Gawin et al. 1 A NEW APPROACH IN THE HYGRO-THERMO- MECHANICAL ANALYSIS OF CONCRETE AT HIGH TEMPERATURE D. Gawin 1 , C.E. Majorana 2 , F. Pesavento 3 , B.A. Schrefler 4 ABSTRACT The prediction of concrete performance at high temperature is of great practical importance, in particular for safety evaluation of various concrete structures during fire or in the case of nuclear accident. In such conditions concrete may be subject to spalling phenomena which hazard the integrity of the construction and which may often lead to collapse of the structure. Designers are increasingly using High Performance Concrete (HPC) and Ultra High Performance Concrete (UHPC) for their significantly improved strength and durability compared to ‘normal’ concrete. Anyway, the drawback of the HPC and UHPC is their higher tendency to experience explosive spalling because of their higher compactness and lower permeability than a ‘traditional’ concrete. Spalling involves thermal, hygral and mechanical processes. Such complex coupled phenomena require numerical simulations implementing new mathematical models. An innovative mathematical model to analyse heat and mass transfer in concrete at high temperature and resulting mechanical behaviour including damage effects and some results of computer simulation of a UHPC structure are presented and discussed. INTRODUCTION To analyse hygro-thermal phenomena in porous media two different approaches are used: phenomenological- and mechanistic ones. In phenomenological approach moisture and heat transport are described by diffusive type differential equations with temperature- and moisture content- dependent coefficients. The model equations are often obtained by means of Irreversible Phenomena Thermodynamics. The model coefficients are 1 Ph.D. Department of building Physics and Building Materials, Technical University of Lodz, Al. Politechniki 6, 93-590 Lodz, Poland 2 Associate Professor, Department of Structural and Transportation Engineering, University of Padua, via Marzolo, 9, I-35131 Padova, Italy 3 Ph.D. Candidate, Department of Structural and Transportation Engineering, University of Padua, via Marzolo, 9, I-35131 Padova, Italy 4 Full Professor, Department of Structural and Transportation Engineering, University of Padua, via Marzolo, 9, I-35131 Padova, Italy