FAQ

Q: Why do we need porous pavements?
A: There is an increasing concern about the environmental impacts of urbanization. Impervious coverage such as rooftops and roadways are replacing wooded natural areas. Impervious cover prevents infiltration and creates excess runoff from storms by not allowing rainfall to come in contact with the natural soil. It is this infiltration that replenishes the groundwater which in turn provides baseflow for streams. Impervious systems like gutters and storm sewers channel rain water directly to streams and rivers. While paved areas decrease baseflow, they increase flood flows causing erosion and sedimentation downstream. Pollutant loading is also increased leading to a degradation of water quality and aquatic habitat. Best Management Practices (BMPs) like pervious concrete in conjunction with infiltration beds are techniques that can be both aesthetically pleasing and cost effective with the purpose of protecting and improving water resources.

Q: How do porous pavements work?
A: Porous pavements, such as pervious concrete and porous asphalt, have functionality and workability similar to that of regular concrete and asphalt. However, the porous mix lacks fine particles found in regular mixes. This creates a significant amount of void space which allows water to flow relatively unobstructed through the pavement. Once through the pavement, the water is flows into the infiltration bed beneath, which is filled with rock, producing a 40% void space for storage of the runoff until it is infiltrated into the soil.

Q: Who benefits from porous pavements?
A: Porous pavements and infiltration beds benefit everyone in the entire watershed by increasing the amount of runoff entering back into the ground water table and thus the amount of clean water available for human consumption. It replenishes the ground water table while at the same time decreasing the amount of runoff entering streams during large storm events, virtually eliminating all potential for downstream flooding events.

Q: How do pervious concrete and porous asphalt differ?
A: The obvious difference is that concrete uses stone and sand aggregates, while asphalt is a petroleum based product.  However this study looks to identify further differences in durability, ability to improve water quality, and sustained ability to infiltrates stormwater.  Check out our reports to see some of these differences.

Q: Is the groundwater table at risk of pollution from the infiltrating stormwater?
A: Pollution is possible if an infiltration BMP is not placed in the proper area. Siting of this BMPs is critical to their success. These systems should not be used in areas where high-levels of contaminants are present. The Villanova Pervious Concrete / Porous Asphalt Comparison Site is a good example of a clean site; the major source of pollution is from the automobiles that park in the parking lot. Stormwater is naturally cleaned as it moves through the soil stratum, so small amounts of metals and nutrients can be cleaned out and retained in the soil just as if they were infiltrated through pervious grass or wooded areas. Parking lots, driveways, and walkways are ideal locations for these infiltration systems.

Q: Is it better to infiltrate contaminants rather than putting them in the river?
A: Yes. Organisms and processes in the soil break down many of the contaminants.

Q: How strong are porous pavements compared to standard pavements?
A: Porous pavements do sacrifice a certain amount of strength for their permeability.  It is not recommended that porous pavements be used in high traffic areas traversed by heavy trucks.  However, the mix provides sufficient strength for parking bays, as is designed in this study.

Q: What is under the porous pavement?
A: The first layer is 1 foot minimum of AASHTO #2 (4 in. stone), then a geotextile filter fabric, and finally the original soil.

Q: How thick is the porous pavement?
A: The concrete is 6 inches thick, while the asphalt is 2.5 inches thick.

Q: What size storm is the site designed for?
A: The site is designed to infiltrate 6 inches of rainfall.

Q: What is the average rainfall for the area?
A: The annual rainfall for this area is approximately 45 inches. 90% of the annual rainfall is 2.14 inches or less.

Q: Where did funding for the project from come from?
A: Funding for this project was provided by the US EPA, Prince George's County MD, and the RMC Research Foundation.

Q: How big is the drainage area?
A: Approximately 7,800 square feet, half of which drains to the concrete and half of which drains to the asphalt.

Q: How do the two pavements differ in installation?
A: The pervious concrete was poured in two four-hour days by a crew of six people.  The porous asphalt was installed in one three-hour day by a five person crew.  For more details on the installation, see the construction page of this website.

Q: What is the design life of the BMP?
A: With proper maintenance, similar uses of pervious surfaces are still effective after 20 years so there is no reason to expect different results here. The worst case of failure could be caused by either a spill or the rocks beds beneath the pervious concrete filling with fine particles.

Q: What maintenance is required?
A: The pervious concrete surface should be vacuum swept or power washed every six months.

Q: Do porous pavements require alternate means of snow removal?
A: Due to the coarse finish on the pavement, plows may catch and chip the pavement. Brushes and other light duty snow removal (hand shovels) are more desirable. In addition, the excess use of sand or cinder should be avoided because they clog the pavements.

Q: Is the loss of material a problem?
A: No. Because of the nature of the material, some loss of material is bound to happen. This does not affect the performance of the BMP.

Q: How do the lysimeters work?
A: Lysimeters draw moisture from the surrounding soil through a pervious cup. This is accomplished by applying a vacuum which creates the necessary pressure gradient to allow moisture transfer through the soil.

Q: How accurate is the pressure transducer?
A: The pressure transducer measures to an accuracy of 1/1000 of an inch.

Q: How is flow over the weir measured?
A: By knowing the elevation of the water going over the weir and the angle of the weir, flow can be calculated using the V-notch weir equation. 

Q: What is downstream of the site?
A: The site is located at the headwaters of Mill Creek which in turn flows into the Schuylkill River.

Q: What kind of traffic does the site get?
A: The site experiences faculty parking year round.  It is not located in an area that is used for driving lanes or areas with heavy truck traffic.

Q: Could the water in the beds freeze?
A: Yes, that is possible.

Q: What are the effects of freeze/thaw?
A: Freeze/thaw is not a problem due to the large pore size of the pervious concrete. Moisture does not stay in the concrete or asphalt long enough to cause a problem.