STUDY ON INTERFACIAL FAILURE MECHANISM OF STEEL AND STEEL FIBER REINFORCED CONCRETE COMPOSITE STRUCTURE UNDER DIFFERENT LOADING CONDITIONS

An innovative method to replace steel reinforcement cage by steel fiber to form the steel and steel fiber reinforced concrete has been proposed to solve the construction problems of steel reinforced concrete structures, such as the position conflict between steel and steel bars, and the difficulty of concrete placement. The push-out tests of 36 specimens and four-point bending tests of 13 specimens were carried out, the interfacial failure of the steel and steel fiber reinforced concrete composite structures was studied under axial force and bending moment, and then the internal force transfer and failure mechanism of the steel and steel fiber reinforced concrete were analyzed under different loading conditions. The bridging effect is formed by the pull-out behavior of steel fibers at concrete cracks, which restrains the crack development and improves the tensile properties of concrete after cracking; as a result, the problem of poor interfacial bond behavior caused by the decrease of cover thickness is solved which occurs between the steel and steel fiber concrete, and the interfacial bonding failure is delayed or even avoided. Under the action of axial force, the interface between the steel and steel fiber reinforced concrete is extruded due to Poisson's ratio effect and the concrete is under tension in two orthogonal directions, which is the main cause of damage and cracking of steel fiber reinforced concrete. The damage degree of steel fiber reinforced concrete is directly related to the interfacial bonding behavior, and is also affected by the Poisson's ratio of steel. In the four-point bending test, the bond cracks concentrate in the mid-span area where only bending moment exists. The internal force transfer between the steel and steel fiber reinforced concrete and the interfacial tensile stress are the main reasons for the large number of bond cracks. Bond crack appears at the tip of steel flange and grows from the tips toward the concrete surface until it reaches the outer surface of the steel fiber reinforced concrete. The trapezoidal failure surface of the steel fiber reinforced concrete cover is finally formed in the mid-span area.