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Investigation of compressive bond behavior of steel and fiber reinforced polymer bars embedded in recycled… Moallemi Pour, Sadaf 2016

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  Investigation of Compressive Bond Behavior of Steel and Fiber Reinforced Polymer Bars Embedded in Recycled Aggregate Concrete  by  Sadaf Moallemi Pour  B.Sc., University of Tehran, 2012  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)   March 2016    © Sadaf Moallemi Pour, 2016        The undersigned certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis entitled:    Investigation of Compressive Bond Behavior of Steel and Fiber Reinforced Polymer Bars Embedded in Recycled Aggregate Concrete  Submitted by             Sadaf Moallemi Pour                   in partial fulfillment of the requirements of   The degree of       Master of Applied Science                                           .  Dr. M. Shahria Alam, Faculty of Applied Science/School of Engineering Supervisor, Professor (please print name and faculty/school above the line)  Dr. Abbas S. Milani, Faculty of Applied Science/School of Engineering Supervisory Committee Member, Professor (please print name and faculty/school in the line above)  Dr. Kasun N. Hewage, Faculty of Applied Science/School of Engineering Supervisory Committee Member, Professor (please print name and faculty/school in the line above)  Dr. Sunny R. Li, Faculty of Applied Science/School of Engineering University Examiner, Professor (please print name and faculty/school in the line above)   External Examiner, Professor (please print name and university in the line above)   April 19, 2016 (Date submitted to Grad Studies)      iii  Abstract Over the past several years, one of the important issues concerning the construction industry has been sustainable development. Therefore, usage of sustainable building materials and the reuse and recycling of previously used building materials have been considered in many areas. Recycling of construction and demolition (C&D) wastes could prevent their costly disposal which is hazardous to the environment. Utilization of demolished concrete as a coarse aggregate, often termed as recycled coarse aggregate (RCA) for producing industry quality concrete, could lead to a solution to the environmental conservation and pave the way towards sustainability by reducing the consumption of natural aggregate. During the development of this new generation product, it is essential to investigate the structural properties of reinforced RCA concrete to promote its application, in the construction industry. Besides, fiber reinforced polymer (FRP) composite rebars have been widely used in construction instead of steel rebars. Their non-corrosive nature and high tensile strength are the main reason of this replacement. Performance of reinforced concrete members at serviceability and ultimate limit states is controlled by the bond between reinforcement and concrete which is a critical design parameter. This research aims to evaluate RCA application in structural concrete and investigate the bond properties between reinforcing steel/FRP bars and RCA concrete. Bond performance of 35 MPa RCA concrete was assessed through an experimental plan. Five RCA replacement percentages and different diameters of reinforcing bars were used. Effects of embedment length and concrete cover to bar diameter ratio on the bond strength were moreover considered. It was observed that under constant mix proportions, an increase in the bar size and the embedment length to bar diameter ratio leads to a reduction in the bond strength due to growth of voids trapped between the bar surface and concrete. Like regular concrete, larger concrete cover helps improving the bond behavior. On average, acceptable bond strengths were observed in RCA samples which were close to control results. In addition, smaller bar diameter and larger concrete cover can improve bond behavior of specimens embedded with FRPs in comparison with steel ones.  iv  Preface Major portions of this study have been submitted for possible publication in peer-reviewed technical journals and conference proceedings as listed below. The author carried out all experimental work, analyses of results, and writing of the initial draft of all papers listed below. The contributions of her research supervisor is consisted of providing guidance, supervision, and helping in the development of the final versions of the publications. Publications arising from the work presented in this dissertation are listed as follows: Moallemi Pour, S., and Alam, M.S. 2015. "Effect of recycled concrete aggregates on the bond behavior of steel reinforced concrete," CONMAT'15 The Fifth International Conference on Construction Materials. Moallemi Pour, S., and Alam, M.S. 2015. "Study of bond behavior between steel bars and recycled aggregate concrete," CICM 2015 First International Conference on Advances in Civil Infrastructure and Construction Materials. Moallemi Pour, S., and Alam, M.S., Milani, A. S. 2016. "Improved Bond Equations for Fiber Reinforced Polymer Bars in Concrete," submitted to Materials. Moallemi Pour, S., and Alam, M.S. 2016. "Investigation of compressive bond behavior of steel rebar embedded in recycled aggregate concrete," submitted to Structures. Moallemi Pour, S., and Alam, M.S. 2016. "Experimental study on bond performance of steel bars embedded in recycled concrete aggregates," submitted to Journal of Materials in Civil Engineering. Moallemi Pour, S., and Alam, M.S. 2016. "Influence of recycled concrete aggregates on compressive bond performance of GFRP bars in concrete," submitted to Journal of Composites for Construction. v  Table of Contents 1.1 General ....................................................................................................................... 1 1.2 Objective of Study ...................................................................................................... 3 1.3 Research Significance ................................................................................................ 4 1.4 Thesis Outline ............................................................................................................ 5 2.1 General ....................................................................................................................... 6 2.2 What Is RCA? ............................................................................................................ 6 2.2.1 Properties ............................................................................................................ 8 2.2.2 Current Usage ..................................................................................................... 9 2.3 What Is FRP? ........................................................................................................... 11 2.3.1 Properties .......................................................................................................... 12 2.3.2 Current Usage ................................................................................................... 13 2.4 Bond Mechanism...................................................................................................... 14 2.4.1 Factors Affecting Bond Behavior ..................................................................... 17 2.5 Bond Behavior between RCA Concrete and Steel Reinforcement .......................... 19 2.6 Bond Behavior between Normal Concrete and FRP Reinforcement ....................... 19 2.7 Evaluation of Bond Strength .................................................................................... 22 2.7.1 Pull-out Test ...................................................................................................... 22 2.7.2 Beam Test ......................................................................................................... 22 vi  2.7.3 Push-out Test .................................................................................................... 23 2.8 Bond Strength and Development Length Equations in Design Codes..................... 24 2.8.1 Steel Reinforced Concrete ................................................................................ 24 2.8.2 FRP Reinforced Concrete ................................................................................. 26 3.1 Materials ................................................................................................................... 28 3.2 Mix Proportions........................................................................................................ 30 3.3 Preparation of Specimens ......................................................................................... 31 3.4 Push-out Test Setup .................................................................................................. 35 4.1 Compressive Strength .............................................................................................. 38 4.2 General Description.................................................................................................. 39 4.3 Bond Strength ........................................................................................................... 42 4.4 The Effect of Bar Diameter on Bond Strength ......................................................... 44 4.5 The Effect of Embedment Length on Bond Strength ............................................... 44 4.6 The Effect of Concrete Cover on Bond Strength ..................................................... 47 4.7 The Effect of RCA Replacement Level on Bond Strength ...................................... 49 4.8 Sensitivity Analysis .................................................................................................. 51 4.9 Comparison of Measured and Predicted Bond Strength .......................................... 53 5.1 General Description.................................................................................................. 57 5.2 The Effect of Bar Diameter on Bond Strength ......................................................... 60 5.3 The Effect of Embedment Length on Bond Strength ............................................... 62 5.4 The Effect of Concrete Cover on Bond Strength ..................................................... 62 5.5 The Effect of RCA Replacement Level on Bond Strength ...................................... 65 5.6 Sensitivity Analysis .................................................................................................. 67 5.7 Comparison of Measured and Predicted Bond Strength .......................................... 69 5.8 Comparison Study between Steel and FRP Bars Bond Behavior ............................ 74 6.1 Conclusion ................................................................................................................ 81 6.2 Limitations of This Study ......................................................................................... 83 vii  6.3 Future Work ............................................................................................................. 83 viii  List of Tables Table 2-1.      Bond strength related to bond condition for failure by splitting of the  concrete ........................................................................................................... 25 Table 2-2.      FRP-concrete bond equations .......................................................................... 27 Table 3-1.      Physical properties of NCA and RCA ............................................................. 29 Table 3-2.      GFRP  bars properties ...................................................................................... 29 Table 3-3.      Mix proportions of concrete (kg/m3)............................................................... 31 Table 3-4.      Push-out test variable parameters and specimen details for second series  of tests ............................................................................................................. 35 Table 4-1.      Average bond strength of  steel specimens ...................................................... 43 Table 4-2.      Analysis of variance table for steel specimens ................................................ 52 Table 5-1.      Average bond strength of GFRP specimens .................................................... 60 Table 5-2.      Analysis of variance table for GFRP specimens ............................................. 68             ix  List of Figures Figure 2-1.     a) Natural coarse aggregates, b) Recycled coarse aggregates ........................... 7 Figure 2-2.     Microscopic view of different types of coarse aggregate; a) natural coarse aggregate, b) recycled coarse aggregate (Huda, 2014) ..................................... 7 Figure 2-3.     Different types of FRP rebar ........................................................................... 12 Figure 2-4.     Stress-strain plots of FRP (Quayyum, 2010) .................................................. 13 Figure 2-5.     Average flexural bond stress in bottom and top rebar in a beam, a) beam;  b) moment diagram; c) bar forces ................................................................... 15 Figure 2-6.     Cracking and damage mechanisms in bond: a) Side view of a member showing shear crack due to bar pullout, b) Cross-sectional view of a  concrete member showing splitting cracks between bars and through  the concrete cover, (Quayyum, 2010) ............................................................. 16 Figure 2-7.     Typical bond-slip response curve, (Quayyum, 2010) ..................................... 17 Figure 2-8.     Schematic of different pull-out and beam test specimens  (Quayyum & Rteil 2010) ................................................................................ 23 Figure 2-9.     Schematic of push-out test setup ..................................................................... 24 Figure 3-1.     Sieve analysis of coarse aggregate combinations for different concrete  mixes ............................................................................................................... 30 Figure 3-2.     Sieve size analysis for the fine aggregates used .............................................. 30 Figure 3-3.     Completed wooden rebar straightening cap for the 4" diameter concrete cylinder moulds ............................................................................................... 33 Figure 3-4.     One example of the de-bonded rebars with the embedment ratio of 5 for a cylinder size of 100×200mm .......................................................................... 33 Figure 3-5.     Specimens for compression and push-out test after casting ............................ 34 Figure 3-6.     Specimens for compression and push-out test after demolding ...................... 34 Figure 3-7.     Fully assembled push out testing apparatus for steel reinforced specimens ... 36 Figure 3-8.     Draw Wire Sensor used in the push-out test ................................................... 36 Figure 3-9.     Steel plates for testing the GFRP specimens ................................................... 37 Figure 3-10.   Fully assembled push out testing apparatus for GFRP reinforced  specimens ........................................................................................................ 37 x  Figure 4-1.     Concrete compressive strength with age ......................................................... 39 Figure 4-2.     Failure of samples ........................................................................................... 40 Figure 4-3.     Bond stress versus slip response in 200 mm concrete cylinder ...................... 41 Figure 4-4.     Bond stress versus slip response in 300 mm concrete cylinder ...................... 41 Figure 4-5.     Transfer of force through bond ....................................................................... 42 Figure 4-6.     Relationship between bond strength and bar diameter, a) control mix,  b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix,  e) 100% RCA mix ........................................................................................... 45 Figure 4-7.     Relationship between bond strength and embedded length, a) control mix,  b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix,  e) 100% RCA mix ........................................................................................... 46 Figure 4-8.     Relationship between bond strength and concrete cover, a) control mix,  b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix,  e) 100% RCA mix ........................................................................................... 48 Figure 4-9.     Relationship between bond strength and recycled aggregate replacement  level in 200 mm concrete cylinder with le⁄db =5. a) bond strength,  b) normalaized bond strength.......................................................................... 49 Figure 4-10.   Relationship between bond strength and recycled aggregate replacement  level in 200 mm concrete cylinder with le⁄db =10. a) bond strength,  b) normalaized bond strength.......................................................................... 50 Figure 4-11.   Relationship between bond strength and recycled aggregate replacement  level in 300 mm concrete cylinder with le⁄db =5. a) bond strength,  b) normalaized bond strength.......................................................................... 50 Figure 4-12.   Relationship between bond strength and recycled aggregate replacement  level in 300 mm concrete cylinder with le⁄db =10. a) bond strength,  b) normalaized bond strength.......................................................................... 51 Figure 4-13.   Effect of different factors on bond strength; a) concrete cover to bar  diameter ratio, b) embedment length to bar diameter ratio,  c) RCA replacement percentage ..................................................................... 53 xi  Figure 4-14.   Calculated bond stress versus measured bond stress according to  a) Orangun formula, b) Darwin formula, c) ACI 318 fomula, and  d) CEB-FIP formula ........................................................................................ 54 Figure 4-15.   Comparison of measured and predicted bond stress values for small  specimens with le⁄db=5, embedded with a) 20M, and b) 15M bars ................ 55 Figure 4-16.   Comparison of measured and predicted bond stress values for small  specimens with le⁄db=10, embedded with a) 20M, and b) 15M bars .............. 55 Figure 4-17.   Comparison of measured and predicted bond stress values for large  specimens with le⁄db=5, embedded with a) 20M, and b) 15M bars ................ 56 Figure 4-18.   Comparison of measured and predicted bond stress values for large  specimens with le⁄db=10, embedded with a) 20M, and b) 15M bars .............. 56 Figure 5-1.     Bond stress versus slip response in 200 mm concrete cylinder ...................... 58 Figure 5-2.     Bond stress versus slip response in 300 mm concrete cylinder ...................... 58 Figure 5-3.     Failure of FRP samples with a) splitting failure; b) splitting and  push-out failure; c) push-out failure ............................................................... 59 Figure 5-4.     Relationship between bond strength and bar diameter, a) control mix,  b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix,  e) 100% RCA mix ........................................................................................... 61 Figure 5-5.     Relationship between bond strength and embedded length, a) control mix,  b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix,  e) 100% RCA mix ........................................................................................... 63 Figure 5-6.     Relationship between bond strength and concrete cover, a) control mix,  b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix,  e) 100% RCA mix ........................................................................................... 64 Figure 5-7.     Relationship between bond strength and recycled aggregate replacement  level in 200 mm concrete cylinder with le⁄db =5. a) bond strength,  b) normalaized bond strength.......................................................................... 66 Figure 5-8.     Relationship between bond strength and recycled aggregate replacement  level in 200 mm concrete cylinder with le⁄db =10. a) bond strength,  b) normalaized bond strength.......................................................................... 66 xii  Figure 5-9.     Relationship between bond strength and recycled aggregate replacement  level in 300 mm concrete cylinder with le⁄db =5. a) bond strength,  b) normalaized bond strength.......................................................................... 67 Figure 5-10.   Relationship between bond strength and recycled aggregate replacement  evel in 300 mm concrete cylinder with le⁄db =10. a) bond strength,  b) normalaized bond strength.......................................................................... 67 Figure 5-11.   Effect of different factors on bond strength; a) concrete cover to bar  diameter ratio, b) embedment length to bar diameter ratio, c) RCA replacement percentage ................................................................................... 69 Figure 5-12.   Calculated bond stress versus measured bond stress according to  a) ACI 440, b) CSA S806, c) CSA S6, d) Lee et al.  and e) Okelo et al......... 71 Figure 5-13.   Comparison of measured and predicted bond stress values for small  specimens with le⁄db=5, embedded with a) 20M, and b) 13M bars ................ 72 Figure 5-14.   Comparison of measured and predicted bond stress values for small  specimens with le⁄db=10, embedded with a) 20M, and b) 13M bars .............. 73 Figure 5-15.   Comparison of measured and predicted bond stress values for large  specimens with le⁄db=5, embedded with a) 20M, and b) 13M bars ................ 73 Figure 5-16.   Comparison of measured and predicted bond stress values for large  specimens with le⁄db=10, embedded with a) 20M, and b) 13M bars .............. 74 Figure 5-17.   Bond stress versus slip response of control specimens in a) 200 mm  and b) 300 mm concrete cylinder.................................................................... 76 Figure 5-18.   Bond stress versus slip response of 30% RCA specimens in a) 200 mm  and b) 300 mm concrete cylinder.................................................................... 77 Figure 5-19.   Bond stress versus slip response of 50% RCA specimens in a) 200 mm  and b) 300 mm concrete cylinder.................................................................... 78 Figure 5-20.   Bond stress versus slip response of 70% RCA specimens in a) 200 mm  and b) 300 mm concrete cylinder.................................................................... 79 Figure 5-21.   Bond stress versus slip response of 100% RCA specimens in a) 200 mm  and b) 300 mm concrete cylinder.................................................................... 80  xiii  List of Symbols, Abbreviations  𝐴𝑏 Area of longitudinal reinforcement, mm2 𝐶 Concrete cover, mm 𝑑𝑏 Bar diameter, mm 𝑑𝑐𝑠 Smallest of the distance from the closest concrete surface to the center of the bar being developed or two-thirds the center-to-center spacing of the bars being developed, mm        𝑓𝑐′ Compressive strength of concrete, MPa 𝑓𝐹 Stress developed in the rebar, MPa 𝑘1 Bar location factor 𝑘2 Concrete density factor   𝑘3 Bar size factor 𝑘4 Bar fiber factor 𝑘5 Bar surface profile factor 𝑙𝑒 Embedment length of reinforcing bar, mm 𝑃𝑚𝑎𝑥 Maximum push-out load, N α Significance level 𝜏 Average bond stress, MPa 𝜏𝑚𝑎𝑥 Average maximum bond strength of reinforcing bar in concrete, MPa AFRP Aramid fiber reinforced polymers ANOVA Analysis of variance C&D Construction and demolition CFRP Carbon fiber reinforced polymers FRP Fiber reinforced polymers GDP Gross domestic product GFRP Glass fiber reinforced polymers LEED Leadership in energy and environmental design NCA Natural concrete/coarse aggregate RCA Recycled concrete/coarse aggregate xiv  Acknowledgements I would like to extend my sincerest thanks to my supervisor, Dr. Sharia Alam from the Civil Engineering Department at the University of British Columbia, for his guidance, advice, and encouragement throughout these past years of my research. I particularly thank my committee members, Dr. Abbas S. Milani and Dr. Kasun Hewage, whose valuable guidance helped me advance my research quality. The current study is supported by Natural Sciences and Engineering Research Council of Canada (NSERC). Appreciation is also extended to OK Builders Ltd. and BP composites Ltd. for the supply of research materials and and useful guides. I would also like to thank many other individuals who had a direct hand in helping with my research. Namely, the Civil Engineering Department lab technicians, Alec Smith  and Ryan Mandau. I would like to thank the dedicated individuals at the University of British Columbia Engineering Machine Shop for their technical skills in the fabrication of various testing components used throughout this research.  Finally, to the excellent group of graduate students in the research group who extended their time and effort during my project whether it involved shovelling, sieving, casting, design checking, batching, or just helping to keep things in perspective, specially Dr. Mohammad nouroz Islam, Dr. Frashad Hedayati Dezfuli, Muntasir Billah, Sumaiya Binte Huda, Philip Chan, Eric Larssen, Alexander Melnyk, Chungha Lee, Yongjoon Lee, and  Yujia Chen. I would like to thank you all, this work would not have been possible without you. Finally, It is particularly important to thank my husband, Nima, for his support throughout my graduate studies and my parents, whom I feel to be the key source of inspiration for all my achievements. xv  Dedication    To my family, Without whom none of my success would be possible  1   Introduction 1.1 General The sustainability and durability of concrete buildings and infrastructures are very challenging issues in construction industries around the world. The preservation of the environment and conservation of natural resources have been some of the main concerns for the past several years.  One of the most widely used building materials in the world is concrete. This popularity is because of its mechanical properties, durability, cost effectiveness, and availability. The statistics showed the annual average concrete production rate to be around 1 ton per human based on the world population (Medina et al., 2014). Every year, millions of tons of concrete waste due to the demolition and deconstruction are being produced in construction industry. Due to the lack of proper planning and processing plants for recycling these materials, they are not reused for new construction, rather sent directly to the landfill (Silva et al., 2014).  In 2008, both Canadian concrete and cement industry invested more than $3.2 billion on Canada's gross domestic product (GDP) and 28.1 million cubic meter of concrete was produced (Butler et al, 2011). Concrete is composed of cement, sand, coarse aggregate, water, and other type of additive admixtures. Up to 75% of the total concrete volume is made of coarse aggregate. Therefore, finding an economical and sufficient source of good quality coarse aggregate is vital in concrete industry. Unfortunately, the new investigations have illustrated the reduction of sustainable sources of natural aggregate in some regions of Canada (Butler et al., 2011). This may lead to an increased material cost for the Canadian concrete industry. Moreover, in terms of availability, there are inadequate sources of high quality aggregate in the vicinity of many regions with increasing construction demand and urban growth. One of the potential and effective solutions for the increasing construction demand is to produce and use recycled concrete aggregate (RCA) which is crushed concrete produced by demolished concrete structures. Reduced cost of coarse aggregate and a lower CO2 emission due to the reduced transportation, and a decrease in the amount of construction wastes stored in landfills are results of reusing demolished concrete structures (Huda & Alam, 2014). 2  Moreover, the companies can achieve the Leadership in Energy and Environmental Design (LEED) credits by means of exploiting RCA as a structural material. However, the mechanical properties of this type of concrete might be affected by the quality variation due to crushing process used to generate RCA in comparison to original concrete (Butler et al., 2011). Inconsistency in the characteristics and properties of RCA is the main engineering concern which is hindering the use of this material (Younis & Pilakoutas, 2013). Compressive strengths of concrete made of RCA in many instances have been found equal to ordinary concrete. Huda and Alam (2014) also showed that even repeated recycled concrete of different generations can achieve the target strength. Some researchers demonstrated that good mechanical properties can be achieved by using a proper mix proportioning method and adjusting total cement content of concrete and recycled aggregates (Fathifazl et al., 2012).  Although steel rebars are among the most widely used types of reinforcement in concrete, their corrosion due to exposure to sea water, industrial chemicals, deicing salts, moisture and other aggressive environments accelerates the concrete deterioration. This could happen, e.g., in bridge decks and piers, ports, marine structures and pavements. According to the growing concern on durability of concrete structures, fiber reinforced polymers (FRPs) have been used as an alternative type of reinforcement (Lee et al., 2008). These materials consist of fibers embedded in polymeric resin. There are different kinds of fibers and resins that are commonly used in FRPs. Glass FRP (GFRP), carbon FRP (CFRP) and aramid FRP (AFRP) are the examples of the most common FRP bars in the current global market of construction. Their lightweight, high tensile strength, non-corrosive nature, and non-conductivity are main advantages of this type of reinforcements. They can be produced with smooth surface, or with deformation like ribbed bars and surface treatments such as sand coated bars, in order to improve the FRP-concrete bond characteristics (Cosenza & Manfredi, 1997; Okelo & Yuan, 2005). The performance of reinforced concrete members, including resistance against bending, shear, and torsion is closely correlated to their bond behavior. The interfacial bond between a rebar and concrete has the most important role in transferring loads from the concrete to the reinforcement.  Studies which are allocated to identifying the consequence of using RCA on the compressive bond strength between concrete and steel reinforcement, and 3  the usage of RCA in structural concrete applications have not been adequately established. The relation between the RCA and the deformed rebar under the equivalent mix proportion has not been determined (Xiao & Falkner, 2007).   It can be concluded that the amount of replacement of natural concrete/coarse aggregate (NCA) by RCA is a vital factor to be evaluated in the estimation of recycled aggregate concrete mechanical properties (Folino & Xargay, 2014). According to previous studies, concrete made of RCA exhibits the potential to resist environmental exposures (Huda & Alam, 2015). Hence, RCA can be recommended for use in structures that are exposed to open air and freeze-thaw conditions (Kim & Yun, 2014). Many studies (Achillides & Pilakoutas 2004; Aiello et al. 2007; Rossetti & Galeota 1995; Arias et al. 2012; Benmokrane et al. 2002; Hao et al. 2009) have been carried out to investigate the bond behavior of FRP reinforced concretes. However, because of different types of commercially available FRP bars and variation in their effective parameters such as type of fiber, fiber volume, type of resin, fiber orientation, rate of curing, and service temperature, a plenary model for predicting the bond strength has not been determined yet.  1.2 Objective of Study The overall objective of this study is to provide a sustainable solution for development in construction industries by using fiber reinforced polymers as reinforcement and replacing the natural aggregates by recycled concrete aggregates. Although extensive studies have been performed on the mechanical properties of RCAs and FRPs, works remain to be done to incorporate RCA concrete and FRP rebar in structural application of concrete. Currently, in construction industries, there is no guideline for producing and designing the concrete members using RCA. The  objective  of  this current investigation is  to  determine the possibility of using RCA  as  a  partial replacement for coarse aggregate in concrete and comply with CSA class C-1 standards (Canadian Standards Association, 2014b) for  concrete for  structural  applications.   This study seeks to provide a concrete that is acceptable in terms of bond behavior between concrete and FRP rebar. The bond features of the RCA to the steel and FRP reinforcement must be examined based on an experimental program. In this regard, bond 4  behavior of RCA concrete embedded with steel and FRP bars will be compared. This project will assist the concrete industries in mass production of RCA concrete reinforced with FRP rebar and with different specified strengths. In particular, the proposed research has the following specific objectives:   Designing concrete mix proportions of particular strength with different percentage of recycled concrete aggregate for structural applications.   Studying the compressive bond between RCA concrete and steel/FRP rebar by means of push-out test and comparing their bond performance and also comparing the experimental results to guidelines prediction values.  Evaluating the role some of effective factors such as RCA replacement level, concrete cover, bar diameter, and embedment length on the bond behavior.  1.3 Research Significance Sustainable construction and infrastructure is an integral part of Canada’s Science & Technology Strategy. Based on an investigation in 2007 about 60% of Canada’s infrastructure is going to reach the end of its service life very soon and will require decommissioning (Mirza, 2007). As a result, a substantial amount of C&D wastes will be produced, which will create a massive load on the landfills. The present  research  aims  to  exploit  this  waste  and  turn  it  into  new  products. This  study  will  help  Canada  save  substantial amounts  of  natural  resources  by  recycling/reusing  C&D wastes and  thus, contribute to the  Canadian  economy  by  improving  the  gross domestic product (GDP).  Moreover, one of the important factors that restrict the life expectancy of reinforced concrete structures is the corrosion of steel reinforcing rebars. Especially in North America, high temperature fluctuation and usage of deicing salts intensify the damage in parking garages and on bridge decks. Indeed, severe environmental conditions occur for roads and bridges subjected to North America’s freeze-thaw cycles and heavy salt applications (El-Salakawy et al., 2005). In addition, great rehabilitation costs and traffic disruption appear due to the expansive corrosion of steel which causes cracking and spalling of concrete bridge decks (Yunovich & Thompson, 2003).  5  The current investigation intends to evaluate FRP materials as reinforcement in combination with normal and RCA concrete. The combination of recycled aggregates and FRP reinforcements can be used to construct new structures. For example recycled aggregates from an old demolished bridge can be used to build a new one in cases that a good quality natural aggregate resource is not available in the vicinity of the area. Also using FRP bars as a non-corrosive material can provide a more durable bridge. Material supply industries like ready-mix  concrete industries, construction firms, government organizations  including  municipalities,  and  federal and  provincial  levels  are  involved  with  rehabilitation of old concrete infrastructures or the construction of new ones, which would directly benefit from the outcomes of the proposed investigation. This study will help Canada evaluate the performance of FRP and RCA and consequently, encourage construction companies to use these materials, and contribute to the Canadian economy by minimizing rehabilitation and material costs.  1.4 Thesis Outline  This thesis is organized in seven chapters. Chapter 1 gives an overview of the research. Chapter 2 provides a general introduction on recycled aggregate concrete and fiber reinforcement polymer and their application, and also reviews the available literature on the bond between concrete and reinforcement. This chapter discusses the effect of different parameters on the bond behavior of steel and FRP rebars in concrete. In addition, the available code equations to predict the peak bond stress (bond strength) are presented. It highlights the gaps in the available literature on the compressive bond behavior of rebars in concrete and thereby, sets the research objectives. Chapter 3 describes the experimental procedure along with the results of aggregate property and fresh and hardened properties of 35 MPa concrete made of RCA having different replacement levels. Chapter 4 and 5 present the results of the accomplished tests for specimens embedded with steel and FRP rebars and discuss the influence of different factors including bar diameter and embedment length, concrete cover, and RCA replacement level on the bond behavior. The results obtained from sensitivity analysis has been discussed in this chapters as well. Chapter 6 presents the conclusions derived from this study and provides recommendations for future research directions.  6   Literature Review 2.1 General  The following chapter presents an overview of recycled concrete aggregates (RCAs) and fiber reinforced polymers (FRPs) properties. The use of FRP instead of steel reinforcement and RCAs as a replacement of natural coarse aggregate (NCA) in a new type of concrete (i.e., RCA concrete) are also discussed in this chapter. Afterwards, bond mechanism, testing methods, a summary of previous findings and research gaps are presented.   2.2 What Is RCA? After demolishing a concrete structure (i.e., concrete pavement, building, bridge, etc.), large pieces of concrete remain. Then, by usage of hydraulic shears, torches and electromagnets, any existing steel reinforcement is removed and separated. According to ACI Committee 555 recommendation, the production process of RCA includes removal of any deleterious material such as glass, plastic, plaster, oil droppings, wood, steel, clay, etc. Once the aggregates have been separated, an adequate particle size distribution necessary for quality RCA concrete production can be provided by jaw crushers (ACI 555, 2001). Recycled aggregates can be classified based on ASTM C 33 or CSA A23.1-09 standards. Afterwards, RCA’s strength, water absorption, specific gravity, sulphate content and alkali-silica reaction potential must be tested before being considered for use as an aggregate (Butler, 2012). Figure 2-1 demonstrates a picture of natural and recycled coarse aggregates used in this study. Figure 2-2 shows the microscopic view of different types of coarse aggregate. These figures reveal that pore and crack was not found in the case of natural aggregate under the microscope. On the other hand, the crack and other damages increased in recycled coarse aggregates and their quality degraded compared to natural aggregates which might influence the mechanical and durability properties of recycled coarse aggregate concrete (Huda & Alam, 2014). RCA’s density and water absorption characteristics should be accounted for the mixture proportioning design to ensure adequate workability and strength. It has been recommended by Tam and Tam, (2009) that RCAs from different sources should be separately crushed and 7  classified rather than processed in a combined form which could lower its overall quality and limit its applications. According to some experiments performed by Nagataki et al. (2004), high-quality coarse RCA with reduced adhered mortar content can be produced from crushed laboratory concrete blocks, using a jaw crusher and an impact crusher followed by further mechanical grinding (Butler, 2012; Nagataki et al., 2004).   Figure 2-1. a) Natural coarse aggregates, b) Recycled coarse aggregates  Figure 2-2. Microscopic view of different types of coarse aggregate; a) natural coarse aggregate, b) recycled coarse aggregate (Huda, 2014) (a) (b) 8  2.2.1 Properties 2.2.1.1 Properties of RCA The properties of RCA such as gradation, shape, and texture significantly influence the properties of recycled aggregate concrete. Their shape and texture are likely to vary over a wide range due to different sources. It has been reported that the gradation and attached mortar content of recycled aggregates are not influenced by the crushing strength and the age of parent concrete (Katz, 2003). Katz (2003) reported that the adherent mortar content of coarse aggregate is about 6.5%. RCAs may be sieved to any variety of gradations, depending on their application. Well-graded RCA can be produced using a jaw crusher (Hansen, 1986). In primary crushing stage, the size of recycled aggregate is dropped down to 50mm. While transferring from primary to secondary crusher, all types of metal impurities are removed by using electromagnets. During secondary crushing process, particle size is reduced to 14-20 mm (Corinaldesi et al., 2002). Due to presence of attached mortar on the surface of recycled aggregate, their specific gravity is less than that of natural aggregate which is around 2.7 (Katz, 2003; Salem et al., 2003). A higher absorption capacity of between 3% and 8% is observed in the recycled coarse aggregate due to the attached mortar and their higher porosity. NCA has a lower absorption capacity which is around 0.3% (Katz, 2003). The weathering resistance and the quality of aggregate can be determined by their abrasion resistance. According to previous researches, recycled aggregate has abrasion resistance of 20% to 45% which is about 12% lower than that of natural aggregate (Sagoe-Crentsil et al., 2001). Also, it has been reported that the replacement level of recycled aggregate does not affect the abrasion resistance of aggregate (Abou-Zeid et al., 2005). 2.2.1.2 Properties of fresh RCA concrete Sagoe-Crential et al. (2001) found that compared to recycled aggregates which are produced for laboratory work, commercially produced recycled aggregates are smoother and spherical which increases the workability of produced recycled aggregate concrete. On the 9  other hand Higher absorption capacity of recycled aggregate can result in stiffer and less workable concrete mixes compared to normal concrete (Salem et al., 2003). Leite et al. (2013) observed that RCA requires 5-10% extra free water in order to achieve the same concrete mix workability. Therefore workability is considerably influenced by the quality of recycled aggregate (Leite et al, 2013).  The consistency and workability of fresh concrete can be specified by the Slump value. Consistency refers to the ease with which concrete flows. It is used to indicate degree of wetness. The concrete slump test is an empirical test that measures workability of fresh concrete. Based on research done by Topcu and Sengel (2004) under a fixed water cement (w/c) ratio, with an increase in recycled aggregate replacement level, the workability and slump value of RCA concrete will reduce. However, same slump values can be obtained for conventional concrete and the one with partial RCA replacement, after adjusting the required amount of water (Poon et al., 2004; Topçu & Şengel, 2004). It has been reported that the air content of fresh concrete made of recycled aggregates is higher than fresh conventional concrete made of natural aggregates. This means that there is a higher amount of entrapped air in RCA concrete compared to normal one (Katz, 2003; Salem et al., 2003).  Current Canadian Standards Association (CSA) has a specific section on the use of RCA or RCA concrete. In CSA, section A23.1-14-O states that some parameters of recycled aggregates including durability characteristics, deleterious materials, potential alkali-aggregate reactivity, chloride contamination, and workability characteristics of resulting concrete should be assessed in a similar manner compared to natural normal-density aggregates (Butler, 2012). 2.2.2 Current Usage It has been more than half a century that recycled aggregates have been used in concrete production. European countries have established recycling waste industries and have been employing construction and demolition waste for construction purposes after the World War ІІ. Every year, only 30% of 200 million tons of demolition wastes generated in Europe is being recycled (Tabsh & Abdelfatah, 2009). Nowadays, European Commission on Management of Construction and Demolition Waste supports the recycling and reusing of C&D wastes. The 10  target level of recycling C&D waste is 50% to 90% among European Union countries (Tabsh & Abdelfatah, 2009). Some of them have effectively achieved a recycling rate of higher than 70% such as Ireland, Germany, Netherland, and Denmark (Huda, 2014; Jeffrey, 2011). In USA, 30-40% of produced virgin aggregates are being used for pavements, maintenance and construction works for roads. The rest amount of natural aggregates is required by structural applications, which is about 60-70% (Huda, 2014). Initially, using recycled aggregate as base or filler materials in the road construction industry was limited (Robinson et al., 2004). Currently, production of recycled aggregate is carried out by natural aggregate producers, debris recycling centre, and contractors in USA. The results of a geological survey conducted in 2000 indicate that approximately 100 million tons of recycled concrete aggregates (RCA) are produced in US (Robinson et al., 2004). These RCAs are mainly used for base materials. Another part of recycled concrete aggregates is used for asphalt pavements, riprap, and new concrete productions (Nantung & Jiang, 2005). It is reported that in every ton, $11 can be saved by replacing natural aggregate by recycled concrete aggregate  (Huda, 2014; Smith et al., 2008). Similar to USA, Ministry of transportation of Ontario (MTO) in Canada did not encourage the use of RCA in the past. However, they started to combine natural and recycled aggregates and use them for the sub-base and base of concrete pavement. Miller (2005) estimated that only 3% of aggregates used in Ontario is RCA (Huda, 2014; Miller, 2005). Japan government established the recycling law in 1991. After this initiative, reusing of waste concrete and application of recycled aggregate was increased by 48% and reached 96% in 2000. They were mostly used as sub-base materials for concrete pavement (Kawano, 2003). In Taiwan, after a devastating earthquake in 1999, about 30 million tons of construction and demolition waste was generated during rehabilitation of infrastructures. 80% of those waste was successfully recycled and 30% of RCA was used as pavement base (Huda, 2014; Raoet al., 2007). A limited number of studies have been carried out to evaluate the usage of RCA as complete or partial replacement of natural aggregate in reinforced concrete or structural concrete applications. Shear strength of RCA concrete, high strength RCA concrete, bond 11  strength of RCA concrete reinforced with steel bars, and seismic performance of RCA concrete were the main subjects of these studies (Ajdukiewicz & Kliszczewicz, 2002; Choi & Kang, 2008; Fathifazl, 2008; González-Fonteboa & Martínez-Abella, 2007; Xiao & Falkner, 2007). However, the new J-Cube Capital Mall in Singapore (completed in 2011) and Enterprise Park at Stapleton, North America are some of few case studies of actual structures built using RCA (Butler et al., 2014; Butler, 2012).  2.3 What Is FRP? Strong fibers embedded in a resin matrix are components of fiber reinforced polymers (FRP). The stiffness of these composite materials is provided by fibers which generally carry most of the applied tension loads. Glass, carbon, and aramid are three common types of fibers used in structural engineering applications. The fiber concentration of FRPs used in construction industries is usually greater than 30% by volume. The bond between fibers, their protection, and transfer of forces from fiber to fiber through shear stresses are provided by a thermosetting matrix such as epoxies, polyesters and vinylesters (ACI 440.1R, 2006; Quayyum, 2010). Structural reinforcing FRP bars are intended for use as concrete reinforcing in areas where steel reinforcing has a limited life span due to the effects of corrosion. Furthermore, they are useful in locations where electrical or magnetic transparency is required. Figure 2-3 shows two types of carbon and glass fiber reinforced polymers (CFRP and GFRP) with smooth and sand coated surfaces.  12   Figure 2-3. Different types of FRP rebar 2.3.1 Properties Manufacturing methods, constituents, surface treatments, and specific formulations are the factors that influence the properties of the currently available FRP systems. CSA S806, ACI 440.3 and different ASTM standards provide experimental procedures to determine the properties of the FRP composite materials and products. In general, high strength, non-corrosive nature, light weight, fatigue resistant, non-magnetic, electrical insulation and small creep deformation are some of FRP characteristics that make them suitable to be used in the construction industry. As it can be seen from Figure 2-4, all FRP systems exhibit linear elastic tensile stress-strain behavior with no yielding. Their behaviors are highly dependent on the direction of fibers. They have lower modulus of elasticity compared to steel except for some carbon fiber reinforced polymers (Quayyum, 2010).  In most of cases, the surfaces of FRP bars are plain, smooth and there are no well-defined ribs on them like in deformed steel bars. Therefore the bond between FRP rebars and concrete results from chemical adhesion and the friction between broken concrete particles and 13  rebars rough surface. Usually in order to improve composite action of FRP reinforced concrete and bond strength, a surface treatment technique such as sand coating is used.  Just after FRP rebars manufacturing, sand particles are attached to them using resin (Goraya et al., 2011).  Figure 2-4. Stress-strain plots of FRP (Quayyum, 2010) 2.3.2 Current Usage For the last 20 years, composite materials have developed into economically and structurally feasible construction materials used in buildings and bridges. Today, structural engineering is using FRP materials in a variety of forms including reinforcement material for new concrete construction, strengthening material for existing structures, and structural members for new construction. FRP material in new construction can serve as internal rebars, prestressing tendons, and stay-in-place formwork. FRP rebars can be produced with either sand coated, helically wound spiral outer surface, indented, braided, or with ribs (Quayyum, 2010). In order to eliminate corrosion, FRP prestressing tendons were first used in Europe in 1980s. However, their usage is still hindered since the conventional steel anchor could not be used in combination with FRP prestressing tendons due to the low transverse strength of the 14  FRP tendons (Erki & Rizkalla, 1993; Nanni et al., 1998). Columns and beams made from FRP tubular shapes and filled with concrete and FRP stay-in-place formwork has been studied for some years (Dieter et al., 2002; Fam & Rizkalla, 2002; Mirmiran et al., 2000; Ozbakkaloglu & Saatcioglu, 2007; Ringelstetter et al., 2006). FRP composite materials are used in order to increase existing flexural, shear, or confinement strengths of concrete, steel, masonry and timber structures. These materials are applied either as prestressing tendons, pre-manufactured rigid FRP strips adhesively bonded to the surface of the structure, or hand layup sheets that consists of in situ forming of FRP composite on the surface of the structural member using flexible, dry FRP sheets and a polymer resin (Quayyum, 2010). 2.4 Bond Mechanism  For an efficient and reliable transfer of the force between materials in reinforced concrete, the presence of a good interfacial bond is vital. As seen in Figure 2-5, if we consider a length of beam between to cracks when a moment acting at the section, the stress or force in reinforcement bar changes from point to point along the length of the bar. Therefore, bond stresses must be present for the bar to be equilibrium (Wight & MacGregor, 1997). The bond mechanism consists of three parts (Quayyum, 2010): (1) mechanical interlocking occurring on the textures on the rebar surface, (2) frictional forces due to roughness of the bar surface and surrounding concrete, (3) chemical adhesion between the bar and concrete. The combination of these forces can be determined an outward component (radial splitting force) and a shear component, parallel to the bar that is the effective bond force.  15   Figure 2-5. Average flexural bond stress in bottom and top rebar in a beam, a) beam; b) moment diagram; c) bar forces  According to ACI code, on each side of that section by embedment length, hooks, and anchorage, or a combination of those shall be provided so that the calculated tension or compression in reinforcing bars at every section of reinforced concrete members will be developed (Wight & MacGregor, 1997). If the rebar is anchored long enough in the concrete or there is enough confinement (concrete cover or transverse reinforcement) in the element, the bond failure can be prevented. The radial and tangential stresses developed along the bar length will be less than the concrete capacity in this situation. Therefore, the bar can achieve its design tensile strength. In the case of inadequate anchorage length of the rebar or insufficient confinement to the concrete, radial and shear forces may be higher than the concrete capacity which can result in bond failure (ACI 408, 2003). There are different types of bond failures. The splitting of the concrete surrounding the reinforcing bar without reinforcing bar rupturing is called splitting failure. This failure occurs when the surrounding concrete and/or the transverse reinforcement are not enough to resist the pressure due to loads applied on the bar. The bars exert radial pressure on the nearby concrete which initiates cracking that are both perpendicular and parallel to the reinforcement. Splitting failure leads to a crack at the concrete-rebar interface that propagates towards the surface resulting in the failure of the concrete by concrete cover splitting (Quayyum, 2010). c) a) b) 16  When the bar pulls out of the concrete without concrete splitting or without bar rupturing, a pull-out failure is happened. In this case, the radial forces from the bar being loaded are lower than what the surrounding concrete and/or transverse reinforcement can resist. Nevertheless, tangential forces are higher than the resistance of the concrete which leads to shearing along a surface at the top of the ribs around the bars (Quayyum, 2010). Figure 2-6 demonstrates cracking pattern in different bond failures.   Figure 2-6. Cracking and damage mechanisms in bond: a) Side view of a member showing shear crack due to bar pullout, b) Cross-sectional view of a concrete member showing splitting cracks between bars and through the concrete cover, (Quayyum, 2010) In both bond failure modes, relative slippage of the rebar to the concrete happens. The concrete is well confined in the case of pull-out failure. Therefore, the radial splitting cracks need more energy to reach the outer surface of the concrete. So pull-out failure occurs at higher bond strengths than the splitting failure. Bond behavior between reinforcing bar and the surrounding concrete can be represented by stress-slip relationship. Furthermore, the required anchorage length to achieve the desired strength of the reinforcing bar can be determined by this relationship (Quayyum, 2010). Bond force-slip or bond stress-slip curves are generated to better understand the stress-slip relationship and nature of bond response. A typical bond force versus slip curve is shown in Figure 2-7. The bond force-slip and bond stress-slip curves are initially very sharp mainly due to adhesion and mechanical bond between the reinforcement and the surrounding concrete. After bond failure the curve will drop dramatically and as loading continues, cracks begin to propagate which causes the curve to gradually level off (Quayyum, 2010).  (a) (b) 17   Figure 2-7. Typical bond-slip response curve, (Shahriar Quayyum, 2010)  2.4.1 Factors Affecting Bond Behavior According to previous studies, some of the main parameters that influence bond behavior include concrete compressive strength, concrete cover, bar’s diameter, shape (the cross section, or the roughness of the surface in case of FRP bars), and embedment length (Quayyum, 2010). It has been reported that the specimens embedded with deformed bars (Deformed bar is usually given ridges for better mechanical anchoring reinforced concrete.) demonstrated much higher bond strength compared to plain bars (Xiao & Falkner, 2007). Results show that the rebar size and also the embedment length to the diameter ratio of rebar have an inverse relation with the bond strength. Moreover, larger concrete cover led to an increase in the bond strength (Arias et al., 2012; Choi et al., 2012; Hossain et al., 2012;  Quayyum, 2010; Tighiouart et al., 1998). Main parameters that influence the bond behavior are:  (1) Concrete compressive strength; as mentioned before, both splitting and pullout modes of failures are related to the tensile and shear strength of the concrete. Consequently, they are affected by the compressive strength of concrete. ACI Committee 408 (1992) has stated that the tensile strength of concrete is almost related to the square root of the compressive strength of concrete. Therefore, the bond capacity have also a good 18  correlation with the square root of compressive strength of concrete ( √𝑓𝑐′) (ACI 408, 2003; ACI 440.1R, 2006). (2) Concrete cover; the concrete affects the bond failure mechanism because it increases the bond strength due to providing the rebar’s confinement. When the member does not have adequate concrete cover, bond failure occurs through splitting of the concrete. Therefore, splitting failure is prevented or delayed by sufficient concrete cover (ACI 440.1R, 2006).  (3) Bar diameter; the bond capacity is reduced by increasing the bar diameter. One of the reasons is that larger diameter bars require longer embedment lengths to develop the same bond stress. (Quayyum, 2010). (4) Embedment length; it is observed that the initial bond stiffness of bars increases when the embedment length is long (e.g. 12 times the bar diameter). However, the stress is distributed along a longer length as an increase is happened in the embedment length. Therefore, there will be a decrease in bond capacity. It is assumed to be a result of the nonlinear distribution of bond stress on the larger embedment lengths. Also, the rate of growth of bond stress is greater for smaller than for larger embedment lengths (Ehsani et al., 1995; Quayyum, 2010). (5) Bar cast position; upward moving of air and water and getting trapped under the rebar during the horizontal placement of concrete have a negative effect on the bond strength (Quayyum, 2010). (6) Type of fiber; based on CSA S806-02, AFRP bars show weaker bond behavior than CFRP and GFRP bars. The bond strength of CFRP and GFRP bars can be considered almost the same (Quayyum, 2010).  (7) Type of rebar surface; although in general it has been observed that the deformed bars have a better bond behavior than bars with smooth surface, no definite relation between rebar surface and bond strength has been determined (Quayyum, 2010).  (8) Transverse reinforcement; confinement provided by transverse reinforcement limits the growth of splitting crack. So, occurring of the bond failure required a bigger force (Quayyum, 2010). 19  2.5 Bond Behavior between RCA Concrete and Steel Reinforcement By means of Pullout test which is a very common way of measuring the bond strength and studying the effect of different factors on bond behavior. Xiao and Falkner (2007) investigated the bond between concrete with different RCA replacement level and two types of plain and deformed rebars. They stated that the specimens embedded with deformed bars demonstrated much higher bond strength compared to plain bars. Since in ribbed and indented rebars mechanical interlocking or bearing resistance is developed. According to the results, bond behavior of RCA concrete embedded with deformed rebar was very similar to normal concrete having same deformed reinforcement (Xiao & Falkner, 2007).  In addition, one of the effective factors on concrete-steel behavior is the size of RCA aggregates (Kim & Yun, 2013). Based on experimental work described by Kim and Yun (2013), there is a rise in bond strength when the maximum size of RCA aggregate decreases. The aggregates settlement due to the shape and size of coarse aggregates could affect the friction and mechanical interlock between the bar and the surrounding concrete. These results indicate that less air voids remain in the concrete when smaller size of aggregates has been used. As a result, the compressive strength of concrete is increased. Hence, the compressive strength of RCA concrete can be improved by use of smaller RCA which has less compressive strength than natural aggregate concrete. Similar to previous studies, Prince and Singh (2013) found out that the bond between deformed steel and RCA concrete is acceptable. In some cases, RCA bond strength was higher than normal concrete bond having same type of reinforcement, when the RCA replacement percentage increases by 50%. Therefore, RCA have the potential to be widely used in construction industry (Prince & Singh, 2013). 2.6 Bond Behavior between Normal Concrete and FRP Reinforcement  Pullout test does not have the ability to simulate an actual concrete element. It is due to no transverse reinforcement in pullout specimens and the concrete cover is usually larger than in real RC members. However, it provides good comparative information about the relative bond of FRP rebars with respect to steel ones. Generally under same conditions in terms of bar diameter, concrete strength, concrete cover, and embedment length, the FRP rebars have 20  demonstrated weaker bond performance than that of steel bars. In general they have lower bond strength and higher slippage at failure (Larralde & Silva-Rodriguez, 1993). As it was mentioned before, the FRP bars are produced with different surface treatments. Okelo and Yuan (2005) described that by means of surface deformations similar to what are applied to steel bars, such as external helicoidally strand and deep grooves, the bond performance can be enhanced compared to a smooth surface (Okelo & Yuan, 2005). Deformed FRP rebars have the contribution of mechanical interlocking that will result in a higher peak bond stress (Aiello et al., 2007). Furthermore, sand coating and surface texture are other acceptable solutions to improve the bond behavior between FRP bars and concrete. Therefore, in addition to concrete cover thickness, embedment length, and concrete compressive strength, type of bar’s surface also will influence the mode of bond failure (Okelo & Yuan, 2005). The weakness of resin matrix in FRPs will result in shearing of the matrix material from the fibers and accordingly, de-bonding between concrete and reinforcement. According to the results reported by Achillides and Pilakoutas (2004), if the compressive strength of concrete is less than 15 MPa, the debonding will occur in surrounding concrete. While, for concrete strengths higher than 30 MPa, the bond failure happens at critical interface between successive layers of fiber of the FRP bar. Hence, the chemical adhesion and the shear strength of resin matrix are two effective factors on the bond failure mode (Achillides & Pilakoutas, 2004). The bond tests will help us estimate the minimum development length needed in design guidelines of concrete structures. The specific shape of the bar deformations and surface treatments of rebar have not taken into account in ACI 440-2001. Therefore, this guideline recommends a conservative development length for concretes reinforced with FRP rebar (Malvar et al., 2003). The experimental results reported by Benmokrane et al. (2002) confirm that for sand coated and ribbed CFRP bars, the theoretical development lengths suggested by ACI 440.1R-01 are about 3.5 times larger than experimental values. Moreover, the new generation CFRP rebars have the potential to be used as reinforcement in concrete structures (Benmokrane et al., 2002). 21  According to previous studies on the interfacial bond strength between the FRP bars and normal concrete, the surface treatment (As discussed before, to facilitate the bond between the finished bar and concrete, FRP bars have a surface treatment usually sand coating.), deformation, and shape of FRP rebar have significant influence on bond behavior (Arias et al., 2012; Choi et al., 2012; Hossain et al., 2012; Quayyum, 2010; Tighiouart et al., 1998).  Furthermore, matrix, fibers, and fiber/matrix interface in FRP bars can be influenced by environmental attack. Hence the degradation of resin matrix or the fibers can affect the bond behavior of FRP rebar (Robert & Benmokrane, 2010). The effect of various environmental exposures was experimentally evaluated in previous papers.  Bond  behavior  of  FRP  bars  embedded  in  concrete  represents  that  acid  environment  can reduce the bond strength up to almost 20% and in some cases this value is even smaller than the steel (pH=2) (Zhou et al., 2011). According  to  some  of  the  experiments,  although  increasing  the  number  of  freeze-thaw cycles can diminish the bond capacity, a maximum of 6% reduction was observed as a marginal change (Colombi et al., 2010; Laoubi et al., 2006). The results illustrate that the bond   behavior   of   FRPs   varies depending on the level of temperature. Bond strength  has  no  significant  reduction  when  the  temperature  is  less  than  60ᵒC.  However, as the temperature reaches about 200ᵒC (in case of fire), the bond strength drops by 80-90%. As a consequence of the growth in temperature, the resin matrix becomes weak which result in shearing of the matrix material from the fibers. Deterioration of the mechanical properties of the matrix material at temperatures above the specific polymer’s glass transition temperature, is the reason behind the loss of bond (Katz & Berman, 2000; Masmoudi et al., 2011). Some of researchers show that individual effect of freeze-thaw cycles could be marginal compared to applying sustained loads simultaneously with freeze-thaw cycles and fatigue loading (Colombi et al., 2010; Laoubi et al., 2006). The common result of all studies indicates that the properties of the surface or the external layer of FRP rebar is as effective as their diameter and concrete cover thickness. Therefore, the influence of various environments,  especially thermal effect at  high  temperatures, should  be  taken  into  account  in  order  to  use  the  FRP  bars  as  reinforcement  in concrete structures. 22  2.7 Evaluation of Bond Strength Depending on the subject of study, different test methods are used to evaluate the bond strength between rebar and concrete. Appropriate bond test is defined by considering various factors including size and geometry of rebar, type of concrete and FRP, and quantity of specimens. The pullout test and beam test are two main methods that have been used by researchers (Galati et al., 2006; Laoubi et al., 2006; Zhou et al., 2011). When reinforcements are subjected to compression, the three-dimensional stress state of the concrete is different. Concrete is in compression in the direction parallel to the bar when the rebars are under compression load. While in the case of bars subjected to tension, concrete is in tension parallel to the bar. Splitting of concrete cover happens in two cases. Push-out test is a test that can help to calculate the compression bond and development lengths ( Li et al., 2013). 2.7.1 Pull-out Test The pullout test is more common due to its simple testing apparatus. A pullout test is a method for determining the bond strength of reinforced concrete by measuring the maximum tensile forces that it can resist. As it can be seen in Figure 2-8(a), this method contains a rebar embedded in the center of concrete specimen. The rebar is usually pulled out by means of a center-hole jack. In a pullout test, the bond strength is determined by the load that the rebar can resist. Currently more complicated samples are suggested by the standard called beam-end specimens for comparing the bond strength (ASTM A944, 2010). 2.7.2 Beam Test The beam test is typically less popular than pullout test (Arezoumandi et al., 2013). Bond strength measured from beam test is lower in comparison with pullout test due to the lack of concrete cracking and local flexural load in pullout test. Also, the cover of concrete thickness surrounding the reinforcing rebar is larger in pullout test (Al-Sulaimani et al., 1990). In the beam specimens, the concrete covering the rebar in the lower part of cross section is under tension stress which varies along the span length. It causes a reduction in the bond strength and leads to the concrete cracking. Thus, the beam tests give a realistic bond stress values which can be considered as the real behavior of concrete members under bending load 23  (Tighiouart et al., 1998). Figure 2-8(c & d) shows a schematic of beam test setup. In order to represent full-size concrete member, both beam anchorage and beam splice specimens can be used. These types of specimens can be used to obtain realistic data on development and splice length. A member with a flexural crack and a known bonded length can be simulated by anchorage specimens. Splice specimens are easier to fabricate compared to anchorage specimens. Usually in these specimens, the splice length overlap is located in the region of constant moment between the two loading points (Butler, 2012).   Figure 2-8. Schematic of different pull-out and beam test specimens (Quayyum & Rteil 2010) 2.7.3 Push-out Test In some cases the rebar in concrete is under compression load such as columns. Also there are some cases that a concrete member is under cyclic loading. The push out test can be used to gather the information required for determining the compressive bond stress and slippage relationship of reinforced concrete. As it can be seen in Figure 2-9, push out test specimen is similar to pull-out one. The rebar will be pushed out from the concrete and the bond stress and slippage relationship can be observed by recording the load and slippage 24  behaviors. String potentiometer/LVDT will be used at the bottom of the rebar in order to determine the slippage level at the fracture. The deformation of rebar under pull-out and push-out test is different. In pull out test the bar is under tension load, therefore, because of the Poisson effect the bar diameter will slightly decrease. On the other hand, the rebar is under compression in push-out test method. So, the Poisson effect will result in a slight increase in bar diameter and it will rise the chance of concrete splitting failure.   Figure 2-9. Schematic of push-out test setup 2.8 Bond Strength and Development Length Equations in Design Codes 2.8.1 Steel Reinforced Concrete  The discussion presented in previous sections studied the effect of different factors such as concrete cover, rebar’s diameter, embedment length, and concrete compressive strength on the bond behavior of concrete members. Previous research works mentioned that bond stress has a good correlation with the concrete compressive strength raised to the 0.5 power (𝑓𝑐′0.5) (ACI 318, 2008; ACI 408, 2003; Canadian Standards Association, 2014b; Darwin et al., 1992; Orangun et al., 1977). However, some studies suggested that the best representation for the 25  effect of concrete comressive strength on bond behavior is provided by 𝑓𝑐′0.25 (ACI 408, 2003; Darwin et al., 2005; Li et al., 2013; Zuo & Darwin, 2000).  Previous studies also represented the bond strength between reinforcing bars under tension and regular concrete (Darwin et al., 1992; Kim & Yun, 2014; Orangun et al., 1977). Orangun et al. (Orangun et al., 1977) proposed a function as: 𝜏 = 0.083045√𝑓𝑐′(1.2 + 50𝑑𝑏 𝑙𝑒⁄ + 3𝐶 𝑑𝑏⁄ ) (2.1) where 𝜏 is bond strength, 𝑓𝑐′ is concrete compressive strength, 𝑑𝑏  is rebar dimeter, 𝑙𝑒  is the embedment length, C is concrete cover. Darwin et al. (Darwin et al., 1992) provided a very similar formula as: 𝜏 = 0.083045√𝑓𝑐′(1.06 + 75𝑑𝑏 𝑙𝑒⁄ + 2.12𝐶 𝑑𝑏⁄ ) (2.2) In addition, based on suggested equations for calculating the development length of deformed bars by ACI-318 (ACI 318, 2008) and CSA A23.3 (Kim & Yun, 2013), the minimum essential bond strength can be found as: 𝜏 = 𝑓𝑦𝐴𝑏 𝑙𝑒𝜋𝑑𝑏⁄  (2.3) where 𝑓𝑦 and 𝐴𝑏 denote the specified yield strength and area of rebar, respectively.  Table 2-1 shows, the bond strength, 𝜏, between the ribbed bars and the surrounding concrete according to the CEB-FIP model code (Special Activity Group 5, 2010) for a monotonic loading. Table 2-1. Bond strength related to bond condition for failure by splitting of the concrete Unconfined concrete Good bond conditions All other bond conditions 2.5√𝑓𝑐′ 1.25√𝑓𝑐′ 26  2.8.2 FRP Reinforced Concrete For comparative purposes in the present study, five bond models for FRP-concrete are considered; three models are based on standard codes including CSA S6-06, CHBDC 2006; and ACI 440.IR-06 (ACI Committee 440-2006), and two models are based on recent research works (Lee et al., 2008; Okelo & Yuan, 2005). Some factors such as bar location (k1), concrete density (k2), bar size (k3), bar fiber (k4), bar surface profile (k5), have already been considered in the CSA S806 model for FRPs.  In CSA S6, besides the bar location factor, the modulus of elasticity of FRP and steel, the area of transverse reinforcement, their spacing, and the flexural strength of concrete (usually 0.4√𝑓𝑐′) have been taken into account (Hossain et al., 2012).  Based on 151 test specimens comprising concretes with compressive strength varying from 29 to 60 MPa and GFRP bars of 6 to 19 mm, Okelo and Yuan (2005) suggested an equation to relate the bond capacity of GFRPs to the concrete compressive strength.  Lee et al. (2008) performed experiments on 54 specimens. The concrete compressive strength (25-92 MPa) and GFRP rebar with sand-coated and helically wrapped external surfaces were used in their experimental program (Lee et al., 2008). All the above equations are summarized in Table 2-2.          27  Table 2-2. FRP-concrete bond equations Standard codes and literatures Bond models for FRP-concrete ACI 440-1R-06 𝜏𝑏 = (0.033 + 0.025𝑐𝑑𝑏+ 8.3𝑑𝑏𝑙𝑒)√𝑓𝑐′ CSA S806-02 𝜏𝑏 =𝑑𝑐𝑠√𝑓𝑐′1.15𝑘1𝑘2𝑘3𝑘4𝑘5𝜋𝑑𝑏 CSA S6-06 𝜏𝑏 =0.4𝑑𝑐𝑠√𝑓𝑐′0.45𝑘1𝑘4𝜋𝑑𝑏 Okelo & Yuan (2005) 𝜏𝑏 = 14.70√𝑓𝑐′𝑑𝑏 Lee et al. (2008) 𝜏𝑏 = 3.3(𝑓𝑐′)0.3   28   Methodology Among the methods discussed in the previous chapter, the push-out test was chosen for studying the compressive bond behavior in this investigation since similar to pull-out tests which have been widely adopted, this method, provides an economical and simple manner to represent the concept of anchoring a bar, and also an easy solution for evaluation and comparison of the bond performance of various concrete mixtures and reinforcing bars (Achillides & Pilakoutas, 2004). Two experimental series of push-out tests were performed to investigate the bond behavior of steel and FRP bars in concrete structures. In this chapter, the studied parameters, the mechanical properties of the considered materials, the specimen preparation, the experimental test setup, and the testing procedure are presented. 3.1 Materials The concrete mixes consisted of cement, water, fly ash, recycled concrete aggregate (RCA), natural coarse aggregate (NCA), natural fine aggregate, water reducer, and air entrainment. In this investigation, GU type Portland cement was used. Sea sand was used as fine aggregate which conformed to CSA A23.2 (Canadian Standards Association, 2014a). Sieve analysis has been done for both coarse and fine aggregates (see Figure 3-1 and Figure 3-2). The maximum size for the natural and recycled coarse aggregates was 25 mm. Figure 3-1 indicates that the aggregate gradation curve of coarse aggregate combinations for all mixes used in this study meet the CSA limits. It ca be seen from Figure 3-2 that fine aggregates gradation was in the CSA standard’s range. Okanagan Builders Supplies Ltd provided the required RCA from a demolished RC bridge. Strength, porosity, and absorption of fine and coarse aggregate affect the strength of concrete. Since all the recycled aggregates used in this study were from the same source, there was no inconsistency in their characteristics and properties. As it can be seen from Table 3-1, absorption capacity of NCA is 0.8%, whereas the absorption capacity of RCA is 4.35%, i.e. five times to the capacity of RCA.  Deformed steel bars are the most common form of rebar used in a range of residential, commercial and infrastructure applications. Their surface is often patterned to form a better bond with the concrete. Therefore, deformed steel bars with average yield strength of 400 MPa 29  were adopted for this study. Nominal diameters of the bars were 16 mm (15M) and 19.5 mm (20M). Sand coated GFRP rebars were provided by BP Composites Ltd. in two sizes of 13 mm and 20 mm. GFRP bars had a 40 GPa modulus, and ultimate tensile strength of 950 MPa. BP Composites Ltd. uses the highest quality corrosion resistant Vinylester resin and fiberglass materials. Table 3-2 shows the properties of GFRP bars produced by BP Composites Ltd. Table 3-1. Physical properties of NCA and RCA  Coarse aggregate Bulk density (kg/m3) Apparent specific gravity Water absorption capacity (%) Moisture content (%) Natural 1633 2.63 0.8 1.31 Recycled 1389 2.41 4.35 1.91  Table 3-2. GFRP  bars properties  Properties Units #4 #6 Nominal size mm 13 20 Ultimate tensile strength MPa 948.6 997.7 Guaranteed ultimate tensile strength (ASTM D7205 / CAN/CSA-S806) MPa 845.4 884.3 Guaranteed tensile force (ASTM D7205/ CAN/CSA-S806) kN 109.1 251.1 Modulus of elasticity GPa 45.6 51.0 Ultimate elongation % 2.1 2.0 Bond strength (ACI 440.3R B3) MPa 12.7 14.4 Transverse shear strength (ACI 440.3R B4) MPa 209.7 190.3 Transverse shear force (ACI 440.3R B4) kN 54.1 108.1 Weight / Length g/m 259 611   30   Figure 3-1. Sieve analysis of coarse aggregate combinations for different concrete mixes   Figure 3-2. Sieve size analysis for the fine aggregates used  3.2 Mix Proportions Mixture proportions for NCA and RCA concretes were designed with a target strength of 35 MPa. A similar mix proportion was used for all concrete mixtures. Natural coarse aggregates were replaced with recycled aggregates by weight, based on the desired RCA replacement level. The weight ratio of RCA to the total coarse aggregate indicates RCA 0204060801000 5 10 15 20 25 30% Passing WeightSieve Size (mm) Control30% RCA50% RCA70% RCA100% RCACSA Upper LimitCSA Lower Limit0204060801000 2 4 6 8 10 12% Passing WeightSieve Size (mm)Fine AggregateCSA Upper LimitCSA Lower Limit31  replacement level in concrete mixture. As it can be seen in Table 3-3, the weight combinations used in the first stage of this study were: 100% NCA (control mixture), 70% NCA + 30% RCA, 50% NCA + 50% RCA, 30% NCA + 70% RCA, and 100% RCA.  In order to keep the water-cement ratio (w/c) around 0.4, the volume of mixing water varied across all the mixtures due to varying the amount of absorbed water (Table 3-3). The workability of fresh concrete was determined by means of the standard slump test setup according to CSA A23.2-5C (Canadian Standards Association, 2014a) and the initial slump values were 60+/-10 mm. Based on CSA A23.2-4C (Canadian Standards Association, 2014a), air content of fresh concrete was in the range of 4 to 6%. Table 3-3. Mix proportions of concrete (kg/m3)  RCA replacement percentage (%) Cement (kg) Fly Ash (kg) Fine Aggregate (kg) Natural Coarse Aggregate (kg) Recycled Coarse Aggregate (kg) Mixing Water (ml) 0 300 80 735 1030 0 148 30 300 80 735 721 309 154 50 300 80 735 515 515 158 70 300 80 735 309 721 160 100 300 80 735 0 1030 160  3.3 Preparation of Specimens The common way of examining the tensile bond strength is pull-out test in which the rebar is pulled out from the concrete. In this study, the push-out test was chosen for investigating the compressive bond behavior in order to simplify the test and due to easiness of fabrication. Despite of some differences in details, the global responses of force and slip is similar to that of pull-out test (Choi et al , 2011). The bond resistance obtained from these specimens is the least realistic because they are not able to exactly model bond conditions which might appear in actual construction concrete elements. However, push-out tests offer a simple means of comparative study of the bond behavior for various concrete mixtures and 32  reinforcing bars. Furthermore, previous experiments revealed that the compressive bond performance is similar to the tensile bond performance (Li et al., 2013). Concrete mixes with coarse aggregate compositions of 30%, 50%, 70%, and 100% RCA were compared to a control batch made entirely using natural coarse aggregate. In order to observe the effect of the cover to bar diameter ratio on bond stress, two different sizes of cylinders (Ø100×200 mm and Ø150×300 mm) were considered. In addition, rebar diameter and embedment length are important factors in calculating the bond stress between the rebar and concrete due to the effect of embedded surface area on the strength of the bond. Thus, 13M, 15M, and 20M rebars and le⁄db ratios of 5 and 10 were used in the tests. The le⁄db ratio is the ratio of embedment length to the diameter of the bar. According to ASTM A944 (Standard test method for comparing bond strength of steel reinforcing bars to concrete using bean-end specimens) a minimum of two test cylinders are required to determine the bond for a specific specimen. Thus, to ensure reliability of results and reduce the unexpected errors, three identical specimens were cast for each category. Nine cylinders (Ø100×200 mm) were also cast in order to identify the concrete mixtures strength after 7, 28, and 56 days. The push-out specimens were cast in a vertical position using plastic molds. In order to properly perform the push-out test, the rebar was held vertically in the center of the concrete cylinder using a specially designed wooden cap shown in  Figure 3-3. The cap was designed to fit tightly over the concrete cylinder mold and hold the rebar vertically in the center of the cylinder. Soft plastic wrap were used along the unbonded length of the rebar to break the contact between the concrete and the embedded rebar (see Figure 3-4). The specimens were demolded after 24 hours of casting, and then cured in the moist curing chamber for 28 days. Figure 3-5 and Figure 3-6 demonstrate the casted and demolded specimens. The following protocol was adopted for the sample identities: 30-10M-4/8-5. The first numeral denotes the RCA replacement level. The second and third numbers denote the rebar size and the size of the cylinder in inches, respectively. The last part stands for the ratio of embedment length to the diameter of the bar.  33    Figure 3-3. Completed wooden rebar straightening cap for the 4" diameter concrete cylinder moulds                                           Figure 3-4. One example of the de-bonded rebars with the embedment ratio of 5 for a cylinder size of 100×200mm  34    Figure 3-5. Specimens for compression and push-out test after casting    Figure 3-6. Specimens for compression and push-out test after demolding     35  Table 3-4. Push-out test variable parameters and specimen details for second series of tests Parameters %RCA 0%, 30%, 50%, 70%, 100% Steel Rebar Size 15M and 20M GFRP Rebar Size 13M and 20M Cylinder Size (mm) 100 × 200 and 150 × 300 le/db Ratio 5 and 10  3.4 Push-out Test Setup In order to perform the push-out tests, Instron stiff frame testing machine was used. Figure 3-7 shows the fully assembled push-out test apparatus. A stand was constructed with a hole at the top with a size larger than the largest rebar’s diameter to allow the rebar to pass through during the compression. The stand was made of solid steel in order to ensure that it could withstand repeated testing. The experiment was carried out by applying uniform loading force which was increased gradually. The tests were performed in a displacement control mode at a rate of 3 mm per min.  In addition, it was necessary to minimize the platen friction and provide a uniform contact. Therefore, a stiff steel restrainer plate with 10 mm thickness was used on top of the rebar. The test was started by applying force and pushing the embedded rebar downward through the concrete. To determine the displacement of reinforcement bar relative to its surrounding concrete, a draw wire sensor (Figure 3-8) was used.  The wire was attached to the bottom of the rebar. A mounting plate was made to protect the draw wire sensor during the push out test.  The plate was made of aluminum and equipped with a pulley so that the draw wire sensor can be mounted outside the compression test stand. In order to calibrate the draw wire sensor, the voltage output was measured at zero and 130mm.  The change in voltage across the range of extension was then monitored to measure the extension of the sensor during testing and the corresponding slippage of the rebar. A National Instruments (NI) data acquisition system was used to capture the sensor’s signal. The push-out tests were continued until the surrounding concrete split or slippage of about 10 mm occurred. 36   Figure 3-7. Fully assembled push out testing apparatus for steel reinforced specimens   Figure 3-8. Draw Wire Sensor used in the push-out test 37  In order to test GFRP specimens, two still plates were prepared. These plates transfer applied load just to rebar cross section and push the rebar through surrounding concrete. Figure 3-9 and Figure 3-10 show the plates and test setup for GFRP specimens.   Figure 3-9. Steel plates for testing the GFRP specimens   Figure 3-10. Fully assembled push out testing apparatus for GFRP reinforced specimens 38   Steel Reinforced Specimens Test Results  4.1 Compressive Strength In general, a reduction has been reported in compressive strength of concrete made of RCA in comparison to conventional concrete in previous studies (Kim & Yun, 2013). Some of the several reasons for this diminution could be an increased concrete porosity, a weak interface bond between RCA aggregate and cement matrix or the bond between RCA and NCA. However, It has been reported that, on an average, the RCA replacement level of up to 30% had minor effect on compressive strength (Corinaldesi & Moriconi, 2007; Li, 2008). In some cases, the early age strength of RCA mix was slightly higher than the control mix because of the presence of unhydrated mortar attached to RCA (Corinaldesi & Moriconi, 2007; Li, 2008). In this study concrete compressive strengths of different mixes were measured after 7, 28, and 56 days of curing. The results are shown in Figure 4-1. The mixes with up to 70% RCA replacement have shown acceptable 28-day strength compared to the control mix and target compressive strength. At RCA replacement level of 0%, the compressive strength after 56 days is about 12% higher than that after 28 days while, at RCA mixes, the rate of strength development is about 5%. The lower bulk density of RCA, adhered mortar content, and weak interfacial transition zone are thought to be the main reasons behind the slightly strength growth of RCA concrete mixes. Error bars are graphical indications of the error, or uncertainty in a reported measurement. Error bars represent standard error in Figure 4-1. The less wide error bars sign that there is more confidence in a particular value.  39   Figure 4-1. Concrete compressive strength with age 4.2 General Description Most of the specimens demonstrated splitting failure. This failure mode occurs when bond stress drops suddenly due to cracking of concrete cover and splitting along the length of reinforcing bar (see Figure 4-2). The Poisson effect can lead to a slight enlargement in bar diameter since the rebar is under compression load. This will result in increasing the developed radial stresses on the concrete and concrete splitting.   Specimens with normal and recycled aggregate concrete showed almost similar pattern for bond slippage behavior. Figure 4-3 and Figure 4-3 show bond stress versus slip response of some of the push-out specimens. Generally, a sharp ascending occurs in the first stage of bond behavior where chemical adhesion is predominant. For most of the specimens this phase remains linear up to peak loads. Mechanical interlock appears to have more contribution in bond resistance in the second stage. The descending branch of the bond behavior was almost linear for most of the specimens. Some of the larger specimens embedded with small bars (15M) had descending response with a smaller slope in this phase. Therefore, the specimen with a larger concrete cover to diameter ratio showed a much more ductile behavior. On an average, the slips corresponded to the peak loads in the range of 0.011 – 1.55 mm.  010203040500 30 50 70 100Compressive strength (MPa)RCA replacement level (%)7 days28 days56 days40  As can be seen in Figure 4-3 and Figure 4-4, the concrete mix with 50% RCA replacement shows a higher bond capacity due to its better compressive strength. Studies performed by Xiao and Falkner (Xiao & Falkner, 2007) and , Prince and Singh (Prince & Singh, 2013) confirmed the observed results of bond stress-slip relationship in this study. This experiment represents the similarity of bond behavior of deformed steel rebars embedded in RCA concrete and conventional concrete.   Figure 4-2. Failure of samples    41    Figure 4-3. Bond stress versus slip response in 200 mm concrete cylinder  Figure 4-4. Bond stress versus slip response in 300 mm concrete cylinder  05101520250 0.5 1 1.5 2Bond stress (MPa)Slippage (mm)0%-100x200-20M-530%-100x200-20M-550%-100x200-20M-570%-100x200-20M-5100%-100x200-20M-50102030400 0.5 1 1.5 2 2.5 3 3.5Bond stress (MPa)Slippage (mm)0%-150x300-20M-530%-150x300-20M-550%-150x300-20M-570%-150x300-20M-5100%-150x300-20M-542  4.3 Bond Strength Bond strength is defined as the maximum local horizontal shear force per unit area of the bar perimeter. According to the assumption that the bond stress has uniform distribution along the embedment length of rebar in concrete (Figure 4-5), the average bond strength acting on the surface of the rebar can be determined by the following equation:  𝜏𝜋𝑑𝑏𝑙𝑒 + 𝐴𝑏𝑎𝑟  𝑓𝐹 = 𝐴𝑏𝑎𝑟(𝑓𝐹 + ∆𝑓𝐹) (4.1) where, τ = average bond stress (MPa); fF = stress developed in the rebar (MPa); db = diameter of the rebar (mm); le = embedment length of the rebar (mm); Abar = area of one rebar (mm2). From Equation 4.1, the resulted bond strength in push-out specimens can be derived as: 𝜏𝑚𝑎𝑥 = 𝑃𝑚𝑎𝑥 (𝜋𝑑𝑏𝑙𝑒)⁄  (4.2) where 𝜏 is the average maximum bond strength in MPa, 𝑃𝑚𝑎𝑥 is the maximum push-out load in N. 𝑙𝑒 and 𝑑𝑏 represent the embedded bar length and the diameter of the bar in mm, respectively. Average values of the bond strength for all test specimens are listed in Table 4-1. The performance of the influencing factors is presented in the following sections.     Figure 4-5. Transfer of force through bond 43   Table 4-1. Average bond strength of  steel specimens Specimen 𝑓𝑐′  (MPa) τ max (MPa) Specimen 𝑓𝑐′  (MPa) τ max (MPa) 0-15M-4/8-5 42 27.1 50-20M-4/8-5 35.5 21.8 0-15M-4/8-10 42 18.7 50-20M-4/8-10 35.5 14.8 0-15M-6/12-5 42 43.8 50-20M-6/12-5 35.5 35.6 0-15M-6/12-10 42 25.0 50-20M-6/12-10 35.5 19.2 0-20M-4/8-5 42 20.0 70-15M-4/8-5 34 28.1 0-20M-4/8-10 42 14.5 70-15M-4/8-10 34 18.6 0-20M-6/12-5 42 34.4 70-15M-6/12-5 34 47.5 0-20M-6/12-10 42 19.7 70-15M-6/12-10 34 22.3 30-15M-4/8-5 34 25.1 70-20M-4/8-5 34 14.3 30-15M-4/8-10 34 20.3 70-20M-4/8-10 34 9.1 30-15M-6/12-5 34 44.5 70-20M-6/12-5 34 35.6 30-15M-6/12-10 34 21.8 70-20M-6/12-10 34 17.9 30-20M-4/8-5 34 14.5 100-15M-4/8-5 32 26.8 30-20M-4/8-10 34 12.5 100-15M-4/8-10 32 17.7 30-20M-6/12-5 34 33.2 100-15M-6/12-5 32 44.8 30-20M-6/12-10 34 17.5 100-15M-6/12-10 32 21.5 50-15M-4/8-5 35.5 31.8 100-20M-4/8-5 32 13.3 50-15M-4/8-10 35.5 16.8 100-20M-4/8-10 32 11.2 50-15M-6/12-5 35.5 44.7 100-20M-6/12-5 32 27.8 50-15M-6/12-10 35.5 24.5 100-20M-6/12-10 32 18.7     44  4.4 The Effect of Bar Diameter on Bond Strength Based on the test results, under equivalent concrete mix proportions, specimens with larger bar diameter and embedment length had lower bond capacity. It has been reported that as the diameter and length of the rebar which are in contact with concrete increase, the amount of bleed water, which is trapped between the bar surface and concrete, increases. This leads to a higher amount of voids and a reduction in the contact area (Quayyum, 2010). Hence, the average bond stress transferred to the surrounding concrete reduces when the bonded length and surface area between two materials increase. As observed from Figure 4-6, the bond strength of 20M rebar embedded in concrete is about 22% less than that in the similar sample with 15M rebar while considering control and 50% RCA mix. However, this reduction is about 30% for 30% RCA mix, and 36% for 70% and 100% RCA mixes due to their lower compressive strength.  4.5 The Effect of Embedment Length on Bond Strength In addition to voids creation, the stress is distributed along a longer length with an increase in the embedment length to bar diameter ratio. Hence, there is a decrease in the bond capacity. Moreover, the embedment length equal to 10db results in 30% to 45% less bond strength compared to 5db while considering similar samples (see Figure 4-7). This reduction can be seen in all mixes and for all rebar sizes. Thus, the embedment length in RC member plays a critical role in providing a better bond strength. When there is a concrete cover with lower thickness, the reduction in the bond due to the embedment length increase diminishes.    45   Figure 4-6. Relationship between bond strength and bar diameter, a) control mix, b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix, e) 100% RCA mix     0102030405015M 20MBond strength (MPa)Rebar size0- 4/8- 50- 4/8- 100- 6/12- 50- 6/12- 100102030405015M 20MBond strength (MPa)Rebar size30- 4/8- 530- 4/8- 1030- 6/12- 530- 6/12- 100102030405015M 20MBond strength (MPa)Rebar size50- 4/8- 550- 4/8- 1050- 6/12- 550- 6/12- 100102030405015M 20MBond strength (MPa)Rebar size70- 4/8- 570- 4/8- 1070- 6/12- 570- 6/12- 100102030405015M 20MBond strength (MPa)Rebar size100- 4/8- 5100- 4/8- 10100- 6/12- 5100- 6/12- 10a)c)b)e)d)46   Figure 4-7. Relationship between bond strength and embedded length, a) control mix, b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix, e) 100% RCA mix    010203040505 10Bond strength (MPa)Embedment length to bar diameter ratio0- 15M- 4/80- 20M- 4/80- 15M- 6/120- 20M- 6/12010203040505 10Bond strength (MPa)Embedment length to bar diameter ratio30- 15M- 4/830- 20M- 4/830- 15M- 6/1230- 20M- 6/12010203040505 10Bond strength (MPa)Embedment length to bar diameter ratio50- 15M- 4/850- 20M- 4/850- 15M- 6/1250- 20M- 6/12010203040505 10Bond strength (MPa)Embedment length to bar diameter ratio70- 15M- 4/870- 20M- 4/870- 15M- 6/1270- 20M- 6/12010203040505 10Bond strength (MPa)Embedment length to bar diameter ratio100- 15M- 4/8100- 20M- 4/8100- 15M- 6/12100- 20M- 6/12a)b) c)e)d)47  4.6 The Effect of Concrete Cover on Bond Strength In general, large diameter (≥ 150 mm) cylinder specimens exhibit better bond resistance due to a better confinement. The same trend was observed for majority of push-out specimens. Figure 4-8 demonstrates a maximum of about 50% increase in the bond resistance for 20M rebar when concrete cover increases from 40 to 65 mm. Similar findings were observed in the literature (Butler et al., 2014; Li et al., 2013). Larger concrete cover to bar diameter ratio (≈ 4.5) for 15M bars was adequate for a good bond condition. The influence of different factors on the bond behavior of concrete made of RCA is similar to conventional concrete. Based on push-out tests performed by Li et al., (2013) similar factors that affect the bond strength in conventional concrete were presented (Li et al., 2013).   48   Figure 4-8. Relationship between bond strength and concrete cover, a) control mix, b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix, e) 100% RCA mix   010203040501 2 3 4 5Bond strength (MPa)Concrete Cover to bar diameter ratio0- 15M- 50- 15M- 100- 20M- 50- 20M- 10010203040501 2 3 4 5Bond strength (MPa)Concrete Cover to bar diameter ratio30-15M 530- 15M- 1030- 20M- 530- 20M- 10010203040501 2 3 4 5Bond strength (MPa)Concrete Cover to bar diameter ratio50- 15M- 550- 15M- 1050- 20M- 550- 20M- 10010203040501 2 3 4 5Bond strength (MPa)Concrete Cover to bar diameter ratio70- 15M- 570- 15M- 1070- 20M- 570- 20M- 10010203040501 2 3 4 5Bond strength (MPa)Concrete Cover to bar diameter ratio100- 15M- 5100- 15M- 10100- 20M- 5100- 20M- 10a)c)b)d) e)49  4.7 The Effect of RCA Replacement Level on Bond Strength The test results indicate that specimens made of concrete mixes with 30%, 50%, 70%, and 100% RCA replacement levels have similar bond behavior compared to control samples. Hence, the use of RCA will have no significant effect on the bond resistance of concrete compared to regular concrete. Traditionally, it has been proved in literatures and design expressions that representation of concrete properties effect on bond strength using the square root of compressive strength is adequate for concretes less than 55 MPa strength (ACI 408, 2003). In order to explain it more clearly, all the bond strength test results are also plotted by normalizing the value with square root of fc’ (√𝑓𝑐′). According to Figure 4-9 to Figure 4-12 , on average, among the specimens with the same size and same embedment length, the concrete mix of 50% RCA replacement have the best bond capacity compared to control mix. It can be concluded that replacement of natural aggregate with recycled concrete aggregate up to 50% can lead to noticeable bond capacity that is comparable to regular concrete (Figure 4-9 to Figure 4-12). These results can be confirmed by similar studies (Prince & Singh, 2013; Kim & Yun, 2014; Xiao & Falkner, 2007).  Therefore, similarity in overall bond capacity and behavior of concrete mixes made of RCA and normal aggregate portrays the possibility of using recycled concrete aggregates in structural applications.   Figure 4-9. Relationship between bond strength and recycled aggregate replacement level in 200 mm concrete cylinder with le⁄db =5. a) bond strength, b) normalaized bond strength 01020300% 30% 50% 70% 100%Bond strength (MPa)RCA percentage15M20M02460% 30% 50% 70% 100%RCA percentage15M20M𝜏    (a) (b) 50    Figure 4-10. Relationship between bond strength and recycled aggregate replacement level in 200 mm concrete cylinder with le⁄db =10. a) bond strength, b) normalaized bond strength    Figure 4-11. Relationship between bond strength and recycled aggregate replacement level in 300 mm concrete cylinder with le⁄db =5. a) bond strength, b) normalaized bond strength      051015200% 30% 50% 70% 100%Bond strength (MPa)RCA percentage15M20M012340% 30% 50% 70% 100%RCA percentage15M20M𝜏    010203040500% 30% 50% 70% 100%Bond strength (MPa)RCA percentage15M20M01234567890% 30% 50% 70% 100%RCA percentage15M20M𝜏    (a) (a) (b) (b) 51   Figure 4-12. Relationship between bond strength and recycled aggregate replacement level in 300 mm concrete cylinder with le⁄db =10. a) bond strength, b) normalaized bond strength 4.8 Sensitivity Analysis The effects of different parameters of concrete cover to bar daimeter ratio ( 𝐶 𝑑𝑏⁄ ), embedment length to bar diameter ratio ( 𝑙𝑒 𝑑𝑏⁄ ), as well as RCA replacement percentages were investigated on the bond performance of steel reinforced samples through an ANOVA sensitivity analysis. To reject the null hypothesis during statistical analysis, a significance level of α = 0.05 was considered. Based on two different bar diameter and two concrete cylinder size, we consider 4 level for  𝐶 𝑑𝑏⁄ . Moreover, 5 level for RCA replacement percentages and 2 level for  𝑙𝑒 𝑑𝑏⁄ . So, a total number of 40 runs were considered for ANOVA analysis. As results show in Table 4-2, the bond strength of specimens with steel bars have a more significant relationship with the two of main factors including concrete cover and embedment length to bar diameter ratio. The P-value and percent contribution of each factor were calculated for data experimental results. The P-value for the parameters 𝐶 𝑑𝑏⁄ ,  𝑙𝑒 𝑑𝑏⁄ ,  were found to be less than 0.0001, whereas RCA percentage showed a P-value of 0.55 which is more than α = 0.05. Similarly, a percent contribution of about 48%, and 38%, were found for 𝐶 𝑑𝑏⁄  and  𝑙𝑒 𝑑𝑏⁄   respectively.  This percentage for RCA replacement level was nearly one percent. According to the above results and other statistical plots of effects in Figure 4-13, the main factors including bar diameter, embedment length, and concrete cover have significant effect on bond strength. However, it can be concluded that RCA replacement percentages had very negligible influence on bond performance compared to other factors. Moreover, the effect of 05101520250% 30% 50% 70% 100%Bond strength (MPa)RCA percentage15M20M012340% 30% 50% 70% 100%RCA percentage15M20M𝜏    (a) (b) 52  increase in concrete cover, embedment length, and RCA level obtained from previous sections can be confirmed by outcomes of sensitivity analysis . Table 4-2. Analysis of variance table for steel specimens Source Sum of Squares Mean Square p-value  Percent contribution Model 3591.2 448.9 < 0.0001  A-C/d 1970 656.67 < 0.0001 48% B-l/d 1567.95 1567.95 < 0.0001 38% C-RCA 53.25 13.31 0.5506 1% Residual 533.39 17.21   Cor Total 4124.59        53   Figure 4-13. Effect of different factors on bond strength; a) concrete cover to bar diameter ratio, b) embedment length to bar diameter ratio, c) RCA replacement percentage 4.9 Comparison of Measured and Predicted Bond Strength Although all existing bond models are applicable to normal concrete, Figure 4-14 shows that the predicted values of bond suggested by Orangun et al. (Orangun et al., 1977), Darwin et al. (Darwin et al., 1992), ACI 318 (ACI 318, 2008), and CEB-FIP (Special Activity Group 5, 2010) are significantly lower than those obtained from experiments. The comparison of measured and predicted bond strength values by different codes and literatures are shown in Figure 4-15 to Figure 4-18. It can be concluded that all the previous codes and literature (a) (c) (b) 54  models gave conservative results compared to experimental results. In most of the cases, ACI 318 had the closest result to the measured value. CEB-FIP model shows better results for large specimens, since the concrete cover was thick enough to provide a good bond condition. Orangun and Darwin models display very similar results based on their formulas. Since almost same terms and factors were considered in both models.  Figure 4-14. Calculated bond stress versus measured bond stress according to a) Orangun formula, b) Darwin formula, c) ACI 318 fomula, and d) CEB-FIP formula For small (100 × 200 mm) specimens the CEB-FIP formula bond prediction was based on unconfied concrete withoutgood bond condition. So in some cases, CEB-FIP formula showed the least adequate results compared to other models. In case of small specimens embedded with 20M bars, due to inadequate concrete cover, the measured bond strength was amaller than the ACI 318 code prediction. In specimens reinforced with 15M bars, tested bond strengths were a little higher and comparable to codes models (see Figure 4-15). CEB-FIP formula bond prediction was based on unconfied concrete with a good bond condition for large concrete cylinders. Hence, ACI 318 and CEB-FIP models demonstrate the closest results to the measured bond for large samples. In general, as the RCA replacement level increases, the 01020304050600 10 20 30 40 50 60Bond (calc) (MPa)Bond (test) (MPa)ACI 31801020304050600 10 20 30 40 50 60Bond (calc) (MPa)Bond (test) (MPa)Orangun01020304050600 10 20 30 40 50 60Bond (calc) (MPa)Bond (test) (MPa)Darwin01020304050600 10 20 30 40 50 60Bond (calc) (MPa)Bond (test) (MPa)CEB-FIP(a) (b) (c) (d) 55  difference between the measured bond and the predicted values reduces. It may be suggested that the RCA replacement effect can be considered in calculating the concrete compressive strength since 𝑓𝑐′ is one of the important factors in all bond equations.   Figure 4-15. Comparison of measured and predicted bond stress values for small specimens with le⁄db=5, embedded with a) 20M, and b) 15M bars  Figure 4-16. Comparison of measured and predicted bond stress values for small specimens with le⁄db=10, embedded with a) 20M, and b) 15M bars  01020300 30 50 70 100Bond strength (MPa)RCA percentage20M-testOrangun et al.Darwin et al.ACI 318CEB-FIP0102030400 30 50 70 100Bond strength (MPa)RCA percentage15M-testOrangun et al.Darwin et al.ACI 318CEB-FIP010200 30 50 70 100Bond strength (MPa)RCA percentage20M-testOrangun et al.Darwin et al.ACI 318CEB-FIP01020300 30 50 70 100Bond strength (MPa)RCA percentage15M-testOrangun et al.Darwin et al.ACI 318CEB-FIP(a) (b) (a) (b) 56   Figure 4-17. Comparison of measured and predicted bond stress values for large specimens with le⁄db=5, embedded with a) 20M, and b) 15M bars    Figure 4-18. Comparison of measured and predicted bond stress values for large specimens with le⁄db=10, embedded with a) 20M, and b) 15M bars  0102030400 30 50 70 100Bond strength (MPa)RCA percentage20M-testOrangun et al.Darwin et al.ACI 318CEB-FIP010203040500 30 50 70 100Bond strength (MPa)RCA percentage15M-testOrangun et al.Darwin et al.ACI 318CEB-FIP01020300 30 50 70 100Bond strength (MPa)RCA percentage20M-testOrangun et al.Darwin et al.ACI 318CEB-FIP01020300 30 50 70 100Bond strength (MPa)RCA percentage15M-testOrangun et al.Darwin et al.ACI 318CEB-FIP(a) (b) (a) (b) 57   FRP Reinforced Specimens Test Results  5.1 General Description  The dominant failure mode for specimens reinforced with GFRP bars was splitting failure. However, there were some cases in which the GFRP bars were pushed out of concrete with no cracks. This happened in large cylinders which had more concrete cover and confinement. The push-out failure accrued to specimens made of recycled concrete, mostly 100% RCA mix. Hence, it can be concluded that on average, recycled aggregates can result in more ductility. Figure 5-3 shows different failure mode of GFRP samples.  Figure 5-1 and Figure 5-2 illustrate bond versus slip behavior of some of GFRP specimens. The same trend of stress-slippage response has been observed for GFRP push-out specimens compared to pull-out experimental results reported by previous researchers (Malvar et al., 2003). In the beginning of test, the chemical adhesion responsible for bond in GFRP samples, were between concrete and sand applied to the surface of bar. The friction and the mechanical bond between concrete and reinforcement characterize the next stage after which the chemical bond is lost. In cases with larger concrete cover, the second stage of bond behavior became distinguishable. An increase in the rate of slip led to a non-linear response with a small increase in the bond stress. After reaching the peak of the bond stress, a sudden drop of the bond stress can be observed, after which the value remains at a constant slip. On average, 50% RCA mix had the closest results to the control mix. These results can be explained by the compressive strength of concrete mixes. In comparison to the control mix, specimens made of RCA had larger slip values corresponded to peak bond. The range of slippage values at the time of failure was 0.2 mm – 6 mm. Table 5-1 indicates the average compressive bond strength of GFRP push-out specimens.     58    Figure 5-1. Bond stress versus slip response in 200 mm concrete cylinder  Figure 5-2. Bond stress versus slip response in 300 mm concrete cylinder   051015200 0.4 0.8 1.2 1.6 2Bond stress (MPa)Slippage (mm)0%-100x200-20M-530%-100x200-20M-550%-100x200-20M-570%-100x200-20M-5100%-100x200-20M-50510152025300 2 4 6 8Bond stress (MPa)Slippage (mm)0%-150x300-20m-530%-150x300-20M-550%-150x300-20M-570%-150x300-20M-5100%-150X300-20M-559   (a)  (b)  (c) Figure 5-3. Failure of FRP samples with a) splitting failure; b) splitting and push-out failure; c) push-out failure   60  Table 5-1. Average bond strength of GFRP specimens Specimen 𝑓𝑐′  (MPa) τ max (MPa) Specimen 𝑓𝑐′  (MPa) τ max (MPa) 0-13M-4/8-5 42 25.5 50-20M-4/8-5 35.5 15.4 0-13M-4/8-10 42 13.2 50-20M-4/8-10 35.5 7.2 0-13M-6/12-5 42 25.6 50-20M-6/12-5 35.5 18.8 0-13M-6/12-10 42 14.3 50-20M-6/12-10 35.5 11.9 0-20M-4/8-5 42 15.2 70-13M-4/8-5 34 15.1 0-20M-4/8-10 42 8.5 70-13M-4/8-10 34 8.8 0-20M-6/12-5 42 23.2 70-13M-6/12-5 34 22.9 0-20M-6/12-10 42 9.3 70-13M-6/12-10 34 13.4 30-13M-4/8-5 34 20.6 70-20M-4/8-5 34 10.1 30-13M-4/8-10 34 9.7 70-20M-4/8-10 34 7.1 30-13M-6/12-5 34 28.5 70-20M-6/12-5 34 18.9 30-13M-6/12-10 34 13.5 70-20M-6/12-10 34 12.1 30-20M-4/8-5 34 13.5 100-13M-4/8-5 32 16.1 30-20M-4/8-10 34 7.0 100-13M-4/8-10 32 9.3 30-20M-6/12-5 34 17.6 100-13M-6/12-5 32 24.3 30-20M-6/12-10 34 9.4 100-13M-6/12-10 32 11.6 50-13M-4/8-5 35.5 18.1 100-20M-4/8-5 32 13.8 50-13M-4/8-10 35.5 9.7 100-20M-4/8-10 32 7.0 50-13M-6/12-5 35.5 24.2 100-20M-6/12-5 32 16.9 50-13M-6/12-10 35.5 12.3 100-20M-6/12-10 32 10.5  5.2 The Effect of Bar Diameter on Bond Strength In terms of bar size, the results indicate the reduction in bond strength when the bar diameter increases. On average, this reduction is about 27% for the control mix (see Figure 5-4). While the concrete mixes with RCA replacement levels equal to and greater than 50% show about 20% bond strength reduction when 20M GFRP bars are implemented. Achillides and Pilakoutas (2012) stated that thinner bars developed greater adhesion with the surrounding concrete than thicker bars because larger diameter bars lose their adhesive bond earlier due to more trapped water between concrete the rebar surface. Other researchers also reported similar results (Achillides & Pilakoutas, 2004; Arias et al., 2012; Benmokrane et al., 2002; Cosenza et al., 1997; Okelo & Yuan, 2005).  61   Figure 5-4. Relationship between bond strength and bar diameter, a) control mix, b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix, e) 100% RCA mix    010203013M 20MBond strength (MPa)Rebar size30- 4/8- 530- 4/8- 1030- 6/12- 530- 6/12- 10010203013M 20MBond strength (MPa)Rebar size50- 4/8- 550- 4/8- 1050- 6/12- 550- 6/12- 10010203013M 20MBond strength (MPa)Rebar size70- 4/8- 570- 4/8- 1070- 6/12- 570- 6/12- 10010203013M 20MBond strength (MPa)Rebar size100- 4/8- 5100- 4/8- 10100- 6/12- 5100- 6/12- 10010203013M 20MBond strength (MPa)Rebar size0- 4/8- 50- 4/8- 100- 6/12- 50- 6/12- 10a)c)b)e)d)62  5.3 The Effect of Embedment Length on Bond Strength As can be seen in Figure 5-5 an increse in embedment length in all specimens reinforced with GFRP bars leads to a drop in the average developed bond strength. This trend was the same among all the concrete mixes. The nonlinear distribution of bond stress along the bar is more pronounced in the case of larger embedment lengths and can be the reason of such respnse. The average reduction value was about 45% for all different control and RCA mixes. These results are also confirmed by (Achillides & Pilakoutas, 2004; Benmokrane et al., 2002; Cosenza et al., 1997).  5.4 The Effect of Concrete Cover on Bond Strength The result of a better confinement due to a thicker concrete cover for GFRP specimens was higher bond strength. Figure 5-6 demonstrates an average of 30% growth in the peak bond stress for RCA concrete mixes, whereas this amount is about 16% for control specimens. It can be concluded that RCA replacement level does not have noticeable influence on bond increase ratio due to the concrete cover enlargement. Findings observed by Quayyum (2010) shows the same trend for normal concrete embedded with FRP rebar (Quayyum, 2010). It is understood that using RCA in concrete mix reinforced with FRP bars leads to a similar bond behavior compared to conventional concrete. Therefore, there is a potential to combine these two materials for structural concrete applications while considering the effect of rebar and concrete properties.  63   Figure 5-5. Relationship between bond strength and embedded length, a) control mix, b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix, e) 100% RCA mix  01020305 10Bond strength (MPa)Embedment length to bar diameter ratio0- 13M- 4/80- 20M- 4/80- 13M- 6/120- 20M- 6/1201020305 10Bond strength (MPa)Embedment length to bar diameter ratio30- 13M- 4/830- 20M- 4/830- 13M- 6/1230- 20M- 6/1201020305 10Bond strength (MPa)Embedment length to bar diameter ratio50- 13M- 4/850- 20M- 4/850- 13M- 6/1250- 20M- 6/1201020305 10Bond strength (MPa)Embedment length to bar diameter ratio70- 13M- 4/870- 20M- 4/870- 13M- 6/1270- 20M- 6/1201020305 10Bond strength (MPa)Embedment length to bar diameter ratio100- 13M- 4/8100- 20M- 4/8100- 13M- 6/12100- 20M- 6/12a)c)b)e)d)64   Figure 5-6. Relationship between bond strength and concrete cover, a) control mix, b) 30% RCA mix, c) 50% RCA mix, d) 70% RCA mix, e) 100% RCA mix    01020301 2 3 4 5Bond strength (MPa)Concrete Cover to bar diameter ratio0- 13M- 50- 13M- 100- 20M- 50- 20M- 1001020301 2 3 4 5Bond strength (MPa)Concrete Cover to bar diameter ratio30- 13M- 530- 13M- 1030- 20M- 530- 20M- 1001020301 2 3 4 5Bond strength (MPa)Concrete Cover to bar diameter ratio50- 13M- 550- 13M- 1050- 20M- 550- 20M- 1001020301 2 3 4 5Bond strength (MPa)Concrete Cover to bar diameter ratio70- 13M- 570- 13M- 1070- 20M- 570- 20M- 1001020301 2 3 4 5Bond strength (MPa)Concrete Cover to bar diameter ratio100- 13M- 5100- 13M- 10100- 20M- 5100- 20M- 10a)c)b)d) e)65  5.5 The Effect of RCA Replacement Level on Bond Strength Since the concrete compressive strength of the batches varied, modified (normalized) bond strengths are also shown in the figures for comparison purposes. Figure 5-7 to Figure 5-10 show that all the specimens made of different levels of recycled aggregates had acceptable bond results compared to control ones. Maximum reduction in the bond strength due to RCA usage was about 30% for GFRP specimens with 70% and 100% RCA concretes.  In some cases, 30% and 50% RCA concrete mixes displayed even about 15% higher bond than normal concrete. Thus, replacing half of the natural coarse aggregates by recycled ones has minor influence on the bond performance of FRP reinforced concrete elements. Moreover, by adjusting the detail of concrete mixes, better compressive and bond strength can be achieved even for concretes made of 100% RCA. The range of average bond stresses obtained in this study is very similar to pull-out test results reported in previous studies (Achillides & Pilakoutas, 2004; Benmokrane et al., 2002). In this investigation different concrete batches were mixed and produced for each concrete mix proportion. Therefore, the bond obtained for each mix and for each bar size does not show same trend. The fluctuation in the graphs is due to variation in concrete compressive strength of batches. When the concrete compressive strength is higher for a specimen, the obtained bond strength is greater than similar specimens with different concrete mixes and compressive strengths. For example, in Figure 5-7a, by increasing RCA% from 30 to 70, the bond strength first increases then decreases for 20M bar but 13M bar had a slightly different trend. A definite trend for the effect of RCA replacement cannot be seen for neither of steel nor GFRP specimens. It is thought that the main reason behind this is the mixing problem during the experimental work. Due to unavailability of a large enough mixer, each concrete mix was produced using five small mixers. Therefore, there was some inconsistency in concrete batches for each concrete mix that affect the properties, compressive strength, and bond performance of specimens casted with same concrete mix.  66   Figure 5-7. Relationship between bond strength and recycled aggregate replacement level in 200 mm concrete cylinder with le⁄db =5. a) bond strength, b) normalaized bond strength    Figure 5-8. Relationship between bond strength and recycled aggregate replacement level in 200 mm concrete cylinder with le⁄db =10. a) bond strength, b) normalaized bond strength    05101520250% 30% 50% 70% 100%Bond strength (MPa)RCA percentage13M20M0240% 30% 50% 70% 100%RCA percentage13M20M𝜏    0510150% 30% 50% 70% 100%Bond strength (MPa)RCA percentage13M20M0120% 30% 50% 70% 100%RCA percentage13M20M𝜏    (a) (b) (b) (a) 67   Figure 5-9. Relationship between bond strength and recycled aggregate replacement level in 300 mm concrete cylinder with le⁄db =5. a) bond strength, b) normalaized bond strength    Figure 5-10. Relationship between bond strength and recycled aggregate replacement level in 300 mm concrete cylinder with le⁄db =10. a) bond strength, b) normalaized bond strength   5.6 Sensitivity Analysis Similar ANOVA sensitivity analysis also has been done for specimens reinforced with GFRP bars. The effects of concrete cover to bar daimeter ratio ( 𝐶 𝑑𝑏⁄ ), embedment length to bar diameter ratio ( 𝑙𝑒 𝑑𝑏⁄ ), as well as RCA replacement percentages were investigated on the bond performance of GFRP reinforced samples. The results presented in Table 5-2 shows that similar to steel specimens, two of factors including concrete cover and embedment length to bar diameter ratio have a more significant effect on the bond strength of specimens with GFRP 01020300% 30% 50% 70% 100%Bond strength (MPa)RCA percentage13M20M0240% 30% 50% 70% 100%RCA percentage13M20M𝜏    02468101214160% 30% 50% 70% 100%Bond strength (MPa)RCA percentage15M20M0120% 30% 50% 70% 100%RCA percentage15M20M𝜏    (b) (b) (a) (a) 68  bars. The P-value for the parameters 𝐶 𝑑𝑏⁄ ,  𝑙𝑒 𝑑𝑏⁄ ,  were found to be less than 0.0001, whereas RCA percentage showed a P-value of 0.044. Therefore, according to significance level of α = 0.05, all the factors influence the bond behavior of GFRP reinforced concrete members. However, the effect of RCA replacement percentage is very minimal comred to other factors. It can be concluded that in case of FRP reinforcing, the effect of RCA replacement level would be more compared to steel reinforced concrete members. A percent contribution of about 30%, and 58%, were found for 𝐶 𝑑𝑏⁄ and  𝑙𝑒 𝑑𝑏⁄    respectively.  This percentage for RCA replacement level was nearly three percent. According to the results presented in Figure 5-11, the main factors including bar diameter, embedment length, and concrete cover have significant effect on bond strength. However, it can be concluded that RCA replacement percentages had partial influence on bond performance compared to other factors. So the result discussed in the previous section can be confirmed by ANOVA analysis’s outcomes. Table 5-2. Analysis of variance table for GFRP specimens Source Sum of Squares Mean Square p-value  Percent contribution Model 1171.52 146.44 < 0.0001   𝐶 𝑑𝑏⁄  383.45 127.82 < 0.0001 30%  𝑙𝑒 𝑑𝑏⁄  747.29 747.29 < 0.0001 58% RCA% 40.78 10.2 0.0439 3% Residual 113.61 3.66   Cor Total 1285.14         69   Figure 5-11. Effect of different factors on bond strength; a) concrete cover to bar diameter ratio, b) embedment length to bar diameter ratio, c) RCA replacement percentage 5.7 Comparison of Measured and Predicted Bond Strength All manufactured bond models suggested for FRP reinforced concrete were discussed in chapter 2. The accuracy of models based on the push-out experimental results is displayed in Figure 5-12. In the graphs presented in Figure 5-12, the closest the results to the solid line the more accurate is the model prediction. As it can be seen from Figure 5-12, the predicted values of bond suggested by Lee et al. (2008), Okelo and Yuan (2005), ACI 440-1R-06, CSA S6, and CSA S806 formulas are lower compared to attained experimental results in most of (a) (b) (c) 70  samples. This is because the mentioned formulas are intended for tensile bond strength whereas the experimental results are for compressive bond. Moreover, safety factors have been considered in codes’ models which lead to a reduction in predicted bond values. The comparison of bond strength values measured and predicted by different codes and literatures are shown in Figure 5-13 to Figure 5-16. Although the size and properties of samples affect the results, it can be concluded that generally, previous codes and literature models gave conservative results compared to experimental results. In most of the cases, ACI 440-1R and Lee et al. models had the closest results to the measured values. Okelo and Yuan model have produced poor results for all specimens. 71   Figure 5-12. Calculated bond stress versus measured bond stress according to a) ACI 440, b) CSA S806, c) CSA S6, d) Lee et al.  and e) Okelo et al.       (a) (b) (c) (d) (e) 72  For all the samples CSA S6 and CSA S806 represented similar bond values since almost same factors are considered in both equations.  Since Lee et al. model is just based on concrete compressive strength, their predicted bond values were almost the same regarding the embedment length, cylinder or bar size.  For small and large specimens with embedment to bar diameter ratio of 5, ACI 440-1R and Lee et al. models showed first and second best prediction values compared to experimental results. Since ACI 440-1R model considered all infuencing factors, its predicted bond values became lower than Lee et al. equation for samlland large concrete cylinders with le⁄db=10. Even in some cases the experimental values is lower than Lee et al. model predictions. It means that embedment length to bar diameter ratio is an important factor that has to be considered in bond behavior of FRP reinforced concrete elements. In general these results were almost constant among the different concrete mixes. The predicted bond strength for all the RCA replacement percentages was similar and it is not considered in the existing models for FRP reinforced concretes.    Figure 5-13. Comparison of measured and predicted bond stress values for small specimens with le⁄db=5, embedded with a) 20M, and b) 13M bars  0510150 30 50 70 100Bond strength (MPa)RCA percentage20M-testACI 440CSA S806CSA S6Lee et al.Okelo et al.051015200 30 50 70 100Bond strength (MPa)RCA percentage13M-testACI 440CSA S806CSA S6Lee et al.Okelo et al.(a) (b) 73   Figure 5-14. Comparison of measured and predicted bond stress values for small specimens with le⁄db=10, embedded with a) 20M, and b) 13M bars    Figure 5-15. Comparison of measured and predicted bond stress values for large specimens with le⁄db=5, embedded with a) 20M, and b) 13M bars  0510150 30 50 70 100Bond strength (MPa)RCA percentage20M-testACI 440CSA S806CSA S6Lee et al.Okelo et al.0510150 30 50 70 100Bond strength (MPa)RCA percentage13M-testACI 440CSA S806CSA S6Lee et al.Okelo et al.05101520250 30 50 70 100Bond strength (MPa)RCA percentage20M-testACI 440CSA S806CSA S6Lee et al.Okelo et al.0510152025300 30 50 70 100Bond strength (MPa)RCA percentage13M-testACI 440CSA S806CSA S6Lee et al.Okelo et al.(a) (b) (a) (b) 74   Figure 5-16. Comparison of measured and predicted bond stress values for large specimens with le⁄db=10, embedded with a) 20M, and b) 13M bars 5.8 Comparison Study between Steel and FRP Bars Bond Behavior On Average, the bond strength of specimens reinforced by GFRP bars were about 40% less than same specimens with steel reinforcement in all concrete mixes. In most of the cases, the slippage corresponded to peak bond stress in GFRP specimens were higher than that in steel ones. Therefore, larger concrete cover, and higher concrete compressive strength is required for FRP reinforced concrete elements compared to conventional RC members in order to reach a target bond strength.  For the GFRP rebars, narrower cracking took place along the rebar and on the surface of the concrete. The narrow cracking in the concrete may be an indication of low bearing stresses produced in the concrete by the rebar surface. Adhesion and friction may then be the important bond stress components in the sand coated GFRP rebars tested here. Deformation in steel rebars’ surface was the reason behind the mechanical interlock which had the major contribution to their bond strength. Hence, surface deformations can be very effective means of bond enhancement in FRP reinforced concrete members.  Based on the discussion made in previous chapters, the effect of concrete and rebar properties on the bond performance of specimens embedded with both GFRP and steel are 0510150 30 50 70 100Bond strength (MPa)RCA percentage20M-testACI 440CSA S806CSA S6Lee et al.Okelo et al.0510150 30 50 70 100Bond strength (MPa)RCA percentage13M-testACI 440CSA S806CSA S6Lee et al.Okelo et al.(a) (b) 75  very similar. Furthermore, the rate of change in bond stress due to changing the RCA replacement level in both samples was similar.   As it can be seen from Figure 5-17 to Figure 5-21, the slippage at failure point of most of the small concrete cylinders embedded with GFRP bars are higher than steel ones.  This may be due to the GFRP surface condition. Lack of deformation in the GFRP rebars makes it easy for rebar to slide through concrete without causing cracks and have greater slip value in compared to deformed steel rebars. However this trend cannot be seen in all the large cylinders. Among concrete mixes with 0%, 30%, 50% RCA replacement level, some of the large steel reinforced cylinders displayed higher slippage. It can be concluded that with a thick enough concrete cover, an acceptable ductility can be achieved regardless of reinforcement type.         76   (a)  (b) Figure 5-17. Bond stress versus slip response of control specimens in a) 200 mm and b) 300 mm concrete cylinder     05101520250 0.1 0.2 0.3 0.4 0.5Bond strength (MPa)Slippage (mm)FRP-0%-100x200-20M-5STL-0%-100x200-20M-5FRP-0%-100x200-20M-10STL-0%-100x200-20M-100102030400 0.5 1 1.5 2 2.5 3 3.5Bond strength (MPa)Slippage (mm)FRP-0%-150x300-20m-5STL-0%-150x300-20M-5FRP-0%-150x300-20m-10STL-0%-150x300-20M-1077   (s)  (b) Figure 5-18. Bond stress versus slip response of 30% RCA specimens in a) 200 mm and b) 300 mm concrete cylinder 0510150 0.5 1 1.5 2 2.5Bond strength (MPa)Slippage (mm)FRP-30%-100x200-20M-5STL-30%-20M-100x200-5FRP-30%-100x200-20M-10STL-30%-20M-100x200-10051015202530350 0.5 1 1.5 2 2.5 3 3.5Bond strength (MPa)Slippage (mm)FRP-30%-150x300-20M-5STL-30%-150x300-20M-5FRP-30%-150x300-20M-10STL-30%-150x300-20M-1078   (a)  (b) Figure 5-19. Bond stress versus slip response of 50% RCA specimens in a) 200 mm and b) 300 mm concrete cylinder     05101520250 0.5 1 1.5 2 2.5Bond strength (MPa)Slippage (mm)FRP-50%-100x200-20M-5STL-50%-100x200-20M-5FRP-50%-100x200-20M-10STL-50%-100x200-20M-100102030400 1 2 3 4 5 6Bond strength (MPa)Slippage (mm)FRP-50%-150x300-20M-5STL-50%-150x300-20M-5FRP-50%-150x300-20M-10STL-50%-150x300-20M-1079   (a)  (b) Figure 5-20. Bond stress versus slip response of 70% RCA specimens in a) 200 mm and b) 300 mm concrete cylinder     02468101214160 0.5 1 1.5 2Bond strength (MPa)Slippage (mm)FRP-70%-100x200-20M-5STL-70%-100x200-20M-5FRP-70%-100x200-20M-10STL-70%-100x200-20M-1005101520253035400 1 2 3 4Bond strength (MPa)Slippage (mm)FRP-70%-150x300-20M-5STL-70%-150x300-20M-5FRP-70%-150x300-20M-10STL-70%-150x300-20M-1080   (a)  (b) Figure 5-21. Bond stress versus slip response of 100% RCA specimens in a) 200 mm and b) 300 mm concrete cylinder  0246810121416180 0.5 1 1.5 2Bond strength (MPa)Slippage (mm)FRP-100%-100x200-20M-5STL-100%-100x200-20M-5FRP-100%-100x200-20M-10STL-100%-100x200-20M-10051015202530350 1 2 3 4 5Bond strength (MPa)Slippage (mm)FRP-100%-150X300-20M-5STL-100%-150x300-20M-5FRP-100%-150X300-20M-10STL-100%-150x300-20M-1081   Conclusion and Recommendations 6.1 Conclusion  With the aim of contributing to the experimental database, this study is focused on evaluation of bond behavior between steel/FRP bars and RCA concrete. In this regard, a comprehensive investigation was provided about the role of influencing variables such as rebar diameter size, embedment length and concrete cover thickness. Compressive bond behavior of deformed steel and GFRP bars embedded in the concrete made of RCA has been experimentally studied through push-out tests. The effect of reinforcing rebar and surrounding concrete mix as well as the compressive strength on the bond behavior were reflected in the experimentally derived bond-slip relations. Major conclusions from this study can be summarized as:  The general bond properties showed similar trends for both natural and recycled concretes reinforced with steel or GFRP bars. Considering same concrete mix proportions, the bond capacity has an inverse relation with the bar diameter and the embedment length to bar diameter ratio. The slippage and bond strength increase with increasing the concrete cover due to better lateral confinement. The same behavior has been observed for normal concrete.   The dominant failure mode for both steel and GFRP specimens was slitting failure. However, in some large RCA specimens embedded with GFRP bars, the rebar were pushed out of concrete with no cracks. So, it is thought that using the recycled concrete aggregates and FRP reinforcement may increase the ductility of concrete elements.   Rebar diameter increment led to an average of 30% and 20% reduction in bond strength for steel and GFRP specimens respectfully. The increase in embedment length caused an average of 45% reduction in bond strength for specimens embedded with both GFRP and steel bars. Enlargement of concrete cover resulted in about 50% higher bond strength for steel reinforced specimens. While this growth was about 30% and 16% for control and RCA concrete mixes with GFRP bars respectfully.   In addition to the acceptable compressive strength of concrete produced with RCA, their bond resistances are close to those of the normal concrete. Generally, 50% RCA 82  replacement showed the best performance in terms of both compressive and bond strengths. Also 30% RCA mix showed very good bond performances compared to control ones, due to lower RCA level and better interface bond between coarse aggregates. Hence, by using appropriate replacement levels of RCA, structural concrete with acceptable performance can be produced.  The outcomes obtained from ANOVA sensitivity analysis confirm that the effect of RCA replacement level on bond strength is very trivial compared to other factors such as concrete cover, embedment length, and bar size.  The compressive bond strength obtained from push-out tests were generally higher than the results predicted by guidelines’ formulas. As the amount of recycled aggregate increases in concrete mixes, the bond strength weakens and the difference between experimental and predicted values reduces. It means that guidelines’ formulas do not take into account the effect of recycled aggregates. Since the existence of RCA in concrete mix will weaken the interface bond between natural and recycled coarse aggregate.  Among the existing codes for steel reinforced concrete bond, ACI 318 presented best results comparing to experimental results. CEB-FIP model also showed good predicted values in case of specimens with larger concrete cover and better bond condition. Overall, ACI 440-1R had the closest bond results to push-out test outputs among the models provided for FRP reinforced concrete members.   Although the actual conditions of rebars in flexural reinforced concrete elements are not represented by the push-out test, conducting such tests using GFRP and steel rebars provide comparative information about the relative bond property of the FRP rebars with respect to the steel rebars embedded in normal and RCA concretes. According to the test results reported herein, for specimens with same concrete strength, diameter, and embedment length, the bond strength of GFRP rebars was lower than that of steel rebars. This result is thought to be because of existence of ribs and deformation in steel bar’s surface which will increase the mechanical interlock. Besides, the slip at failure was greater for the FRP rebars compared to the steel ones.  83  6.2 Limitations of This Study This study has the following limitations mainly due the unavailability of proper equipment and time constraint.  During the production of each concrete mix a big enough mixer was not available. Therefore, each concrete mix has been mixed in 5 batches. This leads to some variability and inconsistency among the specimens casted with same concrete mix.  Performing push-out test is not enough in order to achieve a definite answer about whether using RCA concrete in combination with steel or FRP bars is acceptable or not. Some full scale beams should be cast and tested to ensure the reliability of these materials for construction purposes.  The obtained results for GFRP bars in this investigation cannot be generalized to all types of FRP rebars. Since in this study sand coated GFRP bars supplied from one manufacturer have been used, more tests should be performed on other types of FRP bars like CFRP or AFRP with different kinds of surface deformations and treatments. Also it’s better to investigate the FRP bars provided by different manufacturers. As each company has a different kind of sand coating or bar deformations for their FRP bars.    There were also some limitations regarding the number of specimens. In this study just two different sizes of reinforcements and two concrete cover to bar diameter ratio have been examined. 6.3 Future Work According to this study, RCAs have the potentials to be used in structural concrete members. In order to improve the reliability of these materials in construction industry, future studies about the bond behavior of beams and columns made of RCA is recommended. Based on the findings in this study, the interest will grow among researchers toward recycled coarse aggregate concrete as a new generation concrete. Moreover, replacing steel bars with FRP reinforcement in combination with RCA concrete will lead to a sustainable, durable, and cost 84  effective solution for construction industry. Following are some suggestions for future investigation:   Different sources of recycled aggregates should be considered to evaluate the effect of properties, and replacement level of recycled aggregates on the bond performance of RCA concrete.  Since FRP reinforced concrete elements are exposed to outdoor and aggressive environment, the effect of long time exposure should be considered to examine the durability of performance of both steel and FRP reinforced RCA concretes.  Several types of bar can be characterized by various surface configurations, different qualities and quantity of fibers (The fiber volume fraction), and by the use of different resins as binders. These factors control the mechanics of stress transfer by bond between FRP rebars and concrete. Hence, studies are required to determine the effect of the type of fiber, bar geometry, surface roughness, and surface treatment on the bond between reinforcement and RCA concrete.  In this study a total of 240 push-out specimens were tested. In order to conduct a comprehensive study based on the experimental tests, and also develop a standard guideline more experimental works are required on bond behavior of steel or FRP reinforced columns and beam specimens made of RCA concrete. Furthermore, bond performance of high strength RCA concrete reinforced with steel or FRP should be investigated.    85  Bibliography Abou-Zeid, M. N., Shenouda, M. N., McCabe, S. L., & El-Tawil, F. A. (2005). Reincarnation of Concrete. Concrete International, 27(02), 53–59. Retrieved from http://www.concrete.org/publications/internationalconcreteabstractsportal/m/details/i/14241.aspx Achillides, Z., & Pilakoutas, K. (2004). Bond behavior of fiber reinforced polymer bars under direct pullout conditions. 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