Impervious surfaces such as parking lots and paved roads are often the causes of stormwater runoff, carrying residues into streams and waterways. Although detention basins and reservoirs are effective in slowing the rate of runoff, they require additional land. Many consulting engineers and public works officials have turned to porous asphalt pavements to promote infiltration.
What is Porous Asphalt?
Porous asphalt pavement is a specialized asphalt mixture comprised of both fine and coarse aggregates bound together by a bituminous binder. The surface appearance of porous asphalt is similar to traditional asphalt, but it has a rougher texture.
Porous asphalt is a permeable asphalt surface placed over a granular working platform on top of a large stone reservoir. The asphalt surface is made porous by building with open-graded friction coarse aggregate. The layer under the porous asphalt has the storage capacity to hold the collected water.
Uses for Porous Asphalt
Porous asphalt has many uses, including parking lots, driveways, sidewalks, bike paths, and recreational areas like playgrounds, basketball courts, and tennis courts.
Porous asphalt pavements have successfully been used for residential and urban streets.
Highway engineers also use porous asphalt surface mixes. Instead of putting the porous asphalt over a stone bed, they pour it as a thin surface layer on conventional highway pavement. The Rainwater sinks into the porous asphalt pavement surface and settles on an impermeable asphalt layer below. Water is forced to drain off to the side of the highway.
Even in a driving rainstorm, splash and spray from trucks are absorbed into the pavement. Visibility for drivers is so much better that crashes and fatalities can be significantly reduced.
How Porous Asphalt is made
Porous asphalt pavements are constructed in an open stone bed that provides drainage for water. As the water drains through to the porous asphalt into the stone bed, it slowly infiltrates the soil. The stone bed’s size and depth must be designed so that the water level never rises up into the asphalt. This stone bed, often 18–36 inches in depth, provides a sub-base for the porous asphalt paving.
The Durability of Porous Asphalt
With the appropriate design and installation, porous asphalt pavement can have a lifespan of more than 20 years. Maintenance of the asphalt surface requires regular quarterly vacuuming to prevent clogging by sediment and debris.
The pavement surface can be jet-washed or flushed to help maintain surface porosity. Be careful of high-pressure washing
Sand, ash, or salt granules should not be used on snow or ice because they may clog the pores within the pavement. De-icing compounds work well on porous asphalt.
How does porous asphalt compare to regular asphalt?
The performance of porous asphalt pavement is similar to that of most other asphalt pavements. Like other asphalt pavement, it can be designed for many situations.
The cost of a porous pavement installation is higher than the price of conventional asphalt due to its thicker nature.
Porous Asphalt is a pervious pavement designed to allow stormwater infiltration through the surface into the soil below. The water is naturally absorbed and filtered, and the pollutants are removed. In contrast, regular asphalt pavement is an impervious surface that sheds and repels rainfall and surface pollutants. The water is forced to run off paved surfaces directly into nearby storm drains and then into streams and lakes.
Benefits of Porous Asphalt Use
Porous asphalt has many benefits for land development plans because it makes better use of the land and conserves water.
Because of porous asphalt pavement’s open structure, porous asphalt provides a method to replenish water tables and waterways rather than forcing rainfall into storm sewers.
Porous asphalt also reduces demands on storm sewer systems and provides storm-water management systems that promote infiltration and improve the water quality.
The most significant benefit of porous asphalt use is that it reduces the number of pollutants carried to a storm drain or waterway. Land development has always had the detrimental effect of vastly increasing the amount of impervious surface area as the land is bulldozed and paved to create roads and parking lots.
Increased runoff from impervious surfaces causes dangerous floods, severe erosion damage to our stream channels, diminished recharge of groundwater, and degraded habitat for our fisheries. These same impervious surfaces can transport the many pollutants deposited in urban areas, such as nutrients, sediment, bacteria, pesticides, and chloride into local streams and waterways.
During a storm, water runoff flows over the pavement, picking up pollutants such as dirt, grease, and oil, and transports them to streams, waterways, and storm sewer systems. Some of these pollutants can contaminate local drinking water and waterways used by fish and wildlife.
Designers developed porous paving systems so runoff would pass through the pavement into a stone base and into the soil to replenish groundwater supplies. With proper installation and maintenance, permeable paving allows for infiltration of up to 80% of annual runoff volume.
A range of studies have shown that porous asphalt can remove 90-80% of total suspended solids TSS-, significant percentages of total metals (e.g., up to 88% of lead), and up to 90% of hydrocarbons including oil and grease from vehicles.
Research has identified permeable pavements as “cool pavement technology.” Due to their high air-void structure, porous asphalt can mitigate urban heat island effect by reducing stored pavement energy and allowing rapid cooling via evaporation.
For roadways, other significant benefits include reduced noise from tires and engines, increased wet-weather friction, improved wet-weather visibility, less tire spray, and reduced risk of hydroplaning.
Cost benefits of using Porous Asphalt
Because the porous asphalt roads are thicker, they require more materials and often cost more to install. Porous asphalt usually is 20-50% higher in unit material costs compared to traditional asphalt pavement. Despite their higher installation cost, a porous asphalt pavement can actually save you money.
Elimination of stormwater impact fees. Some local governments will charge developers storm-water impact fees when the water run-off from parking lots overflows the storm sewer system. Since porous asphalt helps reduce demands on the storm sewers, these impact fees can be eliminated.
No need for a retention pond. Porous asphalt also saves money because it eliminates the need to put in a storm-water management solution such as a retention pond. The land that would usually be used for a retention pond can be developed as a profitable asset.
Roads can be built faster than using other forms of permeable pavements because porous asphalt can be poured and rolled in less time than what is needed for concrete, which needs to be cut and cured. Less construction time leads to less road closure time and a reduction in labor costs.
Examples of Porous Asphalt used for stormwater management
Porous asphalt pavements have successfully been used
for over 35 years in a variety of climates around the United States.
In 1977, Walden Pond State Reservation in Massachusetts used the material for their visitor’s center parking lot. Since installation, the local water table has been raised, and erosion, pollution, and the need for storm drains or road salt has been reduced. Walden Pond State Reservation continues to use porous asphalt pavement to protect a site that serves as one of our most treasured literary muses “Walden Pond”.
Built in 2007, Pringle Creek in Salem, Oregon, was conceived as a 32-acre eco-friendly community. Its building designs, landscaping, energy systems, and stormwater control scheme rely on roads and parking lots made of porous asphalt pavement. Porous asphalt filters and stores stormwater. This process allows it to percolate back into the ground rather than add to the drainage system load that flows into creeks and rivers.
The system worked well initially, but no one knew how it would perform in severe weather conditions. When a severe storm flooded roads and rivers all over the Salem area in January of 2012, Pringle Creek community experienced nothing more than a puddle or two.
The community’s green stormwater system had protected the environment and saved residents from potentially millions of dollars of storm damage.
Installation of porous asphalt
Porous asphalt pavement does not require proprietary ingredients and does not require special paving equipment. General paving contractors typically have the necessary equipment to install porous asphalt.
Porous asphalt pavements can be installed as a whole or in part alongside of traditional impervious asphalt pavements.
The design for application of porous asphalt consists of at least four layers:
A two to four-inch layer of asphalt.
Porous asphalt consists of bituminous asphalt, which has been screened and the fines reduced. Void space is created to make it permeable to water. The void space of porous asphalt is approximately 16%, instead of the 2 to 3 percent in a traditional asphalt mix. Porous asphalt provides for some pretreatment of runoff.
A 1 to 2-inch filter layer of half-inch crushed aggregate.
Crushed aggregate filter layer aids with pollutant removal and provides stability for the stone reservoir layer during the application of pavement.
A 12-inch minimum reservoir layer of 1 to 3-inch aggregate.
Treated water runoff is stored in the reservoir bed, a highly permeable layer of open-graded clean-washed aggregate with at least 40% void space.
A layer of geotextile material.
A Nonwoven geotextile material between the reservoir bed and un-compacted subsoil helps prevent migration of fines into the stone reservoir, which could clog the system.
Maintenance requirements of Porous Asphalt
The primary incentive for using permeable pavement is to add a hydrological benefit to a roadway or site design. Recent developments in stormwater ordinances across the country have begun to require more significant onsite stormwater infiltration and detention.
To maintain long-term performance of porous asphalt pavements’ stormwater management capabilities, the surface infiltration rates should be inspected annually during rain events to observe any changes in infiltrating storm water effectiveness.
Porous asphalt pavements should be cleaned by vacuuming two to four times a year or power-washed to remove solids and debris that could lead to more permanent clogging.
During the winter months, there are no special requirements for plowing. Deicing chemicals may be used to melt ice and snow from the surface, but the amount of deicing chemicals will be significantly less than for impervious pavements.
Never seal coat or crack seal porous asphalt pavements. If patching is necessary, a conventional mix may be used if less than 10% of the pavement area is affected.
Porous Asphalt will not work for every stormwater project
Soil characteristics, local topography, and climate are three physical factors that must be used in designing porous asphalt pavement. Some designers will recommend that porous asphalt pavement only be used on sites with gentle slopes, permeable soils, a relatively deep water table and bedrock levels.
There are questions and concerns about how much vehicle traffic the permeable roads can withstand. It has primarily been used for slower traffic areas such as parking lots, shoulders of roads, pedestrian and biking paths, driveways, and quieter, residential streets. High traffic areas or areas open to heavier vehicles may be subject to cracking of the pavement.
Dust is a critical environmental factor that should be considered. Blowing dust will clog the pores of the porous asphalt surface and severely restrict percolation through the system’s top layer. Porous asphalt pavement systems should not be used in barren areas with moderate to high winds.
Permeable pavement, such as porous asphalt, is one of the fastest growing technologies in sustainable roadway engineering. The primary incentive for using permeable pavement is to add a hydrological benefit to a roadway or site design. Recent developments in stormwater ordinances across the country have begun to require more significant onsite stormwater infiltration and detention.
Permeable pavement can help reduce the adverse effects of impervious surfaces. Porous pavement can help reduce the negative impact of impervious surfaces. Permeable pavement can reduce the concentration of pollutants by trapping them physically, chemically, or biologically.
With the appropriate design and installation, porous asphalt pavements can provide a cost-effective solution for stormwater management in an eco-friendly manner. As a result, porous asphalt pavements are recognized as a best eco-practice by the U.S. Environmental Protection Agency and many state agencies.