Credit: Chicago DOT
Measuring in-place permeability with a homemade infiltrometer. A fixed volume of water is poured in after the cylinder, with a known end area, is sealed to the surface. The time it takes for the water to drain through into the pavement is used to calculate the permeability rate. This information is compared with the original permeability and prior annual surveys.
The use of pervious asphalt in urban settings is definitely on the upswing, and that’s a good thing. Permeable pavement is an excellent way to reduce stormwater runoff, the load on your local stormwater utility, and the amount of pollutants reaching waterways while also building a long-lasting parking lot or road.
But it’s a relatively new technology, which means there hasn’t been enough time to establish conventional wisdom or universally acknowledged best design or specification practices. The contractors you rely on may have little or no experience with pervious pavement, and there aren’t yet formal manuals per se.
That said, there are a fair amount of pervious asphalt roadways that have been in use for 10 years or more, and guidelines are emerging. Here are a few that I’d like to pass on, based on my experience as a materials engineer and as a founder and owner of a materials engineering firm and quality assurance lab that has been in business for 14 years.
1) Don’t use a national standard. In the absence of good local knowledge, it seems like a national standard would be a good place to start. But that’s not the case. Put simply, one size doesn’t fit all. Local soils, local aggregates, and the local availability of materials like ground tire rubber (GTR), recycled asphalt shingles (RAS), and modified asphalt cement have a huge effect on the practicality and performance of pervious asphalt.
Eventually, as pervious paving matures, national standards will address local variations and become more useful. Until then, you’re better off applying your own expertise and consulting paving contractors in your region to establish your own specifications. Any additional design expenses are likely to be recovered by savings in materials costs. The pavement may even perform better over time, relative to roads designed with national standards.