include an “elaborate and extensive instrumentation system” to monitor their environmental and geotechnical performance, says Tony Sangiuliano, a foundation engineer with MTO’s materials engineering and research office.

McIntosh Perry was the consultant and contract administrator. Brantford-based Liberty Tire was the TDA supplier.

A specialized consultant to the design team was University of Maine professor Dana Humphrey, the author of a standard practice for the use of shredded tires as lightweight fill, retaining wall backfill.

“We couldn’t have done this (the project) without him,” Sangiuliano says.

While the actual construction time was short, the research and design work leading up to the trial project was extensive and lengthy. The embankments will be monitored on a long-term basis to determine if more extensive TDA applications are feasible, he says.

Should the project be deemed a success, however, there could be enormous environmental benefits. The total volume of TDA fill used at the Boundary Road Bridge was equivalent to about 400,000 scrap tires. Each year approximately two- to three-million tires are stockpiled in Ontario.

Such stockpiles are a potential fire hazard, says Sangiuliano, citing the 1990 Hagersville fire in which 14 million tires burned for 17 days.

The roots of the Cornwall project date to 2008 when the Ministry of the Environment asked Waste Diversion Ontario to develop an industry-funded used tire diversion program—which was approved the following year.

As Sangiuliano describes it, the MTO quickly endorsed the program’s objectives as part of an overall policy of supporting the conservation of natural aggregates, while promoting the use of recycled materials in highway construction.

There have been considerable studies on tire-derived aggregates which have been successfully used as engineered fill in several American states since the early 1990s and, more recently, as highway embankment fill in St. Stephen, N.B., he says. As part of its own due diligence, the ministry did its own research and also asked Humphrey to be a consultant. As well, Sangiuliano and a colleague visited the St. Stephen project.

As those efforts were unfolding in the fall of 2009, the materials office also sent out feelers to the ministry’s five regional offices looking for a project where TDA could be used.

“We wanted something that was going to be built fairly soon, not five years away.” There were strict criteria attached to the request, he says.

“As there might be some risk, we were very conservative. It (the project) could not be close to potable water or near watercourses or wetlands and had to be a secondary road, not the 400 series of highways.”

Other conditions included a minimum 50-metre-long embankment section which could accommodate the single, three-metre-thick TDA layer and stable subsurface soil conditions.

Ultimately the Cornwall site was selected. In May 2010 the Ministry of Environment granted the certificate of approval, subject to its own set of conditions such as the need for a long-term groundwater monitoring program.

In October 2010, Aecon Construction was awarded the contract to build the two-span bridge which is comprised of a concrete deck on 1900-millimetre precast/prestressed concrete girders. The embankment construction was the last major item in the construction schedule.

Although the embankments will be monitored for at least five years, the ministry will looking for other possible TDA sites before that period is up, says Sangiuliano.

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