A refined model for the flexural behavior of reinforced UHPC members under sustained loading

The long-term behavior of reinforced Ultra-High Performance Concrete (UHPC) members is significantly influenced by UHPC's creep properties. To predict the long-term performance of structural members, it is essential to have both a material-level creep model and a constitutive model for structural analysis. This paper presents a refined model for the long-term performance of UHPC, incorporating critical nonlinear deformation mechanisms: instantaneous damage, plastic strain, creep strain, and time-dependent damage. The constitutive model for instantaneous damage and plastic strain mechanisms is based on experimental data from short-term cyclic loading tests on UHPC specimens. Additionally, the creep strain components and time-dependent damage evolution laws are derived from well-established linear and nonlinear creep models, validated for cementitious materials through extensive research. The model parameters were meticulously calibrated using creep damage test results from UHPC. The validation process included linear and nonlinear creep tests on UHPC cylinders under sustained stress levels ranging from 0.2fc to 0.66fc, creep failure tests on UHPC cylinders under sustained stress levels from 0.85fc to 0.95fc, and long-term bending tests of reinforced UHPC beams under sustained load. In the linear and nonlinear creep tests, comparison between the calculated ultimate creep coefficients and the experimental results demonstrates the model's validity under low and moderate stress levels. In the creep failure test, a comparative analysis of strain evolution and creep lifetime between experimental and model results confirms the model's validity under high sustained stress levels. Similarly, during the long-term bending test, simulated time-dependent displacement and compressive strain show good agreement with the experimental data, confirming the model's accuracy in predicting the long-term performance of reinforced UHPC members. Furthermore, to streamline the constitutive model, the instantaneous damage effect was simplified and ultimately neglected. A comparison of simulation results between the original model and the simplified version demonstrates that disregarding the instantaneous damage effect is a safe approach for practical engineering applications.