Francesco Pesavento

Modelling of hygro-thermal behaviour of concrete at high temperature with thermo-chemical and mechanical material degradation

Abstract A mathematical model for analysis of hygro-thermal behaviour of concrete as a multi-phase porous material at high temperatures, accounting for material deterioration, is presented. Full development of the model equations, starting from the macroscopic balances of mass, energy and linear momentum of single constituents is presented. Constitutive relationships for concrete at high temperature, including those concerning material damage, are discussed. The classical isotropic non-local damage theory is modified to take into account the mechanical- and the thermo-chemical concrete damage at high temperature. The final form of the governing equations, their discretised FE form, and their numerical solution are presented. The results of two numerical examples, concerning fire performance of 1-D and 2-D HPC structures, are discussed.

Modeling of Water Migration during Internal Curing with Superabsorbent Polymers

The mobility of water in hardening cement paste is an important aspect in view of the effectiveness of internal curing. A mechanistic-type numerical model of cementitious materials is applied for the analysis of water migration kinetics from internal curing agents [superabsorbent polymers (SAP)] into hydrating cement pastes with a low water-to-cement ratio. It is shown that the release of curing water at early age (i.e., during approximately the first day of hydration) allows for a uniform and practically instantaneous distribution of water within the whole volume of cured paste, even if the distances for water migration are as high as 2–3 mm. The evolution of permeability, as a result of the hydration process, is shown to have a major impact on the mobility of water in the cement paste. The depercolation of capillary porosity may substantially inhibit the water transport. The analysis shows that a part of the water first received by the paste in the proximity of the SAP can be later redistributed to a large volume of hardening paste, even after the permeability has become very low.

Modeling Hygro-thermal Performance and Strains of Cementitious Building Materials Maturing in Variable Conditions

A novel model of hygro-thermal performance of cement-based building materials during their maturing, considering evolution of their strength properties and deformations (shrinkage and creep strains), described in terms of effective stress is briefly presented. Creep is described by means of the modified microprestress — solidification theory by Bazant et al., with some modifications to take into account the effects of temperature and relative humidity on the cement hydration. Shrinkage strains are modeled by using effective stresses in the form introduced by Gray and Schrefler, giving a good agreement with experimental data also for low values of relative humidity. Results of three numerical examples based on the real experimental tests are solved to validate the model. They demonstrate its possibilities to analyze both autogenous deformations in maturing cementitious materials, and creep and shrinkage phenomena, in building elements of different age, sealed or drying at various conditions.

Finite element analysis of strain localization in multiphase materials

ABSTRACT Finite element analysis of strain localization in multiphase materials is presented. The multiphase material is modelled as a deforming porous continuum where heat, water and gas flow are taken into account. The independent variables are the solid displacements, the capillary and the gas pressure and the temperature. The modified effective stress state is limited by the Drucker-Prager yield surface. Small strains and quasi-static loading conditions are assumed. Numerical results of strain localization in globally undrained samples of dense sand are presented. A biaxial compression test is simulated assuming plane strain condition during the computations. Vapour pressure below the saturation water pressure (cavitation) develops at localization in case of dense sands, as experimentally observed.

Concrete Exposed to Fire: From Fire Scenario to Structural Response

In this paper a model for the analysis of concrete structures exposed to fire, based on Porous Media Mechanics, is coupled with a computational fluid dynamics model. To show the capability of this strategy the numerical simulation of a simple concrete slab exposed to fire is presented. The thermal loads as well as the moisture exchange between the structure surface and the environment are calculated by means of computational fluid dynamics program. Thanks to this strategy the structural verification is no longer based on the standard fire curves commonly used in the engineering practice, but it is directly related to a realistic fire scenario. With the simple example proposed, it is possible to highlight how the localized thermal load generates a non-uniform pressure rise in the material, which results in an increase of the structure stress state and of the spalling risk. Spalling is likely the most dangerous collapse mechanism for a concrete structure. Numerical results of various sections of the slab exposed to fire are presented, showing the effects of a more realistic distribution of the thermal loads with respect to the ones obtained by using the standard fire curves. This coupling approach still represents a “one way” strategy, i.e. realized without considering explicitly the exchange of boundary conditions from the structure to the fluid. This results in an approximation, but from physical point of view the current form of the solid-fluid coupling is considered sufficiently accurate in this first phase of the research.

Strain localisation simulation in non-isothermal multiphase geomaterials

A coupled finite element formulation for the hydro-thermo-mechanical behaviour of a water saturated and partially saturated porous material has been presented. This model is obtained as a result of a research in progress on the thermo-hydro-mechanical modelling for multiphase geomaterials undergoing inelastic strains. Numerical results of strain localisation in globally undrained samples of dense and medium dense sands have been presented. Vapour pressure below the saturation water pressure (i.e. water cavitation) develops at localisation in case of dense sands, as experimentally observed. A case of strain localisation induced by a thermal load in a sample where the displacements are constrained and evaporation takes place is also analysed.

Mechanics of Ageing—From Building to Biological Materials

In this work, we present a general model for the analysis of concrete and biological tissues as multiphase porous materials, with particular regard to their ageing. Such problems are typically multiphysics ones with overlapping domains where diffusion, advection, adsorption, phase changes, deformation, chemical reactions and other phenomena take place in the porous medium. For the analysis of such a complex system, the model here proposed is obtained from the microscopic scale by applying the Thermodynamically Constrained Averaging Theory (TCAT) which guarantees the satisfaction of the second law of thermodynamics for all constituents both at micro- and macrolevels. Moreover, one can obtain some important thermodynamic restrictions imposed on the evolution equations describing the material deterioration. Two specific forms of the general model adapted to the cases of cementitious and biological materials respectively are shown. Some numerical simulations, aimed at proving the validity of the approach adopted, are also presented and discussed.

Hygrothermal-chemical couplings in degradation of cementitious materials

A general approach to modeling chemical degradation processes in cementitious materials, due to combined action of variable hygral, thermal, chemical and mechanical loads, is presented. Mechanics of multiphase porous media and damage mechanics are applied for this purpose, and kinetics of chemical processes is described with evolution equations based on thermodynamics of chemical reactions. The mass-, energy- and momentum balances, as well as the evolution equations, constitutive relationships are briefly summarized. The mutual couplings between the hygral, thermal, chemical and mechanical processes are considered in the equations. Two examples of the model application for analyzing chemo-hygro-thermomechanical processes in cement based materials are presented and discussed. The first one deals with the salt crystallization during drying of a concrete wall, and the second one concerns calcium leaching from a concrete wall due to chemical attack of pure water in two different thermal conditions: with- and without temperature gradient.

Modelling Chemical Processes in Cement Based Materials by Means of Multiphase Porous Media Mechanics

A general approach to modelling chemical degradation processes in cement based materials, due to combined action of hygro-thermal, chemical and mechanical loads, is presented. Mechanics of multiphase porous media and damage mechanics are applied for this purpose. The mass-, energy- and momentum balance equations, and constitutive and physical relations are briefly presented, and then numerically solved with the finite element method. Several examples of the model application for analysing ions transport and degradation processes of concrete due to chemical attack of pure water, salt crystallisation and alkali-silica reaction are presented and discussed.© 2012 ASME

Multiscale/Multiphysics Model for Concrete

In this paper a general model for the analysis of concrete as multiphase porous material, obtained from microscopic scale by applying the so-called Hybrid Mixture Theory, is presented. The final formulation of the governing equations at macro-level is obtained by upscaling their local form from the micro-scale. This procedure allows for taking into account both bulk phases and interfaces of the multiphase system, to define several quantities used in the model and to obtain some thermodynamic restrictions imposed on the evolution equations describing the material deterioration. Two specific forms of the general model adapted to the case of concrete structures under fire and to the case of concrete degradation due to the leaching process are shown. Some numerical simulations aimed at proving the validity of the approach adopted, are also presented and discussed.