Issue 29

N. Cefis et alii, Frattura ed Integrità Strutturale, 29 (2014) 222-229; DOI: 10.3221/IGF-ESIS.29.19 222 Focussed on: Computational Mechanics and Mechanics of Materials in Italy Damage modelling in concrete subject to sulfate attack N. Cefis, C. Comi Department of Civil and Environmental Engineering, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano, Italy. nicola.cefis@polimi.it , claudia.comi@polimi.it A BSTRACT . In this paper, we consider the mechanical effect of the sulfate attack on concrete. The durability analysis of concrete structures in contact to external sulfate solutions requires the definition of a proper diffusion-reaction model, for the computation of the varying sulfate concentration and of the consequent ettringite formation, coupled to a mechanical model for the prediction of swelling and material degradation. In this work, we make use of a two-ions formulation of the reactive-diffusion problem and we propose a bi-phase chemo-elastic damage model aimed to simulate the mechanical response of concrete and apt to be used in structural analyses. K EYWORDS . Sulfate attack; Concrete; Damage; Finite Element Method; Porous Media. I NTRODUCTION nder particular environmental conditions, some kinds of concrete may be subject to deleterious chemical reactions that cause swelling and micro-cracking, alter the mechanical properties and affect the durability of concrete structures. The chemistry behind different degradation processes in cement based material in aggressive environments has been the subject of a number of publications in the last twenty-five years; a comprehensive up-to-date review can be found e.g. in [1]. The present work focuses in particular on the sulfate attack and the consequent delayed ettringite formation. There are two kinds of sulfate attack: the internal sulfate attack (ISA) and external sulfate attack (ESA). In the first case, the sulfate ions are already present within the material because of the thermal depletion of primary ettringite due to curing at high temperature or to the excessive heat of hydration developed in massive structures, see e.g. [2, 3]. In the second case, the sulfate is present in the environment and diffuses within the material through the porous microstructure; this happens e.g. in foundations, galleries, stores of radioactive waste in contact with sulfate-rich soils, [4, 5]. In both cases, the reaction between the sulfate and hydrated products of the cement leads to the formation of gypsum and of secondary ettringite, [6-8]. The product formed in the hardened paste exerts an internal pressure resulting in the appearance of micro-cracks and material degradation. The kinetics of the reactions and, consequently, the severity of the damage depends on environmental factors (species and concentration of sulfate, pH of the solution, humidity, temperature) and intrinsic material properties (chemical composition of the cement paste, in particular aluminates content, pore distribution, diffusivity properties). The numerical description of these phenomena requires a proper diffusion-reaction model, for the computation of the amount of reaction expansive products and a mechanical model for the prediction of swelling and material damage. In this work we use the coupled model proposed in [7] and further developed in [9] which allows to compute the sulfate molar U