Stormwater management professionals are starting to utilize the various forms of porous pavements as innovative solutions.

It’s taking time, but the on-site water retention potential of porous pavements is sinking in across the country as communities and developers grapple with federal requirements to clean up surface runoff before it gets into the nation’s waterways.

“It is still a very small fraction of all the paving that is being done, but measured on a percentage basis, the growth is exponential,” said Bruce Ferguson, Franklin Professor of Landscape Architecture at the University of Georgia. With more than 25 years of experience on the subject, he is considered one of the nation’s leading authorities on urban watershed management. “Porous pavements are very competitive in reducing the costs of meeting the given requirements, so there is a demand for it,” he said.

Advances in asphalt, concrete, stone pavers and grass-containment systems are showing promise. “Every aspect of these industries is coming along,” Ferguson said. “This is a whole new opportunity.”

Surfaces that allow water to seep through, instead of creating a seal over the land, have caused a change in approach by the traditional industries that do paving work.

“In the Ready-Mix concrete business for instance, a number of people I’ve talked to say they have been in the business for decades and this is the first time that anything new has actually happened,” he said. “It’s something that is moving forward and they have to run to keep up with it.”

Driving the demand are the amendments to the federal Clean Water Act, put in place in 1987, which are forcing communities and developers across the country to rethink the way they have dealt with the situation in the past - laying down hard surfaces as they go.

“Pavements are two-thirds of the potentially impervious built surface areas. If we can make them operate differently, environmentally, then we have done a huge thing for the urban watershed,” Ferguson said.

“By taking care of the problem at the source, even though pervious pavement costs more per square foot than an impervious pavement, when you take into account the cost of not passing the problem down stream, porous pavements are the economical solution,” he said.

In most current instances, so called “non-point source” pollution — oil drippings from engines, heavy metals from brake linings, anti-freeze, and other polluting residue from human activity — washes off parking lots, streets and roadways, and contaminates streams and rivers. Soil erosion is also an issue because instead of soaking naturally into ground, rain and snow melt water becomes concentrated as it streams from developed sites.

It’s taken a while for the requirements to trickle down from the Environmental Protection Agency (EPA) to state and local governments.
 
“We’ve been able to live for many generations before that without having to change our way of thinking. We just lived with conventions for a long time. That did change with the law change,” relates Ferguson. “I’m not a guy who is in favor of big government regulations, but apparently it did take the heavy hand of federal law to make this penetrate into every single municipality in the country.”


Consistent Concept
The various pervious pavement systems have the same basic make up: a surface that can sustain traffic but allow water to pass through, and a base made up of aggregate which creates voids to hold the water that passes through the surface layer, creating a reservoir until the water can soak into the soil below or flow laterally through perforated piping. Many incorporate a geo-textile material on the soil, which works to keep the reservoir layer from filling up with finer material.

Asphalt and concrete products also rely on same-sized aggregate — smaller stones — in their mixes which create the void that allow the water to pass their surface products. Stone block systems allow water to pass through the spaces between the pavers, and the grass systems combine hard materials to sustain traffic weight with grass-growing cells, which allow the water to pass through to the reservoir space below.

“It’s fundamentally simple and it’s fundamentally not changing over time. We’re getting a lot better with the details, but the fundamentals are not changing,” Ferguson said.  

“The news has been terrific,” said Ferguson of research that shows that porous pavements do an even better job of on-site stormwater cleansing then first believed. Since the 1970s, it’s been established that allowing stormwater to seep into on-site soils cleans stormwater runoff of pollutants better than having them concentrate in detention basins or storm sewers and flushed into surface water. Microbes in those on-site soils break down the pollutants.

Research at Coventry University in England has found that microbes on the surface of the rocks in the reservoir layer are doing the job early on.

“There is, in fact, a micro ecosystem in the aggregate of the base course. It’s a mighty thin layer, but there is a very large surface area where these things exist,” Ferguson explained.

The interior of the pavement is great habitat for the micro ecosystem. It’s moist, the temperature is moderate compared to the surface. Pollutants have a long residency time in that layer and stick to the rocks where the microbes can flourish to do their work.

“The bloom, as they call it, ingests the oils and biodegrades them. They reduce these complex hydrocarbons into simpler components. The oil ceases to exist as a pollutant. It’s the best possible thing that could happen in there,” Ferguson said. “The soil turns out to be the back-up system if the pavement structure should get overloaded somehow.”


Porous Asphalt
Officials in the asphalt industry say they have made some good improvements in their porous asphalt product, which were first tried out in the 1970s.

“There are a number of projects out there that are well over 20-years old and still performing,” said Kent Hansen, Director of Engineering for the National Asphalt Pavement Association (NAPA).

Ferguson said he’d like to see the asphalt industry do more to promote the concept over traditional hard surfacing.

A unique characteristic of asphalt is that its petroleum-based binder can soften and become mobile in the heat.

“When it gets hot on a summer day, it will flow, maybe only one micrometer a day, but when you leave it out there for six or eight years, it can flow down off the surface particles of aggregate,” Ferguson said, potentially becoming a clogging layer.
Hansen said those issues, while speculative, are being addressed.
“We’ve learned about polymer-modified asphalts. It makes it tougher and allows us to put a thicker film of asphalt on the aggregate stone in the mix,” he said. “In some cases we can use fibers that help the aggregate hold on to the liquid binder while it’s still hot so it doesn’t drain down.”

Such advances are drawing interest.

“Three years ago when we started working with this, we’d get very little interest. We did a few training courses. But every year the interest grows,” relates Hansen. “I see it being used more and more in parking lots. That’s going to remain the dominant use any kind of porous pavement system.”

Asphalt is the 500-pound gorilla in the paving world when it comes to market share, capturing about three-quarters of the huge amount of hard-surfacing work in the country. The remaining quarter goes almost entirely to the concrete industry. The block industry takes a fraction of the remainder according to Ferguson.

“The asphalt industry is complacent because they are figuring to sell their asphalt no matter what. They’re number one, so they don’t need to try harder,” Ferguson said.
Hansen states that the market does not allow for complacency.

“With the current emphasis on sustainability and such a demand for pavement to help in the stormwater management of projects,” relates Hansen, “it requires us to continue educating on how our products can meet these demands. In addition to holding events in almost a dozen cities to about 1,500 people from New Jersey and Florida in the east to Oregon on the west. We are constantly making personal and trade show presentations.”

Hansen explains, however, there is reason for caution. Porous pavements are not always the answer. The soils have to be right and the projected load requirements of the project have to be right for them to work. “We don’t want to use them in a situation where they are going to have problems. We need to make sure we build them right to begin with.”

“It’s great technology,” he said. “I’m a firm believer in this. If you have one mistake, people see that. You could have a hundred successes, but they see the one problem.”


Pervious Concrete
Dan Huffman, Managing Director of National Resources for the National Ready Mixed Concrete Association (NRMCA), said the difference in the concrete industry’s porous pavement product starts with the name. It’s “pervious” concrete, a product that dates back more than a century as a structural building material. Some buildings in Europe dating back to just after World War II contain the product, not as pavement, but are still standing today, he said.

“We have to differentiate for any decision-maker the differences between the types of porous pavements they can select,” he said. The concrete product relies on portland cement to bind the mostly same-sized stones in its rock-hard matrix. The removal of sand from conventional pavement materials (concrete and asphalt) is essentially the means by which those materials become significantly less dense and hence, porous.  

“The rigidity that only portland cement can provide as an aggregate binder is the most critical element in allowing pervious concrete to uniquely maintain its void structure and corresponding porosity under load. The structural integrity of portland cement is also largely unaffected by the weathering affects of temperature, sun, and moisture,” he said.

According to Huffman the U.S. EPA has a funded study to compare the asphalt and concrete products to be conducted at Villanova University this summer.

“The EPA already recommends the use of pervious concrete among its recommended BMPs for stormwater management. We have the support of the federal government, from an environmental perspective,” he said. “Congress is also looking further at the use of pervious concrete pavement related to stormwater management.

“The people who are looking at this are large and sophisticated owners on much larger projects. They perceive the benefits from a public relations opportunity by being more environmentally focused,” he said. The technology can also contribute to LEED (Leadership in Energy and Environmental Design) certification for projects.

Among the dynamics holding utilization of the concept back is a general lack of understanding of the technology at the local-agency level. “When they don’t mention it within their regulations or best management practices an owner can get discouraged,” Huffman said.

NRMCA and the Portland Cement Association (PCA) have teamed with Futurus Communications, the publisher of Sustainable Land Development Today, to help educate members of a project development and design team by conducting workshops throughout the country on utilizing pervious concrete as a stormwater management solution.


Porous Pavers
Ferguson said of all the porous pavement systems out there, the concrete block industry has been way ahead of the others.

“They have their act completely together at the moment,” he said.

Helga Piro is General Manager SF Concrete Technology Inc., in Mississauga, Ontario, Canada, a 12-year-old company and subsidiary of SF-Kooperation in Germany. Her company licenses concrete producers to manufacture interlocking paving stones, permeable pavers, and segmental retaining wall systems in Canada and the United States.

“I see a huge increase this year. I think the whole environmental issue is in front for everyone. That is the reason these kinds of pavements are in so much demand,” she said.

“I would say, just comparing 2006 to 2007, I see an increase of 40-percent already, just in the first half of this year.”

The primary benefit pavers pose is flexibility.

“It’s a pavement that consists of single stone pavers compared to a poured concrete that is not flexible, the load that you apply cannot disperse onto the base layers,” she said.

Her company offers an array of products, some with interlocking blocks and some with sand in the joints. Some can be used for driveways, industrial parking lots, walkways and plazas, while other products are specifically designed for road traffic she said.

An advantage with pavers over poured products is that they can be taken up to gain access to the rock reservoir below.

“In the worse case scenario you can exchange the base material and provide new material on top of the base and replace the paving stones without any damage,” she said.


Flexible Concrete that Grows Grass
Nick Jansson is an engineer with California-based Soil Retention, a regional company. “This year we are experiencing very large growth,” he said.

One product, Drivable Grass, incorporates concrete cubes in a two-foot by two-foot grid system. After it is laid over the sub grade layers, a planting medium fills in the spaces on the surface to grow grass.

“Only being an inch and a half thick and having that reinforcement, it’s a flexible concrete pavement system,” Jansson said. “This is a wet cast system that comes in a mat, so the installation of the product can be a lot easier than installing a paver.”

The system works well for uses from fire lanes, driveways, and parking lots, to golf cart paths. It also is a more aesthetically pleasing alternative to rip rap in drainage ditches and works well with bioswales.

It comes with an added benefit of having additional water purification capability due to its growing component.

“The grass will biodegrade pollutants on its own and by filtering down through the sub base, also do cleansing of that water,” he said.

“You run stormwater through a nine foot, grass lined swale, it will treat up to 95% of the pollutants by the time it runs through the product, and that’s without any infiltration. That is just up at the surface,” he said.


Wrap it All Up
The potential uses for porous pavements abound almost everywhere because paving is done on many low traffic areas, according to Ferguson.

“It’s the parking lots, residential streets, residential driveways, sidewalks and so on. That is where most of these materials go. That is where most of the good is to be done.”

That’s why, claims Ferguson,  porous pavements are potentially the most important development in urban watersheds since the invention of the automobile.

“That sounds like a big statement, but it’s easy to make. All these pavements that we are building these days and the pollutants that are coming off of them are the result of the automobile,” he concluded. “That’s why we have all these pavements. And now, we can positively impact the watersheds by utilizing these new technologies.” SLDT