Micropore structure characterization of the bitumen emulsion-based cold in-place recycling mixture considering water gradient migration condition through multiple XCT scanning

Bitumen Emulsion-based Cold In-place Recycling (BE-CIR) has been widely used all around the world due to its superior environmental benefits. Unlike the hot mix asphalt (HMA), BE-CIR mixture presents a unique moisture migration behavior after compaction. The migration of water can alter the micro-pore structure of the BE-CIR mixture and then change its mechanical performance. Therefore, it is essential to characterize the variation of internal structure, especially the air voids, in depth direction during the curing process of BE-CIR mixture. A one-way evaporation method was developed to simulate the field moisture migration of BE-CIR mixture specimens in laboratory. To track the internal structure change with time, multiple X-ray Computed Tomography (XCT) scanning examinations were performed on the lab-prepared specimens with three different curing periods (0 h, 60 h, and 153 h). The effects of curing temperature and initial moisture content on the micropore structure development were also investigated. The morphology variations of air voids including the content, number, volume distribution and void gradation in the depth direction with curing time were further characterized. The results indicate that there is a 0.5% difference in air void content between the top and bottom of the BE-CIR mixture. The BE-CIR mixture's micropore structure evolved over the course of curing with two key characteristics: a fast increase in the number of small voids and a sustained development of large voids. The development of more tiny pores and the gradient properties of these pores, which predominated during the curing time of 60 to 153 h, were a reflection of the internal migration of free water. The initial moisture content mainly affects the magnitude of the large void variation during the curing time of 0 to 60 h, while the curing temperature affects the proportion of pore increase between the two curing periods. The outcomes can provide a better understanding on the dynamic volumetric characteristics of the BE-CIR pavement over the curing process.