This is the accepted version of the following article: [Pros, A., Diez, P. and Molins, C. (2012), Modeling steel fiber reinforced concrete: numerical immersed boundary approach and a phenomenological mesomodel for concrete-fiber interaction. Int. J. Numer. Meth. Engng, 90: 65–86. doi:10.1002/nme.3312], which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1002/nme.3312/abstractSteel fiber reinforced concrete (SFRC) allows overcoming brittleness and weakness under tension, the main drawbacks of plain concrete. The influence of the fibers on the behavior of SFRC depends on their shape, length, slenderness, and also on their orientation and distribution into the plain concrete. The goal of this paper is to develop an ad hoc numerical strategy to account for the contribution of the fibers in the simulation of the mechanical response of SFRC. In the model presented, the individual fibers immersed in the concrete bulk are accounted for in their actual location and orientation. The selected approach is based on the ideas introduced in the immersed boundary (IB) methods. These methods were developed to account for 1D (or 2D) solids immersed in 2D (or 3D) fluids. Here, the concrete bulk is playing the role of the fluid and the cloud of steel fibers is acting as the immerse boundary (that is, a 1D structure in a 2D or 3D continuous). Thus, the philosophy of the IB methodology is used to couple the behavior of the two systems, the concrete bulk and fiber cloud, precluding the need of matching finite element meshes. Note that, considering the different size scales and the intricate geometry of the fiber cloud, the conformal matching of the meshes would be a restriction resulting in a practically unaffordable mesh.\ud In the proposed approach, the meshes of the concrete bulk and fiber cloud are independent, and the models are coupled imposing displacement compatibility and equilibrium of the two systems. In the applications presented here, the concrete bulk is modeled using a standard nonlinear damage model. The constitutive model for the fibers is designed to account for the complex interaction between fibers and concrete. The fiber models are based on the previous investigations describing the concrete-fiber interaction and its dependence on the factors identified to be relevant: shape of the fiber (straight or hooked) and angle between the fiber and crack plane.Peer ReviewedPostprint (author's final draft

Double punch test is used to indirectly assess the tensile strength of plain\ud concrete, ft. For this normalized test, the tensile strength is obtained as a function of the failure load, P, which is expressed as f_t = F(P). Different authors have proposed different expressions for the relation F(·), yielding scattered values of f_t. None of these alternatives is universally recognized as being more suitable than the others. In fact, these expressions are mainly based on elastic models considering the maximum tensile stress under the load P and ft is obtained as an output of the linear model. A numerical\ud simulation allows using models in which f_t is an input of the material model and the corresponding failure load P is obtained associated with each value of f_t. In the present work, double punch test is simulated numerically considering two alternatives for modeling plain concrete accounting for damage\ud and cracking: (a) the nonlocal Mazars damage model and (b) an heuristic crack model including joint elements in an a priori defined crack pattern.\ud Numerical results are validated with experimental data and compared with the analytical expressions available in the literature.Preprin