# A Coarse-Grained Molecular Model for Simulating Self-Healing of Bitumen
###### Abstract  
The longevity of asphalt pavements is a key focus of road engineering, which closely relates to the self-healing ability of bitumen. Our work aims to establish a CGMD model and matched force field for bitumen and break through the limitations of the research scale to further explore the microscopic mechanism of bitumen self-healing. In this study, a CGMD mapping scheme containing 16 kinds of beads is proposed, and the non-bond potential energy function and bond potential energy function are calculated based on all-atom simulation to construct and validate a coarse-grained model for bitumen. On this basis, a micro-crack model with a width of 36.6 nm is simulated, and the variation laws of potential energy, density, diffusion coefficient, relative concentration and temperature in the process of bitumen self-healing are analyzed with the cracking rate parameter proposed to characterize the degree of bitumen crack healing. The results show that the computational size of the coarse-grained simulation is much larger than that of the all-atom, which can explain the self-healing mechanism at the molecular level. In the self-healing process, non-bonded interactions dominate the molecular movement, and differences in the decreased rate of diffusion among the components indicate that saturates and aromatics play a major role in self-healing. Meanwhile, the variations in crack rates reveal that healing time is inversely proportional to temperature. The impact of increasing temperature on reducing healing time is most obvious when the temperature approaches the glass transition temperature (300 K). bitumen; molecular dynamics; coarse-grained; force field; self-healing +
Footnote †: journal:  
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bitumen and, subsequently, [PERSON] and [PERSON] [2; 3] a four-component model. Researchers have investigated the mechanism of self-healing using these molecular models. Scanning electron microscope imaging of bitumen microcracks in combination with MD simulations by [PERSON] et al. [4] demonstrated that temperature, crack width, and the degree of aging are major factors affecting self-healing. [PERSON] et al. [5] determined an optimal self-healing temperature, while [PERSON] et al. [6] proposed a six-component model of a bitumen healing agent. It was found that crack width, temperature, state of molecular aggregation, and aggregate influence self-healing. [PERSON] et al. [7; 8] performed MD simulations using the four-component molecular model with short-term aging and a healing agent. It was seen that as the bitumen microcracks entirely vanish, the sample volume begins to diminish. According to this study, healing depends on the diffusion of bitumen and healing agent into the cracks. [PERSON] [9] described the self-healing process as a combination of surface wetting and surface diffusion. Based on MD, [PERSON] et al. [10] studied the self-healing of crumb rubber-modified bitumen and found that self-healing decreases with increased rubber content. [PERSON] et al. [11] simulated the self-healing process in a 3D microcrack model and studied the effect of carbon-based nanomaterials as a healing agent. It was found that adding carbon nanotubes can effectively improve the self-healing of ordinary bitumen. [PERSON] et al. [12] used the residual oil extracted from soybeans to regenerate aged bitumen, enhancing its rheological properties. The activation energy of self-healing in aged bitumen was estimated by MD simulations. Presently, research on bitumen self-healing is limited to all-atom MD simulations, which reduces the scale of the computational sample and makes it impossible to analyze self-healing at larger scales. In this paper, we propose to bridge the gap between the micro and the macro scales by doing larger-scale simulations using Coarse-Grained Molecular Dynamics (CGMD). CGMD is routinely used in the study of petroleum and hydrocarbon compounds, which are the main chemical components of bitumen.