Study of the effect of laboratory-simulated ageing on the damage resistance of polymer-modified bitumens in intermediate climates

Polymer additives are widely used to improve the performances of road bitumens. However, although their advantages are well recognised, there are still drawbacks and concerns that limit the widening of their usage. One of these concerns is the lack of understanding of their ageing-inhibitor properties. This work focuses on the study of the effect of laboratory-simulated ageing on the microstructure and the rheological response of different polymer-modified bitumens (PMB). A single base bitumen has been modified with different concentrations of two polymers: an elastomer (Styrene-Butadiene-Styrene – SBS), and a plastomer (Ethyl-Vinyl Acetate – EVA). In the preparation of the SBS modified bitumens, the presence of sulphur as cross-linker has been varied to evaluate the effects of the use of cross-linker on their ageing resistance. The bitumens have been laboratory aged in the RTFO and the PAV. The microstructure of the unaged and aged bitumens has been investigated through fluorescence microscopy and Gel Permeation Chromatography (GPC). On the other hand, the rheological response has been investigated by linear viscoelastic characterisation, the Multiple Stress Creep and Recovery (MSCR) test, and the Linear Amplitude Sweep (LAS) test. The artificial ageing susceptibility of the SBS modified bitumens depends on the use of sulphur as cross-linker and the polymer concentration. More specifically, the SBS network degrades in the RTFO and the PAV. The degradation of the polymer phase is clearly visible in the fluorescence microscopy, and it is detected in the GPC. The deterioration of the polymer network contributes to mitigating the effects of the PAV-oxidative hardening of the bitumen phase on the rheological response. The use of cross-linker and the increase in the polymer concentration improve the ageing-inhibitor effect of the SBS. The EVA polymer shows a low ageing susceptibility and a low affinity with the base bitumen irrespective of the polymer concentration. As a consequence, the rheological response of aged EVA modified bitumens does not seem to show relevant deviations from one of the unmodified bitumens except for the cumulative damage resistance at high polymer concentration. Besides PMBs, this work includes a section focused on the laboratory–simulated ageing of SBS modified mastics. An unmodified bitumen and different SBS modified bitumens (i.e., different polymer concentrations) have been mixed with a single filler concentration. The mastics have been aged by using multiple cycles of PAV. The rheological response of the mastics has been investigated by the MSCR test. Results show that the variation with ageing of the resistance to the accumulation of permanent deformation of mastics shows a similar trend to one of the corresponding bitumens. The correspondence of the findings between the bitumen and mastic scales appears to be promising and support the approach followed in this work to study PMBs