Farid Benboudjema

Effects of early-age thermal behaviour on damage risks in massive concrete structures

At early age, temperature in massive concrete structures may reach over 70°C because hydration is an exothermic chemical reaction. Temperature evolution (increasing followed by decreasing temperature) can lead to damage risks in the short and long term. Firstly, cracking due to self-restrained strains (essentially thermal strains) can occur (which increase the transport properties and so the kinetics of degradation); and secondly, delayed ettringite formation can appear. In addition, if autogenous and thermal strains are restrained, compressive stresses and then tensile stresses increase, which can cause crossing cracks. However, this paper will not deal with these phenomena. In the first part, sensitivity to delayed ettringite formation and early age cracks by self-restrained strains are studied with regards to the environmental conditions by a numerical approach. This part shows that the external temperature has a significant impact on the maximal temperature reached, but that the temperature difference between the core and the surface is mainly impacted by the wind velocity. Then, a parametric study on the effect of the variation of thermal properties at early age has been achieved and shows that it needs to be taken into account. Finally, visco-elastic mechanical calculations show the impact of thermal property variation on the stresses generated by self restraint.

Creep Consideration Effect on Meso-Scale Modeling of Concrete Hydration Process and Consequences on the Mechanical Behavior

At an early age, hydration of cement leads to a reduction in volume (caused by Le Chatelier contraction) that induces autogenous shrinkage. In addition, hydration is an exothermic reaction, and an increase in temperature occurs (followed by a decrease). Because autogenous shrinkage arises only in cement paste, and because the coefficient of thermal expansion may be different between cement paste and aggregates, strain incompatibilities lead to an internal self-equilibrated state of stress. Depending especially on the concrete mix, initial cracking may occur at the cement-paste scale leading to a modification of the global concrete behavior. In this paper, finite-element simulations using a mesoscopic mesh are performed to access to the internal stresses and damage state resulting from the hydration process. Calculations take into account the autogenous shrinkage of the cement paste and the differential thermal behavior between cement paste and aggregates. The influences of basic creep strains, thermal boundary conditions, and concrete mix (by a simplified approach) are studied, and it is shown that for ordinary concrete, hydration leads to a slight reduction in elastic stiffness and tensile strength when creep is taken into account. However, for high-performance concrete, a significant reduction in the elastic stiffness and tensile strength is expected (with respect to the potential tensile strength) even if creep is taken into account.

Study of electrical resistivity: variability assessment on two concretes: protocol study in laboratory and assessment on site

This article presents electrical resistivity measurement on concrete. The first part presents the study of a lighter protocol for measuring electrical resistivity, allowing investigations of a large number of specimens, without degradation of the quality of final results. The second part is based on measurements made on two concretes cast regularly during one year each, on two sites. Measurements done to assess the different variability sources distinguish variability linked to the measurement of those related to the material. The third part of the article compares the laboratory measurements to those made on site. Finally, all the results reflected the interests of the resistivity measurement because of low variability obtained, and consistency between results. Les travaux présentés portent sur la mesure de résistivité électrique sur les bétons. Une première partie présente l’étude d’un protocole de mesure électrique allégé, qui a permis de traiter un grand nombre d’éprouvette sans que la qualité finale des résultats n’en soit altérée. La seconde partie repose sur les mesures faites sur deux bétons coulés régulièrement sur deux chantiers durant une année. Les mesures réalisées permettent d’évaluer les différentes sources de variabilité des résistivités en distinguant les variabilités inhérentes à la mesure de celles liées au matériau. Une troisième partie de l’article confronte les mesures en laboratoires à celles réalisées sur site. L’ensemble des résultats traduit finalement l’intérêt de la mesure de résistivité du fait des faibles niveaux de variabilités obtenus, et de leur cohérence entre les différents résultats.

Modelling the influence of temperature on accelerated leaching in ammonium nitrate

This study proposes two models for accelerated leaching in ammonium nitrate under variable temperature. The first approach assumes a thermoactivated linear relationship between degradation and the square root of time. The second approach introduces, within a phenomenological model, the thermoactivation of diffusion and hydrates solubility. These two approaches enable one to analyse accelerated leaching tests to identify the concrete variability in order to get rid of the influence of temperature. Cette étude consiste à proposer deux modélisations de la lixiviation accélérée au nitrate d’ammonium sous température variable. La première approche suppose une relation linéaire thermoactivée entre la dégradation et la racine carrée du temps. La seconde approche introduit dans un modèle phénoménologique la thermoactivation de la diffusion et de la solubilité rétrograde des hydrates. Ces deux approches permettent de dépouiller des essais de type DANA afin d’identifier la variabilité du béton en s’affranchissant de l’influence de la température.

Influence of the time of demoulding on adhesion of cement-based mortar modified with cellulose ether

Industrial mortars have complex mix design with polymers such as cellulose ether. The present paper deals with the influence of drying on the adhesion of a mortar on a concrete substrate. The main properties of the studied mortar (drying shrinkage, mechanical properties and microstructure) were investigated. The system “mortar/substrate” was studied by means of pull-off tests and the cracking of the interface due to restrained shrinkage was monitored by digital image correlation. While the time of demoulding has a beneficial effect on cement hydration and mechanical properties, it appears that the drying shrinkage is increased when this time is longer. This seems in contradiction with results found in the literature on basic cement-based materials. In the case of a sandblasted substrate, this results in a cracking of the interface between mortar and concrete and a lower adhesion. The influence of time of demoulding on free shrinkage is discussed.

Influence of the spatial variability of leaching kinetics parameters on the lifespan of a concrete structure

This works aims at studying, through the Monte Carlo method, the influence of the leaching kinetics parameters spatial variability on the lifespan of a concrete structure. The considered structure is a tunnel for nuclear waste storage. It is observed that the expected value for the lifespan estimated when considering the material spatial variability is significantly lower than the lifespan estimated with a single simulation considering uniform parameter fields, equal to the expected value for each parameter. Le but de cette étude est d’évaluer, par la mise en œ uvre de la méthode de Monte Carlo, l’influence de la variabilité spatiale des paramètres les plus influents par rapport à la cinétique de dégradation en lixiviation sur la vitesse de propagation dans une structure en béton du front de dissolution de la portlandite. La structure considérée est un tunnel de stockage de déchets radioactifs. Nous observons que l’espérance de la durée de vie estimée en tenant compte de la variabilité spatiale des propriétés du béton est sensiblement inférieure à la durée de vie estimée en considérant des champs uniformes, égaux à l’espérance des paramètres considérés.

Analysis of cracking due to shrinkage restraint on the mechanical behaviour of reinforced concrete

Shrinkage may lead to tensile stresses and cracking in reinforced concrete structures due to several mechanisms: gradients of drying shrinkage, restraint of autogeneous and drying shrinkage by reinforcement or concrete members of different thickness or ages etc. This contribution focuses on the effect of drying shrinkage on the behavior of concrete tie members, with a focus on the restraint by reinforcement. It is showed that, if it is not taken into account, numerical simulations show an overestimation of cracking and force at first crack when submitted to uniaxial tension when compared to experimental results. Besides, an appropriate interface model should be also introduced in order to be relevant.

Study of Shrinkage Restraint Effects at Early-Age in Alkali-Activated Slag Mortars

Alkali-activated materials are being increasingly studied nowadays as hydraulic binders. In order to be enrolled in different civil engineering applications, several properties must be characterized. This study focuses on their cracking risk by shrinkage restraints. The current paper summarizes the experimental and numerical results of a project assessing the development of early age properties of an alkali-activated slag mortar. First, an experimental campaign was held in order to quantify hydration heat release and hydration kinetics, to determine the time evolution of the Young modulus and the tensile strength and to measure free shrinkage strains, all in autogenous conditions. Second, two modelling approaches for mechanical properties and shrinkage development were compared. The results showed that the classical approach to characterize the hydration kinetics based on semi-adiabatic calorimetry results isn’t suitable for the studied binder because of slow hydration and low heat release. Numerical work was finally conducted in order to predict stress development of a massive structure of alkali-activated slag mortar subjected to internal (self) and external strains’ restraints (at mesoscopic and macroscopic scales).

Coupling Between Leaching and Mechanical Behaviour of Concrete

In the case of a radioactive waste disposal, concrete containment structures must be studied over extended periods during which it is necessary to account for a possible degradation by calcium leaching due to on-site water. This phenomenon affects the microstructure of concrete and then is coupled with the mechanical behaviour of concrete. The effect of leaching on the static behaviour and then the possible influence of cracks on leaching will be considered. But there is also a possible coupling due to tertiary creep. In this case, failure occurs eventually. And finally, a probabilistic approach is used with the leaching and tertiary creep models to evaluate the lifetime of a concrete structure subjected to chemical and mechanical loading.

Modeling Concrete at Early Age Using Percolation

The prediction of early age behavior of cementitious materials is of particular importance when it comes to the prediction of the crack occurring risks. Amongst the most important parameters that define the hydrating material are its elastic properties and the changes in volume that arise due to the very reaction of hydration. On a discrete generated microstructure, a percolation – type approach is applied. A forest fire algorithm allows taking into account the binding role played by the hydrates, and it reveals a threshold of hydration below which the rigidity of the concrete is negligible. The evolution of elastic characteristics is obtained by using a homogenization method applied to the percolated microstructure. Autogenous shrinkage is assumed to be due to the rise of a capillary pressure, the latter itself being a consequence of the hydration reaction. The capillary pressure is obtained from a model for desorption isotherm and is applied to the deformable skeleton corresponding to the percolated microstructure. Using this approach and Biot’s theory, it is possible to compute the autogenous shrinkage and its evolution around the threshold of percolation.