Nicolas Burlion

Induced anisotropic permeability due to drying of concrete

Structural strength, porous space, and permeability of concrete are strongly affected by mechanical, hydrous, and thermal loading. These various loadings may lead to drying shrinkage, one of the main characteristics of this type of material, which has to be involved in the behaviour modelling and experimental investigations being the subjects of this paper. Experimental devices and principal parameters studied are first presented. Drying shrinkage and loss of mass in time were measured on prismatic samples while uniaxial compression tests were performed on cylindrical samples. Gas permeability tests, carried out on a concrete cylinder 30 mm in diameter, form the second part of this study. The samples used for these measurements were cored from each prismatic sample at the end of 10 months or 2 years of drying, either from the transverse direction of sample (privileged direction of drying) or from the longitudinal direction. Gas permeability procedure, using micropulse test technique, is described as well as the experimental process. Experimental results are finally commented on and discussed with a view on induced anisotropy due to desiccation. Such an anisotropy is clearly observable in permeability, which is also increasing with drying time.

Mise en oeuvre d'une modélisation élasto-plastique endommageable du béton : Calculs numériques dans le cadre d'un benchmark EDF

ABSTRACT An elastoplastic and damageable model for concrete is presented. This model, based on concepts of isotropic damage developed by Mazars, and on non-associated plasticity, was developed in the laboratory of Mechanics of Lille, implemented in the MPPSAT finite elements code of the laboratory, and used for the various benchmark tests proposed by EDF. The basic concepts of the model are developed, respectively for the plastic part and for the damage one. Identification of all the model parameters is detailed. Criticism of numerical results is made and shows the importance of well modelling tension softening for concrete material.

Porosity of concrete: influence of test conditions and material variability

The prediction of the service lifetime of civil engineering structures needs an appropriate comprehension and quantification of the physical, chemical and mechanical processes associated with the durability of structural reinforced concrete. In particular, the porosity is an essential property involved in degradation of concretes. Also, the development of a probabilistic approach could enhance estimations of service lifetime. Experimental tests are therefore performed in the framework of project APPLET and aim at quantifying variabilities linked to concrete itself and its setting in a real structure. A spatial variability and the effect of the drying protocol for the measurement of porosity have been highlighted. La prédiction de la durée de vie d’un ouvrage de génie civil requiert une compréhension et une quantification appropriée des processus et grandeurs physiques, chimiques et mécaniques associés à la durabilité des bétons armés de structure. En particulier, la porosité ouverte est un élément important dans les processus de dégradation des bétons. D’autre part, le développement d’une approche probabiliste permettrait d’affiner les estimations de durée de service. A cet effet, des essais réalisés dans le cadre du projet APPLET visent à déterminer les variabilités susceptibles d’être induites par le protocole expérimental, mais aussi les variabilités liées au béton et à sa mise en œuvre dans un ouvrage réel. Une analyse de ces variabilités est conduite, et met en évidence une variabilité spatiale du matériau ainsi que le rôle majeur des protocoles de séchage retenus pour la détermination de la porosité.

Comportement différé des mortiers soumis au séchage et effet induit sur la perméabilité

ABSTRACT This study aims to give information, on the one hand, about the evolution of drying shrinkage according to mortar composition, and on the other hand, about the effects of hygro-mechanical microcracking on permeability of cementitious materials. It also deals with the influence of the interstitial fluid nature on the permeability. The results obtained demonstrate that the drying shrinkage of the two studied mortars versus weight loss shows three characteristic phases shown in literature. They also put in light, through the study carried out on the mortar with W/C=0.5, that the permeability of dried samples increases after creep while that in drying creep remains constant, compared to the dried samples without creep.

Experimental Study of the Water Desorption and Drying Shrinkage of Cement-Based Materials with Thin Slices

The aim of the present study is to experimentally investigate the water vapor desorption and drying shrinkage of cement-based materials during a step-by-step desorption process, with the objective to link drying shrinkage to water desorption. Water vapor desorption isotherms of two cement pastes and two mortars (with water-to-cement ratio of 0.5 and 0.8) were studied using thin slices with three thicknesses 1 mm, 2 mm and 3 mm. Drying shrinkage was measured with 2 mm slices for each material. The slice thickness has an impact on the relative mass variation, but the difference between 2 mm slices and 3 mm slices is very small or even negligible. The results obtained in the present study are compared with a former study on larger prismatic beams. The curing condition influences the water desorption isotherm: the water content in the present study is always higher at each relative humidity after a longer curing period with thin slices. The size effect is pronounced for drying shrinkage: the drying shrinkage obtained with slices is always bigger than larger prismatic beams. The correlation between drying shrinkage and water mass loss shows two almost linear parts with different slopes for both cement pastes and exhibits three characteristic phases for both mortars.

Effects of inclusion stiffness on the cracking of cement-based composites under drying: a numerical study

Abstract This paper focuses on the evolution of shrinkage cracking in a geometrically simplified concrete, i.e. a composite made of cement matrix including the glass spheres. In the first part, the laboratory samples at different levels of drying are examined experimentally. In order to identify the occurrence of cracking as well as its evolution in a direct and visual way, a non-destructive method called X-ray microtomography is used. The distribution of cracks is clearly identified, including their location and shape by the observation of cross section of samples. Based on the experimental investigations, a special coupled elastoplastic damage model is proposed to describe the mechanical behaviour of the composite. The obtained results satisfactorily reproduced the stress distribution and evolution in studied composite under drying and gave a better understanding of mechanical mechanism of shrinkage cracking. In order to study the influence of matrix behaviour and inclusion stiffness, a series of parametric studies are also performed by using a hydromechanical coupling code, incorporating the proposed constitutive model. One notices that the crack distribution in the sample depends strongly on the stiffness of inclusion and the mechanical behaviour of cement matrix.

First cycle of desorption and sorption isotherms of carbonated and non-carbonated mortars and concretes using accelerated protocol

Abstract When a cementitious material is in contact with ambient air, CO2 enters the material by the porous network and through the cracks, reacts with the CSH hydrates and Ca (OH)2. These carbonation reactions depend on saturation degree of the material, it does not occur in a fully saturated or totally dry material. Thus, knowledge of isotherms, both for water sorption and desorption, is of interest in modelling the long-term behaviour of these materials. The objective of this paper is to obtain desorption and adsorption isotherms in a short time, and to study the influence of carbonation on the isotherms using an accelerated experimental protocol using thin disc materials of 37.5 mm of diameter and 2 mm thick. The results show that isotherms can be obtained in a significantly reduced time, on the basis of near equilibrium state for each relative humidity levels. The analysis of the distribution of pore radii obtained from the isotherms shows that carbonation causes an increase of the volume of capillary pores while dramatically decreasing the connected porosity of hydrates.

Extraction and analysis of pore solution of cement paste: desaturation and resaturation cycles using a non-destructive method by gas injection

Abstract The aim of the present study is to extract the pore solution from cement pastes and mortars with high W/C ratio by a non-destructive method. The sample is placed in a hydrostatic cell under a confining pressure and an inert gas under high pressure is applied at one end to force a movement of interstitial water. As a result of the non-destructive nature of this method, successive extractions were carried out on cement pastes with W/C ratio equal to 0.7: the first one is directly performed on three initially saturated samples and the second one after drainage and resaturation by two chemically different solutions to observe the exchanges between the saturating solution and cementitious matrix. The chemical analysis results of K+, Na+, SO42−, Mg2+, Ca2+, Li+ and Sr2+ show that the volume of extracted water is representative of concentrations of the various ionic species present in a pore solution of cement pastes.

Numerical Simulation of Sorption-Desorption Isotherme for Cement-Based Materials

One of the most important issues in civil engineering about structures is their durability. A structure during his lifetime faces different environmental situations, like different relative humidities, and different cycles of drying and saturation will be imposed on it, so it is very critical to characterize materials behavior against these changes. Some informations are needed to predict it, one of these informations is desorption and sorption isotherm. In cement based materials just as cement paste, there remain generally extremely fine pores, consequently obtaining isotherms experimentally is considered as a substantially time-consuming experience. Therefore, it is of interest to model the isotherm from a numerical point of view. This physical phenomenon has been a field of study of intense researches, however, there are not many models relying on true physical effects which take place during drying and saturation. In the present work, in order to model a porous media, a numerical morphology of the geometry of the porous media has been presented by means of a random field excursion through the selection of a random field and a threshold. Random field parameters are defined based on experimental values. Once the porous media has been reproduced, the morphological analysis must be done in order to model water transfers.

Why is it necessary to use a damage model to simulate the mechanical behavior of concrete under drying and leaching

ABSTRACT Short and long-term behaviors of concrete structures are of importance when dealing with durability aspects. One particular interesting aspect is the change in microstructure of concrete submitted to drying or leaching processes, and a potentially induced cracking pattern. Experimental investigations which focus on the effect of the dimensions of aggregates over cracks are proposed in this paper. The samples are submitted to accelerated leaching or to drying, and the evolution of their microstructure is regularly recorded by means of X-ray microtomography. A method is then proposed to study the cracking pattern, which appears to be greatly dependant of the size of inclusions. In particular, it appears that a larger diameter of inclusions tends to increase the crack opening.