BEHAVIOR OF STEEL FIBER-REINFORCED GEOPOLYMER …
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United Arab Emirates University United Arab Emirates University
Scholarworks@UAEU Scholarworks@UAEU
Theses Electronic Theses and Dissertations
4-2021
BEHAVIOR OF STEEL FIBER-REINFORCED GEOPOLYMER BEHAVIOR OF STEEL FIBER-REINFORCED GEOPOLYMER
CONCRETE MADE WITH RECYCLED CONCRETE AGGREGATES CONCRETE MADE WITH RECYCLED CONCRETE AGGREGATES
Abdalla Ahmed Moallim Hussein
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Recommended Citation Recommended Citation Hussein, Abdalla Ahmed Moallim, "BEHAVIOR OF STEEL FIBER-REINFORCED GEOPOLYMER CONCRETE MADE WITH RECYCLED CONCRETE AGGREGATES" (2021). Theses. 811. https://scholarworks.uaeu.ac.ae/all_theses/811
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ii
Declaration of Original Work
I, Abdalla Ahmed Moallim Hussein, the undersigned, a graduate student at the United
Arab Emirates University (UAEU), and the author of this thesis entitled “Behavior Of
Steel Fiber-Reinforced Geopolymer Concrete Made With Recycled Concrete
Aggregates”, hereby, solemnly declare that this thesis is my own original research
work that has been done and prepared by me under the supervision of Dr. Hilal El-
Hassan, in the College of Engineering at UAEU. This work has not previously been
presented or published, or formed the basis for the award of any academic degree,
diploma or a similar title at this or any other university. Any materials borrowed from
other sources (whether published or unpublished) and relied upon or included in my
thesis have been properly cited and acknowledged in accordance with appropriate
academic conventions. I further declare that there is no potential conflict of interest
with respect to the research, data collection, authorship, presentation and/or
publication of this thesis.
Student’s Signature: Date: __30/05/2021_________
iii
Copyright
Copyright © 2021 Abdalla Ahmed Moallim Hussein
All Rights Reserved
iv
Advisory Committee
1) Advisor: Dr. Hilal El-Hassan
Title: Associate Professor
Department of Civil & Environmental Engineering
College of Engineering
2) Co-advisor: Prof. Tamer El Maaddawy
Title: Professor
Department of Civil & Environmental Engineering
College of Engineering
v
Approval of the Master Thesis
This Master Thesis is approved by the following Examining Committee Members:
1) Advisor (Committee Chair): Hilal El-Hassan
Title: Associate Professor
Department of Civil and Environmental Engineering
College of Engineering
Signature Date
2) Member: Bilal El-Ariss
Title: Associate Professor
Department of Civil and Environmental Engineering
College of Engineering
Signature Date
3) Member: Mahmoud Reda Taha
Title: Professor
Department of Civil Engineering
Institution: University of New Mexico, USA
Signature Date
May 31, 2021
May 31, 2021on behalf of external examiner
vi
This Master Thesis is accepted by:
Dean of the College of Engineering: Professor James Klausner
Signature Date
Dean of the College of Graduate Studies: Professor Ali Al-Marzouqi
Signature Date
Copy ____ of ____
24/6/2021
vii
Abstract
Industrial by-products and recycled concrete aggregates (RCA) have the potential to
fully replace cement and natural aggregates, respectively, in the production of concrete
rather than being discarded wastefully into landfills or stockpiles. Yet, their combined
use in the development of a novel RCA geopolymer concrete has been limited to non-
structural purposes, owing to the inferior mechanical and durability properties of said
concrete. To improve the properties of geopolymer concrete made with RCA, steel
fibers may be added to the mix. This research aims to study the feasibility of reutilizing
locally available industrial solid wastes and RCA in geopolymer concrete for structural
applications. A combination of ground granulated blast furnace slag and fly ash were
used to form a blended precursor binding material. The mechanical properties of steel
fiber-reinforced geopolymer concrete made with RCA were studied through testing
for compressive strength, splitting tensile strength, flexural properties, and modulus of
elasticity. In turn, the durability performance was assessed using water absorption,
sorptivity, bulk resistivity, and abrasion resistance. Experimental test results highlight
the ability to fully replace natural aggregates with RCA in blended geopolymer
concrete incorporating 2% steel fibers, by volume. Compared to the control mix made
with no RCA and steel fibers, such concrete provided superior mechanical and
comparable durability performance. Additionally, new tensile softening relationships
were established from the experimental test data and using inverse finite element
analysis. Three-dimensional finite element (FE) models were developed to simulate
and predict the shear behavior of steel fiber-reinforced RCA geopolymer concrete
beams. Based on regression analysis of FE results, a simplified empirical equation that
accounts for the compressive strength of concrete and steel fiber volume fraction was
established to predict the nominal shear resistance of steel fiber-reinforced geopolymer
concrete beams.
Keywords: Geopolymer, recycled concrete aggregate, steel fibers, performance
evaluation.
viii
Title and Abstract (in Arabic)
الركام من والمصنوعة الصلب بألياف المسلحة الجيوبوليمرية الخرسانة سلوك
تدويره المعاد الخرساني
ص الملخ
( تدويرها المعاد الخرسانة ومجموعات الصناعية الثانوية على RCAالمنتجات القدرة لديها )
استبدال الأسمنت والركام الطبيعي بالكامل ، على التوالي ، في إنتاج الخرسانة بدلا من التخلص
منها في مكبات النفايات أو المخزونات. ومع ذلك ، فإن استخدامها المشترك في تطوير الخرسانة
سبب الخصائص الميكانيكية الجديدة اقتصر على الأغراض غير الهيكلية ، ب RCAالجيوبوليمرية
والمتانة الرديئة للخرسانة المذكورة. لتحسين خصائص الخرسانة الجيوبوليمرية المصنوعة من
RCA يمكن إضافة ألياف فولاذية إلى المزيج. يهدف هذا البحث إلى دراسة جدوى إعادة استخدام ،
و محليا المتوفرة الصناعية الصلبة ال RCAالنفايات الخرسانة للتطبيقات في جيوبوليمرية
الإنشائية. تم استخدام مزيج من خبث الفرن العالي الحبيبي والرماد المتطاير لتشكيل مادة ربط
سلائف مخلوطة. تمت دراسة الخواص الميكانيكية للخرسانة الجيوبوليمرية المقواة بألياف الصلب
نشقاقي وخصائص الانحناء من خلال اختبار مقاومة الانضغاط وقوة الشد الا RCAالمصنوعة من
والامتصاصية الماء امتصاص باستخدام المتانة أداء تقييم تم ، المقابل في المرونة. ومعامل
على القدرة على الضوء التجريبية الاختبارات نتائج تسلط التآكل. ومقاومة السائبة والمقاومة
رية المخلوطة التي تحتوي على في الخرسانة الجيوبوليم RCAاستبدال الركام الطبيعي بالكامل بـ
٪ ، من حيث الحجم. بالمقارنة مع مزيج التحكم المصنوع من عدم وجود 2ألياف فولاذية بنسبة
مماثلة. RCAألياف ومتانة متفوقا ميكانيكيا أداء توفر الخرسانة هذه فإن ، فولاذية وألياف
القوانين التأسيسية للضغط والانفعال من بالإضافة إلى ذلك ، تم إنشاء علاقات تليين الشد الجديدة و
نماذج تطوير تم المعكوسة. المحدودة العناصر تحليل وباستخدام التجريبية الاختبار بيانات
ix
الجيوبوليمرية للخرسانة القص سلوك لمحاكاة الأبعاد ثلاثية المحدودة المقواة RCAالعناصر
، تم إنشاء معادلة تجريبية مبسطة تمثل قوة FEبناء على تحليل الانحدار لنتائج بألياف الصلب.
الضغط لكسر حجم ألياف الخرسانة والصلب للتنبؤ بمقاومة القص الاسمية لعوارض الخرسانة
الجيوسيلية المسلحة بألياف الصلب.
: الجيوبوليمر ، الركام الخرساني المعاد تدويره ، الألياف الفولاذية ، تقييم مفاهيم البحث الرئيسية
.الأداء
x
Acknowledgements
I would like to thank God for giving me the faith and strength to successfully complete
my research. I am so grateful to my advisor Dr. Hilal El-Hassan, as he is an outstanding
professor and excellent researcher. I thank him for all that he taught me and for the
support he has provided through my work. Also, I would like to express my sincere
gratitude to my co-advisor professor Tamer El-Maaddawy for his outstanding
guidance, constructive advice, and valuable assistance throughout the study. I would
like to thank all my family members and friends for their continuing support and
encouragement throughout the entire study. Special thanks to the examination
committee members for their time in reviewing this thesis.
My great thanks to my colleague Jamal Medljy for his outstanding help. The technical
assistance provided by the laboratory staff for the experimental works is appreciated.
My sincere appreciation to Eng. Abdelrahman Alsallamin, Eng. Tarek Salah and Mr.
Faisal Abdulwahab, and other staff for their help and technical assistance throughout
this research. In addition, special thanks are extended to ADEK and UAEU for
providing me with financial support to complete this research work under grant
number 21N209.
xi
Dedication
To my beloved parents and family
xii
Table of Contents
Title ............................................................................................................................... i
Declaration of Original Work ...................................................................................... ii
Copyright .................................................................................................................... iii
Advisory Committee ................................................................................................... iv
Approval of the Master Thesis ..................................................................................... v
Abstract ...................................................................................................................... vii
Title and Abstract (in Arabic) ................................................................................... viii
Acknowledgements ...................................................................................................... x
Dedication ................................................................................................................... xi
Table of Contents ....................................................................................................... xii
List of Tables ............................................................................................................. xv
List of Figures ........................................................................................................... xvi
List of Abbreviations ................................................................................................ xix
Chapter 1: Introduction ................................................................................................ 1
1.1 Overview .................................................................................................... 1
1.2 Scope and Objectives ................................................................................. 2
1.3 Outline and Organization of the Thesis ..................................................... 3
1.4 Research questions ..................................................................................... 4
Chapter 2: Literature Review ....................................................................................... 6
2.1 Introduction ................................................................................................ 6
2.2 Concrete and the environment ................................................................... 6
2.3 Background on geopolymers ..................................................................... 8
2.4 GGBS-fly ash blended geopolymer composites ........................................ 9
2.5 Geopolymer concrete with recycled aggregate ........................................ 14
2.6 Geopolymer concrete reinforced with steel fiber ..................................... 16
2.7 Shear behavior of geopolymer concrete................................................... 19
2.8 Research Significance .............................................................................. 21
Chapter 3: Experimental Program.............................................................................. 23
3.1 Introduction .............................................................................................. 23
3.2 Test program ............................................................................................ 23
3.3 Material properties ................................................................................... 25
3.3.1 Precursor binding material ............................................................... 25
3.3.2 Coarse aggregates ............................................................................. 29
3.3.3 Fine aggregates ................................................................................. 31
3.3.4 Chemical activators .......................................................................... 33
xiii
3.3.5 Steel fibers ........................................................................................ 33
3.3.6 Superplasticizer ................................................................................ 33
3.4 Geopolymer concrete mixture proportions .............................................. 34
3.5 Sample preparation .................................................................................. 35
3.6 Performance evaluation............................................................................ 37
3.6.1 Compressive strength ....................................................................... 37
3.6.2 Modulus of elasticity ........................................................................ 38
3.6.3 Splitting tensile strength ................................................................... 38
3.6.4 Flexural strength ............................................................................... 39
3.6.5 Water absorption .............................................................................. 40
3.6.6 Sorptivity .......................................................................................... 40
3.6.7 Bulk electric resistivity .................................................................... 41
3.6.8 Abrasion resistance .......................................................................... 42
Chapter 4: Experimental Results and Discussions ..................................................... 43
4.1 Introduction .............................................................................................. 43
4.2 Mechanical Properties .............................................................................. 43
4.2.1 Compressive Strength ...................................................................... 43
4.2.2 Compressive stress-strain response .................................................. 52
4.2.3 Modulus of Elasticity ....................................................................... 55
4.2.4 Splitting tensile strength ................................................................... 59
4.2.5 Flexural performance ....................................................................... 63
4.3 Durability properties ................................................................................ 81
4.3.1 Water absorption .............................................................................. 81
4.3.2 Sorptivity .......................................................................................... 83
4.3.3 Bulk resistivity ................................................................................. 86
4.3.4 Abrasion resistance .......................................................................... 88
Chapter 5: Numerical Modelling ............................................................................... 94
5.1 Introduction .............................................................................................. 94
5.2 Geometry of the beam .............................................................................. 94
5.3 Finite Element Modeling ......................................................................... 96
5.4 Material Constitutive Laws ...................................................................... 98
5.4.1 Plain Geopolymer Concrete ............................................................. 98
5.4.2 Reinforcing steel ............................................................................ 100
5.4.3 Steel Plates ..................................................................................... 101
5.4.4 Steel Fiber-Reinforced Geopolymer Concrete ............................... 101
5.5 Results and discussion ........................................................................... 108
5.5.1 Load-deflection curves ................................................................... 108
5.5.2 Crack Patterns and Failure Modes ................................................. 112
5.5.3 Peak Load ....................................................................................... 123
Chapter 6: Conclusions ............................................................................................ 126
6.1 Introduction ............................................................................................ 126
6.2 Limitations ............................................................................................. 127
xiv
6.3 Conclusions ............................................................................................ 127
6.4 Recommendations for Future Studies .................................................... 130
References ................................................................................................................ 132
Appendix .................................................................................................................. 142
xv
List of Tables
Table 1: Experimental Test Matrix ............................................................................ 24
Table 2: Chemical composition of as-received materials .......................................... 26
Table 3: Physical properties of coarse aggregates ..................................................... 31
Table 4: Mixture proportions of geopolymer concrete (kg/m3) ................................. 35
Table 5: Percent increase in cube compressive strength with time. .......................... 46
Table 6: Cylinder and cube compressive strength of 28-day geopolymer
………...concrete ....................................................................................................... 51
Table 7: Equations relating the modulus of elasticity to the compressive
………...strength ........................................................................................................ 58
Table 8: Ratio of tensile splitting strength to compressive strength .......................... 60
Table 9: Equations relating splitting tensile strength and compressive
………...strength ........................................................................................................ 62
Table 10: Flexural performance test results ............................................................... 63
Table 11: Flexural and cylinder compressive strength of 28-day geopolymer
………….concrete ..................................................................................................... 71
Table 12: Equations relating flexural strength and cylinder compressive
………… strength ...................................................................................................... 72
Table 13: Water absorption and initial rate of absorption of geopolymer
………….concrete ..................................................................................................... 86
Table 14: Input parameters of monitoring points ...................................................... 97
Table 15: Mechanical properties of the geopolymer concrete mixes ........................ 99
xvi
List of Figures
Figure 1: SEM micrographs of (a) GGBS and (b) fly ash ......................................... 27
Figure 2: XRD spectrum of (a) GGBS and (b) fly ash .............................................. 28
Figure 3: Particle size distribution of (a) GGBS and (b) fly ash ............................... 29
Figure 4: Particle size distribution of different aggregate percentages ...................... 30
Figure 5: Particle size distribution of dune sand ........................................................ 32
Figure 6: Dune sand (a) SEM micrograph and (b) XRD spectrum ........................... 32
Figure 7: Materials used in casting geopolymer concrete.......................................... 36
Figure 8: Geopolymer specimens after demolding .................................................... 37
Figure 9: Development of cubic compressive strength of concrete mixes
…………made with SF (a) 0%, (b) 1%, and (c) 2% ................................................. 44
Figure 10: Cubic compressive strength for (a) 1-day, (b) 7-day and (c) 28-day
………….geopolymer concrete ................................................................................. 48
Figure 11: 28-day cylinder compressive strength of geopolymer concrete ............... 50
Figure 12: Relationship between cube and cylinder compressive strengths of
…………..geopolymer concrete ................................................................................ 51
Figure 13: Typical compression stress-strain curves of cylinder concrete
…………..specimens with RCA (a) 30%, (b) 70%, and (c) 100% ........................... 52
Figure 14: Typical compression stress-strain curves of cylinder concrete
…………..specimens with SF (a) 0%, (b) 1%, and (c) 2% ....................................... 54
Figure 15: Modulus of elasticity of concrete mixes with different RCA
…………..replacement percentages and SF volume fractions .................................. 56
Figure 16: Modulus of elasticity of concrete mixes as a function of
…………..compressive strength ................................................................................ 57
Figure 17: Experimental versus predicted modulus of elasticity ............................... 58
Figure 18: Splitting tensile strength of 28-day geopolymer concrete ........................ 59
Figure 19: Correlation between splitting tensile strength and compressive
…………..strength ..................................................................................................... 62
Figure 20: Experimental versus predicted splitting tensile strength .......................... 62
Figure 21: Typical load-deflection curves of geopolymer concrete mixes
…………..with SF (a) 0%, (b) 1%, and (c) 2% ......................................................... 65
Figure 22: Typical load-deflection curves of cylinder concrete specimens
…………..with RCA (a) 30%, (b) 70%, and (c) 100% ............................................. 68
Figure 23: Flexural strength of 28-day geopolymer concrete .................................... 71
Figure 24: Relationship between flexural and cylinder compressive strength
…………..of 28-day geopolymer concrete................................................................ 72
Figure 25: Experimental versus predicted flexural strength ...................................... 72
Figure 26: Deflection at peak load of concrete mixes with various …..…
…………..(a) RCA replacement percentage and SF volume fractions and (b)
…………..cylinder compressive strength .................................................................. 74
xvii
Figure 27: Residual strength of concrete mixes with various (a) RCA
…………..replacement percentage and SF volume fractions and (b) cylinder
…………..compressive strength ................................................................................ 76
Figure 28: Flexural toughness of concrete mixes with various (a) RCA
…………..replacement percentage and SF volume fractions and (b) cylinder
…………..compressive strength ................................................................................ 78
Figure 29: Equivalent flexural strength ratio of concrete mixes with various
…………..(a) RCA replacement percentage and SF volume fractions and (b)
…………..cylinder compressive strength .................................................................. 80
Figure 30: Effect of RCA and SF percentages on the water absorption of
…………..28-day geopolymer concrete .................................................................... 82
Figure 31: Absorption of concrete mixes over time: (a) SF 0%; (b) SF 1%;
………….(c) SF2% ................................................................................................... 84
Figure 32: Bulk resistivity of 28-day blended geopolymer concrete mixes .............. 87
Figure 33: Relationship between bulk resistivity and each of water absorption
…………..and compressive strength ......................................................................... 88
Figure 34: Abrasion resistance of geopolymer concrete made with SF (a) 0%,
………….(b) 1%, and (c) 2% .................................................................................... 89
Figure 35: Abrasion resistance of geopolymer concrete mixes made with
………….RCA (a) 30%, (b) 70%, and (c) 100% ...................................................... 91
Figure 36: Relationship between abrasion mass loss and cylinder compressive
…………..strength ..................................................................................................... 93
Figure 37: Modeled beam cross-section view (dimensions in mm) .......................... 95
Figure 38: Modeled beam elevation view (dimensions in mm)................................. 96
Figure 39: Typical FE model for geopolymer concrete beam ................................... 96
Figure 40: Constitutive laws of plain concrete: (a) compressive hardening
…………..law; (b) compressive softening law; (c) tensile softening law
…………..(Alkhalil and El-Maaddawy, 2017; Awani et al., 2016) .......................... 99
Figure 41: Steel reinforcing bars (a) Stress-strain relationship and ……
…………..(b) Arrangement of steel reinforcement in the FE models..................... 101
Figure 42: Typical compression stress-strain constitutive law of
…………..CC3DNonLinCementit2User ................................................................. 102
Figure 43: Concrete prism model used for inverse analysis .................................... 103
Figure 44: Inverse analysis results of R30SF1: (a) experimental and predicted
…………..load-deflection curves and (b) corresponding tension function ............. 103
Figure 45: Inverse analysis results of R30SF2: (a) experimental and predicted
…………..load-deflection curves and (b) corresponding tension function ............. 104
Figure 46: Inverse analysis results of R70SF1: (a) experimental and predicted
…………..load-deflection curves and (b) corresponding tension function ............. 105
Figure 47: Inverse analysis results of R70SF2: (a) experimental and predicted
…………..load-deflection curves and (b) corresponding tension function ............. 106
Figure 48: Inverse analysis results of R100SF1: (a) experimental and predicted
…………..load-deflection curves and (b) corresponding tension function ............. 107
xviii
Figure 49: Inverse analysis results of R100SF2: (a) experimental and predicted
…………..load-deflection curves and (b) corresponding tension function ............. 108
Figure 50: Load-deflection response of concrete models with SF (a) 0%,
…………..(b) 1%, (c) SF 2% .................................................................................. 109
Figure 51: Load-deflection response of concrete models with RCA (a) 30%,
…………..(b) 70%, and (c) 100% ........................................................................... 111
Figure 52: R0SF0 FE models: (a) crack patterns, (b) maximum principal
…………..strains, and (c) minimum principal strains ............................................. 114
Figure 53: R30SF0 FE models: (a) crack patterns, (b) maximum principal
…………..strains, and (c) minimum principal strains ............................................. 115
Figure 54: R30SF1 FE models: (a) crack patterns, (b) maximum principal
…………..strains, and (c) minimum principal strains ............................................. 116
Figure 55: R30SF2 FE models: (a) crack patterns, (b) maximum principal
…………..strains, and (c) minimum principal strains ............................................. 117
Figure 56: R70SF0 FE models: (a) crack patterns, (b) maximum principal
…………..strains, and (c) minimum principal strains ............................................. 118
Figure 57: R70SF1 FE models: (a) crack patterns, (b) maximum principal
…………..strains, and (c) minimum principal strains ............................................. 119
Figure 58: R70SF2 FE models: (a) crack patterns, (b) maximum principal
…………..strains, and (c) minimum principal strains ............................................. 120
Figure 59: R100SF0 FE models: (a) crack patterns, (b) maximum principal
…………..strains, and (c) minimum principal strains ............................................. 121
Figure 60: R100SF1 FE models: (a) crack patterns, (b) maximum principal
…………..strains, and (c) minimum principal strains ............................................. 122
Figure 61: R100SF2 FE models: (a) crack patterns, (b) maximum principal
…………..strains, and (c) minimum principal strains ............................................. 123
Figure 62: Peak loads of geopolymer concrete beams ............................................. 124
Figure 63: Predicted versus experimental shear resistance ...................................... 125
xix
List of Abbreviations
AAS Alkaline Activator Solution
AR Abrasion Resistance
b Width of Specimen
BR Bulk Resistivity
d Depth of Specimen
DS Dune Sand
Ec Modulus of Elasticity of Concrete
f’c Cylinder Compressive Strength of Concrete
fcu Cube Compressive Strength of Concrete
FE Finite Element
FEM Finite Element Modelling
fr Flexural Strength
fsp Splitting Tensile Strength
f150 100
Residual Flexural Strength at L/150
f600 100
Residual Flexural Strength at L/600
GC Geopolymer Concrete
GGBS Ground Granulated Blast Slag
I Absorption
L Length of Specimen
NA Normal Aggregate
NC Normal Concrete
NMS Nominal Maximum Size
OPC Ordinary Portland Cement
xx
RT,150 100 Equivalent Flexural Strength Ratio
RCA Recycled Concrete Aggregate
SCM Supplementary Cementitious Material
SEM Scanning Electron Microscope
SF Steel Fiber
SH Sodium Hydroxide
SP Superplasticizer
SS Sodium Silicate
SSD Saturated Surface Dry
T150 100 Flexural Toughness
vf Steel Fiber Volume Fraction
Vn Nominal Shear Capacity
WA Water Absorption
XRD X-ray Diffraction
XRF X-ray Fluorescence
δp Peak Deflection
ν Poisson’s Ratio
1
Chapter 1: Introduction
1.1 Overview
Aging infrastructure and superstructures entail continuous renovation and
replacement, resulting in increases in demand for new concrete. The ability to recycle
concrete of construction and demolition waste and reutilize industrial solid by-
products as respective aggregates and binder in concrete provides a sustainable
solution to the pressing issues of depletion of natural resources and emissions of
carbon dioxide. Recycled concrete aggregate (RCA) has been proposed as a viable
alternative to natural coarse aggregates. However, its utilization in high-grade
applications as structural concrete is limited due to its inferior physical, mechanical,
and durability properties compared to those of natural aggregates (Akbarnezhad et al.,
2011; Hansen and Boegh, 1985). Nevertheless, numerous studies have been conducted
to improve the performance of RCA concrete by treating these aggregates or altering
the concrete mix design. These methods require excessive energy or cement, which
are not economic, environment-friendly, or feasible on construction sites.
Other efforts have been made to alleviate cement-induced CO2 emissions by
developing environment-friendly binders as inorganic alkali-activated geopolymer.
Upon incorporation into concrete, geopolymer promises to emit much less CO2 in its
production than conventional cement-based concrete (Jiang et al., 2014). Still, this
sustainable material provides a solution to one part of the problem, i.e. cement. To
further stimulate sustainable development, there is a pressing need to collectively
utilize RCA and geopolymer binder in producing concrete. The limited studies that
investigated RCA geopolymer concrete reported inferior mechanical and durability
2
performance compared to conventional concrete (Hu et al., 2019; Kathirvel and
Kaliyaperumal, 2016; Parthiban and Saravana Raja Mohan, 2017; Tang et al., 2019).
To promote its adoption by the construction industry, geopolymer concrete made with
100% RCA should present comparable results to its conventional cement-based
counterpart. One means is the novel incorporation of steel fiber reinforcement into the
geopolymer concrete mix. However, none of the available studies have evaluated the
mechanical and durability properties of geopolymer concrete made with RCA and steel
fibers.
1.2 Scope and Objectives
The main aim of this study is to evaluate the performance of GGBS-fly ash
blended geopolymer concrete made with different proportions of RCA and steel fibers.
Mechanical and durability properties are assessed using standardized test procedures.
Flexural strength test results are then be employed to develop tensile softening
relationships. Three-dimensional numerical models are subsequently developed to
examine the shear performance of beams made of the so-produced concrete. The
specific objectives of this work are as follows:
• Examine the effect of RCA replacement and steel fiber incorporation on the
mechanical properties of blended geopolymer concrete.
• Investigate the influence of different RCA and steel fiber proportions on the
durability performance of blended geopolymer concrete.
• Propose correlation equations among the properties of steel fiber-reinforced
RCA geopolymer concrete and compare them to codified equations.
• Develop tensile softening relations for steel fiber-reinforced RCA geopolymer
concrete.
3
• Examine the structural shear behavior of steel fiber-reinforced RCA
geopolymer concrete using numerical Finite Element Modeling (FEM).
1.3 Outline and Organization of the Thesis
The research work carried out in this thesis is organized into six chapters as
follows:
Chapter 1 provides a brief introduction and a summary of the research
objectives to be addressed throughout the thesis. It also includes an outline,
organization, and research significance of this work.
Chapter 2 presents a detailed and comprehensive literature review on the
available studies investigating geopolymer concrete. Topics will include fundamental
knowledge on geopolymers, the use of RCA in geopolymer concrete, and the use of
steel fiber in geopolymer concrete.
Chapter 3 presents details of the characteristics of the as-received materials,
mixture proportioning, sample preparation. It also comprises the testing methodologies
employed to evaluate the mechanical and durability performance of geopolymer
concrete.
Chapter 4 highlights the experimental test results. Compressive, splitting
tensile, and flexural strength, compressive stress-strain curves, modulus of elasticity,
flexural load-deflection curves, water absorption, sorptivity, and abrasion resistance
of various geopolymer concrete mixes are presented and discussed. Correlations
among these properties are also furnished.
4
Chapter 5 shows the details and results of the numerical FE modeling of the
shear behavior of blended geopolymer concrete beams made with different RCA and
steel fiber proportions.
Chapter 6 provides a summary of the research findings, general conclusions
and limitations of the completed work, and recommendations for future studies on steel
fiber-reinforced RCA geopolymer concrete.
1.4 Research questions
The replacement of natural aggregates by RCA has negatively impacted the
performance of geopolymer concrete. The addition of steel fibers is a promising
solution that may reverse this effect. However, there is a lack of knowledge about the
mechanical and durability performance of steel fiber-reinforced RCA blended
geopolymer concrete. Accordingly, this work aims to provide answers to the following
research questions:
• What is the feasibility of producing cement-free GGBS-fly ash blended
geopolymer concrete made with 100% RCA and steel fibers as a sustainable
alternative to conventional concrete?
• What is the effect of RCA replacement on the mechanical and durability
properties of blended geopolymer concrete?
• What is the influence of the addition of steel fibers on the mechanical and
durability properties of blended geopolymer concrete incorporating RCA?
• How are the mechanical and durability properties of blended geopolymer
concrete incorporating RCA and steel fibers correlated among each other?
5
• What are the tensile softening relations of geopolymer concrete made with
RCA and steel fibers?
• How do RCA replacement and steel fiber addition affect the structural shear
behavior of GGBS-fly ash blended geopolymer concrete beams?
6
Chapter 2: Literature Review
2.1 Introduction
A comprehensive literature review was conducted to summarize and discuss
the available experimental studies on geopolymer concrete. First, a short background
on the production of concrete is presented. Then, emphasis was placed on topics
including the geopolymerization process, use of GGBS and fly ash in producing
geopolymer concrete, RCA geopolymer concrete, and steel fiber-reinforced
geopolymer concrete. The research significance is also highlighted at the end of this
chapter.
2.2 Concrete and the environment
Concrete production is among the world’s fast-growing industries. It is
considered one of the most widely used materials globally with 15 billion tons and
around one cubic meter per capita being produced per year (US Geological Survey,
2016). According to current global statistics, Portland cement, the main binder in
concrete, is estimated to be produced in the range of 4.8 billion tons (Statistica, 2021).
Due to the high demand for concrete, one of its main resources, limestone, is expected
to reach an acute shortage within 25 to 50 years (Aleem and Arumairaj, 2012; Kline
and Kline, 2015). Indeed, Portland cement is manufactured by mixing specific
quantities of limestone and clay at elevated temperatures. The process consumes 1.6
kg of raw material, requires 3.7 MJ of energy, and emits 1 kg of CO2 per kg of cement
produced (Afkhami et al., 2015; Thakur and Wu, 2000). In fact, the cement industry
alone is accountable for 5-7% of the global CO2 emissions (Benhelal et al., 2013;
Davidovits, 1994), leading to an increase in the concentration of CO2 in the atmosphere
7
(Earth System Research Laboratory, 2013; Herzog et al., 2000). As a result, cement
production is becoming a critical global issue from an ecological, social, and
environmental standpoint.
To alleviate the consumption of natural resources and emission of CO2 from
cement manufacturing, scientists and environmentalists propose the incorporation of
supplementary cementitious material (SCMs) in concrete mixes. In most cases, these
SCMs are pozzolanic and non-pozzolanic industrial solid wastes. If integrated into
concrete mixes, they could have a dual benefit of reducing cement usage while also
usefully disposing of solid industrial by-products. Of these materials, fly ash and
ground granulated blast furnace slag (GGBS) are commonly used pozzolans. While
fly ash is an industrial waste from coal power plants, GGBS is a by-product of the
production of steel. Their incorporation in cement-based concrete has been reported to
improve the overall performance (Mehta, 2006).
Construction and demolition wastes management is another global pressing
issue. The massive amounts of debris generated in demolition, construction, and
renovation of structures are stretching the landfill capacity and inducing economic
leakages (Stoner and Wankel, 2008). In the United Kingdom, 20 million tons of debris
is deposited in landfills every year. Of this material, over 30% and 50% are masonry
and concrete respectively (Environmental Resources Limited, 1980). Recycling
construction and demolition wastes offers a sustainable solution to reduce the
consumption rate of landfill sites and natural resources (Kong et al., 2010; Poon and
Chan, 2007). For the past few decades, waste concrete has been reprocessed and reused
as aggregate. The so-produced recycled concrete aggregate (RCA) consists of 65-70%
original aggregate and 30-35% original cement paste by volume (Zhang et al., 2015).
8
A lifecycle inventory development study for the use of RCA in the UAE reported lower
global warming potential with RCA compared to NA (Alzard et al., 2021). However,
the use of RCA in structural concrete has been restricted due to its lower compressive
and tensile strength in comparison to those of normal concrete (Ikea et al., 1988). RCA
concrete also experiences more drying shrinkage and inferior durability properties
(Akbarnezhad et al., 2011; Hansen and Boegh, 1985).
2.3 Background on geopolymers
Davidovits (1991) introduced the word “geopolymer” as a form of inorganic
polymeric material. It is part of a larger family of materials known as alkali-activated
materials. They are typically formed through the activation of aluminosilicate-rich
precursor binding agents using alkaline solutions, including sodium silicate, sodium
hydroxide, potassium silicate, and/or potassium hydroxide. Typically, sodium or
potassium solutions were mixed to create the alkaline activator solution (AAS), but
single activators have also been used (Fernández-Jiménez and Puertas, 2003; Palomo
et al., 1999).
The chemical composition of the precursor dictates whether a certain
geopolymer was categorized with a Ca-Si or Al-Si system (Li et al., 2010). The first
system is a resultant of the alkali-activation of calcium-based binders such as GGBS.
It involves the hydration of calcium oxide in the presence of alumina to produce
calcium aluminosilicate hydrate (C-A-S-H) gel. The activation reaction generally
produces a material with limited workability, high strength, and drying shrinkage
(Aydın and Baradan, 2014; El-Hassan and Elkholy, 2019; El-Hassan and Ismail, 2018;
El-Hassan et al., 2018). Conversely, the second system develops due to the reaction of
fly ash, silica fume, metakaolin, and kaolinite clay. In the case of fly ash, the reaction
9
involves dissolving silica followed by coagulation, exothermic condensation, and
crystallization to produce sodium aluminosilicate hydrate (N-A-S-H) gel. Fernández-
Jiménez and Puertas (2003) concluded that the composition of fly ash had an impact
on the geopolymerization reaction, where class F fly ash was most suitable for its
optimization. However, this reaction was dependent on heat curing at 60-80°C and was
significantly retarded at ambient conditions. As such, its adoption for cast-in-situ
applications was limited. Nevertheless, several attempts have been made to combine
these two systems into a blended system made of GGBS and fly ash to reduce
shrinkage-induced cracks and eliminate the need for heat curing in the first and second
systems, respectively. The reaction products of this combined system highlight the
coexistence of C-A-S-H and N-A-S-H with a higher degree of cross-linking (Yip et
al., 2005).
2.4 GGBS-fly ash blended geopolymer composites
As noted in the previous section, several researchers mixed GGBS and fly ash
to form a blended geopolymer binder. Such a binding material has been utilized in
several past work. While several studies investigated the addition of slag to fly ash-
based geopolymers (slag-to-fly ash ratio < 1), limited investigations examined the
incorporation of fly ash to slag-based geopolymers (slag-to-fly ash ratio > 1).
Puertas et al. (2000) examined the mechanical performance behavior of fly
ash/GGBS geopolymer pastes. Results showed that alkaline activator concentration
and fly ash-to-GGBS ratio were the main contributors to strength development. The
curing temperature was found to have a less pronounced effect. The mixture made with
equal proportions of GGBS and fly ash and 10 M sodium hydroxide and cured at 25°C
could attain a compressive strength of about 50 MPa.
10
Nath and Sarker (2014) studied the effect of adding GGBS to fly ash-based
geopolymer concrete binder. The objective was to produce geopolymer concrete
mixtures suitable for curing at ambient temperature. GGBS and fly ash were activated
in an alkaline solution made of sodium silicate and sodium hydroxide. Results showed
that fly ash-based geopolymer concrete could be proportioned with GGBS for
desirable workability and setting time. In addition, 30% replacement of fly ash with
GGBS led to a compressive strength of 55 MPa.
Sofi et al. (2007) examined the mechanical properties of fly ash/slag-based
geopolymer concrete. Splitting tensile and flexural strengths were similar to the
models illustrated by the Australian standard AS3600 (2009) for conventional cement-
based concrete. While the difference between the splitting tensile and the flexural
strength of the GPC mixes was found to be about 2 MPa, the strength gains were
similar.
Deb et al. (2014) reported an increase in compressive and tensile strength and
a decrease in workability upon adding 10-20% GGBS into fly ash-based geopolymer
concrete. In fact, compressive strength could reach up to 51 MPa in geopolymer mixes
made with 20% GGBS and 80% fly ash and cured at 20°C. Codified equations of ACI
318 and AS3600 provided accurate predictions of the tensile strength but were slightly
more conservative for samples that were cured at elevated temperatures.
Prusty and Pradhan (2020a) investigated the use of GGBS on the strength and
corrosion resistance of fly ash-based geopolymer concrete. Results showed that the
slump decreased with GGBS addition due to the higher water demand of such particles.
The strength at 7 days was almost 80% higher upon replacing fly ash by GGBS with
lower potential values compared to fly ash-based geopolymer concrete.
11
Mehta et al. (2020) optimized the mixture proportions of fly ash-based
geopolymer concrete incorporating up to 20% GGBS. The optimum AAS-to-fly ash
ratio, sodium silicate to sodium hydroxide ratio, total aggregate content, and molarity
of sodium hydroxide were found to be 0.55, 2.5, 70%, and 10 M, respectively. The
addition of 20% GGBS increased the strength to approximately 65 MPa with 3-day
strength achieving 92-99% that at 28 days.
Bellum et al. (2020) studied the effect of GGBS addition on the modulus of
elasticity of fly ash-based geopolymer concrete. Results showed that 70% replacement
of fly ash by GGBS led to the highest compressive strength and modulus of elasticity.
Respective values could reach up to 38 MPa and 20 GPa.
Prusty and Pradhan (2020b) studied the effect of different mixture proportions
on the compressive, splitting tensile, and flexural strength of fly ash geopolymer
concrete. Optimization results noted that the replacement of 45% fly ash by GGBS
along with a sodium hydroxide molarity of 14 M and sodium silicate to sodium
hydroxide ratio of 1.5 were ideal to provide maximum strength.
Garanayak (2020) evaluated the mechanical performance of alkali-activated
fly ash GGBS paste cured in ambient conditions. Mixture proportions involved varying
the GGBS and fly ash proportions and sodium hydroxide molarity. The maximum
strength obtained was around 89 MPa with 30% fly ash and 12M sodium hydroxide
molarity.
Shang et al. (2018) reported that the incorporation of GGBS in fly ash-based
geopolymer enhanced the early performance. Yet, it was found critical to blend the
12
two precursors to balance setting time, fluidity, volume stability, strength, and chloride
permeability.
Rafeet et al. (2019) reported an increase in strength when GGBS was
incorporated into fly ash-based geopolymer pastes without the need for oven curing.
In turn, such mixes required lower activator content, which resulted in lower cost and
environmental footprint while maintaining high compressive strength.
Yazdi et al. (2018) studied the mechanical and transport properties of GGBS
fly ash blended concrete. The compressive and flexural strength of blended mixes
could reach up to 100 and 10 MPa, respectively. The porosity decreased as more
GGBS was incorporated into the mix. Nevertheless, increasing GGBS beyond 50%
did not seem to have any significant impact on the performance.
The resistance to weathering and chloride penetration of fly ash GGBS blended
geopolymer concrete was investigated (Lee et al., 2019). Samples cured in outdoor
conditions for 180 days could reach a compressive strength of 53 MPa with continuous
growth in strength. Conversely, indoor curing could attain a strength of 67 MPa.
Reddy et al. (2018) developed and validated a mix design procedure for fly
ash-GGBS blended geopolymer concrete. The ratio of fly ash to GGBS was set to 7:3,
while the solution to binder ratio varied between 0.4 and 0.8. Compressive strength
was found to decrease with higher solution content but the workability, characterized
by the slump, increased. Yet, the effect of the water-to-cement ratio on the compressive
strength of conventional concrete seemed to be more severe than the impact of the
solution-to-binder ratio on that of blended geopolymer concrete.
13
An amorphous mix design framework was developed for GGBS fly ash
blended geopolymer (Lau et al., 2019). Through the optimization technique, it was
found that the optimal Si/Al and (Na+2Ca)/Al ratios were 2.3 and 3.2, respectively.
The obtained compressive strength could reach up to 69 MPa.
Samantasinghar and Singh (2020) examined the impact of curing regime on fly
ash-GGBS blended geopolymer. Results showed that heat curing was essential for
mixes without GGBS, while those with GGBS experienced microcracks. Further,
autoclaving and microwave radiation presented the highest strength development
among the different regimes.
Other work investigated the influence of GGBS incorporation on the
performance of fly ash-based geopolymer mortar and lightweight concrete (El-Hassan
and Ismail, 2018; El-Hassan et al., 2017; Ismail et al., 2017). Experimental test results
highlighted an increase in mechanical properties, including compressive strength and
modulus of elasticity, as more fly ash was replaced by GGBS. Also, the performance
was less impacted by heat curing with higher GGBS incorporation.
The effect of the curing regime was also investigated by (El-Hassan et al.,
2018, 2021). The authors reported that a combination of air and water curing was ideal
for mixes made with GGBS and fly ash. However, the impact of curing was found to
be less apparent as more fly ash was incorporated into the mix. Nevertheless,
compressive strength of at least 40 MPa could be achieved regardless of curing regime.
Other work studied the effect of fly ash replacement on GGBS-based
geopolymer concrete (El-Hassan and Ismail, 2018; Ismail et al., 2014). Major findings
included the superior performance of blended geopolymers compared to counterparts
14
made with a single precursor. With fly ash contents exceeding 50%, the mixes tended
to have poor early performance and required heat curing, while mixes with no fly ash
were not workable and set quickly. Thus, a blend of the two was deemed ideal for
optimum performance.
2.5 Geopolymer concrete with recycled aggregate
To further enhance the sustainability of geopolymer concrete, several attempts
were made to replace natural aggregates (NA) with recycled concrete aggregate
(RCA). Most studies focused on the impact of such replacement on mechanical
properties, including compressive strength, flexural strength, splitting tensile strength,
and elastic modulus. In general, it was found that the geopolymer concrete
incorporating RCA had inferior performance compared to counterparts made with NA.
This was primarily due to the weak bond between the mortar and RCA and the porous
nature of RCA (Salesa et al., 2017). A more in-depth review of the studies is shown in
this section.
Shi, X.S. et al. (2012) studied the mechanical properties of fly ash geopolymer
concrete containing 50% and 100% recycled coarse aggregate as a replacement of
natural coarse aggregate. Based on the results, the geopolymer concrete comprising
RCA had a compressive strength and elastic modulus higher than counterpart OPC
concrete containing RCA with a better interfacial transition zone. Also, it was observed
that the mechanical properties decreased as the RCA content increases.
Nuaklong et al. (2016) examined the effect of recycled aggregate on the
strength and durability of high calcium fly ash-based geopolymer concrete. A
comparison was made with crushed limestone aggregates. Experimental findings
15
showed that geopolymer concrete made with RCA could reach a strength of up to 38
MPa, but were slightly lower than counterparts made with crushed limestone
aggregates. Further, the higher molarity of sodium hydroxide resulted in more durable
geopolymer concrete.
Kathirvel and Kaliyaperumal (2016) examined the effect of recycled concrete
aggregate (RCA) on the properties of GGBS geopolymer under ambient curing. The
compressive strength development of mixes made with up to 50% RCA replacement
is associated with the enhanced packaging and filling impact of RCA. However, higher
RCA replacement had a negative impact on strength, sorptivity, and chloride diffusion.
Shaikh (2016) investigated the mechanical and durability properties of fly ash
geopolymer concrete made with recycled coarse aggregate from local demolition and
waste. RCA replacement was set as 15, 30, and 50%, by weight. Results highlighted
decreases in compressive strength, splitting tensile strength, and modulus of elasticity
when 50% RCA was incorporated into the geopolymer concrete mix. Furthermore, the
durability, including sorptivity and chloride penetration, was negatively affected by
the replacement of NA by RCA.
Mesgari et al. (2020) investigated the properties of geopolymer concrete and
Portland cement concrete with varying contents of geopolymer RCA, namely 0, 20,
50, and 100% replacement. Respective reductions of 14, 1, and 3% in the compressive
strength, modulus of elasticity, and flexural strength are associated with the
replacement of natural aggregates with geopolymer RCA up to 20%. Higher RCA
content of 100% led to 33, 26, and 21% lower respective properties.
Xie et al. (2019a) examined the influence of RCA replacement on the
performance of GGBS-metakaolin blended geopolymer concrete. Results highlighted
16
decreases of up to 35% in compressive strength with a 75% increase in a slump when
100% NA was replaced by RCA. A lower compressive toughness was also reported
upon the incorporation of RCA in the mix.
Hu et al. (2019) studied the combined effect of GGBS and RCA on the
performance of fly ash geopolymer. The addition of GGBS to the mix reduced the
workability while the incorporation of RCA increased it. Further, the replacement of
100% RCA reduced the compressive strength, modulus of elasticity, splitting tensile
strength, and flexural strength of mixes made with 30% GGBS by 27, 22, 22, and 26%,
respectively. Yet, it should be noted that reductions were higher when geopolymer was
made only with fly ash.
Xie et al. (2019b) and Xie et al. (2019c) replaced 100% NA by RCA in GGBS-
fly ash geopolymer concrete. An increase in a slump was reported upon the addition
of RCA owing to the additional water required for absorption. Conversely, the
compressive strength and modulus of elasticity decreased by 16 and 21%, respectively.
2.6 Geopolymer concrete reinforced with steel fiber
Geopolymer concrete has been advocated as a sustainable alternative to
conventional cement-based concrete. Despite its impressive performance, geopolymer
concrete has very low tensile and flexural properties with brittle characteristics. For
conventional concrete, fiber reinforcement has been proposed to counter the weak
properties while enhancing ductility. Similarly, fibers, especially steel, have been
suggested to improve performance. The following section discusses the work that
utilized steel fibers in geopolymer concrete and mortar.
17
Guo and Pan (2018) added various fibers to geopolymer concrete consisting of
fly ash and GGBS as a binary geopolymer matrix. The effect of volume contents and
type of fibers including basalt fiber, polypropylene fiber, and steel fiber on the
mechanical properties of geopolymer was investigated. Among the various fibers, the
flexural and tensile strength of a geopolymer concrete improved significantly with
steel fiber.
Bernal et al. (2010) conducted a study on the effect of steel fibers on the
mechanical properties of GGBS-based geopolymer concrete. Results highlighted that
the use of steel fibers has reduced the compressive strength but significantly improved
the splitting tensile and flexural strengths. Water absorption and porosity were reduced
by approximately 20% in steel fiber-reinforced geopolymer mixes in comparison to
mixes without fibers.
Devika and Nath (2015) examined the impact of steel fibers on the mechanical
flexural behavior of geopolymer concrete beams. Results show that the addition of
steel fibers transformed brittle geopolymer concrete into a more ductile one, while also
significantly improving tensile strength, tensile strain, and toughness.
Al-Majidi et al. (2017) developed a steel fiber-reinforced, ambient-temperature
cured geopolymer concrete for in-situ applications. Fresh and mechanical properties
were measured. Experimental results showed that steel fiber addition reduced the
compressive strength of the geopolymer with 10 and 20% GGBS. However, it was
significantly improved at higher GGBS content compared to the respective mixes
without steel fibers.
Islam et al. (2017) focused on the development of sustainable concrete
geopolymer using palm oil fuel ash and GGBS as binders and oil palm shell as coarse
18
aggregates. The hooked-end steel fiber was used in this study to investigate the impact
resistance of the concrete. The results indicated that the addition of 0.5% steel fibers,
by volume enhanced the splitting tensile and flexural strengths of fiber reinforced
geopolymer concrete by up to 38 and 44%, respectively, compared to the plain
counterparts. The addition of 0.5% steel fiber also increased the first crack load by
1.5–3.5 times.
The effect of adding steel fibers on the mechanical properties of GGBS-based
geopolymer concrete made with 0, 25, and 50% fly ash was investigated (El-Hassan
and Elkholy, 2019; Elkholy and El-Hassan, 2019). The compressive, splitting tensile,
and flexural strengths were reported to have increased by up to 30, 31, and 25%,
respectively, upon the incorporation of up to 2%, by volume. Codified equations could
be employed for such concrete after applying a modification factor for some mixes.
Liu et al. (2020) studied the influence of steel fiber and silica fume on the
mechanical and fracture performance of ultra-high performance geopolymer concrete.
Upon the addition of steel fibers, the workability of said concrete decreased while the
modulus of elasticity, compressive strength, splitting tensile strength, fracture energy,
flexural strength, and stress intensity factor increased.
Guo and Xiong (2021) studied the resistance of GGBS fly ash blended
geopolymer concrete reinforced with steel fibers to sulfate corrosion and drying-
wetting. The compressive strength of concrete made with 0.4%, by volume, steel fibers
was approximately 68 MPa after 15 durability cycles.
Gülşan et al. (2019) conducted a study on self-compacting fly ash geopolymer
concrete with up to 1% steel fiber, by volume. The addition of steel fibers reduced the
fresh concrete properties, including slump flow, flow time, V-funnel flow time, and L-
19
box passing ability. Yet, it enhanced the bond resistance, flexural strength, fracture
toughness, and stress intensity factor, owing to its bridging effect.
Their and Özakça (2018) incorporated nanosilica and steel fibers in fly ash
geopolymer concrete. The combined incorporation of these two additives increased
the compressive strength to up to 57 MPa with improved water penetration resistance.
While 1% steel fiber volume fraction seemed to generally have a positive impact,
higher percentages caused a decrease in durability properties, including sorptivity and
water penetration.
Shaikh and Hosan (2016) examined the performance of steel fiber-reinforced
geopolymer concrete after being exposed to elevated temperatures. Results showed
that steel fibers allowed for high residual compressive and splitting tensile strength
post-exposure with limited cracking and spalling. Models to predict the performance
were also developed.
Khan et al. (2018) examined the mechanical properties of high-strength
geopolymer concrete reinforced with spiral and hooked-end steel fibers. The inclusion
of steel fibers reduced the workability but provided higher compressive strength
compared to plain counterparts. Additionally, a multi-fold increase in the load-carrying
capacity, toughness, and post-peak residual strength.
2.7 Shear behavior of geopolymer concrete
The shear behavior of steel fiber-reinforced geopolymer concrete was
investigated in past work. Visintin et al. (2017) evaluated the shear capacity of eight
geopolymer concrete beams without stirrups. The beams were designed with a
variation in reinforcement ratio and span-to-depth ratio (a/d). The findings of the direct
shear tests illustrate that the shear-friction properties for the geopolymer concrete used
20
in the experimental investigation fall within the range of shear-friction properties of
conventional cement-based concrete.
Ng et al. (2013) investigated the shear behavior of steel fiber-reinforced
geopolymer concrete beams. Five series of 250 mm deep by 120 mm wide geopolymer
concrete beams spanning 2250 mm without stirrups were tested with varying quantities
and types of steel fibers. End-hooked and straight steel fibers varied from 0 to 1.5%,
by volume. The beams have a span-to-effective depth ratio of 3.7. Results showed that
the cracking load and shear strength increased significantly with the increase of steel
fiber volume fraction, while the rate of crack growth and crack widths decreased.
Chang (2009) studied nine fly ash-based geopolymer concrete beams with
2000 mm length. The crack patterns and modes of failure were found to be generally
similar to Portland cement concrete beams. The methods of calculations that were
typically used in the case of reinforced Portland cement concrete beams were
applicable in predicting the shear strength of reinforced geopolymer concrete beams.
The provisions of the code are generally conservative and safe to predict the shear
strength of geopolymer concrete beams.
Mo et al. (2017) evaluated the shear performance of steel fiber reinforced
cement-based and geopolymer oil palm shell lightweight aggregate concrete. Various
volume fractions of steel fibers were added for the cement-based and geopolymer
concrete. Experimental test results indicated that the shear resistance of geopolymer
concrete beams increased with the addition of steel fibers. The shear capacity existing
prediction equations for steel fiber-reinforced lightweight concrete was demonstrated
to be conservative for the steel fiber-reinforced cement-based and geopolymer
concrete.
21
Yacob et al. (2019) evaluated the shear behavior of fly ash geopolymer
concrete. The crack propagation and load-deflection response were similar in
geopolymer concrete as conventional cement-based concrete, while the crack patterns
were different. Response 2000 equations could conservatively predict the strength of
the investigated concrete beams with better predictions in the beams incorporating
shear reinforcement.
2.8 Research Significance
Aging infrastructure and superstructures entail continuous renovation and
replacement resulting in increases in demand for new concrete and supply of non-
renewable aggregates. The ability to recycle concrete of construction and demolition
waste and reutilize industrial solid by-products provides a sustainable solution to the
pressing issues of depletion of natural resources and emissions of carbon dioxide. RCA
has been proposed as a viable alternative to natural aggregates. However, it has only
been used in non-structural applications due to its inferior strength and durability
properties. Other efforts have been made to alleviate CO2 emissions by developing
sustainable construction materials as geopolymer concrete. Unlike normal concrete,
geopolymer is cement-free and emits much less CO2 in its production. As such, the
incorporation of these two sustainable solutions promises to further enhance the
sustainability of the construction industry.
Few studies have investigated the performance of geopolymer concrete made
with 100% RCA. Results generally showed a reduction in mechanical and durability
properties upon complete replacement of NA by RCA. Yet, it has been reported that
100% RCA replacement could not be attained without compromising the performance.
Additionally, limited studies have reported an improvement in the properties of
22
geopolymer concrete made with natural aggregates upon the addition of steel fibers.
As such, it seems that steel fibers may enhance the performance of geopolymer
concrete made with 100% RCA. However, such a study has yet to be investigated.
There is also a lack of studies on the shear performance of steel fiber-reinforced RCA
geopolymer concrete.
This research aims to fill this gap by assessing the feasibility of producing a
geopolymer concrete made with 100% RCA and steel fibers. Unlike past work, which
mainly focused on fly ash-based geopolymers, this work will utilize a blended binder
of GGBS and fly ash to eliminate the need for heat curing and reduce shrinkage cracks
associated with fly ash- and GGBS-based geopolymers, respectively. The mechanical
and durability properties of such innovative and sustainable concrete will be evaluated.
In addition, a finite element model will also be developed to evaluate the structural
shear performance of beams made of the so-produced concrete. Through experimental
investigation and in-depth analysis of results, this research will provide evidence on
the ability to fully replace natural coarse aggregates with RCA and cement with
industrial waste materials to produce an innovative and sustainable structural
geopolymer concrete.
23
Chapter 3: Experimental Program
3.1 Introduction
This chapter highlights the detailed experimental program carried out to
evaluate the effect of recycled concrete aggregate (RCA) and steel fibers (SF) on the
performance of geopolymer concrete made with a blend of GGBS and fly ash (1:1).
To attain the research objectives, this work was divided into three phases. In the first
phase, the characteristics of the as-received material were determined following
standardized test procedures. A description of sample preparation and mixture
proportioning was then provided. In the second phase, trial mixes were proportioned
to obtain a specific design compressive strength. Subsequently, natural aggregates
were replaced by different quantities of recycled concrete aggregates alongside steel
fibers. The mechanical and durability properties of such hardened geopolymer
concrete were assessed through extensive testing, including water absorption,
sorptivity, compressive strength, splitting tensile strength, flexural strength, modulus
of elasticity, bulk resistivity, and abrasion resistance. Furthermore, the third phase
encompassed the development of analytical finite element models to evaluate the shear
performance of various geopolymer concrete mixes incorporating different quantities
of RCA and SF.
3.2 Test program
At the early stages of this research, several trial mixes were designed to obtain
a desired cube compressive strength of 35 MPa, which is typically used for structural
applications in the United Arab Emirates. This step was critical, as unlike conventional
cement-based concrete, limited research on the mix design procedure of blended
24
geopolymer concrete was available. The trial mixes and associated cylinder
compressive strength are presented in Table A1 of Appendix A. The obtained 35-MPa
control mix served as a benchmark. Subsequently, the natural aggregates (NA) were
replaced by RCA with the incorporation of different volume fractions of steel fibers.
In fact, a total of 10 geopolymer concrete mixtures were proportioned, as shown in
Table 1. The mixes were divided into 4 main groups based on RCA replacement
percentage, by mass of total aggregate. Group [A] included the control mix made with
100% NA without steel fiber. Group [B], [C], and [D] comprised mixes consisting of
30, 70, and 100% RCA replacement, respectively, with the addition of 0-2% SF, by
volume.
Table 1: Experimental Test Matrix
Group
Mix
Number
Mix
Designation
RCA
Replacement
(%)
Fiber Reinforcement
0% 1% 2%
Group A Mix 1 R0SF0 0 x
Group B
Mix 2 R30SF0
30
x
Mix 3 R30SF1 x
Mix 4 R30SF2 x
Group C
Mix 5 R70SF0
70
x
Mix 6 R70SF1 x
Mix 7 R70SF2 x
Group D
Mix 8 R100SF0
100
x
Mix 9 R100SF1 x
Mix 10 R100SF2 x
25
3.3 Material properties
The main materials used in this study included GGBS, fly ash, natural
aggregates, recycled concrete aggregates, dune sand, chemical activators, and
superplasticizer. It is empirical to evaluate the properties of these materials prior to
incorporation in geopolymer concrete mixtures.
3.3.1 Precursor binding material
The geopolymer precursor binding materials were in the form of GGBS and
fly ash. The GGBS was provided by Emirates Cement Company, while the fly ash was
sourced from Ashtech Ltd. Table 2 presents the chemical composition using X-ray
fluorescence (XRF) of the binding materials. While the GGBS was predominantly
made of calcium oxide (CaO) and silica (SiO2), the fly ash mainly comprised silica
(SiO2) and alumina (Al2O3). Their respective Blaine fineness were 4250 and 3680
cm2/g, whereas their corresponding specific gravity were 2.70 and 2.32. Furthermore,
Figure 1 highlights the morphology of the as-received binding materials using
scanning electron microscopy. Spherical and irregular particles were noticed in fly ash
and GGBS micrographs, respectively. The X-ray diffraction (XRD) patterns of Figure
2 show that the binders comprised mainly of quartz and mullite with traces of gehlenite
and hematite in GGBS and fly ash, correspondingly. Additionally, their particle size
distribution is shown in Figure 3. The respective particle sizes of GGBS and fly ash
were in the ranges of 2-80 µm and 0.2-40 µm.
26
Table 2: Chemical composition of as-received materials
Oxide component Material (%)
GGBS Fly ash Dune sand
CaO 42.0 3.3 14.1
SiO2 34.7 48.0 64.9
Al2O3 14.4 23.1 3.0
MgO 6.9 1.5 1.3
Fe2O3 0.8 12.5 0.7
Loss in ignition 1.1 1.1 0.0
Others 1.1 10.5 16.0
Blaine Fineness (cm2/g) 4250 3680 5760
Specific gravity 2.70 2.32 2.77
27
(a)
(b)
Figure 1: SEM micrographs of (a) GGBS and (b) fly ash
28
(a)
(b)
Figure 2: XRD spectrum of (a) GGBS and (b) fly ash
29
(a)
(b)
Figure 3: Particle size distribution of (a) GGBS and (b) fly ash
3.3.2 Coarse aggregates
Coarse aggregates used in this research were natural aggregates (NA) and
recycled concrete aggregate (RCA). The NA was in the form of crushed dolomitic
limestone sourced from Ras Al Khaimah, UAE, and combined 30% of 10 mm and
70% of 20 mm aggregates. The combined aggregate, referred to as NA hereafter had
a nominal maximum size (NMS) of 20 mm. Conversely, the RCA was collected from
0
20
40
60
80
100
0.1 1 10 100
Cu
mu
lati
ve
pas
sin
g (
%)
Particle size (µm)
0
20
40
60
80
100
0.1 1 10 100
Cum
ula
tive
pas
sing (
%)
Particle size (µm)
30
Al Dhafra Recycling Industries (ADRI) that recycled construction and demolition
waste from old concrete structures with unknown compressive strength. The NMS of
the RCA was 25 mm. Figure 4 shows the grading curves for different mixes of NA
and RCA used in this study. It is worth noting that they all satisfy the requirements
and limits of ASTM C33 (ASTM, 2016). The physical properties of NA and RCA are
summarized in Table 3. The water absorption, Los Angeles (LA) abrasion mass loss,
soundness mass loss, and fineness modulus of the former were higher than the latter,
while its specific gravity and dry-rodded density were lower. This signifies that RCA
is indeed of weaker nature than NA. Also, it is worth noting that all measured
properties were within the typical limits given by the ASTM standards.
Figure 4: Particle size distribution of different aggregate percentages
0
10
20
30
40
50
60
70
80
90
100
1 10 100
Cum
ula
tive
Pas
sing (
%)
Particle Size (mm)
100%NA
30%RA 70%NA
70%RA 30%NA
100%RA
31
3.3.3 Fine aggregates
Locally abundant desert dune sand served as the fine aggregates in this work.
Its fineness modulus, dry-rodded density, specific gravity, and surface area were 1.45,
1663 kg/m3, 2.77, and 116.8 cm2/g respectively. The grading curve of dune sand,
shown in Figure 5, indicates that most of the particles are within the range of 100-600
µm. Figure 6 presents the SEM micrograph and XRD spectrum of dune sand. It is
mainly composed of irregular particles representing quartz with some traces of calcite,
ferric oxide, and aluminum oxide.
Table 3: Physical properties of coarse aggregates
Physical Property, Unit Standard Test NA RCA
Fineness modulus ASTM C136 6.82 7.44
Specific gravity ASTM C127 2.82 2.63
Water absorption, % ASTM C127 0.70 6.63
Dry-rodded density, kg/m3 ASTM C29 1635 1563
Los Angeles abrasion, % ASTM C131 16.0 32.6
Specific surface area, cm2/g ASTM C136 2.49 2.50
Soundness (MgSO4), % ASTM C136 1.20 2.78
32
Figure 5: Particle size distribution of dune sand
(a)
(b)
Figure 6: Dune sand (a) SEM micrograph and (b) XRD spectrum
0
20
40
60
80
100
10 100 1000
Per
cen
tage
pas
sin
g (
%)
Particle size (µm)
33
3.3.4 Chemical activators
To activate the precursor binding materials, an alkaline activator solution
(AAS) was formulated as a mixture of sodium silicate (SS) and sodium hydroxide
(SH). The SS solution was Grade N with a chemical composition of 26.3% SiO2,
10.3% Na2O, and 63.4% H2O. Conversely, the SH solution was prepared by dissolving
97%-pure SH flakes in a specific amount of water to obtain molarity of 14 M. This
molarity was chosen based on past research that optimized the reaction efficiency
(Kanesan et al., 2017; Patankar et al., 2014; Sani et al., 2016). Yet, it should be noted
that the SH solution was prepared 24 hours prior to casting to allow for the heat
associated with the exothermic reaction to dissipate.
3.3.5 Steel fibers
Double hooked-end steel fibers from Bekaert were employed in this study. The
fibers had a specific gravity, mean diameter, mean length, and aspect ratio of 7.9, 0.55
mm, 35 mm, and 65, respectively (Bekaert, 2012).
3.3.6 Superplasticizer
A polycarboxylic ether polymer-based superplasticizer (SP) was supplied by
BASF Chemicals Company under the brand name Masterglenium Sky 504. It was
employed in this work to maintain adequate fresh geopolymer concrete workability
without affecting the mechanical properties (Montes et al., 2012; Palacios and Puertas,
2005).
34
3.4 Geopolymer concrete mixture proportions
The mixture proportions of geopolymer concrete mixtures are shown in Table
4. The control mixture (R0SF0) has been designed using trial and error to attain a
cylinder compressive strength of 35 MPa concrete. Another nine mixes were prepared
with varying RCA replacement percentages and steel fiber volume fractions. Mixtures
were designated by RxSFy, where x and y represent the RCA replacement percentage
and steel fiber volume fraction, respectively. For all mixes, equal portions of GGBS
and fly ash were used, i.e. 125 kg/m3. The purpose of using both precursor binders was
to eliminate the need for heat curing associated with fly ash-based geopolymer and
reduce the drying shrinkage related to alkali-activated slag. Such a blended system also
enhanced the performance of geopolymer concrete due to the co-existence of calcium
aluminosilicate hydrate (C-A-S-H) and sodium aluminosilicate hydrate (N-A-S-H)
gels (El-Hassan and Elkholy, 2019; Ismail and El-Hassan, 2018; Yip et al., 2005).
Dune sand content was kept constant at 910 kg/m3. Due to the difference in specific
gravities between NA and RCA, slight changes in the mix design were made with each
coarse aggregate replacement percentage (30, 70, and 100%). The AAS-to-binder ratio
was fixed at 0.60 with an SS-to-SH ratio of 1.5. Superplasticizer was added as 2% of
the total binder mass, while steel fibers were incorporated between 0 and 2%, by
volume.
35
Table 4: Mixture proportions of geopolymer concrete (kg/m3)
Mix
No.
Mix
Designation
Binder Aggregates
Activator
Solution
SP SF GGBS
Fly
ash NA RCA
Dune
sand SS SH
1 R0SF0 125 125 1210 0 910 90 60 5 0
2 R30SF0 125 125 847 346 910 90 60 5 0
3 R30SF1 125 125 847 346 910 90 60 5 78
4 R30SF2 125 125 847 346 910 90 60 5 156
5 R70SF0 125 125 363 798 910 90 60 5 0
6 R70SF1 125 125 363 798 910 90 60 5 78
7 R70SF2 125 125 363 798 910 90 60 5 156
8 R100SF0 125 125 0 1137 910 90 60 5 0
9 R100SF1 125 125 0 1137 910 90 60 5 78
10 R100SF2 125 125 0 1137 910 90 60 5 156
3.5 Sample preparation
Blended geopolymer concrete was prepared and cast in the laboratory at
respective temperature and relative humidity of 24°C ± 2°C and 50% ± 5%. Figure 7
shows the materials prior to mixing. The SH solution was first was prepared by mixing
the SH flakes with a specific amount of water to obtain the 14 M molarity. Once the
heat dissipated, the SH solution was mixed with the SS solution to formulate the AAS.
The heat from the secondary reaction was also allowed to dissipate. This two-step
process was carried out 24 hours prior to incorporation into the concrete mix. The dry
components, including NA and/or RCA, dune sand, fly ash, and GGBS were mixed
for 3 minutes. It is worth noting that the coarse aggregates were used in saturated
surface dry condition. The prepared AAS was then gradually added followed by the
superplasticizer to the dry components and mixed for another 3 minutes to attain a
36
homogeneous and uniform mix. The obtained freshly-mixed geopolymer concrete was
cast in two to three layers into 100 mm diameter x 200 mm height cylinders, 150 mm
diameter x 300 mm height cylinders, 100 mm cubes, and 100 mm height x 100 mm
width x 500 mm length prisms. Samples were then compact-vibrated on a vibrating
table for a period of 10 seconds. Compacted geopolymer concrete samples were
covered with a plastic sheet for 24 hours at ambient conditions, then demolded and
kept in ambient conditions until testing age. The process served to simulate on-site
construction scenarios while excluding water curing. Figure 8 presents some of the
demolded samples for a geopolymer concrete mix.
Figure 7: Materials used in casting geopolymer concrete
37
Figure 8: Geopolymer specimens after demolding
3.6 Performance evaluation
The mechanical properties of blended geopolymer concrete made with RCA
and SF were evaluated through the compressive strength, splitting tensile strength,
flexural strength, and modulus of elasticity. Conversely, the durability performance
was assessed using water absorption, sorptivity, bulk resistivity, and Los Angeles
abrasion. These tests were employed to evaluate the geopolymer concrete’s ability to
withstand abrasive forces and penetration of aggressive ions as an indication of its
durability. For each test, three replicate samples per mix were tested to obtain an
average.
3.6.1 Compressive strength
The concrete compressive strength test was performed using a Wykeham
Farrance machine with a loading capacity of 2000 kN and at a loading rate of 7 kN/s.
Cubes (100 mm) were employed to evaluate the development of compressive strength
(fcu) at the ages of 1, 7, and 28 days according to BS EN 12390-3 (British Standard,
38
2009). Furthermore, the cylinder compressive strength (f’c) was evaluated using 28-
day cylinders (100 mm diameter x 200 mm height) in accordance with ASTM C39
(ASTM, 2015).
3.6.2 Modulus of elasticity
The modulus of elasticity (Ec) of 28-day geopolymer concrete was obtained
according to ASTM C469 (ASTM, 2014). A Wykeham Farrance machine with a
loading capacity of 2000 kN was used. Four 60-mm-long strain gauges were installed
at mid-height on diametrically opposite sides of the circumference of 100 mm diameter
x 200 mm height cylinder to record the compression strain. A 500-kN compression
load cell was also used to determine the compressive load which was applied at a
loading rate of 7 kN/s. The load and strain were recorded using a data acquisition
system. Then, the compressive stress-strain response was plotted. The modulus of
elasticity was obtained as the slope of the chord connecting the stress corresponding
to 40% of the ultimate stress (S2) and that corresponding to a strain of 0.00005 (S1).
Equation 1 was employed in the calculation of the modulus of elasticity.
Ec=S2 - S1
ε2 - 0.00005
(1)
3.6.3 Splitting tensile strength
The tensile strength of 28-day blended geopolymer concrete made with RCA
and SF was indirectly measured through the splitting tensile strength (fsp) test of ASTM
C496 (ASTM, 2011). Cylinders of 150 mm diameter and 300 mm height were used.
The load was applied at a loading rate of 1 kN/s across the entire length of the
specimen. The splitting tensile strength was calculated using Equation 2.
39
fsp
=2P
πDL
(2)
Whereas P is the compressive load at failure in N, L is the length of the cylinder in
mm, and D is the diameter of the cylinder in mm.
3.6.4 Flexural strength
The modulus of rupture or flexural strength of geopolymer concrete made with
different proportions of RCA and SF was measured following the four-point bending
test of ASTM C1609 (ASTM, 2019b). The test was conducted on 28-day 100 mm
height x 100 mm width x 500 mm length prisms by means of an electro-hydraulic
servo-controlled machine at a loading rate of 1 kN/s. A linear variable displacement
transducer (LVDT) measured the mid-span deflection, while the load was recorded via
a load cell. The load-deflection curve was utilized to determine the peak strength or
flexural strength (fp), peak load deflection (δp), and residual strengths f150
100and f
600
100
corresponding to the loads at 0.75 and 3mm deflections. For all strength variables, the
strength was calculated using Equation 3.
f =PL
bd2
(3)
Where P is either the peak load (Pp), P150 100and P600
100 in N; L is the span length of the
specimen in mm, b is the width of the specimen in mm; and d is the depth of the
specimen in mm.
The load-deflection curves were also employed in calculating the geopolymer
concrete capacity to absorb energy or flexural toughness, denoted as T150 100. The value
of T150 100 was determined as the area under the load-deflection curve to a deflection of
L/150 and then used in finding the equivalent flexural strength ratio, RT,150 100 , as per
Equation 4.
40
RT,150 100 =
150 x T150 100
fpbd
2 x 100% (4)
3.6.5 Water absorption
The water absorption of blended geopolymer concrete was evaluated as per the
standard procedure of ASTM C642 (ASTM, 2013a). The test was conducted on 28-
day concrete disc specimens of 100 mm diameter and 50 mm height. The specimens
were dried in an oven at 105°C for 24 hours until a mass change < 0.5% was attained.
The recorded mass was denoted as “oven-dried mass”. The specimens were then
immersed in water for 24 hours, and the mass of the SSD sample was recorded as “SSD
mass”. The water absorption was quantified as the change in mass as a function of the
oven-dried mass, as per Equation 5.
Water absorption (%) =SSD mass (g) - Oven-dried mass (g)
Oven-dried mass (g)×100%
(5)
3.6.6 Sorptivity
The sorptivity is an indirect measure of the material’s short-term durability, as
it relates to the tendency of a material to absorb and transmit water and other liquids
by capillarity action. The sorptivity test was performed on 28-day concrete disc
samples, similar to those used in the water absorption test, following the procedure of
ASTM C1585 (ASTM, 2013c). Prior to testing, specimens were vacuum-saturated and
preconditioned as per ASTM C1202 (ASTM, 2012). The circumferential and top sides
of the samples were then sealed with adhesive tape. This allowed water to penetrate
from the bottom side only while ensuring no evaporation takes place during the test.
Sealed samples were then weighed and placed on supports resting at the bottom of a
water-filled pan. The water only reached 1-3 mm above the supports. The rate of water
41
absorption was determined by measuring the increase in the mass of a specimen as a
function of time. In fact, the mass of the geopolymer concrete specimen was recorded
at 1, 5, 10, 15, 20, 30, and 60 minutes and then after every hour until 6 hours. The
absorption (I) was then calculated as the change in mass divided by the product of the
cross-sectional area of the test specimen and the density of water, as per Equation 6.
Subsequently, the initial rate of water absorption value (mm/s1/2) was determined as
the slope of the line of the best fit of absorption against the square root of time (s1/2).
As per ASTM C1585 (ASTM, 2013c), the test shall be repeated if the regression
coefficient, R2, was less than 0.98.
I (mm) =Change in mass at time t(g)
Exposed area (mm2) × Density of water ( g mm3)⁄
(6)
3.6.7 Bulk electric resistivity
The resistivity of the concrete to the diffusion of chloride ions due to electrical
current is represented by the bulk electric resistivity. In fact, a higher resistivity is
indicative of higher protection against steel corrosion, as noted by ACI 222R-01 (ACI
Committee 222R-01, 2001). A 28-day cylinder (100 mm diameter x 200 mm height)
was preconditioned according to ASTM C1202 (ASTM, 2012) and saturated in
preparation for testing as per ASTM C1760 (ASTM, 2019a). The bulk resistivity was
carried out by placing the sample between two conductive plates with pre-attached
soaked sponges and using a Giatec RCON. Equation 7 was employed to determine the
bulk resistivity in Ω.cm.
Bulk Resistivity (Ω.cm) =Applied voltage (V) × (Avg. sample diameter (mm))
2
1273.2 × Current at 1 minute (mA) × Avg. sample length (mm) (7)
42
3.6.8 Abrasion resistance
To evaluate the blended geopolymer concrete resistance to abrasive, friction,
and rubbing action, the abrasion resistance was determined. It is an indication of the
probable future durability of the geopolymer concrete incorporating different
quantities of RCA and SF (ACI Committee 201.2, 2016; Mehta, 2006). For this test, a
Los Angeles (LA) abrasion machine was utilized to measure the mass loss in 28-day
geopolymer concrete due to abrasive and impact forces in accordance with the
procedure of ASTM C1747 (ASTM, 2013b). The mass of disc specimens (50 mm
height x 100 mm diameter) was recorded before and after every 100 revolutions for a
total of 500 revolutions. The abrasion resistance was determined as the percent mass
loss of the sample as a function of the initial mass, as per Equation 8.
Mass Loss (%) =Final Mass - Initial Mass
Initial Massx 100%
(8)
43
Chapter 4: Experimental Results and Discussions
4.1 Introduction
After determining the properties of the as-received material, the feasibility of
replacing NA by 100% RCA with the incorporation of steel fibers is investigated
through mechanical and durability performance evaluation. This chapter provides
evidence of such feasibility by testing the blended geopolymer concrete for
compressive strength, splitting tensile strength, flexural performance, modulus of
elasticity, water absorption, sorptivity, bulk resistivity, and abrasion resistance.
4.2 Mechanical Properties
4.2.1 Compressive Strength
The strength development profile of blended geopolymer concrete made with
different proportions of steel fibers and RCA replacement is studied by testing cube
samples for compressive strength (fcu) at the ages of 1, 7, and 28 days. For each mix
and test age, three cube specimens were tested. The results are shown in Figure 9. The
compressive strength of the control mix (R0SF0) at 1, 7, and 28 days was 9.1, 28.6,
and 35.3 MPa, respectively, representing corresponding increases of 214 and 23%
from 1 to 7 days and 7 to 28 days, as shown in Table 5. This shows that the
geopolymerization reaction mainly occurred within the first 7 days, owing to an
accelerated reaction of GGBS and production of calcium aluminosilicate hydrate and
calcium-silicate-hydrate gels (Al-Majidi et al., 2016; Chi, 2012; Davidovits, 2008;
Palomo et al., 1999). Temuujin et al. (2009) also concluded that the high molarity of
the SH solution may improve the reaction efficiency at an early age. Nevertheless,
strength increase was also noted between 7 and 28 days, signifying a continuous
44
reaction of the fly ash and the development of sodium-aluminosilicate-hydrate gel. An
analogous finding was noted by Ismail et al. (2014). Similar strength increase trends
were noted for other mixes irrespective of RCA replacement percentage and SF
volume fraction. Yet, it is worth noting that the increase in strength over time generally
decreased as more steel fiber was added to the geopolymer concrete mix. In fact, the
cube compressive strength increased by 210, 157, and 151%, on average, from 1 to 7
days for mixes made with 0, 1, and 2% SF, by volume. Conversely, the same mixes
showed a 12, 14, and 13% increase in strength from 7 to 28 days. This indicates that
the addition of steel fiber to the geopolymer concrete was mostly impactful at an early
age of 1 day.
(a)
Figure 9: Development of cubic compressive strength of concrete mixes made with
SF (a) 0%, (b) 1%, and (c) 2%
0
10
20
30
40
50
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Com
pre
ssiv
e S
tren
gth
, ƒcu
(M
Pa)
Time (days)
R0SF0R30SF0R70SF0R100SF0
Control Strength = 35.3 MPa
45
(b)
(c)
Figure 9: Development of cubic compressive strength of concrete mixes made with SF
(a) 0%, (b) 1%, and (c) 2% (continued)
The effect of RCA replacement percentage and steel fiber on the 1-day
compressive strength of blended geopolymer concrete is shown in Figure 10(a).
Generally, the replacement of NA by RCA did not have a significant impact on the 1-
day strength, owing to a possibly sufficient interfacial bond between the geopolymeric
0
10
20
30
40
50
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Co
mp
ress
ive
Str
ength
, ƒ
cu (
MP
a)
Time (days)
R30SF1
R70SF1
R100SF1
Control Strength = 35.3 MPa
0
10
20
30
40
50
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Com
pre
ssiv
e S
tren
gth
, ƒcu
(M
Pa)
Time (days)
R30SF2
R70SF2
R100SF2
Control Strength
=35.3 MPa
46
mortar and coarse aggregates (NA and RCA). Furthermore, the addition of steel fibers
enhanced the 1-day compressive strength. Compared to that of the plain counterparts,
it increased by, on average, 36 and 67%, when 1 and 2% steel fibers, by volume, were
incorporated into the geopolymer concrete mixes.
Table 5: Percent increase in cube compressive strength with time.
Mix
No.
Mix
Designation
Increasea
(%)
Increaseb
(%)
1 R0SF0 215.8 23.1
2 R30SF0 191.3 8.5
3 R30SF1 203.0 9.1
4 R30SF2 198.5 12.5
5 R70SF0 342.7 12.1
6 R70SF1 163.7 15.4
7 R70SF2 134.6 2.8
8 R100SF0 95.2 15.3
9 R100SF1 104.4 18.3
10 R100SF2 120.6 24.3
aIncrease in fcu from 1 to 7 days
bIncrease in fcu from 7 to 28 days
Figure 10(b) presents the 7-day compressive strength of blended geopolymer
concrete with different RCA replacement percentages and SF volume fractions. While
the replacement of NA by 30% RCA did not have a substantial effect on the 7-day
strength, higher replacements of 70 and 100% reduced the strength by 12 and 15%,
respectively. This loss of mechanical performance could be attributed to the weak
47
aggregate to geopolymeric paste interfacial bond and rough, porous nature of the RCA
(Salesa et al., 2017). Nevertheless, the strength was improved by, on average, 12 and
28%, upon adding 1 and 2% steel fibers, by volume. In fact, the 7-day strengths of
mixes made with 100% RCA were comparable to that of the control with the addition
of at least 1% steel fiber volume fraction.
Figure 10(c) highlights the changes in the 28-day cube compressive strength of
GGBS-fly ash blended geopolymer concrete. Values ranged between 28.1 and 43.4
MPa. While the control mix with 100% NA had a 28-day strength of 35.3 MPa, those
of mixes that replaced NA by 30, 70, and 100% RCA were 31.0, 28.2, and 28.1 MPa,
respectively. Clearly, a loss in strength was noted with RCA replacement. Similar
findings were reported in geopolymer concrete made with a single precursor, i.e. class
C fly ash, class F fly ash, or GGBS (Kathirvel and Kaliyaperumal, 2016; Nuaklong et
al., 2016; Shi, X. S. et al., 2012). However, such loss could be countered by adding 1
and 2% SF, by volume. Indeed, the addition of these quantities of SF increased the 28-
day strength of the plain RCA geopolymer concrete mixes by, on average, 11 and 24%.
This is mainly due to the densification of the matrix and reduction of the pore space,
as evidenced by the water absorption and sorptivity results presented later in the thesis.
In addition, steel fibers may have enhanced the structural integrity of the geopolymer
concrete, owing to its bridging effect. Other studies on NA-based geopolymer concrete
reported similar enhancements in compressive strength upon the addition of steel
fibers (El-Hassan and Elkholy, 2019; Islam et al., 2017). As a result, it could be
concluded that the full replacement of NA by RCA (100%) is possible in steel fiber-
reinforced geopolymer concrete incorporating at least 1% steel fibers, by volume,
while sustaining a limited loss (< 6%) in the 28-day compressive strength.
48
(a)
(b)
Figure 10: Cubic compressive strength for (a) 1-day, (b) 7-day and (c) 28-day
geopolymer concrete
0
10
20
30
40
50
60
30 70 100
1-D
ay C
om
pre
ssiv
e S
tren
gth
(M
Pa)
RCA (%)
SF0SF1SF2
Control Strength
= 9.1 MPa
0
10
20
30
40
50
60
30 70 100
7-D
ay C
om
pre
ssiv
e S
tren
gth
(M
Pa)
RCA (%)
SF0SF1SF2
Control Strength
= 28.6 MPa
49
(c)
Figure 10: Cubic compressive strength for (a) 1-day, (b) 7-day and (c) 28-day
geopolymer concrete (continued)
The effect of different RCA percentages and SF volume fractions on the 28-
day cylinder compressive strength of geopolymer concrete is depicted in Figure 11.
Similar to the 28-day cube compressive strength results, increasing the RCA
replacement percentage led to a decrease in the 28-day cylinder compressive strength.
In fact, 30, 70, and 100% RCA replacement resulted in 26, 51, and 54% respective
reduction in strength. This is possibly owed to the old adhered mortar surrounding the
RCA that created a weak interfacial zone with the geopolymeric paste. It is also likely
due to the poor quality and abundancy of cracks and voids of the RCA (Kachouh et
al., 2019b). Yet, the RCA-associated loss in strength seemed to be more pronounced
in the cylinder samples rather than the cube counterparts. Furthermore, the addition of
1 and 2% steel fibers, by volume, enhanced the strength of the RCA plain concrete by
up to 79 and 174%, respectively. As such, it is clear that the adverse impact of RCA
could be reversed by the addition of steel fibers.
0
10
20
30
40
50
60
30 70 100
28-D
ay C
om
pre
ssiv
e S
tren
gth
(M
Pa)
RCA (%)
SF0SF1SF2
Control Strength =
35.3 MPa
50
Figure 11: 28-day cylinder compressive strength of geopolymer concrete
The f′c/fcu ratio is shown in Table 6. The ratio of the control mix made with
100% NA and no steel fiber was 0.67. It decreased as more NA was replaced by RCA
in plain concrete. This signifies that the difference between cube and cylinder
compressive strength increased as more RCA was incorporated into the geopolymer
concrete mix. Apparently, the confinement effect of cubes under compression became
more critical with RCA replacement. Nevertheless, this ratio increased as more steel
fiber was added to the mix. In fact, it increased, on average, to 0.69 and 0.80 with 1
and 2% steel fiber volume fractions. Based on these results, it is apparent that a
relationship exists between the cylinder and cube compressive strengths of blended
geopolymer concrete at the age of 28 days, as illustrated in Figure 12. The relationship,
shown in Equation 9, indicates the ability to predict f’c from fcu (or vice versa) with
reasonable accuracy (R2 = 0.90).
f'c = 1.70fcu – 35.09 (9)
0
10
20
30
40
50
60
30 70 100
Co
mp
ress
ive
Str
ength
, ƒ
'c (
MP
a)
RCA (%)
SF0SF1SF2
Control Strength =
27.7 MPa
51
Table 6: Cylinder and cube compressive strength of 28-day geopolymer concrete
Mix
No.
Mix
Designation
f’c
(MPa)
fcu
(MPa) f’c / fcu
1 R0SF0 27.7 35.3 0.78
2 R30SF0 17.4 31.0 0.56
3 R30SF1 31.1 37.7 0.82
4 R30SF2 34.1 43.4 0.79
5 R70SF0 11.6 28.2 0.41
6 R70SF1 19.5 31.7 0.68
7 R70SF2 31.9 36.8 0.87
8 R100SF0 10.9 28.1 0.39
9 R100SF1 18.9 33.3 0.57
10 R100SF2 25.3 34.7 0.73
Figure 12: Relationship between cube and cylinder compressive strengths of
geopolymer concrete
f'c = 1.703fcu - 35.092
R² = 0.90
0
5
10
15
20
25
30
35
40
45
0 10 20 30 40 50
Cyli
nder
com
pre
ssiv
e st
ren
gth
(M
Pa)
Cube compressive strength (MPa)
52
4.2.2 Compressive stress-strain response
The effect of different SF volume fractions on the compressive stress-strain
behavior is examined by comparing mixes with the same RCA replacement, as shown
in Figure 13. The plots were obtained based on the procedure of section 3.6.2. The
peak stress increased by an average of 91, 73, and 72% for every 1% steel fiber volume
fraction added to the concrete mix incorporating 30, 70, and 100% RCA, respectively.
Furthermore, the strain at peak load increased, on average, 73 and 104% when 1 and
2% steel fibers were incorporated into the RCA mixes, by volume. Clearly, the
addition of steel fibers enhanced the compressive behavior and increased the
deformability of RCA blended geopolymer concrete, evidenced by the increase in peak
strain.
(a)
Figure 13: Typical compression stress-strain curves of cylinder concrete specimens
with RCA (a) 30%, (b) 70%, and (c) 100%
0
5
10
15
20
25
30
35
40
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Str
ess
(MP
a)
Strain (µε)
R30SF0
R30SF1
R30SF2
53
(b)
(c)
Figure 13: Typical compression stress-strain curves of cylinder concrete specimens
with RCA (a) 30%, (b) 70%, and (c) 100% (continued)
Figure 14 investigates the influence of RCA replacement percentage on the
compressive stress-strain behavior of blended geopolymer concrete. Regardless of the
SF volume fraction, the increase in RCA replacement led to a decrease in the peak
stress and slope of the stress-strain curve. This is primarily owed to the weaker
performance of RCA and lower modulus of elasticity of such mixes. Furthermore, the
0
5
10
15
20
25
30
35
40
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Str
ess
(MP
a)
Strain (µε)
R70SF0
R70SF1
R70SF2
0
5
10
15
20
25
30
35
40
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Str
ess
(MP
a)
Strain (µε)
R100SF0
R100SF1
R100SF2
54
peak strain generally increased as more RCA was incorporated into the mix. In
conclusion, it can be noted that steel fiber addition to RCA blended geopolymer
concrete promoted higher peak stress and strain compared to plain RCA counterparts.
(a)
(b)
Figure 14: Typical compression stress-strain curves of cylinder concrete specimens
with SF (a) 0%, (b) 1%, and (c) 2%
0
5
10
15
20
25
30
35
40
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Str
ess
(MP
a)
Strain (µε)
R0SF0R30SF0R70SF0R100SF0
0
5
10
15
20
25
30
35
40
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Str
ess
(MP
a)
Strain (µε)
R30SF1R70SF1R100SF1
55
(c)
Figure 14: Typical compression stress-strain curves of cylinder concrete specimens
with SF (a) 0%, (b) 1%, and (c) 2% (continued)
4.2.3 Modulus of Elasticity
The modulus of elasticity, Ec (in GPa), refers to the ability of a material to
sustain a sustained stress as the strain increases within the elastic limit. It was
determined by analyzing the stress-strain curves in accordance with ASTM C469
(ASTM, 2014). Figure 15 illustrates the modulus of elasticity of 28-day blended
geopolymer concrete mixes with different RCA replacement and SF volume fractions.
An increase in RCA replacement percentage led to a decrease in the modulus of
elasticity. In fact, 30, 70, and 100% RCA replacement in plain concrete decreased Ec
by 17, 36, and 63%, respectively. This reduction could be attributed to the weak and
porous structure of RCA, porous old adhered mortars, microcracks in the RCA, and
weak interfacial bond between the old mortar and aggregate. Other work on fly ash-
based geopolymer reported similar results (Shaikh, 2016). On the other hand, as the
SF volume fraction increased, the modulus of elasticity increased. For 30, 70, and
0
5
10
15
20
25
30
35
40
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Str
ess
(MP
a)
Strain (µε)
R30SF2
R70SF2
R100SF2
56
100% RCA replacement, the addition of 1% SF to plain counterparts increased Ec by
12, 3, and 23%, respectively, while 2% SF led to respective increases of 17, 12, and
41%. Such an increase in modulus is owed to an increase in compressive strength and
stress, S2, of Equation 1. Based on these results, it can be concluded that the effect of
RCA replacement on the modulus of elasticity was more prominent than the addition
of SF.
Figure 15: Modulus of elasticity of concrete mixes with different RCA replacement
percentages and SF volume fractions
The modulus of elasticity results showed herein were correlated to the 28-day
cylinder compressive strength. The developed analytical relationship is in the form of
Equation 10 and is shown in Figure 16. It is clear that a low correlation (R2 = 0.45)
exists between the two mechanical properties. This is due to the significant impact of
RCA replacement on the modulus of elasticity. Accordingly, a regression model
involving the RCA replacement percentage and f’c was developed. Equation 11
presents this relationship, where f’c is the 28-day cylinder compressive strength in MPa
and RCA is the RCA replacement percentage. Yet, it is mainly valid for f’c values
0
2
4
6
8
10
12
14
0 10 20 30 40 50 60 70 80 90 100
Modulu
s of
Ela
stic
ity (
GP
a)
RCA (%)
SF0SF1SF2
57
between 10 and 40 MPa. From the coefficients of Equation 11, it is clear that Ec and
f’c share a proportional relationship, signifying that an increase in f’c leads to higher Ec
values. In contrast, Ec and RCA replacement percentage are inversely proportional,
indicating that RCA has a negative impact on Ec.
Ec = 1.91√fc
'
(10)
Ec = 0.091f’c – 0.065RCA + 10.845 (11)
Figure 16: Modulus of elasticity of concrete mixes as a function of compressive
strength
The obtained relationship in Equation 11 is compared to those developed by
ACI Committee 318 (2014), CEB-FIP (Comité euro-international du béton and
Federation International de la Precontrainte, 1993), and AS3600 (2009). The codified
equations are presented in Table 7. Figure 17 depicts the predicted values of Ec versus
the experimental ones. With scatter plots converging around the 45°-line, it can be
noted that the model presented herein as Equation 11 was more accurate at predicting
the values of Ec. While the codified equations of ACI 318 and CEB-FIP significantly
Ec = 1.91√f'cR² = 0.45
0
2
4
6
8
10
12
14
0 1 2 3 4 5 6 7
Modulu
s of
Ela
stic
ity (
GP
a)
Compressive Strength, √f'c (MPa)
58
overestimated Ec, the model proposed by AS3600 only slightly overestimated it. In
fact, the error between the experimental and predicted Ec from equation of AS3600
increased as more RCA and SF were incorporated into the geopolymer concrete.
Clearly, the codified equations are unsuitable for the geopolymer concrete produced
in this work.
Table 7: Equations relating the modulus of elasticity to the compressive strength
Reference Modulus of Elasticity (Ec)
ACI Committee 318 0.043w1.5 f’c0.5
AS3600 0.024w1.5 (f’c0.5+0.12)
CEB-FIP 9979.4f’c0.33
Note: w = density of concrete (kg/m3)
f’c = compressive strength (MPa)
Figure 17: Experimental versus predicted modulus of elasticity
0
5
10
15
20
25
30
35
0 5 10 15 20 25 30 35
Pre
dic
ted E
c(G
Pa)
Experimental Ec (GPa)
Equation 11
ACI 318
AS3600
CEB-FIP
59
4.2.4 Splitting tensile strength
The splitting tensile strength (fsp) was utilized as an indirect method to evaluate
the tensile performance of 28-day GGBS-fly ash blended geopolymer concrete made
with RCA and SF, as highlighted in Figure 18. The results show a similar trend to that
of the cylinder compressive strength, where an increase in RCA replacement
percentage in plain geopolymer concrete led to a decrease in splitting tensile strength.
In fact, the values of fsp were reduced by 17, 37, and 47%, when 30, 70, and 100%
RCA replaced NA, respectively. Shaikh (2016) and Hu et al. (2019) reported a similar
adverse effect of RCA on fsp of geopolymer concrete made with fly ash or a blend of
fly ash and GGBS. In comparison, f’c decreased by 26, 51, and 54 for the same RCA
replacement percentages. This indicates that RCA replacement was more influential
on the compressive strength rather than splitting tensile strength, as noted as an
increase in the fsp-to-f’c ratio of Table 8.
Figure 18: Splitting tensile strength of 28-day geopolymer concrete
0
1
2
3
4
5
6
30 70 100
Ten
sile
Sp
litt
ing S
tren
gth
, ƒ
sp(M
Pa)
RCA (%)
SF0
SF1
SF2
Control
Strength
= 2.185 MPa
60
The effect of adding SF was also investigated. Figure 18 shows that the
addition of 1 and 2% SF, by volume, increased the splitting tensile strength by, on
average, 134 and 230% respectively, compared to the plain concrete counterparts. In
fact, fsp could reach up to 4.4 and 4.9 MPa compared to 2.2 MPa for the control. Other
work on NA-based geopolymer concrete reported similar enhancements in fsp when SF
was added to the mix (Bernal et al., 2010; El-Hassan and Elkholy, 2019; Islam et al.,
2017). Furthermore, the ratio of fsp-to-f’c (Table 8) increased with steel fiber addition,
which indicates that the steel fibers were more influential on fsp rather than f’c. It is
clear that the adverse impact of NA replacement by RCA on the splitting tensile
strength could not only be reversed by SF addition but could also exceed that of the
control. This is primarily owed to the steel fibers’ ability to bridge the microcracks and
increase the energy requirements for crack propagation.
Table 8: Ratio of tensile splitting strength to compressive strength
Mix
No.
Mix
designation
f'c
(MPa)
fsp
(MPa)
fsp / f'c
(%)
1 R0SF0 27.7 2.2 7.6
2 R30SF0 17.3 1.8 10.3
3 R30SF1 31.1 4.3 14.1
4 R30SF2 34.0 4.9 14.5
5 R70SF0 11.6 1.3 11.7
6 R70SF1 19.4 2.9 15.1
7 R70SF2 31.8 4.8 15.2
8 R100SF0 10.9 1.1 10.6
9 R100SF1 18.9 2.7 14.6
10 R100SF2 25.2 4.1 16.5
61
The splitting tensile strength of geopolymer concrete is usually predicted
through codified equations that it to the 28-day cylinder compressive strength. Such
equations are presented in Table 9. Nevertheless, an equation was developed to relate
these two properties, as per Equation 12 and shown in Figure 19. Based on the
correlation coefficient, R2, it is possible to predict the splitting tensile strength from
the cylinder compressive strength with reasonable accuracy (R2 = 0.80). To improve
the accuracy of the relationship between these two properties, the SF volume fraction
was added, owing to its clear impact on fsp. Accordingly, Equation 13 was developed
with a higher regression coefficient, R2, of 0.98.
fsp = 1.40√fc
' - 3.53
(12)
fsp = 0.74√fc
' + 0.98SF - 1.30
(13)
The experimental splitting tensile strength was plotted against that predicted
from the codified equations as well as Equations 12 and 13. Figure 20 shows that
models developed by ACI 318, AS3600, and CEB-FIP (ACI Committee 318, 2014;
AS3600, 2009; Comité euro-international du béton and Federation International de la
Precontrainte, 1993) could predict fsp with reasonable accuracy until a value of 3 MPa
after which these codes underestimated it. With a non-conventional blended
geopolymer concrete made with RCA and steel fibers, it can be concluded that codified
equations cannot be employed for the prediction of the splitting tensile strength.
62
Table 9: Equations relating splitting tensile strength and compressive strength
Reference Tensile strength (fsp)
ACI Committee 318 0.56 f’c0.5
AS3600 0.36 f’c0.5
CEB-FIP 0.30 f’c0.67
Figure 19: Correlation between splitting tensile strength and compressive strength
Figure 20: Experimental versus predicted splitting tensile strength
fsp = 1.40√f'c - 3.54
R² = 0.80
0
1
2
3
4
5
6
0 1 2 3 4 5 6 7
Spli
ttin
g t
ensi
le s
tren
gth
(M
Pa)
Compressive strength, √f'c (MPa)
0
1
2
3
4
5
6
0 1 2 3 4 5 6
Pre
dic
ted f
sp(M
Pa)
Experimental fsp (MPa)
Equation 12Equation 13ACI 318AS3600CEB-FIP
63
4.2.5 Flexural performance
The flexural performance of GGBS-fly ash blended geopolymer concrete
having various RCA replacement percentages and SF volume fractions is characterized
by the peak strength, peak deflection, residual strength, toughness, and equivalent
flexural strength ratio. Results are summarized in Table 10 and are discussed in detail
in the following sections.
Table 10: Flexural performance test results
Mix
Designation fp δp f
600
100 f
150
100 T150
100 RT,150 100
Unit MPa mm MPa MPa J %
R0SF0 3.5 0.32 - - 1.8 0.8
R30SF0 2.6 0.35 - - 2.5 1.4
R30SF1 4.7 0.56 3.48 2.64 28.1 9.0
R30SF2 6.4 1.08 - 5.51 57.4 13.5
R70SF0 1.7 0.49 - - 2.7 2.4
R70SF1 3.0 0.79 - 2.03 25.7 12.8
R70SF2 5.0 1.20 - 4.41 45.0 13.5
R100SF0 1.5 0.55 - - 2.9 2.9
R100SF1 2.9 0.86 - 2.03 22.0 13.4
R100SF2 4.0 1.44 - 2.70 44.0 16.5
64
4.2.5.1 Load-deflection curves
The flexural performance is evaluated using the load-deflection curves. Figure
21 illustrates the effect of RCA replacement percentage on the flexural behavior of 28-
day blended geopolymer concrete mixes made with different steel fiber volume
fractions. While three samples were tested per mix, one representative curve was
presented. Plain geopolymer concrete mixes are shown in Figure 21(a). The load
increased in a pseudo-elastic mode until failure. The increase in RCA replacement
caused a decrease in the slope of the load-deflection curve. In fact, 30, 70, and 100%
RCA were associated with respective decreases of 2, 55, and 66%, owing to a reduction
in the modulus of elasticity with RCA replacement.
Figures 21(b-c) illustrate the load-deflection curves of steel fiber-reinforced
RCA blended geopolymer concrete. These curves consist of two main parts. The first
part represents the increase until a peak load is attained. Its slope is controlled by the
modulus of elasticity of the concrete specimen. Clearly, the increase in RCA
replacement did not significantly change the slope regardless of steel fiber volume
fraction. In the second part of the curve, the post-peak softening behavior was
observed. Such a phenomenon was not noted in plain concrete samples. A post-peak
tail was noted for steel fiber-reinforced concrete mixes, owing to the incorporation of
steel fiber reinforcement. In fact, these steel fibers improved the integrity of the
geopolymer concrete by reducing crack development and propagation through their
bridging effect. The RCA replacement percentage seemed to have a limited effect on
the rate at which the residual strength was attained, while the value of the residual
strength was relevant to the peak strength.
65
(a)
Figure 21: Typical load-deflection curves of geopolymer concrete mixes with SF (a)
0%, (b) 1%, and (c) 2%
0
2
4
6
8
10
12
14
16
0 1 2 3 4 5
Lo
ad (
kN
)
Deflection (mm)
R0SF0
R30SF0
R70SF0
R100SF0
66
(b)
(c)
Figure 21: Typical load-deflection curves of geopolymer concrete mixes with SF (a)
0%, (b) 1%, and (c) 2% (continued)
The effect of adding steel fibers on the flexural load-deflection curves was
studied. For mixes made with 30% RCA [Figure 22(a)], the addition of 1 and 2% steel
fibers, by volume, increased the peak load by 43 and 158%, respectively, compared to
the plain counterpart. The deflection at peak load for these mixes also increased by
two and six times. However, the slope did not seem to be significantly impacted by the
addition of steel fibers.
0
2
4
6
8
10
12
14
16
0 1 2 3 4 5
Lo
ad (
kN
)
Deflection (mm)
R30SF1R70SF1R100SF1
0
2
4
6
8
10
12
14
16
0 1 2 3 4 5
Load
(kN
)
Deflection (mm)
R30SF2R70SF2R100SF2
67
Figure 22(b) shows the mixes with 70% RCA and different SF volume
fractions. The peak load, peak deflection, and slope of the first part of the curve
increased as more SF were incorporated into the mix. Such findings were associated
with the higher modulus of elasticity of SF-reinforced mixes. Additionally, past
research has reported that the bond between the matrix and steel fiber may have
delayed crack formation as the applied load was lower than the peak load (Bencardino
et al., 2013; Yoo et al., 2015). After the peak load was reached, cracks formed in the
geopolymeric matrix. Yet, these cracks were bridged by the steel fibers, thus, delaying
further crack propagation. Furthermore, the slope of the descending branch of the load-
deflection curve was nearly the same for 1 and 2% steel fiber volume fractions.
The effect of steel fiber addition on geopolymer concrete made with 100%
RCA is depicted in Figure 22(c). The load-deflection curves were similar to those of
mixes incorporating 70% RCA, except that the peak load was lower. Thus, it can be
concluded that the first part of the curve was mainly impacted by the RCA content,
while the second part was dependent on the SF volume fraction. Although RCA had a
negative impact on the flexural performance of blended geopolymer concrete, its effect
could be countered by SF addition. Not only that, but it could also enhance the flexural
performance beyond the control mix made with 100% NA.
68
(a)
(b)
Figure 22: Typical load-deflection curves of cylinder concrete specimens with RCA
(a) 30%, (b) 70%, and (c) 100%
0
2
4
6
8
10
12
14
16
0 1 2 3 4 5
Lo
ad (
kN
)
Deflection (mm)
R30SF0
R30SF1
R30SF2
0
2
4
6
8
10
12
14
16
0 1 2 3 4 5
Load
(kN
)
Deflection (mm)
R70SF0R70SF1R70SF2
69
(c)
Figure 22: Typical load-deflection curves of cylinder concrete specimens with RCA
(a) 30%, (b) 70%, and (c) 100% (continued)
4.2.5.2 Flexural strength
The flexural strength or modulus of rupture (fr) of 28-day GGBS-fly ash
blended geopolymer concrete made with different proportions of RCA and SF was
investigated. Table 11 presents the results. Similar to the splitting tensile strength, an
increase in RCA replacement percentage led to a loss in flexural strength. In fact,
replacing NA by 30, 70, and 100% RCA in plain geopolymer concrete reduced the
flexural strength by 25, 51, and 43%, respectively. Such decreases were analogous to
those reported in f’c. This indicates that RCA incorporation has a similar effect on fr
and f’c, as noted in the fr-to-f’c ratio of Table 11.
Figure 23 presents the flexural strength of steel fiber-reinforced blended
geopolymer concrete mixes made with different RCA replacement percentages. On
average, the addition of 1 and 2% steel fibers, by volume, increased the flexural
strength by 81 and 165%, respectively, compared to plain counterparts. Similar
0
2
4
6
8
10
12
14
16
0 1 2 3 4 5
Lo
ad (
kN
)
Deflection (mm)
R100SF0R100SF1R100SF2
70
conclusions were reported in the case of reinforcing NA-based geopolymer concrete
made with GGBS or a blend of GGBS and fly ash with steel fibers (Bernal et al., 2010;
Devika and Nath, 2015; El-Hassan and Elkholy, 2019; Islam et al., 2017). Using the
ratio of fr-to-f’c (Table 11), it seems that the steel fiber addition was more influential
on fr than f’c. Additionally, the adverse effect of RCA on fr could be countered by SF
addition. In fact, it is possible to produce blended geopolymer concrete mixes made
with 100% RCA and 2% steel fiber, by volume, with slightly superior flexural
performance than the 100% NA control.
The flexural strength results were correlated to those of the 28-day cylinder
compressive strength, as shown in Figure 24. The resultant analytical model, in the
form of Equation 14, can be used to predict the fr from f’c with reasonable accuracy
(R2 = 0.90). Nevertheless, an attempt was made to utilize the codified equations of
ACI 318, AS3600, and CEB-FIP (ACI Committee 318, 2014; AS3600, 2009; Comité
euro-international du béton and Federation International de la Precontrainte, 1993),
shown in Table 12, to predict the flexural strength. Figure 25 displays the results. The
equations provided by ACI 318 and AS3600 provide relatively good predictions with
values below 4 MPa, whereas CEB-FIP was more accurate above 4 MPa. Still, the
developed equation seems to be most suitable with an average error of 8%, while those
of ACI 318, AS3600, and CEB-FIP were 21, 22, and 25%, respectively.
fr = 1.59√fc
' – 3.92
(14)
71
Figure 23: Flexural strength of 28-day geopolymer concrete
Table 11: Flexural and cylinder compressive strength of 28-day geopolymer concrete
Mix
No.
Mix
designation
f'c
(MPa)
fr
(MPa)
fr / f'c
(%)
1 R0SF0 27.7 3.5 12.6
2 R30SF0 17.3 2.6 15.2
3 R30SF1 31.1 4.7 15.1
4 R30SF2 34.0 6.4 18.8
5 R70SF0 11.6 1.7 15.0
6 R70SF1 19.4 3.0 15.4
7 R70SF2 31.8 5.0 15.7
8 R100SF0 10.9 1.5 13.8
9 R100SF1 18.9 2.9 15.3
10 R100SF2 25.2 4.0 15.8
0
1
2
3
4
5
6
7
30 70 100
Fle
xu
ral S
tren
gth
, ƒ
r(M
Pa)
RCA (%)
SF0SF1
SF2
Control Strength =
3.50 MPa
72
Figure 24: Relationship between flexural and cylinder compressive strength of 28-
day geopolymer concrete
Table 12: Equations relating flexural strength and cylinder compressive strength
Reference Flexural strength (fr)
ACI Committee 318 0.62f’c0.5
AS3600 0.60f’c0.5
CEB-FIP 0.81f’c0.5
Figure 25: Experimental versus predicted flexural strength
y = 1.59√f'c - 3.92
R² = 0.90
0
1
2
3
4
5
6
7
0 1 2 3 4 5 6 7
Fle
xu
ral st
ren
gth
(M
Pa)
Compressive strength √f'c (MPa)
0
1
2
3
4
5
6
0 1 2 3 4 5 6
Pre
dic
ted
fr(M
Pa)
Experimental fr (MPa)
Equation 14
ACI 318
AS3600
CEB-FIP
73
4.2.5.3 Deflection
The structural ductility of blended geopolymer concrete made with different
proportions of RCA and SF was evaluated using the mid-span deflection. Based on the
results of Table 10, the values of the peak deflection (δp) ranged from 0.32 to 1.44 mm
with the respective lowest and highest deflections associated with the 100% NA
control mix and that made with 100% RCA and 2% SF, by volume. Apparently, steel
fiber incorporation into geopolymer concrete increased its deflection capacity. Similar
findings in conventional cement-based concrete associated this increase to steel fibers’
bridging effect and ability to reduce crack propagation (Gao and Zhang, 2018).
However, while ASTM C1609 (ASTM, 2019b) reports that first and peak deflection
corresponding to first and peak load are typically reported, the first load and deflection
were not present in the load-deflection curves of Figures 21 and 22. Therefore, the
focus of the work presented herein was on the peak deflection, δp.
Figure 26(a) highlights the effect of RCA replacement and SF incorporation on
the peak deflection. It is clear that an increase in the replacement of NA by RCA led
to an increase in the value of δp. Indeed, for plain mixes, the replacement of NA by 30,
70, and 100% RCA increased the peak deflection by 9, 53, and 72%, respectively.
Similar trends were noticed for steel fiber-reinforced mixes. Furthermore, the addition
of SF on the deflection of GGBS-fly ash blended geopolymer concrete was
investigated. Steel fiber volume fraction of 1 and 2% resulted in, on average, 59 and
171% respective increases in δp.
Figure 26(b) presents the change in peak deflection (δp) as a function of 28-day
cylinder compressive strength (f’c). For plain geopolymer concrete, the peak deflection
values did not exceed 0.55 mm. Moreover, concrete having higher compressive
strength, i.e. lower RCA replacement percentage, was characterized by a lower peak
74
deflection. Conversely, δp of steel fiber-reinforced counterparts with 1 and 2% steel
fibers could reach up to 0.86 and 1.44 mm, respectively. Furthermore, for a specific
compressive strength value, the addition of steel fibers significantly increased the
value of δp. Thus, it is apparent that the peak deflection is proportional to the RCA
replacement percentage and SF volume fraction and inversely proportional to the
compressive strength. Similar relationships were developed for conventional cement-
based RCA concrete reinforced with steel fibers (Gao et al., 2019; Gao and Zhang,
2018; Kachouh et al., 2020a; Yoo et al., 2015).
(a)
Figure 26: Deflection at peak load of concrete mixes with various (a) RCA
replacement percentage and SF volume fractions and (b) cylinder compressive
strength
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0 10 20 30 40 50 60 70 80 90 100
Def
lect
ion a
t pea
k (
mm
)
RCA Replacement (%)
SF0
SF1
SF2
75
(b)
Figure 26: Deflection at peak load of concrete mixes with various (a) RCA
replacement percentage and SF volume fractions and (b) cylinder compressive
strength (continued)
4.2.5.4 Residual strength
The concrete residual capacity at specific deflections after cracking is
characterized by the residual strength (ASTM, 2019b). In this work, the residual
strengths were determined at two deflection points, namely L/600 and L/150. As
shown in Table 10, no residual strength was reported for the plain concrete mixes, as
they did not experience any post-peak behavior. Also, the residual strengths of mixes
having the peak deflections larger than L/600 were left empty. As such, the main focus
of this work was on the residual strength at L/150, f150
100.
The effect of RCA replacement percentage and SF volume fraction on the
residual strength f150
100 was studied. Figure 27(a) shows that the replacement of NA by
RCA led to a decrease in the value of f150
100. Yet, such a decrease was more apparent
with a 2% steel fiber volume fraction with values dropping from 5.5 to 2.7 MPa when
RCA replacement increased from 30 to 100%. Furthermore, the incorporation of
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0 10 20 30 40
Def
lect
ion
at
pea
k (
mm
)
Compressive strength, f'c (MPa)
SF0
SF1
SF2
76
higher steel fiber volume fractions in the geopolymer concrete mix resulted in higher
residual strength. At 30, 70, and 100% RCA replacement, the increase of SF from 1 to
2%, by volume, led to 109, 117, and 33% respective increases in f150
100. Clearly, the
residual strength was positively impacted by the addition of steel fiber but negatively
affected by RCA replacement. Similar conclusions were reported in conventional
cement-based RCA concrete incorporating steel fibers (Gao et al., 2019; Gao and
Zhang, 2018; Kachouh et al., 2020a).
Figure 27(b) presents the relationship between residual strength and 28-day
cylinder compressive strength. For each steel fiber volume fraction, an increase in f’c,
i.e. decrease in RCA replacement, resulted in an increase in f150
100. However, this
increase was more pronounced at 2% steel fiber, by volume. In fact, f150
100 for 1 and 2%
SF increased by an average of 0.05 and 0.32 MPa, respectively, for every 1 MPa
increase in f’c.
(a)
Figure 27: Residual strength of concrete mixes with various (a) RCA replacement
percentage and SF volume fractions and (b) cylinder compressive strength
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0 10 20 30 40 50 60 70 80 90 100
Res
idu
al s
tren
gth
f1
50
(MP
a)
RCA Replacement (%)
SF1
SF2
77
(b)
Figure 27: Residual strength of concrete mixes with various (a) RCA replacement
percentage and SF volume fractions and (b) cylinder compressive strength
(continued)
4.2.5.5 Flexural toughness
The flexural toughness is a measure of concrete’s energy absorption capacity.
Table 10 summarizes the toughness of blended geopolymer concrete incorporating
different quantities of RCA and SF. Values ranged between 1.8 and 57.4 J. It seems
that the addition of SF and replacement of NA by RCA increased the toughness.
Figure 28(a) presents the relationship between the flexural toughness and
varying mixture proportions, i.e. RCA replacement percentage and SF volume
fraction. For plain concrete, increasing the RCA content had an insignificant impact
on the toughness. Yet, it was more impactful as steel fiber was added to the mix. In
fact, the toughness decreased from 28 to 22 J and 57 to 44 J when RCA replacement
increased from 30 to 100% in mixes made with 1 and 2% steel fibers, by volume,
respectively. Additionally, the effect of steel fiber incorporation was investigated. The
toughness of plain blended geopolymer concrete mixes increased by, on average, 9
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0 10 20 30 40
Res
idu
al s
tren
gth
f1
50
(MP
a)
Compressive strength, f'c (MPa)
SF1
SF2
78
and 18 times when 1 and 2% steel fiber volume fractions were added, respectively.
This is primarily owed to the bridging effect of steel fibers and their ability to reduce
crack propagation and increase flexural performance.
The flexural toughness is studied with respect to the cylinder compressive
strength in Figure 28(b). For plain geopolymer concrete, an increase in strength, i.e.
lower RCA replacement percentage, led to a slight increase in toughness. Yet, this
effect amplified upon incorporating 1 and 2% steel fibers, by volume, with respective
toughness increase by 0.5 and 1.5 J for every 1 MPa increase in strength. Furthermore,
it is worth noting that plain and steel fiber-reinforced mixes having the same
compressive strength had very different toughness values. For example, a 20-MPa
concrete had the toughness of 2.5 and 25 J with 0 and 1% steel fibers. Clearly,
toughness was significantly more influenced by the incorporation of steel fibers than
RCA replacement.
(a)
Figure 28: Flexural toughness of concrete mixes with various (a) RCA replacement
percentage and SF volume fractions and (b) cylinder compressive strength
0
10
20
30
40
50
60
70
0 10 20 30 40 50 60 70 80 90 100
Fle
xura
l T
oughnes
s T
15
0(J
)
RCA Replacement (%)
SF0
SF1
SF2
79
(b)
Figure 28: Flexural toughness of concrete mixes with various (a) RCA replacement
percentage and SF volume fractions and (b) cylinder compressive strength
(continued)
4.2.5.6 Equivalent flexural strength ratio
Table 10 presents the equivalent flexural strength ratio (RT,150 100 ). Values for
plain geopolymer concrete ranged between 0.8 and 1.9%, while those of steel fiber-
reinforced RCA counterparts reached up to 26.3%. This shows that steel fiber addition
and RCA replacement had a positive impact on the ratio.
Figure 29(a) illustrates the effect of SF addition and RCA replacement in more
detail. The replacement of NA by 30, 70, and 100% RCA increased the ratio by 100,
112, and 137%, respectively, compared to the NA-based control. This is attributed to
the increase in toughness and decrease in peak load in plain RCA concrete mixes. Yet,
such a positive impact on RT,150 100 was more pronounced in steel fiber-reinforced
concrete. In fact, mixes made with 1 and 2% steel fiber, by volume, noted average 12
and 16% increases in RT,150 100 for every 10% NA replaced by RCA. Furthermore, the
0
10
20
30
40
50
60
70
0 10 20 30 40
Fle
xu
ral T
ou
gh
nes
s T
15
0(J
)
Compressive Strength, f'c (MPa)
SF0
SF1
SF2
80
effect of steel fiber on the equivalent flexural strength ratio was evaluated. For 1 and
2% steel fiber volume fractions added to the geopolymer concrete mixes, the value of
RT,150 100 increased by, on average, 6 and 9 times, regardless of the amount of NA being
replaced by RCA. This highlights the much more significant impact of SF on the RT,150 100
compared to that of RCA replacement.
(a)
(b)
Figure 29: Equivalent flexural strength ratio of concrete mixes with various (a) RCA
replacement percentage and SF volume fractions and (b) cylinder compressive
strength
0
2
4
6
8
10
12
14
16
18
0 10 20 30 40 50 60 70 80 90 100
Equiv
alen
t R
atio
RT
,15
0(%
)
RCA Replacement (%)
SF0
SF1
SF2
0
2
4
6
8
10
12
14
16
18
0 10 20 30 40
Eq
uiv
alen
t R
atio
RT
,15
0(%
)
Compressive Strength, f'c (MPa)
SF0
SF1
SF2
81
The relationship between equivalent flexural strength ratio and cylinder
compressive strength is presented in Figure 29(b). It is clear that RT,150 100 tended to
decrease with higher strength and more RCA replacement. For plain concrete, RT,150 100
decreased by an average of 0.05% for every 1 MPa increase in compressive strength.
Yet, this trend was more prominent as more steel fiber was incorporated into the mix.
For steel fiber-reinforced geopolymer concrete, every 1 MPa increase in compressive
strength resulted in an average 0.4% decrease in RT,150 100 . Furthermore, plain and steel
fiber-reinforced mixes having equal compressive strength had different equivalent
flexural strength ratios. For instance, a 20-MPa concrete with 0 and 1% steel fiber, by
volume, had RT,150 100 values of 1.2 and 12%, respectively. Thus, it can be concluded that
the incorporation of steel fibers had a more significant effect on the equivalent flexural
strength ratio than RCA replacement.
4.3 Durability properties
4.3.1 Water absorption
The durability of concrete can be evaluated by the rate at which harmful agents
penetrate the concrete. In fact, concrete undergoes deterioration and damage due to the
ingress of moisture or other aggressive liquids through the interconnected pores. As
water is the primary carrier of aggressive ions, the ability of concrete to absorb water,
characterized by its water absorption, can give a good indication of its durability
(Mehta, 2006). Figure 30 illustrates the results of water absorption tests at the age of
28 days. It is observed that the water absorption increased with an increase in RCA
replacement. While the control specimen (R0SF0) had the lowest water absorption of
3.8%, mixes made with 30, 70, and 100% RCA had values of 5.0, 6.6, and 6.8%,
82
respectively. This represents 31, 73, and 76% respective increases in the water
absorption compared to the control mix. Such a finding correlates well with the
decrease in mechanical properties, including compressive, splitting tensile, and
flexural strength. It can therefore be noted that the water absorption increased by 7%,
on average, for every 10% NA replaced by RCA, owing to the porous structure of the
adhered mortar to the RCA and additional void sites in the RCA.
In addition, Figure 30 shows that the water absorption decreased as more SF
was incorporated into the mix. The addition of 1 and 2% SF volume fractions
compared to concretes without SF resulted in average reductions in water absorption
of 4 and 13%, respectively. Clearly, the incorporation of steel fibers into blended
geopolymer concrete made with RCA densified the matrix, leading to a decrease in the
water absorption and an increase in mechanical performance. Bernal et al. (2010)
reported similar findings for GGBS-based geopolymer concrete made with NA and
steel fibers.
Figure 30: Effect of RCA and SF percentages on the water absorption of 28-day
geopolymer concrete
0
1
2
3
4
5
6
7
8
0 10 20 30 40 50 60 70 80 90 100
Wat
er A
bso
rpti
on (
%)
RCA (%)
SF0
SF1
SF2
83
4.3.2 Sorptivity
Sorptivity is a commonly used test to indirectly assess the durability of
concrete, as it relates to the tendency of concrete to absorb and transport water by
capillary action through its microstructure. In fact, the rate of absorption, i.e. sorptivity,
of water depends on the strength of capillary forces, the permeability of the concrete,
the porosity of the concrete, and structure and distribution of the pores (Mehta, 2006;
Neville, 1996). Sorptivity is categorized into two types: initial and secondary. The
initial and secondary sorptivity are governed by the sorption processes through the
large capillary and small gel pores, respectively (Martys and Ferraris, 1997). Because
water occupies the former pores faster than the latter ones, the initial sorptivity is
typically higher than the secondary sorptivity. Accordingly, this work only focused on
the initial sorptivity.
To find the rate of absorption or sorptivity, the change in weight of a concrete
sample over the square root of time is plotted as a scatter line graph, as shown in Figure
30. Within the first 60 minutes, the slope of the sorptivity curve was higher than the
remaining time of exposure. This is mainly a result of the corresponding early and late
filling of large and small pores. Figure 31(a) presents the sorptivity curves of plain
concrete mixes without SF. An increase in RCA replacement percentage led to an
increase in the slope and to higher water absorption, owing to the poor quality and
porous nature of the RCA and its adhered old mortar. It is also possible that RCA may
have cracks and fissures that had developed during the manufacturing process.
The effect of SF on the rate of water absorption is shown in Figure 31(b-c).
Higher steel fiber volume fractions led to lower slopes and water absorption.
Apparently, the steel fibers restricted the movement of water and occupied the larger
84
void space in the geopolymer concrete structure. Other work noted similar conclusions
in steel fiber-reinforced conventional concrete (Ramadoss and Nagamani, 2008; Şanal,
2018). As such, the sorptivity decreased.
(a)
(b)
Figure 31: Absorption of concrete mixes over time: (a) SF 0%; (b) SF 1%; (c) SF2%
0
1
2
3
4
5
6
7
8
9
10
0 15 30 45 60 75 90 105 120 135 150
I (m
m)
Time (√s)
R0SF0R30SF0R70SF0R100SF0
0
1
2
3
4
5
6
7
8
9
10
0 15 30 45 60 75 90 105 120 135 150
I (m
m)
Time (√s)
R30SF1
R70SF1
R100SF1
85
(c)
Figure 31: Absorption of concrete mixes over time: (a) SF 0%; (b) SF 1%; (c) SF2%
(continued)
Table 13 summarizes the sorptivity results of 28-day GGBS-fly ash blended
geopolymer concrete mixes. The sorptivity values of plain concrete increased as more
NA was replaced by RCA. Compared to the NA-based control, the replacement of 30,
70, and 100% RCA increased the sorptivity by 40, 80, and 136%, respectively.
Conversely, steel fiber incorporation decreased the sorptivity of the RCA-based
mixtures, as they may have filled the geopolymer concrete voids. On average, the
addition of SF by 1 and 2%, by volume, resulted in a decrease in the sorptivity by 5.3%
and 13.9 %, respectively, compared to that of RCA-based plain concrete mixes. It is
possible that the steel fibers may have reduced the absorption and sorptivity, owing to
an improvement in the bond within the binding matrix (Tamrakar, 2012). Similar
findings were reported in conventional cement-based steel fiber-reinforced NA and
RCA concrete (Kachouh et al., 2019a; Ramadoss and Nagamani, 2008; Şanal, 2018).
0
1
2
3
4
5
6
7
8
9
10
0 15 30 45 60 75 90 105 120 135 150
I (m
m)
Time (√s)
R30SF2R70SF2R100SF2
86
Table 13: Water absorption and initial rate of absorption of geopolymer concrete
Mix
No.
Mix
Designation
RCA
(%)
SF
(%)
Water
Absorption
(%)
Initial Rate of
Absorption
(mm/√s)
1 R0SF0 0 0 3.80 0.025
2 R30SF0 30 0 4.96 0.035
3 R30SF1 30 1 4.52 0.034
4 R30SF2 30 2 4.28 0.033
5 R70SF0 70 0 6.57 0.045
6 R70SF1 70 1 6.24 0.043
7 R70SF2 70 2 5.41 0.038
8 R100SF0 100 0 6.7 0.059
9 R100SF1 100 1 6.77 0.054
10 R100SF2 100 2 6.12 0.047
4.3.3 Bulk resistivity
The concrete bulk resistivity is an indirect measure of the concrete durability.
Due to the conductive nature of steel fibers and the generation of unrepresentative
results, mixes incorporating 1 and 2% SF volume fractions were not considered in the
analysis. Accordingly, the results of the plain blended geopolymer mixes made with
different RCA replacement percentages were considered, as shown in Figure 32.
Generally, a decreasing resistivity trend was noticed as more RCA replaced NA with
values ranging between 2.0 and 4.6 kΩ.cm. In fact, the bulk resistivity was found to
decrease by 30, 52, and 57% compared to the control mix when RCA replacement was
30, 70, and 100%, respectively. Apparently, the most significant impact was noticed
87
up to a replacement of 70%; however, a higher replacement percentage of 100% had
limited effect. This is possibly due to the increase in the pore space of the geopolymeric
structure when 70% of NA was replaced by RCA to the extent that higher proportions
of RCA replacement did not cause a further reduction.
Based on these findings, it was believed that the bulk resistivity was related to
the 28-day water absorption and cylinder compressive strength. As such, correlations
between the former and latter two properties were developed, as shown in Figure 33.
Strong linear correlations exist between bulk resistivity and each of water absorption
and compressive strength with high correlation coefficients, R2, of 0.98. These
relationships are in the form of Equations 15 and 16, where BR and WA are the bulk
resistivity in kΩ.cm and water absorption in percent, respectively. They may be used
to predict the values of WA and f’c from bulk resistivity, which is a non-destructive
test.
f’c = 5.72BR (15)
WA = -1.15BR + 8.96 (16)
Figure 32: Bulk resistivity of 28-day blended geopolymer concrete mixes
0.0
1.0
2.0
3.0
4.0
5.0
6.0
R0SF0 R30SF0 R70SF0 R100SF0
Bu
lk R
esis
tivit
y (
kΩ
.cm
)
Mix Designation
88
Figure 33: Relationship between bulk resistivity and each of water absorption and
compressive strength
4.3.4 Abrasion resistance
The abrasion resistance of concrete is directly related to the mass loss after
exposing the samples to 500 revolutions in an LA abrasion machine. It is mainly
impacted by the mechanical properties of the concrete and the hardness of the
aggregates. In this work, the mass loss of blended geopolymer concrete mixtures with
different RCA and SF proportions was recorded every 100 revolutions until 500
revolutions to monitor the mass loss profile, as shown in Figure 34. In general, the
highest rate of mass loss was noted within the first 300 revolutions after which the rate
tended to decrease. Figure 34(a) presents the results of the plain concrete mixes. While
the control mix had a mass loss of 82% after 500 revolutions, those of mixes made
with 30, 70, and 100% RCA replacement were 82, 95, and 100%, respectively. This is
primarily owed to the higher porosity and inferior properties of RCA, the weak bond
between the old and new paste, and the porous structure of the adhered mortar. Other
f'c = 5.72BR
R² = 0.98
WA = -1.15BR + 8.96
R² = 0.98
0
1
2
3
4
5
6
7
8
0
5
10
15
20
25
30
0 1 2 3 4 5
Wat
er A
bso
rpti
on
(%
)
Com
pre
ssiv
e S
tren
gth
(M
Pa)
Bulk Resistivity (kΩ.cm)
89
work on conventional concrete made with RCA noted similar findings (Kachouh et al.,
2019a; Wu et al., 2011). Steel fiber-reinforced blended geopolymer concrete mixes
made with different RCA replacement percentages are shown in Figure 34(b-c). An
analogous trend to that of plain concrete was noted, whereby an increase in RCA
replacement led to an increase in the abrasion mass loss, i.e. less resistance to abrasive
loads. It can thus be concluded that replacing NA with RCA had a negative impact on
the abrasion resistance of GGBS-fly ash blended geopolymer concrete.
(a)
Figure 34: Abrasion resistance of geopolymer concrete made with SF (a) 0%, (b) 1%,
and (c) 2%
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
Abra
sion M
ass
Loss
(%
)
Number of Revolutions
R0SF0R30SF0R70SF0R100SF0
90
(b)
(c)
Figure 34: Abrasion resistance of geopolymer concrete made with SF (a) 0%, (b) 1%,
and (c) 2% (continued)
Figure 35 shows the effect of adding steel fiber reinforcement on the abrasion
resistance of blended geopolymer concrete made with 30, 70, and 100% RCA. For
mixes incorporating 30% RCA, the addition of 1 and 2% SF, by volume, reduced the
abrasion mass loss by 28 and 38%, respectively, compared to the plain counterpart.
Conversely, mixes made with 70 and 100% RCA showed respective reductions of up
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
Ab
rasi
on
Mas
s L
oss
(%
)
Number of Revolutions
R30SF1R70SF1R100SF1
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
Abra
sion M
ass
Loss
(%
)
Number of Revolutions
R30SF2
R70SF2
R100SF2
91
to 33 and 19% upon adding SF. This shows that the addition of steel fibers led to
improved geometric integrity and a more densified geopolymer concrete structure
capable of resisting abrasive and impact loading more effectively. In fact, the mix
made with 100% RCA and 2% steel fiber volume fraction showed comparable
abrasion resistance to that of the control mix.
(a)
(b)
Figure 35: Abrasion resistance of geopolymer concrete mixes made with RCA (a)
30%, (b) 70%, and (c) 100%
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
Abra
sion M
ass
Lo
ss (
%)
Number of Revolutions
R30SF0
R30SF1
R30SF2
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
Ab
rasi
on M
ass
Loss
(%
)
Number of Revolutions
R70SF0
R70SF1
R70SF2
92
(c)
Figure 35: Abrasion resistance of geopolymer concrete mixes made with RCA (a)
30%, (b) 70%, and (c) 100% (continued)
Based on the obtained results, it seems that the abrasion mass loss and
compressive strength are related. As such, these two properties were correlated using
a linear regression model. Figure 36 shows that an inversely proportional relationship
exists. It is shown in the form of Equation 17, where AR represents the abrasion
resistance mass loss in percent. Using this equation, it is possible to predict the
abrasion resistance of blended geopolymer concrete made with RCA and SF with
reasonable accuracy (R2 = 0.83).
AR = -1.70f'c + 116.35 (17)
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
Ab
rasi
on
Mas
s L
oss
(%
)
Number of Revolutions
R100SF0
R100SF1
R100SF2
93
Figure 36: Relationship between abrasion mass loss and cylinder compressive
strength
AR = -1.70f'c + 116.35
R² = 0.83
0
10
20
30
40
50
60
70
80
90
100
10 15 20 25 30 35 40
Ab
rasi
on
mas
s lo
ss (
%)
Compressive Strength (MPa)
94
Chapter 5: Numerical Modelling
5.1 Introduction
This chapter examines the effect of varying SF volume fraction and RCA
replacement percentages on the shear behavior of GGBS-fly ash blended geopolymer
concrete beams. The nonlinear analysis was conducted using the finite element (FE)
software ATENA. This software was used for the simulation and modeling of the
reinforced geopolymer concrete as a cost- and time-effective alternative to laboratory
testing. Flexural strength test results are employed to develop tensile softening
relationships using an inverse FE analysis. Ten three-dimensional (3D) FE beam
models were created to simulate and evaluate the shear behavior of concrete beams
made of the 10 mixes of the current study. The developed tensile softening
relationships and other concrete mechanical properties measured experimentally
served as input data in the FE analysis. The modeled reinforced geopolymer concrete
beams were designed to fail in shear. The results obtained from the FE analysis
included load-deflection curves, crack patterns, failure modes, and peak loads.
5.2 Geometry of the beam
The structural behavior of ten beams, representing the 10 geopolymer concrete
mixes, were investigated utilizing the suggested ATENA finite elements model. They
were designed in such a way to induce a shear mode of failure in the left segment.
Each beam had a span length of 2000 mm and a shear span-to-depth ratio (a/h) of 2.4.
All beams had a rectangular cross-section with a width and height of 150 and 250 mm,
respectively. The beams had no shear reinforcement within the shear span region,
while the remaining parts of the beams had 8 mm diameter stirrups spaced at 75 mm
95
distance except last stirrups at both edges with 50 mm spacing. The load was located
at one-third the length of the beam from the support. The longitudinal tensile and
compressive steel reinforcements consisted of 2 No. 20 (20 mm in diameter) and 4 No.
20 (20 mm in diameter) steel bars, respectively. The concrete cover to the center of the
steel reinforcement was 38 mm, rendering an effective depth of d = 198 mm. The steel
plates at the load and support points were 150 x 150 x 20 mm (length x width x
thickness). The dimensions and reinforcement details of geopolymer concrete beams
are illustrated in Figures 37 and 38.
Figure 37: Modeled beam cross-section view (dimensions in mm)
96
Figure 38: Modeled beam elevation view (dimensions in mm)
5.3 Finite Element Modeling
The mechanical properties of concrete were used as input data for defining the
behavior of the geopolymer material. Modeled beams had the same dimensions and
steel reinforcement but different mechanical properties based on the SF volume
fraction and RCA percentage. A typical 3D un-deformed shape of the developed model
is shown in Figure 39.
Figure 39: Typical FE model for geopolymer concrete beam
97
An iterative solution procedure based on the standard Newton-Raphson
method was implemented in the FE analysis in ATENA. The modeled beams were
loaded by a displacement-controlled incremental vertical loading method at the middle
of the top steel loading plate surface. Each step had a 0.1 mm change in the
displacement. The load at the middle of the top plate and the mid-span displacement
were monitored. The concrete beams were modeled using solid 3D brick macro-
elements with 8 nodes. The concrete element size was taken as 25 mm. The load and
support plates were modeled using solid 3D brick macro-elements. They were
connected to the beam through fixed contacts. The end support plates were restrained
from movement in the transverse and vertical directions (y and z directions,
respectively). These restrictions were applied by means of a support line placed at the
middle of the bottom surface of the plate. The end support plates were free to move in
the longitudinal direction (x-direction).
To obtain the numerical data, several monitoring points were added to the FE
models. The numerical values for the applied load, deflection, and steel strains were
attained utilizing these monitoring points. Table 14 shows the input parameters for all
types of monitoring points utilized in the FE models. The type and value specify the
required measurement which will be monitored nearest to the coordinates provided in
the location input. The component number indicates the direction of the monitored
value. Components 1, 2, and 3 represent the directions in X, Y, and Z, respectively.
Table 14: Input parameters of monitoring points
Title Type Value Item
Load Value at node Reaction Component 3
Deflection Value at node Displacement Component 3
98
5.4 Material Constitutive Laws
The software provides built-in material constitutive models that require
minimal user input. Furthermore, it provides alternative models where the user has
space to manually adjust constitutive models of the materials.
5.4.1 Plain Geopolymer Concrete
For concrete properties, the actual concrete strengths measured experimentally
were used for the geopolymer concrete beams. The CC3DNonLinCementitious2
concrete material model of the FE package was used to simulate the plain concrete
(NA and RCA). The properties entered in ATENA included modulus of elasticity (Ec),
cubic compressive strength (fcu), cylinder compressive strength (f’c), uniaxial tensile
strength (ft), assumed as 0.6fr, and a constant Poisson’s ratio (ν) of 0.2. Table 15 present
the mechanical properties of the mixes based on specimens tested in Chapter 4. Figure
40 shows the material constitutive laws of plain concrete in tension and compression.
Although the trend of the constitutive models of the plain geopolymer concrete was
assumed the same as that of a conventional concrete, key parameters such as
compressive strength, tensile strength, and Young’s modulus of the geopolymer
concrete that were measured experimentally were used as input data in the analysis.
99
Table 15: Mechanical properties of the geopolymer concrete mixes
Mix
No.
Mix
Designation
fcu
(MPa)
f’c
(MPa)
Ec
(GPa)
ft
(MPa)
1 R0SF0 35.3 27.7 12.6 2.1
2 R30SF0 31.0 17.4 10.4 1.6
3 R30SF1 37.7 31.1 11.6 2.8
4 R30SF2 43.4 34.1 12.2 3.8
5 R70SF0 28.2 11.6 8.1 1.0
6 R70SF1 31.7 19.5 8.3 1.8
7 R70SF2 36.8 31.9 9.1 3.0
8 R100SF0 28.1 10.9 4.7 0.9
9 R100SF1 33.3 18.9 5.8 1.7
10 R100SF2 34.7 25.3 6.7 2.4
(a)
Figure 40: Constitutive laws of plain concrete: (a) compressive hardening law; (b)
compressive softening law; (c) tensile softening law (Alkhalil and El-Maaddawy,
2017; Awani et al., 2016)
100
(b)
(c)
Figure 40: Constitutive laws of plain concrete: (a) compressive hardening law; (b)
compressive softening law; (c) tensile softening law (Alkhalil and El-Maaddawy,
2017; Awani et al., 2016) (continued)
5.4.2 Reinforcing steel
The finite element modeling of steel reinforcement is much simpler than
concrete modeling, where the steel reinforcing was connected between nodes. Steel
reinforcement was modeled as one-dimensional discrete reinforcement embedded into
the concrete beam. All reinforcing steel bars were assumed to have a bilinear stress-
strain relation modeled using the (CCReinforcement) material model, as shown in
Figure 41(a). The yield strength and modulus of elasticity were assumed to be 550
101
MPa and 200 GPa, respectively, for longitudinal and shear reinforcement. Figure 41(b)
presents a typical arrangement of steel reinforcement in the FE models.
5.4.3 Steel Plates
In the finite element models, steel plates have been added at the locations of
the support and load to prevent stress concentration problems. The steel plates were
modeled using the (CC3DElastIsotropic) material model with an elastic modulus
equal to 200 GPa and a Poisson ratio of 0.3.
(a)
(b)
Figure 41: Steel reinforcing bars (a) Stress-strain relationship and (b) Arrangement of
steel reinforcement in the FE models
5.4.4 Steel Fiber-Reinforced Geopolymer Concrete
To account for the presence of steel fibers on the material constitutive laws,
CC3DNonLinCementit2User has been used. Figure 42 shows a typical compressive
stress-strain of concrete adapted in this study. By testing concrete prisms under four-
point bending and applying an inverse FE analysis, the tensile function of SF
reinforced concretes was developed. The tension function (tensile softening) is needed
102
for modeling the response and post-cracking behavior of steel fiber-reinforced
geopolymer concrete beams. To establish a tension function by an inverse analysis, the
load-deflection responses of the tested four-point bending prisms, shown in Chapter 4,
were utilized. Then, ATENA software was used to perform numerical analysis on the
prism through the 3D FE model, as shown in Figure 43. In ATENA, the
CC3DNonLinCementit2User model allows users to define the tensile properties and
softening behavior of concrete with steel fibers. Specific tensile parameters were set
in the prism model as an initial procedure for inverse analysis. The numerically
predicted load-deflection response was then compared to the load-deflection response
obtained from the four-point bending test results of Chapter 4. The input tensile
function was adjusted with many iterations until the load-deflection attained from the
tension function was similar to that of the experimental test. Figures 44-49 show the
numerical and experimental load-deflection curves of the prisms having SF-reinforced
geopolymer concrete mixes with the corresponding tension function.
Figure 42: Typical compression stress-strain constitutive law of
CC3DNonLinCementit2User
0
0.2
0.4
0.6
0.8
1
1.2
0 0.002 0.004 0.006 0.008
Norm
aliz
ed S
tres
s, f
/f' c
Strain
103
Figure 43: Concrete prism model used for inverse analysis
(a)
(b)
Figure 44: Inverse analysis results of R30SF1: (a) experimental and predicted load-
deflection curves and (b) corresponding tension function
0
3
6
9
12
15
0 1 2 3 4 5
Load
(kN
)
Deflection (mm)
Exp. R30SF1
Pred. R30SF1
0
0.2
0.4
0.6
0.8
1
1.2
0 0.05 0.1 0.15
No
rmal
ized
Str
ess,
f/
f t
Strain
104
(a)
(b)
Figure 45: Inverse analysis results of R30SF2: (a) experimental and predicted load-
deflection curves and (b) corresponding tension function
0
3
6
9
12
15
0 1 2 3 4 5
Load
(kN
)
Deflection (mm)
Exp. R30SF2Pred. R30SF2
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6
Norm
aliz
ed S
tres
s,
f/f t
Strain
105
(a)
(b)
Figure 46: Inverse analysis results of R70SF1: (a) experimental and predicted load-
deflection curves and (b) corresponding tension function
0
3
6
9
12
15
0 1 2 3 4 5
Load
(kN
)
Deflection (mm)
Exp. R70SF1Pred. R70SF1
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6
Norm
aliz
ed S
tres
s,
f/f t
Strain
106
(a)
(b)
Figure 47: Inverse analysis results of R70SF2: (a) experimental and predicted load-
deflection curves and (b) corresponding tension function
0
3
6
9
12
15
0 1 2 3 4 5
Load
(kN
)
Deflection (mm)
Exp. R70SF2Pred. R70SF2
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6
Norm
aliz
ed S
tres
s, f/
f t
Strain
107
(a)
(b)
Figure 48: Inverse analysis results of R100SF1: (a) experimental and predicted load-
deflection curves and (b) corresponding tension function
0
3
6
9
12
15
0 1 2 3 4 5
Load
(kN
)
Deflection (mm)
Exp. R100SF1Pred. R100SF1
0
0.2
0.4
0.6
0.8
1
1.2
0 0.05 0.1 0.15
Norm
aliz
ed S
tres
s,
f/f t
Strain
108
(a)
(b)
Figure 49: Inverse analysis results of R100SF2: (a) experimental and predicted load-
deflection curves and (b) corresponding tension function
5.5 Results and discussion
5.5.1 Load-deflection curves
The effect of RCA percentage on the the load-deflection relationships of the
25 mm mesh models is examined through comparing mixes with the same SF
percentages, as illustrated in Figure 50. Conversely, the impact of SF volume fraction
on the load-deflection relationships is evaluated by comparing mixes with the same
RCA replacement, as depicted in Figure 51. The numerical load-deflection curves
shown in these figures are stopped shortly after reaching the ultimate load.
0
3
6
9
12
15
0 1 2 3 4 5
Load
(kN
)
Deflection (mm)
Exp. R100SF2
Pred. R100SF2
0
0.2
0.4
0.6
0.8
1
1.2
0 0.05 0.1 0.15
Norm
aliz
ed S
tres
s,
f/f t
Strain
109
Figure 50(a) shows that concrete beam models made without SF exhibited a
load decay at the onset of crack initiation. After cracking, the deflection increased at a
higher rate. It is apparent that the increase in replacement percentage of RCA resulted
in a significant decrease in the slope. In fact, the replacement of 30, 70, and 100%
RCA resulted in a 9, 19, and 30% reduction in slope, owing to a lower modulus of
elasticity, as noted in the results of Chapter 4. Figures 50(b-c) present the load-
deflection curves of GGBS-fly ash geopolymer concrete made with SF and RCA.
Similar to plain concrete counterparts, higher RCA replacement led to a decrease in
slope. Yet, in these concrete mixes, a slight change in slope occurred at deflections
between 1-2 mm and 2-3 mm for 1 and 2% steel fiber volume fractions, respectively.
Such a phenomenon is associated with the improved integrity of the geopolymer
concrete due to steel fiber incorporation, leading to a reduction in crack development
and propagation.
(a)
Figure 50: Load-deflection response of concrete models with SF (a) 0%, (b) 1%, (c)
SF 2%
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7
Load
(kN
)
Deflection (mm)
R0SF0
R30SF0
R70SF0
R100SF0
110
(b)
(c)
Figure 50: Load-deflection response of concrete models with SF (a) 0%, (b) 1%, (c)
SF 2% (continued)
The impact of steel fiber addition on the load-deflection curves was
investigated in Figure 51. Regardless of the RCA replacement percentage, the
incorporation of steel fibers led to an increase in the slope. In addition, it resulted in
an increase in peak deflection. This is especially apparent in mixes made with 70 and
100% RCA. Such findings were associated with the higher modulus of elasticity of
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7
Load
(kN
)
Deflection (mm)
R30SF1
R70SF1
R100SF1
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7
Load
(kN
)
Deflection (mm)
R30SF2
R70SF2
R100SF2
111
SF-reinforced mixes while previous studies noted a delayed crack formation due to the
bond between the matrix and steel fiber (Bencardino et al., 2013; Yoo et al., 2015).
Even after these cracks formed, steel fibers bridged them and delayed their further
propagation. It could be thus concluded that although RCA had a negative effect on
the shear performance of GGBS-fly ash blended geopolymer concrete, its impact could
be countered by SF addition.
(a)
Figure 51: Load-deflection response of concrete models with RCA (a) 30%, (b) 70%,
and (c) 100%
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7
Load
(kN
)
Deflection (mm)
R30SF0
R30SF1
R30SF2
112
(b)
(c)
Figure 51: load-deflection response of concrete models with RCA (a) 30%, (b) 70%,
and (c) 100% (continued)
5.5.2 Crack Patterns and Failure Modes
At every applied load step, the ATENA program records a crack pattern. The
developed crack patterns, minimum and maximum principal strains recorded
numerically by ATENA at the peak load for the test models are illustrated in Figures
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7
Lo
ad (
kN
)
Deflection (mm)
R70SF0
R70SF1
R70SF2
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7
Load
(kN
)
Deflection (mm)
R100SF0R100SF1R100SF2
113
52-61. The minimum width of the displayed crack in FE models was set to be 0.1 mm.
Concrete beams without steel fibers experienced either one single diagonal shear crack
within the shear span connecting the support and load points or a diagonal shear crack
connected to a longitudinal crack along the tensile steel reinforcing bars. The concrete
beams without steel fibers eventually failed in a diagonal tension or shear tension mode
of failure. Apparently, the addition of steel fibers restricted the growth of shear cracks
and delayed the shear failure. Similar findings were reported in the modeling of
cement-based conventional concrete incorporating steel fibers and RCA (Kachouh et
al., 2020b). Furthermore, concrete beams with steel fibers exhibited a shear-tension
mode of failure, which included a shear crack in the vicinity of the load point with an
angle of inclination of 45 to 60o connected to a longitudinal crack running parallel to
the tensile steel reinforcing bars. This shear-tension mode of failure usually occurs in
concrete beams without stirrup, as the absence of stirrups’ confinement promotes
development of horizontal cracking along the tension reinforcement.
114
(a)
(b)
(c)
Figure 52: R0SF0 FE models: (a) crack patterns, (b) maximum principal strains, and
(c) minimum principal strains
115
(a)
(b)
(c)
Figure 53: R30SF0 FE models: (a) crack patterns, (b) maximum principal strains,
and (c) minimum principal strains
116
(a)
(b)
(c)
Figure 54: R30SF1 FE models: (a) crack patterns, (b) maximum principal strains,
and (c) minimum principal strains
117
(a)
(b)
(c)
Figure 55: R30SF2 FE models: (a) crack patterns, (b) maximum principal strains,
and (c) minimum principal strains
118
(a)
(b)
(c)
Figure 56: R70SF0 FE models: (a) crack patterns, (b) maximum principal strains,
and (c) minimum principal strains
119
(a)
(b)
(c)
Figure 57: R70SF1 FE models: (a) crack patterns, (b) maximum principal strains,
and (c) minimum principal strains
120
(a)
(b)
(c)
Figure 58: R70SF2 FE models: (a) crack patterns, (b) maximum principal strains,
and (c) minimum principal strains
121
(a)
(b)
(c)
Figure 59: R100SF0 FE models: (a) crack patterns, (b) maximum principal strains,
and (c) minimum principal strains
122
(a)
(b)
(c)
Figure 60: R100SF1 FE models: (a) crack patterns, (b) maximum principal strains,
and (c) minimum principal strains
123
(a)
(b)
(c)
Figure 61: R100SF2 FE models: (a) crack patterns, (b) maximum principal strains,
and (c) minimum principal strains
5.5.3 Peak Load
The numerical finite element model was applied to evaluate the shear behavior
of the SF-reinforced RCA geopolymer concrete beams. The peak loads of such beams
are compared in Figure 62. The replacement of NA by 30, 70, and 100% RCA led to
124
8, 32, and 33% respective decreases in the peak load. This is aligned with other
mechanical properties and owed to the generally weaker and more porous nature of
the RCA, as explained in Chapter 4. Nevertheless, this adverse impact could be
countered by the addition of steel fibers. Results showed that the peak load increased
with the incorporation of steel fibers. In fact, the addition of 1 and 2% steel fibers, by
volume, to RCA geopolymer mixes increased the peak load, on average, by 34 and
65%, respectively. Based on these findings, it can be concluded GGBS-fly ash blended
geopolymer concrete could be made with 100% RCA without compromising shear
behavior, subject that 2% steel fibers, by volume, be incorporated into the mix.
Figure 62: Peak loads of geopolymer concrete beams
The experimental shear resistance results were correlated to those of the 28-
day cylinder compressive strength (f’c in MPa) and steel fiber volume fraction (vf in
%) to introduce an empirical equation to predict the nominal shear resistance (Vn in N)
of GGBS-fly blended geopolymer concrete beams. As such, an analytical relationship
was developed using multivariable linear regression in the form of Equation 18, where
0
20
40
60
80
100
120
30 70 100
Pea
k L
oad
(kN
)
RCA Replacement (%)
SF0
SF1
SF2Control = 80.2 kN
125
b and d are the width and effective depth of the beam, respectively. The feasibility of
utilizing this newly-proposed equation in predicting the nominal shear resistance is
investigated through Figure 63. It can be seen that the scatter plot mainly converges
around the 45°-line, indicating a good accuracy of Equation 18. In fact, the error
between the experimental and predicted values did not exceed 8%.
Vn = 0.366bd√fc
' + 2.41vf
(18)
Figure 63: Predicted versus experimental shear resistance
0
20
40
60
80
0 20 40 60 80
Pre
dic
ted S
hea
r R
esis
tance
(kN
)
Experimental Shear Resistance (kN)
126
Chapter 6: Conclusions
6.1 Introduction
The main aim of this study is to investigate the feasibility of reutilizing locally
available industrial solid wastes and RCA in geopolymer concrete for structural
applications. A combination of ground granulated blast furnace slag (GGBS) and fly
ash were used to form a blended precursor binding material. To advance the use of
RCA in structural geopolymer concrete, steel fiber reinforcement was incorporated at
different volume fractions. This proposed geopolymer concrete promises to offer an
innovative and sustainable solution to ever-increasing global environmental issues,
including the consumption of non-renewable natural resources, emission of carbon
dioxide, and production of industrial waste materials.
The research work comprised experimental testing to study the effect of steel
fibers and RCA on the performance of GGBS-FA blended geopolymer concrete. The
experimental program involved the testing of ten geopolymer concrete mixes made
with different RCA replacement percentages (0, 30, 70, and 100%) and SF volume
fractions (0, 1, and 2%). The mechanical and durability properties under investigation
included compressive and splitting tensile strength, flexural performance, modulus of
elasticity, absorption, sorptivity, abrasion resistance, and bulk resistivity. Additionally,
numerical models that characterize the shear behavior of GGBS-FA blended
geopolymer concrete made with different proportions of RCA and steel fibers were
developed. The FE models accounted for the nonlinear behavior of concrete in tension
and compression adopting realistic materials laws. It analyzed the failure modes, crack
patterns, load-deflection response, and failure ultimate loads. The main findings of this
work and recommendations for future work are outlined in this chapter.
127
6.2 Limitations
The findings of this thesis are limited to the types of GGBS and fly ash
employed herein. The coarse and fine aggregates were dolomitic limestone and dune
sand. The alkaline activator solution was formulated from grade N sodium silicate and
14M sodium hydroxide. The steel fibers utilized were in the form of double hooked
end steel fibers with specific dimensions. Furthermore, the numerical results were
limited to the specific dimensions of the beams considered in the analysis. Variations
in the chemical compositions or physical properties of the mixture components or
beam dimensions may result in different outcomes than those presented in this work.
6.3 Conclusions
The following conclusions can be drawn based on the experimental testing and
numerical modeling result:
• The cube compressive strength at the ages of 1, 7, and 28 days were reduced
by up to 37, 14, and 20%, respectively, upon RCA replacement, compared to
the control mix. Yet, this loss of cube compressive strength could be countered
by adding steel fibers. In fact, the addition of up to 2% steel fibers, by volume,
increased the 1-, 7-, and 28-day cube compressive strength by as much as 67,
28, and 24% in comparison to the plain counterparts. This signifies the superior
impact of steel fibers at the age of 1 day. As such, it was possible to produce a
GGBS-FA blended geopolymer concrete made with 100% RCA and at least
1% steel fiber, by volume, while sustaining a limited loss (<6%) in cube
compressive strength.
• The 28-day cylinder compressive strength presented similar results as that of
the 28-day cube strength. A linear regression model correlating these two
128
properties was developed. With a correlation coefficient, R2, of 0.90, it was
deemed possible to predict one of these properties from the other with
reasonable accuracy.
• The compressive stress-strain curves noted a decrease in peak stress and an
increase in peak strain due to RCA replacement. Conversely, the addition of
steel fibers increased the peak stress and peak strain, leading to enhanced
deformability due to the bridging effect of steel fibers. The slope of the curves,
i.e. modulus of elasticity, decreased with RCA replacement and increased with
steel fiber incorporation. Yet, the adverse impact of the former was more
prominent than the positive effect of the latter.
• Experimental test results of the splitting tensile strength of GGBS-FA blended
geopolymer concrete followed a similar trend to the 28-day cylinder
compressive strength. Indeed, the replacement of NA by RCA resulted in up to
47% decrease in fsp. On the other hand, the addition of steel fibers increased
the value of fsp by up to 230% compared to the plain counterpart. Using the
ratio of fsp-to-f’c, it was noted that the RCA replacement was more influential
on f’c, while steel fiber incorporation was more impactful on fsp. Results
showed that it is possible to replace NA by 100% RCA while adding at least
1% steel fiber, by volume, without compromising the splitting tensile strength
of GGBS-FA blended geopolymer concrete.
• The flexural performance of GGBS-FA blended geopolymer concrete was
characterized by peak strength, peak deflection, residual strength, toughness,
and equivalent flexural strength ratio. The increase in RCA replacement caused
decreases in the slope of the load-deflection curve, fr, and f150
100, increases in δp
and RT,150 100 , and insignificant change in T150
100. On the other hand, the addition of
129
steel fibers increased all the flexural characteristics. Yet, it should be noted that
the positive impact of steel fiber incorporation surpassed the negative influence
of RCA replacement. As such, RCA could be used as the sole aggregate in
GGBS-FA blended geopolymer concrete subject that 2% steel fiber volume
fraction is used.
• The 28-day cylinder compressive strength (f’c) of GGBS-FA blended
geopolymer concrete was correlated to several mechanical and durability
properties, including the modulus of elasticity, splitting tensile strength, and
flexure strength. With R2 > 0.90, all regression models were noted to present
good correlations among the properties. As such, it is possible to predict these
properties from f’c with reasonable accuracy. Conversely, codified equations
were less accurate in predicting these properties.
• The abrasion mass loss continuously increased over the first 300 revolutions
after which the curve tended to plateau. The RCA replacement led to higher
mass loss due to abrasion, owing to the weaker RCA and more porous nature
compared to NA. In turn, the addition of steel fibers reduced the abrasion mass
loss to the point that the value of the mix made with 100% RCA and 2% steel
fiber volume fraction was comparable to that of the NA-based control mix.
Also, it was noted that abrasion mass loss was inversely proportional to f’c. The
correlation model (R2 = 0.87) could be used to predict the former from the latter
with reasonable accuracy.
• The bulk resistivity of GGBS-FA blended geopolymer concrete varied between
2 and 4.6 kΩ.cm with lower values being associated with mixes made with
higher RCA replacement percentages. The bulk resistivity was aligned with the
28-day cylinder compressive strength and water absorption. As such,
130
correlation models were developed to predict the 28-day compressive strength
and water absorption from this non-destructive test with reasonable accuracy
(R2 > 0.98).
• The water absorption and sorptivity were proportional to the RCA replacement
with values increasing by up to 76 and 136%, respectively, compared to the
NA-based control mix, owing to the porous nature of the aggregates. Yet, the
absorption decreased with steel fiber addition, providing evidence to the
densification of the matrix and enhancement of the mechanical properties.
• Tensile softening relations for GGBS-fly ash blended geopolymer concrete
made with RCA and steel fibers were established based on inverse analyses of
the experimental four-point bending/flexure data. Without these relations, it is
not possible to simulate the behavior of such geopolymer concrete beams using
finite element analysis.
• Three-dimensional finite element models were developed and used to simulate
the nonlinear shear behavior of geopolymer concrete beams with different
RCA percentages and steel fiber volume fractions. The use of steel fibers
improved the deflection response and the peak loads of the modeled beams.
• Based on a multivariable linear regression analysis of the numerical shear
resistance of the modeled beams, an analytical relationship was established to
predict the nominal shear resistance of geopolymer concrete beams having
different cylinder compressive strengths and steel fiber volume fractions.
6.4 Recommendations for Future Studies
This research work investigated the feasibility of utilizing geopolymer concrete
as an alternative to ordinary cement concrete and can be considered a promising step
131
towards implementing geopolymer concrete in structural applications. However,
further research is recommended for future studies as follows:
• Investigate the effect of different steel fibers with various geometry, lengths,
and aspect ratios.
• Examine the effect of different contents and types of superplasticizers on the
workability of GGBS-FA blended geopolymer concrete made with RCA and
steel fibers.
• Study the resistance of GGBS-FA blended geopolymer concrete to elevated
temperatures, seawater exposure, and acid and sulfate attack.
• Evaluate the performance of GGBS-FA blended geopolymer concrete
incorporating other waste materials in ternary and quaternary mixes.
• Conduct laboratory testing to verify the results of the numerical models of the
geopolymer concrete beams developed in the current study.
• Perform a lifecycle assessment analysis to verify the feasibility of utilizing
GGBS-FA blended geopolymer concrete in structural applications.
132
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Appendix
Table A1. Mixture proportions of trial mixes
Mix #
Component (kg/m3) Total
(kg/m3) GGBS FA SS SH RCA NA DS SP SF
1 250 250 179 71 0 1000 650 10 0 2410
2 250 250 179 71 0 1000 650 10 156 2566
3 250 250 179 71 1000 0 650 10 0 2410
4 250 250 179 71 1000 0 650 10 156 2566
5 225 225 161 64 0 1100 600 9 0 2384
6 225 225 161 64 0 1100 600 9 156 2540
7 225 225 161 64 1100 0 600 9 0 2384
8 225 225 161 64 1100 0 600 9 156 2540
9 200 200 132 88 0 1160 675 8 0 2463
10 200 200 132 88 1160 0 675 8 0 2463
11 200 200 132 88 1160 0 675 8 156 2619
12 180 180 119 80 0 1180 675 7 0 2422
13 180 180 119 80 1180 0 675 7 0 2422
14 180 180 119 80 1180 0 675 7 156 2577
15 180 180 130 86 0 1160 695 7 0 2438
16 180 180 130 86 1160 0 695 7 156 2594
17 160 160 115 77 0 1180 708 6 0 2406
18 160 160 115 77 1180 0 708 6 156 2562
19 150 150 108 72 0 1180 740 6 0 2406
20 150 150 108 72 1180 0 740 6 156 2562
21 125 125 90 60 0 1210 910 5 0 2405
22 125 125 90 60 1210 0 910 5 156 2561
143
Table A2. Mixture ratios of trial mixes
Mix #
Ratios
SF (%) FA/GGBS AAS/B SS/SH CA/DS RCA/DS Agg/B SP/B (%)
1 1.00 0.50 2.52 1.54 0.00 3.30 2.00 0.00
2 1.00 0.50 2.52 1.54 0.00 3.30 2.00 2.00
3 1.00 0.50 2.52 0.00 1.54 3.30 2.00 0.00
4 1.00 0.50 2.52 0.00 1.54 3.30 2.00 2.00
5 1.00 0.50 2.52 1.83 0.00 3.78 2.00 0.00
6 1.00 0.50 2.52 1.83 0.00 3.78 2.00 2.00
7 1.00 0.50 2.52 0.00 1.83 3.78 2.00 0.00
8 1.00 0.50 2.52 0.00 1.83 3.78 2.00 2.00
9 1.00 0.55 1.50 1.72 0.00 4.59 2.00 0.00
10 1.00 0.55 1.50 0.00 1.72 4.59 2.00 0.00
11 1.00 0.55 1.50 0.00 1.72 4.59 2.00 2.00
12 1.00 0.55 1.49 1.75 0.00 5.15 2.00 0.00
13 1.00 0.55 1.49 0.00 1.75 5.15 2.00 0.00
14 1.00 0.55 1.49 0.00 1.75 5.15 2.00 2.00
15 1.00 0.60 1.51 1.67 0.00 5.15 2.00 0.00
16 1.00 0.60 1.51 0.00 1.67 5.15 2.00 2.00
17 1.00 0.60 1.50 1.67 0.00 5.90 2.00 0.00
18 1.00 0.60 1.50 0.00 1.67 5.90 2.00 2.00
19 1.00 0.60 1.50 1.59 0.00 6.40 2.00 0.00
20 1.00 0.60 1.50 0.00 1.59 6.40 2.00 2.00
21 1.00 0.60 1.50 1.50 0.00 8.00 2.00 0.00
22 1.00 0.60 1.50 0.00 1.50 8.00 2.00 2.00
144
Table A3. Compressive strength development of trial mixes
Mix #
Compressive Strength (MPa) Development (%)
1-day 7-day 28-day 1 to 7 1 to 28
1 41.13 66.03 73.07 60.54 77.66
2 39.00 76.00 87.13 94.87 123.41
3 30.67 45.27 48.07 47.60 56.73
4 29.93 41.10 42.27 37.32 41.23
5 43.33 79.67 76.63 83.87 76.85
6 38.47 78.10 81.90 103.02 112.89
7 20.57 32.50 37.83 58.00 83.91
8 25.40 41.60 39.83 63.78 56.81
9 39.50 59.70 64.10 51.14 62.28
10 23.66 30.40 35.10 28.48 48.35
11 27.86 33.75 40.86 21.14 46.66
12 32.60 62.00 62.00 90.18 90.18
13 21.11 33.44 31.40 58.41 48.74
14 25.30 39.22 39.40 55.02 55.73
15 35.34 58.10 62.00 64.40 75.44
16 23.86 36.20 37.00 51.72 55.07
17 33.76 57.76 67.20 71.09 99.05
18 23.28 36.89 41.10 58.46 76.55
19 25.00 51.00 58.83 104.00 135.32
20 15.86 33.26 37.83 109.71 138.52
21 9.07 28.64 35.30 215.76 289.20
22 12.51 24.42 28.10 95.20 124.62
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