Flexural capacity of steel-FCB bar-reinforced coral concrete beams

Steel-fiber composite bar (SFCB) is a new technique with steel core embedded into the fiber-reinforced polymer (FRP) bars, which solve the shortcomings of poor ductility and low stiffness of FRP-reinforced concrete members. Based on the experimental study, flexural behavior of a new type concrete, SFCB bars-reinforced coral concrete beams, is investigated. The deflection, failure mode, tensile strain, and ultimate flexural capacity of this new type of concrete beam are studied in detail through a series four-points bending tests. The results show that the failure process of SFCB-reinforced coral concrete beam can be divided into three main stages: elastic, cracking, and failure. Because exterior layer of SFCB bar is able to continue to withstand tensile stress after its steel core yield, hence, the flexural rigidity and flexural capacity of the beams both increase. The flexural stiffness of SFCB bars-reinforced coral concrete beams is also increased, and the load-deflection relation is more or less nonlinear. By using SFCB bars, the stiffness of the beams can be increased by about 20%. However, under high stress state, the relative slip between the carbon fiber and the concrete as well as that between the steel core and the carbon fiber cause tensile stress loss which weakened the flexural resistance of the SFCB bar-reinforced coral concrete beam. The bonding performance between the round surface steel core and the carbon fiber is obviously weaker than that of the threaded surface steel core, and the steel core slip is more likely to occur under the higher tensile stress. When Using ACI440.1R-15 and GB50608-2010 to calculate the flexural capacity of the SFCB beams, the ratios of the theoretical value and the measured value from tests are 1.07 and 1.12, respectively. By introducing two new factors k1 and k2, a formula was developed through modification of the existing ones from ACI440.1R-15 and GB50608-2010 to calculate the ultimate flexural capacity of SFCB bar reinforced concrete beam which agrees as high as 93-99% with the experimental results.