Surface modification of steel fibers using chemical solutions and their pullout behaviors from ultra-high-performance concrete

Abstract

To improve the pullout resistance of conventional straight steel fiber from ultra-high-performance concrete (UHPC), its surface was modified using various chemical solutions. Calcium carbonate (CaCO3) particles were precipitated on the surfaces of the steel fiber in an aqueous solution. The morphology and particle size of CaCO3 were controlled by ethylenediaminetetraacetic acid (EDTA) and investigated via scanning electron microscopy; further, its chemical composition was verified through energy-dispersive X-ray spectroscopy. The test results indicate that longer duration of immersion in a solution of CaCl2 and Na2CO3 yield larger CaCO3 particles on the surface owing to Ostwald ripening. The addition of EDTA changes the morphology of the CaCO3 particle and decreases its grain size from 16.2 to 3.1 μm. In order to investigate the effects of the precipitated CaCO3 particles on the fiber pullout resistance, single fiber pullout test was carried out; the modified steel fibers were embedded in UHPC matrix. The CaCO3 particles precipitated on the fiber surface improved the pullout resistance of the steel fiber from the UHPC. The smaller CaCO3 particles were more effective for enhancing the pullout resistance. The average bond stress and pullout work were increased by up to 15% and 37%, respectively, owing to the precipitated CaCO3 particles. The EDTA solution could peel off the surface of steel fiber regularly and render it rough; thus, it greatly increased the pullout resistance (more than 80%) than the plain fiber.