Laboratory and field ageing of SBS modified bitumen: Chemical properties and microstructural characterization

Abstract

Styrene-butadiene-styrene (SBS) modified bitumen is a widely-used civil engineering material with a variety of applications. The properties and durability of bitumen greatly depend on its chemical composition. Due to its organic nature bitumen is highly prone to oxidation. The main cause of bitumen oxidation is the interplay of sunlight (photo-oxidation), atmospheric oxidation with reactive oxygen species (ROS), mechanical ageing and thermal oxidation. However, current standardized ageing methods, such as rolling thin film oven test (RTFOT) and pressure ageing vessel (PAV) do not include photo-oxidation and lack realistic ageing simulation. In this work, different laboratory aging procedures like PAV, RTFOT and UV radiation are applied to SBS modified binder. At the same time field aged bitumen with 19 years service in the road is analysed. The surface has been treated with UV radiation and ROS, resulting in deformation of surface microstructures and change in chemical composition. Since ageing mainly affects the surface of bitumen, a layer study on field aged bitumen samples was conducted to demonstrate ROS penetration depth. Fluorescence spectroscopy indicated that the ROS penetration mainly affected the first centimetres of the surface and a rapid decrease of ROS diffusion could be discovered. Fluorescence excitation-emission matrix (FEEM) results show that the overall fluorescence intensity decreases with ageing. The link between chemical and morphological change due to ageing could be given with fluorescence microscopy. The colloidal nature of bitumen exerts in the microstructural composition, which undergoes undergo a major chemical and morphological change with progressive ageing. We recorded the first fluorescence image of the typical surface microstructures with previously unattainable resolution with fluorescence microscopy, highlighting fluorescing Catana and Peri phase of the surface domains while Para phase exhibited less fluorescence intensity. The link between chemical and morphological change could be given with atomic force microscopy (AFM). Finally, AFM and fluorescence microscopy indicated that photo-oxidation promoted a gradual deformation of the chemically inhomogeneous surface structures toward shrink in size, deformation of the catana pattern and a drastic increase in microstructure height compared to neat bitumen, which encourages the importance of inclusion of atmospheric photo-oxidation to standardized bitumen testing methods. • Ageing trends of 19 years field aged unaged and laboratory aged bitumen. • Gradual decrease of reactive oxygen penetration depth into the bulk material. • Photooxidation causes major structural and chemical changes on the surface features. • Fluorescence microscopy demonstrates fluorescing catana and peri phase.