Performance Optimization and Sustainability Assessment of Reclaimed Asphalt Pavement Modified with Waste Engine Oil for Flexible Pavement Applications

Every time a road is built, it relies on two main ingredients: crushed stone and a sticky, black binder made from crude oil. This binder, known as bitumen, holds the stones together to create a smooth surface for vehicles. For decades, the construction industry has relied on fresh stone and new oil to build these flexible roads. However, this approach is becoming increasingly expensive and environmentally damaging. The world is running out of easy-to-access crude oil, and digging up new stone depletes natural resources. At the same time, when old roads are torn up, the resulting material, called reclaimed asphalt pavement, often ends up in landfills. This creates a double problem: we are wasting a valuable resource while continuing to consume more raw materials.

To solve this, engineers are looking at ways to recycle these old road materials. The challenge is that the bitumen in old roads has aged and hardened over time, becoming brittle and prone to cracking. If builders simply mix this old, brittle material with new stone, the resulting road would likely fail quickly. To fix this, researchers have begun testing a method of "rejuvenating" the old binder. They use waste engine oil, a common pollutant from cars and machinery, to soften the hardened bitumen. The idea is to turn two waste products—old road chunks and used oil—into a new, high-quality road surface. A recent study by researchers at Ajay Kumar Garg Engineering College in India explored exactly how well this combination works, aiming to find the perfect recipe that balances road strength with environmental benefits.

The researchers set out to create a new type of road mixture using reclaimed asphalt pavement and waste engine oil. They started by gathering their materials: standard bitumen used for new roads, crushed stone, and the reclaimed asphalt pavement taken from old, damaged roads. They also collected waste engine oil, which they planned to use as a rejuvenating agent to restore the flexibility of the aged binder. The team prepared several different batches of road mix. Some were traditional mixes made entirely of new materials, while others contained varying amounts of the old reclaimed asphalt. In the mixes with reclaimed asphalt, they added a small amount of waste engine oil, specifically two percent by weight, to see if it could soften the old binder enough to make the new mix workable.

To test these mixes, the team used a standard engineering procedure known as the Marshall mix design. This involves heating the materials, mixing them together, and pressing them into a specific shape to create a test sample. They then placed these samples in a machine that pushes down on them to measure how much weight they can hold before breaking, a property called stability. They also measured how much the sample deforms under that pressure, known as flow, and checked the amount of tiny air pockets trapped inside the mixture. By testing different amounts of bitumen and different percentages of reclaimed asphalt, they were able to map out which combination created the strongest and most durable road surface.

The results showed that the traditional road mix, made with new materials, performed very well, but the researchers found a way to match that performance using recycled materials. They determined that the ideal amount of bitumen for their mix was just over five percent. When they introduced the reclaimed asphalt and the waste engine oil, they discovered a specific recipe that worked best. A mixture containing twenty percent reclaimed asphalt and two percent waste engine oil proved to be the sweet spot. This specific blend achieved a stability of 1006 kilograms, which is nearly identical to the strength of the traditional mix. The amount of air trapped inside the mixture was also perfect, sitting at just over four percent, which is the target for a dense, durable road surface.

Perhaps the most significant finding was not just about strength, but about cost and sustainability. The researchers calculated that by using this twenty percent reclaimed asphalt and two percent waste engine oil mixture, the cost of building a cubic meter of road dropped by more than fourteen percent. This saving comes from using less new stone and less new bitumen. Furthermore, the study highlighted that this approach helps solve two environmental problems at once: it keeps old road materials out of landfills and prevents waste engine oil from contaminating soil and groundwater. The waste oil acted as a chemical bridge, replenishing the lost oils in the aged binder and allowing the old and new materials to work together seamlessly.

The study did not stop at just building the mix; the researchers also used statistical tools to ensure their findings were reliable. They used regression analysis, a method of finding patterns in data, to predict the best bitumen content, and their predictions matched their physical experiments almost perfectly. They also created a performance index that weighed the road's strength, its flexibility, and its cost against each other. In this comprehensive ranking, the mix with twenty percent reclaimed asphalt and two percent waste engine oil came out on top. It offered the best balance of being strong enough to handle traffic, flexible enough to resist cracking, and cheap enough to make economic sense.

While the results are promising, the researchers were careful to note the limits of their work. They confirmed that this specific mixture works well under the conditions they tested, but they acknowledged that real-world roads face many more challenges, such as heavy truck traffic, extreme weather, and long-term aging. The study did not test how this mix would hold up over many years in the field, nor did it analyze the microscopic chemical changes in the binder in extreme detail. These are areas for future research. However, the current findings provide a strong foundation, suggesting that it is possible to build roads that are not only cheaper and stronger but also kinder to the environment.

Ultimately, this research demonstrates a practical path toward a circular economy in road construction. Instead of viewing old roads and used oil as waste, the study shows they can be valuable resources. By carefully blending twenty percent of reclaimed asphalt with a small amount of waste engine oil, engineers can create a road surface that performs as well as one made from virgin materials. This approach reduces the need to dig up new stone and drill for new oil, while simultaneously cleaning up two major waste streams. The work suggests that the future of road building may lie not in finding new resources, but in smarter ways of using the ones we already have.