The impact of bio-oil on the structure, rheology, and adhesion properties of lignin-modified asphalt

This research investigates the efficacy of bio-oil as a sustainable modifier for lignin-modified asphalt (LMA), aiming to enhance its performance characteristics. Utilizing Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), the study analyzes the chemical and microstructural changes induced by bio-oil in LMA. Rheological properties were evaluated using Dynamic Shear Rheometry (DSR), revealing that the addition of 5%–10% bio-oil to LMA significantly reduced stiffness and brittleness, improving ductility and fatigue resistance. For instance, LMAs with 10% bio-oil demonstrated a fatigue life at 2.5% strain comparable to unmodified asphalt. Additionally, bio-oil inclusion increased adhesive strength between asphalt and aggregates, enhancing moisture resistance. Low-temperature properties assessed by dynamic mechanical analysis (DMA) showed improved flexibility and thermal crack resistance with bio-oil addition. These findings underscore the potential of bio-oil in developing high-performance, sustainable asphalt binders, contributing to the advancement of eco-friendly road construction materials.


Introduction
Asphalt binders are commonly used in pavement construction due to their strong adhesion properties, flexibility, and durability [1][2][3][4].However, unmodified asphalt binders can suffer from performance issues such as rutting, fatigue cracking, and thermal cracking.Modifiers such as polymers and biomaterials have been studied to improve the properties of asphalt binders [5].Lignin, which is an abundant biomaterial found in all vascular plants, has shown promise as an asphalt modifier [6].Lignin contains a complex aromatic structure with polar functional groups that can enhance the stiffness, rutting resistance, and thermal stability of asphalt binders [7].However, some studies have found that high lignin contents negatively impact the fatigue life and low temperature cracking resistance of asphalt.
Bio-oils comprised of plant-based pyrolysis oils offer new potential as sustainable modifiers and partial replacements for petroleum-based asphalt [8].Bio-oils contain lighter fluid fractions similar to maltenes in asphalt, which can supplement the asphalt's fluidity and improve workability and compactability [9].Research has shown that bio-oils reduce the viscosity-temperature susceptibility of asphalt binders, enhancing performance at cold temperatures [10].However, bio-oils have exhibited negative effects on high temperature properties due to their softer consistency.Recent research by Li et al [11] has focused on the application of rejuvenators to improve the rheological and mechanical properties of asphalt binders and mixtures, which aligns with our investigation into bio-oil as a sustainable modifier.Additionally, Ding and Hesp [12] have quantified crystalline wax in asphalt binders using variable-temperature Fourier-transform infrared spectroscopy, providing insights into binder composition and performance.Further, recent advancements in the use of phase change materials in asphalt binders, as reviewed by Guo et al [13], could offer valuable context for understanding the thermal properties of bio-oil modified asphalts.Moreover, Eltwati et al [14] explored the synergistic effects of SBS copolymers and aromatic oil on asphalt binders containing reclaimed asphalt pavement, which parallels our study's focus on sustainable modifiers.
This study aims to investigate the combined use of lignin and bio-oil as asphalt modifiers.It is hypothesized that the bio-oil may counteract some of the drawbacks of using lignin alone at high dosages, while the lignin can shore up the high temperature deficiencies of using bio-oil alone.The impact of bio-oil on the structure, rheological properties, and adhesion of lignin-modified asphalts will be analyzed through chemical, microstructural, and mechanical testing.
FTIR will characterize the functional groups and chemical interactions in the modified binders.SEM will provide insights into the microstructure and dispersion of components.Rheological properties including complex modulus, phase angle, rutting factor, fatigue life, and thermal cracking resistance will be quantified using dynamic shear rheometry (DSR), multiple stress creep and recovery (MSCR), linear amplitude sweep (LAS), and DMA.Ductility and force-ductility measurements will evaluate binder cohesion.Adhesion strength between binder and aggregate will be directly measured.
By elucidating the mechanisms through which bio-oil influences the morphology and performance of lignin-modified asphalts across a spectrum of temperatures and loading conditions, optimal synergistic combinations can be identified.The use of sustainable biomaterials can reduce reliance on environmentally detrimental petroleum-based resources.This study will provide insights into developing high-performance sustainable asphalt binders using lignin and bio-oil as primary constituents.The knowledge gained can enable deployment of renewable bio-asphalts to advance pavement sustainability and resiliency while meeting demanding engineering requirements.

Materials and methods
The asphalt binder used in this study was a PG 64-22 obtained from a local supplier in China.The physical properties of the PG 64-22 bitumen were shown in table 1.It contained 63.2% carbon and 29.8% oxygen with low sulfur and ash contents.The lignin had a median particle size of 45 μm, density of 1.25 g cm −3 , and softening point of 127 °C.The bio-oil was produced from fast pyrolysis of pine wood waste at a temperature of 500 °C and obtained from a pyrolysis facility.It was a dark brown liquid with a smoky odor and had a viscosity of 50 cP at 40 °C and density of 1.18 g cm −3 .The bio-oil contained primarily phenolic compounds along with high levels of levoglucosan and acetic acid.Basalt aggregates locally sourced from a quarry were washed and sieved into size fractions meeting Superpave specifications.
The LMAs were produced by blending the asphalt binder with 5%, 10%, 15%, and 20% lignin by weight of the binder.The components were mixed at 165 °C (The chosen temperature of 165 °C is consistent with standard practices for asphalt modification to ensure proper mixing and homogenization.This temperature allows the asphalt binder to reach a sufficiently fluid state for effective incorporation of lignin and bio-oil without causing thermal degradation of the bio-oil or lignin [15].)using a high-shear mixer for 30 min (The mixing duration of 30 min was selected based on preliminary trials that indicated this time frame was adequate for achieving uniform dispersion of lignin particles within the asphalt matrix.Longer durations did not significantly improve dispersion but increased the risk of oxidative aging [16]) at 4000 rpm (The high-shear mixing at 4000 RPM was determined to be effective in breaking down lignin agglomerates and ensuring a homogeneous blend with the asphalt binder.This RPM was chosen to balance between sufficient shear forces for dispersion and avoiding excessive mechanical degradation of the bio-oil [17]).Bio-oil was then added to the LMAs at dosages of 5%, 10% and 15% by weight to create lignin & bio-oil modified asphalts (LOBMAs).They were mixed using the same 165°C temperature and high-shear protocol.Prepared binder samples were then conditioned using the rolling thin film oven (RTFO) test following ASTM D2872 to simulate short-term aging.For long-term aging, the pressure aging vessel (PAV) test was performed on RTFO-aged binders based on ASTM D6521.The chemical functional groups of the binders were analyzed using a Thermo Scientific Nicolet iS5 FTIR with an ATR module.A JEOL JSM-6490LV scanning electron microscope was utilized for microstructural analysis.Rheological testing was conducted on an Anton Paar MCR 302 Dynamic Shear Rheometer.Complex modulus (G * ) and phase angle (δ) were measured using a parallel plate (25 mm diameter, 1 mm gap) over temperatures from 30 °C-80 °C and frequencies of 0.1-100 rad s −1 .The rutting factor (G * /sinδ) was determined at 64 °C.MSCR tests were performed as per ASTM D7405 at 64 °C with 0.1 and 3.2 kPa stresses.LAS tests were run from 0.1%-30% strain at 10 Hz and 20 °C as per AASHTO TP101.Ductility was measured using ASTM D113 on conditioned samples.Force ductility tests followed ASTM D217 to determine tensile properties.Dynamic mechanical analysis was conducted using a TA Instruments DMA Q800 in tension mode at a frequency of 10 Hz over temperatures ranging from −20 °C to 30 °C.
The adhesive bond strength between aggregates and binder was quantified through a simple adhesion test.The adhesion test procedure was designed based on principles from ASTM D3625 (Standard Practice for Effect of Water on Bituminous-Coated Aggregate Using Boiling Water) and ASTM D4541 (Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers), adapted to our specific experimental setup.Basalt aggregates were coated with binder samples and conditioned at 25 °C for 72 h.They were then immersed in water at 60 °C for 30 min followed by cooling to 25 °C for 10 min.The coated aggregates were prodded with a needle tip to evaluate the degree of stripping and adhesion failure under wet conditions.The visual inspection criteria are shown in table 2.

FTIR results
The FTIR spectra of the neat, LMA, and LOBMA binders are shown in figure 1(A).The virgin binder displayed peaks at 2920 cm −1 and 2850 cm −1 corresponding to aliphatic C-H stretching [18].A carbonyl peak was visible at 1695 cm −1 , indicative of the presence of ketones or aldehydes.For the LMAs, additional absorptions were observed which can be attributed to the chemical structure of lignin [8].Specifically, new peaks appeared at 1512 cm −1 , 1423 cm −1 , and 1270 cm −1 .The peak at 1512 cm −1 corresponds to aromatic ring vibrations, which are characteristic of the phenolic structure of lignin.The peak at 1270 cm −1 is attributed to C-O stretching vibrations in the methoxyl groups of lignin.The peak at 1423 cm −1 is associated with aromatic C-H bending vibrations [19].Additionally, a broad hydroxyl peak was observed from 3200-3400 cm −1 , which is indicative of O-H stretching vibrations.This broad peak is characteristic of the hydroxyl groups present in lignin's structure.With increasing lignin content, the intensity of the aromatic lignin peaks increased proportionally as the aliphatic fractions were diluted [20].The carbonyl index reduced from 0.312 in the neat binder to 0.192 in the 20% LMA (figure 1(B)).This indicates the lignin provided an anti-oxidative effect.Incorporation of bio-oil led to growth of the aliphatic C-H peaks reflecting the higher aliphatic content [21].The carbonyl index increased with more bio-oil to 0.242 and 0.276 for 10% and 15% bio-oil in the 20% LMA.This shows bio-oil increased oxidation rates.The FTIR results demonstrate lignin conferred structural changes in chemical functionalities Table 2. Visual inspection criteria for adhesion.

Delamination: The presence of any separation between the binder and aggregate surface Rating 5
No delamination observed.Rating 4 Very minor delamination observed at the edges.Rating 3 Minor delamination observed at multiple points along the edges.Rating 2 Moderate delamination observed, with some areas showing complete separation.Rating 1 Significant delamination observed, with large areas showing complete separation.Stripping: The extent to which the binder has stripped off the aggregate surface.

Rating 5
No stripping observed.Rating 4 Very minor stripping observed at the edges.Rating 3 Minor stripping observed at multiple points along the edges.Rating 2 Moderate stripping observed, with some areas showing exposed aggregate.Cracks: The presence of visible cracks in the binder coating on the aggregate.Rating 1 Significant stripping observed, with large areas showing exposed aggregate.Rating 5 No cracks observed.Rating 4 Very minor cracks observed at the edges.Rating 3 Minor cracks observed at multiple points along the edges.Rating 2 Moderate cracks observed, with some areas showing significant cracking.Rating 1 Significant cracks observed, with large areas showing extensive cracking.
and composition [22].The reducing carbonyl index with more lignin evidences its antioxidative nature.Addition of bio-oil counteracted this effect to some degree by contributing more oxidizable light fractions.

SEM results
The microstructure of the virgin, LMA, and LOBMA binders was investigated through SEM imaging as shown in figure 2. The unmodified binder displayed a smooth, homogeneous morphology.The LMAs showed a coarser microstructure with distributed lignin particles.The size, shape, and dispersion of the lignin phase depended on its dosage [23] (table 3).At 5% lignin content, small spherical lignin particles around 2-5 μm in diameter were dispersed in the asphalt matrix.As the lignin increased, the particles became more angular and irregular in shape and reached sizes over 30 μm.
The lignin dispersion was enhanced with more bio-oil, transitioning from semi-continuous phase separation to discrete particulate dispersion [23].This change in morphology indicates the bio-oil improved miscibility between the hydrophobic asphalt and hydrophilic lignin phases.With 15% bio-oil in the 20% LMA, the lignin particles were reduced to less than 10 μm and more uniformly distributed.The SEM analysis reveals incorporation of lignin significantly alters the microstructure of the binder.The lignin can act as a dispersed rigid filler phase that enhances mechanical properties [24].However, poor miscibility with the asphalt matrix at high dosages causes poorer dispersion and phase separation [25].Addition of bio-oil improves the integration of the lignin particles by increasing compatibility between the two phases.This allows for a more homogeneous composite blend microstructure [26].

Rheological properties
The G * and δ were measured across a range of frequencies from 0.1-100 rad s −1 at temperatures of 30 °C to 80 °C.Master curves of G * and δ versus frequency were constructed at a reference temperature of 54 °C using the time-temperature superposition principle.Figure 3 shows the master curves comparing the neat, LMA, and LOBMA binders.The virgin binder displayed relatively low G * values and high δ, reflecting its more viscous behavior.Adding lignin increased the complex modulus and decreased the phase angle due to the stiffening effect of the solid lignin particles.At a given frequency, G * increased while δ decreased with more lignin [27].The 20% LMA exhibited the highest modulus, approaching elastic solid-like response at high frequencies.This confirms the reinforcing nature of lignin.Incorporation of bio-oil into the LMAs counteracted the above trends.The G * values reduced while the δ increased back towards the neat binder.The binders became more viscous  with the 5%-15% bio-oil.This demonstrates the softening impact of the bio-oil's maltene fractions on the lignin-stiffened matrix [28].However, the moduli remained above the virgin binder indicating the composites still maintained improved mechanical properties.Similar results were observed in the phase angle master curves in figure 3(b).The phase angle decreased with increasing lignin content reflecting more elastic behavior.Addition of bio-oil homogenized the multi-phase system and raised the δ back closer to the unmodified binder [29].The lowest phase were achieved with 10%-15% bio-oil in the 10%-15% LMAs.This optimal balance provided reinforcement from the lignin coupled with enhanced compatibility between phases [30].
Figure 4 shows the rutting factor (G * /sinδ) versus temperature for the different binders.The rutting factor is a measure of the high-temperature stiffness and resistance to permanent deformation of asphalt binders.A higher rutting factor indicates better resistance to rutting, which is critical for maintaining pavement integrity under heavy traffic loads and high temperatures.The rutting factor decreased exponentially with increasing temperature.The lignin drastically increased the rutting factor and high temperature stiffness.However, the LMA became very brittle above 15% lignin content as the rutting factor exceeded 6000 kPa.Addition of bio-oil lowered the rutting factor but maintained it well above the virgin binder up to approximately 10% bio-oil content.This confirms the synergistic effect where bio-oil preserves the heightened rutting resistance imparted by the lignin while resolving the excessive brittleness issue [31].When bio-oil is incorporated into LMA, it introduces lighter maltene fractions into the asphalt binder.These maltene fractions are known to reduce the overall stiffness of the binder by increasing its fluidity and workability.This is evident from the reduction in G * and an increase in δ observed in rheological tests, as shown in figure 3. The decrease in G * indicates a softer binder, while the increase in δ suggests a more viscous behavior.The softer consistency of the bio-oil-modified binder improves its ability to deform elastically under load, which is beneficial for fatigue resistance and lowtemperature performance.However, this increased elasticity and reduced stiffness also mean that the binder is more susceptible to permanent deformation at high temperatures, leading to a lower rutting factor.
The MSCR test evaluated high temperature rutting resistance through the binders' recoverable and nonrecoverable strain responses.The percent recovery (R%) and non-recoverable creep compliance (Jnr) were measured at 64 °C under 0.1 kPa and 3.2 kPa stresses.Higher R% indicates more elastic deformation, while lower Jnr values represent higher rutting resistance [32].
Figure 5 shows the R% results for the different binders.The lignin drastically increased the elastic deformability, with R% exceeding 70% at 0.1 kPa for the 15% and 20% LMAs.However, at 3.2 kPa the R% dropped below 10% as the stiff lignin particles experienced permanent deformation under heavier loads.Addition of 5%-10% bio-oil was able to restore the R% closer to the virgin binder levels under both stresses [33].This demonstrates the bio-oil imparted more viscoelastic response to counter the lignin's brittleness [34].
Examining the J nr data in figure 6 further verifies the synergistic effects.The Jnr declined over 75% with addition of lignin, showing the improved rutting resistance.But above 10% lignin, the Jnr began rising again as the composites became excessively stiff.The bio-oil ameliorated this problem, providing the optimal rutting resistance with 10% bio-oil in the 10%-15% LMAs.The softer bio-oil phases enhanced elasticity and resilience to permanent deformation [35].This lead to the lowest Jnr values, translating to the best anticipated rutting performance.
The MSCR results and the rutting factor both provide insights into the rutting resistance of asphalt binders, but they measure different aspects of the material's behavior.This distinction is essential for reconciling the observed discrepancy where the rutting resistance appears to increase with the addition of bio-oil in the MSCR results, while the rutting factor shows a decrease.The key to reconciling the differences between the rutting factor and MSCR results lies in understanding that they measure different aspects of rutting resistance.While the rutting factor focuses on high-temperature stiffness, MSCR evaluates the binder's ability to recover from deformation and resist permanent strain under cyclic loading.The addition of bio-oil softens the binder, reducing its stiffness as reflected in the lower rutting factor.However, this softening also enhances the binder's elasticity and recovery properties, leading to better performance in the MSCR test.The improved viscoelastic balance and stress distribution contribute to this enhanced performance, demonstrating that bio-oil can effectively mitigate permanent deformation despite a decrease in high-temperature stiffness.
The fatigue resistance of the binders was assessed through LAS tests.The number of cycles to failure (Nf) was measured at strain levels of 2.5% and 5% to evaluate fatigue life.Figure 7 shows the effects of lignin and bio-oil on the binder fatigue performance.The lignin addition substantially reduced the fatigue life of the binder.At 2.5% strain, the Nf declined over 60% for the 20% LMA compared to the virgin binder.As the lignin content increased, the composite became more brittle and prone to fatigue damage under cyclic loading [36].At 5% strain, the LMAs experienced over 90% lower fatigue life than the unmodified binder.
Incorporating bio-oil produced a remarkable recovery in fatigue resistance.The 10% bio-oil LMAs showed comparable or improved fatigue life over the neat binder at both strain levels.The bio-oil's soft fractions imparted flexibility and damage tolerance to the lignin-reinforced system.This integration of components created an optimized composite with enhanced rutting resistance and fatigue life compared to the virgin binder  [37].Analyzing the percent recovery data from the MSCR tests provides supporting evidence for the fatigue performance trends [38].The drop in R% for the LMAs at higher strains and stresses helps explain the decreased fatigue resistance.The bio-oil was able to maintain the R% under these demanding conditions, conferring improved resilience to cyclic loads and fatigue damage.

Force ductility test results
The ductility and tensile cohesion of the binders were evaluated using force ductility tests.The failure strain, maximum tensile force, and fracture energy were measured and compared between the different binders.The lignin addition substantially reduced the ductility and cohesive strength of the binders as shown in figure 8.The failure strain decreased over 50% for the 20% LMA compared to the virgin binder.The lignin's stiff particulate phase limited binder extensibility and cohesion [39].The maximum tensile force showed an initial increase with 5%-10% lignin due to reinforcement but then declined at higher contents as brittleness dominated.
Addition of bio-oil counteracted the ductility loss from the lignin, restoring the failure strain and fracture energy closer to the neat binder.The 5%-10% bio-oil LMAs achieved optimal levels approaching a 100% increase in failure strain and a 50% increase in fracture energy versus the neat binder.This extraordinary synergy demonstrates the bio-oil's light fractions plasticized the lignin network while the lignin contributed strength.However, excess bio-oil beyond 10% diluted the composite properties.The 15% bio-oil LMAs showed reductions in maximum tensile force and fracture energy compared to the 10% bio-oil formulations.This confirms an optimal ratio of lignin to bio-oil exists for maximizing ductile and cohesive properties [40].The biooil restores ductility lost by lignin embrittlement while the lignin boosts tensile strength decreased by softening from bio-oil [41].

DMA test results
The low temperature thermal and mechanical properties of the binders were investigated using DMA.The storage modulus (E′) and glass transition temperature (Tg) were measured from −20 °C to 30 °C.Higher E′ indicates greater stiffness, while higher Tg represents an ability to resist thermal cracking at lower temperatures.Figure 9 shows the E′ curves for selected binders.The lignin drastically enhanced the storage modulus, increasing by over 5 times at −20 °C for the 20% LMA versus the virgin binder.However, the shape of the E′ curve became much steeper with more lignin.This reflects growing brittleness and thermal cracking susceptibility at low temperatures [42].
Adding 5%-10% bio-oil to the LMAs maintained the elevated modulus while reducing the slope of the curve.The bio-oil plasticity improved low temperature flexibility and cracking resistance.The Tg showed a consistent trend, with the highest values around −13 °C to −15 °C achieved for the 10%-15% LMAs with 5%-10% bio-oil (table 4).This synergistic balance between components enhanced stiffness for load resistance yet preserved viscoelastic behavior for thermal cracking tolerance [43].

Adhesion test results
The moisture-induced adhesive failure between the binders and aggregate was evaluated through a simple qualitative adhesion test.Basalt aggregates coated with the different binders were subjected to moisture conditioning.The degree of stripping and loss of adhesion was visually inspected (table 5).The virgin binder exhibited moderate stripping and failure around the aggregate edges.The lignin significantly improved moisture resistance, with the LMAs showing no stripping even at high moisture exposure.This enhancement stemmed from stronger mechanical interlocking and adsorption between the rigid lignin particles and aggregate surface.However, the most remarkable performance was achieved in the 5%-10% bio-oil LMAs.These binders demonstrated superior adhesion with no debonded regions even after prolonged water immersion.The synergistic effect of the miscible bio-oil coupled with lignin crosslinking appears to optimally enhance binder adhesion.Chemical interactions between the polar functional groups in the bio-oil and aggregate likely contributed further to the improved interfacial adhesion [44].But excessive bio-oil beyond 10% deteriorated moisture resistance.The 15% bio-oil LMAs showed partial edge stripping as the bio-oil diluted the composites.This demonstrates an optimal ratio between lignin and bio-oil is necessary to maximize moisture stability and adhesive properties [45].Overall, the study reveals that using bio-oil and lignin in tandem could produce asphalt binders with exceptional moisture damage resistance.
The results clearly demonstrate the synergistic effects of combining lignin and bio-oil to produce optimized composite modified binders.The LMAs improved the high temperature rutting factor, tensile strength, low temperature stiffness, and moisture resistance compared to the virgin binder.However, the lignin addition compromised the fatigue life, ductility, and thermal cracking resistance.The over-reinforcement created by the stiff lignin particles caused excessive brittleness issues.
Introducing 5%-10% bio-oil into the LMAs counteracted these deficiencies while maintaining the positives.The LOBMAs achieved the best balance of properties.They exceeded the rutting factor, ductility, fracture energy, fatigue life, and adhesion of even the neat binder.The low temperature and stiffness properties remained heightened versus the virgin binder as well.This synergistic enhancement resulted from the bio-oil's compatibility effect.It improved the integration and dispersion of the lignin particles to optimize the composite structure.The softer bio-oil fractions plasticized the system to resolve embrittlement.And its polar functionality appeared to assist interfacial adhesion.There was no evidence of the bio-oil worsening moisture sensitivity despite containing more polar groups.Excessive bio-oil levels beyond 10% diluted properties as the lighter fractions dominated.But overall, the study proves bio-oil can amplify the benefits of lignin modification while mitigating its drawbacks.Using bio-oil and lignin together provides a pathway to high-performance sustainable binders that outperform conventional asphalt.
The results indicate several mechanisms may contribute to the synergistic effects of bio-oil on ligninmodified asphalt performance: Compatibilization Effect: The bio-oil enhances the compatibility and miscibility between the hydrophobic asphalt and hydrophilic lignin phases [46].This improves lignin dispersion and unifies the composite, optimizing its morphology and properties.
Plasticization Effect: The light bio-oil fractions soften and plasticize the composite network [47].This reduces brittleness issues from lignin over-reinforcement and restores ductility and fracture resistance.Compositional Optimization: The bio-oil supplements the asphalt's resin and aromatic contents diluted by the lignin [48].This reconstitutes the bitumen composition for balanced properties.
Moisture Resistance: Bio-oil's polar compounds may enhance chemical adhesion to aggregates while the lignin provides mechanical interlocking [50].This amplifies moisture damage resistance.

Conclusion
In this study, we explored the impact of bio-oil on the structure, rheology, and adhesion properties of LMA.The findings reveal that the incorporation of 5% to 10% bio-oil significantly enhances the overall performance of LMA.Using FTIR and SEM, we observed that bio-oil improved the compatibility and dispersion of lignin within the asphalt matrix.Rheological tests demonstrated that the addition of bio-oil reduced the composite's stiffness and brittleness, leading to better ductility and fatigue resistance.For instance, LMAs with 10% bio-oil displayed comparable or improved fatigue life at 2.5% strain compared to the neat asphalt.Additionally, bio-oil was found to enhance the adhesive bond strength between asphalt and aggregates, reducing moisture sensitivity.In terms of low-temperature performance, LMAs with bio-oil showed improved flexibility and thermal crack resistance.However, it was observed that the incorporation of bio-oil increased oxidation rates, as indicated by the rise in carbonyl index with more bio-oil content (figure 1(B)).This outcome suggests a potential trade-off between improved mechanical properties and increased susceptibility to oxidative aging.To mitigate this aging effect, future research could explore the addition of antioxidants to the binder formulation.Antioxidants can help neutralize free radicals and slow down the oxidation process, thereby enhancing the durability of bio-oil modified asphalts.Another approach could involve modifying the asphalt binder formulation by incorporating stabilizers or other additives that enhance the resistance to oxidative aging.By optimizing these formulations, it is possible to achieve a more durable and sustainable asphalt binder that leverages the benefits of both lignin and bio-oil.Overall, this study proves the significant potential of combining bio-oil and lignin in enhancing the properties of conventional asphalt, particularly in terms of sustainability and environmental friendliness.By optimizing the proportion of bio-oil and lignin, high-performance bio-based asphalts can be developed, marking a significant step towards sustainable and resilient road construction.

Figure 2 .
Figure 2. SEM micrographs of neat and modified binders.

Figure 3 .
Figure 3. Master curves of (A) complex modulus and (B) phase angle for binders.

Figure 9 .
Figure 9. Storage modulus versus temperature from DMA testing.

Table 3 .
Particle size distribution of lignin phase.