Plasticity based stress–strain model for concrete confinement

Concrete confinement has an important role in reinforced concrete columns, especially for strengthening applications and in hybrid construction. It is well known that confinement increases axial strength and ultimate strain of concrete. However, existing confinement models are extremely dependent on the type of confinement, i.e., passive or active; and the materials of the confining jacket; i.e., steel or fiber reinforced polymers (FRPs). In this paper, a unified stress–strain model is proposed to evaluate any type of uniform confinement. The model is based on the theory of plasticity, with a key feature on the use of plastic volumetric strain as the hardening parameter. New equations are proposed to define the hardening function in the ascending and descending branches. Initially, a three-dimensional plasticity algorithm was implemented in a nonlinear finite element analysis. The algorithm was validated against a series of test data with different states of stresses, including uniaxial or biaxial compression and uniform confinement. Upon validation of the plasticity model, its equations were rearranged in such a way that the behavior within a strain increment could be described totally using simple equations for uniaxial compression and uniform confinement. Consequently, an explicit stress–strain model was developed and verified against test data for any type of uniform active or passive confinement with any confining material. The procedure is incremental, while no iterative process is necessary. Due to the simplicity of this model, it can be easily implemented in a spreadsheet, so as to consider many parts of the response curve, if so desired. Finally, the model was validated against a representative set of experiments with favorable results.