Microstructural changes in bitumen at the onset of crack formation

For the first time the initiation and early stages of damage and microcrack growth in bitumen under mechanical loading conditions have been observed on the micrometer length scale by atomic force microscopy (AFM). Bitumen films have been applied on flexible substrates. Tensile loading of the substrate leads to straining of the substrate itself as well as the bitumen film that has been attached to it. The surface microstructure of the bitumen film has been studied with AFM for three loading levels: no loading, moderate (5% strain) and high loading (10% strain) conditions. The initiation and onset of propagation of microcracks have been observed for the moderately loaded specimen. It is found that damage in the shape of crazes and cracks does only occur in one of the two microstructural phases of bitumen, i.e. in the ellipse-shaped domains. Subsequent application of higher loading levels leads to complete embrittlement and fragmentation of the elliptical domains into lamellar structures which tend to align and orient in a direction perpendicular to the direction of loading.