4.6.6 Construction

Installation procedures for permeable paving systems are different from conventional pavement. For successful application of any permeable paving system, the following guidelines should be followed:

Qualified manufacturers, installation contractors and suppliers
Material manufacturers must have experience with producing proper mix designs for pervious concrete or porous asphalt and make materials that comply to national standards. Permeable interlocking concrete pavement and other factory produced materials should conform to national product standards. Installation contractors must be adequately trained, have substantial and successful experience with the pavement product, and adhere to material specifications for proprietary systems. Installation contractors should provide information showing successful application of permeable pavements for past projects and recommended certification, if available, for the specific type of permeable pavement. Suppliers must have experience with producing proper mix designs for pervious Portland cement concrete or porous hot-mix asphalt. Substituting inappropriate materials or installation techniques will likely result in structural or hydrologic performance problems or failures.

Sediment and erosion control during construction and long-term
Erosion and introduction of sediment from surrounding land uses should be strictly controlled during and after construction to reduce clogging of the void spaces in the subgrade, base material, and permeable surface. Muddy construction equipment should not be allowed on the base material or pavement, sediment laden runoff should be directed to treatment areas (e.g., settling ponds and swales), and exposed soil should be mulched, planted, and otherwise stabilized as soon as possible. Construction sequencing for proper installation and minimizing erosion and sediment inputs is critical for project success. Long-term operation and maintenance plans that consider the physical setting, timing, and equipment needs should be developed during the design phase.

Poor quality installations are most often attributed to not following guidelines, structural or flow management problems, or failures are likely without qualified contractors and correct application of specifications.

4.6.6.1 Installation guidelines
This section provides general installation guidelines for the subgrade, storage reservoir/aggregate base, and geotextiles (optional) for all types of permeable pavements. Following the general guidance, specific installation guidelines for porous asphalt, pervious concrete, PICP, and grid systems are provided.

Subgrade

Careful attention to subgrade preparation during installation is required to balance the needs for structural support while maintaining infiltration capacity. For all permeable pavements, relative uniformity of subgrade conditions is necessary to prevent differential settling or other stress across the system.

On sites where the topsoil is removed and native sub-soil exposed, no compaction may be required for adequate structural support while protection of the subgrade from compaction is necessary to retain infiltration capacity. For applications with heavy truck traffic, some soil subgrade compaction may be necessary for structural support. The effect of compaction on subgrade permeability will vary significantly depending on soil type. For example, the permeability of a coarser textured sand may be affected minimally while the permeability of finer textured soils will likely be significantly degraded for a given compaction effort. Effects of compaction on soil permeability can be assessed by conducting laboratory Proctor density tests on subgrade soils from the proposed permeable pavement site. Soils in test areas can be compacted to various density levels through field measurements and the resulting permeability measured using ASTM test methods. See Determining subgrade infiltration rates under Section 2.3.3: Soil and Subsurface Hydrology Characterization for more detail on test procedures.

To properly prepare and maintain infiltration capacity and structural support on permeable pavement subgrades, use the following procedures:
  • During and after grading, excessive construction equipment or material stockpiling should not be compacted more than the recommended compaction value. The following guidelines should be used to prevent excessive compaction and maintain infiltration capacity of the subgrade:
    • Final grading should be completed by machinery operating on a preliminary subgrade that is at least 12 inches higher than final grade or structures to distribute equipment load (e.g., steel plates or aggregate base material). Final excavation then proceeds as machinery is pulling back and traveling on preliminary grade as final grade is excavated.
    • To prevent compaction when installing the aggregate base, the following steps (backdumping) should be followed: 1) the aggregate base is dumped onto the subgrade from the edge of the installation and aggregate is then pushed out onto the subgrade; 2) trucks then dump subsequent loads from on top of the aggregate base as the installation progresses.
    • Avoid subgrade preparation during wet periods (soil compaction increases significantly if soil is wet).
    • If machinery must access the final grade, limit the access to a specific travel way that can be tilled before application of the base aggregate or place heavy steel plates on subgrade and limit traffic to the protective cover.
    • NOTE: allowing heavy machinery on permeable paving subgrades during wet or saturated conditions will result in deep compaction (often 3 feet) and cannot be compensated for by shallow tilling or ripping soil (Balousek, 2003).
  • If using the pavement system for retention in parking areas, excavate the subgrade level to allow even distribution of water through the aggregate base and  maximize infiltration across the entire parking area (Cahill, Adams, and Marm, 2003).
  • Immediately before placing base aggregate and pavement, remove any accumulation of fine material (if present) with light equipment and scarify soil to a minimum depth of 6 inches to prevent sealing of the subgrade surface.
  • Excavate the subgrade with level steps. The step length will vary depending on slope, flow control goals, and cost. Excavating level steps is most applicable for parking lots where the pavement surface is also stepped. While the subgrade is excavated level, the pavement surface should maintain a minimal slope of 1 to 2 percent.
Storage reservoir/aggregate base
The open-graded aggregate base provides: 1) a stable base that distributes vehicular loads from the pavement to the subgrade; 2) a highly permeable layer to disperse water downward and laterally to the underlying soil; and 3) a temporary reservoir that stores water prior to infiltration into the underlying soil or collection in underdrains for conveyance (WSDOT, 2003).

Aggregate base material is often composed of larger aggregate (1½ to 2½ inches). Smaller stone (leveling or choker course) may be used between the larger stone and the pavement depending on pavement type, working surface required to place the pavement, and base aggregate size (see sections below on specific pavement type and leveling or choker course guidelines). Typical void space in base layers range from 20 to 40 percent (WSDOT, 2003 and Cahill, Adams, and Marm, 2003). Depending on the target flow control standard, groundwater and underlying soil type, retention or detention requirements can be partially or entirely met in the aggregate base. Aggregate base depths of 6 to 36 inches are common depending on pavement type, structural design, storage needs, and environmental factors such as cold weather (WSU-PSP, 2012).

Flexible pavements (e.g., porous asphalt and permeable pavers) require proper aggregate base material for structural stability. Rigid pavements (pervious concrete) do not require an aggregate base for structural stability; however, a minimum depth of 6 inches is recommended for stormwater storage and providing a uniform surface for applying pervious concrete (WSU-PSP, 2012).

Increasing aggregate base depth for stormwater storage provides the additional benefit of increasing the strength of the overall pavement section by isolating underlying soil movement and imperfections that may otherwise be transmitted to the wearing course (Cahill, Adams, and Marm, 2003). For more information on aggregate base material and structural support, see Section 4.6.2.2: Types of permeable pavement for aggregate base recommendations by specific pavement type.

Geotextile and geogrids (optional)

If geotextile is used between the subgrade and base aggregate:
  • Use geotextile recommended by the manufacturer’s specifications and recommendations of the geotechnical engineer for the given subgrade soil type and base aggregate.
  • Extend the fabric up the sides of the excavation in all cases. This is especially important if the base is adjacent to conventional paving surfaces. The fabric can help prevent migration of fines from densegraded base material and soil subgrade to the open graded base. Geotextile is not required on the sides if concrete curbs extend the full depth of the base/subbase.
  • Overlap adjacent strips of fabric at least 24 inches. Leave enough fabric to completely wrap over small installations (e.g., sidewalks) or the edge of larger installations adequately to prevent sediment inputs from adjacent disturbed areas. Secure fabric outside of storage bed (see Figure 4.6.9).
  • Following placement of base aggregate and again after placement of the pavement, the filter fabric (if used) should be folded over placements and secured to protect installation from sediment inputs. Excess filter fabric should not be trimmed until site is fully stabilized (U.S. Army Corps of Engineers, 2003).

Porous hot-mix asphalt
Aggregate base/storage bed installation
  • Stabilize area and install erosion control to prevent runoff and sediment from entering storage bed.
  • Geotextile fabric (optional): See above discussion on geotextile fabric installation.
  • Install base aggregate in maximum of 8-inch lifts and lightly compact each lift. Compact complete aggregate base with a minimum 10-ton vibratory roller. Use a 13,500 pound force (lbf) plate compactor with a compaction indicator in places that can’t be reached by roller compactor. Make two passes with the roller in vibratory mode and two passes in static mode until there is no visible movement of the aggregate. Moist aggregate will compact more thoroughly than dry aggregate. Do not crush the aggregate during compaction. Compacted aggregate subbase and base should not rut under aggregate delivery trucks or other construction equipment.
  • Use back dumping method described previously in this section to protect the subgrade from compaction.
  • If used, install choker course evenly over surface of course aggregate base and compact.
  • Behind asphalt delivery trucks and in front of asphalt installation, rake out ruts caused by delivery trucks to provide a uniform surface and pavement depth.

Pavement or wearing course installation
  • The porous asphalt pavement installations use the same equipment and similar procedures as conventional asphalt with three notable differences:
  • Mixing temperature should be 260-280 F and 240-260 F for lay down. Air temperature should be no lower than 45 F and rising.
  • The stiffer performance grade for the bituminous asphalt cement adheres more to delivery trucks and installation machinery; accordingly, additional time is necessary to clean equipment.
  • Porous asphalt aggregate base and choker courses are relatively uniform gradations and low in fine material. As a result, equipment operating on the aggregate base will cause more rutting than on more densely graded base material for conventional pavement and will require more hand labor to smooth ruts and prevent areas where the pavement is either too thin or too thick.

General installation
  • Install porous asphalt system toward the end of construction activities to minimize sedimentation. The subgrade can be excavated to within 6 to 12 inches of final subgrade elevation and grading completed in later stages of the project (Cahill, Adams, and Marm, 2003).
  • Erosion and introduction of sediment from surrounding land uses should be closely controlled during and after construction. Erosion and sediment controls should remain in place until area is completely stabilized with soil amendments and landscaping.
  • Insulated covers over loads during hauling can reduce heat loss during transport and increase working time (Diniz, 1980). Temperatures at delivery that are too low can result in shorter working times, increased labor for hand work, and increased cleanup from asphalt adhering to machinery (personal communication between Curtis Hinman and Leonard Spadoni, 2004).
  • Rising water in the underlying aggregate base should not be allowed to saturate the pavement (Cahill, Adams, and Marm, 2003). A positive overflow (elevated drain) can be installed to ensure that the asphalt top course is not saturated from excessively high water levels in the aggregate base.

Minimum infiltration rate for the porous hot mix asphalt
The minimum infiltration rate for newly placed porous asphalt should be 200 inches per hour. Use ASTM C1701/C1701M-09 to test infiltration rates at locations representative of the pavement finished product at a maximum rate of 5,000 square feet per test.

Portland cement pervious concrete
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 infiltration rates, subgrade preparation, and aggregate base placement relevant to the region and developed from national experience.

Aggregate base/storage bed installation
  • Stabilize area and install erosion control to prevent runoff and sediment from entering storage bed.
  • Geotextile fabric (optional): See above discussion on geotextile fabric installation in this Section.
  • Install coarse aggregate in maximum of 8-inch lifts and compact each lift (U.S. Army Corps of Engineers, 2003). Use back dumping method described previously in this Section to protect the subgrade from compaction.
  • If utilized, install a 1- to 2-inch leveling course (typically No. 57 AASHTO crushed, washed stone) evenly over surface of coarse aggregate base and lightly compact to stabilize to provide a more stable, uniform working surface and reduce variation in thickness.

Pavement installation
  • See testing section below for confirming correct mixture and proper installation.
  • With the correct water content, the delivered mix should contain a cement paste that smoothly covers all the aggregate particles while at the same time the paste does not slide or drain off the particles. The paste should adhere the aggregate particles.
  • Pervious concrete mix should be placed within 60 minutes of water being introduced to the mix, and within 90 minutes of using an extended set control admixture (ACI 522.1-08) or an admixture recommended by the manufacturer.
  • Adding water in the truck at the point of discharge of the concrete should be allowed to attain optimum mix consistency, workability, placement, and finish (ACI 522.1-08).
  • Base aggregate should be wetted to reduce moisture loss and improve the curing process of pervious concrete.
  • Concrete should be deposited as close to its final position as possible directly from the truck, using a conveyor belt or hand or powered carts (pervious concrete mixes are stiff and cannot be pumped).
  • Several screed and compaction methods can be used, including low frequency vibrating truss screeds, laser screeds, and hand screed that levels the concrete at above form (typically 3∕8- to ¾-inch). The surface is then covered with 6-millimeter plastic and a static drum roller is used for final compaction (roller should provide approximately 10 pounds per square inch vertical force). A method that is becoming more prevalent and that has advantages for quality of finish and speed are rotating Bunyan screeds or hydraulically powered screeding drums that provide proper compaction at the finished elevation and a nearly-finished surface in one operation (see Figure 4.6.10). Hydraulically operated screeding drums come in various lengths and diameters.
  • Placement widths should not exceed 15 feet unless contractor can demonstrate competence with test panels or previous installations to install greater widths.
  • High frequency vibrators can seal the surface of the concrete and should not be used.
  • Jointing: Shrinkage associated with drying is significantly less for pervious than conventional concrete. Accordingly, control joints are optional. If used, spacing of joints should follow the rules for conventional concrete and should typically be spaced at maximum 15- to 20-foot intervals. Joint depth should be ¼ to 1∕3 the depth of the pavement thickness. Control joints can also facilitate a cleaner break point if sections become damaged or are removed for utility work.

Curing
  • Due to its porous, open structure, pervious concrete dries rapidly. If curing is not controlled, the bond between the aggregate becomes weak and structural integrity will be seriously compromised. Curing is, therefore, a critical step in pervious concrete installation and the following steps should be carefully planned and implemented (ACI 522.1-08):
  • Completely cover surface and edges with 6-millimeter plastic within 20 minutes of concrete discharge. The surface and edges should remain entirely covered for the entire curing time.
  • Curing time: 7 days for pervious concrete with no additives and 10 days for mixtures that incorporate supplementary cementitious materials, such as fly ash and slag (ACI 522.1-08).
  • Secure all edges adequately so that the plastic cannot be dislodged during cure time. Lumber, reinforcing bars, and concrete blocks can be used to secure the plastic continuously along the perimeter. If wooden forms are used, riser strips can be nailed back in place to secure plastic. Do not use dirt, sand or other granular material on the plastic because the sediment may wash or spill into the pores of the concrete during rainfall or removal of plastic (ACI 522.1-08).

Note that admixtures are now becoming available that reduce or eliminate the need to cover the pavement installation with plastic. Consult ACI 522.1-08, industry representatives, and suppliers for recommendations.

Quality control, testing and verification
The following provides a summary of quality control in ACI 522.1-08. Quality control and testing procedures to verify proper placement include test panels, fresh and hardened density, and average compacted thickness of the installation. It is critically important to require adequate National Ready Mix Concrete Association (NRMCA)-certified placement personnel and contractor experience for the installation (see ACI 522.1-08 for more details). There are currently no generally accepted standardized methods to test compression or flexural strength of pervious concrete, and tests used for conventional concrete are not applicable due to the high variability in strength within the porous structure of pervious concrete and should not be used for verification (ACI 522.1-08).

The contractor should place test panels using mix proportions, materials, personnel, and equipment proposed for the project. Test the fresh and hardened density and thickness of the test panel(s). See the current version of ACI 522.1-08 for test procedures and tolerances. If the test panel is outside acceptable limits for one or more of the verification tests, the panel should be removed and replaced at the contractor’s expense. If the test panel is accepted it may be incorporated into the completed installation.

Obtain a minimum 1 cubic foot sample for fresh density testing for each day of placement (see ACI 522.1-08 for test procedures and tolerances).

Remove 3 cores per 5,000 square feet not less than seven days after placement to verify placement hardened density and thickness. See ACI 522.1-08 for test procedures and tolerances. If the tested portion of the installation is outside acceptable limits for 1 or more of the verification tests, the installation is subject to rejection and should be removed and replaced at the contractor’s expense unless accepted by the owner (WSU-PSP, 2012).

Minimum infiltration rate for the pervious concrete pavement
The minimum infiltration rate for newly placed pervious concrete should be 200 in/hr. Use ASTM C1701/C1701M-09 to test infiltration rates of the test panel and at locations representative of the pavement finished product at a maximum rate of 5,000 square feet per test.

Permeable interlocking concrete pavement
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 in the ICPI PICP Installer Program should perform installations. The following provides construction guidelines that apply broadly to permeable interlocking concrete pavers.

Aggregate base/storage bed installation
  • Stabilize area and install erosion control or diversion to prevent runoff and sediment from entering aggregate sub-base, base, and pavers. Prevent sediment from contaminating aggregate base material if stored on-site.
  • If using the base course for retention in parking areas, excavate subgrade level to allow even distribution of water and maximize infiltration across entire parking area.
  • Geotextile fabric (optional):
  • Geotextiles are recommended on the sides of excavations where a full-depth concrete curb is not used to prevent erosion of adjacent soil into the aggregate base. The fabric should extend at least 1 foot onto the subgrade bottom. A minimum overlap of 1 foot is recommended for well-drained soils and 2 feet for poor-draining soils (Smith, 2011).
  • The use of geotextiles on the bottom of the subgrade excavation is optional.
  • See above discussion on geotextile fabric installation in this Section.
  • Install No. 2 stone in 6-inch lifts. Use back dumping method described previously in this Section to protect subgrade from compaction. Compact with at least 4 passes of a 10-ton steel drum vibratory roller or a 13,500 lbf plate compactor. The first two passes should be with vibration and the final two passes should be static. Consolidation of the subbase is improved if the aggregate is wet. Compaction is complete when there is no visible movement in the sub-base as the roller moves across the surface (Smith, 2011).
  • The No. 57 stone base can be spread as one, 4-inch lift. Compact with at least 4 passes of a 10-ton steel drum vibratory roller or a 13,500 lbf plate compactor. The first two passes should be with vibration and the final two passes should be static. The No. 57 stone should be installed moist to facilitate proper compaction.
  • Adequate density and stability are developed when no visible movement is observed in the base as the roller moves across the surface (personal communication between Curtis Hinman and Dave Smith ICPI). If field testing is required, a nuclear density gauge can be used on the No. 57 base in backscatter mode; however, this type of test is not effective/appropriate for the larger No. 2 sub-base stone. A non-nuclear stiffness gauge can be used to assess aggregate base density as well (Smith, 2011).
  • Asphalt stabilizer can be used with the No. 57 and/or the No. 2 stone if additional bearing support is needed, but should not be applied to the No.8 aggregate. To maintain adequate void space, use a minimum of asphalt for stabilization (approximately 2 to 2.5 percent by weight of aggregate). An asphalt grade of AC20 or higher is recommended. The addition of stabilizer will reduce storage capacity of base aggregate and should be considered in the design (Smith, 2000).

Bedding layer installation
  • Install 2 inches of moist No. 8 stone for the leveling or choker course over compacted base. Screed and level No. 8 stone to within ±3∕8-inch over 10 feet surface variation. The No. 8 aggregate should be moist to facilitate movement into the No. 57 stone. Keep construction equipment and foot traffic off screed bedding layer to maintain uniform surface for pavers.

Paver installation
  • Pavers should be installed immediately after base preparation to minimize introduction of sediment and to reduce the displacement of bedding and base material from ongoing activity (Smith, 2000).
  • Place pavers by hand or with mechanical installer. Paver joints are filled with No. 8, 89 or 9 stone. Spread and sweep with shovels and brooms (for small jobs) or small track loaders and power brooms or sweepers (for larger installations). Fill joints to within ¼-inch and sweep surface clean for final compaction to avoid marring pavers with loose stones on the surface.
  • To maximize efficiency and reduce cost of mechanical installation, consult with the supplier to deliver pavers in layers that will be picked up by the installation machine in the final installed pattern.
  • For installations over 50,000 square feet that are installed with mechanical equipment, consult with the paver manufacturer to monitor paver dimension and consistency of paver layers so that layers continue to fit together appropriately throughout installation.
  • Cut pavers along borders should be no smaller that than 1∕3 of a whole paver if subject to vehicle loading.
  • NOTE: Do not use sand to fill paver openings or joints unless specified by the manufacturer. Sand in paver openings and joints can clog easily and will significantly reduce surface infiltration and system performance if system is not specifically designed for sand.
  • Compact pavers with a 5,000 lbf, 75-90 Hz plate compactor. Use a minimum of two passes with each subsequent pass perpendicular to the prior pass.
  • If aggregate settles to more than ¼-inch from the top of the pavers, add stone, sweep clean, and compact again. The small amount of finer aggregate in the No. 8 stone will likely be adequate to fill narrow joints between pavers in pedestrian and vehicular applications. Sweep in additional material as required. ASTM No. 89 or 9 stone can be used to fill spaces between pavers with narrow joints. In all cases, however, the bedding material should be ASTM No. 8 stone (Smith, 2011).
  • For vehicular installations, proof roll with at least two passes of a 10-ton rubber-tired roller.
  • Do not compact pavers within 6 feet of unrestrained edges (Smith, 2011).
  • The PICP installation contractor should return to the site after 6 months from completion of the work and provide the following if necessary: fill paver joints with stones, replace broken or cracked pavers, and re-level settled pavers to specified elevations. Any rectification work should be considered part of original bid price with no additional compensation.

For detailed design guidelines and a construction specification see Permeable Interlocking Concrete Pavements (Smith, 2011).

Plastic or concrete grid systems
Aggregate base/storage bed installation
  • Stabilize area and install erosion control to prevent runoff and sediment from entering storage bed.
  • If using the base course for retention in parking areas, excavate storage bed level (if possible) to allow even distribution of water and maximize infiltration across entire parking area (terrace parking area if sloped).
  • Geotextile fabric (optional): See above discussion on geotextile fabric installation in this Section.
  • Install aggregate in 6-inch lifts maximum. Use back dumping method described previously in this to protect subgrade from compaction.
  • Compact each lift of dense-graded aggregate base to 95 percent standard proctor. (Note: For dense-graded bases in light traffic applications, only standard proctor density is required. Modified proctor requires more compactive force and expense and is not needed for the light loads to which grid pavements are constructed.
  • For open-graded aggregate bases, compact with a minimum 10-ton roller with the first two passes in vibratory mode and the last two in static mode until there is no visible movement of the aggregate.

Top course installation
  • Grid should be installed immediately after base preparation to minimize introduction of sediment and to reduce the displacement of base material from ongoing activity.
  • Place grid with rings up and interlock male/female connectors along unit edges.
  • Install anchors if not integral to the plastic grid. Higher speed and transition areas (e.g., where vehicles enter a parking lot from an asphalt road) or where heavy vehicles execute tight turns will require additional anchors.
  • Aggregate fill should be back dumped to a minimum depth of 6 inches so that delivery vehicle exits over aggregate. Sharp turning on rings should be avoided.
  • Aggregate fill
    • Spread gravel using power brooms, flat bottom shovels or wide asphalt rakes. A stiff bristle broom can be used for finishing.
    • If necessary, aggregate can be compacted with a plate compactor to a level no less than the top of the rings or no more than ¼-inch above the top of the rings (Invisible Structures, 2003).
  • Grass systems
    • Spread sand or soil using power brooms, flat bottom shovels or wide asphalt rakes. A stiff bristle broom can be used for finishing.
    • Lay sod or seed. Grass installation procedures vary by product. Consult manufacturer or supplier for specific grass installation guidelines.
  • Provide edge constraints along edges that may have vehicle loads (particularly tight radius turning). Cast-in-place or pre-cast concrete is preferred.
  • Concrete grids require edge restraints along edges in all applications. Plastic grids require restraints when exposed to vehicles. Edge restraints for concrete or plastic grids in such applications should be cast-in-place or pre-cast concrete.


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Figure 4.6.9
Filter fabric placed under this pervious concrete sidewalk is wrapped over the pavement and secured to protect the installation during construction. Source: Curtis Hinman



Fig4-6-10_6.3.21 bunyan screed
Figure 4.6.10
Hydraulically operated Bunyan screed compacts and provides the finished elevation in one preparation. Source: SvR Design, courtesy of Low Impace Develeopment Technical Guidance Manual for Puget Sound (2012)



Fig4-6-11_6.3.26 mechanical ecostone install
Figure 4.6.11
Mechanical paver installation. Source: Curtis Hinman