## 3 Results and Discussion
### Bending Test  
#### 3.2.1 Stress-Strain Curves under Static Loading  
The initial crack of CNFPUC under bending load occurred at the bottom of the maximum tensile stress. As shown in Figure 5, the stress-strain curves of CNFPUC at different ambient temperatures showed that, with the increase of ambient temperature, the deformation of CNFPUC under the same static load increased, the peak failure strain increased, the ultimate flexural tensile strength decreased, and the flexural tensile modulus decreased slightly. The bending tensile strain was 5100-6000 \(\mu\varepsilon\), and the ultimate strength was up to 26.65 MPa. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Number** & **1** & **2** & **3** & **4** & **5** & **6** & **Mean Value** \\ \hline Strength (MPa) & 3.42 & 3.17 & 3.58 & 3.26 & 3.74 & 3.68 & 3.47 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Bond strength results. According to experimental observations, the CNFPUC bending tensile stress and strain curve could be divided into three stages. The slope of each stage was fitted linearly, and the results are shown in Table 8. Stage I: The strain range was between 0-1000 \(\upmu\epsilon\), the stress increased linearly with the increase of strain at various ambient temperatures, the range of variation was high, and the slope of each curve had almost no change with the increase of temperature. The data points fell within the range of one time the standard deviation. Stage II: The strain range was between 1000-2000 \(\upmu\epsilon\). Compared with Stage I, the stress at various ambient temperatures increased linearly with the increase of strain, and the increase reduced. The slope decreased slightly with the increase of temperature. Stage III: The strain range was from 2000 \(\upmu\epsilon\) to specimen failure. The slope decreasing amplitude of stress-strain curves increased significantly at various ambient temperatures. The curve was relatively flat and had a tendency towards plasticity. The slope of the curve decreased more obviously with the increase of temperature. The data points were discrete. The tensile strength of \(-40\)\({}^{\circ}\)C was close to the glass transition temperature, the tensile strength was increased, and a small part of the data points fell outside one time the standard deviation. As the temperature increasing, the cementitious action decreased, the strength weakened, and the deformation increased. When the temperature reached 40 \({}^{\circ}\)C, the molecular chains of polyurethane elastomers had lower potential energy, more flexibility, and more space of movement of chain segments between molecules. Under the same stress condition, the corresponding strain was larger. Additionally, under the same strain condition, a relative stress of 40 \({}^{\circ}\)C was lower than that of other temperatures. Different from the brittle failure of plain concrete, the neutral axis of the member decreased after the plain concrete bottom bonded to CNFPUC. In the process of loading, CNFPUC limited the development of concrete cracks and delayed the time of cracking at the lower edge of concrete. Because the elastic modulus of CNFPUC was lower than that of concrete, cracks first appeared at the lower edge of concrete and gradually extended to the neutralizing axis. After that, vertical cracks began to appear in the underside of the CNFPUC, and eventually transverse fractures occurred on the bottom of the CNFPUC and broke away from the concrete, as shown in Figure 6.  
Figure 5: Bending tensile stress-strain curve. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Stage** & \(-\)**40 \({}^{\circ}\)C** & \(-\)**20 \({}^{\circ}\)C** & **0 \({}^{\circ}\)C** & **20 \({}^{\circ}\)C** & **40 \({}^{\circ}\)C** \\ \hline I & 8.74(1) & 8.58(1) & 8.32(1) & 7.96(1) & 7.58(1) \\ II & 6.92(0.98) & 6.73(0.97) & 6.61(0.96) & 6.51(0.95) & 6.37(0.95) \\ III & 4.01(0.99) & 3.51(0.98) & 3.77(0.97) & 3.43(0.98) & 3.18(0.94) \\ \hline \hline \end{tabular} \(0\) represents determination coefficient of fitting.  
\end{table}