Performance Evaluation of Stress-Absorbing Layer Mixtures Incorporating High-Content Oil-Rich RAP Fine Aggregate

Abstract

The utilization of oil-rich reclaimed asphalt pavement fine aggregate (O-RAP), characterized by its high asphalt content, which has inherent compatibility with the high asphalt demand of stress-absorbing layer (SAL) mixtures, enables significant recycling rates, thereby promoting resource efficiency and a promising pathway for sustainable infrastructure development. This study provides a comprehensive evaluation of the rheological properties of recycled asphalt binder through dynamic shear rheometer and bending beam rheometer tests. Furthermore, the pavement performance of SAL mixtures was systematically assessed via fatigue testing, moisture susceptibility evaluation, rutting resistance analysis, and overlay testing. The results indicate that increasing the O-RAP content enhances the complex shear modulus while reducing the phase angle, suggesting improved stiffness but reduced flexibility of the binder. In SAL mixtures, higher O-RAP content was associated with decreased fatigue life, moisture stability, and low-temperature cracking resistance, yet it contributed to improved resistance to reflective cracking and high-temperature rutting. Pearson correlation analysis further revealed that the fatigue life of the binder exhibits a strong positive correlation with creep rate and significant negative correlations with creep stiffness modulus, high-temperature stability, and reflective cracking resistance. These findings underscore the viability of high-content O-RAP incorporation in SAL mixtures as a technically sound and environmentally sustainable strategy for low-carbon pavement construction, offering significant reductions in virgin material consumption and associated carbon emissions.

1. Introduction

With growing global emphasis on sustainable development, the utilization of recycled materials in road engineering has become a significant research focus [1,2,3,4]. Asphalt pavement, being a predominant component in road infrastructure, inevitably undergoes aging and develops defects such as rutting and cracking under prolonged exposure to traffic loads and environmental factors [5,6]. These deterioration processes necessitate periodic maintenance and rehabilitation. During such interventions, the existing pavement structure typically requires milling and resurfacing, generating substantial quantities of reclaimed asphalt pavement (RAP). The recycling and reuse of RAP in road engineering has gained considerable attention due to its demonstrated environmental benefits, economic advantages, and resource conservation potential [7,8].

In recent years, widespread research has been conducted globally on the utilization of RAP in various pavement structural layers [9]. RAP, because it contains a certain amount of old asphalt, can be used as an unbound material or as a partial substitute for new asphalt mixtures in the base and sub-base layers of pavements [10,11]. Research has shown that incorporating an appropriate amount of RAP enhances the high-temperature stable performance of mixtures while reducing material costs and construction energy consumption [12]. However, excessive RAP content may increase the risk of low-temperature cracking and compromise pavement durability, necessitating optimized gradation design and modification techniques [13]. For intermediate and upper layers, the application of RAP is subject to more stringent technical requirements [14]. Research has shown that moderate RAP content can improve rutting resistance and long-term performance, whereas high RAP content may adversely affect binder adhesion and fatigue life performance [15]. Consequently, recycled asphalt mixtures with high RAP content often require supplementary measures, such as blending with virgin asphalt or rejuvenators, to enhance adhesion and aging resistance [16].

Due to its highly agglomerated nature and considerable variability, RAP requires processing by fine crushing and screening. Advanced recycling of processed RAP has emerged as a promising approach for high-value recovery [17]. Fine separation technology adopts physical centrifugal stripping and multi-point driven high-frequency excitation sieving technology to realize accurate separation and precise grading of RAP, which is divided into 3~5 grades of different specifications [18]. O-RAP is a material produced from RAP after fine separation, with more aged asphalt wrapped around the surface, and the asphalt content is usually higher than that of ordinary RAP, which has a high recyclable value [19]. The stress-absorbing layer (SAL), a critical structural component for mitigating reflective cracking in pavements, typically uses high-viscosity and high-elasticity asphalt materials with increased binder content to enhance crack resistance [20]. Given their increased aged asphalt content and larger specific surface area, O-RAP demonstrates considerable potential for SAL applications, although research in this area remains limited. Existing studies suggest that optimized incorporation of O-RAP can improve interlayer bonding properties while significantly reducing the need for virgin asphalt [21]. However, excessive RAP content can compromise the adhesion properties of the SAL, adversely affecting interlayer cohesion and fatigue resistance [22]. To date, research on high-RAP content in SAL remains scarce, with most previous studies focusing on incorporation rates typically below 30% for conventional RAP. The application of specially processed O-RAP at very high levels (e.g., exceeding 50%) represents a significant challenge and an under-explored area. This lack of knowledge regarding the maximum feasible incorporation rate and the comprehensive performance trade-offs at high O-RAP content constitutes a critical research gap. Meanwhile, the O-RAP has a high content of old asphalt whose adhesion, rheological properties, and degree of aging directly affect the overall performance of the SAL.

The rational application of O-RAP can reduce the demand for virgin asphalt, thereby lowering resource consumption and carbon emissions associated with asphalt pavement construction, and improving overall resource utilization efficiency [23]. Furthermore, the use of a high percentage of O-RAP has been shown to effectively reduce road maintenance costs and enhance material efficiency [24]. With appropriate rejuvenation treatment, the adhesion characteristics of O-RAP can be improved, contributing to a more stable interfacial structure within the stone mastic asphalt (SAL) mixture. Given that the production of virgin asphalt and the mining of aggregates are energy-intensive processes—and major contributors to carbon emissions in the construction sector—the incorporation of recycled materials such as RAP represents not only an effective waste management strategy but also a crucial pathway toward decarbonizing infrastructure development.

To address the challenges of limited incorporation rates and unclear performance relationships associated with O-RAP, this study systematically investigates its application in SAL mixtures based on advanced separation technology. While previous research has largely focused on conventional RAP in asphalt layers, this work specifically targets the high-value utilization of O-RAP—a challenging material due to its high aged asphalt content—in SAL mixtures, which inherently require high binder content. Consequently, this research pushes the boundaries by targeting an unprecedented O-RAP incorporation rate of 70%. The key innovations of this research include (i) determining the maximum feasible O-RAP incorporation rate (70%) in SAL while satisfying performance specifications; (ii) clarifying the comprehensive performance trade-offs and correlations through a multi-scale experimental methodology; and (iii) quantifying the substantial environmental benefits achieved through this high-recycling strategy.

The research methodology comprises three key components. First, the recycled asphalt binder was characterized at varying content levels to elucidate the influence of RAP content on rheological properties and fatigue characteristics. Secondly, comprehensive scientific evaluations were carried out to evaluate the reflective crack resistance, high-temperature stability, low-temperature cracking resistance, water stability, and fatigue life of SAL, alongside a comparative analysis against conventional mixtures. Finally, Pearson correlation analysis was employed to establish quantitative relationships between various performance indicators. The complete research process is illustrated in Figure 1.

2. Materials and Methods

2.1. Materials

The SBS-modified asphalt (penetration grade 40–60) used in this study was supplied by China Petroleum & Chemical Corporation. All asphalt properties were evaluated in accordance with the Chinese Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011) [25], with detailed specifications presented in

Table 1. Property indexes of SBS-modified asphalt.

Properties	Results	Methods
Ductility (5 cm/min, 5 °C) (cm)	32.3	T 0605 [25]
Penetration (25 °C, 100 g, 5 s) (0.1 mm)	53.4	T 0604 [25]
Softening point (Ring & ball) (°C)	83.0	T 0606 [25]
Dynamic viscosity (135 °C) (Pa·s)	2.3	T 0625 [25]
Solubility (%)	99.3	T 0607 [25]
Elastic recovery (25 °C) (%)	93.4	T 0662 [25]
Density (15 °C, cm)	1.032	T 0603 [25]

. O-RAP was obtained from the surface layer of a 15-year-old highway pavement by advanced separation technology. The gradation characteristics and asphalt content of the O-RAP are summarized in

Table 2. The O-RAP grades and asphalt content [26].

Mesh Size (mm)	4.75	2.36	1.18	0.6	0.3	0.15	0.075	Asphalt Content
Passing rate (%)	100.0	90.9	72.2	48.8	31.7	24.9	19.5	7.0

[26]. Aged asphalt was extracted from the O-RAP using a rotary evaporator apparatus. To counteract the potential brittleness and aging effects of the high O-RAP content, two specialized additives were employed: a proprietary rejuvenator and a high-elasticity modifier. The dosage of the rejuvenator was set at 6% by mass of the aged asphalt in the O-RAP, a ratio determined through preliminary tests to effectively restore the penetration and ductility of the recovered aged asphalt to a level comparable to the virgin SBS asphalt. The high-elasticity modifier was incorporated at a fixed dosage of 5% by mass of the total binder (manufacturer’s recommendation), which was optimized to enhance elasticity without compromising the workability of the high-RAP-content mixture. The fundamental properties of these additives are systematically presented in

Table 3. Performance index of direct injection high elasticity modifier.

Properties	Results
Appearance	Grainy, uniform, satiation
Single mass particle (g)	0.021
Density (g/cm3)	0.959
Melt index (190 °C, 2.16 kg, g/10 min)	7.19
Ash content	0.102

. The rejuvenator, typically composed of maltenes and oil-based compounds, aims to restore the chemical balance of the aged asphalt by replenishing lost lighter oils, thereby improving its flexibility and adhesion. Detailed chemical composition analysis can be found in our previous research [7,26]. The high-elasticity modifier, a polymer-based additive, is designed to enhance the overall elastic recovery and crack resistance of the blended binder system, which is critical for the stress-absorbing function of the SAL.

2.2. Preparation of Samples

The mixture design process was conducted following standardized procedures. The optimal gradation for the SAL mixtures was determined by the Marshall compaction method, with the resulting aggregate gradation shown in Figure 2. The design asphalt-stone ratio was 7.0%.

2.2.1. Preparation of Binder

Three RAP incorporation rates were investigated (30%, 50%, and 70%). The recycled asphalt binder content was calculated based on the actual aged asphalt content in the O-RAP. Aged asphalt was recovered from O-RAP by an automated asphalt extractor coupled with rotary evaporation. The reclaimed asphalt was then blended with virgin SBS-modified asphalt. Rejuvenator content was proportionally added based on the aged asphalt content. A high-elasticity modifier was incorporated at 5% of total binder mass (manufacturer’s recommended content) [27].

2.2.2. Preparation of SAL Mixtures

To ensure proper coating and avoid further aging, the virgin aggregates and SBS-modified asphalt were heated to 175 ± 5 °C, while the O-RAP was heated to a lower temperature of 140 ± 5 °C to mitigate fuming and agglomeration. The mixing procedure was rigorously controlled: first, the pre-heated virgin aggregates were mixed with the high-elasticity modifier for 1.5 min to allow for initial dispersion; then, the pre-heated O-RAP and the rejuvenator were added and mixed for an additional 1.5 min to ensure the rejuvenator could begin diffusing into the aged asphalt film; finally, the SBS-modified asphalt was introduced and mixed for 90 s to form a homogeneous SAL mixture. According to the test need to load the SAL mixtures into the corresponding mold for molding. The content of O-RAP in the prepared SAL mixtures was 0%, 30%, 50% and 70%, respectively. For each formulation, a minimum of three replicate specimens were tested, and the results are presented as the mean value ± standard deviation.

2.3. Test Methods

2.3.1. Recycled Asphalt Binder Test

Temperature Scanning Test

In this study, the rheological behaviors of recycled asphalt binder were evaluated by a dynamic shear rheometer (DSR, Anton Paar MCR 302, Anton Paar Gmbh, Graz, Austria). To characterize the rheological behavior of the recycled asphalt binder, temperature-sweep testing was performed in strain-controlled mode, with the complex shear modulus (G*) and phase angle (δ) serving as critical evaluation indicators.

Linear Amplitude Scanning Test

Fatigue resistance of the recycled asphalt binder was evaluated using the LAS protocol, which involved an initial frequency sweep (0.2 Hz–30 Hz) and subsequent strain-controlled amplitude sweep (0.1–30% strain) at 25 °C. The linearly increasing strain amplitude in the second phase allowed for characterization of the binder’s fatigue failure behavior.

BBR Test

To characterize the low-temperature rheological behavior of the recycled asphalt binder, BBR testing (Cannon TE-BBR) was conducted. Test standards refer to AASHTO T313 [28]. The evaluation focused on two critical parameters, namely (1) creep stiffness (S) at 60 s and (2) the m-value, which collectively reflect the material’s resistance to thermal cracking at low temperatures.

2.3.2. SAL Mixture Test

Overlay Test

The experimental procedure was conducted in strict compliance with the TEX-248-F specification from Texas, USA [29]. For the SAL mixtures, initial cylindrical specimens (150 mm diameter × 115 ± 5 mm height) were prepared via rotary compaction. Additionally, cylindrical tests were cut, and specimens measuring 150 mm × 76 mm × 38 mm were fabricated. These rectangular specimens were subsequently bonded to two tensile discs using epoxy resin, as illustrated in Figure 3, for subsequent testing purposes.

Road Performance Test

The performance evaluation of SAL mixtures was conducted following JTG E20-2011 test specifications [25], incorporating five critical assessments: (1) high-temperature rutting resistance, (2) low-temperature flexural capacity, (3) moisture susceptibility via immersion Marshall test, (4) freeze–thaw durability through splitting tests, and (5) fatigue characteristics using four-point bending methodology.

Pearson Correlation

Pearson correlation was used to verify the correlation between the rheological properties, fatigue properties, road performance, and anti-reflective cracking performance of recycled asphalt binder and SAL mixtures. In Pearson correlation analysis, the R quantifies the linear relationship between variables, with stronger associations indicated by larger absolute values (Equation (1)). The range of R spans from −1 to 1, where values approaching these extremes demonstrate perfect negative or positive correlations, respectively.

$$R={\displaystyle \frac{{\displaystyle \sum _{i=1}^n(x_i-\overline{x})(y_i-\overline{y})}}{\sqrt{{\displaystyle \sum _{i=1}^n{(x_i-\overline{x})}^2}}\sqrt{{\displaystyle \sum _{i=1}^n{(y_i-\overline{y})}^2}}}}$$

(1)

where $x_i$ and $y_i$ are customized variables, and $\overline{x}$ and $\overline{y}$ are the average values of the customized variables, respectively.

2.4. Environmental Benefit Assessment

To quantify the sustainability benefits of using O-RAP, a simplified life cycle assessment (LCA) approach was adopted. The calculation focused on the reduction in virgin asphalt binder and aggregate consumption, which are the primary sources of environmental impact in asphalt production. The embodied carbon savings were estimated based on the substituted mass of virgin materials and their respective carbon emission factors derived from the literature. The assessment was conducted for the SAL mixture with 30%, 50%, 70% O-RAP content compared to the virgin asphalt mixture (0% RAP). The theoretical method employs emission factors to calculate gas emissions from various components of asphalt pavements. Based on emission factors and characterization factors, pavement emission calculation models are derived as shown in Equations (2) and (3).

$$E_C=E_W+E_S+E_Z$$

(2)

$$E_{ci}={\displaystyle {\sum }_{i=1}^n{C}_i}\times A{D}_i\times E{F}_i$$

(3)

where E_W represents gas emissions during the raw material production phase, E_S represents gas emissions during the transportation phase, E_Z represents gas emissions during the mixture mixing and construction phase, i denotes the i-th phase of the pavement life cycle, C_i denotes the gas-specific factor for the i-th gas, AD_i denotes the activity level for the i-th process step, and EF_i denotes the emission factor for the i-th process, respectively.

3. Results and Discussion

3.1. Performance Analysis of Asphalt Binder

3.1.1. High-Temperature Properties

The DSR test results reveal a temperature-dependent reduction in G* for all binder types, as shown in Figure 4a, maintaining consistent thermal susceptibility throughout the experimental range. A clear positive correlation emerges between RAP content and G* magnitude, with the 70% RAP blend showing superior stiffness properties, a characteristic attributed to the predominant effect of aged binder components. The δ analysis indicates significantly greater viscous behavior in virgin asphalt compared to recycled formulations, as shown in Figure 4b. The observed inverse relationship between RAP concentration and δ values suggests progressive enhancement of elastic response with higher incorporation of aged material, demonstrating a systematic transition from viscous to elastic-dominated rheological performance [30]. This transition is attributed not only to the presence of aged asphalt but also to the effective blending between the virgin SBS-modified asphalt, the rejuvenator, and the aged asphalt. The rejuvenator partially restores the colloidal structure of the aged asphalt, while the SBS from both the virgin binder and the high-elasticity modifier forms a continuous polymer network that integrates the recycled components, leading to the enhanced elastic response.

3.1.2. Fatigue Properties

As shown in Figure 5, rheological analysis at 25 °C reveals parabolic stress–strain characteristics across all formulations: virgin binder and recycled variants incorporating 30%, 50%, and 70% reclaimed asphalt pavement (RAP) material. When shear strain is below 10%, the response of all samples remains consistent, characterized by viscoelastic behavior. As shear strain increases, each asphalt binder shows a distinct inflection point where shear stress begins to decline after a period of stabilization, indicating the onset of internal damage. Furthermore, the yield stress decreases with increasing RAP content, suggesting that higher levels of aged asphalt reduce the ductility and damage resistance of the recycled asphalt binder [31].

The fatigue performance characteristics, including damage parameters and service life, were derived from rheological property analysis and amplitude sweep testing, as shown in

Table 4. Calculation results of fatigue parameters of different asphalt binders.

Parameter	Virgin Asphalt	30%RAP	50%RAP	70%RAP
Nf (2.5%)	1.15 × 1012	8.05 × 1011	4.57 × 1010	4.80 × 109
Nf (5%)	7.02 × 1011	1.54 × 1011	9.01 × 109	9.32 × 108

. As the RAP content increases, the fatigue life (N_f) of the recycled asphalt binder reduces progressively, indicating a diminished capacity to resist damage under repeated loading. This trend is attributed to the aging of the asphalt, which adversely affects the fatigue resistance of the asphalt binder. At maximum expected strain levels of 2.5% and 5%, all four types of asphalt binder exhibit consistent trends in fatigue life, suggesting that although incorporating higher amounts of aged asphalt improves resource utilization, it inevitably compromises certain mechanical properties. Nevertheless, the recycled asphalt binders still maintain a relatively high level of fatigue resistance.

Notably, the N_f exhibits a reduction of several orders of magnitude with increasing RAP content, particularly at the 5% strain level. While this indicates a significant decline in the material’s resistance to damage under high-strain conditions, the practical implications of this reduction must be interpreted within the context of realistic pavement service environments. In actual pavement structures, the strain levels experienced by the asphalt binder layer are typically much lower, often below 0.1%. At these lower, more representative strain levels, the fatigue life of all mixtures, including the 70% RAP variant, would be substantially higher and likely sufficient for many design life requirements. Therefore, the drastic reduction observed here primarily defines the performance boundary of high-RAP mixtures under extreme loading. It underscores the importance of limiting the maximum strain in pavement design when utilizing high-RAP content binders, rather than precluding their use altogether. The feasibility of high-RAP mixtures is thus confirmed for applications where traffic-induced strains are controlled, aligning with the performance-based specification philosophy.

3.1.3. Low-Temperature Properties

Figure 6 presents the low-temperature rheological properties of recycled asphalt binders. The creep stiffness modulus (Figure 6a) exhibits a significant increase with decreasing temperature (from −12 °C to −18 °C), demonstrating enhanced material rigidity under colder conditions. This trend signifies a progressive transition from viscoelastic to glassy behavior, resulting in greater brittleness and reduced flexibility at subzero temperatures. At the same temperature, asphalt binders with higher RAP content exhibit higher creep stiffness modulus. Despite the consistent trend of creep stiffness modulus increase with decreasing temperature across all samples, the creep stiffness modulus at −18 °C exceeds the Superpave specification limit of 300 MPa for all binders. However, the creep stiffness modulus remains within acceptable limits at −12 °C. Figure 6b demonstrates an inverse relationship between temperature and creep rate, with the m-value decreasing systematically as the test temperature declines from −12 °C to −18 °C. This phenomenon is attributed to the restricted molecular mobility within the recycled asphalt system under low-temperature stress, where the macromolecular chains are less capable of rearranging spatially [32]. As a result, the asphalt binders exhibit limited stress relaxation capacity [33]. At −18 °C, the creep rate of all four asphalt binders falls below the Superpave specification threshold of 0.3. The experimental results reveal that while increased RAP content enhances creep stiffness, it does not significantly alter the low-temperature performance grade (PG) of the recycled binder, suggesting distinct mechanisms govern these properties.

3.2. Performance Analysis of SAL Mixtures

3.2.1. Anti-Cracking Performance Analysis

As shown in Figure 7, the variation curves of maximum load versus test cycles for the four SAL mixtures exhibit similar trends. The load decreases rapidly in the initial stage, followed by a more gradual decline as the number of cycles increases. The initial maximum load increases with a higher content of O-RAP; however, the load subsequently declines over the test duration and eventually falls below that of the new SAL mixture. This indicates that prolonged cyclic loading significantly affects the reflective cracking resistance of SAL mixtures. The test results are shown in

Table 5. Overlay test results.

Parameter	Virgin Asphalt	30%RAP	50%RAP	70%RAP
Test cycles N (Times)	1200	1200	1200	1200
Load loss rate R (%)	68.42	74.84	79.22	83.51
The maximum load of the first cycle F (kN)	1.38	1.87	2.46	3.34
Critical fracture energy Ec (J/m2)	1.28	1.49	1.84	2.08
Total fracture energy G (J/m2)	594.31	608.53	621.54	632.28

. After 1200 cycles, all SAL mixtures exhibited a load loss rate (R-value) below 93%. Specifically, the R-values for the SAL mixtures with 0%, 30%, 50%, and 70% RAP content were 68.42%, 74.84%, 79.22%, and 83.51%, respectively. The SAL mixture containing 70% RAP exhibited the highest initial load in the first cycle, reaching 3.34 kN, which reflects improved initial adhesion and interlocking among aggregates. This improvement is attributed to the high asphaltene content in the aged asphalt from the O-RAP, which contributes to the formation of a highly viscous recycled asphalt binder when blended with virgin asphalt and additives. Furthermore, both the critical fracture energy and total fracture energy follow a trend consistent with the maximum load in the first cycle. These parameters are highest for the SAL mixture containing the highest RAP content, indicating that greater force and energy are required to initiate reflective cracking at the early stage. This highlights the potential of O-RAP to enhance the early-stage crack resistance performance of SAL mixtures [34].

3.2.2. High-Temperature Performance Analysis

Figure 8 presents the comparative high-temperature performance characteristics of the four SAL mixtures. The dynamic stability values for the SAL mixtures with 0%, 30%, 50%, and 70% RAP content were 3216 Time/mm, 3514 Time/mm, 3852 Time/mm, and 4035 Time/mm, respectively. The control mixture without RAP exhibited the lowest dynamic stability, while the dynamic stability increased progressively with a higher content of O-RAP. The experimental data demonstrate that O-RAP modification substantially improves the high-temperature stability of SAL mixtures, as evidenced by enhanced rutting resistance. The observed improvement in high-temperature performance is primarily attributed to the highly viscous nature of the aged asphalt present in the O-RAP, which has undergone long-term oxidation [35]. The elevated viscosity of RAP-modified binders contributes to improved structural stability in SAL mixtures, particularly at higher recycling rates (50–70% RAP content). This leads to a stronger resistance to plastic deformation and improved rutting performance under repeated loading at high temperatures.

3.2.3. Low-Temperature Crack Resistance Analysis

Figure 9 presents the comparative low-temperature cracking resistance evaluation of the four SAL mixtures. The bending failure strains at low temperature for the SAL mixtures containing 0%, 30%, 50%, and 70% RAP were 3526 με, 3216 με, 2987 με, and 2765 με, respectively—all exceeding the specification limit of 2500 με. Among them, the SAL mixture without RAP exhibited the highest failure strains, indicating superior resistance to low-temperature cracking. The experimental data reveal an inverse correlation between O-RAP content and failure strain, indicating progressively reduced low-temperature cracking resistance with higher incorporation rates. This trend implies that higher RAP content may contribute to increased brittleness in the SAL mixtures, where increased recycled material content corresponds with diminished flexibility and enhanced thermal cracking susceptibility in stone matrix asphalt [36]. The decline in ductility is primarily attributed to the hardened and highly viscous nature of the aged asphalt in the O-RAP, which limits the deformation capacity of the SAL mixtures and intensifies stress concentrations under thermal or mechanical loading.

3.2.4. Water Stability Performance Analysis

The test results of the four SAL mixtures are shown in Figure 10. The MS₀ values for mixtures containing 0%, 30%, 50%, and 70% RAP were 96.51%, 94.52%, 92.13%, and 90.24%, respectively, while the corresponding tensile strength ratio (TSR) values were 94.32%, 92.47%, 88.74%, and 87.50%. All values meet the specification requirements of MS₀ greater than 85% and TSR greater than 80%, indicating acceptable water stability across all SAL mixtures. However, a clear decreasing trend in water stability is observed with increasing content of O-RAP. Particularly at higher RAP contents, the SAL mixtures become more vulnerable to water-induced damage and the effects of freeze–thaw cycles. This reduction in performance is primarily attributed to the presence of aged asphalt in the O-RAP, which exhibits diminished adhesion properties. The weaker bonding between the aged and virgin asphalt compromises the interfacial cohesion, thereby reducing the resistance to water infiltration. Additionally, the incorporation of a high content of O-RAP may alter the aggregate structure of the SAL mixtures, leading to changes in the aggregate-on-aggregate contact and reducing the overall asphalt-aggregate adhesion [37]. As a result, the aggregate is more prone to stripping under the influence of water or freeze–thaw conditions. The observed decline in moisture resistance with higher RAP content can be further explained by the competitive adhesion between the aged and virgin binders. Although the rejuvenator improves the cohesion of the aged asphalt itself, the interfacial transition zone between the partially blended aged asphalt film and the virgin binder may remain a weak point, susceptible to water intrusion and damage.

3.2.5. Fatigue Performance Analysis

Figure 11 presents the comparative fatigue resistance evaluation of the four SAL mixtures. The fatigue lives of the SAL mixtures containing 0%, 30%, 50%, and 70% RAP were 351,600, 331,424, 305,201, and 270,235 cycles, respectively—all satisfying the technical requirement of 250,000 cycles. The SAL mixture with virgin asphalt exhibited the longest fatigue life. However, as the content of O-RAP increased, the fatigue life of the SAL mixtures declined progressively. In particular, when the RAP content exceeded 50%, the reduction in fatigue life became more pronounced. This degradation in fatigue performance is primarily attributed to the aged asphalt in the O-RAP, which is harder and less ductile. The reduced flexibility leads to increased internal stress concentrations within the SAL mixtures under repeated loading. As a result, the SAL mixtures become more susceptible to the initiation and propagation of microcracks, ultimately accelerating fatigue damage and reducing service life [38].

3.3. Correlation Analysis

The Pearson correlation analysis (Figure 12) revealed that the N_f of the recycled asphalt binder is strongly positively correlated with the water stability, low-temperature crack resistance, and fatigue performance of the SAL mixtures. This confirms that binders with superior fatigue life enhance the overall durability of the mixture [39].

The most critical finding, however, lies in the strong negative correlations observed between the binder’s N_f and two key performance metrics of the SAL: its anti-reflective cracking resistance and high-temperature stability. This indicates a significant performance trade-off: while a high-N_f binder improves durability, it may simultaneously compromise the structural integrity of the interlayer. The lower creep stiffness associated with a higher N_f likely increases susceptibility to deformation under traffic and thermal loading. This softening can lead to stress concentrations at the interface, reducing its ability to resist reflective cracking and rutting [40]. Therefore, optimizing the O-RAP content and binder formulation must carefully balance these competing effects to ensure satisfactory performance across all critical parameters.

3.4. Environmental Impact Analysis

The environmental assessment, based on a simplified LCA model, reveals a compelling advantage for O-RAP utilization.

Table 6. Environmental benefit calculation for O-RAP SAL mixture (per tonne).

Component		Virgin Asphalt	30%RAP	50%RAP	70%RAP
Material Consumption	O-RAP Used (kg)	0	300	500	700
	Virgin Asphalt (kg)	70.0	49.0	35.0	21.0
	Virgin Aggregate (kg)	930.0	651.0	465.0	279.0
Reduction vs. Baseline	Virgin Asphalt Saved (kg)	-	21.0	35.0	49.0
	Virgin Aggregate Saved (kg)	-	279.0	465.0	651.0
CO2-eq Emissions (kg)	From Virgin Asphalt	37.80	26.46	18.90	11.34
	From Virgin Aggregate	4.65	3.26	2.33	1.40
	From O-RAP Processing	0.00	24.00	40.00	56.00
Total Emissions		42.45	53.72	61.23	68.74
Net Carbon Reduction		-	−11.27	−18.78	−26.29

details the material consumption and carbon emissions for SAL mixtures with varying O-RAP content per tonne [41]. The results demonstrate a linear relationship between O-RAP content and resource conservation. Compared to the conventional mixture (0% RAP), the use of 30%, 50%, and 70% O-RAP reduces the consumption of virgin SBS-modified asphalt binder by 30.0%, 50.0%, and 70.0%, respectively. Similarly, the demand for virgin aggregate is reduced by 30.0%, 50.0%, and 70.0%. Most importantly, this substitution translates into significant carbon footprint reduction. The net carbon reduction values of −11.27 kg, −18.78 kg, and −26.29 kg of CO₂-equivalent per tonne of mixture for 30%, 50%, and 70% RAP content, respectively, quantify the substantial environmental benefit achieved by avoiding the energy-intensive production of virgin materials. This analysis confirms that high-content O-RAP recycling is a powerful strategy for decarbonizing pavement construction, with the environmental payoff increasing directly with the recycling rate. This high-value recycling strategy in the surface layer can be synergistically combined with sustainable practices in other pavement layers to maximize system-wide benefits. For instance, research by Jaffar et al. [42] demonstrated that sustainable modifiers could effectively improve the stiffness behavior of weak subgrade soil, thereby enhancing the foundational support for the entire pavement structure. This integrated approach not only conserves virgin materials and reduces carbon emissions across multiple pavement layers but also potentially extends the service life of the entire system, offering a comprehensive pathway toward decarbonizing pavement construction.

4. Conclusions

This study comprehensively evaluated the viability of incorporating high-content O-RAP into SAL mixtures, leading to the following principal conclusions:

(1)

Binder Performance Modification: The introduction of O-RAP effectively modifies the rheological properties of the asphalt binder, shifting its behavior toward greater rigidity and elasticity. Critically, this enhancement in high-temperature performance is achieved without adversely affecting the low-temperature performance grade, confirming the binder’s suitability for use in varied climates.

(2)

Mixture Performance and Trade-offs: The use of O-RAP in SAL mixtures presents a definable performance trade-off. While it significantly enhances high-temperature stability and rutting resistance, it concurrently increases mixture brittleness, leading to reductions in low-temperature crack resistance, moisture stability, and fatigue life. Nonetheless, a viable incorporation threshold of up to 70% O-RAP has been established, as mixtures at this level continue to meet standard performance specifications, successfully balancing recycling goals with functional requirements.

(3)

System-Level Correlation: The strong correlation between binder N_f and key mixture properties underscores that binder-level performance is a reliable predictor of mixture behavior. The identified negative correlation between N_f and the mixture’s rutting resistance reveals a critical performance contradiction that must be actively managed during the material design phase to optimize overall pavement durability.

(4)

Demonstrated Environmental Efficacy: The integration of O-RAP provides substantial and quantifiable environmental benefits. The 70% O-RAP mixture achieves an approximate 70% reduction in virgin material consumption and a net carbon emission reduction of 26.29 kg CO₂-eq per ton, establishing high-content O-RAP recycling as a powerful strategy for decarbonizing pavement construction.

In summary, this research validates the technical feasibility and environmental necessity of utilizing high-content O-RAP in SAL applications. The findings provide a scientific basis and practical guidance for industry to adopt this technology, enabling a transition towards more resource-efficient and low-carbon pavement infrastructure without compromising structural performance.