Microstructural investigation of Hot Mix Asphalt (HMA) mixtures using Digital Image Processing (DIP)

It is well known that Nominal Maximum Aggregate Size (NMAS) significantly affects several mechanical properties of asphalt pavement layers both at low and high temperatures. Two different aggregate NMAS, 9.5 mm and 12.5 mm, are commonly used for surface layer mixture. Most of the past research studies investigated the effect of different aggregate NMAS on asphalt mixture properties through experimental testing, while the impact of NMAS on asphalt mixtures microstructure has received little attention. In this paper, asphalt mixtures with two NMAS, 9.5 mm or 12.5 mm, two air voids percentages, 4% and 7%, and two binder contents, 4% and 7%, were prepared. Then, the effect of aggregate NMAS on materials properties was investigated by performing low temperature creep test of asphalt mixture through the Bending Beam Rheometer (BBR) and advanced Digital Image Processing (DIP) analysis of two-dimensional images of asphalt mixture combined with numerical evaluations of 2- and 3- point correlation functions. As a result, asphalt mixtures containing smaller NMAS showed higher stiffness compared to those prepared with larger aggregates at low temperature. The spatial distributions of aggregates, asphalt mastic, and air voids were not significantly affected by the addition of aggregates with different NMAS. However, an increase in Auto Correlation Length (ACL) was observed for asphalt mixtures having smaller NMAS, which suggests that low temperature creep properties are associated to ACL.