Characterization of the drying behaviour of asphalt emulsion

Abstract The process of drying of asphalt emulsion largely dominates the development of their and the mechanical properties of emulsion-based mixtures for pavement structures. However, the drying behaviour of asphalt emulsions was not widely considered because of the complex physico-chemical interactions driving it. The theory of emulsion drying is used to reveal every drying stage of asphalt emulsion as well as the corresponding physical meaning in this work, and the influencing factors of the drying behaviour are analysed, such as temperature, thickness and asphalt content. The results indicated the presence of two types of drying of asphalt emulsions, including no-skin and skin-formed drying. The process of drying without skin can be divided into three stages: (1) the stage with continuously high rate of evaporation which persists to about 69 to 75% of asphalt volume fraction, (2) the intermediate stage in which the evaporation rate rapidly decreases, and (3) the final stage in which the evaporation rate is drastically lower and gradually decreasing over time. The no-skin drying, process was slightly accelerated by the increase in the content of asphalt, drastically prolonged by the increase in the thickness of layer, and drastically shorten by the increase in the drying temperature. However, by increase in the asphalt content, layer thickness, and drying temperature, the skin was easily formed, which greatly prolonged the entire process. The maximum packing density of asphalt droplets (ϕm) was recognised as an important factor for the drying process, and was obviously reached by the most of specimens during the second stage of drying. Based on the results of gravimetric measurements, an original method for calculating ϕm was proposed, which facilitates judging the quality of asphalt emulsion from the perspective of drying.