Impact of Aging on the Microstructure of Asphalt Binder Modified with Antioxidant Additives and Copolymers

Abstract

The effect of long-term aging on asphalt binders mixed with antioxidant additives and copolymers demonstrated potential for retarding oxidative aging at the nanoscale based on atomic force microscopy (AFM) characterization. Two antioxidants (Redicote and Solprene) were effective in retarding the aging of asphalt binders (PG 64-22 and PG 76-22) from a previous study. This new complementary study further evaluates the change in micromechanical properties and microstructural phases of antioxidant-modified binders subjected to aging in the pressure aging vessel (PAV) to simulate the harsh aging conditions. The results were compared to those of asphalt binders extracted from field cores of two test sections in Qatar. Microstructural analysis based on the change in the percent area of the different phases from adhesion maps indicated that long-term aging caused a significant quantitative change in various surface phases of test binders. The dispersive phase area (measured from adhesion maps) increased due to aging with an associated increase in the number of so-called bee structures. This was accompanied by an increase in micromechanical adhesion and stiffness of the asphalt surface zone. An increased number of bee structures surrounded by higher stiffness regions acted as nuclei for crack initiation, eventually leading to fatigue cracking under repetitive loading. Aging also increased the surface roughness parameters of the asphalt binders and caused an overall sharpness on the asphalt surface. However, both antioxidants and copolymers retarded the increase in relative stiffness in laboratory-aged binders compared to field-aged binders.