Micromechanics of the Effects of Mixing Moisture on Foamed Asphalt Mix Properties

Abstract

The mixing moisture content (MMC) is an important mix design variable in pavement full-depth reclamation (FDR) with foamed asphalt. It has been known for many decades that MMC in the granular material subjected to foamed asphalt treatment can affect asphalt dispersion. This paper investigates the micromechanics of this phenomenon by combining direct microstructure observations and conventional laboratory testing. It was found that for a typical loose moist granular material with a considerable amount of fine particles, the agglomeration state evolves through a series of states as the MMC increases. If the MMC is high, the fine particles become saturated and form a pastelike substance that coats larger aggregate particles, creating large agglomerations with low surface area to volume ratios. Inferior asphalt dispersion is resulted for mixes in this state. Theories for wet agglomeration processes are used to explain the observations on the basis of thermodynamic equilibrium. Fracture face image analysis for tested indirect tensile strength specimens confirmed this observation. Laboratory tests also found that mixes with inferior asphalt dispersion due to suboptimal MMC yield low strength and low stiffness. These effects are particularly significant for granular materials with higher fines content. The main practical implication of the findings is that relatively dry loose granular mixes are preferred for foamed asphalt dispersion, and a balance should be sought between asphalt dispersion and mix compactability. The existing postulation that high MMC aids foamed asphalt dispersion is therefore dismissed.