## 8 Conclusions  
In general, the addition of PPF enhanced the Poisson's ratio, peak strain, peak stress, and elastic modulus of concrete at room temperature. In comparison to regular low-slump concrete, the most considerable enhancement in Poisson's ratio was observed in the PL3C0.5 group, whereas the most substantial improvements in peak strain, peak stress, and elastic modulus were seen in the PL3C1.5 group, with enhancements of 11.13%, 15.6%, 19.6%, and 21.65%, respectively. The changes in several parameters of Poisson's ratio, peak strain, peak stress, and modulus of elasticity after high temperature are related to the amount and length of PPF admixture and depend on the change in porosity, release of vapor pressure, and the degree of material deterioration. The PL15C0.5 group had the maximum peak strain after 600-800 \({}^{\circ}\)C, with an increase of 36.66% in the average peak strain compared to the fiber-free concrete group, while the minimum was found in the PL3C0.5 group with a 7.17% decrease in average peak strain, compared to the fiber-free concrete group. This indicates that longer PPF has a toughening effect on concrete at low admixture levels, and the peak strains of PPF concrete all increased to varying degrees after high temperatures. The largest increase in peak stress after 600 \({}^{\circ}\)C was in the PL9C0.5 group, which was 6.70% higher than the fiber-free concrete group, and the largest decrease was in the PL9C1.5 group, which was 29.41% lower. Overall, the presence of PPF reduces the peak stress of concrete, and the weakening effect brought about by the increase in content is more obvious than the weakening effect produced by the increase in length. A significant weakening of the modulus of elasticity occurred at 400 \({}^{\circ}\)C. In contrast, the addition of fibers was able to increase the modulus of elasticity of the concrete, to some extent, with the degree of reduction and increase depending on the change in porosity due to PPF. The Poisson's ratio increased by 5.8% and decreased by 8.68%, respectively, compared to the fiber-free concrete. The compressive strength and splitting tensile strength of PPF concrete at room temperature were significantly influenced by both the lengths of fibers and the amounts of admixture. The strength exhibited a decreasing trend with increasing PPF length. At an admixture amount of 5.0 kg/m\({}^{3}\), the fiber specimens with a length of 19 mm showed the most significant drop, with a compressive strength drop of 8.2% compared to normal concrete, while the effect of admixture on strength demonstrated an initial increase followed by a decrease with higher admixture levels. The maximum compressive strength and splitting tensile strength were achieved at 48.5 MPa and 3.9 MPa, respectively, with a PPF fiber length of 3 mm and an admixture amount of 1.0 kg/m\({}^{3}\). This resulted in a strength increase of 17.15% and 25.72% compared to concrete without PPF. The change in compressive and splitting tensile strengths of PPF concrete under high temperature follows the same pattern as that at room temperature. Under high temperature conditions, PPF with smaller content can significantly improve the mechanical properties of concrete after high temperature. The compressive strength of low collapse polypropylene fiber concrete after high temperature shows a trend of rising and then falling at the same temperature with increase in the fiber admixture. When the temperature was 800 \({}^{\circ}\)C and the fiber admixture was 0.5 kg/m\({}^{3}\), the compressive strengths of 3 mm, 9 mm, 15 mm and 19 mm reached their maxima of 9.65%, 11.33%, 7.90% and 2.87% over ordinary concrete. The effect of PPF on the compressive strength of concrete was not obvious with the increase of fiber length, and the rate of strength reduction in PPF concrete with a large admixture was much higher than that caused by the length of PPF.