Enhancing the Properties of Sulfate-Resisting Cement

Sulfate-resisting cement is used in construction works when a sulfate attack is probable. This type of cement withstands sulfates due to its low C3A content. On the other hand, the decrease in the quantity of C3A leads to a reduction in the rate of early strength development. To overcome this problem, a hardening accelerator was added to the cement mix. To estimate the effect of the accelerator on some properties of hardened cement, compressive and flexural strength, and drying shrinkage tests were carried out. Four series of cement mortar mixes were made. The hardening accelerator was not added to the control series, while it was added to the others with three different percentages (of cement mass) of 0.5, 1, and 1.5%. The results revealed that the hardening accelerator enhances the compressive strength of all mortar ages while it slightly promotes flexural strength only at early ages. It was also observed that the hardening accelerator strongly inhibits the drying shrinkage strain

Analyse par approche numérique mésoscopique des propriétés mécaniques résiduelles du béton après un chargement de type fluage

International audience Les modèles existants pour estimer le fluage du béton basés sur une modélisation rhéologique, supposent généralement que le béton est un matériau homogène. Par conséquent les incompatibilités de déformations entre la pâte de ciment et les granulats lorsque le béton est soumis à un chargement de type fluage ne peuvent pas être prise en compte alors qu'elles peuvent être à l'origine d'une microfissuration. Ces microfissures peuvent entrainer une diminution du module d'élasticité et de la résistance du béton, et une augmentation de la quantité de déformation par fluage sous le même niveau de chargement. Cette augmentation des déformations de fluage peut être considérée comme la source de non linéarités avec le niveau de contrainte. Pour étudier l'influence de ces microfissures, un modèle visco-élastique-endommageable a été adopté pour le calcul de fluage en utilisant une technique de maillage mésoscopique. Les résultats montrent qu'une part importante des non linéarités peut être expliquée par les microfissurations liées aux incompatibilités de déformations et que l'approche mésoscopique permet de reproduire fidèlement des essais de fluage jusqu'à la rupture. </div

Adding Polymer to Mortar Mixture According to the Procedure Recommended by ASTM C 1439

Polymers are considered one of the admixture kinds which they are used to improve the qualit  and the performance of mortar and concrete. I  this study the Styrene Butadiene Rubber (SBR) was used as a polymer to see its effects on the mortar. The (SBR) is considered as a water resistance polymer and bonding agent. Four cement mortar mixes were made in this study, the first one was considered as a reference mix which is not include (SBR) while the other mixes were included (SBR) in deferent percentages. The (SBR) was added as a percentage of cement weight. The amount of mixing water was varied to achieve the same flow which was accomplished in the reference mix throughout carrying out flow table test. Flow table test was adopted according to ASTM C 1439-99 (1) recommendations. For each mix, two prisms were prepared to carry out the drying shrinkage test, nine cubes for the compressive strength test and three cubes for the total water absorption test. It was found that when the quantity of (SBR) increases, the physical properties of mortar under study (drying shrinkage, compressive strength and total water absorption) deteriorate which is adverse to the proposal of advantages due to adding (SBR). This result leads to that the recommendation of ASTM C 1439-99 (1) that adopt the flow table as criteria of adjusting the amount of adding water accompanying with deferent percentages of (SBR) is not adequate

Influence des déformations de fluage sur les propriétés mécaniques résiduelles du béton

L'objectif de cette thèse est d’étudier l'influence du fluage sur les propriétés mécaniques résiduelle du béton. En effet, les sections des éléments d’une structure précontrainte sont majoritairement soumise à une contrainte de compression. En raison de la relaxation des contraintes (due au fluage du béton notamment) et de la charge appliquée, des contraintes de traction peuvent apparaitre au cours de la vie de l’ouvrage. Ainsi, il est nécessaire de connaître la valeur des propriétés mécaniques résiduelles après l'effet de fluage.Le travail se concentre sur le fluage propre (sans échange hydrique avec le milieu environnant) et il comprend une partie expérimentale et une partie numérique. Dans la partie expérimentale, des essais de fluage en compression et en traction indirect (essai Brésilien) ont été effectués sur le béton en utilisant différents niveaux de chargement appliqué à différents âges du béton. A la fin de chaque essai de fluage, un essai quasi statique a été réalisé sur le même béton pour déterminer l'effet du fluage sur les propriétés mécaniques résiduelles. Afin de vérifier l'effet d’une précharge en compression sur la résistance en traction, comme dans le cas d’une structure précontrainte vieillissante ou soumise à un chargement accidentelle, une campagne d’essai Bresilien avec un préchargement quasi statique en compression a également été réalisée. Les résultats montrent que le comportement après fluage dépend fortement de l’âge de chargement et du sens de sollicitation. Numériquement, un modèle mésoscopiques a été utilisé pour simuler le fluage et les propriétés mécaniques après fluage. En effet, sous un chargement constant seule la pâte de ciment flue tandis que les agrégats agissent comme un obstacle à ce fluage. Par conséquent, les contraintes de traction se produisent à l'interface pâte de ciment/agrégats conduisant à microfissures dans cette zone Les déformations viscoélastique sont correctement reproduite à différents niveaux de chargement mais la modification du comportement résiduel du béton ne peut pas être uniquement expliqué par la micro-fissurationThe aim of this study is to evaluate the effect of creep on the concrete mechanical properties evolution. In prestress structures, in the first concrete is completely subjected to compressive stresses. With time, due to relaxation of the prestressed cables and applied load, some tensile stresses may take place in the concrete. So, it is necessary to know the value of the residual mechanical properties after creep effect.The work which concerns only with basic creep (where there is no drying), includes an experimental part and a numerical part. In the experimental part, compressive and tensile (Brazilian) creep tests were carried out on the concrete using different loading levels that applied at different ages of concrete.At the end of each creep test a strength test was carried out on the same concrete for determining the creep effect on the residual mechanical properties. To evaluate the effect of quasi-instantaneous loading in compression and then in tension, some preloading compressive tests were carried out. These tests were accomplished especially to distinguish the effect of quasi-instantaneous effect from that of creep. A quasi-instantaneous compressive test was achieved until a given level and then a Brazilian strength test was carried out to evaluate the effect of compressive preloading on the tensile strength.Numerically, a mesoscopic model was developed for simulating creep and mechanical properties after creep had taken place. The reason for which this mesoscopic technique was used is related to the fact that under the considered loading level of concrete, only the cement paste creeps while the aggregate act as an obstacle to this creep. As a result, tensile stresses arise at cement past-aggregate interface leading to micro cracks at this zone. With this technique, creep strain can generate some damage at mesoscale without any coupling between creep and damage as in sence phenomenological models

Residual concrete strength after sustained load: Experimental results and modelling approach

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Experimental Study of the Creep Effect on the Mechanical Properties of Concrete

Available researches regarding the effect of a sustained load on concrete are limited and sometimes contradictory. In the specific context of prestressed concrete and more generally for all other concrete structures, the effect of creep on the residual mechanical properties of concrete must be closely studied in order to accurately estimate the residual load capacity of a structure. In this study, therefore, sealed concrete specimens were subjected to sustained compressive and tensile loads; then, at the end of each creep test, the mechanical properties were investigated. Results revealed that when applied at a young age (1 month), the compressive creep load leads to an improvement in both compressive strength and elastic modulus. Conversely, when the load is applied at a later age (3 months), the creep strain acts to lower strength while it has almost no effect on the elastic modulus. The tensile creep was also studied for a single loading age (1 month); creep at this low loading level was found to increase tensile strength yet exerted a negligible visible effect when applied at a high loading level. Hence, the most important conclusion of this study is that the effect of creep on mechanical properties of concrete strongly depends on both loading age and loading direction

Residual concrete strength after sustained load: Experimental results and modelling approach

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