Significance of pH Variance in Predicting Chemical Reaction in Hot Mix Asphalt

Abstract

Hot mix asphalt (HMA) is a combination of siliceous, calcareous, or intermediate aggregate materials from selected sources, blended with formulated asphalt binders from various asphalt binder producers. These components are modified by additional additives to enhance the bonding of the binders and aggregates. Concern over moisture damage (stripping), defined as the loss of strength and durability in asphalt mixtures when the binder separates from the aggregate, has prompted years of research into a variety of failure mechanisms. Prominent in these failure mechanisms is the pH at the binder–aggregate interface. This technical paper addresses the pH variance of individual components and loose blended HMA mixes from 17 selected material sources and 2 asphalt binder sources to assess the pH of any chemical reactivity. The main objective of the paper is to determine whether pH variances of individual components or mixed HMA materials (asphalt binder, aggregate, and additives) point to suspect components or combinations that cause a chemical reaction and, thus, instability or moisture-induced stripping at the aggregate interface. This research may point to alkali-silica and alkali-carbonate reactivity of aggregates as potential contributing causes of moisture-induced damage in HMA mixes. Statistical analysis is used to interpret the interaction of the independent variables and indicate contributing components correlating with the pH variance and chemical reaction.