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1 SUMMARY Investigation of Short-Term Laboratory Aging of Neat and Modified Asphalt Binders The objective of NCHRP Project 9-36 was to select, refine, and validate an improved pro- cedure for the short-term laboratory aging of asphalt binders for use in a purchase specifica- tion such as AASHTO M320, Standard Specification for Performance-Graded Asphalt Binder. The selected procedure would be considered to replace the Rolling Thin Film Oven Test (RTFOT, AASHTO T240) that was selected during the Strategic Highway Research Program (SHRP) based on previous experience. The following were major considerations in the selection of the improved procedure: 1. The short-term conditioning procedure should be equally applicable to neat and modified materials. 2. It should mimic the physical changes that occur in hot mix asphalt (HMA) mixes condi- tioned in accordance with AASHTO R30. 3. It should include a method to quantify binder volatility. 4. If possible, it should be extendible to long-term aging. The general approach adopted for NCHRP Project 9-36 was to improve existing technolo- gies rather than develop a completely new aging procedure. The project started with a review of existing binder aging procedures to identify viable candidate methods for possible improvement. Two viable methods were identified: the Stirred Air Flow Test (SAFT) and the Modified German Rotating Flask (MGRF). From the review it was determined that both the SAFT and the MGRF are relatively inexpensive, easy to perform, applicable to both neat and modified binders, and--based on available literature--can reasonably reproduce 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 conducted to choose one of these methods for further development. 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 Pressure Aging Vessel (PAV, AASHTO R28). From this study, the SAFT was selected for further development. The addi- tional development for the SAFT included (1) a volatile collection system (VCS) study to design an improved system for quantifying the volatility of binders tested in the SAFT and (2) a SAFT optimization study to determine operating parameters for the commercial ver- sion of SAFT so that it would reproduce the level of aging obtained with the RTFOT for neat binders. The commercial version of the SAFT used a different heating system and control than the prototype SAFT reported in the literature. The final study conducted in NCHRP Project 9-36 was a verification study. In this study, the properties of binders aged in both the SAFT and the MGRF were compared to proper- ties of binders aged in the RTFOT and the properties of binders from mixtures that were
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2 short-term oven aged in accordance with the performance testing procedure in AASHTO R30. Initially, only the SAFT was included in the study, but the study was expanded to include the MGRF. The results of the verification study were the basis for the final recom- mendations for short-term aging that are the primary product of NCHRP Project 9-36. The key findings from these studies are summarized below. Selection Study It was not feasible to develop a long-term aging version of the MGRF or SAFT. At 100°C, the maximum temperature considered viable for a long-term aging test, the MGRF does not generate a moving film and a number of attempts to modify the apparatus by adding scrapers and balls or rollers was not successful. Adequate mixing of air was also a serious problem at 100°C for the SAFT and a number of impeller designs were evaluated in order to improve the mixing efficiency and, consequently, 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, twice the duration of the current PAV test. Volatile Collection System (VCS) Study The SAFT included a VCS to collect volatiles from the binder during short-term aging. The VCS was considered an improvement over the current mass change procedure used in the RTFOT, but the reported low mass of volatiles collected with the VCS, one tenth of the mass loss in the RTFOT procedure, prompted a review of the VCS developed for the SAFT. This review confirmed that the original design of the VCS was inadequate and that volatiles were passing through the VCS. After considerable trial and error, a VCS based upon absorbents commonly used for chromatographic studies was found to be effective for collecting the volatiles produced during the SAFT procedure. This system includes hydro- carbon and moisture traps on the inlet side of the SAFT vessel and a 100-mm-long resin bed and molecular sieve filters on the outlet to collect hydrocarbons and water, respectively. It also was found that the majority of the volatiles collected are water, not hydrocarbons. SAFT Optimization Study An unexpected finding from early work with the commercial 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 ves- sel wall that was in direct contact with the heating mantle. The commercial SAFT uses an oven to heat the vessel and limits the oven temperature to 176°C. This finding led to an extensive optimization study to establish operating parameters appropriate for the commer- cial SAFT. Operating parameters for impeller speed, airflow rate, and aging time were devel- oped to provide a residue that best approximated the rheological properties of the RTFOT residue for PG 58-XX binders. Verification Study The study to verify the equivalency of the SAFT and MGRF relative to the RTFOT was conducted in the following two parts: 1. RTFOT verification experiment where rheological properties of binders aged in the SAFT and MGRF were compared to rheological properties of binders aged in the RTFOT and
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3 2. Oven-aged mixture experiment where rheological properties of short-term aged binders were compared to properties back-calculated from oven-aged mixtures. The RTFOT Verification Experiment included comparisons of high-temperature contin- uous grades, master curve parameters, and aging indices for SAFT and MGRF residues to those for RTFOT residues. These comparisons showed that the MGRF and RTFOT provided similar aging. The aging for the SAFT relative to the RTFOT was binder dependent, becom- ing significantly less as the binder stiffness increased. In the oven-aged mixture experiment, properties of the short-term aged binders back- calculated from dynamic modulus 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. This experiment showed that 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. However, there was a rea- sonable relationship between rankings of aging susceptibility measured by the RTFOT and the MGRF and that measured by 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 approximately the same as that for the modified binders for AASHTO R30, RTFOT, and MGRF condition- ing. 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 modified binders. Based on the findings summarized above, the MGRF was 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 rhe- ological properties for typical polymer-modified binders. The ranking of the aging suscep- tibility of binders for both the MGRF and the RTFOT correlated well with the ranking of aging susceptibility from mixtures that were short-term oven aged for 4 hours at 135°C in accordance with AASHTO R30. Although rheological properties 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. A modification to the AASHTO M320 mass change criteria would be needed if the MGRF is to be used in its current form. The SAFT, on the other hand, is not an acceptable replacement 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. Additionally, 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. 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 the MGRF. The ranking of binder aging was similar for AASHTO R30, the RTFOT, and the MGRF. Future research was proposed to calibrate short-term aging procedures for binders and mixtures. The research completed in NCHRP Project 9-36 showed a difference between the short-term binder and mixture aging procedures, with the mixture procedure providing somewhat greater levels of aging. It should be possible to calibrate the binder and mixture procedures to provide similar levels of aging. This proposed research should include evaluations of plant-produced mixtures to ensure that the procedures produce representative levels of actual construction aging.
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4 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 suc- cessful. Adaptation of the SAFT to long-term aging was more successful. Through changes in the design of the SAFT impeller, it was demonstrated that equivalent PAV aging could be accomplished in approximately 40 hours, twice the time required for the current PAV. As a result, it was recommended that 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 temperatures repre- sentative of aging during the service life of the pavement. Future research beyond NCHRP Project 9-36 was proposed to adequately address long- term aging. This research should include work with the PAV and other alternatives that may be identified in the future. It should generally be directed at establishing operating condi- tions 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 current PAV produces aged binders with viscosities that are in reasonable agreement with estimates from this model for a time of 10 years and moderate mean annual air tem- perature 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. Another consideration in NCHRP Project 9-36 was an alternate to the current RTFOT mass change procedure for quantifying binder volatility. The SAFT included a VCS that used an air- cooled condenser to collected vapors produced during aging. With appropriate glassware, the MGRF also could be modified to use a similar 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 chromatographic studies was devel- oped. This system can be adapted to the MGRF, but not the RTFOT. Although the redesigned VCS represents an improvement over the mass change measurement currently used in the RTFOT and the MGRF, its implementation was not proposed. Instead, consideration should be given to separating the measurement of binder volatility from the short-term aging proce- dure. This study clearly showed that only a small mass of hydrocarbon volatiles are collected during short-term aging. A simple mass change test performed under heat and vacuum 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 laboratories. It is proposed that this test be pursued for use with both the RTFOT and the MGRF. In addition to investigating the vacuum volatility test described, further development of the MGRF was proposed. Consideration should be given to an investigation of 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 appli- cation in binder quality control testing and in designing mixtures with high percentages of reclaimed asphalt pavement. Consideration also should be given to conducting a formal ruggedness test for the current MGRF procedure to identify appropriate tolerance for the testing conditions. Factors that should be considered include bath temperature, rotational speed, flask submersion depth, airflow rate, flask angle, binder quantity, duration, and the need for a bath cover to better control temperature.