PERVIOUS CONCRETE PAVEMENTS

When it Rains … Pervious Concrete Pavements Drain

The conflict between development and water quality issues – local, and statewide – continues to make news.  As more land in the southeastern portion of the state is paved, a greater amount of rainwater is falling on impervious surfaces such as parking lots, driveways, sidewalks, streets, and playgrounds, rather than soaking into the soil.

The resulting runoff has triggered a host of problems in the eight-county area Schmitz Ready Mix serves, including erosion, floods, water table depletion, and the pollution of local rivers and lakes.  As rainwater or melt-water from snow rushes across pavement surfaces, it picks up many pollutants, from oil, grease, and chemical fertilizers, to deicing salts.

The Environmental Protection Agency has identified storm water runoff as a leading cause of pollutants in Southeastern Wisconsin waterways.

Concrete Solutions

As local communities struggle to strike a balance between economic development and environmental issues, innovative construction methods and materials can provide concrete solutions for controlling storm water runoff.  Rather than building low-volume pavements with conventional pavement materials, an increasing number of environmentally-conscious communities, municipalities, and businesses across North America have switched to pervious concrete pavements – pavements offering the inherent durability and low life-cycle costs of a typical concrete pavement while retaining storm water runoff and replenishing local watershed systems.

Instead of causing runoff, when it rains, a pervious concrete pavement drains.

Pervious concrete is a special type of concrete pavement that allows rain and snowmelt to pass through it, thereby reducing water runoff from the pavement, and ultimately, the amount of untreated runoff discharged into storm sewers.  Typically, pervious concrete is placed over an 8 to 24-inch, highly permeable subbase layer of open-graded crushed stone, which acts as a storage reservoir for runoff.  Filter fabric is often employed to separate soil particles and runoff pollutants from the subbase.

Pervious concretes look a lot like large rice crispy treats and consist of specially-formulated mixtures of portland cement, open-graded coarse aggregate, and water.  Target water/cementitious ratios are usually 0.35 to 0.45, with enough interconnected void space (15 to 25%) to allow rapid percolation of liquids through the installed pavement.  To meet project strength and void content requirements, varying amounts of fine aggregate material can be added to the mix.

As in conventional concrete pavement applications, air entrainment improves the freeze-thaw durability of pervious concretes.  But there are currently no standard ASTM field-testing methods for verifying air content in pervious concrete mixes.

Pervious concrete pavement solutions often save development dollars, including reduced requirements for drainage systems and detention ponds, and can allow for greater site utilization and more effective land use.  For urban settings, where detention ponds are neither desired nor practical, pervious concrete parking areas place a minimum of runoff stress on municipal storm/sewer systems.

In-depth discussion of technical and installation information for pervious concrete pavements is included in the National Ready Mixed Concrete Association (NRMCA) publication, Pervious Concrete. A read-only version of the publication can be accessed by clicking here.

An online brochure listing the benefits of using pervious concrete pavements, also developed by NRMCA, can be accessed by clicking here. Schmitz Ready Mix is a proud and active member of NRMCA.

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