Abstract:
Fatigue in the Hot Mix Asphalt (HMA) pavements is an important distress mainly caused due
inadequate and deficient initial pavement structure and bituminous mixture design. This study
investigates the fatigue characteristics of four selected wearing course gradations, including
two gradations NHA A and NHA B adopted from local agencies and two gradations adopted
from Superpave (SP-1) and Asphalt Institute manual series (MS-2), using the indirect tensile
fatigue test under stress controlled mode subjected to the conditions prevailing in Pakistan. The
material encompassed in the experimental design include aggregate from Margalla quarry and
two bitumen grades of different stiffness, categorized according to penetration grade of 40/50
and 60/70. Marshall Mix design approach was used to define the optimum bitumen content to
be used for each of the eight combinations of HMA mixtures which was further used to
fabricate samples for the performance testing using Superpave gyratory compactor. The
indirect tensile fatigue test was conducted on the prepared samples at 25 °C subjected to
repeated haversine loading at a series of stress levels, ranging from 2000N to 5500N, in
Universal Testing Machine (UTM-25). The results from the performance testing were screened
and used to develop fatigue curves for each of the eight different HMA mixtures using the
stress/strain approach, relationship between number of cycle to failure and initial strain.
Further, the data for all the HMA mixtures was used collectively to develop a non-linear
regression model including the bitumen viscosity, optimum bitumen content and the resilient
modus as additional variables. In order to relatively characterize the different HMA mixtures,
it was found that MS-2 gradation performed much better than any other gradation following
the same trend by using either softer bitumen (60/70) or the stiffer bitumen (40/50). The results
also revealed that using a stiffer bitumen (40/50) caused the number of cycles to failure for the
HMA mixtures to increase by 2 to 3 times than using the softer bitumen (60/70) in the stresscontrolled
conditions.
