Water diffusion in polyurethane grouting materials and its effect on compressive strength

The mechanical properties of polyurethane (PU) grouting materials are susceptible to the influence of water during underground grouting. To elucidate the diffusion mechanism of water within the material and the variation in compressive strength after water immersion. Scanning Electron Microscopy (SEM) was employed to analyze the water diffusion mechanism in polyurethane grouting materials from a microscopic perspective. Furthermore, a series of laboratory experiments were conducted to investigate the effects of factors such as density, surface condition, immersion duration, water pressure, saline environment, and ambient temperature and humidity on the water absorption. Additionally, the compressive strengths before and after immersion were compared and analyzed. The results indicate that the water absorption decreases as a quadratic polynomial function of density (R² = 0.98). Specifically, at densities of 0.30 g/cm³ and 0.42 g/cm³, the water absorption was merely 4.38% and 1.28%, respectively. Compared with the original surface (water absorption of 15.77%), sealing treatment reduces the rate by 44.32%, whereas surface peeling treatment increases it by 151.11%. Moreover, as the water pressure increases by 2 MPa, the water absorption rises gradually with a diminishing marginal increase. The most drastic variation in water absorption occurs within the initial 24 h; thereafter, for every additional 24 h of immersion, the increment remains below 0.5% and progressively decelerates. In saline environments, the water absorption of specimens with various densities increases by 22%, and the water absorption exhibit an upward trend with elevated environmental temperature and humidity. Notably, compressive strength both before and after absorption increases with density (R²>0.88). In the density range of 0.070–0.112 g/cm³, the material exhibits a relatively higher water absorption. Due to the supporting effect of the internal water, the compressive strength experiences a marginal increase of approximately 4.56%. These findings can provide theoretical support for the design and durability evaluation of underwater grouting reinforcement in underground engineering.