Aggregate Properties and the Performance of Superpave-Designed Hot-Mix Asphalt (2005) / Chapter Skim
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The consensus properties -- including coarse aggregate angularity, flat and elongated particles, fine aggregate angularity, and sand equivalent -- were chosen to ensure the aggregate quality was sufficient to provide satisfactory HMA performance for the design traffic level. Specification values were to be uniform throughout the United States without regard for locally available materials.
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New innovations in aggregate testing were also examined, especially those related to aggregate shape, angularity, and texture: To accomplish the research objectives, five tasks were conducted: • Task 1: State of the Practice, • Task 2: Survey of Ongoing Research, • Task 3: Survey of Agency Specifications, • Task 4: Review of Performance Data from Field Test Sections and Full-Scale Accelerated Testing, and • Task 5: Final Report. CONSENSUS AGGREGATE PROPERTIES The Superpave method includes four consensus aggregate properties: coarse aggregate angularity, flat and elongated particles, uncompacted voids in fine aggregate, and sand equivalent.
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Flat and Elongated Particles While the asphalt industry believes that excessive flat and elongated particles are undesirable, perfectly cubical aggregates may also be undesirable. The Superpave method specifies ASTM D4791, "Standard Test Method for Flat Particles, Elongated Particles, or Flat and Elongated Particles," to evaluate aggregate shape with criteria for a maximum percentage (10% by weight)
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are probably more restrictive than necessary from a performance standpoint. Fine Aggregate Angularity To measure the angularity of fine aggregate, the Superpave method specifies AASHTO T304, "Uncompacted Void Content in Fine Aggregate, Method A." The test is included to ensure that there is sufficient internal friction -- resulting from particle shape, angularity, and texture -- to provide rut-resistance in the HMA.
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Based on the research evaluated, the test for uncompacted voids in fine aggregate appears to be a reasonable screening tool for fine aggregates with respect to their rutting potential. The research supports the fact that some crushed fine aggregates with uncompacted voids contents between 43% and 45% can be used to produce rut-resistant mixtures.
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The Superpave method currently specifies the sand equivalent test (AASHTO T176) to identify clay-like fines in fine aggregate.
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Micro-Deval Test The Superpave method did not specify a test method to evaluate the abrasion of aggregates under traffic, although the sulfate soundness test evaluates disintegration of aggregates caused by environmental exposure. Several studies have evaluated the micro-deval test for inclusion as a durability test for aggregates.
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EFFECT OF CRUSHING OPERATIONS ON AGGREGATE PROPERTIES The implementation of the Superpave method impacted the aggregate industry. Some of the areas that highlighted the importance of crushing operations include the following: • Requirements for coarse aggregate angularity for high-traffic pavements; • Increased emphasis on particle shape and specifications for flat and elongated particles using the 31 ratio; • Uniform utilization of product sizes, particularly with the preference for coarsegraded Superpave mixes early in the implementation process; and • The divergent requirements for aggregate properties for different end users (HMA, hydraulic cement concrete, and base)
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These smaller nominal maximum aggregate sizes are typically used in surface mixes where stresses are higher. Particle shape is affected by the geology of the aggregate as well as the crushing process.
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Two areas selected for immediate research are an investigation of alternatives to, and specification limits for, the uncompacted voids in fine aggregate tests and the development of performance relationships and criteria for the new imaging methods to measure aggregate shape, angularity, and texture. However, even where the test method may not be in question, there is still room for additional research to refine specification limits for such tests as coarse aggregate angularity, flat and elongated particles (both of which could one day be replaced by imaging)
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From page 11... ...
White Paper on Fine Aggregate Angularity. Vulcan Materials Company, 2002.
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