## 4 Test Results and Analysis of Compressive Strength
Moreover, the PPF pores increase the path of micro-crack expansion, causing the strength of concrete after high temperature to be much lower than that of concrete at normal temperature. Increasing fiber length leads to higher porosity and larger capillaries, which weaken the effective load-bearing cross-sectional area of concrete. Similarly, increasing the number of fibers also leads to higher porosity, which weakens the effective load-bearing area, while providing more paths for micro-cracks to develop. As shown in Figure 13a, the maximum compressive strength was achieved for 3 mm, 9 mm, 15 mm, and 19 mm when the fiber admixtures were 0.5 kg/m\({}^{3}\), which is 4.21%, 6.29%, 0.86%, and 0.32% higher than that of normal concrete, respectively. The compressive strength reduced by 17.64%, 15.43%, 24%, and 27.04% at a 5.0 kg/m\({}^{3}\) fiber admixture. Figure 13b shows that the residual strength of PPF concrete after 800 \({}^{\circ}\)C was consistent with that after 600 \({}^{\circ}\)C. The effect of PPF type showed less dispersion in strength, and the trend of the curve was closer, indicating that the effect of PPF type on the compressive strength of concrete kept decreasing at higher temperatures. The macro-cracks which developed from micro-cracks and the calcification damage of the material are the main causes of the decrease in strength. When the fiber content is small, PPF can significantly improve the mechanical properties of concrete after high temperature. The compressive strength of low collapse polypropylene fiber concrete after high temperature showed a trend of rising and then falling at the same temperature with increase in fiber admixture. When 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%, representing improvement over ordinary concrete, The effect of PPF on the compressive strength of concrete due to increase in fiber length was not obvious. According to the literature [58], the residual strength of ordinary PPF-free concrete is highest when exposed to high temperatures. However, the conclusion from literature [53] that the variation of PPF type has an insignificant impact on the fluctuation of residual compressive strength, differs significantly from the conclusion of the low collapse degree PPF concrete tests in this paper. In addition, the literature [59; 60] has shown that the compressive strength of ordinary PPF concrete with a PPF admixture of 3.6 kg/m\({}^{3}\) is higher than that of PPF concrete. The conclusion that the compressive strength of concrete is higher than that of PPF concrete slightly differs from the results of this paper, which indicate that different collapse degrees have inconsistent effects on the performance of PPF concrete. Therefore, the study of low collapse degree in this paper is practically and realistically significant. In the present experimental study, the residual compressive strength of PPF concrete significantly decreased when the admixture amount exceeded 0.5 kg/m\({}^{3}\), regardless of fiber length, and the rate of strength decrease significantly accelerated as the fiber length increased. The maximum residual compressive strength was achieved with a dosing rate of 0.5 kg/m\({}^{3}\) and a fiber length of 9 mm, which differs greatly from the study of PPF concrete with conventional collapse at room temperature. The research in this paper shows that PPF can significantly improve the mechanical properties of concrete after high temperature when the fiber content is small under high temperature conditions, and the effect of high dosing of PPF on the mechanical properties of concrete after high temperature is more significant. This is consistent with the change of compressive strength at room temperature.