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OCR for page 62
62 1. The binder aging that occurs when a mixture is short-term show promise for future development as a replacement for conditioned in a forced-draft oven for 4 hours at 135°C the PAV. Unfortunately, it does not appear that short-term per AASHTO R30 generally exceeds the aging that occurs binder aging procedures can be adapted to long-term aging in the short-term binder aging procedures. Hardening because it is very difficult to provide sufficient mixing of air measured by the change in the Christensen-Anderson with the binder at temperatures considered reasonable for crossover frequency was greater for AASHTO R30 com- simulating long-term aging. Attempts to modify the MGRF pared to the RTFOT, SAFT, and MGRF. Binder aging to improve mixing by adding scrapers and balls or rollers indices also were greater for AASHTO R30 compared to were not successful. Adaptation of the SAFT to long-term the three short-term binder aging procedures. aging was more successful. Through changes in the design of 2. Based on an analysis of aging index rankings, short-term the SAFT impeller, researchers demonstrated that equivalent binder aging in the RTFOT and MGRF provided similar PAV aging could be accomplished in approximately 40 hours, rankings as short-term mixture aging in accordance with twice the time required for the current PAV. AASHTO R30. The ranking of binders aged in the SAFT Another consideration in NCHRP Project 9-36 was an alter- did not correlate well with the AASHTO R30 rankings. nate to the current RTFOT mass change procedure for quanti- fying binder volatility. The SAFT included a VCS that used an An interesting and unexpected finding from both of the air-cooled condenser to collect vapors produced during aging. experiments in the verification study was that for the binders With appropriate glassware, the MGRF also could be modified tested, the average aging of the neat binders was approxi- to use a VCS. Based on the VCS study, it was concluded that the mately the same as that for the modified binders for AASHTO air-cooled condenser was inadequate because it only collected R30, RTFOT, and MGRF conditioning. This finding is in a small amount of the volatile compounds generated during the contrast with other studies that have reported less aging in the test. An improved VCS that incorporates a resin bead filter RTFOT for modified binders. For the SAFT, the average and a molecular sieve that are commonly used in chromato- aging of the neat binders was greater than that of the modi- graphic studies was developed. This system can be adapted to fied binders. 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 4.2 Conclusions similarly. There was no difference in average aging indices The primary objective of NCHRP 9-36 was to identify and between neat and modified binders in any of the three tests. For verify a test procedure that can be used as a replacement the specific aggregate used, AASHTO R30 aged the binders for the RTFOT. Based on the findings summarized above, more than the RTFOT and MGRF. The ranking of binder aging the MGRF is considered an acceptable replacement for the was similar for AASHTO R30, the RTFOT, and the MGRF. RTFOT. For neat binders, MGRF and RTFOT conditioning produced similar rheological properties. MGRF and RTFOT 4.3 Proposals for Future Action conditioning also produced similar rheological properties for typical polymer-modified binders. The ranking of the aging Based on the experiments and analyses completed in susceptibility of binders for both the MGRF and RTFOT cor- NCHRP Project 9-36, the following proposals for future related well with the ranking of aging susceptibility from mix- action are made. tures that were short-term oven-aged for 4 hours at 135°C in accordance with AASHTO R30. Although rheological prop- 1. The MGRF is a viable alternative to the RTFOT for short- erties were the same, mass change in the MGRF is less than in term binder aging. For a wide range of neat and modified the RTFOT, averaging approximately 40 percent of the binders, the researchers demonstrated that rheological RTFOT mass change for the binders tested in this study. properties for MGRF and RTFOT residue are the same. The SAFT, on the other hand, is not an acceptable replace- Additionally, the MGRF and RTFOT provide ranking of ment for the RTFOT for a wide range of binders. There is a short-term aging susceptibility that is similar to that for significant difference in the rheological properties of SAFT- mixtures aged in a forced-draft oven in accordance with conditioned and RTFOT-conditioned neat binders, and the AASHTO R30. difference is more apparent for higher stiffness binders. Addi- 2. A modification to the mass change criterion in AASHTO tionally, there is poor correlation in the ranking of the aging M320 is needed if the MGRF is used in its current form. susceptibility of binders as measured by the SAFT and as The mass change in the MGRF is approximately 40 per- measured by oven-aged mixtures. cent of the mass change in the RTFOT; therefore, the mass A consideration in selecting a replacement for the RTFOT change criterion for MGRF residue should be ± 0.40 per- was that the test, or at least the associated equipment, should cent to be equivalent to ± 1.0 percent for the RTFOT.
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63 3. Different conditioning procedures are needed for short- tion, and the need for a bath cover to better control and long-term aging. In general, procedures designed to temperature. expose binder to air at plant mixing temperatures are not 6. Additional research to calibrate short-term aging proce- capable of mixing air with the binder at the lower temper- dures for binders and mixtures should be considered. The atures representative of aging during the service life of the research completed in NCHRP Project 9-36 showed a dif- pavement. ference between the short-term binder and mixture aging 4. Consideration should be given to separating the measure- procedures, with the mixture procedure providing some- ment of binder volatility from the short-term aging proce- what greater amounts of aging. It should be possible to dure. This study clearly showed that only a small mass of calibrate the binder and mixture procedures to provide hydrocarbon volatiles is collected during short-term aging. similar levels of aging. This research should include eval- A simple mass-change test performed under heat and vac- uations of plant-produced mixtures to ensure that the uum could be developed to quantify binder volatility. The procedures produce representative levels of actual con- test could be developed to adapt equipment (scale, pans, struction aging. and vacuum oven) already available in binder testing labo- 7. Additional research should be considered to adequately ratories. It is recommended that this test be pursued for use address long-term binder aging. Future research into with both the RTFOT and MGRF. long-term aging should include work with the PAV and 5. The following additional development for the MGRF other alternatives that may be identified in the future. should be considered: This work should generally be directed at establishing · Replace the mass-change measurements in the cur- operating conditions for simulated laboratory aging tests rent MGRF procedure with the vacuum volatility test that reproduce the degree of aging that occurs in field outlined above. pavements for typical binders. Mirza and Witczak's Global · Investigate modifying the MGRF test to allow aging of Aging Model, while highly empirical, provides an estimate different volumes of binder. One of the advantages of of site-specific aging based on an analysis of historical data. the RTFOT is that it can be used to condition small Work in NCHRP Project 9-23 that was reviewed during quantities of binder. This has practical application in NCHRP Project 9-36 shows that the PAV operated at binder quality control testing and in designing mixtures 100°C for 20 hours under 304.6 psi (2.1 MPa) air pressure with high percentages of recycled asphalt pavement. produces aged binders with viscosities that are in reason- · Conduct a formal ruggedness test for the current pro- able agreement with this model for a time of 10 years and cedure to identify appropriate tolerances for the testing moderate mean annual air temperature conditions. Based conditions. Factors that should be considered include on this finding, the potential for the development of a long- bath temperature, rotational speed, flask submersion term aging procedure that represents a reasonable period depth, airflow rate, flask angle, binder quantity, dura- of service in the field is encouraging.