Abstract
An attempt is made to understand the chemical composition, oxidation mechanisms, and property changes of asphalt binders before and after oxidative aging using molecular dynamics (MD) simulations. Unoxidized and oxidized asphalts are subjected to different compressive and tensile stress rates, and moisture contents at room temperature. Results show that density, energy, and viscosity of the oxidized asphalt are higher than the unoxidized asphalt, indicating hardening and rheological property changes of asphalt after oxidation. Both the unoxidized and oxidized asphalts deform more and fail faster with an increase in stress rates, especially under tensile stress. The oxidized asphalt is stronger than the unoxidized asphalt under mechanical stress. Moisture inclusion affects viscosity more by decreasing the viscosity of the oxidized asphalt faster compared to the unoxidized asphalt. The viscosity of the oxidized asphalt is lower than that of the unoxidized asphalt above 5% moisture inclusion. This indicates that oxidized asphalt pavement might be exposed to more moisture-induced damage.
TopBackground
MD simulation computes, using a computer, the motion of a series of atoms, their co-ordinates in three-dimensional space, and their connectivity in a system as a function of time. It solves the classical equations of motion of N atoms interacting via a potential U, the sum of the pair potential of all the atoms. In Cartesian coordinates, the equation of motion is simply Newton’s second law of motion:
(1) where,
ri is the position of an atom
i,
mi is the mass of the atom,
ai is the resulting acceleration of the atom,
vi is the resulting velocity of the atom, and
t is time. The force
Fi acting on the atom is defined as the negative gradient of the potential
U:
(2)(3) where,
U is a combination of intramolecular and intermolecular potentials which describes the energies associated with both bonded interactions within the molecule and molecules being in a particular conformation relative to each other, and
rij is the distance between the interacting sites for the pair of atoms
i and
j.
In general, the above equations of motion are solved using Verlet integration algorithm. The most widely used for integrating molecular dynamics equations is the velocity Verlet algorithm, which gives positions, velocities and accelerations at the same time and does not compromise precision (Leach, 2001). This method is used in this study. The position and velocity of atom i at the next step in time is determined by the following equations:
(4)(5) where,
dt is the time step.
Key Terms in this Chapter
Moisture Damage: Moisture damage causes the debonding of asphalt-aggregate mixture, which also causes the rheological property changes of asphalt such as viscosity.
Molecular Dynamics Simulation: This 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 force.
Monte Carlo Algorithm: It is a randomized algorithm which uses a degree of randomness as part of its logic to run under certain time until one of the runs produces an output that can be verified to be correct.
Functional Groups: Functional groups are specific groups of atoms in molecules that are responsible for the characteristic chemical reactions and properties of those molecules.
Potential Energy: This is the energy stored in an object due to its position in a force field. In a system, potential energy refers to the configuration of the system in a force field.
Force Field: Force field in chemistry is used to describe the potential energy of a system of molecules and atoms in the form of mathematical functions and parameters of mathematical functions.
Asphalt Oxidative Aging: Asphalt oxidative aging is an inevitable oxidation process when asphalt is exposed to the atmospheric oxygen, since asphalt is a natural organic end product of ancient living organisms which is subject to chemical oxidation. Oxidative aging causes hardening of asphalt which leads to embrittlement of asphalt pavement.
Composition of Asphalt: Asphalt is composed of saturates, aromatics, resins, and asphaltenes, four fractions, with an increase of polarity. The four fractions are mainly comprised of carbon and hydrogen with heteroatoms such as oxygen, nitrogen, and sulfur. Except for saturate fraction, the other three fractions are aging fractions.