# Experimental Study on Flexural Properties of Polyurethane-Cement Composites under Temperature Load
## 6 Results and Discussion
### Destruction Mode  
The damage forms of the bending test of polyurethane cement beams at different temperature levels are shown in Figure 24. The forms of damage sections at different temperatures are different, mainly showing two types of damage sections. One is the damage section at temperatures of \(-\)50 \({}^{\circ}\)C, \(-\)40 \({}^{\circ}\)C, \(-\)10 \({}^{\circ}\)C, 0 \({}^{\circ}\)C, 10 \({}^{\circ}\)C, and 20 \({}^{\circ}\)C, which shows an irregular fold shape; the other is the damage section at temperatures of \(-\)30 \({}^{\circ}\)C and \(-\)20 \({}^{\circ}\)C, which shows the regular rectangular shape, and the fracture surface overlaps with the loading axis. In the process of temperature rise from \(-\)50 \({}^{\circ}\)C to 20 \({}^{\circ}\)C, the specimen damage form is brittle damage, and the damage section is in the loading point position of the beam, i.e., the span position. The damage of the specimens at 30 \({}^{\circ}\)C was ductile and ductile, and more small vertical cracks appeared at the lower edge near the span section. The damage of the specimens at 40 \({}^{\circ}\)C and 50 \({}^{\circ}\)C is in the form of flexible damage. The deformation of the specimens is excellent, but the ability to bear the load is inferior, and many folds appear at the lower edge near the span section. No apparent cracks appear when the damage occurs. According to the load-displacement curve and damage form of the polyurethane cement beamlet bending test, it can be seen that Figure 25 represents three damage modes of polyurethane cement at different temperature levels. The first damage mode load-displacement curve shows linear growth before reaching the ultimate load, and then the curve suddenly turns and drops sharply when reaching the ultimate load. The polyurethane cement beamlet instantly loses its bearing capacity and fractures, called Type I brittle damage. The second damage mode load-displacement curve before reaching the ultimate load, first linear growth and then nonlinear growth, the internal reorganization of the material, mechanical properties change, after reaching the ultimate load, there is a process of decline in bearing capacity. Finally, loss of bearing capacity, fracture, and polyurethane cement beams have a certain ductility, known as Type II elastic-plastic damage. In the third damage mode load-displacement curve, before reaching the ultimate load,there is first a tiny section of the curve to maintain a linear growth trend, and then a large section of the nonlinear growth curve, and finally, basically remain as a horizontal line, the bearing capacity no longer increases. However, the deformation is still occurring, and the curve has a prominent platform period, known as Type III flexible damage. Figure 23: Load-displacement diagram of polyurethane cement at different temperature levels: (**a**) Load-displacement curve for \(-\)50 \({}^{\circ}\)C-50 \({}^{\circ}\)C; (**b**) Load-displacement curve below 20 \({}^{\circ}\)C (including, 20 \({}^{\circ}\)C); (**c**) Load-displacement curve above 20 \({}^{\circ}\)C.  
Figure 24: _Cont._  
The failure mode of polyurethane cement is explained from the microscopic point of view, and the internal polyurethane stress model under three failure modes is established, as shown in Figure 26. In the bending test, the lower part of the polyurethane cement beam specimen is mainly subjected to tensile stress. As a class of crystalline mixture, the cement particles have good compressive capacity but weak tensile properties, and the polyurethane  
Figure 25: Three failure modes of load–displacement curves. colloid mainly bear the tensile stress under external loading.