The performance of hot mix asphalt (HMA) is largely determined by the characteristics of its constituents, asphalt binder and aggregate. In developing the Superpave mix design method, the Asphalt Research Program (1987–1993) of the Strategic Highway Research Program (SHRP) targeted the properties of asphalt binders and hot mix asphalt (HMA) and their effects on pavement performance. Other than asphalt-aggregate adhesion and its consequences to moisture damage, the study of the aggregate's contribution to pavement performance was purposefully excluded from the program. Yet, SHRP researchers were required to produce aggregate gradation and aggregate property specifications without the benefit of experimentation to support or verify their formulation.
Thus, in lieu of a formal aggregate research program, a group of acknowledged experts in the areas of aggregate production and behavior and HMA mix design developed, through the use of a modified Delphi approach, the set of recommended aggregate criteria in the original Superpave mix design method (Report SHRP-A-407, Strategic Highway Research Program, National Research Council, Washington DC, 1994) that were subsequently specified in AASHTO MP2, Superpave Volumetric Mix Design
. This specification presented aggregate gradation control points and restricted zone boundaries as well as requirements for aggregate consensus properties such as coarse aggregate angularity and sand equivalent.
Since the conclusion of SHRP in 1993, laboratory research has continued to better define the value of the aggregate criteria in AASHTO MP2, with the goal of discarding redundant or ineffective requirements in favor of those that significantly influence HMA performance. For example, NCHRP Project 4-19, "Aggregate Tests Related to Asphalt Concrete Performance in Pavements," identified aggregate properties that relate well to HMA performance in laboratory testing; a follow-on project is validating, through accelerated load tests and in-service pavement studies, these properties as determinants of HMA field performance. Project 9-14, "Investigation of the Restricted Zone in the Superpave Aggregate Gradation Specification," found that compliance with the restricted zone requirement is not necessary when an HMA mix design meets all other relevant criteria in AASHTO MP2. Project 4-30, "Test Methods for Characterizing Aggregate Shape, Texture, and Angularity," will address how the shape, texture, and angularity characteristics of aggregates affect HMA performance.
In 2001, the tonnage of Superpave-designed HMA used by the states in pavement construction had reached an estimated 80 percent of the total. This widespread use provides opportunities to evaluate through field pavement surveys how the current Superpave aggregate criteria influence performance. For example, Brown and co-authors have reported on the performance of 44 pavements constructed with Superpave-designed HMA between 1992 and 1998 [E.R. Brown, D. Decker, R. Mallick, and J. Bukowski, Superpave Construction Issues and Early Performance Evaluation, Proceedings of the Association of Asphalt Paving Technologists
, Volume 68, 613 (1999)]. In addition, similar evaluations are available from controlled field experiments and accelerated performance tests conducted in this period, including MnRoad, the LTPP SPS-1, 5, and 9 studies, WesTrack, FHWA ALF, CalAPT, National Pooled Fund Study No. 176, and others.
Research is required to review the body of knowledge gained on Superpave aggregate criteria since 1993 for any demonstrable relationships to HMA pavement performance. The review should include, but is not limited to, the following key issues: performance limits on the use of rounded natural sands in HMA mixes for varying traffic levels; relationship of aggregate properties to mix performance for various Ninitial
, and Nmax
levels; relationship of aggregate properties to HMA compaction issues such as tenderness; relationship of nominal maximum aggregate size to layer thickness and in-place density and permeability; innovative methods to measure aggregate properties such as angularity, specific gravity, and VMA; relationship of fine aggregate and mineral filler properties to asphalt mastic and HMA mix performance; aggregate effects on HMA permeability; effects of crushing equipment and operations types on aggregate properties; and sensitivity of performance tests (e.g., dynamic modulus, asphalt pavement analyzer, and beam fatigue) to aggregate properties.
The objective of this project is to conduct a review of the technical literature to identify consensus, source, and other aggregate properties that significantly impact HMA performance.
Accomplishing this objective will require at least the following tasks:
(1) Conduct an in-depth, critical review of the literature dealing with the development, evaluation, and validation of the Superpave aggregate criteria in AASHTO MP2 and other proposed criteria. Identify those criteria for which experimental results demonstrate positive relationships with HMA performance, and estimate the significance of any such relationships. (2) Survey research in progress, such as NCHRP Projects 4-19(2) and 4-30. Identify any aggregate criteria for which results to date demonstrate positive, significant relationships with HMA performance. (3) Survey current state aggregate specifications, identify deviations from the original Superpave aggregate criteria, and document the reasons for the deviations and their effect on HMA performance. (4) Identify and review available sources of materials and performance data for field pavement sections, including those in the LTPP SPS-1, 5, and 9 experiments and other controlled and uncontrolled experimental field projects and test tracks. Analyze the data to accept or reject relationships between aggregate criteria and HMA performance identified in Tasks 1 and 2. (5) Prepare a final report that summarizes findings, draws conclusions, and documents the results of the project including (1) a table presenting currently specified or other proposed Superpave aggregate criteria, their probable relationship to HMA mix and pavement performance, and recommendations for their retention or adoption in current specifications; (2) a table of redundant or ineffective aggregate requirements and criteria recommended for removal from current specifications; (3) a table of aggregate properties not suitable for specification use, but which have value for other purposes in HMA mix design; and (4) objective and scope statements for possible future research projects to further validate key aggregate specification criteria.
Product Availability: NCHRP Report 539: Aggregate Properties and the Performance of Superpave-Designed Hot-Mix Asphalt.