Exploration of the phase composition of activated calcined clay-slag mixtures using 27Al and 29Si MAS NMR spectroscopy

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Failure risk of recycled aggregates concrete

In the sustainable development context, the use of demolition waste increases in the building industry. Recycled aggregates from the demolition of concrete structures are then mixed in new concrete. Furthermore, the performance evaluation of a these recycled aggregate concrete (RAC) mix is not only limited to the determination of its conventional mechanical properties. The failure risk of concrete elements in structures made from RAC needs a comprehensive analysis. For that, a study on the fracture process of RAC has been performed and compared with that of normal concrete of the same structural class. Acoustic emission technique and digital correlation method have been used to follow the cracking process. Also, a new modelling approach for the fracture behaviour of RAC at the mesoscopic scale has been developed. It has taken into account the old attached mortar surrounding recycled aggregates. Results show that RAC has a more brittle behaviour than ordinary concrete and the numerical analysis shows that cracks growth through the recycled aggregates which have brittleness behaviour

Modélisation des attaques de sulfate dans le béton

Dans ce travail, on se propose d'étudier la durabilité des matériaux du génie civil dans leur environnement, et plus principalement aux problèmes liés à la dégradation du béton soumis à des attaques de sulfate. Ce problème est sujet à de nombreuses interrogations sur l'origine de la dégradation des bétons dans de telles conditions. Nous nous intéressons à la formation d'ettringite lors de la réaction chimique entre une solution concentrée de sulfate de sodium et les constituants du béton, développant une pression sur la surface des pores de la pâte de ciment. Lorsque cette pression est suffisamment importante, on observe l'apparition de fissures dans la microstructure du béton. On note alors que le comportement du matériau cimentaire est fortement lié à la compréhension fine de l'évolution des propriétés de la microstructure. Sur la base d'essais expérimentaux, une étude numérique est réalisée pour modéliser les attaques de sulfates dans des bétons testés en laboratoire

Extended Round Robin Testing program of COST Action TU1404 – lessons learned from the initial experimental phase

The extended round robin testing program (RRT + ) is used in the Working Group 1 of the COST Action TU1404 as a fundamental mechanism: i) to validate advanced, non- standardised experimental techniques for testing cement-based materials and structures, ii) to benchmark different sustainable variations of concrete mixes prepared with mineral admixtures, recycled materials and/or by-products, and iii) to obtain input data for a range of concrete properties which could serve designers and engineers to better predict lifespan, durability, and serviceability of concrete struct ures. With a total of 45 laboratories from Europe, Japan and Canada, performing over 50 test methods on the same concrete mix, it presents one of the most extensive initiatives for joint testing of cement-based materials. The RRT + is divided into two phases: the initial and main experimental phases. During the initial phase, an ordinary concrete mix is prepared using the same constituting materials and following identical preconditioning, preparation, conditioning and test procedures. Even though the framework is identical and potential external causes of deviations are limited, concrete is prepared in different laboratories and some scatter in results can be expected. This paper describes the observations during the initial experimental phase and discusses methods including statistical analysis performed to understand the scatter and results obtainedEDF, France, CEVA Logistics, Austria and Germany, OeBB Infra, Austria, Staten Vegvesen, Norway and Schleibinger Gerate, German

A performance based approach for durability of concrete exposed to carbonation

International audienceCarbonation is a widespread degradation of concrete and may be coupled with more severe degradations. In order to change from prescriptive requirements to performance based specifications for durability, through the equivalent performance concept, it is necessary to find relevant performance tests and indicators. Concrete mixtures of the reported study were designed complying with required binder contents and water–cement ratios to investigate the effects of binder composition, aggregate type and curing conditions. Early drying severely affected performances, as well as cement replacement by low-calcium fly ash. Aggregates had indirect effects on actual water content and curing. Porosity, gas permeability and chloride diffusivity were found as unreliable indicators for carbonation, since they only characterise compactness of concrete. An accelerated carbonation test is proposed as a performance test, as it is sensitive enough and results were consistent with natural carbonation in the studied exposure conditions. Another indicator, based on chloride diffusivity/initial CaO content ratio, could provide useful data for given aggregate mixes and curing conditions, as it takes into account compactness of concrete and chemical reactivity of binder

Suivi ultrasonique de la prise de bétons à teneur élevée en additions minérales

info:eu-repo/semantics/publishe

Effets d'un agent compensateur de retrait sur la microstructure et les déformations libres des bétons

National audienceThe use of shrinkage reducing admixture (SRA) has been suggested to improve concrete performance in terms of lower risk of cracking related to drying shrinkage. In this paper, the influence of SRA on plastic shrinkage, long term shrinkage, mechanical characteristics as well as concrete pore structure were investigated. Samples of concrete were prepared with two water-binder ratios (w/b) to design ordinary and high strength concrete. The effect of the shrinkage-reducing admixture was studied by adding 1 % to the total mass of binder, while keeping the other parameters constant. The results indicate that SRA reduced plastic shrinkage, and reduce the 7 day drying shrinkage for w/c=0.65 and w/c=0.43 concrete mixtures by up to 56% to 31% respectively and the 70 day drying shrinkage by up to 33% to 25% when the specimens were cured at relative humidity of 50%. In fact, the SRA modified the pore structure increasing the total porosity and eliminating the percentage of larger pores with diameters ranging from 0.13 to 1µm. When concerned with drying of concrete, the larger pores are the first ones to lose their internal water and consequently change the RH levels which can explain the reduction in drying shrinkage that occurs when SRA are used in concrete.Les ouvrages du génie civil sont victimes de l'apparition de microfissures dans le béton au jeune âge due aux phénomènes de retraits de la pâte de ciment. L'objectif de ce travail est d'étudier l'effet d'un agent compensateur de retrait, sous forme d'adjuvant, sur les déformations libres du béton. Ainsi trois axes ont été suivis : le retrait plastique, le retrait du béton durci, ainsi que l'analyse des propriétés mécaniques. Les essais sont réalisés sur deux formulations ayant deux rapports eau sur ciment (E/C) différents : 0,65 et 0,43. L'effet de l'adjuvant a été étudié en l'ajoutant au béton à hauteur de 1% en masse du liant tout en conservant les autres paramètres de composition constants. L'amplitude du retrait de dessiccation est diminuée de 57% après 7 jours de durcissement pour le béton ayant un E/C=0,65 et 31% pour le béton avec E/C=0,43 à la même échéance. A l'âge de trois mois, cette différence diminue à 33% et 25% pour les deux bétons respectivement. En effet, une légère augmentation de la porosité totale du béton a été mesurée, ainsi qu'une diminution du pourcentage de pores les plus larges de diamètre compris entre 0,13 et 1µm. Ces pores étant les premiers à perdre l'eau interne, cette modification du réseau poreux peut expliquer la diminution du retrait du béton durci