ForConstructionPros.com, MD
11-08-07
Pervious Concrete FAQs
Green Building
Philip Kresge, National Ready Mixed Concrete Association
Pervious concrete is not new. Studies show that pervious concrete was first used
in 1852. However, pervious concrete is enjoying a new popularity, to the point
that pervious concrete pavement is the hottest topic in today's sustainable
development design community.
As a senior director of national resources for the National Ready Mixed Concrete
Association and a member of the NRMCA's National Accounts promotion
program, I have given numerous presentations on pervious concrete. These
seminars have been attended by architects, engineers and specifiers as well as
representatives from local and state regulatory agencies. Nine out of 10 requests
I receive are for information regarding some aspect of pervious concrete.
While nearly everyone understands the concept of pervious concrete and its
contributions to reduction of stormwater runoff and pollutant removal, I am
continually asked the following questions: "Where has pervious concrete been
used? How does pervious concrete hold up to freezing and thawing? What about
clogging? Does my state's environmental regulatory agency accept/approve
pervious concrete for stormwater management?"
In short, the answers are, "everywhere, very well, no problem and depends." But
you deserve better answers than these, so let me address each of them
separately.
Where has pervious concrete been used?
Pervious concrete pavements have been identified dating back to 1985. There
may be older existing projects still waiting to be identified, though. The confusion
is that back then the product was known by such names as no-fines concrete,
enhanced porosity concrete (EPC) and even "popcorn concrete." The NRMCA is
in the process of compiling a national database of pervious concrete projects.
Of the over 250 projects currently listed in the database, 60 percent are located
in the Southeast United States, with the majority being in North Carolina. Another
20 percent are located in California. The remainder is spread across the country
and includes northern regions such as Pennsylvania, Ohio, Vermont and
Minnesota.
The projects vary in size and function and include sidewalks, cart and walking
paths, driveways and even local streets. The lion's share, however, are parking
lots. This is not surprising since it is where pervious concrete can best provide its
most desirable benefits of reducing quantity and improving quality of stormwater
runoff. The sizes of these lots range from as small as several parking spaces to
projects of one or more acres. The largest site in the database is a 7-acre
parking lot in Westminster, Md. Additionally, a 71⁄2-acre project is soon to be
completed in Williamsburg, Va.
The fact that most of these pervious sites are located in more temperate climates
can be attributed to the apprehension that spawns our second inquiry.
The freeze-thaw question?
Many design professionals have the initial perception that, because of its
porosity, pervious concrete would have little or no freeze-thaw durability. And yet,
it is actually the porosity that accounts for pervious concrete's ability to withstand
severe cold climates.
Due to the average 20 to 25 percent void structure of pervious concrete,
stormwater passes directly through the pavement and into the aggregate
subbase and the subgrade below. The pavement itself is rarely in a fully
saturated state. In the event that there is moisture present at freezing
temperatures, the void structure of the pervious concrete provides ample room
for expansion, minimizing the undue pressures that would normally be exerted on
the concrete.
Researchers at Iowa State University recently completed a study to develop an
optimum cold weather mix design for pervious concrete. Numerous concrete
mixes, with varying chemical and mineral admixtures, were subjected to freezethaw testing per ASTM C 666, Standard Test Method for Resistance of Concrete
to Rapid Freezing and Thawing. The samples were fully saturated and
experienced four to five freeze-thaw cycles per day for a full 300 cycles. Results
of the testing showed that the inclusion of a small amount of fine aggregate, as
well as air entraining admixture, provided the best freeze-thaw durability with less
than 2 percent loss of mass after 300 freeze-thaw cycles.
The NRMCA conducted a field study of 10 pervious concrete pavements, ranging
from 2- to 13-years-old, in various freeze-thaw climates. The most moderate
climate in the study area experienced an average of 50 freeze-thaw cycles per
year while the most severe averaged 210 freeze-thaw cycles, with an average
below freezing for 60 days. The results of the study (available at www.nrmca.org)
show pervious concrete that is partially saturated should have "sufficient voids for
the movement of water and thus demonstrate good freeze-thaw resistance." The
key point here is that total saturation should be avoided for optimum freeze-thaw
durability.
What about clogging?
The majority of pervious concrete pavements will function very well with little or
no maintenance. However, there may be instances where sand, dirt, leaves and
other debris may infiltrate the void structure of the pervious concrete and inhibit
its permeability. In most cases, the clogging is limited to the first 1 to 11⁄2 in. of
the pavement thickness. Routine cleaning can help avoid this situation and
restore better than 90 percent of original permeability.
A recent study by the University of Central Florida supports this claim. The
research, conducted at the Stormwater Management Academy and funded by
the Ready Mixed Concrete Research and Education Foundation, looked at eight
pervious parking lots ranging in age from 6- to 20-years-old. These lots had seen
little or no maintenance since their construction. The cleaning techniques
investigated were pressure washing, vacuum sweeping and a combination of
these two methods.
Pressure washing dislodges the clogging particles, washing a portion offsite and
flushing the remaining portion through the pavement surface. Vacuum sweeping
dislodges the dirt and debris by means of the sweeping action and removes them
via the vacuum. Results of the UCF study show that utilizing either pressure
washing or vacuum sweeping can improve the infiltration rate of clogged
pervious concrete by 90 percent. Cleaning with a combination of the two
methods actually caused a 200 percent increase in infiltration rates over the
benchmark infiltration rates of the clogged samples.
However, the best maintenance practice appears to be prevention. Proper design
and construction of pervious concrete should include consideration of the
drainage of surrounding areas to prevent the flow of potentially clogging
materials onto the pavement surface. Whenever possible, drainage of all
unpaved areas should be away from the pervious concrete pavement.
Additionally, landscaping materials, such as mulch, should not be stored on the
pavement even temporarily.
Your state's regulatory agency and pervious
In 1972, the National Pollutant Discharge Elimination System (NPDES) program
was established under the authority of the Clean Water Act. This regulation
requires communities and public entities that own and operate a municipal
separate storm sewer system (MS4) to obtain an NPDES permit for stormwater
discharges. As a part of the permitting process, the permit applicant must
establish a best management practice (BMP) for stormwater management. The
U.S. Environmental Protection Agency has approved several BMPs for
stormwater management, and pervious concrete is among them.
Though pervious concrete is not new, its use in stormwater management is a
relatively new concept to the engineering community. As such, projects utilizing
pervious concrete are undergoing special scrutiny. Most states include porous
pavements, in general, as an accepted BMP but are reluctant to give blanket
approval for all projects utilizing pervious concrete. Instead, they provide
approval on a case-by-case basis, usually through special provisions to existing
regulations.
Tom Evans, promotion director for the Maryland Concrete Promotion Council,
explains the process: "If you were going to build a road that crosses a stream,
you can either build a bridge or place a culvert with backfill for an at-grade
crossing. Either way, you must submit a completed design for approval. You can't
just say, 'I'm going to put in a bridge.' The same applies for pervious concrete
pavement. You can't just say, 'I'm using pervious concrete' and expect blanket
approval. You need to submit a completed design with full details."
To facilitate a smoother approval process, it helps to do your homework. The
common concerns of regulatory agencies deal with pervious concrete's freezethaw durability, infiltration capabilities and pollutant removal efficacy. Research
has been conducted on all of these issues, and the results are readily available.
Dr. Heather J. Brown of Middle Tennessee State University and coordinator for
the Concrete Industry Management Program has completed Pervious Concrete
Research Compilation: Past, Present, and Future. Funded by the RMC Research
and Education Foundation, the compilation serves as an index of texts, case
studies and research projects for all aspects of pervious concrete including
construction techniques, durability and maintenance, hydrological and
environmental design, concrete mix design, specifications, test methods, and
structural design and properties. The report is available online through the
Foundation's website, www.rmc-foundation.org. Other supporting reports and
documents are available through the NRMCA's website
www.PerviousPavement.org.
There are many resources for in-depth information on pervious concrete, but
perhaps the best resource is your state or local ready mix concrete association.
Contact them for more assistance on identifying existing local projects,
assistance with the approval/permitting process, or to become a certified
pervious concrete contractor.