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1Â Â 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 Specification for Performance-Graded Asphalt Binder Using Multiple Stress Creep Recovery (MSCR) Testâinclude criteria for tests on residue from laboratory condi- tioning 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 procedure 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). This report documents research conducted by the National Cooperative Highway Research Program (NCHRP) to improve laboratory binder conditioning methods to accurately simulate the short-term and long-term aging of asphalt binders and to 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 research concerning short-term binder conditioning conducted in NCHRP Project 09-61 included: 1. Critical evaluation of AASHTO T 240, 2. Identification of candidate-improved short-term conditioning procedures, 3. Design, execution, and analysis of a laboratory experiment comparing rheological properties of binder conditioned using AASHTO T 240 and the candidate improvements to binder recovered from asphalt mix conditioned using laboratory procedures that were calibrated to field production in NCHRP Project 09-52, and 4. Analysis of a survey of practicing technicians concerning binder leakage in AASHTO T 240 and needed improvements to AASHTO T 240. The major concerns with AASHTO T 240 are: (1) binders of different consistencies could be treated differently because the film thickness and its renewal vary with the consistency of the binder, and (2) some heavily modified binders crawl out of the container during conditioning. The primary conclusion drawn from this research was that none of the candidate procedures that were evaluated represent an improvement over AASHTO T 240 for conditioning asphalt binder to simulate the aging that occurs during the production of hot mix asphalt (HMA). For binders with a wide range of viscosities, aging indices from AASHTO T 240 were not significantly different than aging indices for binder recovered from short-term conditioned loose mixtures, and the ratio of the AASHTO T 240 aging S U M M A R Y Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging
2 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging index to the recovered loose-mix-conditioned aging index was not a function of the viscosity of the binder at the conditioning temperature. Binder leakage did not occur for any of the binders when conditioned at 163Â°C per AASHTO T 240, including a ground-tire-rubber (GTR)-modified binder and several heavily modified binders. The survey of practicing technicians documented that binder leakage occurs in about 4Â percent of samples tested, and it occurs for both neat binders as well as modified binders, with GTR and latex-modified binders being particularly susceptible to binder leakage. Some agencies report no binder leakage, while neighboring agencies report high instances of binder leakage. This suggests the need for specific equipment and technique evaluation rather than wholesale changes to AASHTO T 240. The survey of highway agency technicians also revealed users are generally satisfied with AASHTO T 240, but they would like to see additional improvement, primarily in the areas of temperature measurement, calibration, and binder recovery. Another conclusion drawn from the NCHRP Project 09-61 short-term research is static thin film conditioning of 12.5Â g of binder in a standard PAV pan at 163Â°C for 85Â minutes is acceptable for short-term conditioning asphalt binder to simulate the aging that occurs during the production of HMA. For binders with a wide range of viscosities, aging indices from static thin film conditioning were not significantly different than aging indices for binder recovered from short-term conditioned loose mixtures, and the ratio of the static thin film aging index to the recovered loose-mix-conditioned aging index was not a function of the viscosity of the binder at the conditioning temperature. If 12.5Â g, 20-hr, 2.1Â MPa (megapascal) PAV conditioning is adopted for long-term conditioning, then using the same film thickness and equipment for short- and long-term conditioning offers the potential to simplify laboratory conditioning. The research concerning long-term binder conditioning conducted in NCHRP Project 09-61 included: â¢ Evaluation of cracking data from long-term pavement performance (LTPP) sections not subjected to heavy loads to determine the target in-service aging to be simulated in the laboratory, â¢ Critical evaluation of AASHTO R 28 and alternate procedures, â¢ Preliminary evaluation of novel long-term aging approaches based on acoustic, sonic, and ultrasonic mixing, â¢ Design, execution, and analysis of an experiment to determine the range of PAV oper- ating parameters needed to simulate near-surface aging for hot and cool climates, â¢ Field calibration of 12.5Â g, 20-hr, 2.1Â MPa PAV conditioning to simulate 10Â years of aging in the top 1Â inch of pavement, and â¢ Sensitivity experiment to confirm the practicality of using 12.5Â g, 20-hr, 2.1 MPa long- term conditioning and to determine the approximate magnitude of changes to inter- mediate and low-temperature performance grading resulting from the increased aging simulated by 12.5Â g, 20-hour, 2.1Â MPa long-term conditioning. The analysis of cracking data from the LTPP Specific Pavement Study Number 8 (SPS-8), Study of Environmental Effects in the Absence of Heavy Loads, concluded cracking, apparently due to changes in binder properties, occurred after approximately 10 to 12Â years of aging. Cracking occurred at approximately the same age for 7Â in and 4Â in thick asphalt layers and for all environmental zones. Based on this conclusion, 10Â years was selected as the target age for long-term laboratory conditioning. The primary improvement to AASHTO R 28 identified by the evaluation of long-term conditioning procedures was to increase the amount of aging simulated by the PAV to better match the properties of binders near the surface of the pavement after approximately
Summary 3Â Â 10Â years in service. Feasibility experiments found novel approaches of using acoustic, sonic, and ultrasonic mixing to accelerate oxidation reactions were not successful; therefore, further development work investigated changing PAV operating parameters to accelerate aging. The conclusion drawn from the PAV operating parameters experiment was that it is possible to approximate near-surface field aging by varying the operating parameters of the PAV. For PAV conditioning at 100Â°C, 20Â hours of conditioning using a mass of 12.5Â g is approximately equivalent to 40Â hours of conditioning using a mass of 50.0Â g. Both are approximately equivalent to 10Â years of field aging at a depth of 0.75Â in for the pavements at sites in Arizona and Minnesota. The conclusion drawn from the long-term calibration experiment is that it is feasible to approximate near-surface field aging of 10Â years using 12.5Â g, 20-hr, 2.1Â MPa PAV conditioning at temperatures between 85Â°C and 115Â°C, depending on the climate at the site of the pavement. Analysis of the 7,415 weather stations from the United States and Canada included in LTPPBind 3.1 concluded PAV conditioning temperature of 100Â°C covered 41Â percent of the weather stations, while PAV conditioning temperatures of 95Â°C and 105Â°C each covered an additional 20Â percent of weather stations. The current temperature range of 90Â°C to 110Â°C included in AASHTO R 28 covers 98Â percent of the weather stations. The sensitivity experiment concluded that the residue for 12.5Â g, 20-hr, 2.1Â MPa PAV conditioning using the temperatures determined from the calibration experiment is signi- ficantly more aged than the residue from standard AASHTO R 28 conditioning. Using the current AASHTO M 320 and AASHTO M 332 criteria, increases in continuous low- grade temperatures ranged from 0.3Â°C to 13Â°C, while increases in continuous intermediate- grade temperatures ranged from 1Â°C to 10Â°C. Changes to performance grading criteria are needed to implement 12.5Â g, 20-hr, 2.1Â MPa PAV conditioning in performance grading. The following suggestions were made based on the research conducted in NCHRP Project 09-61 and the following three scenarios for the performance grading specifications: 1. Conditioning for Current Performance Grading. No changes to binder conditioning are recommended if current performance grading criteria are maintained. AASHTO T 240 should continue to be used for short-term conditioning, and AASHTO R 28 should continue to be used for long-term conditioning. This stems from the conclusions from NCHRP Project 09-61 that: (1) the various short-term procedures that were evaluated do not represent an improvement over AASHTO T 240 for conditioning asphalt binder to simulate the aging that occurs during the production of HMA, and (2) residue from 12.5Â g, 20-hr, 2.1Â MPa long-term PAV conditioning is significantly more aged than residue from AASHTO R 28 conditioning. 2. Conditioning for Adoption of DTc Criterion. There is growing interest in adding DTc on residue from 40-hr PAV conditioning as a specification criterion to address crack- ing related to asphalt binder aging. If this criterion is added to AASHTO M 320 and AASHTO M 332, consideration should be given to using 12.5Â g, 20-hr, 2.1Â MPa PAV conditioning instead of 50.0Â g, 40-hr, 2.1Â MPa PAV conditioning. This emanates from the conclusion from NCHRP Project 09-61 that 12.5Â g, 20-hr, 2.1Â MPa PAV condition- ing yields residue with rheological properties similar to 50.0Â g, 40-hr, 2.1Â MPa PAV conditioning allowing residue for intermediate- and low-temperature grading and the 40-hr DTc criterion to be conditioned in 20Â hours using a single run of the PAV. AASHTO T 240 would continue to be used for short-term conditioning. To facilitate adoption of this suggestion, a draft appendix to AASHTO R 28 for 12.5Â g condition- ing and a commentary for the draft appendix were prepared. The primary obstacles to adopting 12.5Â g, 20-hr, 2.1Â MPa PAV conditioning are: (1) the stricter leveling
4 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging requirement for the pans in the PAV and (2) the need to form the thin film under nitrogen at 135Â°C before PAV conditioning to ensure modified binders produce a uniform thin film. 3. Conditioning for Revised Performance Grading Criteria. The greatest potential use of the research from this project is in conjunction with changes to binder performance grading criteria that account for the additional aging simulated by 12.5Â g, 20-hr, 2.1Â MPa long-term PAV conditioning. The 12.5Â g, 20-hr, 2.1Â MPa PAV conditioning temperatures as a function of climate recommended from the NCHRP Project 09-61 research simulate near-surface aging after 10Â years in service. When combined with appropriate criteria, this conditioning allows binder grading to be based on properties when nonâload associated cracking begins to occur in pavement systems. Since only 12.5Â g samples will be used, this approach can also take advantage of the conclusion from the NCHRP Project 09-61 research that short-term conditioning can be conducted using 12.5Â g in a standard PAV pan conditioned for approximately 85Â minutes at 163Â°C. Using the same film thickness for short- and long-term conditioning has the potential to simplify laboratory operations. To facilitate the additional research required to adopt this sugges- tion, a draft AASHTO Standard Practice titled Provisional Standard Practice for 0.8Â mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and an associated com- mentary were prepared. The primary obstacle to adopting this practice for conditioning asphalt binder is additional research on pavements with documented performance is required to revise intermediate- and low-temperature criteria to be compatible with the 12.5Â g, 20-hr, 2.1Â MPa long-term PAV conditioning. Another obstacle is fewer binders can be conditioned in the same run of the PAV, which may require more PAV conditioning equipment to meet current laboratory throughput.