Photo: Matteo Pettinari

New road surface saves fuel and cuts carbon emissions

Thursday 12 Mar 15


Matteo Pettinari
DTU Compute

CO2 Energy Efficiency—COOEE

  • The project was conducted as a partnership involving the Danish Road Directorate, Roskilde University, DTU, and NCC Roads, and received financing from Innovation Fund Denmark in the amount of DKK 13.8 million in the period 2011–14.
  • Scientific coordinator: Professor Jeppe Dyre, Roskilde University
  • Project Manager: Bjarne Schmidt, the Danish Road Directorate

The more resistance tyres encounter on the road surface, the more fuel the vehicle consumes. That is why developing new types of asphalt to reduce rolling resistance helps save money and cut environmental impact.

Road no. 145 is a stretch of the Danish road network where the Danish Road Directorate is currently testing a new type of asphalt that contains smaller stones—which means less rolling resistance. The new asphalt has been developed in collaboration with partners including DTU.

The project was launched in 2011 as a reaction to rapidly rising carbon emissions and the associated changes in the Earth’s climate.

Transport is a significant element in this context: one quarter of Denmark’s total energy consumption is attributable to ferrying people and goods around the road network; and of this energy, no less than one third is devoted to overcoming rolling resistance.

The Danish Road Directorate therefore identified appreciable potential in working with the road surface in an attempt to reduce rolling resistance. This resulted in the establishment of the Cooee (CO2 emission reduction by exploitation of rolling resistance modelling of pavements) project, which has recently been concluded with extremely promising results.

“We succeeded in creating an asphalt surface that cuts fuel consumption by 3–5 per cent, without adversely affecting road holding. The success is attributable in no small part to truly amazing input from researchers at DTU and Roskilde University (RUC), as well as excellent involvement from NCC,” relates Bjarne Schmidt, Project Manager, from the Danish Road Directorate.

Smaller stones mean less resistance
Roads are constructed in multiple layers. The base layer is made up of sand, gravel and various other materials designed to assure stability, act as a drain and counteract frost formation. On top of this lies a 25–30 cm bearing layer bound in asphalt, and at the very top is the ‘wearing layer’, which consists of 3–4 cm of asphalt. The top layer is the strongest and most expensive layer and, of course, it determines the driving sensation—whether the tyres run smoothly across the surface, while maintaining a sufficiently strong grip on the road.

One way to reduce rolling resistance is to use smaller stones in the asphalt. The standard size is 11 mm, so in this project the researchers decided to reduce the size to 6–8 mm. However, care must be taken as this alters the balance of the asphalt and it is, of course, essential to maintain sufficient friction to prevent the tyres from skidding across the surface.

Illustration: DTU
Researchers systematically examined the mechanical properties of asphalt in the laboratory before testing the new surface on selected stretches of road.

The right asphalt mix
“We wanted to change the stone size, so we needed to identify a new ‘recipe’ for the ratio of stones to adhesive—bitumen, mortar and other types of filler—which would ensure that the stones were distributed evenly, and that, above all, the surface could withstand at least 15 years of use. If a surface has to be replaced or repaired sooner, then it is simply not viable. Instead of taking a trial and error approach, we chose to associate researchers with the project, people who could apply scientific tools to the task, both at fundamental research level and with regard to clarifying the mechanical properties of the asphalt.”

Investigated at multiple levels
Professor Jeppe Dyre from Roskilde University was responsible for the fundamental research, and examined how the bitumen molecules changed when subjected to different loads and temperatures, and how they adhere to the stone material.

Matteo Pettinari and Huan Feng from DTU Civil Engineering investigated the mechanical properties of the composite material. Using the mathematical model DEM, the researchers were able to simulate the effect of changing the shape and positioning of the stones; they also tested the composition of bitumen with a variety of fillers.

“You could say that the stones constitute the rigid skeleton of the asphalt, while the mix of bitumen and polymer fillers—which have viscoelastic properties—can be compared to the muscles in a body. The pressure from the tyres compresses the asphalt, and it is crucial to safety that this does not create permanent tracks where water can collect. Similarly, the stones must remain in place in the surface and must not crack or wear down too quickly,” explains Matteo Pettinari.

The size of the stones was the only aspect that remained fixed, while all the other ‘ingredients’ in the asphalt were changed and tested.

“Our objective was to understand the material in depth, and to describe how the properties alter under a variety of conditions,” says Matteo Pettinari.

Asphalt meets reality
As the third link in the development chain, the new asphalt has been laid on selected stretches of road in Denmark. The first trial was run in 2012, and thus far it has resulted in fuel savings of fully four per cent with the same friction properties as standard asphalt.

The project is thus a success, and if the new asphalt is applied throughout Denmark, it should have far-reaching and beneficial consequences for the environment. The measured fuel savings are likely to correspond to at least 64 million litres of fuel per year. This, in turn, would mean cutting emissions into the atmosphere by 160,000 tonnes of CO2 and 76 tonnes of nitrous oxides (NOx) per year.

Edited article from DYNAMO no. 40, DTU's quarterly magazine in Danish.