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Putting a Pretty Face on Pervious Concrete
In the 1960s, when pervious concrete pavement first made its appearance
in North America, it simply didn’t catch on widely. Over the
next several decades, while there were many installations of these
pavements, they were localized to certain regions.
In today’s environmentally conscious climate, the benefits
of pervious concrete to sustainability have reinvigorated interest
in these free-draining pavements. And though it’s appealing
for its technical benefits, pervious concrete hasn’t necessarily
been attractive…until now. Renewed attention to pervious pavements
has led some people to experiment with improvement of the surface
aesthetics—and spawned a new type of concrete sometimes referred
to as “architectural pervious concrete” (APC).
Most pervious concrete has a sort of popcorn look about it: Uniformly
sized aggregate, falling within a narrow aggregate gradation, leads
to a concrete with open void areas that promote fast drainage of
large volumes of water. Some like the organic, rough-textured look
of plain pervious concrete. But for those who want a different look,
there are several options for enhancing the aesthetics. Treatments
include color, stamping, jointing, and grinding.
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| This APC cylinder shows that the
material has a smooth texture that is fine enough to be stamped
yet still absorptive. |
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| This test panel is made with architectural
pervious concrete containing 100% fines—no coarse aggregate—and
as a result, the surface can accommodate a stamped finish. |
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| Grinding the surface of traditional pervious
concrete pavement creates a smoother finish and an interesting
texture. |
Color is a fairly simple addition to pervious concrete: Mineral pigments
mixed with gray or white cement give an overall color to the paste,
which coats aggregate particles. Color works with both traditional
pervious concrete and APCs. APCs are typically made with some or even
100% fine aggregate. While mixes that contain fines do not have the
same large void structure as traditional pervious, they are still
absorptive enough to keep walkways dry and recharge ground water.
APCs allow for smoother, stampable finishes, unlike traditional pervious
concrete. Pervious concretes that contain fines are safer for walking
in any kind of shoes, including high heels. Traditional pervious,
which cannot be stamped, can be ground to provide a smoother surface—and
a different appearance.
Mixtures require the use of a special admixture that allows the
addition of a high percentage of fine aggregate, even as much
as
100% fines. The admixtures stabilize the air voids in the mixes
to maintain adequate infiltration rates. Some APCs act a little
like a sponge: It absorbs water and holds it, allowing some to
percolate
into the ground and as much as 40% to evaporate back to the atmosphere.
That is different from traditional pervious. Some admixtures supposedly
have both strengthening and self-consolidating
properties for added workability and ease of placement.
The California
and Nevada Cement Association (CNCA) also weighs in on decorative
treatments for pervious concrete pavements. They note that integral
coloring agents can be added to the mix or water-based concrete
stains can be sprayed onto the surface after installation. CNCA
recommends against the use of solvent-based coatings or sealers
because these materials can clog voids in the pavement. Although
patterns can’t easily be stamped into traditional pervious
concrete—it’s too stiff and doesn’t have the right
texture—it is possible to score decorative lines into the
surface.
| Pervious Concrete Comparisons |
| |
Aggregate |
Strength,psi |
Flexural strength, psi |
Percolation rate, in/hr |
| Traditional pervious |
(limited fine aggregate)
4.75 to 19.0 mm or
1.18 to 9.5 mm or
0.300 to 9.5 mm
|
500 to 4000
(average 2500)
|
150 to 550 |
288 to 770
(up to 1650 measured in the lab)
|
| Percocrete |
4.75 mm top size* or
75% at 4.75 mm/25% at 9.5 mm* or
50% at 4.75 mm/50% at 9.5* mm
|
2400 to 2900
3200 to 3400
up to 3900
|
Not given |
>200 |
| StoneyCrete |
Down to 1.5 mm |
Not given |
20% to 50% greater than generic pervious mixes |
Not given |
| Ecocreto |
Down to 3 mm |
5000 at 28 days
(3000 in 24 hours)
|
300 |
Not given |
| *Sand is taken to be smaller than 9.5 mm, with
only 5% of particles allowed larger than 4.75 mm. Pea gravel
is taken to be 9.5 mm top size. |
Colored Pervious Arrives in Washington State
 |
| Sultan, Washington, is home to a 20-home
subdivision built with pervious concrete for the road, driveways,
and colored sidewalks. |
In 2006, the City of Sultan, Wash. completed a 20-home residential
demonstration project. Stratford Place included 800 yd3
(600 m3) of pervious concrete to pave 32,000 ft2
(3000 m2), replacing traditional concrete for a road, connecting
driveways, and associated sidewalks. Integral color was added to the
sidewalk mix. Tinting the concrete not only improved the appearance
of the walkways, it delineated then. Because curbs and gutters are
not necessary for storm water control with pervious pavements, the
color difference visually separates the road from the sidewalk, making
the road safer for pedestrians.
This was only the second public road in Washington State to be
built with pervious pavement. Besides the storm water benefits,
using pervious pavement saved two lots for development, which would
otherwise have been needed for use as catch basins and vaults. Doing
away with the traditional storm water system and related items ultimately
saved the developer money and was better for the environment. Rick
Cisar, Sultan engineer, was so impressed with the Stratford Place
results that he plans to use pervious pavement for a city street.
He says pervious pavement resolves drainage problems, protects critical
areas and stream corridors, and helps to protect adjacent properties
from encroaching storm water.
Another project in Washington State was a residence on Whidby Island.
The use of two colored pervious concrete mixes for a small access
road and parking area really improved the appearance of the pavement.
References and Technical Resources
Davis, Kelly, “Learning Your APCs,” Concrete Producer
Magazine, Hanley-Wood LLC, Addison, Illinois, September 2006,
5 pages.
ACI 522, Pervious Concrete, ACI Committee Report 522R-06,
The American
Concrete Institute, Farmington Hills, Michigan, 2006, 25 pages.
(This provides technical information on pervious concrete's application,
design methods, materials, properties, mixture proportioning, construction
methods, testing, and inspection.)
Tennis, Paul D., Leming, Michael L., Akers, David J., Pervious
Concrete Pavements,
Engineering Bulletin 302, Portland Cement Association, Skokie, Illinois,
2004, 36 pages.
Stormwater
Management with Pervious Concrete Pavement (IS334P), American
Concrete Pavement Association, Skokie, IL, 2006, 6 pages.
PCA’s
Concrete Technology section on pervious concrete
NRMCA/PCA,
Pervious Concrete Pavement Web site
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