Experimental and theoretical investigation on axial tensile behavior of ultra-high performance concrete (UHPC) with recycled steel fibers from waste tires

Ultra-high performance concrete (UHPC) is renowned for the superior durability and mechanical properties; however, its environmental impact and high cost limit extensive promotion. By partially or completely replacing industrial steel fiber (ISF) with recycled steel fiber (RSF) from waste tires, a cost-effective and sustainable UHPC (RSUHPC) has been developed. This paper aims to research the tensile response and constitutive relations of RSUHPC incorporating RSF and ISF. A series of experimental analyses, including slump tests, compression tests, fiber pullout tests, and direct tensile tests, were presented to assess the workability, fiber-matrix bonding performance, and tensile behavior of RSUHPC. The findings revealed that RSF has the potential to replace ISF and using RSF as toughening materials can prepare RSUHPC with excellent mechanical properties and good workability. While increased RSF or ISF content improves compressive strength, it reduces workability. Additionally, the pullout response of single RSF embedded in UHPC matrix is positively correlated with the embedded length and deformation degree. RSF exhibits favorable pullout behavior, which is intermediate between the pullout properties of ISF with varying aspect ratios. The hybridization of RSF and ISF significantly enhances tensile strength and toughness of RSUHPC, with an optimal mixing ratio yielding better performance than UHPC reinforced solely with ISF. Furthermore, based on the theoretical analysis of bonding behavior of single fiber, a tensile constitutive model that can predict the tensile response of RSUHPC was developed, offering an accurate stress-strain relationship and supporting the efficient use of RSUHPC in practical engineering applications.