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Non-woven geotextiles (filter fabrics) are typically used to prevent fines in the subgrade from migrating up into the stone recharge bed from the subbase.

The Pennsylvania Department of Environmental Protection uses the following example of a typical specification for this material:


Heat-set or heat-calendared fabrics are not permitted.

Stone recharge bed and choker course

Aggregate for the stone recharge bed needs to be clean, uniformly graded, crushed stone, with minimum voids of 40 percent. To make sure the aggregate does not have excessive fines that could clog the bed, an additional grading requirement of 0 to 2 percent passing the No. 100 sieve is recommended.

In many cases AASHTO No. 3 stone is specified; however, other aggregate gradations such as AASHTO No. 1, No. 2, and smaller have also been used successfully.

Optional choker course
In many cases when using an AASHTO No. 3 stone for the recharge bed, an AASHTO No. 57 stone has worked well as a choker course. The choker course should be placed no more than one inch thick and be sufficient to fill the voids of the recharge bed stone in order to provide a smooth paving surface. A number of contractors have reported that they have found no advantage to using a choker course and have successfully constructed pavements without this course. Therefore, the choker course may be considered optional.

Porous asphalt surface

Open-graded asphalt pavement mixes are used to surface porous asphalt parking lots. These mixes are also used for surfaces on highways in many states.

In most cases, the mixes are made with polymer-modified asphalt and in some cases fibers. The polymer-modified asphalt helps to reduce draindown and improve the high-temperature performance of the mix (resistance to scuffing). Fibers are another way to reduce draindown. In draindown, which sometimes occurs during construction, liquid asphalt cement migrates to the bottom of the load of asphalt pavement material. Modern construction methods and materials prevent this from happening.

Open-graded mixes may be designed using the Superpave or Marshall methods with requirements for higher air voids to assure permeability. While modified asphalts should be used for most applications, these are not always necessary, or practical. One example of a recent successful project using a non-polymer-modified asphalt is the Port of Portland’s terminal 6 project where a 35-acre porous pavement was constructed using an unmodified asphalt cement which was two grades stiffer than what would normally be used in that geographic area. This project is performing well.

There are a number of guides and specifications available for porous asphalt mixes, including NAPA's publication Design, Construction, and Maintenance of Open-Graded Asphalt Friction Courses (order number IS-115). ASTM International’s D7064, Standard Practice for Open-Graded Friction Course (OGFC) Mix Design, is also useful.

The definitive technical document on porous asphalt pavements is NAPA's Porous Asphalt Pavements for Stormwater Management (order number IS-131). To order, click here.

Many state DOTs have their own specifications and state asphalt pavement associations can offer assistance as well. State DOT specifications have been developed for local climates and materials and contractors are familiar with them.