Stress-strain model for FRP-confined concrete subject to arbitrary load path

Most existing stress-strain models for fiber reinforced polymer (FRP) confined concrete are applicable to monotonic loading or complete cyclic loading where unloading/reloading is a continuous process between envelope curve and zero stress. However, load cycling is mostly incomplete or partial in practical problems where reverse of loading occurs before the previous load cycle is completed, and this unloading and reloading process is random. Cyclic model allowing for random partial load cycling is rare, if any, due to the lack of experimental data, and the insufficient understanding on the behavior of confined concrete under partial cyclic loading. In this study, FRP confined concrete cylinders were tested under six typical cyclic load patterns. The test results reveal that the partial cyclic stress-strain behavior is different from that under full cyclic loading. The key parameters determining unloading/reloading curves, such as reloading modulus, plastic strain, and tangent unloading modulus are related to loading history and axial stress level. It is found that the key parameters will change, or accumulative damage to concrete will occur, only under certain condition which is defined as effective partial cyclic loading in this work. A simple method is proposed to define the effective partial cyclic loading. Based on an existing complete cyclic model, a stress-strain model for FRP confined concrete under random partial cyclic loading is developed in this work. The proposed model performs well under different load patterns.