Effect Of Aggregate Structure Research Articles

Study on the shear strength of asphalt mixture by discrete element modeling with coarse aggregate morphology

High shear strength is crucial for asphalt pavements to resist permanent deformation at high temperatures, which is largely determined by the skeletal structure of coarse aggregates. In this study, four size ranges of polyhedral coarse aggregate clumps were built to simulate the morphology and dimensional characteristics of coarse aggregate. Subsequently, virtual triaxial shear tests were simulated using discrete element models. Three gradation types were analyzed to study the effects of aggregate structure, granularity composition, and coarse aggregate modulus on the shear strength of asphalt mixtures. The results show that it is feasible to obtain accurate morphological features of coarse aggregates by controlling the minimum diameter of the filled spheres in a polyhedral coarse aggregate box when constructing a discrete element model. The virtual triaxial shear test model developed in this study can be effectively utilized to simulate the shear mechanical properties of asphalt mixtures under various confining pressures. The numerical simulation results indicate that the cohesion of asphalt mixture AC-13 is the best, while the internal friction angle and shear strength of SMA-13 are the highest. The ranking of factors that affect the cohesion and internal friction can be summarized as: aggregate structure ≫ coarse aggregate modulus > granularity composition, while the order of factors that affect the shear strength is: aggregate structure ≫ granularity composition > coarse aggregate modulus. This indicates that the aggregate structure is crucial for the high-temperature performance of asphalt mixtures. This study addressed the issue of high variability in laboratory test results for macroscopic properties of asphalt mixtures and analyzed the skeleton role of coarse aggregate in asphalt mixtures from a mesoscopic perspective. The findings parameters for explaining the strength mechanism. The discrete element models also facilitate the optimization of asphalt mix design with reduced labor and time consumption on laboratory testing.

Time-resolved laser-induced incandescence on metal nanoparticles: effect of nanoparticle aggregation and sintering

This work examines the excessive absorption and anomalous cooling phenomena reported in laser-induced incandescence measurements on metal nanoparticles by considering the effects of aggregate structure and sintering. Experimental investigations are conducted on iron and molybdenum aerosols, which have different melting points, and thus respond differently to the laser pulse. Although aggregation enhances the absorption cross-section of the nanoparticles and allows for higher peak temperatures, this enhancement does not fully explain the observed excessive absorption. Furthermore, as the aggregates of refractory metals such as molybdenum cool, they may sinter through gradual grain boundary diffusion; this change in structure alters their absorption cross-section, manifesting as a rapid drop in the pyrometric temperature, which could explain the anomalous cooling reported for this metal.

Effect of aggregate structure on load-carrying capacity and deformation resistance of porous asphalt concrete based on discrete-element modelling

ABSTRACT Porous asphalt concrete (PAC) constitutes large air voids and coarse aggregate, forming a unique aggregate structure. The open pore structure is advantageous in creating space for stormwater runoff to infiltrate into an underground basin and reduce tyre-road noise. However, this aggregate structure compromises the load-carrying capacity and deformation resistance of the mixture. Achieving balance between permeability and bearing capacity of PAC has been a major concern. This study evaluates effects of the aggregate structure on bearing capacity and the deformation resistance of PAC. Gradation-based framework was first applied to examine different gradations of PAC. Discrete-element models of PAC with varying gradations were developed to evaluate its performance and characteristics of aggregate skeleton. A laboratory wheel track test was conducted to validate findings from the DEM analysis. It was found that gradation that meets the interlock check of the framework produced a mixture with optimal performance. More coarse aggregate provides additional strength but lacks sufficient smaller-sized particles to form a new skeleton when it breaks down. More fine aggregate diminishes the load-carrying capacity and deformation resistance of the mixture as no stable skeleton could be formed.

Modified Prediction Model for Thermal Conductivity of Spherical Nanoparticle Suspensions (Nanofluids) By Introducing Static and Dynamic Mechanisms

Nanofluids possess significantly increased thermal conductivity that conventional heat conduction theories and models for nanoparticles–liquid suspensions cannot explain, which provides new scientific challenges. The aim of this study is to establish a modified prediction model for thermal conductivity of nanofluids that takes into account more comprehensive mechanisms. Based on effective medium theory, the role of dynamics and static mechanisms including the absorption liquid layer at the liquid/nanoparticle interface, the effect of aggregate structure of nanoparticles, and the impact of random motions of nanoparticles have been considered to establish the model. The parameters in the model have definite physical meaning and are more precise. For instance, the thermal conductivity of nanoparticles, kp, is replaced with thermal conductivity of nanoparticles aggregate, and the volume fraction of nanoparticles along with the absorption layer in nanofluids ϕ is used instead of the volume fraction of aggregat...

Effects of Aggregate Structure on the Dissolution Kinetics of Citrate-Stabilized Silver Nanoparticles

Aggregation and dissolution kinetics are important environmental properties of silver nanoparticles (AgNPs), and characterization of the interplay between these two processes is critical to understanding the environmental fate, transport, and biological impacts of AgNPs. Time-resolved dynamic light scattering was employed to measure the aggregation kinetics of AgNPs over a range of monovalent electrolyte (NaCl) concentrations. The fractal dimensions (Df) obtained from aggregation kinetics and static light scattering were found to be dependent on the aggregation mechanism and, in accord with expectation, varied from 1.7 for diffusion-limited cluster aggregation to 2.3 for reaction-limited cluster aggregation. An aggregation-dissolution model, in which the proportion of accessible reactive sites on primary particles as well as the aggregate size and Df are assumed to be key determinants of reactivity, is found to provide an excellent description of the decline of normalized rate of dissolution of AgNPs during aggregation for different NaCl concentrations. This model also provides fundamental insights into the factors accounting for the observed change in rate of dissolution of AgNPs on injection into seawater thereby facilitating improved prediction of the likely toxicity of AgNPs in the marine environment.

Effects of Aggregate Structure and Dimension of Carbon Nanotubes on the Mechanical, Electrical and Thermal Properties of Rubber Composites

Effects of Aggregate Structure and Dimension of Carbon Nanotubes on the Mechanical, Electrical and Thermal Properties of Rubber Composites

リグノフェノールのタンパク質吸着能に対する凝集構造の影響

リグニンを基材とするフェノール系高分子・リグノフェノールの優れたタンパク質吸着能について，これまでは分子構造との関係が検討されてきた．本報では，タンパク質が吸着するのは，凝集により高次構造を形成したリグノフェノール分子集合体，さらにはその凝集体であることに注目し，凝集構造が吸着能に与える影響を検討した．リグノフェノールとして，リグニンにp-クレゾールを導入したリグノクレゾールを，タンパク質としては牛血清アルブミン(BSA)を用いた．微粉化したリグノクレゾールの水中における二次的な凝集はBSA吸着量に影響を与えなかった．一方，物理的なプレスや，異なる貧溶媒中での再沈殿により，BSA吸着量は明確に減少した．リグノフェノールは，分子構造の制御以外に，プレスや沈殿溶媒の選択など簡単な手法で高次構造の制御を行うことができ，それによりタンパク質吸着能などの機能性のコントロールが可能である．

Investigation on the effects of gradation and aggregate type to moisture damage of warm mix asphalt modified with Sasobit

Moisture damage is one of the major concerns for warm-mix asphalt (WMA) because it is hypothesised that the reduction in mixing temperature could adversely affect the moisture sensitivity of WMA. The aggregate type and their properties are also one important factor affecting the moisture damage of WMA. This study focused on understanding the effect of aggregate structure and its physical properties on the mechanism of moisture damage in WMA. This study evaluated the moisture sensitivity of hot-mix asphalt (HMA) and WMA as reflected in the permanent deformation. The effects of aggregate gradation and aggregate types on the moisture damage were also evaluated. The results from this study show that WMA is more resistant to permanent deformation than HMA as indicated by lower permanent strain rate and higher flow number (FN). Gradation has an effect on the permanent deformation in granite and slag mixes due to the difference in aggregate properties. WMA is more sensitive to moisture damage than HMA as shown in lower ratios of FN and permanent deformation rate. Fine-graded mixtures appear to have greater resistance to moisture damage than coarse-graded mixtures for both HMA and WMA. The result could be due to the different structure of air void distribution in the mixes of different gradation. It was found that the proper selection of aggregate type and aggregate structure could reduce the moisture damage problem of WMA.

Viscosity and aggregation structure of nanocolloidal dispersions

In this work, the effects of nanoparticle size, particle volume fraction and pH on the viscosity of silicon dioxide nanocolloidal dispersions are investigated. Both size and pH are found to significantly affect nanocolloid viscosity. Two models are used to study the effect of aggregate structure on the viscosity of the nanocolloidal dispersion. The fractal concept is introduced to describe the irregular and dynamic aggregate structure. The structure of aggregates, which is considered to play an important role in viscosity, is affected by both intermolecular and electrostatic forces. The particle interaction is primarily affected by particle distance and becomes stronger with decreasing particle size and increasing volume fraction. The aggregate structure is also affected by the pH of the solution. Studying the relationship between pH and zeta-potential shows that with the neutralization of charges on the particle surface and decreasing electrical repulsion force, the particle interaction becomes dominated by attractive forces and the aggregates form a more compact structure.

A DEM-based analysis of the influence of aggregate structure on suspension shear yield stress

This study theoretically examined the effect of aggregate structure on the suspension shear yield stress. The aggregation process of colloidal particles was simulated using the discrete element model (DEM) combined with the well-known DLVO theory. The predicted aggregate structural characteristics, namely the coordination number and inter-particle forces were then used in a modified version of the Flatt and Bowen mechanistic model [6] to calculate the corresponding suspension yield stress. The effect of key parameters such as solid volume fraction, suspension pH and ionic strength on the aggregate structure and hence the yield stress of the suspension was investigated.The results showed that the yield stress increased significantly under conditions that were favourable for formation of complex net-like aggregate structures, such as high solid volume fractions, pH values near the iso-electric point, and high ionic strengths. In such cases, the mean coordination number reached a maximum value which was considered to be dependent on the particle size and size distribution. The suspension yield stress exhibited a power law dependency on the solid volume fraction. The interconnected network structure developed at high solid volume fractions was found to be the major contributing factor to the observed high suspension yield stress. As the particle–particle repulsion became significant, a decrease in both the number of bonds and the mechanical bonding strength of the aggregate structure was observed. That was considered to be responsible for the reduction in the suspension yield stress. The suspension yield stress became independent of the suspension ionic strength when the ionic strength exceeded the critical coagulation concentration. Satisfactory agreements were obtained between simulation results and the published experimental data.

When structure means conservation: Effect of aggregate structure in controlling microbial responses to rewetting events

Rewetting events after a drought produce a pulse of soil respiration (the “Birch Effect”) that leads to a loss of carbon from soil, especially in Mediterranean ecosystems. Two main hypotheses have developed to explain the Birch effect: the “metabolic explanation”, based on the rapid consumption of intracellular osmolytes previously accumulated to survive to dry conditions, and the “physical explanation”, based on the consumption of carbon made accessible by physical destruction of internal structures of the soil.Here, we compared the respiration response of intact and crushed 9–4 mm aggregates from a California grassland soil under two different rewetting schemes: (1) successive short dry/wet events and (2) increased drought periods followed by a single rewetting. In intact aggregates, both microbial biomass and respiration rates were relatively stable through both experimental treatments. In crushed aggregates, through multiple short dry/wet cycles, both respiration rate and microbial biomass increased, while as drought length increased, biomass was unaffected but the magnitude of the following rewetting pulse increased. A mechanism that explains both these results is that crushing aggregates exposes occluded particular material that must be degraded into an immediately bioavailable form for microbes to take it up and metabolize it. Nitrification was generally higher in intact than crushed aggregates, suggesting the importance of physical association between nitrifiers and resources in regulating overall soil nitrification.This work suggests that physical processes are most important in driving respiration pulses through multiple rewetting cycles and that the physical association of organisms, substrates, and mineral particles are critical in controlling the functioning of the “microbial landscape”.

Effects of aggregate structure on hot-mix asphalt rutting performance in low traffic volume local pavements

The objective of this study is to evaluate the effect of mix gradations associated with the Superpave restricted zone on rutting potential specifically for low traffic volume roadways. Although the elimination of the restricted zone requirement in Superpave mix design is highly recommended, some questions still remain unanswered as the research conclusions supporting the elimination of the restricted zone were largely made for medium to high traffic volume roadways, where aggregates are highly crushed and of good quality. The applicability of such research conclusions based on high traffic volume mixes needs to be verified for low volume mixes because many states in the US use non-crushed local aggregates for low traffic volume pavements, which might be related with aggregate gradation. This paper summarizes the research findings obtained from a systematic approach consisting of (1) statistical analyses of pre-existing data accumulated for quality assurance purposes, (2) experimental investigations based on the statistical analysis results, and (3) in-field investigation of the rutting performance of low traffic volume pavement. The comparison and analysis results indicate that similar to that for medium to high traffic volume pavements, the restricted zone is not a controlling factor affecting hot-mix asphalt rutting performance for low traffic volume local pavements. The fineness of aggregate gradation rather than the restricted zone seems to be a factor that affects rutting performance.

Effect of aggregate structure on VOC gas adsorption onto volcanic ash soil

The understanding of the gaseous adsorption process and the parameters of volatile organic compounds such as organic solvents or fuels onto soils is very important in the analysis of the transport or fate of these chemicals in soils. Batch adsorption experiments with six different treatments were conducted to determine the adsorption of isohexane, a gaseous aliphatic, onto volcanic ash soil (Tachikawa loam). The measured gas adsorption coefficient for samples of Tachikawa loam used in the first three treatments, Control, AD (aggregate destroyed), and AD-OMR (aggregate destroyed and organic matter removed), implied that the aggregate structure of volcanic ash soil as well as organic matter strongly enhanced gas adsorption under the dry condition, whereas under the wet condition, the aggregate structure played an important role in gas adsorption regardless of the insolubility of isohexane. In the gas adsorption experiments for the last three treatments, soils were sieved in different sizes of mesh and were separated into three different aggregate or particle size fractions (2.0–1.0 mm, 1.0–0.5 mm, and less than 0.5 mm). Tachikawa loam with a larger size fraction showed higher gas adsorption coefficient, suggesting the higher contributions of macroaggregates to isohexane gas adsorption under dry and wet conditions.

A new open-source software developed for numerical simulations using discrete modeling methods

The purpose of this work is to present the development of an open-source software based on a discrete description of matter applied to study the behavior of geomaterials. This software uses Object Oriented Programming techniques, and its methodology design uses three different methods, which are the Discrete Element Method (DEM) [F. Donzé, S.A. Magnier, Formulation of a three-dimensional numerical model of brittle behavior, Geophys. J. Int. 122 (1995) 790–802, F. Donzé, S.A. Magnier, L. Daudeville, C. Mariotti, Numerical study of compressive behaviour of concrete at high strain rates, J. Engrg. Mech. (1999) 1154–1163], the Finite Element Method (FEM) [J. Rousseau, E. Frangin, P. Marin, L. Daudeville, Discrete element modelling of concrete structures and coupling with a finite element model, Comput. Concrete (in print), S.P. Xiao, T. Belytschko, A bridging domain method for coupling continua with molecular dynamics, Comput. Methods Appl. Mech. Engrg. 193 (2004) 1645–1669] and the Lattice Geometrical Method (LGM) [J. Kozicki, Application of discrete models to describe the fracture process in brittle materials, Ph.D. thesis, Gdańsk University of Technology, 2007, J. Kozicki, J. Tejchman, 2D lattice model for fracture in brittle materials, Arch. Hydro-Engrg. Environ. Mech. 53 (2) (2006) 71–88, J. Kozicki, J. Tejchman, Effect of aggregate structure on fracture process in concrete using 2D lattice model, Arch. Mech. 59 (4–5) (2007) 365–384, J. Kozicki, J. Tejchman, Modelling of fracture process in concrete using a novel lattice model, Granul. Matter (in print), doi: 10.1007/s10035-008-0104-4]. These methods are implemented within a single object-oriented framework in C++ using OOP design patterns. The bulk of the original work consisted mainly of finding common objects which will work for these different modeling methods without changing a single line of the C++ code. With this approach it is possible to add new numerical models by only plugging-in the corresponding formulas. The advantages of the resulting YADE framework are the following: (1) generic design provides great flexibility when adding new scientific simulation code, (2) numerous simulation methods can be coupled within the same framework like for example DEM/FEM and (3) with the open-source philosophy, the community of users collaborate and improve the software. The YADE framework is a new emerging software, which can be downloaded at the http://yade.wikia.com webpage.

Effect of aggregate structure on fracture process in concrete using 2D lattice model

The 2D lattice model was used to analyse fracture processes in concrete at the meso–level. Concrete was described as a three–phase material (aggregate, interfacial transition zone and cement matrix). The calculations were carried out for concrete specimens subject mainly to uniaxial extension. The effect of the aggregate density was investigated. In addition, a deterministic size effect was studied. The advantages and disadvantages of the model were outlined.

Effects of aggregate structure and organic C on wettability of Ustolls

Soil wettability is especially important for rainfed agriculture in climates with a dry period during the growing season. The effect of aggregate structure and soil organic C content on wettability of soil aggregates was determined for grassland (grass) and tilled fields (tillage). Soil organic C, plastic limit, aggregate total porosity, and wettability at 100 mm (rapid wetting) and 300 mm (slow wetting) water tension were measured on soil at 0–0.2 m depth. Natural aggregates from tillage and grass were compared to soil pellets formed by remolding aggregates. At both tensions, wettability of grass aggregates was significantly greater than that of tillage aggregates ( P ≤ 0.001). Pellets were significantly less wettable than natural aggregates at 300 mm tension and during the initial wetting at 100 mm tension, but became significantly more wettable with time at 100 mm tension. Cumulative water uptake during 60 min exceeded the initial total porosity of pellets and natural tillage aggregates, suggesting incipient failure (formation of microcracks) during fast wetting. Grass aggregates contained twice as much organic C as tillage aggregates (26 g kg −1 versus 13 g kg −1). Organic C was linearly and positively related to plastic limit, total porosity, and the wettability of natural aggregates at 300 mm tension. At 100 mm tension, organic C was negatively related to wettability of natural aggregates under grass, but unrelated to wettability under tillage. Aggregate wettability was positively related to organic carbon content, except when the arrangement of soil constituents reduced or prevented incipient failure and soil dispersion during rapid wetting resulted in cumulative water uptake (60 min) similar to initial aggregate total porosity. Organic C increased wettability of grass aggregates when compared to tillage aggregates and also stabilized natural aggregates during fast wetting (100 mm tension). Both soil organic C content and aggregate structure were key factors controlling aggregate stability and wettability.

Investigation on effects of aggregate structure in water and wastewater treatment

The fractal structure and particle size of flocs are generally recognized as the two most crucial physical properties having impact on the efficiency of operation of several unit processes in water and wastewater treatment. In this study, an experimental investigation is undertaken on the effect of aggregate structure in water and wastewater treatment in Hong Kong. The fractal dimension of the resulting aggregate is employed as a measure of the aggregate structure. Small angle light scattering technique is used here. Different amounts of polymers are mixed to bacterial suspensions and the resulting structures are examined. The addition of polymer may foster aggregate formation by neutralization of the bacterial surface charge and enhance inter-particle bridging. The aggregation behavior may affect the efficiency of certain water and wastewater treatment processes such as dewatering and coagulation. The impacts of aggregate structure on two representative processes, namely, ultra-filtration membrane fouling and pressure filter dewatering efficiency, are studied. It is found that the looser flocs yield a more porous cake and less tendency to foul whilst more porous filter cakes yield more ready biosolids dewatering.

Soil Aggregate Structure Effects on Dielectric Permittivity of an Andisol Measured by Time Domain Reflectometry

Various types of soil physical properties are affected by texture and structure. Our objective was to determine aggregate structure effect on the soil dielectric property of an Andisol measured by time domain reflectometry (TDR). The relationships between volumetric water content (θ) and dielectric permittivity (ε) for both a wet-sieved aggregate and its crushed sample were examined and compared. In the θ–ε relationship for 0.1- to 2.0-mm-diam. wet-sieved aggregates, the gradient of the θ–ε curve moderately changed at a volumetric water content (critical water content), although this property disappeared when we crushed the aggregate structure. Furthermore, the critical value corresponded to the water content of the plateau in a bimodal-type water retention curve. We suggest that effects of aggregate structure on a soil's dielectric property are involved in the aggregate sizes, the configuration of water in aggregates, the processes of water filling in intra- and interaggregate pores, and the low ε value of bound water adsorbed on soil surfaces.

Soil Aggregate Structure Effects on Dielectric Permittivity of an Andisol Measured by Time Domain Reflectometry

Various types of soil physical properties are affected by texture and structure. Our objective was to determine aggregate structure effect on the soil dielectric property of an Andisol measured by time domain reflectometry (TDR). The relationships between volumetric water content (θ) and dielectric permittivity (ε) for both a wet‐sieved aggregate and its crushed sample were examined and compared. In the θ–ε relationship for 0.1‐ to 2.0‐mm‐diam. wet‐sieved aggregates, the gradient of the θ–ε curve moderately changed at a volumetric water content (critical water content), although this property disappeared when we crushed the aggregate structure. Furthermore, the critical value corresponded to the water content of the plateau in a bimodal‐type water retention curve. We suggest that effects of aggregate structure on a soil's dielectric property are involved in the aggregate sizes, the configuration of water in aggregates, the processes of water filling in intra‐ and interaggregate pores, and the low ε value of bound water adsorbed on soil surfaces.

Effect of Aggregate Structure on Rutting Potential of Dense-Graded Asphalt Mixtures

Superpave® mix design, as the name suggests, was developed to provide superior-performing pavements for the transportation industry. However, researchers and designers in their quest to provide such mixtures have encountered problems with some design specifications, such as the restricted zone. Superpave recommends that designers avoid grading through the restricted zone to eliminate tender mixes and gradations that may be too close to the maximum density line. But studies of the performance of the restricted zone have indicated that dense-graded mixtures that violate this requirement have provided good performance. It has also been reported that mixtures that are graded below the restricted zone (BRZ) have poorer rutting performance than those that are graded above the restricted zone (ARZ) or through the restricted zone. It is also known that the minimum voids in mineral aggregates (VMA) requirement, which was established to ensure durability, is very difficult to achieve. It also leads to overasphalting in BRZ mixtures and compromises their resistance to rutting. The effect of aggregate structures on rutting performance of mixtures is examined. Seven limestone and three granite mixtures were used. Six were BRZ mixtures and four were ARZ mixtures. After the behavior of the mixtures was investigated, it was concluded that BRZ mixtures developed different aggregate structures than ARZ mixtures and that the performance of mixtures depended on their aggregate structures. It was observed that the high VMA requirement caused overasphalting in BRZ mixtures, hence their dismal rutting performance compared with ARZ mixtures.