Transactions, SMiRT-24 BEXCO, Busan, Korea - August 20-25, 2017 Division IX (include assigned division number from I to X) A MOISTURE-DEPENDENT THERMAL STRAIN CONSTITUTIVE MODEL FOR CONCRETE Giacomo Torelli 1 , Parthasarathi Mandal 2 , Martin Gillie 3 1 PhD Candidate, School of MACE, The University of Manchester, Manchester, UK 2 Senior Lecturer, School of MACE, The University of Manchester, Manchester, UK 3 Reader, School of MACE, The University of Manchester, Manchester, UK ABSTRACT This paper presents a thermal strain constitutive model for concrete, able to capture the effects of the moisture content of the material on the mechanical behaviour under compressive loads and high transient temperatures. The model is the first to account for the effect of moisture content on both load induced thermal strain (LITS) and free thermal strain (FTS) for high temperatures. Experimental evidence shows that the deformability of loaded concrete under transient temperatures is strongly influenced by the moisture content of the material at the beginning of the thermal transient. Hence, the drying process taking place in concrete structures prior to a thermal transient significantly affects the performance of the material in case of transient thermal. With this in mind, a new constitutive relationship is presented where the two components of the thermal strain, LITS and FTS, are formulated as a function of the water content of the material at the beginning of the thermal transient. Specifically, LITS is modelled as the sum of a moisture-independent component, named Transient Strain (TS), and a moisture dependent component, named Transient Drying Creep (TDC). Similarly, the FTS is obtained by explicitly modelling its moisture-independent component, Pure Free Thermal Strain (PFTS), and its moisture-dependent components, named Transient Shrinkage (TS) and Transient Swelling (TSW). The presented constitutive model is verified and validated against transient tests performed on concrete specimens having different initial moisture contents. INTRODUCTION Accurate understanding and modelling of the concrete material behaviour under transient thermal conditions is crucial for a reliable assessment of the effects of thermal loads on bulk concrete structures, particularly if a certain level of performance is required in the case of accidental situations. This is the case for safety-related nuclear structures, such as prestressed concrete pressure vessels [1]. When heated in absence of mechanical load, concrete presents an isotropic expansion usually referred to as Free Thermal Strain (FTS). However, experimental evidence shows that if concrete is subjected to a compressive load while heated, a different thermal strain is measured [2]. The difference in between the thermal strain measured in the case of mechanically loaded concrete and the FTS is commonly defined as Load Induced Thermal Strain (LITS) [2,3]. Such strain component has been proved to be mainly irrecoverable on cooling or unloading [2]. Besides, previous studies have reported that when concrete is loaded uniaxially, a contractive LITS develops in the direction of the load, while a dilative LITS appears along the directions perpendicular to the load [4,5], and that LITS is a markedly confinement-dependent phenomenon (Kordina, Ehm, & Schneider, 1986; Petkovski & Crouch, 2008). Such features have been included by the authors in two constitutive models meant to be used for temperatures up to 250°C and 500°C respectively (Torelli, Gillie, Mandal, & Tran, 2016; Torelli, Mandal, Gillie, & Tran, 2017).