Thirsty asphalt and concrete are becoming more durable.
Permeable pavement. Pervious pavement. Porous pavement. No matter which term is used to describe them, urban surfaces that allow water to run through them to the soil below rather than pool on top of them and/or runoff of them are one promising solution to improving stormwater management and “an increasingly adopted tool in the green stormwater infrastructure toolbox,” as a recent study put it.
The benefits of sponge-like roads, sidewalks and parking lots are significant. They include urban flood mitigation, recharged groundwater supplies, improved water quality due to fewer pollutants entering local waterways, and, for drivers, improved traction during rain events.
Perhaps no one knows the advantages of permeable pavement — both asphalt and concrete —better than Mark Palmer, a senior transportation project manager with global architecture and engineering firm HDR and former city engineer and stormwater engineer for Puyallup, Washington. Puyallup and neighboring Tacoma are “the pioneers in replacing impervious surfaces with porous pavements,” said Palmer.
A key frontier: durability. Palmer said a “major shift” in focus, from infiltration capacity to durability of permeable pavement, has been underway in the drizzly Pacific Northwest.
“The primary focus of porous pavements for the first 10, 15 years of my experience with it has been on preventing clogging and keeping the pores of the pavement draining,” said Palmer. “There was a lot less focus on making it competitive with other pavements and making it a more durable, long-last pavement.”
Today, he added, “That’s where we’ve started focusing our efforts.”
The emphasis on durability has led to new compaction standards for porous hot mix and warm mix asphalt (PHMA/PHWA). Materials with a lower carbon footprint, such as recycled asphalt shingles, have been experimented with and introduced into the mix, as have higher grades of asphalt, which help to improve rut resistance.
While the up-front costs for materials and installation are higher for permeable pavement in some cases, Palmer believes it pays for itself in the end due to lower maintenance needs.
“I’m still disappointed that I haven’t been able to convince more jurisdictions and agencies to jump on board, especially with all the benefits in countering the ill effects of urbanization."
“Since pervious pavements can definitely meet the average lifecycle expectancy and can do it with obviously less maintenance — crack sealing, pothole repair and all that — it’s going to be a cost-effective pavement compared to others,” said Palmer. “The incremental cost of porous asphalt is not that much different, while pervious concrete is almost identical.”
The Green Building Alliance notes another financial benefit of permeable pavement: It can reduce the need for stormwater installations such as retention basins.
Increased recognition of the water quality benefits of porous asphalt has helped elevate its appeal, at least regionally, according to Palmer. He credits research that showed the effectiveness of porous asphalt at removing particulate pollutants including coarse sediment, nitrogen, motor oil and heavy metals, particularly lead and zinc, from runoff. The authors concluded, “Porous asphalt pavements appear to act as large filtration systems, with particulate pollutants removed at the highest rates.”
On the pervious concrete front, the use of internal curing additives such as HydroMax has dramatically improved workability. “Huge improvements” in porous concrete technology has also “allowed for standard curing, more durable saw-cut joints and tooling for curb ramps and other sloped surfaces,” Palmer noted.
Still, Palmer said acceptance of the water quality benefits of permeable pavement by governmental powers remains a challenge, as does the acceptance of their “relative costs, maintenance and environmental benefits” by transportation designers, managers and maintenance staff.
“I’m still disappointed that I haven’t been able to convince more jurisdictions and agencies to jump on board, especially with all the benefits in countering the ill effects of urbanization,” said Palmer.
“Change is hard, and people are naturally resistant to it unless they are early adopters,” he added.
Based in New York City, Matt Hickman writes about cities, sustainable design and the intersection between the natural world and the built environment.