Shear Strength Properties of G1 Aggregate

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Motivation for Research

At present the Standard Specifications for Road and Bridge Works for State Road Authorities, COLTO (1998), which is used in the South African road construction industry, makes little direct provision for assessing the surface properties of crushed rock materials which may affect the material’s shear strength properties. The material specification for G1 aggregate is summarised on Table 3602/1 (page 3600-2) of COLTO (1998) and makes reference to only two attributes related to particle shape or texture:  Flakiness Index: “Flakiness index, determined in accordance with TMH1 method B3, shall not exceed 35 on each of the -26,5 + 19 mm fraction and the -19 + 13,2 mm fraction.”  Fractured Faces: “All faces shall be fractured faces.”
To date the shortcomings of assessing particle shape and texture properties comprehensively have been emphasised by many authors and the difficulty of deriving a single parameter to quantify these properties has been emphasised (e.g. Semmelink, 1991). This has specific reference to crushed rock materials to be utilised for base (or even sometimes sub-base material) in road construction. The COLTO (1998) specifications make provision for the shape of the aggregate (in terms of the flakiness index) as well as crushing properties of the aggregate material, such as aggregate crushing value (ACV) and 10 % Fines Aggregate Crushing Value (10 % FACT). Criticism has been raised, however, against the conventional means of assessing the plate-based flakiness index (Anochie-Boateng, 2010; Fernlund, 2005).
The fundamental problem seems to stem from the fact that to date new methods or equipment have not been able to effectively refine or study the particle texture and/or shape properties. In addition, no consideration is given to the impact the crushing equipment or process used in the quarry for production of the aggregate has on the surface texture of the rock aggregate or the relationship between this and the performance of the material in roads, both during construction and in-service. The main reason for this is the difficulty in quantifying surface texture, an issue which appears to plague the industry on a worldwide level. With all of the above in mind, it is clear that there is a lack of comprehensive quantification and/or evaluation of the surface textural properties of a material used in road layer works, more specifically, crushed rock aggregates. Assessment of the shear strength properties of construction materials has also been somewhat limited to date. This is largely due to the fact that the shear strength properties of any given aggregate will be affected by the shape and texture of the particles making up the sample. Equipment for quantifying the shear strength of aggregate materials is also of limited availability. This research therefore seeks to address both issues by investigating particles properties and shear strength properties with the aid of new equipment. In order to address the shortcomings described above, a new approach was developed towards assessing crushed rock aggregate used for G1 material in road construction in South Africa. The motivation behind the research was to develop an accurate method of assessing particle shape and texture properties, which has not been achieved in the South African industry before. Experimental research was undertaken to determine whether a new approach could be developed which was successful or promising enough to justify the pursuit of further, extensive research. The research was further expanded to determine whether the newly derived method could be related to the shear strength properties of the G1 materials, with the motivation being that a better understanding of shear strength properties would improve pavement engineering. This research overlaps and shares interests with a number of current research projects being conducted by the Council for Scientific and Industrial Research (CSIR) in South Africa, utilising a new and innovative three dimensional laser-based scanning system to study the shape and surface properties of construction materials.
The research carried out here, as well as associated research being conducted by the CSIR, share a number of common goals and utilise some of the same bulk crushed rock material samples. Literature Review Existing literature sources outlining experimental methods to determine particle shape or texture characteristics of aggregate materials are reviewed. Although the majority of these methods are only briefly described a range of aggregate properties which may affect the shape and texture properties of the crushed aggregate materials tested are identified. In essence, the strength of an aggregate material used in road layer-work construction can be related to its shear strength properties (in combination with its compacted density and prevailing moisture content). Adequate shear strength is required to ensure satisfactory performance of aggregate in layer works (Maree, 1979). The shear strength properties are related to Coulomb’s Law, as applied to general soil mechanics and is discussed extensively by Cernica (1982). The shear strength of a material, as described by the Mohr-Coulomb failure criteria, can be calculated as indicated in Equation 2.1: …

Table of Contents :

• Declaration
• Abstract
• Acknowledgements
• List of Symbols and Abbreviations
• 1. Introduction
  • 1.1 Motivation for Research
  • 1.2 Aim of Research
• 2. Literature Review
  • 2.1 Flakiness Index
    • 2.1.1 Flakiness Index Test Description
    • 2.1.2 Criticism of the Flakiness Index
  • 2.2 Effects of Particle Shape and Surface Texture
    • 2.2.1 Particle Shape and Surface Texture
  • 2.3 Inherent Influences on Particle Shape and Texture
    • 2.3.1 Geological Origin
    • 2.3.2 The Effects of Micro-cracking
  • 2.4 External Influences on Particle Shape and Texture
    • 2.4.1 Effects of Blasting
    • 2.4.2 Pre-Crushing and Crushing
    • 2.4.3 Grain Crushing
  • 2.5 Advanced Methods of Assessing Particle Shape Properties
    • 2.5.1 Angularity Index
    • 2.5.2 Aggregate Imaging System
    • 2.5.3 Image Analysis
    • 2.5.4 Principal Component Analyses
    • 2.5.5 Laser Profiling
    • 2.5.6 Videographer – Shadowgraph
    • 2.5.7 Videographer – Automation
    • 2.5.8 Laser Scanning

• 3 Methodology
  • 3.1 Introduction
  • 3.2 Sample Selection
  • 3.3 Sample Preparation
    • 3.3.1 Concept of Sample Preparation for Laboratory Analyses
    • 3.3.2 Synthesis of Samples One to Five
    • 3.3.3 Synthesis of Sample Six
    • 3.3.4 Microscope Thin Section of Fines Substitute Mixture
  • 3.4 Laboratory Tests
    • 3.4.1 Grading Analysis
    • 3.4.2 Compaction Properties
    • 3.4.3 Sample Preparation and Vibration Table Compaction
    • 3.4.4 Multi-Stage Tri-axial Tests
    • 3.4.5 Tri-axial Tests
  • 3.5 Mineralogy Review
  • 3.6 Laser Scanning
    • 3.6.1 Equipment and Data Capture
    • 3.6.2 Sample Selection for Scanning
    • 3.6.3 Data Application and Processing
• 4. Results and Analysis
  • 4.1 Laboratory Sample Preparation
    • 4.1.1 Sample Synthesis
    • 4.1.2 Control Grading
    • 4.2.2 Particle Crushing
  • 4.2 Practical Aspects Related to Particle Scanning
    • 4.2.1 Advantages of Scanner System
    • 4.2.2 Limitations of Scanner System
  • 4.3 Model Development
    • 4.3.1 Working Model One
    • 4.3.2 Working Model Two
    • 4.3.3 Comparison of Models
  • 4.4 Detailed Models
    • 4.4.1 Data Analyses
    • 4.4.2 Model Development Procedure
  • 4.5 Method Application
  • 4.6 Influence of Different Material Types
  • 4.7 Shear Strength Properties of G1 Aggregate
    • 4.7.1 Tri-axial Tests
    • 4.7.2 Mohr-Coulomb Failure Criteria and Mohr Circles
  • 4.8 Comparison between Multi-Stage and Conventional Tri-axial Tests
• 5 Application
  • 5.1 Model Application – Modified Dolerite
    • 5.1.1 Flakiness Index
    • 5.1.2 Particle Texture
  • 5.2 Correlation Between Shear Strength Properties and Particle Properties
• 6 Conclusions and Findings
  • 6.1 Particle Grain Crushing
  • 6.2 Development and Application of Scanner-based Models
    • 6.2.1 Scanner Properties and Attributes
    • 6.2.2 Preliminary Models
    • 6.2.3 Working Scanner-based Models
  • 6.3 Flakiness Index
  • 6.4 Shear Strength Properties
    • 6.4.1 Shear Strength of Aggregate
    • 6.4.2 Aggregate Texture Value and Shear Strength Properties
  • 6.4.3 The Use of Multi-Stage Tri-Axial Tests
  • 6.5 Final Conclusion
• 7 Recommendations and Further Work
  • 7.1 Data Base Compilation
  • 7.2 Flakiness Index
  • 7.3 Aggregate Texture Value and Shear Strength Properties
  • 7.4 Multi-Stage Tri-Axial Test Method
• 8 References
  • Addendum B: Thin Section Review of Rock Structures

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