(from the June/July Editor's Note in AsphaltPro Magazine)
This is an opinion piece. So here’s my opinion. If you’re going to build a pavement that allows proper stormwater management, the right material to use is hot mix asphalt (HMA) or warm mix asphalt (WMA). I’m basing this opinion on my subjective bias and some pertinent facts, the latter of which I’ll outline now.
First, the idea of using porous asphalt pavements precludes using pervious concrete structures. There’s sound reasoning behind this. I’m not telling you anything new when I remind you that one of the elements in the design of a typical concrete pavement is a steel structure or grid. But consider this: Rebar doesn’t play nicely with water, so allowing stormwater to filter through a concrete pavement, trickling playfully across rusting infrastructure is unwise. Thus the concrete industry left this internal structure out of its pervious concrete design.
As it turns out, some pervious concrete sections placed in Denver metropolitan parking lots looked “unstructured” enough to members of the Urban Drainage and Flood Control District (UDFCD) back in 2008 that they asked Thompson Materials Engineers, Inc., to check out a few of the failing pervious concrete sections. In June 2008, engineers cut samples from four areas. One of the areas was fine, exhibiting no signs of distress. This area, Site A, was used as the control. The other three areas they chose exhibited signs of distress from “minimal” to “significant.”
You can read the entire findings for yourself in the technical paper of Project CT14,571-356 titled Pervious Concrete Evaluation Materials Investigation Denver, Colorado in the downloads section of http://www.udfcd.org/, (or try http://tinyurl.com/PerviousPDF) but one of the sections that stood out to me alluded to concrete’s inability to serve as a viable option in stormwater management.
"Our data indicates elevated chloride concentrations in the bottom portion of the samples for two of the sites. The other two sites exhibit the elevated chloride concentrations near the surface of the sample. Deicing salts (e.g., chlorides) are deleterious to concrete. They are absorbed into the concrete as it dries, and the absorbed salt strongly attracts water during subsequent wet weather events. If the ambient temperature is cold enough, and the sample does not have sufficient drainage capabilities, the water freezes in spite of the deicer, and will contribute to accelerated deterioration from freeze-thaw conditions."
Luckily, the asphalt industry has an answer for agencies and owners who want to control stormwater in an environmentally responsible manner. It’s called porous asphalt and these structures have been constructed, tested and proved since the late 1970s, according to the National Asphalt Pavement Association (NAPA). Researchers have shown that by designing and constructing a porous asphalt pavement properly, you have a pavement that doesn’t fail. Period.
Another plus researchers have found for these pavements is a reduction in winter maintenance costs. Snow and ice naturally melt more quickly on a porous pavement. If you find it necessary to apply deicing compounds such as salt or liquid deicer, you can reduce quantities from past maintenance practices and you don’t have the fear of negative reactions found with concrete pavements. Researchers warn agencies and public works departments not to use sand or ash on the surface because clogging of the open graded friction course can occur, thus negating the infiltration ability of the structure. So there’s another winter maintenance cost savings porous asphalt offers.
Of course there are oodles more reasons to select a porous asphalt pavement for stormwater management, and NAPA offers publications that outline these. The association also offers publications that assist engineers in designing proper porous asphalt pavement structures. You can find these publications at http://www.hotmix.org/.
Now, what kind of publication would AsphaltPro be if we left you with just this idea? For this special Best Paving Practices issue, you can turn to page 24 to read a professional engineer’s article on how to construct a porous asphalt pavement—from the subbase up.
Stay Safe,
Sandy Lender, Editor (sandy at theasphaltpro dot com)
Tags: AsphaltPro Magazine, Asphalt Pro Magazine, porous asphalt, pervious concrete, construct porous asphalt pavement, stormwater management
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