A Wisconsin community sees benefits in a different mix of asphalt.
by Stuart Macdonald, Chair, National Assoc. of State Trail AdministratorsNovember 01, 2011
While not a new material, “porous asphalt” is gaining interest among trail and greenway planners. Porous asphalt refers to a mix that adds a significant amount of airspace, or voids, to the gravel and petroleum binders that make up asphalt paving.
The material is promising because it combines the ability to drain away moisture with a firm surface for wheeled users. Porous asphalt may also save on maintenance costs by hastening snowmelt and improving drainage. And finally, there is evidence that a porous asphalt overlay may be the right treatment for blacktop trails that are subject to cracking.
As the search for the perfect trail surface continues, many different materials have been used on rail trails, creek-side greenways, and park trail systems. The City of Middleton, Wisconsin is a good example. The city manages a typical community trail system along the Pheasant Branch Creek Corridor Trail, which connects two major arterial roadways on opposite ends of the city. Two school properties adjoin the trail corridor, which is also used by bicycle commuters, joggers, dog walkers, senior citizen exercise groups, the high school cross country team, and families looking for a natural setting.
The challenge was to improve an existing 1.2-mile crushed rock trail along the creek bed. This heavily trafficked route suffered from washouts, severe erosion, flooding, steep grades, and blind corners, as well as three dangerous stream crossings which were impassable in winter months. Project goals centered on creating a year-round, fully accessible, safe, and environmentally friendly multi-use trail to better serve the entire community.
Construction for the improvement project was done in the summer of 2009. Portions of the trail were re-graded to improve accessibility. The width of the paved trail was designed to a 10’ minimum with a 4’ crushed limestone shoulder, where topography allowed, to accommodate those who prefer a natural running surface. Porous asphalt was then used to pave the entire length of the trail.
Penni Klein of Middleton’s Public Lands Department championed the idea of using porous asphalt as the best solution for the trail. Blake Theisen, a landscape architect with SAA Design Group, Inc., was part of the design team that worked on the project. “You really can’t tell the trail is over two years old,” Theisen told us, “it looks brand new even after two Wisconsin winters.”
Some of the benefits of using porous asphalt:
The asphalt mixture includes recycled shingles, carpet fibers, and rubber, all from a local source. Rubber tires, in particular, are suitable for adding to porous asphalt. The materials are ground up as part of the emulsion which gives flexibility to the surface. In addition, any asphalt paving that may be removed can be ground up and recycled in the new mix.
Maintaining a year-round usable trail surface is important to community trail systems. Snow removal, however, is a big expense where winters are cold and overcast. In Middleton, trail managers monitored three trail sections comparing existing asphalt and crushed rock trails with a section of porous asphalt. It was clear that ice and snow tended to linger on the traditional surfaces while melting away quickly on the porous asphalt surface. Apparently, as some melting takes place during the day, the water re-freezes on the impervious asphalt. The porous material is able to absorb a great deal of water as it melts, gradually leaving the surface ice-free.
Preserving the existing wetland area was an essential aspect of the project. Many concerns were raised about possible leaching or pollution from the porous material. Several studies have looked at the water quality treatment that occurs at the geotextile soil interface and concluded that removal of most pollutants is very good. Porous asphalt also provides credits towards LEED Certification of construction projects.
Production cost for porous asphalt for this project was $10-$15 more per ton than regular asphalt (AC, fiber, rubber, and polymer additive). However, the porous material spreads 10-12% farther than regular asphalt because of the large air voids. The conclusion was the cost difference is minimal.
By reducing the need for winter plowing, labor, and equipment, Middleton’s cost savings amount to $3,500 per year compared to regular asphalt trails. Maintenance costs for the crushed rock trail are about $5,000 per year compared to only $300 per year for the porous pavement. The trail is cleaned a couple of times a year with sweepers or blowers to remove accumulations of leaves or dirt.
The City of Middleton has found another promising use for porous asphalt as an overlay for existing asphalt trails that have begun to deteriorate. While the old impermeable layer will not let much moisture through, the real benefit is the flexibility of the porous overlay. Even where unstable soils keep shifting the trail base, cracks do not seem to show through the top layer of porous asphalt.
Trail visitors have been happy with the trails. Density tests show that the porous asphalt mixture is approximately 20% softer than standard asphalt or concrete, which may reduce stress on runners’ joints. The trail also provides an accessible surface for strollers and wheelchairs, meeting the “firm and stable” criteria.
Porous asphalt (“Open Graded Asphalt Concrete”) is created by eliminating the smaller, graduated sizes of crushed rock and using a uniform-sized aggregate. The particle size is typically 1/2” or 3/8” with a goal of 18% air voids. A slurry of recycled rubber and polymer binder replaces much of the petroleum used in standard asphalt mixes. Impervious asphalt and porous asphalt both use the same mixing and application equipment.
Porous asphalt mixes were developed in the 1930s and 1940s by State DOTs. With the development of geotextiles, the current design process has been used since the 1980s. The most common application is for large parking lots and alleys where urban runoff can be greatly reduced.