Walnut Shell Modified Bitumen: A Sustainable Approach to Improving High-Temperature Performance
How finely ground walnut shell powder can enhance rutting resistance in Performance-Graded bitumen
Introduction
The development of sustainable, high-performance road materials is becoming increasingly important in modern pavement engineering. As road infrastructure faces demanding traffic loads and changing climatic conditions, researchers are exploring alternative materials that can improve bitumen performance while making productive use of agricultural waste.
One promising approach is the use of finely ground walnut shell powder (WSP) as a bio-based modifier for Performance-Graded (PG) bitumen. Research conducted by the technical team at Bernet Pars Bitumen investigates how walnut shell powder affects the rheological properties of bitumen, particularly its resistance to permanent deformation at high temperatures.
The findings indicate that walnut shell powder can improve high-temperature performance, although its influence on low-temperature flexibility requires careful consideration.
What Is Walnut Shell Modified Bitumen?
Walnut Shell Modified Bitumen is a bituminous binder incorporating finely ground walnut shell powder as a reinforcing material. Walnut shells are an agricultural by-product that can be processed into a fine powder and evaluated for use in asphalt-related applications.
Unlike conventional polymer modification, which relies on polymeric additives, this approach explores the use of a waste-derived material to alter the physical and rheological behavior of the bitumen matrix.
The objective is to investigate whether this material can improve selected performance characteristics while contributing to the more productive use of agricultural by-products.
Research Methodology
The study evaluated the effect of walnut shell powder on a Performance-Graded PG 64-22 base binder.
The walnut shells were obtained from a local processing plant, washed, dried at 105°C for 24 hours, and ground using a ball mill. The resulting powder was sieved to achieve a maximum particle size of 75 micrometers.
Two modified binders were prepared using walnut shell powder at concentrations of 3% and 7% by weight of the total binder. Their properties were compared with those of the unmodified base bitumen.
The evaluation included conventional binder tests and rheological assessments designed to investigate performance at high, intermediate, and low temperatures.
Key Testing Methods
- Dynamic Shear Rheometer (DSR): Used to evaluate rheological behavior at high and intermediate temperatures, including parameters associated with rutting and fatigue performance.
- Bending Beam Rheometer (BBR): Used to assess low-temperature creep stiffness and stress-relaxation behavior.
- Penetration, softening point, and viscosity tests: Used to examine changes in consistency, temperature sensitivity, and flow behavior.
These tests help characterize the binder’s response to different temperature conditions. However, binder-level results alone do not establish the performance of a finished asphalt mixture in the field.
Key Findings: How Walnut Shell Powder Affects Bitumen Performance
1. Improved High-Temperature Performance
The DSR results showed an increase in complex shear modulus (G*) and a decrease in phase angle (δ) following the addition of walnut shell powder. The resulting increase in the G*/sin δ parameter indicated improved resistance to permanent deformation under the evaluated test conditions.
The study also reported an increase in the continuous high-temperature performance grade. The binder containing 7% walnut shell powder met the requirements for the PG 70-XX high-temperature classification in the reported evaluation, compared with the original PG 64-22 base binder.
This improvement is attributed to the reinforcing effect of the rigid walnut shell particles within the bitumen matrix.
2. Changes in Conventional Properties
The addition of walnut shell powder reduced penetration while increasing the softening point and viscosity.
These changes indicate a harder and more viscous binder. Such characteristics can be beneficial for high-temperature stability, but their practical significance depends on the intended application, processing conditions, and the balance between stiffness and flexibility.
3. Intermediate-Temperature Performance
The study recorded an increase in the G*·sin δ parameter as walnut shell powder content increased.
Although this reflects a change in the binder’s rheological response, it does not by itself confirm improved fatigue resistance. The result requires careful interpretation and further investigation, particularly through asphalt mixture testing under repeated loading.
4. Low-Temperature Performance: An Important Consideration
The BBR results revealed a potential trade-off between high-temperature stiffness and low-temperature flexibility.
As walnut shell powder content increased, creep stiffness increased while the m-value decreased. For the 7% blend evaluated at -18°C, the reported stiffness exceeded 300 MPa and the m-value fell below 0.300.
These findings indicate a potential reduction in the binder’s ability to relax thermal stresses, which may increase susceptibility to low-temperature cracking.
For this reason, the modifier content should not be selected solely on the basis of high-temperature performance. Climatic conditions, project requirements, and the complete performance-grade specification must also be considered.
The Role of Walnut Shell Powder in Sustainable Pavement Engineering
Using agricultural by-products in road construction materials offers an opportunity to explore alternative resources and support more efficient material utilization.
Walnut shell powder is particularly interesting because it can act as a physical reinforcing component within the bitumen matrix. The research describes the modification mechanism as primarily physical rather than the result of a chemical reaction.
However, the sustainability benefits of this approach should be assessed through further investigation of material sourcing, processing energy, production costs, durability, and the environmental performance of the resulting asphalt mixtures.
The findings provide a basis for continued research into the potential use of agricultural waste-derived modifiers in bituminous materials.
Why Performance Testing Matters
A change in a single binder property does not necessarily translate into improved road performance under every condition. A suitable modified bitumen must provide an appropriate balance between high-temperature stability, intermediate-temperature behavior, and low-temperature cracking resistance.
For this reason, selecting a modifier requires systematic testing and consideration of the climatic and traffic conditions associated with the intended project.
At Bernet Pars Bitumen, technical evaluation and research into material performance contribute to the development of bituminous solutions informed by laboratory evidence and application requirements.
Conclusion
The research conducted by Bernet Pars Bitumen demonstrates the potential of finely ground walnut shell powder to improve the high-temperature performance of PG bitumen.
The addition of walnut shell powder increased stiffness-related performance indicators and improved resistance to rutting under the evaluated conditions. At the same time, the low-temperature results highlighted the importance of maintaining an appropriate balance between stiffness and thermal stress relaxation.
Walnut shell modified bitumen represents a promising area for further investigation at the intersection of material engineering and agricultural waste utilization. Additional testing at the asphalt mixture level and evaluation under different climatic conditions will be important in determining its practical applications.
Research team
- Amirhossein Akbari — R&D Manager, Bernet Pars Bitumen
- Hamidreza Ramezani — QC Manager, Bernet Pars Bitumen
- Taha Movahed — QA Expert, Bernet Pars Bitumen
Research-based content prepared from the study published in October 2025.
About Bernet Pars Bitumen
Bernet Pars Bitumen develops and supplies conventional and modified bitumen products for infrastructure and road construction applications, with a focus on technical evaluation, product quality, and international supply.
