Aggregate Properties and the Performance of Superpave-Designed Hot-Mix Asphalt (2005)

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74 CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS The results of this review have emphasized the difficult nature of conducting research to relate aggregate properties and HMA performance. It is difficult to isolate the effects of the aggregate properties from other interactions with grada- tion and mixture volumetric properties. It appears as if the shortcomings of a single property related to rutting resistance can be overcome by other supporting properties. These interactions emphasize the need for laboratory per- formance tests for HMA mixtures. If performance tests are adopted that have criteria in which agencies are confident, the overall performance of the mixture could be assessed instead of relying solely on component screening tests—for exam- ple, if the blend uncompacted voids in fine aggregate were 43% for a given mixture to be placed on a high-volume road, the rutting properties of this mixture could be tested (at the contractor’s expense) to show whether the mix should pro- vide acceptable performance. 6.1 CONSENSUS AGGREGATE PROPERTIES The consensus aggregate properties have been adopted by the majority of the responding agencies. F&E, specified by 79% of the responding agencies, has the lowest level of implementation. 6.1.1 Coarse Aggregate Angularity The research revealed the following about coarse aggre- gate angularity: • Increased coarse aggregate fractured faces provide increased rutting resistance. Increased particle index value or uncompacted voids in coarse aggregate also provide increased rutting resistance. The latter combine the effect of shape, angularity, and texture. • The current Superpave specification levels for coarse aggregate angularity have been adopted by 39% of the agencies that specify ASTM D5821 or an equivalent. Five states have more stringent criteria; and four states, less stringent requirements. • There is little research to support the need for two frac- tured face counts in excess of 95%. 6.1.2 Fine and Elongated Particles The research revealed the following about F&E: • Extreme levels (>10% 51 ratio) of F&E are most likely undesirable in HMA. • Increased levels of F&E increase aggregate breakdown during handling, mixing, and placement. • The current test for F&E, ASTM D4791, is extremely variable (multilaboratory coefficient of variation of 35.3% for the 31 ratio); however, precision improves as the ratio of maximum-to-minimum dimension decreases. • Seven states currently specify the 31 ratio; and one province, the 41 ratio. A specification of a maximum of 20% of particles exceeding the 31 ratio has been adopted by five states. • Research has been unable to establish that between 20% and 40% F&E exceeding the 31 ratio is detrimental to HMA performance. In fact, some level of F&E may be desirable to meet minimum VMA requirements. • If ASTM D4791 continues to be used, specifications should be developed for the 21 or 31 ratio to improve the precision of measurements. Up to 40% F&E exceed- ing the 31 ratio does not appear to be detrimental to pavement performance. Therefore a specification level of up to 40% particles exceeding the 31 ratio may be appropriate. • Imaging methods have been developed to accurately and precisely measure coarse aggregate shape, texture, and angularity and fine aggregate shape. The results with these methods have not yet been correlated with the per- formance of HMA. • Research should be conducted to relate digital means for measuring aggregate shape, texture, and angularity to pavement performance as replacements for both coarse aggregate angularity and F&E tests. 6.1.3 Fine Aggregate Angularity The research revealed the following about FAA: • Currently, the Superpave mix design system does not address the shape, texture, and angularity of the material

75 that passes the 4.75-mm (No. 4) sieve and is retained on the 2.36-mm (No. 8 sieve). It does not appear that any of the current consensus property tests can be used to address this size fraction. • Uncompacted voids in fine aggregate, AASHTO T304, appears to be a reasonable screening test for fine aggre- gate blends. Numerous other tests have been investi- gated, but to date none consistently show a better rela- tionship with performance. • Of agencies that specify AASHTO T304 or an equiva- lent, 51% have adopted the specification criteria recom- mended for the Superpave mix design system; 21% of state agencies specify more stringent criteria. • There are materials with uncompacted voids contents in the range of 43% to 45% that test as false negatives. These materials have a demonstrated history of field performance under high traffic even though they do not meet the uncompacted voids content specifications for high traffic. • Research should be conducted to relate the CAR test to the rutting performance of HMA. Borderline aggregates that fail the current fine aggregate uncompacted void content specifications for high traffic but that provide good performance should be investigated in-depth as part of this study. • Of state agencies, 46% continue to limit natural sand content by specifications. Limits between 10% and 15% are most common. 6.1.4 Sand Equivalent No recent research has been able to corroborate the rela- tionship between clay-like particles, identified by the sand equivalent test, and moisture damage in the laboratory. How- ever, the phenomenon that produces this type of failure may be difficult to duplicate in the laboratory. The methylene blue value appears to be a better indicator of harmful clays in fine aggregate than is the sand equivalent test. There is concern by some agencies that the test is not suitable for routine specifications. 6.2 SOURCE PROPERTIES Following are the conclusions reached regarding source properties. 6.2.1 LA Abrasion The research revealed the following about LA abrasion: • LA abrasion is related to aggregate breakdown during handling, mixing, placement, and compaction. There appears to be no relationship between LA abrasion and long-term abrasion or wear of the pavement surface. • LA abrasion is specified by 96% of responding agencies. • A maximum LA abrasion loss of 40% is the most com- mon specification level. 6.2.2 Sulfate Soundness The research revealed the following about sulfate soundness: • Of the responding agencies, 66% specify sodium sul- fate soundness and 31% specify magnesium sulfate soundness. • A maximum sodium sulfate soundness loss of 12% is specified by the majority of the agencies using that procedure. • Magnesium sulfate soundness loss and micro-deval abra- sion loss are highly correlated. The micro-deval test is also related to abrasion of particles in the pavement. • The micro-deval test is more precise than the sulfate soundness tests. • The micro-deval test should replace sulfate soundness test for measuring aggregates’ resistance to abrasion, wet- ting and drying, and slaking. Research may need to be conducted to identify specifications for specific aggre- gate types, similar to those used by Ontario. • States prone to freeze-thaw cycles should consider a freeze-thaw test, such as AASHTO T103, in addition to the micro-deval abrasion loss 6.3 GRADATION The restricted zone—included in the original Superpave mix design system—was demonstrated to be unnecessary. It has been removed from most current Superpave specifications. Accelerated testing at the 2000 NCAT Test Track indicates no difference in the rutting performance of coarse-graded ver- sus fine-graded Superpave mixtures. 6.4 AGGREGATE PRODUCTION Following are the conclusions reached regarding source properties: • In addition to crusher type, aggregate particle shape can be improved by – Running the crusher with a full or choked feed cavity to promote interparticle crushing. – Operating crushers in closed circuits where a recircu- lating feed can be used to fill the crusher cavity. – Reducing the reduction ratio, reducing the feed size, or increasing the circulating load.

76 6.5 LONG-TERM PAVEMENT STUDIES AND ACCELERATED TESTING There is also a need to emphasize the collection and report- ing of aggregate property data for both in-service pavements and accelerated loading facilities. More effort needs to be placed on capturing aggregate property data in national stud- ies related to HMA performance. – Adjusting the close-side setting approximately equal to the desired product size. • Cubical particles can be produced without the use of impact-type crushers. • The aggregate industry faces divergent requirements for aggregate shape. A change in shape may improve the aggregate for one application, but may hinder its use in another application.