Influence of Test Configuration on the Bond–Slip Behavior of Hooked-End Steel Fibers in Concrete: Quantity, Inclination, and Spacing

The objective of this study is to assess the influence of test configuration on the pullout response of hooked-end steel fibers embedded in a cementitious matrix and to analyze how variations in quantity, inclination, and spacing affect discrete–explicit numerical simulations. The experimental campaign was conducted using dog-bone-shaped specimens with variables of number of fibers (one, two, and four), fiber inclination (0°, 15°, and 30°), and spacing (7 mm and 14 mm), with 133 specimens tested (19 per configuration). The results obtained showed that fiber inclination significantly influences pullout behavior, with higher inclinations (up to 30°) increasing pullout loads (PL1 and PL2 being the maximum pullout and the intermediate pullout load values, respectively) but also leading to fiber rupture in approximately 21% of cases. Closely spaced fibers (7 mm) demonstrated enhanced load transfer compared to wider spacing (14 mm), particularly in setups with multiple fibers. Increasing the number of fibers reduced variability in pullout results, providing more consistent data. Numerical simulations effectively capture fiber–matrix interactions, with load–CMOD curves generally aligning with experimental data. However, discrepancies in the fR1 parameter highlighted the need for further calibration to improve accuracy in modeling early cracking stages. These findings underscore the importance of fiber configuration in optimizing pullout performance and the potential for refining numerical models to better predict post-cracking behavior in steel fiber-reinforced concrete.