The increasing incidence of urban fires poses significant threats to structural integrity, underscoring the urgent need for concrete materials with enhanced mechanical properties post-fire. Incorporating recycled waste steel fibers (WSF) from industrial byproducts into concrete not only bolsters its crack resistance but also advances circular economy principles by transforming industrial waste into valuable resources. Although a large amount of research has focused on native steel fiber-reinforced concrete, there is still a lack of systematic exploration on the optimal dosage and effectiveness of waste steel fibers in slowing down the strength degradation of concrete after high-temperature action. In this study, two grades of concrete (C40 and C60) containing 0%, 1%, and 2% WSF by volume were subjected to heating cycles ranging from 200 °C to 800 °C. Post-cooling evaluations encompassed mass loss quantification, cube compressive strength testing (using 100 mm³ specimens), and splitting tensile tests conducted at a loading rate of 0.1 MPa/s. Results indicated that mass loss escalated to 11% at 800 °C, with C60 experiencing a 12% higher loss compared to C40. Compressive strength decreased by 15% for every 200 °C increment; however, the inclusion of 1% WSF significantly minimized this degradation, preserving 44.5% (for C40) and 37.8% (for C60) of the original strength at 800 °C. Notably, the splitting tensile strength of 1% WSF-reinforced C60 concrete exceeded that of plain concrete by 39.4% after exposure to 800 °C, demonstrating its superior crack-bridging capabilities.

In order to explore the axial compression performance of external steel rib ring restraint waste-steel-fiber-reinforced concrete-filled steel tubes (ERWCFSTs), 18 short-column axial compression tests were conducted. The effects of the number of rib rings, rib ring spacing, rib ring setting position, and waste steel fiber (WSF) content on the axial compression performance of the columns were analyzed. The results show that the concrete-filled steel tube (CFST) short columns with rib rings were strengthened, the specimens were mainly characterized by drum-shaped failure, and the buckling was concentrated between the rib rings. Without rib ring specimens, the steel tube is unable to resist the rapid increase in lateral expansion, leading to buckling initiation near the bottom of the specimens. The columns with rib rings exhibited a minimum increase of 32.5% and a maximum increase of 53.17% in load-bearing capacity compared to those without rib rings, with an average improvement of 37.78%. The columns achieved the best ductility when the rib ring spacing was 50 mm. When the rib ring spacing remained constant, columns with a number of rib rings no less than the height-to-diameter ratio (H/D) demonstrated more uniform stress distribution and optimal confinement effects. For a fixed number of rib rings, specimens with rib ring spacing between H/8 and H/4 showed significant improvements in both load-bearing capacity and ductility. The confinement effect was better when the rib rings were positioned in the middle of the column height rather than near the ends. The incorporation of WSF resulted in a minimum increase of 2.86% and a maximum increase of 10.49% in column load-bearing capacity, indicating limited enhancement. However, WSF improved the ductility performance of the columns by at least 10%. Combined with theoretical analysis and experimental data, a formula for calculating the bearing capacity of ERWCFSTs was established.

Polypropylene (EPP) concrete offers advantages such as low density and good thermal insulation properties, but its relatively low strength limits its engineering applications. Waste steel fibers (WSFs) obtained during the sorting and processing of machining residues can be incorporated into EPP concrete (EC) to enhance its strength and toughness. Using the volume fractions of EPP and WSF as variables, specimens of EPP concrete (EC) and waste steel fiber-reinforced EPP concrete (WSFREC) were prepared and subjected to cube compressive strength tests, splitting tensile strength tests, and four-point flexural strength tests. The results indicate that EPP particles significantly improve the toughness of concrete but inevitably lead to a considerable reduction in strength. The incorporation of WSF substantially enhanced the splitting tensile strength and flexural strength of EC, with increases of at least 37.7% and 34.5%, respectively, while the improvement in cube compressive strength was relatively lower at only 23.6%. Scanning electron microscopy (SEM) observations of the interfacial transition zone (ITZ) and WSF surface morphology in WSFREC revealed that the addition of EPP particles introduces more defects in the concrete matrix. However, the inclusion of WSF promotes the formation of abundant hydration products on the fiber surface, mitigating matrix defects, improving the bond between WSF and the concrete matrix, effectively inhibiting crack propagation, and enhancing both the strength and toughness of the concrete.