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Pervious Concrete
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Pervious Concrete and Freeze-Thaw
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| Pervious concrete pavement bike path in Lakewood
Park, Lakewood, OH. (Photo courtesy of Collinwood Concrete,
February 2006) |
Pervious concrete is one of the hottest topics in land development
today. As owners, architects, land developers, and concrete professionals
become familiar with its benefits, the interest in pervious concrete
continues to grow. The use of pervious concrete pavements provides
a solution to new requirements under Environmental Protection Agency
regulations that call for decreasing the amount of surface water runoff
and initially treating the runoff.
Pervious concrete is made of cementitious materials, water, admixtures,
and narrowly graded coarse aggregate. Very little or no fine aggregate
is used in the mixture. With just enough cement paste to coat the
aggregate, a system of interconnected voids (15 to 35%) is created
resulting in a highly permeable concrete that drains very quickly.
By allowing water to pass directly through the concrete, the amount
of surface water runoff is reduced dramatically. It can also be used
as part of a system to reduce the level of pollution contained in
storm water that is captured in the pervious pavement.
Pervious pavements have been used for years throughout the warmer
climates of the United States with excellent results. However, in
climates prone to severe freeze-thaw cycles, some are hesitant to
use pervious concrete pavements until it has been proven that pervious
concrete can be made to resist freeze-thaw damage.
Resistance of any concrete to freezing and thawing depends on the
permeability, the degree of saturation, the amount of freezable
water, the rate of freezing, and the average maximum distance from
any point in the paste to a free surface where ice can form safely.
The rate of freezing in most applications is dictated by the local
climate. Entrained air may help protect the paste as well.
Perhaps the most important aspect in designing pervious concrete
pavements for freeze-thaw areas is avoiding, or at least limiting,
saturation, especially during the time of year when freezing can
be expected. It is possible to design pervious concrete pavements
to control the degree of saturation and the average maximum distance
to a free surface. Proper subbase design and preparation are keys
to pulling rainwater, ice, and snowmelt away from the pavement and
ensuring suitable drainage.
Replacing as little as 7% of the coarse aggregate with fine aggregate
increases the freeze-thaw resistance; however, there will be a reduction
in voids of a few percent (Kevern 2006 and Mata 2008). In addition,
the paste (or mortar) should be protected by using air-entraining
admixtures to create a sufficient air-void system. Kevern, Wang,
and Schaefer (2008) found that the coarse aggregate properties play
a large role in providing freeze-thaw durability, with absorption
values below 2.5% being of greatest impact. The National Ready Mixed
Concrete Association (NRMCA 2004) has developed guidelines for using
pervious concrete in areas prone to freeze-thaw conditions.
NRMCA Recommendations
Dry Freeze and Hard Dry Freeze
Dry freeze areas are those parts of the country that undergo a number
of freeze-thaw cycles (15+) annually in which there is little precipitation
during the winter. If the ground stays frozen as a result of a long
continuous period of average daily temperatures below freezing,
the area is referred to as hard dry freeze area. Since pervious
concrete is unlikely to be fully saturated in this environment,
no special precaution is necessary for successful performance of
pervious concrete. However, a 100- to 200mm (4- to 8-in.) thick
layer of clean aggregate base below the pervious concrete is recommended
as an additional storage for the water. Many parts of the western
United States at higher elevations come under this category.
Wet Freeze
This includes areas of the country that undergo a number of freeze-thaw
cycles annually (15+) and there is precipitation during the winter.
Since the ground does not stay frozen for long periods, it is unlikely
that the pervious concrete will be fully saturated. No special precaution
is necessary for successful performance of pervious concrete, but
a 100- to 200mm (4- to 8-in.) thick layer of clean aggregate base
below the pervious concrete is recommended. The middle part of the
eastern United States falls under this category.
Hard Wet Freeze
Certain wet freeze areas where the ground stays frozen as a result
of a long continuous period of average daily temperatures below
freezing are referred to as hard wet freeze areas. These areas may
have situations where the pervious concrete becomes fully saturated
because frozen soil will have very low water permeability. The frost
penetration depth (depth at which the temperature is at 0 °C
[32 °F]) varies throughout the country. To design the pervious
concrete pavement for freeze-thaw resistance the following is suggested
by NRMCA.
- Calculate the frost penetration depth in your area. In the
Washington, D.C., area, for example, it is about 75 cm (30 in.).
- Calculate 65% of that. The Federal Aviation Administration
(FAA) says that the top 65% should contain non-frost-susceptible
materials and the bottom 35% may be in frost susceptible subgrade.
It should be noted that the FAA uses the 65% limitation to prevent
frost heave. In this case, the key factor is water infiltration.
This is about 50 cm (19.5 in.) for the 75 cm (30 in.) frost penetration
depth.
- Provide pervious concrete pavement plus aggregate base equal
to the number calculated. For a 50-cm (19.5-in.) calculation,
a 15-cm (6-in.) thick pervious concrete pavement over a 35-cm
(13.5-in.) thick aggregate base would be sufficient. The aggregate
base must consist of clean well draining open graded aggregate
base with less than 1.5% finer than 0.02 mm (0.5 mm).
If the frost depth is very high, for example 250 cm (100 in.) in
North Dakota, additional measures can be taken to reduce the chances
of a fully saturated pervious concrete pavement. A perforated PVC
pipe can be placed in the aggregate base to capture all the water
and let it drain. Pervious concrete in a freeze-thaw environment
should always be air-entrained to provide additional protection.
High Ground Water Table
Pervious concrete is not recommended in freeze-thaw environments
where the ground water table rises to a level less than 90 cm (3
ft) from the top of the surface or where substantial moisture can
flow from higher ground.
Deicing Chemical Usage
Deicing chemicals used to maintain an ice-free, safe pavement surfaces
for dense pavements may be used on pervious pavements; however,
in many cases deicers may not be required to maintain a slip resistant
surface. Snow fall followed by thawing temperatures allows snow
melt to pass through the pavement so rapidly that liquid water is
not available at the pavement surface to be refrozen as an ice coating.
Ice-free pavement surfaces aid safety for pedestrians and vehicles.
With appropriate plowing and limited use of deicers, moisture is
removed from the pavement surface, again preventing moisture from
freezing at the surface and causing icy conditions. As any melting
from the use of deicers occurs, the melt passes downward into the
pavement and in many cases leaves behind some un-dissolved deicer
making it available to future snow and ice events. Dosages of deicing
chemicals have been reported at reduced rates up to 70% (Houle 2008).
This leads to reduced use of deicing chemicals and clear safe pavement
surfaces with minimum cost for winter maintenance.
Summary
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| Example application of pervious concrete:
colored pervious concrete walkway in Bainbridege Island, WA.
(IMG15586) |
The benefits of pervious concrete pavements are well known, but
concerns over the freeze-thaw resistance may prevent many designers
from using pervious concrete in colder climates. There have been
several pervious concrete pavement projects in dry and wet freeze
areas demonstrating good field performance over several years. Research
on freeze-thaw resistance of pervious concrete pavement is ongoing
across the United States. Pervious pavements should be placed by
experienced installers and the structure and surrounding details
should be designed to accommodate the anticipated water flow and
drainage requirements.
References
Houle, Kristopher M., Winter
Performance Assessment of Permeable Pavements, Masters
Thesis, University of New Hampshire, September 2008, 142 pages.
Karthik Obla, NRMCA, Personal communication on May 18, 2006.
Kevern, John Tristan, Mix Design Development for Portland Cement
Pervious Concrete in Cold Weather Climates, Master’s
Thesis, Iowa State University, Ames, Iowa, 2006, 155 pages.
Kevern, John T.; Wang, Kejin, and Schaefer, Vernon R., The
Effect of Coarse Aggregate on the Freeze-Thaw Durability of Pervious
Concrete, SN3063, Portland Cement Association, Skokie,
Illinois, USA, 2008, 29 pages.
Mata, Luis Alexander, Sedimentation of Pervious Concrete Pavement
Systems, PhD Dissertation, North Carolina State University,
Raleigh, North Carolina, 2008. Also available as PCA SN3104.
Mindess, S.; Young, J. F.; and Darwin, D., Concrete, Prentice
Hall, Upper Saddle River, New Jersey, 2003.
NRMCA, Freeze-Thaw Resistance of Pervious Concrete, National
Ready Mix Concrete Association, Silver Spring, Maryland, 2004, 17
pages.
Portland Cement Association, Pervious
Concrete: Hydrological Design and Resources (CD), CD063,
Skokie, Illinois, 2006.
Portland Cement Association, Pervious
Concrete at the LEED™-Certified East Atlanta Library
(video), CD067, Skokie, Illinois, 2006.
Tennis, P. D., Leming, M. L., and Akers, D. J., Pervious
Concrete Pavements, EB302, Portland Cement Association,
Skokie, Illinois, and National Ready Mix Concrete Association, Silver
Spring, Maryland, 2004, 25 pages.
Storm
Water Phase II Final Rule: An Overview, EPA 833-F-00-001,
Fact Sheet 1.0, US Environmental Protection Agency, Office of Water,
January 2000, 4 pages. Available at:
Southeast Cement Association, Pervious
Concrete Pavements Website
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