DESIGN OF LID SYSTEMS - Low Impact Development and Sustainable Stormwater Management - page 138

control. Although the pervious surface and underlying soils below the storage
bed allow filtration of most pollutants, care must be taken to prevent infiltra-
tion in areas where toxic or contaminated materials are present in the underlying
soils or within the stormwater itself. When designed, constructed, and maintained
according to the guidelines, pervious pavement with underlying infiltration sys-
tems can dramatically reduce both the rate and volume of runoff, recharge the
groundwater, and improve water quality.
Pervious Bituminous Asphalt
Pervious bituminous asphalt pavement was first studied in the early 1970s at
the Franklin Institute in Philadelphia [5] and consists of standard bituminous
asphalt in which the fines have been screened and reduced, allowing water to
pass through small voids. Pervious asphalt is placed directly on the stone subbase
in a single 3.5- to 4-in. lift that is lightly rolled to a finish depth of 2.5 to 3 in.
Because pervious asphalt is standard asphalt with reduced fines, it is similar
in appearance to standard asphalt [6]. Recent research in open-graded mixes
for highway applications has led to additional improvements in pervious asphalt
through the use of additives and higher-grade binders. Pervious asphalt is suitable
for use in any climate where standard asphalt is appropriate [7].
Pervious Portland Cement Concrete
Pervious portland cement concrete, or pervious concrete, was developed by the
Florida Concrete Association in the 1990s [8, 9] and has seen the most widespread
application in Florida and other southern areas. Like pervious asphalt, pervious
concrete is produced by substantially reducing the number of fines in the mix
in order to establish voids for drainage. Like other types of pervious pavements,
pervious concrete should always be underlain by a stone subbase designed for
stormwater management and should never be placed directly onto a soil subbase.
Whereas pervious asphalt is very similar in appearance to standard asphalt,
pervious concrete has a coarser appearance than its conventional counterpart
(Figure 7-10) and a trowel-swept finish cannot be achieved. Care must be taken
during placement to avoid overworking the surface and creating an impervious
layer. Pervious concrete has proven to be an effective stormwater management
BMP. Another potential advantage of pervious concrete is the option of intro-
ducing color to the mix. The industry now offers a variety of hues and tints
that can allow a pervious concrete installation to better integrate with its adja-
cent landscape. Additional information pertaining to pervious concrete, including
specifications, is available from the Florida Concrete Association and the National
Ready Mixed Concrete Association [10].
Pervious Paver Blocks
Pervious paver blocks [11] consist of interlocking units (often concrete) that pro-
vide some portion of surface area that may be filled with a pervious material such