Maintaining Porous Pavement

Over the last five years, pervious pavement, also known as porous asphalt and porous concrete has increased its share of preferred material for municipals and others looking to reduce pollutants in groundwater runoff. It has also become a solution for those with limited land by freeing up space that would otherwise be used for stormwater management.
The National Asphalt Pavement Association (NAPA) attributes the pervious pavement’s increase in popularity to recent changes in stormwater regulations, which prompted many consulting engineers and public works officials to seek information about them.

HOW IT WORKS

Stormwater Center defines it as, “Porous pavement is a permeable pavement surface with an underlying stone reservoir that temporarily stores surface runoff before infiltrating into the subsoil. This porous surface replaces traditional pavement, allowing parking lot runoff to infiltrate directly into the soil and receive water quality treatment.”
According to NAPA, “the technology is really quite simple. The secret to success is to provide the water with a place to go, usually in the form of an underlying, open-graded stone bed. As the water drains through the porous asphalt and into the stone bed, it slowly infiltrates into the soil. The stone bed size and depth must be designed so that the water level never rises into the asphalt. This stone bed, often 18 to 36 inches in depth, provides a tremendous subbase for the asphalt paving.”
This means there are millions of tiny holes in the surface. If you think about a pumice stone, the more you use it, the smoother it gets as thousands of dead cells fill up the holes. Dirt and other debris can fill up those holes in the asphalt and clog it up, which means keeping it clean is a priority.

MAINTENANCE

Cahill Associates, an engineering consulting firm in Penn., has constructed hundreds of parking lots around the country. In a document prepared for San Diego County, entitled “Porous Pavement Operation and Maintenance Protocol” Cahill recommends “vacuuming porous asphalt and concrete pavement with a vacuum sweeper on a biannual basis. Acceptable types of vacuum sweepers include the Elgin Whirlwind and the Allianz Model 650.
“Though much less effective than pure vacuum sweepers, regenerative air sweepers, such as the Tymco Model 210, Schwarze 348, Victory, and others, are sometimes used. These units contain a blower system that generates a high velocity air column, which forces the air against the pavement at an angle, creating a ‘peeling’ or ‘knifing’ effect. The high volume air blast loosens the debris from the pavement surface, then transports it across the width of the sweeping head and lifts it into the containment hopper via a suction tube. Thus, sediment and debris are loosened from the pavement and sucked into the unit.”
Cahill does not recommend simple broom sweepers for porous pavement maintenance.
When the pavement has become significantly clogged, Cahill recommends washing with clean, low-pressure water, followed by immediate vacuuming. “Combinations of washing and vacuuming techniques have proved effective in cleaning both organic clogging as well as sandy clogging. Research in Florida found that a power head cone nozzle that concentrated the water in a narrowly rotating cone worked best.”
They warn that if the pressure is too great, contaminants may be driven further into the porous surface and encourage maintenance crews to determine the most effective strategy of cleaning their porous installations.
“For smaller installations, such as sidewalks, plazas, or small parking lots, walk-behind vacuum units may prove most effective. Though these units can be loud and somewhat messy to the operator due to the lack of dust suppression, they are also relatively easy to operate and inexpensive. Examples of acceptable walk-behind units include the Billy Goat models, the 5700 industrial-strength Scrubber by Tennant, and the sidewalk class vacuum sweepers made by Nilfisk, Advance and Hako. If walk-behind units are used, it is recommended that the scrub pressure be kept relatively low. The dirtiest areas may need to be power washed after scrubbing to get out the dirt that has been deeply ground in.”

WINTER MAINTENANCE

“A porous pavement system with subsurface aggregate bed has superior snow melting characteristics than does standard pavement. Therefore, ice and light snow accumulation are generally not as problematic. However, snow will accumulate during heavier storms.
“Abrasives such as sand or cinders should not be applied on or adjacent to the porous pavement. Snow plowing is necessary for significant snow accumulation, but should be done carefully (i.e. by setting the blade slightly higher than usual, about an inch).
“Standard road salt is acceptable for use as a deicer on porous pavement, although a non-toxic, organic deicer, applied either as a blended, magnesium chloride-based liquid product or as pretreated rock salt, is recommended. Acceptable liquid deicers include Magic-O, Ice B’ Gone, Ice Ban, and Geomelt, among others. Magic Salt is an example of an acceptable pretreated salt product. Other acceptable deicer alternatives to standard sodium chloride include calcium chloride, magnesium chloride, potassium chloride, urea, and calcium magnesium acetate. Follow supplier recommendations when applying deicers to pavement.”

story by Jennifer Taylor

RESOURCES
National Asphalt Pavement Association: www.asphaltpavement.org
Elgin: www.elginsweeper.com
Allianz: www.allianzmadvac.com
Tymco: www.tymco.com
Schwarze: www.schwarze.com
Billy Goat: www.billygoat.com
Tennant: www.tennantco.com
Nilfisk: www.nilfisk.com
Advance: www.advance-us.com
Hako: www.hako.co.uk
Magic-O: www.magicsaltproducts.com
Ice B’ Gone: www.icebgmagic.com
Ice Ban: www.ecosurfacesolutions.com
Geomelt: www.snisolutions.com