Pervious Concrete
Buildings Home >
Sustainable Design >Pervious
Concrete The Smart Stormwater Solution
by Craig L. Morrison
It’s Time To Rethink Concrete
 |
| Pervious has to be poured very dry. It’s
not unusual to have to pull the material down the shoot. |
You know the stuff: impervious to water, channels runoff. But what
happens when—without sacrificing strength or durability—water
drains right through it? Consider if roads and driveways, sidewalks
and parking lots could let rain wash directly into the ground, where
it’s naturally filtered on its way to our aquifers. No runoff,
no drains, no catch basins, detention vaults or piping systems.
No kidding.
It’s called pervious concrete, and I think that it’s
about to change how we build things.
The Big Idea
Imagine concrete without the fine sand, and you have the coarse
aggregate texture of a grey Rice Krispies Treat, with a void structure
of 14 to 18 percent. Thick, rigid and good for 40-plus years; it
does require a special blend, and there are some tricky installation
issues, but the results have been proven to more than pay for themselves.
That wasn’t the case just seven to eight years ago when the
product first emerged in the Northwest. Land was less expensive
then, and stormwater issues were less at the forefront. When my
engineer, Noel Higa, Higa Burkholder Associates, brought pervious
to my attention in 1999, only a few forward-thinking scientists,
engineers, mixing plants and governmental planners were looking
into its viability—no jurisdiction in Washington State had
approved its use. That was then…
Pervious concrete is a two-part on-site stormwater management system
consisting of the concrete pavement and a coarse gravel retention
layer for stormwater storage. The system retains all water—including
all pollutants—on site. And, because the layers are porous,
air is present and micro-organisms flourish, eating away pollutants
and other non-desirable elements in the water.
Today, this system has been used successfully throughout most of
the rest of the country, but it seems that a major test case was
needed to demonstrate its advantages in our region. I never intended
to be a pioneer, but I’m excited about what we’ve learned.
The Ccritical Test
Stratford Place, a 20-lot plat in the City of Sultan, Wa., became
our test. My company, CMI Homebuilders, bought it in 2002 and then
ran into neighborhood stormwater issues. After considering the problem
from all angles, we determined that creating an entire pervious
stormwater management system would make the design financially viable
by eliminating stormwater piping, catch basins and detention systems
in favor of additional units. What’s more, the system would
be far superior environmentally.
Without realizing how audacious our proposal was, we submitted it
to City Planner Rick Cisar and City Engineer Jon Stack. After we
all did some learning together, these two men proved invaluable
in helping us secure the City of Sultan approval to use pervious
for the entire development, including roadways, parking areas, sidewalks
and driveways, as well as footing and roof stormwater.
There was just one catch: I had to give the city my personal guarantee
that all 32,000 square feet of product would work for a certain
number of years. As a result, I reluctantly became something of
an expert—and then my own supplier—to ensure the project’s
success, and started a new company, Pervious Concrete Inc., to supply
the pervious infrastructure at Stratford Place.
Through research, experimentation and testing, we figured out how
to control and place the mix to accommodate differences in temperature,
windspeed and other variables, load by load. We learned that proper
curing is critical, and how to ensure it.We found that most ready-mixed
concrete plants in western Washington can produce one or more pervious
mixes, and that we can tint the mix for better wayfinding.
Most importantly, we developed a consistent and repeatable placement
process (and are, at present, the only ACI-certified pervious flatwork
technicians and finishers in Washington State, outside of Vancouver).
And, while we were at it, we saved some $260K over a traditional
system.
Within 21 days after pouring, the road held up to all heavy concrete
and lumber trucks as well as general traffic, and it took water
as fast as a garden hose could flow, with virtually no water spread.
Despite heavy rains, there has been a complete absence of ponding
or overflow. In fact, not one drop of water left the site during
a record seven-week rainfall.
The Unique Challenges
Yes, pervious is trickier than standard concrete. System design
is site-specific, for example, and requires a soils survey and stormwater
calculations that factor in the percability and characteristics
of native soils.
If the pervious is too wet or overworked during placement, the voids
between the stone are reduced or eliminated, and it won’t
drain. If the concrete is too dry, it’s impossible to get
proper compaction for cross section strength, and if curing precautions
aren’t tightly maintained pervious concrete can fail. On the
other hand, when the site is properly prepared, you can significantly
reduce your potential for costly errors.
Care should be taken to keep the surface free from silty or clay-like
material, and to avoid clogging it with sand, top-soil, beauty bark
and other debris. Chemical cleansers are not recommended; plan to
use plain water to flush the pervious pavement voids, and to sweep
or vacuum one to two times a year to remove soil and debris.
The Major Advantages
Pervious concrete eliminates stormwater detention vaults, ponds
and piping systems, which are not only the most time-consuming and
costly elements in plat development, they can take up a couple of
otherwise profitable lots. Getting those lots back can often pay
for the entire system.
Environmentally, it just makes good sense to let rainwater directly
recharge our groundwater. By eliminating untreated stormwater and
runoff, the system mitigates “first flush” pollution
and protects our streams, watersheds and ecosystems in much the
same way as bioswale and natural soil drainage and filtration, rather
than concentrating pollutants by channeling stormwater.
Pervious doesn’t get as hot as standard concrete, which reduces
heat island effects, and it provides a higher albedo surface reflectivity
index (0.35 or higher).
In addition, pervious concrete has been designated an LID (Low Impact
Development) tool for stormwater management and a BMP (Best Management
Practice) by the EPA.
Concerned about plugging? Don’t be. Properly placed pervious
takes water at more than 200 inches per hour. From every viewpoint—dollars,
aesthetics, environment and performance—pervious concrete
makes good sense. Costs are coming down, acceptance is rising, and
my bet is that we’re all going to see a lot more of this smart
stormwater solution.
Craig L. Morrison is the owner and president of both CMI Homebuilders
and Pervious Concrete in Snohomish. He thanks the Smokey Point Concrete
team and GM Scott Mickels for their dedication and hard work, Bruce
Chattin from WACA, and especially the City of Sultan, Planner Rick
Cisar, Engineer Jon Stack, Public Works Director Connie Dunn, Building
Official Craig Bruner, the mayor and the city council for their
support of Stratford Place.
|  |
|