Widespread
misconception exists in the industry about pervious pavement systems,
specifically about their functionality in cold-weather environments. The
prevalent belief is that pervious pavements are not an effective stormwater
management option for cold-weather climates because of concerns related to
diminished permeability during freezing and that the material is not durable
enough to withstand freeze-thaw conditions. Cold climates are typically very
hard on constructed systems, and naturally, questions should arise about the
effectiveness of pervious pavements in these environments—especially due to
concerns about freezing of the filter media.
However, according
to Dr. Robert Roseen, director of the University of New Hampshire Stormwater
Center (UNHSC), stormwater management systems using infiltration and filtration
mechanisms, if properly designed, can work well in cold-weather environments. He
has been leading a four-year research effort focused on monitoring the
year-round performance of a porous asphalt placement that was installed on the
UNH campus. In addition, the UNHSC is hoping to shed light on the functionality
of pervious concrete by testing a large placement that was also installed on the
university campus in August 2007—the first major pervious concrete parking
facility in New England. The purpose and function of the UNHSC is to evaluate
the range of stormwater treatments systems available to designers, including
proprietary and nonproprietary systems. The UNHSC is funded by the Cooperative
Institute for Coastal and Estuarine Environmental Technology and the National
Oceanic and Atmospheric Administration.
Findings from the
porous asphalt study have demonstrated functionality that exceeds conventional
practices by measures of both water quality and
hydraulics.
Porous Asphalt Study
Design
and Durability. The principal
cause of parking lot pavement breakdown in northern climates is freeze-thaw
cycling. Parking lots in these regions typically have a lifespan of about 15
years. By design, an open-graded, well-drained porous pavement system
incorporating significant depth will have a longer life cycle from reduced
freeze-thaw susceptibility and greater load-bearing capacity than conventional
parking lot pavements. “Design guidelines for freeze-thaw consideration reflect
frost depth ranges from 48 to 52 inches from coast to inland,” says Roseen. “For
porous pavements, greater depth of frost is not the concern, but rather, the
increase in the rate of cycling between freeze and thaw. This rate is highest
near the coast.”
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Photo: UNHSC |
| Steady-state infiltration with 30 gallons per minute from a 2-inch hose |
This UNHSC system
included porous asphalt at the surface, a stone choker course immediately below,
and an underlying layer of finer filter material consisting of sand and gravel.
“The mix design for porous asphalts has been in use for decades as an
open-graded friction course—a pavement mix with a void content commonly in the
18 to 20% range,” says Roseen. In low-permeable soil, Roseen says, the filter
course will need to be underlain by a drainage layer consisting of stone and
drainage pipe. “For the UNHSC site, this drainage layer was constructed in order
to monitor and sample the water that passes through the porous asphalt system,” he says.
Hydraulics. Monitoring infiltration rates for the porous
asphalt placement, the UNHSC found excellent performance results year round.
However, one of the most significant findings of the study was the winter
hydraulic functionality of the placement. Indeed, the study determined that
surface infiltration rates were not negatively impacted from frost penetration
but were actually higher during winter months as compared to the summer.
“The strong winter performance of the pervious asphalt placement was the
opposite of what we expected to find,” says Roseen. “One of the most significant
results from this study was that frozen filter media and freeze-thaw were not an
issue. The well-drained nature of the parking lot sub-base ensures that the void
space remains open, even during periods of prolonged freezing. While the filter
may indeed freeze, it does not freeze solid, and infiltration capacity is
preserved.”
The UNHSC collected
monthly surface infiltration rates for three years and found a repeating trend
of oscillating infiltration rates of about 2,000 centimeters per hour in the
winter to 1,000 centimeters per hour in the summer. “Our suspicion is that the
seasonal variation is likely caused by the asphalt binder, which is the weak
link in the system,” says Roseen. “It is petroleum based and becomes sticky and
tacky during the hot summer months, likely causing swelling and reduced pore
size. Then, when the winter season arrives, the cycle is reversed. Our data over
three winters and summers show that this is a reoccurring cycle.” More
importantly, infiltration capacity remained high during the winter, even when
there was significant frost penetration—sometimes in excess of 12 inches. “The
porous asphalt does freeze; however, it generally freezes as a porous medium and
not a solid block. Freezing rain and rain on snow can freeze the material at the
surface, but minor salting and plowing at such times can return the surface to
high infiltration,” says Roseen.
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Photo: John Kuell |
Construction of a "hybrid" parking lot in New London, NH |
Throughout the research period, the study found that
surface runoff did not occur from the parking lot, even though the Northeast
region experienced an increase in extreme storm events. “We witnessed two
100-year storm events during the monitoring period and have never seen surface
runoff, only runoff through the subdrainage system as designed,” says Roseen.
The net water balance for the site was a 25% reduction in volume, with little or
no runoff during the hottest months. “This was accomplished for a site with
relatively poor infiltrating soils where infiltration as a stormwater management
tool is often not considered,” he says.
The study also
examined clogging of the surface and the resulting decline in infiltration
rates. Roseen says clogging has two causes: the first is surface particulates
and the second is a combination of liquid binder and surface particles, which
can permanently clog an asphalt system. Both causes can be addressed—the first
through routine cleaning approximately two to four times per year, depending on
the frequency of use, and the second through the appropriate selection of a
binder and admixtures to minimize binder draindown. Moreover, Roseen states that
if 99% clogging were to occur, the infiltration rate would still be greater than
10 inches per hour, which is more efficient than most sand and soil
mediums.
Water
Quality. The UNHSC also
monitored the quality of water draining at the base of the pervious asphalt
system. Temperature, conductivity, dissolved oxygen, pH, and turbidity were
monitored every five minutes in addition to automated sampler collections during
storms. Water samples were sent to a certified lab for analysis of water
quality. Results showed exceptional water-quality performance for the porous
asphalt system with no seasonal variation. Typical performance efficiencies
exceeded 95% for total suspended solids, total zinc, and total petroleum
hydrocarbons in the diesel range, and approximately 42% for total phosphorous.
“The porous asphalt design is distinctive in its use of a medium-grained sand
for a reservoir base and filter course,” says Roseen. “This refinement enhances
its effectiveness in treating water quality.”
As expected,
nitrogen removal did not occur, as the placement was a nonvegetated system;
however, if nitrogen were a concern, it could be addressed with a small
vegetated system located at the subdrain outlet.
Winter
Maintenance. From a winter
maintenance perspective, the UNHSC also compared the porous asphalt placement
with a nearby nonporous parking lot, evaluating salt application rates; the
degree (percentage) of snow and ice cover; and the friction factor, which is
measured by a standardized test method. “In northern climates, winter pavement
maintenance requires a substantial effort and entails substantial cost,” says
Roseen. “Routine plowing, anti-icing, and deicing is common practice for more
than four months out of the year. Therefore, maintenance strategies for
improving efficiency and effectiveness of winter practices are
valuable.”
Research findings
showed that salt application for porous asphalt could be reduced by 75%, based
on snow and ice cover. “With only 25% of the salt, the snow and ice cover on the
porous asphalt was the same as on the dense-mix asphalt parking lot,” says
Roseen. “And even with no salt, porous asphalt has higher frictional resistance
than dense-mix asphalt with 100% of the normal salt application,” he says.
“Therefore, a sizable reduction in salt application rate is possible for porous
asphalt without compromising braking distance or increasing the chance of
slipping and falling.”
Porous Concrete Study
For the recently
started porous concrete study, the UNHSC is working in conjunction with the
Northern New England Concrete Promotion Association, the Northeast Cement
Shippers Association, and others who are contributing materials and installation
costs. The research team will examine pervious concrete’s performance in
relation to treating water quality, reducing the volume of runoff, and
minimizing the need for salting and sanding during winter months. The team will
also test how pervious concrete holds up to freeze-thaw
conditions.
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Photo: John Kuell |
The 26,000-square-foot lot has 11,000 square feet of pervious
concrete. |
“What is
important to note is that the UNHSC project is not being run by the concrete
industry,” says Jon Kuell, executive director of the Northern New England
Concrete Promotion Association. “It is being conducted by an independent testing
center, measuring system performance for water quality and quantity. The study
will evaluate the effectiveness of the technology in cold environments for
treating pollutants such as automotive fluids, phosphorous, and even what’s
coming down in the rain itself.”
Kuell explains pervious concrete’s capacity to work
and perform in cold environments by the “igloo effect,” where a shelter can be
built out of snow yet contain a comfortable and warm interior. “In a cross
section of a pervious concrete installation, you have an open-graded structure
over a sub-base, which then sits atop the natural soil bed,” says Kuell. “The
sub-base warms the pervious concrete above through the pure connection.”
The UNHSC project
will give researchers a forum and the opportunity to show the industry that
pervious concrete can function effectively in cold weather conditions. “Nobody
has done this kind of experiment in the Northeast before,” says Kuell. “In a
way, the Northeast is acting as a beneficiary for all the other parts of the
country. It is important to shed light on pervious concrete’s effectiveness in
cold climates.”
Design
Criteria. At the UNHSC, the
makeup of the sub-base in the newly installed pervious concrete placement is
considerably different from what is typical of most pervious pavement systems.
The system being tested consists of a media of large coarse stone with
considerable void space and storage capacity, in addition to a sand filter layer
that enhances the system’s overall performance and provides substantial and
improved water-quality benefits. “A typical sub-base media consisting of 1- to
3-inch stones functions more like a detention pond,” says Roseen. “However,
after adding a sand layer, the overall placement begins to function as a
filtration system.” The sand filtration media consists of a chemical and a
physical process. “There is chemical absorption to the sand particles in
addition to mechanical filtering—small pore spaces that work to catch the
sediment load,” says Roseen.
Kuell says there are
three important criteria for constructing pervious concrete projects, especially
in cold environments. First, he says, the concrete should consist of a quality
product and contain a proper mix design. “You don’t want too wet of a mixture,
because the cement paste could leach down through the sub-base into the soil and
reduce infiltration rates. On the other hand, you want to make sure the mixture
isn’t too dry, because raveling could occur,” he says.
The second criterion
is related to the subbase. “When selecting the subbase material, it is important
to look at the structural loading requirements and the water-quality benefits,”
says Kuell. “It must equally be able to support cars and treat water.” He says
the sub-base should be able to provide storage for an extreme storm event with
enough permeability to effectively infiltrate water to the native soils. It
should also be designed to drain well and effectively work with a total drawdown
of no more than five days. “In freeze-thaw environments, you don’t want standing
water resting for too long a time,” he says. A system with subdrains and
well-drained materials will ensure that it doesn’t.
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Photo: John Kuell |
Travel lanes were constructed of conventional concrete. |
The third criterion
is making sure contractors are educated about the material and proper placement
techniques so that the end result is a high-quality, durable pavement capable of
lasting 20 to 40 years.
According to Kuell,
the void content in pervious concrete installations is so high that it is
possible to add fines into the mix without affecting infiltration rates. “If a
pavement will be receiving heavier point loads, such as tractor-trailer traffic,
we can put some of the sand back in the mix, which will help increase our
compressive strengths,” he says. “With a void content of 20 to 35% in standard
pervious concrete, there is considerable room to add fines and still maintain
void contents that will accommodate up to 10 times the amount of runoff for a
100-year rain event for most areas.”
Kuell says the UNHSC
project will be very important in terms of its perceptions among industry
professionals. “With this study, we will be able to refer to a large-sized
placement and show empirical evidence of its performance,” says Kuell. “However,
it is still going to take plenty of convincing. Still, I’m confident that once
the engineering and architectural community can actually see the evidence, they
will begin to accept it.”
Differences Between Porous Asphalt and Pervious
Concrete
There are some
notable differences between porous concrete and porous asphalt. The mix
production of porous asphalt is a bit tougher than that of pervious concrete;
however, Roseen says, it is easy to install, and nearly any qualified installer
can manage it. Pervious concrete, on the other hand, is fairly simple to produce
but is tricky to install and generally requires certified installers for its
placement. Other differences are related to the actual color of the substance.
Because concrete is lighter, it absorbs less heat, contributing less to the
urban heat island effect than traditional asphalt, which is responsible for
adding heat to many cities. For the same reason, porous asphalt will perform
better in the winter, as it will be warmer and promote greater deicing. Another
advantage to concrete is that less nighttime lighting will be required because
the lighter-colored surface reflects more light.
Incentives for Using Pervious Pavement Systems
In keeping with EPA
requirements for Phase II of the National Pollutant Discharge Elimination System
(NPDES) and the total maximum daily load (TMDL) program, most municipalities are
requiring improved stormwater management. These practices are designed to limit
additional contaminant loading, and in some instances, to reduce existing
loadings, as well as to limit effective impervious cover. “This is a high
standard that simply cannot be met with most conventional stormwater management
practices using curbs and gutters and stormwater ponds and swales,” says
Roseen.
Public heath
concerns over mosquito-born illnesses such as West Nile virus and eastern equine
encephalitis are also driving more interest in using stormwater management
practices that do not include standing bodies of water like wetlands or ponds.
“Pervious pavements and other low-impact development [LID] systems that do not
have permanent pools of water are increasingly being considered as primary
treatment options due to these disease concerns,” says Roseen.
However, beyond a
regulation and health standpoint, pervious pavement is attractive in terms of
its stormwater management benefits. For instance, in addition to removing
contaminants, it is very effective at reducing the thermal impact of stormwater.
“During the summertime, runoff can get very hot, sometimes as high as 120
degrees,” says Roseen. “When you have stormwater at that temperature flowing
into a groundwater-fed stream that is at 65 degrees, severe impacts can occur in
the aquatic ecosystem. Pervious pavements can significantly cool
runoff.”
Lower winter
maintenance is another distinct advantage of pervious pavement. Melting snow and
ice can infiltrate directly into the porous material to facilitate faster
melting.
“Along with less
plowing and deicing, the need for constant sanding and salting in the winter
months is greatly reduced,” says Roseen. This translates into greater safety.
Standing water does not collect on the surface of a porous pavement system,
which eliminates the formation of ice.
Stormwater
management options such as pervious pavement that offer subsurface treatment are
also becoming economically more attractive because of their efficient use of
space. “Developers and business owners are willing to pay more for subsurface
stormwater management because these areas can also be used for parking or other
purposes,” says Roseen. And, with pervious pavement, the need for large
detention ponds and other costly stormwater management systems is eliminated,
allowing for even more parking space.
Additionally,
many state regulations that require LID are also requiring some level of
infiltration of stormwater runoff. In Rhode Island, for example, LID systems are
required statewide for new and redevelopment beginning in July 2008. “They are
really pushing the envelope,” says Roseen. “The Rhode Island Sound watershed is
fairly developed with a high degree of imperviousness in some areas. It has
almost reached build out, and as such, stormwater management in that area is
being driven by water quality and water supply. Pervious pavement and other LID
systems will be an essential tool in Rhode Island for improving the overall
quality of the water.”
For
redevelopment, pervious pavement systems are not as competitive or cost
effective, because existing infrastructure is typically already in place, and
pervious pavement requires that an appropriate sub-base be installed anew.
“However, it can help reverse impairments commonly associated with urban waters,
which are often conditions for redevelopment,” says Roseen. “In areas with
impaired water where EPA enforcement is active, redevelopment cannot add
additional contaminants, so innovative BMPs, such as pervious pavement, will
need to be considered. That is exactly what is being required in Providence, RI,
by the EPA.”
Costs
In addition to the
misperceptions about its effectiveness in cold climates, another barrier to
pervious pavement’s acceptance into the industry is the belief that it is much
more expensive. However, a life-cycle-cost analysis shows that pervious pavement
systems are more economical in the long run.
“Normal parking
lots made from impervious pavement typically last 12 to 15 years in northern
climates where freeze-thaw is prevalent, while pervious pavement lots can last
more than 30 years,” Roseen says. “The recent reconstruction of the pervious
concrete lot on the UNH campus is more costly than repaving, but over the long
term, UNH will see a return on this investment. UNH Transportation Services
understands this, and their commitment is crucial to making this technology
demonstration happen.” UNH Transportation Services is also planning to use more
pervious pavements in future parking lots.
For new
development, Roseen says that although material costs are approximately 20 to
25% more than those for traditional pavement, the total project cost for these
systems with reduced stormwater infrastructure is comparable to standard
pavement applications with which stormwater infrastructure is
required.
“The
more these systems are used, the more affordable they will become,” says Roseen.
“Pervious pavement systems also need to be considered as high-performance
stormwater management. Given proper design and oversight, they can be
affordable, effective solutions to stormwater management. They are important
strategies for communities wanting development, while still providing protection
for watershed health.”