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56 6.1 Introduction Because the design and functionality concepts of permeable pavements are quite different from those of conventional pavements, the construction of these pavements is also quite differ- ent. In constructing a pavement that drains internally rather than by diverting the flow of water to a stormwater system, meticulous production and installation of specially designed materials are required. As such, it is critical that the owner agency or its engineering consultant retain only highly experienced and trained contractors for the job and that they establish and implement effective specifications, construction plans, and testing/inspection procedures to ensure a quality product. As described in previous chapters, each permeable pavement type includes some features that are common to the other pavement types (such as base/subbase reservoir aggregate) and other features that are unique to that pavement typeâin particular, the surface layer. Thus, it follows that some of the construction activities associated with the three pavement types are similar, whereas others are unique to a certain type. Table 14 summarizes the various components of construction and their applicability to each type of permeable pavement system. The remainder of this chapter discusses key considerations in permeable pavement construction, beginning with the development of detailed construction documents and extending through the complete construction of the as-designed pavement. Chapter 7 discusses operation and maintenance issues. 6.2 Plans, Specifications, and Estimates As the design of the permeable pavement project materializes, construction details must be prepared that provide the basis for contractor bidding and construction execution. These details largely consist of plans, specifications, and estimates. The plans depict the locations and limits of the permeable pavement project, the cross-sectional details of the pavement system, and the standard details of the various pavement system components. The specifications provide the governing procedures and requirements for the materials to be used and installed, as well as the quality of workmanship expected and the consequences of deficient levels of materials and workmanship quality. The estimates reflect the expected quantities of material and the rates of application or coverage. Guide specifications for porous asphalt, pervious concrete, and PICP pavements are available and should be used as needed to develop the specifications for the subject project. Pertinent guide specifications, as discussed in Chapter 5, include state, industry, and federal standards. Industry and state standards provide plan details specific to permeable pavements, such as edge restraint and tie-in details between pavement sections, as shown in Figures 17 and 18. C h a p t e r 6 Construction Considerations
Construction Considerations 57 Activity/Component Permeable Pavement Type Porous Asphalt Pervious Concrete PICP Plans, specifications, and estimates Experienced contractors and producers Contractor certifications N/A1 Pre-construction meeting Install erosion and sediment controls Test strip construction Optional2 Subgrade preparation Excavation Finishing to design elevations and grades Soil density and infiltration testing Per specs Per specs Per specs Reservoir layer construction Geotextile for soil filtering of excavated sides Geotextile for soil filtering of underlying subgrade Geomembrane for subgrade impermeable barrier Underdrain installation Open-graded base/subbase aggregate placement Reservoir layer density and stiffness testing 3 4 Optional (per plans/specs) Optional (per plans/specs) Per specs 3 4 Optional (per plans/specs) Optional (per plans/specs) Per specs 3 4 Optional (per plans/specs) Optional (per plans/specs) Per specs Choke-stone/filter layer construction Placement of choke-stone layer for leveling of reservoir layer prior to paving or preventing migration of fines from filter layer (if used) into reservoir layer Filter layer for added pollutant removal capability Optional (per plans/specs) Optional (per plans/specs) Optional (per plans/specs) Optional (per plans/specs) Optional (per plans/specs) Optional (per plans/specs) Permeable pavement construction Treated permeable base (ATPB or CTPB) paving Porous asphalt paving Pervious concrete placement and finishing PICP bedding placement PICP paver installation PICP joint filler application Optional (per plans/specs) N/A N/A N/A N/A Optional (per plans/specs) N/A N/A N/A N/A N/A N/A N/A 1May consider âGoldâ certification. 2Test strip recommended. 3Unless impermeable liner is specified. 4A separation aggregate layer is sometimes used. Table 14. Permeable pavement construction activities/components. 6.3 Qualified Material Producers and Contractors As with all pavements, successful construction of a properly designed permeable pavement requires the use of experienced and qualified material manufacturers/suppliers and installation contractors. The fact that permeable pavements require the use of more specialized materials and more meticulous material placement procedures compared to conventional pavements only heightens the need for expert producers and installers. During the planning and design stages, it is important to determine the availability of local enterprises capable of successfully constructing permeable pavement. Ideally, such entities will have a track record of projects with which they have been involved and where they have met or exceeded the clientâs expectations for construction. The performance of their projects should
58 Guidance for Usage of permeable pavement at airports also be carefully considered in the process since materials and construction quality are major determinants of performance. Material manufacturers must have experience in producing proper mix designs for porous asphalt and pervious concrete and in making materials that comply with national standards (Carlson et al. 2013). Similarly, PICP manufacturers must have experience producing pavers that conform to national product standards. Contractor certification is a common requirement in an increasing number of pervious con- crete and PICP specifications and should be a prerequisite for all potential bidders of these types of pavements in airport applications. Source: CH2M (2014). Figure 17. Edge restraint detail. Source: CH2M (2014). Figure 18. Tie-in detail to existing pavement.
Construction Considerations 59 â¢ Pervious concrete. ACIâs pervious concrete pavement specification addresses contractor qualifications as follows: âThe contractor shall employ no less than one NRMCA-certified pervious concrete craftsman who must be on site, overseeing each placement crew during all concrete placement, or the contractor shall employ no less than three NRMCA-certified Pervious Concrete Installers, who shall be on site working as members of each placement crew during all concrete placement, or the contractor shall employ no less than five NRMCA-certified Pervious Concrete Technicians, who shall be on site working as members of each placement crew during all concrete placement unless otherwise specifiedâ (ACI 2013). â¢ PICP. The guide specification in the Permeable Interlocking Concrete Pavements: Design, Speci- fication, Construction, and Maintenance manual stipulates that job foremen should have a current certificate from the ICPI Concrete Paver Installer Certification program and a record of completion from the PICP Specialist Course on best construction practices (Smith 2015). Because open-graded asphalt mixtures have a long history of use (both as surface courses and base courses), and the process of producing and placing them is fairly straightforward, the need for contractor certification for porous asphalt pavement construction is low. While NAPA maintains that any qualified asphalt pavement contractor can construct such pavements, and virtually any asphalt plant can produce the required material (NAPA 2009), due diligence is necessary to ensure that the contractor possesses skilled and trained workers. Although not a requirement, NAPA offers the Diamond Commendations program, which identifies a higher level of training and certification for producers and contractors. Although not specific to permeable pavements, this certification can provide help in distinguishing among producers and contractors. 6.4 Pre-Construction Planning Prior to the start of construction, the owner agency or its engineering consultant should undertake several planning activities that will help lead to a successful permeable pavement project. First and foremost is to review the contract documents and note any changes brought about by updated geotechnical information. New information on soil permeability and density profiles, as well as groundwater levels, may necessitate a change in the pavement system design, which in turn could affect construction (e.g., the need for a thicker reservoir layer or inclusion of underdrains corresponding to lower soil permeability). Specifications included in the contract documents should include the requirement for a pre-construction meeting between the owner agency or its engineering consultant and the organizations involved in the construction of the permeable pavement (e.g., general contractor, subcontractors, material producers and manufacturers, testing labs). In addition to discussing the methods and schedule for accomplishing all phases of construction, this on-site meeting should clearly define the standards of workmanship and the level of material and construction quality specified for the project, as well as the inspection and testing activities that will be per- formed to ensure a high-quality permeable pavement system. Furthermore, in addition to dis- cussing changes in design brought about by updated soil information, a number of other items should be discussed and addressed. These include the following: â¢ Erosion and sediment control. Installation of temporary stormwater and erosion controls is needed to protect the permeable pavement (and material stockpiled for use in the pavement) from soil sediment, which could clog the pavement surface or fill the voids of the stone reservoir (Hansen 2008, ASCE 2015). Such controls should remain in place until all disturbed areas that could allow soil sediment to reach the permeable pavement are stabilized (e.g., via vegetation or paving) (Caltrans 2014). â¢ Protection of pavement from traffic. To the maximum extent possible, construction equip- ment should be restricted to those units needed to properly place, shape, and consolidate the
60 Guidance for Usage of permeable pavement at airports pavement layers. Use of heavy equipment and equipment with narrow tires must be avoided to prevent over-compaction and permeability reduction (Caltrans 2014, Hansen 2008). Pro- cedures should be developed and issued to prevent the tracking of soil and other particles onto the pavement by construction equipment. â¢ Construction timing and weather limitations. The timing of permeable pavement construc- tion is largely governed by the potential for adverse weather conditions. Permeable pavements must never be installed in rain or snow or when the prepared subgrade is saturated or frozen (Virginia Department of Environmental Quality 2011). In addition, so that pervious concrete and porous asphalt mixes can be satisfactorily placed, consolidated, and cured, temperatures must not be excessively high or low. â¢ Construction sequencing and staging. Porous pavement construction should take place late in the project schedule, so that most of the dirtier work (e.g., grading and landscaping) is completed first (Hansen 2008). Staging should be planned such that equipment and vehicular traffic are kept off the pavement to the greatest extent possible. â¢ Materials management. An efficient and rational plan for stockpiling, delivery, and place- ment of paving materials must be developed to ensure minimum pavement disturbance and maximum quality and consistency of the in-place materials. â¢ Paving schedule. Information on the anticipated timeframe for construction (of each compo- nent of the pavement system) and the expected rates of daily production will enable all parties to better plan and prepare for their respective activities (e.g., excavation, grading, material production, paving, inspection and testing, and traffic control). â¢ Test strip. If required by the specifications, a test strip must be built to demonstrate the contractorâs ability to construct the permeable pavement to the standards defined in the con- tract. The location and dimensions of the test strip should be determined, and the condi- tions for accepting the strip and approving the construction procedures should be clearly defined. Typical test strip sizes for pervious concrete and PICP pavements are 10 ft Ã 30 ft and 10 ft Ã 10 ft, respectively. â¢ Contingency plans. A practical set of contingency plans must be developed that describes the actions to be taken should certain design- or construction-related issues arise. 6.5 Subgrade Preparation Subgrade preparation consists of excavating the subgrade soil, leveling the grade to promote uniform infiltration, and grading and shaping the soil as necessary to the design elevations and grades. For designs that include terracing (i.e., the practice of constructing the subgrade layers in level steps or infiltration beds), soil or fabric barriers/berms must be constructed to serve as internal check dams that prevent lateral water flow and promote infiltration (ASCE 2015). Subgrade excavation is typically performed using excavators or backhoes. Where possible, these pieces of equipment should be operated from areas outside of the proposed permeable pavement limits in order to prevent over-compaction of the soil and a reduction of infiltration (ASCE 2015). Where operation of equipment on the subgrade is unavoidable, tracked vehicles or units with low tire pressures (<4 psi) may be acceptable. Additionally, if over-compaction occurs as a result of the equipment, subgrade remediation to a depth of at least 8 in. will be necessary. This can be achieved via scarification (York rake, loader with bucket teeth) or tilling. The final prepared subgrade should be true to grade and free of rocks, ruts, and over- compacted areas. If required by the specifications, density measurements should be taken and should indicate adherence to the specified density level. Installed erosion and sediment con- trols should be checked for effectiveness in protecting the excavated area (and any material stockpiles).
Construction Considerations 61 6.6 Reservoir Construction The construction of the reservoir layer involves installing any geosynthetic materials and underdrains that are part of the design, placing and compacting the open-graded aggregate sub- base layers, and applying a choker/base course as required. Prior to and during the construction process, continuous checks should be made to ensure that the erosion and sediment controls are in place and fully serving their purpose. Any noticeable debris or sediment on the individual in-place layers should be removed prior to the placement of the next layer, and no layer should be constructed on a layer that is muddy, saturated, or frozen (ACI 2010). Large-scale permeable pavement installations should include one or more observation wells for monitoring the length of time required for the reservoir layer to fully drain between storms (ASCE 2015). These wells, which consist of perforated PVC pipe 4 to 6 in. in diameter, should be installed below the bottom of the reservoir layer at locations no closer than 3 ft from the perimeter of the permeable pavement system (Virginia Department of Environmental Quality 2011). Each pipe should be anchored into the subgrade and kept vertical during placement of the stone reservoir. The top of each pipe should be fitted with a lockable cap that is flush with the paved surface. (In the case of PICP, the cap can also be located just beneath a paver block.) 6.6.1 Geotextiles Geotextiles are typically placed vertically against the walls of the excavated soil, but in some cases may also be placed horizontally atop the prepared subgrade. The purpose of the geotextile is to prevent the intrusion of the native soils into the aggregate reservoir layer (ASCE 2015). All geotextiles should be placed in accordance with the manufacturerâs standards and recommenda- tions, with adjacent strips of the material overlapping downslope by at least 16 in., or 24 in. for poor-draining, weaker soils (University of New Hampshire Stormwater Center 2014). Side-slope geotextiles should be placed vertically against the excavated sidewall, with the bottom portion of the strip extending at least 1 ft horizontally atop the subgrade and the top portion extending at least 4 ft beyond the edge of the excavation (Carlson et al. 2013). The top portion should be temporarily secured outside the reservoir bed to prevent sediment migration into the bed. Following placement of the reservoir layer aggregate and again after placement of the permeable pavement, the fabric should be folded over each respective placement and then resecured outside the reservoir bed. Excess fabric present following the placement of pavement should only be trimmed after the site is fully stabilized. In cases where a concrete curb extends the full depth of the reservoir layer, geotextile is not required on the sides. 6.6.2 Geomembranes For permeable pavement systems, an impermeable liner (or geomembrane) may be used in areas where the movement of stormwater into the existing soil subgrade is not desired, such as on soils with a high shrink/swell potential or for sole-source aquifer protection. If included in the design, geomembranes should be installed at the specified locations in accordance with the manufacturerâs instructions (Smith 2015). It is often recommended that a layer of sand be placed beneath the geomembrane to prevent tears or punctures from the aggre- gate that will be placed on top of it. Once installed, the geomembrane should be tested for leaks, with special attention to seams and pipe penetrations. 6.6.3 Underdrains Underdrains consist of perforated PVC pipes 4 to 6 in. in diameter and are used for conveyance/ overflow purposes. They are a requirement for no-infiltration design and are commonly required
62 Guidance for Usage of permeable pavement at airports for a partial-infiltration design, where lower-permeability soils are unable to drain the stormwater fast enough, and a portion of the water must be conveyed to a storm drain system. Underdrains are not typically required for a full-infiltration design involving high-permeability soils but may be used for overflow. Underdrains are typically installed near the bottom of the reservoir layer but can be placed at an elevated level within the reservoir layer. The frost depth needs to be considered during design to determine their elevation. A minimum of 2 in. of aggregate below the drain pipes should be placed in order to provide a buffer from the subgrade soil. A minimum of 6 in. of aggregate cover above the pipes is needed for protection against construction equipment and vehicular loadings. The underdrains should slope down toward the discharge point (e.g., outfall or storm sewer pipe, catch basin) at a slope of 0.5% or greater (Virginia Department of Environmental Quality 2011). In addition, the up-slope end of underdrains should be capped, and there should be no perforations within 1 ft of a connection with a drainage structure. Underdrain installation should conform to the construction plans and specifications. Cleanouts also need to be installed according to construction plans and specifications. Checks of the final pipe elevations, slopes, and connections, and for any potentially crushed pipes, should be made to ensure that the system will function as designed. 6.6.4 Base/Subbase Reservoir Aggregate The collective lifts of base/subbase reservoir aggregate must be sufficiently seated and compacted for strength yet remain largely open for drainage and retention of water. The clean, washed, open- graded aggregate making up the layers should be placed according to the plans and specifications. Typical lift thicknesses are between 6 and 8 in. (Indianapolis Department of Public Works 2009). However, 8- to 12-in. lifts may be acceptable for very thick reservoirs, provided that the lifts can be adequately compacted. The material for each lift should be dumped at the edge of the reservoir bed and then carefully spread and shaped using track-type (or low-contact-pressure) equipment (Hansen 2008). Care must be exercised to avoid damaging any previously installed geosynthetic materials, underdrains, and observation wells. Regular checks should be made to ensure that the aggregate does not segregate during the spreading and shaping operations. To facilitate spreading, shaping, and compaction, the aggregate should be kept in a moist state. Shaping and rolling patterns should begin on the lower side of the subbase and prog- ress to the higher side, while lapping the roller passes parallel to the centerline (University of New Hampshire Stormwater Center 2014). For larger-sized aggregate (e.g., No. 2 or No. 3), two passes of a 10-ton steel vibratory roller in static mode are generally adequate; however, roll- ing should continue until there is no visible movement of the aggregate (Minnesota Pollution Control Agency 2016). In areas that rollers cannot reach, compaction should be achieved using a vibratory plate compactor capable of least 13,500 lbf and equipped with a compaction indicator. Available documents do not have a consensus on density testing: some sources recommend it while others do not. The lack of fine material and typically large top size of the reservoir aggregate can make density testing ineffective. Some gradations that would be used for the reservoir do still allow density testing to be determined, and the use of a nuclear density gauge (ASTM D2922, Standard Test Methods for Density of Soil and Soil-Aggregate In-Place by Nuclear Methods) is generally cited using the âtargetâ density method. The infiltration rate of the compacted subbase can be determined using the double-ring infiltrometer test (ASTM D3385) or an approved alternate (University of New Hampshire
Construction Considerations 63 Stormwater Center 2014). The infiltration rate should be no less than 2.5 to 15 in./h at 95% standard Proctor compaction. 6.6.5 Choke-Stone Course A choke-stone course is used to level out the top of a reservoir layer and provide a smooth, level surface for the permeable pavement (ASCE 2015). A 4-in. choke-stone layer composed of clean, washed aggregate is the standard application for PICP (Smith 2015), whereas a 1- to 2-in. layer of the same is adequate for single-layer porous asphalt pavement (ASCE 2015), if used. The layer can be placed and compacted in one lift, and again the aggregate should be moist during compaction for better consolidation. To achieve density, at least two passes of a 10-ton vibratory roller in vibratory mode (typically high frequency and low amplitude) followed by two passes in static mode should be made, until there is no visible movement of the aggregate (ASCE 2015). (Note: fewer passes may be needed with the thinner applications of choker/base course.) Nuclear density testing on the No. 57 aggregate is possible if done in backscatter mode (Smith 2015). A base stiffness gauge may also be used to assess compacted base density. Often, because this layer is quite thin, density testing is not required. 6.6.6 Filter Layer Construction While not a typical design, permeable pavement systems can be designed to include a filter course between the aggregate reservoir and choke stone for the purpose of providing additional water quality treatment (ASCE 2015). The thickness of this layer can range from between 8 and 12 in., and the material composing it is generally a poor-graded or coarse sand (Virginia Department of Environmental Quality 2011). To prevent the migration of sand particles into the reservoir layer, the filter layer should be underlain by a thin (3-in.) choke-stone layer. The construction of a filter layer requires meticulous quality assurance due to its two key functions of filtration and load bearing. Each of these functions is greatly affected by layer compaction, with filtration adversely affected by over-compaction, and load-bearing capacity sacrificed by under-compaction. Filter layer compaction should result in 90% to 95% standard Proctor density per ASTM D698 and a hydraulic conductivity of 5 to 30 in./h per ASTM D2434 (ASCE 2015). Following construction of the reservoir layer, with or without the inclusion of a filter layer, curbs, gutters, and associated drainage structures should be installed, as designed and specified. The curbs and gutters provide edge restraint for the permeable pavement layer that is yet to be constructed. During this sequence of construction, it is recommended that the completed reservoir layer be protected from contamination or clogging by placing a geotextile over its surface. 6.7 Porous Asphalt Paving Porous asphalt is placed in a single layer or multiple layers depending on the specified thick- ness. NAPA IS 131 (Hansen 2008) provides construction recommendations for porous asphalt pavement. Guidance from IS 131, as well as the ASCE Permeable Pavements manual (ASCE 2015), has been compiled in the sections that follow to cover the construction of the pervious concrete layer on a properly constructed reservoir layer and properly prepared subgrade. Placement of ATPB will be similar to the placement of the porous asphalt surface, so the following sections apply to ATPB construction as well. An additional source for construction guidance of the ATPB
64 Guidance for Usage of permeable pavement at airports is IPRFâs Stabilized and Drainable Base for Rigid Pavement (Hall et al. 2005). As with other layers and materials, all construction activities should be performed in accordance with the specifications and plans. 6.7.1 Installation Porous asphalt needs to be mixed and hauled properly to avoid segregation. Typical ranges of manufacturing and laydown temperatures for porous asphalt mixes are as follows: â¢ HMA: 300Â°F to 350Â°F; 250Â°F to 275Â°F. â¢ WMA: 260Â°F to 300Â°F; 22Â°F 5 to 250Â°F (Washington State Department of Transportation 2012). However, a porous asphalt parking lot project at Stewart International Airport demonstrated that allowing the mix to cool closer to 250Â°F provided better compaction results (Louie et al. 2011). Placement of the porous asphalt layer(s) should be done using a track paver (Hansen 2008). The ambient air temperature should be at least 45Â°F and rising (Carlson et al. 2013), and in no case should the materials be installed on wet aggregate or treated bases. Compaction activities should be in accordance with the specifications and can generally commence once the mix has cooled sufficiently so as to not push or displace under loading. Two to three passes with an 8- to 10-ton static steel-wheel roller are generally recommended for achieving a target air void range of 18% to 22% (ASCE 2015). Additional passes of a lighter roller may be required to remove roller marks, particularly in the final surface. 6.7.2 Tack Coat One key difference in the placement of porous mixes is in the use of prime and tack coats. A prime coat is required on top of unbound layers for conventional pavement because moisture must be prevented from penetrating further into the pavement structure and causing damage over time. In conventional HMA paving, tack coats are applied between paving lifts to ensure bond between layers. In permeable pavements, so that there is no barrier to the flow of water through the pavement structure into the reservoir layer, a prime coat must not be used, and most refer- ences also indicate that a tack coat should not be used. However, if the surface is not completely clean prior to placing the next lift, a light tack coat can be considered. If the porous asphalt is placed on CTPB, such as for the Culpeper apron and Richmond shoulder projects, a light tack coat could also be beneficial for bonding the two different material types. Slippage of layers due to aircraft trafficking has been a common problem at airports; therefore, at least a light tack coat should be used for aircraft applications. The application rate should be monitored closely to ensure that permeability is not decreased. 6.7.3 Testing and Opening to Traffic After final rolling of each layer, the permeability of the pavement should be tested. This can be accomplished by applying clean water at a rate of at least 5 gal/min over the surface (Virginia Department of Environmental Quality 2011). All water must infiltrate directly, without puddle formation or surface runoff. Infiltration testing of the completed porous asphalt pavement should be performed using ASTM C1701. The recommended minimum infiltration rate is 200 in./h (Carlson et al. 2013). As with the reservoir aggregate, density testing of porous asphalt can be ineffective because of the open gradation. However, there also appears to be no consensus for testing porous asphalt. IS 131 does not indicate density testing, but the UFGS requires either laboratory testing of cores
Construction Considerations 65 or nuclear density testing. There may be nondestructive testing methods that could be used to verify layer strengths, but these have not been proven. The completed pavement (such as shown in Figure 19) should be to the elevations, grades, and surface tolerances specified in the construction plans. Traffic should be restricted for the first 48 h following completion to allow the pavement to cure and stiffen (ASCE 2015). 6.8 Pervious Concrete Paving Pervious concrete is placed in a single layer to the specified thickness, as determined by its inherent structural and hydrological properties and the design traffic loadings. ACI 522.1-13 (ACI 2013) is the current national standard for specification of pervious concrete pavement. Key guidelines from ACI, as well as the ASCE Permeable Pavements manual (ASCE 2015), have been compiled in the sections that follow to cover the construction of the pervious concrete layer on a properly constructed reservoir layer and properly prepared subgrade. NRMCAâs Pervious Concrete Construction Checklists (NRMCA, n.d.) provide detailed construction checklists that can be useful. The following discussion also applies to CTPB since it will be placed in a similar manner. IPRFâs Stabilized and Drainable Base for Rigid Pavement (Hall et al. 2005) provides an additional reference for construction guidance. As with other layers and materials, all construction activities should be performed in accordance with the specifications and plans. 6.8.1 Installation The pervious concrete mix should be placed within 60 min of water being introduced to the mix, and within 90 min of using an extended set control admixture (ACI 2013). The material should be deposited as close to its final position as possible directly from the truck, using a conveyor belt or via hand or powered carts. (Note: pervious concrete mixes are stiff and cannot be pumped.) The underlying reservoir aggregate should be in a moist condition at the time the pervious con- crete is installed (ASCE 2015). The concrete mix can be placed using various screeding devices, including hand screeds, low-frequency vibrating truss screeds, laser screeds, rotating Bunyan screeds, and hydraulically powered screeding drums (ACI 2013). The latter two devices have the advantage of providing proper compaction at the finished elevation and a nearly finished surface in one operation. The former devices level the concrete at above form (typically 0.375 to 0.75 in.), and then require the use of a static drum roller (capable of 10 psi vertical force) for Source: Campbell and Paris (2014b). Figure 19. Finished porous asphalt surface.
66 Guidance for Usage of permeable pavement at airports final compaction. Because high-frequency vibrators can seal the surface of the concrete, they should not be used. Control or contraction joints are optional for pervious concrete (ACI 2013). However, if used, the spacings and depths should be the same as for conventional concreteâ15- to 20-ft intervals for spacing and one-fourth to one-third the slab thickness for depth. The joints should be con- structed using a rolled joint former, also known as a âpizza cutterâ (ASCE 2015) (see Figure 20). Saw-cutting joints is problematic because of the created slurry potentially clogging some of the pavement, and it would require removing the curing sheeting. 6.8.2 Curing Curing is a critical step in pervious concrete construction due to the rapid drying that can occur as a result of the materialâs porous nature (ACI 2013). This rapid drying can weaken the bond within the aggregate, which in turn can weaken the structural integrity of the concrete. To provide maximum curing, the entire pervious concrete surface and edges should be covered with 6-mm plastic within 20 min of concrete discharge. All edges of the plastic should be adequately secured using lumber, reinforcing bars, or concrete blocks (ACI 2013), or by stapling the plastic to the wood construction forms (ASCE 2015). The use of sheeting material can be difficult in the airside environment because this environment is often open to winds, propeller wash, and jet blast. A surface sealant, as used with conventional PCC, is ineffective with pervious concrete because of the open surface. A 7-day curing time is recommended for pervious concrete with no additives, and a 10-day cur- ing time is recommended for mixes with supplementary cementitious materials (e.g., fly ash, slag). 6.8.3 Testing and Opening to Traffic Infiltration testing of the completed pervious concrete pavement should be performed using ASTM C1701. The recommended minimum surface infiltration rate is 200 in./h (Carlson et al. 2013). The completed pavement should be to the elevations, grades, and surface tolerances specified in the construction plans. Traffic should be restricted until curing of the pavement is complete, Courtesy of NRMCA; Â© NRMCA. Figure 20. Pervious concrete pavement joint formation using âpizza cutter.â
Construction Considerations 67 and truck traffic should not be permitted for at least 14 days following construction (ASCE 2015). For aircraft traffic, the pervious concrete needs to reach full design strength before opening to traffic, which may take longer than 14 days. 6.9 PICP Construction PICP construction consists of three steps: (1) placement of the bedding layer, (2) paver instal- lation, and (3) joint filling. The bedding layer provides a smooth, level surface on which the paver units can be placed. The concrete paver units placed in tight-knit configurations provide a strong, durable surface over which a broad range of traffic can pass. The joint filler aggregate pro- vides interlock between the pavers so that the surface responds more integrally to traffic loadings. The ICPI Permeable Interlocking Concrete Pavements manual (Smith 2015) is the national standard for specification of PICP. Key guidelines from this document, as well as the ASCE Permeable Pavements manual (ASCE 2015), have been compiled in the sections that follow to describe the construction of the PICP layer on a properly constructed aggregate reservoir and properly prepared subgrade. As with other layers and materials, all construction activities should be performed in accordance with the specifications and plans. 6.9.1 Bedding Layer The bedding layer consists of 1.5 to 2 in. of washed No. 8 aggregate, a crushed stone with a 0.375-in. nominal maximum aggregate size. This material should be placed in a moist state over the choke-stone layer, and then screeded and leveled for paver unit installation. To maintain permeability throughout the system, the bedding layer should not be compacted. Various sizes and types of screeds are available for use, ranging from hand screeds, to manual- or machine-powered bucket screeds, to modified asphalt spreaders with laser guidance systems (Smith 2015). The surface of the fully screeded bedding layer should meet the requirements of the specifications. The typical surface tolerance is Â±0.375 in. over 10 ft. 6.9.2 Paver Unit Installation Paver units should be placed immediately after final screeding of the No. 8 bedding layer (Smith 2015). Paver installation can be by hand or with mechanical equipment (Figure 21), Source: FHWA (2015). Figure 21. Mechanical installation of concrete paver units for PICP.
68 Guidance for Usage of permeable pavement at airports with the former being most appropriate for small projects and the latter for larger projects due to much higher rates of installation. Pavers should be installed in the patterns and joint widths shown in the construction plans, and straight pattern lines should be maintained at all times (ASCE 2015). Gaps at the edges should be filled with cut units, with the caveat that any cut unit that will be subject to traffic should not be cut to smaller than one-third of a whole unit. 6.9.3 Joint Filling and Compaction Paver unit joints should be filled to the top with No. 8 aggregate or with a finer gradation conforming to No. 89 or No. 9 (Smith 2015). This material should be swept into the joints, with any excess aggregate removed from the surface. Compaction and seating of the pavers should be done using a low-amplitude (75 to 90 Hz) plate compactor capable of at least a 5,000-lbf compaction force. At least two passes should be made across the entire surface, with each pass overlapping the previous pass by several inches. Because compaction will cause some settlement of the joint filler aggregate, additional material should be swept into the joints to a level approximately 0.25 in. below the paver surface. (Note: a third application may be necessary 6 months after construction.) As with the bedding layer, the final PICP surface should meet the requirements of the specifications, with the typical surface tolerance being Â±0.375 in. over 10 ft. For pavements subject to aircraft traffic, a sealer is necessary to prevent loss of the joint sand from the effects of repeated jet blast and propeller wash (McQueen et al. 2003). The sealer will also prevent the ingress of water, oils, and fuel through the joint sand into the bedding sand.