# The Shear Behavior of the Curved Interface in Polyurethane-Concrete Composite Structures
## 3 Results and Discussion
### Microstructure Analysis of Specimens with Different Interface Shapes  
In this study, secondary electron imaging was employed to analyze the microscopic morphology of the interface region between polyurethane and concrete. Scanning was conducted on composite specimens with both curved and planar interfaces, as well as curved specimens with varying polyurethane densities, to investigate the effects of interface  
Figure 19: Shear residual strength of curved composite specimens: (**a**) variation in residual strength with polyurethane densities for specimens with different arc diameters, and (**b**) variation in residual strength with arc diameter for specimens with different polyurethane densities.  
Figure 18: Variation in peak shear displacement with arc diameter for curved composite specimens with different polyurethane densities.  
shape and polyurethane density on the microstructure. The study further analyzed the influence mechanism of microstructural changes on interfacial bonding performance. Prior to testing, 150 mm specimens were cut, and the central region of each specimen was selected as the scanning sample. Composite specimens were sliced into thin sections approximately 10 mm thick, which were then gently fractured at the center to prevent cutting scratches from affecting the interface's microstructure. The samples were cleaned, dried, and coated with a conductive layer. The application of a metal film not only prevented charging and discharge effects but also minimized electron beam damage to the sample, increased the generation of secondary electrons, and enabled the acquisition of clear images.  
Taking curved and planar polyurethane-concrete composite specimens with a polyurethane density of 0.30 g/cm\({}^{3}\) as examples, the SEM scans of their interfacial microstructures are shown in Figure 20a,b, respectively. It can be observed that the interfacial microstructures of both curved and planar specimens are similar, consisting of polyurethane, polyurethane shear layer, and concrete parts from top to bottom. The pore structure of the polyurethane shear layer is smaller and less dense compared to the polyurethane part, indicating that the polyurethane shear layer has not fully formed a dense pore structure, which can be used as a basis for structural layering. Additionally, it is found that the pores at the top of the curved interface are more concentrated and smaller in volume compared to the sides of the curved interface. In contrast, the pore structure in the interfacial region of the planar specimen is more uniformly distributed. This suggests that the shape of the polyurethane-concrete interface significantly affects the distribution of the polyurethane structure in the interfacial region, resulting in notable differences in interfacial shear performance between the specimens. Furthermore, the polyurethane pore structure is tightly integrated with the concrete hydrates, with some regions even showing polyurethane pores embedded in the concrete structure. This indicates a strong interlocking effect between the polyurethane and concrete, potentially accompanied by chemical reactions.  
SEM scan images in Figure 21 depict the overall microstructures of the interface regions of polyurethane-concrete composite specimens with densities of 0.30 g/cm\({}^{3}\), 0.55 g/cm\({}^{3}\), and 0.80 g/cm\({}^{3}\). It is evident that with the increasing polyurethane density, the individual pore volume of polyurethane gradually decreases, and the number and density of pore structures diminish. Simultaneously, the thickness range of the polyurethane shear layer expands with higher polyurethane densities.  
Figure 20: SEM scan images of the interfacial microstructures of curved and planar specimens: (**a**) curved specimen, and (**b**) planar specimen.