Effect Of Aggregate Size Distribution Research Articles

Mesoscopic discrete modeling of compression and fracture behavior of concrete: Effects of aggregate size distribution and interface transition zone

This paper presents a mesoscopic discrete modeling approach for studying compression and fracture behavior of concrete. A modified piecewise linear sieve curve was proposed to generate irregularly shaped aggregates following specific particle size distributions, and then a random particle model and the parallel bond model were implemented within a 3D framework of discrete element model (DEM). To characterize the interface transition zone (ITZ) which is generally the weak region in concrete, a reduction factor was employed to adjust the strength and consider the effect of ITZ on the mechanical response of concrete. The numerical modeling was calibrated and validated by comparing simulation results with experimental data from compression, splitting tensile, and dog-bone tests. The results demonstrate that the random aggregate model provides accurate representations of the concrete mesostructure, and the discrete model can effectively capture the effects of aggregate size distribution and ITZ on mechanical properties and crack propagation of concrete.

Effect of aggregate size distribution and confining pressure on mechanical property and microstructure of cemented gangue backfill materials

The use of gangue, cementitious materials, and water mixed to make cemented gangue backfill material (CGBM) can achieve solid waste recycling while reducing environmental problems caused by its accumulation. In this paper, the fractal dimension of particle size distribution (PSD) and the confining pressure were investigated on the compressive strength, elastic modulus, stress–strain behavior, dilatancy deformation, and failure mode of CGBM using uniaxial compression, conventional triaxial compression, and microscopic scanning tests. The mechanism of the PSD fractal dimension on the mechanical properties of CGBM was revealed from a microscopic perspective. The results demonstrate that the compressive strength and elastic modulus of CGBM are quadratic polynomial and positively linearly related to the PSD fractal dimension and confining pressure respectively, with the PSD fractal dimension characterizing the maximum compressive strength of CGBM ranging between 2.4150 and 2.6084. The volume strain variation of CGBM diminishes as the PSD fractal dimension grows and increases dramatically when the confining pressure rises. The PSD fractal dimension has a quadratic polynomial relationship with both the cohesive force and the internal friction angle of CGBM. A reasonable PSD fractal dimension can optimize the microstructure of CGBM, reduce the distribution of defects such as microcracks and micropores, and enable hydration products to effectively fill the defects, guaranteeing that the CGBM has sufficient load-bearing capacity.

Effect of aggregate size distribution on soil moisture, soil-gas diffusivity, and N2O emissions from a pasture soil

Grazed pastures rich in nitrogen (N) from ruminant urine and fertilizer inputs are significant sources of nitrous oxide (N2O), a highly potent greenhouse gas. Diffusion-controlled emission of N2O from pasture systems can be described by soil-gas diffusivity (Dp/Do), and its dependency on soil physical properties and soil moisture dynamics. But studies linking soil aggregation, soil moisture variation, Dp/Do and N2O emissions are lacking. Using coarse (2–4 mm) and fine (<0.2 mm) aggregates, and seven different combinations thereof, the effect of soil aggregate size distribution on soil–water characteristic (SWC), Dp/Do and N2O fluxes in a pastoral soil were investigated. Sieved-repacked samples, with varying fine aggregate fractions (F = 0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 1.0) were saturated with KNO3 (1800 μg mL−1) solution and systematically drained to nine different matric potentials (−1 kPa to −10 kPa), followed by an air-dry step (−30 kPa). At each potential, Dp/Do and N2O fluxes were measured. The measured SWC and SWC-derived pore-size distributions showed clear bimodal pore structures in all combinations. The highest and lowest total porosities were observed with F = 0 and 0.7, respectively. The lowest N2O peak flux was observed with F = 0.7 which also had the lowest Dp/Do, while the highest flux among all combinations was observed in F = 1.0 at Dp/Do = 0.002. Peak N2O flux varied with Dp/Do dynamics that were in turn a function of inter-aggregate pore drainage. Initially increasing the fine fraction is speculated to have enhanced nitrifier-denitrification while further increases in the fine fraction, which lowered N2O peak emissions, were likely due to a shift from nitrifier-denitrification to denitrification and associated N2O consumption or entrapment.

Experimental investigation of the effects of aggregate size distribution on the fracture behaviour of high strength concrete

This paper examines the influence of different aggregate size distributions on the fracture behaviour of high strength concrete. Three-point bend test was performed on 63 notched beams casted using three aggregate size distributions and two water to binder ratios. The total fracture energy, GF, and critical stress intensity factor, KIC, were used to determine the fracture characteristic of concrete. The results show that the values of GF decrease substantially with increasing coarseness of aggregate grain structure, λ. Values of KIC also decreased but demonstrated only limited dependence on λ. In contrast, reducing the total w/b ratio substantially increases the value of KIC but had no measurable effect on GF.

Impact of aggregate packing on dynamic modulus of hot mix asphalt mixtures using three-dimensional discrete element method

Aggregates are the major component of hot mix asphalt (HMA) mixtures. The properties of aggregates and the way of aggregate packing have important influence on the performance of HMA mixtures. Because the dynamic modulus is considered as the most important HMA property influencing the field fatigue and rutting performance of a flexible pavement and are used in the mechanistic-empirical pavement design guide for determining the stress–strain responses of the pavements, a study of the impact of aggregate packing on dynamic modulus will provide insight on the HMA mix design and the material performance evaluation. This paper studies the effect of aggregate size distribution and angularity distribution on dynamic modulus using a 3D discrete element method (DEM). Angular particles are generated using an image-based ball-clumping approach which requires significantly reduced number of balls and is capable of capturing the particle shape and angularity effect. These particles are assigned to the DEM dynamic modulus specimen based on the angularity distributions of the actual experimental specimen. A 3D DEM dynamic modulus model is thus established and calibrated using experimental data. This calibrated model is further used to evaluate how the different aggregate packing due to the change of the proportions of aggregate particles and the change of aggregate angularities can result in the change of dynamic modulus.

Investigation of effects of aggregate size on the fracture behavior of high performance concrete by acoustic emission

As an important component of concrete, aggregate has a significantly influence the fracture behavior of high performance concrete. In this paper, the effects of aggregate size distribution on the fracture properties of high performance concretes with strength ranging from 50 MPa to 80 MPa were investigated under three-point-bending tests. Acoustic emission (AE) technique with three-dimensional orientation feature was applied to study the effect of maximum aggregate size ( d max) on fracture properties and the facture process zone (FPZ) at the crack tip. The results showed that the fracture energy of concrete increased with the increase of d max. The larger the size of the aggregate, the more significant the deflection of propagating crack and the greater the FPZ can form. That AE hits had a very good relation indicating its potential to measure inner damage of concrete during cracking.

Effects of primary particle diameter and aggregate size distribution on the temperature of soot particles heated by pulsed lasers

Temperature histories of nanosecond-pulsed laser-heated soot particles of different primary particle diameters and different aggregate sizes were calculated using an aggregate-based heat transfer model. Relatively low laser fluences were considered to ensure maximum particle temperatures were below about 3800 K to avoid soot particle sublimation. After the laser pulse, the temperature of soot particles in larger aggregates decreases more slowly than that of particles in smaller aggregates due to the increased shielding effect. For a given aggregate size, the temperature of particles of smaller diameter decays faster as a result of a larger surface area-to-volume ratio. The effective temperature of soot particles in the laser probe volume was calculated based on the ratio of thermal radiation intensities of soot particles at 400 and 780 nm to simulate the experimentally measured soot particle temperature using two-color optical pyrometry. The effect of aggregate size distribution of soot particles on the effective particle temperature was investigated under different initial temperatures.

One-dimensional solute transport modelling in aggregated porous media. Part.2. Effects of aggregate size distribution : Hayot, C; Lafolie, F J HydrolV143, N1/2, March 1993, P85–107

One-dimensional solute transport modelling in aggregated porous media. Part.2. Effects of aggregate size distribution : Hayot, C; Lafolie, F J HydrolV143, N1/2, March 1993, P85–107

One-dimensional solute transport modelling in aggregated porous media Part 2. Effects of aggregate size distribution

The influence of various aggregate size distributions on solute transport is analysed from a quantitative and qualitative point of view. Uniform, normal, log-normal and discrete aggregate size distributions are considered. The model described in Part 1 is used to calculate the breakthrough curves. The second-order moments and the sum of squared differences are used to compare the responses calculated with the distributions and with the mean radius. The influence of a distribution depends on the velocity, and consequently on the proportion of aggregates in situations of local equilibrium or physical non-equilibrium. Breakthrough curves calculated from normal distributions, whatever their spread, do not differ significantly from those calculated using the mean radii, while slightly skewed narrow distributions generate significant differences. Thus, the skewness and not only the spreading of the distribution seem to modify the spreading of solute. The problem of using a unique ‘mean radius’ instead of a distribution is also considered. It is shown that the problem does not have a simple answer. For a given distribution, the existence of an acceptable mean radius depends strongly on the contrast between the mobile phase residence time and the distribution of characteristic diffusion times. Also, intuitively, acceptable mean radii are more easily found for narrow distributions. In most cases, it appears that a mean radius cannot be used instead of the distribution. The analysis of the way the distributions modify the breakthrough curves suggests that simple bimodal distributions presenting fast- and slow-reacting sites could be used in lieu of complete distributions.

Monitoring crystallization experiments using dynamic light scattering: Assaying and monitoring protein crystallization in solution

Various analytical techniques may be valuable as precrystallization assays for monitoring and screening various protein crystal growth conditions. Dynamic light scattering (DLS) offers unique capabilities for such studies. DLS provides a measure of the translational diffusion coefficient, D, of the protein molecules. Assuming a spherical model for the protein, its hydrodynamic diameter, d, can then be estimated. DLS can be used to monitor crystallization events as a function of time, creating the possibility of a dynamically controlled crystal growth experiment under previously characterized solution conditions. Variation of d as a function of particular solution properties such as protein concentration, pH, precipitant concentration, or temperature can also be determined. Early in the evolution of a supersaturated solution of macromolecules, monomers assemble to form aggregates which lead either to crystals or to precipitates. In an obvious terminology it is convenient to refer to the former as craggs (precrystalline aggregates) and the latter as praggs (preprecipitating aggregates). The protein concentration dependence of d should be correlated with the thermodynamic equilibria governing these two types of aggregates and has been suggested as a way to differentiate between praggs and craggs in solution. Although other factors may also lead to a concentration dependence of d, these are in most cases expected to be small, relative to the effect of aggregate size distribution. This capability would allow one to make early predictions regarding the outcome of a particular, unknown crystallization condition on the basis of a quantitative experimental measurement. The protein concentration dependence of d has been used for several proteins to differentiate between the two types of aggregates.

Solute transport in aggregated media: Aggregate size distribution and mean radii

An aggregate model was employed to study the effects of aggregate size distribution (ASD) on solute transport in a column. Instantaneous local sorption equilibria and nondegradation were assumed. The aggregate model was solved using an effective new iterative numerical scheme. Three ASDs from the literature were used in the calculations. The effects of aggregate size and size distribution can not be separated and are influenced by the flow regimes of the system. A mean variance radius was derived. This mean variance radius depends on the internal diffusion and mass transfer coefficient of the aggregates. It provided the best approximation among other types of mean aggregate radii used in the literature. Multiple mean variance radii should be used to represent the ASDs.

Tinting strength of carbon black

The tinting strength of carbon black (i.e., its efficiency in decreasing reflectance when mixed with a white pigment) is of practical importance in the paint industry and is also used in quality control of carbon black manufactured for use in rubber. The relative tinting strength of a carbon black is proportional to its specific absorbance, which we calculate by Mie theory from the refractive index of carbon black and the size of the independent structural units; namely, the fused aggregates, which as a first approximation are treated as solid spheres containing the same volume of carbon. This volume depends primarily on the particle size, and to a lesser but significant extent on the “structure” or number of particles per aggregate, which is calculated from the vehicle absorption at the critical pigment volume concentration. The refractive index of carbon black is estimated from various published data as m = 1.84 (1− 0.46 i). The Mie calculations show that the tinting strength is virtually independent of solid-sphere size below a certain value (approximately at a size parameter of α = 0.7); at larger sizes, the tinting strength diminishes gradually, and eventually becomes nearly proportional to 1/α. The calculated behavior is in good agreement with experimental findings. Calculations of the wavelength dependence of tinting strength show that carbon blacks of small solid-sphere size are of brownish tint “tone,” while those of large size are bluish, in agreement with experimental data. The effect of aggregate size distribution is discussed.