In order to accurately measure the internal stress-strain curve of plain concrete specimens and confined concrete specimens under compression, a new measurement method is proposed, which adopts conventional strain gauges to measure the internal strain data of the specimens, and the micro soil pressure box with ultra-large range is developed to measure the internal stress of the specimens. The uniaxial compression tests of 3 plain concrete specimens and 9 confined concrete specimens are completed, and the macroscopic failure process of the specimens and the stress-strain curves at different internal points are obtained. Combined with the experimental results, the accuracy of the calculation results of several classical confined concrete constitutive models is compared, and a modified constitutive model is proposed. Solid finite element analysis is used to analyze the stress-strain curves at different points inside the specimens, and the prediction accuracy of different constitutive models is compared. On this basis, nonlinear finite element analysis is used to verify the quasi-static test of RC columns, and the accuracy of different constitutive models in the nonlinear analysis at the component level is compared and analyzed. The results show that the measurement method proposed in this study can accurately measure the stress-strain data internal the concrete. The calculation results of the modified constitutive model proposed in this study are in the best agreement with the test results, and have a wide range of applications, which can be applied to the measurement of internal stress-strain curves of other different types of specimens.

The accuracy of a constitutive model for confined concrete largely relies on its capability to capture concrete's dilatancy behavior. In this paper, a non -orthogonal flow rule (NFR) is used to reasonably characterize the concrete's volume change law in relation to the stress state without the necessity for a plastic potential function. Then, the non -orthogonal plastic model is implemented in the finite element (FE) software ABAQUS using the implicit stress update algorithm, which employs the line search method and the numerical consistent tangent stiffness matrix to ensure the robustness and computational efficiency of FE analysis. Finally, FE simulations are performed to evaluate the constitutive model's capabilities in actively and passively confined concrete. In the latter case, steel tubes restrict the concrete's lateral deformation. An analysis and discussion are conducted regarding the impact of dilatancy behavior on concrete -filled steel tube (CFST) columns. The consistency between experimental data and simulation results demonstrates that the FE modeling with a non -orthogonal constitutive model provides an effective tool to describe the behavior of confined concrete.