Shear Behavior of Curved Concrete Structures Repaired with Sustainability-Oriented Trenchless Polymer Grouting

Urban underground infrastructure is increasingly challenged by material aging, environmental degradation, and structural deterioration. In response, trenchless polymer grouting technologies employing sustainability-oriented two-component foaming polymers have attracted growing attention. To investigate shear behavior at the polymer–concrete interface, this study conducted direct shear tests on two types of composite interface geometries—curved and planar—formed by bonding two-component foaming polymer to concrete substrates. Five polymer densities (0.33, 0.42, 0.51, 0.58, 0.66 g/cm3), three concrete strengths (C20, C30, C40), three normal stress levels (0.3, 1.0, 2.0 MPa), three shear rates (0.5, 2.0, 5.0 mm/min), and three interface sizes (100, 150, 200 mm) were examined. The results show that both interface types undergo five characteristic stages under shear. Across identical parameter levels, curved interfaces consistently exhibited higher peak shear strength and larger peak displacement than planar ones. When the polymer density is identical, the peak shear strength and displacement of curved specimens are about 1.38 and 1.43 times those of planar specimens, respectively. Similarly, for specimens with the same concrete strength, normal stress, and shear rate, the corresponding ratios of peak shear strength and displacement are about 1.14 and 1.55, 1.96 and 1.43, and 1.43 and 1.36, respectively. Within the tested ranges, the shear stress increases with polymer density, concrete strength, and normal stress, and generally decreases with shear rate. The shear displacement decreases with polymer density, concrete strength, and shear rate, and generally increases with normal stress. As the specimen size increases, the peak shear strength and peak shear displacement of the curved specimens first increase and then decrease, whereas for the planar specimens, the peak shear strength exhibits a nonlinear increasing trend. These findings provide valuable insights to promote sustainable underground infrastructure rehabilitation.