Design Considerations for Vegetated Permeable Pavement
Flexible Concrete Mat
A precast mat unit with a network of concrete pads cast around a polymer grid that flex and conform to irregular ground surface contours.
Basic Product Composition: Wet cast concrete mat with an engineered grid cast inside. Individual pads are intended to crack at the joints and grid is designed to allow for long-term settlements. Individual mats are butted up similar to conventional pavers. The concrete can use recycled material such as fly ash or slag.
Flexible or Rigid: Flexible
Typical Infill and Bedding Course: Specifies 80% sand and 20% organic infill and bedding course. Bedding course is typically 2 inches thick.
Typical Void Width: 1.5 inches
Typical Unit Load-Bearing Surface Area: 40%
Typical Dimensions: 24 inches x 24 inches x 1.5 inches
Measure of Performance: ASTM. No specific ASTM for this type of product.
Testing: Manufacturer's reports: Permanent Deflection and Performance Study Sept. 2005 – Feb. 20067; Runoff 911 Test Section Performance8
Limitations: Steel track vehicle areas. No certified installation program.
Concrete Grid Slab
Concrete grid slabs are a cast-in-place, monolithic pavement with voids from forms, with steel reinforcement.
Basic Product Composition: Cast-in-place, monolithic, pervious concrete pavement that is continuously reinforced with steel. The concrete can use recycled material such as fly ash or slag.
Flexible or Rigid: Rigid
Typical Infill and Bedding Course: Infill not specified; typically set on 1 inch of bedding sand
Typical Void Width: 4 inches
Typical Unit Load-Bearing Surface Area: Not available
Typical Dimensions: continuous slab, 5.5 inches thick
Measure of Performance: ASTM, ACI. No specific ASTM for this type of product.
Testing: Common concrete strength testing and reinforcement per ASTM. Manufacturer's report: One-day test 1986 structural load test with Grumman ladder truck 23,000 lb per axle. RAM testing 1994—lateral load testing of the protruding concrete surface posts.9 One-day test 1980 City of El Segundo Fire Department.
Limitations: Contoured installations. Accessibility to utilities below installation. Long-term oxidation of rebar. Lack of continuous bedding course for root zone.
Most people know these as the waffle-like, commonly available individual concrete block pavers with voids.
Basic Product Composition: Dry cast modular concrete blocks. Some products include steel reinforcement to allow for heavier vehicles (typical spec for any modular block is the length / thickness <= 4). The concrete can use recycled material such as fly ash or slag.
Flexible or Rigid: Individual units are rigid.
Typical Infill and Bedding Course: Top soil infill or sand, if non-vegetated; ½ inch to 1 inch of bedding sand
Typical Void Width: 3 inches
Typical Unit Load Bearing Surface Area: 61%
Typical Dimensions: 24 inches x 24 inches (L x W) or less. Minimum thickness of 3.125 inches
Measure of Performance: ASTM, ICPI, NCMA
Testing: Common concrete strength testing. Several light-duty case studies through academic research available. No published field studies with heavy vehicles found.
Limitations: Steel track vehicle areas. Heavy loads on larger units without reinforcement (based on ASTM 936) resulting in cracking at surface from base or subgrade settlement. Elaborate contoured installations. Lack of continuous bedding course for root zone.
Plastic Geocells
Reinforced plastic cells are made of a recycled plastic—high-density polyethylene (HDPE)—the #2 category for recycling.
Basic Product Composition: Plastic. Most commonly HDPE plastic in modular tray units or standard roll sizes covering 108 to 538 square feet. Rings connected by tensile members or a network of square or hexagonal cells.
Flexible or Rigid: Flexible and rigid
Typical Infill and Bedding Course: Infill varies between top soil and sand. No bedding course is typically specified. Rolled units require staking into the base or subbase material.
Typical Void Width: 2 to 3 inches
Typical Unit Load-Bearing Surface Area: 5 to 13%.
Typical Dimensions: 1 to 1.5 inches thick. Varying dimensions for trays and roll sizes.
Measure of Performance: None at this time.
Testing: Different laboratory tests per manufacturer for sand filled and unfilled compression tests. A.G. Wassenaar Geotechnical compression resistance/load-bearing capacity test 1991. No published heavy vehicle field tests.
Limitations: Steel track vehicle areas. Memory in the material once disturbed. Movement of plastic under stress (plastic is susceptible to thermal expansion /contraction). Strength of narrow sidewalls in a saturated environment. Lack of continuous bedding course for root zone. No common measure of performance or certified installation program.
Green Infrastructure Initiatives
Other decisions that should be part of the initial design process are whether the project is pursuing LEED® and SITES® (in development), other pavement design factors that might be important for your project type, potential first and life-cycle costs will be, and the level of commitment that the client plans for maintenance. The goals of reducing stormwater runoff and improving stream health are inherent in both green building/green infrastructure rating systems such as LEED® and SITES® (in development). Cities and regional planning agencies often provide BMPs with details and specifications within their green building/green infrastructure development standards and/or guidelines. Vegetated permeable pavement can be used in bioswales, bioretention areas, or rain gardens, for adding to the site's biomass index (BMI), or to prevent erosion, recharge the groundwater, or reduce UHI.
In New York's Green Infrastructure Plan (2011): A Sustainable Strategy for Clean Waterways the goal is to reduce by 10 percent its combined sewer overflow through the use of retention and infiltration by 2020. Within nearly each land-use type in this large metropolitan area, from streets and sidewalks to parks and parking lots, there are opportunities to use permeable pavement to achieve that 10 percent goal. The City of Chicago, through its Green Alley Handbook, and extensive green infrastructure design efforts, lists “Permeable Paving” as one of its preferred materials, being “most effective in areas closer to Lake Michigan that are underlain with sandy, permeable soils…Permeable pavement may have aesthetic and marketing advantages over conventional pavement, depending on the materials selected. Vegetated pavers, in particular, could substantially improve the aesthetic appeal of paved areas...(and) effective in reducing the 'urban heat island' effect.” To this city, permeable pavement is particularly appropriate for “…overflow and special event parking, driveways, utility and access roads, emergency access lanes, fire lanes, and alleys.”
The City of Seattle Public Utilities (SPU) Department and Office of Sustainability recommends use of permeable pavement for their Stormwater Facility Credit (SPU) program. The SPU program grants discounts on drainage bills for private stormwater systems that “reduce stormwater flow and/or provide water quality treatment…” Permeable pavement is among the stormwater structures that qualify for up to a 50 percent credit. In theState of Virginia's Stormwater Design Specification #7: Permeable Pavement (Version 1.8 3/1/11), BMPs are enumerated at length primarily for permeable pavements. The specification details the properties for each type of pavement, material specifications, maintenance recommendations, and construction installation sequence. This type of BMP information is common among those cities and regions leading the way in the use of permeable pavements.
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