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.

1

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

2

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.

3

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.

4

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.

5

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

6

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

11

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

15

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.

18

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

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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

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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

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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.