Granular Skeleton Research Articles

Soil microstructural changes induced by suffusion: x-ray computed tomography characterization

Suffusion is one mechanism of internal erosion, which occurs in gap-graded or broadly graded soils when the fine particles are detached and transported by the seepage flow through the void space formed by the granular soil skeleton. Suffusion is therefore a particle scale mechanism. During this microscale, the initial soil fabric may change due to both fines migration and coarse grains rearrangement, leading to an increase/decrease of global/local porosity and hydraulic conductivity, besides of a probable appearance of heterogeneity, which can, in turn, impact the mechanical behaviour of the eroded soil. In the literature, suffusion test results give only a macroscopic point of view and fail to quantify the effect of suffusion at the scale of the soil's induced heterogeneities. In this paper, x-ray tomography is used to get microscopic observations of soil sample microstructure evolution during a suffusion test. The results reveal that suffusion is not a homogeneous process; the removal of fine particles takes place mainly around the soil sample circumference leading to a higher void ratio at the periphery. Besides, the inter-granular void ratio decreases significantly but almost uniformly throughout the sample owing to the progressive collapse and reorganization of the coarse grains induced by the loss in fines.

Micromechanics of Liquefaction in Granular Materials

A discrete element method (DEM) implementation is developed to study the micromechanics of liquefaction in granular materials. In a liquefaction event, the pore water acts as a cushion between the grains, reducing the contact and friction forces and the overall soil strength. The proposed model reproduces this phenomenon by introducing the effect of pore water as a constraint over the DEM particles' mechanics. The DEM particles will suffer resistance to any displacement changing the pore volume, which takes into account the very small compressibility of water. It is found that this constraint is enough to simulate soil liquefaction under quasistatic deformation. Finally, it is shown that the initial density of the granular skeleton, defined by the number of contacts between grains, plays a critical role in determining if the soil will liquefy or not. This critical value opens the possibility of treating liquefaction in soils as a bifurcation problem.

A Theoretical Packing Density Model (TPDM) for ordered and disordered packings

In previous papers, the 4-parameter Compressible Packing Model (CPM) was theoretically developed and validated for the packing density of binary mixtures of spheres and crushed aggregate particles. The 4-parameter CPM is now called the Theoretical Packing Density Model (TPDM). The TPDM is a self-consistent packing density model which is now validated for both disordered and ordered packings. Thanks to the compaction index K which is representative of the packing process efficiency and thanks to the critical cavity size ratio from which the loosening effect appears, the TPDM is successfully applied to densest binary crystalline structures (K = 100) and to disordered ternary mixtures of spherical, round aggregate and crushed aggregate particles (from K = 4.7 to K = 15). The critical cavity size ratio can be selected in the range between 0 for irregular particles with strong surface roughness and 0.2 for frictionless spherical particles. The TPDM predictions are excellent for the peak packing density obtained for each of the densest binary crystalline structures identified. The TPDM also proves its efficiency for disordered ternary mixtures from the analysis of 408 results published in the literature when compared with the PAL (Prior, Almeida, Loureiro) model and with the 3-parameter Particle Packing Model (3PPM, Kwan et al.). The question of optimization of ternary mixtures is finally addressed. A simplified model is proposed for the purpose of building the densest ternary mixture. The analytical expressions for the highest packing density and for the corresponding volume fractions of each granular class are established. Thanks to its theoretical wall effect and loosening effect functions, the TPDM appears to be a useful tool for optimizing the granular skeleton in all circumstances: whatever its packing process (simple pouring or highly sophisticated packing process) and whatever the material (crushed aggregate particles with strong surface roughness or frictionless spherical particles).

Storage of aerobic granular sludge embedded in agar and its reactivation by real wastewater.

Aerobic granular sludge (AGS) was preserved using an agar embedding method to maintain its stability. No obvious damage was imposed on the granular appearance during 30 days of cold and dry storage, but the granular microstructure had an uneven surface with a large number of holes. The results were consistent with the extinction of microbial communities and the monitored consumption of extracellular polymeric substances, in which granular specific oxygen utilization rate and mixed liquor volatile suspended solids/mixed liquor suspended solids ratio, respectively, decreased by 72.4% and 62.5% during storage. A mass conversation calculation indicated that the loss of granular mass was 1.6393 g. An offensive odour was smelled during storage, and the results indicated that a material transformation and mitigation were involved between AGS and the gas phase. Although the granular structure was destroyed to a certain extent, no obvious damage was imposed on the granular skeleton during storage. After it was aerated again after a feeding with real wastewater, the residual skeleton served as a carrier for the rapid proliferation of microorganisms, and good granular properties were obtained after 11 days of reactivation.

Effects of adding sisal and glass fibers on the mechanical behaviour of concrete polymer

In this study, we investigated the influence of the addition of sisal and glass fibres on the mechanical properties of polymer concrete (PC). These types of concrete are used in many modern civil engineering applications. The prismatic specimens sized according to ASTM C580-02 were elaborated with a PC constituted by 14% constant mass of polyester resin matrix, a granular skeleton based on sand and powder marble. The reinforcement with 60% sand and 26% marble powder adopted in this investigation is the best formulation found in previous authors work. This composition was reinforced by 1 and 2% of sisal and glass fibres, the first one having lengths of 6 mm or 12 mm, however, the second unidirectional cut into bands. These specimens were subjected to 3-point bending monotonic loading. The results obtained were discussed and compared with those obtained for control beams without fibres reinforcement. It is important to note that the incorporation of the glass fibre contributes to an increase of the ultimate load of the polymer composite material produced, however, the addition of the sisal fibre lead to its decreases. In addition, the incorporation of 2% of sisal fibre having 6 mm length leads to a reduction of 26% of the mass of the specimens.

Effects of adding sisal and glass fibers on the mechanical behaviour of concrete polymer

In this study, we investigated the influence of the addition of sisal and glass fibres on the mechanical properties of polymer concrete (PC). These types of concrete are used in many modern civil engineering applications. The prismatic specimens sized according to ASTM C580-02 were elaborated with a PC constituted by 14% constant mass of polyester resin matrix, a granular skeleton based on sand and powder marble. The reinforcement with 60% sand and 26% marble powder adopted in this investigation is the best formulation found in previous authors work. This composition was reinforced by 1 and 2% of sisal and glass fibres, the first one having lengths of 6 mm or 12 mm, however, the second unidirectional cut into bands. These specimens were subjected to 3-point bending monotonic loading. The results obtained were discussed and compared with those obtained for control beams without fibres reinforcement. It is important to note that the incorporation of the glass fibre contributes to an increase of the ultimate load of the polymer composite material produced, however, the addition of the sisal fibre lead to its decreases. In addition, the incorporation of 2% of sisal fibre having 6 mm length leads to a reduction of 26% of the mass of the specimens.

A mix-design method for lightweight aggregate self-compacting concrete based on packing and mortar film thickness theories

This paper puts forward a simple mix design method for lightweight aggregate self-compacting concrete (LWASCC) based on the packing and mortar film thickness (MFT) theories. The mix design method is composed of two simple stages: optimization of granular skeleton and optimization of cementitious materials composition. The former is conducted according to ASTM C 29 by packing different amounts of coarse and fine aggregates. The latter is carried out by using the minimum water requirement method. To verify its applicability for LWASCC, five mixture proportions with varying MFT (i.e., 1.4, 1.6, 1.8, 2.0 and 2.2mm) were designed. The results showed that the proposed mix method is an effective method to develop LWASCC. The flow-ability increases with the increase of MFT. The compressive strength at 28d and 56d shows a contrary variation tendency compared with its early ages, which is attributed to the stiffness difference between aggregates and mortar at different age. The splitting tensile strength of MT-1.4 is higher than that of MT-1.6. With the increase of MFT ranging from 1.6mm to 2.2mm, the splitting tensile strength increases due to the decrease in the quantity of aggregate content and the positive effect of MFT on the aggregate/paste interface.

Optimal strengthening of particle-loaded liquid foams.

Foams made of complex fluids such as particle suspensions have a great potential for the development of advanced aerated materials. In this paper, we study the rheological behavior of liquid foams loaded with granular suspensions. We focus on the effect of small particles, i.e., particle-to-bubble size ratio smaller than 0.1, and we measure the complex modulus as a function of particle size and particle volume fraction. With respect to previous work, the results highlight a new elastic regime characterized by unequaled modulus values as well as independence of size ratio. A careful investigation of the material microstructure reveals that particles organize through the network between the gas bubbles and form a granular skeleton structure with tightly packed particles. The latter is proven to be responsible for the reported new elastic regime. Rheological probing performed by strain sweep reveals a two-step yielding of the material: The first one occurs at small strain and is clearly attributed to yielding of the granular skeleton; the second one corresponds to the yielding of the bubble assembly, as observed for particle-free foams. Moreover, the elastic modulus measured at small strain is quantitatively described by models for solid foams in assuming that the granular skeleton possesses a bulk elastic modulus of order 100 kPa. Additional rheology experiments performed on the bulk granular material indicate that this surprisingly high value can be understood as soon as the magnitude of the confinement pressure exerted by foam bubbles on packed grains is considered.

Multi-physical properties of a structural concrete incorporating short flax fibers

An experimental investigation was undertaken on the physical characterization of a flax fiber-reinforced concrete (FFRC) in both fresh and hardened state. The objective of this study is to provide guidance for the mix-design of these FFRC. The study was conducted from two points of views: improving the workability of the concrete in a fresh state and improving the flexural strength in the hardened state. Several parameters have been studied independently as fiber length, fiber content, or the paste content. The characterization of flax fibers highlighted a high water absorption capacity which must be taken into account for the concrete mix-design. In addition, the flax fibers significantly impact the compactness of granular skeleton. For the characterization of concrete, testing in the fresh state showed a significant decrease of the workability of concrete with the addition of flax fibers. However, the use of shorter fibers, allows to reduce this damaging influence on the fresh concrete workability. Moreover, increasing the paste content allows compensating this fluidity loss. In the hardened state, the increase of the fiber content enhances the flexural strength, but a decrease of the compressive strength is observed. A greater porosity of the concrete was also observed with the incorporation of flax fibers. An increase in porosity was also observed when increasing the paste content.

A behavioural framework for fibre-reinforced gravel

The mechanical behaviour of granular materials is known to be influenced by the addition of fibres. However, most previous research has been carried out on materials with relatively small grains (sands), and its application to larger grains is not well documented. This paper reports an investigation into the mechanical behaviour of a fibre-reinforced granular material with a relatively large grain size corresponding to one-third and one-fifth scale railway ballast. The investigation was carried out by means of triaxial tests incorporating a full-field, image-based deformation measurement technique; this enabled detailed observations to be made of each triaxial test specimen during shearing. The test data demonstrate the benefits of random fibre reinforcement for aggregates that have a relatively large grain size. Analysis taking into account the effect of fibre tension on the effective stresses experienced by the granular skeleton provides new insights into the mechanisms of reinforcement in larger sized granular materials.

Elasticity of particle-loaded liquid foams.

Mixing solid particles with liquid foam is a common process used in industry for manufacturing aerated materials. Desire for improvement of involved industrial processes and optimization of resulting foamed materials stimulates fundamental research on those complex mixtures of grains, bubbles and liquid. In this paper, we generate well-controlled particle-loaded liquid foams and we determine their elastic behavior as a function of particle size (6-3000 μm) and particle volume fraction (0-6%). We focus on both the elastic modulus exhibited by the material at small strain and the strain marking the end of the linear elastic regime. Results reveal the existence of a critical particle-to-bubble size ratio triggering a sharp transition between two well-defined regimes. For small size ratios, the behavior is governed by the mechanical properties of the solid grains, which have been proved to pack in the shape of a foam-embedded granular skeleton. In contrast, bubbles elasticity prevails in the second regime, where isolated large particles contribute only weakly to the rheological behavior of the foamed material. The modeling of elasticity for each regime allows for this transition to be normalized and compared with previously reported particle size-induced effects for foam drainage (Haffner et al. J. Colloid Interface Sci., 2015, 458, 200-208) and solid foam mechanics (Khidas et al., Compos. Sci. Technol., 2015, 119, 62-67). This highlights that rheology and the other properties of particle-loaded foams are subjected to the same size-induced morphological transition.

Micro-mechanical investigation of the effect of fine content on mechanical behavior of gap graded granular materials using DEM

In this paper, we present a micro-mechanical study of the effect of fine content on the behavior of gap graded granular samples by using numerical simulations performed with the Discrete Element Method. Different samples with fine content varied from 0% to 30% are simulated. The role of fine content in reinforcing the granular skeleton and in supporting the external deviatoric stress is then brought into the light.

Calendering of free-standing electrode for lithium-sulfur batteries with high volumetric energy density

The lithium-sulfur (Li-S) batteries are potential candidates for next-generation power sources with very high-energy-density. The rational integration of three dimensional (3D) carbon skeletons into sulfur cathode is an efficient and effective route to full use of conductive scaffolds with chemical and mechanical stability and ability to accommodate large amount of active materials. Herein, a 3D free-standing electrode composed of nitrogen-doped porous graphene/sulfur composite granular and super-long carbon nanotube (CNT) skeleton is proposed for Li-S batteries with high volumetric energy density. With a facile calendering process, the mechanical properties, bulk conductivity, and the pore distribution of the flexible graphene@S-CNT electrodes were tuned. The high rate capability, long-life stability, and high volumetric energy density of Li-S batteries are highly depended on the nanostructure evolution of composite carbon/sulfur electrode. A high rate capability of 40% retention of discharge capacity of 2.0 C against that at 0.05 C, a high cyclic stability of 0.1%/cycle within 180 cycles, and a high volumetric energy density over 850 Wh L−1 are demonstrated with the pressed graphene@S-CNT electrode. This work unfolded a general strategy to modulate the bulk structure of electrodes, which is an efficient route towards Li-S batteries with high volumetric energy density.

Influence of particle lattice effect on stability of suspensions: application to self-consolidating concrete

One of the parameters influencing the stability of a granular skeleton in a fluid is particle-size distribution (PSD). This phenomenon partially originates from the particle lattice effect (PLE) where in a given fluid the sedimentation behavior of one particle or a group of particles is modified in the presence of other particles. The PLE is of particular interest for the design of highly flowable concrete in which given the high fluidity of the paste, segregation of coarse aggregate is of concern. In the present study, the stability of several groups of bidisperse and polydisperse spherical glass particles (3–19 mm in diameter) suspended in limestone filler pastes designed with different rheological properties is investigated. Test results show that regardless of the PSD in the suspension, the PLE of any size-class is proportional to the volume fraction of such class. The main contribution of PLE to the enhancement of the stability of the overall system can be attributed to the stabilization of individual fine classes as the volume fractions of such classes are increased, instead of simply the interaction between different particle classes. Two indices are proposed to quantify the PLE potential of a given PSD and to predict the risk of segregation of a mixture of particles suspended in a yield stress fluid. The predictions made by the segregation index are shown to be feasible to apply to self-consolidating concrete (SCC) mixtures.

Micromechanical modelling of bituminous materials’ complex modulus at different length scales

The aim of this work is to establish a multi-scale modelling technique usable in the study of the complex viscoelastic properties of asphalt mixes. This technique is based on a biphasic approach. At each scale, the heterogeneous media is considered as a two-phase material composed of granular inclusions with linear elastic properties and a matrix of bituminous materials exhibiting linear viscoelastic behaviour at small strain values. In this approach, the homogeneous equivalent properties of biphasic composites are transferred from one scale of observation to the next, higher scale of observation. The viscoelastic properties of the matrix and the elastic properties of the aggregates serve as the input parameters for the numerical models. The generalised Maxwell rheological model is used to describe the viscoelastic behaviour of the matrix. Thanks to the rheological properties of bitumen and the elastic properties of the aggregates, the viscoelastic properties of mastic, mortar and hot mix asphalt (HMA) as bituminous composites can be, respectively, estimated using a micromechanical finite element model. Random inclusions of varying sizes and shapes are generated in order to construct the granular skeleton. A cyclic loading was imposed on the top layer of the digital model. The dynamic modulus of the pre-cited bituminous composites, obtained from the presented multi-scale modelling process while passing from the bitumen to the HMA scale, is validated by comparison with experimental measurements when possible. Concerning our results, we have found that at low temperature (−10 °C), the predicted dynamic modulus is satisfactorily comparable to the experimental measurements. On the other hand, an acceptable gap between predicted numerical results and experimental data takes place when the temperature increases.

Influence of cement matrix porosity on the triaxial behaviour of concrete

This article focuses on the influence of the cement matrix porosity on the triaxial behaviour of concrete in the presence of high confining pressures. It offers new experimental results on two concretes, respectively a high performance concrete (HPC) and a low performance concrete (LPC). These results complement recent studies on ordinary concrete (OC), in using the same large-capacity triaxial press. This paper serves to better characterise the behaviour of these 3 concretes, whose porosities are very different in terms of volume and size; moreover, it emphasises that the contribution of cement matrix strength (mainly linked to its porosity) decreases at high confining pressures and that concrete behaviour is governed by the granular skeleton, even when the capillary porosity is low or the entrapped air porosity is high. To reach this level of residual behaviour, higher confining pressures are needed for HPC, even if the amount of porosity compacted is smaller.

Environmental evaluation of concrete made from recycled concrete aggregate implementing life cycle assessment

Recycled concrete aggregates from demolition constitute one of the largest waste streams within the developed countries. These study aims to quantify environmental impacts associated with mixing compositions of concrete made of waste materials by using LCA. Environmental performances of natural, recycled and mixed 20-mm concrete samples, formulated with the same mechanical strength regarding the functional unit, were evaluated. Eight millimeter concrete samples, formulated with natural or recycled (concrete or terracotta brick) aggregates – with the same volume composition of the granular skeleton for apparent concrete application regarding the functional unit – were also studied. The LCA results are presented using various impact assessment methods, according to both EN 15804 and NF P 01–010 standards. Recycled samples present good environmental behavior, even if recycled materials (sand and aggregates) involve different operations (crushing against extraction, etc.). The terracotta 8-mm concrete sample presents low environmental impacts in comparison with the other 8-mm concrete samples. This sample exhibits a low aggregate density, which decreases transport impacts, and good mechanical strengths, which improves its lifetime.

Self-Compacting Concrete Formulation by Measuring Excess Paste Volume

Concrete is composed of a liquid phase (paste) and of a solid phase (aggregates with fixed gravel/sand ratio), the concrete self-compacting properties come necessarily from those of the paste. The present research is the continuity of a first phase of the testing already conducted, which resulted in obtaining an optimal self-compacting cement paste composition. This paste will be used to prepare a self-compacting concrete (SCC), while passing from the scale of the cement paste to that of the concrete, by injecting wet aggregate to the self-compacting paste. The excess paste theory was used to determine the thickness of the paste coating each aggregate with a given diameter of constituting granular skeleton, then generalized for the determination of the quantity of total paste allowing the flow of the concrete by decreasing frictions between the grains of its granular skeleton. This approach was also experimentally validated. The influence of the granular distribution was minimized by the use of the approach based on the determination of the average diameter of the aggregates. This required the determination of a homothetic factor “K” similar for all concretes with different aggregate grading. Formulation of a self-compacting concrete passes initially by the determination of a sufficient quantity of paste allowing its flow without frictions between its aggregates and to balance the mixture by the quantity of water retained by the aggregates. The self-compacting concrete characteristics would come from those of the cement paste which composes it.

OPTIMIZATION OF PERCENTAGES OF STEEL AND GLASS FIBER REINFORCED CONCRETE

Cementitious matrices are the fragile materials that possess a low tensile strength. The addition of fibers randomly distributed in these matrices improves their resistance to cracking, substantially. However, the incorporation of fibers into a plain concrete disrupts the granular skeleton and quickly causes problems of mixing as a result of the loss of mixture workability that will be translated into a difficult concrete casting in site. This study was concerned on the one hand with optimizing the fibers reinforced concrete mixes in the fresh state, and on the other hand with assessing the mechanical behaviour of this mixture in the hardened state, in order to establish a compromise between the two states . In this paper optimization of fibers by using different percentages in steel and glass fiber reinforced concrete of grade M 70 have been studied. It optimizes 1.5% for steel Fiber content and 1% for glass fiber content by the volume of cement is used in concrete.

Estudio para la optimización de la composición de un HACFRA (hormigón autocompactante reforzado con fibras de acero) estructural

The interest of HACFRA (self compacting concrete reinforced with steel fibers), is the combination of the residual strength increase and cracking decrease compared to plain concrete by the introduction of steel fibers in the mass with the advantages of the self-compacting. The paper presents an analysis of the influence of different components of the HACRFA and provides their selection, refered to the granular skeleton and to different steel fiber types and amount, in order to obtain an optimization of its features and structural behavior.