Page 24 - Nov-Dec2017 Vol35 No5

STATEWIDE
Alternative Pavement Designs
In neighborhoods across the state
driveways, sidewalks, and patios have
been constructed with the tried and
true materials of asphalt, concrete and
grouted tiles or stones. These impervious
surfaces cause rainwater to immediately
runoff instead of being absorbed by the
ground. This extra water can cause pond-
ing in yards and can mean increased pol-
lution and reduced groundwater supplies
for your community. Build a greener,
more environmentally-conscious yard
by using one of the following alterna-
tive pavement designs for landscaping
projects.
The first of the alternative pavements
presented is permeable concrete. This
is specially mixed concrete product
with most of the “fines” removed from
the mixture. Eliminating the small
aggregates, like sand and crushed stone,
creates pores that permit water to travel
through the concrete, thus allowing
stormwater to be filtered, detained, or
infiltrated into the ground. The benefit
to this product is that it can be used to
reduce very large percentages of runoff.
It can be designed to decrease from 60
percent to 100 percent depending on the
volume of the stone reservoir underneath
the pavement or the use of an underdrain
pipe to carry away excess flows. The
system is only limited by the subbase
configuration as infiltration rates of the ent shapes, sizes and wall thicknesses.
material can be as high as 140 to 600
inches per hour. This design also has the
benefit of filtering out some nutrients
and most of the oils, heavy metals and
bacteria in the runoff due to the amount
of surface area or void space of the
material.
The drawback to this option is that it
requires a little more knowledge about
the volumes and flow rates that of each
of the various sublayers and ground will
24 TPW November/December 2017
handle. The installation process is critical intermittent use, this grid system can
to the proper functioning of the concrete;
therefore it is recommended that the av-
erage landscaper seek out an experienced
and certified contractor for the work.
For guidance on contractor selection the
Tennessee Concrete Association or the
National Ready Made Concrete Associa-
tion are excellent references. The cost will boast of being made from 100 percent
depend on the quantity and mix design
of the concrete used and the type and
depth of base materials used. A rough
price is about $4 per square foot for the
forms, materials, delivery and placement
of the concrete and another $2 per square
foot, or so, for subgrade excavation and
materials.
Has your dog worn a path through
your yard? Do you have an area where
guests pull onto your lawn for overflow
parking? Thinking about paving, but
worried about the water generated and
the loss of your beautiful grass? A second
paving alternative is Plastic Reinforcing
Grids. These can also be called Flexible
Plastic Pavement or Grass Paver
Matrixes.
Basically, these are prefabricated rolls
or squares of plastic material around
1-inch thick consisting of “cells” or holes. Many of the grid manufacturers sell
There are a multitude of companies that
market different cell designs with differ-
Selection will vary depending on the type
of traffic expected, from pets, horses,
lawn equipment to passenger cars. You
can even buy a brand that is rated for
helicopters or private planes in case you
expect some extra special guests.
The lattice-like openings are filled with
gravel and left as is or filled with soil or
sand and then seeded or sodded. The
grid prevents the material from mov-
ing and protects the roots from being
crushed. For areas that receive light or
By Chris Masin P.E.
reduce runoff and reduce pollution by
providing infiltration and treatment as
the water percolates through the soil. An-
other big benefit is that the area will have
a reduced heat island effect as grass does
not absorb as much heat as traditional
hardscapes. Many of the products offered
recycled plastic and are ADA compliant.
Installation is fairly easy, but as with
our previous pavement alternative, po-
rous concrete, the system will be limited
to the flow of the base material. Flow
rates for this product can range from 60
to 320 inches per hour, so the manufac-
turers recommend at least a 6-inch layer
of course washed stone (3/4 inch dia. or
TDOT Grade E). A suitable base would
also include a 6-inch layer of a mixture
of 2/3 crusher run or #57 stone and 1/3
masonry sand. If the area is large or is ex-
pected to collect run-off from other areas
(or you live in the clay-rich, poor-per-
colating Memphis area) an under drain
of 4-inch plastic perforated pipe can be
added below the sub-grade. If the grid is
to be filled with soil or sand for grass, a
layer of filter fabric should be placed over
the base to stop the base from clogging.
the material with a layer of filter fabric
already attached to the bottom. The grid
is then rolled down or the individual
squares snapped together. The tops of the
cells should be 1/4 inch to 1/2 inch below
the adjacent ground or hard surfaces.
The cells or grid are then filled with fine
aggregate (3/8 inch or pea gravel) if it
is to be left exposed or filled with good
top soil or sand if it is to be seeded or
covered with sod.
The resulting spaces should be use-
ful, require little maintenance and add
to the natural aesthetics that our yards
Permeable Concrete