Jean-Michel Torrenti

Modelling of leaching in pure cement paste and mortar

The leaching of cement-based materials is analysed through experimental and numerical results. From the experimental point of view, the leaching processes of a pure cement paste and a mortar are characterised by the degraded depths and the cumulative amount of leached calcium at different times. From the mathematical point of view, the leaching is modelled with the mass balance equation of calcium in the liquid phase. Material properties useful for a numerical resolution of this equation are presented and the finite volume method is applied for the numerical simulations. Comparisons between experimental and predicted results show the validity of the simple model used to describe the leaching phenomenon and the ability of the finite volume method to support high nonlinearities.

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.

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.

Restrained shrinkage of massive reinforced concrete structures: results of the project CEOS.fr

Within the CEOS.fr national research project, several experiments on massive concrete structures were conducted to improve the knowledge on the cracking phenomenon. In this paper, experiments where deformations at early age are restrained are presented. Testing bodies are I-shaped and two largely dimensioned steel struts are placed laterally between the two transverse heads to prevent almost any shrinkage. Three testing bodies were realized: RG8, the reference one; RG9, with a reduced reinforcement and RG10, with an increased cover. A full set of measurement was used for auscultation of these beams during early age. Optical long base fibres gave information on the relative displacement of the central part of the beam. Local measurements of strains in concrete were given thanks to Vibrating Wire Extensometers. Gauges on rebars produced data of the strain on the first reinforcement layer, and the force in struts was monitored. With this, the force and stresses in concrete and rebars could be deduced. This huge amount of data allows verifying the phenomenology of the concrete. Various hypotheses were analysed to explain the strain measured and the corresponding forces in each component during specific period of early age. A first analysis of the cracking process shows that the cracks could appear for stresses below the tensile strength.

Développement d'un conteneur pour l'entreposage de déchets nucléaires: résistance au choc

ABSTRACT The development of a High Integrity Container (HIC) made of Reactive Powder Concrete (RPC) for interim storage of radioactive waste is presented, namely its impact performance. RPC was chosen for durability reasons, then the remaining integrity of the container after a drop test was assessed numerically. The constitutive model developed is suited to describe the behaviour and damage of concretes under high loading rate. RPC is modelled in the frame of degrading plasticity with viscous hardening, its parameters (related to the probable casting mode) have been identified with direct tensile tests at quasi-static to high loading rates. The computations carried out with varied parameters are described. The results agree well with results of 1/3 scaled prototypes. Thus the steps of development of a scale 1:1 prototype container are considered.

Basic creep of concrete-coupling between high stresses and elevated temperatures

The prestressed concrete confinement vessel is the third and last barrier in French Nuclear Power Plants (NPP). In case of a severe accident (loss of cooling agent of the reactor for instance), pressure and temperature will increase in the nuclear vessel (+0,5 MPa and 180 °C during 2 weeks). Due to elevated temperatures, the evolution of basic creep will be accelerated. High compressive stresses would appear and induce higher delayed strains and damage. The modelling of basic creep and its couplings with temperature is then very important for the safety of the structure (tightness of the concrete vessel). Here, we present a model considering the following elements: a coupling between creep and damage is introduced, kinetics of basic creep is affected by temperature by the means of an Arrhenius thermo-activation, damage due to the increase in temperature (under stress or not) is taken into account. The model is compared with the available experimental results. This work is a part of the MACENA project.

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.

Interest of the probabilistic approach for the equivalent durability concept

The construction sector is the largest industrial consumer of raw materials. Like all materials, concrete has an important role to play in sustainable development through its contribution to energy saving and durability of constructions. In this context, the performance-based approach to concrete durability appears to be one of the essential tools. It enables the use of new constituents and innovative concrete mixes, while improving the level of safety for the end user. The difficulty of the performance-related design method lies in choosing the threshold value. In the equivalent durability concept the main difficulty is to define the reference concrete. In this article we propose a methodology which can be used to take into account the variability of the concrete properties when a performance-based approach is applied. The proposed methodology allows the interest of using some statistical aspects to be discussed. Le secteur de la construction est le plus grand consommateur industriel de matières premières. Comme tout matériau, le béton a un rôle important à jouer dans le domaine du développement durable à travers sa contribution aux économies d’énergies et à la durabilité des constructions. Dans cette optique l’approche performantielle de la durabilité apparait comme étant un outil essentiel. Ce concept permet l’utilisation de nouveaux constituants et la formulation de bétons innovants tout en améliorant le niveau de sûreté pour les utilisateurs finaux. Dans le cas d’une approche performantielle basée sur le respect de seuils, toute la difficulté réside dans leur détermination. En ce qui concerne l’approche performantielle comparative, c’est la définition du béton de référence qui constitue le point essentiel. Dans cet article nous proposons une méthodologie permettant de prendre en compte la variabilité des propriétés de durabilité des bétons dans le cas d’une approche performantielle de la durabilité. La méthodologie proposée montre l’intérêt d’aller vers la prise en compte des aspects statistiques.

Cracking Risk and Regulations

This chapter is focused on the cracking risk at early ages. After general considerations about cracking, the cracking risk prediction is discussed. Two main ways to assess this risk are considered: through an evaluation of the tensile stresses and through an evaluation of the strains. Finally, the evaluation of crack opening at early ages and the reinforcement design in regulations are presented.