Flexural beam fatigue testing of asphalt mixtures has
been used for nearly forty years in the pavement industry. Since the development of the test, the
definitions of initial and failure stiffness have not been verified or validated
in any comprehensive study. The main
objective of this study is to validate the criteria used to define the initial
and the final stiffnesses in flexure fatigue testing. In this study, extensive flexure fatigue
tests were performed on five typical dense-graded mixtures and an asphalt
rubber gap-graded mixture. An optimization
approach was used, in which different initial and failure conditions were
assumed. Fatigue models were developed
using linear regression curve fitting and the conditions that produced the best
fit were selected. Both the
phenomenological and the dissipated energy approaches were used. Test results conclusively indicated
that the initial stiffness should be defined at cycle number 50. In addition, when a phenomenological approach for fatigue is employed, the
fatigue failure stiffness should be taken at 50 percent of the initial
stiffness. A stiffness degradation model was developed, which provided
an independent proof that failure occurs when the stiffness of the beam is
reduced to 50 percent of the initial stiffness.
This model represents a basic material property at which damage
accumulation in the mixture has produced an inability of the mix to resist
further damage independent of the mode of loading. In contract to
the tensile strain-failure approach, data analysis with the energy approach
showed that fatigue failure stiffness, taken at 30 percent of the initial
stiffness, provided identical fatigue energy failure regardless of constant
stress or strain mode of loading. The
results show that the phenomenological
and the energy approaches provide different definitions of failure and the test
should be consistent with the method of analysis used.
Hot mix asphalt, asphalt
rubber, flexural beam fatigue, fatigue failure criteria, initial stiffness,
failure stiffness, stiffness degradation model.
