comparative study for strengthening techniques of …library.iugaza.edu.ps/thesis/117617.pdf ·...

Comparative Study for Strengthening Techniques of

RC Beams Using Concrete Jackets and Steel Plates

حزمة اخلرسانية باستخذام القمصان ألساليب تقوية األ دراسة مقارنه

واأللواح املعذنيةاخلرسانية

By

Qasem Khalaf

Supervised by

Prof. Mohamed Ziara

In partial fulfillment of the requirement for the degree of

Master of Science in Civil Engineering. Rehabilitation and Design of Structures

February 2015

I

ABSTRACT

In comparison with the option of “demolish and rebuild buildings”, sustainability can be

better achieved by extending the life spans of existing structures. Rehabilitation of

structures results in less construction waste materials, reserves natural resources,

minimizes adverse environmental effects, saves time, saves cost, etc. The structural

behavior of reinforced concrete beams strengthened with concrete jackets or steel

plating has been investigated in the undertaken research both theoretical and

experimental. The main aim of the research is to demonstrate the best among the

investigated strengthening techniques. The experimental work included testing of 26

reinforced concrete beams divided into two series. The first series contains 13 beams

proposed to have inadequate flexural capacity and the purpose is to increase it. While,

the second series contains 13 beams with low shear capacity because of inadequate

amount of stirrups and the purpose is to prevent the shear failure. Strengthening

operation has been accomplished using concrete jackets or steel plates with different

configurations. The test variables included the type of strengthening (shear or flexural),

the technique of strengthening (concrete jacketing, steel plating) and the type of

connections to prevent laminar shear between the old concrete and the strengthening

elements (mechanical or chemical). As predicted, the strengthened beams exhibited

different structural behavior upon loading up to failure. The beams strengthened with

mechanically bonded concrete jackets showed best structural behavior. The beams

strengthened with chemically bonded concrete jackets showed good results, especially

in the case of three-face jackets. The beams strengthened with chemically bonded steel

plates exhibited sudden brittle failure upon debonding of the plates from the beams at

ultimate load levels. The beams strengthened with mechanically connected steel plates

have not achieved the expected level of enhancement because of pre-mature debonding

of the steel plate or buckling. Better results may be achieved by using a combination of

mechanical and chemical bonding especially at the end of jacket or plate. Steel straps

bonded at the outerface of the beam can prevent the shear failure and recover the

flexural capacity while continuous steel plates bonded to both sides of the beam can

only delay the shear failure with increasing in flexural capacity. In conclusion,

strengthening of existing beams can be effectively achieved using practical techniques,

which will increase the life spans of the beams and thus enhance the sustainability of

existing structures.

II

ملخص الرسالة

انز هشآد انمبئخ انزحم ن صبدح لذسحفئ ػهخ إػبدح انجبءخبس اإلصانخ ػذ يمبسزب يغ

إػبدح رؤم إ .ؼزجش انخبس األفضم ف كثش ي األحال االفزشاض ػشبرذذ رؤد إن

حبفع ػه اناسد انطجؼخ مهم ي األثبس انسهجخ ػه انشآد مهم ي اناد انسزخذيخ

انزصشف ظشخ يخجشخ ػ دساسخ ػم ف زا انجحث لذ رىانجئخ فش انلذ انبل.

ماح ثبسزخذاو انمصب انخشسبخ األناح انؼذخ. اإلشبئ نألحضيخ انخشسبخ انسهحخ ان

ظبس األسهة األفضم ي ث أسبنت انزمخ انزجبح ف ز انذف األسبس نزا انجحث إ

حضاو خشسب يمسخ إن سهسهز. انسهسهخ األن 62انذساسخ. انؼم انخجش رض اخزجبس

حضاو ثحبجخ نضبدح لذسح رحهب نم االحبء ثب انسهسهخ انثبخ رحز ػه 31رحز ػه

سح رحهب نم انمص انذف يغ االبس ػ طشك ل حضاو رؼب مصب ف لذ 31

انمص. ز األحضيخ رى رمزب ثبسزخذاو انمصب انخشسبخ األناح انؼذخ ثئبد يخزهفخ.

أ انمص( رمخ انزمخ )انمصب انخشسبخ أ األناح االحبءيزغشاد انجحث ع انزمخ )

ث انؼصش انجذذ انحضاو األصه )يكبكخ أ كبئخ(. كب يزلغ شثظرمخ انانؼذخ(

فئ رنك حست ع خالل انزحم نغبخ االبسخ يخزهفخ إشبئد ماح أثذد رصشفباألحضيخ ان

انزمخ خ انز رى سثطب يكبكب ثبألحضيخ األصهماح ثاسطخ انمصب انخشسبخ . األحضيخ ان

كزنك فئ .ػه اخزالف ئبرب أثذد أفضم رصشف ماح ثبسزخذاو انمصب انخشسبخ األحضيخ ان

انز رى سثطب كبئب ثبألحضيخ األصهخ أثذد رصشفب جذا خصصب ف حبل انمصب راد

ابسد ثشكم ماح ثبسزخذاو األناح انؼذخ انز رى سثطب كبئب انثالثخ أج. األحضيخ ان

انزحم انمص. األحضيخ يشحهخيفبجئ ش ػذ افصبنب ػ انحضاو األصه ػذ صنب

ماح ثبسزخذاو األناح انؼذخ انز رى سثطب يكبكب نى رحمك انسز انطهة ي انزحسان

. زبئج أفضم سثب رزحمك ػذ انجغ يب ث رنك إيب ثسجت االفصبل انسجك أ اثبء األناح

أسهث انشثظ انكبك انكبئ خصصب ػذ بخ انمص انخشسب أ انهح انؼذ.

يغ انحضاو اسزخذاو أطاق يؼذخ يهصمخ ػه اإلطبس انخبسج نهحضاو انخشسب ثئيكب

ثب األناح انحضاو نم االحبء يغ اسزؼبدح سؼخي االبس ػ طشك انمص انخشسب

ثؼض انؼذخ انزصهخ انهصمخ ػه جبج انحضاو انخشسب فجئيكبب فمظ رؤجم االبس يغ

خ ي انك ثشكم فؼبل أ رزحمك ئاألحضيخ انمب أ رمخانخالصخ رحس ف انمح. ف

ذ انؼش االفزشاض نهحضاو انخشسب ثبنزبن انز ثئيكبب رذ سهخ ثبسزخذاو رمبد ػهخ

رحس لذسح انزحم نهشآد انمبئخ.

III

ACKNOWLEDGMENT

The author would like to express his sincere appreciation to his research adviser Prof.

Mohamed Ziara for his help and guidance in the preparation and development of this

work. The constant encouragement, support and inspiration he offered were

fundamental to the completion of this research.

The author would like to thank the discussion committee members, Dr. Samir Shehada

and Dr. Ali Tayeh for their comments and suggestions. The author’s sincere thanks

also go to the technicians in the material and soil lab of the Islamic University of Gaza

for their supports throughout the research work.

The author’s acknowledgements would not be completed without mentioning his work

team either in the stage of preparing or testing the samples for being very cooperative

and making it an enjoyable work place. The author would like to thank his parents for

their love and support.

IV

Table of Contents

ABSTRACT .................................................................................................................... I

II .................................................................................................................... ملخص الرسالة

LIST OF TABLES ...................................................................................................... IX

LIST OF FIGURES ......................................................................................................X

ABBREVIATIONS .................................................................................................. XIII

CHAPTER 1: INTRODUCTION ................................................................................ 1

1.1 INTRODUCTION ......................................................................................................... 1

1.2 THE NEED FOR REHABILITATION .............................................................................. 1

1.3 REHABILITATION NEEDS IN GAZA STRIP .................................................................. 2

1.4 DAMAGES IN BEAMS IN GAZA STRIP ........................................................................ 2

1.5 STATEMENT OF THE PROBLEMS ................................................................................ 3

1.6 STRENGTHENING TECHNIQUES ................................................................................. 4

1.7 STRENGTHENING OF RC BEAMS ............................................................................. 4

1.7.1 Jacketing of Beams ......................................................................................... 4

1.7.2 Jacketing by Post-Tensioning Concrete ......................................................... 6

1.7.3 Span Shortening .............................................................................................. 6

1.7.4 Increasing Reinforcement ............................................................................... 6

1.7.5 External Post-Tensioning ............................................................................... 8

1.8 COMPARISON BETWEEN RC JACKETING AND STEEL PLATING OF BEAMS ................. 8

1.9 RESEARCH SCOPE, OBJECTIVES AND LIMITATIONS .................................................. 8

1.9.1 The Aim ........................................................................................................... 8

1.9.2 The Objectives ................................................................................................ 9

1.9.3 Scope and Limitations .................................................................................... 9

1.10 UNIQUE FEATURES OF THE RESEARCH ................................................................... 9

1.11 METHODOLOGY ...................................................................................................... 9

1.11.1 Literature review ........................................................................................ 10

1.11.2 Test program and materials ....................................................................... 10

1.11.3 Analysis of samples ..................................................................................... 10

1.11.4 Constructing the Samples and applying strengthening .............................. 10

1.11.5 Testing of samples: ..................................................................................... 10

1.11.6 Analysis and discussion: ............................................................................. 10

1.11.7 Conducting comparison .............................................................................. 10

1.11.8 Conclusions and recommendations ............................................................ 10

1.12 OUTLINE OF THE THESIS ....................................................................................... 11

CHAPTER 2: LITERATURE REVIEW.................................................................. 12

2.1. INTRODUCTION ...................................................................................................... 12

2.2 STRENGTHENING RC BEAMS BY CONCRETE JACKETING ........................................ 12

V

2.2.1 Concrete as Strengthening Material ............................................................. 14

2.2.2 Applications .................................................................................................. 15

2.2.3 Advantages and Disadvantages .................................................................... 15

2.2.4 Installations .................................................................................................. 15

2.2.5 Review of Investigations ............................................................................... 16

2.3 STRENGTHENING RC BEAMS BY STEEL PLATES ..................................................... 19

2.3.1. Steel Plates as Strengthening Material ....................................................... 19

2.3.2. Applications ................................................................................................. 19

2.3.3. Advantages and Disadvantages ................................................................... 20

2.3.3.1 Advantages and Disadvantages of Chemically Bonded Steel plates ..... 20

2.3.3.2 Advantages and Disadvantages of Mechanically Connected steel plates

............................................................................................................................ 21

2.3.4. Installation ................................................................................................... 21

2.3.5 Review of Investigations ............................................................................... 22

2.4. ANCHORING AND BONDING OF INTERFACES .......................................................... 26

2.4.1 Bonding Techniques ..................................................................................... 27

2.4.1.1 Roughening of Concrete Surface ........................................................... 27

2.4.1.2 Mechanical Connectors .......................................................................... 28

2.4.1.3 Chemical Adhesives ............................................................................... 28

2.4.1.4 Selecting a Structural Adhesive ............................................................. 29

2.4.1.5 General Properties of Adhesives ............................................................ 30

2.4.1.6. Epoxy .................................................................................................... 31

2.4.6 Anchor and Bond Strength ........................................................................... 31

CHAPTER 3 DESIGN OF CONCRETE JACKETED AND STEEL PLATED

BEAMS ........................................................................................................................ 33

3.1 INTRODUCTION ....................................................................................................... 33

3.2 BASIC PRINCIPLES IN DESIGN OF STRENGTHENED RC BEAMS ................................ 33

3.2.1 Proper Reinforced Concrete Beams ............................................................. 33

3.2.2 Strengthened RC Beams ............................................................................... 34

3.2.3 Stresses Transfer at the Interfaces ................................................................ 35

3.2.4 Compatibility of Strengthened Beam Materials ........................................... 35

3.2.4.1 Physical Compatibility ........................................................................... 36

3.2.4.2 Mechanical Compatibility (Stiffness and Strain). .................................. 36

3.3 STRUCTURAL DATA OF THE ORIGINAL BEAM. ...................................................... 37

3.4 DESIGN OF BEAMS STRENGTHENED BY CONCRETE JACKETING .............................. 38

3.5 DESIGN OF BEAMS STRENGTHENED BY STEEL PLATES ........................................... 41

3.5.1 Modes of Failure of Beams Strengthened by Steel Plates ............................ 41

3.5.1.1 Adhesively Bonded Plates ..................................................................... 42

3.5.1.2 Bolted Plates .......................................................................................... 43

3.5.2 Stiffness Limits of Steel Plate ........................................................................ 43

3.5.3 Section Strength of Tension Face Plated Beam (Flexural Strengthening) ... 44

3.5.4 Section Strength of beam Strengthen by Straps of Steel Plates at the Sides

(shear strengthening) ............................................................................................. 46

VI

3.5.5 Section Strength of Beam Strengthen by Continuous Steel Plates at the Beam

Web (Shear Strengthening) .................................................................................... 47

3.6 BONDING DESIGN ................................................................................................... 53

3.6.1. Mechanical Anchorage to Concrete ............................................................ 54

3.6.2 Adhesively Bonding to Concrete. .................................................................. 54

3.7 CONCLUDED REMARKS .......................................................................................... 57

CHAPTER 4 TEST PROGRAM ............................................................................... 58

4.1 INTRODUCTION ....................................................................................................... 58

4.2 DEFINITION OF SAMPLES SECTIONS ........................................................................ 58

4.2.1 Original sample: ........................................................................................... 58

4.2.2 First series: Flexural Strengthening ............................................................. 61

4.2.3 Second Series: Shear Strengthening ............................................................ 62

4.3 EXPERIMENTAL WORK ........................................................................................... 63

4.3.1 First Series: Flexure Samples: ..................................................................... 63

4.3.1.1 Original Specimens: ............................................................................... 63

4.3.1.2 Addition of Mechanically Connected RC jacket at the Tension Side

(AF0, AF1and AF2) ........................................................................................... 63

4.3.1.3 Addition of Chemically Bonded RC jacket at the Tension Side (BF1,

BF2). .................................................................................................................. 64

4.3.1.4 Addition of Mechanically Connected Steel Plate to the Tension Side

(AF3, AF4): ........................................................................................................ 64

4.3.1.5 Addition of Chemically Bonded Steel Plate to the Tension Side (BF3,

BF4): .................................................................................................................. 65

4.3.2 Second Series: Shear Examination: ............................................................. 66

4.3.2.1 Original Specimens: 13 Specimens as Detailed in Test Program ....... 66

4.3.2.2 Addition of Mechanically Connected U Shape Concrete Jacket with

Additional Steel Cage (AS1, AS2) .................................................................... 66

4.3.2.3 Addition of U Shape Reinforced Concrete Jacket to Roughened

surface and Partially Painted with Chemical (ES1) ........................................... 67

4.3.2.4 Addition of U Shape Reinforced Concrete Jacket without Additional

Connection (BS3). .............................................................................................. 68

4.3.2.5 Addition of Chemically Bonded shape Plain Concrete Jacket (BS1,

BS2). .................................................................................................................. 68

4.3.2.6 Addition of Mechanically Connected Steel Plate to the beam Sides

(AS3, AS4). ........................................................................................................ 68

4.3.2.7 Addition of Chemically Bonded Steel Plate to the beam Sides (BS3,

BS4). .................................................................................................................. 68

4.3.2.8 Addition of External Straps of Steel Plate Chemically Bonded around

the beam outer-face (BS6) ................................................................................. 69

4.4 MATERIAL PROPERTIES .......................................................................................... 69

4.4.1 Concrete ........................................................................................................ 69

4.4.2 Steel Bars ...................................................................................................... 70

4.4.3 Steel Plates ................................................................................................... 71

VII

4.4.4 Chemical Adhesives ...................................................................................... 71

4.4.4.1 EPICHOR 1768 ..................................................................................... 71

4.4.4.2 Sikadur®-31 CF ..................................................................................... 72

4.4.4.1 Sikadur®-32 ........................................................................................... 72

4.5 ANALYSIS OF SAMPLES .......................................................................................... 73

4.5.1 First Series: Flexure Examination ............................................................... 73

4.5.1.1 Original Section ..................................................................................... 74

4.5.1.2 Concrete Jacketed Section ..................................................................... 74

4.5.1.3 Steel Plated Section ................................................................................ 77

4.5.1.4 Monolithic Section ................................................................................. 79

4.5.2 Second Series: Shear Examination ........................................................... 80

4.5.2.1 Original Section ..................................................................................... 80

4.5.2.2 Concrete Jacketed Section (U jacket). ................................................... 81

4.5.2.3 Concrete Jacketed Section (∩ shape) ..................................................... 82

4.5.2.4 Steel Straps ............................................................................................. 84

4.5.2.5 Steel Plated Beam (Side Plates). ............................................................ 85

4.5.2.2 Monolithic Section (Simulation of Jacketed Section) ........................... 85

4.5.3 Summary of Theoretical Results ................................................................... 86

CHAPTER 5 RESULTS AND DISCUSSION ....................................................... 87

5.1. INTRODUCTION ...................................................................................................... 87

5.2. FIRST SERIES: FLEXURAL SAMPLES ....................................................................... 87

5.2.1 Control Beams: ............................................................................................. 87

5.2.2. Addition of Mechanically Connected RC Jacket to the Tension Side (AF0,

AF1, and AF2). ...................................................................................................... 88

5.2.3. Addition of Chemically Bonded RC Jacket to the Tension Side (BF1, BF2)

............................................................................................................................... 92

5.2.4. Addition of Mechanically Connected Steel Plate to the Tension Side (AF3,

AF4) ....................................................................................................................... 94

5.2.5. Addition of Chemically Bonded Steel Plate to the Tension Side (BF3, BF4)

............................................................................................................................... 95

5.3. SECOND SERIES: SHEAR SAMPLES. ........................................................................ 98

5.3.1. Control Beams: ........................................................................................... 98

5.3.2. Addition of U Shape RC Jacket with Additional Mechanical Connection

(AS1, AS2) .............................................................................................................. 99

5.3.3. Addition of Chemically Bonded Shape Plain Concrete Jacket (BS1, BS2)

............................................................................................................................. 101

5.3.4 Addition of U Shape Reinforced Concrete Jacket to Roughened surface and

Partially Painted by Adhesive(ES1) .................................................................... 103

5.3.5 Addition of U Shape RC Jacket without additional connection (only "friction

+ new stirrups") (BS3)......................................................................................... 104

5.3.6. Addition of Steel Plate Mechanically Connected to the Sides of the Original

Beam(AS3, AS4) ................................................................................................... 105

5.3.7. Addition of Chemically Bonded Steel Plate to the beam Sides (BS4,BS5) 108

VIII

5.3.8. Addition of External Straps from Steel Plate Chemically Bonded to the out-

surface of the Original Section(BS6) ................................................................... 109

5.4. COMPARISON BETWEEN THE STRENGTHENING TECHNIQUES: .............................. 111

5.4.1. Load Capacity ............................................................................................ 111

5.4.2. Stiffness and Deflection at SLS .................................................................. 112

5.4.3. Stiffness and Failure Mode at ULS ............................................................ 117

5.4.4 Skills and Time of Construction. ................................................................ 119

5.4.5 Concrete Jacketing Versus Steel Plating ................................................... 120

5.4.6 Mechanical Versus Chemical Bonding ...................................................... 120

5.5 SUMMARY OF RESULTS: ....................................................................................... 121

CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS ....................... 124

6.1. INTRODUCTION .................................................................................................... 124

6.2 CONCLUSIONS ...................................................................................................... 124

6.3. RECOMMENDATIONS ........................................................................................... 128

6.3.1. Practical Recommendations ...................................................................... 128

6.3.2. Recommendations for Future Research dick ............................................. 128

REFERENCES .......................................................................................................... 130

IX

LIST OF TABLES

TABLE 2. 1: STRUCTURAL AND PRACTICAL REMARKS ABOUT CONCRETE JACKETING (16) .................... 13

TABLE 2. 2: TECHNIQUES OF BONDING IN THE STUDY (15) ............................................................ 17

TABLE 2. 3: TEST RESULTS (31) .............................................................................................. 22



TABLE 2. 6: TEST RESULTS FOR SERIES A (WITHOUT SHEAR REINFORCEMENT) BEAMS (34) .................. 25

TABLE 2. 7: TEST RESULTS FOR SERIES B (WITH SHEAR REINFORCEMENT) BEAMS (34) ........................ 25

TABLE 2. 8: : GENERAL REQUIREMENTS OF PATCH REPAIR MATERIALS FOR COMPATIBILITY (47) ........ 30

TABLE 2. 9: BONDING STRENGTHS OF STRUCTURAL ADHESIVE (1) .................................................. 30

TABLE 3. 1: EXPERIMENTAL AND THEORETICAL RESULTS (30) ......................................................... 52

TABLE 3. 2: MEASURED TO PREDICTED BOND STRENGTH RATIOS(CHEN ET AL. (2001)) ..................... 56

TABLE 4. 1: SAMPLE DESCRIPTION ............................................................................................. 60

TABLE 4. 2: COMPRESSIVE STRENGTH FOR THE FIRST MIX ............................................................. 69

TABLE 4. 3: COMPRESSIVE STRENGTH FOR THE SECOND MIX ......................................................... 70

TABLE 4. 4: FC' FOR THE ORIGINAL AND ADDITIONAL CONCRETE .................................................... 70

TABLE 4. 5: YIELD AND ULTIMATE STRENGTH FOR STEEL BAR (10MM) ............................................ 71

TABLE 4. 6: YIELD AND ULTIMATE STRENGTH FOR STEEL PLATE .................................................... 71

TABLE 4. 7: PROPERTIES OF EPICHOR 1768 ............................................................................. 71

TABLE 4. 8: PROPERTIES OF SIKADUR®-31 CF ........................................................................... 72

TABLE 4. 9: PROPERTIES OF SIKADUR®-32 ............................................................................... 72

TABLE 4. 10: SUMMARY OF THEORETICAL RESULTS .................................................................... 86

TABLE 5. 1: DEFLECTION AND STIFFNESS AT SLS OF THE FIRST SERIES ............................................ 113

TABLE 5. 2: DEFLECTION AND STIFFNESS AT SLS OF THE SECOND SERIES....................................... 113

TABLE 5. 3: DEFLECTION, STIFFNESS AND FAILURE MODE AT ULS OF THE FIRST SERIES .................... 117

TABLE 5. 4: TABLE: DEFLECTION, STIFFNESS AND FAILURE MODE AT ULS OF THE SECOND SERIES ...... 118

TABLE 5. 5: CONSTRUCTION TIME AND THE DEGREE OF SKILLS FOR THE FIRST SERIES ....................... 119

TABLE 5. 6: CONSTRUCTION TIME AND THE DEGREE OF SKILLS FOR THE SECOND SERIES ................... 119

TABLE 5. 7: SUMMARY OF THE RESULTS ................................................................................. 121

X

LIST OF FIGURES

FIGURE 1. 1: CRACK ALONG STEEL BAR DUE TO STEEL CORROSION (5) .............................................. 2

FIGURE 1. 2:VERTICAL FLEXURAL CRACKS (5) ............................................................................. 3

FIGURE 1. 3: DIAGONAL SHEAR CRACK (6) .................................................................................. 3

FIGURE 1. 4:THE MAIN CATEGORIES OF RETROFITTING METHODS (4)................................................. 4

FIGURE 1. 5: DIFFERENT CONFIGURATION OF CONCRETE JACKETS (8) ................................................ 5

FIGURE 1. 6: ONE FACE JACKETING (COMPRESSION SIDE) (8) ........................................................... 5

FIGURE 1. 7: THREE-FACES JACKETING (U JACKET) (10) ................................................................. 5

FIGURE 1. 8: PREPARING STEEL AND TENDONS (6) ........................................................................ 6

FIGURE 1. 9: CASTING AND POST-TENSIONING ............................................................................. 6

FIGURE 1. 10: SPAN SHORTENING BY ADDING CONCRETE OR STEEL COLUMN (8) .................................. 6

FIGURE 1. 11: DIFFERENT FORMS OF EXTERNAL REINFORCEMENT FOR SHEAR STRENGTHENING (8) .......... 7

FIGURE 1. 12: MECHANICAL AND CHEMICAL BONDING OF EXTERNAL REINFORCEMENT FOR FLEXURAL

STRENGTHENING (9) ....................................................................................................... 7

FIGURE 1. 13: FLEXURAL STRENGTHENING BY STEEL PLATE (12) ..................................................... 7

FIGURE 1. 14: FLEXURAL STRENGTHENING BY FRP SHEET (13) ....................................................... 7

FIGURE 1. 15: EXTERNAL POST-TENSIONING (6) ........................................................................... 8

FIGURE 2. 1: FAILURE LOADS OF BEAMS TESED BY EL-EBWEINI AND ZIARA (7) .................................. 16

FIGURE 2. 2: FAILURE LOAD OF TESTED BEAMS BY RAVAL AND DAVE (15) ...................................... 18

FIGURE 2. 3: ARANGMENT AND SPACING OF STEEL STRAPS (35) ..................................................... 26

FIGURE 3. 1: THE IDEALIZED SYSTEM OF BONDING (51). ................................................................. 34

FIGURE 3. 2: STRAIN AND STRESS DISTRIBUTION AT ULTIMATE CONDITION FOR PROPER SECTION (50). ... 34

FIGURE 3. 3: : COMPONENTS OF STRENGTHENED BEAMS ............................................................... 34

FIGURE 3. 4: STRESSES AT THE INTERFACES (52) ......................................................................... 35

FIGURE 3. 5: EFFECTS OF MISMATCHING ELASTIC MODULI ............................................................ 37

FIGURE 3. 6: STRAIN AND STRESS DIAGRAMS OF PLATED BEAM AT THE TENSION FACE (41) ................ 37

FIGURE 3. 7: DIFFERENCE IN STRAIN CURVATURE BETWEEN BEAM SECTION AND SIDE PLATE (29) ......... 37

FIGURE 3. 8: APPLYING FLEXURAL THEORY TO FULL-JACKETED SECTION ALTUN (22) ........................ 38

FIGURE 3. 9: JACKETING IN ONE FACE (TENSION FACE) ................................................................. 39

FIGURE 3. 10: JACKETING IN THREE FACE (U SHAPE) ................................................................... 39

FIGURE 3. 11: COMPRESSIVE FORCE PATH MACALEVEY ET AL. (37) ............................................... 40

FIGURE 3. 12: SLANT SHEAR RESULTS (37) ................................................................................ 40

FIGURE 3. 13: COMPRESSIVE FORCE PATH AT A SIMPLY SUPPORTED END (37) ................................... 41

FIGURE 3. 14: MODES OF FAILURE OF STEEL PLATE (57) ............................................................... 42

FIGURE 3. 15: FLEXURAL STRENGTHENING- RECOMMENDED DIMENSION LIMITS (33) ........................ 44

FIGURE 3. 16: SHEAR STRENGTHENING- RECOMMENDED DIMENSION LIMITS (33) .............................. 44

FIGURE 3. 17: STRAIN AND STRESS FOR DEFORMED SECTION (33) .................................................. 45

FIGURE 3. 18: STRAIN AND STRESS FOR INTACT SECTION (STEEL PLATED) (33) ................................. 45

FIGURE 3. 19: SHEAR STRENGTHENED BEAMS BY BONDED STRAPS.................................................. 46

FIGURE 3. 20: DIFFERENCE IN CURVATURE IN BOLTED STEEL PLATE AT BEAM SIDES (41)..................... 48

FIGURE 3. 21: DIFFERENCE IN CURVATURE IN ADHESIVELY BONDED STEEL PLATE AT BEAM SIDES (41) .. 48

FIGURE 3. 22: PLATED CONCRETE BEAM UNDER SHEAR LOADING (23) ............................................ 49

FIGURE 3. 23: FURTHER MODIFIED STRESS DISTRIBUTION IN PLATED BEAM (23) ............................ 49

FIGURE 3. 24: EQUILIBRIUM SECTION OF SIDE STEEL PLATED BEAMS (30) ......................................... 50

FIGURE 3. 25: BARNES MODEL (30) ......................................................................................... 50

FIGURE 3. 26: NOTATIONS OF BARNES MODEL (30) .................................................................... 51

FIGURE 3. 27: BEAM REINFORCEMENT DETAILS. (30) ................................................................... 51

FIGURE 3. 28: DEFLECTION AT LOAD POINT OF TESTED BEAMS (30) ................................................ 52

FIGURE 3. 29: TENSION FORCE IN THE BARS ............................................................................... 53

XI

FIGURE 3. 30: TENSION FORCE IN THE STEEL PLATE ..................................................................... 53

FIGURE 3. 31: SHEAR STRESS AT THE BONDED AREA (65).............................................................. 55

FIGURE 4. 1: THE OBJECTIVES OF THE TEST PROGRAM .................................................................... 58

FIGURE 4. 2: ORIGINAL FOR FLEXURE SAMPLES .......................................................................... 58

FIGURE 4. 3: ORIGINAL FOR SHEAR SAMPLES............................................................................. 58

FIGURE 4. 4: SAMPLES DISTRIBUTION ....................................................................................... 59

FIGURE 4. 5: JACKETING FOR FLEXURE USING SHEAR CONNECTORS (AF1, AF2, AF0) ......................... 61

FIGURE 4. 6: JACKETING FOR FLEXURE USING ADHESIVES (BF1,BF2) ............................................. 61

FIGURE 4. 7: STEEL PLATING FOR FLEXURE USING SHEAR CONNECTORS (AF3,AF4) ........................... 61

FIGURE 4. 8: STEEL PLATNG FOR FLEXURE USING ADHESIVES (BF3,BF4) ........................................ 61

FIGURE 4. 9: STRENGTHENING FOR SHEAR BY CONCRETE JACKETING .............................................. 62

FIGURE 4. 10: STRENGTHENING FOR SHEAR BY SIDE STEEL PLATES ................................................. 62

FIGURE 4. 11: STRENGTHEING FOR SHEAR BY STEEL STRAPS (BS6) ................................................. 62

FIGURE 4. 12: STEEL CAGE OF THE ORIGINAL SAMPLE OF THE FIRST SERIES ...................................... 63

FIGURE 4. 13: STEEL CAGE OF THE UNDERLAY ........................................................................... 63

FIGURE 4. 14: DRILLING HOLES IN THE TENSION SIDE IN THE ORIGINAL BEAM ................................... 63

FIGURE 4 . 15: STICKING SHEAR CONNECTORS ............................................................................ 64

FIGURE 4. 16: SETTING THE NEW STEEL CAGE ............................................................................ 64

FIGURE 4. 17: DRILLING HOLES IN STEEL PLATE ......................................................................... 64

FIGURE 4. 18: FIXING STEEL PLATE .......................................................................................... 65

FIGURE 4. 19: STEEL CAGE OF THE ORIGINAL SAMPLE OF THE SECOND SERIES ................................... 66

FIGURE 4. 20: DETAILING OF THE NEW STIRRUPS AND CONNECTORS ............................................... 66

FIGURE 4. 21: HOLES DISTRIBUTION ........................................................................................ 66

FIGURE 4. 22: FIXING THE CONNECTORS AND THE NEW STIRRUPS ................................................... 67

FIGURE 4. 23: THE NEW STIRRUPS DETAILING ............................................................................ 67

FIGURE 4. 24: HOLES DISTRIBUTION OF ES1 .............................................................................. 67

FIGURE 4. 25: FIXING STIRRUPS AND LONGITUDINAL REINFORCEMENT ............................................ 68

FIGURE 4. 26: PARTS OF STEEL STRAP ...................................................................................... 69

FIGURE 4. 27: WELDING AND BONDING STEEL STRAPS................................................................. 69

FIGURE 4. 28: LOADING SYSTEM OF THE FIRST SERIES .................................................................. 73

FIGURE 4. 29: ORIGINAL SECTION FO FLEXURAL STRENGTHENING .................................................. 74

FIGURE 4. 30: JACKETING AT THE TENSION FACE ........................................................................ 74

FIGURE 4. 31: STEEL PLATES AT THE TENSION FACE .................................................................... 77

FIGURE 4. 32: FLECTURAL MONOLITHIC SECTION........................................................................ 79

FIGURE 4. 33: LOADING SYSTEM FOR SECOND SERIES .................................................................. 80

FIGURE 4. 34: THE ORIGINAL SECTION FOR SHEAR STRENGTHENING ................................................ 80

FIGURE 4. 35: U JACKET FOR SHEAR STENGTHENING ................................................................... 81

FIGURE 4. 36: ∩JACKET FOR SHEAR STRENGTHENING .................................................................. 82

FIGURE 4. 37: SECTION OF BEAM STRENGTHENED STEEL STRAPS .................................................... 84

FIGURE 4. 38: SHEAR MONOLITHIC SECTION .............................................................................. 85

FIGURE 5. 1:FAILURE MODE AND CRACK PATTERN OF CF1.............................................................. 87

FIGURE 5. 2: FAILURE MODE AND CRACK PATTERN OF CF2 .......................................................... 87

FIGURE 5. 3: LOAD-DEFLECTION RELATIONSHIP OF CF1 ............................................................... 88

FIGURE 5. 4: FAILURE MODE AND CRACK PATTERN OF AF0 AND CF0 .............................................. 90

FIGURE 5. 5: FAILURE MODE AND CRACK PATTERN OF AF1 ........................................................... 90

FIGURE 5. 6: FAILURE MODE CRACK PATTERN OF AF2 ................................................................. 90

FIGURE 5. 7: LOAD-DEFLECTION RELATIONSHIP OF AF0 COMPAIRED WITH CONTROL BEAM CF0 ........... 91

FIGURE 5. 8: LOAD-DEFLECTION RELATIONSHIP OF AF1 AND AF2 COMPAIRED WITH CONTROL BEAM CF2

AND MONOLITHICLY CASTED BEAM MF ............................................................................ 91

FIGURE 5. 9: FAILURE MODE CRACK PATTERN OF BF1 ................................................................. 93

FIGURE 5. 10: FAILURE MODE (POST ULTIMATE) AND CRACK PATERN OF BF2 ................................... 93

XII

FIGURE 5. 11: LOAD-DEFLECTION RELATIONSHIP OF BF2 AND BF2 COMPARED WITH CONTROL BEAM CF2

AND MONOLITHICLY CASTED BEAM MF ............................................................................ 93

FIGURE 5. 12: FAILURE MODE AND CRACK PATTERN OF AF3 ......................................................... 94

FIGURE 5. 13: FAILURE MODE AND CRACK PATTERN OF AF4 ......................................................... 95

FIGURE 5. 14: LOAD-DEFLECTION RELATIONSHIP OF AF4 AND AF3 COMPARED WITH MF AND CF2 ...... 95

FIGURE 5. 15: FAILURE MODE OF BF3 ...................................................................................... 96

FIGURE 5. 16: CRACK PATTERN OF BF3 .................................................................................... 96

FIGURE 5. 17: LOAD-DEFLECTION RELATIONSHIP OF BF3 COMPARED WITH MF AND CF2 .................... 97

FIGURE 5. 18: FAILURE MODE OF BF4 ...................................................................................... 97

FIGURE 5. 19: LOAD-DEFLECTION RELATIONSHIP OF BF4 ............................................................. 97

FIGURE 5. 20: FAILURE MODE AND CRACK PATTERN OF CS1 ......................................................... 98

FIGURE 5. 21: FAILURE MODE AND CRACK PATTERN OF CS2 ......................................................... 98

FIGURE 5. 22: LOAD-DEFLECTION RELATIONSHIP OF CS1 AND CS2 ................................................ 99

FIGURE 5. 23: FAILURE MODE AND CRACK PATTERN OF AS1 ....................................................... 100

FIGURE 5. 24: FAILURE MODE AND CRACK PATTERN OF AS2 ....................................................... 100

FIGURE 5. 25: LOAD-DEFLECTION RELATIONSHIP OF AS1 AND AS2 COMPARED WITH THE CONTROL BEAMS

.............................................................................................................................. 100

FIGURE 5. 26: FAILURE MODE AND CRACK PATTERN OF BS1 ....................................................... 101

FIGURE 5. 27: LOAD-DEFLECTION RELATIONSHIP OF BS1 ........................................................... 102


FIGURE 5. 29: LOAD-DEFLECTION RELATIONSHIP OF BS2 COMPARED WITH THE CONROL BEAM .......... 103

FIGURE 5. 30: FAILURE MODE AND CRACK PATTERN OF ES1 ....................................................... 103

FIGURE 5. 31: LOAD-DEFLECTION RELATIONSHIP OF ES1 COMPARED WITH THE CONTROL BEAMS ....... 104


FIGURE 5. 33: LOAD-DEFLECTION RELATIONSHIP OF BS3 COMPARED WITH THE CONTROL BEAMS ....... 105

FIGURE 5. 34: FAILURE MODE OF AS3 .................................................................................... 106

FIGURE 5. 35: FAILURE MODE OF AS4 .................................................................................... 106

FIGURE 5. 36: LOAD-DEFLECTION RELATIONSHIP OF AS4 AND AS4 ............................................. 107

FIGURE 5. 37: LOAD-DEFLECTION RELATIONSHIP OF AS4 COMPARED WITH CS1 AND CS2 ................. 107

FIGURE 5. 38: FAILURE MODE OF BS4 .................................................................................... 108

FIGURE 5. 39: FAILURE MODE OF BS5 .................................................................................... 108

FIGURE 5. 40: LOAD-DEFLECTION RELATIONSHIP OF BS4 AND BS5 COMPARED WITH CONTROL BEAMS 109


FIGURE 5. 42: LOAD-DEFLECTION RELATIONSHIP OF BS6 COMPARED WITH THE CONTROL BEAMS ....... 110

FIGURE 5. 43: PERCENTAGE OF ENHANCMENT OVER THE CONTROL BEAM FOR FIRST SERIES ............... 111

FIGURE 5. 44: PERCENTAGE OF ENHANCMENT OVER THE CONTROL BEAM FOR SECOND SERIES............ 112

FIGURE 5. 45: COMPARATIVE STIFFNESS FOR THE FIRST SERIES AT THE SLS ................................... 114

FIGURE 5. 46: COMPARATIVE STIFFNESS FOR THE SECOND SERIES AT THE SLS ................................ 114

FIGURE 5. 47: COMPARATIVE LOAD-DEFLECTION CURVES FOR THE FIRST SERIES ............................. 115

FIGURE 5. 48: .: COMPARATIVE LOAD-DEFLECTION CURVES FOR THE SECOND SERIES ....................... 116

XIII

ABBREVIATIONS

ACI American Concrete Institute

IUG Islamic University of Gaza

JIS Japanese Industrial Standards

FRP Fiber Reinforced Polymers

RC Reinforced Concrete

CFP Compressive Force Path

IC Crack-induced debonding

CDC Critical Diagonal Crack

1

CHAPTER 1: INTRODUCTION

1.1 Introduction

The well-designed reinforced concrete systems exhibit excellent behavior through several

decades. However, several factors affect the reinforced concrete structures causing

decrease in load carrying capacity, especially environmental and mechanical factors.

Structural rehabilitation represents an important aspect of the construction industry and

its significance is increasing because the concrete structures are forever expanding and

aging and more of the available resources are being used to maintain it. It is becoming

both environmentally and economically preferable to repair or strengthen the structures

rather than replacement, particularly if rapid, effective and simple strengthening methods

are available (1; 2). Reinforced concrete (RC) beams are significant structural members

in load transferring process in structural skeleton. Therefore, they should afford satisfied

structural performance. The load carrying capacity of beams may decrease by many

reasons such as design errors and increase of loads. Existing beam members that are

deficient with respect to flexural or shear capacity are costly to demolish and reconstruct.

An efficient, cost-effective means of strengthening existing concrete beams is needed so

an unsafe or unusable structure can once again be utilized. There are several

strengthening methods for RC beams, each with different advantages, disadvantages and

practical limitations, so engineers should have the ability to choose the best according to

some criteria (2).

Little information is available and insufficient code guidelines are accessible for

strengthening concrete structures (1). In fact, most repair and strengthening designs are

based on the assessment of engineers only and, often, empirical knowledge and current

practice have an important role in the decisions to be made. Therefore, it is imperative

that researches should be done in purpose of providing reliable knowledge about

rehabilitation techniques.

In this research, the work is to investigate the behavior of different strengthening

techniques to propose the most effective for the rehabilitation of beams. This has

accomplished by strengthening RC beams by RC jackets and steel plates to increase

flexural and shear capacity using mechanical and chemical bonding techniques. These

methods are evaluated according to different characteristics in order to help structural

engineers to choose the most appropriate solutions. Recommendations are given, based

on theoretical and experimental study supported by published studies.

1.2 The Need for Rehabilitation

Generally, the need to rehabilitate buildings and structural elements such as beams may

arise at any time from the beginning of the construction phase until the end of the service

life. There are a number of different aspects contributing to the degradation of reinforced

concrete structures (2; 3; 4):

1. Physical: water and moisture transport, freezing, shrinkage, fatigue, abrasion,

early age cracking etc.

2

2. Mechanical: excessive loading, vibration, explosion, settlement, impact etc.

3. Chemical: de-passivation of reinforcement, chloride intrusion, corrosion,

4. Design phase deficiencies: calculation errors, poor detailing.

5. Construction phase deficiencies: workmanship mistakes, material inelegances.

6. Service life phase: accidents (such as collisions, fire, explosions), earthquakes,

changing in the structure functionality, the development of more demanding code

requirements.

1.3 Rehabilitation Needs in Gaza Strip

Structures in Gaza Strip are mainly reinforced concrete structures. Gaza Strip is a coastal

area. According to the survey study executed by Abu-Hamam (5), the damages in

existing building in Gaza strip are mainly due to environmental conditions, which result

in deterioration of concrete and corrosion of steel reinforcement. Also other damages are

associated with design and construction errors, poor quality concrete, fire accident and

Israeli military attacks. Design and construction errors represent 28% of the natural

causes of the assessment, while the need for upgrading the structures represent 10%.

Deterioration represents the greatest percentage as motivation for rehabilitation (49%).

Mainly, the reason is that Gaza strip is a coastal region. According to the researcher,

rehabilitation works, including repair of structural defects and strengthening of some

elements were recommended for about 33% of defects. This visibly implies that the field

of rehabilitation work is considerable in Gaza strip. The rehabilitation works include

strengthening and repairing of structural and non-structural member. As long as this is

true, reliable and applicable repairing and strengthening methods are needed.

1.4 Damages in Beams in Gaza Strip

The deficiency of RC beams is generally due to the unexpected loads, corrosion (Figure

1. 1) and upgradation of load standards. Vertical (Figure 1. 2) and diagonal (Figure 1. 3)

cracks which may result from overload, section deficiency, or/and low strength materials.

According to the survey carried by Abu-Hamam (5) which conducted in 2008, 9.6% of

the deficiencies in Gaza strip are structural cracks in slabs and drop beams.

Figure 1. 1: Crack along steel bar due to steel corrosion (5)

3

Figure 1. 2:Vertical flexural cracks (5)

Figure 1. 3: Diagonal shear crack (6)

Many researchers have been studied several types of damages of beams. El_Ebweini and

Ziara conducted a study about the effect of corrosion on RC beams (7). While, Owayda

and Shihada studied three types of damages in beams, over loading cracks,

honeycombing, and spalling of concrete cover due to elevated temperature (8). These

researches include execution and testing of suggested repair of the damaged beams.

1.5 Statement of the Problems

The function of the structures and structural members at the serviceability and ultimate

limit state exposed to be at lower levels due to several reasons such environmental

exposure, design errors, accidents, etc. There is significant need around the world for

rehabilitation since it is the best option in many cases. In addition, a survey study in Gaza

strip refer to the need of the rehabilitation in Gaza strip. Therefore, the field of

rehabilitation engineering needs reliable, economical an applicable techniques for

repairing and strengthening. The reliable strengthening technique should result in stable

system during the service life of the structural element. In addition, strengthening system

should behave in a ductile manner at the ultimate limit state.

In this research, the focus will be on examining two of strengthening techniques of

beams, which are steel plate bonding and partial concrete jacketing. This research

proposes answers to the following questions (research problems):

1. How the strengthened beams by bonding steel plates and concrete jacketing

behave in the serviceability and ultimate states? Is it better in the case of

monolithically casted beams?

2. Can the strengthening by the adopted techniques provide additional shear and

flexural capacity?

3. Is the type of bonding a significant factor in strengthening process?

4. Can the beam reach its shear or flexural capacity when the bonding is done by

chemical adhesives only? And is the mechanical bonding provide better results?

5. Is the partial concrete jacketing better than bonding steel plates theoretically and

experimentally?

6. Can the one-face jacketing provide good behavior always? And is the three-face

jacketing better?

These issues and others will be discussed through theoretical and experimental study for a

set of small-scale beams strengthened by concrete jacketing and steel plate as detailed in

this thesis.

4

1.6 Strengthening Techniques

Basically, the strengthening techniques for reinforced concrete structures can be divided

into:

1. Addition of new structural elements;

2. Strengthening of the existing structural elements.

Techniques of structural strengthening are various such as enlarging sectional area,

adding reinforcements, pre-stressed retrofit, changing load path, sticking steel plates and

encasing members with steel (1). Figure 1. 4 summarizes the common categories and

types of strengthening techniques (4):

1.7 Strengthening of RC Beams

As they most common structural elements, beams and slabs maintain the largest

retrofit workload. A lot of researches and case studies are available about strengthening

beams by different techniques. Following are some of the techniques of strengthening RC

beans.

1.7.1 Jacketing of Beams

Jacketing of beams involves enlarging one or more side in the section. Several forms of

concrete jacketing are shown in Figure 1. 5, Figure 1. 6 and Figure 1. 7.

Figure 1. 4:The main categories of retrofitting methods (4)

5

Figure 1. 5: Different configuration of concrete jackets (8)

Figure 1. 6: One face jacketing (compression side) (8)

Figure 1. 7: Three-faces jacketing (U jacket) (10)

6

1.7.2 Jacketing by Post-Tensioning Concrete

Figure 1. 8 and Figure 1. 9 illustrate using post-tensioning concrete in jacketing.

1.7.3 Span Shortening

This method involves shortening beam span by adding concrete or steel column to reduce

the bending moment. This can be done also by adding transverse beam at the middle of

the existing beams (see Figure 1. 10).

1.7.4 Increasing Reinforcement

External reinforcement can be bonded chemically or mechanically to the surface of the

beam to increase its load carrying capacity. The reiforcement added to the web vertically

or diagonally if the strengthening is needed for shear (Figure 1. 11), while added to the

tension side of the beam when the strengthening needed for flexure (Figure 1. 12). The

common reinforcement used for these purposes is steel plates (Figure 1. 13) and Fiper

Reinforced Polymers (FRP) plates or sheets (Figure 1. 14).

Figure 1. 8: Preparing steel and tendons (6)

Figure 1. 9: Casting and post-tensioning

Figure 1. 10: Span shortening by adding concrete or steel column (8)

7

Figure 1. 11: Different forms of external reinforcement for shear strengthening (8)

Figure 1. 12: Mechanical and chemical bonding of external reinforcement for flexural

strengthening (9)

Figure 1. 13: Flexural strengthening by steel

plate (12)

Figure 1. 14: Flexural strengthening by FRP

sheet (13)

8

1.7.5 External Post-Tensioning

External post-tensioning systems utilize high strength tendons, strands and steel bars, and

are used to increase or restore load-carrying capacity (Figure 1. 15). Post-tensioning is

ideal for situations that require significant load capacity increases or deflection and/or

crack control.

Figure 1. 15: External post-tensioning (6)

1.8 Comparison between RC Jacketing and Steel Plating of Beams

Engineers may face difficulties to choose the best strengthening technique for beams. The

most important criteria for comparison between the techniques are:

1. Availability of rehabilitation materials.

2. The cost of the rehabilitation materials.

3. The need of skilled workmanship.

4. The ability to provide the needed load capacity.

5. Behavior of the beam at failure.

In this research, a comparison based on detailed knowledge extracted from theoretical

and experimental study has been done between some systems of RC jacketing and steel

plating of RC beams, basically with regard the following features:

1. Crack pattern and the nature of strengthening system failure:

a. Monolithic Flexural failure.

b. Monolithic Shear failure.

c. Bond failure.

d. Delamination of concrete cover.

2. The percentage of increasing in load capacity.

3. The construction time and degree of skills needed for samples fabrication.

1.9 Research Scope, Objectives and Limitations

1.9.1 The Aim

The structural behavior of reinforced concrete beams strengthened with concrete jackets

or steel plates has been investigated in the undertaken research both theoretical and

experimental. The main aim of the research is to demonstrate the best among the

investigated strengthening techniques

Tendo

ns

Anchorage device

Existing beam

9

1.9.2 The Objectives

This research:

1. Reviewed theoretical study of the strengthening systems applied to the samples.

2. Presented experimental investigation for concrete jacketed and steel plated beams.

3. Presented a comparison between the strengthening systems applied to the

samples.

4. Provided recommendations for future research

5. Provided a practical recommendation in strengthening RC beams using concrete

jacketing and steel plating.

1.9.3 Scope and Limitations

This research studied two strengthening techniques, which are; concrete jacketing and

steel plating of beams. This study is theoretical and experimental. The samples are small

scale and simply supported. Reinforced concrete beams with rectangular sections was

used.

1.10 Unique Features of the Research

1. Variety of strengthening systems of RC jacketing and steel plating of RC beams.

2. Studying a package of variables in the same circumstances.

3. The presentment in a comparative way.

1.11 Methodology

To achieve the objectives of this research, the following tasks were executed:

Conclusions and recommendations

Conducting comparison

Analysis and discussion.

Testing of samples

Applying the strengthening

Constructing the Samples.

Analysis of samples.

Material properties.

Test program

Literature review

11

1.11.1 Literature review

Reviewing the literature related to strengthening beams using concrete jacketing and

steel plating in order to extract important points that help in conducting this research.

1.11.2 Test program and materials

Selecting and specifying the properties of the set of samples and the materials used to

construct them to serve the achievement of the aim of this research.

1.11.3 Analysis of samples

Analyzing the samples in order to obtain the theoretical values of the load carrying

capacities.

1.11.4 Constructing the Samples and applying strengthening

Constructing the original samples and then executing the strengthening operation using

the material specified previously.

1.11.5 Testing of samples:

Testing the samples using the loading system which is available in the Islamic

University in Gaza, and recording the results during testing.

1.11.6 Analysis and discussion:

Analysis and discussion of tests results and stating explanations for the results.

1.11.7 Conducting comparison

Conducting comparison between the strengthening techniques in order to specify the

precedence of them.

1.11.8 Conclusions and recommendations

Producing conclusions after analysis and discussion and then presenting

recommendations for practice and future research.

11

1.12 Outline of the Thesis

The following is a brief description of the contents of each chapter in the thesis:

Chapter 1 mainly contained Statement of the problems, methodology, Research Scope, Objectives and limitations

Chapter 2 reviews the use of RC jacketing and steel plating to strengthen RC beams

through a literature survey and evaluates the commonly used retrofit materials, properties

and application procedures. The content is placed within the framework of the knowledge

and the aim of this thesis.

Chapter 3 contained different design approaches of beams strengthened by concrete

jackets and steel plates.

Chapter 4 describes the experimental program, including the first and the second series

and analysis of the samples. The mechanical properties of the materials, reinforcement

steel, concrete, steel plate and epoxy are presented.

Chapter 5 presents the results of the experimental program, including the effects of

various parameters as well as different failure modes. The overall behavior of concrete

beams strengthened with various systems of RC jacketing and steel-plating techniques is

discussed. In addition, a comprehensive comparison between the adopted systems is

performed according the declared criteria.

Chapter 6 summarizes the thesis with a retrospective view on the research study and

draws conclusions from the work. Recommendations for future research are also

highlighted in this chapter.

12

CHAPTER 2: LITERATURE REVIEW

2.1. Introduction

Beams are very important structural elements in the structural system of buildings. They

may need to be strengthened to increase or recover their flexure and shear capacity as a

result of structural deficiency. Structural deficiency can be caused by details errors,

increase of load or environmental reasons such as reinforcement corrosion.

Strengthening of reinforced concrete beams is one of the important tasks normally

associated with the maintenance of concrete structures. The load carrying capacity of the

strengthened beams will increase if monolithic action exists between the existing beams

and the strengthening materials. The monolithic action may be achieved by using either

chemical bonding materials (epoxy resin adhesive, etc.) or mechanical shear connectors

at the interface between the strengthening materials and the existing beam and with

proper end anchorage. Strengthening R.C beams by externally bonded steel plates or R.C

jacketing is the most traditional method. Strengthening of reinforced concrete beams by

RC jacketing is a well-established and frequently used technique. It involves increasing

size of the existing reinforced concrete section by adding more reinforcement and

concrete. This method may be easier and cheaper compared to other approaches (14; 2).

Steel plates are one of the most common materials for strengthening of reinforced

concrete beam. It is very effective for increasing the flexural and shear capacity of

reinforced concrete beam. Strengthening by steel plate is a popular method due to its

availability, cheapness, uniform materials properties (isotropic), easy to work, high

ductility and high fatigue strength (1). A literature review gathered from different

references and articles relating to the use of externally bonded steel plates and R.C

jacketing for shear and flexural strengthening of concrete beams is included in this

chapter. The main purpose of the literature review in this and the following chapter is to

flatten the way for building the hypothesis and test program for this study.

2.2 Strengthening RC Beams by Concrete Jacketing

Jacketing has been considered as one of the important methods for strengthening and

repairing of RC beams (15). Traditionally, concrete has been used to jacket RC beams in

most of the applications. Concrete jacketing could be accomplished by either of the

following methods (14):

1. Bonding of hardened concrete to hardened concrete, typically associated with the

use of precast units in repair and strengthening.

2. Casting of fresh concrete to hardened concrete using an adhesive bonded joint

forming a part of the structure requiring composite action.

In the second method concrete poured into a form, containing the newly placed additional

reinforcement around the beam after the surface of the beam has been roughened by

trimming and chipping. The concrete jacketing enhances strength and stiffness and

improves resistance against seismic loads and enhances the durability of the element also,

and it can be applied to any type of RC structures such as residential blocks, industrial

structures, and bridges that may be damaged due to environment or accidents. Waghmare

(16) summarized structural and practical remarks about concrete jacketing stated by

Teran and Ruiz (17). These remarks presented in Table 2. 1 (16).

13

Table 2. 1: Structural and practical remarks about concrete jacketing (16)

Item Description

Properties of concrete jacket Match with the concrete of the existing

structure.

Compressive strength greater than that of the

existing structures by five N/mm2 or at least

equal to that of the existing structure.

Minimum width for jacket

8 cm if concrete cast in place or 4 cm for

shotcrete

Longitudinal reinforcement Percentage of steel on the jacket should be

limited to 50% of the total area of the

composite section.

Shear reinforcement Ignore the effect of existing shear

reinforcement

New reinforcement should have 135o hooks

and at each corner of the tie there must be at

least one longitudinal bar.

The bar used for the tie should have at least 8

mm diameter

Multiple piece ties can be used.

Depth of jacketed beam: The

following items should be taken

into consideration before

choosing the final depth of

jacketed beam

Span/depth ratio

Story height

Ductile behavior

Shear stress at the interface Provide adequate shear transfer mechanism to

assure monolithic behavior.

A relative movement between both concrete

interfaces (between the jacket and the existing

element) should be prevented.

Chipping the concrete cover of the original

member and roughening its surface may

improve the bond between the old and the new

concrete.

Connectors Distributed uniformly around the interface,

avoiding concentration in specific locations.

It is better to use reinforced bars (rebar)

anchored with epoxy resins of grouts.

14

2.2.1 Concrete as Strengthening Material

In the field of repair and strengthening concrete structures one of the following types of

concrete systems can be used in concrete jacketing: (18; 3):

1. Conventional Concrete: Pouring concrete around the member to be strengthened

with additional steel reinforcement properly anchored to the existing section.

Ordinary concrete jacketing requires formwork and is time consuming due to long

curing time. Furthermore, it is difficult to achieve a dense mix in constrained

conditions. Adhesion is also an issue, especially for overhead applications.

2. Sprayed Concrete (Shotcrete): A mixture of Portland cement, sand, and water “shot”

into place by compressed air. Pneumatically projecting concrete on to the reinforced

(usually with wire mesh) and prepared surface of the member being strengthened

with a spray gun. A variety of additives and admixtures are also introduced to

expedite strength gain, reduce rebound, reduce water requirement, curb shrinkage

and improve adhesion. The grading of aggregates is critical in sprayed concrete

due to the absence of external vibration and the reduction in the quantity of coarse

aggregates as a result of rebound. Shotcrete does not require formwork and is

useful to retrofit large areas in a relatively short period of time. But, the operation

is very messy with enormous loss of sprayed materials, resulting not only wastage

of materials, but an unsightly-rough surface finish too. It is not economical for

small areas of retrofit due to high setup and machinery costs. This technique

needs good skills and experience of nozzle operators.

3. Pre-Packed Aggregate Grouting: Pumping of cementitious grout into washed/

graded coarse aggregates placed with properly anchored reinforcement around the

member to be strengthened in a tightly sealed formwork. It is one of the better

ways of jacketing a concrete member as it results in a dense mix with good

surface finish. Drying shrinkage of this concrete is approximately half that of the

conventional concrete. This type used in large projects and when the conventional

placing of concrete is difficult such as underwater.

4. Ferrocement: commonly constructed of hydraulic cement mortar reinforced with

closely spaced layers of continuous and relatively small diameter wire mesh.

Because of no formwork is required, ferrocement is especially suitable for

structures with free-form shapes like curves.

5. Fiber-reinforced concrete: conventional concrete with either metallic or

polymeric fibers added to achieve greater resistance to plastic shrinkage.

6. Silica-fume concrete: silica fume (a by-product of the manufacture of silicon and

ferrosilicon alloys) when added to concrete will increase compression strength

and decrease permeability.

7. Polymer-impregnated concrete (PIC): a hydrated Portland-cement that has been

impregnated with a monomer that is subsequently polymerized.

8. Polymer-modified concrete: Portland cement and aggregate combined at the

time of mixing with organic polymers that are dispersed in water.

9. Polymer concrete: composite material in which the aggregate is bound together

in a dense matrix with a polymer binder.

15

2.2.2 Applications

R.C. jacketing technique is usually proposed in preference to other repair methods in the

following cases (19):

1. When the volume of repair material required is such that hand application or

spraying concrete (shotcrete) is not appropriate.

2. For repairs of concrete damaged by steel corrosion to protect the bars from

future corrosion by restoration of an alkaline environment, similar to the original

concrete.

3. In areas where the repair must contribute to structural strength at high

temperatures.

4. When an exposed concrete finish must be maintained.

5. When highly specialized workmanship is not available. The similarity with

traditional cast-in-place concrete makes this method relatively easier to use than

most repair techniques.

2.2.3 Advantages and Disadvantages

Strengthening using reinforced concrete have advantages as following:

1. The material is mechanically and physically compatible with the original

material.

2. Significantly enhance the strength and the stiffness.

3. Concrete is durable material.

4. Normal skills is needed.

However, it have some disadvantages like following:

1. Heavy weight and large dimensions.

2. Relatively long construction period.

3. Need evacuation of occupants.

2.2.4 Installations

When executing concrete jacketing the following considerations should be taken into

account (3; 14):

1. In most circumstances, and particularly in cases of concrete deterioration by

corrosion, durable repairs will be obtained by cutting out the concrete all around

the original reinforcement to an extent that can allow a good cleaning of the back

of the bar.

3. The concrete substrate must be thoroughly clean and mechanically sound to

provide a rough, aggregate exposed concrete surface.

4. The exposed reinforcing bars should be cleaned to the standards of new

construction.

5. The anchorage and cover of additional steel bars must comply with the

appropriate code provisions.

6. Bonding agents are usually recommended by repair manuals on the grounds of

improving adhesion.

3. The concrete mix should provide a reasonably high workability for satisfactory

placement and compaction.

16

5. The formwork must allow good concrete placing and compaction in the recasting

process, preventing the risk of trapping pockets of air. In most cases it has to be

built up in stages as the work proceeds.

2.2.5 Review of Investigations

El-Ebweini and Ziara (7) repaired formerly corroded beams by removing concrete

cover and anchoring two bars 10mm by shear connectors then casting 40mm of new

concrete as following:

Normal concrete without bonding agent: beams (R1-B1) and (R1-B2).

Normal concrete + bonding agent (epoxy resin concrete bonding agent used for

bonding wet cementitious material to existing cementitious surface): beams (R2-

B1) and (R2-B1).

Shrinkage compensated cementitious precision grout: beams (R3-B1) and (R3-

B1).

Figure 2. 1: Failure loads of beams tesed by El-Ebweini and ziara (7)

The results of the study shown in Figure 2. 1. The beams (Co.B1, Co.B2) and (CB1,

CB2) denoted to the corroded and control beams respectively. The researchers concluded

that the repaired beams showed good ductility behavior during the flexural test and

performed as sound constructed beams especially in term of flexural capacity and crack

development. The flexural capacity of the repaired beams increased by percent of 47.3%

compared with the control beams and by a percent of 105% compared with the corroded

beams.

Mahdy et al. (20) conducted an experimental study on beams strengthened by

three-faces RC jackets (U-shape, 50mm at the bottom and 37.5 at each side of the beam)

with and without additional stirrups. The strengthened beams of additional stirrups

exhibit typical failure with a ductile manner and with enhancement in strength reach

233% of the control beam. While, the strengthened beam without additional stirrups fail

in brittle manner and by separation of the added concrete layer with strength

enhancement reach 132% of the control beam.

17

Ziara (21) examined the effect of adding new overlay of steel fiber reinforced

concrete (SFRC) using chemical and mechanical bonding. The researcher concluded that

the epoxy bond could not prevent the inter-laminar shear failure in the beams

strengthened using chemical bonding such that the failure occurred by separation cracks

at the common interface surface between the original beam and the SFRC overly.

However, the beams of this type which are with additional stirrups reached their full

flexural capacity. On the other hand, the beams strengthened by mechanical bonding

failed monolithically and in ductile manner after reaching their full flexural capacities.

Altun (22) studied the effect of concrete jacketing of preloaded beams on the

flexural behavior. The main aim of this study is to prove that the concrete jacketing can

recover and enhance the load bearing capacity of the beam even if it reach the yielding

state. The original beams have been loaded until yielding then jacketed by 50mm

reinforced concrete layer in all beam faces after trimming off the outer clearance part

between the stirrups and the outer edge of these beams. The strengthened beams failed

typically with strength enhancement reach to 153% of the theoretical value.

Raval and Dave (15) made experimental investigation on jacketed RC beams

with 60 mm thickness all-round RC jackets using different techniques of bonding as in

Table 2. 2. All jacketed beams failed typically with significant enhancement in strength

as shown in Figure 2. 2.

Table 2. 2: Techniques of bonding in the study (15)

No. Beam code Bonding

4. C Control RC beam

5. QSD Smooth surface + dowel connectors.

6. QSB Smooth surface + bonding agent.

7. QSDB Smooth surface + dowel connectors + bonding agent.

8. QSM Smooth surface without bonding.

9. QPD Chipped surface + dowel connectors.

10. QPB Chipped surface + bonding agent.

11. QPDB Chipped surface + dowel connectors + bonding agent.

12. QPM Chipped surface without bonding

18

Figure 2. 2: Failure load of tested beams by Raval and Dave (15)

Chalioris and Pourzitidis (23) conducted experimental study on shear-damaged

reinforced concrete beams using self-compacting concrete jacketing. Five beams were

constructed and subjected to monotonic loading in order to develop shear failure. The

damaged specimens were restored using relatively thin reinforced jackets and retested by

the same four-point bending loading. The self-compacting concrete jacket was applied by

encasing the bottom width and both vertical sides of the initially loaded beams (U-formed

jacketing) with a small thickness (25mm) and small diameter of steel bars (∅5) and U-

formed stirrups. Test results and the comparisons between the experimental behavior of

the beams indicated that the examined jacketing technique is a reliable rehabilitation

method since the capacity of the retrofitted beams was fully restored or improved with

respect to the initial specimens. The increase in load bearing capacity varied from 35% to

200% for the retrofitted beams with respect to the corresponding initial beams.

AL-Kuaity (24) conducted an experimental study on the behavior and strength of

reinforced concrete T-beams before and after strengthening by using reinforced concrete

jacket. Four full-scale beams were first loaded to certain levels of ultimate capacity (0,

60%, 77%, 100% of failure load). After formation of cracks or failure, the beams were

strengthened by 50mm reinforced concrete jackets and tested again up to failure. The

main objective of this study was to recover the full capacity of the beams which failed by

flexure and to strengthen the cracked beams. In addition, it aimed to investigate the effect

of loading condition on beams before repair on the ultimate capacity after repair. The

main factor considered here is the effect of the level of loading percentages (percentages

of ultimate load before repair) on the strength and behaviors of the beam after repair. Test

results showed that the repairing by reinforced jacketing can effectively restore more than

150% of the full flexural capacity of the original beam. In addition, reinforced jacket can

effectively increase the ultimate capacity of cracked T-beam after repair up to 250%.

Furthermore, the use of reinforced jackets for the cracked or failed beams is greatly

improved the serviceability, deformation behavior, cracking behavior as well as ductility

of T- beams compared to those of the original beams. The researcher concluded that the

ultimate flexural strength of T-beams failed by flexure and repaired by reinforced

concrete jackets can accurately be predicted using conventional ultimate strength method

of reinforced concrete. The investigation showed the effectiveness of jacketing method in

restoring the flexural strength of T-beams.

19

2.3 Strengthening RC Beams by Steel Plates

In recent years, sticking steel reinforcement method has been developed for structural

retrofitting and repairing (1). Steel plates are one of the most common materials for

strengthening of reinforced concrete beams (25). Research work in the performance of

members strengthened by steel plates was pioneered simultaneously in South Africa and

France in the 1960s (26). It is very effective for increasing the flexural and shear capacity

of reinforced concrete beam. Strengthening by steel plate is a popular method due to its

availability, cheapness, uniform materials properties (isotropic), easy to work, high

ductility and high fatigue strength. This method had been used to strengthen both

buildings and bridges in countries such as Belgium, France, Japan, Poland, South Africa,

Switzerland and United Kingdom (25). This technique involves enhancing strength

(shear, flexure, compression) or improving stiffness of deficient reinforced concrete

members by bonding steel plates of calculated thickness with adhesives and anchors to

the existing sections. Forces can be transmitted to the external plates from the RC

structure through an adhesive bond, bolts or wrapping. Plates can be placed on any

surface of the beam or slab and they can have any shape such as flat plates, channels or

angle sections. On the other hand, Steel plate bonding is a cumbersome process requiring

extensive work and drilling in the existing section. Steel plates are hard to lift and need to

be tailor made to suit to the as-built dimensions of the members resulting in surface finish

is unsightly and steel plate retrofit is prone to corrosion over time.

2.3.1. Steel Plates as Strengthening Material

To mobilize the additional steel strength with low deformations of the strengthened

element it is convenient the use of a low tensile strength steel, such as the Fe360 (11).

According to Japanese Industrial Standards (JIS), SS 400 (234MPa) is a standard material

used for steel plating. Khair Al-Deen Bsisu et. al. had used two types of steel plate which

are high strength galvanized steel plates (HSGS plates); a material that combines high

strength to weight ratio and corrosion resistance due to galvanization, and normal

strength steel plates (NSS plates) which have a more ductile behavior (27). In general, the

quality of steel plates shall be indicated by their tensile strength and other strength

properties, Young's modulus and other deformation properties, and thermal properties

and other material characteristics. Steel plates must be those for which weldability and

bonding with adhesives can be ensured when necessary (1). The surface of steel plates

must be suitably protected to prevent their quality from changing over time.

2.3.2. Applications

The strategy of sticking steel plates, which is widely used in retrofit practice for its

shortcut and convenience in construction, is particularly applicable to:

1. For the elements with good quality but insufficient reinforcement, the external

reinforcement's addition is more adequate technique (11).

2. Flexural members such as beams and slabs, but unsuitable for axial compression

members and small eccentricity members (1).

3. Where there is not evident of on-going deterioration, for example due to

reinforcement corrosion (28).

https://www.google.ps/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CBsQFjAA&url=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FJapanese_Industrial_Standards&ei=7li_VISoEufQygO78IHwDA&usg=AFQjCNGCVemc7BSRA9PZ9MVat-5hcwAKHQ&bvm=bv.83829542,d.bGQ

21

4. For bridge deck applications, consideration should be given to the effect of the

plates on headroom and on the effect of damage to the soffit from vehicle impact.

2.3.3. Advantages and Disadvantages

The experience has shown that bonding steel plate to the surface of the RC beams and

slab is inexpensive and efficient method for strengthening and stiffening (29). Steel plate

not only acts as externally bonded reinforcement to the concrete section but it also

improves the moment of inertia (stiffness) of the composite (concrete-steel) section. The

in situ rehabilitation or upgrading of RC beams using bonded steel plates has been proven

in the field to control flexural deformations and crack widths, and to increase the load-

carrying capacity of the member under service load for ultimate conditions. It is

recognized to be an effective, convenient and economic method of improving structural

performance (26). In addition to this, steel plating have the following advantages (1; 30):

1. Possible increase in the beam ductility. When sufficient plate is effective then

shear failure will be less sudden and more gradual (plate controlled).

2. Short construction period, little or no downtime due to fast hardening of adhesive.

3. Simple process, fast and convenient construction.

4. It may be possible to strengthen the structure whilst it is still in use.

5. Relatively small increase in the size and weight of the existing section.

However, although the technique has been shown to be successful in practice, it also

has disadvantages (26; 30).

1. Uncertainty regarding the durability and the effects of Corrosion.

2. Weight of the plates (transporting, handling and installing).

3. Extensive shoring is required to hold the steel plates in the position while the

adhesive cures.

4. In the case of splicing, welding at the joints would destroy the adhesive bond

5. Relatively labor intensive.

2.3.3.1 Advantages and Disadvantages of Chemically Bonded Steel plates

Advantages and disadvantages of steel plates bonded using structural adhesives (30).

a) Advantages 1. Generally uniform stress distribution.

2. Adhesive layer provides corrosion protection to the face of the steel plate

nearest the beam.

3. Smooth external surface.

4. Good control of surface cracking.

b) Disadvantages 1. May still need bolts to contribute to anchorage and avoid peeling stresses.

2. Additional bolts may be required for temporary propping and squeezing out of

the excess adhesive when large plates are employed.

3. Limited evidence of impending failure, with possible explosive peeling of the

plate.

4. In-situ concrete surface preparation to high standards for the application of the

adhesive.

5. Relies on the tensile strength of the concrete at the surface-limited area

available in short shear span beams.

6. Distortion of the plates during grit blasting.

21

2.3.3.2 Advantages and Disadvantages of Mechanically Connected steel plates

Following are the advantages and disadvantages of steel plates bonded using bolts or

dowels (30):

a) Advantages 1. Avoids any uncertainty over long term durability of adhesives.

2. Overcomes the problem of peeling associated with high stress concentrations

at the ends of adhesive joints.

3. Stainless steel plates or non-ferrous coatings can be used to resist corrosion.

4. Load transfer from the plate utilizes the sub-surface compressive strength of

the concrete.

5. Through bolting can provide containment for the concrete core by allowing a

triaxial stress state to develop.

6. High post-failure capacity.

7. Only minimal on site surface preparation required.

b) Disadvantages 1. Temporary weakening of the beam due to drilling of the bolt holes, especially

if any links are cut; bolt anchors must be positioned within the main bars.

2. Corrosion of the external plate on its internal face.

3. The time and labour costs associated with drilling the numerous bolt holes.

4. Aesthetic appearance of bolts and plates.

2.3.4. Installation

When concrete jacketing method needed to be used, the following parameters are

important:

1. To allow the additional steel mobilization for the service loads the service loads

must be removed from the structures during the strengthening execution.

1. The concrete surface must be well prepared to receive the epoxy and the surface

of the steel plate must also be well cleaned and polished to a high standard using

grit blasting. A high roughness is inconvenient because it lead to an elevate resin

thick (11).

2. The epoxy must be carefully selected for both concrete and steel.

3. After mixing the two parts of the adhesive (resin and hardener) the potlife should

be observed according the manufacturer instruction.

4. The entire steel plate surface in contact with the concrete must be covered with

epoxy.

5. The thickness of the adhesive may controlled carefully by metal spacers. The

epoxy resin should allowed to cure for 14 days in all cases prior to testing.

6. When mechanical shear connectors used, high attention should be done during

drilling holes specially regarding to the position of embedded reinforcement.

22

To minimize the possibility of corrosion (26):

1. All chloride-contaminated concrete should be removed prior to bonding

2. The plates must be subjected to careful surface preparation, storage and the

application of resistant priming systems.

3. The integrity of the primer must be periodically checked.

2.3.5 Review of Investigations

Goldar et al. (31) examined RC beams strengthened by steel plates bonded at the

tension face by expansion bolts. Yielding strength of steel plate of 3mm thick equal 307

N/mm2 and for steel plate 6mm thick =327 N/mm2. The researchers used 6mm bolts for

plates of thickness 3mm and 10mm bolts for plates of thickness 6mm. They used the

concept of flexural theory to calculate the analytical values of ultimate loads. The test

results shown in Table 2. 3. For bolted steel plate (3 mm thick) attached to the bottom

face of the beam, the failure load was 95 KN i.e. about 132% of the reference beam load

and more than 87% of the predicted load. The predicted load was 108.87KN; the

difference in predicted and experimental load was 13.87KN due to the failure of bolts.

For bolted steel plate (6mm thick) attached to the bottom face of the beam, the failure

load was 126 KN, which was about 175% of the reference beam load and about 80% of

the predicted load. The predicted load was 158.36KN, the difference in predicted and

experimental load was 32.36KN and the researchers said that this possibly due to the slip

occurred at the interface of two material i.e. external plate and beam surface. For bolted

steel plate (3mm thick) attached to both side faces of the beam, the failure load was 108

KN, which was about 150% of the reference beam load and about 90% of the predicted

load. The predicted load was 120.586KN; the difference in predicted and experimental

load was 12.586KN, the researcher said that this possibly due to slippage occurred at the

interface of the two material i.e. external plate and beam surface.

Table 2. 3: Test results (31)

23

Huovinen (32) conducted an experimental study on concrete beams strengthened

by steel plate attached on the tension side of the beam bonded by two types of chemical

adhesives. The thickness of the plate varied (2, 5and 10 mm). The width and length of the

plates were 100mm and 280 respectively. The distance between plate end and support

(cut-off distance) is 100mm. The test results shown in Table 2. 4. The steel plates was

yielded in the beams strengthened by the plates of thickness of 2 and 5mm (except beam

no. 7) but in the beams strengthened by 10 mm thick plates the plates loosened before the

yield phase. The test results showed that the ratio between the width and thickness of the

bonded plate ought to be greater than 20. The researcher also stated that anchor bolts can

also be used to avoid plate separation at the ends of the plates. However, the bolts have

no effect on the load bearing capacity before the concrete above the plate has begun to

crack.


Avr. Ultimate

load (KN)

glue

type

Avr.Yield strength

(MPa)

Plate thickness

(mm) Beam

69 --- --- --- control 1

79 --- --- --- control 2

101.3 type1 176 2 1

103.6 type1 176 2 2

96.6 type2 176 2 3

94.6 type2 176 2 4

146.6 type1 219 5 5

144.6 type1 219 5 6

101.5 type2 219 5 7

138.5 type2 219 5 8

167.7 type1 343.5 10 9

47.7 type1 343.5 10 10

159.8 type2 343.5 10 11

114 type2 343.5 10 12

Ajeel and Ghedan (33) studied the effect of replacing internal tension bars of RC

beams by external steel plates chemically bonded at the tension face on their cracking

pattern, structural deformations and ultimate strength. The length of steel plates is

1400mm with varied thickness and yielding strengths. The researchers used the

conventional method to design the steel plated beams. The test results shown in Table 2.

5. The researchers concluded that the beams which reinforced with external steel plates

showed beam action and composite behavior until failure and can be used successfully

instead of internal reinforcement, they concluded this for replacement ratios of removed

bars to the original tension bars equal to 33% and 67% , the results is better in the case of

wider and thinner steel plates. There are, however, limitations to plate thickness beyond

which shear/bond failure occurs without the beams achieving their full flexural strength.

24


BHAGAT and BHUSARI (34) examined the effect of strengthening of RC

beams by external steel plates chemically bonded at the sides of them on the

enhancement of the shear capacity through an experimental study. The experimental

study is conducted on 26 RC beams divided in two series of beams i.e. without and with

internal shear reinforcement. The investigated variable in this research was the effect of

thickness and depth of the steel plates on ultimate shear strength. In the first series (no

stirrups) the control beam was failed by (diagonal-tension) and the shear failure

prevented in only 45% of the samples (5 samples). While the failure mode in the rest

beams was (shear-compression). The increase in shear strength was at least between

108% -186% of the strength of the control beam, this range refer to the difference in

thickness (2, 2.5 and 3mm) and depth (40, 60, 80 and 100mm) i.e. the section area (As)

of the steel plate. In the second series (inadequate distance between stirrups), the control

beam was failed by (flexure-shear), most of strengthened beams was failed by this mode

(73% of the beams), while the rest failed by (diagonal-tension). The increase in shear

strength was at least between 117% to 166% of the strength of the control beam, this

range refer to the difference of the section area (As) of the steel plate. Table 2. 6 and

Table 2. 7 show the data of the samples and test results. The researchers concluded that

increasing the plate depths and thickness increases the ultimate shear strength of all the

beams except one beam may be due to the defects in bonding operation, which resulted in

not so perfect bond of steel plates to concrete. However, this relationship between the

ultimate load and depths or thickness of steel plate is very normal so that the effect is a

result of increasing of the area of steel plate section. In addition, the variety of depths or

thickness has no touchable effect on the mode of failure of the samples.

25

Table 2. 6: Test results for series A (without shear reinforcement) beams (34)

Table 2. 7: Test results for series B (with shear reinforcement) beams (34)

26

Altin et. al. (35) studied the effect of steel straps chemically bonded to the

outer face of shear deficient RC beams. The researchers used different

arrangement and spacing as shown in Figure 2. 3. The researchers concluded that

strengthened specimens showed similar behavior to a control specimen up to

flexural yield. The flexural capacity of the control beams without shear

reinforcement was 61% of the full capacity while the capacity of beams with

straps of steel plate reach to 92.5% with improvement in stiffness and ductility.

The type of steel member and its arrangement on the beam were among the

effective parameters directing the ductility behavior and determining the failure

mode. The displacement ductility ratio was increased when the spacing of the steel

straps was decreased. The increase in the bonding area on the shear span reduced

the propagation of shear cracks significantly.

Figure 2. 3: Arangment and spacing of steel straps (35)

2.4. Anchoring and Bonding of Interfaces

Shear stresses arise in a repaired concrete structure due to both shear forces and

differential shrinkage. In order to prevent failure in the interface between old and new

concrete, there must be sufficient shear bond strength. Stresses due to differential

shrinkage are neglected since the real shear bond strength is several times larger than the

design shear strength in available design codes (36).

In design of strengthening work, the typical problem that should be solved is the transfer

of shear forces between the old concrete and the new material applied for strengthening.

There are factors affect the joint strength and performance such as:

1) The quality of substrate preparation.

2) The technique of bonding (mechanical or chemical).

3) Quality of the bonding (material and application).

4) The nature of the strengthening material (compatible with the old material or

not).

5) Type of loading (cyclic or static).

6) Exposure and environmental conditions (temperature, salts, etc.).

Generally, there are some considerations should be taken into account for a good bonding

(22):

1) Good choice of a suitable bonding technique.

2) Appropriate design of the joint.

3) Adequate preparation of the bonded surfaces.

27

4) Good choice of a suitable adhesive.

5) Controlled fabrication of the joint.

6) Protection from unacceptably hostile conditions in service (heat).

7) Post-bonding quality assurance.

2.4.1 Bonding Techniques

A good bond between old and new concrete is necessary for a successful repair of

concrete structures. In general, the techniques of bonding the strengthening materials to

the surface of RC beams can be one or combination of the following (14):

2.4.1.1 Roughening of Concrete Surface

This method can be adequate only for concrete-to-concrete bonding and more practical in

small repair work rather than strengthening. It is referred to some researchers that surface

preparation by suitable methods can provide acceptable shear strength of the bond (14).

However, in practice there is a risk for of zero adhesion for the more poor removing

methods. The slant shear tests demonstrated that the failure of a concrete-to-concrete

interface is sudden (37). The bond is usually determined through pure tension tests, e.g.,

the common pull off test. However, in most applications, the shear bond strength is more

interested (36). The most common methods of surface preparation are (3):

1. Mechanical preparation: This technique consists of mechanically removing thin

layers of surface concrete using such equipment as impacting tools (breakers,

scabblers), grinders, and scarifier. Depending on the equipment used, a variety of

surfaces may be obtained.

2. Abrasive preparation: This technique consists of removing thin layers of surface

concrete using abrasive equipment such as sandblasters, shotblasters, or high

pressure water blasters.

In Gaza strip only simple grinder and manual methods available but these methods cannot

be reliable, so the site engineers should construct samples for testing the bond efficiency

before applying the any method. There are various tests to examine the efficiency of the

interface bonding such as direct tension, direct shear, pull out testing, etc. In general the

following actions are normally needed for shear forces (14):

a. The substrate must be cleaned from any contamination.

b. The substrate should not contain microcracks.

c. The substrate surface should have a good roughness (this is the critical stage which

needs lab tests for ensure the bonding efficiency).

d. The substrate surface should be pre-watered but superficially dry at the time of

casting.

Surface Preparation techniques and their efficiency have been studied by several

researchers. While, Perez et al. (38) studied the correlation between the roughness of the

substrate surface and the debonding risk and concluded that repaired beams of a substrate

with a rough surface permit to achieve a monolithic behavior of the repaired system but

the increase of roughness does not enhance the bond strength. The researchers stated that

to insure monolithic behavior, surface treatment must produce a minimal adhesion and

must induce a certain level of roughness and if the roughness is higher than a certain

threshold, the debonding risk decrease rapidly and monolithic behavior will reached (38).

Abu-Tair et al. (39) studied a new method of quantifying surface roughness and compared

it with a method reported in the literature. Also Santos et. al. (40) presented a state-of-the-

art review on roughness quantification methods for concrete surfaces.

28

2.4.1.2 Mechanical Connectors

Steel-to-concrete or concrete-to-concrete connections can be accomplished through the

use of several types of anchorage systems. Anchorage to concrete is well-known and has

detailed design procedures in the available codes, for example (ACI-appendixD). The

bolts and dowels play a good role in transferring the shear from the RC member to the RC

jacket or steel plate. Bolt (or dowels) shear connectors, are ductile connections. For steel

plate connection it referred to (Oehlers and Bradford 1995, 1999) that mechanical shear

connectors, such as dowels or bolts shear connectors behave in a similar fashion to stud

shear connectors in composite steel and concrete design. Hence, it is recommended that

rules for the design of stud shear connectors can be applied directly and which are given in

national codes for composite steel and concrete beams (41). Mechanical Connectors can

be divided into two general types, bonded and expansion anchors (3):

3. Bonded anchors include both grouted (headed bolts or a variety of other shapes

installed with a cementitious grout) and chemical anchors (usually threaded rods

set with a two-part chemical compound that is available as glass capsules, plastic

cartridges, tubes, or bulk). These anchor systems develop their holding capacities

by the bonding of the adhesive to both the anchor and the concrete at the wall of

the drilled hole.

4. Expansion anchor systems: include torque-controlled, deformation-controlled, and

undercut anchors. These anchors develop their strength from friction against the

ide of the drilled hole, from keying into a localized crushed zone of the concrete

resulting from the setting operation, or from a combination of friction and keying.

For the undercut anchors, strength is derived from keying into an undercut at the

bottom of the drilled hole.

Mechanical bonding can be achieved by dowels from traditional reinforcement or

structural bolts which can used for all applications. The most important properties for the

dowels or bolts are tensile and shear strength. Bolts typically used for structural joining

applications can be classified based on three ASTM specifications: A307, A325, and

A490 (38).

2.4.1.3 Chemical Adhesives

The main structural adhesives specifically formulated for use in the construction industry

are epoxy and unsaturated polyester resin systems, both thermosetting polymers (26). In

general the categories of structural adhesives are epoxies (one and two part formulations);

Acrylics (and two-step formulations); Urethanes (two part formulations) and

Cyanoacrylates (“instant adhesives”). The common structural adhesive is an epoxy-based

material mixed with proportional curing agent, flexibilizer, plasticizer and in some cases

fillers (43; 44). The use of structural adhesives in the manufacture of load-bearing

components has grown extensively in recent years. It is stated in the literature that the

feasibility of bonding concrete with epoxy resins was first demonstrated in the late 1940s,

and since the early 1950s adhesives have become widely used in civil engineering (26).

The desirable qualities which adhesive bonding allows in comparison with more

traditional joining techniques such as riveting and welding include (45):

a. Allowance of a relatively uniform stress distribution, resulting in improved

fatigue performance.

b. The ability to join dissimilar substrate materials which, due to their dielectric

nature, minimizes the possibility of electrolytic corrosion between dissimilar

metals.

29

c. Allows the joining of thin-gauge metals to each other resulting in the

availability of lightweight structures exhibiting high strength to weight ratios.

d. Allows both increased design flexibility and the ability to fabricate complex

shapes.

e. The possibility of reduced production costs in comparison to welding and

riveting

2.4.1.4 Selecting a Structural Adhesive

Regardless of the routes chosen to select structural adhesives to test, the key is testing; no

final decision should be made without specific validation testing. However, key

principles can be used to select a set of adhesives to test. Structural adhesives should be

chosen with the end use requirements. Once these are known, the proper adhesive can be

selected by matching the requirements to the different processing and performance

characteristics of different structural adhesives. In particular, end use conditions to

consider include (46):

1. Expected conditions during end use:

Temperature: how hot? how cold?

Humidity: will the material be exposed to rain? To salt water?

UV exposure: will the joint be exposed to the sun and can the UV penetrate the

substrates to reach the adhesive?

2. Chemical resistance required:

Fluids (motor oil, gasoline, diesel fluid, jet fuel): will these contact the joint?

Cleaning solutions (weak acids and bases): will the joint be cleaned frequently?

Are there specialized chemicals which may contact the bonded part?

Will contact be continual (e.g. in a filtering assembly) or only occasional?

3. Cleanliness / Environmental issues during production and end use:

Outgassing, ionics, corrosion potential: is the part being bonded sensitive to these

issues?

Toxicity, disposal: are there regulations that come into play? Will the adhesive be

used in food packaging or a medical device?

4. Mechanical Challenges

Impact, vibration: will the bonded part be subject to high impact or vibrational

forces in use?

Stress type and magnitude: how high are the stresses on the bondline? What types

of stresses will the bondline experience?

For a final selection, however, testing and validation is always recommended. Typically,

overlap shear tests of some sort are done to determine the strength of the adhesive on

particular substrates or under environmental conditions, and peel tests are also common.

The exact details of the test should be based on the particular project; do not rely solely

on the results in the manufacturer’s technical data sheets. However, the most carefully

chosen adhesive may not give acceptable application performance if the bonding surfaces

have not been properly prepared or the joint has been poorly designed (46).

31

2.4.1.5 General Properties of Adhesives

In general, the more relevant resin properties for this type of application (strengthening)

are the viscosity, the pot life, the hardening time, the elasticity modulus and the strength

(11). Manufacturer provides a wide range of bonding agents with various properties

according to the nature of the strengthening materials. The correct choice and proper use

of repair materials is critical to the achievement of long service life for repaired

structures. The designer and prospective user (engineers or contractors) of the materials

should be equipped with performance criteria (shown in Table 2. 8) that provide a

rational analytical tool for selecting the appropriate materials for a particular repair

situation (47). Table 2. 9 presents structural requirements of bonding between different

surfaces (1).

Table 2. 8: : General Requirements of Patch Repair Materials for Compatibility (47)

Table 2. 9: Bonding strengths of structural adhesive (1)

31

2.4.1.6. Epoxy

Epoxy resins are wide family of materials and have the best mechanical performance

characteristics of all the resins. The most demanding strength applications use epoxy

almost exclusively. It has excellent strength and hardness, very good chemical heat and

electrical resistance. Disadvantages include higher cost, processing difficulty (quantities

of resin and hardener need to be measured precisely). Also, often heat curing is required.

Epoxy systems are used in applications like aerospace, defense, marine, sports

equipment, adhesives, sealants, coatings, architectural, flooring and many others (48; 44).

The advantages of epoxy resins over other polymers as adhesive agents for civil

engineering use can be summarized as follows (1):

a. Epoxy resin has high adhesiveness and good bond strength with most materials

such as metal, concrete, ceramics and glass.

b. Epoxy resin has good processing property and stable storage performance. It can

be prepared as thick paste or thin grouting materials, whose curing time would be

adjusted appropriately according to needs.

c. Cured epoxy adhesive has excellent physical and mechanical properties, corrosion

resistance capacity, and small curing contraction.

d. Epoxy material has relatively low cost, non-toxicity and plentiful material

resources.

e. Formulation can be readily modified by blending with a variety of materials to

achieve desirable properties.

f. Can be thixotropic for application to vertical surfaces.

2.4.2 Anchor and Bond Strength

The strength of bonds executed by chemical adhesive depend mainly on the correct

choice of the adhesive and the level of following the manufacturer instructions. However,

laboratory tests must be done to specify the bond strength especially for this type of

works (strengthening). In the case of mechanical anchor the strength and long-term

performance are dependent on a variety of factors that must be evaluated for the specific

anchor to be used. Some factors to be considered include (3):

1. Material strength (yield and ultimate),

2. Hole diameter and drilling system used,

3. Embedment length,

4. Annular gap between the anchor and the drilled hole for post-installed anchors,

5. Concrete strength and condition,

6. Type and direction of load application (static, dynamic, tension, shear, bending,

or combined loading),

7. Spacing to other anchors and edges,

8. Temperature (for chemical anchors),

9. hole cleaning,

10. Mode of failure of the anchor system (concrete breakage, steel breakage, slip, or

pullout),

11. Environmental conditions for moisture and corrosion resistance, and creep.

32

For all anchor systems, installation instructions should be followed to insure proper

anchor performance. Site testing for verification of performance is recommended for

critical applications. For chemical anchors, tests should be performed to determine the

long-term creep performance at the highest expected service temperature. There are many

types of tests for anchor and bond strength such as direct tension test, shear test and pull-

off test.

33

CHAPTER 3 DESIGN OF CONCRETE JACKETED AND STEEL

PLATED BEAMS

3.1 Introduction

This chapter aiming to present the basic principles and different approaches of designing

the strengthened beams by concrete jackets and steel plates in order to help in more

understanding the special considerations of design like these beams. Designing the

strengthened section need special considerations. Design the strengthening systems

includes the ultimate Limit States and the Limit states verifications. In the strengthening

design the serviceability Limit States verification, the existent damages and the lower

stiffness of the strengthened elements must be considered. A reduction of materials and

cross sections area and inertia is usually adopted because the uncertainties related to the

existent damage simulation and, as a consequence of assessment, by the possibility to

reduce the materials strength uncertainty (11). When the system of strengthened beam

need to be designed, mainly, three issues should be considered:

a) The structural condition of the original beam.

b) The contribution of the new material in flexural or shear enhancement.

c) The efficiency of bonding between the old and new material.

3.2 Basic Principles in Design of Strengthened RC Beams

Before design concrete jacketed and steel plated RC beams, basic principles should be

understood by designers. The beam section components must act consistently with each

other to guarantee the composite action. The proper RC beams mainly consist of

concrete and longitudinal reinforcement which surrounded by concrete and no efforts

needed to ensure the bonding between them, only casting suitable and workable concrete.

While, in the case of strengthened beams, different materials attached at one side or more

of the outer faces of RC beams. Bonding the new to the old material is critical and need

enough attention. The main difficulty in strengthening process is bonding the new

material to the old material. Duthinh and Starnes (49) stated that according to the survey

of Boncci (1996), 64% of the failure of the elements strengthened by FRP occurs by de-

bonding (brittle behavior), while 22% of the cases failed by the rapture of the FRP

material. The uncertainty in strengthened beams arise a query: Is it possible to estimate

the load carrying capacity for strengthened concrete beams, in flexure and shear by using

available design code or some modifications and considerations are needed? and can the

strengthened beam reaches its ultimate capacity before de-bonding of new materials?

3.2.1 Proper Reinforced Concrete Beams

The reinforcement in a reinforced concrete structure, such as a steel bar, has to undergo

the same strain or deformation as the surrounding concrete in order to prevent

discontinuity, slip or separation of the two materials under load (50). Maintaining

composite action requires transfer of load between the concrete and steel. The direct

stress is transferred from the concrete to the bar interface to change the tensile stress in

the reinforcing bar along its length. This load transfer is achieved by means of bond

(anchorage) and is idealized as a continuous stress field that develops in the vicinity of

the steel-concrete interface. In a reinforced concrete element bond develops through the

action of several mechanisms in the vicinity of the concrete-steel interface. At the scale of

the reinforcing steel, the bond response may be defined by continuous stress and

deformation fields. Figure 3. 1 shows the idealized system of bonding between

34

reinforcement bar and concrete. Stimulation of bond mechanisms results in the

development of bond stress in the direction parallel to the axis of a reinforcing bar and

radial stress in the direction perpendicular to the bar axis (51).

Figure 3. 1: The idealized system of bonding (51).

In proper RC beams, the flexural theory is applicable. The main assumption in the

flexural theory is that the sections perpendicular to the axis of bending which are plane

before bending remain plane after bending (Figure 3. 2). In other words, strain profiles

remain linear at all times. In addition, ductility cannot be maintained unless some

conditions checked, mainly the ratio of longitudinal reinforcement (50).

Figure 3. 2: Strain and stress distribution at ultimate condition for proper section (50).

3.2.2 Strengthened RC Beams

If beams need to be strengthened three properties need to be considered, the properties of

materials used for rehabilitation, the properties of existing materials, and the interaction

between new and existing material (2). Figure 3. 3 show the components of strengthened

beam.

Existing

beam

Bonding

New material

Figure 3. 3: : Components of strengthened beams

35

There are some factors govern the monolithic behavior of the strengthened section should

be taken in consideration at design stage. The most critical factor and the most important

stage of strengthening process is the bonding between the strengthening material and the

substrate. If the bonding executed in good level of quality and become not critical, the

mechanical compatibility is the important factor in the short term while the physical

compatibility is the important factor in the long term. If all considerations which ensure a

good bonding and compatible materials are taken into account, then the composite action

can be guaranteed.

The de-bonding mechanisms of the new materials and their compatibility with the old

material should be well understood. Consequently, well-known procedures in level of

design and practice to assure stability and satisfied yielding of strengthening system,

good flexural stiffness and acceptable failure mode should be acknowledged. The use of

proper procedures in repair and rehabilitation are critical to success, yet these procedures

are not nearly as well defined by codes and standards as those for new construction (47).

3.2.3 Stresses Transfer at the Interfaces

In general the strengthened RC beams subjected to a combined bending/shear load,

resulting in shear and normal (peeling) stresses between the concrete and the externally

bonded strengthening layer,

Figure 3. 4 (52).

Figure 3. 4: stresses at the interfaces (52)

3.2.4 Compatibility of Strengthened Beam Materials

Compatibility is a measure of the matching of physical, chemical, electrochemical and

dimensional properties between the repair materials and the substrate (47). Materials of

proper reinforced concrete beams have been experienced that they are compatible

physically and mechanically. This is because the thermal compatibility and good bonding

between reinforcement bars and concrete (50). On the other hand, the systems of

strengthened beams which consist of different layers of different materials need detailed

study about the compatibility with each other. The efficiency of the strengthening

systems in beams can be achieved only if the old and new materials are compatible and

the bonding between them is efficient.

36

3.2.4.1 Physical Compatibility

1. Dimensional compatibility (2): The compatibility of dimensional stability means

that the repair materials and the existing concrete will undergo a compatible

drying shrinkage under the influence of temperature, moisture, loading, and other

environment changes. Dimensional incompatibility will cause severe problems for

repair patches. For example, if a mortar is used as the repair material, the

shrinkage of the fresh mortar will create stress concentration at the point of

contact with the existing concrete. The stress concentration could be normal stress

or shear stress depending on the orientation of the repaired area. The normal stress

concentration at the interface may cause separation, and the shear stress

concentration may cause de-bonding between the two materials. Hence, the

shrinkage of repair mortar must be limited.

2. Thermal compatibility (2): If the thermal expansion of existing concrete differs

significantly from the thermal expansion of the new concrete, cracks may occur

due to a change in environmental temperature in the surface of repaired concrete

or along the interface of the old and new concrete. As a result, it is necessary to

choose a repair material with the same coefficient of thermal expansion as the old

concrete, or as close to the coefficient of the old concrete as possible.

3.2.4.2 Mechanical Compatibility (Stiffness and Strain).

Irregular distribution of stress is often caused by differential stiffness in repair

systems. Stiffness of a material is measured by its modulus of elasticity. A material with

low modulus of elasticity generally deforms more under the same load than a material

with a higher modulus of elasticity. The volumetric compatibility is reflected in uneven

distribution of stress due to the different Young’s moduli of new and old concretes.

When materials with widely different moduli are in contact with each other and under

loading, the large difference in the stiffness will result in uneven distribution of the

stress and thus lead to failure of either the material with low modulus of elasticity or the

material with high modulus of elasticity, depending on the strengths of the materials (2).

If the strength of the new material with higher modulus is not sufficient to bear the load,

the new materials may fail earlier than the old concrete. This is one of the important

mechanisms that are responsible for pre-matured failures of repair work in the new

material (2). Figure 3. 5 shows the effects of mismatching elastic moduli of the new and

the old material.

37

Figure 3. 5: Effects of mismatching elastic moduli

Often, in the case of strengthening beams using concrete, the stiffness compatibility can

be controlled whatever is the position of the new concrete jacket. While in the case of

steel plating, if the plate at the tension face, with respecting some considerations the

beam section can behave monolithically (and then confirm the flexural theory (

Figure 3. 6)), but if the plate at the sides of the beam there is a significant difference in

stiffness between concrete section and the steel plate (

Figure 3. 7) so consequently difference in strain curvature.

Figure 3. 6: Strain and Stress Diagrams of Plated Beam at the tension face (41)

Figure 3. 7: Difference in strain curvature between beam section and side plate (29)

3.3 Structural Data of the Original Beam.

It is very important to take into account the original stress situation before applying the

strengthening to the beam. Only in that case an appropriate design is possible (53).The

structure strengthening design must be proceeded by a strength assessment of the existent

structure. This involves a compilation related to the build erection and design, a structural

38

inspection and a load capacity evaluation (11). The first step is to evaluate the current

condition of the concrete structure. This evaluation may include a review of available

design and construction documents, structural analysis of the structure in its deteriorated

condition, review of structural instrumentation data, review of records of any previous

repair work accomplished, review of maintenance records, visual examination,

destructive (core drilling) and nondestructive testing, and laboratory analysis of concrete

samples. Upon completion of this evaluation step, the team making the evaluation should

have a thorough understanding of the condition of the concrete structure and have

insights into the causes of any deterioration or distress noted. Additional information on

conducting surveys may be found in the reports of ACI Committees 201, 207, and 325

(3). In some practical applications the flexural or shear capacity is only partly increased

as the structure is not supported by a hydraulic jack during the installation of the

strengthening layer. Consequently, the strengthened section only provides enhanced

capacity for the live and imposed loads.

3.4 Design of Beams strengthened by Concrete Jacketing

Since the new material is concrete then the materials of concrete jacketed RC beams

are compatible physically and mechanically. Therefore, in this case these systems have

high potential for the applicability of the flexural theory especially if effective bonding

and good quality of strengthening process have been achieved. AL-Kuaity (24), have

strengthened RC T-beams by this method and design them using conventional code,

testing have shown conservative results and ductile failure. Altun (22) generated the

equation of flexural capacity of full-jacketed section using flexural theory (Figure 3. 8) as

following:

Figure 3. 8: Applying flexural theory to full-jacketed section Altun (22)

Figure 3. 9 and Figure 3. 10 show two examples of flexural theory application on

concrete jacketed beams. Design equations can be generated as following:

39

1. Effective depth and reinforcement ratio :

dav =

and

2. Shear strength:

Vc = √

bw dav

, Av = Av1 + Av2

3. Flexural strength:

C =

) +

), or

)

MacAlevey et al. (37), proved that jacketed RC beams can be designed as

monolithic beams if monolithic behavior guaranteed. They stated that the monolithic

behavior of the jacketed beam is ensured if critical portions of the interface between the

two concretes possesses adequate strength (i.e. the interface in the region where it is

crossed by the inclined branch of the compressive force path (CFP) (Figure 3. 11 and

b

d1

d2

As1

As2

0.003

NA

b

d2 d1

As1 As2

0.003

NA

Figure 3. 9: Jacketing in one face (tension face)

Figure 3. 10: Jacketing in three face (U shape)

41

Figure 3. 13)). The researchers described the development of a model that can be used for

the design of jacketed reinforced concrete beams. This model is based on the use of the

compressive force path concept to describe the behavior of a monolithic reinforced

concrete beam, and a Mohr-Coulomb approach to describe the behavior of the interface

between the new and old concretes.

Figure 3. 11: Compressive Force Path MacAlevey et al. (37)

The researchers developed the model using the results of a series of tests on slant

shear prisms and simply supported and continuous jacketed beams.

Figure 3. 12 shows the results of the slant shear tests on concrete with a 28-day fcu

of 35 N/mm2.

Figure 3. 12: Slant shear results (37)

The values of the cohesion c and the angle of friction Φ obtained as a result of the slant

shear tests on various grades of concrete of "moderate roughness" result in the following

Mohr-Coulomb equations:

2 7 σtan 48° , or cu 25 N/mm2(1) 3 5 σtan 58° , or cu =30 N/mm2 (2) 3 2 σtan 50° , or cu =35 N/mm2 (3) 4 5 σtan 4]0 , or cu = 40 N/mm2 (4) 3 8 σtan 50° , or cu = 50 N/mm2 (5)

41

Figure 3. 13: Compressive Force Path at a simply supported end (37)

A summary of the recommended design approach is as follows:

1. Proportion the reinforcement as for a monolithic.

2. Detail the beam so that the additional longitudinal reinforcement should be fully

anchored at simple supports and at the points of contraflexure of continuous

beams.

3. Check the capacity of the interface in the region where the compressive force path

crosses it (using Eqn. 1, 2, 3, 4, or 5, as appropriate).

The model produced by the researchers seems rational but it needs more detailing and

deeper study. However using this approach implies that anchoring the new and the old

concrete at the region of the inclined leg of the CFP is enough for composite action.

Also Ziara (54) used the concept of CFP. The researcher add new layer of

reinforced concrete at the compression face and weld the new and the old stirrups only in

the region of inclined legs of the CFP. The samples behave monolithically at all loading

levels. Moreover, the researcher examined adequacy of welding of all stirrups and

concluded that this did not result in any additional enhancement in the structural

behavior. In addition the researcher measured the strain of one strengthened and found

that the strain diagram is typical this ensure that the concrete jacketed beams can be

designed using available method. According to the researcher, all beams was failed

monolithically and the extra strength measured reached up to 154% of the capacity of the

existing beams.

3.5 Design of Beams Strengthened by Steel Plates

Design considerations of steel plated RC beams differ according to:

1. Bonding technique (adhesive or bolt)

2. Position of plate (tension face or side web)

The first hypothesis, which we can start with, is, if the steel plate (or any new layer) is

properly attached to the RC beam, the strain will be consistent with strain compatibility

theory which state that cross sections that are plane before bending remain plane after

bending. Therefore, the composite section can be designed by the procedures of the

monolithic beam. However, to guarantee this assumption some considerations should be

taken into account to ensure the monolithic behavior.

3.5.1 Modes of Failure of Beams Strengthened by Steel Plates

Tests show that steel plate stuck at the bottom of a beam can achieve yield strength when

damaged (1). In proper reinforced concrete beams, as load increases, the reinforced beam

is damaged when concrete has been crushed after yielding of the reinforcement. The ideal

42

behavior of steel plated beams is yielding of the new steel plate and failure in a ductile

manner. This is the desired mode of failure since the increased load capacity by the

strengthening system can be reached. However, some experiments showed that when

such beams was destroyed, the steel plate was still below the yield strength (1).

3.5.1.1 Adhesively Bonded Plates

Jumaat and Alam (55) conducted a research about the problems associated with

bonding methods of strengthening reinforced concrete beams. They stated that the

adhesively bonded plates are highly susceptible to premature deboning. They also

mentioned that the problem of preventing premature de-bonding of adhesively bonded

plates is an extremely complicated problem. research has shown that there are three

mechanisms of de-bonding which will be referred to as 1. Flexural peeling: when moment (curvature) is applied to a plated beam the plate

tries to stay straight including cracks in at the plate end which propagate inward.

2. Shear peeling: the sliding or rotation of the critical diagonal crack causes the

debonding crack to start at the base of the diagonal crack outward the plate.

3. Axial peeling: Axial peeling occurs when a plate spans across a flexural or shear

crack, where it can be seen that wherever a flexural crack touches the plate a

debonding crack along the edge of the plate occurs. If debonding did not occur,

where the plate crossed the crack, the plate would, in theory, be subjected to an

infinite strain, which of course cannot occur.

Ali et al. (56), mentioned similar to this statement and produced rules for peeling

resistance. These three mechanisms appear in several forms such as shown in (

Figure 3. 14) (53).

Figure 3. 14: Modes of failure of steel plate (57)

The reduction in strengthening effectiveness is associated with a change of failure mode

from crack-induced debonding to plate end failure (2). Plate end failure (premature

43

failure) is an extremely important problem because invariably de-bonding of adhesive

joints is a brittle and catastrophic failure mechanism (55). Many researchers study the

solution of this problem and produce some limits which can help in avoidance of end

plate debonding. Some of these limits can be like follwong:

a) End plate anchorage: Ignoul et al. stated that if the force capacity of the

connection is insufficient, an additional mechanical anchorage, such as bolts or

external stirrups, has to be provided (53).

b) Effect of Member Size: Swamy et al. (58) suggested two tentative design criteria

for plated beams to ensure their full flexural capacity and ductility at failure: the

first is the plate-width-to-thickness ratio should not be less than 50; further, the

neutral axis depth should not be greater than 0.4 times the effective depth.

a) Cut-off Distance: Jones et al. (59) studied the effect of the plate cut-off distance

(from the plate edge to the support) on premature debonding and stated that for

practical values of cut-off distance a considerable increase in the average bond

stress will exist in the region of the plate end. As the cut-off distance increases,

significant reduction in strengthening effectiveness can be observed, with the

failure mode changing from crack-induced-bonding to plate end failure so when

the cut-off distance is sufficiently small (i.e. the plate end is sufficiently close to

the support), the failure load and maximum plate strain are essentially

independent of cut-off distance (2). Adhikary et al. (60) tested beams strengthened

by long plates (short cut-off distances) and short plates(longer cut-off distances)

and concluded that the failure mode changes from plate yielding to debonding if

the plate is short and cut at greater distance from the support. The possibility of

debonding failure can be minimized if the plate is cut at the location very close to

the support

b) Effect of steel plate stiffness: As the plate becomes stiffer, the plate

concentration of end stresses becomes more severe (2). The value of EI of steel

plate can be reduced by reducing the plate thickness and then increasing the width

to maintain the area of plate.

3.5.1.2 Bolted Plates

The problems which may arise in the case of using bolts or dowels in bonding steel plates

to concrete can be like following (41):

1. As with stud shear connectors in composite steel and beams, bolt shear connectors

can fail by fracturing due to excessive slip, which can be easily prevented by

designing for full shear connection.

2. Placing the side plate partly in the compression region can increase the ductility of

the beam but the plate is then susceptible to buckling. A simple solution to

prevent plate buckling is by restricting the plate to the tension zone of the section.

3. The bolts shear connectors apply a concentrated load to the reinforced concrete

beam, which may cause split for the concrete.

3.5.2 Stiffness Limits of Steel Plate

Barnes et. al. (30) stated that it is important to realize that the maximum enhancement

obtained by retrofitting shear plates is limited by the anchorage capacity of the

44

connection. The required capacity of the connection is governed by the plate stiffness;

increasing the plate thickness increases the connection requirement. By applying strain

compatibility it can be seen that increasing the thickness of the plate increases the total

force carried by the plate at any given load. Therefore, when insufficient anchorage is

provided, increasing the plate thickness can reduce the beam load at which the failure of

connection will occur. In addition, the interaction of the plate depends on the stiffness of

the adhesive layer, this need to be evaluated to understand possible changes in the force

distributions along the critical section (30). Gomes and Appleton (11) stated that when

the connection is only guaranteed by the resin a steel plate with a maximum 5mm thick

and 200mm width is recommended. The resin thick must be got between 1 and 3mm. A

higher resin thick leads to a lower bond capacity. In addition, they recommend limits for

steel plate dimensions as shown in Figure 3. 15 and Figure 3. 16 (11).

Figure 3. 15: Flexural strengthening- recommended dimension limits (11)

Figure 3. 16: Shear strengthening- recommended dimension limits (11)

3.5.3 Section Strength of Tension Face Plated Beam (Flexural Strengthening)

Adhesively bonding steel plate at the tension side of RC beams increase the flexural

capacity and stiffness. In addition, Ali et. al. (29) stated that the presence of tension

plates delays the formation and resist the propagation of a diagonal crack at a given

location and constructed a formula of shear load on the steel plate which caused by

diagonal cracking (29). Adhesively bonding plate to tension face of RC beam can

produces a plated beam with full interaction and, hence, the composite plated beam can

be analyzed or designed using all the conventional procedures that are available for RC

a)Without metallic anchors b)With metallic anchors

a) Without metallic anchors b) With metallic anchors

45

structure (55). However, there are a part at the end of the plate the elastic theory don’t

apply (59), so some consideration should be done to prevent premature de-bonding

especially in the plate end as discussed above.

Gomes et al. stated that two different models can be used to evaluate the section

strength for adhesively bonded steel plates (11):

1. Modeling of damage in the initial section and the additional materials behavior,

including the bond between the new and old material (Figure 3. 17).

Figure 3. 17: Strain and stress for deformed section (11)

2. Simplified method based on the monolithic action of the system components. The

strengthened beam is designed as a new structure, assuming that there are no

damage and the connection between the materials is perfect (Figure 3. 18).

Figure 3. 18: Strain and stress for intact section (steel plated) (11)

Bolted steel plate at the tension face can provide ductile behavior at the ultimate limit

state but it cannot provide stiffness as well as adhesively bonded plates. However,

Bonding plates only by dowels or bolts does not produce complete and uniform bonding,

so in this case the strengthened beam has very little chance to behave according the

flexural theory. Goldar et al. (31) examined beams strengthened by bolted steel plates of

several thickness and found that the ultimate load of these beam less than the theoretical

values by 10-20% and referred this to two reasons, bolt failure and slippage at the

interface (31). This indicate that there are doubts that these beams follow the flexural

theory. For all these reasons, the technique of bonding steel plates at the tension face of

46

RC beams can perfectly accomplished by chemical adhesive bonding with mechanical

anchorage at the end of plate.

Referring to the aforementioned knowledge about mechanical or chemical bonding of

steel plate important fact can be concluded; that the combination between the two

techniques can ensure the monolithic behavior of the steel plated RC beams. There are

additional two things ensure the monolithic behavior:

Reducing the cut-off distance of the plate.

Using thin and wide plate.

If these considerations are taken into account and monolithic action is assumed,

flexural theory can be applied and then the following equations can be generated for

tension face steel plated beams:

a) Flexural Strength

) +

)

C =

b) Shear Strength

Vn = (Vc1 + Vs1)

Vc = √

bwdav

, Av = (Av1 + Av2)

3.5.4 Section Strength of beam Strengthen by Straps of Steel Plates at the

Sides (shear strengthening)

Shear capacity can be increased by attaching straps of steel plates as shown in Figure 3.

19.

Figure 3. 19: Shear strengthened beams by bonded straps

47

If good bonding is guaranteed the conventional provisions can be used:

∅Vn = ∅(VC + Vs + Vs2)

Altin et al. (35) used different model to calculate the contribution of steel straps in shear

capacity:

3.5.5 Section Strength of Beam Strengthen by Continuous Steel Plates at the

Beam Web (Shear Strengthening)

This type of sections has a very low potential for the applicability of flexural

theory, so that for this position and orientation of steel plate the difference in stiffness is

significant and consequently incompatible strain (Figure 3. 20 and Figure 3. 21 ). This is

ensued in the case of bonding using bolts such that the bonding not uniform and buckling

of steel plate can be occur. However, Barnes et. al. (30) , mentioned that for steel plated

beams (at beam sides), the flexural capacity can be calculated using strain compatibility

(plastic analysis) if full interaction of the plate is assumed within the mid-span and the

normal assumptions for elementary beam theory are applicable. While for shear capacity,

they stated that it can be calculated using the equilibrium of forces method (30).

48

Figure 3. 20: Difference in curvature in bolted steel plate at beam sides (41)

Figure 3. 21: Difference in curvature in adhesively bonded steel plate at beam sides (41)

More than one researcher have been tried to construct models simulate the

contribution of continues steel plate in the shear strength of the side steel plated beams

(29; 30; 61). The ultimate shear strength of reinforced concrete beams with web-bonded

continuous horizontal steel plates is computed by adding the contributions from concrete,

internal shear reinforcement and the external steel plates (34). Nominal shear strength

(Vn) of plate-bonded beam is given as follows:

Vn = VC + Vs + VP

VC and Vs can be calculated using the available codes, while for VP the researchers use

Barnes et al. (61) constructed a model for side steel plate contribution in shear

strength of RC beams. The basic principle is that the as the diagonal crack form begin to

transfer the transverse tensile stress to the plate then an effective length of plate resist

these stresses. Figure 3. 22 show the transverse tensile stresses and Figure 3. 23 show the

modified stress distribution.

49

Figure 3. 22: Plated concrete beam under

shear loading (11)

Figure 3. 23: Further modified stress

distribution in plated beam (11)

The researchers proposed the following model and for design:

1. The contribution of steel plate in shear capacity Vp:

2. The area of both side plates Ap:

The researchers stated that analysis of the results from the beam tests has led to the

proposal of the following parameters:

BHAGAT et. al. (30) stated that the expression for shear contribution of web-

bonded continuous steel plates to the shear strength of beam (VP) is given by summing up

the shear stresses in steel plates over its depth and thickness:

La

51

VP =

fyp hp tp Where,

fyp: Yield strength of steel plate,

hp: Depth of the steel plate,

tp: Thickness of steel plate

Barnes et. al. (30), proposed a method of analysis for beams strengthened by

adhesive bonded and bolted steel plates at the beam webs, this method based on the

equilibrium of forces along the critical section. The forces which keep the critical section

in equilibrium are shown in Figure 3. 24.

Figure 3. 24: Equilibrium section of side steel plated beams (30)

After applying equilibrium to the critical section shown in Figure 3. 24, the

researchers generated the following equations (followed by equation numbers according

to the original article) (Figure 3. 25), the notations related to the model are shown in

Figure 3. 26.

Figure 3. 25: Barnes model (30)

51

This model had been examined by the researchers through excremental study on nine

RC beams strengthened by steel plate attached at beam sides using two methods,

adhesive bonding and bolting. They use 16mm bolts and tow part epoxy resin. Figure 3.

27 show the reinforcement details of the original beam. Table 3. 1, summarize beams

details, material properties, experimental and theoretical results. Figure 3. 28 show

deflection at load point for samples. The researchers found that the results of the

proposed model showed a good correlation with the tests results and concluded that the

model is conservative because the average of the theoretical values less than the

experimental values as shown in table. They assured that these results guaranteed only in

the case of good execution of bonding and good arrangement of bolts.

Figure 3. 27: Beam reinforcement details. (30)

Figure 3. 26: Notations of Barnes model (30)

52

Table 3. 1: Experimental and theoretical results (30)

Figure 3. 28: Deflection at load point of tested beams (30)

53

3.6 Bonding Design

The key in design of an effective strengthening solution using concrete jacketing or

externally bonded plates is the bonding strength, and substantial research has been carried

out on this issue. In the strengthened section, the stresses will be transferred from part to

anther of the section through the bonding system, which should be sustainable.

There are different approaches for calculation the shear stress in the interface

between the new and the old layer according to the position and the type of additional

layer as following:

1. In REHABCON, ANNEX I "strengthening with reinforced concrete" (14) stated

that in the case of attaching layers at the tension face. The shear flow may be

estimated as large as the increment of the tension force in the bars included in the

repaired part of the section (see Figure 3. 29).

Shear stresses = 2(As Fy)/ (bonding area)

2. In the case of using steel plates, Gomes el al. used shear stress equal to the

stresses generated in the attached steel plate (see Figure 3. 30).

3. In the plated beams, the stress distribution at a region in the end of the plate differ

about the stress in the rest area of the plate. This region should be exposed to

special treatment as discussed before.

4. In REHABCON, ANNEXI " strengthening with reinforced concrete" (14) It is

sugeted a method to calculate the mean longitudinal sliding shear per unit length

in the case of repair at the compression side can e.g. be found in the european

standard ENV 1992-1-1:1991 (eurocode 2, chapter 4.3.2.5).

5. In the case of attaching the new layer at the sides of the beam the stresses equal

the applied shear divided to the bonding area.

Shear stresses = (Vu)/ (bonding area)

Asfy (for flexural bar of repaired beam)

Asfy (for the additional plate)

Figure 3. 29: Tension force in the bars

Figure 3. 30: Tension force in the steel plate

54

3.6.1. Mechanical Anchorage to Concrete

Anchorage to concrete using shear connectors is well-known and included in more than

one of the conventional codes for example in ACI: (chapter 17: composite concrete

flexural members), (appendix D: anchoring to concrete) and (section 12.13: Development

of web reinforcement).

3.6.2 Adhesively Bonding to Concrete.

To guarantee that the failure will not occur in the adhesive layer the bond strength of the

adhesive must exceeds the permissible shear stress in the concrete. If the sliding shear

exceeds the shear bond dowels has to be used (14). In general, the shear stress in the

interface should be less than the tensile strength of concrete (fct). The anchorage capacity of

the adhesive bonded plate is generally limited by the tensile capacity of the concrete at the

adhesive/concrete interface (30). Gomes et al. (11) presented equations for checking the stress

in the adhesive layer as following:

a. Connection without bolts:

b. Connection with bolts: = * ≤

+ n ,

= min (fct, 2 MPa)

This model consider that the bond is effective at the whole area of the steel plate

while many researchers tried to prove that the fact somewhat different. Chen et al. (62)

stated that a very important aspect of bond behavior is that there exists an effective bond

length beyond which an extension of the bond length cannot increase the bond strength.

This is a fundamental difference between the anchorage design of an externally bonded

plate and an internal reinforcement for which a sufficiently long anchorage length can

always be found, so that the full tensile strength of the reinforcement can be achieved.

Many researchers conduct experimental and analytical studies to examine the anchorage

failure mechanism and construct models for shear stress at this region and effective

anchorage length (29; 30; 63; 61).

Gemert (64) examined the stresses in steel plates bonded to a rectangular plain

concrete prism in a double shear test. The tensile force in the steel plate was found to

decay exponentially toward the anchored end of the plate. At higher loads, the

distribution of the tensile force became more and more even in the initial bond zone but

the shear stresses in the adhesive layer not equivalent with the tensile forces. This means

that practically no force was transferred from the plate to the concrete in this zone,

because the cracking of the concrete near the applied load shifted the active bond zone to

new areas farther away from the loading point. Chen and Teng (62) stated that the shift

of the active bond zone means that at any one time, only part of the bond is effective.

That is, as cracking in the concrete propagates, bond resistance is gradually lost in the

zone near the load, but in the meantime it is activated farther away from the load. The

implication is, then, that the anchorage strength cannot always increase with an increase

in the bond length, and that the ultimate tensile strength of a plate may never be reached,

however long the bond length is. This leads to the important concept of effective bond

length, beyond which any increase in the bond length cannot increase the anchorage

strength, as confirmed by many experimental studies (62). Ming and Ansari (65) derived

a simple and rational model as following (see Figure 3. 31):

55

Figure 3. 31: Shear stress at the bonded area (65)

In which, Pu is the ultimate load, Ld is the development length, bp is the width of

the FRP fabrics. c is the shear strength of concrete specimen, which is believed to be

equal to 15% of the compressive strength of concrete, fc.

Chen and Teng (62) reviewed the current anchorage strength models for both

FRP-to-concrete and steel-to-concrete bonded joints under shear. The researchers

assessed these models with experimental data collected from the literature in order to

specify the deficiencies of all existing models. The authors proposed a new model based

on an existing fracture mechanics analysis and experimental observations. The

researchers stated that the new model is suitable for practical application in the design of

FRP-to-concrete as well as steel-to-concrete bonded joints. In addition, the researchers

compared their model to models of previous research and found that the new model is

more realistic as shown in Table 3. 2 They use the same data collected from the literature

for comparison. The new model can be summarized as following:

Where:

bc = width of concrete member;

bp = width of bonded plate;

L = bond length;

Le = effective bond length;

Ep = Young’s modulus of bonded plate;

tp = thickness of bonded plate;

56

Table 3. 2: Measured to Predicted Bond Strength Ratios(Chen et al. (2001))

The researchers stated that the existing test data suggest that the main failure mode is

concrete failure under shear, occurring generally at a few millimeters from the concrete-

to-adhesive surface. The bond strength, therefore, depends strongly on the concrete

strength. In addition, the plate-to-concrete member width ratio has a significant effect.

Thin stiff plates should be used to make the best use of the tensile strength of the bonded

plate. However, these models for direct tension (pure shear in the interaction) and does

not consider the interaction of shear and bending.

Note: (16) refer to the new model

57

3.7 Concluded Remarks

After reviewing the literatures related to concrete jacketing and steel plating of RC beams

in chapter 2 and chapter 3, the following points can be remarked:

1. Referring to the aforementioned reviewing about using concrete jackets or steel

plates and about using mechanical or chemical bonding it is noticed that, in the

literature, there are not a comprehensive study focusing on comparison between

the behavior of beams strengthened by concrete jackets and steel plates using

mechanical and chemical bonding. This comparison presentation is needed for

engineers to help them in choose the optimum system of RC beam strengthening.

2. For active the utilization of all the materials of composite beam and consequently

obtaining monolithic behavior, the following items should be realized:

Executing bonding and anchorage with satisfied level of quality and

ensuring all practical procedures which prevent separation and plate end

peeling.

Good design for shear stresses between the old and the new surfaces.

Good selection of chemical adhesive type.

Designing the strengthened section to be tension controlled section.

3. It can be said that the concrete jacketed sections can be designed as monolithic

beam by available codes.

4. It can be said that it is enough to bond only the region of the inclined paths of

CFP in the case of concrete jacketed beams.

5. There are doubts about if the steel plated beams comply the flexural theory. Many

researchers tried to study their behavior and construct models for design their

sections. However, beams strengthened by steel plate at the tension face have

more potential to comply the flexural theory especially if:

The end of the plates good anchored.

The cut-off distance decreased.

Stiffness compatibility achieved.

6. Decreasing the cut-off distance (the distance between the end of steel plate and

the support) may lessen the chance of plate debonding.

7. The stiffness incompatibility can be overtaken by using lesser thickness and larger

widths of the steel plate section (plate-width-to thickness ratio at least equal 50).

58

CHAPTER 4 TEST PROGRAM

4.1 Introduction

To reach the proposed objectives of this thesis, which stated in the first chapter, an

experimental test program was conducted. The main objectives of this test program

presented in Figure 4. 1 :

Figure 4. 1: The objectives of the test program

To achieve these objectives, an experimental test program consisted of designing,

constructing, strengthening and testing for flexural and shear of twenty-six reinforced

concrete beams was carried out. The test program consist of two series; the first for

flexural strengthening while the second for shear strengthening. Figure 4. 4 and Table 4.

1 show the sample distribution and description.

4.2 Definition of Samples Sections

4.2.1 Original sample:

The original sample section is 150 X 150 in dimension, and provided with flexural and

shear reinforcement as in Figure 4.2 and Figure 4.3.

1 • Conducting theoritical study.

2 • Preparing the original samples.

3 • Aplying the strengthing to the old section.

4 • Testing the strengthened samples.

5 • Taking data(failure load, load defliction, load of first crack, crack width).

6 • Analysing and discusing the test results.

7 • Making comparasion between theorical and expermental results.

8 • Making comparasion between strengthening methods.

9 • Presenting conclusions and recommendations for practice and research filed.

∅6@50

150 mm

150 mm

2 ɸ 6

2 ɸ 10

150mm

150 mm

2 ɸ 6

2 ɸ 10

Figure 4. 3: Original for shear samples Figure 4. 2: Original for flexure samples

59

original Specimens

26 Specimens

increasing flexural strength

13 Specimen

Beam control

3 Specimens

Monolithic section

1 Specimens

Adding Concrete Jacket

5 Specimens

Mechanical connectors

3 Specimens

Chemical adhesive

2 Specimens

Adding Steel Plate

4 Specimens

Mechanical connectors

2 Specimens

chemical adhesive

2 specimens

increasing shear strength

13 Specimen

Control Beam

2 Specimens

Adding Concrete Jacket

6 Specimens

Mechanical +Stirrups

2 Specimens

Chemical No Stirrups

2 Specimens

No additional connection

+Stirrups 1 Specimen

Surface roughening with some addhesive+Stirrups

1 Specimens

Steel plate

5 Specimens

Tow side steel plates Mechanicaly connected

2 Specimens

Tow side steel plates chemically connected

2 Specimens

External steel plate straps chemically connected

1 Specimens

Figure 4. 4: Samples distribution

61

Table 4. 1: Sample description

Connection

Type

Strengthening

Type

Sample No

First series: Flexural strengthening

----- No Strengthening CF0, CF1,

CF2

1.

Monolithic

section

----- MF 2.

Dowels Concrete Jacket/Tension face AF0,AF1,

AF2

3.

Adhesive Concrete Jacket/Tension face BF1, BF2 4.

Dowels Steel Plate/Tension face AF3, AF4 5.

Adhesive Steel Plate/Tension face BF3, BF4 6.

Second series: Shear strengthening

------ No Strengthening (without stirrups) CS1 7.

------ No Strengthening (with stirrups @5cm) CS2 8.

Dowels Bottom U layer (with additional stirrups) AS1, AS2 9.

Adhesive Top ∩ layer (without additional stirrups) BS1, BS2 10.

Partial

adhesive and

friction

Bottom U layer with additional stirrups ES1 11.

No connection Bottom U layer with additional stirrups BS3 12.

Dowels Steel plate at the two sides of the beam AS3, AS4 13.

Adhesive Steel plate at the two sides of the beam BS4, BS5

14.

Adhesive

partially

External steel plate straps BS6 15.

61

4.2.2 First series: Flexural Strengthening

This series contained jacketing the original beam by 100mm of RC using mechanical (Figure 4.

5) and chemical (Figure 4. 6) bonding and plating by 3mm thick steel plates using mechanical

(Figure 4. 7) and chemical bonding (Figure 4. 8).

2 ɸ 10 100mm

2 ɸ 10

2 ɸ 6

150 mm

150 mm

2 ɸ 10 100mm

2 ɸ 10

2 ɸ 6

150 mm

150 mm ∅6@50

∅6@50

100 mm

150 mm

2 ɸ 10

2 ɸ 6

100 mm

150 mm

2 ɸ 10

2 ɸ 6

∅6@50

Figure 4. 6: Jacketing for flexure using

adhesives (BF1,BF2)

Figure 4. 5: Jacketing for flexure using

shear connectors (AF1, AF2, AF0)

Figure 4. 7: Steel plating for flexure using

shear connectors (AF3,AF4)

Figure 4. 8: Steel platng for flexure using

adhesives (BF3,BF4)

Steel plate, width =120mm,

Thick. = 3mm, length = 1050mm



62

4.2.3 Second Series: Shear Strengthening

This series contain jacketing by concrete (Figure 4. 9), attaching steel plates at both sides of the

beam Figure 4. 10 and chemically bonded steel straps Figure 4. 11

a) Bonding by connectors (AS3,AS4)

2 ɸ 6

2 ɸ 10

150 mm

150 mm

b) Bonding by adhesive (BS4,BS5)

150 mm

150 mm

2 ɸ 6

2 ɸ 12

Concrete jacketing samples

b) Stirrups only (ES1,BS3)

`

150 mm

150 mm

2 ɸ 6

2 ɸ 10

3 ɸ 6

150 mm

150 mm

a) Bonding by connectors (AS1,AS2)

50mm 50mm 250 mm

200 mm

2 ɸ 6

2 ɸ 10

c) Bonding by adhesive (BS1,BS2)

Figure 4. 9: strengthening for shear by Concrete jacketing

Figure 4. 10: Strengthening for shear by side steel plates

Figure 4. 11: Strengtheing for shear by steel straps (BS6)

150 mm

150 mm

Steel straps, width =20mm,

Thick. = 2mm each 45mm





2 ɸ 10

3 ɸ 6

63

4.3 Experimental Work

This section present the procedures of samples construction to be ready for testing

4.3.1 First Series: Flexure Samples:

4.3.1.1 Original Specimens: 13 specimens as detailed in test program

1. Preparing the reinforcement cage as following (Figure 4. 12):

Figure 4. 12: Steel cage of the original sample of the first series

2. Preparing forms of 150 X 150 X 1250 mm, in dimension.

3. Adjusting the steel cage inside the form and then casting the ready mixed concrete.

4. Curing the specimens after 24 hours.

5. Three samples tested as control beams and the others were strengthened.

4.3.1.2 Addition of Mechanically Connected RC jacket at the Tension Side (AF0,

AF1and AF2).

1. Preparing the reinforce cage as following (Figure 4. 13):

Figure 4. 13: Steel cage of the underlay

2. Preparing holes (10cm in depth and 10mm in diameter) in the tension side of the original

beam (Figure 4. 14):

∅6@50

2 ɸ 6

2 ɸ 10

110 mm

110 mm

∅6@5

2 ɸ 6

2 ɸ 10

110 mm

6 mm

Figure 4. 14: Drilling holes in the tension side in the original beam

64

3. Sticking the shear connector onto the holes using Sikadure31-CF, but using EPICHOR

1768 fo the sample AF0 (shear connector = 20cm: 10cm through the old concrete and

10cm each 8cm through the new concrete) (see Figure 4 .15).

Figure 4 .15: Sticking shear connectors

4. Setting the new steel cage and adjusting the system inside the form and then casting the

ready mixed concrete (see Figure 4. 16)

Figure 4. 16: Setting the new steel cage

5. Testing the specimens and recording the results.

4.3.1.3 Addition of Chemically Bonded RC jacket at the Tension Side (BF1, BF2).

1. Preparing of the reinforcement cage as in previous samples.

2. Painting Sikadure-32, in the tension side of the original side just before casting.

3. Setting the new steel cage an adjusting the system inside the form and then casting the

ready mixed concrete.

4.3.1.4 Addition of Mechanically Connected Steel Plate to the Tension Side (AF3,

AF4):

1. Preparing of the steel plates with dimensions of 12.5 X 1050 X 3 mm.

2. Drilling holes on tension side of the beam each 8.5cm center-center within the area of

steel plate (12 pair of holes).

3. Taking marks on the steel plate and then drilling at the same positions of beam holes

(Figure 4. 17).

Figure 4. 17: Drilling holes in steel plate

65

4. Sticking the shear connector onto the holes using Sikadure31-CF, (15cm: 10cm through

the old concrete and 5cm welded in the other connector and steel plate.

5. Fixing the steel plate at its place and pressing it to the concrete (Figure 4. 18).

Figure 4. 18: Fixing steel plate

6. Bending the connectors and welding them to each other and to the steel plate.

7. Testing the sample.

4.3.1.5 Addition of Chemically Bonded Steel Plate to the Tension Side (BF3, BF4):

1. Preparing of the steel plates with the dimensions of 1050 X 105 X 3 mm.

2. Preparing of the concrete and steel plate surfaces (roughening and brushing but not to

standards level).

3. Mixing and plastering the adhesive (Sikadure31-CF) onto the steel plate.

4. Laying the steel plate at the pre-marked place at the tension side of the beam.

5. Testing the samples.

66

4.3.2 Second Series: Shear Examination:

4.3.2.1 Original Specimens: 13 Specimens as Detailed in Test Program

1. Preparing of the reinforce cage as shown in (Figure 4. 19), except one beam reinforced

like the beams in the first series:

Figure 4. 19: Steel cage of the original sample of the second series

2. Preparing forms have dimensions of 150 X 150 X 1250 mm.

3. Adjusting the steel cage inside the form and then casting the ready mixed concrete.

4. Curing the specimens after 24 hours.

5. Two samples tested as control beams and the others were strengthened.

4.3.2.2 Addition of Mechanically Connected U Shape Concrete Jacket with

Additional Steel Cage (AS1, AS2)

1. Preparing of the stirrups and shear connectors as shown in Figure 4. 20.

Figure 4. 20: Detailing of the new stirrups and connectors

2. Drilling holes and fixing the connectors and the new stirrups as shown in Figure 4. 21:

Stirrups: 2 pieces

Adhesive

Welding

Connectors: 2 pieces

25cm: 15cm in the old concrete

20cm: 10cm in the old concrete

The tension side 11 hole/1 hole each 9cm, for connectors

12 hole/1 hole each 9cm, for connectors

12 hole/1 hole each 9cm, for stirrups

Figure 4. 21: Holes distribution

67

3. Fixing the connectors and the new stirrups using Sikadure31-CF (Figure 4.22) :

Figure 4. 22: Fixing the connectors and the new stirrups

4. Adjusting the system inside the form and then casting the ready mixed concrete.


4.3.2.3 Addition of U Shape Reinforced Concrete Jacket to Roughened surface

and Partially Painted with Chemical (ES1)

1. Preparing of the stirrups and shear connectors (Figure 4. 23).

2. Drilling holes and fixing the connectors and the new stirrups as the following (Figure 4.

24):

3. Fixing the new stirrups using Sikadure31-CF and then longitudinal reinforcement

(Figure 4. 25):

12 hole/1 hole each 9cm, for stirrups

Additional stirrups: 2 pieces

Bonded by epoxy

Welding

Figure 4. 23: The new stirrups detailing

Figure 4. 24: Holes distribution of ES1

The tension side

68

Figure 4. 25: Fixing stirrups and longitudinal reinforcement

4. Roughening and partially painting by Sikadur-32, of concrete surface.

5. Adjusting the beam inside the form and then casting the ready mixed concrete.


4.3.2.4 Addition of U Shape Reinforced Concrete Jacket without Additional

Connection (BS3).

The procedures of construction here similar to the previous samples but without any aditional

bonding or connection (only the connection from stirrups)

4.3.2.5 Addition of Chemically Bonded shape Plain Concrete Jacket (BS1, BS2).

1. Roughening the concrete surface (but not to standards)

2. Full-Painting the top face and the two sides of the beam with Sikadur-32.

3. Adjusting the beam inside the form and then casting the ready mixed concrete.


4.3.2.6 Addition of Mechanically Connected Steel Plate to the beam Sides (AS3,

AS4).

1. Preparing two steel plates 120 X 1000 X 2 mm in dimension for each beam.

2. Drilling holes in concrete and steel plate.

3. Sticking shear connectors into beam using Sikadur31-CF (20cm: 15cm through the

beam section and 5cm bended and welded to the plate at both the sides of beam 9cm

center-center(11 pair of connectors)).

4. Welding shear connectors to each other and to the steel plate.

5. Testing.

4.3.2.7 Addition of Chemically Bonded Steel Plate to the beam Sides (BS3, BS4).

1. Preparing two steel plates 100 X 1000 X 2 mm in dimension for each beam.

2. Roughening the surfaces of steel plate and concrete.

3. Plastering the steel plate by the paste of Sikadur3-CF.

4. Positioning the steel plates on the suitable place on the two sides of the beams.

5. Pressing on the plate by weights until drying of the adhesive.

6. Testing after at least days.

The tension side

69

4.3.2.8 Addition of External Straps of Steel Plate Chemically Bonded around the

beam outer-face (BS6)

1. Preparing stirrups from 20mm width steel plates as shown in (Figure 4. 26):

2. Sticking the steel straps each 4.5 cm center to center by the paste of Sikadure31-CF.

3. Welding and bonding the pieces of stirrups (see Figure 4. 27).

4.4 Material Properties

Following sections contains the properties values of the used materials (concrete, steel bars,

steel plates and chemical adhesives).

4.4.1 Concrete

Table 4. 2 and

Table 4. 3 show the compressive strength for the first mix (average = 33.46 Mpa) and the

second mix (average = 24.2 MPa) respectively. Table 4. 4 shows the samples with the related

compressive strength.

Table 4. 2: Compressive strength for the first mix

fc'

MPa

Fcu

MPa

Compression

force KN

Unit

weight

H

mm

W

mm

L

mm

No

33.68 42.1 425 3.39 100 101 103 1.

30.48 38.1 385 2.28 101 100 101 2.

35.12 43.9 443 2.31 100 101 102 3.

34.56 43.2 432 2.255 100 100 102 4.

30.64 38.3 402 2.455 105 100 102 5.

15

cm 7.5 cm

8 cm

Welding

Adhesive

Figure 4. 26: Parts of steel strap

Figure 4. 27: Welding and bonding steel straps

71

33.36 41.7 421 2.27 100 101 102 6.

33.46

Table 4. 3: Compressive strength for the second mix

fc'

MPa

Fcu

MPa

Compression

force KN

Unit

weight

H

mm

W

mm

L

mm

No

21.92 27.4 277 2.31 101 100 102 1.

23.84 29.8 304 2.27 102 100 102 2.

23.6 29.5 301 2.28 102 100 103 3.

27.44 34.3 350 2.15 100 102 102 4.

24.2

Table 4. 4: fc' for the original and additional concrete

Added layer Original Sample No.


1.

----- 11.32 CF1 2.

----- 63.6 CF2 3.

24.2 MF 4.

24.2 33.46 AF0, AF1, AF2 5.

24.2 33.46 BF1, BF2 6.

----- 24.2 AF3, AF4 7.

----- 33.46 BF3, BF4 8.

Second series: Shear strengthening ------ 33.46 CS1 9.

------ 24.2 CS2 10.

24.2 ------ MS 11.

24.2 33.46 AS1, AS2 12.

24.2 33.46 BS1, BS2 13.

24.2 33.46 ES1 14.

24.2 33.46 BS3 15.

------ 24.2 AS3, AS4 16.

------ 33.46 BS4, BS5 17.

------ 33.46 BS6 18.

4.4.2 Steel Bars

The steel reinforcing bars used for the construction of the beams consisted of 6mm diameter

steel bar were used for both stirrups and secondary top reinforcement. In addition, 10 mm

diameter steel bar were used for main bottom reinforcement. Samples from the 10 mm

reinforcing bars were tested using the standard tension test, the results shown in Table 4. 5. The

average value of fy of the 10 mm bar is 380 MPa.

71

Table 4. 5: Yield and ultimate strength for steel bar (10mm)

Ultimate

force KN

Yielding

force KN

Section area

(mm)

No.

54 38 10 1

53 38 10 2

53.5 38 10 Average

4.4.3 Steel Plates

Table 4. 6 shows the data and the result of steel plate samples. The average value of fy of

the steel plate is 310 MPa.

Table 4. 6: Yield and ultimate strength for steel plate

Fu

MPa

Fy

MPa

Length after

failure

Ultimate

force KN

Yielding

force KN

Thickness

(mm)

Width

(mm)

Length

(mm)

420 312 110 62 46 2.95 50 80

420 320 109 61 47 2.95 49.8 80

400 299 111 61 46 3.05 50.4 80

410 310

4.4.4 Chemical Adhesives

4.4.4.1 EPICHOR 1768

"EPICHOR 1768" is two-component solvent free, clear epoxy product. It can be mixed with

graded sand to be used as fixer of dowels in concrete and repairing mortar. The properties of

"EPICHOR 1768" is presented in Table 4. 7. This material used only for connecting shear

connectors of sample AF0.

Table 4. 7: Properties of EPICHOR 1768

Property Value Density 2.1 gm / cm3 for mortar epoxy

Modulus of Elasticity 2320N/mm2

Compressive Strength 54.0 N/mm2 (After 7 days) ASTM (C 579

Method B) Flexural strength @ 7 days (BS 6319) 42 N/mm2

Tensile strength @ 7 days 2.90 N/mm2 (After 7 days) ASTM (C 301)

Bond strength for Mortar epoxy (Resin + Hardener + filling)

9.70 N / mm2

Bond strength For pure epoxy (Resin + Hardener)

6.7 N / mm2

72

4.4.4.2 Sikadur®-31 CF

Sikadur®-31 CF is an epoxy material used to bond steel to concrete, its properties presented in

Table 4. 8.

Table 4. 8: Properties of Sikadur®-31 CF

Property Value

Density 1.94 kg / litre approx.

Volume solids 100% (solvent free)

Modulus of Elasticity 2.0 GPa approx.

Compressive Strength 69 - 79 N/mm2

Flexural strength @ 7 days (BS 6319) 25-30 MPa approx.

Tensile strength @ 7 days (BS 6319) 14-24 MPa approx.

Adhesion to concrete >4 MPa approx. (concrete failure all grades)

Adhesion to sandblasted steel @ 3 days 13-17 MPa approx.

4.4.4.3 Sikadur®-32

Sikadur-32 is a high performance bonding agent based on a 2-component solvent free epoxy

resin ideally suited to a wide range of building and civil engineering applications. Table 4. 9

shows some of physical and mechanical properties of Sikadur®-32.

Table 4. 9: Properties of Sikadur®-32

Property Value

Density 1.4 kg / litre approx.

Volume solids 100% (solvent free)

Secant Flexural Modulus of Elasticity (BS

6319) @ 7 days 2.0 GPa approx.

Flexural strength @ 7 days (BS 6319) 28 MPa approx.

Tensile strength @ 7 days (BS 6319) 13 MPa approx.

Adhesion to concrete >3 MPa approx. (concrete failure all grades)

Adhesion to sandblasted steel @ 10 days 20 MPa approx.

73

4.5 Analysis of Samples

This section present the analysis of samples in order to know their shear and flexural capacities

and the ultimate loads according to the adopted loading system. Following are some items and

assumptions should be ensured:

1. The analysis done using the real values of material properties.

2. The dimensions and the area of reinforcement of the original beam are proposed (as

mentioned before) and then analyzed.

3. The dimensions and the area of reinforcement of the additional concrete jackets of shear and

flexural strengthened beams are also proposed (as cleared before) and then analyzed.

4. The beams with flexural strengthening by steel plates designed to have the same value of

flexural capacity of the concrete jacketed beams.

5. The monolithic sample designed to have the same value of flexural capacity of the

strengthened beams.

6. The flexural capacity of side steel plated beams cannot be calculated using the flexural

theory because of the mechanical incompatibility.

7. The sample strengthened by external straps designed using the normal procedures.

Assumptions:

1) The bonding is perfect.

2) The flexural theory is applicable in the section of strengthened beam.

3) There is high certainty in load and material so the load and reduction factors would not

be used in design.

4.5.1 First Series: Flexure Examination

The loading system and shear-moment diagram for the first series are shown in Figure 4. 28.

P KN 1100mm

2P

75mm 75mm

75mm

55

0

2

1100mm

55

0

75mm

P KN

550P KN.mm

Figure 4. 28: Loading system of the first series

74

4.5.1.1 Original Section

The section of the original section shown in Figure 4. 29:

1) Flexural Capacity(for fc'=33.46)= 6.68 KN.m, The applied load 2P = 24.30 KN

2) Flexural Capacity(for fc'=24.2) = 6.52 KN.m, The applied load 2P = 23.7 KN

4.5.1.2 Concrete Jacketed Section

The section of the jacketed beam shown in Figure 4. 30. The analysis is presented in the

following steps:

1) Section Details :

As1 = (2 ɸ 10) = 157 mm2

As2 = (2 ɸ 10) = 157 mm2

= 150 – 20 –6 – 5 = 119mm

d2 = 250 – 20 – 6 – 5 = 219 mm

dav = (d1+d2)/2 = (119+219)/2 = 169 mm

ρ

= 0.0124

2 ɸ 10 100m

m

Bonding by adhesive

2 ɸ 10

2 ɸ 6

150 mm

150 mm

2 ɸ 10 100m

m

Bonding by shear connector

2 ɸ 10

2 ɸ 6

150 mm

150 mm ∅6@50

∅6@50

150 mm

150 mm

2 ɸ 6

2 ɸ 10

Figure 4. 29: Original section fo flexural strengthening

Figure 4. 30: Jacketing at the tension face

75

2) Flexural Capacity :

) +

)

C =

=

= 32.9 mm

= 57 380 9

) + 57 380 2 9

)

= 18.50 KN.m

This value of flexural strength will be used to obtain the area of steel plate and the area of

steel of the monolithic beam for comparison.

3) The applied load 2P:

Mn = Mu (without using reduction factor)

Referring to figure 4.12 Mu= 550p = 18500 KN.mm so, p = 33.64 KN and 2P = 67.29 KN.

4) Shear capacity:

Vn = (Vc1 + Vs1)

Vc = √

bwdav

For Normal weight concrete, λ=1.0

Vc = √

150*169 = 24.44 KN < 12.06

=

= 72.59KN ∅6 @ 5cm)

Vn = 24.44 + 72.59 = 97 KN

5) Check of yielding:

=

= 0.0124

2 ɸ 10 80

2 ɸ 10

2 ɸ 6

100 mm

120 mm ∅6@50

0.005

0.003

NA

76

0 3 9

0 3 9 0 85

= 0.0239 > 0.0124 tension controlled section.

6) Bond Design

The force which should be transferred by the bonding system = the increment of tensile

force of flexural reinforcement of the strengthened beam = Asfy = 157*380 = 59660 N

a. Mechanical Bond (Shear Connectors)

According to (ACI: D.6.1.2) the nominal strength of a single anchor or group of anchors

in shear, Vsa, shall not exceed:

Vsa = n0.6As (dowel) X fut (dowel) (equation ACI: D-20)

According to (ACI: D.6.2.1) The nominal concrete breakout strength, Vcb , in shear of a

single anchor or group of anchors shall not exceed: For shear force perpendicular to the

edge on a single anchor equation (ACI: D-21):

From D.6.2.6 equation (D-28):

Ψed,V = 0.79

From D.6.2.7: ψc,V = 1.4

From D.6.2.8: ψh,V = 1

According to (ACI: D.6.2.2) The basic concrete breakout

strength in shear of a single anchor in cracked concrete,

Vb , shall not exceed:

le = hef for anchors with a constant stiffness over the full length of embedded section,

such as headed studs and post-installed anchors with one tubular shell over full length of

the embedment depth or, le ≤ 8da in all cases

According to (ACI: D.8.5) The value of hef for an expansion or undercut post-installed

anchor shall not exceed the greater of 2/3 of the member thickness and the member

thickness minus (4 in = 10.16 cm)

Le = hef =2/3 * 150mm = 100 mm

77

Le ≤ 8da = 8*6mm = 48mm

Vb = (0.66*(48/6)0.2

√6)*1* √33 46 * (100)1.5

= 14174.26N

Vcb = 7739.15 N

Vsa = 0.6*520*113 = 8817.12 N, the smaller will be used

Number of dowels = 59660/7739.15 = 7.7 for each side of the beam

26 dowels will be used for AF0, and 30 dowel for AF1 and AF2

b. Chemical Bond (Adhesive Layer)

Shear stress in the adhesive layer = (bonded area)/2Asfy

= 2* 59660 / (1250*150) = 0.64 MPa

Tensile strength of concrete = 2.4 MPa, ok

4.5.1.3 Steel Plated Section

The section of the steel plated beam shown in Figure 4. 31. The analysis is presented in the

following steps:


Steel plate thickness = 3mm.

= 150 – 20 –6 – 5 = 119mm

d2 = 150 + 1.5 = 151.5mm

dav = (119 + 151.5)/2 = 135.25

As2 = area of steel plate section which will be calculated in the next step.

2) Area of steel plate:

) +

)

∅6@50

100 mm

150 mm

2 ɸ 10

2 ɸ 6

Bonding by adhesive

100 mm

150 mm

2 ɸ 10

2 ɸ 6

Bonding by dowels

∅6@50

Figure 4. 31: Steel plates at the tension face

78

Mn = Mn of the jacked beam

C =

=

= 22.75 + 0.118

18500000 = 57 380 9

) + 3 0 50 75

)

Solving this equation using C = 22.75 + 0.118 we obtain:

– 3347.13 + 931767.85 = 0

Using √

,

For (fc' = 24.2) As2 = 335 mm2,

For (fc' = 33.46) As2 = 302 mm2

But actually it is used As2 = (125mm – 20mm "hole opening" = 105) width X 3 mm

thickness = 315 mm2

But actually it is used As2 = 105 width X 3 mm thickness = 315 mm2

For (fc' = 24.2):

C = 22.75 + 0.118A_s2= 22.75 + 0.118 3 5 = 59.92

57 380 9

) + 3 5 3 0 5 5

)= 17.9 KN. M

For (fc' = 33.46):

C = 16.45+ 0.085 = 16.45+ 0.085*315 = 43.23

57 380 9

) + 3 5 3 0 50 75

)= 18.9 KN. M



For (fc' = 24.2) Referring to figure 4.12 Mu= 550p = 17900, P = 32.55 KN, 2P = 65.1 KN

For (fc' = 33.46) Referring to figure 4.12 Mu= 550p = 18900, P = 34.36 KN, 2P = 68.73 KN

4) Shear Capacity:

Vn = (Vc1 + Vs1)

Vc = √

bwd

For Normal weight concrete, λ=1.0

Vc = √

150*151.5 = 21.9 KN

79

=

= 57.1 KN ∅6 @ 5cm)

Vn = 21.9 + 57.1 = 79 KN


As,av =

= 227.99

=

= 0.0112

Average fy =

= 333.28

0 3 9

0 3 9 0 85

= 0.0197 tension controlled section.

6) Bond Design

The force which should be transferred by the bonding system = the increment of tensile force

of flexural reinforcement of the strengthened beam = As*fy = 157*380 = 59660 N

a. Mechanical Bond (Shear Connectors)

Referring to section 4.4.1.2 item (6), 24 dowel was used

b. Chemical Bond (Adhesive Layer)

Shear stress in the adhesive layer = (bonded area)/2Asfy = 2*59660/ (1050*105) = 1.08

MPa

Tensile strength of concrete = 2.4 MPa, ok

4.5.1.4 Monolithic Section

The monolithic section of the flexural strengthened beam shown in Figure 4.32. (flexural

capacity = 18.5 KN)

1) Area of steel plate = (3ɸ 10 = 235.5 mm2), Mn = 18.3KN.m

2) The applied load 2P = 66.55 KN

2 ɸ 6

150 mm

250 mm

3ɸ 10

Figure 4. 32: Flectural monolithic section

81

4.5.2 Second Series: Shear Examination

The loading system and shear-moment diagram for the second series are shown in Figure 4.33.

4.5.2.1 Original Section

The original section for shear strengthening will be without stirrups but at the extremes and the

middle to hold the longitudinal reinforcement. The section of the original beam shown in Figure

4. 34

1) Flexural Capacity = 6.682 KN.m

2) The applied load 2P = 48.6 KN

150mm

150 mm

2 ɸ 6

2 ɸ 10

100mm

m

275m

m

P 2P

500mm

1050mm

P

275mm

100mm

2P

1050mm

500mm

100mm 100mm

275P KN.mm

P KN

P KN

Figure 4. 33: Loading system for second series

Figure 4. 34: The original section for shear

strengthening

81

4.5.2.2 Concrete Jacketed Section (U jacket).

The section of the jacketed beam shown in Figure 4. 35. The analysis is presented in the

following steps:


As1 = (2 ɸ 10) = 157 mm2

As1 = (3 ɸ 6) = 84.78 mm2

= 150 – 20 –6 – 5 = 119mm

d2 = 200 – 20 – 6 – 3 = 171 mm

dav = (119+171)/2 = 145 mm

ρ

= 0.0111

Fc' = (3*33.46+2*24.2)/5 = 29.76 MPa


) +

)

C =

=

= 17.1 mm

) +

)

= 57 380 9

) + 84 78 380 7

)

= 11.94 KN.m = 11940 KN.m

This value of flexural strength will be used to obtain the area of steel plate and the area of

steel of the monolithic beam for comparison.



b) Stirrups only

150 mm

150 mm

2 ɸ 6

2 ɸ 10

150 mm

150 mm

a) Bonding by connectors

51mm 51mm

Figure 4. 35: U Jacket for shear stengthening

82


4) Shear capacity:

∅Vn = ∅(Vc1 + Vc2 + Vs + Vs2)

(Vc1 + Vc2) = √

bwd =

√

250*171 = 39 KN

Vs1 = 0

=

= 40.81KN (∅6 @ 9cm)

Vn = 39 + 40.81 = 79.81 KN > 43.49 KN


=

= 0.0144

0 3 9

0 3 9 0 85


6) Bond design

The force which should be transferred by the bonding system = the increment of tensile force

of flexural reinforcement of the strengthened beam = As*fy = 157*380 = 59660 N

a. Mechanical bond (shear connectors)

The force which should transferred by the sides = (Vu + (

)* 2Asfy)

= 43530 + (

)* 2*157*380 = 123076.67N

Number of dowels = 123076.67/7739.15 = 15.9, 24 dowels was used at the sides beside

the connection available from bonded stirrups.

Stress at the bottom face = ( (

) * Asfy) = (

)* 2*157*380 = 39773.33 N

Number of dowels = 39773.33 /7739.15 = 5.1, 12 dowels was used at the bottom side.

4.5.2.3 Concrete Jacketed Section (∩ shape)

The section of the ∩ shape jacketed beam shown in Figure 4.36. The analysis is presented in

the following steps:

250 mm

200 mm

2 ɸ 6

2 ɸ 10

Figure 4. 36: ∩jacket for shear strengthening

83


As2 = (2 ɸ 10) = 157 mm2

= 200 – 20 –6 – 5 = 169mm

ρ

= 0.00929

Fc' = (33.46 + 24.2)/2 = 28.83MPa


)

C =

=

= 11.46

)

= 157*380(169 –

= 9.79 KN.m




4) Shear capacity:

∅Vn = ∅(Vc1 + Vc2 + Vs + Vs2)

(Vc1 + Vc2 ) = √

bwd =

√

250*169 = 38.57 KN

Vs1 = 0

Vn = 37.8 KN > 35.60 KN


=

= 0.0037

0 3 9

0 3 9 0 85


6) Bond design (chemical bond)

Compression force at the top = tension force at the bottom

The stress at the sides = (Vu + (

)* 2Asfy)/bonding area

= (35680 + (

)* 2*157*380)/ (2*150*1250) = 0.307MPa < fct ok

84

Stress at the top face = ((

) * Asfy) = (

)* 2*157*380/ (150*1250) = 0.2Mpa < fct ok

4.5.2.4 Steel Straps

The section of the beam strengthened by straps shown in Figure 4. 37. The analysis is presented

in the following steps.


As = (2 ɸ 10) = 157 mm2

(of steel straps) 2( 20mm X 2 mm) 80mm2 each 45 mm.

= 150 – 20 –6 – 5 = 119mm

ρ

= 0.0088


)

C =

=

= 17.76 mm

)

= 57 380 9

)

= 6.65 KN.m




4) Shear capacity:

(Vc) = √

bwd =

√

150*119 = 16.29 KN

150 mm

150 mm

Steel straps, width =20mm,

Thick. = 2mm

Figure 4. 37: Section of beam strengthened steel straps

85

Vs ( for steel plate) = 79.33-16.29 (Vc of the original section) = 63.04 KN

=

65 58

Vn = 16.29 + 65.58 = 81.87 KN


=

= 0.0088

0 3 9

0 3 9 0 85


4.5.2.5 Steel Plated Beam (Side Plates).

These beams assumed be not confirm with the flexural theory.

4.5.2.1 Monolithic Section (Simulation of Jacketed Section)

The monolithic section of the shear strengthened beam shown in Figure 4. 38.

1) Area of steel = (3ɸ 10 = 235.5 mm2), Mn = 13.8KN.m

2) The applied load 2P = 100.4KN

250 mm

200 mm

2 ɸ 6

3 ɸ 10

.

Figure 4. 38: Shear monolithic section

86

4.5.3 Summary of Theoretical Results

Table 4. 10 present the summary of the theoretical results of load carrying of the samples.

Table 4. 10: Summary of theoretical results

Theoretical load

capacity KN

Fc' Added layer Fc' Original Sample 19.

First series: Flexural strengthening 24.30 ----- 11.32 CF1 20.

23.72 ----- 63.6 CF2 21.

66.55 24.2 MF 22.

AF0 23.

67.29 24.2 33.46 AF1, AF2 24.

67.29 24.2 33.46 BF1, BF2 25.

65.1 ----- 24.2 AF3, AF4 26.

68.73 ----- 33.46 BF3, BF4 27.

Second series: Shear strengthening 48.6 ------ 33.46 CS1 28.

47.42 ------ 24.2 CS2 29.

100.4 24.2 ------ MS 30.

86.84 24.2 33.46 AS1, AS2 31.

21.2 24.2 33.46 BS1, BS2 32.

86.84 24.2 33.46 ES1 33.

86.84 24.2 33.46 BS3 34.

------ 24.2 AS3, AS4 35.

------ 33.46 BS4, BS5 36.

48.36 ------ 33.46 BS6 37.

87

CHAPTER 5 RESULTS AND DISCUSSION

5.1. Introduction

In the previous chapter, the detailing, designing and construction procedures of the samples are

presented clearly. After preparing the samples they have been exposed to loading testing using

the loading device of the laboratory of Islamic University of Gaza (IUG). During testing, data

and results were recorded for all the samples. In this chapter the results and their explanation are

presented. The chapter accomplished by comparative presentation of the results.

5.2. First series: Flexural samples

5.2.1 Control Beams:

CF0, (the control beam of the sample AF0) failed in flexure with ultimate load equal 40.05KN.

CF1, CF2 (the control beams of AF1 and AF2) failed in flexure and provided ultimate load of

36.423KN and 36.124KN respectively. Figure 5. 1 and Figure 5. 2 show the sample after testing

and the crack pattern for CF1, CF2 respectively. Figure 5. 3 show the load-deflection curves of

the two samples. Average deflection for the tow samples at SLS equal 5.63 mm while the

maximum deflection equal 24.5mm.

Figure 5. 2: Failure mode and Crack pattern of CF2

Figure 5. 1:Failure mode and crack pattern of CF1

88

5.2.2. Addition of Mechanically Connected RC Jacket to the Tension Side (AF0,

AF1, and AF2).

Figure 5. 4, Figure 5. 5 and Figure 5. 6 show the samples AF0, AF1 and AF2 after testing and

their crack pattern respectively.

The sample AF0 (see Figure 5. 4, and its related control beam shown in same figure) failed

in pure flexure without any separation between the old and the new sections until the ultimate

load. The value of ultimate load is 105.18KN, which equal 290% of the ultimate load of the

control beams. This result ensure that using procedures of anchorage and curing with good level

of quality can grantee the composite action of the beam strengthened by concrete layer at the

tension face. The load-deflection curve show that the beam failed in a ductile way.

The sample AF1 (Figure 5. 5) loaded to the load of 32.2 (48.3% of the theoretical value)

and unloaded accidentally (electricity). In the second loading, the sample failed at load 69KN

which equal 270% of the ultimate load of the control beams and equal 102.6% of the theoretical

value. The first crack was flexural and crossed the whole section without separation. The first

sign of separation was at the load of 62.1KN.

In the case of sample AF2 (Figure 5. 6) the first crack was flexural and crossed the whole

section without separation. The first sign of separation was at the load 69KN. The value of

Figure 5. 3: Load-deflection relationship of CF1

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30

Load

KN

Deflection mm

CF1

CF2

89

ultimate load is 78.361KN, which equal 216% of the ultimate load of the control beams. In AF1

and AF2, the separation occur after at least two monolithic flexural cracks crossed the system,

this mean that the dominant mode is flexural failure. The separation in this type of systems not

so critical such that the anchorage is deep not like in the case of using chemical adhesive. The

superiority of AF0 over the samples AF1 and AF2 may be for one or more of the following

reasons:

1. The samples AF1 and AF2 exposed to accidental unloading after significant loads.

2. The difference in the adhesive nature. The adhesive used for the first sample AF0

contain sand as a filler that may provide more friction and better interaction with the

concrete surface.

3. The number of holes in the last two samples more than the first, which mean less

distances between center-center.

4. The first sample exposed to perfect curing rather than the last two samples, this may has

an effect.

5. The additional concrete casted in the sample AF0 more workable than used in the

samples AF1 and AF2. The workability of the additional concrete is very important

especially in the case of congestion resulted from the new steel cage and shear

connectors.

The experimental value of the ultimate load of the perfect sample (AF0) is larger than the

theoretical by 157.3% and the worst case (AF2) the experimental is larger than the theoretical

by 116.2% this give indication that using the traditional procurers to predict the theoretical

value of the ultimate load for beam strengthened by this method is very conservative.

The deflection for AF0, AF1 and AF2 at the SLS equal 8mm, 3.2mm and 3.7mm

respectively while at ULS equal 30.8mm, 19.48mm and 11mm respectively. Figure 5. 7 show

the load-deflection relationship of the sample AF0 compared with the control beam. Figure 5. 8

show comparison between load-deflection curves of the samples AF1 and AF2 and the control

beam. The figures show enhancement in the deflection in SLS. The extra ductility of AF0 over

the other two samples may refer to that the concrete used in this sample has lower strength and

lower w/c ratio. Detailed analysis of deflection will be presented in sections 5.4.2 and 5.4.3.

91

Figure 5. 4: Failure mode and crack pattern of AF0 and CF0

Figure 5. 5: Failure mode and crack pattern of AF1

Figure 5. 6: Failure mode Crack pattern of AF2

AF0

CF0

91

Figure 5. 7: Load-deflection relationship of AF0 compaired with control beam CF0

Figure 5. 8: Load-deflection relationship of AF1 and AF2 compaired with control beam

CF2 and monolithicly casted beam MF

0

20

40

60

80

100

120

0 10 20 30 40

Load

KN

Deflection mm

CF0

AF0

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

Load

KN

Deflection mm

AF2

CF2

AF1

MF

92

5.2.3. Addition of Chemically Bonded RC Jacket to the Tension Side (BF1, BF2)

Figure 5. 9 and Figure 5. 10 show the samples BF1 and BF2 after testing and their crack pattern

respectively.

In the sample BF1 (Figure 5. 9) the first crack was flexural and crossed the whole section

without separation and was at load 50.6 KN. The first sign of separation was at the load

62.1KN. The value of ultimate load is 91.195KN, which equal 251.4% of the ultimate load of

the control beams.

In the case of sample BF2, the first crack was flexure (at 46KN). The flexural cracks were

growing up until they reached and stopped at the interface, which start separate at load 73.6KN.

This sample exposed to post-loading after ultimate load to in order to show the behaviors of the

beam in this stage of loading. The old and the new section have fully separated and each of

them act independently. However, both sections still tied with each other by the pressure of the

supports, this continued until crunching the bearing area at the supports as shown in figure 5.9.

The value of ultimate load is 85.58KN, which equal 236% of the ultimate load of the control

beams.

In the samples AF1,AF2,BF1 and BF2 the system still work and each part share in carrying

the total load even after two parts are completely separated due to lateral support (such that the

strengthening layer continue beyond the supports, and in the case of AF1 and AF2 the shear

connectors are deep), in this case bearing area at the supports is the critical. There is enough

anchorage beyond the supports prevent the complete separation of the bottom layer and provide

some adhesion. This explain the failure of the anchorage above the left support of the two

samples BF1 (Figure 5. 9) and BF2 (Figure 5. 10) and above the right support of the samples

AF1(Figure 5. 5) and AF2 (Figure 5. 6). The deflection for BF1 and BF2 at the SLS equal

3.65mm and 4.43mm respectively while at ULS equal 18mm and 11.7mm respectively.

Figure 5. 11 show the Load-deflection relationship of BF1 and BF2 compared with MF and

control beam. Figures show enhancement stiffness in SLS. The load-deflection curve of BF1

show that the system act similar to the monolithic sample and show good level of ductility even

after separation. In the case of BF1, the bonding of the chemical adhesive still efficient until

satisfied stage and enough signs of failure before reaching the ultimate state. Detailed analysis

of deflection will be presented in sections 5.4.2 and 5.4.3.

93

Figure 5. 10: Failure mode (post ultimate) and crack patern of BF2

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

Load

KN

Deflection mm

BF1

BF2

CF2

MF

Figure 5. 9: Failure mode Crack pattern of BF1

Figure 5. 11: Load-deflection relationship of BF2 and BF2 compared with control beam

CF2 and monolithicly casted beam MF

94

5.2.4. Addition of Mechanically Connected Steel Plate to the Tension Side (AF3,

AF4)

Figure 5. 12 and Figure 5. 13 show the samples AF3 and AF4 after testing and their crack

pattern. The sample AF3 failed in pure flexure without failure in shear connectors until the

ultimate load. The value of ultimate load is 51.60KN, which equal 142% of the ultimate load of

the control beams. The situation in sample AF4 is similar to AF3 and the ultimate load was

51.5KN.

However, the average of experimental value of the ultimate load of the two samples is

equal 79.3% of the theoretical value. This reduction in the system capacity can be explained that

the interaction between the two surfaces not in the whole area of the plate, which cannot delay

the widening of the flexural cracks like in the case of chemical adhesive. This reason ensure that

the composite action have not achieved so the load capacity of the beam strengthened by

mechanically attached steel plates to the tension face of the beam cannot be calculated using the

flexural theory. The relative movement between the concrete section and the steel plate is not

possible because the bonding designed for full shear connection. The deflection for AF3 and

AF4 at the SLS equal 3.95mm and 4.05mm respectively while at ULS equal 21mm and 24mm

respectively. Figure 5. 14 show load-deflection relationship of AF3 and AF4 compared with MF

and the control beam. The figure show some enhancement in stiffness at SLS over the control

beam but less than the stiffness of the monolithic sample. Detailed analysis of deflection will be

presented in sections 5.4.2 and 5.4.3.

Figure 5. 12: Failure mode and Crack pattern of AF3

95

Figure 5. 13: Failure mode and Crack pattern of AF4

Figure 5. 14: Load-deflection relationship of

AF4 and AF3 compared with MF and CF2

5.2.5. Addition of Chemically Bonded Steel Plate to the Tension Side (BF3, BF4)

The two samples BF3 (Figure 5. 15 and Figure 5. 16) and BF4 (Figure 5. 18) was failed by the

peeling of the steel plate at the end with rupture of the concrete cover because the generated

shear stresses are larger than the tensile strength of concrete. The rapture of concrete proving

that the problem not in the bond. In BF3, the ultimate load is 72.146KN, which equal 199% of

the ultimate load of the control beams and exceed the theoretical value by 5%.

The superiority of BF3 over AF3 and AF4 (mechanical bonding) in term of strength can be

explained that adhesive bonding allowed the composite behavior from the section and

transferred the force to the plate until the sudden separation occurred. The uniform attachment

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

Load

KN

Deflection mm

AF3

AF4

CF2

MF

96

maximizes surface crack control under the plate. However, the anchorage capacity is limited by

the tensile capacity of the concrete below the adhesive. Stress concentrations at the ends of the

plate (end of the shear span) initiated plate peeling. However, AF3 and AF4 better than BF3 in

the failure mode.

The sample BF4 unloaded accidentally at the load 54.28KN (81.6% of the theoretical).

Then the sample reloaded until it uploaded also accidentally at 17.12KN. After times of loading

and unloading no cracks or signs of failure were appeared. The sample then reloaded until

failure at 51KN. The reduction in the capacity in this sample can explained that the state of the

sample after each loading and unloading differ from the original state of the sample especially

that the first loading reach to 81.6% of the expected capacity. The repetition of loading and

unloading often weakened the adhesive layer.

The load-deflection curves of the two samples BF3 and BF4 (Figure 5. 17 and Figure 5. 19

respectively) show that they still stiff until the ultimate state. This mode is not desired so that no

signs developed before the sudden failure. The deflection for BF3 at the SLS equal 3.92mm

while at ULS equal 6.7mm. Figure 5. 17 show load-deflection relationship of BF3 compared

with MF and control beam. Figure 5. 19 show load-deflection relationship of BF4 for the last

loading. The figures show enhancement in stiffness in SLS. Detailed analysis of deflection will

be presented in sections 5.4.2 and 5.4.3.

Figure 5. 15: Failure mode of BF3

Figure 5. 16: Crack Pattern of BF3

97

Figure 5. 17: Load-deflection relationship of BF3 compared with MF and CF2

Figure 5. 18: Failure mode of BF4

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

Load

KN

Deflection mm

BF3

CF2

MF

Figure 5. 19: Load-deflection relationship of BF4

0

10

20

30

40

50

60

0 1 2 3 4 5 6

Load

KN

Deflection mm

BF4

98

5.3. Second Series: Shear Samples.

5.3.1. Control Beams:

The control beams CS1 (without internal stirrup) failed in shear with some flexural cracks and

with ultimate load of 53.88KN and. Figure 5. 20 show the sample after testing and the crack

pattern for CS1. The deflection for CS1at the SLS equal 4.5mm while at ULS equal 20mm.

Figure 5. 20: Failure mode and Crack pattern of CS1

In the case of CS2 (original section with internal stirrups similar to the details of the original

sample of the first series) (Figure 5. 21) the ultimate load is 74.253KN. Figure 5.22 show the

Load-deflection curves of the two control beams. The deflection for CS2 at the SLS equal

5.8mm while at ULS equal 17.5mm.

Figure 5. 21: Failure mode and Crack pattern of CS2

99

Figure 5. 22: Load-deflection relationship of CS1 and CS2

5.3.2. Addition of U Shape RC Jacket with Additional Mechanical Connection

(AS1, AS2)

The sample AS1 (Figure 5. 23) failed in flexure with ultimate Load equal 98.62 KN which

equal 183% of the ultimate of the control beam (CS1: without stirrups) and equal 113.6% of the

theoretical value. The sample AS2 (Figure 5. 24) failed in flexure with ultimate Load equal

101.19 KN which equal 188% of the ultimate of the control beam (CS1) and equal 116.6% of

the theoretical value .

In the two samples the shear failure was prevented and both of them failed in flexure

without separation between the old and the new layers until the ultimate state. In other words,

the composite action work until failure, which lead to the next point: The comparison with the

theoretical calculations indicate that using the flexural theory to predict the capacity of the

beams strengthened by this method is conservative.

Load-deflection curves of the samples is compared with the control beams (Figure 5. 25)

and show the enhancement in flexural stiffness. The deflection for AS1 and AS2 at the SLS

equal 6.9mm and 6.6mm respectively while at ULS equal 21mm and 18mm respectively.

Detailed analysis of deflection will be presented in sections 5.4.2 and 5.4.3.

0

10

20

30

40

50

60

70

80

0 5 10 15 20 25

Load

KN

Deflection mm

CS1

CS2

111

Figure 5. 23: Failure mode and Crack pattern of AS1

Figure 5. 24: Failure mode and Crack pattern of AS2

Figure 5. 25: Load-deflection relationship of AS1 and AS2 compared with the

control beams

0

20

40

60

80

100

120

0 5 10 15 20 25

Load

KN

Deflection mm

AS1

AS2

CS1

CS2

111

5.3.3. Addition of Chemically Bonded Shape Plain Concrete Jacket (BS1, BS2)

The sample BS1 (Figure 5. 26) loaded and then unloaded accidentally at load 68.5KN (96.3%

of the theoretical capacity) without any appearance of failure signs. Then it was reloaded and

failed at load equal 40.22KN. In this sample, the failure was in shear with some flexural cracks.

The second sample BS2 (Figure 5. 28) also unloaded accidentally at load 64.4KN(90% of the

theoretical capacity) then it reloaded and failed at 73.69KN (103.5% of the theoretical

capacity). In this sample, the shear failure was prevented and the failure was in flexure. The

difference in behavior can be explained that in the case of first sample (BS1) the adhesive layer

affected largely by the loading and unloading so in the second loading the shear crack (diagonal

tension) began in the original section then transferred to the new layer through the remaining

bonding. While in the second sample the adhesive layer abided until failure (but not like the

case if no preloading occur) which helped in maintaining of the composite action and

subsequently the work of strengthening layer.

Figure 5. 27 show the deflection behavior in the two stages of BS1, in the first stage the

beam was stiff until the end of loading while it behaved in a ductile manner when failed in

second stage. Figure 5. 29 show the flexural stiffness of BS2 compared with the control beam.

Despite the occurrence of loading and unloading the samples display satisfied results. The

deflection for BS2 at the SLS equal 3.6mm while at ULS equal 18.1mm. Detailed analysis of

deflection will be presented in sections 5.4.2 and 5.4.3.

Figure 5. 26: Failure mode and Crack pattern of BS1

112

Figure 5. 27: Load-deflection relationship of BS1

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10

Load

KN

Deflection mm

BS1, stage1

BS1, stage2


113

5.3.4 Addition of U Shape Reinforced Concrete Jacket to Roughened surface and

Partially Painted by Adhesive(ES1)

The shear failure was prevented in sample ES1 and the failure was in flexure (Figure 5. 30). The

sample provided ultimate load equal 82.83 KN (95.4% of the theoretical capacity). This value

larger than the control sample CS1 (without internal stirrups) by 35% and larger than the control

sample CS2 (with internal stirrups) by 12%.

The load-deflection curves (Figure 5. 31) show that the flexural stiffness for this sample

(ES1) similar to the control sample CS1 (without internal stirrups) and CS2 (with internal

stirrup) with little enhancement in stiffness at SLS and the ductility at the ULS. The deflection

for ES1 at the SLS equal 5.15mm while at ULS equal 23mm. Detailed analysis of deflection

will be presented in sections 5.4.2 and 5.4.3.

Figure 5. 29: Load-deflection relationship of BS2 compared with the conrol

beam

0

10

20

30

40

50

60

70

80

0 5 10 15 20 25

Load

KN

Deflection mm

BS2

CS1

Figure 5. 30: Failure mode and Crack pattern of ES1

114

Figure 5. 31: Load-deflection relationship of ES1 compared with the

control beams

5.3.5 Addition of U Shape RC Jacket without additional connection (only "friction

+ new stirrups") (BS3)

In the sample BS3 (Figure 5. 32) the shear failure was prevented and the sample failed in

flexure with ultimate load equal 103.8KN (119.6% of the theoretical capacity). Although it

seems not logic that this sample (without additional connection) provide load capacity more

than the samples (AS1, AS2, and ES1) in which additional connection was used, but there is

persuasive explanation as following:

1. The connection provided by additional stirrups only (which connected into the old

section) is enough and the excessive drilling of holes in the sample (in the case of AS1

and AS2) for connecting additional stirrups and shear connection at the sides and the

bottom somehow weaken the original sample.

2. The surface preparation in the case of ES1 wasnot enough and made inverse action

through loosening the surface. Deeper and more quality controlled surface preparation

may was needed.

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25

Load

KN

Deflection mm

ES1

CS2

CS1

115

3. In addition, the reason may be problem in construction of sample. This reason is weak

due to the satisfied level of quality control.

Figure 5. 33 show the comparison between load-deflection curves for BS3, CS1 and CS2.

Through this comparison it is clear the enhancement in term of stiffness and strength in BS3.

The deflection for BS3 at the SLS equal 6.12mm while at ULS equal 28mm. Detailed analysis

of deflection will be presented in sections 5.4.2 and 5.4.3.


Figure 5. 33: Load-deflection relationship of BS3 compared with the control beams

5.3.6. Addition of Steel Plate Mechanically Connected to the Sides of the Original

Beam(AS3, AS4)

The samples AS3 (ultimate load =64.36KN) and AS4 (ultimate load = 70.05KN) after testing

are shown in Figure 5. 34 and Figure 5. 35 respectively. In both the samples, the shear failure

0

20

40

60

80

100

120

0 5 10 15 20 25 30

Load

KN

Deflection mm

BS3

CS1

CS2

116

was delayed but was not prevented. This can be referred to the mechanical incompatibility

between the section of concrete and section of steel plates. This incompatibility often referred to

unsuitable dimensions and configuration of the steel plates. In addition, the ununiformed

attachment of the plates let the plates to be buckled so the plate stopped to yield normally.

Despite the reduction in capacity (less than the control sample CS2 (with internal stirrups) by

10%), but the existence of steel plate:

1. Enhanced the ultimate load of the original beam (CS1: without stirrups) by 25%

(average of the two samples).

2. Gained the beam more ductility at the ULS.

As shown in the load-deflection curves (Figure 5. 36) there are two stages (stiffness then

ductility without transition stage). This refer to that the beam failed suddenly at end of the

stiffness stage (at load 58KN) and the ductility stage started by yielding of the bulked portions

of the steel plate. The deflection for AS3 and AS4 at the SLS equal 4.9mm and 3.8mm

respectively while at ULS equal 24mm and 21mm respectively. Figure 5. 37 show the

comparison between load-deflection curves of AS3 and the control beams CS1 and CS2.

Detailed analysis of deflection will be presented in sections 5.4.2 and 5.4.3

Figure 5. 34: Failure mode of AS3

Figure 5. 35: Failure mode of AS4

117

Figure 5. 36: Load-deflection relationship of AS4 and AS4

Figure 5. 37: Load-deflection relationship of AS4 compared with CS1 and CS2

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50

Load

Deflection

AS3

AS4

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50

Load

KN

Deflection mm

AS4

CS2

CS2

118

5.3.7. Addition of Chemically Bonded Steel Plate to the beam Sides (BS4,BS5)

The samples BS4 (ultimate load =107KN) and BS5 (ultimate load = 91.15KN) after testing are

shown in Figure 5. 38 and Figure 5. 39 respectively. As in the previous two samples (AS3 and

AS4), the shear failure in samples BS4 and BS5 was also delayed but was not prevented with

larger increment of capacity enhancement. This can be referred also to the mechanical

incompatibility between the concrete section and steel plates section. This incompatibility often

referred to unsuitable dimensions and configuration of the steel plate. However, the uniformed

attachment of the plates maximize the crack control and provide more prorogation of shear

failure than the samples AS3 and AS4 (mechanical attached steel plates). The enhancement is in

the ultimate load such that the average of the capacity of the two samples (BS4 and BS5) larger

than the control sample CS1 (without internal stirrups) by 84% and larger than the control

sample CS2 (with internal stirrups) by 33.4%.

Although the larger load capacity of the samples BS4 and BS5, but the flexural stiffness

is more worse than both the control beams and the samples AS3 and AS4 (mechanical attached

steel plates) as shown in the load-deflection curves (Figure 5. 40) such that the beams BS4 and

BS5 still stiff until the ULS and the failure was sudden. The deflection for BS4 and BS5 at the

SLS equal 6.4mm and 8mm respectively while at ULS equal 13mm and 14.5mm respectively.

Figure 5. 37 show the comparison between load-deflection curves of BS4, BS5 and the control

beams CS1 and CS2. Detailed analysis of deflection will be presented in sections 5.4.2 and

5.4.3.

Figure 5. 38: Failure mode of BS4

Figure 5. 39: Failure mode of BS5

119

Figure 5. 40: Load-deflection relationship of BS4 and BS5

compared with control beams

5.3.8. Addition of External Straps from Steel Plate Chemically Bonded to the out-

surface of the Original Section(BS6)

The shear failure was prevented in sample BS6 and the failure was by flexure (Figure 5. 41)

with partial separation in some straps (only 4straps), this separation occur at the ultimate state.

The sample provide ultimate load equal 77.23 KN. This value larger than the control sample

CS1 (without internal stirrups) by 31% and larger than the control sample CS2 (with internal

stirrups) by 10%. The enhancement in the original sample (CS1: without stirrups) was by

recovering the flexural capacity with little enhancement in the ductility at the ULS. These

results ensure the successful of this method in preventing shear failure but with no significant

enhancement in strength.

The load-deflection curves (Figure 5. 42) show that this sample (BS6) stiff at SLS and

ductile at the ULT, this behavior very near to the control sample CS2 (with internal stirrup).The

deflection for BS6 at the SLS equal 6.7mm while at ULS equal 31.28mm. Figure 5. 42 show the

0

20

40

60

80

100

120

0 5 10 15 20 25

Load

KN

Deflection mm

BS4

CS1

CS2

BS5

111

comparison between load-deflection curves BS6 and the control beams CS1 and CS2. Detailed

analysis of deflection will be presented in sections 5.4.2 and 5.4.3.


Figure 5. 42: Load-deflection relationship of BS6 compared with the

control beams

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25 30 35

Load

KN

Deflection mm

BS6

CS2

CS1

111

5.4. Comparison between the strengthening Techniques:

In this section a comprehensive comparison will be made between the adopted strengthening

techniques according the theoretical and experimental data obtained in this research. The

preference in the strengthening techniques can be specified by many criteria and can be cleared

by comparison between them in similar circumstances. Here a comparison is presented through

some criteria, which are:

1. Load capacity.

2. Stiffness at the SLS.

3. Failure mode and stiffness at the ULS.

4. Time and skills need.

5.4.1. Load Capacity

The main purpose of strengthening in the first series is increasing the load capacity of the beam.

In the second series the main purpose was preventing shear failure. The strengthened samples

designed initially to carry the same load considering that the composite action will maintained

until the ultimate state. The difference in load carrying capacities referred to some reasons,

which prevent the composite action to be maintained until the ULS. Some considerations can be

taken in account to overtake these faults. Figure 5. 43 and Figure 5. 44 show the compression

between the samples in the base of load capacity for the first and the second series respectively.

Figure 5. 43: Percentage of enhancment over the control beam for first series

CF1

; 10

0%

CF2

; 99

.20

% M

F; 2

54

.50

%

AF1

; 26

8.4

0%

AF2

; 21

5.1

6%

BF1

; 25

0.4

0%

BF2

; 25

4.5

0%

AF3

; 14

3%

AF4

; 14

2.5

0%

BF3

; 19

9%

BF4

; 19

7.5

0%

0%

50%

100%

150%

200%

250%

300%

112

Figure 5. 44: Percentage of enhancment over the control beam for second series

5.4.2. Stiffness and Deflection at SLS

The value of stiffness considered to be the slope of the line between (0, 0) and the coordination

of the value of 60% of the ultimate load. Figure 5. 45 and Figure 5. 46 show the comparison

between the samples on the base of stiffness at the SLS for the first and the second series at the

SLS respectively. Table 5. 1 and Table 5. 2show the deflection, stiffness and increasing of

stiffness over the control beam at SLS of the first and the second series respectively. Figure 5.

47 and Figure 5. 48 show the comparative load-deflection curves for the first and the second

series respectively. Larger values of stiffness means larger stiffness. In general and as shown in

the figures and the tables, the following pointes can be noted:

1. The concrete jacketed beams have more potential to be stiffer at the SLS than the steel

plated beams due to the larger stiffness of the added layer.

2. In general the samples in which chemical adhesion have more potential to be stiffer at

the SLS due to the crack control occurred by the layers of the adhesive.

CS1

; 10

0%

MS; 1

47

.23

%

AS1

; 18

3.0

4%

AS2

; 18

7.8

0%

BS1

; 12

4.3

5%

BS2

; 13

6.4

4%

ES1; 1

53

.73

%

BS3

; 19

2.6

5%

AS3

; 11

9.4

5%

AS4

; 11

9.4

9%

BS4

; 19

9.4

4%

BS5

; 16

9.1

7%

CS2

; 13

7.8

0%

BS6

; 14

3.3

4%

0%

50%

100%

150%

200%

250%

113

Table 5. 1: Deflection and stiffness at SLS of the first series

Stiffness increasing

over control beam

(%)

Stiffness at SLS Deflection at SLS Service Load Sample

--- 5.83 3.75 21.852 CF1

--- 5.43 4 21.684 CF2

--- 4.15 5.8 24.03 CF3

108 12.1 4.6 55.632 MF

90 7.89 8 63.108 AF0

122 12.94 3.2 41.4 AF1

118 12.71 3.7 47.016 AF2

157 15 3.65 54.72 BF1

99 11.6 4.43 51.348 BF2

44 7.84 3.95 30.96 AF3

40 7.64 4.05 30.912 AF4

90 11.05 3.92 43.29 BF3

Table 5. 2: Deflection and stiffness at SLS of the second series

Stiffness increasing

over control beam

(%)

Stiffness at SLS Deflection at SLS Service Load Sample

---- 7.19 4.5 32.328 CS1

20 8.58 6.9 59.172 AS1

28 9.2 6.6 60.714 AS2

71 12.29 3.6 44.214 BS2

35 9.66 5.15 49.698 ES1

42 10.18 6.12 62.28 BS3

10 7.89 4.9 38.616 AS3

54 11.07 3.8 42.03 AS4

41 10.08 6.4 64.476 BS4

-4 6.84 8 54.69 BS5

7 7.69 5.8 44.55 CS2

-3 6.92 6.7 46.338 BS6

114

Figure 5. 45: Comparative stiffness for the first series at the SLS

Figure 5. 46: comparative stiffness for the second series at the SLS

CF1,2

MF

AF0

AF1,2 BF1,2

AF3,4

BF3

0

2

4

6

8

10

12

14

CS1

AS1

BS1,2 ES1

BS3 AS3,4

BS4,5

CS2

BS6

0

2

4

6

8

10

12

115

Figure 5. 47: Comparative load-deflection curves for the first series

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35

Load

KN

Deflection mm

BF3

AF3

AF1

BF1

MF

AF0

116

Figure 5. 48: .: Comparative load-deflection curves for the second series

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35

Load

KN

Deflection mm

CS1

CS2

AS1

BS2

AS3

BS4

BS3

BS6

ES1

117

5.4.3. Stiffness and Failure Mode at ULS

The ductile failure is basic demand in beam design, this give caution and enough signs of failure

before the critical state is reached. Table 5. 3 and Table 5. 4 show the values of deflection and

stiffness at ULS and the manner by which the samples failed for the first and the second series

respectively. The stiffness calculated as the slope of the line between the 60% of load and the

ultimate load. Larger values of stiffness means less ductility. Referring to the control beams in

the two series there is no enhancement in ductility but the stiffness values are low and

satisfaction regarding to the values of the ultimate loads except the samples in which only

chemical adhesives used. These samples failed in sudden and brittle manner. As shown in the

tables the samples of chemical adhesion connection have the larger potential for brittle and

sudden failure. This can be handled by using the mechanical connectors beside the chemical

adhesion.

Table 5. 3: Deflection, stiffness and failure mode at ULS of the first series

Failure mode Stiffness at

ULS

Deflection at

ULS (mm)

Sample NO.

Ductile, Flexural failure 0.69 25 CF1 1.

Ductile, Flexural failure 0.73 24 CF2 2.

Ductile, Flexural failure 1.62 15.7 MF 3.

Ductile, Flexural failure 2.77 18 AF0 4.

Ductile ,Monolithic

flexural failure then

partial separation

1.85 30.8 AF1 5.

Ductile ,Monolithic


partial separation

1.7 19.48 AF2 6.

Ductile, Monolithic


partial separation

4.3 11 BF1 7.

Brittle, Complete

separation

2.55 18 BF2 8.

Ductile, Flexural failure 4.71 11.7 AF3 9.

Ductile, Flexural failure 1.22 21 AF4 10.

Brittle and sudden, Plate

end peeling with concrete

cover delamination

1.04 24 BF3 11.

Brittle and sudden, Plate

end peeling with concrete

cover delamination

10.39 6.7 BF4 12.

118

Table 5. 4: Table: Deflection, stiffness and failure mode at ULS of the second series

Failure mode Shear failure Stiffness at

ULS

Deflection

at ULS

Sample NO.

Ductile, Shear

failure

Occurred 1.4 20 CS1 1.

Ductile, Flexural

failure

Not occurred ------ ------ MS 2.

Ductile, Flexural

failure

Prevented 2.8 21 AS1 3.

Ductile, Flexural

failure

Prevented 3.56 18 AS2 4.

Ductile, Shear

failure

Not prevented but delayed

------- ------ BS1 5.

Ductile, Flexural

failure

Prevented 2.04 18.1 BS2 6.

Ductile, Flexural

failure

Prevented 1.86 23 ES1 7.

Ductile, Flexural

failure

Prevented 1.9 28 BS3 8.

Brittle-Ductile,

Shear failure


1.35 24 AS3 9.

Brittle-Ductile,

Shear failure


1.63 21 AS4 10.

Brittle, Shear

failure


6.52 13 BS4 11.

Brittle, Shear

failure


5.61 14.5 BS5 12.

Ductile, Flexural

failure

Not occurred 2.54 17.5 CS2 13.

Ductile, Flexural

failure

Prevented 1.26 31.28 BS6 14.

119

5.4.4 Skills and Time of Construction.

Table 5. 5 and Table 5. 6 present comparison between the samples in term of the construction

time and the degree of skills for the first and the second series respectively.

Table 5. 5: Construction time and the degree of skills for the first series

Time and skills Sample NO.

Normal time and skills CF1,

CF2

1.

normal time and skills MF 2.

Higher demand of time and skills AF0,

AF1,

AF2

3.

Immediate demand of time and skills BF1,

BF2

4.

Higher demand of time and skills AF3,

AF4

5.

Immediate demand of time and skills BF3,

BF4

6.

Table 5. 6: Construction time and the degree of skills for the second series

Time and skills Sample NO.

Normal time and skills CS1 1.

Normal time and skills MS 2.

Higher demand of time and skills AS1, AS2 3.

Immediate demand of time and skills BS1, BS2 4.

Higher demand of time and skills ES1 5.

Higher demand of time and skills BS3 6.

Higher demand of time and skills AS3, AS4 7.

Immediate demand of time and skills BS4, BS5 8.

Normal time and skills CS2 9.

Immediate demand of time and skills BS6 10.

121

5.4.5 Concrete Jacketing Versus Steel Plating

In general and as shown in the previously presented figures and tables that the beams

strengthened by additional reinforced concrete layers have more potential to be stiffer at the

SLS and more ductile at the ULS than beams strengthened by steel plates. In addition, these

beams have more ability to be stable until providing its full capacity. This can be explained by:

1. The concrete layers provide larger volume than steel plate so more stiffness can be

gained.

2. The added reinforced concrete similar to the original material so mechanical

compatibility is ensured.

3. In the samples of concrete jacketed beams there are lateral anchorage such that the

strengthening layer continue beyond the supports and subsequently no end peeling

occur.

4. Casting concrete to the old concrete premise complete touch between the old and the

new section. Complete touch helps in maintaining the composite action. In the case of

steel plating the complete touch achieved only by chemical adhesive.

While, in the case of beams strengthened by steel plates, in general, steel plating of concrete

beams, either by shear connectors or chemical adhesive, could not give results as good as

concrete jacketed beams in term of strength or flexural stiffness. However, beams strengthened

by steel plating have high potential to success if some considerations taken into account to

prevent:

1. End peeling of the steel plate in the case of chemically attached steel plates.

2. Relative movement between the concrete surface and steel plate.

3. Buckling of steel plate in the case of shear strengthening by steel plate attached

mechanically at the sides of beam.

5.4.6 Mechanical Versus Chemical Bonding

1. In the case of bonding concrete to concrete by chemical adhesives, there are high risk

for brittle failure as in sample BF2. The risk will be higher if there is no lateral

connection. While in the case of mechanical bonding the risk is very low even after

separation due to the action of deep anchorage which provided by shear connectors.

2. In the case of success of chemical adhesive, the results will be better than using shear

connections, this is due to the weakening occurred in the original beam after drilling

holes, which reduce the stiffness of the original beam and made internal cracks.

Subsequently, this accelerate the failure when it begin. In addition, the adhesive layer

help in controlling both the flexural and shear cracks.

3. The configuration of strengthening system play important role in success of bonding, the

beams strengthened by concrete jacketed in three faces provide extra bonding area over

the one-face jacketing.

4. In the case of bonding steel plate to concrete using chemical adhesive, flexural stiffness

was high noticeably due to the important role of adhesive layer in delaying the

propagation of either flexural or shear cracks. However, the failure of these beams was

sudden and brittle. While in the case of mechanical bonding, the state is differing. When

using mechanical bonding of the side plates, the portions between shear connectors was

buckled. Although the steel plated beams using mechanical bonding provided lesser

strength, the failure was ductile in these beams.

121

5.5 Summary of Results:

Summary of the results is presented in Table 5. 7

Table 5. 7: Summary of the results

Failure modes Ratio Of

Test

Ultimate

Load

Ultimate Load

(KN)

Connection

Type

Strengthening

Type

Sample No

% Exp./The. Exper. Theo.


Flexural(ductile) 100% 149.9% 36.42 24.30 ----- No Strengthening CF1 1.

Flexural(ductile) 99.3% 152.4% 36.14 23.72 ----- No Strengthening CF2 2.

----- No Strengthening CF0 3.

Flexural(ductile) 254.6% 139.4% 92.72 66.55 Monolithic

section

----- MF 4.

Flexural(ductile) 290.6 157.3% 105.81 Dowels Concrete Jacket/Tension

face AF0 5.

Monolithic flexural then partial

separation

189.5% 102.6% 69* 67.29 Dowels Concrete Jacket/Tension

face AF1 6.


separation

215.2% 116.5% 78.36 67.29 Dowels Concrete Jacket/Tension

face AF2 7.


separation

250.5% 135.6% 91.2 67.29 Adhesive Concrete Jacket/Tension

face BF1 8.

Full separation of the underlay (in

the post-ultimate loading).

235% 127.2% 85.58 67.29 Adhesive Concrete Jacket/Tension

face BF2 9.

Flexural(ductile) 141.7% 79.3% 51.6 65.1 Dowels Steel Plate/Tension face AF3 10.

122

Continue Table 5.7


Test

Ultimate

Load

Ultimate Load

(KN)

Connection

Type

Strengthening

Type

Sample No


Flexural(ductile) 142.5% 79.3% 51.52 65.1 Dowels Steel Plate/Tension face AF4 11.

Plate end peeling with concrete

cover delamination (brittle and

sudden)

198.2% 105% 72.15 68.73 Adhesive Steel Plate/Tension face BF3

12.

Plate end peeling with concrete

cover delamination(brittle and

sudden)

149.1% 79% 54.28* 68.73 Adhesive Steel Plate/Tension face BF4 13.

Second series: Shear strengthening

Shear failure 100 110.9% 53.88 48.60 ------ No Strengthening

(without stirrups) CS1 14.

Flexural failure 183.1 113.6% 98.62 86.84 Dowels Bottom U layer (with

additional stirrups) AS1 15.

Flexural failure 187.9 116.6% 101.19 86.84 Dowels Bottom U layer (with

additional stirrups) AS2 16.

Shear failure 127.2 96.3% 68.5* 71.2 Adhesive Top ∩ layer (without

additional stirrups) BS1 17.

Flexural failure 136.8 103.5% 73.69* 71.2 Adhesive Top ∩ layer (without

additional stirrups) BS2 18.

Flexural failure 153.8 95.4% 82.83 86.84 Partial adhesive

and friction

Bottom U layer with

additional stirrups ES1 19.

Flexural failure 192.7 119.6% 103.8 86.84 No connection Bottom U layer with

additional stirrups BS3 20.

Shear failure with steel plate

buckling

119.5 ------ 64.36 ------ Dowels Steel plate at the two

sides of the beam AS3 21.

123

Continue Table 5.7


Test

Ultimate

Load

Ultimate Load

(KN)

Connection

Type

Strengthening

Type

Sample No


Shear failure with steel plate

buckling

130.1 ------ 70.05 ------ Dowels Steel plate at the two

sides of the beam AS4 22.

Shear failure with sudden separation

of the steel plate

199.5 ------ 107.46 ------ Adhesive Steel plate at the two

sides of the beam BS4

23.

Shear failure with sudden separation

of the steel plate

169.2 ------ 91.15 ----- Adhesive Steel plate at the two

sides of the beam BS5

24.

Flexural failure 137.9 153.6% 74.25 48.36 ------ No Strengthening (with

stirrups @5cm) CS2 25.

Flexural failure 143.4 160.1% 77.23 48.24 Adhesive

partially

External steel plate

straps BS6 26.

CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS

6.1. Introduction

A comparative study about concrete jacketing and steel plating of RC beams was

presented in this research. The test program of the current study have been detailed in

the third chapter of this thesis. After testing the samples, the result have been

analyzed and discussed in the fourth chapter of this thesis. This chapter contains the

answers of the research questions, which mentioned in the first chapter. Additional

conclusions and recommendations will be presented in this chapter. These

conclusions based on the adopted range of the configurations in this study for

concrete jacketing and steel plating of RC.

6.2 Conclusions

The results presented in this research lead to the following conclusions:

1. In general, if it is needed to differentiate between concrete jacketing and steel

plating it could not be said that one is better than the other infinitely. Each

technique has a kind of superiority over the other according to some criteria.

The same thing can be said for the differentiation between mechanical and

chemical bonding.

2. Using procedures of anchoring, casting and curing with good level of quality

can grantee the composite action of beams strengthened by any configuration

(which adopted in this research) of concrete jacketing, while additional

procedures needed over the adopted configuration in the case of using steel

plates.

3. In the range of the adopted configurations in this research, strengthening

beams in flexure by concrete jacketing is better in term of strength and

stiffness than using steel plate. Other configurations may let steel plating

better than concrete jacketing like anchoring the end of adhesively bonded

plates.

4. Using three-face concrete jackets can prevent shear failure while in the case of

attaching steel plates at the two sides of the RC beams the shear failure

delayed only. In addition to prevention or delay of shear failure, these

techniques enhance strength, stiffness at SLS and ductility at ULS in the case

of concrete jacketing. While, sticking straps of steel plates using suitable

adhesive at the outer face of RC beams can prevent shear failure and recover

the potential strength without significant increasing in strength or

enhancement in stiffness at SLS.

5. In the range of adopted configurations and arrangement in this research, the

concrete jacketing can achieve a level of enhancement in flexural stiffness

better than the case of steel plating.

6. Mechanical bonding (shear connectors) is better than chemical bonding when

the strengthening material is concrete not steel plate. In the case of bonding

steel plates, both techniques have advantage compensate the disadvantage of

the other. Chemical adhesive provide complete and uniform bonding while

125

shear connectors provide deep anchorage. In all cases, the combination

between the two techniques of bonding is the best choice in term of

mechanical behavior.

7. Although using chemical adhesive can guarantee higher stiffness at SLS and

can abide until failure, there are higher risk for separation and sudden failure.

This risk is higher in the case of steel plating. This problem can be overtaken

by additional anchorage (shear connectors) at the ends of the beam or plate.

8. In noticeable number of cases it is observed that the beams in which chemical

adhesive used, can provide more capacity than which bonded by mechanical

bonding (shear connectors) this can be explained that drilling holes in the

original beam to fix the shear connectors may weaken the original beam and

effect on the behavior of strengthening system, this may because one or more

of the following:

a. Holes decrease the stiffness of the original beam.

b. Drilling operation may cause formation of invisible internal cracks.

c. Rarefaction of the bonding between reinforcement and concrete during

drilling.

9. Lateral anchorage of the new concrete layer is important such that the system

can yield even after separation of the two layer and then the critical is the

lateral anchorage. The main purpose of the lateral anchorage is preventing end

peeling and sever failure specially in the case of using chemical adhesive.

10. This study proved that in general, using three-face concrete jackets is better

than one-face concrete jackets.

11. Beams strengthened by one layer at the tension face using mechanical

bonding (shear connectors) can behave monolithically until and after ultimate

loads. The chance is greater in the case of three-face jacketing.

12. Beams strengthened by one RC layer at the tension face using chemical

bonding (adhesives) can behave monolithically until failure but they have

high potential for separation before and after ultimate loads. The risk is higher

in the absence of additional anchorage at the end of the layer and lower in the

case of three-face jacketing.

13. In all beams strengthened by concrete jacketing, the experimental value of

their capacity is larger than the theoretical by 118-158%. This give indication

that using the traditional procurers (flexural theory) to predict the theoretical

value of the ultimate load for these beams is very conservative. There are

doubts about the validation of flexural theory to predict the capacity of beams

strengthened by steel plates. In all cases, this issue need more detailed study.

14. Preloading the beam strengthened by concrete jacket has not high effect on

the capacity or failure mode but it may accelerate the separation and weaken

the adhesive layer. The risk is higher in the case of using only chemical

adhesive without additional anchorage at the end of the concrete jacket or

steel plate.

15. Adding 100mm of reinforced concrete layer at the tension face of beam

(mechanically or chemically bonded) can enhance the stiffness at all load

levels and can increase the load capacity to 190-290% of the ultimate load of

126

the control beams with increase in stiffness reach 157% over the stiffness of

the control beam.

16. Despite the beams strengthened by steel plate bonded to the tension face using

shear connectors yielded capacity exceed the control beam by 42% with

acceptable enhancement in flexural stiffness in all load levels (reach 44% over

the stiffness of the control beam), these beams was not yielded the expected

capacity (only 77% of the theoretical capacity). This may refer to:

a. The interaction between the two surfaces not in the whole area of the

plate.

b. This type of composite beams is not follow the flexural theory.

17. In spite of the premature failure of the beams strengthened by steel plate

attached to the tension face by chemical adhesive but these beams can yield

capacity reach 99% over the control beams and 9% over the theoretical value

and with enhancement in stiffness in service life reach 90% over that of the

control beam. Additional anchorage for the plate end can guarantee more

yielding and can prevent catastrophic failure. These results ensure the

superiority of complete interaction provided by chemical adhesives, which

increase the stiffness and delay the widening and propagation of flexural

cracks. In addition, results show that the repeated loading and unloading of

these beams weaken the adhesive layer but this need more study.

18. Strengthening beams by three-faces reinforced concrete jackets (50mm thick

U shapes) with additional mechanical bonding can prevent shear failure,

enhance the flexural stiffness and provide extra strength reach to 88% over the

original beam and 16% over the theoretical capacity with enhancement in

serviceability stiffness reach 28% over which for the control beam. These

beams can behave monolithically and there capacity can conservatively be

guessed using flexural theory. .

19. Strengthening beams by three-faces plain concrete jackets (50mm thick ∩

shapes) with chemical adhesive bonding can prevent shear failure, enhance

the flexural stiffness and provide extra strength reach to 38% over the original

beam and 3% over the theoretical capacity with enhancement in serviceability

stiffness reach 71% over the stiffness of the control beam. Despite they have

higher potential for separation especially in the case of repeated loading and

unloading, these beams can behave monolithically and their capacity can

conservatively guessed using flexural theory.


U shapes) with surface toughening (by manual grinder) and partial painting of

suitable adhesive can prevent shear failure, enhance the flexural stiffness

(service life stiffness 35% over the control beam) and provide extra strength

reach to 35% over the original beam. Despite this beam can behave

monolithically, the ultimate load is 5% lower than the theoretical capacity.


U shapes) with additional mechanical bonding (only the action of bonded

external stirrup) can prevent shear failure, enhance the flexural stiffness

(service life stiffness 42% over the control beam) and provide extra strength

reach to 94% over the original beam and 19% over the theoretical capacity.

127

These beams can behave monolithically and there capacity can conservatively

be guessed using flexural theory.

22. Referring to the points 19 to 21 the following points can be concluded:

a. The connection provided by additional stirrups only (which connected

into the old section) is enough.

b. The excessive drilling of holes in the sample for connecting additional

stirrups and shear connection at the sides and the bottom somehow

weaken the original sample.

c. If the surface is not prepared in a suitable quality, the case of surface

will not be as good as the case of either intact or well-prepared

surfaces.

23. In the range of the adopted distributions and configurations, strengthening

beams by steel plate mechanically connected to the sides cannot prevent shear

failure but can delay it, and can enhance the flexural stiffness at SLS and

provide extra strength reach to 25% over the original beam with enhancement

in service life stiffness reach 54% over that of the control beam. These beams

cannot behave monolithically.

24. In the range of the adopted distributions and configurations, strengthening

beams by steel plate chemically connected to the sides cannot prevent shear

failure but can delay it, and can enhance the flexural stiffness at SLS (reach to

41% over the control beam) and provide extra strength reach to 84% over the

original beam. These beams cannot behave monolithically and fail suddenly. 25. Sticking straps of steel plates at the outer face of the beams by suitable

adhesive and good distribution, can prevent shear failure and recover the

flexural capacity with enhancement in flexural stiffness at ULS, but can't

enhance the stiffness at SLS .

26. Although the effect of adhesive type did not studied deeply, it can be said that,

for sticking shear connectors there are signs that the adhesives which

contained sand as filler are better than those not contained due to the

additional friction.

27. Through studying the results of tests it can be ensured that the following items

have significant effect on the behavior of strengthened beams (but need more

study) :

a. Repeated loading and unloading of the strengthened beam.

b. The difference in the adhesive nature.

c. Drilling holes in the beam.

d. Workability and quality of new concrete and quality of curing after

concrete hardening.

e. In general, quality of strengthening construction.

128

6.3. Recommendations

6.3.1. Practical Recommendations

According to the experimental and theoretical study, which have been conducted in

this study, the following items can be recommended:

1. To precede the technique of concrete jacketing before the choice of steel

plating if there is not any other considerations.

2. For safe consequences and better results, it is recommended to use a

combination of mechanical and chemical bonding especially in the case of

strengthening RC beams by steel plates. This procedure guarantee prevention

of sudden separation and plate end peeling in the case of attaching steel plate

chemically bonded to the surface to the RC beam.

3. When concrete jacketing is used, if any considerations prevent the

combination between mechanical and chemical bonding it is recommended to

precede the choice of mechanical bonding.

4. To pay high attention to the quality of new materials or strengthening

procedures (anchoring, casting and curing).

5. To precede the three-face concrete jacketing before the choice of one-face

concrete jacketing if there is not any other considerations.

6. If the required is prevention of shear failure without increasing capacity its

recommended to stick straps of steel plates using suitable adhesive at the outer

face of RC beams in similar distribution of internal stirrups.

7. To use additional anchorage at the end of concrete jacket and steel plate.

8. To use the traditional procedures (flexural theory) to predict the theoretical

value of the ultimate load for concrete jacketed beams.

6.3.2. Recommendations for Future Research

In the field of experimental work related to this research it's recommended to:

1. Conduct experimental work to study the optimum combination between

mechanical and chemical bonding. The questions are;

a. What the level of combination which guarantee the yielding of steel

plate.

b. To which limit the shear connectors can be minimized to guarantee

satisfied level of safety and cost. Minimizing shear connectors is

important to decrease the level of weakening in the original beam.

2. Study the behavior of other configurations of steel plating, the question is;

which one can assure the composite action? this mean:

a. Overtaking the peeling of plate end and buckling of steel plate.

b. Overtaking the sudden failure of adhesive layer or sudden separation

of steel plate.

c. Yielding of steel plate until failure.

3. Study the effect of the following factors on the behavior of strengthened beams:

129

a. Repeated loading and unloading of the strengthened beam.

b. The difference in the adhesive nature.

c. Drilling holes in the beam.

d. Workability and quality of new concrete and quality of curing after

concrete hardening (In general, quality of strengthening construction).

4. Study the effect of preloading of the original beam to service loads on the

behavior of the strengthened beam.

5. Study the effect of conducting the strengthening operation during loading

(load simulate the dead loads only).

131

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