Enhancing interfacial bond performance of steel fibers embedded in ultra-high-performance concrete through surface dezincification and chelation treatments

The flexural and tensile strength of ultra-high-performance concrete (UHPC) depend on the interfacial bond performance between the steel fibers and UHPC matrix. This study proposed a novel dezincification-chelation modification method for steel fibers, leveraging the dezincification effect of brass coating and the adsorption of chelating agents. To investigate the modification mechanisms, efficiency for fibers of varying diameters, and compare improvements, tests were conducted on dezincification, chelation, and dezincification-chelation methods. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) revealed that dezincification exposed the cold-drawn grooves of steel fibers, while chelation refined and expanded these grooves. Atomic force microscopy (AFM) showed that all treatments increased surface roughness, with dezincified-chelated fibers achieving a maximum 21.5-fold increase. All the surface treating methods improved the pullout behavior of the steel fibers, the optimal chelation and dezincification-chelation modification times of 6 h and 9 h, respectively, were independent of fiber size. The average bond strengths of the dezincified and dezincified-chelated short steel fibers increased by 45.3 % and 52.9 %, respectively, and the pullout energy increased by 96.6 % and 53.8 %, respectively, compared to the blank steel fiber. The average bond strengths of the dezincified and dezincified-chelated long steel fibers increased by 46.1 % and 64.3 %, respectively, and the pullout energy increased by 78.5 % and 70.8 %, respectively. Previous studies confirmed our findings that if the improvement in the root mean square roughness of chelated-modified steel fibers should exceed 10.0 times that of untreated steel fibers the pullout performance will decline.