March 8, 2012

ONTARIO MINISTRY OF TRANSPORTATION

Slip Form Construction workers install a plastic covering over the controlled section of the pervious parking area to cure the concrete at a Ministry of Transportation of Ontario parking lot in Brampton.

FEATURE | Roadbuilding

Ontario Ministry of Transportation evaluates pervious concrete in Brampton

DAN O’REILLY

correspondent

Drivers parking in a recently completed parking lot in Brampton are participants in a long-term Ministry of Transportation (MTO) evaluation on the performance and environmental benefits of pervious concrete pavement.

A 190-millimetre deep, 866-square-metre controlled section of the new lot at the Williams Parkway/Highway 410 intersection was built with the concrete to allow stormwater and contaminates — such as oil dripping off cars — to drain naturally into the ground rather being directed into storm drains.

Underneath the concrete is a 300-mm-deep layer of Granular O and below the granular is a 450-mm layer of SSM fill, says Neil Johnston, estimator/project manager with Woodbridge-based Slip Form Construction, the paving subcontractor. The material supplier was Dufferin Concrete, a division of Holcim (Canada) Inc.

“We did five passes over three days,” says Johnston. Fermar Paving was the general contractor overseeing the construction of the entire lot, the rest of which was built with standard concrete.

While there will be no formal monitoring, regular maintenance inspection of the parking lot will assess how well the concrete performs, says Becca Lane, head of MTO’s foundations and pavement section.

This is the second pervious paving parking lot the Ministry has built. The first was constructed in 2007 at Guelph Line and Highway 401 as a demonstration project.

But the Brampton lot is in a more built-up area and could yield more conclusive results, she says.

“Permeable pavements offer the greatest benefit to urban areas where existing stormwater management facilities are challenged to accommodate additional surface runoff.”

Pervious concrete pavement isn’t feasible for major expressways. On the other hand, parking lots are the ideal location to install and evaluate the material because, for the most part, they’re occupied by relatively light-weight vehicles, she explains.

Pervious concrete pavement is recognized by the United States Environmental Protection Agency as a best management practice for stormwater management and has been used in other jurisdictions such as California, which is a leader in its development, says Lane.

Pervious concrete is typically a zero slump, open-graded material consisting of Portland cement, coarse aggregate, little or no fine aggregate, admixtures and water.  The lack of a substantial amount of fine aggregate results in a high void content — typically between 15 to 25 per cent — which provides a free draining pavement layer, she said.

And that drainage ability can be the source for a host of environmental and financial dividends.

“Depending on the project design, the stormwater that enters the pervious concrete pavement can be allowed to percolate into the underlying soil, avoiding the need for a retention basin (or stormwater management pond). By allowing water to drain directly into the subgrade, there is better recharging of the groundwater.”

Other advantages include the elimination of potential mosquito-breeding sites, which, in turn, helps to control the spread of the West Nile virus.

But the material also has its challenges and limitations. “You have to get the right mix. If it is too dense it won’t allow the stormwater to flow through.”

As part of its long-term evaluation, the Ministry is working with two universities. The University of Waterloo is analyzing different pervious concrete mixes, while the University of Guelph is conducting research that includes determining the correlation between the use of the concrete and water quality.

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