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Fatigue characteristics of asphalts

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Fatigue can be defined as: ‘The phenomenon of fracture under repeated or fluctuating stress having a maximum value generally less than the tensile strength of the material’. However, this has been generally

accepted as referring to tensile strains induced by traffic loading, and, because other means of generating tensile strains in a pavement exist, a better definition may be: ‘Fatigue in bituminous pavements is the phenomenon of cracking. It consists of two main phases, crack initiation and crack propagation, and is caused by tensile strains generated in the pavement by not only traffic loading but also temperature variations and construction practices’. Under traffic loading, the layers in a flexible pavement are ubjected to flexing that is virtually continuous. The size of the strains is dependent on the overall stiffness and nature of the pavement construction, but analysis confirmed by in situ measurements has suggested that tensile strains of the order of 30 × 10−6 to 200 × 10−6 for a standard wheel load occur. Under these conditions, the possibility of fatigue cracking exists.