Equivalent Damage Factors Based on Mechanistic-Empirical Pavement Design

One of the key objectives in pavement design and analysis is to determine pavement life under given structural, environmental, and traffic conditions. The AASHTO 1993 Design Guide estimates pavement life in terms of the number of equivalent single axle loads (ESALs). Its design equation was established through empirical analysis primarily based on AASHTO Road Test in Ottawa, Illinois, in late 1950’s. Recent NCHRP has sponsored a comprehensive research study that develop the guide for the “Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures”, commonly referred to as the M-E Design Guide. This new guide provides a convenient and more accurate way to determine pavement performance as a function of time or the number of vehicle or axle repetitions under different failure criteria. In this study, two commonly used failure criteria in flexible pavement, 0.5 in. surface rutting and 10 percent fatigue cracking, were evaluated for estimating pavement performance under various conditions. The concept of Equivalent Damage Factor (EDF) was used to quantify and compare pavement performance as a function of increasing axle loads. A series of models expressing EDF as a function of relevant variables affecting pavement life were formulated and estimated. It is demonstrated that the models developed and their parameter implications agree with previous research findings and engineering judgment and, in addition, allows for the quantification of the effects of these relevant variables. The usefulness of the proposed models can be summarized as follows: 1) the damage on pavements by a given axle load can be easily quantified, 2) equivalent loads for different axle configurations can be determined, and 3) the models enable a quick and approximated estimation of pavement performance under changing distributions of axle configurations and loads.