The Importance of Mixing and Compaction Moisture Content for Foamed Bitumen Stabilisation of Marginal Gravel Materials

Foamed bitumen stabilisation is a valuable technology that allows standard, as well as marginal (or non-standard) materials to be significantly improved by incorporating a relatively low (2%–4%) content of bitumen and a lower (1%–2%) content of a cementitious binder. The mixing/compacting moisture content is critical to the workability and compaction of these materials during construction, with most materials being compacted at approximately 70% of the optimum moisture content associated with the unstabilised granular material. However, some marginal materials have a higher portion of fine particles, greater plasticity, and higher reactivity to moisture, and these often require additional mixing/compaction moisture. One standard granular material and three typical marginal materials were stabilised with foamed bitumen at mixing moisture contents set to 70% and 85% of the granular material optimums. After standard laboratory production, compaction, and curing, specimens were tested for modulus and for tensile strength. For the most marginal granular materials, the visual tightness and uniformity of the samples was clearly improved by the additional mixing moisture, and both the modulus and tensile strengths increased significantly. In contrast, for the standard granular material, the increased mixing moisture adversely effected both the modulus and tensile strength. It was concluded that when marginal materials are being stabilised with foamed bitumen, a workability trial should be performed to ensure the mixing moisture content is set appropriately. Otherwise, the standard 70% of granular optimum rule will likely create an unworkable material in the field.