## 7 Added-Values Given by the Use of Nanoparticles  
There are numerous examples showing that the addition of nanoparticles within the bitumens or asphalts confers specific properties of certain added values. For example, hot mix asphalt mixtures modified with carbon nanofibers can give piezoresistive responses [86]. Modification of bitumens with nanoclays (montmorillonite) reduces their flammability. In this way, bitumen can move from a \"fuel\" classification to a \"self-extinguishing\" classification [87]. The proposed  
Figure 4: Panel **(A)**: (a) 3D representation of cup-stacked graphene layers in a single CNF, and (b) Simplified schematic of stacked-cup carbon nanofiber helical structure with inset showing TEM image of inclined orientation of grapheme planes along the side of structure with respect to nanofiber axis. Image taken from [76]. Courtesy of Elsevier. Panel **(B)**: Schematic of size and morphology distribution of various nanotubular structures that shows the main difference in the structure between carbon nanofibers and carbon nanotubes. (a) single wall carbon nanotube, (b) multi walled carbon nanotube, (c) carbon nanofiber, (d,e) carbon fibers of different complexity. Reprinted from [68]. mechanism of action is interesting: nanoclays can catalyse char-forming reactions producing charred residues which are able to delay the escape of volatile products and the penetration of oxygen thus hindering combustion. Nano-zinc oxide can also behave like an antistripping agent in hot mix asphalt (HMA) with limestone and granite as aggregates [88]. It was found, in fact, that nano-ZnO decreases the acidity of the bitumen thus favouring the adhesion between the bitumen itself and acid aggregates, such as granite. In this way, nano-ZnO improves the adhesion between the bitumen and the aggregates, especially in the case of moisture-conditioned samples. Another very interesting added value provided by the addition of nanoparticles is the protection against aging. Aging effect, indeed, is particularly severe in surface layers that are exposed to environmental conditions such as UV radiation, moisture, oxygen, and temperature changes [89]. In this ambience, even if there are examples of uses of very expensive materials like silver nanoparticles [90] to contrast oxidative aging, nano-TiO\({}_{2}\) and nano-CaCO\({}_{3}\) were found to have a possible antioxidant action [55]. However, another interesting effect is that provided by semiconductor nanoparticles in the screening of UV-radiation. Thus, nano-ZnO was found to give good resistance of modified bitumens to the UV aging, although [PERSON] et al. [91] found that their effect depends on the bitumen nature. The potential of ZnO nanoparticles to increase the resistance of bitumen to ultraviolet aging was also studied by [PERSON] et al. [64] and [PERSON] et al. [65]. Since some semiconductor nanoparticles have some problems in chemical compatibility with the apolar bitumen [51], other authors modified their surface to better disperse nanoparticles within the bituminous matrix [65; 67]. The problems of obtaining a better distribution of nanoparticles come above all from the chemical reasoning that asphalt concretes are heterogeneous materials: there are composed of aggregates, nanoparticles, bitumen and pores; sometimes there are also networks, so the mechanical properties depend also on the form and the distribution of the aggregates. Some more comments are given below, in Section 8.2 entitled \"How to Probe the Distribution of Nanoparticles: Nanotomography as One of the Techniques of Election\". However, in the case of semiconductor nanoparticles, we want to stress another aspect which in our opinion has to be taken into account for. Semiconductor nanoparticles have basically two effects whose relative contributions are in complex competition: (i) their semiconductor nature allows absorption of those radiations whose energies are above the energy gap value and (ii) the size in the nano-regime allows radiation scattering within the sample.