On (not completely) solid ground: Rain-friendly concrete

by Rebekah Bovenmyer

You’re circling the mall parking lot, rain pounding your windshield. The only space you can find is way in the back—in the middle of a big puddle. By the time you’re inside the mall, your pants and feet are soaked. Great. At least you’re getting a free car wash while you shop, right?

Where does all that dirt from your car go? Into storm drains—those grates or openings on curbs that lead to pipes underground. And not just dirt: trash, motor oil, and antifreeze from vehicles on the parking lot are also carried down the storm drains. Eventually it all ends up in nearby rivers, streams, and lakes.

Making a splash

Parking lots can be a source of pollutants in storm water because there’s so much pavement in one place where vehicles park for hours and leak little bits of fluid. Those pollutants have nowhere to go until it rains and washes them off into a storm drain. If it rains too much, the water goes into a big pond the store had to build for excess rainwater.

The Environmental Protection Agency is calling for a more environmentally-friendly way to manage storm water from parking lots, and companies that need big parking lots want to save money by not building that drainage pond on their property.

A concrete strainer

A relatively new kind of concrete pavement, called pervious concrete, could help solve the problem. Pervious is a word similar to porous, meaning it’s not completely solid. There are gaps in it where liquid can pass through like a strainer.

Pervious concrete doesn’t use as much sand or other fine ingredients to fill in the gaps. It ends up looking like a Rice Krispie bar without the marshmallow goo.

Pervious concrete lets rainwater soak through it into the soil beneath instead of into storm drains or flooding parking lots.

Regular concrete, on the other hand, is completely solid, and no water can get through it. To help water drain off, it's slightly sloped. Otherwise, water puddles on top until it dries.

How it works

A pervious concrete pavement is usually 6 inches thick. Underneath the pavement is a layer of medium-sized rocks. Water drips through the holes in the pavement to the rocks. Some of the water evaporates from the rock bed. What’s left soaks into the soil.

In case there are several heavy rains close together, a pipe in the rock bed drains excess water into the storm drain.

Dirt and pollutants get stuck in the gaps in the pervious pavement and the rock layer instead of being washed into storm drains. Pervious concrete gets vacuumed or pressure washed about once a year to make sure the holes in the concrete don’t get too clogged up.

No salt added

Besides filtering water, pervious concrete has a few other benefits.

For highways, the uneven surface makes it quieter than regular concrete (see the Green Scene for more). When it's raining, there's also much less water spray from fast
moving vehicles because the tires are riding on the aggregate tops instead of through water that's puddled on the surface.

It also doesn’t need salt and sand in the winter because the surface already provides enough texture to keep cars and people from sliding. Pervious concrete is warmer so ice melts quicker (and drips into the rock bed), keeping the pavement drier.