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A Trickle Down Approach to Storm Runoff Space-Saving Alternative to Retention Ponds
November, 2008
By Oon-Soo Ooi
Environmental and municipal authorities are starting to view the parking lots, roadways and pedestrian paths surrounding most large buildings with a critical eye. Runoff from precipitation flows quickly along these routes, and can overload storm water systems and streams, possibly causing flooding.
Storm water management ponds provide one avenue to manage this problem, but they add development costs and divert land from other purposes. Alternatively, pervious concrete offers a way for rainwater to drain through paved surfaces into the soil and, from there, into the ground water.
This is a form of concrete that uses little or no sand, but rather 5- to 14-milimetre (mm) stone particles that are coated with a paste-like mixture that allows pores to develop so that water flows down from the surface.
This creates a void within the pavement's mass that typically allows more than 100 litres of water per minute to drain through each square metre. This provides higher drainage capacity than the loading normally encountered in the majority of rainstorms.
Pervious concrete is usually placed in a thickness of 150 to 200 mm (or more if required) depending on the type of applications. Some of the benefits include:
* Less risk of flash floods, which can cause risk to life and property;
* Reduced need for storm water management measures such as storm sewers and retention ponds, skimmers, pumps and pipes. Reduced need for retention ponds can allow more area for other purposes, such as more parking spaces;
* Surface water contaminated with hydrocarbons, antifreeze and other compounds spilled on the parking lot surface can flow into the subsurface environment, allowing more chance for treatment through soil chemistry and biological processes;
* Replenished groundwater and aquifers due to water flowing into the soil instead of surface water bodies;
* Compared to asphaltic surfaces on roofs, roads and parking lots, the lighter colour and lower-density pervious concrete means more reflection of heat, reducing the heat-island effect;
* Improved tire traction on pervious concrete surface due to the drier surface in wet weather;
* Reduced tire noise due to minimized contact of tires with solid surfaces; and
* Improved visibility of road surfaces in wet weather due to the drier surfaces.
APPLICATIONS & MAINTENANCE
Pervious concrete has been used in North America for three to four decades, but for many years application was generally limited to the construction of permeable bases, sub-bases or tennis court surfaces. More recently, its potential to support environmental sustainability has been recognized.
The US Environmental Protection Agency introduced storm water regulations earlier this decade, which spurred building owners and developers to consider the use of pervious concrete in their projects. With increased knowledge about how to design and maintain it - particularly in relationship to freeze-thaw cycles - it is now starting to be applied in Canada, and is practical in most of Canada's inhabited latitudes. It typically works best in low-traffic situations such as parking lots, low-volume streets and roads, driveways, sidewalks, swimming pool decks and golf cart paths.
The amount of paste applied to the particles is one of the most critical variables in pervious concrete. Too little paste and the concrete is unstable and unable to bear the wear-and-tear of real-world uses, but, with too much paste, the interconnected pores are too restricted to allow enough water through. The paste should also have air entrainment for freeze-thaw durability.
Experience has shown that a soil type with enough percolation to support a septic tank system will be compatible with pervious concrete. It may be necessary to lay down a layer of crushed stone or aggregate before installing the pervious concrete if the existing soil does not have good drainage capacity.
A 150- to 300-mm thick layer of drainable aggregate base - containing gravel pieces of 19 mm or greater - or an equivalent subsurface drainage system - should be present below the layer of pervious concrete to allow water that infiltrates the pervious concrete to discharge properly. Without an effective subsurface drainage system, the water would be trapped in the pores of the pervious concrete and this can cause damage when exposed to freeze-thaw cycles. Therefore, it is critical that pervious concrete only be used on ground that has good subsurface drainage.
It is also important to ensure that the pores in the pervious concrete are not clogged, as this reduces the rate of water infiltration. Maintenance personnel should be aware of the need to keep the surface clear of anything that would clog the pores in the concrete. Regular sweeping and vacuuming will keep the surface clean and reduce clogging.
Grass or other vegetation can potentially cause clogging so landscape design should keep it separate from pervious concrete surfaces. Rock salt and sand can also cause clogging, so it may be best to reduce or avoid these de-icing methods in winter. (See related story, page ?)
STANDARDS DEVELOPMENT
Despite its increasing popularity, the construction industry in Canada and the United States does not yet have standard procedures to design, construct, qualify and evaluate pervious concrete. Local construction practices and materials result in different standards and procedures across North America.
Researchers in both private and public sectors are now focusing on engineering, construction, testing and evaluation of pervious concrete. Notably, the American Concrete Institute (ACI) recently published a reference document (ACI 522R-06) and design specifications (ACI 522.1-08) to guide pervious concrete work.
In 2007, ASTM International (ASTM) began developing standard methods for testing and evaluating pervious concrete. The ASTM test standards being developed at this time include: the testing of concrete density and void content; the rate of surface infiltration; compressive strength; and flexural strength of pervious concrete. It is expected that the ASTM will approve the first of these standard test methods before the end of 2008.
In Canada, the design and construction of concrete structures is generally governed by requirements of the Canadian Standards Association (CSA). The 2009 edition of CSA A23.1/A23.2 Concrete Materials and Methods of Concrete Construction - which is updated about every five years - will include a new section on pervious concrete.
Application of pervious concrete should with the development and availability of ACI, ASTM and CSA standards for the product. These standards will provide design professionals, concrete suppliers and contractors with the information they need for successful applications. Consistent standards also give potential clients - such as municipalities - a better means of judging whether a person who claims expertise in pervious concrete is able to produce a product that will perform as expected.
Oon-Soo Ooi, M.Sc.E., P.Eng. is an Associate and Senior Materials Engineer in Golder Associates' Greater Vancouver office. He serves on several technical committees of ACI, ASTM, CSA and the BC Ready-Mixed Concrete Association. He can be reached at Ooi@golder.com or 604.591.6616.
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