Creating Reservoirs Under Roads And Parking Lots

Pervious concrete, a type of concrete without sand, can drain hundreds of inches of rain per hour, allowing water to filter into groundwater stores below, rather than run off into storm drains. Landscape architect Cheryl Sullivan discusses the pros and cons of this eco-friendly paving option.

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IRA FLATOW, host: You're listening to SCIENCE FRIDAY. I'm Ira Flatow.

(SOUNDBITE OF SONG, "BIG YELLOW TAXI")

JONI MITCHELL, SINGER: (Singing) They paved paradise, put up a parking lot with a pink hotel, a boutique and a swinging hot spot.

FLATOW: 1970 was the year of the first Earth Day, the same year Joni Mitchell wrote her environmental lament "Big Yellow Taxi," about paving over paradise and putting up a parking lot. When Joni Mitchell sang about paving paradise to put up that parking lot, I'm guessing she wasn't thinking of pervious cement, pervious concrete, a more sustainable type of concrete already in use all over the country. Water runs right through it. Yup. It's sturdy enough to pave parking lots and driveways, but so porous, looks just like a rice crispy treat. So rainwater trickles right through it, down to the ground below, rather than running off and flooding storm drains and sewers.

It also allows you to actually feed the trees that may have roots running down there and all kinds of stuff underground, returning the water. There are other sustainable paving ideas out there, too, like cement that sucks up greenhouse gasses. Or if you got that sticky gunk left over from making biofuel, why not use that as a binder for roads rather than asphalt? My next guests can answer those questions and many more about the brave new world of pavement.

Joining me to talk about these technologies are Cheryl Sullivan, senior landscape architect at the Cunningham Engineering Corporation in Davis, California. Welcome to SCIENCE FRIDAY.

CHERYL SULLIVAN: Hi. Thanks.

FLATOW: Hi, there. John Harvey is professor of civil and environmental engineering at the University of California, Davis. He's also director of the University of California Pavement Research Center. He joins us by phone. Welcome to SCIENCE FRIDAY.

JOHN HARVEY: Good afternoon.

FLATOW: Now, Cheryl, I saw that demonstration, and actually on our website, at sciencefriday.com, we have a video of all these water pouring through the pavement. How does that work?

SULLIVAN: Well, how does it work? Well, the difference between regular concrete and pervious concrete is that pervious concrete doesn't contain any sand. It just has cement, rock and water in it. So all the little rocks are coated with this slurry and then put in to a form and tamped down, and all the holes remain. Pervious concrete will have anywhere from 15 to 20 percent voids, or air pockets, within it. So that's how the water drains through it, and it goes in to a gravel base underneath.

FLATOW: Is it being used in many places?

SULLIVAN: It's being used all over the place. It's been in Florida for over 30 years now. It's used up in the Northwest - or Northeast and Northwest, so it's in cold country and dry country and wet country. It's, as my grandma used to say, it's the best thing since sliced bread. It's amazing stuff, but it's so simple.

FLATOW: Yeah. And is it strong enough, though, to be used as highway paving, for example?

SULLIVAN: We don't use it in highway paving. It doesn't take the high-traffic speeds. It will ravel off. The little rock pieces will be spurred off with the high speed. It's used in parking lots, driveways, low-speed alleys, like in condominium complexes or apartment complex or even in these - the urban infields where we have a lot of low-speed alleys. And then school grounds can use it. Hospitals are using it. It's all over the place.

FLATOW: How much water, can you give us an idea, can go through this stuff?

SULLIVAN: Sure. A piece of pervious concrete can have 400 inches of water pass through it in an hour.

FLATOW: Wow.

SULLIVAN: So it doesn't clog. So...

FLATOW: So even if it got coated with grime from the road, you know, from car drippings and things like that.

SULLIVAN: No. Say, it's Seattle. Seattle, I think, the highest - their highest rainstorm is like five inches in 24 hours. So even if 90 percent of the concrete were blocked up - you know, someone had drop sand in it or dirt or gunk - it still wouldn't be - you would still have enough storage capacity and enough perviousness for that water to not stay on the surface of that parking area or that roadway. So it takes a heck of a lot of stuff to clog up a pervious pavement.

FLATOW: Right. John Harvey, can it then act as sort of a reservoir and store any of that water?

HARVEY: Yeah. The idea, there's two things that you design the pavement for. One is the hydraulic infiltration, as Cheryl was talking about, and we've actually - and the second thing is the structural capacity. Up to now, mostly folks have been putting it down just for cars, because as Cheryl mentioned, it hasn't been designed for the heavier trucks. We've been doing some recent calculations and some research where we tightened the mix down a little bit, make it not quite so permeable, but able to handle a lot of the storms that we have and then increase the structural capacity.

And we're looking at some potential applications of retrofitting the shoulder of the roads so that then they can carry some trucks. They can't carry the high-speed, but maybe it can carry some low-speed trucks. And then that pavement, if you put water in it, has been looked at as a way to cool the environment - cool the local environment down around the pavement due to the evaporative cooling.

FLATOW: So you might just actually not wait for the rain, but you might spray water on this pavement, and it just evaporates a little bit and cools it off.

HARVEY: Yeah. As a colleague of mine - I was talking to him earlier today. You know, that's an old technology used around the world, is toss some water out on the pavement, and you get the evaporative cooling effect. So there has been some research done by the Public Works Research Institute in Japan, where they've been taking the irrigation water from the medians, the landscaping in-between - in the middle of a boulevard, and dropping that water down into the pavement in the late afternoon, and then getting a pretty substantial evaporative cooling during that peak heat time.

FLATOW: I tell you, we have this post on our Facebook at SCIENCE FRIDAY - our SCIFRI Facebook page, and we got so much response from this idea. And it's just - you know, it's amazing. But people were worried about it clogging up. They were worried about, will it freeze? Will it, you know, will it expand if there's water in there? Will that expand and break up the pavement in the winter?

HARVEY: I think that - I'd like to add one thing to what Cheryl said, is also, besides pervious concrete, there's also pervious asphalt. There are pervious, interlocking concrete blocks. And we actually have our parking lot at our research site as - we just have gravel, and it's fine for low volumes of low-speed traffic. There's been some research at the University of New Hampshire that has looked at some - and also I believe Cunningham has done some work in - and some other folks in Pennsylvania, some other freezing environments.

And it has not been a major problem from what I've seen, because the water actually is down in the gravel, which doesn't really have that big a problem with it, and down next to the soil. So it's...

SULLIVAN: Right. And it all depends on the soil mix and the, you know, what the geotechnical engineers said, has to be the depth of the gravel below. But when it's designed properly, there isn't the cracking, the freeze/thaw issues that you have with regular concrete, because as John was saying, it just - the water melts and just flows down into it. So the rock isn't coated with water.

FLATOW: So it's...

SULLIVAN: So it's great for a parking like(ph), like in Denver. When it snows, it starts to melt. The parking lot's dry. You're not left with these icy puddles that you have on a - you know, a regular - or a conventional asphalt or concrete parking lot.

FLATOW: And what happens if there's a gasoline or an oil spill? Will you just sink right through into the ground? Or...

SULLIVAN: Yeah, it would. That's why we don't recommend putting it in gas stations, because if you do have a spill like that, you're going to have to take it out and remove it, in all likelihood.

HARVEY: Kind of the way we thought about that, particularly for - if we're going to be retrofitting a shoulder of a highway pavement, what happens now when there's a gasoline spill? A lot of it runs off the road and runs into the soil on the shoulder of the road. So, in a way, we're actually capturing it in that shoulder or in that pavement. And then there is the - a lot of - as long as you're not going to affect the groundwater, we've been talking to some of our environmental engineers that a lot of the bacteria, as long as you capture it and hold it there, it may not be as big a problem as you might think, compared to the, you know, the regular pavement.

FLATOW: Somebody sent us a note saying that in their neighborhood where they have it installed, it has to be vacuum-cleaned once a month to get the dust out of the pores.

SULLIVAN: Well, typically, they recommend vacuuming once or twice a year. Perhaps some people may be overly concerned about clogging issues. If you do have this open matrix that dust can fall through and then get deposited - you know, you have six inches of concrete and maybe six or 12 inches of gravel underneath it. Unless someone is dumping, you know, a truckload of sand or soil on top of it, it's not really going to clog it.

FLATOW: All right. Now, we've talked so much about it, how can you get your own?

SULLIVAN: Well, I would certainly recommend that if it's a homeowner, that they hire a contractor who has been a certified pervious concrete installer. Because there is a special way to do it, and if you don't do it correctly, it won't work, or work as well. And there are a lot of contractors out there now, concrete contractors, who are certified in the application of it. So you can't go down to the, you know, the local Home Depot and pick up a bag of pervious concrete mix.

FLATOW: That wasn't my question. And John, are there any newer types of paving material that people are looking at?

HARVEY: Well, one of the big questions in the paving world is the quest for what we call the third binder. Mostly, we use for binders in pavements in the surface area asphalt or cement. And asphalt coming from oil refining, has a lot of environmental considerations, impacts. And cement also is basically the burning of limestone, which also has - produces quite a bit of CO2, as well. And so the quest has been for a third binder. There's a lot of work going on right now.

Some demonstration projects, things like growing algae and processing the algae to try and extract an asphalt-type binder using vegetable-based binders. I've even seen one paper about using pig manure, but I'm not - for each - and then in the cement area, there's a new type of cement which is - there's a company in California that is demonstrating how to capture the CO2 out of the flue of a power plant and then process that using saltwater and some other really innovative chemical processes.

And they can produce aggregate. They can produce what we call supplementary cementitous materials, and even some types of cement from that. So they're actually taking the CO2 out of the stack of a power plant and converting that into road-building materials.

FLATOW: Wow. Wow. That's - so it's not a boring topic, talking - watching the road dry.

(SOUNDBITE OF LAUGHTER)

SULLIVAN: No. It's a terrific topic.

FLATOW: All right. I want to thank you both for taking time to be with us today.

HARVEY: All right.

SULLIVAN: OK. Thanks a lot, Ira.

FLATOW: And good luck to you.

HARVEY: Thank you.

FLATOW: You're welcome. Cheryl Sullivan, the senior landscape architect at the Cunningham Engineering Corporation in Davis, California. John Harvey, professor of civil and environmental engineering at the University of California, Davis. This is SCIENCE FRIDAY, from NPR.

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