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9 2.1 Introduction Permeable pavements can be constructed using a variety of surface materials. These permeable surfaces are typically placed over an aggregate base/subbase reservoir, which collects the water that infiltrates through the surface. Although permeable pavement systems are often designed for soils with high permeability, the systems can also be designed for other conditions. This chapter provides a general overview of permeable pavement systems, while Chapter 3 discusses the application of these systems at airports. 2.2 Permeable Pavement Systems The use of permeable pavement systems is a structural stormwater management practice designed to manage the quantity and quality of stormwater runoff (ASCE 2015). Permeable pavements are alternative paving surfaces that allow stormwater runoff to filter through voids in the pavement surface into an underlying stone reservoir, where the runoff is temporarily stored or infiltrated (Virginia Department of Environmental Quality 2011). As shown in Figure 1, a typical permeable pavement system cross-section includes a permeable pavement surface layer on top of open-graded aggregate base/subbase reservoir layers, which serve to retain or detain stormwater and support traffic loads (ASCE 2015). The thickness of this reservoir layer is deter- mined by structural and hydrologic design analyses (Virginia Department of Environmental Quality 2011). As illustrated in Figure 2, there are typically four major categories of permeable pavement: ⢠Porous asphalt. ⢠Pervious concrete. ⢠PICP. ⢠Others (such as grid pavement systems). The type of permeable pavement material selected and the design of the full pavement system will be dependent on the site goals and the particular use for the pavement (ASCE 2015). The reservoir system can be designed for full stormwater infiltration, partial infiltration, or no infiltration (Smith 2015); these are discussed in Chapter 4. The most common system, full infiltration, directs water through the base/subbase reservoir and infiltrates it into the soil sub- grade. A full-infiltration system is typically used in areas with high-permeability soils and generally does not require underdrains (ASCE 2015). Partial-infiltration systems rely on drainage of the base/subbase into the subgrade soil and drainage pipes to direct excess water to a sewer or stream. No-infiltration systems are required when the soil has very low permeability and low strength (Smith 2015) or to restrict groundwater recharge into soil with contamination. An impermeable C h a p t e r 2 Types of Permeable Pavements and Their Benefits
Figure 1. Generic permeable pavement cross-section. Porous asphalt1 Pervious concrete1 PICP2 Grid pavement system (plastic or concrete)3 1Source: Applied Pavement Technology, Inc. 2Source: EPA.gov website, https://www.epa.gov/green-infrastructure/ what-green-infrastructure, accessed 5/3/17. 3Source: EPA.gov website, https://buildingdata.energy.gov/sites/ default/files/styles/slideshow_grid/public/nv_data/projectfiles/ project_902/IMG_2154.jpg?itok=1aeMUwOU, accessed 5/3/17. Figure 2. Typical permeable pavement surfaces.
types of permeable pavements and their Benefits 11 liner (e.g., geotextile, clay barrier) between the reservoir course and subgrade is included to prevent infiltration of stormwater into the subgrade (ASCE 2015). 2.3 Porous Asphalt Porous asphalt pavements include one or more layers of porous asphalt underlain by a choke- stone layer (or treated base layer) and aggregate base/subbase reservoir (Figure 3). Layer depth is based on structural load, stormwater requirements, and frost depth requirements (ASCE 2015). Porous asphalt typically consists of conventional HMA or warm-mix asphalt (WMA) with sig- nificantly reduced fines (aggregate passing the No. 4 sieve), resulting in an open-graded mixture that allows water to pass through an interconnected void space. Additives and higher-grade binders are often used to improve durability and reduce the potential of draindown of the asphalt. Porous asphalt is similar in appearance to conventional asphalt pavement, although generally coarser in texture. The porous asphalt surface void space typically ranges from 18% to 25% (conventional HMA has around 5% air voids), and surface permeability ranges from 170 to 500 in./h (ASCE 2015). 2.4 Pervious Concrete Pervious concrete consists of a hydraulic cementitious binding system (e.g., Portland cement) combined with an open-graded aggregate to produce a rigid, durable pavement. Pervious concrete is typically placed over a choke-stone layer (or treated base layer) and aggregate base/subbase reservoir (Figure 4). Pervious concrete pavement typically has 15% to 25% interconnected void space and a surface permeability of 300 to 2,000 in./h (ASCE 2015). Overall thickness of the permeable pavement system is determined based on hydrologic design, vehicle loading, and frost depth considerations. A minimum thickness of 12 in. in freezeâthaw climates is typical (ASCE 2015). 2.5 Permeable Interlocking Concrete Pavement PICP consist of manufactured concrete units that form permeable voids and joints when assembled into a laying pattern (ASCE 2015). The joints allow stormwater to flow into a crushed stone aggregate bedding layer and base/subbase reservoir that support the pavers (Figure 5). The Figure 3. Typical porous asphalt system cross-section.
12 Guidance for Usage of permeable pavement at airports Figure 5. Typical PICP cross-section. joints typically make up 5% to 15% of the paver surface area and maintain surface permeability of 400 to 600 in./h (ASCE 2015). PICP typically includes a small-sized aggregate bedding layer below the pavement surface and on top of the choke-stone layer to ensure a level surface for the pavers/grids (ASCE 2015). 2.6 Others 2.6.1 Grid Pavements Grid pavements are composed of concrete or plastic open-celled paving units. The cells or openings penetrate the full thickness so they can accommodate aggregate, topsoil, or grass (ASCE 2015). Concrete and plastic grid pavements (Figure 6) typically include a small-sized aggregate bedding layer below the pavement surface and on top of the choker course to ensure a level surface for the pavers/grids. Surface void space ranges from 20% to 75% (ASCE 2015). Surface permeability depends on the fill material and ranges from 30 to 40 in./h, 200 to 400 in./h, and 1 to 2 in./h for sand, aggregate, or grass fill, respectively (ASCE 2015). Figure 4. Typical pervious concrete pavement system section.
types of permeable pavements and their Benefits 13 2.6.2 Pervious Pavers Pervious paver pavement consists of unit paving typically made of a combination of uncrushed or crushed pea-sized stones bound together with a polymer or cement (ASCE 2015). These pavers differ from PICP systems in that the pavers themselves are permeable and, as such, the permeability is related to the entire paver surface and not limited to the open joints between them. The paver matrix typically contains 20% to 40% voids (ASCE 2015). Pervious pavers generally are rectangular or square. Depending on the intended use and specific design goals, pervious pavers may be situated over a permeable aggregate base/subbase reservoir (Figure 7). In some cases, the pavers may be designed to be placed directly over the underlying soil subgrade (ASCE 2015). The paving units vary in thickness. A 2-in. minimum thickness for polymer-bound units can be used for areas of light traffic load, including pedestrian areas. With the proper base/subbase depth specifically designed to support the required loads, the pavers can support periodic use by emergency or utility vehicles. Paver thickness can be increased up to 4 in. for use in heavier or higher-frequency traffic areas, including parking lot entrances and exits (ASCE 2015). Figure 6. Typical grid paving unit cross-section. Figure 7. Typical cross-section for a pervious paver system.
14 Guidance for Usage of permeable pavement at airports 2.6.3 Rubber Overlay Pavement Rubber overlay pavements are a type of permeable pavement made from a mix of recycled rubber granules (0.24 to 0.4 in. in size), dry aggregate, and a proprietary binder (ASCE 2015). When bound together into the overlay matrix, they form a permeable surface with open voids. Rubber overlay pavements have flow-through rates of as high as 400 in./h (ASCE 2015). If properly mixed, the binder enables the rubber to resist degradation from transmission fluid, brake fluid, hydraulic fluid, salt water, oil, chlorine, ozone, bromine, muriatic acid, and other reactive materials. Rubber overlay pavements also demonstrate elastic properties that help them resist cracking (ASCE 2015). The pavement material is typically poured in place by hand using specialized labor and equipment. 2.6.4 Rubber Composite Permeable Pavers A rubber composite paver is a lightweight paver made from up to 95% post-consumer recycled materials, such as scrap tires and plastic (ASCE 2015). While the pavers themselves are not per- meable, they can be placed with open joints between them and backfilled with small aggregate to create a permeable surface. The pavers are roughly one-third the weight of standard concrete pavers and are available in a variety of shapes and sizes. 2.7 Benefits and Concerns of Permeable Pavements 2.7.1 Environmental, Operational, and Economic Benefits Permeable pavements offer a number of environmental, operational, and economic benefits. However, there are also risks associated with their use. 2.7.1.1 Environmental Benefits Permeable pavements have historically been known for the following environmental benefits: ⢠Reduced stormwater volume (infiltration systems), reducing flows to storm sewer systems and streams. ⢠Increased groundwater recharge (infiltration system). ⢠Decreased and delayed peak discharge. ⢠Reduced pollutants and improved water quality (by filtering and capture). ⢠Reduced urban heat island effect. An additional potential benefit in the airport environment is the possibility of reducing wildlife attractants. At some airports, wildlife such as birds nest in open drainage ponds and create the risk of bird strikes. Eliminating the open water source and providing subsurface water storage can reduce the number of birds and other wildlife on the airport. The removal of open water sources in public areas also reduces safety risks (such as accidental drowning). Permeable pavements also generally require no (or at least less) deicing (FHWA 2012). Water does not stay on the surface, so icing does not generally occur. Therefore, less deicer product is introduced into the environment. While the environmental benefits of permeable pavements are many, there are also risks. The primary environmental risk for permeable pavement is accidental chemical spills, such as spills of hydraulic or engine oils. An impervious pavement surface allows a window of opportunity to collect chemicals if there is a spill. However, once a fluid is spilled on permeable pavement,
types of permeable pavements and their Benefits 15 it is going to drain into the permeable pavement structure. For a full-infiltration system, this provides a nearly direct access to the subgrade. Mitigation of a chemical spill once it reaches the subgrade can be costly. If the permeable pavement is a lined system, chemical spills may not be as significant an issue. While the chemical does enter the permeable pavement structure, it will be transported to the outflow, and the outflow can be designed with a separator, filter, or other collection system to contain potential spills. Lined permeable pavement systems could actually be a benefit for chemical collection: lined systems could potentially be used for aircraft deicer collection. Aircraft deicing chemicals need to be collected and, therefore, are problematic for the use of unlined permeable pavement systems. 2.7.1.2 Operational Benefits Permeable pavements also provide operational benefits. These pavements reduce the risk of surface ponding and, therefore, the risk of hydroplaning, making travel safer during rain events (FHWA 2012). As mentioned previously, permeable pavements do not generally ice over because the water does not remain on the surface. Therefore, there is a lower risk of skidding on ice. There are typically fewer snow removal and deicing operations overall than for impervious surfaces (FHWA 2012). However, snow removal procedures can be different from those for other pavements, and this requires changes in operational procedures (e.g., raising plow blades) that personnel need to be aware of. Some operations may require restrictions with permeable pavements. Fueling would not be allowed near the permeable pavement, and the location for maintenance activities may need to be restricted, such as by requiring maintenance to be performed in maintenance buildings or hangars. These limitations could be made part of the leasing agreement if the activities are tenant related. The users of the pavement need to be aware of what the pavement is and how their actions can influence performance of the permeable system. Maintenance and repair operations will also be different from those for conventional pave- ment areas. Chapter 7 provides more in-depth discussion of maintenance and repair activities. To maintain permeability, periodic vacuuming of the surface will be required, which may not be common maintenance work at some airports. This will require appropriate maintenance equipment that an airport may not currently own. Pavement repairs will also be different from those for conventional pavements. Patching methods and surface treatments applied elsewhere should not be applied to permeable pavements. 2.7.1.3 Economic Benefits Initial construction costs for permeable pavement are often considered to be higher than those for conventional pavement, but these costs are often offset by a reduction in other traditional stormwater structures. Some of the reasons for the initial cost difference are as follows: ⢠Unfamiliarity with permeable pavement materials/construction. With unfamiliar materials and techniques, contractors are likely to increase their unit costs to cover contingencies. Suppli- ers are also likely to charge more for the materials because they are not typically common products and require manufacturing changes to produce a comparatively small quantity of material. ⢠Additional high-quality material. The base/subbase reservoir required for permeable pavement can be thick, depending on local climate and design requirements. A thick reservoir will require greater excavation because of the pavement depth. Aggregate used for the reservoir is typically a high-quality material. Therefore, the aggregate has a higher price. Although standard pave- ments may be just as thick, particularly if there are frost depth requirements, the non-frost
16 Guidance for Usage of permeable pavement at airports materials used may not be as costly as the base/subbase reservoir aggregate. A filter fabric may also be required in the permeable pavement design but may not be needed for a conventional pavement. Although the initial construction costs of permeable pavements may be higher than those for conventional pavements, the cost comparison needs to take into consideration items beyond just pavement materials. That is, the cost comparison should include the difference in overall construction items, such as catch basins and stormwater pipes. These items are likely minimal for a runway or taxiway project, but apron pavements often have extensive stormwater drainage systems. Permeable pavement will require fewer of the traditional drainage features, so the cost savings from a reduction in those materials may offset the higher initial construction cost of the permeable pavement materials. Permeable pavement typically satisfies any required water quality obligations, thus reducing the need for additional water quality measures. For Wittman Field in Oshkosh, Wisconsin, Givens and Eggen (2012) compared costs between a permeable pavement structure and a conventional pavement structure with a drainage system to carry stormwater to a drainage pond. The drainage pond in this case had to be relatively far from the project site (a half mile) because of existing airport facilities and the availability of land. For cost comparisons, the designer determined the cost for a conventional pavement and a drainage pond constructed adjacent to the taxiway (if there had been available land). The analysis showed that the permeable pavement design cost was twice that of the baseline cost, but providing a stormwater system to reach the pond a half mile away was ten times the baseline cost. Although Paine Field (Everett, Washington) did not conduct a cost analysis for its apron pave- ment project, the difference in costs was apparent during planning. If the airport had added an impervious surface, it would have had to do extensive upgrades to the drainage system to meet county stormwater management requirements. By using permeable pavement and not increasing the impermeable surface area, upgrades to the drainage system were not required. Similar subjective cost analyses were considered in the Richmond and Culpeper case study projects (discussed in Appendix Bâonline at www.TRB.org). The Richmond project would have required installation of stormwater piping and structures over a significant distance, extending through existing paved facilities. This work was not needed with the use of permeable pavement. The Culpeper projectâs location at the airport would have required the airport to purchase adjacent land outside of the current airport property or would have required the installation of a drainage system to move water across the airport to where there was open space. Both options would have been costly. The use of permeable pavement avoided expensive land acquisition or drainage work. In a similar fashion, the use of permeable pavement can potentially increase avail- able leasable property because the property would not be tied up as surface drainage facilities. 2.7.1.4 Funding A concern expressed in survey responses for this study was the ability to fund permeable pavement projects. Permeable pavements are not currently a standard FAA pavement design, and obtaining FAA funds for any project incorporating this type of pavement would likely be difficult. Securing federal funding through the FAA would require obtaining a Modification of Standards (MOS) in accordance with FAA Order 5300.1, Modifications to Agency Airport Design, Con- struction, and Equipment Standards. The case study projects were funded with non-FAA funds. The actual funding mechanisms included the use of airport funds (generated from landing fees and lease revenues) and outside grants, such as from the Virginia Department of Ecology and Virginia Department of Agriculture.
types of permeable pavements and their Benefits 17 2.7.2 Performance Expectations and Concerns Permeable pavements need to meet design requirements for their expected design lives, just as other pavements do. They also need to meet similar functional and safety requirements as standard pavements. The hydrologic expectation is to maintain the minimum design permeability, storage capacity, water quality, and other design requirements over the planned life of the pavement. The most commonly expressed concern is maintaining permeability. Studies have shown that vacuum sweeping can maintain permeability of the pavement over time (Suozzo and Dewoolkar 2012). Other steps should be taken during the design process to maintain hydrologic function as well, such as limiting run-on, particularly from areas that may be a source of sediments, and not using sand for winter maintenance. Proper construction (discussed in Chapter 6) and using the right materials (discussed in Chapter 5) are also essential to long-term performance. Structural performance is generally measured by the presence of rutting or cracking due to loading. Rutting can occur in porous asphalt due to rutting in the subgrade or one of the pave- ment layers. Fatigue cracking can also occur in the porous asphalt. Pervious concrete pavement is designed based on cracking from the bottom up due to loadings (i.e., for cracking not to occur until the anticipated design loading). While permeable pavement structural design has historical data for vehicular applications, the ability of permeable pavements to support heavier loads and higher tire pressures associated with aircraft loadings is not well documented. Airport permeable pavement projects identified during this research are discussed in Chapter 3. While there have been permeable pavements in areas with potential aircraft loadings (shoulder pavements), it is not known if aircraft have actually loaded these pavements. The Culpeper project in the case studies was recently completed, so no performance data are yet available, and it is for general aviation aircraft (lighter loads). A safety concern with using permeable pavements for aircraft areas is the potential production of foreign object debris (FOD). Because of the open nature of the surface, permeable pavements can be more susceptible to raveling and damage from abrasion, particularly with turning wheel movements. Materials selection, mix design, and construction play primary roles in surface performance.
