Introducing a nitrogen conditioning to separate oxidative from non-oxidative ageing effects of hot mix asphalt

Bitumen used as a binder is subjected to ageing that leads to increased stiffness and brittleness. For the simulation of long-term ageing of hot mix asphalt (HMA), most methods are based on conditioning at high temperatures to increase oxidation. This paper investigates the impact of non-oxidative / thermal and oxidative effects on long-term ageing of asphalt mix specimens in the laboratory and aims at separating these effects. Therefore, specimens are placed in a heating cabinet at different temperatures ranging from +60°C to +160°C. One part of the specimens is exposed to the air atmosphere, while the other part is stored under nitrogen atmosphere (N-VAPro). Thereby, oxidative ageing is avoided for the nitrogen-stored specimens and the combination of thermal effects can be separated from oxidative effects. The changes in the material behaviour of HMA as a result of the different ageing regimes are investigated by stiffness tests at intermediate temperatures. Furthermore, Dynamic Shear Rheometer (DSR) tests are carried out on virgin, aged binder (RTFOT, RTFOT + PAV) as well as on recovered bitumen of both conditioning groups. The results indicate that conditioning temperatures must be kept below 110°C to prevent the asphalt mixtures from non-oxidative ageing effects. However, atmospheric air conditioning for HMA specimens is only suitable as a long-term ageing procedure for temperatures above 110°C to increase the oxidation rate to an efficient rate. This threshold temperature can also be seen as the limit value for lab-ageing procedures that should not be exceeded to avoid non-oxidative ageing effects that cannot occur in the field due to limited maximum temperatures. Furthermore, a double exponential increase with the ageing temperature on the binder level was shown.