Laboratory Characterization of HMA Mixes Subjected to Indirect Tensile Fatigue Test

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.