Moisture migration characterization of bitumen emulsion-based cold-mix asphalt mixture over curing based on capacitive measurement

Bitumen emulsion-based cold in-place recycling has been widely used all around the world due to its superior environmental benefits. During the mixing process of cold-mix asphalt (CMA) mixtures, extra water is introduced to enhance workability and ensure uniform coating. The limited understanding of moisture migration in the curing process has restricted the broader application of this technology in pavement maintenance. Therefore, this paper aims to develop a novel moisture migration characterization method for CMA mixtures over curing based on capacitive measurement. A semi-sealed laboratory curing method was employed to simulate practical construction and the moisture migration in the depth direction and radial direction of CMA mixtures was analyzed by a self-designed three-layer circular capacitance sensor (CCS) during the curing period. It is found that the capacitance values of asphalt specimens measured by CCS show a good “U” curve trend. Two parameters ($C_s$ and ${\epsilon }_m$) for evaluating moisture migration in depth direction were proposed. The variation of $C_s$ and ${\epsilon }_m$ demonstrated the gradient moisture distribution in the depth direction during the first stage of curing. And the rate of moisture consumption was uniform during the second stage. All three layers of CMA-13 exhibited a gradient distribution in the radial direction without side waterproof layer, while the imaging within the circular cross-section is uniform in semi-sealed curing condition, with no gradient distribution. These findings suggest that electrical capacitance method could provide a tool for characterizing and visualizing the moisture migration in CMA mixture non-destructively.