Effect of hot-compacted waste nylon fine aggregate on compressive stress-strain behavior of steel fiber-reinforced concrete after exposure to fire: Experiments and optimization

Although using polymeric waste as a substitution for stone aggregate in concrete can greatly contribute to diminishing environmental issues, it leads to the degradation of many mechanical features of concrete. To approach this issue, using upgraded waste granules can improve these degraded features. Therefore, for the first time in this work, through hot-compacting nylon waste, the compressive stress–strain performance of fiber-reinforced concrete incorporating upgraded nylon waste was assessed before and after heating. To do this, 81 concrete samples were made and subjected to the axial compressive test, with test variables of the volume of the upgraded nylon waste (0, 10, and 20%) substituting for sand, the volume fraction of steel fibers (0, 0.75, and 1.25%), and the heating temperature (20, 300, and 600 °C). Then, parameters contributing to the compressive behavior of concrete including the compressive strength, strain at peak stress, elastic modulus, ultimate strain, toughness index, and stress–strain graph were evaluated, the surface appearance and failure mode were examined, microstructural observations were conducted, and prediction models of the mechanical features were proposed. Moreover, the residual compressive strength values were assessed versus the estimations of the ACI 216, EN 1994–1-2, and ASCE codes. The results demonstrated that by raising the heat, notable degradation occurred in the mechanical features of the concretes, and the greatest drops occurred after 600 °C, such that compressive strength and elastic modulus on average dropped by 61 and 84%, respectively, and the ultimate strain increased by 2–9 folds. Further, adding nylon granules to the concrete mixtures lowered the mechanical features of the samples while introducing steel fibers improved them. Afterward, two stress–strain models were proposed to capture the post-heating compressive behavior of concrete. These models demonstrated proper agreement with the experimental results. Finally, the response surface method (RSM) was employed to present an optimum solution for the design parameters by maximizing the compressive strength and toughness index of concrete.