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A coupled thermomechanical mesoscale model for concrete under heating is presented. When considering the heterogeneous structure under coupled loads, complex macroscopic material properties can be modelled using simple constitutive relations. For instance, damage evolution is directly driven by the incompatibility of thermal strains between matrix and aggregates. Without prescribing
fc = f(T), a decline in compressive strength with rising temperatures will be shown.
Concrete is a complex material. Its properties evolve over time, especially at early age, and are dependent on environmental conditions, i.e. temperature and moisture conditions, as well as the composition of the material.
This leads to a variety of macroscopic phenomena such as hydration/solidification/hardening, creep and shrinkage, thermal strains, damage and inelastic deformations. Most of these phenomena are characterized by specific set of model assumptions and often an additive decomposition of strains into elastic, plastic, shrinkage and creep components is performed. Each of these phenomena are investigated separately and a number of respective independent models have been designed. The interactions are then accounted for by adding appropriate correction factors or additional models for the particular interaction. This paper discusses the importance of reconsider even in the experimental phase the model assumptions required to generalize the experimental data into models used in design codes. It is especially underlined that the complex macroscopic behaviour of concrete is strongly influenced by its multiscale and multiphyscis nature and two examples (shrinkage and fatigue) of interacting phenomena are discussed.
Compressive strength of concrete is highly temperature dependent. Using experimentally obtained relations as input for numerical simulations is problematic. A more accurate and reliable material model results from taking the coupling between thermal and mechanical behaviour into account. In this contribution, a coupled finite element solution on mesoscale geomtries is shown to exhibit a loss in compressive strength at higher temperatures. This is purely a result of the incompatible expansion of mortar matrix and aggregates, with no explicit temperature dependency of the employed constitutive models.
Porous media flow is an important aspect of geomechanics and material behaviour of concrete under heating and drying. We will present a model for multiphase nonisothermal flow and its solution using the finite element method. In contrast to single phase models (e.g.), which have to consider vaporization or condensation as a temperate dependency on the thermal capacity, the phase changes will be explicitly considered. More complex models have been proposed (e.g.), which also capture the multiphase nature of the flow field. These models, however, also include the coupling to a mechanics field, with many of the constitutive relations of the flow field dependent on the specific damage formulation employed. The resulting complexity of the constitutive models leads to a high number of experiments and difficult calibration procedures to determine their parameters.
The simulation of the moisture distribution under fast and slow heating is presented. The constitutive relations employed here do not assume any particular damage-mechanical model and will therefore allow free choice when mechanical coupling is desired. The resulting pore pressure field is an important prerequisite for the modeling of concrete spalling. Suitable numerical methods to achieve optimal convergence will be discussed.
By using direct coupling between the parts of a multiphysics problem, the models for each part can be simplified, making calibration and sensitivity calculations easier. This is also important for more complicated problems, where the need for simpler material laws that handle coupled phenomena becomes more evident. In particular, this contribution is an essential step towards the modeling of concrete spalling.
Spalling of concrete structures is a serious issue for their safety. A better understanding of the pore water distribution and state during a fire is a prerequisite for numerical approaches to such problems.
Temperature-driven water transport in concrete consists of multiple phenomena, such as convection, diffusion, adsorption and dehydration. Distinguishing the different influences experimentally is difficult because typically they cannot be disentangled.
A common experimental setup approximates a one-dimensional flow, and places temperature and pressure gauges along the propagation direction. For direct information about the water content inside a sample, methods such as NMR or neuron radiography are necessary.
A multiphase model for the flow in porous media is presented, with dehydration and changes in the pore size distribution taken into consideration. NMR measurements for temperature-driven flow have been performed. The numerical and experimental results are compared for water transport at temperatures below the critical point. Since both the finite-element model and the experiment allow the distinction between adsorbed, capillary and bulk water, a more fine-grained view of the pore water state is obtained.
A three-phase transport model for high-temperature concrete simulations validated with X-ray CT data
(2021)
Concrete exposure to high temperatures induces thermo-hygral phenomena, causing water phase changes, buildup of pore pressure and vulnerability to spalling. In order to predict these phenomena under various conditions, a three-phase transport model is proposed. The model is validated on X-ray CT data up to 320 ◦C, showing good agreement of the temperature profiles and moisture changes. A dehydration description, traditionally derived from thermogravimetric analysis, was replaced by a formulation based on data from neutron radiography. In addition, treating porosity and dehydration evolution as independent processes, previous approaches do not fulfil the solid mass balance. As a consequence, a new formulation is proposed that introduces the porosity as an independent variable, ensuring the latter condition.