HOMOGENIZATION OF TRANSPORT PROCESSES AND HYDRATION PHENOMENA IN FRESH CONCRETE

The problem of hydration and transport processes in fresh concrete is strongly coupled and non- inear, and therefore, very difficult for a numerical modelling. Physically accurate results can be obtained using fine-scale simulations, which are, however, extremely time consuming. Therefore, there is an interest in developing new physically accurate and computationally effective models. In this paper, a new fully coupled two-scale (meso-macro) homogenization framework for modelling of simultaneous heat transfer, moisture flows, and hydration phenomena in fresh concrete is proposed. A modified mesoscalemodelisfirstintroduced. Inthismodel, concreteisassumedasacompositematerialwithtwo periodically distributed mesoscale components, cement paste and aggregates. A homogenized model is then derived by an upscaling method from the mesoscale model. The coefficients for the homogenized model are obtained from the solution of a periodic cell problem. For solving the periodic cell problem, two approaches are used – a standard finite element method and a simplified closed-form approximation taken from literature. The homogenization framework is then implemented in MATLAB environment and finally employed for illustrative numerical experiments, which verify that the homogenized model provides physically accurate results comparable with the results obtained by the mesoscale model. Moreover, it is verified that, using the homogenization framework with a closed-form approach to the periodic cell problem, significant computational cost savings can be achieved

Coupled heat transport and Darcian water flow in freezing soils

summary:The model of coupled heat transport and Darcian water flow in unsaturated soils and in conditions of freezing and thawing is analyzed. In this contribution, we present results concerning the existence of the numerical solution. Numerical scheme is based on semi-implicit discretization in time. This work illustrates its performance for a problem of freezing processes in vertical soil columns

Temperature analysis of lightweight aggregate concrete slab members at elevated temperatures for predicting fire resistance

This paper is focused on lightweight aggregate concrete with expanded clay and its issues according to fire resistance. The procedures and recommendations for the calculation of fire resistance of lightweight aggregate concrete structures are presented. These procedures are based on nowadays methods for normal-weight concrete structures. These methods are modified in order to cover specific parameters and properties of lightweight concrete structures

Assessment of structures repeatedly exposed to thermal loading and extinguishing water: a case study of a firefighting training facility

When designing a new structure or assessing an existing one, the risk of fire and its effect on the structure must be considered. Structures are usually assessed for fire resistance at the design stage – i.e. before the possible exposure to fire. If structure is exposed to fire during its service life, a post-fire assessment must be conducted in order to evaluate whether the structure is still safe and reliable for use. The post-fire assessment is conducted quite regularly; however, the assessment is usually conducted for structures exposed to fire only once. This paper presents an interesting and unique case study of a post-fire structural analysis of a firefighting training facility exposed to cyclic fire loading and the effect of extinguishing water. The main conclusion of the study is that though means of protection are recommended, the structure still has a sufficient load-bearing capacity and can continue being used as a firefighting training facility in the future. Aside from the specific conclusions for the investigated structure, this paper presents the best practices and methods for the post-fire assessment of structures exposed to repeated fire loading, and can thus be used as a guidance by other engineers and researchers interested in this topic

Anisotropy of magneto-optical spectra in ultrathin Fe/Au/Fe bilayers

Molecular beam epitaxy grown Au(5 nm)/Fe(1.1 nm)/Au(t)/Fe(1.1 nm)/Au(100 nm)/Fe(1 nm)/MgO(001) structures with t between 1 and 2.5 nm were the subject of optical and magneto-optical (MO) ellipsometry studies at photon energies between 1.5 and 5.4 eV. Both the optical ellipsometry and MO polar Kerr rotation spectra can be explained with a multilayer model based on the bulk optical and MO data for Fe and Au. However, the longitudinal MO Kerr effect spectra depart from the model prediction showing an additional well-resolved structure between 3 and 4 eV. This demonstrates a stronger effect of interfaces on the electronic properties at in-plane magnetization in ultrathin magnetic films. On a macroscopic level the effect can be accounted for by a reduced symmetry of the linear MO tensor