FAQ

Q: Why do we need pervious concrete?
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 does pervious concrete work?
A: Pervious concrete has functionality and workability similar to that of regular concrete. However, the pervious concrete mix lacks the sand and other fine particles found in regular concrete. This creates a significant amount of void space which allows water to flow relatively unobstructed through the concrete. Once through the concrete, the water flows into the infiltration bed beneath, which is filled with rock. The rock has a 40% void space allowing for the storage of the runoff until it is infiltrated into the soil.

Q: Who benefits from pervious concrete?
A: Pervious concrete and infiltration beds benefit everyone in the entire watershed by increasing the amount of runoff entering into the ground water table and thus increasing 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 the potential for downstream flooding.

Q: What are some possible applications for pervious concrete sites?
A: Parking lots, driveways, and walkways with relatively low vehicle traffic are ideal locations for these infiltration systems.

Q: What are the specifications for the pervious concrete mix?
A: General pervious concrete mix specifications can be found at the following locations:
http://www.pervious.info/
http://www.perviousconcrete.com/

Q: How strong is pervious concrete compared to regular concrete?
A: Manufacturers claim that pervious concrete can be designed to have the same compressive strength as regular concrete. The pervious concrete for this site was designed with a compressive strength of 3,000 psi. Failure of our first pour was attributed to a strength below 1,000 psi. The second pour had a strength near 3,000 psi. It should be noted that the material became less pervious as the strength increased. For our site, compressive strength was not as important as the issue of porosity.

Q: What is under the pervious concrete?
A: The first layer is 2 inches of a small diameter choker stone followed by 4 feet of AASHTO #2 (4 in.) stone, a geotextile filter fabric and finally the undisturbed natural soil.

Q: How thick is the pervious concrete?
A: Four inches.

Q: What size storm is the site designed for?
A: The site is designed to infiltrate 2 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 Pennsylvania 319 Nonpoint Source Program.

Q: How much did both the constructions cost?
A: Link to Budget / Funding.

Q: How big is the drainage area?
A: Approximately 57,000 square feet or about 1.3 acres.

Q: What is the design life of the BMP?
A: With proper maintenance, similar uses of other types of pervious surfaces are still effective after 20 years.  We are experiencing problems on this site, but it was experimental in nature and before the industry had standards.  Many areas are in good shape, and others show the effect of weathering, and snowplow damage (raised areas damaged).

Q: What maintenance is required?
A: The pervious concrete surface should be vacuum swept periodically depending on use.

Q: Does pervious concrete require alternate means of snow removal?
A: Due to the coarse finish on the concrete, plows will catch and chip raised areas.  This should be avoided in the design and pour.  Modification of snow plows with a small roller has worked in other areas.

Q: Is the loss of stones a problem?
A: No. Because of the nature of the material, some loss of stones 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 the average infiltration rate?
A: The average infiltration rate is approximately 0.2 inches per hour through the stone beds.  It does vary with temperture.

Q: How long does it take the beds to empty for different size storms?
A: At maximum bed capacity, it usually takes around 2 days for the beds to empty.

Q: Is the groundwater table at risk of pollution from the infiltrating stormwater?
A: The soil is an excellent filter for many pollutants, but some dissolved chemicals such as chloride are not removed.  The contributing watershed should be checked to make sure there are no contaminants that could pollute the groundwater.  If precautions are taken and these systems are not used in high risk areas, then infiltrating stormwater can be safe and beneficial.

Q: What contaminants are prevalent at Villanova's Pervious Concrete Site?                                      A: The Villanova Pervious Concrete Site is a good demonstration of a clean site, with the only contaminants being acid rain and copper from the gutters, both of which decrease with infiltration through the subsurface soils.

Q: Why are lysimeters not tested for total suspended solids (TSS)?
A: Stormwater is naturally filtered as it moves through the soil stratum so particulate matter is cleaned out and retained in the surface of the soil.  Therefore filtration is not necessary and there is no reason to test for total suspended solids.

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 mostly experiences pedestrian traffic with some light vehicular traffic from maintenance vehicles and during moving in and out of the dorms. Therefore the risk of spills is low.

Q: What do students think of the site?
A: The general feeling has been positive. They are supportive of innovations which help the environment.

Q: Could the water in the beds freeze?
A: No. The frost depth in this area is around 3 feet, which will have no effect on the water in the beds.

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 long enough to cause a problem.