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Porous pavements move stormwater efficiently

Porous pavements move stormwater efficiently

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    Cahill Associates Photo

    In this Morris Arboretum parking lot at the University of Pennsylvania, Philadelphia, conventional/impervious asphalt for the driveway surface was used at right, and pervious asphalt was used in the parking bays.

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    Kent R. Hansen

    Porous asphalt pavement cross-sectionTypical porous asphalt pavement consists of an open-graded asphalt surface (2½ inches), a top filter course of small aggregate, a deep stone reservoir course, and a bottom of filter fabric placed on uncompacted natural soils.

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    The parking lot at Morris Arboretum at the University of Pennsylvania, Philadelphia, is saturated during a hurricane. The rain runs off the standard asphalt (right) and drains into the pervious part (left).

The bottom of the recharge bed must be level to allow infiltration over the entire bed bottom. For sloped sites, parking areas may be terraced to minimize both excavation and the quantity of stone recharge beds required. The terraced parking would be connected using conventional pavement surface with runoff directed to the porous pavement. Care must be taken to avoid compacting the bed bottom, since this will reduce the permeability of the soil.

Non-woven geotextile lines the bed bottom and sides to allow infiltration and prevent subgrade soils from moving into the stone recharge bed. When the stone recharge bed is used to provide stormwater management for impermeable surfaces such as roads and roofs, the stormwater is conveyed directly to the stone bed and perforated pipes are used to distribute water in the stone bed.

A thin layer—one to two inches thick—of clean, single-size ½-inch stone (AASHTO No. 57) is placed on top of the stone reservoir. This layer is important since its purpose is to lock up the stone surface of the reservoir bed, providing a firm paving platform. A 2- to 4-inch layer of open-graded asphalt is used for the surface. As the name implies, the gradation of the mix is open, with only a small percentage of sand. A number of state and federal standards may be used to specify open-graded mixes. Guidance on these mixes is available in the National Asphalt Pavement Association (NAPA) publication IS-115: Design, Construction, and Maintenance of Open-Graded Asphalt Friction Courses.

Using a standard local or state DOT open-graded friction course (OGFC) mix, if available, is recommended since contractors will be familiar with the design, production, and construction of these mixes. The mix requirements should be checked to ensure the following requirements are met:

  • Minimum air voids of 18% to ensure surface drainage
  • Minimum asphalt content of 6% by weight of mix for durability
  • Maximum draindown of 0.3% (ASTM D6390).
  • Open-graded asphalt mixes used for porous pavements historically have used unmodified binders with good results. However, because open-graded pavements are more susceptible to scuffing, it is recommended that the binder be one or two grades stiffer than that used for conventional mixes. Polymer-modified asphalt, asphalt rubber, and fibers have proven beneficial for OGFC used as thin surfaces on highways, and may prove beneficial for porous pavements.

    KEEPING COST IN MIND

    When comparing the costs of porous pavements to conventional pavements, it is important that the comparison include the costs of the alternative stormwater management strategies. The cost of the porous pavement and stone recharge bed will typically be higher than that for a conventional dense-graded pavement, primarily due to the amount of material required for the stone recharge bed. However, this cost difference is generally offset by the significant reduction in stormwater pipes and inlets. Additionally, because porous pavement is designed to incorporate into the topography of a site, there is generally less earthwork and no deep excavations.

    Proper construction practices are critical to the performance of porous pavements. Protecting the bed from contamination during and after construction is of particular importance. Building a porous pavement late in the construction sequence helps reduce the chances of contamination. It is critical that sediment control remain in place until vegetation is well established.

    — Kent R. Hansen, P.E., is the director of engineering for the National Asphalt Pavement Association, Lanham, Md.

    Building a better bed

    A successful porous pavement application depends on a number of factors. Consider the following when planning such an application:

  • Avoid compacting the subgrade. Excavate using track equipment and/or a backhoe-loader. The bottom of the bed must be flat.
  • Keep sediment control in place during and after construction until vegetation is established to avoid contaminating with sediment-laden runoff.
  • Construct the porous pavement as late in the construction schedule as possible to avoid contamination.
  • Keep truck movement over the stone bed to a minimum during placement of open-graded friction course (OGFC).
  • OGFC should be placed hot, but will probably need to cool some before compaction starts.
  • Do not remove sediment controls until vegetation is established.
  • Vacuum-sweep porous pavement surfaces twice per year with an industrial vacuum sweeper.
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