Moisture and confinement dependent thermal strain constitutive models for concrete subjected to high temperatures

Abstract

The aim of this work is to generate a strong impact on the industry and safety of society by improving existing concrete thermal strain models for the numerical assessment of nuclear Prestressed Concrete Pressure Vessels (PCPV) in the case of fault of their water cooling system. In this context, this work presents three novel thermal strain models for concrete subject to multiaxial loads and transient high temperatures. The three models are developed incrementally and are characterized by an increasing degree of complexity. The first model captures the confinement-dependent behaviour of concrete heated to temperatures up to 250 °C, temperatures representative for PCPVs subject to partial fault of their water cooling system. The effects of the stress confinement on the behaviour of heated concrete are modelled through a novel approach that allows the deformability of the material to be expressed as a function of the triaxiality of the stress state. The second model represents an extension of the first model to temperatures up to 500 °C, temperatures representative for PCPVs subjected to complete fault of their water cooling system. Finally, the third model includes the dependency of the concrete thermal strains on the moisture content of the material. Such a dependency is introduced by explicitly modelling the moisture-dependent components of the concrete thermal strain and expressing them as a function of the material moisture content prior to heating. First, the proposed models are implemented numerically within the open source finite element package Code_Aster. Then, they are verified and validated against published transient tests. The results indicate that the models presented here capture the behaviour of heated concrete better than existing models. Finally, the models are employed to assess the structural behaviour of a typical nuclear PCPV subject to both partial and complete faults of its water cooling system. It is found that accurate modelling of concrete thermal strains is essential to capture the loss in prestress taking place in PCPVs subject to fault conditions. Moreover, it is shown that the loss in pretension can be significantly underestimated if the dependency of the concrete thermal strains on stress confinement and material moisture conditions is not explicitly modelled.

Date of Award	1 Aug 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Parthasarathi Mandal (Supervisor) & Martin Gillie (Supervisor)