Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Fraser, Tanya Leigh
Master of Applied Science - MASc
University of British Columbia
Fabric Reinforced Cementitious Matrix (FRCM) is a relatively new material used for strengthening applications of reinforced concrete structures. Several studies reported in the literature have indicated that the bond between FRCM layers and the concrete substrate is one of the main factors that affect the performance of the system. Few studies have attempted a combined approach of analyzing the surfaces between the three layers of this composite (fibres-matrix-concrete). An investigation of the bond behavior is studied in this thesis with an emphasis on the fibre-matrix and the concrete-matrix interfaces. The efficacy of surface preparation methods and the existing concrete strength were used to evaluate the bond at the concrete-matrix interface. The fibre-mortar width ratio was used to analyze the bond at the fibre-matrix interface. This experiment was completed in two phases: 72 concrete blocks (150x150x165mm) in double shear testing and 12 small concrete beams (150x150x500mm) in flexural testing. For double shear, the varying parameters included 1) surface preparation (untreated, grinding, sandblasting, shotblasting, and combination), 2) fibre-mortar width ratio (1:1 and 1:2) with fibre widths of 120mm and 60mm, and 3) concrete strength (30MPa and 50MPa). The double shear test results indicated that the bond strength increased with enhanced methods of surface preparation. Surface grooving reduced the debonding response, and combined surface preparation methods were effective at eliminating debonding failures. Also, the fibre-mortar ratio of 1:2 exhibited higher bond strength and reduced bond failure than the 1:1 ratio. The concrete strength had an insignificant effect on the bond strength and failure mode. For flexural specimens, the surface preparation method was examined. The surface preparation had a significant effect on the bond strength, and all failures occurred at the fibre-matrix interface. The bond strength was on average 44.5% higher for the flexural than for the double shear specimens.
INTERFACIAL BOND STRENGTH OF FABRIC REINFORCED CEMENTITIOUS MATRIX: EFFECT OF SUBSTRATE PROPERTIES AND FIBRE-MORTAR RATIO by Tanya Leigh Fraser B.A.Sc., University of British Columbia, Kelowna, 2018 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Civil Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) April 2020 © Tanya Leigh Fraser, 2020 ii The undersigned certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis entitled: INTERFACIAL BOND STRENGTH AND FAILURE MODE ANALYSIS OF FABRIC REINFORCED CEMENTITIOUS MORTAR: EFFECT OF SUBSTRATE PROPERTIES AND FIBRE-MORTAR RATIO Submitted by: Tanya Fraser in partial fulfillment of the requirements of the degree of Master of Applied Science Dr. Ahmad Rteil, School of Engineering Supervisor, Professor (please print name and faculty/school above the line) Dr. Abbas Milani, School of Engineering Supervisory Committee Member, Professor (please print name and faculty/school in the line above) Dr. Shahria Alam, School of Engineering Supervisory Committee Member, Professor (please print name and faculty/school in the line above) Dr. Rudolf Seethaler, School of Engineering University Examiner, Professor (please print name and faculty/school in the line above) iii ABSTRACT Fabric Reinforced Cementitious Matrix (FRCM) is a relatively new material used for strengthening applications of reinforced concrete structures. Several studies reported in the literature have indicated that the bond between FRCM layers and the concrete substrate is one of the main factors that affect the performance of the system. Few studies have attempted a combined approach of analyzing the surfaces between the three layers of this composite (fibres-matrix-concrete). An investigation of the bond behavior is studied in this thesis with an emphasis on the fibre-matrix and the concrete-matrix interfaces. The efficacy of surface preparation methods and the existing concrete strength were used to evaluate the bond at the concrete-matrix interface. The fibre-mortar width ratio was used to analyze the bond at the fibre-matrix interface. This experiment was completed in two phases: 72 concrete blocks (150x150x165mm) in double shear testing and 12 small concrete beams (150x150x500mm) in flexural testing. For double shear, the varying parameters included 1) surface preparation (untreated, grinding, sandblasting, shotblasting, and combination), 2) fibre-mortar width ratio (1:1 and 1:2) with fibre widths of 120mm and 60mm, and 3) concrete strength (30MPa and 50MPa). The double shear test results indicated that the bond strength increased with enhanced methods of surface preparation. Surface grooving reduced the debonding response, and combined surface preparation methods were effective at eliminating debonding failures. Also, the fibre-mortar ratio of 1:2 exhibited higher bond strength and reduced bond failure than the 1:1 ratio. The concrete strength had an insignificant effect on the bond strength and failure mode. For flexural specimens, the surface preparation method was examined. The surface preparation had a significant effect on the bond strength, and all failures occurred at the fibre-matrix interface. The bond strength was on average 44.5% higher for the flexural than for the double shear specimens. iv LAY SUMMARY The use of concrete repair materials has expanded with the growing age of current infrastructure, with concrete being one of the most used construction materials. The cost of replacing whole concrete sections is economically unsustainable and presents environmental concerns regarding the carbon footprint of new and waste construction. Repair of concrete with the use of externally applied materials is a commendable option. High strength fibres embedded in a mortar bonding matrix known as fabric reinforced cementitious matrix (FRCM) are one of the new materials designed to address these civil engineering challenges. FRCM can be easily applied to the outside surface of deteriorating concrete sections by providing increased load carrying capacity. To provide this capacity increase, the bond between the FRCM and concrete must be fully analyzed to ensure its use as a safe and reliant material. An analysis on the bond behaviour of FRCM is investigated, and from the results achieved, simple guidelines have been developed to avoid debonding failure. v PREFACE This thesis is based on the original, experimental work completed in whole by the author in the School of Engineering at the Okanagan campus of the University of British Columbia. The author was responsible for completing all aspects of the project including literature review, material acquisition, experimental work, data collection and analysis, and thesis writing. Considerable guidance and supervision throughout the thesis work were contributed by Dr. Ahmad Rteil. Major portions of chapter 3 and 4, and a minor portion of chapter 2, have been submitted to peer-reviewed journals and conferences for publication. The following is a list of publications based on the thesis: • Fraser, T., & Rteil, A. (2019). Effect of Surface Roughness on FRCM-Concrete Bond. The 14th Internationsl Fiber Reinforced Polymer For Reinforced Concrete Structures (FRPRCS). June 5-7, Belfast, Ireland. (Presented) • Fraser, T. and Rteil, A. 2020 Effect of Substrate Properties and Fibre-Mortar Ratio on the Interfacial Bond Performance of FRCM. International Institute for FRP in Construction (IIFC), July 1-3, Istanbul, Turkey. (Accepted) • Fraser, T. and Rteil, A. 2020. Analysis of Bond Stress of FRCM-Strengthened Notched Concrete Beams Subjected to Bending Bond Test. The 8th International Conference on Advanced Composite Materials in Bridges and Structures (ACMBS-VIII), August 18-20, Sherbrooke, Canada. (Accepted) • Fraser, T., & Rteil, A. (2020). Effect of the Fibre-Mortar Width Ratio on the Fibre-Matrix Bond of FRCM. ASCE: Journal of Composites for Construction (Submitted) • Fraser, T., & Rteil, A. (2020). Comparative Analysis of Double Shear and Flexural Bond Test Methods for FRCM. Composites Part B: Engineering (Submitted) vi TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii LAY SUMMARY .......................................................................................................................... iv PREFACE ....................................................................................................................................... v TABLE OF CONTENTS ............................................................................................................... vi LIST OF FIGURES ........................................................................................................................ x LIST OF TABLES ........................................................................................................................ xv LIST OF ACRONYMS .............................................................................................................. xvii ACKNOWLEDMENTS ............................................................................................................ xviii DEDICATION ............................................................................................................................. xix CHAPTER 1: INTRODUCTION ............................................................................................... 1 1.1 Research Motivation ..................................................................................................... 1 1.2 Fabric Reinforced Cementitious Matrix ....................................................................... 3 1.3 Thesis Layout ............................................................................................................... 4 CHAPTER 2: LITERATURE REVEIW .................................................................................... 6 2.1 Externally Bonded Materials ........................................................................................ 6 2.2 FRCM System .............................................................................................................. 7 2.3 Material Properties ....................................................................................................... 8 2.3.1 Mortars .................................................................................................................. 8 2.3.2 Fabrics ................................................................................................................... 9 vii 2.3.3 Composite System ............................................................................................... 10 2.3.4 Advantages .......................................................................................................... 12 2.3.5 Limitations .......................................................................................................... 13 2.4 FRCM Strengthening Applications ............................................................................ 13 2.5 FRCM Bond Failure ................................................................................................... 15 2.6 Bond Test Methods ..................................................................................................... 16 2.7 Factors Affecting Bond .............................................................................................. 18 2.7.1 Bonded Area ........................................................................................................ 19 2.7.2 Fibre Properties ................................................................................................... 20 2.7.3 Number of Layers................................................................................................ 21 2.7.4 Anchorage ........................................................................................................... 22 2.7.5 Surface Preparation ............................................................................................. 23 2.8 Research Needs ........................................................................................................... 27 2.9 Objectives ................................................................................................................... 27 2.10 Problem Scope ............................................................................................................ 28 CHAPTER 3: EXPERIMENTAL METHODOLOGY ............................................................. 29 3.1 Test Matrix ................................................................................................................. 29 3.2 Test Specimens ........................................................................................................... 31 3.3 Materials ..................................................................................................................... 33 3.3.1 Concrete .............................................................................................................. 33 viii 3.3.2 Fabric and Matrix ................................................................................................ 34 3.4 Specimen Preparation ................................................................................................. 37 3.4.1 Specimen Casting ................................................................................................ 37 3.4.2 Surface Preparation ............................................................................................. 39 3.4.3 Fabric Preparation ............................................................................................... 43 3.4.4 Strain Gauges ...................................................................................................... 44 3.4.5 FRCM Application .............................................................................................. 45 3.5 Test Setup ................................................................................................................... 47 CHAPTER 4: EXPERIMENTAL RESULTS AND DISCUSSION ........................................ 50 4.1 Double Shear Bond Testing ........................................................................................ 50 4.1.1 Phase 1: General .................................................................................................. 50 4.1.2 Test Results ......................................................................................................... 52 4.1.3 Failure Modes ...................................................................................................... 55 4.1.4 Bond Stress .......................................................................................................... 72 4.1.5 Strain Gauge Analysis ......................................................................................... 81 4.2 Flexural Bond Testing ................................................................................................ 95 4.2.1 Phase 2: General .................................................................................................. 95 4.2.2 Failure Detection ................................................................................................. 95 4.2.3 Test Results ......................................................................................................... 96 4.2.4 Failure Modes ...................................................................................................... 97 ix 4.2.5 Bond Stress ........................................................................................................ 104 4.3 Comparison of Double Shear and Flexural Bond Testing ........................................ 105 4.3.1 Failure Modes .................................................................................................... 106 4.3.2 Bond Stress ........................................................................................................ 106 CHAPTER 5: CONCLUSION ................................................................................................ 109 5.1 Conclusion ................................................................................................................ 109 5.1.1 Experiments ....................................................................................................... 109 5.1.2 Design Recommendation .................................................................................. 111 5.2 Research Contribution .............................................................................................. 111 5.3 Limitations ................................................................................................................ 112 5.4 Future Research Scope ............................................................................................. 113 REFERENCES ........................................................................................................................... 114 APPENDICES ............................................................................................................................ 122 Appendix A: Application Procedures ..................................................................................... 122 Appendix B: Load Displacement Graphs................................................................................ 124 Appendix C: Statistical Analysis............................................................................................. 136 x LIST OF FIGURES Figure 1.1: Current methods of concrete repair systems ................................................................ 3 Figure 2.1: Different fabric types of FRCM [ (Escrig, Gil, Bernat-Maso, & F., 2017) © with permission from the publisher)] .................................................................................................... 10 Figure 2.2: Idealized tensile stress-strain behavior of FRCM [(Caggegi, et al., 2017) © with permission from the publisher)] .................................................................................................... 11 Figure 2.3: Experimental vs. theoretical trilinear stress-strain curve [(Caggegi, et al., 2017) © with permission from the publisher)] .................................................................................................... 12 Figure 2.4: Types of FRCM bond failure modes [ (Bencardino, et al., 2018) © with permission from the publisher)] ...................................................................................................................... 16 Figure 2.5: Typical bond test setup [ (Mukhtar & Faysal, 2017), © with permission from the publisher)] ..................................................................................................................................... 17 Figure 2.6: Different material orientations [ (Escrig, Gil, Bernat-Maso, & F., 2017) © with permission from the publisher)] .................................................................................................... 20 Figure 2.7: Setup for single shear bond test with FRP bar anchor [(Awani, El-Maaddawy, & Ismail, 2017) © with permission from the publisher)] ............................................................................. 22 Figure 2.8: CSP rating for chips ................................................................................................... 24 Figure 2.9: Orientation of grooving concrete surface [ (Mostofinejad & Mahmoudabadi, 2010) © with permission from ASCE)] ...................................................................................................... 25 Figure 3.1: Double shear test specimens ....................................................................................... 32 Figure 3.2: Flexural test specimen ................................................................................................ 33 Figure 3.3: Carbon textile dimensions .......................................................................................... 35 Figure 3.4: Tools used for casting concrete specimens ................................................................ 37 xi Figure 3.5: Concrete block specimen preparation for double shear samples ............................... 38 Figure 3.6: Concrete block specimen preparation for flexure samples ........................................ 39 Figure 3.7: Concrete block surfaces after roughening .................................................................. 40 Figure 3.8: Flexure sample with CSP 3 ........................................................................................ 40 Figure 3.9: Double shear samples with grooves on untreated, CSP 3 and CSP 6 ........................ 41 Figure 3.10: Flexure samples with grooves on untreated and CSP 3 ........................................... 42 Figure 3.11 Rounded edges on anchor location ............................................................................ 43 Figure 3.12: Fabric lengths for double shear (front) and flexure (top) specimens ....................... 44 Figure 3.13: Strain gauge locations .............................................................................................. 45 Figure 3.14: FRCM Application for blocks .................................................................................. 46 Figure 3.15: FRCM application process for beams ...................................................................... 46 Figure 3.16: CAD rendering of double shear block setup ............................................................ 47 Figure 3.17: Actual test setup for Phase 1 .................................................................................... 48 Figure 3.18: CAD rendering of flexure beam setup ..................................................................... 49 Figure 3.19: Actual test setup for Phase 2 .................................................................................... 49 Figure 4.1: Failure planes for fabric rupture and FRCM-debonding failure ................................ 50 Figure 4.2: Failure planes for interlaminar-debonding and slippage failure ................................ 51 Figure 4.3: Examples of rupture fabric (RF) failure mode ........................................................... 57 Figure 4.4: Typical load-displacement curves for specimens with ruptured fabric failure (RF) .. 58 Figure 4.5: Number of rupture fabric (RF) failures by specimen type ......................................... 59 Figure 4.6: Examples of interlaminar debonding (ID) failure ...................................................... 61 Figure 4.7: Typical load-displacement curves for specimens with interlaminar-debonding (ID) failure ............................................................................................................................................ 62 xii Figure 4.8: Number of interlaminar-debonding (ID) failures modes based on specimen type .... 63 Figure 4.9: Percentage of interlaminar debonding (ID) failures for each fibre width .................. 64 Figure 4.10: Examples of FRCM-debonding (FD) failure mode ................................................. 66 Figure 4.11: Typical load-displacement curve for specimens with FRCM-debonding (FD) failure....................................................................................................................................................... 67 Figure 4.12: Number of FRCM-debonding (FD) failures by specimen type ............................... 68 Figure 4.13: Percentage of FRCM-debonding (FD) failures per group........................................ 69 Figure 4.14: Examples of slippage (SL) failure mode .................................................................. 70 Figure 4.15: Typical load displacement curves for specimens with slippage (SL) failure ........... 71 Figure 4.16: Number of slippage (SL) failures by specimen type ................................................ 72 Figure 4.17: Average fibre-matrix bond stress at failure for all 72 specimens ............................. 74 Figure 4.18: Average fibre-matrix bond stress at failure for bond failure specimens .................. 75 Figure 4.19: Average concrete-matrix bond stress at failure for all 72 specimens ....................... 78 Figure 4.20: Average concrete-matrix bond stress at failure for bond failure specimens ............ 79 Figure 4.21: Strain distribution profiles for specimens with ruptured fabric (RF) failure mode .. 82 Figure 4.22: Generalized strain distribution for ruptured fabric (RF) failure ............................... 83 Figure 4.23: Strain distribution profiles for specimens with interlaminar-debonding (ID) failure....................................................................................................................................................... 85 Figure 4.24: Generalized strain distribution trend for interlaminar-debonding (ID) failure ........ 86 Figure 4.25: Strain distribution profiles for specimens with FRCM-debonding (FD) failure ...... 88 Figure 4.26: Generalized strain distribution trend for FRCM-debonding (FD) failure ................ 89 Figure 4.27: Strain distribution profiles for specimens with slippage (SL) failure ...................... 91 Figure 4.28: Generalized strain distribution trend for slippage (SL) failure ................................ 92 xiii Figure 4.29: Average maximum strain and average design strain for each failure mode ............ 93 Figure 4.30: Failed beam under 3-point bending bond test .......................................................... 96 Figure 4.31: Stress distribution for a cracked rectangular beam .................................................. 97 Figure 4.32: Number of failures per specimen type ..................................................................... 98 Figure 4.33: Examples of slippage (SL) failure mode .................................................................. 99 Figure 4.34: Load-displacement curve for typical slippage failure ............................................ 100 Figure 4.35: Examples of slippage/rupture (SL/RF) failure mode ............................................. 101 Figure 4.36: Load-displacement curve for slippage/rupture (SL/RF) failure ............................. 102 Figure 4.37: Example of shear crack (SH) failure mode ............................................................ 103 Figure 4.38: Load-displacement curve for shear crack (SH) failure .......................................... 103 Figure 4.39: Average concrete-matrix bond stress for flexural specimens ................................ 104 Figure 4.40: Comparison of concrete-matrix bond stress for double shear and flexural specimens..................................................................................................................................................... 107 Figure B.1: DS-R0-S50-W60...................................................................................................... 124 Figure B.2: DS-R0-S30-W60...................................................................................................... 124 Figure B.3: DS-R0-S50-W120.................................................................................................... 125 Figure B.4: DS-R0-S30-W120.................................................................................................... 125 Figure B.5: DS-R0G-S50-W60 ................................................................................................... 126 Figure B.6: DS-R0G-S50-W60 ................................................................................................... 126 Figure B.7: DS-R0G-S50-W120 ................................................................................................. 127 Figure B.8: DS-R0-S30-W120.................................................................................................... 127 Figure B.9: DS-R3-S50-W60...................................................................................................... 128 Figure B.10: DS-R3-S30-W60.................................................................................................... 128 xiv Figure B.11: DS-R3-S50-W12.................................................................................................... 129 Figure B.12: DS-R3-S30-W120.................................................................................................. 129 Figure B.13: DS-R3G-S50-W60 ................................................................................................. 130 Figure B.14: DS-R3G-S30-W60 ................................................................................................. 130 Figure B.15: DS-R3G-S50-W120 ............................................................................................... 131 Figure B.16: DS-R3G-S30-W120 ............................................................................................... 131 Figure B.17: DS-R6-S50-W60.................................................................................................... 132 Figure B.18: DS-R6-S30-W60.................................................................................................... 132 Figure B.19: DS-R6-S50-W120.................................................................................................. 133 Figure B.20 DS-R6-S30-W120 ................................................................................................... 133 Figure B.21: DS-R6G-S50-W60 ................................................................................................. 134 Figure B.22: DS-R6G-S30-W60 ................................................................................................. 134 Figure B.23: DS-R6G-S50-W120 ............................................................................................... 135 Figure B.24: DS-R6-S30-W120.................................................................................................. 135 Figure C.1: Q-Q plot for double shear specimens ...................................................................... 137 Figure C.2: Q-Q plot for flexure specimens ............................................................................... 138 xv LIST OF TABLES Table 3.1: Double shear bond test matrix ..................................................................................... 30 Table 3.2: Flexure bond test matrix .............................................................................................. 31 Table 3.3: Concrete mix design for double shear specimens (Phase 1)........................................ 34 Table 3.4: Concrete mix design for flexure specimens (Phase 2) ................................................. 34 Table 3.5: FRCM fibre properties ................................................................................................. 36 Table 3.6: FRCM mortar properties.............................................................................................. 36 Table 3.7: FRCM composite properties ........................................................................................ 36 Table 4.1: Group designation by surface preparation method ...................................................... 52 Table 4.2: Group R0 double shear test results .............................................................................. 53 Table 4.3: Group R0G double shear test results ........................................................................... 53 Table 4.4: Group R3 double shear test results .............................................................................. 54 Table 4.5: Group R3G double shear test results ........................................................................... 54 Table 4.6: Group R6 double shear test results .............................................................................. 55 Table 4.7: Group R6G double shear test results ........................................................................... 55 Table 4.8: Failure modes per group .............................................................................................. 56 Table 4.9: Factor information for the ANOVA test on fibre-matrix bond stress ......................... 76 Table 4.10: ANOVA for the fibre-matrix bond stress (all specimens) ......................................... 77 Table 4.11: Factor information for the ANOVA test on concrete-matrix bond stress ................. 80 Table 4.12: ANOVA of the concrete-matrix bond stress (all specimens) .................................... 80 Table 4.13: Proposed design recommendations............................................................................ 94 Table 4.14: Test results of the flexural bond specimens ............................................................... 97 Table 4.15: One-way ANOVA of bond stress at the concrete-matrix interface ......................... 105 xvi Table 4.16: Failure modes of the double shear and flexure bond specimens ............................. 106 Table 4.17: Factor information for the ANOVA test on concrete-matrix bond stress ............... 108 Table 4.18: ANOVA for the concrete-matrix bond stress (double shear vs flexural specimens)..................................................................................................................................................... 108 Table C.1: Kruskal Wallace test for failure mode effect on failure load .................................... 136 Table C.2: Descriptive statistics for double shear specimens ..................................................... 137 Table C.3: Descriptive statistics for flexure specimens .............................................................. 138 xvii LIST OF ACRONYMS ACI American Concrete Institute ASCE American Society of Civil Engineers ASTM American Society for Testing and Materials CSA Canadian Standards Association DS Double shear F Flexure FRC Fibre reinforced concrete FRCM Fabric reinforced cementitious mortar FRP Fibre reinforced polymer PBO Polyphenylene benzobisoxazole RC Reinforced concrete TRC Textile reinforced concrete TRM Textile reinforced mortar xviii ACKNOWLEDMENTS The extensive effort required to complete an experimental project of this caliber would not be possible without the substantial contributions from the engineering academic community. I must acknowledge the efforts alone from my supervisor and advocate Dr. Ahmad Rteil have significantly contributed to the success of this project and of my academic career. His thoughtful approach, knowledge and on-going support throughout the timeline were crucial in aiding my completion of this thesis, amongst many other research initiatives over the last four years. If not for his consistent belief in my capabilities, I likely would have never undertaken a master’s degree or contributed to the research industry. I thank you for motivating me to achieve my goals. The many components throughout this experiment would not have been possible without the financial and technical contribution of some local businesses and staff at the School of Engineering. I am especially grateful to Kelowna Ready Mix for donating concrete materials. Some of the most meaningful acknowledgments go to people who personally devoted their time, effort and consistent support. Thank you, Alec Smith, Kim Nordstrom, Luis Diaz, Durwin Bossy, and Tim Giesbrecht, for your continuous support in the Structures Lab at UBC Okanagan. I also would like to personally thank my dear friend, research partner, and concrete captain Dylan Kennedy, your extensive wisdom and enthusiasm went a long way in helping me finish this project, one that could not have been completed without your help. I would lastly like to thank my former research mentor and friend Devin Walline for your assistance and motivation in the early stages of my project. xix DEDICATION This dedication is whole-heartedly deserved by my family; I thank you for your time, efforts and considerable support. First and foremost, my parents and true idols Terry and Lisa, as my success would not be where it stands today without your continuous assistance and passion for my academic career. To my loving sister Tara, for always helping me through the times when I needed it most. Lastly, to my caring partner, Torrin, I thank you for your consistent motivation to obtain my master’s degree, of which I am gracious for your support. You inspire me everyday to work hard and achieve my goals in life. 1 CHAPTER 1: INTRODUCTION 1.1 Research Motivation Modern reinforced concrete (RC) was developed during the 19th century which enabled numerous applications involving mass concrete such as tall buildings, bridges and dams and currently it is among the commonly used materials such as steel, timber and plastics (Narayanan, 2013). Due to its versatile application and admirable properties, concrete takes the lead on the most used construction material worldwide today (Qeshta, Shafigh, & Jumaat, 2016). Although highly durable, concrete was once mistaken to be impenetrable. However, this misconception became prevalent as concrete deterioration took place. Roughly 40-50% of the construction industry is targeted towards the repair and restoration of RC structures in the US, as service lives of concrete built in the 20th century are deteriorating after 10-20 years (Narayanan, 2013). Excessive loading, exposure conditions, reinforcement corrosion, inadequate design and seismic events are just a few causes that create a negative condition for concrete materials, leading to potential failure. The increasing knowledge of the negative effects seen with concrete behaviour has led to the growing concrete repair and rehabilitation industry. Presently, concrete structures’ state of repair is commonly broken down into an annual infrastructure report per capita. In Canada, the Project Steering Committee (PSC) completes a 3-year report card on all infrastructure applications across the country. The Canadian Infrastructure Report card categorizes infrastructure into a scale of very poor, poor, fair, good and very good. In 2016, among the very poor, poor and fair conditions, yield a staggering replacement cost of bridges and buildings of CAD $45 billion (PSC, 2016). In 2019, the infrastructure report card updated the scale for bridges, and the percentage of bridges that were in very poor, poor or fair condition 2 increased from 26% to 38% from 2016 (PSC, 2019). The scale has not been updated for buildings and the cost replacement analysis is based on the 2016 report card. The American Society of Civil Engineers (ASCE) completes a 4-year report on civil infrastructure conditions in the US which evaluates deficient areas, and way to improve their current infrastructure status. The recent 2017 report states an estimated US $123 billion is required for bridge rehabilitation needs (ASCE, 2017). Inevitably, the immense cost of replacement for concrete structures is unsustainable, as it requires the use of raw materials and high energy levels in addition to generating waste (Narayanan, 2013). Therefore, replacing whole structures is not a viable option, and thus, solutions for repair is a better alternative. Aside from traditional repair strengthening techniques such as structural repair mortars, shotcrete, section enlargement, post-tensioning, near-surface mounting (NSM), and steel jacketing; other externally retrofitted repair solutions are available today (Qeshta, Shafigh, & Jumaat, 2016). The use of an external strengthening system to increase the performance of concrete structures can be guaranteed both in terms of strength and durability (Ombres, 2015). These include fibre reinforced polymers (FRP), ferrocement and fabric reinforced cementitious matrix (FRCM). FRP and ferrocement are considered “conventional” materials of externally bonded (EB) systems alongside EB steel, whereas FRCM is considered an “unconventional” EB material, alongside sprayed FRP and wire mesh epoxy (Qeshta, Shafigh, & Jumaat, 2016). A design tree of the typical schematic of concrete repair methods can be seen in Figure 1.1. 3 Figure 1.1: Current methods of concrete repair systems 1.2 Fabric Reinforced Cementitious Matrix FRCM is a two-part composite material made of continuous fibres (fabric) reinforced with a cementitious mortar (matrix) which is externally bound to masonry and concrete structures with the purpose of increasing the structural capacity. When a successful bond is achieved between the FRCM and the surface of the member, forces are transferred from the concrete to the highly resistant fibres, thus increasing the members’ capacity. Falling under a new class of EB composite materials (Figure 1.1), FRCM has gained recent popularity in the research community as it overcomes some of the drawbacks associated with other materials, and provides additional benefits such as vapor permeability, enhanced chemical compatibility, and fire protection with concrete and masonry structures (Bencardino, et al., 2018) (Younis & Ebead, 2018) (Awani, El-Maaddawy, & Ismail, 2017) (Ombres, 2015). While FRCM is a good alternative, there are some constraints concerning minimal research availability and additional bond-related failures, such as failure within the mortar layer and the at the fibre interface. Due to the novelty of the material, experimental and analytical research is underway to create a generalized design provision (Ceroni & Salzano, 2018) as there is currently only one guideline produced in 2013 by ACI (ACI 549.4R-4 13, 2013). Regarding bond failure, a recent study compiled FRCM-bond tests and reported that 87% of the 347 shear-bond tests failed by debonding, whether at the concrete-matrix interface or the fibre-matrix interface. As reported in the state-of-the-art review by Bencardino et al. (2017), FRCM-strengthened concrete in shear and flexural beam upgrading, column confinement, and seismic upgrading for beams and beam-column joints have shown effective performance in all systems, although, bond failure was the predominant failure mode. To adequately strengthen concrete members with FRCM, a complete understanding of how the bond behaves between the concrete and FRCM matrix, and between the matrix and fibres is an area of research that is still under investigation. Externally bonded systems like fibre reinforced polymers (FRP) and ferrocement have similar bond requirements and through the many contributions to literature, these complications have preventative measures through recent design codes and guidelines (ACI 440.2R-17, 2017) (ACI 549.1R-18, 2018). It is essential to fulfill the same level of research for bond behaviour with FRCM composites (ACI 549.4R-13, 2013). These include bonded area, type of material, number of layers, surface preparation, adequate application procedure, existing concrete strength, loading conditions and anchorage. While many of these factors have been explored, the underdeveloped areas needing research include defining appropriate methods of surface preparation for given applications, comparing bond failure modes with varying loading conditions, and understanding the bond behaviour of the three layers in this composite. 1.3 Thesis Layout This thesis will evaluate the bond behaviour of fabric reinforced cementitious mortar. The layout of the thesis is broken down into 5 chapters. 5 • Chapter 1 presents the description of the problem in the concrete repair industry and highlights the need for researching economical repair solutions. • A comprehensive literature review is presented in Chapter 2 highlighting FRCMs history and application practices, advantages and disadvantages, behaviour under flexure and shear loading and lastly a review of its bonding properties. Based on the literature, the objectives of this study are presented in Chapter 2. • Chapter 3 outlines the methodology of this experimental program along with the materials used, specimen preparation and specifications for the test setup and instrumentations implemented. • Experimental results and analysis are presented in Chapter 4 highlighting the resulting failure modes, bond stress and strain for the two test setups used. Statistical analysis was conducted to confirm the results of this experimental program. • Chapter 5 concludes the thesis with an emphasis on research findings and recommendations for further implementation. The limitations of this research are identified to align with recommendations for future work in this field. 6 CHAPTER 2: LITERATURE REVEIW 2.1 Externally Bonded Materials Before reviewing the literature of fabric reinforced cementitious mortar (FRCM), a brief understanding of similar externally bonded strengthening materials is presented. Fibre reinforced polymer (FRP) is a two-part material which can be externally applied to concrete structures to increase the concrete’s capacity. FRP is composed of a continuous fibre sheet bound by an organic adhesive, and has gained popularity due to admirable properties (Sezen, 2012) (Hollaway, 2010). Some of the advantages to FRP is its high strength to weight ratio, ease of application, non-corrosive nature, and well-developed design codes (Ahmed, Bakay, & Shrive, 2009) (Hollaway, 2010). FRPs high strength to weight ratio correlates with ease of handling and fast application rates for a variety of uses. FRPs non-corrosive nature aids in extending the lifetime of a structure and internal corrosion resistance is significantly increased. Another advantage of FRP is the extensive research available which aids as a guide for various uses and application methods, and the material properties are readily available to consumers. Despite the extensive attributes, there are some disadvantages to note. When FRP composites reach high temperatures, the mechanical properties are significantly reduced, as the material will lose its hardness (Hollaway, 2010). Additionally, the resin used can be expensive, as well as harmful to laborers applying the material. Proper safety methods must be implemented to reduce improper inhaling of chemical fumes. More disadvantages may include difficult bonding between resin and substrate at low temperatures or wet surfaces (Hollaway, 2010). Ultimately this led to the introduction of using an inorganic adhesive as seen with FRCM. 7 Ferrocement is another two-part composite material externally bonded to concrete composed of a steel wire mesh with a cement-based composite used to strengthen concrete structures (ACI 549.1R-18, 2018). Improvements in many structural engineering applications such as resistance to fracture, fatigue performance, and increased tensile and flexural strength are of the many benefits of ferrocement materials (Masood & Arif, 2014). Alongside ease of application, better crack prevention, and relatively inexpensive cost make this EB material admirable for the structural repair industry. Commonly, the square meshes have a wire spacing anywhere between 6 and 25mm with variable wire diameter between 0.5 and 2mm. While ferrocement has proven to be effective, problems associated with corrosion of the steel remain an issue (Nanni, Arboleda, & De Caso, 2016). The corrosion factor and durability of ferrocement is a function of water/cement ratio, concrete cover, and thickness of the mortar layer. While ferrocement can be highly effective in strengthening concrete structures, according to a study on various repair techniques (Qeshta, Shafigh, & Jumaat, 2016), methods involving placement in-situ a thin layer of cementitious material require adequate surface preparation and anchors to ensure adequate bond. 2.2 FRCM System FRCM may also be known in the literature based on application, as textile reinforced concrete (TRC) and textile-reinforced mortar (TRM) (Caggegi, et al., 2017), (Awani, El-Maaddawy, & Ismail, 2017) (Raoof, Koutas, & Bournas, 2016). TRC typically refers to methods involving new construction, and TRM was the former name before FRCM became generally adopted in the late 2010s (Caggegi, et al., 2017), although it is still used in some studies. The two-part composite consists of the fabric, which is comprised of fibres aligned in a grid and may be termed as textile, mesh, or sheet and is bound to the concrete with a cementitious paste that may be termed as the mortar, agent or adhesive. For consistency, this document will use the terms textile or 8 (fabric/fibres) and mortar or (matrix). FRCM composites have been under investigation over the last fifteen years but have gained popularity only recently due to their performance under axial, shear and flexure loading, proving to be useful in strengthening concrete and masonry members (Bencardino, et al., 2018) (Ceroni & Salzano, 2018). Several experimental studies have been analyzed for the effectiveness of strengthening reinforced concrete (RC) members in shear, flexure, column confinement, and seismic upgrading. With upcoming research, the potential to phase FRCM from an experimental stage to the construction industry is underway, and a few construction projects have already been implemented. As reported in ACI 549.4R-13, a few projects include 1) the strengthening of a roof slab that required an opening passage of air ducts that operated at temperatures beyond the allowable limit of conventional FRP systems, 2) the repair of a structural deficiency lining a reinforced concrete tunnel in Greece, 3) and a concrete support base confinement for the trestle of a railway bridge in New York, as a breathable material was required (ACI 549.4R-13, 2013). These projects are just a few of many potential applications for the use of FRCM in the foreseeable future. 2.3 Material Properties The FRCM materials can be broken into two categories of mortars and fabrics, which contain individual properties as well as composite properties. 2.3.1 Mortars The inorganic matrix composition includes water, sand, and cement paste and may include short fibres to reduce cracking due to plastic shrinkage, as well as achieve a better bond with the fabric (Carozzi & Poggi, 2015). Pozzolanic or geopolymers may also be used to control fresh properties, as cited in Donnini and Corinaldesi (2017) but should be kept to less than 5% by weight 9 to maintain fire-proofing (Tumialan & Luca, 2014). Finely graded sands (< 0.5 mm) are used to aid the workability and the impregnation of the mortar between the fibres. Hydraulic cement is used, and the compressive strength of the mortar ranges from 20 to 40 MPa, depending on the cement properties and water/cement ratio. The mortars must contain the following properties: non-shrinkable, highly viscous (for easement of application on vertical or overhead surfaces), good workability and sufficient shear strength to prevent debonding (Tetta, Koutas, & Bournas, 2014). 2.3.2 Fabrics The textiles may be made up of carbon, glass, basalt, polyparaphenylene benzobisoxazole (PBO) fibres (Figure 2.1), or hybrid systems (Younis & Ebead, 2018) (Tumialan & Luca, 2014), all with varying properties. However, for strengthening, carbon and PBO are most commonly present in the literature (Bencardino, et al., 2018). The fibres have their function to resist tensile forces and are very strong, with typical tensile strengths of 3000MPa, 5000MPa and 5800MPa for basalt, carbon, and PBO, respectively (Donnini & Corinaldesi, 2017). Being strong and lightweight makes them an admirable material to use for strengthening concrete. The fibres may be woven together, or bonded with an agent (ACI 549.4R-13, 2013) (Tumialan & Luca, 2014) yielding the spacing between the openings, which are typically not larger than 20 mm. The textile is woven in a grid to allow the cementitious mortar to penetrate the openings, to achieve a good bond between the fibres and the concrete. The orientation of the fibres openings’ will largely contribute to the success of the bond with the mortar (Younis & Ebead, 2018). Additionally, the fibres are typically coated in resin and/or sand to improve the bond with the mortar (Tetta, Koutas, & Bournas, 2014). 10 Figure 2.1: Different fabric types of FRCM [ (Escrig, Gil, Bernat-Maso, & F., 2017) © with permission from the publisher)] 2.3.3 Composite System Although the fabrics and matrix have individual properties that contribute to the success of the composite material, the ultimate performance indicator is determined by the mechanical properties of the FRCM system. The composite properties can be defined by the response of FRCM coupons via clevis-grip tensile testing as per AC434 guidelines: “Acceptance criteria for FRCM and steel reinforced grout (SRG) systems” as published by the International Code Council (ICC) Evaluation service in 2013 (Santis, Hadad, Basalo, Felice, & Nanni, 2018). The response of the FRCM system can be characterized by a stress-strain behaviour that follows three branches with varying slopes (Figure 2.2), which can be described as three phases; initial uncracked section (phase 1), transition phase (phase 2), and lastly the cracked section (phase 3). The tensile properties c) PBO d) Glass a) Basalt b) Carbon 11 are governed by the behaviour of the uncracked section (phase 1: line OA Figure 2.2) of the cementitious matrix. This stage corresponds to the first linear portion of the curve up to the transition phase (phase 2: line AB Figure 2.2), characterized by a high modulus of elasticity due to the properties of the mortar. As mentioned, phase 2 is the transition phase, is governed by cracking of the mortar, where the transfer of forces gradually shifts to the individual fibres, when the load overcomes the capacity of the matrix strength. This load shift causes the decrease in stiffness (slope of the line) for Youngs Modulus. The final phase (phase 3: line BC Figure 2.2), is characterized by a cracked section, where the transfer of forces is almost completely governed by the textile up until failure. Figure 2.2: Idealized tensile stress-strain behavior of FRCM [(Caggegi, et al., 2017) © with permission from the publisher)] 12 The three stages contribute to varying Young’s modulus, commonly referred to in the literature as E1, E2, and E3 (Santis, Hadad, Basalo, Felice, & Nanni, 2018) (Caggegi, et al., 2017). A study reported by Caggegi et al. (2017) completed 35 tensile tests to characterize values of E1, E2, and E3 and found averages of 4824GPa, 14GPa, and 117GPa, respectively, for carbon-FRCM and 108GPa, 32GPa, and 61GPa, respectively, for aramid-FRCM. An example of one of the stress strain curve for FRCM coupon testing from Caggegi et al. (2017) is presented in Figure 2.3. Figure 2.3: Experimental vs. theoretical trilinear stress-strain curve [(Caggegi, et al., 2017) © with permission from the publisher)] 2.3.4 Advantages Many of the advantages of FRCM are comparable to that of FRP including high strength to weight ratio, corrosion resistance, ease of application and relatively inexpensive cost (ACI 549.4R-13, 2013) (Sneed, D’Antino, & Carloni, 2014), (Raoof, Koutas, & Bournas, 2016), (Awani, El-Maaddawy, & Ismail, 2017), (Santis, Carozzi, Felice, & Poggi, 2017), (Younis & Ebead, 2018), (Bencardino, et al., 2018) (Ceroni & Salzano, 2018). Additionally, the inorganic mortar matrix responds better at high temperatures, so fire performance is superior compared to other strengthening materials. The application procedure is also beneficial as the installation can be performed on uneven, wet surfaces, and no toxic fumes will be present to the worker, as was seen 13 in the epoxy resin component of former FRP materials (Santis, Carozzi, Felice, & Poggi, 2017). In addition, unlike FRP which is a thermosetting material, FRCM can be removed if required with relative ease which is beneficial for potential construction errors. 2.3.5 Limitations Aside from the benefits mentioned, in a critical state of the art review by Oluwafunmilayo et al. (2017), there are still some notable drawbacks of FRCM largely due to the age of this material and minimal research available. It has been found that the studying the bond of FRCM is complex (Carozzi & Poggi, 2015), due to the additional failure patterns compared to other strengthening systems, including slippage of the textile and interlaminar debonding (failure in between layers). Moreover, the ultimate performance is largely based on the proper application technique, which can be hard to control in given a construction site (Santis, Carozzi, Felice, & Poggi, 2017). In terms of longevity, it is difficult to analyze the long-term performance of FRCM at this stage, due to a lack of available studies, and this is an area that needs to be investigated (Bencardino, et al., 2018). These systems are relatively new, and although there is some literature available, more research is required to confirm the design procedure for future use of these materials. 2.4 FRCM Strengthening Applications FRCM can be used to enhance concrete structural performances most commonly in flexure and shear and can be applied to beams, bridge decks, columns, girders, slabs and masonry walls (Koutas, Tetta, Bournas, & Triantafillou, 2019) (Ceroni & Salzano, 2018) (Awani, El-Maaddawy, & Ismail, 2017) (ACI 549.4R-13, 2013). In terms of flexural strengthening, the system is applied to the tension face with the fibres aligned longitudinally, providing a transfer of forces from the tension chords of the concrete to the FRCM matrix. This method increases the flexural capacity 14 by increasing the tensile strength of the member, thus maximizing its longevity. Strengthening members with FRCM can increase the flexural capacity by up to 120%, and even a 245% increase was reported (Awani, El-Maaddawy, & Ismail, 2017). However, ACI 549 guidelines limit the increase of flexural strength for design to only 50% of the un-strengthened member. This is to safeguard against catastrophic failure due to unforeseen events. In short, the strengthened members should not fully rely on the external system and must have independent resistance to external loads. In terms of shear strengthening, the system involves applying the material with fibres oriented perpendicular to the main axis of the beam (Koutas, Tetta, Bournas, & Triantafillou, 2019). The section wrapping schemes which have been tested are the full section wrap, the U-wrap, and side bonding. While the full wrap is the most effective, it is not suitable for use in all situations, for example, beams cast into slabs or bridge decks. Reports have indicated that non-anchored and anchored U-wrap increased concrete beams shear capacity by up to 150% and 190%, respectively (Tetta, Koutas, & Bournas, 2014). Similarly, to flexure, ACI 549 guidelines limit the increase of shear strength for design to only 50% of the un-strengthened member. Three general failure modes were observed when FRCM was used as a strengthening system: tension failure (steel yield or fibre rupture followed by concrete crushing), compression failure (concrete crushing before steel yielding) and bond failure (slippage, FRCM-debonding, interlaminar-debonding, and delamination) (ACI 549.4R-13, 2013). In the state-of-the-art review by Awani et al. (2017), the failure modes of several experimental studies that were focused on flexure and shear enhancement, noted that debonding is the most common failure mode. 15 2.5 FRCM Bond Failure Generally, the loss of composite action is a common failure mode for any externally bonded strengthening system, which is directly related to lack of force transfer between the different materials. The type of bond failure, whether it be at the concrete-matrix interface or the fibre-matrix interface, is dependent on several factors. Those that affect the bond capacity of the strengthening system include the number of layers, type of fibres, type of mortar, concrete strength, anchorage, reinforcement ratio, and existing corrosion (Awani, El-Maaddawy, & Ismail, 2017). All these factors may contribute to the several bond-related failure modes seen with FRCM. For FRCM, the four types of bond related failures reported in the literature are slippage (fabric slips/pulls from matrix), interlaminar-debonding (fibre-matrix interface, or within matrix layer) FRCM-debonding (concrete-matrix interface), and delamination (cover loss or separation) (Bencardino, et al., 2018) as seen in Figure 2.4. When slippage occurs, the load overcomes the bond strength between the fabric and mortar, causing it to pull the fibres out while the mortar remains intact with the concrete (Figure 2.4). Interlaminar-debonding is similar but characterized by a fracture surface within the mortar layer at the fibre-matrix interface (Figure 2.4). FRCM-debonding failure occurs at the concrete-interface either when the whole mortar section debonds from the concrete substrate, or even a partial section (Figure 2.4). Delamination occurs when the failure is within the concrete substrate (Figure 2.4) and is the least common bond failure mode for FRCM. 16 Figure 2.4: Types of FRCM bond failure modes [ (Bencardino, et al., 2018) © with permission from the publisher)] If the bond fails either at the concrete-matrix interface or the fibre-matrix interface, then the load transfer to the FRCM system decreases, which renders the strengthening system, leading to a potential failure of the RC member. Currently, there are few guidelines for avoiding debonding with FRCM composite systems, such as those provided by an FRCM manufacturer or the ACI 549.4R-13. With the help of experimental research, the major contributing factors affecting the bond behavior have been identified, along with failure modes and ways to mitigate the loss of composite action for FRCM material systems. 2.6 Bond Test Methods Two methods commonly used for bond testing that has been adopted for FRCM are single and double-lap shear test (Figure 2.5). These two tests are widely used, as they are easy to set up and can evaluate the bond stress in terms of surface shear stress. Another bond test that has been conducted with FRP systems is the three-point bending test where a notch is created to create a a) Slippage b) FRCM-Debonding c) Interlaminar-Debonding d) Delamination 17 location to control debonding at the point of maximum stress. This method has been proven effective for FRP (Zheng, Dai, & Fan, 2016) (Mukhtar & Faysal, 2017), so it could also be adapted for FRCM and compared with other bond test results. The benefits of the beam test are that the notch induces cracking at the notch location, initiating bond failure and simplifies instrumentation. Having a three-point bending test reduces the need for a constant moment region and increases the shear span, reducing the chance of shear failure (Eveslage, Aidoo, & Bro, 2011). Figure 2.5: Typical bond test setup [ (Mukhtar & Faysal, 2017), © with permission from the publisher)] In literature, it was observed that the bond failure type varies depending on the type of test being performed. Namely, bond, flexure or shear tests, whereas rupture within the matrix is commonly reported for column confinement when the column experiences buckling failure (Awani, El-Maaddawy, & Ismail, 2017). The most common failure mode seen in single or double-lap shear bond tests is slippage and interlaminar debonding, according to (Younis & Ebead, 2018), (Carozzi, 18 et al., 2017). Although these are the most common type of failure, it is difficult to overcome this in laboratory tests, as the individual properties of the mortar and fibres play an important role in this failure type (Younis & Ebead, 2018). FRCM-debonding may be less commonly reported than the former two, but can occur when either of the following conditions is not met: the surface of the concrete is not properly prepared for application, failure to apply the material correctly, or when the bonded length is inadequate for the load (Younis & Ebead, 2018). It should be noted that the rupture of the fabric is often seen with shear bond tests within the unbonded region, due to the nature of the test having the free fibre region for setup purposes. The rupture failure develops when the fibres disconnect outside of the bonded region. In flexure and shear tests, failure at the fibre-matrix interface and the concrete-matrix interface are observed. In flexure tests, FRCM-debonding is more common at the ends of the beam, known as ‘end-debonding’ (Ombres L. , 2012) and interlaminar-debonding propagates from the region of maximum moment. In shear tests, FRCM-debonding was found with more than 2 layers of FRCM due to increased flexural rigidity of the FRCM composite, and slippage occurred with 1 layer at locations crossing shear cracks (Awani, El-Maaddawy, & Ismail, 2017). In some cases rupture of the fabric may occur within the matrix in shear and flexure testing. In flexure tests, it may occur along the length of the bonded region, and for shear, at locations crossing shear cracks. 2.7 Factors Affecting Bond A state-of-the-art review was completed by Awani et al. (2017) on the response of strengthening with FRCM in shear, flexure, and bond behavior. The various studies compiled on bond behaviour cover various parameters that may affect the bond between FRCM and concrete. 19 The major factors tested were the bonded area (length, width) material type (carbon, glass, and PBO), number of layers, anchorage type and surface preparation. 2.7.1 Bonded Area Younis & Ebead (2018), Ombres (2015), Sneed et al. (2014) determined that an increase with bonded length was accompanied by an increase in bond strength. Younis & Ebead determined that an increase in bond length from 75mm to 100mm resulted in an increase in bond capacity of 194% for carbon, 137% for PBO fibre, and 181% for glass fibre. Ombres reported the average failure load increased from 2.4kN to 3.5kN with one layer at 150mm to 200mm in length, whereas with 250mm length, the average failure load was 3.3kN. This indicates that the effective length for this study was 200mm. With two layers, the same effective length was determined, as the average failure load at 100mm was 7.1kN, at 200mm was 8.1kN, and at 250mm was 7.3kN. Sneed et al. noted an increase in the average failure loads of 1.5kN, 2.2kN, 3.0kN, and 2.5kN with lengths of 100mm, 150mm, 200mm, and 250mm, respectively, coinciding with an effective bond length of 200mm found previously. The width of the bonded area does not show a significant effect on FRCM composite systems, according to Sneed et al. (2014), which differs from that of FRP systems and is often ignored in later FRCM research experiments. However, Ombres (2015) found that the bonded width had an impact on bond stress, where specimens with the same bond length showed higher bond stresses in widths of 50mm compared to widths of 70mm. This was explained to be the result of non-uniformity of force distribution across the fibres (Ombres, 2015), which is a phenomenon that often occurs with this material when the mortar does not penetrate the fibre bundles evenly (Awani, El-Maaddawy, & Ismail, 2017). These tests were completed using the same width value for the mortar and the fibres, meaning that, the same area of fibres is the same 20 area of mortar used for each test. Thus, the effect of increasing the mortar ratio in the two-part component has not yet been explored. 2.7.2 Fibre Properties The four materials used for FRCM systems are carbon (C-FRCM), polyparaphenylene benzobisoxazole (PBO-FRCM), glass (G-FRCM), and basalt (B-FRCM). However, basalt is not often used in current experiments and is henceforth removed from this discussion. Material type and orientation of the fibres have an effect on the success of mechanical interlock between fibres and the matrix (Younis & Ebead, 2018) (Awani, El-Maaddawy, & Ismail, 2017) (Hashemi & Al-Mahaidi, 2010). A study completed by Hashemi & Al-Mahaidi (2010) analyzed the fabric textiles with and without openings. They showed that the bond strength increased by 30% with the adoption of textiles with spacing between fibre bundles. This created the need for FRCM materials to adapt to an open system as opposed to the closed sheets seen with FRP materials. FRCM textiles have varying orientations depending on the manufacturer. For example, Figure 2.6 shows three types of orientation seen with the different material types, carbon, glass, and PBO. Figure 2.6: Different material orientations [ (Escrig, Gil, Bernat-Maso, & F., 2017) © with permission from the publisher)] 21 These fabrics were used in a study completed by (Younis & Ebead, 2018), where the fibre type and bonded length were analyzed. The PBO system exhibited the highest bond capacity than its carbon and glass counterpart. The failure modes recorded were FRCM-Debonding, interlaminar-debonding, and fibre rupture for PBO, carbon, and glass, respectively. Interestingly, although PBO had the highest bond strength, it also had an FRCM-debonding failure. This can be explained by the orientation of the fibre openings. The PBO was uni-directional, and the openings of the fibres for were larger in the weft direction than the bi-directional carbon fibres, which provided an increase in the mechanical interlock between the matrix and the fibres in the direction of loading. The results for the glass system (rupture failure) can be attributed to the lower mechanical properties of this material type. 2.7.3 Number of Layers The number of layers of FRCM systems influences the ultimate capacity as well as the strain in the fibres. Few studies have been reported with this factor. D’Ambrisi (2013) reported that lower strains were found with FRCM systems with two layers than with one layer, and the debonding fabric strain for two layers was roughly 85% of that with one layer (D'Ambrisi, Feo, & Focacci, 2013). Sneed et al. (2014) reported the same result of a decrease in fabric strains in addition to a shift of failure mode from slip to FRCM debonding from 1 to 2 layers (Sneed, D’Antino, & Carloni, 2014). While the number of layers appears to have little effect on the bond strength, increasing the number of layers increases the mechanical interlock between layers of mortar and fibres, thus shifting the failure mode from the fibre-matrix interface to the concrete-matrix interface. Understanding how to control the failure mode through experiments like this is integral to coordinate a better design. While increasing the number of layers may change the failure mode, 22 FRCM debonding is not a preferred failure type as it is more brittle than slippage (Awani, El-Maaddawy, & Ismail, 2017). 2.7.4 Anchorage Anchoring systems can be effective at delaying the debonding failure mode. Hashemi and Al-Mahaidi (2010) tested an anchorage system on a single-lap shear bond test with grooves cut laterally in the block and held into place with FRP strips (Figure 2.7). The results of this setup yielded higher bond capacity, improving the bond between the matrix and substrate (Hashemi & Al-Mahaidi, 2010). Results showed an increase of 15%-25% in bond capacity in specimens with anchorage compared to the control. Figure 2.7: Setup for single shear bond test with FRP bar anchor [(Awani, El-Maaddawy, & Ismail, 2017) © with permission from the publisher)] There are other studies that analyzed various anchor systems and their effect on bond strength under flexural loading conditions. Hashemi & Al-Mahaidi (2012) analyzed the same anchor system used in the shear bond test previously mentioned, on a beam under flexural loading. Results indicated that anchoring FRCM in flexure tests only improved the bond capacity by 5% and still resulted in end-debonding (Hashemi & Al-Mahaidi, 2012), i.e., the same anchorage did not have the same result with different loading conditions. Another mechanical anchor system proposed again by Hashemi & Al-Mahaidi (2012), where steel pins were installed into the concrete prior to 23 applying the FRCM, and the layers were installed between steel plates, which were tightened to the pins. This method of anchoring again only increased the peak load by 5%; however, it shifted the failure mode from FRCM-debonding to concrete crushing (Hashemi & Al-Mahaidi, 2012). U-wrap anchors have also been explored by few researchers. The U-wrap anchor is set to resist out-of-plane peeling effect, however, is ineffective at resisting slippage of the fibres at the matrix interface (Sneed, Verre, Carloni, & Ombres, 2016). It was confirmed by all authors that more investigation is needed to confirm the effectiveness of anchors in FRCM systems. 2.7.5 Surface Preparation Surface preparation is rarely involved as a studied parameter that affects bond performance, although, is commonly mentioned to play an important role for an adequate bond between the concrete and the matrix (Younis & Ebead, 2018) (Ceroni & Salzano, 2018) (Raoof, Koutas, & Bournas, 2016) (Awani, Refai, & El-Maaddawy, 2015). Moreover, it is a guideline that surface preparation is a requirement for externally applied concrete strengthening systems, to ensure a good bond (ACI 549.1R-18, 2018) (ACI 440.2R-17, 2017) (ACI 549.4R-13, 2013) (ICRI, 2013). Surface preparation generally consists of surface roughening or surface grinding. Surface roughening is a requirement for improving the bond performance, as the exposure of some aggregate will aid in the adhesion for the mortar mix to impregnate the concrete. The International Concrete Repair Institute (ICRI) established a scale system to evaluate the surface roughness with the use of Concrete Surface Profiling (CSP) chips (Figure 2.8). Standard practice dictates that concrete should be roughened to a CSP level of 6 to 9.The chips are rated on a scale of 1-10 (Figure 2.8) to evaluate the level of roughening required for the respective surface. 24 Figure 2.8: CSP rating for chips Surface roughening may be completed by means of wire-brushing (CSP level 1-2) sandblasting and low-pressure water-jetting (CSP level 3-4), shot-blasting and high-pressure water jetting (level 5-6), hydro-demolition and jackhammering (CSP level 7-8), and scarification (CSP level 9-10) (Courard, Piotrowski, & Garbacz, 2014) and is determined depending on the location and ease of accessibility. Surface sandblasting is more common with FRCM systems based on literature. Surface grooving is completed with a mechanical grinder and may be added to the beam laterally, longitudinally, or diagonally or a combination thereof (Figure 2.9). The action of adding 25 grooves is a cleaner and more cost-effective way to tend to surface preparation of concrete. Flat surface grinding of the concrete surface is another method which reaches a CSP level 2, however, is different from adding grooves to the concrete. Additionally, ACI 549 states that surface treatments may be completed according to the accessibility of the member (ACI 549.4R-13, 2013). Grinding is easier for hard to reach members and requires little labor and equipment. Using this method, as opposed to sandblasting, could be beneficial as the sandblasting material eventually wears down; therefore, more material will be required, hence affecting cost. Alternatively, using both methods could potentially increase bond strength. Figure 2.9: Orientation of grooving concrete surface [ (Mostofinejad & Mahmoudabadi, 2010) © with permission from ASCE)] Aside from FRCM applications, according to several studies completed on bond behaviour between old-new concrete joints, surface preparation significantly impacts bond strength and failure mode (Courard, Piotrowski, & Garbacz, 2014) (Santos, Santos, & Dias-da-Costa, 2012) (Júlio, Branco, & Silva, 2005). In one report, Courard, Piotrowski & Garbacz (2014) studied eight surface preparation techniques amongst other parameters with new-old concrete sections. It was reported that there was an increase in bond strength with stronger methods of surface preparation such as water jetting and hydro demolition, whereas bond strength was lower with milder methods such as scarification and wire brushing. Santos et al. (2012) studied the condition of the substrate a) Laterally b) Longitudinally c) Diagonally 26 concrete layer, moisture condition, preparation of substrate (smooth, wire brushing or shotblasting), presence of bonding agent, and condition of added concrete layer. Overall, it was reported that the shear bond strength increased with the smooth specimens to those with wire brushing, and higher still for dry surface shotblasting, with a decrease with wet surface shotblasting. In addition to new and old concrete bonds, studies showed that surface preparation is crucial to ensure an adequate bond between FRP-concrete sections (Chena, et al., 2019) (Liu, 2014) (Mostofinejad & Mahmoudabadi, 2010) (Ahmed, Bakay, & Shrive, 2009). One study completed by Liu (2014) analyzed two surface profiles (smooth and rough) by means of sandblasting the rough profile and left-as-cast for the smooth profile. These profiles were studied with varying FRP epoxies and determined that the rough surface profiles increased bond strength, and failure was within the epoxy layer as opposed to concrete failure. Mostofinejad & Mahmoudabadi (2010) analyzed surface grinding as opposed to surface roughening and found that it significantly improved the strength of the system by means of adding grooves longitudinally at a depth of 10 mm. There are few experiments where surface preparation is a studied parameter with FRCM systems. One study compared a sandblasted surface (roughening level not specified) to a non-treated surface accompanied by other parameters with PBO Fibres. Although sandblasting played a limited role in influencing peak load in this study, it was reported that surface preparation is recommended to avoid sudden debonding (D’Antino, Sneed, Carloni, & Pellegrino, 2015). This one study investigated minimal parameters around the topic of surface preparation, and the effect substrate characteristics have on the failure modes of FRCM composites requires further analysis. Including the degree of roughening and substitute methods such as adding surface grooves, hydro 27 demolition, water jetting, wire brushing, etc. Another study evaluated grinding a surface grid into the concrete, compared with the sandblasting method, and confirmed that both methods gave consistent results, indicating that both methods were adequate for surface preparation (Raoof, Koutas, & Bournas, 2016). Based on the above discussion, it could be concluded that surface preparation is a factor and should be further investigated for FRCM applications. 2.8 Research Needs From this literature review, it is observed that there is a lack of research focused on generalising the behaviour of the bond at both the fibre-matrix interface and the concrete-matrix interface. Due to the increased complexity with the additionally failure modes, it is integral to create design provisions based on all failure modes. These research contributions will be in favour of the current state of repair needed for concrete structures, including bridges and piers, beams, columns, girders, slabs, and masonry systems to achieve safety requirements of updated design codes and seismic events. Regarding the fibre-matrix interface, failure is most pronounced but the hardest to control as it is largely dependent on the geometry of the fibres, and the compatibility of the matrix. One parameter that can be analyzed to reduce bond failure is the fibre-mortar ratio. Regarding the concrete-matrix interface, adequate surface preparation and the existing properties of the concrete substrate can be effective in increasing the strength of the bond between FRCM and the concrete. 2.9 Objectives The goals of this thesis follow two major objectives: 1. To understand the bond behaviour of FRCM at the fibre-matrix interface and the concrete-matrix interface based on substrate, fibre and matrix properties 2. To present design recommendations for avoiding bond failure 28 2.10 Problem Scope The focus of this research is to investigate the use of carbon-FRCM (C-FRCM) as a strengthening system with a focus on the bond behaviour at the fibre-matrix interface and the concrete-matrix interface. • Evaluate how surface preparation methods (untreated, sandblasting, shotblasting surface grinding, and combination methods) affect the bond strength and failure mode at the concrete-matrix interface • Examine if the existing concrete strength influences the bond strength and the failure mode at the concrete-matrix interface • Determine if the fibre-mortar width ratio (1:2 and 1:1 mortar: fibres) influences the bond strength and failure mode at the fibre-matrix interface • Compare the bond strength and failure modes of two different types of bond test methods, namely the double shear bond test and the flexural bond test • To contribute to current guidelines for preventing bond failure, for the safe and effective use of this material as a strengthening solution. 29 CHAPTER 3: EXPERIMENTAL METHODOLOGY 3.1 Test Matrix To achieve the set objectives, a two-phase experimental program was designed. Phase 1 is comprised of 72 concrete blocks tested in a double shear pullout test, whereas Phase 2 contained 12 small beams in a 3-point flexure notched beam bond test. Phase 1: A total of 24 specimens with 3 repeats yielding 72 concrete blocks were cast and tested in double-shear to follow general procedure for bond testing. The parameters investigated were: surface preparation method (untreated, surface grooving, sandblasting (CSP level 3), shotblasting (CSP level 6), and combination), concrete strength (30MPa and 50MPa) and fibre-mortar width ratio (1:1 and 1:2 with fibre widths of 60mm and 120mm). The surface preparation was designed to analyze the bond performance of different methods, and to determine if dual methods could be used to enhance bond strength. The untreated method is considered the control of the surface preparation methods and is thus given the denoted value of 0. Surface grooves were completed on the control surfaces (untreated), to see if the surface grinding is effective on a smooth surface, as well, grooves were combined with the sandblasted and shotblasted surfaces to see if dual methods performed better. The concrete strength was completed as an added variable to see if it influenced the bond strength at the concrete-matrix interface. The strength of 30MPa was selected as this is a commonly used design strength in industry, and the 50MPa was chosen to mimic that of semi-high strength concrete, also to be comparable with the compressive strength of the cementitious matrix which was 52MPa. The fibre-mortar width ratio was completed to determine if the increased mortar surrounding the fibre bundles (1:2) would influence the bond strength and failure mode compared to the equal fibre-mortar width (1:1). The levels of 60mm and 30 120mm were chosen due to simplicity of the block geometry and to aim for different widths that have not been tested in the literature. All these factors are considered fixed variables, including surface preparation. Careful control was taken to ensure consistency among surface preparation profiles with the use of the concrete surface profiling chips. Table 3.1 lists each of the specimens tested with a specimen ID following the format DS-RxG-Sy-Wz where DS = double shear, Rx = roughness level (x = 0, 3, 6); G is for groove (present or absent); Sy = concrete strength (y = 30MPa or 50MPa) and Wz = fibre width (z = 60mm or 120mm). Table 3.1: Double shear bond test matrix Group Group ID Surface Preparation Method Concrete Strength (MPa) Fabric Width (mm) R0 DS-R0-S30-W60 Untreated 30 30 60 DS-R0-S30-W120 120 DS-R0-S50-W60 50 50 60 DS-R0-S50-W120 120 R3 DS-R3-S30-W60 Sand Blasting 30 30 60 DS-R3-S30-W120 120 DS-R3-S50-W60 50 50 60 DS-R3-S50-W120 150 R6 DS-R6-S30-W60 Shot Blasting 30 30 60 DS-R6-S30-W120 120 DS-R6-S50-W60 50 50 60 DS-R6-S50-W120 120 R0G DS-R0G-S30-W60 Untreated & Surface Grooving 30 30 60 DS-R0G-S30-W120 120 DS-R0G-S50-W60 50 50 60 DS-R0G-S50-W120 120 R3G DS-R3G-S30-W60 Sand Blasting & Surface Grooving 30 30 60 DS-R3G-S30-W120 120 DS-R3G-S50-W60 50 50 60 DS-R3G-S50-W120 120 R6G DS-R6G-S30-W60 Shot Blasting & Surface Grooving 30 30 60 DS-R6G-S30-W120 120 DS-R6G-S50-W60 50 50 60 DS-R6G-S50-W120 120 31 Phase 2: A total of 4 specimens with 3 repeats yielding 12 small beams were cast and tested in a 3-point notched beam bending bond test to analyze if different loading conditions influenced the bond strength and failure modes compared to Phase 1. This group was a small subset of specimens with one factor, the surface preparation parameter: surface roughening (untreated, sandblasted (CSP level 3), and surface grooving). The fibre-mortar ratio of 1:1 and concrete strength of 30MPa were held constant in this phase to compare with those same parameters (120mm fibre width and 30MPa) from the double shear specimens. This theory was designed to determine if 1) surface preparation, and 2) different loading conditions influenced the bond strength and failure mode. Table 3.2 lists each of the specimens tested with a specimen ID in the format F-RxG where F = flexure, Rx = roughness level (x = 0 or 3); G is for groove (present of absent). The FRCM spanned across a notch and anchored on one side to analyze only half of the beam that contained the same parameters of the blocks from Phase 1. Table 3.2: Flexure bond test matrix Group Group ID Surface Preparation Method Concrete Strength (MPa) Fabric Width (mm) F-R0 F-R0-S30-W120 Untreated Sand Blasting Surface Grinding Sand Blasting & Surface Grinding 30 30 120 F-33 F-R3-S30-W120 120 F-R0G F-R0G-S50-W60 30 30 120 F-R3G F-R3G-S50-W120 120 3.2 Test Specimens The specimens used in this program were designed to satisfy the requirements for double shear bond testing and to replicate the parameters in small beams with the same bonded area of 150x165mm. 32 Phase 1: The concrete specimens made for this experiment were 150x150x165mm blocks, bonded with FRCM on both sides. The fibre bundles were cut at widths of 60mm and 120mm; however, the bonded area of the mortar remained constant of the blocks surface area, with 15mm of mortar on each edge of the block considered negligible to reduce edge effects (Figure 3.1), yielding the 1:2 ratio (60:120) and 1:1 ratio (120:120). Figure 3.1: Double shear test specimens Phase 2: The beam dimensions were cast as 150x150x500mm with a bonded area on one side of the beam of 150x165mm (Figure 3.2), with 50mm unbonded region for the notch. The non-tested end of the beam was anchored to ensure the failure would occur along the bonded region. For this part of the experiment, only the width to mortar ratio of 1:1 was selected as 120mm. 33 Figure 3.2: Flexural test specimen 3.3 Materials 3.3.1 Concrete Phase 1: The two concrete strengths used were designed to have a compressive strength of 30MPa and 50MPa to replicate that of potential exiting conditions and whether strength influences bond performance. The concrete was cast in the School of Engineering Structures Laboratory, and the mix proportions are listed in Table 3.3. Standard concrete cylinders of dimensions 100mm in 34 diameter and 200mm in length were cast and tested at 3, 7, and 28 days as per CSA A23.2-9C (CSA-A23.1/A23.2., 2014) with an average compressive strength of 26.4MPa, 30.0MPa and 31.4MPa respectively for the 30MPa mix and 37.2MPa, 46.1MPa and 48.6MPa for the 50MPa mix. Table 3.3: Concrete mix design for double shear specimens (Phase 1) Material 30MPa 50MPa Units Cement 432 617 kg/m3 Coarse Aggregate 889 889 kg/m3 Fine Aggregate 744 592 kg/m3 Water 206 222 kg/m3 Superplasticizer 0 180 mL/100kg cement Phase 2: The concrete strength was not a variable for Phase 2 and hence, was kept constant at 30MPa. Table 3.4 shows the mix design used for the beams. Standard concrete cylinders of 100mm in diameter x 200mm in length were tested at 3, 7, and 28 days as per with an average compressive strength of 27.8MPa, 32.7MPa and 34.3MPa (CSA-A23.1/A23.2., 2014). Table 3.4: Concrete mix design for flexure specimens (Phase 2) Material Amount Cement 432 kg/m3 Coarse Aggregate (14mm) 889 kg/m3 Fine Aggregate 744 kg/m3 Water 206 kg/m3 Superplasticizer 0 mL/100kg cement 3.3.2 Fabric and Matrix The FRCM composite strengthening system consisted of a unidirectional carbon textile with cementitious mortar of which was available in the market. The fabric textile consisted of 35 unidirectional carbon fibres woven in the warp (strong) direction with glass fibres in the weft (weak) direction. The spacing between fibre bundles was 15x30mm in the strong direction (Figure 3.3), which acts to increase the strength of the bond between the fibres and the mortar. Figure 3.3: Carbon textile dimensions The manufacturer defines the mortar as a one-component, shrinkage compensated cementitious matrix with a four-hour fire rating. This mortar is made specifically to be applied with its carbon-fibre component for structural reinforcement applications. The mortar and fibre properties as provided by the manufacturer are readily available and presented in Table 3.5 and Table 3.6, and the composite properties in Table 3.7. To determine the cementitious mortar’s compressive strength, standard 50mm cubes were cast and tested at 3, 7, and 28 days of curing as per ASTM C109. The values recorded were 31.5MPa, 39.8MPa, and 51.0MPa respectively (ASTM C109, 2016) 36 Table 3.5: FRCM fibre properties Fibre Material Amount Warp Spacing 15 mm Weft Spacing 30 mm Equivalent Dry Fabric Thickness 0.157 mm Weight 280 g/m2 Ultimate Tensile Strength 450* kN/m Axial Stiffness 30,000* kN/m Cross sectional area 157* mm2/m *Per meter width Table 3.6: FRCM mortar properties Mortar Material Amount Compressive Strength 52* MPa Drying Shrinkage 0.09 % Tensile Modulus 26.8 GPa Direct Tensile Bond Strength 2.7* MPa Direct Shear Bond Strength 2.1* MPa Slant Shear Bond Strength 18.1* MPa Unit Weight 2,240 kg/m3 *Values are recorded at 28-day test Table 3.7: FRCM composite properties Composite Material Amount Cracked Tensile Modulus 49,000 MPa Ultimate Tensile Strain 1.1 % Ultimate Tensile Strength 885 GPa Lap Tensile Strength 834* MPa Thickness per Layer 13 mm *per 30cm lap 37 3.4 Specimen Preparation 3.4.1 Specimen Casting The concrete used for this experiment was prepared, mixed, and cast using equipment provided by the UBC School of Engineering Structures Lab (Figure 3.4). The concrete was mixed with a standard 0.07m3 mixing drum. Metal beam molds of 150x150x500mm were used for the double shear and flexure specimens, as they were readily available and provided adequate dimensions for the desired samples. Standard cylinder forms of 100mm in diameter and 200mm in length were used for testing the concretes strength at later test dates specified in Section 3.3.1. The two phases were prepared at different stages of the study. Figure 3.4: Tools used for casting concrete specimens Phase 1: The first phase required eight pours to accommodate the large volume of concrete required to make 72 block specimens. The concrete pouring took place over two days, with four pours per day for the two strengths provided. The concrete was mixed on-site and poured into the a) Mixing drum b) Beam molds c) Cylinder molds 38 forms. Tamping rods and a hand-held vibrator were used to ensure adequate compaction for the cylinders and beam forms following CSA A23.2 guidelines (CSA-A23.1/A23.2., 2014). The concrete for the double shear specimens was air-cured in laboratory settings for 24hrs before demolding, and all the specimens were wrapped in wet burlap and plastic sheet to retain water as they were moist cured for the 28 days. After the 28 days, the samples were air cured. The 150x150x165mm blocks were constructed by cutting the concrete beams into equal thirds with a concrete saw (Figure 3.5). Figure 3.5: Concrete block specimen preparation for double shear samples Phase 2: Following the first samples, the concrete was mixed on-site, and the same procedures as described in Phase 1 were followed. The second phase only required one day of pouring spread over four pours to accommodate the volume needed for 12 beams and 9 cylinders. The notches of the beams were made at the casting stage with a 50x150mm piece of high-density foam that was placed in the beam mold before concrete casting. After 24 hours, the beams were demolded, and the foam pieces were cut out, leaving a notched space in the beam. The concrete a) Moist cure in burlap/sheet b) Concrete saw c) Final cut blocks 39 was cured in a moist curing room for 28 days. After 28 days, the specimens were removed and air-cured in laboratory settings (Figure 3.6). Figure 3.6: Concrete block specimen preparation for flexure samples 3.4.2 Surface Preparation One of the goals of this research is to determine whether the roughening influences the bond strength, or the failure mode of concrete strengthened with FRCM. Therefore, sandblasting, shotblasting and surface grooving methods were used and a combination thereof to compare with untreated surfaces. 3.4.2.1 Surface Roughening The surface of the samples was prepared to meet the requirements of the designed test matrix (CSP level 3 and CSP level 6). Surface roughening was completed using a sandblasting cabinet for CSP 3, and a pressurized abrasive blaster with copper steel slag for CSP 6. The untreated samples were left as cast. Each sample was carefully examined and compared with the respective ICRI scale to ensure conformity among all the samples. An example of the two levels 40 of roughening of Phase 1 samples are shown in Figure 3.7. This method was repeated for the Phase 2 samples, but only sandblasting was performed (Figure 3.8). Figure 3.7: Concrete block surfaces after roughening Figure 3.8: Flexure sample with CSP 3 a) Double shear sample with CSP 3 b) Double shear sample with CSP 6 41 3.4.2.2 Surface Grooving The alternative method of surface preparation used for this experiment was via surface grinding. A mechanical grinder was used to cut the grooves into the surface of the concrete blocks longitudinally with respect to the loading plane. Two grooves were cut at 10mm across by 10mm deep, to replicate a similar experiment performed with FRP (Mostofinejad & Mahmoudabadi, 2010). Surface grooving was completed as an isolated method on an untreated surface (untreated/grinding) and completed as a combined method with sandblasting (CSP 3) and shotblasting (CSP 6). Therefore, the two groups of combined methods that were completed are: sandblasting/grinding and shotblasting/grinding (Figure 3.9). This method was completed for the Phase 2 flexure samples, but only with sandblasting/grinding and untreated/grinding (Figure 3.10). Figure 3.9: Double shear samples with grooves on untreated, CSP 3 and CSP 6 42 Figure 3.10: Flexure samples with grooves on untreated and CSP 3 3.4.2.3 Phase 2 Anchor An anchor was added to the untested side of the beam to ensure that the FRCM would fail along the desired test area. The anchor chosen was a simple full wrap of bi-directional FRCM. The bi-directional layer is more flexible than the unidirectional and can resist forces in both directions. The sharp edges of the beam were mechanically smoothed to create a rounded profile of a radius of roughly 10mm (Figure 3.11), before applying the U-wrap, to avoid failure due to stress concentrations. 43 Figure 3.11 Rounded edges on anchor location 3.4.3 Fabric Preparation Phase 1: The fabric was cut into lengths of 830mm, to allow 165mm of bonded length on each side of the blocks and 500mm free length for the double shear test setup (250mm each side). The fabric was cut at widths of 60mm and 120mm to account for the width variable (Figure 3.12). Phase 2: The fabric was cut into lengths of 350mm to allow space on each end of the beam for the support placement with 50mm unbonded length over the notch and 135mm bonded length for anchor region. An additional 50mm distance away from each support was provided to minimize the effect of the compression region on the bond section (Figure 3.12). 44 Figure 3.12: Fabric lengths for double shear (front) and flexure (top) specimens 3.4.4 Strain Gauges KYOWA (type KFG-5-120-C1-11) strain gauges (SG) were placed on the fibres prior to application to measure the strain of the fibres under loading. Four locations along the fibres were chosen, outside the bonded length (SG: 0 Figure 3.13), 45mm in the bonded area on the centermost fibres (SG :1 Figure 3.13), 150mm in the bonded area from the loaded end on the centermost fibres (SG: 2 Figure 3.13) and 75mm from the loaded end on the outermost fibres (SG: 3 Figure 3.13). This aids in understanding the non-uniformity of stress along the fibres bundles when the mortar does not penetrate the bundles evenly. A thin layer of epoxy was applied to seal the fibres’ surface at the strain gauges locations to smooth the region. This location heavily sanded to create a smooth surface for the epoxy to adhere to. The strain gauges had 5mm gauge lengths were used, and a layer of wax was coated to protect the gauge during casting and testing. The strain gauges were applied and tested prior to applying the FRCM onto the concrete. Strain gauge procedures can be found in (Appendix A: Application Procedures). 45 Figure 3.13: Strain gauge locations 3.4.5 FRCM Application The FRCM application followed the standard practice as guided by ACI 549.1R-13 and supplier guidelines, alongside several research experiments (Appendix A: Application Procedures). Following the application, the mortar was cured for several hours before soaked burlap and plastic sheet were placed over the mortar. The FRCM was cured for several days in this condition before being removed and remained at laboratory conditions for at least 28 days before testing. Phase 1: For the double shear specimens, each side of the block had to be applied on different days, to allow the first side to dry completely. After the first side was applied, the next day the block was flipped over, and the layer was applied on that side. (Figure 3.14). 46 Figure 3.14: FRCM Application for blocks Phase 2: The FRCM application for the flexure specimens followed the same steps (Appendix A: Application Procedures), although there was slight variation with the beam and anchor set up. After saturation, the layer of FRCM was applied to the concrete (Figure 3.15), the anchor had to be fixed in place. This process was completed in one sitting while rotating the beam to complete each side of the anchored end. Figure 3.15: FRCM application process for beams a) Concrete saturation b) First mortar layer c) Final mortar layer a) Beam saturation b) FRCM and anchor mortar c) U-wrap Anchor 47 3.5 Test Setup Phase 1: An MTS was used to test the double shear specimens. The specimens were axially loaded in a double shear test with steel plates connected to grips. A curved support on the top plate helped to avoid localized stress points along the fibres, and uniformly distributed the loads to the bonded regions. The specimen was tested under displacement controlled at approximately 2mm/min until failure was detected. Failure detection response was defined when a 25% drop in the load was recorded or when the sample failed suddenly causing the test to stop. LabView software was used in unison to record and tabulate data from the strain gauges, while the data acquisition software recorded the axial load and displacement. Cameras were placed around the specimen to locate failure at any area. A view of the test setup cross-sections along with the actual image can be seen in Figure 3.16 - Figure 3.17. Figure 3.16: CAD rendering of double shear block setup 48 Figure 3.17: Actual test setup for Phase 1 Phase 2: The specimens were tested in 3-point bending in a universal compression test machine at a span length of 450mm under displacement controlled (2mm/min) until failure was detected. Failure detection response was based on a 25% drop in maximum load. LabView software was used in unison to record and tabulate data from the strain gauges while the Instron machine recorded the flexure load. Cameras were placed around the specimen to locate failure along the length of the beam. Specifically, one was placed facing the anchored region, one along the notch, and along the full span of the beam. Concrete markers were used to identify cracks that occurred throughout the duration of the test. A view of the test setup cross-sections along with the actual image can be seen in Figure 3.18 - Figure 3.19. 49 Figure 3.18: CAD rendering of flexure beam setup Figure 3.19: Actual test setup for Phase 2 50 CHAPTER 4: EXPERIMENTAL RESULTS AND DISCUSSION 4.1 Double Shear Bond Testing 4.1.1 Phase 1: General The test matrix provided in Table 3.1 was designed to determine the effect of the bond strength and failure mode of concrete blocks strengthened with FRCM strips in a double shear bond test given varying parameters. The measured parameters were comprised of surface preparation method (untreated, sandblasting (CSP 3), shotblasting (CSP 6), surface grooving, and combination), concrete strength (30MPa and 50MPa), and fabric-mortar width ratio (1:1 (fibre width of 60mm) and 1:2 (fibre width of 120mm)). The performance indicators were the maximum load at failure, failure mode, and the corresponding effect on stress and strain distribution of the composite section. The typical failure modes recorded from this study included rupture of the fabric outside of the bonded region (Rupture Fabric: RF), bond failure at the concrete-matrix interface (FRCM-Debonding: FD) (Figure 4.1), bond failure at the fibre-matrix interface (interlaminar debonding: ID), and fibres slipping from the matrix (slippage: SL) (Figure 4.2). Figure 4.1: Failure planes for fabric rupture and FRCM-debonding failure 51 Figure 4.2: Failure planes for interlaminar-debonding and slippage failure Failure response detection was based on a 25% drop in the load from the maximum load recorded or when sudden failure caused the machine to stop, as seen with some of the RF or FD failure modes. Overall, both the bond failure and the rupture failure modes were commonly present in the double shear test specimens. Other than a few pure de-bonded plates (full area of the matrix separated from the surface of the concrete), all specimens eventually ruptured, regardless of the primary failure mechanism. The failure mechanisms were determined via physical inspection of the specimens during and after the test, and by analyzing the data recorded. During the test, the specimens were carefully observed for cracking either along the surface of the mortar or cracking that fully extended to the concrete surface. Also, any ruptured fibres that may have occurred before failure were recorded, which were easily identified by the loud cracking or snapping noises throughout the test. Physical examinations after the test included: 1) tapping along the mortar area with a hammer to determine if there was any interlaminar debonding within the matrix layers and 2) measuring the length of the fibre distance from the bonded region to determine if any slip occurred. Several of the bond failure specimens had early onset bond failure, eventually leading to rupture. The early failure can be explained as any loss of the bond between the matrix-concrete or matrix-fibres resulted in a decrease in the bonded area, thus resulting in increased tension forces 52 along the fibres, triggering the rupture failure. Each specimen had three repeats, and many of the repeats had inconsistent failure mode. Due to the variability among the repeats, each specimen was analyzed individually for failure mode and bond stress/strain. 4.1.2 Test Results The results were categorized into 6 groups based on the variables that were investigated. Namely, surface preparation group and varying concrete strength and fibre-mortar width ratio to efficiently analyze the results of the 72 double shear specimens. Each sequential group has 12 specimens, with increasing concrete surface area by roughening or grinding; group R0: untreated, group R0G: surface grooving, group R3: sandblasting, group R3G: sandblasting/grinding, group R6: shot-blasting, group R6G: shot-blasting/grinding (Table 4.1). During testing, the samples were not nested, in other terms they were tested at random and not via the groups presented. Table 4.1: Group designation by surface preparation method Group R0 Group R0G Group R3 Untreated Surface grooving Sandblasting Group R3G Group R6 Group R6G Sandblasting and grooving Shotblasting Shotblasting and grooving The test results are presented in Table 4.2 to Table 4.7. The load presented (P) is the maximum load recorded by the data acquisition system at failure. The average shear bond stress of the fibre-matrix interface and the concrete-matrix interface is calculated by: 𝜏𝑓𝑚 =𝑃2𝐿𝑊𝑓 and 𝜏𝑐𝑚 =𝑃2𝐿𝑊𝑚 respectively, where (P) is the maximum load recorded, (L) is the length of the bonded region parallel to the loading plane (165mm), Wf is the width of the fabric (60mm or 120mm), and 53 Wm the width of the mortar (150mm), refer to Figure 3.1 for fabric dimensions. The load (P) is divided by two assuming half of the load goes into each side of the fabric. Table 4.2: Group R0 double shear test results Group Specimen Label Maximum Load, P (kN) Bond Stress, 𝝉𝒇𝒎 (MPa) Bond Stress, 𝝉𝒄𝒎 (MPa) Failure Mode R0 DS-R0-S50-W60-1 14.4 0.73 0.29 FD DS-R0-S50-W60-2 20.1 1.01 0.41 FD DS-R0-S50-W60-3 3.7 0.19 0.07 FD DS-R0-S30-W60-1 16.6 0.84 0.34 FD DS-R0-S30-W60-2 17.5 0.88 0.35 FD DS-R0-S30-W60-3 25.6 1.29 0.52 ID DS-R0-S50-W120-1 28.5 0.72 0.57 FD DS-R0-S50-W120-2 33.2 0.84 0.67 RF DS-R0-S50-W120-3 12.7 0.32 0.26 FD DS-R0-S30-W120-1 38.3 0.97 0.77 ID DS-R0-S30-W120-2 21.7 0.55 0.44 FD DS-R0-S30-W120-3 25.5 0.65 0.52 FD Table 4.3: Group R0G double shear test results Group Specimen Label Maximum Load, P (kN) Bond Stress, 𝝉𝒇𝒎 (MPa) Bond Stress, 𝝉𝒄𝒎 (MPa) Failure Mode R0G DS-R0G-S50-W60-1 27.6 1.40 0.56 ID DS-R0G-S50-W60-2 20.8 1.05 0.42 ID DS-R0G-S50-W60-3 26.1 1.32 0.53 RF DS-R0G-S30-W60--1 16.0 0.81 0.32 RF DS-R0G-S30-W60-2 11.1 0.56 0.22 FD DS-R0G-S30-W60-3 20.0 1.01 0.40 SL DS-R0G-S50-W120-1 29.7 0.75 0.60 FD DS-R0G-S50-W120-2 38.8 0.98 0.78 ID DS-R0G-S50-W120-3 27.3 0.69 0.55 ID DS-R0G-S30-W120-1 33.7 0.85 0.68 ID DS-R0G-S30-W120-2 26.1 0.66 0.53 RF DS-R0G-S30-W120-3 22.9 0.58 0.46 FD 54 Table 4.4: Group R3 double shear test results Group Specimen Label Maximum Load, P (kN) Bond Stress, 𝝉𝒇𝒎 (MPa) Bond Stress, 𝝉𝒄𝒎 (MPa) Failure Mode R3 DS-R3-S50-W60-1 28.6 1.44 0.58 RF DS-R3-S50-W60-2 21.7 1.10 0.44 RF DS-R3-S50-W60-3 14.0 0.71 0.28 FD DS-R3-S30-W60-1 11.4 0.58 0.23 RF DS-R3-S30-W60-2 21.1 1.07 0.43 ID DS-R3-S30-W60-3 21.1 1.07 0.43 ID DS-R3-S50-W120-1 25.4 0.64 0.51 RF DS-R3-S50-W120-2 28.4 0.72 0.57 ID DS-R3-S50-W120-3 32.2 0.81 0.65 FD DS-R3-S30-W120-1 39.7 1.00 0.80 RF DS-R3-S30-W120-2 29.0 0.73 0.59 RF DS-R3-S30-W120-3 34.2 0.86 0.69 ID Table 4.5: Group R3G double shear test results Group Specimen Label Maximum Load, P (kN) Bond Stress, 𝝉𝒇𝒎 (MPa) Bond Stress, 𝝉𝒄𝒎 (MPa) Failure Mode R3G DS-R3G-S50-W60-1 26.3 1.33 0.53 RF DS-R3G-S50-W60-2 22.9 1.15 0.46 RF DS-R3G-S50-W60-3 24.0 1.21 0.48 ID DS-R3G-S30-W60-1 20.0 1.01 0.40 RF DS-R3G-S30-W60-2 20.3 1.03 0.41 ID DS-R3G-S30-W60-3 22.5 1.14 0.45 ID DS-R3G-S50-W120-1 36.9 0.93 0.74 ID DS-R3G-S50-W120-2 29.2 0.74 0.59 RF DS-R3G-S50-W120-3 26.6 0.67 0.54 ID DS-R3G-S30-W120-1 32.3 0.82 0.65 ID DS-R3G-S30-W120-2 33.8 0.85 0.68 ID DS-R3G-S30-W120-3 31.7 0.80 0.64 ID 55 Table 4.6: Group R6 double shear test results Group Specimen Label Maximum Load, P (kN) Bond Stress, 𝝉𝒇𝒎 (MPa) Bond Stress, 𝝉𝒄𝒎 (MPa) Failure Mode R6 DS-R6-S50-W60-1 22.5 1.14 0.46 RF DS-R6-S50-W60-2 21.1 1.06 0.43 RF DS-R6-S50-W60-3 22.5 1.13 0.45 SL DS-R6-S30-W60-1 18.6 0.94 0.38 RF DS-R6-S30-W60-2 9.8 0.50 0.20 RF DS-R6-S30-W60-3 22.0 1.11 0.44 SL DS-R6-S50-W120-1 29.2 0.74 0.59 RF DS-R6-S50-W120-2 25.5 0.64 0.52 RF DS-R6-S50-W120-3 33.5 0.85 0.68 RF DS-R6-S30-W120-1 39.6 1.00 0.80 ID DS-R6-S30-W120-2 34.2 0.86 0.69 DB DS-R6-S30-W120-3 23.0 0.58 0.46 RF Table 4.7: Group R6G double shear test results Group Specimen Label Maximum Load, P (kN) Bond Stress, 𝝉𝒇𝒎 (MPa) Bond Stress, 𝝉𝒄𝒎 (MPa) Failure Mode R6G DS-R6G-S50-W60-1 23.8 1.20 0.48 RF DS-R6G-S50-W60-2 23.0 1.16 0.47 RF DS-R6G-S50-W60-3 27.9 1.41 0.56 RF DS-R6G-S30-W60-1 16.5 0.83 0.33 RF DS-R6G-S30-W60-2 22.3 1.12 0.45 RF DS-R6G-S30-W60-3 21.5 1.08 0.43 SL DS-R6G-S0-W120-1 28.2 0.71 0.57 ID DS-R6G-S0-W120-2 27.2 0.69 0.55 SL DS-R6G-S0-W120-3 30.5 0.77 0.62 RF DS-R6G-S30-W120-1 28.7 0.73 0.58 ID DS-R6G-S30-W120-2 27.1 0.68 0.55 ID DS-R6G-S30-W120-3 36.8 0.93 0.74 RF 4.1.3 Failure Modes When comparing bond and tension failure, bond failures were the dominant failure mode with 43 of 72 specimens (59.7%) failing in some form of bond, with the remaining 29 of 72 (40.3%) specimens failing by rupture of the fibres. Of the bond failure modes, interlaminar 56 debonding (ID) failure governs with 23 of 72 (31.9%), followed FRCM-debonding (FD) with 15 of 72 (20.8%), and lastly slippage of the fibres (SL) with 5 of 72 (6.9%). Table 4.8 illustrates a direct view of the failure modes for each respective sample in this study within the respective group described in Section 4.1.2. A non-parametric statistical analysis was completed to determine if the failure mode influences the bond strength, and results can be found in Appendix C: Statistical Analysis. The results indicated that the failure mode has a significant effect on the failure load. Table 4.8: Failure modes per group Group 1 Group 2 Group 3 Untreated (R0) Grinding (R0G) Sandblasting (R3) DS-R0-S50-W60-1 FD DS-R0G-S50-W60-1 ID DS-R3-S50-W60-1 RF DS-R0-S50-W60-2 FD DS-R0G-S50-W60-2 ID DS-R3-S50-W60-2 RF DS-R0-S50-W60-3 FD DS-R0G-S50-W60-3 RF DS-R3-S50-W60-3 FD DS-R0-S30-W60-1 FD DS-R0G-S30-W60-1 RF DS-R3-S30-W60-1 RF DS-R0-S30-W60-2 FD DS-R0G-S30-W60-2 FD DS-R3-S30-W60-2 ID DS-R0-S30-W60-3 ID DS-R0G-S30-W60-3 SL DS-R3-S30-W60-3 ID DS-R0-S50-W120-1 FD DS-R0G-S50-W120-1 FD DS-R3-S50-W120-1 RF DS-R0-S50-W120-2 RF DS-R0G-S50-W120-2 ID DS-R3-S50-W120-2 ID DS-R0-S50-W120-3 FD DS-R0G-S50-W120-3 ID DS-R3-S50-W120-3 FD DS-R0-S30-W120-1 ID DS-R0G-S30-W120-1 ID DS-R3-S30-W120-1 RF DS-R0-S30-W120-2 FD DS-R0G-S30-W120-2 RF DS-R3-S30-W120-2 RF DS-R0-S30-W120-3 FD DS-R0G-S30-W120-3 FD DS-R3-S30-W120-3 ID Group 4 Group 5 Group 6 Shotblast/grind (R6G) Sandblast/grind (R3G) Shotblast (R6) DS-R3G-S50-W60-1 RF DS-R6-S50-W60-1 RF DS-R6G-S50-W60-1 RF DS-R3G-S50-W60-2 RF DS-R6-S50-W60-2 RF DS-R6G-S50-W60-2 RF DS-R3G-S50-W60-3 ID DS-R6-S50-W60-3 SL DS-R6G-S50-W60-3 RF DS-R3G-S30-W60-1 RF DS-R6-S30-W60-1 RF DS-R6G-S30-W60-1 RF DS-R3G-S30-W60-2 ID DS-R6-S30-W60-2 RF DS-R6G-S30-W60-2 RF DS-R3G-S30-W60-3 ID DS-R6-S30-W60-3 SL DS-R6G-S30-W60-3 SL DS-R3G-S50-W120-1 ID DS-R6-S50-W120-1 RF DS-R6G-S50-W120-1 ID DS-R3G-S50-W120-2 RF DS-R6-S50-W120-2 RF DS-R6G-S50-W120-2 SL DS-R3G-S50-W120-3 ID DS-R6-S50-W120-3 RF DS-R6G-S50-W120-3 RF DS-R3G-S30-W120-1 ID DS-R6-S30-W120-1 ID DS-R6G-S30-W120-1 ID DS-R3G-S30-W120-2 ID DS-R6-S30-W120-2 FD DS-R6G-S30-W120-2 ID DS-R3G-S30-W120-3 ID DS-R6-S30-W120-3 RF DS-R6G-S30-W120-3 RF * RF = Rupture Fabric ID = Interlaminar-Debonding FD = FRCM-Debonding SL= Slippage 57 4.1.3.1 Fabric Rupture The most common failure mode observed was rupture of the failure (RF) outside of the bonded region, where, overall, 29 of 72 specimens (40.3%) resulted in this failure pattern. This failure type is characterized by excessive cracking of the fibres resulting in a sudden drop in the load, or when the sudden rupture of the fabric width detached the entire specimen from the test machine. Rupture failure could occur at any length along the fabric area, such as near the loaded end, along the length or immediately before the bonded region (Figure 4.3). Rupture failure governed when there was no other failure mechanism such as cracking of the mortar, or loss of bond at either the concrete-matrix interface or the fibre-matrix interface. Rupture failure was easy to analyze, and in some cases, the abrupt energy release when the fabric failed caused simultaneous chips of mortar to fly off, particularly when failure occurred near the bonded region (Figure 4.3). These cases were carefully reviewed through the videos during the post-test analysis. Figure 4.3: Examples of rupture fabric (RF) failure mode Full rupture at loaded end Partial rupture along length Rupture at bonded end 58 The load-displacement curves of four typical specimens are presented in Figure 4.4. The trends of ruptured specimens either followed a smooth curve until failure occurred, due to the rupture of either multiple bundles or the whole fabric which resulted in a sudden drop in the load to a lesser value or 0. Alternatively, in some cases a few fibres within the bundles experienced early failure, resulting in multiple small drops in the load-displacement curve (Figure 4.4). The remaining load-displacement curves for each specimen group can be found in Appendix B: Load Displacement Graphs. Figure 4.4: Typical load-displacement curves for specimens with ruptured fabric failure (RF) According to these results, two factors can initiate rupture failure, 1) when a strong, uniform bond is achieved between the concrete, fibres, and mortar. In this case, the bond strength overcomes the tensile strength of the fibres, thus resisting the load until the fibres fail, and 2) non-uniformity of stress along the longitudinal fibres creates localized stress points either at the loaded end or immediately at the bonded region, causing early onset rupture. The latter failure causes 05101520253035400 1 2 3 4 5 6 7 8 9 10Force (kN)Displacement (mm)DS-R3-S50-W60-1 DS-R0G-S30-W120-2 DS-R6-S30-W120-3 DS-R3G-S30-W60-1Early ruptured fibres Sudden fabric failure Final ruptured fibres 59 variation in the recorded peak loads found for the samples that failed in rupture, which ranged from 9.8kN to 28.6kN for specimens with 60mm width, and from 23kN to 39.7kN for specimens with 120mm width. However, the majority of the samples with rupture failure averaged a peak load of 23.9kN and 30.1kN for 60mm width and 120mm width, respectively. A breakdown of the number of rupture failures by specimen type and divided by the concrete strength parameter is presented in Figure 4.5. Each group contains 12 specimens as was presented in Table 4.8. Figure 4.5: Number of rupture fabric (RF) failures by specimen type Overall, specimens showed an increasing trend of rupture failures with increasing roughness level via surface preparation method (Figure 4.5), and this can be evaluated within each respective group. Out of groups: R0, R0G, R3 and R3G (untreated, surface grinding, sandblasting, and sand/grinding), group R0 had the lowest number of rupture failures with 1 of the 12 specimens (8.3%) in this group. Group R0G had 3 of the 12 specimens (33.3%), group R3 had 6 of the 12 specimens (50%), and group R3G had 4 of the 12 specimens (25%) with rupture failures. More rupture failures were found with the specimens that contained shotblasting methods as seen with R6 and R6G (shotblasting and shot/grinding), with R6 having 8 of the 12 specimens (66.7%) and R6G having 7 of the 12 specimens (58.3%) with rupture failures. The changing trend of rupture 0123R0 R0G R3 R3G R6 R6GNumber of RF Failures50MPaW60 W1200123R0 R0G R3 R3G R6 R6GNumber of RF Failures 30MPa W60 W12060 failures can be attributed to the roughness level of the concrete surface. The samples with a smooth, non-treated surface resulted in lower bond capacity between the mortar and the concrete, as expected, thus shifting the failure mode from rupture to bond. Moreover, specimens with increased bond capacity due to the increased roughness level, resulted in fewer bond failures as the bond resisted the load until the ultimate capacity of the fibres was reached. Within each respective group, the fibre-mortar width ratio showed a marginal effect with slightly more failures found with specimens having 60mm widths than with the 120mm widths. Overall, out of the 29 rupture failures, 17 had the 60mm width (58.6%), and 12 had the 120mm width (41.4%). This could be attributed to the higher fibre-mortar width ratio as seen with the 60mm fibre width (1:2) as opposed to the 120mm fibre width (1:1). The higher mortar ratio attributes to a better bond at the fibre-matrix interface, shifting the failure mode from bond to rupture for specimens with a 1:2 ratio (60mm fibre width). The groups with concrete strength of 50MPa showed a slightly higher number of rupture failures, (17 of the 29 specimens (58.6%)) compared to specimens with 30MPa concrete streangth (12 of the 29 specimens (41.4%)), respectively (Figure 4.5). This could be attributed to a slight increase in bond performance found with the increased concrete strength (50MPa), resulting in more rupture failures as opposed to bond failure. 4.1.3.2 Interlaminar Debonding The second most common failure mode was interlaminar debonding (ID), which occurred at the fibre-matrix interface or within the 5mm mortar layer. This failure mode was typically characterized by the propagation of cracks along the surface of the mortar, either at the loaded end of the bonded region, or along the edges of the mortar and concrete block (Figure 4.6). Visible failure of the physical specimens was possible, where splitting of the mortar from the fabric was apparent, or when cracking occurred during the test that did not propagate to the concrete-matrix 61 interface (Figure 4.6). Other methods were taken to confirm this failure mode, such as physically examining the specimen to determine whether there was internal debonding that was not visible from the outside. The physical examination included tapping along the mortar face and edges with a hammer, to detect any variation in sound that would indicate debonding may have occurred. While this method was only necessary for a few specimens, it proved to be effective. Figure 4.6: Examples of interlaminar debonding (ID) failure Partial fibre-matrix separation Interface separation Cracking along fibre-matrix interface 62 Graphical examination of data also aided in confirming this failure method. Any irregularities (small dips or fluctuations) in the load-displacement curves were accounted for by cracking of the mortar and confirmed with the videos during the post-test analysis. Figure 4.7 presents the trends for the load-displacement curves for four typical specimens. Large drops in the curves are a result of larger sections of the matrix losing bond, redistributing the loads to the newly unbonded fibres, causing the curves to increase again (Figure 4.7). This trend may repeat itself until the load dropped 25% from the maximum load, or until the fibres ruptured. In other cases, the curves represented tiny fluctuations at the maximum load, which was a result of the mortar cracking until failure occurred (Figure 4.7). The complete set of load-displacement curves can be found in Appendix B: Load Displacement Graphs. Figure 4.7: Typical load-displacement curves for specimens with interlaminar-debonding (ID) failure Additionally, all specimens with this failure mode eventually ruptured or were triggered to rupture immediately after a bond failure occurred. This failure mode had the largest variability in 05101520253035400 1 2 3 4 5 6 7 8 9 10Force (kN)Displacement (mm)DS-R3G-S30-W120-3 DS-R3-S30-W60-2 DS-R6G-S50-W120-1 DS-R0G-S50-W60-1Loss of matrix bond Matrix cracking 63 terms of failure loads, however, with respect to the failure mode, some trends were observed. The number of ID failures by specimen type that failed are shown in Figure 4.8. Groups consisted of 12 specimens, such as presented in Table 4.8. Figure 4.8: Number of interlaminar-debonding (ID) failures modes based on specimen type When evaluating the method of surface preparation in groups: R0, R0G, R3, and R6 (untreated, grinding, sandblasted, and shotblasted) and the combination methods in groups R3G and R6G (sand/grinding, shot/grinding), the following observations were made. The majority of ID failures were observed with groups R0G and R3 and R3G (grinding, sandblasting, and sand/grinding). Ultimately, within the middle spectrum of surface roughness, which leads to an interesting observation. While the substrate preparation method should not have a direct effect on this failure mode, the substrate preparation method increases the available contact area, thus, increasing the surface area of the mortar in contact with the concrete, shifting the failure mode from FD to ID. When the bond strength at the concrete surface is typically low as is the case for group R0 (untreated), the load transfer mechanism is weakened, thus increasing the number of FRCM-debonding (FD) failures and resulting in fewer ID failures. On the other hand, when the bond strength was increased with enhanced roughness (CSP: 3), or with the presence of grooves, 0123R0 R0G R3 R3G R6 R6GNumber of ID Failures50MPaW60 W1200123R0 R0G R3 R3G R6 R6GNumber of ID Failures30MPa W60 W12064 the load transfer is resisted at the concrete-matrix interface, shifting the failure mode to dominant ID failure. When looking at groups R6 and R6G (shotblasting, shot/grinding), the bond strength at the concrete-matrix interface is the highest having the highest surface area in contact with the mortar. For these groups, a dominant fibre rupture (RF) failure mode was observed, with fewer ID failures. When evaluating the concrete strength effect on ID failures, no major trend was observed. However, the 30MPa group had more ID failures, with 14 of the 23 (60.9%) and the latter 9 (39.1%) failures for 50MPa (Figure 4.8). If the bond at the concrete-matrix is weakened by a lower concrete strength, the load is redistributed throughout the matrix, causing failure within the mortar layer. Thus, the higher number of specimens with 30MPa failing with ID failure mode is attributed to this factor. When evaluating the effect of fibre-mortar width ratio, the number of ID failures was dominated by those with a fibre width of 120mm compared to those with 60mm fibre width (Figure 4.9). Figure 4.9: Percentage of interlaminar debonding (ID) failures for each fibre width 0%5%10%15%20%25%Untreated Grinding Sandblast Sand/Grind Shotblast Shot/GrindPercentage of ID failuresW60 W12065 This observation is interesting, as the higher fibre-mortar width ratio for the 60mm fibre width (1:2 fibre to mortar ratio) resulted in fewer failures at the fibre-matrix interface. Therefore, confirms that increasing the mortar area relative to the fibre area would result in lower stresses in the mortar, thus, fewer ID failures. While this analogy is not practical for flexural strengthening of beams where the width is dependant on the beam dimensions, this application can be useful in scenarios where shear strengthening is required with strips of fibres, or, for flexural strengthening of slabs. Additionally, this theory should be further investigated by increasing mortar thickness or fibre layers, and larger fibre width sections where a 1:2 ratio is not practical. 4.1.3.3 FRCM-Debonding The third most common failure mode was FRCM-debonding (FD), characterized as the bond failure at the concrete-matrix interface, resulting in 15 of the 72 (20.8%) failures of this type. This failure mode was detectable by one of two causes: 1) progressive cracking of the mortar along the concrete surface, causing a sudden drops in load to either below 75% of the maximum or until the fibres eventually ruptured 2) full detachment of the matrix, resulting in a sudden drop in load. In the former case, some specimens only had smaller areas of mortar detachment from the substrate, leading to an increase in stress along the remaining bonded fibres which resulted in fibre rupture. A full detachment of the mortar occurred in only 3 of the 72 specimens: DS-R0-S50-W60-3, DS-R0-S50-W120-3, and DS-R0G-S30-W60-2. In general, FD failures occurred with only partial debonding. Examples of some FD failures are presented in Figure 4.10 . 66 Figure 4.10: Examples of FRCM-debonding (FD) failure mode The load displacement graphs for FD failure typically followed a smooth curve until cracking, followed by sudden and often several drops in the load (Figure 4.11). In some cases, early matrix cracking occurred along the concrete interface, however, the composite system resisted the load for some time later. Most of the FD failure specimens resulted in rupture failure after the initial FD failure mechanisms were present, apart from the full pate detachment. This occurred because when the mortar separates from the matrix, the load redistributed unevenly to the fibres, causing rupture. The final failure load depended on the initial location of the loss in Interface separation Full plate debond Partial debonding 67 bond. The complete set of load-displacement graphs can be found in Appendix B: Load Displacement Graphs. Figure 4.11: Typical load-displacement curve for specimens with FRCM-debonding (FD) failure FD failures were generally governed by a lower average peak load than other failure modes. Apart from specimen DS-R0G-S50-W120-1 (Figure 4.11), which resulted in a failure load of 30kN. This specimen was fully bonded until near the end of the test, when a sudden debonding occurred in the upper left portion of the bonded area, leading to an immediate fibre failure. The number of failures per specimen type, categorized by the concrete strength is shown in Figure 4.12. 05101520253035400 1 2 3 4 5 6 7 8 9 10Force (kN)Displacement (mm)DS-R0-S50-W120-3 DS-R0-S30-W60-2 DS-R3-S50-W60-3 DS-R0G-S30-W120-3Debonding at the concrete surface Early matrix crack Sudden FRCM-plate failure 68 Figure 4.12: Number of FRCM-debonding (FD) failures by specimen type This failure mode was dominated by group R0, with 9 of the 12 (75%) specimens in that group failing by this mode. Followed by R0G with 3 of the 12 (25%) specimens, group R3 with 2 of the 12 (16.7%) specimens, and group R6 with 1 of the 12 (8.3%) specimens resulting in FD failures (Figure 4.12). With an untreated concrete surface, the resulting concrete-matrix bond strength is low, leading to most failures occurring at the concrete-matrix interface. Increasing the roughness increases the bond strength, shifting the failure to other modes. When comparing groups R0 and R0G (untreated vs. untreated/grinding), surface grinding does influence this failure mode, with fewer failures at the concrete surface due to the increased mortar-concrete surface area available for bonding. When comparing groups R0G with R3 (grinding vs. sandblasting), a relatively comparable result was achieved, as both having a similar number of FD failures. This result follows the observation made by (Raoof, Koutas, & Bournas, 2016) that surface grinding could be used as an alternative to sandblasting for some applications. Alternatively, when evaluating the results of group R3G, R6 and R6G, (sand/grind, shotblast, shot/grind) these methods proved to be effective in eliminating the failure at the concrete-matrix interface (Figure 4.13). Thus, alternative methods that reach the same level of roughening as per ICRI guidelines could achieve the bond strength required to delay debonding failure with the addition of grooves. 0123R0 R0G R3 R3G R6 R6GNumber of FD Failures50MPaW60 W1200123R0 R0G R3 R3G R6 R6GNumber of FD Failures30MPa W60 W12069 Figure 4.13: Percentage of FRCM-debonding (FD) failures per group The concrete strength had no effect on specimens with the FD failure mode, with 8 of the 15 (53.3%) failures had 50MPa concrete strength and 7 of the 15 (46.7%) failures had 30MPa concrete strength (Figure 4.12). While it was expected that the higher bond strength would result in less failures at this interface, as this group was dominated by group R0 (untreated), this factor alone contributed to most of the FD failures. The fibre-mortar width ratio also had no effect on this failure mode with 8 of the 15 (53.3%) specimens failing by FD had 120mm fibre width and 7 of the 15 (46.7%) specimens failing by FD had 60mm fibre width (Figure 4.12). This type of failure is solely governed by the quality of bond at the concrete-matrix interface, concluding that surface preparation is the governing factor regarding FD failure mode. 4.1.3.4 Fibre Slippage The least common failure mode observed was characterized by pure slippage of the fibres from the matrix, with only 5 of the 72 (6.9%) specimens failing in this manner. Failure was 0.0%10.0%20.0%30.0%40.0%50.0%60.0%70.0%80.0%Untreated Grinding Sandblast Sand/Grind Shotblast Shot/GrindPercentage of FD failureFD70 detected by visual detection of fibres slipping from the mortar, where cracking along the mortar was not present (Figure 4.14). This type of failure occurred when mortar does not fully penetrate the fibre openings, causing the textile section to pull out. This failure is often accompanied by interlaminar bond failure, as the failure occurs along the fibre-matrix interface. However, in the literature, a distinction between interlaminar-debonding failure and slippage failure has been reported, and thus, should not be combined into one failure mode (Awani, El-Maaddawy, & Ismail, 2017) (Bencardino, et al., 2018). Figure 4.14: Examples of slippage (SL) failure mode Graphical analysis of the load-displacement curves was used to confirm the slip failure mode, as the stiffness of the curve collectively decreases (Figure 4.15). In some cases an initial slip caused a drop in load, followed by another increase in load however with reduced stiffness. In most cases slip occurred until some fibres ruptured, causing failure of the specimen. The maximum loads recorded for the slippage failures ranged from 20.0kN to 27.2kN, with an average of 22.5kN. The complete set of load-displacement curves for all samples can be found in Appendix B: Load Displacement Graphs. The number of slip failures by specimen type can be found in Figure 4.16. 71 Figure 4.15: Typical load displacement curves for specimens with slippage (SL) failure Typically, slip displacements were not more than 5mm, except for specimen DS-R0G-S30-W60-3 where the slip was recorded as 10mm and is characterized by the long tail at the end of the curve (Figure 4.15). The other slip specimens eventually lead to rupture failure. Meaning that, although small slips were measured, partial bond of the section remained intact and due to the non-uniform distribution along the fibres, some bundles ruptured. The number of SL failures by specimen type that failed are shown in Figure 4.16. Groups consisted of 12 specimens, such as presented in Table 4.8. 0510152025300 1 2 3 4 5 6 7 8 9 10Force (kN)Displacement (mm)DS-R6-S50-W60-3 DS-R6G-S50-W120-2 DS-R0-S30-W60-3 DS-R6-S30-W60-3Initial slip Final slip 72 Figure 4.16: Number of slippage (SL) failures by specimen type Failures occurred in groups R0G, R6, and R6G (surface grinding, shotblasting, and shot/grinding) (Figure 4.16). However, the failure is purely the fibres detaching from the matrix, and thus, the method of substrate preparation should have no effect. This correlates with the assumption made in section 4.1.4.2, where increased roughness/grinding strengthens the bond at the concrete interface, therefore shifting the major failure modes from FD to ID to RF, and in some cases to SL. When evaluating the fibre-mortar width ratio, another interesting observation was made. Most slip failures, 4 of the 5 (80%) specimens occurred with 60mm fibre widths, and the remaining 20% with 120mm fibre widths. This result correlates with the smaller area for the mortar to penetrate the fibre openings of the 60mm width as compared to 120mm width. Concrete strength had no effect on this failure mode, with 2 of the 5 specimens having 50MPa concrete strength and 3 of the 5 specimens having 30MPa concrete strength. 4.1.4 Bond Stress The bond stress, as mentioned in Section 4.1.3, is categorized via two surfaces, the concrete-matrix interface, and the fibre-matrix interface. In terms of the bond stress experienced at each interface (with the different surface area), it is essential to identify this distinction, as it is affected 0123R0 R0G R3 R3G R6 R6GNumber of SL Failures50MPaW60 W1200123R0 R0G R3 R3G R6 R6GNumber of SL Failures30MPa W60 W12073 by the parameters tested. The concrete strength is directly related to the bond stress experienced at the concrete-matrix interface being a function of the concrete’s properties. The fibre-mortar width ratio is directly related to the fibre-matrix interface, being a function of the fibre’s properties. The surface preparation influences the bond stress experienced at the both the concrete-matrix interface (more directly), and the fibre-matrix interface (more indirectly) as it is a function of the mortar’s surface area. As mentioned in Section 4.1.4.2, the strength of the overall system is enhanced as surface preparation methods are improved, shifting the failure modes and in turn, the increasing bond strength. 4.1.4.1 Fibre-Matrix Bond Stress As discussed in Section 4.1.4, the shear bond stress (𝜏𝑓𝑚) at the fibre-matrix interface is directly related to the fibre-mortar width ratio parameter. To eliminate the effect of the concrete strength, the fibre-matrix interfacial bond stress was normalized by the square root of the concrete strength and plotted against each group via the surface preparation method (Figure 4.17). The reasoning for using the square root of the concrete strength is because the tensile (weak) strength of the concrete is proportional to the square root of its’ respective compressive strength. This plot contains the average of all 72 specimens for 60mm and 120mm fibre widths regardless of the failure mode. The fibre width of 60mm was consistently higher than its 120mm counterpart (Figure 4.17). Fibre widths of 60mm have a fibre-mortar width ratio of 1:2, whereas the 120mm has a fibre-mortar width ratio of 1:1. It could be expected that a higher fiber width (120mm) would result in higher strength, as it would endure more load-carrying capacity. However, it is apparent from this study that the higher ratio (smaller width of 60mm) results in higher bond strength. This is confirmed with the discussion in Section 4.1.3.2, where the higher fibre-mortar ratio resulted in fewer failures at this interface. 74 Figure 4.17: Average fibre-matrix bond stress at failure for all 72 specimens With respect to surface preparation, an overall increasing trend can be noted for groups 1 to 6 (R0, R0G, R3, R3G, R6, R6G). This is attributed to the enhanced bond strength resulting from surface preparation and surface grinding. As mentioned in Section 4.1.4, while the surface preparation parameter is located at the concrete-matrix interface, it has an indirect effect on the fibre-matrix interface, as it is a function of the matrix’s available surface area. Group R3G (sandblast/grind) had the highest bond strength for both fibre-mortar ratios of 60mm and 120mm, and group R0 (untreated) had the lowest bond strength. However, large variability was observed with the bond stress at the fibre-matrix interface. This variability is likely associated with the unavoidable factors such as the FRCM application related to the mortar consistency and operation irregularity. Or, the non-uniformity of stress along the fibres such as unequal fabric strip geometry, or slight offset in loading. While these factors are monitored and carefully controlled in a 0.0000.0500.1000.1500.2000.250Untreated Grinding Sandblast Sand/Grind Shotblast Shot/Grind60 120 Linear (60) Linear (120)𝜏 𝑓𝑚 𝑓′ 𝑐75 laboratory environment, some instances are inevitable. Additionally, the variability in failure modes affects the overall bond stress. When analyzing only the specimens that failed in bond, the analysis was repeated for all modes of bond failure (Figure 4.18), and a similar conclusion was observed with Figure 4.17. Those with fibre 60mm widths were generally higher except for that of group R3 (sandblasting), where the average normalized bond stress for 120mm and 60mm widths was 0.135MPa and 0.125MPa, respectively (Figure 4.18). Otherwise, the normalized bond stress values were relatively the same for Figure 4.17 and Figure 4.18, (all 72 specimens vs. 43 bond failure specimens), proving the accuracy of the data. A slight increase in the trendline found for the 60mm fibre widths in Figure 4.18, further confirming the previous discussion, that by decreasing the fibre width, and thus, increasing mortar ratio which yields slightly higher bond strength. Figure 4.18: Average fibre-matrix bond stress at failure for bond failure specimens 0.0000.0500.1000.1500.2000.250Untreated Grinding Sandblast Sand/Grind Shotblast Shot/Grind60 120 Linear (60) Linear (120)𝜏 𝑓𝑚 𝑓′ 𝑐76 4.1.4.2 Statistical Analysis of the Fibre-Matrix Interface To determine if the factors tested had a statistically significant effect on the bond stress, an analysis of variance (a = 0.05) was completed. The factor information is shown in Table 4.9, where all the variables were fixed factors. The ANOVA tests was the General Linear Model (balanced multiple ANOVA), of which the results are given in Table 4.10. The normality of the data was confirmed with a Q-Q plot and descriptive statistics test, which the results can be found in Appendix C: Statistical Analysis. The dependant variable for this test was the fibre-matrix bond stress, and the independent variables were the surface preparation group, the fibre-mortar width ratio and the concrete strength. The results indicate the fibre width factor to be significant, with a P-value of 0.024 < 0.05. This correlates with the discussion in Section 4.1.4.1 regarding the width effect on the fibre-matrix bond stress. The surface preparation method did not have a significant effect on the bond stress at the fibre-matrix interface, with a P-value of 0.174 > 0.05. Also, the concrete strength did not show any significance on the bond stress, with a P-value of 0.366 > 0.05. The interaction of the factor levels is calculated in the test and implemented in the results of the individual factors. Table 4.9: Factor information for the ANOVA test on fibre-matrix bond stress Factor Type Levels Values SURFACE PREP Fixed 6 1, 2, 3, 4, 5, 6 WIDTH Fixed 2 60, 120 STRENGTH Fixed 2 30, 50 77 Table 4.10: ANOVA for the fibre-matrix bond stress (all specimens) Source DF Adj SS Adj MS F-Value P-Value Significant? SURFACE PREP 5 0.31654 0.06331 1.60 0.174 No WIDTH 1 0.21301 0.21301 5.37 0.024 Yes STRENGTH 1 0.03719 0.03719 0.94 0.336 No Error 64 2.53632 0.03963 Lack-of-Fit 16 0.78890 0.04931 1.35 0.205 Pure Error 48 1.74742 0.03640 Total 71 3.10305 4.1.4.3 Concrete-Matrix Bond Stress As discussed in Section 4.1.4, the shear bond stress (𝜏𝑐𝑚) at the concrete-matrix interface is directly related to the concrete strength parameter. To eliminate the effect of the fibre width, the concrete-matrix interfacial bond stress was normalized by the respective fibre width (Figure 4.19). This plot contains all 72 specimens, regardless of the failure mode. Typically, specimens with 50MPa yielded higher bond strength than those with 30MPa, except for specimens in R0 (untreated) (Figure 4.19). This is counter-intuitive, as with lack of any surface preparation, it was expected that specimens with a higher concrete strength would contribute to a better bond overall. However, as has been recorded in the literature (Awani, Refai, & El-Maaddawy, 2015), the bond strength between concrete composites is mostly dependant on the removal of the laitance layer of the concrete surface. The laitance is the thin layer of weak concrete at the surface due to consolidation and bleeding of water. So, regardless of the strength of the concrete, this layer creates a barrier, thus creating more variability with the results for group R0 (untreated). 78 Figure 4.19: Average concrete-matrix bond stress at failure for all 72 specimens As discussed in Section 4.1.4, the bond stress at the concrete-matrix interface is directly affected by surface preparation, as it is a function of the matrix’s available surface area. Overall, an increasing trend is found for both concrete strengths as the method of surface preparation increases from group 1 to 6 (R0, R0G, R3, R3G, R6, R6G). This is attributed to the enhanced bond strength at the concrete-matrix interface from higher levels of surface preparation. Minimal variability is presented in terms of bond stress for groups 4 to 6, whereas a larger scatter in the data is observed in group 1 and 2, where the surface was not subject to sandblasting or shot blasting. When only the specimens that failed in bond the analysis was repeated for all modes of bond failure (43 specimens) (Figure 4.20), a similar conclusion was observed when comparing to the 72 specimens in Figure 4.19. The trendline slopes of the normalized average bond stress is relatively the same for Figure 4.19 and Figure 4.20. However, the 30MPa specimens bond stress 0.0000.0030.0050.0080.010Untreated Grinding Sandblast Sand/Grind Shotblast Shot/Grind50 30 Linear (50) Linear (30)𝑡 𝑓𝑚𝑊79 increased slightly as seen in Figure 4.20, as a result of removing the rupture failure specimens. Specimens with 50MPa concrete strength typically had more specimens failing in rupture which typically failed at higher loads, therefor reducing the overall bond stress in Figure 4.20. This indicates that the effect that the concrete strength has on the bond stress at the concrete-matrix interface may be low. Figure 4.20: Average concrete-matrix bond stress at failure for bond failure specimens 4.1.4.4 Statistical Analysis of the Concrete-Matrix Interface To determine if the factors tested had a statistically significant on the bond stress, an analysis of variance (a = 0.05) was completed. The factor information is presented in Table 4.11, and all the variable were considered fixed variables. The ANOVA tests is General Linear (balanced multiple ANOVA), of which the results are given in Table 4.12, respectively. The normality of the data was confirmed with a Q-Q plot and descriptive statistics test, which the results can be found 0.0000.0030.0050.0080.010Untreated Grinding Sandblast Sand/Grind Shotblast Shot/Grind50 30 Linear (50) Linear (30)𝑡 𝑓𝑚𝑊80 in Appendix C: Statistical Analysis. The dependent variable for these tests was the concrete-matrix bond stress, and the independent variables were the surface preparation group, the fibre-mortar width ratio, and the concrete strength. The results indicated that the width factor to be significant, with P-value of 0 (Table 4.12). In addition, the surface preparation factor generated slight statistical significance with a P-value = 0.048 < 0.05. Although this signifies that the relationship may not be as strong as the width factor, the surface preparation factor is still considered statistically significant for the concrete-matrix bond stress. This result is due to the increase in bond strength as the surface preparation techniques increase. The concrete strength did not have any effect on the concrete-matrix bond stress, with a P-value of 0.719 > 0.05, which correlates with the results found in section 4.1.4.2, where the concrete strength was also insignificant. The interaction of the factor levels was calculated in the test and implemented in the results of the individual factors. Table 4.11: Factor information for the ANOVA test on concrete-matrix bond stress Factor Type Levels Values SURFACE PREPARATION Fixed 6 1, 2, 3, 4, 5, 6 WIDTH Fixed 2 60, 120 STRENGTH Fixed 2 30, 50 Table 4.12: ANOVA of the concrete-matrix bond stress (all specimens) Source DF Adj SS Adj MS F-Value P-Value Significant? SURFACE PREPARATION 5 0.13829 0.027657 2.37 0.048 Yes WIDTH 1 0.69057 0.690573 59.16 0.000 Yes STRENGTH 1 0.00153 0.001528 0.13 0.719 No Error 64 0.74705 0.011673 Lack-of-Fit 16 0.18155 0.011347 0.96 0.509 Pure Error 48 0.56550 0.011781 Total 71 1.57744 81 4.1.5 Strain Gauge Analysis The strain gauge distribution along the length of the fibres was observed with the four strain gauges applied to the fibres (Figure 3.13). Three of the strain gauges were applied on the centermost fibre bundle, and one on the outermost fibre bundle. The three strain gauges loaded on the centermost bundle were denoted as “unbonded,” “middle,” and “bottom,” where unbonded was taken at 0mm from the bonded end as it is located just outside of the bonded region. The middle strain gauge was installed at 45mm from the loaded end, and the bottom strain gauge was 120mm from the loaded end. The strain gauge on the outermost bundle was denoted as “side,” which was 75mm from the loaded end. As discussed in Section 3.4.2, the strain gauges were applied to one of three repeats for every group. The strain distribution profiles were plotted for each failure mode. The strain gauge distribution was completed at 20% intervals of the maximum load. Any missing strain gauge on the plots indicates the gauge was broken prior to testing, and due to being embedded within the matrix could not be repaired. The concrete strength parameter is negligible in terms of how it may affect the strain on the fibres, as this parameter will only affect the failure mode or the bond stress at the concrete-matrix interface. This parameter was removed from further discussion on the strain gauges. 4.1.5.1 Fabric Rupture The six strain gauge profiles for ruptured fabric (RF) failure are presented in Figure 4.21. 82 Figure 4.21: Strain distribution profiles for specimens with ruptured fabric (RF) failure mode Typically, the strain distribution profiles of specimens that failed by fabric rupture followed a similar trend of large strain between the unbonded and middle strain gauges (0mm and 45mm 02000400060008000100000 45 75 120a) DS-R0G-S50-W60-3 (26.1kN) 02000400060008000100000 45 75 120b) DS-R6-S50-W120-3 (33.5kN) 02000400060008000100000 45 75 120c) DS-R6-S30-W120-3 (23.0kN) 02000400060008000100000 45 74 120d) DS-R6G-S50-W60-3 (27.9kN) 02000400060008000100000 45 75 120e) DS-R6G-S50-W120-3 (30.5kN)02000400060008000100000 45 75 120f) DS-R6G-S30-W120-3 (36.8kN) Distance from the loaded end (mm) Increments of maximum load (%) 83 from the loaded end, marginal strain recorded for the side gauge (75mm from loaded end) and no strain experienced at the bottom gauge (120mm from loaded end) (Figure 4.22). When a good bond between the concrete, the matrix and the fibres was achieved, the load was resisted by the composite system until failure. As a result, strain was generally experienced at all stages along the bonded area until failure, with a steeper slope found for the strain gauges closer to the loaded end. Figure 4.22: Generalized strain distribution for ruptured fabric (RF) failure Specimens presented in Figure 4.21 (a), (b), (c), and (d) follows this generalized trend, with exception of specimens in Figure 4.21 (e) and (f). Generally, at the start of the test (20% and 40%), the unbonded (0mm) and middle (45mm) strain gauges were picking up the load causing the steeper strain curves observed at these locations. As the load increases, the side (75mm) strain gauge starts to pick up load, signifying the load distribution across the bonded area, and thus, indicating perfect bond between the matrix, fibres, and concrete. In some cases, the bottom strain gauge picks up load near the end of the test (80% and 100%). There was some variation in the 84 general trend, such as seen with profile in Figure 4.21 (b) and (f). Although the strain profile in Figure 4.21 (b) has lower total strain, the profile follows the same general trend. The failure load for this specimen was 33.5kN, indicating that this likely due to non-uniformity of stress along the fibres, where some of the bundles may have experienced more strain, however, not the bundle with the strain gauges. This correlates with the strain profile in Figure 4.21 (f), where the unbonded and middle strain gauges experienced the same strain until failure, and the side gauge broke at 80% maximum load. Higher strain was recorded for the two specimens with a 60mm fibre width, (Figure 4.21 (a) and (d)), which is a result of the higher stress recorded for specimens with the 60mm fibre width compared to those with the 120mm fibre width as was discussed previously in Section 4.1.4. 4.1.5.2 Interlaminar-Debonding The eight strain profiles for interlaminar-debonding failure mode generally had a similar strain profile and are presented in Figure 4.23. 85 Figure 4.23: Strain distribution profiles for specimens with interlaminar-debonding (ID) failure 02000400060008000100000 45 75 120a) DS-R0-S30-W60-3 (25.6kN) 02000400060008000100000 45 75 120b) DS-R0G-S50-W120-3 (27.3kN)02000400060008000100000 46 75 120c) DS-R3-S50-W60-3 (22.9kN) 02000400060008000100000 45 75 120d) DS-R3-S30-W120-3 (34.2kN) 02000400060008000100000 45 75 120e) DS-R3G-S50-W60-3 (24.0kN)02000400060008000100000 45 75 120f) DS-R3G-S30-W60-3 (22.5kN)02000400060008000100000 45 75 120g) DS-R3G-S50-W120-3 (26.6kN)02000400060008000100000 45 75 120h) DS-R3G-S30-W120-3 (31.7kN) Distance from the loaded end (mm) Increments of maximum load (%) 86 Typically, the strain distribution followed roughly parallel strain profiles along the fibre length as the load increased (Figure 4.24). Relatively proportional strain is experienced at the unbonded strain gauge (0mm from the loaded end), the middle strain gauge (45mm from the loaded end) the side strain gauge (75mm from the loaded end) and in most cases the bottom strain gauge (120mm from the loaded end). This general distribution is a result of the failure mode. When there is a loss of the bond between the matrix and the fibres, the fibres will experience more strain at the newly unbonded locations due to not being embedded in the matrix. Therefore, as the load increases, the resulting strain has relatively equal slopes, which are experienced along the whole length of the fibres throughout the test, including the bottom strain gauge (120mm). This profile is highly generalized due to the large number of broken strain gauges observed, which correlates with this failure mode as well. When the matrix experiences a loss of bond surrounding the strain gauge, the redistribution of stress along the fibres is often strong enough to break the strain gauge at that location. Figure 4.24: Generalized strain distribution trend for interlaminar-debonding (ID) failure 87 This generalized trend follows that of the strain profiles in Figure 4.23 (a), (b), (c), (g), with minor exception for the profile in Figure 4.23 (f) and (h). However the strain profiles in Figure 4.23 (d) and (e) do not closely follow this trend. The strain profile in Figure 4.23 (d), higher strain was recorded, and this curve follows more closely with that of a ruptured specimen. However, some of the interlaminar-debonding (ID) specimens exhibited an ID failure mechanism such as cracking of the matrix, followed very closely by rupture failure, which is the case for this specimen. Specimen in Figure 4.23 (e) experiences no strain at the side (75mm) and bottom strain gauges (120mm), indicating non-uniform distribution of forces along the fiber area. The minor exception seen with the strain profile in Figure 4.23 (f) and (h) is due to the lack of strain experienced at 20% maximum load, however, may have picked up load shortly after 20%. Otherwise, these two profiles follow the general trend. On average, the maximum strain recorded was 5000 micro-strain. The maximum strain is relatively equal for that of the fibre widths of 60mm and 120mm. As discussed in Section 4.1.4.1, fibre widths of 60mm typically had higher stress than fibre widths of 120mm, however, higher stress does not always relate to higher strain because stress is measured as the average along the length and not the actual peak stress. Also, the strain largely depends on the location of the failure within the matrix, and how the failure mechanism affects the strain values at that location. 4.1.5.3 FRCM-Debonding The 6 strain gauge profiles for FRCM-debonding (FD) failure are presented in Figure 4.25. 88 Figure 4.25: Strain distribution profiles for specimens with FRCM-debonding (FD) failure Specimens with FRCM-debonding failure mode followed a typical strain distribution shown in Figure 4.26. The trend typically had high levels of strain for the unbonded strain gauge 02000400060008000100000 45 75 120a) DS-R0-S50-W60-3 (3.7kN) 02000400060008000100000 45 75 120b) DS-R0-S50-W120-3 (12.7kN) 02000400060008000100000 45 75 120c) DS-R0-S30-W120-3 (25.5kN)02000400060008000100000 45 75 120d) DS-R0G-S30-W120-3 (22.9kN) 02000400060008000100000 45 75 120e) DS-R3-S50-W60-3 (14.0kN) 02000400060008000100000 45 75 120f) DS-R3-S50-W120-3 (32kN)Increments of maximum load (%) Distance from the loaded end (mm) 89 (0mm from the loaded end), with a steeper decline to the middle strain gauge (45mm from the loaded end). Little or no strain experienced at the side strain gauge (75mm from the loaded end), and typically, no strain was experienced at the bottom strain gauge (120mm from the loaded end) (Figure 4.26). Few strain gauges were broken for these groups, due to the cracking of the matrix and leaving large sections of unbonded mortar. Typically, failure at the concrete-matrix interface was close to the loaded end, therefore, there was no force distribution to the bottom end of the bonded area. Only the top part was fully engaged, resulting in the higher strains recorded for the unbonded strain gauge (0mm from the loaded end), and failure generally occurred at lower loads, resulting in the lack of strain experienced at the side and bottom strain gauges (75mm and 120mm from the loaded end). Figure 4.26: Generalized strain distribution trend for FRCM-debonding (FD) failure Although variations were noted in the peak strains recorded, this generalized trend follows that of the strain profiles shown in Figure 4.25 (b), (c), (e), and (f), with minor exception in Figure 90 4.23 (a) while the strain profile in Figure 4.23 (d) does not follow this trend. For strain profile in Figure 4.23 (d), roughly equal yet low strain is experienced in all gauges. This specimen’s failure plane was on the side of the block that was not instrumented with strain gauges. As the failure was not located near the strain gauges, spikes in the curves were not recorded, and generally all strain gauges experienced some strain until the specimen finally failed. Specimen with the strain profile in Figure 4.23 (a) showed equal strain values for the unbonded (0mm) and middle (45mm) strain gauges. This is due to the full debonding of the mortar in the upper portion of the block, resulting in the same force (and thus, strain) for both those locations. Specimen with strain profile in Figure 4.23 (c), appears to follow a different trend than the generalized profile, as mentioned, however although the peak at the unbonded gauge was not recorded, the remainder of the curve follows this trend with much lower values of strain. A peak in the strain was observed at 100% of the maximum load for the side strain gauge (75mm) resulting in higher strain than the middle (45mm) strain gauge. This is a good example of non-uniformity of force distribution often seen along the bonded length. Moreover, the variability in terms of peak strain at the maximum load was due to of the nature of this failure mode. Many of the specimens had FRCM-induced failure mechanisms of early cracking or separation of the matrix from the concrete due to lack of surface preparation. This occurrence often happened at lower loads, and whether that mechanism caused immediate failure of the specimen or prolonged failure, the result presents variability in the failure loads recorded, and thus the strain. Once again, the maximum strain is relatively the same for fibre widths of 60mm and 120mm, and as discussed in Section 4.1.5.2, the strain depends on the location of the failure within the matrix. 4.1.5.4 Slippage The four strain gauge profiles for slippage (SL) failure are presented in Figure 4.27. 91 Figure 4.27: Strain distribution profiles for specimens with slippage (SL) failure The generalized trend for this failure mode is presented in Figure 4.28. This trend shares similar characteristics with the generalized trend for interlaminar-debonding failure, however, with two noticeable differences, higher and relatively parallel strains values were recorded for the unbonded and middle strain gauges (0 and 45mm from the loaded end). A noticeable change in the slope of the curve was noted for the side and bottom strain gauges (75mm and 120mm from the loaded end), and on average, strain experienced at all stages. This is attributed to the nature of slip failure. The fibres pull from the matrix, the gauges closer to the loaded end (unbonded (0mm) and middle (45mm)) experience similar strain, proportional to the distance. As the test proceeds, 02000400060008000100000 45 75 120a) DS-R0-S30-W60-GY-3 (20.0kN) 02000400060008000100000 45 75 120b) DS-R6-S50-W60-3 (22.5kN)02000400060008000100000 45 75 120c) DS-R6-S30-W60-3 (22kN) 02000400060008000100000 45 75 120d) DS-R6G-S30-W60-3 (21.5kN)Increments of maximum load (%) Distance from the loaded end (mm) 92 forces are distributed to the lower half of the block, and thus, the strain gauges farther away from the loaded end (side and bottom) pick up the load. Figure 4.28: Generalized strain distribution trend for slippage (SL) failure Although variations were noted in the peak strains recorded, this generalized trend follows the profiles presented in Figure 4.27 (a), (b), (c) and (d), with slight variation. All the profiles have a fibre width of 60mm, and while the maximum strain was not uniform throughout, the slip can occur on either face of the concrete block, meaning that for the lower maximum strains recorded, the slip occurred on the face without strain gauges. In terms of surface preparation, if a good bond was achieved at the concrete-matrix interface, the load is resisted by the strength at the fabric-matrix interface. In these cases, the bond at the fibre-matrix interface was hindered, causing the fibres to pull out and yielding these strain profiles. 93 4.1.5.5 Effective Design Strain The generalized strain profiles as presented in Sections 4.1.5.1 to 4.1.5.4 were determined by the strain distributions for each failure mode. The maximum average and standard deviation of the strain for the individual failure modes that followed the generalized trend is presented in Figure 4.29, and the recommended design strain follows (𝜀𝑑𝑒𝑠𝑖𝑔𝑛 = 𝜀𝑓𝑢 − 1𝑆𝑇𝐷), as per the ACI 549 guideline on designing with FRCM (ACI 549.4R-13, 2013). This chart yield the design strain for each failure mode: RF - 6000 micro strain, SL - 4000 micro strain, ID - 3000 micro strain, and FD - 1000 micro strain (Figure 4.29). Figure 4.29: Average maximum strain and average design strain for each failure mode For the rupture failure, most of the specimens had surface preparation methods with shot blasting for a CSP level 6 (shotblast and shot/grind). To design for a rupture failure mode recommended, a CSP level 6 should be achieved, and the design strain should be limited to 6000 micro strain (Table 4.13). For the slip failure mode, again, most of the specimens had preparation FD, 1061ID, 3019SL, 4179RF, 62630200040006000800010000FD ID SL RFMicrostrain (mε)Average Strain Standard Deviation Design Strain94 methods yielding a CSP level 6 surface. However, with one having a surface grooving method and few data points to support the slippage failure, the achievable surface preparation is taken as a CSP level 3. The design strain for this failure mode should be limited to 4000 micro strain (Table 4.13). For the interlaminar-debonding failure mode, most of the specimens had a surface preparation method with a CSP level 3 (sandblast and sand/grind), and the resulting design strain should be limited to 3000 micro strain (Table 4.13). As slippage and interlaminar-debonding failures both occur at the fibre-matrix interface, the design can be optimized with the fibre-mortar width ratio. Slippage failure favored the 1:2 ratio, and interlaminar-debonding favored the 1:1 ratio and thus, these failures can assume their respective ratio (Table 4.13). The FRCM-debonding failure mode is not recommended for any design purpose, however, if no preparation of the concrete surface is taken, the limited design strain is 1000 micro strain (Table 4.13). Additionally, grooves can be added to the surface of the concrete to avoid debonding and can improve the bond strength. If the design strain per failure mode is not met, the designer can choose to change the area of the FRCM applied for strengthening. It should be emphasized that the proposed design recommendations are based solely on the specimen results from this study. Table 4.13: Proposed design recommendations Preparation Level FM Ratio Design Strain (mε) Expected Failure Mode CSP level 6 NA 6000 Rupture CSP level 3 1:2 4000 Slippage 1:1 3000 Interlaminar-debonding No treatment NA 1000 FRCM-debonding 95 4.2 Flexural Bond Testing 4.2.1 Phase 2: General The test matrix provided in Table 3.2 was designed to determine the effect of bond strength and failure mode on small concrete beams strengthened with FRCM strips in 3-point bending given varying parameters. This experiment was designed to check if different loading conditions would affect the bond behaviour, therefore, only one parameter was examined: surface preparation. To compare with groups F-R0, F-R0G, F-R3 and F-R3G of the double shear specimens, the fibre-mortar width ratio and the concrete strength parameters were held constant at 120mm fibre width and 30MPa concrete strength. The performance indicators were the maximum load at failure, failure mode, and the corresponding effect on stress and the resulting strain distribution of the composite section. The most common failure modes recorded from this study included failure by fibres slipping from the matrix (slippage: SL), with two accompanied by partial rupture of the fabric (slippage/ruptured fabric: SL/RF) and one failure was recorded with a shear failure of the concrete (shear crack: SC). One sample, F-R3G-3, which was omitted from the analysis of this study due to machine malfunction during testing. 4.2.2 Failure Detection Failure response detection was based on a drop in the failure load to below 75% of the maximum load recorded, or when the test was manually stopped due to major slip failure, causing a slow decline from the maximum load. Sudden failure was not recorded for any specimens other than the omitted sample, F-R3G-3. The failure mechanisms were determined via physical inspection of the specimens during and after the test, and by analyzing the data recorded. During the test, the specimens were carefully observed for cracking either along the surface of the mortar 96 or cracking that fully extended to the concrete surface; however, few cracks were observed with the mortar. The notch within the beam initiated concrete cracking, which is intended to initiate a region to control debonding for this type of bond test (Figure 4.30). Once the initial cracking of the concrete began, the fibres began slipping from the mortar, which was visible throughout the duration of the test. Figure 4.30: Failed beam under 3-point bending bond test 4.2.3 Test Results The test results are presented in Table 4.14. The load presented (P) is the maximum load recorded by the data acquisition system at failure while (F) is the resulting tension force in the FRCM, which was calculated using cracked section analysis for a rectangular concrete section (Figure 4.31). Shear bond stress of the concrete-matrix interface was determined as 𝜏𝑐𝑚 =𝐹𝐿𝑊𝑚, where (L) is measured as the length of the bonded region parallel to the loading plane (165mm), Cracking along the beam at notch location 97 and Wm is the width of the mortar (150mm). The notch in the beam was ignored in the calculations, as it was outside of the bonded region. Table 4.14: Test results of the flexural bond specimens Group Specimen Label Max Load, P (kN) Tension Force F, (kN) Bond Stress, 𝝉𝒄𝒎 (MPa) Failure Mode F-R0-1 24.1 26.4 1.06 SL F-R0 F-R0-2 21.0 28.6 1.16 SL F-R0-3 14.2 21.8 0.88 SL/RF F-R0G-1 34.5 18.4 0.74 SL F-R0G F-R0G-2 37.4 16.0 0.65 SL F-R0G-3 28.6 10.8 0.44 SL F-R3-1 30.9 23.6 0.95 SH F-R3 F-R3-2 29.4 22.4 0.91 SL F-R3-3 36.1 27.6 1.11 SL F-R3G-1 21.7 20.4 0.82 SL F-R3G F-R3G-2 30.1 23.0 0.93 SL/RF F-R3G-3* 18.5 - - - * Specimen omitted from analysis Figure 4.31: Stress distribution for a cracked rectangular beam 4.2.4 Failure Modes The dominant failure mode of the flexural test specimens was slip, with 8 of the 11 specimens (73%) failing by pure slippage of the fibres, followed by the failure of slippage accompanied by ruptured fibres (SL/RF) with 2 of the 11 (18%) specimens with this failure mode, and lastly 1 of 98 the 11 (9%) specimens with concrete shear failure mode. The number of failures per specimen type is presented in Figure 4.32. Figure 4.32: Number of failures per specimen type Slip failure was dominant in all surface preparation method groups, with surface grinding samples (F-R0G) having all three failures be in this form of failure. The shear crack failure was in the sandblasting group (F-R3) and shows good bond was achieved, and the RF/SL failures appear to not favor any surface preparation group, with one in the untreated group (F-R0) and the other in the combination of surface grinding and sandblasting group (F-R3G). No failures were recorded at the concrete-matrix interface, which was expected to occur with samples that had no surface preparation. This result leads to the conclusion that under bending, the bond between the concrete and the mortar exceeds that of the fibre-matrix interface. 4.2.4.1 Slippage The slip (SL) failure dominated the flexural test group, which is interesting as this was the least reported failure mode for the double shear specimens. Non-uniform distribution along the fibres occurs when the mortar lacks to effectively penetrate the fibre openings. Fibre bundles are 0123R0 R0G R3 R3GNumber of FailuresSL/RF Failure SL Failure SH FailureF-R0 F-R0G F-R3 F-R3G 99 concentrated within the matrix, and relative slip may occur between the inner and outer bundles, until the capacity of the fibre-matrix bond is reached, which is apparent for the flexure samples. This creates the variability in failure loads recorded for specimens with SL failure. Failure occurred without the presence of mortar cracks, and fibre slips were recorded up to 40mm (Figure 4.33). The SL, or SL/RF failure mode occurred in all specimen types regardless of surface preparation. However, the surface grinding samples (F-R0G) had a consistent failure of this type, with all three repeats failing in this manner with similar maximum loads, averaging 33.5kN. Figure 4.33: Examples of slippage (SL) failure mode The load-deflection curve for typical slip samples are presented in Figure 4.34. One slip sample was taken from each group to show the distribution of the curve. The curves are characterized by initial cracking of the concrete at the notch site, which initiates debonding, followed by a sudden drop in load as the slip is initiated and begins to pull from the matrix until the test was stopped. During the initial cracking phase, when the concrete cracks a noticeably drop Approx. 15mm Approx. 25mm 100 in the load is noted in the curves, then the loads are redistributed, increasing the load onto the fibres which pick up the load and the curves increase. This is repeated for every concrete crack that occurs until a different failure mechanism is introduced. In this case, the failure mechanism introduced at the peak load (Figure 4.34) is the loss of bond between the fibre and the matrix area, causing the fabric section to pull out. The individual load-displacement curves for all SL specimens can be found in Appendix B: Load Displacement Graphs. Figure 4.34: Load-displacement curve for typical slippage failure 4.2.4.2 Slippage/Rupture Two specimens, F-R0-3 and F-R3G-2 failed by slippage failure accompanied by rupture of fibres. These two samples are also examples of the non-uniformity of forces distributed among the fibre bundles, often seen within the fibre area as it depends on the mortars ability to effective 05101520253035400 5 10 15 20 25Load (kN)Displacement (mm)F-R3-2 F-R0-1 F-R0G-3 F-R3G-1Beam split/initial slip Continuous fibre slippage Initial concrete cracking at notch location 101 penetrate the fibre openings (Figure 4.35). In this case, causing some fibres to rupture as they are overstressed, which was not noted in the other 8 pure slippage specimens. Figure 4.35: Examples of slippage/rupture (SL/RF) failure mode The load-deflection curves for these samples are displayed in Figure 4.36. The load-displacement curves are characterized by the initial cracking of the concrete at the notch location, similar to what was discussed in Section 4.2.4.1, followed by a drop in load due to the initial fibre slip, with continuous slippage and followed by ruptured fibres, yielding the drops in load on the tail end of the curves. While the curves are similar in appearance, F-R0 had a lower load of just 14.1kN, whereas F-R3G reached a maximum load of 28.6kN. In the case of F-R0G, the fibres were fully engaged and resisted a higher maximum load until the bond between the fibres and the matrix failed. Specimens F-R0-3 had the lowest recorded load of all the specimens. This result is an example of non-uniformity of stress along the fibres, which often occurs depending on how the matrix penetrated the openings evenly. The full width of the fibre section may not have been fully engaged, causing the fabric area to slip sooner. Major slip (Approx. 40mm) Major slip (Approx. 15mm) Ruptures 102 Figure 4.36: Load-displacement curve for slippage/rupture (SL/RF) failure 4.2.4.3 Shear Crack Only one sample, F-R3-1, failed within the concrete beam. This failure was characterized by a shear crack that initiated from the location of load application at the center of the beam and propagated through the concrete towards the middle of the bonded region. It should be noted that while the FRCM provided flexural reinforcement, the beam had no shear reinforcement. The cross-section of the beam split into two, separating the FRCM throughout the middle of the section (Figure 4.37). At the location of the FRCM separation, all the fibre bundles had ruptured under the load. This specimen shows an example of bond strength overcoming the shear strength of the concrete, where the bond was resisted at both the concrete-matrix interface and the fibre-matrix interface. 051015202530350 5 10 15 20 25 30 35Load (kN)Displacement (mm)F-R0-3 F-R3G-2Concrete cracking at notch location Fibre ruptures Slip Beam split/initial slip 103 Figure 4.37: Example of shear crack (SH) failure mode The load displacement curve for this specimen (Figure 4.38) is characterized by a high peak load with a sudden drop as the crack fully extended through the beam. The tail end of the curve declined as the beam was continuously separated until the load dropped below 75% of the maximum. Figure 4.38: Load-displacement curve for shear crack (SH) failure 051015202530350 5 10 15 20 25 30Load (kN)Displacement (mm)F-R3G-1Shear crack Shear crack FRCM separation 104 4.2.5 Bond Stress The bond stress at the concrete-matrix interface was analyzed as there was no change in the fibre width for flexural specimens and is presented in Figure 4.39. The largest bond stress is observed with the surface grinding (F-R0G) group, followed closely by sandblasting (F-R3) and then the combination of grinding and sandblasting (F-R3G). This decreasing trend was not expected to occur; however, most of the specimens shared the same failure mode of SL or SL/RF. This failure mode had high variability in terms of failure load, and thus, the bond strength. As mentioned in Section 4.1.4.2 and 4.2.4, the fibre bundles are concentrated, and their effectiveness is dependent on location and compatibility within the matrix, creating non-uniform distribution of forces. This phenomenon is widely recognized in the literature (Bencardino, et al., 2018) (Awani, El-Maaddawy, & Ismail, 2017) (Caggegi, et al., 2017) (D’Antino, Sneed, Carloni, & Pellegrino, 2015) (Tran, Stitmnnaithum, & Ueda, 2014). While the surface preparation showed an increase in bond strength for double shear specimens, here it is not the case. Figure 4.39: Average concrete-matrix bond stress for flexural specimens 0.611.030.990.880.000.200.400.600.801.001.20Untreated Grinding Sandblast Sand/grindBond Stress (MPa)Surface Preparation GroupF-R0 F-R0G F-R3 F-R3G105 4.2.5.1 Statistical Analysis of Bond Stress To determine if the effect of surface preparation on bond stress is significant under flexural loading, a one-way ANOVA (a = 0.05) was performed, with the surface preparation method (fixed variable) as the input variable and the fibre-matrix bond stress as the output variable. The normality of the data was confirmed with a Q-Q plot and descriptive statistics test, which the results can be found in Appendix C: Statistical Analysis. For the omitted specimen, a dummy variable with the average of the two specimens was made to balance the data. The results are presented in Table 4.15. The results from this analysis of variance indicated that the surface preparation has a statistically significant effect on bond stress for these specimens, with a P-value of 0.01 < 0.05. Table 4.15: One-way ANOVA of bond stress at the concrete-matrix interface 4.3 Comparison of Double Shear and Flexural Bond Testing The goal of this research study was to determine if similar parameters of using fabric-reinforced cementitious mortar (FRCM) as a concrete strengthening material were affected by loading conditions. As discussed in Section 2.5, the failure modes often varied when analyzed under flexural and shear loading. Due to the lack of bond testing under flexural loading for FRCM research, the second phase of this experiment was conducted to determine if the failure mode and bond strength were different than that of the double shear bond testing. Source DF SS MS F-Value P-Value Significant? Between Groups 3 0.141537 0.047179 7.142484 0.011886 Yes Within Groups 8 0.052843 0.006605 Total 11 0.19438 106 4.3.1 Failure Modes A direct comparison can be drawn from the double shear groups R0, R0G, R3, and R3G with the 120mm width and 30MPa strength, which have identical parameters to the flexural specimens (Table 4.16). As discussed in Section 4.1.4 and 4.2.4, the failure mode was significantly affected, as slippage was the most common failure mode for flexural samples and not common for double shear samples. While the failure mode is inherently different, there is a similar relationship with the failure mode being at the fibre-matrix interface. For double shear specimens, this was commonly interlaminar bond failure, whereas pure slippage was common for the flexure specimens. Interestingly, failure at the concrete-matrix interface was completely avoided with flexure specimens, even for the untreated samples. These results, however, are the first flexure bond test of its kind for FRCM materials and need further investigation to confirm this theory, and more testing to avoid bond failure at the fibre-matrix interface needs to be completed. Table 4.16: Failure modes of the double shear and flexure bond specimens DS-R0-1 ID DS-R0G-1 ID DS-R3-1 RF DS-R3G-1 ID DS-R0-2 FD DS-R0G-2 RF DS-R3-2 RF DS-R3G-2 ID DS-R0-3 FD DS-R0G-3 FD DS-R3-3 ID DS-R3G-3 ID F-R0-1 SL F-R0G-1 SL F-R3-1 SH F-R3G-1 SL F-R0-2 SL F-R0G-2 SL F-R3-2 SL F-R3G-2 SL/RF F-R0-3 SL F-R0G-3 SL/RF F-R3-3 SL F-R3G-3 OMIT 4.3.2 Bond Stress The bond stress at the concrete-matrix interface is plotted for both the flexural and double shear specimens regardless of failure mode (Figure 4.40). The results indicated that the flexure specimens resulted in consistently higher bond strength, 44.5% higher than the double-shear specimens (Figure 4.40) with the same parameters. This indicates that the loading condition 107 influences the bond strength, and the flexural loading has better bond strength than that of flexure loading, which could attribute to the type of failure as well. With double shear specimens, the slippage failure was the least common, but typically occurred when good bond strength was achieved overall and near maximum load, the fibres slipped instead of rupturing. In the case of the flexural specimens, there was little room for the specimens to rupture outside the bonded region, resulting in dominant slippage failure. Figure 4.40: Comparison of concrete-matrix bond stress for double shear and flexural specimens 4.3.2.1 Statistical Analysis of Bond Stress To determine if the effect on bond stress is statistically significant, a two-way ANOVA (a=0.05) was completed for the bond stress at the concrete-matrix interface. The loading type and surface preparation method were the input variables and the bond stress as the output variable (Table 4.17), of which the factors are fixed. The results of this analysis are presented in Table 4.18. 0.000.200.400.600.801.001.201.40Untreated Grinding Sandblast Sand/grindAverage Concrete-Matrix Bond Stress (MPa)F DS Linear (F) Linear (DS)108 The results of this analysis for the concrete-matrix bond stress indicated the loading conditions, the surface preparation and the interaction of these two factors are considered statistically significant. With the loading condition having a P-value of 0 < 0.05, the surface preparation having a P-value of 0.012 < 0.05 and the interaction resulting in a P-value of 0.036 < 0.05. This indicates that loading conditions should be considered when experimenting with the bond stress for FRCM, as it depicts significance on all levels of comparison from failure mode, to the factors tested and the response on the bond strength. Further experiments should be explored to confirm the current bond test methods and standardize these test methods for varying loading conditions when analyzing the bond behaviour of FRCM. Table 4.17: Factor information for the ANOVA test on concrete-matrix bond stress Factor Type Levels Values Loading Fixed 2 Flexure, Double Shear Preparation Fixed 2 R0, R0G, R3, R3G Table 4.18: ANOVA for the concrete-matrix bond stress (double shear vs flexural specimens) Source DF SS MS F-Value P-Value Significant? Loading (F vs. DS) 1 0.458214 0.45821 31.4645 0.0000 Yes Surface Prep 3 0.221398 0.0738 5.0676 0.0118 Yes Interaction 3 0.15917 0.05306 3.6433 0.0355 Yes Within 16 0.233006 0.01456 Total 23 1.071789 109 CHAPTER 5: CONCLUSION 5.1 Conclusion This thesis was designed to determine the effect of the bond strength and failure mode of 72 concrete blocks strengthened with FRCM strips in a double shear bond testing, and 12 beams strengthened with FRCM in 3-point flexural bending given varying parameters. The tested parameters for double shear tests were comprised of surface preparation method (untreated, sandblasting, shotblasting, surface grinding, and combination), concrete strength (30MPa and 50MPa), and fabric-mortar width ratio (1:1 and 1:2). The tested parameters for flexural tests were based solely on surface preparation method (untreated, sandblasting, shotblasting, surface grinding, and combination). The performance indicators were maximum load at failure, failure mode, and the corresponding effect on stress and strain distribution of the composite section. It should be emphasized that the results of this thesis are within the range of the tested specimens from this study. The following presents the conclusions that can be drawn from this test matrix: 5.1.1 Experiments • Surface preparation had a significant effect on the resulting failure modes of the double shear specimens, with a decreasing number of concrete-matrix bond failures as the level of surface preparation increased. This parameter has no effect on the flexural specimens in terms of failure mode. • The surface preparation parameter was determined as a statistically significant parameter for the double shear specimens and the flexural specimens when considering the bond stress at the concrete-matrix interface when analyzing all specimens regardless of failure mode. 110 • The fibre-mortar ratio of 1:2 resulted in fewer failures at the fibre-matrix interface than the 1:1 ratio, with 34.7% of the specimens with interlaminar debonding failure mode having fibre widths of 60mm and 65.2% having fibre widths of 120mm. This result indicates that increasing the ratio reduces the failure at the fibre-matrix interface. • The fibre-mortar ratio was considered a statistically significant parameter for the double shear specimens when considering the bond stress at the fibre-matrix interface and the concrete-matrix interface. Increasing the fibre-mortar ratio from 1:1 to 1:2 increases the bond strength by up to 31.1% when analyzing all specimens regardless of failure mode, and 13.6% when analyzing specimens that failed in all types of bond failure. • The concrete strength parameter was considered insignificant in terms of failure mode, bond stress, and strain for the double shear specimens. • For double shear specimens, in terms of fibre-mortar width ratio, specimens with 60mm width had, on average, 6.1% higher maximum strains than specimens with 120mm width. Four generalized strain profiles are presented based on failure mode. • From the results obtained, the recommended surface preparation method to avoid bond failure is as follows: o To avoid failure at the concrete-matrix interface: minimum CSP level 3 is recommended o To avoid failure at the fibre-matrix interface: CSP level 6 is recommended o Surface grooving may be added to the concrete to increase the bond strength 111 • Flexural specimens reported the bond strength, on average was 44.5% higher than that of double shear specimens. This result indicates that in one layer of FRCM has higher strength in flexure than direct shear. • The change in loading condition from double shear to flexure for bond testing showed a statistically significant effect on the bond strength when comparing the same parameters. This indicates that the type of bond test performed greatly influences the bond strength. • The failure modes were greatly influenced by the change in loading conditions from double shear to flexural specimens. Flexural specimens resulted in dominant SL or SL/RF failures (90.1%), whereas the double shear specimens resulted in dominant FD, ID, and RF failures with only 8.3% SL failures. 5.1.2 Design Recommendation • From the results obtained, the recommended design strain for each failure mode is as follows: Preparation Level FM Ratio Design Strain (mε) Expected Failure Mode CSP level 6 NA 6000 Rupture CSP level 3 1:2 4000 Slippage 1:1 3000 Interlaminar-debonding No treatment NA 1000 FRCM-debonding 5.2 Research Contribution This initiative taken with this research effort contributes to the research and knowledge on FRCM focused on FRCM and its use as a structural upgrading system and implementation. The 112 flexural loading conditions, the comparison between the double shear and the flexural loading, multiple surface preparations, and fibre-mortar width ratio, is the first effort of its kind completed. This work presents significant findings into the effect of surface preparation and fibre-mortar ratio on reducing bond-related failure modes and increasing bond strength, a large problem associated with this material. Bond failure can be reduced by first selecting the appropriate surface preparation method for a given application to eliminate bond failure at the concrete-matrix interface, and select a fibre-mortar width ratio that will eliminate or reduce the possibility of bond failure at the fibre-matrix interface. 5.3 Limitations Although completed with great success, few limitations should be addressed throughout the experiment. For Phase 1, the large number of samples contributed to increased variability in the consistency of surface preparation and FRCM application techniques, one that was closely looked after but could not be perfected. The target concrete strengths of 50MPa and 30MPa resulted in 48MPa and 36MPa, of which the actual values were used for the analysis and could have contributed to the lack of effect or inconsistency found with the concrete strength analysis. From a data perspective, more instrumentation such as LVDTs to analyze load-slip displacement, and without the loss of several strain gauges would make for more accurate results, as well as anchoring one side of the block to ensure failure would occur on one side, making it easier during testing. For Phase 2, the small number of samples could make for better comparison between Phase 1 and Phase 2. The results could have been improved without the loss of the one omitted sample in Phase 2. Considering both phases, further exploration into using more numerical data for analysis would make for interesting results. Such as more fibre-mortar width ratios with the same concrete strength for a better comparison amongst groups, in addition to more constant variables within groups as 113 the test matrix designed had several alternating variables. Additionally, these tests should be completed with different materials that have different properties and may eliminate failure at the fibre-matrix interface completely. Lastly, it could have proven beneficial to measure the surface preparation in terms of a quantifiable value, by measuring the depth of the depth of the roughness, or by measuring the change in volume and present it in terms of an increase in surface area. 5.4 Future Research Scope The current research work contributed to promising outcomes involving the bond behaviour of FRCM materials. The results helped fill voids in the literature, such as the use of alternative surface preparation techniques and fibre-mortar width ratio to reduce debonding failure seen with this material. 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Jounral of Engineering Mechanics, 142(3). doi:10.1061/(ASCE)EM.1943-7889.0001021 122 APPENDICES Appendix A: Application Procedures The recommended FRCM application procedure is given as: 1) The concrete surface must be clean from any debris or dust 2) The concrete surface must be soaked prior to the FRCM application during the cleaning process 24 hours prior to application and again several hours prior 3) Apply a 5mm thick layer of mortar to the surface of the concrete 4) Firmly place a fabric layer onto the mortar 5) Place another 5mm layer on top of the fabric 6) Each subsequent layer or mortar should be wet between layers to ensure consistency 7) After the final layer is applied the surface can be smoothed with a trowel. The recommended strain gauge application procedure is given as: 1) The location for the strain gauges was measured, marked, and then the surface was sanded down flat using a pneumatic drill and flap disk. 2) The surface was then sanded smoother using a 320-grit general use sandpaper to provide a uniform surface to affix the gauges. 3) The flat section was cleaned used high-pressure air and acetone to remove any FRP particles or dust. 4) A thin coat of general use strain gauge cement was applied to the surface of the rod and the strain gauge was applied while pressure was applied for a minute using a plastic sheet as per the cement manufacturers recommendations (Tokyo Sokki Kenkyujo, 2017). 123 5) A strain gauge terminal was applied directly next to the strain gauge using the same strain gauge cement and the process as in step 4. 6) The lead wires from the strain gauges were shaped towards the terminal and soldered down using rosin flux and soldering wire. 7) Long wires were soldered to the other side of the terminal to provide a connection to the data acquisition (DAQ) system. 8) The strain gauge and wires were sealed with an insulating compound to prevent electrical interference, and further sealed with beeswax to prevent and damage during the bonding process. 9) The strain gauges were tested from the far end of the wires using a handheld voltmeter to ensure the gauges still read 120 +/- 0.5 Ω meaning that no damage had occurred during the application process. 124 Appendix B: Load Displacement Graphs Figure B.1: DS-R0-S50-W60 Figure B.2: DS-R0-S30-W60 05101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R0-S50-W60-GN-1 DS-R0-S50-W60-GN-2 DS-R0-S50-W60-GN-305101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R0-S30-W60-GN-1 DS-R0-S30-W60-GN-2 DS-R0-S30-W60-GN-3125 Figure B.3: DS-R0-S50-W120 Figure B.4: DS-R0-S30-W120 05101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R0-S50-W120-GN-1 DS-R0-S50-W120-GN-2 DS-R0-S50-W120-GN-305101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R0-S30-W120-GN-1 DS-R0-S30-W120-GN-2 DS-R0-S30-W120-GN-3126 Figure B.5: DS-R0G-S50-W60 Figure B.6: DS-R0G-S50-W60 05101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R0-S50-W60-GY-1 DS-R0-S50-W60-GY-2 DS-R0-S50-W60-GY-30510152025300 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R0-S30-W60-GY-1 DS-R0-S30-W60-GY-2 DS-R0-S30-W60-GY-3127 Figure B.7: DS-R0G-S50-W120 Figure B.8: DS-R0-S30-W120 05101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R0-S50-W120-GY-1 DS-R0-S50-W120-GY-2 DS-R0-S50-W120-GY-305101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R0-S30-W120-GY-1 DS-R0-S30-W120-GY-2 DS-R0-S30-W120-GY-3128 Figure B.9: DS-R3-S50-W60 Figure B.10: DS-R3-S30-W60 05101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R3-S50-W60-GN-1 DS-R3-S50-W60-GN-2 DS-R3-S50-W60-GN-305101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R3-S30-W60-GN-2 DS-R3-S30-W60-GN-2 DS-R3-S30-W60-GN-3129 Figure B.11: DS-R3-S50-W12 Figure B.12: DS-R3-S30-W120 05101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm) DS-R3-S50-W120-GN-1 DS-R3-S50-W120-GN-2 DS-R3-S50-W120-GY-305101520253035400 1 2 3 4 5 6 7 8 9 10Load(kN)Displacement (mm) DS-R3-S30-W120-GN-1 DS-R3-S30-W120-GN-2 DS-R3-S30-W120-GN-3130 Figure B.13: DS-R3G-S50-W60 Figure B.14: DS-R3G-S30-W60 0510152025300 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R3-S50-W60-GY-1 DS-R3-S50-W60-GY-2 DS-R3-S50-W60-GY-305101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm)DS-R3-S30-W60-GY-1 DS-R3-S30-W60-GY-2 DS-R3-S30-W60-G-3131 Figure B.15: DS-R3G-S50-W120 Figure B.16: DS-R3G-S30-W120 05101520253035400 1 2 3 4 5 6 7 8 9 10Load(kN)Displacement (mm) DS-R3-S50-W120-GY-1 DS-R3-S50-W120-GY-2 DS-R3-S50-W120-GY-305101520253035400 1 2 3 4 5 6 7 8 9 10Load(kN)Displacement (mm) DS-R3-S30-W120-GY-1 DS-R3-S30-W120-GY-2 DS-R3-S30-W120-GY-3132 Figure B.17: DS-R6-S50-W60 Figure B.18: DS-R6-S30-W60 133 Figure B.19: DS-R6-S50-W120 Figure B.20 DS-R6-S30-W120 05101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm) DS-R6-S50-W120-GN-1 DS-R6-S50-W120-GN-2 DS-R6-S50-W120-GN-305101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm) DS-R6-S30-W120-GN-1 DS-R6-S30-W120-GN-2 DS-R6-S30-W120-GN-3134 Figure B.21: DS-R6G-S50-W60 Figure B.22: DS-R6G-S30-W60 135 Figure B.23: DS-R6G-S50-W120 Figure B.24: DS-R6-S30-W120 05101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm) DS-R6-S50-W120-GY-1 DS-R6-S50-W120-GY-2 DS-R6-S50-W120-GY-305101520253035400 1 2 3 4 5 6 7 8 9 10Load (kN)Displacement (mm) DS-R6-S30-W120-GY-1 DS-R6-S30-W120-GY-2 DS-R6-S30-W120-GY-3136 Appendix C: Statistical Analysis Table C.1: Kruskal Wallace test for failure mode effect on failure load FM N Median Mean Rank Z-Value DB 15 20.0980 25.1 -2.36 ID 23 28.2036 47.2 2.97 RF 29 25.3945 35.8 -0.24 SL 5 21.9773 25.6 -1.21 Overall 72 36.5 Null hypothesis H₀: All medians are equal Null hypothesis Alternative hypothesis H₁: At least one median is different Alternative hypothesis Method DF H-Value P-Value Method Not adjusted for ties 3 11.82 0.008 Not adjusted for ties Adjusted for ties 3 11.82 0.008 Adjusted for ties 137 Figure C.1: Q-Q plot for double shear specimens Table C.2: Descriptive statistics for double shear specimens -4.00-3.00-2.00-1.000.001.002.003.00-3.000 -2.000 -1.000 0.000 1.000 2.000 3.000Q-Q PlotDescriptive Statistics Mean 25.08813 Standard Error 0.873951 Median 25.52842 Mode #N/A Standard Deviation 7.415723 Sample Variance 54.99295 Kurtosis 0.179782 Skewness -0.24255 Range 35.97425 Minimum 3.699913 Maximum 39.67416 Sum 1806.345 Count 72 138 Figure C.2: Q-Q plot for flexure specimens Table C.3: Descriptive statistics for flexure specimens -2.50-2.00-1.50-1.00-0.500.000.501.001.502.00-2.000 -1.500 -1.000 -0.500 0.000 0.500 1.000 1.500 2.000Q-Q PlotDescriptive Statistics Mean 27.6293 Standard Error 2.0548 Median 28.9639 Mode #N/A Standard Deviation 7.1180 Sample Variance 50.6661 Kurtosis -0.4483 Skewness -0.4746 Range 23.2473 Minimum 14.2011 Maximum 37.4484 Sum 331.5514 Count 12
Thesis/Dissertation
eng
Civil Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Interfacial bond strength of fabric reinforced cementitious matrix : effect of substrate properties and fibre-mortar ratio
Text