Thomas Rougelot

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.

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.

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.