# Char from Pyrolysis of Waste Tires to Increase Bitumen Performances
## 1 Introduction  
In the last decades, the ever-increasing traffic demand brought about through the transportation of people and merchandise has elucidated the need of high-performance road pavements. Road pavements are made up of 95% wt/wt crushed stones and inorganicaggregates bound together by a viscoelastic organic-based fluid called bitumen. That being said, the most effective way to obtain better road pavements is to improve their fluidic part, i.e., the bitumen binder, since its role in holding up the whole structure gives it a pivotal role in determining the overall properties of the road pavement. Road pavement characteristics depend therefore on the physico-chemical properties of the bitumen used [1]. Through bitumen modification, the characteristics of a bitumen of low quality can be improved, thereby optimizing economic benefit from industrial raw materials, especially in a situation of deficiency of oil resources [2]. A bitumen modification approach, which deserves more attention, is the blending of the binder with fragments of wasted vehicle tires. This brings about important modification of rheological properties such as an increase in resistance to fatigue and fracture, and in rutting resistance [3; 4]. A more sophisticated approach implies the treatment of bitumen with the solid residue (char) derived via the pyrolysis of waste tires. Pyrolysis is a fundamental process in the reduction in urban waste, which entails a thermal degradation process in the absence of oxygen, which converts wastes into combustible gases [5], an oil rich in hydrocarbons and oxygenated species (used for several applications [6; 7; 8; 9]) and a solid, powdery residue (char) [10; 11], which contains high amounts of carbon. It must also be noted that the absence of oxygen prevents combustion and therefore CO\({}_{2}\) emission. For the purposes of our work, the last (solid) fraction is the most important as it can be well incorporated into bitumen's organic-based structure thanks to its carbonaceous nature [12] and the porous/fibrous structure allows, in principle, enhanced interactions with bitumens [13]. Similar studies showed the great potentialities of char, whose applications need, however, adequate characterization [14]. A recent study demonstrated the use of char as an example of how to use waste in a circular economy view [15]. The scientific idea originating our work lies in the possibility that the waste from urban activities can be treated through pyrolysis, becoming added-value input ingredients for the preparation of bitumens. In this way, bitumens can have stronger resistance upon a temperature increase, suggesting lower maintenance costs of road pavements and eventually a reduction in waste materials. The environmental benefit of this kind of work needs to be emphasized. In fact, efforts to minimize resource input and waste output need to be made [16]. Legislation in fact is leaning toward a transition to a regenerative circular economy [17] and so initiatives fostering raw material re-use and recycling while reducing the production of waste [18] must be encouraged. Production of asphalts with a higher performance coupled with optimal re-use of wastes facilitated through pyrolysis is an ideal match. The use of pyrolysis residues is not trivial, since its characteristics (chemical and morphological) strongly depend on the pyrolysis parameters: the heating rate, the temperature range and the gas residence time. Therefore, these parameters must first of all be defined for reproducible results and, secondly, their effects need to be clarified. Although these parameters have been explored in the literature, to the best of our knowledge, the effect of the gas flowing on the material under the pyrolysis process has not yet been deeply studied. This is important knowledge, which is lacking, since the type of the flowing gas can affect the chemical reactions taking place in the material under pyrolysis, especially at the surface.