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12 CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH 1.1 INTRODUCTION out regard for locally available materials. The specification values for the source properties--including Los Angeles (LA) Mineral aggregates make up between 80% and 90% of the abrasion, sulfate soundness, and deleterious materials--were total volume or 94% to 95% of the mass of hot mix asphalt to be set by the agency. This was done to allow for variances (HMA). For this reason, it is important to maximize the qual- in locally available materials. Finally, the modified Delphi ity of the mineral aggregates to ensure the proper perfor- process identified volumetric properties of the resulting HMA mance of our nation's roadways. The quality of mineral aggre- mix including air voids, voids in mineral aggregate (VMA), gates for road-paving materials has been specified by the voids filled with asphalt, and dust-to-asphalt proportion. The toughness, soundness (durability), cleanliness, particle shape, aggregate bulk specific gravity is required to calculate VMA angularity, surface texture, and absorption. and the aggregate fines content is required to calculate dust- The Superpave mix design method is a product of the Strate- to-asphalt proportion. gic Highway Research Program (SHRP). Research to investi- Prior to the development of the Superpave mix design gate aggregate's contribution to pavement performance was method, the aforementioned aggregate properties had not been intentionally excluded from the SHRP Asphalt Research Pro- applied in concert. Some agencies found that the Superpave gram. Instead, the aggregate gradations and physical proper- aggregate specifications precluded the use of materials with ties included in the Superpave mix design method were devel- long performance histories, particularly with regard to grada- oped through the use of a modified Delphi approach (1). The tion. Other agencies found the consensus aggregate properties Delphi process is designed to ascertain the consensus of a prevented the use of locally available materials. Others ques- group of experts while avoiding some of the negative aspects tioned the precision of certain tests. Determination of aggre- of group dynamics. The Delphi process uses a series of ques- gate bulk specific gravity for calculation of VMA was a con- tionnaires to focus the experts' opinions. The use of ques- cern for quality control/quality assurance (QC/QA) testing tionnaires allows the expert panel to remain anonymous and during production. Since the conclusion of SHRP, these con- prevents strong-willed individuals from steering the group. cerns have resulted in several national research studies and To determine the Superpave aggregate gradations and phys- in numerous smaller studies to better define the need for var- ical properties, SHRP used a modified Delphi process that ious aggregate properties as well as the interaction between included two meetings of the group of experts. In total, the the properties. process included an initial questionnaire, followed by two Three national studies relating aggregate properties to per- face-to-face meetings, followed by two more questionnaires. formance have been completed since the conclusion of SHRP; These are documented in report SHRP A-408 (1). two others are ongoing. NCHRP Project 4-19, "Aggregate The final results of the modified Delphi process used by Tests Related to Asphalt Concrete Performance in Pavements," SHRP included aggregate properties, test methods to deter- (published as NCHRP Report 405: Aggregate Tests Related mine those properties, and specification criteria. Gradation to Asphalt Concrete Performance in Pavements ) evalu- limits were included as part of the aggregate properties. The ated currently used and promising new aggregate tests as they new gradation limits included definitions for nominal max- relate to the performance of HMA. As a result of the study, a imum and maximum aggregate size, control points for var- suite of nine tests was recommended: two of the Superpave ious nominal maximum aggregate sizes (NMAS), and the consensus properties (although one was modified), one of the restricted zone. The remaining aggregate physical proper- source properties, and one of gradation analysis. The remain- ties were divided into two categories: consensus and source ing five tests are different from those currently specified by properties. The consensus properties--including coarse aggre- AASHTO M323. A follow up study, NCHRP Project 4-19(2), gate angularity, flat and elongated particles, fine aggregate "Validation of Performance-Related Tests of Aggregates for angularity, and sand equivalent--were chosen to ensure that Use in Hot-Mix Asphalt Pavements," is ongoing. NCHRP the aggregate quality was sufficient to provide satisfactory Project 9-14, "Investigation of the Restricted Zone in the HMA performance for the design traffic level. Specification Superpave Aggregate Gradation Specification," (published as values were to be uniform throughout the United States with- NCHRP Report 464: The Restricted Zone in the Superpave