G. De Schutter

Finite element simulation of thermal cracking in massive hardening concrete elements using degree of hydration based material laws

In massive hardening concrete elements, early age thermal cracking due to the heat of hydration might occur. At Ghent University, a simulation procedure is developed, based on the degree of hydration as a fundamental parameter. This parameter is related with the microstructural development during cement hydration. Accurate finite element simulations are obtained for the problem of early age thermal cracking, applying a staggered analysis. The time dependent material behaviour is implemented by means of a Kelvin chain. The cracking behaviour is implemented using a smeared cracking approach with non-linear softening behaviour. The results of the model are verified experimentally.

Random particle model for concrete based on Delaunay triangulation

In this paper, an efficient simulation method for obtaining a random particle model for concrete is outlined. First, the ‘take-and-place method’ and its extension, the ‘directed searching process’, are discussed briefly and the shortcomings are indicated. A new method, the ‘divide-and-fill method’, appears to be more convenient, especially when only a small computer is used. In this simulation method the available space (two-dimensional) is divided in separate areas, using a Delaunay triangulation. These areas are filled with particles taking into account a given grading curve and gravel content. Comparison with physical concrete sections, obtained by means of image analysis, shows that the results of this method closely represent reality. The divide-and-fill method also yields a finite-element mesh in a quasi-automatic way.ResumeCette publication traite d’une méthode de simulation efficace pour l’obtention d’un modèle à particules aléatoires pour le béton. On donne d’abord un aperçu de la méthode ‘prenez-et-placez’ et de son extension, la méthode du ‘procédé de placement dirigé’.La méthode prenez-et-placez produit un modèle numérique à troi dimensions. On réalise la structure composite en plaçant les granulats dans un ordre de grosseur décroissante. Les principles qui sont à l’origine de cette méthode sont d’une simplicité remarquable. Néanmoins on n’obtient de bons résultats qu’à l’aide d’un ordinateur assez puissant, même en considérant le ‘procédé de placement dirigé’. Ce procédé, qui est une forme primaire d’intelligence artificielle, tient compte des zones occupées pour la sélection de la position des granulats.Une méthode nouvelle qui, de plus, permet l’utilisation d’un PC ordinaire, est la méthode ‘divisez-et-remplissez’. Cette méthode donne un modèle de simulation à deux dimensions et procède en deux étapes. D’abord, on divise la section en triangles de Delaunay. Ensuite, on remplit ces triangles de particules en tenant compte de la distribution granulométrique et de la teneur en gravier. La courbe granulométrique bidimensionnelle peut être calculée facilement à partir de la courbe tridimensionnelle. La triangulation de Delaunay peut être réalisée sur la base d’un algorithme ‘divisez-et-vainquez’, avec un procédé dit ‘bubble’ et contrôle ‘swap’. La méthode divisez-et-remplissez donne quasi automatiquement un maillage d’éléments finis. La géométrie des granulats naturels roulés peut être décrite par une loi morphologique, basée sur la méthode développée par Beddow et Meloy.

Advanced monitoring of cracked structures using video microscope and automated image analysis

Traditionally, the monitoring of cracks in buildings is performed by very simple techniques yielding only limited information. By means of a video microscope in combination with automated image analysis, an advanced and more precise analysis method of cracks in structures is obtained. The system was evaluated during laboratory tests on concrete beams and slabs. Furthermore, on site monitoring has been evaluated by measurements in a historical building in Ghent. The results were verified by means of traditional measuring techniques. For in situ monitoring of cracked structures, the combination of video microscope and automated image analysis seems to be a user-friendly and very accurate technique.

Multi-disciplinary characterization and monitoring of sandstone (Kandla Grey) under different external conditions

Nowadays there is an increase of imported natural building stones, often used as replacement of local, more traditional building stones. The durability of these traditional stones is generally well known; however, when new imported materials are used, it is essential to study their behaviour under the current and local climatological conditions to predict their weathering resistance. In addition to water exposure, these building materials have to be resistant to significant mechanical stress created by the imbibition of de-icing salt solutions, frequently used during winter in Western Europe, with temperature periodically changing from freeze to thaw conditions. Porous network modifications related to the materials’ chemical composition are very complex when different forces are acting on the stone itself. Therefore it is crucial to determine the internal structure of the building stone under changing external conditions with and without the presence of de-icing salts, to understand the influence of these additional salts. In this paper, particular attention was paid to the multi-characterization of compact Kandla Grey laminated sandstone, a building stone frequently imported from India to Belgium recently. Traditional as well as highly advanced research techniques were used for the characterization and monitoring of changes under different external conditions. This study demonstrates that the structural characteristics of the laminations have an effect on the frost resistance of the stone and its response to salt weathering. From the experiments carried out, it can be concluded that Kandla Grey can be sensitive to frost and salt weathering under the current climatic conditions in Western Europe.

Degree of hydration-based creep modeling of concrete with blended binders: from concept to real applications

The mechanical behavior of hardening concrete is to a large extent determined by the evolving microstructure as a result of the hydration process. For traditional binder systems, consisting of Portland cement or blast furnace slag cement, the degree of hydration is known to be a fundamental parameter in this respect, enabling a detailed study and accurate prediction of the early-age mechanical behavior, including basic creep. Nowadays, in view of improved sustainability of cementitious materials, binder systems tend to become more complex, consisting of a blend of different powders. As the hydration process and microstructure development are influenced by the inclusion of powders into the binder, the question is raised whether the degree of hydration concept is still applicable to concrete based on complex blended binder systems. In this paper, some experimental results are summarized and the application to real structures is illustrated. Basic creep of hardening concrete with complex blended binders can still be modeled following the degree of hydration concept.

Multicompound Model for the Hydration of Portland Cement–Fly Ash Binders

In this study, traditional reaction equations (based on the theory of Powers) are applied to the hydration reactions of portland cement and the pozzolanic reaction of fly ash separately. Moreover, portland clinker is considered as a mixture of four minerals, each with its own sensitivity to the presence of fly ash. The kinetics of the reactions of each clinker mineral have been analyzed by fitting generally known models such as the Avrami and Jander equations to isothermal heat measurements on pastes of cement, fly ash, and water. The proposed model therefore consists of different stages, in which nucleation, phase-boundary, and diffusion reactions become rate controlling. The kinetics of the pozzolanic reactions have been described with similar equations, implementing parameter values based on measured selective dissolution data. Fly ash may accelerate the reaction of a clinker mineral, while at the same time it can decelerate another mineral. Depending on the relative proportions of the clinker minerals, the method can explain the apparent contradiction found in literature related to the acceleration or deceleration effect of fly ash on the cement hydration.

Half-scale test: an important step to demonstrate the feasibility of the Belgian supercontainer concept for disposal of HLW

The paper presents results of a half-scale test performed by ESV EURIDICE, an Economic Interest Grouping between the Belgian Nuclear Research Centre (SCK.CEN) and the Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS). The primary objective of the test was to assess the feasibility of constructing the Supercontainer and to provide experimental data to validate modelling calculations obtained using the finite element program HEAT/MLS. The test focused on the early-age behaviour of the concrete matrix materials and the practical aspects of construction. Generally, the results obtained from the half-scale test confirm that it is feasible to construct the Supercontainer with currently available techniques. The results also validate scoping calculations obtained earlier with the finite element model. These finding contribute an important step to demonstrate the feasibility to construct the Supercontainer and to validate the Belgian Supercontainer concept proposed by ONDRAF/NIRAS for disposal of high level waste (HLW) in Belgium.

Toward absolute durability performance criteria: preliminary case study of a sea lock

In order to achieve durable concrete structures, concrete code provisions typically require a minimum cement content, a maximum water cement ratio, and a minimum strength class. These ‘deemed-to-satisfy’ rules are mainly based on long-term practical experience. However, as there is a clear trend to use new binder types, considering alternative cement replacing and environment-friendly powders, this prescriptive approach can be criticized. Classical definitions such as cement content and water cement ratio are open to heavy debate in case of the alternative binder systems. Furthermore, the relation between durability performance of concrete based on new binder systems and the prescriptive parameters (cement content, water cement ratio, strength) is not without criticism either. The equivalent concrete performance concept (ECPC) offers a first step to a more soundly based evaluation of durability requirements for new binder types. Nevertheless, ECPC also has its limitations, as it is based on comparative testing still considering a deemed-to-satisfy reference concrete. A more fundamental solution can be based on absolute durability performance for the concrete to be applied in a structure. This performance can be checked in laboratory conditions (potential performance) as well as on the completed structure (as-built performance). In this study, some preliminary research results are reported in view of defining absolute durability criteria for a sea lock to be constructed.

The Effect of Print Parameters on the (Micro)structure of 3D Printed Cementitious Materials

The extrusion-based 3D printing method is one of the main additive manufacturing techniques worldwide in construction industry. This method is capable to produce large scale components with complex geometries without the use of an expensive formwork. The main advantages of this technique are encountered by the fact that the end result is a layered structure. Within these elements, voids can form between the filaments and also the time gap between the different layers will be of great importance. These factors will not only affect the mechanical performance but will also have an influence on the durability of the components. In this research, a custom-made 3D printing apparatus was used to simulate the printing process. Layered specimens with 0, 10 and 60 min delay time (i.e. the time between printing of subsequent layers) have been printed with two different printing speeds (1.7 cm/s and 3 cm/s). Mechanical properties including compressive and inter-layer bonding strength have been measured and the effect on the pore size and pore size distribution was taken into account by performing Mercury Intrusion Porosimetry (MIP) tests. First results showed that the mechanical performance of high speed printed specimens is lower for every time gap due to a decreased surface roughness and the formation of bigger voids. The porosity of the elements shows an increasing trend when enlarging the time gap and a higher printing speed will create bigger voids and pores inside the printed material.

Experimental determination of chloride penetration in concrete with real cracks

It is well known that the presence of cracks in reinforced concrete structures in aggressive environments accelerates rebar corrosion. The influence of real cracks in concrete structures on the penetration of chlorides and the resulting service life is being investigated in this study. Investigations are carried out at the Magnel Laboratory for Concrete Research of Ghent University in Belgium within a bilateral agreement with Politehnica University of Timisoara, Romania. Non-steady state migration tests are realized according to NT BUILD 492 using an electrical field and real cracks in order to determine the chloride profile. Samples with different crack patterns obtained by drilling from a reinforced concrete slab exposed to a simulated accidental failure of the central support and subsequent vertical loading until collapse have been used in the study in order to provide a more realistic image of the geometry of the cracks. The crack widths are measured using the optical microscope. The chloride penetration depth is measured with a colorimetric method on each specimen and the nonsteady state diffusion coefficients are determined. For evaluating the parameters which have the most influence on chloride migration on the samples used in this experiment, a two-level factorial experiment is designed and carried out. The results obtained provide a better understanding of the diffusion process when dealing with concrete structures with real cracks. Kljucne reci