Characterization of Conventional HMA Mixtures using Simple Performance Testing Protocols

Abstract

In this research Simple Performance Testing of HMA mixes is carried out using Asphalt Mix Performance Tester (AMPT). The three candidate tests include dynamic modulus |E*|, flow number (FN) and flow time (FT) tests which are conducted on two wearing course gradations (SP A and NHA A) and two base course gradations (SP B and DBM). The bitumen binder used are of two penetration grades which are 40/50 and 80/100 grade. Dynamic modulus (E*) test was conducted on four different temperatures i.e. 4.4°C, 21.1°C, 37.7°C and 54.4°C and six different loading frequencies i.e. 25Hz, 10Hz, 4Hz, 1Hz, 0.5Hz and 0.1 Hz. Flow Number and Flow Time tests were conducted at a single temperature of 54.4ºC and a stress level of 300 Kpa. Optimum binder contents were obtained using Marshal Mix design method and the samples for performance testing were prepared using Superpave Gyratory Compactor (SGC) and then cored and trimmed to the specified dimensions. The |E*| test results were subjected to non-linear optimization technique to develop stress-dependent master curves which revealed that penetration grade of bitumen significantly influence the stiffness of mixtures. Two level factorial design of experiment technique was utilized to find the simultaneous effect of independent variables and their interaction on the response. Three factors were found to have a significant effect on the values of dynamic modulus i.e. temperature, frequency and binder viscosity. Mixture prepared using 80/100 pen grade binder were more temperature susceptible. Fatigue parameter was calculated using the dynamic modulus and phase angle values and the results revealed that fatigue parameter value is low for mixtures with 80/100 binder which means that mixtures with 80/100 binder are less fatigue susceptible. Performance modeling was also carried out to develop regression equations to predict the values of dynamic modulus using temperature, frequency, viscosity of the binder and NMAS as independent variables for both wearing and base course mixtures with a coefficient of determination (R2) value of 0.84 and 0.86 respectively. Flow number and flow time results were also analyzed to determine the rutting susceptibility of mixtures. The mixtures prepared using 40/50 binder accumulated less strains as compared to the 80/100 binder making them less rut susceptible.