Porous asphalt—the pavement that cleans

Porous asphalt - the pavement that cleansBy Dave Johnson, P.E.

Pavement designers have traditionally attempted to create pavements that are impervious to water and air. It has long been recognized that such pavements resist premature aging from moisture damage and oxidation.

Sometimes asphalt pavements compact under traffic and consequently rut. For this reason, a design air void content of 4 percent has long been targeted to avoid these distresses. All of this gets turned on its head when the design is for a porous pavement.

Porous pavements are designed, as the name implies, to allow for the movement of water through the structure. Two principle uses for this specialty pavement are fairly common. The first application is as a surface for highways of all traffic volumes. In this configuration it is known as a porous friction course or an open-graded friction course.

The second application, and the focus of this article, is a part of an integrated stormwater management plan for a parking lot or subdivision. In this application, stormwater is allowed to infiltrate through the pavement into a single-sized stone reservoir where it will then infiltrate into the subsoil. By moving through these structures the water is filtered and cleaned.

Applicable uses
As is the case with most engineered systems, a porous pavement has its place in the engineer’s or architect’s toolbox. Some of the potential application opportunities for porous pavements are parking lots, urban areas, locations with limited space for drainage structures, a facility seeking LEED® certification and anywhere conditions warrant its consideration. On the other hand, areas that may call for caution include locations that are at risk to plugging due to sanding operations for winter maintenance; adjacent to dust producers such as beaches or tilled fields; areas with a naturally high water table or areas with low permeable in-situ soils.

Design considerations
Design considerations for a potential porous pavement installation should begin with an evaluation of the local conditions. The designer should determine the depth to groundwater at the site and determine the permeability of the native soils. Sufficient cover above the seasonally high groundwater table should be at least two feet. Recommendations on the appropriate range for permeability vary from the broadest of 0.1-10 inches/hour to a narrower 0.5-3 inches/hour. It is advisable to investigate the presence of any drinking water wells. Typically this is not an issue for an urban installation, but sufficient distance should be maintained in the event there is a well in the area. If so, a minimum 100 foot offset from the well is desirable.

Discussion is warranted regarding the design of the recharge bed. Sizing of the bed will be dependent on the design storm which, according to the EPA, is typically a 6-month, 24-hour event, although local requirements need to be verified. A typical recharge bed is 1-3 foot deep. These are constructed with an awareness of the need to maintain a design void space of approximately 40 percent. Attention to this need should steer contractors to the use of equipment and practices that minimize compaction. Aggregate is typically a crushed AASHTO No. 3 stone and is built in lifts of about 8-inch thicknesses. The bed is usually lined with a geotextile fabric to prevent the infiltration of fine material into the recharge bed which could clog it. On top of the recharge bed a choker material may be placed to create a more suitable platform for paving operations. This material will typically be either a 1/2- or 3/4-inch crushed rock and will be about 3 inches in thickness. Choker material should be drainable, so it too is primarily a single-size aggregate.

For the design of the porous hot mix two primary differences from a typical hot mix stand out. First, to achieve the desired porosity and thus fluid flow, 15-20 percent air voids are sought. This is accomplished by choosing a lab compaction level of about 50 gyrations on a Superpave gyrator compactor, and the second difference is an engineered gradation that is gap graded. For assistance on selecting such a gradation, most state DOT specifications for open-graded friction courses are useful. Nominal maximum aggregate of either 1/2- or 3/8-inch with a maximum percent passing a #4 sieve of 35 percent, 15 percent for #8 and 2 percent for the #200 sieve should create proper hydraulic conductivity. Optimum asphalt binder tends to produce high film thicknesses so polymer-modified binders and perhaps the introduction of fibers may be warranted to combat drain down.

Constructing porous pavements
Construction of porous pavements is best done with a gentle hand. The goal is not to beat the materials into a confined state. Rather, it is to seat the materials to provide adequate structure to carry the loads while maintaining sufficient openness to allow for fluid flow. No specialty equipment is needed to construct porous pavements. In selecting a paver, a track paver with their softer footprint is advisable. Generally, only one steel drum roller is needed. It is to make two to four passes on each lift in static mode. This single roller is performing both breakdown and finishing operations. Attention to the rolling behavior may indicate that letting the material cool some before rolling could be needed. The goal of compaction is to maintain adequate void space while seating the material.

Maintaining porous pavements
Just as the design and construction of porous pavements are unique, so are the maintenance requirements for these surfaces. Failures of porous pavements generally come from clogging due to inadequate maintenance. To alleviate the possibility of clogging, cleaning operations are needed. Vacuuming the surface twice a year with a pressure washing follow-up will generally suffice. Porous pavements should never receive a sealcoat. If significant additional construction is to occur in the area, delay the placement of the porous pavement until after this is completed if possible. Patching and crack filling operations should also occur as is typical for conventional pavements in the area. A recommended maximum of 10 percent of the surface area can be sealed with patches or sealant.

In areas that require snow removal, porous pavements again have unique requirements. Sanding operations must be avoided as they will clog the pavement’s pores. Lighter use of deicing salts than is typical for the area is also advisable for two reasons. First, porous pavements tend to deice faster naturally so they do not need as much salt to aid with deicing. Second, aggressive salting can affect the salinity of local aquifers via the recharge mechanism.

Benefits to storm water quality
Research in Europe and stateside has consistently confirmed the environmental advantages of porous pavement. According to a University of New Hampshire study, porous pavements are highly effective at removing suspended solids, phosphorous, zinc and petroleum hydrocarbons. Similar results have been documented by other research over the last 20 years. With the slow release from the recharge bed, aquifers are provided with a renewal source. Moreover, with the use of the parking area as its own drainage facility, traditional drainage via curb and gutter, drop inlets, etc. is not required.

The differences between traditional hot mix asphalt and porous asphalt pavements are notable. The criteria in which they are designed, the techniques employed during construction and their maintenance while they are in service standout as key differences. The benefits of porous pavements are well documented. Pollutant removal, reduced need for traditional drainage structures and aquifer recharge have all been noted. With proper attention in the design, construction and maintenance stages of its life, a porous pavement can offer quality service for many years.