Determination of Creep Compliance and Tensile Strength of Hot-Mix ...

INTRODUCTIONWith the Missouri Department of Transportation (MoDOT) beginning to fullyimplement the new Guide for Mechanistic-Empirical (M-E) Design of New andRehabilitated Pavement Structures (1), the need existed for various types of testingof hot-mix asphalt (HMA) used by MoDOT in its flexible pavements. The AmericanAssociation of Highway and Transportation Officials (AASHTO) test method T 322-07 (2) is utilized to determine HMA properties that are needed as inputs to the M-EPavement Design Guide (M-E PDG) software.Two HMA properties derived from AASHTO T 322-07 are creep compliance andtensile strength. Creep compliance is defined as time-dependent strain per unitstress while indirect tensile (IDT) strength is best defined by what its name implies;HMA strength when subjected to tension. Both properties are determined using theIDT method; i.e. a cylindrically shaped specimen is loaded in compression across itsdiameter thus indirectly causing tension in opposite directions perpendicular to andbeginning at the line of loading. As HMA is considered a visco-elastic material, creepcompliance and tensile strength are not only dependent on the HMA mix constituentproperties, constituent proportions, and compacted mix properties (e.g. % air voids),both are also temperature dependent. Additionally, creep compliance is dependenton the load/unload duration and tensile strength is dependent on load rate.The contract was started when T 322-03 (3) was the current version for determiningcreep compliance and tensile strength using IDT methods. T 322-07 was publishedin the summer of 2007. Some changes to T 322-03 were in response to resultspublished in the National Cooperative Highway Research Program (NCHRP) Report530 (4). Especially in the context of M-E PDG inputs, creep compliance and tensilestrength determination has been a moving target and, thus, experts (seeAcknowledgements) were contacted in regard to how these properties are actuallybeing obtained in practice. It is fair to say that there were about as many methodspromoted and opinions expressed as there were contacts. Nonetheless, T 322-07was adhered to as closely as possible, with a few exceptions (see TechnicalApproach section).MoDOT contracted with Missouri University of Science and Technology (MissouriS&T) to perform the creep compliance and tensile strength testing on several HMAmixes used in wearing (surface) courses throughout the state. Test results areneeded by MoDOT to calibrate the M-E PDG thermal (low-temperature) crackingdistress models to local conditions; e.g. locally available HMA mix constituents.1