## 4 Test Results and Analysis of Compressive Strength  
The relationship between PPF admixture and compressive strength of concrete at room temperature is shown in Figure 12. The number at the end indicates the average compressive strength that could be achieved by a specimen. It can be seen that the PPF admixture significantly increased the compressive strength of concrete. The compressive strength decreased as the PPF fibers became longer for the same PPF content, while an increase in PPF content for the same PPF fiber length led to an initial increase and then a decrease in the compressive strength. The peak point of increase occurred at the lower PPF content, which was in agreement with the findings of [PERSON] et al. [55]. The appropriate amount of PPF fiber length and admixture can increase the compressive strength of concrete because the bridging effect of PPF is greater than the negative effect of porosity. A small amount of admixture and short length of PPF fibers can fill the internal pores of concrete. However, an increase in fiber length and admixture leads to a decrease in the apparent density and an increase in porosity, which results in a decrease in strength. Additionally, too long fibers may form tangles inside, which decreases the uniformity of distribution, also increases porosity. The compressive strengths of PPF concrete with PPF fiber lengths of 3 mm and 9 mm were highest at a PPF admixture of 1 kg/m\({}^{3}\), which increased by 17.15% and 14.85%, respectively, compared to ordinary concrete. The compressive strength of PPF concrete with PPF fiber lengths of 15 mm and 19 mm reached a maximum at a PPF admixture of 0.5 kg/m\({}^{3}\), which increased by 11.25% and 11.2%, respectively. With an increase in PPF admixture, the compressive strength of 19 mm long PPF concrete was initially lower than that of ordinary concrete, while the compressive strength of 15 mm long PPF concrete was only 1.37% higher than that of ordinary concrete. When the PPF admixture amount reached 5 kg/m\({}^{3}\), the compressive strength of PPF concrete with 9 mm length was 2.67% lower than that of ordinary concrete, while the compressive strength of PPF concrete with 3 mm length was 6.52% higher than that of ordinary concrete. This indicates that the effect of polypropylene fiber admixture on the mechanical properties of concrete after high temperature is more significant compared to the fiber length, due to the greater porosity brought about by the large admixture, compared to that brought about by the length [56]. The effect of PPF admixture on the compressive strength of concrete at high temperatures is illustrated in Figure 13. After being exposed to temperatures of 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C, relatively few cracks were observed on the surface of PPF concrete specimens. This is largely due to the fact that PPF, after melting and vaporizing at high temperatures, creates many holes inside the concrete, increasing its internal connectivity. As a result, water vapor and heat can escape more easily, reducing vapor pressure and providing more free space. This also acts as a thermal shock absorber, reducing the damage to the micro-structure of the  
Figure 11: The relation between Poisson’s ratio and temperature. concrete [57]. In contrast, the strengths of plain concrete after being exposed to maximum temperatures of 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C were only 68% (28 MPa), 35% (14.3 MPa), and 22% (9.1 MPa) of that at room temperature. The compressive strength of PPF concrete showed a trend of increasing and then decreasing with increase of the fiber admixture, except for the compressive strength of concrete with a fiber length of 9 mm, which reached its maximum with the same admixture. The law of compressive strength follows a decreasing trend with the increase of length. This is likely because the pores of PPF after high-temperature melting and vaporization cause an increase in porosity, which reduces the area of the concrete matrix subjected to load.