by Fernando Pagés
In a natural landscape, most of the rain that falls on the
ground seeps through the soil and replenishes the water table.
But when rain hits conventional pavement, it washes off —
along with oil and other contaminants — into a nearby
municipal storm sewer system or directly into the watershed.
Pervious concrete pavement, however, is different: It acts like
gravel and allows water to drain right through it into the
Because of its environmental benefits and a regulatory climate
that makes strict stormwater regulations likely in the near
future, pervious concrete is generating a lot of buzz. So when
the time came to pave a private roadway in a small subdivision
that I'm building in Lincoln, Neb., I was eager to give it a
Like regular concrete, pervious concrete consists of cement,
gravel, and water. Unlike regular concrete, it contains little
— or no — sand. As a result, it's filled with voids
that allow water to percolate through its surface at up to 18
gallons per minute per square foot of concrete (see Figure
Figure 1. Made largely without fine
aggregates, pervious concrete has a porous structure that
allows up to 18 gallons of water per minute per square foot to
flow through it — yet the material can have a compressive
strength of more than 3,000 psi when fully cured.
When stormwater collects and passes through pervious concrete
pavement, large sediments are filtered out, hydrocarbons and
other pollutants are biologically degraded by bacteria
colonizing the concrete, and heavy metals settle toward the
bottom. As the water filters through the stone base supporting
the pavement and then through the soil underneath, bacteria
continue to digest pollutants; by the time the water reaches
the aquifer, it's been scrubbed clean.
In essence, pervious pavement — along with its stone base
— serves as a giant sponge, absorbing rainwater and
runoff and buffering flows into municipal storm sewers and
nearby streams. It can reduce site drainage requirements and is
recommended as an effective way to reduce runoff by the LEED
(Leadership in Energy and Environmental Design) green-building
rating system. In some municipalities, the use of pervious
concrete can contribute to the required impervious-to-pervious
surface ratios established by the Clean Water Act, in effect
lowering stormwater fees and allowing for more square footage
of building and parking.
Originally tested for flood-control purposes in Florida,
pervious concrete has been adapted for use in cold climates,
too. One advantage in those applications is that the porous
structure of the concrete allows heat from the ground to rise
and liquefy snow from underneath. The water drains through the
pavement, leaving behind a drier surface that requires less
snow and ice removal.
Working on Site
The logistics of making pervious concrete are tricky. For this
highly porous concrete to provide a durable surface, the
mixture must contain an exact proportion of aggregates and
water plus special chemical admixtures, be properly mixed
during delivery, and be expertly placed. Pervious pavement is
not so much a concrete product as it is a paving system
consisting of soil, subgrade, and — of course — the
It's not a job for amateurs, so to build my pervious alley I
hired one of the best paving contractors in my area,
Constructors Inc. For technical help I turned to the Nebraska
Concrete and Aggregates Association. And the largest ready-mix
supplier in Nebraska, NEBCO, offered its labs to help develop a
climate-appropriate mix. NEBCO employees even poured an
experimental test patch at the plant to make sure everything
would go smoothly on my site.
Subgrade prep. Since water drains through pervious
concrete, it must have somewhere to go. Pervious concrete can
sometimes be laid on grade when placed over sandy soils, but
most soils require a gravel bed of at least 6 inches thick;
this bed acts as a drainage plane that holds the stormwater as
it slowly percolates into the soil (Figure 2). In my area
— where the soil is high in clay and water freezes in
winter — we wanted to be sure that any moisture in the
concrete would immediately drain, so we overexcavated the
subgrade by 12 inches. We rolled out a layer of nonwoven
geotextile against the soil to prevent the gravel bed from
becoming silted with mud, then put down 10 inches of clean,
lightly compacted No. 57 stone (1 1/2-inch to 3/4-inch crushed
limestone) (Figure 3).
Figure 2. Pervious concrete requires a
thick, well-drained gravel layer underneath, particularly in
colder climates where trapped water can freeze and crack even
the sturdiest pavement.
Figure 3. When designing a pervious
concrete surface, a free-draining substrate is the key to good
performance. The author used 10 inches of stone because the
soil beneath had high clay content.
For insurance, we also installed three rows of French drains
that connected to the storm sewer system. If unusually high
precipitation causes the thick gravel bed to flood, the storm
sewer will serve as an emergency overflow valve.
While conventional concrete is always pitched to drain, the
gravel bed under pervious concrete must remain level to
maximize water retention and percolation into the soil. In our
case, this requirement was an advantage, since we were having
trouble establishing a water-shedding grade for our
400-foot-long alley. Using pervious concrete, we were able to
pour the road low and level and still manage surface flows from
adjacent driveways and lawns.
For more information on pervious concrete, contact the
National Ready Mixed Concrete Association
(888/846-7622, www.nrmca.org), which offers a number of
publications and reports on pervious concrete. Also,
the American Concrete Institute's manual "Pervious
Concrete" (248/848-3700, www.aci-int.org) contains
technical information on all aspects of working with
the material. In addition, many state concrete
aggregate associations offer pervious-concrete training
and certification, administered in conjunction with the
A climate-suitable mix. A typical yard of pervious
concrete contains coarse 3/8-inch (pea gravel) aggregate, six
or seven sacks of portland cement, and water in a
water-to-cement ratio of .25-to-.30 (which is about a third
drier than standard concrete). Because of the low water
content, pervious concrete is a difficult mix to work with: It
can't be pumped, it doesn't pour out of the truck or spread
easily, and its high porosity allows air to get in and
hydration to take place quickly. To combat these
characteristics, concrete designers include admixtures to slow
hydration, stabilize the mix, and provide a slightly longer
working time. Even so, a pour must be placed, finished, and
covered within 20 minutes.
On this job, the admixtures included air entraining agents.
These chemicals add microscopic air pockets to the cement paste
so that moisture has a place to expand, which prevents cracking
in freezing temperatures. To improve workability, water
reducers and viscosity modifiers were added, too. The objective
was to have a practical mix that was relatively easy to place
and that would yield a pavement with a 15 percent to 20 percent
void structure — somewhat resembling a rice cake.
The ready-mix company also added a small percentage of fines
— in this case, fly ash (a recycled material derived from
coal-burning power plants) — to increase compressive
strength. We wanted to make sure that the concrete surfaces
would not crumble over time; none of us wanted to come back to
tear out and replace the pavement.
Proper placement and curing. Pouring pervious concrete
demands a highly coordinated effort. Getting the dry, virtually
zero-slump mix out of the truck and down the chute, then
spreading and striking it — all within 20 minutes —
requires plenty of manpower and crew members who know what
In this case, one worker directed the chute while forcing the
stiff mix down with his hand. As the concrete spilled to the
ground, the crew raked and shoveled hard to spread it like
heavy, damp gravel from end to end. A mechanical vibratory
screed, set 1/2 inch above the surface, came right behind,
striking the concrete level (Figure 4).
Figure 4. Fresh pervious concrete is dry
and stiff, and requires plenty of manpower to place (top). A
vibratory screed followed by a roller consolidates the mix
(bottom). No floating, troweling, edging, or control joints are
necessary (unless wanted for aesthetic reasons), which helps
offset the higher costs for labor and subgrade
Next, instead of floating and troweling, the pavement was
simply consolidated with a steel pipe roller. With pervious
concrete, the surface profile is not critical, as dimples and
dips will not become birdbaths on a leaky surface. On the other
hand, overworking the surface can seal it and defeat the
purpose. "Strike, screed, and roll" is all that's needed.
Immediately after compacting with the steel roller, workers
fogged the concrete with a soybean-based curing compound (C2
Products, 800/575-5988, www.c2products.com), then covered the
surface with polyethylene sheeting, all within 15 minutes of
placement (Figure 5).
Figure 5. To slow hydration, newly placed
pervious concrete is sprayed with a soybean-based curing
compound (A, B), then quickly covered with polyethylene
sheeting (C). The sheeting is weighted down with sandbags and
left in place for seven days (D), and the paving is allowed to
fully cure for 21 days before being opened up to vehicle
Because of pervious concrete's highly porous structure and very
low water content, moisture evaporates from it quickly, which
can compromise the cure. Spraying water on the surface to slow
hydration is not an option because it could cause the cement
paste to settle along the bottom, leaving an impermeable layer
below and a lean, crumbly crust above.
By the yard, pervious concrete doesn't cost much more than
conventional concrete; in fact, since there's no finishing
involved, it's a little cheaper to install. With no floating,
finishing, edging, or cutting, my paving contractor was able to
install our 14-foot-wide by 400-foot-long strip in about four
hours. On the other hand, we found that the extra site work
involved in building the drainage bed almost doubled our
overall project cost.
We also found that construction dirt presents a big challenge.
To keep tracked mud from clogging the concrete, we used a lot
of plastic and did a great deal of shoveling and cleaning. A
concrete surface needs regular maintenance, as well —
sweeping, vacuuming, or power-washing — to remove normal
dirt and debris that otherwise would accumulate in the
Today, after many careful inspections and several freezing
nights, the concrete in this project appears to be performing
as designed. Rainwater never puddles and the roadway dries off
within minutes. Moreover, a recent 2-inch snowfall simply
disappeared, no plowing required.
Fernando Pagés Ruiz is a building
contractor in Lincoln, Neb.