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NUMERICAL MODELING OF FIBRE-REINFORCED CONCRETE

ABSTRACT

This paper reports the findings of a study into the behaviour of steel-fibre-reinforced concrete (SFRC) using non-linear finite-element analysis and existing experimental data. The overall aim of the research work is to formulate a reappraisal of the way in which stresses, deformations and cracking of such structural elements are predicted at present under both static and dynamic loading and how these predictions can be used to influence design decisions. The literature survey that preceded the work helped identify major shortcoming in the way SFRC mechanical properties are classified and presented at the moment and the lack of a unified approach to selecting a suitable model for general analysis and design purposes. There is also a clear gap in the literature on the application of SFRC constitutive models to study the potential of applying SFRC to enhance the seismic response of a structure and to assess the potential ductility and energy absorption capacity of such composites.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

CHAPTER ONE

INTRODUCTION
1. BACKGROUND OF THE PROJECT
1. PROBLEM STATEMENT
1. AIM OF THE STUDY
1. SCOPE OF THE STUDY
1. APPLICATION OF THE STUDY
1. PROJECT ORGANISATION

CHAPTER TWO

LITERATURE REVIEW

OVERVIEW OF FIBER-REINFORCED CONCRETE (FRC)
- REVIEW OF THE STUDY
- THE EVALUATION OF LONGITUDINAL SHEAR IN REINFORCED CONCRETE MEMBERS
- HISTORICAL BACKGROUND OF FIBRE REINFORCED CONCRETE
- REVIEW OF DIFFERENT TYPES REINFORCED CONCRETE

CHAPTER THREE

METHODOLOGY

MATERIAL MODELS FOR SFRC
- ABAQUS CONSTITUTIVE MODELS ADOPTED
- CALIBRATION OF THE NUMERICAL MODELS

CHAPTER FOUR

RESULT AND DISCUSSION

CHAPTER FIVE

CONCLUSION
- RECOMMENDATION
- REFERENCES

CHAPTER ONE

1.0 INTRODUCTION

Fibre-reinforced concrete (FRC) is a cement-based composite material reinforced with discrete, usually randomly distributed, fibres. Fibres of various shapes and sizes produced from steel, synthetics, glass, and natural materials can be used. However, for most structural purposes, steel fibres are the most used of all fibre materials, whereas synthetic fibres (e.g. polypropylene and nylon) are mainly used to control the early cracking (plastic-shrinkage cracks) in slabs [2005]. Fibre reinforcement mainly enhances the post-cracking properties of concrete and leads to a more ductile material behavior. The increased ductility is due to the ability of the fibres to transfer tensile stresses across a cracked section, potentially leading to a reduction in crack widths. The extent of the crack-width reduction depends on the amount of fibres added as well as their physical properties (e.g. surface roughness and chemical stability) and mechanical properties (e.g. tensile strength).

Typical applications where steel fibres may be used as sole reinforcement include slabs on grade and tunnel linings. In other applications the steel fibres are used as a complement to the conventional reinforcement, where, in some cases, the amount of conventional reinforcement can be reduced. Extensive research has been carried out by technical committees in several countries, such as RILEM TC 162-TDF [2003], and CNR-DT 204/2006 [2002], which has resulted in recommendations/guidelines for design of steel-fibre-reinforced concrete (SFRC). Although the use of SFRC in structural applications is already a common practice within the construction field, generally accepted design methods have not yet been established. Due to this, many engineers are hesitant to use SFRC. If the technique with fibre-reinforced concrete is to be further developed and accepted by practicing engineers, the concrete community should agree about the design methods to use, refine them, and introduce them in codes.

The structural response of SFRC elements is characterised by its tensile strain-softening behaviour. A number of available constitutive models for SFRC have been identified such as those proposed by RILEM, Barros, Lok, Tlemat and others. The main characteristics of the models have been closely studied. Non-linear finite-element analysis was used to calibrate these models and, ultimately, one model has been selected for the subsequent parametric studies on SFRC under seismic conditions. This was achieved by incorporating the models into ABAQUS (smeared cracking and brittle cracking) models for concrete and then carrying out comparisons between ABAQUS predictions and existing experimental data on SFRC beams. This paper summarizes the first phase of the work which focused on static loading. Further calibration is underway to compare the numerical predictions with the results of full-scale tests on SFRC beams and column-beam joints under cyclic and seismic loading.

1.1 BACKGROUND OF THE STUDY

Combining concrete with dispersed “fibres” consisting of grains of steel left-overs is an idea patented already in 1874 by the American A. Berard, thus creating a new more ductile material. Today steel and synthetic fibres are used for both non-structural and structural purposes. Although it has been found that adding fibres to concrete mainly enhances the post-cracking properties in terms of a more ductile behaviour and reduced crack widths (e.g. Stang & Aarre [1992] and Löfgren [2005]), it still remains to show that these enhanced mechanical properties can be predicted with reasonable accuracy and that they can be incorporated into design methods.

Many attempts have been made to develop methods/models which can predict the behaviour (especially in bending) of fibre-reinforced concrete members, e.g. Zhang & Stang [1998], Lok & Xiao [1999] and Casanova & Rossi [1997]. Two main approaches may be identified: (1) obtaining material properties by testing, or (2) estimating material properties on a theoretical basis (e.g. [1999]). The obtained material properties may be used in e.g. FEM analyses (often based on fracture mechanics), e.g. [Zhang & Stang, 1998], or analytical models e.g. based on the non-linear hinge concept as in [1997]. One theory based on the theoretical approach is that if the pull-out behaviour of one fibre can be described, then by considering different factors, the behaviour of a specimen with randomly distributed fibres can be described. These factors can be e.g. the shape and size of the concrete specimen, the orientation and amount of fibres added, and the geometrical and mechanical properties of the fibres, e.g. length and cross-sectional shape. To estimate how effective the fibres are in a certain FRC mixture, a so-called fibre-efficiency factor can be estimated. One proposed method to consider this factor for differently shaped and sized specimens can be found in Voo and Foster [2003] and also in the Norwegian guidelines [2006]. In general the fibre-efficiency factor depends on the number of fibres that bridge a crack (calculated or estimated) and the fibre orientation (i.e. the fibre alignment with reference to the crack surface). Additionally, it considers the thickness and height of the specimen in relation to the fibre geometry and amount of fibres added. In a small flat body (1-D) this factor is 1.0, compared to an infinitely large body (3-D), where it theoretically may be estimated as 0.5.

To derive the material properties used in the currently available design methods, several test methods can be used, although the bending test is the most frequently recommended. Since this test method yields results in the form of flexural capacities (bending load–CMOD or bending load–deflection) (CMOD = crack-mouth-opening displacement), and the calculation methods usually require direct tensile capacities, a translation method is needed. A question which arises is how to perform this translation in the best possible way. All of the reviewed design methods propose similar methods for how the flexural strength can be translated into direct tensile strength. The differences between the methods can be found in the coefficient used to translate flexural stress into tensile stress and in the way measured crack opening is translated into strain. In one respect the Italian method takes a step forward compared to the other methods, by the introduction of a so-called characteristic length l_cs, which depends on the calculated average crack spacing, or either the height of the tensile zone if conventional reinforcement is present or without rebars l_cs = the cross-section height [2006].

1.2 PROBLEM STATEMENT

The problem of using concrete includes poor tensile strength, low strain of fracture and formwork requirement. The major disadvantage is that concrete develops micro cracks during curing. It is the rapid propagation of these micro cracks under applied stress that is responsible for the low tensile strength of the material. Hence fibres are added to concrete to overcome these disadvantages.

The addition of fibres in the matrix has many important effects. Most notable among the improved mechanical characteristics of Fibre Reinforced Concrete (FRC) are its superior fracture strength, toughness, impact resistance, flexural strength resistance to fatigue, improving fatigue performance is one of the primary reasons for the extensive use of Steel Fibre Reinforced Concrete(SFRC)in pavements, bridge decks, offshore structures and machine foundation, where the composite is subjected to cyclically varying load during its lifetime.

The main reasons for adding steel fibres to concrete matrix is to improve the post- cracking response of the concrete, i.e., to improve its energy absorption capacity and apparent ductility and to provide crack resistance and crack control. Also, it helps to maintain structural integrity and cohesiveness in the material. The initial researches combined with the large volume of follow up research have led to the development of a wide variety of material formulations that fit the definition of Fibre Reinforced Concrete.

Steel fibre’s tensile strength, modulus of elasticity, stiffness modulus and mechanical deformations provide an excellent means of internal mechanical interlock. This provides a user friendly product with increased ductility that can be used in applications of high impact and fatigue loading without the fear of brittle concrete failure. Thus, SFRC exhibits better performance not only under static and quasi-statically applied loads but also under fatigue, impact, and impulsive loading.

1.3 AIM OF THE STUDY

The main aim of this work is to analyse the numerical modeling of fibre reinforced concrete. Mechanical properties and durability of fiber reinforced concrete.

1.4 SCOPE OF THE STUDY

The usefulness of fiber reinforced concrete (FRC) in various civil engineering applications is indisputable.

Fiber-reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers

This study presents numerical modeling of fibre reinforced concrete. Mechanical properties and durability of fiber reinforced concrete.

1.5 APPLICATION OF FIBER REINFORCED CONCRETE

Fiber reinforced concrete has so far been successfully used in slabs on grade, architectural panels, precast products, offshore structures, structures in seismic regions, thin and thick repairs, crash barriers, footings, hydraulic structures and many other applications. Fiber Reinforced Concrete (FRC) is gaining attention as an effective way to improve the performance of concrete. Fibers are currently being specified in tunneling, bridge decks, pavements, loading docks, thin unbonded overlays, concrete pads, and concretes slabs. These applications of fiber reinforced concrete are becoming increasingly popular and are exhibiting excellent performance.

1.6 PROJECT ORGANISATION

The work is organized as follows: chapter one discuses the introductory part of the work, chapter two presents the literature review of the study, chapter three describes the methods applied, chapter four discusses the results of the work, chapter five summarizes the research outcomes and the recommendations.

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