Influence of shrinkage-reducing admixtures on plastic and long-term shrinkage

International audienceThe use of shrinkage-reducing admixtures (SRA) has been suggested to improve concrete performance in terms of lower risk of cracking related to drying shrinkage. Various forms of SRA are commercially available and they may act through different mechanisms. Some SRA mainly acts on drying and weight loss leading to shrinkage. In this paper, the influence of a liquid 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-to-cement ratios (w/c) 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. At early age, the results indicate that the SRA lead to the same plastic shrinkage for w/c = 0.65 while it reduced the plastic shrinkage by 25% for w/c = 0.43. Drying shrinkage was assessed from 1 day on hardened concrete. The SRA reduced the 7 day drying shrinkage for w/c = 0.65 and w/c = 0.43 concrete mixtures by up to 56%–31%, respectively, and the 70 day drying shrinkage by up to 33%–25% when the specimens were cured for 24 h then stored at relative humidity of 50%. At equal water-to-cement ratios, the SRA is seen to be more efficient in reducing drying shrinkage at early ages. These findings suggest that the SRA is most effective when internal relative humidity is relatively high or when higher porosity exists in the material. In fact, the SRA modified the pore structure increasing the total porosity and eliminating the percentage of larger pores with diameters ranging from 300 to 1000 nm. When concerned with drying of concrete, the larger pores are the first ones to lose their internal water and consequently change the RH levels where capillary stresses are the main cause of shrinkage. Thus, this phenomenon may contribute in the reduction of drying shrinkage that occurs when this liquid SRA is used in concrete

DEVELOPMENTS AND STANDARDISATION OF FLOWABLE CONCRETE REINFORCED WITH FIBRES FOR STRUCTURAL DESIGN, UPDATE OF fib TG 4.3

The fib Model Codes aim at integrating in a single document the relevant knowledge for structural design with concrete. Fibre reinforced concrete is already integrated in fib Model Code 2010 as a general category of materials. The group of flowable concrete consists of clusters of different types of concrete among others Self-Compacting Concrete, Ultra-High-Performance Concrete and Strain-Hardening Cementitious Composite. Being highly flowable is the distinguishing characteristic, flowable concrete might contain or not contain fibres. Although the fibre contribution on the structural level can be assessed on short-term, the structural behaviour also depends on the behaviour of the fibres and the matrix in which they are embedded. fib Task Group 4.3 worked on identifying and characterising different types of flowable concrete and discusses in a fib bulletin the most relevant aspects with regard to mix design, manufacturing, material performance and structural behaviour of flowable concrete which can allow innovative applications to be developed and realised. This paper discusses recent developments with regard to flowable concrete in a broader perspective and addresses the progress with regard to standardisation

Performance-based design and carbonation of concrete with high fly ash content

International audienceThis paper reports on an investigation of the durability of high-volume fly ash concrete exposed to carbonation (XC class in EN 206-1 Standard). The reduction of cement content is assumed to mitigate CO2 emissions due to the production of Portland cement clinker. The durability of concrete is assessed through a performance-based approach. First, a performance-based method was applied to design a concrete mixture with more than 50% substitution of cement by fly ash. To cope with the high viscosity of the so-obtained mixture, an air-entraining agent was used. Due to its low cement content, the so-obtained mixture does not comply with the prescriptive requirements of the European standard (EN 206-1). Thus, according to the equivalent performance concept, its potential durability has to be proved through comparison with reference mixtures complying with prescriptive standard requirements. The performance-based evaluation of the durability of the designed mixture and reference mixtures (with Portland cement and blended cement) consisted in porosity and accelerated carbonation tests. The influence of curing mode and thermal treatment duration before carbonation test were also investigated. Whatever the curing mode, the designed mixture was found as resistant to accelerated carbonation as the reference mixture containing blended cement

Carbonation of concrete with high substitution of cement by blast furnace slag: influence of interaction between drying and hydration

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Béton recyclé au jeune âge.

International audienceCe chapitre présente la méthodologie et les résultats d’une étude expérimentale portant sur le comportement au jeune âge de bétons incorporant des granulats de béton recyclé (GBR). Il s’intéresse plus particulièrement à la mesure du coefficient et de la cinétique d’absorption d’eau des gravillons de GBR, au comportement du béton frais en condition de séchage normal et sévère, et à l’évaluation des propriétés du béton en cours de durcissement. Les gravillons de GBR utilisés dans cette étude, fournis par le Projet National RECYBETON, présentent une absorption d’eau élevée. Leur taux de saturation atteint 90 % dans les deux premières heures, et plus rapidement encore si les gravillons sont immergés dans la pâte de ciment fraîche. Le suivi de leur absorption d’eau au sein de la pâte fraîche a également montré qu’une partie de l’eau absorbée pouvait se libérer en cas d’exposition à un séchage sévère. Le passage d’un régime de séchage normal à un régime de séchage sévère accroît le taux d’évaporation, mais n’augmente pas le ratio « retrait/perte de masse ». Toutefois, de plus grandes amplitudes de retrait sont observées en raison d’un retrait plastique accéléré qui, dans ce cas, se développe avant la prise.Le concept de pression d’entrée air a été utilisé pour analyser la sensibilité à la fissuration du béton frais. La valeur de cette pression a été estimée pour divers bétons, avec ou sans gravillons de GBR. Une bonne corrélation entre la pression d’entrée d’air et la dépression capillaire au moment de la fissuration a été trouvée. Le rapport eau efficace sur liant (Eeff/L) influence directement la pression d’entrée d’air. Ce rapport (Eeff/L) prend en compte l’état initial de saturation des gravillons de GBR.Entre la prise et 24 heures, l’évolution du retrait plastique, du module d’élasticité et de la résistance à la traction a été suivie en continu. La saturation initiale en eau des gravillons de GBR a une influence limitée sur ces propriétés. Des essais de retrait empêché seraient nécessaires pour analyser l’influence du taux de GBR sur la sensibilité à la fissuration, et pour estimer la relaxation

Nb-Doped TiO2TiO_{2} Nanofibers for Lithium Ion Batteries

Niobium-doped nanofibers elaborated by facile, single-step electrospinning present a higher rate capability in electrochemical cycling experiments than nondoped materials. This is attributed to the reduction of Li+ diffusion path lengths and enhanced intimate interparticle contact, in combination with improved intraparticle conductivity. Niobium doping has a significant effect on the electronic structure and provokes a substantial decrease in particle size

A study of the surface acidity of acid-treated montmorillonite clay catalysts

Abstract The surface acidity of a series of commercial Süd Chemie acid-treated montmorillonite clays (K-catalysts) has been evaluated by a wide range of complementary experimental techniques. The different methods applied allow a rather complete characterisation of the surface acidity providing a complete picture of the Lewis/Brønsted acid strength/density of the surface sites. IR data show that the Brønsted sites on these catalysts are relatively weak and provide evidence for a slight increase of the strength and the density of Brønsted sites in the order K5 < K10 ∼ K20 < K30 in full agreement with the trend in iso-butene conversion, which is a measure of the strength and/or the abundancy of Brønsted sites. The apparent contradiction of these data with those obtained from the ammonia adsorption and iso-propanol conversion experiments can be explained by the structural and chemical modification of the clays upon acid treatment