Adaptation of Austroads mechanistic-empirical pavement design for tropical climates

Abstract

Many road agencies have employed conventional empirical pavement design methods, such as the American Association of State Highway and Transportation Officials (AASHTO) and Transport Research Laboratory (TRL) Overseas Road note 31 guidelines, to design and analyse flexible pavements. Empirical pavement design methods utilise empirical formulae based on past experiments conducted in extreme weather conditions. They have significant drawbacks permitting limited freedom for pavement designers, leading to material constraints in road projects. Such restrictions may result in high costs for hauling materials from far sites, leading to increased costs for road construction projects. Road agencies have identified the advantages of mechanistic-empirical (M-E) pavement design methods that accommodate available materials resulting in economical designs. However, performance models given in ME methods are derived under laboratory conditions and require calibrations to the field. Austroads is one of the most recognised M-E guidelines widely used in Australia and New Zealand. It accompanies a user-friendly and time-efficient software packageCIRCLY. This research was focused on adapting the Austroads M-E pavement design guidelines for tropical climates. Austroads suggests two performance models, each for fatigue cracking and subgrade rutting. This research focused on calibrating the default performance models proposed by Austroads by finding the most suitable damage exponent (b) and shift factor (SF) for tropical climates. Calibrating the default fatigue performance model was only considered. Data from two A-class roads were obtained for model development and model validation. Cumulative damage factor (CDF) representing the accumulated fatigue damage was estimated at varying damage exponents (b) and shift factors (SF) using the mechanistic design software-CIRCLY. The alligator cracking index (ACI) suggested by the Federal Highway Administration (FHWA), quantifying the fatigue damage level, was calculated using visual surveys to detect and classify pavement distresses. It could be observed that the CDF have a strong relationship with the ACI. The most robust relationship between the ACI and the CDF could be observed at the damage exponent (b)= 5.1 and the shift factor (SF)= 2.5. CDF values were computed for the model validation by inputting the calibrated damage exponent (b=5.1) and shift factor (SF=2.5) to the mechanistic design software- iii CIRCLY. ACI values were predicted by substituting these calculated CDF values for the derived regression model. When the observed ACI values were plotted against the predicted ACI values, it could be noticed that the best-fit curve for the observed ACI and predicted ACI has R2 = 0.99 and slope = 1.007. This observation emphasizes the accuracy of the derived model, thereby justifying its use for the design and analysis of flexible pavements for tropical climatic conditions. Therefore, it can be justified that adapting the Austroads mechanistic pavement design guideline for tropical climates is highly favourable by modifying the default performance relationships. The use of mechanistic tools to design and analyse pavements was strongly emphasised during the analysis steps, ensuring the reliability of such tools in pavement design and analysis. As the M-E design procedure allows the designer to use available materials, adapting the M-E design procedure will address the material constraint in road construction projects and deliver more cost-effective designs. This will finally help reduce unwanted expenditures and improve the economic aspect of road construction projects. Keywords: Mechanistic-Empirical (M-E) Pavement Design, Austroads, Modified failure functions, Alligator Cracking Index (ACI), CIRCLY, Cumulative Damage Factor (CDF), Tropical climates