Review of porous asphalt pavements in cold regions: the state of practice and case study repository in design, construction, and maintenance

Porous asphalt pavement is a sustainable infrastructure tool used to benefit urban resilience. This paper summarizes the design, construction, and maintenance practices of porous asphalt pavements (PAPs) specific to cold regions. It includes discussions on the structural design considering frost depth and frost heave of subgrade soils, material selection and design for adequate freeze-thaw durability, construction of PAPs in cold weather, winter maintenance of PAPs for snow and ice control, and performance deterioration caused by other winter activities such as studded tires. Distinguished from other review works on this topic, the major contributions of this review paper employ case studies of PAPs to address design, construction, and maintenance concerns of PAPs in cold regions. These projects have demonstrated the success of using PAPs in cold regions and design practitioners can refer to these case studies for the new design and installation of PAPs in cold regions.


Introduction
Permeable pavement is a unique class of pavement that functions as both a structural vehicular and pedestrian surface, but also as a stormwater management tool to reduce surface runoff. The most common permeable pavements include Porous Asphalt Pavement (PAP), Pervious Concrete (PC), Permeable Interlocking Concrete Pavement (PICP), and plastic grid-stabilized systems, however, a variety of proprietary options and related technologies also exist. The environmental and sustainable benefits of PAPs include, but are not limited to, the reduction of storm runoff volume, recharge of groundwater, improved water quality, and improvement of driving safety and quality by reducing hydroplaning tendency and noises [6,63]. In cold regions, case studies showed that PAPs had a faster snow melting rate than impermeable pavements and could prevent formation of black ice on pavement surfaces, which help reduce the use of salt in winter maintenance [23,32,50,57,64]. The reduction of ice build-up improves driving safety and transportation resilience after snowstorms along with lessening environmental impacts of excess salt application. With these benefits, PAP is considered as a green infrastructure technique to improve urban resilience [55] and has been used in various applications, such as parking lots, low-volume traffic roads, highvolume state routes, highway permeable shoulders, and permeable pavements at airports.
The PAPs are designed to allow water to flow through the full depth of the pavement into soil subgrade and/or underdrain pipes. A typical PAP structure with the water flow pathway is illustrated in Fig. 1a consisting of a porous asphalt (PA) surface course, a bedding course, and a stone reservoir course (aka stone recharge bed) often placed over the geotextile fabric and uncompacted or lightly compacted soil subgrade (after [6]). This fulldepth permeable pavement structure is designed to have different water flow pathways from pavements with open-graded friction course (OGFC) as shown in Fig.  1b, which consists of a permeable wearing OGFC mixture laid on the top of a dense-graded asphalt, base layer, and compacted subgrade. Although these two types of pavement have different pavement structures and water flow paths, the asphalt mixtures (PA and OGFC mixtures) used as wearing courses are similar.
Despite PAPs categorized as a low impact development technology for stormwater management, not all sites are suitable to install PAPs. The constraints include local regulatory requirements, geotechnical risks (e.g. Karst area, sinkholes, expansive soil), minimum depth to bedrock or seasonal groundwater table, groundwater contamination risk (e.g. potential chloride contamination due to the application of salt for deicing), and subgrade soil properties [6,21]. The checklist developed by ASCE [6] shows the common design considerations for permeable pavements, which could be reviewed by designers in the feasibility study phase. In addition, a feasibility decision matrix was developed by Hein [21] to evaluate the suitability of PAPs as permeable shoulders. This decision matrix could be adapted by state and local agencies to evaluate the feasibility of PAPs for parking lots or residential streets as well, after adjustments of the design goals and weightings of design considerations.
In cold regions, additional considerations in design, construction, and maintenance of PAPs include (1) structure design with consideration of frost depth and frost heave of subgrade soils; (2) material selection and design for adequate freeze-thaw durability; (3) construction of PAPs in cold weather conditions, (4) winter maintenance of PAPs for snow and ice control; and (5) pavement performance deterioration caused by winter activities such as plowing and use of studded tires. Therefore, this article mainly focuses on the syntheses and discussions on these topics and briefly summarizes the general guidelines for the design, construction, and maintenance of PAPs.
The objectives of this review article are to synthesize the state of practice and establish a case study repository of PAPs to address the above-mentioned concerns of design, construction, and maintenance of PAPs in cold regions. This review work searched database includes Transport Research International Documentation (TRID), American Society of Civil Engineers (ASCE) library, publications by National Asphalt Pavement Association (NAPA) and Federal Highway Administration (FHWA), standard specifications of northern States Departments of Transportation (DOTs) in the U.S., and Google Scholar. The literature searches and reviews mainly focus on the most recent 10-year publications (2010-2020) on PAPs in cold regions, albeit the earliest publication on PAPs cited in this paper is traced back to Thelen and Howe [57]. Distinguished from other literature review publications on PAPs in cold regions, e.g. Weiss et al. [63], the specific contribution of this review work uses 14 PAP projects, among which 12 projects are installed in cold regions, to illustrate state-of-practice on the design, construction, and maintenance of PAPs in cold regions. Engineering practitioners can refer to these case studies for the new design, installation, and maintenance of PAPs in cold regions.

Thickness design of porous asphalt pavements in cold regions
The thickness design of PAPs in cold regions includes considerations for hydrologic design, hydraulic design, structural design, and base thickness verification with the frost depth requirements.

Hydrologic and hydraulic designs
The primary goal of the hydrologic design is to determine the peak flow rate (e.g. Rational method) or depth (e.g. Curve Number method) of the stormwater run-off so that the designed stone reservoir course has adequate thickness and storage capacity to retain the desired amount of stormwater temporarily. The stone reservoir is often designed to hold the respective stormwater, however, if flooding protection is a design requirement, then a larger storage volume may be required. The hydraulic design is primarily to estimate the drawdown time of the retained stormwater drain off from the stone reservoir course to the subgrade so that the designed PAP can restore capability for the consequent precipitation events. The drawdown time can be estimated based on the saturated flow theory [21] or unsaturated flow theory in soils [12]. The recommended requirements on drawdown time for PAPs range from 12 a , 24 b , or 48 c to 72 h ([17] a [6]; b [63]; c ). When the subgrade soil has a low permeability that leads to a longer drawdown time, underdrain pipes are designed and used for overflow control in a permeable pavement with partial-infiltration or no-infiltration to subgrade soils [6,53]. The hydrologic and hydraulic designs and use of underdrain pipes can also prevent prolonged saturation and freeze-thaw damage of base and surface course in cold regions.

Structural design
The structural design of PAPs is conducted to determine the thickness of the PA course and verify the total thickness of the stone reservoir course and PA surface course is adequate to support designed traffic loads. The American Association of State Highway and Transportation Officials (AASHTO) empirical design method (aka AASHTO 1993 Design Method) is the most common method to design PAPs [63]. Schwartz and Hall [54] illustrate the design procedure and provide examples of using the AASHTO 1993 Design Method for PAPs. The performance-based mechanistic-empirical design approach was also developed for PAPs [28,33]. The catalog-type design procedure was developed to implement this design method and used to design the PAP parking lots in California State University, Long Beach Campus [51]. The designed PA parking lots have a 20 cm (7.9-in.) PA layer with a 5.1 cm (2-in.) choker layer and a 50 cm (19.7-in.) stone reservoir course.
Another pavement thickness design method to verify the structural sufficiency of PAP is the Soil Factor Design Method, which is mainly adopted by the Minnesota Department of Transportation (MnDOT) for the stateaid flexible pavement projects [67]. This method was used to design PAPs as Cells 86 and 88 in the MnROAD low-volume test road [32].
The asphalt-treated permeable base (ATPB) is an optional layer for a PAP. However, this asphalt-stabilized layer has a higher layer coefficient and structural strength than the unbounded choker layer or stone reservoir base. Therefore, an ATPB is often used in the PAP for driveways with the high-volume annual average daily traffic (AADT) to reduce the base thickness. For example, the PAP installed for the State Route 87 in Chandler, Arizona had a 15.2 cm (6-in.) PA wearing course, 15.2 cm (6-in.) ATPB, and 20.3 cm (8-in.) stone reservoir course, and used for AADT of 30,000 in the early of 1990s [24]. The PAP project installed for Maine Mall Road in South Portland, Maine, also used ATPB layer to increase the structural strength of the PAP with an AADT of 16,750 [44]. This PAP had a 7.6 cm (3-in.) PA wearing course, 15.2 cm (6-in.) ATPB, a minimum of 38.1 cm (15-in.) stone reservoir course, and 15.2-30.5 cm (6 to 12-in.) sand filter layer [44].
Design of porous asphalt pavement with frost depth requirements PAP applications in cold climates are uniquely beneficial for the protection of permafrost, however, susceptible to frost damage from the direct introduction of water into the pavement base structure. The symptoms of pavement distress due to the frost damage include frost heave of the subgrade, significant loss of supportive strength of the thawed subgrade, and the consequent performance deteriorations in terms of cracking and raveling in pavements. The frost damage occurs when all of three necessary conditions exist simultaneously, which include (1) presence of water in the subgrade, (2) frostsusceptible soil, and (3) frost depth of sub-freezing temperatures (lower than 0°C (32°F)) exceeding the thickness of pavement to subgrade [60]. For PAPs, the presence of water in the subgrade is difficult to avoid and limits the primary benefit, as the stormwater is designed to infiltrate into the subgrade that is even likely saturated. The design of PAPs shall focus on the conditions of frost-susceptible soils and frost depth. Schwartz and Hall [54] listed the frost potential of different soil types, varying from non-frost-susceptible soil to very high frost-susceptible soil that should be replaced. Extreme caution must be exercised to design PAPs in the areas where the frost-susceptible subgrade soils exist and historically cause frost damages for impervious pavements.
To avoid the third necessary condition in terms of frost depth, the principle is to design the pavement with adequate layer thicknesses, including surface, base, and subbase (if used), to meet the frost depth requirement. There are two design guidelines used for PAPs to meet this requirement. One is to strictly follow the principle that the total designed thickness of the pavement exceeds the local frost depth. This guideline becomes debatable, as several PAPs, which have the total designed thickness much lower than the local frost depth but exhibited no frost damages in cold regions [17]. Thus, the second design guideline recommends the total thickness of PAP above the subgrade shall exceed 65% of the local frost depth [59]. Table 1 summarizes four PAPs with the reported data of pavement structure, frost depth, frost heave, and early pavement performances, which are used to assess design guidelines of frost depth for PAPs. The consensus observations were that the frost depth was shallower in a PAP than the corresponding impermeable pavement during the freezing periods. When the air temperature warmed up during the thawing periods, the subgrade soils beneath a PAP thawed earlier and faster than soils beneath impermeable pavements [8,23]. For the PAP installed in Luleå, Sweden, the subgrade soil was frost-susceptible, and the measured frost depth exceeded the pavement structure, which caused frost heave. However, the PAP exhibited lower and uniformly distributed frost heave (1-2 cm) than the frost heave (7-8 cm) in the impermeable pavement during the extremely cold winter [8]. The frost heave was lower in the thicker PAP with a base thickness of 100 cm (39.4-in.) than that was measured in the PAP with a base of 60 cm (23.6-in.). For MnROAD test sections, the measured frost depth also exceeded total pavement thickness, which caused noticeable seasonal heave. While for the PAP parking lot installed at Durham, New Hampshire, USA, and the PAP residential streets installed at Robbinsdale, MN, USA, the measured maximum frost depth was lower than or close to the thickness of PAP so that there was no visually observed frost heave. Based on these case studies, it concludes that the PAP has a low risk of frost damage due to lower frost depth and heave. It is advisable to recommend that in the areas with extreme or high frost-susceptible soils (e.g. PAP in Luleå, Sweden), the total designed thickness of the pavement shall be adequate to exceed or close to the local frost depth. Otherwise, the design guideline of the total thickness of PAP exceeding 65% of the local frost depth could be used [6].

Subgrade soils
The types of subgrade soil greatly affect the performance of PAPs in cold regions, although PAPs have been successfully implemented in most soil types, including clays with low permeability and high frost-susceptibility. As shown in Table 1, PAPs installed in the MnROAD and Robbinsdale, MN, USA explored the influence of subgrade soil types, sand and clay, on the frost depth and structural supports. The PAP installed on the sandy subgrade had a shallower frost depth than that on the clayed subgrade [32,64]. This observation was mainly attributed to the higher permeability and pronounced effect of air insulation in the sandy subgrade. As a result, the PAP over the sandy subgrade exhibited more effectiveness on the ice control and snow melting than the PAP overlaid on the clayey subgrade [64]. The sandy subgrade had similar back-calculated stiffness as the clayey subgrade in spring and summer, while the clayey subgrade exhibited higher stiffness during the freezing seasons. However, the clayey subgrade had less structural support than the sandy subgrade, evidenced by the lower back-calculated resilient modulus of stone reservoir base and higher longitudinal strain responses [32].

Geosynthetics
The geosynthetic material, such as geotextile fabric, geogrid, and geomembrane, are placed individually or jointly between native soils (subgrade soil and/or excavated soil) and stone reservoir course for different purposes in different applications. The non-woven geotextile fabric is used in the full-infiltration or partial-infiltration design for filtering purposes to prevent sediment-laden runoff from subgrade and/or excavated soils contaminating and clogging the stone reservoir course [17,21]. In cold regions, a graded filter blanket is recommended to replace geotextile fabric to serve as a filtering and capillary barrier to mitigate frost damage [59].
The geogrid is used to stabilize subgrade soil and distribute loads uniformly over the structurally weak soils [6]. The use of geogrid can also reduce the thickness of PAPs [43] and increase the life of PAPs that are built over the saturated and uncompacted subgrade soils [20]. The geogrid and non-woven geotextile fabric could be used together, such as the PAP project at the Cuyahoga Valley National Park, Ohio, USA [65]. This PAP has a 7.6 cm (3-in.) PA, 5.1 cm (2-in.) choker with No. 57 limestones, and 22.9 cm (9-in.) stone reservoir layer, which was overlaid on the top of a geogrid and nonwoven geotextile fabric. Another PAP project that used geotextile fabric and geogrid jointly was installed in Decatur Street, Olympia, Washington. The geogrids were placed at the top and bottom of a 30.5 cm (12-in.) stone reservoir course to meet the designed equivalent single axle loads (ESALs) requirement [20].
A geomembrane is an impermeable material, which is used for the no-infiltration design of the permeable pavements to prevent runoff from entering into the subgrade and/or excavated soils. The no-infiltration permeable pavements are used in areas with expansive soils or highpotential frost susceptibility soils [6,21]. The typical material of a geomembrane includes polyvinyl chloride (PVC), ethylene propylene diene monomer (EPDM), or high-density polyethylene (HDPE) [21]. The project installed at Sylvan Avenue, City of Ann Arbor, Michigan, is a no-infiltration PAP design. This project used an impervious PVC fabric over the clayed subgrade to direct stormwater into a manhole to resolve water ponding and icing problems on the pavement surface [27]. The designed PAP has a 7.6 cm (3 in.) PA, a 45.7-61.0 cm (18-24 in.) stone reservoir course, a woven geotextile fabric, and a 15.2 cm (6 in.)-thick sand filter layer with underdrain pipes overlaid on an impervious PVC liner.

Stone reservoir aggregates
The aggregate used for the stone reservoir course shall be selected to fulfill the hydrologic and structural requirements to retain the stormwater runoff and support traffic loads. The freezing damage of aggregate is minimal because the water in the stone reservoir course freezes rarely, and in case this happens, the large voids in the stone reservoir course provide spaces for water expansion without causing damage [57].

Choker aggregates
The choker course (aka bedding course) is designed to interlock large stone reservoir aggregates and provide a smooth and stable platform for PA placement.  [17,63].

Asphalt treated permeable base (ATPB)
The asphalt treated permeable base (ATPB) is a viable bedding course to substantially increase the structural loading capacity and provide a stable platform for PA construction. Table 3 shows the synthesis of specifications on ATPB from northern DOTs, including Minnesota (Section 2363), Pennsylvania (Section 360), Missouri (Section 302), New York (Section 401), and Oregon (Section 00743), and general specification provisions by American Public Works Association, Washington State, Local Agency General Specification Provisions [5], and a project-based specification. The No. 57, No. 67, or similar gradation is commonly used as the aggregate gradation in an ATPB mixture. The APTB mixture is designed with lower asphalt binder content (typically 2.0-4.5%) than the PA mixture (typically 6.0-6.5%). Thus, ATPB works as a cost-effective practice to reduce the total thickness of a PAP [43] and is typically used for the high-volume traffic road, such as projects of the State Route 87 in Chandler, Arizona [24] and Maine Mall Road in South Portland, Maine [44]. The performance grade of the asphalt binder used in an ATPB mixture varies in specifications. The polymermodified asphalt binder is increasingly used in the ATPB mixture. The binder content varies from 2.0% to 4.5%, which should uniformly coat on aggregates at a minimum percentage of 90% or 95% tested using the AASHTO T195 and has no excessive binder drained off from aggregates. The specification on the use of warm mix additive and/or recycled materials (e.g. reclaimed asphalt pavement and recycled asphalt shingle) in the ATPB mixture also differs among the state agencies and shall conform to the local experience.
Moisture stripping of ATPB might be a concern when it is used as a drainable base that is placed over an

Porous asphalt
The mix design of a PA mixture generally follows the design guidance of the OGFC mixture where primary concerns include draindown stability and rapid oxidation. Tables 4 and 5 summarize aggregate quality requirement and mixture design of PA based on national design guidelines from NAPA publication [29] and ASTM D7064/D7064M-08 [7], as well as design guidance of PA mixtures in cold regions from state agencies [42,45], a state asphalt pavement association [36], a local general special provisions [5], the research university [59], and a project-based design provision [44]. The aggregate quality tests generally include the Los Angeles abrasion test, fractured faces, flat and elongated particles, durability, and soundness test. The PA mixture is typically designed with the binder content higher than 5.5%, air voids higher than 16%, and binder draindown less than 0.3%. The typically designed number of gyrations (N des ) for the PA mixture is 50. The moisture susceptibility owing to freeze-thaw cycles is one of the durability concerns for PA mixtures  No. 16      used in cold regions. The modified AASHTO T283 is typically used to determine the retained tensile strength of a PA mixture after five freeze-thaw cycles instead of one cycle. The tensile strength ratio (TSR) higher than 80% is used as the threshold value to control moisture damage of PA mixtures. In case the designed PA mixture fails to meet the moisture susceptibility requirement, a liquid anti-stripping additive and/or hydrated lime can be used to improve compatibility between aggregates and asphalt binder [7]. The polymer-modified asphalt binder is also recommended to use in cold regions to improve the freeze-thaw durability of PA mixtures [32,52]. Another durability concern for a PA mixture is the raveling due to reductions of cohesive and adhesive strengths caused by the oxidization of asphalt binder. The Cantabro test is used to assess the PA mixtures' resistance to abrasion. The laboratory accelerated aging protocol includes 85°C for 5-day aging [29] or 60°C for 7-day aging [7] for the compacted PA specimens. The maximum mass loss values for unaged and aged specimens tested by the Cantabro abrasion test are 20% and 30%, respectively. The use of a polymer-modified asphalt binder and increasing the binder content are two effective practices to improve the durability and raveling resistance of PA mixtures [29]. However, such high binder content could lead to the binder draindown during the construction phase and summer seasons, which cause voids clogging and permeability declination at the lower part of mixtures [23,69]. The addition of fiber in a PA mixture becomes a common practice to minimize the binder draindown ( [3,19,47]; and [68]), improve durability by allowing higher binder content [1,14], and increase resistance to rutting and cracking of PA mixtures [1,35].

Construction
Subgrade, geosynthetics, stone reservoir course, and choker The preparations of subgrade, placement of geosynthetics, and installations of stone reservoir course and choker in cold regions follow the common construction guidance of permeable pavements in other areas. The details can be referred to the publication by ASCE [6].

ATPB and PA mixtures
The best practices of ATPB and PA installations in cold regions or under cold weathers are summarized in Tables 6 and 7 according to the construction sequences of ATPB and PA mixtures. The air and surface temperatures should be higher than 10°C to place ATPB and PA mixtures ([45, 59], and [42]). This constrains the construction season of APTB and PA mixtures in cold regions. The warm mix asphalt technology can help to extend the paving season, allow long haul distance, and compact asphalt mixtures at lower temperatures than hot mix asphalt, which is recommended to use in cold regions. The silo storage time shall be minimal to reduce the oxidation of mixtures. When ATPB or PA mixture is placed, the thickness of one lift shall be less than 10.2 cm (4-in.), but higher than twice the maximum • Traffic is allowed after a 72-h cure [42] Tack Coat • Not allowed on the newly-placed and clean ATPB course [5,40] • Light tack coat (0.02 gal/yd 2 residual asphalt) may be used when the ATPB course is dirty [5] Zhang and Kevern Journal of Infrastructure Preservation and Resilience (2021) 2:4 Table 7 Construction requirements of porous asphalt in cold regions
Silo Storage • ≤ 4 h in the non-insulated silo and 8 h in the insulated silo [5] • ≤ 2 h in Maine Mall project [44] • ≤ 1.5 h in MnROAD test sections [32] Hauling • Haul vehicles with a clean and smooth dump bed coated with a non-petroleum asphalt release agent [29,59] • Avoid long haul distance [59]; • WMA technology can be used to extend the paving season and allow long haul distance.
• Avoid segregation when PA mixtures are deposited from haul vehicles [42] Placement Thickness • One lift for small projects [6] • Two lifts for thicker cross-section [6] or large projects in cold weathers [59] • ≤ 10.2 cm (4-in.) for one lift [42,45] • ≥ twice the maximum aggregate size in PA for one lift [42] Roller Selection • Smooth double steel-wheeled roller with a weight of 8-10 tons [6,17] • An additional small roller with a weight of 1-1.5 tons could be used for finishing rolling to achieve a smooth finished surface [6,59] • The pneumatic-tired roller is prohibited ( [29], [36], and [40]) Rolling temperature • Sufficient cool to resist roller [42] and avoid over compaction [9] • Curing time before opening to traffic • Cure at least 24 h ( [17,36], and [40]) • Cure at least 48 h [6,59] • Cure at least 72 h [42] Zhang and Kevern Journal of Infrastructure Preservation and Resilience (2021) 2:4 Page 11 of 17 aggregate size. The steel-wheeled roller is used and operated in a static mode to compact APTB and PA mixtures. Pneumatic tire roller is prohibited. The APTB or PA mixture shall be sufficiently cool and stable before compaction. The rolling pattern shall be developed to compact ATPB and PA mixtures in such a way as to achieve balanced hydraulic performance (higher voids are preferred) and mechanical strength (lower voids are preferred). A density gauge is recommended to monitor the in-place density and air voids during the compaction of the PA mixture and ensure to achieve balanced performances [69]. Adequate curing time is needed to cool down ATPB and PA mixtures before opening to traffic.

Maintenance
The maintenance for PAPs involves (1) corrective maintenance for distresses on pavement surfaces; (2) preventive and restorative maintenance for permeability, and (3) winter maintenance for snow and ice control in cold regions.

Corrective maintenance for surface distresses
The localized patch is commonly used to fix PA raveling in small areas, which is one of the typical PAPs' distresses as reported by Hossain et al. [24] and Lebens and Troyer [32]. Raveling is caused by many factors, such as materials (e.g. lower binder content, high air voids, and oxidation), construction (e.g. temperature segregation, insufficient compaction, and weak interface bonding between surface and underlying layers), and repetitive vehicle loads [13,22]. When PA mixture is readily available, it is the preferred patching material to maintain hydraulic performance, especially when the patching area is larger than 10% of the permeable surface [17]. A densegraded asphalt mixture is an alternative patching material for small areas. However, a dense-graded asphalt mixture is not recommended to patch at locations where the water flow will converge and not be able to flow around the patched areas [48]. Corrective maintenance is also needed to repair wearing damages caused by studded tires or snow chains in cold regions where they are allowed to use in winters. Al-Rubaei et al. [2] reported that studded tires created excessive fines to clog PAPs, but studded tire damage on PAPs is rarely reported in the literature. Since PAPs use the OGFC mixture as the surface course, this type of asphalt mixture is very susceptible to wearing damages caused by studded tires [34] and snow chains [4]. This was the reason that led to the discontinuity of using OGFC for high-traffic volume roads in cold regions, e.g. Washington [4] and Oregon [39]. One of the potential methods to reduce studded tire wearing on PAPs is to design PA mixtures using aggregates with larger size and higher abrasion resistance [41]. Therefore, in cold regions, the volume of vehicles equipped with studded tires and snow chains should be considered as an additional factor in the feasibility study of a PAP project.

Preventive and restorative maintenances for clogged permeable surfaces
Preventive and restorative maintenances, such as mechanical sweeping, pressure washing, and vacuum sweeping, are used individually or jointly to restore the infiltration of clogged permeable surfaces due to debris (e.g. dust, sand, or vegetation) or debris-laden flow. Low infiltration can be also caused by improper mix design, over compaction, or surface densification (e.g. rutting). In these locations, preventive and restorative maintenances cannot restore the low infiltration. Figures 2 and 3 illustrate the effectiveness of these maintenance methods on the permeability restoration of PAPs in short-term and long-term service life. Figure 2a shows the infiltration of PAPs in MnROAD test sections maintained by vacuum sweeping [32] and Fig. 2b shows two PAPs in Canada maintained by dry and wet vacuum sweeping [10], hand sweeping, pressure washing, and low/high suction vacuum sweeping ( [16], and [15]). These projects showed that the infiltration rates of PAPs decreased dramatically in the early stage, even though vacuum sweeping and/or pressure washing are used, which could only restore the permeability of PAPs to a very limited extent.
However, these non-destructive maintenances are necessary to maintain the long-term infiltration performance of PAPs, which are demonstrated in Fig. 3 using two long-term PAP projects in northern Sweden. The PAP in Haparanda (Fig. 3a) was maintained by mechanical sweeping only without vacuuming, and the PAP in Luleå was maintained by the mechanical and vacuum sweeping and pressure washing to clean the applied fine gravel and dust after each winter until 2005/2006. Consequently, the high-pressure washing, vacuum sweeping, or combination of these two methods worked more effectively to restore permeability of PAP in Luleå than PAP in Haparanda in both 2011 and 2015 maintenances. Therefore, Al-Rubaei et al. [2] concluded that regular and sufficient maintenance is critical to retain the longterm permeability of PAPs. This conclusion is also applicable to maintain the long-term hydraulic performance of pervious concrete pavement [25].
When PAPs are severely clogged and non-destructive cleaning methods cannot restore the infiltration of PAPs to meet hydrologic design goals, the surface milling can be used as the restorative maintenance method. Typically, the milling depth is down to the choker layer and a new PA course is repaved [11]. The shallow milling was attempted and reported recently by Winston et al. [66], which is considered as a rational restorative method since the clogging materials are mainly accumulated on the top of permeable pavements [2]. Winston et al. [66] tested shallow milling at various depths of 0.5 cm, 1.5 cm, and 2.5 cm (0.2 in., 0.6 in., and 1 in.) for the PAP in Luleå, Sweden, which has a total thickness of PA surface of 4.5 cm (1.8-in.). The debris control and removal during and after the milling process are crucial to prevent the clogging of the milled surface. This study removed debris after milling using pressure washing. As shown in Fig. 3b, the shallow milling method was more effective than nondestructive cleaning methods to restore permeability, and the milling to the depth of 2.5 cm (0.6 in.) was the only restorative maintenance method that can restore the infiltration to the rate of this PAP when it was newly paved [66].

Winter maintenance
Winter maintenance treatments, such as sanding, snow plowing, and applications of anti-icing and deicing additives, are used to improve traction and eliminate snow and ice on pavement surfaces to provide safe driving conditions. Sanding is not recommended or even prohibited for PAPs [2,6,11,17,21], because the applied fine gravels or sands and their crushed products by vehicles can cause severe clogging and significant reduction of the surface permeability [2,10,15]. When sands have to be applied, it is recommended to use relatively large-size sands with minimal dust [2], and vacuum sweeping should be performed to remove sands and debris [6].  1 The infiltration rate of this project is calculated based on the average testing results along the center of the pavement. 2 The initial infiltration rate of this project is not available and the dashed line is extrapolated Mechanical sweeping without vacuuming should be avoided, because a regenerative street sweeper may spread out sands or debris over the permeable surfaces to cause further clogging [31].
The snow plowing is operated as routine practice for PAPs after snow events. However, plow damages on PAPs were reported by Briggs [9] and Lebens and Troyer [32]. Practices to reduce plow damages include the use of nonmental blades [11], the use of wide blades, avoidance of back-blading [30], and raise of the blade. However, Hein [21] mentioned special blades are not necessary, and UNHSC [58] did not recommend raise of blades. The plowed snow should not be piled on permeable pavements [11], because snow plowing also collects dust and debris that can clog permeable pavements as snow melts [6]. In case the snow piling on permeable pavements is unavoidable, vacuum sweeping is recommended to clean the permeable surfaces after snow melts.
Salts are commonly applied on impermeable pavements before and after snowstorm events to control and reduce compacted snow and black ice. Application of salts has to be reduced for PAPs because salts can flow with snowmelt water and pollute underground water in full-infiltration and partial infiltration applications. Indeed, it is found that salts can be significantly reduced for winter maintenance of PAPs in cold regions. This conclusion is supported by case studies of the PAP parking lot in the University of New Hampshire (UNH) Campus, and PAPs installed in Robbinsdale, MN. The UNH parking lot study tested various salt application rates on both PA and dense-graded asphalt parking lots. The statistical analysis indicated a salt reduction of 64%-77% on PAP was feasible to maintain equivalent or better surface conditions, which were quantified by the coverage area of snow and ice and pavement skid resistance [23,50]. Wenck Associates [64] monitored and compared plowed but unsalted PAPs with plowed and salted/unsalted dense-graded asphalt pavements. It was found that the unsalted PAP had a comparable area of uncovered surfaces by snow and ice, but a few to several hours of time lag were observed between salted dense pavement and unsalted PAP, depending on the conditions of air temperature and solar radiation. The salt reduction for PAPs could be attributed to two reasons at least. The first one is that the PAPs warm up more rapidly than the conventional asphalt pavement as air temperature increases [8,50]. This leads to a faster snow melting rate on PAPs than conventional pavements, which was observed in MnROAD test sections, especially in the event of light snowfall followed by a sunny day [32]. The second attribution is that, except for freezing rain events, salts are not needed to prevent black ice on PAPs, because the melting snow and ice can drain off from PAPs promptly without standing water ( [23,57], and [50]). Thus, the PAP is a viable option to reduce salt application for winter road maintenance in cold regions.

Conclusions and recommendations
In cold regions, the PAP works as a green infrastructure tool to improve the resilience of cities and winter transportation systems. This paper synthesized case studies, literature, and specifications to address specific concerns of design, construction, and maintenance of PAPs in cold regions. These concerns include frost damages of PAPs, freeze-thaw durability of ATPB and PA mixtures, construction of PAPs in cold weathers, and maintenances of PAPs for permeability and snow and ice control in winters. The conclusions and recommendations include: (1) Based on the established case study repository in this paper, the type of subgrade soil (sand or clay) greatly affects the structural and thermal performance of PAPs in cold regions. In general, PAPs exhibited a lower risk of frost damage than impervious pavements due to lower frost penetration depth and rapid response to warm air temperatures. In areas with low risks of frost damages, the design of the total thickness of PAP exceeding 65% of the local frost depth could be used. While in areas with high frost-susceptible soils, it is recommended to design the total thickness of PAP equivalent to the local frost penetration depth.
(2) The guideline of the mix design of PA mixture in cold regions is summarized in this work following the national design guidance, specifications of northern state DOTs and state asphalt pavement association in cold regions of the U.S., and research outcomes from universities. Generally, the use of polymer-modified asphalt binder with higher binder content and/or anti-stripping additives is recommended to improve the freeze-thaw durability and reduce the raveling of PA mixtures. Warm mix asphalt technology is also recommended for PA mixtures in cold regions. The uses of these additives increase the material and construction costs of PAPs. However, potential savings can be achieved from stormwater management, environmental benefits, and winter maintenance operations. Thus, the life cycle cost analysis for PAPs needs to be conducted with considerations of urban and transportation resilience benefits and impacts of climate change and extreme events, such as floods and snowstorms, on road infrastructures.
(3) Regular restorative maintenance methods, such as mechanical sweeping, pressure washing, and vacuum sweeping, can only restore the permeability of PAPs partially, but these practices are critical to maintain the longterm performance of PAPs and meet hydrologic design goals. Shallow surface milling is an emerging and promising method to effectively restore the permeability of PAPs, while methods of collection and cleaning of dust need to be developed to minimize clogging after shallow milling. Future works are also needed to explore other innovative design and cleaning methods for PAPs to maintain long-term permeability.
(4) Sanding should be prohibited for winter maintenance of PAPs. Snowplowing could be operated routinely after snow events. PAPs allow lower salts application for snow and ice control due to the rapid responses to ambient temperatures and prompt drainage of meltwater. To understand the fundamental mechanism of rapid responses to air temperature and fast thawing of snow and ice on PAP surfaces, research works are needed to develop theoretical analyses to investigate the internal heat transfer process in PAPs based on heat transfer and fluid flow in a porous medium.