BACHELOR OF SCIENCE IN CIVIL ENGINEERING...
Transcript of BACHELOR OF SCIENCE IN CIVIL ENGINEERING...
UNIVERSITY OF NAIROBI
A Sustainable Approach to Fibre Reinforced Concrete Use
By Balala Heikal Ghazy, F16/35750/2010
A project submitted as a partial fulfilment for the requirement for the
award of the degree of
BACHELOR OF SCIENCE IN CIVIL ENGINEERING
2015
Dedication
I dedicate this work to my family who have stood by me through the best and worst of times.
Thank you.
Acknowledgements
I would like to specially thank my supervisor Dr. (Eng) J. Mwero for his patience and valuable
help in guiding and supporting me throughout the duration of this project. I am grateful to all the
other lecturers who have helped me gain the necessary knowledge and skills required during my
five years at the University of Nairobi.
The experimental testing of this research would not have been possible without the technical
assistance of Mr. Martin, Mr. Muchina and Mr. Nicholas. To my classmates Abdulqadir
Mohamed and Hatim Dossaji, thank you for giving a hand in the lab, it made the whole
experience more valuable in deed.
I thank Henkel Industries for providing some of the materials required for this project.
Lastly, I wish to thank my family and friends for their love and support.
Table of Contents
Dedication ....................................................................................................................................... i
Acknowledgements ....................................................................................................................... ii
List of Tables ................................................................................................................................ vi
List of Figures ............................................................................................................................... vi
List of Charts .............................................................................................................................. viii
Abstract ......................................................................................................................................... ix
Chapter One – Introduction ........................................................................................................ 1
1.1 Background ........................................................................................................................... 1
1.2 Fibre Reinforced Concrete (FRC) ......................................................................................... 1
1.3 Historical Development of Fibre-Reinforced Concrete ........................................................ 2
1.4 Some Advantages and Disadvantages of FRC ...................................................................... 3
1.5 Practical Applications of Fibre Reinforced Concrete ........................................................... 4
1.5.1 Sisal Fibres ..................................................................................................................... 4
1.5.2 Steel Fibres ..................................................................................................................... 4
1.5.3 Glass-reinforced Plastic .................................................................................................. 4
1.6 Objective ............................................................................................................................... 4
1.6.1 General Objective ........................................................................................................... 4
1.6.2 Specific Objectives ......................................................................................................... 5
1.10 Scope and limitations of study ............................................................................................ 5
Chapter Two - Literature Review ............................................................................................... 6
2.1 Properties of Fibre Reinforced Concrete ............................................................................... 6
2.1.1 Type of fibre ................................................................................................................... 6
2.1.2 Aspect Ratio ................................................................................................................... 6
2.1.3 Quantity of fibres ............................................................................................................ 7
2.1.4 Orientation of the fibres. ................................................................................................. 7
2.1.5 Fibre size......................................................................................................................... 7
2.2 Mechanics of Fibre in FRC ................................................................................................... 7
2.3 Concept of Toughness ........................................................................................................... 9
2.4.1.3 Sisal Fibre .................................................................................................................... 9
2.4.2 Glass-Reinforced Plastic Fibres .................................................................................... 11
2.4.3 Steel Fibres ................................................................................................................... 11
2.5 Workability.......................................................................................................................... 12
2.7 Conclusion ........................................................................................................................... 13
Chapter Three - Methodology ................................................................................................... 14
3.1 Materials and Sampling ....................................................................................................... 14
3.1.1 Plain Concrete............................................................................................................... 14
Mix Proportion ...................................................................................................................... 14
3.1.2 Fibres ............................................................................................................................ 15
3.2 Characterization .................................................................................................................. 16
3.3 Material Quantities .............................................................................................................. 17
3.3.1 Cubes ............................................................................................................................ 17
3.3.2 Cylinders ....................................................................................................................... 18
3.3.3 Quantity Calculations ....................................................................................................... 18
3.4 Batching of materials/Method of mixing ............................................................................ 19
3.5 Tests .................................................................................................................................... 20
3.5.1 Aggregate Crushing Test .............................................................................................. 20
3.5.2 Flakiness Index Test ..................................................................................................... 22
3.5.4 Fresh Concrete Tests .................................................................................................... 25
3.5.5 Hardened Concrete Tests .............................................................................................. 29
Casting, demoulding and curing ............................................................................................ 30
Chapter Four - Results and Discussion ..................................................................................... 34
4.1 Aggregate Crushing Test ..................................................................................................... 34
4.2 Flakiness Index .................................................................................................................... 34
4.3 Grading Curves ................................................................................................................... 35
4.4 Fresh Concrete Tests ........................................................................................................... 37
4.4 Slump Test........................................................................................................................... 37
4.5 Compaction Factor Test ...................................................................................................... 39
4.6 Hardened Concrete Tests .................................................................................................... 40
4.6.1 Compressive Strength Results ...................................................................................... 40
4.6.2 Tensile Strength Test .................................................................................................... 43
Chapter Five - Conclusion and Recommendations.................................................................. 45
Conclusions ............................................................................................................................... 45
Recommendations ..................................................................................................................... 46
References .................................................................................................................................... 47
Appendices ................................................................................................................................... 49
Appendix A - Aggregate grading results................................................................................... 49
Appendix B – Slump and Compaction factor results ................................................................ 51
Appendix C – Strength test results ............................................................................................ 51
Appendix D – Figures ............................................................................................................... 54
List of Tables
Table 2.1: Properties of Natural Fibres..........................................................................................12
Table 3.1: Mass of dry ingredients required..................................................................................21
Table 3.2: Degrees of workability for fresh concrete and their uses.............................................29
Table 4.1: Flakiness Index test results...........................................................................................36
Table A1: Coarse aggregate grading results..................................................................................45
Table A2: Fine aggregate grading results......................................................................................45
Table A3: Grading requirements for coarse aggregates according to BS 882: 1992.....................46
Table A4: BS and ASTM requirements for grading of fine aggregate..........................................46
Table A5: Slump test and Compaction factor tests results............................................................47
Table A6: Compressive strength results (7 day strength)..............................................................48
Table A7: Compressive strength results (28 day strength)............................................................49
Table A8: Tensile strength results (28 day strength).....................................................................50
List of Figures
Figure 2.1: Schematic load-deflection relationship of fibre reinforcement...................................9
Figure 2.2: Bridging action of reinforcing fibres in a concrete beam under application of a point
load................................................................................................................................................9
Figure 2.3: Available steel fibre shapes in the market today.........................................................15
Figure 3.1: Preparation of fibreglass..............................................................................................18
Figure 3.2: Preparation of sisal fibres............................................................................................19
Figure 3.3: Random samples of steel fibre chosen for characterisation........................................19
Figure 3.4: Random fibreglass chosen for characterisation...........................................................19
Figure 3.5: Material batching.........................................................................................................21
Figure 3.6: Balling up of glass-reinforced plastic fibres................................................................22
Figure 3.7: Schematic drawing of ACV Test apparatus................................................................23
Figure 3.8: ACV Test apparatus....................................................................................................23
Figure 3.9: Flakiness index test apparatus (metal thickness gauge and sample)...........................25
Figure 3.10: Sieving of coarse aggregate being carried out...........................................................26
Figure 3.11: Types of slump that can be obtained.........................................................................28
Figure 3.12: Slump Test Apparatus...............................................................................................30
Figure 3.13: Compaction Test Apparatus......................................................................................32
Figure 3.14: Demoulding of the test samples and labelling..........................................................32
Figure 3.15: Curing the test samples in the curing tank................................................................33
Figure 3.16: Compression Test Machine.......................................................................................34
Figure 3.17: Illustration of the splitting tensile test.......................................................................35
Figure 4.1: Flakiness Index test being performed..........................................................................37
Figure 4.2: Fine aggregate grading (wet sieving)..........................................................................39
Figure 4.3: Typical fine aggregate test sieve.................................................................................39
Figure 4.4: Slump Test for sisal-reinforced concrete.....................................................................42
Figure 4.5: Sisal-reinforced concrete cube tested to failure..........................................................43
Figure 4.6: Fibreglass-reinforced concrete cube tested to failure..................................................43
Figure A1: use of steel fibre-reinforced concrete as a paving material.........................................53
Figure A2: Shotcrete tunnel lining.................................................................................................53
Figure A3: Rock slope stabilisation...............................................................................................53
Figure A4: Fibreglass used to reinforce formwork panels made of cement..................................54
Figure A5: US Corps of Engineer’s Dolosse.................................................................................54
Figure A6: Pavilion Bridge in Zaragoza, Spain.............................................................................54
List of Charts
Chart 2.1: Typical stress-strain curves for fibre-reinforced concrete............................................10
Chart 2.2: Relationship between slump, paste volume fraction and fibre content by volume......16
Chart 4.1: Coarse aggregate grading curve..................................................................................................38
Chart 4.2: Fine aggregate grading curve........................................................................................38
Chart 4.3: Variation of slump values with fibre type and content.................................................41
Chart 4.4: Variation of compaction factor with fibre type and content.........................................42
Chart 4.5: 7 day strength development curve................................................................................44
Chart 4.6: 28 day strength development curve..............................................................................45
Abstract
The present trend in concrete technology is to increase its strength and durability of concrete and
at a much lower cost. Sustainable engineering is steadily being incorporated into the Kenyan
construction industry. This will result in higher quality structures and infrastructure at a much
lower cost while maintaining an eco-friendly approach to construction.
Since concrete is a brittle material, reinforcement with fibres of different kinds has shown to
improve the mechanical properties of plain concrete. The fibres used in this report included sisal
fibres, which were obtained from natural sources, steel fibres were extracted from used tyres and
glass-reinforced plastic (fibreglass) having a composition of glass (non-pollutant) and plastic
which could easily be obtained from recycled plastic.
The aim of this research was to investigate the use of the above mentioned fibres as
reinforcement in concrete, to determine their mix proportions, method of mixing, handling and
placing techniques. Tests were done to determine the change in properties of fresh concrete upon
addition of varying percentages of the fibres. Compression and tensile tests were done to
determine the mechanical properties of the fibre-reinforced concrete. The experimental
investigations were done on cylinders and cubes with 0%, 1%, 2% and 3% fibre content by
weight of cement.
By testing the cubes and cylinders, we found out how both the tensile and compressive strengths
were affected. It was determined that the steel fibres were the most effective in enhancing both
the compressive and tensile strength of concrete. Sisal showed poor results with only very slight
increment in both compressive and tensile strength. Fibre glass showed promising results with
intermediate values between the steel and sisal fibres.
A common problem experienced during the experiments was the balling up of fibres, especially
the sisal and fibreglass. More research needs to be carried out on coming up with technique or
admixtures that could reduce balling up and deterioration of fibres in the concrete mix.
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Chapter One – Introduction
1.1 Background
Concrete is the most widely used man-made construction material in the world. It is obtained by
mixing cementitious materials, water, aggregate and sometimes admixtures in required
proportions. Fresh concrete or plastic concrete is freshly mixed material which can be moulded
into any shape, hardens into a rock-like mass know as concrete. The hardening is because of a
chemical reaction between water and cement, which continues for a long period leading to
increase in strength with age.
Concrete is weak in tension. Micro-cracks begin to generate in the matrix of a structural element
at about 10% to 15% of the ultimate load, propagating into macro-cracks at 25% to 30% of the
ultimate load. Consequently, plain concrete members cannot be expected to sustain large
transverse loading without the addition of continuous-bar reinforcing elements in the tensile zone
of supported members such as beams and slabs. The developing micro-cracking and macro-
cracking still cannot be arrested or slowed by the use of continuous reinforcement. The function
of such reinforcement is to replace the function of the tensile zone of such a section. The addition
of randomly spaced discontinuous elements should aid in arresting the development or
propagation of the micro-cracks that are known to generate structural failure [2]
.
1.2 Fibre Reinforced Concrete (FRC)
Fibre reinforced concrete (FRC) is Portland cement concrete (or mortar) reinforced with more or
less randomly distributed fibres. Several types of fibres have been used to reinforce cement-
based matrices. Fibres can be synthetic organic (e.g. polypropylene or carbon), synthetic organic
(steel or glass), natural organic (cellulose or sisal) or natural inorganic (asbestos). Although
asbestos-cement products have been widely used in the past, they are rapidly being replaced by
other cement-based composites due to the health hazards associated with airborne asbestos
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fibres. Natural organic fibres are a popular form of concrete reinforcement in less-developed
countries like Kenya for economic reasons.
Introduction of fibres in plain concrete enhances many of the engineering properties such as
toughness, impact and fatigue resistance, flexural/compressive/tensile strength. It also increases
the materials resistance to cracking which would lead to water and chloride ingress. When
concrete cracks, the randomly oriented fibres arrest crack formation and propagation. This
significantly improves the durability of concrete structures [6]
. In order for FRC to be a viable
construction material, it must be able to compete economically with existing reinforcing systems.
The failure modes of FRC are either bond failure between fibre and matrix or material failure. A
careful optimization of the fibres has to be determined. If the fibres have a weak bond with the
concrete matrix, they can slip out at low loads and do not contribute in bridging the cracks. If the
bond is too strong, the fibres may break before they dissipate energy. Therefore to bridge the
micro-cracks it is necessary to have a large number of short fibres.
GRP in concrete can be appreciated more so in coastal regions where corrosion is a constant
threat to steel structures and steel-reinforced concrete structure. This is due to the high
temperature and humidity levels along with high soil salinity.
1.3 Historical Development of Fibre-Reinforced Concrete
Fibres have been used to reinforce brittle materials from time immemorial, dating back to the
Egyptian and Babylonian eras, if not earlier. Straws were used to reinforce sun-baked bricks and
mud-hut walls, horse hair was used to reinforce plaster, and asbestos fibres have been used to
reinforce portland cement mortars [4]
.
In recent times, large scale commercial use of asbestos fibres in a cement paste matrix began
with the invention of the Hatschek process in 1898. Asbestos cement construction products were
widely used but due to the health hazards associated with the asbestos fibres, alternate fibre types
were introduced throughout the 1960s and 1970s.
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Research in the late 1950s and early 1960s by Romauldi and Batson (1963) and Romauldi and
Mandel (1964) on closely spaced random fibres, primarily steel fibres, heralded the era of using
the fibre composite concretes we know today [3]
. By the 1960s, steel-fibre concrete began to be
used in pavements, in particular. Other developments using bundled fibreglass as the main
composite reinforcement in concrete beams and slabs were introduced by Nawy et al. (1971) and
Nawy and Neuwerth (1977). From the 1970s to the present, the use of steel fibres has been well
established as a complementary reinforcement to increase cracking resistance, flexural and shear
strength, and impact resistance of reinforced concrete elements both in situ cast and precast [6]
.
Initial attempts at using synthetic fibres (nylon, polypropylene) were not successful as those
using glass or steel fibres. However, better understanding of the concepts behind fibre
reinforcement, new methods of fabrication, and new types of organic fibres have led researchers
to conclude that both synthetic and natural fibres can successfully reinforce concrete.
1.4 Some Advantages and Disadvantages of FRC
Advantages
Compared to plain concrete, FRC has more strength and durability.
Minimises cavitation/erosion damage in structures such as sluice-ways, navigation locks
and bridge piers among others where high velocity flows are encountered.
Using FRC reduces the thickness of structural elements such as slabs. This leads to
decrease in overall cost.
In bridges, FRC helps reduce catastrophic failures.
Additional flexibility.3.
Disadvantages
The fabrication process of incorporating fibres into the cement matrix is labour intensive
and costlier than the production of plain concrete.
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1.5 Practical Applications of Fibre Reinforced Concrete
1.5.1 Sisal Fibres
Used in manufacture of roof tiles, corrugated sheets, pipes, silos and water tanks.
Used for low-cost house construction.
Used for making cement composite panel lining, eaves, soffits.
1.5.2 Steel Fibres
Used as industrial floor slabs and as external paving.
In railways, SFRC has been used to replace the track bed slab (Thameslink, London)
Shotcrete (sprayed concrete) has been used in slope stabilisation, tunnelling and repair
work.
In hydraulic structures, to provide resistance to cavitation and erosion.
1.5.3 Glass-reinforced Plastic
GRP pipes: they are rigid, light weight, have high tensile strength and are not susceptible
to corrosion. Smooth internal surface entails smaller head loss to flow of water.
1.6 Objective
1.6.1 General Objective
The key objective of this project is to develop concrete mixes reinforced with selected fibres and
thus to investigate their effect on the engineering properties of concrete and to make a
comparison between the fibres as regards to their performance in the concrete matrix after
undergoing the required tests.
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1.6.2 Specific Objectives
1. To characterise the fibres.
2. To determine the compressive and tensile strengths of the fibre-reinforced concrete cubes
and cylinders.
3. To determine the most efficient fibre considering technical, economical and sustainability
factors.
1.10 Scope and limitations of study
The first constraint is the financial constraint. Fibres such as carbon and kevlar are expensive to
obtain and thus the research would be well over the intended budget. Using the locally available
fibres is fairly a cheap affair but the expense is attributed to the number of cubes, cylinders and
beams that need to be tested to obtain clear details from the resultant graphs
Other than the financial constraint, there’s the time constraint. Important properties of concrete
such as creep will not be covered in this work due to the long duration required to obtain the
required results.
This paper therefore covers only three types of fibres:
Glass-reinforced plastic fibres
Sisal fibres
Steel fibres
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Chapter Two - Literature Review
2.1 Properties of Fibre Reinforced Concrete
Concrete properties are mainly affected by its composites, namely; aggregates, water and the
cement. FRC is further affected by the fibres in the concrete matrix.
2.1.1 Type of fibre
Quality of the fibre to be used in FRC must possess the following properties:
The fibre surface must have good adhesion with the concrete matrix
The modulus of elasticity of the matrix must be less than that of the fibre for efficient
transfer of stress. A low modulus fibre (Nylon or polypropylene) imparts more energy
absorption while a high modulus fibre (Steel, glass or carbon) imparts strength and
stiffness [2]
.
They must be sufficiently short, fine and flexibly to permit transportation, mixing and
placing.
They must be sufficiently strong and robust to withstand the mixing process, during
working life of the FRC; they should provide required tensile capacity and resistance to
crack formation.
2.1.2 Aspect Ratio
Aspect Ratio is defined as the ratio of the fibre length to its diameter. The value of aspect ratio
varies from 30 to 150. Increase in aspect ratio leads to an increase in the strength and toughness
of the fibres up till the value of 100.
7
2.1.3 Quantity of fibres
The quantity of fibres in the concrete matrix is measured as a percentage of the overall weight of
the concrete. As the volume of the fibres increase, so does the strength and toughness of the
concrete up to an optimum value from where it starts reducing.
2.1.4 Orientation of the fibres.
The main practice when dealing with FRC is to distribute the fibres in random directions as this
has proved to be the most efficient way in resisting crack propagation.
2.1.5 Fibre size
The fibre size plays a significant part in determining the stage at which fibres are effective during
cracking. Research suggests that fine micro fibres (with diameters less than 0.05mm) can
increase the elastic limit and strength of concrete by bridging micro-cracks, whilst macro fibres
(typically with diameters greater than 0.5mm) can improve post-peak toughness by bridging
larger cracks. This observation has led to the practice of fibre hybridisation where material
performance is optimized by combining fibres of varying size and moduli to control the cracking
process at different stages in loading [6]
.
2.2 Mechanics of Fibre in FRC
FRC in flexure undergoes a tri-linear (OA, AB, BC) deformation behaviour as in Figure. Once
the matrix is cracked, the applied load is transferred to the fibres that bridge and tie the crack to
keep it from opening further. As the fibres deform, additional narrow cracks develop, and
continued cracking of the matrix takes place until the maximum load reaches point B of the load-
deflection diagram. During this stage, debonding and pullout of some of the fibres occur, but the
8
yield strength in most of the fibres is not reached. In the falling branch, BC, of the diagram,
matrix cracking and fibre pullout continue. If the fibres are long enough to maintain their bond
with the surrounding gel, they may fail by yielding or by fracture of the fibre element, depending
on the size and spacing [7]
.
Figure 2.1 Schematic load-deflection relationship of fibre reinforcement
Figure 2.2 Bridging action of reinforcing fibres in a concrete beam under application of a
point load
The bond between the fibres and portland cement matrices is a critical factor in determining
strength properties of fibre-reinforced concrete structural elements. It has been shown from
research that the pull-out capacity of a group of randomly oriented fibres decreases drastically
when the number of fibres pulling out from the same area increases. The surface characteristics
of the fibres also play a major role in the fibre pull-out [18]
.
9
2.3 Concept of Toughness
Toughness is defined as the area under a load deflection (or stress-strain) curve. As can be seen
in figure, adding fibres to concrete greatly increases the toughness of the material. That is, fibre-
reinforced concrete is able to sustain load at deflections or strains much greater than those at
which cracking first appears in the matrix.
Chart 2.1: Typical stress-strain curves for fibre-reinforced concrete
2.4.1.3 Sisal Fibre
Sisal fibre is extracted from the leaves of Agave Sisalana. These fibres are built up of about 100
individual fibre cells in a cross section, bound together by hemicelluloses, lignin, and pectin. As
indicated in table, sisal fibre is strong compared to most other natural fibres. However, like so
many other natural fibres, sisal fibre does have certain durability problems associated with its use
as concrete reinforcement.
Available information indicates that sisal fibres can be added directly to concrete during mixing
or worked into concrete after placement. When adding sisal fibres during concrete mixing, the
fibres reportedly have a tendency to clump up. The setting time was also noticeably affected by
10
the presence of the sisal fibre, presumably due to the retarding effect of organic impurities that
leach from the fibres.
Research conducted by Gram has indicated that composites reinforced with sisal fibre can be
manufactured with flexural strengths in excess of the cracking strength of the cement matrix. As
indicated in figure, composites with 2% sisal fibre by volume (placed by hand and aligned) and
not subjected to any cycles of simulated weathering possess significant flexural strength and
ductility. Gram hypothesized that this durability problem was caused by chemical decomposition
of the sisal fibre in the alkaline environment of the cement matrix [4]
.
Table 2.1: Properties of Natural Fibres
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2.4.2 Glass-Reinforced Plastic Fibres
GRP offers a combination of properties seldom found in other materials – high strength and
dimensional stability, with low weight. However, GRP has an inherent limitation. It has a low
modulus of elasticity (stiffness). It is composed of E-type glass which is the most common form
of fibreglass [4]
.
If GRP is to be successfully used in load bearing components in construction, its structural form
must be chosen as to overcome the apparent lack of stiffness in the overall structure. The
required rigidity of the structure is then derived from its shape rather than from the material – the
strength of the structure is only a function of the structure of the material. The extent to which a
simple change in geometry can impart stiffness would be evident from the fact that a corrugated
GRP sheet exhibits greater rigidity and load bearing capability than a flat sheet of similar
thickness.
The lightweight characteristics and improved tensile strength of GRP as compared with concrete
led to a recent research program to study the viability of its use as a structural material (Ferreira,
Branco et al., Viegas, Cian and Della Bella).
2.4.3 Steel Fibres
SFRC is a cement-based material reinforced with short steel fibres. When steel fibres are added
to a concrete mix, they are randomly distributed and act as crack arrestors. Debonding and
pulling out of fibres require more energy, giving a substantial increase in toughness and
resistance to cyclic and dynamic loads.
The purpose of steel fibre reinforcement and conventional reinforcement are distinctly different.
Steel fibres are added to concrete mainly to influence the way in which concrete cracks as it fails.
Micro-cracks form when concrete is loaded. Subsequently, the micro-cracks coalesce to form
macro-cracks. Fibres can bridge cracks during loading and, hence, influence mechanical
performance. Steel fibres do provide the concrete with a significant post-cracking strength.
12
Steel fibres have a tensile strength typically 2-3 times greater than traditional fabric
reinforcement and a significantly greater surface area (for a given mass of steel) to develop bond
with the concrete mix [17]
.
Experimental investigations were done and the results obtained from the uniaxial compression
tests with fibre reinforced concrete revealed a slight increase in the compression strength,
stiffness and strain at peak load.
Figure 2.3: Available steel fibre shapes in the market today
2.5 Workability
The primary factors that could affect workability are the paste volume, the fibre content and the
fibre aspect ratio. Typically, the fibres decrease the slump, but this does not necessarily make the
fibres mixes harder to compact with vibration. Fibres do make mixes drier due to their high
specific area.
Johnston gives the results of Pfeiffer and Soukatchoff, who did tests regarding the effect of paste
volume on workability. They assessed slump in terms of paste volume fraction and fibre content
by volume. Their work was with steel fibres, but the results are likely to be qualitatively similar
13
to what is seen in glass-reinforced plastic fibres and sisal fibres. The results are presented in the
following graph.
Johnston also gives the results of Edgington, Hannant, and Williams, who did tests correlating
the steel fibre aspect ratio to workability. In their tests, they assessed vibration time required for
placement compared to fibre aspect ratio and volume. For each aspect ratio, there was a distinct
limit beyond which an increase in fibre content caused a dramatic decrease in workability [4]
.
Chart 2.2: Relationship between slump, paste volume fraction and fibre content by volume
2.7 Conclusion
In conclusion, for the effective use of fibre in hardened concrete:
Fibres should be significantly stiffer than the matrix, i.e. have a higher modulus of
elasticity than the matrix.
Fibre content by volume must be adequate.
There must be a good fibre-matrix bond.
Fibre length must be sufficient.
Fibres must have a high aspect ratio, i.e. they must be long relative to their diameter.
It should be noted that published information tends to deal with high volume concentrations of
fibre. However, for economic reasons, the current trend in practice is to minimize fibre volume,
in which case improvements in properties may be marginal.
14
Chapter Three - Methodology
3.1 Materials and Sampling
3.1.1 Plain Concrete
3.1.1.1 Aggregates
Both fine (river sand) and coarse aggregate (10 mm ballast) were obtained from the university.
Sampling of the aggregates was done by a process known as quartering.
3.1.1.2 Cement
Cement was provided by the university. Bamburi Portland Cement 32.5N was used throughout
the study for preparation of concrete mixes.
3.1.1.3 Water
Water is essential for the hydration of Portland cement to take place. The water used was potable
water obtained from the laboratory taps. A water cement ratio of 0.5 was used for all concrete
mixes.
Mix Proportion
M25 mix design was used in the preparation of concrete mixes. The mix ratio of the ingredients
is 1:1.5:3 which represents cement:sand:ballast respectively. This mix design yields a 7-day
strength of 17 N/mm2 and a 28-day strength of 25 N/mm
2
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3.1.2 Fibres
3.1.2.1 Fibreglass
The fibreglass was obtained from Henkel industries located in the Industrial Area in Nairobi.
About 600 grams was enough for the experiment. The fibreglass came as a mat and had to be
plucked down to the required fibres. Random fibres from the fibreglass heap were selected for
characterization.
Figure 3.1: Preparation of fibreglass
3.1.2.2 Sisal Fibres
Sisal fibres were obtained from sisal rope cut into the required lengths which vary from 40 to 50
mm. they were obtained from a local store. Thereafter, the sisal fibres had to be untwined to
obtain the individual fibres required for the experiment. Similar characterization to that of the
fibreglass was done.
16
Figure 3.2: Preparation of sisal fibres and steel fibres.
3.1.2.3 Steel Fibres
The steel fibres were obtained from a used tyre. The rubber of the tyre was burnt to obtain the
steel reinforcement wire in the tyre. The steel wire was then cut to appropriate lengths as
required by the experiment. It ranged from 40 to 50 mm as was with the sisal fibres. The inter-
twined diameter of the fibres was obtained using a micrometer screw gauge. The fibre quantity
was obtained as a percentage of the weight of cement. The quantities were to be 1%, 2% and 3%.
3.2 Characterization
Characterization was done on the fibres to determine the average lengths of each fibre type and
their diameters. The steel fibres obtained from used tyres were intertwined with smaller diameter
steel fibres to make stronger, durable steel wire. The fibreglass too was composed of smaller
fibres attached together due to adhesion between the individual strands of fibreglass.
A few strands of sisal fibres were picked at random and their lengths measured. The average
length was found to be approximately 45 mm. Similarly, the steel fibres had and average length
of 50 mm. The fibreglass individual fibres were the most variable. They had an average length of
54 mm.
17
The diameter of fibreglass ranged from 5-25 micrometers while the sisal fibres ranged from 100-
300 microns.
Figure 3.3 and 3.4: Steel and fibreglass characterization
3.3 Material Quantities
3.3.1 Cubes
Cube size of 100x100x100 mm was used
Volume of one cube = 0.001 m3
Density of concrete = 2400 Kg/m3
Mass of concrete required for one cube = V x D = 0.001 x 2400 = 2.4 Kg
Total number of cubes required = 2 cubes for averaging, 3 types of fibre to be tested (sisal,
fibreglass and steel), 3 percentages to be considered (1%, 2% and 3%), cubes at 7 and 28 day
strength development = 36 cubes in total.
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3.3.2 Cylinders
Cylinder size of 150 mm diameter and 300 mm height was used.
Volume of one cylinder = 0.053 m3
Mass of one cylinder = 12.72 Kg
Total number of cylinder required = 2 cylinders for averaging, 3 types of fibre to be tested, 3
percentages to be considered, cylinders at 28 day strength required = 18 cylinders in total.
3.3.3 Quantity Calculations
Mix design = class 25 concrete (cement: sand: ballast = 1: 1.5: 3)
A water cement ratio of 0.5 to be used
A wastage factor of 1.15 was included in the calculations to account for 15% wastage during the
laboratory tests.
Mass of required ingredients = ratio of ingredient x total mass of concrete x 1.15
The quantities tabulated below are for each fibre type.
Table 3.1: Mass of dry ingredients required
Material Calculation Mass required (Kg)
Cement 1/5.5 × 105.12 × 1.15 21
Sand 1.5/5.5 × 105.12 × 1.15 33
Ballast 3/5.5 × 105.12 × 1.15 66
19
Note: the above quantities were divided into three batches each representing quantities for either
1%, 2% or 3% fibre content.
3.4 Batching of materials/Method of mixing
The above materials are to be done in three batches. The first batch contains 1% of fibre content
by weight of cement; the second and the third batch contain 2% and 3% of the same respectively.
Figure 3.5: Material batching
The first ingredients to go into the mixer are the dry ingredients. The coarse aggregate, followed
by fine aggregate and finally cement were added into the mixer. The mixer was activated and the
dry ingredients were allowed to mix for about 3 minutes after which the water was added a litre
at a time.
Once the ingredients had mixed properly into fresh concrete, the fibres were then to be added. As
described earlier, balling up of the fibres is the formation of large clumps of entangled fibres that
may occur during the mixing process. To prevent balling up of the fibre, the fresh concrete and
20
the fibres were mixed with a spade as opposed to the fixed three paddle system that the
laboratory mixing system uses. The fibres were added slowly while folding them into the mix
which successfully distributed the fibres randomly into the concrete mix.
Figure 3.6: Balling up of glass-reinforced plastic fibres
3.5 Tests
3.5.1 Aggregate Crushing Test
The aggregate crushing value (ACV) test is prescribed by BS 812-110: 1990 and BS EN 1097-2:
1998.
Apparatus
An open ended steel cylinder of nominal 150mm internal diameter with a plunger and
base plate as shown in fig 9.7. The surface in contact with the aggregate shall be
machined and case hardened.
Round steel tamping rod, 16mm diameter , 600mm in length.
Balance accurate to 1.0g of capacity of at least 3kg.
Test sieves of sizes 14mm, 10mm and 2.36mm.
A compression testing machine capable of applying a force of 400KN at a uniform rate of
loading so that this force is arrived at in 10min.
21
A cylindrical metal measure of internal dimensions, 115mm diameter by 180mm depth.
Figure 3.7 and 3.8: ACV test apparatus
Method
The sample was to be dried in an oven at 100 to 110°C for 4 hours before testing.
The cylinder of the test apparatus was put in position on the base plate, and the test
sample of the aggregate was added in 3 layers of approximately equal depth, each layer
being tamped 25 times by the rounded end of the tamping rod distributed evenly over the
surface of the layer. The rod was dropped from a height of approximately 50mm above
the surface of the aggregate.
The surface was then levelled off using the rod and the plunger was inserted so that it
rested horizontally on the surface.
The cylinder with the test sample and plunger were placed in position on the compression
testing machine. It was then loaded at a uniform rate of 40kN/min up to a maximum of
400kN. This was done by moving the control of the compression machine slowly with
22
respect to the timer to ensure gradual compression. The loading was then released after
10min.
The crushed material was then removed by holding the cylinder over a clean tray and
hammering on the outside with a rubber mallet to enable the sample to fall freely on the
tray. The fine particles adhering to the inside of the cylinder, the base plate and the
underside of the plunger were transferred to the tray by means of a stiff brush. The whole
sample on the tray was sieved in the 2.36mm test sieve.
The ratio of the mass of material passing the sieve to the total mass of sample gave the
aggregate crushing value.
3.5.2 Flakiness Index Test
Apparatus
Test sieves: 63, 50, 37.5, 28, 20, 14, 10 and 6.3
Metal thickness gauge as in figure
Balance of sufficient capacity and accurate to 0.5% of the mass of the test sample.
Method
The samples were sieved with the appropriate sieves.
A minimum of 200 pieces of each size fraction was tested and weighed.
In order to separate the flaky materials, each fraction was gauged for thickness on the
thickness gauge.
The amount of flaky material passing the gauge was weighed to an accuracy of 0.5% of
the test sample.
23
Figure 3.9: Flakiness index test apparatus: metal thickness gauge and sample.
3.5.3 Grading (Sieve Analysis)
3.5.3.1 Coarse and Fine Aggregate Grading
The process of dividing a sample of aggregate into fractions of same particle size is known as a
sieve analysis, and its purpose is to determine the grading or size distribution of the aggregate.
A sample of air-dried aggregate is determined by shaking or vibrating a nest of stacked sieves,
with the largest sieve at the top, for a specified time so that the material retained on each sieve
represents the fraction coarser than the sieve in question but finer than the sieve above.
24
Figure 3.10 and 3.11: Fine aggregate grading (wet sieving) shown on the left and a typical
fine aggregate test sieve.
The tables in Appendix A3 and A4 list the sieve sizes normally used for grading purposes
according to BS 812-103.1: 1985, BS EN 933.2: 1996 and ASTM C 136-06. Also shown are the
previous designations of the nearest size. To be noted, 4 to 5 mm is the dividing line between the
fine and coarse aggregate. It is important to use aggregate with a grading such that a reasonable
workability and minimum segregation are obtained in order to produce a strong, durable and
economical concrete.
Figure 3.12: Sieving of coarse aggregate being carried out
25
From the sieve analysis results, grading curves are plotted for cumulative passing against the
sieve sizes in logarithmic scale.
3.5.4 Fresh Concrete Tests
3.5.4.1 Slump Test
The slump test does not measure workability of concrete but is very useful in detecting variations
in the uniformity of a mix of given nominal proportions.
Method
The mould for the slump tests is a frustum of a cone, 300 mm high. It is placed on a smooth
surface with the smaller opening at the top, and filled with concrete in three layers. Each layer is
tamped 25 times with a standard 16 mm diameter steel rod and the top surface struck off by
means of a rolling motion of the tamping rod. The mould must be firmly held against its base
during the entire operation.
After filling, the cone is slowly lifted and the unsupported concrete will then slump. The
decrease in the height of the centre slumped concrete is called slump. If instead of slumping
evenly all rounds as in true slump, one half of the cone slides down an inclined plane, a shear
slump is said to have taken place, and the test should be repeated. If the shear slump persists, this
would indicate a lack of cohesion of the mix.
The slump tests were carried out for all the three fibres to be tested at each of their different
content in the concrete mix.
26
Figure 3.13: Slump Test Apparatus
3.5.4.2 Compaction factor test
There is no generally accepted method of directly measuring workability i.e. the amount of work
necessary to achieve full compaction. The best test available is use an inverse approach where
the degree of compaction achieved by a standard amount of work is determined.
The degree of compaction, called the compacting factor, is measured by the density ratio, i.e. the
ratio of the density actually achieved in the test to the density of the same concrete fully
compacted.
The table on the following page indicates the various degrees of workability that can be obtained,
their corresponding ranges of slump values, compacting factors and the uses for which the
particular concrete is suitable.
27
Method
The test is done in accordance to BS 1881: Part 2: 1970. The apparatus consists essentially of
hoppers, each in shape of a frustum of a cone, and only one cylinder, the three being above one
another. The hoppers have hinged doors at the bottom. All inside surfaces are polished to reduce
friction.
The upper hopper is filled with concrete, this being placed gently so that at this stage no work is
done on the concrete to produce compaction. The bottom door of the hopper is then released and
the concrete falls into the lower hopper. This is the smaller than the upper hopper one and is
therefore filled to overflowing and thus always contains approximately the same amount of
concrete in a standard state; this reduces the influence of the personal factor in filling the top
hopper. The bottom door of the lower hopper is released and the concrete falls into the cylinder.
Excess concrete is cut by two floats slid across the top of the mould, and the net weight of the
concrete in the known volume of the cylinder is determined.
The density of the concrete in the cylinder is now calculated, and this density divided by the
density of the fully compacted concrete is known as the compacting factor.
28
Table 3.2: Degrees of workability for fresh concrete and their uses
29
Figure 3.14: Compaction Test Apparatus
3.5.5 Hardened Concrete Tests
Tests on hardened concrete include
Compression test
Splitting tensile test
The most common of all tests is the compressive tests, partly because it is easy to make and
partly because many of the desirable characteristics of concrete are related to its strength. The
strength tests can be classified into mechanical tests to destruction and non-destructive tests. This
paper is concerned with only the destruction tests.
The two main objectives of tests are control of quality and compliance with specifications.
30
Casting, demoulding and curing
The test was done in conformity with BS 1881: Part 3: 1970. The specimens were to be cast in
100×100×100 mm iron cube moulds. The mould is separate to its base and therefore must be
clamped together with bolts during casting to prevent leakage of cement paste.
The inside surfaces of the moulds were covered with oil to prevent the development of bond
between the mould and the concrete. The mould was then filled in three layers, each layer being
compacted by not less than 35 strokes of a 25 mm tamping rod. After compaction, the top
surface of the cube was finished by means of a trowel. The test specimens were then left
undisturbed for 24 hours, being protected against dehydration and any form of disturbance.
After 24 hours, the cube moulds were removed and the cubes marked with details that indicate;
the type of mix, date of casting, duration of curing and the day of crushing. The cubes were to be
cured at prescribed ages such as at 7 days, 14 days and 28 days for the purpose of generating a
strength development curve. This was done for cubes with high content fibre of each type (i.e.
glass, steel and sisal fibres) in addition to the control.
Figure 3.15: Demoulding of the test samples and labelling.
31
Figure 3.16: Curing the test samples in the curing tank.
3.5.5.1 Compressive test (cube crushing)
After the cubes have attained the required age, the cube was placed with its faces in contact at
right angles with the platens of the testing machine. A constant rate of loading of 15 MPa/min
(15N/mm2/min) was applied on the specimen to failure. The readings from the dial gauge were
then recorded. The crack patterns on each cube were recorded by means of photographs.
The compressive strength is given by:
σ =
Where: P = Applied load
A = Area of loading
32
Figure 3.17: Compression Test Machine
3.5.5.2 Tensile strength
The standard cylinder mould is 150 mm in diameter, 300 mm long with a clamp base.
Cylindrical specimens were made from the same mixes as those for the cubes; however, only 4
specimens were to be casted. The concrete was compacted in three layers using a 16 mm steel
rod. The top surface was finished by means of a trowel.
The specimens were then left undisturbed for 24 hours after which they were placed in a curing
tank. A total of 4 specimens were cast. (3 fibre reinforced specimens, 1 control.)
Splitting tensile test
The specimens are placed in a horizontal position with the curved part of the cylinder being
plane with the end plates of the compression testing machine. The loading was the applied at a
constant rate until failure. The readings were observed and recorded.
33
Figure 3.18: Illustration of the splitting tensile test
The tensile strength is given by the formula:
=
Where: = tensile strength
F = load at failure
D = diameter of specimen
L = length of specimen
34
Chapter Four - Results and Discussion
4.1 Aggregate Crushing Test
Weight of pan = 345 g
Weight of pan + sample (passing) = 835 g
Weight of pan + sample (retained) = 2555 g
ACV =
x 100 =
x 100 = 18.15%
There is no explicit relation between the aggregate crushing value and the compressive strength
but, in general, the crushing value is greater for a lower compressive strength. For crushing value
of over 25 to 30, the test is rather insensitive to the variation in strength of weaker aggregates.
This is so because, having been crushed before the full load of 400 kN has been applied, these
weaker materials become compacted so that the amount of crushing during later stages of the test
is reduced.
For structural concrete, the required ACV value is specified as a value below 45% would be
sufficient. The result obtained was 18.15% signifying that the sample obtained was adequate in
relation to its ACV value.
4.2 Flakiness Index
Weight of pan = 113 g
Table 4.1: Flakiness index test results
Sieve Size (in) Mass Passing (g) Mass Retained (g)
1/2 to 3/8 45.2 68.2
3/8 to 1/4 182.9 257.2
Total 228.1 325.4
35
Flakiness Index =
x 100 =
x 100 = 41.2%
BS 882: 1992 specifies a flakiness index limit of 40% for crushed gravel or partially crushed
gravel. These types of aggregate lower the workability of the concrete mix. They can also
adversely affect the durability of concrete as they tend to orient in one plane, with water and air
voids forming underneath. Flaky aggregates also affect the bond in a concrete matrix. The larger
surface area of a more angular aggregate provides greater bond. Therefore the samples available
for testing were not desirable to use in a concrete mix.
Figure 4.1: Flakiness Index test being performed
4.3 Grading Curves
In the following grading curves, the blue curve represents the grading curve of the sample tested,
while the curves in red and green represent the lower and upper limits to the envelop
respectively.
36
Chart 4.1: Coarse aggregate grading curve
Chart 4.2: Fine aggregate grading curve
0
10
20
30
40
50
60
70
80
90
100
0.01 0.1 1 10 100
Cu
mu
lati
ve
Pass
ing (
%)
Sieve Size (mm)
0
10
20
30
40
50
60
70
80
90
100
0.01 0.1 1 10 100
Cu
mu
lati
ve
Pass
ing (
%)
Sieves (mm)
37
Grading of aggregates governs the amount of voids that must be filled with paste as well as the
surface area that needs to be coated with paste. A smooth gradation curve decreases the voids
between the aggregate particles. Because the voids must be filled with a mixture of cement and
water, it therefore decreases the amount of cement required. Cement is usually the most
expensive component in concrete, so minimising the amount needed makes the mix more
economical.
The larger the aggregate particle the smaller the surface area to be wetted per unit mass. Thus,
extending the grading of aggregate to a larger maximum size lowers the water requirement of the
mix so that, for specified workability and richness of the mix, the water/cement ratio can be
reduced with a consequent increase in strength.
The grading limits are intended to provide a fairly dense packing of aggregate particles to
minimize the cement paste requirement. From the obtained results the blue curve indicates the
grading curve of the samples taken. It is below the minimum acceptance curve meaning there are
a substantial amount of void spaces. This would affect the overall strength of the concrete matrix
and would result in an increased amount of cement paste required.
4.4 Fresh Concrete Tests
4.4 Slump Test
The workability of the concrete mix was affected on addition of the fibres. The mixture became
stiffer as the fibre content was increased. Addition of the fibres led to a decrease in workability.
This was highly noticeable upon addition of the sisal fibres.
Fibreglass resulted in the most workable mix as compared to the other fibres with the highest
slump of 35 mm on addition of 1% fibres. Addition of both 3% sisal and steel fibres reduced the
slump the most, and thus the workability, giving a slump of only 18 and 19 mm respectively.
Generally, fibres do make mixes somewhat drier due to their specific surface area. The stiffness
of the fresh concrete could also be explained with relation to the structure of the different fibrous
38
materials being investigated. Fibreglass exhibits a more flexible structure as compare to the
stiffer more irregularly shaped structure of the sisal fibres. Moreover, sisal fibres being a natural
fibre tend to absorb the water in the concrete mix further making the mixture stiffer. The
stiffness brought about by the steel fibres could be due to mechanical action of the fibres holding
the fresh mix in place.
Steel fibres, due to their much higher density, lowered the workability of the mix. It was stiffer
than fibreglass but more workable than the mix with sisal fibres.
Chart 4.3: Variation of slump values with fibre type and content.
The concrete mix would require more water to obtain better workability as the percentage of
each fibre increases for better strength. Admixtures can also be used to enhance workability and
0
5
10
15
20
25
30
35
40
1 2 3
Slu
mp (
mm
)
Fiber Content (%)
fiberglass
sisal fiber
steel fiber
39
strength properties of concrete. However, these would not be effective when applied to concrete
mixes when the critical limit of fibre content has been exceeded.
4.5 Compaction Factor Test
Compaction Factor =
The compaction factor was observed to decrease with increase in fibre content. This is attributed
to the fibres in the mix, their distribution interrupting the movement of concrete particles during
vibration and subsequently prevented packing of the particles into a closely packed
configuration. A relationship can thus be established between the slump, compaction factor and
the fibre content: an increase in slump resulted in a lower compaction factor with increase in
fibre content. The fibreglass concrete mix showed consistently higher compaction factors than
the other two fibres. This could be due to their flexible nature allowing densification of the mix.
Chart 4.4: Variation of compaction factor with increase in fibre type and content
0.68
0.7
0.72
0.74
0.76
0.78
0.8
0.82
0.84
0.86
0.88
1 2 3
Co
mp
acti
on
Fac
tor
Fibre Content (%)
Fibreglass
Sisal Fibre
Steel Fibre
40
Figure 4.4: Slump Test for sisal-reinforced concrete
4.6 Hardened Concrete Tests
4.6.1 Compressive Strength Results
The effect of fibre content on compressive strength of concrete at 7 and 28 days are shown in
chart 4.5 and chart 4.6 respectively. The addition of fibres shows an increase in compressive
strength from the 7th
day to the 28th
day as compared with the control.
At 7 days, addition of 1% fibreglass and sisal fibre content resulted in increments of 23% and 6%
of compressive strength respectively. Addition of 2% fibreglass content resulted in a decrease of
20%. At 3% fibre content, the fibreglass and sisal fibres had decreased the compressive strength
of the fibre-reinforced concrete to below that of the control. The fibreglass strength had
decreased by 46% and sisal fibres decreased by 34% with reference to the control. A 1% addition
of steel fibres resulted in an 64% increase in compressive strength. This gradually decreased to
53% and 31% on addition of 2% and 3% steel fibre content respectively. At 7 days, the optimum
fibre content for the three fibres was at 1%.
At 28 days, a 1% addition of fibreglass showed a 12% increase in strength while a 2% and 3%
addition of the same showed a decrease of 9% and 19% decrease in strength respectively. A
41
similar case was seen with the sisal fibres. Steel fibres showed the greatest increase in
compressive strength at 28 days, with an increment of 41%. The optimum content again was at
1% for the three fibres.
Figure 4.5 and 4.6: Crushed Sisal-reinforced concrete cube (shown on the left) and crushed
fibreglass-reinforced concrete cube.
Chart 4.5: 7 day strength development curve
0
5
10
15
20
25
30
35
0 0.5 1 1.5 2 2.5 3 3.5
Com
pre
ssiv
e st
ren
gth
(N
/mm
2)
Fiber content (%)
Fiberglass
Sisal Fiber
Steel Fiber
42
Chart 4.6: 28 day strength development curve.
At 28 days, sisal fibres showed a higher variation in change of strength. A 1% addition of the
fibres resulted in a 4% increase in compressive strength and further addition of the sisal fibres
resulted in a relatively high decrease in strength of 11% and 32% respectively.
Steel as expected, gave the highest strength values due to its high tensile strength. Fibreglass,
showed greater potential in increasing the strength of concrete than sisal did. Sisal fibre being
thicker and denser than the fibreglass could have interrupted the bonding of the concrete matrix
thus interfering with the concrete strength.
It is observed that increasing the fibre content increases the strength up to an optimum point,
from which further increase in fibre content results in a decrease in strength. High strength was
achieved when then fibre content was between 1% and 2% while the strength started to decrease
between 2% and3%.
0
5
10
15
20
25
30
35
40
45
0 0.5 1 1.5 2 2.5 3 3.5
Co
mp
ress
ive
stre
ng
th (
N/m
m2)
Fiber Content (%)
Fiberglass
Sisal Fiber
Steel Fiber
43
Increase in fibre content past the optimum value decreases the mechanical and chemical bonding
of the concrete matrix. Entrapped air due to the presence of fibre content above the optimum
content also contributes in strength reduction of the concrete matrix.
4.6.2 Tensile Strength Test
Addition of fibres, in some cases, resulted in an increase in the tensile strength of concrete as
was with the compressive strength. Steel fibres showed the highest increase in tensile strength
with an increment of 68% on addition of 1% steel fibre content. The strength decreased by 21%
on addition of 2% steel fibre content. At 3% fibre content, the strength decreased by a further
5%. From chart 4.7, the optimum fibre for steel fibres was determined to be 1%.
For sisal and fibreglass, the optimum content was determined to be at around 2%. This resulted
in a tensile strength increment of 8% and 10% respectively. In comparison with the steel fibres,
this can be considered to be negligible. At 3% fibre content, the tensile strength of fibreglass and
sisal had decreased by 10.4% and 12% respectively with reference to the control.
The high tensile strength of the steel-fibre reinforced concrete can be attributed to the high
tensile strength of the steel fibres and a high pull out force. This strength was transferred to the
concrete matrix through bond stresses. Due to the ‘bridging’ action of the steel fibres on the
crack formed at failure, extra strength was imparted in to the otherwise weak concrete in tension.
Sisal fibres and fibreglass reinforced concrete showed lower tensile strengths because of their
much lower tensile capacities as compared to the steel fibres. Their pull out force could have also
contributed to their weak performance.
The bond formed between the fibres and the concrete matrix also plays a big role in increasing
tensile strength of fibre-reinforced concrete. Deformed steel fibres may have given a much
higher overall tensile capacity of the fibre-reinforced concrete. Further studies on the use of
fibre-reinforced concrete can consider such factors.
44
Chart 4.7: 28 day tensile strength development curve
Figure 4.7: Sisal-reinforced concrete cylinder at failure
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 0.5 1 1.5 2 2.5 3 3.5
Ten
sile
Str
ength
(N
/mm
)
Fiber Content (%)
Fiberglass
Sisal Fibers
Steel Fibers
45
Chapter Five - Conclusion and Recommendations
Conclusions
1. The workability of fresh concrete was found to decrease with increase in fibre content. Sisal
fibre was the most effective in decreasing the workability as attributed to the low slump values
observed.
2. The addition of fibres at 1% by weight causes a significant enhancement in early as well as
long term compressive strength of concrete. The maximum improvement in 28 day-strength was
observed to be with the 1% addition of steel fibres which caused an increment of 14% in
compressive strength. Fibreglass showed an increment of 12% while sisal showed the least of the
three with a 4% increment with 1% addition of fibres by weight.
3. The addition of 1% fibres by weight causes a considerable improvement in the early and long
term tensile strength. Addition of 1% steel fibres caused the highest increment in tensile strength
with an addition of 68% to the tensile strength. Fibreglass and Sisal fibres showed increments
10% and 7.4% respectively with the addition of 2% fibre content.
4. An optimum fibre content was determined for each fibre type and the strength test performed.
For the compressive strength test, all fibres showed an optimum content of 1%. For the splitting
tensile test, steel fibres showed an optimum fibre content of 1% while fibreglass and sisal fibres
showed an optimum content of 2%.
5. The critical length and aspect ratio play a crucial role in determining the strength enhancing
properties of the fibre. Due to time and resource constraints, the tests were performed without
consideration on varying the aspect ratio and lengths of the fibres.
6. The steel fibres showed greatest increment in the mechanical strength of concrete while Sisal
fibres showed lowest increment in mechanical strength.
7. The fibres showed sufficient crack arrest properties. Sisal fibres arrested the most cracks
followed by steel fibres and glass-reinforced plastic which showed the least.
46
Recommendations
1. A number of tests could not be performed due to time, equipment and financial restraints.
These include chemical resistance, shrinkage and creep, water absorption, toughness (impact and
abrasion), etc. these tests would enable a more decisive conclusion to be made on the use of
fibres for modern construction applications.
2. Glass-reinforced plastic contains a high percentage of plastic. Glass fibres are much stronger
than plastic fibres hence if the percentage of the glass fibres can be increased, at the same time
considering the cost factor, promising results could be attained.
3. One of the main problems encountered during addition and mixing of the fibres into the
concrete mix was balling up of the fibres. Research should be carried out to come with an
additive or mixing and placing techniques that will prevent the entanglement of these fibres.
4. The tests carried out in this research paper involved fibres with a specific length and diameter.
These were chosen from literature study of past investigations done. Further tests should be
done, varying the aspect ratios of the fibres and determining the optimum aspect ratio and
content.
5. Further encouragement should be made on the use of sustainable materials for construction.
47
References
1. Belaguru, P.N., and Shah, S.P. (1992). Fibre-Reinforced Cement Composites, McGraw-Hill,
Inc.
2. Daniel, J.I; Roller, J.J. (1992). Fibre Reinforced Concrete SP 39.01T, Portland Cement
Association, Skokie.
3. Bentur, A., and Mindess, S. (1990). Fibre Reinforced Cementitious Composites, Elsevier
Applied Science.
4. Hannant, D.J. (1978). Fibre Cements and Fibre Concretes, John Wiley and Sons.
5. US-Sweden Joint Seminar (1986). Steel Fibre Concrete, Elsevier Applied Science Publishers
Ltd.
6. (2002) State-of-the-art Report on Fibre Reinforced Concrete, American Institute of Concrete
Committee ACI 544.1R-96.
7. Newman, J. and Choo, B.S. (2003). Advanced Concrete Technology, Elsevier Ltd.
8. Neville, A.M and Brook, J.J. (2001). Concrete Technology, Pearson Education Limited,
Edinburg Gate, England.
9. Neville, A.M (1981). Properties of Concrete, 3rd
Edition, Longman Scientific and Technical.
10. BS 1881-102: 1983. Testing Concrete - Method for determination of slump, British Standard
Institute.
11. BS 1881-103: 1983. Testing Concrete – Method for determination of compacting factor,
British Standard Institute.
12. BS 1881-108: 1993. Testing Concrete – Method for making test cubes from fresh concrete,
British Standard Institute.
13. 1881-110: 1993. Testing Concrete – Method for making test cylinders from fresh concrete,
British Standard Institute.
48
14. BS 1881-116: 1983. Testing Concrete – Method for determination of compressive strength of
concrete cubes, British Standard Institute.
15. BS 1881-117: 1983. Testing Concrete – Method for determination of tensile splitting
strength, British Standard Institute.
16. The Concrete Society (2007). Technical Report No. 63: Guidance for the Design of Steel-
Fiber-Reinforced Concrete.
17. Meyers, D.S. (2006). Fiber-reinforced concrete and bridge deck cracking, Oklahoma.
18. Naaman, A.E. (1976). Pull-Out Mechanism in Steel Fiber-Reinforced Concrete, Journal of
the Structural Division, Illinois, Chicago.
49
Appendices
Appendix A - Aggregate grading results
Table A1: Coarse aggregate grading results
Sieve size (mm) Retained mass
(gm) % Retained (%)
Cumulative
passed
percentage (%)
Acceptance Criteria
Min(%) Max (%)
20 0 0.0 100.0
14 0 0.0 100.0 100
10 114 10.0 90.0 85 100
5 780 68.5 21.4 0 25
2.36 244 21.4 0.0 0 5
Σ = 1138
Table A2: Fine aggregate grading results
Sieve size
(mm)
Retained mass
(gm)
% Retained
(%)
Cumulative
passed
percentage (%)
Acceptance Criteria
Min(%) Max (%)
14 0 0.0 100.0
10 4 1.6 98.4 100
4.76 5 1.9 96.5 89 100
2.36 12 4.7 91.9 60 100
1.18 21 8.1 83.7 30 100
0.6 73 28.3 55.4 15 100
0.3 107 41.5 14.0 5 70
0.15 28 10.9 3.1 0 15
0.075 4 1.6 1.6
254
BS 882: 1992 and ASTM C 33-03 specify the grading limits for fine and coarse aggregate as
shown in the following tables.
50
Table A3: Grading requirements for coarse aggregates according to BS 882: 1992
Table A4: BS and ASTM requirements for grading of fine aggregate
51
Appendix B – Slump and Compaction factor results
Table A5: Slump test and Compaction factor tests results
Fibre Content
(%)
Slump (mm) Uncompacted
Weight (Kg)
Compacted
Weight (Kg)
Compaction
Factor
Control (0%) 42 20.6 22.6 0.91
Fibreglass
1 35 17.6 21.7 0.87
2 31 19.0 22.1 0.86
3 28 17.5 21.1 0.83
Sisal Fibres
1 26 18.7 22.3 0.81
2 25 18.1 21.8 0.79
3 19 15.52 19.9 0.75
Steel Fibres
1 23 18.8 22.9 0.80
2 20 18.72 23.4 0.79
3 18 17.9 23.3 0.76
Appendix C – Strength test results
Table A6: Compressive strength results (7 day strength)
Failure Load (kN) Compressive Strength
P/A (N/mm2) Fibre content
(%)
Sample 1 Sample 2 Average value
Control (0) 150 170 160 16.0
52
Fibreglass
1 210 185 197.5 19.75
2 160 170 165 16.5
3 110 95 102.5 10.25
Sisal fibre
1 130 140 120 17.0
2 160 135 147.5 14.75
3 90 120 105 10.5
Steel fibre
1 320 270 295 29.5
2 250 240 245 24.5
3 230 190 210 21.0
Table A7: Compressive strength results (28 day strength)
Failure Load (kN) Compressive Strength
P/A (N/mm2) Fibre content
(%)
Sample 1 Sample 2 Average value
Control (0) 295 310 302.5 30.25
Fibreglass
1 400 330 365 36.5
2 285 265 275 27.5
3 260 230 245 24.5
Sisal fibre
1 300 330 315 31.5
2 285 255 270 27.0
3 215 195 205 20.5
53
Steel fibre
1 450 400 425 42.5
2 390 340 365 36.5
3 330 300 315 31.5
Table A8: Tensile strength results (28 day strength)
Failure Load (kN) Tensile Strength
(N/mm2) Fibre content Sample 1 Sample 2 Average value
Control (0%) 200 180 190 2.69
Fibreglass
1% 190 220 205 2.90
2% 200 220 210 2.97
3% 180 160 170 2.41
Sisal fibre
1% 190 200 195 2.76
2% 200 210 205 2.90
3% 160 175 167.5 2.37
Steel fibre
1% 310 330 320 4.53
2% 200 260 230 3.25
3% 190 210 200 2.82
54
Appendix D – Figures
Figure A1: use of steel fibre-reinforced concrete as a paving material
Figure A2 and A3: shotcrete tunnel lining (left picture) and rock slope stabilisation
55
Figure A4 and A5: Some applications of fibre reinforced concrete. The image on the left
shows GFRC slab formwork panels made of cement. Dolosse are shown on the other image.
Figure A6. The figure shows the ‘Pavilion Bridge’ in Zaragoza, Spain. The architect, Zaha
Hadid, chose glass fibre reinforced concrete to envelop the bridge; she covered the outer
skin of the building with 29,000 triangles of GFRC.