Performance Degradation and Fatigue Life Prediction of Hot Recycled Asphalt Mixture Under the Coupling Effect of Ultraviolet Radiation and Freeze–Thaw Cycle

Abstract

In actual service, asphalt pavement is subjected to freeze–thaw cycles and ultraviolet radiation (UV) over the long term, which can easily lead to mixture aging, enhanced brittleness, and structural damage, thereby reducing pavement durability. This study focuses on the influence of freeze–thaw cycles and ultraviolet aging on the performance of recycled asphalt mixtures. Systematic indoor road performance tests were carried out, and a fatigue prediction model was established to explore the comprehensive effects of recycled asphalt pavement (RAP) content, environmental action (ultraviolet radiation + freeze–thaw cycle), and other factors on the performance of recycled asphalt mixtures. The results show that the high-temperature stability of recycled asphalt mixtures decreases with the increase in environmental action days, while higher RAP content contributes to better high-temperature stability. The higher the proportion of old materials, the more significant the environmental impact on the mixture; both the flexural tensile strain and flexural tensile strength decrease with the increase in environmental action time. When the RAP content increased from 30% to 50%, the bending strain continued to decline. With the extension of environmental action days, the decrease in the immersion Marshall residual stability and the freeze–thaw splitting strength became more pronounced. Although the increase in RAP content can improve the forming stability, the residual stability decreases, and the freeze–thaw splitting strength is lower than that before the freeze–thaw. Based on the fatigue test results, a fatigue life prediction model with RAP content and freeze–thaw cycles as independent variables was constructed using the multiple nonlinear regression method. Verification shows that the established prediction model is basically consistent with the change trend of the test data. The research results provide a theoretical basis and optimization strategy for the performance improvement and engineering application of recycled asphalt materials.

1. Introduction

In the high-altitude region of western China, the pavement’s service environment tends to induce damage such as cracks, ruts, loose spalling, frost heave, and frost boiling. This is attributed to significant altitude differences, cold climate, temperature variations exceeding 20 °C, and intense ultraviolet radiation, combined with frequent geological disasters, such as frozen soil freeze–thaw cycles and earthquake-induced landslides [1,2]. Icy and snowy weather further exacerbates the risks of skidding and structural damage. Asphalt mixtures have long been exposed to this complex service environment, where multiple environmental factors interweave, including frequent temperature fluctuations, continuous oxidation, intense ultraviolet radiation, and persistent water erosion [3,4]. These factors pose a severe challenge to the long-term durability of RAP. The combined effect of these environmental factors significantly impacts the performance of asphalt mixtures, thereby presenting numerous technical obstacles to the long-term application of RAP in road engineering.

There exist significant performance discrepancies between recycled asphalt mixtures (RAMs) and conventional asphalt mixtures, and these are particularly evident in cold regions and areas with intense ultraviolet radiation [5,6]. The high-temperature stability, low-temperature crack resistance, and water stability of RAMs are highly sensitive to environmental factors, with these properties being prone to degradation under combined environmental stresses, thereby shortening the pavement service life [7,8]. In the alpine regions of western China, asphalt pavements under freeze–thaw cycles and UV radiation are subjected to dual impacts: volume expansion–contraction from repeated water freezing–thawing and photo-oxidation-induced aging. This not only disrupts the mixture structure but also accelerates pavement peeling and loosening, further compromising mixture durability [9,10]. Ren et al. [11] revealed that UV radiation primarily impairs the low-temperature crack resistance of asphalt mixtures, while freeze–thaw cycles predominantly reduce tensile strength, thus identifying freeze–thaw cycles as the primary cause of performance degradation. He et al. [12] designed a simulation method for coupled freeze–thaw–UV aging, demonstrating that coupled aging induces far more significant performance degradation than single-factor aging, with UV radiation exerting a greater impact on physical properties than freeze–thaw cycles. Current research on environmental effects on asphalt and recycled asphalt mixtures has predominantly focused on single factors (e.g., freeze–thaw cycles, UV radiation) influencing mixture road performance. However, existing studies have insufficiently addressed the unique freeze–thaw cycle characteristics of western alpine regions and lack systematic research on the performance of RAMs under the synergistic effect of freeze–thaw cycles and UV aging.

At present, the focus is primarily on the fatigue failure criteria and fatigue life calculation methods for asphalt mixtures. However, such models do not fully account for the viscoelastic properties of recycled asphalt. Fatigue life is influenced by multiple factors, including RAP content, regenerant type, temperature, and loading frequency [13,14,15]. Yin et al. [16] found that high RAP content (≥30%) typically leads to a reduction in fatigue life, though regenerants can mitigate this drawback by restoring the bonding strength of aged asphalt. Shi et al. [17] summarized how the key factors affecting the fatigue performance of recycled asphalt include RAP content, new asphalt type, regenerant type, aggregate characteristics, environmental erosion (ultraviolet radiation and water damage), and interface compatibility between old and new asphalt. Research on the fatigue characteristics of recycled asphalt mixtures in road engineering primarily employs various test methods, including indirect tensile, four-point bending, and semi-circular bending. Currently, research on the fatigue performance of recycled asphalt mixtures mainly focuses on factors such as RAP content, load magnitude, and load frequency. However, these studies have not fully explored the comprehensive influence of internal and external factors on fatigue performance. A systematic theoretical framework for the influence mechanism of environmental coupling effects and recycled material composition remains to be established. In particular, issues such as how to accurately predict the service life of recycled asphalt mixtures in the specific road service environments (freeze–thaw cycles, ultraviolet radiation) in high-altitude areas and how to implement scientific pavement maintenance measures remain unresolved.

In summary, it is of great theoretical significance and practical value to explore the performance degradation law of recycled asphalt mixtures under extreme conditions and establish a corresponding fatigue life prediction model.

2. Design and Synthesis of Materials

2.1. Materials

• Asphalt

First, 70# matrix asphalt was selected as the new asphalt, and its performance indexes are shown in

Table 1. 70 # matrix asphalt test results.

Test Items	Result of Survey	Technology Index	Test Method
Penetration (25 °C, 100 g, 5 s)/0.1 mm	72	60–80	T0504-2011
Penetration index PI	−1.1	−1.5–+1.0	T0604-2011
Softening point/°C	47.0	≥46	T0606-2011
60 °C Dynamic viscosity/Pas	222	≥180	T0620-2011
Ductility (15 °C)/cm	>150	≥100	T0605-2011
Wax content (distillation method)/%	2.0	≤2.2	T0615-2011
Flash point/°C	>300	≥260	T0611-2011
Solubility/%	99.96	≥99.5	T0607-2011
Density (15 °C)/(g/cm3)	1.039	-	T0603-2011
Quality change after RTFOT/%	−0.038	≤±0.8	T0609-2011

.

2.

Technical properties of the new aggregate

The basalt was used as coarse and fine aggregates. The test results for each index are shown in

Table 2. The performance index of basalt coarse and fine aggregate.

Specification	10~15	5~10	3~5	0~3	Technical Requirement
Relative density of bulk volume/(g/cm3)	2.681	2.604	2.621	2.722	≥2.50
Apparent specific gravity/(g/cm3)	2.723	2.704	2.672	2.750	≥2.60
Water absorption/%	0.714	1.330	0.602	0.825	≤3.0
Aggregate crushing value/%	18.5				≤28
Adhesivity	2				≥4
Abrasion value/%	18				≤30

.

3.

Technical properties of mineral powder

Ground limestone powder was selected as the ore powder, and its physical performance indicators are shown in

Table 3. Measured indexes and technical requirements of ore powder.

Item	Unit	Measured Value	Technical Requirement
Apparent density	t/m3	2.82	≥2.8
Specific surface area	m2/kg	401	≥350
Fluidity ratio	%	97	≥90
Water content	%	0.4	≤1.0
SO3	%	0.32	≤4.0
CL-	%	0.01	≤0.02
Loss on ignition	%	0.23	≤2

.

4.

Technical properties of RAP

Since the old aggregate in RAP needs to participate in the construction of the skeleton structure of the recycled asphalt mixture, indexes such as the crushing value, Los Angeles wear value, and needle-like particle content should be consistent with the performance requirements of the new asphalt mixture. The test results are shown in

Table 4. Technical properties of old aggregate and old asphalt.

Performance Index	Result of Survey	Technical Requirement
The volume relative density of wool/(g/cm3)	2.763	-
Apparent relative density/(g/cm3)	2.851	≥2.6
Asphalt density (15 °C)/(g/cm3)	1.028	-
Los Angeles wear value/%	12.4	≤28
Crushing value/%	12.2	≤26
Water absorption/%	0.45	≤2.0
Needle flake particle content/%	7.1	≤12
RAP oil-stone ratio/%	4.8	-
Softening point of old asphalt/°C	63.5	≥46
Old asphalt 15 °C ductility/cm	19.6	≥100
Old asphalt 25 °C penetration/0.1 mm	24.9	60–80

.

5.

Regenerant performance and dosage determination

In this study, Runqiang-RA102 high-performance asphalt rejuvenator produced by Nanjing Subote New Material Co., Ltd., Nanjing, China was selected, as shown in Figure 1, and its technical indicators are shown in

Table 5. Technical indexes of regenerant.

Performance Index	Test Value	Specification Requirement
Viscosity at 60 °C/(mm2/s)	82	50~175
15 °C density/(g/cm3)	0.98	-
Flash point/(°C)	240	≥220
Saturate content/(%)	20.5	≤30
Aromatic content/(%)	64.4	-

.

2.2. Asphalt Mixture Component Design

The RAP material selected in this study has an asphalt content of 4.8%, and the old asphalt exhibits a high degree of aging (the 25 °C penetration is 2.49 mm). Its performance can be improved by adding a regenerant, making it suitable for selecting a higher blending ratio. Mixture ratios with four different RAP contents (0%, 30%, 40%, 50%) were designed. Based on the screening data of new aggregates and old RAP aggregates, the gradations of the two were reasonably combined to ensure that the synthetic gradation met the median requirements of the AC-13 gradation.

According to existing research, 4.9% was selected as the median value of the asphalt content. The asphalt contents were set to 3.9%, 4.4%, 4.9%, 5.4%, and 5.9% to prepare five groups of Marshall specimens, for which relevant tests and calculations were carried out. The stability, flow value, porosity, gap rate, and saturation index were obtained. Finally, under the conditions of different RAP contents, the proportion of new and old asphalt in the recycled asphalt is shown in

Table 6. Different RAP contents in old and new recycled AC-13 asphalt.

RAP Content/%	The Best Oil/Stone Ratio/%	Proportion of the New Asphalt Mixture	Total Asphalt Content/%	Old Asphalt Content/%	New Asphalt Content/%
0	4.9	1	4.67	0	4.67
30	4.98	0.7	4.74	1.37	3.37
40	5.02	0.6	4.78	1.84	2.94
50	5.1	0.5	4.85	2.3	2.55

.

2.3. Preparation Process of Recycled Asphalt

The aged asphalt was placed in an oven and heated at 135–150 °C until it was completely melted (to avoid secondary aging caused by excessive temperatures). During this process, air bubbles and impurities in the asphalt were removed through gentle stirring with a glass rod. The regenerant was heated to 60–80 °C (to prevent volatilization at high temperatures) to minimize the temperature difference with the asphalt, thereby avoiding asphalt agglomeration due to sudden cooling during mixing. The preheated regenerant was slowly poured into the aged asphalt, while an electric stirrer (e.g., a high-speed shear mixer) was used to stir at a speed of 1000–3000 r/min for an initial 5–10 min, ensuring that the regenerant was preliminarily dispersed in the asphalt. The stirring speed was then increased to 3000–5000 r/min, and shearing was continued for 20–30 min. Throughout this period, the asphalt temperature was maintained at 120–130 °C. The shearing force broke down the aged structure of the asphalt, facilitating full integration between the regenerant and asphalt molecules. Finally, the speed was reduced to 500–1000 r/min, and stirring was continued for 5–10 min to eliminate bubbles generated during mixing, resulting in a more uniform and stable recycled asphalt system.

2.4. Freeze–Thaw Cycle and UV Aging Test Design

This study designed a freeze–thaw–ultraviolet aging test scheme by integrating the typical climatic characteristics of the alpine region in the western study area and the settings of freezing–thawing and ultraviolet aging conditions in the relevant literature. The aim is to authentically simulate the service degradation behavior of asphalt mixtures in an environment with severe diurnal temperature differences and intense ultraviolet radiation. A self-made ultraviolet aging test chamber was utilized (as shown in Figure 2).

The test conditions were set as follows:

(1)

Saturated Water Treatment: Rationale

Before the freeze–thaw cycle and ultraviolet aging test, the prepared specimens were immersed in water at 25 °C for 24 h to achieve saturation.

(2)

Freeze–Thaw Cycle Design: Rationale

Freezing Stage: Specimens were placed in a −18° C freezer for 12 h to simulate winter low-temperature freezing, ensuring that internal moisture in the asphalt mixture froze and induced frost heave effects. Melting Stage: Specimens were immersed in 25 °C water for 6 h to simulate diurnal ice–snow melting and water re-infiltration, as well as erosion caused by temperature fluctuations [18,19].

(3)

UV Aging Conditions: Design Rationale

Since ultraviolet light with a wavelength shorter than 290 nm is absorbed by the ozone layer in the atmosphere when sunlight passes through it, the ultraviolet aging of asphalt pavement is mainly caused by UVA [20,21]. Therefore, the light source used in the indoor ultraviolet aging test was a high-pressure mercury lamp, the radiation range was 400 nm-320 nm, and the main wavelength was 365 ± 20 nm. According to an investigative report on ultraviolet radiation in northwest China [22,23], it can be seen that the winter half-year (October–March) is basically about 40–50 Mw/m², and the daily average time of ultraviolet radiation is 11–12 h. Therefore, the UV irradiation intensity was set to 50 Mw/m² for 12 h, with the temperature being maintained at 30 °C to simulate sunshine aging under sunny conditions.

(4)

Test Cycle Design: Rationale

Each environmental test cycle spanned 24 h, comprising three stages: first, specimens were frozen at −18 °C for 12 h to simulate nighttime/winter low-temperature frost heave, followed by 6 h of UV irradiation (controlled wavelength and intensity) to mimic solar aging; finally, 6 h of immersion was conducted in 25 °C water to reproduce water erosion from diurnal ice–snow melting. These three stages formed a complete freeze–thaw + UV environmental cycle that was equivalent to the damage process of actual roads under typical diurnal cycles in alpine regions, defined as one environmental effect (see Figure 3).

(5)

Cycle Number Setting: Rationale

Current domestic and international standards for the number of freeze–thaw cycles in asphalt mixtures are inconsistent, with most studies using 1–10 cycles. Other research indicates that specimen performance stabilizes after 6–8 freeze–thaw cycles. Considering the actual freeze–thaw cycles experienced by asphalt pavements and test feasibility/resource constraints, the numbers of environmental effects were set to 0 (control group), 2, 4, 6, and 8 times (denoted as E0, E2, E4, E6, E8).

3. Test Method

This study focuses on the influence of freeze–thaw cycles and ultraviolet aging on the performance of a recycled asphalt mixture. A systematic indoor road performance test is carried out, and a fatigue prediction model is established to explore the comprehensive influence of the RAP content, environmental action time, and other factors on the performance of the recycled asphalt mixture. A technical roadmap of this study is shown in Figure 4. The equipment used in this study is shown in

Table 7. Equipment statistics.

Test Types	Instrument	Producer
Needle penetration	Automatic penetration instrument	Shanghai Changji SYD-2801, Shanghai, China
Softening point	Automatic softening point meter	Shanghai Li Tao XCQ-24, Shanghai, China
Ductility	Digital dilatometer	Shanghai electron WQD-1A/400, Shanghai, China
Wheel tracking test	Hamburg rutting instrument	UK Cooper‘s CRT-WTIM, Glasgow, UK
Beam bending test	Asphalt mixture beam bending tester	Shanghai Changji SYD-0715, Shanghai, China
Immersion Marshall test	Marshall stability tester	Hebei Construction LWD-5, Shijiazhuang, China
Freeze–thaw splitting test	Asphalt mixture splitting test machine	Tianjin Jianyi FY716-3, Tianjin, China
Indirect tensile fatigue test	Multifunctional test system (UTM)	IPC Australia UTM-25, Brisbane, Australia

.

4. Test Results and Discussion

4.1. Determination of Regenerant Dosage

The amount of regenerant is generally between 5% and 12% [24,25]. After experimental verification, this range can balance performance and cost. When it is lower than 6%, the performance cannot be significantly improved; at more than 12%, it may lead to excessive changes in performance. In this study, four regenerant contents of 6%, 8%, 10%, and 12% were selected to carry out a ratio test on RAP asphalt. The conventional technical indicators, such as the penetration, softening point, and ductility of the recycled asphalt, were tested. The relevant test results are provided in

Table 8. Old asphalt performance indexes with different regenerant contents.

Index	Unit	Base Asphalt	Old Asphalt	Regeneration Agent Dosage (%)
				6	8	10	12
Penetration	0.1 mm	72	24.9	37	64	78	96
Softening point	°C	47	63.5	60.1	56.7	48.3	45.2
Ductility (15 °C)	cm	>150	19.6	37.8	90.2	109.6	>100

Note: The content of the regenerant refers to the mass fraction of the regenerant in old asphalt.

.

It can be observed from the data in Table 8 that as the regenerant dosage increases from 6% to 12%, the penetration value rises from 37 (0.1 mm) to 96 (0.1 mm), demonstrating that the regenerant effectively reduces the hardness of aged asphalt. The softening point, a key indicator of asphalt’s high-temperature performance, exhibits the following trend after regenerant addition: with the increase in regenerant content, the softening point gradually decreases from 60.1 °C to 45.2 °C, indicating that the regenerant dilutes the heavy components in aged asphalt. The ductility increases from 37.8 cm to >100 cm, highlighting that the regenerant significantly improves the low-temperature ductility of aged asphalt. When the regenerant content is 10%, the penetration approaches the level of base asphalt, the softening point is 48.3 °C (close to that of base asphalt), and the ductility reaches 109.6 cm, meeting the low-temperature performance requirements of most roads. Through analysis and comparison, when the regenerant dosage is 10%, the penetration, softening point, and ductility achieve an optimal equilibrium, effectively restoring the comprehensive performance of recycled asphalt. Therefore, this study determines the optimal regenerant dosage to be 10%.

4.2. High Temperature Performance

In Figure 5 and Figure 6, it is evident that the dynamic stability increases as the RAP content rises from 0% to 50%, while the deformation rate gradually decreases. This phenomenon is attributed to the significantly higher rigidity and bonding strength of aged asphalt in RAP compared with new asphalt. Its strong bonding capacity effectively inhibits the plastic deformation of the mixture under high-temperature conditions, thereby enhancing the dynamic stability and reducing the deformation rate. This indicates that incorporating RAP has a positive effect on improving the mixture’s high-temperature performance, with this advantage becoming more pronounced after multiple environmental actions.

Table 9. Recycled AC-13 under freeze–thaw cycles.

Environment Simulation/Times	Mixture Type	Deformation at 45 min d1/mm	Deformation at 60 min d2/mm	Dynamic Stability DS Times/mm	Deformation Rate PRD /%
0	0% RAP	3.542	3.925	1645	7.66
	30% RAP	3.138	3.43	2158	5.84
	40% RAP	3.046	3.297	2510	5.02
	50% RAP	2.875	3.11	2681	4.70
2	0% RAP	3.665	4.073	1544	8.16
	30% RAP	3.657	3.978	1963	6.42
	40% RAP	3.436	3.725	2180	5.78
	50% RAP	3.419	3.689	2333	5.4
4	0% RAP	3.889	4.351	1364	9.24
	30% RAP	3.727	4.096	1707	7.38
	40% RAP	3.698	4.044	1821	6.92
	50% RAP	3.689	3.997	2045	6.16
6	0% RAP	3.904	4.425	1209	10.42
	30% RAP	3.834	4.257	1489	8.46
	40% RAP	3.816	4.198	1649	7.64
	50% RAP	3.747	4.076	1915	6.58
8	0% RAP	4.27	4.843	1099	11.46
	30% RAP	4.025	4.479	1388	9.08
	40% RAP	3.986	4.408	1493	8.44
	50% RAP	3.678	4.058	1658	7.6

shows that as the environmental actions increase from E0 to E8, the dynamic stability of mixtures with all RAP contents significantly decreases, and the deformation rate gradually increases. Notably, even under the E8 conditions, the dynamic stability still meets the specification requirements. This trend reveals that the mixture’s high-temperature stability is negatively correlated with the number of environmental actions, i.e., the longer the environmental exposure, the poorer the high-temperature deformation resistance, yet the mixture maintains good high-temperature stability after repeated environmental actions. The degradation of high-temperature performance stems from volume changes caused by water within the mixture during freeze–thaw cycles, leading to reduced binding force between aggregates and the expansion of micro-cracks. This cyclic stress weakens the mixture’s overall skeleton structure, decreases its bearing capacity, and significantly reduces the dynamic stability [26]. Simultaneously, ultraviolet-induced oxidation of asphalt causes gradual hardening and brittleness, leading to a loss of viscoelasticity and further aggravating the permanent deformation of the mixture. The synergistic effect of freeze–thaw cycles and ultraviolet aging significantly reduces adhesion between asphalt and aggregates, making the mixture more susceptible to permanent deformation under high temperatures. Physical damage from freeze–thaw cycles and chemical degradation from ultraviolet aging are the primary causes of reduced high-temperature performance in recycled asphalt mixtures, with their combined effect accelerating performance deterioration [27].

4.3. Low Temperature Performance

It can be seen in

Table 10. Low temperature trabecular bending test results of regenerated AC-13.

Simulation/Times	Mixture Type	Maximum Load/KN	Mid-Span Deflection/mm	Flexural Tensile Strength RB/MPa	Maximum Bending Strengthb/
					Measured	Stipulation
0	0% RAP	0.90	0.62	8.56	2796	≥2300
	30% RAP	0.97	0.56	9.27	2518
	40% RAP	1.02	0.55	9.72	2488
	50% RAP	1.09	0.51	10.37	2315
2	0% RAP	0.87	0.60	8.27	2714
	30% RAP	0.98	0.54	8.96	2443
	40% RAP	1.00	0.52	9.53	2318
	50% RAP	1.06	0.50	10.05	2267
4	0% RAP	0.83	0.59	7.95	2637
	30% RAP	0.90	0.52	8.58	2358
	40% RAP	0.97	0.50	9.24	2259
	50% RAP	1.01	0.48	9.65	2147
6	0% RAP	0.81	0.56	7.68	2518
	30% RAP	0.84	0.51	8.01	2279
	40% RAP	0.89	0.49	8.52	2194
	50% RAP	0.98	0.44	9.29	1987
8	0% RAP	0.75	0.55	7.15	2478
	30% RAP	0.81	0.48	7.69	2153
	40% RAP	0.88	0.47	8.39	2101
	50% RAP	0.90	0.41	8.53	1848

that the flexural strength of the recycled asphalt mixtures increases with the increase in RAP content, while their maximum flexural strain exhibits a decreasing trend. This variation in mechanical properties ultimately leads to the deterioration of the material’s low-temperature crack resistance. In-depth analysis indicates that the hardening effect of aged asphalt in RAP due to long-term service is the primary cause of the mixture’s enhanced tensile properties, but it also reduces the material’s deformability [28]. Additionally, the surface of the RAP aggregate becomes rougher after use and aging, enhancing its adhesion with new asphalt, which further contributes to improving the mixture’s overall strength.

Analysis of the test data (Figure 7 and Figure 8) reveals that both the maximum flexural strain and flexural strength of ordinary asphalt mixtures and recycled mixtures with different RAP contents show a continuous decline with the accumulation of environmental action cycles. This performance degradation is primarily attributed to the synergistic damage effect of freeze–thaw cycles and ultraviolet radiation; freeze–thaw cycles induce micro-cracks within the mixture, especially in high-RAP-content mixtures, where the brittleness of old asphalt amplifies micro-crack expansion and weakens the material’s overall strength. Ultraviolet aging accelerates the aging of both new and old asphalt, causing them to harden and become brittle, reducing the mixture’s ductility and crack resistance, and leading to a decrease in the maximum flexural tensile strain. Aging also weakens the bonding force between asphalt and aggregate, making the mixture more prone to cracking under bending loads. This superposition effect results in a more significant downward trend in both flexural tensile strength and maximum flexural tensile strain [29].

Further analysis shows that the initial ultimate flexural strain of all mixture types exceeded the technical standard of 2300 με before experiencing environmental actions. However, with the accumulation of environmental effects, mixtures with different RAP contents exhibit distinct performance degradation characteristics. After E8, the flexural strain of the 0% RAP recycled mixture still remains above the standard limit; when the RAP contents are 30%, 40%, and 50%, their flexural strains fall below the standard values after E6, E4, and E2, respectively. This indicates that high-RAP-content mixtures may fail to meet performance standards within a relatively short period. This phenomenon is mainly attributed to the frost heave stress caused by water freezing in the internal voids of the mixture at low temperatures, particularly water immersed in the interface between the asphalt and aggregate, which exacerbates internal structural damage. As RAP content increases, the number of new–old asphalt interfaces rises, further aggravating damage under frost heave stress and leading to a more significant reduction in failure strain [29].

4.4. Water Stability

(1)

Marshall Immersion Test

As shown in

Table 11. Immersion Marshall test results of regenerated AC-13.

Number of Cycles/Times	Mixture Type	Marshall Stability MS/kN	Immersion Stability MS1/kN	Residual Stability MS0/%
				Measured Value	Specification Requirements
0	0% RAP	12.38	10.89	87.96	≥80
	30% RAP	12.87	11.12	86.40
	40% RAP	13.27	11.39	85.83
	50% RAP	14.14	11.91	84.23
2	0% RAP	11.26	9.79	86.94
	30% RAP	12.04	10.27	85.30
	40% RAP	12.19	10.36	84.99
	50% RAP	13.23	10.94	82.69
4	0% RAP	10.98	9.36	85.25
	30% RAP	11.87	10.08	84.92
	40% RAP	12.05	10.12	83.98
	50% RAP	12.87	10.43	81.04
6	0% RAP	10.26	8.36	81.48
	30% RAP	11.07	8.87	80.13
	40% RAP	12.25	9.72	79.35
	50% RAP	13.24	10.04	75.83
8	0% RAP	10.23	8.25	80.65
	30% RAP	10.66	8.35	78.33
	40% RAP	12.67	9.78	77.16
	50% RAP	13.87	10.36	74.69

and Figure 9, the stability of the 0% RAP mixture decreased from 12.38 kN to 10.23 kN, and that of the 50% RAP mixture decreased from 14.14 kN to 13.87 kN. The decrease in Marshall stability (MS) for high-RAP-content mixtures under freeze–thaw cycles was relatively small, indicating that RAP addition enhances the mixture’s skeleton structure, enabling it to maintain high compressive strength under short-term environmental action [30]. The overall immersion stability (MS1) also declined with increasing environmental actions; the MS1 value of the 0% RAP mixture dropped from 10.89 kN to 8.25 kN, and that of the 50% RAP mixture decreased from 11.91 kN to 10.36 kN. High-RAP-content mixtures showed good resistance to water damage at the initial stage (E0–E4), but the decrease in immersion stability significantly intensified after E6, suggesting that the durability of high-RAP-content mixtures is profoundly affected by the environment.

Figure 10 illustrates that as the number of environmental actions increased, the MS0 of different RAP contents displayed a downward trend, with the decline rate increasing notably with higher RAP content. For example, the residual stability (MS0) of the 0% RAP mixture decreased from 87.96% to 80.65%, while that of the 50% RAP mixture dropped from 84.23% to 74.69%. Under the E6 conditions, the MS0 of the 40% RAP mixture fell below the standard requirement of 80%; with E8, the MS0 of the 50% RAP mixture also failed to meet the standard. This indicates that the water damage resistance of high-RAP-content mixtures significantly deteriorates under long-term environmental actions, primarily due to the increased brittleness of aged asphalt and the degradation of the mixture’s internal interface structure.

Under the combined effects of freeze–thaw cycles and ultraviolet aging, the interface between the aggregate and asphalt is more prone to peeling. Meanwhile, after water invades the internal pores of the recycled asphalt mixture, repeated freezing and thawing during cycles gradually loosen the aggregate–asphalt interface structure, exacerbating water damage [31,32]. When the RAP content is controlled at 30%–40%, it provides certain skeletal support for the mixture, initially maintaining good water stability. However, when the RAP content exceeds 50%, the mixture’s internal bonding force decreases significantly, and water damage intensifies, largely due to the overall performance degradation caused by excessive aged asphalt content.

(2)

Freeze–thaw Splitting Test

The splitting strength primarily depends on the viscosity and elasticity of asphalt, as well as the aggregate properties and mix ratio of the mixture [33,34]. Freeze–thaw cycles weaken the internal skeletal structure of the asphalt mixture and the adhesive performance at the asphalt–aggregate interface, leading to a reduction in tensile strength. Test data from Table 11 and Figure 11 show that both the unfreeze–thaw splitting strength (RT1) and freeze–thaw splitting strength (RT2) of recycled asphalt mixtures decrease significantly with an increase in the number of environmental actions. For the 0% RAP mixture, the RT1 value dropped from the initial 0.9867 MPa to 0.7646 MPa, and RT2 decreased from 0.8538 MPa to 0.6031 MPa. For the 50% RAP mixture, RT1 fell from 1.3409 MPa to 1.1352 MPa, and RT2 declined from 1.0918 MPa to 0.8127 MPa, demonstrating the significant impact of freeze–thaw cycles on the mixture’s tensile properties.

The variation in the freeze–thaw splitting strength ratio (TSR) reflects the asphalt mixture’s resistance to water damage under freeze–thaw conditions [35]. A higher TSR indicates better tensile performance and water stability after freeze–thaw cycles. As shown in Figure 12, the TSR of all mixtures gradually decreases with increasing environmental actions, and the decline is more rapid for higher RAP contents. For example, the TSR of the 0% RAP mixture decreased from 86.53% to 78.88% (a drop of 8.84%), while the 30% RAP mixture saw a TSR reduction from 84.21% to 75.39% (10.47%), and the 50% RAP mixture experienced a decrease from 81.42% to 71.59% (12.07%). Notably, at E8, the TSR of the high-RAP-content (40%) mixture fell below 75%, indicating that the reduced toughness of aged asphalt in RAP fails to provide sufficient adhesion, making the mixture more susceptible to spalling under freeze–thaw cycles and leading to decreased splitting strength. The accumulation of environmental effects causes repeated expansion–contraction of internal moisture, exacerbating the degradation of interface bonding between asphalt and aggregate and significantly reducing water damage resistance.

In summary, an appropriate RAP content (30%) can maintain good water stability, but the durability decline should be monitored as environmental actions increase. High-RAP-content mixtures (40%–50%) exhibit significantly reduced water stability after freeze–thaw cycles, necessitating measures such as anti-stripping agents or modified asphalt to enhance water damage resistance. In cold and freeze–thaw-prone regions, high-RAP-content recycled asphalt mixtures should be avoided to improve pavement service life and durability.

4.5. Fatigue Performance

(1)

Effect of RAP content on fatigue life

In Figure 13 and

Table 12. Recycled AC-13 fatigue life times.

RAP Content/%	Stress Ratio	Brake Fatigue Life/Times	Logarithm of Fatigue Life
0	0.4	12546	4.0985
30		11231	4.05042
40		7368	3.8673
50		5268	3.7216

, the results indicate that the slope of the logarithmic fatigue life curve for recycled asphalt mixtures with RAP contents ranging from 0% to 30% is gentler than that of mixtures with RAP contents between 30% and 50%. This suggests that at low RAP contents, the fatigue performance of recycled asphalt mixtures remains relatively stable. Evidently, 30% RAP content represents a critical threshold; once exceeded, the fatigue performance of recycled asphalt mixtures deteriorates abruptly.

(2)

Influence of Stress Level on Fatigue Life

In order to analyze the variation law of the fatigue life of recycled asphalt mixtures with different stress ratios, the test data are summarized in

Table 13. The fatigue life times of regenerated AC-13 under different stress ratios (10 Hz, 15 °C).

RAP Dosage/%	Stress Ratio	Fatigue Life Times/Times	Logarithm of Fatigue Life
0	0.3	72,387	4.859660578
	0.4	12,546	4.098505283
	0.5	6439	3.808818425
	0.6	2346	3.370328008
30	0.3	49,398	4.693709366
	0.4	11,231	4.050418427
	0.5	4950	3.694605199
	0.6	1597	3.203304916
40	0.3	38,327	4.583504827
	0.4	7868	3.895864351
	0.5	3262	3.513483957
	0.6	1264	3.101747074
50	0.3	24,384	4.38710495
	0.4	5968	3.775828814
	0.5	1778	3.249931757
	0.6	876	2.942504106

.

Figure 14 and Figure 15 illustrate the linear fitting law of the logarithmic fatigue life for recycled asphalt materials with varying RAP contents. Analysis of the test data reveals that when the stress ratio ranges from 0.3 to 0.6 as the RAP content gradually increases from 0% to 30%, 40%, and 50%, the evolution of anti-fatigue performance exhibits a significant linear pattern. The data in

Table 14. A summary of the regression parameters of the fatigue curve.

RAP Dosage/%	K	n
0	6.175	4.75
30	6.05	4.77
40	5.94	4.82
50	5.77	4.85

show that the regression coefficient for various types of recycled asphalt mixtures exceeds 0.930, demonstrating the model’s high reliability.

Combined with the data, the following observations can be made: (1) at different stress ratios, the fatigue life attenuation parameter K for specimens with 0% and 30% RAP content exhibits a similar variation pattern. As the RAP content increases, the K value of the new asphalt mixture specimens is higher than that of recycled asphalt mixture specimens with other RAP contents. This feature indicates that incorporating recycled materials significantly reduces the mixture’s fatigue durability. (2) For every 10% increase in the recycled material proportion, the index term n in the fatigue equation systematically increases and exceeds the n value of the new asphalt mixture, suggesting that RAP incorporation makes recycled asphalt mixtures more sensitive to stress levels than new asphalt mixtures.

4.6. Establishment of Fatigue Prediction Model

To accurately evaluate the actual fatigue performance of recycled asphalt mixtures, a prediction model incorporating RAP content (R), stress ratio (C), and environmental action cycles (E) should be established. The experimental data demonstrate that logN_p maintains a linear relationship with C with varying R and E, indicating a logarithmic correlation between N_p and C. By adjusting model coefficients, the influences of R and E on N_p can be reflected. Based on this, the fatigue life prediction model described in Equation (1) is proposed.

$$\left\{\begin{array}{l}\ln N_f=a+b\ln C\\ a=f_1(R,E)\\ b=f_2(R,E)\end{array}\right.$$

(1)

In the formula: N_f is the number of fatigue life, times; c is the stress ratio; r is the content of RAP, %; e is the number of environmental effects, times; a and b are the model parameters related to R and E, respectively.

(1)

RAP content

Based on the fatigue performance test data of the recycled asphalt mixtures, a fatigue life prediction model was constructed to characterize the dosage effect under temperature conditions of 15 °C and a loading frequency of 10 Hz. By analyzing the correlation between different RAP ratios (30%–50%) and the loading stress ratio, the quantitative relationship between the fatigue life and stress level was established using the power function model N_p = a·C^b, as depicted in Figure 16. The results show that the model can accurately reflect the fatigue response characteristics of recycled asphalt mixtures with variable RAP contents and under multi-stress conditions, with a significant goodness of fit (R² > 0.95), which verifies the adaptability of the model parameters to the evolution of the material properties. Finally, this study determines the specific values of parameters a and b in the fatigue equation under different RAP content conditions, as presented in

Table 15. Fitting values of fatigue equation parameters a and b under different RAP content.

RAP Dosage/%	0	30	40	50
a	75.67	125.78	64.886	63.73
b	−5.69	−4.96	−5.299	−4.93

.

By analyzing the variation trend of fatigue equation parameters a and b with the RAP content in Table 15, the parameter variation curve shown in Figure 16 can be obtained.

Figure 16 shows that when the RAP content (R) is less than 30%, the parameters a and b change linearly with R, and the linear regression model a = A₁ + B₁*x, b = A₁ +B₂* x can be used for fitting. When the RAP content reaches or exceeds 30 %, the relationship between parameters a and b and R becomes nonlinear. At this time, the quadratic polynomial model a = C1 + D₁*x + E₁*x², b = C₂ + D₂*x + E₂*x² can be used to obtain more accurate fitting results. The specific parameter values of each fitting equation are detailed in Table 15 and

Table 16. The fitting value of parameter a.

Parameter Value	A1	B1	C1	D1	E1
a	75.67	1.670	666.89	−26.9977	0.29869

.

Figure 17 shows that when the RAP content (R) is less than 30%, the parameters a and b change linearly with R, and the linear regression model a = A₁ + B₁*x, b = A₁ +B₂* x can be used for fitting. When the RAP content reaches or exceeds 30 %, the relationship of parameters a and b with R becomes nonlinear. At this time, the quadratic polynomial model a = C₁ + D₁*x + E₁*x², b = C₂ + D₂*x + E₂*x² can be used to obtain more accurate fitting results. The specific parameter values of each fitting equation are detailed in Table 16 and

Table 17. The fitting value of parameter b.

Parameter Value	A2	B2	C2	D2	E2
b	−5.69	−0.024	0.305	−0.2817	0.00354

.

In the fatigue prediction model, when the loading frequency is 10 Hz and the temperature is 15 °C, the variation characteristics of parameters a and b with the RAP content are as follows in Equations (2) and (3):

$$a=\left\{\begin{array}{l}75.67+1.670R(R<30\%)\\ 666.89-27.01R+0.29869R^2(R\ge 30\%)\end{array}\right.$$

(2)

$$b=\left\{\begin{array}{l}-5.69-0.024R(R<30\%)\\ 0.305-0.2817R+0.00354R^2(R\ge 30\%)\end{array}\right.$$

(3)

(2)

The number of cycles

Based on the test data, the following reference conditions were selected: R = 30%, temperature T = 15 °C, and loading frequency H = 10 Hz. According to the correlation law between the fatigue life and the number of environmental actions revealed by the test, the power function form Nf = a*Cb is used to fit the fatigue life and the loading stress ratio with different environmental actions. The relevant prediction results are shown in Figure 18.

Table 18. Fitting values of a and b parameters.

Number of Cycles/Times	0	2	4	6	8
a	125.839	48.753	26.678	30.637	126.677
b	−4.96	−5.67	−5.906	−5.37	−3.325

lists the fatigue fitting parameters a and b obtained under different environmental conditions. The relationship of the parameters a and b with the number of environmental cycles is drawn in Figure 19, and it can be seen that the change trend is approximately in line with the cubic function relationship. Therefore, the cubic polynomial is used to fit a = A₃ + B₃*E + C₃*E₂ + D₃*E₃, b = A₄ + B₄*E + C₄*E₂ + D₄*E₃, and the parameters of the fatigue equation are shown in

Table 19. The fitting value of parameter a.

Parameter Value	A3	B3	C3	D3
a	124.481	−40.722	2.01	0.386

and

Table 20. The fitting value of parameter b.

Parameter Value	A4	B4	C4	D4
b	−4.966	−0.348	−0.017	0.0107

.

Under the condition of high RAP content (R ≥ 30%), according to the variation characteristics of the number of environmental effects, the fatigue prediction model for recycled asphalt mixtures is as follows in Equation (4):

$$\left\{\begin{array}{l}a=124.481-40.722E+2.01E^2+0.386E^3\\ b=-4.966-0.348E-0.017E^2+0.0107E^3\end{array}\right.$$

(4)

(3)

RAP content and number of cycles

The influences of three variables on a and b need to be considered at the same time; the following Equations (5) and (6) can be assumed:

$$a=P_1+P_{2}R+P_3{R}^2+P_{4}E+P_5{E}^2+P_6{E}^3$$

(5)

$$b=P_1+P_{2}R+P_3{R}^2+P_{4}E+P_5{E}^2+P_6{E}^3$$

(6)

In this study, the professional numerical analysis software LSTOPT (LS-OPT 7.0)was employed for parameter optimization. This platform integrates the Levenberg–Marquardt algorithm with a global optimization method. Aiming at the fatigue characteristics of recycled asphalt mixtures, a nonlinear regression model between the stress ratio and fatigue life was established. The objective function was constructed using the least squares method, enabling accurate calibration of the model coefficients in Equations (5) and (6). The detailed parameter estimation results are documented in

Table 21. The parameter values of the fatigue prediction equation.

Model Parameter	P1	P2	P3	P4	P5	P6
a	791.371	−27.01	0.0298	−40.722	2.01	0.386
b	−4.661	−0.281	0.00354	−0.348	−0.017	0.0107

.

Through sorting and analysis, a fatigue life prediction model suitable for high-RAP-content (R ≥ 30%) recycled asphalt mixtures is finally obtained; the specific form is shown in Equation (7).

$$\begin{array}{l}\ln N_f=791.32-27.01R+0.0298R^2-40.722E+2.01E^2+0.386E^3\\ -\left(4.661+0.281R-0.00354R^2+0.348E+0.017E^2-0.0107E^3\right)\ln \left({\displaystyle \frac{1}{C}}\right)\end{array}$$

(7)

4.7. Model Validation

The parameters can be substituted into Formula (7), and then the correlation model between the fatigue life Nf and the load stress ratio C of recycled asphalt mixtures can be established. The model can systematically reflect the influence mechanism of different values of R and numbers of environmental actions E on the durability of materials, and empirical research can be conducted through comparative analysis of Figure 20 and Figure 21.

As shown in Figure 20 and Figure 21, when the stress ratio is in different ranges, the predicted value of the model is highly consistent with the measured data, and the variation law of the two shows significant synchronization. The research shows that the prediction method based on multi-parameter coupling not only effectively verifies the reliability of the model, but its prediction accuracy also meets the requirements of engineering applications.

In summary, the fatigue life prediction model proposed in this study exhibits a fitting degree of 0.9 between the prediction results and the test data, demonstrating outstanding adaptability under complex working conditions. The successful construction of this model not only addresses the key technical challenges in the performance evaluation of recycled asphalt materials but also provides an economical and efficient numerical analysis tool for green road construction, holding significant practical value for engineering.

5. Conclusions

This research analyzed the variations in the road performance of mixtures with different RAP contents under freeze–thaw cycles and ultraviolet aging. Through data analysis, a fatigue life prediction model of recycled asphalt mixture with R, C, and E as variables was established, and finally, the prediction effect of the model was evaluated. The main conclusions are as follows.

(1)

The incorporation of an appropriate amount of RAP can enhance dynamic stability and reduce the initial-stage rutting deformation rate. However, after continuous freeze–thaw cycles, the high-temperature deformation resistance gradually declines. Under low-temperature conditions, both the flexural tensile strength and maximum flexural tensile strain decrease with the increase in the number of freeze–thaw and ultraviolet aging cycles; particularly when the RAP content is high, the low-temperature durability of the material deteriorates more significantly. Additionally, the immersion stability and freeze–thaw splitting residual stability continue to decline, indicating that the water damage resistance of high-RAP-content mixtures is notably weakened after multiple freeze–thaw cycles.

(2)

A comprehensive analysis of multiple road performance indicators of recycled asphalt mixtures shows that the sensitivity to the number of environmental action cycles is most pronounced at low temperatures, followed by water stability, while high-temperature performance is the least affected.

(3)

The fatigue life decreases significantly with the increase in RAP content. At 0% and 30% dosages, the fatigue curve parameter K values are similar. When the dosage exceeds 30%, the K value of the new material is significantly higher than that of the recycled material. The fatigue curve index n value of the recycled material is generally larger than that of the new material and increases with the rise in RAP content.

(4)

A fatigue life prediction model based on the RAP content and the number of environmental action cycles was constructed. The model can accurately predict the fatigue life of recycled asphalt mixtures under the specified conditions, with a high fitting accuracy (R² > 0.9). The model’s verification demonstrates its good applicability and reliability.

These research results can promote the dual benefits of thermal regeneration technology in environmental and economic aspects, reduce the life cycle cost by reducing waste asphalt emissions and new material consumption, help achieve the “double carbon” goal, and promote the large-scale application of thermal regeneration technology in areas with extreme climates.