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5Â Â Background Problem Asphalt binders experience aging that occurs in two distinct stages under quite different con- ditions: short-term during construction (plant mixing, storage, placement, and compaction) and long-term during the service life of the pavement. During construction, aging occurs at high temperatures where there is an opportunity for the asphalt binder to both oxidize and lose volatile compounds. In contrast, aging during the service life of a pavement occurs at a much lower temperature where oxidation is the primary aging mechanism. There is little volatile compound loss during the service life of a pavement. Since aging of asphalt binder significantly influences the performance of asphalt mixtures, the specifications for performance graded (PG) asphalt binderâAASHTO M 320, Standard Specification for Performance-Graded Asphalt Binder and AASHTO M 332, Standard Specifi- cation for Performance-Graded Asphalt Binder Using Multiple Stress Creep Recovery (MSCR) Testâinclude criteria for tests on residue from laboratory conditioning procedures intended to simulate the aging that occurs during construction and the service life of the pavement. The aging that occurs during construction is simulated using the short-term conditioning proce- dure AASHTO T 240, Standard Method of Test for Effect of Heat and Air on a Moving Film of Asphalt Binder (Rolling Thin-Film Oven Test). The aging that occurs during the service life of the pavement is simulated using the long-term conditioning procedure AASHTO R 28, Standard Practice for Accelerated Aging of Asphalt Binder Using a Pressurized Aging Vessel (PAV). Both conditioning procedures were selected during the Strategic Highway Research Program (SHRP) based on previous experience and limited validation studies. The resources available to the SHRP asphalt research program did not allow an extensive validation of either the short- or long-term conditioning procedures. Although experience with AASHTO T 240 and AASHTO R 28 has generally been positive, several concerns have been identified. For AASHTO T 240, or Rolling Thin-Film Oven Test (RTFOT), some users reported that stiff, modified binders do not properly flow within the rotating container, violating the basic premise of the procedure (i.e., that the binder is exposed to heated air in a continuously moving thin film). Users also reported that the binder can climb out of the container. The primary concern with AASHTO R 28 is the conditioning is not severe enough to simulate long-term aging that occurs near the pave- ment surface. Poor cracking performance of some pavements incorporating asphalt binder with new additives and recycled asphalt shingle binder has led researchers and practitio- ners to question the effectiveness of AASHTO R 28 when applied to some emerging asphalt technologies. C H A P T E R Â 1
6 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging Objective and Scope The objectives of National Cooperative Highway Research Program (NCHRP) Project 09-61 were to: (1) develop practical laboratory conditioning methods to accurately simulate the short- term (from production to placement) and long-term (in-service) aging of asphalt binders and (2) calibrate the improved procedures to the aging that occurs during mixture production, transport, and placement as well as during the service life of the pavement structure. The scope of work included four major parts: â¢ Critical evaluation of AASHTO T 240, AASHTO R 28, and other laboratory conditioning procedures that have been proposed as alternates, including those in use or proposed for use other countries; â¢ Identification and selection of improved laboratory conditioning procedures for further evaluation considering accuracy and ease of implementation; â¢ Design, execution, and analysis of a series of experiments to further develop the selected laboratory conditioning procedures and to relate them to the aging that occurs during construction and the service life of the pavement; and â¢ Preparation of materials to aid in implementing the improved laboratory conditioning procedures. Organization This report contains three chapters in addition to this, ChapterÂ 1 Background. The research approach is described in ChapterÂ 2, which includes: 1. Important considerations for laboratory conditioning of asphalt binders, 2. Selection of tests and analyses for judging the equivalency of field aging and laboratory conditioning, 3. Evaluation of short- and long-term conditioning procedures, 4. Selection of conditioning procedures for further evaluation, and 5. Design and analysis of experiments to further develop the selected short- and long-term conditioning procedures. ChapterÂ 3 presents the major findings from the experiments and discusses the application of those findings to improve laboratory binder conditioning procedures. The final chapter, ChapterÂ 4, presents conclusions drawn from the work completed in NCHRP Project 09-61, proposals for improving current binder conditioning practices, and recommendations for additional research and development.