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DEVELOPMENT OF POLYPROPYLENE FIBER AS CONCRETE
REINFORCING FIBER
by
RICKY NOVRY RATU
B. Eng. (Civil), Universitas Sam Ratulangi, 1993
M.Sc. (Wood Science), The University of British Columbia, 2009
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
MASTER OF APPLIED SCIENCE
in
THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES
(CIVIL ENGINEERING)
THE UNIVERSITY OF BRITISH COLUMBIA
(Vancouver)
April 2016
© Ricky Novry Ratu, 2016
ii
ABSTRACT
The objective of this research is to produce polypropylene fibers with improved interface
bonding with a concrete matrix. The Laboratory Mixing Extruder paired with the Randcastle
fiberline drawing device was used for producing fiber from polypropylene (PP) chips. A target
diameter of 0.5 mm fiber was obtained from a 2-stage process in the production line. The effort
to improve the fiber surface by applying aluminum oxide sol-gel coating was unsatisfactory
due to the failure of the coating materials to adhere to the fiber. Incorporating silica fume (SF)
powder in the fiber extrusion process enhanced fiber properties. Silica fume co-extruded PP
(SFPP) fiber has different characteristics in appearance, flexibility and surface roughness. Most
importantly, the co-extrusions produced significance improvements in surface characteristics.
Silica fume particles caused significant changes in the surface roughness of the fiber and
contributed to the improved bonding performance in a cement-based matrix. The inclusion of
the extruded fibers in a concrete matrix also improved the flexural toughness. Additional
testing was conducted to examine the performance of extruded fiber in preventing plastic
shrinkage cracking. Fiber reinforced mortar containing RPP and SFPP fibers were evaluated.
Based on total crack area reduction efficiency, and crack width reduction efficiency SFPP
fibers performed better than RPP fibers. These results indicate that the objective of developing
a concrete reinforcing fiber using laboratory equipment was successfully achieved. The
inclusion of silica fume particles in the extrusion process significantly changed the properties
of the fiber and therefore contributed to the performance of these extruded fibers in the concrete
matrix.
iii
PREFACE
This thesis is original, unpublished, independent work by the author, Ricky Novry Ratu,
under the supervision of Professor Nemkumar Banthia.
iv
TABLE OF CONTENTS
ABSTRACT .......................................................................................................................... ii
PREFACE ............................................................................................................................ iii
TABLE OF CONTENTS ..................................................................................................... iv
LIST OF TABLES .............................................................................................................. vii
LIST OF FIGURES ............................................................................................................ viii
ACKNOWLEDGEMENTS .................................................................................................. xi
Chapter One1
GENERAL INTRODUCTION1
1.1 Introductory Remarks ................................................................................................ 1
1.2 Research Objective .................................................................................................... 2
1.3 Study Outline ............................................................................................................. 2
Chapter Two4
FIBERS IN CONCRETE4
2.1 Introduction ............................................................................................................... 4
2.2 Factors Affecting Concrete Cracking ......................................................................... 5
2.3 Types of Fiber in Concrete Application ...................................................................... 8
2.4 Significance of Polypropylene Fibers ........................................................................12
2.4.1 Polypropylene Material .........................................................................................12
2.4.2 Polypropylene Fiber ..............................................................................................14
2.4.3 Properties of Polypropylene Fiber .........................................................................16
2.5 Application of Polypropylene Fiber in Concrete........................................................17
v
2.6 Review on Surface Modification of Polypropylene Fiber ..........................................24
2.7 Summary ..................................................................................................................27
Chapter Three29
DEVELOPMENT OF POLYPROPYLENE FIBER29
3.1 Introduction ..............................................................................................................29
3.2 Fiber Extrusion System .............................................................................................30
3.2.1 Laboratory Mixing Extruder ..................................................................................30
3.2.2 Randcastle Extrusion Lines ...................................................................................31
3.3 Fiber Extrusion Processes .........................................................................................34
3.3.1 Material Preparation ..............................................................................................34
3.3.2 LME Setup and Operation Parameter ....................................................................35
3.3.3 Limitation and Controls .........................................................................................38
3.4 Surface Modification of the Fiber .............................................................................41
3.4.1 Aluminum Oxide Coatings ....................................................................................41
3.4.1.1 Sol gel preparation .........................................................................................41
3.4.1.2 Coating of fiber ..............................................................................................42
3.4.2 Silica Fume Co-extruded PP Fiber .........................................................................44
3.4.2.1 Surface characteristics of extruded fiber .........................................................47
3.5 Bonding Performance and Tensile Testing ................................................................50
3.5.1 Bonding Performance of Extruded Fibers ..............................................................50
3.5.2 Comparison of Extruded Fibers .............................................................................56
3.6 Summary ..................................................................................................................63
Chapter Four64
COMPARATIVE FLEXURAL STRENGTH OF MIXTURES CONTAINING
EXTRUDED PP FIBERS64
4.1 Introduction ..............................................................................................................64
4.2 Experimental Design .................................................................................................64
4.2.1 Materials and Mixtures Proportion ........................................................................64
4.2.2 Preparation of Test Specimens ...............................................................................66
4.2.3 Experimental Setup for Flexural Toughness and Testing Procedure .......................68
vi
4.3 Experimental Results and Discussions ......................................................................71
4.3.1 Compressive Strength ............................................................................................71
4.3.2 Flexural Testing ....................................................................................................72
4.3.2.1 Fracture mode ................................................................................................72
4.3.2.2 Flexural response ...........................................................................................74
4.3.2.3 Flexural toughness .........................................................................................75
4.4 Summary ..................................................................................................................78
Chapter Five79
PLASTIC SHRINKAGE PERFORMANCE OF EXTRUDED FIBERS REINFORCED
OVERLAY79
5.1 Introduction ..............................................................................................................79
5.2 Experimental Design .................................................................................................79
5.2.1 Materials, Mixtures Proportion and Casting ...........................................................80
5.2.1.1 Substrate base.................................................................................................80
5.2.1.2 Overlay mortar ...............................................................................................84
5.2.2 Preparation of Test Specimens ...............................................................................85
5.2.3 Testing Procedure and Crack Assessment ..............................................................87
5.3 Experimental Results and Discussions ......................................................................92
5.3.1 Crack Development ...............................................................................................92
5.3.2 Extruded Fibers Performance ................................................................................93
5.4 Summary ..................................................................................................................97
Chapter Six99
GENERAL CONCLUSIONS AND SUGGESTION FOR FURTHER RESEARCH99
6.1 General Conclusions .................................................................................................99
6.2 Suggestion for Further Research ............................................................................. 101
REFERENCES.................................................................................................................. 103
APPENDIX ....................................................................................................................... 110
vii
LIST OF TABLES
Table 2.1: Types of cracking in concrete structures (Source: Concrete, p 507, Mindess et.al.,
1996) ............................................................................................................................. 7
Table 3.1: Material characteristics of polypropylene chips ...................................................35
Table 3.2: Extrusion parameter ............................................................................................37
Table 3.3: Bonding performance of extruded PP fibers ........................................................62
Table 4.1: Mixture Proportion .............................................................................................65
Table 4.2: Compresive strength data ....................................................................................72
Table 4.3: Average flexural toughness parameter according to ASTM C1609 ......................77
Table 5.1: Mixture proportion ..............................................................................................81
Table 5.2: Mix proportion of overlay mortar ........................................................................84
Table 5.3: Crack analysis .....................................................................................................96
viii
LIST OF FIGURES
Figure 2.1: Image of fibers crack bridging ............................................................................ 4
Figure 2.2: Plastic shrinkage crack on beam specimens ........................................................ 6
Figure 2.3: Different types of steel fibers .............................................................................. 8
Figure 2.4: Various types of carbon fibers (a, b, c) and glass fibers (d, e) .............................. 9
Figure 2.5: Various types of synthetic fibers (a-g) and some natural fibers (h-j) ...................11
Figure 2.6: Various types of polypropylene fiber product ....................................................15
Figure 3.1: The layout of Laboratory Mixing Extruder (LME) .............................................31
Figure 3.2: Randcastle fiberlines drawers (Slow drawer, left; Fast drawer, right; and the oven,
middle) .........................................................................................................................32
Figure 3.3: Actual image of fiber drawing showing the setting of the devices used ..............33
Figure 3.4: Sample of polypropylene chips used in this experiment .....................................34
Figure 3.5: Extruded fiber was pulled to the godet roll .........................................................36
Figure 3.6: Typical amorphous PP fiber produced using LME .............................................37
Figure 3.7: Polypropylene fiber with a final size of 0.5 mm diameter, 50 mm length ...........38
Figure 3.8: Layout of the extrusion system ..........................................................................39
Figure 3.9: Comparison of extruded PP fiber: Amorphous state (lower) and Semi Crystalline
(upper) .........................................................................................................................40
Figure 3.10: Aluminum isopropoxide powder (left) and PVA powder (right) .......................41
Figure 3.11: Refrigerated incubator shaker (left), and Aluminus oxide sol gel (right) ...........42
Figure 3.12: Comparison of uncoated and coated PP fiber ...................................................43
Figure 3.13: Surface image of uncoated (left) and coated (right) PP fiber at 20x magnification
.....................................................................................................................................44
Figure 3.14: Silica fume application on the surface of PP fiber ............................................46
Figure 3.15: Proportion of PP chips and silica fume powder prior mixing (left); Uncoated and
SF coated PP chips (right) ............................................................................................46
Figure 3.16: Surface of PP chips at 20x magnification: Uncoated (right) and SF coated (right)
.....................................................................................................................................47
Figure 3.17: Fiber extrusion process showing SF co-extruded PP fiber ................................48
Figure 3.18: Silica fume co-extruded PP fiber (Amorphous, left and semi-crystalline, right) 49
Figure 3.19: Microscope image of SF co-extruded PP fiber at 5x magnification ..................49
ix
Figure 3.20: Confocal microscope image of the surface SFPP fiber at 20x magnification
(Normal exposure, left and high contrast, right) ............................................................50
Figure 3.21: Dogbone-shaped molds for fiber pull out testing ..............................................51
Figure 3.22: Dogbone-shaped specimens prior to testing .....................................................51
Figure 3.23: The lay out of pull out testing apparatus ...........................................................52
Figure 3.24: Images of pull out specimens placed in its grip prior (upper) and during (lower)
testing ..........................................................................................................................53
Figure 3.25: Pull out load - end slip relationship performance of uncoated fiber ..................54
Figure 3.26: Pull out load - end slip relationship performance of Al2O3 coated fiber ............55
Figure 3.27: Typical failure pattern of coated fiber during pull out test ................................55
Figure 3.28: Pull out load - end slip relationship of SFPP fiber ............................................56
Figure 3.29: Extruded amorphous PP fiber, 1.5 mm diameter (center); Final product (0.5 mm
diameter) semi-crystalline PP fiber: SFPP (bottom left) and RPP (top right), ................57
Figure 3.30: Microscope image of extruded amorphous PP fibers ........................................58
Figure 3.31: Microscope image of semi-crystalline extruded PP fibers ................................59
Figure 3.32: Images of EDS spectrum of minerals on the surface of extruded PP fibers: RPP
(top) and SFPP (middle) ...............................................................................................60
Figure 3.33: Tensile strength of extruded PP fiber ...............................................................61
Figure 3.34: Lay out setting of strength evaluation of the fiber ............................................61
Figure 3.35: Pull out load - end slip relationship of extruded PP fibers ................................62
Figure 4.1: Extruded PP fibers 0.5 mm diameter, 50 mm length. RPP (left), SFPP (right) ....65
Figure 4.2: Sample calculation of mixture ingredients of FRC .............................................66
Figure 4.3: Pan mixer used (left) and cast specimens (right) ................................................67
Figure 4.4: Image of beam and cylinder specimens in curing rack........................................68
Figure 4.5: Testing set up showing Instron machine, data acquisition panel and computer ...68
Figure 4.6: Beam specimens with deflection fixture (yoke) ..................................................69
Figure 4.7: Test set up for determining cylinder compression strength .................................71
Figure 4.8: Images of specimens of each mix after testing. RPP (left) and SFPP (right) .......72
Figure 4.9: Typical fracture mode in concrete beam.............................................................73
Figure 4.10: Images of fiber bridging at the exposed cracks .................................................73
Figure 4.11: Load - Deflection curve Mix 1 with regular PP fiber ........................................74
Figure 4.12: Load - Deflection curve Mix 2 with SF co-extruded PP fiber ...........................75
Figure 4.13: Averaged flexural response of FRC containing extruded fibers ........................76
Figure 5.1: Dimension of substrate base (source: Gupta, Thesis 2008) .................................80
Figure 5.2: Sample calculation of mixture ingredients of concrete base for shrinkage tests ..82
Figure 5.3: Molds for substrate base ....................................................................................83
x
Figure 5.4: Image of base specimens in curing room............................................................83
Figure 5.5: Extruded PP fibers, 0.5 mm diameter, 50 mm length. RPP (lef) and SFPP (right)
.....................................................................................................................................84
Figure 5.6: Sample calculation of overlay mortar .................................................................85
Figure 5.7: Molds for plastic shrinkage testing showing substrate base placement ...............86
Figure 5.8: Repair overlay specimens after finishing and before starting the test ..................87
Figure 5.9: Environmental chamber showing the placement of specimens ...........................88
Figure 5.10: Specimens after demolding ..............................................................................89
Figure 5.11: Image of cracked specimens after testing and tools used for measuring the crack
.....................................................................................................................................90
Figure 5.12: Crack progression on plain overlay specimen #1 ..............................................92
Figure 5.13: Crack progression on SFPP fiber reinforced overlay specimen #1 ....................93
Figure 5.14: Complete set of overlay specimens after testing ...............................................94
Figure 5.15: Crack mapping ................................................................................................95
Figure 5.16: Crack control efficiency of RPP, SFPP and PVAPP .........................................97
xi
ACKNOWLEDGEMENTS
1 I would like to thank all the people who helped and encouraged me during my graduate
studies at the Department of Civil Engineering, Faculty of Applied Science the University
of British Columbia. 2
3 I especially want to thank my supervisor, Professor Nemkumar Banthia, for his support,
valuable advice and encouragement throughout the course of my research. I also thank
him for giving me the opportunity to participate in various seminars, meetings and
conferences including the regular ACI - BC Chapter meetings, EFCECM 2014, and
CONMAT 2015. 4
5 I would like to thank Professor Frank Ko, the leader at the Advanced Fibrous Materials
lab, for allowing me to work using his lab facilities and for his generous comments about
my research outcome. Also, many thanks to his group members, especially Dr. Heejae
Yang and Dr. Yuqin Wan, for their help during fiber production in the AMPEL lab. 6
7 Technicians in the Machine Shop at the Department of Civil Engineering have my thanks
for supporting my academic research. Special thanks to Mr. Harald Schrempp for helping
with all the technical problems in the lab and for keeping the “LME” in good shape. Also,
thanks to Ms. Paula Parkinson in Environmental lab for helping me with the chemical
related work. 8
9 I also thank Dr. Sidney Mindess for his time, constructive comments and approval as a
second examiner of this thesis. 10
11 I am grateful to all members of the Materials research group for making the lab a pleasant
place to work. Special appreciation goes to Ms. Jane Wu for helping with SEM work and
arranging the equipment schedule availability for everyone. Also thanks to Dr. Obinna
Onuaguluchi for giving feedback and correction for some parts of my thesis manuscript.
Thanks to my former fellow graduate students Sudip Talukdar, Tasnuba Islam, Sahar
Ranjbar and Qiannan Wang for their help in my early years in the research group and for
their friendship. My thanks also to Dr. Cristina Zanotti, Negar Roghanian, Brigitte Goffin,
Mohammed Farooq and all the members of this wonderful group. 12
13 I also acknowledge the involvement financial contributions of Natural Sciences and
Engineering Research Council (NSERC) of Canada. 14
15 My deepest gratitude goes to my family; my wife, Mitsi Singal and the boys, Leri, Valdi
and Verrel for their support, love and prayer throughout my life. 16
17 Most overall, praise be to God forever and ever. He has made everything beautiful in its
time.
18 "There is surely a future hope for you, and your hope will not be cut off (Proverbs 23:18)"
1
1 Chapter One
GENERAL INTRODUCTION
1.1 Introductory Remarks
In general, fiber has become an integral part of concrete application. Vast ranges of
materials have been tested such as steel, carbon, glass, plastic, polypropylene, nylon, and even
natural materials such as cotton. In general, the introduction of fibers into the concrete matrix
was found to significantly alter the brittle tension response of the concrete material.
Before cracking the addition of fibres has little effect. However, even small amounts of
fiber addition leads to significant increases in the post-cracked toughness and ductility of
concrete (Shah and Rangan, 1971). As well, significant improvements in crack control can be
achieved, with a reduction in crack width and crack spacing in the concrete (Banthia et al.,
1993; Banthia and Gupta, 2006). The smaller crack widths and increased abrasion resistance
promotes an improvement in the long-term serviceability of the structure by preventing the
ingress of chemicals and water that can have deleterious effects (Johnston, 2001).
Synthetic fiber, such as polypropylene fiber, is gaining popularity due to its low cost and
non-corrosive nature. This type of fiber is of particular interest due to its corrosion resistance
relative to steel, resistance to alkali attack, relatively low cost, and durability with a long
service life. Polypropylene fibers can also be made into a variety of cross-sectional shapes and
can be designed with different surface finishes, allowing for further improvement in bond
properties (Wang et al., 1987).
However, its hydrophobic nature is a major drawback and this still needs to be
overcome. Polypropylene fibers are not expected to bond chemically in concrete matrix, but
bonding has been shown to occur by mechanical interaction. The effort to explore and optimize
2
its potential both in academic research and industrial development has been tremendously
increased in the past decade.
In this thesis, the effort to improve polypropylene material as a concrete reinforcing fiber
is described. The possibility to improve the performance of interface bonding between fiber
and concrete matrices by surface modification is explored. This includes the application of sol
gel coating and silica fume (SF) particles inclusion in the fiber extrusion process.
1.2 Research Objective
The purpose of this research is to explore the optimum performance of polypropylene
fiber in concrete application by improving its bond properties with a matrix.
This process includes:
Developing an extrusion process of polypropylene fiber
Optimizing the settings of the equipment used for extruding the fiber
Applying a coating layer for surface modification of the fiber
Developing a novel procedure in fiber production by incorporating supplementary
materials as fillers in the extrusion process
Testing the performance of the fibers including bond, flexural performance and
plastic shrinkage crack resistance
1.3 Study Outline
This Chapter provides an introduction and the rationale for the study as well as the
general outline of the study. Chapter 2 reviews the relevant literature on fiber reinforced
concrete, including types of fiber, the application of fibers in concrete and efforts to maximize
the benefit gained from fiber inclusion. Chapter 2 also reviews the process of producing
polypropylene fiber.
3
In Chapter 3, the development of polypropylene fiber is described. The process of
production using a laboratory scale extruder and drawing equipment is explained. The attempt
to modify the surface of the fiber using aluminum oxide sol gel and its bonding performance
with concrete mortar is discussed. Chapter 3 also discusses the addition of silica fume and
polyvinyl alcohol particles in extruding process of the fiber. The comparison between these
co-extruded fibers and regular extruded fiber including surface characteristics and performance
in concrete matrix is deliberated. Other properties such as tensile strength of the fiber are also
determined.
Chapter 4 and Chapter 5 describe experiments that were carried out to test the
performance of the fibers in flexural response (Chapter 4) and plastic shrinkage cracking
(Chapter 5). The experimental method and test results of both tests are discussed. Finally,
General conclusions and Recommendations for further research are summarized in Chapter 6.
4
2. Chapter Two
FIBERS IN CONCRETE
2.1 Introduction
The use of fibers to reinforce concrete materials is a well-known concept. It has been
practiced since ancient times, with straw mixed into mud bricks and horsehair in mortars. Straw
was used to reinforce sun-baked bricks and horsehair was used to reinforce masonry mortar
and plaster (ACI 544). The concept of fiber reinforcement of cement based materials using
asbestos started with the invention of the Hatschek process in 1898. Later, glass fibers were
proposed as reinforcement of cement paste and mortar (Biryukovich et al., 1965). In modern
times, the choice of fibers can vary from synthetic organic materials such as polypropylene or
carbon, synthetic inorganic such as steel or glass, natural organic such as cellulose or sisal to
natural inorganic asbestos.
Using fibers in concrete matrices addresses the issue of cracking in cement based
materials. Concrete is considered to be a relatively brittle material with a low tensile strength
compared to its compressive strength. When subjected to tensile stresses, unreinforced
concrete will crack and fail. The use of fibers modifies properties of concrete both in plastic
and hardened stages and results in a more durable concrete.
Figure 2.1: Image of fibers crack bridging
5
Fiber-reinforced concrete (FRC) has become an important material in the construction
of buildings and other structures. Reinforcing fiber’s ability to support load after cracking
(Figure 2.1) and to reduce the brittleness of concrete has positive effects on the structural
performance of concrete.
This section describes the general role of fibers in improving concrete performance and
the involvement of polypropylene fibers, in particular. The properties of polypropylene
material and the production process of the fiber are also presented. Finally, studies
incorporating polypropylene fiber in concrete applications are reviewed.
2.2 Factors Affecting Concrete Cracking
Cracks can develop due to a number of reasons. The main causes are low tensile
strength of concrete, intrinsic volumetric instability and deleterious chemical reactions.
Concrete is a brittle material and is prone to cracking in the plastic as well as the hardened
stage.
Plastic shrinkage occurs due to the loss of moisture from the concrete surface in its
plastic state. This state is defined as the first 24 hours after cement hydration begins. When the
rate of water evaporation from the surface of the concrete exceeds its bleeding rate, the surface
begins to dry resulting in high capillary stress near the concrete surface (Cohen et al., 1990).
Since concrete is very weak to tension, especially in its plastic stage, a volume change can
cause the surface to crack.
Plastic shrinkage cracks (Figure 2.2) are short cracks that occur before final finishing
on days when wind, a low humidity, and a high temperature occur. Surface moisture evaporates
faster than it can be replaced by rising bleed water, causing the surface to shrink more than the
interior concrete. Because the interior concrete restrains shrinkage of the surface concrete,
stresses can develop that exceed the concrete's tensile strength resulting in surface cracks.
Plastic shrinkage cracks are of varying lengths and are spaced from a few centimeters up to 3
m apart and often penetrate to mid-depth of a slab (PCA, 2001).
6
Figure 2.2: Plastic shrinkage crack on beam specimens
Cracks that occur after hardening are usually the result of drying shrinkage, thermal
contraction, or subgrade settlement. After hardening, if there is loss of water concrete will
shrink due to the volume change, which if restrained by the subgrade and reinforcement will
crack. Because of the evaporation of moisture in concrete, the tensile stresses that are confined
to the surface tension of the water are transferred to the capillary walls. This tension in the
capillary walls causes the shrinkage of concrete (Brown et al., 2001).
A major factor influencing the drying shrinkage properties of concrete is the total water
content of the concrete. As the water content increases, the amount of shrinkage increases
proportionally. Large increases in the sand content and significant reductions in the size of the
coarse aggregate increase shrinkage because total water content is increased and smaller coarse
aggregates provide less internal restraint to shrinkage. This causes tensile stress to develop in
hardened concrete causing the concrete to crack.
Cracking can be also the result of one or a combination of factors such as subgrade
settlement, thermal contraction, restraint (external or internal) to shortening, and applied loads.
Settlement cracks may develop over embedded items, such as reinforcing steel, or adjacent to
forms or hardened concrete as the concrete settles or subsides. Settlement cracking results from
insufficient consolidation, high slump, or a lack of adequate cover over embedded items.
7
Thermal expansion and contraction can also cause cracking. Concrete has a coefficient
of thermal expansion of approximately 10 x 10-6 per °C. Concrete placed during hot midday
temperatures will contract as it cools during the night. A 22 °C drop in temperature between
day and night would cause about 0.7 mm of contraction in a 3 m length of concrete, sufficient
to cause cracking if the concrete is restrained (PCA, 2001).
Cracks can also be caused by freezing and thawing of saturated concrete, alkali-
aggregate reactivity, sulfate attack, or corrosion of reinforcing steel. However, cracks from
these sources may not appear for years. Proper mix design and selection of suitable concrete
materials can significantly reduce or eliminate the formation of cracks and deterioration related
to freezing and thawing, alkali-aggregate reactivity, sulfate attack, or steel corrosion (PCA,
2001).
Mindess et al. (2003) lists the types of cracking in concrete structures as shown in Table
2.1.
Table 2.1: Types of cracking in concrete structures (Source: Concrete, p 507, Mindess et.al.,
1996)
Nature of Crack Cause of Cracking Remarks
Large, irregular, frequently with height
differential
Inadequate support,
overloadingSlabs on ground, structural concrete
Large, regularly spacedShrinkage cracking, thermal
cracking
Slabs on ground, structural concrete,
mass concrete
Coarse, irregular "map cracking" Alkali-silica reaction Extrusion of gel
Fine, irregular "map cracking" (crazing)Excessive bleeding, plastic
shrinkage
Finishing too early, excessive
troweling
Fine cracks roughly parallel to each
other on surface of slabPlastic shrinkage Perpendicular to direction of wind
Cracks parallel to sides of slabs
adjacent to joints (D-cracking)
Excessive moisture contents,
porous aggregates
Deterioration of conctere slab due to
destruction of aggregates by frost
Cracks above and parallel to
reinforcing barsSettlement cracking
Structural slabs due to consolidation of
plastic concrete around reinforcing
bars near upper surface
Cracking along reinforcing bar
placements, frequently with rust
staining
Corrosion of reinforcementAggravated by the presence of
chlorides
8
2.3 Types of Fiber in Concrete Application
According to ACI 544.1R-96, State-of-the-Art-Report of Fiber Reinforced Concrete, a
wide variety of fibers have been incorporated into concrete. The basic fiber categories are steel,
glass, synthetic and natural fiber materials. For each application it needs to be determined
which type of fiber is optimal for concrete application. The selection of the type of fiber is
guided by the properties of the fiber such as diameter, specific gravity, Young’s modulus,
tensile strength and the extent these fibers affect the properties of the cement matrix.
Figure 2.3: Different types of steel fibers
Steel Fibers
The introduction of steel fiber in concrete can increase the resistance to fatigue, impact, blast
or seismic events. The main advantage of using steel fiber in concrete is to increase the post-
peak load carrying capacity of concrete after initial cracking. Steel fibers intended for
reinforcing concrete are defined as short, discrete lengths of steel having an aspect ratio from
about 20 to 100 and that are sufficiently small to be randomly dispersed in an unhardened
concrete mixture using usual mixing procedures (ACI 544.1R-96). Steel fibers have a
relatively high strength and modulus of elasticity, and their bond to concrete matrix can be
9
enhanced by mechanical anchorage and surface roughness. Therefore, the fibers were modified
to various types including hooked end, crimped, deformed and enlarged-end fibers. Tensile
strength of steel fibers is in the range of 345 – 1200 MPa and the ultimate elongation in the
range of 0.5 – 3.5% (ACI 544.1R-96). Figure 2.3 shows various types of steel fibers in concrete
application.
Carbon fibers
Carbon fiber (Figure 2.4; a-c) is defined as a fiber containing at least 92 wt % carbon. Carbon
fibers are extremely thin fibers which are 0.005 - 0.010 mm in diameter and are generally used
in shorter lengths. The density of carbon fiber is very low compared to steel (Chung D D L,
1992). Carbon fibers have high tensile strength, low thermal expansion, good abrasion
resistance and stability at high temperatures with relatively high stiffness which makes them a
popular material in industries such as aerospace, civil engineering, and military (Huang X,
2009). However, this type of fiber is expensive compared to other types of fiber such as glass
or synthetic.
Figure 2.4: Various types of carbon fibers (a, b, c) and glass fibers (d, e)
10
Glass Fibers
Glass fibres (Figure 2.4; d, e) are produced in a process in which molten glass is drawn in the
form of filaments, through the bottom of a heated platinum tank or bushing. The structure of
the reinforcing glass fibres has required the development of special technologies to incorporate
the fibres into the matrix (Bentur and Mindess, 2007). The elastic modulus of glass fiber is
found to be approximately one third of steel. However, when compared to carbon fiber, glass
fiber elongates much more before failure. The main disadvantage of glass fibers is their
sensitivity to an alkaline environment. In recent years, attempts have been made to improve
the alkali resistance of glass fibers. These fibers are called AR-glass.
Synthetic fibers
The synthetic polymeric fibers used in the construction industry include acrylic, aramid,
carbon, nylon, polyester, polyethylene, and polypropylene fibers (Figure 2.5; a-g). All these
fibers have a high tensile strength, but most of these fibers have a relatively low modulus of
elasticity. As the diameters of polymeric fibers are of the order of micrometers, their high
length-to-diameter ratios are useful in fiber reinforced concrete. The major disadvantages of
polymeric fibers are a low modulus of elasticity, poor bond with cement matrix, and a low
melting point. Their bond to the cement matrix can be improved by twisting several fibers
together or by treating the fiber surface.
11
Figure 2.5: Various types of synthetic fibers (a-g) and some natural fibers (h-j)
Natural fiber
Fibers produced by plants, animals and geological processes are known as natural fibres
(Figure 2.5; h-j). Researchers have used natural fibers as an alternative for steel or synthetic
fibres in composites including cement paste, mortar and concrete to increase their strength
properties. Some of the best known natural fibers are sisal, coconut, sugarcane bagasse,
plantain, jute, bamboo, palm, banana, hemp, flax, and cotton. Natural fibres are cheap and
locally available in many countries. Thus their use as reinforcement material for improving the
properties of composites costs little. One of the disadvantages of using natural fibers is that
they have a high variation in their properties (Li et al., 2006).
12
2.4 Significance of Polypropylene Fibers
The principal reason for incorporating fibers into a cement matrix is to increase the
toughness and tensile strength, and improve the cracking resistance of the resultant composite.
The real advantage of adding fibers is that when fibers bridge these cracks and undergo pullout
processes, the deformation can continue only with a further input of energy from the loading
source. Reinforcing fibers stretch more than concrete under loading. Therefore, the composite
system of fiber reinforced concrete is assumed to work as if it was non-reinforced until it
reaches first crack strength. It is from this point that fiber reinforcement takes over and holds
the concrete together. With reinforcing, the maximum load carrying capacity is controlled by
fibers pulling out of the composite.
A substantial amount of research has been done to evaluate the properties of fiber
reinforced concrete. Test data have been obtained for concrete reinforced with polypropylene
fibers at volume percentages ranging from 0.1 % - 10.0 %. The material properties of
polypropylene fiber reinforced concrete are somewhat variable, depending greatly on fiber
concentration and the properties of the fiber.
The effectiveness of the polypropylene fiber as concrete reinforcement depends on the
bond between the fiber and the matrix. Although polypropylene fibers are characterized by low
elastic modulus and poor physicochemical bonding with cement paste, it is quite apparent that
the load carrying ability of a structure under flexural loading may be considerably increased
(Brown et al., 2001).
2.4.1 Polypropylene Material
Polypropylene (PP) is a versatile thermoplastic material, which is produced by
polymerizing monomer units of propylene molecules into very long polymer molecules or
chains in the presence of a catalyst under carefully controlled heat and pressure (Brown et al.,
2002). Polypropylene is one of the fastest growing classes of commodity thermoplastics, with
a market share growth of 6 – 7 % per year. The volume of polypropylene produced is exceeded
13
only by polyethylene and polyvinyl chloride. The moderate cost and favorable properties of
polypropylene contribute to its strong growth rate (Maier and Calafut, 1998).
Polypropylene is extremely hard and stiff and is brittle at very low temperatures. It
gradually becomes softer and more flexible as the temperature increases until it softens beyond
the range of usefulness. The crystalline structure of the polymer undergoes a major change at
the melting point. The high melting point of polypropylene provides resistance to softening at
elevated temperatures. Standard grades of polypropylene can withstand continuous service
temperatures of over 107 ºC and over 121 ºC for short periods of time (Maier and Calafut,
1989).
Amorphous regions of the PP resin undergo a glass transition at temperatures between
- 35 and 26 ºC. This transition depends on the heating rate, thermal history and microstructure
and measurement method. Molecules and segments of polymer chains above the glass
transition temperature vibrate and move in non-crystalline polymer regions. The normal
temperature range within which PP is most commonly used is limited by the crystalline melting
point on the high side and by the glass transition temperature on the low side (Brown et al.,
2001).
The mechanical properties of polypropylene are strongly dependent on time,
temperature and stress. Furthermore, it is a semi-crystalline material, so the degree of
crystallinity and orientation also affects the mechanical properties. Also the material can exist
as homopolymer, block copolymer and random copolymer and can be extensively modified by
fillers, reinforcements and modifiers.
Polypropylene is a thermoplastic and hence softens when heated and hardens when
cooled. It is hard at ambient temperatures and this inherent property permits economical
processing techniques such as injection molding or extrusion. The softening point or resistance
to deformation under heat limits its service temperature range. If the product has a wide
working temperature range, then the coefficient of linear expansion becomes significant. The
coefficient of linear expansion of polypropylene is higher than most commodity plastics but is
less than that of polyethylene (Maier and Calafut, 1989).
When polypropylene is exposed to high temperatures within its maximum operating
temperatures, a gradual deterioration takes place. This effect is known as thermal aging. Aging
14
temperature varies from 70 °C to 135 °C depending upon the degree of stability of the fiber
and the expediency of the test. A 50 percent loss in fiber strength and elongation or the
toughness factor is generally taken as the end of the induction period and is considered as a
relative measure of polymer stability at test temperature (Maier and Calafut, 1998).
Polypropylene has a high resistance to chemical attack due to its non-polar nature. The
term non-polar refers to the bond between atoms. The atoms of each element have a specific
electronegativity value. The smaller the difference between the electronegativity values of the
atoms in a bond, the smaller will be the polarity of the bond. When this difference is small the
material is said to be non-polar (Maier and Calafut, 1998).
Many chemical attacks are more severe at higher temperatures and at higher
concentrations of the chemical reagent. In general, polypropylene is resistant to alcohols,
organic acids, esters and ketones. Contact with copper and copper alloys accelerates oxidation,
particularly in the presence of fillers and reinforcements. Also the water absorption is low and
this is because of the non-polar nature of the material (Brown et al., 2001).
2.4.2 Polypropylene Fiber
Polypropylene chips can be converted to fiber by traditional melt spinning processing.
Melt spinning is a process in which the molten polymer is forced through a spinneret. The
molten polymer emerges from the spinneret as continuous strands of fiber that are cooled using
water or air current. The fibers are then drawn by heating to a temperature close to the melting
point before being stretched. This process reduces the cross section and aligns orientation in
fibers, resulting in increased tensile strength (Maier and Calafut, 1989).
Polypropylene fibers are available in two different forms: Monofilaments and Multi-
filaments. Monofilaments are ribbons of polypropylene composed of a single extruded
filament produced by melt spinning followed by water quenching. Sizes of monofilaments
range from 105 – 865 microns. Monofilaments are used in weaving stiffer products such as
rope or twine. Monofilament fibers are characterized by highly reflective and translucent
surface, limited absorption capacity, high stiffness and good tensile strength.
15
Several individual monofilaments that are ≤ 105 microns are grouped into a single
continuous bundle to produce multifilaments. Multifilament fibers are characterized by its
flexibility, light weight and hydrophobic nature.
Polypropylene fibers are also produced as continuous cylindrical monofilaments that
can be chopped to specified lengths or as films and tapes that can be fibrillated to form the
fibrils of rectangular cross-section. Fibrillated means the polypropylene film is slit so it can be
expanded into an open network of fibers. Figure 2.6 shows the various types of polypropylene
fiber products.
Figure 2.6: Various types of polypropylene fiber product
Polypropylene fibers are composed of crystalline and non-crystalline regions. Fiber
spinning and drawing may cause the orientation of both crystalline and amorphous regions.
Polypropylene fibers are characterized by their light weight, good resilience, good thermal
stability, high strength, and favorable elongation properties (Maier and Calafut, 1989).
16
2.4.3 Properties of Polypropylene Fiber
Specific gravity of PP fiber is 0.90 – 0.91 gm cm-3. Because of its low specific gravity,
PP yields the greatest volume of fibre for a given weight. This high yield means that PP fiber
provides good bulk and cover, while being lighter in weight. Polypropylene is the lightest of
all fibres and is also lighter than water. It is 34 % lighter than polyester and 20 % lighter than
nylon.
Polypropylene fiber has the lowest thermal conductivity of any natural or synthetic
fiber. Polypropylene fibres retain more heat for a longer period of time. And, it remains flexible
at temperatures around -55 °C. The melting point of polypropylene is about 165 °C and while
it does not have a true softening point temperature, the maximum processing temperature of
the fiber is approximately 140 °C.
Prolonged exposure to elevated temperatures will cause degradation of the fiber, but
anti-oxidants are incorporated in polypropylene fibers to protect them during processing and
at normal service temperatures. Nevertheless, this temperature is sufficiently high for the fiber
to be processed satisfactorily in almost all normal manufacturing processes (Brown et al.,
2001).
Polypropylene has the best resistance of any common fibre to the action of most types
of chemicals and is affected only by the most aggressive acids and oxidizing agents. The fiber
is unaffected by most acids, alkalis, and salts. Polypropylene fiber is not affected by bacteria
or micro-organisms. It is also moth-proof and rot-proof and is inherently resistant to the growth
of mildew and mold (Brown et al., 2001).
17
2.5 Application of Polypropylene Fiber in Concrete
This section includes literature on various researches that evaluate the performance of
fiber reinforced concrete that employed polypropylene fiber in concrete composite.
Bayasi and Zeng (1993) investigated the properties of fiber reinforced concrete with
polypropylene fibers. Different length and volume fraction of fibrillated PP fiber were added
into the mixtures. The authors concluded that addition of PP fiber tended to increase the water
permeability of concrete. Fibers had a relatively small favorable effect on compressive strength
and compressive toughness of concrete when ½ inch and ¾ inch fibers were used, respectively.
They also concluded that for volumes equal to or less than 0.3 %, ¾ inch long fibers were more
favorable for enhancing the post-peak resistance, but for 0.5 % volume, ½ inch long fibers
were more effective.
Kakooei et al. (2012) evaluated the effect of polypropylene fibers on the properties of
reinforced concrete structure. In their study the influence of different amount of polypropylene
fibers content on concrete properties were investigated by measuring permeability, electrical
resistivity and compressive strength. They found that concrete compressive strength increased
proportionally with the increase in volume ratio of PP fiber. They concluded that the presence
of PP fibers had caused delay in the degradation process by reducing permeability, reducing
the amount of shrinkage and expansion of concrete that can significantly affect the lifespan of
the structure. They also concluded that electrical resistivity of concrete with fiber ratio of 1
and 1.5 kg m-3 had higher values in comparison with other samples.
Mechanical properties of PP fiber reinforced concrete and the effects of pozzolanic materials
were investigated by Alhozainy et al. (1996). Collated fibrillated PP fiber at volume fraction
ranging from 0.05 % to 0.3 % were added to the mixture containing different composition of
cementitious binder including cement, fly ash, silica fume and slag. The authors summarized
that polypropylene fiber had no statistically significant effect on the compressive strength and
toughness of conventional concrete. Moreover, PP fiber had no effects on the flexural strength,
however the addition of 0.1 %, 0.2 % and 0.3 % volume fraction of fibers increased the flexural
18
toughness by 44 %, 271 % and 387 %, respectively. Additionally, PP fibers increased the first
crack and failure impact resistance of concrete. They also concluded that, while pozzolans
generally reduce the impact resistance of concrete, the positive interactions between PP fiber
and pozzolans lead to enhanced impact resistance of fibrous concrete with pozzolans.
The effect of PP fiber reinforcement on the properties of fresh and hardened concrete in the
Arabian Gulf environment was studied by Al-Thayib et al. (1998). They applied commercial
polypropylene fibers in 20 mm fibrillated bundles into separate concrete mixture with different
water-cement ratios. The effect of fiber addition on plastic shrinkage, drying shrinkage and
mechanical properties including compressive, tensile and flexural strength of the mixtures,
were assessed. They found that the inclusion of polypropylene fibers eliminates the plastic
shrinkage cracking in slabs subjected to temperature as high as 46 ºC, however, this was not
the case for the drying shrinkage. They also found that the inclusion of PP fiber slightly
improved the tensile and flexural strength but did not improve the compressive strength of
concrete. The authors concluded that the PP fiber did not help in reducing the strength loss of
concrete that occurs due to curing in hot weather condition.
Soroushian et al. (1995) evaluated plastic shrinkage cracking of polypropylene fiber reinforced
concrete. They summarized that polypropylene fiber reduced the total plastic shrinkage crack
area and maximum crack width at 0.1 percent fiber volume fraction. They also concluded that
different PP fiber volume fraction (0.05, 0.1, 0.2 percent) had statistically similar effects on
the total plastic shrinkage crack area and the maximum crack width. Moreover, longer fibers
produced less cracks at 0.1 and 0.2 percent fiber volume fractions and smaller maximum crack
width at 0.05 percent fiber volume fractions, when compared with the shorter fibers.
Soroushian et al. (1995) compared the mechanical properties of concrete materials reinforced
with polypropylene or polyethylene fiber. They found that PP fibers at 0.1 percent volume
fraction as well as PE fibers at 0.025 and 0.025 percent volume fraction had negligible effect
on the flexural strength of concrete; only 0.1 percent volume fraction of PE fibers could
improve flexural strength. They also concluded that 0.05 percent of PE fiber volume fraction
produced impact strengths comparable to those with 0.1 PP fibers volume fraction in concrete.
19
Aly et al. (2008) evaluated the effect of PP fibers on shrinkage and cracking of concretes. They
employed a commercial PP fiber in the form collated fibrillated fiber bundles of 19 mm length
with different volume fractions fibers ranging from 0.05 to 0.5 % in the mixtures. They
concluded that increasing dosages of PP fiber in concrete caused small but consistent increases
of the overall total shrinkage strain of concrete. The increases in shrinkage are notable in
concretes without any curing (exposed at 1-day). In concretes with 7-days moist curing, the
shrinkage differences are not significant. The authors also concluded that concrete mixtures
that incorporated PP fiber are more permeable and hence more vulnerable to drying, as
evidenced by more moisture lost during the period of drying than the companion mixtures
without fibers.
Banthia and Gupta (2006) investigated the influence of PP fiber geometry on plastic shrinkage
cracking in concrete. Four types of PP fibers, three monofilament and one fibrillated fiber type,
with different volume fractions were added into separate concrete overlay mixtures. They
concluded that PP fibers are highly effective in controlling plastic shrinkage cracking in
concrete. The addition of fibers reduced the total crack area, maximum crack width and the
number of cracks. They also stated that the effectiveness of fiber reinforcement increases when
fiber volume fraction increases.
Studying crack growth resistance of hybrid fiber reinforced cement composites, Banthia and
Nandakumar (2003) employed two types each of steel and polypropylene fibers in mortar mix.
Monofilament or fibrillated PP fiber were combined with crimped or flattened end steel fiber
in different volume fractions. The authors summarized that the use of a secondary PP micro-
fiber even at nominal dosage rates appeared to be highly effective in enhancing the efficiency
of deformed steel fibers in concrete. They also concluded that the monofilament fiber appeared
to be more effective than the fibrillated fiber.
Poon et al. (2004) studied the compressive behavior of fiber reinforced high-performance
concrete subjected to elevated temperature. Combination of steel (hooked) and polypropylene
(19 mm length) fibers with different volume fractions were added into the mix. Those authors
20
concluded that PP fibers slightly increased the specific toughness and compressive strength of
the concrete for unheated specimens, however, they resulted in a quicker loss of the
compressive strength and toughness after exposure to the elevated temperatures. They also
stated that the combined use of PP fiber and steel fiber showed little benefits compared with
the use of steel fibers only.
Pull-out behaviour of PP fibers from cementitious matrix was investigated by Singh et al.
(2004). Thin strips of polypropylene, 50 mm long and have a rectangular cross-section of 1.25
x 0.2 mm were used. They concluded that with the increase in embedded length, fiber abrasion
effect becomes prominent and results in an increase in pullout load in the frictional sliding
zone of the pullout. When applying mechanical indentation of the surface of the fiber, the
authors also concluded that the bond strength between PP fibers and cement matrix increased
by a factor of three with optimum level of dent modification.
Sukontasukkul et al. (2010) evaluated the post-crack flexural response and toughness of FRC
after exposure to high temperature. They investigated three types of fibers; steel,
polypropylene and polyethylene, at three different volume fractions. The specimens were
exposed to three different temperatures: 400 ºC, 500 ºC, and 800 ºC. The authors concluded
that prior to the peak, the response was entirely dominated by the response of concrete matrix.
The post-peak flexural response of FRC was affected by two factor: the level of temperature
and the type of FRC. For the PP and PE FRC, because of the evaporation of the fiber, large
drops of load-deflection responses were observed. That was not the case of SRFC.
Effect of exposure to elevated temperature of PP FRC was also examined by Bayasi and Al
Dhaheri (2002). Specimens containing fibrillated PP fiber, 19 mm long, in different volume
fraction ranging between 0.1 to 0.3 % were exposed to different temperature with different
duration. They found that exposure to elevated temperature caused the ultimate flexural
strength and the post-peak flexural strength of PP fiber concrete to decrease, this became more
pronounced as temperature increased and the length of duration increased.
21
Hsie et al. (2008) investigated the mechanical properties of PP hybrid fiber-reinforced
concrete. The combination of coarse monofilament PP fibers and staple PP fiber with different
volume fractions were evaluated. They concluded that the performance of hybrid FRC was
better than that of single FRC. Comparing with the strength of plain concrete, the compressive
strength of PP hybrid FRC, splitting tensile strength, and modulus of rupture, increased by
17.31 %, 13.35 % and 24.60 %, respectively.
Toutanji H A (1999) evaluated the properties of PP fiber reinforced silica fume expansive-
cement concrete. The fibrillated PP fibers, ranging between 6 and 51 mm long, were added to
the mix at 0.1, 0.3 and 0.5 % volume fraction. The author found that the use of 5 % silica fume
resulted in improving the bond strength between the repair materials to the old substrates. The
rate of increase in bond strength decreased with increasing SF content from 5 to 10 %.
However, the use of PP fiber resulted in an increase in bond strength especially for the mixtures
with 10 % silica fume. Moreover, increasing PP fiber volume fraction resulted in an
improvement in post-peak flexural strength of fiber reinforced silica fume expansive-cement
concrete. The author also concluded that the addition of PP fiber caused an adverse effect on
the chloride permeability of expansive-cement concrete.
Toutanji et al. (1998) investigated the chloride permeability and impact resistance of PP fiber
reinforced silica fume concrete. Different length and volume fraction of the fibrillated PP fiber
were added into separate mixtures containing different contents by weight of silica fume. They
concluded that the incorporation of PP fibers increased the permeability of concrete specimens
containing no silica fume. Reducing fiber length from 19 to 12.5 mm, with an equivalent
volume fraction, resulted in a decrease in the permeability of plain and silica fume concrete.
They also found that the addition of silica fume enhanced the impact resistance of PP fiber
concrete, but had no effect on the unreinforced concrete.
Mazaheripour et al. (2011) studied the effect of PP fiber on the properties of fresh and hardened
lightweight self-compacting concrete. Different volume percentages of 12 mm long PP fiber
were added into the mixture. The authors summarized that the presence of PP fibers greatly
decreases the slump flow. Increasing the volume percentage of PP fibers reduces the filling
22
height in U-box test. They also stated that PP fibers did not have an impact on the compressive
strength and elastic module of the composites. Moreover, splitting tensile strength, and flexural
strength were increased as volume percentage of PP fibers increased.
Zeilml et al. (2006) added 1.5 kg/m3 PP fiber, 18 µm diameter and 6 mm length, to the concrete
mixture. They performed permeability test on specimens with no heat treatment and after pre-
heating to the temperature ranging from 80 to 600 ºC to evaluate the influence of PP fiber in
spalling behaviour of in-situ concrete. They concluded that at pre-heating temperatures lower
than 140 ºC, the permeability of concrete was three to four times larger than that of plain
concrete with decreasing differences for increasing temperatures. For temperature between 140
and 200 ºC, the difference between the permeability of concrete with and without PP fibers
increased. Hence, in the case of the tested in-situ concrete, the effect of melting of PP fiber had
equal impact as the difference in the low temperature permeability.
Kawashima et al. (2011) studied the effect of PP FRC composite and steel FRC for enhancing
the seismic performance of bridge column. Monofilament PP fibers with a diameter of 42.6
µm and length of 12 mm were added to the mixture in 3 % volume fraction (PFRC). Steel fiber
with hooked end, 0.55 mm diameter and 30 mm length, were added in separate mix (SFRC).
The column had a 400 mm by 400 mm square cross section with rounded corner and an
effective height of 1680 mm. The quasi static loading experiment was conducted under
displacement control. The authors concluded that PFRC showed superior damage mitigation
performance than SFRC because the higher deformation capacity of polypropylene fiber than
steel fiber that mitigated rupture of fibers under repeated large crack opening and closing of
concrete. They also found that the PFRC and SFRC column exhibited similar flexural strength
and ductility capacity with RC column while experiencing less damage on the cover and core
concrete. The advantage of using PFRC is in the reduced damage which could allow the
structure to remain serviceable even after a strong earthquake.
The effect of polypropylene fiber on durability of concrete composite containing fly ash and
silica fume were studied by Zhang and Li (2003). They mixed single short PP fibers in different
length (10-15 mm and 15-20 mm) with volume fraction ranging from 0.06 % to 0.12 % to the
23
mix containing a constant volume of fly ash and silica fume content. They found that addition
of PP fiber decreased the workability of the matrices. With the increase of fiber volume
fraction, both the slump and slump flow were decreasing gradually. PP fiber had a great
restricted the dry shrinkage of concrete containing fly ash and silica fume. They also found
that the presence of PP fiber in the composite reduced the carbonation depth considerably.
Carbonation depth of the concrete decreased gradually with the increased of fiber volume
fraction. They finally concluded that freeze-thaw resistance of PP FRC containing FA and SF
was slightly increased compared to plain concrete.
The durability properties of polypropylene fiber reinforced fly ash concrete were also inspected
by Karahan and Atis (2011). The fibrillated PP fiber were added into the concrete mix in
different volume fractions. The mix containing 15 % and 30 % fly ash as cement replacement
in mass basis were prepared by modifying the control Portland cement concrete. They
concluded that influence of PP fiber on compressive strength and elastic modulus was
insignificant. Porosity, water absorption and sorptivity coefficient values increased with the
increase of fly ash and fiber contents for all concrete mixtures. They also concluded that the
presence of PP fiber and fly ash in concrete, regardless separately or together, reduced drying
shrinkage. Freeze-thaw resistance of PP fiber concrete was found to slightly increase when
compared to concrete without fibers.
Manolis et al. (1997) examined the dynamic properties of polypropylene fiber reinforced
concrete slabs. Three different volume fractions (0 %, 0.1 % and 0.5 %) of 19 mm fibrillated
polypropylene fiber were added into the mixtures. They concluded that the inclusion of PP
fibers significantly improved the impact resistance of concrete slabs without affecting the
natural frequency. They also found that the static compression and flexural strength decreased
with increasing fiber content.
Fracture behaviour of polypropylene fiber reinforced concrete under biaxial loading was
investigated by Elser et al. (1996). Concrete mixes consisted of two different length (10 and
20 mm) and two different volume fraction (0.1 % and 0.5 %) of fibrillated polypropylene fiber.
They concluded that the peak shape and peak height of the load/displacement curves of FRC
24
with uniaxial and biaxial loading did not vary much compared to plain concrete. The fiber
reinforcement only becomes effective during strain softening.
Kodur et al. (2003) examined the effect of strength and fiber reinforcement on fire resistance
of high-strength concrete (HSC) column. Steel and polypropylene fibers were added into
separate mixtures. They concluded that the addition of steel and polypropylene fiber in HSC
column could improve the ductility of HSC column and increased their fire endurance. They
also concluded that the presence of polypropylene fibers in HSC columns could reduce spalling
and enhance their fire resistance.
2.6 Review on Surface Modification of Polypropylene Fiber
The attempt to modify the surface properties of polymer to make it hydrophilic has been
studied by many researchers. Some treatments previously reported in literature for fibers in
concrete application include coating, plasma based treatment, acid and other chemical
treatments. These surface treatments may cause an interfacial reaction between the fiber and
cementitious matrix.
Titanium dioxide (TiO2), Polyvinyl alcohol (PVA) and Aluminum oxide (Al2O3) were
among the coating agents that had been employed in order to modify the wetting characteristics
of polypropylene fabrics or fibers. Increasing the hydrophilic rate on the fiber surface could
possibly improve the bond performance between fiber and cement matrix interface.
Coating of TiO2 nanoparticles on polypropylene fiber was studied by Szabová et al.
(2009). The polypropylene nonwoven (PPNW) surface were first activated by a pressure
plasma treatment and then coated using nanoscaled TiO2 in water dispersion and chitosan
dispersion. SEM was used to compare the surface of the specimen. They found that the surface
of polypropylene fiber was rougher using water dispersion technique.
Natarajan and Moses (2012) studied the surface modification of polyester fabric using
PVA in alkaline medium. They concluded that the wetting behavior of PVA treated fabric
25
increases considerably due to the good linkage between PET and PVA. The water contact angle
of PVA treated PET fabric is found to be much less that the control PET fabric itself.
Xiao et al. (2009) reported that Al2O3 sol gel coating improved the wetting properties
of polyester fabrics. They concluded that Al2O3 particles deposited on the surface of the fabrics
might form hydrogen bonds with water when they contact water. Also, the porous network of
the Al2O3 particles may facilitate the adsorption of water, thus the water adsorption of PET
fabric was significantly improved.
Liu et al. (2012) studied the application of TiO2/PVA coating on low cost polyester
filter cloth. They reported that the nano TiO2 enhances the interaction between PVA and
polyester membrane, forms a more hydrophilic surface, and drastically reduces the contact
angel with water. The authors concluded that the PVA/TiO2 modification drastically reduced
the contact angle of the filter membrane because of the rich –OH groups from PVA and TiO2.
Xu et al. (2012) also reported that the application of 60 nm nano TiO2 layers on the
surface of PET, PP and viscose fibers significantly affect the surface properties of the fiber.
The authors concluded that the increases of surface energy was more prominent in viscose
fiber due to the presence of more hydroxyl group. Relatively smoother and higher surface
energy properties of PET over PP lead to more uniform film and better adsorption to TiO2
clusters.
Surface characterization of plasma-treated polypropylene fibers was also studied by Wei
(2004). The author treated the laboratory produced PP fibers with cold gas plasma and the
treated fibers were characterized using an X-ray photoelectron spectrometer (XPS), atomic
force microscope (AFM) and an environmental scanning electron microscope (ESEM). The
author concluded that the surface properties of polymer fibers can significantly alter the surface
properties of polymer fibers by changing their surface physical and chemical features.
Fialova et al. (2012) investigated the influence of atmospheric plasma treatment on
wetting properties of polypropylene and the cohesion of PP fibers to cement matrix. They
found that plasma treatment enhances the wettability of polypropylene fibers. In their test, the
results of restrain shrinkage test indicate that cohesion of plasma treated PP fibers has been
significant improved.
26
Huang et al. (2006) studied the morphology and dynamic contact angels of PP fibers
treated with plasma. The treatment was performed with oxygen and argon at a pressure of 15
Pa. The authors concluded that the plasma treatment can considerably reduce the contact angle
and significantly improve the wettability of PP fibers. They also found that the surface
roughness is the main reason for reducing the receding contact angle, while the advancing
contact angle is more related to the surface properties of the fibers.
Surface modification of PP fiber for hydrophilicity enhancement was also investigated
by Mercado-Cabrera et al. (2013). They treated the PP fiber using a non-thermal dielectric
barrier discharge plasma and the fiber characterization was evaluate using SEM, AFM and A-
ray diffraction. The authors found that the hydrophilicity capacity of the fiber increased after
the treatment with non-thermal plasma. The AFM and SEM revealed morphological changes
on the PP surface.
Pei et al. (2003) performed surface treatments of PP fibers to optimize their reinforcing
efficiency in cement composites. Gamma rays from a 60Co source preirradiation-induced graft
copylymerization of acrylic acid onto subdenier monofilament PP fiber. To maintain the
original mechanical properties of PP fiber, the grafting yield was controlled in the range of 3.5-
5.0%. The treated fiber or untreated fiber were mixed in a similar mortar composition. The
authors summarized that using SEM the interfacial bonding between the treated suddenier
monofilament PP fibers and the cementitious matrix can be improved. They also found that
the PP fibers grafted with acrylic acid enhanced both the compression strength and flexural
strength.
Ning et al. (2010) studied the modification of PP fibers by acrylic acid and its influence
on the mechanical property of cement mortar. The authors concluded that modified PP fibers
can improve their enhancement effect on cement mortar. The surface of modified fibers grafted
acrylic acid and the hydrophilic performance of fiber were improved. They also summarized
that the interface conjugation between fibers and basal body were increased from macroscopic
and the mechanical properties of specimens improved.
López-Buendía et al. (2013) modified the surface of PP fiber by treating the fiber with
an alkaline surface treatment and alkaline precursors in order to increase the adhesion of PP
fibers to concrete. They used PP fiber with a diameter of 0.74 mm and 40 mm long. Chemical
27
modification of the surface treated PP fiber was characterized by IR spectroscopy and surface
analysis of the fiber was performed by X-ray photoelectron spectroscopy. The authors
concluded that treated fibers exhibit higher performance compared to the standard concrete or
when untreated PP fibers are added to concrete.
2.7 Summary
The most common reason for cracking in concrete is its low tensile strength. The
reinforcement of concrete using fibers has proved to be an efficient and economical way to
mitigate this problem. Different fiber types such as steel, glass and synthetic have been used
as reinforcement for concrete. Incorporating synthetic fibers helps to reduce thermal and
shrinkage cracks. Addition of steel fibers enhances the ductility performance, post-crack
tensile strength, fatigue strength and impact strength of concrete structures. Many attempts had
been made to provide the advantages and benefits of using fiber reinforced concrete for a
variety of applications.
The reason for the popularity of the polypropylene fibers is because of the versatility of
the material. It has a good combination of properties, cheaper than many other materials and it
can be manufactured using various techniques. These benefits are derived from the very nature
and the structure of polypropylene. Polypropylene fibers are produced from homopolymer
polypropylene resin in a variety of shapes and sizes, and with different properties. The main
advantages of polypropylene fibers are their alkali resistance, relatively high melting point of
165 ˚C and the low cost of the material.
Based on the reviewed literature in this chapter, the effect of polypropylene fibers on the
properties of hardened concrete varies depending on the type, length and volume fraction of
fiber. It is also depends on the mixture design and the nature of concrete materials used. The
effect of PP fiber on compressive, flexural and tensile strength as well as on toughness and
elastic modulus is not quite clear. In some cases the addition of PP fiber has been reported to
decrease the ultimate strength of hardened concrete. However, the general results are that
permeability, abrasion, impact resistance and are all significantly improved by the addition of
28
polypropylene fibers. PP fiber is also found to be highly effective in controlling plastic
shrinkage cracking of concrete.
The other factors that control the performance of the composite material are physical
properties of the reinforced concrete and matrix including the strength of the bond between
fibers and matrix. The chemical properties of the fiber, in terms of their inertness or reactivity
with the surrounding environment and the mechanical properties play an important role in
determining the bonding characteristics of the fiber and the composite as they may or may not
from a chemical or mechanical bond between the fiber and matrix. This is important because
some fibers pull out easier than others when used as reinforcing elements and will affect the
toughness of the concrete structures.
It is generally accepted that there is no physiochemical adhesion between PP fibers and
cement, given that PP fiber have a hydrophobic surface and a lower modulus of elasticity than
that of the matrix. During recent years, numerous surface modification techniques toward
improving interfacial strength between PP fibers and cement matrices have been developed.
The attempt to enhance the hydrophilicity of PP fiber in order to improve the bonding with
concrete matrix is necessary. Therefore this thesis focuses on the development of an improved
polypropylene fiber in order to enhance its performance in the composite.
29
3. Chapter Three
DEVELOPMENT OF POLYPROPYLENE FIBER
3.1 Introduction
Polypropylene (PP) fibers are increasingly being used to enhance the toughness, energy
absorption capacity and to reduce the cracking sensitivity of cement composites. However, one
of the disadvantages of PP fiber application in FRC is its low interfacial bonding with
cementitious matrix compared to other types of synthetic fibers. The poor bonding of PP fiber
is due to its low surface energy traceable to its hydrophobic behavior and smoothness and its
low roughness.
Therefore improvement in surface roughness of PP fiber could alter the interfacial
bonding between fiber and matrix hence enhance the benefit of using this material in concrete.
The improvement in surface roughness of the fiber could possibly be done by surface
modification using a filler, coating or related techniques.
In this chapter, the development of a technique of producing polypropylene fiber is
described. The devices used for producing the fiber including extrusion machine, drawing unit,
along with ancillary equipment are also described.
This chapter describes the attempt to improve PP fiber performance in concrete matrix
by modification of the fiber surfaces. It is well known that PP fiber has a poor bonding
performance compared to other fibers such as steel fiber or glass fiber and therefore the
modified surface properties of PP fiber which would ultimately improve fiber-matrix interface
in concrete applications. This investigation includes the application of aluminum oxide coating
on PP fiber and also silica fume inclusion in the extrusion process of fiber production.
The extruded fibers properties were also examined. This includes tensile strength, and
pull-out performance using a single fiber pull-out testing method.
30
3.2 Fiber Extrusion System
This section describes the equipment used to produce the fiber. The final product of the
fiber was obtained in two-stage process. The first stage is producing the fiber in amorphous
state using extruder device. The function of the extruder is to heat the polymer material to a
homogenous melt and to pump it though the die at a constant rate. Because polymer extrusion
is a continuous process, the melt preparation device must be capable of a constant output.
The second stage is to obtain the final product by drawing the amorphous state fiber
through the heating system to produce a crystalline state fiber. This stage involved using a set
of drawing equipment capable of running at different speed settings. The process involves
stretching the fiber while it is still soft inside the oven chamber, in a controlled temperature,
and then collecting the fiber at the godet roll. More details of this fiber production are
described below.
3.2.1 Laboratory Mixing Extruder
The device used for extruding the fiber was the Dynisco Polymer Test LME model
LME-120 (Figure 3.1). This device has two temperature controllers, one each for the rotor and
the header and also contains a speed control for adjusting the rotor speed. The other component
was a dial gauge which indicates the clearance between the rotor’s surface and the header inner
side. The LME has a cooling system connecting the hopper to water supply and drain. This
arrangement helps to prevent the polymer chips from melting inside the hopper. As mentioned
earlier, this device is used to obtain the amorphous fiber.
The procedure of extruding the fiber is explained as follow: the polypropylene chips
material is placed in a cooled hopper where it falls onto the hot surface of a cylindrical rotor.
As the rotor turns, the PP chips are dragged against the inclined surface of the stationary scroll
and move toward the outlet die. As the material collects in the radial gap, it is compressed by
the converging space between the scroll surface and the end of the header case.
The material is melted through heat conduction created by the mechanical work of the
turning rotor. When sufficiently melted, the specimen moves to the axial gap where it is
31
rotationally sheared between the end of the rotor and the inside case. This motion causes a
centripetal pumping effect, enabling the polymer to flow to the outlet die and exit though the
nozzle.
Figure 3.1: The layout of Laboratory Mixing Extruder (LME)
3.2.2 Randcastle Extrusion Lines
In order to obtain the desired final product, the amorphous PP fiber was further drawn
to obtain the crystalline PP fiber with the final size of 0.5 mm diameter. A pair of Randcastle
32
drawing stand model No. RCP-MSS Godet were used. The incorporated oven with precision
temperature controller for accurate drawing was set between the drawers (Figure 3.2).
The draw stands consist of low (10.2 Feet Per Minute, FPM) and high (42 FPM) speed
designs. Each godet has a 5.5 inch diameter, ambient, friction roll 3.055 inches wide, and is
chrome plated. A separator roll is mounted above the friction roll allowing for multiple wraps.
The separator roll is mounted on air bearings for minimal drag. Each control panel includes
the DC drive with tach feedback, 10 turn pot, start stop controls and a digital display showing
the actual speed in operation.
The oven consists of a 12 inch long heat chamber, insulated access door, mounting
plate with leveling pads, and a single zone digital temperature controller with a thermocouple.
Figure 3.2: Randcastle fiberlines drawers (Slow drawer, left; Fast drawer, right; and the
oven, middle)
In fiber drawing process, the amorphous state fibers were first re-arranged at the slow
speed godet and then the fiber end was pulled across the chamber and dragged through the
outtake opening and then finally secured and collected at the fast speed godet (Figure 3.3). The
main reason for re-arranging the amorphous fiber at the slow speed godet before further
33
drawing is to prevent the fiber being entangled during the drawing process. Furthermore, the
fiber arrangement should be done properly in order to maintain the consistency of the drawing
process when the fiber enter the oven. Poor arrangement of the fiber could result in delay in
production due to fiber fracture or inconsistent diameter of the final product.
The procedures of fiber drawing are summarized as follows:
Arrange the fiber at the slow speed godet
Set the oven temperature
Run the slow speed godet at minimum speed
Drag the fiber to the high speed godet through the oven chamber
Run the high speed godet
Slowly increase the speed of both godet accordingly to its setting speed in order to get
the target diameter of the fiber
Figure 3.3: Actual image of fiber drawing showing the setting of the devices used
34
3.3 Fiber Extrusion Processes
The function of the extruder is to heat the plastic material to a homogenous melt and to
pump it though the die at a constant rate. Because polymer extrusion is a continuous process,
the melt preparation device must be capable of a constant output. This section describes the
process of fiber production from material preparation to polypropylene fiber final product.
3.3.1 Material Preparation
In this project, raw polypropylene materials were supplied by Reliance Industries Ltd.
India. The commercial name of this material is Repol AS I 60N Homopolymer and has a round
shaped white color chips with a maximum of 4 mm diameter size (Figure 3.4).
Figure 3.4: Sample of polypropylene chips used in this experiment
This homopolymer polypropylene material was graded as a staple fiber and for
multifilament application. The material was stored in dry condition at temperature below 50
ºC and protected from direct sunlight. There is no additional action required in preparing this
35
material for extrusion process. Some typical characteristics of the material are given in Table
3.1 below.
Table 3.1: Material characteristics of polypropylene chips
Properties value
Melt flow rate 16 g/ 10 min
Tensile strength at yield 37 MPa
Elongation at yield 11 %
Flexural Modulus 1500 MPa
Notched Izod Impact Strength 25 J/m
Heat Deflection Temperature 105 ºC
3.3.2 LME Setup and Operation Parameter
The initial settings of the LME to produce polypropylene fiber are as follow:
- Rotor temperature : 175 ± 3 °C
- Header temperature : 175 ± 10 °C
- Motor speed : 30 RPM
- Nozzle hole size : 2.0 mm
During the extrusion process both rotor and header temperature were set at the upper
range of polypropylene melting point, which is 175 °C. Notice that the temperature fluctuates
in both settings within the range of ± 3 °C in the rotor and ± 10 °C in the header during
production. The initial setting of the motor speed was 30 RPM and the nozzle has 2.0 mm
diameter orifice. These settings were able to produce the fiber at a constant speed.
It should be noted that the feeding process of polypropylene chips into the hopper
should be maintained at persistent rate in order to not only continuously produce a consistent
amorphous fiber with constant diameter but also to keep the polypropylene polymer flowing
36
out from the nozzle without breaking. Figure 3.5 shows the fiber being extruded and collected
at the slow speed godet.
Figure 3.5: Extruded fiber was pulled to the godet roll
In order to match the speed of the flowing polymer out of the LME nozzle, the speed of
the godet roll at the lower speed drawer was set to 2.84 cm/sec. This extrusion process produce
an amorphous material which was pulled to the godet roll and hot drawn to a size of 1.4 ± 0.2
mm diameter. The extruded amorphous fiber has a smooth surface and a slightly off white color
(Figure 3.6).
37
Figure 3.6: Typical amorphous PP fiber produced using LME
The amorphous fiber was further drawn and collected at the high speed godet roll
through the oven to produce semi crystalline fiber with a final size of 0.5 ± 0.2 mm diameter
(Figure 3.7). The speed of the godet roll of the higher speed drawer was set to 15.0 FPM or 7.62
cm/sec. The temperature of the oven was set at 175 ± 20 °C. Table 3.2 shows the details of
setting parameters of the extrusion equipment.
Table 3.2: Extrusion parameter
Description Setting Output
LME motor 30 RPM
Low speed drawer 5.6 FPM (2.84 cm/sec) 1.4 ± 0.2 mm diameter
Amorphous PP fiber
Oven temperature 175 ± 20 °C
High speed drawer 15.0 FPM (7.62 cm/sec) 0.5 ± 0.2 mm diameter
Semi crystalline PP fiber
Drawing ratio 2.6 – 2.8
38
The final fiber product, with smooth surface and transparent color, were then manually
cut into desired lengths (Figure 3.7).
Figure 3.7: Polypropylene fiber with a final size of 0.5 mm diameter, 50 mm length
3.3.3 Limitation and Controls
The purpose of this research was to produce concrete reinforcing fiber from PP chips in
a controlled system from its initial stage to the final product. The result shows that the
Laboratory Mixing Extruder (LME) could be used for producing the fiber. However, the
production line was limited to the single fiber extrusion process.
Production was delayed as a result of these two major factors: slow process of drawing
and the need to produce fiber with uniform diameter and tensile strength. Those factors were
highly depends on several aspects including the speed settings of the motor, feeding process
onto hopper, temperatures fluctuation during extrusion in LME, speed setting of the godet at
slow speed draw stand, temperature fluctuation during drawing at the oven chamber, and speed
39
setting of the godet in high speed draw stand. The aforementioned factors should be kept in
constant synergy in order to obtain a consistent and uniform result.
The setting parameter of both the LME and drawers component could be optimized more
in order to obtain the desired properties (i.e diameter) of the fiber. Further extrusion process
was successfully implemented by doubling the speed setting parameter of both rotor and the
godet. First stage of the production, setting speed of the rotor was increased to 60 RPM and
accordingly the speed of the godet was adjusted to 5.6 cm/sec in order to accommodate the
flowing polymer out from the nozzle.
Figure 3.8: Layout of the extrusion system
Figure 3.8 shows the complete fiber extrusion system. The devices used to produce the
amorphous fiber are marked in dashed-line circle and the devices used to obtain the crystalline
fiber are marked in solid-line circle. This two-stage production was successful in producing the
fibers. The typical amorphous fiber obtained from the first stage and crystalline fiber obtained
from second stage of production are shown Figure 3.9.
40
Figure 3.9: Comparison of extruded PP fiber: Amorphous state (lower) and Semi Crystalline
(upper)
41
3.4 Surface Modification of the Fiber
In this section, the attempt to modify the surface of extruded fiber using Al2O3 sol gel
coating is described. This experiment includes materials preparation to produce sol gel and also
the coating process of the fiber. Furthermore, the inclusion of silica fume and PVA inclusion
on the extruding process are also discussed.
3.4.1 Aluminum Oxide Coatings
3.4.1.1 Sol gel preparation
The procedure of preparing Aluminum oxide sol gel was based on the method developed
by Yoldas (1975) and then modified by Xiao et al. (2009). The Al2O3 sol was prepared by
mixing the precursor aluminum ixopropoxide (Al(OCH(CH3)2)3) (Sigma-Aldrich, St.Louis,
MO, USA) with ethanol water suspension. Thereafter, Polyvinyl alcohol (PVA) (Figure 3.10,
right) was added to the suspension and the whole mixture was continuously and mechanically
stirred for 12 hours and the stable sol were then acquired after 12 h of mixing.
Figure 3.10: Aluminum isopropoxide powder (left) and PVA powder (right)
42
The procedures and steps involved in preparation of Al2O3 sol (Figure 3.11; right) are as
follows:
The precursor aluminum isopropoxide was mixed with the ethanol-water mixture at
80°C with high speed stirring
The ethanol and water were mixed in a volume ratio of 1:1
The water and aluminum isopropoxide were mixed in a molar ratio of 100:1
The reaction time was about 1h
Thereafter the temperature was increased to 90 °C
Adjust the pH value of the liquid to 2.5 using hydrochloride acid.
Add PVA
Aluminum isopropoxide and PVA weight ratio of 1:2
Mix (using refrigerated incubator shaker, Figure 3.11; left) at temperature 90 °C for
12 h
Figure 3.11: Refrigerated incubator shaker (left), and Aluminus oxide sol gel (right)
3.4.1.2 Coating of fiber
Once the sol gel is formed, 50 mm long polypropylene fiber were then prepared for coating.
43
The procedures and steps involved in the coating process are as follows:
Fiber was washed in ethanol then rinsed in distilled water
Fiber were then dried at 40 °C in oven
Immersed on fiber in prepared sol for 30 min
Drying of fiber in the oven at 90 °C for 40 min
Figure 3.12: Comparison of uncoated and coated PP fiber
44
Figure 3.12 shows the differences in uncoated and coated fiber. The coated fiber
become reddish and its surface became rougher. It can be seen that the sol gel layer was
successfully applied and fully covered the surface areas of the fiber.
Microscope image of the surface of both coated and uncoated fibers is shown in Figure
3.13. The surface characteristic of both fibers at 20 x magnification were slightly different. The
application of aluminium oxide coating on fiber created a slightly rough surface on the fiber
surface.
Figure 3.13: Surface image of uncoated (left) and coated (right) PP fiber at 20x magnification
3.4.2 Silica Fume Co-extruded PP Fiber
The main reason for incorporating silica fume particles in fiber extrusion process is to
change the surface characteristics of the fiber. It is assumed that if SF particles are introduced
to the heated surface of the fiber they will somehow stick and create a rougher fiber’s surface
and therefore improving the bonding performance of the fiber. Furthermore, the addition of
silica fume material in the fiber’s surfaces might increase the pozzolanic reaction when they
came into contact with cement matrix and accordingly increases the bonding characteristic of
the composite.
In this experiment, the three options evaluated were:
45
i. SF inclusion on crystalline state PP fiber
The first option is to apply silica fume (SF) particles on the fiber surface of the fiber
immediately after the final drawing process (semi-crystalline state). The application of this
method was done when the fiber is brought out from the oven in its crystalline state, prior to
fiber collecting at high speed godet. The surface of the extruded fiber was still hot when SF
particles were sprayed onto it. In this method, it is assumed that the particles will blend into
the surface of fiber through the spraying mechanism pressure. However, it was observed that
most of SF particles did not stick into the surface of the fiber after the spraying process and
therefore this method was not utilized in this study.
ii. SF inclusion on amorphous state PP fiber
The second option is to apply the SF powder on the fiber surface immediately after the
fiber exits from the nozzle of the extruder (amorphous state). Two different approaches were
performed:
The first method is to manually apply SF particles on the fiber surface. This was done
by applying a gentle pressure by hand in order to make the powder sticks to the fiber, however
it was observed that the particles could not entirely cover and uniformly stick on the surface of
the fibers. Moreover, huge quantities of SF materials were wasted during this trial.
In the second method, SF powder was placed in a small container and the extruded
fibers were passed through the container (Figure 3.14, left). The powder stuck to the surface
of the amorphous fiber and changed color to light grey (Figure 3.14, right). However, it was
noticed that some silica fume powder fell off and disappeared during the drawing process. The
final product of extruded fiber seemed to only have a little or even no sign of silica fume on its
surface. Due to the insignificant coating this method was also discarded.
46
Figure 3.14: Silica fume application on the surface of PP fiber
iii. SF inclusion during pre-extrusion process of PP fiber
The third option is to mix silica fume particles with PP chips prior to extrusion. It was
assumed that due to the higher melting point properties of silica fume particle compared to PP
polymer, the particle will embed in the melting polymer during the extrusion process and
thereby changing the characteristics of the extruded fiber.
The mixing ratio of SF particles and PP chips is 1:10 by weight (Figure 3.15, left). Both
materials were placed into a plastic container and gently mix until they were properly mixed.
Consequently, the white colour of the PP chips became greyish as the SF particles stuck to the
surface of PP chips (Figure 15, right). The blended materials were then sent to the LME for
extrusion.
Figure 3.15: Proportion of PP chips and silica fume powder prior mixing (left); Uncoated and
SF coated PP chips (right)
47
Microscope images were also obtained to evaluate the changes on the surface of the
chips. It can be seen that SF particles covered the surfaces of the chips and changed the color
from white to light grey. It was also observed that some particles were deposited into some
tiny holes that exist on the chip’s surface (Figure 3.16).
Figure 3.16: Surface of PP chips at 20x magnification: Uncoated (right) and SF coated (right)
3.4.2.1 Surface characteristics of extruded fiber
Amorphous fiber
There were significant differences in the appearance of the extruded fiber when it came
out from the nozzle. Figure 3.17 shows the amorphous PP fiber is being extruded from the
LME and collected at the slow speed godet.
48
Figure 3.17: Fiber extrusion process showing SF co-extruded PP fiber
The silica fume pre-coated PP chips produced a dark grey colour amorphous fiber
contrasting the semi-transparency of regular PP fiber (Figure 3.18, left). Moreover, the surface
of the extruded fiber was also rougher than regular PP fiber. Silica fume particles were well
distributed in the amorphous fiber during extrusion process and were evenly distributed on the
surface of the fiber, as can be seen from the consistency of color produced by extruded fiber.
Note that the uniformity of the fiber produced in this amorphous state plays a significant role
when the fiber further drawn to its crystalline state.
Furthermore, the inclusion of silica fume particles in the extrusion process also changed
the stiffness of the extruded fiber. This inclusion made the extruded fiber less flexible
compared to regular PP fiber.
49
Figure 3.18: Silica fume co-extruded PP fiber (Amorphous, left and semi-crystalline, right)
Semi-crystalline fiber
When the drawing process continued to the high speed godet to produce semi-
crystalline fiber, the result showed that the appearance of the final product of SF co-extruded
PP (SFPP) fiber changed color to light grey (Figure 3.18, right). Nevertheless, the surface
roughness was still higher than the regular fiber. Using an optical microscope, it was observed
that some SF particles were still stuck on the surface of the fiber and therefore contribute to
the roughness of the final fiber (Figure 3.19).
Figure 3.19: Microscope image of SF co-extruded PP fiber at 5x magnification
Silica fume
particle
50
More detailed observation on the surface characteristics of SF co-extruded PP fiber
were then performed using a confocal microscope (Figure 3.20). At 20 x magnification, it can
be seen that the silica fume particles were randomly distributed and filled some areas on the
surface of the fiber and therefore this distribution increased the surface roughness of the fiber.
Moreover, groups of particles or one big particle, that deposit inside the fiber, created an
uneven surface in some parts of the fiber, this also contributed to the increase of the surface
roughness of the extruded fiber.
Figure 3.20: Confocal microscope image of the surface SFPP fiber at 20x magnification
(Normal exposure, left and high contrast, right)
3.5 Bonding Performance and Tensile Testing
3.5.1 Bonding Performance of Extruded Fibers
Single fiber pull out tests were conducted to evaluate the pullout behaviour of the fiber
in mortar matrix. The testing procedure followed the method developed by Banthia (1990).
Small dogbone-shaped specimens measuring 25.4 x 78 mm with contour at both ends were
used in the test. The contoured shape was applied in order to prevent the bearing contact from
interfering with the fiber – matrix interface.
51
The specimens were prepared by positioning each fiber in dogbone-shaped mold
(Figure 3.21), 25 mm embedded length in each side separated by plastic separator in the
middle. The 50 mm fiber were placed horizontally at 0 degree orientation in the mold.
Figure 3.21: Dogbone-shaped molds for fiber pull out testing
In this series of tests a constant mixing ratio of Water : Cement : Sand (0.5 : 1 : 1.9)
was used. All ingredients were mixed together in a small Hobart mixer for around 15 minutes.
After mixing, the mortar was poured in prepared molds and lightly vibrated for about 30 s. For
each condition 10 replicates were cast. After casting the specimens were covered with plastic
sheets and then stored at room temperature at about 23 °C. They were demolded 24 h after
casting and then stored in the curing room until age of 14 days for further testing. Figure 3.22
shows the specimens prior to testing.
Figure 3.22: Dogbone-shaped specimens prior to testing
52
Testing apparatus
Figure 3.23 shows the lay out of the testing equipment. The dogbone-shaped grips were
installed to a horizontal shaft panel in which the load cell was fixed in one end and a motor on
the other. Also, a LVDT system was attached to the grip for measuring the displacement during
test. Moreover, a speed control panel was connected to an electric motor and linked to
computer system for controlling the device operation. All devices were connected to a data
acquisition panel and computer system to record all data during the pull out test.
Figure 3.23: The lay out of pull out testing apparatus
The dogbone-shaped specimen was placed in test fixture (Figure 3.24). This set up
allows holding the specimen in place without applying additional lateral pressure on the fiber.
After positioning and centering the specimen, it is tightly gripped within the fixture which is
attached to the load cell. The horizontal deformation of the grip system, was measured by an
attached LVDT. The response of the testing results were recorded using data acquisition
system linked to a computer during testing.
53
Figure 3.24: Images of pull out specimens placed in its grip prior (upper) and during (lower)
testing
Aluminum Oxide coated fiber
Figures 3.25 and 3.26 show the bonding performance of both uncoated and coated
fibers in a cement matrix, respectively. The pull out load and the end slip behaviour of uncoated
extruded fiber showed similar trend of a typical pull out resistance of commercially available
polypropylene fiber material as shown in Figure 3.25.
54
Figure 3.25: Pull out load - end slip relationship performance of uncoated fiber
In Figure 3.26, the pull out load and the end slip behaviour of Al203 coated fiber showed
insignificant results. The best result of coated fiber (specimen 4) performance were similar
with the average pull-out of uncoated one, however the rest of the coated specimens show
lower shear resistance than the uncoated one. The insignificant bonding performance of the
coated fiber in cement matrix is believed to be the factor of poor bonding between the coating
layer and the fiber.
0
5
10
15
20
25
30
0 1 2 3 4 5 6 7 8 9 10
Pu
ll o
ut
load
(N
)
End slip (mm)
Fiber 1
Fiber 3
Fiber 4
Fiber 5
Fiber 6
Fiber 7
Fiber 8
Fiber 9
Fiber 10
Average
55
Figure 3.26: Pull out load - end slip relationship performance of Al2O3 coated fiber
It is observed that during testing, the coating layers peeled off thereby reducing fiber
resistance to pull-out. Hence slip along the fiber occurred. Figure 3.27 shows the typical failure
of the coating during pull out test. Due to the poor bonding performance of this fiber, no further
actions was taken.
Figure 3.27: Typical failure pattern of coated fiber during pull out test
-
5,00
10,00
15,00
20,00
25,00
30,00
0 1 2 3 4 5 6 7 8
Pu
ll-o
ut
load
(N
)
End Slip (mm)
Coated_fiber2
Coated_fiber3
Coated_fiber4
Coated_fiber5
Coated_fiber6
Coated_fiber7
Coated_fiber8
Coated_fiber9
Coated_fiber10
Average
56
Silica fume co-extruded fiber
Figure 3.28 shows the pull out load and the end slip relationship of SF co-extruded PP
fiber. It can be seen that there is a significant improvement in bonding performance. It is likely
that the surface roughness of the fiber might have contributed to the improvement. Pozzolanic
reaction between SF and Ca(OH)2 in the cement matrix may also be a primary bonding
mechanism.
Figure 3.28: Pull out load - end slip relationship of SFPP fiber
3.5.2 Comparison of Extruded Fibers
In this section the characteristics and properties of all extruded fibers are compared and
discussed.
0
5
10
15
20
25
30
35
40
45
50
0 5 10 15
Pu
ll o
ut
load
(N
)
End Slip (mm)
SF101
SF102
SF103
SF104
SF105
SF106
SF107
Average
Control Regular PP
57
Surface characteristics
Figure 3.29 shows the two types of extruded fibers that were successfully produced
with LME and Randcastle fiber line equipment using the same production set up. The center
image presented both amorphous fibers side by side. The regular fiber was on the right and SF
co-extruded PP fiber was on the left.
Figure 3.29: Extruded amorphous PP fiber, 1.5 mm diameter (center); Final product (0.5 mm
diameter) semi-crystalline PP fiber: SFPP (bottom left) and RPP (top right),
It can be noticed that the colors and the surface roughness were quite different among
both fibers. As mentioned earlier, SF co-extruded PP fiber showed darker color different from
the regular extruded fiber. The dark color of silica fume particles that blended well with
polypropylene chips during the extrusion process is the main reason for the change of color.
It should be noted that the surface roughness of SF co-extruded PP fiber was also higher
than the regular one. The silica fume particles that embedded inside the polymer or underneath
58
the fiber outer surface layer caused an increases in surface roughness of this extruded fiber.
The detail of this observation could be seen in Figure 3.30.
Figure 3.30: Microscope image of extruded amorphous PP fibers
The images in bottom left and top right (Figure 3.29) show the crystalline state of SF
co-extruded PP fiber and regular PP fiber, respectively. The difference in term of color and
surface appearance among those fiber were more explicit. The SF co-extruded PP fiber retain
the darker color (greyish), the regular PP fiber now become transparent.
Furthermore, the surface roughness of all fibers also retained their amorphous
characteristics. Again, the surface roughness of SF co-extruded PP fiber was more dominant
compared to regular PP fiber. Figure 3.31 shows the comparison of surface characteristics of
crystalline state extruded fibers at 5x magnification. The presence of silica fume particles that
embedded inside the fiber caused the solid greyish color (right) different from the transparent
(left) in appearance of the fibers.
59
Figure 3.31: Microscope image of semi-crystalline extruded PP fibers
SEM analysis
In order to further examine the surface characteristics of the extruded fibers, SEM
analysis were performed. Figure 3.32 reveals the composition of minerals that exist on the
surface of each type of extruded fiber. The distribution and intensity of the defined elements
over the scanned area were also attached in each corresponding image.
For example, carbon (C) and aluminum (Al) were occupied 95.7 % and 4.3 %,
respectively, surface of the regular PP fiber as shown on the upper image. Moreover, the
presence of silica constituent (Si) and Manganese (Mg) were confirmed and detected at 1.1 %
and 0.2 %, respectively, alongside with C (96.3 %) and Al (2.3 %) on the surface of SF co-
extruded PP fiber as shown in the lower image.
60
Figure 3.32: Images of EDS spectrum of minerals on the surface of extruded PP fibers: RPP
(top) and SFPP (middle)
Tensile strength
The tensile strength test results of the both types of extruded fibers is presented in
Figure 3.33. There is no significance difference in tensile strength among those fibers in both
amorphous and crystalline states. However, it can be seen that at the amorphous state, the
tensile strength of regular fiber was slightly higher than that of SF co-extruded fiber. It might
be related to the fact that some SF particles were trapped inside the polymer during the
extrusion process and therefore affected their tensile strength.
61
Figure 3.33: Tensile strength of extruded PP fiber
On the other hand, in the crystalline state, the tensile strength of SF co-extruded PP
fiber was slightly higher than that of regular PP fibers. It is assumed that the presence of silica
fume particles might affect the degree of crystallinity achieved in the drawn fiber. However,
the degree of crystallinity of this type of fiber did not significantly increase. Figure 3.34 shows
the typical breaking pattern of crystalline state fiber.
Figure 3.34: Lay out setting of strength evaluation of the fiber
41,85
327,79
38,22
332,94
AMORPHOUS SEMI CRYSTALLINE
Regular SF co-extruded
62
Bonding performance
The average pullout load-slip profiles for the tested series of each type of fiber are
shown in Figure 3.35. As stated in previous section, SF co-extruded PP fibers showed
significant increase in pull out resistance compared to regular one. This figure reveals that the
pull-out performance of SF co-extruded PP fiber was almost double than that of the regular PP
fiber. The detail of pull-out performance of both fibers was presented in Table 3.3.
Figure 3.35: Pull out load - end slip relationship of extruded PP fibers
Table 3.3: Bonding performance of extruded PP fibers
Regular PP SF co-extr. PP
Average Energy (N-mm) 82.14 154.5
Average Peak strength (MPa) 0.45 0.65
Average Equivalent strength (MPa) 0.17 0.31
0
0,2
0,4
0,6
0,8
1
0 2 4 6 8 10 12
Bo
nd
str
ess
(MP
a)
End slip (mm)
Regular PP
SF co-extruded
63
3.6 Summary
This Chapter presented a discussion on the development of a technique of producing
polypropylene fiber with modified surface properties which would ultimately improve fiber-
matrix interface in concrete applications.
To produce the fiber in a continuous manner production, a Laboratory Mixing Extruder
(LME) paired with a set of Randcastle fiber extrusion line was used. The slow fiber extrusion
process is the only drawback in this production technique. However further studies should be
taken to address this issue.
The characteristics of the two types of extruded fibers varied in terms of appearance,
color and flexibility. While the regular PP fiber (RPP) has a smooth transparent surface and is
flexible, the SF co-extruded PP (SFPP) fiber is greyish and less flexible.
The tensile strength of extruded PP fibers was approximately 330 MPa. The inclusion
of silica fume in the extrusion process might affect the strength of the fiber. However the
difference was insignificant both in amorphous and crystalline state of the fiber.
In this study, the coating of Al2O3 sol on polypropylene fiber was also examined. The
coating layer was successfully applied. However, the result shows that the performance of the
Al2O3 coated fiber in single fiber pull out testing was unsatisfactory. This might be due to poor
adhesion of coating material to the fiber which could have caused the coating to peel off from
fiber during testing.
The inclusion of silica fume particles in the extrusion process of PP fibers significantly
improved the surface characteristics of the extruded fiber. This improvement is assumed as a
main contributor in improving the bonding performance of this type fibers in a cementitious
matrix. Additional tests are required to obtain more comprehensive information on the
performance of silica fume co-extruded PP fiber in a concrete matrix.
64
4 Chapter Four
COMPARATIVE FLEXURAL STRENGTH OF MIXTURES
CONTAINING EXTRUDED PP FIBERS
4.1 Introduction
In this chapter, the flexural performance of Fiber Reinforced Concrete (FRC)
containing extruded and co-extruded polypropylene fibers is examined and discussed. Two
types of extruded fibers, as described in the previous chapter, were added into the concrete
mixture. Two separate FRC mixes were prepared and the flexural performance was evaluated
according to ASTM C1609 - Standard Test Method for Flexural Performance of Fiber-
Reinforced Concrete (Using Beam with Third-Point Loading).
This test method evaluates the flexural performance of FRC based on parameters
derived from the load-deflection curve obtained by testing a simply supported beam under
third-point loading. The first-peak strength, peak load, and residual strength determined by this
test method reflect the behavior of FRC under quasi static flexural loading. The area under the
load-deflection curve, which is an indicator of the energy absorption capacity of specimens
was also used in comparing the performance of various FRC mixtures.
4.2 Experimental Design
This section includes the mix design, preparation of test specimens and testing procedure
for the beam.
4.2.1 Materials and Mixtures Proportion
Portland cement Type GU was used in all concrete mixes. Fine aggregates of local
natural river sand and gravel coarse aggregates with a maximum size of 19 mm were used. The
65
water used for making concrete mix was tap water from the University of British Columbia.
The mix proportions of the concrete are given in Table 4.1. With a water to cement ratio of
0.5, the compressive strength of this mix was expected to reach 40 MPa at 28 days.
Table 4.1: Mixture Proportion
Materials kg/m3
Cement (kg) 370
Water (w/c=0.5) 185
Coarse Aggregate 940
Fine Aggregate 820
Figure 4.1 shows the two types of extruded polypropylene fibers used: regular PP
(RPP) and silica fume co-extruded PP (SFPP). The dimensions of the fiber were 0.5 mm in
diameter and 50 mm long. They were added into the concrete mixtures at a volume fraction of
1 %. A sample of calculation of the ingredients for FRC mix design with SFPP fiber is shown
in Figure 4.2.
Figure 4.1: Extruded PP fibers 0.5 mm diameter, 50 mm length. RPP (left), SFPP (right)
66
Figure 4.2: Sample calculation of mixture ingredients of FRC
4.2.2 Preparation of Test Specimens
Two separate batches of concrete using the same mix design for each batch were
prepared. The two types of extruded polypropylene fiber, at 1 % by volume of concrete, were
added separately in each mix. For each of the two batches, five 100 mm x 100 mm x 350 mm
beams for performing flexural tests were cast along with eight 100 mm x 200 mm cylinders
for compressive strength determination according to ASTM C192.
A counter-current motion type pan mixer was used for mixing. Sand and coarse
aggregates were added to the mixer and mixed suitably to provide a well-mixed mass. The
Portland cement was then added to the batch and mixed for around three more minutes.
FRC
Fresh concrete weight 2315 kg/m3 S.G % vol
kg/m3 g kg pbw Air 0.03
Cement 370.0 10,122.65 10.12 1 3.15 0.117
Water 185.0 5,061.32 5.06 0.50 1 0.185
Fine Agg 820.0 22,433.97 22.43 2.22 2.62 0.313
Coarse Agg 940.0 25,716.99 25.72 2.54 2.66 0.353
Fly ash (type F) - - - - 2.4 -
Silica Fume - - - - 2.26 -
Superplastizer 0 - - - 1.15 -
0.999
w/c 0.5000
Specimen
Diameter
or Width
(mm)
Height
(mm)
Length
(mm)number
total vol.
(liter)
B1 100 100 350 5 17.50
75*150 cylinder 75 150 8 5.30
100*200 cylinder 100 200 0 -
Slump test 150 300 0 -
Air content 200 200 0 -
20% 27.36
Fiber TypeCast Vol.
(m3)
SG
(kg/m3)Fiber (g)
0.0273585 920 251.70
Casting date:
Total (plus
March 6, 2015 SF-PP
SF-PP fiber
extra)
Single Vol.
(liter)
6.2800
5.2988
1.5700
0.6623
3.5000
Volume calculation
0.000273585
Fiber Vol.
(m3)Volume Fractions
1.00%
67
The required amount of polypropylene fiber was then added in the mixer and mixed for
an additional two minutes. Figure 4.3 (left) shows the composition of the mixture after the
fibers were added in the mixer. In order to ensure that the fibers are well distributed in the mix,
fibers were added gradually.
Approximately two thirds of the water was added and mixed for two minutes to obtain
an even distribution. The remaining water was then added to the batch and mixed for three
minutes by the end of which the concrete was ready to pour.
Figure 4.3: Pan mixer used (left) and cast specimens (right)
The prepared molds were placed on top of a vibrating table and concrete was poured to
fill half of the volume of each mould and then vibrated to consolidate. The remaining concrete
mix was then added to sufficiently fill the mold and vibration continued to cause the concrete
to consolidate in the mold.
After casting, all test specimens were finished with a steel trowel as shown in Figure
4.3 (right). Immediately after finishing, the specimens were covered with a plastic sheet to
minimize the moisture loss, and allowed to cure for 24 hours. All test specimens were stored
at room temperatures at about 23 ± 2 °C.
They were demolded 24 hours after casting and then stored in the curing room for 28
days (Figure 4.4). Three of the compressive strength test cylinders from each mix were taken
out after seven days to assess their compressive strength.
68
Figure 4.4: Image of beam and cylinder specimens in curing rack
4.2.3 Experimental Setup for Flexural Toughness and Testing Procedure
A closed-loop, fatigue-rated Instron 8802 testing machine was used. The ASTM C1609
test setup is shown in Figure 4.5. A “third-point loading” fixture is used with two support
points below the beam specimen and two loading noses on the top of the beam specimen.
Figure 4.5: Testing set up showing Instron machine, data acquisition panel and computer
In order to eliminate the spurious deformation arising from crushing or support
settlement, a deflection fixture (yoke) was installed around the specimen. This fixture is also
69
needed so that only the net deformation at the neutral axis of the specimen is meassured. In
addition, two Linear Variable Displacement Transducers (LVDTs) were mounted on each
sides of the specimen to measure net deflection.
Figure 4.6 shows the deflection fixture set up as well as the specimen support and
position of the loading points.
Figure 4.6: Beam specimens with deflection fixture (yoke)
The output of the transducers was averaged together to provide the net deflection
measurement. This configuration ensures accurate measurement of mid-span deflection and
minimizes errors due to concrete specimen twisting, seating or crushing. The LVDTs also
provided feedback to the servo-valve for closed-loop control during testing.
The test procedure was as follows:
Prepare the specimen carefully and record its dimensions prior to the test
Mount the deflection fixture frame on the sample. Set the LVDT on each side of the
specimen and ensure that there is enough travel for them to record the sample deflection
at the mid-point
Load the specimen on the third-point loading fixture
70
Run the test at the specified net deflection rates as measured at the mid-span point of
the beam
Record the test data
71
4.3 Experimental Results and Discussions
4.3.1 Compressive Strength
A Forney FX 600 series testing machine was used to measure the compressive strength
of concrete mixtures (Figure 4.7) as per ASTM C-39: Standard Test Method for Compressive
Strength of Cylindrical Concrete Specimens. The compressive strengths of all mixtures were
obtained at 7 and 28 days after casting. The loading rate for each test was maintained at 0.42
MPa/sec.
Figure 4.7: Test set up for determining cylinder compression strength
Table 4.2 presents the compressive strength of each mix at 7 and 28 days of testing. As
expected, compressive strength of the mix at 28-day was approximately 42 MPa. The standard
deviations were 0.65 MPa and 0.41 MPa for 7-day and 28-day test, respectively. It can be seen
that the addition of fibers into concrete mixtures has no significant effect on the compressive
strength.
72
Table 4.2: Compresive strength data
Mix
Compressive strength (MPa)
7-day 28-day
Regular PP fiber 31.42 42.16
SF co-extruded PP fiber 30.13 41.35
SD 0.65 0.41
4.3.2 Flexural Testing
4.3.2.1 Fracture mode
In general, the location of the fracture in all tested specimens occurred within the middle
third of the span of the beam as shown in Figure 4.8.
Figure 4.8: Images of specimens of each mix after testing. RPP (left) and SFPP (right)
73
Figure 4.9 shows the typical failure of specimen after testing and Figure 4.10 reveals
the exposed crack in specimen showing the fiber bridging.
Figure 4.9: Typical fracture mode in concrete beam
Figure 4.10: Images of fiber bridging at the exposed cracks
74
4.3.2.2 Flexural response
Figure 4.11 and 4.12 show the flexural response of both mixes contain RPP and SFPP
fibers, respectively. Typical flexural response of both FRC mixes clearly showed a two-peak
response. The first peak indicated the flexural strength of concrete, while the second peak
showed the ability of the fiber additive to sustain increases in load once the first cracks
occurred.
Figure 4.11: Load - Deflection curve Mix 1 with regular PP fiber
75
Figure 4.12: Load - Deflection curve Mix 2 with SF co-extruded PP fiber
For both fiber concrete mixes a noticeable drop in strength appeared right after the first
peak and crack. Once the concrete starts to crack, fibers require quite a large deformation to
occur before the fibers are stretched enough and begin to pick up the load. Generally, the post-
crack part of the load-deflection response was used for characterizing the fiber’s ability to
withstand load after cracking. In this experiment, the second peak load was found to increase
in both FRC mixes; however, FRC with SFPP fiber seemed to perform better than RPP FRCs
as seen by the higher post-peak load.
4.3.2.3 Flexural toughness
Flexural toughness is defined as the post-crack energy absorption ability of fiber
reinforced materials. It can be calculated using the area under the load-deflection curve up to
the specified deflection. In general, FRC produced from concrete with similar strength will
exhibit similar first crack toughness. This is because the first peak response depends entirely
on the concrete strength.
76
The comparison of averaged flexural toughness of all mixes is given in Figure 4.13.
The typical flexural response of plain concrete was also included. It can be seen that there are
improvements in flexural toughness when the fiber was introduced to the concrete mixture. It
also can be seen that the average flexural maximum load of the concrete in the three FRC mixes
is similar at approximately 16 kN.
Figure 4.13: Averaged flexural response of FRC containing extruded fibers
A detailed performance presented in Table 4.3 as per ASTM C-1609. Although, the
peak load (Pmax) of FRC containing RPP and SFPP were quite similar at 16.29 and 16.20 kN,
respectively, and thereby reflected to the beams peak strength (fmax) at 4.89 and 4.86 MPa for
RPP and SFPP, respectively. Nevertheless, it is revealed that FRC containing SFPP fibers
performed better than FRC containing RPP.
77
The load values corresponding to ½ mm of net deflection at mid span (P600) were 4.88
and 6.02 kN for FRC containing RPP and SFPP, respectively. This numbers indicates the
residual strength of the specimens in their corresponding stress (f600) of 1.47 and 1.81 MPa,
respectively.
Moreover, the residual strength of FRC specimens containing SFPP fiber obtained at 2
mm of net deflection at mid-span (f150) was 2.11 MPa. This value is 19.89 % higher compared
to that of FRC specimens containing RPP fiber (1.76 MPa). The corresponding value for
toughness parameter revealed a similar trend. The T150 value of FRC specimens containing
SFPP fiber was 13.7 Joules. This value is 18.1 % higher compared to that of FRC specimens
containing RPP fiber (11.6 Joules). Therefore it can be concluded that concrete containing
SFPP fiber performed better than that of concrete containing RPP fiber in flexural testing.
Table 4.3: Average flexural toughness parameter according to ASTM C1609
Parameter Regular PP FRC
SF co-extruder PP
FRC
Pmax (kN) 16.29 16.20
fmax (MPa) 4.89 4.86
P600 (kN) 4.88 6.02
f600 (MPa) 1.47 1.81
P150 (kN) 5.88 7.04
f150 (MPa) 1.76 2.11
T150 (J) 11.6 13.7
78
4.4 Summary
This chapter describes the performance of FRC containing two types of extruded
polypropylene fibers subjected to flexural loading. Two separate mixes containing 1% volume
fraction of each type of fiber were tested according to ASTM C1609 method.
From the results, it can be concluded that the addition of the extruded fibers improves
the ability of concrete to withstand flexural loading to greater crack openings. Both mixes
showed the improvement of the specimen in withstanding the load after the first cracks occurs.
The results indicate that the strength obtained at ½ mm of net deflection in FRC beam
containing SFPP was 23.13 % higher than FRC containing RPP. Similar trend also occurred at
2 mm of net deflection. The strength of FRC containing SFPP was 19.89 % higher than FRC
containing RPP. Moreover, the toughness parameter of FRC specimens containing SFPP fiber
was 18.1 % higher compared to that of FRC specimens containing RPP fiber.
Therefore it can be concluded that FRC containing SF co-extruded PP fiber performed
better than FRC mixtures containing regular PP fiber.
79
5 Chapter Five
PLASTIC SHRINKAGE PERFORMANCE OF EXTRUDED
FIBERS REINFORCED OVERLAY
5.1 Introduction
Plastic shrinkage occurs at an early age, before the concrete has hardened. This type of
shrinkage typically occurs because of poor curing conditions leading to the evaporation of
water and hence generation of high capillary stresses. Plastic shrinkage depends on two
primary factors: the rate at which surface water forms and its evaporation rate. When the
evaporation rate from top surface of the concrete exceeds the bleed rate at which water rises
from the concrete, the top surface dries out. The addition of fiber in concrete has been reported
to improve the performance in preventing plastic shrinkage cracking in concrete.
In this Chapter, the performance of extruded fiber addition in concrete mortar overlay
in preventing plastic shrinkage cracking is evaluated. The testing procedure was based on the
method and techniques developed by Banthia and Gupta (2007).
5.2 Experimental Design
This section includes the mix design, preparation of test specimens and testing
procedure. In this experiment, a layer of fresh concrete mortar is placed directly on a fully
hardened substrate. The dimensions of the substrate bases were 40 x 95 x 325 mm and were
made from high strength concrete. This concrete base has surface protrusions as shown in
Figure 5.1 below.
80
Figure 5.1: Dimension of substrate base (source: Gupta, Thesis 2008)
5.2.1 Materials, Mixtures Proportion and Casting
5.2.1.1 Substrate base
Portland cement Type GU was used in the concrete mixes. Fine aggregates of local
natural river sand with a fineness modulus of 2.65 and gravel coarse aggregates with a
maximum size of 12 mm were used. The water used for making the concrete mix was the tap
water from the University of British Columbia. The mix proportion of the base concrete was
identical with the experiment conducted by Banthia and Gupta (2009) and is given in Table
5.1. Using this mix design, the compressive strength of concrete was expected to reach 85 MPa
at 28 days.
81
Table 5.1: Mixture proportion
Materials kg/m3
Cement 535.5
Silica Fume 59.5
Water (w/c=0.317) 166.6
Coarse Aggregate 809.2
Fine Aggregate 809.2
Superplasticizer 1.61
Figure 5.2 shows the calculation of the mixtures proportion. It can be seen that the
water binder ratio is very low at 0.285 and therefore the mixture includes superplasticizer to
maintain good workability during mixing and concrete placement. In addition, to reduce the
chances of breakage during handling and to enhance the linear stiffness, 2 steel rebars with 10
mm diameter and 275 mm length were placed along the length of the substrate.
82
Figure 5.2: Sample calculation of mixture ingredients of concrete base for shrinkage tests
A counter current motion type pan mixer was used for mixing. Sand and coarse
aggregates were added to the mixer and mixed suitably to provide a well mixed mass. The
Portland cement was then added to the batch and mixed for around three more minutes.
Approximately two thirds of the water was added and mixed for two minutes to obtain
an even distribution. Superplasticiser and the remaining water were then added to the batch
and mixed for three minutes at the end of which the concrete was ready to pour. The dosage of
superplasticizer could be increased until the mix reach a good workability.
HSC
Fresh concrete weight 2383.22 kg/m3 S.G % vol
kg/m3 g kg pbw Air 0.02
Cement 535.5 13,195.79 13.20 1 3.15 0.170 16.72
Water 166.6 4,105.36 4.11 0.31 1 0.167 16.39
Steel Fibre 7.85 - 0.00
Silica fume 59.5 1,466.20 1.47 0.11 0.10 2.2 0.027 2.66
F. Agg 809.2 19,940.31 19.94 1.51 2.64 0.307 30.15
C.Agg 809.2 19,940.31 19.94 1.51 0.60 2.5 0.324 31.84
Silica Sand - - - - - 2.62 - 0.00
Recycled Glass - - - - -
2.48 - 0.00
Recycled Quartz - - - 2.48 - 0.00
Superplastizer3.22 79.35 0.08 0.01 1.15 0.003 0.28
1.017
w/c 0.3171 0.100
w/b 0.2854 Σ b 595.0 kg/m3 -
Specimen
Diameter
(Width)
(mm)
Height
(mm)
Length
(mm)
Single Vol.
(liter)number
total vol.
(liter)
Base 95 40 325 1.235 9 11.12
75*150 cylinder 75 150 0.66234375 0 -
100*200 cylinder 100 200 1.57 6 9.42
Slump test 150 300 5.29875 0 -
Air content 200 200 6.28 0 -
20% extra) 24.64 Total (plus
Casting date: June 8 2015 Base
sf content
fa content
Volume calculation
83
Figure 5.3: Molds for substrate base
The prepared molds (Figure 5.3) were placed on top of a vibrating table and the rebars
were placed accordingly. Concrete was poured to fill each mold and then vibrated to
consolidate. After casting, all test specimens were finished with a steel trowel. Immediately
after finishing, the specimens were covered with a plastic sheet to minimize the moisture loss,
and allowed to cure for 24 hours. All test specimens were stored at room temperatures at about
23 ± 2 °C and 50% RH. They were demolded 24 hours after casting and then stored in the
curing room for at least 60 days before being used in tests (Figure 5.4).
Figure 5.4: Image of base specimens in curing room
84
5.2.1.2 Overlay mortar
Portland cement Type GU and fine aggregates of local natural river sand were used.
The water used for making mortar overlay was the tap water from the University of British
Columbia. The mix proportions of the overlay mortar are given in Table 5.2.
Table 5.2: Mix proportion of overlay mortar
Materials kg/m3
Cement 800
Water 400
Fine Aggregate 400
Two types of extruded polypropylene fibers were added to the overlay mortar: regular
(RPP) and silica fume co-extruded (SFPP) fiber as shown in Figure 5.5. The dimensions of the
fiber were 0.5 mm in diameter and 50 mm long. They were added into the mortar mixtures at
a volume fraction of 0.2 %.
Figure 5.5: Extruded PP fibers, 0.5 mm diameter, 50 mm length. RPP (lef) and SFPP (right)
A sample calculation of the ingredients for the overlay mortar with SFPP fiber is shown
in Figure 5.6.
85
Figure 5.6: Sample calculation of overlay mortar
5.2.2 Preparation of Test Specimens
Three separate batches of mortar using the same mix design for each batch were
prepared. The two types of extruded polypropylene fibers at 0.2 % by volume of mortar were
added separately to each mix and one batch of the mixture was plain mortar as a control. A
regular pan mixer (30 L capacity) was used for mixing all ingredients. Sand and Portland
cement were added to the mixer and mixed for around three minutes. The water was then added
and mixed for an additional three minutes to obtain an even distribution. The required amount
of polypropylene fiber was then added into the mixer and mixed for an additional two minutes
at the end of which the overlay mortar was ready to be poured.
Overlay Mortar
Fresh mortar weight 1600 kg/m3 S.G % vol
kg/m3 g kg pbw Air 0.03
Cement 800.0 6,912.00 6.91 1 3.15 0.254
Water 400.0 3,456.00 3.46 0.50 1 0.400
Fine Agg 400.0 3,456.00 3.46 0.50 2.64 0.152
Coarse Agg - - - - 2.66 -
Fly ash (type F) - - - - 2.4 -
Silica Fume - - - - 2.26 -
Superplastizer 0 - - - 1.15 -
0.835
w/c 0.5000
Specimen
Diameter(
Width)
(mm)
Height
(mm)
Length
(mm)number
total vol.
(liter)
B1 100 60 400 3 7.20
75*150 cylinder 75 150 0 -
100*200 cylinder 100 200 0 -
Slump test 150 300 0 -
Air content 200 200 0 -
20% 8.64
Fiber TypeCast Vol.
(m3)
SG
(kg/m3)Fiber (g)
0.0086400 920 15.90
Total (plus extra)
Casting date: Aug 6th 2015 SF Co-Ext PP
Volume calculation
Single Vol.
(liter)
2.4000
0.6623
1.5700
5.2988
6.2800
Volume FractionsFiber Vol.
(m3)
SF-PP fiber0.20% 0.0000173
86
Figure 5.7: Molds for plastic shrinkage testing showing substrate base placement
For each batch, three identical specimens of the repair overlay were prepared using the
procedures developed by Banthia and Gupta (2009). A fully cured substrate base was first
placed in the mold measuring 100 x 100 x 375 mm (Figure 5.7). All three molds were
positioned inside the environmental chamber at the same distance from the heating source. A
60 mm deep repair overlay was then poured over the substrate base and finished with a trowel
(Figure 5.8).
87
Figure 5.8: Repair overlay specimens after finishing and before starting the test
5.2.3 Testing Procedure and Crack Assessment
As mentioned earlier, an enviromental chamber (Figure 5.9) developed at UBC Civil
Engineering materials lab was used to evaluate plastic shrinkage cracking on overlay mortar
(Banthia and Gupta, 2007). A chamber made of clear plastic, measuring 1390 x 1290 x 280
mm, could accommodate three specimens in parallel. Each specimen was placed 800 cm away
from the heating source. Heated air is allowed to escape the chamber through three 240 x 175
mm openings. The chamber contains three heating fans at one end capable of circulating air to
the other end at a rate of about 0.016 m3/s.
88
Figure 5.9: Environmental chamber showing the placement of specimens
The chamber is equipped with digitally adjustable humidity and temperature controllers
capable of recording and maintaining humidity to ± 1% and the temperature to ± 1 °C. These
controllers regulate the power supply to the heaters (with fans) as necessary to maintain a
constant temperature and humidity in the chamber. In this test, a temperature of 50 °C was
chosen which results in a relative humidity of 5 % and produces an evaporation rate of
approximately 1.0 kg/m2/h from the specimens’ surface (Gupta, 2008).
The chamber was prepared and then fans were started until it reached a steady state of
temperature and humidity at which the specimens were placed and positioned at their
designated spots inside the chamber. The placement of all specimens should be kept at the
same distance from the fan in order to maintain a uniform drying of their surfaces during
testing.
89
After 2 hours, the chamber was opened and the sides of the mold of each specimen
were removed to expose the specimens to a uniform state of drying (Figure 5.10). The chamber
was then closed and temperature was maintained and RH monitored for the next 22 hours. For
the first four hours after demolding, the surface of each specimen was photographed at one
hour intervals to record the cracks development during that period. A final image was also
taken at the end of the testing period.
Figure 5.10: Specimens after demolding
A magnifier with 23 mm diameter lens was used to characterize the cracks developed
in the overlay (Figure 5.11) at the end of testing period. Crack width was measured at 10
locations for every 100 mm of crack length of the surface of the specimen. The magnifier lens
used had an accuracy of 0.01 mm and is equipped with an LED light source to precisely
measure the crack on the surface of specimens.
90
Figure 5.11: Image of cracked specimens after testing and tools used for measuring the crack
After measuring and quantifying the cracks on each specimen, the total crack area,
average of crack width and crack control efficiency were determined using the same method
provided by Banthia and Gupta (2007) as follows:
Total crack area (Atotal) was obtained by summing over all cracks in a specimen
𝐴𝑡𝑜𝑡𝑎𝑙 = ∑ 𝑊𝑖 𝑙 𝑖
𝑖=𝑛
𝑖=1
where :
Wi = average crack width of the ith crack,
l i = length of the ith crack
n = number of cracks observed in a test.
91
For each specimen, the maximum crack width (Wmax) was also measured.
The crack width control efficiency (ηwidth) and the crack area control efficiency (ηarea)
were calculated using the following formulae:
𝜼𝒘𝒊𝒅𝒕𝒉 =(𝑊𝑚𝑎𝑥, 𝑝𝑙 − 𝑊𝑚𝑎𝑥, 𝑓𝑟)
𝑊𝑚𝑎𝑥, 𝑝𝑙 100
𝜼𝒂𝒓𝒆𝒂 =(𝐴𝑡𝑜𝑡𝑎𝑙, 𝑝𝑙 − 𝐴𝑡𝑜𝑡𝑎𝑙, 𝑓𝑟)
𝐴𝑡𝑜𝑡𝑎𝑙, 𝑝𝑙 100
where :
Wmax,pl = the average of maximum crack widths observed in the three plain overlay
mortar specimens
Wmax,fr = the average of maximum crack widths observed in the three fiber
reinforced overlay mortar specimens
Atotal,pl = the average of total crack areas observed in the three plain overlay mortar
specimens
Atotal,fr = the average of total crack areas observed in the three fiber reinforced
overlay mortar specimens
92
5.3 Experimental Results and Discussions
5.3.1 Crack Development
Figure 5.12 and Figure 5.13 show the typical crack development on the surface of the
specimen of plain and SFPP fibers reinforced overlay, respectively. It can be seen that cracks
started to appear 1 h after demolding and then significantly grow in the first 4 hours.
Figure 5.12: Crack progression on plain overlay specimen #1
93
Figure 5.13: Crack progression on SFPP fiber reinforced overlay specimen #1
5.3.2 Extruded Fibers Performance
As previously mentioned, 0.2 % volume fraction of extruded polypropylene fibers were
added into overlay mortar to evaluate their influence on plastic shrinkage cracking. Two
separate mixes containing each type of fiber, RPP and SFPP, and one plain mortar mix, were
evaluated.
Figure 5.14 shows the influence of fiber reinforcement on crack pattern on the overlay
specimens after the final hour of being exposed to drying in the environmental chamber. It can
be seen that the influence of fiber reinforcement was apparent. The crack area decreased when
94
fibers are introduced to the mix. Moreover, addition of fibers were also capable in controlling
the crack width development on the surface of overlay mortar.
Figure 5.14: Complete set of overlay specimens after testing
Figure 5.15 shows the map of crack distribution on the surface of all specimens based
on their length and width. In this image the measured crack length was represented by the
quantity of printed number in each measurement bar. For instance, if the bar has 3 printed
numbers then the length of the crack is 3 cm. Furthermore, the measured crack width was
95
represented by the color scale from white (smaller) to red (wider) as also indicated with the
numbered value (in mm) inset the bar.
Figure 5.15: Crack mapping
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Based on the color scheme shown in Figure 5.15, plain and RPP overlay mixes
developed bigger cracks than SFPP or PVAPP fiber reinforced overlay mixes. The total
number of visible cracks developed on the surface of the three overlay mortar specimens for
each mixes ranged from 11 to 15.
The crack measurement and analysis are summarized and presented in Table 5.3.
Table 5.3: Crack analysis
The mix with SFPP fibers performed better compared to RPP in preventing shrinkage
cracking. Although the average number of cracks on the specimens’ surface in all mixes were
relatively similar, the total crack areas of FRM with SFPP fiber were significantly decreased.
Furthermore, the average crack width on the surface of the overlay mortar decreased
when fibers were introduced into the mix. The maximum crack width in all three specimens
of each overlay mortar batch were 1.1, 0.9 and 0.6 mm for plain, RPP, and SFPP, respectively.
The average crack width for each corresponding mix were 0.4, 0.35 and 0.21 mm, respectively.
Figure 5.16 presents the efficiency of fiber addition in controlling the total crack area
and crack width of the overlay mortar. It can be seen that fiber addition was effective in
controlling cracks in all mixes. Results indicated that SFPP performed better than RPP in
controlling the total crack area and crack width. The efficiency of SFPP in preventing total
cracking area was 60.54 %. The comparable numbers for RPP were 19.25 %. Moreover, mix
with SFPP reduced the crack width by 45.45 % efficiency compared to RPP by 18.18 %.
Average Average Maximum Average Crack area Crack width
Fiber type Volume number crack area crack width crack width control control
fraction of cracks (mm2) (mm) (mm) efficiency efficiency
No fiber 0 4 129.22 1.10 0.40 - -
Reg PP 0.20% 3.7 104.35 0.90 0.35 19.25 18.18
SF PP 0.20% 5 51.00 0.60 0.21 60.54 45.45
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Figure 5.16: Crack control efficiency of RPP, SFPP and PVAPP
5.4 Summary
The purpose of the experiments performed here was to evaluate the performance of
overlay mortar containing two types of extruded polypropylene fibers on plastic shrinkage
cracking. Two separate mixes containing 0.2% volume fraction of each type of fiber were
evaluated alongside one mix of plain concrete as control.
From the results in this chapter it can be concluded that the addition of the extruded
polypropylene fibers was effective in controlling plastic shrinkage cracking in concrete. In
general, both types of extruded fibers reduce the maximum crack width and total crack area.
Mix containing SF co-extruded PP fiber showed significant capability in reducing total
crack area and crack width on the surface of overlay mortar specimens. For instance, the
introduction of SFPP fiber reduced the total crack area of the specimens approximately 61 %.
98
Meanwhile the introduction of RPP fiber reduced the total crack area of the specimens by 19
%. Moreover, SFPP performed better in reducing the average crack width by 46 % compared
to RPP by 18 %.
99
6 Chapter Six
GENERAL CONCLUSIONS AND SUGGESTION FOR
FURTHER RESEARCH
6.1 General Conclusions
The aim of the work presented in this thesis was to produce fiber materials that could be
used as a reinforcements in concrete applications. Polypropylene (PP) chips material graded
as fiber and staple application were used in this experiment. Even though polypropylene fibers
are among the most used fibers in concrete application, their performance has limitation in
terms of interface bonding with concrete matrix. This research explores methods to address
this limitation. The techniques and methods used in this research were based on the
understanding that improving the fiber performance should be attempted from the first step of
production until the desired final product is obtained.
The Laboratory Mixing Extruder (LME) paired with Randcastle fiberline (RFL) drawing
device, equipment used in this research, was able to produce polymer fiber from polypropylene
chips. PP chips raw material was supplied by Reliance Industries and was shipped directly
from their plant in India. A target diameter of 0.5 mm fiber was obtained from a 2-stage process
in the production line. An amorphous state fiber with a larger diameter was obtained from the
extrusion process using LME and then a crystalline state fiber was obtained from the drawing
process using RFL, as explained in Chapter 3. The settings of equipment were optimized to
maintain the consistency and uniformity of extruded fiber. However, it was observed that the
slow drawing process affected the fiber production rate.
An attempt to modify the surface of the fiber in order to improve its performance was
described in Chapter 3. Attempts to apply a thin layer of Aluminum Oxide sol gel on the
extruded fiber were successful; however, the pull-out performance of these coated fibers in
concrete matrices was unsatisfactory. This was a result of failure in adhesive bonding between
the coating layer and the fiber during testing.
100
Incorporating silica fume (SF) powder in the fiber extrusion process enhanced fiber
properties. The SF co-extruded fiber had a different appearance, flexibility and surface
roughness. It was also interesting to see the change in color of the extruded polymer. Initially
a dark grey amorphous fiber extruded from LME that changed to a light grey crystalline fiber
at the end of the production process.
However, the most important part was the improvement in surface characteristics of the
fiber. Silica fume particles were blended well with the polymer during the extrusion process
and formed a rough-surfaced amorphous fiber. This characteristic was still noticeable on the
surface of the crystalline fibers. Pull out testing revealed that compared to regular PP (RPP)
fiber, bonding performance of SF co-extruded PP (SFPP) fiber in concrete matrix was
significantly better. This improvement was attributed to the improved surface roughness of the
fiber and potential pozzolanic reaction. Using SEM, the presence of silica on the fiber’s surface
was confirmed.
Additional testing was performed (Chapter 4) to evaluate the performance of FRC
containing the extruded fibers. In this test, the flexural toughness response of three separate
FRC mixes containing different types of extruded fibers was evaluated according to ASTM
C1609. The results from this experiment show that fibers inclusion improved the flexural
response of all FRC specimens compared to plain concrete. Results also indicated that flexural
toughness of FRC contained SFPP fiber was increased by 19 % compared to those of FRC
containing RPP fibers.
To evaluate the extruded fibers performance further, plastic shrinkage testing as
described in Chapter 5 was performed. Two types of extruded fibers, with a volume ratio of
0.2 %, were added into two separate overlay mortar mixtures, and their performance in
preventing the cracks which occur during the early age of concrete was evaluated.
Comparing the cracks formed on the surface of fiber reinforced overlay mortar (FRM)
with plain overlay specimens revealed that there was a significant improvement in preventing
crack development in mixtures containing these fibers. Results showed that the total crack
areas on specimens containing RPP or SFPP fibers were smaller than those of plain overlay
mortar specimens.
101
In terms of crack area reduction efficiency, specimens containing SFPP fibers performed
better than that of specimens containing RPP fibers. FRM containing SFPP fiber marked 60.54
% efficiency compared to 19.25 % of FRM containing RPP fibers. Moreover, the mix with
SFPP reduced the crack width by 45.45 % efficiency compared to RPP by 18.18 %. Based on
the results of this experiment, it can be concluded that the addition of extruded fibers improved
the performance of FRM overlay mixture in preventing plastic shrinkage cracking.
This research successfully achieved its goal of developing a concrete reinforcing fiber
using laboratory equipment. The use of silica fume particles in the co-extrusion process
significantly changed the properties of the extruded fiber. The increased surface roughness and
pozzolanic reaction were believed to contribute to the increased interfacial bonding
performance in concrete composites. Additionally, based on the experiments described in
Chapter 4 and 5, the silica fume co-extruded fiber increased the flexural response of FRC and
also performed better than regular fiber in preventing plastic shrinkage cracking. Therefore, it
can be concluded that SFPP extruded fibers have a promising capability in enhancing the FRC
performance and therefore could be useful for concrete applications in the future.
6.2 Suggestion for Further Research
The extruded fibers produced using Laboratory Mixing Extruder and Randcastle
fiberline equipment were adequate as a starting point for exploring all possibilities in
improving fiber properties. For example, silica fume particles were chosen with a specific
reasons for modifying the surface of the fibers. The results showed improvement in the
extruded fibers. The improvement of the extruded fibers in bonding performance with concrete
matrix was correlated to its surface properties. However, the mechanisms underlying the
improvements of bonding are unclear. Further research is needed to determine the probability
of an extra pozzolanic reaction or other chemical interaction occurring in the fiber matrix
interface, if there are any, during the curing period of the composite.
102
Furthermore, if the improvement of interface bonding between fibers and concrete
matrix was solely affected by the roughness of the extruded fibers then perhaps other types of
particles might also be worth examining. Recycled glass, silica sands, fly ash or industrial
sands with a maximum size of 0.1 mm, for example, could be used as a filler material in the
extrusion process. Cost wise, those materials are less expensive than the previously examined
material.
Other properties of extruded fiber such as surface roughness or fiber flexibility were
visually noted to vary in both types of extruded fibers. Nonetheless, the consistency of the
roughness on the fiber’s surface in SFPP fiber was unknown. In this research, the effort to
measure the degree of roughness and the flexibility of each type of fiber were neglected due to
the time frame limitation of this project. Again, if those properties are the main contributor of
the improvement in bonding performance then further evaluation is recommended to obtain
more comprehensive results related to their properties.
The tensile strength of extruded fiber reached approximately 330 MPa during
production. One of the factors affecting the tensile strength of extruded fiber is the degree of
crystallinity achieved during the drawing process. For this particular material, the process
depends on factors such as temperature setting in the oven, the draw speed ratio, the diameter
of amorphous fiber and the final diameter of the extruded fiber. Based on the equipment
settings in the fiber production process reported in this thesis, there is still more room to
improve the properties of the fibers, including their tensile strength by optimizing the
aforementioned factors.
103
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APPENDIX
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