Background: In the Superpave mix design method, specified volumetric mix properties are evaluated at a design air void content of 4 percent regardless of the traffic loading and climatic conditions. However, because of local experience and concerns about achieving adequate hot-mix asphalt (HMA) durability and performance, some agencies have modified this requirement. For example, the Arizona Department of Transportation specifies a design air void content of 5 percent to guard against the possibility of pavement rutting in the high ambient temperatures experienced in large portions of the state. The Quebec Ministère des Transports allows the design air void content to vary between 4 and 7 percent in order to meet a target value of effective volume of asphalt within specified limits for voids in mineral aggregate (VMA) and voids filled with asphalt (VFA).

Therefore, research, including a laboratory study, is needed to evaluate how such variations from the present Superpave criterion for design air void content may affect HMA durability and performance.

Objective: The objective of this research is to recommend for future field validation the range of design air void content, within the context of the Superpave mix design method, required for adequate durability and resistance to permanent deformation and fatigue cracking of dense-graded HMA.

Tasks: Accomplishing this objective will require at least the following tasks.

PHASE I (1.) Evaluate the state of current practice for selecting a design air void content through (a) a critical review of the literature, (b) a review of relevant research in progress with emphasis on NCHRP Project 9-25, and (c) a survey of current agency Superpave design air void content requirements differing from the value of 4 percent at N_design gyrations specified in Table 5 of AASHTO MP2, Standard Specification for Superpave Volumetric Mix Design. Based on this evaluation, suggest a range of design air void content for laboratory evaluation. (2.) Develop a detailed work plan for a statistically designed laboratory experiment to evaluate how variation in design air void content, within the range suggested by Task 1, influences HMA durability, performance, and, if possible, compactibility. In the experiment design, consider at a minimum the following factors: VMA, asphalt content, compactive effort, climate, and aggregate gradation and other properties. Coordinate this experiment design with that of the experiment planned for Project 9-25 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. (3.) Submit within 6 months of the effective date of the contract an interim report of the findings of Tasks 1 and 2. The research agency will be required to meet with the project panel approximately 1 month later to obtain NCHRP approval of the work plan before beginning Task 4.

PHASE II (4.) Conduct the laboratory experiment according to the work plan approved in Phase I. (5.) Analyze the results of the laboratory experiment. Recommend for future field validation the range of design air void content, within the context of the Superpave mix design method, required for adequate durability and resistance to permanent deformation and fatigue cracking of coarse and fine dense-graded HMA in various climates. (6.) Prepare a suggested work plan and budget for a future experiment (coordinated with that developed in Phase II of NCHRP Project 9-25) to validate the recommended design air void content range with field data. In the plan, include any suggested revisions to mix design procedures in AASHTO MP2 necessary to accomplish the validation. (7.) Prior to submission of the final report, meet with the NCHRP to present a concise letter report of the findings of Tasks 4, 5, and 6. (8.) Submit a final report that summarizes findings, draws conclusions, and documents the results of Phases I and II. In the report, present (1) the recommended range of design air void content required for adequate HMA durability and resistance to permanent deformation and fatigue cracking and (2) the suggested work plan and budget for an experiment to validate the design air void content range with field data.

Product Availability: The project final report is available as NCHRP Report 567, Volumetric Requirements for Superpave Mix Design.