Types of permeable pavement

The following section provides design guidelines for porous asphalt, pervious concrete, a permeable interlocking concrete pavement, and a plastic grid system. Each product has specific design requirements and each site has unique characteristics and development requirements. Accordingly, qualified engineers and allied design disciplines, as well as association and manufacturer specifications, should be consulted for developing specific permeable paving systems.

Porous hot-mix asphalt
Porous hot or warm-mix asphalt is similar to standard hot or warm-mix asphalt; however, the aggregate fines (particles smaller than No. 30 sieve) are reduced, leaving a matrix of pores that conduct water to the underlying aggregate base and soil (Cahill, Adams, and Marm, 2003). Porous asphalt is commonly used for light to medium duty applications, including residential access roads, driveways, utility access, parking lots, and walkways; however, porous asphalt has been used for heavy applications, such as airport runways (with the appropriate polymer additive to increase bonding strength), auto storage at ports, and highways (Hossain, Scofield, and Meier, 1992). Properly installed and maintained porous asphalt should have a structural service life that is comparable or longer than conventional asphalt (WSU-PSP, 2012).

Early applications of porous asphalt were subject to fairly rapid decline of infiltration rates and surface raveling. The primary cause of these problems was inadequate binder strength and associated drain-down of the binder from higher to lower elevation in the pavement. As a result, the binder coating and cohesion between the surface aggregate is reduced and the aggregate dislodges from vehicle wear. The additional binder moving downward in the pavement then collects just below the asphalt surface as it thickens from entrained particles lodged in the pores and as temperatures decline from the surface. The additional binder forms a layer that clogs the porous asphalt pores and reduces infiltration.

The following provides specifications and installation procedures for porous asphalt applications where the wearing top course is entirely porous, the base course accepts water infiltrated through the top course, and the primary design objective is to significantly or entirely attenuate storm flows.

Applications include but are not limited to: parking lots, residential access and collector roads, light arterial roads, pedestrian and bike paths, and utility access.

Soil infiltration rate

  • Surface flows directed from adjacent areas to the pavement surface or subgrade can introduce excess sediment, increase clogging, result in excessive hydrologic loading, and should only be considered with particular attention to sediment control, infiltration capacity of the subgrade, and adequate maintenance.
  • See Section 2.3.3: Soil and Subsurface Hydrology Characterization, for guidelines on determining subsurface infiltration rates.



Aggregate base/storage bed material
  • Minimum base depth for structural support should be based upon hydrologic modeling to determine storage capacity needed and structural pavement design consideration.
  • Maximum depth is determined by the extent to which the designer intends to achieve a flow control standard with the use of a below-grade storage bed. Aggregate base depths of 12 to 24 inches are common depending on storage needs.
  • Several aggregate gradations can be used for a porous asphalt base. For a successful installation, the aggregate should: 1) have adequate voids for water storage (20 to 40 percent voids is typical); 2) be clean and have minimal fines (0 to 2 percent passing the 200 sieve maximum); and 3) be angular and have adequate fractured face to lock together and provide structural support (70 percent minimum and 90 percent preferred for fractured face). Two example aggregate guidelines are provided below:
    • WSDOT Permeable Ballast (9-03.9(2) ¾ to 2.5 inches) with a 1- to 2-inch deep choker course consisting of the same aggregate gradation that is use for the pavement wearing course (see below).
    • ¾- to 1½-inch, clean coarse, crushed rock aggregate with 0 to 2 percent passing the 200 sieve. This gradation provides a uniform working surface and does not require a choker course. However, additional attention during installation of the pavement is required (see below).
Pavement or wearing course materials
Material availability may vary regionally and mix design may vary for the materials. The following references for mix design may be appropriate for design in eastern Washington.
  • National Asphalt Pavement Association Information Series 131.
  • 2012 Ecology SWMMWW, Volume V, BMP T5.15.
  • 2005 Ecology SWMMWW, Volume III, Appendix III-C.
  • Client Assistance Memo 2215, Seattle Department of Transportation.

Example aggregate gradation and bituminous asphalt cement guidelines that have been used successfully in the Puget Sound region are provided below and in in the Section 6.3.2 of the 2012 SWMMWW.

  • Thickness:
    • Porous asphalt has a slightly lower structural contribution than conventional asphalt. Follow National Asphalt Pavement Association literature on the structural contribution and recommended asphalt pavement thicknesses.
    • Parking lots: 2 to 4 inches typical, 3 inches minimum recommended.
    • Residential access roads and arterials: 4 to 6 inches typical.
  • Aggregate gradation:
      U.S. Standard Sieve Percent Passing
      3/8”  70-90
       4  20-40
       8  10-20
       40  7-13
       200  0-3
    A small percentage of fine aggregate is necessary to stabilize the larger porous aggregate fraction. The finer fraction also increases the viscosity of the asphalt cement and controls asphalt drainage characteristics.
  • Bituminous asphalt cement:
    • Content: 6.0-6.5 percent by weight of total (dry aggregate) mix. Performance Grade (PG): 70-22. Do not use an asphalt cement performance grade less than 70-22 for open graded, porous asphalt mixes (Note: supplies of PG 70-22 may be limited in the winter season).
    • Drain-down: 0.3 percent maximum according to ASTM D6390-11.
    • An elastomeric polymer can be added to the bituminous asphalt cement to reduce draindown (Note: PG 70-22 and stiffer PG grades usually contain and elastomeric polymer).
    • Fibers can be added and may prevent draindown.
    • Anti-stripping agent: as water moves through the porous asphalt pavement, the asphalt emulsion contact with water increases compared to conventional impervious asphalt. An antstripping agent reduces the erosion of asphalt binder from the mineral aggregate and is, therefore, recommended for porous asphalt. A qualified products list of anti-stripping additives is available from WSDOT under Standard Specification: 9-02.4. Use an approved test for anti-strip such as AASHTO T 283-07 Standard Method of Test for Resistance of Compacted Asphalt Mixtures to Moisture-Induced Damage or the Hamburg test.
    • Total void space should be approximately 16 to 25 percent per ASTM D3203/D3203M-11 (conventional asphalt is 2 to 3 percent).
    Backup systems for protecting porous asphalt systems See Section Common components design, for backup or overflow guidelines and 4.6.6: Construction, for construction techniques.

    Portland cement pervious concrete
    Pervious Portland cement concrete is similar to conventional concrete with reduced or no fine aggregate (sand). The mixture is a washed crushed or round coarse aggregate (typically 3/8 or ¼-inch), hydraulic cement, admixtures (optional), and water. The combination of materials form an agglomeration of course aggregate surrounded and connected by a thin layer of hardened cement paste at the points of contact. When hardened, the pavement produces interconnected voids that conduct water to the underlying aggregate base and soil. Pervious concrete can be used for various light to heavy duty applications supporting low to moderate speeds. Properly installed and maintained concrete should have a structural life comparable to conventional concrete (ACI 522.1-08).

    Pervious concrete pavement is a rigid system and does not rely to the same degree as flexible pavement systems on the aggregate base for structural support. Designing the aggregate base will depend on several factors, including project specific stormwater flow control objectives (retention or detention storage), costs, and regulatory restrictions. As with other permeable pavement systems, deeper aggregate base courses (e.g., 12 to 24 inches) can provide important benefits including significant reduction of above ground stormwater retention or detention needs and uniform and improved subgrade support (FCPA, n.d.). See Section 4.6.4: Runoff Model Representation and Appendix C: Sizing of LID Facilities, for more information on flow modeling guidance.

    The following provides design guidelines that apply broadly to pervious concrete pavements. Design of pavements should be performed by experienced engineers with geotechnical and traffic data for the particular site. Industry standards, materials, and methods specific to pervious concrete should be followed. Over the past several years, pervious concrete mixes that include proprietary additives have been developed with varying degrees of success. The following section examines standard concrete mix design characterized by washed course aggregate (e.g., ¼ or 3/8-inch), hydraulic cement, admixtures (optional), and water with no proprietary ingredients.

    ACI 522.1-08 is the current national standard for specification of pervious concrete pavement. This manual defers to the current version of ACI 522.1-08 for developing pervious concrete pavement specifications. Included below are specific sections of ACI 522.1-08 relevant to this design manual and additional guidelines for infiltration rates, subgrade preparation, and aggregate base placement specific to this region and developed from national and local experience.

    : parking lots, driveways, sidewalks, trails, promenades, utility access, commercial parking, and residential roads.

    Soil infiltration rate:

    • See Section 2.3.3 for guidelines on determining subsurface infiltration rates.
    • Soils with lower infiltration rates (e.g., < 0.3 inches
    • per hour) may require under-drains or elevated drains to prevent periodic saturated conditions within 6 inches of the bottom of the aggregate base (interface of the subgrade and aggregate base).
    • Surface flows directed from adjacent areas to the pavement surface or subgrade can introduce excess sediment, increase clogging, and result in excessive hydrologic loading; therefore, special attention should be paid to sediment control and infiltration capacity of the subgrade, and adequate maintenance.
    • On extremely poor soils with low strength and very low infiltration rates, use an impermeable liner with under-drains.



    Aggregate base/storage bed materials

    • The minimum base depth should be based on structural design consideration.
    • Maximum depth is determined by the extent to which the designer intends to achieve a flow control standard with the use of a below-grade storage bed. Aggregate base depths of 6 to 18 inches are common when designing for retention or detention.
    • The coarse aggregate layer varies depending on structural and stormwater management needs. Typical placements are crushed washed aggregate and include WSDOT Permeable Ballast (9-03.9(2) ¾ to 2.5 inches). Do not use round rock where perimeter of the base aggregate is not confined (e.g., sidewalk placed above grade). Round rock will easily move or roll from the perimeter of the aggregate base, creating weak voids with no structural support for the pavement.
    • The concrete can be placed directly over the coarse aggregate or an open graded leveling course (e.g., ½-inch to US sieve size number 8 or AASHTO No. 57 crushed washed stone), which may be placed over the larger stone for final grading to provide a more stable, uniform working surface and reduce variation in thickness.

    Pavement materials
    The following guidelines provide typical ranges of materials for pervious concrete. Proper mix design and the resulting performance of the finished product depends on the specific aggregate used and proper cement content and water-cement ratios determined by that aggregate. Consult the qualified concrete supplier, local jurisdiction specifications, and ACI 522.1-08 for developing final mix design.
    • Pavement thickness:
      • Parking lots: 5 to 9 inches typical.
      • Roads: 6 to 12 inches typical.
    • Unit weights: 120-135 pounds per cubic foot ± 5 percent typical. Pervious concrete is approximately 70-80 percent of the unit weight of conventional concrete) (FCPA, n.d.).
    • Void content: 18-20 percent ± 3-5 percent typical per ASTM C138/C13813-M (interconnectivity of voids and, therefore, infiltration rates are inadequate below 15 percent) (ACI 522.1-08). Void content is measured indirectly by determining fresh (wet) concrete density using ASTM C138/C13813-M or ASTM C1688/CC168813-M and is a secondary measure reflecting strength and permeability of the hardened concrete. Acceptable permeability, strength and appearance is primarily determined by the test panel (see Quality control, testing and verification section below), which in part includes comparing unit weights of the accepted test panel cores and finished work cores.
    •  Water cement ratio: 0.26-0.45 provides the optimum aggregate coating and paste stability. Water content is a critical design element of pervious concrete. If too dry, cohesiveness and cement hydration efficiency may be reduced. If too wet, the cement paste may drain down and result in a weak upper structure and clog the lower portion of the pavement (ACI 522.1-08).
    • Total cementitious material content: for the development of strength and void structure total cementitious material content should be determined by the supplier and identified in the mix design submittal. Total cementitious content will range from 470-564 pounds per cubic yard. The optimum content is entirely dependent on aggregate size, void content and gradation (ACI 522, 2010).
    • Aggregate: gradations are typically either single-sized coarse aggregate or gradations between ¾- and 3/8-inch. In general the ¼-inch crushed or round produces a slightly smoother surface than coarser aggregate. Aggregate should meet requirements of ASTM D448-12 and C33/C33M-12. Aggregate moisture at mixing is important to produce adequate workability and prevents draining of paste (ACI 522.1-08).
    • Portland cement: Type I or II conforming to ASTM C150/C150M-12, C595/C595M-13 or C1157/C1157M-11. Supplementary cementitious materials such as fly ash, ground blast furnace slag and silica fume can be added to Portland cement. Testing material compatibility is strongly recommended (ACI 522.1-08).
    • Admixtures: water reducing/retarding, viscosity modifiers and hydration stabilizers can be used to increase working time and improve the workability of the pervious concrete mix.
    • Water: Use potable water.
    • Fibers may add strength and permeability to the placed concrete, are recommended, and can be used as an integral component of the concrete mix.

    Backup systems for protecting pervious concrete systems
    See Section Common components design, for backup or overflow guidelines and 4.6.6: Construction, for construction techniques.

    Permeable interlocking concrete pavement
    Permeable interlocking concrete pavers are designed with various shapes and thicknesses from high-density concrete to allow infiltration through a built-in pattern of openings or joints filled with aggregate. Pavers are typically 3? inches thick for vehicular applications and pedestrian areas may use 2? inches thick units (Smith, 2011). When compacted, the pavers interlock and transfer vertical loads to surrounding pavers by shear forces through aggregate in the joints (Pentec Environmental, 2000). Interlocking pavers are placed on open graded sub-base aggregate topped with a finer aggregate layer that provides a level and uniform bedding material. Properly installed and maintained, high-density pavers have high load bearing strength and are capable of carrying heavy vehicle weight at low speeds. Properly installed and maintained pavers should have a service life of up to 40 years (Smith, 2011).

    The Interlocking Concrete Pavement Institute (ICPI) provides technical information on best practices for PICP design, specification, construction, and maintenance. Manufacturers or suppliers of particular pavers should be consulted for materials and guidelines specific to that product. Experienced contractors with a certificate from the ICPI PICP Installer Program should perform installations. This requirement should be included in project specifications. The following design guidelines apply broadly to permeable interlocking concrete pavers.

    Applications: Industrial and commercial parking lots, industrial sites that do not receive hazardous materials, utility access, low speed (< 40 mph) residential access roads, driveways, patios, promenades, and walkways.

    Soil infiltration rate:
    • See Section 2.3.3: Soil and Subsurface Hydrology Characterization for guidelines on determining subsurface infiltration rates.
    • Surface flows directed from adjacent areas to the pavement subgrade can introduce excess sediment, increase clogging, result in excessive hydrologic loading, and special attention should be paid to sediment control, infiltration capacity of the subgrade.

    • Open graded subase: No. 2 stone.
    • Open graded base: No. 57 stone.
    • Bedding course: No. 8 stone, typically.
    • Soils should be analyzed by a qualified professional for infiltration rates and load bearing, given anticipated soil moisture conditions.
    • The ICPI recommends a minimum CBR of 4 percent (96-hour soak per ASTM D1883-07e2 or AASHTOT 193) to qualify for use under vehicular traffic applications (Smith, 2011).
    • See Section Installation guidelines, for construction techniques to reduce compaction.


    Aggregate base/storage bed materials
    • Minimum sub-base thickness depends on vehicle loads, soil type, stormwater storage requirements, and freeze-thaw conditions. Typical sub-base depths range from 6 to 24 inches. ICPI recommends base/sub-base thicknesses for pavements up to a lifetime of 1 million, 18,000 lb equivalent single axle loads (ESALs). For example, at lifetime ESALs of 500,000 with a CBR of 5 percent, the sub-base (ASTM No. 2 stone) should be 18 inches and the base (ASTM No. 57 stone) thickness should be 4 inches. Increased aggregate sub-base thicknesses can be applied for increased stormwater volume storage. See ICPI guidelines for details on base thickness and design (Smith, 2011).
    • Minimum sub-base depth for pedestrian applications should be 6 inches (Smith, 2011).
    • See Figure 4.6.8 for aggregate sub-base, base, bedding course, and paver materials.
    • The sub-base and base aggregate should be hard, durable, crushed stone with 90 percent fractured faces, a Los Angeles (LA) Abrasion of < 40 (ASTM C131-06 and C535-12) and a design CBR of 80 percent (Smith, 2011).

    Edge restraints

    The type of edge restraint depends on whether the application is for pedestrian, residential driveway or vehicular use. For vehicular installations, use a cast-inplace curb (typically 9 inches deep) that rests on the top of the sub-base, or one that extends the full depth of the base and sub-base. If the paver installation is adjacent to existing impervious pavement, the curb should extend to the full depth of pavement and aggregate base to protect the impervious installation base from excessive moisture and weakening. If the concrete curb does not extend the full depth an impermeable liner can be used to separate the two base materials (Smith, 2011).

    Cast-in-place concrete curbs or dense-graded berms to provide a base to secure spiked metal or plastic edge restraints can be used for pedestrian and residential driveway applications. An additional option for pedestrian and light parking application is a subsurface concrete grade beam with pavers cemented to the concrete beam to create a rigid paver border.

    Backup systems for protecting PICP systems
    See Section Common components design, for backup or overflow guidelines and 4.6.6: Construction, for construction techniques.

    Plastic or concrete grid systems
    Plastic or concrete grid systems come in several configurations. The goal for all plastic grid systems is to create a stable, uniform surface to prevent compaction of the gravel or soil and grass fill material that creates the finished surface. Of all the permeable paving systems, grid systems have the largest void space available for infiltration in relation to the solid support structure. Flexible grid systems conform to the grade of the aggregate base, and when backfilled with appropriate aggregate top course, provide high load bearing capable of supporting fire, safety, and utility vehicles. These systems, when properly installed and maintained, are not impacted by freeze-thaw conditions found in eastern Washington and have an expected service life of approximately 25 years (Bohnhoff, 2001).

    : Typical uses include alleys, driveways, utility access, loading areas, trails, and parking lots with relatively low traffic speeds (15-20 mph maximum).



    Aggregate base/storage bed materials
    • Minimum base thickness depends on vehicle loads, soil type, and stormwater storage requirements. Typical minimum depth is 4 to 6 inches for driveways, alleys, and parking lots (less base course depth is required for trails) (personal communication between Curtis Hinman and Andy Gersen, 2004). Increased depths can be applied for additional storage capacity if needed to meet flow control goals.
    • Typical base aggregate is a sandy gravel material typical for road base construction.
    • Example aggregate grading:
      U.S. Standard Sieve Percent Passing
       1"  100
       3/4"  90-100
       3/8"  70-80
       #4  55-70
       #10  45-55
       #40  25-35
       200  3-8

    Aggregate fill for aggregate systems
    • Aggregate should be clean, washed, and hard angular stone typically 3/16- to ½-inch.

    Aggregate fill for grass systems
    • For plastic grids, sand (usually with a soil polymer or conditioner), sandy loam or loamy sand are typical fill materials.
    • For concrete grids, fill the openings with topsoil.

    Backup systems for protecting grid systems

    Figure 4.6.5
    Porous asphalt at Yakima Kiwanis Park. Source: AHBL, Inc.

    Figure 4.6.6
    Porous asphalt section. Source: CleanWater Services and AHBL, Inc.

    Figure 4.6.7
    Pervious concrete. Source: HDR Engineering.

    Figure 4.6.8
    PICP section detail. Source: CleanWater Services and AHBL, Inc.