Facilities Management (Managing Assets)
Pervious Concrete: A Mixed Blessing?
- By Pieter van der Have
- September 1st, 2016
We construct new
buildings on our campuses
even as we raze few of the old
ones. Each building has substantial roof surfaces
that keep rain from naturally percolating
into native soils. New or additional parking lots
may be constructed to support the voracious
parking needs of building occupants, along
with various pedestrian walks and other hard surfaces. Collectively,
these hard surfaces have an undesirable impact on the natural
ecology, essentially forcing stormwater to collect into substantial
runoffs, ultimately funneled into areas not intended to handle undesirable
quantities of runoff created by significant storm events.
Various federal and local agencies have established legislation
and resulting guidelines intended to control, if not eliminate,
stormwater runoff. By establishing stormwater fees, many
jurisdictions “encourage” property owners to provide appropriate
detention/retention facilities. This is a huge step in the
right direction, in that it helps avoid situations where properties
“downstream” from our campuses are flooded by runoff, avoids
overtaxing storm-sewer systems, and (important!) reduces pollution
of our streams and waterways.
One available option for dealing with this challenge is to install
pervious concrete on parking lots, roadways and even pedestrian/bicycle paths. What exactly is pervious concrete?
The Makeup of Pervious Pavement
Pervious concrete, originally conceived a century and a half ago,
is an interesting concept that continues to offer substantial promise.
Comprised of coarse aggregate, water, portland cement and various
admixtures, this system is intended to allow water to percolate into
the soils below. In encouraging this to occur, property owners avoid
sending that same stormwater downstream. When designed and
installed correctly, in the appropriate locations, these systems can be
very effective. In fact, if properly designed and constructed, they can
eliminate the need for detention/retention basins. This could reduce
the unwanted commitment of sparse real estate.
The concept of the design for this product is hardly rocket
science. In contrast to traditional concretes, pervious concrete
has (should have) voids between the aggregates, allowing water
to pass through. Afterwards, precipitation enters into a substrate
of larger aggregate, six inches or more in depth. This layer should
be designed and constructed in such a way as to allow temporary
storage of any water that enters the system. The proper design and
thickness of this layer, along with the size of the aggregate used,
is largely dependent on the type of native soils underneath: clay
requires a different design than does sand.
What Makes It Work, or Not Work
The continued success of any installation relies on three areas of
expertise: design, construction and maintenance. If any of these are
performed in a manner or at a level not appropriate to the unique installation
and site, the system will not behave in the desired manner.
I am closely aware of two specific installations in my area. One
is working very well, while the other one not so much. I had nothing
to do with the first one (it is off-campus). I argued against the use of
this product in the second location — but was outvoted, based on
the rationale that using pervious pavement was believed to be a cost-effective
and “PC” way of obtaining the desired level of LEED points.
The installation with which I am familiar was a new parking lot
constructed in two phases, by different subcontractors. One phase
is significantly more successful than is the other after only five years
of use. Why is that, when they were (theoretically) constructed using
the same technical design specifications? Both phases received
exactly the same level of maintenance: proper snow removal techniques
and periodic vacuuming to eliminate voids from filling up
with crud, etc. One part of the parking lot is so bad that it is unsafe
for pedestrians, and is sarcastically referred to as a place to go four-wheeling.
It needs immediate replacement, and likely with another
pervious application in order to again qualify for LEED recognition.
Factors that could lead to failure include the impact of the local
climate, materials not meeting specifications and poor installation
processes. Since these two phases were installed months apart, a drastic
difference in ambient air temperatures could have had an effect.
The failure of this installation is not unique. Literature is
rampant concerning other locations where these applications have
failed. There are, of course, also reports of successes. We should
not be frightened away from considering pervious concrete, but
must be sensitive to location, type of use, climate, proper design
and extraordinarily careful installation and proper maintenance,
with the ultimate benefit of protecting the environment.
This article originally appeared in the September 2016 issue of College Planning & Management.
Pete van der Have is a retired facilities management professional and is currently teaching university-level FM classes as well as doing independent consulting. He can be reached at email@example.com.