Pervious Concrete: A Mixed Blessing?

• By Pieter van der Have
• 09/01/16

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 College Planning & Management September 2016 issue of Spaces4Learning.