Numerical Simulation of Steel Fibre Reinforced Concrete Girders Subjected to Cyclic Loads

Engineering structures of plain (PC) and steel fibre reinforced concrete (SFRC) are often exposed to cyclic loadings. Thereby, fatigue causes progressively increasing strains accompanied by degradation of strength and stiffness and may lead to failure. Although the cyclic behaviour of both materials appears comparable in principle, fibres are stated to have a dual impact on fatigue life. It has to be distinguished between effects being helpful, since fibres bridge cracks and retard their growth, or unfavourable ones, since fibres initiate micro cracking. To assess the fatigue performance of SFRC girders with or without additional rebar at all intermediate steps up to failure, elasto-plastic damage theory serves. Therein PC and SFRC are idealized homogenously on a macroscopic level. Based on the envelope concept and a split of strains in inelastic and plastic portions, isotropic but time-dependent material damage parameters are derived for concretes containing steel fibres of different types, dosages, orientations and bond behaviours. The evolution of damage corresponding to s-shaped creep curves is expressed in terms of related numbers of cycles to failure. Thereby, fibres’ post-cracking response and stress levels strongly influence damage states. Transfer to absolute numbers of cycles to failure is achieved by SN-curves considering fibres’ dual effect by a new toughness index. For numerical analysis of a girder’s time-variant deformations and stress-redistributions, the equations are implemented in the Finite-Element software Abaqus. Time discretisation is done via a jump-in-cycles procedure yielding to progressively updated, but degraded stress-strain relations per integration point. Truss elements discretely model rebars and are coupled to the concrete solids using embedded modelling techniques. Verification is done by recalculations of experimental data of SFRC beams tested at Ruhr-University Bochum and other experiments taken from the literature. The results agree well on average indicating a beneficial effect of fibres on the fatigue performance of structures subjected to flexural fatigue.