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6 inconsistent, and/or redundant specifications. Any modifica- HMA composition on rut resistance, fatigue resistance, perme- tion in the current Superpave specifications should address ability, and age hardening. This presentation should give pave- not only the performance of the resulting mixtures, but also ment engineers the specific information they need to evaluate the clarity and efficiency of the resulting specification. potential modifications in their Superpave specifications. Inter- pretation of the research findings (given in Chapter 3) is also Research Objective described in general and flexible terms, for two reasons. First, many agencies have already implemented a variety of changes The research objective for NCHRP Project 9-25 is stated in in Superpave, so a number of such changes are discussed in the RPS: Chapter 3 of this report as an aid to agencies that are evaluating the effectiveness of these modifications. Second, some agencies The objective of this research is to develop recommended mix may be considering changes in Superpave requirements, but the design criteria for VMA, VFA, or calculated binder film thickness, as appropriate, to ensure adequate HMA durability and resist- nature of such changes will no doubt vary depending on cli- ance to permanent deformation and fatigue cracking for coarse mate, traffic and the nature of local materials; therefore, practi- and fine, dense-graded mixes in the context of the Superpave mix cal application of the findings of NCHRP Projects 9-25 and design method. 9-31 must consider a variety of scenarios. It is acknowledged that some agencies may be quite happy with the performance of The research objective for NCHRP Project 9-31 is also HMA produced according to existing specifications and thus stated in its RPS: may feel no need for modifying their requirements for VMA, air voids, and related factors. The objective of this research is to recommend for future field validation the range of design air void content, within the con- text of the Superpave mix design method, required for adequate Research Approach durability and resistance to permanent deformation and fatigue cracking of dense-graded HMA. The initial phase of both NCHRP Projects 9-25 and 9-31 involved a review of literature and current practice. Because NCHRP Project 9-25 was initiated prior to Project 9-31, the Scope of Study literature review for the latter project was essentially an exten- The laboratory testing for this research was limited in the sion and refinement of the Project 9-25 literature review. RPS for NCHRP Project 9-25 to 9.5-, 12.5- and 19-mm nom- Much of the literature review focused on studies in which an inal maximum aggregate size (NMAS) mixtures. Therefore, attempt was made to relate volumetric properties to one or the laboratory work did not involve 25- and 37.5-mm NMAS more performance-related properties. Phase I of NCHRP mixtures, and the findings of the report tend to be focused Project 9-31 included a survey of current practice, in which more on the properties and performance of surface-course the manner that state highway agencies were specifying mixtures rather than on base course mixtures. However, it is Superpave mixtures was reviewed and summarized. believed that most of the findings presented in this research The laboratory testing performed as part of NCHRP Proj- are applicable to all HMA, regardless of the aggregate size. ects 9-25 and 9-31 involved a range of procedures designed to The RPSs for both projects required two phases: Phase I, provide information relating various aspects of HMA per- involving a review of literature and current practice; and formance to mixture composition. Laboratory tests were per- Phase II, involving laboratory testing and data analysis. Nei- formed on a variety of HMA mixtures composed of four ther RPS contemplated using sections at test tracks, test roads, different aggregate types, three different aggregate gradations, or other forms of accelerated pavement testing in performing and four different binders. Laboratory tests performed on the research. Therefore, mixture evaluations performed dur- these mixtures included repeated shear at constant height ing this research were limited to laboratory tests. However, to (RSCH), uniaxial fatigue, permeability, uniaxial compressive verify the results of this research, significant use was made of strength, indirect tensile (IDT) strength, and dynamic mod- data previously published from several test tracks/test roads, ulus before and after long-term oven conditioning. The including WesTrack, the Minnesota Road Research Project results of these tests were analyzed using a variety of methods, (MnRoad), and the NCAT test track (2, 5, 6). typically including an initial graphical analysis, followed by Because climate, type and amount of traffic loading, and sub- an in-depth statistical analysis. grade soil types vary enormously across North America, some In general, existing models for relating mixture perform- flexibility is desirable in HMA specifications. For this reason, the ance to volumetric composition were found to be inappro- findings of this report (given in Chapter 2) are presented in gen- priate for use in NCHRP Projects 9-25 and 9-31. In some eral terms--equations, graphs, and summary statements cases this was because of inaccuracy of the model; in other describing clearly the effect of changing a particular aspect of cases, it was because the model was developed using obsolete
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7 parameters, such as asphalt softening point temperature or mixture performance from mixture composition, plots were ductility. Some models, such as the well-known Witczak developed showing different aspects of performance as a model for HMA stiffness, rely heavily on parameters such as function of VMA, air void content, and related characteristics. aggregate gradation data that cannot be directly related to These plots and the underlying analyses were further analyzed mixture volumetric composition (7). Because of the short- and summarized in terms of typical effects of changing VMA, comings of existing models for estimating mixture perform- VTM, VBE, and related factors on performance. These spe- ance, newer models were developed or existing models cific findings are presented in Chapter 2. The final stage in the refined during the course of NCHRP Projects 9-25 and 9-31. analysis involved interpreting these findings in terms of prac- As much as possible, these mathematical models were tical applications to HMA mix design technology. This dis- selected (or designed) to reflect reasonable theoretical and/or cussion is presented in Chapter 3. The most significant physical models for the given mode of distress. The fatigue sections of this chapter involve discussion of how recent model proposed in this research, for example, is based largely changes in HMA mix design have affected pavement per- on continuum damage theory, reduced through mathematics formance, discussion of how possible modifications of Super- and calibration with laboratory data to a simple formula pave requirements might affect performance, and general comparable with traditional empirical equations for flexural guidelines for modifying HMA specifications to improve fatigue life. fatigue resistance and durability. As discussed under Scope of To refine the models developed from analysis of laboratory Study, the findings and recommendations of this research data and to verify their validity in application to field data, in have been presented in a format designed to provide for some most cases they were applied to other data sets generated in flexibility in implementation so that when modifying their independent research. These data sets included performance Superpave specification, agencies can effectively address local data from WesTrack, MnRoad and the NCAT test track and conditions and materials. As suggested in NCHRP report also permeability data collected in a study by the Florida DOT guidelines, the body of this report includes only the most on the permeability of Superpave mixtures (2, 3, 5, 6). After important technical information and related findings, con- refining/verifying the proposed models for estimating clusions, and recommendations.