# Experimental Study on Flexural Properties of Polyurethane-Cement Composites under Temperature Load
## 6 Results and Discussion
### Flexural Strength
The first manifested as a change from brittle to viscoelastic with a cut-off point temperature of \(-\)30 \({}^{\circ}\)C, and the second manifested as a change from viscoelastic to plastic with a cut-off point temperature of 30 \({}^{\circ}\)C. Since the bending and tensile strength of the polyurethane cement composite undergoes a sharp decrease in the process of increasing from 20 \({}^{\circ}\)C to 30 \({}^{\circ}\)C, it can be considered that a specific temperature of increasing from 20 \({}^{\circ}\)C to 30 \({}^{\circ}\)C, recorded as (20 \({}^{\circ}\)C, 30 \({}^{\circ}\)C), is the demarcation point between viscoelasticity and plasticity of the polyurethane cement composite. It can be seen from Figure 20 that the load-displacement curve shows a linear growth trend when the temperature is lower than 20 \({}^{\circ}\)C. The polyurethane cement beam undergoes brittle damage when the ultimate load is reached. The load-displacement curve does not show a plateau period and a slow decreasing section. However, it shows a sharp decrease, indicating that the polyurethane cement composite material mainly shows brittle characteristics when the temperature is lower, and the bending and tensile deformation ability is inferior. At this time, the breaking load and bending and pulling strength of polyurethane cement beams are more significant; however, the bending and pulling strength increases after the temperature is lower than \(-\)30 \({}^{\circ}\)C, which is because of the brittleness of polyurethane cement composites is very large at this time. As the temperature decreases, the breaking load increases, and the displacement decreases so that the bending and pulling strength increases. When the temperature is higher than 20 \({}^{\circ}\)C, the polyurethane cement composites mainly show viscous characteristics, the ability to bend and tensile deformation is perfect, brittle damage will not occur. The form of damage is bending plastic damage with good ductility, which shows that the load-displacement relationship curve at 30 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C is not linearly changing. A plateau period will appear before reaching the ultimate load. After the ultimate load, the load-displacement curve decreases slowly, an apparent decreasing section appears, and the bending and tensile strength of polyurethane cement composites at this stage is minimal, and the resistance to deformation is significant. Polyurethane cement, as an organic-inorganic composite, has the properties of both organic polyurethane colloid and inorganic cement particles. The cement is mainly a crystalline structure formed by the coalescence of particles such as C\({}_{2}\)S and C\({}_{3}\)S, with a reticulated internal distribution, which mainly provides compressive strength. Polyurethanecolloid is made of polyether polyol soft chain segments and isocyanate hard chain segments polymerized into a polymeric substance with long molecular chains and interwoven distribution of soft and hard chain segments, which has good tensile properties and directly determines the flexural strength of polyurethane cement. The principle of bending strength of polyurethane cement is explained from the microscopic molecular point of view. The soft chain segment of polyurethane colloid presents a rubbery state, which provides low-temperature performance; the hard chain segment presents a glassy or semi-crystalline state, which provides high-temperature performance [52]. Reflecting the temperature performance of polyurethane is mainly expressed in the glass transition temperature \(\mathrm{T_{g}}\), the magnitude of \(\mathrm{T_{g}}\) depends on the binding force between macromolecules and the length of molecular chains, so the slender and soft chain segments have a more obvious effect on the \(\mathrm{T_{g}}\) of polyurethane cement [53]. Table 9 lists the \(\mathrm{T_{g}}\) of several common types of polyurethane colloids [54].