Investigation on Nano-modification and Thermomechanical Properties of Asphalt Materials

Abstract

Long-term durability is the greatest concern for asphalt pavement. A variety of thermomechanical degradations, including cracking, rutting, swelling and depression, can significantly affect the durability of asphalt pavement during service. Improved thermomechanical performance of asphalt is needed to reduce the occurrence of flaws and failures. Functionalized graphene nanosheets has the potential to become the next generation of modifiers for asphalt pavements that require long-term durability. However, the thermomechanical properties, thermal conductivity and self-healing properties of functionalized graphene-modified asphalt nanocomposites, as well as the reinforcing mechanisms of incorporated graphene nanosheets, are still unclear at this stage. The use of modeling tools helps reveal the working mechanisms of the nanomaterials.

In this research, the thermomechanical properties of graphene and functionalized graphene-modified asphalt nanocomposites are investigated using molecular dynamics simulations. The shear capacity of asphalt nanocomposites at different temperature levels is evaluated through the pullout test, and the details of interactions between functionalized graphene and asphalt matrix are characterized. Results show that hydrogen bonding plays an important role in the reinforcement of asphalt nanocomposites. Polar components of asphalt interact more easily with the functional groups of functionalized graphene sheet by π-π stacking, while the nonpolar components interact with modifiers via mechanical entanglement. Graphene sheet with hydroxyl functional groups presents more effective reinforcement due to stronger hydrogen bonding interactions. The shear resistance of asphalt nanocomposites decreases with increasing temperature due to the increment of intermolecular distance and free volume size. The in-depth understanding of reinforcing mechanisms helps to provide information on more favorable functional groups, which is beneficial for the manipulation and design of durable asphalt pavement materials using nano-engineering.

The effects of functional groups and defects on the thermal conductivity of graphene-modified asphalt are systematically investigated by molecular dynamics simulations. Results show that the functionalized graphene is beneficial to interfacial thermal conductivity, and the butyl group increases twofold due to its interfacial penetration compared to the pristine one. The incorporation of functionalized groups can promote phonon transport at the asphalt–graphene interface. The predominant factor determining the thermal conductivity of asphalt nanocomposites changes from the interface-dominant mode to the filler-dominant mode when crossing the critical size threshold. These findings can provide valuable information on the role of functional groups and defects in the thermal conductivity of asphalt nanocomposites, which is beneficial for developing asphalt pavements with advanced energy harvesting ability.

The self-healing properties of pure asphalt and graphene-modified asphalt are investigated through molecular dynamics simulations. Asphalt bulk with two crack widths and locations for graphene is introduced, and the molecular interactions among asphalt components and the graphene sheet are analyzed. The results show that the location of graphene significantly affects the self-healing behaviors of asphalt. Graphene near the crack surface can greatly accelerate the self-healing process by interacting with the aromatic molecules through π-π stacking, whereas graphene at the top area of the crack tip has only a minor impact on the process. The self-healing process of asphalt undergoes reorientation for asphaltene, polar aromatic and naphthene aromatic molecules, as well as the bridging effect of saturate molecules between the crack surface. This in-depth understanding of the self-healing mechanism contributes to the knowledge of enhancement of self-healing properties, which will help to develop durable asphalt pavements.

The durability of asphalt is considerably affected by the increasing amount of asphaltene aggregation after aging. The mesoscopic behavior of asphaltene plays a significant role in the formation of microstructures and agglomerates. A new coarse-grained model is developed to investigate the formation mechanism of aggregated microstructures in asphaltene; disk-like building blocks are adopted as the basic unit in the coarse-grained model. The forcefield parameters based on Gay-Berne potential are derived from the full atomistic simulations and applied in the coarse-grained simulations. The results show that randomly orientated asphaltene disks gradually adjust their positions and orientations during equilibrium process, and ultimately reach the jammed state with different columnar structures containing packed asphaltene disks. The packing fraction and the microstructures of agglomerates are affected by the aspect ratio of asphaltene disks and the aging treatments, which include heating and compression. The newly developed coarse-grained model is capable of characterizing the evolution of asphaltene microstructures, and it helps to understand the formation mechanism of asphaltene agglomerates. The in-depth analysis reveals the influencing factors on asphaltene agglomerates, which can enlighten the solutions for aggregation problems of asphaltene system in asphalt industry.

Date of Award	4 Aug 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Denvid LAU (Supervisor)