The addition of steel fiber improves concrete''s tensile strength and shear strength. It also decreases concrete''s brittleness through increase in toughness and energy dissipation capacity. Thus, studies on steel fiber-reinforced concrete (SFRC) have been actively conducted in Korea and overseas. This study aims to evaluate the effect of aspect ratio and tensile strength of steel fiber on the mechanical properties of normal-strength and high-strength SFRC.
In the preliminary tests, uniaxial compression tests were conducted by applying strain control speed at rates of 0.05, 0.1, 0.3, 0.5, and 1.0 mm/min in a universal material testing machine, thereby evaluating the effect of a loading rate on the compression behavior of SFRC. In the tests, 35 MPa-grade SFRC and steel fibers (aspect ratio was 64 and tensile strength was 1,200 MPa) were used.
In this test, four types of steel fibers were used with variables of normal and high-strength SFRC mixes, aspect ratio, and tensile strength to evaluate the mechanical properties of eight SFRC mixes. In addition, general concrete specimens without mixing steel fiber were fabricated. For axial characteristics, compressive strength, elastic modulus, and strain were evaluated, and the compressive toughness and compressive toughness index were evaluated based on the stress?strain curve. For flexural characteristics, initial crack strength and flexural strength were evaluated, and flexural toughness and flexural toughness index were evaluated based on the load?deflection curve.
The compressive behavior evaluation was conducted using a cylindrical specimen of 150-mm diameter and 300-mm height to ensure a uniform distribution of 60-mm steel fiber. A compressormeter was installed at 100-mm around the center to measure the compressive strain in the center of the specimen. Two linear wire extensometers were installed to measure the strain through the average value.
For the flexural behavior evaluation, tests were conducted using a universal material testing machine with 200-kN capacity. A third-point loading bending test was conducted, in which a distance between loading points was set to 450 mm. The cross-section of the flexural specimen was set to 150 × 150 mm considering the fiber''s dispersion, and two linear wire extensometers connected to the yoke were installed to evaluate the deflection in the center.
The conclusions obtained through test results and analysis are as follows:
1) As a loading rate decreased, compressive strength and compressive toughness tended to decrease, but the effect of loading rate on the compressive toughness index was minimal. Post-peak behavior was clearly evaluated for the 35-MPa-grade SFRC showed at a mix rate of 0.75%, but the 60 MPa SFRC exhibited brittle failure after the maximum load, which made the evaluation of the post-peak behavior difficult. Thus, additional studies are required about the compressive evaluation technique of high-strength SFRC.
2) This study found that the mechanical properties due to the steel fiber mix had a minimal effect on the modulus of elasticity and compressive strength, but the compressive toughness and compressive toughness index appeared to increase when using steel fiber whose aspect ratio and tensile strength were high. The flexural behavior evaluation results showed that the efficiency of the high-strength steel fiber was high in the high-strength SFRC, which was closely related to the failure pattern of steel fiber.
3) This study analyzed a correlation of compressive and flexural toughness indexes of SFRC to estimate the compressive toughness index of SFRC, which was difficult to be evaluated in a relative sense, and derived a proposed equation accordingly. The proposed equation is expected to be useful for verifying the restrained effect of SFRC in column and coupling beam members in the future.