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Suggested Citation:"Research Results Digest 324." National Academies of Sciences, Engineering, and Medicine. 2007. Simulating the Effects of Hot-Mix Asphalt Aging for Performance Testing and Pavement Structural Design. Washington, DC: The National Academies Press. doi: 10.17226/23238.
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Suggested Citation:"Research Results Digest 324." National Academies of Sciences, Engineering, and Medicine. 2007. Simulating the Effects of Hot-Mix Asphalt Aging for Performance Testing and Pavement Structural Design. Washington, DC: The National Academies Press. doi: 10.17226/23238.
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Suggested Citation:"Research Results Digest 324." National Academies of Sciences, Engineering, and Medicine. 2007. Simulating the Effects of Hot-Mix Asphalt Aging for Performance Testing and Pavement Structural Design. Washington, DC: The National Academies Press. doi: 10.17226/23238.
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Suggested Citation:"Research Results Digest 324." National Academies of Sciences, Engineering, and Medicine. 2007. Simulating the Effects of Hot-Mix Asphalt Aging for Performance Testing and Pavement Structural Design. Washington, DC: The National Academies Press. doi: 10.17226/23238.
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Suggested Citation:"Research Results Digest 324." National Academies of Sciences, Engineering, and Medicine. 2007. Simulating the Effects of Hot-Mix Asphalt Aging for Performance Testing and Pavement Structural Design. Washington, DC: The National Academies Press. doi: 10.17226/23238.
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Page 5
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Suggested Citation:"Research Results Digest 324." National Academies of Sciences, Engineering, and Medicine. 2007. Simulating the Effects of Hot-Mix Asphalt Aging for Performance Testing and Pavement Structural Design. Washington, DC: The National Academies Press. doi: 10.17226/23238.
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Suggested Citation:"Research Results Digest 324." National Academies of Sciences, Engineering, and Medicine. 2007. Simulating the Effects of Hot-Mix Asphalt Aging for Performance Testing and Pavement Structural Design. Washington, DC: The National Academies Press. doi: 10.17226/23238.
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Suggested Citation:"Research Results Digest 324." National Academies of Sciences, Engineering, and Medicine. 2007. Simulating the Effects of Hot-Mix Asphalt Aging for Performance Testing and Pavement Structural Design. Washington, DC: The National Academies Press. doi: 10.17226/23238.
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Research Results Digest 324 October 2007 BACKGROUND Environment plays a significant role in determining the properties of hot mix as- phalt (HMA) as a function of time, which in turn affects the performance of HMA pave- ments. The major environmental factors that affect HMA material properties include the changes of temperature and moisture over time. Research conducted in Strategic High- way Research Program (SHRP) Project A-005 clearly demonstrated the effect of environmental temperature on the age hardening characteristics of asphalt binders. The project concluded that higher mean annual air temperatures result in relatively higher rates of aging than the cooler cli- mates. The research also showed the effect of other parameters, such as volumetric properties and the location of the HMA layer in the pavement system on asphalt mix aging. Higher air void contents result in greater oxidation and hence more stiffening of the HMA mix. HMA layers located deeper in the pavement are not in direct con- tact with air; as a result, the oxidation of the asphalt binder in these layers is reduced. Therefore, the stiffening of the HMA mix is inversely proportional to the depth at which it is located in the pavement system. Hardening of the original asphalt binder due to the plant mixing and laydown (short- term aging) and normal in situ aging (long- term aging) are extremely complex phe- nomena because of the numerous factors influencing the rate of aging. While the mechanism of aging is complex, its impact on pavement performance is generally un- derstood. Short- and long-term aging result in hardening of the asphalt binder with time and a gradual, concomitant increase in the dynamic modulus (stiffness) of the HMA mix over time. This hardening of the asphalt binder and HMA mixture can lead to the devel- opment of several types of distress, which may ultimately lead to the failure of the pavement system. These distresses include fatigue cracking, low-temperature cracking, and non-load-associated cracking failure due to random, irregular surface cracking. Two laboratory procedures were de- veloped in SHRP Project A-003 to simu- late the hardening potential of asphalt binders and HMA mixes. These procedures SIMULATING THE EFFECTS OF HOT MIX ASPHALT AGING FOR PERFORMANCE TESTING AND PAVEMENT STRUCTURAL DESIGN This digest summarizes key findings from Part 1 of NCHRP Project 9-23, “Environmental Effects in Pavement Mix and Structural Design Systems,” conducted by Arizona State University, Tempe, Arizona. The Part 1 final report was authored by W. N. Houston, M. W. Mirza, C. E. Zapata, and S. Raghavendra and is available online as NCHRP Web-Only Document 113. Subject Areas: IIB Pavement Design, Management, and Performance and IIIB Materials and Construction Responsible Senior Program Officer: Edward T. Harrigan NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

are available today as (1) AASHTO Standard Prac- tice R 28, “Accelerated Aging of Asphalt Binder Using a Pressurized Aging Vessel” (PAV), and (2) AASHTO Standard Practice R 30, “Mixture Condi- tioning of Hot-Mix Asphalt.”1 These practices have proven of great value in HMA mix design, but, due to constraints on resources and time in SHRP, these prac- tices have certain limitations. SCOPE OF THE RESEARCH AASHTO Standard Practice R 28 (hereafter re- ferred to as “AASHTO R 28”) requires testing of asphalt binder samples at the following conditions: • Asphalt binder aged using AASHTO T 240, “Effect of Heat and Air on a Moving Film of Asphalt (Rolling Thin-Film Oven Test).” • PAV aging time = 20 hours. • Air pressure = 2.10 MPa. • Aging temperature = 90°C, 100°C, or 110°C, depending on the climatic conditions being simulated. For long-term mixture conditioning for mechan- ical property (i.e., performance) testing, AASHTO Standard Practice R 30 (hereafter referred to as “AASHTO R 30”) specifies: • HMA mixture aged in a forced draft oven. • Aging time = 5 days. • Aging temperature = 85°C. Potential limitations associated with these two practices can be summarized as follows: • Although the mean annual air temperature (MAAT) in the United States varies over an approximate range of 35°C to 75°C, AASHTO R 28 specifies only three PAV temperatures and AASHTO R 30 specifies only one oven- aging temperature to represent this wide range of MAAT. • AASHTO R 28 fails to specify clear “cut- off” points between the climatic conditions represented by each possible PAV aging temperature. • AASHTO R 28 represents the expected aging of the asphalt binder over a period of 5 to 10 years, while AASHTO R 30 represents the expected aging of the HMA mix over a period of 5 to 7 years. These ranges are wider than de- sirable, and prediction of aging at any other time during the life of the pavement is not pos- sible with the practices as presently written. • The laboratory conditions are similar for every asphalt binder. However, the aging potential of a binder depends on its physicochemical prop- erties, which ideally should be accounted for in the practices. • The aging simulated by the practices does not account for the influence of volumetric proper- ties, in particular variation in air void content, which has a significant influence on oxidative aging. Part 1 of NCHRP Project 9-23, “Environmental Effects in Pavement Mix and Structural Design Sys- tems,” was conducted to (1) verify the work done under the SHRP Projects A-002, A-003, and A-005 in developing these two practices for simulating the real-world aging of asphalt binders and HMA mixes and (2) provide guidance on the significance of the limitations of the practices noted above. To accom- plish these goals, a program of laboratory testing and parametric analyses summarized below was con- ducted at the Advanced Pavement Laboratory at Arizona State University on asphalt binders and HMA cores obtained from LTPP and other field sites across the United States. Detailed information of the conduct and results of these experiments is con- tained in NCHRP Web-Only Document 113. EXPERIMENTS AND ANALYSES Correction Factor for Binder Recovery This experiment was carried out to develop a cor- rection factor to account for the changes in the binder viscosity that may occur due to extraction and re- covery of asphalt binder from HMA. Verification of AASHTO R 28 In order to verify the existing AASHTO R 28, the viscosities of asphalt binders aged in the labora- tory were determined and compared to the viscosities of asphalt binders extracted from field cores, the age of which were established from construction records. Comparisons between laboratory- and field-aged binders were used to verify the practice. 2 1AASHTO R 28 was developed from Provisional Standard PP 1, and AASHTO R 30 was developed from Provisional Standard PP 2.

Improvement of AASHTO R 28 and Model Development Possible improvements to AASHTO R 28 were developed in this experiment. A key goal was de- velopment of an improved predictive model cali- brated to asphalt binder type and specific field aging conditions. Calibration of the Predictive Model with Field Data The improved prediction model was calibrated with field data. Cores were obtained from MnRoad, WesTrack, and Arizona DOT field experiments. Asphalt binders were extracted from these cores, re- covered, and tested with the dynamic shear rheome- ter to determine their viscosity. The viscosity of the asphalt binders was predicted for the given set of field aging conditions with the improved model. Predicted viscosities were then compared with the measured viscosities, and the model was calibrated to best fit the available field data. Validation of the Improved Predictive Model The improved predictive model was validated with an independent set of LTPP data, using a pro- cedure similar to that used in the previous calibra- tion experiment. Cores were taken at eight LTPP sites, with the field aging conditions obtained from the DATAPAVE database. However, no original asphalt binders were available from these sites, and the rolling thin film oven (RTFO) viscosities were generally not available from the database, so the RTFO viscosities needed as input to the predictive model were estimated using A-VTS values obtained from the Mechanistic-Empirical Pavement Design Guide software, where A and VTS are regression constants in the following equation relating asphalt binder viscosity in centipoise and temperature in degrees Rankine: Parametric Study of the Improved Predictive Model A sensitivity analysis was conducted of the cal- ibrated and validated predictive model. Outputs were loglog logη = + ( )A TRVTS generated with a matrix covering the possible range of input values, and constraints to be applied on the input parameters were determined. Verification of the AASHTO R 30 Long-Term Conditioning Protocol The long-term conditioning protocol in Sec- tion 7.3 of AASHTO R 30 is intended to simulate the in situ oxidative aging that occurs in HMA mixes during 5–7 years of pavement service. In this experiment, the dynamic modulus of HMA speci- mens aged in the laboratory was determined. These modulus values were compared with those of field cores, whose ages were known from construction records. The protocol was verified through com- parison of the laboratory-aged specimens with the field-aged cores. Analysis and Correlation of Laboratory-Aged and Field-Aged Data This analysis was conducted in the hope of making the AASHTO R 30 long-term mixture- conditioning protocol more reflective of the volu- metric properties of the HMA and the specific envi- ronmental conditions to which it is exposed. FINDINGS AND RECOMMENDATIONS FOR AASHTO STANDARD PRACTICE R 28 Findings Correction Factor for Binder Recovery There was no practical, significant difference between the viscosities of original and recovered binders. Thus, the particular binder recovery proce- dure employed in the project had no significant effect on binder viscosity. No correction factor to the recovered binder vis- cosity was necessary. Verification of AASHTO R 28 The performance grade of the asphalt binder (in terms of stiffness), MAAT, and the mix air void content should be considered when deciding the PAV aging temperature required to produce a PAV- aged binder with the same viscosity as a field-aged binder. 3

AASHTO R 28 should be expanded to include the effects of field aging conditions and the detailed volumetric properties. AASHTO R 28 can also be improved to better simulate the aging that occurs over a specific time period. Calibration of the Improved TPAV Predictive Model with Field Data An improved model (Equation 1) for predic- tion of the required PAV aging temperature is more accurate than the values in Tables 1 and 2 of AASHTO M 320 and may be valuable addendum to AASHTO R 28. Validation of the Improved TPAV Predictive Model with Field Data Equation 1 provided reasonable PAV aging tem- peratures when actual field aging conditions were used as input. Higher MAAT, air void content, and aging time yield higher PAV-aged temperatures, as expected. Parametric Study of the Improved TPAV Predictive Model The parametric study of Equation 1 yielded no significant limitations or constraints on the input values. Recommendations for Future Work Further refinement of the improved TPAV predic- tive model (Equation 1) should be accomplished with data from additional field sites. In this research, binder viscosity was found to remain more or less constant with depth in the pave- ment; the differences found were not practically sig- nificant. This unexpected result should be further examined and tested. TPAV RTFO C = + ×( ) × ° 2 132432 0 193560 60 2 . . loglog , η MAAT t aging ⎛ ⎝⎜ ⎞ ⎠⎟ × ( ) + − × ln . . log 109 9632 78 2945 log . , ηRTFO C60 2 0 44544 ° ( ) ⎧ ⎨ ⎪⎪⎪ ⎩ ⎪⎪⎪ ⎫ ⎬ ⎪⎪⎪ ⎭ ⎪⎪⎪ × 5 0 378370×( )VAorig. (1) Recommendations for the Implementation of the Improved TPAV Predictive Model Equation 1 yields a continuum of PAV tempera- tures that may fall below 90°C or exceed 110°C in specific instances, while the operation of commercial PAVs is typically limited to temperatures between 90°C and 110°C. One means to remedy this situation is to estimate the field aging time taging for an expected combination of binder viscosity, air void content, and MAAT at one or more of the standard PAV temper- atures (90°C, 100°C, and 110°C). This can be done by inverting Equation 1 to Equation 2 to yield taging: Suggested Revisions to AASHTO R 28 In order to demonstrate the possible implemen- tation of Equations 1 and 2 within AASHTO R 28, ten cities in various climatic regions within the United States were chosen. Values of MAAT for these cities were calculated with the Mechanistic- Empirical Pavement Design Guide software, and an original air void content of 8 percent was used in all cases. Typical binder grades were selected based on the maximum and minimum temperatures found for the cities from LTPPBIND V2.1. RTFO viscosities corresponding to the selected asphalt binder performance grades were calculated using A-VTS values in the Mechanistic-Empirical Pave- ment Design Guide software for Level 3 designs. With these values input to Equation 1, the PAV aging temperatures required to simulate 5, 10, 15, and 20 years of aging were predicted. Equation 2 was then used to estimate the field aging time in months simulated by PAV aging temperatures of 90°C, 100°C, and 110°C. Based on these two sets of estimates presented in Tables 1 through 3, recom- mended revisions to AASHTO R 28 were developed and are presented in Table 4. t T VA aging PAV orig = × ⎛ ⎝⎜ ⎞ ⎠⎟ exp . .0 445445 0 378370 − + × ( ) ⎛ ⎝ ⎜⎜⎜ ° 109 9632 78 2945 60 2 . . loglog , ηRTFO C ⎜⎜⎜ ⎞ ⎠ ⎟⎟⎟⎟⎟⎟ + × 2 132432 0 193560 6 . . loglog , ηRTFO 0 2 ° ( ) × ⎛ ⎝⎜ ⎞ ⎠⎟ ⎛ ⎝ ⎜⎜⎜⎜⎜⎜⎜⎜⎜ ⎞ ⎠ ⎟⎟⎟⎟⎟⎟⎟⎟⎟ C MAAT (2) 4

5Table 1 Summary of input data used in the prediction loglog RTFO MAAT VAorig Binder Viscosity Site (F) (%) PG cP @ 60°C Barrow, AK 12.2 8 46–46 0.6851 Fargo, ND 42.7 8 58–34 0.7289 Billings, MT 47.7 8 58–34 0.7289 Chicago, IL 52.8 8 58–28 0.7289 Washington, DC 55.2 8 64–22 0.7572 San Francisco, CA 56.8 8 58–10 0.7265 Oklahoma City, OK 60.6 8 64–16 0.7577 Dallas, TX 66.7 8 64–16 0.7577 Las Vegas, NV 68.9 8 70–10 0.7839 Phoenix, AZ 74.4 8 76–16 0.8061 Table 2 Predicted PAV aging temperatures Site 5 years 10 years 15 years 20 years Barrow, AK 85 87 88 89 Fargo, ND 93 97 100 102 Billings, MT 95 100 103 105 Chicago, IL 97 102 105 107 Washington, DC 97 102 106 108 San Francisco, CA 99 104 107 110 Oklahoma City, OK 99 105 109 111 Dallas, TX 102 108 112 115 Las Vegas, NV 102 109 113 116 Phoenix, AZ 104 112 116 119 Table 3 Estimated field aging times (months) Site PAV 90°C PAV 100°C PAV 110°C Barrow, AK 328 7671 179561 Fargo, ND 37 179 860 Billings, MT 29 123 524 Chicago, IL 23 88 340 Washington, DC 26 90 310 San Francisco, CA 19 69 250 Oklahoma City, OK 21 66 210 Dallas, TX 17 49 144 Las Vegas, NV 19 50 134 Phoenix, AZ 18 43 104

CONCLUSIONS AND RECOMMENDATIONS FOR AASHTO STANDARD PRACTICE R 30 Long-term HMA mix aging in situ is a complex process that is influenced by several factors—most critically, HMA mix properties and external envi- ronment—that should be considered when simulat- ing long-term aging in the laboratory. Warmer tem- peratures are generally associated with increased rates of oxidation, all other factors being equal. Sim- ilarly, higher air void contents in the mix will result in a higher oxidation rate, because more mix is in contact with the circulating air. In addition, the change in air voids under traffic can significantly affect mix aging. This change in air void content is directly de- pendent on mix type and traffic level. While the mechanism of aging is complex and not fully understood, its impact upon pavement per- formance is generally straightforward. Short- and long-term aging results in hardening of the asphalt binder with time. This binder hardening leads to a gradual increase of the rigidity and stiffness of the HMA mix with time, expressed as an increase in its dynamic modulus. This change in modulus, in turn, leads directly to a variable set of changing stress, strain, and deflection patterns within the pavement structure as the pavement system “ages.” Stiffer HMA mixes have increased susceptibility to cracking and fracture, which leads to the development of other distress types and, ultimately, to failure of the pave- ment system. SHRP proposed a laboratory procedure to sim- ulate long-term field aging of HMA mixes. This procedure was adopted by AASHTO in Standard Practice R 30. According to the practice, the pro- cedure “. . . is designed to simulate the aging the compacted mixture will undergo during seven to ten years of service.” However, as was the case for AASHTO R 28, the procedure in AASHTO R 30 does not take into account the effects of HMA mix properties and environmental factors on the aging process. This research carried out a verification of AASHTO R 30 that was similar to that conducted for AASHTO R 28. Field samples were obtained from three sites in the United States: in Arizona, Minnesota (MnRoad), and Nevada (WesTrack). These sites represented a broad range of environ- mental conditions. In addition, the pavement sec- tions were constructed under strict quality control standards, and each site had multiple sections repre- senting different binder and mix properties. Key observations and findings of this portion of the project are presented in the next section. Observation and Findings for AASHTO R 30 The field sites selected were adequate to verify the laboratory procedure in AASHTO R 30 to sim- ulate the long-term age hardening behavior of HMA mix, but did not provide enough data to improve that laboratory procedure or develop a new, more accu- rate and precise one. The data that were collected from these sites varied and seemed unable to ac- count for all significant variables. Plant mixes obtained from the three sites were compacted in the laboratory and then aged at 80°C, 85°C, and 90°C for 5 hours. The dynamic modulus 6 Table 4 Recommended provisional protocol MAAT Recommended PAV Aging Temperature (°C) Site (F) 5 years 10 years 15 years 20 years Barrow, AK 12.2 85 85 90 90 Fargo, ND 42.7 95 95 100 100 Billings, MT 47.7 95 100 105 105 Chicago, IL 52.8 95 100 105 105 Washington, DC 55.2 95 100 105 110 San Francisco, CA 56.8 100 105 105 110 Oklahoma City, OK 60.6 100 105 110 110 Dallas, TX 66.7 100 110 110 115 Las Vegas, NV 68.9 100 110 115 115 Phoenix, AZ 74.4 105 110 115 120

of the aged specimens was determined at two tem- peratures and six loading frequencies. For all mixes tested, increased aging temperature resulted in higher values of dynamic modulus. This result was expected because higher temperatures will result in relatively more aging and thus an increase in mix stiffness. Based upon this observation, warmer climates will result in more aging than the cooler climatic regions, all other factors being equal. This result contradicts the existing procedure in AASHTO R 30 that em- ploys only one standard aging temperature irrespec- tive of the climatic region. In general, dynamic modulus values of the laboratory-aged specimens were greater than those of field-aged specimens. This result may be because a more dramatic stiffness profile was found for the field specimens than for the laboratory specimens. That is, field aging, and thus mix stiffness, is very pronounced at the surface, but then rapidly falls off, yielding a lower average or effective stiffness for the entire core. For laboratory-aged specimens, the aging profile is relatively uniform through the entire spec- imen, giving higher stiffnesses for these specimens. AASHTO R 30 suggests that the long-term lab- oratory aging procedure corresponds to 7–10 years of aging in the field, irrespective of the environmen- tal and mix properties. All sites selected for this proj- ect were 7–10 years old, and the air void contents varied from 4 percent to 12 percent. Based on the data collected, the laboratory stiffness values were higher than the field stiffness values except for the WesTrack sections that had 8 percent and 12 percent air voids. This observation suggests that for air void contents less than 8 percent, laboratory aging is more severe than aging experienced in situ in the field. How- ever, higher air void contents resulted in more se- vere field aging, which in time resulted in higher stiffness for the field samples compared with labo- ratory specimens. This latter observation suggests that a simple linear relationship may not exist be- tween the air void contents of the field samples and specimens compacted in the laboratory for the pur- pose of simulating field aging. Laboratory speci- mens are compacted approximately to the design air void content. Recommendations for Further Research Modifications to AASHTO R 30 are needed to more accurately predict the aging characteristics of HMA mixes. Additional data are needed from con- trolled field experiments, and the data should be obtained at several different points of time. Aging prediction should be examined as a func- tion of the thickness of the HMA layer. That is, thicker pavements will result in lower effective dy- namic modulus values than thinner pavements, which age more uniformly with depth. For example, a 50-mm-thick HMA layer will age differently than a 150-mm-layer under the same conditions. Thus, the laboratory aging procedure should account for the asphalt layer thickness. Air void content is a critical factor that should be considered in any improved laboratory practice for simulating the long-term field aging of HMA mixes. In this study, field stiffness values were generally larger than those of laboratory-compacted specimens when the air void contents were greater than 8 per- cent. The reverse was observed when air void con- tents were less than 8 percent. The field sites selected for future research should include sections with a wider range of air void content than that used herein. 7

Transportation Research Board 500 Fifth Street, NW Washington, DC 20001 These digests are issued in order to increase awareness of research results emanating from projects in the Cooperative Research Programs (CRP). Persons wanting to pursue the project subject matter in greater depth should contact the CRP Staff, Transportation Research Board of the National Academies, 500 Fifth Street, NW, Washington, DC 20001. COPYRIGHT PERMISSION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FTA, or Transit Development Corporation endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP.

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TRB's National Cooperative Highway Research Program (NCHRP) Research Results Digest 324: Simulating the Effects of Hot-Mix Asphalt Aging for Performance Testing and Pavement Structural Design summarizes the results of an NCHRP effort that examined the limitations associated with provisional protocols on hardening potential of asphalt binders and mixes, and explored ways to enhance the predictive capabilities of these protocols. Detailed information on the conduct and results of these experiments was published as NCHRP Web-Only Document 113.

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