# Shear Properties of the Interface Between Polyurethane Concrete and Normal Concrete
## 3 Results
### Load-Displacement Curve  
The interface strain values at two locations of the oblique shear specimen were measured using strain gauges at the failure interface. The average of three test results was taken as the strain measurement for the specimen. The load-displacement curve of the PUC-NC interface is shown in Figure 9. Typically, brittle (sudden) interface failure is defined as the abrupt shear of the PUC-NC interface, accompanied by minimal interface slip at failure, with the load-displacement curve showing no horizontal yield or descending phase. In contrast, ductile interface failure is characterized by significant interface slip at the PUC-NC interface failure, exhibiting a certain shear resistance beyond the ultimate load, with a prolonged yield or descending phase in the load-displacement curve. Figure 9 illustrates that the shear load-displacement curves for all PUC and ordinary NC specimens are similar. The response of the shear load-displacement curve is generally linear until the applied load reaches its maximum value. After reaching the maximum load, the shear load sharply decreases to zero. The entire failure process is brittle, and the interface load-strain curves for all groups essentially do not exhibit a yielding or descending phase, as failure occurs at the PUC-NC interface or the substrate. The oblique shear strength (\(\tau\)) of the PUC-NC interface can be calculated using Equation (1):  
\[\tau=\frac{\mathrm{P_{u}}}{\mathrm{S}}\times\sin\alpha \tag{1}\]  
In the equation, S is the specimen's surface area (mm\({}^{2}\)), \(\mathrm{P_{u}}\) is the ultimate load value (N), and \(\alpha\) is the angle between the shear plane and the horizontal plane, where \(\alpha=45^{\circ}\) in this study. Figure 9: Load–displacement curves for PUC–NC interface: (**a**) grooved group; (**b**) notched group; (**c**) adhesive coating; (**d**) steel fiber. The method for determining the shear stiffness of the PUC-NC interface follows the secant modulus approach used in calculating the shear stiffness of bolt connections. Specifically, an appropriate interface shear force is selected from the linear region of the load-displacement curve for each specimen, and the shear stiffness is approximated by dividing the selected shear force by the corresponding displacement. According to the load-displacement curves shown in Figure 9, the interface load-displacement curves for all groups exhibit an approximately linear behavior before reaching 75% of the ultimate load (\(\mathrm{P_{u}}\)). Therefore, the secant modulus corresponding to 0.7 \(\mathrm{P_{u}}\) is used to calculate the shear stiffness of the PUC-NC interface, i.e., 0.7 \(\mathrm{P_{u}}\) divided by the corresponding interface displacement (\(\mathrm{S_{0.7\mathrm{P_{u}}}}\)), as shown in the following equation:  
\[\mathrm{K}=\frac{0.7\mathrm{P_{u}}}{2\mathrm{S_{0.7\mathrm{P_{u}}}}} \tag{2}\]  
The average values of the measured interface shear strength and shear stiffness for each group of specimens are summarized in Table 7. For ease of comparison, the interface shear strength and shear stiffness for each group are presented as bar charts in Figure 10.  
Table 7 presents the shear strength of the PUC-NC interfaces for specimens with different surface treatments. Ranked from the highest to the lowest interface shear strength, the groups are ZM, NJJ, GXW, and KC. The ZM and NJJ groups exhibit higher interface shear strengths, with average values exceeding 4 MPa. Among them, the ZM group performs the best, primarily due to the high interface roughness, which increases the contact area and frictional resistance at the interface. The maximum bond strength reaches 5.28 MPa, representing a 230% increase compared to the GH control group. When the surface roughness increases from 0 mm to 2 mm, the bond strength rapidly increases by 162.5%. However, when the roughness value increases from 2 mm to 3.5 mm, the growth in bond strength becomes relatively smaller, with a corresponding growth rate of 25.71%.