A confinement-dependent load-induced thermal strain constitutive model for concrete subjected to temperatures up to 500 °C

This paper proves that, given a Load-Induced Thermal Strain (LITS) curve able to accurately describe the uniaxial LITS development for a specific type of concrete and temperatures up to 500 °C, a more accurate prediction of the 3D LITS state is obtained through a confinement-dependent 3D implementation than through the classic confinement-independent approach. In particular, a new model is presented, obtained by extending to 3D a fourth order polynomial LITS derivative function in a way that allows the effects of the stress confinement to be taken into account. For comparison purposes, the same fourth order polynomial LITS derivative function is also implemented in 3D through the classic confinement-independent modelling approach. These constitutive relationships are adopted to model transient experiments performed on concrete subjected to constant uniaxial and biaxial compressive loads. The results show that the confinement-dependent modelling approach gives a better prediction of the LITS state developing in the case of biaxial compression than the classic confinement-independent approach. Finally, the validated confinement-dependent model is applied to evaluate the LITS-related stress redistribution taking place in a typical Prestressed Concrete Pressure Vessel (PCPV) subjected to heating to 500 °C and cooling back to the normal operating temperature of 50 °C. It is found that including the effects of three dimensional LITS behaviour has significant effects on the predicted stress states.