Morphology Characterization of Foam Bitumen and Modeling for Low Temperature Asphalt Concrete

Abstract: Development of new asphalt technologies to reduce both energy consumption and CO2 production has attracted great interest in recent years. The use of foam bitumen, as one of them, is attractive due to the low investment and production cost. Formation and decay of foam bitumen is a highly dynamic temperature dependent process which makes characterization difficult. In this thesis, new experimental tools were developed and applied for characterizing the foam bitumen during the hot foaming process. One of the main goals of this study was to improve understanding and characterization of the foam bitumen formation and decay. X-ray radiography was used to study the formation and decay of foam bitumen in 2D representation. The main parameters influencing foam bitumen formation, i.e. water content and temperature, were also investigated. The results demonstrate the influence of water content on the morphology and expansion of foam bitumen bubbles. Adding more water in the foaming process leads to quick collapse of bubbles and intensifies coalescence of foam bitumen. The morphology of bubble formation depends on the types of bitumen used. Moreover, theoretical investigation based on the 3D X-ray computed tomography scan dataset of bubble merging showed that the disjoining pressure increased as the gap between the bubbles in the surface layer (foam film) decreased with time and finally was ruptured. The speed of the bubbles also increased with time when the distance between the bubbles decreased.  A high-speed camera was applied for examining the foam bitumen stream right at the nozzle revealing that foam bitumen at a very early stage contains fragmented pieces of irregular size rather resembling a liquid than foam. The residual water content in the decaying foam bitumen was determined by thermogravimetric analysis. The result demonstrated that residual water content depends on the initial water content, and was found to be between 38 wt% and 48 wt% of the initial water content of 4 wt% to 6 wt%. Furthermore, the influence of water content on the foam bitumen and the asphalt mixture was investigated. The influence of the water content in combination with compaction temperature was studied using gyratory compaction method. The investigation revealed that the Marshall stability is more influenced by compaction temperature compared to water content.Moreover the influence of viscosity and surface tension on bubble shape and rise velocity of the bubbles using level-set method was implemented in finite element method. The modeling results were compared with bubble shape correlation map from literature. The results indicated that the bubble shapes are more dependent on the surface tension parameters than to the viscosity of the bitumen, whereas the bitumen viscosity is dominant for bubble rising velocity.