Permeable, or porous, paving is a type of paving that allows water to drain through to the ground below. Water can then directly infiltrate on-site, rather than being diverted to a drain for off-site treatment.
There are three basic types of permeable paving:
- Permeable monolithic paving, such as porous asphalt or porous concrete, looks like a standard paved surface.
- Paving block systems look like bricks or other blocks, but allow water to drain between the modules.
- Turf support structures make earth/grass mesh more resilient. These systems come either as gridded porous sheets that are added just below the ground layer or as a fiber mesh or other ground stabilizing material that is tilled into the ground and makes the turf increasingly stable as the groundcover grows.
There are several factors that go into choosing a paving type and, in many cases, permeable paving can be used in place of impervious paving. Site condition, soil types, use patterns, aesthetics, cost considerations, and other criteria can affect the choice of paving type. Permeable paving is especially useful for light duty paving areas like pedestrian and bike paths or for seasonal/occasional use areas like auxiliary parking lots and emergency vehicle right-of-ways and turnarounds. Permeable paving blocks, permeable asphalt, and permeable concrete are designed for the heaviest use. They are meant for daily, low-speed use in parking lots, driveways, residential drives, and similar areas. Generally, turf support structures are intended for infrequently used areas, such as event grounds, fire lanes, overflow or event parking, and other spaces where there is little regular traffic. Grass pavers have similar applications, though are designed to absorb wear directly on the paver surface, allowing heavier-duty use. See the Notes on Use section for additional information.
The use of permeable paving decreases peak flow rates in area streams and rivers during a rain event because it allows water to infiltrate into the ground on-site. This infiltration reduces the amount and speed of water leaving the site during a rain event.
The urban heat island effect occurs where paving, other man-made surfaces, and environmental changes cause urban areas to be hotter than the surrounding area. This effect changes weather patterns and the environmental conditions for plants and animals. It can also increase the magnitude of heat waves in cities and the need for air-conditioning, thereby increasing energy consumption. Permeable paving and other "cool paving" options decrease the heat island effect and, if implemented at a large scale, could mitigate the urban heat island effect impact of paving.
Notes on Use
Cost, site conditions, use patterns, and aesthetics are considerations for determining when permeable paving is appropriate for a particular project.
Permeable asphalt or permeable paving tiles are options for areas with limited, low-speed traffic, such as parking lots or driveways. The performance quality of these systems is primarily due to underground drainage layers. Apart from the aesthetics and wear pattern of the top layer, there seems to be little performance difference between various monolithic and the modular components. Cost and aesthetics, though, vary significantly between products. There is some concern over the use of interlocking block systems with plows, as the edge of the plow blade can catch and damage the pavers. Permeable paving may not be suitable for high-traffic areas and areas that are regularly sanded or plowed, but this depends on the type of permeable paving and the necessity of sanding or plowing.
Permeable pavement can be used in mid- to high-traffic areas where traditional paving might be used. There are some issues to keep in mind, regarding siting and the maximum allowable slope. Permeable paving should not be used in locations like industrial yards that contain toxic chemicals, gas station lots, or other sites that pose a groundwater contamination risk from the infiltration of toxic chemicals.
Turf-reinforcing systems are slightly less robust than the other two options. This resilience can vary with the system, as some grass or turf paving options are advertised as being quite tough. They are best used for areas that need to be resilient when wet, but aren't used on a daily basis. They are appropriate for overflow parking, pedestrian pathways, and other low-traffic areas.
Grass pavers are a hybrid of permeable pavers and turf-reinforcement systems. Grass Pavers are interlocking pavers with holes large enough for grass to grow through. These can be used for seldom-used areas like parking lots, fire lanes, and other low- to medium-traffic areas.
Maintenance of permeable areas is somewhat different than traditional paving. Permeable monolithic systems need to be washed, swept, and/or vacuumed occasionally to prevent clogging. Accordingly, the use of sand or ash is not advised. Grass pavers need to be irrigated and mowed like any other lawn system, though the voids can be filled with sand instead of grass to avoid concerns about maintaining a lawn or killing grass through overuse. Apart from necessary repairs, grass reinforcing systems can be treated like other turf systems, with regular mowing and lawn care.
Options and Analysis
Permeable Paver Types
As noted above, permeable paving systems can be roughly divided into three categories: modular or block based systems, monolithic systems like permeable asphalt or concrete, and turf stabilization systems. The substrates for the modular systems and the monolithic systems are similar. The information below is intended for guidance--consult an installer or product information for specific installation instructions. Note that some grass reinforcing mats are called grass pavers by manufacturers. Here, the term grass pavers is used to describe hard-surface concrete or concrete-like blocks that create a pavement grid, where turf or grass in installed in the holes in the system.
Monolithic systems consist of either an asphalt or concrete pour-in-place surface that does not have small aggregate, allowing drainage through the surface. Because consistency in this mix is more difficult to achieve than standard surfaces, experienced installation is recommended. Monolithic permeable paving systems consist of:
- Subgrade: Some recommend compacting soil for load bearing capacity, others recommend un-compacted soil for drainage.
- Geotextile fabric: Lines the infiltration bed, preventing dirt migration between the subgrade and the gravel.
- Coarse Aggregate: Clean, uniformly graded coarse aggregate.
- Small Aggregate: Tops the coarse aggregate and prevents the porous paving from draining into the coarse aggregate.
- Pervious paving layer: The wear layer of the system, provides the usable surface.
Modular Systems and Grass Pavers
Modular systems are typically constructed with highly drainable layers below a layer of blocks that are designed to allow water to pass through joints between them. Modular permeable paving systems or grass pavers consist of:
- Gravel layer: A layer of gravel allows for uniform distribution and drainage of water.
- Sand Layer: Allows for distribution of load and drainage.
- Geotextile fabric: Optional in many systems, this provides support and prevents migration of interstitial soil to under layers.
- Block layer: A layer of blocks with sand or soil in the joints between the blocks.
Turf Stabilization or Grass Reinforcing Systems
Grass reinforcing system components vary greatly based on the system. Consult manufacturer for specifics. The system outlined below is for plastic turf or gravel reinforcing panels. Consult the manufacturer for till-in net reinforcing recommendations. Grass paver systems consist of:
- Subgrade: Recommendations vary on whether to compact subgrade.
- Gravel drainage layer: Provides temporary water storage and drainage potential for the system.
- Sand layer: Clean, sharp sand provides drainable bearing surface for system.
- Reinforcement and soil/sand layer: The layer consists of plastic reinforcing panels filled with sand or soil and a thin topping layer of soil. When properly installed, the turf grows over the height of the plastic reinforcing panels and is indistinguishable from a non-reinforced lawn.
Rainwater Catchments / Storage / Drainage Systems
Rainwater catchment and storage can be achieved with the above systems by introducing an impervious layer above the subgrade and grading the system to drain to an intended point (for extraction or to go into another system of stormwater management). The volume of gravel layer also needs to be increased to contain the additional water if the system is used for water storage.
Paving Type Cost/sf (installed) Asphalt $0.50 - $1.00 Porous Concrete $2.00 - $6.50 Grass/Gravel Pavers $1.50 - $5.75 Interlocking Concrete Paving Blocks $5.00 - $10.00
The life of permeable paving products is sometimes thought of as significantly less than traditional paving options, although lifetime varies greatly with paving type, installation, appropriate use, and maintenance. There are some installations that function after 20+ years of operation.1
Permeable paving is becoming increasingly available as an alternative to traditional paving, though finding installers is sometimes difficult in smaller markets. Though the installation of permeable asphalt and concrete is similar to standard concrete and asphalt, some installers have complained that it is more difficult to create a workable mixture without a fine aggregate component. Permeable pavers are widely available and the installation should not prove problematic, though it is important to ensure that, if the sub-grade is not supposed to be compacted, it is not compacted during installation. Consult with local retailers and subcontractors to ensure supply and installation expertise related to the various paving options.
The material impact of each of these methods depends on the material that is used in the manufacturing. For permeable concrete or asphalt, the material use concerns are similar to regular asphalt and concrete, as they contain the same materials and are manufactured in the same way, with the exception of the omission of certain sized aggregate.
Future Recycling & Waste Generation
There is a wide variety of products that are used in permeable paving. Asphalt and concrete can, for the most part, be reground into aggregate or base, allowing reuse, often on-site. Pavers can be reused or ground, depending on their condition and available opportunities. The extraction of tilled-in mesh elements may be difficult, as this requires separation of the plants and dirt from the reinforcement.
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