Permeable Interlocking Concrete Pavers
Their Role In Green Construction
We all understand the value of permeable pavers in terms of their ability to give us a hardscape surface that allows water to seap into the water table. But we thought it would be nice to to share some specific information based on our experience with permeable pavers, which will allow you to highlight the important pros and cons related to this product for your customers, specifically in terms of its role in the arena of sustainable design. Below is a general outline of the main points related to this subject, and it should be a quick and easy way to and to put some of the basic facts and figures at your fingertips. If you wish to print this outline, it will also be available on our website as well.
The outline describes, in general, the basic problems with impervious surfaces, what Low Impact Development (LID) is, and how it seeks to address this problems with impervious surfaces, the role of permeable pavers in Low Impact Development, some specific facts about how permeable pavers function in eco-friendly construction, how they conform with ADA standards, and some pointers on how to use permeable pavers to score points in LEED projects.
Problems with Impervious Surfaces: Surfaces that do not permit the penetration of surface water cause two main problems:
1. Overworked storm drains, causing excess, untreated water to run to rivers and lakes.
2. Suspended sediments and heavy metals run directly into storm drains without being filtered by the natural system.
Low Impact Development (LID): A design strategy with the goal of "maintaining and enhancing the pre-development hydrologic regime of urban and developing watersheds." (Taken from the Low Impact Development Center website: www.lowimpactdevelopment.org) One of the goals of Low Impact Development is to reduce the percentage of impermeable surfaces in new construction.
Permeable Pavers Role in LID: Permeable pavers allow water to pass through the upper layer of the pavement and then enter the sub-base reservoir, which acts like a sub-surface infiltration basin. Storm water is therefore treated through filtration, volatilization, microbial activity, storage and ion exchange. The storm water then slowly infiltrates into the sub-grade soils, instead of overunning or storm drains and/or adversely affecting the water supply.
Infiltration: There is considerable debate on the infiltration rate of permeable pavers. The infiltration rate must be calculated based on the overall infiltration that occurs once the pavers have been installed (i.e., when the infiltration rate of sub-surfaces and soil are all factored in.) One study showed a mean infiltration rate of 3.5 inches of water per hour, for clean pavers (free of debris.) This study is here:
http://www.bae.ncsu.edu/stormwater/PublicationFiles/ICPIreport2004.pdf
Some suppliers and manufacturers will claim much higher infiltration rates, including the rate you would expect from the pavers alone (before they are set on soil.) Beware of unreasonably high infiltration claims.
Exfiltration: Rainfall may exceed the exceed the infiltration rate of the soil below. For different densities of soil, different exfiltration methods may be utilized in order to completely re-route water:
Full exfiltration: For use when the soil is expected to fully exfiltrate all of the surface water. All of the water exfiltrates into the soil below, with no additional sub-surface drainage.
Partial exfiltration: For use when the soil may not exfiltrate all the surface water. In this case, a sub-surface perforated pipe is used to redirect the water.
No exfiltration: For use when the soil below has low or no permeability. In this case, an impermeable membrane is used at the bottom and sides of the system, in addition to a sub-surface perforated drain pipe.
Reduction of Pollutants: The aggregate filters and sub-grade soils the allow sedimentation to occur, and contribute through bacterial treatment of the pollutants and cation exchange. Growth of "good" bacteria has been found on established aggregate bases. in addition, because water immediately enters the permeable surface, it maintains a lower temperature, which means that unnaturally heated water does not enter the water habitats of wildlife, where it can cause shock or death.
Where Not to Use Permeable Pavers: Pavers should be installed at least 100 feet from water supply wells, streams and wetlands. They should not be installed in facilities where the by-products of products may contaminate groundwater, such as industrial facilities that store
hazardous wastes, or gas stations.
In addition, permeable pavers are highly subject to clogging when exposed to sediments or fines. The effectiveness of permeable pavers can be significantly reduced due to clogging. (As a side note, permeable pavers should be cleaned on a recurring basis in order to maintain maximum permeability.)
Compliance with Americans with Disabilities Act: There is no express rule for paver spacing in order to satisfy ADA requirements. However, the rule of thumb is that the spacing should be a vertical maximum spacing of 1/2".
LEED Credits:
Credit 6.1 and 6.2: Stormwater Design - 2 possible points (for achieving the appropriate percentage of permeable surface.)
Credit 7: Heat Island Effect Non-Roof - 1 possible point (for using light colored, high albedo materials)
MR Credit 4: Recycled content - 2 possible points (when using pavers that contain fly ash or blast furnace slag)
MR Credit 5: Regional materials - 2 possible points (when using materials that are extracted, processed, and manufactured regionally.
This article is intended as a general overview only. There are a myriad of other factors that we can explore in relation to permeable pavers, and we have included some useful links for those of you who wish to explore the subject in more detail. We always welcome your comments and feedback.
The above article was based in part on information found at the following web links, which you may find useful:
http://www.lowimpactdevelopment.org/
http://www.bae.ncsu.edu/stormwater/PublicationFiles/ICPIreport2004.pdf
http://www.icpi.org/faq/48
http://www.aecdaily.com/en/1647234
