# Shear Properties of the Interface Between Polyurethane Concrete and Normal Concrete
## 3 Results
### Load-Displacement Curve
Conversely, the KC group shows the lowest interface shear strength (less than 2 MPa) because the large coarse aggregate in the PUC, which affects the flow of colloidal material into the grooves, results in poor interface bonding. This is a key factor influencing the PUC-NC interface shear strength. For the KC group, the relatively smooth interface or lower roughness, combined with the absence of bonding agents, results in lower interface shear strength. When the groove depth is 2 mm, the shear strength increases by 21.25%. From these results, substrate roughness significantly affects bond strength. However, increasing roughness only substantially enhances interface performance within a certain range. Beyond this range, further roughness increases do not result in significant bond strength improvements. This is attributed to the change in the relative contribution of mechanical interlocking and chemical bonding forces as the roughness increases, which results in a reduced enhancement of bond strength beyond a certain roughness level. Similar phenomena have been reported in the study by [PERSON] et al. (2012), who observed that bond strength first increases with roughness before decreasing. A critical roughness value exists where bond strength reaches its maximum. At low temperatures, interface bonding performance typically improves with increasing roughness. This is likely because bond strength at low temperatures is primarily provided by mechanical interlocking forces. Therefore, increasing surface roughness is an important method to improve shear bond strength. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Specimen** & **P\({}_{\mathbf{u}}\)** & **S\({}_{\mathbf{0.7 Pu}}\)** & **Shear Strength** & **Shear Stiffness** & **Average Shear** \\  & **(KN)** & **(mm)** & **(MPa)** & **(kN/mm)** & **Strength (MPa)** & **Stiffness (kN/mm)** \\ \hline GH & 31.44 & 0.0630 & 1.6 & 174.1 & & \\
33.25 & 0.0720 & 1.7 & 161.4 & 1.6 & 171.3 \\  & 31  
The NJJ group utilizes the bonding agent to enhance the bond performance between the PUC and the adjacent NC materials. Generally, the contact area, mechanical interlocking, and frictional resistance are increased for both the ZM and NJJ groups. The maximum interface bond strength of the NJJ-1.5 group is 4.35 MPa, a 190.6% increase compared to the GH group. However, when the amount of bonding agent exceeds 1.5 g, the bond strength decreases. The GXW group has a lower interface bond strength, but the average strength still exceeds 2.5 MPa, mainly due to the adhesion of steel fibers, which enhances the bond strength and mechanical interlocking at the interface. At a steel fiber content of 4%, the shear strength reaches its maximum value of 3.05 MPa, an increase of 90.06% compared to the GH group. For steel fiber contents between 0% and 2%, the shear strength increases by 71.8%; from 2% to 4%, the increase slows, with a corresponding growth rate of 10.71%; however, when the steel fiber content increases from 4% to 6%, the shear strength decreases by 43.28%. Therefore, within a certain range of interface agents and steel fiber content, interface performance can be significantly improved. Excessive fibers may lead to agglomeration, diminish effective mechanical interlocking, and introduce defects (such as voids) that impair interfacial bonding, altering the shear load-displacement behavior by reducing the peak strength and stiffness. Figure 11 shows a scanning electron microscope image of PUC reinforced with steel fibers. Irregular voids around the steel fibers were observed, likely resulting from air entrapment during fiber placement or insufficient infiltration of the matrix slurry into the fiber gaps. Figure 11 presents a high-magnification SEM image, revealing distinct gaps and small voids at the fiber-matrix interface, with localized cracks propagating along the fiber axis.