Used car tires are buried or thrown away worldwide, which is a severe threat to the environment. Therefore, recycling discarded tires is an important issue. This research assessed the possibility of utilizing steel fibers recycled from discarded tires and industrial steel fibers as effective concrete reinforcement at ambient temperature and after exposure to high temperatures. A three-dimensional mesoscale modeling was done to comprehend the behavior of fiber-reinforced concrete. In the mesoscale model, concrete is a composite material consisting of four phases: aggregate, mortar matrix, interfacial transition zone (ITZ), and fibers. The actual shape of the particles has been used to model the aggregate. To investigate the effect of the shape of the aggregate, their performance was compared with ellipsoidal particles. Also, the effect of ITZ thickness on uniaxial compressive strength has been examined. The results indicate that considering aggregates with actual shape and ITZ significantly impacts mesoscale modeling and cannot be ignored. In the end, with the help of the concrete damaged plasticity model, homogeneous responses of fiber-reinforced concrete were obtained on a macroscale, and its effect on improving the performance of structural members, including a steel–concrete composite beam, was investigated. The results also exhibited that adding recycled steel fibers increased the concrete slab\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\'s load-bearing capacity, energy absorption, and ductility. In general, the results show that mesoscale modeling is an effective tool for understanding the mechanical behavior of fiber-reinforced concrete and has been able to provide comparable results with experimental studies.

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