An experimental study was undertaken to develop an extension of Mohr-Coulomb and Newman models to predict the failure of concrete subjected to elevated temperatures. Concrete specimens of 83×166-mm dimensions were prepared and subjected to various temperature levels of 300, 500, and 700°C. The tests were carried out in triaxial cell using various confining stress levels. The applied confining pressures varied between 1.4 and 24 MPa, corresponding to 3 and 54% of the uniaxial compressive strength. Test results indicated that the increase in temperature and/or confining pressure lead to a significant change in the mechanical properties of concrete. Furthermore, the Mohr-Coloumb and Newman models did not provide good fitting of the experimental data obtained on concrete subjected to high temperature of up to 700°C. A dehydrate index is introduced and used to define new variables in the existing models. The modified models are shown to adequately predict the residual strength of concrete that has been...

A constitutive model to describe the triaxial load-response spectrum of plain concrete in both tension and shear was developed. The inelastic phenomena are described using the plastic flow with direction determined by the gradient of the plastic potential. A new plastic potential is introduced and experimentally fitted to ensure better estimate of the load direction. This approach allows to control the inelastic dilatancy in terms of the inelastic deformation of the material. By overlaying the plastic potential on modified Etse and Willam’s yield surface (both defined on the Haigh–Westergaard coordinates), the results showed that the two curves do not undergo similar stress states for a given strength level. It is, therefore, necessary that each surface goes through the current stress state to ensure adequate evaluation of normal vectors. A closed-form solution to accurately predict the triaxial stress state in concrete has been proposed. The predictive capabilities of the proposed model are evaluated by comparing predicted and measured stresses. The proposed model is shown to be accurate in predicting stress state of concrete.