Determination of Equivalent Axle Load Factors on the Basis of Fatigue Criteria for Flexible and Semi-Rigid Pavements

This paper presents the analysis of equivalent axle load factors on the basis of fatigue criteria used in various mechanistic-empirical methods of pavement design. It specifies the formulas of fatigue criteria for flexible pavements (asphalt fatigue cracking and structural deformation criteria) and for for semi-rigid pavements (fatigue cracking criteria for cement treated bases - CTB). These criteria were used to derive the formulas to calculate the equivalent axle load factors. The numerical results of calculation are given. It was found that equivalency factors are closely related to the mathematical form of fatigue criteria. The formulas for calculating the equivalent axle load factors for flexible pavements resemble the well known "fourth power equation", except that the exponent it ranges from 3.3 to 5.0 m relation to the fatigue criteria used. The formulas for semi-rigid pavements are much more complex. The equivalent axle load factors differ in relation to pavement structure and depend on bending strength or limit strains of cement treated base and on configuration of semi-rigid pavement (thicknesses and material of layers and subgrade and their moduli). There are considerable differences between the equivalent axle load factors derived for semi-rigid pavements from different fatigue criteria used in various design methods. If an exponential formula, similar in form to the "fourth power equation", was used the exponent n derived from various design method ranged from 4 to 20 and even more. The formulas were derived assuming linear relationships between stresses and strains induced in layers of pavement structure and the axle load value. This assumption has been verified for certain pavement structures.