Chelate effect on fiber surface morphology and its benefits on pullout and tensile behaviors of ultra-high-performance concrete

To develop optimum engineered steel fibers as reinforcement for ultra-high-performance concrete (UHPC), three different types of steel fibers having various geometries, such as circular straight (C), non-twisted triangular (T0), and singly twisted triangular (T1), were considered. The surface of the steel fibers was also modified using an electrolyte solution comprising ethylenediaminetetraacetic acid (EDTA). The surface morphology was quantitatively evaluated, and the pullout and tensile behaviors of UHPC with the steel fibers were examined. The roughness of the fiber surface increased with the duration of immersion in EDTA electrolyte solution up to 9 h, and the roughness parameter increased by approximately 10 times. The C fiber absorbed the highest pullout energy (632.1 mJ), followed by the T0 and T1 fibers, whereas the T1 fiber most effectively increased the tensile strength and specific energy of UHPC, followed by the C or T0 fiber. The surface treatment efficiently enhanced both the pullout and tensile performance of UHPC with the C and T0 fibers owing to the increased surface roughness, whereas it deteriorated the tensile performance of UHPC with the T1 fibers. Steel fiber types that produced severe matrix spalling at the inclined condition or those that were ruptured in UHPC had a higher possibility of inferior tensile performance than those with minor matrix spalling and nonrupture. As an optimal reinforcement strategy of UHPC, 6-h treatment of C and T0 fibers or the pristine T1 fiber was recommended, which helped to achieve tensile strengths of 17.5–20.4 MPa and specific energies of 106.7–113.0 kJ/m³.