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The major flexible pavement failures are fatigue cracking, rutting, and
thermal cracking.
The fatigue cracking of flexible pavement is due to horizontal tensile
strain at the bottom of the asphaltic concrete.
The failure criterion relates allowable number of load repetitions to
tensile strain and this relation can be determined in the laboratory fatigue test on asphaltic concrete specimens.
Rutting occurs only on flexible pavements as indicated by permanent deformation
or rut depth along wheel load path.
Two design methods have been used to control rutting: one to limit the vertical
compressive strain on the top of subgrade and other to limit rutting to a tolerable
amount (12 mm normally).
Thermal cracking includes both low-temperature cracking and thermal fatigue
cracking.
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