Low-temperature fracture resistance of plant-mixed heat recycled asphalt mixture based on SCB

To investigate the low-temperature crack resistance of plant-mixed heated recycled asphalt mixture, we conducted a semi-circular bending (SCB) test to analyze the effects of the reclaimed asphalt pavement (RAP) content, salt concentration, and loading rate on the crack resistance at −15 °C. The experimental results show that the fracture energy (G f) of the recycled asphalt mixture decreases with an increase in RAP content and salt concentration but increases with an increase in loading rate. When the RAP content is 0%, the Gf of the specimen reaches its maximum value, while when the RAP content is 50%, Gf reaches its minimum value. When the loading rate is 10 mm s−1, the destruction of coarse aggregates in the specimen is much greater than the destruction mode at a loading rate of 2 mm s−1. Meanwhile, as the RAP content increases, the influence of chloride salt on the low-temperature crack resistance of the recycled asphalt mixture fluctuates. According to the quadratic fitting of RAP content and G f, it can be concluded that a lower loading rate is more helpful in analyzing the degradation mechanism of reclaimed asphalt mixture by salt, and a loading rate of 5 mm s−1 ha−1s−1 a significant impact on the degradation trend of the recycled asphalt mixture. Finally, by establishing the relationship between the Mohr-Coulomb expression and the ultimate tensile stress, we obtained the cohesive force (c) and further analyzed the degradation mechanism of the recycled asphalt mixture under the loading rate and chloride salt. However, the effect of RAP content on c is influenced by various factors, which should be considered comprehensively. These research results enrich the mechanism analysis of the low-temperature cracking performance of plant-mixed heated asphalt mixture and provide theoretical guidance for enhancing the low-temperature cracking resistance design of asphalt pavement.


Introduction
Since 1981, when the State Council of China determined that the national highway network would consist of 12 primary routes, 28 longitudinal axes, and 30 horizontal axes, the main national expressway network has been completed.Over the course of 40 years of development, China has achieved a total expressway mileage of 160,000 kilometers.Notably, the completion and opening of the HZMB (Hong Kong-Zhuhai-Macao Bridge) symbolize China's infrastructure projects reaching a world-class standard.Asphalt mixtures are widely used all over the world thanks to their excellent properties like low noise, high comfort and short construction time [1].Asphalt is the most common mixture used to surface roads because of its smooth, safe and durable qualities.It is less noisy and less expensive to maintain than concrete pavements.The United Kingdom (UK) has about 95% of its roads paved with asphalt [2].This kind of pavement structure is one of the main structural forms of long-life pavement for heavy-duty traffic in China.It has a high bearing capacity, good driving comfort, and low maintenance and repair costs [3].However, due to meteorological conditions, traffic loads, and long-term aging , many roads' performance is gradually becoming inadequate to meet current user requirements.Thus, continuous renovation and maintenance are necessary.During this process, a substantial amount of reclaimed materials from asphalt pavement (RMAP) is generated.According to incomplete statistics, more than 10 million tons of RMAP are produced annually [4].If these materials are not reused, they not only waste valuable resources but also harm the environment.Simultaneously, in recent years, the nation has adopted the concept of sustainable development as part of its overall development perspective [5], Sustainable infrastructure construction has become a research hotspot [6].The research on utilizing secondary resources such as solid waste can effectively improve the supply pressure of high-quality natural stone [7].It is well known that the construction & demolition waste (CDW) is generated in urban construction, and its recycling has become the sustainable development strategy of many cities [8,9].Therefore, it has become critical to develop AM with long-life durability, resource conservation, and reduced environmental impact through the design of materials [6].The reuse of these waste materials and maximizing their value is closely tied to national development strategy, the imperative of creating a more environmentally friendly China, and achieving high-quality development.
At present, numerous engineering examples have demonstrated that hot central plant recycling technology offers advantages in terms of wide applicability and high construction quality compared to other regeneration technologies [10,11].As a result, many studies commence their analysis of the degradation mechanism of recycled asphalt mixtures in various environments by focusing on hot central plant recycling technology.Due to the relatively recent inception of research on road regeneration technology in China, it is still in the stage of practical project application and lacks a systematic theoretical framework [12].Consequently, accurately assessing the comprehensive performance of plant-mixed heat-recycled asphalt mixtures in actual projects remains challenging.Of utmost concern are issues related to low-temperature cracking and fatigue of recycled asphalt mixtures.In order to improve the crack resistance of asphalt mixture, scholars from various countries have carried out extensive research [13,14].Wang et al concluded that the low-temperature crack resistance of recycled asphalt mixtures decreases with a higher percentage of reclaimed asphalt pavement (RAP) and found that the use of a rejuvenating agent (RA) helps mitigate the decline in crack resistance in medium-low temperature environments [15].Through an extensive series of experiments, Wang et al determined that the low-temperature cracking performance of recycled asphalt mixtures primarily depends on the performance of the asphalt binder, and that its resistance to low-temperature cracking deteriorates significantly with an increasing proportion of reclaimed asphalt pavement (RAP) .Additionally, some scholars have employed the semi-circular bending (SCB) test to ascertain that the amount of RAP exerts a significant influence on the crack resistance of recycled asphalt mixtures [15].Due to these challenges, scholars have sought to enhance the utilization rate of reclaimed asphalt pavement (RAP) by focusing on improving material performance.Their efforts aim to maximize RAP content while ensuring adequate low-temperature anti-cracking performance.These efforts include secondary modification of regenerated SBS-modified asphalt, the incorporation of rejuvenating agents (RA), and adjustments to mixing conditions [16,17].However, despite achieving certain results, these findings have not significantly increased the utilization rate of RAP.This limitation primarily stems from the complex geographical conditions in China, which impose various constraints on the promotion and utilization of recycled asphalt mixtures.Additionally, most scholars have primarily conducted isolated analyses, examining either internal factors (such as RAP content and material properties) or external factors (such as temperature and loading rate) that influence the strength of recycled asphalt mixtures .The lack of a comprehensive factor analysis has hindered the ability to systematically reflect the actual road performance of asphalt mixtures, making it difficult to fully comprehend the damage mechanisms and degradation processes of recycled asphalt mixtures.
Building upon this foundation, this paper combines the climatic characteristics of coastal areas in Jiangsu Province.It takes RAP content, salt concentration, and loading rate as variables, utilizing fracture energy to analyze the low-temperature cracking performance of recycled asphalt mixtures under the combined influence of internal and external factors.Concurrently, leveraging the Mohr-Coulomb theory, we establish the relationship between the Mohr-Coulomb expression and ultimate tensile stress to derive cohesion (c).This enables a comprehensive analysis of the low-temperature cracking mechanisms in recycled asphalt mixtures.These findings provide a theoretical framework to support the practical engineering applications of recycled asphalt mixtures in coastal areas.

Materials
In this paper, RAP was obtained from the surface layer of an old expressway.The RAP underwent a sifting process to remove impurities, and the asphalt was then extracted.Initially, trichloroethylene was employed to soak the RAP, dissolving the asphalt on the surface of the aggregates.Furthermore, a high-speed centrifuge was used for aggregate extraction.The recovered asphalt and trichloroethylene mixture underwent distillation.Figures 1-3 shows the details.As per the approach employed by researchers like Alavi [18], short-term aging of the extracted bitumen was conducted to reduce the trichloroethylene content to the lowest possible level.To effectively restore the three major properties of aged asphalt, a rejuvenating agent (RA) was introduced.The RA diffused into the aged asphalt to replenish the lost light components [19], thereby rejuvenating the performance of the aged asphalt [20,21].Heavy oil from 70# road asphalt was used in the process.Table 1 displays the main technical parameters of the RA, while table 2 presents the test results for the three properties of the rejuvenated binders.Both coarse and fine aggregates consist of limestone from Huainan, and the mineral powder in the mixture is also derived from limestone.All of these materials comply with the requirements outlined in the Chinese Technical Specifications for Highway Asphalt Pavement Recycling and Technical Specifications for Highway Modified Asphalt Pavement [22,23].
Table 2 reveals that the extracted bitumen has undergone a certain degree of aging, primarily evidenced by a significant decrease in penetration and ductility, failing to meet the requirements specified in [24].Therefore, the addition of RA to the aged asphalt is imperative to restore its aging performance, enabling the excessively brittle and stiffened aged asphalt to regain its flexibility.Previous research findings have demonstrated that an 8% RA content (by mass fraction, representing 8% of the aged asphalt's total mass) effectively restores the performance of aged asphalt and offers significant economic benefits.
Based on the aforementioned data, it becomes evident that the influence of RA on softening point and penetration remains relatively constant, exhibiting minor fluctuations within a small range.However, its impact on ductility is more pronounced.As the amount of RAP increases, ductility continues to decline, indicating that  restoring the ductility of aged asphalt cannot be achieved solely by adding a fixed proportion of RA.The aged asphalt exhibits severe brittleness.

Test design
Based on the actual situation in Jiangsu Province, located in the eastern coastal region of China, this paper examined the lowest winter temperatures in Jiangsu over the past 20 years.It was found that the lowest winter temperatures in this area typically hover around −10 °C, with occasional dips to −15 °C, recorded in some areas during 2016.Consequently, for testing the cracking performance of recycled asphalt mixture, the environmental temperature was set at −15 °C, with a freeze duration of 6 h.In low-temperature conditions, asphalt mixture predominantly experiences brittle failure, and the loading curve remains relatively stable [20,24].This stability facilitates the analysis of the impact of other variables on the low-temperature cracking resistance of recycled asphalt mixture.
Additionally, due to the frequent occurrence of sea fog throughout the year in coastal areas [21], the study delved into the damage mechanism of RAP caused by chlorine salt.The concentration of chlorine salt (0%, 10%, and 20%) was taken as a variable, and salt solutions were prepared based on the designated concentrations.The testing procedure involved soaking in a constant-temperature water bath at 25 °C for 12 h, followed by indoor drying at 25 °C for another 12 h.This cycle was repeated 30 times.
Furthermore, considering the existing research indicating a moderate correlation between loading rate and asphalt mixture strength [25], with its impact on the performance of recycled asphalt mixture not systematically investigated, the loading rate was introduced as a variable (2 mm/s, 5 mm s −1 , and 10 mm s −1 ).
To thoroughly analyze the degrading trend of RAP on the low-temperature crack resistance of recycled asphalt mixture, RAP content was varied at levels of 0%, 10%, 20%, 30%, 40%, and 50%, with 4 replicate specimens prepared for each variable.The gradation type AC-13 was adopted.To mitigate the substantial variability of RAP and eliminate agglomerations, the particle size of the AC-13 gradation was screened, and a single gradation curve was optimized to minimize gradation errors (as shown in table 3).The total asphalt consumption for the recycled asphalt mixture was determined through the Marshall test, as indicated in table 4.
Generally, the low temperature cracking resistance and high temperature rutting resistance are the focus of pavement performances, which are often affected by the its volumetric performances [26][27][28].One way to evaluate the low temperature cracking resistance of asphalt mixtures is by semi-circular bending (SCB) test [29].The Semi-Circular Bending (SCB) test, known for its effectiveness in evaluating the low-temperature crack resistance of recycled asphalt mixture [30], was selected.Semicircular specimens were cut from the standard Marshall specimens.The Marshall specimen was first halved along its diameter, and each semicircle was subsequently divided into two along its height.As a result, each small semicircular specimen had a diameter of 101.6 mm and a height of 31.75±1mm.The loading configuration is illustrated in figure 4, with the distance between two fulcrums at the bottom of the specimen set at 0.8 times the diameter of the specimen (S/D = 0.8).The notch had a length of 10 mm and a width of 1.5 mm.

Fracture energy analysis
Fracture energy serves as an index for assessing the low-temperature crack resistance of asphalt mixtures.It closely approximates the actual stress conditions on the pavement and provides a comprehensive reflection of the pavement's low-temperature crack resistance [20,31,32].
( ) Where: W f is the work of fracture (N•mm); P is the applied load (N); u is the linear displacement corresponding to the load (mm).
Where: G f is fracture energy (J•mm −2 ); r is the radius of the sample (mm); a is the length of the notch (mm); t is the thickness of the sample (mm).It can be observed from table 4 that the fracture energy (G f ) of the recycled mixture exhibits a clear pattern under various variables.In other words, regardless of the concentration of chlorine salt in the specimen, G f decreases as the RAP content increases, and there is a positive correlation between G f and the loading rate.

Analysis of the influence of RAP content on fracture energy
The G f directly measures the low-temperature crack resistance of the specimens.It serves as a parameter indicating their resistance to low-temperature cracking.As shown in table 5, the G f of the specimens all reaches its maximum when the RAP content is 0%, and when the RAP content reaches 50%, the G f reaches its minimum value.Furthermore, the G f consistently decreases with increasing RAP content, the mechanical compatibility between the aged RAP binder and the fresh binder becomes a critical factor.The aged binder in RAP may have undergone oxidation and stiffening over time.This can lead to a weaker bond between RAP and the new binder, reducing overall cohesion and fracture resistance.highlighting the significant impact of RAP content on the specimens' G f .
The strength of the asphalt mixture primarily relies on the cohesive force of the asphalt and the frictional resistance of the aggregate.Therefore, the performance of the rejuvenated binder and recycled aggregate plays a  crucial role in the ability to resist low-temperature cracking.Additionally, this test is conducted at −15 °C, resulting in a strictly brittle failure mode [20], characterized by sudden fracture-loss of bearing capacity upon reaching the failure point.The adverse effect of recycled asphalt mixture on low-temperature crack resistance becomes even more pronounced at lower environmental temperatures.Due to the inability to completely remove aged asphalt on the surface of the aggregate, the aged asphalt remains incompletely regenerated due to the severe volatilization of light components and high viscosity .As a result, it remains in a semi-fused state when blended with virgin asphalt [10,33,34].With an increase in RAP content, the uneven miscibility between aged asphalt and virgin asphalt intensifies, forming a weak interface [35].Simultaneously, the asphalt exhibits brittleness and stiffness in low-temperature environments [4], leading to inferior adhesion between aged asphalt and aggregate compared to virgin asphalt [36].This significantly impairs the low-temperature resistance of the specimens.
Furthermore, the surface of RAP is coated with aged asphalt mortar.Even when the extracted aggregate is heated in an oven at 115 °C for 4 h before mixing, a certain amount of moisture remains in RAP.During the high-temperature mixing process, the excessive heat causes the asphalt on the surface of the recycled aggregate to melt, while the moisture within the aggregate evaporates.However, some water molecules, influenced by the aged asphalt, fail to evaporate and accumulate on the aggregate's surface.This reduces the adhesion of the recycled asphalt mixture, thereby diminishing its low-temperature crack resistance to some extent.4 illustrates a positive correlation between G f and the loading rate.As the loading rate increases, the G f of the recycled asphalt mixture gradually rises.This phenomenon is attributed to the fact that at higher loading rates, the energy accumulated within the specimen is not released quickly, a higher loading rate can result in more concentrated stresses at certain points within the material, These localized stress concentrations can promote crack initiation and propagation.resulting in a 'delay' error.This delay leads to an increase in peak load and subsequently, an increase in G f .Upon failure, a sudden release of energy occurs, accompanied by a 'bang' sound.

Analysis of the influence of loading rate on fracture energy Table
In contrast, A lower loading rate distributes the load more evenly, reducing the likelihood of stress concentrations and cracks.at lower loading rates, stress concentration is more likely to occur at the specimen's initial tiny defects.With the accumulation of energy, micro-cracks can rapidly expand, forming a path of failure [34,37].Observing the failure crack of the specimen, when the loading rate is 2 mm s −1 , as shown in figure 5(a), the cracks primarily follow the interface between asphalt mortar (asphalt and mineral powder) and the combination of asphalt fine material (asphalt, fine material, and mineral powder) [35].This behavior is mainly due to the fact that the strength of asphalt mortar and the bond strength between asphalt mortar and aggregate are much lower than the strength of the coarse aggregate [25], resulting in the formation of a weak interface.Consequently, the cracks bypass the aggregate [15].
However, when the loading rate reaches 10 mm s −1 , as shown in figure 5(b), cracks no longer avoid the larger aggregate particles.Consequently, the fracture surface of the recycled asphalt mixture is often dominated by the failure of the coarse aggregate under higher loading rates .
Based on the above analysis, even in a −15 °C environment where the asphalt mixture becomes brittle, the instantaneous tensile strength of the coarse aggregate remains higher than the instantaneous cracking strength of the asphalt mortar at lower loading rates (2 mm s −1 ) [24,38].The high loading rate (10 mm/s) becomes a significant contributing factor to the failure of the coarse aggregate.

Analysis of the influence of salt concentration on fracture energy
To analyze the influence of salt on the recycled asphalt mixture more intuitively, the G f is drawn as a line chart, as shown in figure 5.
It can be observed from figure 6 that the erosion of chloride salt has a certain degree of influence on the G f of the specimens.The G f of the recycled asphalt mixture generally decreases with an increase in chloride salt concentration.Salt ions can chemically interact with the asphalt binder, potentially causing its degradation.This chemical interaction can result in the formation of voids or weakened bonds within the mixture, reducing its cohesion and fracture resistance.As the RAP dosage increases, regardless of the loading rate, the G f curve of the recycled asphalt mixture intersects at 40%-50% RAP content.This suggests that the degradation of specimens due to salt exhibits some fluctuation with increasing RAP content.
This phenomenon primarily arises from the fact that the surface-aged asphalt on the larger aggregate particles in the recycled asphalt mixture possesses a high viscosity and forms a strong interface [36].This characteristic makes it challenging for the chloride ion-containing salt solution to penetrate effectively, thereby reducing salt-induced erosion damage to some extent.Consequently, increasing the RAP content can enhance the salt erosion resistance of the recycled asphalt mixture.

Fitting RAP content and fracture energy
Table 5 presents the fitting results, where 'y' represents the fracture energy, and 'x' denotes the RAP content.The fitting results reveal that in a −15 °C environment, the G f of the recycled asphalt mixture follows a quadratic polynomial trend with respect to RAP content [15,39].Analyzing the change in the slope of fracture energy, it becomes apparent that at a low loading rate (2 mm s −1 ), the increase in RAP content results in a more pronounced influence from 10% chloride salt on the degradation trend of the recycled asphalt mixture.However, at medium and high loading rates (5 mm s −1 and 10 mm s −1 ), the chloride salt concentration has minimal effect on the degradation trend of the specimens.This suggests that the degradation trend of lowtemperature performance in the recycled asphalt mixture is influenced by the loading rate.To thoroughly analyze the degradation mechanism caused by salt concentration, it is advisable to employ a lower loading rate.
Furthermore, the degradation trend in crack resistance of the recycled asphalt mixture due to pure water exposure appears to progress at a slower pace.Huang et al discovered that immersion in the salt solution led to partial dissolution of the light components in the asphalt and a decrease in gelatin content, weakening the bond between the aggregate and interface.
The above analysis indicates that salt exacerbates the deterioration of low-temperature cracking resistance in the recycled asphalt mixture.Additionally, table 6 highlights that a loading rate of 5 mm s −1 exerts a more significant influence on the degradation trend of crack resistance in the recycled asphalt mixture.This suggests that in real-world road applications, lower loading rates do not necessarily favor road durability.

The change law of the cohesive force
The strength of the asphalt mixture primarily consists of two components: cohesive force due to the presence of asphalt and internal friction resistance resulting from the presence of aggregate.In the experimental condition of three-point bending fracture, the compressive stress within the asphalt mixture is predominantly supported by the internal friction resistance formed by the framework and embedded forces.Tensile stress is primarily carried by the cohesive force between the asphalt mortar and the mineral material.ANSYS analysis was employed to create a 1.5 mm wide and 10 mm long notch in the center of the bottom of the specimens.Most areas of the specimen experience tensile stress, with pressure concentration observed only at three constraint positions [24].
The low-temperature performance of recycled asphalt mixture is significantly influenced by the properties of the rejuvenated binder and recycled aggregate.Therefore, it is essential to investigate the relationship between cohesive force and the ultimate tensile stress of recycled asphalt mixture.In this test, a single gradation was employed for RAP with varying dosages, and the specimens were fabricated under identical test conditions, allowing the assumption of the same internal friction angle [10].This paper utilizes the Mohr-Coulomb theory to analyze the strength of recycled asphalt mixture, with cohesive force serving as the analysis parameter for strength theory, indirectly assessing the fusion performance of the recycled asphalt mixture.The formula of Mohr-Coulomb is provided in equation (3).
Annotations: The internal friction angle j is assumed to be constant, then , and the SCB test is a three-point bending pull, so s 3 = 0 and s s = , 1 0 namely: Where: c is cohesive force (MPa).P Max is the maximum load (N); t is the thickness of the specimen (mm); r is the radius of the sample (mm).
According to the relationship between loading rate and ultimate tensile stress, the equation y = kx + b was employed to fit the correlation curve.The equation y = s 0 was established to investigate the change pattern of 'c'.The results are presented in tables 7, 8, and 9.

Analysis of the law of c changing with loading rate
Figure 7 illustrates a noticeable variation pattern for 'c'.When the RAP content remains constant, 'c' increases with an increase in loading rate, aligning with Qian's law, which solves 'c' and the internal friction angle by establishing joint equations .Therefore, from a loading rate perspective, it can be inferred that the internal friction angle has minimal impact on the strength of the recycled asphalt mixture in this test, further validating the earlier inference.Furthermore, the trend of the curves reveals that when the salt concentration is 20%, the curve's trend remains nearly identical under different loading rates.However, when the salt concentration is 0%, the variation pattern of 'c' differs across various loading rates.This suggests that after 30 cycles of salt erosion, within the range of the six RAP dosages set in this experiment, 20% chlorine salt reduces the disparity in the cohesion curve and stabilizes the trend in the 'c' curve under the three loading rates.

Analysis of the law of c changing with salt concentration
Figure 8 reveals the sensitivity of 'c' in recycled asphalt mixture to salt erosion.In comparison to pure water, salt erosion reduces 'c' in the recycled asphalt mixture, aligning with the detrimental impact of salt erosion on conventional asphalt mixtures.According to Fick's law, the saline water entering the asphalt leads to a saltingout effect , causing the continuous precipitation of saturated and aromatic components from the asphalt mortar.As salt concentration increases, the salting-out effect strengthens, resulting in a higher leaching of light components from the asphalt.This process accelerates asphalt aging and diminishes 'c'.
Furthermore, when saline water infiltrates the interior of the asphalt mortar, Cl -and Na + ions are partially retained within the asphalt voids and weak interfaces due to the influence of asphalt components [40].As salt is hydrophilic, residual chlorine salts continue to absorb water, further weakening 'c'.Consequently, at a loading rate of 2 mm s −1 , the reduction in 'c' in the recycled asphalt mixture caused by 20% chlorine salt is more pronounced than that caused by other concentrations.However, under the same loading rate, a comparison and analysis of the 'c' curves for 10% and 20% salt concentrations reveals that the disparity value between the 'c' curves for these two concentrations decreases gradually with an increase in loading rate.This phenomenon suggests that a high loading rate (10 mm s −1 ) conceals the effect of high salt concentration (20%) on 'c'.In other words, 'c' varies with the increase in RAP content, indicating that 'c' is significantly influenced by the RAP content.The 'c' is derived by establishing the relationship between the Mohr-Coulomb theory and ultimate tensile stress.Based on the earlier assumptions, it was assumed that the internal friction angle remains constant.Consequently, it can be inferred that ultimate tensile stress and internal friction angle are the primary factors contributing to the fluctuation of 'c' with increasing RAP content.Furthermore, according to the P Max values, the P Max of the recycled asphalt mixture increases with the rise in RAP content.This can be attributed to the increased hardness of the recycled asphalt mixture due to the addition of RAP [41], leading to a corresponding increase in P Max .Consequently, ultimate tensile stress also increases, signifying that ultimate tensile stress exhibits a certain pattern with the increase in RAP content.Additionally, a linear function is employed to fit the ultimate tensile stress, with a high R 2 value, rendering the influence of fitting errors negligible.
In light of the above analysis, it can be summarized that RAP content has a substantial impact on 'c' in recycled asphalt mixture, attributable to two primary reasons.Firstly, during particle size screening, the presence of both strong and weak agglomerations in RAP cannot be eliminated.These agglomerates are reorganized during the stirring and compaction processes, resulting in gradation instability that affects 'c'.Secondly, there is considerable uncertainty in the fusion of virgin asphalt and aged asphalt during asphalt regeneration.As RAP content increases, evaluating the miscibility effect becomes increasingly challenging.Therefore, a comprehensive consideration of the influence of RAP content on 'c' in recycled asphalt mixture is warranted.

Conclusions
An SCB test was conducted to assess the crack resistance of heat-recycled asphalt mixture at −15 °C.The relationship between the Mohr-Coulomb theory and ultimate tensile stress was established, and obtaining the cohesive force (c) was deemed feasible.The effects of RAP content, salt concentration, and loading rate on the low-temperature crack resistance of recycled asphalt mixture were analyzed using fracture energy and 'c'.Several key findings can be summarized as follows: (1) The inclusion of RAP significantly reduced the G f (fracture energy) of the recycled asphalt mixture, and G f decreases as the RAP content increases.When other variables are held constant, G f is positively correlated with the loading rate, and the crack path of the specimen is influenced by the loading rate.At a loading rate of 2 mm s −1 , the failure path of the specimen bypasses the coarse aggregate.However, if a loading rate of 10 mm s −1 is used, the coarse aggregate on the fracture surface is disrupted.
(2) Salt has a detrimental effect on the low-temperature crack resistance of recycled asphalt mixture, and an increase in RAP content mitigates the damage caused by salt to specimens to some extent.Quadratic fitting of G f and RAP content reveals the following pattern: the degradation trend of salt concentration on the lowtemperature crack resistance of recycled asphalt mixture is influenced by the loading rate.To conduct a more detailed analysis of the deterioration mechanism of salt concentration, a lower loading rate should be adopted.Furthermore, the loading rate of 5 mm s −1 ha −1 s −1 the most significant impact on the deterioration trend of the specimens.
(3) At the same salt concentration and RAP dosage, c increases with an increase in the loading rate, while salt erosion weakens the cohesion (c) of the reclaimed asphalt mixture.Moreover, a high loading rate (10 mm s −1 ) masks the impact of high salt concentration (20%) on c.However, due to the uneven mixing of virgin asphalt and aged asphalt, as well as the easy recombination of strong and weak agglomerates during the mixing and compaction process, the gradation becomes unstable.Therefore, with an increase in RAP content, c fluctuates significantly, requiring a comprehensive analysis.
In conclusion, for the analysis of low-temperature crack resistance in recycled asphalt mixtures, the primary recommendation is to use G f as the evaluation criterion.If it is necessary to analyze the degradation trends of recycled asphalt mixtures at different chloride salt concentrations, a comprehensive analysis based on multiple indicators should be conducted, and lower loading rates should be employed.In engineering applications, it is advised not to exceed a RAP content of 30%, as higher proportions of RAP can affect the stability of recycled material performance.

Figure 1 .
Figure 1.Contrast the extracted material with the new material.

Figure 3 .
Figure 3.Comparison of strong and weak agglomeration.

Figure 4 .
Figure 4. Schematic diagram of loading of SCB specimen.

Figure
Figure Crack path of recycled asphalt mixture.

Figure 6 .
Figure 6.Effect of salt concentration on G f .

Figure 8 .
Figure 8.Effect of salt concentration on c.

Table 1 .
Properties of the rejuvenating agent.

Table 2 .
Properties of the asphalt.

Table 4 .
Asphalt contents of recycled mixtures.

Table 5 .
G f of SCB specimens.

Table 6 .
Fitting results between fracture energy and RAP contents of the recycled mixture.