Synergistic crack control by reinforcement and steel fibers in self-compacting SFRC beams: DIC characterization and crack-width prediction

To clarify the synergistic effects of reinforcement ratio and steel fiber volume fraction on the flexural crack resistance and deformation capacity of steel fiber reinforced self-compacting concrete (SFRSCC) beams, four-point bending tests were conducted on beams with different reinforcement ratios and fiber contents. Combined with digital image correlation (DIC)-based full-field strain measurements, moment-curvature analysis, and crack-parameter statistics, the cracking behavior, crack evolution, load-carrying capacity, and ductility were systematically investigated. The results show that steel fibers modify the cracking mechanism through crack bridging, promoting the transition from localized cracking to a distributed multi-crack pattern, and thereby improving crack control at the serviceability limit state (SLS) in terms of crack width, crack spacing, and crack depth. When the steel fiber volume fraction increased from 0.52% to 1.05%, the ultimate moment of lightly reinforced beams increased by only 1.7%-2.3%, whereas the cracking moment Mcr and the serviceability-related moment M0.2mm increased by 32.3%-57.0% and 64.0%-86.2%, respectively, indicating that the primary contribution of steel fibers lies in enhancing service performance rather than ultimate flexural capacity. In contrast, increasing the reinforcement ratio alone improved load-carrying capacity but reduced deformation capacity. A moderate reinforcement ratio combined with an appropriate fiber dosage achieved the best overall balance among crack control, stiffness retention, and ductility, and exhibited service-stage crack resistance close to that of the UHPC reference group. Based on these findings, an analytically grounded crack-width prediction model for reinforced SFRSCC beams was developed and validated against the experimental data, providing a theoretical basis and experimental support for improving relevant design methods and code provisions.