The Effect of the Temperature–Humidity Coupling Cycle on the Performance of Styrene Butadiene Styrene Polymer-Modified Asphalt Mastic

Abstract

To study the variation laws and effects of asphalt mastic under the cooperative interaction of different temperatures and humidities, cyclic conditions for different temperature ranges were set to conduct indoor experimental simulations of thermal–humidity coupling cycles. Firstly, the macroscopic performance changes in styrene butadiene styrene polymer (SBS)-modified asphalt mastic were evaluated by the penetration test, softening point test, ductility test, Brookfield rotational viscosity test, and double-edge notched tensile (DENT) test; then, the mechanism of performance changes was explored from the perspective of chemical composition by combining this with Fourier transform infrared spectroscopy (FTIR). The research results show that with the increase in thermal–humidity coupling cycles, SBS-modified asphalt mastic exhibited aging phenomena such as hardening and embrittlement, and its macroscopic performance deteriorated; under the same test conditions, the interval with a higher temperature difference had a greater impact on the performance of the mastic; the sulfoxide index (I_S=O) of SBS-modified asphalt mastic increases after thermal–humidity coupling cycles, while the isoprene index (I_B) decreases.

1. Introduction

Asphalt pavements have many advantages, such as ride comfort, noise reduction, smoothness, and ease of maintenance, and are widely used in the construction of high-grade highways in China [1]. China’s seasonal frost regions experience alternating periods of high temperatures and rainfall in summer and cold and dry conditions in winter. Such extreme environments subject asphalt pavements to the long-term coupling effects of temperature and humidity, making them susceptible to aging caused by natural elements such as sunlight, air, and rainwater [2,3]. This causes the asphalt pavement to deteriorate, such as through loosening, peeling, and potholes, especially in hot and rainy areas, severely affecting their service life and performance [4,5]. These environmental factors shorten the service life of asphalt pavements [6], increase their maintenance frequency and cost, and affect traffic safety.

SBS is a polymer that combines rubber elasticity and resistance to permanent deformation, so it has advantages such as good elasticity, high tensile strength, and good low-temperature deformation performance and is widely used as an asphalt modifier [7,8,9]. Previous studies have shown that when the base asphalt and SBS act together, they can effectively improve the elasticity and cause resistance to the permanent deformation of the asphalt material, reducing its temperature sensitivity and greatly improving the performance of the asphalt pavement [10,11]. At the same time, research has also found that the dosage of the SBS modifier has different effects on asphalt, and an excessive or insufficient dosage may also be detrimental to asphalt modification [12,13,14]. During the service life of pavements, the alternating effects of traffic loads and environmental factors make SBS-modified asphalt more susceptible to aging, which is caused by the interaction of the oxidation of aged asphalt and degradation of copolymers, leading to changes in the chemical structure of mastics [6,15]. Studying the aging performance of asphalt has become one of the most challenging issues in pavement engineering [16,17,18], and many researchers have studied the effects of SBS modification on improving the mechanical properties of asphalt; however, few have studied their effects on the aging durability of asphalt.

Meanwhile, research on the variation laws of asphalt and asphalt mixtures under various environmental conditions is also one of the current research hotspots, and scholars at home and abroad have conducted certain studies on this. Ding et al. [19] studied the effect of various environmental conditions on the performance of asphalt mastics and found that the permeability and ductility of asphalt decrease with aging, the softening point increases, the fatigue resistance gradually decreases under loading, and the elastic properties are ultimately damaged due to fatigue. Mingyuan Chen et al. [4] studied the fatigue performance of asphalt under different water aging conditions and established a model to predict the fatigue change in asphalt after water aging. Cheng Y et al. [20] evaluated the rheological characteristics of asphalt and asphalt mastic under freeze–thaw cycles using established mathematical models and the viscoelastic continuum damage theory. It can be observed that research on asphalt mostly centers on the analysis of physical and chemical changes or the introduction of various modern testing techniques to detect and characterize the material structure and properties of asphalt [21,22]. Research on the effects of temperature change is rare, and the combined action of moisture on SBS-modified asphalt mastic under changing climatic conditions indicates the need for further study.

This article attempts to adopt a simulation test method that better reproduces the temperature and humidity coupling in natural environments, considering different temperature and humidity cycling conditions, conducting conventional tests, Brookfield rotational viscosity tests, and double-edge notched tensile (DENT) tests to evaluate the performance variation in SBS-modified asphalt mastic, and combining Fourier transform infrared spectroscopy (FTIR) to explore its performance change mechanism from a chemical composition perspective. This study aims to find the influence of temperature and humidity coupling cycles on the performance of SBS asphalt mastic with the hope of providing a reference for optimizing pavement design and improving the performance of asphalt materials. The specific research plan is shown in Figure 1.

2. Materials and Methods

2.1. Raw Materials

This article adopts SBS-modified asphalt, the technical indicators for which are shown in

Table 1. Properties of SBS-modified asphalt.

Properties	Experimental Value	Normative Value	Test Method
Penetration (25 °C, 100 g, 5 s)/0.1 mm	46.1	40~60	T 0604
Softening point/°C	85.6	≥60	T 0606
Ductility (5 °C, 5 cm/min)/cm	24.6	≥20	T 0605

, meeting the requirements of JTGF40-2004 [23] “Technical Specifications for Highway Asphalt Pavement Construction”. The mineral powder is limestone powder, the technical indicators of which are shown in

Table 2. Properties of mineral filler.

Properties		Experimental Value	Normative Value	Test Method
Apparent density/(g∙cm−3)		2.706	≥2.50	T 0352
Water content/%		0.4	≤1	T 0332
Hydrophilic coefficient		0.4	<1	T 0353
Mineral powder plasticity index (%)		2.8	<4	T 0354
Particle range	<0.6 mm passage rate/%	100	100	T 0351
	<0.15 mm passage rate/%	91.6	90~100	T 0351
	<0.075 mm passage rate/%	81.7	75~100	T 0351

, meeting the requirements of JTGE42-2005 [24] “Technical Specifications for Highway Engineering Aggregate Testing Procedures”.

2.2. Asphalt Mastic Sample Preparation

Heat the SBS-modified asphalt in a 170 °C oven until it reaches a flowing state, then pour it into a 180 °C oil bath pot. Place the mineral powder in a 105 °C oven for 4 h to remove moisture, weigh out an amount of mineral powder equal to 1:1 with the asphalt by mass, gradually add it to the asphalt multiple times, and mix it using a high-shear mixer at a speed of 1000 r/min for 30 min. Pour the prepared asphalt mastic into a mold to produce the required samples.

2.3. Temperature and Humidity Coupling Cycle Test

Referring to relevant studies [20,25], indoor-simulated temperature and humidity coupling cycle tests were conducted. For the purpose of studying the variation law of asphalt mastic performance under different conditions, the experiments set different temperature cycling ranges (−20 °C to 60 °C, −20 °C to 40 °C, 0 °C to 60 °C, 0 °C to 40 °C). Among them, the low-temperature conditions in the temperature and humidity coupling cycle are controlled by a low-temperature box, and the high-temperature and high-humidity conditions are controlled by a constant temperature water bath. The asphalt mastic samples are placed in the low-temperature box for continuous freezing for 12 h and then placed in a constant temperature water bath for 12 h, constituting one cycle. Following the above steps, different numbers of temperature and humidity coupling cycles are repeated 3 times, 6 times, 9 times, and 12 times to complete the experiment. After the cycle ends, the asphalt mastic samples are taken out and left to stand at room temperature for subsequent routine tests. The cone penetration test is shown in Figure 2.

2.4. General Tests

With reference to JTG E20-2011 [26], “Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering”, the penetration, softening point (ring and ball method), and ductility of asphalt mastics after temperature and humidity coupling cycles at 15 °C and 40 °C are measured.

According to existing studies [27,28], conventional penetration tests on asphalt mastics are prone to data distortion. Therefore, cone penetration tests on asphalt mastics at 15 °C and 40 °C are conducted to evaluate the variation law of low-temperature shear performance and high-temperature shear performance after temperature and humidity coupling cycles. The test method refers to the penetration test method in JTG E20-2011, “Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering”, with the standard needle replaced by a cone needle. The test apparatus is shown in Figure 3. The cone penetration of asphalt mastics under different conditions is obtained, and their shear strength τ (kPa) is calculated using Equation (1).

$$\mathsf{\tau }=\frac{981Q{\cos }^2\left(\frac{\alpha }{2}\right)}{\pi h^2\tan \left(\frac{\alpha }{2}\right)}$$

(1)

In the above formula, Q is the total mass of the tapered needle, connecting the rod and weights, g; α is the taper needle tip angle, (°); and h is the taper, 0.1 mm.

2.5. Brinell’s Rotational Viscosity

With reference to JTG E20-2011 [26], “Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering”, the Brookfield rotational viscometer was used to test the Brookfield viscosity of SBS-modified asphalt mastics at 135 °C, 145 °C, 155 °C, 165 °C, and 175 °C under temperature and humidity coupling cycles, and their viscoelastic activation energy was calculated.

2.6. Fourier Infrared Spectroscopy Test

Fourier infrared spectroscopy test was performed on asphalt mastics under different test conditions to observe changes in the vibration spectra of major functional groups and further explore the mechanism of performance degradation under temperature and humidity coupling cycles. The experiment utilized a Nicolet 6700 Fourier transform infrared spectrometer with a testing range of 4000 to 500 cm⁻¹, a resolution of 4 cm⁻¹, and 32 scans.

2.7. Double-Edge-Notched Tension Testing (DENT)

This paper adopts double-edge-notched tension testing (DENT) to evaluate the mid-temperature crack resistance performance change in asphalt mastics under the action of temperature and humidity coupling cycles. The experiment was implemented with reference to the AASHTO TP 113-15 [29] standard, using the main instrument of the universal material testing machine, with a test temperature of 25 °C and a tensile rate of 50 mm/min. For the same type of asphalt mastic, three specimens with different tape widths (5 mm, 10 mm, 15 mm) were prepared using homemade molds for testing, as shown in Figure 4.

3. Analysis of Test Results

3.1. Conventional Test

3.1.1. Penetration

The results of the 15 °C and 40 °C shear strength τ of asphalt mastic under different working conditions are shown in Figure 5.

After cycling SBS-modified asphalt at different temperatures, it was placed at 15 °C and 40 °C for shear strength testing. From Figure 5, it can be observed that there are differences in the increase in shear strength of the mastic under different temperature differences, and the increase in shear strength of the mastic in the temperature difference range with larger cycles is more significant.

Table 3. Increment of shear strength at different temperature ranges.

Temperature Range	15 °C Shear Strength Increment	40 °C Shear Strength Increment
−20~60 °C	47.54%	50.9%
−20~40 °C	36.32%	18.1%
0~60 °C	32.05%	45.7%
0~60 °C	24.35%	15.7%

shows the increase in shear strength at different temperatures. Specifically, when the number of cycles is 12, compared with the control group that did not participate in temperature and humidity coupling cycles, the shear strength of SBS-modified asphalt mastic increased by 47.54%, 36.32%, 32.05%, 24.35% at temperatures ranging from −20 °C to 60 °C, −20 °C to 40 °C, 0 °C to 60 °C, and 0 °C to 40 °C, respectively, and the shear strength at 40 °C increased by 50.9%, 18.1%, 45.7%, 15.7%, respectively. With the progression of temperature and humidity coupling cycles, the shear strength of asphalt mastics in different temperature cycle ranges showed an upward trend, indicating that with the progression of temperature and humidity coupling cycles, the content of light components reduced, the elastic components of SBS-modified asphalt mastic increased, and the mastic became harder and more brittle.

Figure 5b illustrates that at a test temperature of 40 °C, the increase in the shear strength of SBS-modified asphalt mastic under the −20 °C to 40 °C temperature range cycle is significantly smaller than the increase in shear strength of the mastic under the 0 °C to 60 °C temperature range cycle. This indicates that under higher temperature range cycles of temperature and humidity coupling, the hardening phenomenon of SBS-modified asphalt mastic becomes more pronounced.

3.1.2. Softening Point

The softening point test results of asphalt mastic under the action of temperature and humidity coupling cycles are shown in Figure 6.

From Figure 6, it can be seen that under the same cycle temperature range, with the increase in the number of temperature and humidity coupling cycles, the softening point of SBS-modified asphalt mastic changes minimally and does not show a consistent pattern of change, which correlates with related research results. This indicates that it is difficult to characterize the possible changes in its high-temperature shape with the softening point. This may be because the change in the softening point of SBS-modified asphalt is determined by the comprehensive action of the SBS modifier and base asphalt. The SBS modifier forms a kind of net structure inside the SBS-modified asphalt mastic, wrapping the asphalt and making the asphalt structure more stable. The temperature and humidity coupling cycle causes less damage to the network structure of the SBS modifier, resulting in insignificant changes in the softening point.

3.1.3. Ductility

The ductility test results of asphalt mastic under the action of temperature and humidity coupling cycles are shown in Figure 6.

From Figure 7, it can be observed that under the same cycle temperature range, with the increase in the number of temperature and humidity coupling cycles, the trend of elongation changes in SBS-modified asphalt mastic is consistent, showing a decreasing trend. In addition, the level of impact of different temperature differences on the elongation of SBS-modified asphalt mastic varies. The larger the cycle temperature difference, the greater the decrease in mastic elongation. Moreover, under the same temperature difference, the impact of temperature and humidity coupling cycles on the elongation of mastic in the high-temperature range is more severe than that in the low-temperature range. This indicates that temperature and humidity coupling cycles lead to a decrease in the low-temperature crack resistance of the mastic, and the greater the temperature difference and temperature, the more pronounced the diminish in the low-temperature crack resistance of the mastic. This is mainly because temperature and humidity coupling cycles cause aromatic components to transform into resin and asphalt, resulting in the hardening of SBS-modified asphalt mastic and a decrease in low-temperature ductility. The greater the temperature difference and the higher the temperature range, the more pronounced this phenomenon becomes.

3.2. Bostwick Rotary Viscosity Test

The results of the Bostwick rotary viscosity test of asphalt mastic under the action of temperature and humidity coupling cycles at 135 °C, 145 °C, 155 °C, 165 °C, and 175 °C are shown in Figure 8.

The results in Figure 8 show that the viscosity of the mastic increases with the number of cycles at the same test temperature. The increase in viscosity after temperature and humidity coupling cycles is more pronounced at lower test temperatures, and the increase in viscosity is smaller as the test temperature increases. The most significant change in the viscosity of the mastic occurs when cycling within the range of −20 °C to 60 °C, and as the temperature difference in the cycling temperature range decreases, the increase in viscosity of the mastic decreases. This is because, after temperature and humidity coupling cycles, the light components of SBS-modified asphalt mastic volatilize or transform into heavy components, increasing the proportion of heavy components, leading to increased intermolecular movement resistance within the mastic and resulting in increased viscosity. In temperature ranges with larger differences, the reaction rate is faster.

The Arrhenius equation can be used to characterize the relationship between chemical reaction rates at different temperatures. SBS-modified asphalt mastic undergoes complex chemical reactions during temperature and humidity coupling cycles, causing viscosity changes. The reaction process of SBS-modified asphalt mastic during temperature and humidity coupling cycles can be characterized by establishing an Arrhenius equation with viscosity as a parameter.

The Arrhenius equation reflects the relationship between the viscosity and temperature of SBS-modified asphalt mastic.

$$\lg (\eta \left(T\right))=\lg \left(K\right)+\frac{E_{\eta }}{2.303RT}$$

(2)

In the above formula, η(T) is the viscosity at a temperature of T, Pa∙s; K is the material constant; R = 8.314 J/(mol∙K), R is the Boltzmann constant; and E_η is the activation energy, J/mol.

E_η refers to the minimum energy required for the flow units (i.e., molecular segments) of polymer materials to overcome barriers and transition from their original positions to nearby “holes” during the flow process. According to existing research [30,31,32], the activation energy E_η of asphalt mastic can characterize the ease of flow and temperature sensitivity of the mastic. By plotting lg(η(T)) and 1/T according to Equation (2) and performing linear regression on the data, the slope of the resulting straight line, which represents E_η, can be obtained. Refer to

Table 4. Calculation results of visco-flow activation energy.

Temperature Range	Number of Cycles	$\mathbf{lg}(\mathit{\eta }\left(\mathit{T}\right))-1/\mathit{T}$ Relational Regression Model	Coefficient of Determination R2	p	Eη/(kJ∙mol−1)
−20~60 °C	0	y = 3923.8x − 8.67418	0.990	0.0003	75.13
	3	y = 3965.9x − 8.74436	0.993	0.0001	75.94
	6	y = 3978.1x − 8.73747	0.994	0.0001	76.17
	9	y = 4011.8x − 8.79068	0.995	0.0001	76.81
	12	y = 4062.6x − 8.88664	0.994	0.0001	77.79
−20~40 °C	0	y = 3923.8x − 8.67418	0.990	0.0003	75.13
	3	y = 3941.0x − 8.69152	0.996	0.0001	75.46
	6	y = 3955.1x − 8.6973	0.999	0.0001	75.73
	9	y = 3983.2x − 8.74042	0.999	0.0001	76.27
	12	y = 4005.6x − 8.77071	0.999	0.0001	76.7
0~60 °C	0	y = 3923.8x − 8.67418	0.990	0.0003	75.13
	3	y = 3945.7x − 8.69562	0.997	0.0000	75.55
	6	y = 3964.8x − 8.7142	0.998	0.0000	75.91
	9	y = 3981.6x − 8.73365	1.000	0.0000	76.24
	12	y = 4010.3x − 8.78018	0.999	0.0000	76.79
0~40 °C	0	y = 3923.8x − 8.67418	0.990	0.0003	75.13
	3	y = 3939.0x − 8.68776	0.997	0.0000	75.42
	6	y = 3944.9x − 8.68674	0.999	0.0000	75.53
	9	y = 3964.4x − 8.71990	0.998	0.0000	75.91
	12	y = 3981.5x − 8.74841	0.998	0.0000	76.23

for the results.

As shown in the above table, under different temperature and humidity coupling cycle numbers, the relationship between lg(η(T)) and 1/T for the SBS-modified asphalt mastic can be expressed as a linear relationship with determination coefficients (R2) all greater than 0.990 and p values all less than 0.05. In different cycling temperature ranges, with the increase in temperature and humidity coupling cycle numbers, the E_η values all show an upward trend, but the magnitudes of such increases vary. The mastic E_η shows the largest increase when cycled within the −20 °C to 60 °C range, reaching 77.48 kJ/mol after 12 cycles, followed by a slightly lower increase when cycled within the −20 °C to 40 °C and 0 °C to 60 °C ranges, and the smallest increase is observed when cycled within the 0 °C to 40 °C range, with E_η reaching 76.05 kJ/mol after 12 cycles. This may be because SBS-modified asphalt mastic, after undergoing temperature and humidity coupling cycles, generates polar oxygen-containing functional groups, such as sulfonyl groups in a high-temperature and high-humidity environment, leading to an increase in molecular weight and intermolecular forces, resulting in an increase in the energy required for asphalt molecule flow; in a high-temperature and high-humidity environment, the composition changes, with some light components volatilizing and others transforming into heavy components, causing the asphalt to thicken. The interaction force between the modifier and the asphalt, as well as the interfacial viscous effect, become stronger. The higher the energy barrier that the asphalt material needs to overcome when moving, the more E_η increases after temperature and humidity coupling cycles.

3.3. The Force–Displacement Curves for SBS-Modified Asphalt Mastic for Different Numbers of Cycles

Based on the test outcomes in Figure 8, it can be determined that the alteration in performance of SBS-modified asphalt mastic within the temperature range of −20 °C to 60 °C is notably significant. Consequently, the SBS-modified asphalt mastic subjected to temperatures ranging from −20 °C to 60 °C was designated as the focal point of this investigation.

Utilizing the DENT test findings, a load–displacement curve was graphed, as depicted in Figure 9, facilitating the determination of the critical crack tip opening displacement (CTOD) of the specimen in accordance with Equations (3)–(7). Prior research [32] suggests a strong association between CTOD values and the fatigue properties of asphalt blends, underscoring the capacity of CTOD values to delineate the fracture resistance of asphalt under medium temperatures.

The total fracture energy of SBS-modified asphalt mastic is mainly composed of basic fracture energy and plastic deformation energy, which can be determined by the area under the load–displacement curve as follows:

$$W_t=W_e+W_P=w_e\times LB+\beta w_P\times B{L}^2$$

(3)

In the above formula, $W_t$ is the total fracture energy, kJ; $W_e$ is basic fracture energy, kJ; $W_p$ is plastic deformation energy, kJ; $W_e$ is fracture ratio basic energy, kJ∙m⁻²; $W_p$ is fracture specific plasticity work, kJ∙m⁻³; $B$ is specimen thickness, mm; $L$ is specimen ligament width, mm; and $\beta$ is a geometrically defined parameter that characterizes the configuration of the plastic zone.

$$w_t=W_t/BL$$

(4)

Associative Equations (3) and (4) can be obtained as follows:

$$w_t=w_e+\beta w_p\times L$$

(5)

A linear fit can be made to the $W_t$ fracture ratio fundamental and L ligament width according to Equation (5), which yields a straight-line intercept of $W_e$ and a slope of $\beta w_p$.

Based on the peak load of the glued pulp specimen with a ligament width of 5 mm and the geometry of the specimen, the net sectional stress of the specimen can be calculated according to Equation (6):

$${\sigma }_n=P_{peak}/(\overline{B} \overline{L})$$

(6)

In the above formula, ${\sigma }_n$ is net sectional stress; $P_{peak}$ is peak load.

CTOD can be calculated by the following equation:

$${\delta }_t=w_e/{\sigma }_n$$

(7)

In the above formula, ${\delta }_t$ is CTOD, mm.

The crack tip opening displacement, also known as CTOD. Due to the high correlation between CTOD value and fatigue performance of asphalt mixtures. Therefore, the CTOD value can characterize the medium temperature fracture resistance of asphalt.

Observing Figure 9 reveals that the load–displacement curve of SBS-modified asphalt mastic exhibits three distinct stages of fluctuation. In the first stage (or segment), the load increases rapidly with displacement. In the second stage (ab segment), after reaching the yield load, the specimen continues to stretch with displacement while the load decreases slowly. In the third stage (bc segment), the load drops sharply until the specimen fractures. This phenomenon could arise due to the formation of a cross-linked network structure by the SBS modifier within the mastic.

With the rise in the quantity of temperature–humidity coupling cycles, the failure deformation of the mastic decreases, and the peak load increases, indicating that after the temperature–humidity coupling cycle, the SBS-modified asphalt mastic becomes harder, and its ductility deteriorates. As shown in

Table 5. DENT results for SBS-modified asphalt mastic under different cycle times.

Number of Temperature and Humidity Coupling Cycles	we/(kJ∙m−2)	R2	CTOD/(mm)
0	17.673	0.9962	17.03
6	13.330	0.9728	11.99
12	11.501	0.9984	9.57

, the CTOD of the SBS-modified asphalt mastic decreases with the increase in the number of temperature–humidity coupling cycles. The lower the CTOD, the closer the mastic is to a brittle state, making it more susceptible to permanent damage. The temperature–humidity coupling cycle diminishes the mid-temperature fracture resistance of the SBS-modified asphalt mastic, with the magnitude of this impact escalating proportionally with the number of cycles, aligning with the analysis findings regarding peak load and failure deformation. On the one hand, this may be due to the increased number and size of internal pores in the SBS-modified asphalt mastic under the combined action of temperature and humidity, providing channels for water to enter the mastic and ultimately destroying the internal structure of the mastic. On the other hand, it is because hydrophilic polar molecules and some water-soluble substances in the SBS-modified asphalt mastic combine with water, causing the mastic to become brittle and hard [33,34].

3.4. Changes in Chemical Composition

The Fourier transform infrared spectrometer was used to test SBS-modified asphalt, limestone powder, and SBS-modified asphalt mastic, and the SBS-modified asphalt mastic after 12 cycles at different cycling temperatures was tested. The test results are shown in Figure 10.

The SBS-modified asphalt mastic did not produce new characteristic absorption peaks. Its characteristic absorption peak is the superposition of the characteristic absorption peaks of SBS-modified asphalt and limestone powder, indicating that the contact between limestone powder and SBS-modified asphalt is mainly physical adsorption, and no clear chemical reaction occurred. Combined with Figure 10, it can be seen that under the action of temperature and humidity coupling cycles, the chemical composition of limestone powder was stable, while alterations in the characteristic functional groups of the mastic primarily stemmed from changes in the characteristic functional groups of the asphalt. Therefore, the change rate of asphalt characteristic functional groups can be used to characterize the influence of temperature and humidity coupling cycles on the mastic.

The changes in the carbonyl absorption peak at 1700 cm⁻¹, sulfinyl absorption peak at 1031 cm⁻¹, and butadiene absorption peak at 966 cm−1 can be used to characterize the aging degree of SBS-modified asphalt [35,36]. In accordance with the Lambert–Beer law, the summation of peak areas within the fingerprint region ranging from 2000 to 600 cm⁻¹ served as the reference point. The carbonyl index (I_C=O), sulfite index (I_S=O), and butadiene index (I_B) of the mastic before and after cycling were determined using Equations (8)–(10). Subsequently, quantitative analysis was conducted on the alterations in the chemical compositions of the SBS-modified asphalt mastic, both pre- and post-temperature–humidity coupled cycling, with the outcomes presented in

Table 6. Calculation results of the characteristic functional group index of the FITR spectrum of SBS-modified asphalt mastic under different test conditions.

Temperature Cycle Interval	Uncycled	−20~60 °C	−20~40 °C	0~60 °C	0~40 °C
IB	0.00686	0.00282	0.00435	0.00385	0.00548
${\mathrm{I}}_{\mathrm{S}=\mathrm{O}}$	0.01632	0.03171	0.02495	0.02454	0.02499

.

$${\mathrm{I}}_{\mathrm{C}=\mathrm{O}}=\frac{A_{1700}}{{\displaystyle \sum A_{2000\text{-}600}}}$$

(8)

$${\mathrm{I}}_{\mathrm{B}}=\frac{A_{966}}{{\displaystyle \sum A_{2000\text{-}600}}}$$

(9)

$${\mathrm{I}}_{\mathrm{S}=\mathrm{O}}=\frac{A_{1031}}{{\displaystyle \sum A_{2000\text{-}600}}}$$

(10)

In the above formula, A_2000-600 is the peak area of the asphalt fingerprint region in the range of 2000~600 cm⁻¹.

$$A_{2000\text{-}600}=A_{1600}+A_{1456}+A_{1376}+A_{1162}+A_{1031}+A_{966}+A_{861}+A_{810}+A_{744}+A_{722}$$

Combined with Figure 9 and Table 6, it can be seen that there is no carbonyl absorption peak at 1700 cm⁻¹ in the SBS-modified asphalt mastic before and after the temperature and humidity coupling cycle. This may be because the experimental environment is relatively mild for the generation of carbonyl under the action of the temperature and humidity coupling cycle, resulting in a small amount of carbonyl generation that is not observed. In different temperature cycling intervals, the I_S=O of SBS-modified asphalt mastic shows the same trend of change. As the number of temperature and humidity coupling cycles increases, the value of I_S=O demonstrates a continual rise. Comparing this with the uncycled mastic, the I_S=O content of the mastic after 12 cycles in the −20 °C to 60 °C, −20 °C to 40 °C, 0 °C to 60 °C, and 0 °C to 40 °C intervals increased by 94.30%, 52.88%, 50.37%, and 53.12%, respectively, indicating an increase in the sulfoxide content of SBS-modified asphalt mastic after temperature and humidity coupling cycles. The increase in the number of temperature–humidity coupling cycles and temperature difference exacerbates the aging of SBS-modified asphalt mastic to varying degrees, which is consistent with the laws observed in previous conventional tests, viscosity tests, and mechanical tests. This is because, under the action of high temperature and oxygen, hydrogen peroxides are formed in the mastic. As an oxidant, it oxidizes sulfur-containing functional groups, such as sulfides and thiols in asphalt molecules, into sulfoxide functional groups.

Meanwhile, the I_B value decreases with the increase in the number of temperature–humidity coupling cycles, and the I_B value of the mastic in the −20 °C to 60 °C range decreases the most by 58.89% during cycling, indicating that the SBS modifier undergoes degradation under the action of temperature and humidity. Water molecules interact with SBS polymer molecules, accelerating the cleavage of the C=C double bonds in the polybutadiene segments of the SBS modifier, thus disrupting the network structure formed by SBS.

4. Conclusions

Under the action of temperature–humidity coupling cycles, with the increase in cycle times, the physical indicators of SBS-modified asphalt mastic change are mainly manifested as increased shear strength, decreased ductility, insignificant changes in the softening point, decreased viscosity, and the increased activation energy of asphalt viscosity flow. This indicates that under the temperature–humidity coupling cycle environment, the macroscopic performance indicators of SBS-modified asphalt mastic deteriorate, manifesting aging phenomena such as decreased flowability and increased brittleness.

When the same temperature–humidity coupling cycle times are applied, the aging phenomenon of SBS-modified asphalt mastic becomes more pronounced with a larger temperature difference in the cycling range; under the same temperature difference conditions, SBS-modified asphalt mastic cycled at higher temperature ranges exhibits more significant changes in high-temperature shear strength and ductility. Therefore, special attention should be paid to the service status of asphalt pavements in regions with large temperature differences, such as Xinjiang and Tibet, in actual environments.

The results of the DENT test show that with the increase in temperature–humidity coupling cycle times, the failure deformation and CTOD value of SBS-modified asphalt mastic decrease, and the peak load increases, indicating that the temperature–humidity coupling cycle weakens the mid-temperature fracture resistance of SBS-modified asphalt mastic.

The FITR test results indicate that under temperature–humidity coupling cycles in different temperature ranges, there is no carbonyl absorption peak at 1700 cm⁻¹, possibly due to the mild environment for carbonyl generation; the sulfoxide index, which characterizes the aging of SBS-modified asphalt mastic, increases, while the isoprene index decreases, and the greater the temperature difference in the cycling interval, the more pronounced the changes. The increase in temperature–humidity coupling cycle times and temperature differences exacerbates the aging of SBS-modified asphalt mastic to varying degrees, and the FITR test results are consistent with the macroscopic performance results.