The minimum voids in mineral aggregate (VMA) requirement for dense-graded hot mix asphalt (HMA) was proposed by McLeod in the late 1950s to ensure reasonable durability (hereinafter used to mean resistance to aging) of HMA. Minimum VMA was based on a binder content of 4.5 percent, which was considered the minimum binder content necessary for adequate durability.
Prior to the introduction of the Superpave mix design method, more than 80 percent of the dense-graded HMA used in the United States had aggregate gradations above the maximum density line or the restricted zone (a zone residing along the maximum density line from the 0.3 mm sieve to either the 2.36 or the 4.75 mm sieve through which it is usually undesirable for the gradation to pass) recommended in the Superpave mix design method. McLeod's minimum VMA requirement worked reasonably well for such relatively fine, dense-graded mixtures. However, most Superpave-designed mixes for high-traffic-volume pavements are coarse, dense-graded mixes with a gradation falling below the maximum density line and near the lower control point at the 2.36 mm sieve. Such gradations have low surface areas for coating with binder, resulting in calculated binder film thicknesses that are significantly higher than the 6 to 8 microns generally obtained for mixes designed before the introduction of the Superpave method.
Recent studies conducted by the National Center for Asphalt Technology (NCAT) (see, for example, NCAT Report 98-1," A Critical Review of VMA Requirements in Superpave"
and references therein) indicate that Superpave-designed mixes with gradations near the lower control point at the 2.36 mm sieve have calculated binder film thicknesses in the 10- to 12-micron range, significantly greater than that considered satisfactory in the past for HMA durability. NCAT results suggest these relatively coarse Superpave-designed mixes have lower resistance to permanent deformation because of their increased binder film thickness compared with mixes with gradations above the maximum density line. Moreover, in report FHWA-RD-99-134
, "Performance of Coarse-Graded Mixes at WesTrack-- Premature Rutting,"
the premature failure of several WesTrack pavement sections was partially attributed to "...high binder contents due to high VMA values..." and, by extension, to excessive binder film thickness.
The Superpave mix design method also specifies a range for voids filled with asphalt (VFA) that is interrelated with VMA and percent air voids; this effectively establishes a maximum VMA requirement. There is a critical need to evaluate these VMA and VFA requirements and their interrelationship for Superpave-designed coarse and fine-graded mixes. Recommended VMA or VFA requirements should be sensitive to the range of design equivalent single-axle loads (ESALs) and nominal maximum aggregate sizes currently included in the Superpave mix design system.
Further, there is a need to evaluate whether specifying a minimum and maximum calculated binder film thickness is more appropriate than specifying a range of VMA or VFA or both. This also requires consideration of the method used to estimate the surface area of different size aggregate particles used to calculate binder film thickness and which particle sizes are coated by a binder film as opposed to acting as a filler in the binder. There is a lack of background research for the most frequently used surface area factors presented in Table 6.1 of the Asphalt Institute Manual Series 2, Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types
(Sixth Edition - 1994).
The objective of this research is to develop recommended mix design criteria for VMA (voids in minimal aggregates), VFA (voids filled with asphalt), or calculated binder film thickness, as appropriate, to ensure adequate HMA durability and resistance to permanent deformation and fatigue cracking for coarse and fine, dense-graded mixes in the context of the Superpave mix design method.
It is anticipated that the research will encompass at least the following tasks.
(1) Conduct an in-depth, critical literature review to identify the effects of VMA, VFA, and calculated binder film thickness on the durability (i.e., resistance to aging) of HMA and its resistance to permanent deformation and fatigue cracking. Specifically, include the results of the FHWA-administered National Pooled Fund Study # 176 , the papers and reports referenced in Special Note J of the Research Problem Statement for Project 9-25, and the references therein. In the review, also address the effectiveness of the methods available for estimating aggregate surface area and calculating binder film thickness. (2) Using appropriate methods such as finite element analysis, analyze how calculated values of VMA (VFA) and binder film thickness relate to aggregate properties such as particle shape and surface texture, aggregate structure, and mix factors such as air voids content and asphalt binder content known to affect HMA durability and its resistance to permanent deformation and fatigue cracking. In the analysis, examine whether VMA (VFA) or calculated binder film thickness is the more sensitive measure of changes in these aggregate and mix factors. (3)
Using the results of Tasks 1 and 2, develop a detailed, updated work plan for a statistically designed laboratory experiment to confirm the expected effects of VMA (VFA) or calculated binder film thickness on the durability of HMA and its resistance to permanent deformation and fatigue cracking. If calculated binder film thickness is selected, also propose and justify a method for estimating aggregate surface area. In the experiment design, consider dense-graded mix designs prepared with (1) at least two nominal maximum aggregate sizes, including 9.5 and 19 mm; (2) aggregate gradations near the upper control points, near the maximum density line, and near the lower control points; (3) a wide range of VMA (VFA) or calculated binder film thickness; and (4) several aggregates of substantially different mineralogical composition and asphalt absorption. (4) Submit within 6 months of the effective date of the contract an interim report of the findings of Tasks 1 through 3. In the report, present (1) key findings of the Task 1 literature review, (2) the results of the Task 2 analysis, and (3) the Task 3 work plan for the laboratory experiment.
(5) Conduct the approved Task 3 work plan. Analyze the experimental results and recommend ranges (i.e., maximum and minimum limits) of VMA (VFA) or calculated binder film thickness required to obtain adequate HMA durability and resistance to permanent deformation and fatigue cracking with respect to aggregate gradation, nominal maximum aggregate size, and design ESALs, as appropriate. Determine whether VFA requirements are redundant if both a minimum and a maximum VMA are specified. If calculated binder film thickness is selected as a requirement, recommend a method to determine this parameter. Insofar as possible, independently verify the recommended limits with results from other laboratory and field studies (e.g., NCHRP Projects 9-14 and 9-20). (6) Develop a work plan and budget for possible research under a separate project to (1) confirm the results of Task 5 with mixes of nominal maximum aggregate size greater than 19 mm and (2) validate the results of all the laboratory testing through accelerated pavement testing, full-scale field testing, or both. (7) Prepare a final report that summarizes findings, draws conclusions, and documents the research products, including (1) the recommended ranges (i.e., maximum and minimum limits) of VMA (VFA) or calculated binder film thickness required to obtain adequate HMA durability and resistance to permanent deformation and fatigue cracking; (2) if limits on binder film thickness are recommended, a suggested method for estimating aggregate surface area and calculating binder film thickness; and (3) the suggested work plan and budget for an experiment to confirm the recommended limits for other nominal maximum aggregate sizes and validate them with field data. Prepare recommended new or revised specifications, methods, or practices for review and possible action by the TRB Superpave Mixture and Aggregate Expert Task Group and the AASHTO Highway Subcommittee on Materials.
The project final report is available as NCHRP Report 567: Volumetric Requirements for Superpave Mix Design.