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New study on asphalt mixture can pave the way to a reduction in CO₂ emissions

Photo: cowi

​A reduction of the pavement’s rolling resistance can reduce CO₂ emissions from the transportation sector, because less fuel will be needed to get the wheels moving. A recent PhD has provided a new tool for optimizing the asphalt mixture.

​25 percent of the energy consumption in Denmark is related to road transportation of people and goods. One third of that energy is actually used to overcome the rolling resistance. In other words, both money and CO₂ emissions can be saved by reducing the rolling resistance of the pavement.

Basically the trick is to use smaller stones in the asphalt mixture, but how to do that without undermining the performance of the pavement?
Huan Feng, pavement specialist at COWI, gives part of the answer in his recent Phd project ”Modeling of asphalt mixture – A Discrete Element Method (DEM) to study the viscoelastic behaviour of asphalt mixture.”

”The project focuses on establishing a scientific background for novel pavement types and asset management solutions that minimize the rolling resistance for cars and trucks, and eventually attains the goal of reducing CO2 emission from the transportation sector,” explains Huan Feng.

3-5 percent reduction in fuel consumption

With the increasing demands for heavy loads, large traffic volume and new tendering schemes, pavement design is now moving towards more mechanistic based design methodologies with the purpose of producing long lasting and high performance pavements.

The mathematical model developed by Huan Feng is time saving compared to lab tests and has already been applied to a project by Vejdirektoratet  (the Danish road authority). The results indicate that the fuel consumption can be reduced by 3-5 percent without undermining the road grip.

However, it will take further studies to determine fully on the viscoelastic behavior of the asphalt when changing the mix design, Huan Feng points out.

“A road consist of many layers and we have only focused on the top 3-4 centimeters, which is the strongest and most expensive part and it is the top layer that decides how it feels to drive on the road. But when you change the mixture in the top layer it also has an impact on the lower layers, and this impact needs to be investigated as well before any final conclusions can be made,” explains Huan Feng.

Aiming for sustainable solutions

Thomas Mejer, Vice President at COWI Highways and Airports International, considers the new findings to be an important first step towards a more sustainable transportation sector:

 “There is a great need for new infrastructure not least in Africa where we have several projects. We are aiming at the most sustainable solutions and reducing CO₂ emissions while maintaining high performance and durability are key components to achieve that. In other words, the mathematical modeling holds a great potential for optimizing the sustainability of new roads,” says Thomas Mejer.


​The developed mathematical model is able to capture the viscoelastic properties of asphalt mixtures at real traffic loading frequencies.

It is possible to access the internal mechanical response of the material at meso-scale inside the flexible pavement structure, and hence the internal stress distribution and relative velocities could be easily monitored and analyzed.

Based on the developed model, the effect of aggregate gradation, aggregate shape and air void content on the viscoelastic behavior of asphalt mixture was studied, which provide a better insight into the internal mechanical properties of asphalt mixture.

Furthermore, a feasible way to study the correlation between the amounts of dissipated energy and the rolling resistance of asphalt mixtures could be obtained through the developed DEM model.

It was found that usage of relative smaller stones is helpful for reducing rolling resistance without sacrificing the overall performance of asphalt mixture.

LAST UPDATED: 22.04.2017