Characterisation of Foam Bitumen Quality and the Mechanical Properties of Foam Stabilised Mixes
The use of foam bitumen stabilisation is rapidly growing worldwide because of its environmentally friendly benefits and its high field-performance properties. The foam bitumen quality is assessed by two empirical parameters, namely, half life time and expansion ratio. Despite the simplicity of measurements of these two parameters, several discrepancies between the classification of the foam and its actual quality were observed. Such inconsistencies in the current classification system have led to the proposal of a new method to characterize the quality of the foam, which was developed in this study. The proposed method uses the Brookfield rotational viscometer to measure the viscosity of foam at different time intervals. The average foam viscosity over the first 60 seconds of foaming is used as a measure of the quality of the foam. This parameter is analogous to the foam index. Foam index combines both expansion ratio and time of decay of the foam, while the new parameter combines foam viscosity over the first 60 seconds of foam decay. The foamant water content that provides the lowest average viscosity is considered the optimum foamant water content. In addition to foam characterization, the mechanical properties of foam-stabilized mixes were measured and compared to those of hot mix asphalts.