Assessment of steel fiber corrosion in ultra-high-performance concrete and its implication on interfacial bond and tensile behaviors

Abstract

Until now, a lot of efforts have been made to increase the strength of concrete, but there were many problems to be solved because of the problem that brittleness increases when the strength of concrete increases. To solve this problem, discontinuous steel fibers that reinforce concrete after cracking were frequently adopted. When the fibers are bonded to high-strength concrete, a strong adhesion through fiber bridging occurs, which can improve ductility, fracture toughness and energy absorption capacity, thus negating this brittle failure. In particular, the recently developed ultra-high -performance fiber reinforced concrete (UHPFRC) has the advantage of optimizing the granular mixture of low water-binder ratio that leads to homogenization of microstructure, and integrating a large amount of iron fiber to provide excellent strength and ductility. Therefore, it has been argued that UHPFRC is less susceptible to corrosion than conventional steel in concrete structures. However, UHPFRC is known to have an extremely steep increase in autogenous shrinkage, making it easy for shrinkage cracks to occur in the manufacturing stage, and the propagated chloride ions in the crack UHPFRC is observed in depth. Therefore, corrosion of steel fibers in concrete needs a lot of investigation. In this study, the purpose is to investigate the effect of steel fibers exposed to 3.5% NaCl solution on ultra-high-performance concrete. The experimental method is divided into fiber pullout test and direct tensile test. In addition, there are two types of direct tensile test: For single cracks, the tensile performance evaluation of ultra-high-performance fiber-reinforced concrete with single cracks was investigated according to the relationship between the crack width and exposure duration. For this purpose, a total of five residual crack opening displacement was applied to a 3.5% NaCl solution. In addition, the exposure period considered four periods of 0,4,10 and 20 weeks. As a result of the experiment, the longer the exposure period and the wider the width of the crack, the more severely corroded the steel fiber and the tensile performance of UHPFRC with pre-crack occurred further deteriorated. In the case of tensile specimens with a crack width of less than 20μm, no significant deterioration was observed even after exposure to NaCl solution for 20 weeks, and the degree of oxidation of the steel fiber surface was also minor. However, the tensile strength of specimens with a crack width of 50μm or greater was reduced when exposed to the NaCl solution for 20 weeks. Multiple cracks were classified into two types of steel fibers (e.g., straight and twisted), two levels of pre-strain (e.g. 0.45 % and 0.6 %) causing different microcracking properties. And four immersion durations 0, 4, 10 and 20 weeks were investigated. As a result of the experiment, the steel fibers embedded in the multi-cracked UHPC were corroded by the penetration of the NaCl solution, and the degree of corrosion increased as the longer immersion duration. In addition, it was investigated that the tensile performance of both straight and twisted steel fibers is improved when appropriate corrosion proceeds. However, it was investigated that UHPC reinforced with twisted fibers was more susceptible to corrosion effects than UHPC reinforced with straight fibers. Given the same immersion period, less microcracks and smaller forming, the steel fibers are moderately corroded, resulting in better tensile performance. Steel fiber corrosion has a greater effect on energy absorption capacity than tensile strength, regardless of fiber type and microcrack properties.