Asphalt binder micro-characterization and testing approaches: A review

Journal Pre-proofs Asphalt binder micro-characterization and testing approaches: a review Mengya Zhang, Peiwen Hao, Shi Dong, Yan Li, Gaoang Yuan PII: DOI: Reference:

S0263-2241(19)31119-4 https://doi.org/10.1016/j.measurement.2019.107255 MEASUR 107255

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Measurement

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20 August 2019 31 October 2019 7 November 2019

Please cite this article as: M. Zhang, P. Hao, S. Dong, Y. Li, G. Yuan, Asphalt binder micro-characterization and testing approaches: a review, Measurement (2019), doi: https://doi.org/10.1016/j.measurement.2019.107255

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Asphalt binder micro-characterization and testing approaches: a review Mengya Zhanga, Peiwen Haoa*, Shi Donga, Yan Lia, Gaoang Yuana a

Key Laboratory of Road Structure & Material, Ministry of Communication, PRC, Chang’an University, Xi’an 710064, China

* Corresponding author. Tel.: +86 29 62630337

E-mail address: [email protected]. (P. Hao). Abstract Asphalt binder is a complex material whose chemical composition and structure determine its properties. Fourier Transform infrared spectroscopy (FTIR), Nuclear Magnetic Resonance (NMR), Gel Permeation Chromatography (GPC) and Atomic Force Microscopy (AFM) have been widely used as an efficient tool for detecting and analyzing asphalt binders at the molecular level. This article focuses on elucidating the typical application of those technologies in exploring asphalt binder properties and introduces the basic principle of these methods and recent highlights; sets forth problems impacting the microscopic test results translating to physical properties; points the direction of future studies on asphalt fingerprinting. The findings in this research show that microscopic testing approaches can be successfully used to study the relationship between microstructure and properties of the asphalt binders. This review is to provide guidance and reference for researchers to find the relationship between micro and macro properties of the asphalt binder. Keyword: asphalt binder; microstructure; microscopic testing approach.

1. Introduction Asphalt binder is a complex chemical composite. Different types of constituent molecules coexist in the asphalt binder molecules with stable structures [1]. The interaction between those different molecular groups plays an essential role in the properties of asphalt binder materials. Changes in four major chemical compositions (asphaltenes, resins, aromatics, saturates- “SARA”) fundamentally alter the structure and properties of asphalt binder (such as the physical, mechanical and rheological properties) [2,3]. The identification of these compositions and the investigation of their properties have always been in the focus of the scientific community. The chemical structure (e.g., chemical component, fraction polarity, molecular weight, phase changes, and molecules interaction) and morphology of asphalt binder can be changed as a result of preparation, mixing, and laying on the road, further influencing its properties [4,5]. For instance, the addition of polymers can change the microstructure and the properties of asphalt binder at the same time. To better understand the properties, we need to look into the chemical structure of asphalt binder materials from the aspect of material “fingerprint”. The asphalt binder fingerprint refers to the nanometer-scale differences that will be inherited to the macroscopic properties of asphalt binder. Thus, researching the chemical structure of asphalt binder or the asphalt binder fingerprint can help to develop the relationship between microstructure and properties of asphalt binders in the future works. The microscopic testing approaches can provide detailed and valid information for the study of the asphalt binder properties and compositions. Fourier Transform Infrared Spectroscopy (FTIR) has been used to characterize the asphalt binder chemical composition [6-18]. By determining the various functional groups in the asphalt binder, an understanding of its history can be obtained. Also, polymer additives can be identified by quantifying and analyzing the changes in the chemical bond. Nuclear Magnetic Resonance (NMR) is a useful tool for investigating the structural characterization of asphalt compositions, such as percentages of aromatic, methyl carbons, ring carbons, and aromatic-carbon ratio et al. [19]. Gel permeation chromatography (GPC) separates mixtures according to their molecular size. Meanwhile, the aging performance of asphalt binder can be predicted by chromatograms. When NMR, FTIR, and GPC information is combined, structures of asphalt binder can be calculated and analyzed. Atomic Force Microscopy (AFM) reveals the morphology of characteristic and reproducible appearance of SARA constituents [20-28]. Also, it is possible to compare the mechanical properties of virgin, aged and modified asphalt by using the appropriate AFM mode. Optical microscopy techniques, such as fluorescence microscopy, scanning electron microscope (SEM), X-ray diffraction can also be used in exploring asphalt binder and its mixture properties. For example, fluorescence microscopy has been widely used in characterizing the dispersion state and morphology of modifiers. SEM can characterize the asphalt and its mixture profile and damage morphology. Also, the molecular structure of organic reagent can be recognized by using X-ray diffraction. Considering it can be a separate issue to discuss in the future study. Therefore, this article does not review any optical microscopy techniques. These microscopic testing approaches provide insight into the molecular interactions which reflect the real difference in asphalt binder chemical structure that cannot be distinguished by conventional engineering performance evaluation indicators. Some people assumed that the chemical properties of the asphalt binder could predict its properties. While others believe that the knowledge of the chemical structure has limited help for understanding asphalt properties, they point out that the advanced modern analytical techniques always provide average results that are not easily converted into mechanical properties due to the two significant difficulties: 1) as the composition of asphalt binder is complex and difficult to separate and characterize accurately. So even if the structure is known in principle, the exact structure is unknown; 2) the asphalt chemistries, reactions, and phase changes occur at the microscale and nanoscale, and modern analytical techniques are hard to obtain full information [29]. Therefore, the development of material characterization and computer technology can certainly help understanding asphalt binder at the micro-levels. However, whether it can be used to predict the macroscopic performance of asphalt binder and its mixtures accurately

remains to be explored. The properties of asphalt binders are greatly affected by its chemical ingredients and structure. The research goal of this paper is to study these correlations and trends though different microscopic testing approaches, as presented by the proposed framework in Fig.1. In the first section, the chemical compositions and separation techniques of asphalt binder are summarized. Then, the structure of the asphalt binder and its history are introduced. The limitation of understanding of colloidal structure is also emphasized. Finally, the applications of microscopic testing approaches in exploring asphalt binder properties are introduced, and the results in the relationship between microstructure and properties of asphalt binders are reviewed.

Fig. 1 The study on the relationship between microstructure and macro-properties.

2. The composition and structure of asphalt binders 2.1 Asphalt Composition The chemical composition varies with the source and batch of crude oil used in the production of asphalt binder [30]. That is the reason why the structure of asphalt binders cannot be characterized by certain molecules [31]. However, their chemical compositions can be classified according to different properties of molecules. By using this classification, it can help us to discover the relationship between the chemical structure and properties of the asphalt binder. There are two common methods for separating asphalt binders, respectively, for the chemical precipitation method and selective adsorption-desorption (chromatographic) method [32]. In both ways, asphalt binder is considered as a chemical continuum, without discontinuity of mass, polarity, and aromatic content of its compositions [33]. The insoluble part of asphalt binder in pentane or heptanes constitutes the asphaltenes, while the soluble portion is called maltenes which can be further separated into mixtures with differing solubility properties [34]. White et al. [35] summarized the four representative compositions of asphalt binder, commonly known as “SARA” (saturated, aromatic, resin, and asphaltene) fractions. Their characteristic parameters are listed in table 1. Table 1. Characteristic parameters of four main asphalt fractions [33][36-39] Oil Group Resins Asphaltenes Saturates Aromatics Colorless or lightly Yellow to dark red Dark brown to black solid Dark brown to colored liquid liquid or semi-solid black powder Description

Low polarity Weight percent range (wt. %) Average molecular weight (g/mol) Density (g/cm3) Composition aromatic

High polarity

5–20

30-65

30-45

5-20

600

800

1100

600-3500

0.9 0

1.0 0.25

1.07 1.15 0.42 0.5 Heterocyclic (NSO) Includes compounds such as acids, High molecular Chemical compositions Aliphatic compounds mono-aromatics and bases phenolics, naturally weight complex polycyclic aromatics occurring compounds matrix (humic acids)

The compositions of asphalt binder can be divided into distinct molecular groups based on the size of the molecules and different solubility [39,40]. Both polarity and molecular weight are related to the properties of asphalt binder [41]. The interactions of polar or polarizable composition have significant effects on the viscosity and rheological properties of asphalt binder. Zenke [42] and Weigel et al [43]. already determined a positive relation between the asphaltene content and the softening point. Sultana [44] found that an increase in asphaltenes content may result in improved high-temperature properties without any significant increase in stiffness at low temperature. Molecular weight has an important influence on the property of asphalt binder [45], especially in the application of recycled asphalt materials to road work [46]. Molecular weight, for example, is a major factor controlling the diffusion process between virgin and recycled asphalt [47-49]. 2.2 Asphalt Structures In structural terms, Rosinger proposed the colloidal structure of asphalt [50]. However, a detailed description of the asphalt colloidal structure was published by Nellensteyn in 1923 [33]. In the 1940s, J. P. Pfeiffer and co-workers [51] further developed a colloid model. According to these theories, the difference in rheological properties between sol and gel asphalt can be found, as shown in Figure 2. Sol asphalt exhibits Newtonian behavior, whereas gel asphalt shows non-Newtonian mechanical behavior. Between these two extremes, most of the asphalt binder has intermediate behavior due to the sol-gel mixing structure.

Fig. 2. Sol (a) and gel (b) colloidal model of asphalt binder [51]

Modern colloid theory from Yen T. F. [52] and Peramanu S. [53] is that the colloidal model in which asphaltenes are thought to condense into a micellar structure surrounded by other high-polar material (the resins) and dispersed through the maltene phase (see Fig.3). A large number of scholars confirmed that asphaltenes form micelles in organic solvents [54,55], in crude oil [56] and in asphalt binder [57,58] by using Small Angle X-ray Scattering (SAXS) and Small-Angle Neutron Scattering

(SANS) tests.

Fig. 3. The colloidal structure of asphalt binder.

However, the structure of the asphalt binder remains a somewhat controversial issue. In the American Strategic Highway Research Program (SHRP) of the 1990s, some researchers pointed out that asphalt binder is a pure homogeneous fluid, this view was later known as the Dispersive Polar Fluid (DPF) model [59]. They denied the colloidal hypothesis based on a series of arguments, some of which were valid, such as the absence of elastic plateau for gel asphalt binders, while others were theoretical hypotheses due to the lack of scientific evidence [33]. It is worth mentioning that the monotonic time dependence and the unimodal relaxation spectrum found for asphalt binder viscoelasticity was considered to be the main argument to support the homogeneous model [59]. The most significant difference between these two models is that the DPF model considers the asphalt binder to be a pure continuous phase, whereas colloidal model recognizes asphalt as two phases: one is continuous low polarity phase, and the other is dispersed high polarity phase. The disappearance of bee domains upon heating and a re-appearance upon cooling proved that asphalt binder is not homogenous in bulk [60]. Moreover, the mechanism of polymer modified asphalt binder is difficult to understand in a homogeneous framework structure. In contrast, the colloidal model gives an explanation based on the polymer-asphalt micelle repulsion. Therefore, a colloidal treatment of the microstructure would be more reasonable. 3. Chemical characterization approaches of asphalt binder 3.1 Fourier Transform Infrared Spectroscopy FTIR is used for identifying asphalt binder functional groups or chemical bonds. The infrared spectrum is an energy level transition diagram of the chemical bonds or functional groups in asphalt binder at specific wavelengths thus helping to infer the structure of the substance [61,62]. After the baseline correction of the original spectrum, FTIR can qualitatively and quantitatively analyze organic components by statistical algorithms. The qualitative analysis involves identifying known substances by comparing the sample spectra to a standard spectrum. Also, unknown objects can be identified in the same way. In terms of the quantitative analysis, the concentration, location, and shape parameters of these characteristic absorption peaks can be deduced from the content of certain components by using the Beer-Lambert Law [63,64]. The application of infrared spectrum in asphalt binder can be divided into four categories. 1) Asphalt binder grade and source recognition. As shown in Figure 4, Weigel [65-67] investigated 32 asphalt binder samples of different aging states, grades, and oil source. It is clear that most asphalt binders give very similar identical spectra and it is difficult to determine any particular functional group based on a specific peak, peak intensity, or peak shape. Pierp et al. [68] used a similar method to identify the source of asphalt binder by detecting the content of branched aliphatics, aromatic, sulfur, and other elements. However, it is impossible to distinguish between asphalt samples, such as their refineries, based solely on the evaluation of absorption peaks. The chemometric methods give way for getting more information due to the consideration of

almost the entire spectra [69]. Partial Least Squares Regression (PLSR) used to establish the prediction model of the different chemical composition of the asphalt binder. The Principle Component Analysis (PCA) and the Linear Discriminant Analysis (LDA) are always used for secondary treatment of asphalt binder infrared spectra, and as a result, asphalt binder can be easily classified. Fig. 5 shows a clear separation of the refinery from Weigel [65].

Fig. 4. FTIR spectra of the 32 asphalt samples [65].

Fig. 5. The LDA results of 13 samples [65]

2) Modifier recognition The FTIR method can effectively identify and quantitatively analyze additives in the asphalt binder, such as styrene-butadiene-styrene copolymer (SBS), Polyurethane (PU), crumb rubber, WMA additives, and rejuvenator, etc. For example, Zhao [70] found that the SBS modified asphalt spectrum is a composition graph of the infrared spectrum of virgin asphalt with SBS modifier. A similar conclusion was drawn by Nivitha et al. [71]. Two individual absorption peaks at 966 cm-1 and 699 cm-1 due to the bending vibration of -CH=CH- and C-H are attributed to butadiene and styrene, respectively. Besides, Infrared spectroscopy can also be used to determine the modifier content, such as a certain correspondence exists between SBS characteristic peak area and SBS content. Comparing to virgin asphalt, the absorption peaks at 3400 cm−1 and 2682 cm−1 reflect the existence of warming agents, which attributed to amines and amino ions, respectively [ 72 ]. The infrared

absorption peaks of typical additives shown in Table 2. Table 2. A summary of FTIR spectrum absorption peaks of modifier Polymer additive Functional group Wavenumber, cm-1 Species Aromatic C-H 699 Polystyrene block Styrene-butadiene copolymers Trans-alkene C-H 965-970 Butadiene block Polyurethane

Crumb Rubber

Evotherm DAT

C=O C-H N-H N-H C-H S=O N-H N-H+ C-O N-O

1726 2270 3330, 1530 1458 2850, 2923 1031 3400 2682 1248 1546

Urethane Free isocyanate Amines Amines Methylene Rubber vulcanization Amines Amino ions Carboxylic Nitro

Reference Zofka A. et al. (2013) [73] Bazmara B. et al. (2019) [74] Chen Z.X. et al. (2019) [75] Yu H. et al. (2016) [72]

3) Modification mechanism recognition Infrared spectroscopy can also provide a basis for analyzing the modification mechanism by identifying the differences in the absorption peaks of the asphalt compositions and functional groups. The SBS modified asphalt exhibited physical interaction due to the absence of new peaks or peak position shift [70,71]. The C-H rocking with 1 cm-1 shift in PE modified asphalt compared to the pure PE polymer, which proved that the PE modification process was a result of the combination of chemical and physical effects [71,76]. The amines absorption peak of crumb rubber is between 3000 cm-1 and 3500 cm-1. After mixing with the asphalt binder, the characteristic peak disappeared, and a new peak at 3300 cm-1 confirmed the chemical reaction between the asphalt binder and the crumb rubber [71]. However, Kudva [77] hold a contrary conclusion that crumb rubber modification process was the physical effect. The modification mechanism can be inferred from the infrared spectrum, but it is difficult to quantify each interaction, especially for composite modified asphalt. 4) Aging recognition Benzylic ketones, sulfoxides, and free hydroxyl radicals were the three main products of asphalt binder oxidation [78-82]. Besides, Petersen and Glaser [83] re-examined the theory of aging kinetics and found that the formation of alcohol also played an important role in asphalt oxidation, especially in asphalt binders with higher sulfur content. The peak area of the infrared spectra of aged asphalt at 1700 cm−1 increased significantly. Lamontagne et al. [84] performed a more detailed analysis of the absorption peaks around this region. Carboxylic acid and ketone were the main products during the aging process of asphalt. As shown in Fig.6, the stretching vibration absorption peaks of the carbonyl and sulfoxide functional groups were increasing with the aging severity [85]. As a consequence, the FTIR approach has been proved as an important tool to study the aging process of the asphalt binder by testing the type and content of functional groups [86-93]. Aging behavior of the asphalt binder can be pursued by quantifying the change of absorption peaks of the functional groups attributed to the oxidation. Also, the impact of preparation temperature and mixing time of mix production can be quantitatively evaluated based on the application of FTIR in ATR mode [94]. Hofko [95] proved that mix production temperature has a stronger impact on the formation of sulfoxide structures than for carbonyl structures. However, only the Carbonyl Index of unaged or aged samples (both RTFOT and PAV aged) shown significant difference, but not in terms of Sulfoxide Index.

Fig. 6. FTIR spectra of PG 70-22 asphalt after laboratory and field aging (note: the field aging sample is extracted from different parts of field cores on the right) [85]

3.2 Nuclear Magnetic Resonance Analysis NMR has been used in the petroleum industry for many years [96,97], and the study of fluid behavior in rocks began in the 1950s. Recently, NMR became a potent and versatile tool for asphalt binder characterization. It can be used to detect phenomena at different length scales, ranging from molecules and colloids to macroscopic. The application of NMR in asphalt binder can be divided into two aspects: 1) Structural analysis The NMR spectrum can characterize the structure of asphalts compositions based on molecular weight and density, refractive index, or hydrocarbon percentage. Chemical elements can be identified by measuring chemical shifts [98-101]. Analysis and interpretation of NMR spectrum can provide the following useful information for understanding the structure of molecules and compounds [102-104]  The number of peaks shows the types of proton present in the molecule.  The chemical shift values, i.e., the positions of the peaks indicate the state of the molecular structure.  The relative height of the peaks represents the number of protons of each kind.  The number of peak splits represents the number of protons on adjacent atoms. Ramsey et al. were the first to use hydrogen-1 NMR (1H NMR) characterizing the asphalt structure [105]. The asphalt structure could be calculated from the 1H NMR, FTIR, and GPC result from the B-L method. The derivation methods and formulas of the Brown-Ladner (B-L) method are not discussed in this review. The research and application of 1H NMR spectra was the main direction before the 1970s. After the 1970s, with the advent of the Fourier Transform Spectrometer, the research on carbon-13 nuclear magnetic resonance (13C NMR) was carried out rapidly. The percentages of methyl carbons, aromatic carbons, bridged carbons, aliphatic carbons, ring carbons, paraffinic chain lengths, naphthenic carbons, and other parameters can be obtained from the results of 1 H NMR and 13C NMR analysis [106]. 1H NMR is more important than 13C NMR in terms of structural analysis due to its high sensitivity. However, one big disadvantage of 1H NMR is that all carbon skeleton structures are derived indirectly from different types of hydrogen structures, which inevitably leads to some errors. Therefore, the combination of 1H NMR and 13C NMR improves the accuracy of the analysis [107].

A more detailed characterization of asphalt structure can be obtained by combining elemental analysis, infrared analysis, and nuclear magnetic resonance spectrum [108,109]. Dreeskamp et al. [110] indicated that the H/C ratio calculated by 13C NMR was consistent with that derived from elemental analysis. Betancourt et al. published further evidence that the elemental composition and the 1H NMR spectra show very similar quantities of hydrogen in the different structural fragments (e.g., olefinic, aromatic, aliphatic, or phenolic) [111]. Numerous studies have shown that NMR spectroscopy can be used to predict complex organic molecular structures. Compared with other structural analysis methods, NMR spectrometry quantitative method presents advantages: selective recognition, rapidity, simplicity, and quantitative determination of aliphatic and aromatic hydrogens in asphalt binder. 2) Aging evaluation The researchers also found a good correlation between the NMR parameters and the asphalt viscosity. As is known, the viscosity increases with the aging degree of asphalt binder [112-114]. Therefore, chemical reactions of aged asphalt can be inferred from NMR studies to some extent. Siddiqui [115] found that isomerization and dehydrogenation types of reactions occurred during aging [116,117]. In terms of the composition of asphalt, the aromatics in the asphalt are converted to resins, and resins change to asphaltenes during aging. This difference in structure and composition can be used to identify whether asphalt was aging, which further shows that NMR measurement is an effective way to understand the aging process. NMR image includes two parts: low frequency (High field nuclear magnetic resonance (HF-NMR)) and high frequency (Low Field Nuclear Magnetic Resonance (LF-NMR)). HF-NMR is mainly used to test the chemical structure of molecules, while LF-NMR is mainly used for providing dynamic information between molecules. As a tool for evaluating the asphalt mix aging, LF-NMR can selectively measure the asphalt without being significantly affected by the voids and solid compounds. That is, the aggregate in the asphalt concrete will not be detected [118]. Studies showed that NMR transverse relaxation time T2, and Relative Hydrogen Index (RHI) was highly correlated with asphalt viscosity [119]. Menapace et al. [120] found that the T2 distributions of the sample tended to higher values as the temperature increased, whereas the peaks shifted towards lower T2s when the samples cool to room temperature (Fig. 7). It proved that the reduction in T2 relaxation time and a decrease of cumulative amplitude were the indications of asphalt viscosity increasing. Bryan [121] pointed out that RHI (AIasphalt/AIwater) can also be used to evaluate aging behavior. RHI was an indicator of the chemical build-up, which represented the hydrogen nuclei per unit of weight [122]. Aged asphalt showed a lower RHI value comparing to the unaged samples [123].

Fig. 7. T2 distributions of an asphalt sample [120].

LF-NMR can detect the aging tendency of asphalt cores without extracting asphalt. For now, portable NMR equipment has appeared in the market, which could allow the nondestructive measurements of pavement aging directly in the field. Therefore, this method provides advantages to evaluate aging properties and future behaviors of asphalts in roadwork.

3.3 Gel Permeation Chromatography In 1955, Lathe and Ruthven [124] first developed size exclusion chromatography. Almost ten years later, GPC was developed by Moore J.C. for separating polymer molecules according to their size and shape in solution [125]. It can be used to analyze polymer and its homologs which have the same chemical properties but different molecular volumes. The molecular weight of polymers can be defined in many ways, including the number average molecular weight (Mn), the weight average molecular weight (Mw) (see molar mass distribution), the size average molecular weight (Mz), or the viscosity molecular weight (Mv). It is worth noting that the molecular weight reflects the agglomerated microstructures in the asphalt binder, rather than the strictly chemical molecules. The application of GPC in asphalt binder can be divided into two aspects: 1) Structural analysis and pavement performance prediction by molecular size. According to the results of GPC test, the apparent molecular weight chromatography of elution components can be divided into three regions: large molecular size (LMS), medium molecular size (MMS), and small molecular size (SMS) based on elution volume [126-129].The elution time as abscissa and signal intensity as ordinate, the area under the curve indicates the molecules injected into the GPC system. The molecular size decreases gradually from left to right (see Fig. 8).

Fig. 8. Gel permeation spectrogram of asphalt.

Studies indicated that asphalt binder could be divided into oil and colloidal components according to the molecular weight [130]. Also, the LMS and SMS regions have been proved important in predicting pavement performance [131-133]. Jennings et al. [134] proposed that the LMS content in the asphalt binder had a critical point, about 20%, beyond which the performance of the asphalt binder was unfavorable. Beak et al. [135] studied the molecular weight distribution of rubber modified asphalt before and after aging by using GPC. It was found that the complex modulus (G*) of rubber modified asphalt was correlated with its LMS percentage during aging. Wahhab et al. [136] increased the asphalt content of low MW to improve the ductility of asphalt at intermediate temperatures. As a conclude, GPC can be used to predict the molecular structure of asphalt binder [137], investigate the rheological properties of different asphalt binders and study the performance of its mix [138]. 2) Asphalt aging analysis The viscosity is an indicator of aging performance. At the micro-level, the interaction of molecules along with the molecular weights codetermine the viscosity of asphalt. Specifically, the increase in the polarity of the medium leads to an increase in the intermolecular traction during aging. Light molecules aggregate to form a larger molecular structure. The appearance of degradation of macromolecules and crosslinking of small molecules can be observed from GPC test results of aged asphalt. For example, asphaltenes experienced dissociation, isomerization, and fragmentation during PAV aging [115]. The molecular weight interval shifts to

higher molecular weight distribution. Further studies found that the LMS ratio of aged to unaged samples was consistent with the aging time [127,128]. Kim et al. [139] used CPG to estimate the aging process of asphalt mixture without binder extraction. The results proved that the viscosity increased with the percentage of LMS in the asphalt binder. Moschopedis and Speight [140] conducted a detailed study on asphalt oxidation. The molecular weight of asphaltene increased with the temperature rising. The H/C, N/C, and S/C ratio decreased while the O/C increased. The results indicated that the oxygen uptake of asphaltene was the same as the sulfur and nitrogen release amount [140]. The similar trend was observed in resins, but their H/C increased. This trend due to the conversion of aromatics into resins. Those results confirmed aging leads to a decrease in aromatics content and subsequent increase in the content of polar fractions (resin and asphaltenes). So, the change in mechanical properties caused by asphalt aging can be explained by the composition and molecular weight distribution (MWD). However, substances with similar molecular weights and different chemical structures are unlikely to be completely separated by gel permeation chromatography [141]. Since the molecular weights of most of the chains are too close, GPC separation can only show a broad peak. The GPC method may not give the correct molecular weight, but the relative value. For example, asphalt binders with similar four compositions may have similar GPC chromatograms, but their mechanical properties are very different. Alternatively, there may be fundamental differences between the two asphalt GPC chromatograms with similar mechanical properties [142]. In addition, these factors will lead to an erroneous estimation of the aging degree because of the degradation and crosslink between the modifier and asphaltene [143]. In order to overcome these difficulties, Themeli et al. [144] introduced a new parameter which called hereafter the aging molecular distribution shift (AMDS). It translated the global degree of molecular associations during aging. The quantitative analysis results showed that the modified asphalt presented a better aging performance than pure petroleum asphalt of equivalent grade. 3.4 Atomic Force Microscopy AFM was invented by Binning and Quate [145], which was used for material property and microstructure surface analysis of samples at the micro and nano levels [146,147]. The AFM has a nanometer-sized tip at the end of a cantilever that probes the height of the material. The height of the probe that corresponds to the changes in surface-tip interaction. The cantilever will shift by Hooke's law. Usually, this shift causes a difference in the reflected signal from the laser source shining on the back of the cantilever. The surface topography can be obtained after computer processing. Figure 9 shows a schematic diagram of AFM scanning the surface of the asphalt binder. Regarding AFM imaging models, there are five general modes: contact, non-contact, tapping, pulsed force, peak force tapping, and lateral force microscope model. Different imaging modes have different effects on the mechanical properties and microscopic morphology of the captured sample surface. The advantages and disadvantages of varying AFM techniques are not discussed in this paper. The possible application of AFM on asphalt microstructure characterization, mechanical properties evaluation, and aging recognition are briefly described.

Fig. 9. the schematic diagram of AFM

1) Microtopography characterization Loeber et al. [148] was the first researcher who defined the small wavy shaped structures called “bee structure” in the asphalt binder by using AFM. Based on the work of Loeber, researches adopted image analysis software to study the “bee” microstructures. Four different phases were defined as Catana, Peri, Para, and Sal phases based on varying sizes of the microstructures (see Fig.10). These four phases have different elastic modulus, adhesion, and refractive index. Bee structure (or Catana phase) is the irregular soft part of the asphalt surface. The resin (or Peri phase), which surrounds the bee structure, is relatively hard. The Para phase is flatter and softer than Catana phase, which distinguishes the Peri phase from the flat regions of the surface. Sometimes, small quasi-spherical domains are finely dispersed in Para phases, known as the Sal phases. The chemical composition of bee structures has long been controversial and can be broadly divided into two categories: asphaltene fraction or the crystallizing waxes. The composition of bee-structures is reviewed in Table 3. Although AFM has made progress in asphalt morphology characterization, whether the microstructure observed on asphalt surface represents its bulk structure remains controversial. Fischer et al. [149] demonstrated that it was possible to analyze asphalt’s internal microstructure by AFM. While Blom et al. [150] hold a different view that bee structure was a surface phenomenon which is not present in the bulk phase volume of samples.

Fig. 10 The asphalt phases detected by AFM [151] Table 3. A summary of past studies on the chemical composition of asphalt “bee structure” by AFM Imaging Researchers Asphalt source AFM mode Bee structure temperature Loeber et al. [148] Contact mode, Room A Gel asphalt Asphaltene fraction (1996) Tapping mode temperature Pauli et al. [152] 8 SHRP asphalts Room Tapping mode Asphaltene fraction (2001) (different origins) temperature Non-Contact Jäger et al. [170] Room 5 asphalts mode, Pulsed Asphaltene fraction (2004) temperature force mode Masson et al. [172] 13 asphalts (different Room Metallic cations and the size and Tapping mode (2006) origins) temperature shape of the molecular planes Schmets et al. [153] 8 SHRP asphalts Room Tapping mode Wax crystals (2010) (different origins) temperature Contact mode, Zhang et al. [154] SK-70 asphalt from Lateral force Room Asphaltene fraction (2011) Korea microscope, temperature Tapping mode

De Moraes et al. [183] (2010)

Asphalt cement

Pauli et al. [155] (2011)

8 SHRP asphalts (different origins)

Tapping mode Tapping mode, Contact mode, Lateral force microscope

Room temperature, Thermal cycles 25-75℃ Room temperature, Thermal cycles 25-43℃ Room temperature, Thermal cycles 25-120℃

2 asphalts from the Middle East and Tapping mode European crudes, respectively 4 asphalts Prabir KD et al. [157] Thermal cycles (PEN:70/100, different Tapping mode (2013) 30-60℃ origins) Soutry et al. [156] (2011)

Wax crystals

The interaction between waxes and the remaining binder components Wax provides the nucleation centers for the bee-phase; asphaltene fractions work as an affinity to other species such as wax Wax crystals

Asphaltene contents steer the size of the bee-phase; metal content affects Nahar et al. [158] 3 asphalts Thermal cycles the area fraction of the bee-phase; Tapping mode (2013) (different origins) 25-90℃ the wax is related to a hysteresis effect during the heating-cooling cycle Wax is the nucleation center of “bee Fischer et al. [159] Q8 asphalt Thermal cycles structure”, and asphaltene fraction is Tapping mode (2013) (PEN:70/100) 30-65℃ the nucleation sites for crystals and aggregates Room 4 asphalts Soenen et al. [160] temperature, (PEN:70/100, different Tapping mode Wax crystals (2014) Thermal cycles origins) 25-80℃ Yang et al. [161] Room 2 SBS modified asphalt Tapping mode Wax crystals (2015) temperature Chen et al. [162] Room Asphaltenes and microcrystalline Asphalt (PEN:70/80) Tapping mode (2018) temperature wax Room Blom et al. [150] 5 asphalts (different Tapping-Mode temperature Paraffin wax crystals (2018) wax content) 20℃ Note: PEN represents penetration grade, 0.1mm

2) Aging recognition In general, asphalt pavement is subject to aging due to long-term effects of natural factors. The surface features of the asphalt binder changed after aging, which can reflect the aging characteristics. AFM is a nondestructive measurement for obtaining the surface microstructure and microphysical parameters. Therefore, it became an effective tool to study the microstructure of asphalt during different aging processes. Many scholars have confirmed that aging had a significant effect on the spatial variations of the sample mechanical properties [163,164]. Xu et al. [165] analyzed three different kinds of asphalt binder (50#-PEN:40/60, 70#-PEN:60/80, and 90#-PEN:80/100) and found that the bee-like structures in 50# asphalt binder became more but shorter with an increase in PAV aging duration. The micro-topography of 70# and 90# asphalt binder did not appear to change substantially (see Fig.11). Wu et al. [166] attributed the increasing bee structures to the generation of asphaltene-micelle structures during aging. Zhang et al. [167] found that the nano‐ parameters of bitumen (e.g., the area ratio of bee‐ like structure, roughness, and maximum amplitude) shown a trend of increasing gradually during aging, but the increasing rates become smaller and smaller.

Fig. 11. Variation trends of bee structures of 50#, 70#, and 90# asphalt binder. [165]

3）Molecular-level properties evaluation. AFM can provide asphalt binder surface morphology and mechanical properties data at nanoscale levels [168,169]. A surprising correlation between the chemical and mechanical properties was found among various asphalt binder phases [170-174]. Munir D. et al. [175] explored the effects of three different warm mix asphalt (WMA) additives on the nanostructure, adhesive, and cohesive properties of asphalt binder. It was found that the effects of WMA additives on bee structure were different. The adhesion forces of the asphalt binder increased with the addition of WMA additives prior to moisture conditioning. By using three-dimensional AFM images, Zhang et al. [176] found that the convex structure was uniformly distributed in the continuous phase of the virgin asphalt, whereas convex structure agglomerated to varying degrees in the modified asphalt. Therefore, AFM can be used to identifying different additives effects based on the changes in different phases. Macroscopic mechanical properties of asphalt binder (e.g., stiffness, viscoelasticity, plasticity) and rheological properties can also be investigated by using AFM. Table 4 showed that the moduli of the flat area on the asphalt surface (valley area) with different aging durations were smaller than that of the bee structure area, while the maximum adhesion force showed an opposite trend. The changes in the chemical compounds during aging can affect the mechanical properties of the different domain. The aging process led to an association of asphaltenes and formed asphaltene micelle structure [177]. The increased micelle structure formed a network in the asphalt binder, which improved the softening point and viscosity of asphalt [178]. In addition, a tighter bond between the probe and asphalt can be

found due to the negative charges carried by asphaltenes which increased the attractive force between silicon probe ions and asphaltenes [179]. Pauli et al. [180] indicated that crystallization was a primary mechanism affecting asphalt low-temperature properties. The shape and size of the wax crystals vary with the type of asphalt as well as temperature history [181]. Therefore, the detection of wax crystals in asphalt by AFM can help researchers better predict the low- and mid-temperature properties of asphalt binder. Table. 4. A summary of the mechanical properties of asphalt binder with different aging method Maximum adhesion force Modulus (MPa) (nN) Asphalt AFM Researches Aging method binder mode Valley Bee Valley Bee area structure area structure PEN:40/60 12.11 3.57 73 127 Accelerated Xu et al. Peak-force aging at 65℃ PEN:60/80 12.16 3.22 80 116 [165] QNM for 720 (h) PEN:80/100 12.31 4.93 75 125 RTFOT (at PEN:60/80 8.01 3.98 900 550 163℃ for Wang et al. Peak-force 85min) + PAV [182] QNM SBS modified (at 100℃ for 15.01 11.85 1334 1400 asphalt 20h) Note: Peak Force Quantitative Nano Mechanical (QNM) is a surface force mapping technique with high spatial resolution, which simultaneously measures topography and mechanical property maps of asphalt.

AFM can provide useful information at the micro level. However, in many cases, some factors have greatly influenced the interpretation of images results [155]:  The image shows the asphalt morphology, but whether the microstructure represents the bulk constituents is uncertain;  Hysteresis effects, an increase in temperature can significantly change the appearance of the sample surface [160,183];  Inhomogeneity of asphalt surfaces, the imaged area may not represent the overall surface characteristics;  Experimental/instrumental factors, including contamination of the cantilever or probe tip, variations in setpoints, and incorrectly set gains in the feedback loop can cause significant changes in the image. After repeated scans, anomalies such as the apparent phase inversion, the changes in the resolution [184] and the disappearance of the bee-structures [148] are often due to these factors;  Sample variables (e.g., film thickness, solution concentration, and solvent selection) significantly affect imaging results. Sometimes no visible microstructure changes can be obtained. Some concluded that the bee structure sizes vary from asphalt to asphalt and differences in sensitivity to short-term aging [185]. While others believed that aging process features phase scattering, gathering, and disintegration. Asphalt binder turned to a single-phase material [162] or an ideally mixed state during aging [186,187]. The experimental findings of Nahar underpin this conclusion, the microstructure of different asphalt samples gradually disappeared with the increase of temperature, and the complete “melting” of these microstructures occurred above a certain temperature (60-90°C) [187]. 4. Summary and recommendation for future work Previous studies strongly support the view that the properties of the asphalt binder depend on its microstructure. The advents of material characterization promote the study and quantify the influence of such chemical structure on the performance of asphalt binder at the micro and macroscopic levels. This review looks back to the application of four microscopic testing approaches (FTIR, NMR, GPC,

and AFM) in exploring microstructure of asphalt binder and mechanical properties. The following conclusions are summarized: 1) FTIR can be used for identifying asphalt binder functional groups and determining the molecular structure of the asphalt binder. By the determination of various chemical functional groups in an asphalt binder, its origin and modifier can be recognized. By identifying the differences in the absorption peaks of the asphalt compositions and functional groups in an asphalt binder, the modification mechanism can be analyzed. Also, FTIR can rapidly evaluate the aging degree of asphalt by determining the proportion of oxidation products. Thus, FTIR results could be employed to find the influence of chemical groups on the microstructure and properties of asphalt binders through appropriate statistical treatment. 2) The average structural parameters of the asphalt binders can be obtained by combining NMR and GPC. Besides, the mechanisms of aging can be inferred by LF-NMR. The Relative Hydrogen Index (RHI) obtained from LF-NMR can be used to evaluate the aging behavior of the asphalt binder, and the T2 shifts express the change in the asphalt binder viscosity. 3) GPC is a useful tool for predicting molecular model of asphalt and studying the performance of asphalt and its mix. The change in molecular weight distribution is related to the change of properties of asphalt binder (e.g., aging, rheological properties) and microstructure (e.g., dissociation, isomerization, and fragmentation). However, the GPC results in an error when the molecular weight difference of the tested object is small. So, a new parameter called hereafter the aging molecular distribution shift (AMDS) is introduced to overcome these difficulties. 4) AFM is a useful tool for understanding the microstructure and performance characteristics of the asphalt binder. From image information, asphalt's morphology could be divided into four phases: para, sal, peri, and catana (bee-structure). Aging can significantly increase the spatial variation of sample mechanical properties, especially the change of bee-structure. Also, the mechanical properties of asphalt binder (e.g., elastic modulus, hardness, adhesion, and healing properties) are related to multiple asphalt phases. Moreover, it is worth noting that some technical difficulties, such as tip contamination, sample preparation, hysteresis effects, and data acquisition, etc., will lead to image interpretation. Table 5 summarizes the advantages and disadvantages of microscopic testing approaches for understanding the microstructure of asphalt binder from parts of past studies. Table. 5 Summaries of the microscopic testing approaches. Technique Index Advantage Fourier Functional group All wavelengths are collected Transform simultaneously Infrared Spectroscopy (FTIR) Nuclear Structural 1. Ability to simultaneously detect Magnetic parameters and quantify multiple components in a Resonance single spectrum (NMR) 2. Identifying aging trends in asphalt cores without binder extraction (LF-NMR) Gel Permeation Molecular weight 1. GPC can perform detailed Chromatography structural analysis by molecular size (GPC) 2. Quick and relatively easy to estimate molecular weight and distribution Atomic Force Micromorphology 1. 3D surface profile Microscopy & Mechanical 2. Atomic resolution (AFM) properties

Disadvantage 1. Most asphalts give a similar spectrum 2. Baseline correction is needed to make data 1. High requirement for sample uniformity 2. Unable to provide any spatial resolutions close to the feature size present in asphalt 1. Some asphalt compositions are easily combined in solution 2. Dissolution in a solvent may induce important structural modifications 1. Single scan image size 2. Many factors can cause the error

The characterization of the asphalt binder properties at nano and micro scales is an indispensable research method to obtain asphalt fingerprints, from the colloidal structure and molecular interaction

to mechanical properties. Although these outcomes are very encouraging, the study in asphalt microscopy has some defects and still needs further investigation. There are still many questions about the relationship between the microscopic observation results and the macroscopic properties of the asphalt binder. On the one hand, due to the complex chemistry of asphalt binder, current microscopic testing approaches have limitations in the chemical composition characterization of the asphalt binder. On the other hand, the average results provided by advanced modern analytical techniques cannot directly represent the physical properties or performance properties. Combining microscopic testing approaches with computer simulation technology and statistical algorithm may present an avenue to linking the microstructures to macro-properties of asphalt binder. Based on these relationships, the physical, rheological, mechanical behavior of the asphalt binder can be estimated by its chemical composition and structure. With this knowledge, the fundamental transformation of the materials development mode becomes possible, such as the directional modification of the asphalt binder properties or the derivation of the requirements for additives. Furthermore, the aging behavior of asphalt binder can be estimated and thus, the durability of asphalt pavements predicted. The relationships concerning adhesion behavior allow a targeted selection of asphalt binder and aggregates to produce durable asphalt mixture.

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