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Permeable Pavers FAQs
Permeable Pavers from Unit Paving
Permeable interlocking concrete pavement (PICP) consists of manufactured concrete units that reduce stormwater runoff volume, rate, and pollutants. The impervious units are designed with small openings that create permeable joints. The openings typically comprise 5% to 15% of the paver surface area and are filled with highly permeable, small-sized aggregates. The joints allow stormwater to enter a crushed stone aggregate bedding layer and base that supports the pavers while providing storage and runoff treatment. PICPs are highly attractive, durable, easily repaired, require low maintenance, and can withstand heavy vehicle loads. The picture shows installed pavers in a Seattle, Washington residential neighborhood. PICP significantly reduces the total amount of impervious surface and runoff.
How is a PICP surface cleaned and how often?
The openings in the surface of PICPs will require periodic removal of detritus and sediment trapped by the small sized crushed stone. Dirt is typically removed by a vacuum-sweeping street cleaning machine. Cleaning is done when the pavement surface and detritus are dry and can be loosened by sweeping and vacuuming. The frequency of cleaning will vary with the use of the pavement and position of sediment, leaves, etc. from adjacent areas.
A North Carolina State University study has shown that the initial surface infiltration rate of PICPs can be as high as 2,000 inches of rainfall per hour. For reference a 100 year storm may produce 4-8 inches per hour with 43 inches over 24 hours holding the record for the greatest one-day rainfall in the US. Per ASTM standards, a permeable project Unit Paving installed 3 years ago still has an average infiltration rate of 175 in./hr. That is 30 times greater than the 4-8 in./hr a 100 year storm may produce. We expect the system to maintain greater than 100 in./hr for many years to come.
Other research has shown that near initial surface infiltration rates can easily be restored through cleaning and replacement of the initial 3/4 to 1 in. (18 to 25 mm) depth of small stones in the openings of PICPs. For highly clogged pavement openings, the stones can be removed with vacuuming and replaced with clean material. This is a distinct maintenance advantage over pervious concrete and porous asphalt pavements.
Is there any benefit to using PICPs on low-infiltration soils such as some types of clays?
Yes. If soil infiltration is slow (generally under 0.5 in./hour or 4 x 10-6 m/sec), perforated plastic pipe drains at the bottom of the base can remove excess water while still allowing some of the water to infiltrate into the soil. Digging trenches up to 4ft deep into the soil often allows access to soils with infiltration rates up to 100 times greater than those found at the surface. The drainage rate for the water contained in the base is typically no greater than 24 hours. Over practically impervious soils or high bedrock, an impervious pond liner can be used to detain, filter and release the water through drain pipes. Regardless of the rate of soil infiltration, the filtering action of the open-graded base can reduce water pollutants.
Does the surface conform to the Americans with Disabilities Act (ADA) requirements?
Yes. ADA Design Guidelines require that surfaces be firm, stable and slip resistant. PICP designs can provide a firm and stable surface for visually impaired persons and those using wheeled mobility devices. If the openings in the surface are not desired, solid units can be used in areas subject to disabled persons. Such areas might include designated spaces in parking lots.
ADA recommends that the static coefficient of friction for flat surfaces along accessible routes be 0.6 and 0.8 for ramps. ADA advisory material recommends various test methods to assess surface slip resistance. PICPs can meet slip ADA resistance requirements using test methods recommended in ADA advisory literature. For additional information on these requirements, see ICPI Tech Spec 13 – Slip and Skid Resistance of Interlocking Concrete Pavements.
How well does the pavement perform in freeze and thaw conditions?
PICPs have been in service for years in freezing climates and have performed very well. Many projects throughout Canada, the northern U.S., the United Kingdom and Germany speak to the durability of these pavement systems in cold climates, and their ability to accept snowplows and salts without paver damage. In order to ensure high durability in freezing climates, the paving units should conform to the requirements of ASTM C 936 in the U.S. or CSA A231.2 in Canada. Both of these product standards include tests for freeze thaw durability. Freeze thaw cycles throughout the southeast are limited due to the lack of deep freezes and snow melts compared to northern climates. The southeast
When the sun and temperature are right, ice and snow on PICPs can melt and immediately soak into the pavement surface. Water does not collect on the surface and re-freeze. This reduces slipping hazards. Sand should not be used for foot or tire traction on PICP. Deicing salts can be used. After plowing, melting of any remaining snow can occur if the temperature rises above freezing. This reduces dependency on deicing salts as it eliminates ice from forming on the surface with the added benefit of reducing salt contamination in the groundwater. Water in the base typically should drain within 24 hours. It’s unlikely that ice will form in the base within this time period should temperatures drop below freezing. If the water does freeze before draining, there is adequate space for the ice to expand within the open-graded base, thereby minimizing the risk of heaving. Should soil heaving occur, the pavement surface is flexible and should not be damaged from minor upward movement or from resettlement during a thaw.
What intensity and duration of storms can be managed?
It depends on the amount of water that drains onto the PICP, the depth (and storage capacity), the base, the infiltration rate of the soil under an open-graded base and the presence of drain pipes within an open-graded base. PICPs are intended to manage water quantities and pollutants from smaller, more frequent storms such as those with a return period of 10 years or less. These storms tend to be shorter in duration and often have the highest concentrations of pollutants. PICPs are not intended to control flooding from larger, infrequent rainstorms.
Should a dense-graded or open-graded aggregate base be used under PICPs?
An open-graded base is standard use for permeable pavers because it has water storage capacity (void space between the aggregates) of typically 30% to 40%. Open-graded bases will vary in thickness due to water filtration needs and overall design parameters. The stones in the base course range from 2-6 in. (50-150 mm) in size. The bedding stone is used directly under the concrete pavers and is ¾ in. (18 mm) stone. The bedding stone depth is typically 1-2 inches but should never exceed 4 inches to minimize lateral shift down the road. The ¾ in. (18 mm) stone is also used as the joint filler between the permeable pavers. The base course and bedding material maximize storage, filtering, and treatment of pollutants in stormwater runoff entering the pavement surface. Open-graded bases are preferred because of the storage and waste water treatment benefits.