Mechanical behavior of functionally graded concrete flexural members

Abstract

Deformation capacity is essential for the ability of reinforced concrete flexural members to withstand earthquakes. To improve this capacity, this study introduced functionally graded concrete flexural members (FGCFMs) with ductile failure modes and multiple plastic regions. In FGCFMs, a graded distribution of the flexural strength is achieved by incorporating a graded hybrid reinforcement of fiber-reinforced polymer (FRP) and steel. Multiple plastic regions can develop when the graded distribution of the flexural strength aligns with the external moment. Pushover tests validated the formation of these plastic regions, showing an increase in deformability of up to 147 %, an improvement in ductility of 51.8 %, and an energy dissipation performance of 314 % compared with conventional reinforced concrete flexural members. The tests revealed two failure modes of the specimens. Specifically, when failure occurred in the grade reinforced only with steel bars, the specimen failed in the ductile mode. However, in the grade reinforced with steel and FRP bars, the failure mode was predominantly brittle. The tests revealed that the key factors leading to the formation of multiple plastic regions were the alignment between the flexural strength of the sections and the external moment, as well as the significant increase in flexural strength provided by the FRP bars after the steel bars yielded. A comprehensive parametric analysis further examined the impact of the grade height on the overall deformation capacity and development of multiple plastic regions. Optimal deformability in the FGCFMs was achieved when the grade height reached specific threshold values, effectively balancing the flexural strength with the external moment and resulting in large displacements and a ductile failure mode.