Effect of loading on water distribution and migration characteristics of hardened cement paste with different water content

The service life of concrete structures is significantly influenced by the coupled effects of mechanical forces and environmental conditions, with axial pressure and environmental humidity being two of the most prevalent factors. Despite extensive research, the water migration behavior of hardened cement paste (HCP) with varying water content under axial pressure remains poorly understood. This study introduces a novel axial pressure-controlled Hydrogen Nuclear Magnetic Resonance (¹H NMR) system, investigating in-situ monitoring of strain changes and water distribution in HCP with different water contents during loading at various stress levels. The results showed that a reduction in water content would reduce the interlayer spacing of C–S–H, thereby increasing the densification of C–S–H gel and enhancing the mechanical properties of cement-based materials. The critical sliding point between C–S–H layers occurs at an average interlayer spacing of 1.89 nm. Under axial compressive loading, the C–S–H gel is compressed, causing some gel pores to reorganize into interlayer pores. Consequently, the interlayer water content increases while the gel water content decreases. As the stress level rises, the interlayer water content gradually increases, reaching its maximum when the stress level equals or exceeds the critical stress. At this point, the water migration behavior transitions from fully reversible to partially reversible. These findings provide valuable insights into the coupled effects of mechanical loading and water migration in HCP, which are crucial for predicting the long-term performance and durability of concrete structures in diverse environmental conditions.