Delayed ettringite formation in concrete is likely to lead to swelling and cracking in structures which have undergone eartly age heating to a temperature of over 65°c. Application of a method that accelerates this process has made it possible to study the impact of cement properties on DEF.

The impact of cement parameters on Delayed Ettringite Formation

https://hal.archives-ouvertes.fr/hal-01007361/document

The effect of natural pozzolan on delayed ettringite formation of the heat-cured mortars

https://hal.insa-toulouse.fr/hal-01878729/document

Effects of restraint on expansion due to delayed ettringite formation

Delayed Ettringite Formation (DEF) is seen as a form of internal sulfate attack of cementitious materials, caused by early age heating to a temperature higher than 70°C. In this paper, the effect of fly ash on delayed ettringite formation of heat-cured cement-based mortars was investigated. To fulfil the aim of this study, a portion of cement was replaced by class F-fly ash, with three different dosages (10, 20 and 30%). The mortars were heat-cured at early-age, and the tests of expansion, mechanical strength, dynamic elastic modulus, mercury porosity and thermogravimetric analysis were carried on these mortars along a period of 650 days. Additionally, Scanning Electron Microscopy (SEM) observations were realized. The results obtained highlighted the mitigation effects of fly ash on DEF: A replacement of the cement used with 20% to 30% of fly ash was efficient to eliminate the long-term swelling due to DEF. This positive impact was explained by the combined effects of the high Al2O3 content of fly ash and the portlandite consumption induced by the pozzolanic reactions.

The effect of the partial cement substitution with fly ash on Delayed Ettringite Formation in heat-cured mortars

La Reaction Sulfatique Interne (RSI) est une pathologie du beton pouvant affecter les materiaux soumis a un echauffement au-dela de 65°C. Elle consiste en une formation d'ettringite dans le materiau durci et conduit a son gonflement. Il s'en suit une fissuration et une degradation des performances mecaniques pouvant poser des problemes d'integrite structurelle a l'instar de la Reaction Alcali-Granulat (RAG) a laquelle elle est frequemment couplee in situ. Lorsqu'un ouvrage est atteint, il convient de poser un diagnostic, evaluer son aptitude au service, predire son evolution et mettre en uvre des methodes de reparation. Ceci necessite une comprehension fine des effets de la RSI a l'echelle microscopique et a l'echelle de l'ouvrage. De nombreuses etudes experimentales et theoriques ont ete menees pour determiner les mecanismes mis en uvre et les parametres influencant la RSI. Toutefois, la complexite des phenomenes rend delicate la transposition de ces connaissances a l'echelle de la structure. Les approches macroscopiques semblent donc plus adaptees a ce type de probleme. Pour mettre au point ces approches, il est necessaire de comprendre en detail les effets de la pathologie a l'echelle du materiau et de la structure. Cette these decrit les resultats d'une etude de laboratoire basee sur des essais sur eprouvettes pour caracteriser les couplages entre les gonflements et l'humidite, la temperature et l'etat de contraintes. Ces travaux ont egalement ete l'occasion d'etudier les couplages entre RAG et RSI. En parallele, des suivis dimensionnels et hydriques de poutres soumises a des conditions d'exposition a l'humidite controlees ont permis de constituer une base de donnees des effets structurels de la RSI. La confrontation de ces essais menes conjointement a l'echelle du materiau et de la structure fournit des donnees permettant de mettre au point des methodes de re-calcul des ouvrages et de les valider en confrontant leurs predictions aux resultats experimentaux

Analyse sur structures modèles des effets mécaniques de la réaction sulfatique interne du béton

This article, which deals with the study of the microscopic modifications of DEF-affected materials, has two main objectives The first one is to study the influence of sample preparation on the microcracks of specimens observed in Scanning Electron Microscopy (SEM) From the results of this study, it can be concluded that direct observations of the samples are inappropriate because they produce cracks that are not ascribable to the pathology It is therefore preferable to use an indirect technique of observation such as the replica technique This technique was developed in the 1980s and used to study the evolution of microcracking due to mechanical damage to the concrete The results presented in this article show that it is possible to use this technique to study the microcracking associated with delayed ettringite formation The second objective is to study ettringite formation during the swelling of DEF-affected specimens The main results show that secondary ettringite is initially formed in the paste–aggregate interface even at low levels of expansion For these levels of expansion, there is no trace of secondary ettringite or microcracks observable in the paste However, for higher expansions, cracks filled with ettringite appear in the paste

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Microscopic observations of samples affected by delayed ettringite formation (DEF)

https://hal.archives-ouvertes.fr/hal-01006828/document

Nordine Leklou

Papers

Microscopic observations of samples affected by delayed ettringite formation (DEF)

Expansion of five-year-old mortars attributable to DEF: Relevance of the laboratory studies on DEF?