Fiber bridging in polypropylene-reinforced high-strength concrete: An experimental and numerical survey

Fracture process of fiber-reinforced concrete notched beams is investigated here. Polypropylene macrosynthetic fibers are utilized for reinforcing concrete specimens, and a high-strength mix design is used to produce strong bonds between the embossed polypropylene fibers and the cementitious matrix of beams. Considering different locations for the notch, this study focuses on bridging mechanism under different conditions using both experimental and numerical approaches. First mode of fracture occurs due to opening of crack faces. This mode of failure is simulated by imposing symmetric boundary conditions on middle-notched beams. Inducing the notch with an offset from the middle, mixed-mode condition is achieved, wherein a combination of opening and sliding of crack surfaces occurs. Plain and reinforced concretes are used to cast each setup of test in order to analyze the bridging effects in different cases. Nonlinear behavior of the cementitious matrix is reproduced numerically using a continuum damage model, and instead of the common phenomenological representation of fibers (e.g., via cohesive crack method), fibers are explicitly modeled in direct numerical simulations. Once the nonlinear response of fibers is obtained, the method can provide valid responses in general loading conditions. This feature distinguishes the proposed approach from the cohesive crack method, wherein the contribution of opening mode, shearing mode, and even tearing mode in three-dimensional cases are required for simulating a general mixed-mode test. Different distributions with random locations and random orientations of fibers are generated in order to assure the objectivity of results. It is found that the prevailing dissipative mechanism within the reinforced specimens is resulted by the sliding resistance of fiber-matrix interfaces. In addition, owing to the high tensile strength of polypropylene fibers, instead of sudden cracking due to fiber rupture, ductile post-peak responses with tremendous amount of energy dissipation are obtained. Low elastic modulus of polypropylene fibers, on the other hand, leads to negligible change in pre-peak responses as the fibers are added to the mixtures.

Automated summary

The fracture process of fiber-reinforced concrete notched beams is investigated in this study. Polypropylene macrosynthetic fibers are used to reinforce the concrete specimens, and a high-strength mix design is applied. The results show that the sliding resistance of fiber-matrix interfaces is the predominant dissipative mechanism within the reinforced specimens, leading to ductile post-peak responses due to the high tensile strength of polypropylene fibers.