Elsevier

Powder Technology

Volume 325, 1 February 2018, Pages 530-544
Powder Technology

Post-breakage changes in particle properties using synchrotron tomography

https://doi.org/10.1016/j.powtec.2017.11.039Get rights and content

Highlights

  • Various granular materials studied using 4D Synchrotron Micro-Computed Tomography

  • Fractal distribution demonstrating a competition between cushioning and size effects

  • Morphology evolution demonstrating a reversal trend with increasing stress

  • Particle shape evolution due to breakage obeying universality

Abstract

Granular recycled Construction and Demolition (C&D) materials reused in pavements, roads, and embankments experience particle breakage causing serious issues, such as settlement, during their service life. Particle breakage is of paramount importance for understanding the behaviour of particulate materials used not only in pavements, roads, and rail tracks, but also in the oil and gas industry and mineral processing. The engineering characteristics of a granular assembly is closely dependent on properties altered by breakage. Changes in particle properties of uniformly graded and spectrum of C&D specimens due to breakage under compressive loads were studied using three-dimensional Synchrotron Radiation-based Micro-Computed Tomography. The high-resolution 4D imaging was used to scan the inner body of the granular assemblies under compression. The resulting images were subsequently processed using a variety of techniques, including image thresholding, filtering, and segmentation, to identify and label each fragment in the assemblies. The fractal distribution of granular assemblies demonstrated that breakage becomes dominant in smaller particles rather than larger particles, where an increase in newly generated fine fragments brings about high coordination number surrounding the larger particles. More prominently, the results of particle morphology evolution showed a reversal trend as the stress increased. The C&D particles tended to generate more spherical fragments with higher aspect ratio, although by increasing the stress this tendency totally reversed. In addition, it has been found that the general trend of changes in particle shape obeys universality. In other words, for the materials tested; same generic evolution by increasing stress was observed irrespective of the material types or sizes.

Introduction

Recycled Construction and Demolition (C&D) waste materials are particulate materials produced during the construction and demolition of buildings and structures or commercial and industrial activities. C&D materials were recognized to have suitable geotechnical properties to be reused as pavement subbase/base materials in Victoria, Australia [1]. Recycling and reusing of waste materials leads to a decline in the demand for limited natural resources and simultaneously lowers disposal cost into the landfills. Among various types of C&D materials, crushed basaltic Waste Rock (WR), Recycled Concrete Aggregate (RCA), and Crushed Brick (CB) are of interest in this study (Fig. 1). Crushed WR originates from surface excavation of Quaternary aged basaltic rock, whose placement normally occurs near the surface to the west and north of Melbourne, Australia [2]. Waste Rock is by-product of excavation activities for residential subdivisional development and drainage lines as well as other subsurface infrastructure. Traditionally, WR, excavated during site preparation, would have been disposed as waste into landfill [3]. RCA and CB are by-products of construction and demolition activities of buildings and structures.

The response of particulate layers under traffic loading is normally characterized by the resilient modulus test. However, the true nature of the deformation mechanism of aggregates in pavement layers is still not fully understood [4] leading to continuous maintenance and rehabilitation work in the pavement industry. It has been accepted that the deformation of granular materials under loading is the consequence of three major mechanisms: consolidation, particle rearrangement, and breakage [5]. The consolidation mechanism is the alteration in compressibility of particle assemblies while the particle rearrangement mechanism includes sliding and rolling of particles. The breakage mechanism is the crushing that occurs when the applied load exceeds the strength of the particles. Crushing is a progressive process that can initiate at relatively low stresses, change the soil fabric and packing gradually, and cause serious issues such as settlement and reduction in hydraulic conductivity of the soil [6], [7]. In addition, the engineering characteristics of a granular assemblage, including friction angle, shear strength, and constitutive behaviour, have been shown to be dependent on properties altered by breakage [8]. Particle crushing causes dramatic changes in particle size and shape [9]. Considering the inherent links between particle size and shape, it is reasonable to hypothesize that wherever size matters, shape has to be important too [10], [11]. A review of the latest literature reveals that although the measurement, description and application of particle shape have recently experienced a great leap forward, studies on particle shape in 3D are limited. The most likely reason for this shortcoming is the fact that 3D particle shape is more difficult to measure and quantify [12], [13].

Recently, Discrete Element Modelling (DEM) has been used in a number of studies to explore the micromechanical breakage behaviour, which mostly provides information on the evolution of the particle size distribution during breakage [8]. Some studies have utilised agglomerate-based models to study breakage (e.g. [14], [15]), but they were unable to quantify the particle shapes forming as a result.

Non-destructive testing methods have also recently become popular in fields such as material sciences and geomechanics to study granular materials at particle-scale. Among different methods, wave-based techniques such as X-ray Micro-Computed Tomography (μCT) are emerging as a powerful tool to study a wide range of materials in terms of deformation and density as exemplified in [16], [17], [18], [19], [20]. X-ray computed tomography has been used to observe the interior microstructure of samples without physically penetrating its surface. 3D CT images offer rich information about the entire specimen in contrast with point-wise data [18], [21], [22]. The recent advances in X-ray tomography, with synchrotron sources and sensitive detectors, have provided a powerful tool to obtain much finer spatial resolution of geomaterials, such as particle-scale characterization of sand undertaken by [23], [24]. Nevertheless, investigations into the evolution of particle properties due to breakage, in particular morphology evolution, have been hampered due to the time-consuming image processing and segmentation procedure of CT images, where thousands of particles and newly generated smaller fragments have to be segmented and labelled accurately.

This study aims to gain a better understanding of soil behaviour in relation to particle-scale damage by examining the changes in the grading and particle morphology of specimens. Synchrotron Radiation-based X-ray Micro-Computed Tomography (SR-μCT) was used as a powerful means for analysing particle breakage and subsequent changes in granular C&D assemblies at particle-scale, impossible to achieve using conventional laboratory tests. A synchrotron light is a monochromatic highly culminated X-ray source that produces a beam with a high flux and a specific energy level [25], [26]. The synchrotron beam utilised in this study, in contrast with laboratory scanners, provides a comparatively rapid scanning technique than other available CT scanners and is highly efficient given the sequential loading-imaging cycles required to characterise progressive deformation and breakage. An in-situ particle assembly compression apparatus was also designed to carry out crushing tests on different kinds of C&D assemblies under confinement during scanning (Fig. 2).

Uniformly graded and spectrum of C&D materials with particle diameters (d) ranging from 0.425 to 4.75 mm, were scanned under 0, 5, 10 and 15 kN constraint compressive loading (i.e. 0, 10.2, 20.4, and 30.6 MPa). The particle size and shape distribution after each loading sequence were obtained from precise 3D reconstruction of 2159 CT horizontal slices using Avizo 9 package. Image processing techniques, including image thresholding, filtering, and segmentation, were used to segment and label each fragment in the granular assemblies.

Section snippets

Tested materials

To identify mineralogical and microstructural aspects of C&D particles, Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) were used. A Scanning Electron Microscope scatters a beam of electrons to scan samples. The electron beam interacts with atoms in the specimen and produces a variety of signals containing microstructural information of sample surfaces [27]. Fig. 3 shows a selection of SEM images of WR, RCA, and CB particles captured at a voltage level of 20 kV

Synchrotron radiation-based X-ray micro-computed tomography

A synchrotron source consists of main components shown in Fig. 4. Firstly, a barium cathode is heated to nearly 1000 °C in an electron gun in order to generate electrons. Secondly, in an accelerator, the electrons are accelerated to 99.99% of the speed of light. Thirdly, the electrons are transferred to the first ring, i.e. booster ring, where their energy is increased/boosted using a radio frequency current of 3 GHz. Fourthly, electrons circulate for about 30–40 h in the second ring, i.e. storage

3D reconstruction

Locally adaptive thresholding was used to segment particles in the images. This method uses a variable thresholding value based on local image characteristics rather than using a single global value [35]. To increase the accuracy of the segmentation process, images went through different pre- and post-processing techniques using erosion or dilation morphological operations (a summary of different techniques can be found in [36]) to remove artefacts in the original image. Nevertheless, extensive

Fractal distribution

The initial particle size distribution is one of the main factors controlling the macro-mechanical behaviour of a particulate assemblage. More importantly, the manner in which the particle size distribution changes during loading can assist in predicting the material behaviour, particularly the potential for further breakage [39]. The particle size distribution of the confined C&D assemblies under different one dimensional constraint compressive loading is illustrated in Fig. 6, Fig. 7, Fig. 8.

Conclusion

Particle breakage results in changes in properties of granular materials, particularly in the particle size distribution and particle morphology. These alterations affect the macro-mechanical behaviour of particulate systems and most prominently, the material's crushing strength against further breakage. Various granular materials, in terms of mineralogy, microstructure, and particle size (i.e. C&D specimens) under different loading levels, were studied using cutting-edge synchrotron

Acknowledgment

This research was undertaken on Imaging and Medical Beam Line (IMBL) at the Australian Synchrotron, Victoria, Australia. The authors would like to acknowledge Dr. Anton Maksimenko, Dr. Robert Acres, and the other beam scientists at Australian Synchrotron for their support during our experiments.

References (54)

  • G. McDowell et al.

    The fractal crushing of granular materials

    Journal of the Mechanics and Physics of Solids

    (1996)
  • W.K. Hartmann

    Terrestrial, lunar, and interplanetary rock fragmentation

    Icarus

    (1969)
  • M. Takei et al.

    Time-dependent behavior of crushable materials in one-dimensional compression tests

    Soils Found.

    (2001)
  • T. Afshar et al.

    Impact of particle shape on breakage of recycled construction and demolition aggregates

    Powder Technol.

    (2017)
  • A. Arulrajah et al.

    Geotechnical and Geoenvironmental properties of recycled construction and demolition materials in pavement subbase applications

    J. Mater. Civ. Eng.

    (2013)
  • J. McAndrew et al.

    Regional guide to Victorian

  • A. Arulrajah et al.

    Resilient moduli response of recycled construction and demolition materials in pavement subbase applications

    J. Mater. Civ. Eng.

    (2013)
  • M. Luong

    Mechanical Aspects and Thermal Effects of Cohesionless Soils under Cyclic and Transient Loading

    (1982)
  • F. Lekarp et al.

    State of the art. I: resilient response of unbound aggregates

    J. Transp. Eng.

    (2000)
  • F. Lekarp et al.

    State of the art. II: permanent strain response of unbound aggregates

    J. Transp. Eng.

    (2000)
  • G. Miao et al.

    Breakage and ultimate states for a carbonate sand

    Géotechnique

    (2013)
  • G.-C. Cho et al.

    Particle shape effects on packing density, stiffness, and strength: natural and crushed sands

    J. Geotech. Geoenviron.

    (2006)
  • J.P.D. Bono et al.

    DEM of triaxial tests on crushable cemented sand

    Granul. Matter

    (2014)
  • M.B. Cil et al.

    3D evolution of sand fracture under 1D compression

    Géotechnique

    (2014)
  • M. Takahashi et al.

    Microstructure in shear band observed by microfocus X-ray computed tomography

    Geotechnique

    (2004)
  • K.A. Alshibli et al.

    Assessment of localized deformations in sand using X-ray computed tomography, ASTM

    Geotech. Test. J.

    (2000)
  • T.M. Evans

    Microscale physical and numerical investigations of shear banding in granular soils

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