Effect of different types of fibers on the microstructure and the mechanical behavior of Ultra-High Performance Fiber-Reinforced Concretes

International audienceThis study investigates the effect of adding different types of fibers on the microstructure and the mechanical behavior of cementitious composites, in particular on UHPC. These fibers were distinguished mainly by their differing nature (steel, mineral and synthetic), their dimensions (macroscopic or microscopic), and their mechanical properties. The microstructure of the specimens was examined by using SEM observation and by measuring the porosity, the intrinsic permeability and the P-wave velocity. The mechanical behavior under loading has been studied using a uni-axial compression test which combines the gas permeability and the acoustic emission (AE) measurement. This work focuses on the cracking process under mechanical loading. The experimental results show that the fiber has a relatively slight influence on the compressive strength and elastic modulus of concrete, except for the steel fiber which improves the strength because of its intrinsic rigidity. However, The addition of fiber significantly reduces the lateral strain at peak loading and increases the threshold of initial cracking (σk-ci) and that of unstable cracking (σk-pi). Therefore, the fibers clearly restrain the cracking process in concrete under the mechanic loadin

Effects of thermal damage on physical properties and cracking behavior of ultrahigh-performance fiber-reinforced concrete

International audienceIn this work, we study the impact of thermal damage on the physical and mechanical properties of ultrahigh-performance fiber-reinforced concrete (UHPFRC), especially on their cracking process under compressive loading. Four mixtures of UHPFRC were prepared using identical composition but reinforced with different types of fibers: mineral fibers (Steel or Wollastonite) or organic fibers (PP or PVA) and compared with that without fibers (UHPC). To induce a thermal damage on UHPFRC, the samples were subjected to temperatures ranging from 150 to 400 °C. After each degradation stage, the gas permeability and the P-wave velocity were measured. The mechanical behavior under loading has been studied using a uniaxial compression test which combines the gas permeability and the acoustic emission measurement. The results show that the melting of organic fibers at approximately 180 °C builds a tunnel across the cement paste and increases brutally the gas permeability. At 400 °C treatment, a decrease of compression strength by 30 % and of Young modulus by approximately 60 % was observed. However, we can see that the thermal damage results a decrease in the threshold of initial cracking (rk-ci) and that of unstable cracking (rk-pi), and this can be explained by the initiation of new cracks and their coalescence

Durabilité d’un ciment composé à base de mâchefer de Tefereyre (Niger): absorption capillaire, porosité accessible à l’eau et attaque acide

In this study, we evaluate the durability characteristics of composite cement with coal bottom ash produced by SONICHAR (Société Nigérienne de Charbon). The study firstly consists in the determination of the pozzolanic activity index of the coal bottom ash and the optimum rate to use in the composite cement. Thereafter an evaluation of capillary absorption, porosity accessible to water and resistance to sulfuric and nitric acid attack of mortar prepared made with the composite cement is performed. The results showed that the coal bottom ash powder obtained by grinding can partially substitute Portland cement at a rate of 15% enable to develop a CEM II A composite cement according to EN 197-1 Standard.Dans la présente étude, nous évaluons les caractéristiques vis-à-vis de la durabilité d’un ciment composé à base de mâchefer de charbon produit par la SONICHAR (Société Nigérienne de production d’énergie à base de  charbon minéral). L’étude consiste d’abord en la détermination de l’indice d’activité pouzzolanique du mâchefer et du taux optimal à utiliser pour mettre en place un ciment composé. Par la suite une évaluation du taux d’absorption capillaire, de la porosité accessible à l’eau et à la résistance à l’attaque acide sulfurique et nitrique de mortier confectionné à base du ciment composé élaboré est effectuée. Les résultats montrent que la poudre de ce mâchefer obtenu par broyage peut substituer partiellement le ciment Portland à un taux de 15% permettant d’élaborer un ciment composé de type CEM II A selon la norme EN 197-1

Transfer behaviour and durability of cementitious composites containing polycarbonate plastic wastes

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Conception, caractérisation physico-mécanique et durabilité de nouveaux matériaux de construction à caractère environnemental (application dans la technologie des constructions)

Notre travail de recherche s inscrit dans le contexte du développement durable. Il étudie les potentialités de valorisation de déchets plastiques comme nouvelle source de matière pour le secteur de la construction et tente d élaborer de nouveaux matériaux composites incorporant des granulats plastiques. Pour élaborer ces composites, trois types de plastiques (PC, PET1 et PET0.1) ont été introduits dans des mortiers cimentaires par substitution à des pourcentages volumiques définis de sable (3%, 10%, 20% et 50%). Les mortiers obtenus ont été caractérisés par l étude de leurs propriétés microstructurales, physiques et mécaniques. Leur durabilité a été également étudiée à l aide d essais de dégradation physique, thermique et chimique. Les résultats obtenus montrent que l introduction des granulats plastiques dans les mortiers cimentaires entraîne une modification de la microstructure, une augmentation de la porosité connectée et de la perméabilité, et une diminution de la résistance en compression et en traction. En contrepartie de cette baisse de résistance, l incorporation de granulats plastiques donne des matériaux plus légers, significativement plus ductiles, qui manifestent une grande capacité de déformation et une faible aptitude à la fissuration de retrait. Ces mortiers composites présentent également un réel intérêt du point de vue de l isolation thermique et acoustique car leur conductibilité est plus faible et la propagation des ondes acoustiques y est ralentie. L ensemble des résultats présentés permet de conclure que dans de nombreuses applications où la résistance à la fissuration due aux déformations imposées est un critère de performance, l incorporation de granulats plastiques peut constituer une solution pertinente pour améliorer la durabilité. Les bétons de granulats plastiques représentent en définitive une voie réelle de valorisation du plastique en fin de vie.Our research is part of sustainable development. It studies the potential use of waste plastics as a new source of material for construction sector and attempts to develop new composite materials incorporating plastic aggregates. To elaborate these composites, three types of plastic (PC, PET1 and PET0.1) were introduced into cement mortars by replacing defined volume percentages of sand (3%, 10%, 20% and 50%). The mortars obtained were characterized by studying their microstructural, physical and mechanical properties. Their durability has also been studied using the tests of physical, thermal and chemical degradation. The results obtained show that the introduction of plastic aggregates in cementitious mortars causes a change in the microstructure, an increase in connected porosity and permeability, and a decrease in compressive and tensile strength. In counterpart of this reduction in strength, the incorporation of plastics aggregates gives lighter materials, significantly more ductile, which show a large deformation capacity and low ability to shrinkage cracking. These mortar composites also show a real interest in terms of thermal and acoustic insulation because their conductivity is lower and the acoustic wave propagation is slowed. All results presented allow concluding that in many applications where resistance to cracking due to imposed deformations is a performance criterion, the incorporation of plastic aggregates can be an effective solution to improve the durability. The Concretes with plastic aggregates represent ultimately a real path for plastic valorization at the end of life.RENNES-INSA (352382210) / SudocSudocFranceF

Effect of MgO-Based Expansive Agent on the Cement-Based Mortar Behavior

International audienceAs shrinkage can affect the concrete durability, several solutions were designed to limit this deformation. One of them consists to add expansive agents like MgO-based products. The aim of the present research work is to better understand the effect of MgO-based expansive agent on the self-healing potential of cementitious materials. At 28 days old, Portland cement mortars with different contents of expansive agent (0%, 5% and 10%) are pre-cracked by means of splitting test and stored under water. Self-healing is monitored using 2D and 3D tests: optical microscope observations and water permeability measurements respectively. Mechanical resistances and deformations of the studied mortars are also monitored and their microstructures are characterized by means of thermogravimetric analysis. The results show that mortars containing MgO-based expansive agent present a better self-healing capacity. It can be due to its swelling capacity decreasing/deleting shrinkage deformations and the formation of supplementary products like brucite. © 2021, RILEM

Delayed Deformations of Na- and K- Sulphates Activated Blast-Furnace Slag Mortars

International audienceAlkali sulphates are chemical products that can be used to activate and improve the performance of mineral additions, like Blast-Furnace Slag (BFS) and Fly Ash (FA). This study investigates the effect of two alkali sulphates (sodium sulphate and potassium sulphate) on delayed deformations of mortars containing a high content of BFS. Autogenous shrinkage is monitored after 2 days and basic creep tests are conducted on 28-day-old specimens loaded at 30% of their compressive strength. Their microstructure is characterized by using several experimentations, such as internal relative humidity measurements, thermogravimetry analysis and a water porosity test. A numerical model in terms of C-(A)-S-H content evolution is applied to quantify the degree of hydration advancement. The results show that the use of sodium sulphate leads to a higher autogenous shrinkage, but to a lower specific creep. This is linked to the fact that the mortars activated with sodium sulphate show a faster hydration rate and a lower porosity, which results in higher capillary pressures. However, this also limits the movement of water required for creep development. © 2023, The Author(s), under exclusive license to Springer Nature Switzerland AG

Self-healing Capacities of Mortars with Crystalline Admixtures

International audienceThe aim of this research study consists of determining the self-healing capacities of cement-based materials incorporating Crystalline Admixtures (CA) such as permeability and shrinkage reducers. Mortars with three different types of CA were studied. At 28 days old, specimens were cracked by means of a three-point bending test to obtain a single crack characterized by a width varying from between 120 and 200 μ m. Thereafter, the specimens were kept under water and the self-healing process was monitored by means of the crack width and area measurements at 35 and 120 days after cracking. From these first experimental results, it appears that specimens without CA and with calcium sulphate are characterized by a higher healing rate. This difference of behavior between the mortar mixtures is probably related to their microstructure. To confirm this hypothesis, their hydration products and their porosity were characterized at 28 days. © 2022, The Author(s), under exclusive license to Springer Nature Switzerland AG

Beneficial use of waste plastics as fine aggregate substitutes in civil engineering cementitious materials

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Properties of cementitious materials incorporating thermoplastic wastes as aggregates

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