Abstract Manager

Granular base and subbase layers constitute major portions of flexible pavement structures in Canada. Canadian experience has shown that thicker and good quality granular base and subbase materials improve pavement performance due to increased load carrying capacity, improved drainage from pavement subsurface and added protection from frost and freeze/thaw related damages. The AASHTOWare Pavement ME Design (PMED) software has been designed to predict several key distresses in pavements for varied inputs of traffic loads, layer materials, subgrade and climate. However, past studies have shown that this software is unable to adequately consider the influence of the granular base/subbase materials when compared to past performance experience. As part of continued design trials since 2007 to assess the suitability of the PMED software, the Transportation of Association of Canada (TAC) ME Pavement Design Subcommittee undertaken this study in 2025 to verify the findings of earlier studies using the latest version of the software. This study included two granular base and two granular subbase materials each having unique stiffness, gradation and other physical properties. Two different thickness of each layer material were also employed. Design trials were completed across eleven Canadian climatic regions, while traffic loading, asphalt layers, and subgrade conditions remained unchanged in all these trials.

The preliminary analyses of results indicate that rutting is more influenced by climatic condition than by granular base/subbase layer with negligible effects of thickness and stiffness, and with some inconsistencies. Thicker and stronger granular base and subbase generally reduce bottom-up fatigue cracking (BUFC). Changes in granular base thickness and stiffness result in negligible or inconsistent effects on the predicted top-down fatigue cracking (TDFC). Similarly, variations in subbase thickness show minimal or inconsistent influence, while its stiffness show no effect on the predicted TDFC. Observed trends indicate that thicker and weaker granular base and subbase result in increased rutting in the respective layer. However, a thicker base reduces rutting in the subbase layer, whereas a weaker base material shows negligible or inconsistent effects on it. Subgrade rutting decreases with increased thickness and stiffness of base/subbase, which is consistent with the expected pavement behaviour.

The objective of this paper is to present the details of these trials, results, analyses and findings to assist highway agencies and practitioners in evaluating the suitability of PMED software in terms of predicted influence of base and subbase material type and layer thickness on pavement performance.