Differences in Predicted Flexible Pavement Performance Using the AASHTOWare Pavement-ME—ESALs versus Axle Load Spectra

Traffic loading is one of the most critical inputs in the pavement design process. Load characterization includes the magnitude and the number of load repetitions for each axle configuration. Historically, procedures for axle load characterization in pavement design include: (a) fixed traffic load, (b) fixed vehicle/axle, and (c) variable traffic and vehicle/axle. Generally, the first procedure is adopted for airport pavements, the latter both characterizations are used for the design of highway pavements. In a fixed vehicle/axle procedure, the thickness design is based on the number of repetitions of a standard vehicle or axle load, usually the 18-kip single axle load for highway pavements. If the axle load is different from 18-kip or consists of tandem or tridem axles, it is converted to 18-kip single axle repetitions by a load equivalency factor (LEF). Finally, for variable traffic and vehicle/axle, both axles and their repetitions are considered separately and required axle load spectra (ALS) for different axle types. The concept of equivalent single axle load (ESAL) was developed in AASHTO design guide while mechanistic-empirical design method (Pavement-ME) uses ALS to characterize axle loadings and repetitions. The primary objective of the paper is to evaluate the effects of different load characterizations (ESAL versus ALS) on flexible pavement performance. The approach includes analysis of a standard flexible pavement section by using the axle load spectra from 41 weigh-in-motion (WIM) stations across Michigan. The ESALs were estimated from the ALS for all the WIM stations. Subsequently, the traffic load and repetitions were modeled as ESALs in the Pavement-ME. Flexible pavement performance (fatigue cracking, rutting, and IRI) was predicted by using the site-specific ALS and ESALs. Pavement damage and performance measures were compared between ALS and ESALs to establish the impact of load characterization on the flexible pavement performance. The results show that ALS demonstrated different damage and predicted flexible pavement performance as compared to ESALs.