## 3 Classification of Pavements
### Fatigue Damage to the Asphalt Pavement  
The useful life of a pavement, in terms of service and performance, is related to its correct dimensioning which seeks to avoid the occurrence of structural defects such as cracking and subsidence. These two are the main defects of asphalt pavements, traditionally called flexible pavements. Cracking can be caused by phenomena such as fatigue in which intermittent loads cause cracks, which is progressive damage. Sinks are due to a mechanism called permanent deformation. These defects are caused by two different modes of mechanical demand like repeated bending (responsible for the fatigue of the material) and simple compression, in the development of the service [113, 114, 115, 116, 117, 118, 119, 120]. In a mechanistic methodology for pavement design, the stresses and deformations required by the structure are calculated and compared with the permissible stresses and deformations of the materials to be used, which are obtained through laboratory tests or mechanistic-empirical equations [121]. Thus, by evolving from an empirical approach to a rational or mechanistic-empirical one, it becomes possible to calculate stresses and deformations that will lead to the correct functioning of pavements in the face of previously ignored or poorly understood defects. Fatigue cracking and permanent deformation are the main acceptance and/or evaluation criteria of the pavement structure. They correspond to the most common defects that most affect the usefulness of a pavement throughout its useful life [122, 123, 124, 125]. The well-dosed and well-executed asphalt layer, at the beginning of its useful life, is subject to a small permanent deformation that is characterized more by compaction than by the viscous characteristics of the asphalt binder [126]. This initial deformation must be small or unimpressive. Even reduced, it allows for closing of the asphalt mix with a small reduction in the volume of voids. Permanent set caused by asphalt binder viscosity associated with particulate lubrication is most pronounced over the life of the asphalt mix. Obviously, in poorly dosed asphalt mixtures and with unsuitable binders for the climate and demanding traffic, the deformations caused by lubrication and viscosity can be important at the beginning of the useful life [127, 128, 129]. Asphalt coatings have high surface temperatures due to the black color of the asphalt binder [130, 131]. The surface of the mixture absorbs large amounts of solar radiation that the coating stores as heat. The heat stored throughout the day is transmitted from the coating to the lower layers of the soil by conduction. During the day, soils become considerably warmer than the environment. During the night, the heat stored in the lower layers is transmitted to the coating and this radiates the excess heat to the environment. Thus, the surface temperatures of the coating are even higher than those of surfaces covered with Portland concrete or with vegetation [132]. The wheel tread sag (Automated Traffic Recorders (ATR)) observed in a pavement is the sum of the permanent deformation portions experienced by all its constituent layers, these being subgrade, subbase, base, and cladding [133, 134]. This non-recoverable deformation is a function of several variables such as weather and traffic conditions, together with the level of stress supported by the layer. However, due to the proximity of the load and the high stiffness compared to other layers of pavement, the asphalt coating is usually responsible for most of the magnitude of the total permanent deformation of the structure. This is especially evident on roads with heavy traffic, featuring vehicles with high axle loads and high tire pressure. Thus, the role of each constituent of the asphalt mixture (asphalt binder, aggregates and volumetry) in the phenomenon of permanent deformation will be discussed below. There are two main mechanisms of fatigue cracking in asphalt pavements, being top to bottom and bottom to top [135].