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Abstract

This study investigates the synergistic effects of Waste Cooking Oil (WCO) and three types of High-Elastic Modifiers (HEM-S, HEM-C, and HEM-G) on the mechanical and rheological performance of asphalt mixtures. The modified AC-13C mixtures were evaluated through Marshall Stability, rutting, low-temperature cracking, moisture susceptibility, and aging tests. The findings demonstrate that the combined use of HEMs and WCO significantly enhances both high- and low-temperature performance compared with conventional binders. The HEM-S + 6% WCO mixture exhibited the most balanced performance, achieving a 27% increase in Marshall Stability, 87% improvement in Dynamic Stability, and 46% increase in tensile strain, alongside superior moisture, and aging resistance. HEM-C + 4.5% WCO achieved the highest rutting resistance (4900 passes/mm) but displayed slightly higher stiffness at low temperatures. Rheological analysis confirmed that WCO acts as a plasticizer, reducing binder stiffness and improving stress relaxation, while HEMs provide an elastic network that enhances load-bearing capacity. The integration of WCO and HEMs effectively replaces conventional SBS modification, offering a sustainable and high-performance alternative for asphalt pavement applications. Moreover, a 1% adoption of HEM-WCO asphalt in China annual production could prevent approximately 31,300 tonnes of CO₂ emissions and recycle over 6,200 tonnes of waste oil annually, underscoring the environmental and circular-economy potential of this technology.

Keywords

Asphalt mixtures novel high elastic modifiers waste cooking oil rutting resistance low temperature cracking

Introduction

The continuous expansion of transportation infrastructure and the escalation of traffic volumes, axle loads, and environmental extremes have created an urgent demand for high-performance and sustainable asphalt pavements. Asphalt mixtures remain the dominant paving material worldwide due to their durability, recyclability, and ease of maintenance.^1,2 However, conventional asphalt binders derived from petroleum resources are prone to rutting at high temperatures, cracking at low temperatures, and oxidation-induced aging, which collectively reduce pavement service life and increase maintenance costs.^3–5 To overcome these deficiencies, polymer modification has become a standard practice in asphalt technology, significantly improving rutting resistance and elasticity.⁶ Among the various polymers, styrene–butadiene–styrene (SBS) is the most extensively used modifier due to its proven ability to enhance viscoelastic recovery and structural stability. Nevertheless, SBS-modified binders exhibit several limitations, including high production costs, thermal instability at mixing temperatures exceeding 180°C, susceptibility to oxidative aging, and dependence on non-renewable petrochemical feedstocks.⁷ These drawbacks motivate the search for more sustainable, cost-effective, and environmentally responsible alternatives that maintain or surpass the performance of SBS systems.⁸

References 1–8 Recycling waste materials in road construction is an essential strategy to mitigate environmental pollution and safeguard public health. In China, approximately 5 million tons of waste cooking oil (WCO) are generated annually, of which 2 to 3 million tons are illicitly reintroduced into the food supply chain, posing serious risks to both physical and mental well-being.^9–11 WCO reduces binder stiffness by 15-20% at −12°C, improving low-temperature cracking resistance, which translates to a 20% reduction in fracture energy in mixture tensile tests. Similarly, nano calcium carbonate and nano hydrated lime modifiers increase G*/sinδ by 21% at high temperatures, correlating with a 68% improvement in rutting resistance in wheel-tracking tests. This synergy ensures that binder-level improvements directly enhance mixture performance, unlike SBS-modified asphalt, where binder stiffness often compromises low-temperature properties. These findings, supported by a recent study¹² establish a robust connection between rheological enhancements and mixture-scale outcomes, validating the WCO-HEMs approach.^13,14 Research on alternative modified asphalt mixtures has emerged as a critical prerequisite for achieving sustainability in pavement engineering. Empirical evidence indicates that incorporating novel modifiers into asphalt mixtures can significantly enhance mechanical performance parameters such as rutting resistance, fatigue life, and moisture susceptibility.¹⁵ These improvements contribute to longer service lives, reduced maintenance interventions, and lower consumption of virgin materials, thereby minimizing environmental impacts and improving the long-term resilience of transportation infrastructure.^16–18 The synergistic integration of WCO and HEMs presents an innovative strategy to achieve both high performance and sustainability in asphalt mixtures. WCO can improve compatibility and dispersion of HEMs within the binder matrix by reducing viscosity, while the elastic polymer network can offset the softening effect of WCO, thereby maintaining structural strength. This complementary mechanism is expected to enhance both high-temperature rutting resistance and low-temperature crack resistance two competing performance aspects often difficult to balance. Furthermore, the use of WCO contributes to resource recovery and greenhouse gas reduction, aligning with the goals of green infrastructure and carbon neutrality.^16–18

The performance of asphalt mixtures is critical for ensuring long-lasting pavements under diverse conditions.¹⁹ Traditional SBS-modified asphalt improves rutting resistance but often struggles with low-temperature cracking and long-term aging. The WCO-HEMs approach addresses these limitations by enhancing mixture-level properties. WCO, as a rejuvenator, restores the viscoelastic properties of aged bitumen, reducing low-temperature cracking susceptibility by 15-20%, as measured by indirect tensile strength tests. SBS and graphene-based modifiers, improve rutting resistance by 25-30%, with wheel-tracking tests showing a rut depth reduction from 8 mm to 5.5 mm at 60°C compared to SBS-modified mixtures. Additionally, WCO-HEM mixtures exhibit superior moisture susceptibility, with tensile strength ratios (TSR) of 0.90–0.95 in wet conditions, compared to 0.85 for SBS-modified asphalt.²⁰ Freeze–thaw durability is also enhanced, with WCO-HEM mixtures retaining 80% of their stiffness after 10 cycles, compared to 70% for conventional mixtures. Long-term aging tests indicate that WCO-HEM mixtures maintain flexibility 20% better than SBS-modified asphalt of simulated aging, linking improved binder rheology to enhanced mixture durability.^21–24

High-elastic modifiers (HEMs) are an emerging class of engineered additives designed to provide high tensile recovery, superior thermal stability, and enhanced resistance to permanent deformation. Some HEMs have already been applied in large-scale road rehabilitation projects in China, including the G235 (Xin-Hai transportation corridor), the S341 (Yi-li line), and the S356 (Jiangbei riverside corridor).^25,26 Their demonstrated field performance highlights their practical relevance, but systematic laboratory evaluation particularly when combined with bio-based rejuvenators such as WCO remains limited.

To address the lack of systematic research on sustainable alternatives to conventional polymer-modified asphalt pavements, this study focuses exclusively on the performance of asphalt mixtures incorporating innovative combinations of three high-elastic modifiers (HEM-S, HEM-C, and HEM-G) with varying dosages of waste cooking oil (WCO). Each of these high-elastic modifiers has been engineered for field applications, offering unique structural benefits that can improve load-bearing capacity, deformation resistance, and long-term durability of the mixture. The main objective of this research is to identify the optimal HEM–WCO combination capable of achieving a balanced improvement in rutting resistance, cracking resistance, moisture durability, and aging stability. The outcomes of this study aim to provide practical design guidelines for sustainable pavement construction and to demonstrate that bio-polymer synergy can serve as a viable, environmentally friendly alternative to conventional SBS-modified asphalt systems. Specifically, the research investigates whether (i) the chemical architecture of HEMs (epoxy-grafted SBS vs olefin copolymers) governs compatibility with WCO, (ii) an optimal WCO dosage exists that maximizes ductility without compromising elasticity, and (iii) a single HEM-WCO formulation can deliver holistic performance improvements across all critical service conditions.

Methodology and Materials

This study examines the influence of high-elastic modifiers (HEMs) and waste cooking oil (WCO) on the performance of AC-13C asphalt mixtures. Figure 1 presents the overall research framework, outlining the key stages of the study, which include material characterization, mixture design, specimen preparation, and performance evaluation. The methodology was conducted in accordance with standard specifications to ensure reliability and comparability of results.

Figure 1.

Research framework.

Materials

Base Asphalt Binder

The 70# base asphalt was selected as it is the standard binder for surface courses in Chinese highway projects, ensuring the relevance and practical applicability of this study. The physical properties of the base asphalt were determined in accordance with AASHTO M320 standards²⁷ and are presented in Table 1.

Table 1.

Technical properties of base bitumen.

Test	Results	Requirement	Standard
Penetration (25°C)	68	60-80	ASTM D5
Penetration index	0.1	−1.5 + 1.5	-
Softening point (°C)	47	≥ 46	ASTM D36
Ductility (cm)	>150	≥ 100	ASTM D113
Solubility (%)	99.8	≥ 99.5	ASTM D2042
RTFO (short-term aging	+0.2	≤ ± 0.8	ASTM D2872

High Elastic Modifiers

Three types of novel high-elastic modifiers (HEMs) HEM-S, HEM-C, and HEM-G were utilized in this study to investigate their distinct influences on asphalt performances shown in Figure 2. These modifiers were sourced from Jiangsu Jicui Advanced Polymer Materials Research Institute, Guolu Gaoke Engineering Technology Institute Co., Ltd, and Jiangsu Transportation Technology Institute Co., Ltd, and were selected based on their advanced formulation chemistry and potential to enhance elasticity, thermal stability, and rutting resistance. HEM-S is an SBS-based matrix grafted with a mono-component epoxy and reactive tackifier. The epoxy groups introduce additional cross-linking sites, while the tackifier improves interfacial bonding between the polymer and asphalt, enhancing elasticity, dispersion, and moisture resistance. HEM-C is a thermoplastic polymer functionalized with linear graft groups and compounded with compatibilizing additives. This structure promotes molecular entanglement and the formation of a spatial polymer network, providing superior compatibility with asphalt and exceptional thermal stability. HEM-G is a thermosetting-type polymer blended with synthetic resin, forming a dense cross-linked network. This high crosslink density restricts molecular chain mobility, resulting in greater stiffness and viscosity but reduced low-temperature flexibility. The differing chemical architectures epoxy-tackified SBS (HEM-S), thermoset resin blend (HEM-G), and functionalized thermoplastic (HEM-C) are critical in explaining their performance differences in rutting resistance, cracking resistance, and aging behaviour, Melt flow index (MFI) and crystallinity data for the modifiers were provided by the respective manufacturers, these parameters are critical for understanding the processability and phase behavior of the modifiers during blending and long-term storage as detailedin Table 2.^25,28

Figure 2.

High elastic modifiers.

Table 2.

Physicochemical properties of high-elastic modifiers.

Property	HEM-S	HEM-C	HEM-G	Methods
Appearance	Green granules	White irregular particles	Grey composite granules	ASTM D1535
Density (g/cm³)	0.950	0.980	0.976	ASTM D792 (23°C)
Particle size (μm)	10-1800	8-1600	3000-4000	Laser diffraction (ISO 13320)
MFI (g/10 min)	8.5	4.3	10.2
Crystallinity (%)	<5	25	<5
Thermal stability (°C)	209	305	275	TGA (5% mass loss)

Aggregate

The AC-13 gradation was designed in accordance with ASTM D3515 for dense-graded surface mixtures.^29,30 Aggregates were characterized using ASTM standards to ensure compliance with critical performance parameters.

Tables 3–5 provides a summary of the test results and performance indices based on the established testing standards.

Table 3.

Technical indices of coarse aggregate.

Index	Result	Technical standard
Abrasion loss (%)	16	≤28
Crushing value (%)	20	≤30
Apparent density	2.785	≥2.70
Water content (%)	0.75	≤2.0
Elongated particle content (%)	10.5	≤15

Table 4.

Technical properties of fine aggregate.

Technical indicator	Results	Requirements
Clay content (%)	1.7	≤3
Specific gravity	2.63	≥2.50
Sand equivalent (%)	73	≥60
Sturdiness (%)	5.2	≤12

Table 5.

Mineral powder technical parameters.

Material characteristic	Results	Requirements
Apparent density (g/cm³)	2.70	>2.5
Hydrophilic coefficient	0.79	<1
Moisture content	0.18	<1
Plasticity index	2.8	<4
D₁₀ (μm)	3.4	-
D₅₀ (μm)	30.8	-
D₉₀ (μm)	69.6	-

Waste Cooking Oil

The waste cooking oil (WCO) utilized in this study was sourced from local restaurants, after multiple uses and subjected to a purification process to remove impurities and food residues,³¹ the WCO is primarily composed of unsaturated fatty acids, particularly monounsaturated lipids, which contribute to its liquid state under ambient conditions.³² The key properties of WCO are summarized in Table 6.

Table 6.

Basic properties of WCO.

Test	Result	Standard
Flash point (°C)	310	ASTMD93
Fire point (°C)	335	-
Carbon residue (%)	0.30	ASTM D189
Viscosity (Pa·s)	0.175	ASTM D4402
Melting point (°C)	0.5	ASTM D5440

Asphalt Mixture Specimen Preparation

AC-13C asphalt mixture specimens were prepared as shown in Figure 3 following a two-stage process. In the first stage, modified asphalt binders were produced by blending 70# base asphalt with waste cooking oil (WCO) and high-elastic modifiers (HEMs). The base asphalt was heated to 150 ± 2°C, after which pre-filtered WCO (1 µm sieve) was incorporated at dosages of 3%, 4.5%, and 6% by weight of the binder levels selected based on established literature^33,34 indicating this range optimizes rejuvenation without compromising high temperature stability, and mixed at 3000 r/min for 10 min. Subsequently, pulverized HEMs (<0.3 mm) were added at 4%, 6%, and 8% of binder weight dosages recommended by the manufacturers as optimal for field performance and workability, and the mixture was subjected to high shear mixing at 5000 r/min for 40 min at 180 ± 2°C to ensure full dispersion.

Figure 3.

Preparation process of asphalt mixture.

In the second stage, AC-13C asphalt mixtures were fabricated. Limestone aggregates, pre-heated to 160 ± 5°C and graded according to AC-13C specifications, were blended with the modified binders at the optimum asphalt content of 5.2% (by total mixture weight). The mixing process was conducted mechanically for 90 s to achieve uniform coating of aggregates. Short-term aging was carried out at 135°C for 4 h, after which the mixtures were compacted to a target air void content of 7.0 ± 0.5%.

Preparation of Modified Asphalt

Design of Asphalt Mixture

Asphalt mixture is a composite material consisting of modified asphalt binder and mineral aggregates. Its overall performance is influenced not only by the properties of the binder but also by the gradation of the aggregates. In this study, a fine-graded AC-13C asphalt mixture was selected in accordance with the AASHTO M323-20 standard as shown in Figure 4.

Figure 4.

Gradation AC-13.

Mixing and Compaction Temperature for Asphalt Mixtures

The binder viscosity was measured for each group of asphalt at different temperature 135°C,155°C and 165°C as shown in Figure 5. According to AASHTO T 316-21.^35,36 The mixing temperature for the modified asphalt mixtures was determined to ensure adequate fluidity for the binder to properly coat the aggregates without excessive aging. The compaction temperature was selected to provide optimal workability for field or laboratory densification, ensuring the binder would not undergo thermal degradation. The mixing temperature ranged from 135°C to 155°C, and the compaction temperature was set at 155°C. These temperatures were chosen based on AASHTO T 316-21, ensuring the binder had the appropriate viscosity for proper mixture preparation. The OAC for each asphalt mixture was determined using the Marshall Mix Design method in accordance with ASTM D1559, incorporating High-Elastic Modifier (HEM) and Waste Cooking Oil (WCO) modifications. The OAC values varied from 5.2 % for the control mixture to 5.5 % for the HEM-G-modified mixture. The selection criteria for OAC were based on a combination of performance indicators, including peak Marshall stability, maximum stability, target air void content of 4 %, and a tensile strength ratio (TSR) exceeding 85 %. This viscosity-based characterization confirms that all HEM-WCO binders achieved suitable rheological properties for uniform mixing, compaction, and polymer dispersion prior to mixture fabrication.

Figure 5.

Rotational viscosity of modified asphalt.

The Optimum Asphalt Content (OAC) for the different asphalt mixtures was determined by analysing the volume index and other related parameters, as presented in Table 7. This table summarises the corresponding test data and calculation results for each of the five asphalt mixtures.

Table 7.

Optimal asphalt content and corresponding volumetric parameters.

Technical indicators	Control	HEM-S	HEM-C	HEM-G	Specifications
Air voids (%)	4.1	3.9	4.0	3.8	3-5 (AASHTO R 35)
VMA (%)	14.8	15.2	15.0	15.4	≥14 (AASHTO M 323)
VFA (%)	72.3	74.1	73.5	14.78	60-75 (AASHTO M 323)
Stability (kN)	11.2	13.8	12.5	14.2	≥8 (ASTM D6927)
Flow (mm)	3.2	3.5	3.3	3.7	2-4 (ASTM D6927)
Dust/Binder ratio	1.15	1.08	1.12	1.05	0.9-1.2 (AASHTO M 323)

Tests Methods 2.4.2 Marshal Stability and Flow Test

The Marshall stability and flow test was conducted in accordance with ASTM D6927 to evaluate the mechanical performance of the HEM-WCO-modified asphalt mixtures.^37,38 This test assessed the mixtures’ resistance to deformation (stability) and flexibility (flow) under loading, ensuring compliance with standard specifications for pavement applications. Specimens were prepared by compacting mixtures using the Marshall hammer with two rounds of 75 blows per side to achieve a target air void content of 7.0 ± 0.5%, followed by short-term aging at 135°C for 4 hours (AASHTO R 30). Three replicates per formulation were tested to ensure statistical reliability. The Marshall stability and flow tests were conducted at 60°C under a constant loading rate of 50.8 mm/min (ASTM D6927), with stability recorded as the maximum load resistance (kN) and flow measured as the vertical deformation (mm) at failure.

Rutting Resistance Test

The rutting resistance of the asphalt mixture was evaluated in accordance with AASHTO T324.³⁹ Rectangular beam specimens (300 mm × 300 mm × 50 mm) were prepared using the wheel-tracking method and conditioned in a controlled environmental chamber at 60°C for 5 hours prior to testing. A wheel load of 0.7 MPa was applied to the specimens at a rolling speed of 48 passes per minute (24 double passes per minute). The rutting deformation was measured at two specific time intervals 45 and 60 minutes. These values were then used in equation (1) to calculate the Dynamic Stability (DS), which serves as a significant performance indicator for evaluating the rutting resistance of asphalt mixtures, simulating long-term pavement deformation under controlled temperature, and loading conditions.

D S = \frac{(t_{2} - t_{1}) \times N}{(d_{2} - d_{1})} \times C_{1} \times C_{2}

(1)

DS = Dynamic Stability (passes per mm per minute).

T₁, T₂ = Time points for rut depth measurement (at 45 min and 60 min).

N = Number of wheel passes.

D₂ = Final rut depth at T₂ (mm).

D₂ = Reference or initial rut depth (mm).

C₁, C₂ = Correction factors C₁ is test machine type factor, 1.0, C₂ is the specimen-related coefficient.

Low-Temperature Indirect Tensile Testing

To evaluate the resistance of HEM-WCO modified asphalt mixtures to thermal cracking, low-temperature indirect tensile (IDT) tests were conducted following AASHTO T 322 (Semi-Circular Bend Test) and ASTM D6931 standards.⁴⁰ This test characterizes the fracture properties of asphalt mixtures under thermally induced stresses. The maximum load and maximum horizontal deformation at peak load were recorded. Subsequently, (R_T), (εT), and (S_T) were calculated using equations (6) and (7) Additionally, P is the maximum applied load (N), μ is assumed to be 0.25. The splitting tensile strength, failure tensile strain, and failure stiffness modulus were determined using equations (2) and (3).

R_{T} = \frac{0.006287 P}{h}

(2)

$ε_{T} = X (0.307 + 0.936 μ / (1.35 + 5 μ))$

S_{T} = \frac{P (0.27 + 1.0 μ)}{h . X}

(3)

R_T = Splitting tensile strength

εT = failure tensile strain.

S_T = Rupture modulus

h = Specimen height (mm).

μ = Poisson’s ratio.

X = Horizontal deformation.

Immersion Marshall Test

The moisture susceptibility of HEM-WCO modified mixtures was evaluated using an AASHTO-adapted Immersion Marshall Test protocol. Marshall specimens compacted to 7.0 ± 0.5% air voids were divided into two subsets: (1) Control group tested immediately after 60°C water bath conditioning for 30 ± 5 min (MS₀), and (2) Conditioned group submerged in 60°C water for 48 ± 1 h prior to testing (MS₁). Specimens were loaded at 50.8 mm/min (ASTM D6927), with Immersion Residual Stability (IRS).⁴¹ The Marshall Stability (MS) of each asphalt mixture group has been evaluated before and after the high-temperature ageing technique to evaluate resistance to ageing. The Marshall stability was calculated using Equation (4), and the Marshall Stability Ratio (MSR) was subsequently determined using Equation (5).

I R S = \frac{{M S}_{c o n d i t i o n e d}}{{M S}_{u n c o n d i t i o n e d}} \times 100

(4)

R M S R = \frac{{M S}_{1}}{M S} \times 100

(5)

RMSR = Marshall Stability Ratio after aging.

MS₁ = stability (kN) of aged specimens.

MS = stability (kN) of unaged specimens.

Long-Term Aging Resistance

The aging simulation was conducted in two stages. Short-term aging was performed by placing the mixed asphalt material on an iron tray in a forced-draft oven at 135 ± 5°C for 4 hours, with manual stirring every hour to ensure uniform oxidation consistent with standard practice for mixture-level aging (AASHTO R 30). For long-term aging, the short-term aged Marshall specimens were conditioned in the same oven at 90 ± 5°C for 5 days. This protocol is a widely adopted laboratory method to accelerate oxidative aging through thermal oxidation, effectively simulating 5-10 years of field aging by promoting the conversion of maltenes to asphaltenes, which governs binder hardening and embrittlement.⁴² However, it is acknowledged that this approach does not replicate all field aging mechanisms, such as UV radiation, moisture infiltration, freeze -thaw cycling, or traffic-induced fatigue. These factors can significantly influence aging kinetics and cracking behaviour in real pavements.

6Binder Rheological Characterization

To evaluate the fundamental viscoelastic properties of the modified binders and establish a mechanistic link between binder modification and mixture performance, dynamic shear rheometer (DSR) and bending beam rheometer (BBR) tests were conducted in accordance with AASHTO M320 and AASHTO T313, respectively.

Unaged binders were tested in oscillatory shear mode using a dynamic shear rheometer (Anton Paar MCR 302) equipped with a 25 mm parallel plate geometry. Complex shear modulus (G∗) and phase angle (δ) were measured over a temperature range of 64–76°C at a frequency of 10 rad/s to determine the high-temperature performance grade based on the criterion G∗/sinδ≥1.00 kPa. Following rolling thin-film oven (RTFO) aging (AASHTO T240), the same test was repeated to assess post-processing aging effects. Additionally, pressure-aging vessel (PAV) aged samples (AASHTO R28) were subjected to DSR testing at 25°C to calculate the fatigue parameter G∗⋅sinδ, with lower values indicating improved fatigue resistance.

Low-temperature performance was evaluated using the Bending Beam Rheometer. PAV-aged binders were loaded onto a beam mold and cooled to target temperatures (−12°C and −18°C). A constant load of 100 ± 4 mN was applied for 240 seconds, and the resulting deflection was used to compute creep stiffness (S) and relaxation rate (m -value). Acceptable low-temperature performance requires S < 300 MPa and m > 0.3. These binder-level tests provide critical insight into the synergistic mechanisms of HEM-WCO modification prior to mixture fabrication.

Results

Marshall Test

As shown in Figures 6 and 7, all asphalt mixtures satisfy the Marshall design criteria stipulated in the specification, with stability values exceeding 8 kN and flow values within the range of 1.5-4.0 mm. The base asphalt (BA) exhibited a stability of 12.5 kN and a flow value of 3.6 mm, representing the reference mechanical performance. Incorporation of High Elastic Modifiers (HEM) in combination with Waste Cooking Oil (WCO) significantly changed the load bearing and deformation characteristics of the mixtures. The HEM-S+6%WCO formulation achieved the highest stability of 15.9 kN, corresponding to an approximate 27% increase over BA, while maintaining a flow value of 3.7 mm. This indicates that the enhanced structural integrity was not accompanied by excessive embrittlement, thus preserving sufficient deformation capacity to mitigate cracking under traffic and thermal stresses. The HEM-C+4.5%WCO mixture recorded a stability of 13.6 kN with a flow of 3.4 mm, demonstrating a moderate yet well balanced improvement in both rutting resistance and flexibility. Conversely, the HEM-G+3%WCO mixture yielded a stability of 12.6 kN and a markedly reduced flow of 2.9 mm, observing limited enhancement in load capacity but a noticeable increase in stiffness, which may elevate the risk of fatigue or low-temperature cracking.

Figure 6.

Results of the marshall stability test.

Figure 7.

Flow value experimental results.

The significant performance of HEM-S+6%WCO can be attributed to a synergistic physical-chemical interaction between the modifier and the waste cooking oil. The WCO, rich in unsaturated fatty acids, acts as a bio-based plasticizer. It infiltrates the asphalt matrix, increasing the free volume between molecules and enhancing molecular mobility, which improves flexibility and low-temperature performance. Concurrently, the HEM-S, with its functionalized polymer architecture (epoxy and reactive tackifier grafted on an SBS core), forms a continuous three-dimensional elastic network within the binder. This network enhances the mixture’s load-bearing capacity and resistance to deformation. The optimal dosage of 6% WCO appears to facilitate the dispersion of HEM-S and tune the viscoelastic balance of the binder, resulting in high stability without excessive brittleness. The observed increase in Marshall stability and maintained flow value are direct manifestations of this synergistic effect.

The 27% increase in Marshall stability and 65.6% rise in Marshall modulus for HEM-S+6%WCO exceed typical enhancements reported for conventional SBS-modified asphalt mixtures (+15-25% stability) and are comparable to high-dosage HEM systems (+60% modulus). Similarly, the 46% improvement in low-temperature tensile strain significantly surpasses most WCO-rejuvenated recycled asphalt mixtures, which typically report +10–30% strain recovery. These comparisons confirm that the HEM–WCO synergy delivers above-average, practically meaningful improvements relative to current sustainable modification strategies.⁴³

Marshall Modulus (T = MS/FL)

The Marshall modulus (kN/mm) reflects mixture resistance to permanent deformation under load. As shown in Figure 8, the trend in T mirrors the stability response but emphasizes formulation-dependent stiffness: the base asphalt (BA) exhibits T = 3.46 kN/mm, while HEM-modified, WCO-containing mixtures show clear gains HEM-S+6%WCO 5.73 kN/mm (+65.6%), HEM-C+4.5%WCO,4.36 kN/mm (+26.0%), and HEM-G+3%WCO: 4.20 kN/mm (+21.4%) relative to BA. The highest T for HEM-S+6%WCO indicates the strongest resistance to high-temperature deformation, consistent with a synergistic effect in which the elastic network supplied by HEM-S is optimally tuned by WCO to improve aggregate binder cohesion and load transfer. The more moderate increases for HEM-C and HEM-G at lower WCO dosages suggest that both modifier chemistry and oil content govern the viscoelastic balance; sub-optimal WCO levels under-soften the polymer matrix and yield smaller stiffness gains. While elevated T is desirable for rutting control, excessive stiffness may penalize fatigue and thermal cracking; hence, HEM-S+6%WCO provides the most advantageous stiffness enhancement among the tested blends, whereas HEM-C+4.5%WCO and HEM-G+3%WCO offer more conservative, potentially better-balanced designs for climates or structures where cracking risk is critical.

Figure 8.

Marshall modulus experimental results.

Rutting Test and Dynamic Stability Analysis

In the rutting test as shown in Figure 9 the BA mixture exhibited the highest rut depth, reaching approximately 3.45 mm after 2500 passes. In contrast, the HEM-modified mixtures demonstrated significantly reduced rut depths, indicating enhanced resistance to permanent deformation at high temperatures. Among them, HEM-S showed the smallest final rut depth (1.50 mm), while HEM-G (1.96 mm) and HEM-C (1.98 mm) displayed intermediate performance.

Figure 9.

Rut test data diagram.

To quantitatively assess deformation progression, slope fitting of the rutting curves was conducted for the primary stage (0-500 passes) and secondary stage (500-2500 passes). The BA mixture exhibited the highest slope in both stages, with 0.00293 mm/pass in the primary stage and 0.00095 mm/pass in the secondary stage, indicating rapid deformation accumulation throughout the test. In contrast, HEM-S displayed the lowest slopes (0.00202 mm/pass primary, 0.00024 mm/pass secondary), demonstrating its superior capacity to suppress rut growth, particularly in the secondary stage. HEM-C (0.00232/0.00037 mm/pass) and HEM-G (0.00219/0.00042 mm/pass) showed intermediate reductions in rutting rate compared to BA. The lower secondary-stage slopes of all HEM mixtures confirm their ability to maintain structural integrity under prolonged loading.

The DS values is shown in Figure 10, further support these findings. The BA mixture achieved the lowest DS (1970 time/mm), falling well below the modified mixtures. The highest DS was recorded for HEM-C+4.5%WCO (4900 time/mm), approximately 2.5 times greater than BA, indicating a substantial enhancement in rutting resistance. HEM-S+6%WCO (3685 time/mm) and HEM-S+3%WCO (3200 time/mm) also exhibited notable improvements, exceeding BA by 87% and 62%, respectively. Although the addition of WCO changed the ranking compared to the pure HEM results favoring HEM-C over HEM-S in DS performance all modified mixtures still met the specification requirement of ≥2800 time/mm for high-temperature rutting resistance.

Figure 10.

Results of dynamic stability.

The enhanced low-temperature cracking resistance of the HEM-S+6%WCO mixture is a result of the combined action of WCO and HEM. The WCO plasticizes the binder, reducing its stiffness and increasing its ability to relax thermal stresses, as evidenced by the significant increase in tensile strain (εT). Meanwhile, the HEM provides an elastic network that helps to distribute stress and resist fracture. The synergy is such that the WCO prevents the HEM-modified binder from becoming too stiff at low temperatures, while the HEM network prevents excessive softening, maintaining overall strength. This balance is crucial for thermal cracking resistance, as it allows the mixture to withstand both thermal contraction and loading without failure.²³

The dynamic stability (DS) values of the HEM-WCO modified mixtures, particularly HEM-C+4.5%WCO (4900 passes/mm), not only meet but exceed the typical performance of conventional polymer-modified asphalts. For instance, SBS-modified asphalt mixtures typically achieve DS values in the range of 2500-4000 passes/mm. Similarly, asphalt mixtures modified with waste cooking oil (WCO) alone or in combination with other modifiers generally exhibit DS values between 1800 and 3500 passes/mm. The superior DS performance of the HEM-C+4.5%WCO formulation (4900 passes/mm) represents a 22.5% improvement over the upper bound of conventional SBS-modified asphalt and a 40% improvement over the upper bound of WCO-modified systems, demonstrating the exceptional high-temperature stability achieved through the synergistic modification.⁴⁴

These results collectively indicate that high elastic modifiers significantly improve the rutting performance of asphalt mixtures, and that the incorporation of WCO can further enhance performance trends depending on the base modifier type. The superior performance of HEM-C+4.5%WCO observed a synergistic effect between the polymer network and the rejuvenator, enhancing elasticity and load distribution. The improved DS and reduced rut depths align with the high-temperature rheological behavior observed in modified binders, confirming that optimized HEM-WCO formulations can substantially enhance resistance to permanent deformation under high-temperature conditions.

Analysis of Low Temperature Performance

As shown in Figure 11, the low-temperature indirect tensile testing results at −10°C of the asphalt mixtures are compared with the base asphalt (BA). The splitting tensile strength (R_T), tensile strain (εT), and damage stiffness modulus (S_T) are presented. Compared with BA, the R_T value of the asphalt mixture increases significantly from 2.5 MPa to 3.40 MPa after adding 6%WCO + HEM-S, representing the most pronounced enhancement in load-bearing capacity at low temperature. Simultaneously, the εT value increases from 4.45 × 10⁻³ to 6.50 × 10⁻³, reflecting a substantial improvement in deformation capacity, which is highly beneficial for suppressing thermal cracking. The S_T value rises only moderately from 1253 MPa to 1357 MPa, observing that the mixture gains ductility without excessive stiffening.

Figure 11.

Low-temperature mechanical performance of asphalt binders: (a) tensile strain, (b) damage stiffness modulus, and (c) splitting tensile strength.

For the HEM-C+4.5%WCO mixture, the R_T value reaches 3.20 MPa and εT improves to 6.18 × 10⁻³, both higher than BA. However, the S_T value attains the maximum of 1411 MPa, indicating that although the mixture has good strength and deformation ability, the increased stiffness may intensify thermal stress accumulation, thereby raising the risk of cracking under severe cooling.

In the case of HEM-S+3%WCO, the RT value is 3.09 MPa, εT reaches 5.80 × 10⁻³, and S_T is 1354 MPa. Although the degree of improvement is less significant than with higher dosages, the mixture still demonstrates clear superiority over BA, offering a balanced enhancement of low-temperature properties. The low-temperature performance of the HEM-S+6%WCO mixture, evidenced by a 46% increase in tensile strain (from 4.45 × 10⁻³ to 6.50 × 10⁻³) compared to the base asphalt, stands out when contextualized with existing studies. Conventional polymer-modified asphalt systems typically report improvements in tensile strain ranging from 20% to 40%. Similarly, bio-rejuvenated mixtures using waste oils or bio-based modifiers show improvements between 15% and 35%.^45–47 The 46% enhancement achieved by HEM-S+6%WCO not only exceeds these typical ranges but also highlights the effectiveness of the HEM-WCO synergy in mitigating low-temperature cracking, a critical failure mode in cold climates.

Overall, all three modified mixtures exhibit superior cracking resistance compared with BA at −10°C. Among them, HEM-S+6%WCO provides the most favorable balance, simultaneously enhancing tensile strength and ductility while maintaining moderate stiffness, thus showing the strongest ability to resist thermal cracking. This superior performance can be ascribed to the synergistic effect of WCO and HEM. The presence of WCO introduces small, highly mobile molecules into the asphalt system, which increase intermolecular spacing, reduce van der Waals interactions, and improve molecular mobility, thereby enhancing ductility at low temperatures. In parallel, HEM contributes elastic reinforcement, enabling the binder to sustain external loading without premature fracture. Consequently, WCO primarily enhances the deformability of the asphalt, while HEM provides structural elasticity and load resistance, and their combined modification significantly strengthens the low-temperature cracking resistance of the asphalt mixtures.

Rheological Test BBR Results

The low-temperature cracking resistance of the modified asphalt binders was assessed using the Bending Beam Rheometer (BBR). As shown in Figure 12 (a) and (b), the inclusion of waste cooking oil (WCO) resulted in a significant reduction in creep stiffness (S) and an increase in the relaxation rate (m) across all formulations, confirming its role as an effective plasticizer that enhances molecular mobility and stress relaxation. At −12°C, the HEM-S + 6% WCO formulation exhibited a creep stiffness of 210 MPa and an m-value of 0.41, meeting the Superpave criteria (S ≤300 MPa, m ≥ 0.30). This indicates excellent flexibility and stress dissipation at critical low temperatures. In comparison, the base asphalt displayed a creep stiffness of 295 MPa and an m-value of 0.29, failing the m-value criterion, indicating a higher susceptibility to thermal cracking. The HEM-C + 4.5% WCO formulation showed moderate improvement (S = 265 MPa, m = 0.34), while the HEM-G + 3% WCO formulation demonstrated less favourable performance (S = 275 MPa, m = 0.32), observed limited compatibility between the thermoset structure of HEM-G and the plasticizing effect of WCO.

Figure 12.

(a) Creep stiffness (S -value) (b) Relaxation rate (m -value).

At −18°C, only the HEM-S + 6% WCO formulation met both Superpave criteria (S = 255 MPa, m = 0.37), qualifying it for a PG low-grade of −22°C, a significant achievement for a bio-rejuvenated binder system. All other formulations failed due to either excessive stiffness (BA, HEM-G) or inadequate relaxation rate (HEM-C). These binder-level findings correlate with the superior mixture-level performance described in the analysis of low temperature performance section. The HEM-S + 6% WCO mixture exhibited a 46% increase in tensile strain (εT = 6.50 × 10⁻³) at −10°C compared to base asphalt (εT = 4.45 × 10⁻³). This improvement is attributed to the combined effects of WCO, which reduces intermolecular cohesion and increases free volume, enhancing ductility, and the epoxy-grafted SBS core of HEM-S, which forms a continuous elastic network that resists fracture initiation and propagation. The high m-value further indicates that this network facilitates effective internal stress relaxation under thermal contraction, preventing crack formation.

In contrast, the higher stiffness, and lower m-values of HEM-G+3%WCO are associated with reduced tensile strain (εT = 5.80 × 10⁻³) and an increased risk of cracking, reinforcing the importance of selecting appropriate modifier chemistry for low-temperature applications. The BBR test results provide direct evidence that the HEM-S + 6% WCO formulation achieves a favourable viscoelastic balance, combining sufficient elasticity to resist deformation and adequate flexibility to accommodate thermal shrinkage, making it the optimal choice for climates with severe winter conditions.

DSR Test Results

To evaluate the high-temperature performance of the modified binders and establish a mechanistic link to mixture-scale rutting resistance, dynamic shear rheometer (DSR) tests were conducted on unaged samples in accordance with AASHTO M320. Fatigue factor (G∗) were measured over a temperature range of 58-82°C at a frequency of 10 rad/s. The parameter G∗/sinδ was calculated to assess rutting resistance, with values above 1.00 kPa indicating compliance with Superpave high-temperature grading requirements.

As shown in Figure 13, all HEM-WCO modified binders exhibit significantly higher G∗/sinδ than base asphalt across the tested temperature range. At 64°C, all formulations meet the PG 70 requirement (G*/sinδ >1.00 kPa), but only HEM-C+4.5%WCO satisfies the PG 76 criterion (G*/sinδ >1.85 kPa). Specifically, HEM-C+4.5%WCO achieves 1.85 kPa at 64°C, meeting the PG 76 requirement, which aligns with its exceptional rutting resistance test (DS = 4900 passes/mm). HEM-S+6%WCO (1.65 kPa) and HEM-G+3%WCO (1.30 kPa) also demonstrate substantial improvements over base asphalt (0.85 kPa), confirming that the synergistic effect of HEMs and WCO preserves or enhances elasticity under thermal stress.

Figure 13.

High-temperature rutting factor of unaged asphalt binders.

The trend in G∗/sinδ with increasing temperature reflects the relative stiffness of each formulation: HEM-C, with its functionalized thermoplastic architecture, forms a dense entangled network that resists flow, while HEM-S’s epoxy-grafted structure provides strong interfacial bonding without excessive rigidity. These findings validate the hypothesis that HEM-WCO composites achieve a favourable balance between elasticity and cohesion, translating directly into enhanced pavement durability.

To evaluate the low-temperature cracking resistance and fatigue potential of the modified binders, DSR tests were conducted on PAV-aged samples at temperatures ranging from 0°C to 25°C. The parameter G∗/sinδ was used to assess the risk of fatigue damage, with lower values indicating better performance.

As shown in Figure 14, the addition of waste cooking oil (WCO) significantly reduces G∗/sinδ at low temperatures, confirming its role as an effective plasticizer that enhances molecular mobility and stress relaxation capability. Notably, the HEM-S + 6% WCO formulation achieves the lowest G∗/sinδ values (2500 kPa at 20°C) among all tested binders, indicating superior fatigue resistance. In contrast, base asphalt exhibits higher values (e.g., 4000 kPa at 20°C), suggesting greater susceptibility to crack initiation and propagation under repeated loading.

Figure 14.

Low-temperature and fatigue performance of PAV-aged asphalt binders.

The HEM-C + 4.5% WCO mixture shows moderate improvement (G∗/sinδ = 3000 kPa at 20°C), while HEM-G + 3% WCO performs worse (G∗/sinδ = 3500 kPa at 20°C), suggesting limited compatibility between the thermoset structure of HEM-G and the plasticizing effect of WCO. These findings are consistent with the mixture-scale performance: the HEM-S + 6% WCO mixture demonstrated the highest tensile strain (6.50 × 10⁻³) at −10°C, confirming its ability to deform under thermal stress without fracturing.

Moisture Susceptibility

The moisture susceptibility of asphalt mixtures was evaluated using the Marshall Stability (MS, MS1) before and after moisture conditioning, with the Immersion Retained Stability (IRS%) representing the retained strength.

From Figure 15, the base asphalt mixture (BA) exhibits an IRS value of 84.5%, which reflects moderate resistance to water-induced damage. The incorporation of HEM-S + 6% WCO significantly improved the IRS to 89.7%, indicating superior moisture resistance compared to the control. This improvement can be attributed to the synergistic effect of the highly elastic modifier (HEM-S) and waste cooking oil (WCO), which enhance the binder aggregate adhesion and reduce moisture-induced stripping at the interface.

Figure 15.

Marshall stability (MS and MS1) and Immersion Residual Stability (IRS) of AC-13C asphalt mixtures.

In contrast, the mixture with HEM-C+4.5%WCO shows a reduction in IRS to 84.2%, nearly equivalent to BA. Although the stability values remain high, the lower IRS observed that this composition is less effective in resisting water damage. Similarly, the HEM-G+3%WCO mixture demonstrates the lowest IRS at 81.3%, highlighting weaker moisture resistance. This indicates that lower dosages of WCO combined with HEM-G may not sufficiently improve the moisture sensitivity of the asphalt mixture, possibly due to limited interaction with the binder film and insufficient hydrophobicity.

The results indicate that HEM-S+6%WCO offers the best balance of stability and moisture damage resistance, making it the most effective modifier among the tested groups. The IRS values above 80% for all mixtures still satisfy general moisture susceptibility requirements, but the differences highlight the importance of optimizing both the type of high-elastic modifier and the WCO content.

Freeze Thaw Splitting Strength (TSR)

As shown in Figure 16, the freeze thaw splitting test results show that the Tensile Strength Ratio (TSR) of the asphalt mixtures follows a trend consistent with the IRS findings. The base asphalt mixture (BA) exhibits a TSR of 83%, which serves as the reference. The incorporation of HEM-S+6%WCO leads to a significant improvement, with the TSR reaching 88.6%, representing a 6.7% increase relative to BA. This enhancement demonstrates that the synergistic effect of HEM-S and WCO improves the cohesion of the binder and its adhesion to aggregates, thereby effectively mitigating moisture induced deterioration under freeze-thaw cycles.

Figure 16.

Splitting tensile strength (RT₁ and RT₂) and Tensile Strength Ratio (TSR).

In contrast, the TSR values of HEM-C+4.5%WCO and HEM-G+3%WCO are 81.5% and 80%, respectively, corresponding to slight decreases of 1.8% and 3.6% compared with BA. These results indicate that at the tested dosages, the modification with HEM-C and HEM-G combined with WCO does not provide sufficient resistance against freeze-thaw damage, suggesting a less favorable balance between binder elasticity and the softening effect of WCO.

According to the specification standard of TSR ≥80%, all mixtures meet the minimum requirement. However, only HEM-S+6%WCO achieves both high TSR and IRS values simultaneously, confirming its superior performance in terms of water stability. The results further highlight that the moisture resistance of asphalt mixtures is strongly influenced by the type of high-elastic modifier and the dosage of WCO. Optimized formulations are therefore essential to maximize the positive contribution of WCO while maintaining adequate elastic recovery from the polymer network.

Analysis of Long-Term Ageing Performance

Effects of Long-Term Ageing on Performance at High Temperatures

As shown in Figure 17 Long-term aging significantly changes the high temperature behavior of asphalt mixtures due to the continuous oxidation of lighter components and the transformation of maltenes into resins and asphaltenes. This process results in increased binder viscosity, higher stiffness, and a reduction in ductility, which enhances resistance to permanent deformation but compromises flexibility. As observed in our study, after long-term aging, the Marshall stability of all mixtures exhibited a noticeable increase. This is mainly because the light fractions volatilize or oxidize into larger molecular weight asphaltenes, leading to a harder binder and improved load-bearing capacity at elevated temperatures. The base asphalt (BA) mixture showed the highest increase in stability after aging however, the modified mixtures with high-elastic modifiers (HEM) and waste cooking oil (WCO) demonstrated a more stable RMSR trend.

Figure 17.

Effect of aging on marshall stability ratio.

In particular, the RMSR values of HEM-modified mixtures were closer to 100% compared with the base asphalt, indicating improved resistance to performance variation under aging conditions. When WCO was incorporated alongside HEM, the RMSR values further decreased toward 100%, suggesting that the combined modification improves the anti-aging capacity at high temperatures. This can be attributed to the synergistic mechanism where HEM provides an elastic network structure, while WCO promotes dispersion and partial rejuvenation of lighter asphalt components, resulting in a more uniform system after oxidation. Among the different formulations, the HEM-G+3%WCO mixture showed the most stable high-temperature performance after long-term aging, with its RMSR approaching 100% and minimal reduction in stability compared to unaged conditions. This demonstrates that the addition of WCO in controlled amounts can mitigate excessive hardening during aging and improve the durability of HEM-modified systems under high-temperature service environments.

Overall, these results confirm that the combination of HEM and WCO not only enhances initial sustainability and flexibility but also significantly improves the aging resistance of asphalt mixtures, maintaining favorable rutting resistance while controlling the degree of hardening over long service periods.

Effects of Long-Term Aging on Performance at Low Temperatures

As shown in Figure 18, the RT values of all four asphalt mixtures remained relatively stable, indicating that the incorporation of HEM and WCO did not significantly affect tensile strength. The base asphalt mixture exhibited an ITSR value of 119.5%. With the addition of HEM-S+6%WCO and HEM-C+4.5%WCO, the ITSR values increased to 126% and 123%, respectively, demonstrating enhanced resistance to moisture damage. In contrast, the HEM-G + 3% WCO mixture showed a reduced ITSR of 114%, lower than the base asphalt, suggesting inferior water resistance. This behaviour may be attributed to the ability of WCO to replenish light fractions lost during asphalt processing, thereby improving adhesion, while the degree of compatibility between WCO and different HEM types influences the overall performance. Therefore, it can be concluded that the synergistic use of WCO and HEM not only sustains anti-rutting performance but can also improve moisture stability when the dosage and modifier type are appropriately selected.

Figure 18.

Effects of long-term aging on performance at low temperatures.

Integrated Performance Comparison

Table 8 summarizes the performance comparison of the HEM-WCO modified asphalt mixtures, showcasing their superior properties compared to base asphalt (BA). Notably, the HEM-S + 6% WCO formulation demonstrated the most significant improvements in Marshall Stability (+27%) and Dynamic Stability (+87%), while maintaining an optimal flow value and low-temperature cracking resistance. On the other hand, HEM-C + 4.5% WCO exhibited the highest Dynamic Stability (+148%) but showed less pronounced improvements in low-temperature performance. These results highlight the trade-off between high rutting resistance and the potential risk of increased stiffness, especially in mixtures with lower WCO content like HEM-G + 3% WCO.⁴⁸

Table 8.

Comprehensive performance comparison of hem-WCO modified asphalt mixtures.

Property	BA	HEM-S + 6% WCO	HEM-C + 4.5% WCO	HEM-G + 3%WCO
Marshall stability (kN)	12.5 ± 0.3	15.9 ± 0.4	13.6 ± 0.3	12.6 ± 0.2
Flow (mm)	3.6 ± 0.1	3.7 ± 0.1	3.4 ± 0.1	2.9 ± 0.1
Marshall modulus (kN/mm)	3.46	5.73	4.36	4.20
Rutting resistance (mm)	3.45	1.50	1.98	1.96
DS (passes/mm)	1970	3685	4900	3200
(εT)	4.45 × 10⁻³	6.50 × 10⁻³	6.8 × 10⁻³	5.80 × 10⁻³
Splitting tensile strength (MPa)	2.5	3.4	3.2	3.09
IRS	84.5%	89.7%	84.2%	81.3%
TSR	83%	88.6%	81.5%	80%
RMSR	-	98.5%	96.2%	99.5%
ITSR	119.5%	126%	123%	114%

Results shows in Table 9 that HEM-C+4.5%WCO achieves the highest CPI (0.96), driven by its exceptional rutting resistance (DS = 4900 passes/mm). However, it exhibits relatively lower moisture resistance (IRS = 84.2%) and moderate low-temperature ductility (ε_t = 6.18 × 10⁻³), revealing a rutting-focused trade-off. In contrast, HEM-S+6%WCO delivers the most balanced performance (CPI = 0.934), with no metric falling below 75% of the best value particularly excelling in low-temperature strain (ε_t = 6.50 × 10⁻³) and moisture resistance (IRS = 89.7%, TSR = 88.6%). HEM-G+3%WCO shows the lowest CPI (0.863) due to compromised rutting and moisture performance.

Table 9.

Composite Performance Index (CPI) for each mixture.

Mixture	DS (NORM)	E_t (NORM)	IRS (NORM)	RMSR (NORM)	CPI (AVG)
HEM-S + 6% WCO	0.75	1.00	1.00	0.98	0.93
HEM-C + 4.5% WCO	1.00	0.95	0.94	0.96	0.96
HEM-G + 3% WCO	0.65	0.89	0.91	1.00	0.86

Estimated Sustainability Benefits

A first-order estimate of the sustainability benefits of the High Elastic Modifier and Waste Cooking Oil (HEM–WCO) assuming only 1% adoption within China’s annual hot-mix asphalt (HMA) production demonstrates measurable waste-diversion and greenhouse-gas (GHG) reduction potential. China total HMA production is approximately 200 million tonnes per year, with a typical surface course binder content of 5.2%, equivalent to around 10.4 million tonnes of bitumen annually. Incorporating 6% WCO by binder mass corresponds to roughly 0.62 million tonnes of WCO per year. Therefore, if only 1% of national production adopted the HEM–WCO approach, approximately 6,200 tonnes of waste cooking oil would be repurposed instead of entering sewage systems or being illegally reused for cooking.¹¹

If 1 tonne of WCO combustion emits 2.74 tonnes of CO₂ equivalent, preventing this disposal pathway would avoid approximately 17,000 tonnes of CO2 emissions annually. Moreover, replacing the conventional 4% styrene–butadiene styrene (SBS) polymer modifier in the same binder volume would eliminate the need for roughly 4,200 tonnes of virgin SBS, which carries a cradle to-gate emission intensity of about 3.2 t CO₂ per tonne. This substitution would yield an additional 13,400 tonnes of CO₂ savings per year.

In addition, the plasticising effect of WCO enables a mixing temperature reduction of about 5°C, decreasing thermal energy demand by approximately 3%. Given an average energy use of 0.19 Megajoule per kilogram of asphalt mix, this translates to an energy saving of roughly 11.4 Terajoule annually for 2 million tonnes of HMA, equivalent to a further 900 tonnes of CO₂ reduction. Overall, adopting HEM–WCO asphalt for just 1% of China’s HMA production could prevent approximately 31,300 tonnes of CO₂ equivalent emissions per year while recycling about 6,200 tonnes of hazardous waste oil. These benefits scale proportionally with market share and provide compelling, quantifiable evidence of the technology’s sustainability and circular-economy value^11,49–52

Conclusion

This research comprehensively evaluated the performance of asphalt mixtures modified with novel high-elastic modifiers (HEMs) and waste cooking oil (WCO), demonstrating significant mechanical and environmental benefits. The key conclusions are as follows.

The combined modification of WCO and HEMs produces a distinct synergistic effect, improving both deformation resistance and flexibility. WCO enhances molecular mobility and low-temperature ductility, while HEMs establish a stable elastic network that strengthens load-bearing capacity.

The HEM-S + 6% WCO formulation achieved the most balanced performance, excelling in Marshall stability, dynamic stability, low-temperature cracking resistance, and moisture durability. The HEM-C + 4.5% WCO mixture provided the highest rutting resistance, while HEM-G + 3% WCO exhibited moderate improvement.

DSR and BBR tests confirmed that the optimized HEM–WCO systems meet Superpave criteria, providing enhanced high-temperature stiffness and excellent low-temperature flexibility (S ≤300 MPa, m ≥ 0.30). The results align with mixture-scale performance trends.

Long-term aging tests showed stable mechanical properties and improved resistance to oxidative hardening, while moisture susceptibility (IRS >85%, TSR >80%) verified strong adhesion and durability under wet and freeze–thaw conditions.

Incorporating WCO into asphalt mixtures not only enhances pavement performance but also contributes to environmental sustainability. Even limited adoption of HEM–WCO technology could significantly reduce greenhouse gas emissions and repurpose hazardous waste oil.

The HEM–WCO modification presents a practical, high-performance, and eco-friendly alternative to SBS-modified asphalt, aligning with modern goals of green and intelligent pavement construction. Future studies should extend to field-scale validation, fatigue life modelling, and long-term performance monitoring under real traffic and climate conditions.

Practical Recommendations for Practitioners

Based on the comprehensive performance evaluation, specific recommendations can be made for pavement engineers and designers, tailored to different climatic and traffic conditions:

For High-Temperature Climates and Heavy-Traffic Areas: In regions prone to high temperatures and experiencing significant rutting due to heavy axle loads (e.g., urban highways, desert environments), the HEM-C + 4.5% WCO formulation is strongly recommended. Its exceptional dynamic stability (4900 passes/mm) provides superior resistance to permanent deformation, making it the optimal choice for maximizing pavement durability under these demanding conditions.

For Cold Climates and Regions with Thermal Cycling: In areas subject to low temperatures and freeze-thaw cycles where thermal cracking is the primary concern, the HEM-S + 6% WCO mixture is the preferred option. This formulation delivers the most balanced overall performance, offering excellent low-temperature flexibility (a 46% increase in tensile strain), high moisture resistance (IRS = 89.7%, TSR = 88.6%), and adequate rutting resistance. It is ideal for climates that experience significant seasonal temperature variations.

General Dosage Guidelines: For optimal results across various applications, WCO dosages between 4.5% and 6% by binder weight, combined with HEM dosages in the range of 4–6%, are suggested. These ranges provide a favourable balance of workability during construction, mechanical performance, and sustainability benefits, while avoiding the excessive softening associated with higher WCO doses or the brittleness from lower ones.

Limitations and Future Work

The study was limited to laboratory-scale testing under controlled conditions; actual field performance under traffic and environmental variability remains unverified.

The WCO source variability (composition, degree of oxidation, and purification quality) may influence long-term performance consistency.

The dosage range was restricted to 3–6% WCO and 4–8% HEMs; broader optimization could identify more cost-effective or temperature-specific formulations.

The life cycle assessment (LCA) and cost-benefit analysis were estimated but not empirically validated through a full-scale environmental and economic assessment.

These recommendations provide practical guidance for pavement designers to select the most appropriate HEM-WCO formulation based on local climate conditions, traffic loading, and performance requirements, ensuring optimized asphalt mixture performance throughout the service life of the pavement.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is supported by National Natural Science Foundation of China (Grant No: 51878168).

ORCID iD

Syed Khaliq Shah

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Synergistic enhancement of asphalt mixtures with novel high-elastic modifiers and waste cooking oil

Abstract

Keywords

Introduction

Methodology and Materials

Materials

Base Asphalt Binder

High Elastic Modifiers

Aggregate

Waste Cooking Oil

Asphalt Mixture Specimen Preparation

Preparation of Modified Asphalt

Design of Asphalt Mixture

Mixing and Compaction Temperature for Asphalt Mixtures

Tests Methods 2.4.2 Marshal Stability and Flow Test

Rutting Resistance Test

Low-Temperature Indirect Tensile Testing

Immersion Marshall Test

Long-Term Aging Resistance

6Binder Rheological Characterization

Results

Marshall Test

Marshall Modulus (T = MS/FL)

Rutting Test and Dynamic Stability Analysis

Analysis of Low Temperature Performance

Rheological Test BBR Results

DSR Test Results

Moisture Susceptibility

Freeze Thaw Splitting Strength (TSR)

Analysis of Long-Term Ageing Performance

Effects of Long-Term Ageing on Performance at High Temperatures

Effects of Long-Term Aging on Performance at Low Temperatures

Integrated Performance Comparison

Estimated Sustainability Benefits

Conclusion

Practical Recommendations for Practitioners

Limitations and Future Work

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References