## 1 Introduction  
Self-compacting concrete (SCC) is a novel high-performance concrete (HPC) with superior characteristics. Studies show that SCC can spread easily under its weight into tight and congested areas without segregating or bleeding while achieving good consolidation [1]. Considering the high workability and high durability of SCC, it has been widely used in tunnels and other underground structures [2; 3; 4; 5]. Compared with conventional concretes, SCC is usually prepared with lower water-to-binder ratios and higher amounts of filler materials [6; 7; 8]. Generally, SCC has low porosity and high resistance to permeability. Despite these advantages, SCC is susceptible to spalling under fire conditions. This shortcoming mainly originates from the relatively low permeability and dense matrix of SCC [9; 10; 11]. Accordingly, the main challenge that confines the application of SCC is to improve the resistance time of the concrete structure in the case of fire. This issue is of significant importance to increase the structure safety in catastrophic accidents and maintain the protective properties of SCC. Spalling is commonly characterized by the fierce or non-fierce breaking off of layers of concrete from the structure surface when it is exposed to abruptly rising temperatures, which is a normal condition in fires [12]. Based on the destruction mechanism, the spalling phenomenon can be primarily divided into four categories, including corner, aggregate, surface, and explosive spalling. It should be indicated that explosive spalling is the most severe spalling mechanism, which usually takes place fiercely and significantly degrades the material properties. Further investigations reveal that concrete spalling is a complex phenomenon which is affected by numerous factors such as concrete strength, moisture content, porosity, density, heating rate, and fiber reinforcement [13; 14; 15]. It is worth noting that the two most common reasons for the spalling of concrete at high temperatures are enhanced pore pressure build-up originating from the vaporization of the free and chemically bound water as well as thermal stresses originating from high-temperature gradients along with the concrete depth [16; 17; 18]. The performed investigations demonstrate that polypropylene fibers (PF) can effectively prevent HPC spalling at high temperatures [19; 20; 21; 22]. This mainly originates from the PF melting when the temperature inside the concrete exceeds 165-173 \({}^{\circ}\)C. In this case, the vapor pressure inside the concrete can be released via the inter-connected space, which appears after the PF melting. In this regard, [PERSON] et al. [22] showed that for the same water-to-binder ratio, similar porosity and pore size distribution can be obtained in HPC. Accordingly, PF can be used in SCC to improve the spalling resistance in case of fire. This idea has been studied and verified [23; 24; 25]. In this regard, EC2 [26] recommends that adding 2 kg/m\({}^{3}\) of PF to the concrete effectively reduces the spalling. Further investigations reveal that lower amounts of PF, such as 0.05-0.15% by volume, are also effective in increasing the concrete resistance to high temperature spalling [20]. The fire resistance of the concrete is not only highly resistant to the spalling phenomenon, but it also maintains high residual mechanical properties after heating to elevated temperatures. [PERSON] and [PERSON] [24] carried out experiments on six SCC mixtures with different concrete strengths and PF amounts. The obtained results showed that high-strength SCC is more prone to spalling, while the addition of PF effectively prevents high-temperature spalling. However, it was found that the mechanical properties of the concrete deteriorate after the inclusion of PF in SCC. It is concluded that it is not a good idea to enhance the fire resistance of SCC by adding PF only.