In the November 2013 issue of The Construction Specifier, we published the article, “Controlling Stormwater at the Source,” by Katie McKain, ASLA, MLA, MUD. David R. Smith, CSI, of the Interlocking Concrete Pavement Institute (ICPI), wrote in about what he felt were some inaccuracies; we then shared his comments with the author.
I read with interest Ms. McKain’s article. She made some inaccurate statements about permeable interlocking concrete pavement (PICP) that require correction. This includes misstating PICP longevity at seven to 15 years. Surface longevity is 30+ years since the pavers have an average compressive strength of 55,158 kPa (8000 psi) and render high durability even in winter climates. The longevity of the open-graded aggregate base/subbase and soil subgrade regarding infiltration capacity depends on the sediment load, as with all permeable pavements. There are PICP parking lot installations in the Toronto area constructed in the late 1990s still in service today.
Another misstatement is PICP is susceptible to damage and movement in cold climates. On the contrary, PICP provides a high level of stability and durability when subject to deicing materials and snow plows. Countless applications in parking lots, alleys, and streets in the Chicago area, Toronto, New England, and Minnesota demonstrate winter durability. Further evidence can be found in a report by the University of New Hampshire Stormwater Center, which monitored PICP for two years in a road and parking lot.
Contrary to this article, PICP does not have low load-bearing capacity; it can support truck traffic. PICP has been used in residential and downtown streets and in port applications here and overseas. Charles City and downtown West Union, Iowa, did not select PICP because it has longevity of “seven to 15 years.”
PICP is being used to replace impervious streets and alleys in green infrastructure projects. It is achieving the goal Ms. McKain notes at the end of her article on making the shift from impervious to pervious streets, and those with truck traffic.
Regarding structural capacity of PICP and load-bearing capacity, the California Department of Transportation (Caltrans) has published design guidelines on porous asphalt, pervious concrete, and PICP. The structural design guidelines for PICP in it go to maximum loads of Caltrans Traffic Index of ‘9,’ or about a million 18,000-lb equivalent standard axle loads. That represents heavy trucks seen on collector streets.
Finally, like most permeable pavements, PICP requires routine surface vacuuming maintenance. For PICP owners who would normally vacuum impervious parking lots or streets, cleaning permeable pavement represents little increased expense.
In her article, Ms. McKain noted high installation costs for PICP are due to a deep base. Water storage requirements would require the same open-graded base/subbase thickness for other permeable pavements, so there is often little additional expense for PICP base/subbase compared to others. Installation costs vary geographically for all permeable pavements. Machine installation is used to place PICP in most commercial and municipal applications, thereby keeping costs for the pavers, jointing, and bedding stone to around $4 to $6/sf, depending on the site area and configuration. These costs can be competitive with other types of permeable pavement. Open-graded base/subbase reservoir costs would be similar to other permeable pavements.
For more information on PICP, I’d recommend checking out the January 2012 issue of The Construction Specifier for the article, “Capturing the Rain: Permeable Interlocking Concrete and Stormwater” by Julie Rapoport, Ph.D., PE, LEED AP, or visiting www.icpi.org.
Thank you for your time and consideration.
Katie McKain, ASLA, MLA, MUD, responds.
In my article, I examined different types of permeable paving systems to be able to broadly compare their advantages and disadvantages so the reader could understand each system at a glance. I am a firm believer in all the permeable paving systems I mentioned, but I also trust each system has a certain place in design and no one system rises above the others to handle all circumstances.
In my experience, within the range of seven to 15 years, pavers installed in a permeable system begin to move if they have not yet already. (Please note, my article was about permeable paver systems—it was not about non-permeable paver systems that are hard-set with a type of mortar between the cracks.) It seems unreasonable to think thousands of small pavers unbound together would not budge over this course of time considering all the elements involved—load, compaction, weather, snowplow activity, freeze-thaw, settling, etc. With that said, a little movement can be tolerable here and there; it is major movement that becomes a maintenance issue.
Movement of pavers is greatly dependent on the design and thickness of the sub-base. In fact, all permeable systems heavily depend on the sub-base, and design varies with the load required, soil type, average precipitation, drainage, and overall temperatures—it is important to have the sub-base design engineered properly.
While there does not seem to be documented proof PICP systems have a tendency to last longer than 15 years, I don’t think it is out of the question that some will. I believe it is hard to say any permeable system is expected to last longer than 15 years for many reasons, the most obvious being these systems have neither been around nor studied properly for that long yet. I know of a few documented void-structured concrete systems where Grasscrete was used that have been around for more than 20 years, but with so few installed back then and the inability and lack thereof to effectively track projects, it is a hard claim to make.
I would hate to underrepresent a product, especially as I am a firm believer in PICP, so I was happy to look into it further. I read one study by the late Dr. Brian Shackel, a visiting professor at many acclaimed universities and expert on concrete pavers, and he thought it to be appropriate to adopt a 20-year lifecycle for PICP. Other studies show the longevity of a well-designed PICP system really depends on the use—a residential sidewalk, driveway, or patio with light use and load is more likely to last longer than an application supporting heavy vehicles and substantial use, for example. As far as categorizing the longevity, I think it is hard because each system varies so much, but I believe it is safe to say PICP will last more than 15 years, and is suitable to use for long-term needs. I am eager to see results 10 years from now of PICP systems that are currently being tracked from the late 1990s.
PICP load-bearing capacity is another interesting topic to me, because as a designer I believe these systems perform their best and are most economical with low loads. PICP is very pedestrian-friendly and handles light traffic well with little maintenance. However, PICP can and does handle heavy loads as evidence of it being installed at fire stations and other places where emergency vehicles require access. Again, these types of systems have not yet had the chance to stand the test of time (and I would love to see them do just that), but currently are functioning well and expected to continue on that pace.
I would like to retract my statement saying PICP has low load-bearing capacity—it can indeed have high load-bearing capacity and I apologize if I mislead anyone. As a designer, I certainly have my preference for how I choose a permeable pavement, and since each system has strengths and weaknesses to consider, I hope my article was helpful to those weighing options. The bottom line is realistic permeable paving options are out there and they are affordable—we need to use them.