Asphalt aging is a complex process in which asphalt binders undergo important modifications of their structure and composition through processes such as oxidation, formation/breaking of weak bonds (chemical aging), ordering of similar molecules, evaporation of volatiles (physical aging) due to temperature, pressure, air, and other environmental factors. These modifications have dramatic effects on binder and mixtures? mechanical properties such as stiffness, harness, adhesion, and cohesion. Understanding how and what structural modifications affect the asphalt binder and mixtures? mechanical properties has remained unexplored, largely because these modifications occur at the atomic to molecular scales. This research offers a novel approach to understating the link between asphalt molecular structure and mechanical behavior through nanoscale experimentation and advanced molecular modeling. In this research, molecular dynamic models will be developed, validated, and employed to predict rheological and mechanical properties determined in the laboratory using nano experimentation, in addition to other traditional methods.
This project is an integration of asphalt chemistry and mechanics, which will be integrated in an educational program that will improve the quality of engineering education. The molecular dynamics simulation will be supported through industry collaboration, which will strengthen the PI?s on-going research activities, and benefit the industry from broadening its product applications. This study may result in a general understanding of aging at a molecular scale, which is applicable to a wide range of construction materials. The integration of research and teaching will be addressed by involving graduate students, increasing diversity in engineering education, outreaching to high school students, development of new courses, and broad dissemination of research results.
Asphalt aging is a complex process in which asphalt binders undergo important modifications of their structure and composition through processes such as oxidation (chemical aging), ordering of similar molecules, evaporation of volatiles (physical aging) due to temperature, pressure, air, and other environmental factors. These modifications have dramatic effects on binder and mixtures’ mechanical properties such as stiffness, harness, and adhesion. This study determines how and what structural modifications affect the asphalt binder and mixtures’ mechanical properties through a fundamental study of integrated nano- and molecular characterization and modeling of asphalt binder. In essence, a nanoindenter is employed to determine stiffness and hardness of asphalt binders and asphalt concrete before and after aging. Molecular Dynamic (MD) simulations are conducted on model asphalt binders to determine the change in molecular structure and composition under the influence of aging factors such as temperature, pressure, and oxidation. It is shown that binder rheological properties such as stiffness increases and phase angle and creep compliance decrease due to aging. Based on aging index defined by complex shear modulus (G*), increase in percent polymer results in decrease in aging index value of polymer modified binders. At low temperature, aging has significant effects on G* and difference in creep compliance of unaged and aged binder is small compared to that at high temperature. This confirms that temperature significantly affects aging. For both unmodified and modified binders, hardness and reduce modulus increase exponentially due to aging. Atomic force microscopy test does not show significant difference in phase values between aged and unaged binders. Nanoindentation tests on unaged asphalt binders failed because the indenter tip is unable to detect and establish the contact surface. It is essential that a tip establish a contact surface before proceed. Based on the Oliver-Pharr prediction of elastic modulus and hardness, it is shown that as the dwell time increases the value of both apparent elastic modulus and hardness decrease in aged binder. Noindentation tests are successfully conducted on mastic phase as well as the aggregate phase of an asphalt concrete . Aged mastic showed lower elastic modulus and higher hardness value than those of unaged mastic. MD simulation results verify that for oxidation mechanism, both chemical and physical properties of asphalt before and after oxidative aging are dominated by functional groups. MD simulation results also verify that Ketones and sulfoxides are responsible for the asphalt hardening and the loss of saturates, aromatics and resins are offset by the increase of asphaltenes. Density temperature curves before and after oxidative aging via MD simulation indicate that oxidative aging change the glass transition of asphalt and causes the hardening of asphalt. Higher energies in the oxidative aged asphalt prove the formation of strong interacting components that are responsible for increasing viscosity and altering complex flow properties. This study has made significant improvements in aging test methods and our understanding of the molecular to nano to micro to macro scale behavior of asphalt concrete due to aging, which will play vital roles in the design, construction, and maintenance of roadway pavements years to come. This project has resulted in supporting two Ph.D. students, three M.S. students, and two undergraduate students. Several journals and conference papers were published and presented based on the outcome of this project. This project has made multiple positive impacts on instruction’s laboratory equipment and graduate course materials. This project has organized workshop and seminars that have educated asphalt engineers and professional about nanotechnology based solution to yet unsolved problems such as this. This project is a unique example which embraces the elements of lower length scale by nano-experiments and molecular models. In this GOALI project, molecular dynamics simulation is supported through collaboration with CULGI, a computational chemistry company with an office based in Albuquerque, New Mexico specializing in molecular modeling with applications in colloid, polymer, and surfactant. Such industrial collaboration has strengthened the PI’s on-going research activities, and benefited the industry from broadening its software applications to new areas such as asphalt.
