CONTRIBUTION TO STATE OF KNOWLEDGE

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LITERATURE STUDY

INTRODUCTION

The literature study is divided into two sections, the first focussing on historical literature and the second focussing on current literature. Historical literature includes work that has been published and has been tested by other researchers over the years. Historical literature related to fundamental knowledge regarding materials, resilient behaviour and resilient response modelling is overviewed. Current literature includes work that has been completed since January 2011 under the SAPDM Project, but that is not publicly available at this stage. The work that is included is mainly project literature. The intention of the SAPDM sponsors is to publish work upon the completion of the project. The current literature related to resilient behaviour and resilient response modelling is overviewed. It is important to unpack and understand the keywords in the title of this thesis. Therefore, this historical literature review focuses on keywords in the thesis title as depicted by Figure 2.1.

 HISTORICAL LITERATURE

Natural Material

Introduction

Unbound granular material is of particular importance world-wide and especially in South Africa, since granular material is used in the pavement structure and usually comprises the bulk of the pavement’s structural and foundation layers. This is evident in the quantities of granular material produced and used world-wide. Constantino (2012) summarised information reported to the United States (US) Geological Survey that aggregate production increased overall in the US in the first quarter of 2012. An estimated 216 million metric tons (Mt) of crushed stone were produced and shipped for consumption (increase of 9.4 percent), construction sand and gravel produced and shipped for consumption was 136 Mt (increase of 10 percent) and construction aggregates produced and shipped for consumption was 352 Mt (increase of 9.7 percent) compared with that of the same period in 2011. In South Africa the production of sand and aggregate historically also increased, in 2005 by 5.5 per cent (49 970 kt), in relation to 47 382 kt produced in 2004 (Baloyi, 2006). The annual per capita consumption of construction aggregate (G1 to G10) is about 2 000 kg per capita in South Africa and 10 000 kg per capita in the United States (ASPASA, 2006).

Definition

Natural material is obtained from various geological formations. Each formation has a characteristic upper portion depending on the climate and topography, consisting of various thicknesses of residual weathering products or transported materials. These weathering products or transported materials are defined as stone, gravel or sand. Not all natural materials are suitable for road construction and more often than not, surface materials are considered unsuitable (Steyn and Paige-Green, 2009).

Material properties

Material properties are those characteristics of a material that distinguishes it from other materials. In pavement engineering there are a number of properties that are regarded as essential fundamental characteristics. These characteristics are obtained through routine tests such as:

  • Sieve analysis (grading) to determine the particle size distribution of the material;
  • Atterberg indicator tests to characterise the fine fraction of the material, such as plastic limit,  liquid limit, plasticity index and linear shrinkage;
  • Volumetric properties characterised through for example Apparent Relative Density (ARD) and  Bulk Relative Density (BRD) and water absorption;
  • Gravimetric properties characterised through  Maximum Dry Density (MDD)  and Optimum  Moisture Content (OMC) using a recognised compaction method; and  California Bearing Ratio (CBR) of the material to indicate the materials bearing capacity;

These fundamental characteristics or properties are used to classify a material into a specific group of which the behaviour and limitations when used in pavement structure can be deduced. Section 2.2.1.4 discusses material classification and which properties are used in the classification system mainly referred to in this thesis.

Classification

The term ‘unbound granular material’ refers to the classification of natural material, which has not been modified in any way, as defined in TRH4 (1996) and TRH14 (1985). Unbound granular material is classified from a G1 to G10 according to its fundamental behaviour and strength characteristics (TRH4, 1996, TRH14, 1985).
A G1 quality material is defined as a ‘graded crushed stone’, usually obtained from crushing solid unweathered quarried – or mined rock or boulders. G2 and G3 quality material are obtained by the same process as a G1 quality material, but may contain natural fines not derived from crushing the parent rock. G4, G5 and G6 quality material are defined as ‘natural gravel or a mixture of natural gravel and boulders which may require crushing’. Any of these materials may be modified using for example cement, lime, bitumen or polymers to enhance certain strength characteristics of the material. G7, G8, G9 and G10 quality material are defined as gravel-soil (TRH14, 1985).
In this thesis the term ‘crushed stone’ or ‘crushed aggregate’ is used to refer to G1 to G3 quality material and ‘natural material’ will refer to G4 to G10 quality material. ‘Unbound granular material’ refers to both crushed stone and natural material (i.e. G1 to G10).
Table 2.1 gives an abbreviated summary of the criteria used to classify unbound granular materials in terms of TRH 4 and TRH 14, as well as indicating the approximate similar AASHTO classification for the same material.
Classifying a material that has been in use for some time may be more complex. Jooste et al. (2007) developed a material classification approach for consistent classification of pavement materials based on the TRH 4 (1996) and TRH14 (1985) classification system. In this approach, all available information is used to give a consistent, rational and objective assessment of a material class. Utilising all the information, the certainty that a material belongs to a particular material class is determined using Fuzzy Logic and Certainty Theory. Certainty theory in essence provides the framework from which hypotheses can be tested using vague and uncertain evidence (Jooste et al., 2007). In the application of this approach, a Certainty Factor (CF) associated with a specific test is assigned. This is done by defining a simple distribution of the data set for that specific test/parameter using the 10th, median and 90th percentile statistics. Most pavement materials tests provide only a partial indication of material behaviour, and therefore a certainty factor is assigned to each test. This certainty factor represents the confidence in the ability of a test to serve as an accurate indicator of material performance in the pavement layer (0 represents poor and 1 absolute confidence) (TG 2 Appendix A, 2009). Table
The result of this approach is a Design Equivalent Materials Class (DEMAC). ‘The DEMAC denotes a material that exhibits shear strength, stiffness, durability and flexibility properties similar to a newly
constructed material of the same class. The DEMAC implies that the material may not meet the exact specifications for a particular material class, but in terms of behaviour the material is similar’
(TG2, 2009).
In this thesis material are classified using the TRH 4 (1996) and TRH14 (1985), DEMAC (Jooste TG2, 2009) and AASHTO (1928) systems.

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Levels of material property evaluation

The criteria used in Table 2.1 to classify natural material are based on inherent material properties.
Material properties can be evaluated on three levels, namely performance properties, engineering
properties and fundamental properties (Steyn, 2007).
The most basic level of evaluation is by performance properties. A performance property does not indicate why this specific behaviour occurs, or which properties of the material influence the performance. It does indicate what can be expected from this specific material in this specific state and under the specific conditions. For example, performance properties such as permanent deformation of a G5 material at a certain moisture content, PI and swell are only indicative of that specific scenario and cannot indicate behaviour when a different G5 material is subjected to the specific conditions or the conditions change, higher moisture content, lower PI and lower swell (Steyn, 2007).
Engineering properties of a specific material provide a more detailed understanding of the material and its potential responses to loading conditions or changes, while remaining constant for a wider range of parameters. For example, an engineering property such as the stiffness of a material will remain constant at particular moisture content and density, but change under different loading conditions (Steyn, 2007).
Properties which are not determined by external conditions (such as temperature or load) but always remain the same and directly influence the engineering and performance properties of the material are called fundamental properties. For example, from the grading of a granular material the expected performance as well as engineering properties can be derived (Steyn, 2007). Atterberg Indicator tests include Plasticity Index (PI), Liquid Limit (LL), Grading Modulus (GM), etc. and are often referred to as the fundamental properties of the material.
Classifying material according to engineering or fundamental properties and using that in modelling the behaviour of material is the aim of any good model. Figure 2.2 illustrates schematically the levels of evaluation that can be applied to material properties. In Chapter 4 and 5, linking a material model to fundamental properties will be explored.

 Resilient response

 Basic definitions

There are basic mechanics of material definitions which must be defined to avoid misunderstanding of terms in the following sections. These are as follows:
Elasticity, stress and strain
Elasticity is the physical property of a material by which it returns to its original dimensions during
unloading (i.e. the removal of stress) (Gere, 2001). Stress is defined by the force exerted on a specific area, while strain is the ratio of elongation to length at infinitely small scale (Gere, 2001; Dawson, 2009).
Stiffness
Stiffness is the resistance of a body to deformation by an applied force. Elastic modulus is sometimes used as an indication of the stiffness of a material (Gere, 2001).
Resilience
Resilience represents the ability of a material to absorb and release energy within the elastic range of that material (Gere, 2001).
Elastic modulus / Hooke’s Law
Elastic modulus or modulus of elasticity is described by Hooke’s Law, after Robert Hooke. It describes elastic modulus through the equation σ = E(ϵ) in which σ is stress, ϵ is strain and E a constant of proportionality known as modulus of elasticity. It defines the linear relationship between applied loads (stress) and resulting elongation (strain) (Gere, 2001). It states that strain is directly proportional to stress throughout a materials elastic range, i.e. for stresses below the yield strength. The ideal elastic behaviour of an ideal material is illustrated in Figure 2.3 and denoted by numbers 1 to 4. Number 3 on the curve indicate the elastic limit after which permanent deformation takes place until the material yields at number 4 on the curve. The position of numbers 1 to 4 on the stress strain curve is determined by the stress as the curve dimensions increase with increasing stress (Gere, 2001).

1 INTRODUCTION & BACKGROUND 
1.1 INTRODUCTION.
1.2 BACKGROUND.
1.3 PROBLEM DEFINITION.
1.4 OBJECTIVES
1.5 SCOPE
1.6 CONTRIBUTION TO STATE OF KNOWLEDGE
1.7 RESEARCH PROGRAM.
1.8 STRUCTURE OF REPORT
1.9 REFERENCES
2 LITERATURE STUDY
2.1 INTRODUCTION
2.2 HISTORICAL LITERATURE.
2.2.1 Natural Material .
2.2.1.1 Introduction
2.2.1.2 Definition
2.2.1.3 Material properties
2.2.1.4 Classification
2.2.1.5 Levels of material property evaluation
2.2.2 Resilient response
2.2.2.1 Basic definitions
2.2.2.2 Definition of resilient response for granular material
2.2.2.3 Factors influencing resilient response
2.2.3 Moisture
2.2.3.1 Expressing moisture content
2.2.3.2 Influence of moisture on resilient response
2.2.4 Suction pressure
2.2.4.1 Definition of suction
2.2.4.2 Matric suction
2.2.4.3 Osmotic suction
2.2.4.4 Suction measurement
2.2.4.5 Importance of estimation of resilient response
2.2.5 Modelling
2.2.5.1 Levels of technology for evaluation.
2.2.5.3 Resilient response models
2.2.5.4 Resilient response models incorporating moisture and density.
2.2.6 International researchers
2.2.6.1 Crockford et al. model
2.2.6.2 Lytton model
2.2.6.3 NCHRP Mechanistic Empirical Pavement Design Guide (MEPDG) model
2.2.6.4 George’s Mississippi sub-grade model
2.2.6.5 Long et al. model
2.2.6.6 Liang et al. model.
2.2.6.7 Cary and Zapata
2.2.7 South African researchers
2.2.7.1 Visser’s model
2.2.7.2 Emery’s model
2.2.7.3 Theyse’s 2009 model
2.2.8 Conclusions
2.3 CURRENT LITERATURE
2.3.1 Introduction
2.3.2 Background
2.3.3 SAPDM/B1-A
2.3.3.1 Introduction.
2.3.3.2 Model development
2.3.3.3 Discussion of models
2.3.3.4 Conclusions.
2.4 REFERENCES
3 METHODOLOGY 
3.1 INTRODUCTION.
3.2 RESEARCH DESIGN
3.3 METHODOLOGY
3.4 LIMITATIONS
3.5 REFERENCES
4 VERIFICATION OF RESILIENT RESPONSE MODEL
4.1 INTRODUCTION
4.2 MATERIAL TEST RESULTS
4.3 CALIBRATION OF CORD MODULUS MODEL VARIABLES
4.4 CONCLUSIONS
4.5 REFERENCES
5 THE CORD MODULUS MODEL FOR CRUSHED AND NATURAL UNBOUND MATERIAL
5.1 DISTINCTION BETWEEN CRUSHED AND NATURAL UNBOUND MATERIAL
5.2 CALIBRATION OF CORD MODULUS MODEL VARIABLES FOR CRUSHED AND NATURAL UNBOUND MATERIAL
5.3 PARAMETRIC ANALYSIS OF CALIBRATED CRUSHED AND NATURAL UNBOUND
MATERIAL CORD MODULUS MODELS.
5.4 LINK MODEL TO BASIC MATERIAL PROPERTIES
5.5 LINK MODULUS TO BASIC PROPERTIES THROUGH STATISTICAL DISTRIBUTIONS5
5.6 CONCLUSION
5.7 REFERENCES
6 CONCLUSIONS AND RECOMMENDATIONS
6.1 CONCLUSIONS
6.2 RECOMMENDATIONS
7 REFERENCES 
APPENDICES
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