Frequently Asked Questions (FAQ)
General Information
This page contains Sections on: EPA Requirements, Pervious Concrete, PAR, Federal Green Construction Guide, LEED, LID, Pollution Treatment, Recharging the Aquifer, Parking Under Trees and Cool Communities. (Click your "back-button" to return to this list.)
On April 19, 2007 The Environmental Protection Agency (EPA) signed an agreement to promote the use of Green Infrastructure. Green infrastructure can be used to reduce stormwater discharges and help to restore the natural hydrology, water quality and habitat of urban and suburban watersheds. "These approaches can be used to keep rainwater out of the sewer system so that it does not contribute to a sewer overflow and also to reduce the amount of untreated stormwater discharging to surface waters." Permeable pavements, such as pervious concrete , are considered an integral part of Green Infrastructure. (See the EPA Web Site on Porous Pavement.)
EPA Stormwater Treatment Requirements - The EPA Stormwater Phase II Final Rule requires operators of Municipal Separate Storm Sewer Systems (MS4s) whose population exceeds 50,000 people, to obtain National Pollutant Discharge Elimination System (NPDES) permit coverage because their stormwater discharges are considered “point sources” of pollution. All point source discharges are required under the Clean Water Act to be covered by federally enforceable NPDES permits.
The operator of each MS4 has the flexibility to determine the stormwater Best Management Practice (BMP), and measurable goals, for each minimum control measure that are most appropriate for their district. The chosen BMPs and measurable goals, submitted in the NPDES permit application, become the required stormwater management program for the MS4.
Typically, BMPs for stormwater control has centered around on-site storage (detention) and treatment. For small sites (less than 15-acres) this has taken the form of open-ponds and underground-container systems. Recently, The Federal Government, in the charge of The National Institute of Building Sciences (NIBS), has created a set of guidelines and specifications for use in government buildings defined as the Federal Green Construction Guide. This guide describes the use of pervious/porous pavements that achieve the mandates of the EPA - Clean Water Act, while also complying with the requirements of the USGBC-LEED Certification program.
This web site uses the principles and guidelines of the NIBS Federal Green Construction Guide as a basis for the design and construction of what is defined as a dual-use pervious concrete pavement, combined with a stormwater storage/treatment train. The intent of this system is to design the major portion of the site as a dry detention-pond (infiltration basin). The parking area is then constructed inside the pond. This system also complies with the EPA - Green Infrastructure statement of intent.
Pervious Concrete was first designed and constructed as a pavement, in Florida, during the early 1980s. It was initially approved only as a pavement under trees. The pervious concrete was placed, 4-inches thick, directly on compacted soil. Over time the pavement thicknesses, for parking lots, was increased to 6-inches (by industry consensus).
People quickly saw the advantages of having a rigid permeable pavement, and started using it in applications other than just under the drip-line of trees. Most of these applications (in Florida) were placed directly on compacted soils. In these applications, pervious concrete pavements were designed essentially in the same manner as other impermeable pavements such as asphalt and conventional concrete. Many of these pervious concrete pavements are still in use today (with no overlay requirements).
More recent applications, under the mandates of the EPA and "sustainable-building", have been to place the pervious concrete pavements over a layer of clean gravel/crushed stone. (Review the Federal Green Construction Guide and the EPA Web Site on Porous Pavement.) The stone acts as a structural-base for the pavement, and as the on-site stormwater detention area (reservoir). This is defined as a dual-use application. The pavement area is designed for automobile storage, but it is placed inside the on-site stormwater storage and treatment train instead of on separate property. This dual-use design saves the Developer a considerable amount of money because there is no requirement for conventional stormwater infrastructure - such as stormwater-inlets, stormwater pipes, or for an additional pond.
The properties of pervious concrete, such as a high perk rate of over 200-inches per hour, and a rigid-pavement design, offer the properties and attributes necessary for an effective stormwater management system. Note: See the new NRMCA Video of how fast water is absorbed into the pavement.
Pervious Concrete Composition – Pervious concrete is a mixture of portland cement, coarse aggregates (stone), water and admixtures. Pervious concrete contains little, or no sand, and is sometimes referred to as a “no-fines” concrete. The portland cement and water forms a paste that binds the coarse aggregates together. Only enough portland cement and water is added to the mix to glue the aggregate together at the points of contact, but not enough to fill all the space between the aggregates. A typical pervious concrete mix will contain about 15-25% void space within the concrete sample. Many of the void spaces within the mix will be interconnected, forming channels that allow water and air to pass freely through the mix. (Specifier's Note: Unlike a layer of clean gravel, not all of the void spaces in pervious concrete will be interconnected - due to the presence of the cement-paste. This property will affect the outcome of field-tests for a void-space/permeability relationship.)
Par - Unlike conventional concrete, pervious concrete undergoes many changes during the mixing process, and during transit to the jobsite. The aggregate can become coated with successive layers of the available cement paste. In many cases the paste can harden around the aggregate, and inside the mixer-drum, to the extent that there is insufficient free paste left to glue the aggregate together. The quantity of paste, still in a "plastic" or semi-fluid state, as compared to the quantity of aggregate, can be defined as the paste to aggregate ratio (par). The par of fresh pervious concrete can vary over a short period of time depending on several factors to include: properties of the aggregates, admixtures used, ambient weather conditions or time of travel. Because there are no standard tests to determine the par, the Concrete Contractor must be able to make a determination by visual inspection of the mix when it arrives at the jobsite. He/she should have enough experience to determine if the mix has the correct properties to include par. If a mix arrives at the jobsite with insufficient par, and is used by the Contractor anyway, then the finished pavement will not have sufficient strength to withstand automobile traffic loading, and will result in severe surface raveling.
Specifier's Note: Many of the problems associated with the use of pervious concrete has been as a result of insufficient par. However, many suppliers and contractors don't have the experience necessary to make the visual-test, or adjustments in the mix design. There are currently no standardized tests used to measure the par of fresh pervious concrete at the jobsite, and acceptance of the mix is often based solely on the knowledge and experience of the Contractor. (See "Criteria for Acceptance" on the Details & Specifications page.)
Important: Because of the lack of standardized test for quality control and acceptance of Pervious Concrete, it is critical that whoever is making the decisions for acceptance of both the fresh concrete, and the finished pavement, that they are qualified to do so. The decision-maker must have adequate knowledge and experience to recognize the essential properties of Pervious Concrete, and know how to make field-adjustments. (Note: Both the NRMCA, and the National Institute of Building Sciences, recognize that only NRMCA Certified "Craftsmen" have the 1500-3000 hours of experience necessary as a basis of qualification.)
Importance of Pervious Concrete – Pervious concrete has been used in construction work for more than 100 years. Its first recorded use was in parts of Europe where it was used in the construction of buildings. Today, in the U.S., it is being used primarily as a pavement material for parking lots.
A new interest in pervious concrete is being driven by the demand from the Environmental Protection Agency (EPA). The EPA, “Clean Water Act”, Phase 1 and 2, orders States, Counties and Municipalities to adopt procedures that address stormwater runoff issues. The Sustainable Building program, to include LEED Certification, is also a major driver. The use of pervious concrete can help in addressing those issues.
It is important that each City and County Stormwater Management Team recognize the values and properties of rigid, pervious-concrete pavements in order to properly establish codes for its use, or for LEED Certification. Pervious Concrete Pavements, in conjunction with a clean gravel/crushed-stone base, offer many of the required properties to meet, or exceed, NPDES stormwater permitting requirements. (See the Federal Green Construction Guide and the EPA Web Site on Porous Pavement.)
Magruder Construction is turning gray concrete into green parking lots.
Developers and Engineers may choose to use Pervious Concrete Pavements because:
Federal Green Construction Guide - The National Institute of Building Sciences (NIBS) has been established by the U.S. Congress, and acts as the clearinghouse for many of the design and construction programs used for government projects. The NIBS has put together a program called the Whole Building Design Guide in which the Federal Green Construction Guide for Specifiers is a key part. The guide has been adopted by: The Department of Defense (to include the Corps of Engineers), The Department of Energy, Homeland Security, The Environmental Protection Agency, NASA and The National Park Services. The design-guide can also be used for private-industry Sustainable/Green-Building projects.
This web site uses the principles and guidelines adopted by the NIBS - Federal Green Construction Guide for the design and construction of the (non-proprietary) dual-use pervious concrete pavement system. The "Guide" describes the use of several types of porous/pervious pavements. This site promotes only the use of pervious concrete, and demonstrates why it is the material of choice for porous/pervious pavement and stormwater treatment designs.
LEED Recognition – The U.S. Green Building Council is a national coalition of construction industry entities promoting environmentally responsible and healthy places to live and work. The Council’s LEED program is a rating system that assesses the environmental sustainability of new and existing buildings.
The LEED program also uses a process called commissioning to determine how different sections of a building function together as a whole for optimal energy efficiency. (The commissioning process can be extended to include the parking lot in what is then defined as the whole-building concept.) The LEED-NC rating system evaluates new construction, major upgrades to existing buildings, and building operations and maintenance to classify them at certification levels ranging from “Certified” to “Platinum” based on a point system. Applicants receive points for the environmental friendliness and/or energy consumption of each component for the design and construction with respect to the building’s stormwater maintenance, re-use of building materials, temperature and light reflectance (heat-island effect) and the conservative use of water.
Pervious concrete is recognized as a pavement material within the LEED program that meets several of the Requirements for certification in several categories to include: Sustainable Sites (SS), Water Efficiency (WE) and Materials & Resources (MR). When pervious concrete is used correctly, in the pavement design, then multiple credits may be achieved towards a LEED Certification. (See also the ACI Points for Pervious.)
One of the first "task" items, within the Certification process, is to select a "LEED-Project Team". The team should be composed of individuals from the various project disciplines that have the ability to work together in the "Whole-Building" design approach. Developers, or their Representatives, should include Don Wade as a part of their Design Team to insure the maximum credits that can be achieved by using pervious concrete.
Low Impact Development – Low Impact Development (LID) is an approach to environmentally friendly land use planning. It includes a suite of landscaping and design techniques that attempt to maintain the natural, pre-developed ability of a site to manage rainfall. LID techniques capture water on site, pass it through permeable pavements or vegetation, and let it soak into the ground where it is filtered and can recharge the local water table rather than being lost as surface runoff. An important LID principle includes the idea that stormwater is not merely a waste product to be disposed of, but rather that rainwater is a resource. LID can be applied to new development, urban retrofits, and redevelopment / revitalization projects at many scales. At a small scale, LID techniques can be used to better handle rainfall for a single-family lot through pervious concrete driveways and grass swells. At a larger scale (one to fifteen-acres), proper site design in combination with pervious concrete pavements and infiltration techniques improve rainfall and run-off management.
Pollution treatment – In a parking lot, pollutants, in the form of hydrocarbons and heavy metals, are deposited on the surface. According to the EPA, about 90 percent of the surface pollutants are carried off by the first 1/2 inch to 1-inch of rainfall. This is known as the “first flush”. In a conventional, impervious, parking lot the polluted rainfall flows to a storm drain inlet, and eventually finds its way to our streams, lakes or rivers. Asphalt pavements are not a solution to the stormwater runoff problems - they are part of the problem. It is better to stop the runoff at the source - than to allow it to become runoff.
In the dual-use systems, the permeability of a rigid pervious concrete pavement (typical perk-rate over 200-inches per hour) (see the new NRMCA Video) allows the first-flush to pass through the pavement into a layer of clean gravel/crushed-stone beneath the pavement. The stormwater is temporarily stored in the gravel layer until it has had time to soak into the soil below. The soil of the site then filters and treats the rainfall in the same manner as before the site was disturbed. The rainfall is also spread evenly over the entire parking lot area instead of concentrating it in one spot such as a conventional detention pond. Treatment is also at a greater distance from the water table than with other treatment systems such as conventional detention ponds or precast underground storage/infiltration systems. Typically, the depth of the entire treatment-train is only one-foot (six-inches of pavement and six-inches of gravel). This is important in areas where the water table is near the surface, or where karst-topography may restrict the use of deeper infiltration systems.
Projects utilizing the dual-use systems, as a post-construction BMP, are closer to matching pre-construction conditions than with any other system. In addition, pollution removal is rated at over 80%.
Recharging the Aquifer – Many areas of the U.S. are dealing with water shortages. Yet we allow an abundance of water in the form of rainfall to run into storm sewer systems and bypass the natural process of the rainfall filtering into the aquifer. The more impervious surfaces that are constructed, the less water finds its way back into the aquifer, and the less water we have to drink.
Dual-Use (Parking in the Pond) systems allows the rainfall to percolate back into the soil where it falls, and replenish the aquifer naturally.
Parking Under Trees – Pervious concrete pavements were first approved for parking areas under trees. Trees need water and air to live just like us. Impervious pavements, underneath trees, keep water and air away from the tree roots. Many communities have adopted regulations that do not allow the construction of impervious pavements within the drip-line of trees. This can prove difficult in places where trees are within, or near, roads or parking lots.
The solution; construct the roads and parking lots using pervious concrete. This allows the rainfall and air to pass freely through the pavement to sustain the tree roots. (A layer of gravel under the pavement may not be required for this application.)
Cool Communities – The Cool Communities program works to reduce energy consumption and increase air quality by promoting "cool" strategies of appropriate placement of trees and shrubs and use of reflective roofing and pavements. Partnering with many groups, the program is involved in education and demonstration projects, and incorporating Cool Communities strategies into municipal policy and city ordinances.
Pervious concrete is less able to absorb and store heat than conventional asphalt or concrete pavements. The lower density of the material (15 - 25% void spaces) also helps to reduce the heat storage capacity. The lighter colors of some pervious concrete pavements may also reduce the heat absorbing capacity. In addition, the open void structure in the pavement may allow cooler earth temperatures and evaporating water from below to cool the pavement. These factors allow pervious concrete pavements to approach natural ground cover in heat absorbing and storage capacity. More information can be found at the Georgia Concrete & Products Association web site. LEED Credits can also be achieved by using pervious concrete in specific projects relating to "heat-island" effects.
Contact Don Wade, Program Manager and NRMCA Certified Craftsman, Magruder Construction for additional information.