For some reason, watersheds seem to be popular places to put down pavement. Housing developments, grocery stores, and shopping malls require places to drive and places to park, which means those places end up getting paved. Where water used to infiltrate into the soil, trickle down through natural water-filtering aquifers and rejoin the reservoirs that serve our thirsty civilization, now rain falls on oily pavement. Paved areas prevent water from sinking into the soil beneath them and create deluges of polluted runoff that strip away topsoil, contaminate waterways, and eventually disrupt ecosystems for miles around all paved human developments.
The problem is that concrete is impervious to water (otherwise it wouldn’t work for lining our wonderfully cool and beautiful summer swimming water in our backyard or local pools). Americans are all into preserving the environment as long as it doesn’t interfere with our favorite conveniences, and chief among those conveniences we jealously guard is the convenience of driving everywhere we go. Luckily, there is a simple and brilliant solution that has been here all along.Left: Rain surface runoff seen on standard impervious pavement. Right: Pervious pavement allowing infiltration through to the crushed stone reservoir below.
For the last 30 years builders in rain-soaked areas have used, instead of concrete that is impervious to water,something they call Pervious Concrete. Pervious Concrete is made much the same way as regular concrete, but instead of all of the tiny holes in its matrix of gravel being filled with sand, the concrete is made with only gravel and cement so that space is left where water can penetrate. Pore space can be controlled through varying the recipe, but pore spaces of 15-20% are common. The resulting concrete looks a lot like gray rice crispy treats underfoot. Water flows through Pervious Concrete much in the same way it flows through an aquifer and soaks into the ground below.
Pervious Concrete withstands low-speed traffic like a champ, but the grinding torque of high-speed highway driving sheers off the gravel chunks and eventually tears through the pavement. It’s no good for highways, but highways aren’t the major problem when it comes to watershed loss. The problem confronts you as soon as you open your garage door in the morning and creep out onto your driveway, take that turn onto the urban or suburban streets and begin that morning commute crawling along in local traffic. The problems with runoff are caused in low-speed areas anyway.
Pervious Concrete may not solve all of our world’s water problems, but it promises some very interesting options for finding ways to balance our driving addiction and our need to minimize our impact on the world. Though it’s not new, the popularity and use of Pervious Concrete has been growing like wildfire in recent years. Today you can find it on school campuses, playgrounds, and parking lots from South Florida to Vermont, and its uses and forms are expanding rapidly. Research is taking place all over the country at universities such as the University of California Pavement Research Center and at companies looking to advance the technology and improve the usability of pervious pavements.