Assessment of Reliability and Load Factor Design Approach Against Fatigue Cracking in Flexible Pavements

A reliability-based approach for designing flexible pavements that resist bottom-up and top-down fatigue cracking is developed. The approach uses load factors applied to the nominal fatigue cracking values to obtain failure values in the field. The uncertainty in input parameters is modeled with Weibull and lognormal distribution of the damage index with different values of skewness and coefficient of variation. Field data from the Long-Term Pavement Performance database are used to model the uncertainty in prediction between the nominal (predicted) and actual field cracking. The probability density function of fatigue cracking is found to be asymmetrical; the skewness of the distribution changes with the level of damage. The nonlinear limit state performance function is used to determine the reliabilities and the load factors through Monte Carlo simulation. Differences between the failure values obtained from the standard Gaussian assumption and the simulation are observed. Design charts and load factor tables are developed and can be used with the new Mechanistic-Empirical Pavement Design Guide for designing new and rehabilitated pavements. Finally, the use of load factors in flexible pavement design is presented through practical examples.