Degradation mechanisms of polyurethane grouting materials under quasi-static and cyclic compression loading: Density and size effects

Foamed polyurethane (PU) grouting materials are widely used for road trenchless rehabilitation. However, the density and size effects on their mechanical properties and the microscopic mechanisms under quasi-static and cyclic compression are still unclear. In this paper, the degradation mechanisms of PU grouting materials with different densities and specimen sizes were studied by quasi-static compression, cyclic compression, and macro-micro morphological characterization. As the specimen diameter increased, the compressive strength decreased and the elastic modulus increased. As the diameter-to-height ratio increased, the compressive strength increased and the elastic modulus decreased. The compressive strength/elastic modulus increased as a power function with density. When the stress levels were above the fatigue threshold, cyclic failure occurred. When the stress level reached the fatigue threshold, the loading rate, elastic strain, and loss factor were concentrated at 0.25 s-1, 0.013, and 0.023, respectively. The initial dynamic stiffness increased by 55.94 % compared to the quasi-static stiffness. The fatigue threshold tended to decrease with the increasing specimen diameter and diameter-to-height ratio. For specimens with D20H20, the density effect on the fatigue threshold was not significant. For specimens with D40H80 and D40H40, the fatigue threshold decreased with increasing density. Density and size effects were due to the combined effects of relative skin thickness, cell structure, and matrix content. Moreover, the S-N curve was established.