Biphasic finite element analysis of pullout-controlled behavior in recycled steel fiber reinforced concrete

The structural application of recycled tire steel fibers in concrete remains limited by uncertainties regarding their post-cracking efficiency and by the absence of predictive numerical frameworks capable of representing their structural behavior. This study investigates whether recycled steel fibers can provide structurally efficient reinforcement mechanisms comparable to industrial fibers and proposes a numerical framework capable of reproducing the governing fiber–matrix interaction mechanisms. An experimental–numerical investigation was conducted on recycled fiber reinforced concrete (RFRC) combining mechanical characterization and biphasic finite element modeling. The recycled fibers presented high geometric variability, with lengths ranging from 1.65 to 61.64 mm and diameters from 0.06 to 0.53 mm, promoting intrinsic hybridization and multiscale crack-bridging mechanisms. RFRC with 0.6% fiber content achieved a 55% increase in splitting tensile strength compared to plain concrete and residual flexural strengths f R,1 and f R,4 of 2.91 MPa and 2.07 MPa, respectively, comparable to industrial fiber reinforced concrete produced with similar fiber contents and compressive strengths (∼30–35 MPa). Pullout tests indicated average bond strengths of approximately 12 MPa, with pullout governing the failure mechanism due to the rough surface morphology and high tensile strength of recycled fibers. The proposed biphasic model accurately reproduced the experimental load–displacement behavior, crack evolution, stress redistribution, and residual strength by explicitly incorporating the statistical variability of fiber geometry and orientation. The results demonstrate that recycled tire steel fibers can provide efficient pullout-controlled reinforcement mechanisms when appropriate selection and processing procedures are adopted, supporting future design development and broader implementation of circular solutions in concrete construction.