Are steel fibres able to mitigate or eliminate size effect in shear?

Abstract

This paper reports some recent results of an experimental campaign on fibre reinforced concrete (FRC) beams under shear loading tested at the University of Brescia: nine full scale beams, having a height varying from 500 to 1,500 mm, were tested for investigating the effect of steel fibres on key-parameters influencing the shear response of concrete members, with special emphasis on size effect. All tested members contained no conventional shear reinforcement and different amounts of steel fibres: 0, 0.64 or 1 % by volume. Results show that a relatively low volume fraction of fibres can significantly increase bearing capacity and ductility. The latter determines visible deflection and prior warning of impending collapse, which is not possible in plain concrete beams (without transverse reinforcement). The size effect issue is substantially limited. Results were compared against the shear formulation incorporated in the final draft of fib Model Code, which can be considered a valuable analytical model for FRC beams under shear loading, even in the case of the three deepest elements herein tested.