Tailor-made asphalt

Roads are exposed to many stresses: They have to withstand both increasing traffic frequencies and ever heavier trucks – and this in heat, cold and rain. Over time, cracks, ruts and potholes in the road surface develop, which have to be repaired at great effort and cost. «The goal of asphalt research is therefore to develop road surfaces that withstand the stresses for as long as possible,» explains Martin Zaumanis, asphalt researcher in Empa's Road Engineering / Sealing Components lab, which is led by Manfred Partl.

But there is no such thing as perfect asphalt to suit all locations – a busy road in a moderate climatic zone requires different characteristics than a rarely used mountain road that has to last even during the cold winter. «Modern technologies make it possible to adjust the properties of asphalt in such a way that it can resist cracking, increase skid resistance, ensure water runoff or become quieter,» says Zaumanis.

Asphalt consists of two original materials: Rock of different grain sizes and the binder bitumen, a black sticky mass obtained from crude oil. In this mixture, the proportion of rock takes over the supporting function in the asphalt; the binder ensures, among other things, that the mass holds together and has visco-elastic properties.

Different types of bitumen, the shape and size of the stones used, the ratio between the two basic components, the proportion of pores and additives, but also modern technologies such as processes with lower production temperatures or the use of recycled asphalt influence the properties of the finished material.  But how do you achieve the desired «ideal» asphalt with low susceptibility to deformation and cracking and maximum durability?

The method used is called performance-based mixture design. The manufacturer first defines what properties the asphalt should have and selects the ingredients accordingly in terms of quality and quantity. The finished asphalt mix is then artificially aged and tested for its desired properties. If it does not meet these requirements, the mixture is improved step by step and tested again until it reaches the desired behavior. That sounds simple - but which parameters should be changed if the road should not crack even in very cold weather? What should be done if the road has to withstand heat and heavy loads without building ruts? And how can new resources be preserved and asphalt be recycled?

Martin Zaumanis has taken up the subject. Based on a long list of older and more recent research results, Zaumanis created a matrix that describes the influence of various parameters on six desired properties: Resistance to ruts, moisture, thermal and load-induced cracks, rigidity and workability. «This matrix should help future asphalt designers to fine-tune the properties of the road surface in as few steps as possible,» explains the researcher.

This approach is even more relevant as new mixture types with alternative materials or high amounts of recycling are becoming a priority. This priority area has led to a multi-disciplinary project at Empa funded by the Swiss federal office for the environment led by Lily Poulikakos with team members Martins Zaumanis and Maria Chiara Cavalli from the Road Engineering/Sealing Components lab and Norbert Heeb and Maria Munoz Fernandez of the advanced analytical chemistry lab at Empa evaluating the environmental aspects of high amounts of recycled materials. Maria Chiara Cavalli recently successfully completed her PhD at ETH Zurich under the supervision of Lily Poulikakos and Professor Edoardo Mazza considering the chemistry and mechanics of bitumen with high amounts of recycling and bio-based rejuvenators. The Swiss companies of BHZ and Ammann are further members of this project showing the importance of alternative mixtures for the industry in Switzerland.

An example: Heavy loads can promote ruts as well as load-induced cracks. This also results in an optimization problem in heavily loaded pavements with heavily canalized traffic, for example on the right-hand lanes of motorways, motorway access roads or single lane sections before light signals: a higher proportion of bitumen in the asphalt mix increases the resistance to cracks. However, at the same time the asphalt becomes softer, i.e. more susceptible to rutting. Additives are the right solution here, such as the synthetic rubber SBS. Adding SBS to the bitumen increases both the resistance against ruts as well as load cracks – but at the same time the asphalt becomes more difficult to process.

In the future, asphalt manufacturers will be able to achieve the desired asphalt content in just a few steps thanks to this method – and road users will also benefit: the longer the pavement withstands the load, the less traffic stops due to road traffic work.