Interrelationship between uncompacted void content of aggregates and asphalt concrete properties

It is a well known fact that morphological characteristics of aggregates influence the mechanical response of asphalt concrete (AC). Uncompacted void content (UVC) of aggregate is often related to their morphological characteristics. A detailed experimental study was performed to explore relationship between UVC and mechanical response of AC. Initially, aggregates were artificially smoothened using Los Angles abrasion testing machine. UVC was then evaluated using blended aggregates at five levels of smoothness/texture. Increasing aggregate smoothness resulted in decreased UVC values. Higher UVC was obtained with a finer gradation compared to coarse gradation. Three AC mixtures prepared using these smoothened aggregates were tested for their strength and flow properties. A linear, strong positive correlation was found between UVC and Marshall parameters (stability, Marshall quotient). This can be attributed to interparticle resistance developed during mechanical loading. Parabolic relations were found between UVC and retained Marshall parameters. The retained Marshall parameters was found to be a function of UVC. It is concluded that UVC can be effectively used to capture changes in aggregate morphology and AC response.     

Optimisation of hot mix asphalt performance based on aggregate selection

Researchers over the last four decades have identified and demonstrated the effects of aggregate morphological properties (particularly shape, size distribution, angularity and texture) on the mechanical properties of hot mix asphalt (HMA). Rare studies, however, have clearly established the relationships between the aggregate properties and pavement performance. Therefore, they have not provided methods to optimise aggregate properties at the design stage to improve that performance. This study focuses on understanding the effects of aggregate gradation and type on moisture damage resistance of HMA and on pavement performance as indicated by stiffness and rutting. Results show that basalt aggregate achieves higher moisture susceptibility resistance and stiffness than limestone aggregate. Coarser gradation has the highest permanent deformation, while open gradation 2C provides the lowest moisture damage resistance. Furthermore, dense gradation 4C provides the lowest rutting and the highest stripping resistance. It is indicated that suitable selection of aggregate type and gradation can improve pavement performance and reduce the moisture damage problem of HMA.     

Molecular dynamics simulations of the straining of nanoparticle chain aggregates: the case of copper

Previous studies in our laboratory have shown that individual nanoparticle chain aggregates (NCAs) exhibit unusual mechanical behaviour when under strain inside the transmission electron microscope. NCAs made of various materials (e.g.?carbon, metal oxides and metals) were strained by as much as 100% under tension. The nanoparticles that compose the chains were 5-10?nm in diameter and the chains of the order of 1?μm in length. Such aggregates are of technological importance in the manufacture of nanocomposite materials (e.g.?rubber), aggregate break-up (e.g.?sampling diesel emissions) and chemical-mechanical planarization. The goal of this study was to simulate the mechanical behaviour of chain aggregates with morphological properties similar to those of technological interest. Molecular dynamics (MD) and energy minimization computer simulations are employed to investigate, at the atomic scale, the behaviour of short nanoparticle aggregates under strain and to obtain quantitative information on the forces involved in aggregate straining and fracturing. The interaction potential used is that of copper obtained with the embedded atom method (EAM). Two seven-nanoparticle aggregates are studied, one linear and the other kinked. The seven nanoparticles in both aggregates are single crystals and about 2.5?nm in diameter each. The aggregates are strained along their longest dimension, to the breaking point, at strain rates spanning from 2.5 × 10(7) to 8.0 × 10(8)?s(-1) (MD simulations). The linear aggregate yield strain is about 0.1. The kinked aggregate elastic limit is also about 0.1, but only one-third of the stress develops along the straining direction compared to the linear aggregate. The kinked aggregate breaks at a strain of about 0.5, five times higher than the breaking strain of the linear aggregate. The ability of the kinked aggregate to straighten through combined nanoparticle interface sliding and rotation accounts for the extra strain accommodation. Simulation strain rates are orders of magnitude higher than the experimental ones. However, aggregate behaviour is independent of strain rates over the range studied here. The MD and energy minimization straining gave very similar results. In the elastic regime, the 1/S(11) modulus for the seven-nanoparticle kinked aggregate is about one-fifth of the bulk value. This is due to a combined effect of the small primary particle diameter and the aggregate kinked structure. If this softening behaviour also occurs for nanoparticle aggregates of other materials (e.g.?carbon, silica), nanoparticle aggregates, in some cases, may be strained along with the nanocomposite they reinforce.     

Use of paraffin impregnated lightweight aggregates to improve thermal properties of concrete panels

In this study, the effect of aggregates impregnated with phase change material (paraffin type) on properties of concrete is investigated. The experimental series consists of two stages. The first stage is to investigate the techniques used to impregnate phase changed material (paraffin type) into lightweight aggregates and the properties of aggregates with paraffin inside (PLA). Two impregnation techniques are introduced, (1) heat only and (2) heat and pressure (autoclaving). Using the obtained results, the aggregate with the highest level of impregnation in the shortest time is selected to use in the concrete production process of the next stage. In the second stage, the properties of concrete mixed with non-paraffin and paraffin impregnated lightweight aggregates (PLA) at different proportions are investigated. The experimental series include density and absorption, compressive strength, thermal storage (and insulation) and sound transmission loss. Results in aggregate level show the increase in specific gravity and the decrease in absorption with paraffin inserted into aggregates. In concrete form, the density, compressive strength and sound insulation are found to increase with the PLA replacement ratio. The sound transmission loss, on the other hand, becomes less efficient with increasing PLA replacement ratio.     

Mechanical properties of lightweight ceramic concrete

Concrete produced using a magnesium phosphate binder can exhibit faster strength gain and result in lower overall environmental impacts than concretes produced with Portland cement binders. This paper reports a study to develop and characterize the rheological and mechanical properties of lightweight ceramic concretes (LWCC) that use a magnesium potassium phosphate binder. The aggregate type and the overall mix composition were primary variables in the study. Aggregate types included expanded clay, expanded slate, and expanded shale. Crushed bottom ash aggregate from a local coal-fired thermal generating station was also used. The aggregates of a given material varied by size fraction and by surface characteristics in some cases. The test results showed that increases in the water/binder ratio increased the slump flow but had negligible influence on the setting time. The compressive strength and density of the LWCCs both decreased with increases in the aggregate/binder mass ratio and the water/binder ratio, regardless of the type of lightweight aggregate. The 28 day compressive strength and density ranged from 17 to 36 MPa and 1600 to 1870 kg/m³ respectively. Regardless of the aggregate type, increasing the water/binder ratio also reduced the elastic modulus, modulus of rupture and direct shear strengths. Relationships were developed to directly relate these mechanical properties to the corresponding compressive strengths. The results indicate that LWCCs using a magnesium phosphate binder and lightweight aggregates can be formulated with rheological and mechanical properties suitable for structural applications.     

Characterisation of asphalt pavement surfaces using torque measurements

Continuous increase in traffic volume in the recent years has resulted in greater surface friction loss of asphalt pavements. While the literature presents several ways of aggregate and asphalt mix screening to ensure that surface characteristics are at an acceptable level of friction to control skid-related accidents, the recommended polishing tests using different accelerated polishing devices are time-consuming and to some extent labour-intensive. Therefore, it is imperative that existing polishing devices be improved to shorten the test duration to a level desired by the industry as well as state and federal agencies. This paper aims at addressing this improvement that uses power unit (or motor) to generate energy enough to rotate the polishing disk at constant rotational speed while being pressed against the specimen surface, then read the power needed from the display screen for the following steps. Multiple verification techniques including comparison studies and statistical analyses were used to examine the validity of this improvement. It was found that data collected by the power unit was repeatable and able to precisely detect surface deterioration history for different asphalt mixes with different polish susceptibility in a manner similar to conventional friction and texture-measuring devices. Hence, aggregates and mixes were classified based on their surface frictional properties. Additionally, results from this study correlated well with results from other studies using conventional methods. Most importantly, it was found to be possible to cut down the polishing test time using torque values in lieu of conventional surface quantification methods.     

Characterisation of unbound aggregate materials considering physical and morphological properties

The objective of this paper is to evaluate the factors affecting resilient and permanent deformation behaviour of unbound granular materials, with a focus on the aggregate physical and morphological characteristics. To evaluate the behaviour of base course, repeated load triaxial testing is commonly used to establish the stress-dependent resilient modulus properties of unbound aggregate base and subbase materials. Although resilient modulus of aggregates is a critical input into mechanistic-empirical pavement design methods, the resilient modulus of unbound base material is often estimated from empirical correlations with index properties in the AASHTOWare Pavement ME design procedure for its simplicity. Since actual field stress conditions and resilient modulus stress states are generally quite different from those generated in the empirical test methods, use of an empirical correlation could lead to an unreliable prediction of resilient modulus and permanent deformation. In order to properly assess the stability of an unbound aggregate layer, it is necessary to establish a proper process to understand the factors affecting fundamental and performance-related properties of unbound granular materials. In this study, aggregate samples from four different sources were tested for resilient modulus and Poisson’s ratio measurements using the Precision Unbound Material Analyzer equipment. Morphological or shape properties of aggregate samples were also measured using an image analysis device. The results demonstrate that aggregate physical and morphological properties affect aggregate resilient and permanent deformation. Further, it is suggested that the resilient modulus of the aggregate should not be used as the sole indicator of rutting performance of aggregate base.     

Effects of using screening materials in the graded aggregate base layer of flexible pavements

In this paper, the potential use of screening materials in Graded Aggregate Base (GAB) layer of pavements was investigated using aggregate sources in Georgia. Three content levels of screening materials in GAB, i.e. 0%, 25% and 50%, were studied. Morphological analysis of aggregates and Proctor test were conducted to reveal the characteristics of the modified GAB materials, followed by the California Bearing Ratio (CBR) test and Mechanistic-Empirical Pavement Design Guide software simulation to evaluate how the GAB strength and overall pavement performance change with varying amount of screening materials in the GAB layer. The results indicated that the effect of using screening materials in GAB varies and depends on the amount of screening materials used, which changes gradation, and the aggregate types and sources, which exhibit significantly different morphological properties.     

Microstructural characteristics,porosity and strength development in ceramic-laterized concrete

Interfacial bonding between constituent materials and pore sizes in a concrete matrix are major contributors to enhancing the strength of concrete. In a bid to examine how this phenomenon affects a laterized concrete, this study explored the relationship between the morphological changes, porosity, phase change, compressive, and split tensile strength development in a ceramic-laterized concrete. Varying proportions of ceramic aggregates, sorted from construction and demolition wastes, and lateritic soil were used as substitutes for natural aggregates. Strength properties of the concrete specimens were evaluated after 7, 14, 28 and 91 days curing, but morphological features, using secondary electron mode, were examined only at 7 and 28 days on cured specimens, using Scanning electron microscope (SEM). From all the mixes, selected samples with higher 28 day crushing strength, and the reference mix, were further characterized with more advanced analysis techniques, using the mercury intrusion porosimetry (MIP), thermogravimetric analysis (TGA), X-ray Diffractometer, and SEM (backscatter electron mode-for assessment of the interfacial transition properties between aggregates and paste).The reference mix yielded higher mechanical properties than the concrete containing secondary aggregates, this was traced to be as a result of higher peaks of hydration minerals of the concrete, coupled with its low tortuosity and compactness. However, a laterized concrete mix containing both 90% of ceramic fine and 10% of laterite as fine aggregate provided the optimal strength out of all the modified mixes. Although, the strength reduction was about 9% when compared with the reference case, however, this reduction in strength is acceptable, and does not compromise the use of these alternative aggregates in structural concrete.     

Concrete with recycled aggregates: the Portuguese experimental research

The main objective of this study is to define expedient procedures to estimate the properties of structural concrete that contains recycled aggregates. Experimental results from Portuguese research, most of which supervised by the first author, were used to establish a relationship between some properties of hardened concrete (compressive strength, splitting and flexural tensile strength, modulus of elasticity, abrasion resistance, shrinkage, water absorption, carbonation penetration and chloride penetration) and the density and water absorption of the aggregates’ mixture and also the compressive strength of concrete at the age of 7 days. The workability and density were also analysed for fresh concrete. The graphic analysis of each property shows the relationship between those for recycled aggregate concrete (RAC) mixes and a reference mix using natural aggregates only (RC). The density and water absorption of all the aggregates in the mixture, for each substitution rate, were calculated in order to represent the exact proportion of each type of aggregate (natural and recycled). This method proved to be viable to estimate the variation of the properties of concrete with recycled aggregates by obtaining results for the three parameters mentioned above. This innovative procedure can contribute to increasing the use of recycled aggregates in the construction sector and make it a sustainable activity.     

Multi-scale study of the polishing behaviour of quartz and feldspar on road surfacing aggregate

In this paper, the exact mechanisms as to how petrographic properties of aggregate influence the polishing behaviour quantitatively were identified. Since the aggregate is composed of the different rock-forming minerals, the analyses conducted in this research project focused not only on the aggregate but also on crystals/minerals. The hardest and most abundant rock-forming minerals found on earth – quartz and feldspar – were investigated with polishing tests with the Aachen Polishing Machine on granite aggregates from four sources which exhibit different mineralogical properties. Changes of the aggregate texture and minerals were studied based on texture measurements and skid resistance measurements obtained by means of the Pendulum Test and the Wehner/Schulze device. The influences of mineral composition and crystal size on the changes of the micro-texture and skid resistance of the aggregates were determined by comparing the four granites. The polishing and wearing behaviour of quartz and various feldspars (albite, microcline and plagioclase) were characterised quantitatively by means of a spectral analysis and abrasion analysis. The results show that the micro-texture of exposed minerals is greatly smoothened on wavelengths from 100 to 500 μm. Fine-grained quartz or feldspar crystals tend to be rougher than medium of large-sized crystals. Quartz exhibits more favourable properties than feldspar between 100 and 500 μm; this is especially true for fine quartz crystals. The correlation that more content of coarse-grained quartz leads to high friction value cannot be proved in this research.     

Properties of cold bonded quarry dust coarse aggregates and its use in concrete

In this study, artificial coarse aggregates are prepared by a cold bonding technique. The waste materials, namely, fly ash and quarry dust, are used for the preparation of the cold bonded artificial aggregate. Portland cement is used as the binder material. The independent variables considered for the preparation of the artificial aggregate are cement and fly ash contents. The properties of the artificial aggregate are determined and regression models are proposed for predicting these properties. The strength and workability of concrete containing artificial aggregate is determined. The slump loss of concrete containing artificial aggregate is found to be gradual. The concretes with strengths of up to 30 MPa is prepared using artificial aggregates. The study promotes the use of waste material and supports sustainable construction practices.     

Effects of cold-bonded fly ash aggregate properties on the shrinkage cracking of lightweight concretes

This paper presents an experimental study on the restrained shrinkage cracking of the lightweight concretes made with cold-bonded fly ash lightweight aggregates. Two types of fly ash having different physical and chemical properties were utilized in the production of lightweight aggregates with different strengths. Afterwards, lower strength aggregates were also surface treated by water glass and cement–silica fume slurry to improve physical and mechanical properties of the particles. Therefore, a total of eight concrete mixtures were designed and cast at 0.35 and 0.55 water–cement ratios using four types of lightweight coarse aggregates differing in their surface texture, density, water absorption, and strength. Ring type specimens were used for restrained shrinkage cracking test. Free shrinkage, creep, weight loss, compressive and splitting tensile strengths, and modulus of elasticity of the concretes were also investigated. Results indicated that improvement in the lightweight aggregate properties extended the cracking time of the concretes resulting in finer cracks associated with the lower free shrinkage. Moreover, there was a marked increase in the compressive and splitting tensile strengths, and the modulus of elasticity.     

A new method to determine specific surface area and shape coefficient of a cohesionless granular medium

The specific surface area of soil grains (SSA) influences the physical and chemical properties of soil. Despite the widespread application of the SSA, it is often determined by analytical equations or estimated using empirical relationships and visual comparisons. On the other hand, little attention has been paid, especially to three-dimensional and precise measurements. In this study, the precise SSA and shape coefficient was measured in a group of aggregate with different sizes and shapes using a novel method. Geometrical properties such as volume and surface area were measured for each particle using X-ray micro-computed tomography (µCT) images and image processing. The particle shape coefficient was obtained from the specific surface area and effective diameter. Further, the particle shapes were measured in three distinct characteristics, sphericity, roundness, and roughness. Particle's roundness was measured by Wadell’s formula in the plan outline, which did not show an agreement with the shape coefficient. Nevertheless, as the particle sphericity decreases and roughness increases, the shape coefficient is increased. Hence, the shape coefficient of grains was classified based on the sphericity and the surface roughness. The measured SSA and shape coefficient, for 12 gradations with the three different particle size distribution and four different particle shapes, indicated good agreement with predictive analytical relations for rounded and crushed grains and more dissimilarity for flaky and elongated grains. This novel method can provide the quantitative/qualitative classification for non-spherical particles and also improve our understanding on the properties of granular media.     

Simulation of the optical properties of plate aggregates for application to the remote sensing of cirrus clouds

In regions of deep tropical convection, ice particles often undergo aggregation and form complex chains. To investigate the effect of the representation of aggregates on electromagnetic scattering calculations, we developed an algorithm to efficiently specify the geometries of aggregates and to compute some of their geometric parameters, such as the projected area. Based on in situ observations, ice aggregates are defined as clusters of hexagonal plates with a chainlike overall shape, which may have smooth or roughened surfaces. An aggregate representation is developed with 10 ensemble members, each consisting of between 4-12 hexagonal plates. The scattering properties of an individual aggregate ice particle are computed using either the discrete dipole approximation or an improved geometric optics method, depending upon the size parameters. Subsequently, the aggregate properties are averaged over all geometries. The scattering properties of the aggregate representation closely agree with those computed from 1000 different aggregate geometries. As a result, the aggregate representation provides an accurate and computationally efficient way to represent all aggregates occurring within ice clouds. Furthermore, the aggregate representation can be used to study the influence of these complex ice particles on the satellite-based remote sensing of ice clouds. The computed cloud reflectances for aggregates are different from those associated with randomly oriented individual hexagonal plates. When aggregates are neglected, simulated cloud reflectances are generally lower at visible and shortwave-infrared wavelengths, resulting in smaller effective particle sizes but larger optical thicknesses.     

钢渣沥青混凝土技术及其应用研究进展

钢渣常被当成工业废弃物处置,但其碱性强、棱角丰富,兼具有优异的力学特性,可以改善沥青混凝土的抗水损害、抗高温变形以及耐磨和抗滑等性能,被认为是可替代天然矿质集料的理想筑路材料。近年来道路建设对集料用量的高需求与天然集料短缺之间的矛盾越来越突出,钢渣沥青混凝土技术因而成为备受关注的热点。概述了钢渣的材料特性以及钢渣沥青混凝土的设计与性能,介绍了钢渣沥青混凝土的实际应用情况,研究了钢渣沥青混凝土长期应用后的路面性能变化,最后对钢渣沥青混凝土未来的发展趋势进行了展望。     

AUTOMATED MEASUREMENT OF AGGREGATE INDICES OF SHAPE

Aggregate angularity is a significant factor in determining pavement resistance to rutting. Determination of aggregate angularity by direct means allows a more meaningful study of the mechanisms of pavement deformation. However, direct techniques have not been developed sufficiently to allow a more advanced study of angularity effects on pavement. Utilizing modern digital imaging hardware and image analysis techniques, we report here the development of an automated system for measuring aggregate angularity. Combining high resolution video, image capture hardware, sample transport, and computerized analysis, an instrument was constructed and used to compare various aggregate samples. Differentiation of known good quality aggregate from poorer grades was possible. Shape indices were developed from basic linear measurements, and the Hough Transform technique. These indices appear to have value in quantifying aggregate shape, and in studying the mechanisms of pavement deformation, and possibly the prediction of pavement longevity.     

颗粒形状对堆石料力学特性影响的离散元分析

颗粒形状是影响堆石料力学特性的重要因素,值得深入研究.以爆破堆石料为研究对象,采用离散元软件PFC3D根据真实颗粒形状建立不同球度的单颗粒模型.分别生成不破碎和可破碎试样并进行三轴剪切试验模拟,分析颗粒球度对堆石料力学特性的影响及细观机理.研究结果表明:颗粒球度通过影响试样内颗粒咬合和颗粒破碎,进而影响堆石料力学特性;...     

Effect of curing method on characteristics of cold bonded fly ash aggregates

This paper discusses the influence of normal water curing, autoclaving and steam curing on the properties of a typical class-C fly ash aggregate. The 10% fines value, water absorption, and porosity of aggregates are correlated with SEM and XRD results to understand the influence of various factors and material characteristics. An increase in duration of normal water curing significantly improved the aggregate properties. Autoclaving and steam curing resulted in relatively lower enhancement in the properties as compared to normal water cured aggregate. Between the accelerated curing methods, autoclaved aggregate possessed properties closer to the normal water cured aggregate due to the dense microstructure formation. Continuation of normal water curing, after initially subjecting the aggregates to accelerated curing, exhibited only a marginal improvement in the properties.