Study of the Chemical Processes of Combined Sulfate-chloride Attack on Low-carbon Cementitious Materials

In marine environments, concrete structures are not only exposed to sulfate ions but also to chloride ones, from an early age. This leads to concrete expansion and cracking as well as steel reinforcements corrosion. However, the coupling effect of sulfate and chloride is still not widely studied. When addressed, some researchers showed that chloride ions mitigate the effect of sulfate while others concluded that it accelerates it and vice versa. Therefore, one of the objectives of this study is to observe both the combined and individual effects of chloride on sulfate attack. The second objective is to perceive the resistance of selected additives (fly ash, blast furnace slag, and metakaolin) to a combined attack. Then, powders of pure cement, binary, ternary, and quaternary blended pastes were immersed in sulfate, chloride, and sulfate-chloride solutions at an early age. Results of the characterization showed that the ettringite formation was delayed due to the presence of chloride. However, the presence of sulfate ions accelerated the chloride effect. The incorporation of more than one additive enhanced the samples’ durability

Étude des couplages hydratation-échanges hydriques-carbonatation dans les mortiers modifiés polymères

L exposition des mortiers modifiés polymère utilisés en revêtement de surface à un environnement agressif peut affecter leur durabilité. Parmi ces agents figure le gaz carbonique CO2. Plusieurs travaux se sont penchés sur la carbonatation du béton armé, où le processus aboutit à la corrosion des armatures. A contrario, on trouve peu d études concernant le cas des mortiers modifiés polymères. C est pourquoi, l objectif de cette thèse est de traiter la particularité du processus de la carbonatation dans le cas de ce type de mortier en s appuyant sur une étude expérimentale basée sur les essais de la pulvérisation à la phénolphtaléine, les analyses thermogravimétriques (ATG), la microscopie électronique à balayage (MEB), la porosimétrie au mercure et à l eau ainsi que des essais mécaniques et de mesure de variations dimensionnelles et pondérales. L investigation de la carbonatation est réalisée par le couplage de l indicateur coloré avec des ATG. L effet de la cure sur les cinétiques de carbonatation et les conséquences de celle-ci sur l évolution physico-mécanique ont pu être appréhendés (stabilités dimensionnelles et pondérales, résistances mécaniques, microstructure). Enfin, une modélisation du couplage hydratation-échanges hydriques a été proposée en se basant sur des équations de conservation de masse. Elle a permis de prédire l évolution du degré d hydratation et de la porosité du matériau. La validation du modèle montre une bonne concordance avec les valeurs expérimentales de l hydratation.LA ROCHELLE-BU (173002101) / SudocSudocFranceF

Effectiveness of Non-Destructive Methods for the evaluation of structures affected by Internal Swelling Reactions: Electric, seismic and acoustic methods.

Internal swelling reactions (ISR) (which mainly comprise alkali-silica reaction, alkali-carbonate reaction, delayed ettringite formation, etc) are pathologies that affect the long term durability of concrete structures. The reactions are apparently characterized by series of closely spaced, tight map cracks with wide cracks appearing at regular intervals and excessive tensile stresses in rebars. This phenomenon is alarming for affected structure managers as it deals with people safety and structures operation. Moreover, there is no easily implemented way to stop the reactions. Prediction of concrete expansion, structural degradation and assessment of efficiency andperiodicity of repair works are crucial issues. The aim of this work is to explore the applicability of some Non Destructive Techniques (NDT) of investigations (electric, seismic and acoustic methods) to differentiate the structure zones affected and not affected by the ISR for different exposure conditions. The results highlight that the coupling between some of those ND Methods can reduce the measurement uncertainty and eliminate the factors that may influence the characterisation of the structure parts affected by the ISR. Nevertheless, for the complete structure reassessment, it is desirable to also refer to global geometrical/topographical survey and to destructive analysis methods on some cored samples for chemical analysis, petrography and dimensional measurements

A proposed modeling of coupling carbonation-porosity-moisture transfer in concrete based on mass balance equilibrium

International audienc

Durability of self-compacting rubberized concrete exposed to external sulphate attack

This study evaluated the performance of self-compacting rubberized concrete against external sulphate attack (ESA). cylinders 100 mm in diameter and 220 mm in length of control concrete (no rubber) and rubberized concrete were prepared and tested by visual inspection to identify visible degradation, length and mass variations of specimens, compressive strength, water-accessible porosity, mercury intrusion porosimetry (MIP), and thermal decomposition obtained from thermogravimetric analysis (TGA). Results show that the incorporation of up to 15% rubber enhances the performance against ESA. Rubber reduced the expansion strains and compressive strength. All immersed specimens did not have visible cracks around them. Water porosity was found to increase or decrease versus the time of sulphate immersion. As a result, the impact of rubber on porosity variation is significantly less than the effect of the sulphate-hydrate reaction. Thermogravimetric analysis showed a decrease in portlandite, which is not related to rubber incorporation

A proposed modelling of coupling carbonation-porosity-moisture transfer in concrete based on mass balance equilibrium

International audienc

A Critical Review of Existing Test-Methods for External Sulfate Attack

External sulfate attack (ESA) of cementitious materials has been studied worldwide for a very long time. This physical/chemical interaction between sulfate ions and the cement hardened elements affects the long-term durability of concrete structures: cracking, spalling or strength loss of concrete structures. To study these damaging phenomena, some standardized and non-standardized accelerated aging tests are used to evaluate the performance of cements in sulfate-rich environments. However, these existing methods do not adequately predict field performance and some shortcomings or deficiencies still exist: change of degradation mechanisms when using high concentrations of sulfate, variable boundary conditions and small specimens compared to the real concrete structures. In this work, a critical review of some existing test methods and foreign national standard methods for ESA are presented, analyzed, and discussed. This results in some proposed recommendations for improving these methods to meet the needs of structure managers

Effect of Exposure Conditions on Mortar Subjected to an External Sulfate Attack

This study aims to investigate the influence of exposure conditions on the behavior of mortar subjected to an external sulfate attack (ESA). Three different exposure conditions (full immersion, semi-immersion, and drying/wetting cycles) were tested on mortar prisms made with Portland cement and two w/c ratios (0.45 and 0.6). To monitor degradation, it was necessary to evaluate variations in length (expansion), mass changes, compressive and tensile strengths, changes in the total porosity measured using water accessible porosity tests, and changes in the macroscopic behavior of the samples. Mercury intrusion porosimetry (MIP) was used to determine the size distribution of the pores. It was demonstrated that mixing mortar with the lower w/c ratio of 0.45 results in improved performance against an ESA. This study also demonstrates that the type of exposure to an ESA has no significant effect on the kinetics of sulfate penetration during the exposure period. However, the sample’s surface becomes more cracked when subjected to repeated drying and wetting cycles. For all the considered exposure conditions, expansion occurred in three stages. In stage 1, the reaction product (ettringite) precipitated in large voids, without causing significant expansion (the expansion remained low and stable). During the second stage, the reaction products generated growing internal stress. The final stage of expansion resulted in microcracks, strength losses, and the formation of macropores, which ultimately lead to material failure. The MIP results indicate that major changes in the porosity and pore volume distribution occur at the surface layer in regard to the gel and capillary pore ranges

DEF coupled to thermal history : Recent Advances

Concrack 5, JCI-Rilem International Workshop, TOKYO, JAPON, 24-/04/2017 - 26/04/2017Thermal history conditions susceptible of triggering Delayed Ettringite Formation (DEF), be it at early age due to uncontrolled heat curing or heat generation from hydration, or at a mature stage due to accidental or excessively hot service conditions, have deserved recent experimental investigations. Two concrete mixes with differentsulphates, alkalis and tri-calcium aluminates contents were considered. Expansion of specimens kept under watersaturated conditions after they have been submitted to 60 to 85°C treatments during one to several days have confirmed the risk of DEF occurrence when the specimens have been submitted for a sufficiently long duration toa temperature higher than about 65°C. For a given concrete, parameters describing the sigmoid-shaped expansion curves may be determined from master curves as a function of the 'efficient' thermal energy provided. A pessimum effect related to the expansion amplitude has been quantitatively confirmed. It can be explained by the availability of sulphates on one hand, aluminates on the other hand, depending on the thermal history characteristics. Moreover, differences in the expansion kinetics might be consistent with the critical role of sulphates accessibility to pore water. These results shall improve DEF prevention through better tuned control of thermal history, especially at early age; it can also help in better managing affected structures