Analysis of Adhesive Characteristics of Asphalt Based on Atomic

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Analysis of Adhesive Characteristics of Asphalt Based on Atomic Force Microscopy and Molecular Dynamics Simulation Meng Xu,† Junyan Yi,*,†,‡ Decheng Feng,† Yudong Huang,‡ and Dongsheng Wang† †

School of Transportation Science and Engineering and ‡School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150090, China ABSTRACT: Asphalt binder is a very important building material in infrastructure construction; it is commonly mixed with mineral aggregate and used to produce asphalt concrete. Owing to the large differences in physical and chemical properties between asphalt and aggregate, adhesive bonds play an important role in determining the performance of asphalt concrete. Although many types of adhesive bonding mechanisms have been proposed to explain the interaction forces between asphalt binder and mineral aggregate, few have been confirmed and characterized. In comparison with chemical interactions, physical adsorption has been considered to play a more important role in adhesive bonding between asphalt and mineral aggregate. In this study, the silicon tip of an atomic force microscope was used to represent silicate minerals in aggregate, and a nanoscale analysis of the characteristics of adhesive bonding between asphalt binder and the silicon tip was conducted via an atomic force microscopy (AFM) test and molecular dynamics (MD) simulations. The results of the measurements and simulations could help in better understanding of the bonding and debonding procedures in asphalt− aggregate mixtures during hot mixing and under traffic loading. MD simulations on a single molecule of a component of asphalt and monocrystalline silicon demonstrate that molecules with a higher atomic density and planar structure, such as three types of asphaltene molecules, can provide greater adhesive strength. However, regarding the real components of asphalt binder, both the MD simulations and AFM test indicate that the colloidal structural behavior of asphalt also has a large influence on the adhesion behavior between asphalt and silicon. A schematic model of the interaction between asphalt and silicon is presented, which can explain the effect of aging on the adhesion behavior of asphalt. KEYWORDS: asphalt, adhesive characteristics, atomic force microscopy, molecular dynamics simulations, colloidal structure

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INTRODUCTION Asphalt binder, which is also called bitumen, is the primary binding material used in road pavement. It has been employed for several hundreds of years, and more than 1 billion tonnes of asphalt are consumed every year worldwide. Despite this, we are still unable to accurately predict the performance of asphalt from some of its basic material properties. More fundamental studies from the point of view of materials science are needed to better characterize and predict the service performance of asphalt materials. One of the most popular uses of asphalt is mixing it with mineral aggregate to produce asphalt concrete. In asphalt concrete, asphalt binder plays the role of binding the aggregate together and its mechanical properties, in particular its adhesive characteristics, contribute significantly to the overall performance of pavement.1 In fact, both chemical adsorption and physical adsorption occur between asphalt and aggregate, which together form an adhesive bond. The chemical adsorption usually refers to a reaction between acid and alkali. Asphalt is considered to be acidic, so the aggregate should be alkaline to make its adhesion to asphalt stronger. However, this kind of chemical interaction might not be important because the acidity © XXXX American Chemical Society

of asphalt is low. Furthermore, not all aggregates satisfy requirements for alkalinity against the background of increasingly scarce natural resources. Therefore, physical adsorption plays a more important role in the adhesion between asphalt and aggregate. However, although many types of adhesive bonding mechanisms have been proposed to explain the interaction forces between asphalt binder and mineral aggregate, many of these relied on assumptions.2 There has also been a lack of clarity regarding both the bonding and debonding procedures when asphalt is mixed with mineral aggregate and how the chemical composition and microstructure of asphalt affect its adhesion. For example, some results of interfacial tests suggested that long-term aging could help increase the adhesive strength between thin-film asphalt and aggregate in an adhesion test,3,4 which is inconsistent with common experience. This may partially have been due to the improper preparation and testing method used in the tests, which was linked to reductions in adhesion and cohesion. Received: February 5, 2016 Accepted: April 26, 2016

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DOI: 10.1021/acsami.6b01598 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

In the MD simulation, the key task was to establish a molecular model of asphalt, including the selection of typical molecules and force fields, calculations of proportions and parameters, and so forth. In this respect, Greenfield et al. proposed a molecular model of asphalt,29−31 which has been sufficiently tested and verified. The molecular model used in this study mainly refers to work by Li and Greenfield30 and was modified with respect to the calculation of proportions and force fields to make it suitable for our study. Therefore, the objective of this study was to characterize the adhesive bonding characteristics of the asphalt−aggregate system using AFM testing on the nanoscale and MD simulations on the micro/nanoscale and to identify the key factors that could affect the adhesive bonding characteristics. Specifically, this paper aims to (1) investigate the interactions between typical single molecules of asphalt and silicon; (2) simulate the bonding and debonding procedures for a molecular model of asphalt and silicon, which is a similar process to an AFM force curve test; and (3) conduct a force curve test by AFM to measure the adhesive force that exists between asphalt and silicon.

However, it cannot be ignored that many issues still need to be clarified regarding the adhesion mechanisms. It is important to have a better understanding of the chemistry of binders, because the chemical composition of asphalt often determines its physical and mechanical properties.5 However, asphalt binder is a complex mixture that contains millions of chemical components.6−9 Several methods have been developed to separate asphalt binder into different fractions to obtain a better understanding of its mechanical characteristics, among which the analysis of SARA (saturates, aromatics, resins, and asphaltenes) is frequently used.10 It is believed that these four fractions provide different physical and mechanical properties. Aromatics and saturates are responsible for viscosity, whereas asphaltenes contain polar polyaromatic compounds and contribute to the surface activity and adhesion of asphalt to mineral aggregate.11−13 However, the chemical composition is not the only determining factor that affects the properties of a binder. Because the adhesion between asphalt and aggregate occurs mainly at their interface, the surface microstructure of asphalt is also an important factor. At present, in the most widely accepted theory, the structure of asphalt is described by a colloidal model.5 It is believed that heavy components with high polarity form a core and are dispersed in light components. In the past, research on the adhesive characteristics of asphalt was mainly based on the surface energy method.14,15 However, the surface energy method is a macroscale method, and its measurements are liable to large accidental errors. In recent years, new methods of testing the adhesive characteristics of asphalt have been investigated using atomic force microscopy (AFM).16−18 Since its discovery, AFM19 has become an important tool for imaging the topography and measuring the surface forces of different kinds of materials. A “bee-like’’ microstructure was first proposed by Loeber et al.20 when studying the surface topography of asphalt binder. For investigating the cause of this bee-like structure, some studies have been carried out on the microstructures and components of asphalt.21,22 It was proposed that this structure was mainly determined by the chemical composition of asphalt and had a close relation to its mechanical properties.23,24 It was believed that the bee-like structure resulted from the crystallization of paraffin waxes.24,25 It was also thought that it comprised colloidal particles of asphaltene. 26 Despite its unclear composition, it was generally believed that the bee-like structure may have an important effect on the adhesion performance between asphalt and aggregate. Although AFM could be used to investigate the surface topography and surface forces on the nanoscale, atomic force microscope tips, of which the diameters range from 80 to 120 nm,27 are too large relative to the size of molecules. The topographic images and measurements of phase separation and adhesive force acquired from an AFM test can provide limited information on the interaction between asphalt and aggregate. For better understanding the adhesive characteristics of asphalt, the relationships between the chemical composition and the mechanical properties of asphalt need to be studied and clarified. For determining the relationship between physical adhesion and chemical composition, a detailed study on a smaller scale is needed. Lu and Wang28 investigated nanoscale adhesive characteristics using a molecular dynamics (MD) simulation method and a microscopy technique. However, more detailed analysis is still needed.

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EXPERIMENTAL AND SIMULATION DETAILS

Materials. Asphalt is a very complicated mixture. In most previous studies of the chemical composition of asphalt, the binder was separated into four fractions via an adsorption and solvent separation method (ASTM D4124-09). The four fractions were defined in terms of the SARA components. Because the area of production of asphalt may have an influence on its chemical composition, only one type of asphalt, which had a penetration grade of 80/100, was used in this study. This asphalt came from Panjin, China. It is widely accepted that the aging of asphalt is the main factor that influences its performance properties, including adhesion, in engineering applications. Therefore, asphalt with a penetration grade of 80/100 was aged to different degrees, including short-term aging using a standard rolling thin-film oven test and long-term aging using a pressure aging vessel test. Variations in the contents of the four fractions of asphalt at various degrees of aging were measured and are presented in Table 1.

Table 1. Proportions of the Four Fractions of Asphalt at Various Degrees of Aging aging conditions

sample mass (g)

asphaltenes (%)

saturates (%)

aromatics (%)

resins (%)

virgin short-term long-term

0.4717 0.5057 0.4985

14.89 15.13 17.67

18.88 17.23 12.92

42.21 37.97 27.88

23.63 29.66 41.52

It is clear that the aging of asphalt increased the contents of asphaltenes and resins and decreased the contents of saturates and aromatics. Previous studies reported that carbonyl compounds formed as a result of the aging of asphalt via the oxidation of aromatic compounds in the naphthene aromatic, polar aromatic, and asphaltene fractions.32−34 An increase in the content of polar carbonyl groups results in the stiffening of asphalt binder. In addition to the variations in the chemical fractions and physical hardening of asphalt, the microstructure on the surface of asphalt was also affected during aging and was investigated by AFM in scanning mode, as seen in Figure 1. It is obvious that different phases are present in asphalt. Each phase is believed to have different mechanical properties, such as stiffness or adhesion behavior. The typical feature termed the bee-like structure, which refers to an arrangement of “hills” and “valleys”, is regarded as one crystallizable phase. As mentioned before, there is still much debate on the actual composition of the bee-like structure. Most recent studies have suggested that the bee-like structure results from the crystallization of asphaltenes or paraffins.35 In combination with the B


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Figure 1. Microstructure of asphalt viewed by AFM under different aging conditions, including (a) virgin, (b) short-term aged, and (c) long-term aged asphalt.

Figure 2. Schematic representation of the AFM force curve test. measurement of the contents of fractions and previous reports, the bee-like structure might be a mixture of asphaltenes and paraffins. The

aging of asphalt causes the crystallization and aggregation of paraffins and asphaltenes,36 which must have an effect on the adhesion behavior C


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Figure 3. Structures of representative asphalt molecules. of asphalt. In addition to the adsorption that occurs between the fractions of asphalt and mineral aggregate, differences in the surface roughness of asphalt binder after various aging treatments could also be an important factor when asphalt binder adheres to mineral aggregate, which is also discussed in this study. Atomic Force Microscopy Testing. For investigating the adhesive characteristics of asphalt, AFM force mapping was performed with a Bruker Dimension Icon atomic force microscope in PeakForce mode. As mentioned before, the silicon tip of the atomic force microscope was selected to represent the silicate minerals in aggregate and contact the surface of asphalt binder. A schematic picture can be found in Figure 2. During the procedures of approach and retraction of the tip, a force curve can be plotted, as shown in Figure 2. The size of the adhesive force can be measured directly from the force curve and also used to calculate the adhesive strength. During the force curve test, the peak force is the contact force of the silicon tip on the asphalt surface. To minimize damage to the original structure of the asphalt, the peak force was set at the very small value of 5 nN during the test. The radius of curvature of the monocrystalline silicon tip was measured and found to be 12.46 nm. Molecular Dynamics Simulations of Single Molecules and the Silicon Tip. The characteristics of physical adsorption between the silicon tip and the asphalt were analyzed by an MD simulation method. In this method, we chose the COMPASS force field, which is based on all-atom interactions, and used the open-source large-scale atomic/molecular massively parallel simulator (LAMMPS) to perform parallel MD simulations. Monocrystalline silicon was chosen because of its stable chemical and physical performance and was also selected to be the material of the atomic force microscope tip. A square silicon tip with dimensions of 54.2 × 54.2 Å was made to represent the silicon tip used in the AFM test.

Regarding the model of asphalt used in the simulation, because asphalt contains millions of different types of molecules, it was impossible to determine all of the molecular structures and add them to the molecular model. Only a few typical molecules were considered in this study. According to Li and Greenfield, the molecules shown in Figure 3 can be used to accurately represent and characterize the performance properties of asphalt materials.30 During the study, the first aim was to investigate the characteristics of adhesion between the individual molecules shown in Figure 3 and the silicon tip. An image of the initial model used during the MD simulation can be seen in Figure 4. Molecular Dynamics Simulations of a Composite System of Asphalt Molecules and the Silicon Tip. A composite system, which included all 12 of these typical molecules, was then constructed and used to simulate the adhesion between the asphalt and the silicon tip. In this system, the numbers of asphalt molecules were calculated and adjusted to conform to the measurements of the asphalt fractions,

Figure 4. MD model of a single molecule and the silicon tip. D


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ACS Applied Materials & Interfaces as indicated previously. The numbers of molecules that were used are presented in Table 2. By considering the types and numbers of these

with two free surfaces was formed. The thickness of the membrane was approximately 30 Å (depending on its density). As mentioned before, a square silicon tip with dimensions of 54.2 × 54.2 Å was made to represent the silicon tip used in the AFM test. The asphalt membrane was fixed to a pedestal to prevent it from moving with the tip. An image of the initial model during the MD simulation can be seen in Figure 6.

Table 2. Numbers of Asphalt Molecules Used in the Simulation number of molecules component saturates aromatics resins

asphaltenes

total

type of molecule

virgin asphalt

short-term aged asphalt

long-term aged asphalt

A B A B A B C D E A B C

54 47 110 125 17 16 33 18 23 20 13 16 492

49 43 98 113 26 25 49 27 34 21 14 17 516

36 32 71 81 35 34 67 37 47 24 16 20 500

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RESULTS AND DISCUSSION Characteristics of Adhesion between Single Molecules and Silicon Tip: MD Simulation. The characteristics of adhesion between monocrystalline silicon and the 12 selected typical molecules in asphalt were first analyzed using an MD simulation method. The model of the silicon tip and the single molecules was the same as that shown in Figure 4. The interaction between the silicon tip and the single molecules was recorded during the approach and retraction procedures. The entire simulation procedure can be described as follows. To correspond to the practical situation during the mixing of asphalt and aggregate, the silicon tip was first set to be at 473 K (200 °C) and the asphalt molecules to be at 413 K (140 °C). Next, the molecules were made to move closer at a speed of 0.01 Å/fs until their separation was approximately 5−8 Å. Then, they were allowed to move freely for 15 ps at time steps of 0.1 fs. At the same time, the temperature was allowed to gradually fall to 298 K (25 °C) for the first 5 ps and then remain stable at 298 K for the remaining 10 ps. Finally, the asphalt molecules were moved away from the silicon at the same speed at which they approached. During the entire procedure, curves of the interactive forces plotted against the positions of the molecules with respect to the tip were

molecules, the mass fraction of each component of asphalt in the simulation can be calculated and compared with measured values, as shown in Figure 5. It is clear that the model and experimental results are in good agreement. After determining the contents of the fractions of asphalt for different aging treatments, a relaxation step was initially conducted on the models of asphalt to make the asphalt adopt a stable structure. After relaxation, an asphalt membrane with dimensions of 120 × 120 Å

Figure 5. Comparison of contents of fractions between simulation and measurement for (a) virgin, (b) short-term aged, and (c) long-term aged asphalt. E


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Figure 6. MD model of asphalt and the silicon tip.

method. On the basis of the image information from the simulation, the projected area of each molecule on the silicon tip was defined as being the contact area. In this way, the spatial configuration of the molecule was compressed into a twodimensional plane, and it was assumed that the force was evenly distributed over this area. For calculating the area of the plane containing the point atoms, a further approximation was needed. Here, we set the effective radius of every atom to be 2 Å, as the balance distance of van der Waals forces is approximatly 4 Å. Therefore, the area of the molecule was the sum of the effective areas of each atom. We constructed a rectangle that could enclose all of the atoms (with a radius of 2 Å). Then, a point was randomly generated, and if the distance between the point and any of the atoms was less than 2 Å, it was regarded as being inside the molecule; otherwise, it was outside the molecule. To calculate the area of each molecule, over 2 million such points were generated and, at the same time, the number of points inside the molecule was counted. Therefore, the area of one molecule equaled the area of the rectangle multiplied by the quantity ratio (the number of points inside the molecule divided by the total number of points). The entire procedure of this Monte Carlo method can be found in Figure 9. Then, the adhesive force was deduced from the force− position curve, and the adhesive strength between each molecule and the silicon tip was calculated, as shown in Table 3 and Figure 10. The results indicate that the asphaltenes provided stronger adhesion on average than the other fractions in asphalt, which is consistent with prior experience that asphaltenes significantly contribute to the surface activity and adhesion of asphalt to mineral aggregates. A possible reason is that the atoms of asphaltenes are more densely packed and, at the same time, most of them are distributed on a flat surface. Other molecules with these features, such as aromatic A and resin C, also provided stronger adhesion. Normally, types of molecular moieties that have higher density have higher surface free energy because more external work needs to be done to create a new unit surface area for molecular moieties with higher density. Therefore, molecules with higher surface free energy can exhibit stronger adhesion to aggregate. In this situation, this kind of adhesive strength is mainly attributed to the molecular structure. However, in reality, molecules cannot move freely

recorded. Figure 7 shows a typical force−position curve for a molecule of asphaltene A. The force fluctuated near the force

Figure 7. Force−position curve for a molecule of asphaltene A.

balance point, which resulted from the thermal motion of the molecule during free motion, as in the practical situation. In this process, the maximum adhesive force was characteristic of the adhesive strength of the molecule. Because the 12 selected molecules in asphalt have different spatial molecular configurations and therefore different plane projection areas, this may introduce errors when calculating and comparing the adhesive strength of each molecule. Typical spatial molecular configurations and their corresponding plane projection areas are shown in Figure 8. It is clear that the adhesive strength between each molecule and the silicon tip is determined by the adhesive force and the plane projection area of the molecule. For averaging the adhesive force on a unit projected area, the area of each molecule was calculated using a Monte Carlo

Figure 8. Typical plane projection areas of molecules. F


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Figure 9. Procedure of Monte Carlo method for calculating the area of a molecule.

measurement procedure of an AFM force curve test, as shown in Figure 2, was simulated to obtain theoretical results for adhesion. During the simulation, the tip approached and was then retracted from the free surface of the asphalt following a similar measuring process as in the AFM test. The speed of motion was set at 0.005 Å/fs, and the peak force was controlled at approximately 2 nN. According to a movie of the simulation process, the tip just reached the surface, and the structure of the asphalt was not destroyed under these conditions. The force on the tip was calculated, and the force curves are plotted in Figure 11. On this scale, the detection of force was much more sensitive. The result might exhibit greater randomness because the effect of the inhomogeneity of the surface of the material was more prominent. However, the shapes of the force curves can still provide good information on characteristics of the adhesion of asphalt. The approach curves for short- and long-term aged asphalt display significant fluctuations. One possible reason may be the roughness on the molecular scale on the surface. When the tip approached the asphalt, it touched some molecules first, then pressed them into the body of the asphalt and flattened the surface of the asphalt. In this procedure, a resistance force may have been produced. Moreover, the result also shows that the tip was only significantly affected by the force up to a distance of approximately 10 Å. Little attraction or repulsion was produced beyond this range.

Table 3. Adhesive Force and Strength between Each Molecule and the Silicon Tip molecule

molecular weight

maximum adhesive force (nN)

molecular area (Å2)

adhesive strength (MPa)

saturate A saturate B aromatic A aromatic B resin A resin B resin C resin D resin E asphaltene A asphaltene B asphaltene C

422.9 483.0 464.8 406.8 554.0 573.1 290.4 530.9 414.8 575.0 888.5 707.2

−0.65 −0.53 −0.82 −0.55 −0.68 −0.56 −0.49 −0.49 −0.56 −1.05 −1.13 −1.14

219.90 173.38 209.51 166.68 202.57 201.11 126.32 185.71 178.76 257.85 329.95 293.52

296.70 302.91 389.82 331.24 337.67 278.41 386.50 266.03 311.76 408.45 341.56 388.20

and there are many complex interactions between them. The interactions between different molecules and fractions make the characteristics of adhesion more related to the distribution and organization of atoms. Therefore, a study on a larger scale, which included the 12 typical molecules that represented real asphalt, was conducted. Characteristics of Adhesion between a Composite System of Asphalt Molecules and the Silicon Tip: MD Simulation. On the nanoscale, both MD simulations and an AFM force curve test are effective methods for describing physical adhesion behavior. In this section, the entire

Figure 10. Results of calculations for adhesion of molecules to the silicon tip. G


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Figure 11. Simulation of force−position curves for the silicon tip with (a) virgin, (b) short-term aged, and (c) long-term aged asphalt.

silicon tip. Because adhesive bonding behavior occurs at the interface, variations in the composition of asphalt are believed to change the surface microstructures of asphalt. On larger scales, the effects of the surface microstructures must be more significant. For confirming this assumption, AFM force curve tests were conducted on real specimens of asphalt. Characteristics of Adhesion between Real Asphalt Specimens and Silicon Tip: AFM Tests. The force curve tests were conducted by AFM in PeakForce mode. Some typical force curves are plotted in Figure 12. In fact, asphalt is a very soft material. Regarding its colloidal structure, the surface of asphalt is quite complicated. From the force curves, it was found that the deformation of asphalt was much greater than that of normal crystalline materials such as metals, ceramics, and rocks when the tip was retracted from the surface of the sample. Although the shapes of the force curves in the AFM tests were similar to those in the MD simulation, the distances during approach and retraction were different. The AFM tip began to be attracted when it was tens of nanometers away from the surface of the sample, whereas in the theoretical simulation, this distance was only approximately 10 Å. We believe that the tip moved too rapidly to make the asphalt undergo deformation in the simulation, whereas in the AFM test, the tip was much larger and there was sufficient time for deformation of the asphalt. When comparing the results of the AFM test with those of the MD simulation, the greatest difference lies in the larger size and real surface characteristics (microstructures) of the tested specimens of asphalt. Among these, we believe that the

It can also be seen from Figure 11 that short-term aged asphalt experienced the greatest attraction to the silicon tip, and the attractive force for long-term aged asphalt was a little greater than that for virgin asphalt. Then, the adhesive strengths were calculated based on the maximum adhesive force, as shown in Table 4. Table 4. Adhesive Strengths of Asphalt in MD Simulations asphalt virgin asphalt short-term aged asphalt long-term aged asphalt

maximum adhesive force (nN)

contact area (Å2)

adhesive strength (MPa)

1.84 3.14

2937.64 2937.64

62.64 106.89

2.32

2937.64

78.97

It is interesting to find that the physical adhesive strength between the silicon tip and the asphalt membrane on the nanoscale was much less than that between the silicon tip and a single molecule. This is because the space density of atoms was greatly reduced. As was demonstrated before, asphaltenes provided stronger adhesion. However, in this simulation, the adhesive strength of long-term aged asphalt, which contained the most asphaltene, was not the greatest, being approximately 35% less than that of short-term aged asphalt and approximately 20% greater than that of virgin asphalt. This means that, in addition to physical adsorption (van der Waals forces, electric field forces, etc.), the composition of asphalt began to influence the interaction between the asphalt and the H


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Figure 12. Results of AFM force curve tests on (a) virgin, (b) short-term aged, and (c) long-term aged asphalt.

roughness on the molecular scale in the tested specimens is an important factor that led to the large difference in the positions (deformations). Roughness of the surface is inevitable, and the colloidal structure of asphalt exacerbates it. How the surface microstructure affects adhesion will be discussed later. During the AFM test, as described for the MD simulation, the tip first contacted the bulge and flattened it. When the tip was retracted, some molecules or molecular clusters, especially bulges, could move with the tip under its adhesive force. In virgin asphalt, there are more flowable light components (saturates and aromatics), which move easily when force is applied. With the loss of the flowable light components during aging, the hard core of asphalt (asphaltenes and resins) was exposed and was more likely to touch the silicon tip. The hard cores are more difficult to displace; therefore, long-term aged asphalt underwent a smaller deformation, as indicated in Figure 12. The roughness of the surface of the asphalt also made it impossible to calculate the contact area. Only the maximum adhesive force was calculated, as shown in Table 5. The results indicate that short-term aged asphalt provided the strongest adhesion to the tip and virgin asphalt the weakest adhesion. This is almost the same as the results of the simulation and is also consistent with practical experience. A rough estimate of the contact area is 200−300 nm2; therefore, the adhesive strength was approximately 10−50 MPa, which is less than the result of the simulation. Differences between MD Simulations and AFM Tests. It can be seen that the adhesion in the AFM tests was weaker than that in the MD simulations. One possible reason may

Table 5. Adhesive Strengths of Samples of Asphalt in AFM Tests asphalt virgin asphalt short-term aged asphalt long-term aged asphalt

effective measuring points

average maximum adhesive force (nN)

standard deviation (nN)

26 24

4.58 9.12

0.16 1.31

21

5.93

0.62

relate to the surface microstructure of the real asphalt specimens in the AFM tests. For explaining how the microstructure of asphalt influenced its adhesive properties, a model of the interaction between asphalt and the silicon tip (or mineral aggregate) is proposed in Figure 13 according to some experimental phenomena and characteristics of molecular forces. In this model, the asphalt surface is rugged on the nanoscale because of its colloidal structure. The aggregate surface first comes into contact with bulges of asphalt and flattens them until there is a balance between attraction and repulsion. When the surfaces separate, the bulges undergo deformation and increase the effective adhesion distance. In addition, as seen in Figure 13, parts of the aggregate are attracted by the asphalt, whereas other are repelled. Therefore, the roughness can bring about a balance between attraction and repulsion if the surface is large enough. Finally, the results of measurements of adhesive strength in the AFM tests were lower than those in the MD simulations, which were on a smaller scale. I


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Figure 13. Model of interaction between asphalt and aggregate.

asphalt, which contained more light components, had the worst adhesion performance. All these results imply that the microstructure of asphalt has a larger influence on its adhesive characteristics compared with that of its composition. In general, some traditional methods of improving the adhesion performance of asphalt include increasing the viscosity of asphalt and increasing the roughness of aggregate. The basic principles are very similar to the conclusions reached in this study. For maximizing the adhesive bonding between asphalt and aggregate, the systematic design and selection of materials should be undertaken. The light components in asphalt can increase the adhesion area, whereas the heavy components in asphalt provide better adsorption adhesion. In addition, an appropriate surface microstructure could help improve the interaction between asphalt and mineral aggregate. According to these conclusions, the adhesion performance between asphalt and aggregate can be improved from the point of view of the design and modification of asphalt materials. This is a fundamental task for the Materials Genome Initiative.

This model can accurately explain how the surface microstructure of asphalt influences its adhesive properties, which indicates the importance of an effective contact area. Although long-term aged asphalt contained more asphaltenes, which formed stronger adhesive bonds to the silicon tip, the fact that the deformability of long-term aged asphalt was relatively lower owing to its surface roughness reduced the overall adhesive forces and made long-term aged asphalt have the worst adhesion performance. This could be deduced from Figure 1. During aging, more asphaltenes/paraffins aggregated to form a larger and stiffer bee-like structure, which would reduce the strength of adhesion between the asphalt and aggregate. Therefore, the physical adhesive bonding between asphalt and mineral aggregate is much more complicated and is influenced by the effects of physical adsorption, the surface microstructure of asphalt, the fluidity of the light components in asphalt, and so forth. For example, a sufficient quantity of flowable light components in virgin asphalt could lead to a larger effective contact area, which is helpful for adhesion, but on the other hand, the relatively weak interaction between the light components and mineral aggregate makes virgin asphalt exhibit weak adhesion.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-451-86282120. E-mail: [email protected].

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Notes

CONCLUSIONS AND RECOMMENDATIONS An MD simulation method in combination with AFM force curve tests was employed to study the physical mechanisms of adhesion between asphalt and aggregate from the points of view of composition and microstructure. It was found that molecules with a higher atomic density, higher surface free energy, and a planar structure, such as three types of asphaltene molecules, provided greater adhesive strength in the adhesion of asphalt to mineral aggregate. In addition to the composition of asphalt, its surface microstructure is also important for its adhesive properties. According to the results of MD simulations and AFM tests, short-term aged asphalt provided better adhesion performance than long-term aged asphalt, which contained the largest amount of asphaltene. The crystallization and aggregation of paraffins and asphaltenes in aged asphalt introduced larger and stiffer bee-like structures and had a damaging influence on its adhesion performance. Virgin

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support by the China Postdoctoral Science Foundation (Nos. 2013M541393 and 2015T80357), Heilongjiang Postdoctoral Science Foundation (LBH-Z13084), National Natural Science Foundation of China (Grant Nos. 51408154 and 51408161), Open Fund of Key Laboratory of Road Structure and Material of Ministry of Transport (Changsha University of Science & Technology) (kfj140304), and the Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 2017043).

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REFERENCES

(1) Yu, X.; Burnham, N. A.; Mallick, R. B.; Tao, M. A Systematic AFM-based Method to Measure Adhesion Differences between

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Analysis of Adhesive Characteristics of Asphalt Based on Atomic

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