Design criteria for flexible pavements

The design criteria or distress modes adopted by most analytical and semi-analytical methodologies for flexible pavement design are as follows (a) fatigue of treated layers, that is, the asphalt layer or the hydraulically bound layer not to crack under the influence of traffic and (b) deformation of the subgrade, that is, the subgrade to be able to sustain the traffic without excessive deformation. 

Repetitive tensile strain applied during the pavement s service life will initiate the first crack at the underside surface of the bottom asphalt layer (or hydraulically bound layer), which will propagate to the surface causing surface cracking. Similarly, repetitive compressive strain at the surface of the subgrade will cause subgrade deformation, which will eventually spread to the surface of the pavement, resulting in surface or structural deformation. 

The expression repetition of loading during the pavement s service life implies that the structural condition of the pavement deteriorates with time. In fact, the deterioration mechanism of the pavement is a fatigue phenomenon, related to the number of load repetitions and the magnitude of the exerted strain. 

The number of load repetitions for failure may be determined from fatigue equations by knowing the magnitude of the exerted strain, e. Similarly, from a fatigue equation, the allowable strain may be determined by knowing the number of load repetitions. A fatigue equation has the following mathematical expression  

Fatigue equations for asphalts have been developed by various organisations and research institutes and more information can be found in Section 7.7. 

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California bearing ratio Design criteria for flexible pavements... 

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