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50 Asphalt mixtures have been traditionally designed on the basis of volumetric parameters in which the optimum asphalt content for a given aggregate gradation was dependent upon the compaction effort, AV content, voids in mineral aggregate, and asphalt absorption into aggregate voids. These volumet- ric procedures worked well for agencies and contractors in an era when the components for asphalt mixtures were rela- tively constant. However, in the last three decades, changes in asphalt mixture components, production parameters, and plant design have occurred and, consequently, raised ques- tions of the validity of the current mix design procedures in adequately assessing the volumetric needs of asphalt mix- tures and the physical characteristics required to meet per- formance expectations. This project investigated the need for mix design procedures to consider the impact of these recent changes including binder source, aggregate absorption, WMA technology, recycled material inclusion, plant type, and pro- duction temperature on the volumetric and performance characteristics of asphalt mixtures during production and construction. Phase I Experiment The objectives of Phase I of this project were to (1) develop a laboratory STOA protocol for asphalt loose mix prior to compaction to simulate the aging and asphalt absorption by the aggregate as it is produced in a plant and then loaded into a truck for transport, and (2) identify factors with significant effects on the performance-related properties of short-term aged asphalt mixtures. Laboratory STOA protocols of 2 hours at 275°F (135°C) for HMA and 2 hours at 240°F (116°C) for WMA, as previously recommended in NCHRP Report 763 (Epps Martin 2014), were used to fabricate LMLC specimens for volumetric analysis and performance testing. The simu- lations of asphalt aging and absorption during plant pro- duction and construction by the selected laboratory STOA protocols were evaluated by comparing the binder or mix- ture performance of LMLC specimens to the corresponding PMPC specimens and cores at construction. Additionally, the laboratory test results were used to identify mixture com- ponents and production parameters with significant effects on the performance of short-term aged asphalt mixtures. A guide for conducting experiments to assess the short-term aging of asphalt mixtures produced in the field is presented in Appendix F based upon the techniques developed in this research. Recommended changes to the current AASHTO R 30 short-term aging protocol are given in Appendix G, which shows the modifications in âtrack changesâ format. The basic changes resulting from this project include (1) fixing the compaction temperatures for WMA at 240°F (116°C) and HMA at 275°F (135°C) and (2) conditioning the sample for 2 hours at the compaction temperature regardless of whether the sample is being prepared for volumetric mix design or performance testing. The following conclusions pertain to Phase I of this project, in which 522 LMLC specimens, PMPC specimens, and cores at construction from nine field sites were evaluated. Simulation of Plant Aging ⢠The correlation between LMLC and PMPC specimens in terms of volumetric parameters (i.e., theoretical maximum specific gravity and percentage of absorbed binder) indi- cated that the selected STOA protocols for LMLC speci- mens were able, to a large extent, to simulate the asphalt absorption that took place during production at the plant. ⢠The correlations between LMLC and PMPC specimens and cores at construction in terms of resilient modulus stiffness and dynamic modulus stiffness for a wide range of mixtures indicated that the laboratory STOA protocols used for fabricating LMLC specimens were able to simulate plant aging. ⢠The correlations between LMLC and PMPC specimens for the HWTT resistance parameters (i.e., RRP and rut depth at C H A P T E R 4 Conclusions and Suggested Research
51 5,000 load cycles) also provided evidence that the laboratory STOA protocols produce representative specimens for per- formance testing. HWTT results from construction cores did not correlate well with those for LMLC specimens, pos- sibly due to testing difficulties of thin lifts and the required use of plaster to fit the cores into the HWTT molds. ⢠The correlation in terms of binder continuous performance grades and Fourier transform infrared spectroscopy car- bonyl area values between extracted and recovered bind- ers from LMLC specimens versus PMPC specimens and cores at construction indicated that the selected laboratory STOA protocols produced equivalent binder stiffening and oxidation effects as the plant production and construction processes. ⢠Although it was not an experiment design factor, different binder grades and the use of polymers were included in the study. Across a wide spectrum of grades (PG 58-22 to 76-22), the LMLC MR test results were consistently in line with the PMPC values. One of the most interesting obser- vations concerning polymer modification can be seen in the New Mexico data where a virgin polymer-modified (PG 76-22) mixture had a lower stiffness than a high-RAP content mixture made with a straight PG 64-28 binder. Factor Analysis ⢠Laboratory test results indicated a significant effect on the performance of short-term aged asphalt mixtures from WMA technology. Lower mixture stiffness and decreased rut- ting resistance were observed for WMA mixtures compared to HMA mixtures, possibly due to the reduced production temperature. ⢠Laboratory test results on the effect of production tem- perature and plant type indicated no significant change in stiffness and rutting resistance of short-term aged asphalt mixtures. ⢠Laboratory test results on the effect of recycled materials indicated a significant increase in mixture stiffness even with the use of a softer asphalt binder, although the effect was inconsistent due to the variability of recycled materials in terms of original asphalt mixtures and in-service times and climatic conditions. ⢠A significant effect on the performance of short-term aged asphalt mixtures was observed for aggregate absorption. Asphalt mixtures using low-absorptive aggregates exhibited higher stiffness and better rutting resistance than the mix- tures using high-absorptive aggregates, which was attributed to the thicker effective film thickness in the high-absorptive aggregate mixtures from volumetric compensation in the mixture design process. High aggregate absorption in mix- tures produced greater variability in results than other vari- ables that were studied. ⢠Binder source had a significant effect on the performance of short-term aged asphalt mixtures. Different mixture perfor- mance in terms of stiffness and rutting resistance should be expected from asphalt mixtures using the same PG binders from different sources. Phase II Experiment The objectives of Phase II of this project were to (1) evalu- ate the evolution of performance-related properties of asphalt mixtures through their initial period of field performance, (2) develop a correlation between long-term field aging and laboratory LTOA protocols, and (3) identify mixture com- ponents and production parameters with significant effects on the intermediate- or long-term aging characteristics of asphalt mixtures. To simulate long-term aging in the labora- tory, WMA and HMA LMLC specimens were subjected to a STOA protocol of 2 hours at 275°F (135°C) plus an LTOA protocol of either 5 days at 185°F (85°C) in accordance with AASHTO R 30 or 2 weeks at 140°F (60°C). The following conclusions pertain to Phase II of this proj- ect, in which 512 LMLC specimens and cores acquired at con- struction and 8 to 22 months after construction from seven field sites were evaluated. Quantification of Field Aging and Correlation to Laboratory LTOA Protocols ⢠Cumulative degree-days (32°F [0°C] base) was proposed as a metric to quantify field aging and was able to account for the differences in construction dates and climates for various field sites. ⢠Binder or mixture property ratios (i.e., MR ratio, HWTT RRP ratio, DSR complex modulus ratio, and FT-IR CA ratio), defined as the relative differences of binder or mixture prop- erties of short-term aged versus long-term aged specimens, were used to quantify the evolution of mixture stiffness and rutting resistance and binder oxidation with field aging. ⢠An exponential function was proposed to correlate the binder or mixture property ratios of post-construction cores versus their corresponding CDD values, and a desir- able correlation resulted. ⢠Mixture stiffness results showed that the average MR ratio values for all LMLC specimens with STOA protocol of 2 hours at 275°F (135°C) on loose mixture plus LTOA protocols of 2 weeks at 140°F (60°C) and 5 days at 185°F (85°C) on compacted specimens from the selected field sites were approximately 1.46 and 1.76, respectively. The higher MR ratio values associated with the LTOA protocol of 5 days at 185°F (85°C) versus that of 2 weeks at 140°F (60°C) indicated that mixture aging was more sensitive to aging temperature than to aging time in the LTOA protocol.
52 ⢠Based on the mixture stiffness results, the laboratory STOA protocol of 2 hours at 275°F (135°C) on loose mixture plus LTOA protocols of 2 weeks at 140°F (60°C) or 5 days at 185°F (85°C) on compacted specimens were able to pro- duce mixture aging equivalent to an average of 9,100 and 16,000 CDD, respectively, in the field. This resulted in no change to AASHTO R 30 for the LTOA procedure (i.e., 5 days at 185°F [50°C]), and this is reflected in Appendix G. ⢠The pavement in-service time for each field site correspond- ing to 9,100 and 16,000 CDD was determined. The labora- tory STOA protocol of 2 hours at 275°F (135°C) on loose mixture plus LTOA protocol of 2 weeks at 140°F (60°C) on compacted specimens was equivalent to approximately 7 months in service in warmer climates and 12 months in service in colder climates. As for the same laboratory STOA protocol on loose mixture plus LTOA protocol of 5 days at 185°F (85°C) on compacted specimens, approximately 11 months and 22 months in service were required for warmer climates and colder climates, respectively. Factor Analysis ⢠The effect of WMA technology on mixture stiffness evolu- tion with field aging compared to HMA was categorized into three different scenarios: â Scenario I: The stiffness of HMA cores was always higher than WMA cores, but the difference in stiffness between these two mixtures decreased with field aging. â Scenario II: HMA had higher mixture stiffness com- pared to WMA at the initial aging stage (i.e., construction cores), but the WMA stiffness eventually equaled that of HMA after some time in the field. In other words, there was a catch-up point in the stiffness values of HMA and WMA. â Scenario III: Equivalent mixture stiffness was shown for cores at construction between HMA and WMA, but higher stiffness for post-construction cores was observed for WMA versus HMA. For the majority of the field sites (four out of seven), the MR stiffness evolution with field aging followed Scenario II, indicating that the stiffness of WMA was initially lower than that of HMA, but it was able to catch-up to the stiffness of HMA after a certain in-service time in the field. Ultimately, the stiffness and rutting resistance of WMA mixtures was comparable to HMA. ⢠The critical in-service time when WMA equaled HMA (CDDWMA=HMA) was achieved after 23,000 CDD, which was equivalent to approximately 17 months in service in warmer climates and 30 months in service in colder cli- mates. Field aging of approximately 3,000 CDD was neces- sary for the stiffness of WMA to equal the initial stiffness of HMA (CDDWMA=HMA0), which was equivalent to approxi- mately 2 months for warmer climates and 3 months for colder climates. ⢠Laboratory test results indicated no significant effect of production temperature and plant type on the stiffness and rutting resistance of long-term aged asphalt mixtures. ⢠Laboratory test results for the effect of recycled materials indicated a higher increase in MR stiffness after long-term aging for the control mixtures compared to the RAP/RAS mixtures. The greater sensitivity to aging exhibited by the control mixtures was attributed to the larger amount of vir- gin binder in the mixture, which was likely more susceptible to aging. ⢠Aggregate absorption, specifically the effective binder content in the mix, also had a significant effect on the long-term aging characteristics of asphalt mixtures. A greater rate of stiffen- ing as indicated by the increase in MR stiffness induced by long-term aging for mixtures using high-absorptive aggre- gates was observed compared to the mixtures using low- absorptive aggregates. In addition, mixtures prepared with high-absorptive aggregates exhibited a greater increase in rutting resistance than those using low-absorptive aggre- gates. The greater sensitivity of mixture stiffness and rutting resistance to aging for mixtures using high-absorptive aggre- gates was likely due to the higher volume of effective binder in these mixtures that was available for aging, and the con- tinuous asphalt absorption by the aggregates with time. Suggested Research The following recommendations are made based on the results of this study: ⢠There is a need to further monitor the long-term behavior of the field mixtures from this study to more accurately model their aging. It is recommended that cores be obtained from as many of these field sites as possible for at least one more point in time. The testing could be restricted to MR at 77°F (25°C). ⢠While this research focused on characterizing the aging characteristics of asphalt mixtures over a wide range of factors, only mixture stiffness and rutting resistance were utilized in this study to discriminate asphalt mixtures with different short-term aging characteristics and testing was conducted at one set of temperatures for each test for most of the data. Additional mixture properties such as mois- ture susceptibility, fatigue cracking resistance, and ther- mal cracking resistance need to be considered for further validation and to examine how aging affects the long-term cold-temperature behavior of asphalt mixtures in order to quantify possible embrittlement with time.
53 ⢠Differences in volumetric properties were observed for mixtures using highly absorptive aggregates when com- paring LMLC specimens with the laboratory STOA pro- tocols of 2 hours at 275°F (135°C) for HMA and 2 hours at 240°F (116°C) for WMA and the corresponding PMPC specimens. Therefore, the proposed STOA may not be fully applicable to those mixtures, and additional STOA proto- cols should be explored in future research for achieving equivalent volumetric properties. ⢠Based on the limited amount of MR stiffness results mea- sured for Indiana and Florida LMLC specimens with different LTOA protocols, 3 days at 185°F (85°C) and 2 weeks at 140°F (60°C) produced an equivalent level of mixture aging. This 3-day LTOA protocol might be more practical for simulating the field aging in colder climates. Therefore, there is a need to further explore the LTOA protocol of 3 days at 185°F (85°C) based on additional mixture results. ⢠Early stripping in the HWTT was observed for a substan- tial portion of short-term aged asphalt mixtures in this study. As a consequence, the rut depth measurements for these mixtures were possibly biased from the stripping of the binder from aggregates. Therefore, future research on a more accurate rutting evaluation for softer asphalt mixtures by performing the HWTT at a lower temperature is neces- sary. Additionally, there is a need for appropriate modifica- tions to the HWTT procedure for testing field cores, such as finding a firmer substrate than plaster. ⢠There needs to be a research effort to define a balanced mix design procedure incorporating performance testing. It is recommended that the STOA defined herein should be used for rutting and moisture susceptibility performance testing as this would provide the most severe conditions for those distresses and STOA and LTOA from this project should be used in conditioning asphalt mixtures for crack- ing performance testing.
