This type of fatigue cracking first shows as short longitudinal cracks in the wheel path that quickly spread and become interconnected to form a chicken wire/alligator cracking pattern. These cracks initiate at the bottom of the bituminous layer and propagate to the surface under repeated load applications.
This type of fatigue cracking is a result of repeated bending of bituminous layer under traffic. Basically, the pavement and bituminous layer deflects under wheel loads that results in tensile strains and stresses at the bottom of the layers. With continued bending, the tensile stresses and strains cause cracks to initiate at the bottom of the layer and then propagate to the surface .This mechanism is illustrated in figure below
The following briefly lists some of the reason for the higher tensile strains and stresses to occur at the bottom of the bituminous layer
· Relatively thin and weak bituminous layers for the magnitude and repetitions of the wheel load.
· Higher wheel loads and higher tyre pressures
· Soft spots or areas in unbound aggregates base materials or in the subgrade soil.
· Weak aggregate base/Sub base layers caused by inadequate compaction or increases in moisture contents and or extremely high ground water table.
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