Used Cooking Oil Could Help Turn Old Asphalt Into New Roads, Studies Find
· Free Press Journal
Used cooking oil, usually treated as kitchen waste, could help increase the amount of recycled material used in road construction. Laboratory research has found that waste cooking oil can soften the hardened binder in old asphalt, making it possible to incorporate larger quantities of reclaimed pavement into new asphalt mixtures.
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A 2019 study published in the Journal of Traffic and Transportation Engineering found that asphalt mixtures containing 60% to 100% recycled material could be designed using waste cooking oil as a recycling agent. However, researchers found that the amount of oil was critical, as excessive quantities could reduce some aspects of pavement performance.
Hamed Majidifard, Nader Tabatabaee and William Buttlar conducted the study. Majidifard and Buttlar were affiliated with the University of Missouri, while Tabatabaee was associated with Sharif University of Technology in Iran, where equipment at the Asphalt Pavement Lab was used for testing.
Why aged asphalt needs softening
Reclaimed asphalt pavement (RAP) is old pavement that has been milled and processed for reuse. Recycling it reduces demand for new aggregates and asphalt binders while giving material removed during resurfacing another use.
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Waste cooking oil can counter some of this stiffness by softening the aged binder. The researchers said rising raw material and energy costs made efforts to increase the recycled content of asphalt particularly relevant.
Citing earlier research, they also noted that China produced about 5 million tons of waste cooking oil annually. The material had a flash point above 220 degrees Celsius, an important property when considering its use in hot-mix asphalt.
Tests showed that adding 10% to 16% waste cooking oil improved the aged binder’s low-temperature properties. Its low-temperature grade shifted from about minus 10 degrees Celsius to between minus 22 and minus 28 degrees Celsius.
Oil also affected the production and compaction of recycled asphalt. Adding it allowed some mixtures containing high levels of RAP to achieve the target of 4% air voids, the small spaces left between particles after compaction. Without oil, some mixtures containing high proportions of recycled material failed to reach the same target.
More oil did not always mean better performance
The benefits came with trade-offs. Increasing the amount of oil reduced the mixtures’ resistance to rutting and weakened their moisture resistance.
A mixture containing 16% oil lost about 43% of its wet tensile strength compared with the corresponding mixture without oil, although its moisture susceptibility ratio remained within the range considered acceptable in the study.
Researchers therefore examined crumb rubber produced from recycled tyres as an additional component. Adding rubber improved resistance to rutting and moisture while retaining some benefits of the cooking oil. The combination also produced one of the widest working temperature ranges among the binders tested.
However, the researchers stressed that the appropriate formulation depended on the materials and conditions involved.
Long-term ageing presented another challenge. In one laboratory ageing test, recycled binder containing 2.9% oil reached a specified cold-cracking threshold after 51 hours of simulated ageing, compared with 80 hours for fresh binder.
The study found that susceptibility to ageing varied according to the quantity of recycling agent. Its approach was therefore aimed at identifying an optimum amount of oil by balancing different performance characteristics rather than simply using as much oil as possible.
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More recent research explored the use of waste cooking oil with different recycled asphalt mixtures.
In a 2025 paper published in the Journal of Materials in Civil Engineering, Shobhit Jain and Anush K Chandrappa tested waste cooking oil in mixtures containing varying amounts of recycled material. They assessed properties including rutting, fatigue and durability.
Their laboratory tests found that recycled mixtures containing an optimum amount of waste cooking oil could achieve rutting and cracking resistance comparable to conventional hot-mix asphalt under the conditions tested.
Moisture susceptibility, however, remained a concern. Jain and Chandrappa attributed the mixtures’ increased affinity for moisture to the acidic nature of the waste cooking oil and asphalt examined in their study.
They also tested methods to limit moisture damage. The results showed that both the quantity of oil and the way it was introduced affected performance. Dry addition provided better moisture resistance than the wet method in their tests, while antistripping agents produced further improvement.
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Field testing needed before wider road use
Both the 2019 and 2025 studies were conducted in laboratories. Their findings therefore do not establish how asphalt containing waste cooking oil would perform over years of traffic, changing weather and seasonal temperature fluctuations on actual roads.
The 2019 researchers called for further studies to determine suitable oil quantities and assess long-term performance. The optimum amount could vary according to factors including climate and the condition of the pavement.
More field testing would consequently be required before laboratory results could be used to predict the performance of full-scale road projects.
For now, the studies point to a possible productive use for waste cooking oil. Carefully selected quantities could help soften aged asphalt and allow road mixtures to contain high proportions of recycled pavement. The findings also underline that simply adding more oil does not necessarily produce better asphalt, with performance depending on achieving the right balance.