Concrete strains under transient thermal conditions: A state-of-the-art review

- LITS of concrete and its components are clearly identified and defined.
- LITS is a markedly confinement-dependent phenomenon.
- Multiaxial tests for temperatures higher than 250 °C are needed.
- 3D models capturing LITS dependency on the stress confinement have to be formulated.

Extensive research has been carried out over the past four decades on the behaviour of mechanically loaded concrete under transient thermal conditions. The purpose of this paper is to provide a concise review of the existing experimental and analytical works with a strong focus on the load-induced thermal strain (LITS) component. In order to eliminate ambiguities in definitions, the existing terms used to describe the strain components that develop in concrete under a transient thermal regime are compared and a clear definition of LITS and its components is given. The analysis of the existing experimental work shows that LITS is: a strain occurring only during first heating of loaded concrete to a given temperature; significantly influenced by the moisture flux in the temperature range 100-250 °C; and independent of aggregate type for temperatures up to about 400 °C. Examination of the existing multiaxial test data demonstrates that LITS is the result of markedly confinement-dependent phenomenon and that experiments on concrete subjected to triaxial compression and transient temperatures above 250 °C are needed. In the light of the experimental evidence, for temperatures up to about 400 °C LITS seems to be mainly due to chemical reactions and microstructural changes taking place in the cement paste, such as dehydration, drying and rearrangement of the water molecules within the cement paste. By contrast, for higher temperatures, thermomechanical damage due to thermal incompatibility between cement paste and aggregates is believed to contribute significantly to the development of LITS. Moreover, the necessity for modelling explicitly the LITS component in the case of Heating-Cooling (HC) cycles is discussed. Finally, a review of the main existing uniaxial and multiaxial explicit LITS models is given, and the advantages and drawbacks of each model are outlined.