Asphalt Pavement — Georgia's Green Pavement
Better Water Quality
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
Better Water Quality
Figure
1:
Hydrograph comparison showing how porous pavement reduces peak flow and
total volume of runoff Source: Cahill Associates
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.