Researchers at Jiangsu Province institutions have demonstrated that a composite rejuvenator made from waste cooking oil and waste wood pyrolysis oil can effectively restore the properties of aged asphalt. Published in Case Studies in Construction Materials, the study presents a dual-source bio-oil strategy that addresses the complex structural changes occurring in asphalt binders during long-term service. The team found that these two waste streams offer complementary molecular functions, allowing them to repair different aspects of asphalt degradation simultaneously.
Asphalt pavements undergo significant chemical and physical changes due to heat, oxygen, ultraviolet radiation, moisture, and traffic loading. This aging process leads to the loss of light components and an increase in polar oxygen-containing functional groups. Consequently, the relative content of asphaltenes rises, and the colloidal structural stability decreases, causing the asphalt to harden. This hardening manifests as increased viscosity and reduced low-temperature deformation capacity, which ultimately deteriorates the pavement’s resistance to cracking.
Traditional petroleum-based rejuvenators often rely on non-renewable resources and primarily function through dilution and stiffness reduction. They have limited ability to regulate deeper issues in aged asphalt, such as enhanced polar components and asphaltene cluster aggregation. In contrast, bio-oils offer a renewable alternative. However, bio-oils from different sources vary significantly in molecular composition, meaning their rejuvenation effects are not uniform. Designing a functional rejuvenation system based on the specific compositional characteristics of these bio-oils is crucial for improving structural recovery.
The study utilized waste cooking oil (WCO) and waste wood pyrolysis oil (WWO) as the primary rejuvenating agents. WCO, collected from a restaurant in Jiangsu Province, was pretreated to remove food residues, moisture, and salts. It is primarily composed of fatty acid esters and long-chain aliphatic compounds. These flexible molecular chains are conducive to enhancing molecular mobility within aged asphalt. The WWO, a commercially available product, is a dark-brown liquid containing a greater proportion of oxygen-containing functional groups and aromatic structures compared to WCO.
The researchers prepared aged asphalt samples using the rolling thin-film oven test and the pressure aging vessel test to simulate short-term and long-term aging, respectively. They then created three types of rejuvenated asphalt: one treated with WCO alone, one with WWO alone, and one with a composite of both at a 1:1 mass ratio. The total rejuvenator dosage was fixed at 8 wt% of the aged asphalt, a level identified in previous studies as appropriate for restoring properties without compromising high-temperature stability.
Chemical composition analysis using thin-layer chromatography with flame ionization detection revealed that the composite system effectively restored the balance of SARA fractions. Compared to aged asphalt, the aromatic content of the composite-rejuvenated asphalt increased by 33.92%, while the asphaltene content decreased by 32.20%. The colloidal stability index, a measure of structural balance, increased by 22.50%, approaching the value of virgin asphalt. These changes indicate that the composite system successfully replenished light fractions and improved colloidal structural stability.
Microstructural characterization via scanning electron microscopy showed that aging caused pronounced block-like segmentation and interconnected cracks on the asphalt surface. The addition of WCO reduced the severity of these cracks, attributed to its ability to replenish light fractions and enhance molecular mobility. WWO, on the other hand, exhibited fewer cracks and a looser lamellar structure, suggesting its oxygen-containing aromatic structures weakened asphaltene aggregation. The composite system produced the most uniform and compact surface morphology, with a marked reduction in interconnected cracks.
Rheological tests confirmed that the composite rejuvenator improved both high-temperature and low-temperature performance. At high temperatures, the composite system maintained satisfactory stability while restoring viscoelastic response. The multiple stress creep recovery test showed that the composite system mitigated the excessive softening caused by WCO alone, achieving a more balanced recovery of high-temperature viscoelastic properties. At low temperatures, the bending beam rheometer test indicated that the composite system significantly reduced creep stiffness and increased the m-value, demonstrating improved resistance to thermal cracking.
Molecular dynamics simulations provided insights into the underlying mechanisms of this synergistic rejuvenation. The simulations showed that aging increased the cohesive energy density and reduced the fractional free volume, leading to compact asphaltene aggregation. The composite rejuvenator reversed these trends by weakening excessive asphaltene self-association and enhancing asphaltene-resin interactions. The flexible aliphatic chains in WCO enhanced molecular mobility, while the oxygen-containing aromatic structures in WWO improved compatibility with polar heavy fractions.
The study highlights the importance of functional differentiation in bio-oil rejuvenation. WCO primarily restores molecular mobility by replenishing light components, whereas WWO regulates the association of polar heavy fractions. Their combined use allows for simultaneous improvement of fluidity and colloidal structural stability. This approach offers a sustainable solution for utilizing waste resources in road engineering, though the specific ratio and dosage may need adjustment for different material batches and engineering applications.
The experimental baseline relied on a standardized 90-grade base binder evaluated under Chinese highway engineering specifications, exhibiting an initial penetration of 85 tenths of a millimeter at 25 degrees Celsius, a softening point of 47.1 degrees Celsius, a ductility exceeding 144 centimeters at 15 degrees Celsius, and a dynamic viscosity of 182.3 Pascal-seconds at 60 degrees Celsius. To prepare the bio-oils for incorporation, the raw restaurant waste cooking oil underwent a multi-stage physical purification sequence. The oil was preheated to 60 degrees Celsius, filtered through a 100-micrometer stainless-steel mesh to eliminate suspended debris, settled for 24 hours to separate free moisture, washed with deionized water at an equal volume ratio to eliminate residual culinary salts, and dehydrated under a vacuum of negative 0.08 megapascals at 100 degrees Celsius for two hours, yielding a purified oil with a density of 0.919 grams per cubic centimeter. The commercial pyrolysis oil derived from lignocellulosic waste sawdust and residues presented a dark brown liquid appearance, possessing a density of 0.94 grams per cubic centimeter, an acidic pH of 3.3, and an initial moisture fraction of 5.1 weight percent.
To isolate the true rejuvenating behavior from physical thinning, the research team performed mass-balance calculations on the four chemical fractions determined through thin-layer chromatography with flame ionization detection. Assuming non-reactive dilution across the 100 grams of aged binder and eight grams of rejuvenator, physical mass addition alone would have merely lowered the aged binder’s aromatic fraction from 28.3 percent to 26.20 percent and reduced asphaltenes from 20.5 percent to 18.98 percent. Instead, empirical measurements revealed that the composite system raised the aromatic content to 37.9 percent and reduced measurable asphaltenes to 13.9 percent, proving that dilution accounted for only 23.01 percent of the apparent asphaltene reduction. Qualitative infrared spectroscopy using attenuated total reflectance corroborated these distinct structural inputs. The waste cooking oil introduced characteristic ester carbonyl absorptions around 1740 to 1750 reciprocal centimeters and carbon-oxygen stretching near 1150 reciprocal centimeters, while the wood pyrolysis oil supplied broad hydroxyl responses and aromatic skeletal vibrations, both of which coexisted without destructive interference in the composite blend.
High-temperature dynamic shear rheology performed across 46 to 70 degrees Celsius demonstrated the rheological mechanisms underlying the composite modifier. Thermal aging had stiffened the binder, causing complex shear modulus increases of 71.15 percent at 46 degrees Celsius and 35.94 percent at 70 degrees Celsius alongside phase angle drops exceeding 14 percent. At the 64-degree evaluation threshold, incorporating waste cooking oil alone expanded the phase angle by 11.08 percent over aged asphalt by lubricating molecular segments, but it markedly compromised permanent deformation resistance during multiple stress creep recovery trials. Under a 3.2-kilopascal shear stress, the non-recoverable creep compliance of the single-source cooking oil mixture surged by 105.98 percent to 2.41 inverse kilopascals. In contrast, the composite blend reduced complex shear modulus by 35.66 percent compared to cooking oil alone and 30.10 percent compared to wood oil alone, while maintaining non-recoverable compliance within 7.41 percent below virgin binder values, thereby curbing excessive softening and preserving rutting resistance.
Sub-zero performance assessed via the bending beam rheometer confirmed substantial gains in thermal stress relaxation. Across test temperatures of negative 12, negative 18, and negative 24 degrees Celsius, the aged binder exhibited severe embrittlement, characterized by creep stiffness escalations of up to 132.89 percent and relaxation rate drops of up to 35.71 percent. Introducing the balanced composite rejuvenator lowered creep stiffness values by 41.03 percent, 51.87 percent, and 45.71 percent across the respective temperature steps, while elevating corresponding stress relaxation rates by 28.13 percent, 45.45 percent, and 50.00 percent relative to aged asphalt. These low-temperature improvements were achieved using a total rejuvenator dosage of eight weight percent, contrasting with previous literature where single bio-oil additions of 9 to 20 percent were typically required to restore equivalent thermal cracking resistance. All rheological measurements maintained rigorous reproducibility across three independently prepared replicate specimens.
Molecular dynamics simulations using the COMPASS II force field and twelve-component models clarified this nanoscale synergy. Thermo-oxidative aging caused the asphaltene-asphaltene radial distribution function peak to surge by 21.76 percent to 59.26, while shrinking fractional free volume by 25.00 percent. The composite system reduced this aggregation peak by 17.48 percent to 48.90, expanded free volume by 20.00 percent, and increased the asphaltene cluster radius of gyration by 3.02 percent. However, the researchers emphasized that these simulations reflect physical rearrangements rather than chemical bond breaking, and 4-ethyl-2-methoxyphenol serves as a simplified proxy for complex wood oil. Practical field deployment requires comprehensive quality screening of raw bio-oil batches, alongside mixture-scale evaluations and secondary aging trials.
Subject of Research: Synergistic rejuvenation of aged asphalt using waste cooking oil and waste wood pyrolysis oil
Article Title: Complementary rejuvenation of aged asphalt by waste cooking oil and waste wood pyrolysis oil: Experimental characterization and molecular dynamics insights
Article References: Fan, J., Zhang, Y., Zhang, J., Xie, X., & Wei, Z. (2026). Complementary rejuvenation of aged asphalt by waste cooking oil and waste wood pyrolysis oil: Experimental characterization and molecular dynamics insights. Case Studies in Construction Materials, 25, Article e06689. https://doi.org/10.1016/j.cscm.2026.e06689
Image Credits: AI Generated
DOI: 10.1016/j.cscm.2026.e06689
Keywords: asphalt rejuvenation, waste cooking oil, waste wood pyrolysis oil, molecular dynamics, rheology, construction materials, sustainability, bio-oils, Complementary, rejuvenation, aged, asphalt
News Source: Denise Maddox. (November 9, 2026). Waste Oils Restore Aged Asphalt via Complementary Molecular Roles. Scienmag.
