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Services: Porous Asphalt / Permeable Pavement Design

Read our case study on the Sherman Library project:

Imagine standing in a parking lot during a rainstorm and seeing the rain actually drain through the pavement rather than run across the pavement to a catch basin or that rapidly expanding puddle that is now growing in front of your car door.  This thought may become a reality on one local project that is proposing the use of porous asphalt in its parking area. The Sherman Library which currently exists at the corner of Saw Mill Road and CT Route 37 in Sherman is looking to expand its current building by constructing a new addition which looks to connect existing library with an existing historic barn.  The Sherman Library is also looking to increase the number of parking spaces and provide better pedestrian access to the new facility. Even though the Sherman Library has recently acquired a property adjacent to the current library site for construction, the site is extremely limited by areas of steep slopes and exposed bedrock.  Due to the site limitations and the need to accommodate the new addition, parking areas, and a underground septic system, there was no land left to be set aside exclusively for stormwater management.  It was therefore necessary to develop an engineered solution that could be implemented into one of the new features of the site.   This solution was the use of porous asphalt.

Porous asphalt was selected for the Sherman Library project due to its ability to allow rainwater to infiltrate the pavement surface and percolate into the site’s underlying native soils.  By allowing rainfall to infiltrate into the ground close to where it first hits the ground, the porous asphalt promotes a more “natural” flow of stormwater through the watershed than what is typically experienced with traditional impervious paved areas.  By minimizing the concentration of overland flow, porous asphalt also aids in the reduction of erosion and sedimentation to downgradient streams and waterbodies.  Other benefits of porous pavement include the reduction of thermal impacts to downgradient watercourses and waterbodies, enhancement of water quality, runoff reduction, and groundwater recharge.  In addition, it has also been proven that snow and ice melt with porous asphalt is drastically quicker than traditional pavement resulting in a reduction in the amount of salt needed for winter maintenance.

Background Information:
In a natural environment (such as a forest or grassland), rainfall typically soaks into the ground, filters through the underlying soils, and eventually makes its way to streams, ponds, lakes, and aquifers through underground flow.  When a site is developed with buildings and traditional impervious pavements, the ability for the rainwater to penetrate the ground surface is no longer possible and rain hitting these surfaces becomes runoff.  To address the additional runoff created by a developed site, developers traditionally have spent large amounts of time and resources having engineered designs developed that provide onsite stormwater storage so that runoff flow rates leaving a developed site do not exceed the flow rates that were experienced on site prior to development.  These traditional engineered designs often include land intensive measures such as detention ponds or underground storage systems that temporarily detain runoff on a site during a large rain event.  While these systems are often effective at providing temporary rainwater storage, they also can change the manner in which rainwater travels through a site.   In doing so, rainwater never reaches areas that it would otherwise reach on a vegetated site resulting in a reduction in groundwater recharge and less subsurface flow to downhill waterbodies and watercourses. These particular engineered detention systems often concentrate runoff to one single point through the use of centralized drainage pipe and catch basin systems.   The concentrated flows typically travel at much greater velocities to receiving watercourses and water bodies than flows on sites with “natural” groundcover and vegetation.  These greater velocities experienced by concentrated flows also tend to lead to erosion and transport of eroded sediment and pollutants to receiving watercourses and waterbodies.  In the summertime, flows off of a hot traditional asphalt pavement tend to experience a warming effect that results in higher temperature runoff being carried to downhill receiving watercourse and water bodies.  These thermal affects can have an impact on fish (such as trout) that rely on colder temperatures to survive.

Stormwater Management:
Porous asphalt is similar to conventional asphalt paving materials with the exception that most of the fine grained particles (sand and dust) are left out of the mixture.  By leaving these fine grained particles out of the mix, the asphalt is able to form voids within the porous asphalt structure that become interconnected and allow the passage of rainwater down into the underlying layers making up the porous asphalt cross section.  By allowing stormwater to readily flow through the porous asphalt, stormwater travels in much the same way it would through an area with a natural or vegetated groundcover.  When rainfall first hits a section of porous asphalt, it quickly passes through the asphalt layer into the underlying stone and gravel filter layers that make up the porous asphalt cross section.  These underlying layers consist of crushed stone, gravel, and sand to provide a stable base for the asphalt, provide a means for filtering the rain passing through the porous asphalt, and provide a means for temporarily storing the runoff prior to being infiltrated into the ground.  Sands and gravels tend to infiltrate water rather rapidly due to the large amount of void space that is created when sand particles interact.  These underlying sand and gravel layers have the ability to provide temporary storage of rain water beneath the upper asphalt layer without the need for construction of costly underground stormwater storage systems and associated pipe and inlet structures.

The void ratio of a volume is sand is conservatively around 0.32 and the void ratio of a volume of crushed stone is usually around 0.40.  This means that 32 percent of a volume of sand and 40 percent of a volume of crushed stone is made up of air pockets or voids that are capable of storage of stormwater.  On sites with poorly draining underlying soils, the porous asphalt cross section can be designed with perforated pipe drains, otherwise known as underdrains, that can be set below the pavement and above the bottom of the porous asphalt cross section to provide a means for passing stormwater during unusually large storms.

The use of porous asphalt as a stormwater management measure is effective in providing for both stormwater quality and stormwater quantity.  By allowing stormwater to readily flow through the porous asphalt, stormwater travels in much the same way it would through an area with vegetated groundcover.  When rainfall first hits a section of porous asphalt, it quickly passes through the asphalt layer into the underlying stone and gravel filter layers that make up the porous asphalt cross section.  During this process, sediment and pollutants captured by rainwater that are washed through the asphalt surface are able to be filtered from the stormwater passing through the porous asphalt cross section. This “natural” movement of stormwater enhances stormwater quality prior to it reaching downhill lakes, ponds, and streams.  In addition, this “natural” movement also results in a reduction in thermal impact to downstream receiving waters.  Since water readily drains through the porous asphalt pavement, there is less contact time for rain water to experience the warming effect that would otherwise be experienced on a traditional pavement.

The origins of porous asphalt seem to date back to the late 1960’s when the concept was proposed to “promote percolation, reduce storm sewer loads, reduce floods, raise water tables, and replenish aquifers.”  Throughout the 1970’s, the concept was discussed and refined to a point where the Environmental Protection Agency (EPA) contracted to “determine the capabilities of several types of porous pavements for runoff control, in terms of cost and efficiency.”  Some of the first porous asphalt installations were done in Pennsylvania, Delaware, and Texas.  In 1977, Edmund Thelen and L. Fielding Howe co-authored a design guide for porous pavements for the Franklin Institute in Philadelphia.

Many porous pavement sites have been constructed since the late 1970’s throughout the United States and Europe with both success and failure.  The most common causes of failure to date have been due to fine sediment clogging the pores making up the asphalt cross section.  Another reason for failure in some circumstances has been the installation of improperly mixed asphalt that contains too many fine particles.

There have been findings that while a traditional asphalt pavement parking area can last around 15 years, a properly constructed porous asphalt parking area can last over thirty years in a northern climate.  This is due to the reduction in freeze/thaw stress that is experienced by porous asphalt as compared to traditional pavement.

The porous asphalt design for the Sherman Library Expansion was shaped largely from information provided by publications and workshops that have been provided by the University of New Hampshire Stormwater Center.  The University of New Hamphire Stormwater Center (UNHSC) operates and collects test data from its field site that contains numerous Stormwater “Best Management Practices” (BMP’s).  Best Management Practices (BMP’s) are stormwater measures or techniques that effectively and practically manage a site’s stormwater needs with regard to providing sediment and pollutant removal.  Once the test data is compiled, the UNHSC then compiles the information and assesses each practice’s ability to provide stormwater quality and quantity management.  One of the BMP’s that is tested is a porous asphalt parking area that was constructed in 2004.  This parking lot was constructed next to an identical size traditional impervious pavement parking area.  While the traditional impervious parking lot required piping and catch basins, the porous asphalt parking area required only an underdrain system set above the subbase that would collect and pass water during an unusually large storm.  Based on the comparison of the two lots and collected data, it was found that the porous asphalt parking area performed extremely well with regard to providing sediment, total petroleum hydrocarbon, total phosphorus, and total zinc removal.

On a different part of campus, the University of New Hampshire also constructed a pervious concrete parking area in a location that overlays shallow bedrock.  Since aquifer protection was required above the bedrock, an impermeable HDPE liner was used so no infiltration into the underlying bedrock could take place.  To provide a means for overflow during unusually large storms, the pervious concrete pavement cross section was designed with a system of perforated underdrains.   The design as constructed avoided the need for catch basins and pipe and provides the University of New Hampshire with a means for providing flow rate attenuation and sediment settlement without the need for land intensive surface detention systems.  Evidence that the system can handle large quantities of stormwater can be found on the UNHSC website (www.unh.edu/erg/cstev/pubs_specs_info.htm) which shows a video of a truck dumping large amounts of water over a section of the subject pervious concrete.  As can be seen in the video, the pervious concrete readily drains the water through the porous asphalt to the pavement subbase with no excess runoff from the edge of the parking lot.

The University of New Hampshire Stormwater Center (UNHSC) provides tours of their facility where they walk visitors to each of the Best Management Practices (BMP’s) that they conduct research and testing on and then give a brief discussion as to the effectiveness of each practice with regard to stormwater quantity and quality management.  Following the tour, presentations regarding the performance of BMP’s and the permeable pavements are provided to demonstrate the effectiveness of each measure with regard to pollutant and sediment removal.  These presentations are also available online at the University of New Hampshire Stormwater Center’s website (www.unh.edu/erg/cstev/).  In addition to the presentations, the UNHSC also provides their UNHSC Design Specifications for Porous Asphalt Pavement & Infiltration Beds.  This publication provides the background information and guidelines recommended for properly designing and specifying the various layers making up the porous asphalt cross section. Lastly, the UNHSC also provides recent students’ Master Theses which include in-depth research and analyses regarding various stormwater BMP’s performance.  Several of these theses focus exclusively on porous asphalt.

Click Here if you would like to read the Porous Asphalt Over Bedrock developed for the Sherman Library!

There are many misconceptions about porous asphalt.  Critics of porous asphalt may say that porous asphalt freezes faster than traditional pavement.  Due to the ability for water to pass readily through the porous asphalt section, porous asphalt actually has reduced ice potential and thaws quicker than traditional impervious pavements that are unable to penetrate the pavement surface and instead must travel to a drainage inlet before icing potential is minimized.  Studies by the University of New Hampshire Stormwater Center have actually found that significantly less salt (up to 75% less) is required to treat a porous asphalt pavement as opposed to traditional pavements.  Other critics of porous asphalt argue that porous asphalt requires greater cost for installation and maintenance.  It has been found that porous asphalt parking areas can cost close to the same as the same size parking lot that is installed with traditional pavement.  While the square footage cost of porous asphalt is greater than that of conventional asphalt, the cost increase is often offset by the ability to eliminate costly drainage structures and pipe that are required with traditional asphalt.   Some critics argue that the life span of a porous asphalt parking area is less than that of a traditional asphalt parking lot.  Research has actually found that when properly installed, porous asphalt has a longer life span than traditional asphalt.  This is due to the reduced freeze/thaw action that occurs as a result of porous asphalt freezing as a porous medium rather than a solid block.

In order to promote the long term operation of porous asphalt, it is necessary to inspect and maintain porous asphalt areas.  The most prevalent maintenance concern with porous asphalt is the potential for clogging of the pores making up the porous asphalt pavement section. Fine particles that can clog the pores are deposited on the surface from vehicles, the atmosphere, and runoff from adjacent land surfaces. The frequency of clogging can 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 porous asphalt and other permeable pavements have found high infiltration rates initially, followed by a decrease, and then leveling off with time. To minimize the potential for clogs and to promote the long term infiltration capacity of a porous asphalt pavement section, it is necessary to train maintenance staff regarding the maintenance needs of porous asphalt.

Winter Maintenance
Maintenance of porous asphalt during and after winter snow and ice storms is slightly different than that of conventional pavement.  While both conventional pavement and porous asphalt need to be plowed, the use of sand and salt for deicing and traction varies significantly between the two types of pavement.  While deicers or road salts can be applied to both types of pavement, the application of sand to porous asphalt should be avoided to prevent the clogging of the pores within the porous asphalt pavement.  Despite the lack of sand application to porous asphalt during winter storms for traction, research has found that porous asphalt can provide the same surface friction coefficient as compared to a traditional pavement surface with applied sand.  It has also been found that porous asphalt requires less salt or deicer application than traditional pavement due to the ability of melt water to pass readily through the porous asphalt rather than over a traditional impervious pavement. 

Porous asphalt 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.  Cold weather and frost penetration do not negatively impact surface porous asphalt infiltration rates. Porous asphalt freezes as a porous medium rather than a solid block because permeable pavement systems are designed to be well-drained and infiltration capacity is preserved because of the open void spaces. Due to the reduced freeze-thaw stress as compared to conventional pavement, porous asphalt has been found to experience less of the breakdown associated with freeze/thaw cycling as compared to conventional impervious pavements which leads to a longer lifespan for porous asphalt.

Click Here if you would like to read the Porous Asphalt Maintenance Handbook developed for the Sherman Library!

Porous Asphalt versus Porous Concrete 
Porous asphalt was selected for the Sherman Library Expansion over porous concrete for several reasons.  While both porous asphalt (18-22% void space) and porous concrete (15-22% void space) both provide basically the same function in reducing stormwater runoff and promoting stormwater quality, there are several key distinctions that led to the selection porous asphalt over porous concrete.  The most notable distinction is the ease of construction and cost difference in installing porous asphalt as compared to porous concrete.  While the mix production of porous asphalt is a bit more difficult than that of porous concrete, porous asphalt is much easier to install than porous concrete.  While virtually any qualified installer can install porous asphalt, the installation of porous concrete requires trained and certified installers.  If porous concrete is installed improperly, it can result in low infiltration rates and structural problems.  Due to the need for highly trained installers, porous concrete can sometimes run up to 4 times the price of an equal area of porous asphalt for virtually the same benefit.   While porous concrete does perform better in reducing the “heat island” effect in the summer due to its lighter color and ability to reflect (and not absorb the sun’s heat), it does not perform nearly as well as porous asphalt in promoting ice melt in the winter.

Click Here if you would like to read the Porous Asphalt vs. Porous Concrete analysis developed for the Sherman Library!

Be a Trendsetter

The goal of Low Impact Development (LID) is to maintain and mimic pre-development hydrology through the use of small scale controls integrated throughout the site. The practical use of Low Impact Development (LID) measures is largely dependent on a site’s conditions. Soil permeability, site slopes, and existence of an elevated water table or bedrock are all site characteristics that may limit the use of Low Impact Development (LID) practices. Local land use regulations, public perception, and breaking old habits also present obstacles to the implementation of Low Impact Development (LID) practices. If you are interested in developing a site using Low Impact Development (LID) techniques, contact us at (860) 354-9346 so that we can discuss how Arthur H. Howland & Associates, P.C. can provide you with cost effective, site specific, “green” solutions that you can feel good about.

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