CHAPTER 1
Within the following dissertation I aim to study the use of Fibre Reinforced Concrete within the construction industry.
Over the last decade, fibre reinforced concrete (FRC) has become widely used in different structural and non-structural applications such as pavements, floors, overlays, industrial slabs and shotcrete linings etc where the major concern is toughness and first crack in flexure. It is estimated that more than 150 000 metric tonnes of FRC has been used throughout this duration of time.
Particular focus will be made as to FRC within the construction industry whilst trying to identify what the future hold for this composite material.
The question will be posed at to what the general consensus is within the construction industry in regard to the use of fibre reinforcement within concrete.
1.2 Concrete and Fibre Reinforcement
Concrete is a material that is very strong in compression although is comparatively weak in tension. To compensate for this imbalance in the concrete’s behavior, an appropriate reinforcement must be cast into the concrete to help carry the tensile loads.
Two forms of reinforcement commonly used are Steel Fabric (Rebar) reinforcement and Fibre reinforcement.
Steel Fabric Rebar/Mesh has been used for many years in the construction industry to reinforce concrete and is usually made of carbon steel which is incorporated with ridges to help provide a better bond and adhesion to the concrete. As the rate that both steel and concrete expand and contract is the same this assists in eliminating any possible issues relating to any early signs of cracking within the concrete which could occur if the materials expanded and contracted contradictory to each other. This will assist in enhancing the overall strengthening of the structure.
Rebar comes in various lengths and thicknesses to accommodate different types and sizes of jobs. These bars can be tied together to form a grid or cage, this is extremely effective for larger projects or alternatively mesh can be delivered in a variety of standard sheet sizes to site.
An exciting alternative product to the above which could be used would be that of fibre reinforcement.
The idea of using a fibrous material to provide tensile strength to a material strong in compression but brittle loses itself in the mists of time; in ancient Egypt straw was added to clay mixtures in order to provide brick with enhanced flexural resistance, thus providing better handling properties after the brick had dried in the sun.
Fibre reinforced concrete is a composite material which entails a cementitious hydrated paste which is mixed with small reinforced filaments for instance glass, steel, polymer or carbon fibres which come in various shapes and sizes.
The multiple fibres restructure the energy within the concrete, preventing the procedure of formation and extension of cracks.
This helps to increase the ductility within the concrete whilst sustaining the residual capability during the post cracking period
The fibres within the concrete literally bond the sides of a forming crack together at the same time as preventing any visual damage from becoming apparent
Although FRC is an application that has been used extensively throughout Western Europe and beyond, its use within the UK has been somewhat restricted to the purpose of reinforcement within industrial floors as its main application.
The absence of a recognizably accepted design standard may have an influence on the situation although calls are currently being made for clear industrial guidelines to be certified.
The Concrete Society Technical Report 63 concentrates on any problematic issues and provides guidance for current and future design.
All information and design aspects associated with fibre reinforcement are incorporated within the British Standard Institution.
BS EN 14889, Fibres for concrete –
Part 1: Steel Fibres – Definition, specifications and conformity.
Part 2: Polymer Fibres – Definition, specifications and conformity.
1.3 Aim
The main aim of the following theses is to investigate and evaluate the various properties and functions of Fibre Reinforced Concrete (FRC) throughout the construction industry whilst giving consideration as to what lies ahead for the future of FRC.
1.4 Objectives
To provide a historic overview of Concrete and Fibre Reinforcement.
To identify and analyze the various types of fibre products available for concrete reinforcement.
To establish possible concrete mix performance enhancements associated with FRC.
To explore the Construction Management criteria within the industry in relation to the use of FRC.
To highlight the future possibilities and potential that FRC has within the construction industry whilst seeking the views and opinions of various construction professionals.
1.5 Rationale for Research
There are three main reasons why this particular topic has been chosen for this theses along with the required research which has been carried out, these are:
A personal interest in the material of concrete along with its various functions and applications due to work experience gained in this field.
An interest in new innovations being made available to the construction market which will enhance the overall application of the material.
An aspiration to investigate whilst enhancing my current knowledge on the theory of FRC
1.6 Methodology
Due to the nature of the topic to meet the aims and objectives previously set out for this thesis and to establish the direction for this piece of work information has been obtained from the following sources.
Primary Literature Academic Research Journals (Refereed)
Conferences / Seminars (Referred)
Government Publications
Technical Report Papers
Secondary Literature Construction Journals
Construction Textbooks
Library Search Indexes and Abstracts
Internet / World Wide Web
Library Catalogue
Case Study A case study of fibre reinforcement with regard to
Construction Management criteria will also be analyzed via various site visits prior and during any works being carried out.
This will provide a valuable insight as to how this product is perceived whilst strengthening the case for use in the future.
Questionnaire Questionnaires will also be distributed to various
Professional bodies influenced by the use of FRC within the construction industry.
The methodology adopted is fully discussed afterwards in Chapter 4 along with the analysis of the research findings in Chapter 5.
1.7 Chapter Overview
Chapter 1: Introduction
Provide a brief introduction into the areas of study which has been identified by the author whilst outlining the main aims and objectives to be achieved in order to fulfill the research criteria.
Chapter 2: Background for Study (Literature Search)
A review of the relevant theory and recent / current publications associated with Fibre Reinforced Concrete will be used to obtain the necessary information required to establish the Aim and Objectives which have been prepared by the author.
Chapter 3: Case Study
On site Case Study (tbc)
Chapter 4: Methodology
An evaluation of the types of research and methodology methods carried out in order to achieve the aims and objectives previously stated.
This will describe how and why the chosen research methods had been adopted.
Chapter 5: Data Interpretation and Analysis
This will consist of an analysis of the data and information generated from the questionnaire returns using relevant diagrams, tables and text to illustrate all findings.
Chapter 6: Conclusions and Recommendations
All findings from the research process will be compiled and it will be observed as to whether the main outlined objectives have been realized.
Recommendations will also be given on areas of further research to enhance the material within the theses.
CHAPTER 2
A REVIEW OF FIBRE REINFORCED CONCRETE
2.1 History of Concrete and Fibre Reinforcement.
‘The history of composite materials started in ancient Egypt over 2000 years ago with mud bricks, reinforced with straw fibres.’
(Crowther, 2009)
‘Concrete is the second most widely used material on earth after water’ (The Economist, 2006)
‘Concrete is a building material composed of cement, aggregate sand and water, often with chemical admixtures and other materials’ (Stanley & Bond, 1999).
Its modern development spans less than 200 years with 1824 being the patent date associated with the first Portland cement, according to Binns (2002), which is one of the most important milestones in concrete’s history.
Concrete is an ever present material and its versatility, comparative cheapness and energy efficiency have ensured that it is of great and increasing importance for all types of construction throughout the world’.
Many structures have concrete as their principal material, albeit as a composite with steel to give either reinforced or prestressed concrete, even in those structures where other materials such as steel or timber form the principal structural elements, concrete will normally still have an important role, for example in the foundations.
In its simplest form, concrete is a mixture of cement, water and aggregates in which the cement and water have combined to bind the aggregate particles together to form a monolithic whole. (Illston & Domone, 2001)
(Sutherland, 2009) explains that even though our knowledge and understanding of the material is far from complete, and research continues, concrete has been successfully used in many cultures and in many civilisations.
It is not just a modern material; the oldest concrete discovered so far is in southern Israel, and dates from about 7000 BC. It was used for flooring, and consists of quicklime, made by burning limestone, mixed with water and stone which set into a hardened material.
Fibres mixed into the concrete can provide an alternative to the provision of conventional rebar or steel fabric mesh in some applications. The concept has been in existence for many years (the first patent was applied for in 1874) and it has been used in a range of applications: amongst the first major uses was the patching of bomb craters in runways during World War 2. However, it was during the 1970’s that commercial use of this material began to gather momentum particularly in Europe, Japan and throughout America. (Clark, 2007)
Crowther (2009) reiterates that throughout the 1960’s research by various scientists which included Romauldi and Mandel who were industrial scientists,
In the 1960’s, studies by Industrial scientists Romauldi, Mandel and others, created the hypothetical and experimental foundations to help realise the development of a steel fibre product which could be used to as a means of reinforcing and strengthening concrete.
In the early 1970’s, James Romauldi who was initially involved in establishing the creation of the steel fibre patented the philosophy of steel fibre reinforced concrete, essentially inventing a new material for use within the construction industry.
Throughout the 20th century the appliance of asbestos fibre cement was extremely common and extensively utilised whilst more recently it has continued with the use of highly developed carbon fibre substances which have previously been more associated within the specialised aerospace and automotive industries.
Glass fibres were introduced and research continued into harnessing the properties of synthetic fibres such as polypropylene along with various other plastic materials.
By 2000, the first ‘macro fibre, concrete was ready for production from various suppliers. (Tarmac, 2009)
Fibre reinforced concrete is employed around the world on major construction projects which involve infrastructure works, tunnel linings, marine environments, pre cast and insitu walling systems, low shrinkage thin screeds, sprayed concrete applications and significant reinforcement structures.
Throughout the UK, its general application is in the use of large industrial concrete floors.
Characteristics which are extremely beneficial following the installation of the floors is the improved in impact resistance, this helps minimise any possible unnecessary damage where high volumes of belligerent traffic is expected.
The table below Figure 2 clearly shows how concrete is affected by the addition of fibres in various application fields:
Table 1 – Concrete improvement by fibres
Application field
Strength
Cracking
Wear
Closure systems
Tunnels
Sole plates / cellar walls
Sewer systems / pipes
Precast elements
Toppings / wear layers
Roads / airfield paving
Industrial floors
Figure 1
2.2 Types of Fibre Products used For Concrete Reinforcement
Although there are numerous fibre products on the market the most commonly used fibre types are shown and discussed below giving information on the producer of the fibre, its properties, fibre content in appliance along with the effects of the fibre type within the concrete matrix.
2.2.1 Steel Fibres
‘Concrete containing steel fibres has been shown to have substantially improved resistance to impact and greater ductility of failure in compression, flexure and torsion.’
(ACI Special publication sp-44)
(Bentur & Mindess, 1990) claim that it was throughout the early 1900’s that steel fibres were first introduced into being mixed with concrete to provide the required reinforcement.
The early design of steel fibres was relatively simple and consisted of a rather plain round and smooth design which was cut to the necessary lengths.
Only more recently has the introduction of hooked end, indented etched roughened surface, crimped and new polygonal twisted modern fibres have been introduced whilst the original smooth fibres have mainly disappeared.
More up to date fibres which are available commercially are contrived from drawn stainless steel wire or by a melting and moulding process which produce fibres which have a crescent profile cross section.
(Concrete Society TR 63, (2007)) points out that steel fibres are produced by various processes as discussed above and are supplied in many different shapes and sizes as is shown below in Figure 2. They may either be straight or deformed, however most are round in cross – section with diameters between 25 and 60 mm. Steel fibres have a tensile strength typically 2-3 times greater than that of traditional fabric reinforcement and a significantly greater surface area (for a given mass of steel) to develop bond with the concrete matrix.
Figure 2
(Neville, 1995) points out that the configurations of fibres can be either straight, continuous-deformed, or end formed as shown previous. Initially, straight fibers were the only configuration of steel fibers available. After further investigation it was quickly learned that there bonding potential was limited which restricted there expected contribution to the engineering properties of concrete.
New products were developed to increase the bond between the fibre and concrete, the two best configurations which emerged were: A hooked end, draw wire fibre and a continuously deformed, slit sheet steel fibre.
It was determined that the continuous deformed, slit sheet steel fibre provided better micro macro cracking performance as well as flexural strength enhancement, whereas the end deformed, drawn wire steel fibre performed best post first crack.
As a feature of steel fibre reinforcement concrete its improved ductility helps to greatly enhance the application where impact resistance is important.
The characteristic of fatigue resistance within the concrete is highlighted a being increased by up to 70%.
(Clark, 2007) explains that the addition of steel fibres as supplementary reinforcement in concrete minimise the possibility of spalling of the concrete surface due to any inclement temperatures or weather conditions
Some of the physical characteristics of fibres directly affect key aspects of concrete performance while others are less important. The factors considered to have the strongest influence on the performance of the steel fibre in concrete are:
Bond and Anchorage mechanisms (e.g. straight or deformed shape, end cones or hooked ends)
Fibre length and diameter.
Dosage used in concrete pours. (kg/m3)
Fibre count (number of fibres per kg of fibre), which is a function of fibre size and dosage.
Tensile Strength
Elastic Modulus
(Hannant, D 2002) outlines that fibre reinforcement in concrete act in various ways. Firstly they can remove the formation and development of cracks due to early age plastic settlement and drying shrinkage. Secondly, they may provide a degree of post cracking load carrying capacity.
The mechanisms are as follows:
Steel fibres, being randomly distributed in the concrete, intercept micro – cracks as they form, inhibiting the tendency for them to form into larger cracks.
After cracking, the fibres spanning the crack will provide a residual load carrying capacity. The capacity can be considerable, depending on dosage and the type of fibre used.
Following a shrinkage case study carried out by Propex concrete systems the photos below evaluate the initial 24 hours of curing time comparing a traditional reinforced slab with wire mesh against a fibre reinforced slab.
The traditional (control) slab started to crack within 3 hours whilst the fibre reinforced slab continued to perform whilst maintaining its integrity, this is shown below in Figure 3.
Cracks within the concrete which occur at an early stage will only endanger the reliability whilst preventing the concrete from ever attaining its maximum performance capability.
Figure 3
The explanation for this early age damage to the slab is relatively simple traditional mesh refrains from functioning until there is movement within the slab and the concrete cracks, for there to be any possibility of the fibre mesh to work the slab would have to be designed ensuring that the mesh had been placed In the top third of the slab.
As has been previously mentioned there are various shapes and sizes of steel fibres which are used two examples which are used within the construction of industrial floors are shown below in Figures 4 and 5.
Figure 4 Figure 5
Figure 4 shows four different shapes and sizes of fibre products which include smooth surface, indented etched, roughened surface and round with hooked ends which are the most commonly products used in regards to steel fabric reinforcement. Whereas Figure 5 gives a more in-depth description on the bundled hooked end product.
2.2.2 Synthetic Fibres
‘Straight or deformed pieces of extruded, orientated and cut polymer material, generally polypropylene, that is suitable for mixing in concrete. Macro fibres have a diameter greater than 0.3mm; micro fibres have a smaller diameter’
(ACIFC, 2007)
(Concrete Society’s TR66, 2007) considers that polymer fibres can be described and categorised into the following:
Macro synthetic fibres which are generally greater than 0.3mm in diameter and are used where an increase in post cracking flexural strength is required. Macro synthetic fibres can be incorporated within the design of structural concrete.
Micro Synthetic Fibres are relatively similar to the previously mention Macro synthetic fibres although they are smaller in size along with the inability to fulfil any structural role within any concrete design.
Characteristics of this type of fibre include the ability to reduce the instance of plastic shrinkage cracking along with acting as an enhancement for freeze thaw resistance.
References:
Illston, J.M. and Domone, P.L.J. (2001) Construction Materials – There Nature and Behavior 3rd Edition. P91. Spon Press.
Sutherland, M. (2009) Concrete Engineering International Volume 13 Number 2. Summer 2009. The Concrete Society
Romtech. (2009) Technical Support Products accessed 30th October at http://www.rom.co.uk/products.aspx
Crowther, D. (2009) Concrete for the Construction Industry Journal Volume 43 Number 3 April 2009. The Concrete Society.
Clark, J. (2007) Technical Report No. 63 Guidance for The Design of Steel Fibre Reinforced Concrete. Camberley
Hannant, D, Fibres in concrete – A Perspective. Concrete, Vol. 36, No 8, September 2002, pp 40 – 43.
Tarmac. (2009) ‘Topforce Fibre Reinforced Concrete’ accessed 12 November at www.tarmaconline.com/…/0982%20Tarmac%20Fibre%20Reinforced%20Concrete%20Brochure.pdf.
Propex. (2006) ‘Propex Concrete Systems’ accessed 13 November at www.CS-500_Propex%20Concrete%20Systems%20Brochure_Jul%2006 (1).Pdf.
Anon., (2006). ‘Concrete Possibilities’. Economist Technology Monthly. The Economist, 380 (23 September), p32.
Anon., (1972). American Concrete Institute, an International Symposium: Fibre Reinforced Concrete, Detroit: ACI, 1974. (ACI Special Publication SP-44)
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