Annotated Literature Review for NCHRP Report 640 (2009)

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279 1.75.6 Rehabilitation Practices Caltrans currently only allows removal and replacement for rehabilitating failed or aged OGFC layers. 1.75.7 Performance No performance measures were given. 1.75.8 Structural Design Caltrans has assigned a gravel factor of 1.4 to OGFC mixes. However, the document also states that the contribution of the OGFC to structural capacity is not considered. 1.75.9 Limitations The guidelines provide several instances on where OGFCs should not be utilized which include: • Unsound Pavements: OGFCs should not be considered as a solution for structurally unsound pavements. The existing pavement must be properly prepared prior to placing OGFC layers. • Snowy or Icy Areas: OGFCs should not be placed in snowy or icy areas where tire chains, studded tires or snowplows are commonly used. The guidelines do state, however, that the use of modified asphalt binders may assist in resisting the abrasive forces of tire chains, studded tires and snowplows. • Areas with Severe Turning Movements: OGFCs should not be used in parking areas, intersections, ramp terminals, truck stops, or weigh stations. • Muddy and Sandy Areas: OGFCs should not be used in areas where dirt, debris, mud, or sands can be tracked onto the surface. These materials will clog the OGFC void structure. Areas that may be prone to dirt, debris, mud or sand include agricultural areas, areas near beaches or areas near sand dunes. • Areas Prone to Oil and Fuel Drippings: OGFC should not be used in areas with a high potential for oil or fuel drippings. These drippings can soften the asphalt binder within the OGFC and lead to rapid deterioration. Intersections are an area prone to oil and fuel drippings. • Bridge Decks: OGFC should not be used on bridge decks without special approval. • Digouts and Localized Areas to be Removed and Replaced: OGFC should not be placed in areas where a bath tub effect can be created. • Cold Climate Areas: OGFC should not be placed in cold weather (< 45°F). 1.76 Alvarez, A.E., A. Epps Martin, C.K. Estakhri, J.W. Button, G.J. Glover and S.H. Jung. “Synthesis of Current Practice on the Design, Construction, and Maintenance of Porous Friction Courses.” FHWA TX-06/0-5262-1. Texas Transportation Institute. College Station, Texas. July 2006. 1.76.1 General This synthesis provides an excellent overview on the use of OGFC worldwide. Chapters within the report cover advantages/disadvantages, mix design methods, construction and maintenance practices, and performance.

280 1.76.2 Benefits of Permeable Asphalt Mixtures Benefits listed by the authors were divided into three categories: safety improvements, economic benefits and environmental benefits. Benefits related to safety were predominantly related to wet weather conditions. For this reason, the characteristic permeability of OGFC mixes was highlighted as the most important property related to the safety benefits of OGFC mixes. Because of the ability to drain water from the pavement surface, hydroplaning is basically eliminated. The relatively high permeability of OGFC mixes also greatly reduces splash and spray. Since water drains into the pavement layer, it is not available to create splash and spray. Splash and spray are generally described together; however, the two terms refer to different phenomena. Spray is very fine water particles that are created by rolling tires advancing on wet pavements. Splash is coarser water particles created when tires pass over pools of water on the pavement surface. Both splash and spray contribute to reduced visibility. The reduction in visibility due to splash and spray can be more pronounced than fog because the water droplets in the combined effect of splash and spray have a higher density and are larger in size. OGFCs also reduce glare, especially at night. The high macrotexture of OGFC layers helps to diffuse the reflections of light during both darkness and daylight. This characteristic improves the visibility of pavement markings. This improvement is also obtained during wet weather since less water is available on the pavement surface. Wet frictional properties are also improved when compared to dense-graded HMA. This benefit is importantly achieved at higher speeds on porous layers. However, at lower speeds OGFCs and dense-graded layers perform similarly during wet weather. The authors state that the use of porous mixtures results in reduced fuel consumption due to improved smoothness generally achieved by OGFC. This benefit was the only one identified as related to economics. Additionally, it has been suggested that the rate of tire wear is decreased when driving on OGFC layers. The primary environmental benefit identified by the authors was the reduction in tire/pavement noise levels. An additional benefit mentioned was that PFC mixtures produce a cleaner runoff than dense-graded HMA layers. This is based upon work that showed lower total suspended solids, total metals and chemical oxygen demand measured on runoff from PFC layers. The authors state that the noise reduction capacity of OGFC layers is an important benefit to reduce or control highway noise levels. The noise reducing capabilities of OGFC mixes have motivated the use of porous mixtures in Europe while the safety benefits have been the primary motivation for using these mixes in the US. A number of references were cited that provided various decreases in noise levels when OGFC mixes are utilized. In general, OGFC mixes reduce noise levels by 3 to 6 dB(A) when compared to dense- graded surfaces. Because of the reduced noise, drivers have a higher comfort level when driving.

281 When comparing the noise reduction of porous mixtures to noise barriers on a unit cost basis, porous mixtures are 2.5 to 4.5 times more efficient. Further, OGFC is more advantageous because the noise is reduced at the source (tire-pavement interface). 1.76.3 Materials and Mix Design The authors provide an excellent overview of numerous methods for designing OGFC mixes. Following are brief summaries of each mix design method discussed by the authors. Federal Highway Administration (FHWA) Method In December of 1990, the FHWA provided technical guidance on the use of OGFC. Included within this guidance was a method for designing OGFC. This method is based on the evaluation of surface capacity of the “predominant aggregate fraction” by immersing and draining the aggregate in S.A.E. No. 10 lubricating oil. Aggregate passing the 9.5 mm (3/8 in.) sieve and retained on the 4.75 mm (No. 4) sieve is considered the predominant aggregate fraction. Using the surface constant value (Kc) determined from the percent retained lubricating oil and the apparent specific gravity of t he aggregate fraction, the asphalt binder content is calculated. The next step entails taking into account the volume of asphalt binder and the target air void content (15 percent) so that the percent of fine aggregate can be calculated. If the air void content at the selected asphalt binder content is not 15 percent or higher, then the gradation of the predominant aggregate fraction should be modified. Two additional tests are also required within the FHWA method once the mixture has been designed. First a test to evaluate draindown potential is run in order to establish optimum mixing temperature. Finally, a test for evaluating the mixture’s resistance to moisture susceptibility is conducted. National Center for Asphalt Technology (NCAT) Mixture Design Method NCAT recommended a mix design method for a new-generation OGFC based upon experiences of different states in the U.S., progress in Europe and internal research. This method includes four primary steps: 1) materials selection; 2) selection of design gradation; 3) determination of optimum asphalt binder content; and 4) evaluation of moisture susceptibility. Table 133 provides the recommended requirements for aggregates. NCAT recommended that polymer modified asphalt binders and fibers be used for medium to high traffic volumes and that either polymer modified asphalt binders or fibers be used for low traffic roadways. A single gradation band was recommended form which the design gradation should be developed (Table 134). The design gradation should ensure a high air void content and provide stone-on-stone contact within the coarse aggregate fraction (fraction retained on the 4.75 mm sieve.

282 Table 133: NCAT Recommended Aggregate Properties Parameter Specified Value Los Angeles Abrasion, % 30 max. Fractured Faces, % 90 max with two or more faces 100 max with one face Flat and Elongated Particles, % 5 max at 5:1 20 max at 3:1 Fine Aggregate Angularity 45 min Table 134: NCAT's recommended Gradation Requirements Sieve Size, mm Percent Passing 19.0 100 12.5 80-100 9.5 35-65 4.75 10-25 2.36 5-10 0.075 2-4 Stone-on-stone contact is evaluated by comparing the voids in coarse aggregate of the compacted mixture (VCAMIX) to the voids in coarse aggregate of the coarse aggregate fraction in a dry-rodded condition (VCADRC). The VCADRC is determined in accordance with AASHTO T19, Bulk Density and Voids in Aggregate. When the VCAMIX is less than the VCADRC, the mixture is considered to have stone-on-stone contact. In order to select the design gradation, three trial blends are developed. Each trial blend is then mixed with 6.0 to 6.5 percent asphalt binder and compacted with 50 gyrations of the Superpave gyratory compactor. The design gradation is selected based upon the existence of stone-on-stone contact and the desired air void content (18 to 22 percent). Optimum asphalt binder content (step 3) is selected based upon the results of laboratory tests conducted on mixtures comprised of the desired gradation and various asphalt binder contents. Table 135 summarizes the laboratory tests and specification limits recommended by NCAT. Once a mixture has been successfully designed, it is then subjected to moisture susceptibility testing. A modified Lottman method is recommended that includes five freeze/thaw cycles. A minimum tensile strength ratio of 80 percent is required. Table 135: Specifications for Selecting Optimum Asphalt Binder Content Parameter Specification Draindown Test, % 0.3 max Air voids, % 18 min Cantabro Abrasion on Unaged Samples, % 20 max Cantabro Abrasion on Aged Samples, % 30 max

283 The authors also reported on additional research conducted by NCAT to refine the new- generation OGFC mix design method. Some of this recent research confirmed that the design gyration level, 50 gyrations, is appropriate for the design and control of OGFC. Historically, the Cantabro Abrasion test had been conducted on specimens compacted with the Marshall hammer that are both unaged and aged for 120 hours at a temperature of 85°C (185°F). Work by NCAT confirmed that specimens compacted with the Superpave gyratory compactor could be used for the Cantabro Abrasion test. However, NCAT found the testing of aged specimens not necessary since there were only slight differences between results from unaged and aged samples. Requirements for the Cantabro Abrasion were a maximum of 20 percent loss on unaged samples. Within the original research project, NCAT recommended determining the bulk specific gravity of OGFC samples using volumetric measurements of compacted specimens. Subsequent research recommended using a vacuum sealing method for determining bulk specific gravity. When this method is used, a double bag set up was recommended. Lower air void contents should be expected when using the vacuum sealing method when compared to the volumetric calculation. Therefore, when using the vacuum sealing method, the minimum air void content during design should be 16 percent instead of 18 percent. The final refinement was to reduce the number of freeze/thaw cycles during moisture susceptibility testing from five to one. Texas Department of Transportation Permeable Friction Course Design The Texas Department of Transportation (TxDOT) specifies two types of PFC depending upon the type asphalt binder used in the mix. The two binder types include a polymer modified asphalt binder with a minimum high temperature grade of PG 76-XX or a TxDOT Grade C or B crumb-rubber modified asphalt binder. When using crumb-rubber, a minimum of 15 percent, by mass of asphalt binder, is required. PFC mixes utilizing polymer modified binders include 1 to 2 percent lime and 0.2 to 0.5 percent fibers. Gradation requirements are based upon the type asphalt binder used in the mix. Table 136 provides TxDOT’s gradation requirements and asphalt binder requirements. Aggregates must meet coarse aggregate angularity, deleterious materials, soundness, Los Angeles Abrasion, Micro-Deval and flat and elongated requirements. Table 136: TxDOT Master Gradation Band and Binder Content Sieve Size, mm PG 76-XX Mixtures Crumb-Rubber Mixtures 19.0 100 100 12.5 80-100 95-100 9.5 35-60 50-80 4.75 1-20 0-8 2.36 1-10 0-4 0.075 1-4 0-4 Asphalt Binder Content, % 6.0-7.0 8.0-10.0

284 Samples of the PFC are compacted in the Superpave gyratory compactor using a design number of gyrations of 50. Initially, two replicates for each of three binder contents are mixes and compacted. Optimum asphalt binder content is selected based upon the target air voids of 18 to 22 percent. TxDOT utilizes a minimum asphalt binder content requirement of 6 percent. Mixtures prepared at the selected optimum asphalt binder content are then prepared to evaluate draindown, moisture susceptibility and durability. A maximum draindown of 0.2 percent is specified using a draindown basket. Moisture susceptibility is evaluated by boiling loose mixture in water for 10 minutes and visually evaluating the percentage of stripping immediately after boiling and again after 24 hours. Durability is evaluated using the Cantabro Abrasion test with a requirement of 20 percent loss maximum. Danish Mixture Design Procedure The Danish method for designing porous asphalt utilizes the Marshall hammer for compacting samples. A target air void content of 26 percent is used during design. In order to limit excessive asphalt binder contents, the Danish use a draindown test. For rutting, the Hamburg Wheel Tracking test is conducted while for durability the Rotating Surface Abrasion test is used. The premise of the mix design method is to maximize air void contents to provide improved functional properties while also maximizing asphalt binder content to provide durable wearing layers. To assist in these objectives, the Danish generally utilize SBS modified asphalt binders, cellulose fibers, hydrated lime and limestone filler. The Netherlands Mixture Design Method The design of porous asphalt in the Netherlands is based upon the compaction of samples using 50 blows per face with the Marshall hammer. A minimum air void content of 20 percent is specified for the Marshall compacted specimens. Porous asphalt gradations have maximum aggregate sizes of 11 and 16 mm with a requirement for crushed aggregates. Penetration-graded asphalt binders are used; however, polymer modification is not generally used. Australian Mixture Design Method The design of porous asphalt in Australia includes 80 gyrations of the Australian gyratory compactor. Three maximum aggregate size gradations are used in Australia, 10, 14, and 20 mm. Two classes of porous asphalt are specified in Australia depending upon the expected traffic volume. Type I porous asphalt is suggested for roads with lower traffic volumes while Type II is suggested for higher traffic volumes. For the Type II porous asphalt mixtures, between 0.3 and 0.5 percent fibers are identified as a method for preventing draindown. Modified asphalt binders and hydrated lime are also suggested for the Type II porous asphalt.

285 Minimum asphalt binder contents are established using the Cantabro Abrasion test while air voids and draindown are used to establish a maximum asphalt binder content. Air voids are determined after compaction using 80 gyrations of the Australian gyratory compactor. Table 137 summarizes the requirements for designing porous asphalt in Australia. Table 137: Summary of Design Requirements in Australia Design Property Type I Mixture Type II Mixture Cantabro Abrasion, Unconditioned % 25 max 20 max Cantabro Abrasion, Moisture Conditioned % 35 max Air Voids, % 20 min 20-25 Draindown, % 0.3 max 0.3 max Belgian Mix Design Method In Belgium, aggregate gradations are optimized using a software program entitled PradoWin (Programs for Road Asphalt Design Optimization). Gradations are required to have between 81 and 85 percent “stone fraction” (material larger than 2 mm). The “sand fraction” (material retained on 0.063 mm and passing 2mm) must be between 11 and 13 percent of the aggregates and the filler fraction (passing 0.063) must be between 4 and 6 percent. Maximum and minimum asphalt binder contents are selected based upon volumetric properties and the Cantabro Abrasion loss test. In order to select a maximum asphalt binder content, porous asphalt samples are compacted with the Marshall hammer at varying asphalt binder contents. A minimum of 21 percent air voids is specified. To select a minimum asphalt binder content, the Cantabro Abrasion loss test is conducted. A maximum of 20 percent loss is specified when testing is conducted at 18°C. 1.76.4 Construction Practices During the production of OGFC mixes, special attention has to be paid to the moisture within stockpiled aggregates. Control of the moisture enhances the ability to properly control the temperature and homogeneity of the produced mix. Some agencies have a requirement to maintain the stockpiled aggregates at a moisture content near the saturated –surface dry condition. Additionally, some agencies require that at least two days of reserve aggregates be maintained at the production facility. The authors state that conventional asphalt production plants can be used to produce OGFC. The primary modifications required are a method of introducing fibers and the use of polymer-modified asphalt binders. A fiber feed device is the most common method of introducing fibers. The authors state that pelletized fibers generally include some amount of asphalt binder within each fiber in order to bind the fibers together. When placed in contact with the heated aggregates, the asphalt binder within the pellets

286 melts and allows the fibers to be distributed within the mixture. The authors indicate that the asphalt binder included within the fibers should be considered as part of the asphalt binder in the mixture. When producing with a batch plant with fibers, both the dry and wet mixing times should be increased. This enhances the distribution of the fiber within the mixture. Because almost a single size of aggregate is used to comprise an OGFC’s gradation, all screen decks should be inspected prior to beginning production to prevent hot bins from being overridden. Because of the potential for draindown within OGFCs, close control of production temperatures is needed. Some agencies have limited the mixing temperature to minimize the potential for draindown. Typical maximum temperatures referenced by the author included values from 320 (160) to 347°F (175°C). Spanish standards establish a maximum mixing temperature based upon the type plant used to produce the mixture. The maximum temperature for OGFC produced by drum plants is 311°F (155°C) while for batch plants the maximum temperature is 338°F (170°C). However, some agencies simply recommend a target viscosity for the asphalt binder to be maintained during mixing. The FHWA recommends a target viscosity of 700 to 900 centistokes and within the Design Manual for Roads and Bridges in Britain recommends 0.5 Pa-s. Also because of potential draindown problems, some agencies limit the amount of storage and transportation times. Typical time limits included for storage are between 1 and 12 hours. The FHWA suggested both a time and distance requirement that for combined handling and hauling of OGFC mixtures should not be more than 40 miles or 1 hour. In Britain, a maximum time limit of 3 hours is specified for mixing, placement and compaction. In order to minimize the amount of cooling that takes place during transportation, it is common for agencies to require the use of tarps. Maintaining the heat during transportation will help prevent crusting of the OGFC. In Britain, insulated trucks are also required during transportation. Acceptable release agents are also recommended for truck beds. This is especially true when polymer-modified binders are utilized in the OGFC. OGFC layers should not be considered for correcting surface defects. Prior to placing an OGFC layer, any pavement deficiencies should be corrected. Additionally, areas prone to holding water should be corrected. Areas of the existing pavement that hold water can increase the potential for moisture damage in underlying layers. All OGFC layers should be placed over an impermeable layer by applying an appropriate tack coat and sufficient cross-slope. In Britain, a minimum cross-slope of 2.5 percent is specified. The FHWA suggested a tack coat application of 0.05 to 0.1 gallons per square yard to seal the underlying layer. As with the construction of any HMA layer, the paver should advance continuously with a minimum number of stoppages. Continuous stoppages will result in an OGFC layer

287 that is not constructed smooth. The authors state that the use of a remixing materials transfer device should be considered when constructing OGFC. When a materials transfer device is not used, it is important to minimize the number of cold lumps and crusting that occurs during transportation. When pavers with extendible screeds are used, auger extensions should be used to avoid irregular distribution of the mixture from the center of the paver to the ends of the screed. Prior to beginning paving, the screed on the paver should be heated. Cold screeds will cause excessive pulling of the placed mat. Handwork should be minimized for OGFC. A typical manner of defining acceptable paving conditions is to define a minimum ambient temperature. Within the US, an air temperature of 60°F (15°C) is common. The British Manual of Contract Documents for Highway Works also specifies a maximum wind speed as part of acceptable paving conditions. When compacting OGFC mixtures on the roadway, static steel-wheel rollers are the most common type of roller used. Typically, two to four passes of an 8 to 9 ton roller is appropriate for compacting OGFC. In Britain, five passes is recommended; however, lift thicknesses in Britain are generally 2 inches. Heavy rollers should be avoided as they can lead to excessive breakdown of the aggregates. Pneumatic rollers should not be used because the kneading action of these rollers reduces the drainage capacity by closing the surface voids of the mixture. Vibratory rollers should not be used. The compactive energy created by vibratory rollers can break down the aggregates within the layer. Longitudinal and transverse joints require special attention. Transverse joints should be minimized if at all possible. Where transverse joints are required, they can be formed by using lumber attached to the underlying layer to create a vertical face. When this method is used, no tacking of the vertical face is needed. However, when a saw cut technique is employed, a small amount of asphalt binder is needed to enhance adhesion. The transverse joint can be formed by placing the screed flat on the existing OGFC approximately 1 ft (0.3 m) before the joint. The new mixture should be allowed to advance in the paver until it reaches the front of the screed. Transverse joints should be cross-rolled. A vibratory roller can be used when constructing transverse joints. The preferred method of constructing longitudinal joints is echelon paving. When a cold longitudinal joint is required, the joints should not be located within wheel paths or next to pavement lane markings. Longitudinal joints are constructed by placing mixture approximately 0.06 in (1.5 mm) above the existing mat and compacting the mat. The authors state that longitudinal should be sawed prior to placement of the hot side and that a small amount of asphalt binder should be placed on the cut face to promote adhesion. The sawn face should not be completely covered with asphalt binder as that would block the lateral flow of water. Acceptance of produced and placed OGFC generally entails asphalt binder content and gradation. Only qualitative measures of compaction processes are generally conducted. These qualitative evaluations are generally to assess density material variability and

288 segregation. Essentially all agencies specify a minimum value of smoothness. In Britain, a specified permeability of the OGFC layer is required which is tested before traffic is allowed to pass on the pavement. In Spain, acceptance is based upon air voids within the mixture compared to a reference air void content. 1.76.5 Maintenance Practices Within this report, maintenance was divided into two categories: winter maintenance and surface (general) maintenance. The authors state that maintenance is a fundamental aspect that must be considered for OGFC because maintenance activities on these pavement types are different from for conventional HMA pavements. Open-graded mixtures have a different thermal conductivity than conventional HMA layers. Because of the relatively high air void contents contained in OGFC layers, the flow rate of heat through the mixture is reduced. The authors cite a source that says an OGFC may have 40 to 70 percent of the thermal conductivity of dense-graded HMA layers. Because of the differences in thermal properties, the surface of an OGFC layer during cold weather can be between 1.8 and 3.6°F (1 and 2°C) lower than the surface temperature of a nearby dense-graded layers. This results in earlier and more frequent frost and ice formation on OGFC layers than conventional HMA layers. Therefore, it is expected that winter conditions (frost, ice, snow, etc.) will stay on OGFC for longer periods of time. Formation of hazards like black ice and extended frozen periods are considered the main problem with OGFC layer maintenance within the US. Because of the issues highlighted above, OGFC layers require different winter maintenance practices than dense-graded HMA layers. Some agencies employ the use of pavement condition sensors, meteorological instrumentation, and connecting hardware and software to assist in the decision process for winter maintenance of OGFC. OGFC layers generally require more deicing agents as well as more frequent applications. In Texas, deicing agents are currently considered the most effective winter maintenance treatment followed by liquid deicers and sand. However, the use of sand or other abrasive substances to improve friction contributes to the clogging of OGFC layers. One problem with many deicers is that the materials can flow into the void structure of the OGFC. The authors cite work in Oregon which states they have conducted research that states organic deicers with higher viscosity and electrostatic charges (similar to emulsions) may improve the bonding of the deicing materials to the OGFC surface. In Belgium, “intensive” application of deicing materials to OGFC layers is needed to provide similar conditions between dense-graded and HMA surface layers subjected to snowy weather. Also, more frequent applications and about 25 percent deicing material per application are reported in the Netherlands. The use of liquid chloride solutions has been reported as more effective than salt in the Alpine regions of Italy, Austria, and Switzerland. In Britain, the practice is to apply preventative salting just before a snowfall with more frequent applications of salt compared to dense-graded HMA. They also

289 recommend additional salt near transitions between OGFC and dense-graded surfaces because there is a reduction in the transfer of salt from the OGFC and dense-graded HMA. When snow plowing is utilized (which is promptly after a snow), they utilize plows fitted with rubber edges on the blades to prevent the blades from damaging the OGFC. However, a Japanese study concluded that the performance of OGFC surfaces and dense-graded surfaces were similar during the winter; therefore, winter maintenance practices were similar. Based upon cited work, the authors state that within the US there is no major general maintenance programs employed for OGFC surfaces. About the only maintenance activity that is utilized is the use of fog seals and these are only used by a very few US agencies. The authors also state that there is no quantitative evidence that fog seals are effective preventative maintenance techniques. Some research in Oregon has shown reductions in permeability and changes in pavement friction when fog seals are applied; however, it was concluded that the mixtures maintained some level of permeability and macrotexture which still preserved the ability to reduce draindown potential. The authors state that it is expected that fog seals would extend the life of OGFCs since that provide a small film of unaged asphalt binder at the pavement surface. The authors state that cleaning of OGFC layers to improve permeability is not common practice in the US. The authors state that this indicates that local agencies accept that OGFC functionality is maintained due to the ability to self-clean. Current general maintenance activities in Denmark include cleaning OGFC layers by high pressure water and air suction twice a year. 1.76.6 Rehabilitation Practices Rehabilitation practices within the report can be divided into minor or major rehabilitation activities. Minor rehabilitation practices include crack sealing, pothole repair, and patching of delaminated areas. When repairing potholes or localized areas of distress, the FHWA advises that one must consider the area and drainage continuity when selecting the rehabilitation technique. In Britain, dense-graded patches are limited in size to 1.64 ft by 1.64 ft maximum. Small areas that are in need of patching can be patched with dense-graded materials as the flow can be maintained around small areas. Large areas should be patched with OGFC mixes or other types of mix that allow water to drain through the pavement. Dense-graded HMA placed in large areas will prevent water from draining. When dense-graded HMA is used to patch OGFC layers, the patch should be such that it forms a diamond shape. The diamond should be rotated 45 degrees such that water will flow around the patch. Work in Oregon indicated that machine patches, blade patches, or screed patches with OGFC may be used. This is especially true if some OGFC material still remains in the repair area.

290 Major rehabilitation practices mentioned included mill and inlays. This technique was recommended by Oregon when quantities of patching materials justified this technique. Some DOTs state that mill, recycle and inlay is the preferred major rehabilitation technique. This technique is also used in the Netherlands. In most cases, the milled OGFC is replaced with new OGFC. The practice of placing dense-graded HMA directly over OGFC as a rehabilitation technique is not recommended. Reports from the Netherlands indicate that recycled OGFC kept approximately the same level of permeability and that durability is about the same as new OGFC layers. 1.76.7 Performance The authors divide performance into one of two categories: durability and functionality. Durability includes issues such as moisture sensitivity and aging potential. Functionality considers noise reduction and permeability. The most prevalent distress reported for OGFC layers is raveling. This distress is related to durability. Raveling in OGFC layers is often characterized by its rapid progression. When raveling occurs rapidly, the layer can disintegrate within a few weeks or few months. Raveling can be associated with the aging of the asphalt binder coating the aggregates or from softening of the binder film due to oil and fuel drippings. As the asphalt binder ages (oxidizes and age-hardening), the asphalt binder film coating the aggregates becomes brittle. When this occurs, the binder cannot accommodate the strain from traffic loadings and results in brittle failure. The authors cite work that suggests a critical asphalt binder penetration of approximately 15 (1/10 mm) and softening point of 158°F (70°C) where OGFCs will fail at low temperatures. The same reference states that the addition of hydrated lime and the use of higher asphalt binder contents will result in lower rates of age hardening and prolong the durability of OGFCs. The authors state that understanding the impact of field aging is an important factor in the design of OGFC mixes. They provided several areas where the aging of OGFC mixes is different from typical dense-graded HMA layers: • OGFC layers are placed on pavement surfaces where oxidation rates are higher. • OGFC mixtures, because of their high permeability, will provide better access of oxygen to the asphalt binder film, tending to increase oxidation rates. • Thicker asphalt binder films will serve to reduce the oxygen transport rates into the asphalt binder, thereby, slowing oxidation. • The thicker asphalt binder films in OGFC mixtures will favorably affect the impact of aging on durability compared to dense-graded HMA which have thinner films of asphalt binder. • Fibers used in some OGFCs may act to reinforce the asphalt binder film and minimize the effects of age hardening. • The presence of hydrated lime may retard the effects of binder aging.

291 • Use of polymer-modified asphalt binders may have a beneficial impact on age- related durability. As shown in Table 138, the reported service lives of OGFC mixes is very variable and has a range from 6 to 15 years. One of the factors that most influences the durability of OGFC mixtures is asphalt binder type. The majority of agencies that have reported successful use of OGFC are utilizing modified asphalt binders. Table 138: Typical Reported OGFC Mixture Service Life Typical Mixture Service Life, yrs. Type of Mixture Country 8 or more OGFC United States 13 Rubber-modified OGFC (Arizona) United States 15 OGFC (Wyoming) United States 6 to 8 OGFC (TxDOT Project) United States 7 to 10 Porous Asphalt United Kingdom 7 Porous Asphalt Denmark 8 to 12 Porous Asphalt France The primary performance characteristic related to functionality of OGFC is the high air void content and, thus, permeability. OGFCs will become clogged over time. In an effort to prolong the effect of clogging, some agencies tried to utilize larger aggregate size gradations. Larger aggregate size gradations would result in larger voids. However, major changes in the ability to maintain permeability were not observed. Another method evaluated for maintaining permeability was to design OGFC to higher air void contents. During the 1980’s in Spain, OGFC mixes were design with air void contents between 15 and 18 percent. However, these mixes became clogged after a relatively short time. After 1986, they began designing mixes with air void contents above 20 percent and showed an increased functional life. Permeability was maintained for a longer period of time. Clogging is delayed when suction forces produced by high speed traffic flushes debris from the void structure. Some have suggested that speeds should be higher than 44 mph (70 km/hr) to minimize clogging in OGFC. A minimum speed of 31 mph (50 km/hr) has been reported by others. Determination of permeability in the laboratory and field is an important part of designing mixtures that will maintain permeability. However, the authors state that the measurement of permeability is not widespread. Several mix design methods do include permeability measurements in the laboratory. Different equipment is needed, though, to measure permeability in the field. The common approach to measuring permeability in the field is to measure a discharge rate for a specific volume of water. Several equipment were mentioned within the report including the IVT Swiss Federal Research Institute device, LCS drainometer used in Spain and a device being used by the Belgium Road

292 Research Center. A unique modification to the above described approach of specifying a discharge rate for a specific volume of water is the Zarauz permeameter. With this device, the water falls from a certain height onto the OGFC surface and flows freely into the pavement. Two parameters are mentioned with this test: the maximum radial distance advanced by the water before it penetrates into the OGFC and the total time required for the water to disappear from the pavement surface. 1.76.8 Structural Design Application of OGFC layers within structural design varies widely throughout the world. In Spain and Britain, OGFC and dense-graded layers are considered to provide similar mechanical responses. Similar conclusions were found by Oregon based upon deflection measurements. However, others have stated that the structural layer coefficients for OGFC should be 60 to 70 percent of the magnitude for dense-graded HMA. Based upon laboratory testing, researchers in Argentina indicated that OGFC had about 50 percent of the structural capacity of typical dense-graded layers. Most laboratory research studies that have compared the modulus of OGFC mixes to dense-graded HMA have shown that OGFC is not as stiff as dense-graded HMA. However, many references indicate that rutting is not a problem within OGFC layers. 1.76.9 Limitations The authors highlighted a number of disadvantages related to the use of OGFCs which included reduced performance, high construction costs, winter maintenance, and minimal contribution to pavement structural capacity. As stated above, performance can be categorized as durability and functionality. The primary durability issues are associated with raveling. According to a 1998 survey cited by the authors, service lives of 8 years or more were reported for OGFC and positive experiences were indicated by half of the survey respondents. For functionality, accelerated loss of permeability and noise reducing capability are the primary concern. Based upon work in Spain, OGFC layers with air void contents near 20 percent retained their permeability for 9 years when subjected to medium traffic, whereas, clogging was reported after 2 years in mixes subjected to heavy traffic. Construction of OGFC layers is more costly compared to dense-graded HMA layers. On a per ton basis, OGFC in the US will cost between 10 and 80 percent more than dense- graded HMA. Also, the life span of OGFC will be 50 to 100 percent that of dense-graded layers. The authors state that winter maintenance is considered a significant disadvantage of OGFC layers. Because of the different thermal properties of OGFC layers, larger amounts and more frequent application of winter maintenance activities is required. These activities result in higher maintenance costs for OGFC.