Permeable asphalt is an innovative pavement material that allows stormwater to drain through it, leading to improved stormwater management, water quality, and environmental sustainability.
Unlike traditional impervious asphalt, permeable asphalt contains pores and voids that allow water to infiltrate through the surface into an underlying stone reservoir, where it can infiltrate into the subgrade soils below. This unique characteristic provides numerous engineering, environmental, and economic benefits compared to conventional pavement.
How Permeable Asphalt Works
Permeable asphalt contains a system of interconnected voids that drain water vertically through the pavement surface. It is composed of standard asphalt binder mixed with carefully graded aggregates to form a stable, porous structure. A typical permeable asphalt cross-section is composed of:
- Open-graded asphalt surface course 1.5-4 inches thick
- Aggregate base/reservoir 4-12 inches thick
- Subgrade/native soil
As rainfall lands on the permeable surface, it flows through the porous asphalt and into the aggregate base below. The reservoir stores stormwater temporarily while allowing it to infiltrate the underlying native soil over time. Some permeable pavements have an optional impermeable liner to prevent infiltration in certain scenarios. A key advantage of permeable asphalt is its ability to manage stormwater through detention, infiltration, and evaporation.
Stormwater Management Benefits
Permeable asphalt provides exceptional stormwater management capabilities compared to impervious pavements. Key benefits include:
Reduced Runoff Volumes: By allowing infiltration, permeable asphalt pavements can reduce stormwater runoff volumes significantly:
| Surface Type | Runoff Coefficient |
|---|---|
| Conventional Asphalt | 0.70-0.95 |
| Permeable Asphalt | 0.10-0.50 |
Lower Peak Discharge Rates: Permeable asphalt depressions storage in the pavement system attenuates peak runoff discharge rates:
| Surface Type | Peak Discharge Rate (cfs/acre) |
|---|---|
| Conventional Asphalt | 2-5 |
| Permeable Asphalt | 0.2-2 |
Recharge of Groundwater: Allowing infiltration recharges groundwater supplies and aquifers instead of discharging runoff into surface waters.
Smaller Stormwater Infrastructure: With reduced runoff rates and volumes, the need for retention ponds, storm sewers, and other infrastructure is lessened. This results in cost savings.
Treatment of Stormwater Pollutants: Physical, chemical, and biological processes in the pavement profile remove sediments, metals, nutrients, bacteria, and other pollutants from infiltrating stormwater.
Overall, implementing permeable asphalt is an effective way to restore pre-development hydrology in urban areas and reduce the negative impacts of stormwater runoff.
Water Quality Benefits
As stormwater infiltrates through permeable asphalt, pollutants are filtered out, leading to improvements in downstream water quality. Key water quality benefits include:
- Sediment/TSS Removal: Over 80% of total suspended solids (TSS) are filtered out from stormwater infiltrating permeable asphalt.
- Nutrient Removal: 50-90% reductions in total nitrogen and total phosphorus have been observed.
- Metal Removal: High removal rates of lead (65-98%), zinc (71-82%), and other metals are achievable.
- Bacteria Removal: Fecal coliform bacteria reductions of 70-99% are feasible.
- Hydrocarbon Removal: Oils, grease, and organic pollutants are also effectively treated.
Overall, the filtration effects of permeable asphalt provide substantial stormwater treatment that protects downstream aquatic environments. It can be a vital green infrastructure strategy for stormwater quality control.
Other Benefits
Beyond stormwater management and water quality, permeable asphalt also provides other sustainability benefits, including:
- Flood Control: By reducing runoff volumes and rates, permeable asphalt can mitigate urban flooding issues.
- Erosion Control: Lower discharge velocities prevent erosion and damage to streams/channels.
- Improved Safety: Pavement drains rapidly after rain, reducing hydroplaning risks and improving visibility
- Reduced Heat Island Effects: Open-graded surfaces have lower thermal conductivity, reducing urban heat islands.
- LEED Credits: Permeable asphalt can assist projects in achieving LEED green building credits.
- Aesthetics: Variations in color/texture provide aesthetically pleasing surfaces.
Overall, permeable asphalt is a versatile, eco-friendly pavement material with sustainability advantages beyond just stormwater management.
Design Considerations
While permeable asphalt offers tremendous benefits, it also requires careful design considerations:
- Native Soil Testing: Infiltration rates of subgrade soils must be adequate to drain water through the pavement profile and recharge groundwater.
- Loading/Traffic: Applications are best suited for parking lots, sidewalks, trails, and low to medium-traffic roadways. Heavy traffic can clog pores.
- Site Topography: Slopes should be flat or gentle to allow proper infiltration through the pavement. Maximum slope limits are typically 5%.
- Cold Climate Design: Reservoir base depths should be increased by 18-24 inches for freeze/thaw protection in cold climates.
- Construction: Proper materials, techniques, and QA/QC testing are needed during paving operations. Compaction must be minimized to maintain permeability.
- Maintenance: Periodic vacuum sweeping and preventive maintenance help prevent surface clogging.
With good engineering and construction practices, permeable asphalt can be successfully implemented in appropriate applications.
Comparative Costs
Permeable asphalt tends to have a moderately higher initial construction cost compared to conventional asphalt. However, the ability to significantly reduce stormwater infrastructure needs results in potential cost savings and benefits over the long term. Typical comparative costs are:
| Pavement Type | Initial Construction Cost | Life Cycle Cost |
|---|---|---|
| Conventional Asphalt | $2 – $4 psf | $5 – $10 psf |
| Permeable Asphalt | $4 – $6 psf | $3 – $8 psf |
Although permeable asphalt has a higher upfront cost, smaller stormwater infrastructure needs compared to traditional asphalt and concrete can offset these expenses over the design life. Sustainable pavement selection requires lifecycle cost analysis.
Applications and Case Studies
Permeable asphalt is being used successfully in a wide variety of applications such as:
- Parking lots
- Sidewalks and pathways
- Driveways
- Low-volume roads and alleys
- Tennis courts, basketball courts and playgrounds
- Golf cart paths
Some example project case studies showing the benefits of permeable asphalt include:
Parking Lot Retrofit (Edison, NJ)
- 61,000 sf existing parking lot retrofitted with permeable asphalt
- Reduced runoff volume by over 1 million gallons per year
- Eliminated the need for detention basins and storm sewer infrastructure
- $400,000 construction cost offset by $250,000 savings in stormwater infrastructure
Neighborhood Street Reconstruction (Kensington, MD)
- 7 residential streets (total 4000 lf) rebuilt with permeable asphalt
- Reduced runoff volume by 72% and peak discharge rate by 99%
- Eliminated flooding, potholes, and icy conditions
- Improved neighborhood aesthetics and property values
Drive-Through Restaurant (Orlando, FL)
- 13500 sf restaurant parking built with permeable asphalt
- Onsite infiltration reduced runoff volume by 100%
- Saved $150,000 by eliminating the need for a retention pond
- Enhanced safety through less ponding on rainy days
These case histories demonstrate that permeable asphalt can successfully manage stormwater in a wide range of applications while providing economic feasibility.
Key Takeaways
Permeable asphalt is a versatile, eco-friendly pavement solution that offers superior stormwater management capabilities compared to conventional pavements. It reduces runoff volumes, lowers peak discharges, enhances infiltration and groundwater recharge, and improves downstream water quality through filtration. Permeable asphalt also provides sustainability benefits related to flooding, erosion, safety, urban heat island effects, and more. Although it has a moderately higher initial cost, it is a cost-effective solution over the full life cycle due to savings in stormwater infrastructure needs. With proper design and construction, permeable asphalt can be successfully used for parking areas, roadways, pedestrian facilities, and various other applications. Overall, this innovative material is an excellent green infrastructure tool for sustainable and resilient stormwater management.