Annotated Literature Review for NCHRP Report 640 (2009) / Chapter Skim
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29 1.8 Alderson, A., "The Design of Open Graded Asphalt." Australian Asphalt Pavement Association.
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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.)
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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.
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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)
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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.
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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 densegraded 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.
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