Rheological Deterioration of High Viscosity High Elasticity Asphalt (HVEA) Under the Coupling Effect UV Aging and Salt Freeze-Thaw (SFT) Cycles

Abstract

To investigate the deterioration pattern of the rheological properties of high-viscosity high-elasticity asphalt (HVEA) under UV and salt freeze–thaw (SFT) cycle environments, two snowmelt salts were used for coupled aging tests, along with temperature sweep, bending beam rheological (BBR), and Fourier-transform infrared spectroscopy (FT-IR) tests. The results showed that both snowmelt salts could enhance the high-temperature rutting resistance of HVEA, in which the enhancement effect of NaCl was more significant. With the increase in salt concentration, the BBR stiffness of HVEA decreased and then increased, while the m-value showed the opposite trend, indicating that the addition of snowmelt salt impaired its low-temperature creep performance. Additionally, UV-SFT aging would exacerbate the degradation of low-temperature crack resistance. The temperature sensitivity of HVEA gradually decreased with the drop of viscosity temperature sensitivity (VTS) value; salt corrosion further significantly reduced its temperature sensitivity. UV-SFT aging would significantly weaken fatigue performance of HVEA, especially after 15 cycles. FT-IR test showed that UV-SFT resulted in the enhancement of S=O and C=C characteristic peaks, suggesting that the HVEA underwent oxidization and chemical aging, which increased the low-temperature brittleness.

1. Introduction

As a high-performance asphalt material, high-viscosity high-elasticity asphalt (HVEA) had attracted much attention in the field of road engineering in recent years [1,2]. Compared with conventional asphalt binders, HVEA exhibited excellent rheological properties. For example, its dynamic viscosity at 60 °C can reach tens of thousands of Pa·s (much higher than ordinary modified asphalt), while it had enhanced elastic recovery and deformation resistance, which made HVEA particularly suitable for drainage asphalt pavements, steel bridge decks, and other demanding engineering scenarios. It can also provide excellent rutting resistance, fatigue resistance, and resistance to water damage [3,4,5].

However, the performance of asphalt materials, as typical viscoelastic substances, was highly dependent on environmental conditions [6]. In real service environments, HVEA was inevitably exposed to the coupled effects of several environmental factors at the same time, especially the synergistic effects of UV radiation and salt freeze–thaw cycles. At present, many scholars had unilaterally studied the effects of UV or salt freeze–thaw cycles on the rheological properties of asphalt binder. Li et al. [7] presented the latest research on the UV aging mechanism and anti-aging methods of asphalt binders, pointing out that UV aging of asphalt developed from the surface to the interior over time, and that UV aging resistance of asphalt can be improved by adopting a low penetration asphalt, reducing the void ratio, and using UV stabilizers/absorbers. Feng et al. [8] prepared a variety of anti-aging additives, producing asphalt modified with ultraviolet absorber, antioxidant, and their combination, and investigated its physical properties and aging characteristics. The results showed that the addition of anti-aging agents can improve the toughness of asphalt binder material, and at the same time reduce its softening point and viscosity. The UV absorber and antioxidant can simultaneously improve the thermo-oxidative and photo-oxidative aging properties of asphalt. Yu et al. [9] evaluated the effect of UV radiation on the aging properties of Styrene-Butadiene-Styrene-modified asphalt (SBSMA) binder, and the results showed that UV radiation can seriously damage the network structure of crosslinking in SBSMA and lead to a significant change in its rheological properties. Guo et al. [10] evaluated the feasibility of carbon black (CB) and hindered amine light stabilizers (HALS) as UV additives, and found that UV additives can slow down the aging of asphalt binder. CB, which accounted for 3% of the asphalt mass, significantly and adversely affected the performance of the original asphalt binder, while HALS had good UV-protection properties and had less effect on the original asphalt binder. Li et al. [11] reported the effect of the main wavelength of UV radiation on the aging degradation of asphalt binder material, pointing out that in the wavelength range of 300-380 nm, all UV radiation would lead to pure asphalt aging degradation, and UV aging would change the asphalt’s chemical composition, and at the same time, affect the physical, rheological, and other macroscopic properties of asphalt. Sun et al. [12] investigated the effect of nano-polymer materials on the UV aging performance of asphalt binders, and found that with the extension of UV aging time, the nano-polyamide (nano-PA) modifier can effectively alleviate the aging effect of UV on asphalt binders. Specifically, the PA modifier could enhance the viscoelasticity and thermal stability of asphalt under UV irradiation, inhibit the generation of microcracks on the surface of asphalt, and restrain the generation of carbonyl and sulfoxide groups in asphalt. In summary, it can be seen that the aging effect of UV radiation on asphalt was severe, but most scholars had not carried out UV aging research on the synergistic effect of multi-environmental factors in the actual service scenarios, and there was a lack of connection between the research and the actual multi-factor coupled aging environment.

On the other hand, a few scholars had carried out the decay of asphalt properties under the influence of salt freeze–thaw cycles (SFT). In a study by Pangarova et al. [13], conventional asphalt (SBSMA blended with amine anti-strippers) was subjected to 20wt% concentration of snowmelt salt and tested with 20 SFT cycles, which showed that the combination of the salt solution and freeze–thaw cycles had a destructive effect on the low-temperature performance of the asphalt. Zhang et al. [14] analyzed the effect of salt corrosion environment on asphalt properties by four-component test, Atomic Force Microscope (AFM), and asphalt conventional performance tests, and the results showed that under the salt corrosion environment, the chemical composition of asphalt binder changed significantly, and the micro-morphological characteristics such as surface roughness and area ratio of honeycomb structure decreased. Its high-temperature performance was slightly improved, but fatigue resistance and low-temperature crack resistance were reduced. Cheng et al. [15] also analyzed the creep characteristics and evolution mechanism of asphalt binders under salt erosion conditions and found that salt erosion decreased the proportion of small molecules in asphalt binders and deteriorated the low-temperature performance. Xiong et al. [16] studied the effect of sodium sulfate SFT on the performance of asphalt binder, and found that the internal erosion of asphalt reduced its adhesion, deteriorated the low temperature rheological properties, and concluded that the “salt aging” effect was the main cause of performance deterioration. Wei et al. [17] mechanistically investigated the effect of SFT on asphalt properties and proposed the conclusion that oxidation and polymerization by SFT cycles altered asphalt sulfoxide, aromatic, aliphatic, and sulfonate functional group contents, with no significant change in carbonyl groups. In short, SFT cycles would accelerate the development of damage in asphalt binders.

Although studies had been conducted to investigate the effects of UV aging or SFT cycling on asphalt properties separately, the understanding of the coupled effects of these two factors was still very limited. This study breaks through the research framework of a single aging factor and systematically reveals the synergistic degradation mechanism of HVEA rheological properties by the coupling of UV aging and SFT cycles from both macroscopic and microscopic perspectives, which enriches the theoretical system of asphalt materials coupled with multi-environmental aging factors.

In this study, high-viscosity high-elastic asphalt (HVEA) was prepared, and two snowmelt salts were selected to design the coupling test of UV and salt freeze–thaw (SFT) cycles; in addition, temperature sweep, bending beam rheometer (BBR), and Fourier transform infrared spectroscopy (FT-IR) tests were carried out in order to investigate the deterioration laws of the rheological properties of HVEA.

First, the “Materials and Methods” Section specifies the core materials and details the test program. Subsequently, the “Results and Discussion” Section presents the variation patterns of HVEA’s rheological properties under different UV-SFT coupling conditions Finally, the “Conclusion” Section summarizes the core conclusions of the HVEA’s rheological degradation under coupling.

2. Materials and Methods

2.1. Raw Materials

The matrix asphalt selected for this study was a SK70# asphalt binder, whose basic performance indexes are shown in

Table 1. The main technical indicators of matrix asphalt binder SK70#.

Item		Required Index by Standard	Measured Value
25 °C Penetration (0.1 mm)		60–80	64
Softening point (°C)		$/<$46.0	47.5
15 °C Ductility (cm)		$/<$100	>100
Kinetic viscosity 60 °C (Pa·s)		$/<$180	219
Flash point (°C)		$/<$260	282
Wax content (%)		$/>$2.2	1.4
TFOT	Loss of mass (%)	$/>$±0.8	0.03
	Penetration ratio (%)	$/<$65	67.9
	Residual ductility (10 °C, cm)	$/<$6.0	7.6

.

The elastomer used in this study was compounded from a variety of polymers, and the modified asphalt prepared in this way provided superior high-temperature resistance to deformation, low-temperature resistance to cracking, as well as temperature-sensitive properties, which can improve the asphalt binders’ aging resistance and storage stability. Its technical indicators are shown in

Table 2. The main technical indicators of elastomer.

Indicators	Melt Index (g/10 min)	Specific Gravity (g/cm3)	Elongation (%)	Tensile Strength (MPa)
Measured value	78.3	0.96	790	27.9

.

To produce asphalt and aggregate with excellent adhesion, adding a certain amount of viscosity enhancer was required. The selected tackifier was C₅ hydrogenated resin [18], and its technical specifications are shown in

Table 3. The main technical indicators of C₅ hydrogenated resin.

Indicators	Density (kg/m3)	Flash Point (°C)	Pour Point (°C)	Kinematic Viscosity (mm2/s)	Aniline Point (°C)	Evaporation Loss (%)
Measured value	896.7	192	-30	40.12	93.5	2.2

.

The oil compatibilizer improved the elastomer and asphalt solubility at the same time, as it introduced small molecules into the modified asphalt system to improve the low-temperature performance of asphalt, reduce the consistency and hardness of asphalt, and play a certain plasticizing effect [19,20]. The compatibilizer used in this study was furfural oil, and its technical indicators are shown in

Table 4. The main technical indicators of furfural oil.

Indicators	Specific Gravity	Motion Viscosity (mm2/s at 100 °C)	Flash Point (°C)	Pour Point (°C)	Color Chromaticity	Ash (%)	Aniline Point (°C)
Measured value	1.015	21	226	12	4.6	0.025	50

. The stabilizer of this study was sulfur.

The snow-melting salts used in the study were lower cost and commonly used chlorinated salt-type snow-melting agents, including NaCl and CaCl₂, in concentrations of 3%, 6%, 9%, 12%. The concentration of salts was specifically calculated as the mass of salts divided by the total mass of salts and fresh water.

2.2. HVEA Preparation Process

Firstly, the matrix asphalt was placed in a 150 °C oven (Suzhou Yinbang Energy saving Electric Heating Equipment Co., Ltd., Suzhou, China) and heated to a fluid state. Subsequently, it was transferred to a heating mantle (170 °C) for heat preservation, and the cleaned shearing machine was immersed in the matrix asphalt. The weighed C₅ resin was added into the asphalt, after which the rotating speed of the shearing machine (Hangzhou Jintao Instrument Co., Ltd., Hangzhou, China) was adjusted to 4500 r/min, and shearing was conducted for 5 min. Then, the weighed elastomer was added into the asphalt, and shearing was continued at the same rotating speed for 20 min. Next, the temperature of the heating mantle was adjusted to 175 °C, and the rotating speed was gradually increased to 5000 r/min. Under these temperature and rotating speed conditions, the compatibilizer and stabilizer were added, and shearing was performed for 5 min, respectively. Finally, the preparation of HVEA was completed.

Referring to [20,21], the dosages of C₅ resin, elastomer, furfural oil, and sulfur were specified to be 4%, 6%, 3%, and 0.5%, respectively.

2.3. UV Aging–SFT Cycle Coupling Test Scheme Design

A freeze–thaw (FT) cycle process was set up for 18 h, of which the freezing time and melting time were 12 h and 6 h, respectively. As the project site was in the Western Sichuan Plateau, the freezing temperature was set at −20 ± 2 °C and the melting temperature at 30 °C, according to winter temperature conditions. In winter, the asphalt pavement is covered with ice and snow, which requires the spreading of snow-melting agents to melt the ice and snow. Therefore, on the basis of freeze–thaw cycles, consideration also had to be given to the fact that the salt in the snow-melting agents would remain on the road surfaces, thereby exerting an impact on the pavements. Thus, the test was set to be conducted as salt freeze–thaw (SFT) cycles. According to the local situation, the snowfall and rainfall in winter were relatively low, so the number of SFT cycles experienced was relatively small. Thus, the number of SFT was set to 3, 9, and 15 cycles, respectively.

In the indoor ultraviolet (UV) aging test, the light source used was a high-power mercury lamp. The UV radiation intensity of the Western Sichuan Plateau in winter was approximately 40 to 50 mW/cm². Therefore, the UV irradiation intensity was set to 45 mW/cm², and the irradiation time was consistent with the melting time, which was 6 h. Meanwhile, the temperature was maintained at 30 °C to simulate the UV aging effect under sunny conditions. The thickness of the asphalt samples were about 2 mm, since UV aging of the asphalt binder occurred only on the surface of the asphalt. According to the size of the sample disk, the mass of asphalt per disk was about 120 g. The flowchart is shown in Figure 1.

2.4. Characterization

2.4.1. Temperature Sweep (TS) Test

Asphalt samples underwent the TS test employing a dynamic shear rheometer(Abbreviated as DSR, TA Instruments, New Castle, DE, USA). The test was performed with parallel plates measuring 25 mm in diameter and 1 mm in thickness. The temperature varied from 52 °C to 82 °C at 6 °C intervals, and the angular frequency was adjusted to 10 rad/s. The strain was fixed at 12% under the strain control mode based on the results of the strain sweep test. The high temperature rutting resistance of HVEA binders under the influence of UV-SFT was evaluated based on the rutting factor (G*/sinδ) obtained from the TS test.

Studies have demonstrated that viscosity temperature sensitivity (VTS) accurately characterizes the temperature sensitivity of asphalt binders, in contrast to the penetration index obtained via the penetration test. By plotting the relationship between $logT$ and $log{\eta }^{\prime }$ and performing a linear regression, the absolute value of the slope of the regression line was the VTS, and the calculations are shown in Equations (1) and (2).

$${\eta }^{\prime }={\displaystyle \frac{{\left(sin\delta \right)}^{-4.8628\left|G^{*}\right|}}{\omega }}$$

(1)

$$VTS={\displaystyle \frac{\mathit{log}\left(log{\eta }_2\right)-log\left(log{\eta }_1\right)}{log{T}_{k2}-log{T}_{k1}}}$$

(2)

where ${\eta }^{\prime },{\eta }_1,{\eta }_2$ were viscosities, Pa·s; $\delta$ was phase angle, °; $G^{*}$ was complex module, kPa; $\omega$ was angular frequency, rad/s; and $T_{k2},T_{k1}$ were Kelvin temperatures, K.

2.4.2. Bending Beam Rheometer (BBR) Test

The BBR test was employed to evaluate the low-temperature rheological properties of asphalt binder at −12 °C, −18 °C, and −24 °C. Creep rate (m-value) and stiffness (S) were determined in the test.

2.4.3. Fatigue Performance Test

The fatigue factor G*·sinδ obtained from the TS test based on the dynamic shear rheometer (Abbreviated as DSR, TA Instruments, New Castle, DE, USA) was used to evaluate the fatigue resistance characteristics of the samples.

2.4.4. Fourier Transform Infrared Spectroscopy (FT-IR) Test

FT-IR test was conducted by irradiating the material with infrared light of continuously varying frequencies [22,23]. After absorbing infrared radiation of different frequencies, the material molecules underwent energy level transitions, which resulted in changes in the intensity of transmitted light across different regions. The infrared spectrum—used to reflect the characteristics of molecular vibration and rotation—was obtained by plotting the relationship curve between the transmittance of infrared light and its wavenumber or wavelength. Using information such as the position, shape, and peak value of absorption peaks in the infrared spectrum, it was possible not only to characterize the spatial structure of material molecules but also to identify the chemical bonds and functional groups contained in the material. In this present study, the FT-IR test was performed on asphalt samples to evaluate the effect of UV-SFT cycles coupling on HVEA’s properties.

3. Results and Discussion

3.1. High Temperature Rheological Properties

Since the effect of the two salt solutions on the HVEA samples was difficult to observe clearly when the UV-SFT number was three cycles, the results of nine and fifteen cycles were discussed. The rutting factor results when UV-SFT was nine cycles are shown in Figure 2 and Figure 3.

The rutting factor (G*/sinδ) was the core indicator for evaluating the high-temperature rutting resistance of asphalt. The larger its value, the stronger the ability of asphalt to resist permanent deformation under high-temperature conditions [24,25]. It could be observed from Figure 2 that as the test temperature increased, the rutting factors of all HVEA samples exhibited a significant decreasing trend. This directly indicated that the high-temperature rutting resistance of asphalt deteriorated significantly with the rise in temperature, and this result was highly consistent with objective facts. In actual road engineering, asphalt pavements are prone to rutting distress during high-temperature periods in summer due to the decreased deformation resistance of asphalt. In essence, the increase in temperature caused the intensified movement of asphalt molecules and the shift in viscoelastic properties to viscosity dominance, thereby weakening its ability to resist permanent deformation. With the freshwater group (0%wt) as the control, the HVEA samples treated with the NaCl solution UV-SFT cycle had a significantly larger rutting factor than the control group. This phenomenon proved that in the composite deterioration environment of the UV-SFT cycle, the addition of NaCl had instead greatly improved the high-temperature deformation resistance of HVEA. This was because the intervention of NaCl increased the relative content of resins and asphaltenes in HVEA, which were the core carriers of asphalt’s elasticity and adhesion. The increase in their content could enhance the elastic recovery ability of HVEA and reduce the tendency of permanent deformation at high temperatures [26,27,28]. Meanwhile, during the UV-SFT cycle, the NaCl solution underwent phase change due to temperature fluctuations, and the precipitated salt crystal particles would invade the internal microscopic structure of HVEA, embed in the asphalt molecular gaps, and increase the internal frictional resistance of asphalt, thereby further improving its high-temperature deformation resistance. When the temperature did not exceed 70 °C, the G*/sinδ of HVEA showed a wave pattern of increasing, then decreasing, and then slightly increasing with the rising of NaCl concentration. And when the temperature exceeded 70 °C, the G*/sinδ changed with NaCl concentration in an insignificant pattern. The reason was that the excessive temperature led to a sharp increase in the viscosity of the asphalt, the elasticity percentage decreased dramatically, and the overall viscosity-dominated state of “easy flow” was observed. Specifically, the G*/sinδ was highest at 3% NaCl at temperatures less than 70 °C, indicating that high-temperature rutting resistance was the strongest at this time. In the process of increasing NaCl concentration from 3% to 9%, the G*/sinδ showed a decreasing trend, and the high-temperature rutting resistance correspondingly weakened, because the high-concentration NaCl destroyed the colloidal structure stability of asphalt and changed the interaction of its internal components [29,30]. In addition, the difference value of the rutting factor of HVEA samples treated with UV-SFT of different NaCl solution concentrations continuously decreased as the temperature increased. This phenomenon indicated that the degree of influence of the NaCl concentration on the high-temperature performance of asphalt gradually weakened with the rise in temperature.

As can be seen in Figure 3, the G*/sinδ of the HVEA treated with freshwater was significantly smaller than that of the HVEA treated with CaCl₂ solution under UV-SFT cycle. This comparative result clearly confirmed that the incorporation of CaCl₂ could effectively improve the high-temperature rutting resistance of HVEA in this composite deterioration environment. Meanwhile, it was observed that as the test temperature increased gradually, the difference in rutting factor between the control group and the experimental groups showed a continuous decreasing trend, which implied that the increase in temperature would gradually weaken the regulatory effect of snow-melt salt solution concentration on the high-temperature deformation resistance of HVEA. Especially when the temperature reached 82 °C, the difference in rutting factor between the two groups had shrunk to an extremely small level. Furthermore, under the same test temperature condition, as the concentration of CaCl₂ solution increased gradually, the rutting factor of HVEA exhibited a trend of first increasing and then decreasing. Among them, when the concentration of CaCl₂ solution was 6%, the rutting factor of HVEA reached the peak value, and its high-temperature resistance to permanent deformation was at its best. Whereas, when the concentration increased to 12%, the rutting factor of HVEA dropped to the lowest value, and accordingly, its high-temperature rutting resistance was in the worst state.

By comparing Figure 2 and Figure 3, it was found that the rutting factor of HVEA under the NaCl solution condition was larger than that under the CaCl₂ solution condition. This indicated that under the combined conditions of UV-SFT, NaCl had a more significant impact on the high-temperature rutting resistance of HVEA. At the same test temperature, as the concentration of the NaCl solution increased, the rutting factor of HVEA first decreased and then increased. Whereas, as the concentration of the CaCl₂ solution increased, the rutting factor of HVEA first increased and then decreased. It was observed that the rutting factors under these two soaking solutions exhibited opposite changing trends with the variation in solution concentration.

The G*/sinδ results of HVEA samples under different solutions and UV-SFT cycles are shown in Figure 4 and Figure 5.

According to Figure 4, the G*/sinδ of HVEA samples after 9 cycles of UV-SFT aging was smaller than that of HVEA samples after 15 cycles. This indicated that the greater the number of UV-SFT cycles, the better the high-temperature rutting resistance of the HVEA. This was because as the aging duration increased, the degree of erosion of HVEA by salt and UV radiation deepened, and the change in HVEA performance became more significant. When the temperature was not lower than 58 °C, the rutting factor of HVEA samples subjected to 15 UV-SFT cycles changed slightly with the increase in salt solution concentration. This indicated that at higher temperatures, the greater the number of UV-SFT aging cycles, the smaller the impact of salt solution concentration on the high-temperature rutting resistance of the HVEA. Under the condition of nine cycles of UV-SFT aging cycles, the rutting factor reached its maximum when the salt concentration was 3%. Whereas, under the condition of 15 cycles of UV-SFT aging cycles, the rutting factor peaked when the salt concentration was 12%. It was obvious that when the number of aging cycles was 15 cycles, the difference in rutting factor between the NaCl solution group and the water control group was larger than when the number of aging cycles was 9 cycles. This demonstrated that the greater the number of UV-SFT aging cycles, the more significant the impact of NaCl solution on the high-temperature rutting resistance of the HVEA.

Figure 5 indicates that under nine cycles of UV-SFT cycles, at the same temperature, the G*/sinδ of HVEA first increased and then decreased with the rise in solution concentration. In contrast, under 15 cycles of UV-SFT cycles, at the same test temperature, the G*/sinδ of HVEA first decreased and then increased as the concentration increased. As the number of UV-SFT aging cycles increased, the G*/sinδ of HVEA increased accordingly, but the increment was very small. This suggested that under the CaCl₂ salt solution condition, the number of UV-SFT cycles had little impact on the high-temperature rutting resistance of HVEA. Compared with the freshwater control group, the increment of rutting factor at each concentration under 15 cycles was slightly larger than that under 9 cycles. This indicated that the greater the number of aging cycles, the more significant the impact of CaCl₂ salt solution on the HVEA G*/sinδ was, and the more obvious the improvement in high-temperature rutting resistance became.

3.2. Low Temperature Rheological Properties

As displayed in Figure 6, Figure 7, Figure 8 and Figure 9, as the test temperature decreased, the stiffness (S) of the HVEA increased significantly, while the m-value decreased rapidly. This indicated that the low-temperature flexibility and stress relaxation performance of HVEA deteriorated [31,32,33]. As the concentrations of the two types of salts increased, the stiffness of HVEA exhibited a trend of first decreasing and then increasing, and the m-value showed a trend of first increasing and then decreasing. This suggested that the addition of snow-melt salts deteriorated the low-temperature creep performance of the asphalt. When the salt concentration was constant, as the number of UV-SFT aging cycles increased, the stiffness of HVEA increased and the m-value decreased. This indicated that UV-SFT aging would weaken its low-temperature crack resistance. It is worth noting that the stiffness of HVEA samples under any UV-SFT cycles exceeded 300 MPa and the m-value was lower than 0.3 when the test temperature was lowered to −24 °C. In order to prevent the pavement from cracking due to excessive asphalt rigidity and insufficient deformation cushioning capacity at low temperatures, the U.S. Strategic Highway Research Program (SHRP) specified that the stiffness should be less than 300MPa and the creep rate (m-value) should be greater than 0.3. This indicated that the low-temperature cracking resistance of HVEA was completely unsatisfactory to meet the engineering requirements no matter how many UV-SFT cycles were experienced at −24 °C and that the pavements would face a very high risk of cracking if they were used at this temperature. Moreover, as the test temperature increased, the differences between the HVEA’s stiffness and m-values under different UV-SFT aging cycles decreased. This demonstrated that higher ambient temperatures could alleviate the impact of UV-SFT aging on the low-temperature creep performance of the asphalt. On the other hand, the stiffness and m-values of the HVEA under NaCl and CaCl₂ solutions were relatively similar. This indicated that the two types of salts exerted a comparable influence on the low-temperature performance of HVEA.

3.3. Temperature Sensitivity

A smaller VTS indicated that the asphalt material was less sensitive to changes in temperature [34,35]. As depicted in Figure 10, as the number of UV-SFT cycles increased, the VTS values of HVEA decreased accordingly, which indicated that its temperature sensitivity was reduced. Under the NaCl solution condition, when the number of UV-SFT cycles was nine cycles, the VTS of HVEA showed a slight decrease compared with that of the water control group, suggesting a reduction in temperature sensitivity. However, when the number of UV-SFT cycles increased to 15 cycles, the VTS of HVEA after salt corrosion decreased significantly compared with the water control group. This indicated that the addition of salt at this stage significantly reduced the temperature sensitivity of HVEA. This was because the salt destroyed the three-dimensional network structure of HVEA, which made it impossible to effectively restrict the movement of asphalt molecules. Additionally, the salt particles inside the asphalt could share the impact caused by temperature changes. Therefore, the temperature sensitivity of the asphalt decreased. Under the CaCl₂ solution condition, when the number of UV-SFT cycles was nine cycles, the VTS of HVEA after salt corrosion first increased and then decreased compared with the water control group, meaning that the temperature sensitivity of the asphalt increased initially and then decreased. Moreover, with the increase in salt solution concentration, the VTS continued to decrease. When the number of UV-SFT cycles was 15 cycles, the VTS of HVEA after salt corrosion decreased compared with the water control group, which indicated that the temperature sensitivity of HVEA was significantly attenuated after salt corrosion. In addition, the increase in the number of UV-SFT aging cycles weakened the temperature sensitivity of HVEA, and NaCl had a more severe impact on the temperature sensitivity of HVEA than CaCl₂.

3.4. Fatigue Properties

Due to the similar change rule of UV-SFT fatigue factor (G*·sinδ) results for both NaCl and CaCl₂ solutions, this paper only showed the test results of UV-SFT for NaCl solution, which were depicted in Figure 11 and Figure 12.

The smaller the G*·sinδ of asphalt was, the smaller the dissipated energy per loading cycle was, the less the damage accumulation became, and the longer the fatigue life of the asphalt material was [36,37]. As can be seen from Figure 11 and Figure 12, as the test temperature increased, the G*·sinδ of HVEA decreased gradually, which indicated that the fatigue resistance improved with the rise in temperature. Compared with the freshwater group (0%wt), the G*·sinδ of HVEA after UV-SFT was larger, which suggested that the addition of snow-melt salt significantly weakened the fatigue resistance. At the same test temperature, the G*·sinδ of HVEA first decreased and then increased with the increase in NaCl solution concentration (from 3%wt to 12%wt), and this phenomenon was particularly evident at temperatures of 52 °C and 58 °C. The G*·sinδ of HVEA was at its minimum, but still higher than that of the freshwater group, at a concentration of 9%, and was at its maximum at a concentration of 3%. This indicated that it was not the case that the higher concentration of NaCl exerted a more serious effect on the fatigue performance of HVEA. In addition, the G*·sinδ after 15 cycles was significantly higher than that after 9 cycles, which indicated that the fatigue resistance of HVEA was weakened after UV-SFT cycle. This was because UV led to volatilization of light components within HVEA, oxidative fracture, and cross-linking of macromolecular chains, making HVEA brittle and hard. This, coupled with water intrusion, salt crystal expansion, and contraction due to SFT cycles, destroyed the integrity of the HVEA microstructure and exacerbated the generation of internal defects.

3.5. Chemical Properties

FT-IR test was performed on virgin HVEA and samples of 12wt% NaCl and 12wt% CaCl₂ at 15 cycles, and the results are shown in Figure 13.

According to Figure 13, the characteristic peaks at 2915 cm⁻¹ and 2843 cm⁻¹ were attributed, respectively, to the symmetric and asymmetric stretching vibrations of methylene, and thus the presence of alkyl compounds could be concluded. The peak at 1370 cm⁻¹ resulted from the in-plane bending vibration of C-H in methyl and methylene groups. Both the peaks at 1016 cm⁻¹ and 1030 cm⁻¹ were characteristic peaks of S=O. The peak at 965 cm⁻¹ was the characteristic peak of C=C in the polybutadiene segment of the HVEA [38,39,40].

After UV-SFT, the peak intensity of the sulfoxide group at 1016 cm⁻¹ was significantly higher than that of the virgin HVEA. This was because the sulfur components in the asphalt underwent oxygen-absorbing aging during the UV aging and water aging processes, and reacted with oxygen to form sulfoxide groups. Meanwhile, the intensity of the characteristic peak of C=C in polybutadiene at 966 cm⁻¹ was enhanced after the UV-SFT cycle, indicating an increase in the amount of trans-olefin. This was the result of reactions, such as cis–trans isomerization, chain breaking, and oxidation due to UV-SFT, reflecting the chemical aging of HVEA, and could lead to an increase in its low-temperature brittleness and deterioration of its properties. In addition, the characteristic peaks and their intensities observed in the three types of samples were basically consistent. Therefore, it could be concluded that no new chemical substances were generated in HVEA after UV-SFT aging.

4. Conclusion

This paper investigated the effect of NaCl and CaCl₂ solutions on the deterioration of the rheological properties of high-viscosity high-elasticity asphalt (HVEA) under a UV and salt freeze–thaw (UV-SFT) environment. The main conclusions were as follows:

(1)

Both NaCl and CaCl₂ solutions effectively enhanced the high-temperature rutting resistance of HVEA under the UV-SFT environment, with NaCl being the more significant enhancement in this regard. As the temperature increased, the effect of both salt solution concentrations on the high-temperature performance of HVEA diminished. After UV-SFT cycling, the G*/sinδ of HVEA increased. Moreover, the higher the number of UV-SFT cycles, the greater the increase in the G*/sinδ.

(2)

With the increase in the concentration of either NaCl or CaCl₂, the stiffness of HVEA first decreased then increased and the m-value showed the opposite trend of first increasing then decreasing, suggesting that the addition of snow-melt salts impaired the HVEA’s low-temperature creep performance. When the salt concentration was constant, more UV-SFT aging cycles led to a higher stiffness and a lower m-value, which weakened HVEA’s low-temperature crack resistance.

(3)

As the number of UV-SFT cycles increased, the VTS values of HVEA showed a decreasing trend, indicating that its temperature sensitivity gradually decreased. Salt corrosion significantly weakened the temperature sensitivity of HVEA, and NaCl affected the temperature sensitivity of HVEA to a much greater extent than CaCl₂.

(4)

The addition of snow-melt salt significantly weakened the fatigue resistance of HVEA, but the higher concentration of snow-melt salt impaired the fatigue performance of HVEA more severely. The G*·sinδ after 15 cycles of UV-SFT was significantly higher than that after 9 cycles, indicating a further weakening of the fatigue resistance by UV-SFT cycling.

(5)

After UV-SFT cycling, the intensity of the S=O characteristic peak was significantly higher than that of virgin HVEA, reflecting that the HVEA underwent an aging process. The enhanced intensity of the characteristic peak of C=C reflected an increased trans-olefin content, suggesting that chemical aging had occurred in HVEA, which may have increased its low-temperature brittleness and deteriorated its performance.