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New sidewalk uses porous, flexible pavement

At first glance, the tree at the northeast corner of 8th and K Streets, NE appears to be buried in asphalt. The truth is much more interesting.

To comply with Americans with Disabilities Act regulations, DDOT rebuilt the intersection of West Virginia Avenue NE, 8th Street and K Street in the H Street neighborood. This intersection is a busy transfer point between the 90 and D Metrobus lines and has a lot of foot traffic.

Image from DDOT provided by Veronica Davis.

DDOT has installed modern curb ramps at all pedestrian crossings, and they've repaired the concrete in order to smooth out some potentially dangerous bumps. The elderly and disabled now have a smoother path to get from bus to bus as they travel across town.

(It would be nice if DDOT would restripe the crosswalks with higher-visibility zebra/piano striping, but perhaps that's a subject for another blog post.)

Within the project limits, there were two trees whose roots were lifting the sidewalk. This created dangerous tripping hazards and the narrowed sidewalks made it difficult for those in wheelchairs to use the sidewalk.

The sidewalk before. Image from Google Street View.

Instead of concrete, construction workers have used porous pavement in these areas that extend all the way around each tree. This makes the sidewalks a bit wider, eliminates the need to trim weeds around the base of the trees, and allows more water to filter through the ground to the trees' roots.

It will be interesting to see how well this pavement works, and if we'll be seeing it used more widely around town in the near future.

Geoff Hatchard lived in DC's Trinidad neighborhood. The opinions and views expressed in Geoff's writing on this blog are his, and do not necessarily represent the views of his employer. 


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They've been using it in Georgetown for a bit as well.

by charlie on Jul 17, 2013 11:12 am • linkreport

Hmm. On 7th St. just south of Eastern Market, parts of the sidewalk were replaced with a sort of rubberized matting. Not sure how pervious it is.

by 20002ist on Jul 17, 2013 11:15 am • linkreport

I wish the city had more of the stuff, it is great to run on.

by Richard B on Jul 17, 2013 11:16 am • linkreport

That tree needs a curb extension

by JJJ on Jul 17, 2013 11:28 am • linkreport

There's a whole bunch of this stuff installed out front of the Anacostia Community Boathouse. It's been there for about 2.5 years, and seems to be holding up pretty well.

The one caveat of the stuff (at least at the ACBA installation) is that it cannot hold the weight of a vehicle at all. Drive a heavy pickup truck onto it, and it starts to sink almost immediately. This could be a problem for the numerous city contractors who decide to treat sidewalks like parking spaces.

No clue how the stuff will hold up as that tree and its roots grow. While I'm glad to see DDOT finally acknowledge that bus stops need to actually be able to accommodate people while they stand and wait for the bus, I don't want to see too many treeboxes or grassy areas paved over.

by andrew on Jul 17, 2013 11:43 am • linkreport

That being said, this intersection still sucks for pedestrians, and these improvements don't do much to fix that. Closing off the "stub" of WV Ave would dramatically improve the pedestrian experience at this intersection, and probably also make things a bit nicer for drivers as well..

Really, this whole stretch of WV Ave is a little scary. Just north of here, the area where 9th, L, WV, and Florida all meet is also pretty terrible for pedestrians and drivers alike. There must have been some kind of edict that stated that WV Ave could never ever have any stop signs on it...

by andrew on Jul 17, 2013 11:51 am • linkreport

Porous paving is good, but why did they pour it all the way up to the tree trunk? I can understand the desire to remove bumps, and also to widen the clear path along the sidewalk, but what is the rationale for boxing in the tree on the sides?

Even if it is porous and water can get through to the tree, it's certainly not as porous as dirt/grass.

by Alex B. on Jul 17, 2013 11:51 am • linkreport

So far, I LOVE THIS STUFF! First, it is probably cheaper, more durable (my guess). Second, it's softer to walk on, so it won't bother your knees and feet so bad if you walk a lot. Third, it probably won't absorb and radiate as much heat as rock does. It's a win win!

by Matth on Jul 17, 2013 12:21 pm • linkreport

Oh, and this has made it's way to other places! There are several areas like this on 17th Street in the Dupont Circle neighborhood that have had this treatment done.

by Matt on Jul 17, 2013 12:23 pm • linkreport

It will heave--and if it doesn't stretch, it will crack--just like concrete as the roots expand.

by crin on Jul 17, 2013 12:28 pm • linkreport

It's shown up along Rhode Island in Bloomingdale too. Much nicer!

by Josh on Jul 17, 2013 12:30 pm • linkreport

Could any of you be mistaking this stuff for the rubber sidewalks that are also appearing throughout the city?

AFAIK, porous concrete is no "softer" than regular concrete. The surface is a bit rougher, but it's just as firm.

by andrew on Jul 17, 2013 12:50 pm • linkreport


17th Street has the rubber blocks, which are better for the trees, but still have their problems. Many of them have become warped, causing the edges to stick up; it's very easy to trip on. I'm also not sure if those rubber pieces are porous.

This seems to be actual poured asphalt, which would likely be an improvement.

by Adam L on Jul 17, 2013 12:51 pm • linkreport

Before singing the praises of this material, I want to know a lot more about it and how it will effect the health of that tree. Additionally, I doubt that it is flexible enough to withstand those tree roots when they want to explore for more water and nutrients.

by Thad on Jul 17, 2013 1:23 pm • linkreport

Am I the only one who thinks it looks ugly as sin? There other ways to achieve the same goal that are more asthetically pleasing and that don't cost anymore money.

by Arkie on Jul 17, 2013 2:12 pm • linkreport

I think it looks a little odd but I wonder if that's just because I'm conditioned to think sidewalk = concrete.

by m2fc on Jul 17, 2013 2:25 pm • linkreport

It's not beauitful, but perhaps certain treatments or color schemes could be applied? Certainly much less hazardous than the wildly askew pavers I trip over about once a week.

by Alan B. on Jul 17, 2013 2:30 pm • linkreport

They just poured this stuff on Columbia Road between 19th and 20th NW. Interestingly there are two tree boxes and one is poured up to the trunk of the tree the other has a nice rectangle cut out of it. Maybe they are testing different ways to pour around the trees?

by Stephen on Jul 17, 2013 2:57 pm • linkreport

Hi - I'm with DDOT's Urban Forestry Administration and just wanted to clarify about this flexible, permeable paving product we are beginning to use more frequently to protect trees and reduce stormwater runoff. It's made in part from recycled tires (1/3 of a tire per square foot) and has been used in Europe for years.

It feels firm but is actually flexible enough to bend around roots. However, the reason roots push up under sidewalks is to find water. With water able to penetrate this pavement, roots are not as likely to rise.

It's poured up to the tree trunk to help meet code for sidewalk width in narrow places and to protect the soil. It is poured thinner as it gets closer to the tree so the paving can break away in small bits as the tree slowly expands. Particularly in a heavy foot traffic area like a bus stop, soils can get extremely compacted, stripping trees of access to necessary oxygen, nutrients, and water filtration. So, permeable paving allows water to penetrate while keeping the soil healthy underneath.

There are different colors available (DDOT chose a few from a much wider pallet) and it has been used for slower-speed vehicle travel, such as in parking lots. We're primarily using it in high foot-traffic or extreme tree / sidewalk conflict areas.

by Ian Leahy on Jul 17, 2013 4:14 pm • linkreport

Thanks, Ian. Very interesting and informative!

by Alan B. on Jul 17, 2013 4:16 pm • linkreport

@Ian: How's this stuff being installed? Does it require any sort of buffer to be installed underneath the surface, or is it just a single layer?

by andrew on Jul 17, 2013 4:24 pm • linkreport

@andrew - It sits on a bed of at least 4" crushed concrete or similar aggregate and is installed similarly to asphalt.

Thanks Alan. Glad I could help.

by Ian Leahy on Jul 17, 2013 4:32 pm • linkreport

How much does it cost?

by SJE on Jul 17, 2013 5:25 pm • linkreport

Unfortunately it is both more expensive and less durable. It doesn't mean it isn't great, but you cant put it everywhere without costing a ton.

by Richard B on Jul 17, 2013 6:15 pm • linkreport

@Ian , this looks like a great potential option for my urban driveway, currently badly cracked due to the large tree exploring how much water it can find there. Is this product available to the residential market? Does it have a proper name?

by Julia D on Jul 17, 2013 6:25 pm • linkreport

I can't say the aesthetics of the block have been improved much. It's too bad the grass (using the term loosely) couldn't be preserved.

by Chris S. on Jul 17, 2013 7:05 pm • linkreport

This new material you are seeing around the Mid-Atlantic is called Flexi-Pave ( and as Ian mentioned it is made from recycled tires and stone held together with an elastomeric binder that bends rather than breaks when it is stressed.

Not only does it allow 2500 gallons of water per square foot per hour to pass through it, but microbes in the 23% void spaces eat up contaminants like nitrates and orthophosphates so the water that filters through it is up to 86% cleaner.

It reduces heat island effects and is not affected by freeze thaw. It is cold applied and before installing, the soil underneath is decompacted with a supersonic air tool to increase air and water infiltration and improve the soil structure of the tree's critical root zone.

It can be used in any situation both commercial and residential where traditional paving would be used except main roadways because it is limited to low speed traffic.

The main benefits are that it is flexible and invisible to stormwater. Feel free to read more about it online.

by Noble on Jul 18, 2013 12:28 am • linkreport

It is ugly as sin.

I have a question for the scientists/engineers: I was very excited a few years back about the promise of pourous asphalt, as a way to prevent the polluting runoffs from roads. Since then, I've heard that pourous asphalt is not really all that pourous, especially as a road surface. It gets pretty clogged up in short order.

Is that right?

by Fischy (Ed F.) on Jul 18, 2013 1:17 am • linkreport

You are correct. Porous asphalt and porous concrete have static fixed voids which clog with sediments, therefore they have to be vacuumed regularly to extract the fines. And even with vacuuming only about 80% of those fines can be extracted so eventually they both get clogged.

Flexi-pave is different in that the voids are dynamic and constantly changing size due to thermal expansion and loading. Therefore as they change size and as each rain event occurs the fines work their way through the matrix into the drainage stone beneath. That is why Flexi-pave is considered self cleaning and doesn't require vacuuming like its predecessors. There are studies to prove this, which is why municipalities are moving away from porous concrete and asphalt and choosing Flexi-pave instead.

Cities like Alexandria, DC, New York, Miami, Chicago, Tampa, Key West... counties like Arlington, Fairfax, Montgomery... institutions like Walt Disney World, the Smithsonian, Lockheed Martin... are all using Flexi-pave throughout their jurisdictions because they recognize the benefits.

Beauty however, is in the eye of the beholder. It does come in 28 colors so perhaps you might like one of the others.

Thanks for your thoughtful question.

by Noble on Jul 18, 2013 6:55 am • linkreport

@Noble -- Thank you for your answer. On the aesthetics, perhaps "ugly" isn't the most appropriate word choice. I just meant that it lacks the character of the concrete it's replacing and it is jarring to see the asphalt poured right up to the tree. I suppose, that if it works as advertised, it might be preferable to the barren patch of dirt that typically surrounds sidewalk trees, even if it's less attractive than the pavers.

by Fischy (Ed F.) on Jul 18, 2013 7:43 am • linkreport

Very cool and great since it's better for the tree and for people with mobility issues. It is a bit jarring to see the ground paved right up to the tree, but I think that's just a matter of it looking "different" - the dirt patches around trees aren't nice and turn into mud pits when it rains.

I think DDOT would be well-served by putting up an informational sign here about what the stuff is and how it works. I assume people will call to complain about how somebody poured asphalt all over the poor tree box!

by MLD on Jul 18, 2013 8:30 am • linkreport

You two are both spot-on. This new paving material is so completely different than everything that has come before that our construction techniques and installation parameters are new and different as well. Remember this is not asphalt, this is 3/8" rubber granules from recycled tires mixed with similar sized granite chips applied with a special binder in a "cold pour". The challenge is to get people to think outside the proverbial box with these new and improved tree boxes.

Consider that Flexi-pave does not girdle the tree trunk since the rubber granules self-prune as the tree grows larger. Therefore we are able to decompact the soil and then pave over the barren ground right up next to the tree to protect it from further foot traffic compaction. The trees prefer this to bare dirt or weeds. The Flexi-pave acts like permanent mulch over the critical root zone allowing water and air to pass through but preventing rutting, compaction or contamination.

I agree, visually it takes some getting used to though since we are so accustomed to seeing 4x10 tree boxes filled with weeds or trash.

Great idea about putting up signs explaining what this is and why it it extends up to the tree. The Georgetown tree pits already have signs on them and signs for these new areas will be forthcoming as well.


by Noble on Jul 18, 2013 10:04 am • linkreport

what of symbiotic organisms?

(insects, companion plants, good bacteria, micorhyzzae, etc.)

by DB on Jul 19, 2013 8:05 am • linkreport

Excellent question!

The biological component of the soil is often the most missunderstood and neglected. Arborists have long been fertilizing trees but many studies have shown that man made fertilizers can actually make trees more susceptable to insect and disease attacks!

Compost tea on the other hand, which contains millions of beneficial microorganisms such as bacteria, nematodes, protozoa, etc improves the soil fertility naturally by putting back the beneficial microorganisms that are present in healty uncompacted forest soils but are often absent in heavily compacted urban soils.

Compost tea and mycorrhizae can be added to the soil before the paving is installed. They can also be added later by just pouring it through the porous paving.

by Noble on Jul 19, 2013 11:42 am • linkreport

Next time they should instead build sidewalks around the trees . That area had to have sidewalks when the trees was just growing (30-100 years ago) so there should be no reason to why there was not a consideration about the tree growing and providing more space for it.

A) Remove the tree
B) extend the sidewalk the right (in the process purchasing the land from the homer owner)
C) extend the sidewalk around the tree on all sides

by kk on Jul 20, 2013 2:59 am • linkreport

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