Porous, Flexible Pavement: Where The Rubber Meets The Roots

A type of porous, flexible sidewalk pavement that’s been used in places like Yellowstone National Park and Arlington National Cemetery is likely coming to Foxhall Village in city action to protect trees and smooth passage for pedestrians.

Erupted sections of concrete sidewalk near some trees in the village have been cut out by city transportation department crews and packed with gravel. Workers said they expect the sites will be paved with the rubbery granite material touted as being easier on tree roots and people feet.


AWAITING NEW-FASHIONED PAVING: A tree-side section of P Street walkway, after having erupted concrete removed, is targeted for installation of a new type of flexible, porous paving intended to aid tree root access to water and air and smooth passage for pedestrians, including people in wheelchairs. Photo Credit: John A. Bray

At least 350,000 square feet of the material have been poured in Washington, D.C., over the past five years, according to Noble Atkins, founder of Capitol Flexi-Pave.

Conventional pavement blocks tree roots from what they need to thrive.

“When you allow air and water to go through,” Atkins said, “there is no need for the roots to rise up and break the sidewalk.”

Atkins, who spent 22 years as an urban forester in D.C. and other locations around the country, said the company uses a pavement that is much like a layer of mulch, which also helps shield roots from extreme temperatures and compaction.

He said his tree protection projects, including jobs at the White House and Supreme Court, showed him the potential of the new pavement and he “brought it to town.” The company uses a paving system developed by KBI. The pavement is made with binder and bits of granite and used tire. The system also has been used on walkways around the Old Faithful geyser at Yellowstone National Park and some sidewalks at Arlington National Cemetery.

Working Out Kinks

KBI CEO and Founder Kevin Bagnall spent nearly 30 years in an offshoot of the recycled tire business, playground surface installation. Then he started looking for a way to create a material that would break the conventional asphalt and concrete paving mold. He also wanted it to have an environmental application.

“But it couldn’t need to be a tree-hugger product,” Bagnall said in a phone interview from Montana after completing another phase of the Yellowstone paving project. “It had to have a very valuable approach and acceptance within the tire recycling industry,” Bagnall said, adding that he targeted uses where conventional paving could not perform, and knew that extensive testing would be required for success. “It had to have a place within the construction industry.”

“The aforementioned is a very hard task,” said Bagnall, who is from England and moved to Florida in 1991.

A variety of porous pavements, made of multiple types of materials, have been developed over the years for different conditions. For example, use of a porous asphalt layer on highway pavement surfaces, mainly in the south, to aid drainage started in the 1980s, according to Michael Heitzman, Assistant Director and Senior Researcher at the National Center for Asphalt Technology at Auburn University.

What type of pavement gets used where depends on many factors. Heitzman said that shipping costs might deter use of crumb rubber if the paving location is too far from the scrap tire processing facility. But he said that production of such porous paving mixes requires a refined system, with the stone and rubber particles sized, proportioned, and blended with the binder in a way that the material does not become tightly packed.

On one level, Bagnall notes, porous paving provides potential for more buildable space. On another, Bagnall sees continuing urban growth and increasing population density, with accompanying expansion of the waste stream and demand for fresh water.

“We do not leach anything, hence, we can be used next to some very sensitive environmental areas,” Bagnall said of the material. “We are totally inert. We are totally benign.”

Robert Horton, a D.C. transportation department project engineer, said a formal report on the use of the paving system in the city would probably be completed by early 2017. Asked about his view so far, Horton said, “It’s been excellent.”

Atkins said the paving cost ranges from $10 to $20 per square foot, depending on conditions.

To make the material, tires get crushed into quarter-inch granules, Atkins said. It’s colored and mixed with granite bits, and bound with a special adhesive that maintains flexibility.

“It’s poured as we make it on site,” Atkins said, noting the process employs a truck like a small cement mixer. “It cures very quickly. So we don’t mix great quantities at one time.”

The pavement maintains its porosity and flexibility by being used and rained on, according to Atkins. He said other porous pavements tend to clog over time and require vacuuming. “The truth is, hardly anybody actually ever vacuums them,” he said. “They all end up becoming like regular concrete.”

Atkins said that it’s not known how long it ultimately will last, since the technology has only been around for about 16 years. “It lasts as long as traditional pavement or maybe longer,” he said.

The spongy pavement is unsuited to stand up to use for roads, according to Atkins. But it fits its niche and is softer to walk on. “It’s better on the joints,” Atkins said. “Perhaps it could be extended to the entire sidewalk. Everything takes time.”


At Yellowstone, crumbling asphalt paths, accumulating used tires, and directives to tread more lightly on the ecosystem prompted a shift in the approach to paving. Lynn Chan, a National Park Service landscape architect and program coordinator for sustainability in Yellowstone, told me that asphalt paths, while inexpensive and easy to spread, tend to crack in the extreme thermal area heat and winter cold.

“Once it breaks up, it tends to get kicked and sometimes even thrown intentionally into the geysers and hot pools,” Chan said. “It’s really significant to us that we keep them in their natural state.”

With few trees in the area of the geyser, the pavement’s flexible aspect makes little difference when it comes to tree roots, but it’s useful in accommodating stress from ice, said Chan, who noted that it does not leach oil like asphalt. There also is the desire to have the rain percolate where it falls, which reduces erosion from runoff.

In two phases, starting in 2015, the park has covered about 1,400 feet of roughly 9-foot wide path with the pavement. “It looks like an asphalt path, quite honestly, but it’s not,” Chan said, who has worked at the park for two decades. “It’s a great walking surface.”

In the past, discarded tires from the park’s fleet of hundreds of vehicles were buried in a northern Montana landfill. But now they go to western Washington State, where they get pulverized and then returned in bags to the park for incorporation in the paving material, according to Chan, who said some tires from communities around Yellowstone also have gone into the mix.

The new pavement doesn’t amount to even a tiny fraction of the paths in the park, which covers roughly 3,468 square miles, mainly in Wyoming.

“It’s just one small piece of our sustainability program,” Chan said, who said the park service is grappling with how to preserve the place while accommodating a growing number of visitors. Annual park visitation has risen from 2.8 million people in 2005 to just more than 4 million in 2015, according to park service statistics. “That is very challenging on many fronts, including all of the infrastructure,” Chan said.

National Park Service funding reached $3.1 billion in 2014, a 3 percent drop from 2005 when adjusted for inflation, including an 8 percent decrease in money annually appropriated by Congress, which make up on average 88 percent of the service’s budget, according to a December 2015 Government Accountability Office report. The rest comes from fees, donations and other sources, which increased over the same period by 39 percent. The report noted that the number of park units in the system has risen from 390 in 2006 to 409 in 2015.

Chan said partnerships with private companies are key. For example, Michelin annually donates about $300,000 worth of tires and maintenance expertise for the park’s fleet, according to the company. Chan said the company also has provided funds for the path project.

“We probably couldn’t have done this without the partnership with Michelin,” she said.

Such partnerships are aided by Yellowstone Forever, a foundation that has raised more than $100 million since 1996 to support the park, including through research and education projects, according to Jeff Augustin, the organization’s vice president of strategic partnerships.

Of course, the park could just keep putting down asphalt. “That certainly is one solution,” Chan said. “I try to look into a lot of different alternative path treatments and I would say all are more expensive than asphalt,” Chan said. “But we’re hoping the benefit proves that it’s worthwhile.”

The pavement continues to be assessed. “We’re really picky about our products,” Chan said. “They have to satisfy many of our staff for different reasons. I think the challenge is: What is the best practice?”


At Arlington National Cemetery, the paving was installed for sidewalks along Eisenhower Drive in 2012 and along Meigs Avenue near the James Tanner Amphitheater in 2014, according to Col. Michael D. Peloquin, Chief of Engineering at Arlington.

Peloquin said that use of the material was more about porosity than flexibility. “There are all sorts of permits we have to have for storm water runoff,” Peloquin said. “You get some credit for it being a semi-porous material.”

“There was a big push, particularly back at that time, for government facilities working toward different levels of accreditation,” Peloquin said, referring to certifications under the Leadership in Energy & Environmental Design (LEED) program organized by the U.S. Green Building Council.

“There was interest in showing that facilities were being environmentally responsible.”

“It’s performed as we anticipated,” Peloquin told me. “It seems to have held together pretty well.”

At least for the near term, trees are not positioned in places where roots would be close enough to test the flexibility of the pavement, according to Peloquin.

Driveway Launches New Way To Pave


KEVIN BAGNALL: The chief executive officer and founder of KBI at Yellowstone National Park, where the flexible, porous paving he developed is in use on walkways. Photo Credit: Provided By Kevin Bagnall

The computer tech industry has its lore of the garage-incubated business phenomenon.

Kevin Bagnall thought outside that box – just outside.

In 2001, he mixed old tire chips and granite bits and a binder, and poured it out on his driveway in Clearwater, Florida.

“Lo and behold, we have the world’s first flexible, porous paving,” Bagnall told The Hoe.org in an interview. But would he get a bounce or would his industrial granola bar fall flat?

A coal miner’s son from the north of England, Bagnall spent many years in Europe as an installer of playground surfaces made of recycled tires and moved to United States in 1991. But he saw a bigger horizon in reducing the flow of old tires into places like landfills.

Bagnall, 56, said he knew there were many possible uses for his formulation of stone, tire granule and binder. He said chemists helped him develop the binder, which he said is a benign type of urethane.

“The engineering community at that time didn’t really understand it and neither was there a market for it,” he said. “Guess what? We’ve made a product that now has a distinctive place in infrastructure development.”

Infrastructure means more to Bagnall than just construction. It also means natural resources, one in particular. For example, the walkways around Old Faithful geyser that are paved with his porous material allow water to return to the aquifer rather than run off and cause erosion. Similar dynamics are in play in cities around the world.

“As urbanization is increasing, which it is, the population is increasing; the waste stream is increasing; the fresh water supply is not increasing,” Bagnall said, his tone something akin to that of Robert Shaw’s Quint as he told his “Jaws” crew that what they faced was no ordinary fishing expedition. “We had better take care of that fresh water supply.”

That’s partly why he calls what might be considered an unglamorous aggregate an “absolute winner.” “We’ve taken that increased urbanization waste product, i.e., tires, and put it into water conservation,” Bagnall said. “That is massive.”

Bagnall did not aspire to the work of his father, uncles and cousins in the big undertaking of Grimethorpe, the village of his upbringing.

“That’s shaft mining. No, my father wouldn’t allow me to do it,” Bagnall said. “He basically, not booted me out, but, we’ll just say: ‘You’re not going down the coal mine. You need to leave.’ I can’t thank him enough for that.”

He entered the British Royal Navy and served as a diver.

KBI also bears the initials of his father, Kenneth. “I told him I was going to name a company after our initials,” Bagnall said, “and he loved it.”

Bagnall said he intends to “evangelize the engineering community. We’re not salesman. We’re educators. And the next generation of engineers is really bound by an understanding of next generation infrastructure.”

© 2016 John A. Bray