When it rains, it's porous: Why paving with pervious concrete matters
November 07, 2007
They paved paradise
And put up a parking lot.
It's still true that "You don't know what you've got till it's gone." Yet, environmental concerns, including questions about what we as individuals and communities can do to make a difference, have only increased in number and complexity since Joni Mitchell penned her anthem of loss, “Big Yellow Taxi.”
Some choices are simple. One choice in particular can make lasting, if invisible, environmental improvements in all of our communities and in the world: Choosing to pave paradise with pervious concrete, a fairly new form of that most ancient of natural building materials, may just help save it.
Pervious concrete is a term that applies to various blends of Portland cement, aggregates, admixtures, and water yet typically very few (if any) “fines,” or sand and other fine particles. Once placed and hardened, pervious concrete creates a hard, durable paving with permanent interior air pockets or “voids” that allow water to permeate. Known and utilized for 100 years in Europe, where it was originally used as structural insulation, and for about 30 years in the United States, pervious concrete can be as strong and durable as regular concrete, able to carry the weight of both cars and trucks for 50 years.
Beyond its environmental benefits, pervious pavements are safer for drivers and pedestrians, especially in areas with high rainfall. Pervious concrete drains and redirects water rather than allowing it to puddle, thereby reducing the risk of hydroplaning and tire spray. For pedestrians, it decreases the danger of rainy-day slips and falls. And little or no maintenance is required. If the interior voids of pervious concrete get clogged with debris or sediment, power washing or vacuuming will remedy the situation.
The Protective Power of Pervious Concrete
As strange as the notion seems at first, concrete can protect and enhance rather than damage the natural environment. Water passes through pervious concrete at the rate of 400 inches per hour, though some calculate the “flow” as between three to five gallons (or more) per minute per square foot of surface area. In either case this typically exceeds the flow rate needed to prevent runoff and its sometimes damaging side effect, flooding. So pervious concrete can reduce or eliminate the need for storm drains and water retention ponds in housing and other developments, an obvious economic saving for cities and businesses.
Yet its environmental benefits are even greater. Rather than preventing the infiltration of water into soil, like most paving surfaces, pervious concrete “captures” rainwater in its network of voids and allows rain to percolate into the underlying soil. By retaining storm water runoff, pervious concrete can also help replenish and recharge local watershed systems.
And if roads, driveways, parking lots, tennis courts, and pool areas were paved with durable pervious concrete rather than impervious materials, toxic pollution reaching streams, rivers, and oceans—the “big flush” pollution effect following storms—would almost be eliminated. Because pervious concrete supports a rich microbial soil environment, pollutants including hydrocarbons, nitrates, and phosphates are easily digested. To further improve groundwater quality, crushed, reused concrete layered beneath the pervious concrete can serve as a natural water filter.
In addition to preventing polluted runoff in waterways and oceans and helping to recharge local watersheds, pervious concrete channels more rainwater to tree roots and landscaping, so there is less need to irrigate.
Pervious systems can also be used to actively collect and store runoff for later use as irrigation water. An ingenious example of this approach is Finley Stadium in Chattanooga, Tennessee, where strips of pervious concrete are integral to the stadium’s storm water management plan. The parking area was designed so that rainwater runoff drains toward the pervious concrete. The runoff then flows, via an underground collection system, to an onsite holding tank. From there the water is recycled to water the stadium’s grass playing field, reducing the long-term cost of maintaining it.
Pervious concrete also helps reduce the “urban heat-island effect,” or the tendency of paved and developed areas to absorb solar energy and radiate heat, increasing the overall temperature of urban areas. Because they are light in color and feature an open structure, pervious pavements don’t absorb, retain, then radiate heat as intensely as do dark asphalt surfaces. Pervious pavement also conducts cooler earth temperatures to the surface, helping to cool the pavement.
The high porosity of pervious concrete gives it other useful characteristics. For one thing it offers excellent thermal insulation—making it useful for the walls of buildings—and for another, it has good acoustical properties, perfect for barrier walls or soundproofing.
So if you opt to pave your own corner of paradise with pervious concrete, keep in mind that can also use it to put up a parking lot.
Kim Weir, who holds degrees in Environmental Studies and Analysis and in Creative Writing is an active member of the all-volunteer Chico Sustainability Group, which develops and supports sustainability initiatives for Chico, California and beyond. She is a member of the Society of American Travel Writers and author of the environmentally focused Moon Handbooks: Northern California and other travel guides.
Photo credit: Tanya Komas