Advanced Self-Healing Asphalt Reinforced by Graphene Structures: An Atomistic Insight

Deterioration under daily vehicle loadings and variant environmental conditions, and the aging of asphalt during service result in degradation or even structural failures, i.e., cracking and raveling, which can further weaken the durability of asphalt pavements. The inherent response of asphalt to repair micro-cracks and voids automatically helps it recover from damages and restore strength¹. This self-healing capability can considerably extend the service life of asphalt, save costs on maintenance, and reduce the emission of greenhouse gases^2,3. The self-healing behavior of asphalt generally depends on several influencing factors, including its chemical composition, the degree of damage, and environmental conditions⁴. The improved self-healing capability of asphalt that can fully heal damage within a short period is desired; this has attracted extensive research interest in better mechanical performance and durability for asphalt pavements within civil engineering.

Novel methods to improve the self-healing capability of asphalt mainly include three approaches – inducing heating, encapsulation healing, and incorporating nanomaterials – which can be applied individually or simultaneously^5,6. Inducing heating can significantly improve the mobility of asphalt and activate its self-healing for recovery⁷. The self-healing technology of asphalt by inducing heating can be ascribed to the assisted self-healing technique, which indicates that the self-healing properties of asphalt are improved by external stimuli. The objective of adding the steel wool fibers is to enhance the electrical conductivity so as to increase the healing capacity of the asphalt binder⁸. The approach to induce heat is to expose these electrically conductive fibers to the high-frequency alternating electromagnetic field, which can induce eddy currents, and the heat energy can diffuse into the asphalt binder by the conductive fibers⁹. The steel wool fibers enhance not just the electrical conductivity but also the thermal conductivity, both of which can positively affect the self-healing properties of asphalt. However, it is challenging to select the proper mixing time for fibers¹⁰. The length of fibers decreases with increased mixing time and influences thermal conductivity, while the decreased mixing time leads to clusters of fibers and impedes the mechanical properties of asphalt⁹. The encapsulation method can supply light components of aged asphalt such as aromatics and saturates and refresh the self-healing capability of asphalt^11,12. However, this is a once-only treatment, and the healing materials cannot be replenished after the release. With the development of nanotechnology, nanomaterials have become promising modifiers for enhancing asphalt-based materials. Asphalt binders incorporated with nanomaterials present better thermal conductivity and mechanical properties¹³. Graphene with excellent mechanical performance and high thermal performance is regarded as an excellent candidate to improve the self-healing ability of asphalt^14,15,16,17. The increased healing properties of graphene-modified asphalt can be attributed to the fact that graphene increases the capacity of the asphalt binder to be heated and produce heat transfer inside the asphalt binder, which means that graphene-modified asphalt can be heated more rapidly and reach up to higher temperature than pure asphalt¹⁸. The generated heat can be transferred throughout the graphene-modified asphalt at a faster speed than that through pure asphalt. The crack region of the asphalt binder can be influenced easily and healed faster by the heat flow with higher temperature and higher heating capacity. The self-healing reaction will begin if the energy that is equal to or larger than the healing activation energy exists at the crack surface of the asphalt¹⁹. Graphene can improve the thermal activation healing performance and accelerate the healing rate of asphalt^19,20. Besides, graphene can save heating energy up to 50% during the healing process, which can benefit energy efficiency and reduce maintenance costs²¹. As a microwave-absorbent material, graphene is reported to improve the healing ability of asphalt during the rest period of microwave heating²². It is expected that the addition of graphene into asphalt will improve not only the mechanical performance but also the self-healing and energy-saving capacity, which requires in-depth knowledge of the self-healing mechanism.

Self-healing at the nanoscale is mainly due to the wetting and diffusion of asphalt molecules at the fractured faces²³. As asphalt consists of various polar and non-polar molecules, its self-healing capability is strongly related to molecular interactions and movements of asphalt molecules of different components¹. However, current research mainly relies on experimental techniques to quantify macroscopic mechanical properties, which causes missing information in the change of microstructures and the interactions among asphalt molecules when trying to understand the healing mechanism. The reinforcing mechanism of graphene in the self-healing capability of asphalt is also unclear at this stage. Molecular dynamics (MD) simulations play an influential role in investigating molecular interactions and motions of nanocomposite systems, and link microstructural deformation with molecular interactions and movements^{24,25,26,27,28,29,30,31}. MD simulations have become more and more popular for analyzing material behaviors that cannot be accessed easily by experiments^32,33. Existing studies have shown the feasibility and availability of MD simulations in asphalt systems; the cohesion, adhesion, aging, and thermomechanical properties of asphalt and asphalt composites can be explored by MD simulations^34,35,36,37. The self-healing behaviors of asphalt can also be predicted by MD simulations^38,39,40. Therefore, it is believed that the investigation using MD simulations is an effective way to understand both the self-healing and reinforcing mechanisms.

The objectives of this study are to investigate the self-healing behaviors of pure asphalt and graphene-modified asphalt nanocomposites and to understand the role of graphene in improving the healing capacity of asphalt through MD simulations. The self-healing simulations of pure asphalt and graphene-modified asphalt composites are carried out by introducing cracks into the initial structures. The self-healing capabilities are characterized by the contour of atom numbers, the reorientation and entanglement of molecules at the fractured face, and the mobility of asphalt components during the self-healing processes. By investigating the healing efficiency of graphene at different sites, the reinforcing mechanism of graphene contributing to the self-healing abilities of asphalt is unveiled, which can help with the monitoring of nanofillers in an optimal way and thus enable the life extension of asphalt pavements. An investigation of the self-healing capacity at the atomistic scale can provide an efficient way to develop advanced asphalt-based materials for future research.

According to asphalt chemistry, asphalt consists of various types of hydrocarbons and non-hydrocarbons with different polarity and shapes, which can mainly be divided into the four components of asphaltene, polar aromatics, naphthene aromatics, and saturates^41,42. Asphaltene molecules are relatively larger and heavier than other molecules in asphalt, with a mean atomic mass of roughly 750 g/mol and a molecular diameter in the range of 10-20 Å. It has been widely accepted that asphaltene is composed of large aromatic cores that contain heteroatoms and are surrounded by different lengths of alkyl groups⁴³. A modified asphaltene molecule is constructed, as shown in Figure 1a. The molecules of polar aromatics and naphthene aromatics are constructed based on the polarity and the element ratio of asphalt molecules, with benzobisbenzothiophene (C₁₈H₁₀S₂) representing the polar aromatic molecule and 1,7-dimethylnaphthalene (C₁₂H₁₂) chosen as the representative naphthene aromatic molecule, as shown in Figure 1b–c. N-docosane (n-C₂₂H₄₆) is constructed as shown in Figure 1d. The parameters listed in Table 1 for asphalt molecules are selected and used to meet the desired criteria, including the elemental mass fraction, the atom ratio, and the aromatic/aliphatic ratio, of real asphalt from experiments⁴¹. The same mass ratio has been defined in our previous studies, and the other thermomechanical properties like density, glass transition temperature, and viscosity are in good agreement with experimental data of real asphalt³⁶. The molecular structure of graphene applied in this study is shown in Figure 1e. The adopted graphene sheet in this study has no defect and no fold compared to that of the real case, while the real graphene sheet usually has several defects such as atomic vacancies and Stone-Wales defects⁴⁴, and some of the graphene sheets can be folded during the mixing process in the asphalt matrix⁴⁵. These imperfect situations are not considered in this study, since we focus on the effect of the site of the graphene sheet on the self-healing properties and choose it as the only variable. The variables of graphene sheets in terms of the defects and folded cases will be the focus of our future studies. The mass ratio of graphene to asphalt in this study is 4.75%, which is the normal situation (<5%) for graphene modified asphalt in the experiment^46,47.

Figure 1: Chemical structure. The atomistic models of (a) asphaltene molecule (C₅₃H₅₅NOS), (b) naphthene aromatic molecule (C₁₂H₁₂), (c) polar aromatic molecule (C₁₈H₁₀S₂), (d) saturate molecule (C₂₂H₄₆), (e) graphene, and (f) pure asphalt. For the atomistic asphalt model, the carbon, oxygen, nitrogen, sulfur, and hydrogen atoms are shown in gray, red, blue, yellow, and white, respectively. Please click here to view a larger version of this figure.

Table 1: Overall components of pure asphalt model and graphene-modified asphalt model.

With respect to the protocol described below, two types of wedge-like cracks with different sizes are inserted into the middle of the asphalt model with a blunt crack tip and two parallel crack surfaces, while the middle-top area of the asphalt bulk remains intact. Two crack widths are chosen as 15 Å and 35 Å, as shown in Figure 2a–b. The reason for selecting 15 Å is that the crack width should be wider than the cutoff of 12 Å to avoid the early self-healing of asphalt molecules during the equilibrium process while investigating an extreme case for a small crack. The reason for selecting 35 Å is that the crack width should be wider than the length of the saturate molecules of 34 Å in order to prevent the bridging effect. The height of the crack is 35 Å, the same as the box width, and the depth of the crack is 70 Å, the same as the box length. In the real situation, the observed micro-crack sizes can be varied in the range from several micrometers to several millimeters, which is far larger than the length scale we are modeling here. Normally, the length scale in MD simulation is limited to the scale of 100 nm, which is still several orders of magnitude smaller than the real crack size. However, the cracks initiate at the nanoscale and grow into macroscale cracks with continuous deformation⁴⁸. The understanding of the self-healing mechanism at the nanoscale can help to prevent the growth and further propagation of the crack at the macroscale. Even though the selected crack sizes are in the range of nanometer, the results can still be influential and applicable to explore the self-healing behaviors of asphalt molecules. There are two locations for the graphene sheets in the crack areas: one is on top of the crack tip and the other is perpendicular to the left crack surface. It has been found that these are the most common positions for graphene in graphene-modified nanocomposites with cracks⁴⁹.

Figure 2: The self-healing schemes for pure asphalt and graphene-modified asphalt. The self-healing model of pure asphalt with a crack width of (a) 15 Å and (b) 35 Å. The self-healing model of graphene-modified asphalt with the graphene sheet is located (c) at the top of the crack tip and (d) perpendicular to the crack surface. Please click here to view a larger version of this figure.

In MD simulations, the intramolecular and intermolecular interactions in the asphalt nanocomposites are described by the Consistent Valence Forcefield (CVFF)⁵⁰, which works well with asphalt and graphene-based materials. The functional form of CVFF is expressed as the following expression:

1

Here, the total energy E_total is composed of the bonded energy terms and the non-bonded energy terms. The bonded interactions consist of the covalent bond stretching, the bond angle bending energy, the torsion angle rotation, and the improper energies as expressed in the first four terms. The non-bonded energy includes an LJ-12-6 function for the van der Waals (vdW) term and a Coulombic function for the electrostatic interactions. CVFF has been widely employed in simulating asphalt materials^51,52. The simulated physical and mechanical properties such as density, viscosity, and bulk modulus are in good agreement with the experimental data, which demonstrates the reliability of CVFF⁵¹. CVFF is not only suitable for inorganic materials, but it has also been successfully employed in structures consisting of organic and inorganic phases such as asphalt-silica⁵² and the system of epoxy-graphene⁵³. In addition, the interfacial interactions between graphene and asphalt can be characterized by CVFF^36,54. Since the major part in selecting forcefield is to determine the asphalt-graphene interface, the non-bonded interactions described by CVFF are more reliable, which is also considered in our previous study³⁶. Overall, the forcefield CVFF is adopted in this study. The partial charges for different kinds of atoms are calculated by the forcefield-assigned method.