Grey Correlation Analysis between Macro Mechanical Damage and Meso Volume Characteristics of SBS Modified Asphalt Mixture under Freeze-Thaw Cycles

Abstract

The effect of freeze–thaw (F–T) in the seasonal frozen area would lead to damage to asphalt pavement. After water enters asphalt pavement, the water in voids would expand at a lower temperature, which could change the void content and number, affecting the macro mechanical properties of the asphalt mixture. The rapid development of CT scanning and digital image processing (DIP) provides powerful technical support for the research of asphalt mixture meso volume characteristics. In this paper, the mechanical properties of basalt fiber reinforced asphalt mixture subjected to F–T cycles were tested at different temperatures to clarify the decay law of mechanical properties under F–T cycles. Then, the meso images of the asphalt mixture under various F–T cycles could be obtained by using CT tomography. Based on DIP technology, the meso characteristic parameters of CT images for asphalt mixture were extracted, and the development of asphalt mixture freeze–thaw damage was further analyzed. The test results showed that with the F–T cycle, the macro mechanical properties of the asphalt mixture rapidly declined in the early stage of the F–T cycle and gradually tended to be flat. There would be serious damage inside the asphalt mixture in the late stage of the F–T cycle. The damage to the mechanical properties of the asphalt mixture under the F–T cycle can be attributed to the change in the internal mesostructure of the asphalt mixture. Based on the grey relational analysis theory, the formation of the connected void was the main factor affecting the damage in the early stage of the F–T cycle, while the formation of new voids mainly affected the later development of F-T damage.

1. Introduction

Considering the action of moisture and temperature, asphalt pavement in seasonal frozen areas suffers freeze–thaw (F–T) damage, which is essentially a process of damage accumulation inside asphalt materials determined by internal and external factors [1,2,3,4]. After the long-term F–T cycle, asphalt and aggregate of asphalt pavement would peel off, and the damage continues to accumulate, resulting in looseness, peeling, potholes, and so on [5,6,7,8]. Therefore, there will be varying degrees of mechanical performance degradation in asphalt pavement located in the seasonal frozen area due to the F–T effect.

F–T damage of asphalt mixture has attracted a lot of researchers at home and abroad, and some related research work has been carried out mainly by comparing and analyzing various properties of asphalt mixture before and after the action of the F–T cycle to explore the F–T damage characteristics [9,10,11,12]. Huang et al. [13] studied the effect of the F–T cycle on AC and SMA-graded asphalt mixture and discussed the three-dimensional damage criterion of the asphalt mixture during the F–T cycle through triaxial tests. The results showed that SMA has better F–T resistance. Cheng et al. [14] used diatomite and basalt fiber to improve the performance of asphalt mixture in the seasonal freezing area, analyzed the influence of F–T cycle on its strength and strain energy through indirect tensile test, and proposed the stress ratio of the linear zone and nonlinear zone as the evaluation index. Gong et al. [15] used nanomaterials to improve the F–T resistance of asphalt mixture and established a gray prediction model through volume parameters and mechanical properties to discuss the F–T resistance of modified asphalt mixture. You et al. [16] used the interface bond strength test to evaluate the impact of asphalt mixture durability under the F–T cycles and fully considered the change of bond performance between aggregate and asphalt during the F–T cycle. Wu et al. [17] studied the water stability and water permeability of asphalt mixture under the F–T cycle and evaluated the degree of correlation using the gray correlation entropy analysis theory, which is of great significance to the research on early water damage and F–T damage of asphalt mixture. Lovqvist et al. [18] proposed a multi-scale mechanical model to link F–T damage with mechanical property damage. Based on the energy damage and cohesion model, the impact of F–T cycles was simulated to evaluate the extent of F–T damage. Although many studies have been performed to characterize the F–T damage of asphalt mixture from the perspective of macro mechanical properties, it is difficult to reveal the F–T damage mechanism. In view of the rapid development of CT scanning technology and digital image processing (DIP) technology, some researchers began to discuss the F–T damage mechanism of asphalt mixture from the meso perspective [19,20,21,22,23,24]. Liu et al. [25] used CT and DIP technology to reconstruct the three-dimensional microstructure of asphalt mixture, used an indirect tensile test to determine its tensile strength, and proposed a research method for asphalt mixture performance mechanism based on the evolution of pore fractal dimension. Ji et al. [26] analyzed the internal structure of asphalt mixture CT images before and after freezing and thawing, and the results showed that its porosity and moisture content had a good correlation with the indirect tensile strength of the asphalt mixture. Ahmad et al. [27] used CT scanning technology to discuss the relationship between the micro characteristics of porous asphalt mixture and its performance and quantitatively analyzed the relationship between its pore characteristics, elastic modulus, and water permeability through the microstructure.

Much research has been carried out on the F–T damage and its attenuation law of asphalt mixture. However, the damage evolution research is not comprehensive and systematic. Then, SMA-13 specimens with basalt fiber were first designed and subjected to 0-21 F–T cycles. The load-displacement curves under compression and tensile modes of the asphalt mixture were measured to investigate the influence of F–T cycles on the mechanical properties and fracture characteristics of the asphalt mixture. At the same time, the meso images of asphalt mixture under various F–T cycles could be obtained by using CT tomography, which could be further extracted for the meso characteristic parameters. Finally, the grey correlation analysis was adopted to analyze the influence of meso characteristic parameters on the F–T damage of the asphalt mixture. The general objectives of this research are to analyze the change rule of mechanical properties and meso volume characteristics of SBS-modified asphalt mixture under freeze–thaw cycles. The specific objective of this research is to explore the correlation between macro mechanical properties and meso volume characteristics of SBS-modified asphalt mixture, and the research innovation is that the damage mechanism of macro mechanical properties for asphalt mixture is revealed from the perspective of micropore characteristics. The research framework of this study is shown in Figure 1.

2. Materials and Methods

2.1. Raw Materials and Specimen Preparation

Due to its reliable performance and resistance to various conditions, SBS-modified asphalt is widely applied as pavement bitumen [28]. The SBS-modified asphalt used in this study was produced in Yingkou, China, and its properties are summarized in Table 1. All properties meet the requirements in Technical Specification for Construction of Highway Asphalt Pavements (Chinese Standard JTG F40–2004 [29]). In this research, basalt and limestone were used as aggregate, and mineral filler, respectively, and the basic physical parameters are presented in Table 2, Table 3 and Table 4. All parameters meet the requirements of Chinese Standard JTG F40–2004. The basalt fiber with a length of 6 mm and diameter of 13 μm was selected as the fiber stabilizer, and the basic performances were listed in Table 5. The stone mastic asphalt (SMA) asphalt mixture investigated in this study is the same as the mixture investigated by Wang et al. [30]. The aggregate gradation, reported in Figure 2, is typical for pavement layers placed in Chinese motorways. Meanwhile, the sieve analysis of aggregate was performed by ASTM C136, and the nominal maximum aggregate size (NMAS) of SMA mixtures is 13.2 mm. Before the preparation of the asphalt mixture, the mixing and compaction parameters of the loose asphalt mixture were obtained by the Superpave gyratory compaction (SGC) according to Wang et al. [31]. Moreover, the optimum asphalt binder content of the asphalt mixture was determined by the volumetric parameters of the asphalt mixture. The total binder content (SBS bitumen) is 5.7% by aggregate weight for all the mixtures. Cylindrical specimens with dimensions of ∅150 mm × 170 mm were fabricated by the gyratory compactor at the rotation angle of 1.25° and a vertical pressure of 600 kPa. Finally, the cylindrical specimens with dimensions of ∅100 mm × 150 mm were prepared by using a core drilling machine. Subsequently, a high-precision cutting machine was utilized to cut horizontally from the upper and lower ends of the specimen. These SMA-13 specimens were treated by F–T cycles and then tested for uniaxial compression and tensile as well as CT scanning tests.

2.2. Experimental Procedure

In this study, the detail of the F–T cycle treatment is as follows: the freezing temperature is −18 °C for 16 h, and the thawing temperature is 60 °C for 8 h. Then, the strain-controlled uniaxial compression test was conducted at 50 °C to evaluate the compressive strength of the asphalt mixture. The strain-controlled splitting test was carried out at −10 °C to evaluate the crack resistance of the asphalt mixture. The dynamic indirect tensile test at 20 °C was adopted to evaluate the tensile strength of the asphalt mixture. Therefore, the high-temperature uniaxial compression test, low-temperature splitting test, and indirect tensile stiffness modulus (ITSM) test were carried out to analyze the macro mechanical properties of the asphalt mixture. To investigate the effects of the freeze–thaw cycles on the pore structure of the asphalt mixture, CT scanning and image processing technology were carried out for asphalt mixtures subjected to different freeze–thaw cycles.

2.3. CT Scanning Test

The resulting 60 mm height cylinder specimen was cored to obtain the final cylinder with 60 mm height and 100 mm diameter for CT scanning. A Simens SOMATOM Scope computed tomography scanner was employed for the scanning of asphalt mixtures, with 110 kV as scanning voltage and 110 mA as current. By scanning the sample from top to bottom, more than 100 sectional projection images were obtained. Figure 3 displays the working schematic diagram of the X-ray CT scanning system.

In the image processing, the raw file of image sequence was imported based on the digital image processing technique, and the original slice images were converted into an 8-bit grayscale image. In the process of image acquisition and storage, due to various factors, the image quality is often affected, and it is inevitable that noise will be mixed, affecting the accuracy of the image. How to reduce or eliminate the noise points in the image and ensure the clarity of the details, such as the AOI of the image, is the focus of DIP technology. The remove outliers function was first selected to remove the over-bright regions through radius and threshold. Then the over-dark areas, such as little pores, were removed. For the processed image, through the median filter, the image blur caused by the linear filter can be overcome, and the edge information of the image can be better preserved. The median filtering algorithm was adopted to eliminate random noise in the image to obtain a clearer grayscale image. The grey level of the CT image corresponds to the density of material components one by one. The darker the color is, the greater the density is. Then the contrast between the aggregate and asphalt mortar was then enhanced using adaptive histogram equalization. The equalized image was used for the segmentation of each phase of the asphalt mixture. Based on the previous DIP technology [32,33], Figure 4 shows the filtered and enhanced image.

3. Results and Discussion

3.1. Analysis of Macro Mechanical Properties

Figure 5 shows the results of the uniaxial compression test, splitting test, and indirect tensile stiffness modulus test of asphalt mixture with basalt fiber under F–T cycles. It can be seen from Figure 5a that with the increase of F–T cycles, the uniaxial compressive strength of the asphalt mixture with basalt fiber gradually decreases. Compared with the control group without F–T treatment, the loss rates of the high-temperature uniaxial compressive strength of asphalt mixture with basalt fiber after 3, 6, 9, 12, 15, 18, and 21 F–T cycles are 6.74%, 9.76%, 10.98%, 11.87%, 15.04%, 17.24%, and 24.05%, respectively. In the early stage of the F–T cycle, the uniaxial compressive strength of the asphalt mixture decreases rapidly and then tends to be flat; when the F–T cycle reaches 15~18 cycles, the loss rate of uniaxial compressive strength increases.

It can be seen from Figure 5b that with the increase of F–T cycles, the splitting strength of the asphalt mixture with basalt fiber gradually decreases. At the same time, compared with the control group without F–T treatment, after 3, 6, 9, 12, 15, 18, and 21 F–T cycles, the loss rates of the low-temperature splitting strength of asphalt mixture with basalt fiber are 8.27%, 14.71%, 18.43%, 19.96%, 21.78%, 28.52%, and 31.56%, respectively. In the early stage of the F–T cycle, the low-temperature splitting strength of the asphalt mixture continues to decline and gradually tends to be flat, and after 12~15 F–T cycles, the splitting strength decreases significantly. It can be seen that the development law of F–T damage in the low-temperature splitting test is similar to the above high-temperature uniaxial compression test.

According to the test results in Figure 5c, the indirect tensile stiffness modulus of asphalt mixture with basalt fiber decreases gradually with the increase of F–T cycles. At the same time, compared with the control group without F–T treatment, after 3, 6, 9, 12, 15, 18, and 21 F–T cycles, the loss rates of dynamic indirect tensile stiffness modulus of asphalt mixture with basalt fiber are 8.29%, 11.26%, 14.67%, 19.73%, 21.29%, 24.14%, and 25.41%, respectively. With the F–T cycle increasing, the indirect tensile stiffness modulus of the asphalt mixture continues to decline and gradually tends to be flat.

3.2. Analysis of Meso Volume Characteristics

3.2.1. Meso Characteristic Parameters

According to the volume characteristics of asphalt mixture, four parameters, including air void content, connected void content, void number, and average void diameter, are used to quantitatively describe and evaluate the internal mesostructure of asphalt mixture specimen under F–T cycle and the influence of F–T cycles.

• Air void content

The air void content of the asphalt mixture reflects its compactness to some extent. The higher the density, the lower the air void content of the asphalt mixture, and vice versa. Generally, according to the highway grade, the air void content of the asphalt mixture needs to be controlled during the compaction process. The pavement performance of the asphalt mixture is closely related to the air void content. The smaller the air void content, the worse the permeability of the asphalt mixture, and the asphalt would be easy to be squeezed out of the ground in use. The larger the air void content is, the more water and other impurities would easily penetrate the asphalt mixture and cause blockage, and its strength would also be easily affected. In the CT images, the air void content of the asphalt mixture can be calculated as follows:

Air void content = Void area/Total area × 100%,(1)

• Connected void content

Connected void content is connected with each other between voids and the edge of the end branch of voids, which can be regarded as a void cluster. Generally, the connectivity of the internal voids of asphalt mixture specimens is characterized by the connected void content, which is related to its water permeability.

Connected void content = Connected void area/Total area × 100%,(2)

• Void number and average void diameter

The void number and average void diameter are important geometric parameters of the asphalt mixture mesostructure. In the F–T cycle, there will be new voids in the interior of the asphalt mixture, and there are also voids constantly connected with each other, which can be characterized by the parameter of void number. Therefore, the void number parameter can reflect the formation and development of its internal damage to a certain extent. The average void diameter reflects the size of the void, which is related to air void content and void number.

Average void diameter = 2 × (Void area/(Void number × π))^−1/2,(3)

3.2.2. Air Void Content

The air void content of the internal structure of asphalt mixture with basalt fiber under F–T cycles varies with the interval of 0.6 mm along the vertical height of the specimen, as shown in Figure 6. It can be seen from the figure that under different F–T cycles, the air void content of the asphalt mixture with basalt fiber has a similar distribution law along the specimen height. At the top and bottom of the specimen, the air void content is large. And the air void content overall presents a distribution trend of larger at both ends and smaller in the middle. At the same time, with the increase of F–T cycles, the air void content of the asphalt mixture with basalt fiber becomes larger and larger, which indicates that the F–T cycle treatment has a significant impact on the porosity of the asphalt mixture. This is consistent with the finding obtained by Wu et al. [34]. After 0, 9, and 21 freeze–thaw cycles, the air void content of the asphalt mixture changes from 3.82% to 5.50% and finally to 6.64%. The increase of air void content can be attributed to the continuous generation of new voids in the asphalt mixture. According to the results of CT image processing, the F–T cycle causes new voids inside the asphalt mixture specimen, and the existing voids gradually expand and develop, which eventually leads to the continuous development of F–T damage in the asphalt mixture.

3.2.3. Connected Void Content

Figure 7 shows the distribution changes of the connected void content of the internal structure of asphalt mixture with basalt fiber with the interval of 0.6 mm along the vertical height of the specimen under F–T cycles. It can be seen from the observation in Figure 7 that the change of connected void content of asphalt mixture with basalt fiber is more complex under the same F–T cycle. By comparing the connected void content of the asphalt mixture under different F–T cycles, it can be seen that the connected void content of the asphalt mixture increases gradually with the F–T cycle. After 0, 9, and 21 F–T cycles, the air void content of the asphalt mixture changes from 2.68% to 3.87% and finally to 5.17%. The increase in connected void content is mainly attributed to the development of connected voids. The F–T cycle will lead to the continuous generation of new voids in the asphalt mixture, which will increase the air void content. At the same time, the connectivity between existing voids and between existing voids and new voids inside the asphalt mixture will gradually develop, and its connected void content will also increase, which will gradually increase the F–T damage of the asphalt mixture. This finding was also reported by Gong et al. [33]. When the F–T cycle reaches a certain degree, the connection rate between voids slows down, and the F–T damage mainly shows as new voids or microcracks.

3.2.4. Void Number

Figure 8 shows the distribution of the void number of the internal structure of the asphalt mixture with basalt fiber under F–T cycles with an interval of 0.6 mm along the vertical height of the specimen. As shown in the figure, the change of the void number in the asphalt mixture with basalt fiber in the height direction of the specimen is still complex under the same F–T cycle. However, on the whole, the void number decreases first and then increases with the increase of F–T cycles, consistent with the literature [35]. After 0, 9, and 21 F–T cycles, the void number in the asphalt mixture decreases from 203 to 109 and then increases to 323. This is because in the early stage of the F–T cycle, although new voids are constantly produced in the asphalt mixture, the void number is gradually reduced due to the connectivity between voids and the development of larger connected voids. With the gradual development of the internal damage of asphalt mixture, when the F–T cycle goes to a certain extent, the new voids inside the asphalt mixture increase significantly, the generation of new voids dominates, and the void number increases sharply, which also shows that the F–T damage of asphalt mixture is getting worse and worse.

3.2.5. Average Void Diameter

Figure 9 shows the distribution of the average void diameter of the internal structure of asphalt mixture with basalt fiber under F–T cycles with an interval of 0.6 mm along the vertical height of the specimen. It can be observed from the figure that the average void diameter of asphalt mixture with basalt fiber generally increases with the increase of the F–T cycle, initially showing an increasing trend and later showing a changing trend of first decreasing and then increasing. This is because in the early stage of the F–T cycle, although the damage caused by the F–T treatment inside the asphalt mixture is increasing, the existing voids are connected to each other and develop into larger connected voids. Compared with the formation of new voids, the connected void plays a dominant role, and the void number gradually decreases, which makes the average void diameter increase; With the increase of the F–T cycle, the new voids inside the asphalt mixture increase continuously, and the average void diameter of asphalt mixture decreases in the late F–T period.

3.3. Grey Correlation Analysis between Macro Mechanical Damage and Micro Characteristics

Grey correlation analysis is one of the most widely used models in grey system theory, which has been widely used to evaluate the development trend and correlation of various factors in the uncertainty system. The grey correlation analysis method does not require a high number of samples and does not need a typical factor distribution law. It can effectively extract the main factors affecting the system.

Through previous literature research [6,36], it was found that the changes in the internal mesostructure of the asphalt mixture caused by the F–T cycle can affect the macro mechanical properties of the asphalt mixture. Therefore, in order to clarify the main factors of meso characteristic parameters that influence the mechanical properties of asphalt mixture under the F–T effect, this study adopted the grey correlation analysis method. The change rates of air void content, connected void content, void number, and average void diameter are regarded as the comparison sequence, and the loss rates of uniaxial compressive strength, low-temperature splitting strength, and dynamic indirect tensile stiffness modulus are regarded as the reference sequence. The correlation degree of the asphalt mixture subjected to different F–T cycles with respect to the asphalt mixture without F–T treatment is calculated. The flowchart of the grey correlation analysis is shown in Figure 10. The calculation results of the grey correlation degree between the reference sequence and comparison sequence are shown in Figure 11.

From Figure 11, it can be seen that under different F–T cycles, the change rates of air void content, connected void content, void number, and average void diameter have different effects on the loss rates of compressive strength, splitting strength, and ITSM.

• The order of influence factor on the compressive strength loss of asphalt mixture with basalt fiber from high to low is: void number > air void content > connected void content > average void diameter. The change of void number is the main factor affecting the compressive strength loss of the asphalt mixture with basalt fiber. The correlation between the void number and the compressive strength could be observed and analyzed by combining the above results. It can be found that the void number decreases at the initial stage of the F–T cycle. At this time, the secondary influencing factor for the loss rate of compressive strength, namely, air void content, dominates. When the F–T cycle treatment reaches about 9~12 cycles, the change rate of the void number increases, and the generation of new voids dominates. Similar laws can be found in the loss rate of uniaxial compressive strength. In the early stage of the F–T cycle, the compressive strength of the asphalt mixture decreases and then tends to be flat. When the F–T cycle reaches 12 cycles, the decline rate gradually increases.

• As for the loss rate of splitting strength, the correlation degree of influence factor from high to low is: air void content > connected void content > void number > average void diameter. The change of air void content is the main factor affecting the loss of splitting strength of the asphalt mixture with basalt fiber. The correlation between air void content and splitting strength can be analyzed by combining the above results. The internal air void content of the asphalt mixture increases gradually with the F–T cycle, and the F–T damage continues to form. The change rate of air void content is fast and slow. The observation on the loss rate of low-temperature splitting strength of asphalt mixture shows that the splitting strength decreases continuously, and the loss rate is fast and slow, which is more consistent with the change of air void content.

• For the loss rate of ITSM, the correlation degree of influence factor from high to low is: connected void content > air void content > void number > average void diameter. The increase in connected void content is the main factor affecting the loss rate of ITSM. Therefore, the above results can be combined to analyze and explain the loss rate of ITSM from the perspective of the change of connected void content. With the F–T cycle, the change of internal connected void content of asphalt mixture gradually increases, and the change of connected void content slows down due to the generation of new voids. At the same time, according to the change of ITSM, the loss rate gradually increases with the increase of F–T cycles, and the change amplitude tends to be gentle. It can be seen that the change rate of connected void content is similar to the loss rate of ITSM.

From the above analysis, it can be seen that the changes in different meso characteristic parameters of the asphalt mixture have different effects on its mechanical damage. In order to comprehensively evaluate the meso characteristic correlation factors corresponding to mechanical property attenuation of asphalt mixture under F–T cycles, the following comprehensive correlation index (r_iC) is used for comprehensive analysis of compressive strength, splitting strength, and ITSM.

(4)$r_{iC}=\frac{1}{m}{\displaystyle \sum _{i=1}^m{r}_i}$

where r_iC is the comprehensive grey correlation degree between the loss rates (compressive strength, splitting strength, ITSM) and the meso characteristic parameters, r_i is the single correlation degree between the loss rates (compressive strength, splitting strength, ITSM) and the meso characteristic parameters, respectively, and m is the number of comparison sequences. The calculation result of the comprehensive grey correlation degree is shown in Figure 12.

It can be seen from Figure 12 that the changes in the internal air void content, connected void content, and void number of asphalt mixture have a significant effect on the loss rate of compressive strength, splitting strength, and ITSM caused by F–T cycles, and their corresponding comprehensive grey correlation degrees are similar. However, compared with these three parameters of air void content, connected void content, and void number, the average void diameter has slightly less impact on the macro mechanical properties of the asphalt mixture with basalt fiber.

4. Conclusions

As a typical viscoelastic composite with a void structure, the macro performance damage of the asphalt mixture is actually the result of the accumulation and development of damage behavior at the meso scale. In order to analyze the influence and mechanism of F–T cycles on the mechanical properties of asphalt mixture, the variation of mechanical strength and internal void characteristics were investigated based on the compressive and tensile tests, CT scanning, and DIP technology of SMA-13. The following conclusions are obtained throughout the entire investigation:

(1) The mechanical test results showed that the compressive strength and tensile strength gradually decrease with the increase of F–T cycles and the tested strength values rapidly declined in the early stage of the F–T cycle and gradually tended to be flat.

(2) Based on CT scanning and DIP, the extracted meso characteristics can be used to quantitatively analyze the F–T evolution of internal structure. With the increase of F–T cycles, the internal air void and connected void content gradually increased, and the void number decreased first and then increased, while the average void diameter showed a trend of increasing first and then decreasing on the whole.

(3) Through the grey correlation analysis, it is proved that the damage to the mechanical properties of the asphalt mixture under the F–T cycle can be attributed to the change in the internal mesostructure of the asphalt mixture. The formation of the connected void was the main factor affecting the damage in the early stage of the F–T cycle, while the formation of new voids mainly affected the later development of F–T damage.

The CT scanning test is a fairly new technique that has been adopted to characterize the damage of the asphalt mixture. This study focused on the correlation analysis between mechanical damage and void characteristics through a CT scanning test and found the main influencing factor of early damage and later damage of asphalt mixture by using the grey correlation analysis method. Considering that the structural stability of the asphalt mixture is not only related to aggregates but also to asphalt binder, the superglues such as epoxy asphalt and PU would greatly influence its mechanical strength. The asphalt binder types would be taken into consideration, and the corresponding influences and quantitative analysis will be further addressed.

Author Contributions

Conceptualization, W.W.; Methodology, W.W. and J.L.; Validation, J.L.; Formal Analysis, W.X. and J.L.; Investigation, W.X. and J.L.; Writing—Original Draft Preparation, W.W.; Writing—Review & Editing, W.X. and J.L.; Project Administration, W.W.; Funding Acquisition, W.W. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. The research framework of this study.

Figure 2. Aggregate gradations of investigated mixtures.

Figure 3. The working schematic diagram of X-ray CT scanning system.

Figure 4. Pre-processing of CT images.

Figure 5. The macro mechanical properties results: (a) Compression strength, (b) Splitting strength, (c) Indirect tensile stiffness modulus.

Figure 6. Distribution of internal air void content of asphalt mixture under F–T cycles.

Figure 7. Distribution of internal connected void content of asphalt mixture under F–T cycles.

Figure 8. Distribution of the internal void number of asphalt mixture under F–T cycles.

Figure 9. Distribution of internal average void diameter of asphalt mixture under F–T cycles.

Figure 10. Correlation between mechanical damage and meso characteristics of asphalt mixture.

Figure 11. Correlation between mechanical damage and meso characteristics of asphalt mixture.

Figure 12. Comprehensive correlation between mechanical damage and meso characteristics of asphalt mixture.

Table 1

Technical properties of SBS-modified asphalt.

Parameters	Unit	Values	Standards
Penetration	0.1 mm (@ 25 °C, 100 g, 5 s)	72	T0604
Ductility	cm (@ 15 °C, 5 cm/min)	45	T0605
Softening point	°C	60.5	T0606
Density	g/cm3	1.018	T0603
Flash point	°C	262	T0611
Rolling thin film oven test (RTFOT)
Mass loss	%	−0.094	T0609
Penetration ratio	% (@ 25 °C)	66.9	T0609

Table 2

Basic physical parameters of coarse aggregate.

Parameters		Unit	Values	Standard Limits	Standards
Crushing value		%	13.6	≤26	T0316
Los Angeles abrasion value		%	17.9	≤28	T0317
Apparentspecificgravity	13.2 mm	—	2.836	≥2.6	T0304
	9.5 mm		2.805
	4.75 mm		2.726
Waterabsorption	13.2 mm	%	0.6	≤2.0	T0304
	9.5 mm		0.28
	4.75 mm		0.7

Table 3

Basic physical parameters of fine aggregate.

Parameters	Unit	Values	Standard Limits	Standards
Apparent specific gravity	—	2.73	≥2.5	T0328
Water absorption	%	0.64	—	T0304
Angularity (flow time)	s	39.7	≥30	T0345
Sand equivalent	%	68	≥60	T0334

Table 4

Basic physical parameters of mineral filler.

Parameters		Unit	Values	Standard Limits	Standards
Apparent density		t/m3	2.712	≥2.5	T0352
Hydrophilic coefficient		—	0.63	<1	T0353
Water content		%	0.3	≤1	T0103
Plastic index		%	2	<4	T0354
Granular composition	<0.6 mm	%	100	100	T0351
	<0.15 mm		92.5	90~100
	<0.075 mm		81.8	75~100

Table 5

Basic performances of basalt fiber.

Parameters	Unit	Values
Length	mm	6
Diameter	µm	13
Specific gravity	g/cm3	2.55~2.65
Tensile strength	MPa	≥3000
Elongation at break	%	3.2

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