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Abstract (summary): High-performance fibre-reinforced cementitious composites (HPFRCC) are cementitious materials with improved mechanical characteristics and enhanced durability properties relative to conventional concrete. Most contain only one type of fibre that benefits performance within a limited range of deformation. Hybrid steel fibre-reinforced concrete (HySFRC) attempts to take advantage of different fibres by combining them. This dissertation examines the synergistic effects between steel fibres of different geometries, from the material to the structural level under shear-dominated conditions, by examining mixtures with the same total fibre volumetric ratio. At the material level, tests on cubes, cylinders, dogbones and prisms under bending were performed. A novel dogbone configuration suitable for FRC was also developed. The benefits of fibre hybridization on the shear behaviour and cracking properties of HySFRC with conventional steel reinforcement were evaluated using fourteen panel specimens. The test results showed that fibre hybridization benefited performance both at the serviceability limit state (SLS) and at the ultimate limit state (ULS) relative to the monofibre counterparts. These benefits in the mechanical performance may help reduce congestion of conventional reinforcement. Moreover, hybridization offers flexibility in the constitutive characteristics of the basic mechanical properties of concrete, which is useful for targeted concrete designs. An investigation on modelling HySFRC in shear was also conducted. Analysis identified the need for the development of a model that accurately captures the enhanced tensile post-cracking behaviour due to fibre bridging within the framework of the Disturbed Stress Field Model (Vecchio, 2000). The proposed formulations build on the Simplified Diverse Embedment Model (SDEM) developed by Lee, Cho and Vecchio (2013). Advancements include the consideration of interaction between fibres that are pulled-out simultaneously, the synergistic influence of short fibres on the pull-out behaviour of the longer fibres, and the fibre-matrix interactions that influence the bond-slip behaviour of a single fibre. Verification studies on structural elements were performed using the finite element analysis program VecTor2.