A review of top-down cracking in asphalt pavements: Causes, models, experimental tools and future challenges

In the last decades, a new type of distress has been observed more and more frequently on asphalt pavements. This distress, ascribable to fatigue failure, has been named top-down cracking (TDC) because it consists in longitudinal cracks that initiate on the pavement surface and then propagate downwards. A series of surveys recently carried out on Italian motorways highlighted that TDC can affect up to 20%–30% of the slow traffic lane. Therefore, in order to achieve a better understanding of such distress, this paper reviews causes, models and experimental tools and highlights future challenges for TDC. The literature review indicates that TDC can evolve on the pavement surface in three stages (i.e., single crack, sister cracks, alligator cracking) and, below a certain depth, the cracks can form angles of 20°–40° with respect to the vertical plane. Even though multiple factors contribute to TDC development, thick pavements are more likely to fail due to TDC induced by tire-pavement contact stresses, especially in the presence of open-graded friction courses (OGFCs). Moreover, in literature there are several TDC models based on mechanics (e.g., fracture mechanics or continuum damage mechanics) which allow a rigorous study of crack initiation and propagation. Future challenges include the identification of a reliable and feasible test method, among those proposed in literature, to study the TDC performance of asphalt mixtures and the implementation of TDC in pavement management systems (PMSs) through the definition of criteria for TDC recognition in the field as well as for the rehabilitation depth evaluation. Finally, more research is needed for open-graded asphalt mixtures, which present critical drawbacks in terms of TDC.