Porous Brochure
Porous Brochure
Porous Brochure
ASPHALT
PAVEMENTS
Porous asphalt is
an environmentally
friendly tool
for stormwater
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
Stormwater management tools can 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 open-graded surface over an underlying
stone recharge bed. The water drains through the porous asphalt and into the
stone bed, then, slowly, infiltrates into the soil. Many contaminants are removed
as the stormwater passes through the porous asphalt, stone recharge bed, and
soils through filtration and microbial action.
DESIGN
WATER QUALITY
Porous pavements are highly
effective in reducing pollution in
stormwater runoff from pavements.
Cahill reports that, although sampling on porous pavement systems
has been limited, the available data
indicate a high removal rate for total
suspended solids (TSS), metals,
and oil and grease.3
Table 1 shows the pollution removal efficiency for a porous parking lot constructed at the University
of New Hampshire (UNH) in 2004.4
The University reports that
The water quality treatment performance of the porous asphalt
lot generally has been excellent.
It consistently exceeds EPAs
recommended level of removal of
total suspended solids, and meets
regional ambient water quality criteria for petroleum hydrocarbons
and zinc. Researchers observed
limited phosphorus treatment and
none for nitrogen, which is consistent with other non-vegetated
infiltration systems.
PAVEMENT STRUCTURE
From the bottom up, the standard
porous asphalt pavement structure
consists of:
n An uncompacted subgrade to maximize the infiltration rate of the soil.
n A geotextile fabric that allows water
to pass through, but prevents migration of fine material from the subgrade into the stone recharge bed.
n A stone recharge bed consisting
of clean single-size crushed large
stone with about 40 percent voids.
This serves as a structural layer and
also temporarily stores stormwater
as it infiltrates into the soil below.
TABLE 1
Pollution removal efficiencies
Treatment
Total
Total Phosphorus
Total Zinc
System
Suspended Solids
(% Removal)
(% Removal)
(% Removal)
Porous
Pavement
99
38
96
Total Petroleum
Hydrocarbons
in the Diesel Range
(% Removal)
99
FIGURE 1
Typical porous pavement cross section
Figure 1: Typical Porous Pavement Cross Section
n
n
Frost
In the past it has been recomended that the bottom of the recharge
bed should exceed the depth of frost
penetration in the region where the
porous pavement is to be installed.
More recently this has come into
question, since a number of porous
pavements have been installed in
freezing climates with total depths
much shallower than this. These
include pathways at Swarthmore
College, Pennsylvania, and a parking lot at Walden Pond Visitor Center,
FIGURE 2
Terraced porous parking
Figure 4: Terraced porous parking
FIGURE 3
Roof leaders can be connected directly to the subsurface infiltration bed
(Based on Cahill Associates illustration)
Porous pavements are not normally designed to store and infiltrate all stormwater from all storms.
Therefore, it will be necessary to
include overflow devices to prevent
the water from rising into and over
the porous asphalt surface (Figure 4).
Paths
FIGURE 4
Example of stone edge for alternate path
to bed and overflow device
MATERIALS
Geotextile (Filter Fabric)
FIGURE 5
Porous asphalt path
Figure 10: Porous Asphalt Path
Outlet Structure
Porous Asphalt
Overflow
Weir
Stone bed
Outlet
Geotextile
Perforated Pipe
CONSTRUCTION
GUIDELINES
The following are some general
guidelines for construction of porous
pavements:
n The site area for the porous pavement should be protected from
excessive heavy equipment running
on the subgrade, compacting soil,
and reducing permeability.
n Excavate the subgrade soil using
equipment with tracks or over-
sized tires. Avoid narrow rubber
tires as they compact the soil and
reduce its infiltration capabilities.
n As soon as the bed has been
excavated to the final grade, the
filter fabric should be placed.
n Install drainage pipes if required.
n Place aggregate for the stone
recharge bed, taking care not to
damage the filter fabric. Aggregate
should be dumped at the edge
of the bed and placed in layers
of 8 to12 inches using track
equipment. Compact each
lift with a single pass of a light
roller or vibratory compactor.
FIGURE 6
The photos show a parking lot at University of New Hampshire one hour after plowing,
with a close-up of the porous asphalt portion of the lot at bottom. A 75 percent reduction
in salt application was possible: that is, with only 25 percent of the salt, the snow and ice
cover on the porous asphalt was the same as on conventional dense-mix asphalt.
n
n
Post-Construction Practices
MAINTENANCE
To prevent clogging of porous
pavements it is recommended that
they be vacuum swept twice per
year. As previously discussed, it is
also very important that sanding not
be used for winter maintenance.
SUMMARY
Porous asphalt pavements provide excellent parking lots and roads. They tend not to exhibit
cracking and pothole formation problems. The surface wears well. Porous asphalt has been
proven to last for decades, even in extreme climates, and even in areas with many freeze-thaw
cycles.5 Using the underlying stone bed to manage stormwater for adjacent impervious areas
such as roofs provides even more benefits.
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
Additional Resources
The definitive guide for porous
pavements is NAPAs Porous
Asphalt Pavements for Stormwater Management: Design,
Construction, and Maintenance
Guide (IS-131).
For porous asphalt mixes, consult NAPAs Design, Construction, and Maintenance of OpenGraded Asphalt Friction Courses
(IS-115).
For photos and videos visit
www.porouspavement.net.
Visit http://store.hotmix.org