Evaluation of Moduli and Structural Coefficients of South Carolina’s Asphalt Base Mixtures

Abstract

South Carolina, like many states, is moving toward implementation of AASHTOWare Pavement ME Design. In South Carolina, asphalt base mixtures are commonly referred to as black base. Dynamic modulus values are a significant input required for asphalt mixtures. Moduli for the actual mixes that will be used to construct the pavement are unlikely to be available at the time the pavement is designed. Consequently, catalogues of values or prediction models based on state specification ranges must be used for design. Dynamic modulus tests were performed on four production mixtures, representing six of the seven districts of South Carolina, according to AASHTO TP 79-15. These data can be used as the beginning of a catalogue of asphalt base moduli. Comparisons between predicted moduli, generated with both the Witczak and Hirsch models, and measured data indicate the inclusion of reclaimed asphalt pavement must be accounted for in the selection of binder grade used in prediction models to accurately predict moduli. Mixes with greater than 30% binder replacement required a two-grade high-temperature and a one-grade low-temperature increase in stiffness. South Carolina will continue to use the 1972 AASHTO pavement design guide until Pavement ME is implemented. Use of asphalt base mix for pavements designed with the 1972 pavement design guide has declined in South Carolina in favor of options that appear to be less expensive. However, if the structural coefficients used for the materials are inaccurate, pavements designed with different layers will not perform similarly. According to the measured moduli, it is recommended that South Carolina increase the structural coefficient for asphalt base mixtures to 0.44.