**1. Introduction**

Asphalt concrete, due to its advantages of easy construction, driving comfort and low noise, is widely used as a paving material throughout the world [1,2]. The appearance of asphalt concrete distress is mainly due to the occurrence of aging under the conditions of heat, oxygen, ultraviolet and water [3]. Thermal-oxygen aging occurs mainly during the construction period, including mixing, paving, etc. Its mechanism mainly includes the volatilization and oxidation of light components, the condensation of saturates and the degradation of macromolecules in asphalt [4]. UV radiation has the most significant effect on asphalt concrete aging during service. The molecular chains in asphalt absorb enough energy from the UV wavelengths to cause the bonds to break [5]. Additionally, water and the oxygen in the water can cause the alternating occurrence of asphalt oxidation, dissolution and migration [6]. In different regions with different climatic conditions, asphalt concretes are subjected to the diverse service environment [7]. Asphalt concrete in different areas are exposed to various corrosive media, such as salt, acid and alkali [8–11]. Asphalt

**Citation:** Zou, Y.; Pang, L.; Xu, S.; Wu, S.; Yuan, M.; Amirkhanian, S.; Xu, H.; Lv, Y.; Gao, X. Investigation of the Rheological Properties and Chemical Structure of Asphalt under Multiple Aging Conditions of Heat, UV and Aqueous Solution. *Materials* **2022**, *15*, 5711. https://doi.org/10.3390/ ma15165711

Academic Editor: Simon Hesp

Received: 20 July 2022 Accepted: 15 August 2022 Published: 19 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

aging is the main cause of asphalt concrete aging [12]. Therefore, there are a large number of studies are concerned with the aging of asphalt, but preliminary research mainly focused on single factor of asphalt aging due to the complexity of asphalt composition [13–15].

With the deepening of research, multiple factors are gradually being considered in the simulation of asphalt aging. Abouelsaad and White pointed out that the performance of hot mix asphalt mixture gradually weakened with the increase of coupling aging time [16]. Tan and Li investigated the coupled effect of thermal-oxygen–UV on asphalt and found the effect of coupled aging was more obvious than thermal-oxygen aging, which showed a rapid decay of asphalt properties [17]. Li et al. supposed that the coupled ageing mechanism of UV and different aqueous solution mainly includes three channels, including the excitation and break of asphalt molecules and the dissolution and separation of the organic component in asphalt [18]. It was Ilaria and Eyad who found that the combined effect of UV–heat–oxygen–moisture caused a portion of the asphalt to become soluble and dissolved, while the rest of the asphalt showed cracking [19]. Zhang et al. concluded that coupled aging effect of heat–UV–water was considered to have a significant effect on the viscoelasticity and high-temperature performance of warm mix asphalt [20]. These atmospheric factors had a very important role in the degradation and microstructural evolution of asphalt. However, the above studies simulate the simultaneous action of multiple factors on asphalt, and the interaction and contribution of each factor is not clear. Because the aging mechanism of the three kinds of aging methods is various, so is the effect behavior on properties of asphalt. The study of asphalt aging mechanism is important not only for predicting the longevity of asphalt, but information about asphalt aging can help correctly restore asphalt properties with recycled asphalt pavement (RAP). The industries all over the world are looking for solutions to use 100% RAP, which can significantly improve economic and ecological outcomes [21]. Therefore, it is of great interest to investigate the aging performance of asphalt under multiple conditions of heat, UV and aqueous solution.

Changes in asphalt properties are mainly caused by changes in the internal chemical composition and structure. The combination of EA and FTIR can complement each other and facilitate a more accurate analysis of the microstructure of asphalt after aging [22]. Therefore, the aging performance of asphalt under multiple conditions of heat, UV and aqueous solution was investigated in this study using the characterization of the rheological properties, element composition and chemical structure of asphalt, and the performance evolution law was discussed. The research program is illustrated in Figure 1. This study adopted sequentially Thin Film Oven Test (TFOT), UV aging test and hydrostatic immersion test to conduct different aging test on 70 A. The multiple aging conditions are divided into three levels, including heat–UV coupling, UV–solution coupling and UV–water cycle. After aging, Dynamic Shear Rheometer (DSR) test and Bending Beam Rheometer (BBR) test were employed to observe the change of rheological properties, including high-temperature rutting resistance and low-temperature cracking resistance. Additionally, the major element composition and the characteristic functional groups of asphalt were detected by EA test and FTIR test, respectively. The results of EA test and FTIR test were combined to analyze the changes in chemical composition before and after aging and to investigate the aging mechanism of the properties changes.

**Figure 1.** Research program. **Figure 1.** Research program.

#### **2. Materials and Experiments 2. Materials and Experiments**

*2.1. Materials*

*2.1. Materials* 2.1.1. Asphalt

2.1.1. Asphalt Base asphalt with 60/80 penetration grade (simply referred as 70 A) employed in this study was obtained from Hubei Guochuang Road Material Technology Co., Ltd. (Wuhan, Base asphalt with 60/80 penetration grade (simply referred as 70 A) employed in this study was obtained from Hubei Guochuang Road Material Technology Co., Ltd. (Wuhan, China). The basic physical properties were illustrated in Table 1.

China). The basic physical properties were illustrated in Table 1. **Table 1.** Physical properties of 70 A.


Ductility (10 °C/5 °C) cm >100 ASTM D-113 [25] 2.1.2. Preparation of Aqueous Solution

Solubility (trichloroethylene) % 99.5 ASTM D-2042 [26] 2.1.2. Preparation of Aqueous Solution Four kinds of aqueous solutions with different media were prepared, including distilled water, 10 wt% NaCl saline solution, pH 3 acid solution and pH 11 alkali solution, to simulate asphalt immersed in various aqueous solution. The 10 wt% NaCl saline solution Four kinds of aqueous solutions with different media were prepared, including distilled water, 10 wt% NaCl saline solution, pH 3 acid solution and pH 11 alkali solution, to simulate asphalt immersed in various aqueous solution. The 10 wt% NaCl saline solution was obtained by dissolving solid sodium chloride with distilled water. The distilled water was used to dilute the mixed solution of sulfuric acid and nitric acid with a molar ratio of 9:1, and the pH 11 alkali solution was prepared by dissolving a certain amount of solid sodium hydroxide. The pH value of which was monitored by a precision pH meter.

was obtained by dissolving solid sodium chloride with distilled water. The distilled water was used to dilute the mixed solution of sulfuric acid and nitric acid with a molar ratio of 9:1, and the pH 11 alkali solution was prepared by dissolving a certain amount of solid

#### *2.2. Aging Simulation Test of Asphalt 2.2. Aging Simulation Test of Asphalt*

Figure 2 illustrates the sample preparation procedure under the multiple conditions of heat, UV and aqueous solution following six steps: Figure 2 illustrates the sample preparation procedure under the multiple conditions of heat, UV and aqueous solution following six steps:


**Figure 2.** Sample preparation procedure. **Figure 2.** Sample preparation procedure.

#### *2.3. Characterization of Asphalt 2.3. Characterization of Asphalt*

The rheological properties, chemical structure and element composition of asphalt were characterized by DSR, BBR, EA and FTIR, and the related information of the instruments as shown in the Table 2. These tests were performed in accordance with relevant specifications. The rheological properties, chemical structure and element composition of asphalt were characterized by DSR, BBR, EA and FTIR, and the related information of the instruments as shown in the Table 2. These tests were performed in accordance with relevant specifications.

BBR [28]

DSR [27]

EA [29]

FTIR [30]

FTIR [30]


Smartpave 102 Stain: 0.5%

Ostfildern, Germany

Shanghai Changji Geological

Smartpave 102 Stain: 0.5%

Anton Paar Co., Ltd. Temperature: 30–80 °C

SYD-0627 Load: 980 ± 50 mN

Instrument Co., Ltd. Temperature: −6, −12, −18 °C

Shanghai, China Span length: 102 mm

Frequency: 10 rad/s

Heating rate: 2 °C/min

Plate diameter: 25 mm

Plate gap: 1 mm

Langenselbold, Germany

#### **Table 2.** Relevant information of instruments. Frequency: 10 rad/s Anton Paar Co., Ltd. Temperature: 30–80 °C

*Materials* **2022**, *15*, x FOR PEER REVIEW 5 of 20

**Test Instruments Origin Test Parameters**

**Table 2.** Relevant information of instruments.

*Materials* **2022**, *15*, x FOR PEER REVIEW 5 of 20

**Test Instruments Origin Test Parameters**

*Materials* **2022**, *15*, x FOR PEER REVIEW 5 of 20

**Table 2.** Relevant information of instruments.

**Table 2.** Relevant information of instruments.

#### 2.3.1. DSR Test 2.3.1. DSR Test

ment performance of asphalt mixture [31]. The high-temperature rheological property of asphalt samples was characterized by a temperature sweep using DSR with strain control mode. About 0.8 g of asphalt sample was used to prepare the cylinder with a diameter of 25 mm and a height of 1 mm. The complex modulus (G\*) and phase angle (δ) of asphalt are the main parameters. The high-temperature rheological property is usually

Nicolet 6700 Chip: KBr Thermo Fisher Scientifific Scanning range: 4000–400 cm−<sup>1</sup> Waltham, MA, USA Scan time: 64 times 2.3.1. DSR Test As a kind of viscoelastic material, asphalt is sensitive to changes in temperature and load. The rheological property of asphalt has a vital effect on its processability and pavement performance of asphalt mixture [31]. The high-temperature rheological property of asphalt samples was characterized by a temperature sweep using DSR with strain control mode. About 0.8 g of asphalt sample was used to prepare the cylinder with a diameter of 25 mm and a height of 1 mm. The complex modulus (G\*) and phase angle (δ) of asphalt are the main parameters. The high-temperature rheological property is usually Nicolet 6700 Chip: KBr Thermo Fisher Scientifific Scanning range: 4000–400 cm−<sup>1</sup> Waltham, MA, USA Scan time: 64 times 2.3.1. DSR Test As a kind of viscoelastic material, asphalt is sensitive to changes in temperature and load. The rheological property of asphalt has a vital effect on its processability and pavement performance of asphalt mixture [31]. The high-temperature rheological property of asphalt samples was characterized by a temperature sweep using DSR with strain control mode. About 0.8 g of asphalt sample was used to prepare the cylinder with a diameter of 25 mm and a height of 1 mm. The complex modulus (G\*) and phase angle (δ) of asphalt are the main parameters. The high-temperature rheological property is usually As a kind of viscoelastic material, asphalt is sensitive to changes in temperature and load. The rheological property of asphalt has a vital effect on its processability and pavement performance of asphalt mixture [31]. The high-temperature rheological property of asphalt samples was characterized by a temperature sweep using DSR with strain control mode. About 0.8 g of asphalt sample was used to prepare the cylinder with a diameter of 25 mm and a height of 1 mm. The complex modulus (G\*) and phase angle (δ) of asphalt are the main parameters. The high-temperature rheological property is usually As a kind of viscoelastic material, asphalt is sensitive to changes in temperature and load. The rheological property of asphalt has a vital effect on its processability and pavement performance of asphalt mixture [31]. The high-temperature rheological property of asphalt samples was characterized by a temperature sweep using DSR with strain control mode. About 0.8 g of asphalt sample was used to prepare the cylinder with a diameter of 25 mm and a height of 1 mm. The complex modulus (G\*) and phase angle (δ) of asphalt are the main parameters. The high-temperature rheological property is usually characterized by the rutting factor (G\*/sinδ) to evaluate the rutting resistance [32]. The ratio of G\*/sinδ before and after aging is defined as the Rutting factor Aging Index (RAI), which can quantify the influence of aging conditions on the high-temperature performance of asphalt, as shown in Equation (1) [33].

Mode: C/H/N/S

$$\text{RAI} = \frac{\text{G}\_{\text{aging}}^{\*} / \sin \delta\_{\text{aging}}}{\text{G}\_{\text{viring}}^{\*} / \sin \delta\_{\text{viring}}} \tag{1}$$

where G\*virgin and G\*aging represent the G\* of asphalt before and after aging, respectively, Pa; and δvirgin and δaging represent the δ of asphalt before and after aging, respectively. The greater the RAI, the more distinct the aging effect.

#### 2.3.2. BBR Test

As the evaluation parameters of cracking resistance, the creep Stiffness modulus (S) and creep rate (m-value) of asphalt were obtained by BBR test at low temperature, as shown in Equations (2) and (3) [34]. The sample size was 127 mm × 12.7 mm × 6.35 mm. The higher the S, the worse the low temperature ductility. The m-value represents the change rate of S, and the greater the value, the higher the relaxation rate and the more excellent the low temperature performance.

$$S(t) = \frac{PL^3}{4bh^3\Delta(t)}\tag{2}$$

$$m(t) = B + 2\mathcal{C}[\lg(t)]^2\tag{3}$$

where *S*(*t*) represents the creep stiffness at 60 s, MPa; *P* represents the test load, mN; *L*, *b* and *h* represent the span length, width and depth of samples, mm; ∆(*t*) represents the

deflection of samples at 60 s; and *B* and *C* represent the regression coefficients of lg[*S*(*t*)] and lg[*m*(*t*)].

#### 2.3.3. EA Test

The variation of the element composition of asphalt was detected by elemental analyzer under multiple condition. The analysis principle is dynamic adsorption–desorption TCD measurement program [35]. The C, H, N, S and O elements are the main elements of asphalt, and the test mode of C, H, N and S elements was selected. The asphalt with a weight of 5 mg is burned under pure oxygen conditions, and then the gas produced by the combustion is measured. After homogenization, the gas is separated in chromatography within the separation zone and finally measured in the detection zone. Since the content of hetero atoms in asphalt is too low to be ignored, the content of O is obtained by subtracting the content of C, H, N and S elements from 100%.

The density method was employed to analyze the structural composition of asphalt. Three important indexes, the molar ratio of hydrogen–carbon (n(H)/n(C)), aromatic carbon ratio (*fA*) and condensation index (*C<sup>I</sup>* ), were selected. The larger the n(H)/n(C), the more saturated hydrocarbons in the asphalt and the less the aging degree and vice versa, as shown in Equation (4) [36].

$$\ln(\text{H})/\text{n(C)} = 11.92 \times \left[ \omega(\text{H})/\omega(\text{C}) \right] \tag{4}$$

where ω(H) and ω(C) represent the mass fraction of hydrogen and carbon atoms in all elements of asphalt, respectively, %.

The *f<sup>A</sup>* reflects the ratio of aromatic carbon atoms to total carbon atoms. The larger the *fA*, the more ring structure, especially the aromatic rings, which means that there are more macromolecules and a greater degree of aging, as shown in Equations (5)–(8).

$$
\rho = 1.4673 - 0.0431\omega(\text{H})\tag{5}
$$

$$M\_{\mathbf{c}/\rho} = 1201/[\rho \times \omega(\mathbf{C})] \tag{6}$$

$$\left(M\_{\mathbf{c}/\rho}\right)\_{\mathbf{c}} = M\_{\mathbf{c}/\rho} - 6[100 - \omega(\mathbf{C}) - \omega(\mathbf{H})]/\omega(\mathbf{C})\tag{7}$$

$$f\_A = 0.09 \left( M\_{c/\rho} \right)\_\mathcal{c} - 1.15 \text{n(H)} / \text{n(C)} + 0.77 \tag{8}$$

where *ρ* represents the density of asphalt, g/cm<sup>3</sup> , and *Mc*/[*yellow*]*<sup>ρ</sup>* and (*Mc*/*<sup>ρ</sup>* ) *c* represent the molar volume of individual carbon atom before and after correction, respectively, L/mol.

The *C<sup>I</sup>* represents the molecule condensation degree, and the larger the *C<sup>I</sup>* , the greater the molecule condensation degree and the more complex the ring structure, as shown in Equation (9).

$$\mathbf{C}\_{I} = \mathbf{2} - \mathbf{n}(\mathbf{H})/\mathbf{n}(\mathbf{C}) - f\_{A} \tag{9}$$

#### 2.3.4. FTIR Test

Under the multiple aging condition, the chemical structure of asphalt was detected by FTIR with OMNIC 6.2 software (Thermo Fisher). Firstly, asphalt with a weight of 0.1 g was dissolved by CS<sup>2</sup> to prepare 5 wt% asphalt–CS<sup>2</sup> solution. Then, two drops were placed on the KBr chip with a glue dropper, and the CS<sup>2</sup> was completely vaporized by infrared light. Finally, the prepared samples were undertaken for the FTIR test. The carbonyl group and sulfoxide group are the main products of asphalt oxidation, and their indexes (*IC*=*<sup>O</sup>* and *IS*=*O*) are usually used to quantify the aging degree of asphalt, which can be calculated using Equations (10) and (11) [37].

$$I\_{\mathbb{C}=O} = A\_{1700\text{cm}^{-1}} / \Sigma A\_{2000-600\text{cm}^{-1}} \tag{10}$$

$$I\_{\rm S=O} = A\_{1030\text{cm}^{-1}} / \Sigma A\_{2000-600\text{cm}^{-1}} \tag{11}$$

where *A*1700cm <sup>−</sup><sup>1</sup> and *A*1030cm <sup>−</sup><sup>1</sup> represent the area of carbonyl group and sulfoxide group at the 1700 cm−<sup>1</sup> and 1030 cm−<sup>1</sup> , respectively, and <sup>Σ</sup>*A*2000−600cm <sup>−</sup><sup>1</sup> represents the total area from 2000 cm−<sup>1</sup> to 600 cm−<sup>1</sup> . The higher the indexes, the greater the degree of aging.

#### **3. Results and Discussion**
