Asphalt concrete (AC) is created by mixing liquid asphalt binder with solid aggregates. When asphalt binder is mixed with aggregates, it creates a thin film (about 7-16 microns thick) around the aggregate surface. Fine aggregate (smaller than 0.075 mm size) get entrapped within the binder film or mixed with the binder, and create a phase called mastic. Both asphalt film and mastic play significant roles in aging of asphalt pavement. To this day, tests were performed mainly on the bulk liquid asphalt, but not on the film or mastic. In this study, for the first time, nanoindentation tests were done on asphalt film and mastic to identify the aging behavior of asphalt.
This study identifies aggregate, binder film and mastic phases using indentation depth resolution and creep response techniques. Creep compliance response is used to identify an individual phase. The results show that out of 100 indentations, only 30% of the indentations could be made on the mastic phase of the AC and about 5% to 9% could be made on the binder phase of the AC. The identification study confirmed that the binder phase also exists in an AC sample.
In this study, AC samples were aged using a draft oven for 2 days, 5 days, 10 days, 15 days and 20 days at 85 °C temperature. Load-displacement data was analyzed using a spring-dashpot-rigid body model to determine stiffness and hardness. Stiffness and hardness data were further analyzed as a function of time and modeled using a kinetic energy model. One of the key parameters of the kinetic energy model is the activation energy of aging. Results show asphalt binder’s activation energy is much lower (about 50%) than that of asphalt mastic. This indicates that mastic reacts with oxygen at a much slower rate than asphalt binder.
This study attempts to correlate the available voids in an AC sample using nanoindentation hardness and viscoelastic relaxation parameter. AC samples were prepared with an increasing order of air voids from 7% to 20% and aged for four years at room temperature and atmospheric conditions. Binder of the aged AC samples was extracted and recovered using the centrifuge and distillation process. The oxidative aging of asphalt binder samples was quantified using a Fourier Transformed Infrared Spectroscopy (FTIR) analysis of carboxyl and sulfoxide functional groups. Oxidation measured for ketones increases linearly with the increase of pore content in AC. Nanoindentation creep behavior was evaluated from the load-displacement behavior and then fitted in the viscoelastic model to quantify the viscoelastic relaxation time parameters. Results show the porous AC undergoes more aging compared to a dense AC sample. Oxidation measured for ketones increases linearly with the increase of pore content in AC. Nanoindentation relaxation and retardation time increase with the increase in effective voids in the AC samples. This means higher air voids in AC causes damage to the binder which is reflected by reduction in the stress-relaxing capacity. The viscoelastic relaxation time of the AC sample with 20% air voids is 7.5 times more than that AC sample with 7% air voids. The viscoelastic relaxation behavior is compared for the mastic phase of asphalt concrete as well. The results show that the relaxation time increases only 3.9 times for 13% change in air voids.
Nanoindentation test results can be significantly affected by film thickness. Therefore, this study investigates how asphalt film thickness changes by an increase in aging time. Results show that film thickness reduces exponentially as a function of aging. After 10 days thickness reduces to 50% of its original thickness. To investigate the reason behind this reduction, aged films were tested in FTIR for chemical changes. Results show C=O and S=O increase with aging. The coagulation of the polar fractions in asphalt binder creates a thinner asphaltic film due to aging. The coagulation of the polar fraction creates a larger stiffer form of asphalt molecule, which is also known as asphaltene fraction of asphalt binder. Chromatographic analysis on aged asphalt binder shows stiffer asphaltene fraction increases with aging of asphalt.
Pavement, Asphalt, Nanoindentation, Aging, Concrete, Nanomechanics
Level of Degree
First Committee Member (Chair)
Rafiqul A. Tarefder
Second Committee Member
Third Committee Member
Fourth Committee Member
Faisal, Hasan M.. "Understanding Asphalt Concrete Aging Using Nanoindentation." (2018). https://digitalrepository.unm.edu/ce_etds/214