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30 Conclusions and Suggested Research Several different factors affect the aging of an asphalt mixture, including aggregate absorption, asphalt aging, recycled material (RAP and RAS) type and content, and WMA technology. Factors such as plant type and produc- tion temperature [within 30°F (17°C)] have also been noted in the literature as having critical effects. The complex inter- actions of these factors in asphalt mixture design makes it challenging to know how they will affect pavement behavior with respect to aging. This project focused on connecting performance characteristics from laboratory testing results to several different mixture factors over time and creating stiffness-based global aging models for asphalt mixtures. Conclusions Aging Models In NCHRP Project 09-52, a global model of mixture aging was suggested based on approximately 3 years of data, and the aging protocols for LTOA in that project mimicked only about 1 to 2 years of field aging. In NCHRP Project 09-52A, the field aging time was extended to 4 to 5 years, and a dif- ferent LTOA was explored and resulted in mixture aging that was projected to occur at 7 to 13 years, depending on climate. In NCHRP Project 09-52, the aging models were divided into two climatic zones (warm and cold) because the global model of MR stiffness versus CDD as shown in Figure 9 indicated that the mixture aging behavior in cold climates was different than that in warm climates. Iowa, Wyoming, and South Dakota fell into the colder climate category, and New Mexico, Texas I, Texas II, Florida, and Indiana fell into the warmer climate. Based on the MR stiffness results, the colder climate field cores were less stiff than those from warmer climates at equal CDD levels, which could be the result of lower rates of aging during winter temperatures and, in some instances, the use of lower-temperature PG asphalts. The rate of aging of the mixtures when plotted against CDD was not dependent on the climate because both models had power exponents of 0.18. Factor Analyses Asphalt Absorption Asphalt absorption can contribute to mixture aging by two mechanisms: (a) the reduction of effective asphalt as it is drawn into the aggregate pores; and (b) the possible selective drawing of lighter ends of the asphalt deeper into the aggregate, leaving the heavier and more brittle portion of the asphalt as the effective binder. The Florida field site had two levels of aggregate water absorption, in which granite had lower absorption than limestone. The absorbed water content was 0.6% for granite and 3.7% for limestone. The total binder content for granite and limestone mixtures was 5.1% and 6.8%, respectively. The Florida mixture with lime- stone had a slightly lower MR stiffness value than the granite mixture at construction, but this difference decreased as the mix aged in the field. At the field site in Iowa, a lower- absorption and a higher-absorption limestone were used. The lower-absorption limestone had an absorbed water content of 0.9% and a total binder content of 5.0%; the higher-absorption limestone had an absorbed water content of 3.2% and a total binder content of 7.0%. The plot of high- absorption MR stiffness versus low-absorption MR stiffness showed an approximately 8% offset from the line of equality for the combined Indiana and Florida data, which indicated that aggregate absorption did have a significant effect on the stiffness of asphalt mixtures at the Florida and Iowa field sites. Figure 16 illustrated that the lower-absorption asphalt mix- tures at both locations had higher stiffnesses, which is the opposite of what was expected. It appears that the volumet- ric mixture design over-compensated for the use of highly absorptive aggregates as shown by the design asphalt content in both of these field sites (Newcomb et al. 2015). C H A P T E R 4
31 Asphalt Source It has long been known that asphalt from different sources may age at different rates. The Texas II field site was built specifically to examine the effects of asphalt source on mix- ture aging. Both mixtures examined during this study were HMA produced in a DMP. One was HMA produced with Binder V, and the other was produced with Binder A. Both binders appeared to be very similar during PG grading. Both were m-controlled for low-temperature properties and were within 2°C for high-temperature grading. The intermediate temperature grade was slightly different, with Binder A being 3°C higher than Binder V. Both mixtures started at similar MR stiffness values at construction, but a substantial separa- tion appeared at 42 and 51 months. The plot of Binder A MR stiffness versus Binder V MR stiffness (Figure 19) showed an 18% offset from the line of equality. Therefore, the factor of asphalt sources had a very significant effect on the stiffness of the asphalt mixture compared with all other factors except RAP/RAS content. Recycled Materials The introduction of recycled materials into asphalt mix- tures is expected to produce a greater stiffness compared with a virgin mixture. Mixtures with RAP/RAS consistently had higher MR stiffness values than those without RAP/RAS. The RAP in the New Mexico field site made the asphalt mixture stiffer initially, and it stayed consistently greater than the virgin mixture over time. The Texas I site also had both RAP/RAS and virgin mixtures. The Texas I mixtures had a similar MR stiffness trend as the New Mexico mixtures. Based on the plot of the virgin mixture MR stiffness versus RAP/RAS mixture MR stiffness for both the New Mexico and the Texas I field sites (Figure 21), the offset from the line of equality was about 30%. Therefore, the factor of recycled materials had the greatest effect on the stiffness compared with the other factors studied in this project. WMA Technology While WMA mixtures, for the most part, had lower stiff- nesses than HMA mixtures over the course of the project, there were cases in which the WMA mixtures were stiffer or were virtually indistinguishable from HMA mixtures, such as South Dakota, New Mexico, Texas I, and Indiana. Although researchers looked for explanations for what made sites have higher or lower WMA stiffness, there was no clear reason. Neither the type of WMA technology, the use of RAP/RAS, nor the climate could explain the differences. The plot of HMA MR stiffness versus WMA MR stiffness in Figure 24 showed that the trendline had an 11% offset from the line of equality, so the conclusion was that the factor of WMA tech- nologies had a significant effect on the stiffness of field cores at these field sites. Therefore, the research team concluded that WMA mixtures did not age as much in general and were better at aging up to about 5 years after construction. Some sites did exhibit a tendency for HMA and WMA to converge as the age increased. Production Temperatures Changes of 30°F (17°C) did not produce significant differ- ences in the asphalt mixtures in Iowa and Wyoming initially and at the first sampling interval in NCHRP Project 09-52. This trend continued in NCHRP Project 09-52A. IDEAL-CT The use of MR or dynamic modulus in the development of aging models is useful for input to mechanistic-empirical performance analysis; however, most asphalt fatigue models assume that as stiffness increases, cracking resistance increases. However, increasing stiffness may indicate greater brittleness, and cracking resistance decreases with time and aging. With the material remaining from different sampling times and field sites, researchers performed a simple CT to see if there was a possibility of capturing the effect of aging on crack- ing resistance. Zhou et al. (2017) proposed IDEAL-CT, which is performed at 77°F (25°C) on cylindrical samples that are 150 mm in diameter by 62.5 mm high at a rate of 50 mm/min. The CT index, used to determine the crack resistance, is a function of the area under the load-displacement curve from the initiation of loading to the end of loading, and the slope of the post-peak curve at a point that is 75% of the peak load. A high CT index indicates better cracking resistance and less cracking in the field. According to Zhou et al.âs research, a laboratory specimen with a 7% AV content is expected to have a CT index of 75 for a typical dense-graded asphalt mix and 150 for an excellent crack-resistant mix. The field cores in this project had various AV content because the conditions varied at different field sites. The field cores were selected for IDEAL-CT to cover a range of CDDs, AV content, and MR stiffness. The CT index results were com- pared with CDD and MR stiffness in Figure 35 and Figure 36. The CT index tended to decrease as CDD increased. The Texas I field site was the oldest field site in this project, and the CT index on its 60-month post-construction field cores was only 3. More data are needed to create a better global model for cracking resistance but based on the IDEAL-CT results in this project, there is an indication that this may be possible in a future project. A study on the deterioration of mixture cracking resistance with time is suggested, because it
32 might provide more information on how to mitigate cracking in the long term. Suggested Research This project validated global aging models using MR stiff- ness and CDD with various factors in asphalt mixture design. The 40th parallel north divided the field sites into colder and warmer climates. The global models of stiffness in colder and warmer climates had better statistical fits. Therefore, a study of the effects of specific climates on aging needs to be considered in the future. The main changes to aging protocols suggested herein are to AASHTO R 30 for STOA and, potentially, LTOA. AASHTO R 30 recommends an STOA of 2 hours of condi- tioning at the compaction temperature. For STOA, the research team suggests a conditioning of the loose mix at 275°F (135°C) for 2 hours for HMA and 240°F (116°C) for 2 hours for WMA. This simplifies the process by eliminating the need to adjust oven temperatures. Currently, AASHTO R 30 recommends LTOA conditioning be conducted at the compaction temperature for 4 hours. While this achieves some early aging, it has little to do with field LTOA. Getting beyond 5 years of field aging in NCHRP Project 09-52A required oven aging the loose mix at 185°F (85°C) for 5 days. This was also based on a limited amount of material. While realistic, this may not be practical for many applications. This would be a good topic of discussion for the AASHTO Subcommittee on Materials to consider. The factor of asphalt source was found to have a signifi- cant effect on asphalt mixture performance in aging. Three sets of field cores were collected at different dates from the Texas II field site and tested for MR stiffness. The stiffness of the two mixtures showed that the data points diverged quickly for the two binders (A and V) and reached a plateau at about 44 months after construction. The mixture made with Binder A was significantly stiffer than that made with Binder V. There could be a broader effort to examine the effects of different asphalt sources on the rates of aging in different climates. In this study, the standard AASHTO M320 PG grading did not show the field aging in the typical testing regime, so another approach needs to be tried. One method is to determine the difference in critical temperature (ÎTc) between the stiffness and m-value from the low continuous grades from BBR testing. This parameter to evaluate cracking potential in asphalt binder is related to age and reflects the flexibility and ductility of the material. Cracking resistance with aging needs to be more thoroughly investigated. The results in this study were based on cores from various projects with leftover samples that had been stored in the laboratory and that had a higher-than-desirable range of AVs. The testing, however, did show a tendency for cracking resistance to decrease over several years. A more controlled experiment with different asphalt binder sources would be desirable to confirm this finding. It would also be desirable to develop a relationship between aging conditions in the laboratory and field aging.
