Multiscale compressive behavior and constitutive modeling of self-skinning polyurethane grouting materials: synergistic effects of density and dimensional

In trenchless rehabilitation of infrastructures, self-skinning polyurethane grouting materials (SSPUGMs) are tailored to meet different strength requirements and defect sizes within infrastructures. However, the synergistic effects of specimen size parameters and skin preservation on their compressive behavior — particularly regarding constitutive models — remain poorly understood. In this work, the compressive performance, failure mechanisms, and damage constitutive models were investigated through a multiscale method combining macro-micro experimental characterization, molecular dynamics simulations, and theoretical analysis accounting for density differences and size effects. The results showed that macroscopic compression failure modes involved core fragmentation, skin rupture, collapse, and through-cracks. Microscopically, the cell fracture rates at the failure section approached 100%. Compressive deformation induces nanovoid nucleation and enhances interchain sliding within the SSPUGMs matrix. The elastic, yielding, and failure stages are predominantly governed by density and diameter-to-height ratio (ranging from 0.5 to 2.0), with specimen size exhibiting negligible influence. To quantitatively capture these stress-strain relationships, a segmented constitutive model was developed, along with a simplified model incorporating density and size for practical engineering applications, achieving a mean prediction error below 11% compared to experimental data. This work offers valuable insights for engineering applications of SSPUGMs.