## 4 Discussion  
### Effect of Thermal Conditions on Dynamic Mechanical Properties of Polypropylene Fiber Concrete  
The average dynamic compressive strength and average elastic modulus of polypropylene concrete under different temperature levels and different impact pressure levels are showed in Figure 9.  
Figure 8: Dynamic stress–strain curves of polypropylene fiber concrete in different conditions. (**a**) Impact air pressure = 0.4 MPa, average impact = 5.5 m/s; (**b**) impact air pressure = 0.6 MPa, average impact = 7.5 m/s; (**c**) impact air pressure = 0.8 MPa, average impact = 9.5 m/s.  
Figure 9: Effect of thermal conditions on dynamic mechanical properties of polypropylene fiber concrete. (**a**) Average dynamic compressive strength of polypropylene concrete under different conditions. (**b**) Average elastic modulus of polypropylene concrete under different conditions.  
It can be observed from Figure 9a that under the action of temperature load, the dynamic compressive strength of polypropylene fiber concrete was not monotonically linear as the temperature grade increased. For example, with an impact air pressure 0.4 MPa (average impact velocity = 5.5 m/s), compared with the case at 25 \({}^{\circ}\)C, the dynamic compressive strength of polypropylene fiber concrete at 200 \({}^{\circ}\)C increases by 10.2%, and compared with the case at 200 \({}^{\circ}\)C, the dynamic compressive strength of polypropylene fiber concrete at 400 \({}^{\circ}\)C, 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C decreases by 8.73%, 31.14% and 72.59%, respectively. It seemed that there was a threshold temperature grade; when the temperature grade was lower than this threshold, the dynamic strength of polypropylene fiber concrete increased as the temperature grade increased, and when the temperature grade was higher than this threshold, the opposite situation takes place. In this experiment, the threshold was between 200 \({}^{\circ}\)C and 400 \({}^{\circ}\)C. It also can be observed from Figure 9b that the elastic modulus of polypropylene concrete under different temperature grades decreased as the thermal grade increased and that it reached its minimum at 800 \({}^{\circ}\)C. Although the dynamic elastic modulus showed an improvement when the impact pressure level increased, the strain rate effect of the elastic modulus is not as obvious compared to the strain rate effect of the dynamic compressive strength.  
Other relevant studies from the literature [56; 57; 58; 59; 60] have used microscopic observation technology to study the internal components, structure, and other microscopic aspects of fiber concrete exposed to high temperatures. The research results can explain the experimental phenomenon well: before the threshold temperature, as the temperature increases, part of the water that had been combined with the concrete decomposed due to the high temperatures, promoting the further hydration of the unhydrated cement particles [60]. The polypropylene fibers gradually dissolved, and the dissolved polypropylene fibers played a role in filling the original pores, improving the strength. When the temperature exceeded a certain range, due to the continuous escape of the water in the concrete, hydrated calcium silicate gel and calcium hydroxide decomposed and dehydrated at different temperatures, and the original microcracks continued to develop under the high-temperature conditions, resulting in the original dense structure gradually becoming looser over time, resulting in reduced strength.