Study on the Modification Mechanism and Rheological Properties of Bio-Oil-Based Composite-Modified Material for TOP-DOWN Crack Treatment in Long-Life Pavement

Abstract

To address the durability limitations of conventional crack sealants under coupled extreme temperatures and traffic loads in long-life pavements, a bio-oil composite-modified patching material was developed using 90# base asphalt as the matrix, synergistically modified with crumb rubber (CR) and epoxidized soybean oil (ESO). To resolve the contradictory requirements for high elasticity and thermal expansion/contraction coordination in sealants, ESO was introduced; its polar epoxy groups optimize phase compatibility and promote low-temperature stress relaxation without restricting thermal deformability. Rheological evaluations revealed that the optimal system (OPT) successfully extended the service temperature window from PG 76–−24 °C (baseline) to PG 82–−24 °C, significantly enhancing its adaptability to extreme climatic fluctuations. At −24 °C, OPT exhibited a reduced creep stiffness (S) of 164 MPa and an increased creep rate (m) of 0.312, with a cracking resistance ratio (k) as low as 525.6; the quantitative significance of these metrics lies in granting the sealant superior stress relaxation capacity, enabling it to accommodate dynamic crack widening without interfacial debonding or brittle fracture. Fatigue testing via time sweeps demonstrated that N_f50 reached 2890 cycles, highlighting robust long-term resistance against high-frequency shear strains induced by tire edges. Micro-mechanistic analyses (FTIR, TG/DTG, and DSC) confirmed that the modification is primarily driven by physical blending. The elevation of the thermal decomposition threshold (T5%) to 302.4 °C and the residue at 600 °C to 44.8% provide a critical safety margin for high-temperature construction heating, preventing thermal degradation. Furthermore, the glass transition temperature (T_g) decreased to approximately −35.2 °C. These findings establish a rigorous quantitative and mechanistic framework for designing sustainable, high-performance patching materials for resilient pavement maintenance.

1. Introduction

In recent years, pavement maintenance management has increasingly transitioned from reactive rehabilitation toward preventive maintenance strategies that emphasize full life-cycle performance. Concurrently, the drive towards green and low-carbon transformations within the transportation sector has accelerated the development of sustainable materials. Specifically, the utilization of renewable bio-based modifiers and the valorization of solid waste resources, such as waste tire rubber, are widely recognized as effective approaches for reducing environmental impact while enhancing material sustainability [1,2]. Within this context, the development of pavement distress mitigation materials capable of withstanding coupled climatic actions and traffic loading, while simultaneously satisfying engineering performance requirements and promoting resource recycling, has become a critical research priority in pavement maintenance engineering.

Among various distresses, top-down cracking has emerged as a critical mode of failure, particularly in long-life asphalt pavements, representing one of the most prevalent and rapidly propagating early-stage distresses; once cracks penetrate through the surface layer, they facilitate water infiltration, which, under freeze–thaw cycling and repeated vehicular loading, accelerates moisture-induced damage and structural fatigue, eventually leading to secondary distresses including potholes, raveling, and alligator cracking [3]. To suppress crack propagation and delay structural deterioration, crack sealing has been extensively incorporated into preventive maintenance systems. The Federal Highway Administration (FHWA) in the United States has categorized crack sealing as a representative preventive maintenance activity and has provided systematic recommendations regarding timing, material selection, and construction practices [4]. In engineering practice, hot-applied crack sealants are widely adopted owing to their construction adaptability and post-installation sealing stability [5]. Nevertheless, service behavior of crack sealants is governed by pronounced temperature–load coupling characteristics, under which adequate resistance to flow and shear deformation at elevated temperatures is required to prevent bleeding and pump-out, whereas sufficient ductility and stress relaxation capacity at low temperatures are essential to mitigate brittle fracture and interfacial debonding. In addition, adhesion durability and resistance to fatigue-induced peeling must be maintained under combined temperature cycling and traffic loading, rendering synergistic performance across wide temperature range a central objective in formulation design [6].

Research efforts targeting performance synergy of crack sealants have generally evolved along two primary directions. One line of investigation focuses on polymeric or rubber modification, through which elastic contribution is enhanced to improve resistance to high-temperature deformation, followed by development of performance-oriented evaluation methodologies under service conditions. For instance, research conducted by National Research Council (NRC) of Canada extended bending beam rheometer (BBR) framework to characterize low-temperature behavior of crack sealants, thereby establishing rheological evaluation concepts specifically tailored for sealing materials [7]; Virginia Transportation Research Council (VTRC) and other institutions in the United States further developed and validated performance-based testing matrices and grading thresholds for hot-applied sealants, integrating crack sealant BBR (CSBBR), dynamic shear, and adhesion tests into unified framework that facilitates correlation between laboratory indices and field performance. Domestic studies have frequently employed orthogonal experimental designs to optimize composite formulations, with emphasis placed on contributions of individual modifiers to high-temperature, low-temperature, and overall performance. Another line of research emphasizes rejuvenators or bio-based oils, which, by regulating viscoelastic characteristics of binder system, aim to reduce temperature susceptibility and enhance low-temperature relaxation while maintaining workability and adhesion performance. Increasing attention has also been devoted to short-term aging induced by construction heating; for example, investigations into effects of different aging temperatures under field construction conditions on viscoelasticity, physicochemical properties, and thermal behavior of crack sealants have provided basis for defining construction temperature windows and durability assessment [8,9].

Recently, hybrid modification systems combining crumb rubber (CR) and bio-based oils have gained significant traction [10,11]. While existing literature demonstrates that CR provides a robust elastic network for high-temperature stability and bio-oils effectively rejuvenate aged binders to enhance low-temperature flexibility, their specific application to crack-patching materials remains underexplored. A distinct research gap exists regarding the fatigue durability of these composite sealants under the localized, high-strain cyclic shear typical of top-down cracking [12,13]. Furthermore, unlike prior investigations that treat bio-oils merely as generic softening agents, this work focuses on how the unique polar epoxy functional groups in epoxidized soybean oil (ESO) mechanistically interact with the swollen CR network. Although the epoxy rings possess inherent physical rigidity, their strong polarity effectively anchors high-molecular-weight polymer chains and enhances interfacial compatibility with the asphaltene phase. This synergy facilitates superior thermal deformation coordination without compromising the integrity of the reinforced elastic network.

Using 90# base asphalt as the continuous viscoelastic matrix, an optimal composite formulation (OPT) was developed by synergistically incorporating 20 wt% CR and 10 wt% ESO, supplemented by a minor dosage of anti-stripping agent (ASA) for interfacial stability. From a performance evaluation perspective, DSR and BBR were employed to characterize rheological resistance to high-temperature shear and low-temperature creep-relaxation, respectively, while the Performance Grading (PG) framework was introduced to verify engineering applicability under extreme climatic fluctuations. Furthermore, time-sweep fatigue tests were conducted to quantify damage evolution under cyclic loading. At the mechanistic level, FTIR, TG/DTG, and DSC were utilized to construct a multi-dimensional evidence chain encompassing molecular structure, thermal stability, and glass transition behavior. This systematically elucidates the synergistic mechanism of “CR-swelling network reinforcement” and “ESO-mediated polar compatibility optimization,” providing a robust cross-scale methodological framework and quantitative support for sustainable pavement maintenance.

2. Materials and Experimental Methods

2.1. Raw Materials and Formulation Design

In the present study, 90# base asphalt was selected as the matrix binder for the patching material to provide a continuous viscoelastic matrix and ensure fundamental adhesion performance. To establish a rigorous commercial performance benchmark, styrene-butadiene-styrene (SBS)-modified asphalt (utilizing a standard commercial dosage of approximately 4.5 wt% SBS was utilized as the control group, representing the prevailing industry standard for high-performance crack sealants. The composite modification system primarily consists of crumb rubber (CR) derived from waste tires and epoxidized soybean oil (ESO). Upon absorbing light fractions from the asphalt, CR undergoes volumetric swelling to form an elastic particle-network structure, thereby significantly enhancing the intermediate-to-high temperature deformation resistance and energy dissipation-recovery characteristics of the material [14]. In this study, ESO was selected as a bio-based modifier. It is crucial to clarify that the ‘epoxy value of 6.31%’ listed in

Table 1. Raw materials.

Material Name	Model	Manufacturer	Key Indicators	Description
Base Asphalt	90#	Panjin Bafang Industrial Co., Ltd. (Panjin, China)	Penetration: 72 (0.01 mm); Softening point: 59 °C; Ductility at 5 °C: 40 cm.	Serves as patching material matrix, providing requisite adhesion and elastic framework.
Crumb Rubber (CR)	CR	Hebei Hezhen Rubber Co., Ltd. (Shijiazhuang, China)	60-mesh	Induces elastic networks via swelling, augmenting high-temperature stability and recovery.
Epoxidized Soybean Oil (ESO)	ESO	Jinan Ningsheng Trade Co., Ltd. (Jinan, China)	Epoxy value 6.31%; Flash point 290 °C; Acid value 0.45 mg KOH/g	Replenishes light fractions, optimizing phase compatibility and facilitating low-temperature stress relaxation.

indicates the concentration of polar epoxy functional groups grafted onto the flexible triglyceride backbone of the soybean oil, rather than the addition of rigid thermosetting epoxy resins. These polar groups are expected to replenish light components and optimize phase compatibility, thereby aiming to effectively reduce temperature susceptibility and promote stress relaxation at low temperatures.

To ensure comprehensive pavement performance and preparation stability of the composite system, specific auxiliary additives were incorporated into the formulation. To reinforce interfacial resistance against moisture damage, a minor dosage of non-amine anti-stripping agent (ASA, manufactured by Hebei Shengkang Chemical Co., Ltd., Hengshui, China) was introduced to improve the wettability and adhesion retention at the asphalt–aggregate interface; its primary function is oriented toward adhesion enhancement rather than altering the bulk rheological behavior. Concurrently, a penetrant (JFC, penetrating power ≤ 5 s, manufactured by Shandong Defeng Chemical Co., Ltd., Linyi, China) was added to mitigate risks of agglomeration and segregation, while a vulcanizing agent (PDM, initial melting point ≥ 195 °C, mass loss on heating ≤ 0.5%, manufactured by Dongguan Xiangfa Rubber and Plastic Materials Co., Ltd., Dongguan, China) was utilized synergistically to further strengthen the elastic recovery and structural stability of the system.

The optimal composite formulation (OPT) was determined through preliminary physical property screening experiments. Specifically, a 20 wt% dosage of crumb rubber (CR) was selected as it represents the typical critical threshold for establishing a continuous elastic network structure within the asphalt matrix. Concurrently, the 10 wt% dosage of ESO was optimized to maximize low-temperature stress relaxation without inducing excessive softening, thereby preserving the requisite high-temperature rutting resistance. To systematically elucidate the respective contributions of single modifications versus synergistic composite effects on the rheological characteristics and adhesion stability of the patching material, four material groups—SBS-modified asphalt (control), 20% CR-only modification, 10% ESO-only modification, and the OPT composite—were comparatively evaluated. The core raw materials utilized in this study and their respective manufacturers are detailed in Table 1.

2.2. Preparation Process of Bio-Oil Composite-Modified Patching Materials

Bio-oil composite-modified patching materials were synthesized via a high-shear dispersion process coupled with constant-temperature development. Procedures utilized electric constant-temperature blast ovens, electric stirrers, and high-speed shear mixers (FLUKO FM300, Shanghai, China), supplemented by constant-temperature heating jackets to maintain thermal stability within a strict tolerance of ±2 °C throughout fabrication. Initially, 90# base asphalt was preheated at 165 °C for 2 h to achieve a homogeneous fluid state. Upon transferring the binder to the heating jackets, crumb rubber (CR) was introduced under low-speed stirring (500 r/min) for 30 min to initiate the absorption and swelling of light fractions, thereby mitigating subsequent agglomeration risks. Subsequently, to ensure optimal particle refinement without inducing thermo-oxidative degradation, a three-stage continuous shearing program was executed at 165 ± 2 °C utilizing the FM300 mixer at a consistent rate of 7000 r/min for a total of 30 min: (1) pre-shearing for 5 min; (2) incorporating epoxidized soybean oil (ESO) with continuous shearing for 10 min; and (3) adding the anti-stripping agent (ASA) for the final 15 min of shearing. This multi-stage procedure ensures consistent mixing energy and highly uniform dispersion within the asphalt phase. Post-shearing, the specimens underwent a development phase under low-speed stirring for 1.5 h to promote microstructural rearrangement and physical network construction, ultimately yielding stable composite-modified patching material samples. To ensure rigorous comparative analysis, the single-modified control groups (CR-only and ESO-only) were synthesized following identical thermal and mechanical protocols, simply omitting the respective absent components during the specified shearing stages. The preparation process flow chart of the bio-oil composite-modified patching materials is illustrated in Figure 1.

2.3. Experimental Program

(1)

Dynamic Shear Rheological Testing: Temperature and frequency scans were performed utilizing a Dynamic Shear Rheometer (DSR). When viscoelastic materials are subjected to external loading, a phase difference arises between peak stress and strain, facilitating the calculation of the complex shear modulus (G*). While G* quantifies deformation resistance, the phase angle (δ) characterizes viscoelastic proportions. The governing fundamental rheological equations are formulated as Equations (1)–(5) [15,16,17,18].

$$G^{*}={\displaystyle \frac{\tau (t)}{\gamma (t)}}={\displaystyle \frac{{\tau }_0\sin (\omega t)}{{\gamma }_0\sin \omega t}}={\displaystyle \frac{{\tau }_0}{{\gamma }_0}}=(\cos \delta +i\sin \delta )$$

(1)

$$G^{*}=\left|G^{*}\right|(\cos \delta +i\sin \delta )=G^{\prime }+i{G}^{″}$$

(2)

$$G^{\prime }={\displaystyle \frac{{\tau }_0}{{\gamma }_0}}\cos \delta =\left|G*\right|\cos \delta$$

(3)

$$G^{″}={\displaystyle \frac{{\tau }_0}{{\gamma }_0}}\sin \delta =\left|G*\right|\sin \delta$$

(4)

$$\left|G*\right|=\sqrt{(G^{\prime 2}+G^{″2})}$$

(5)

where G* represents complex shear modulus (Pa); τ₀ and γ₀ denote stress and strain amplitudes, respectively; ω is angular frequency; t is time; δ signifies phase angle; G′ and G″ represent storage and loss moduli, respectively.

(2)

Low-temperature Rheological Testing: Bending Beam Rheometer (BBR) tests were executed on specimens (125 mm × 12.5 mm × 6.25 mm) at temperatures of −12 °C, −18 °C, and −24 °C for 60 s creep duration. Resultant creep stiffness modulus S and creep rate m were utilized to evaluate low-temperature stress relaxation capacity and cracking susceptibility [18].

(3)

Performance Grading (PG): DSR high-temperature and BBR low-temperature indices were adopted to define service temperature windows [19]. Continuous grading temperatures (T_c) for complex modulus and creep stiffness were calculated via logarithmic interpolation according to Equation (6):

$$T_C=T_1+\left({\displaystyle \frac{\lg \left(P_S\right)-\lg \left(P_1\right)}{\lg \left(P_2\right)-\lg \left(P_1\right)}}\right)\left(T_2-T_1\right)$$

(6)

where T_c represents the continuous grading temperature; T₁ and T₂ denote test temperatures; P_s signifies the target threshold (1.0 kPa for high temperature and 300 MPa for low temperature); and P₁ and P₂ are experimental values at T₁ and T₂, respectively. For the creep rate m, continuous grading temperatures were derived through linear interpolation as shown in Equation (7), where P_s is set at 0.30:

$$T_C=T_1+\left({\displaystyle \frac{P_S-P_1}{P_2-P_1}}\right)\left(T_2-T_1\right)$$

(7)

(4)

Fatigue Performance: Cyclic shear loading was applied via DSR time scans at 25 °C and 10 Hz under constant temperature, frequency, and strain control to monitor G* attenuation. Fatigue life was quantified as critical number of cycles (Nf₅₀) corresponding to 50% reduction in initial G*, with energy dissipation and damage evolution analyzed to elucidate fatigue mechanisms [20].

(5)

Microscopic and Thermal Characterization: Functional group and structural features were investigated via FTIR. Thermal decomposition stages and residue content were determined through Thermogravimetric/Derivative Thermogravimetric (TG/DTG) analysis. Differential Scanning Calorimetry (DSC) was employed to obtain the glass transition temperature (T_g), providing thermal evidence for variances in low-temperature relaxation [21].

3. Rheological Properties Study

3.1. High- and Low-Temperature Rheological Properties

3.1.1. High-Temperature Rheological Properties

Under high-temperature conditions, patching materials for top-down cracks must concurrently satisfy the resistance requirements against flow, surface tracking, and severe shear deformation. As illustrated in Figure 2, rising temperatures cause G* to decrease while it increases across all systems, indicating a typical viscoelastic transition from elastic-dominant to viscous-dominant behavior. Within identical temperature ranges, CR systems exhibit significantly higher G* compared to SBS and ESO counterparts, suggesting that elastic particle phases formed via rubber swelling provide superior shear load-bearing and recovery capacities. Conversely, ESO systems show lower G* and higher δ due to dilution and viscosity-adjusting effects of bio-oil, leading to reduced high-temperature stiffness and elastic proportion.

As illustrated in Figure 2, rising temperatures cause G* to decrease while δ increases across all systems, indicating a typical viscoelastic transition from an elastic-dominant to a viscous-dominant state. Specifically, the ESO-only modified system exhibits a relatively low complex modulus with minimal variation across the temperature sweep; this low temperature sensitivity is attributed to the bio-oil acting as a substantial light-fraction replenisher, which dilutes the matrix but limits structural stiffening. In contrast, the CR-only system significantly elevates the overall G* profile without fundamentally altering the temperature sensitivity slope of the base asphalt, as the rubber particles primarily provide dispersed elastic reinforcement. Notably, the OPT composite sealant demonstrates excellent temperature sensitivity and maintains a high G* even at 82 °C. This difference arises because the binder, after composite modification, transforms into a stable multiphase colloidal system. The swollen CR particles create a robust elastic network that effectively counteracts the dilution effect of the bio-oil, while ESO’s polar epoxy groups prevent phase segregation, ensuring that the composite binder maintains structural integrity and avoids excessive softening at elevated temperatures.

Further evaluation utilized the rutting factor G*/sinδ derived from frequency scan data, which were processed to construct master curves based on the time–temperature superposition principle (TTSP). Shift factors were calculated and fitted using the Williams-Landel-Ferry (WLF) equation to translate data at various temperatures to a reference temperature. As a critical SHRP performance indicator, a higher G*/sinδ denotes enhanced rutting resistance, with AASHTO standards requiring G*/sinδ > 1.0 kPa for unaged binders. All DSR measurements were performed in triplicate, displaying minimal error bars with coefficients of variation (CV) strictly maintained below 5%, thereby ensuring the statistical robustness and reproducibility of the rheological evaluations [22,23].

Analysis of Figure 3 reveals that CR systems demonstrate marked |G*| elevation in low-frequency regions, indicating robust structural support against slow high-temperature deformation. OPT systems maintain high low-frequency |G*| without excessive stiffness increase at high frequencies, exhibiting a “balanced” spectrum that reconciles high-temperature stability with low-temperature coordination. Rational composite ratios enable systems to maintain high elastic support while avoiding construction difficulties associated with excessive viscosity.

3.1.2. Low-Temperature Rheological Properties

Under low-temperature conditions, patching materials must execute rapid creep and stress relaxation to mitigate thermal stress concentration and brittle fracture risks. Bending Beam Rheometer (BBR) tests conducted at −12 °C, −18 °C, and −24 °C provided creep stiffness modulus S (60 s) and creep rate m (60 s), characterizing low-temperature stiffness and relaxation capacity, respectively. Similar to the DSR testing, all BBR measurements were performed in triplicate. The coefficients of variation (CV) for both S and m values were strictly maintained below 5%, confirming the statistical reliability and reproducibility of the low-temperature evaluations.

As presented in

Table 2. BBR low-temperature creep indices.

Materials	S (−12 °C)	S (−18 °C)	S (−24 °C)	m (−12 °C)	m (−18 °C)	m (−24 °C)
SBS	129	270	616	0.38	0.327	0.216
CR	28	111	337	0.516	0.427	0.334
ESO	76.4	170	302	0.309	0.25	0.186
OPT	15.2	56.5	164	0.461	0.418	0.312

, decreasing temperatures result in increased S and decreased m for all systems, reflecting restricted molecular motion and attenuated viscoelastic relaxation. CR systems demonstrate significantly reduced S and enhanced m across all temperatures (e.g., S = 337 MPa, m = 0.334 at −24 °C), indicating that elastic networks formed via swelling facilitate deformation coordination and provide channels for stress release. Notably, OPT composite systems achieve the lowest S and highest m (S = 164 MPa, m = 0.312 at −24 °C), exhibiting synergistic “low-stiffness and high-relaxation” characteristics.

To comprehensively evaluate “stiffness-relaxation” coupling, low-temperature cracking resistance ratio k (k = S/m) was introduced, whereby lower k values indicate superior combined performance [24,25,26].

Calculated results in

Table 3. Low-temperature crack resistance ratio.

Materials	k (−12 °C)	k (−18 °C)	k (−24 °C)
SBS	339.5	825.7	2851.9
CR	54.3	260.0	1009.0
ESO	247.2	680.0	1623.7
OPT	33.0	135.2	525.6

confirm that OPT maintains the lowest k value (525.6 at −24 °C), fulfilling performance requirements and validating its low-temperature advantages. ESO optimizes system compatibility to maintain viscoelastic flow, while CR-induced elastic structures improve deformation coordination, achieving mutually reinforcing effects in OPT formulations. This pronounced stress relaxation capacity is particularly crucial for mitigating top-down cracks, as the surface layer of long-life pavements experiences the most severe thermal gradients and environmental aging, demanding superior low-temperature deformation coordination from the patching materials.

3.2. Performance Grading (PG)

In strict accordance with the AASHTO M320 standard, the performance grading (PG) methodology was adopted to evaluate the service temperature windows of the patching materials [27,28,29]. DSR rutting factors were utilized for establishing high-temperature limits, while BBR creep indicators characterized low-temperature performance. High-temperature grades were defined by the temperature at which G*/sinδ ≥ 1.0 kPa, whereas low-temperature limits were determined based on the concurrent criteria of S (60 s) ≤ 300 MPa and m (60 s) ≥ 0.30. It should be noted that while the standard PG framework traditionally incorporates an intermediate-temperature fatigue cracking parameter (G*sinδ), patching materials in long-life pavements are primarily subjected to localized, high-strain shear. Therefore, a more mechanistically rigorous evaluation of fatigue durability was conducted independently via cyclic time sweeps, as detailed in Section 3.3.

Grading results, summarized in

Table 4. PG grading results.

Material	PG Grade	Description
SBS	76~−24	Control group
CR	82~−18	Significant high-temp improvement; limited low-temp gain
ESO	64~−24	Low-temp maintenance; insufficient high-temp support
OPT	82~−24	Synchronized high- and low-temperature enhancement

, reveal distinct modification trajectories whereby CR primarily facilitates high-temperature grade elevation, whereas ESO focuses on low-temperature relaxation improvement. Given that BBR testing was limited to a minimum of −24 °C, materials satisfying criteria at this point were assigned a −24 °C grade. Notably, OPT composite system achieves a synergistic expansion of the service window to 82–−24 °C, effectively satisfying the “high-temperature stability and low-temperature flexibility” requirements essential for high-performance patching materials in complex climates.

3.3. Fatigue Characteristics

Accordingly, Dynamic Shear Rheometer (DSR) strain and time scans were performed to characterize cyclic damage evolution. Initially, strain scans were conducted at 25 °C and 10 Hz to identify linear viscoelastic (LVE) limits via dynamic modulus response, whereby a 5% strain level was selected for time scans to balance testing efficiency with damage characterization sensitivity. Subsequent time scans applied cyclic shear loads under constant temperature, frequency, and strain, recording attenuation of complex shear modulus G* relative to cycle number [30]. To enhance comparability across systems with varying initial moduli, the normalized dynamic modulus Gnorm (Gnorm = G*/G0) was introduced. This normalization is a widely adopted approach in asphalt fatigue evaluation to isolate the damage evolution rate from inherent initial stiffness differences [31]. As illustrated in Figure 4.

As illustrated in Figure 4, the dynamic moduli for all systems decrease with increasing strain. However, the optimal composite formulation (OPT) exhibits a broader linear viscoelastic (LVE) limit and a more gradual, controlled attenuation trajectory at higher shear strains, indicating superior microstructural stability and deformation resistance under large shear deformations. Consequently, a 5% strain level was selected for the time sweeps. This strain level strategically exceeds the LVE limit of the materials to accelerate damage evolution, while reasonably simulating the typical heavy-load tire shear deformations experienced in the top-down cracking zone of long-life pavements. Although this single-strain empirical approach does not constitute a full Viscoelastic Continuum Damage (VECD) model, it provides a highly reliable relative index (Nf₅₀) for comparing the cyclic shear durability of the sealants. Utilizing this 5% strain level, constant temperature and frequency time scans for the OPT system yielded the evolution curves of characterization parameters, as shown in Figure 5.

Experimental results indicate that N_f50 for the OPT system reaches 2890 cycles, demonstrating robust damage resistance and significant potential for durable service under cyclic shear. Analyzing from a structure–performance perspective, CR particles absorb light fractions to form elastic networks within the matrix, providing continuous structural support and deformation recovery during cyclic loading to inhibit rapid modulus decay. Concurrently, ESO regulates viscosity and improves compatibility, facilitating reduced local stress concentration while enhancing stability of viscoelastic relaxation and energy dissipation. These components achieve complementary synergy in OPT formulation, enabling material to balance energy dissipation and structural recovery during cyclic shear, thereby effectively delaying damage accumulation and enhancing fatigue durability.

4. Microscopic and Thermal Characterization

4.1. Thermal Stability and Phase Transition Analysis

4.1.1. Thermogravimetric (TG) Analysis

The thermal stability of the composite-modified patching materials during heating was systematically evaluated utilizing a TGA4000 thermogravimetric analyzer (PerkinElmer, Inc., Waltham, MA, USA). Tests were conducted under nitrogen atmosphere (100 mL/min) with a heating rate of 10 °C/min across a temperature range of 30–600 °C.

TG/DTG profiles across four systems (SBS, CR, ESO, and OPT) exhibit consistent morphological characteristics, whereby a single primary weight loss stage occurs predominantly within the 250–500 °C range (

Table 5. Thermal degradation and mass loss of modified asphalts.

Sample	T5% (°C)	T70% (°C)	Mass Loss (%)	Residue at 600 °C (%)
SBS	295.3	463.9	83	17
CR	300.3	471.5	77.7	22.3
ESO	281	459.1	83.6	16.4
OPT	302.4	465.9	55.2	44.8

). This stage primarily involves volatilization of light fractions coupled with thermal degradation of structural components [32]. To facilitate quantitative comparison, temperature corresponding to 5% mass loss (T_5%) was adopted as the initial thermal decomposition threshold, while temperature at 70% mass loss (T_70%) characterized late-stage degradation.

As can be seen from Figure 6, results indicate that OPT system significantly elevates T_5% to 302.4 °C and drastically increases residue content at 600 °C to 44.8%. Notwithstanding ESO’s tendency to slightly lower early decomposition thresholds, CR-induced structural reinforcement effectively mitigates overall mass loss, thereby enhancing high-temperature structural retention and thermal stability essential for construction heating and service conditions.

4.1.2. Differential Scanning Calorimetry (DSC) Analysis

The glass transition temperature (T_g) was determined via DSC2500 (TA Instruments, New Castle, DE, USA) to characterize the critical transition from glassy to viscoelastic states, whereby lower T_g values denote superior chain mobility and stress relaxation at low temperatures. Specimens were analyzed between −70~120 °C, with thermal history eliminated via a standardized heating–cooling–reheating cycle [33].

As can be seen from Figure 7, experimental profiles reveal a T_g hierarchy of OPT < ESO < CR < SBS, with specific values for OPT reaching approximately −35.2 °C. While ESO single-modification significantly reduces T_g via light component replenishment, CR introduces structural reinforcement that slightly elevates the transition region. The optimal composite formulation (OPT) achieves synergistic benefits whereby CR-provided structural support is maintained while ESO-promoted relaxation is fully released. This thermal phase transition behavior quantitatively correlates with the macroscopic low-temperature rheological indicators obtained from BBR testing. The significant depression of T_g to −35.2 °C provides the thermodynamic foundation for the observed reduction in creep stiffness (S = 164 MPa) and the enhancement of the creep rate (m = 0.312) at −24 °C. From a mechanistic perspective, the lower T_g indicates that the amorphous regions of the polymer chains within the OPT system retain sufficient free volume and segmental molecular mobility at extreme sub-zero temperatures. This prevents the binder from transitioning prematurely into a brittle, glassy state, thereby confirming that the “low-stiffness and high-relaxation” characteristics of OPT effectively suppress low-temperature brittle cracking risks in pavement applications.

4.2. FTIR Analysis

Fourier Transform Infrared Spectroscopy (FTIR) was conducted utilizing an Agilent Technologies 4300 Handheld spectrometer (Santa Clara, CA, USA) equipped with an Attenuated Total Reflection (ATR) accessory. Testing was performed across a wave number range of 400–4000 cm⁻¹. By comparing spectra of SBS, CR, ESO, and optimal composite OPT systems, variations in characteristic absorption peak positions, shapes, and intensities were analyzed to detect emergent functional groups or displacement of existing ones, thereby elucidating modification mechanisms from a molecular perspective [33]. Spectra for the four materials are illustrated in Figure 8.

Analysis of the base asphalt indicates a primary composition of saturated hydrocarbons, aromatic hydrocarbons, resins, and minor heteroatomic components. Following CR incorporation, the spectra remain largely consistent with the base binder without distinct new peaks, suggesting that the modification occurs primarily via physical swelling through the absorption of light fractions. While retaining original peaks, ESO-modified systems exhibit a weak characteristic C=O absorption at approximately 1750 cm⁻¹, reflecting light component replenishment. The OPT spectra represent a superposition of CR and ESO characteristics without the emergence of prominent independent functional group peaks. It is important to note that while no robust new covalent bonds were detected, this does not preclude the existence of weak chemical interactions. These results suggest that the composite modification mechanism of OPT is not exclusively a simple mechanical blend, but is rather dominated by physical network construction coupled with weak intermolecular interactions (such as dipole–dipole interactions or hydrogen bonding) induced by the polar groups of ESO. This interaction effectively facilitates the wetting and dispersion of CR within the matrix, thereby inhibiting phase separation and ensuring system stability.

Synthesizing FTIR, TG/DTG and DSC results, composite modification mechanisms of OPT are systematically deconstructed across three interrelated, logically progressive dimensions: functional group evolution, thermal stability regulation, and low-temperature pavement service performance. At the fundamental functional group level, the absence of prominent new characteristic absorption peak post-modification verifies that composite modification proceeds primarily via physical blending and weak intermolecular interactions, with no formation of new covalently bonded structures that would alter inherent chemical backbone of base binder. Moving to thermal stability regulation, CR, acting as structural reinforcing phase within blend matrix, elevates T_5% and T_70% alongside increased pyrolysis residue content, while ESO simultaneously optimizes phase compatibility and component dispersion; their synergistic effects reduce overall mass loss and enhance high-temperature structural integrity of modified binder. Finally, for low-temperature pavement service performance, significant T_g reduction and enhanced stress relaxation facilitated by ESO are coupled with elastic network support from CR, a synergistic match that enables modified material to retain superior deformation coordination and relaxation capacity even under harsh low-temperature service conditions.

5. Conclusions

In this study, the wide-temperature rheological responses, PG grading, fatigue damage evolution, and microscopic/thermal mechanisms of crack-patching materials modified with crumb rubber (CR) and epoxidized soybean oil (ESO) were systematically investigated. The principal conclusions are as follows:

(1)

High-Temperature Reinforcement: CR absorbs light fractions to form an elastic network, significantly increasing the complex shear modulus and rutting factor. This provides robust structural support against high-temperature shear deformation.

(2)

Low-Temperature Relaxation: ESO’s polar epoxy groups optimize viscoelasticity and reduce stiffness without compromising deformability. This grants the OPT system superior stress relaxation capacity at −24 °C, preventing brittle fracture.

(3)

Broadened Service Window: The synergistic interaction between CR and ESO successfully broadens the performance grade from PG 76–−24 °C to PG 82–−24 °C, fulfilling the stringent requirements for long-life pavement sealants in extreme climates.

(4)

Exceptional Fatigue Durability: Time sweep tests reveal an Nf₅₀ of 2890 cycles for the OPT system. The composite network effectively dissipates energy, demonstrating exceptional durability against high-frequency shear strains from tire edges.

(5)

Physical Synergistic Mechanism: FTIR, TG/DTG, and DSC confirm that the modification is driven by physical blending and weak intermolecular interactions. High thermal stability (44.8% residue at 600 °C) ensures construction safety, while a lowered T_g improves flexibility.

(6)

Limitations and Future Work: Current findings are limited to laboratory rheological and thermal analyses. Future studies must incorporate RTFO/PAV aging simulations, moisture susceptibility and aggregate adhesion tests, and full-scale field validations to assess long-term in situ performance.