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6 CHAPTER 2 Research Approach 2.1 Overview aging obtained with the RTFOT. The last study that was con- ducted in NCHRP Project 9-36 was the verification study. In Figure 2-1 presents a flowchart for the project. The general this study, the properties of binders aged in both the SAFT and approach adopted for NCHRP Project 9-36 was to improve the MGRF were compared to properties of binders aged in the existing technologies rather than develop a completely new RTFOT and the properties of binders from mixtures that aging procedure. The project started with a review of existing were short-term oven-aged in accordance with the perform- binder aging procedures to identify viable candidate methods ance-testing procedure in AASHTO R30. Initially, only the for possible improvement. Two viable methods were identified: SAFT was included in the study, but the study was expanded the Stirred Air Flow Test (SAFT) and the Modified German to include the MGRF. The verification study served as the Rotating Flask (MGRF) (1, 2). Figures 2-2 and 2-3 are schemat- basis for the final proposals for short-term aging that are the ics of these two devices. The SAFT uses air blowing to simulate primary product of NCHRP Project 9-36. Each of the studies short-term aging of the binder. Air from a nozzle submerged shown in Figure 2-1 is described in greater detail in the sections in the binder is dispersed in the binder by an impeller mounted that follow. to an external motor. The SAFT also includes a condenser to collect volatile compounds that are released during the aging process. The MGRF uses a rotary evaporator similar to that 2.2 Identify Viable specified in ASTM D5404 and AASHTO T319 for recovery of Candidate Methods binders after solvent extraction. In the MGRF, air is intro- duced into the rotating flask to age the binder. The exhaust 2.2.1 Ideal Aging Procedure gases from the MGRF are not collected; mass change measure- for Specification Testing ments are used to quantify binder volatility. From the review, A critical review of existing binder aging procedures was it was determined that both the SAFT and the MGRF are rel- conducted to identify viable candidates for improvement in atively inexpensive, easy to perform, applicable to both neat NCHRP Project 9-36. To guide this critical review, the fol- and modified binders, and--based on available literature-- lowing requirements for ideal short- and long-term aging can reasonably reproduce the level of aging that occurs in the procedures for specification testing in the United States were RTFOT. However, it was not clear from the review if either developed. test could be extended to long-term aging. The selection study was, therefore, conducted to choose one of these methods for further development. The selection study investigated whether, 1. Both long-term and short-term aging must be simulated. at a temperature of 100C, either test can adequately mix air The aging of asphalt binders, whether in the field or during with stiff modified binders to produce a level of aging simi- accelerated laboratory aging, is a very complex process that lar to that obtained in the PAV. From this study, the SAFT has received considerable attention from researchers for was selected for further development. The additional devel- many years. It is generally agreed that the aging process opment for the SAFT included (1) a volatile collection system occurs in two distinct steps: (1) during construction (plant (VCS) study to design an improved system for quantifying the mixing, placement, and compaction) and (2) during the volatility of binders tested in the SAFT and (2) a SAFT opti- service life of the pavement. During construction, the aging mization study to determine operating parameters for the occurs at an elevated temperature, and there is opportunity SAFT so that for neat binders, it would reproduce the level of for the asphalt binder to both oxidize and to lose volatile

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7 Identify Viable Candidate compounds. In contrast, aging during the service life of a Methods pavement occurs at a much lower temperature where oxi- dation is the primary aging mechanism. There is relatively little volatile compound loss during the service life of a pavement. Therefore, the ideal binder aging procedure Selection Study must accommodate both the short-term aging that occurs (Evaluate Extendibility to during the construction process as well as the long-term Long-Term Aging) aging that occurs during the service life of a pavement. Stirred Air Modified German 2. Simulated laboratory aging must be conducted at two Flow Test Rotating Flask temperatures, one representing conditions at the hot mix plant and the other as close as possible to pavement service temperatures. Short-term aging is simulated with Volatile Collection System the RTFOT. In this test the asphalt binder is exposed to a Study stream of air at 163C, which is representative of mixing and compaction temperatures. For a pavement, maximum service temperatures range from 58 to 70C. Research con- ducted during the SHRP asphalt research program clearly SAFT Optimization Study demonstrated that the aging mechanisms that occur in the laboratory during simulated aging change significantly when the aging temperature rises above approximately 110C. This finding limits the extent to which temperature can be used to accelerate the simulation of long-term aging. Verification Study Consequently, the short- and long-term aging must be conducted at different temperatures. Previous studies also indicate that the long-term aging mechanism, and its asso- Recommendations ciated kinetics, is more reliably simulated when the accel- erated aging is conducted as close as possible to the service Figure 2-1. Flowchart for NCHRP Project 9-36. temperature. Figure 2-2. Schematic of the prototype Stirred Air Flow Test (1).

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8 Figure 2-3. Schematic of the Modified German Rotating Flask (2). 3. The short-term aging process must include a procedure 4. Sufficient material must be available for characterizing for capturing and measuring volatile loss. During the the physical properties of the asphalt binder after short- mixing and compaction process, some of the material of and long-term aging. The ideal aging procedure, as envi- lighter molecular weight can vaporize and escape into the sioned within the context of this project, will be used for atmosphere. This is, in part, essentially an extension of the specification purposes. The specifications for asphalt distillation process. As well, components--primarily oils binders are based upon physical properties. Consequently, used in the preparation of modified asphalt binders--also the ideal aging procedure must yield sufficient material to may vaporize and escape into the atmosphere. The RTFOT characterize the physical properties of the asphalt binder. provides a means of measuring mass change, which includes Adequate material must be available after the short- and the loss of mass resulting from volatilization as well as mass long-term aging steps. gain that can result from oxidation. Volatile loss is more 5. The long-term aging must be completed within 46 hours. significant to the paving industry than mass change. Whereas the short-term aging can be conducted at tem- Volatile loss should be more directly related to "blue smoke" peratures representative of the hot mix plant and at real- than mass change, wherein excessive vapors are lost to the istic times (less than 4 hours), the long-term aging must atmosphere, causing an environmental problem. Further, be accelerated by some means in order to meet the needs volatile loss provides better control than mass change over of both the producers and users of the asphalt binder. Cur- certain undesirable refining practices (for example, when a rently, both pressure and temperature are used to acceler- hard base stock is "cut" with oils). Therefore, the ideal ate the aging process in the PAV. This results in a 20-hour binder aging procedure should capture the volatile com- test. During the SHRP asphalt research program, a 144- pounds that are lost during the simulation of short-term hour test conducted at 60C was proposed. Industry aging. For the purposes of a specification test, it is not nec- strongly objected to this protocol and, consequently, the essary to characterize these materials, but simply to collect aging temperature was increased to 100C. It is clear that and weigh them. Including such a procedure in the short- a specification test that can be used for quality control and term aging process also should be of benefit to research acceptance purposes, and that is applicable to U.S. prac- studies where it may be desirable to characterize the nature tice, cannot be of 1-week duration. Recognizing that of the volatile compounds. there must be a compromise between test reliability and

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9 test length, the long-term aging test must be completed ter curve. This was demonstrated with both laboratory- within approximately 46 hours (2-day cycle time for and field-aged materials. Thus, aging kinetics can be cap- equipment). As discussed with the NCHRP project panel, tured by using a very limited rheological testing protocol. limiting the long-term aging procedure to a 1-day test The ideal aging procedure will allow sampling at multiple may be too restrictive in terms of developing a reliable aging times so that aging kinetics can be included in a long-term aging test. On the other hand, a test that requires future specification. more than 2 days to complete will be unacceptable to the 8. The aging test procedure must not expose the operators industry. to unsafe or hazardous conditions. Safety is a key issue that 6. The short- and long-term aging procedure must be must be considered in any laboratory procedure. Of pri- applicable to both neat and modified asphalt binders. mary concern with respect to an ideal aging test is the aging One of the primary reasons for NCHRP Project 9-36 is that atmosphere during the long-term test. Some researchers the RTFOT is not applicable to some modified binders. have suggested the use of pure oxygen as the aging envi- Some modified asphalt binders do not roll uniformly ronment, but there are concerns over the safety of the use within the bottle during the test, thereby negating a basic of pure oxygen in commercial laboratories. Further, the assumption of the test (i.e., that during the test the asphalt use of high pressure, regardless of atmosphere, is to be binder is exposed, in the form of a thin film, to the stream avoided if possible. of air). A second problem with the RTFOT is that some 9. Equipment for the aging procedure must have a reason- modified binders tend to "crawl" out of the bottle during able cost, be reliable, easy to operate and clean, and be the test. These problems were addressed during NCHRP 9- configurable for both the short-and long-term aging 10 by placing steel rods in the RTFOT bottles. Subse- procedures. There are a number of other features that an quent work by FHWA showed that the steel rods were not ideal procedure must embody. Without elaborating, the effective. During the long-term aging process, the asphalt equipment must be reasonably priced, reliable, easy to binder must form a film, or be stirred in a manner that is operate, and easy to clean (especially with respect to the equally applicable to neat or modified binders. This is use of solvents). Although the operating conditions for especially of concern during the simulation of long-term short-term aging and long-term aging must be different, aging where oxidation is the aging mechanism. In order this does not necessarily militate against the use of the same for the long-term test to be equally applicable to neat and equipment for both short-term aging and long-term aging. modified binders, the availability of oxygen must be inde- Various scenarios are possible, and the same equipment pendent of the type of binder. might be used for both long-term and short-term aging, but 7. The long-term aging test must accommodate binder- the temperature may be changed. On the other hand, the and modifier-specific aging kinetics. The rate of harden- equipment may be the same but operated under different ing, whether in the field or in the laboratory under accel- conditions as, for example, a container switched between erated conditions, is binder-specific. Different binders are replicate devices operated at different temperatures. affected differently by changes in temperature or length of exposure during accelerated long-term aging. In other A summary listing of the key features of the ideal aging pro- words, aging kinetics are binder-specific. The current PAV cedure for specification testing is presented as follows. This list test may simulate the aging that occurs in 5 years for one was used to guide the review of existing tests and the selection binder and 10 years for another binder. The fact that the of viable procedures. The ideal aging procedure should PAV test does not treat all binders equally was well known to researchers during the SHRP asphalt research program. Simulate both long-term and short-term aging; To further complicate the matter, aging kinetics are also Be performed at two temperatures, one representing condi- modifier-specific and differ from the aging kinetics of tions at the hot mix plant and the other representing, as close asphalt binders. To characterize aging kinetics, physical as possible, service temperatures; property measurements must be made at multiple times Include a procedure for capturing and measuring volatile or temperatures, or a combination thereof. Although it is loss during short-term aging; unrealistic to expect that a complete characterization of Provide sufficient material for characterizing the physical aging kinetics will be part of a future specification, the properties of the binder after short- and long-term aging; ideal long-term aging test should, as a minimum, provide Complete long-term aging within 46 hours; for sampling at multiple aging times. During the SHRP Be applicable to both neat and modified asphalt binders; asphalt research program, researchers noted that the aging Accommodate binder- and modifier-specific aging kinetics process caused a shift in the rheological master curve as in the long-term aging; opposed to a change in the shape of the rheological mas- Be safe and technician-friendly; and