Surface Free Energy Theory for Evaluating Moisture Damage in Expandable Polyurethane Grouting Materials

The resistance properties of the interface between polyurethane(PU) grouting materials and rocks subjected to water erosion are the theoretical basis for evaluating the long‐term performance, which, however, is not clear yet. Herein, surface free energy (SFE) theory is used to analyze the mechanism of water damage at the interface between the polyurethane and rock. The surface energy parameters of various PU grouting materials with different densities, limestone, and granite rock materials are tested. The adhesion and peeling work indices are calculated by the SFE theory. The results show that water brings little influence on PU grouting materials. The lower the density of PU is, the better the hydrophobicity is. The increase in the density of PU promotes adhesion between the PU and rock with and without water. Moreover, density increases the difference between SFEs of PU–rock and water–rock, decreasing the detachment tendency of PU over the rocks. Lithology also affects the adhesion of PU and rocks. PU–granite composite exhibits the largest adhesion work and smallest peeling work, which indicates that it has stronger water damage resistance. Finally, the validity of the surface energy theory is proved by mechanical, bond tests and the application.