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There is much debate on the expression used for the critical chloride threshold as well as on its value for breakdown of the protective layer of steel for reinforced concrete. In fact, this concept suggests that the breakdown is only driven by dissolution of the protective layer. A reactive transport model including dissolution and precipitation of solid species with their kinetics is then used in order to simulate the ingress deleterious substances in a concrete exposed to seawater and the chemical degradation of the oxides and hydroxides present in the protective layers covering a steel rebar. The numerical results confirm that most of the oxides are thermodynamically very stable even after a long period of exposure, especially in the inner layer. This finding suggests that corrosion initiation depends on protective layer thickness and history. These results also provide a sound explanation why a wide scatter of values of critical chloride threshold values is reported in literature.

Usually the regulation defines the concentration of sulfates to consider aggressive soil or water. There are different standards to evaluate the resistance of concretes to the external sulfate attack. Most of these tests are based on the immersion of samples in solutions with high concentration of sulfates. But in the field, the concretes are also exposed to wet–dry cycles and the crystallization of salts can occur. In this work, 65 concretes manufactured with sulfate-resisting cements are tested. The cores were tested during 1 year submerged in sodium and calcium solutions for analyzing aspect, expansion, and variation of weight. After this, the cores were exposed to the same solutions semi-immersed for 2.5 years. After being semi-submerged for 1 year, some of the concretes revealed/showed alterations. After 2.5 years, around 15% of cores showed cracks and loss of material, due to formation of ettringite and thaumasite.

In real structures, gradients between inner and surface zones, which depend on the mix-design, as well as on curing and environmental conditions, are critical issues for durability, in particular when the mixture incorporates supplementary cementitious material (e.g. fly ash (FA)). This paper deals with pore structure and moisture content gradients, investigated not only on concrete samples in lab conditions, but also on reinforced concrete (RC) structural elements exposed for several years to three different outdoor environments (temperate climate, tidal zone in marine environment, as well as road and cold environment) and on full-scale structures (bridges). The experimental study was launched a lot of years ago within the framework of the large-scale and long-term “BHP 2000” French national project. The various mixtures studied (28-day cylinder compressive strengths ranging from 20 to 130 MPa) include high-performance concretes (HPCs) with or without silica fume (SF), as well as normal strength FA concretes. MIP pore size distributions obtained on the RC structural elements have been compared to lab results. In addition, in order to investigate the long-term drying(-wetting) behavior of the various types of materials studied here, water vapor desorption isotherms, internal relative humidity (RH), and degree of saturation profiles have been measured, respectively, by the saturated salt solution method, RH sensors, and gamma-ray attenuation on lab samples or on cores extracted from the elements and bridges at various times. The data have been analyzed as a function of the exposure conditions and of the mix-parameters. For example, the impact of early age external drying, carbonation, as well as freezing and thawing cycles in the presence of deicing salts has been highlighted on the field data. In addition, good performance of SF-HPCs and mature FA-concretes has been pointed out in the outdoor conditions considered.

The main cause of premature deterioration of reinforced concrete structures is the corrosion of steel bars, induced by chloride ions (for example in marine environment) and/or by carbonation (atmospheric CO2). At the same time, environmental-induced degradations of concrete can also affect the structure, such as sulphate attack. This can lead to the formation of ettringite, inducing expansion inside the materials and finally degradation. Carbonation, chloride and sulphate ingress are usually studied separately in the literature. This is not representative of in-situ conditions since they can occur at the same time and can have an influence on each other. In this paper, chloride ingress are studied for concretes and cement pastes partially carbonated or/and in presence of sulphate in chloride contact solution. The mixtures contain OPC alone or with supplementary cementitious materials (SCM). SCMs are here pozzolanic materials (Fly Ash or Metakaolin) or alkali-activated materials such as ground granulated blast furnace slag (GGBS). The materials, partially carbonated (2 months in chamber at 1.5 % of CO2) or not, are put in contact with chloride solutions in presence of sulphate. This study focuses on the apparent chloride diffusion coefficients, as well as chloride binding isotherms which are obtained by the profile method. In addition, some aspects of the microstructure and of the pore structure are investigated, by using Mercury Intrusion Porosimetry and 27Al NMR, in order to better understand the results obtained relatively to the apparent chloride diffusion coefficients and to the chloride binding. Chloride ingress increases when sulphates are present in the contact solution for all cement materials tested (partially carbonated or not). In addition, chloride ingress is faster when the material is partially carbonated before contact with chloride solution. It appears that both carbonation or presence of sulphate decrease chloride binding, thus explaining the results. The results show an evolution of the properties as a function of the cement replacement ratio by SCMs.

Long-term studies under various natural exposure conditions are still needed in order to understand and quantify ageing and durability of concrete structures, in particular in the case of mixtures which incorporate supplementary cementitious materials (e.g. fly ash). With this purpose, a large-scale and long-term experimental program was initiated a lot of years ago by the LPC network within the framework of the French national project “BHP 2000.” From then, further researches have been carried out in order to complement the program. The present paper focuses on chloride penetration in concrete samples in lab conditions and in reinforced concrete (RC) structural elements exposed for 10 years to two different natural environments (tidal zone in marine environment, as well as road and cold environment). Numerous mixtures (28 day cylinder compressive strengths ranging from 20 to 130 MPa) were studied in these various conditions, e.g. high-performance materials with or without silica fume (SF), as well as normal-strength fly-ash (FA) concretes. Carbonation and chloride penetration depths, along with total chloride concentration and degree of saturation profiles, have been measured on the RC structural elements and analyzed as a function of the exposure conditions and of the mix-parameters. In addition, the chloride data have been compared to lab results. Apparentchloride diffusion coefficients, as well as chloride binding isotherms and capacities, have also been assessed on these structural elements and compared to lab results. Good consistence between results in field and in lab conditions has been highlighted, except when carbonation has affected the field results. In addition, the good performance of SF-HPCs and mature FA concretes has been pointed out in the tested outdoor conditions, as well as that of the mature ground granulated blast furnace slag concretes tested in lab.

The influence of the chemical activity effects on moisture transport in the presence of ions in cementitious materials is studied. Wetting-drying tests are carried out on various hardened concretes (sound and fully carbonated) with water. Moisture profiles are experimentally assessed by gamma-ray attenuation. A multispecies and multiphase numerical model, in which the Lin and Lee approach is applied to compute the chemical activity coefficients of water in presence of ions, is used to simulate the transport of ions and moisture. The drying kinetics is analyzed to assess the intrinsic permeability of the material by inverse analysis. Experimental profiles are compared to the simulations to illustrate the relevance of the modeling.

Deterministic approaches for the prediction of the carbonation penetration have limitations related to the high variability of the input data. This paper deals with two carbonation models which are developed in the framework of a probabilistic approach. The random input data are presented, i.e. materials, environment and engineering design properties. The evaluation of the time-dependent probability of carbonation-induced reinforcement corrosion is assessed by calculating the reliability index for three studied concretes. This probabilistic approach provides a framework for a sensitivity analysis which enables one to identify the most influencing model parameters which have to be determined carefully in order to improve the accuracy of the prediction and to determine the parameters whose variability should be reduced to enhance the durability. Furthermore, a probabilistic approach provides a tool to design the thickness of the concrete cover in order to avoid oversizing.Les modélisations déterministes de la carbonatation présentent des limites, essentiellement liées à la forte variabilité des données d’entrée des modèles. Le présent article porte sur deux exemples de modèle de carbonatation qui sont assortis d’un cadre probabiliste. Les données d’entrée des modèles sont présentées et assorties de lois de probabilité. La sécurité, vis-à-vis du risque de carbonatation, est donneé avec un niveau de confiance. L’approche probabiliste offre aussi un cadre pour une analyse de sensibilité de ces deux modèles permettant de dégager les paramètres les plus influents qui devront être déterminés avec soin pour améliorer la justesse de la prédiction et ceux pour lesquels la variabilité devra être réduite afin d’améliorer la durée de vie. De plus, les modèles probabilistes constituent des outils d’aide au dimensionnement de l’enrobage, notamment lorsque les approches normatives conduisent à un surdimension-nement.

ABSTRACTIn this paper, the liquid water permeability has been investigated by various methods in laboratory conditions. A comparison has been performed between the “intrinsic” permeabilities thus obtained. The values have also been compared to the results obtained from gas permeability measurement. The results point out that the concept of intrinsic permeability, independent of the permeating fluid, is not relevant for cementitious materials, when liquid water is regarded, except maybe for highly permeable ones. The various physicochemical processes likely to occur during the tests can explain the significant differences observed. The apparent chloride diffusion coefficient has been assessed by means of diffusion and electrical migration tests. The differences recorded have been discussed. They can be explained in particular by the various microstructural changes that occur during these two types of tests, as illustrated by MIP and 29Si NMR results.

ABSTRACTA modelling of drying of concrete is presented. « Intrinsic » liquid water (Kl) and gas (Kg) permeabilities are distinguished, since the concept of intrinsic permeability, which is independent of the fluid nature, is not relevant for a cementitious material. New laws for gas transfers are introduced. Thus, a law gives the effective diffusion coefficient of water vapour as a function of porosity and degree of liquid water saturation. In the same way, a new function, expressing the relative permeability to gas according to this degree of saturation, is calibrated thanks to experimental results. In order to describe the global movement of gas, viscous and slip flows are taken into account according to the Klinkenberg's concept. A numerical study shows, on the one hand, that a gas depression (below the atmospheric gas pressure) can be observed and, on the other hand, that transfers of water in the gas phase can significantly contribute to the drying of cementitious materials in addition to liquid water transport by capillarity. Simplified approaches are presented. Finally, a calibration of Kl is carried out thanks to drying experiments.

ABSTRACTThe modeling of concrete carbonation needs physical parameters well determined, especially those of the real carbonation front and the quantity of calcium carbonate formed after carbonation. This paper proposes a comparison between different carbonation measurement methods. Obtained results are slightly different from used method. Furthermore, the interest and the limit of each technique are showed.

ABSTRACTThis work deals with the influence of the superficial microcracking on concrete carbonation. Three types of concrete with strengths ranging between 25 and 60 MPa were subjected to different thermo-hygrometric treatments to create a superficial microcracking, then, to accelerated carbonation tests for 3 months. After their period of preconditioning treatments, the quantification of the superficial microcracking and the measure of the water absorption of the concrete were carried out to observe correlations with carbonation depth. Results show that the influence of the superficial microcracking is strongly dependant of the concrete strength and begins to be significant when concrete strength reaches about 50 MPa.

ABSTRACTThis paper deals with the comparison between the steady state and the non steady state measurement of chloride diffusion coefficient. Thanks to the comparison, the non steady state method appears to be a quick and reliable method to evaluate the chloride diffusion coefficient.

ABSTRACTThis study, achieved as a part of the research project “Transfers in concrete and durability of structures”, deals with comparative measurements between two methods which evaluate the degree of hydration of different hardened Portland cement pastes: classical measurement of the chemically bound water, and image analysis on polished sections by scanning electron microscope in backscattered mode (SEM—BSE). The water/cement ratio ranging from 0.25 to 0.6 and time from 28 days to 2 years, are the two majors parameters of this study. The results indicate a good agreement between the two methods. Assumptions, advantages and limitation of each method are discussed, especially, when applied to other types of cements and to concrete.

ABSTRACTThis work deals with the influence of the W/C ratio on the cement hydration and on the chemical characteristics and the microstructure of hydrated compounds. Results obtained with various techniques show that the W/C ratio has a significant influence on the structural characteristics of the hydrated cement pastes. Whatever the time of hydration, it appears that the materials have a different behaviour when W/C is higher than 0.40. This limit could correspond to a “percolation threshold” of water within the capillary pore network. When W/C increases, the amount of calcium hydroxide, referred to a unit mass of initial cement, increases whereas the CaO/SiO2of C-S-H has a tendency to decrease. Moreover, for the autogenous deformations recorded during hydration, a swelling phase is exhibited before shrinkage phase with high W/C ratios. In the case of low W/C ratios, only shrinkage is observed.

ABSTRACTAs part of the research project “Transfers in concrete and durability of structures”, the microstructural characterization of cementitious materials stands as a basic stage for the study of their durability. Numerous methods of investigation have been carried out for this characterization. In this paper are presented the experimental results obtained on a broad range of hardened cement pastes: SEM observations, evaluation of Ca(HO) 2and CaCO3amounts by TGA, water porosity measurement, determination of pore size distribution by MIP, evaluation of degree of hydration, and quantification of microcracking by image analysis. The characteristics thus pointed out have been interpreted as a function of the mix-parameters, and analyzed within the scope of prediction of transport properties.

ABSTRACTOne of the objectives of the LCPC research project “Transfers in concretes and durability of structures” is the development of methods of determination of transport properties. These parameters play a key role in the evaluation of the durability of cement-based materials. In this paper, a method to assess the liquid water permeability of weakly permeable concretes is proposed. It is about an indirect method combining model and experiment. A precise analysis of the moisture transfers has also been carried out on the basis of this modelling. The conclusions obtained, relative to the mechanisms involved during the drying of weakly permeable materials, are also presented here.

ABSTRACTThis paper presents an experimental study about the gas permeability of different concretes versus their saturation degree. The hydric state affects indeed the transport parameters like permeability, which has a large influence on the penetration of gaseous aggressive agents and so on durability of concretes. The experimental results deal with the gas permeability measurement, saturation degree obtained by weighing or gamma-ray measurement and the pore size distribution by mercury intrusion porosimetry. The analysis points out the influence of the pre-conditioning method on the moisture gradients in the concrete samples and the influence of the concrete microstructure on the saturation degree evolution during drying. Finally, this study underlines the effect of the concrete saturation degree on the gas permeability variation.

ABSTRACTThis work has been carried out as part of the LCPC research project OA9 « Transfers in concrete and durability of structures » on the basis of a very large experimentation achieved on a concrete slab and samples during their drying. For this purpose, we have implemented a very complete set of investigation methods. In addition to deformations, many parameters (microstructural, thermal, hydric ones…) have been measured and their evolution has been followed up since the early age and for several months. Important gradients of the properties have been made obvious and measured from the middle to the slab drying surface.

Several enzyme activities were measured in extracts from acclimated and nonacclimated maize (Zea mays) root tips at pH 6.5 and 7.5, corresponding to cytoplasmic pH in anaerobiosis or aerobiosis, respectively, to determine what causes the decline of the glycolytic flux observed in anoxia in nonacclimated tips. We found that phosphorylation of hexoses by kinases was a major limiting step of glycolysis in anoxia. When fructose was substituted for glucose, glycolysis was slightly enhanced and survival improved, but neither matched that of acclimated tips. Decrease of kinase activities was not the result of proteolytic degradation but was more likely the result of inhibition by internal factors (low pH and low ATP). There was no evidence of induction during the hypoxic pretreatment of isoenzymes better adapted to the anoxic cellular environment. Maintenance of the glycolytic flux in acclimated tissues is explained by a combination of a rise in kinase activities and decreased inhibition resulting from a higher cytoplasmic pH and ATP content. The behavior of intact root tips is discussed in comparison with the behavior of excised root tips.