Although not a new technology, pervious concrete (it was first used in 1852) is receiving renewed interest. The high flow rate of water through a pervious concrete pavement allows rainfall to be captured and to percolate into the ground, reducing storm water runoff, recharging groundwater this unique ability of pervious concrete offers advantages to the environment, public agencies, and building owners by controlling rainwater on-site and addressing storm water runoff issues. Depending on local regulations and environment, a pervious concrete pavement and its sub-base may provide enough water storage capacity to reduce the need for retention ponds, rain-water harvesting projects and other precipitation runoff containment strategies. This provides more efficient land use and is one factor that has led to a renewed interest in pervious concrete. Other applications that take advantage of the high flow rate through pervious concrete include drainage media in parking lots, tennis courts, greenhouses, and pervious base layers under heavy-duty pavements.
This paper includes applications, uses, installation, and case study of pervious concrete.
Keywords: Pervious concrete, Environment, Ground Water Recharge etc.
1.1 What is Pervious Concrete?
Pervious concrete is a mixture of coarse aggregate, Portland cement, water and little to no sand. A typical pervious concrete pavement has a 15-25% void structure and allows 125 to 325 litres of water per minute to pass through each square metre.
Carefully controlled amounts of water and cementitious materials are used to create a paste that forms a thick coating around aggregate particles without flowing off during mixing and placing. Using just enough paste to coat the particles maintains a system of interconnected voids.
The result is a very high permeability concrete that drains quickly. Due to the high void content, pervious concrete is also lightweight, 1600 to 1900 kg/m3.
After placement, pervious concrete resembles popcorn. Its low paste content and low fine aggregate content make the mixture harsh, with a very low slump. The compressive strength of pervious concrete is limited since the void content is so high. However, compressive strengths of 3.5 to 27.5 MPa are typical and sufficient for many applications.
1.2 Pervious Concrete Pavements
Pervious Concrete can be successfully used for pavements and is environmentally friendly paving material that offers the durability of standard concrete while retaining stormwater runoff and replenishing local watershed systems
Paved surfaces are everywhere in urban areas today that most of us give little thought to the impact they have on water quality and the health of the environment. But here’s the sobering reality: As more available land area in the country gets paved over, a larger amount of rainwater ends up falling on impervious surfaces such as parking lots, driveways, sidewalks, and streets rather than soaking into the soil. This creates an imbalance in the natural ecosystem and leads to a host of problems including erosion, flash floods, water table depletion, and pollution of rivers, lakes, and coastal waters as rainwater rushing across pavement surfaces picks up everything from oil and grease spills to deicing salts and chemical fertilizers.
A simple solution to avoiding these problems is to stop installing the impervious surfaces that block natural water infiltration into the soil. But few of us are ready to give up our paved roads, driveways, and parking lots. Rather than building them with conventional concrete or asphalt, more and more communities, municipalities, and businesses are switching to pervious concrete or porous pavement, a material that offers the inherent durability and low life-cycle costs of a typical concrete pavement while retaining stormwater runoff and replenishing local watershed systems.
1.3 Where Pervious Concrete is Being Used?
While pervious concrete pavements may be new to some areas of the country, Florida and other southeastern states have been installing them since the 1970s to control runoff, erosion, and flooding. Florida has been a leader in the construction of pavements using pervious concrete, according to the Florida Concrete & Products Association. Hundreds of projects have been completed statewide, with many pavements in service for more than 10 years.
More recently, pervious concrete has been embraced by the West Coast for its environmental benefits. For example, pervious concrete is helping communities in California and Washington restore groundwater supplies and reduce pollution of coastal waters, which can endanger fragile aquatic ecosystems and even swimmers. In a study conducted by researchers at the University of California, Irvine, contaminated runoff has been linked with waterborne illnesses in surfers in urban areas.
In the Pacific Northwest, most of the annual precipitation comes from rainfall events of less than an inch. A stormwater management system using pervious concrete can be very effective at reducing total runoff and increasing the amount of filtered groundwater, according to Glacier Northwest, the area’s largest concrete supplier.
“It’s a great system in that it provides both day-to-day mitigation as well as catastrophic containment,” says Andy Youngs, a technical representative with the California Nevada Cement Promotion Council and a specialist in stormwater management. “With the change in EPA stormwater runoff regulations, pervious concrete has really come into play because of its use in controlling pollution. In California, the concrete industry knew about pervious concrete, but didn’t understand the fit and what a great product it is. It took some time to get everyone onboard, but now we’re starting to hit critical mass and see the use of pervious concrete explode.”
In California, pervious concrete is primarily being used to pave parking lots, says Youngs. But he notes that its also becoming a practical alternative for subdivision streets, sidewalks, and golf cart paths.
More homeowners are using pervious concrete as well, to eliminate puddling, prevent erosion, and save the expense of tying into local storm sewer systems. Typical applications include driveways, walkways, pool decks, and patios.
1.4 Limitations of Pervious Concrete
While this thirsty pavement is an excellent option for certain situations, it may not always be a viable choice.
Because pervious concrete has a rough-textured, honeycombed surface, moderate amounts of surface raveling are normal. This can be a problem on heavily traveled roadways.
“The main reason pervious concrete is not used for high-traffic pavements, such as highways, is surface raveling,”
says Youngs, who notes that tire sheer can loosen the aggregate at the surface. One potential solution being looked at is to grind down the pavement surface about half an inch.
Special attention must also be given to the overall design of the pavement system in order for pervious concrete to perform as intended. Proper engineering of the substrate beneath the pavement is essential, since it must be able to temporarily store the water while it percolates into the soil. An initial soils site survey and site-specific stormwater calculations should be performed by a stormwater management engineer.
2. Benefits of Pervious Concrete Pavements
2.1 Economic Benefits
In general, initial costs for pervious concrete pavements are higher than those for conventional concrete or asphalt paving. But total costs can be substantially lower.
The material itself is only a little more expensive, but we tend to install pervious concrete thicker than regular concrete, says Youngs. The reason is that we know the water is going to go through and saturate the subgrade underneath. So we have to design for a weaker subgrade. With a pervious parking lot, we may go 6 inches thick versus 4 inches for conventional concrete.
But he adds that when you compare overall installation and life-cycle costs, pervious concrete is the clear winner. You just cant look at per square foot costs. You have to look at overall system costs, he says. For parking lot owners, pervious concrete is a sustainable product that actually saves them money. It ends up being less expensive than a conventional parking lot.
Among the reasons why:
• Lower installation costs
According to the Center for Watershed Protection, installing traditional curbs, gutters, storm drain inlets, piping, and retention basins can cost two to three times more than low-impact strategies for handling water runoff, such as pervious concrete. Projects that use pervious concrete typically don’t need storm sewer ties-ins, which eliminates the cost of installing underground piping and storm drains. Grading requirements for the pavement are also reduced because there is no need to slope the parking area to storm drains.
• Permits the use of existing sewer systems
Pervious concrete may also reduce the need for municipalities to increase the size of existing storm sewer systems to accommodate new residential and commercial developments. Cities love pervious concrete because it reduces the need to rebuild storm sewer systems when new developments go up, says Youngs.
• Increased land utilization
Because a pervious concrete pavement doubles as a stormwater management system, there is no need to purchase additional land for installing large retention ponds and other water-retention and filtering systems. That means developers and property owners can use land more efficiently and maximize the return on their investment.
• Lower life-cycle costs
Pervious concrete is a sustainable paving material, with a life expectancy equal to that of regular concrete. Most parking areas, when properly constructed, will last 20 to 40 years, according to the Southern California Ready Mixed Concrete Association.
2.2 Environmental Benefits
Stormwater runoff is a leading source of the pollutants entering our waterways. According to the U.S. Environmental Protection Agency, about 90 percent of surface pollutants are carried by the first 1-1/2 inch of rainfall. Stormwater drains don’t typically channel this polluted runoff to treatment facilities, but instead convey it directly into local water bodies. This can increase algae content, harm aquatic life, and require expensive treatments to make the water potable.
According to Youngs, the big three pollutants in urban runoff are sediment (dirt and debris), heavy metals (from the brake linings of cars), and hydrocarbons. One source of hydrocarbons is the oil that drips onto pavements from vehicles. But the primary contributor is asphalt. Studies have shown that 90 to 95 percent of the hydrocarbons in urban runoff is from the binder and sealer used for asphalt pavements, he says.
To address these serious pollution concerns, the EPA and many local municipalities and regional watershed authorities are tightening environmental regulations and requiring more stringent stormwater management practices. Pervious concrete is becoming one of the most viable solutions. Here are some of the reasons why.
A pervious concrete pavement can:
• Reduce the amount of untreated runoff discharging into storm sewers.
• Directly recharge groundwater to maintain aquifer levels.
• Channel more water to tree roots and landscaping, so there is less need for irrigation.
• Mitigate pollutants that can contaminate watersheds and harm sensitive ecosystems.
• Eliminate hydrocarbon pollution from asphalt pavements and sealers.
In addition to stormwater control, pervious concrete pavements aid in reducing the urban heat-island effect. Because they are light in color and have an open-cell structure, pervious concrete pavements don’t absorb and store heat and then radiate it back into the environment like a typical asphalt surface. The open void structure of the pervious pavement also allows cooler earth temperatures from below to cool the pavement.
The lighter color of concrete is also beneficial from an energy-savings standpoint. Because the concrete is reflective, the need for lighting at night is reduced.
Beyond helping the environment, pervious concrete pavements are also safer for drivers and pedestrians. Because pervious concrete absorbs water rather than allowing it to puddle, it reduces hydroplaning and tire spray. In California, parks are installing pervious concrete pathways to provide disability access for people in wheelchairs.
3. Installation of Pervious Concrete
Pervious concrete is delivered to the jobsite by conventional ready-mix trucks and placed within standard forms. Because pervious concrete is thicker in consistency than regular concrete, a vibrating mechanical screed is used to level it off. Vibration is followed by compaction with a heavy steel roller to attain greater strength.
Because pervious concrete has a low water content, curing is especially critical. After placement, the concrete is misted with water and then covered with plastic sheeting and kept damp for at least 7 days to allow full hydration of the cement.
Often paving crews can complete pervious concrete jobs faster than when installing regular concrete. That’s because pervious concrete doesn’t need to be worked with a bull float or trowel, since these finishing operations can seal off the pavement surface and decrease water penetration.
Ten Strategies for Ensuring a Successful Pervious Concrete Installation
Pervious concrete has many wonderful properties, but without an experienced contractor and ready-mix supplier, a failed installation is possible. Pervious concrete is difficult to place and finish properly. The right mix and proper compaction and curing are critical to success. Attention to the following steps will help improve the performance and durability of your pervious pavement installation.
1. Design the pavement system to prevent saturation of the pavement during freeze-thaw cycles.
2. In freezing climates, prevent water runoff from buildings or adjacent impervious pavements from draining onto frozen pervious concrete.
3. Pervious concrete pavement systems should only be used where the underlying soil percolates well or when there is a subbase drainage system.
4. To get the proper mix, work with an experienced ready-mix supplier and insist on 600 pounds per cubic yard of cementitious material (no more than 50 pounds per cubic yard of fly ash), a water-cement ratio of 0.26 to 0.30, aggregate at ½-inch maximum size (3/8 inch is best), and no fine aggregate smaller than #4. This should result in a concrete with a unit weight of 120 pcf with voids at 20% after compaction.
5. Adding 5 ounces per hundredweight of cement of hydration stabilizer (not retarder) is essential to prevent hydration before placement; some contractors also like to use a viscosity modifier and a mid-range water reducer.
6. Contractors must have experience working with pervious concrete, since it is different than conventional concrete. The National Ready Mixed Concrete Association (NRMCA) offers a training program, but installers should also have some hands-on experience.
7. Proper compaction—to a void ratio of 12% to 20%—is essential. This can be achieved with a roller screed followed by cross rolling with hand rollers that weigh 40 pounds per foot.
8. Don’t overwork edges or joints. Joints should be cut with a “pizza cutter” roller, not with a saw.
9. Curing must begin within 10 minutes after concrete placement and continue for at least 7 days. During the interval between screeding and placing plastic sheeting, spray evaporation retarder onto the surface.
10. If a pattern is to be stamped into pervious concrete, stamp through the plastic curing film using open-faced stamps.
4. Some Pervious Concrete Projects
4.1 Pervious Paving Installed at City of Glendale, AZ Park and Ride Project
As the valley of the sun continues its urban sprawl at an exponential rate, planners involved with the development of Maricopa County are faced with the inevitable overcrowding of roadways and the effects of such growth on the environment. The use of sustainable products in construction is becoming very popular and the City of Glendale, Arizona is seizing opportunities to use these products.
4.2 Use of Pervious Concrete Eliminates Construction Costs
At the Stratford Place residential project, the community of Sultan, Wash., discovered how a paving application that looks like a Rice Krispies Treat(R) can eliminate puddles on their streets and sidewalks forever. Pervious concrete provides Stratford Place with improved storm water drainage for hardscapes throughout the development.
Pervious Concrete Eliminated Costs:
• Traditional storm water catch basins, embeds, and piping infrastructure, labor ($175,000)
• Need for detention vaults with lid
• Interior plat curbing ($37,000)
• Asphalt roadway system ($48,000)
• Developer estimate of traditional storm water system ($460,000)
• City/county future maintenance of roadway and storm system
Pervious Concrete Site Benefits
• Builder reclaimed two additional lots versus land used for detention vaults, ponds, perimeter structures. Lot values: $100,000 each
• City receives two additional real estate tax parcels
• Develop cost for pervious concrete system—approx. $196,000
• Net savings to developer—approx. $264,000
• Rain water from roofs directed to recharge area under pervious concrete street
• Eliminated untreated storm water and creates zero runoff
• Directly recharges ground water, mimics natural infiltration of site
• Mitigates “first flush” potential by reducing pollutant loading
According to Chattin, interest in pervious concrete is growing but remains difficult to get placed due to slow acceptance in local jurisdictions. “We want to reduce or eliminate runoff,” says Chattin. “We have 10-12 pervious residential subdivisions—more than any other area of the country.”
Growing Pervious Concrete Awareness
At the 2007 American Institute of Architects (AIA) National Convention and Design Exposition, pervious concrete will be among the cement-based applications for sustainable development highlighted by the Portland Cement Association (PCA). The organization will have a full pervious concrete display to demonstrate how this innovative technology filters stormwater, while providing a solid base for sidewalks, roadways and other hardscapes.
“The cement industry prides itself on providing the building community with numerous environmentally responsible solutions,” said Dave Shepherd, director of sustainable development for the Portland Cement Association.
“Pervious concrete is a prime example of how the industry continues to use new technologies and research to develop practical, sustainable applications.”
5. Further scope
There is much more scope for research work to impart sufficient smoothness to pervious concrete pavement surface. Grinding of surface or providing porous plaster may be tried.
Above experimentation realized the feasibility of pervious concrete to use as a pavement. This is to mention that imparting strength and workability to concrete mix without (or very little) fine aggregate is not impossible. Better results can be achieved by using admixtures to reduce setting time of concrete mass. Use of fly ash, silica fume, glass fibres, carbon fibres, polypropylene fibres can be very advantageous to improve performance of the pervious concrete.
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