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Investigation of Short-Term Laboratory Aging of Neat and Modified Asphalt Binders (2011)

Chapter: Chapter 4 - Conclusions and Recommendations

« Previous: Chapter 3 - Findings
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Suggested Citation:"Chapter 4 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2011. Investigation of Short-Term Laboratory Aging of Neat and Modified Asphalt Binders. Washington, DC: The National Academies Press. doi: 10.17226/14613.
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Page 60
Page 61
Suggested Citation:"Chapter 4 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2011. Investigation of Short-Term Laboratory Aging of Neat and Modified Asphalt Binders. Washington, DC: The National Academies Press. doi: 10.17226/14613.
×
Page 61
Page 62
Suggested Citation:"Chapter 4 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2011. Investigation of Short-Term Laboratory Aging of Neat and Modified Asphalt Binders. Washington, DC: The National Academies Press. doi: 10.17226/14613.
×
Page 62
Page 63
Suggested Citation:"Chapter 4 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2011. Investigation of Short-Term Laboratory Aging of Neat and Modified Asphalt Binders. Washington, DC: The National Academies Press. doi: 10.17226/14613.
×
Page 63

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

61 for chromatographic studies was found to be effective for col- lecting the volatiles produced during the SAFT procedure. This system includes hydrocarbon and moisture traps on the inlet side of the SAFT vessel and a 3.9-in.-long (100-mm-long) resin bed and molecular sieve filters to collect hydrocarbons and water, respectively, on the outlet side. It also was found that the majority of the volatiles collected are water, not hydrocarbons. 4.1.3 SAFT Optimization Study An unexpected finding from early work with the commer- cial version of the SAFT was that the degree of aging in the commercial SAFT was significantly less than that obtained with the prototype SAFT. The difference was attributed to rapid aging in the prototype at the vessel wall that was in direct contact with the heating mantle. The commercial SAFT uses an oven to heat the vessel and limits the oven tempera- ture to 176°C. This finding led to an extensive optimization study to establish operating parameters appropriate for the commercial SAFT. Based upon a Plackett-Burman statistical experiment design that included impeller speed, airflow rate, and aging time as variables, the study team found that the following operating parameters provided a residue that best approximated the rheological properties of the RTFOT residue for PG 58-XX binders: • 163°C aging temperature, • 2,000 mL/min airflow, • 1,000 rpm impeller speed, • 50-minute aging time, • 250-g sample mass, and • Vacuum degassing per AASHTO R28 after short-term aging in the SAFT. The degassing step was added because the researchers found that the bubbles entrapped in the SAFT residue signif- icantly affect its measured rheological properties. Although the degassing significantly affected the properties of the binder, the study team deemed it necessary because of the error otherwise caused by the presence of air bubbles. 4.1.4 Verification Study The study to verify the equivalency of the SAFT and MGRF relative to the RTFOT was conducted in the follow- ing two parts: • RTFOT verification experiment where rheological prop- erties of binders aged in the SAFT and MGRF were com- pared to rheological properties of binders aged in the RTFOT. • Oven-aged mixtures experiment where rheological proper- ties of short-term aged binders were compared to properties back-calculated from oven-aged mixtures. 4.1.4.1 RTFOT Verification Experiment The RTFOT verification experiment included compar- 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 comparisons of the continuous high-temperature grade after short-term aging showed the MGRF results in the same high- temperature grade as the RTFOT over the range of binders tested. The maximum difference in the short-term-aged, high-temperature continuous grade between MGRF- and RTFOT-aged binders was 1.8°C, and the difference was not a function of the high-temperature grade of the binder over the range from 56°C to 86°C. The short-term aged, high-temperature continuous grade for SAFT-conditioned binders, on the other hand, was similar to that for RTFOT- aged binders between 58°C and 60°C, but was as much as 6°C (one grade level) lower for higher stiffness binders. Variability of the short-term-aged, high-temperature con- tinuous grade for the MGRF was somewhat higher than for the RTFOT and SAFT. The continuous high-temperature grade analysis only inves- tigated the effect of the aging procedures on high-temperature rheology. The master curve analysis compared the rheology of the binders over their entire stiffness range. This analy- sis showed the MGRF aging produced similar changes in the Christensen-Anderson master curve parameters as RTFOT aging, while the changes for SAFT aging were different. This indicates that the MGRF aging produces similar changes in the structure of the binder as RTFOT aging, but the SAFT results in different structural changes. The aging index, the ratio of short-term aged G* to unaged G* for a particular test temperature and frequency of loading, is another way to evaluate the aging procedures over a range of binder stiffnesses. This analysis showed the aging indices to be similar for the MGRF and RTFOT, but those for the SAFT were lower. 4.1.4.2 Oven-Aged Mixture Experiment In the oven-aged mixture experiment, properties of the short-term-aged binders back-calculated from dynamic mod- ulus tests on oven-aged mixtures were used to compare the degree of aging that occurs in the RTFOT, SAFT, and MGRF to that occurring in AASHTO R30. The comparisons were based on changes in Christensen-Anderson master curve model parameters and aging indices. This experiment pro- duced the following findings:

62 1. The binder aging that occurs when a mixture is short-term conditioned in a forced-draft oven for 4 hours at 135°C per AASHTO R30 generally exceeds the aging that occurs in the short-term binder aging procedures. Hardening measured by the change in the Christensen-Anderson crossover frequency was greater for AASHTO R30 com- pared to the RTFOT, SAFT, and MGRF. Binder aging indices also were greater for AASHTO R30 compared to the three short-term binder aging procedures. 2. Based on an analysis of aging index rankings, short-term binder aging in the RTFOT and MGRF provided similar rankings as short-term mixture aging in accordance with AASHTO R30. The ranking of binders aged in the SAFT did not correlate well with the AASHTO R30 rankings. An interesting and unexpected finding from both of the experiments in the verification study was that for the binders tested, the average aging of the neat binders was approxi- mately the same as that for the modified binders for AASHTO R30, RTFOT, and MGRF conditioning. This finding is in contrast with other studies that have reported less aging in the RTFOT for modified binders. For the SAFT, the average aging of the neat binders was greater than that of the modi- fied binders. 4.2 Conclusions The primary objective of NCHRP 9-36 was to identify and verify a test procedure that can be used as a replacement for the RTFOT. Based on the findings summarized above, the MGRF is considered an acceptable replacement for the RTFOT. For neat binders, MGRF and RTFOT conditioning produced similar rheological properties. MGRF and RTFOT conditioning also produced similar rheological properties for typical polymer-modified binders. The ranking of the aging susceptibility of binders for both the MGRF and RTFOT cor- related well with the ranking of aging susceptibility from mix- tures that were short-term oven-aged for 4 hours at 135°C in accordance with AASHTO R30. Although rheological prop- erties were the same, mass change in the MGRF is less than in the RTFOT, averaging approximately 40 percent of the RTFOT mass change for the binders tested in this study. The SAFT, on the other hand, is not an acceptable replace- ment for the RTFOT for a wide range of binders. There is a significant difference in the rheological properties of SAFT- conditioned and RTFOT-conditioned neat binders, and the difference is more apparent for higher stiffness binders. Addi- tionally, there is poor correlation in the ranking of the aging susceptibility of binders as measured by the SAFT and as measured by oven-aged mixtures. A consideration in selecting a replacement for the RTFOT was that the test, or at least the associated equipment, should show promise for future development as a replacement for the PAV. Unfortunately, it does not appear that short-term binder aging procedures can be adapted to long-term aging because it is very difficult to provide sufficient mixing of air with the binder at temperatures considered reasonable for simulating long-term aging. Attempts to modify the MGRF to improve mixing by adding scrapers and balls or rollers were not successful. Adaptation of the SAFT to long-term aging was more successful. Through changes in the design of the SAFT impeller, researchers demonstrated that equivalent PAV aging could be accomplished in approximately 40 hours, twice the time required for the current PAV. Another consideration in NCHRP Project 9-36 was an alter- nate to the current RTFOT mass change procedure for quanti- fying binder volatility. The SAFT included a VCS that used an air-cooled condenser to collect vapors produced during aging. With appropriate glassware, the MGRF also could be modified to use a VCS. Based on the VCS study, it was concluded that the air-cooled condenser was inadequate because it only collected a small amount of the volatile compounds generated during the test. An improved VCS that incorporates a resin bead filter and a molecular sieve that are commonly used in chromato- graphic studies was developed. This system can be adapted to the MGRF, but not the RTFOT. For the binders used in this study, AASHTO R30, the RTFOT, and the MGRF treated the neat and modified binders similarly. There was no difference in average aging indices between neat and modified binders in any of the three tests. For the specific aggregate used, AASHTO R30 aged the binders more than the RTFOT and MGRF. The ranking of binder aging was similar for AASHTO R30, the RTFOT, and the MGRF. 4.3 Proposals for Future Action Based on the experiments and analyses completed in NCHRP Project 9-36, the following proposals for future action are made. 1. The MGRF is a viable alternative to the RTFOT for short- term binder aging. For a wide range of neat and modified binders, the researchers demonstrated that rheological properties for MGRF and RTFOT residue are the same. Additionally, the MGRF and RTFOT provide ranking of short-term aging susceptibility that is similar to that for mixtures aged in a forced-draft oven in accordance with AASHTO R30. 2. A modification to the mass change criterion in AASHTO M320 is needed if the MGRF is used in its current form. The mass change in the MGRF is approximately 40 per- cent of the mass change in the RTFOT; therefore, the mass change criterion for MGRF residue should be ± 0.40 per- cent to be equivalent to ± 1.0 percent for the RTFOT.

3. Different conditioning procedures are needed for short- and long-term aging. In general, procedures designed to expose binder to air at plant mixing temperatures are not capable of mixing air with the binder at the lower temper- atures representative of aging during the service life of the pavement. 4. Consideration should be given to separating the measure- ment of binder volatility from the short-term aging proce- dure. This study clearly showed that only a small mass of hydrocarbon volatiles is collected during short-term aging. A simple mass-change test performed under heat and vac- uum could be developed to quantify binder volatility. The test could be developed to adapt equipment (scale, pans, and vacuum oven) already available in binder testing labo- ratories. It is recommended that this test be pursued for use with both the RTFOT and MGRF. 5. The following additional development for the MGRF should be considered: • Replace the mass-change measurements in the cur- rent MGRF procedure with the vacuum volatility test outlined above. • Investigate modifying the MGRF test to allow aging of different volumes of binder. One of the advantages of the RTFOT is that it can be used to condition small quantities of binder. This has practical application in binder quality control testing and in designing mixtures with high percentages of recycled asphalt pavement. • Conduct a formal ruggedness test for the current pro- cedure to identify appropriate tolerances for the testing conditions. Factors that should be considered include bath temperature, rotational speed, flask submersion depth, airflow rate, flask angle, binder quantity, dura- tion, and the need for a bath cover to better control temperature. 6. Additional research to calibrate short-term aging proce- dures for binders and mixtures should be considered. The research completed in NCHRP Project 9-36 showed a dif- ference between the short-term binder and mixture aging procedures, with the mixture procedure providing some- what greater amounts of aging. It should be possible to calibrate the binder and mixture procedures to provide similar levels of aging. This research should include eval- uations of plant-produced mixtures to ensure that the procedures produce representative levels of actual con- struction aging. 7. Additional research should be considered to adequately address long-term binder aging. Future research into long-term aging should include work with the PAV and other alternatives that may be identified in the future. This work should generally be directed at establishing operating conditions for simulated laboratory aging tests that reproduce the degree of aging that occurs in field pavements for typical binders. Mirza and Witczak’s Global Aging Model, while highly empirical, provides an estimate of site-specific aging based on an analysis of historical data. Work in NCHRP Project 9-23 that was reviewed during NCHRP Project 9-36 shows that the PAV operated at 100°C for 20 hours under 304.6 psi (2.1 MPa) air pressure produces aged binders with viscosities that are in reason- able agreement with this model for a time of 10 years and moderate mean annual air temperature conditions. Based on this finding, the potential for the development of a long- term aging procedure that represents a reasonable period of service in the field is encouraging. 63

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 709: Investigation of Short-Term Laboratory Aging of Neat and Modified Asphalt Binders provides a proposed method of testing for short-term laboratory aging of neat and modified asphalt binders using the modified German rotating flask as an alternative to the rolling thin film oven test.

The following appendixes A-E to NCHRP Report 709 are only available in electronic format:

Appendix A: Binder Aging Bibliography

Appendix B: Selection Study Report

Appendix C: Volatile Collection System Study Report

Appendix D: SAFT Optimization Study Report

Appendix E: Verification Study Report

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