PERMEABLE PAVEMENT SYSTEMS INTRO
An Introduction To Permeable Interlocking Concrete Pavement
As urbanization increases, so does the concentration of pavements, buildings and other impervious surfaces. These surfaces generate additional runoff and pollutants during rainstorms causing streambank erosion, as well as degenerating lakes and polluting sources of drinking water. Increased runoff also deprives groundwater from being recharged, decreasing the amount of available drinking water in many communities. Recreational opportunities from lakes, streams and rivers decline from the impacts of urban runoff. Commercial fishing productivity can decline in estuaries and bays thereby negatively impacting regional economies.
In response to environmental and economic impacts from stormwater runoff, U.S. federal law mandates that states control water pollution in runoff through the National Pollutant Discharge Elimination System (NPDES). Among many things, the law requires that states and localities implement best management practices (BMPs) to control non-point source pollution in runoff from development. BMPs can include storage, filtration and infiltration land development practices. Infiltration practices capture runoff and rely on infiltration through soils, vegetation, or aggregates for the reduction of pollutants. Detention ponds are a common example of BMP used to hold, infiltrate, and release stormwater. Infiltration trenches are another that reduce stormwater runoff and pollution and replenish groundwater. All of these BMPs provide some treatment and reduction of runoff pollutants.
Pavement that Detains and Infiltrates Runoff
Like infiltration trenches, permeable interlocking concrete pavements (PICPs) are highly effective in providing infiltration, detention and treatment of stormwater pollution. The base can be designed to filter, treat, and slowly release water into a storm sewer or watercourse while providing a walking and driving surface. PICPs answer the call from municipal regulations that limit the amount of impervious cover flowing into storm drains working at capacity, or when sites have limited space for detention ponds.
PICPs have been widely used across Europe, especially Germany since the early 1990s. In America, The U.S. Environmental Protection Agency and several state agencies consider PICPs an infiltration BMP. An increasing number of cities, counties, and states are incorporating them into land development and runoff standards, low-impact development guidelines and design manuals on stormwater control. With proper design, material selection, construction and routine maintenance, PICP is a sustainable low-impact BMP used by landscape architects, architects, engineers, developers and public agency staff.
Should a dense-graded or open-graded aggregate base be used under PICPs?
An open-graded base is most commonly used because it has water storage capacity (void space between the aggregates) of typically 30% to 40%. The stone sizes in open-graded bases can be as large as 3 in. (75mm) and as small as 1/4 in. (6mm). There is typically a thinner layer of small stone sizes (6mm to 1mm) used for bedding directly under the concrete pavers. The bedding and base bedding material maximizes storage, filtering, and treatment of pollutants in stormwater runoff entering the pavement surface. Open-graded bases are preferred because of the storage and treatment benefits
Dense-graded bases are occasionally used under PICPs. They may be used in areas of concentrated wheel loads from truck traffic. While there is additional structural support, most of the runoff from common rainstorms is stored in the bedding material and within the openings in the pavement surface. Maximum stormwater storage and infiltration benefits, however, come from PICP with an open-graded base.
What intensity and duration of storms can be managed?
That depends on the amount of water that drains onto the PICP, the depth (and storage capacity) the base, the infiltration rate of the soil under an open-graded base and the presence of drain pipes within an open-graded base. PICPs are intended to manage water quantities and pollutants from smaller, more frequent storms such as those with a return period of 10 years or less. These storms tend to have the highest concentrations of pollutants. PICPs are not intended to control flooding from larger, infrequent rainstorms.
Are PICPs eligible for LEED credits?
Yes, they can be under the U.S. and Canadian Green Building Councils (USGBC and CGBC) guidelines. PICPs typically can meet the requirements for Conservation of Material and Resources, Recycled Content under the USGBC LEED for new construction where at least 20% of the building products should be manufactured within a radius of 500 miles (800km) of the project. Most paving units are locally manufactured and delivered to projects within 500 miles (800km). To find the closest manufacturer or distributor, visit www.icpi.org
and conduct a search for producers in the 'Find a Member' section.
PICPs can meet the LEED credit requirements under Sustainable Sites. These requirements limit site disruption and water pollution by managing stormwater. The pavements can reduce runoff-generating impervious cover and decrease the rate and quantity of runoff. PICPs meet these credits through the filtering action of the base that reduces total suspended solids and phosphorous in runoff, as well as other pollutants.
PICPs can also meet the sustainable sites requirement to reduce urban heat islands (thermal gradient difference between developed and undeveloped areas) and minimize impact on microclimate, as well as human and wildlife habitat. This is accomplished through increased albedo (a measure of the solar energy reflected from a surface) or use of a pavement system with less than 50% imperviousness. PICPs have substantially higher reflectivity than conventional asphalt pavement and can meet the requirement for less than 50% imperviousness. For additional information on U.S. or Canadian LEED credits see www.usgbc.org
How well does the pavement perform in freeze and thaw conditions?
PICPs have been in service for years in freezing climates and have performed adequately. Many projects throughout Canada and the northern U.S., in the United Kingdom and Germany speak to the durability of these pavement systems in cold climates, as well as their ability to accept snowplows and salts without paver damage. In order to ensure high durability in freezing climates, the paving units should conform to the requirements of ASTM C 936 in the U.S. or CSA A231.2 in Canada. Both of these product standards include tests for freeze-thaw durability.
When the sun and temperature are right, ice and snow on PICP can melt and immediately soak into the openings in the pavement surface. Water does not collect on the surface and re-freeze. This reduces slipping hazards. Obviously, sand shouldn't be used for foot or tire traction on PICPs. De-icing salts can be used. After plowing, melting of any remaining snow can occur if the temperature moves above freezing. This will help eliminate ice from forming and reduce salt contamination in groundwater.
Since the pavement base temporarily stores rainfall, will the base heave and damage the pavement surface when frozen?
Water in the base typically should drain within 24 hours. It's unlikely that ice will form in the base within this time period should temperatures drop below freezing. If the water does freeze before draining, there should be adequate space for the ice to expand within the open-graded base as it freezes, thereby minimizing the risk of heaving. Should soil heaving occur, the pavement surface is flexible and should not be damaged from minor upward movement or from resettlement during a thaw.
Does the surface conform to ADA requirements?
Yes, ADA Design Guidelines require that surfaces be firm, stable and slip resistant. PICP designs can provide a firm and stable surface for visually impaired persons and those using wheeled mobility devises. If the openings in the surface are not desirable, solid units can be used in areas subject to disabled persons. Such areas might include designated spaces in parking lots.
ADA requires that the static coefficient of friction for flat surfaces along accessible routes be 0.6 and 0.8 for ramps. ADA advisory material recommends various test methods to assess surface slip resistance. PICPs can meet slip ADA resistance for requirements using test methods recommended in ADA advisory literature. For additional information on these see ICPI Tech Spec 13 - Slip and Skid Resistance of Interlocking Concrete Pavements. This and other technical bulletins are available at www.icpi.org
Is there any benefit to using PICPs on low-infiltration soils such as some types of clays?
Yes, if soil infiltration is slow (generally under 0.5 in./hour or 1/3 x 10-2 m/sec), perforated plastic pipe drains at the bottom of the base can remove excess water while still allowing some of the water to infiltrate into the soil. The drainage rate for the water contained in the base is typically no greater than 24 hours. For practically impervious soils or high bedrock, an impervious pond liner can be used to detain, filter and release the water through drainpipes. Regardless of the rate of soil infiltration, the filtering action of the open-graded base can reduce water pollutants.
All permeable pavements require periodic surface cleaning. How is a PICP surface cleaned and how often?
The openings in the surface of PICPs will require periodic removal of detritus and sediment trapped by the small sized crushed stone. Dirt is typically removed by a vacuum-sweeping street cleaning machine. Cleaning is done when the pavement surface and detritus are dry and can be loosened by sweeping and vacuuming. The frequency of cleaning will vary with the use of the pavement and deposition of sediment, leaves, etc. from adjacent areas. Cleaning should be done at least once a year, and the surface monitored during the early life of the pavement so that a regular cleaning schedule can be established.
A North Carolina State University study has shown that the initial surface infiltration rate of PICPs can be as high as 2000 in./hour (5080cm/hour). Other research has shown that near initial surface infiltration rates can be restored through cleaning and replacement of the initial 3/4 to 1 in. (20 to 25 mm) depth of small stones in the openings of PICPs. For highly clogged pavement openings, the stones can be removed with vacuuming and replaced with clean material. This is a distinct maintenance advantage over monolithic permeable pavements.
What about high heel shoes?
Solid pavers can be introduced into PICPs in pedestrian areas to accommodate a variety of shoes including high heels.
Permeable Pavement Resources
The Interlocking Concrete Pavement Institute (ICPI) offers a manual, Permeable Pavement Interlocking Concrete Pavements
that covers selection, design, specification, construction, and maintenance. It synthesizes literature on infiltration trenches, porous asphalt pavement, research on and practical experience with permeable interlocking concrete pavements. ICPI's manual is essential for design professionals and municipal authorities that regulate stormwater runoff. It can be purchased at www.icpi.org