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The asphalt mastic-aggregate interface has a significant impact on the service performance of asphalt mixtures. In this article, we are going to discuss how to improve the overall performance of the asphalt by giving a test on the asphalt molecular adhesion.
asphalt mixtures
The adhesion behaviors and mechanisms between asphalt mastic and aggregate were investigated using molecular dynamic (MD) simulations in this study.
First, when developing and validating the asphalt mastic model, the actual mass ratio of filler to asphalt (F/A) condition was considered.
Second, the molecular arrangement characteristics of polar components on the aggregate substrate were examined using the radial distribution function (RDF), relative concentration (RC), and mean square displacement methods (MSD).
Third, the interfacial adhesion ability of asphalt and aggregate was quantitatively evaluated using work of adhesion.
The coupling effect of moisture and temperature on interfacial adhesion behaviors was investigated to investigate the adhesion failure characteristics of the asphalt-aggregate interface.
The results show that the thermodynamic properties can be used to validate the accuracy of the asphalt mastic model.
Because of silica particles' high polarity, the self-aggregation degree of polar components in base asphalt could be significantly increased with the addition of silica particles, resulting in a configuration change from "parallel arrangement" to "stack distribution."
Because of the adsorption effect of silica particles, the polar components in asphalt mastic have a more uniform distribution state and less mobility than base asphalt.
Silica particles with residual charges could significantly increase the electrostatic energy of the asphalt mastic-aggregate interface, improving adhesion between asphalt mastic and aggregate.
In contrast to the base asphalt-aggregate interface, temperature increases the work of adhesion at the asphalt mastic-aggregate interface.
When compared to base asphalt, asphalt mastic is more sensitive to interfacial moisture damage. The findings will help researchers better understand the micro adhesion mechanism at the asphalt mastic-aggregate interface.
Asphalt mixtures are extremely vulnerable to damage during long-term service due to environmental factors and vehicle loads.
Prior research has shown that the most fragile area in asphalt mixtures, the asphalt-aggregate interface, has a significant impact on the overall performance of asphalt mixtures.
Moisture, aging, and temperature, for example, would significantly weaken the bonding strength between the asphalt and the aggregate, reducing the serviceability of asphalt mixtures.
To reduce the occurrence of various distresses and extend the useful life of asphalt pavement, it is critical to fully understand adhesion behaviors and improve the adhesion capacity of the asphalt-aggregate interface.
Fundamental theories such as mechanical theory, chemical theory, weak boundary theory, and thermodynamic theory are currently being used to explain the adhesion mechanism between asphalt and aggregate.
Adhesion, according to mechanical theory, is the mechanical interlock between the aggregate surface and the asphalt binder.
asphalt powder
Aggregates with porous and slightly rough surfaces, according to general consensus, could improve mechanical interlocking and encourage better adhesion with the asphalt binder.
According to chemical theory, the chemical reaction between acid functional groups in asphalt and alkaline active components on the aggregate surface contributes a significant portion of the interfacial bonding ability.
The selective adsorption effect, according to the weak boundary theory, causes a region of the interface to have low adhesion strength, which is why the interface's adhesive failure occurs there.
According to thermodynamic theory, the process of energy exchange and eventual equilibrium between asphalt and aggregate is primarily responsible for the formation of interfacial bonding strength.
The adhesion properties of the asphalt-aggregate interface could be quantified using a variety of methods, most notably the boiling method, which has been widely used as a standard procedure.
The asphalt-aggregate interfacial adhesion behaviors have been quantified from a multi-scale perspective using AFM, surface free energy method, and dynamic shear rheometer (DSR).
It should be noted, however, that these theories and methods do not fully explain the molecular-scale micro adhesion behaviors of asphalt and aggregate.
As a result, a thorough investigation of the interfacial adhesion mechanism is urgently required in terms of the properties of interfacial nanostructures and molecular arrangement under various conditions.
Molecular dynamic (MD) simulation is a useful method for examining the mechanical, thermodynamic, and structural properties of materials. Significant efforts have been made to investigate the interfacial adhesion behaviors of asphalt and aggregates, particularly for asphalt materials.
The adhesion behaviors of the asphalt-aggregate interface, in general, are specifically dependent on the quantitative evaluation of the aggregation and distribution characteristics of asphalt components on the aggregate surface.
Luo et al. used mean square displacement (MSD) and the diffusion coefficient to investigate the effect of temperature on the diffusion of asphalt components on the surface of mineral aggregate.
The results showed that the interaction of asphalt constituents and Al2O3 aggregate could make asphalt constituent diffusion temperature independent. Huang et al. investigated the characteristics of asphalt component diffusion on mineral surfaces.
asphalt driveway
The findings suggested that polar fractions, such as asphaltene and resin, could cause physical adsorption of asphalt on the surface of the aggregate, which is the root cause of interfacial adhesion behaviors.
According to Guo et alanalysis .'s of the relative concentration (RC) of asphalt components on the aggregate surface, the majority of asphalt component distribution was parallel to the surface of minerals [28].
Dong et al. used the radial distribution function (RDF) and radial correlation coefficient (RC) to quantify the aggregation and distribution characteristics, as well as the nanostructure of the asphalt-aggregate interface.
Polar component aggregation, according to the findings, would result in increased adhesion capacity to withstand interfacial damage.
Xu et al. used stress-separation responses to quantitatively assess the bonding strength of the asphalt-aggregate interface, and they concluded that the large air voids created in the bulk asphalt would cause the adhesive to fail.
Numerous studies have also been conducted to investigate the variables that influence interfacial adhesion behaviors as determined by MD simulation.
Wang et al. studied the effects of temperature and moisture on the asphalt-quartz aggregate interface stress. According to the findings, as moisture content or temperature increased, interface stress decreased.
In wet and dry environments, aged asphalt exhibits different adhesion behaviors, and Xu et al. discovered that moisture significantly reduced the adhesion strength between aged asphalt and aggregate.
In their study of the impact of moisture and aging on the nanostructure and adhesive energy of the asphalt-aggregate interface, Sun et al. discovered that aging and moisture could bring asphaltene clusters closer together.
asphalt texture
The moisture separating the asphaltene and resin from the mineral surface would reduce the interfacial adhesive energy.
Gao et al. investigated the bonding and debonding behaviors of asphalt with four different types of minerals, including quartz, calcite, albite, and microcline, in relation to moisture.
The findings revealed that the chemistry of the mineral surface had a direct impact on the resistance of the asphalt-aggregate interface to moisture damage and that the non-bonding interaction energy of alkali minerals was stronger under moist conditions.
According to Liu et alresearch.'s into the interaction mechanism between asphalt and steel slag, steel slag would provide a stronger electrostatic interaction with asphalt because it contains a significant amount of Ca2+, which could improve interfacial adhesion ability.
Furthermore, when developing simulation models, careful consideration should be given to the addition and mixing of material components, as this will affect the adhesion properties of engineering materials.
As previously stated, numerous studies have been conducted to investigate the molecular-scale influences on the interfacial adhesion behaviors and bonding strength of the asphalt-aggregate interface. These findings support further research into these behaviors.
An asphalt mixture is a type of dispersion system with a multi-level spatial network structure that consists of three different dispersion systems: a coarse dispersion system (asphalt mixture), a subdivided dispersion system, and a differential dispersion system (asphalt mastic).
As the dispersed phase in an asphalt mixture, coarse aggregate is distributed throughout the asphalt mortar. Fine aggregate is dispersed as the dispersed phase in asphalt mortar, which is composed of asphalt binder and mineral filler.
Asphalt mastic is essential for bonding aggregates and filling voids in asphalt mixtures. This has a significant impact on the performance of asphalt mixtures as pavements.
Asphalt Adhesion
The physical and rheological properties of asphalt mastic are currently being studied using MD simulation technology. Zhu et al. investigated the thermodynamic and mechanical behavior of the asphalt mastic molecular model for the first time when silica molecules were added.
Li et al. developed an asphalt mastic model with a low silica particle content and discovered that the selective adsorption of silica particles and polar components in asphalt is a potential cause of the physicochemical interaction.
Furthermore, using MD simulations, the thermodynamic and mechanical properties of asphalt modified with nanoparticles such as nano-hydrated lime, nano-ZnO, graphite nanoplatelets, and carbon nanotubes were investigated.
Long et al. used MD simulation to investigate the interfacial adhesion properties of asphalt mastic modified with nano-silica.
According to the findings, nano-silica particles used as mineral filler would slightly increase the interfacial adhesion between nano-silica-modified asphalt and aggregate [48].
The studies would benefit both the development of molecular models of asphalt mastic and the understanding of its mechanical properties at the molecular level.
In conclusion, previous research using MD simulations improved our understanding of the behaviors and adhesion mechanism at the asphalt-aggregate interface.
However, developing molecular models of asphalt mastic in a real filler to asphalt ratio (F/A) condition and investigating the corresponding thermodynamic behaviors of asphalt mastic have received relatively little attention to date.
Furthermore, there aren't enough studies on the adhesion mechanisms and behaviors of the asphalt mastic-aggregate interface from the perspectives of interfacial nanostructure and molecular arrangement.
Based on the molecular arrangement characteristics of asphalt mastic on the aggregate surface.
the aggregation and distribution behaviors of asphalt mastic on the aggregate surface were investigated, as well as the interfacial adhesion mechanism between asphalt mastic and aggregate with the coupling effect of moisture and temperature.
To begin, the thermodynamic properties of asphalt mastic and base asphalt were compared in order to establish and validate the asphalt mastic model under actual F/A conditions.
An asphalt mastic-aggregate interface model was developed to study the aggregation and distribution behaviors of polar components on the aggregate substrate from the perspectives of interfacial nanostructure and molecular arrangement based on radial distribution function (RDF), relative concentration (RC), and mean square displacement (MSD).
asphalt testing
Finally, the coupled effects of temperature and moisture on interfacial adhesion behaviors were considered in a quantitative evaluation of the adhesive strength of the asphalt mastic-aggregate interface.
The study should provide some insight into how asphalt mastic and aggregate adhere to one another, in addition to providing a more logical and accurate method for assessing the interfacial damage resistance of the asphalt mixture.
The thermodynamic properties of two types of asphalt material models, namely density, glass-transition temperature (Tg).
and cohesive energy density (CED), were discussed to determine whether the molecular simulation method using the COMPASS force field parameters is appropriate for describing the properties of base asphalt and asphalt mastic.
The density of asphalt, a key thermodynamic characteristic, is frequently used in MD simulations to assess the accuracy of molecular models.
The densities of base asphalt and asphalt mastic tend to be 0.998 g/cm3 and 1.371 g/cm3, respectively, at 500 ps, which is the point at which the densities of base asphalt and asphalt mastic were calculated during the NPT ensemble for 500 ps at 1 atm, as shown in Figure 3.
The calculated base asphalt density, as shown in Table 2, is reasonably close to the results.
Further investigation revealed that the use of higher hydrogen, more carbon, but less sulfur, and a lack of heteroatoms in the asphalt binder models would be the primary causes of the simulated results being lower than the true density [64].
Furthermore, as the optimization time increases, the cell length of the models tends to stabilize. The cell lengths of base asphalt and asphalt mastic are 44.8 to 37.8 and 58.4 to 44.4, respectively.
It goes without saying that the addition of filler particles would significantly increase the density and model size of base asphalt.
MD simulations were used in this study to thoroughly examine the adhesion behaviors of the asphalt mastic-aggregate interface from the standpoint of molecular arrangement characteristics and interfacial nanostructures.
The following are the key findings based on the simulated results:
The asphalt mastic model under the real F/A condition is created and validated by comparing the thermodynamic properties of asphalt mastic and base asphalt.
The presence of silica particles improves the self-aggregation behaviors of polar components significantly.
Because of the high polarity of silica particles, the molecular arrangement of polar components changes from "parallel arrangement" to "stack distribution," lowering the van der Waals energy.
Because of their ability to adsorb polar components, silica particles have the potential to significantly alter the distribution of polar components in base asphalt.
Polar components adsorb around evenly dispersed silica particles in asphalt mastic, resulting in a more uniform distribution state and lower mobility than base asphalt.
The addition of silica particles with residual charges significantly increases the electrostatic energy of the asphalt mastic-aggregate interface, which strengthens the adhesion between asphalt mastic and aggregate.
According to the moisture-temperature coupling effect, moisture could significantly worsen the adhesion between asphalt and aggregate, particularly at the asphalt mastic-aggregate interface.
In contrast to the base asphalt-aggregate interface, temperature increases the work of adhesion at the asphalt mastic-aggregate interface.
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