March 13, 2009

Pervious concrete requires careful installation that differs from methods used for conventional concrete, a need addressed by a one-day Ready Mixed Concrete Association of Ontario training course.

Kortright job will put pervious concrete to the test

Green-friendly benefit targeted in parking lot job

DON PROCTER

correspondent

The Kortright Centre for Conservation in Kleinburg is testing the waters, so to speak, with the installation of 12,000 square feet of pervious concrete for its parking lot this spring.

The porous concrete is seen as green-friendly because it allows storm water to drain through it to the soil below, rather than diverting it into the region’s underground stormwater infrastructure. The concrete contract will be done by Lafarge Construction, starting in April or May, depending on weather.

While pervious concrete has been around for the better part of a decade, it is making a play for bigger piece of the market these days because of the growing emphasis on sustainable design and construction.

Along with its draining abilities, another benefit is that it contributes to better air quality by lowering atmospheric heating through its light colour and low density. That decreases the heat island effect (temperature increases of up to 4 Celsius) created by dark pavement (asphalt), explains John Hull, president of the Ready Mixed Concrete Association of Ontario.

But a pervious concrete parking lot is not just green, it is cheap. Or at least it can be, when compared to the price of an asphalt parking lot and its associated stormwater management costs, says Hull. Furthermore, land ordinarily devoted to costly stormwater management practices with an asphalt lot can be developed or preserved when pervious concrete is selected instead, says Hull.

Because pervious concrete doesn’t rely on stormwater infrastructure, it is well suited to retail malls such as big box centres and residential subdivisions, he says, adding the porous surface might get the OK in some areas of municipalities where asphalt parking lots are no longer permitted.

Pervious concrete is comprised of coarse aggregate, cement materials and water. Its porous structure (15-30 percent voids in hardened state) is a result of its low sand content, says Hull, noting flow rates of at least 200 litres of water per minute pass through each square metre of pervious pavement when it is properly installed.

Typically, a parking lot consists of a 150-millimetre thick topping of pervious material over a 300-millimetre base of open graded granular.

The base acts as a reservoir for storm water which is gradually filtered down into ground soil, helping to nourish the surrounding green landscape and replenish aquifers.

While installation is not difficult, placing and finishing methods are not like conventional designs.

The low-sump product doesn’t require concrete pumps, drainage pipes or skimmers for grading, says Hull. “It is put down flat with an asphalt spreader or fixed forms and a roller screed is used to place it.”

Because curing starts immediately a polyethylene sheet is placed over the surface immediately.

Hull says while it is straightforward to install, if improperly placed serious problems can result. In one instance, an inexperienced contractor overly compacted it, minimizing its draining ability.

To prevent installation mistakes the RMCAO offers a one-day training course (registered to the National Ready Mix Concrete Association in the U.S.) for contractors. To date, about 150 people have completed the course.

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