Water: Best Management Practices
Porous Asphalt Pavement
Porous asphalt, also known as pervious, permeable, "popcorn," or open-graded asphalt, is standard hot-mix asphalt with reduced sand or fines and allows water to drain through it. Porous asphalt over an aggregate storage bed will reduce stormwater runoff volume, rate, and pollutants. The reduced fines leave stable air pockets in the asphalt. The interconnected void space allows stormwater to flow through the asphalt as shown in Figure 1, and enter a crushed stone aggregate bedding layer and base that supports the asphalt while providing storage and runoff treatment. When properly constructed, porous asphalt is a durable and cost competitive alternative to conventional asphalt.
Porous asphalt can be used for municipal stormwater management programs and private development applications. The runoff volume and rate control, plus pollutant reductions, allow municipalities to improve the quality of stormwater discharges. Municipal initiatives, such as Portland's Green Streets program (Figure 2), use porous asphalt to reduce combined sewer overflows by infiltrating and treating stormwater on site. Private development projects use porous asphalt to meet post-construction stormwater quantity and quality requirements. The use of porous asphalt can potentially reduce additional expenditures and land consumption for conventional collection, conveyance, and detention stormwater infrastructure.
Porous asphalt can replace traditional impervious pavement for most pedestrian and vehicular applications. Open-graded asphalt has been used for decades as a friction course over impervious asphalt on highways to reduce noise, spray, and skidding. Highway applications with all porous asphalt surfacing have been used successfully for highway pilot projects in the United States, but, generally, porous asphalt is recommended for low volume and low speed applications (Hossain et al., 1992). Porous asphalt performs well in pedestrian walkways, sidewalks, driveways, parking lots, and low-volume roadways. The environmental benefits from porous asphalt allow it to be incorporated into municipal green infrastructure and low impact development programs. The appearance of porous asphalt and conventional asphalt is very similar. The surface texture of porous asphalt is slightly rougher, providing more traction to vehicles and pedestrians.
Siting and Design Criteria
Porous asphalt should be designed and sited to intercept, contain, filter, and infiltrate stormwater on site. Several design possibilities can achieve these objectives. For example, porous asphalt can be installed across an entire street width or an entire parking area. The pavement can also be installed in combination with impermeable pavements or roofs to infiltrate runoff. Several applications use porous asphalt in parking lot lanes or parking stalls to treat runoff from adjacent impermeable pavements and roofs. This design economizes porous asphalt installation costs while providing sufficient treatment area for the runoff generated from impervious surfaces. Inlets can be placed in the porous asphalt to accommodate overflows from extreme storms. The stormwater volume to be captured, stored, infiltrated, or harvested determines the scale of permeable pavement required.
|Table 1. Asphalt Mix |
|Sieve Size||% Passing|
|Percent bituminous asphalt 5.75-6.0% by weight|
Porous asphalt comprises the surface layer of the permeable pavement structure and consists of open-graded coarse aggregate, bonded together by bituminous asphalt. A typical reduced fines asphalt mix is shown in Table 1. Polymers can also be added to the mix to increase strength for heavy load applications. The thickness of porous asphalt ranges from 2 to 4 inches depending on the expected traffic loads. For adequate permeability, the porous asphalt should have a minimum of 16% air voids. Additional subsurface components of this treatment practice are illustrated in Figure 3 and include the following (NAPA, 2008):
- Choke course - This permeable layer is typically 1 - 2 inches thick and provides a level and stabilized bed surface for the porous asphalt. It consists of small-sized, open-graded aggregate.
- Open-graded base reservoir - This aggregate layer is immediately beneath the choke layer. The base is typically 3 - 4 inches thick and consists of crushed stones typically 3/4 to 3/16 inch. Besides storing water, this high infiltration rate layer provides a transition between the bedding and subbase layers.
- Open-graded subbase reservoir - The stone sizes are larger than the base, typically ¾ to 2 ½ inch stone. Like the base layer, water is stored in the spaces among the stones. The subbase layer thickness depends on water storage requirements and traffic loads. A subbase layer may not be required in pedestrian or residential driveway applications. In such instances, the base layer is increased to provide water storage and support.
- Underdrain (optional) - In instances where porous asphalt is installed over low-infiltration rate soils, an underdrain facilitates water removal from the base and subbase. The underdrain is perforated pipe that ties into an outlet structure. Supplemental storage can be achieved by using a system of pipes in the aggregate layers. The pipes are typically perforated and provide additional storage volume beyond the stone base.
- Geotextile (optional) - This can be used to separate the subbase from the subgrade and prevent the migration of soil into the aggregate subbase or base.
- Subgrade - The layer of soil immediately beneath the aggregate base or subbase. The infiltration capacity of the subgrade determines how much water can exfiltrate from the aggregate into the surrounding soils. The subgrade soil is generally not compacted.
The same equipment can be used for mixing and laying permeable asphalt as conventional asphalt. The method for laying the asphalt will also be similar. During compaction of the asphalt, minimal pressure should be used to avoid closing pore space. Vehicular traffic should be avoided for 24 to 48 hours after pavement is installed.
Specific Design Considerations and Limitations
The load-bearing and infiltration capacities of the subgrade soil, the infiltration capacity of the porous asphalt, and the storage capacity of the stone base/subbase are the key stormwater design parameters. To compensate for the lower structural support capacity of clay soils, additional subbase depth is often required. The increased depth also provides additional storage volume to compensate for the lower infiltration rate of the clay subgrade. Underdrains are often used when permeable pavements are installed over clay. In addition, an impermeable liner may be installed between the subbase and the subgrade to limit water infiltration when clay soils have a high shrink-swell potential, or if there is a high depth to bedrock or water table (Hunt and Collins, 2008).
Measures should be taken to protect permeable pavement from high sediment loads, particularly fine sediment. Appropriate pretreatment BMPs for run-on to permeable pavement include filter strips and swales. Preventing sediment from entering the base of permeable pavement during construction is critical. Runoff from disturbed areas should be diverted away from the permeable pavement until they are stabilized.
Several factors may limit permeable pavement use. Porous asphalt has reduced strength compared to conventional asphalt and will not be appropriate for applications with high volumes and extreme loads. It is not appropriate for stormwater hotspots where hazardous materials are loaded, unloaded, stored, or where there is a potential for spills and fuel leakage. For slopes greater than 2 percent, terracing of the soil subgrade base may likely be needed to slow runoff from flowing through the pavement structure.
Key Siting and Maintenance Issues:
The most prevalent maintenance concern is the potential clogging of the porous asphalt pores. Fine particles that can clog the pores are deposited on the surface from vehicles, the atmosphere, and runoff from adjacent land surfaces. Clogging will increase with age and use. While more particles become entrained in the pavement surface, it does not become impermeable. Studies of the long-term surface permeability of porous asphalt and other permeable pavements have found high infiltration rates initially, followed by a decrease, and then leveling off with time (Bean, et al., 2007a). With initial infiltration rates of hundreds of inches per hour, the long-term infiltration capacity remains high even with clogging. When clogged, surface infiltration rates usually well exceed 1 inch per hour, which is sufficient in most circumstances for the surface to effectively manage intense stormwater events (ICPI, 2000). Permeability can be increased with vacuum sweeping. In areas where extreme clogging has occurred, half inch holes can be drilled through the pavement surface every few feet or so to allow stormwater to drain to the aggregate base. A stone apron around the pavement connected hydraulically to the aggregate base and subbase can be used as a backup to surface clogging or pavement sealing.
Due to the well draining stone bed and deep structural support of porous asphalt pavements, they tend to develop fewer cracks and potholes than conventional asphalt pavement. When cracking and potholes do occur, a conventional patching mix can be used. Freeze/thaw cycling is a major cause of pavement breakdown, especially for parking lots in northern climates. The lifespan of a northern parking lot is typically 15 years for conventional pavements; porous asphalt parking lots can have a lifespan of more than 30 years because of the reduced freeze/thaw stress (Gunderson, 2008).
In cold climates, sand should not be applied for snow or ice conditions. However, snow plowing can proceed as with other pavements and salt can be used in moderation. Porous asphalt has been found to work well in cold climates as the rapid drainage of the surface reduces the occurrence of freezing puddles and black ice. Melting snow and ice infiltrates directly into the pavement facilitating faster melting (Gunderson, 2008).
Cold weather and frost penetration do not negatively impact surface infiltration rates. Porous asphalt freezes as a porous medium rather than a solid block because permeable pavement systems are designed to be well-drained; infiltration capacity is preserved because of the open void spaces (Gunderson, 2008). However, plowed snow piles should not be left to melt over the porous asphalt as they can receive high sediment concentrations that can clog them more quickly.
Permeable pavements do not treat chlorides from road salts but also require less applied deicers. Deicing treatments are a significant expense and chlorides in stormwater runoff have substantial environmental impacts. Reducing chloride concentrations in runoff is only achieved through reduced application of road salts because removal of chloride with stormwater BMPs is not effective. A porous asphalt lot installed at the University of New Hampshire required 25% of the salt routinely applied to other impervious asphalt lots for equivalent deicing. No salt application was required for the porous pavement to have an equivalent friction factor and traction than normally treated conventional pavements because porous pavement has higher frictional resistance than conventional pavement (UNHSC, 2007).
All permeable pavements, including pervious asphalt, are on-site stormwater management practices and will have the same or very similar effectiveness with regards to the reduction of the volume and rate of stormwater runoff as well as pollutant concentrations. Porous asphalt, pervious concrete, and permeable pavers all have the same underlying stormwater storage and support structure. The only difference is the permeable surface treatment. The choice of permeable surface is relevant to user needs, cost, material availability, constructability, and maintenance, but it has minimal impact on the overall stormwater retention, detention, and treatment of the system.
Permeable pavement transforms areas that were a source of stormwater to a treatment system and can effectively reduce or eliminate runoff that would have been generated from an impervious paved area. Because it reduces the effective impervious area of a site, permeable pavement should receive credit for pervious cover in drainage system design. The infiltration rate of properly constructed pervious asphalt and base generally exceeds the design storm peak rainfall rate; the subsoil infiltration rate and base storage capacity are the factors determining stormwater detention potential. Table 2 provides monitored reductions in stormwater volumes via storage and infiltration.
|Table 2. Volume Retention of Permeable Pavements|
|Application||Location||Soil Type||Underdrain||Volume Retention|
|Parking lot||State College, PA||--||--||Retained the 25 yr - 24 hr storm|
|Park Lot*||Durham, NH||Clay||Underdrain||25%|
|Permeable Interlocking Concrete Pavers|
|Residential street||Auckland, New Zealand||Clay||Yes||60%|
|Field and laboratory tests||Guelph, Ontario, Canada||--||--||90%|
|Parking lot||Swansboro, NC||Sandy soil||No||100%|
|Parking lot||United Kingdom||Impermeable liner installed||Yes||34% - 45%|
|Parking lot||Renton, WA||---||No||100%|
|Parking lot||Kinston, NC||Clay||No||55%|
|Residential streets and sidewalk||Sultan, WA||--||--||100%|
|Parking lot||Kingston, NC||Clay||No||99.9%|
|*System designed to collect infiltrated stormwater in underdrain for monitoring purposes. (Legret, M. & Colandini, 1999)(Cahill et al., 2003)(Roseen and Ballestero, 2008)(Fassman and Blackbourn, 2006)(Bean, et al., 2007a)(Bean et al., 2007b)(Pratt, 1999)(Booth and Leavitt, 1999)(Brattebo and Booth, 2003)(Collins, et al., 2008)(WA Aggregates & Concrete Association, 2006)(Collins, et al., 2008)|
Permeable pavement reduces pollutant concentrations through several processes. The aggregate filters the stormwater and slows it sufficiently to allow sedimentation to occur. The subgrade soils are also a major factor in treatment. Sandy soils will infiltrate more stormwater but have less treatment capability. Clay soils have a high cation exchange capacity and will capture more pollutants but will infiltrate less. Also, studies have found that in addition to beneficial treatment bacteria in the soils, beneficial bacteria growth has been found on established aggregate bases. In addition, permeable pavement can process oil drippings from vehicles (Pratt et al., 1999). Table 3 provides measured pollutant removals from pervious pavement structures.
|Table 3. Monitored Pollutant Removals of Permeable Pavement|
|Highway (friction course only)||Austin, TX||94%||76-93%||43%|
|Parking lot||Durham, NH||99%||Zn: 97%||TP: 42%|
|Permeable Interlocking Concrete Pavers|
|Driveways||Jordan Cove, CT||67%||Cu: 67%|
|Parking lot||Goldsboro, NC||71%||Zn: 88%||TP: 65%|
|Parking lot||Renton, WA||---||Cu: 79%|
|Parking lot||King College, ON||81%||Cu: 13%|
|Parking lot||Tampa, FL||91%||75-92%||--|
|(Barrett et al., 2006)(UNHSC, 2007)(Bean, et al., 2007b)(Clausen and Gilbert, 2006)(Van Seters/TRCA 2007)(Rushton, 2001)|
Permeable pavement water quantity and pollutant reduction characteristics such as 80 percent total suspended solids reductions can qualify it to earn credits under green or sustainable building evaluation systems such as Leadership in Energy and Environmental Design (LEED®) and Green Globes. Credits also can be earned for water conservation and conservation of materials by utilizing some recycled materials and regional manufacturing and resource use.
Several factors influence the overall cost of porous asphalt:
- Material availability and transport - The ease of obtaining construction materials and the time and distance for delivery.
- Site conditions - Accessibility by construction equipment, slope, and existing buildings and uses.
- Subgrade - Subgrade soils such as clay may result in additional base material needed for structural support or added stormwater storage volume.
- Stormwater management requirements - The level of control required for the volume, rate, or quality of stormwater discharges will impact the volume of treatment needed.
- Project size - Larger porous asphalt areas tend to have lower per square foot costs due to construction efficiencies.
Costs vary with site activities and access, porous asphalt depth, drainage, curbing and underdrains (if used), labor rates, contractor expertise, and competition. The cost of the porous asphalt material ranges from $0.50 to $1 per square foot (NCHRP, 2005).
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Interlocking Concrete Pavement Institute, Permeable Interlocking Concrete Pavements - Design, Specification, Construction, Maintenance, Third Edition, 2000.
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Van Seters, T., Performance Evaluation of Permeable Pavement and a Bioretention Swale Seneca College, King City, Ontario, Interim Report #3, Toronto and Region Conservation Authority, Downsview, Ontario, May 2007.
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