Features Columns Education
Pervious concrete finally recognized

Pervious concrete is a performance-engineered material using the components of a conventional Portland cement concrete (coarse aggregate, cement, water, admixtures and little or no fines) at a very low slump.

Pervious concrete is a performance-engineered material using the components of a conventional Portland cement concrete (coarse aggregate, cement, water, admixtures and little or no fines) at a very low slump.

Pervious concrete is a performance-engineered material using the components of a conventional Portland cement concrete through which water can easily pass.

These elements combine to produce a hardened material with connected pores ranging in size from 2 – 8mm through which water can easily pass. The void content varies from 15 – 25%, with typical compressive strength of 2.5 – 28MPa. Its porous structure allows a flow of at least 200 litres of water per minute to pass through each square metre of pervious pavement when it is properly installed, but flow rates can be substantially higher. This type of pavement can handle far more water than most rainstorms can produce.

Pervious concrete in a parking lot application.

Directly underneath the Pervious Concrete Pavement, is an underlying temporary stone reservoir that allows the first flush of storm water runoff to percolate into the ground where soil chemistry and biology can then “treat” the polluted water. The advantage of using this technology is that it turns the paved area into a stormwater management tool. Instead of contributing to runoff problems, pavement can become part of the solution.

By returning rainwater to the soil where it belongs, pervious concrete replenishes water tables and aquifers without adding to water pollution. Pervious concrete is recognized by the U.S. Environmental Protection Agency as a best practice for stormwater management. Stormwater runoff from traditional parking lots can send as much as 90% of the automotive pollutants, such as oil and other hydrocarbon liquids, directly into storm drains, rivers and streams – and our water supply. But pervious concrete captures rainfall and lets it percolate into the ground, where natural soil chemistry and biology can treat the polluted water.

Pervious concrete can also contain recycled material, and is manufactured locally. Under the Canada Green Building Council’s (CaGBC’s) Leadership in Energy and Environmental Design (LEED Canada) program, using pervious concrete as a stormwater management system allows for points in a number of areas. These include Sustainable Sites (SS) Credit 6.1, Stormwater Management: Rate and Quantity; Sustainable Sites (SS) Credit 7.1: Heat Island Effect: Non-Roof; Materials and Resources (MR) Credit 4: Recycled Content; and Materials and Resources (MR) Credit 5: Regional Materials.

Pervious Concrete in Canada
The Cement Association of Canada is sponsoring a three-year research study by the University of Waterloo to look at some of the technical issues surrounding pervious concrete, with the goal of achieving maximum durability in Canada’s freeze/thaw winter environment.

Susan Tighe at the University of Waterloo is in charge of the research. “We are actually working on a study to develop a design, construction, maintenance and management guide for pervious concrete pavement usage in Canada,” she explains. “In addition to working with the CAC, we are also working with several public and private sector partners to ensure that our work is useful to practicing engineers and designers.” As well as testing an innovative product, the project is developing better tools for the purpose. “We’ve designed unique sensors that track water through the pavement structure, temperature, and as well some civil engineering structural testing equipment,” adds Tighe.

The project has constructed several test sections across Canada, primarily parking lots and driveways. The three-year study is about halfway complete, and a report and refined Canadian guidelines are expected in 2010.

In Ontario, several pervious concrete projects have been completed over the past two years, with the largest scheduled for this summer at Centennial College’s Progress Campus. The Toronto Region Conservation Authority is also considering pervious concrete at their Earth Rangers Centre, part of the Kortright Centre for Conservation in Kleinberg, near Woodbridge, Ontario. The Earth Rangers Centre is a 5.8 square kilometre wild animal treatment, rehabilitation and education facility. Its design uses concrete to achieve multiple green objectives, including green roofs, energy conserving masonry walls, radiant floor hearing, and pervious pavement.

A good bet for winter use
In addition to the rainwater management benefits, the new pervious lot may provide a safer surface for winter users. University of Missouri-Kansas City Civil Engineering professor Dr. John Kevern noted that the high permeability of pervious concrete reduces or eliminates ponding of melt water after a thaw in winter, limiting the potential for ice formation and reducing the need for salt and sand applications. Also, the peaks and valleys in the surface of pervious concrete pavement improve traction by leaving room for water, snow, and ice to accumulate below the surface.

Another recent and interesting finding, according to Dr. Kevern, is that a pervious concrete surface is warmer than adjacent pavement, but is cooler underneath. Due to the porous nature of the pavement, heat accumulates at the top and the air voids below provide insulation. It has been observed that in the winter pervious concrete surfaces can still melt snow and ice even when the air temperature is too cold to melt ice using salt.

So far, pervious concrete has stood up well to harsh winters. When properly installed, it tends not to heave or form potholes, as the base doesn’t get saturated when ice forms. Reports from the Chicago area, for example, and Durham, New Hampshire, where a pervious concrete parking lot was installed by the University of New Hampshire in 2007 for research purposes, have been glowing and have prompted enquiries from many municipalities.

Proper installation is the key
To get the full benefit of pervious pavement, the contractor must have the knowledge and skill to mix and install it properly. Pervious concrete is different from conventional concrete. As John Hull, president of the Ready Mixed Concrete Association of Ontario (RMCAO) explains, the design must be appropriate for local soil conditions, the mix proportions must be accurate and curing operation have to be handled carefully.

With the different nature of pervious concrete in mind, the RMCAO urges contractors, consultants, concrete producers and anyone else working in a supervisory capacity during all concrete placement to be certified by NRMCA’s Pervious Concrete Contractor Certification Program to ensure quality placement. As the local sponsoring group, RMCAO provides this day-long training course several times per year. This course is also being presented in western Canada and in the Atlantic provinces, all under the auspices of the Ready Mix Concrete Association and the Cement Association of Canada. RMCAO also recommends that current pervious concrete specifications be followed.

New Canadian guidelines in 2010
With innovative uses of technology, there are always questions that need to be answered, especially in the demanding Canadian climate. Field history has shown that sand and salt do not substantially impair drain-ability and annual vacuuming easily restores permeability. Snowploughing may damage the surface depending on how tight the void structure is, but again field history demonstrates that less snow is retained on a pervious surface and frequent ploughing may not even be necessary. As for the freeze-thaw cycle that wreaks havoc on asphalt, the voids in pervious pavement are too large to induce sufficient pressure to break the concrete. If the system is well designed, water will drain away, quickly minimizing the conditions for freezing.

This article was submitted by the Cement Association of Canada.