Development of Novel Curvilinear Steel Fibers for Improving Tensile Behavior of Ultra-High-Performance Concrete

Abstract

The brittleness of concrete can be alleviated in terms of the tensile performance by incorporating discontinuous fibers in the matrix. In this manner, the tensile stress can be redistributed through the fibers after initial matrix cracking to avoid rapid matrix failure. Ultra-high-performance concrete (UHPC) has extremely high compressive strength and durability compared to conventional concrete due to its high particle packing density using only fine aggregates, but because it is still brittle material, incorporating steel fibers can also reinforce tensile performance. The composites of ultra-high-performance concrete in which such fibers are incorporated is called ultra-high-performance fiber-reinforced concrete (UHPFRC). Tensile behaviors after improved cracking is closely related to the interfacial bond properties between the steel fibers and the matrix, and a number of studies have been conducted using fiber pullout tests to investigate interfacial bonds, and direct tensile and flexural tests to verify the effect of steel fiber incorporation. However, according to a recent study, since the UHPC differs from the conventional concrete in a considerable part such as compressive and tensile strengths, density, and mixed materials, the conventional steel fibers commercially used for conventional concrete were not highly effective for UHPC matrix. In this study, a new curvilinear steel fiber was proposed to optimize for the enhancing the tensile performance of UHPFRC. To the best of the authors’ knowledge, no study has been published so far on the curvature effect of steel fibers in UHPC on the pullout behavior. Therefore, we first investigate the fiber pullout behavior of curvilinear straight steel fibers in UHPC according to the radius of curvature. The behavior of the UHPFRC composites was evaluated through direct tensile test to confirm the effect on the curvature of the curvilinear steel fiber. In this study, five types of steel fibers including straight steel fibers and curvilinear steel fibers with 10, 15, 25, and 50 mm radius of curvature were evaluated. In the fiber pullout test, two inclination angles of 0 ° and 45 ° were evaluated and in the direct tensile test, steel fibers of 1.5% (by volume fraction) were incorporated into the UHPC. Based on results of the fiber pullout test, as the curvature of the curved steel fiber increases, the average and equivalent bond strengths tend to be improved regardless of the inclination angle. However, highly curved steel fibers with inclination angles exhibited low equivalent bond strength due to matrix damage. In the direct tensile test, curvilinear steel fibers with a radius of curvature of 25 and 50 mm showed a clear improvement in tensile strength such as tensile strength, strain capacity, and g-value as curvature increased. For the fibers with a radius of curvature 10 and 15 mm, Tensile performance was rather degraded due to the loss of orientation of the fibers and the increase of fiber balls. Tensile test results showed a relatively high correlation with the equivalent bond strength results of the inclined fiber in the pullout test. Based on the comprehensive results of fiber pullout and direct tensile test, the increase in curvature of the curvilinear steel fiber has the effect of improving bond strengths and tensile performance, and the use of curved steel fiber with a radius of curvature of 25 mm was most effective for enhancing the tensile performance of UHPFRC. Therefore, it is expected that curvilinear steel fibers can be proposed as a novel tensile reinforcing material for UHPC.