A Numerical Model for the Optimization of Concentrated Suspensions for Sustainable Concrete Proportioning

Concrete has a large environmental impact due to CO2 emissions related to cement manufacturing and the consumption of natural aggregates. More sustainable concretes can be developed, replacing part of the cement with mineral admixtures or natural aggregates with recycled ones. However, recycled materials are less regular than natural ones, and using new deposit changes concrete properties, which necessitates the re-optimization of mixture proportions. For small/medium-size waste deposits, the expensive experimental work needed to adapt concrete formulation containing these particular wastes is not profitable, which prevents from their valorization. The aim of this study is to develop a numerical model to optimize the mixture proportions of concentrated suspensions based on very limited entry data. In the model, spheres of small radii are seeded in the porosity and allowed to swell until reaching a target radius/density. On monosized suspensions, it is shown that the ratio between the number of random displacements to the number of particles varies with density, following a classical viscosity–density relationship, which allows identification of the packing fraction. The model is extended to bidisperse systems, with the viscosity of the whole suspension calculated by combining the viscosities of each granular class. The model is applied to bidisperse systems of size ratios 4:1 and 2:1 with varying proportions of large particles. The optimum proportions identified numerically are compared successfully to experimental results from the literature

Modélisation DEM de l'agglomération des fines minérales en suspension: influence du gradient de vitesse, de la minéralogie et du pH sur la taille des agglomérats.

National audienceABSTRACT. A discrete element model (DEM) is used to study the deformation, restructuring and fragmentation of agglomerates of fine particles subjected to velocity gradient and to characterize, at steady state, the solid fraction of the flocs. The contact, van der Waals, electrostatic and hydrodynamic forces are considered. The results show that in the steady state, the average size of flocs decreases with increasing shear rate and the fractal dimension of flocs is independent of the velocity gradient in the range of shear rates studied. Moreover, the average flocs size depends on the mineralogy of fines and increases with the Hamaker constant. The pH of solution has little influence on the average size and morphology of the flocs in the pH range examined. MOTS-CLÉS : fines minérales, demande en eau, dimension fractale, cisaillement, DEM.RÉSUMÉ :Une modélisation par éléments discrets (DEM) est utilisée afin d'étudier la déformation, la restructuration et la fragmentation des agglomérats de fines minérales en suspension soumis à un gradient de vitesse et de caractériser, à l'équilibre, la compacité de ces flocs. Les forces de contact, de Van der Waals, électrostatiques et hydrodynamiques sont prises en compte. Les résultats montrent, qu'à l'équilibre, la taille moyenne des flocs diminue avec l'augmentation du taux de cisaillement tandis que la dimension fractale des flocs est dépend peu du gradient de vitesse dans la gamme des taux de cisaillement étudiés. Par ailleurs, la taille moyenne des flocs dépend de la minéralogie des fines et augmente avec la constante de Hamaker. Le pH de la solution a très peu d'influence sur la taille moyenne et la morphologie des flocs pour les conditions de simulations utilisées

Control of effective water in recycled aggregate concrete using power curves of the mixer

International audienc

Evolution de l’eau efficace lors du malaxage du béton de granulats recyclés

ACTI, ACTN, AFF, COM, INVInternational audienc

A Numerical Model for the Optimization of Concentrated Suspensions for Sustainable Concrete Proportioning

International audienceConcrete has a large environmental impact due to CO2 emissions related to cement manufacturing and the consumption of natural aggregates. More sustainable concretes can be developed, replacing part of the cement with mineral admixtures or natural aggregates with recycled ones. However, recycled materials are less regular than natural ones, and using new deposit changes concrete properties, which necessitates the re-optimization of mixture proportions. For small/medium-size waste deposits, the expensive experimental work needed to adapt concrete formulation containing these particular wastes is not profitable, which prevents from their valorization. The aim of this study is to develop a numerical model to optimize the mixture proportions of concentrated suspensions based on very limited entry data. In the model, spheres of small radii are seeded in the porosity and allowed to swell until reaching a target radius/density. On monosized suspensions, it is shown that the ratio between the number of random displacements to the number of particles varies with density, following a classical viscosity–density relationship, which allows identification of the packing fraction. The model is extended to bidisperse systems, with the viscosity of the whole suspension calculated by combining the viscosities of each granular class. The model is applied to bidisperse systems of size ratios 4:1 and 2:1 with varying proportions of large particles. The optimum proportions identified numerically are compared successfully to experimental results from the literature

Influence d’Absorption d'Eau des GBR sur l’Eau Efficace des Bétons Recyclés

ACTI, ACTN, AFF, COM, INVInternational audienc

Swelling and shrinkage of plant aggregates: Experimental and treatment effect

International audienceThe present work focuses on providing a better evaluation and understanding of the swelling and shrinkage behavior of plant aggregates in response to moisture variation. The morphology of the aggregates is analyzed using an optical microscope to measure the percentage of swelling and shrinkage response to the relative humidity variation. The aggregates are exposed to a relative humidity cycle with varying stages allowing swelling and shrinking of the aggregates. An investigation on the effect of salt treatments with NaHCO3 and NaCl, is also conducted to evaluate their impact on the swelling behavior of the plant aggregates studied (wheat straw, rapeseed straw, sunflower bark and sunflower pith). The results show a high percentage of swelling of the sunflower pith, followed by wheat straw, rape straw, and sunflower bark. Treatment with NaHCO3 seems to be effective to reduce the swelling and shrinkage of aggregates with a decrease of 53% unlike NaCl, which has no effect on the change in chemical components. The effect of the lime binder is also studied in order to enhance the understanding of the behaviour and design of biocomposites

Influence of the mix composition on the thixotropy of 3D printable mortars

International audienc

Impact of the initial moisture level and pre-wetting history of recycled concrete aggregates on their water absorption

Initial moisture and pre-wetting method influence on the water absorption of recycled concrete aggregates (RCA) have been studied experimentally. For this purpose, RCA were pre-wetted by three methods: soaking under partial vacuum (simulating long term wetting), soaking under atmospheric pressure, and spraying (simulating short term wetting). The results show that the same initial amount of water in two samples of RCA do not lead to the same total amount of water absorbed by RCA during 5-120 min. The latter depends on the way they have been pre-wetted (either long term or short term). It is suggested that this phenomenon is related to the accessibility filled pores in the different pre-wetting methods. So, the pre-wetting history of RCA could change the amount of total water absorbed by the RCA up to 1%. When mixing and casting concrete are produced with RCA, the corresponding error in the determination of the effective water should have adverse effects on the fresh and hardened concrete characteristics. In this study real applications on mixtures of recycled concrete have been carried out, where the influence of initial moisture content and pre-wetting history of RCA on fresh recycled concrete properties (slump) have been investigated

Water migration during mixing recycled aggregate concrete

ACTI, ACTN, AFF, COM, INVInternational audienc