## 1 Introduction
[PERSON] and [PERSON] [20] found that adding 1% volume fraction of polypropylene fibers was more effective in preventing concrete spalling but had a minimal impact on the bonding characteristics of concrete. [PERSON] et al. [21] discovered that adding polypropylene fibers can significantly reduce the loss of tensile strength in high-strength concrete, and the mass loss rate is proportional to the fiber content. [PERSON] et al. 's research [22] showed that adding polypropylene and steel fibers can improve not only the bonding performance, but also the mechanical properties, of an ultra-high-performance concrete (UHPC) repaired cementitious composite system at high temperatures. It can also inhibit the high-temperature bursting and spalling of concrete. Meanwhile, [PERSON] et al. 's results [23] demonstrated that adding a 0.2% volume fraction of polypropylene fibers to beam specimens of ultra-high-performance concrete cured at room temperature can effectively reduce the bursting of the beam surface under fire conditions. [PERSON] and [PERSON] [24] suggested that the aspect ratio of polypropylene fibers is the critical factor that affects the spalling effect of UHPC concrete. Thinner and longer polypropylene fibers are better at preventing spalling. When the fiber content is below 3 kg/m\({}^{3}\), UHPC can be prevented from peeling, and the critical value of the aspect ratio of peeling is not less than 300. Self-compacting concrete (SCC) is a current research hotspot. [PERSON] et al. 's research [25] showed that a small amount of polypropylene fiber can effectively inhibit the explosive spalling of self-compacting concrete. [PERSON] et al. [26] found that, compared with steel and glass fibers, polypropylene fibers can more effectively reduce the slump of concrete, and the mechanical properties of polypropylene fiber concrete with a volume fraction of 0.5% are optimal after being exposed to 800 \({}^{\circ}\)C. [PERSON] et al. [27] found that when the mixing ratio of polypropylene and steel fibers is 1:0.2, the dynamic splitting performance and tensile toughness of concrete at high temperatures can be significantly improved. [PERSON] et al. [28] showed that when the amount of added polypropylene fiber does not exceed 2.5 kg/m\({}^{3}\), the alkaline components in the concrete decrease with increase in temperature, and the degree of decrease increases with the increase in the amount of polypropylene fiber. [PERSON] [29] pointed out that when the addition of polypropylene fiber was 0.25% volume fraction, the impact strength of foamed concrete increased by 300% compared to a blank control group. To reduce the bursting and peeling of concrete shield tunnel segments under fire and to improve durability, [PERSON] et al. [30; 31; 32; 33; 34; 35] investigated the fire performance of concrete shield tunnel segments with polypropylene fibers and their mixtures. [PERSON] et al. [36] observed that the hybrid mixture of polypropylene and micro-steel fibers exhibited good impact resistance at high temperatures. [PERSON] and [PERSON] [37] stated that adding 0.5% PPF significantly improved various mechanical properties of concrete at high temperatures. In summary, research on the effect of high temperature on PPF concrete mainly focuses on the following: the inhibitory effect of PPF on the bursting phenomenon of different types of concrete (including ultra-high performance concrete (UHPC), Self-compacting concrete (SCC), lightweight concrete, etc. ); the performance of PPF applied in actual structural components (such as reinforced concrete beam-slab structures, concrete shield tunnel lining structures, etc. ); the mixed mechanical properties of PPF with other fibers. No studies were found on the three-factor coupling effect of PPF length, PPF dosage, and temperature on multiple samples. This study conducted mechanical research on PPF concrete with eight dosages and four lengths after high temperature, to more fully understand the working mechanism of PPF in concrete and fill the gap in related research.