## 1 Introduction  
Hybrid fibre-reinforced concrete (HFRC) is a mixture of two or more fibres mixed into the concrete matrix to form a new composite material that can achieve different fibres to complement each other's strengths and advantages. At present, scholars at home and abroad have done research on the mechanical properties of HFRC, and the types of these fibres are steel fibre, polypropylene fibre, alkali-resistant glass fibre, basalt fibre, and polyvinyl alcohol fibre. For example, steel fibre has been used extensively in concrete in recent years. It was made by cutting fine steel wire or cold rolled strip shear with an aspect ratio of 40 to 80. Basalt fibre is made from basalt stone molten at 1450-1500 \({}^{\circ}\)C and drawn at high speed through a platinum- rhodium alloy drawing funnel. It not only has high strength but also has a variety of excellent properties such as electrical insulation, corrosion resistance, and high temperature resistance. As a result, basalt fibre is widely used in many areas such as fibre-reinforced composites, shipbuilding materials, the automotive industry, and civil engineering. Alkali-resistant glass fibre is characterised by its effectiveness in resisting the high alkali substances in cement and its high tensile and flexural strength, and it therefore also belongs to a green reinforcing material used in high-performance reinforced concrete. Polypropylene fibre and polyvinyl alcohol fibre have the advantages of high strength, toughness, chemical resistance, and microbial resistance, so it is commonly used as fibre additive in concrete. [PERSON] [1], [PERSON] [2], and [PERSON] [3] mixed steel and polypropylene fibre into concrete in different proportions to carry out mechanical properties tests, and the results showed that mixing different fibres into concrete could significantly improve the compressive and flexural strengths. [PERSON] [4] mixed steel fibre, polypropylene fibre, and glass fibre into concrete and carried out a semicircular bending test (SCB), which showed that the hybrid fibre enhanced the ductility and load-bearing capacity of the concrete, with the glass fibre enhancing the ductility of the concrete and the steel fibre improving the energy absorption capacity of the concrete. [PERSON] [5] chose to mix alkali-resistant glass fibre and coconut bark fibre into concrete to study the effect of hybrid fibre on the mechanical properties of concrete, and the test results showed that hybrid fibre can enhance the compressive strength and tensile strength of concrete; the impact resistance had also improved. Based on the above research, it has been shown that HFRC has incomparable advantages over single-fibre-reinforced concrete, and it improves the mechanical properties of concrete in many aspects and multiple dimensions. In recent years, the acceleration of urbanization in various countries has brought about the impact that many dilapidated residential buildings have been demolished, resulting in a large amount of construction waste, which is extremely detrimental to environmental protection. However, research has shown that recycled coarse aggregate obtained by crushing waste concrete from construction waste can replace natural coarse aggregates in the preparation of recycled aggregate concrete (RAC), and the application and development of recycled concrete technology is an important measure to realise the resourcefulness of construction waste and to ensure the sustainable development of construction resources [6]. Existing research results show that the compressive strength and splitting tensile strength of recycled concrete decrease as the replacement ratio of recycled coarse aggregate increases [7; 8]. The shape of the uniaxial compressive stress-strain curve of recycled concrete is basically similar to that of natural concrete, but the peak stress and elastic modulus decrease significantly as the replacement rate of recycled coarse aggregate increases [9; 10].