# A Review on Bitumen Rejuvenation: Mechanisms, Materials, Methods and Perspectives
## 3 Mechanisms of Ageing and Rejuvenating
### Rejuvenation  
A rejuvenator has been defined as that agent capable of restoring the original rheological properties of a bitumen. Therefore, it is assumed that the primary action of a rejuvenator should be to bring the bitumen to lower stiffness and viscosity and higher ductility. Despite the above description of bitumen ageing showing different and somehow interconnected mechanisms, taking place, simultaneously, the very first action to restore the original bitumen rheological properties is to shift back the ratio between solid asphaltenes and fluid maltenes to higher maltene contents. However, a rheological rejuvenating agent can exert its action in different sub-mechanisms (see the schematic diagram of Figure 1):  
1. Softening (usually called fluxing) agent: flux oil, lube stock, slurry oil, etc. can lower the viscosity of the aged binder;
2. Real rejuvenator: it helps to restore the physical and chemical properties [33].  
As it will be seen (see Section 4), literature does not make a clear distinction among the types of mechanism exerted by the specific rejuvenator: this name is usually given to any king of additive, which allows a certain restoring of the original rheological properties, no matter the effective mechanism involved. Instead, we are keen on making a clear distinction: given the aim of this contribution to go deeper into the details and to make clear all the aspects involved, it will be now defined a rheological rejuvenator as any type of additive causing a certain restoring of the rheological characteristics. In the specific, if the rejuvenator is also able to restore even the bitumen inner structure, then it will be defined as a real rejuvenator. If, on the other hand, the rheological rejuvenator makes the bitumen more ductile and less viscous/brittle by simply furnishing oily components to maltene without restoring the original complex structure (hierarchical structures of asphaltenes), then it will be called as softening (more often fluxing) agent. It can be seen that in this distinction, the term \"rheological rejuvenator\" has a more general sense and is associated with the concept of \"rejuvenator\" usually present in the literature. For this reason, in the following, when simply naming \"rejuvenator\" it will be referred to rheological rejuvenator, to resemble the classical meaning of rejuvenator usually adopted. Instead, when dealing with real rejuvenator effect, the terminology \"real rejuvenator\" will be stressed. Rheological rejuvenators are, however, usually based on oils: lubricating oil extracts and extender  
Figure 1: Scheme showing the main processes involved in bitumen ageing and its rejuvenation.  
oils. They contain an adequate amount of maltene constituents, naphthenic or polar aromatic fractions, which re-balance the composition of the aged binder in favour of such compounds or those usually lost during construction and service. Rejuvenator must fulfil two requirements:  
1. They should have a high proportion of aromatics, which are necessary to keep the asphaltenes dispersed;
2. They should contain a low content of saturates, which are highly incompatible with the asphaltenes.