Annotated Literature Review for NCHRP Report 640 (2009)

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29 1.8 Alderson, A., “The Design of Open Graded Asphalt.” Australian Asphalt Pavement Association. CR C5151. November 1996. 1.8.1 General Alderson provides a comprehensive report on the design and construction practices for using open graded asphalt mixes in Australia. Starting with a review of expected benefits of open graded asphalt mixes, Alderson indicates which properties are critical for achieving desired performance and then describes the mix design and construction practices required for obtaining those properties. The author provides test data from different types of pavement which show the noise reduction benefit of open-graded mixes. Regarding mix design, Alderson mentions that the design asphalt binder content is determined on the basis of air voids, abrasion loss and draindown criteria, and provide some recommended values. The important properties are sufficient binder content and proper type of binder for resisting abrasion loss, and binder draindown. The air voids criterion is important for achieving the noise reduction and water removal properties. The use of polymer modification and fiber for enhancement of binder properties and content has been mentioned. Alderson mentions that for production and construction, a lower temperature is necessary to avoid draindown, and the use of static steel-wheel drum rollers with a few passes is necessary for proper compaction. Alderson recommends using layer thicknesses of more than 2.5 times and less than 4 times the nominal aggregate size. Alderson cautions that to avoid damage in underlying layers, open-graded mixes must be placed only on impervious mixes, such as those with less than 5 percent air voids. 1.8.2 Benefits of Permeable Asphalt Mixtures Alderson lists the following benefits for open-graded asphalt mix: 1) Provide better wet weather skid resistance, 2) Reduce both external and within vehicle noise, 3) Reduce splash and spray, 4) Improve visibility of road surface markings, specifically in wet weather; and 5) Provide a smooth riding surface. 1.8.3 Materials and Mix Design Alderson provides information on the two types of open-graded asphalts used in Australia, and their applicability. These are summarized in Table 12.

30 Table 12: Open-Graded Asphalt Used In Australia Pavement traffic volume Grade Description Commercial vehicles/lane/day Equivalent standard axles Type I For modest level of performance at minimum cost; higher binder content or modified binders not used < 500 < 5 * 106 Type II Best performance; High binder contents, polymer modified binders and fibers used; (mix designs described in Alderson’s report are mostly applicable to this type.) > 500 > 5 * 106 Alderson then provides information on nominal aggregate gradation and mix design, which are summarized in Table 13.

31 Table 13: Mix Design of Open-Graded Asphalts in Australia Parameter Selection Recommendation Aggregate Nominal size Based on performance and layer thickness; available in 10, 14 and 20 mm nominal sizes, with 10 mm being most common For proper compaction and to avoid instability, should be placed in layer thickness at least 2.5 times and maximum 4 times the nominal size of the mix economic to consider the largest nominal size consistent with performance and layer thickness; design thickness can be considered on the basis of accommodating run-off from periodic high intensity rain storms. Quality/ Properties Aggregates meeting standards for wearing courses are applicable. Require high quality aggregates – strong, cubical, good microtexture, affinity for asphalt, clean and with crushed faces Mineral fillers Durability problems can be avoided by preventing loss of adhesion of asphalt binder to aggregates; hydrated lime improves binder-aggregate adhesion and reduces potential of stripping; also reduces draindown Hydrated lime is preferred; Portland cement and ground limestone can be used. Gradation Selected as a balance between highly permeable (highly gap graded) and a strong aggregate skeleton (less gap graded); minimum air void content should be 20 % Nominal mix size Sieve size, mm 10 mm 14 mm 20 mm Production tolerance 26.5 19.0 13.2 9.5 6.7 4.75 2.36 1.18 0.6 0.3 0.15 0.075 100 83 18 14 6 4 4 3 3 2 100 95 50 27 11 9 8 6.5 5.5 4.5 3.2 100 95 55 30 20 10 8 6 4 3 3 2 Nil ±6 ±6 ±6 ±6 ±5 ±5 ±5 ±5 ±3 ±3 ±1 Asphalt Binder Binder type, modifier and fiber Binder film should provide sufficient cohesion to resist traffic loads; binder should be resistant to oxidation or present in sufficient thickness to prevent oxidative hardening; fibers reduce draindown Modified asphalt binders for improved cohesion and durability, and possible reduction in draindown; Polymers include SBS, SBR, EVA and crumb rubber modifier (CRM); fibers used in 0.3 to 0.5 percent by mass of total mix. Binder content Effective binder content (with consideration of absorption) should be such that criteria for recommended air voids, maximum permissible abrasion loss and draindown are met Use step by step procedure to select gradation, based on recommended asphalt content; change gradation and/or asphalt content to provide optimum air voids; check abrasion loss and binder draindown; check with recommended asphalt binder contents. Alderson provides a description of the steps necessary for conducting mix design. These steps are summarized in this review in Figure 4.

32 Note: All binder contents are effective binder contents, determined on the basis of absorption of aggregates Prepare three specimens at each of three asphalt binder content with selected gradation, using gyratory compactor and 80 cycles Determine maximum density, and bulk density, and calculate air voids Determine asphalt binder content (BCmax) for 20 % air voids Note: Use more gap/close gradation and/or lower/higher binder content to raise/lower air voids Run Cantabro abrasion test, on each of three unconditioned specimens, using a Los Angeles abrasion machine and 300 revolutions Determine binder content (BCmin) for maximum allowable loss of 20 % for Type II and 25 % for Type I Note: Increase binder content and/or include more intermediate size aggregate and/or use binder with more cohesive properties to decrease loss; if BCmin > BCmax, redesign mix Determine BCeffecive = Average of BCmax and BC min Conduct draindown test with a range of binder content Determine draindown at BCeffective; draindown should be less than 0.3% Note: Redesign mix if draindown > 0.3% Determine BCdesign = BCeffective + draindown Check if BCdesign> recommended binder content Note: If BCdesign<recommended binder content, use recommended value plus draindown at that recommended binder content Figure 4: Steps in Mix Design

33 1.8.4 Construction Practices Alderson provides a set of discussions and recommendations for production and laydown of open-graded asphalt mixes. These discussions and recommendations are summarized in Table 14. Table 14: Production and Laydown of Open-Graded Asphalt Mixes Property Open-graded asphalt mix Factors/Recommendations Production Risk of draindown during production and transportation Lower mixing temperature by 10 to 20oC; follow mix design guidelines. Construction Equipment similar to those use for dense graded asphalt; thin layers and high air void content; joints should allow drainage Rollers should be close behind pavers to compact mix while it is hot; use limited passes with static wheel rollers to avoid crushing or over-compaction; avoid vibratory roller to avoid forcing of binder to surface (and closing voids), and crushing of aggregates, and avoid rubber tired rollers to prevent sticking of materials; tack coating of cold longitudinal joints should be avoided to prevent blocking of drainage; recommend using hot joints. Alderson mentions that to avoid damage to underlying layers, open-graded mixes should be placed on impervious layers only. He indicates that pavements can be considered to be impervious if they have less than 5 percent air voids and have been trafficked for more than a year. If such conditions are not available before the placement of an open-graded mix, Alderson recommends the use of a very heavy tack coat, a fog coat or a 7 mm seal. 1.8.5 Maintenance Practices No information has been provided on maintenance practices. 1.8.6 Rehabilitation Practices No information has been provided on rehabilitation practices 1.8.7 Performance Alderson discusses each performance related property along with specific factors affecting this property. His discussions are summarized in Table 15.

34 Table 15: Performance Related Properties Property Open graded asphalt Factors Skid resistance Better than dense-graded asphalt; potential of hydroplaning is reduced; improved visibility Initially has lower skid resistance due to thicker asphalt binder film; wearing by traffic exposes the aggregates within a time period depending on traffic and environment; precaution needed when placed on low volume high speed roads in cold weather; skid resistance can be measured by the British Pendulum tester Design life Limited life, 8-10 years in terms of structural life, but adequate in terms of retention in surface characteristics Porous structure exposes binder films between aggregates to actions of UV, moisture and oxidation, leading to raveling and failure; important to have binders with adequate properties and binder films of adequate thickness to prevent failures. Water dispersion Significantly better than dense- graded mixes; porosity can get reduced with time due to clogging; oil spills or leakage can lead to destruction of cohesive bonds between aggregates and asphalt binder and mud from tires can clog up pores Important to drain out water that percolates through the surface; underlying surface should be impervious, there should be adequate cross slope and side drains; high speed traffic can help in cleaning the pores; not recommended for intersections to avoid oil spills and also not recommended near quarries or farms to prevent clogging by dust or mud. Noise reduction Reduction of 3dBA is generally made possible by the use of open- graded asphalt; reduction is proportional to air void content and layer thickness; reduction caused by expulsion of air between tire and surface into pores and surface texture; study reports noise levels for two open-graded asphalts as 79.3 and 77.5 dBA, compared to 87.2 dBA for deep grooved Portland cement concrete and 83.4 dBA for dense-graded asphalt. Optimum thickness for noise reduction is 40 mm; noise reduction due to surface texture is reduced with wearing of surface. Strength Structural strength not considered during design of pavement; about half to two thirds of strength of dense- graded asphalt; current practice is to consider modulus of 800 to 1200 MPa. Tests in laboratory should be conducted under confined conditions; wheel tracking tests are suitable for evaluation of rutting potential; fatigue testing can be done with four point bending beam test at low temperatures. 1.8.8 Structural Design No information has been provided on structural design. 1.8.9 Limitations No information has been provided on limitations.