# A Review on Bitumen Rejuvenation: Mechanisms, Materials, Methods and Perspectives
## 4 The State-Of-The-Art
### Images Techniques as Useful Complementary Tools
Over time, some effects can be highlighted: needles are expelled from the matrix phase, disconnected domains only comprising the network phase are born, and the wrinkling in the elliptical domains tends to decrease. The network phase, in this period, has formed a kind of bilayer around the elliptical domains, consisting of 200-300 nm stiffer layer (higher phase shift, light colour in Figure 22(bii)), surrounded by a 1 um somewhat softer layer. Both layers display the typical pattern of randomly oriented needles, typical of the network phase. The higher stiffness layer may be just a denser packed region of the network phase. On the contrary, for the higher concentration of CM1 rejuvenator (25%) the effect of the time on microstructure is less obvious. Indeed, it is possible to notice islands solely consisting of the needle network, as well as the bilayer of 15 needles surrounding the elliptical domains. It is noticeable that, here, the stiffer (white) region of the needle network phase is the more prominent phase. Also, at a 25% concentration of CM1, the wrinkling in the elliptical domains remains over time, though the oscillation amplitude has decreased. Figure 14:1 AFM phase image of aged bitumen (**a**) and aged bitumen doped with 20% of BM1 (**b**) with (i) elliptical domains, (ii) matrix, (iii) tertiary phase and (iv) quaternary phase (adapted from Reference [63]). [PERSON] et al. [64] evaluated the effect of composite rejuvenator in comparison with a common rejuvenator by means of dynamic shear rheometer (DSR) and atomic force microscopy (AFM). The composite rejuvenator, named RRA, was laboratory-prepared by blending the light weight oil rich in aromatics with a chemical compound containing polar group. The common rejuvenator, denoted as CRA, was also prepared in a laboratory. They used for these researches the bitumen SK-70, whose physical properties are listed in Figure 14:2  
The aged binder was prepared through the aging of SK-70 by Thin Film Oven Test (TFOT). Aged bitumen was doped with 4; 6; 8 and 10 wt% of the two kinds of rejuvenator. The results are shown in Figure 14:3 the composite rejuvenator (RRA) has a greater effect with respect to the common rejuvenator (CRA). When the content of RRA is 10 wt%, the values of penetration, ductility, softening point and viscosity are very close to the values of pristine SK-70. Therefore, the Authors believe that the rejuvenator RRA is able to restore the colloidal structure by increasing the aromatics content. In addition, the polar groups of RRA can react with the asphaltene in aged bitumen, decreasing the asphaltenes content themselves. On the contrary, according to them, chemical reactions between the common rejuvenator and asphaltenes molecules do not take place, due to the lack of polar groups in common rejuvenator. \begin{table}
\begin{tabular}{c c} \hline \hline
**Index** & **Value** \\ \hline Penetration (25 \({}^{\circ}\)C, dmm) & 73 \\ Ductility (15 \({}^{\circ}\)C, cm) & \textgreater{}150 \\ Softening point/\({}^{\circ}\)C & 45.2 \\ Viscosity (135 \({}^{\circ}\)C, Pa*s) & 0.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Physical properties of SK-70 bitumen (reprinted from Reference [64] with the permission of Springer Nature). Figure 22: Time evolution of microstructure of aged bitumen doped with CM1. AFM phase images; (**a**) 10% CM1 and (**b**) 25% CM1, (i) after preparation (ii) after seven days (reprinted from Reference [63] with the permission of SAGE Publications). From the rheological analysis, it results that the rutting factor, shown in Figure 24a as a function of temperature, of the aged binder is greater than the virgin one. The addition of 10 wt% of RRA and CRA causes a decrease in the rutting factor, but sample with RRA gives a trend similar to the pristine one. Obviously, the aged bitumen is very brittle and easy to crack under load at low temperature, in fact, has a high fatigue factor (see Figure 24b) compared to virgin bitumen. The poor regenerative power of CRA is evident.