Diffusion Phenomenon between Two Different Bitumens from Mechanical Analysis

The use of reclaimed asphalt pavement (RAP) in hot asphalt mixtures has been investigated widely, mainly to increase the amount of RAP incorporated into new asphalt mixtures as secondary material. A key point to successfully increase the amount of RAP in mixtures is a comprehensive understanding of the blending between different bitumens, such as old and fresh bitumen. This paper introduces a mechanical model to simulate the diffusion phenomenon between two different bitumens on the basis of their rheological properties. A double-layer dynamic shear rheometer (DSR) specimen with a 25 mm diameter and 0.5 mm total thickness in plate-plate configuration was prepared by superposing two 0.25-mm-thick layers, each composed of a different bitumen. The evolution over time of the equivalent shear complex modulus of the whole double-layer specimen was investigated by performing a series of frequency sweeps at 50°C every 30 min. The time-dependent evolution of the equivalent modulus was modeled by considering an intermediate layer, composed of fully blended bitumen, at the interface between the two layers of the two base bitumens. The thickness of the intermediate layer increased as a function of time, due to diffusion. The validity of the model was confirmed by a frequency-independent thickness of the intermediate layer. Extrapolating the results, the model gives an indication of the blending of the two bitumens in the long-term, and the findings may suggest an interaction between different bitumens in asphalt mixtures containing RAP. The results are promising for the development of a more-refined mechanical model.