Development of (half-) warm foamed bitumen mixes: state of the art

Abstract

Foamed bitumen mixes are produced by mixing foamed bitumen with aggregate at ambient conditions. However, the variations in the temperature of mineral aggregate at the time of mixing have a profound influence on the properties of the foamed bitumen mixes produced. A feasibility study previously undertaken to investigate the possible benefits of heating the aggregate moderately (above ambient temperature but below 100°C) before foam treatment, highlighted the improvement of mix properties that is achievable. The properties that can be enhanced include particle coating; mix cohesion and tensile strength, as well as a degree of compaction when compared with the equivalent properties of conventional cold foam mixes (CMA). This has been found to be applicable to, in particular, reclaimed asphalt pavement (RAP) and densely graded crushed aggregates. The feasibility study was undertaken exclusively at laboratory scale. A second phase, more focussed research project was subsequently launched in the Netherlands using the mixes that show the most potential to benefit most from the “Half-warm Foamed Bitumen Treatment” process viz, STAB (crushed stone ashalt concrete used in the Netherlands as a base course layer) and RAP combinations. This investigation was undertaken at laboratory level as well as full scale production trials. This paper discusses the findings of the entire investigation into half-warm foamed (HWF) bitumen treatment, carried out in South Africa and the Netherlands. As special development, the reuse of porous asphalt (PA) RAP in HWF PA, is also discussed. With up to 40% less energy consumption than that required for manufacture than hot mix asphalt (HMA), HWF asphalt mixes can provide comparative monotonic properties at higher test temperatures, similar fatigue properties and a lower phase angle at low loading frequencies. The improvement of mix properties relative to CMA is significant. This indicates that the HWF process holds the potential for successful implementation in pavement layers.