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Practical Method to Determine Strain Level for Cyclic Direct Tension Fatigue Testing in the Asphalt Mixture Performance Tester

This paper presents a practical procedure for estimating strain levels in the asphalt mixture performance tester cyclic direct tension fatigue test and provides practitioners with guidance for selecting a strain value for testing materials with unknown fatigue characteristics. A large variety of plant-produced and laboratory-prepared mixtures were analyzed. These included hot- and warm-mix asphalt, reclaimed asphalt pavement, and recycled asphalt shingles and various gradations and air void contents; all were tested following the AASHTO TP 107 procedure. The experimental results from a segregated material illustrate that AASHTO TP 107 could produce repeatable results and was sensitive to field variations in binder content. Data for satisfactorily performing and poorly performing mixtures were clustered on the plot of cycles to peak phase angle versus actuator strain level and a Black Space diagram evaluated at the AASHTO TP 107 test temperature. Curves of cycles to peak phase angle versus actuator strain level were built from the experimental database, with each curve representing one ideal mixture that followed a particular power function. The dynamic modulus data are used to determine a recommended actuator strain level to start the set of fatigue tests for each cluster.