Figure 1: A typical permeable interlocking concrete pavement cross-section. Image courtesy ICPI.
As sustainable building practices become more common, the principles of low-impact development and stormwater management are seen as increasingly vital to new and retrofit commercial and residential projects. Federal, state, and local mandates, as well as green building certification programs, further drive the need to understand runoff-reduction controls.
When it rains, water that falls on hardscaped surfaces––such as roads, parking lots, and alleyways––generally runs off into storm sewers. Overtaxed systems, particularly during periods of heavy or extended rainfall, can lead to overflowing storm sewers and flooding. In addition, water that rushes off driveways and parking lots into storm sewers is not percolating back into the soil to recharge underground aquifers, can lead to erosion problems at the site or downstream, and can take sediment and chemicals with it.
Permeable interlocking concrete pavements (PICPs), such as CalStar’s Holland pavers and permeable pavers, are widely recognized as an effective means of stormwater control, reducing or eliminating rainwater flowing offsite and helping to direct it back into the ground.
What Are PICPs?
Contrary to some misperceptions, water does not soak through permeable pavers themselves. PICPs consist of solid pavers with narrow openings or gaps between them that are filled with a permeable joint material. These openings allow water to seep through the paving surface and are generally 5 to 15 percent of the surface area.
Figure 2: A cross-section of full-exfiltration installation. Image courtesy ICPI.
Permeable pavements are distinct from pervious concrete, as the latter is designed to have many holes in the body of the concrete itself. Pervious concrete and pervious pavers tend to be weaker than interlocking pavers used in PICP. According to David Smith, technical director for the Interlocking Concrete Pavement Institute (ICPI), many pervious pavers do not meet ASTM standards for solid concrete paving units, such as ASTM C 936-09/C 936M-09, Standard Specification for Solid Concrete Interlocking Paving Units, which requires an average compressive strength of 55 MPa (8000 psi). PICPs also differ from concrete or plastic grid pavers, which have open cells typically filled with soil and grass. Smith notes these surfaces have lower infiltration rates and are limited to light-duty traffic areas.
PICP surfaces do not look much different than traditional interlocking concrete pavements. They can be installed in similar applications, including for vehicular traffic areas, provided the pavers themselves are sufficient thickness (typically 80 mm). For larger residential developments or commercial projects, PICPs are often used in combination with other stormwater management strategies, including natural features such as bioswales and rain gardens.
According to the EPA, PICPs should be able to intercept, contain, filter, and infiltrate stormwater onsite. For example, PICPs can be installed across a street width or parking area, or installed in combination with impermeable pavement to infiltrate runoff and start a treatment train. Various applications use PICP in parking lot lanes or parking stalls to treat runoff from adjacent impermeable pavements and roofs. This economizes installation costs and provides enough treatment area for the runoff made from impervious surfaces. Inlets can be placed in the PICPs to accommodate overflows from extreme storms.
Figure 3: A cross-section of partial-exfiltration installation. Image courtesy ICPI.
There are some places where no permeable pavement, including PICPs, should be used. These include “stormwater hotspots” that may accumulate sediment or chemicals, such as vehicle salvage yards, outdoor unloading facilities in industrial areas, and outdoor liquid container storage areas.
Figure 4: A cross-section of no-exfiltration installation. Image courtesy ICPI.
Step by Step
Permeable pavers take about the same time to install as traditional products. At present, permeable pavement systems can cost more to install than standard pavers, due to additional subsurface preparation; however, many jurisdictions are beginning to offer tax rebates or reduced stormwater fees, which can help offset the additional costs. Likewise, PICPs might save developers money by reducing or eliminating the need for retention ponds and related drainage infrastructure, freeing up valuable land space on the developed properties.
ICPI’s handbook, Permeable Interlocking Concrete Pavements, offers extensive information on design, specifications, construction, and maintenance. Here is an overview of the basic makeup of PICP from ICPI’s handbook, from the top to the bottom layer (see Figure 1).
- Solid pavers with molded joints or openings filled with permeable material
Pavers should comply with ASTM C 936-09, and should be at least 60 mm thick for pedestrian traffic and at least 80 mm thick for significant vehicular traffic.
- Open-graded bedding course
This layer, typically 50 mm thick, provides bedding for the pavers, consisting of small-sized, open-graded aggregate, ASTM No. 8 stone or similar.
- Open-graded base reservoir
This is a 100-mm-thick layer of 9.5- to 25-mm crushed stones (typically ASTM No. 57), which stores water while providing a gradational transition between the bedding above and the sub-base below.
- Open-graded sub-base reservoir
This layer stores stores water among stone sizes of 50 to 75 mm––typically ASTM No. 2, 3, or 4. The thickness of the layers depends on water storage requirements and traffic loads. This layer might not be required for pedestrian and residential driveway applications; instead, the base layer thickness can be increased. Always consult a local engineer with knowledge of regional conditions.
Perforated pipes can help facilitate water removal, if required, when a permeable system is installed over low-infiltration soils. These can drain to a swale, stream, or lake; they can also connect to an underground cistern for water collection.
- Geotextile (optional)
This separates the sub-base from the subgrade to prevent soil migration.
This is the layer of soil beneath the aggregate base or sub-base, generally not compacted so as to allow optimal water infiltration through the soil. If the subgrade is compacted, then the hydrologic design must account for decreased infiltration.
According to the ICPI handbook, PICP can be designed with three types of exfiltration (Figures 2, 3, and 4).
- Fill exfiltration
Water is directed through the base/sub-base and exfiltrates into the soil subgrade––a common application over high-infiltration soils such as gravels and sands. Overflow is directed to swales, bio-retention areas, or storm sewer inlets via surface-level perimeter drains.
- Partial exfiltration
This is a design common to lower infiltration rate soils, such as silts and clays. Perforated pipes above the soil subgrade drain excess water, usually to the storm sewer.
- No exfiltration
This approach is used with soil with low permeability and low strength, sites with insufficient soil depth to filter pollutants before entering groundwater, and fill soils in which water may cause settling and movement. This method functions as a detention pond, with the bottom and sides of the PICP sub-base enclosed with an impermeable liner and geotextile. Water is captured and drains through the sub-base via an underdrain.
CalStar’s Permeable Pavers
CalStar’s offerings for permeable systems include Holland pavers in the tumbled finish installed as part of a permeable system (outlined above) and 80-mm permeable pavers suitable for vehicular traffic. The permeable pavers feature large lugs, which provide 5.9% open area to accommodate a drainage rate of more than 800 inches per hour.
Both the Holland pavers and the permeable pavers are made with CalStar’s proprietary manufacturing process, which incorporates 30% recycled content, uses up to 81% less energy, and emits up to 84% less CO2. Both products contribute up to eight credits under LEED 2009 when installed as part of a permeable system.
Pallets of CalStar pavers can be packaged for mechanical installation.
Beyond green certification points, reducing the strain on storm sewers is a smart move for architects and developers, bringing operational cost savings throughout the building’s life, helping to maintain the health of the surrounding areas, and meeting increasing requirements from local municipalities. From tighter regulations to “green alley” initiatives, reducing runoff is top of mind in many cities. Permeable interlocking concrete pavements provide an appropriate, aesthetically appealing solution.
Diagrams courtesy of ICPI.