Georg Bus, a doctoral student at the Chair of Road Construction, and his team have developed a method of producing asphalt at lower temperatures using foamed bitumen. In tests and on two rural roads in Germany, the warm mix asphalt showed comparable performance to conventional hot mix asphalt.

The main component of asphalt is bitumen, a binding agent that holds the other ingredients together. Like rubber, it becomes liquid at hot temperatures of 150 to 180 degrees Celsius and can weld the components together well. At the same time, this manufacturing process consumes a lot of energy and releases vapors and aerosols that can be harmful to health. Until now, the limit value for the hot processing of bitumen was 10 milligrams per cubic meter. In 2020, an occupational health and safety regulation issued by the Committee for Hazardous Substances of the Federal Institute for Occupational Safety and Health lowered the limit to 1.5 milligrams per cubic meter. A transitional period runs until the end of 2024.

Georg Bus is now investigating a new method of producing asphalt at temperatures 20 to 30 degrees Celsius lower. To ensure that the bitumen can still be processed well, water is added to produce so-called foamed bitumen. This production process is already used in the USA and the Netherlands, but due to the previous occupational exposure limit of 10 milligrams per cubic meter in Germany, it was not necessary to convert conventional hot asphalt. The new limit value now changes the requirement. At the same time, the different origin of the crude oil from which the bitumen binder is obtained influences the foaming properties of the bitumen. As part of the research project, Georg Bus and his colleagues produced 1,000 kilograms of asphalt in the laboratory and tested a variety of recipes for the production of different types of asphalt. Using the optimum mixture, the Bochum team produced foamed bitumen hot asphalt slabs, which they compared with slabs of conventional hot asphalt. To do this, they drilled test specimens out of the slabs and tested their behavior in cold and hot conditions. The team found that the performance of the two types of asphalt was comparable.

On two rural roads in Baden-Württemberg and Rhineland-Palatinate, the asphalts were finally used over a large area as part of a planned renovation project. “Our warm mix asphalt was laid on half of the road sections and hot mix asphalt on the other half,” says Georg Bus. “Both were equally easy to work with.” Samples were taken again and their performance tested several times. Once again, the results were comparable.

With regard to the vapor and aerosol value, a reduction to an average of 2.3 milligrams per cubic metre was achieved for the paver operator with the new hot mix asphalt. Previously, the value was 6.9 milligrams per cubic meter. Although this is still above the new limit value of 1.5 milligrams per cubic meter, Georg Bus and his team have already halved it and at the same time achieved savings in energy and CO₂ emissions.