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166 Conclusions Short-Term Conditioning The research concerning short-term binder conditioning conducted in NCHRP Project 09-61 included the following: 1. A 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 proper- ties 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 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 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 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 GTR-modified binder and several heavily modified binders. The survey of practicing tech- nicians 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. A second conclusion from this research was that further modification of AASHTO T 240 is needed to use this procedure to simulate WMA production. When binder is conditioned using AASHTO T 240 at 135Â°C, aging indices were significantly lower than aging indices C H A P T E R Â 4 Conclusions and Recommendations
Conclusions and Recommendations 167Â Â from binder recovered from short-term conditioned loose mixtures. However, the ratio of the AASHTO T 240 aging index to the recovered loose mix conditioned aging index was not a function of the viscosity of the binder at the conditioning temperature, indicating the bias could be eliminated by increasing the conditioning temperature above 135Â°C or increasing the conditioning time. These modifications are preferred over adding mixing screws and maintaining the temperature at 135Â°C and the conditioning time at 85Â minutes because: (1) no new equipment is required, (2) binder recovery from the mixing screws is difficult, and (3) mass change measurements when using heated containers, as required when using mixing screws, are highly variable. The conclusion drawn from the NCHRP Project 09-61 short-term research is static, thin- film conditioning of 12.5 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. As discussed in more detail below, if 12.5 g, 20-hr, 2.1 MPa 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. Long-Term Conditioning The research concerning long-term binder conditioning conducted in NCHRP Project 09-61 included: (1) evaluation of cracking data from LTPP sections not subjected to heavy loads to determine the target in-service aging to be simulated in the laboratory, (2) a critical evaluation of AASHTO R 28 and alternate procedures, (3) preliminary evaluation of novel long-term aging approaches based on acoustic, sonic, and ultrasonic mixing, (4) design, execution, and analysis of an experiment to determine the range of PAV operating parameters needed to simulate near-surface aging for hot and cool climates, (5) field calibration of 12.5 g, 20-hr, 2.1 MPa PAV conditioning to simulate 10Â years of aging in the top 1 inch of pavement, and (6) a sensitivity experiment to confirm the practicality of using 12.5 g, 20-hour, 2.1 MPa long-term conditioning and to determine the approximate magnitude of changes to intermediate and low-temperature performance grading resulting from the increased aging simulated by 12.5 g, 20-hour, 2.1 MPa long-term conditioning. The analysis of transverse cracking data from the LTPP SPS-8 study, Study of Environmental Effects in the Absence of Heavy Loads, concluded transverse cracking, apparently due to changes in binder properties, occurred after approximately 10 to 12Â years of aging. Transverse cracking occurred at approximately the same age for 7 in and 4 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 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 12.5 g is approximately equivalent to 40Â hours
168 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging of conditioning using a mass of 50.0 g. Both are approximately equivalent to 10Â years of field aging at a depth of 0.75 in for the pavements at the Arizona and Minnesota sites. The conclusion drawn from the long-term calibration experiment is it is feasible to approxi- mate near-surface field aging of 10Â years using 12.5 g, 20-hr, 2.1 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 in the United States and Canada included in LTPPBind 3.1 concluded a 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 the 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 12.5 g, 20-hr, 2.1 MPa PAV condi- tioning using the temperatures determined from the calibration experiment is significantly more aged than the residue from standard AASHTO R 28 conditioning. Using the current AASHTO M 320 criteria, continuous low-temperature grade temperatures increased 0.3Â°C to 13Â°C, while continuous intermediate-grade temperatures increased 1Â°C to 10Â°C. Changes to performance grading criteria are needed to implement 12.5 g, 20-hr, 2.1 MPa PAV conditioning. Proposals Conditioning for Current Performance Grading No changes to binder conditioning are necessary 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 12.5 g, 20-hr, 2.1 MPa long-term PAV conditioning is significantly more aged than residue from AASHTO R 28 conditioning. 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 cracking related to asphalt binder aging (Asphalt Institute 2019). If this criterion is added to AASHTO M 320 and AASHTO M 332, consideration should be given to using 12.5 g, 20-hr, 2.1 MPa PAV conditioning in place of 50.0 g, 40-hr, 2.1 MPa conditioning. This emanates from the conclusion from this project that 12.5 g, 20-hr, 2.1 MPa PAV conditioning yields residue with rheological properties similar to 50.0 g, 40-hr, 2.1 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. As discussed in ChapterÂ 3, with 10 pans available in the PAV, loading two pans with 50.0 g and eight pans with 12.5 g yields approximately 95 g of binder for low- and intermediate-temperature grading and 85 g to 90 g of binder for DTc determination. This is sufficient residue for testing BBR beams at two different temperatures. With this proposal, AASHTO T 240 would continue to be used for short-term conditioning. To facilitate adoption of this proposal, a proposed appendix to AASHTO R 28 for 12.5 g conditioning and a commentary for the proposed appendix were prepared and are included in Appendix A of this report. The primary obstacles to adopting 12.5 g, 20-hr, 2.1 MPa PAV conditioning are: (1) the stricter leveling requirement for the pans in the PAV, and (2) the need to form the thin film under nitrogen at 135Â°C before PAV condi- tioning to ensure modified binders produce a uniform thin film.
Conclusions and Recommendations 169Â Â 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 12.5 g, 20-hr, 2.1 MPa long-term PAV conditioning. The 12.5 g, 20-hr, 2.1 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 12.5 g samples will be used, this approach can also take advantage of the conclusion from the projectâs research that short-term conditioning can be conducted using 12.5 g in a standard PAV pan conditioned for 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 proposal, a proposed AASHTO Standard Practice titled, 0.8Â mm Static Film Short- and Long- Term Conditioning of Asphalt Binder and associated commentary was prepared and is included as Appendix B of this report. 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 12.5 g, 20-hr, 2.1 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. Additional Research and Development Additional research and development are needed to implement the findings and suggestions from NCHRP Project 09-61. Specific recommendations for additional research and develop- ment are listed below. 1. To effectively use 0.8Â mm thin film conditioning in practice requires modification of current PAV systems to meet the stricter levelness requirement of 0.025Â degrees needed to form and condition 0.8Â mm thin films. This will require collaboration between engineers and tech- nicians using the equipment and equipment manufacturers. The process used in NCHRP Project 09-29 to develop the Asphalt Mixture Performance Tester where manufacturers were involved early in the process is an example of how collaboration during equipment specifi- cation development can lead to innovative equipment meeting the needs of the end-user. Manufacturers should be encouraged to develop innovative systems that meet the levelness requirement. Modified equipment from various manufacturers should then be assessed to confirm compliance with the levelness requirement and evaluate the time and effort required to level the pans. 2. A new vacuum oven is also needed to use 20-hour, 0.8Â mm thin film conditioning to produce residue for the 40-hour DTc criterion. This oven is needed to quickly form uniform thin films for many modified binders and to degas the stiffer residue to form acceptable specimens for DSR and BBR testing. The development of vacuum ovens will also require collaboration between engineers and technicians using the equipment and equipment manufacturers. 3. Additional development work is also needed to better define the conditions needed for forming uniform 0.8Â mm thin films of RTFOT residue for modified binders. The fact that some heavily modified binders do not form a film that completely covers the pan was iden- tified late in NCHRP Project 09-61; therefore, only limited effort could be expended on standardizing film formation. The conditions that were determinedâtemperature of 135Â°C, vacuum cycle nitrogen purging, and duration of up to 30Â minutesâwere found to be reason- able for the binders tested. However, other conditions, such as higher temperature for a shorter time may also be acceptable and require less time and effort.
170 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging 4. Implementation of the suggestions in the Proposed AASHTO Practice, 0.8Â mm Static Film Short- and Long-Term Conditioning of Asphalt Binder will require the development of a new oven with accurate temperature and pressure control for performing the short-term conditioning. If properly designed, this oven could also be used to degas PAV residue after conditioning. Again, effective development of this oven will also require collaboration between engineers and technicians who use the equipment and equipment manufacturers. 5. Implementation of the suggestions in the Proposed AASHTO Practice, 0.8Â mm Static Film Short- and Long-Term Conditioning of Asphalt Binder for the performance grading speci- fications, AASHTO M 320 and AASHTO M 332, additional research is needed to define appropriate specification criteria. Using 12.5 g, 20-hr, 2.1 MPa long-term conditioning and the climate-based temperatures developed in NCHRP Project 09-61 will produce residue with properties similar to binder from the top 1 in of pavement aged 10Â years in service. This residue, however, is significantly more aged than that obtained from the standard 50.0 g, 20-hr, 2.1 MPa PAV conditioning. Combining the increased aging with the current grading criteria results in changes of one to two grade levels. Additional research using pavement sections with documented performance is needed to determine if these grade changes are justified or if the criteria should be adjusted to accurately capture actual performance.