It begins with a road in distress. Potholes scatter across its surface, the bitumen cracked by years of sun, rain, and relentless traffic. Most would see waste material destined for a landfill or a low-grade reuse. But a growing number of scientists are asking a different question: what if yesterday’s roads could become tomorrow’s highways, stronger and greener than before?
In an era where governments are pledging net-zero targets and battling rising infrastructure costs, the way roads are built is under scrutiny. Every year, millions of tonnes of reclaimed asphalt pavement, or RAP, are stripped from roads during resurfacing. Too often, this valuable material is underutilised, ending up in low-grade applications or discarded. The cost of asphalt binder (commonly known as bitumen) is nearly equivalent to that of sugar. Now, imagine discarding aggregates coated with tons of this costly “sugar” into landfills. It is difficult to justify such waste, especially when this binder can be reused to recoat aggregates and contribute to the next design cycle of pavements. The environmental toll is heavy, given the energy-intensive process of producing virgin bitumen and the carbon footprint of transporting new materials.
Researchers at the Indian Institute of Technology Roorkee (IIT Roorkee) believe the answer lies in perfecting the recipe for recycled asphalt. Led by first author Ankit Sharma, two complementary studies published in Road Materials and Pavement Design and Journal of Materials in Civil Engineering dig deep into the physics and chemistry of blending old and new binders. Their aim: to create durable, high-performance roads using significant quantities of RAP without compromising on quality.
Why recycling roads is harder than it looks
On the surface, asphalt recycling seems straightforward: mill the old pavement, blend it with fresh binder, add a recycling agent (RA) if needed, combine with new aggregates, and then repave. The problem is that the old binder in RAP is stiff and aged, often with a high performance grade (PG) temperature exceeding 130°C. This means it resists flow and deformation far more than fresh bitumen, making it difficult to blend uniformly.
Most design methods rely on mixing rules. These are mathematical models that predict the properties of the blended binder based on the properties and proportions of RAP binder and RA. But as Sharma’s team points out, traditional mixing rules often assume 100 percent blending, which rarely happens in practice. The result can be misleading predictions of stiffness, viscosity, and long-term durability.
In countries like India, where traffic loads are heavy and climatic conditions extreme, getting this prediction right can mean the difference between a road that lasts two decades and one that fails in five years.
The search for the right formula
The first study in Road Materials and Pavement Design put seven mixing rules to the test, ranging from simple log-log viscosity blending charts to more complex models incorporating binary interaction parameters or viscosity blending indexes. The researchers prepared 19 RAP-RA blends using three RAP sources of varying ages and stiffness, blending them at RAP contents from 0 to 100 percent.
They then measured the blends’ zero shear viscosity (ZSV), rotational viscosity at high temperatures, softening point, and rheological parameters such as complex modulus (|G*|) and phase angle (δ). These measurements were compared to values predicted by each mixing rule.
The results revealed a nuanced picture. While some rules, such as the Chevron equation and the Grunberg & Nissan model, showed excellent correlation between measured and predicted values, others tended to underestimate viscosity at higher RAP contents. This could lead to underestimating the stiffness of the final mix, increasing the risk of cracking under low temperatures or repeated traffic loads.
Linking lab science to real-world performance
The second study, published in the Journal of Materials in Civil Engineering, takes the analysis further by directly correlating rheological properties to a critical performance measure: the Superpave rutting parameter (G*/sin δ). This parameter is widely used in the United States to predict a binder’s resistance to permanent deformation under high temperatures.
Using oscillatory tests and the time–temperature superposition principle, Sharma’s team constructed master curves for each blend and used the Carreau–Yasuda model to estimate ZSV. They found that ZSV increased exponentially with RAP content, a sign of increased stiffness and potentially better rutting resistance. However, they also observed a critical threshold: blends with more than 40 percent RAP showed a distinct change in rheological behaviour, indicating potential brittleness.
Perhaps the most practical outcome of this study was the development of G*/sin δ contour plots. These allow engineers to quickly determine the minimum and maximum RAP content that will meet both rutting and fatigue resistance criteria for a specified target binder grade in a given region or state. It is a tool that bridges the gap between lab measurements and on-site decision-making.
Sustainability meets durability
Why does this matter now? Global asphalt production is a major contributor to greenhouse gas emissions, and the extraction of virgin aggregates puts further strain on natural resources. The ability to use higher percentages of RAP without sacrificing performance could reduce the demand for new bitumen, cut emissions, and lower construction costs.
In India, where rapid urbanisation and highway expansion are driving massive road-building programmes, the potential impact is enormous. Similar benefits could be realised in Europe and North America, where ageing highway networks require continuous maintenance.
By rigorously testing and refining mixing rules, and by providing practical tools such as contour plots, the IIT Roorkee research offers a science-backed pathway to scaling up sustainable road construction.
Tying into the global infrastructure challenge
This research arrives at a time when climate-resilient infrastructure is a hot topic in policy circles. The United Nations has called for more sustainable materials in public works, and the European Green Deal includes targets for resource efficiency in construction. Meanwhile, the United States Federal Highway Administration is actively promoting high-RAP mixes as part of its sustainability initiatives.
Yet challenges remain. Contractors may be hesitant to use higher RAP contents due to concerns about performance warranties. Specifications in many countries are conservative, often capping RAP use at 20 to 30 percent for surface courses. Overcoming these barriers will require not only further research but also changes in policy and industry practice.
The IIT Roorkee studies provide a robust evidence base for updating such specifications. By showing which mixing rules produce reliable predictions and where caution is needed, they help pave the way for more ambitious RAP use targets.
The road ahead
Dr. Ankit Sharma and colleagues stress that their work is not the final word. Field trials will be essential to confirm that lab predictions translate into real-world durability. They also note that RAP properties can vary widely depending on their source, age, and prior service conditions, so site-specific testing is key.
Future research may explore rejuvenators beyond the VG-10 binder used in these studies, or investigate the role of nanomaterials and polymers in enhancing recycled binder performance. Advances in portable rheometers could make on-site testing faster and more practical, further closing the loop between design and execution.
Why it matters to the public
For the average road user, the benefits of this research may seem abstract. But in practical terms, better use of RAP means smoother roads, fewer potholes, and lower maintenance costs borne by taxpayers. It means reduced disruption from roadworks and a smaller environmental footprint from one of the largest infrastructure sectors in the world.
At a time when extreme weather events are testing the resilience of transport networks, ensuring that roads are both durable and sustainable is no longer optional. It is a public good.
The science is advancing, but adoption depends on decision-makers. Policymakers, highway agencies, and contractors should review and, where appropriate, revise specifications to allow higher RAP use based on scientific evidence. Investment in training and equipment for rheological testing could further support the shift.
For citizens, the takeaway is to demand accountability in infrastructure spending and environmental stewardship. The next time your local council resurfaces a road, ask whether they are using recycled materials—and if not, why not.
The road to a greener future may quite literally be paved with the asphalt of the past. The question is whether we have the vision and the will to follow it.
References
Sharma, A., Ransinchung, G. D., & Kumar, P. (2022). Applicability of various mixing rules for hot asphalt recycled binders. Road Materials and Pavement Design, 23(11), 2547–2566. https://doi.org/10.1080/14680629.2021.1985597
Sharma, A., Ransinchung, G. D., Kumar, P., & Raha, S. (2022). Rheological characterization of recycled asphalt binders and correlating the zero shear viscosity to the Superpave rutting parameter. Journal of Materials in Civil Engineering, 34(9), 04022218. https://doi.org/10.1061/(ASCE)MT.1943-5533.0004358
