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International audience; Transient Thermal Deformations (TTD) of High Performance Concrete (HPC) having various temperature and compressive loading histories in sealed conditions are investigated experimentally. The aim is to provide accurate data to quantify the transient thermal strain and to specify the conditions required to give rise to the phenomenon. The results are commented and hypotheses proposed to advise practitioners dealing with concrete structures concerned by the conditions of environment and loading that lead to Transient Thermal Deformation.

International audience

WOS:000423245700005; International audience; This paper presents the corrosion-induced cracking process of a corroded reinforced concrete beam exposed to a chloride environment for 26 years. The cracking maps of the beam were drawn during different corrosion periods. The first corrosion-induced cracks in the compression zone occurred at about the 5th year and then developed significantly. The corrosion-induced cracks in the tension zone appeared at about the 7th year, followed by the stirrup zones in the transversal sections. At about the 14th year, the width of the cracks in the tension zone exceeded that in the compression zone. The cracks in the tension zone became connected almost throughout the span. The top-bar effect, bleeding and the "top surface ponding effect" led to the appearance of corrosion-induced cracks first in the compression zones, while the corrosion-induced cracks in the tension zone increased more significantly in both length and width as a result of the sustained load.

Drinking water pipes can be lined with a cement matrix based on CEM III, which can release aluminium into the water. However, the European Union limits the aluminium concentration in drinking water for public health reasons. In order to study the long-term leaching mechanism of aluminium, semi-dynamic leaching tests using an aggressive solution (demineralised water) or drinking water (mineralised water) were conducted on crushed CEM III pastes maintained at pH 7. They resulted in a very high leaching coefficient, approaching 50%. However, in spite of this strong leaching, the amount of Al leached remained very low, at less than 1.5% of the initial amount of Al in the material. Under these conditions, the leaching of Al from a CEM III paste is complex since it involves several steps, where slag (1st and last steps) or cement paste (2nd step) are the main contributors.

Knowing the water transport phenomena is fundamental for understanding the durability of cement-based materials. The moisture content within cementitious materials is of primary importance because it has a direct influence on mechanical (shrinkage, creep) and chemical (penetration of aggressive agents) degradation. Many models based on the determination of the transfer properties of the material under study already exist but require fitting of the parameters to find the experimental kinetics. Transfer parameters are generally estimated from the empirical approaches developed for soils by Van Genuchten and the water vapour diffusion reduction factor is often calculated via Millington's semi-empirical model. In this paper, we make a proposal to improve the prediction of moisture transfer, through mechanisms such as drying, capillary imbibition and water vapour permeability in cement-based materials, by using a porous network model to access the transfer parameters.

WOS:000367483000024; International audience; Chloride-induced macrocell corrosion of steel is recognized as a frequent and harmful problem affecting reinforced concrete structures. For this specific corrosion mechanism, anodic and cathodic areas at steel-concrete interface are spatially separated. Thereby, at equilibrium, anodic and cathodic potentials are different due to electrical resistivity of concrete, which means there is a potential gradient and a macrocell corrosion current flowing in the concrete volume. Despite some available works on electrochemical modelling of free macrocell corrosion systems in reinforced concrete, the literature is very scarce regarding the response of such systems under polarization. Due to this lack of knowledge, the assessment of steel corrosion rate in reinforced concrete is still commonly based on the uniform corrosion assumption. In this paper, the detailed polarization response of the different components of macrocell systems is studied in steady-state condition on the basis of numerical simulations and the qualitative conclusions are validated by original experiments. Some important results are achieved, with potentially major consequences regarding the non-destructive evaluation of steel corrosion in reinforced concrete. In particular, the distribution of the polarizing current in the macrocell system is found to be dependent on the polarization nature (anodic or cathodic) and magnitude. Moreover, in case of chloride-induced macrocell systems, the apparent linear polarization range is actually much higher than usually assumed and significant differences are observed between anodic and cathodic responses. (C) 2015 Elsevier Ltd. All rights reserved.

This paper presents the application of a new test protocol developed for determining chloride threshold values initiating corrosion in reinforced concrete. The experimental set-up was described in previous work and is based on the intrinsic localized aspect of chloride-induced corrosion. The test protocol is also based on the concept of anodic control of corrosion initiation, and therefore independent of the area of passive steel. The test method is applied to mortar and concrete formulations with different binders and steel surface conditions. Chloride-binding isotherms were also determined to allow the transition from total to free chlorides. It is found that the test protocol is rapid and applicable to all formulations. The experimental results provide an understanding of the influence of binder type, w/b ratio and porosity on chloride-induced corrosion initiation. As expected, the steel surface condition is shown to have an important overall effect on corrosion initiation.

International audience; This paper aims to better understand the mechanisms explaining the superior resistance of calcium aluminate cement (CAC) materials compared to Portland cement (PC) based materials in sewer networks. The bacteriostatic effect of CAC materials on Sulfur Oxidising Bacteria (SOB) is often argued in the literature as a possible mechanism explaining their better resistance. Reactor tests conducted on SOB demonstrated their ability to acclimatise to high aluminium contents. More generally, using a laboratory biodeterioration protocol reproducing aggressive conditions, the nature of the material (CAC or PC with different slag contents) did not significantly affect the SOB selection. Moreover the CAC materials seemed to favour the development of a higher SOB activity (and so acid production) than PC-based systems, leading to more aggressive conditions. Finally, the resistance of CAC materials to biodeterioration appeared to be mainly linked to the intrinsic resistance of phases initially present or precipitated during the deterioration process.

International audience; The effect of aggregate size on ASR expansions has been largely studied and conflicting results exist concerning the aggregate size that leads to the highest ASR expansion. Most of the research works clearly show a pessimum effect of aggregate size on ASR expansion. However, all the results available in the literature were obtained using different experimental conditions and the combined effects of other important parameters, such as specimen size used in the expansion tests, have often been neglected. This paper aims to investigate the combined effect of specimen size and aggregate size on ASR expansion. Experimental results highlight a scale effect, a combination of the effects of aggregate size and specimen size on ASR expansion. This scale effect appears to be influenced by the reactive silica content of the aggregate. Modelling at microscopic level is used to propose a quantification of this effect.

International audience; The aim of this work is to describe a method based on a simple representation of the pore size distribution, which is able to predict hysteresis phenomena encountered in water sorption–desorption isotherms, particularly for cementitious materials. The hysteresis effect due to network constrictivity is taken into account in order to extend models of transfer in porous media to situations involving wetting–drying cycles. This is not achieved in earlier models and their performance in terms of prediction in such conditions is thus limited. The present modeling is based on an idealized pore size distribution. This has three modes, associated with C–S–H pores, medium capillary pores, and large capillary pores including consideration of cracks. The distribution is assessed from the chemical composition of the cement, the formulation of the material, the degree of hydration, the total water porosity and the intrinsic permeability

International audience; The aim of this paper is to point out the water driving effect on the alkali silica reaction (ASR) expansion and particularly when modifications of moisture conditions occur After being submitted to a unidirectional moisture gradient during 14 months, the upper faces of ASR reactive beams were covered by water for 9 months This late water supply on the upper face rapidly produced large expansions, which mainly occurred along the transverse and the vertical directions resulting in large longitudinal cracks Companion nonreactive specimens were kept in the same environmental conditions in order to quantify the basic characteristics of moisture-dependent expansive behaviour of the material. The paper focuses on the effects of late water supply or late drying on already ASR-damaged concrete, which is a significant concern for reallife structures. Both structural effects of late water supply on ASR progress in already damaged structures and interpretation of such phenomena are described (C) 2010 Elsevier Ltd All rights reserved

International audience; This paper deals with the evolution of the corrosion pattern based on two beams corroded by 14 years (beam B1CL1) and 23 years (beam B2CL1) of conservation in a chloride environment. The experimental results indicate that, at the cracking initiation stage and the first stage of cracking propagation, localized corrosion due to chloride ingress is the predominant corrosion pattern and pitting corrosion is the main factor that influences the cracking process. As corrosion cracking increases, general corrosion develops rapidly and gradually becomes predominant in the second stage of cracking propagation. A comparison between existing models and experimental results illustrates that, although Vidal et al.'s model can better predict the reinforcement corrosion of beam B1CL1 under localized corrosion, it cannot predict the corrosion of beam B2CL1 under general corrosion. Also, Rodriguez's model, derived from the general corrosion due to electrically accelerated corrosion experiments, cannot match natural chloride corrosion irrespective of whether corrosion is localized or general. Thus, for natural general corrosion in the second stage of cracking propagation, a new model based on the parameter of average steel cross-section loss is put forward to predict steel corrosion from corrosion cracking.

International audience; The effect of the size of the aggregate on ASR expansion has already been well illustrated. This paper presents a microscopic model to analyze the development of ASR expansion of mortars containing reactive aggregate of different sizes. The attack of the reactive silica by alkali was determined through the mass balance equation, which controls the diffusion mechanism in the aggregate and the fixation of the alkali in the ASR gels. The mechanical part of the model is based on the damage theory in order to assess the decrease of stiffness of the mortar due to cracking caused by ASR and to calculate the expansion of a Representative Elementary Volume (REV) of concrete. Parameters of the model were estimated by curve fitting the expansions of four experimental mortars. The paper shows that the decrease of expansion with the size of the aggregate and the increase of the expansion with the alkali content are reproduced by the model, which is able to predict the expansions of six other mortars containing two sizes of reactive aggregate and cast with two alkali contents. (c) 2009 Elsevier Ltd. All rights reserved.

International audience; This work is a part of an overall project aimed at developing models to predict the potential expansion of concrete containing alkali-reactive aggregates. First, this paper reports experimental results concerning the effect of particle size of an alkali-reactive siliceous limestone on mortar expansion. Special attention is paid to the proportions of alkali (Na2Oeq) in the mixtures and reactive silica in the aggregate. Results show that ASR expansion is seven times larger for coarse particles (1.25–3.15 mm) than for smaller ones (80–160 μm). In mortars for which the two size fractions were used, ASR expansion increased in almost linear proportion to the amount of coarse reactive particles, for two different alkali contents. Then, an empirical model is proposed to study correlations between the measured expansions and parameters such as the size of aggregates and the alkali and reactive silica contents. Starting with the procedure for calibrating the empirical model using the experimental program combined with results from the literature, it is shown that the expansion of a mortar containing different sizes of reactive aggregate can be assessed with acceptable accuracy.

Although more than 10 years of studies on delayed ettringite formation (DEF) have led to consensus in numerous areas of past disagreements, some questions remain experimental work is needed to complete the knowledge of this pathology. Following this objective, this paper studies the influence of pre-existing microcracking, wetting/drying cycles and the type of sulfated addition on DEF in steam cured mortars. The mortar specimens were prepared using an Ordinary Portland Cement and two types of sulfate were added to the mixtures: calcium sulfate (CaSO 4 ) or sodium sulfate (Na 2 SO 4 ). The results confirm the well-known effect of temperature: no expansion was observed in any of the mixtures cured at room temperature. Moreover, no expansion was observed after 800 days for the reference mortar or for the mortar containing calcium sulfate but all the specimens of heat-cured mortars containing sodium sulfate expanded markedly after about 50 days whatever the supplementary treatments applied (thermal shrinkage or wetting/drying cycles). These results show the significant role played by alkalis in the occurrence of delayed ettringite. The supplementary treatments intended to cause prelimiray microcracking of the specimens did not promote expansion but contributed to a slight acceleration of the reaction. The ultimate values of expansion were similar to those obtained with sound mortars.

Effluents, such as liquid manure and silage effluents, stored in silos often made of concrete, contain organic acids that are chemically very aggressive for the cement-based matrix. The pH of liquid manure is comprised between 6 and 8, and the pH of silage effluent is about 4. There has already been much research done on manure’s effect on concrete using aggressive solutions with a pH of or inferior to 4, in order to accelerate alteration kinetics. These studies aimed at simulating liquid manure and silage effluent, equally. The goal of this article is to validate the use of solutions with a pH of 4 to implement accelerated studies on alterations occurring to structures exposed to the acidic part of liquid manure. In this study, the alteration mechanisms of the cement-based matrix produced by two solutions of organic acids with pH of 4 and 6 were compared. At the end of the experiment, carried out on ordinary Portland cement and slag cement pastes, the kinetics of alteration of the cement pastes immersed in the solution with a pH of 4 was ninefold higher than in the solution with a pH of 6. The chemical and mineralogical modifications of the paste were analyzed by electron microprobe, XRD and BSE mode observations. It was shown that the alteration mechanisms of the paste are sensibly identical for both solutions: almost complete decalcification, the disappearance of the crystallized or amorphous hydrated phases and the probable formation of a silica gel containing aluminum and iron, mainly. The differences in alteration mechanisms between the two solutions are minor and mainly concern the stability of the anhydrous phases: C4AF and slag grains. D 2004 Elsevier Ltd. All rights reserved.

International audience; Concrete tensile microcracking due to a mechanical loading enhances the diffusion of aggressive agents from concrete cover to the reinforcements, which leads to the beginning of the propagation period of steel corrosion. The purpose of this work is to quantify the effect of concrete microcracking on CO2 penetration. Concrete carbonation was studied on two 13-year-old reinforced concrete beams subjected to atmospheric carbonation. Indeed, these beams were exposed to the climate of the south-west of France but in a covered place and were always stored in a loaded state. Because the microcracking network is quite impossible to characterise, the tensile stress in the reinforcements was chosen as the main parameter of the microcracking state. This investigation results on a proposed model whose accuracy predicts the increase of the carbonation depth of the concrete in relation to the tensile stress in rebar.

The results obtained on both cement paste samples with and without silica fumes and mortars with and without silica fumes, show that there is an Interfacial Transition Zone (ITZ) effect on the alteration of the mechanical properties due to leaching. © 1997 Elsevier Science Ltd

International audience; This article deals with the correlation between active corrosion of steel in concrete electrode potential measurements obtained by both classical method and bi-electrode method.In a first part, this paper deals with a relative value interpretation method which leaves aside the degree of ambiant hygrometry and the moisture of concrete. This relative value interpretation exhibits a good correlation between the corroded areas of concrete reinforcing rebars extracted from laboratory beams and the potential measurement.In a second part, this paper deals with an improved potential measurement using two reference electrodes. Two kinds of bi-electrode device are presented. In the first place, one reference electrode is fixed and takes the place of the electrical connection to reinforcement. Secondly, the two reference electrodes are moving simultaneously. With the relative value method, it is possible to make direct use of the values measured with both types of bi-electrode device. The results presented show a good correlation between potential mesurement obtained by bi-electrode devices and corrosion of rebars.

International audience