Paving

Pervious Concrete

Pervious concrete resembles Rice Krispie Treats. Pervious concrete is concrete without the "fines" or sand. By eliminating these smaller particles, voids are created that allow water to pass through. Bacteria acting in the voids are said to break down oil and other pollutants.

Compressive strength of pervious concrete ranges from 500 to 4000 PSI, depending on the mix used.

Check out the new virtual tour of pervious concrete in Beaverton Oregon.

When installing pervious concrete, screeding is minimized and surface finishing is eliminated to prevent filling of surface voids. The wet concrete is immediately covered with plastic sheets to facilitate slow curing. Curing time is longer than that for impervious concrete, and may last several weeks. It is important to maintain the plastic cover during this time. The photo to the left shows the installation by a City of Beaverton crew of a StoneyCrete sidewalk on 155th street .

To avoid impacts of freezing a significant bed of gravel or crushed rock is installed below the pervious concrete. Maintenance needs vary by the aggregate size used in the concrete, the depth of the gravel bed, and by the amount and size of debris that collects on the site. Maintenance is done by either vacuuming or power-washing.

Evolution Paving Resources is a local supplier of pervious concrete.

Porous Concrete Pavers Blocks

Porous concrete pavers are an attractive and available choice for paving. Spaces between the interlocking blocks allow water to run through to the gravel base and soil below. Uni-stone is one brand of pavers commonly that you can find at various local sites including the Lucky Lab Brew Pub in Multnomah, Jurgens Park in Tualatin and Clean Water Services Field Operations Center in Beaverton. Periodic vacuuming is recommended maintenance.

Mutual Materials and Willamette Graystone are local suppliers of porous concrete paver blocks.

Porous Asphalt

According to Steve Fancher of the City of Gresham, “The Oregon Department of Transportation has been using pervious asphalt on highways for years. They put it down over impermeable surfaces to collect stormwater and prevent it from puddling on the road. No puddles means no hydroplaning.” The Port of Portland paved 35 acres with porous asphalt on a parking lot used to store Hyundai cars on the Columbia River. We are unaware of any projects in the Tualatin basin.

Porous asphalt is said to cost no more than regular asphalt and is far less expensive than porous concrete. The cost of additional gravel underneath porous asphalt to accomodate drainage may add costs over a standard asphalt application, but this may offset the costs of a piped stormwater system. Asphalt is typically maintained by a seal coat every few years. A seal coat applied to pervious asphalt would render it impervious. Porous asphalt is not appropriate on slopes of more than 6%.

Reinforced Gravels

Gravels reinforced by a polyethylene webwork are installed cost-effectively and very pervious. Such a treatment is limited to areas with low traffic volumes and speeds. The Geoweb® Cellular Confinement System is sold by Presto Products Company.

More Information

Clean Water Services' brochure, The Rain Runs Through It, discusses various pervious pavement installations in the Tualatin basin.