Measuring the Influence of Aggregate Coating on the Workability and Moisture Susceptibility of Cold-Mix Asphalt

Abstract

The purportedly higher moisture susceptibility of emulsified asphalt mixtures relative to traditional hot-mix asphalt is a primary concern limiting their more widespread use. Of the factors contributing to moisture susceptibility in cold-mix asphalt (CMA) that can be controlled by mix designers, aggregate coating is one of the most obvious physical means to reduce this distress. More complete aggregate coating is expected to help limit the moisture susceptibility of CMA by reducing the amount of water that can be absorbed into exposed aggregate. This paper refines a recently developed method to quantify aggregate coating in CMA by using digital imaging analysis and applies the procedure to several aggregate–emulsion systems to isolate factors most directly affecting aggregate coating. After the most significant factors were identified, a regression analysis of the coating matrix was used to develop quantitative models to predict coating of aggregates in CMA as a function of mix design parameters. Models were used to predict several representative levels of aggregate coating, and laboratory-compacted candidate mixtures were assessed for workability and moisture susceptibility by using a compactability parameter and modified tensile strength ratio test, respectively. Results show that the workability and moisture susceptibility of CMA are highly dependent on the level of aggregate coating predicted by the quantitative models; this finding suggests that the imaging procedure can be used not only to reliably predict aggregate coating in CMA but also to develop practical, performance-based limits on plant-produced CMA mixtures in terms of aggregate coating.