## 1 Introduction
Additionally, high SF contents improve the tensile strength of concrete members by improving the bond resistance between the concrete and rebars [41]. Conversely, if an insufficient SF content is used, the tensile strength is not considerably enhanced [42]. In 2008, [PERSON] et al. [43] investigated the performance of FRC beams with four kinds of fibres and two SF contents (0.4 and 1.2%). According to the results, high strength steel twisted fibres perform significantly better than other SF in a higher strength matrix. [PERSON] and [PERSON] [44] evaluated the influence of SF on the bending behaviour of lightweight concrete beams. Therefore, cracks propagate faster in RC members manufactured using lightweight concrete than in normal and high-strength concrete. It was shown that SFs increase the toughness capacity of prismatic concrete beams and their ductility. [PERSON] et al. [45] studied the post-cracking performance of normal and high-strength SF-reinforced concrete beams. The flexural strength, post-peak ductility and deflection capacity significantly increased by using a 1% SF content. [PERSON] et al. [46] evaluated the effect of SF contents on the flexural behaviour of RC beams. For over-reinforced specimens, the post-peak stiffness of the beams was significantly lower than that of normal-weight concrete specimens. [PERSON] and [PERSON] [47] tested SF reinforced concrete beams under predominant torsion and bending. They showed an improvement in the torsional strength, combined torsional-shear-bending strength and crack strength of the concrete when SFs were used. [PERSON] et al. [48] investigated the static and dynamic behaviour of SF RC beams. Improvements were found in the static bending response with different fibre contents compared to those conducted under dynamic loading. [PERSON] et al. [49] determined the structural response of SF RC beams under different loading values. The results indicated that the use of SFs improved the ductility of the beams regardless of the strain rate. Furthermore, the shear strength of the SF-reinforced beams substantially improved. RCA generally represents a way to change a waste creation into a reserve. Consequently, utilisingdemolished supplies as an alternative to natural ones could be a beneficial technique to keep the environment safe. RCA has widely been utilised in non-structural concrete elements and lower feature uses, such as back-fills and lower layers of roads, and natural aggregates have been used in a wide range of structures [50]. Alternatively, SFs not only delay the creation of cracks but also control their growth, and when combined with the post-crack bridging capability of fibres, SFs diminish the width of cracks when concrete is restrained [51]. In the hardened state, the interaction between SFs and mortar plays a bridging role in order to delay the widening of cracks. Therefore, if sufficiently high contents of SFs are provided in the specimens, the tension resistance of the concrete matrix will substantially increase. Furthermore, the bridging role of SFs in the mortar resulted in obtaining the composite performance of concrete that causes a transition from micro-cracks to macro-cracks and prevents increasing the cracks' width. In concrete with a low fibre content, the fibres do not meaningfully enhance the tension and bending resistance of the concrete. Alternatively, in concrete containing a high SF content, the benefits of fibre reinforcement include raising the tension resistance, the strain-hardening response before cracking and a boosted stiffness beyond crack instalment [51]. When the member with fibres undergoes bending, their presence leads to enhancing the shear behaviour of the RC member. Furthermore, using fibres at the cracks' location causes a postponement of the crack extension [52]. Different standards suggest different equations to predict the crack spacing and width in RC beams.