A numerical model for the creep of fiber reinforced concrete

In fiber reinforced concrete (FRC), fibers provide a post-cracking tensile strength to the concrete by taking up tensile forces after matrix cracking. The introduction of FRC in the Model Code has given designers guidelines and rules to use FRC in structural applications. However, the long-term performance and behavior of cracked FRC needs to be accounted for, but no design rules are given in the Model Code. A fundamental understanding of the micro-mechanical behavior is lacking and numerical analysis can provide useful insights. In this paper, the results of a two-phase numerical model with discretely modelled fibers are presented. The finite element model focuses on the long-term behavior of polymeric FRC under sustained uniaxial tensile loading. In the model, the fibers are randomly generated within the bounds of the finite element model. The material model parameters are calibrated from an extensive experimental campaign. It is possible to take into account fiber creep and rupture as well as instantaneous fiber pull-out. The numerical results correspond well to the crack widening response of an FRC element under sustained tensile loading. It is argued that fiber creep is the main contributing creep mechanism of polymeric FRC. For the considered load levels and fiber volume fraction, the average tensile stress of the fibers only amounts to 7% of their tensile strength.