Quantitative characterization of morphological and micromechanical properties of aged asphalt binders using atomic force microscopy: Insights into different testing protocols

This study aims to quantitatively analyse the morphological and micromechanical properties of asphalt binders using Atomic Force Microscopy (AFM) and to investigate the sensitivity of results to changes in the testing protocol. Two asphalt binders with different penetration grades were subjected to various ageing treatments, resulting in changes in their colloidal structure. Two AFM test protocols were defined, differing in method of sample preparation and probe type. Microstructural parameters, including surface roughness, bee structure parameters (number, area, coverage, size), as well as domain areas, were obtained by means of tests carried out in the tapping mode. Additionally, parameters resulting from Force Distance Curve (FDC) tests, such as adhesion force and Derjaguin-Muller-Toporov (DMT) modulus, were calculated. Finally, experimental results were compared to synthetic indices derived from the chemical characterization of bulk asphalt binders, with the ultimate goal of identifying the AFM protocol that better reveals the inherent characteristics of the considered materials. It was found that the evolution in the number of bee structures due to ageing is highly related to the colloidal structure of asphalt binders. Furthermore, the morphology images and microstructural parameters were found to be strongly dependent upon the adopted testing protocols, with protocol 2 generally exhibiting stronger correlations between bee-structure parameters and chemical composition. Notably, the absolute correlation coefficients between bee number and the SARA indices ranged from 0.17 to 0.32 for protocol 1 and 0.71 to 0.77 for protocol 2, while those for bee size ranged from 0.69 to 0.85 and 0.96 to 0.97, respectively. Micromechanical assessment revealed that the amount of polar fractions in the asphalt binder was found to be strongly associated with adhesion force. The results also highlighted the potential need to adjust the target peak force according to the probe type and asphalt binder stiffness to ensure accurate DMT modulus measurements.