Materials for Green Paving Systems

There are three primary technologies of permeable paving surfaces:

permeable segmental concrete or clay pavers (also called PICP),
porous asphalt,
and pervious (porous) concrete.

Grass or turf pavers are another form of pervious surface; however, they are not recommended for primary parking applications or pedestrian surfaces, and are therefore used mostly for overflow parking or fire access lanes. There are also recycled products being developed for use as a pervious surface for pedestrian and vehicular applications, but their history and engineering data have not been documented in all climates and soils, and therefore are not recommended for many applications.

While all permeable pavers allow stormwater to infiltrate through their surface and filter into their aggregates below, the specifics of each technology varies. Permeable pavements can be categorized as either an open joint system, typically with joints that are 1/2” or greater, or a closed joint system, where joints are less than 1/2” and will usually have void openings as well. Permeable paver surfaces are generally supported with an open-aggregate structure that also contains a storage reservoir for stormwater runoff, specifically designed to meet BMP detention requirements. Regardless of the specific technology, research has demonstrated the ability of all permeable pavement systems to substantially reduce urban runoff and the suspended pollutants within it.

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PERMEABLE INTERLOCKING CONCRETE/CLAY PAVEMENT (PICP)

PICP (also called permeable segmental pavers) are non-porous, solid blocks made of brick, stone, clay or concrete, which are set on a highly permeable bedding of stone chips. These solid, high-strength molded units are surrounded by small, highly pervious stone-filled joints and opening voids which receive and infiltrate stormwater downward. These stone-filled joints also help contribute to interlocking and load transfer to neighboring pavers.

Depending on the paving unit pattern and design, PICP joints can vary between 1/8" and 1 1/4" inch. And while PICP has less visible permeable surface area than porous asphalt or pervious concrete, it still provides very high surface infiltration rates – initially over 50-75in/hr. However, due to sediment build-up these initial rates will typically reduce by around 50% in the first 5 years. Over a 20-year period, PICP’s are designed to achieve and maintain a consistent 3in/hr infiltration rate – which more than exceeds most municipal storm design requirements. This aspect of an open-graded permeable pavement design makes its hydrological performance equal to or superior than other pervious surfaces.

Permeable segmental pavers are designed to support traffic and heavy vehicular loads, and are available in a variety of shapes, styles, colors and textures. Select manufacturers design and patent unique bricks suitable for a wide range of uses and applications. Also, a variety of colors, textures and styles are available to suit any architectural character, while simultaneously enhancing the environment.

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POROUS ASPHALT

On the surface, porous asphalt looks similar to traditional asphalt. However, it is actually formulated with larger aggregate and less fine particles. This “open-graded” surface relies on these aggregates bound with traditional asphalt to create a porous mix that not only allows stormwater to drain through, but also supports vehicular traffic.

Porous asphalt is actually mixed at conventional asphalt plants, but fine (small) aggregates are omitted from the mixture. The remaining single-sized aggregate particles leave open voids (typically between 25-35%) that give the material its porosity and permeability. This also allows for its high infiltration rate, with a runoff coefficient that is close to zero. Beneath its surface, porous asphalt utilizes an underlying stone reservoir that then filters water directly into the underlying soil, or through an under-drain system connected to the storm-drain system.

Because porous asphalt is mixed with conventional asphalt, it is typically always black or shades of dark grey, and therefore offers no additional "curb appeal".

PERVIOUS (POROUS) CONCRETE

Pervious (porous) concrete appears on the surface to be grainier and less smooth than traditional concrete pavement. It is actually made by using carefully controlled amounts of water and cement materials that are bound with large aggregate particles to create a porous mix that not only allows stormwater to drain through, but also supports limited vehicular traffic.

Unlike conventional concrete, the pervious concrete mixture contains little or no fine sand, which creates a substantial void space between 25-35%. The single-sized aggregate particles allow for high infiltration rate, with a runoff coefficient close to zero. Pervious concrete also uses an underlying stone reservoir to filter water directly into the underlying soil, or through an under-drain system connected to the storm-drain system.

Because pervious concrete is mixed with conventional concrete, a limited range of colors and textures are available.

GRASS PAVERS

Also called "turf blocks", grass pavers are a type of open-cell unit paver in which the cells are filled with soil and planted with turf or gravel. The voids in these pavers allow water to seep through, providing the necessary nutrients and minerals for the grass or vegetation inside to grow once installed. Comprised of a grid system, which is made of concrete or synthetic materials such as plastic, concrete or gravel, grass pavers distribute the weight of traffic and prevent compression of the underlying soil. Runoff coefficients are similar to grass, typically 0.15 to 0.6.

Grass pavers are appropriate for pedestrian-only areas and for very low-volume, low-speed areas such as overflow parking areas, residential driveways, alleys and parking stalls. Although they aren't as versatile in usage than other forms of permeable pavers, they can significantly improve site appearance by providing vegetation where there would otherwise be only pavement.

THE APPLICATIONS FOR PERMEABLE PAVERS

Ideal in residential streets and commercial parking lots, permeable paving systems can also be used in pedestrian walkways and driveways, overflow parking areas, alleys and parking stalls. However, the recommended uses for the different types of permeable paver systems vary depending on their design.

Typically, these systems can be classified into categories according to open or closed joint permeable pavers, with and without voids. When selecting the appropriate paver to use for a project, it is important to look at long-term performance for both loading and maintenance. In addition, specific ADA regulations require compliance for surface smoothness and site-specific BMP applications. Therefore, when installing permeable pavers for pedestrian use, as well as for vehicles, equipment, and other wheeled applications, all requirements and limitations should also be examined.

Open-Joint Permeable Pavers

Joints in excess of 1/2” on all perimeters
Can be used for parking stall applications, overflow parking, light vehicular use, but limited pedestrian applications (as may not meet specific ADA requirements)
Constructed on open-graded aggregate setting bed, base and sub-base
Most concrete paver shapes may be mechanically installed

Closed-Joint Permeable Pavers

Joints less than 3/8” on all perimeters and may or may not have voids
Can be used for vehicular and pedestrian applications; however only joints >1/8” but < 3/8” can be used with open-graded aggregates
Built on an open-graded aggregate setting bed, base and sub-base
Herringbone patterns recommended for vehicular applications
Long-term maintenance due to clogging is unlikely (joint aggregates cannot be removed through a vacuumed process and will lack ability to reinstate infiltration)
Most concrete paver shapes may be mechanically installed

Closed-Joint Permeable Pavers With Voids

Joints less than 3/8” and opening(s) greater than the joint to facilitate remedial maintenance
Can be used for vehicular and pedestrian applications
Pavers with 1/8” joints and/or with sand in joints should be avoided (can cause premature clogging in void areas and limit infiltration rates as well as necessitate more frequent remedial maintenance cycles)
Pavers with joints > 3/16” and voids < 13mm will provide optimum performance (when top-of-chamfer to top-of-chamfer dimension is less than 17mm at both joint areas and void areas)
Herringbone patterns recommended for vehicular applications
Installed on open-graded aggregate setting beds, base and sub-base with angular chips for the joint and void areas
Granite chips may be used for the joint and void areas (when colored joints are required)
Most concrete paver shapes may be mechanically installed

Learn more about the difference between permeable paver systems (pdf).

AROUND THE COUNTRY AND AROUND THE WORLD …

Permeable paving systems create a visually appealing surface that works to recharge the planet with every storm. And their ability to stand up to intense and frequent freeze-thaw cycles make them suitable for all climates and areas. Today, permeable pavement systems are widely being used for such applications as:

Municipal Buildings - parking lots, walkways and driveways
Colleges and Universities - parking lots and walkways
City Streets
City Alleyways
Pedestrian Pathways
Golf Courses and Country Clubs - cart lanes, walkways, driveways and parking lots
Boat Parking Areas and Ramps
Military Bases
ADA-Approved Parking Stalls (for the disabled)
City Courtyards, Fountain Areas, Parks and more
Ports and Industrial Sites

More and more, permeable pavers prove to be the ultimate solution for the future of urban development - and the ecological sustainability of our planet. Learn more about the specific benefits of each type of permeable pavement system.