Annotated Literature Review for NCHRP Report 640 (2009)

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273 1.74.8 Structural Design No specifics on inclusion within structural design were given. 1.74.9 Limitations Open-graded mixes are not used in high elevation snow zones that require a significant amount of snow plowing. 1.75 “Open Graded Friction Course Usage Guide.” California Department of Transportation. Division of Engineering Services. Materials Engineering and Testing Services-MS #5. Sacramento, California. February 2006. 1.75.1 General This guidelines document prepared by the California Department of Transportation provides an excellent review of OGFC practices in California. The document includes a history of OGFC use in the US, information on materials and mix design, production and construction practices and maintenance practices. The document states that the initial interest in open-graded mixes grew out of an effort to avoid the shortcomings associated with chip seal construction. These shortcomings primarily involved broken windshields from loose aggregates and time constraint problems associated with setting aggregates during a sudden rainstorm. Oregon began experimenting with open-graded mixes during the 1930’s in an effort to improve the frictional properties of pavements. However, problems were encountered that were due to draindown issues. The draindown problems led to areas of asphalt binder rich locations and areas of low asphalt binder. In the 1940’s, the use of open-graded mixes began to increase within the western US. As early as 1944, California began to use this mix type for drainage interlayers and as an alternative to chip seals and slurry seals. In the 1980’s, Oregon accelerated the use of open-graded mixes when they developed their “F-mix.” The authors of this guidelines document define an OGFC as a sacrificial wearing layer. OGFC consists of a relatively uniform aggregate grading with little or no fine aggregates and mineral filler. These mixes are specifically designed to have large air void contents. The guidelines state that OGFC can be used in new construction, major rehabilitation projects and maintenance overlays. Layers of OGFC are sacrificial and are used in areas that experience high traffic volumes and moderate to heavy rainfall. When used as a maintenance overlay over a structurally sound pavement, OGFCs can renew the existing surface in terms of functional performance (i.e., ride quality). When used as a maintenance overlay, OGFC layers can rectify/retard the following distresses: raveling, oxidation, skid problems, hydroplaning, splash and spray, flushing and bleeding, and reflective cracking. The following are specific situations where placement of OGFC wearing layers should be considered:

274 • Hydroplaning: OGFCs should be used in areas that have shown a high potential for wet weather accidents. These mix types will reduce hydroplaning potential by providing drainage channels for water to flow beneath the pavement surface. As a result, good tire-pavement contact will be achieved. • Wet and Nighttime Visibility: OGFCs should be used in areas that have shown a high potential for wet weather and nighttime accidents. OGFCs will minimize the potential for splash and spray. Because the rain infiltrates into the pavement, less water is available at the pavement surface which reduces the amount of reflected light for oncoming traffic. • Skid Resistance: OGFCs should be used when an increased wet friction number is required for high speed areas. Due to the macrotexture within OGFC mixes, they result in higher friction numbers at higher speeds. At medium speeds, OGFCs and dense-graded surfaces may have similar frictional properties; however, the friction numbers do not decrease as rapidly with increasing speed on OGFC surfaces. • Location: OGFC should be used when the project is located adjacent to an existing OGFC such as lane addition projects. Placement of dense-graded layers adjacent to OGFC layers may impede the flow of water through the OGFC layers creating a “bath tub” effect. • Cross Slope: The guidelines state that OGFC should be considered when the cross slope is less than 2 percent and there are two or more lanes in one direction. However, the guidelines also state that OGFC should not be considered a solution to cross slope problems. • Roadway Smoothness: OGFCs should be considered as an alternative to chip seals and slurry seals when correcting minor surface irregularities. OGFCs should not be used to correct major rutting problems or depressions. • Oxidation Reduction: OGFCs should be considered in desert areas as a protective layer against oxidation of the underlying dense-graded layer. In this instance, the OGFC is a true sacrificial layer. • Mitigation of Flushing and Bleeding: OGFCs should be considered as a temporary solution to mitigating flushing and bleeding problems. OGFCs will improve frictional properties and the void structure of this mix allows absorption of the free asphalt binder from bleeding pavements. The guidelines emphasizes that this only provides a short term solution. OGFCs should not be used when life cycle cost analyses suggest significant savings with other surface types. OGFC is more expensive per ton than dense-graded HMA layers. However, OGFCs are generally placed at a lower unit weight (thickness) which partially offsets the higher costs per ton of OGFC. Using modified asphalt binders increases the costs of OGFCs but use of these binders does tend to increase the life expectancy of OGFCs. 1.75.2 Benefits of Permeable Asphalt Mixtures The document states that the most important benefit realized from OGFC is the increase in roadway safety during wet weather. The increased safety is due to providing maximum tire to surface contact during wet weather and a strong contrast in pavement

275 markings. When rain falls on an OGFC surface, the void structure allows the water to penetrate into the layer which allows for maximum contact between the tire and pavement surface. Also, without the water on the pavement surface, the potential for hydroplaning and splash and spray is greatly reduced. 1.75.3 Materials and Mix Design Guidance is provided on the selection of materials as well as mix design considerations. Coarse aggregates used in OGFC mixes should be polish resistant. They should also be hard, angular and abrasion resistant to provide stone-on-stone contact, deformation resistance and low susceptibility to clogging. The aggregate gradation controls the amount of macrotexture and drainage capacity of OGFC layers. Most of the US utilizes a ⅜ in nominal maximum aggregate size gradation while Caltrans allows both ⅜ in and ½ in gradations. In some instances where very high rainfall is expected, a 1 in gradation is sometimes allowed. Since OGFCs have very high in-place air void contents, air and water are able to come in contact with the asphalt binder coating the aggregates. Therefore, some states allow the use of soft asphalt binders to increase durability by prolonging the time for the binder to age-harden. Alternatively, other states use harder asphalt binders to permit higher mixing temperatures, thus, increasing asphalt binder film thickness to resist age-hardening. Caltrans currently allows the use of unmodified asphalt binders as well as polymer and rubber modified asphalt binders within OGFCs. In general, unmodified binders are used in normal applications and modified binders are used in cooler climates. The document states that OGFC durability is a function of initial binder viscosity and film thickness. Thicker asphalt films improve durability and overall performance. To select optimum asphalt binder content, several tests are utilized. First, an approximate asphalt binder content is determined using California Test 303, Method of Test for Centrifuge Kerosene Equivalent and Approximate Bitumen Ratio (ABR). This method utilizes the centrifuge kerosene equivalent (CKE) to determine the approximate asphalt binder content. Next, California Test 368, Standard Method for Determining Optimum Bitumen Content (OBC) for Open Graded Asphalt Concrete, is used to determine optimum. This test entails placing loose mixture prepared at the ABR into “extraction thimbles.” Next a 4 kg mass is placed on top of the mix and the assembled test samples are allowed to stay in an oven at 275°F (135°C) for 30 min ± 5 sec. After the prescribed time period, the mix is removed from the assembly and the amount of asphalt binder remaining in the assembly is measured [a measure of draindown]. This process is again conducted at asphalt binder contents ± 0.7 percent asphalt from the ABR. After all testing, the asphalt binder contents and average drainage at each asphalt content are plotted with asphalt binder content on the y-axis (form provided in the test method). Optimum asphalt binder content is defined as the asphalt binder content that provides 0.4 g of drainage.

276 Caltrans does not design to a minimum air void content. However, the guidance document states that mixes designed as described above would have design air void contents near or above 18 percent. The role of fine aggregates within OGFC is simply to fill the voids between the coarse aggregate particles. However, the fine aggregates should not bulk (push apart) the coarse aggregates. The percent passing the No. 4 (4.75 mm) sieve is limited to 15 percent, by volume. Caltrans specifies the optimum mixing temperature within its standard specifications. Use of anti-strip materials is made on the District level in California. 1.75.4 Construction Practices OGFC can be produced in plants typically used to produce dense-graded HMA. No specific modifications are required. Mixing temperature must be closely controlled in an effort to minimize the potential for draindown problems. Because of the potential for draindown problems, OGFC should not be stored in a silo for more than two hours. When storage silos are used, it is essential to observe the mix to identify if draindown is occurring. Draindown can also occur during mix transportation. During transport, best practices should be used to prevent loss of heat and to protect the mix from the weather. Best practices could include tarping the loads, selecting minimal haul times, utilizing material transfer devices, dumping directly into the hopper, etc. For projects with long haul times, the use of polymer modified binders was recommended to allow for higher mixing temperatures. Only non-petroleum type release agents should be used to coat truck beds. Petroleum based release agents will soften the mix and accelerate deterioration of the pavement. To ensure proper bonding of the new OGFC layer to the existing pavement, a good tack coat is required. The existing pavement surface should be structurally stable prior to placing the OGFC layer. Cracking and rutting should only be minimal, if existent, prior to placing OGFC. Substantial cracking may reflect up and rutting will provide areas where water will accumulate. Table 131 provides guidance on the relationship between ambient temperature and placement temperatures. Additionally, the wind must also be considered when constructing OGFC layers. Wind may reduce the temperature of the existing layer and the OGFC. Handwork should be minimized. Transverse joints are difficult to construct with OGFC. The guidance document states that butt joints work best. Longitudinal joints are constructed in a similar manner as dense-graded HMA layers. Caltrans utilizes a method specification to compact OGFC layers. These specifications indicate that two complete coverages with a static steel-wheel roller should be made for compaction. Vibratory and pneumatic tire rollers should not be used. Vibratory rollers will break

277 down aggregates within the OGFC and pneumatic tire rollers tend to pick up the OGFC. Also, the kneading action caused by pneumatic tire rollers tends to close the voids at the surface of the OGFC. Table 132 presents temperature limits for OGFC which also includes minimum temperatures for rolling. Table 131: OGFC Placement Temperatures Anticipated Atmospheric Temperature Best Practices In General • Follow the requirements in the Standard Specifications and SSPs • Continuously monitor temperature and wind. • Ensure that a tack coat is applied uniformly and at the proper rate. • Compaction of the OGFC pavement is complete after 2 complete coverages of the roller. • Stop operations if the critical temperatures* stated in Table Y cannot be met. Temp. > 70°F • For OGFC with conventional asphalt binder, complete rolling before the temperature of the OGFC reaches 195°F. • For OGFC with polymer modified binder, complete rolling before the temperature of the OGFC drops below 250°F. • For OGFC with asphalt rubber binder, complete the first coverage of the initial breakdown compaction when the temperature of the OGFC is greater than 275°F. Complete breakdown compaction before the temperature of the OGFC drops below 250°F. 55°F < T ≤ 70°F • For OGFC with conventional asphalt binder, complete rolling before pavement reaches 220°F. • For OGFC with polymer modified binder, complete rolling before the temperature of the OGFC drops below 250°F. • For OGFC with asphalt rubber binder, complete the first coverage of the initial or breakdown compaction when the temperature of the OGFC is greater than 280°F. Complete breakdown compaction before the temperature of the OGFC drops below 260°F. 45°F ≤ T ≤ 55°F • Polymer modified binders must be used. • For OGFC with polymer modified binder, complete rolling before the temperature of the OGFC drops below 250°F. Temp. < 45° • OGFC should not be placed. * Critical temperatures are the minimum atmospheric temperature, minimum pavement temperature, maximum aggregate temperature at plant and the mix laydown temperature range.

278 Table 132: OGFC Temperature Limits Type OGFC Minimum Atmospheric Temperature Minimum Pavement Temperature Maximum Aggregate Temperature at Plant Recommended Minimum Breakdown Rolling Temperature Recommended Minimum Finishing Rolling Temperature Conventional (Normal) 70°F * 275°F N/A 195°F Conventional (Cold Temp.) 55°F * 275°F N/A 220°F Polymer Modified 45°F * 325°F N/A 250°F Asphalt Rubber (Normal) 65°F 65°F 325°F 275°F 250°F Asphalt Rubber (Cold Temp.) 55°F - 65°F 55°F 325°F 280°F 260°F 1.75.5 Maintenance Practices The document lists the following as possible distresses encountered in OGFCs • Shear failures in high stress areas • Cracking due to fatigue • Cracking due to reflection from below • Raveling due to oxidation and hardening of the binder • Raveling due to softened binder from oil and fuel drippings • Raveling due to lack of compaction. • Delamination due to improper tack coat application. • Clogging of voids from mud, sand, etc. causing loss of permeability • Rich and dry spots due to draindown of binder during transportation and placement. Proper selection of materials, mix design, construction and project will help to minimize the occurrence of the above listed distresses. When distresses occur, the method of maintenance utilized should avoid obstructing the lateral flow of water through the layer. For instance, crack sealing or patching small failed areas with dense-graded HMA can reduce the flow of water through an OGFC layer. When large areas of patching are involved, OGFC should be used to replace OGFC. OGFC has different thermal properties than typical dense-graded HMA layers. The document states that thermal conductivity is up to 70 percent less than typical dense- graded layers. For this reason, frost and ice will accumulate sooner and last longer on OGFC. This fact is important for maintenance forces to know and understand since it may alter winter maintenance activities. Also, OGFC will require more extensive de- icing measures due to the open nature of the mix.