3D FEM meso-level analysis of sulphate attack in concrete: new results and developments using parallel HP computing.

When concrete is subject to an environment characterised by a high humidity index and rich in sulphate ions, a concrete degradation process may be initiated due to external sulphate attack (ESA). The sulphate penetrating into the concrete activates a series of chemical reactions that lead to the formation of secondary ettringite, which may cause non-uniform volumetric expansions that may in turn generate cracking and ultimately culminate in the disintegration of the sample. Because the cracking may in turn facilitate sulphate penetration, ESA may be considered a coupled chemical-mechanical problem. In this study, the numerical analysis of ESA is conducted using the Finite Element Method by considering the specimen at the meso level composed of larger aggregates embedded in a mortar matrix. Standard continuum finite elements are used to discretize the aggregates and the mortar. Zero thickness interface elements are inserted along all the aggregate-mortar and selected mortar-mortar contacts to represent potential cracks. The diffusion-reaction of sulphate ions (chemical problem) is formulated following Tixier and Mobasher (2003) and Idiart et al. (2011b). Regarding the mechanical problem, the continuum elements are considered linear elastic, while the interface elements behave according to an elasto-plastic law incorporating concepts of fracture mechanics which was initially developed by Carol et al (1997) and later extended for 3D analysis by Caballero et al. (2006). The first part of this thesis deals with the verification and use of DRAC5, a completely parallelized version of the in-house code developed within the materials mechanics group (MECMAT) of the Universitat Politècnica de Catalunya (UPC) which now incorporates MPI and PETSC libraries as well as HDF5 i/o files. This new version of the code, which is used to solve both the mechanical problem and the chemical problem through a staggered scheme, has allowed the analysis of new and more challenging 3D studies, producing realistic results that reflect the 'onion peeling' cracking pattern, similar to what has been observed in the laboratory and in previously studied 2D cases (Idiart, 2009). The second part of the thesis, deals with the development of new numerical solving techniques applicable to this type of mesh. In particular, a solution technique based on substructuring and the Schur complement is applied to the analysis of specimens comprising elements of the continuum exhibiting linear elastic behaviour and interface elements characterized by non-linear (elasto-plastic) behaviour. This new technique, which reduces substantially the number of degrees of freedom that need to be considered during the iterative process, has been preliminarily implemented in DRAC4, a simpler series version of the code, and is tested showing great advantages in terms of solution time for a range of 2D application examples. The development of a new formulation using rigid-plastic interfaces is also initiated. This formulation uses relative degrees of freedom at each pair of interface nodes, and leads to the resolution of a Linear Complementarity Problem. This development allows a further reduction in the degrees of freedom of the problem by only considering the nodes of the interface elements involved in the fracture process.