Pervious concrete, also known as porous, gap-graded, or enhanced porosity concrete, is concrete with reduced sand or fines and allows water to drain through it. Pervious concrete over an aggregate storage bed will reduce stormwater runoff volume, rate, and pollutants. The reduced fines leave stable air pockets in the concrete and a total void space of between 15 and 35 percent, with an average of 20 percent. The void space allows stormwater to flow through the concrete as shown in Figure 1, and enter a crushed stone aggregate bedding layer and base that supports the concrete while providing storage and runoff treatment. When properly constructed, pervious concrete is durable, low maintenance, and has a low life cycle cost. Figure 2 shows a pervious concrete walkway installed at the EPA Headquarters in Washington, D.C.
Pervious concrete can be used for municipal stormwater management programs and private development applications. The runoff volume and rate control, plus pollutant reductions, allow municipalities to improve the quality of stormwater discharges. Municipal initiatives, such as Chicago's Green Alley program, use pervious concrete to reduce combined sewer overflows and minimize localized flooding by infiltrating and treating stormwater on site. Private development projects use pervious concrete to meet post-construction stormwater quantity and quality requirements. The use of pervious concrete can potentially reduce additional expenditures and land consumption for conventional collection, conveyance, and detention stormwater infrastructure. Public and private developments have used pervious concrete, which is a naturally brighter surface than traditional asphalt, to reduce lighting needs and increase nighttime safety.
Pervious concrete can replace traditional impervious pavement for most pedestrian and vehicular applications except high-volume/high-speed roadways. Pervious concrete can be designed to handle heavy loads, but surface abrasion from constant traffic will cause the pavement to deteriorate more quickly than conventional concrete. Pervious concrete has performed successfully in pedestrian walkways, sidewalks, driveways, parking lots, and low-volume roadways. The environmental benefits from pervious concrete allow it to be incorporated into municipal green infrastructure and low impact development programs. In addition to providing stormwater volume and quality management, the light color of concrete is cooler than conventional asphalt and helps to reduce urban temperatures and improve air quality (Grant, et al., 2003) (Vingarzan and Taylor, 2003). Unlike the smoothed surface of conventional concrete, the surface texture of pervious concrete is slightly rougher, providing more traction to vehicles and pedestrians.
Siting and Design Criteria
Pervious concrete should be designed and sited to intercept, contain, filter, and infiltrate stormwater on site. Several design possibilities can achieve these objectives. For example, pervious concrete can be installed across an entire street width or an entire parking area. The pavement can also be installed in combination with impermeable pavements or roofs to infiltrate runoff. Several applications use pervious concrete in parking lot lanes or parking stalls to treat runoff from adjacent impermeable pavements and roofs. This design economizes pervious concrete installation costs while providing sufficient treatment area for the runoff generated from impervious surfaces. Inlets can be placed in the pervious concrete to accommodate overflows from extreme storms. The stormwater volume to be captured, stored, infiltrated, or harvested determines the scale of permeable pavement required. Figures 3 and 4 illustrate some pervious concrete design variations.
Pervious concrete comprises the surface layer of the permeable pavement structure and consists of Portland cement, open-graded coarse aggregate (typically 5/8 to 3/8 inch), and water. Admixtures can be added to the concrete mixture to enhance strength, increase setting time, or add other properties. The thickness of pervious concrete ranges from 4 to 8 inches depending on the expected traffic loads. Additional subsurface components of this treatment practice are illustrated in Figure 5 and include the following (NRMCA, 2008):
- Choke course - This permeable layer is typically 1 - 2 inches thick and provides a level bed for the pervious concrete. It consists of small-sized, open-graded aggregate.
- Open-graded base reservoir - This aggregate layer is immediately beneath the choke layer. The base is typically 3 - 4 inches thick and consists of crushed stones typically 3/4 to 3/16 inch. Besides storing water, this high infiltration rate layer provides a transition between the bedding and subbase layers.
- Open-graded subbase reservoir - The stone sizes are larger than the base, typically 2½ to ¾ inch stone. Like the base layer, water is stored in the spaces among the stones. The subbase layer thickness depends on water storage requirements and traffic loads. A subbase layer may not be required in pedestrian or residential driveway applications. In such instances, the base layer is increased to provide water storage and support.
- Underdrain (optional) - In instances where pervious concrete is installed over low-infiltration rate soils, an underdrain facilitates water removal from the base and subbase. The underdrain is perforated pipe that ties into an outlet structure. Supplemental storage can be achieved by using a system of pipes in the aggregate layers. The pipes are typically perforated and provide additional storage volume beyond the stone base.
- Geotextile (optional) - This can be used to separate the subbase from the subgrade and prevent the migration of soil into the aggregate subbase or base.
- Subgrade - The layer of soil immediately beneath the aggregate base or subbase. The infiltration capacity of the subgrade determines how much water can exfiltrate from the aggregate into the surrounding soils. The subgrade soil is generally not compacted.
Properly installed pervious concrete requires trained and experienced producers and construction contractors. The installation of pervious concrete differs from conventional concrete in several ways. The pervious concrete mix has low water content and will therefore harden rapidly. Pervious concrete needs to be poured within one (1) hour of mixing. The pour time can be extended with the use of admixtures. A manual or mechanical screed set ½ inch above the finished height can be used to level the concrete. Floating and troweling are not used, as those may close the surface pores. Consolidation of the concrete, typically with a steel roller, is recommended within 15 minutes of placement (Figure 6). Pervious concrete also requires a longer time to cure. The concrete should be covered with plastic within 20 minutes of setting and allowed to cure for a minimum of 7 days (NRMCA, 2008).
Specific Design Considerations and Limitations
The load-bearing and infiltration capacities of the subgrade soil, the infiltration capacity of the pervious concrete, and the storage capacity of the stone base/subbase are the key stormwater design parameters. To compensate for the lower structural support capacity of clay soils, additional subbase depth is often required. The increased depth also provides additional storage volume to compensate for the lower infiltration rate of the clay subgrade. Underdrains are often used when permeable pavements are installed over clay. In addition, an impermeable liner may be installed between the subbase and the subgrade to limit water infiltration when clay soils have a high shrink-swell potential, or if there is a high depth to bedrock or water table (Hunt and Collins, 2008).
Measures should be taken to protect permeable pavement from high sediment loads, particularly fine sediment. Appropriate pretreatment BMPs for run-on to permeable pavement include filter strips and swales. Preventing sediment from entering the base of permeable pavement during construction is critical. Runoff from disturbed areas should be diverted away from the permeable pavement until they are stabilized.
Key Siting and Maintenance Issues:
- Do not install in areas where hazardous materials are loaded, unloaded, or stored.
- Avoid high sediment loading areas.
- Divert runoff from disturbed areas until stabilized.
- Do not use sand for snow or ice treatment.
- Periodic maintenance to remove fine sediments from paver surface will optimize permeability.
Several factors may limit permeable pavement use. Pervious concrete has reduced strength compared to conventional concrete and will not be appropriate for applications with high volumes and extreme loads. It is not appropriate for stormwater hotspots where hazardous materials are loaded, unloaded, stored, or where there is a potential for spills and fuel leakage. For slopes greater than 2 percent, terracing of the soil subgrade base may likely be needed to slow runoff from flowing through the pavement structure. In another approach for using pervious concrete slopes, lined trenches with underdrains can be dug across slope to intercept flow through the subbase (ACPA, 2006).
Consistent porosity through the concrete structure is critical to prevent freeze-thaw damage. Cement paste and smaller aggregate can settle to the bottom of the structure during consolidation and seal off the concrete pores. If surface water becomes trapped in pavement voids, then it can freeze, expand, and break apart the pavement. An evaluation of four (4) pervious concrete sites (3 with deterioration and 1 without) in Denver, CO by the Urban Drainage and Flood Control District, found that the larger aggregate size mix exhibited better permeability and less surface deterioration (UDFCD, 2008). The National Ready Mixed Concrete Association also recommends the following precautions to prevent pervious concrete from becoming saturated in regions where hard wet freezes occur (NRMCA, 2004):
- Use 8 to 24 inch thick layer of clean aggregate base below the pervious concrete.
- Attempt to protect the cement paste by incorporating an air-entraining admixture in the mixture.
- Use an underdrain to drain the aggregate base.
The most prevalent maintenance concern is the potential clogging of the pervious concrete pores. Fine particles that can clog the pores are deposited on the surface from vehicles, the atmosphere, and runoff from adjacent land surfaces. Clogging will increase with age and use. While more particles become entrained in the pavement surface, it does not become impermeable. Studies of the long-term surface permeability of pervious concrete and other permeable pavements have found high infiltration rates initially, followed by a decrease, and then leveling off with time (Bean, et al., 2007a). With initial infiltration rates of hundreds of inches per hour, the long-term infiltration capacity remains high even with clogging. When clogged, surface infiltration rates usually well exceed 1 inch per hour, which is sufficient in most circumstances for the surface to effectively manage intense stormwater events (ICPI, 2000). A study of eleven (11) pervious concrete sites found infiltration rates ranging from 5 in/hr to 1,574 in/hr. The sites taking runoff from poorly maintained or disturbed soil areas had the lowest infiltration rates, but they were still high relative to rainfall intensities (Bean, et al., 2007a). Permeability can be increased with vacuum sweeping. In areas where extreme clogging has occurred, half inch holes can be drilled through the pavement surface every few feet or so to allow stormwater to drain to the aggregate base. Many large cuts and patches in the pavement will weaken the concrete structure. Freeze/thaw cycling is a major cause of pavement breakdown, especially for parking lots in northern climates. Properly constructed permeable concrete can last 20 to 40 years because of its ability to handle temperature impacts. (Gunderson, 2008).
In cold climates, sand should not be applied for snow or ice conditions. However, snow plowing can proceed as with other pavements and salt can be used in moderation. Pervious concrete has been found to work well in cold climates as the rapid drainage of the surface reduces the occurrence of freezing puddles and black ice. Melting snow and ice infiltrates directly into the pavement facilitating faster melting (Gunderson, 2008).
Cold weather and frost penetration do not negatively impact surface infiltration rates. Permeable concrete freezes as a porous medium rather than a solid block because permeable pavement systems are designed to be well-drained; infiltration capacity is preserved because of the open void spaces (Gunderson, 2008). However, plowed snow piles should not be left to melt over the pervious concrete as they can receive high sediment concentrations that can clog them more quickly.
Permeable pavements do not treat chlorides from road salts but also require less applied deicers. Deicing treatments are a significant expense and chlorides in stormwater runoff have substantial environmental impacts. Reducing chloride concentrations in runoff is only achieved through reduced application of road salts because removal of chloride with stormwater BMPs is not effective. Road salt application can be reduced up to 75% with the use of permeable pavements (UNHSC, 2007).
All permeable pavements, including pervious concrete, are on-site stormwater management practices and will have the same or very similar effectiveness with regards to the reduction of the volume and rate of stormwater runoff as well as pollutant concentrations. Pervious concrete, porous asphalt, and permeable pavers all have the same underlying stormwater storage and support structure. The only difference is the permeable surface treatment. The choice of permeable surface is relevant to user needs, cost, material availability, constructability, and maintenance, but it has minimal impact on the overall stormwater retention, detention, and treatment of the system.
Permeable pavement transforms areas that were a source of stormwater to a treatment system and can effectively reduce or eliminate runoff that would have been generated from an impervious paved area. Because it reduces the effective impervious area of a site, permeable pavement should receive credit for pervious cover in drainage system design. The infiltration rate of properly constructed pervious concrete and base generally exceeds the design storm peak rainfall rate; the subsoil infiltration rate and base storage capacity are the factors determining stormwater detention potential. Table 1 provides monitored reductions in stormwater volumes via storage and infiltration.
|Table 1. Volume Retention of Permeable Pavements |
| Pervious Concrete
|Residential street and sidewalk
| Permeable Interlocking Concrete Pavers
||Auckland, New Zealand
|Field and laboratory tests
||Guelph, Ontario, Canada
||Impermeable liner installed
||34% - 45%
| Porous Asphalt
||State College, PA
||Retained the 25 yr - 24 hr storm
| *System designed to collect infiltrated stormwater in underdrain for monitoring purposes.
(WA Aggregates & Concrete Association, 2006)(Collins, et al., 2008)(Fassman and Blackbourn, 2006)(Bean, et al., 2007a)(Bean et al., 2007b)(Pratt, 1999)(Booth and Leavitt, 1999)(Brattebo and Booth, 2003)(Collins, et al., 2008)(Legret, M. & Colandini, 1999)(Cahill et al., 2003)(Roseen and Ballestero, 2008)
Permeable pavement reduces pollutant concentrations through several processes. The aggregate filters the stormwater and slows it sufficiently to allow sedimentation to occur. The subgrade soils are also a major factor in treatment. Sandy soils will infiltrate more stormwater but have less treatment capability. Clay soils have a high cation exchange capacity and will capture more pollutants but will infiltrate less. Also, studies have found that in addition to beneficial treatment bacteria in the soils, beneficial bacteria growth has been found on established aggregate bases. In addition, permeable pavement can process oil drippings from vehicles (Pratt et al., 1999). Table 2 provides measured pollutant removals from pervious pavement structures.
|Table 2. Monitored Pollutant Removals of Permeable Pavement |
| Permeable Concrete
| Permeable Interlocking Concrete Pavers
||Jordan Cove, CT
||King College, ON
| Porous Asphalt
|Highway (friction course only)
| (Rushton, 2001)(Bean, et al., 2007b)(Clausen and Gilbert, 2006)(Van Seters/TRCA 2007) (Barrett et al., 2006)(UNHSC, 2007)
Permeable pavement water quantity and pollutant reduction characteristics such as 80% total suspended solids reductions can qualify it to earn credits under green or sustainable building evaluations systems such as Leadership in Energy and Environmental Design (LEED®) and Green Globes. Credits also can be earned for water conservation, urban heat island reduction, and conservation of materials by utilizing some recycled materials and regional manufacturing and resource use. Permeable concrete also allows less lighting to be used when compared to traditional asphalt because its lighter color reflects more light.
Several factors influence the overall cost of pervious concrete:
- Material availability and transport - The ease of obtaining construction materials and the time and distance for delivery.
- Site conditions - Accessibility by construction equipment, slope, and existing buildings and uses.
- Subgrade - Subgrade soils such as clay may result in additional base material needed for structural support or added stormwater storage volume.
- Stormwater management requirements - The level of control required for the volume, rate, or quality of stormwater discharges will impact the volume of treatment needed.
- Project size - Larger pervious concrete areas tend to have lower per square foot costs due to construction efficiencies.
Costs vary with site activities and access, pervious concrete depth, drainage, curbing and underdrains (if used), labor rates, contractor expertise, and competition. The cost of the pervious concrete material ranges from $2 to $7 per square foot (NCHRP, 2005). The material cost of pervious concrete can drop significantly once a market has opened and producers have made initial capacity investments. Eliminating or reducing the use of admixtures, which are a significant cost in construction, can also lower installation costs. When the City of Chicago began using pervious concrete in its Green Alleys program (Figure 7), the cost of pervious concrete was $145 per cubic yard. One year later, after having made the initial investment in the city's pervious concrete market, the price of pervious concrete dropped and was comparable to ordinary concrete at $45 per cubic yard (Saulny, 2007). In addition, the public investment in the pervious concrete market made the product more affordable for smaller development projects.
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