Using Statistical Analysis to Assess the Default Structural Properties of Alabama Department of Transportation Base and Subgrade Materials Used for Flexible Pavement Design

Abstract

Structural properties of materials are important during flexible pavement design since they represent the capacity to withstand traffic loads. It is essential to regularly update the structural inputs for design to achieve optimum pavement designs. Alabama Department of Transportation (ALDOT) updated the structural inputs for hot-mix-asphalt in 2009, but not yet those for base and subgrade materials. Therefore, the goal of this study was to evaluate and update the materials’ structural inputs used by ALDOT for: 1) aggregate base (limestone and sandstone), 2) stabilized base (full-depth reclamation with cement [FDR-C]) and soil-cement [S-C]), and 3) subgrade. For that purpose, numerous laboratory and field-testing results for those materials were used to obtain resilient modulus (M_R) and structural coefficients, which were compared with ALDOT default values using statistical analysis methods. For aggregate base, the M_R of limestone was statistically greater while the M_R of sandstone was statistically lower than the default design values. For stabilized aggregate base, the M_R of FDR-C and S-C were statistically lower than the default values when using 7-day unconfined compressive strength (UCS) and statistically greater when using 28-day UCS. The use of 28-day UCS in existing correlation equations to obtain structural properties of FDR-C and S-C was shown to provide more realistic values and, therefore, is recommended for design purposes. The practical difference between the new base materials properties and the default design values was also evaluated through a case study. For subgrade, new recommended M_R values were provided for all soil types.