DURABILITY OF CONCRETE BEAMS  REINFORCED WITH FIBER REINFORCED POLYMERS

ABSTRACT

 

Externally bonded Fiber Reinforced Polymer Composites (FRP) are widely used for repair and strengthening of concrete members. Despite the widespread use of this technique there is still insufficient knowledge about the long-term durability of FRP systems externally bonded to concrete. FRP bonded concrete members can be exposed to various environmental effects during the lifetime of the structures, as this system is applied for both indoor and outdoor members in buildings and bridges. To ensure the safety of these structures during their service life, durability of the FRP material under such weathering conditions and service loading should be understood.

In this study, long term durability of Glass and Carbon FRP reinforced, pre-cracked concrete beams without steel reinforcement under sustained loading is investigated.

The flexural test results of seventeen beams, which were casted and reinforced with FRP in 2005 are used. Eight of these beams were tested within eight months after the manufacturing date without any weathering or sustained loading exposure by Whitaker (2007). Nine of them were exposed to indoor and outdoor conditions in State College/PA under sustained loading for 72 months, and tested in 2011. During the tests, in addition to deflection and crack opening measurements, 2D Digital Image Correlation (DIC) was used to obtain full field strain and slip distributions. The long term performance of the beams is evaluated by comparing the flexural test results of these unconditioned and 72 months conditioned beams.

Externally bonded FRP sheets and plates introduce failure modes such as debonding and FRP rupture. Therefore, in addition to the ultimate strength, deflections and strain distributions, the failure modes, and interfacial bond strength between the FRP and the concrete are also compared.

In this study, it is observed that;direct exposure to outdoor conditioning causes reduction in the ultimate capacity of FRP reinforced concrete beams.Moreover,the first debonding of the conditioned beams occurred with the tensile rupture of the concrete; also, the moment capacity at the initiation of debonding showed an increasing trend with the increase of the tensile strength of the concrete beams. The furthest debonding propagation before the failure is observed for the outdoor conditioned CFRP and GFRP beams, whose FRP reinforced side is directly exposed to outdoor conditioning.

In terms of local behavior, fracture toughness is obtained by a fracture mechanics analysis in which a local non-linear shear stress-slip model is assumed and its parameters are obtained by curve-fitting to experimental slip versus position data. According to the analysis results, the lowest fracture energies are observed for the indoor CFRP beam which had the largest sustained moment with 47% of the ultimate, during conditioning. For this beam; increasing local fracture energy further away from the pre-crack and especially from the active region of the FRP during sustained loading, resulted in no reduction of the ultimate moment capacity. Among the beams that have the same level of sustained moment, the beams, FRP reinforced sides of which were directly exposed to outdoor conditioning have the lowest fracture energies. In eight out of nine beams local fracture energy increases further away from the pre-crack, the location, where the affect of conditioning and sustained loading is assumed to be the most severe.

Also, the 2D DIC Method, which is more practical compared to the 3D DIC, is found to be a successful method to obtain full-field strain and slip distributions of the FRP sheets that are externally bonded to concrete members, under flexural loading, unless there is a movement of the FRP strip relative to the concrete.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TABLE OF CONTENTS

LIST OF FIGURES …………………………………………………………………………………………….. viii

LIST OF TABLES ………………………………………………………………………………………………. xxi

LIST OF SYMBOLS ………………………………………………………………………………………….. xxiii

ACKNOWLEDGEMENTS ………………………………………………………………………………….. xxv

CHAPTER 1- INTRODUCTION ……………………………………………………………………………… 1

1.1 Background …………………………………………………………………………………………….. 1

1.2 Thesis Objectives …………………………………………………………………………………….. 3

1.3 Thesis Flowchart ……………………………………………………………………………………… 4

CHAPTER 2 – LITERATURE REVIEW………………………………………………………………………. 6

2.1 Bond Behavior …………………………………………………………………………………………. 6

2.1.1 Debonding, Fracture Energy and Fracture Modes 1 and 2 ………………………. 6

2.1.2 Effective Length …………………………………………………………………………………. 9

2.1.3 Mode 2 Fracture and Simplified Models ……………………………………………….. 9

2.2 Effects of Environment on FRP/Concrete Bond Behavior ……………………………. 19

2.2.1 Moisture-Wet-Dry Cycles ………………………………………………………………….. 20

2.2.2 Freeze-Thaw Cycles and Temperature Changes …………………………………… 21

2.2.3 Alkaline and Acidic Environment ……………………………………………………….. 23

2.2.4 UV Radiation……………………………………………………………………………………. 23

2.3 Digital Image Correlation (DIC) ………………………………………………………………… 23

2.3.1 DIC Background ……………………………………………………………………………….. 23

2.3.2 Use of DIC with Externally Bonded FRP ………………………………………………. 27

CHAPTER 3 – REVIEW of PREVIOUS STUDIES WITH SIMILAR BEAMS ……………………… 30

3.1 Brown (2003), Jia et al. (2005) …………………………………………………………………. 30

3.2 Whitaker (2007) …………………………………………………………………………………….. 34

CHAPTER 4 – EXPERIMENTAL PROGRAM …………………………………………………………… 37

4.1 Specimen Description …………………………………………………………………………….. 37

4.2 Beam Matrix………………………………………………………………………………………….. 39

4.3 Test Setup …………………………………………………………………………………………….. 41

CHAPTER 5 – DIC ANALYSIS ………………………………………………………………………………. 46

5.1 Strain Distributions ………………………………………………………………………………… 46

5.1.1 Procedure and Accuracy of Strain Measurements with the DIC Method … 46

5.1.2 Resulting Strain Distributions and 2D Contour Plots by the DIC Method…. 59

5.2 Slip Measurements ………………………………………………………………………………… 62

5.3 Vertical Deflections ………………………………………………………………………………… 66

CHAPTER 6 – EVALUATION OF TEST RESULTS and COMPARISON OF BEAMS WITH

DIFFERENT TYPE of CONDITIONING ………………………………………………………………….. 71

6.1 EFFECT OF CONDITIONING ON GLOBAL BEHAVIOR ……………………………………. 72

6.1.1 Failure Modes ………………………………………………………………………………….. 72

6.1.2 Moments at First Debonding (M_db) …………………………………………………….. 78

6.1.3 Ultimate Moments (M_u) ……………………………………………………………………. 81

6.1.4 Vertical Deflections ………………………………………………………………………….. 85

6.2 Effect of Conditioning on Local Behavior…………………………………………………… 91

6.2.1 Strain and Slip Values at the Initiation of Debonding ……………………………. 92

6.2.2 Shear-Stress Slip Curves with a Non-linear τ-s Model Assumption ……….. 101

CHAPTER 7 CONCLUSIONS and FUTURE RECOMMENDATIONS …………………………… 121

7.1 Durability of FRP Reinforced Concrete Beams …………………………………………. 121

7.1.1 Effect of Conditioning under Sustained Loading In terms of Global Behavior

…………………………………………………………………………………………………………….. 121

7.1.2 Effect of Conditioning under Sustained Loading in Terms of Local Behavior

…………………………………………………………………………………………………………….. 122

7.2 Use of DIC with externally bonded FRP …………………………………………………… 123

REFERENCES…………………………………………………………………………………………………. 124

APPENDIX A – PICTURE MOMENT LEVELS on MOMENT- DEFLECTION DIAGRAMS. . 128

APPENDIX B – STRAIN AND SLIP CONTOURS …………………………………………………….. 134

APPENDIX C-  ANALYTICAL AND EXPERIMENTAL SLIP AND STRAIN COMPARISON … 225

CHAPTER 1- INTRODUCTION

1.1 Background

Externally bonded FRP (Fiber Reinforced Polymer) wet-laid sheets and pultruded plates are being widely used for strengthening and repairing of existing reinforced concrete structures since the mid 1980’s (ACI 440 2R-02). The advantages of FRP materials are: they are lightweight, noncorrosive, and easily applied on existing structures and have higher tensile strength compared to steel. The fibers used in externally bonded FRP systems are typically carbon (CFRP) and glass (GFRP), while the matrix binding the fibers together and attaching the fibers to the concrete is typically epoxy. Concrete structures strengthened or repaired with FRP can be exposed to various environmental effects during the lifetime of the repaired structure. To ensure the safety of these structures during their service life, the durability of the FRP repair system under such conditions and loading should be understood.

Various researchers have studied the effects of natural weathering as well as exposure to controlled laboratory conditions, such as humidity, wet and dry cycles, high temperature, UV radiation, freeze and thaw cycles, and alkali solutions, on concrete members strengthened with externally bonded FRP (Kharbari and Zao 1998, Chajes et al. 1995, Toutanji and Gomez 1998, Lopez et al. 1999, Leung et al. 2001, Murthy et al. 2002, Jia et al. 2005, Tan et al. 2009). Some of these studies included sustained loading (Lopez et al. 1999, Murthy et al. 2002, Jia et al. 2005, Tan et al. 2009). In the above studies, degradations of the bond strength of the FRP-concrete interface is observed.

A common failure mode of concrete elements with externally bonded FRP sheets or plates is debonding at measured strains less than the ultimate tensile strain of the FRP. The debonding phenomenon of the concrete/FRP interface have been studied by researchers using empirical models, finite elements analysis and fracture mechanics (Chen and Teng 2004). In the fracture mechanics approach, which can be used to find the energy required to cause debonding, a local shear stress versus slip relationship (τ-s) at the FRP/concrete interface is hypothesized and used predict the bond behavior of the FRP/concrete interface. In this approach, The area under this shear stress- slip curve is defined as the fracture energy (G_f), the energy required to cause debonding, and directly related with the tensile load that can be carried by the FRP before debonding (Talsten et al. 1997, Yuan et al. 2001, Dai et al. 2005). To determine shear stresses at the FRP/concrete interface, it is helpful to know the strain distribution and/or slip distribution along the length of the FRP sheet (Yuan and Yoshizawa 2001, Kurtz and Helm 2006, and Dai et al. 2005). In the current investigation, long-term durability of pre-cracked concrete beams externally reinforced with glass and carbon FRP under sustained loading is investigated by comparing the flexural test results of unconditioned beams, tested in 2005, with those of identical beams that were exposed to indoor and outdoor conditionings in

State College PA, USA, under sustained loading for 72 months and tested in flexure in 2011. The effects of conditioning are investigated in terms of global and local behaviors. Global behavior includes failure modes, moment capacity before the onset of debonding, ultimate strength, and ultimate deflections. Local behavior includes strain and slip distributions in the FRP reinforcement and the interfacial bond strengths parameters.

Two dimensional Digital Image Correlation (DIC) method is used to measure displacement and strain distributions in the externally bonded FRP reinforcement. The DIC results are used to derive local bond stress-slip parameters that can help improve insight on the durability of externally bonded FRP reinforcements following exposure to typical loads and environments over a six years period. The 2D DIC method is used for the first time in the literature to capture full-field strain and slip distributions of FRP sheets externally bonded to concrete beams under flexural

loading.

1.2 Thesis Objectives

The objectives of this study and the related tasks accomplished are as follows:

1. To understand the effect of indoor and outdoor conditioning under sustained loading to the global behavior of pre-cracked concrete beams that are externally reinforced with FRP sheets.

Related tasks are:

1. Comparison of failure modes, ultimate moment and deflection capacities of GFRP and CFRP reinforced concrete beams with different types of conditioning environment and concrete compressive strength.
2. Comparison of the failure modes, ultimate moment and deflection capacities of the conditioned beams with the unconditioned ones.

2. To understand and quantify the degradation in the bond strength of the conditioned beams.

Related tasks are:

1. Capturing changes in strain and slip values at the initiation of debonding and fracture energies as debonding propagates and shifts away from the pre-crack, where the effect of conditioning under sustained loading is assumed to be most severe.
2. Comparing strain and slip values at the initiation of debonding and fracture energies with the unconditioned beams.

3. To evaluate the 2D DIC method to capture the full-field strain and slip distributions of the FRP sheets that are externally bonded to concrete beams, under flexural loading.

Related tasks are:

1. Checking correlation of the strain values obtained by the 2D DIC method with the strain gages values.
2. Checking correlation of slip values obtained by the 2D DIC method with the slip values obtained by the CMOD gage, located at the pre-crack, assuming the central crack opening is equal to the total slip of both sides.

1.3 Thesis Flowchart

Flowchart of the thesis is provided on the next page.

 

 

Durability of Concrete Beams  Reinforced with FRP Sheets

Experiments (2011)                                          Review of the Results of the Unconditioned Beams (2005)

3 Point Bending Tests		3 Point Bending Tests
2 Indoor, 2 Outdoor Conditioned 40 MPa GFRP		3 Unconditioned 40 MPa GFRP
2 Outdoor, 1 Indoor Conditioned 40 MPa CFRP		3 Unconditioned 40 MPa CFRP
2 Indoor Conditioned 20 MPa GFRP		2 Unconditioned 20 MPa GFRP
Moment-Deflection Data		Moment- Deflection Data
Digital pictures of 30 cm central region of the beams		Strain distribution of 30 cm central region
from bottom and side at every 0.2 kN-m		with Photoelasticity
Full field strain and slip distributions with 2D DIC Bond Strength Parameters using Zhou et al. 2010		Bond Strength Parameters using Yuan 2001
Shear stress-Slip Law		Shear stress- Slip Law

 

                    Comparisons

Global Behavior		Local Behavior
Failure Modes		Strain and Slip Values at the Initiation of Debonding
Ultimate Moment Capacity   Moment-Deflection Diagrams   Moments at First Debonding		Bond Strength Model Paramaters, Fracture Energy

 

                 Conclusions

Direct exposure to outdoor conditioning reduces the bond strength and the ultimate capacity of FRP/concrete systems

 

2D DIC is a working method to obtain full-field strain and slip distributions on the FRP sheets/plates that are externally bonded to concrete

DURABILITY OF CONCRETE BEAMS  REINFORCED WITH FIBER REINFORCED POLYMERS

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