The Interlocking Concrete Pavement Institute makes a case for permeable interlocking concrete pavers.
By David Smith, Technical Director,
Interlocking Concrete Pavement Institute
The last century has seen a tremendous growth in the number of paved surfaces for vehicular traffic. While offering many economic benefits, these impervious surfaces created significant environmental and economic impacts such as stream bank erosion, flooding, and polluted streams, lakes, rivers and estuaries. Fortunately, onsite stormwater management is a priority in site design today. Further, infiltration via permeable pavement is rising as a major tool for onsite stormwater management.
The National Pollutant Discharge Elimination System (NPDES) in the United States for permitting stormwater discharges recognizes the viability of using permeable pavement as a best management practice (BMP). Permeable pavements are recognized as a BMP by the U.S. Environmental Protection Agency, as well as by many provincial, state and local stormwater management agencies. In addition, rating systems for “green” or sustainable building, such as LEED and Green Globes, offer credit to site designs that use permeable pavement. Moreover, permeable pavement has become a cornerstone of low-impact development (LID) design and regulations. A LID goal is to maintain and enhance the pre-development hydrologic regime of urban and developing watersheds.
While there are several ways to make vehicular and pedestrian pavement permeable, the three primary technologies are porous asphalt, pervious concrete and permeable interlocking concrete pavement (PICP). Figures 1-3 in this article illustrate typical cross-sections. PICP has been used in Germany and elsewhere in Europe since the late- 1980s and in North America since 1992. Pervious concrete pavement was first used in Florida in the early-1970s and porous asphalt emerged at the same time.
All permeable pavements have high initial surface infiltration rates and all can immediately infiltrate and store rainfall and runoff from the heaviest of rainstorms. In many cases, runoff is completely eliminated. Permeable pavements rely on the ability of open-graded aggregate in their surfaces and base to receive, store and infiltrate runoff back into the soil beneath. Porous asphalt and pervious concrete pavements with smaller-sized, open-graded aggregate surfaces support wheel loads, while larger aggregate provides a structural base and reservoir that filters stormwater.
Research has demonstrated the ability of all permeable pavements to substantially reduce urban runoff. During the last 15 years, PICP has been researched extensively in North America and overseas. Studies point to significant runoff reductions, as well as lower suspended solids, nutrients and metals. Reports are available upon request from the ICPI.
Pervious concrete and porous asphalt rely on small-sized aggregates bound with asphalt or cement to create a porous matrix that supports vehicular traffic. In contrast, PICP relies on solid, high-strength concrete units to support traffic surrounded by small, highly pervious stone-filled joints to receive and infiltrate stormwater. The stone-filled joints also contribute to interlocking and spreading wheel loads.
Depending on the paving unit design and pattern, PICP joints can vary between 1⁄8 and 1⁄2 inches. (3 and 13 mm). Small-sized aggregate in the joints that allow water to pass through can be somewhat deceptive. While PICP has less visible permeable surface area than porous asphalt or pervious concrete, PICP openings still provide high surface infiltration rates. These rates are well above practically all rainfall intensities, making their hydrological performance equal to or better than other permeable surfaces. The small aggregate in the joints and bedding also facilitates interlock and load transfer to neighboring pavers. Unlike standard interlocking concrete pavement, no sand is used in PICP joints or bedding since it has very low permeability.
Materials and Construction
Porous asphalt and pervious concrete are supplied in a “plastic” state and formed on the job site. This makes them subject to weather. PICP can be installed in freezing weather; however, porous asphalt and pervious concrete cannot. In above-freezing temperatures, plastic asphalt and concrete mixes must be checked regularly by the contractor for consistency and conformance to specifications. These materials impose time limits within which the contractor must work before asphalt cools and concrete cures and stiffens. These time- and temperature-constrained materials rely on a high degree of site control to achieve a successful installation.
In contrast, PICP units are manufactured in a factory and delivered to the site. They are not subject to time and temperature limitations in installation. PICP paving units should comply with national product standards (ASTM C 936 or CSA A231.2). These product standards require manufacture of high compressive strength concrete averaging 8,000 psi (55 MPa). Pervious concrete has a typical compressive strength of about 2,500 to 4,000 psi (17 to 28 MPa). Unlike pervious concrete and porous asphalt, concrete pavers have freeze-thaw durability test methods and requirements within their product standards to help assure adequate field performance in winter conditions. Freeze-thaw durability and higher-strength PICP offers a more durable surface under wheel loads, snowplow abrasion and deicing materials.
Because PICP is ready to install, there is no curing time, making it ready for traffic when placed. PICP is typically mechanically installed with a machine that can place more than 5,000 square feet (500 m2) per machine per day, thereby accelerating construction time. Unlike PICP, traffic must be kept from porous asphalt for 24 hours after installation – seven days for pervious concrete – so that it can cure. Pervious concrete must also be completely covered during this time to contain moisture essential for curing.
PICP presents a much friendlier, human-scale appearance that cannot be achieved with porous asphalt or pervious concrete. It comes in a variety of colors and textures to fit any architecture and building character. Because they are monolithic materials, porous asphalt and pervious concrete provide a narrow range of colors and textures. In most cases, they appear similar to conventional concrete and asphalt, but with a coarser texture. In contrast, PICP makes a visually strong statement about where stormwater is going and the sustainable nature of the site in which it is placed.
Other Sustainable Aspects
In addition to the reduction of stormwater runoff and water pollution, PICP paving units can be made in light colors to increase surface reflection or albedo, thereby reducing temperatures and heat typical to impervious pavements. Light-colored units also can reduce the need for nighttime lighting, thereby conserving electricity. For example, the owners of a PICP parking lot in a suburban Chicago shopping center went with a light-colored material for high reflectivity. This is illustrated in the photo on page 27.
All permeable pavements promote cooling through evaporation and facilitate tree growth by allowing water and oxygen better access to adjacent tree roots. However, PICP sustainability goes further; units can be made with cement substitutes. This helps reduce their carbon footprint. Unlike porous asphalt, concrete materials do not use oil-based products for their binders.
Design for the Disabled
The Americans with Disabilities Act design guidelines require that surfaces in pedestrian access routes be firm, stable and slip-resistant. In addition, surface openings in these areas should not exceed half an inch (13 mm) to promote comfortable travel for disabled persons using wheeled mobility devices such as wheelchairs. When designed and constructed properly, all permeable pavements can meet these requirements. PICP units can be colored (or painted) to indicate pedestrian access routes, as well as parking stalls and lanes for vehicles. The flat surface facilitates easier traversing by disabled persons. Traditional concrete pavers can be combined with PICP to help mark pedestrian access routes for disabled persons, as well as vehicular travel lanes, as a means to increase safety for pedestrians and drivers. In addition, some unit designs provide an exceptionally smooth surface for environments with shopping carts and strollers.
All permeable pavements infiltrate melted snow, thereby reducing snow plowing and the risk of hazardous ice patches. Deicing salts should be used sparingly on all permeable pavements and salts typically don’t remain on the pavement surface. Should deicing materials accumulate, PICP can better resist deterioration because it consists of high-quality concrete. PICP is plowed like any other pavement; no special plows or blades are required. Should it receive damage, however, individual paver units can be removed and reinstated easily. Surface repair is much more difficult on pervious concrete or porous asphalt. Because they are monolithic materials, their original structural capacity is likely reduced after repairing cut pavement.
All permeable pavements require regular inspection and periodic removal of accumulated sediment. For PICP, this is achieved with a vacuum-sweeper. Vacuuming and sweeping is recommended at least once or twice a year. Should there be an accidental spill of dirt, PICP units and stone jointing materials can be removed by a strong vacuum if needed, then cleaned and replaced. This isn’t possible with porous asphalt and pervious concrete.
A significant advantage of PICP is its modular nature. Units can be removed and reinstated if there is a need for base or underground utility repairs, or installation of new pipes or lines. There are no unattractive patches since the same removed pavers are reinstated. This also conserves materials. Small areas of porous asphalt and pervious concrete can be removed and replaced, but there will be an unattractive patch. Moreover, it can be difficult to obtain small quantities of pervious asphalt and porous concrete for patch and repair work, especially during cold winters. This is not the case with PICP because the same units can be re-used after an underground repair. Also, unlike pervious concrete and porous asphalt, PICP will not crack.
Cost and Longevity
Typically, porous asphalt is the least expensive permeable paving material, with pervious concrete and PICP having similar prices. It is important to note, however, that local materials, pavement area and contractor experience influence job pricing. Experienced PICP contractors that follow the Interlocking Concrete Pavement Institute’s construction guidelines should be sought for proposals.
Permeable pavement systems can last more than 20 years while providing an initial high level of surface infiltration even as the surface takes in moderate amounts of sediment. While cleaning frequency depends on the extent of use and deposited dirt, regular surface cleaning of all permeable pavements helps restore and maintain higher infiltration rates. Lifetime infiltration rates on maintained PICP surfaces can typically average 4 to 9 inches (10 to 23 cm) per hour or higher, thereby infiltrating the most intense rainstorms. Among the pavements, PICP, however, provides the best looking, easiest to maintain pavement choice with high in-service durability in a range of climates.
For additional information on permeable interlocking concrete pavement, visit www.permeablepavement.org.