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Hybrid steel fibre-reinforced concrete (HSFRC) composites, constituted from different geometries of steel fibres, benefit from positive performances of different fibre geometries. When applied in segmental tunnels, different aspects of their structural performance should be adequately evaluated to ensure a safe and durable design. In this paper, the flexural behaviour of segmental joints and performance under tunnel-boring machine (TBM) thrust forces for HSFRC segments is separately investigated through an experimental and numerical campaign. Experimental tests are used to evaluate the mechanical characteristics of the HSFRC composites, specifically the nonlinear tensile performance. Numerical models are then developed to study the aforementioned structural aspects in HSFRC segmental tunnels. Currently, guidelines and standards do not provide relationships specifically for the flexural design of segmental joints and TBM jack loads in HSFRC segmental linings. With the intention of meeting this deficit, simplified engineered relations are proposed.

Recent advancements in fiber reinforced concrete (FRC) technology has led to the development of fibrous concrete composites, comprised of fibers with different material and/or geometry, commonly known as hybrid FRC. In one type of hybrid FRC composites, advantageous behaviors of fibers of the same material but with different geometries are gathered in a single FRC mix. The aim of this paper is to develop and validate stress-strain relationships for hybrid steel FRC composites. Six different steel FRC mixes are produced and characterization tests are conducted. Cube, cylindrical and beam specimens are produced for each characterization test corresponding to each of the Steel FRC (SFRC) composites. In this regard, an experimental program is performed to determine the basic engineering properties of SFRC composites using standard compressive, splitting tensile and three-point bending tests. The prescribed procedure of the RILEM guideline, originally developed for non-hybrid FRC, is followed using the obtained experimental results to develop stress-strain behavior models for the SFRC mixes. To validate results for the hybrid SFRC composites, numerical simulations of the 3-point bending tests were performed and compared to that of corresponding experimental results. The results indicated that the proposed stress-strain relationships yield acceptable results for characterizing the behavior of hybrid SFRC composites.