Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction: Phase III Results (2021)

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14 Research Approach Overview The CAI, which is used to relate the pavement temperature history to the required laboratory aging duration, was recalibrated and simplified using original binders, aggregates, and field cores from selected pavement sections. Emphasis now was placed on other mixture types, i.e., WMA mixtures, mixtures containing RAP, and PMA mixtures, which were not considered thoroughly in the initial calibration of the laboratory aging procedure. In addition, replicate field cores from sections included in the initial calibration of the laboratory aging procedure were analyzed to improve the confidence of the field measurements. Figure 6 summarizes the experimental plan to conduct this task. Laboratory-mixed loose mixtures were subjected to laboratory aging at 95°C for prolonged durations in an oven. Mixture samples were removed from the oven periodically and subjected to extraction and recovery, after which the binder AIPs were measured. In addition, binders were extracted and recovered from field cores obtained at various pavement depths from in-service pavements. The AIPs of the field-aged binders were measured and compared to oxidation rates of laboratory-aged binders to determine the labora- tory aging durations that would match the target field AIPs for the specific projects. The project- specific aging durations then were used to recalibrate and simplify the CAI that was proposed in the original NCHRP Project 09-54. The resultant CAI can be used to determine the laboratory aging duration that best matches field aging at any location of interest and pavement depth of interest using hourly pavement temperature data obtained from EICM analysis of MERRA-2 weather data. Test Materials and Field Projects Table 3 provides a summary of the pavement test sections that were selected in this study for the recalibration of the long-term aging protocol. The pavement test sections analyzed were obtained from five projects: FHWA’s Accelerated Loading Facility (ALF), the Manitoba Infra- structure and Transportation (MIT), the National Center for Asphalt Technology (NCAT), the Long-Term Pavement Performance Program (LTPP), and WesTrack. These sections are rep- resented by the original component materials and field cores that were used to recalibrate the CAI. These sections cover a wide range of pavement design, climatic conditions, field core ages, binder and aggregate characteristics, air void contents, asphalt contents, and gradations. The list includes the original NCHRP Project 09-54 materials as well as modern mixtures, i.e., WMA mixtures, mixtures containing RAP, and PMA mixtures. In addition, replicate field cores from sections included in the initial calibration of the laboratory aging procedure were analyzed in this study to improve the integrity of the field measurements. The location, age of the field core, C H A P T E R 3 Refinement of the Climate-Based, Predefined Laboratory Aging Durations

Refinement of the Climate-Based, Predefined Laboratory Aging Durations 15   and the number of cores tested are noted in the table, along with check marks to indicate that the core contained a modern mixture (WMA, RAP, or PMA). Figure 7 shows the geographic coverage of the selected materials and field projects. The figure indicates that a broad range of geographic locations across the United States and in Canada were used to refine the long-term aging procedure. Sample Preparation Methods Asphalt Mixture Aging All of the asphalt mixtures aged in the laboratory were prepared using the component aggre- gate and binder that were used to construct the pavements from which the field cores were obtained. The mixtures were subjected to short-term aging at 135°C for 4 hours in accordance with AASHTO R 30 prior to long-term aging. The long-term oven aging of the loose mixtures was accomplished by separating the mix into several pans such that each pan had a relatively thin layer of loose mix that was approximately equal to the nominal maximum aggregate size (NMAS) of the aged mix. The pans of loose mixture were conditioned in an oven at 95°C and systematically rotated to minimize any effects of an oven temperature gradient and/or draft on the degree of aging. After long-term aging, the materials were taken out of the oven and mixed together to obtain a uniform mixture. Field Core Preparation Full-depth cores were acquired from in-service pavements and the FHWA’s Materials Ref- erence Library. The field cores were wrapped with plastic wrap and placed in a temperature- controlled room to minimize further aging during storage. Following storage, the upper 50 mm of each field core was sliced to create four 12.5-mm-thick discs. The remainder of each field core was cut into 25-mm-thick discs. Special care was taken to avoid the tack coat and prime coat layers when slicing the field cores. The upper four discs were subjected to binder micro extraction and recovery, and the recovered binder was subsequently subjected to rheological testing. Microextraction and Recovery Microextraction and recovery of the asphalt binder from the asphalt mixtures and field cores were undertaken following the procedure proposed by Farrar et al. (2015). This procedure Loose Mix Oven Aging at 95°C Loose Mix Oven Aging Rate at 95°C Field Core log |G*| Measurement Estimated Laboratory Aging Duration Required to Match Field Core log |G*| Predicted Laboratory Aging Duration Required to Match Field Section Age Level Climatic Data of Field Section EICM Simulation and CAI Calculation Compare Figure 6. Proposed experimental plan to refine aging durations.

Project ID Location Age of Field Cores Section # Cores Tested Binder Grade WMA Modified Binder Contains RAP FHWA ALF Virginia 8 years Control-2002-Lane 2 (ACTRL) 2 PG 70-22 SBS-LG-2002-Lane 4 (ASBS) 2 PG 70-28 ü CR-TB-2002-Lane 5 (ACRTB) 2 PG 76-28 ü Terpolymer-2002-Lane 6 (ATerp) 2 PG 70-28 ü MIT Manitoba, Canada 4 years Control (MWC) 3 Pen 150/200 Advera-PTH 14 (MWA) 1 ü Evotherm-PTH 14 (MWE) 1 ü 15% RAP-PTH 8 (M15R) 3 ü 50% RAP-PTH 8 (M50R) 2 ü NCAT Alabama 4 years Control-S9 (NWC) 1 PG 76-22 15% RAP-S8 (N15R) 2 PG 76-22 ü 50% RAP-N10 (N50R) 2 PG 67-22 ü Evotherm-S11 (NWE) 1 PG 76-22 ü 50% RAP with foam-N11 (N50RF) 2 PG 67-22 ü ü Foam-S10 (NWF) 1 PG 76-22 ü LTPP California 7, 8, 14 years SPS-8 ID: 06-A805 (LCA) 3 AR-40 New Mexico 10, 11, 18 years SPS-8 ID: 35-0801 (LNM) 3 AC-20 Texas 11(2), 18 years SPS-8 ID: 48-0802 (LTX) 3 AC-20 Washington 6, 11, 14 years SPS-8 ID: 53-0801 (LWA) 3 AC-20 Wisconsin 8(2), 17 years SPS-8 ID: 55-0806 (LWI) 3 - South Dakota 14, 21 years SPS-8 ID: 46-0804 (LSD) 2 Pen 120/150 WesTrack Nevada 19 years (S4) Fine, Opt. ac%, L AV% (WTFOL) 4 PG 64-22 (S2) Fine, Low ac%, M AV% (WTFLM) 4 (S1) Fine, Opt. ac%, M AV% (WTFOM) 4 (S14) Fine, High ac%, M AV% (WTFHM) 4 (S17) Fine, Opt. ac%, H AV% (WTFOH) 4 (S18) Fine, High ac%, L AV% (WTFHL) 4 (S3) Fine, Low ac%, H AV% (WTFLH) 2 17 years (S39) Coarse, Opt. ac%, L AV% (WTCOL) 2 PG 64-22(S36) Coarse, Opt. ac%, H AV% (WTCOH) 2 Note: SBS-LG = linear grafted styrene-butadiene-styrene; CR-TB = crumb rubber terminal blend; SPS = specific pavement study; ac% = asphalt content; AV% = air void content; L = low, M = medium, H = high. Table 3. Selected sections for recalibration of long-term aging protocol.

Refinement of the Climate-Based, Predefined Laboratory Aging Durations 17   uses a solvent mixture of toluene and ethanol (85:15). The mixture sample size is limited to 200 g to produce approximately 10 g of asphalt binder per extraction, which is adequate for both ATR-FTIR spectrometry testing and DSR testing. To prevent further aging of the binder, the distillation flask was subjected to vacuum pressure of 80.0 ± 0.7 kPa (600 ± 5 mm Hg) under nitrogen gas during the recovery procedure. The recovered samples were then placed in a degassing oven and heated to 130°C for 60 minutes under nitrogen to remove any remaining traces of the solvent. In this study, ATR-FTIR spectrometry testing was conducted following extraction and recovery to ensure that no detectable solvent was present prior to DSR testing. Determination of Rheological Aging Index Properties Frequency sweep tests of the extracted and recovered binders were conducted using an Anton Paar modular compact rheometer (MCR) 302 at frequencies ranging from 0.1 Hz to 30 Hz at 64°C using 8 mm or 25 mm parallel plate geometry. A strain amplitude of 1% was applied at all frequencies. The log |G*| at 64°C and 10 rad/s was used as the rheological AIP for all analyses. Determination of Project-Specific, Hourly Pavement Temperature Histories The MERRA-2 hourly climatic data stations with coordinates and elevations closest to each project location were identified and used to obtain the most accurate weather information available. After the appropriate stations were chosen, 37 years of available weather data (1980 through 2017) were input and analyzed in the EICM to obtain hourly pavement temperature histories as a function of pavement depth. Prior to generating the pavement temperature data for CAI analysis, the sensitivity of the EICM results to pavement structure was analyzed; the results showed negligible sensitivity of the calculated CAI values to the chosen pavement struc- ture. Therefore, a simple pavement structure composed of 9 inches of asphalt concrete over an aggregate base and sub-base was used for all EICM analyses. The sub-layering and nodal struc- ture specified in the Mechanistic-Empirical Pavement Design Guide (MEPDG) were adopted for all EICM analyses (Applied Research Associates 2004). The pavement temperature history Note: AL = Alabama, MB = Manitoba, Canada, NV = Nevada, NM = New Mexico, NC = North Carolina, SD = South Dakota, TX = Texas, VA = Virginia, WI = Wisconsin Figure 7. Locations of materials/projects included in the study.

18 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction: Phase III Results for each year was used to calculate the CAI values, and the averaged data for the 37 years were used in the recalibration effort. The FHWA ALF cores were extracted from sections that had been subjected to high tempera- tures, which were not realistic, in previous FHWA ALF rutting experiments. In the ALF rutting experiments, the pavements were subjected to temperatures up to 74°C for up to 2 months (Gibson et al. 2012). The locations and thermal histories used for the ALF tests were identified in this study for each field core and incorporated into the CAI calculations. Determination of Project-Specific, Oven Aging Durations Least squares optimization was conducted to minimize the error in the measured log |G*| values of the binders extracted and recovered from the loose mixture aging experiments and those predicted by Equation (1) to determine the M value (the fitting parameter that is related to fast reaction reactive material) for each mixture. Equation (1) was then used to determine the aging durations at 95°C that were required to match the log |G*| values of the binders extracted and recovered from the field cores as a function of depth. These project-specific aging durations were used to recalibrate the CAI parameters. Note that project-specific aging durations for some of the RAP and WMA projects could not be calculated. Those RAP and WMA sections, i.e., in the NCAT and MIT projects, contained field cores that had been extracted after only 4 years of in-service aging. The field cores from the other projects were extracted after longer periods of in-service aging. In some instances, the age of the laboratory short-term aged material exceeded the age level of the 4-year-old field cores based on the log |G*| values of the extracted and recovered binder. This finding could suggest that the short-term aging laboratory procedure recommended in AASHTO R 30, which includes loose mixture conditioning at 135°C for 4 hours, may induce more aging than takes place during production and placement. The recommendations from NCHRP Project 09-52, “Short-Term Laboratory Conditioning of Asphalt Mixtures” support this hypothesis. Newcomb et al. (2015) proposed that short-term aging should be refined to include only 2 hours of oven conditioning at 135°C for HMA and 116°C for WMA. The measured laboratory aging duration for condi- tions where the short-term laboratory aging exceeded the field core AIP is reported as 0 days in this study. The CAI values for these sections were calculated and are reported in the Findings and Applications section below. However, instances where the age of the laboratory short-term aged material exceeded the age level of the field cores were not used in the recalibration of the CAI parameters in this study. Findings and Applications Evaluation of Original CAI Parameters The ability of the original CAI parameters given in Equation (4) to yield accurate loose mixture aging durations at 95°C to match given field age levels was evaluated in this study. Figures 8 (a) and (b) present comparisons of the measured laboratory oven aging durations and the original CAI values according to project and depth, respectively. Figure 8 (a) shows the conventional HMA mixtures in blue, the WMA mixtures in red, the mixtures containing RAP in green, and the PMA mixtures in yellow. Figure 8 indicates that most of the data fall above the line of equality (LOE), i.e., they overpredict the required laboratory aging duration. Further- more, the results demonstrate that the most scatter in the data occurs at the two depths closest to the pavement surface. The large amount of scatter in the data at the depth of 0.6 cm is not surprising because the surface conditions could be impacted by UV oxidation and other surface wear mechanisms that are not considered in the CAI calculation. The scatter also reflects the

Refinement of the Climate-Based, Predefined Laboratory Aging Durations 19   large variability in the measurements taken from the field cores. The noted discrepancies in the relationship between the measured aging durations and the values calculated using the original CAI parameters could reflect differences in the MERRA-2 weather data used herein and the MEPDG weather data that were used to calibrate the original CAI values. In addition, the incorporation of the additional projects and the replicate field core testing reflect differences in the data presented herein versus those used in the original CAI calibration. As mentioned, the WMA and RAP projects encompass only 4 years of in-service aging and thus reflect shorter measured and predicted aging durations than most of the other sections. This factor makes it difficult to determine whether the relationship between the laboratory and field aging of the WMA and RAP mixtures differs from that of the conventional mixtures presented in Figure 8 (a). Given the available data, the WMA sections generally align with the HMA sections. However, the mixtures that contain RAP seem to deviate from the virgin 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 M ea su re d D ur at io n at 9 5° C (d ay s) Climatic Aging Index 0.6 cm 1.9 cm 3.2 cm 4.5 cm R2 = 0.34 SE = 6.40 (b) 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 M ea su re d D ur at io n at 9 5° C (d ay s) Climatic Aging Index LTX LNM LSD LWI WTC WTF MWA MWE M15R M50R NC N50R NWE NWF ACTRL ASBS ACRTB ATerp R2 = 0.34 SE = 6.40 (a) Note: blue = conventional HMA mixtures, red = WMA mixtures, green = mixtures containing RAP, yellow = PMA mixtures Figure 8. Comparison of measured laboratory aging durations at 95çC and original CAI values according to (a) project and (b) pavement depth.

20 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction: Phase III Results mixtures shown in Figure 8 (a). All the PMA sections are part of the FHWA ALF project, and Figure 8 (a) shows that data for many of the PMA sections (ASBS, ACRTB, and ATerp) devi- ate from the trends of most of the other sections. As previously noted, the FHWA ALF cores were extracted from sections that were subjected to high temperatures during earlier rutting experiments. In this study, the locations and thermal histories used in the ALF experiments were identified for each field core and incorporated into the CAI calculations. However, these harsh conditions may have led to discrepancies in the relationship between the laboratory and field aging. If the ALF sections are removed from Figure 8, the R2 value increases to 0.50. Initial Recalibration of Climatic Aging Index Parameters Initially, all the project data were used in the recalibration of the CAI parameters except for cases where the measurements for the laboratory short-term aged loose mixtures exceeded those of the field cores based on log |G*| measurements of the extracted and recovered binders. The recalibration effort focused on optimizing the D parameter (the depth correction factor) in Equation (4) at each measured field core depth (i.e., 0.6 cm, 1.9 cm, 3.2 cm, and 4.5 cm). The parameter A (reaction frequency factor) in Equation (4) was set to 1 to simplify the CAI expression. Consideration was also given to recalibrating the Ea value (the fitting parameter) in Equation (4); however, although a limited set of Ea values was tried, changing Ea did not appear to lead to significant improvement in the accuracy of the CAI. Also, iteratively updating Ea was cumbersome due to the large amount of data analyzed (i.e., 37 years of climate data for each section). Least squares optimization was carried out to minimize the error between the mea- sured oven aging durations and those predicted by the CAI to determine the value of D at each measured depth of the field cores. Figure 9 presents the results of the initial recalibration of the CAI. Figure 9 (a) shows the conventional HMA mixtures in blue, the WMA mixtures in red, the mixtures containing RAP in green, and the PMA mixtures in yellow. Figure 9 demonstrates an improvement in the accuracy of the CAI compared to the original parameters that were based on the coefficient of determination, R2 (0.56 versus 0.34), and standard error (5.60 versus 6.40). Figure 9 (b) dem- onstrates that the recalibrated CAI better reflects the depth dependence of field aging than the original CAI. Figure 9 (a) shows that the data for the WMA sections appear to align with those of the con- ventional mixtures. The mixtures that contain RAP all fall under the LOE, suggesting that the relationship between laboratory and field aging may be impacted by the presence of reclaimed materials. Given that short-term aging conducted in accordance with AASHTO R 30 led to age levels that exceeded that of 4-year-old field cores from several of the RAP mixture projects, future research is recommended to investigate the relationship between both laboratory short- term and long-term aging with field aging for RAP mixtures. Approximately half of the ALF sections shown in Figure  9 (a) exhibit outlier behavior. These sections are the terpolymer-modified binder section (ATerp, 11 years old), the control (unmodified) binder section (ACTRL, 8 years old), and the SBS-modified binder section (ASBS, 8 years old). The outlier behavior of these ALF sections could be due to the unrealistically harsh temperatures used in the ALF experiments, making it difficult to infer whether or not polymer modification affects the relationship between laboratory and field aging. Because three of the six ALF sections are outliers, and the ALF tests included unrealistic thermal histories, the CAI parameters were recalibrated a second time in this study without the ALF sections. In addition, the LTPP South Dakota (LSD) section is a clear outlier in Figure 9 (a) and leads to a bias in the CAI calibration that does not represent the other data as well. Therefore LSD was removed from the final recalibration effort to avoid discrepancies with the rest of the data. The reason for the outlier behavior of LSD is unknown.

Refinement of the Climate-Based, Predefined Laboratory Aging Durations 21   Final Recalibration of the Climatic Aging Index A final recalibration of the CAI was carried out to determine the optimal D values in Equa- tion (4) using least squares optimization to minimize the error between the measured oven aging durations and those predicted by the CAI at each depth after excluding the ALF and LSD sections. Figure 10 presents the results of the final CAI recalibration. Figure 10 (a) shows the conventional HMA mixtures in blue, the WMA mixtures in red, the mixtures containing RAP in green, and the PMA mixtures in gray. The PMA sections and LSD sections are shown in gray even though they were not used in the recalibration because of their outlying behavior. A 90% confidence interval is shown in gray above and below the LOE in Figure 10 (a) to describe the uncertainty in the original CAI recalibration results. The results demonstrate that the final recalibration better represents most of the sections than the initial recalibration that used the 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 M ea su re d D ur at io n at 9 5° C (d ay s) Climatic Aging Index 0.6 cm 1.9 cm 3.2 cm 4.5 cm R2 = 0.56 SE = 5.60 (b) 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 M ea su re d D ur at io n at 9 5° C (d ay s) Climatic Aging Index LTX LNM LSD LWI WTC WTF MWA MWE M15R M50R NC N50R NWE NWF ACTRL ASBS ACRTB ATerp R2 = 0.56 SE = 5.60 (a) Note: blue = conventional mixtures, red = WMA mixtures, green = mixtures containing RAP, yellow = PMA mixtures Figure 9. Comparison of measured laboratory aging durations at 95çC and initial recalibration of CAI according to (a) project and (b) pavement depth.

22 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction: Phase III Results ALF sections. The results also reflect the depth dependence of field aging relatively well. The R2 value of 0.78 indicates that the final CAI provides a reasonable estimate of the laboratory aging duration that is required to match a given pavement temperature history and depth of interest; note the improvement in R2 from Figure 8 to Figure 9 can be partially attributed to the removal of the PMA and LSD sections. It should be noted, though, the lack of points beyond an aging duration of 10 days, implying that CAI estimates only the laboratory aging duration below 10 days within reasonable accuracy. More data points are needed in future studies to verify the accuracy of CAI for longer laboratory aging durations. It should be noted that half of the PMA sections fall within the confidence interval in Fig- ure 10 (a), and the other half fall outside the bounds. The good agreement between the LOE and results for the ALF sections that are not outliers suggests that the relationship between lab and field aging may not be impacted by polymer modification. However, the relationship 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 M ea su re d D ur at io n at 9 5° C (d ay s) Climatic Aging Index LTX LNM LSD LWI WTC WTF MWA MWE M15R M50R NC N50R NWE NWF ACTRL ASBS ACRTB ATerp R2 = 0.78 SE = 3.05 (a) 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 M ea su re d D ur at io n at 9 5° C (d ay s) Climatic Aging Index 0.6 cm 1.9 cm 3.2 cm 4.5 cm R2 = 0.78 SE = 3.05 (b) Note: blue = conventional mixtures, red = WMA mixtures, green = mixtures containing RAP, gray = PMA and LSD mixtures Figure 10. Comparison of measured laboratory aging durations at 95çC and final recalibration of CAI according to (a) project and (b) pavement depth (ALF and LSD sections not included).

Refinement of the Climate-Based, Predefined Laboratory Aging Durations 23   between the field and laboratory aging of polymer-modified materials under realistic climate conditions merits further investigation in future work. The data for the WMA sections corre- spond closely to those of the HMA sections, suggesting that the recalibrated CAI is applicable to WMA mixtures. Simplification of the Climatic Aging Index The CAI expression given in Equation (4) was simplified based on the final recalibration, resulting in Equation (15). To arrive at Equation (15), the D values were plotted as a function of depth, which revealed a strong power-law relationship between D and depth, with R2 of 0.98, as shown in Figure 11. The division by 24 in Equation (4) was removed and included in the opti- mization of the revised D values. As previously mentioned, the parameter A in Equation (4) was set to a value of 1 to reduce the variables in the CAI expression. The parameter Ea in Equation (4) was divided by R in Equation (15) to further simplify the CAI expression. ∑= = − − = CAI 0.0437 (15)0.426 1601.167 1 t d eoven T i N i where toven = the laboratory loose mixture oven aging duration at 95°C (days), d = the depth below the pavement surface (cm), and Ti = the hourly temperature (K). Note that temperatures below 20°C are excluded from the CAI calculation. Note that Equation (15) is considered valid only for depths greater than 0.6 cm. Aging Duration Maps Equation (15) was used to calculate laboratory aging durations for three field ages: 4 years, 8 years, and 16 years for all locations in the MERRA-2 weather database (i.e., 2,855 stations). For each field age, the laboratory aging durations were determined at three depths: 6 mm, 20 mm, and 30 mm. The results were used to generate aging duration contour maps. Correspondingly, Figure 12, Figure 13, and Figure 14 show the CAI-determined loose mixture aging durations at 95°C that match 4, 8, and 16 years of aging, respectively. The aging duration maps demonstrate that climate has a significant effect on the laboratory aging duration that matches a given field age. As expected, the aging time increases as one moves from north to south. y = 0.0437d –0.426 R² = 0.98 0 0.01 0.02 0.03 0.04 0.05 0.06 0 1 2 3 4 5 D ep th F ac to r Depth (cm) Figure 11. CAI depth factor as a function of depth.

24 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction: Phase III Results Figure 12. Required oven aging durations at 95çC to match 4 years of field aging for depths of (a) 6 mm and (b) 20 mm. (a) 6 mm Depth (b) 20 mm Depth Legend

Refinement of the Climate-Based, Predefined Laboratory Aging Durations 25   Figure 12. (Continued). Required oven aging durations at 95çC to match 4 years of field aging for depths of (c) 30 mm. (c) 30 mm Depth Figure 13. Required oven aging durations at 95çC to match 8 years of field aging for depths of (a) 6 mm. (continued on next page) (a) 6 mm Depth

26 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction: Phase III Results Figure 13. (Continued). Required oven aging durations at 95çC to match 8 years of field aging for depths of (b) 20 mm and (c) 30 mm. (b) 20 mm Depth (c) 30 mm Depth

Refinement of the Climate-Based, Predefined Laboratory Aging Durations 27   (a) 6 mm Depth (b) 20 mm Depth Figure 14. Required oven aging durations at 95çC to match 16 years of field aging for depths of (a) 6 mm and (b) 20 mm. (continued on next page)

28 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction: Phase III Results Summary The recalibrated CAI can be used to determine the required laboratory aging durations as a function of hourly pavement temperature history and depth with reasonable accuracy (R2 = 0.78) for HMA and for WMA mixtures without RAP. The recalibrated CAI results suggest that the CAI values for WMA materials are commensurate with those for HMA materials. The RAP mixtures show evidence of variation from the virgin mixtures in terms of the effects of aging. Relatively few RAP mixtures were analyzed, and the corresponding field cores were only 4 years old. In some instances, the laboratory short-term aged RAP mixtures that were prepared according to AASHTO R 30 exceeded the age level of the field cores, suggest- ing that refinement of the short-term aging procedure may be necessary prior to investigating the effect of RAP mixtures on the relationship between laboratory long-term aging durations and field aging. Many of the PMA sections evaluated exhibited outlier behavior in terms of the relationship between the measured loose mixture aging duration and that predicted by the CAI, which could be due to the unrealistically harsh thermal history of the ALF rutting test projects from which the field cores were acquired. However, roughly half of the PMA sections still fell within the 90% confidence interval of the recalibrated CAI, suggesting promise for the CAI’s applicability to PMA materials. Future research should further evaluate the effects of PMA and RAP on the relationship between laboratory and field aging using a broader set of materials and field conditions. (c) 30 mm Depth Figure 14. (Continued). Required oven aging durations at 95çC to match 16 years of field aging for depths of (c) 30 mm.