Asphalt-aggregate Interface Research Articles

A molecular insight into asphalt-aggregate interfacial debonding: The role of hydrogen bonds in water molecule migration

In this study, molecular dynamics were employed to compare the angles, lengths, and numbers of hydrogen bonds between water molecules and different aggregates. Compared to quartz, calcite has shorter bond lengths, larger bond angles, and a greater number of bonds with water molecules. The differences in hydrogen bonds lead to the formation of a three-layer structure on the calcite surface by water molecules: ordered, relatively ordered, and disordered, while a two-layer structure—relatively ordered and disordered—is formed on the quartz surface. In the ordered layer, water molecules experience the strongest hydrogen bonding, are neatly arranged, and have low diffusion coefficients. In contrast, the disordered layer is characterized by weaker hydrogen bonding, random arrangement, and higher diffusion coefficients. The movement path of water molecules shows that the debonding of asphalt and aggregate is jointly influenced by the migration behavior of both the underlying and upper-layer water molecules. These findings reveal the microscopic mechanism of water molecules forming a wedge shape at the asphalt-aggregate interface from the perspective of hydrogen bonds, explaining why the constraint of water on the Calcite {214} and Calcite {018} surfaces is stronger compared to Calcite {1 0 4}, but the bonding energy with asphalt decays more rapidly.

Effect of salt erosion on interfacial adhesion of waste tire crumb rubber-modified asphalt mixtures: A molecular dynamics study

In this study, a wetting and migration model was established using the molecular dynamics simulation method. Through in-depth analysis of parameters such as contact angle, mean square displacement, and work of adhesion, the dynamic wetting and migration characteristics of salt solution were explored, and the damage mechanism of waste tire crumb rubber-modified asphalt mixtures under salt erosion was revealed. The results showed that compared with an aqueous solution, the salt solution was more likely to be adsorbed and dispersed on the asphalt surface. This exacerbated its damage. However, the presence of salt ions reduced the wetting effect of the solution on the aggregate surface and reduced the migration rate of the solution at the asphalt-aggregate interface. Significant diffusion of Na+ and Cl- ions occurs during salt solution migration, while SO42- ions mainly accumulate near the aggregate surface. In addition, salt solution migration alters the distribution of asphalt components. This further affects the interface adhesion effect. The inclusion of crumb rubber can effectively reduce the sensitivity of asphalt to salt solutions and enhance its hydrophobicity. Simultaneously, it enhances the adhesion strength and stability of the asphalt. This effectively obstructs the intrusion of water molecules and salt ions.

Evaluation on the diffusion-fusion properties and interfacial mechanical behavior of virgin and aged asphalt

To investigate the regeneration mechanisms of rejuvenated asphalt mixtures, molecular simulation techniques were employed to research virgin and aged asphalt's fusion characteristics and interface mechanical properties. The blended zone was identified based on relative density distribution, and a molecular-scale calculation method for the Degree of binder Blended (DOB) between virgin and aged asphalt was proposed. Finally, the influence of DOB on the mechanical properties of the asphalt-aggregate interface was evaluated through interface pull-out tests. The results revealed that the proposed molecular-scale DOB calculation method effectively assessed the blending effectiveness between virgin and aged asphalt. Higher degrees of asphalt aging made it more challenging for aged asphalt to fuse with virgin asphalt, while elevated temperatures facilitated diffusion fusion between them. Under tensile loading, initial cracks were primarily initiated from the rejuvenated asphalt's blended zone. This region represented the weakest area within the interface system and was prone to fracture under load. The increase in DOB significantly improved the interfacial properties of recycled asphalt, with the increase in DOB the interfacial tensile strength increased by 4%–14% and the fracture energy increased by 10%–19%. However, this enhancement effect diminished with an increased degree of asphalt aging. At the molecular scale, increasing DOB improved the interface strength and deformation resistance of rejuvenated asphalt. The molecular simulation results provide theoretical guidance for evaluating the fusion degree of aged asphalt in RAP materials and determining the utilization efficiency of aged asphalt.

Insight into the salt damage mechanism to the asphalt-aggregate interface in recycled asphalt mixtures at different salt environments using molecular dynamics and DFT simulations

In coastal areas, the complex salt environment poses new challenges to the interfacial stability and service durability of hot recycled asphalt pavements. To investigate the influence mechanism of salt environment on the adhesion properties of asphalt-aggregate interface, this study employed molecular dynamics (MD) simulation and density functional theory (DFT) to analyze the interfacial interaction energy, molecular diffusion behavior, component distribution, and molecular electrostatic property at asphalt-aggregate interface. The results show that as the content of aged asphalt increased, the interaction between asphalt and anions (Cl⁻ and SO₄²⁻) is weaker than that with cations (Na⁺, Mg²⁺ and Ca²⁺). H₂O causes the aromatics, saturates and resins to move away from the interface. Cl⁻ causes the saturates to further move away from the interface, while Na⁺ and Mg²⁺ bring the aromatics closer to the interface. Due to the greater polarity of SO₄²⁻, it moves the resins closer to the interface. With increasing aged asphalt content, the number of hydrogen bonds between asphalt and SiO₂ increases from none to some, thus enhancing the interfacial adhesion. However, this adhesion becomes more susceptible to salts. The aging effect redistributes the charge of asphalt molecules, concentrating negative potentials on the carbonyl and sulfoxide functional groups located on the outside of molecules. This increases the polarity of the asphalt molecules, making them more susceptible to salt ion attack. These findings contribute to the understanding of the adhesion properties of recycled asphalt mixtures in extreme salt environments.

Quantifying the influence of lipophilic carbon chain length on adhesion between emulsified asphalt and acidic aggregates

ABSTRACT Emulsified asphalt technology conserves resources and reduces costs while aligning with the principles of low-carbon and high-quality development. Acidic aggregates demonstrate excellent mechanical properties and are suitable alternatives to alkaline aggregates. This study develops molecular dynamics models and macroscopic experiments to quantify the influence of carbon chain length on the adhesion between emulsified asphalt and acidic aggregate interfaces from a molecular perspective. The results revealed that aromatics and saturates exhibit high sensitivity to variations in carbon chain length. Their adsorption on the SiO2 interface is directly proportional to the carbon chain length. The diffusion coefficient and adhesion parallelly show an initial increase followed by a decrease with the same factors. The van der Waals forces contributed to more than 67% of the adhesion. Water molecules establish hydrogen bonds with the hydroxyl groups on the SiO2 interface, thereby decreasing the adhesion between the emulsified asphalt and the SiO2 interface. The strong hydrogen bonds between free water and asphalt hinder the accumulation of asphalt at the SiO2 interface. In summary, regulating the emulsifier through the carbon chain length and chloride ions enhances the adhesion between the emulsified asphalt and acidic aggregate interface.

Microanalysis of behavior characteristics for asphalt binders on different mineral surfaces based on MD and SEM

To investigate the influence of different minerals on the behavior characteristics of asphalt binders at micro scale, molecular dynamics (MD) and scanning electron microscopy (SEM) were combined to study the interaction of asphalt binders and different mineral surfaces. Firstly, commonly used aggregates were analyzed by X-ray diffraction (XRD), and the main mineral categories were obtained, including quartz, albite, biotite and calcite. Secondly, the asphalt-mineral micro models were established by combining the reasonable asphalt micro model and mineral micro model, and the interface behavior characteristics were analyzed by MD simulation. Finally, the interface structure of asphalt and minerals was observed through SEM testing, which intuitively revealed the behavior characteristics of asphalt on different mineral surfaces. The results show that the relative concentration distribution of asphalt binders at the asphalt-mineral interface is relatively high, and it can be affected by mineral category, surface structure state and asphalt type. The transition zone of biotite, albite and asphalt interface is relatively wide, which means that the influence of biotite, albite and asphalt diffusion characteristics is relatively obvious, and the interface interaction ability is relatively strong. The study reveals the behavior characteristics of asphalt binders on different mineral surfaces, and provides a research basis for exploring the structural stability of asphalt-aggregate interface.

Investigation of damage and crack characteristics of epoxy modified recycled asphalt mixtures based on a three-dimensional meso-heterogeneous model

Epoxy asphalt has excellent fatigue resistance, making it a valuable addition to recycled asphalt mixtures. However, the lack of sufficient understanding of the cracking mechanism in epoxy-modified recycled asphalt mixtures (EMRAM) hinders its widespread application. The purpose of this paper was to investigate the damage and crack characteristics of EMRAM based on a three-dimensional meso-heterogeneous model. The coarse aggregates were first randomly generated using the secondary Delaunay algorithm. Meanwhile, the fracture parameters of each component material and asphalt-aggregate interface in EMRAM were tested through three-point bending tests and direct shear tests, respectively. Cohesive zone model elements were then inserted at the aggregates, asphalt mastic, and interfaces between different materials to establish a three-dimensional meso-heterogeneous model of EMRAM. Finally, based on the established model, the damage degree of each composition material in EMRAM was quantitatively analyzed to identify the vulnerable aspects of EMRAM during the crack propagation stage. Optimization strategies were proposed accordingly. The results demonstrated that the model established in this study can accurately represent the meso-heterogeneous of EMRAM and effectively simulate its cracking behavior. EMRAM was more susceptible to developing internal damage within the aggregates, followed by relatively noticeable damage at the interface and the internal regions epoxy-modified fine aggregate mastic. The cracking resistance of EMRAM showed a trend of initially increasing and then decreasing with an increase in the volume fraction of coarse aggregates with a size above 4.75 mm. The optimal range for the volume fraction of coarse aggregates in EMRAM was within 35–44 %. The fracture energy of EMRAM was improved by more than 30 % after gradation optimization compared to EMRAM with gradation at the median value of AC20. Within the designed service life, EMRAM durable pavement can save at least 30 % more in economic costs compared to ordinary asphalt concrete pavement.

Influence of acidity and alkalinity of water environment on water stability of asphalt mixture: Phase II-microscopic erosion mechanism

Acidity and alkalinity of water environments have been proven to influence the water stability of asphalt mixture. However, the relevant influencing mechanism still remains unclear. To fulfill this research gap, this study conducted a series of microscopic tests, aiming at unveiling the microscopic erosion mechanism of the water environment on the water stability of asphalt mixture for better moisture stability achieved. Specifically, the asphalt-aggregate interfacial transition zone (ITZ) samples were prepared by combining #70 asphalt with limestone and granite, respectively, which were then immersed in water solutions at 3 different pH levels (pH = 3.0, 7.0, and 11.0) for 7 days. The chemical composition and microstructure of the asphalt film on the surface of the ITZ samples pre and post-treatment were scrutinized using the Fourier Transform Infrared Spectroscopy (FTIR) and the Atomic Force Microscope (AFM), while the development of microcracks at the asphalt-aggregate interface were identified and traced utilizing the Scanning Electron Microscope (SEM). The test results indicate that after treatment with acidic and alkaline aqueous solutions, the asphalt film's surface is enriched with polar components such as asphaltenes, resins, and aromatic fractions. The migration of these polar components to the surface of the asphalt film reduces the adhesion between asphalt-aggregate, leading to the formation of microcracks at the asphalt-aggregate interface. The most severe crack development occurs in alkaline water environment, followed by acidic water environment. It is interestingly noted that the adhesion at the asphalt-granite interface is improved under the acidic environment. This improvement may be attributed to the covalent nature of silica, which hinders the adsorption of ions from acidic solution into the asphalt film. This hindrance effectively prevents water intrusion at the asphalt-aggregate interface, thereby reducing the adhesion loss. These findings can help elucidate the mechanism of water damage in asphalt pavement exposed to real-world conditions and enhance its water stability with effective countermeasures proposed.

Effects and molecular mechanisms of jet ablation and fuel corrosion on separation failure of the asphalt-aggregate interface in airport asphalt pavements

To guide the improvement of asphalt pavement durability in airports, the effects and molecular mechanisms of jet ablation and fuel corrosion on separation failure of the asphalt-aggregate interface were investigated in this study using molecular dynamics simulations and density functional theory method. Firstly, tensile separation behaviors of the virgin, aged, and fuel-corroded asphalt-aggregate interfaces were simulated under different loading rates to evaluate their tensile strength and fatigue cracking resistance, and the experimental data were also used to verify the reliability of the simulation results. Subsequently, the molecular mechanisms of jet ablation and fuel corrosion on the separation failure of the asphalt-aggregate interface were analyzed by calculating the work of adhesion, work of cohesion, flowability parameters, and intermolecular binding energy. The results show that the aged specimen has higher ultimate tensile strength and fracture energy, but its flexibility and fatigue cracking resistance are significantly reduced. This is because the polar oxygen-containing groups generated during high-temperature aging enhance the intermolecular binding energy of the asphalt, but this also restricts the diffusion of asphalt molecules, resulting in greater frictional resistance and viscosity of asphalt during shear deformation, and thus loss of mobility and self-healing properties. On the contrary, since the light and medium oils soften and dilute the asphalt, and also weaken the intermolecular binding energy of asphalt, the fuel corrosion reduces the ultimate tensile strength and fracture energy of the asphalt-aggregate interface system, resulting in insufficient overall strength and load-bearing capacity.

A multiscale study of the influence of aggregate type on adhesion at the asphalt–aggregate interface

In order to examine the influence of aggregate type on the asphalt–aggregate interface, the adhesion performance between asphalt and aggregate was investigated at three scales. The pull-out test was conducted on asphalt and three kinds of aggregates at the macroscale to directly estimate the adhesion performance between them. At the microscale, the surface energies between asphalt and limestone, basalt, and andesite were examined through a surface energy experiment, and then the adhesion and debonding work were assessed. At the nanoscale, the interfacial behavior between six mineral compositions (SiO2, CaO, MgO, Al2O3, Fe2O3, and Na2O) of these aggregates and asphalt was determined using molecular dynamics simulation. The correlation of adhesion work at the microscale and nanoscale was further explored to unveil the bonding mechanisms between asphalt and aggregate. The macroscale results indicate that the adhesion force of andesite is slightly greater than that of limestone, probably because andesite has a greater surface roughness than limestone. The microscale results show the best adhesion and water stability between asphalt and limestone. The finding further explains that alkaline aggregates exhibit superior interfacial adhesion to asphalt. The nanoscale results show that the adhesion work of alkaline oxides with asphalt is more significant than that of acidic oxides with asphalt. Limestone and basalt have better adhesion to asphalt due to higher alkaline oxide content, while andesite has poorer adhesion to asphalt due to lower alkaline oxide and higher SiO2 content. The interfacial adhesion work increases from the microscale to the nanoscale by a factor of 2.91–4.75.

Effect of chloride salt migration at interfaces on asphalt-aggregate adhesion: A molecular dynamics study

Asphalt pavements can be damaged by moisture and salt, and the bond strength between asphalt and aggregate is related to the erosion of the salt solution. In this study, considering the migration and diffusion of the solution at the interface, a molecular dynamics modeling method was used to simulate the process where the salt solution erodes the asphalt-aggregate interface, and results show that the solution invades the asphalt-aggregate interface, gradually squeezing the asphalt away from the aggregate surface in a wedge shape, and the interaction force between the asphalt and the aggregate is gradually reduced to 0. The diffusion rate of the solution at the interface is related to the position of the solution. If the solution is farther away from the aggregate, it will be less restricted by the aggregate. NaCl affects the diffusion of water molecules. The greater the concentration of NaCl solution, the lower the height of the wedge formed by the solution at the interface. Compared to quartz, calcite has a stronger limiting effect on the solution, which is manifested by a greater resistance to water damage. Under the attraction of NaCl solution, the polar components are more likely to move toward the solution surface, while the less polar components show different distribution and diffusion states. In this study, we observed the conformational changes in the erosion process of different concentrations of NaCl solutions and calculated the change rule of interaction force, which helps understand the erosion process of salt solutions and provides a new idea for the study of chloride on asphalt pavement spalling.

Investigation of interfacial degradation mechanism and debonding behavior for degraded rubber asphalt-aggregate molecular system using a molecular dynamics method

This study aims to investigate the debonding damage behaviors in rubber asphalt-aggregate interfaces under different rubber degradation degrees from a molecular-scale perspective. The pull-off test was simulated using molecular dynamics (MD) to determine the damage patterns at the asphalt-aggregate interface. The digital image processing (DIP) technique was employed to quantify the percentages of cohesive debonding, and the radial distribution function (RDF) and mean square displacement (MSD) was performed to explain the adhesion and cohesion damage mechanisms. In addition, the pull-out test results were compared with the simulated values to verify the reliability of the asphalt-aggregate molecular model. The findings of this study reveal that the presence of rubber powder molecules enhances the cohesion among asphalt molecules and mitigates damage to the cohesion at the asphalt-aggregate molecular interface. As rubber degradation progresses, the damage mode undergoes a gradual transition from adhesive damage to cohesive damage at the asphalt-aggregate interface. At lower temperatures, there is an increase in the internal aggregation of asphalt molecules, and the debonding rate of asphalt molecules at the mineral-aggregate interface rises from 7.67% to 35.67% following complete rubber degradation. Conversely, at higher temperatures, the asphalt-aggregate molecular system is primarily governed by the bonding of “double bond” and “single bond” groups, with cohesive damage dominating the asphalt matrix. The pull-out test results showed similar trends with the simulated values, indicating that the asphalt-aggregate molecular model in this study was relatively reasonable. The present study provides some guidance on the damage behavior of degraded rubberized asphalt-aggregate interface.

Interfacial adhesion properties and debonding mechanisms in rejuvenated asphalt mixtures

The interfacial strength between asphalt and aggregate significantly affects the macroscopic properties of rejuvenated asphalt mixtures. Investigating the interfacial properties of rejuvenated asphalt mixtures contributes to understanding the adhesion mechanisms and debonding behavior, enhancing the service life of rejuvenated asphalt mixtures. In this study, molecular dynamics methods were utilized to construct interface models of virgin, aged, and rejuvenated asphalt-aggregate systems, subsequent to which asphalt-aggregate interface pull-out tests were conducted. The findings revealed that the exponential-type cohesive zone model (CZM) effectively characterize the stress-strain correlations during asphalt-aggregate interface pull-out tests. The combination of adhesive and cohesive failure is the typical damage modes at asphalt-aggregate interface. Cohesive failure tended to be the main failure mode between aged asphalt-aggregate interface, while asphalt oxidation aging notably reduced the interface tensile strength and hence the cracking resistance of asphalt mixtures. The interface failure patterns between rejuvenated and virgin asphalt were similar, demonstrating an adhesive failure at lower temperatures and a shift towards cohesion failure at higher temperatures. The interface failure usually occurred at lower temperatures especially for aged asphalt, while the incorporation of rejuvenator could effectively mitigate the cracking potential asphalt mixtures. This study simulated the interfacial failure behavior for rejuvenated asphalt mixtures, which can be a theoretical basis of using rejuvenator in recycled asphalt mixtures.

Analysis of asphalt microscopic and force curves under water-temperature coupling with AFM

Asphalt, as a crucial material in road construction, is affected by external environmental factors, among which water and temperature are the primary causes of water damage to asphalt pavements. Different hydro-thermal environments can cause varying types of damage to asphalt or the asphalt-aggregate interface. To analyze the micro-performance changes of different types of asphalt under various hydro-thermal environments, this study conducts tests onPG64–22 matrix asphalt, PG76–22 SBS-modified asphalt and PG82–34 high viscosity asphalt under different hydro-thermal coupling and freeze-thaw cycle conditions using Atomic Force Microscopy (AFM). The analysis is based on the obtained micro-morphology images and mechanical properties, exploring the impact mechanisms of different hydro-thermal environments on various asphalts. Results show that under different hydro-thermal conditions, the three asphalts exhibit different morphological changes: the asphalt surface's “bee-shaped structure” is gradually destroyed after hydro-thermal coupling, while a “disappearance-reappearance” phenomenon is observed under freeze-thaw cycles. Moreover, the roughness of the two modified asphalts shows a different trend from that of the matrix asphalt, indicating that the modifier alters the micro-structure of asphalt to some extent. Under hydro-thermal coupling, the adhesion force of the three asphalts decreases linearly, with matrix asphalt decreasing by 89.2% after 3 hours, SBS modified asphalt by 63.1%, and high-viscosity asphalt by 23.9%, showing that high-viscosity asphalt has better waterproof and heat-resistant performance. During freeze-thaw cycles, the adhesion force between the asphalt and the tip shows varying degrees of recovery. Additionally, there are differences in adhesion force across different areas of the asphalt surface, which tend to diminish with hydro-thermal coupling or freeze-thaw cycles. These results indicate that hydro-thermal coupling and freeze-thaw cycles have different mechanisms of damage to the internal structure of asphalt, providing important data for understanding the damage mechanisms in depth.

Microcosmic mechanism of asphalt-aggregate interface adhesion failure under freeze-thaw cycles based on molecular dynamics

ABSTRACT In order to investigate the micro-mechanisms of adhesive failure at the asphalt-aggregate interface under freeze-thaw cycling, this study presents a novel model for freeze-thaw cycling of asphalt mixtures constructed using molecular dynamics. The model is employed to explore the micro-mechanisms of adhesive failure between asphalt and aggregate surfaces under freeze-thaw cycling conditions. Utilising the four-component model of asphalt, asphalt molecules were constructed, with silicon dioxide and calcium carbonate chosen to represent acidic and alkaline aggregates, respectively. Subsequently, an asphalt-aggregate-water freeze-thaw cycling model was developed. Building upon this foundation, the variations in interface interactions between asphalt and the two types of minerals were analysed under the same freeze-thaw cycling conditions but differing freeze-thaw cycle counts. This analysis was conducted through the examination of Radial Distribution Function (RDF), water molecule coordination numbers, and hydrogen bond quantities at the interfaces between asphalt and the two mineral types. The research findings indicate that acidic aggregates exhibit a greater affinity for water molecules compared to alkaline aggregates. Hydrogen bond interactions exist between aggregates and water molecules, with the hydrogen bond energy being greater than the interactions between asphalt molecules and aggregate surfaces. The cohesive energy decreases with an increasing number of freeze-thaw cycles, leading to a gradual reduction in asphalt viscosity and facilitating the detachment of asphalt from the aggregate surface. Consequently, acidic aggregates tend to absorb water molecules more readily, and water can lower the viscosity of asphalt molecules, ultimately resulting in the macroscopic delamination of asphalt from the aggregate surface. These research outcomes provide valuable theoretical guidance for a deeper exploration of the adhesive failure mechanisms in asphalt-aggregate interactions.

Influence of chloride salt erosion on the adhesion of asphalt-aggregate interfaces considering mineral anisotropy: insights from molecular dynamics.

Asphalt, a polymer mixture composed of various hydrocarbons, is extensively utilized due to its excellent performance. To evaluate the sensitivity of asphalt concrete to chloride salt damage at the nanoscale, considering the anisotropy of aggregate mineral crystal orientation, molecular dynamic simulations were employed to model the interface interactions between asphalt and aggregate mineral components (silica and calcite) under chloride salt exposure. The wetting processes of water droplets and chloride salt droplets on different crystal surfaces of silica and calcite were analyzed. Furthermore, the adhesive energy, decohesion energy, degradation ratio, and energy ratio of the interaction model between asphalt and aggregate mineral interfaces were analyzed to reveal the variations in the interfaces between the two components. The results demonstrated that the anisotropy of the minerals significantly affects the adhesion energy of asphalt and aggregate. Moreover, the surface hydrophilicity of calcite was larger than that of silica. The interfacial adhesion energy of asphalt-calcite was larger than that of asphalt-silica under dry condition and chloride salt attack, and the interfacial adhesion energy of both asphalt and mineral decreased with the increase of chloride salt concentration. The degradation ratio and energy ratio values of asphalt and calcite were larger than those of asphalt and silica, and the anisotropy of the mineral surface affected the degradation ratio and energy ratio values. This study provides a new method and theoretical basis for further research on the damage mechanism and strengthening measures of asphalt-aggregate under chloride salt attack. To investigate the interaction between asphalt and mineral aggregates under chloride salt erosion, models of asphalt, aggregate, and asphalt/aggregate composite systems were constructed using the Amorphous Cell module in Materials Studio 2020 software. Molecular dynamic simulations of the asphalt/aggregate composite system were then conducted using the Forcite module, employing the COMPASS II force field to describe atomic and molecular interactions. Initially, considering the crystal orientation and surface configuration of the aggregate minerals, the impact of mineral anisotropy on the asphalt-aggregate interface model was analyzed. Subsequently, by simulating the contact states of nano water droplets and chloride salt droplets with different mineral surfaces, the wetting characteristics of nano water droplets on silica and calcite surfaces were determined. Lastly, considering the corrosion effect of chloride solution at different concentrations, the adhesion energy, debonding energy, degradation ratio, and energy ratio of the asphalt-aggregate interface interaction model were analyzed.

Investigation on the Diffusion Behavior of Dry Modified SBS at the Asphalt-Aggregate Interface: Molecular Simulation and Experiments

Investigation on the Diffusion Behavior of Dry Modified SBS at the Asphalt-Aggregate Interface: Molecular Simulation and Experiments

Evaluation of styrene butadiene rubber asphalt modification mechanism and adhesion effect based on molecular simulation

Recycled rubber materials represent an environmentally sustainable option as asphalt modifiers. This research delves into the modification effects of Styrene Butadiene Rubber (SBR) asphalt at varying SBR content levels, employing a blend of molecular simulation techniques and laboratory experimentation. A comprehensive molecular model of SBR-modified asphalt is established, and molecular dynamics simulations are executed to scrutinize the thermodynamic attributes, dipole moments, mechanical characteristics, and adhesion properties inherent to SBR asphalt. The outcomes of this inquiry reveal that SBR exhibits notable compatibility with a majority of asphalt molecules, as evidenced by their dipole moments. The introduction of SBR into the asphalt matrix results in the adsorption of a substantial quantity of lightweight components, fostering the formation of a cross-linked network that bolsters the asphalt's resilience against external deformation forces. Furthermore, in terms of adhesion work, asphalt demonstrates heightened affinity with alkaline aggregates, and SBR serves to augment van der Waals interactions at the asphalt-aggregate interface, thereby amplifying interfacial bonding strength. However, when the content of SBR reaches 30%, some SBR molecules will be aggregated in the asphalt, which will lead to phase separation and affect the stability of the colloid. Therefore, it is recommended that the content of SBR should be limited to about 20% in practical engineering applications. In this work, the modification mechanism of SBR was analyzed at the molecular scale and verified experimentally, and the results provide a theoretical basis for the rational use of recycled SBR powder in asphalt applications.

Research on the crack resistance of semi-flexible pavement mixture based on meso-heterogeneous model

Semi-Flexible Pavement (SFP) material, prepared by pouring cement-based grouting material into the voids of a porous asphalt mixture, has good load bearing capacity and excellent resistance to rutting. It is gaining increased applications in areas where permanent deformation of asphalt mixtures is a concern. When applied as the pavement surface course, the major distress observed in SFP is cracking. Currently, there is a lack of comprehensive understanding of the cracking mechanism of SFP. To improve such understanding, this study investigated the effects of fracture parameters of asphalt mortar and asphalt-aggregate interface on the crack resistance of SFP materials through laboratory experiments and numerical simulation using the finite element method. Firstly, a three-point bending test was performed on a beam SFP specimen. A multi-scale mesoscopic heterogeneity model was then established based on the mesoscopic structure of the specimen and calibrated and validated using the load-displacement curves and the cracking process features. With the validated model, the effects of different fracture parameters on the beam cracking process were investigated. It was found that the load-bearing capacity of SFP increased with the increase of the tensile strength of asphalt mortar and asphalt-aggregate interface in a certain range, while the crack propagation rate after crack initiation also increased. An increase in the fracture energy of asphalt mortar and asphalt-aggregate interface delayed the crack propagation rate, but with an attenuative effect with the increase of fracture energy. Based on the actual test conditions, the desirable range of key cracking parameters of SFP materials were recommended.

Influence of acidity and alkalinity of water environments on the water stability of asphalt mixture: Phase I - molecular dynamics simulation

The pH of water environments surrounding asphalt pavements is not constant at 7.0. Previous studies have demonstrated that the erosion of asphalt mixtures is more severe in acidic or alkaline water environments. However, the underlying mechanism remains unclear. To fulfill this research gap, the molecular dynamics (MD) simulation was employed in this study to explore the erosion influence of three different aqueous solutions on asphalt and asphalt-aggregate interface. Asphalt-solution systems and asphalt-aggregate systems under the different water environments were established. Through investigating the nanostructure change of the asphalt and asphalt-aggregate interface, the relative concentration (RC) and mean square displacement (MSD) of asphalt molecules, and the adhesion work between asphalt and aggregate were analyzed. The results show that the nanostructure of asphalt changes under acidic and alkaline water conditions, which further results in the decrease of the asphalt cohesion. In addition, the study revealed that the acidic and alkaline water solutions promote the molecular migration of SARA (saturates, aromatics, resins, and asphaltene) components, leading to the molecule reorganization at the asphalt-aggregate interface, finally affecting the bonding performance of asphalt-aggregate and the water stability of asphalt mixtures. These findings are anticipated to provide new insights into the influence of acidic and alkaline water environments on the water stability of asphalt mixture from the molecular scale perspective.