Porous Asphalt

This page serves as an introduction to the design considerations for porous asphalt when it is part of an MSD-approved stormwater management plan.

MSD has developed details illustrating typical sections for porous asphalt.

Porous Asphalt Design Detail
Porous Asphalt Sign

This document serves as an introduction to the design considerations for Porous Asphalt Pavement when it is part of a stormwater management plan.

The designer should consider the following when designing a porous asphalt pavement system:

1. Porous asphalt pavement may be used to provide water quality volume (WQv), provided it is designed and constructed in accordance with Permeable Pavement Performance Criteria or is used in conjunction with a properly designed infiltration basin/trench, sand filter, or other approved BMP per the 2000 Maryland Stormwater Design Manual. The supplemental BMP shall be located downstream from the porous asphalt system, not underneath it.
2. Designer should reference National Asphalt Pavement Association (NAPA) Information Series 131 for additional guidelines.
3. Consideration should be given to the type of traffic loads that the pervious pavement will be required to endure. An engineer with expertise in pavement design should provide input on this aspect early in the design phase.
4. Porous Asphalt Pavement will not be allowed on sites whose land use or activity generates higher concentrations of hydrocarbons, trace metals or toxicants than are found in typical stormwater runoff. Examples of such “hotspots” are listed in table 2.6 in the 2000 Maryland Stormwater Design Manual.
5. Subgrade should be flat, and where topography requires, terracing of the subgrade is allowed. The volume of water stored behind each terrace will be considered part of the infiltration bed.
6. A minimum of 2 feet vertical clearance should be provided between the bottom of the storage bed and the seasonally high water table.
7. Placing Porous Asphalt Pavement over areas of recent fill or compacted fill is not optimal for infiltration.
8. Provide an overflow system or additional storage to prevent water in the open graded storage bed from rising into the Porous Asphalt Pavement for the 2-yr, 24-hr storm event. Any emergency overflow pipes placed within the subbase should discharge only when the storage volume is exceeded. The horizontal and vertical location of all underdrains and overflow systems shall be determined by the design engineer and shown on the plans. Emergency overflow pipes and underdrains systems shall discharge to a storm water structure or daylight to a stabilized outfall.
9. When Porous Asphalt Pavement is used in conjunction with stormwater runoff volume reduction practices, MSD recommends the suitability and infiltration rates of the underlying soils shall be confirmed by geotechnical evaluation prior to final design. Soil boring logs and a soil survey shall accompany all test results. Designers shall also consider the depth of the Porous Asphalt Pavement system in determining the applicability of the information. If infiltration testing is not conducted prior to design, then the underlying soils shall be assumed to have an infiltration rate of 0.05 in/hr (“default” subgrade saturated hydraulic conductivity). If use of a greater design rate is desired for the purpose of estimating volume reduction capabilities, at least one infiltration test shall be provided for each on site soil type where Porous Asphalt is proposed. In no case shall the frequency of testing be less than 1 per 5000 S.F. of Porous Asphalt. All infiltration test reports shall be submitted to MSD for review prior to final project approval.
10. Observation wells should be provided in low areas within the Porous Asphalt Pavement system, and shall extend to the bottom of the aggregate storage bed. The wells shall consist of a six inch schedule 40 PVC pipe with cast iron frame and cover, as shown on the MSD Porous Asphalt Pavement detail. The number of wells required on an installation will vary on a site specific basis, and will be determined by the design engineer.
11. A permanent sign shall be posted warning that care should be taken during snow plowing; and prohibit the following: resurfacing, the use of sand abrasives for winter tire traction, and the use of power washers.
12. The property owner must execute a Maintenance Agreement with MSD, and the entire Porous Asphalt Pavement area shall be platted in a Reserve Area for Stormwater Management.
13. A Perforated PVC underdrain system will be required. The horizontal and vertical locations shall be determined by the design engineer, depicted on the drawings, and be in general accordance with the MSD Porous Asphalt Pavement detail.
14. Prepared subgrade soil should be separated from the porous subbase. The prepared subgrade soil shall be dry and, where needed, covered with MSD Type 4 filter fabric (such as Mirafi 140N or approved equal) on the vertical sides of the typical section. Separation of the prepared subgrade along the bottom of the section may be provided by MSD Type 4 filter fabric or a well graded filter layer at the discretion of the design engineer.
15. To provide an infiltration bed, the primary underdrain system may be perched or configured as internal water storage within the subbase rock. The perched height is dependent on the infiltration characteristics of the underlying subgrade and the desired storage. The underdrain system shall not be perched if the underlying soils are plastic or plastic soils that have been stabilized with lime. Design the open graded storage bed to completely drain within 12 to 72 hours.
16. The volume in the voids of the storage bed shall be considered the total water storage capacity. A void ratio not greater than 40 percent will be used to calculate storage volume. No storage volume will be allowed in the porous asphalt. The emergency overflow apparatus, if any, should discharge only when the storage volume is exceeded.
17. Subgrade should normally be compacted to a minimum density of 90% to 95% of the theoretical density per AASHTO T 180. However, the designer should be aware that increasing the subgrade density decreases its infiltration capacity. Since soil conditions, grading requirements, and end user demands are site specific, this requirement shall be considered from a case by case standpoint. A geotechnical engineer shall provide input in balancing the infiltration and load bearing demands that the Porous Asphalt Pavement must satisfy.
18. The design of the finished grade of the pavement surface shall take into consideration the Americans with Disabilities Act (ADA) and local code requirements.

The following notes shall be included on plans, details, and specifications for porous asphalt pavement designs proposed for MSD plan approval:

1. The contractor shall verify that the porous asphalt producer that will supply the Porous Asphalt for this project has been prequalified by MSD. The name of the producer and their facility location shall be provided to the MSD Division Inspector prior to construction.
2. If porous pavement area is used for temporary sediment basin during construction, the bed shall be excavated at least one foot above the final elevation of the bed. After the sediment is removed, the bed shall be excavated to final grade before the installation of the porous pavement system.
3. Construct porous pavement late in the project schedule so that all of the dirty work such as grading and landscaping is completed first. Porous pavement and the stone bed shall not be installed until all areas tributary to it are established.
4. Porous pavements must be protected from sediment during and after the paving process. At no time shall sediment or other material capable of clogging the surface be allowed to contact the pavement.
5. Any grade adjusting requiring fill shall be done using an open-graded material, such as the stone subbase.
6. MSD Type 4 filter fabric shall be used on the sides of the stone bed and between the subbase and the storage bed to prevent sediment entry. The filter fabric shall not be installed between layers of aggregate.
7. Stone for storage bed shall be 2″ clean crushed stone with minimum 12″ thick storage bed.
8. Place aggregate for the stone recharge bed with care (not to damage the filter fabric). Aggregate should be dumped at the edge of the bed and placed in layers of 8 to 12 inches using track equipment. Compact each lift with a single pass of a static steel wheel roller. Vibrator plate compactor may be used for areas that cannot be compacted with the steel wheel roller.
9. A thin choker course layer evenly placed over the storage bed is optional. The gradation of the choker course should be selected based on the gradation of the storage bed. If AASHTO No. 3 is used for the storage bed, then AASHTO No. 57 is acceptable for the choker course.
10. Porous asphalt shall be transported in covered, clean dump beds that have been sprayed with a non-petroleum release agent or soap solution to prevent the mixture from adhering to the dump beds. Mineral filler, fine aggregate, slag dust, etc. shall not be used to dust truck beds.
11. Haul distances shall be limited such that porous asphalt shall be placed within 90 minutes of being loaded.
12. The porous asphalt is placed in 2-inch to 4-inch thick lifts using track pavers and normally compacted with only a few (1-4) passes of a 10-ton static roller.
13. Traffic shall be restricted for the first 48 hours or until the placed material has been allowed to cool below 100 F. Use of water to cool the pavement is not permitted.
14. Porous asphalt shall not be placed when the ambient air temperature of the pavement site in the shade away from artificial heat is below 60 F or when the actual ground temperature is below 50 F. The contractor shall not pave on days when rain is forecasted.
15. The full permeability of the pavement surface shall be tested by application of clean water at the rate of 5 gpm over the surface, using a hose or other distribution device. All applied water shall infiltrate directly without large puddle formation or surface runoff and shall be observed by the certifying engineer or his representative.
16. Do not clean the Porous Asphalt pavement surface with high pressure hoses or abrasives. When cleaning is necessary, combination cleaning machines that combine a wet spray and vacuum process has been found to be effective.
17. A permanent sign shall be posted warning that care should be taken during snow plowing; and prohibit the following: resurfacing, the use of sand abrasives for winter tire traction, and the use of power washers.
18. At completion of the project, prior to final dedication, an as-built certification, signed and sealed by a Missouri Professional Engineer shall be provided.

1. Maintenance personnel: Do not seal-coat the porous pavement.
2. Do not use sand or ash for control of snow and ice.
3. Inspect the porous pavements several times in the first few months after construction, and at least annually thereafter.
4. Inspection should be conducted after rain storms to check for surface ponding that might indicate possible clogging.
5. Semi-annual vacuum sweeping is recommended to prevent clogging.
6. Non-porous patching shall not exceed 10 percent of the paved area.
7. Do not store soil or other material capable of clogging the surface on or adjacent to porous pavement.

The Porous Asphalt manufacturer shall be responsible for establishing and maintaining a quality control program. Prior to use on projects requiring MSD approval, the manufacture shall submit five (5) copies of a completed pervious paving application as well as documentation describing the quality control program. The completed application and other documentation shall be submitted to:

MSD BMP Committee
Metropolitan St. Louis Sewer District
2350 Market Street
St. Louis, Missouri 63103-2555

The contractor shall obtain the Porous Asphalt manufacturer’s certification that the Porous Asphalt has been approved by MSD. This certification shall be provided to the MSD Division Inspector. The certification shall include the manufacturer’s name, and state that the Porous Asphalt has been approved by MSD and that the paving materials meet all requirements as evaluated under the manufacturer’s quality control program.

Prior to obtaining a construction permit from MSD to construct the Porous Asphalt for a given project, the engineer providing as-built certification shall verify that the installing contractor has past history demonstrating experience and/or training in installing Porous Asphalt.

At completion of the project, prior to final dedication, an as-built certification, signed and sealed by a Missouri Professional Engineer, shall be provided certifying:

1. The Porous Asphalt system was built as designed.
2. The Porous Asphalt system was installed by a qualified contractor.
3. The Porous Asphalt system installation was witnessed by the certifying engineer or a representative under his direct supervision.