C.E. Majorana

A coupled finite element model for the consolidation of nonisothermal elastoplastic porous media

A coupled finite element model for the analysis of the deformation of elastoplastic porous media due to fluid and heat flow is presented. A displacement-pressure temperature formulation is used for this purpose. This formulation results in an unsymmetric coefficient matrix, even in the case of associated plasticity. A partitioned solution procedure is applied to restore the symmetry of the coefficient matrix. The partitioning procedure is an algebraic one which is carried out after integration in the time domain. For this integration, a two-point recurrence scheme is used. The finite element model is applied to the investigation of nonisothermal consolidation in various situations.

Thermo‐hydro‐mechanical modelling of high performance concrete at high temperatures

This paper presents the physical, mathematical and numerical models forming the main structure of the numerical analysis of the thermal, hydral and mechanical behaviour of normal, high‐performance concrete (HPC) and ultra‐high performance concrete (UHPC) structures subjected to heating. A fully coupled non‐linear formulation is designed to predict the behaviour, and potential for spalling, of heated concrete structures for fire and nuclear reactor applications. The physical model is described in more detail, with emphasis being placed upon the real processes occurring in concrete during heating based on tests carried out in several major laboratories around Europe as part of the wider high temperature concrete (HITECO) research programme. A number of experimental and modelling advances are presented in this paper. The stress‐strain behaviour of concrete in direct tension, determined experimentally, is input into the model. The hitherto unknown micro‐structural, hydral and mechanical behaviour of HPC/UHPC were determined experimentally and the information is also built into the model. Two examples of computer simulations concerning experimental validation of the model, i.e. temperature and gas pressure development in a radiatively heated HPC wall and hydro‐thermal and mechanical (damage) performance of a square HPC column during fire, are presented and discussed in the context of full scale fire tests done within the HITECO research programme.

An approach for modelling concrete spalling in finite strains

A new approach for modelling concrete spalling process is here proposed, taking into account a fully nonlinear-displacement/strain theory able to catch complex interactions between pressure, thermal and mechanical fields. The micro-structural modelling of concrete under fire conditions is derived from a mechanical and thermodynamic consistent theory and it is strictly related to a self-consistent, carefully extracted set of experimental data, in order to make a correct validation and calibration of the numerical F.E. procedures and codes. Even if appearing as a first but successful example, it is shown that a procedure accounting for coupled material and geometric nonlinearities is able to attain valuable and realistic numerical results concerning spalling process in concrete.

Thermal‐fluid flow within innovative heat storage concrete systems for solar power plants

Purpose – The purpose of this paper is to describe an experience of R&D in the field of new technologies for solar energy exploitation within the Italian context. Concentrated solar power systems operating in the field of medium temperatures are the main research objectives, directed towards the development of a new and low‐cost technology to concentrate the direct radiation and efficiently convert solar energy into high‐temperature heat.Design/methodology/approach – A multi‐tank sensible‐heat storage system is proposed for storing thermal energy, with a two‐tanks molten salt system. In the present paper, the typology of a below‐grade cone shape storage is taken up, in combination with nitrate molten salts at 565°C maximum temperature, using an innovative high‐performance concrete for structures absolving functions of containment and foundation.Findings – Concrete durability in terms of prolonged thermal loads is assessed. The interaction between the hot tank and the surrounding environment (ground) is co...

Three‐dimensional modelling of bond behaviour between concrete and FRP reinforcement

Purpose – The purpose of this paper is to investigate the bond behaviour between fiber reinforced polymer (FRP) sheets and concrete elements, starting from available experimental evidences, through a calibrated and upgraded 3D mathematical‐numerical model.Design/methodology/approach – The complex mechanism of debonding/peeling failure of FRP reinforcement is studied within the context of damage mechanics to appropriately catch transversal effects and developing a more realistic and comprehensive study of the delamination process. The FE ABAQUS© code has been supplemented with a numerical procedure accounting for Mazarss damage law inside the contact algorithm.Findings – It has been shown that such an approach is able to catch the delamination evolution during loading processes as well.Originality/value – A Drucker‐Prager constitutive law is adopted for concrete whereas FRP elements are assumed to behave in a linear‐elastic manner, possibly undertaking large strains/displacements. Surface‐to‐surface conta...

Hygrothermal and mechanical model of concrete at high temperature

A computational model allowing for the thermohygrometric and mechanical analysis of concrete structures at high temperature by means of the finite element method is presented. The model includes creep coupled with damage and related cross-effects due to hygrothermal behaviour. A comparison between experimental tests performed by Bažantet al. on concrete cylinders at high temperature and the numerical results obtained from the model presented are shown.RésuméOn présente un modèle pour l’analyse thermohygrométrique et mécanique des structures en béton à haute température au moyen de la méthode des éléments finis. Ce modèle inclut le fluage associé à l’endommagement et les effets reliés au comportement hygrothermique. On présente une comparaison entre des résultats des essais réalisés par Bažantet al. sur des cylindres en béton à haute température et les résultats numériques obtenus du modèle.

A general model for the mechanics of saturated-unsaturated porous materials

A general predictive model for the mechanical analysis of isothermal and non-isothermal saturated-unsaturated porous materials is presented. The model is developed along the lines of Biots theory and applies both for high water content and for low to medium water content in the pore space. Due to the similarity of the matrices in both situations, even if the transfer mechanisms are different, a single computer program can handle all of them. Examples belonging to both domains in the isothermal case as well as to heat and mass transfer in deforming porous media are shown.

Radiation damage evaluation on concrete within a facility for Selective Production of Exotic Species (SPES Project), Italy

Concrete is commonly used as a biological shield against nuclear radiation. As long as, in the design of nuclear facilities, its load carrying capacity is required together with its shielding properties, changes in the mechanical properties due to nuclear radiation are of particular significance and may have to be taken into account in such circumstances. The study presented here allows for reaching first evidences on the behavior of concrete when exposed to nuclear radiation in order to evaluate the consequent effect on the mechanical field, by means of a proper definition of the radiation damage, strictly connected with the strength properties of the building material. Experimental evidences on the decay of the mechanical modulus of concrete have allowed for implementing the required damage law within a 3D F.E. research code which accounts for the coupling among moisture, heat transfer and the mechanical field in concrete treated as a fully coupled porous medium. The development of the damage front in a concrete shielding wall is analyzed under neutron radiation and results within the wall thickness are reported for long-term radiation spans and several concrete mixtures in order to discuss the resulting shielding properties.

Mechanical and durability behaviour of growing concrete structures

Purpose – This paper seeks to analyse 3D growing concrete structures taking into account the phenomenon of body accretion, necessary for the simulation of the construction sequence, and carbon dioxide attack.Design/methodology/approach – A typical 3D segmental bridge made of precast concrete is studied through a fully coupled thermo‐hygro‐mechanical F.E. model. The durability of the bridge is evaluated and carbonation effects are considered. Creep, relaxation and shrinkage effects are included according to the theory developed in the 1970s by Bažant for concretes and geomaterials; the fluid phases are considered as a unique mixture which interacts with a solid phase. The porous material is modelled using n Maxwell elements in parallel (Maxwell‐chain model).Findings – First, calibration analyses are developed to check the VISCO3D model capabilities for predicting carbonation phenomena within concrete and the full 3D structure is modelled to further assess the durability of the bridge under severe condition...

Thermo‐hygro‐mechanical meso‐scale analysis of concrete as a viscoelastic‐damaged material

Purpose – Knowledge of the behavior of concrete at mesoscale level requires, as a fundamental aspect, to characterize aggregates and specifically, their thermal properties if fire hazards (e.g. spalling) are accounted for. The assessment of aggregates performance (and, correspondingly, concrete materials made of aggregates, cement paste and ITZ – interfacial transition zone) is crucial for defining a realistic structural response as well as damage scenarios.Design/methodology/approach – It is here assumed that concrete creep is associated to cement paste only and that creep obeys to the B3 model proposed by Bažant and Baweja since it shows good compatibility with experimental results and it is properly justified theoretically.Findings – First, the three‐dimensionality of the geometric description of concrete at the meso‐level can be appreciated; then, creep of cement paste and ITZ allows to incorporate in the model the complex reality of creep, which is not only a matter of fluid flow and pressure dissipa...