For minimizing stormwater runoff, permeable interlocking concrete pavements are increasingly popular. Learn here about their use, design, construction, and maintenance.

Early permeable pavements typically consisted of stone, concrete, or plastic pavers laid out in a grid pattern with soil and grass between the individual units. Although these systems increased permeability and permitted some rainwater to infiltrate rather than run off, they did not provide substantial structural capacity.

More recently, researchers and industry have designed permeable interlocking concrete pavements (PICPs) to accommodate more frequent and heavier vehicle loading. As a result, PICP applications have expanded to include streets, walkways, driveways, large commercial parking areas, alleys, and roadway shoulders.

A PICP consists of a surface of paving units with joints or openings filled with permeable aggregate. The joints allow water from storms to flow freely into an open-graded aggregate base or subbase reservoir that stores the water before it is released to aquifers, streams, and lakes. For soils with low infiltration rates, perforated drain pipes are often placed in the subbase or subgrade to drain excess water directly to storm sewers or to a surface water feature.

The new permeable interlocking concrete pavements not only have wider applications because they are stronger, but they also address a number of other issues, such as water pollution and excess heat in surface waters, which can be dangerous to aquatic ecosystems.

Varieties of Paver Systems

Concrete and clay brick pavers are available in a variety of shapes, and these generally conform to ASTM product standards. Some involve large paving elements with arched surfaces reminiscent of cobblestones. But the most common segmental types are PICPs consisting of solid concrete paving units that meet the size and concrete properties in ASTM C936 Standard Specification for Solid Concrete Interlocking Paving Units. When assembled into a pattern, they create joints or openings in the pavement surface.

The joints, which are filled with permeable aggregates, permit water to freely enter the subsurface at rates as high as 1,000 inches (2,540 centimeters) per hour. The paving units are compacted on a thin bedding layer of permeable aggregates, which rests over a base and subbase of larger open-graded aggregates.

In vehicular applications, the concrete pavers, bedding, and base layers are typically restrained by a concrete curb.

Geosynthetics such as geotextiles, geogrids, or geomembranes are applied to the subgrade, depending on the structural and hydrological design objectives. Separation geotextiles placed on the sides of the base and subbase prevent the entrance of fines from adjacent soils.

Benefits of PICPs

Like other permeable pavements, PICP systems typically are a component of a treatment train; that is, a series of control measures that infiltrate stormwater on a site and reduce water pollution. An example of a treatment train is a roof garden that drains stormwater into a PICP parking lot, and then overflows into a bioswale. Such designs that combine various control measures are a very effective approach to stormwater management.

When stormwater is treated through a PICP system, the water is filtered so that suspended particles are captured and held within the aggregate bedding, base, and subbase layers. Removing suspended particles from stormwater discharge benefits water quality by reducing attached heavy metals and nutrients that would otherwise enter streams or other surface waters.

Heavy metals such as copper and zinc are often attached to the suspended particles and are introduced into stormwater by weathering roofs, rusting vehicles, and brake linings. In sufficient quantities, these metals can be toxic to aquatic life.

Stormwater, in addition to containing heavy metals, often holds other suspended solids--plant litter, animal waste, and fertilizers--that carry nutrients such as nitrogen and phosphorus. These nutrients can cause excessive and ecologically harmful plant and algae growth in receiving water systems.

Research over the past 20 years, summarized by Eisenberg et al. in the American Society of Civil Engineers (ASCE) book, Permeable Pavements, has demonstrated that permeable pavements are an effective method for managing stormwater runoff and capturing pollutants from urbanized areas. Average design efficiencies for pollutant removal from water exiting PICP drains are on the order of 75 percent for total suspended solids, 45 percent for phosphorus, and as high as 40 percent for nitrogen. Permeable pavement systems typically treat oils through bacterial digestion and metals through adsorption to clay particles. Permeable pavements do not remove chemicals, salts, metals, or nutrients dissolved in the water, except by infiltration into the soil subgrade.

Excessive temperatures are another form of water pollution. During the warmer months, urban environments contaminate stormwater runoff with heat from rooftops and pavements. The thermal energy is conveyed through storm drains to receiving creeks, rivers, and lakes. This process can create sharp and rapid increases in water temperature, which is harmful, and sometimes toxic, to aquatic organisms.

Water retained in the reservoir layers within PICPs has an opportunity to cool down prior to its discharge. Water that infiltrates becomes part of the groundwater supply in the water cycle, further promoting natural temperature balancing.

Another benefit of the use of PICPs in urban areas is that they may offer cost savings where they can conserve land use. They do not require land to be used for runoff detention facilities, and they reduce the need for space to install drainage infrastructure.

Compliance With Stormwater Regulations

PICPs can help achieve compliance with a number of national, regional, State, and local regulations, as well as design requirements for the control of stormwater runoff. These requirements can include implementing some of the following activities:

• Complying with National Pollutant Discharge Elimination System (NPDES) permits.
• Controlling runoff volumes and pollutants from new developments and redevelopments.
• Limiting impervious cover, such as roofs and pavements, as well as the resulting runoff.
• Reducing overflows, especially combined sewer overflows in older urban areas, by regulating the volume of runoff for storage and infiltration.
• Meeting the total maximum daily load requirements for receiving waters.
• Complying with regulations that prescribe the capture and management of specified rainfall depths for improving water quality and reducing runoff volumes. Such regulations are typically expressed as a percentile (for example, 85th percentile or 95th percentile storm depth) as required for U.S. Federal Government facilities in section 438 of the Energy Independence and Security Act.
• Complying with building code requirements. Examples include the California Green Building Standards Code, the International Green Construction Code, American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 189.1, or other codes that require compliance with Leadership in Energy and Environmental Design (LEED^®), Sustainable Sites Initiative™, or similar sustainable design and construction rating systems.

 

Are PICPs Right for Your Next Job?
Site Feature	Considerations
Drainage Path	Assess drainage patterns in the surrounding area to determine possible impacts.
Traffic Type and Patterns	Assess the traffic type and composition. Avoid using in high-traffic areas subjected to repeated loading from buses or heavy trucks. Avoid use where traffic will contaminate the pavement surface with dirt, oil, and grease.
Winter Maintenance	Avoid the use of winter sand, which may clog the pavement. If sand is used, remove it the following spring.
Groundwater Depth	Avoid the use of PICPs in areas where the groundwater is within 2 feet (0.6 meter) of the bottom of the pavement subbase.
Subsurface Conditions	Underground utilities and the presence of bedrock may require special considerations. Utility lines in the base or subbase might require encasement.
Surrounding Land Use	Avoid use in areas where high levels of sediment or other contaminants are generated.
Rainwater Capture and Reuse	Limit the use of deicing chemicals or other contaminants where stormwater is captured for reuse.

 

Determining Project Suitability

Permeable pavements are not suitable for every application. But with proper design, construction, and maintenance, they can provide a low-impact, green alternative worth considering.

Several key factors should be considered when determining whether a permeable pavement system is suitable for a given project. The key factors may not have equal weight relative to their impact on decisionmaking. Typical site characteristics to consider before using a PICP, or other permeable pavements, are the drainage path, traffic types and patterns, winter maintenance, groundwater depth, subsurface conditions, surrounding land uses, and rainwater capture and reuse.

To assist in evaluating the suitability of projects for the use of permeable pavements, researchers developed a project suitability matrix. (See http://onlinepubs.trb.org/onlinepubs/nchrp/docs/NCHRP25-25(82)_FR.pdf.) The matrix includes considerations such as flaws or major design challenges, that have an overriding influence on the decision to move forward with a project. These criteria provide an initial decision screening for the suitability of a site for permeable pavements.

The considerations are weighted the highest to reflect their importance in moving forward with the project. Secondary and other considerations are used to prioritize the suitability of sites but are not generally viewed as factors that remove permeable pavements from consideration. The decision factors are rated, and then the weighted factors are totaled on a scale of 0 to 100. If the score adds up to less than 65, the project is not considered to be a good candidate for permeable pavements. Between 65 and 75, the project can be considered. Scores greater than 75 indicate that the project is well suited for permeable pavements.

For example, the matrix was used to consider a total of seven urban locations in Berkeley, CA, that had suitability scores ranging from 47 to 67. The city eventually selected a section of Allston Way, a downtown street, for a demonstration project. The contractor excavated the existing pavement, which had reached the end of its service life. The workers removed the pavement to the subgrade level and installed a permeable subbase and base, followed by permeable interlocking concrete pavers. The city completed the project in the fall of 2014. Since then, the city has been evaluating the effectiveness of permeable pavements for stormwater management by monitoring outflows through drain pipes in the pavement. To date, only one outflow occurred from a severe storm. All other storms rendered no outflow, thus indicating infiltration into the clay subgrade.

Structural and Hydrological Design

Although there are many well-designed and constructed PICPs, some projects have performed below their design expectations. Therefore, implementing best practices for design, construction, and maintenance is essential.

The design of permeable pavements requires a balance between providing a structurally sufficient pavement to withstand traffic loading while achieving stormwater management and hydrological goals. Structural designers for conventional interlocking pavements and PICPs adopt the flexible pavement design procedures in the 1993 Guide for Design of Pavement Structures by the American Association of State Highway and Transportation Officials. The AASHTO design procedure is relevant because the load distribution and failure modes of PICPs are similar to those for other flexible pavement systems.

The design process includes an analysis of the expected axle loads, followed by characterization of subgrade strength and evaluation of the surface, base, and subbase thicknesses to support the design traffic for the life of the PICP. The structural capacity of the pavement layers in a permeable pavement system is lower than for conventional pavements. The open-graded nature of the surface, base, and subbase layers results in a lower structural capacity and structural layer coefficient than for conventional pavements. As a result, permeable pavements are generally thicker than conventional pavements for the same design traffic.

Hydrological analyses typically involve using a manual or a computerized model to characterize the movement of water through the pavement system under consideration. This includes water infiltrating into the soil subgrade, and outflow through underdrains. In general, hydrological analyses assess the extent to which water volumes can be stored and infiltrated over 48 to 72 hours, with the remainder released out of the pavement structure.

When PICPs are installed over high-infiltration subgrade soils, they may not require an underdrain. Such structures are called full-infiltration designs. Although water storage is available within open-graded aggregates, all of the stormwater that enters the PICP infiltrates into the subgrade until the system drains dry.

Partial-infiltration designs are used where subgrade soils have low infiltration rates. Such soils make it impossible to infiltrate all of the stormwater into the soil within a reasonable period of time, especially during extreme storm events. In these cases, an underdrain is included through which excess water discharges to a surface water feature or underground stormwater collection system. These drains often have raised outlets to detain and infiltrate some water into the soil and then release the remainder.

No-infiltration systems are designed to prevent infiltration into the soil subgrade. This is done for PICP systems installed over expansive or fill soils, or close to buildings, by enclosing the bottom and sides of the pavement structure with geomembrane (that is, an impermeable liner). An outlet pipe penetrating the membrane provides temporary storage and outflow control. This design approach also can be used for water harvesting or for horizontal ground-source heat pumps.

Construction Considerations

Construction techniques and proper maintenance also are critical to the longevity of permeable pavements. Permeable pavement materials are similar to those used for the construction of conventional pavements. The majority of the tests required to determine quality are also the same, except that compliance targets and minimum and maximum values differ.

Similarly, PICPs are manufactured using the same materials as traditional interlocking concrete pavers and do not require onsite curing time, nor are they subject to onsite variability in material mixes. To make the surface permeable, the manufacturer produces paving units with small spacer bars to create widened joints filled with permeable aggregate that permits water to enter.

construction.

Some key construction activities essential to the success of all permeable pavements include the following:

Preconstruction Meeting. A preconstruction site meeting is critical to installing permeable pavements successfully. All contractors and tradespeople must understand how to install the permeable pavement correctly and protect it from damage and contaminants during construction.

Subgrade Preparation. To facilitate water infiltration, guidelines published by State stormwater agencies for the construction of permeable pavement generally recommend not compacting the subgrade. Although an uncompacted subgrade benefits water infiltration, it tends to consolidate when saturated under vehicular loading, causing settlement and possible rutting of the pavement surface. Therefore, the design may need to balance infiltration against compaction.

The design engineer should conduct Proctor compaction tests in a laboratory, and then establish test areas for compaction of the in situ subgrade, followed by infiltration tests. This approach establishes a relationship between soil subgrade density and infiltration, and indirectly the strength of the soil subgrade, in order to sustain traffic loads when saturated.

In addition, the contractor should complete subgrade preparation during dry weather conditions. The contractor also should complete placement and compaction of the open-graded aggregate base and subbase as close together in time as possible to minimize the risk of sedimentation from construction vehicles or nearby erosion. If severe, such sedimentation can compromise water storage and infiltration into the soil subgrade.

Compaction of the Base/Subbase Aggregate Layer. Compaction of the open-graded aggregate is required for all applications. The compaction provides a stable platform for the placement of the surface course, as well as structural capacity for traffic support and prevention of settlement. A 10-ton (9-metric-ton) dual or single vibratory smooth drum roller or a reversible vibratory plate compactor delivering 13,500 pounds (60 kilonewtons) of centrifugal force with a compaction indicator is recommended. Stiffness measurements of the compacted base and subbase aggregates can be accomplished using a lightweight deflectometer used according to ASTM E2835 Standard Test Method for Measuring Deflections Using a Portable Impulse Plate Load Test Device.

Geotextiles. For all applications, these materials are generally placed vertically against the walls of excavated soil to separate the permeable pavement from adjacent soils. Geotextiles are typically nonwoven fabric, and the contractor should protect them from contamination during installation. This is done by keeping equipment from traversing the fabric and preventing sediment from entering the excavated area.

Geomembranes. Typically composed of polyvinyl chloride, ethylene propylene diene monomers, or high-density polyethylene, geomembranes may be used to vertically separate the open-graded base and subbase from adjacent pavements and building structures. Since these materials are impermeable, they prevent water from entering building foundations. In other cases, they may enclose the sides and bottom to create a no-infiltration design for water storage and flow control.

Drainage Features. Depending on the drainage design goals for the permeable pavement, perforated drainage pipes may be used to convey stormwater from high-flow rain events away from the pavement. Other drainage features, such as drain inlets, curb cutouts, and additional subsurface piping, may be designed to accommodate some of the surface or subsurface water flow during significant storms.

Underdrains. The contractor should install a perforated underdrain, if required, in a trench at the lowest point of the permeable pavement subgrade. The pipe is surrounded with open-graded aggregate that offers protection during construction. The underdrain pipe should have a minimum 2 percent slope to an outlet. The designer should select the pipe size and spacing to ensure that the pavement does not flood and become completely saturated during storms, because this can lead to instability and damage under vehicular traffic.

Supplementary Surface Drainage Features. Outlet pipes should not be perforated. Depending on the hydrological design goals for the project, the end of the outlet pipe may be upturned to promote water storage in the aggregate reservoir, increase infiltration, and reduce outlet flow. This is particularly useful for achieving detention (volume capture) objectives. Pipe outlets should be directed to outlet chambers or other protective systems to prevent soil erosion.

Observation Wells. These features typically consist of a perforated plastic pipe placed vertically through the pavement and anchored into the subgrade. They are installed to enable maintenance crews to observe or measure downward drainage over time. Such wells can function as places to remove accumulated sediment if required.

Other Keys to Success

Other important factors for successful installations include the following:

Edge Restraints. One key element is edge restraints to prevent lateral movement of the surface during construction and under traffic. Edge restraints consist of concrete curbs, adjacent pavement surfaces, dense-graded aggregate base (protected by a geomembrane), or landscape architectural features. Edge restraints are particularly important for permeable shoulder applications for vehicles traversing on a shoulder to the traveled lanes. Vehicle wheel loading near an unsupported edge might damage the permeable pavement.

Mechanical Installation. Most PICP projects are machine installed to accelerate construction time while reducing labor costs. An experienced machine operator typically lifts and places about 1 square yard (0.8 square meter) of PICP every 20 seconds, setting it on the screeded bedding layer in its final pattern. This installation rate can be three to five times faster than manual installation. The paving units are compacted, the joints are filled with aggregate, and the paver surface is swept clean and compacted again.

Contractor Certifications and Experience. The production and placement of permeable pavements generally require attention to detail to ensure that a durable pavement is produced. In addition, contractors and tradespeople working at or near the permeable pavement must be cognizant of the need to avoid clogging the pavement surface and joints with particles. Avoiding surface clogging may require the installation of cattle guards and tire washing stations to ensure that the construction traffic does not contaminate the pavement with sediment. For training and certification, the Interlocking Concrete Pavement Institute (ICPI) offers a 1-day PICP specialist course that earns a record of completion. Contractors who take an additional certification course and provide evidence of at least 50,000 square feet (4,645 square meters) of PICP installation experience and continuing education qualify for ICPI Certified Installer and PICP Specialist designation.

Maintenance Inspections

Transportation departments should conduct PICP inspections once or twice annually (preferably after a storm event). Inspection tasks need to include the following:

• Review maintenance and operations records to determine indicators of the need for maintenance.
• Document general site features and take photographs of the pavement.
• Note obvious sources of surface contaminants, such as sediment.
• Identify the extent and severity of any damage or deficiencies (for example, settlement, ponding, and cracked pavers). Document the structural condition and create a pavement condition index using ASTM E2840 Standard Practice for Pavement Condition Index Surveys for Interlocking Concrete Roads and Parking Lots.
• Identify any changes in adjacent land use that might affect runoff from the contributing area, creating potential sources of contaminants that could reduce system permeability.
• Inspect vegetation around the PICP perimeter for cover and soil stability.
• Inspect edge restraints to ensure continued functioning.
• Repair or reinstate damaged edge restraints to correct the resulting movement in the pavers. Repair may require the removal and reinstatement of adjacent paving units.
• Check the observation well(s) and outlet drain(s) to ensure continued water drainage from the pavement structure.
• Check the surface for buildup of sediment in joints. Buildup typically occurs at locations nearest the adjoining impervious pavements.
• If surface water remains longer than 1 hour after a rainstorm, then conduct the ASTM C1781 Standard Test Method for Surface Infiltration Rate of Permeable Unit Pavement Systems to determine the surface infiltration rate.
• Repair localized settlement greater than 0.5 inch (13 millimeters [mm]) and rutted pavement areas.
• Repair outflow features, piping, energy dissipaters that reduce and spread water flow, and erosion protection systems, as required.

The transportation agency should document the results of the inspection to assist in updating the maintenance plan for the PICP system, as well as develop a database of maintenance to meet any NPDES permit requirements. This information should assist in predicting future maintenance needs and can be part of an overall management system for the pavement. Based on the results of the inspection, it may be appropriate to conduct remedial maintenance, particularly if the surface has not been vacuumed regularly.

If the results of the ASTM C1781 test are below 10 inches per hour (254 mm per hour), vacuum the surface to remove sediment trapped in the joints (typically 0.5 to 1 inch, or 13 to 25 mm, deep) using a full or true vacuum machine (not regenerative air). Refill joints with clean aggregate, sweep the surface clean, and test the infiltration rate again per ASTM C1781 to a minimum 50 percent increase or minimum 10 inches per hour (254 mm per hour).

Winter Maintenance

Avoid the use of winter sand for traction. If used, remove with regenerative air cleaning equipment in the spring (regenerative equipment does not evacuate jointing materials):

• Remove snow with a standard plow or snowblowing equipment.
• Stockpile plowed snow onto turf or other vegetated areas and not on the PICP.
• Monitor temperatures and apply anti-icing or deicing materials such as sodium chloride, calcium chloride, or magnesium calcium acetate.

Performance

A substantial number of PICP projects can be found in parking areas, alleys, and roadways, as well as unusual uses of PICPs subjected to heavy loadings by buses and even military tanks. Pavement stability and winter durability of PICPs have been documented in parking lots in Toronto, Ontario, Canada; Chicago, IL; and Vancouver, WA. In addition, municipal DOTs in Los Angeles and Sacramento, CA; Longmont, CO; Washington, DC; Dubuque, IA; St. Louis, MO; Lancaster, PA; and Richmond, VA, have experience with PICPs in alley projects. PICP streets in Berkeley, CA; Atlanta, GA; Charles City and West Union, IA; Moline, IL; Lafayette, IN; and Winchester, VA, have integrated roadways with stormwater management, thereby solving stormwater problems in a cost-effective manner.

Summary

Permeable pavements can be a major contributor to the effective management of stormwater. They provide the opportunity to transform traditional stormwater management into use of a best management practice for capturing, storing, and infiltrating stormwater into the natural surroundings. The benefits include reduced stormwater discharges, as well as improvements to water quality, such as reduced suspended solids and chemical contaminants. Although permeable pavements can be an effective tool, engineers should carefully consider structural and hydrological concerns in their design and construction to ensure that these systems provide cost-effective solutions over their design life.

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David K. Hein, P. Eng., is vice president of transportation at Applied Research Associates, Inc., in Toronto, Canada. He chairs a committee of the American Society of Civil Engineers (ASCE) that is developing a consensus standard for the design, construction, and maintenance of PICPs. He holds a bachelor of applied science degree from the University of Waterloo in Canada.

Samuel S. Tyson, P.E., is a concrete pavement engineer with the Federal Highway Administration’s Office of Asset Management, Pavements, and Construction in Washington, DC. He earned a bachelor’s degree in civil engineering and an M.S. in civil engineering from the University of Virginia.

David R. Smith is the technical director of the Interlocking Concrete Pavement Institute in Chantilly, VA. He holds a master’s degree in urban and regional planning (environmental concentration) from Virginia Tech. He has written a book on PICPs and has contributed to several Federal, State, and municipal guidance documents on permeable pavements, as well as the ASCE book, Permeable Pavements.

For more information, see the FHWA Tech Brief at www.fhwa.dot.gov/pavement/concrete/pubs/hif15006.pdf or contact David Hein at dhein@ara.com or 416–621–9555.