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

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

13 Aggregate Gradation Specification [3]) determined that the restricted zone was a redundant requirement when used in con- junction with the other aggregate and volumetric properties specified in AASHTO M323. Pooled Fund Study 176, “Vali- dation of SHRP Asphalt Mixture Specifications Using Accel- erated Testing,” examined the effects of VMA, fine aggregate angularity (FAA), and gradation on the performance (pri- marily rutting) of HMA. Finally NCHRP Project 4-30, “Test Methods for Characterizing Aggregate Shape, Texture, and Angularity,” was recently awarded. It is expected that this study will provide new methods to more accurately measure the aforementioned properties with improved precision. Evaluation of aggregate properties has been included in some field and accelerated performance studies. The LTPP Program documents the construction and tracks the perfor- mance of numerous HMA pavements. Some of the HMA pavements in the Specific Pavement Studies (SPS) were designed with the Superpave methodology. The SPS-1, -5, and, particularly, -9 sections provide limited data relating aggregate properties to performance. Three test tracks—the MnRoad, WesTrack, and the NCAT Test Track—include Superpave- designed HMA. The NCAT Test Track has a number of dif- ferent aggregate sources and gradations represented by the test sections. Accelerated testing related to aggregate properties has also been conducted by the FHWA’s Accelerated Load- ing Facility (ALF) and the Indiana Department of Transporta- tion (DOT)–Purdue University APT Facility. Based upon performance histories of locally available mate- rials or research projects conducted to address concerns relat- ing to the aggregate specifications included in AASHTO M323, numerous agencies have modified their aggregate specifications for Superpave-designed HMA. Often times this experience is not shared with other agencies that are using similar materials. A consensus among the states, based on their experiences, may indicate the need to alter aggregate test pro- cedures or specifications on a national basis. 1.2 OBJECTIVE The objective of this study (NCHRP Project 9-35) was to review the technical literature and ongoing research to identify the consensus, source, or other aggregate properties that sig- nificantly impact HMA performance. The review also exam- ined the effect of production and crushing operations on aggregate properties. The review concentrated on the effect of aggregate properties on HMA designed using the Super- pave methodology, HMA construction, and HMA perfor- mance. New innovations in aggregate testing were also exam- ined, especially those related to aggregate shape, angularity, and texture. 1.3 SCOPE To accomplish the research objectives, five tasks were con- ducted; their descriptions are as follows: • Task 1—State of the Practice: In Task 1, a literature search and review was conducted to determine the cur- rent state of the practice on the evaluation and valida- tion of the Superpave aggregate criteria and the state of the art on the development of promising new methods of aggregate characterization. The results from Task 1 are presented in Chapter 2. The influence of aggregate crushing operations is also discussed in Chapter 2. • Task 2—Survey of Ongoing Research: In Task 2, a sur- vey was conducted to identify the preliminary findings of ongoing research related to aggregate properties. The results from Task 2 are presented in Chapter 2 in con- junction with the results from the literature review. • Task 3—Survey of Agency Specifications: A survey was conducted of state aggregate specifications for aggregate used in the production of HMA. The survey provides insights into the widespread adoption of the Superpave method and into the aggregate tests that agencies ques- tion. The survey results are summarized in Chapter 3; see also the Appendix. • Task 4—Review Performance Data from Field Test Sec- tions and Full-Scale Accelerated Testing: Results from field test sections and accelerated test sites were reviewed to gather performance relationships for aggregate prop- erties. As expected, there are complex interactions when aggregate properties are altered. These interactions tend to dilute any correlations that may exist; however, trends were identified. The results from the performance review are summarized in Chapter 4. Specific experiments are also discussed in relationship to specific test methods in Chapter 2. • Task 5—Final Report: The final report was prepared to document all of the research conducted in Tasks 1 through 4. Chapters 5 and 6 present (1) future work sug- gested for validating new aggregate shape and texture parameters based on image analysis and evaluation of the CAR test and (2) a summary of the conclusions of NCHRP Project 9-35.