# A Review on Bitumen Rejuvenation: Mechanisms, Materials, Methods and Perspectives
## 4 The State-Of-The-Art
### Images Techniques as Useful Complementary Tools
Figure 15: The comparative plot of fracture index for various asphalt types (reprinted from Reference [62] with the permission of Elsevier). Figure 15 shows that the addition of 10% of the rejuvenator \"B\" could not significantly soften the aged asphalt. However, although 10% of the rejuvenator \"B\" was not completely effective in softening the PAV-aged asphalt, 20% of that rejuvenator shows the highest efficiency in preventing undesirable cracks at low temperatures. Moreover, both A-15% and A-30% asphalts were soft enough to prevent surface fractures. Other authors have also focused their attention on the use of image techniques to better understand the structural changes induced by the use of rejuvenators. Indeed, [PERSON] et al. [5] have used the AFM technique to analyze the materials' surface morphology. They have investigated two types of bitumen which are called by the Authors as asphalt binders, named AAD (PG 58-28) and ABD (PG 58-10), and two types of rejuvenators: one comes from fast and convenience food frying oil, here named WV oil, the other is an aromatic extract containing approximately 75% of aromatic oil and resin compounds with small amount of saturate oil. AFM images are shown in Figure 16.  
The two virgin binders displayed different morphologies: virgin ABD has a dispersed phase with flake-like structures (with an average size of fewer than 2 um in diameter) spreading over a smooth matrix continuous phase; virgin AAD, instead, clearly shows the elliptical domains with \"bee-structures\" (with axes of a few microns). The Authors attribute these differences to the chemical composition of the two samples, in particular, worth of note is the high wax content of the bitumen AAD (1.94%) with respect to the ABD bitumen (0.81%). Upon aging, in ABD, the size of the flake-like microstructures decreased while the quantity increased; on the other hand, in AAD, the \"bee-structures\" are still present with no obvious morphological changes. The addition of the aromatic extract in the aged ABD bitumen, instead, produces marked changes, giving elliptical domains with \"bee-structures\" at the middle of the domains. On the other hand, the use of the aromatic extract in the aged bitumen AAD gives smaller-sized \"bee-structures\". Even, the less noticeable contrast between the 'bee-structures' and the matrix suggests that the amplitude of the undulated \"bee-structures\" is smaller than that of the virgin and aged ones. On the contrary, the addition of WV oil in both aged binders do not produce a significant morphological modification. The Authors conclude that the addition of the rejuvenator can cause big morphological changes, even bigger than those coming from the aging; however, this behavior is not general, depending on the specific rejuvenator. This because the asphalt binders' effect depends on its chemical behavior, and therefore, on the complicated molecular interactions which can establish with the other chemical species in the bitumen (and also the eventual inorganic particles). This is generally true for any additive. This conclusion is in accordance with the clues of a recent paper by [PERSON] et al. [1] which highlights the physical and chemical reasons for this. In this work, the Authors carried out a deep structural investigation by X-ray scattering on bare and additivated bitumen and found that asphaltene clusters hierarchically self-assemble to form aggregates at various levels of complexity, with different sizes up to the micrometer-sized domains dispersed in the maltene, and hold up by interactions of different strengths. The eventual presence of an additive triggers the formation of further intermolecular interaction in competition with those responsible for this self-assembly, causing a change of the size and shape of these aggregates. The potential of the AFM has been exploited by [PERSON] et al.