Civil Engineering ETDs

Author

Jielin Pan

Publication Date

2-1-2016

Abstract

Oxidative aging is a major issue in asphalt pavements that leads to hardening of the asphalt, which further contributes significantly to asphalt pavement embrittlement and eventually results in excessive pavement cracking. In addition, the performance of asphalt binder is also highly related to its service conditions which involve climatic conditions and traffic conditions, such as temperature, moisture and traffic loads. Generally, the mechanical properties of asphalt before and after oxidative aging can be tested in the laboratory; however, the fundamental material science of asphalt before and after oxidative aging is difficult to be investigated in the laboratory regarding chemical composition and property change due to the difficulties of studying the molecular structures and their dynamic behavior in asphalt. Therefore, Molecular Dynamics (MD) simulation method is used in this study to understand how the chemical composition and property changes of asphalt after oxidative aging affect the physical, thermodynamic, rheological and mechanical performance of the asphalt, and how the oxidized asphalt acts under different conditions, such as loading, temperature, and moisture, compared to the unoxidized asphalt. MD simulation is a computational method used to simulate the physical movements of atoms and molecules depending on time and force field chosen under different conditions, such as temperature, pressure, and loading. Asphalt models before and after oxidative aging are developed for MD simulations in this study under different temperatures, loading and moisture contents. Simulation results show that the oxidized functional groups in asphalt molecules increase the strength of intermolecular bonds of asphalt, which further contributes to the hardening of the oxidized asphalt. Specifically, the internal energy changes, especially for higher magnitude of intermolecular and lower kinetic energies, are responsible for the hardening of oxidized asphalt, and the higher potential energies and for oxidized asphalt further proves that oxidation increases the polarity of molecules in asphalt and forms strongly interacting components. The internal property change is consistent with the external physical and rheological property change after oxidation, which is revealed by the increase of density, bulk modulus and viscosity. Considering the mechanical property of asphalt using MD simulations, both the unoxidized and oxidized asphalts deform more and fail faster with an increase in both compressive and tensile stress rates, especially under tensile stresses. However, the oxidized asphalt is stiffer than the unoxidized asphalt, which shows slower and less deformation, further validating the hardening of asphalt after oxidation. Asphalt is significantly susceptible to temperature. Simulation results show that density, and zero shear viscosity of asphalt decrease with an increase in temperature. Isothermal compressibility and bulk modulus also change under different temperatures. Moreover, the density, bulk modulus (inverse of isothermal compressibility) and zero shear viscosity of oxidized asphalt is higher than the unoxidized asphalt proving the hardening of asphalt during oxidation. The different change trends of isothermal compressibility and bulk modulus between unoxidized and oxidized asphalts under different temperatures indicate glass transition behavior changed after asphalt oxidation. According to MD simulations, the moisture impacts on asphalt before and after oxidative aging are significant regarding bulk modulus and zero shear viscosity. Both bulk modulus and zero shear viscosity of asphalt decrease with an increase in moisture content in the asphalt. Moreover, moisture adversely impacts more on oxidized asphalt. The moisture impact on the density of asphalt before and after oxidation is not significant. Density of oxidized asphalt is constantly higher than the unoxidized asphalt under different moisture contents and the density fluctuations of oxidized asphalt is larger. This indicates that asphalt is more sensitive to moisture inclusion after oxidative aging. Laboratory tests are used to validate MD simulation results. Pycnometer method, Dynamic Shear Rheometer (DSR) and nanoindentation are used to test density, zero shear viscosity, and nanomechanical properties including modulus, hardness and viscosity of asphalt binders with different degree of oxidative aging, respectively. Laboratory testing results are consistent with MD simulation results.

Keywords

Molecular Dynamics Simulation, Asphalt Oxidative Aging, Physical, Rheological and Mechanical Properties, Temperature and Mositure Impacts

Sponsors

National Science Foundation (NSF)

Document Type

Dissertation

Language

English

Degree Name

Civil Engineering

Level of Degree

Doctoral

Department Name

Civil Engineering

First Advisor

Tarefder, Rafiqul

First Committee Member (Chair)

Shen, Yu-Lin

Second Committee Member

Ng, Tang-Tat

Third Committee Member

Zhang, Guohui

Fourth Committee Member

Tarefder, Rafiqul

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