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60 CHAPTER 4 Conclusions and Recommendations 4.1 Summary of Findings 4.1.1 Selection Study The general approach adopted for NCHRP Project 9-36 At 100C, the maximum temperature considered viable for was to improve existing binder aging technologies rather a long-term aging test, the MGRF does not generate a mov- than develop a completely new procedure. The project ing film and a number of attempts to modify the apparatus by started with a review of existing binder aging procedures to adding scrapers and balls or rollers were not successful. A identify viable candidate methods for possible improve- long-term aging test in the MGRF would require consider- ment. Two viable methods were identified--the SAFT and ably in excess of 2 days to complete in order to simulate the MGRF. From the review, the study team determined that aging that occurs in the PAV. both the SAFT and MGRF are relatively inexpensive, easy Adequate mixing of air was also a serious problem at 100C to perform, applicable to both neat and modified binders, for the SAFT, and a number of impeller designs were evalu- and--based on available literature--can reasonably repro- ated in order to improve the mixing efficiency and, conse- duce the level of aging that occurs in the RTFOT. However, quently, the degree of aging. These designs improved the it was not clear from the review if either test could be extended mixing and the rate of aging so that aging consistent with the to long-term aging. Therefore, a selection study was con- PAV could be obtained after 40 hours. However, the degree of ducted to choose one of these methods for further develop- aging relative to the PAV was found to be dependent on the ment. The selection study investigated whether at a temperature binder. Unexpectedly, the stiffer polymer-modified binders of 100C either test can adequately mix air with stiff binders aged more relative to the PAV than did the neat asphalt binders. to produce a level of aging similar to that obtained in the PAV. From this study, the SAFT was selected for further development. The additional development for the SAFT 4.1.2 VCS Study included a VCS study to design an improved system for The SAFT included a volatile collection system (VCS) to col- quantifying the volatility of binders tested in the SAFT and lect volatiles from the binder during short-term aging. The an optimization study to determine operating parameters reported small mass of volatiles collected with the original for the SAFT so that it would reproduce the level of aging VCS, one-tenth of the mass lost with the RTFOT procedure, obtained for neat binders with the RTFOT. The last study prompted a review of the VCS supplied with the prototype conducted in NCHRP 9-36 was a verification study. In this version of the SAFT. This review confirmed that only a small study, the properties of binders aged in both the SAFT and amount of volatiles was collected by the air-cooled condenser. MGRF were compared to properties of binders aged in the Additionally, the study team found that condensation of RTFOT and the properties of binders from mixtures that volatiles on the inside of the lid of the SAFT vessel, saturation were short-term oven-aged in accordance with the per- of the air passing through the SAFT with volatiles, and suppres- formance testing procedure in AASHTO R30. The verifica- sion of volatilization caused by air pressure within the SAFT tion study served as the basis for the final recommendations could not explain the relatively small amount of volatiles for short-term aging that are the primary product of NCHRP collected in the original prototype VCS. It was shown that Project 9-36. The major findings from the various studies the design of the original VCS was inadequate, and that conducted in NCHRP Project 9-36 are summarized in the volatiles were passing through the VCS. After considerable following sections. trial and error, a VCS based upon adsorbents commonly used

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61 for chromatographic studies was found to be effective for col- Oven-aged mixtures experiment where rheological proper- lecting the volatiles produced during the SAFT procedure. ties of short-term aged binders were compared to properties This system includes hydrocarbon and moisture traps on the back-calculated from oven-aged mixtures. inlet side of the SAFT vessel and a 3.9-in.-long (100-mm-long) resin bed and molecular sieve filters to collect hydrocarbons RTFOT Verification Experiment and water, respectively, on the outlet side. It also was found that the majority of the volatiles collected are water, not The RTFOT verification experiment included compar- hydrocarbons. isons of high-temperature, continuous-grade Christensen- Anderson master curve parameters and aging indices for SAFT and MGRF residue to those for RTFOT residue. The 4.1.3 SAFT Optimization Study comparisons of the continuous high-temperature grade after An unexpected finding from early work with the commer- short-term aging showed the MGRF results in the same high- cial version of the SAFT was that the degree of aging in the temperature grade as the RTFOT over the range of binders commercial SAFT was significantly less than that obtained tested. The maximum difference in the short-term-aged, with the prototype SAFT. The difference was attributed to high-temperature continuous grade between MGRF- and rapid aging in the prototype at the vessel wall that was in RTFOT-aged binders was 1.8C, and the difference was direct contact with the heating mantle. The commercial SAFT not a function of the high-temperature grade of the binder uses an oven to heat the vessel and limits the oven tempera- over the range from 56C to 86C. The short-term aged, ture to 176C. This finding led to an extensive optimization high-temperature continuous grade for SAFT-conditioned study to establish operating parameters appropriate for the binders, on the other hand, was similar to that for RTFOT- commercial SAFT. Based upon a Plackett-Burman statistical aged binders between 58C and 60C, but was as much as experiment design that included impeller speed, airflow rate, 6C (one grade level) lower for higher stiffness binders. and aging time as variables, the study team found that the Variability of the short-term-aged, high-temperature con- following operating parameters provided a residue that best tinuous grade for the MGRF was somewhat higher than for approximated the rheological properties of the RTFOT residue the RTFOT and SAFT. for PG 58-XX binders: The continuous high-temperature grade analysis only inves- tigated the effect of the aging procedures on high-temperature 163C aging temperature, rheology. The master curve analysis compared the rheology 2,000 mL/min airflow, of the binders over their entire stiffness range. This analy- 1,000 rpm impeller speed, sis showed the MGRF aging produced similar changes in the 50-minute aging time, Christensen-Anderson master curve parameters as RTFOT 250-g sample mass, and aging, while the changes for SAFT aging were different. This Vacuum degassing per AASHTO R28 after short-term indicates that the MGRF aging produces similar changes in the aging in the SAFT. structure of the binder as RTFOT aging, but the SAFT results in different structural changes. The degassing step was added because the researchers The aging index, the ratio of short-term aged G* to unaged found that the bubbles entrapped in the SAFT residue signif- G* for a particular test temperature and frequency of loading, icantly affect its measured rheological properties. Although is another way to evaluate the aging procedures over a range the degassing significantly affected the properties of the binder, of binder stiffnesses. This analysis showed the aging indices the study team deemed it necessary because of the error to be similar for the MGRF and RTFOT, but those for the otherwise caused by the presence of air bubbles. SAFT were lower. 4.1.4 Verification Study Oven-Aged Mixture Experiment The study to verify the equivalency of the SAFT and In the oven-aged mixture experiment, properties of the MGRF relative to the RTFOT was conducted in the follow- short-term-aged binders back-calculated from dynamic mod- ing two parts: ulus tests on oven-aged mixtures were used to compare the degree of aging that occurs in the RTFOT, SAFT, and MGRF RTFOT verification experiment where rheological prop- to that occurring in AASHTO R30. The comparisons were erties of binders aged in the SAFT and MGRF were com- based on changes in Christensen-Anderson master curve pared to rheological properties of binders aged in the model parameters and aging indices. This experiment pro- RTFOT. duced the following findings: