BACKGROUND

Mineral filler plays an important role in the construction and performance of hot mix asphalt (HMA) pavements. Very little attention was given to the study of mineral filler (often referred to as the minus 200 fraction) during the Strategic Highway Research Program, although the Superpave mix design method includes a recommendation on the dust-to-binder ratio. Some field experience suggests that this ratio may be too restrictive. In certain cases, constructibility and performance can be enhanced with the use of additional filler as long as the filler is properly specified.

The nature and quantity of mineral filler are especially important in specialty mixes like stone matrix asphalt (SMA) mixes where the mineral filler, which forms a mastic with the asphalt binder, contributes significantly to compactibility impermeability, and in-service pavement performance. There is also some evidence that certain mineral fillers are incompatible with anti-strip agents and modified binders. Moreover, the composition and reactivity of mineral filler are normally not determined but could have a significant impact on aging and other HMA performance characteristics.

Specification of mineral filler for HMA is given less emphasis in the United States than in Europe. A number of different test methods and more defined criteria are often part of European HMA specifications. Given the importance of mineral filler to the overall performance of HMA as well as the diversity and abundance of mineral fillers available to industry, a significant national study on the testing and specification of mineral filler is clearly warranted.

OBJECTIVES

The objectives of this study are to (1) identify or develop test methods for mineral filler that characterize its mechanical and chemical effects on the performance of mastics (combinations of asphalt binder and mineral filler) and hot mix asphalt (HMA) and (2) recommend specification criteria for mineral filler that optimize HMA performance.

In this project, mineral filler is defined as any material meeting the criteria of AASHTO M 17 (ASTM D 242). The research shall investigate the behavior of mineral matter meeting these criteria as (1) a filler, an extender, or both, and (2) an inert or a reactive component in mastic and HMA. The test methods and related criteria will be applicable to HMA (to include dense-graded HMA, stone matrix asphalt [SMA], and open-graded friction course [OGFC]), and be suitable for use in both mix design and quality control.

Accomplishment of the project objectives will require at least the following tasks.

TASKS

Phase I  (1. )  Conduct a thorough review of the worldwide literature to identify (1) current practices on the specification and use of mineral filler; (2) the nature and characterization of the mechanical and chemical properties of mineral fillers as they relate to the performance of mastics and HMA; (3) test methods currently used to specify mineral fillers and measure their effect on the performance of mastics and HMA; and (4) effects of mineral filler on HMA pavement performance (including rutting, fatigue cracking (bottom-up and top-down), low-temperature cracking, moisture damage, and aging) and constructibility (including tender mix, compactibility, aggregate coating, and surface checking). Submit a letter report to NCHRP identifying candidate test methods with promise to (a) characterize the mechanical and chemical properties of mineral fillers and (b) measure the effect of mineral filler on HMA pavement performance and constructibility and the rationale for their selection.  (2.)  Prepare an updated, detailed work plan, budget, and schedule for a laboratory evaluation program to:
(1) Select one or more test methods from among the promising candidates identified in Task 1 based upon their demonstrated ability to (a) characterize the mechanical and chemical properties of mineral fillers and (b) measure the effect of mineral filler on HMA pavement performance and constructibility;
(2) Evaluate the selected test methods with a broad range of mineral fillers, including those commonly used across the United States, to determine the applicability and reliability of the methods; and
(3) Validate and quantify the effectiveness of the selected test methods with respect to their potential for (a) predicting or measuring HMA pavement performance and constructibility and (b) their use in quality control.
(3.) Within 6 months of the effective date of the contract, submit an interim report of the findings of the Tasks 1 and 2, including an annotated bibliography of the literature review. The research agency will be required to meet with the project panel approximately 1 month later to obtain NCHRP approval of the proposed program before beginning Phase II.

Phase II  (4.) Conduct the work plan approved in Task 3 and, based on the results of the laboratory evaluation, recommend one or more test methods with a demonstrated ability to (a) characterize the mechanical and chemical properties of mineral fillers and (b) measure the effect of mineral filler on HMA pavement performance and constructibility.  (5.) Develop preliminary specification criteria for the test methods recommended in Task 4.  (6.) Assess the sensitivity of the recommended test methods and specification criteria for application in normal construction operations, including quality control. (7.) Prepare a final report that documents results, summarizes findings, draws conclusions, and presents (1) recommended test methods in draft AASHTO format and (2) recommended specification criteria and associated limits for each criterion. In the report, discuss benefits of and obstacles to implementing the recommended test methods and specification criteria for the state departments of transportation and the HMA industry.