Effects of high friction aggregate and PG Plus binder on rutting resistance of hot mix asphalt mixtures

The rutting resistance of hot mix asphalt (HMA) Superpave™ mixes in surface course materials was investigated using asphalt material characterisation tests and a digital imaging processing (DIP) technique. The effects of the type of aggregate, the type of binder and the binder content on rutting resistance were quantified. Two types of aggregate were examined: Superpave™ SP12.5 and high friction SP12.5 FC2. Both a modified (PG Plus) and an unmodified binders were considered at the optimum binder content and the optimum content plus an additional 0.5%. To accurately identify the effect of each variable, the shear upheave of these mixes was also quantified. The DIP technique involved estimating the number of aggregate contacts, the total contact length and internal structure index of two-dimensional images of the experimentally tested samples. The results showed that both the rutting resistance and stiffness of HMA surface mixes were sensitive to aggregate type, binder type and binder content. A high friction aggregate provided a better internal structure characteristic, as well as superior rutting resistance and stiffness for HMA mixes. The use of PG Plus and the addition of 0.5% to the optimum binder content negatively affected HMA stiffness and rutting resistance. However, the levels of rutting resistance for all mixes were acceptable (rut depth < 12.5 mm), even when the shear upheave was considered. Internal structure indices measured by DIP were effective for capturing changes in the internal HMA structure with respect to aggregate type and asphalt cement content.