The simulation of particulate materials packing using a particle suspension model

The behavior of particulate composite materials, such as portland cement concrete, depends to a large extent on the properties of their main constituent—the aggregates. Among the most important parameters affecting the performance of concrete are the packing density and corresponding particle size distribution (PSD) of aggregates. Better packing of aggregates improves the main engineering properties of composite materials: strength, modulus of elasticity, creep and shrinkage. Further, it brings major savings due to a reduction in the volume of binder. A simulation algorithm was developed for the modeling of packing of large assemblies of particulate materials (of the order of millions). These assemblies can represent the real aggregate systems composing portland cement concrete. The implementation of the developed algorithm allows the generation and visualization of the densest possible and loose-packing arrangements of aggregates. The influence of geometrical parameters and model variables on the degree of packing and the corresponding distribution of particles was analyzed. Based on the simulation results, different PSDs of particulate materials are correlated to their packing degree.     

Particle size effect on the strength of rice husk ash blended gap-graded Portland cement concrete

Rice husk ash (RHA) has been used as a highly reactive pozzolanic material to improve the microstructure of the interfacial transition zone (ITZ) between the cement paste and the aggregate in high-performance concrete. Mechanical experiments of RHA blended Portland cement concretes revealed that in addition to the pozzolanic reactivity of RHA (chemical aspect), the particle grading (physical aspect) of cement and RHA mixtures also exerted significant influences on the blending efficiency. The relative strength increase (relative to the concrete made with plain cement, expressed in %) is higher for coarser cement. The gap-grading phenomenon is expected to be the underlying mechanism. This issue is also approached by computer simulation. A stereological spacing parameter (i.e., mean free spacing between mixture particles) is associated with the global strength of the blended model cement concretes. This paper presents results of a combined mechanical and computer simulation study on the effects of particle size ranges involved in RHA-blended Portland cement on compressive strength of gap-graded concrete in the high strength/high performance range. The simulation results demonstrate that the favourable results for coarser cement (i.e., the gap-graded binder) reflect improved particle packing structure accompanied by a decrease in porosity and particularly in particle spacing.     

Linking solid particle packing of Eco-SCC to material performance

The concept of Eco-SCC aims to achieve self-compacting concrete (SCC) design for intermediate compressive strength mixtures that are commonly used in the ready-mix concrete industry. Contrary to many other approaches, in which the reactive cement clinker is replaced by a less reactive component, Eco-SCC involves the reduction of the total powder content. In the approach presented in this paper, the lubricant volume is increased by using a non-reactive filler. An improved particle packing with an enhanced lattice effect can minimize the lubricant demand and enhance the stability of the concrete. The effect of particle-size distribution on stability and performance of optimized Eco-SCC is evaluated. Fresh and hardened properties, including rheological properties, blocking behavior, sedimentation stability, compressive strength development, and drying shrinkage are determined. Test results are correlated to particle packing characteristics of the aggregate and cementitious materials combinations using the gyratory intensive compaction test (ICT) approach. A clear relationship is observed between the particle-packing characteristics and the performance of Eco-SCC.     

A hierarchical approach to simulate the packing density of particle mixtures on a computer

In many fields of materials science it is important to know how densely a particle mixture can be packed. The “packing density” is the ratio of the particle volume and the volume of the surrounding container needed for a random close packing of the particles. We present a method for estimating the packing density for spherical particles based on computer simulations only, i.e. without the need for additional experiments. Our method is particularly suited for particle mixtures with an extremely wide range of particle diameters as they occur e.g. in modern concrete mixtures. A single representative sample from such mixtures would be much larger than can be handled on present standard computers. In our hierarchical approach the diameter range is therefore divided into smaller intervals. Samples from these limited diameter intervals are drawn and their packing density is estimated from a simulated packing. The results are used to “fill” the interstices in the sample from the next larger particle interval. To account for the interaction between particles of different sizes we include larger particles into the sample of smaller ones. The larger ones act as part of the boundary during the packing. Thus we obtain more realistic estimates of how dense a fraction of particles can be packed within the whole mixture. The focus of this paper is on the divide-and-conquer approach and on how the simulation results from the fractions can be collected into an overall estimate of the packing density. We do not go into details of the simulation technique for the single packing. We compare our results to some experimental data to show that our method works at least as good as the classical analytical models like CPM without the need for any experiments.     

Compaction-interaction packing model: regarding the effect of fillers in concrete mixture design

For the design of defined-performance concrete, predicting the material properties of concrete becomes more and more important. To be able to select the right type of fillers and control the water demand in such mixtures, an extension to the compressible packing model was developed to optimize the particle packing of aggregates as well as powders in concrete. Modelling mixtures with particles smaller than 125 μm requires advanced interaction equations, taking due account of surface forces like van der Waals forces, electrical double layer forces and steric forces. In this paper the equations for the newly developed compaction-interaction packing model are presented, including the additional effects of agglomerating particles on the wall and loosening effect. Calculated packing densities are related to the results of compressive strength experiments on 50 mortar mixtures. Higher packing densities leave less space for voids to be filled with water, which reduces the water demand and increases the strength of concrete mixtures. This is shown by the cement spacing concept. The relation between the cement spacing factor and strength can be used as a tool to predict concrete strength in defined-performance concrete mixtures.     

Optimization of a Computer Simulation Model for Packing of Concrete Aggregates

A simulation algorithm was developed for modeling the dense packing of large assemblies of particulate materials (in the order of millions). These assemblies represent the real aggregate systems of portland cement concrete. Two variations of the algorithm are proposed: sequential packing model and particle suspension model. A developed multicell packing procedure as well as fine adjustment of the algorithm's parameters were useful to optimize the computational resources (i.e., to realize the trade-off between the memory and packing time). Some options to speed up the algorithm and to pack very large volumes of spherical entities (up to 10 million) are discussed. The described procedure resulted in a quick method for packing of large assemblies of particulate materials. The influence of model variables on the degree of packing and the corresponding distribution of particles was analyzed. Based on the simulation results, different particle size distributions of particulate materials are correlated to their packing degree. The developed algorithm generates and visualizes dense packings corresponding to concrete aggregates. These packings show a good agreement with the standard requirements and available research data. The results of the research can be applied to the optimal proportioning of concrete mixtures.     

Computer simulation of morphology and packing behaviour of irregular particles, for predicting apparent powder densities

This paper describes a methodology for prediction of powder packing densities which employs a new approach, designated as random sphere construction (RSC), for modelling the shape of irregular particles such as those produced by water atomization of iron. The approach involves modelling an irregular particle as a sphere which incorporates smaller corner spheres located randomly at its surface. The RSC modelling technique has been combined with a previously developed particle packing algorithm (the random build algorithm), to provide a computer simulation of irregular particle packings. Analysis of the simulation output data has allowed relationships to be established between the particle modelling parameters employed by the RSC algorithm, and the density of the simulated packings. One such parameter is η, which is the number of corner spheres per particle. A relationship was established between η (which was found to have a profound influence on packing density), and the fractional density of the packing, fd. Vision system techniques were used to measure the irregularity of the simulated particles, and this was also related to η. These two relationships were then combined to provide a plot of fractional density for a simulated packing against irregularity of the simulated particles. A comparison was made of these simulated packing densities and observed particle packing densities for irregular particles, and a correlation coefficient of 0.96 was obtained. This relatively good correlation indicates that the models developed are able to realistically simulate packing densities for irregular particles. There are a considerable number of potential applications for such a model in powder metallurgy (PM), process control. In combination with on-line particle image analysis, the model could be used to automatically predict powder densities from particle morphology.     

An empirical approach for the optimisation of aggregate combinations for self-compacting concrete

The fresh and hardened properties of self-compacting concrete (SCC) depend on number of factors such as paste composition, paste content, aggregate content, aggregate gradation etc. In the present investigation, the influence of the packing density of aggregates on the properties of SCC was evaluated. Experiments were conducted to measure the packing density for different combinations of aggregates precisely. A ternary packing diagram (TPD) was developed based on the packing density of measured and interpolated data. Considering the limitations in generalising the TPD and the difficulty involved in adopting mathematical models for aggregates, an attempt was made to establish a simple method for the selection of the combination of aggregates resulting in maximum packing density from the particle size distribution of aggregates (represented by the Coefficient of uniformity??C _u). Further, studies were extended to investigate the effect of aggregate packing density on fresh and hardened SCC properties. The results indicate that for a constant paste volume and paste composition, with increase in packing density of aggregates, the fresh properties and the compressive strength of SCC were improved positively. An attempt was also made to identify the influence of 10 different proportions of aggregates having the same packing density on the properties of SCC. The results indicate that at the same aggregate packing density, the fresh concrete properties were influenced significantly by the choice of the aggregate combination, while there was little or no influence on the hardened properties. Furthermore, the experimental data obtained was used for supplementary validation of the existing model (compressible packing model) for predicting the packing density and the fresh behaviour of SCC.     

Defined-performance design of ecological concrete

Energy consumption and CO₂-emission of concrete can be reduced when cement is replaced by secondary materials such as residual products from other industries. However, for the design of such environmentally friendly concretes, predicting its performance is very important. In this article a cyclic design method is presented, which can predict the strength of a concrete mixture based on particle packing technology. In the procedure, the amount of water is estimated from the required workability and calculated packing density. After that, the strength of that mixture is predicted from packing density calculations and the amount of water in the mixture via the cement spacing factor. This cycle is repeated until the mixture composition does not have to be adjusted anymore to comply with the desired performance or strength class. With the presented cyclic design procedure cement contents can be decreased without changing concrete properties in a negative way, thereby saving up to 57 % of Portland cement and reducing CO₂-emission with 25 %. This is shown by experimental results of ecological concrete mixtures tested on compressive strength, tensile strength, modulus of elasticity, shrinkage, creep and electrical resistance. The results confirmed that relationships between cube compressive strength, tensile splitting strength and modulus of elasticity correspond to those for normal concrete. The experimental program showed the possibility to use cube compressive strength as the governing design parameter in the cyclic design procedure for ecological concrete. Furthermore, it is shown how the cyclic design method can be used for defined-performance concrete design.     

Meso-level analysis of moisture flow in cement composites using a lattice-type approach

In this paper, a simple flow model for stimulating mass or heat transfer in heterogeneous materials like concrete is presented. The material is discretized as a regular triangular lattice. The lattice elements are considered as conductive “pipes”. A generated particle structure of concrete is projected on the lattice, where-after different properties are assigned to “pipes” falling in different phases of the composite. Drying of a two-phase composite was analyzed. It is assumed that all mass or heat transfer can be described by means of a single diffusion equation. The comparison with finite element computations is found to be very satisfactory. A parameter analysis has been carried out, showing the effective transport properties of the composite as a function of geometrical or physical properties of the different phases in the heterogeneous material. In particular, the model has been used to study the effect of the permeability of the interfacial transition zone and the effect of using non-saturated porous aggregates on the moisture flow in concrete.     

Granular packing: numerical simulation and the characterisation of the effect of particle shape

The packing of granular particles is investigated using a combined finite-discrete element approach. One of the aims of this paper is to present an application of a recently improved numerical simulation technique for deformable granular material with arbitrary shapes. Our study is focused on the influence of the effect of the particle shape on (1) the emergent properties of a granular pack (packing density, coordination number, force distribution), and on (2) the spatial distribution of the stress. A set of simulations that mimick the sedimentation process is carried out, with varying input parameters, such as contact friction and particle shape. It is shown that the eccentricity of the particles not only significantly influences the final density of the pack but also the distribution of the stress and the contact forces. The presence of surface friction increases the amount of disorder within the granular system. Stress heterogeneities and force chain patterns propagate through the particles more efficiently than for the frictionless systems. The results also suggest that for the monodisperse systems investigated the coordination number is one of the factors that controls the distribution of the stress within a granular medium.     

Effects of silica powder and cement type on durability of ultra high performance concrete (UHPC)

Ultra-high performance concrete (UHPC) achieves extraordinary strength characteristics through optimization of the particle packing density of the cementitious matrix. The dense matrix also promotes exceptional durability properties and is arguably the biggest benefit of the material. A durable concrete enables structures to last longer, reduces the cost of maintenance and helps achieve a significantly more sustainable infrastructure. To assess the durability of UHPC, the performance of several non-proprietary blends are investigated by assessing the materials' resistance to freeze-thaw cycles, ingress of chlorides as well as the presence and distribution of air voids. The main experimental variables are cement type and the quantity of silica powder, which varies from 0% to 25% of the cement weight. All mixes displayed negligible chloride ion penetration and high resistance to freeze-thaw with mass loss well below the limit in over 60 cycles of freeze-thaw. Analysis of the test data indicates that the silica powder content has little influence on performance.     

Engineering property and structural design relationships for new and developing concretes

The paper reports the findings of a study carried out to investigate the effect of new and developing concrete technology solutions,e.g. (i) use of particle packing techniques and fillers to minimise voids, (ii) use of cement additions attained from industrial by-products and (iii) use of high range water-reducing admixtures which enable lower cement contents, on the engineering and structural performance of concrete and implications for structural design. The test programme considered 54 concrete mixes in three series to assess the impact of these on the tensile strength, flexural strength and modulus of elasticity of concrete, and in parallel, 37 mixes to measure these effects on the shear resistance of reinforced concrete beams. The results indicate that the influence of the concretes on compressive strength were generally inproportion to the effects on other engineering properties and were in line with current design assumptions on the behaviour of concrete. Furthermore, EC2 equations for predicting the shear strength of reinforced concrete beams, based on compressive strength, were also found to be appropriate for the range of concrete mixes considered. Overall, the work has demonstrated that new and developing concrete technology solutions can be utilised effectively within the framework of present design procedures and compressive strength is an appropriate parameter for assessing the structural performance of these concretes.     

Investigation on the influence of granular packing on the flow properties of cementitious suspensions

Fresh concrete can be considered as a suspension of grains of various sizes in a continuous fluid phase. The rheological properties of fresh concrete greatly depend on the physical factors, chemical and mineralogical characteristics of the fine components. Interparticle interactions occur during flow and modify the apparent rheological behaviour. Therefore, a thorough understanding of the rheological behaviour of cementitious suspensions with respect to particle packing is required. The objective of this study is to characterise the interrelationship between the flow properties and the particle packing density of the cementitious suspensions. The experimental investigation included Puntke tests for determining the packing density, and rheological tests that were performed in a rheometer for the characterisation of cement and silica fume (C + SF) as well as cement and fly ash (C + FA) mixtures. The effect of the water to powder ratio (w/p ratio) and the packing density on the flow properties of the cementitious suspensions was studied. From the study, it was observed that a good correlation exists between the w/p ratio and the yield value (g) for both C + SF and C + FA mixtures. The packing density shows a marked influence on the value of g for both mixtures, but has less influence on the value of plastic viscosity (h) for C + FA mixture.     

Mix design for self-compacting palm oil clinker concrete based on particle packing

The consumption of waste materials in self-compacting concrete (SCC) in the construction industry will not only help to conserve the natural resources but also promote sustainability in preserving the environment. Palm oil clinker (POC) is a waste by-product from the incineration process of oil palm shells and fibres. They are porous and lightweight in nature, which makes them suitable for use as a lightweight aggregate (LWA). In this study, a new procedure was employed to obtain the mix design based on the particle packing (PP) concept to ensure the fresh and hardened properties of SCC are achieved. The actual packing level of aggregate and paste volume is integrated into the proportioning method to obtain the final mix design. The proposed procedure was verified by evaluating the SCC formed for self-compactability and mechanical properties. Based on the overall performance of fresh and hardened properties, it can be deduced that the procedure satisfied the requirements for SCC. The satisfactory results indicate that the mix design can be employed not only for POC but also for a variety of combinations of aggregate.     

Effect of particle size on the performance of cross-flow microfiltration

The effects of particle size on the cake properties and the performance of cross-flow microfiltration are studied. A particulate sample with a wide size distribution range from submicron to micron is used in experiments. The probabilities of particle deposition are analyzed based on a force analysis. Since themajor forces in determining the particle deposition and packing in the filter cake are different for submicron and micron particles, the particle size plays an important role in the filtration performance. Cake properties, such as mass, porosity and average specific filtration resistance of the cake, are calculated theoretically and compared with experimental data. Except for the overestimation of the mean particle size for about 1 μm, the calculated results of the pseudo-steady filtration rate and cake properties under various operating conditions agree fairly well with the experimental data.     

A New Algorithm for Simulating the Random Packing of Monosized Powder in CIP Processes

A new model for monosized particle packing is developed in this study in simulating CIP powder compaction. The model uses a central growth method by which particles are placed from a central particle outwards until a specified container is filled. The simulation program is able to generate a particle packing of 5000 particles with characteristics that are similar to experimental results with significantly short computational time. A particle packing with a high occurrence of contacts among particles is effectively simulated using this model. It is shown that this work is an improvement in predicting the coordination number of the compact compared with the predictions of previous models. The average coordination number predicted by this model is 7·0 compared to 6·0-6·5 obtained by other simulations.     

我国分布式供能关键技术研究进展

分布式供能技术是我国中长期科技发展规划中能源领域的前沿技术,是实现高效、环保、可靠、智能、多元化供能的先进技术.通过对分布式供能作为我国能源战略需求的必要性和重要意义的阐述,概述了分布式供能的关键技术及其在我国的研究进展,指出微小型动力、动力余热高效利用、多能源互补等系统集成技术是当前重要的研究方向.     

Particle Size Distribution of Limestone Fillers: Granulometry and Specific Surface Area Investigations

Mineral fillers can be defined as “inert materials included in a mix design for some useful purpose” (NF P18-508 Janvier 2012). They can be added to compounds in order to complete a large variety of final properties without increasing costs or to improve specific characteristics like hardness, brittleness, impact strength, compressive strength, softening point, fire resistance, surface texture, electrical conductivity, and so on. In Belgium, locally available limestone fillers are specifically very well adapted for the optimization of particle packing and flow behavior of cementitious pastes in concrete mixes. Limestone fillers may be easily characterized in terms of chemical and mineralogical properties. These properties are fundamental for the study of the behavior of concrete mixes in fresh state and for understanding interactions existing at the level of the interfacial transition zone between aggregates and cement paste. These properties are however insufficiently discriminant and particle size, as well as shape distribution, seem to have a potential influence on physical phenomena which happen during the setting process. The aim of this article is to compare five major techniques used to quantify the size and the shape of limestone fillers particles: laser diffraction scattering, wet sieving, and image analysis for particle size measurement; and BET adsorption and Blaine permeability methods for specific surface area.