Performance of foamed bitumen-stabilised mixtures

Mixtures stabilised with foamed bitumen are commonly made with recycled aggregate, typically reclaimed asphalt planings. These mixtures are influenced by many factors, which require full consideration during the production and construction stages. Untreated recycled material is often unsatisfactory owing to the particle size distribution and high water content. Addressing the grading problem may require the addition of other materials; defining material type and quantity is significant. Foamed bitumen stabilised material is often modified with a hydraulic binder, not only to dry up the mixture, but also to enhance its performance, particularly in its early life. Moisture levels within the material during compaction are very important in aiding the workability of the mixture. Also, the compaction process is very dependent on the compaction energy. The stabilised mixture should be protected from the ingress of water during its early life. Foamed bitumen stabilised mixtures perform successfully when laid as a base layer, and a conventional binder course and surface course can be laid immediately on top. This paper is based on more than ten years of the author's experience in this field. In particular, it highlights the significance of the addition of hydraulic binder, moisture content, compaction level and curing time. Graphical relationships are established to aid mixture design, and can be used to predict mixture performance. Also, analyses of laboratory and site curing times are compared.