Ductile‐to‐brittle transition in fibre‐reinforced concrete beams: Scale and fibre volume fraction effects

The mechanical response of fibre-reinforced brittle-matrix structural elements subjected to bending is discussed in the framework of fracture mechanics. By means of numerical simulations based on the bridged crack model, the flexural behaviour of steel fibre-reinforced concrete (FRC) beams has been investigated, taking into account pull-out or yielding bridging mechanism of the secondary phase. In both cases, the numerical results predict a transition of the global structural behaviour, which can range from ductile to catastrophic. This behaviour is governed by a dimensionless parameter, the brittleness number, NP, in which the effects of the structural size and the fibre volume fraction are included. An experimental campaign is carried out on FRC beams subjected to three-point bending tests, considering three different beam sizes and four different fibre contents. A comparison between experimental tests and numerical simulations shows that the bridging mechanism is due to the fibre slippage into the matrix, rather than the fibre plastic flow. The expected ductileto-brittle transition is found for each beam scale, as predicted by NP. As a consequence, the minimum fibre volume fraction can be defined according to this model, providing an effective structural bearing capacity of FRC structural members.