Evolution of Mechanical Properties and Internal Moisture Behavior of Emulsified Asphalt Cold Recycled Mixtures Based on LF-NMR and EIS Non-Destructive Testing Techniques

During the curing process of emulsified asphalt cold recycled mixtures (ECRM), mechanical strength gradually develops as internal moisture evaporates. However, the relationship between mechanical property evolution and internal moisture content during the curing of ECRM has not been sufficiently investigated. Moreover, accurate and effective non-destructive methods for monitoring internal moisture loss are still lacking. In this study, low-field nuclear magnetic resonance (LF-NMR) and electrochemical impedance spectroscopy (EIS) were employed as non-destructive techniques to characterize the internal moisture behavior of ECRM with different RAP contents and to analyze its correlation with mechanical performance. LF-NMR testing enables direct characterization of the content, spatial distribution, and migration behavior of internal moisture within a specimen. EIS measures the impedance spectra of materials containing conductive phases. Both techniques offer rapid, non-destructive, and continuous measurement capabilities, allowing visualization of moisture distribution within the material.

The study first evaluated the time-dependent evolution (up to 28 days of curing) of key mechanical properties—including abrasion resistance, Marshall stability, indirect tensile strength (ITS), and splitting tensile modulus (STM)—of ECRM with different RAP contents(0%, 30%, 50%, and 70%). Subsequently, LF-NMR was employed to investigate the content, spatial distribution, and time-dependent migration behavior of free moisture within ECRM containing different RAP contents over a 28-day curing period. The study fabricated working electrodes using epoxy resin and steel rods. The electrodes were buried in the center of ECRM Marshall specimens to measure the electrochemical impedance values of the samples. Given the presence and evolution of internal moisture in ECRM, EIS was then used to monitor the electrical resistance of ECRM with different RAP contents over the same curing period, enabling a quantitative analysis of free moisture evolution. Finally, correlations between the mechanical properties of ECRM and its internal moisture characteristics were established. LF-NMR results indicate that, regardless of RAP content, the internal free moisture in ECRM exhibits a similar distribution pattern: approximately 10% in mesopores (<0.01μm), about 10–30% in intermediate pores (between 0.01μm and 0.1μm), and roughly 60–70% in macropores (>0.1μm). As curing time increases, the internal free moisture in ECRM with different RAP contents consistently migrates from mesopores to intermediate and macropores. This migration behavior results from the combined effects of pore structure characteristics, moisture transport mechanisms, and physicochemical interactions. EIS results showed that impedance increased with curing time, and mechanical performance exhibited a positive correlation with moisture loss. The results demonstrate that LF-NMR and EIS measurements are effective methods for investigating internal moisture characteristics and the evolution of mechanical properties in ECRM. The results reveal the distribution characteristics and migration behavior of internal free moisture in ECRM. These features exhibit a universal pattern and are independent of RAP content.

The findings provide technical guidance for accurately determining the curing time and mechanical strength development of cold-mixed asphalt mixtures. The proposed methods offer significant advantages in non-destructive testing and in situ monitoring. The research conclusions provide a solid foundation for studying the performance of cold-mixed asphalt materials and offer effective solutions for non-destructive testing.