Determination of Creep Compliance and Tensile Strength of Hot-Mix Asphalt for Wearing Courses in Missouri

David Newton Richardson, Missouri University of Science and Technology
S. M. Lusher

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Creep compliance and indirect tensile (IDT) strength of hot-mix asphalt (HMA) are the two primary inputs to the low-temperature or thermal cracking module in the new Mechanistic-Empirical Pavement Design Guide (M-E PDG) software. Creep compliance is defined as time-dependent strain per unit stress, while IDT strength is best defined as HMA strength when subjected to tension. AASHTO T 322 test protocol was used as reference for this work. However in preparation for the laboratory work performed at the Missouri University of Science and Technology many experts were consulted as to how IDT creep/strength testing and calculations are actually being performed. Using MoDOT supplied test specimens, six different plantproduced wearing (surface) course mixes were tested. Four mixes were tested at three levels of percent air voids: 4, 6.5, and 9% and two mixes were tested only at 6.5% air voids. Per requirements of the M-E PDG, creep testing was performed at 0, -10, and -20 degrees Centigrade (°C) and IDT strength testing was performed at -10°C. Additional IDT strength testing was performed at 4.4 and 21°C (40 and 70 °F) per MoDOT’s requirements. Poisson’s ratio was determined from the creep testing while tensile failure strain was determined from the IDT strength testing. Trends such as increasing creep compliance and decreasing tensile strength with increasing % air voids and/or temperature were confirmed. The presence of recycled asphalt pavement (RAP) in a mix tended to decrease the creep compliance (increase the stiffness) and increase the tensile strength compared to similar mixes without RAP.