Understanding Internal Structure Characteristics of Foamed Asphalt Mixes with Fracture Face Image Analyses

The rate of use of full-depth reclamation with foamed asphalt to rehabilitate cracked pavements is increasing worldwide. However, the state-of-the-art understanding of foamed asphalt mixes and the state-of-the-practice engineering application are largely empirical because of the lack of in-depth knowledge of this material's internal structure characteristics. A fracture face image analysis framework for quantification of the asphalt mastic phase distribution in foamed asphalt mixes, which is a key microscopic structure characteristic, is described. Fracture face asphalt coverage is a quantitative indicator of asphalt spot distribution on fractured faces and is primarily a function of the asphalt dispersion in the mix, the relative strengths of the asphalt mastic and mineral filler phases, and the boundary conditions of specimen compaction and testing. Fracture face image analyses combined with laboratory testing results are presented. It was found that (a) the strength of the asphalt mastic phase is much less susceptible to moisture conditioning than the strength of the mineral filler phase, (b) filler contents that are too low or too high both negatively influence the effectiveness of asphalt stabilization, and (c) the image analyses show that the impact of asphalt grade on the asphalt distribution in the mix dominates the effectiveness of asphalt stabilization but not the strength of the asphalt cement itself. Additionally, engineers can perform a qualitative fracture face asphalt distribution check at the project level of mix design to diagnose mix problems, for which an interim guideline is proposed.