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5 CHAPTER 1 Introduction and Research Approach The purpose of this report is to present the results of two course mixtures. A study in Florida has documented the rel- closely related projects: NCHRP Project 9-25,"Requirements atively high permeability of Superpave surface-course mix- for Voids in Mineral Aggregate for Superpave Mixtures" and tures (3); NCHRP Project 1-42 has been initiated to evaluate NCHRP Project 9-31, "Air Void Requirements for Superpave the increasing occurrence of top-down cracking in hot mix Mix Design." The objectives of these projects are so closely asphalt (HMA) pavements since the implementation of related that the results cannot be separated in a useful way; Superpave. voids in the mineral aggregate (VMA), air voids, effective A closely related issue is that of design air void content. binder content, voids filled with asphalt (VFA), and other fac- Since the early 1990s, the Marshall mix design system has tors related to mixture composition are interrelated. This allowed design air void content for HMA to vary from 3% to chapter summarizes the objectives of these projects, the scope 5% (4). The Superpave system, as originally developed, spec- of the research performed, and the general approach taken in ified a single design air void content of 4%. In recent years, performing the work. The problem statement and research some agencies have modified the design air void content for objective that follow paraphrase and, in some cases, directly Superpave mixtures in order to improve their performance. quote the research project statements (RPSs) for NCHRP The Arizona DOT, for example, currently specifies a design air Projects 9-25 and 9-31. void content of 5% for Superpave mixtures. Evidence suggests that the composition of HMA--as indi- cated by VMA, air void content (total voids in mix, or VTM), Problem Statement effective asphalt content (VBE), VFA, and the ratio of VBE and Research Objective and/or VMA to aggregate specific surface (often expressed as a binder film thickness)--can affect both rut resistance and Problem Statement durability. Effective and efficient guidelines are needed for Before the advent of the Superpave system of mixture Superpave volumetric composition to ensure that these mate- design and analysis, 80% of the dense-graded HMA produced rials exhibit adequate levels of resistance to rutting, fatigue in the United States used aggregate gradations that passed cracking, and age hardening. above the maximum density gradation--that is, they were An important related issue, besides how composition fine gradations. Under the Superpave system, most mixtures affects the performance of Superpave mixtures and the opti- use coarse gradations that pass below the maximum density mal ranges in composition for different applications, is how gradation. Mixture volumetric requirements developed from to most effectively and efficiently specify these compositions. the 1960s through the 1980s, including VMA and air voids, This problem is complicated by the inter-relationship of vol- were based largely on the performance of fine-graded mix- umetric factors such as VMA, VBE, VTM, and VFA and also tures rather than on the typical coarse-graded Superpave by controversial terminology such as "binder film thickness," mixture. Recent research at the National Center for Asphalt which some engineers believe to be a useful concept in eval- Technology (NCAT) and especially results of the WesTrack uating HMA durability, while others strongly believe it to be study have shown that some coarse-graded Superpave mix- misleading and potentially useless. Developing specifications tures can exhibit very poor rut resistance (1, 2). At the same involving multiple constraints on mixture compositional fac- time, durability problems have been observed in a significant tors, without carefully considering the full range of potential number of pavements constructed using Superpave surface- mixtures and performance, can produce overly complicated,