Rate-Dependent Characteristic of Relaxation Time of Concrete

This study focused on the rate-dependent relaxation time of concrete using the split Hopkinson pressure bar test and the ultrasonic technique. Based on the experimental results, and considering the viscous effect in concrete, the viscoelastic stress-strain relationship was established in terms of the principles of irreversible thermodynamics and the generalized Onsager's principle, providing the convergence of an iterativescheme in terms of the Lipschitz's condition. It was found that the relaxation time of concrete depends on the strain rate. Furthermore, considering the stateequations and damage evolution, a nonlinear viscoelastic constitutive model of concrete coupling damage was proposed. Compared with the experimental results, this model could better characterize the strengthening and softening characteristics of concrete prior to and beyond the strength limit. Further analysis indicated that the viscous effect is dominantly caused by the C-S-H in concrete.