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The ReaxFF molecular dynamics simulations, which can predict chemical reactions, were performed on integral asphalt and individual asphalt molecules at different temperatures and oxygen levels to investigate the oxidation mechanism of asphalt and develop a molecular model suitable for aged asphalt. The simulation of integral asphalt suggests that the main oxidation products of asphalt are C–O, H–O, and S–O bonds. The oxygen level has the greatest influence on the yield of C–O bonds, and the temperature has the greatest influence on the H–O bonds. The simulation of individual asphalt molecules indicated that the oxidation of asphalt is accompanied by the decomposition of the aromatic rings, and thus, the aromaticity of the oxidized asphalt is decreased. Oxidation of asphaltenes starts with the oxygen molecules attacking the aromatic ring and generating a ketone, while the initial reactions of the other components are diverse. In addition, the simulation results were validated with Fourier transform infrared spectroscopy and nuclear magnetic resonance tests and were used to study the effects of oxidation on the characteristics of asphalt. The results suggested that the introduction of oxygen-containing functional groups decreases the component compatibility of asphalt and causes the aged asphalt to be harder and more viscous.

Due to the difficulty of obtaining relaxation modulus directly from experiments, many interconversion methods from other viscoelastic functions to relaxation modulus were developed in previous years. The objectives of this paper were to analyze the difference of relaxation modulus converted from dynamic modulus and creep compliance and explore its potential causes. The selected methods were the numerical interconversions based on Prony series representation. For the dynamic to relaxation conversion, the time spectrum was determined by the collocation method. Meanwhile, for the creep to relaxation conversion, both the collocation method and least squares method were adopted to perform the Laplace transform. The results show that these two methods do not present a significant difference in estimating relaxation modulus. Their difference mostly exists in the transient reduced time region. Calculating the average of two methods is suggested to avoid great deviation of single experiment. To predict viscoelastic responses from creep compliance, the collocation method yields comparable results to the least squares method. Thus, simply-calculated collocation method may be preferable in practice. Further, the master curve pattern is sensitive to the Prony series coefficients. The difference in transient reduced time region may be attributed to the indeterminate Prony series coefficients.

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