Better Water Quality
(Used with the Permission of the Asphalt Pavement Alliance, Lexington, KY)

Porous Asphalt Draining Water Porous asphalt...
• Conserves water
• Allows for better use of land
• Reduces runoff
• Promotes infiltration
• Cleans stormwater
• Replenishes aquifers
• Protects streams

Porous asphalt offers a powerful tool in the toolbox for storm-water management.
In the natural environment, rainfall sinks into soil, filters through it, and eventually finds its way to streams, ponds, lakes, and underground aquifers. The built environment, by way of contrast, seals the surface. Rainwater and snowmelt become runoff which may contribute to flooding. Contaminants are washed from surfaces directly into waterways without undergoing the filtration that nature intended.¹ For these reasons, managing stormwater is a significant issue in land use planning and development. Stormwater management tools can serve to mitigate the impact of the built environment on natural hydrology. Unfortunately, however, they also can lead to unsound solutions such as cutting down stands of trees in order to build detention ponds. Porous asphalt pavements allow for land development plans that are more thoughtful, harmonious with natural processes, and sustainable. They conserve water, reduce runoff, promote infiltration which cleanses stormwater, replenish aquifers, and protect streams.

A typical porous pavement has an opengraded surface over an underlying stone recharge bed.² The water drains through the porous asphalt and into the stone bed, then, slowly, infiltrates into the soil. If contaminants were on the surface at the time of the storm, they are swept along with the rainfall through the stone bed. From there they infiltrate into the subbase so that they are subjected to the natural processes that cleanse water.

Construction and performance
Porous asphalt pavements are fast and easy to construct. With the proper information, most asphalt plants can easily prepare the mix and general paving contractors can install it.

The stone bed, often eighteen to thirtysix inches in depth, provides a tremendous subbase for the pavement. As a result, porous asphalt pavements tend not to exhibit cracking and pothole formation problems. The surface wears well. Under the stone bed is a geotextile which keeps fine particles from moving into the stone bed from below and filling in the spaces. Porous asphalt has been proven to last for decades, even in extreme climates, and even in areas with many freeze-thaw cycles.³

The underlying stone bed can also provide stormwater management for adjacent impervious areas such as roofs and roads. To achieve this, stormwater is conveyed directly into the stone bed, where perforated pipes distribute the water evenly.

Economics
Porous pavement is a sound choice on economics alone. A porous asphalt pavement surface costs approximately the same as conventional asphalt. Because porous pavement is designed to “fit into” the topography of a site, there is generally less earthwork. The underlying stone bed is usually more expensive than a conventional compacted sub-base, but this cost difference is offset by eliminating the detention basin and other components of stormwater management systems. On projects where unit costs have been compared, the porous pavement has been the less expensive option. Porous pavements are therefore attractive on both environmental and economic grounds.¹

An installation at the University of North Carolina in Chapel Hill included parking lots where some sections were constructed from porous asphalt and others used porous concrete. The cost differential was approximately 4:1 – that is, the porous concrete pavement cost four times as much as the porous asphalt pavement.¹

Impact on groundwater
Asphalt pavements are compatible with clean water. Studies show that asphalt pavements and stockpiles of reclaimed asphalt pavement do not leach^.4,5 Contaminants on the surface of pavements tend to become part of runoff, but with a porous pavement, they are washed into the stone bed. From there they flow down into the soil, where beneficial bacteria and other natural processes cleanse them. Data are limited, but indicate a very high removal rate for total suspended solids, metals, and oil and grease.¹ Figure 1 shows the effect of a porous asphalt pavement on the hydrology of a developed site.

Cooler cities
Porous asphalt pavements have been shown to mitigate the urban heat island effect. Open-graded asphalt roads and highways—which use the same surface material as porous parking lots—have been shown to lower nighttime surface temperatures as compared to impervious pavements. In at least one city, the hottest heat signature is at the airport, with its thick, dense, impervious runways.⁶

Comparisons to other asphalt pavements
The surface of a porous asphalt pavement wears well. While slightly coarser than some pavements, it is attractive and acceptable. Most people driving or walking on the pavement will not notice (or believe) that it is porous. Like all asphalt pavements, porous pavements are ADA-friendly.

Environmental applications
Asphalt pavements have been used for many years to enhance water quality. At landfills, asphalt liners and caps keep contaminants from leaking into groundwater. Drinking water reservoirs lined with asphalt pavement have been used in California since the 1950s. Salmon hatcheries and fish rearing ponds in the Pacific Northwest use asphalt liners.⁷

Variations on the theme
Porous asphalt can be used successfully in parking lots, walkways, and playgrounds. Several current suburban projects are exploring its use in subdivision roads. A few porous highways and city streets have been constructed, both in the U.S. and in Europe, and have performed well.

The open-graded asphalt surface used for porous pavements has been used extensively to surface high-volume highways that carry heavy trucks. Its benefits include noise reduction, a decrease in splash and spray kicked up by vehicles in heavy downpours, and mitigation of the urban heat island effect.

REFERENCES
1. Cahill, Thomas H., et al., “Porous Asphalt: The Right Choice for Porous Pavements,” Hot Mix Asphalt Technology, National Asphalt Pavement Association, Lanham, MD, September/October 2003.
2. Jackson, Newt, Design, Construction and Maintenance Guide for Porous Asphalt Pavements (IS-131), National Asphalt Pavement Association, Lanham, MD, 2003.
3. MacDonald, Chuck, “Porous Pavements Working in Northern Climates,” Hot Mix Asphalt Technology, National Asphalt Pavement Association, Lanham, MD, July/August 2006.
4. Kriech, AJ, et. al. “Determination of polycyclic aromatic compounds in asphalt and in corresponding leachate water.” Polycyclic Aromatic Compounds, Taylor & Francis Group, Philadelphia, PA. Volume 22, Numbers 3-4, pp. 517-535. 2002.
5. Townsend, Timothy, and Allen Brantley. Leaching Characteristics of Asphalt Road Waste. University of Florida, 1998. (http://www.hinkleycenter.com/publications/townsend_98-2.pdf, accessed September 1, 2006)
6. Golden, Jay, and Kamil Kaloush, “A Hot Night in the Big City: How to Mitigate the Urban Heat Island,” Public Works, December 2005. (http://www.pwmag.com/industry- news.asp?sectionID=770&articleID=268116, accessed September 5, 2006)
7. Environmental Applications for Hot Mix Asphalt (PR-1). Asphalt Institute, Lexington, KY. Undated.

Iowa’s Second Perpetual Pavement

State DOT officials have approached the industry about building Iowa’s second Perpetual Pavement. The project is nearly one mile in length and encompasses the North Mix-Master in Des Moines where I-35, I-80, and I-235 converge, making this one of Iowa’s highest trafficked junctions.

The state elected to go with the Perpetual Pavement design because it would require only minimal attention in the future which fits into the “get in, get out and stay out” design philosophy.

The contractor, Des Moines Asphalt & Paving Company, winner of the National Asphalt Pavement Association’s 2006 Sheldon G. Hayes award, is scheduled to have the project completed this summer.

Clean Air & Cool Cities
Lower Greenhouse Gases, Lower Fuel Consumption
(Used with permission of the National Asphalt Pavement Association)

The production and placement of asphalt pavements consumes less fuel and produces lower levels of greenhouse gases. According to a recent study, asphalt pavements require about 20 percent less energy to produce and construct than other pavements.(1) Less fuel consumption means less production of carbon dioxide and other greenhouse gases.

Since 1970, the asphalt industry has decreased total emissions from plants by 97 percent while increasing production by 250 percent.(2) Emissions from asphalt plants are so low, the EPA considers them as only minor sources of industrial pollution.(3)

The asphalt industry is also working on ways to reduce the temperatures at which asphalt pavements are produced and placed. Typically, asphalt paving temperatures are in the range of 280 to 320°F. Lowering these temperatures by 50°F or more would save fuel and reduce production of greenhouse gases and other emissions. Working in cooperation with the Federal Highway Administration, state Departments of Transportation, and other key stakeholders, the asphalt industry’s research on several new “warm-mix asphalt” technologies holds great future promise.

Asphalt moves traffic along
When traffic backs up, cars and trucks consume fuel unnecessarily and produce excess emissions. One way to reduce both fuel consumption and emissions is to keep traffic moving along. Asphalt’s speed of construction allows planners and managers a way to fix congestion hot spots and bottlenecks, quickly and cost-effectively—often, all the work can be done at off-peak hours, so that the morning and evening commutes go smoothly. Because a newly rehabilitated asphalt road can be opened for traffic as soon as it has been compacted and cooled, keeping lanes coned off for curing is not necessary.

Driving on smooth roads also saves fuel. Studies at a Nevada test track showed that vehicles driving on smooth roads consumed 4.5 percent less fuel, on average, than on rough pavement.(4) Asphalt can make rough roads smoother, quickly, cost-effectively, and without prolonged road closures.

Urban heat island reduction: how asphalt pavements can help
The urban heat island (UHI) effect—the phenomenon that makes cities 2 to 10°F warmer than nearby rural areas on a hot summer day—is not a black and white issue. Many factors contribute to heat retention in urban areas. And, many strategies for reducing the UHI effect are being explored.(5)

Because pavements cover a large percentage of urban areas, and because improvements to pavements occur more frequently than improvements to buildings, pavement-related strategies for cooling off the city core are of interest.

Some attention has been given to the idea of making pavements more reflective, on the theory that a lighter-colored or more reflective surface may keep things cooler. But on closer look, it is seen that many factors other than color and reflectivity—including pavement thickness and the type of surface used—can influence the way a pavement retains, radiates, and/or releases heat. When and how heat is released is also of importance.

Porous asphalt pavements have been shown to lower nighttime surface temperatures as compared to other pavements. A thermal image taken by satellite (ASTER) over Phoenix in October 2003 (Figure 1) shows that an impervious freeway which has been resurfaced with open-graded asphalt is actually cooler at night than nearby freeways without the asphalt surface. Also influencing the cooling of pavements is the presence of sound walls (which can trap heat), vegetation cover on the adjacent landscape, whether the pavements are at or below grade, and the thickness of the pavement itself. In the same ASTER image, the hottest heat signature is at the airport, where the impervious runways are 23 inches thick.

REFERENCES
1. Gambatese, John A. and Sathyanarayanan Rajendran, “Sustainable Roadway Construction: Energy Consumption and Material Waste Generation of Roadways,” American Society of Civil Engineers, Reston, VA. Proceedings of 2005 Construction Research Congress. (http://www.pubs.asce.org/WWWdisplay.cgi?0520020. Downloaded June 22, 2006.)

2. Report to Members 2001, National Asphalt Pavement Association, Lanham, MD. 2002.

3. Federal Register, February 12, 2002, pp. 6521 ff.
(http://frwebgate.access.gpo.gov/cgi-bin/getpage.cgi?dbname=2002_register&position=all&page=6521,
accessed September 7, 2006.) Also, Federal Register, November 8, 2002, pp. 68124 ff.
(http://frwebgate.access.gpo.gov/cgi-bin/getpage.cgi?dbname=2002_register&position=all&page=68124,
accessed September 7, 2006.)

4. Sime, M., et al. WesTrack Track Roughness, Fuel Consumption, and Maintenance Costs. Tech Brief published by Federal Highway Administration, Washington, DC. January 2000.

5. Golden, Jay, and Kamil Kaloush, “A Hot Night in the Big City: How to Mitigate the Urban Heat Island,” Public Works, December 2005. (http://www.pwmag.com/industry-news.asp?sectionID=770&articleID=268116, accessed September 5, 2006.)

Better Water Quality (Used with the Permission of the Asphalt Pavement Alliance, Lexington, KY)

Better Water Quality
(Used with the Permission of the Asphalt Pavement Alliance, Lexington, KY)