http://www.perviousconcrete.com/what_pervious_factors.htm
Pervious concrete pavement is a unique and effective means to address important environmental
issues and support green, sustainable growth. By capturing stormwater and allowing it to seep into the
ground, porous concrete is instrumental in recharging groundwater, reducing stormwater runoff, and
meeting U.S. Environmental Protection Agency (EPA) stormwater regulations. In fact, the use of
pervious concrete is among the Best Management Practices (BMPs) recommended by the EPA-- and
by other agencies and geotechnical engineers across the country-- for the management of stormwater
runoff on a regional and local basis. This pavement technology creates more efficient land use by
eliminating the need for retention ponds, swales, and other stormwater management devices. In doing
so, pervious concrete has the ability to lower overall project costs on a first-cost basis.
In pervious concrete, carefully controlled amounts of
water and cementitious materials are used to create a paste that forms a
thick coating around aggregate particles. A pervious concrete mixture
contains little or no sand, creating a substantial void content. Using
sufficient paste to coat and bind the aggregate particles together creates
a system of highly permeable, interconnected voids that drains quickly.
Typically, between 15% and 25% voids are achieved in the hardened
concrete, and flow rates for water through pervious concrete are typically
around 480 in./hr (0.34 cm/s, which is 5 gal/ft²/ min or 200 L/m²/min),
although they can be much higher. Both the low mortar content and high
porosity also reduce strength compared to conventional concrete
mixtures, but sufficient strength for many applications is readily achieved.
While pervious concrete can be used for a surprising number of
applications, its primary use is in pavement. This site focuses on the
pavement applications of the material, which also has been referred to as
porous concrete, permeable concrete, no-fines concrete, gap-graded
concrete, and enhanced-porosity concrete.
http://www.concretenetwork.com/pervious/florida_pervious_concrete_guide
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Pervious Concrete Pavement is a water management system.
Its bottom component is a soil base that will let water percolate through it to
the aquifer below. This includes sandy and gravely soils. A layer of clay may
require a system to allow water to seep through to the aquifer below which
could include pipes, French drains or removing some clay. A capable site
engineer's services are a valuable tool at this time. Below is a picture of a
typical well draining soil.
When such soil is not available, some times a crushed concrete or aggregate
base is installed to store water under the pervious concrete paving. This
allows the filtered water to slowly pass through the less cooperative soils by
capillary seepage.
Aggregate Base under Pervious Concrete being poured. In this case the
aggregate prevents a build up of water in the pervious. This is extremely
important during freeze-thaw conditions.
For the system to function properly the correctly designed mixture must be
loaded properly at the ready-mix plant, delivered promptly by a truck driver
who is thoroughly trained in working with pervious ready-mix, and willing to
thoroughly cooperate with the installation crew overseer.
As the trucks arrive to discharge the pervious concrete, they should be
accommodated by a properly leveled base set to correct elevations and thoroughly
compacted. No deep ruts should be created by the truck tires.
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Base preparation includes testing existing undisturbed sub-soil for its ability
to percolate. If the water table is at least 4 ft. below grade and septic fields
work in the area, probably the permeability of the sub soil will be adequate.
The base soil should be appraised by a site engineer. In a few cases this will
involve taking test borings. Again we stress all organic materials such as
roots, grass and trash must be thoroughly removed. Then a compactable,
porous sub-base should be installed, and compacted with a vibratory
compactor to a level of proctor level of approximately 92%. where sandy,
compactable sub-soils are fould they may be converted into a base for
pervious concrete. A properly compacted base will not be damaged by
ready-mix trucks delivering the pervious concrete.
If the soil is slow draining a base of crushed concrete, or other stable
aggregate should be installed and compacted to a level of 95%. This is
usually 6" thick or more.
The
Installed
Base.
While the engineered mixture is
important there are other factors to
consider such as: having a suitable
base of soil, sand or crushed stone;
having a drainable water table,
sufficiently below the pavement is also
important. Other factors are the proper
preparation
of
an
appropriate
compacted, sub-base free of all organic
matter, the correct concrete mixture,
the designed mixing procedure, prompt
placement, finishing and proper curing.
Curing, really means hydration, a
chemical process that requires the
presence of moisture. To prevent
evaporation, Pervious Concrete should
be covered with polyethylene film for a week or more. The sub-base can
consist of sand, crushed stone or a combination of drainable materials. If
crushed stone is used the base can serve as a detention pond.
Pervious concrete is usually delivered to the site in Ready Mix trucks and
mixed at a specified mixing speed to result in 75 to 100 rotations of the
mixing drum before discharge. After the closely controlled amount of water
is added, one hour is usually the maximum time allowed for discharge. The
batch must be placed, rolled, sprayed and covered quickly. This closely
monitored process produces a pavement with unique characteristics. This
allows it to recharge the aquifer beneath, to prevent puddles and to reduce
drainage into storm sewers, streams and adjacent properties.
Some of our competitors claim Pervious Concrete will allow 8 to 12 gallons
per square foot per minute to pass through it. This will depend on the subbase and size and type of aggregate. Our tests with a 3/8" aggregate
indicate that it will allow 3 to 10 gallons of water to pass through it per
minute, compared to 2.5 to 4 gallons for Bahia grass. Larger aggregate in an
appropriate mix design will drain better. In any event,1½" of rainfall, a good
first flush, will represent .6234 gallons, almost five times the average three
gallon minimum, if it fell within 60 seconds, an unusually heavy rainfall. How
well it performs is almost always determined by what lies below it. Total
percolation is mostly dependent upon base construction, aggregate size and
existing soil and water table conditions.
Pervious Concrete resists degradation from stress during curing. Curing is a
chemical process called hydration during which molecular reactions between water
and cement create a unique chemical bond which gives the Pervious Concrete
unrecognized strength. Since the Pervious mixture dries with much less shrinkage
than most other paving concretes, our experience has demonstrated that control
joints are needed much less than with conventional concrete. Once cured it does
not distort under traffic as does asphalt which often develops ruts and ripples under
the hot sun, and the friction from vehicle tires.
Because of its open cell structure, Pervious Concrete reduces the reflection
of heat into the atmosphere. Unlike asphalt it also absorbs much less heat
from the sun (50% to 75%). When it rains it does not heat the storm water
runoff like asphalt, which eventually heats the bodies of water into which it
is discharged. The rain water retained also cools the air above the concrete,
because it acts like perspiration evaporating to cool your skin.
When oil or grease drips on to pervious that same open cell structure
enables volatiles to evaporate, and remaining solids to pass into the soil
below, where microbial activity may consume or convert them. Because of
the heavy, strong, Portland cement paste component, it forms strong
molecular bonds that grow during hydration. Since most aggregate is porous
it forms a mechanical bond as well and creates a pavement with greater
strength and durability. If compaction by vibrating screed and rolling is not
done quickly this mechanical bond will not be achieved and raveling will
occur under traffic.
The appropriate thickness for most pavements is 6". However thicker
pavements may be designed for heavy loads. The skills of concrete
engineers should be put to use in designing heavy use pavements.
http://www.georgiastormwater.com/vol2/3-3-7.pdf
3.3.7.2 Pollutant Removal Capabilities
As they provide for the infiltration of stormwater runoff, porous concrete systems have a high
removal of both soluble and particulate pollutants, where they become trapped, absorbed or
broken down in the underlying soil layers. Due to the potential for clogging, porous concrete
surfaces should not be used for the removal of sediment or other coarse particulate pollutants.
The following design pollutant removal rates are conservative average pollutant reduction
percentages for design purposes derived from sampling data, modeling and professional
judgment.
Total Suspended Solids – not applicable
Total Phosphorus – 50%
Total Nitrogen – 65%
Fecal Coliform – insufficient data
Heavy Metals – 60%
Pollutant removal can be improved through routine vacuum sweeping and high pressure washing,
insuring a drainage time of at least 24 hours, pretreating the runoff, having organic material in the
subsoil, and using clean washed aggregate (EPA, 1999).
The cross-section typically consists of four layers, as shown in Figure 3.3.7-1. The aggregate
reservoir can sometimes be avoided or minimized if the sub-grade is sandy and there is
adequate time to infiltrate the necessary runoff volume into the sandy soil without by-passing
the water quality volume. Descriptions of each of the layers is presented below:
Porous Concrete Layer – The porous concrete layer consists of an open-graded concrete
mixture usually ranging from depths of 2 to 4 inches depending on required bearing strength
and pavement design requirements. Porous concrete can be assumed to contain 18 percent
voids (porosity = 0.18) for design purposes. Thus, for example, a 4 inch thick porous
concrete layer would hold 0.72 inches of rainfall. The omission of the fine aggregate provides
the porosity of the porous pavement. To provide a smooth riding surface and to enhance
handling and placement a coarse aggregate of 3/8 inch maximum size is normally used. Use
GDOT No. 8 coarse aggregate (3/8 to No. 16) per ASTM C 33 or No. 89 coarse aggregate
(3/8 to No. 50) per ASTM D 448. See the GCPA specifications (referenced).
Top Filter Layer – Consists of a 0.5 inch diameter crushed stone to a depth of 1 to 2 inches.
This layer serves to stabilize the porous asphalt layer. Can be combined with reservoir layer
using suitable stone.
Layer – The reservoir gravel base course consists of washed, bank-run gravel, 1.5
to 2.5 inches in diameter with a void space of about 40% (GADOT No.3 Stone). The depth of
this layer depends on the desired storage volume, which is a function of the soil infiltration
rate and void spaces, but typically ranges from two to four feet. The layer must have a
minimum depth of nine inches. The layer should be designed to drain completely in 48 hours.
layer should be designed to store at a minimum the water quality volume (WQ v). Aggregate
contaminated with soil shall not be used. A porosity value (void space/total volume) of 0.32
should be used in calculations unless aggregate specific data exist. .
Bottom Filter Layer – The surface of the subgrade should be an 6 inch layer of sand (ASTM
C-33 concrete sand or GADOT Fine Aggregate Size No. 10) or a 2 inch thick layer of 0.5 inch
crushed stone, and be completely flat to promote infiltration across the entire surface. This
layer serves to stabilize the reservoir layer, to protect the underlying soil from compaction,
and act as the interface between the reservoir layer and the filter fabric covering the
underlying soil.
Filter Fabric – It is very important to line the entire trench area, including the sides, with filter
fabric prior to placement of the aggregate. The filter fabric serves a very important function by
inhibiting soil from migrating into the reservoir layer and reducing storage capacity. Fabric
should be MIRFI # 14 N or equivalent.