# The Shear Behavior of the Curved Interface in Polyurethane-Concrete Composite Structures
## 4 Conclusions  
This study compared the shear behavior of polyurethane-concrete composite specimens with curved and planar interfaces through direct shear tests. The influence of the arc diameter and polyurethane density on shear strength, shear displacement, shear modulus, and residual strength was discussed. SEM was used to analyze the microstructures of curved and planar interfaces. The following main conclusions were drawn:  
1. The shear behavior of the curved composite specimens is significantly higher than that of the planar composite specimens. For curved specimens with a diameter of 500 mm and polyurethane densities of 0.25, 0.33, 0.42, 0.51, 0.58, and 0.66 g/cm\({}^{3}\), the shear strength of the curved composite specimens was, respectively, 1.38, 1.39, 1.38, 1.39, 1.37, and 1.38 times greater than that of the planar composite specimens. The shear stress-displacement curves show similar patterns for both curved and planar specimens, but there are notable differences in the fracture failure stage. Shear failure in the curved specimens occurs more abruptly compared to the stable shear failure observed in planar specimens. 2. The shear strength of the curved composite specimens is significantly greater than that of the planar composite specimens. The shear strength of the composite specimens increases gradually with a higher polyurethane density, although the rate of increase diminishes at higher densities. The shear strength of the curved composite specimens with diameters of 400 mm, 500 mm, and 700 mm is approximately 1.50, 1.39, and 1.10 times that of the planar composite specimens, respectively. 3. The shear strength of the curved composite specimens with different diameters follows a similar trend with the polyurethane density. As the arc diameter of the curved  
Figure 21: SEM scan images of the microstructures of interface regions in polyurethane-concrete composite specimens with different densities: (**a**) density = 0.30 g/cm\({}^{3}\), (**b**) density = 0.55 g/cm\({}^{3}\), and (**c**) density = 0.80 g/cm\({}^{3}\). composite specimens increases, the shear strength gradually decreases and approaches that of the planar composite specimens. 4. The distribution of polyurethane foam pore structures differs significantly between curved and planar composite specimens. In the curved composite specimens, the foam structure at the top of the curved interface is more concentrated, with the volume of bubbles decreasing gradually towards the sides of the curved interface. In contrast, the foam structure in the planar composite specimens is uniformly distributed across the plane. This study reveals that curved composite specimens exhibit significantly higher shear strength compared to planar composite specimens, a finding with substantial engineering implications, particularly in the repair and reinforcement of infrastructure such as tunnels and underground pipelines. This study utilized C30 concrete as the base material for the composites. Although C30 concrete is widely used in construction, varying concrete strength grades may influence the shear behavior of the polyurethane-concrete interface. Future research could explore the impact of concrete strength on the shear behavior of polyurethane-concrete curved interfaces to enhance the generalizability and practical application of the findings. B.L. Supervision, funding acquisition, conceptualization, writing-review and editing. X.W. : validation, data curation, formal analysis, investigation. Y.W. : investigation, data curation. Y.J. : funding acquisition, validation, methodology. J.W. : investigation, software; X.D. : Software, Validation. N.W. : software. All authors have read and agreed to the published version of the manuscript.