# A Review on Bitumen Rejuvenation: Mechanisms, Materials, Methods and Perspectives
## 4 The State-Of-The-Art  
### General Requirements  
As a general requirement, additives should be non-hazardous and stable over a wide range of temperatures, from production to application. In addition, they must not experience any exudation or evaporation, in order to ensure good performance over the designed lifetime of the asphalt pavement. Further specific requirements depend on the local country specification. [PERSON] et al. for example [47], focus on the use of a specific bio-based rejuvenator to produce asphalt concrete using a high amount of RAP without scarifying the mix performance and complying with the Italian specifications. In particular, the paper presents the results from a trial section on a highway connecting Ancona to Perugia, in the center of Italy. In any case the use of \"forbidden\" chemicals should not be the goal of the work: for example, [PERSON] et al. [48] take care in clarifying that the additives they studied in their work (a liquid process oil with a typical viscosity at 40 \({}^{\circ}\)C of 700 mm\({}^{2}\)/s) conform to all current Registration, Evaluation, Authorisation and restriction of Chemicals, (REACH) and Polycyclic Aromatic Hydrocarbons (PAHs) limits. Another additive they used (a tall oil distillate originating from pine trees) is \"not classified as dangerous according to Directive 6/548/EEC and its amendments and according to EU legislation\". In our opinion, any work should clarify the environmental/safety/legal issues connected to the additives presented. In this sense, [PERSON] et al. [49], and [PERSON] et al. [50] show that the addition of some particular bio-additives has a large potential application as reversible fluxing agents in bitumen industry, RAP technology and Warm Mix Asphalt (WMA). They used vegetable oil produced using various raw materials (Rapeseed oils, Soybean oil, Sunflower oil, Linseed oil, etc.) and combined in bituminous binders generally as a modifier/additive (up to 10%). Interestingly, not only they used vegetable oils, but they also perform chemical action to prepare other additives (they use methyl esters of fatty acid obtained by transesterification of vegetable oils). The aspect of a couple the use of raw/cheap materials with chemical actions to increase their performances is intriguing and promising. The chemical actions that can be performed for the optimization of additive performances will be discussed in Section 5. The rejuvenating benefits can be utilized to allow for higher RAP addition using traditional hot mix asphalt (HMA) configuration or in the warm mix asphalt (WMA) technology since the additives, changing the viscosity of the binder, allow for the lowering of the asphalt mixture production temperature. Critical issue related to the application of the additive fluxing of the bitumen is the final viscosity of the bitumen and stiffness of the mixture placed in the pavement. In fact, while fluxing is desired during mixture production, placement and compaction, it is no longer appreciated once the road is open to traffic. Considering the examples of [PERSON] et al. [49], and [PERSON] et al. [50], the suitability criterion for vegetable oils and the corresponding methyl esters for obtaining environment-friendly bitumen fluxes is their reactivity to the oxypolymerization reaction, which raises the viscosity of the stock, thus, contributing to its hardening and drying. The efficiency of polymerization depends on the number of double bonds and their position in the aliphatic chain of fatty acid. Oxypolymerization of the fatty acids present in the vegetable stocks is a multistage process and leads to the crosslinking of their structural units. The principle of using additives susceptible to oxypolimerization has been recently exploited for preparing additives, increasing the bitumen viscosity [51]. This paragraph, from the next sub-paragraph on, is devoted to showing the works involving rejuvenators as additives in bitumen.