Q: Is there a recognized measure, or index, of permeability for paving materials?

A: Pervious concrete and permeable pavers that are properly designed, installed, and maintained have surface infiltration rates of 140+ in/hr. An example of research suggesting this is “Study on the Surface Infiltration Rate of Permeable Pavements,” accessible through the North Carolina State University Web site listed at the end of this article.

Q: How much credit should be given for the pavement as a “pervious” surface?

A: Correctly designed, installed, and maintained pervious pavements have surface infiltration rates higher than that of" "almost any natural soil, and several times greater than the maximum possible rainfall intensity anywhere in the country—" "in other words, greater than anything that is already called “pervious.”

Q: What is involved in maintaining the pavement “right”?

A: If you are in a municipality where sand or cinders are spread on the roads for winter traction, then vacuuming will be" "necessary at least once per year: in the spring, following snowmelt. The key word is vacuuming, with or without simultaneous washing, to lift material out and restore the open, permeable pores.

Q: How long will the system last before it becomes a “non-permeable” surface?

A: Just make sure that your pavement is selected, designed, installed, and maintained correctly. If you are duly careful" "with all these steps, then the installation should be permeable indefinitely."

Q: Pretreatment using a filter strip or vegetated swale is required, right?

A: Absolutely not: Don’t do that! Any upstream soil, even soil that is grassed or mulched, can erode and generate" "pavement-clogging sediment sometime. Adding a grass strip or forebay would just add more erodible upstream soil.

Q: Should porous pavement be avoided where trees are present? Should overhanging trees be removed?

A: The only thing overhanging trees do to porous pavements is deposit their annual drop of organic debris. The debris decomposes to a minute fraction of the volume it started with. Vacuuming might be called for after a number of years, to reopen the pavement’s pores.

Q: What is recommended when you have a large chemical spill or hazardous material spill?

A: The same as if a spill occurred anywhere else in your city: Immediate and complete cleanup is the legal responsibility of the industry that spilled it, at their expense.

Q: What are the risks associated with hydrocarbon (oil) contamination?

A: Letting oil into a porous pavement’s voids is the whole idea in water-quality improvement. In the pavement, naturally occurring microorganisms biodegrade hydrocarbons before they migrate to the bottom of the pavement. The constituents go off as carbon dioxide and water vapor, and very little else; the hydrocarbons cease to exist as water-quality pollutants.

Q: What is the cost difference between standard and porous pavements in the same situation?

A: Pervious concrete costs approximately 20% more than conventional impervious concrete, because of its high cement content and specialized quality control. Permeable pavers cost about the same as pervious concrete. When you use these materials intelligently in a site plan to absorb and treat stormwater, and the municipality gives you credit for their stormwater functions, then the use of porous paving ordinarily reduces total development cost by reducing or eliminating the need for additional stormwater
facilities.

Q: Do you use traditional trench backfill material under porous pavement, or do you use open-graded material instead?

A: The base material must be open-graded (single-sized) aggregate such as ASTM No. 57, so it can store and convey water.

Q: How widespread is this usage? How much (and how rapidly) is it changing? What is the future for widespread adoption?

A: Porous pavements are still a small proportion of all the paving being done in the world, but they are growing at an exponential rate. Developers and suppliers are ready to install these new materials; their motivation is to meet today’s environmental requirements in economical ways.

Q: What can my municipality do to encourage the use of porous pavements?

A: Make sure your municipality is not an unnecessary impediment. When a developer proposes porous paving, give it credit for what it can do to satisfy your stormwater requirements: It reduces impervious cover; lowers the runoff coefficient; and absorbs, detains, and treats stormwater