UNH Porous Asphalt Parking Lot
With the increasing need for cost-effective
and sustainable stormwater treatment technologies, porous
asphalt (PA) has become an attractive option. Porous Asphalt
is a bituminous paving mixture which varies from conventional
asphalt only in the elimination of fine particles from the
aggregate. Eliminating the fine particles 2mm and smaller
creates an 18-22% void space in the asphalt allowing rainfall
to drain through the pavement and into a porous infiltrating
sub-base eliminating surface runoff. PA mix design problems
of the past have largely been resolved. To validate this
new technology, especially in the context of a cold climate,
the UNH Stormwater Center (UNHSC) at University of New Hampshire
(UNH) built a 5,000 sf PA lot beside a conventional dense
mix asphalt (DMA) reference lot in October 2004. Performance
metrics evaluated at the UNHSC include hydraulics, water
quality treatment, pavement condition, surface infiltration
capacity, and frost penetration. A distinct design variance
from typical PA stormwater applications is the use of a
sand/gravel layer below the pavement for water quality treatment
rather than a large stone reservoir, as is the typical specification.
The design consists of a PA pavement
over a porous media reservoir (PMR). The PMR consists of
bank run gravel underlain by crushed stone with an elevated
underdrain. The bank run gravel is a poorly graded sand,
and was selected as a filter course because of its superior
water quality improvement properties. The excavation is
lined on the bottom and sides with a non-woven geotextile
fabric to prevent influx of fines, a practice no longer
recommended. The PA surface perimeter is surrounded by a
protective gravel curtain to prevent offsite fines from
clogging the pavement. This curtain connects to the PMR.
Design guidance was received from National Asphalt Pavement
Association (NAPA) ISI-131, and Cahill Associates and the
actual Mix was produced by Pike Industries in local plants.
Watch
porous asphalt in action here>>
