WHY Use Pervious Concrete?
The proper utilization of pervious concrete is a recognized
Best Management Practice by the U.S. Environmental Protection
Agency (EPA) for providing first-flush pollution
control and storm water management. As regulations further
limit storm water runoff, it is becoming more expensive
for property owners to develop real estate, due to the
size and expense of the necessary drainage systems. Pervious
concrete reduces the runoff from paved areas, which
reduces the need for separate storm water retention ponds
and allows the use of smaller capacity storm sewers. This
allows property owners to develop a larger area of available
property at a lower cost. Pervious concrete also naturally
filters storm water and can reduce pollutant loads entering
into streams, ponds and rivers. Pervious concrete
functions like a storm water retention basin and allows the
storm water to infiltrate the soil over a large area, thus facilitating
recharge of precious groundwater supplies locally.
All of these benefits lead to more effective land use.
Pervious concrete can also reduce the impact of development
on trees. A pervious concrete pavement allows the
transfer of both water and air to root systems allowing trees
to flourish even in highly developed areas.
HOW to Create Pervious Concrete Pavement?
An experienced installer is vital to the success of pervious
concrete pavements. As with any concrete pavement, proper
subgrade preparation is important. The subgrade should be
properly compacted to provide a uniform and stable surface.
When pervious pavement is placed directly on sandy
or gravelly soils it is recommended to compact the subgrade
to 92 to 96% of the maximum density (ASTM D 1557).
With silty or clayey soils, the level of compaction will depend
on the specifics of the pavement design and a layer of
open graded stone may have to be placed over the soil. Engineering
fabrics are often used to separate fine grained
soils from the stone layer. Care must be taken not to overcompact
soil with swelling potential. Moisten the subgrade
prior to concrete placement, and wheel ruts from the construction
traffic should be raked and re-compacted. Moistening
the subgrade prevents pervious concrete from setting
and drying too quickly
Typically pervious concrete has a water to cementitious
materials (w/cm) ratio of 0.35 to 0.45 with a void content
of 15 to 25%. The mixture is composed of cementitious
materials, coarse aggregate and water with little to no fine
aggregates. Addition of a small amount of fine aggregate
will generally reduce the void content and increase the
strength, which may be desirable in certain situations. This
material is sensitive to changes in water content, so field
adjustment of the fresh mixture is usually necessary. The
correct quantity of water in the concrete is critical. Too much
water will cause segregation, and too little water will lead
to balling in the mixer and very slow mixer unloading. Too
low a water content can also hinder adequate curing of the
concrete and lead to a premature raveling surface failure. A
properly proportioned mixture gives the mixture a wet-metallic
appearance or sheen.
A pervious concrete pavement may be placed with either
fixed forms or slip-form paver. The most common approach
to placing pervious concrete is in forms on grade that have
a riser strip on the top of each form such that the strike off
device is actually 3/8-1/2 in. (9 to 12 mm) above final pavement
elevation. Strike off may be by vibratory or manual
screeds, though vibratory screens are preferable. After striking
off the concrete, the riser strips are removed and the
concrete compacted by a manually operated roller that
bridges the forms. Rolling consolidates the fresh concrete
to provide strong bond between the paste and aggregate,
and creates a smoother riding surface. Excessive pressure
when rolling should be avoided as it may cause the voids to
collapse. Rolling should be performed immediately after
strike off.
Jointing pervious concrete pavement follows the same rules
as for concrete slabs on grade, with a few exceptions. With
significantly less water in the fresh concrete, shrinkage of
the hardened material is reduced significantly, thus, joint
spacings may be wider. The rules of jointing geometry, however,
remain the same (See CIP 6). Joints in pervious concrete
are tooled with a rolling jointing tool. This allows
joints to be cut in a short time, and allows curing to continue
uninterrupted.
Proper curing is essential to the structural integrity of a pervious
concrete pavement. Curing ensures sufficient hydration
of the cement paste to provide the necessary strength
in the pavement section to prevent raveling. Curing should
begin within 20 minutes of concrete placement and continue
through 7 days. Plastic sheeting is typically used to
cure pervious concrete pavements.
management plan, the designer should ensure that it is functioning
properly through visual observation of its drainage
characteristics prior to opening of the facility. Questions
have been raised about the freeze thaw durability of pervious
concrete. Even though most of the experience with pervious
concrete has been in warmer climates recently there
have been several pervious concrete projects in colder climates.
Pervious concrete in freeze thaw environment must
not become fully saturated. Saturation of installed pervious
concrete pavement can be prevented by placing the pervious
concrete on a thick layer of 8 to 24 inches (200 to 600
mm) of open graded stone base. Limited laboratory testing
has shown that entrained air may improve the freeze thaw
durability even when the pervious concrete is in a fully saturated
condition. However, the entrained air content cannot
be verified by any standard ASTM test procedure.
EPA recommends that pervious concrete pavement be
cleaned regularly to prevent clogging. Cleaning can be accomplished
through vacuum sweeping or high pressure
washing. Even though pervious concrete and the underlying
soil provide excellent filtration capabilities, all the contaminants
may not be removed. In critical situations to preserve
the quality of ground water, storm water testing is
recommended.
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