Evaluation on the Adhesion Property of Recycled Asphalt Based on the Multi-Scale Experiments

Abstract

The adhesion property has consistently been a critical focus in the utilization of recycled asphalt (RA). This research aimed to elucidate the mechanisms influencing the adhesion property of RA at various scales. Specimens of base asphalt (BA), aged asphalt (AA), and RA were systematically prepared. The impacts of aging and rejuvenators on the nano adhesion property of asphalt were assessed using multi-scale testing methods. The findings revealed that aging adversely affected the adhesive interaction between BA and aggregate, whereas the application of rejuvenators substantially improved this effect. When compared to acidic aggregate of granite, the alkaline aggregate of limestone demonstrated superior adhesion properties with RA. Moreover, the correlation analysis affirmed that mechanical testing across various scales consistently evaluated the trends in the adhesion property of RA with aggregates.

1. Introduction

Over time, asphalt pavements endure prolonged exposure to traffic and environmental conditions, which often results in surface layer thinning and aging [1,2,3]. Consequently, the performance metrics of asphalt pavements decline, necessitating maintenance and repair earlier than their designed lifespan. This premature deterioration generates substantial quantities of recycled asphalt pavement (RAP) [4,5,6]. To address the growing needs for road maintenance and to conserve resources, the recycling of asphalt pavement has emerged as a focal area of research [7,8]. The primary method for recycling involves using rejuvenators to restore the functional attributes of aged asphalt (AA) found in RAP, making the effectiveness of rejuvenators on the service performance of AA a critical aspect of RAP recycling and reuse [9,10].

The aging of asphalt, primarily induced by external factors such as heat, oxygen, ultraviolet radiation, and moisture, leads to changes in its chemical structure and the redistribution of its components [11,12,13]. This process causes the light fractions within the asphalt to convert into heavy fractions, resulting in increased viscosity and a tendency towards elastic deformation, ultimately contributing to cracking and rutting [14]. Additionally, as the asphalt ages and becomes more viscous, its adhesion property with aggregates deteriorate, which can cause the asphalt mixture to loosen and strip, leading to further pavement distress [15,16]. Therefore, improving the adhesion property of AA after rejuvenation significantly affects the performance of reclaimed asphalt (RA) and recycled asphalt mixtures. The adhesion properties of RA have increasingly become a focal point for research [17,18]. In this context, Li [19] conducted an atomic force microscopy (AFM) examination to explore the adhesion between asphalt and aggregate in recycled asphalt mixtures, finding that the addition of cement enhances this interaction. Chen [20] explored the change trend of the adhesion property of RA through AFM and various rheological tests. The results indicated that adhesion between asphalt and aggregates weakened as rejuvenation cycles increased, concurrently reducing RA’s aging resistance. Zhao [21] employed a contact angle test to study the adhesion property of recycled high-viscosity asphalt, revealing that higher ambient temperatures diminished the adhesion property. Zhang [22] utilized AFM to assess the influence of rejuvenators on the adhesion property of AA, discovering an enhancement in adhesion between AA and aggregates due to rejuvenator use. Similarly, Ji [23], using the surface free energy (SFE) test, found that rejuvenators primarily counteracted physical aging and adjusted components to improve AA adhesion property. These studies collectively show significant insights into RA’s adhesion property; however, it primarily focuses on a singular scale, while the analysis of the variation rule of the adhesion property of RA at the multi-scale is lacking. Moreover, the rejuvenator type was single for the research on the adhesion property of RA; thus, the evolution mechanism of the adhesion property of RA at the multi-scale still needed to be developed.

Adhesion properties are crucial for the structural integrity and longevity of asphalt pavements, particularly since aging can significantly impair these properties in asphalt. Restoring these properties in AA through rejuvenation is therefore critically important for the sustained stability of recycled asphalt pavements [24,25]. RA that exhibits superior adhesive qualities contributes to a more robust pavement structure by effectively bonding with aggregates. This bond significantly bolsters the resistance of the pavement to cracking and enhances its durability, ultimately extending the lifespan of the pavement [26]. In addition, this is also conducive to further promoting the application of RA in the practical engineering, reducing the waste of solid waste resources, which provides a considerable economic and environmental benefit. Therefore, it is necessary to conduct deep research on the adhesion properties of RA at multiple scales to further investigate the evolution mechanism of the adhesion properties of RA. In this study, four distinct rejuvenators were used to prepare RA specimens. Subsequently, the variations in the adhesive properties between the RA and aggregates were analyzed at different scales using the pull-off (PO) test, SFE test, and AFM test. Furthermore, a correlation analysis was conducted to explore the relationship between adhesion property indicators across these various scales. The in-depth evaluation of the evolution mechanisms influencing the adhesion property of RA was intended to establish valuable insights into the application and research of RA.

2. Materials and Methods

2.1. Nomenclature

Since several asphalts and test methods were involved in this study, and to facilitate the writing and simplifying of the paper, several asphalts and test methods were named by using abbreviations in this study. To facilitate the analysis, these abbreviations are summarized and exhibited in

Table 1. The abbreviations for several asphalts and test methods.

Abbreviation	Explanation
BA	Base asphalt
AA	Aged asphalt
RA	Recycled asphalt
SFE	Surface free energy
PO	Pull-off
AFM	Atomic force microscopy
RTFOT	Rolling thin film oven test
TFOT	Thin film oven test
PF-QNM	Peak force quantitative nano mechanical
A	Bio-oil-based rejuvenator
B	Mineral oil-based rejuvenator
C	Naphthenic oil-based rejuvenator
D	Waste engine oil-based rejuvenator

.

2.2. Asphalt

The BA utilized was 90# base asphalt, while the AA consisted of milled material sourced from a base asphalt mixture pavement of a roadway that had been in use for over five years. Specifically, when recovering AA from old pavements, the recycled milled material was first crushed to an appropriate particle size. Then, trichloroethylene was chosen as the solvent. The milled material and trichloroethylene were stirred at a temperature of 25 °C, and the mixture containing dissolved AA was separated using a centrifuge. Finally, the AA was extracted from the mixture using a rotary evaporator. The basic properties of both materials are detailed in

Table 2. The basic properties of asphalts.

Asphalt Type	Penetration/(0.1 mm/25 °C)	Ductility/(cm/15 °C)	Softening Point/°C
AA	55	53.17	56.4
BA	84	>100	45
BA after RTFOT aging	61.3	56.9	55.4
Technical specifications of BA [27]	80~100	>100	≥44
Requirements after RTFOT or TFOT aging of BA	penetration ratio ≥ 57%	≥20	-
Test method [28]	T 0604	T 0605	T 0606

.

2.3. Rejuvenator

According to the aging mechanism of asphalt and the theory of component regulation, the aging process in asphalt predominantly occurs as a transition from light to heavy components. Consequently, the rejuvenators currently in use are primarily composed of oils, which are designed to replenish the light components that are depleted in AA [29,30]. For this, four typical oil-based rejuvenators were selected for the preparation of RA in this study, which were the bio-oil-based rejuvenator (A), mineral oil-based rejuvenator (B), naphthenic oil-based rejuvenator (C), and waste engine oil-based rejuvenator (D), and their physical properties are shown in

Table 3. The physical properties of rejuvenators.

Performance Index	A	B	C	D
Density/(g/cm3)	0.925	0.998	1.01	0.963
Viscosity/(Pa·s/60 °C)	0.134	0.127	0.153	0.132
Flash point/°C	225	239	231	236
Viscosity ratio for RA after RTFOT aging	1.86	1.68	1.43	1.56
The absolute value of mass loss rate for RA after RTFOT aging/%	1.27	1.01	0.76	0.93

. It was found that RA with several rejuvenators had lower viscosity ratios and mass loss ratios after RTFOT aging, and also had low viscosity and high flash points, which indicated that several rejuvenators were oil-based rejuvenators with excellent aging resistance. Among them, the RTFOT is a method used to simulate the aging process of asphalt during actual construction and short-term aging. First, 35 g of the asphalt sample is placed in a sample bottle. The sample bottle is then placed in the RTFOT equipment, with the test temperature set to 163 °C, the airflow rate set to 4000 mL/min, and the test time set to 75 min. After the test, the sample taken from the bottle is the RTFOT-aged asphalt. Furthermore, the preliminary exploratory findings from this study indicated that the recommended optimal dosage of several rejuvenators was 4% [31]. This specific dosage was subsequently utilized in the preparation of RA specimens for the subsequent tests described in this study.

2.4. Preparation of RA

To investigate the changes in the adhesion properties of AA before and after rejuvenation and to evaluate the evolution mechanism of RA, this study utilized four different rejuvenators for preparing RA specimens. The preparation process was as follows: Initially, AA was heated in an oven at 150 °C for 2 h until it became flowable. Subsequently, 200 g of AA was measured into a beaker, and a rejuvenator was added at a mass ratio of 4%. The beaker was then placed in an oil bath at 150 °C, and the mixture was manually stirred with a glass rod for 5 min. Finally, the mixture was subjected to high-speed shearing at 2000 rpm for 15 min to complete the preparation of RA. The detailed preparation process is depicted in Figure 1.

2.5. Methods

2.5.1. PO Test

The PO test is employed to assess and characterize the bond strength between asphalt and aggregate at a macroscopic scale, thereby providing insights into the adhesion property of asphalt [32]. To characterize the variation trend of the bond strength of AA before and after rejuvenating, the bond strength between several asphalts and two aggregates was investigated by using the PO test in this study. The test aggregates used in this study were limestone and granite, and both aggregate slates were sanded using sandpaper with 600 abrasive particles per square inch until the slate surface was smooth and level. The slates were first cleaned with water and then dried in an oven at 150 °C for 4 h along with the tie bars until both reached a stable dry weight. Subsequently, 0.2 g of asphalt was evenly spread on the slate surface and heated in the oven until smooth. The tie bars were subsequently affixed to the asphalt and maintained at a temperature of 25 °C for durations spanning 2, 4, and 8 h. The bond strengths were measured, and the samples were designated as PO-2, PO-4, and PO-8 based on the bonding duration. In addition, three repetitive tests were conducted on each asphalt sample. Particularly, based on relevant research experience [33], the bond strength was tested at 25 °C in this study.

2.5.2. SFE Test

The SFE theory suggests that when a liquid is applied to the surface of a material, wetting behavior occurs, and the degree of wetting correlates with the adhesive properties of the material. In this study, the SFE test was employed to meticulously assess the adhesive properties of RA at a microscopic level [34,35]. The SFE test typically involves measuring the contact angle between a liquid and asphalt using the sessile drop method. The test liquids utilized in the experiment encompassed distilled water, glycerol, and formamide. The test aggregates selected for analysis were limestone and granite, and their physical properties are detailed in Figure 2. In addition, three repetitive tests were conducted on each asphalt sample. Post-test, the contact angles between RA and various liquids were measured to compute the cohesive, adhesive, and peeling work of RA, thereby evaluating the adhesion property variation mechanism of RA [36]. The calculation methods are provided in Formulas (1)–(3).

$${\mathrm{W}}_{\mathrm{A}\mathrm{B}}={\mathsf{\gamma }}_{\mathrm{B}}\left(1+\mathrm{c}\mathrm{o}\mathrm{s}\mathsf{\theta }\right)=2\sqrt{{\mathsf{\gamma }}_{\mathrm{A}}^{\mathrm{d}}{\mathsf{\gamma }}_{\mathrm{B}}^{\mathrm{d}}}+2\sqrt{{\mathsf{\gamma }}_{\mathrm{A}}^{+}{\mathsf{\gamma }}_{\mathrm{B}}^{-}}+2\sqrt{{\mathsf{\gamma }}_{\mathrm{A}}^{-}{\mathsf{\gamma }}_{\mathrm{B}}^{+}}$$

(1)

$${\mathrm{W}}_{\mathrm{B}\mathrm{B}}=2{\mathsf{\gamma }}_{\mathrm{B}}$$

(2)

$${\mathrm{W}}_{\mathrm{A}\mathrm{B}\mathrm{W}}=2\sqrt{{\mathsf{\gamma }}_{\mathrm{w}}^{+}}\left(\sqrt{{\mathsf{\gamma }}_{\mathrm{B}}^{-}}+\sqrt{{\mathsf{\gamma }}_{\mathrm{A}}^{-}}\right)+2\sqrt{{\mathsf{\gamma }}_{\mathrm{w}}^{-}}\left(\sqrt{{\mathsf{\gamma }}_{\mathrm{B}}^{+}}+\sqrt{{\mathsf{\gamma }}_{\mathrm{A}}^{+}}\right)+2\sqrt{{\mathsf{\gamma }}_{\mathrm{A}}^{\mathrm{d}}{\mathsf{\gamma }}_{\mathrm{W}}^{\mathrm{d}}}+2\sqrt{{\mathsf{\gamma }}_{\mathrm{W}}^{\mathrm{d}}{\mathsf{\gamma }}_{\mathrm{B}}^{\mathrm{d}}}-2{\mathsf{\gamma }}_{\mathrm{w}}^{\mathrm{d}}-2\sqrt{{\mathsf{\gamma }}_{\mathrm{A}}^{\mathrm{d}}{\mathsf{\gamma }}_{\mathrm{B}}^{\mathrm{d}}}-2\sqrt{{\mathsf{\gamma }}_{\mathrm{A}}^{-}{\mathsf{\gamma }}_{\mathrm{B}}^{+}}-2\sqrt{{\mathsf{\gamma }}_{\mathrm{A}}^{+}{\mathsf{\gamma }}_{\mathrm{B}}^{-}}+4\sqrt{{\mathsf{\gamma }}_{\mathrm{W}}^{-}{\mathsf{\gamma }}_{\mathrm{W}}^{+}}$$

(3)

While ${\mathsf{\gamma }}_{\mathrm{A}}^{\mathrm{d}}$, ${\mathsf{\gamma }}_{\mathrm{B}}^{\mathrm{d}}$, ${\mathsf{\gamma }}_{\mathrm{A}}^{+}$, ${\mathsf{\gamma }}_{\mathrm{B}}^{+}$, ${\mathsf{\gamma }}_{\mathrm{A}}^{-}$, and ${\mathsf{\gamma }}_{\mathrm{B}}^{-}$ represent the dispersion, Lewis acid, and Lewis base components of the SFE for both aggregates and asphalt, ${\mathrm{W}}_{\mathrm{A}\mathrm{B}}$ denotes the adhesion work, ${\mathrm{W}}_{\mathrm{B}\mathrm{B}}$ denotes the cohesive work, and θ denotes the contact angle. ${\mathsf{\gamma }}_{\mathrm{B}}$ stands for the asphalt SFE. The peeling work is indicated by ${\mathrm{W}}_{\mathrm{A}\mathrm{B}\mathrm{W}}$. Additionally, ${\mathsf{\gamma }}_{\mathrm{W}}^{\mathrm{d}}$, ${\mathsf{\gamma }}_{\mathrm{W}}^{+}$, and ${\mathsf{\gamma }}_{\mathrm{w}}^{-}$ are the dispersion, Lewis acid, and Lewis base components of water.

2.5.3. AFM Test

In this study, the evolution of adhesion property of RA at the nano level were explored using AFM. The AFM analysis utilized the peak force quantitative nano mechanical (PF-QNM) mode [37,38], which enabled the assessment of mechanical properties across various asphalt specimens. The procedure involved heating the asphalt in an oven at 150 °C until it reached a flow state, then applying a small amount to the center of a slide. The slide was then placed back into the oven at 150 °C to ensure uniform dispersion and formation of a smooth surface. Afterward, the specimens were cooled in a dryer at 25 °C until solidified. In addition, three repetitive tests were conducted on each asphalt sample. Following the experimental testing phase, the nano adhesion and Derjaguin–Muller–Toporov (DMT) modulus of the asphalt samples were evaluated using the NonoScope Analysis 1.8 software.

3. Results

3.1. PO Test

To examine the changes in adhesion properties of AA before and after rejuvenation, this study conducted the PO test on various asphalts. The adhesion properties of multiple asphalts and different aggregates were evaluated based on PO strength [39], as depicted in Figure 3.

From Figure 3, the PO strength of AA with aggregate was lower than that of BA. This is attributed to the aging effect, which shifts the light components to heavy components within the asphalt, reducing the proportion of viscous components and increasing the tendency towards elastic deformation. Consequently, the adhesion property was diminished, resulting in a decline in PO strength, indicating that aging weakens adhesion. The introduction of a rejuvenator significantly increased the PO strength between RA and aggregate. The rejuvenator, primarily consisting of oil components, replenished the depleted light fractions within AA, thereby restoring the asphalt’s propensity towards viscous deformation and augmenting the bonding strength between the RA and the aggregate. This enhancement manifested notably in the increased PO strength observed at the macroscopic scale. Despite RA demonstrating a significantly higher adhesion property compared to AA, its performance remained marginally inferior to that of BA. This is because the rejuvenator primarily improved the viscoelastic component ratio within AA, leading to its gradual softening and partial restoration of the adhesion property. However, during practical use, asphalt aging involves not only component transfer, but also chemical structure changes. The rejuvenator generally has a limited effect on the chemical structure of AA, resulting in the PO strength of RA being slightly lower than that of BA.

In addition, it was revealed that there were differences in the rejuvenating effect for AA with different rejuvenators. With the incorporation of rejuvenators, the PO strength of several RAs increased by 11.11–31.82%, 12.69–45.45%, 17.46–54.55%, and 14.29–40.91%, and it showed that the naphthenic oil-based rejuvenators had the best restoration on the adhesion property of AA. The superior performance of naphthenic oil in rejuvenating asphalt adhesion properties can be attributed to its comparatively simpler molecular structure [40], facilitating enhanced penetration into RA. This deeper infiltration intensifies the fusion with AA and further enriches the viscoelastic components of AA. Consequently, naphthenic oil exhibits a rejuvenating effect on the AA adhesion property that surpasses other rejuvenators marginally. Notably, the type of aggregate also exerts a substantial influence on the RA adhesion property. Specifically, the PO strength observed with limestone combined with various asphalt types exceeded that with granite, owing to the aggregate’s material properties. Asphalt, inherently acidic due to its composition, which is rich in asphaltic acid and asphaltic anhydride, is predisposed to neutralization reactions with alkaline aggregates, thereby enhancing the adhesion property. Granite, primarily composed of quartz with low-valent internal cations such as Na⁺, is more inclined to be acidic. In contrast, limestone, mainly consisting of CaCO₃ with high-valent internal cations like Ca²⁺, has higher alkalinity than granite, providing a better adhesion property for asphalt. This indicates that limestone is more suitable as an aggregate for RA in practical engineering applications.

3.2. SFE Test

3.2.1. Contact Angle

To assess the microscopic adhesion property of different asphalt samples, this study conducted experiments aimed at measuring the contact angles [41], as depicted in Figure 4.

Based on the findings presented in Figure 4, it was observed that the contact angle of AA with several test liquids showed a significant reduction compared to BA. The measurement of contact angles served as an indicator of the interaction between asphalt and the test liquids; a lower value indicated increased wetting capability of the asphalt. This result suggested that aging processes enhanced the asphalt’s ability to be wetted by the test liquids. Considering that the deterioration of asphalt adhesion property in practical engineering is primarily influenced by water, and the contact angle of distilled water with asphalt was notably higher than other test liquids, this study specifically focused on the contact angle between distilled water and different asphalt types. It was found that the contact angle between AA and distilled water was lower than that of BA, whereas the contact angle between RA and distilled water was higher than that of AA. This indicates that AA is more easily wetted by water, while the use of a rejuvenator in AA increased its resistance to being wetted by water. This suggests that aging reduces asphalt hydrophobicity, and rejuvenators can enhance the hydrophobicity of aged asphalt to some extent. Additionally, the contact angles of both BA and RA were above 90°, while the contact angle of AA was below 90°, indicating that BA and RA are hydrophobic, whereas AA is not. This meant that AA was more prone to water damage; thus, the structural stability of the pavement was destroyed, while the use of a rejuvenator effectively reduced the possibility of water damage of AA and further improved the adhesion property between AA and aggregates in the pavement structure. In addition, there were some differences in the rejuvenating effect of the rejuvenator type on the adhesion property of AA: With the incorporation of the rejuvenators, the contact angle of AA increased by 4.49%, 7.19%, 10.33%, and 8.94%, respectively. It was found that the naphthenic oil-based rejuvenator significantly improved the contact angle of AA, aligning with the results of the PO test.

3.2.2. Cohesive Work

The cohesive work parameter provides a measure of the asphalt’s capacity to withstand cracking when subjected to stress. A high proportion of elastic components in asphalt increases its tendency towards elastic deformation and cracking, which diminishes its adhesive properties. Therefore, cohesive work is a critical index for evaluating these properties [42]. This study calculated the cohesive work for various asphalts, as illustrated in Figure 5.

As shown in Figure 5, the cohesive work of AA decreased significantly compared to BA. The cohesive work parameter is a fundamental metric used to assess the ability of asphalt to withstand cracking: higher values indicate better resistance and stronger adhesion properties. The lower cohesive work of AA suggests reduced cracking resistance compared to BA, likely due to aging effects altering the internal components of AA. This makes the asphalt more prone to elastic deformation and cracking under external forces. However, the incorporation of a rejuvenator markedly increased the cohesive work of RA. This improvement is attributed to the rejuvenator replenishing the lost light components in AA, enhancing its viscoelastic properties and reducing cracking susceptibility. The findings also imply that aging adversely affects the adhesion property of AA, which can be improved by adding a rejuvenator, as is consistent with previous analysis results. Although the cohesive work of RA is significantly higher than that of AA, it remains slightly lower than that of BA. This discrepancy arises because the rejuvenator primarily restores the performance of AA by adjusting its light component ratio, but cannot fully reverse all chemical changes caused by aging. Interestingly, the addition of rejuvenators increased the cohesive work of RA by 92.83%, 91.41%, 93.29%, and 91.41%, respectively. Moreover, the cohesive work of RA is in close proximity to that of BA, indicating that although the rejuvenator cannot entirely restore the adhesion property of AA, it significantly enhances them, thereby bringing the performance of RA nearly on par with that of BA and promoting its application in practical engineering.

3.2.3. Adhesion Work

To comprehensively analyze the rejuvenating effect of the rejuvenator on the adhesion property of AA, this study calculated the adhesion work between various asphalts and aggregates [43], as depicted in Figure 6.

According to Figure 6, it was observed that the adhesion work of AA was markedly lower than that of BA. Adhesion work serves to characterize the bonding properties between asphalt and aggregate, where higher values indicate superior adhesion properties. The analysis revealed that AA exhibited an inferior adhesion property compared to BA, as is consistent with previous findings. This result stemmed from the aging effect, which diminished the fraction of viscoelastic components in AA. As a result of the decrease in viscous components, the bonding ability between asphalt and aggregate was compromised, thereby reducing their adhesion properties. The introduction of a rejuvenator significantly elevated the adhesion work of RA relative to AA, indicating an enhanced adhesion property between AA and the aggregate. This enhancement stemmed from the rejuvenator’s augmentation of the viscoelastic components within AA, resulting in an increase in viscous components that reinforced the bonding between the asphalt and the aggregate. Consequently, there was a substantial improvement in the adhesion work between RA and the aggregate. Specifically, the adhesion work of RA-C was closest to BA, which paralleled the cohesive work calculations. Moreover, the aggregate type significantly influenced the adhesion property, with several asphalt samples exhibiting higher adhesion work with limestone aggregates. This suggests stronger adhesion between limestone and asphalt, likely influenced by the aggregate’s material properties. Asphalt, being acidic, tends to undergo neutralization reactions with alkaline aggregates, enhancing their adhesion. Limestone, with higher alkalinity compared to granite, thereby provides a more robust adhesion property for RA, which is consistent with macroscopic test findings.

3.2.4. Peeling Work

The adhesion work measures the bonding properties between the asphalt and the aggregate under dry conditions. Nevertheless, in practical applications, asphalt pavements often endure water erosion. Therefore, this research utilized peeling work to assess the adhesion properties of asphalt and aggregate under wet conditions [44], as depicted in Figure 7.

The peeling work quantifies the ability to displace the asphalt membrane from aggregate surfaces under aqueous conditions. Higher values indicate greater susceptibility of asphalt mixtures to water damage and poorer adhesion properties. According to Figure 7, AA exhibited higher peeling work compared to BA, indicating that aging reduces asphalt water damage resistance. This is attributed to decreased hydrophobicity of aged asphalt surfaces, enhancing wetting behavior and water infiltration, which compromises structural stability. Additionally, RA showed lower peeling work not only than AA, but also than BA, suggesting that rejuvenator incorporation enhances water damage resistance in AA, surpassing even BA. This was because, although the main component of the oil-based rejuvenator is oil, its internal components usually contain a certain amount of adhesives to consider the comprehensive service performance of RA when it was developed. This was beneficial to further enhancing the water damage resistance of the asphalt. Particularly, as opposed to the change rule of adhesion work, RA-C did not provide the most excellent water damage resistance compared to the other RA. This was because naphthenic oils have a simple chemical structure, although it was favorable to promote their fusion with AA, and their chemical reactivity was low. However, although other oils had complex chemical structures, there were more compounds with reactive groups within the oil, which were more prone to combine with AA, and thus these enhanced the water damage resistance of AA. However, it was observed that the peeling work of RA-C remained slightly lower than BA, suggesting that the naphthenic oil-based rejuvenator met the required specifications. Furthermore, the type of aggregate also significantly influenced the peeling work, with granite demonstrating considerably higher peeling work than limestone when combined with asphalt. This observation implies that granite–asphalt mixtures are more susceptible to water damage and exhibit inferior adhesive properties, aligning with the earlier analysis.

3.3. AFM Test

This study examined the variations in the adhesion property of RA at the nanoscale through an AFM test. The research primarily elucidated the evolution mechanism of adhesion properties through mechanical indices measured via the PF-QNM mode of the AFM test. Given the length of the paper, additional details on asphalt surface morphology were omitted, focusing instead on characterizing the nano adhesion and DMT modulus.

3.3.1. Nano Adhesion

In this study, NanoScope Analysis software was employed to quantify the nano adhesion between the probe and asphalt [45], as depicted in in Figure 8.

From Figure 8, the range of nano adhesion distribution for AA was narrower compared to BA. Nano adhesion reflects the adhesion property of asphalt, where higher values indicate better adhesion. This discovery underscores the detrimental influence of aging on the adhesion property of asphalt. In contrast to AA, RA exhibited a significantly broader distribution range of nano adhesion. This difference arose because aging reduced the viscous component in AA, leading to stiffening of the asphalt and thereby reducing the adhesion properties. The introduction of rejuvenators effectively increased the viscous component of AA, softening the asphalt and partially restoring its adhesion properties. This underscores the role of rejuvenators in improving AA’s adhesion properties. Additionally, acknowledging potential data distribution errors, this study conducted a weighted average calculation of nano adhesion across multiple asphalt samples, as depicted in Figure 8b. Analysis of Figure 8b indicates that BA exhibited the highest nano adhesion, AA demonstrated the lowest, and RA consistently exhibited nano adhesion significantly higher than that of AA, thereby further confirming the beneficial effect of the rejuvenator. Interestingly, the rejuvenator type had a significant effect on the nano adhesion: compared to AA, the nano adhesion of RA increased by 33.04%, 19.41%, 63.53%, and 63.26%, respectively. It was determined that the rejuvenating impact of the naphthenic oil-based rejuvenator on the nano adhesion of AA was the most effective. The nano adhesion of RA-C approached that of BA, as is consistent with findings from previous analyses. In summary, the aging phenomenon prominently deteriorated the adhesion property of asphalt. The introduction of the rejuvenator resulted in a marked improvement in the adhesion property of AA. Although RA exhibited a performance index value marginally lower than that of BA, the rejuvenating effect of the rejuvenator remained highly significant.

3.3.2. DMT Modulus

The DMT modulus values of various asphalts were calculated in this study to further validate the nano adhesion analysis results [46], as depicted in Figure 9.

From Figure 9, the distribution range of the DMT modulus for AA exhibited a noticeable increase compared to BA. The DMT modulus characterizes the degree of elastic deformation in asphalt, with higher values indicating greater susceptibility to elastic deformation and poorer adhesion properties. This discovery underscores the detrimental impact of aging on the adhesion property of asphalt, which is corroborated by findings from nano adhesion analysis. Furthermore, the distribution range of the DMT modulus for RA was markedly lower than that of AA, suggesting that the addition of a rejuvenator reduces the elastic deformation of AA, thereby effectively enhancing its adhesion property. This improvement stems from AA’s inherently higher elastic deformation due to a lower proportion of viscous components, whereas the rejuvenator partially restores these light components in AA, thereby increasing the proportion of viscous components and resulting in RA exhibiting a reduced distribution range of DMT modulus. In addition, to reduce the error caused by the continuous distribution of data, this study performed a weighted average calculation on the DMT modulus values of several asphalts, which are shown in Figure 9b. In Figure 9b, BA demonstrated the lowest DMT modulus, whereas AA exhibited the highest DMT modulus. This finding further underscores the detrimental impact of aging on adhesion properties. The DMT modulus of RA was significantly less than that of AA, indicating a notable difference in their stiffness properties, and the rejuvenator type also exhibited a certain correlation with the rejuvenating effect. The RA-C had the lowest DMT modulus and its value was closest to BA, which implied that the RA-C exhibited the best adhesion property compared to the other RA, and this was the same as the analysis results of the nano adhesion.

3.4. Correlation Analysis

In this study, the mechanical responses of the adhesion property indexes of BA, AA, and RA were investigated at multiscale levels by the PO test, SFE test, and AFM test. Based on the variation trends of these indexes, the evolution mechanism on the adhesion property of AA before and after rejuvenating was evaluated. Based on the test results of several tests, the change rule on the adhesion property at different scales was the same. In addition, although these tests effectively characterized the adhesion property of RA at different scales, there was a lack of effective analysis to reflect the correlation between the indicators at different scales, and during the practical engineering, the operation difficulty of the tests at different scales as well as the environmental requirements of these tests were highly different. Therefore, if the correlation analysis test was used to evaluate the correlation among the different scale indexes, and then a more convenient and effective method was conducted to characterize the adhesion property of RA, it would be beneficial to promote the further application of RA in practical engineering. For this purpose, the present study conducted a correlation analysis across various evaluation metrics, aiming to provide informative insights for investigating the adhesion property of RA.

When performing the correlation analysis on the multiscale indexes, when the bonding time reached 4 h, the increasing trend of the PO strength between the asphalt and the aggregate was not significant, and the limestone showed a more significant adhesion property than granite. As a result, PO-4 from various asphalt–limestone combinations was selected as the macroscopic performance index for this study. Furthermore, for a comprehensive assessment of RA’s adhesion property, the study opted to evaluate both the adhesion work and nano adhesion across multiple asphalt samples, serving as microscopic and nanoscale performance metrics, respectively. The test results of these asphalt samples were subsequently fitted and analyzed, as illustrated in Figure 10.

From Figure 10, it is evident that the R² values of the fitting equations for adhesion property indices across various scales for each RA were consistently above 0.9. The higher R² indicated the precision of the fitted curves established in this study, effectively characterizing the correlation of adhesion property indices across different scales. Moreover, it highlighted a significant linear relationship among these indices. Overall, the mechanical property indices based on the adhesion property exhibited strong correlations across different scales, unaffected by the type of rejuvenator. The study found that changing the rejuvenator type did not alter the evolution pattern of the RA adhesion property significantly. The slopes of the fitting equations for several RAs at different scales were very close, with R² values all significantly exceeding 0.9. This underscored robust correlation among the adhesion property indices of RA across different scales. Despite variations in mechanical test methods and conditions across different scales, there was a consistent approach to assessing the evolution of adhesion properties between RA and the aggregates. This indicated that the PO test could serve as a reliable method for conveniently and effectively evaluating the evolution mechanism of the RA adhesion property, particularly when facing constraints related to cost and test conditions.

Overall, to further promote the application of RA in road engineering, this study investigates the evolution mechanism of the adhesion property of RA prepared with different rejuvenators at different scales. Through correlation analysis, the study further evaluates the relationships between evaluation indicators at different scales, thereby better guiding the resource utilization of waste pavement materials and reducing the construction and maintenance costs of road engineering. It is worth noting that this study only explores the application potential of RA from the perspective of adhesion properties. The practical value of RA in engineering should also be comprehensively evaluated in conjunction with the changes in the pavement performance of recycled asphalt mixtures. Therefore, future research should further investigate the evolution mechanism of the pavement performance of recycled asphalt mixtures to provide a more comprehensive reference for the application of RA in road engineering.

4. Conclusions

The primary objective of this study was to explore the underlying mechanisms responsible for the variations observed in the adhesion properties of RA across multiple scales. Four distinct rejuvenators were selected for preparing RA samples. The investigation utilized the PO, SFE, and AFM tests to explore the influence of aging and rejuvenators on the adhesion properties of AA across various scales. Based on these findings, the correlation between the adhesion property evaluation indices of RA at different scales was explored, and the evolutionary mechanisms influencing the adhesion properties of RA were further evaluated. The following finds were obtained:

(1)

The aging effect degrades the bond strength between AA and aggregate. Conversely, the addition of a rejuvenator effectively enhances their bond strength. Moreover, due to the properties of the aggregate, limestone and asphalt exhibit stronger adhesion properties compared to granite.

(2)

The aging process led to a decrease in the hydrophobicity of asphalt, rendering it more susceptible to water damage and cracking, thereby diminishing its adhesion property. Although the addition of the rejuvenator partially mitigated this adverse effect, the adhesion properties of RA remained slightly inferior to those of BA. Notably, the naphthenic oil-based rejuvenator demonstrated a more pronounced ability to restore the adhesion property of AA.

(3)

The calculation results of the nano mechanical properties showed that the aging effect led to a decrease in the nano adhesion and an increase in the DMT modulus of asphalt, which implied that AA was more inclined to elastic deformation and its adhesion properties were decreased. The incorporation of rejuvenators showed a considerable restoring effect on the nano adhesion property of AA, the performance index values of RA were extremely close to BA, and the rejuvenating effects of several rejuvenators were similar to the macroscopic and microscopic test results.

(4)

The correlation analysis revealed that the adhesion property of RA exhibited a robust relationship across various mechanical properties indices at different scales, which remained consistent regardless of the type of rejuvenator used. The slopes of the regression equations for several RA samples at different scales showed remarkably similar values with high precision. This consistency suggests that, despite variations in mechanical test methods and conditions across different scales, there exists a uniformity in the evaluation of the trends in adhesion properties between RA and aggregates.

(5)

This study investigates the evolution mechanism of the adhesion properties of recycled asphalt at different scales. The research findings can provide a reference for the optimization of recycled asphalt formulations and processes, enhance the durability and performance of pavements, promote the reuse of waste materials, reduce the environmental impact, and lower the costs of road construction and maintenance. Additionally, these findings can aid in driving technological innovation, establishing industry standards, and providing scientific guidance for practical engineering in future research.