A Method of Maximum Thickness for Flexible Pavement Design
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2024-03-01
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Edition:Final Report
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Abstract:Asphalt pavement thickness design, for both new and overlay scenarios, has been evolving since the 1960’s from empirical to mechanistic-empirical design frameworks. Regardless of the design methodology, the fundamental goal of thickness design has been to determine the optimum cross-section to achieve the desired performance under the prevailing conditions (i.e., traffic, climate and materials). Central to that effort has been the calibration of design systems to accurately predict pavement deterioration over time. While calibration can be effective in developing more accurate predictions and, therefore, optimizing cross-sections, most design systems do not have a maximum thickness built into the procedure, which can yield unreasonably thick cross-sections. Figure 1.1 shows a simple example, developed with the AASHTO 1993 Design Guide, where the Asphalt Concrete (AC) layer thickness is plotted against the design traffic, expressed in terms of Equivalent Single Axle Loads (ESALs). Clearly, the AC layer thickness increases indefinitely with traffic to overly thick sections that are likely overdesigned and impractical to build. This problem also exists with mechanistic-empirical design approaches such as the Mechanistic-Empirical Pavement Design Guide (AASHTO, 2008). Each design approach is simply a set of mechanistic and empirical equations that yield recommended design thicknesses given a set of material, structural, traffic, and climate inputs and leaves it to the designer, or written policy, to decide when it is sufficiently thick.
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