Influence of steel fiber modifications on the pullout and tensile behaviors of ultra-high-performance fiber-reinforced concrete

Abstract

This thesis aims to improve the pullout and tensile behavior of UHPFRC through modifications of steel fibers, and the ultimate objective is to achieve enhanced tensile behaviors which allows reducing the steel fiber content. For this, various types of geometrically modified and surface modified steel fibers were employed and developed as reinforcement for UHPFRC, and their influences were experimentally verified based on microstructures and mechanical properties. As geometrically modified fiber, commercial twisted and hooked were employed, and novel half-hooked fibers were developed for UHPFRC. As surfaced modified fiber, previously reported steel fibers treated by acid, zinc phosphating, and nano-silica coating were employed, and CaCO3 coated fibers were developed using CaCO3 synthesis methods. Moreover, novel EDTA-electrolyte treated fibers were developed to overcome the drawbacks of previously reported surface modified fibers. The surface characteristics of the modified steel fibers were investigated using SEM-EDX and AFM analyses, and pullout and tensile behaviors of UHPFRC incorporating the modified steel fibers were examined. The UHPFRC incorporating 0.5 vol.% half-hooked fibers and 1.5 vol.% short straight fibers showed the enhanced tensile performance as compared to the commercial UHPFRC incorporating 2.0 vol.% short straight fibers. The CaCO3 coating on fiber surface significantly improved the pullout and tensile behaviors of UHPFRC incorporating straight steel fibers, owing to enhanced pullout resistance attributed to the accumulating and crumbling of CaCO3 particles. The EDTA-electrolyte treatment for steel fiber significantly improved the pullout and tensile behaviors of UHPFRC incorporating straight steel fibers, owing to enhanced interfacial bond attributed to the furrowed surface. Based on the comparative tensile energy absorption capacity, three types of steel fibers (i.e., EDTA-treated steel fiber for 6h, CaCO3 coated steel fiber, and CaCO3 coated steel fiber with EDTA) provided the improved tensile behaviors that allows reducing 0.5 vol.% steel fiber content.