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Abstract

Conventional bitumen is a byproduct of petroleum refining often used in flexible pavement applications. Due to the rapid depletion of bitumen, its non-renewable nature, and its environmental and economic implications, researchers are seeking an alternative material that is naturally occurring, renewable, poses minimal risks to environmental and human health, and is cost-effective. This study explores castor oil (CO) and rice husk ash (RHA) modified bio-bitumen (CORMBB) as a sustainable alternative binder for flexible pavements. CORMBB was synthesized by partially replacing the base binder with 9 % CO, 6 % RHA, and 85% base binder (BB) and characterized through physical, chemical, structural, and rheological analyses. Physical test findings demonstrated increased penetration and decreased softening point of CORMBB, indicating greater flexibility and reduced thermal rigidity. The storage stability analysis demonstrated that CORMBB achieved difference of softening point (ΔT) values well below the limit, confirming adequate compatibility and homogeneity. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) investigations validated the successful chemical interaction between CO, RHA, and BB. They also disclosed the integration of silica from rice husk ash, enhancing structural rigidity and moisture resistance. Rheological evaluations conducted through, temperature sweep, amplitude sweep, flow sweep, and frequency sweep tests utilizing a dynamic shear rheometer (DSR) demonstrated reduced complex modulus and phase angle values for CORMBB, alongside increased viscosity at low shear rates. This indicates improved elasticity, temperature sensitivity, enhanced rutting resistance, and sustained compaction ease at elevated shear rates. These findings highlight the potential of CORMBB as a renewable and eco-friendly substitute for conventional binders in pavement applications.

Keywords

Bio-bitumen castor oil rice husk ash base binder rheology sustainable pavements

Introduction

The construction of blacktop roadway pavement, which heavily relies on natural resources, is rapidly modernizing. With the rising need for sustainable and renewable alternatives to conventional bitumen, innovations in existing technology and an explosion of novel ones will be expected. Moreover, rising crude oil prices and environmental concerns need to encourage the creation of an alternative to petroleum-based bitumen for the road construction industry in India and globally. The use of bio-bitumen as partial replacement for conventional petroleum-derived bitumen is attracting significant interest recently. Bio-bitumen is an alternative environmentally friendly alternative to conventional bitumen. Produced to reduce the carbon footprint of flexible pavement construction, it is manufactured from waste and other natural resources. Guarin et al.¹ discussed benefits of adopting bio-bitumen instead of conventional bitumen include it being a renewable resource in nature, ecologically benign, less effect on human health, and greater efficiency than conventional bitumen. With second-largest road network worldwide behind the United States, India has vast network of conventional bitumen highways is mostly composed of India has around 6.33 million kilometers of road as of December 2022,² of which roughly 92% were built using conventional bitumen. But environmental issues, changing costs, and crude oil depletion are making the dependence on conventional bitumen even more untenable. According to the Ministry of Road Transport and Highways (MORTH),² the nation needs over 8 million tonnes of bitumen yearly, with local refineries supplying about 5 million tonnes which results in significant imports. With these imports ranging from $3.01 to $3.61 million USD, the infrastructure budget of the nation is under financial strain. Finding sustainable substitutes for bitumen is thus crucial as its growing cost has also raised the general cost of road building. India and other nations are investigating bio-bitumen as a reasonably affordable and sustainable substitute to handle the above stated difficulties. In this regard, bitumen modification prospects for bio-based components like castor oil (CO) and agricultural waste like rice husk ash (RHA) have become very attractive. While RHA, rich in silica, offers the further benefits of utilizing waste and reinforcing the material, CO, a renewable resource with unique chemical properties, has demonstrated it may improve the performance of bitumen.

Castor oil, derived from castor seeds, is a promising bio-oil due to its high triglyceride and hydroxyl content, which enhances the flexibility and lower temperature properties, while reduces viscosity of bio-bitumen.³ As the global leader in CO production, India produces over 1.8 million metric tonnes annually, accounting for approximately 90% of the world supply.⁴ This dominance positions India as a key player in bio-bitumen driven from CO applications. CO has high hydroxyl content improves bonding with conventional bitumen matrix, enhancing resistance to oxidative aging, while its long chain fatty acids contribute viscosity and stability, ensuring durability under varying climate conditions.⁵ Other hand RHA, a byproduct of burning rice husk, is another material with potential for asphalt enhancement. With global rice husk production exceeding 150 million tonnes annually, India contributes approximately 23 million tonnes due to its extensive rice cultivation.⁶ RHA is rich in silica (SiO₂), constituting 92%–95% of its composition, which imparts pozzolanic properties, making it valuable for pavement application.⁷ Utilizing RHA also addresses agricultural waste management challenges, such as improper disposal through burning or dumping, which contributes to pollution. The existing study highlights the environmental benefits of RHA utilization: for example, During et al.⁸ pointing out that utilizing half of annually produced rice husk in cement production industry in the Nepal, it could reduce CO₂ emissions by 808,000 tonnes annually. In addition, RHA has also utilized in road applications: for example, Abdelmagid et al.⁹ utilized RHA in road application and suggested 7.6% RHA as optimal for improving penetration and viscosity, while Arbani and Tahami¹⁰ cautioned against exceeding 15% due to risk of excessive stiffness.

Several studies have demonstrated the potential of bio-bitumen derived from various renewable resources, including: soybean oil,^11,12 wood waste,¹³ waste cooking oil,^14,15 waste vegetable oil,^16–18 and others^19–21 to replace natural bitumen in pavement engineering partially or entirely. These studies have shown that bio-bitumen can improve the performance of pavement materials, enhancing properties such as cracking resistance,²² durability,²³ rutting,²⁴ and moisture damage prevention.^25,26 For instance, bio-bitumen derived from waste cooking oil has been found to improve resistance to cracking and rutting, while bio-bitumen from wood waste and pinewood shows improved durability and elasticity, making it suitable for use in various climatic conditions. Bio-bitumen offers several environmental benefits^27–29 over traditional petroleum-based bitumen. It is produced from renewable resources, reducing dependency on fossil fuels and promoting waste management by utilizing materials like waste cooking oil and wood waste. The use of bio-bitumen can also significantly lower greenhouse gas emissions,³⁰ contributing to global efforts to mitigate climate change. From a performance perspective, bio-bitumen has shown improved resistance to fatigue cracking, durability, and elasticity, making it a promising alternative for road construction. For example, bio-bitumen derived from sugarcane bagasse²⁸ and waste cooking oil¹⁵ has demonstrated enhanced flexibility, reducing the likelihood of cracking under heavy loads and temperature fluctuations. These properties make bio-bitumen a viable alternative for regions with diverse climate conditions, including both high-temperature and cold-weather environments. However, the bio-bitumen industry is still in its nascent stages, requiring further research and technological advancements to optimize production processes and improve the performance of bio-bitumen binder and mixes. The performance of bio-bitumen can vary depending on the type of biomass used, which could affect its reliability and consistency in large-scale applications.

Furthermore, recent studies also support the application of bio-based materials in asphalt binder systems. Zeng et al.³¹ studied on CO-based bio-bitumen, highlighting enhanced penetration, while reduced softening point and ductility, indicating a softening effect. Further, Zeng et al.³² evaluated waste cooking oil and palm oil-based bio-rejuvenators, showing enhanced low temperature performance such as fatigue cracking resistance in rolling thin film oven test (RTFOT) aged binder. Peng et al.³³ explored CO and vegetable oil-based polyurethane modified bio-bitumen, highlighting enhance high temperature performance such as rutting resistance when both oils optimized appropriately, while excessive content may lead to reduced strength. Authors, utilizing response surface methodology to determine the optimal content of CO and vegetable oil in modified bio-bitumen. Complementing these findings, Peng et al.³⁴ conducted a comparative study on various bio-oils, including castor, soybean, straw, gutter, and vegetable oils, concluding that castor and straw oils offer excellent permanent deformation resistance, while gutter and vegetable oils contribute to low-temperature cracking resistance. Zhang et al.³ further investigated castor oil-based bio-asphalt (COBA), revealing that a 5% bio-oil addition balances improved flexibility and moisture resistance with minimal loss in high-temperature performance. In a broader review, Zhang et al.³⁵ emphasized that while bio-oils hold potential in enhancing sustainability, challenges remain in achieving long-term compatibility and performance equivalent to conventional binders.

In addition, recent advances in sustainable pavement materials have highlighted the potential of bio-oils and agricultural wastes as alternative modifiers to reduce reliance on fossil-based bitumen, yet also revealed critical trade-offs that necessitate careful optimization. For example, olive kernel ash (OKA) has been shown to significantly enhance rutting resistance and fracture energy, though excessive dosages (>10 %) reduce low-temperature flexibility,³⁶ while bio-oils derived from spent coffee grounds (SCG) exhibit contrasting performance depending on extraction pathways—pyrolysis oils improving low-temperature flexibility and fatigue resistance, and hydrothermal oils improving high-temperature deformation resistance.³⁷ Similarly, rice bran oil (RBO) used as a rejuvenator increases flexibility but exacerbates rutting susceptibility unless coupled with nano-CaO, where the hybrid system reduces rutting by 13–34 % and restores balance across temperature regimes.²⁴ These findings, supported by other recent studies,^38–40 collectively emphasize that while bio-oils offer environmental and low-temperature benefits, their integration with secondary modifiers—whether polymers, nanoparticles, or siliceous ashes—is crucial for achieving multi-temperature performance and durability.

Additionally, the performance properties of asphalt binders and mixtures can be enhanced using conventional modifiers, including polyphosphoric acid (PPA), warm mix asphalt (WMA) additives,⁴¹ and styrene–butadiene rubber (SBR). The current study evaluates the combined effects of PPA and WMA content on the rheological behavior of SBR-modified binders and stone matrix asphalt S(MA) mixtures.⁴² Modified binders were tested using rotational viscosity, dynamic shear rheometer (DSR), bending beam rheometer (BBR), multiple stress creep recovery (MSCR), and linear amplitude sweep (LAS). The addition of PPA significantly increased the fatigue life of SBR-modified binders. Moreover, SBR and PPA improved rutting resistance, and fatigue endurance of mixtures. However, despite these promising directions, a clear research gap remains in exploring the synergistic interaction of bio-oils with agricultural silica-based ashes within a single hybrid system, particularly with systematic dosage optimization and chemomechanical validation. Addressing this gap, the present study investigates castor oil (CO) combined with rice husk ash (RHA) as a hybrid bio-modifier (CORMBB), leveraging CO’s polar functional groups to improve adhesion and flexibility and RHA’s silica-rich reinforcement to enhance stiffness, moisture resistance, and aging stability.

This study explores the potential of castor oil (CO) and rice husk ash (RHA) as partial replacements for conventional bitumen with the aim of enhancing pavement sustainability, reducing reliance on non-renewable resources, and improving cost-effectiveness. While bio-oils and mineral ashes have been individually investigated, there is a distinct lack of research on the combined effects of CO and RHA in bio-bitumen, particularly with respect to their physical, chemical, structural, and rheological performance. To address this gap, the present work represents the first systematic evaluation of the synergistic influence of CO and silica-rich RHA on bitumen, emphasizing their complementary roles in improving binder characteristics for long-term pavement applications. The objectives are: (i) to develop a sustainable binder by partially substituting VG-30 bitumen with castor oil and rice husk ash modified bio-bitumen (CORMBB), (ii) to characterize its physical, chemical, and rheological properties using standardized test methods, and (iii) to compare the performance of CORMBB against conventional binders to assess its suitability for flexible pavement applications. Figure 1 represents the overall flow chart of present study.

Figure 1.

The overall flaw chart of this study.

Materials and methods

Materials

The conventional bitumen/base binder (BB) used in this study is Viscosity Grade-30 (VG-30) bitumen sourced from Indian Oil Corporation Limited (IOCL) Barauni, Bihar, India. Table 1 presents the basic properties of the BB. RHA is sourced from a local flatted rice mill in Khajauli, Madhubani, Bihar. The characteristics of RHA are summarized in Table 2. Before testing, RHA underwent sieve analysis (Figure 3(a)) to check and ensure an even particle size distribution suitable for modifying bio-bitumen. CO, derived from castor seeds, was obtained from a local oil mill in Khutauna, Madhubani, Bihar.

Table 1.

Technical characteristics of BB.

Parameters	Code used	Results	Specification as per IS:73-2013⁴³
Penetration at 25°C, 0.1 mm, 5 sec	IS: 1203	70.5	>45
Absolute viscosity at 60°C, poises	IS: 1206 (P-2)	2556.7	2400-3600
Softening point, °C	IS: 1205	49.5	>47
Specific gravity	IS: 1201	1.013	NA
Flash point, °C	IS: 1448 (P-69)	242	>220
After thin rolling film oven test (TFOT)
Ductility at 25°C, cm	IS: 1208	83	>40
Mass Loss, %		0.982	<1

Table 2.

Properties of RHA.

Parameters	Code used	Results
Specific gravity	IS: 1201⁴⁴	1.65
Moisture content, %	Oven dry method	0.42
Silica (SiO₂), %	IS: 1527⁴⁵	94.47
Iron oxide (Fe₂O₃), %		1.06
Alumina (Al₂O₃), %		0.89
Magnesia (MgO), %		1.03
Fineness modulus, %	IS: 1727⁴⁶	3.32

Preparation of CORMBB

Castor oil and rice husk ash dosage selection

The dosage levels of 9 % CO and 6 % RHA were adopted based on an earlier optimization study Kumar and Suman,⁴⁷ in which Response Surface Methodology (RSM) with Central Composite Design (CCD) was employed to identify the statistically optimum formulation. Multiple responses including penetration, softening point, specific gravity, Marshall stability, indirect tensile strength, tensile strength ratio, and Cantabro loss were modeled using quadratic regression equations validated by ANOVA. A multi-response desirability approach indicated that 9 % CO and 6 % RHA represented the optimum compromise, balancing workability with enhanced rutting and moisture resistance. Therefore, this composition (CORMBB) has selected for present study.

Preparation of CORMBB

The preparation of CORMBB followed a controlled sequence to ensure consistency and reproducibility. Initially, the base binder (BB) was heated to 130 ± 2°C for 60 minutes to achieve uniform fluidity. Castor oil (9% by weight of BB) was gradually introduced and mixed using a mechanical stirrer at 1500 r/min for 10 minutes to achieve preliminary blending. The mixture was then subjected to high-shear mixing at 1500 r/min at 130 ± 2°C for 20 minutes, producing castor oil modified bio-bitumen (COMBB). Subsequently, rice husk ash (6% by weight of BB, pre-dried at 105°C for 24 h to remove moisture) was slowly added to COMBB in small increments to avoid agglomeration, followed by stirring at 1500 r/min for 15 minutes. A final high-shear mixing step at 1500 r/min and 150 ± 2°C for 20 minutes ensured complete homogenization, resulting in the CO and RHA modified bio-bitumen (CORMBB), as shown in Figure 2. The blended binders were poured into sealed metal containers, cooled to ambient laboratory temperature (25 ± 2°C), and stored in airtight conditions to prevent premature oxidation prior to testing. Short-term aging was simulated using the rolling thin film oven test (RTFOT) in accordance with ASTM D2872. These controlled mixing and storage conditions, adopted from Kumar and Suman⁴⁷ and Guo et al.⁴⁸ respectively, ensured uniform dispersion of modifiers without thermal degradation, thereby allowing the study to isolate the specific effects of CO and RHA on binder performance.

Figure 2.

Schematic diagram illustrating the preparation process of CORMBB.

Methods

Physical tests

Penetration, softening point, mass loss, and specific gravity

The performance of CORMBB was evaluated using standard binder characterization tests. Penetration was determined at 25°C in accordance with IS 1203 (1978),⁴⁴ with three replicate measurements conducted for each sample to ensure repeatability. The softening point, indicative of thermal stability, was measured using the ring-and-ball method as per IS 1205 (1978),⁴⁴ also with triplicate testing for consistency. Short-term aging was simulated using the rolling thin film oven test (RTFOT), carried out at 163°C for 85 minutes following ASTM D2872⁴⁹ and AASHTO T240.⁵⁰ Mass loss during RTFOT was measured in triplicate to confirm repeatability and to quantify the evaporation of volatile fractions.

Storage stability

The storage stability of the formulated binders, including the base binder (BB) and the castor oil and rice husk ash modified bio-bitumen (CORMBB), was evaluated in accordance with the ASTM D7173 protocol. Approximately 50 g of binder was placed in a cylindrical metal tube (25 mm diameter, ∼137 mm length) with a sealed base. The tube was covered at the top with aluminum foil to minimize volatilization during conditioning. The sealed tubes were subjected to thermal storage in a hot-air oven at 163 ± 5°C for 48 h. Following conditioning, the tubes were refrigerated at −10°C for 4 h to enable clean sectioning. Each tube was then cut into three equal parts, and binder specimens from the top and bottom sections were collected for testing. The softening points of these sections were determined, and the difference (ΔT) was calculated as an indicator of storage stability, with values below 2.5°C considered acceptable. To ensure repeatability, all tests were performed in triplicate, and the mean values along with standard deviations are reported.

Chemical characterization test

Fourier transform infrared spectroscopy (FTIR) test

FTIR can analyze the chemical interaction among the castor oil, RHA, and base binder and identify the compounds (functional groups) of substances according to the relative vibration and rotation of molecules. In the present study, the FTIR test is conducted on castor oil, base binder (VG-30) and modified bio-bitumen binder (CORMBB); it has been characterized using the KBr compression method with a Nicolet iS05 FTIR instrument at the Central Research Facility, Indian Institute of Technology (IIT) Delhi, India. The samples are subjected to 32 scans across the wave number range of 400-4000 cm⁻¹, employing a spectrometer resolution of 4 cm⁻¹. The functional group has been identified and analyzed to assess the microstructural changes caused by incorporating RHA and CO into the bitumen matrix. For each binder, three replicates were analyzed, and the mean spectra with ±1 SD envelopes are reported. Furthermore, FTIR spectra of BB and CORMBB were processed using baseline correction and Gaussian peak fitting to isolate functional group regions. The integrated peak areas were normalized by the aliphatic C–H band, and characteristic indices—hydroxyl (A_O-H/A_C-H), carbonyl (A_{C = O}/A_C-H), aromatic (A_{C = C}/A_C-H), and silicate (A_Si–O–Si/A_C-H)—were computed to quantify chemical modifications induced by castor oil and RHA.

X-ray diffraction (XRD) test

The XRD analysis for RHA, base binder, and optimized bio-bitumen binder is conducted at the Centre of Interdisciplinary Research, Motilal Nehru National Institute of Technology (MNNIT), Allahabad (Prayagraj), Uttar Pradesh, India. The samples are finely ground and analysed using an X-ray diffractometer equipped with a Cu-Kα radiation source (wavelength = 1.5406 Å). The scanning has been performed over a 2θ range of 10° to 80° with a scan resolution of 0.0002° and a scanning speed of 10°/min. The diffraction patterns are recorded using Ka radiation to identify the crystalline and amorphous phases in the samples. The peaks have been analysed to assess the structural changes caused by the incorporation of RHA and CO into the bitumen matrix, indicating the degree of crystallinity.

Rheological characterization tests

Rheological characterization was performed to analyse the rheological performance of BB and CORMBB. A DSR (according to AASHTO T315⁵¹) was utilized to conducts frequency sweep tests, amplitude sweep tests, and flow sweep tests on BB and optimized CORMBB. The frequency sweep test was conducted at a temperature of 20°C–90°C (with 10°C intervals) with frequency varying 0.1 to 200 rad/s (0.1Hz–31 Hz). An 8 mm diameter spindle with a 2 mm plate gap was used for the temperature range 20°C–30°C; between 40°C and 90°C, a 25 mm diameter with a 1 mm plate gap was utilized. Master curve of complex modulus (G*) were conducted utilizing time-temperature superposition principal (TTSP)⁵² at reference temperature of 30°C, 40°C, 50°C, and 70°C respectively. A Williams-Landel-Ferry (WLF)⁵² equation was used to calculate shift factors and a Sigmoidal model (SM)⁵² equation was used to model the data. Equations (1)–(3) shows mathematical representation of reduced frequency (f_r) equation, WLF equation, and SM model, respectively.⁵² Figure 3 represents the WLF fitted model at the reference temperature of 30°C. The amplitude sweep test was performed at a temperature of 30°C, and at fixed frequency of 1 Hz with oscillating strain varying 1%-100%, to determine the linear viscosity region of both binders and access damage resistance, stiffness, and elasticity. A 25 mm parallel plate geometry with 1 mm gap was used. The complex modulus (G*), phase angle (δ), rutting parameter (G*/sin δ), and complex viscosity (η*) were recorded under increasing strain amplitude. The flow sweep test was employed at a temperature of 60°C-90°C (with 10°C intervals) with shear rate (1/s) varying 0.001-1000, to access shear-thinning behavior of both binders. A 25 mm diameter with 1 mm plate gap was used. The complex viscosity (η*) was recorded with increasing shear rate. Furthermore, temperature sweep tests were conducted on both BB and CORMBB under unaged and aged conditions. Short-term aging was simulated using the rolling thin film oven test (RTFOT) in accordance with ASTM D2872. The temperature sweep was carried out over a range of 46°C to 82°C, at increments of 6°C, using a constant angular frequency of 10 rad/s (1.59 Hz). For each condition, the complex modulus (G*) and phase angle (δ) were measured, and the rutting parameter (G*/sin δ) was calculated to assess high-temperature performance. All rheological and performance tests were conducted on binders subjected to appropriate aging protocols according to AASHTO T315.⁵¹

\log (f_{r}) = \log (f) - \log (a_{T})

(1)

Where f is the actual frequency and a_T is the shift factor at temperature T.

\log (a_{T}) = - \frac{C_{1} (T - T_{r})}{C_{2} + (T - T_{r})}

(2)

Where T_r is the reference temperature and C₁ and C₂ are the empirical calculated constants.

Log (G^{*}) = δ + \frac{α}{1 + \exp [β + γ . \log f_{r}]}

(3)

Where G* is the complex modulus, f_r is the reduced frequency, and

δ

α, β, γ

are model parameters calculated via non-linear regression utilizing the Levenberg-Marquardt algorithm.

Figure 3.

WLF model fitted curve at reference temperature of 30°C.

Results and discussion

Physical test results

Penetration, softening point, mass loss, and specific gravity

The data illustrated in Figure 4 provide a comparative analysis of the physical properties of BB and CORMBB. The penetration of CORMBB is found to be 97.5 (0.1 mm at 25°C), which is approximately 27.69% higher than BB 70.5 (0.1 mm at 25°C), as illustrated in Figure 4(a), suggesting that it is a softer material. The observed phenomenon can be attributed to the softening effect of CO, which generally decreases binder stiffness and enhances flexibility at standard temperature. Conversely, the softening point (illustrated in Figure 4(b)) of CORMBB is found to be 40.5°C, which is approximately an 18.18% reduction compared to BB (40.5°C). This indicates a decrease in resistance to deformation at elevated temperatures, a phenomenon frequently observed with the introduction of CO. Evaluating the balance between enhanced low-temperature performance and diminished high-temperature stability for particular pavement applications is essential.

Figure 4.

The physical properties of BB and CORMBB: (a) penetration, (b) softening point, (c) mass loss, and (d) specific gravity.

The results regarding mass loss were also promising. CORMBB exhibited a lower value of 0.631% compared to BB, which had a value of 0.982%, as illustrated in Figure 4(c). This suggests improved thermal stability during the aging process. This may be associated with the RHA, potentially functioning as a stabilizer or contributing to inhibiting oxidation. The specific gravity of CORMBB is found to be 1.038, exhibiting a slight increase of 1.66% compared to BB, which has a specific gravity of 1.021, as illustrated in Figure 4(d). This increase can likely be linked to the mineral content provided by the RHA. While it is improbable that performance will be significantly impacted, it may influence mixed design calculations. The findings regarding the physical properties indicate that the CORMBB improves flexibility and aging resistance, albeit with a trade-off in thermal stiffness. The equilibrium among these properties will be contingent upon the specific application of the binder.

Storage stability

The storage stability results (Table 3) confirm that both BB and CORMBB exhibit softening point between upper and lower part (ΔT) values well below the ASTM D7173 limit of 2.5°C, demonstrating adequate compatibility. Mechanistically, the slightly higher ΔT observed for CORMBB (0.50°C) compared to BB (0.23°C) is explained by the complex interplay between castor oil and RHA: castor oil, being a low-viscosity bio-oil, softens the matrix and enhances molecular mobility, while the silica-rich RHA provides structural reinforcement that resists sedimentation of heavier fractions. This dual action minimizes phase separation, maintaining uniformity even under thermal storage. Previous studies have highlighted similar trends. For example, Zhang et al.¹⁵ demonstrated that vegetable oil–modified binders require secondary stabilizers such as mineral ash to maintain phase homogeneity during prolonged storage. Likewise, Abdelmagid et al.⁵³ reported that optimized bio-oil/ash composites achieved ΔT values below 2.5°C, with RHA contributing to both chemical compatibility and physical stabilization. These consistent findings validate that the optimized CO–RHA combination in CORMBB offers sufficient storage stability and homogeneity, ensuring its practical viability in paving applications.

Table 3.

The storage stability test results.

Binder	Section	Trial 1 (°C)	Trial 2 (°C)	Trial 3 (°C)	Mean (°C)	SD (°C)
BB	Upper	52.8	53.0	52.9	52.9	0.10
BB	Lower	52.6	52.8	52.6	52.6	0.12
ΔT _BB	-	0.19	0.20	0.29	0.23	0.06
CORMBB	Upper	51.0	50.9	49.9	50.6	0.61
CORMBB	Lower	50.1	50.2	50.0	50.1	0.10
ΔT _CORMBB	-	0.89	0.69	−0.10	0.5	0.53

Chemical characterization

FTIR results

Fourier Transform Infrared (FTIR) spectroscopy was performed to characterize the chemical composition of the base binder (BB) and CORMBB. The spectra (Figure 5) revealed clear modifications in the functional groups after incorporation of 9 % CO and 6 % RHA. The broad peak at 3440 cm⁻¹ (O–H stretching) intensified due to hydroxyl groups in castor oil, enhancing polarity and binder–aggregate adhesion.⁵⁴ Strong absorptions at 2923 and 2885 cm⁻¹ (C–H stretching) correspond to aliphatic hydrocarbons from triglycerides, improving compatibility and workability. Peaks at 1460 and 1370 cm⁻¹ (CH₂/CH₃ bending) reflected increased hydrocarbon content, enhancing flexibility and crack resistance, while the 723 cm⁻¹ band confirmed long-chain hydrocarbons, contributing to ductility and fatigue resistance.⁵⁴ Castor oil often introduces polar functional groups such as hydroxyls (–OH), carbonyls (C = O), esters, and sometimes acids. These increase binder polarity, enhance hydrogen-bonding with both bitumen matrix (asphaltenes/resins) and with filler surfaces (e.g. silica in ash), improving adhesion and reducing moisture damage. For example, Hassanjani et al.⁵⁵ showed that bio-oil extraction method (pyrolysis vs hydrothermal) impacts the distribution of these polar groups; higher polarity led to higher temperature and deformation resistance when combined with mineral modifier.

Figure 5.

FTIR result of base binder (VG-30) grade bitumen and modified bio-bitumen.

A distinct carbonyl peak at 1746 cm⁻¹ (C = O stretching) was more pronounced in CORMBB, validating ester linkages formed during modification, whereas the 1220–1160 cm⁻¹ region (C–O stretching) exhibited variations, indicating consumption or rearrangement of ester/ether groups that improve oxidation resistance. Peaks at 1340–1370 cm⁻¹ (C–H bending), 1020–1070 cm⁻¹ (Si–O stretching from RHA), and 865 cm⁻¹ (aromatic C–H bending) remained largely unchanged, confirming retention of the silica network and the aromatic backbone of the base binder. To provide quantitative confirmation, Gaussian peak deconvolution was applied and data is represented in Table 4. The hydroxyl index (A_O–H/A_CH) decreased from 0.689 (BB) to 0.366 (CORMBB), suggesting hydroxyl involvement in esterification. The carbonyl index (A_{C = O}/A_CH) fell sharply from 0.069 to 0.014, indicating consumption/rearrangement of carbonyl groups and improved oxidation stability. The aromatic index (A_{C = C}/A_CH) also reduced slightly (0.027 to 0.014), while the silicate index (A_Si–O–Si/A_CH) remained of the same order (0.025 to 0.016), confirming stable RHA contribution. These quantitative indices confirm that CORMBB undergoes chemical modifications (esterification, hydroxyl interaction, silicate integration) while preserving the hydrocarbon framework, providing a mechanistic basis for improved adhesion, flexibility, and durability.

Table 4.

FTIR peak assignments and quantitative analysis of BB and CORMBB (Areas from gaussian fits after baseline correction; ratios normalized by C–H area).

Functional group	Wavenumber (cm⁻¹)	Peak area (BB)	Peak area (CORMBB)	Ratio BB A/A_CH	Ratio CORMBB A/A_CH	Remarks
O–H stretching (hydroxyl)	∼3430-3440	0.458	0.240	0.689	0.366	Enhanced hydroxyl content from castor oil; improves polarity and binder–aggregate adhesion
C–H stretching (aliphatic)	2920-2885	0.665	0.657	1.000	1.000	Long-chain hydrocarbons from castor oil triglycerides; improves compatibility and workability
C = O stretching (carbonyl/ester)	∼1734-1746	0.046	0.009	0.069	0.014	New ester linkages from CO incorporation; enhances oxidation resistance and aging stability
C = C stretching (aromatic)	∼1629-1600	0.018	0.009	0.027	0.014	Modified aromatic interactions; contributes to structural stability
C–H bending (CH₂/CH₃)	∼1460	-	-	-	-	Increased hydrocarbon flexibility; improves crack resistance at low temperatures
CH₂ rocking (long-chain hydrocarbons)	∼723	-	-	-	-	Confirms additional alkyl chains; enhances ductility and fatigue resistance
Si–O–Si stretching (silica, RHA)	∼1070, 1020	0.017	0.011	0.025	0.016	Confirms mineral contribution of RHA; aids rigidity and thermal
Aromatic C–H bending	∼865	-	-	-	-	Unchanged aromatic backbone of VG-30 binder

XRD results

X-ray diffraction (XRD) analysis was conducted to examine the crystallinity and amorphous nature of the base binder and the modified bio-bitumen binder. Figure 6(a) illustrates the XRD analysis of RHA, revealing that the ash is primarily in an amorphous form, as evidenced by a broad peak at a 2θ angle of 22°, confirming the presence of amorphous silica.⁷ Alongside amorphous silica, crystalline phases such as silica oxide (S) (dominant), diaspore (D), magnetite (M), alumina oxide-K (A), lime (L), and periclase (P) were also identified. The high silica content of RHA enhances stiffness and flow resistance in COMBB, while the secondary phases contribute to durability, adhesion, and chemical resistance. Figure 6(b) presents the XRD analysis of the BB (VG-30) and CORMBB. The VG-30 binder exhibits a predominantly amorphous structure with low-intensity diffraction peaks, characteristic of unmodified bitumen. However, the optimal-CORMBB shows pronounced crystalline peaks between 15° and 30° (2θ angle), indicating the successful incorporation of crystalline silica (SiO₂) from RHA. The increased crystallinity of the modified binder enhances structural rigidity, contributing to improved mechanical performance. Additionally, CO acts as a bio-modifier, improving compatibility and dispersion of RHA particles within the binder matrix, creating a more structured and uniform network.

Figure 6.

XRD results of (a) RHA and (b) BB and CORMBB.

Rheological behavior

Temperature sweep test

The temperature–rheology response of BB and CORMBB (Figure 7) reveals clear differences in stiffness and viscoelastic balance, both before and after aging. The complex modulus (G*) declined with temperature for all binders, yet CORMBB consistently showed lower| values than BB in both aged and unaged states, confirming higher flexibility; aging increased G* in both cases due to oxidative hardening, but the magnitude of stiffening was smaller in CORMBB, indicating improved aging resistance. Similarly, phase angle (δ) rose with temperature, but CORMBB maintained a lower δ than BB, reflecting greater elasticity and rutting resistance. Mechanistically, castor oil, rich in long-chain triglycerides, softens the binder matrix by disrupting asphaltene–maltene associations, while RHA provides a siliceous micro-filler skeleton that adsorbs polar fractions and counteracts excessive softening, leading to a more balanced rheology. This synergistic “softener + reinforcer” effect justifies why CORMBB retains viscoelastic equilibrium after aging. Recent studies corroborate these findings: bio-oil modifiers lower binder stiffness but enhance workability and fatigue life, while secondary inorganic additives restore rigidity and mitigate oxidative aging, as observed in rice-bran oil/nano-CaO systems,⁵⁶ waste cooking oil,¹⁵ and hybrid bio-oil.⁴⁸ These reports, together with the present results, confirm that hybridized bio-bitumen strategies not only balance rutting and fatigue performance but also improve aging durability, making CORMBB a viable sustainable alternative to conventional binders.

Figure 7.

The complex modulus and phase angle of the BB and the CORMBB (both unaged and aged): (a) the complex modulus of BB and CORMBB and (b) the phase angle of BB and CORMBB.

Rutting parameter

The rutting parameter (G*/sin(δ)) values of both binders decrease monotonically with temperature, reflecting thermo-rheological softening and the growing viscous contribution at higher temperatures—consistent with the trends in Figure 8 where G* falls and δ rises with temperature. As expected, RTFOT-aged specimens exhibit higher G*/sin(δ) than their unaged counterparts for both BB and CORMBB, because aging simultaneously increases G* and reduces δ (Figure 7), thereby elevating the rutting parameter. At a given temperature, aged CORMBB shows higher G*/sin(δ) than BB (unaged and aged), indicating better high-temperature rutting resistance after short-term aging. For the unaged condition, CORMBB maintains a higher G*/sin(δ) than unaged BB up to ∼65°C, after which the curves converge, implying that the CO-driven plasticization begins to dominate over the RHA micro-filler reinforcement at the highest test temperatures. Mechanistically, this behavior aligns with the combined evidence: (i) FTIR indices (reduced carbonyl and aromatic indices; stable silicate index) indicate chemical adjustments and silicate integration that temper oxidative stiffening while preserving network integrity; (ii) in Figure 7 CORMBB exhibits lower δ (more elastic response) at comparable temperatures, which raises G*/sin(δ) even when G* is modestly lower; and (iii) RHA’s silica-rich, high-surface-area particles provide a load-bearing micro-skeleton and adsorption sites that curtail flow under shear, particularly evident after RTFOT when polar oxidation products would otherwise promote viscous dissipation in BB. Collectively, these results indicate that CO + RHA synergy shifts the rutting–fatigue trade-off favorably at service temperatures: CORMBB retains elasticity (lower δ) and limits aging-induced softening, thereby sustaining a higher G*/sin(δ) than BB in the critical temperature window. In practical terms, and in line with AASHTO M320 high-temperature criteria (unaged ≥1.0 kPa; RTFOT-aged ≥2.2 kPa) shown in the Figure 8, the observed elevations in G*/sin(δ) for aged CORMBB suggest greater margin against rutting at the same grade temperature, while the convergence of unaged curves above ∼65°C flags the upper bound where CO plasticization begins to offset RHA reinforcement—a dosing and grade-selection consideration for field application.

Figure 8.

The rutting parameter (G*/sin(δ)) of BB (before short-term aging and after short-term aging) and CORMBB (before short-term aging and after short-term aging).

Amplitude sweep test

The current study conducts an amplitude sweep test at 30°C and 1 Hz using DSR testing, assessing the complex modulus (G*), phase angle (δ), rutting parameter (G*/sin δ), and complex viscosity (η*) while varying oscillation strain (%) from 0 to 100, and compares the viscoelastic properties of BB and CORMBB. Figure 9 presents the amplitude sweep test results of the BB conducted at 30°C and 1 Hz, illustrating the variation of (a) complex modulus (G*), (b) phase angle (δ), (c) rutting resistance parameter (G*/sinδ), and (d) complex viscosity (η*) with oscillatory strain. In Figure 9(a), the binder displays a plateau in G* up to approximately 5% strain, characterizing the linear viscoelastic region (LVER). Beyond this threshold, G* begins to decline significantly, signalling the initiation of structural degradation and transition into the Nonlinear Viscoelastic Region (NLVER). Correspondingly, the phase angle (δ) in Figure 9(b) gradually increases with strain, indicating a shift towards more viscous behavior, thus confirming viscoelastic nonlinearity onset.

Figure 9.

Amplitude sweeps test results of BB at 30°C and 1 Hz: (a) complex modulus, (b) phase angle, (c) rutting parameter, and (d) complex viscosity.

The rutting parameter (G*/sinδ), as shown in Figure 9(c), decreases markedly after the LVER, with the fitted curve achieving an R² = 0.98, signifying a reliable predictive trend in performance reduction. Additionally, the complex viscosity (η*), illustrated in Figure 9(d), also decreases with increasing strain, further confirming reduced resistance to deformation under shear stress. These results align with findings by Zhang et al.³ and Guarin et al.,¹ who noted that similar degradation trends in unmodified bitumen arise from polymer chain disentanglement and microstructural breakdown under increased strain amplitudes. Such nonlinear viscoelastic responses under cyclic loading are critical for characterizing a rutting potential of and long-term performance of binder in pavement structures.⁵⁷ Figure 9 demonstrates the amplitude sweep test outcomes for CORMBB, showing how (a) complex modulus (G*), (b) phase angle (δ), (c) rutting resistance parameter (G*/sinδ), and (d) complex viscosity (η*) vary with oscillatory strain. In contrast to the BB (Figure 10), the CORMBB exhibits a broader LVER, as seen in Figure 10(a), spanning two distinct ranges—indicating improved flexibility and enhanced tolerance to deformation without microstructural damage.

Figure 10.

Amplitude sweeps test results of CORMBB at 30°C and 1 Hz: (a) complex modulus, (b) phase angle, (c) rutting parameter, and (d) complex viscosity.

However, the sharper drops in G* beyond ∼1% strain signify a quicker transition into the NLVER. This suggests an early onset of structural breakdown, possibly due to reduced cohesive interactions caused by the addition of CO and RHA. Figure 10(b) reveals a steep rise in δ, reaching values above 85°, highlighting a pronounced transition toward viscous behavior, likely influenced by the plasticizing effect of castor oil, which reduces intermolecular friction and cohesion. Figure 10(c) shows a notable decline in the rutting parameter G*/sinδ after the LVER, with a relatively low regression coefficient R² = 0.78, indicating a more irregular performance pattern than the BB. This variability may stem from microstructural heterogeneity introduced by CO and RHA components. Additionally, Figure 10(d) shows a steeper decline in (η*), reflecting a greater sensitivity to strain, which can be attributed to the lower network density and softening effect of the CO-rich matrix. Mechanistically, the polar groups in CO disrupt intermolecular interactions within the BB. At the same time, the pozzolanic RHA particles contribute stiffness at low strain levels but may act as stress concentrators at higher strains. This dual-phase behavior is consistent with prior literature,^1,3 emphasizing the balance between softening and reinforcing contributions in bio-modified binders. The CORMBB binder demonstrates enhanced elasticity at low strains and increased viscous dissipation under large deformations. It is suitable for applications demanding greater flexibility and crack resistance,⁵⁸ though potentially less rutting resistance than VG-30 bitumen (BB).

Flow sweep test

Figure 11 illustrates the flow sweep test results for the base binder (BB) and castor oil and rice husk ash modified bio-bitumen (CORMBB), including a temperature range of 60°C-90°C. This test provides critical insights into the complicated viscosity characteristics of binders at varying shear rates a range of 0.001 s⁻¹–1000 s⁻¹, emulating the mixing and compaction conditions seen in pavement construction. At 60°C, both BB and CORMBB exhibit shear-thinning behavior, as seen by a progressive decrease in complex viscosity with an increase in shear rate. Nonetheless, CORMBB exhibits much greater viscosity at low shear rates than BB. The heightened resistance to flow may be ascribed to the synergistic effects of castor oil, which promotes network formation via polar contacts, and rice husk ash, which serves as a stiffening agent. These results correspond with a previous study by Kumar et al.⁵⁹ transformed a VG-30 base binder with Mesua Ferrera seed cover waste, evaluating the viscosity of bio-bitumen at different bio-oil contents and finding that viscosity decreased as bio-oil content increased. This trend was also observed by Sun et al.,⁶⁰ who found that waste cooking oil reduced the viscosity of a 40/60 penetration grade binder, enhancing its workability at lower temperatures. As the temperature rises to 70°C, 80°C, and 90°C, a significant reduction in complex viscosity is seen for both binders, indicating their thermo-rheological characteristics. Despite the narrowing viscosity differences between BB and CORMBB at increasing temperatures, CORMBB regularly demonstrates marginally higher values, especially at reduced shear rates. The remaining stiffness indicates the structural stability the changed binder retains, possibly attributable to the synergistic effects of bio-oil plasticization and ash-induced reinforcing. Guarin et al.¹ showed the same findings, emphasizing the advantages of integrating reactive modifiers to improve the thermal stability of bio-binders. At shear rates above 10 s⁻¹, indicative of standard mixing and compaction conditions in practice, the viscosity curves for BB and CORMBB align. This convergence indicates similar workability in hot-mix asphalt manufacture. In low-shear conditions, such as slow traffic or static loads, the increased viscosity of CORMBB may enhance rutting resistance and structural integrity. The flow sweep findings highlight the possibility of CORMBB as a feasible substitute for traditional bitumen. Its capacity to sustain elevated viscosity at reduced shear rates while preserving workability under operational shear circumstances demonstrates a balanced performance profile. These results endorse using sustainable pavement-material modifiers, providing ecological and technical benefits.

Figure 11.

Flow sweep test results of BB and CORMBB.

Frequency sweep test

The frequency sweep results for both BB and CORMBB, presented in Figure 12, provide further insight into the rheological behavior of the binders across a wide range of loading frequencies and temperatures. Figures 12(a) and (c) illustrate the complex modulus (G*) for BB and CORMBB, respectively. In both cases, the modulus increases with frequency, which is characteristic of viscoelastic materials where elastic behavior dominates at higher loading rates. However, BB consistently exhibits higher G* values than CORMBB across all temperatures, indicating superior stiffness and deformation resistance. These results are consistent with findings reported by Zhang et al.,³⁵ where higher stiffness was linked to better load-bearing capacity and rutting resistance. As temperature increases from 20°C to 90°C, a gradual reduction in modulus is observed for both binders, which is expected due to thermal softening. Notably, the rate of decline is steeper for CORMBB, especially at elevated temperatures, highlighting its greater temperature susceptibility. This trend reinforces the earlier understanding that although rice husk ash imparts rigidity, the influence of castor oil in the bio-bitumen matrix dominates under thermal exposure. Figures 12(b) and (d) depict the corresponding phase angle (δ) for BB and CORMBB, respectively. The phase angle generally decreases with increasing frequency, reflecting a transition from viscous to elastic behavior. BB shows a broader range of phase angle values, with lower values at higher frequencies, indicating a more pronounced elastic response.

Figure 12.

Complex modulus and phase angle based on frequency sweep test: (a) complex modulus of BB, (b) phase angle of BB, (C) complex modulus of CORMBB, and (d) phase angle of CORMBB.

Conversely, CORMBB maintains higher phase angles across the frequency spectrum, suggesting that it remains more viscous in nature under identical conditions. This difference in phase angle behavior is critical for performance evaluation. A lower phase angle at higher frequencies, as seen in BB, implies better resistance to permanent deformation under traffic loading. In contrast, the relatively higher phase angle of CORMBB suggests greater energy dissipation and more viscous flow, which may benefit fatigue resistance but could compromise rutting resistance under heavy traffic and high-temperature conditions. These observations support earlier findings by Sun et al.,^61,62 emphasizing the significance of phase angle as an indicator of field performance. In essence, the frequency sweep analysis highlights the trade-off inherent in bio-modified binders. While CORMBB offers environmental benefits and may enhance certain performance aspects like fatigue resistance, its higher temperature susceptibility and lower stiffness necessitate careful consideration in high-stress pavement applications. These outcomes justify the need for further optimization in bio-bitumen formulations, as also suggested by Guarin et al.,¹ to balance performance and sustainability goals in asphalt binder design.

Master curve of complex modulus

Figure 13 shows the complex modulus (G*) master curves of BB and CORMBB at reference temperature of 30°C, 40°C, 40°C, and 70°C. These master curves were generated using the sigmoidal model, demonstrating apparent differences in viscoelastic behavior between BB and CORMBB across various reduced frequencies. The reference temperatures were selected to access the stiffness characteristics over a wide range of in-service pavement temperature.⁶³ At the lower reference temperature of 30°C (Figure 13(a)), both BB and CORMBB show comparable trends at lower frequencies, where complex modulus values remain relatively low, indicating a predominantly viscous response. However, BB exhibits significantly more complex modulus values at higher reduced frequencies than CORMBB. This shows BB’s greater elastic response under fast loading circumstances, consistent with results by Wang et al.,⁵⁴ who found that higher complex modulus values signify superior stiffness and resilience to deformation under rapid load application. The variations between BB and CORMBB become increasingly apparent when reference temperatures (40°C and 50°C, Figures 13(b) and (c)) rise. Over the whole frequency spectrum, BB maintains better modulus values at 40°C. This tendency implies that the plasticizing action of castor oil causes CORMBB to undergo more notable softening with rising temperatures. Zhang et al.³ also noted similar discoveries wherein molecular interactions lowering intermolecular forces inside the bitumen caused decreases in stiffness with bio-oil addition. The viscoelastic response continues to vary at 50°C; BB shows a somewhat larger modulus than CORMBB throughout all frequencies, verifying the modified bio-bitumen’s temperature sensitivity. The difference at high temperatures emphasizes the need for modifiers such as rice husk ash, whose pozzolanic qualities are supposed to improve stiffness but are inadequate at this higher temperature to completely match the BB performance.¹ At 70°C (Figure 13(d)), the complex modulus of both binders finally drastically reduces, suggesting a strong viscous reaction resulting from the severe softening at high temperatures. Still, BB shows higher modulus values than CORMBB, reaffirming its somewhat better high-temperature performance. The reduced stiffness of CORMBB at high temperatures corresponds with other research showing that increased viscous flow causes bio-modified binders to typically have lower complex moduli.^1,62,64 Overall, the master curve evaluations unequivocally show that while CORMBB has benefits as a sustainable substitute, it shows less high-temperature rigidity than BB. Therefore, implementing CORMBB might need further modification or optimization for improved high-temperature performance in practical pavement projects.

Figure 13.

Master curve of complex modulus of BB and CORMBB reference temperature: (a) at 30°C, (b) at 40°C, (c) at 50°C, and (d) at 70°C.

Conclusions

This work replaced 15% of the BB with CORMBB and a thorough assessment including physical, chemical, structural, rheological, and bituminous concrete mix performance criteria. Physical characteristics were examined, including penetration, softening point, mass loss, specific gravity, and storage stability. Using FTIR and XRD studies, chemical and structural properties were investigated; rheological behavior was evaluated by temperature sweep test, amplitude sweep test, flow sweep test, and frequency sweep test using DSR. Finally, the master curve of complex modulus of both BB and CORMBB analyzed. The results indicate that the incorporation of CO and RHA yields positive outcomes in several key parameters, particularly in enhancing flexibility, moisture resistance, rutting resistance, cracking resistance, and durability-related properties. Based on the findings of this study, the following conclusions are drawn.

• Physical characteristics indicated that CORMBB showed enhanced flexibility but lowered thermal stiffness by displaying a 27.69% greater penetration and an 18.18% lower softening point than BB. Suggesting superior thermal stability and denser composition, CORMBB also showed decreased mass loss with aging (0.631% vs 0.982%) and a 1.66% rise in specific gravity (1.038 vs 1.021).

• The storage stability results confirm that both BB and CORMBB meet the ASTM D7173 requirement, with ΔT values well below the 2.5°C threshold, demonstrating adequate compatibility. Although CORMBB exhibited a slightly higher ΔT than BB, the synergistic action of castor oil and silica-rich RHA minimized phase separation and maintained homogeneity during thermal storage. These findings, consistent with earlier studies, validate that the optimized CO–RHA combination ensures sufficient stability and reinforces the practical applicability of CORMBB as a sustainable pavement binder.

• New peaks in CORMBB connected with C = O, C–O, and S = O functional groups were shown by FTIR spectra, thereby verifying the chemical interaction between CO, RHA, and BB. Reducing oxidative stability, workability, and binder matrix strength enhanced the plasticizing impact of CO and the pozzolanic action of RHA.

• Unlike the amorphous structure of BB, the XRD study revealed higher crystallinity in CORMBB with prominent diffraction peaks between 15° and 30° 2θ, matching with silica phases from RHA. More rigidity and moisture resistance are results from this structural strengthening.

• The temperature sweep test confirmed that CORMBB, through the synergistic effect of castor oil and RHA, achieves greater flexibility, elasticity, and aging resistance than BB, with higher rutting parameter values indicating improved high-temperature performance and suitability as a sustainable pavement binder.

• The amplitude sweep test at 30°C and 1 Hz revealed that CORMBB exhibits a broader Linear Viscoelastic Region and enhanced flexibility compared to BB but shows greater sensitivity to strain and reduced rutting resistance due to early structural breakdown and increased viscous behavior, validating its suitability for flexible pavement applications requiring improved crack resistance.

• The frequency sweep test confirmed that BB consistently has higher complex modulus values across all temperatures and frequencies, suggesting better stiffness and rutting resistance, and the flow sweep test confirmed that CORMBB exhibits a higher complex viscosity at low shear rates compared to BB, particularly at 60°C, indicating enhanced resistance to deformation under slow or static loading conditions. Both BB and CORMBB demonstrate shear-thinning behavior, but their viscosity profiles converge at higher shear rates, suggesting similar workability during mixing and compaction.

Limitations, recommendations, and scope for future work

While offering meaningful insights into the optimization of castor oil and rice husk ash content and the chemical, structural, and rheological behavior of CORMBB, this study is limited to laboratory-scale testing. It does not extend to field validation under diverse climatic and traffic conditions. It also focuses on a single bio-binder composition (CORMBB) without exploring varying dosages or long-term durability characteristics such as aging and moisture damage. To address these limitations, future investigations should consider a broader range of temperatures, modifiers, and blend ratios and incorporate long-term aging protocols and field trials. Employing advanced rheological tools like VECD and LAS models will enable more comprehensive fatigue prediction. Further research may also explore the integration of nanomaterials or polymers to enhance performance, assess environmental impacts through life cycle analysis, and support the formulation of performance-based specifications to facilitate the field implementation of sustainable binders like CORMBB in modern pavement systems.

Footnotes

Acknowledgements

The authors gratefully acknowledge the support of the National Institute of Technology Patna, India, for providing academic guidance throughout the research. Special thanks are extended to the Indian Institute of Technology Delhi (IIT Delhi) and Motilal Nehru National Institute of Technology Allahabad (MNNIT Allahabad), Prayagraj, Uttar Pradesh, for granting access to laboratory facilities and permitting the execution of essential experimental tests. The authors also appreciate the assistance provided by the laboratory staff at these institutes during material preparation and testing procedures. The first author would like to express sincere gratitude to Dr Sanjeev Kumar Suman for his valuable supervision and continuous encouragement during the course of this study.

Author contributions

The authorship of this study is ordered as follows, based on their respective contributions. Sanjeev Kumar, Ph.D., served as the lead contributor in all major aspects of the research, including conceptualization, data curation, formal analysis, methodology development, validation, and both the original drafting and critical revision of the manuscript. Sanjeev Kumar Suman, Associate professor, contributed in a supporting capacity to conceptualization, data curation, formal analysis, methodology, and visualization, while providing lead supervision and offering equal input in validation. He also supported the writing of the original draft and the review and editing process.

Declaration of conflicting interests

The authors declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Sanjeev Kumar

Sanjeev Kumar Suman

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Rheological behavior and chemical characterization of bio-bitumen modified by castor oil and rice husk ash: Sustainable alternatives to conventional bitumen

Abstract

Keywords

Introduction

Materials and methods

Materials

Preparation of CORMBB

Castor oil and rice husk ash dosage selection

Preparation of CORMBB

Methods

Physical tests

Penetration, softening point, mass loss, and specific gravity

Storage stability

Chemical characterization test

Fourier transform infrared spectroscopy (FTIR) test

X-ray diffraction (XRD) test

Rheological characterization tests

Results and discussion

Physical test results

Penetration, softening point, mass loss, and specific gravity

Storage stability

Chemical characterization

FTIR results

XRD results

Rheological behavior

Temperature sweep test

Rutting parameter

Amplitude sweep test

Flow sweep test

Frequency sweep test

Master curve of complex modulus

Conclusions

Limitations, recommendations, and scope for future work

Footnotes

Acknowledgements

Author contributions

Declaration of conflicting interests

Funding

ORCID iDs

References