Cracking Performance Evaluation of Asphalt Mixtures in Virginia
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Authors
El Hajj, Habib
Issue Date
2023
Type
Thesis
Language
Keywords
Alternative Title
Abstract
Asphalt plays an essential role in various transportation and infrastructure projects worldwide. Throughout its production and service life, asphalt binder undergoes aging influenced by factors like temperature and exposure to oxygen. Oxidative aging, for instance, alters the properties of the asphalt binder, leading to increased stiffness and a consequent decrease in resistance to cracking. This study aims to establish a long-term aging protocol specifically tailored for evaluating the cracking resistance of asphalt mixtures in Virginia. The research comprised two phases. The first phase focuses on establishing a practical aging protocol for dense-graded surface mixtures (SM) A and D that can be implemented in mix design and acceptance. According to Virginia Department of Transportation (VDOT), the A designation corresponds to mixtures with an equivalent single axle load (ESAL) of 0 to 3 million while the D designation corresponds to an ESAL range of 3 to 10 million. The second phase aims to establish initial test criteria for the CT index based on the current criteria for short-term aged asphalt mixtures. Given the challenges with aging of compacted specimens (e.g., aging gradient within the specimen), loose asphalt mixture aging was employed in this study. Eleven SMs from Virginia were selected, each comprising different combinations of reclaimed asphalt pavement (RAP), asphalt binder grade and content, aggregate gradation, and recycling agent (RA). Preliminary observations from 3 of the 11 SMs suggest a critical aging duration of 1 day at 95°C following 4 hours of aging at compaction temperature (4H_1D). This aging level demonstrates the ability to differentiate between the cracking resistance of the evaluated asphalt mixtures. Equivalent accelerated aging durations of 6 and 8 hours at compaction temperature (6H_0D, 8H_0D) were evaluated and found to simulate the 4H_1D aging duration. To set the criteria for CTindex at the critical aging found in the study, a thorough analysis of the test data was done. This analysis involved a combination of test parameters obtained from the different binder and mixture testing. Glover-Rowe parameter (GRP) was plotted against the CTindex for all mixtures and the cracking limits were obtained in terms of CTindex limits. Additionally, the CTindex was plotted against the aging duration and the cracking limits were also found in terms of aging duration.
The resulting CTindex limits for cracking were about 71 for an onset of cracking corresponding to a GRP of 180 kPa and 43 for significant cracking corresponding to a GRP of 600 kPa. Furthermore, the aging durations for cracking were 0.26 days (4 hrs at compaction temperature followed by 2.24 hrs at 95C) for onset of cracking (i.e., GRP equal to 180 kPa) and 1.41 days (4 hrs at compaction temperature followed by 29.84 hrs at 95C) for significant cracking (i.e., GRP equal to 600 kPa). The drop in CTindex after 4 hours of aging was calculated for each aging level and was found to have a strong correlation with the GRP of the recovered asphalt binder from the asphalt mixture at the respective aging level. A 10.8% drop in CTindex after 4 hours of aging corresponds to the asphalt binder onset of cracking (GRP of 180 kPa) and a further drop reaching 29.8% corresponds to the significant cracking level (GRP of 600 kPa). These preliminary findings will be verified in a follow up study using the remaining 8 SMs from Virginia.