# A Review on Bitumen Rejuvenation: Mechanisms, Materials, Methods and Perspectives
## 1 Introduction to Bitumen and Ageing  
Asphalts are well-known materials used for road pavement throughout the world. They are heterogeneous systems where one phase is constituted by macro-meter sized inorganic particles called aggregates, and the other one is the binding agent (bitumen). The latter, in turn, is a micro heterogeneous complex viscous fluid constituted by nano-meter sized aggregates of polar molecules (asphaltenes) hierarchically organized with different levels of aggregations [1], and dispersed in a more apolar continuous phase of saturated paraffins, aromatic oils and resins called maltene [2; 3]. Asphalts are, therefore, biphasic systems, with the predominant phase (93-96%, _w/w_) made by the macro-meter sized inorganic aggregates (size from microns to millimeters) hold together by small amounts of binding bitumen which constitutes the second phase. Although the bitumen constitutes the minor part of the asphalt, it however plays the most important role, giving consistency to the overall material, which is necessary for practical purposes: even slight changes in the bitumen will affect the overall properties of asphalt. Even the usability of asphalt can be traced back to the properties of its bitumen. This work will focus the attention on bitumen as a key ingredient in asphalt. Where necessary, however, some extensions will cover asphalt and related aspects. The aggression of chemicals normally presents in the environment, or ageing can oxidize some of the organic components in the bitumen so that the increase in polar functional groups can cause immobilization of an excessive number of macromolecules and ultimately bitumen embrittlement and asphalt cracks. Other processes causing ageing are loss of volatiles and changes in a molecular organization driven by the spontaneous tendency to reach a stable (equilibrium) thermodynamic state, which obviously depends on the conditions [4]. As a consequence of the mentioned processes taking place in bitumen, the final asphalt is susceptible to fracturing or cracking in thermal or mechanical stresses. Ageing of bitumen, therefore, constitutes a serious problem. Aged asphalt can be re-used in mixtures with new binders, answering the economic need for low-cost production and fully facing obvious environmental concerns. Therefore, recent research has been focused on reclaimed asphalt pavements (RAPs), which really are environmentally friendly alternatives and constitute an economically viable way to afford the costs of binder and aggregates. However, their use is still limited (less than 20% in the new mixtures), due to their low rheological performances (high stiffness and low stress relaxation ability [5] which can cause unexpected premature failure [6]. To overcome these problems, opportune actions must be taken to improve the mechanical and chemical properties of aged bitumen or RAP/bitumen mixtures. The addition of opportune additives is certainly one of the most effective. For example, a compound able to tune the red-ox state of the polar molecules contained in the bitumen can avoid the degrading process. The restoring of a favorable asphaltene/maltene relative ratio by providing more maltene is another solution, since the viscosity of bitumen is related to the fraction of asphaltene [7]. Another strategy can be directed to the stabilization of the supramolecular aggregates mainly made by asphaltene formed in the maltene phase [8; 9] acting on their interfacial tension and consequently better dispersing them. In all these cases, a rejuvenator is usually dealt with. With this work, the recent studies carried out in this field will be highlighted in order to shed light on the possible mechanisms of actions of a rejuvenator and to furnish a panoramic view considering both theoretical considerations and applicative aspects. This works will show the state-of-the-art in the use of rejuvenators in bitumen, taking care to highlight also some applications to the rejuvenation of recycled asphalt for a more complete view of the problematics.