TEST OF PRESTRESSED CONCRETE T-BEAMS RETROFITTED FOR … · 2016. 5. 17. · Test of Prestressed...

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TEST OF PRESTRESSED CONCRETE T-BEAMS RETROFITTED FOR SHEAR AND FLEXURE USING

CARBON FIBER REINFORCED POLYMERS

Alison Agapay

Ian N. Robertson

Prepared in cooperation with the: State of Hawaii Department of Transportation Highways Division

and U.S. Department of Transportation Federal Highway Administration

Research Report UHM/CEE/04-08 August 2004

Technical Report Documentation Page 1. Report No.

2. Government Accession No.

3. Recipient's Catalog No. 5. Report Date August 2004

4. Title and Subtitle Test of Prestressed Concrete T-Beams Retrofitted for Shear and Flexure using Carbon Fiber Reinforced Polymers 6. Performing Organization Code

7. Author(s) Alison Agapay, Ian N. Robertson

8. Performing Organization Report No. 10. Work Unit No. (TRAIS)

9. Performing Organization Name and Address Department of Civil and Environmental Engineering University of Hawaii at Manoa 2540 Dole St. Holmes Hall 383 Honolulu, HI 96822

11. Contract or Grant No. HiDOT Project #46507

13. Type of Report and Period Covered Final

12. Sponsoring Agency Name and Address Hawaii Department of Transportation Highways Division 869 Punchbowl Street Honolulu, HI 96813

14. Sponsoring Agency Code

15. Supplementary Notes Prepared in cooperation with the U.S. Department of Transportation, Federal Highway Administration

16. Abstract In 1997, a precast prestressed T-Beam in the Ala Moana Shopping Center Parking Garage was strengthened in flexure using carbon fiber reinforced polymer (CFRP). When the old parking garage was demolished in June 2000 to make way for a new multilevel parking garage, this beam and two control beams were salvaged and transported to the University of Hawaii at Manoa Structural Testing Laboratory for testing. This report presents testing of the strengthened beam and a control beam. It also describes the retrofit procedures during field application of the CFRP strips, beam recovery, and preparation for laboratory testing. In addition, a step-by-step analysis of the predicted strengths is presented. To ensure flexure failure, the beams were retrofitted in shear with CFRP. Two types of wrapping scheme were used and anchorage was provided for the shear retrofit. The left half of each beam was retrofitted with 3” wide double layer CFRP stirrups. The right half of each beam was retrofitted with 12” wide CFRP sheets. After flexural testing, each half of each beam was recovered for shear testing. Flexural test results indicate that the CFRP strengthening provided a 71% increase compared with the control specimen without reducing the beam’s ductility. The flexural capacity of the strengthened beam was 21% greater than predicted by ACI 440R-02. The two T-Beam tests with CFRP sheets for shear strengthening produced 7% and 16% increases in the shear capacity when compared with the control beam without CFRP shear strengthening. These increases are below the 42% increase predicted by ACI 440R-02. Because of conservatism in the estimate of concrete and internal steel stirrup contribution to the shear capacity, the failure shear strength of the beams with CFRP sheets was still slightly greater than the ACI 440R-02 prediction for ultimate shear capacity. The shear tests indicated delamination of the CFRP stirrups and sheets occurring prior to the maximum shear load. Anchorage at the top and bottom of the beam web helped prevent complete delamination of the CFRP; however further anchorage development is required to improve the strength of the CFRP shear retrofit.

17. Key Word Prestressed T-beam Carbon Fiber Reinforced Polymer Flexural Strengthening Shear Strengthening

18. Distribution Statement

19. Security Classif. (of this report) Unclassified

20. Security Classif. (of this page) Unclassified

21. No. of Pages 268

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

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ABSTRACT

In 1997, a precast prestressed T-Beam in the Ala Moana Shopping Center Parking

Garage was strengthened in flexure using carbon fiber reinforced polymer (CFRP).

When the old parking garage was demolished in June 2000 to make way for a new

multilevel parking garage, this beam and two control beams were salvaged and

transported to the University of Hawaii at Manoa Structural Testing Laboratory for

testing. This report presents testing of the strengthened beam and a control beam. It also

describes the retrofit procedures during field application of the CFRP strips, beam

recovery, and preparation for laboratory testing. In addition, a step by step analysis of the

predicted strengths is presented.

To ensure flexure failure, the beams were retrofitted in shear with CFRP. Two types

of wrapping schemes were used and anchorage was provided for the shear retrofit. The

left half of each beam was retrofitted with 3” wide double layer CFRP stirrups. The right

half of each beam was retrofitted with 12” wide CFRP sheets. After flexural testing, each

half of each beam was recovered for shear testing.

Flexural test results indicate that the CFRP strengthening provided a 71% increase

compared with the control specimen without reducing the beam’s ductility. The flexural

capacity of the strengthened beam was 21% greater than predicted by ACI 440R-02. The

failure shear strength of the beams with CFRP sheets was slightly greater than the ACI

440R-02 prediction. The shear tests indicated delamination of the CFRP stirrups and

sheets occurring prior to the maximum shear load. Anchorage at the top and bottom of

the beam web helped prevent complete delamination of the CFRP; however further

anchorage development is required to maximize the strength of the CFRP shear retrofit.

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AKNOWLEDGEMENTS

This report is based on a Masters thesis prepared by Alison Agapay under the

direction of Dr. Ian Robertson. This research project was funded by research grant No.

46507 from the Hawaii Department of Transportation (HDOT) and the Federal Highway

Administration (FHWA). The contents of this report reflect the views of the authors,

who are responsible for the facts and accuracy of the data presented herein. The contents

do not necessarily reflect the official views or policies of the State of Hawaii, Department

of Transportation or the Federal Highway Administration. This report does not constitute

a standard, specification or regulation.

In addition to the primary sponsors, a number of individuals and companies have

made significant contributions to this research project. The authors would like to

acknowledge and thank the following:

Drs. Gregor Fischer and Si-Hwan Park for their effort in reviewing this report.

Timothy Goshi for assisting with the construction of the test frame, and in preparing the

beams for testing. Kainoa Aki for programming the LAB VIEW data acquisition system,

monitoring the instrumentation during each test, and assistance with installing the CFRP

retrofit. Gaur Johnson for assistance preparing the beams for testing and recording crack

development. Stephanie Fung for assistance with data recording during the tests.

Laboratory technicians, Andrew Oshita and Miles Wagner, for their advice during

construction of the test frame, and general assistance during laboratory testing.

The authors are extremely grateful to Adriano “A. B.” Bortolin of Sika Products,

USA, for providing valuable information concerning the original CFRP application, at

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which he was the Sika representative. Sika Products, USA, also donated all of the CFRP

materials required for laboratory shear retrofit of the test beams. Brian Ide of Martin and

Chock, Inc., the structural engineer responsible for the original CFRP strengthening

design, provided detailed information regarding the design and installation of the original

flexural strengthening. The authors are also indebted to Chandler Rowe and his

colleagues at Plas-Tech Ltd., Honolulu, Hawaii, for donating their labor and expertise in

the repair of the beams and for installation of the shear retrofit materials.

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TABLE OF CONTENTS

AKNOWLEDGEMENTS........................................................................................................... IV

LIST OF FIGURES..................................................................................................................... XI

LIST OF TABLES.................................................................................................................... XIX

1 INTRODUCTION ................................................................................................................ 1

1.1 BACKGROUND ................................................................................................................ 1

1.2 OBJECTIVE...................................................................................................................... 3

1.2.1 Use of CFRP for flexural strengthening .................................................................... 4

1.2.2 Use of CFRP for shear strengthening ........................................................................ 5

1.3 SUMMARY ...................................................................................................................... 6

2 LITERATURE REVIEW .................................................................................................... 9

2.1 INTRODUCTION............................................................................................................... 9

2.2 CFRP FLEXURAL STRENGTHENING ............................................................................... 9

2.3 CFRP SHEAR STRENGTHENING ................................................................................... 14

2.4 CHAPTER SUMMARY .................................................................................................... 19

3 ALA MOANA BEAM REPAIR ........................................................................................ 21

4 BEAM RECOVERY AND REPAIR................................................................................. 25

4.1 INTRODUCTION............................................................................................................. 25

4.2 EPOXY CRACK REPAIR................................................................................................. 27

4.3 EPOXY MORTAR SPALL REPAIR................................................................................... 28

4.4 TOP SLAB REPLACEMENT ............................................................................................ 31

5 TEST SETUP AND BEAM LAYOUT.............................................................................. 35

5.1 INTRODUCTION............................................................................................................. 35

5.2 TEST APPARATUS......................................................................................................... 35

5.3 BEAM SHEAR RETROFIT............................................................................................... 41

5.3.1 CFRP Shear Stirrups................................................................................................ 41

5.3.2 CFRP Shear Sheets .................................................................................................. 49

5.4 BEAM TEST CONFIGURATION AND INSTRUMENTATION .............................................. 54

5.4.1 T-Beam 1 Layout and Instrumentation.................................................................... 54

5.4.2 T-Beam 1L Layout and Instrumentation ................................................................. 58

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5.4.3 T-Beam 1R Layout and Instrumentation ................................................................. 60

5.4.4 T-Beam 2 Layout and Instrumentation.................................................................... 64

5.4.5 T-Beam 2L Layout and Instrumentation ................................................................. 76

5.4.6 T-Beam 2R1 Layout and Instrumentation ............................................................... 79

5.4.7 T-Beam 2R2 Test Setup and Layout ....................................................................... 81

6 MATERIAL PROPERTIES.............................................................................................. 83

6.1 CONCRETE COMPRESSIVE STRENGTHS ........................................................................ 83

6.2 TOP SLAB CONCRETE MODULUS OF RUPTURE ............................................................ 85

6.3 STEEL REINFORCEMENT TENSILE STRENGTHS ............................................................ 85

6.4 CFRP MATERIAL PROPERTIES..................................................................................... 86

6.5 CFRP PULL-OFF TESTS ................................................................................................ 87

7 THEORETICAL BEAM STRENGTHS .......................................................................... 89

7.1 NOTATION .................................................................................................................... 89

7.2 FLEXURAL STRENGTH OF T-BEAM 1............................................................................ 94

7.3 FLEXURAL STRENGTH OF T-BEAM 2 (W/CARBODUR STRIPS) ...................................... 96

7.3.1 Flexural Capacity of a Reinforced Concrete Beam with CFRP .............................. 96

7.3.2 Nominal Flexural Capacity of a Prestressed Concrete Beam with CFRP ............... 99

7.3.3 Calculation of the Predicted Flexural Strength of T-Beam 2 ................................ 104

7.4 SHEAR STRENGTH OF T-BEAM 1 (WITHOUT SHEAR RETROFIT) ................................. 112

7.5 SHEAR STRENGTH OF T-BEAM 2R1 (PLAIN CONCRETE) ............................................ 119

7.6 SHEAR STRENGTH OF T-BEAM 1L (W/CFRP STIRRUPS)............................................ 122

7.7 SHEAR STRENGTH OF T-BEAM 2L (W/CFRP STIRRUPS)............................................ 126

7.8 SHEAR STRENGTH OF T-BEAM 1R (W/CFRP SHEETS)............................................... 130

7.9 SHEAR STRENGTH OF T-BEAM 2R2 (W/CFRP SHEETS) ............................................ 134

8 RESULTS AND DISCUSSION ....................................................................................... 139

8.1 T-BEAM 1 RESPONSE ................................................................................................. 139

8.1.1 ACI 318 Predicted Flexural Capacities ................................................................. 141

8.1.2 Slab Reinforcement Strain Gage Readings............................................................ 144

8.1.3 Concrete Strain Gage Readings ............................................................................. 148

8.1.4 Vertical Deflection................................................................................................. 148

8.1.5 Strains in the CFRP stirrups and sheets ................................................................. 148

8.2 T-BEAM 2 RESPONSE ................................................................................................. 152

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8.2.1 Failure mechanism for T-Beam 2 .......................................................................... 157

8.2.2 Slab Reinforcement Strain Gage Readings............................................................ 160

8.2.3 Vertical Displacement from LVDT Readings ....................................................... 161

8.2.4 Carbodur Strip Strain Gages.................................................................................. 166

8.3 ACI 440 VERSUS EXPERIMENTAL MOMENT CAPACITY............................................ 182

8.4 SHEAR STRENGTH OF T-BEAM 2R1 (PLAIN CONCRETE) ............................................ 184

8.5 SHEAR STRENGTH OF T-BEAM 1L (CFRP STIRRUPS) ............................................... 189

8.5.1 Measured strain in the CFRP stirrups .................................................................... 195

8.6 SHEAR STRENGTH OF T-BEAM 2L (CFRP STIRRUPS) ............................................... 206

8.6.1 Strain Gage Readings for CFRP Stirrups .............................................................. 213

8.7 SHEAR STRENGTH OF T-BEAM 1R (CFRP SHEETS) .................................................. 220

8.7.1 Strain Gage Readings for CFRP Sheets................................................................. 225

8.7.2 Carbodur strip strain gages .................................................................................... 225

8.8 SHEAR STRENGTH OF T-BEAM 2R2 (CFRP SHEETS) ............................................... 231

8.8.1 Strain Gage Readings attached on CFRP Sheets ................................................... 236

8.9 COMPARISON OF THE SHEAR STRENGTHS OF THE T-BEAMS TESTED IN SHEAR........ 240

8.10 ACI 440 VERSUS EXPERIMENTAL SHEAR CAPACITIES ............................................. 242

9 SUMMARY AND CONCLUSION ................................................................................. 245

9.1 SUMMARY .................................................................................................................. 245

9.2 CONCLUSIONS ............................................................................................................ 246

9.2.1 Flexure Tests.......................................................................................................... 246

9.2.2 Shear Tests............................................................................................................. 247

10 APPENDIX A.................................................................................................................... 249

REFERENCES .......................................................................................................................... 267

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LIST OF FIGURES

FIGURE 1-1 DAMAGED BEAM IN THE ALA MOANA PARKING GARAGE PRIOR TO 1997 REPAIR ..... 2

FIGURE 1-2 ELEVATION OF REPAIRED T-BEAM SHOWING THE TOP FLANGE AND JOISTS............... 3

FIGURE 3-1 CROSS SECTION OF T-BEAM FOR CARBODUR STRIP INSTALLATION ......................... 22

FIGURE 3-2 SURFACE PREPARATION PRIOR TO CFRP APPLICATION............................................ 23

FIGURE 3-3 SIKA CARBODUR STRIPS BEING PREPARED FOR INSTALLATION ................................ 23

FIGURE 3-4 APPLICATION OF CFRP STRIPS ................................................................................. 23

FIGURE 3-5 WET LAY-UP ANCHORAGE WRAP AT END OF CFRP STRIPS ...................................... 24

FIGURE 3-6 SIKA WET LAY-UP WRAPS AT CONCRETE SPALLS...................................................... 24

FIGURE 3-7 REPAIRED BEAM IN SERVICE ..................................................................................... 24

FIGURE 4-1 SAW CUTTING COLUMN CAPITAL TO REMOVE PRECAST PRESTRESSED BEAM .......... 26

FIGURE 4-2 CABLE SUPPORT DURING BEAM REMOVAL................................................................ 26

FIGURE 4-3 INJECTION PORT PLACEMENT ................................................................................... 27

FIGURE 4-4 EPOXY INJECTION...................................................................................................... 28

FIGURE 4-5 PRIMING THE REPAIR CONTACT SURFACE ................................................................. 29

FIGURE 4-6 FORMING THE REPAIR AREA...................................................................................... 29

FIGURE 4-7 POURING EPOXY MORTAR PATCH MATERIAL ............................................................ 30

FIGURE 4-8 COMPLETED SPALL REPAIR ....................................................................................... 30

FIGURE 4-9 TYPICAL T-BEAM CROSS-SECTION............................................................................ 32

FIGURE 4-10 T-BEAM 1 FLANGE LAYOUT .................................................................................... 33

FIGURE 4-11 T-BEAM 1 FLANGE REINFORCEMENT LAYOUT........................................................ 33

FIGURE 4-12 PLYWOOD BLOCK-OUTS FOR CFRP SHEAR STIRRUPS............................................. 34

FIGURE 4-13 T-BEAM 1 TOP FLANGE CONCRETE PLACEMENT..................................................... 34

FIGURE 5-1 SCHEMATIC LAYOUT OF TEST SETUP ....................................................................... 36

FIGURE 5-2 DETAIL OF FINAL TEST FRAME DESIGN.................................................................... 38

FIGURE 5-3 TEST FRAME DETAILS ............................................................................................... 39

FIGURE 5-4 TEST FRAME UNDER CONSTRUCTION (SHORT COLUMNS)........................................ 40

FIGURE 5-5 COMPLETED TEST FRAME WITH T-BEAM 1 READY FOR LOADING ........................... 40

FIGURE 5-6 SHEAR RETROFIT LAYOUT FOR T-BEAM 1 ................................................................ 43

FIGURE 5-7 SLOTTED HOLES IN TOP SLAB FOR CFRP SHEAR STIRRUPS....................................... 44

FIGURE 5-8 CONCRETE SURFACE PREPARATION USING NEEDLE GUN .......................................... 44

FIGURE 5-9 ROUGHENED WEB FOR 3” WIDE STIRRUPS ................................................................ 44

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FIGURE 5-10 CFRP STIRRUPS BEING SATURATED WITH SIKADUR HEX 300................................ 47

FIGURE 5-11 SIKA 30 HI MOD GEL EPOXY USED TO PREPARE SURFACE FOR CFRP.................... 47

FIGURE 5-12 3” WIDE DOUBLE LAYER CFRP STIRRUPS WRAPPED THROUGH SLOTTED HOLES ... 47

FIGURE 5-13 CFRP STIRRUPS IN PLACE, BOTTOM TRIMMED AFTER CURING............................... 48

FIGURE 5-14 INSTALLATION OF STEEL TUBE ANCHORAGE FOR 3” WIDE CFRP STIRRUPS .......... 48

FIGURE 5-15 COMPLETE INSTALLATION OF ANCHORAGE FOR 3” WIDE CFRP STIRRUPS ............ 49

FIGURE 5-16 ROUGHENED WEB AND HI MOD GEL APPLICATION FOR 12” WIDE SHEETS ............ 50

FIGURE 5-17 CFRP SHEETS SATURATED WITH SIKADUR HEX 300.............................................. 50

FIGURE 5-18 INSTALLATION OF CFRP SHEETS AS SHEAR REINFORCEMENT ............................... 50

FIGURE 5-19 SIKA 30 HI MOD GEL EPOXY BED AT RE-ENTRANT CORNER OF CFRP SHEETS...... 52

FIGURE 5-20 TUBE ANCHORAGE SET IN EPOXY BED AT RE-ENTRANT CORNER OF CFRP SHEETS 52

FIGURE 5-21 TELESCOPE SPLICE IN STEEL TUBE ANCHORAGE..................................................... 53

FIGURE 5-22 COMPLETE INSTALLATION OF MECHANICAL ANCHORAGE FOR 12” CFRP SHEETS 53

FIGURE 5-23 T-BEAM 1 LAYOUT AND INSTRUMENTATION ......................................................... 56

FIGURE 5-24 T-BEAM 1 IN THE LOAD FRAME READY FOR TESTING ............................................. 57

FIGURE 5-25 T-BEAM 1 READY FOR TESTING – CENTER VIEW..................................................... 57

FIGURE 5-26 LAYOUT AND INSTRUMENTATION OF T-BEAM 1L.................................................. 59

FIGURE 5-27 T-BEAM 1L READY FOR TESTING ............................................................................ 60

FIGURE 5-28 FLEXURAL STRENGTHENING OF T-BEAM 1R USING PRE-CURED CARBODUR STRIPS

.............................................................................................................................................. 62

FIGURE 5-29 T-BEAM 1R LAYOUT AND INSTRUMENTATION....................................................... 63

FIGURE 5-30 T-BEAM 1R READY FOR TESTING............................................................................ 64

FIGURE 5-31 STEEL REACTION BRACKETS AT BOTH ENDS OF T-BEAM 2..................................... 65

FIGURE 5-32 T-BEAM 2 TOP FLANGE REINFORCEMENT AND STRAIN GAGE LAYOUT................... 66

FIGURE 5-33 T-BEAM 2 SLAB LAYOUT PRIOR TO CONCRETE POUR ............................................. 67

FIGURE 5-34 T-BEAM 2 BEING PREPARED FOR SHEAR RETROFIT.................................................. 68

FIGURE 5-35 BEAM SHEAR RETROFIT LAYOUT FOR T-BEAM 2 .................................................... 70

FIGURE 5-36 SIKA 30 HI MOD GEL EPOXY BEING APPLIED TO THE CFRP STIRRUPS FOR EVEN

SEATING OF THE ANCHORAGE ANGLES ................................................................................. 71

FIGURE 5-37 CFRP ANGLES INSTALLED AS ANCHORAGE FOR CFRP SHEAR REINFORCEMENT .. 71

FIGURE 5-38 ELECTRICAL RESISTANCE STRAIN GAGES INSTALLED ON THE CARBODUR STRIPS.. 72

FIGURE 5-39 T-BEAM 2 LAYOUT AND INSTRUMENTATION ......................................................... 74

FIGURE 5-40 T-BEAM 2 TEST SETUP............................................................................................. 75

FIGURE 5-41 T-BEAM 2L BEING PREPARED FOR SHEAR TESTING ................................................ 76

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FIGURE 5-42 T-BEAM 2L LAYOUT AND INSTRUMENTATION....................................................... 78

FIGURE 5-43 T-BEAM 2L IN TEST SETUP...................................................................................... 78

FIGURE 5-44 WEDGE ANCHORS INSTALLED ON PRESTRESSED STRANDS AT END OF T-BEAM 2R1

.............................................................................................................................................. 79

FIGURE 5-45 T-BEAM 2R1 LAYOUT AND INSTRUMENTATION..................................................... 80

FIGURE 5-46 T-BEAM 2R1 IN TEST SETUP.................................................................................... 80

FIGURE 5-47 T-BEAM 2R2 LAYOUT AND INSTRUMENTATION..................................................... 82

FIGURE 5-48 T-BEAM 2R2 IN TEST FRAME.................................................................................. 82

FIGURE 6-1 CONCRETE CORE SAMPLE TAKEN FROM A T-BEAM WEB .......................................... 84

FIGURE 6-2 DYNA Z16 PULL-OFF TESTER ................................................................................. 87

FIGURE 6-3 TYPICAL LOCATIONS OF CFRP PULL-OFF TESTS....................................................... 87

FIGURE 7-1 CROSS-SECTION OF T-BEAM 1 .................................................................................. 94

FIGURE 7-2 STRESS AND STRAIN DISTRIBUTION OF A REINFORCED CONCRETE BEAM WITH CFRP

UNDER FLEXURE AT ULTIMATE LIMIT STATE CONDITION ..................................................... 97

FIGURE 7-3 STRESS AND STRAIN DISTRIBUTION OF A PRESTRESSED CONCRETE BEAM UNDER

FLEXURE AT THE INITIAL CONDITION (PRIOR TO APPLICATION OF CFRP).......................... 100

FIGURE 7-4 STRESS AND STRAIN DISTRIBUTION OF A PRESTRESSED CONCRETE BEAM WITH CFRP

UNDER FLEXURE AT ULTIMATE LIMIT STATE CONDITION ................................................... 102

FIGURE 7-5 T-BEAM 2 TRIBUTARY WIDTH AT ALA MOANA PARKING GARAGE ....................... 105

FIGURE 7-6 SECTION PROPERTIES OF THE PRECAST PRESTRESSED BEAM SECTION.................... 106

FIGURE 7-7 SECTION PROPERTIES OF THE COMPOSITE SECTION ................................................ 107

FIGURE 7-8 STRESS AND STRAIN DISTRIBUTIONS FOR T-BEAM 2 AT THE INITIAL CONDITION .. 108

FIGURE 7-9 STRESS AND STRAIN DISTRIBUTIONS FOR T-BEAM 2 AT ULTIMATE STATE

CONDITIONS ........................................................................................................................ 109

FIGURE 7-10 T-BEAM 1 LAYOUT FOR SHEAR STRENGTH CALCULATION ................................... 112

FIGURE 7-11 SHEAR CAPACITY AND SHEAR DIAGRAM OF T-BEAMS 1 AND 2............................ 118

FIGURE 7-12 T-BEAM 2R1 LAYOUT AND SHEAR AND MOMENT DIAGRAMS .............................. 119

FIGURE 7-13 T-BEAM 1L LAYOUT AND SHEAR AND MOMENT DIAGRAMS................................. 122

FIGURE 7-14 CROSS SECTION OF T-BEAM 1L SHOWING CFRP STIRRUP LAYOUT ..................... 124

FIGURE 7-15 T-BEAM 2L LAYOUT AND SHEAR AND MOMENT DIAGRAMS................................. 126

FIGURE 7-16 CROSS SECTION OF T-BEAM 2L ............................................................................ 128

FIGURE 7-17 T-BEAM 1R LAYOUT AND SHEAR AND MOMENT DIAGRAMS ................................ 130

FIGURE 7-18 CROSS SECTION OF T-BEAM 1R SHOWING CFRP SHEET LAYOUT ........................ 132

FIGURE 7-19 T-BEAM 2R2 LAYOUT AND SHEAR AND MOMENT DIAGRAMS .............................. 134

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FIGURE 7-20 CROSS SECTION OF T-BEAM 2R2 SHOWING CFRP SHEETS .................................. 136

FIGURE 8-1 T-BEAM 1 READY FOR FLEXURAL TESTING............................................................. 139

FIGURE 8-2 MID-SPAN MOMENT – DISPLACEMENT RELATIONSHIP FOR T-BEAM 1.................. 142

FIGURE 8-3 T-BEAM 1 MID-SPAN CONDITION CORRESPONDING TO SIX DUCTILITY LEVELS...... 143

FIGURE 8-4 T-BEAM 1 SLAB REINFORCEMENT STRAIN GAGE READINGS................................... 145

FIGURE 8-5 STRAIN READINGS FOR STRAIN GAGES 4-6 (T-BEAM 1) ......................................... 146

FIGURE 8-6 STRAIN READINGS FOR STRAIN GAGE 1-4 (T-BEAM 1) ........................................... 147

FIGURE 8-7 REPRESENTATION OF THE VERTICAL DEFLECTION OF T-BEAM 1 FROM LVDT

READINGS............................................................................................................................ 150

FIGURE 8-8 STRAIN READINGS FROM GAGES ATTACHED TO CFRP STIRRUPS AND SHEETS (T-

BEAM 1) .............................................................................................................................. 151

FIGURE 8-9 T-BEAM 2 READY FOR FLEXURAL TESTING............................................................. 152

FIGURE 8-10 MID-SPAN MOMENT-DISPLACEMENT RELATIONSHIP FOR T-BEAM 2 .................. 155

FIGURE 8-11 T-BEAM 2 CONDITION CORRESPONDING TO SIX DUCTILITY LEVELS .................... 156

FIGURE 8-12 FLEXURE-SHEAR CRACK FORMED OUTSIDE OF THE LEFT LOAD POINT (T-BEAM 2).

............................................................................................................................................ 157

FIGURE 8-13 DELAMINATION OF CARBODUR STRIPS INITIATING AT THE FLEXURE-SHEAR CRACK

............................................................................................................................................ 159

FIGURE 8-14 CARBODUR STRIPS DELAMINATED FROM BEAM AND PULLING OUT OF CFRP WRAP

ANCHOR .............................................................................................................................. 159

FIGURE 8-15 STRAIN READINGS FOR SLAB REINFORCEMENT STRAIN GAGE (T-BEAM 2) .......... 162

FIGURE 8-16 STRAIN READINGS FOR STRAIN GAGES 4-6 (T-BEAM 2) ....................................... 163

FIGURE 8-17 STRAIN READINGS FOR STRAIN GAGES 1-4 (T-BEAM 2) ....................................... 164

FIGURE 8-18 REPRESENTATION OF THE VERTICAL DEFLECTION OF T-BEAM 2 FROM LVDT

READINGS............................................................................................................................ 165

FIGURE 8-19 T-BEAM 2 BEAM SOFFIT SHOWING LOCATION OF STRAIN GAGES ......................... 166

FIGURE 8-20 STRAIN READINGS OF STRAIN GAGES 7-9.............................................................. 168

FIGURE 8-21 STRAIN READINGS OF STRAIN GAGES 25-27.......................................................... 169






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FIGURE 8-27 WEB-SHEAR CRACKS FORMING AWAY FROM THE MID-SPAN OF BEAM AS

CONFIRMED BY CARBODUR STRAIN READINGS................................................................... 175

FIGURE 8-28 STRAIN READINGS FOR GAGES ON THE FIRST CARBODUR STRIP ........................... 176

FIGURE 8-29 STRAIN READINGS FOR GAGES ON THE SECOND CARBODUR STRIP ....................... 177

FIGURE 8-30 STRAIN READINGS FOR GAGES ON THE THIRD CARBODUR STRIP .......................... 178

FIGURE 8-31 STRAIN READINGS ON THE FIRST CARBODUR STRIP CORRESPONDING TO THE SIX

DUCTILITY LEVELS.............................................................................................................. 179

FIGURE 8-32 STRAIN READINGS ON THE SECOND CARBODUR STRIP CORRESPONDING TO THE SIX


FIGURE 8-33 STRAIN READINGS ON THE THIRD CARBODUR STRIP CORRESPONDING TO THE SIX


FIGURE 8-34 PLOT OF NORMALIZED ACI 440 PREDICTION AND EXPERIMENTAL MOMENT

CAPACITIES ......................................................................................................................... 183

FIGURE 8-35 TEST SETUP AND SHEAR SPAN OF T-BEAM 2R1 .................................................... 184

FIGURE 8-36 SHEAR-DISPLACEMENT RELATIONSHIP FOR T-BEAM 2R1.................................... 186

FIGURE 8-37 T-BEAM 2R1 CONDITION AT CRITICAL STAGES DURING THE TEST....................... 187

FIGURE 8-38 FAILURE OF STEEL SHEAR REINFORCEMENT AT FAILURE SHEAR CRACK.............. 188

FIGURE 8-39 SHEAR REINFORCEMENT ANCHORAGE FAILURE AT BASE OF WEB ........................ 188

FIGURE 8-40 TEST SETUP OF T-BEAM 1L (CFRP STIRRUPS) ..................................................... 189

FIGURE 8-41 SHEAR-DISPLACEMENT RELATIONSHIP FOR T-BEAM 1L...................................... 192

FIGURE 8-42 T-BEAM 1L CONDITION AT VARIOUS STAGES IN THE SHEAR-DISPLACEMENT

RESPONSE............................................................................................................................ 193

FIGURE 8-43 FLEXURAL FAILURE OF T-BEAM 1L...................................................................... 194

FIGURE 8-44 DELAMINATION OF CFRP STIRRUPS FROM T-BEAM 1L ....................................... 194

FIGURE 8-45 STRAIN READINGS FROM STRAIN GAGE 25............................................................ 197






FIGURE 8-51 STRAIN READINGS ON STRAIN GAGE 30 ................................................................ 203

FIGURE 8-52 STRAIN READINGS FROM STRAIN GAGES 28-30 FROM THE FIRST TEST OF T-BEAM

1L........................................................................................................................................ 204

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FIGURE 8-53 STRAIN READINGS FROM STRAIN GAGES 28-30 FROM THE SECOND TEST OF T-BEAM

1L........................................................................................................................................ 205

FIGURE 8-54 T-BEAM 2L TEST SETUP ........................................................................................ 207

FIGURE 8-55 INITIAL CONDITION OF T-BEAM 2L BEFORE TESTING........................................... 207

FIGURE 8-56 SHEAR – DISPLACEMENT CURVE FOR T-BEAM 2L................................................ 209

FIGURE 8-57 T-BEAM 2L CONDITION AT VARIOUS STAGES DURING TESTING ........................... 210

FIGURE 8-58 SHEAR FAILURE AND TENDON END ANCHORAGE SLIP........................................... 211

FIGURE 8-59 RUPTURE OF GFRP ANGLE AT THRU-BOLTS......................................................... 211

FIGURE 8-60 AREAS OF CFRP DELAMINATION ON T-BEAM 2L ................................................ 212

FIGURE 8-61 BUCKLING OF CFRP STIRRUPS AT FAILURE....................................................... 212

FIGURE 8-62 STRAIN READINGS FROM STRAIN GAGES 1 AND 7 ................................................. 214



FIGURE 8-65 STRAIN READINGS FROM STRAIN GAGES 4 AND 10 ............................................... 217



FIGURE 8-68 T-BEAM 1R TEST SETUP........................................................................................ 220

FIGURE 8-69 SHEAR-DISPLACEMENT CURVE FOR T-BEAM 1R.................................................. 222

FIGURE 8-70 T-BEAM 1R CONDITION AT VARIOUS STAGES DURING SHEAR TESTING ............... 223

FIGURE 8-71 T-BEAM 1R CONDITION AT CFRP DELAMINATION .............................................. 224

FIGURE 8-72 T-BEAM 1R CONDITION AT FAILURE..................................................................... 224

FIGURE 8-73 STRAIN READINGS FROM STRAIN GAGES 13-16..................................................... 226




FIGURE 8-77 STRAIN READINGS ON THE CARBODUR STRIPS FROM STRAIN GAGES 30-32.......... 230

FIGURE 8-78 T-BEAM 2R2 TEST SETUP...................................................................................... 231

FIGURE 8-79 SHEAR-DISPLACEMENT CURVE FOR T-BEAM 2R2................................................ 233

FIGURE 8-80 T-BEAM 2R2 CONDITION AT SEVEN STAGES......................................................... 234

FIGURE 8-81 DELAMINATION OF THE CFRP SHEET ................................................................... 235

FIGURE 8-82 RUPTURE OF GFRP ANGLES AT THRU-BOLTS ....................................................... 235

FIGURE 8-83 STRAIN READINGS FROM STRAIN GAGES 1-3......................................................... 237



xvii

FIGURE 8-86 SHEAR-DISPLACEMENT CURVES FOR ALL SHEAR TESTS....................................... 241

FIGURE 8-87 NORMALIZED ACI 440 PREDICTIONS VERSUS EXPERIMENTAL SHEAR CAPACITIES

............................................................................................................................................ 243

xix

LIST OF TABLES

TABLE 6.1 CONCRETE COMPRESSIVE STRENGTH AND MODULUS OF ELASTICITY...................... 84

TABLE 6.2 MODULUS OF RUPTURE TEST ..................................................................................... 85

TABLE 6.3 STEEL REINFORCEMENT TENSILE STRENGTHS........................................................... 86

TABLE 6.4 CFRP MATERIAL PROPERTIES.................................................................................... 86

TABLE 6.5 PULL-OFF TEST RESULTS............................................................................................ 88

1

CHAPTER 1

1 INTRODUCTION

1.1 Background Carbon Fiber Reinforced Polymers (CFRP) has become a valuable material for

repairing and retrofitting damaged or deficient structures. Numerous research studies

have shown that CFRP sheets or strips bonded to the concrete surface can substantially

increase flexural, shear and compressive strength of concrete members. The literature

review in Chapter 2 summarizes a number of research studies on the use of CFRP for

flexural and shear strengthening of concrete beams.

During a routine structural inspection of the Ala Moana Parking Garage in the late

1990’s, a number of large flexural cracks were noted on the bottom of a precast

prestressed concrete beam along with severe spalling damage to the beam ledges (Figure

1-1). The beam spans 30’ from center to center of supporting columns. The beam is a

precast prestressed T-Beam supporting joists and a slab acting as the top flange of the

beam (Figure 1-2). The damaged beam was repaired in 1997 using CFRP materials.

Chapter 3 provides a description of the original beam repair.

In 2000, the portion of the Ala Moana Parking Garage around this beam was

demolished to allow for a new multilevel parking structure. This beam, along with two

identical undamaged beams, was salvaged in June 2000 for testing in the University of

Hawaii at Manoa Structural Testing Laboratory (UHM-STL). During demolition, beam

recovery, and transportation, minor damage was caused to the beam webs. This damage

2

was repaired and the top slab reinstated at UHM-STL prior to testing. Chapter 4

describes the beam recovery and repair operations.

The research program reported here involved flexural and shear testing of two of the

beams salvaged from the Ala Moana Parking Garage. One un-strengthened beam was

used as the control specimen. This beam is referred to as T-Beam 1. The second beam is

the strengthened beam referred to as T-Beam 2. The third beam will be used in a future

research study on CFRP shear retrofit of cracked beams.

Considerable instrumentation was installed to monitor the beams during testing. The

layout of this instrumentation for each test, and the precise test setup for each loading

condition, are described in detail in Chapter 5. Material properties for the beams and

CFRP materials are presented in Chapter 6. Predicted strengths of the control and

strengthened beams are presented in Chapter 7. The results of the flexural and shear

testing are presented in Chapter 8. Finally Chapter 9 presents a summary and

conclusions for this research project.

Figure 1-1 Damaged beam in the Ala Moana parking garage prior to 1997 repair

3

30'

BEAM ELEVATION

Figure 1-2 Elevation of repaired T-beam showing the top flange and joists

1.2 Objective The primary objective of this research program was to evaluate the use of CFRP

material as a retrofit for damaged or deficient prestressed concrete beams. The repaired

beam, T-Beam 2, was the first application of CFRP material for strengthening of a

concrete structure in the State of Hawaii. The Hawaii Department of Transportation

(HDOT) funded this research program in order to evaluate the use of CFRP for retrofit of

deficient bridge structures across the state. Because of similarities between the Ala

Moana beams and typical prestressed AASHTO bridge girders, the results of this study

should provide valuable insight into the likely performance of bridge girders retrofitted

with CFRP materials.

Very few tests have been conducted on field applied CFRP after exposure to service

conditions. The precast prestressed beam tested in this study was retrofitted for flexure

with CFRP applied under field conditions. It was then in service for three years and

4

spent an additional eighteen months exposed to exterior environmental conditions prior to

testing.

Initial theoretical strength calculations for T-Beam 1 and T-Beam 2 indicated that the

addition of CFRP to increase the flexural capacity of T-Beam 2 resulted in a shear critical

failure mode if the beam were tested under the proposed laboratory conditions. To

reduce the potential for a shear failure and ensure the desired flexural failure of T-Beam

2, the shear spans of both beams were increased for the laboratory loading and CFRP

shear retrofit was applied prior to flexural testing. Two shear retrofit techniques were

employed on each beam, namely external CFRP shear stirrups on the left half of the beam

and CFRP sheets on the right half of the beam. Subsequent to flexural testing, each of

the beam shear spans was tested in shear to evaluate the performance of the CFRP shear

retrofit.

1.2.1 Use of CFRP for flexural strengthening

Prior laboratory research studies have shown that external application of CFRP in the

tension zone of a flexural member can dramatically increase the flexural capacity of the

member (Chapter 2 Literature Review). The externally applied CFRP adds to the tensile

capacity of the existing internal non-prestressed or prestressed tension reinforcement,

thereby increasing the flexural capacity. The high tensile strength of CFRP materials

provides significantly increased capacity for relatively small amounts of added material.

The relatively high modulus of elasticity of CFRP also enhances the post-cracking

flexural stiffness of the member. Care must be taken not to increase the flexural tension

reinforcement to the point where compression failure of the concrete governs the flexural

strength of the beam.

5

T-Beam 2 was retrofitted with three pre-cured carbodur strips epoxied to the bottom

of the beam. The ends of these tension strips were restrained at the ends of the beam by

means of uni-directional CFRP fabric wraps. Chapter 5 provides a detailed description of

the flexural strengthening of T-Beam 2.

1.2.2 Use of CFRP for shear strengthening

Shear strengthening using CFRP has proved effective in a number of prior

experimental studies (Chapter 2 Literature Review). In many of these previous studies,

the CFRP shear reinforcement was bonded to the beam web without any anchorage at the

top and bottom of the web. Failure of the CFRP shear retrofit was usually the result of

de-bonding of the CFRP from the concrete surface. The relatively low tensile strength of

the surface concrete controls the pull-off strength and therefore precipitates the

debonding failure. Few studies have considered mechanical anchorage of the CFRP

shear reinforcement at top and bottom of the beam. Such anchorage may improve the

performance of the shear reinforcement and so was used in the tests reported here.

Two types of shear strengthening were installed on T-Beam 1 and T-Beam 2. The

left half of each beam was retrofitted with external CFRP stirrups placed at 12” on center,

between the internal 3/8” diameter steel stirrups. Each stirrup consisted of a 3” wide

double layer of CFRP fabric extending from the bottom of the beam, up the side of the

web, through a slot in the top slab, continuously over the top of the web and down the

other side of the beam. In order to prevent premature delamination of the CFRP stirrups

at the re-entrant corner at the bottom of the web, mechanical anchorage was provided in

the form of steel tubes or GFRP angles with steel bolts through the beam web.

6

The right half of each beam was retrofitted in shear with 12” wide single layer sheets

of CFRP epoxied to both sides of the beam web. The shear sheets were installed at 18”

on center so as to leave a 6” gap between the sheets to allow moisture and vapor to

escape from the beam concrete without affecting the bond between CFRP and concrete.

Each sheet extended from the bottom of the web to the soffit of the top slab. In order to

prevent premature delamination of the CFRP sheets at the re-entrant corners at the

bottom of the slab and at the bottom of the web, mechanical anchorage similar to that

used for the shear stirrups was installed at both locations. A detailed description of the

shear retrofit of each beam is provided in Chapter 5

1.3 Summary The objectives of this research program are summarized below.

1. To determine the increase in flexural strength of the precast T-Beam as a result of

the field applied CFRP flexural reinforcement. The performance of the

strengthened T-Beam 2 will be compared with the control specimen, T-Beam 1,

and with the results of previous experimental studies.

2. To evaluate the use of CFRP in the form of stirrups and sheets for shear retrofit of

prestressed concrete beams. The effect of mechanical anchorage provided at the

re-entrant corners of the CFRP shear reinforcement will also be evaluated.

3. Compare shear test results with previous experimental studies and with the

strength predicted by the recently published ACI 440R-02 committee report on

the use of externally bonded FRP.

7

4. To determine the failure mechanism of the CFRP flexural and shear retrofit

systems used on the T-Beams.

5. To recommend appropriate CFRP flexural and shear retrofit methods for

prestressed concrete bridge girder beams.

9

CHAPTER 2

2 LITERATURE REVIEW

2.1 Introduction Since the late 1980’s, research into the use of Fiber Reinforced Polymers (FRP) for

external repair and retrofit of concrete flexural members has progressed rapidly.

Researchers have considered various types of FRP, different application techniques and

various loading conditions. The majority of this research has been performed in the

laboratory on small-scale reinforced concrete specimens. Limited research is available

on the performance of field applied FRP and on the application of FRP materials to

prestressed concrete beams. This chapter presents some of the recent research using

Carbon Fiber Reinforced Polymer (CFRP) for flexural and shear strengthening of

reinforced concrete members.

2.2 CFRP Flexural Strengthening Numerous research programs have studied the performance of CFRP as flexural

strengthening for reinforced concrete beams. These studies considered different methods

of wrapping the CFRP onto the concrete member. Some also investigated the effect of

rate of loading on the performance of the strengthened concrete member. Research has

also been performed on the use of CFRP as confinement for partially corroded concrete

members.

Spadea et al.1 studied two methods of adding CFRP for flexural strengthening of

reinforced concrete members. In the first method, two layers of CFRP were bonded to

the tension face of the specimen and wrapped up the vertical faces of the beam to one

10

third of the beam height. In the second method, four layers of CFRP were bonded in the

same manner as before with the addition of end anchorage wraps near the supports that

extended to the top of the vertical faces of the beam. Both CFRP strengthening methods

produced higher flexural strengths than the control beam, reaching the calculated

theoretical strengths. However the flexural ductility of the beams was reduced compared

with the un-strengthened control specimen. The first application method exhibited a

sudden failure mode due to debonding of the laminate. This method showed a loss in

ductility of between 45% and 65%. The second method showed a sudden failure mode

due to shear at the supports. Although the end anchorages were effective at restraining

debonding of the flexural FRP, shear cracks near the supports opened up and the internal

shear stirrups ruptured, precipitating a brittle shear failure. This second method showed a

loss in ductility of between 25% and 40%.

Spadea et al.2 also tested a number of beams retrofitted with only one sheet of CFRP

laminate bonded to the tension zone of the specimens. Testing included a control

specimen, a strengthened specimen without end anchorage, and a strengthened specimen

with anchorage at the ends of the CFRP near the supports. The retrofitted specimens

were less ductile than the control specimen. Their ductility was reduced by 30-65%

compared with the control beam. Results showed that the strengthened specimens

without end anchorage failed suddenly with the debonding of the laminate. Specimens

with the anchored laminate performed very well with increases in flexural capacity of

30% to 70% and did not exhibit premature debonding of the CFRP. The specimens with

anchored CFRP were more ductile than those without anchorage.

11

GangaRao and Vijay3 conducted a study of 24 beams that compared different types of

FRP flexural retrofit. They divided the beams into six groups. The first group included

the control specimens. The second group of beams had steel plates bonded to the tension

face for flexural strengthening. The rest of the groups were retrofitted with CFRP wraps

with different anchorage systems. Some of the beams were subjected to bending to

induce flexural cracking prior to strengthening with CFRP. The third and fourth groups

had CFRP wraps extending 90% up the vertical faces and along the full length of the

beam. The fifth and sixth groups had CFRP wrapped up the full height of the vertical

faces and along the full length of the beam. All of the repaired specimens showed

improvements in flexural performance. The beams retrofitted with CFRP performed

better than the steel-plate reinforced beams. Some of the specimens failed in shear so the

theoretical bending moments were not reached. The performance of repaired damaged

beams and repaired undamaged beams was similar.

Fanning and Kelly4 tested ten rectangular beams that were 9’-10” in length. Two

beams were used as control specimens. Eight of the beam specimens were retrofitted

with CFRP composites of different lengths in the tension zone of the beam. For the beam

retrofitted with CFRP over the full span length, the ends of the CFRP were anchored at

the supports. No anchorage was provided for shorter CFRP retrofits. Results showed an

increase of 40% in overall stiffness and a minimum of 50% increase in ultimate load for

the beams retrofitted with CFRP over the full span length. Specimens with shorter

lengths of CFRP showed no significant increase in strength. Beams without anchorage

failed due to CFRP plate peel-off.

12

Shahawy et al.5 experimented on full-scale T-girders that were preloaded at different

stress levels and then repaired with two different CFRP wrapping techniques. Both

wrapping techniques had no additional anchorage. The first wrapping technique

consisted of two layers of CFRP on the tension face extending four to six inches up the

sides of the beam web. The second wrapping technique consisted of two layers of CFRP

on the tension face with wraps extending up the full side of the beam web. The

preloading of the beams had no effect on the performance of the CFRP. The partially

wrapped specimens had less strength than the fully wrapped specimens. In addition,

failure of the partially wrapped specimens resulted from sudden delamination of the

CFRP.

White et al.6 conducted a study of reinforced concrete beams subjected to a high rate

of loading. A total of nine beams were tested with one being the control specimen. Eight

of the specimens were retrofitted with two types of CFRP. The first was an S-type

consisting of pultruded CFRP laminates. The second was an R-type consisting of prepreg

sheets. No anchorage was provided for the CFRP on any of the specimens. A number of

loading cycles were used. These included one slow stroke to failure, one fast stroke to

failure, one stroke to 150 kN at a slow rate followed by a fast stroke from 150 kN to

failure, and twelve fast loading cycles to 120 kN followed by fast loading to failure.

Specimens subjected to the high rate of loading had an increase of 5% in flexural

capacity, stiffness, and energy absorption over slowly loaded beams.

Masoud et al.7 investigated the retrofit of corroded reinforced beams using CFRP.

The beams were cracked under typical service load conditions. An electrical current was

passed through the longitudinal reinforcements and wet-dry cycling applied to the beam

13

to accelerate corrosion of the reinforcing bars. Two techniques were used to repair the

beams with CFRP. The first involved fully wrapping the tension face and both sides of

the beam with CFRP. A horizontal strip of CFRP was placed at the top edge of the wrap

on both sides of the beam web for anchorage. The second scheme was the same as the

first with the addition of one longitudinal CFRP sheet added to the tension face. The

objective of this repair was to confine the concrete cover as opposed to actual flexural

strengthening. Results showed that an increase in strength was observed for all of the

strengthened specimens under monotonic loading. The test also showed that

strengthened specimens had better fatigue life over un-strengthened specimens.

Bonacci and Maalej8 also investigated the repair of corroded reinforced concrete

beams with CFRP before and after sustained loads were applied. The beams were

divided into groups where one group had more corrosion than the other. The beams were

retrofitted with CFRP on the tension face of the beam. Some had no anchorage while

others had anchorages provided at the ends and at mid-span of the beam. Results showed

that the CFRP repair increased the beam strength and decreased deflection. Delamination

of the CFRP started in the mid-span region and continued to the supports.

The CFRP flexural retrofit applied to the T-Beams in this research program has

similarities to many of the examples described in the literature. Pre-cured CFRP strips

were bonded to the tension face of the beam over its full length. Hand lay-up CFRP

sheets wrapped up the sides of the web were used to provide anchorage at the ends of the

tension strips. These beams were therefore expected to perform similar to those in the

literature review. In contrast to the specimens presented in the literature review, the T-

14

Beams in this program are precast prestressed concrete beams as opposed to the

reinforced beams used in most prior research.

Because of the predicted increase in flexural strength due to the addition of CFRP

flexural strengthening, T-Beam 2 might experience a shear failure prior to flexural

failure. Since the primary intent of this project was to determine the flexural

performance of the field-applied CFRP, it was important to prevent a premature shear

failure. Two types of CFRP shear retrofit were investigated on T-Beam 1 so as to

validate their effectiveness before applying them to T-Beam 2. The two shear retrofit

systems consisted of CFRP stirrups and sheets as described later in this report. The

following section provides a review of literature on experimental programs investigating

FRP shear retrofit of concrete beams.

2.3 CFRP Shear Strengthening A number of laboratory experimental research studies have been performed on shear

strengthening of reinforced concrete beams using CFRP materials. These studies

considered different wrapping configurations of CFRP on the concrete surface. Some of

the configurations used CFRP fully wrapped on all four faces of a rectangular beam,

CFRP on the sides and bottom of the beam, and CFRP on the beam sides only. Studies

also considered CFRP bonded at different angles on the sides of the beam. Some of these

studies investigated different anchorage systems to provide better attachment between the

CFRP and the concrete.

Sheikh et al.9 investigated 5/6th scale models of beams from a building that was

damaged by unexpected loads during the first two years of service. The beams were cast

with a haunch to simulate being framed into the walls and to force shear failure to occur

15

within the shallow section of the beam. The repaired beam was completely wrapped on

all sides of the shallow section to prevent shear failure. The beams were subjected to a

single point load at the edge of the haunch section. The control beam failed in shear at a

load of 1,700 kN while the retrofitted beam failed in flexure at a load of 2,528 kN. The

failure of the beam changed from a brittle shear failure to a more ductile flexural failure.

The mid-span deflection of the repaired specimen was 10 times greater than that of the

control specimen.

Czaderski10 experimented with specimens retrofitted with prefabricated L-shaped

CFRP stirrups as shear reinforcement. A total of five specimens were tested of which

two were control specimens. One control specimen was a beam that had internal steel

shear reinforcement while the other did not have internal steel shear reinforcement. The

remaining three specimens were retrofitted with the L-shaped stirrups spaced equally

along the length of the shear span and overlapped on the bottom of the beam. The

specimens were subjected to increasing static loading until failure. The test results

showed an increase of the shear strength with the beams retrofitted with the L-shaped

stirrups. The retrofitted specimens also exhibited greater ductility than the control

specimens. The bottom overlapped ends of the L-shaped stirrups tended to separate from

one another at failure.

Chaallal et al.11 tested reinforced concrete beams retrofitted with CFRP stirrups

bonded to the sides of the beam at various angles. Three groups of beams were tested.

The first group had internal steel shear reinforcement but wasn’t retrofitted with CFRP

stirrups. The second group did not have enough internal steel shear reinforcement and

wasn’t retrofitted with CFRP stirrups. The third group was the same as the second group

16

but it was retrofitted with CFRP stirrups at 90 degrees to the horizontal and 45 degrees to

the horizontal. The CFRP retrofit was applied to the two vertical faces of the beams.

The beams were subjected to four-point bending. Results showed an increase in strength

of about 70 percent and increased stiffness for the repaired beams. The 45-degree CFRP

stirrups performed better than the 90-degree CFRP stirrups. Failure occurred due to

delamination of the CFRP stirrups from the surface of the concrete. For more extreme

loading, U-shaped retrofit stirrups were suggested.

Triantafillou12 tested eleven reinforced concrete beams strengthened with CFRP

stirrups at various angles on the vertical sides of the beam. Two beams were used as

control specimens. Three of the beams were fitted with CFRP stirrups oriented at 45

degrees to the horizontal. The rest of the beams were fitted with CFRP stirrups oriented

at 90 degrees to the horizontal. Internal steel shear reinforcement was not included in

order to force shear failure in each specimen. The beams were loaded in four-point

bending. Results showed an increased in shear strength between 65 and 95 percent over

the control specimens. Failure was initiated by shear cracking followed by peeling of the

CFRP shear stirrups. Results also showed that the 45 degrees CFRP shear reinforcement

was more effective than the vertical CFRP shear reinforcement due to the fibers being

more nearly perpendicular to the shear cracks.

Al-Sulaimani et al.13 conducted research on sixteen beams with various

configurations of CFRP shear reinforcement. The beams were divided into four groups.

The first group was used as the control. All of the retrofitted beams were preloaded until

shear cracks formed. The load was then released and the beams were repaired. The first

retrofit method consisted of bonding CFRP stirrups to the sides of the beam in the shear

17

span area. The second retrofit method consisted of CFRP sheets bonded to the whole

sides of the beam. The final retrofit method consisted of CFRP sheets that continuously

wrapped on the bottom side of the beam along the full span length. The specimens were

all tested in four-point loading. Results showed that the stirrups and sheets bonded to the

sides of the beams produced a similar increase in strength. They also had similar failure

modes where both delaminated at the bottom of the member. The CFRP sheets that

wrapped at the bottom side of the beam prevented shear failure and caused the specimens

to fail in flexure. The continuity of the wrap repair reduced the stress concentrations that

were present in the stirrups and sheets. All forms of shear repair also increased the beam

stiffness.

Schuman and Karbhari14 conducted research on half-scale cantilever T-Beams

retrofitted for shear with wet lay-up CFRP. They investigated the effect and benefits of

anchoring the CFRP shear stirrups to the side of the beam. Two types of CFRP retrofit

were considered. The first consisted of U-Shaped CFRP stirrups bonded to the bottom

and sides of the beam. The second consisted of L-Shaped CFRP stirrups that were placed

in an offset configuration so as not to overlap on the bottom of the beam. All shear

stirrups were anchored at the top of the web using steel plates and expansion anchors

embedded into the top slab. Five sets of specimens were tested with different anchorage

configurations. The first specimen was the control. The second specimen was retrofitted

with the U-Shaped stirrups without anchorage. The third specimen was retrofitted with

Offset L-Shaped stirrups with 3/8” diameter anchor bolts extending 4” into the top slab.

This anchor embedment did not extend past the internal steel stirrups. The fourth

specimen was also retrofitted with the Offset L-Shaped stirrups using 3/8” diameter

18

anchor bolts extending 6” into the top slab. The embedment of the anchors was now

deep enough to pass the internal steel stirrups and slab reinforcement. The last specimen

was also retrofitted with the Offset L-Shaped stirrups but using ½” diameter anchor bolts

extending 6” into the top slab. The test results showed that there is little or no benefit in

using CFRP shear stirrups without anchorage. The third specimen only showed a slight

increase in strength and ductility. The fourth and fifth specimens showed considerable

increase in strength, ductility, and stiffness. The test also showed that there was a strong

dependence on both the anchor size and embedment depth of the anchorage system.

Many of the studies described above relate to the type of shear retrofit used for the T-

Beams in this research program. However, most of the studies found in the literature

were applied to reinforced concrete beams without internal steel shear reinforcement,

while the T-Beams in this study are prestressed beams with internal steel shear

reinforcement. Based on the literature review, two shear retrofit systems were

considered, consisting of CFRP stirrups and sheets. The CFRP stirrups were installed on

both vertical sides of the T-Beam web and wrapped over the top of the web through slots

in the top slab. The CFRP sheets were installed on the two vertical sides of the T-Beam

web. The shear retrofit was not extended around the soffit of the beam so as not to

introduce additional restraint to the field-applied CFRP flexural retrofit. The shear

retrofits were anchored at top and bottom of the web by means of steel tubes or GFRP

angles with bolts passing through the web. More detail of the shear retrofit systems is

provided in Chapter 5.

19

2.4 Chapter Summary Numerous research projects have investigated flexural strengthening of reinforced

concrete members using various CFRP retrofit systems. All of the repaired specimens

showed some increase in flexural capacity. Specimens that were damaged prior to

strengthening behaved like specimens that were not damaged prior to strengthening. End

anchorages helped prevent early debonding of the CFRP, thus increasing the flexural

strength. Failure of the beam was generally associated with delamination of the CFRP.

There have also been many research projects involving reinforced concrete members

retrofitted with different CFRP shear strengthening configurations. Beams that were

retrofitted with CFRP on just two sides did not perform as well as beams that had

continuous CFRP on three sides or with a complete wrap. Specimens that were

retrofitted with CFRP at an angle performed better than specimens that had CFRP

attached vertically. The retrofitted beams generally failed upon delamination of the

CFRP from the concrete surface. However, the addition of effective anchorage for the

ends of the CFRP increased the shear capacity and ductility of the concrete member.

Failure now depends on the anchorage and not on delamination of the CFRP.

21

CHAPTER 3

3 ALA MOANA BEAM REPAIR

In 1997, during a structural inspection of the parking garage at the Ala Moana

Shopping Center, significant flexural cracking and ledge spalls were noted on one of the

precast prestressed T-Beams supporting the elevated parking level. This beam was

repaired using epoxy-modified mortar to repair the spalls and CFRP materials to improve

the flexural capacity. The repair was designed by Martin and Bravo Structural Engineers,

Honolulu, Hawaii. Figure 3-1 shows a cross section of the beam strengthening using

three pre-cured CFRP carbodur strips. These strips extended the full length of the beam

soffit and anchored at the ends by 6” wide double ply wet lay-up CFRP wraps extending

up both sides of the beam web.

The repairs were performed by Concrete Coring of Hawaii in the presence of the

Martin and Bravo design engineer and a representative from the CFRP supplier, Sika

Products USA. A rotary grinder was used to remove paint and weak concrete paste

before installation of the CFRP material (Figure 3-2). This surface preparation is

necessary to provide a suitable bond between the CFRP and the concrete. The result was

a smooth surface similar to ICRI-CSP surface profile 215. A slightly higher surface

profile such as ICRI-CSP 3-4 is normally specified for CFRP applications.

Once the concrete surfaces were prepared, three 4” wide pre-cured CFRP Sika

carbodur strips and two 6” wide Sika Wrap Hex 103C uni-direction sheets were prepared

for installation (Figure 3-3). A uniform thin layer of Sikadur 30 Hi-Mod Gel two-part

22

epoxy was applied to the soffit of the beam. The pre-cured CFRP strips were pressed

onto this epoxy layer using a roller (Figure 3-4). The 6” wide wet lay-up sheets were

saturated in Sikadur Hex 300 two-part epoxy and then applied at each end of the beam to

anchor the flexural strips (Figure 3-5). In addition, Sika Wrap Hex 103C was used to

wrap the epoxy mortar patches at the ledge spalls at third points along the span (Figure

3-6). Once the epoxy had cured, the beam was painted with exterior quality latex paint

and put back into service (Figure 3-7).

Figure 3-1 Cross section of T-Beam for carbodur strip installation

23

Figure 3-2 Surface preparation prior to CFRP application

Figure 3-3 Sika carbodur strips being prepared for installation

Figure 3-4 Application of CFRP strips

24

Figure 3-5 Wet lay-up anchorage wrap at end of CFRP strips

Figure 3-6 Sika wet lay-up wraps at concrete spalls

Figure 3-7 Repaired beam in service

25

CHAPTER 4

4 BEAM RECOVERY AND REPAIR

4.1 Introduction The portion of the old Ala Moana Parking Structure containing the repaired T-Beam

was demolished in June 2000 to make way for a new multilevel parking structure.

During demolition, the repaired T-Beam and two nominally identical un-repaired T-

Beams were salvaged for testing. The top slab forming the flange of the T-Beams was

removed, along with the transverse joists, to facilitate shipping of the salvaged beams.

This removal was performed using a demolition rig with large hydraulic pincer. Removal

of the top slab over the precast beam web occasionally resulted in spalling of concrete at

the top of the web. These spalls were repaired once the beams were delivered to UHM-

STL as described later in this chapter. The top slab was also reinstated at UHM-STL.

Removal of each beam was performed using the demolition rig and a large diameter

concrete saw. The precast prestressed web section of each beam was removed by saw

cutting through the cast-in-place column capital at each end of the span (Figure 4-1).

This enabled recovery of the entire precast section of the beam without damaging any of

the prestressing steel or altering the prestress in the beam. During saw cutting, the beam

was supported by two cables from the demolition rig (Figure 4-2). Because of the

negative bending induced by the self-weight of the beam, flexural cracking occurred in

one of the beam webs. These cracks were repaired once the beam was delivered to

UHM-STL as described later in this chapter.

26

Figure 4-1 Saw cutting column capital to remove precast prestressed beam

Figure 4-2 Cable support during beam removal

27

4.2 Epoxy Crack Repair During handling of one of the control specimens, negative bending cracks formed in

the beam web. These cracks were not anticipated to affect the flexural strength of the

beam during testing since they did not extend into the prestressed bulb at the base of the

beam. However, they would affect the stiffness of the beam during flexural testing and

may jeopardize the shear strength of the beam and so were repaired prior to replacement

of the top slab. The cracks were repaired by Plas-Tech Ltd., Hawaii, using a Sika epoxy

injection system. Injection ports were epoxied onto the crack at 2-inch intervals and the

surface of the crack was sealed with Sika epoxy modified mortar (Figure 4-3). The

cracks were then injected using Sika epoxy modified mortar and allowed to cure before

handling the beam (Figure 4-4).

Figure 4-3 Injection Port Placement

28

Figure 4-4 Epoxy Injection

4.3 Epoxy Mortar Spall Repair During demolition of the top slab and supporting joists, portions of the beam webs

were damaged by concrete spalls. None of the spalls affected the internal flexural and

shear reinforcement in the precast prestressed beams, however, in order to restore the

beams to their original condition, these spalls were repaired prior to replacement of the

top slab.

The spall repairs were performed by personnel from Plas-Tech Ltd., Hawaii, using a

Sika epoxy mortar patching system. The contact surface was primed using Sika epoxy

modified mortar (Figure 4-5) and formed with plywood (Figure 4-6). A two part Sika

epoxy modified mortar was mixed with clean silica sand and poured into the form (Figure

4-7). The patch material was allowed to cure before removal of the form (Figure 4-8).

29

Figure 4-5 Priming the repair contact surface

Figure 4-6 Forming the repair area

30

Figure 4-7 Pouring epoxy mortar patch material

Figure 4-8 Completed spall repair

31

4.4 Top Slab Replacement To facilitate transportation of the beams from the Ala Moana Shopping Center to

UHM-STL, the top slab and supporting joists were removed during demolition. In order

to replicate the T-Beam behavior of the in-situ beam, a concrete flange was poured onto

the recovered precast beams prior to testing. The effective flange width suggested by the

ACI 318–02 Building Code for the in-situ condition is 77.5 inches. Because of

limitations of the test frame used to test the beams, the flange width over half of the beam

was reduced to 66 inches (Figure 4-10). This slight reduction in flange width was not

anticipated to affect the beam flexural performance.

The 4.5 inches thick top flange was reinforced according to the original design

documents for the parking garage (Figure 4-9). The slab reinforcement consisted of two

layers of grade 60 #3 reinforcing bars running both transverse to the beam axis and

longitudinally. In the transverse direction, the bottom bars were spaced at 12” on center

and the top bars were spaced at 7” on center (Figure 4-11). In the longitudinal direction,

the bottom bars were spaced at 6” on center and the top bars were spaced at 12” on center

(Figure 4-11). Some of the transverse reinforcing bars were relocated to avoid the 3”

openings formed for the proposed CFRP shear reinforcing stirrups. These openings were

formed by means of plywood block-outs (Figure 4-12). The block-outs were located at

approximately 12 inches on center so as to fall between the original internal steel shear

reinforcement.

In order to monitor strains in the top slab reinforcement, six electrical resistance strain

gages were bonded to the surface of longitudinal bars in the top slab. Four strain gages

32

were located along the top center longitudinal bar while two gages were located on slab

top bars to one side of the beam web at mid-span (Figure 4-11).

The existing steel shear reinforcement in the precast beams consisted of double leg #3

reinforcing bars at a nominal spacing of 12 inches on center along the full length of the

beam. Each bar consisted of a straight vertical section in the beam web, with no hook or

anchorage at the bottom of the beam, and a 90 degree bend into the top slab at the top of

the beam. During demolition of the top slab, some of the 90 degree bend extensions had

been removed or damaged. These stirrups were repaired by welding #3 bar extensions to

reinstate the original anchorage into the top slab.

The concrete was supplied by Hawaiian Cement Ready Mix and poured in place. For

T-Beam 1, the top slab was poured directly on the precast beam web (Figure 4-13). For

T-Beam 2, Corr Bond was applied to the top of the beam web to improve the bond

between the top of the precast beam and the concrete slab. The top slab concrete was wet

cured for 7 days and then exposed to the laboratory environment while waiting for

testing.

(10) 38"Ø Stress-relieved prestress strands 51

2"

MIDSPAN BEAM SECTION

1'-412"

2-leg #3 Stirrups @ 12" o.c.

5'-6"

512"

412"

1'-612"

2 38"1 78"

1 34"Slab Reinforcement

Figure 4-9 Typical T-Beam cross-section

33

14'-212"

5'-6"

9'-912"

3" TYP

6'

3" wide cfrp stirrups (typ)

Figure 4-10 T-Beam 1 flange layout

14'-212"

5'-6"

9'-912"

3" TYP

6" TYP

1' TYP

6'


Bottom Reinforcement Layout

3'-1012"

14"

14'-212"

4'-2"

5'-6"


1'-11"

12"

1 234"

9'-912"

3" TYP

6

5

4

1012"

2'-012"

31' TYP

7" TYP

6'

12"

1'-314"

Top Reinforcement Layout and Strain Gage Locations

Figure 4-11 T-Beam 1 flange reinforcement layout

34

Figure 4-12 Plywood block-outs for CFRP shear stirrups

Figure 4-13 T-Beam 1 top flange concrete placement

35

CHAPTER 5

5 TEST SETUP AND BEAM LAYOUT

5.1 Introduction This chapter describes the test setup and specimen details for the prestressed concrete

T-Beams tested in this program. The testing apparatus was constructed specifically for

this project, but was configured so that it could be modified and re-used for future testing

in the UHM-STL. The CFRP shear strengthening applied to each T-Beam to preclude

premature shear failure is described, along with all instrumentation placed on the beams

during testing. The test routine and data recording procedures are also presented.

5.2 Test Apparatus In the Ala Moana Shopping Center Garage, the prestressed T-Beams supported

transverse joists at 7 feet on center, which in turn supported the parking level slab. The

majority of the load applied to the T-Beams was therefore applied by the transverse joists

at 3.5 feet on either side of the beam mid-span. If the repaired T-Beam with flexural

CFRP were tested under point loads at the locations of the original joists, the beam might

fail prematurely in shear. In order to avoid this shear failure, additional CFRP shear

strengthening was applied to both beams as described later in this chapter. In addition,

the load points were relocated to 2 feet either side of the beam mid-span so as to increase

the shear span.

The T-Beams in this program were tested under 4-point loading. Figure 5-1 shows

the original schematic of the test setup. Based on the anticipated flexural capacity of the

strengthened beam, a 200,000 lb capacity hydraulic actuator supported by a two-post

36

frame was initially considered for the test setup. However, since the beams were to be

retrofitted for shear, it was decided that, subsequent to flexural testing, each of the shear

spans would be tested to determine the shear capacity of the two retrofit systems. This

would require a 300,000 lb capacity hydraulic actuator supported by a four-post frame.

The final test frame configuration is shown in Figure 5-2 and Figure 5-3.

TestBeam

TEST SETUP ELEVATIONA

Spreader Beam

Load Cell

Load Frame

200 Kip Actuator

A

SECTION "A-A" Figure 5-1 Schematic Layout of Test Setup

The four-post frame consists of four 4” x 6” steel tube columns with a 1.75” diameter

high-strength threaded steel rod inside each column. The rods extend from the top of the

frame and pass through the 2’ thick laboratory strong-floor. They were pre-tensioned to

approximately 25% of their capacity before testing. The 300,000 lb 30” stroke hydraulic

actuator is suspended from a steel cross-head at the top of the four tube columns. The

cross-head was constructed from two W24 wide flange beams welded side-by-side and

supported on double W12 wide flange beams supported on the tube columns (Figure 5-3).

The frame was designed so that during testing of the T-Beam specimens, the compression

force applied by the actuator is transferred by the cross-head to the four 1.75” diameter

37

high-strength steel rods. Numerous web stiffeners were installed in the W24 and W12

beams to facilitate this load transfer.

In order to test the T-Beams for this project, the four-post frame had to extend 19 feet

above the laboratory strong-floor (Figure 5-2). However, this encroached on the travel of

the overhead crane and so limited the use of the laboratory on the far side of the frame.

In order to retain the four-post frame for future testing, but also maintain full crane

function, the tube steel columns were spliced at 2/3rd height. Figure 5-4 shows the test

frame in the short configuration during construction. Figure 5-5 shows the test frame

with the column extensions in place. In order to stabilize the frame, adjustable cross-

bracing was installed above the test specimen (Figure 5-5). A load spreader beam was

fabricated from two W24 wide flange beams to distribute the actuator load to the load

points at 2 feet either side of the beam mid-span (Figure 5-3). W8 wide flange beams

were used to fabricate pin and roller supports for each end of the test beam (Figure 5-3).

Fabrication of the test frame was performed over an 8 month period in UHM-STL.

38

22'

2'-0"

Detail 2

15'-4"

2'-412"

2'

Detail 1

6'-7"

6"

2'

2"1'

9"1"

Detail 3

6'6" 6"

512"

512"

1'6"

1"

12"

1'-012"

Figure 5-2 Detail of Final Test Frame Design

39

6'

1"

6"1'

2'-0"

6"6" 3'

6"6"

DETAIL 1 - CROSS-HEAD FRAMING

Actuator Swivel Head2-W24 Beams

2-W12 Beams (Typ)

Plate Stiffeners

W8x48

1/2" PlateStiffeners

8" 2'

3"

812"

1/2"

6"

1'1'-6"

DETAIL 2 - PINNED SUPPORT

Steel PlateStiffeners

4'-0" Cement Grout

2'-0"

2-W24 Beams

Swivel HeadLoad Platen

DETAIL 3 - SPREADER BEAM Figure 5-3 Test Frame Details

40

Figure 5-4 Test Frame under Construction (Short Columns)

Figure 5-5 Completed Test Frame with T-Beam 1 ready for loading

41

5.3 Beam Shear Retrofit T-Beam 1 was the first beam to be prepared for testing as the control specimen.

During the demolition of Ala Moana Parking Garage, the original top slab and joists were

removed to facilitate shipping of the beams to UHM-STL. The top slab was

reconstructed as described in Chapter 4 to act as the top flange and compression zone of

the beams. In order to avoid premature shear failure of the strengthened beam, CFRP

shear reinforcement was installed on T-Beam 2 prior to flexural testing. Two types of

shear strengthening were considered for this application. In order to evaluate these two

options, they were applied to T-Beam 1 and tested in shear after completion of the

flexural test.

The two shear retrofit systems installed on T-Beam 1 consisted of 3” wide double

layer CFRP stirrups installed on the left half of the beam and 12” wide single layer CFRP

sheets installed on the right half of the beam (Figure 5-6).

5.3.1 CFRP Shear Stirrups CFRP shear stirrups were used to retrofit the left half of T-Beam 1 (Figure 5-6). Each

stirrup consisted of a double layer of 3” wide CFRP unidirectional fabric extending from

the bottom of the beam on one side of the web, through the top slab and down the other

side of the web. In a normal application, the stirrup would extend under the beam soffit

to form a lap splice, so as to create a continuous hoop. However, since the primary intent

of this program was to evaluate the original field application of the CFRP carbodur

flexural strengthening, the additional restraint provided by full hoop stirrups would have

altered the flexural performance of T-Beam 2.

42

To maintain continuity at the top of the beam, the stirrups were passed through slots

fabricated in the top slab. In a field application, this would require cutting slots in the top

slab to facilitate the stirrup installation. To prevent premature delamination of the

stirrups at the re-entrant corner at the bottom of the web, steel tubes with thru-bolts were

installed to restrain the stirrups.

The layout for the shear retrofit of T-Beam 1 is shown in Figure 5-6. The existing

internal #3 reinforcing stirrups were located at approximately 12” on center. In order to

avoid conflicts at the top of the beam, the 3” wide double layer CFRP stirrups were

located between the existing steel stirrups. This resulted in a 12” center-to-center spacing

for the 3” wide CFRP stirrups. The contribution of these stirrups to the shear capacity of

the beam is computed in Chapter 7. Plywood saddles were used as block-outs to create

the slots in the top slab. After the concrete slab had set, the saddles were removed using

a pneumatic chipping hammer and power drills. The plywood saddles proved difficult to

remove, and so high density Styrofoam was used to create the saddles for T-Beam 2.

Figure 5-7 shows the slotted holes after the saddles were removed. The top of the web at

the slotted holes had small holes due to entrapped air under the saddles. These holes

were filled with epoxy mortar prior to installation of the CFRP stirrups (Figure 5-11).

A pneumatic needle gun was used to roughen the surface of the web to remove the

surface cement paste and improve the bond between the CFRP and the concrete. For

each of the 3” wide double layer CFRP stirrups, a 3” wide strip of concrete was

roughened on both sides of the beam web (Figure 5-8). The resulting surface condition

was similar to ICRI surface profile #415.

14'-212"

24'

CONCRETE SLAB

TBEAM 1

Left Span (3" wide double layer CFRP Stirrups)

5'-6"

3" wide CFRP stirrups (typ)

2'

7"Left Support

412"

Internal #3@12" shear stirrups (typ)

1'-512"

1'-034"

1112"

834"

10"

1014"

1'-112"12"

1'-012" 91

2" 1114"

1' TYP

CL

Right Span (12" wide CFRP Sheets)

6'

9'-912"

3" TYP

Right Support6" TYP 1'-6"

Figure 5-6 Shear retrofit layout for T-Beam 1

43

44

Figure 5-7 Slotted holes in top slab for CFRP shear stirrups

Figure 5-8 Concrete surface preparation using needle gun

Figure 5-9 Roughened web for 3” wide stirrups

45

Before installation of the CFRP materials, a total of twenty-four 3” wide CFRP strips

were cut from standard 24” wide CFRP unidirectional material. Each 3” wide CFRP

stirrup contained 12 CFRP tows. The strips were saturated in Sikadur Hex 300 epoxy in

preparation for installation (Figure 5-10). Sika 30 Hi Mod Gel epoxy was applied to the

roughened surfaces of the concrete as a bonding agent (Figure 5-11). It was also used to

fill the air holes on top of the web. The 3” wide double layer CFRP stirrups were then

installed from the top of the slab going through the slotted holes and down the sides of

the web. A roller was used to press the CFRP into place and remove any air bubbles

between the CFRP and the concrete surface. Figure 5-12 shows the slotted holes with the

3” wide double layer CFRP installed in place. After installation, the epoxy was allowed

to cure for five days before the excess bottom CFRP was removed with a grinder (Figure

5-13).

In order to prevent premature delamination of the CFRP stirrups at the re-entrant

corner at the base of the web, mechanical anchorage was provided in the form of steel

tubes with thru-bolts passing through the beam web. Three quarter inch diameter holes

were drilled through the web midway between the CFRP stirrups. The steel tube was

provided in two sections rather than a continuous member so as not to enhance the beam

bending capacity. The two sections of tube were 1.25” x 1.25” and 1.5” x 1.5” with 1/8”

wall thickness. These pieces fit inside each other to form a telescopic joint midway along

the stirrup half of the beam. Thru-bolts made from 5/8” diameter threaded steel rod were

used to anchor the steel tubes on either side of the web. Before installation of the tube

steels, a bed of Sika 30 Hi Mod Gel epoxy was spread over the bend in the CFRP so as to

46

provide even load transfer between the CFRP stirrup and steel tube. The nuts were snug

tightened and the epoxy allowed to cure for five days (Figure 5-14 and Figure 5-15).

47

Figure 5-10 CFRP stirrups being saturated with Sikadur Hex 300

Figure 5-11 Sika 30 Hi Mod Gel epoxy used to prepare surface for CFRP

Figure 5-12 3” wide double layer CFRP stirrups wrapped through slotted holes

48

Figure 5-13 CFRP stirrups in place, bottom trimmed after curing

Figure 5-14 Installation of steel tube anchorage for 3” wide CFRP stirrups

49

Figure 5-15 Complete installation of anchorage for 3” wide CFRP stirrups

5.3.2 CFRP Shear Sheets

The right half of T-Beam 1 was retrofitted with 12” wide CFRP sheets as shear

reinforcement (Figure 5-6). The surface preparation for the CFRP sheets involved

roughening of the concrete surface of the web and a 4” return under the top slab using a

needle gun (Figure 5-16). Sika 30 Hi Mod Gel Epoxy was then applied for each 12”

CFRP sheet (Figure 5-16). The first 12” CFRP sheet was located 18” from the right

support, just inside the thickened anchorage zone of the prestressed beam. The sheets

were spaced at 18” on center so as to leave a 6” gap between sheets to allow for moisture

in the beam to escape.

50

Figure 5-16 Roughened web and Hi Mod Gel application for 12” wide sheets

Figure 5-17 CFRP sheets saturated with Sikadur Hex 300

Figure 5-18 Installation of CFRP sheets as shear reinforcement

51

The 12” wide CFRP sheets were cut from the standard 24” wide CFRP roll and

saturated with Sikadur Hex 300 epoxy (Figure 5-17). Sika 30 Hi Mod Gel epoxy was

applied to the roughened surfaces of the concrete as a bonding agent (Figure 5-16). The

CFRP sheets were then installed on the web (Figure 5-18). A roller was used to press the

CFRP onto the concrete and to remove air bubbles.

At the top of the web, four inches of the 12” CFRP sheet was bonded underneath the

slab as a return so that mechanical anchorage could be installed at both the top and

bottom of the web to prevent premature delamination of the CFRP sheets at the re-entrant

corners. A 2” x 4” timber was used to hold the CFRP in place under the slab while it was

being rolled onto the web. After installation, the epoxy materials were allowed to cure

for five days. The excess CFRP at the bottom of the beam was then removed with a

grinder (Figure 5-18).

Anchorage of the CFRP sheets was provided using steel tubes and threaded rods as

described earlier for the CFRP stirrups. A bed of Sika 30 Hi Mod Gel epoxy was placed

in the CFRP bends at all anchorage locations so as to provide even load transfer from the

CFRP to the steel tubes (Figure 5-19). The bottom anchorage was identical to that used

for the stirrups except that the threaded rod thru-bolts were located at the center of the 6”

gap between CFRP sheets (Figure 5-20). A telescopic joint of the steel tube is shown in

Figure 5-21. For the top anchorage, a 2” x 1.5” by 1/8” thick continuous steel tube was

used. There was no need to splice the tube since its contribution to the compression

flange is negligible. Thru-bolts made from 5/8” diameter threaded steel rod were used to

anchor the tube steel on either side of the web (Figure 5-22). The nuts were snug

tightened and the epoxy allowed to cure for five days.

52

Figure 5-19 Sika 30 Hi Mod Gel epoxy bed at re-entrant corner of CFRP sheets

Figure 5-20 Tube anchorage set in epoxy bed at re-entrant corner of CFRP sheets

53

Figure 5-21 Telescope splice in steel tube anchorage

Figure 5-22 Complete installation of mechanical anchorage for 12” CFRP sheets

54

5.4 Beam Test Configuration and Instrumentation Both T-Beam 1 and T-Beam 2 were instrumented with strain gages installed on the

top slab reinforcement, slab and precast beam concrete, and on the CFRP shear

reinforcement. In addition, dial gages and LVDTs (linear variable displacement

transducers) were used to monitor the vertical deflection of the beams. After testing each

beam in flexure, the remaining beam halves were tested in shear to evaluate the ultimate

performance of the CFRP shear retrofit schemes. These half beams were also

instrumented extensively to monitor the CFRP strains.

In the sections below, each beam test configuration and instrumentation layout is

presented in detail. The results of the tests are presented in Chapter 8.

5.4.1 T-Beam 1 Layout and Instrumentation

T-Beam 1 was tested under four point loading over a span of 24 feet (Figure 5-23). It

was supported at each end by pinned supports. It was loaded with two point loads

located 4’-3” apart, centered at the beam mid-span. The load was applied in

displacement control. Displacement increments started at 0.005” and increased to 0.025”

as the test progressed.

Figure 5-23 shows the layout and instrumentation for T-Beam 1. A total of 25 strain

gages were installed on the beam. Strain gages 1-6 were installed on the longitudinal

reinforcement in the concrete top slab. These gages were Micro-measurement electrical

resistance gages CEA-06-250UN-350 bonded to the surface of the reinforcement

following the manufacturer’s instructions. Strain gages 7-21 were Micro-measurement

electrical resistance gages EA-06-20CBW-120 designed for bonding to concrete. They

were installed on the top slab concrete surface and along the bottom of the beam to

55

determine the curvature of the beam. Data from these strain gages were used in another

project to evaluate a strain-based deflection monitoring system16. Strain gages 22-25

were installed on the CFRP shear reinforcement to monitor strains in the CFRP during

flexural testing.

Three LVDTs were supported on independent uni-strut frames so as to record the

vertical deflection of the top slab (Figure 5-23). LVDT3 was placed at mid-span of the

beam, but slightly to one side of the beam centerline so as to avoid interference with the

steel spreader beam. The other two LVDTs were located in the right shear span as shown

in Figure 5-23. Dial gages were also installed on the concrete top slab directly over the

supports to record any settlement of the supports during testing.

Figure 5-24 shows T-Beam 1 in the test frame immediately prior to testing. Figure

5-25 shows a close-up view of the center section of T-Beam 1 prior to testing.

24'

8'-112"

8'-112"

9'-912"

12'

4'-3"

2'

5'-6"

DIAL GAGE

Left Support

412"

14'-212"

CONCRETE SLAB

134"

8'-1"

8'

9

5'-134"

1'-834"

7 8

5'-3"

112"

1'-10"14

2'-3"1'-31

2"

251'-4"

241'-5"

7

134"

15

8

10

T-BEAM 1

11

3" TYP

1817

10

16101

2" 2"

9

112"

19

11

LVDT 3

P CL P

6'

Right Support

DIAL GAGE

1'-312"

5'-4"

12

2'-112"

13

112"

5'-314"

3'-9"3'

23

6'-9"

112"

20

12

2'-212"

21

1'-5"

22

13

LVDT 1LVDT 2

Left Span (3" wide double layer CFRP stirrups) Right Span (12" wide CFRP sheets)

Figure 5-23 T-Beam 1 Layout and Instrumentation

56

57

Figure 5-24 T-Beam 1 in the load frame ready for testing

Figure 5-25 T-Beam 1 ready for testing – center view

58

5.4.2 T-Beam 1L Layout and Instrumentation

After flexural failure of T-Beam 1 at mid-span, the left half of the beam, which was

retrofitted in shear with 3” wide double layer CFRP stirrups, was tested as T-Beam 1L

(Figure 5-26). It was supported on a 10’ span and subjected to two point loads located

1’-2” apart, centered at mid-span.

Several of the strain gages from T-Beam 1 were still functioning after the flexural test

and were monitored during testing of T-Beam 1L. Strain gages 7-9 and 14-16 were the

original strain gages attached to the concrete surface. Additional strain gages were

installed on the CFRP shear stirrups to monitor their performance. Several of the CFRP

stirrups were selected and the strain gages located based on likely shear crack locations.

Strain gages 24-30 were attached to the CFRP retrofit (Figure 5-26).

Three LVDTs were located on the top slab to measure the vertical displacement of the

specimen. LVDT3 was placed at mid-span of the beam. LVDT1 was placed directly

over the left support, while LVDT2 was placed at quarter span (Figure 5-26).

Initially, T-Beam 1L was supported with two pin supports. However, during testing,

significant deformation of the supports indicated that they were experiencing inward

lateral load due to shortening of the span. The resulting tension in the bottom of the

beam would likely affect the shear and flexural capacities of the beam. The beam was

unloaded and one of the supports modified to provide a roller support. The beam was

then re-loaded with one pinned support and one roller support as shown in Figure 5-26.

Figure 5-27 shows T-Beam 1L in the test frame ready for testing.

1 1/2" x 1 1/2" x 1/8"hollow tube steel

1 1/4" x 1 1/4" x 1/8" hollow tube steel

6"

2'

10'

912"

1'-6"

1'-2"

412"

roller support

LVDT 1 LVDT 2 LVDT 3

P PCL

5'2'-3"

3'-712"

2'-9"1'-9"

25 24 26 27

1'-6"

1'-512"1'-4"1'-5"

2829 30

1' TYP3'

2'1'-2"

11"1'-31

2"

existing shear stirrups (typ)

T-BEAM 1L

T-BEAM 1L CROSS SECTION

1'-412"

5'-6"51

2"

512"

2'-412"

Hollow tube steel anchorsnuts & bolts

Sika 30 Hi Mod Gel epoxy

3" double layer CFRP stirrups

pinned support

Figure 5-26 Layout and Instrumentation of T-Beam 1L

59

60

Figure 5-27 T-Beam 1L ready for testing

5.4.3 T-Beam 1R Layout and Instrumentation

T-Beam 1R is the right half of T-Beam 1 recovered after flexural failure at mid-span.

It was retrofitted with 12” wide CFRP sheets with 6” spacing between sheets. Based on

the test results from T-Beam 1L, it was determined that additional flexural strengthening

was necessary to ensure failure in shear. Therefore, three pre-cured carbodur strips were

installed under the soffit of T-Beam 1R. Technicians from Plas-Tech Ltd. installed the

pre-cured carbodur strips (Figure 5-28).

A flexural retrofit for T-Beam 1R was necessary due to the flexural failure observed

in T-Beam 1L. Three 4” wide pre-cured carbodur strips were installed as flexural

reinforcement on the soffit of the beam. Prior to installation, the beam soffit concrete

was roughened with a pneumatic needle gun. Sika 30 Hi Mod Gel epoxy was used as the

bonding agent. It was applied on the surface of the concrete and on the carbodur strips.

The strips were then placed along the beam soffit and pressed into place with a roller.

61

The ends of the CFRP carbodur strips were wrapped with Sika Wrap Hex 103C uni-

direction fabric to resist end delamination. These anchorage sheets were saturated with

Sika Hex 300 epoxy for the wet lay-up application. The end supports were located under

these CFRP wraps to enhance anchorage of the carbodur strips. Figure 5-28 shows the

installation of the pre-cured carbodur strips and CFRP wraps on T-Beam 1R.

T-Beam 1R was supported over a 10’-2 ½” span by a pinned support and a roller

support (Figure 5-29). Two point loads were applied to the top flange as described for T-

Beam 1L. A total of 32 strain gages were installed on the beam. Strain gages 1-12 were

installed on the steel tubes to determine the level of strain in the anchorage tubes during

testing. Strain gages 13-25 were installed on the CFRP shear sheets to monitor their

performance during shear testing. Strain gages 27-29 were the original concrete strain

gages installed on T-Beam 1. Strain gages 30-32 were installed on the three carbodur

strips at mid-span to monitor the tension strains in the flexural retrofit. Three LVDTs

were also installed to monitor vertical deflection of the beam. LVDT3 was placed at

mid-span of the beam. LVDT1 was placed directly over the right support and LVDT2

was placed at quarter span. Figure 5-29 shows the instrumentation and layout of T-Beam

1R. Figure 5-30 shows T-Beam 1R in the test setup ready for testing.

62

T-Beam 1R prepared for carbodur strip installation by Plas-Tech technicians

End anchorage provided by Sika Wrap Hex 103C wraps

Completed Sika Wrap Hex 103C anchorage at each end of beam

Final T-Beam 1R retrofitted in flexure

Figure 5-28 Flexural strengthening of T-Beam 1R using pre-cured carbodur strips

1 1/2" x 1 1/2" x 1/8"Steel tube

2" x 1 1/2" x 1/8" Steel tube

Spliced steel tubes

CFRP overlap frombottom of beam

Steel tube anchorswith thru-bolts

2'-412"Sika 30 Hi Mod Gel epoxy

12" CFRP sheets

A

2'-412"

1 1/4" x 1 1/4" x 1/8" Steel tube

252423

5'-114"

2'-4"

10'-212"

1'-2"

BOTTOM OF BEAM

11"

16151413101

2"4

16 3/4"17

19185

B

6

2 3

T-BEAM 1R

323130

1134"

6" TYP

712" 20

2221

10 11

7 8

P PCL

2"

1'-6"

2612

9

Roller Support

LVDT 1LVDT 2LVDT 3

Pinned Support

212"

1'

634"

134"2"6"

134"

214"

CFRP sheets wrapped around soffit to restrain

precured carbodur strips1'-41

2"512"

5'-6"512"

Pre-cured carbodur strips

Steel tube anchorswith thru-bolts

Pre-cured carbodur strips1'-41

2"

512"

5'-6"512"

Sika 30 Hi Mod Gel epoxy

12" CFRP sheets

SECTION A SECTION B Figure 5-29 T-Beam 1R Layout and Instrumentation

63

64

Figure 5-30 T-Beam 1R ready for testing

5.4.4 T-Beam 2 Layout and Instrumentation

T-Beam 2 was the strengthened beam with the field-applied pre-cured carbodur strips.

Due to the presence of pre-cured carbodur strips attached to the beam soffit with CFRP

wraps at each end, the test setup and layout of T-Beam 2 was slightly different to that

used for T-Beam 1. In order to avoid increasing the end anchorage of the CFRP carbodur

strips, the support reactions could not be located under the ends of the beam as was the

case for T-Beam 1. Instead, steel plate supports were fabricated and attached to the ends

of the beam using an epoxy bond, expansion anchors and prestress anchors on the tendon

extensions (Figure 5-31). The existing CFRP that was wrapped around the repaired

ledges at the third points of the beam span were removed so that the only anchorage for

the carbodur strips was at the ends of the beam. The slab reinforcement layout is the

same as used for T-Beam 1. Six strain gages were attached to the slab longitudinal

reinforcement at the same locations as T-Beam 1 (Figure 5-32).

65

Left End Pinned Support Steel Reaction Bracket Right End Roller Support

Figure 5-31 Steel reaction brackets at both ends of T-Beam 2

A bonding agent (Corr Bond) was applied to the top of the precast beam web to

improve the bond between the top of the web and the concrete slab. The concrete slab

dimensions were 4.5” thick, 5’-4” wide, and 24’-10” long. The width of the concrete slab

was 2” less than the slab on T-Beam 1 to allow for additional clearance in the load frame.

High density Styrofoam was used to create the saddle blockouts for the 3” wide CFRP

stirrups (Figure 5-33). This simplified the construction and demolition of the saddles

compared with the plywood saddles used in T-Beam 1. Figure 5-32 shows the concrete

slab layout and reinforcement of T-Beam 2. Figure 5-33 shows the slab reinforcement

and high density foam blockouts prior to pouring.

66

24'-10"

5'-4"

3" wide CFRP stirrups (typ)

Top Slab Layout

1' TYP24'-10"

5'-4"6" TYP

Slab Bottom Reinforcement Layout

3'-1112"

24'-10"

5'-4"

1'-11" 1'-11"

1'

1'

4

5

6

321

7" TYP

1' TYP

Slab Top Reinforcement and Strain Gage Layout

Figure 5-32 T-Beam 2 top flange reinforcement and strain gage layout

67

Figure 5-33 T-Beam 2 slab layout prior to concrete pour

T-Beam 2 was strengthened in bending at the Ala Moana Parking Garage. The pre-

cured carbodur strips were anchored at both ends of the beam with CFRP wraps.

Because of the concrete damage at third points along the span of the beam, it was also

wrapped with CFRP at these locations adding additional restraint to the carbodur strips.

The CFRP wraps at the third point locations were removed prior to the shear retrofit to

release the additional restraints on the pre-cured carbodur strips. Figure 5-34 shows the

locations where additional CFRP wraps were removed prior to the laboratory beam shear

retrofit.

68

Figure 5-34 T-Beam 2 being prepared for shear retrofit

The beam shear retrofit layout for T-Beam 2 is shown in Figure 5-35. The beam

shear retrofit had basically the same procedure as T-Beam 1. After the concrete slab was

cured, the CFRP blockouts were removed to form the slotted holes. The Styrofoam

blockouts were much easier to remove than the plywood blockouts used in T-Beam 1.

Locations of the existing reinforcing stirrups were recorded prior to the slab pour so

that the CFRP shear stirrups could be placed between them. The locations of the CFRP

stirrups and sheets were roughened to produce better bonding and remove surface cement

paste. The mechanical anchorage for the shear retrofit was the same layout as for T-

CFRP wraps removed

69

Beam 1, except that Glass Fiber Reinforced Polymer (GFRP) angles were used instead of

the steel tubes (Figure 5-36 and Figure 5-37). These angles were 3” x 3” with a thickness

of ¼”. Because they are non-corrosive, these GFRP angles are ideally suited for retrofit

of structures exposed to external weather conditions.

The 3” wide double layer CFRP stirrups and 12” wide CFRP sheets were prepared

and installed in the same manner as the wet lay-up method described for T-Beam 1.

There were a total of twenty-two 3” wide double layer CFRP stirrups and fourteen 12”

wide CFRP sheets.

After the CFRP shear retrofit was installed and allowed to cure, the anchorage angles

were installed. Since the angles stiffness is significantly lower than the steel tubes used

for anchorage in T-Beam 1, it was assumed that they would not contribute much to the

bending strength of the beam. They were therefore installed as continuous sections for

each of the beam half spans. Three quarter inch holes were drilled through the web

between the CFRP shear stirrups and sheets, avoiding the internal steel stirrups. Sika 30

Hi Mod Gel epoxy was used for the load transfer between the CFRP and the anchorage

angles. Figure 5-36 shows the epoxy being applied on the CFRP. Figure 5-37 shows the

completed installation of the GFRP angle anchorages.

5 '-4 "

S E C T IO N B

2 4 '-1 0 "

2 5 '-8 12 "

C O N C R E T E T O P S L A B

T -B E A M 2 E L E V A T IO N

B E A M S O F F IT

1 '

2 '-4 12 "

S E C T IO N A

1 '-4 12 "

E x is tin g C F R P w ra p p e d a ro u n d b e a m e n d to re s tra in f ie ld -a p p lie d

c a rb o d u r s tr ip s

S ik a 3 0 H i M o d

e x is tin g p re -c u re dc a rb o d u r s tr ip s

G e l e p o x y

3 " D o u b le la y e r C F R P s t ir ru p s

1 '-4 12 "

5 '-4 "

7 "

1 0 "

5 '-4 "

A

L e ft S u p p o rt

2 '

4 12 "

1 '-1 "

1 0 "

B

E x is t in g s h e a r s t ir ru p s ( ty p )1 '-1 "

1 ' 1 '

1 '-4 "8 "

1 ' 1 '

1 1 "1 '

1 ' 1 '

1 0 " 1 '-2 "

1 '1 '

1 '1 '-1 "

2 '-4 12 "2 '-4 1

2 "1 2 " C F R P s h e e ts

S ik a 3 0 H i M o d G e l e p o x y

3 " x 3 " x 1 /4 " G F R P a n g le sT h ru -b o lts

e x is tin g p re -c u re dc a rb o d u r s tr ip s

5 12 "

e x is t in g p re -c u re dc a rb o d u r s tr ip s

S E C T IO N C

1 '-4 12 "

5 12 "

3 " x 3 " x 1 /4 " G F R P a n g le ss e c u re d w ith T h ru -b o lts

5 '-4 "

6 "

5 12 "

C

1 '

6 "

1 '-1 "1 '

6 "1 ' 1 '

1 '-1 "1 '

C L

1 '-2 "

1 '1 ' 6 "

1 1 "

6 " 1 '

1 0 " 1 '-3 "

5 12 "

A

1 0 "

1 '1 ' 6 "

1 '-1 " 1 0 "

6 "R ig h t S u p p o rt

L e ft S p a n (3 " w id e d o u b le la y e r C F R P s t ir ru p s ) R ig h t S p a n (1 2 " w id e C F R P s h e e ts )

3 " w id e C F R P S tir ru p s ( ty p )

Figure 5-35 Beam shear retrofit layout for T-Beam 2

70

Figure 5-36 Sika 30 Hi Mod Gel epoxy being applied to the CFRP stirrups for even seating of the anchorage angles

3” wide CFRP stirrups with anchorage angles 12” wide CFRP sheets with anchorage angles in place

Figure 5-37 CFRP angles installed as anchorage for CFRP shear reinforcement

71

72

T-Beam 2 was supported with a pin and roller. It was supported outside the ends of

the precast beam rather than underneath the beam. Supporting the beam underneath the

CFRP wrapped ends would have added extra restraining force to the pre-cured carbodur

strips. Therefore, two steel plates were fabricated and attached at the ends of the beam

(Figure 5-31). They were attached to the beam by means of epoxy, expansion bolts and

anchoring them to the prestressed strands with prestressed anchors. The pin and roller

supports were placed underneath these steel brackets.

Most of the instrumentation of T-Beam 2 was focused on the pre-cured carbodur

strips. A total of 27 strain gages were installed. Strain gages 1-6 were installed on the

longitudinal reinforcement in the concrete slab. The rest were installed on the carbodur

strips. Strain gages 7-27 were installed on the pre-cured carbodur strips. Three rows of

strain gages were attached on the same location of the three pre cured carbodur strips.

Figure 5-38 shows a row of strain gages installed on the pre-cured carbodur strips.

Figure 5-38 Electrical resistance strain gages installed on the carbodur strips

73

In addition, three LVDTs were placed on top of the concrete slab to measure the

vertical deflection of the beam. There were no dial gages placed on the support because

no settlement of the supports was experienced during testing of T-Beam 1. Figure 5-39

shows the overall layout and instrumentation of T-Beam 2.

T-Beam 2 was tested on a 25’-8 ½” span from pinned support to roller support. It has

a longer span than T-Beam 1 due to the supports being located under the steel brackets

outside the beam ends. It was loaded with two point loads located 4’-3” apart centered

on the beam mid-span. The loading routine was the same as that used for T-Beam 1.

Figure 5-40 shows T-Beam 2 in the load frame ready for testing.

25'-812"

12'-5"

8'-112"

LVDT 211"

Left Support

1'-412"

2'

412"

178"

1'1'-1" 10" 1' 1'1' 1' 1'

TBEAM 2

1' 1' 1' 6"1' 1' 1'6"

Existing shear stirrups (typ)

3'

1'-2"

3'

134"

7"10" 8" 1'-4"

3'

262527

2"

11' 11"

21'-1" 10"

232224

201921

5 & 64

BOTTOM OF BEAM

CL

1'-1"

4'-3"

1'-1"1'

2"

P

LVDT 1

3

3"

1'

2'-112"

181617 14

1315

P

1'-1"1' 1'

Right Support1'6"6" 1' 1' 6" 6"6" 1'

6'-9"3'-9"

3'

1'-1"10"1'-2"

LVDT 31'-3"

111012

10" 10"

879

Left Span (3" wide double layer CFRP stirrups) Right Span (12" wide CFRP sheets)

Figure 5-39 T-Beam 2 Layout and Instrumentation

74

75

Figure 5-40 T-Beam 2 test setup

76

5.4.5 T-Beam 2L Layout and Instrumentation

After flexural testing of T-Beam 2, the left-hand section of the beam was recovered

and re-tested in shear as T-Beam 2L. This section had shear retrofit consisting of 3” wide

double layer CFRP stirrups. The pre-cured carbodur strips on the soffit of the beam had

delaminated during flexural testing of T-Beam 2. These strips were reinstated with a new

layer of Sika 30 Hi Mod Gel epoxy to maintain the increased bending capacity of the

beam. Wet lay-up Sika wraps were installed at each end of the carbodur strips to

improve anchorage. In addition, to simulate typical shear retrofit, the CFRP stirrups were

extended around the bottom of the beam by splicing a 3” wide double layer of Sika Wrap

with 4” minimum overlap at the bottom of the original CFRP stirrups. Figure 5-41 shows

T-Beam 2L ready for retrofitting with the carbodur strips and CFRP wraps extensions.

Figure 5-41 T-Beam 2L being prepared for shear testing

77

Because of the off-center flexural-shear failure of T-Beam 2, the left section of the

beam was considerably shorter than T-Beam 1L, the left section of T-Beam 1. T-Beam

2L was supported by pin and roller supports placed as close as possible to the ends of the

beam, creating a span length of 7’-5”. The right support was directly below the wrapped

end of the carbodur strips, while the left support was beyond the end of the strips. The

beam was loaded by means of a single point load acting on an area measuring 16” by 21”

placed off-center to create a 1.5:1 shear span to depth ratio in the right side of the beam

(Figure 5-42). Because of the inclined prestress strands in the left side of the beam, the

shear capacity of this portion is greater than the right side. Shear failure was therefore

anticipated, and occurred, in the right portion of the beam. A total of 12 strain gages

were installed on the first two 3” wide double layer CFRP stirrups adjacent to the right

support. Six strain gages were placed on stirrups on the front of the beam and the other

six were placed at the same locations on the stirrups on the back of the beam (Figure

5-42). An LVDT was placed at the loading location to measure the vertical deflection.

During loading, a shear crack formed between the left support and the end of the

carbodur strips on the soffit of the beam. In order to avoid premature failure, the load

was removed and the left support relocated to bear directly under the CFRP wrap at the

ends of the carbodur strips (Figure 5-42). The beam was reloaded until shear failure

occurred in the right shear span. Figure 5-43 shows T-Beam 2L in the test frame ready

for testing.

78

1'-4"

P & LVDT

BOTTOM OF BEAM

T-BEAM 2L6'-9"

7'-5"

Left SupportSecond Loading

First Loading

2'

10" 8"7"

Existing prestressstrands

Existing shearstirrups (typ)

Right Support

1'-1"11"1'

654

321

10" 1'-2"2'-10" 1'-4" 3'-3" 2'-3"

1"6"

1112"

1'-1" 10" 1' 1' 1' 1'512"

434"

278"

412"

121110

987

1-6 Front strain gages7-12 Back strain gages

Figure 5-42 T-Beam 2L Layout and Instrumentation

Figure 5-43 T-Beam 2L in test setup

79

5.4.6 T-Beam 2R1 Layout and Instrumentation

After testing T-Beam 2 in flexure, the right portion of the beam was recovered and

used for two shear tests, designated T-Beam 2R1 and T-Beam 2R2. T-Beam 2R1 was

performed on the center section of the original T-Beam 2 to determine the original shear

capacity of the beam without any shear retrofit. Two of the 12” wide CFRP shear retrofit

sheets were removed to allow for shear failure in the left shear span (Figure 5-45). The

carbodur strips providing flexural strengthening were still intact on this portion of the

beam. In order to prevent anchorage slip of the prestressing strands at the damaged left

end of the beam, wedge anchors were installed on each strand as shown in Figure 5-44.

T-Beam 2R1 was simply supported on a 12’-8½” span by pin and roller supports

(Figure 5-45 and Figure 5-46). The load was applied on a 16” by 21” area located off-

center so as to induce a shear failure in the un-retrofitted section of the beam. An LVDT

was placed at the loading location to measure the vertical deflection. No strain gages

were monitored during testing of this un-retrofitted beam section.

Figure 5-44 Wedge anchors installed on prestressed strands at end of T-Beam 2R1

80

Left Support

BOTTOM OF BEAM

Existing shear stirrups (typ)1' 11" 10" 10" 10"1'1' 1'-1" 1'-1" 1'-2" 1'-3" 1'-1"

P & LVDT

1'1' 6" 6"6" 1' 1'6" 1' 6"

1'-4"3'-712"

1'Right Support

12'-812"

TBEAM 2R1

Figure 5-45 T-Beam 2R1 Layout and Instrumentation

Figure 5-46 T-Beam 2R1 in test setup

81

5.4.7 T-Beam 2R2 Test Setup and Layout

T-Beam 2R2 was the second shear test performed on the right hand section recovered

from the T-Beam 2 flexural test. T-Beam 2R2 was performed to evaluate the 12” wide

CFRP shear reinforcement sheets installed on the right half of T-Beam 2. In order to

simulate typical shear retrofit, the first two CFRP sheets from the left support were

extended around the soffit of the beam by means of spliced 12” wide CFRP sheets as

shown in Figure 5-47. The prestressed strands at the left end of the beam were anchored

by means of wedge anchors similar to T-Beam 2R1 shown in Figure 5-44. The pre-cured

carbodur strips were still intact on the soffit of this portion of the beam. No additional

repair was needed.

T-Beam 2R2 was simply supported over a 9’-0” span with pin and roller supports as

shown in Figure 5-47. A total of nine strain gages were installed on the first two 12”

wide CFRP sheets from the left support. T-Beam 2R2 was loaded on a 12” x 36” area

located at mid-span, simulating a line load. An LVDT was placed at the loading location

to measure vertical deflection. Figure 5-47 shows the instrumentation and layout of the

beam. Figure 5-48 shows T-Beam 2R2 in the test frame ready for testing.

82

B O T T O M O F B E A M

4 '-6 "

E x is t in g s h e a r s t ir ru p s ( ty p )

9 'T -B E A M 2 R 2L e ft S u p p o r t R ig h t S u p p o rt

1 1 " 1 0 " 1 0 " 1 0 "1 '-1 "1 '-2 " 1 '-3 "

P & L V D T

1 '8 1

2 "4 "

1 12 "

3 "

1 ' 6 " 1 '1 ' 6 " 6 " 1 ' 1 '6 " 6 "

1 " 1 12 "

5 "

123

2 "

8974

564 1

2 "4 "4 "

3 12 "

3 12 "

4 12 "

Figure 5-47 T-Beam 2R2 Layout and Instrumentation

Figure 5-48 T-Beam 2R2 in Test Frame

83

CHAPTER 6

6 MATERIAL PROPERTIES

Material properties of the T-Beam concrete, reinforcing and prestressing steel, and

CFRP carbodur strips and wet lay-up wrap were determined through coupon testing

performed after the T-Beam tests. These material properties were required for strength

calculations in Chapter 7, Theoretical Strengths of T-Beam 1 and T-Beam 2.

6.1 Concrete Compressive Strengths Two compressive strengths were determined for each T-Beam. The compressive

strength of the concrete used in the top slabs was determined using 6” diameter by 12”

long concrete cylinders cast when the T-Beam top slabs were poured. They were tested

in compression on the same day as the flexural tests on the T-Beams. The compressive

strengths of the precast prestressed beams were determined by testing concrete cores

taken from the web and anchorage blocks. After all testing had been performed on the T-

Beams, 4”diameter by 5.5” long concrete cores were drilled from un-cracked sections of

the precast beam web and anchorage blocks using a core drill as shown in Figure 6-1.

The cores were tested in compression and the resulting strengths adjusted according to

ASTM C42-99 due to their non-standard cylinder size. From these compressive

strengths, the modulus of elasticity of the concrete was estimated using the expression,

(ksi) 1000 40 += cfE 17. Table 6.1 shows the average and standard deviation for

concrete compressive strengths and corresponding modulus of elasticity values

determined from these tests.

84

Figure 6-1 Concrete core sample taken from a T-Beam web

Table 6.1 Concrete Compressive Strength and Modulus of Elasticity

Top Slab, 6” x 12” cylinder Precast Beam, 4” x 5.5” cores

Avg. fc (psi)

Std. Dev. (psi)

No. of samples

Avg. fc (psi)

Std. Dev. (psi)

No. of samples

T-Beam 1 5396 121 3 8413 329 6

T-Beam 2 9023 276 4 8397 555 4

Modulus of Elasticity, E = 40√fc + 1000 (ksi)

Top Slab Precast Beam

T-Beam 1 3938 4669

T-Beam 2 4800 4665

85

6.2 Top Slab Concrete Modulus of Rupture Two rectangular beams were cast during the pouring of the concrete top slab for T-

Beam 2 to perform modulus of rupture tests according to ASTM C78-99. The beams

were 6” x 6” x 18” long and loaded at third points along the span. Table 6.2 lists the

results of the rupture tests.

Table 6.2 Modulus of Rupture Test

Beam Load (lbs) M (lb-in) fr (psi)

1 6625 19875 552

2 9525 28575 794

Avg 673

6.3 Steel Reinforcement Tensile Strengths Internal shear stirrups and prestressing strands were recovered from the T-Beams

after all tests were complete. The shear stirrups were two-legged #3 deformed

reinforcing bars. The prestressing strands were 3/8” nominal diameter seven-wire stress-

relieved strands with a design nominal tensile strength of 250 ksi. Coupons of these

materials were prepared and tested in tension to determine their yield and ultimate

strengths. Table 6.3 lists the yield and ultimate strengths of the shear stirrups and

prestressing strands.

86

Table 6.3 Steel Reinforcement Tensile Strengths

Yield Stress Ultimate Stress

Description No. of samples

Avg. fy (ksi)

Std. Dev. (ksi)

Avg. fu (ksi)

Std. Dev. (ksi)

Shear stirrups 9 50.9 1.27 73.1 3.31 T-Beam 1

Prestress strands 272

Shear stirrups 50.9 1.27 73.1 3.31 T-Beam 2

Prestress strands 272

6.4 CFRP Material Properties The 4” wide pre-cured carbodur strips and the Sika Wrap Hex 103C uni-direction wet

lay-up material were tested after all T-Beam tests were complete. Samples of the

carbodur strips were recovered from locations where they appeared undamaged and still

in good condition. Double layer 12” x 12” wet lay-up samples of the Sika Wrap Hex

103C were made at the same time as the shear retrofit of the beam webs. Coupons of

these materials were cut and tested in tension to determine their tensile strength and

modulus of elasticity. Table 6.4 lists the tensile strength and modulus of elasticity of

these materials.

Table 6.4 CFRP Material Properties

Tensile Strength Modulus of Elasticity

CFRP Material Avg. fCFRP (ksi) Avg. ECFRP (ksi)

Carbodur strips 406 23900

Sika Wrap Hex 103C 139 10600

87

6.5 CFRP Pull-off Tests Pull-off tests were performed on the CFRP to determine the bond strength between

the CFRP and the precast concrete. Tests were performed on the CFRP shear stirrups,

CFRP shear sheets and the pre-cured carbodur strips. The tests were made at locations

where the concrete was un-cracked and the bond between CFRP and concrete was still

intact. The tests were performed using the DYNA Z16 pull-off tester shown in Figure

6-2. Figure 6-3 shows typical locations where the pull-off tests were performed. In all

cases, failure occurred in the concrete substrate, and not in the CFRP or the epoxy bond.

Figure 6-2 DYNA Z16 Pull-Off Tester

Figure 6-3 Typical locations of CFRP pull-off tests

88

Table 6.5 lists the pull-off test results from T-Beam 1 and T-Beam 2. All of the pull-

off tests exceeded the 200 psi minimum recommended by the ACI 440R-02 report for

CFRP installation18.

Table 6.5 Pull-off Test Results

No. of

Samples Stress (psi)

Std. Dev. (psi) Comment

T-Beam 1R 9 462 283 Concrete failure T-Beam 1

T-Beam 1L 3 696 167 Concrete failure

T-Beam 2 T-Beam 2R2 7 563 138 Concrete failure

89

CHAPTER 7

7 THEORETICAL BEAM STRENGTHS

This chapter presents predicted strength calculations for the T-Beams tested in this

program. In the first section of this chapter, the flexural strength of T-Beam 1 is

predicted using the ACI 318-02 Building Code. The flexural strength capacity of T-

Beam 2 with CFRP flexural strengthening is predicted using the ACI 440R-02. The

strain-compatibility methodology proposed by ACI 440R-02 for non-prestressed beams is

presented in detail. Adjustments are proposed for application of the ACI 440R-02

methodology to prestressed concrete beams. This methodology is then applied to the T-

Beam 2 section properties.

The rest of the chapter presents shear strength predictions for the original T-Beam

without retrofit and for the CFRP retrofitted beams. The ACI 440R-02 approach to

predicting the contribution of CFRP shear reinforcement is introduced. Shear strength

predictions are presented for T-Beam 2R1 (concrete beam without CFRP shear retrofit),

T-Beam 1L and T-Beam 2L (with CFRP shear stirrups), and T-Beam 1R and T-Beam

2R2 (with CFRP shear sheets). The predicted strengths are compared with the observed

strengths in Chapter 8.

7.1 Notation

fff wntA = = area of CFRP external reinforcement

sA = area of non-prestressed steel reinforcement

cpA = area of precast prestressed concrete section

csA = area of concrete slab

90

ccA = area of composite section

fffv wntA 2= = area of CFRP reinforcement within spacing s

psA = total area of prestressed strands ca 1β= = depth of equivalent rectangular stress block

b = width of the compression flange c = depth of the neutral axis

EC = environmental reduction factor d = centroidal depth of a non prestressed reinforcement measured from

top of beam fd = depth of CFRP shear reinforcement

pd = centroidal depth of prestressed strands measured from top of beam e = eccentricity of the prestressed tendons

ce = eccentricity of the prestressed tendons at center

ee = eccentricity of the prestressed tendons at support

cE = modulus of elasticity of the concrete slab

fE = tensile modulus of elasticity of CFRP

pE = modulus of elasticity of prestressed concrete

psE = modulus of elasticity of prestressed tendons

sE = modulus of elasticity of steel

cf = measured compressive strength of concrete '

cf = specified compressive strength of concrete

bsf = bottom stress level in the concrete slab of a prestressed concrete section due to the dead load of the concrete slab

bpf = stress level in the bottom of the prestressed beam section due to the prestressing force and dead load of beam

cef = stress due to prestress at tension fiber

cpf = stress level in the prestressed beam section at the level of the prestressed tendons

df = stress due to un-factored dead load at tension fiber

fef = effective stress in the CFRP; stress level attained at section failure

fuf = design ultimate tensile strength of CFRP *fuf = ultimate tensile strength of the CFRP material as reported by the

manufacturer pf = stress level of the prestressed tendons attained at section failure

pcf = compressive stress in concrete at centroid of composite section

pef = effective prestressing stress of the prestressed tendons due to eP

pif = initial prestressing stress of the prestressed tendons

91

psf = stress in prestressed reinforcement at nominal strength

puf = ultimate strength of prestressing tendons

sf = stress in non-prestressed steel reinforcement

tpf = stress level in the top of the prestressed beam section due to the prestressing force and dead load of beam

tsf = stress level in the top concrete slab of a prestressed beam section due to the dead load of the concrete slab

yf = specified yield strength of non-prestressed reinforcement h (i.e. ch ) = overall thickness of member

cI = moment of inertia of composite prestressed section

pI = moment of inertia of precast prestressed beam section

1k = modification factor applied to vκ to account for the concrete strength

2k = modification factor applied to vκ to account for the wrapping scheme

l = member span length eL = active bond length of CFRP laminate

crM = cracking moment

dM = moment at section due to un-factored dead load (self-weight of precast and topping)

dpM = dead load moment of the prestressed concrete beam

dsM = dead load moment of the concrete slab of a prestressed concrete beam

maxM = maximum factored moment at section due to external loads (not including dead load)

nM = nominal flexural capacity n = number of plies of CFRP reinforcement

cn = modular ratio s = shear stirrup spacing

fs = CFRP shear reinforcing spacing

eP = effective prestressing force of the prestressed tendons

bc

cbc y

IS = = bottom section modulus of the composite section

b

pbp y

IS = = bottom section modulus of the prestressed beam

bs

cbs y

IS = = section modulus to the bottom of the concrete slab of a composite

section

92

t

ptp y

IS = = top section modulus of the prestressed beam

ts

cts y

IS = = top section modulus of the composite section

t = thickness of the concrete slab ft = nominal thickness of one ply of the CFRP reinforcement

cV = nominal shear strength provided by concrete

cwV = nominal shear strength provided by concrete when diagonal cracking results from excessive principal tensile stress in the web

dV = shear force at section due to un-factored dead load (self-weight of precast and topping)

fV = nominal shear strength provided by CFRP stirrups

iV = factored shear force at section due to externally applied loads occurring with maxM

nV = nominal shear capacity

pV = vertical component of effective prestress at section

sV = nominal shear strength provided by steel stirrups

fw = width of the CFRP reinforcing plies

dw = self-weight of prestressed beam and concrete slab

dpw = self-weight of prestressed beam

dsw = self-weight of concrete slab

bsy = distance from the centroid of gravity of a composite prestressed beam section to the bottom of concrete slab

by = distance from the centroid of gravity of a prestressed beam section to the bottom of the prestressed beam section

bcy = distance from the centroid of gravity of a composite prestressed beam section to the bottom of the composite prestressed beam section

ty = distance from the centroid of gravity of a prestressed beam section to the top of the prestressed beam section

tsy = distance from the centroid of gravity of a composite prestressed beam section to the top of concrete slab

1β = ratio of the depth of the equivalent rectangular stress block to the depth of the neutral axis

bε = strain level in the concrete substrate developed by a given bending moment (tension is positive)

biε = strain level in the concrete substrate at the time of the CFRP installation (tension is positive)

93

cuε = maximum usable compressive strain of concrete (0.003)

feε = effective strain level in CFRP reinforcement; strain level attained in section failure

fuε = design rupture strain of CFRP reinforcement

sε = strain level in the non-prestressed steel reinforcement

bpε = initial strain level in the bottom prestressed concrete substrate due to bpf (i.e. strain level in the concrete substrate at the time of the CFRP installation)

cpε = initial strain level of prestressed concrete substrate at the level of the prestressed tendons

bsε = initial strain level in the bottom concrete slab substrate of the prestressed concrete due to bsf

pε = strain level of the prestressed tendons attained in section failure

peε = effective strain level of the prestressed tendons due to pef

pyε = yield strain level of the prestressed tendons

tpε = initial strain level in the top prestressed concrete substrate due to

tpf

tsε = initial strain level in the top concrete slab substrate of the prestressed concrete due to tsf

mκ = bond dependent coefficient for flexure

vκ = bond reduction coefficient

p

psp bd

A=ρ = ratio of prestressed reinforcement

pγ = factor for type of prestressing tendon

fψ = additional CFRP strength-reduction factor

94

7.2 Flexural Strength of T-Beam 1

The predicted nominal flexural capacity of T-Beam 1 was based on ACI 318-02

building code. The mid-span cross-section of T-Beam 1 is shown in Figure 7-1.

(10) 38"Ø Stress-relieved prestress strands 51

2"


1'-412"

Slab Reinforcement2-leg #3 Stirrups @ 12" o.c.

b = 66"

512"

412"

1'-612"

2 38"1 78"

1 34" dp = 24.65"

Figure 7-1 Cross-section of T-Beam 1

The nominal flexural capacity was calculated using the following formula,

2

−= adfAM ppspsn

where

−+−= )'(

'1

1

ωωρβγ

pc

pup

ppups d

dff

ff (ACI 318-02, Eq. 18-3).

Since there was no mild tension steel in the T-Beam and the effect of the compression

steel in the flange was negligible, psf was simplified to,

'

11

−=

c

pup

ppups f

fff ρ

βγ

95

where 65.01000

)4000'(05.085.0 1 ≥−−= cfβ .

The flexural capacity of T-Beam 1 was predicted using the measured material

property values. For 5396' =cf psi for the concrete slab (Table 6.1), 78.01 =β . The

area of one 3/8” diameter seven wire stress-relieved prestressing strand (grade 250) is

0.080 in2. The area of ten strands is therefore 80.0=psA in2. The width of the flange is

"66=b (Figure 7-1). The depth of the centroid of the prestressed strands is "65.24=pd .

For stress-relieved tendons, 4.0=pγ . Based on coupons tests, the ultimate strength of the

prestressing tendons is 272=puf ksi (Table 6.3). Substituting gives:

269396.5

27265.2466

80.078.04.01272 =

−= x

xf ps ksi.

From internal force equilibrium, the depth of the concrete compression block is:

"71.066396.585.0

26980.085.0 ' =

×××=

×

×=bf

fAac

psps .

Note that 'cf for the equivalent rectangular stress block is based on the concrete slab

concrete cylinder tests. The depth of the equivalent rectangular stress block is less than

the thickness of the concrete slab, therefore the nominal flexural capacity of T-Beam 1 is

given by:

ft.-kip 436 in-kip 5228271.065.2426980.0

2==

−×=

−×= adfAM ppspsn

96

Using the nominal material strengths of 4000' =cf psi for the top slab and

250=puf ksi for the prestressed strands, the nominal flexural capacity of T-Beam 1 is

397=nM kip-ft.

7.3 Flexural Strength of T-Beam 2 (w/carbodur strips) The ACI 440R-02 report suggests a methodology for computation of the nominal

flexural capacity of a reinforced concrete beam retrofitted in flexure with CFRP bonded

to the tension surface. The calculation is based on the ultimate limit state condition of the

beam’s stress and strain. However, the ACI 440R-02 report does not consider prestressed

concrete members. An understanding of the stress and strain distribution of the

reinforced concrete beam was helpful in the development of the equations used for

calculating the nominal flexural capacity of a prestressed concrete beam retrofitted with

CFRP in flexure.

7.3.1 Flexural Capacity of a Reinforced Concrete Beam with CFRP19

The stress and strain distribution of a typical reinforced concrete beam retrofitted with

CFRP is shown in Figure 7-2. The ACI 440R-02 procedure used to arrive at the nominal

flexural strength of the beam satisfies the strain compatibility and force equilibrium of

Figure 7-2. It also considers potential controlling failure modes as compressive concrete

crushing or CFRP debonding.

97

(Neutral Axis)d

sε

c

εcu

ff s

f fe

sffe

0.85

β c1

fc'

b

h

εb

biεε fe

Figure 7-2 Stress and strain distribution of a reinforced concrete beam with CFRP

under flexure at ultimate limit state condition

To determine the flexural strength of the beam, several equations must be satisfied by

trial and error. For an assumed depth of the neutral axis, c, the strain level of the CFRP is

computed as:

fumbicu cch εκεεε ≤−

−= fe (ACI 440R-02, Eq.9-3)

where,

>≤

≤≤

−

=

1000000for 90.050000060

1

1000000for 90.02000000

160

1

fffffu

ffff

fu

m

tnEtnE

tnEtnE

ε

εκ (ACI 440R-02, Eq.9-2).

The left side of ACI 440R-02 equation 9-3 is based on strain compatibility of the

beam section assuming concrete crushing, while the right side represents the CFRP

debonding failure mode. If the left side of the equation controls, the failure mode of the

section is concrete crushing, while debonding governs if the right side of the equation

controls. The concrete failure strain level is usually taken as 0.003.

98

The initial strain level in the concrete at the level of the CFRP, biε , is computed

considering the load experienced by the beam immediately prior to application of the

CFRP. Usually, this load is the dead weight of the beam and any supported slab. It is

appropriate to subtract the initial strain level from the total strain level to get the effective

strain level of the CFRP. Unless the beam is shored to relieve some of the existing dead

load, this initial strain level must be considered when computing the strain in the CFRP.

A bond dependent coefficient, mκ , is provided in the calculation as a safety factor against

CFRP debonding.

Since the tensile stress-strain relationship for the CFRP is linear until failure, the

stress level in the CFRP is given by:

feffe Ef ε= (ACI 440R-02, Eq. 9-4).

Based on the strain level of the CFRP and the initial strain level of the concrete, the

strain level of the non-prestressed steel reinforcement is determined using strain

compatibility of the beam section as:

( )

−−+=

chcd

bifes εεε (ACI 440R-02, Eq. 9-8).

Assuming perfectly elastic-plastic behavior for the non-prestressed steel, the stress

level in the steel is given by:

ysss fEf ≤= ε (ACI 440R-02, Eq. 9-9).

Having determined the stresses in the CFRP and steel reinforcement for the assumed

neutral axis depth, c, internal force equilibrium is checked using:

99

bffAfA

cac

fefss'1 85.0

+== β (ACI 440R-02, Eq. 9-10).

The equivalent rectangular stress block (Whitney stress block) is used to estimate the

compressive stress in the concrete compression zone for both potential failure modes. If

the value of c determined from ACI 440R-02 equation 9-10 differs from the assumed

value, the new value of c is used as the next assumed c and the process repeats. Iteration

of these equations continues until the neutral axis depth determined from ACI 440R-02

equation 9-10 agrees with the assumed value. The nominal flexural capacity of the CFRP

retrofitted concrete beam is then determined as:

−+

−=

22ahfAadfAM feffssn ψ (ACI 440R-02, Eq. 9-11).

A reduction factor of 85.0=fψ is applied to the flexural strength contribution of the

CFRP reinforcement.

7.3.2 Nominal Flexural Capacity of a Prestressed Concrete Beam with CFRP

Since the ACI 440R-02 equations were developed for non-prestressed concrete

beams, it was necessary to modify these equations to determining the nominal flexural

capacity of a prestressed concrete beam retrofitted with CFRP. The modified system is

also based on strain compatibility and force equilibrium of the prestressed member. The

equations differ from those presented in Section 7.3.1 due to the presence of a

prestressing force in the steel and concrete, and the difference in stress-strain response of

prestressing steel compared with non-prestressed reinforcement.

The stress and strain distributions in the beam at both initial and final conditions are

considered in this derivation. The initial condition represents the beam condition at the

100

time the CFRP retrofit was applied. Usually, the stress and strain distribution in the beam

at the initial condition is a function of the level of prestress and the dead weight on the

beam. A typical stress and strain profile of a prestressed concrete beam at the initial

condition is shown in Figure 7-3. In order to generalize to a composite T-Beam section,

this derivation considers a composite section with a non-prestressed top slab.

(Neutral Axis)h

d

b

εcp

εbp

peε

c

tsε

p

tsf

pef f cp

bsεtpε bsfftp

bpf

t

Figure 7-3 Stress and strain distribution of a prestressed concrete beam under

flexure at the initial condition (prior to application of CFRP)

The stress levels in the concrete at the initial condition are:

p

c

ts

dsts E

ESM

f ×−

= p

c

bs

dsbs E

ESM

f ×−

=

bs

ds

tp

dp

tp

e

cp

etp S

MS

MS

ePA

Pf −−+

−=

bc

ds

bp

dp

bp

e

cp

ebp S

MSM

SeP

AP

f ++−−

=

and ( ) bppbptp

cp fdhthff

f +−×

−−

=

101

The corresponding strain levels in the concrete at the initial condition are:

c

tsts E

f=ε

c

bsbs E

f=ε

p

tptp E

f=ε

p

bpbp E

f=ε

( ) bppbptp

cp dhth

εεε

ε +−×

−−

=

In addition to the stress and strain levels in the concrete, the stress and strain of the

prestressed tendon at the initial condition are:

cp

epe A

Pf =

ps

pepe E

f=ε

where eP is the effective prestress after losses at the time of installation of the CFRP.

These values make up the stress and strain profile of Figure 7-3. It is likely that the

majority of the precast prestressed concrete beam section will be in compression at the

initial condition. In particular, the bottom fibers may be subjected to significant

compression at the time of FRP application, as opposed to the small tensile strain in the

bottom fibers for a non-prestressed beam.

Once the initial condition has been determined, the stress and strain profiles for the

final condition are developed as shown in Figure 7-4. The final condition is the ultimate

limit state of the beam in flexure.

102

feε

dh p

b

c

(Neutral Axis)

t

peε εppfpf

bpεfef fef

β

cpε

cuε

c1

0.85 'fc

Figure 7-4 Stress and strain distribution of a prestressed concrete beam with CFRP

under flexure at ultimate limit state condition

At the ultimate limit state, the beam section experiences tension from the neutral axis

to the bottom of the section. The dotted line represents the initial strain condition of the

prestressed concrete. Since the prestressing tendons were bonded to the concrete, the

additional elongation of the tendons started at the initial condition. In addition, the CFRP

elongation also started at the initial condition when the whole section was still in

compression.

To arrive at the nominal flexural strength of the prestressed concrete beam, several

equations are developed which must satisfy the strain compatibility and force equilibrium

of Figure 7-4. Also, the concrete strain levels must be checked according to the mode of

failure, namely concrete crushing or CFRP debonding. As before, this new set of

equations is satisfied by iteration. For an assumed depth c, the strain level of the CFRP is

computed from:

fumbpcu cch εκεεε ≤+

−= fe .

103

Note that bpε and biε have the same meaning, although the first is for a prestressed

beam while the second is for a non-prestressed beam. They both represent the initial

strain at the bottom concrete fiber before the CFRP was applied. In the case of the

prestressed beam, the strain in the CFRP at the ultimate limit state is the sum of this

initial concrete compressive strain and the strain corresponding to crushing of the

compression concrete at the top of the beam.

Since the tensile stress-strain relationship for the CFRP is linear until failure, the

stress level in the CFRP is given by:

feffe Ef ε= .

Based on the initial tensile strain in the prestressing steel and the compressive strain

in the concrete at the level of the prestress steel centroid, the strain level of the

prestressed steel at the ultimate limit state is determined from strain compatibility as:

pecpp

cup ccd

εεεε ++

−= .

The strain level of the concrete at the centroid of the prestressing steel and the

effective prestress strain in the prestressing steel due to the prestressing force minus

losses, are added to the strain induced in the prestressed tendons at the ultimate bending

capacity. The stress corresponding to this strain must be determined from the stress-

strain relationship for the prestressing steel.

If pyp εε ≤ , then ppp Ef ε= .

104

If pyp εε > , then pf is determined from the stress-strain relationship for

the prestressing steel with a limit of psp ff ≤ , where,

×+−= ''

1

1c

fuf

c

pup

ppups f

fbhA

ff

ff ρβγ

.

This expression for psf takes into account the contribution of the CFRP flexural

reinforcement.

With the stresses in the CFRP and prestressing steel determined for the assumed

neutral axis depth, c, internal force equilibrium is checked using:

bffAfA

cac

fefpps'1 85.0

+== β .

Iteration of these equations is required until the neutral axis depth determined from

this equation matches with the assumed value. Once satisfied, the nominal flexural

capacity of the CFRP strengthened prestressed concrete beam computed as:

−+

−=

22ahfAadfAM feffpppsn ψ .

7.3.3 Calculation of the Predicted Flexural Strength of T-Beam 2

This section presents the computation of the nominal flexural capacity of T-Beam 2

using this modified ACI 440R-02 procedure for prestressed beams. In its initial condition

in the Ala Moana Parking Garage, the beam supported a tributary width of 30’ over a 30’

span. The dead load supported by the beam at the time of retrofit application is assumed

to be the self-weight of the precast beam and the weight of 30’ tributary width of slab.

Figure 7.5 shows the cross-section of the beam used to calculate this dead load.

105

AcsAcp

30'

Figure 7-5 T-Beam 2 tributary width at Ala Moana Parking Garage

The initial dead load on the beam was therefore determined as follows: Unit weight of concrete: 150 lb/ft3

Weight of precast beam: 198=cpA in2 206150144198 =×=dpw lb/ft

Weight of concrete slab: 16205.41230 =××=csA in2 16881501441620 =×=dsw lb/ft

Dead load moment: 1781000

128

242068

22

=×== xlwM dp

dp kip-in (precast)

760100012

24301688

24

22

=×== xlwM ds

ds kip-in (concrete slab)

Note that the precast self-weight is supported by the precast member over a span of

24 feet, while the topping slab was added once the precast beam was installed on the

column capitals, representing a span length of 30 feet. The precast section was shored

during addition of the topping slab. Any continuity at the supports has been neglected to

simplify the computation of dead load moments.

The initial stress level in the prestressing steel at the time of CFRP application was

determined as follows:

With 250=puf ksi, and 5.18725075.075.0 =×== pupi ff ksi. Assuming 20% prestress loss, 1505.18780.080.0 =×== pipe ff ksi. Therefore, 12080.0150 =×== pspee AfP kips

106

Material properties of the prestressed beam from Table 6.1 are:

9023' =cf psi (concrete slab) and 8397' =cf psi (precast)

4800=cE ksi (concrete slab) and 4665=pE ksi (precast)

Section properties: Precast Section:

24"15"

9"

cgp

cgs e = 5.15"

Figure 7-6 Section properties of the precast prestressed beam section

10075=pI in4 "9=by "15=ty

11199

10075 ===b

pbp y

IS in3 672

1510075 ===

t

ptp y

IS in3

107

Composite Section (Flange transformed to equivalent width):

64" x 1.03= 66"

cgc

19.4"

9.1"4.6"

Figure 7-7 Section properties of the composite section

45843=cI in4 "4.19=bcy "6.4=bsy "1.9=tsy

23634.19

45843 ===bc

cbc y

IS in3 9966

6.445843 ===

bs

cbs y

IS in3

50381.9

45843 ===ts

cts y

IS in3 03.1

46654800 ===

p

cc E

En

Initial Condition Stresses:

155.003.15038

760 −=×−=×−

=p

c

ts

dsts E

ESM

f ksi

079.003.19966

760 −=×−=×−

=p

c

bs

dsbs E

ESM

f ksi

9966760

672178

67215.5120

198120 −−+−=−−+

−= x

SM

SM

SeP

AP

fbs

ds

tp

dp

tp

e

cp

etp

027.0076.0265.092.0606.0 −=−−+−= ksi

2363760

1119178

111915.5120

198120 ++−−=++−

−= x

SM

SM

SeP

AP

fbc

ds

bp

dp

bp

e

cp

ebp

677.0322.0159.0552.0606.0 −=++−−= ksi

108

( ) bppbptp

cp fdhthff

f +−×

−−

=

( ) 573.0677.065.245.285.45.28677.0027.0 −=−−×

−+−= ksi

and 150=pef ksi. Initial Condition Strains (10-6):

3.320000323.04800

155.0 −=−===c

tsts E

fε 5.160000165.04800

079.0 −=−===c

bsbs E

fε

79.500000579.04665

027.0 −=−===p

tptp E

fε 145000145.0

4665677.0 −=−===

p

bpbp E

fε

5260005260.028500150 ====

ps

pepe E

fε

( )

( ) 123000123.0000145.065.245.285.45.28

000145.000000579.0 −=−=−−×

−+−=

+−×

−−

= bppbptp

cp dhth

εεε

ε

Figure 7-8 and Figure 7-9show the stress and strain profiles across the mid-span

cross-section for T-Beam 2 in flexure at the initial and ultimate loading conditions

respectively.

Strain (10 )

32.3

cgc5.79 16.5 0.027

150123145

Stress (ksi)

0.1550.079

0.5730.677-6

5260

Figure 7-8 Stress and strain distributions for T-Beam 2 at the initial condition

109

Strain (10 )

neutral axiscgc

εp

εfe

Stress (ksi)

123145

3000

pf

0.85f'c

ffe

-6

5260

Figure 7-9 Stress and strain distributions for T-Beam 2 at ultimate state conditions

After iteration the neutral axis depth converged to c=0.93”. The following

calculations show the final iteration loop.

Strain level of the CFRP at the ultimate limit state is:

fumbpcu cch εκεεε ≤+

−= fe

where the CFRP debonding failure mode coefficient is:

>≤

≤≤

−

=inlbtnE

tnE

inlbtnEtnE

fffffu

ffff

fu

m

/1000000for 90.050000060

1

/1000000for 90.02000000

160

1

ε

εκ

and inlbinlbtnE ff /1000000/1123300047.0239001 >=××= ,

therefore: 90.050000060

1 ≤

=

fffum tnEε

κ

where: 014.023900

40685.0*

==== xE

fCEf

f

fuE

f

fufuε from CFRP tests.

Therefore, 90.053.01123300500000

014.0601 ≤=

=

xmκ

and, 00742.0014.053.0 == xfumεκ .

110

Finally, 00742.0089.0000145.093.0

93.05.28003.0 fe >=+

−=+

−= bpcu c

ch εεε

The stress level in the CFRP is then:

17700742.023900 =×== feffe Ef ε ksi. The strain level in the prestressed steel at the ultimate limit state is:

082.000526.0000123.093.0

93.065.24003.0 =++

−=++

−= pecp

pcup c

cdεεεε .

This strain exceeds the yield strain of the prestressing steel assumed to be 01.0=pyε ,

therefore the stress in the prestressing steel must be determined from the stress-strain

curve for the tendons. Referring to the stress-strain relationship suggested by Nawy

(2003) for stress-relieved 250 ksi tendons, the stress at 045.0>pε is 250=pf ksi20.

Checking psf :

65.060.01000

)40009023(05.085.01000

)4000'(05.085.0 1 <=−−=−−= cfβ

Therefore, 65.0 1 =β .

+−= '

1 '1

c

fuf

c

pup

ppups f

fbhA

ff

ff ρβγ

246023.9

4065.2864

564.0023.9

25065.2464

8.065.04.01250 =

×

×+×

×−= ksi

Therefore, 250246 <== psp ff ksi. Checking the assumed neutral axis depth c:

"604.0)64)(023.9(85.0177564.02468.0

85.0 '1 =×+×=+

==bf

fAfAca

c

fefppsβ

111

Therefore, "93.065.0604.0

1

===βac which agrees with the assumed depth c.

The nominal flexural capacity of T-Beam 2 is then given by,

ftkinkip

ahfAadfAM feffpppsn

−=−=+=

−××+

−×=

−+

−=

5997185239347922604.05.28177564.085.0

2604.065.242468.0

22ψ

.

112

7.4 Shear Strength of T-Beam 1 (without shear retrofit)

The shear strength of T-Beam 1 was calculated using the ACI 318-02 provisions for

shear strength of composite prestressed concrete beams. T-Beam 1 spanned 24 ft and

supported two point loads at 25.5 inches from mid-span as shown in Figure 7-10. The

shear strength analysis and shear profile along the length of the beam are developed in

this section.


1' TYP

24'

2'

412"

TBEAM 1

4'-3"P P

CL

Figure 7-10 T-Beam 1 layout for shear strength calculation

Concrete T- Beam properties: Prestressed Beam Section: fc' 8413:= psi fpu 250000:= psi fpi 187500:= psi

fpe 150000:= psi A ps 0.8:= in2 cgs c 3.85:= in from bottom cgs e 10.5:= in from bottom Eps 28500:= ksi fy 50900:= psi for stirrups h 24:= in b f 66:= in b w 5.5:= in L 24:= ft Topping Slab: fct' 5396:= psi hc 28.5:= in d p 24.65:= in Pe fpe Aps×:= Pe 120000= lb

113

1. Calculate the location of the section centroid:

i. prestressed section ii. composite section A cp 198:= in2 Ep 4669:= ksi prestressed Ec 3938:= ksi topping

y b 9:= in n cE cE p

:=

nc 0.84=

y t 15:= in A cc 445:= in2 y bc 18.6:= in

Ip 10075:= in4 yts 9.9:= in Ic 43149:= in4

SbpIpyb

:= Sbp 1119.44:= in3 SbcIc

ybc:= Sbc 2319.84= in3

StpIpyt

:= Stp 671.67:= in3 Sts

Icyts

:= S ts 4358.48:= in3

ec yb cgs c−:= ec 5.15= in Eccentricity of prestress tendons at center ee yb cgs e−:= ee 1.5−= in Eccentricity of prestress tendons at support 2. Compute concrete shear capacity based on flexure-shear cracking, Vci:

wdAcp 66 4.5×+

1440.150×:= Wd 0.52= klf Assuming normal weight concrete

Beam self weight plus topping self weight Wu 1.2Wd:= Wu 0.62= klf

Pl431.6

:= kips Live load at each load point corresponding to flexural failure of the beam.

P u 1.6 P l:= Pu 43= kips

Vci x( ) 0.6λ fc'× bw× dp x( )× Vd x( )+Vi x( ) Mcr x( )×

Mmax x( )+ 1.7λ fc'× bw× dp x( )×≥:=

Mmax λ 1.0:= for normal weight concrete. dp x( )

24.65 18−5.67

x 18+ x 5.67<if

24.65 x 5.67≥if

:=

x 1 2, 10..:= ft Compute shear capacity at 1 ft intervals from support to load point.

Vu x( )W u L×

2P u+ W u x×−:= Factored shear force at section x.

Vd x( ) WdL2

x−

×:= Shear force at x due to un-factored dead load.

Vi x( ) Vu x( ) Vd x( )−:= factored shear force at x due to external load.

114

x12

3

4

5

6

7

8

9

10

=

Vd x( )5.675.16

4.64

4.13

3.61

3.09

2.58

2.06

1.55

1.03

=

Vu x( )49.8149.19

48.57

47.95

47.33

46.71

46.09

45.47

44.86

44.24

=

Vi x( )44.1344.03

43.93

43.82

43.72

43.62

43.52

43.41

43.31

43.21

=

Vu0 Vu 0( ):= Vu0 50.42= k Factored shear force at support Vd0 Vd 0( ):= Vd0 6.19= k Un-factored shear force due to dead load at support

Md x( )Wd x×

2L x−( )×:= Moment at section x due to un-factored dead load

Mu x( ) Vu 0( ) x×Wu x2×

2−:= Moment at section x due to factored load

Mmax x( ) Mu x( ) Md x( )−:= Maximum factored moment at section x due to external

loads (not including dead load)

x12

3

4

5

6

7

8

9

10

=

Mu x( )50.1299.61

148.49

196.75

244.39

291.41

337.82

383.6

428.77

473.31

=

Md x( )5.93

11.34

16.24

20.63

24.49

27.84

30.68

33

34.8

36.09

=

Mmax x( )44.1988.27

132.25

176.13

219.9

263.57

307.14

350.6

393.96

437.22

=

Mcr x( )Iccybc

6 fc'× fce x( )+ fd x( )−( )×:= fd

ft kips kips kips

ft k ft− kips ft−k ft−

115

Eccentricity of prestress at section x, 4 strands harped at section x=5.67 ft from support

e x( ) eeec ee−

5.67x×+

x 5.67<if

5.15 x 5.67≥if

:=

ec 5.15=

ee 1.5−=

fd x( )12000Md x( )

Sbp:= Stress due to un-factored dead load at section x

fce x( )Pe

Acp

Pe e x( )×

Sbp+:= Stress due to prestress at tension fiber at section x

Mcr x( )Ic

12000ybc6 fc'× fce x( )+ fd x( )−( )×:= Cracking moment

x

12

3

4

5

6

7

8

9

10

=

e x( )-0.330.85

2.02

3.19

4.36

5.15

5.15

5.15

5.15

5.15

=

fce x( )570.99696.71

822.44

948.16

1073.89

1158.12

1158.12

1158.12

1158.12

1158.12

=

fd x( )63.56121.6

174.11

221.09

262.55

298.47

328.87

353.75

373.09

386.91

=

Mcr x( )204.49217.57

231.73

246.95

263.24

272.58

266.7

261.89

258.15

255.48

=

Mmax x( )44.1988.27

132.25

176.13

219.9

263.57

307.14

350.6

393.96

437.22

=

Vci x( ) max0.6 λ× fc'× bw× dp x( )×

1000Vd x( )+

Vi x( ) Mcr x( )×

Mmax x( )+

1.7λ fc'× bw× dp x( )×

1000,

:=

x

12

3

4

5

6

7

8

9

10

=

dp x( )19.1720.35

21.52

22.69

23.86

24.65

24.65

24.65

24.65

24.65

=

Vi x( )44.1344.03

43.93

43.82

43.72

43.62

43.52

43.41

43.31

43.21

=

Mcr x( )204.49217.57

231.73

246.95

263.24

272.58

266.7

261.89

258.15

255.48

=

Mmax x( )44.1988.27

132.25

176.13

219.9

263.57

307.14

350.6

393.96

437.22

=

Vci x( )215.72119.85

88.12

72.44

63.17

55.66

47.83

41.95

37.39

33.74

=

ft in psi psi k

ft kips

116

3. Compute concrete shear capacity based on web shear cracking, Vcw Vcw x( ) 3.5λ fc'× 0.3fpc x( )+( ) bw× dp x( )× Vp x( )+:= Vp

PeAps fpe×

1000:=

Pe 120= kips fpe 150000= psi

cgs e 10.5= in at the support cgs c 3.85= in at center Vertical component of effective prestress at section x is:

Vp x( )Pe cgs e cgs c−( )×

68x 5.67≤if

0 x 5.67>if

:=

y ybc ybp−:= y 9.6= in distance between centroid of precast and composite

sections

fpc x( )Pe

Acp

Pe e x( )× y×

Ip−

Md x( ) y× 12×

Ip+:=

Vcw x( )3.5 λ× fc'× bw× dp x( )×

10000.3 fpc x( )× bw× dp x( )×+ Vp x( )+:=

where λ 1= for normal weight concrete.

x

12

3

4

5

6

7

8

9

10

=

dp x( )19.1720.35

21.52

22.69

23.86

24.65

24.65

24.65

24.65

24.65

=

fpc x( )0.710.64

0.56

0.48

0.39

0.34

0.37

0.39

0.42

0.43

=

Vp x( )11.7411.74

11.74

11.74

11.74

0

0

0

0

0

=

Vcw x( )68.0969.11

69.65

69.66

69.11

57.17

58.49

59.57

60.41

61.01

=

ft ksi kips kips

117

4. Shear Strength of Concrete, Vc: Vc x( ) min Vcw x( ) Vci x( ),( ):= Shear strength of concrete x

12

3

4

5

6

7

8

9

10

=

Vci x( )215.72119.85

88.12

72.44

63.17

55.66

47.83

41.95

37.39

33.74

=

Vcw x( )68.0969.11

69.65

69.66

69.11

57.17

58.49

59.57

60.41

61.01

=

Vc x( )68.0969.11

69.65

69.66

63.17

55.66

47.83

41.95

37.39

33.74

=

5. Compute concrete shear capacity of steel stirrups, Vs:

Two - leg #3 stirrups @ 12 in o.c. Although the bottoms of the stirrups are not

anchored with a hook, the full capacity of the stirrups is assumed in these calculations.

As 0.22:= in2 s 12:= in

Vs x( )

As fy× dp x( )×

1000s:=

6. Compute combined concrete and steel stirrup shear capacity, Vn: Vn x( ) Vc x( ) Vs x( )+:= x

12

3

4

5

6

7

8

9

10

=

Vc x( )68.0969.11

69.65

69.66

63.17

55.66

47.83

41.95

37.39

33.74

=

Vs x( )17.8918.99

20.08

21.17

22.27

23

23

23

23

23

=

Vn x( )85.9888.1

89.73

90.83

85.44

78.67

70.83

64.95

60.39

56.74

=

ft kips kips kips

kipsft

118

7. Plot the Shear Capacity Profile for T-Beams 1 and 2

Figure 7-11 shows the applied shear diagrams for T-Beams 1 and 2 based on their

flexural capacities, and the shear strengths for the T-Beams without CFRP shear

reinforcement. The shear applied to T-Beam 1 exceeds the beam capacity over a 2 feet

distance adjacent to the load points. For T-Beam 2, the applied shear exceeds the beam

capacity for 5 feet on either side of the point loads. It was concluded that the shear

capacity of both T-Beams should be increased by means of a CFRP shear retrofit to

reduce the potential for shear failure during the flexural testing.

Shear Capacity Profile of T-beam1

0102030405060708090

100110120130140150160170180190200210220

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Span length of beam (ft)

Shea

r Fo

rce

(kip

s)

Vci(x)Vcw(x)Vc(x)Vs(x)Vn(x)T-beam1 Shear DiagramT-beam2 Shear Diagram

Figure 7-11 Shear capacity and shear diagram of T-Beams 1 and 2

119

7.5 Shear Strength of T-Beam 2R1 (plain concrete)

The shear capacity of T-Beam 2R1 was determined according to ACI 318-02

provisions for non-composite prestressed concrete beams. It was analyzed as a non-

composite section because the dead weight of the beam was neglected. The nominal

shear capacity is based on contributions from the concrete and steel stirrups. Figure 7-12

shows T-Beam 2R1 test layout at failure, along with the corresponding shear and bending

moment diagrams.

3'-712"

12'-812"

1'

TBEAM 2R1

1' 1'

6"

6"1'1'

6" 6" 6"

1'

Pu=185 k


122.5

62.5

V(kips)

M(kip-in)

5329

1'-934"

Vu=122.5

Mu=2664

Figure 7-12 T-Beam 2R1 layout and shear and moment diagrams

120

T-Beam 2 Section Properties:

8397' =cf psi 5.5=wb in 65.24=pd in

1119=bpS in3 45843=cI in4 198=cpA in2

4.19=bcy in 120=eP kips 0.1=λ NWC Concrete Shear Strength: cV is lesser of ciV or cwV Calculation of ciV :

5.122=uV kips

2664=uM kip-in

16.11119

15.5120198120 =+=+= x

SeP

AP

fbp

e

cp

epe ksi

404016.11000

839764.19

45843)6( ' =

+=+= pec

bc

ccr ff

yI

M kip-in

pwcu

crupwcci dbf

MMV

dbfV '' 7.16.0 λλ ≥+=

19318645.72664

40405.1221000

65.245.583976.0 =+=×+×××=∴ ciV kips

1.211000

65.245.583977.1 =×××≥ kips

Therefore, 193=ciV kips. Calculation of cwV :

606198

120000 ===cp

epc A

Pf psi

0=pV

( ) ( ) 1.681000

65.245.56063.083975.33.05.3 ' =×××+=++= ppwpcccw VdbffV λ kips

Therefore, 1.68=cV kips.

121

Strength of Shear Stirrups: Two - leg #3 stirrup @ 12 in o.c, assuming full anchorage at top and bottom of web: 22.0=vA in

2 12=s in

23121000

65.245090022.01000

===x

xxs

dfAV pyv

s kips

Therefore, the nominal shear capacity of T-Beam 2R1 is:

1.91231.68 =+=+= scn VVV kips.

122

7.6 Shear Strength of T-Beam 1L (w/CFRP stirrups)

The shear capacity of T-Beam 1L was determined according to ACI 318-02 and ACI

440R-02 guidelines. The concrete and existing steel stirrup contributions to the total

shear strength were computed based on the ACI 318-02 code. The CFRP shear retrofit

contribution was computed according to the recommendations in the ACI 440R-02

report. Figure 7-13 shows the T-Beam 1L test layout at failure, and the corresponding

shear and bending moment diagrams.

412"

Mu=2319

M(kip-in)

87.5

10'

Vu=87.5

2'-212"

2'

6"

V(kips)

1' TYP

TBEAM 1L


1'-2"

Pu=87.5 k Pu=87.5 k

Figure 7-13 T-Beam 1L layout and shear and moment diagrams

123


8413' =cf psi 5.5=wb in 65.24=pd in

1119=bpS in3 43149=cI in4 198=cpA in2

6.18=bcy in 120=eP kips 0.1=λ NWC

Concrete Shear Strength: cV is lesser of ciV or cwV Calculation of ciV :

5.87=uV kips

2319=uM kip-in

16.11119

15.5120198120 =+=+= x

SeP

AP

fbp

e

cp

epe ksi

396816.11000

841366.18

43149)6( ' =

+=+= pec

bc

ccr ff

yI

M kip-in

pwcu

crupwcci dbf

MMV

dbfV '' 7.16.0 λλ ≥+=

15715046.72319

39685.871000

65.245.584136.0 =+=×+×××=ciV kips

1.211000

65.245.584137.1 =×××≥ kips


606198

120000 ===cp

epc A

Pf psi

0=pV

( ) ( ) 2.681000


Therefore, 2.68=cV kips.

124

Strength of Shear Stirrups: Two - leg #3 stirrups @ 12 in o.c., assuming full anchorage at top and bottom of the web: 22.0=vA in

2 12=s in

23121000

65.245090022.01000

=×

××==s

dfAV pyv

s kips

Contribution of CFRP shear retrofit ( fV ) based on ACI 440R-02 report procedure:

1'-412"

52"

df=24.65" 2'

512"

cgs

Figure 7-14 Cross section of T-Beam 1L showing CFRP stirrup layout

CFRP Stirrups Properties: Two sided stirrups: 85.0=fψ (ACI 440R-02, Table 10.1) 004.0≤= fuvfe εκε (ACI 440R-02, Eq. 10-6b) "65.24=fd 2=n plies

"12=fs "3=fw "039.0=ft

10600=fE ksi 139* =fuf ksi 85.0=EC

125

Calculation of fV :

f

ffefvf s

dfAV = (ACI 440R-02, Eq. 10-3)

feffe Ef ε= (ACI 440R-02, Eq. 10-5)

fffv wntA 2= (ACI 440R-02, Eq. 10-4)

75.0486

21 ≤=fu

ev

Lkkk

ε (ACI 440R-02, Eq. 10-7)

011.010600

13985.0*

=== xE

fC

f

fuefuε (ACI 440R-02, Eq. 8-4)

( ) ( )"924.0

10600000039.0225002500

58.058.0 ===xxEnt

Lff

e (ACI 440R-02, Eq. 10-8)

64.140008413

400032

32

'

1 =

=

= cf

k (ACI 440R-02, Eq. 10-9)

925.065.24

924.0265.2422 =−=

−= x

dLd

kf

ef (ACI 440R-02, Eq. 10-10)

75.0262.0011.0486

924.0925.064.1486

21 ≤===x

xxLkkk

fu

ev ε

therefore 262.0=vk

004.0003.0011.0262.0 ≤=== xk fuvfe εε therefore 003.0=feε

8.31003.010600 === xEf feffe ε ksi

468.03039.0222 === xxxwntA fffv in2

6.3012

65.248.31468.0 === xxs

dfAV

f

ffefvf kips

Therefore, the nominal shear capacity of T-Beam 1L is:

( ) 11726232.686.3085.0232.68 =++=++=++= xVVVV ffscn ψ kips.

126

7.7 Shear Strength of T-Beam 2L (w/CFRP stirrups)

The shear capacity of T-Beam 2L was determined according to ACI 318-02 and ACI




report. Figure 7-15 shows the T-Beam 2L test layout at failure, and the corresponding


3'-3"

6'-9"

1'

2'-2"

2'

412"

1'-4"

TBEAM 2L

1'10"1'-1" 1'

Pu=289 k

existing prestressstrands

1'

existing shearstirrups (typ)

Mu=2360

Vu=121

M(kip-in)

V(kips)

121

168

1'-712"

Figure 7-15 T-Beam 2L layout and shear and moment diagrams

127


8397' =cf psi 5.5=wb in 65.24=pd in

1119=bpS in3 45843=cI in4 198=cpA in2


Concrete Shear Strength:

cV is the lesser of ciV or cwV . Calculation of ciV :

121=uV kips

2360=uM kip-in

16.11119

15.5120198120 =+=+= x

SeP

AP

fbp

e

cp

epe ksi

404016.11000

839764.19

45843)6( ' =

+=+= pec

bc

ccr ff

yI

M kip-in

pwcu

crupwcci dbf

MMV

dbfV '' 7.16.0 λλ ≥+=

21420745.72360

40401211000

65.245.583976.0 =+=×+×××=ciV kips

1.211000

65.245.583977.1 =×××≥ kips


606198

120000 ===cp

epc A

Pf psi

128

0=pV

( ) ( ) 1.681000


Therefore, 1.68=cV kips. Strength of Shear Stirrups: Two - leg #3 stirrups @ 12 in o.c., assuming full anchorage at top and bottom of the web: 22.0=vA in

2 12=s in

23121000

65.245090022.01000

=×

××==s

dfAV pyv

s kips

Contribution of CFRP shear retrofit ( fV ) based on ACI 440R-02 procedure:

df=24.65"

512"

1'-412"

512"

cgs

2'

3.85" Figure 7-16 Cross section of T-Beam 2L

CFRP Stirrups Properties: Completely wrapped: 95.0=fψ (ACI 440R-02, Table 10.1) fufe εε 75.0004.0 ≤= (ACI 440R-02, Eq. 10-6a) "65.24=fd 2=n plies

129

"12=fs "3=fw "039.0=ft

10600=fE ksi 139* =fuf ksi 85.0=EC

Calculation of fV :

f

ffefvf s

dfAV = (ACI 440R-02, Eq. 10-3)


fffv wntA 2= (ACI 440R-02, Eq. 10-4)

011.010600

13985.0*

=== xE

fC

f


008.0004.0011.075.0004.075.0004.0 ≤=≤=≤= xfufe εε therefore 004.0=feε

4.42004.010600 === xEf feffe ε ksi

468.03039.0222 === xxxwntA fffv in2

8.4012

65.244.42468.0 === xxs

dfAV

f

ffefvf kips

Therefore, the nominal shear capacity of T-Beam 2L is:

( ) 1308.38231.688.4095.0231.68 =++=++=++= xVVVV ffscn ψ kips.

130

7.8 Shear Strength of T-Beam 1R (w/CFRP sheets)

The shear capacity of T-Beam 1R was determined according to ACI 318-02 and ACI




report. Figure 7-17 shows the T-Beam 1R test layout at failure, and the corresponding


1'-6"10'-21

2"

V(kips)

M(kip-in)

Mu=3553

TBEAM 1RVu=131

2'-318" 6" TYP

131

1'-2"

Pu=131 k Pu=131 k

Figure 7-17 T-Beam 1R layout and shear and moment diagrams

131


8413' =cf psi 5.5=wb in 65.24=pd in

1119=bpS in3 43149=cI in4 198=cpA in2



131=uV kips

3553=uM kip-in

16.11119

15.5120198120 =+=+= x

SeP

AP

fbp

e

cp

epe ksi

396816.11000

841366.18

43149)6( ' =

+=+= pec

bc

ccr ff

yI

M kip-in

pwcu

crupwcci dbf

MMV

dbfV '' 7.16.0 λλ ≥+=

15314646.73553

39681311000

65.245.584136.0 =+=×+×××=ciV kips

1.211000

65.245.584137.1 =×××≥ kips


606198

120000 ===cp

epc A

Pf psi

0=pV

132

( ) ( ) 2.681000



2 12=s in

23121000

65.245090022.01000

=×

××==s

dfAV pyv

s kips

Contribution of CFRP shear retrofit ( fV ) based on ACI 440R-02 code:

df=20.15"2'

1'-412"

512"

512"

cgs

3.85" Figure 7-18 Cross section of T-Beam 1R showing CFRP sheet layout

CFRP Sheets Properties: Two sided sheets: 85.0=fψ (ACI 440R-02, Table 10.1) 004.0≤= fuvfe εκε (ACI 440R-02, Eq. 10-6b) "15.20=fd 1=n ply

133

"18=fs "12=fw "039.0=ft

10600=fE ksi 139* =fuf ksi 85.0=EC Calculation of fV :

f

ffefvf s

dfAV = (ACI 440R-02, Eq. 10-3)


fffv wntA 2= (ACI 440R-02, Eq. 10-4)

75.0486

21 ≤=fu

ev

Lkkk

ε (ACI 440R-02, Eq. 10-7)

011.010600

13985.0*

=== xE

fC

f


( ) ( )"38.1

10600000039.0125002500

58.058.0 =××

==ff

e EntL (ACI 440R-02, Eq. 10-8)

64.140008413

400032

32

'

1 =

=

= cf

k (ACI 440R-02, Eq. 10-9)

86.015.20

38.1215.2022 =×−=

−=

f

ef

dLd

k (ACI 440R-02, Eq. 10-10)

75.0364.0011.0486

38.186.064.1486

21 ≤=×

××==fu

ev

Lkkk

ε therefore 364.0=vk

004.0004.0011.0364.0 ≤=×== fuvfe k εε therefore 004.0=feε

4.42004.010600 =×== feffe Ef ε ksi

936.012039.0122 =×××== fffv wntA in2

4.4418

15.204.42936.0 =××==f

ffefvf s

dfAV kips

Therefore, the nominal shear capacity of T-Beam 1R is:

( ) 1297.37232.684.4485.0232.68 =++=×++=++= ffscn VVVV ψ kips.

134

7.9 Shear Strength of T-Beam 2R2 (w/CFRP Sheets)

The shear capacity of T-Beam 2R2 was determined according to ACI 318-02 and

ACI 440R-02 guidelines. The concrete and existing steel stirrup contributions to the total

shear strength were computed based on the ACI-318 code. The CFRP shear retrofit


report. Figure 7-17 shows the T-Beam 2R2 test layout at failure, and the corresponding


V u = 1 4 2

M u = 3 4 0 8

1 1 "

1 '

4 '-6 "

e x is tin g sh ea r s tirru p s ( ty p )

9 '

V (k ip s )

M (k ip -in )

1 4 2

1 0 "

1 '

6 "

1 4 2

1 '-3 "

1 '

1 '

T B E A M 2 R 2

1 '

6 " 6 "

1 '-2 "1 0 "

6 "

1 '

1 '-1 "81

2 "

P u = 2 8 4 k

6 "

1 0 "

Figure 7-19 T-Beam 2R2 layout and shear and moment diagrams

135


8397' =cf psi 5.5=wb in 65.24=pd in

1119=bpS in3 45843=cI in4 198=cpA in2



142=uV kips

3408=uM kip-in

16.11119

15.5120198120 =×+=+=

bp

e

cp

epe S

ePAP

f ksi

404016.11000

839764.19

45843)6( ' =

+=+= pec

bc

ccr ff

yI

M kip-in

pwcu

crupwcci dbf

MMV

dbfV '' 7.16.0 λλ ≥+=

17516845.73408

40401421000

65.245.583976.0 =+=×+×××=ciV kips

1.211000

65.245.583977.1 =×××≥ kips


606198

120000 ===cp

epc A

Pf psi

0=pV

136

( ) ( ) 1.681000

65.245.56063.083975.33.05.3 ' =+=++= xxxVdbffV ppwpcccw λ kips


2 12=s in

23121000

65.245090022.01000

=×

××==s

dfAV pyv

s kips

Contribution of CFRP shear retrofit ( fV ) based on ACI 440R-02 code:

3.85"1'-41

2"

512"

df=20.15"

512"

cgs

2'

Figure 7-20 Cross section of T-Beam 2R2 showing CFRP sheets

CFRP Sheets Properties: Three sided sheets: 85.0=fψ (ACI 440R-02, Table 10.1) 004.0≤= fuvfe εκε (ACI 440R-02, Eq. 10-6b) "15.20=fd 1=n ply

137

"18=fs "12=fw "039.0=ft

10600=fE ksi 139* =fuf ksi 85.0=EC Calculation of fV :

f

ffefvf s

dfAV = (ACI 440R-02, Eq. 10-3)


fffv wntA 2= (ACI 440R-02, Eq. 10-4)

75.0486

21 ≤=fu

ev

Lkkk

ε (ACI 440R-02, Eq. 10-7)

011.010600

13985.0*

=== xE

fC

f


( ) ( )"38.1

10600000039.0125002500

58.058.0 =××

==ff

e EntL (ACI 440R-02, Eq. 10-8)

64.140008397

400032

32

'

1 =

=

= cf

k (ACI 440R-02, Eq. 10-9)

93.015.20

38.115.202 =−=

−=

f

ef

dLd

k (ACI 440R-02, Eq. 10-10)

75.0394.0011.0486

38.193.064.1486

21 ≤=×

××==fu

ev

Lkkk

ε therefore 394.0=vk

004.0004.0011.0394.0 ≤=×== fuvfe k εε therefore 004.0=feε

4.42004.010600 =×== feffe Ef ε ksi

936.012039.0122 =×××== fffv wntA in2

4.4418

15.204.42936.0 =××==f

ffefvf s

dfAV kips

Therefore, the nominal shear capacity of T-Beam 2R2 is:

( ) 1297.37231.684.4485.0231.68 =++=×++=++= ffscn VVVV ψ kips. (Note: CFRP shear contribution was identical for T-Beam 1R and T-Beam 2R2).

139

CHAPTER 8

8 RESULTS AND DISCUSSION

This chapter presents the flexural and shear results of the T-Beams tested in this

program. In the first section of this chapter, the flexural results of T-Beam 1 and T-Beam

2 are presented and compared with the predicted strengths covered in Chapter 7. In the

rest of the chapter, the shear results of T-Beam 2R1, T-Beam 1L, T-Beam 2L, T-Beam

1R and T-Beam 2R2 are presented and compared with the predicted strengths covered in

Chapter 7. In addition, the beam response and failure modes are discussed in detail.

8.1 T-Beam 1 Response Figure 8-1 shows the initial setup for testing T-Beam 1 in flexure. The beam was

tested over a simply supported span of 24 feet with two equal line loads applied at 2’-1½”

either side of mid-span. A detailed description of the test setup and instrumentation is

provided in Chapter 5.

Figure 8-1 T-Beam 1 ready for flexural testing

140

The bending moment at mid-span of T-Beam 1 is plotted against the mid-span

deflection in Figure 8-2. The plot shows the T-Beam 1 test result and the ACI 318-02

predicted flexural capacity based on nominal material properties and the flexural capacity

based on measured material properties. Six significant stages in the beam response are

identified on the plot. Crack propagation in the mid-span region of the beam at each of

these stages is shown in Figure 8-3.

The first flexural cracks were observed at mid-span at the bottom of the beam at a

bending moment of 196 kip-ft (Figure 8-3, Stage 1). As the load increased, these cracks

extended up into the web and new cracks formed below the load points. Apparent

“yielding” of the prestressing steel occurred between a mid-span deflection of 0.30 and

0.50 inch as indicated by the change of slope of the moment-displacement response after

0.50 inch displacement (Figure 8-2, Stage 2). Based on the initial stiffness of the beam

and the peak load capacity, an estimate of the “yield” displacement, y∆ , is shown in

Figure 8-2. The flexural ductility of the beam at subsequent stages is determined by

comparison with this displacement. The mid-span flexural cracks continued to open as

the load was increased (Figure 8-3, Stages 3, 4 & 5). The beam exceeded the ACI 318-02

capacity based on nominal material properties at a deflection of 1.6 inches, representing a

ductility level of 4.69 (Figure 8-2 Stage 4). The beam reached an ultimate bending

moment of 424 kip-ft at a mid-span deflection of u∆ = 3.06 inches before rupture of one

or more of the prestress strands resulted in a sudden drop in load (Figure 8-2). This

represents a ductility of 9.0 when compared with y∆ . Complete flexural failure occurred

when the remaining prestressed strands ruptured at the center crack (Figure 8-3, Failure).

141

8.1.1 ACI 318 Predicted Flexural Capacities

The flexural capacities of T-Beam 1 predicted by the ACI 318-02 code using nominal

and measured material properties are shown in Figure 8-2. The predicted nominal

flexural capacity was based on the nominal material properties assumed in the original

design of the beam ( 4000' =cf psi and 250=puf ksi). Subsequent to beam failure,

concrete cores and steel coupons were recovered and tested as described in Chapter 6.

Based on these actual measured material properties, the ACI 318-02 flexural capacity

was recomputed. Calculations of the predicted flexural capacities are provided in

Chapter 7.

The beam reached the predicted nominal flexural capacity of 397 kip-ft at 4.69 y∆

(Figure 8-2, Stage 4). The beam continued to carry load beyond this point reaching an

ultimate bending moment of 424 kip-ft. This ultimate strength represented an increase of

7% over the ACI 318-02 code nominal capacity. The beam never reached the predicted

flexural capacity of 436 kip-ft based on actual material properties. This predicted

strength was 3% higher than the ultimate strength of the beam. The ACI 318-02

predicted flexural capacity based on actual material properties therefore provided a

reasonable estimate of the flexural capacity of the beam.

Moment-Displacement Curve

1 - Cracking

2 - "Yielding"

FAILURE3 - Ductility 3.42

4 - Ductility 4.695 - Ductility 8.2

∆y = 0.339 ∆u = 3.063.42∆y 4.69∆y 8.2∆y0

50

100

150

200

250

300

350

400

450

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5Midspan Vertical Displacement (in)

Mom

ent (

kip-

ft)

T-Beam 1 Test ResultMn (ACI 318-02) - ActualMn (ACI 318-02) - Nominal1 - Cracking2 - "Yielding"3 - Ductility 3.424 - Ductility 4.695 - Ductility 8.2FAILURE

Ductility: µ = ∆u / ∆y = 9.03

Figure 8-2 Mid-span Moment – Displacement Relationship for T-Beam 1

142

Stage 1 - Cracking Stage 2 – “Yielding” Stage 3 – Ductility 3.42

Stage 4 – Ductility 4.69 Stage 5 – Ductility 8.2 Stage 6 - FAILURE

Figure 8-3 T-Beam 1 mid-span condition corresponding to six ductility levels

143

144

8.1.2 Slab Reinforcement Strain Gage Readings

Six electrical resistance strain gages were attached to the longitudinal reinforcement

in the concrete slab. The gage locations are described in Chapter 5 and shown in Figure

8-4. The strains recorded by these gages during flexural testing of T-Beam 1 are plotted

against the applied mid-span moment in Figure 8-4. These readings indicate that the top

reinforcing strains did not exceed 1200 microstrain in compression, indicating that the

extreme fiber compression in the concrete slab was well below the assumed failure strain

of 3000 microstrain. This confirms the theoretical computations showing that the beam

failure is controlled by yielding of the steel and not compression failure of the concrete.

Figure 8-5 shows the strain distribution across the flange at mid-span for each of the 6

stages identified in Figure 8-2. As expected, the strains increase with increasing applied

load. It is also evident that the strains are almost constant across the full width of the

flange, indicating that the entire flange width is effective.

Figure 8-6 shows the strain readings from strain gages 1-4 plotted along the half span

of the beam. These strain profiles are again plotted at each of the six stages noted in

Figure 8-2. Theoretically the strain in the compression zone should be constant between

the load points and decrease linearly between the load point and the support. This trend

is visible in Figure 8-6, however gages 1 and 4 deviates from the expected response.

Gage 4 was located slightly lower in the flange than gage 3, and appears to be affected by

crack propagation into the flange at the final stages of the test. The lower than expected

strains at gage 1 may indicate inadequate protection of the gage during concrete

placement resulting in failure of the bond between the gage and the reinforcing steel.

Slab Reinforcement Readings

050100150200250300350400450

-1400 -1200 -1000 -800 -600 -400 -200 0

Strain (10^-6)

Mom

ent (

kip-

ft)Strain 1 (End-Center of Slab)Strain 2 (Next to End)Strain 3 (Next to MidSpan)Strain 4 (Midspan)Strain 5 (Off Center)Strain 6 (Off-Center Edge)

5'-6"

9'-912"14'-21

2"

3" TYP

6'1' TYP

7" TYP

1 2 3 4

5

6

12" 3

4" 14"

12"

1'-314"

1012"

4'-2"1'-11" 3'-101

2" 2'-012"


Figure 8-4 T-Beam 1 slab reinforcement strain gage readings

145

Slab Reinforcement Strain Gage Readings for 4-6

4

5

6

-1200

-1000

-800

-600

-400

-200

00 3 6 9 12 15 18 21 24 27 30 33

Right Half Span of T-Beam 1 cross section (in)

Stra

in (1

0^-6

)

1 - Cracking2 - "Yielding"3 - Ductility 3.424 - Ductility 4.695 - Ductility 8.2FAILURE

4 5 6

12" 1'-33

4"2'-21

4"412" 11

4"

Figure 8-5 Strain readings for strain gages 4-6 (T-Beam 1)

146


1

2

3

4 (Center Line)

Support

-1400-1200-1000

-800-600-400-200

00 20 40 60 80 100 120 140

Left Half Span of T-Beam 1 (in)

Stra

in (1

0^-6

) 1 - Cracking2 - "Yielding"3 - Ductility 3.424 - Ductility 4.695 - Ductility 8.2FAILURE


3'-1012"

14'-212"

24'

5'-6"

412"

2'

4'-2"1'-11"

1 2

234"23

4"1 2

6'

9'-912"

4'-3"

4

2'-012"

3

212"31

4"3 4

CL

P P

Figure 8-6 Strain readings for strain gage 1-4 (T-Beam 1)

147

148

8.1.3 Concrete Strain Gage Readings

Several 2” gage length strain gages were installed on the concrete surface of T-Beam

1 as described in Chapter 5. These strain gages were installed as part of another research

project to develop a strain-based deflection monitoring system16. The intent of this

project was to measure the beam curvature so as to determine the deflected shape by

double integration of this curvature. Once the concrete cracks in tension, the bottom

strain gages no longer represent the average strain in the beam, so this strain-based

deflection system is only effective while the beam is un-cracked. Once cracks form in

the tension concrete, the strain in the concrete between these cracks deviates from that

anticipated by beam theory.

8.1.4 Vertical Deflection

The vertical deflection of T-Beam 1 was recorded by three LVDTs (linear variable

displacement transducers) installed on the top slab as described in Chapter 5. In addition,

dial gages were installed at each end of the beam to monitor any support settlement. The

dial gage readings indicated negligible settlement at the supports. The LVDT readings on

one side of the beam span were mirrored to produce a complete deflected shape at each of

the six stages identified in Figure 8-2. The resulting deflected shapes are shown in Figure

8-7. These deflection profiles confirm that the majority of the beam curvature is

concentrated between the load points due to significant cracking in this region. This is

particularly evident for the final stages prior to failure.

8.1.5 Strains in the CFRP stirrups and sheets

Although shear failure was not anticipated to control the failure of T-Beam 1, four

strain gages were attached to the CFRP stirrups and sheets as described in Chapter 5.

149

These strain gages were installed to monitor the vertical strain in the shear retrofit close

to the supports. Figure 8-8 shows that the strains in the CFRP shear retrofit were small

throughout the test, never exceeding 21 microstrain. No shear cracking was observed in

the shear spans during testing of T-Beam 1.

Vertical Displacement from LVDTs

Dial Gage

Mirror Image

Mirror Image

LVDT 3

LVDT 2

LVDT 1

Dial Gage0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 40 80 120 160 200 240 280Beam Span Length (in)

Ver

tical

Disp

lace

men

t (in

)

1 - Cracking2 - "Yielding"3 - Ductility 3.424 - Ductility 4.695 - Ductility 8.2FAILURE

4'-3"

12'LVDT 3

24'

412"

2'


DIAL GAGE

P

6'-9"

LVDT 1LVDT 2

3'-9" DIAL GAGE

PCL

Figure 8-7 Representation of the vertical deflection of T-Beam 1 from LVDT readings

150

Strain gage readings on frp stirrups and sheets

0

50

100

150

200

250

300

350

400

450

-8000 -6000 -4000 -2000 0 2000 4000 6000 8000

Strain (10^-6)

Mom

ent (

kip-

ft) Strain 22 (Right Edge CFRP Sheet)

Strain 23 (Second Right Edge CFRP Sheet)

Strain 24 (Third Left Edge CFRP Stirrup)

Strain 25 (Second Left Edge CFRP Stirrup)


Left Span (3" wide double layer CFRP stirrups)24'

412"

2' 1'-4"24

1'-312"

252'-3"

1'-5"

4'-3"

Right Span (12" wide CFRP sheets)

1'-312" 3'

23 1'-5"22

CL

P P

Figure 8-8 Strain readings from gages attached to CFRP stirrups and sheets (T-Beam 1)

151

152

8.2 T-Beam 2 Response

T-Beam 2 was tested under the same loading conditions as T-Beam 1. However, the

span length was increased to 25 feet and 8 ½ inches because the supports were located

under steel brackets bolted to the ends of the beam. This was necessary to prevent the

support condition from providing additional restraint to the ends of the carbodur strips on

the beam soffit. Figure 8-9 shows the test setup for T-Beam 2. More detailed

information on the test setup and instrumentation of T-Beam 2 is provided in Chapter 5.

Figure 8-9 T-Beam 2 ready for flexural testing

The mid-span bending moment-displacement response of T-Beam 2 is plotted in

Figure 8-10 along with the response recorded for T-Beam 1. The plot also shows the

153

flexural capacity predicted by the modified ACI 440R-02 procedure using measured

material properties.

Six significant stages in the beam response are identified on this plot with associated

damage conditions shown in Figure 8-11. For ease of comparison, the response of T-

Beam 1 and the ACI 318-02 predicted flexural capacity is also plotted in Figure 8-10.

The first flexural cracks were observed at mid-span at the bottom of the beam at a

bending moment of 217 kip-ft (Figure 8-11, Stage A). As the load increased, these

flexural cracks extended up into the web and new flexural-shear cracks formed below and

outside the load points. Based on the change in slope of the moment-displacement

response, “yielding” of the prestressing steel was considered to occur between mid-span

deflections of 0.30 and 0.50 inch. Based on the intersection of the initial and final

stiffness tangents, the “yield” displacement was defined as 43.0=∆ y inch (Figure 8-10).

Subsequent to “yielding”, the stiffness of T-Beam 2 with CFRP carbodur strips exceeded

the stiffness of T-Beam 1 without the carbodur retrofit.

As the load increased, the flexural cracks between the load points and the flexural-

shear cracks outside the load points continued to open (Figure 8-11, Stages C, D, & E).

The stiffness of the beam remained relatively constant through these three stages. Failure

occurred at a bending moment of 725 kip-ft with a maximum mid-span deflection of 4.03

inches when the carbodur strips delaminated from the beam soffit over the left half of the

span (Figure 8-11, Failure).

Based on the definition of “yield” displacement shown in Figure 8-10, the ultimate

ductility of T-Beam 2 was 9.37. Both in terms of ductility and total mid-span deflection,

154

T-Beam 2 response was more ductile than that for T-Beam 1. The addition of CFRP

carbodur strips as tension reinforcement has not reduced the ductility as observed in some

prior research studies (Chapter 3). This is attributed to the relatively low reinforcement

ratio for the original prestressed beams and to the presence of anchorage wraps to prevent

premature delamination at the ends of the CFRP strips.

During the flexural test of T-Beam 2, the response was similar to that for T-Beam 1

up to the “yield” point. Initially, T-Beam 2 was less stiff than T-Beam 1 as a result of the

longer span for T-Beam 2. However, the post-yielding stiffness of T-Beam 2 was greater

than T-Beam 1 and did not degrade as rapidly. At stage C in Figure 8-10, T-Beam 2

supported the same load that caused failure in T-Beam 1, but at a third of the mid-span

deflection. The cracks in T-Beam 2 at this stage (Figure 8-11, Stage C) were

significantly shorter and smaller than those at the same load in T-Beam 1 (Figure 8-3,

Stage 5). The CFRP flexural reinforcement was instrumental in reducing the crack sizes

and limiting the deflection at the nominal moment capacity of the control beam.

The ACI 440R-02 procedure was modified for prestressed beams in Chapter 7. This

procedure predicted a flexural capacity of 599 kip-ft for T-Beam 2. T-Beam 2 exceeded

this bending moment at a mid-span deflection of 2.5 inches. The beam supported an

ultimate moment of 725 kip-ft, which is 21% greater than ACI 440R-02 predicted

moment capacity. This ultimate capacity also represents a 71% increased over the

ultimate capacity of T-Beam 1, while the ACI 440R-02 procedure suggests the increase

to be around 37% compared with the ACI 318-02 predicted flexural capacity based on

measured material properties.

Moment-Displacement Curve

A - Cracking

FAILURE

B - "Yielding"

C - Ductility 2.42

D - Ductility 4.98

E = Ductility 6.74

FAILURE (Max Load)

1

23 4

5

∆y = 0.43 ∆u = 4.032.42∆y 4.98∆y 6.74∆y0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Midspan Vertical Displacement (in)

Mom

ent (

kip-

ft)

T-Beam 2 Test ResultMn (ACI 440R-02) - ActualT-Beam 1 Test ResultMn (ACI 318-02) - ActualA - CrackingB - "Yielding"C - Ductility 2.42D - Ductility 4.98E = Ductility 6.74FAILURE (Max Load)

Ductility: µ = ∆u / ∆y = 9.37

Figure 8-10 Mid-span Moment-Displacement Relationship for T-Beam 2

155

A – Cracking B – “Yielding” C – Ductility 2.42

D – Ductility 4.98 E – Ductility 6.74 FAILURE

Figure 8-11 T-Beam 2 condition corresponding to six ductility levels

156

157

8.2.1 Failure mechanism for T-Beam 2

Failure of T-Beam 2 occurred when the CFRP strips delaminated from the bottom of

the beam over the left shear span. This delamination appeared to initiate at the base of a

flexure-shear crack that had formed just outside the left load point (Figure 8-12).

Vertical offset in the soffit of the beam on either side of this crack may have contributed

to the initiation of delamination. In addition, large strain differential between the CFRP

strips and the flexurally cracked concrete would also have contributed to deterioration of

the bond between CFRP and concrete.

Figure 8-12 Flexure-shear crack formed outside of the left load point (T-Beam 2).

158

For the first 18 inches from the delamination initiation point, the failure occurred in

the surface concrete, with a thin layer of concrete remaining attached to the CFRP strips

(Figure 8-13). Beyond this point, the CFRP strips separated from the epoxy, likely

because of the increased angle of peeling as the CFRP stripped away from the beam

soffit. The delamination occurred rapidly and extended from the flexure-shear crack to

the end of the CFRP strips, which pulled half way out of the CFRP fabric wrap anchors

(Figure 8-14). There was no tendency for delamination to initiate at the end of the

carbodur strips as had been reported in some laboratory studies, however the anchor

wraps were not sufficient to prevent pull-out once delamination had initiated elsewhere.

Figure 8-12 and Figure 8-13 show the bottom of the CFRP shear stirrups adjacent to

the failure crack. The stirrups were not continuous around the soffit of the beam so as not

to add anchorage to the carbodur strips that was not present in the field application being

evaluated. However, it was evident that the bottom bulb of the T-Beam has split open

with the ledge sections rotating outwards with the CFRP stirrups still attached. The

bottom of the bulb, with carbodur strips, was then free to move downward, causing

delamination of the flexural CFRP. Had the CFRP shear stirrups been continuous around

the soffit of the beam, they may have prevented this splitting of the bottom bulb and

delayed the delamination of the carbodur strips.

In some of the subsequent shear test on the beam halves with CFRP shear retrofit,

continuity of the stirrups and sheets was instated by splicing additional wraps around the

soffit of the beam.

159

Front side of beam Back side of beam

Figure 8-13 Delamination of carbodur strips initiating at the flexure-shear crack

Figure 8-14 Carbodur strips delaminated from beam and pulling out of CFRP

wrap anchor

160

8.2.2 Slab Reinforcement Strain Gage Readings

A total of six strain gages were attached to the longitudinal reinforcement in the top

slab of T-Beam 2. The strain gage locations are described in Chapter 5, and were

relatively close to those used in T-beam1. Strain readings from these strain gages are

plotted in Figure 8-15. The maximum strain is around 900 microstrain, which is similar

to the maximum observed in T-Beam 1. This suggests that the concrete in the

compression block is well below the assumed failure strain of 3000 microstrain and that

tension failure, and not concrete crushing, should govern beam failure. This confirms the

ductile flexural failure observed for T-Beam 2.

These top slab strain readings were plotted across the mid-span section and along the

length of the beam to illustrate the strain distribution in the top slab. Profiles are plotted

for each of the six stages identified in the moment-displacement curve (Figure 8-10).

Figure 8-16 shows the strain distribution across the slab at the mid-span section based

on strain readings from strain gage 4-6. The shape of the distribution is similar to that

observed in T-Beam 1. Strain gage 4 is set lower in the slab than gages 5 and 6, resulting

in lower strain readings. However the gages illustrate the nearly uniform distribution of

stress across the full width of the slab. This confirms that the full width of the flange was

effective in compression.

Figure 8-17 shows the distribution of strain along the left half of the beam based on

strain readings from gages 1-4. Strain gage 2 was damaged during the concrete pour and

did not provide reliable strain readings. The remaining gages show the expected decrease

in strain from a maximum at the load point to zero at the supports. The strains at gage 4

were low because its location was lower in the slab than the other gages.

161

8.2.3 Vertical Displacement from LVDT Readings

Three LVDTs were used to record vertical deflections of the top of the concrete slab

during testing. Since no support settlement was observed in the test of T-beam1, dial

gages were not placed at the supports for T-Beam 2. Figure 8-18 shows the beam

deflection at each of the previously identified load stages using the three LVDT readings

and their mirror image on the other half of the beam.

Slab Reinforcement Readings

050100150200250300350400450500550600650700750

-900 -800 -700 -600 -500 -400 -300 -200 -100 0

Strain (10^-6)

Mom

ent (

kip-

ft)

Strain 1 (End-Center of Slab)Strain 2 (Next to End)-DamagedStrain 3 (Next to MidSpan)Strain 4 (Midspan)Strain 5 (Off Center)Strain 6 (Off-Center Edge)


7" TYP

1' TYP5'-4"

24'-10"

4

5

6

3211'

1'

1'-11"3'-1112"1'-11"

Figure 8-15 Strain readings for slab reinforcement strain gage (T-Beam 2)

162


4 (Center Line)

56

-700

-600

-500

-400

-300

-200

-100

00 4 8 12 16 20 24 28 32

Right Half Span of T-Beam 2 cross section (in)

Stra

in (1

0^-6

)

A - CrackingB - "Yielding"C - Ductility 2.42D - Ductility 4.98E - Ductility 6.74FAILURE (Max Load)

1' 1' 2"

4 5 6

412"


163


1

3

4 (Center Line)

Support

Slab

Slab

-900

-700

-500

-300

-100

0 20 40 60 80 100 120 140Left Half Span of T-Beam 2 (in)

Stra

in (1

0^-6

)


3'-1112"

24'-10"

25'-812"


5'-4"1'-11"

12 (Damaged)

412"

2'

2"134"

21

1'-11"3 4

178"

4'-3"

2"3

4

P P

CL


164

Vertical Displacement from LVDTs

Support

Mirror Image

Mirror Image

LVDT 1

LVDT 2

LVDT 3

Support

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 50 100 150 200 250 300Beam Span Length (in)

Ver

tical

Dis

plac

emen

t (in

)


2 5 '- 8 12 "

e x is t in g s h e a r s t i r r u p s ( ty p )

4 12 "

2 '

1 2 '- 5 "L V D T 1

4 '- 3 "P P

6 '- 9 "3 '- 9 "

L V D T 3L V D T 2

C L

Figure 8-18 Representation of the vertical deflection of T-Beam 2 from LVDT readings

165

166

8.2.4 Carbodur Strip Strain Gages

T-Beam 2 was retrofitted in flexure with three carbodur strips. Strain gages were

attached to each of these strips to monitor the longitudinal strains during flexural testing.

The gages were installed on each strip at the locations shown in Figure 8-19.

2'-112"

2322

3'

24

3'

2625

1'-412"

27 212019

3' 3'8'-112"

CL

171618 15

1413

Third StripSecond StripFirst Strip

111012

789

Figure 8-19 T-Beam 2 beam soffit showing location of strain gages

These strain gages all provided reliable readings throughout the test. Strain gages 7-9

and 25-27 registered very small strains because they were far from mid-span and close to

the supports (Figure 8-20 and Figure 8-21). Strain gages 10-12 and 22-24 were virtually

symmetric about mid-span (Figure 8-22 and Figure 8-23). They recorded small strains up

to a mid-span bending moment of 520 kip-ft, after which the strains increased linearly to

a maximum of 2800 microstrain. Strain gages 19-21 also recorded small strains until the

mid-span bending moment reached 340 kip-ft. At this point the strains increased linearly

to a maximum of 8500 microstrain (Figure 8-24). This high strain reflects their position

close to the left load point. Note that no corresponding strain gages were installed on the

right half of the beam because of the transverse wrap covering the carbodur strips.

Although this wrap had been removed from the sides of the beam to avoid restraining the

carbodur strips, it was not removed from the strips themselves so as to avoid damaging

the carbodur material.

Strain gages 16-18 and 13-15 show almost identical behavior because they are below

or between the load points and therefore represent sections subjected to the same bending

167

moment (Figure 8-25 and Figure 8-26). Strain gages 13-15 recorded small strains up to a

mid-span bending moment of 220 kip-ft, which corresponds to the first flexural cracks

observed in the mid-span region. The slope then decreased significantly but remained

constant until the maximum strains of around 10000 microstrain were recorded

immediately prior to beam failure. Gages 16-18 experienced similar response except that

the change in slope was delayed until flexural cracks developed under the point load at

250 kip-ft mid-span moment.

The six stages identified in the moment-displacement curve are also plotted on the

moment-strain relationships. In all cases the change in slope of the strain diagrams was

the result of formation of cracks in the beam soffit at or near the strain gage locations.

This cracking initiated at mid-span and under the point loads, but slowly spread towards

the supports as the load increased (Figure 8-27).

Figure 8-28 to Figure 8-30 show the strains in all gages attached to a particular

carbodur strip. Figure 8-31 to Figure 8-33 show the profile of strain along the length of

each carbodur strip corresponding to the six stages identified earlier. As noted above, the

strains were highest within the point loads and decreased towards the supports.

Carbodur Strip Strain Gages 7-9

AB

C

D

E

FAILURE

050

100150200250300350400450500550600650700750

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

Strain (10^-6)

Mom

ent (

kip-

ft)

Strain Gage 7Strain Gage 8Strain Gage 9ABCDEFAILURE

3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

121011

987First Strip

Second StripThird Strip21

Figure 8-20 Strain readings of strain gages 7-9

168


AB

C

D

E

FAILURE

050

100150200250300350400450500550600650700750

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

Strain (10^-6)

Mom

ent (

kip-

ft)


3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

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169


AB

C

D

E

FAILURE

050

100150200250300350400450500550600650700750

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

Strain (10^-6)

Mom

ent (

kip-

ft)


3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

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170


AB

C

FAILURE

D

E

050

100150200250300350400450500550600650700750

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

Strain (10^-6)

Mom

ent (

kip-

ft)


3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

121011

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171


AB

FAILURE

C

D

E

050

100150200250300350400450500550600650700750

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000Strain (10^-6)

Mom

ent (

kip-

ft)


3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

121011

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172


AB

C

D

E

FAILURE

050

100150200250300350400450500550600650700750

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

Strain (10^-6)

Mom

ent (

kip-

ft) Strain Gage 16Strain Gage 17Strain Gage 18ABCDEFAILURE

3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

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173


AB

C

D

E

FAILURE

050

100150200250300350400450500550600650700750

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

Strain (10^-6)

Mom

ent (

kip-

ft) Strain Gage 13Strain Gage 14Strain Gage 15ABCDEFAILURE

3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

121011

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174

Web-shear racks that formed on left span Web-shear cracks that formed on right span

Flexure-shear cracks started under the point load. At higher loading, web-shear cracks formed away from the mid-span.

Figure 8-27 Web-shear cracks forming away from the mid-span of beam as confirmed by carbodur strain readings

175

Carbodur Strip Strain Gages on the First Strip

050

100150200250300350400450500550600650700750

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Strain (10^-6)

Mom

ent (

kip-

ft) Strain Gage 25

Strain Gage 22

Strain Gage 19

Strain Gage 16

Strain Gage 13

Strain Gage 10

Strain Gage 7

3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

121011

987First Strip


Figure 8-28 Strain readings for gages on the first carbodur strip

176

Carbodur Strip Strain Gages on the Second Strip

050

100150200250300350400450500550600650700750

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000Strain (10^-6)

Mom

ent (

kip-

ft)

Strain Gage 26

Strain Gage 23

Strain Gage 20

Strain Gage 17

Strain Gage 14

Strain Gage 11

Strain Gage 8

3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

121011

987First Strip


Figure 8-29 Strain readings for gages on the second carbodur strip

177

Carbodur Strip Strain Gages on the Third Strip

050

100150200250300350400450500550600650700750

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000Strain (10^-6)

Mom

ent (

kip-

ft) Strain Gage 27

Strain Gage 24

Strain Gage 21

Strain Gage 18

Strain Gage 15

Strain Gage 12

Strain Gage 9

3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

121011

987First Strip


Figure 8-30 Strain readings for gages on the third carbodur strip

178

Gages on First Strip

Support Gage 25

Gage 22

Gage 19

Gage 16

Gage 13

Gage 10

Gage 7 Support

0

2000

4000

6000

8000

10000

12000

0 50 100 150 200 250 300

Beam Span (in)

Stra

in (1

0^-6

)


3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

121011

987First Strip


Figure 8-31 Strain readings on the first carbodur strip corresponding to the six ductility levels

179

Gages on Second Strip

Support Gage 26

Gage 23

Gage 20

Gage 17

Gage 14

Gage 11

Gage 8 Support

0

2000

4000

6000

8000

10000

12000

0 50 100 150 200 250 300

Beam Span (in)

Stra

in (1

0^-6

)


3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

121011

987First Strip


Figure 8-32 Strain readings on the second carbodur strip corresponding to the six ductility levels

180

Gages on Third Strip

Support Gage 27

Gage 24

Gage 21Gage 18

Gage 15

Gage 12

Gage 9Support

0

2000

4000

6000

8000

10000

12000

0 50 100 150 200 250 300

Beam Span (in)

Stra

in (1

0^-6

)


3'3'

27

1'-412"

2526

24

3'

2223

1920

8'-112"

13141518

2'-112"

1617

3'

121011

987First Strip


Figure 8-33 Strain readings on the third carbodur strip corresponding to the six ductility levels

181

182

8.3 ACI 440 Versus Experimental Moment Capacity

The moment capacity of T-Beam 2 was compared with the ACI 440R-02 prediction

using the same approach as Lyle Nakashima in his Masters Report21. Figure 8-34 from

his report shows the normalized nominal moment capacities of previous tests on FRP

retrofitted concrete beams compared with the ACI 440 report predictions. The nominal

moment values are normalized with respect to the beam cross-section dimensions. The

45-degree datum represents a one-to-one agreement between the predicted and

experimental results. For all specimens the experimental results exceed the ACI 440

predictions. The failure moment capacity of T-Beam 2 with CFRP carbodur strips is also

plotted in Figure 8-34. Because of the large flange width, the normalized moment

capacity is considerably lower than the tests performed on rectangular sections. The

experimental bending capacity exceeded the ACI 440R-02 prediction as indicated by the

point falling below the 45 degree datum.

ACI 440 Vs. Experimental Moment Capacities

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

Experimental Mn/bfdp2 (ksi)

AC

I 440

Mn/

b fd p

2 (ksi)

Spaeda Shahawy FanningBonacci Swamy T-Beam 2GangaRao White Design Datum

Figure 8-34 Plot of Normalized ACI 440 prediction and experimental moment capacities

183

184

8.4 Shear Strength of T-Beam 2R1 (plain concrete)

T-Beam 2R1 represents a shear test of the right hand portion of T-Beam 2 with

internal steel stirrups but no externally applied CFRP shear reinforcement. It was loaded

so as to induce a shear failure in the area without CFRP shear stirrups or sheets shown in

Figure 8-35. The test section had a span to depth ratio of about 1.5. A detailed

description of the test setup and instrumentation is provided in Chapter 5.

Figure 8-35 Test setup and shear span of T-Beam 2R1

The shear force applied to the test shear span is plotted against the vertical

displacement at the applied load in Figure 8-36. The ACI 318-02 predicted shear

strengths provided by concrete, Vc, and concrete plus internal steel stirrups, Vc+Vs, are

also plotted for comparison with the test result. Three stages were selected in the

response and identified in Figure 8-36. The beam condition at each of these stages is

shown in Figure 8-37.

The first diagonal tension crack formed in the test span under an applied shear force

of 58 kips (Figure 8-37, Initial Crack). It initiated in the web as a web-shear crack. As

the load increased, additional diagonal cracks formed adjacent to the original crack. This

zone of diagonal cracking extended downwards toward the left support and up to the

185

soffit of the concrete slab. By the time the applied shear reached 86 kips, the diagonal

crack zone extended from the support to the soffit of the top slab (Figure 8-37, Applied

Load of 130 kips). The width of the cracks continued to open until the beam reached its

maximum load capacity at an applied shear of 122 kips which is 34% greater than the

predicted ACI 318-02 nominal shear capacity (Figure 8-37, FAILURE).

Figure 8-38 and Figure 8-39 show the failure shear zone after removal of the loose

concrete. The shear failure crossed three of the internal steel stirrups. Only the center

stirrup reached its full capacity and failed in tension at the shear zone (Figure 8-38). The

stirrup that crossed the shear zone close to the bottom of the web failed due to anchorage

pull-out because of the lack of hook anchorage at the bottom of the web (Figure 8-39).

Better anchorage of this vertical stirrup in the original construction may have increased

the shear capacity. The shear stirrup that crossed the shear zone at the top of the web did

not fail, but was unable to prevent propagation of the shear crack through the top slab

(Figure 8-38). The tension reinforcement consisting of internal prestressing strands and

externally applied carbodur strips were deformed when shear failure occurred but did not

fail (Figure 8-38 and Figure 8-39).

In spite of inadequate anchorage of the shear stirrups at the bottom of the web, the

shear capacity of the original prestressed concrete beam well exceeded that predicted by

the ACI 318-02 code. This may be attributed to the relatively high concrete strength and

general conservatism in the ACI 318-02 shear estimate. In addition, the shear span to

depth ratio of around 1.5 restricted the shear failure to a limit portion of the beam.

T-Beam 2R1 Shear-Displacement Curve

Vc (ACI 318-02)

Vc + Vs (ACI 318-02)

Initial Crack

Applied Load @ 130 kips

FAILURE

0

20

40

60

80

100

120

140

160

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Vertical Displacement @ Applied Load (in)

Shea

r (k

ips)

Figure 8-36 Shear-Displacement relationship for T-Beam 2R1

186

187

Front Side Back Side

Initial Shear Crack


Applied load of 130 kips


FAILURE

Figure 8-37 T-Beam 2R1 condition at critical stages during the test

188

Figure 8-38 Failure of steel shear reinforcement at failure shear crack

Figure 8-39 Shear reinforcement anchorage failure at base of web

189

8.5 Shear Strength of T-Beam 1L (CFRP Stirrups)

T-Beam 1L was retrofitted in shear with CFRP stirrups. The test setup and layout of

T-Beam 1L are shown in Figure 8-40. The beam was supported on pinned supports at

both ends of the span. During testing, it was noted that the supports were resisting

longitudinal movement of the bottom of the beam, thereby introducing a net compression

in the beam. The beam was unloaded and a roller support installed at the right support.

The beam was reloaded with the second test supported on pinned and roller supports at

the two ends. A more detailed description of the test setup is given in Chapter 5. Both

results from the first and second loadings are presented below. Although the beam

experienced significant shear cracking, the shear capacity of the section was not reached

before flexural failure of the beam at mid-span. Subsequent shear test specimens were

retrofitted in flexure with carbodur strips to prevent premature flexural failure.

Figure 8-40 Test setup of T-Beam 1L (CFRP stirrups)

190

The shear-displacement relationships for each loading are plotted in Figure 8-41. Six

significant stages are noted on the response. The predicted shear capacities from ACI

318-02 and ACI 440R-02 are also shown. The beam condition at each of the highlighted

stages is shown in Figure 8-42 through Figure 8-44. On the front of the beam, the black

lines refer to the cracks observed during the flexural test of T-Beam 1. The red lines

indicate the shear cracks resulting during T-Beam 1L testing. On the back of the beam,

the opposite color scheme applies with the shear cracks indicated in black.

The first visible diagonal tension crack occurred at a shear force of about 50 kips

(Figure 8-42, Initial Crack). Existing cracks from prior flexural testing of T-Beam 1 also

increased in size as the load increased. With increasing load, flexural cracks formed at

mid-span while additional shear cracks formed parallel to the first diagonal tension crack

(Figure 8-42, Applied Load of 148 kips). The beam reached a maximum shear force of

85 kips (Figure 8-42, Applied Load of 170 kips) before failing in flexure at mid-span

(Figure 8-43).

Although the full shear capacity of the beam was not achieved, a number of

observations were made regarding the performance of the CFRP shear stirrup retrofit. At

a shear load of 64 kips, portion of a CFRP stirrup delaminated from the concrete surface

(Figure 8-44, D-0.45). The delamination extended as the load increased. A second

CFRP stirrup started delaminating at an applied shear of 78 kips (Figure 8-44, D-0.60).

The effect of this delamination on the strains in the CFRP stirrups is discussed in Section

8.5.1.

Delamination initiated at uneven sections of the concrete web. Because of deviations

in the stirrup alignment, tension developing in the CFRP stirrup resulted in out-of-plane

191

loads on the bond between the CFRP and concrete surface. Better preparation of the

concrete surface may have reduced this tendency, however, deviations in the CFRP

alignment are to be expected during typical installation. This delamination would

probably have resulted in the complete debonding of the shear stirrups had it not been for

the continuity provided at the top of the stirrups by wrapping the CFRP through the top

slab. In addition, the steel tube anchorage at the bottom of the web prevented pealing of

the stirrups at the re-entrant corner. Since this delamination occurred before reaching

even the nominal capacity of the un-retrofitted beam, it is likely that without adequate

anchorage, the CFRP stirrups would not have contributed to the shear capacity of the

beam.

T-Beam 1L Shear-Displacement Curve

Vc (ACI 318-02)

Vc + Vs (ACI 318-02)

Vc + Vs + ψfVf (ACI 440R-02)

Initial Crack1st Delam

Applied Load @ 148 kips2nd Delam


FAILURE

0

20

40

60

80

100

120

140

160

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Midspan Vertical Displacement (in)

Shea

r (k

ips)

TBeam1L 72202TBeam1L 72302

Figure 8-41 Shear-Displacement relationship for T-Beam 1L

192

193

Front Side Right Back Side Left

Initial Shear Crack


Applied Load of 148 kips



Figure 8-42 T-Beam 1L condition at various stages in the shear-displacement response

194

FAILURE

Figure 8-43 Flexural failure of T-Beam 1L

Delamination points

Figure 8-44 Delamination of CFRP stirrups from T-Beam 1L

195

8.5.1 Measured strain in the CFRP stirrups

A total of seven strain gages were installed on the CFRP stirrups on the front side of

the beam. The strain gage readings were plotted against the applied shear in Figure 8-45

through Figure 8-53. Because of the additional shear capacity provided by the inclined

prestress strands in the left shear span, shear cracks only occurred in the right shear span

of T-Beam 1L. The strains recorded in the CFRP stirrups on the left shear span were

therefore very small throughout the test (Figure 8-45 to Figure 8-48), while those on the

right shear span recorded significant strains in the CFRP stirrups (Figure 8-49 to Figure

8-51).

Figure 8-52 shows the strains recorded during the first test of T-Beam 1L in the three

strain gages located on the CFRP stirrups in the failure shear span. The formation of the

first diagonal tension crack at a shear of 20 kips corresponds with the rapid increase in

strain recorded by strain gage 30. As the shear load increased to 40 kips, the crack

propagated through the next stirrup causing increased strains in strain gage 29. Strain

gage 28 also indicates increased strain after 40 kips, but increases more rapidly after 60

kips load when additional shear cracks formed in the web.

As the load increased, the strain gages recorded increasing strains to a maximum of

3250 microstrain. At a shear force of 78 kips, the first two CFRP stirrups from the right

support delaminated from the concrete surface. The strains recorded by gages 29 and 30

dropped slightly as the reduction in stiffness of the stirrups transferred some of the shear

force to the concrete and internal stirrup mechanism. Strain gage 28 continued to

measure higher strains until flexural failure of the beam because delamination did not

occur on this stirrup.

196

The strain measurements recorded during the second loading of T-Beam 1L indicate

that the CFRP stirrups supported load throughout the test because the beam was already

cracked and the end two stirrups had already delaminated during the first loading (Figure

8-53).

Strain 25

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000 3500Strain (10^-6)

Shea

r (k

ips)

TBeam1L 72202

TBeam1L 72302

2425 2726 28

1'-2"

3029

CL

Figure 8-45 Strain readings from strain gage 25

197

Strain 24

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000 3500Strain (10^-6)

Shea

r (k

ips)

TBeam1L 72202

TBeam1L 72302

2425 2726 28

1'-2"

3029

CL


198

Strain 26

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000 3500

Strain (10^-6)

Shea

r (k

ips)

TBeam1L 72202

TBeam1L 72302

2425 2726 28

1'-2"

3029

CL


199

Strain 27

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000 3500

Strain (10^-6)

Shea

r (k

ips)

TBeam1L 72202

TBeam1L 72302

2425 2726 28

1'-2"

3029

CL


200

Strain 28

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000 3500

Strain (10^-6)

Shea

r (k

ips)

TBeam1L 72202

TBeam1L 72302

2425 2726 28

1'-2"

3029

CL


201

Strain 29

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000 3500

Strain (10^-6)

Shea

r (k

ips)

TBeam1L 72202

TBeam1L 72302

2425 2726 28

1'-2"

3029

CL


202

Strain 30

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000 3500

Strain (10^-6)

Shea

r (k

ips)

TBeam1L 72202

TBeam1L 72302

2425 2726 28

1'-2"

3029

CL

Figure 8-51 Strain readings on strain gage 30

203

CFRP Readings on Strain Gages 28-30 Right Side (T-Beam 1L 72202)

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000 3500

Strain (10^-6)

Shea

r (k

ips)

Strain Gage 28Strain Gage 29Strain Gage 30

2425 2726 28

1'-2"

3029

CL

Figure 8-52 Strain readings from strain gages 28-30 from the first test of T-Beam 1L

204

CFRP Readings on Strain Gages 28-30 Right Side (T-Beam 1L 72302)

0

20

40

60

80

100

0 500 1000 1500 2000 2500 3000 3500

Strain (10^-6)

Shea

r (k

ips)

Strain Gage 28Strain Gage 29Strain Gage 30

2425 2726 28

1'-2"

3029

CL

Figure 8-53 Strain readings from strain gages 28-30 from the second test of T-Beam 1L

205

206

8.6 Shear Strength of T-Beam 2L (CFRP Stirrups)

T-Beam 2L was the left shear span of the flexural test of T-Beam 2, recovered for

evaluation of the CFRP stirrups. Because the failure of T-Beam 2 resulted from a

flexure-shear crack just outside the left load point, the remaining section of beam to be

tested as T-Beam 2L was only one third of the original beam length. This complicated

the shear testing of this section since mid-span loading would have resulted in a shear

span to depth ratio less than 1.5. In addition, flexural cracking had already lead to

debonding of the prestressing strands at the right end of T-Beam 2L.

The beam was loaded off center to produce a 1.5 shear span to depth ratio and induce

a shear crack in the right side of the beam. The beam was also retrofitted with carbodur

strips for additional flexural reinforcement. This beam was considered completely

wrapped because the bottom end of the stirrups was continued under the beam soffit with

additional CFRP wraps. During loading of the beam, a crack developed between the left

support and the end of the carbodur strips added for flexural strengthening. In order to

avoid a premature failure at this location, the beam was unloaded and the left reaction

moved inward to bear directly below the end of the carbodur strips. Figure 8-54 shows

the test setup for T-Beam 2L. A more detailed description is covered in Chapter 5. The

initial condition of the beam before testing is shown in Figure 8-55. The existing cracks

are the result of the original T-Beam 2 flexural test.

207

Figure 8-54 T-Beam 2L test setup


Figure 8-55 Initial condition of T-Beam 2L before testing

208

The shear-displacement response is plotted in Figure 8-56. The predicted shear

capacities from ACI 318-02 and ACI 440R-02 are also plotted. Three significant stages

during the test were identified and are indicated on the response curve. The beam

condition at each of these stages is shown in Figure 8-57. The figure shows both front

and back sides of the beam.

Stirrup delamination was observed at an applied load of 149 kips which corresponds

to a shear of 70 kips (Figure 8-57, Applied Load of 149 kips). At an applied load of 191

kips, which corresponds to a shear of 90 kips, shear cracks had extended and additional

stirrup delamination was noted (Figure 8-57, Applied Load of 191 kips).

The beam reached an ultimate shear capacity of 120 kips and failed as the shear

cracks opened near the support (Figure 8-57, FAILURE). The beam never reached the

ACI 440R-02 predicted shear capacity of 130 kips. This premature failure was attributed

to anchorage slip of the prestress tendons at the right end of the beam (Figure 8-58). The

CFRP angles ruptured at the thru-bolts during shear failure (Figure 8-59). A close up

view of the delamination of the CFRP stirrups at failure is shown in Figure 8-60 and

Figure 8-61.

T-Beam 2L Shear-Displacement Curve

Vc (ACI 318-02)

Vc + Vs (ACI 318-02)



Applied Load @ 191 kipsFAILURE

0

20

40

60

80

100

120

140

160

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Vertical Displacement @ Applied Load (in)

Shea

r (ki

ps)

First LoadingSecond Loading

Figure 8-56 Shear – Displacement curve for T-Beam 2L

209

210






FAILURE

Figure 8-57 T-Beam 2L condition at various stages during testing

211

Figure 8-58 Shear failure and tendon end anchorage slip

Figure 8-59 Rupture of GFRP angle at thru-bolts

212

Figure 8-60 Areas of CFRP delamination on T-Beam 2L

Figure 8-61 Buckling of CFRP stirrups at FAILURE

213

8.6.1 Strain Gage Readings for CFRP Stirrups

There were two groups of readings taken from the strain gages because the beam was

loaded twice. The two readings were combined to produce a single strain response. A

total of twelve strain gages were installed on the CFRP stirrups. Six were installed on the

front of the beam and the other six were installed on the back. The strain gage readings

were plotted against the applied shear in Figure 8-62 to Figure 8-67.

The strain results indicate that shear cracks occurred close to strain gages 1, 8, 3 and

9, and 6 and 12. This can be verified by the pictures shown in Figure 8-58 and Figure

8-61. The other gages recorded small strains throughout the test until immediately prior

to failure. Delamination of the CFRP is evident from the sudden decrease in strain

readings in gages 1, 8, 9, 6, and 12.

Strain Gages 1 & 7

0

20

40

60

80

100

120

140

160

-7000 -5000 -3000 -1000 1000 3000 5000

Strain (10^-6)

Shea

r (k

ips)

Strain Gage 1 - First LoadingStrain Gage 1 - Second LoadingStrain Gage 7 - First LoadingStrain Gage 7 - Second Loading

6'-9"7'-5"

P & LVDT

2'-10"

Second LoadingFirst Loading

2'-3"3'-3"

121110

654

321

987

Strain Gages 7-12 Back SideStrain Gages 1-6 Front Side

Figure 8-62 Strain readings from strain gages 1 and 7

214

Strain Gages 2 & 8

0

20

40

60

80

100

120

140

160

-7000 -5000 -3000 -1000 1000 3000 5000

Strain (10^-6)

Shea

r (k

ips)


6'-9"7'-5"

P & LVDT

2'-10"


2'-3"3'-3"

121110

654

321

987



215

Strain Gages 3 & 9

0

20

40

60

80

100

120

140

160

-7000 -5000 -3000 -1000 1000 3000 5000

Strain (10^-6)

Shea

r (k

ips)


6'-9"7'-5"

P & LVDT

2'-10"


2'-3"3'-3"

121110

654

321

987



216

Strain Gages 4 & 10

0

20

40

60

80

100

120

140

160

-7000 -5000 -3000 -1000 1000 3000 5000Strain (10^-6)

Shea

r (k

ips)

Strain Gage 4 - First Loading

Strain Gage 4 - Second Loading

Strain Gage 10 - First Loading

Strain Gage 10 - Second Loading

6'-9"7'-5"

P & LVDT

2'-10"


2'-3"3'-3"

121110

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987



217

Strain Gages 5 & 11

0

20

40

60

80

100

120

140

160

-7000 -5000 -3000 -1000 1000 3000 5000

Strain (10^-6)

Shea

r (k

ips)


6'-9"7'-5"

P & LVDT

2'-10"


2'-3"3'-3"

121110

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218

Strain Gages 6 & 12

0

20

40

60

80

100

120

140

160

-5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 5000Strain (10^-6)

Shea

r (k

ips)


6'-9"7'-5"

P & LVDT

2'-10"


2'-3"3'-3"

121110

654

321

987



219

220

8.7 Shear Strength of T-Beam 1R (CFRP Sheets)

T-Beam 1R was the right half of T-Beam 1 which was strengthened in shear with

CFRP sheets. To evaluate the shear capacity of the CFRP sheet retrofit, the beam was

subjected to two point loads as described in Chapter 5. In order to prevent the premature

flexural failure observed in T-Beam 1L, this section of beam was retrofitted with

carbodur strips as additional flexural reinforcement. The shear sheets were not continued

around the soffit of the beam, so the shear retrofit was considered to be a two-sided

application. Figure 8-68 shows the test setup for T-Beam 1R. A more detailed

description is provided in Chapter 5.

Figure 8-68 T-Beam 1R test setup

221

The shear-displacement response is plotted in Figure 8-69 along with the predicted

shear capacities from ACI 318-02 and ACI 440R-02. Six significant stages are identified

on the response curve and the beam condition at each of these stages is shown in Figure

8-70 to Figure 8-72.

The initial shear cracks developed at a shear of 68 kips (Figure 8-70, Actuator Disp of

0.35”) which was close to the ACI 318-02 predicted shear capacity of the concrete, Vc.

No CFRP delamination was observed on this stage. As the load increased, more shear

cracks formed and extended (Figure 8-70, Actuator Disp of 0.55”). The ACI 318-02

predicted nominal shear capacity of the beam was reached at the same shear of 91 kips.

CFRP sheet delamination was observed at a shear of 104 kips (Figure 8-70, Actuator

Disp of 0.70”). At a shear of 116 kips, the tube steel anchors lifted off the epoxy bedding

(Figure 8-71, Actuator Disp of 0.85”). The CFRP sheet delamination increased at a shear

of 124 kips (Figure 8-71, Actuator Disp of 1.00”), below the ACI 440R-02 predicted

nominal shear capacity. Without the steel tube anchors, the CFRP sheet may have

delaminated completely resulting in premature failure. However, because of the

anchorage, the beam was able to exceed the ACI 440R-02 predicted capacity. The beam

reached an ultimate shear of 131 kips which was 2% greater than the ACI 440R-02

nominal shear capacity and 44% greater than the ACI 318-02 nominal shear capacity

(Figure 8-72, FAILURE). The failure of the beam caused a large shear crack extending

from the support through the concrete top slab. Although the steel tube anchors were

deformed during failure, no bolt failures or steel rupture occurred.

T-Beam 1R Shear-Displacement Curve

Vc (ACI 318-02)

Vc + Vs (ACI 318-02)


Actuator Disp @ 0.35"



Actuatro Disp @ 0.85"


FAILURE

0

20

40

60

80

100

120

140

160

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1Midspan Vertical Displacement (in)

Shea

r (k

ips)

Figure 8-69 Shear-Displacement curve for T-Beam 1R

222

223

Left side – Front view Right side - Back view

Actuator Disp of 0.35”



Left side Delaminate - Left side Delaminate - Back side


Figure 8-70 T-Beam 1R condition at various stages during shear testing

224

Tube Lift Actuator Disp @ 0.85” CFRP Delaminate Actuator Disp @ 1.00”

Figure 8-71 T-Beam 1R condition at CFRP delamination


FAILURE

Figure 8-72 T-Beam 1R condition at failure

225

8.7.1 Strain Gage Readings for CFRP Sheets

There were a total of 14 strain gages on the CFRP sheets. Seven strain gages were

installed on each side of the beam. Figure 8-73 through Figure 8-76 plots these strain

readings against the applied shear. Strain gages 13-15 recorded smaller strains than

gages 17-19 because the shear cracks passed through the bottom of the first CFRP sheet

and further up through the second sheet. Strain gage 18 indicates increasing strain due to

a shear crack. At a shear of approximately 65 kips, strain gage 17 shows a rapid increase

in strain to match the strains recorded by gage 18. This indicates delamination of the

CFRP sheet between gages 17 and 18. The same occurs to gage 19 at a shear of

approximately 105 kips. After this load, the CFRP is completely delaminated and its

performance relies entirely on the anchorage provided by the steel tubes at the top and

bottom of the web. Flexibility in this anchorage system resulted in a drop in the sheet

strains.

8.7.2 Carbodur strip strain gages Three strain gages were installed on the carbodur strip at mid-span. Figure 8-77

shows a plot of these strains against the applied shear. The strain readings are very

similar to those recorded at mid-span of T-Beam 2 during flexural testing. The

significant change of slope corresponds to formation of a mid-span flexural crack at a

mid-span moment of 260 kip-ft, which is similar to that observed for T-Beam 2.

First Left Side FRP

0

20

40

60

80

100

120

140

160

-4000 -3500 -3000 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500 3000

Strain(10^-6)

Shea

r (k

ips)

Strain Gage 13-Top

Strain Gage 14 - Middle

Strain Gage 15-Bottom

Strain Gage 16-Slope11

10'-21"

2219 654 1516 10

1817

214131

1'-2"

321207

CL

25 1226

2423

98

Figure 8-73 Strain readings from strain gages 13-16

226

Second Left Side FRP

0

20

40

60

80

100

120

140

160

-4000 -3500 -3000 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500 3000

Strain(10^-6)

Shea

r (k

ips)

Strain Gage 17-Top



11

10'-21"

2219 654 1516 10

1817

214131

1'-2"

321207

CL

25 1226

2423

98


227

Second Right Side FRP

0

20

40

60

80

100

120

140

160

-4000 -3000 -2000 -1000 0 1000 2000 3000

Strain(10^-6)

Shea

r (k

ips)

Strain Gage 20-Top


Strain Gage 22-Bottom11

10'-21"

2219 654 1516 10

1817

214131

1'-2"

321207

CL

25 1226

2423

98


228

First Right Side FRP

0

20

40

60

80

100

120

140

160

-4000 -3500 -3000 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500 3000

Strain(10^-6)

Shea

r (k

ips)

Strain Gage 23-Top



Strain Gage 26-Slope11

10'-21"

2219 654 1516 10

1817

214131

1'-2"

321207

CL

25 1226

2423

98


229

Carbodur Strip Strains

Actuator Disp @ 0.35" Actuator Disp @ 0.55"




FAILURE

0

20

40

60

80

100

120

140

160

0 1000 2000 3000 4000 5000 6000

Strain(10^-6)

Shea

r (k

ips)

Strain 30-Third StripStrain 31-Second StripStrain 32-First Strip

32 First Strip31 Second Strip30 Third Strip

CL

Figure 8-77 Strain readings on the carbodur strips from strain gages 30-32

230

231

8.8 Shear Strength of T-Beam 2R2 (CFRP Sheets)

T-Beam 2R2 was the second test of the right hand portion of T-Beam 2 recovered

after flexural testing. The left end of this beam had already been tested to determine the

shear capacity of the prestressed beam without CFRP shear retrofit. T-Beam 2R2 was

tested to evaluate the CFRP shear sheet with full continuity around the beam soffit. It

was loaded by a line load applied at mid-span. The beam was retrofitted with carbodur

strips for additional flexural reinforcement. Wedge anchors were placed on the ends of

the prestressed tendons to prevent tendon slip.

CFRP wraps were used to extend the previous shear sheets around the soffit of the

beam. According to ACI 440R-02, this represents a three-sided retrofit. Figure 8-78

shows T-Beam 2R2 in the test frame. A detailed description is provided in Chapter 5.

Figure 8-78 T-Beam 2R2 test setup

232

The shear-displacement response is plotted in Figure 8-79 along with the predicted

shear capacities from ACI 318-02 and ACI 440R-02. Seven significant stages during the

test are identified and the condition of the beam at each of these stages is shown in Figure

8-80.

The initial shear cracks occurred at a shear of 59 kips (Figure 8-80, Applied Load of

119 kips). This is also when the first delamination of the CFRP sheets was observed. As

the load increased, the shear crack opened and extended up to the soffit of the top slab.

Delamination of the CFRP sheets also extended as the load increased. The majority of

the shear cracks formed in the left span of the beam. The beam reached an ultimate shear

of 142 kips (Figure 8-80, Ultimate Shear – Back Side). This represented an increase of

16% over the capacity of T-Beam 2R1 tested without CFRP shear strengthening. This is

significantly less than the 42% increase predicted by the ACI 440R-02 procedure.

However, the failure shear capacity still represents a 10% increase over the ACI 440R-02

predicted nominal shear capacity.

Delamination of the CFRP sheets occurred soon after development of shear cracks

below the CFRP as shown in Figure 8-81. Without the GFRP angle anchorage at the top

and bottom of the web, it is possible that the sheets would have lost their capacity. After

extensive delamination, the GFRP angles deformed and eventually ruptured at the thru-

bolts (Figure 8-82).

T-Beam 2R2 Shear-Displacement Curve

Vc (ACI318-02)

Vc + Vs (ACI318-02)



Applied Load @ 227 kipsApplied Load @ 235 kips

Applied Load @ 239 kipsApplied Load @ 247 kips

Ultimate Shear

FAILURE

0

20

40

60

80

100

120

140

160

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1Midspan Vertical Displacement (in)

Shea

r (k

ips)

Figure 8-79 Shear-Displacement curve for T-Beam 2R2

233

Applied Load of 119 kips Applied Load of 227 kips Applied Load of 235 kips


Applied Load of 247 kips Ultimate Shear – Back Side FAILURE

Figure 8-80 T-Beam 2R2 condition at seven stages

234

Figure 8-81 Delamination of the CFRP sheet

Figure 8-82 Rupture of GFRP angles at thru-bolts

235

236

8.8.1 Strain Gage Readings attached on CFRP Sheets

There were a total of 9 strain gages on the CFRP sheets on the right side of the beam

because shear failure was anticipated in this shear span. Figure 8-83 through Figure 8-85

show plots of the measured strains against the applied shear. Strain gages 4-9 recorded

the highest strains because of shear cracks passing below the second CFRP shear sheet.

Initial shear cracks formed at 59 kips and additional shear cracks formed in the same area

as the load increased. The strain readings highlight the delamination of the CFRP sheets.

Gages 5 and 6 record increasing strains due to a shear crack below the CFRP. Further

from the crack, gage 4 records very small strains until a shear load of 110 kips. At this

point the strain in gage 4 suddenly increases to match that in gages 5, indicating

delamination of the CFRP sheet between gages 4 and 5. The sudden decrease in strain in

the CFRP sheets after delamination is due to the flexibility of the GFRP angles allowing

relaxation of the sheets. More of the shear load is therefore transferred to the concrete

and internal stirrups system leading to failure of the beam.

Load vs. Strains on First FRP Stirrup

0

20

40

60

80

100

120

140

160

-3000 -2000 -1000 0 1000 2000 3000 4000 5000 6000

Strains (10^-6)

Shea

r (k

ips)

Strain Gage 1 - First FRP, top

Strain Gage 2 - First FRP, middle

Strain Gage 3 - First FRP, bottom

P & LVDT @ MIDSPAN

9'

9231

87

654


237

Load vs. Strains on Second FRP Stirrup

0

20

40

60

80

100

120

140

160

-3000 -2000 -1000 0 1000 2000 3000 4000 5000 6000

Strains (10^-6)

Shea

r (k

ips)

Strain Gage 4 - Second FRP, left side, top

Strain Gage 5 - Second FRP, left side, middle

Strain Gage 6 - Second FRP, left side, bottom

P & LVDT @ MIDSPAN

9'

9231

87

654


238

Load vs. Strains on Second FRP Stirrup

0

20

40

60

80

100

120

140

160

-3000 -2000 -1000 0 1000 2000 3000

Strains (10^-6)

Shea

r (k

ips)

Strain Gage 7 - Second FRP, right side, topStrain Gage 8 - Second FRP, right side, middleStrain Gage 9 - Second FRP, right side, bottom

P & LVDT @ MIDSPAN

9'

9231

87

654


239

240

8.9 Comparison of the Shear Strengths of the T-Beams Tested in Shear

Figure 8-86 shows the shear-displacement response for all of the shear test beams. T-

Beam 2R1 was a shear test of the prestressed concrete beam without CFRP shear

strengthening. This provided a control shear strength, which exceeded the ACI 318-02

predicted strength by 34%.

T-Beams 1L and 2L were strengthened in shear with CFRP stirrups. Due to flexural

failure, T-Beam 1L did not reach its shear capacity. To prevent premature flexural failure

all other shear test specimens were strengthened in flexure using CFRP carbodur strips

bonded to the soffit of the beam. T-Beam 2L failed in shear but did not achieve the

strength predicted by the ACI 440R-02 approach. This was attributed in part to

debonding and slip of the prestressing tendons at the end of this portion of T-Beam 2. To

avoid this tendon slip in subsequent tests, wedge anchors were installed on the strand

extensions.

T-Beam 1R and T-Beam 2R2 with CFRP sheets for shear strengthening exceeded the

ACI 440R-02 predicted nominal shear capacity. T-Beam 1R represented a 2% increase

over the ACI 440R-02 predicted nominal shear capacity while T-Beam 2R2 represented

an increase of 10%. The expected increase from the ACI 318-02 predicted nominal shear

capacity to the ACI 440R-02 predicted nominal shear capacity was 42%. In these shear

tests, the majority of the increased strength is the result of conservatism in the ACI 318-

02 strength predictions and only part of the increase results from the addition of the

CFRP sheets.

Shear-Displacement Curve

Vc (ACI 318-02) For Stirrups and Sheets

Vc + Vs (ACI 318-02) For Stirrups and Sheets

Vc + Vs + ψfVf (ACI 440R-02) For Sheets and T-Beam 2L

Vc + Vs + fVf (ACI 440R-02) For T-Beam 1L Two Sided

0

20

40

60

80

100

120

140

160

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1Vertical Displacement @ Applied Load (in)

Shea

r (k

ips)

T-Beam 2R1 T-Beam 1L 72302T-Beam 2L - Second LoadingT-Beam 1RT-Beam 2R2

Figure 8-86 Shear-Displacement curves for all shear tests

241

242

8.10 ACI 440 Versus Experimental Shear Capacities

The shear capacities of the shear tests were normalized with respect to the section

dimensions and plotted in Figure 8-87. The figure also includes the results of prior

research compiled by Lyle Nakashima in his Masters Report21. The 45-degree datum

represents a one-to-one agreement between the predicted and experimental results. For

all specimens except T-Beam 2L, the experimental results exceeded the ACI 440R-02

predictions. As noted earlier, tendon slip in T-Beam 2L during testing resulted in a

reduction in the shear capacity. The shear capacities of both T-Beam 1R and T-Beam

2R2 with CFRP sheets as shear reinforcement exceeded the ACI 440R-02 predicted

values.

ACI 440 Vs. Experimental Shear Capacities

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

Experimental Vn/bwdp (ksi)

AC

I 440

Vn/

b wd p

(ksi)

Chaallal Xiao Czaderski Triantafillou ChajesAl-Sulaimani T-Beam 1R (Sheets) T-Beam 2R2 (Sheets) T-Beam 2L (Stirrups) Design Datum

Figure 8-87 Normalized ACI 440 predictions versus experimental shear capacities

243

245

CHAPTER 9

9 SUMMARY AND CONCLUSION

9.1 Summary

This research study involved flexural and shear testing of two precast prestressed T-

Beams salvaged from the Ala Moana Parking Garage. One un-strengthened beam was

used as the control specimen referred to as T-Beam 1. The second beam was the

strengthened beam referred to as T-Beam 2. This beam had been strengthened in 1997

using CFRP carbodur strips epoxy bonded to the soffit of the beam because of a number

of flexural cracks and severe spalling to the beam ledges.

Initial theoretical strength calculations for T-Beam 1 and T-Beam 2 indicated that the

addition of CFRP to increase the flexural capacity of T-Beam 2 resulted in a shear critical

failure mode if the beam were tested under the proposed laboratory conditions. To

reduce the potential for a shear failure and ensure the desired flexural failure of T-Beam

2, the shear spans of both beams were increased for the laboratory loading and CFRP

shear retrofit was applied prior to flexural testing. Two shear retrofit techniques were

employed on each beam, namely external CFRP shear stirrups on the left half of the beam

and CFRP sheets on the right half of the beam. In order to prevent premature

delamination of the CFRP shear retrofits at the re-entrant corners at the top and bottom of

the beam web, mechanical anchorage was provided in the form of steel tubes and GFRP

angles with steel bolts through the beam web. Subsequent to flexural testing, each of the

beam shear spans was tested in shear to evaluate the performance of the CFRP shear

retrofit and mechanical anchorage.

246

9.2 Conclusions

9.2.1 Flexure Tests

• Sika CFRP pre-cured carbodur strips epoxy bonded to the soffit of the

strengthened beam significantly increased the flexural strength over that of the

control beam without reducing the beam ductility.

• There was no apparent degradation of the CFRP strips, CFRP fabric wrap anchors

or epoxy bonding agent during the five years of field exposure between

application in 1997 and testing in 2002.

• The ACI 440R-02 strain compatibility procedure for estimating the flexural

strength of concrete beams with externally bonded CFRP is conservative for the

condition tested here.

• The failure bending strength of the retrofit beam was 21% greater than that

predicted by the ACI 440R-02 report procedure.

• The failure bending strength represented a 71% increase compared with the

control specimen, while the ACI 440R-02 predicted a 37% increase when

compared with the ACI 318-02 predicted flexural capacity of the control

specimen.

247

9.2.2 Shear Tests

o The shear capacity of the prestressed T-Beam without CFRP shear strengthening

exceeded the ACI 318-02 predicted strength by 33%.

o The two T-Beam tests with CFRP sheets produced 7% and 16% increases in the

shear capacity when compared with the beam without CFRP shear strengthening.

These increases are below the 42% increase predicted by ACI 440R-02.

o The failure shear strength of the beams retrofitted with CFRP sheets was slightly

greater than the ACI440R-02 prediction.

o In all shear tests, delamination of the CFRP stirrups and sheets occurred prior to

the maximum shear load. Without adequate anchorage at the top and bottom of

the beam web, the CFRP would have been ineffective.

o The steel tubes provided better anchorage than the GFRP angles.

o Future research studies in anchorage system design are necessary to maximize the

effectiveness of CFRP shear retrofit systems, particularly when applied to

prestressed concrete beams with existing internal shear reinforcement.

249

10 APPENDIX A

“Field Retrofit of Prestressed Concrete T-Beam Using CFRP”

by I. N. Robertson, A. A. Agapay, and L. M. Nakashima

Technical Paper presented

at the

ACI Fall Convention, September 2003

in

Boston, Massachusetts

and published in

“Field Applications of FRP Reinforcement: Case Studies”

Editors: Sami Rizkalla and Antonio Nanni, SP-215, ACI, 2003

251

Field Retrofit Of Prestressed Concrete T-Beam Using CFRP

Ian N. Robertson, Alison A. Agapay, Lyle M. Nakashima

Synopsis In 1997, a precast, prestressed T-beam in the Ala Moana Shopping Center parking garage, in Honolulu, Hawaii, was strengthened in flexure using carbon fiber reinforced polymer (CFRP) strips epoxy bonded to the soffit of the beam. When the parking garage was demolished in June 2000, this beam and two control beams were salvaged and brought to the University of Hawaii for testing. This paper presents the retrofit procedures used during field application of the CFRP strips. It also describes the beam recovery and preparation for laboratory testing. The test program and results of the flexural testing of both unstrengthened and strengthened beams under four-point loading are presented in detail. The CFRP retrofit significantly increased the flexural capacity of the beam while also increasing its flexural ductility. The failure moment was well in excess of the nominal moment capacity predicted using the strain-compatibility procedure described in the ACI 440R-02 report.

Keywords: T-beam, strengthening, prestressed concrete, carbon fiber, fiber reinforced polymers, field application

252

Ian Robertson, teaches structural engineering design courses and performs experimental research investigating the performance of concrete and steel structures under dynamic, cyclic and long-term loading conditions. He is a registered professional structural engineer in the State of Hawaii and has been involved in structural engineering design and research for over 20 years. Alison Agapay, is a Graduate Research Assistant in the Structural Engineering Laboratory at the University of Hawaii. The project reported here constitutes the research component of his Masters Thesis. He constructed the 4-post load frame used to test the T-beams and performed all testing and result analysis for this study. Lyle Nakashima, is a design engineer with Mitsunaga and Associates, a structural engineering consulting company in Honolulu, Hawaii. He performed an extensive literature review on FRP strengthening, assisted in the beam recovery operation, and computed the theoretical beam strengths reported in this paper as part of his Masters program in structural engineering at the University of Hawaii.

INTRODUCTION During a routine structural inspection of the Ala Moana Shopping Center Parking Garage in Honolulu, Hawaii, it was noted that one of the prestressed T-beams had substantial spalling damage to the beam ledges and a large flexural crack across the bottom flange of the beam. The beam is a precast prestressed inverted T-beam supporting joists and a slab, which acts as the top flange for the T-beam (Figure 1). The beam was repaired in 1997 using CFRP pre-cured strips bonded to the beam soffit to augment the flexural capacity (Figure 2). This was the first use of FRP materials for structural retrofit in the State of Hawaii. CFRP wet lay-up wrap and epoxy mortar were used to repair the damaged beam ledges. CFRP wraps were also provided at the ends of the span to restrain the end of the CFRP tension strips.

The repairs were performed by Concrete Coring of Hawaii using Sika Carbodur pre-cured CFRP strips epoxied to the bottom flange of the beam. The repairs were performed following standard manufacturer’s instructions, with no thought that the beam would be tested at some future date. In additional to the retrofit beam, an identical unstrengthened beam was tested as a control specimen. This is one of the first tests of a field installed FRP retrofit after an extended field service period. The Hawaii Department of Transportation (HDOT) funded this research program in order to evaluate the potential for FRP retrofit of deficient bridge structures in the state.

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RESEARCH SIGNIFICANCE

Field application of FRP materials for strengthening of concrete members has increased significantly subsequent to extensive laboratory testing of retrofit techniques (GangoRao and Vijay, 1998; Fanning and Kelly, 2001; Shahawy et al, 2001; Spadea et al, 2002 and many others). Very few of these field applications are available for testing after exposure to service conditions. This paper presents the results of flexural testing of a prestressed concrete T-beam strengthened in the field with CFRP pre-cured strips and CFRP wet layup. The results are an important link between traditional laboratory testing and the performance of field installed FRP strengthening.

FIELD APPLICATION OF CFRP PRE-CURED STRIPS The CFRP retrofit was designed in 1997 by Martin and Bravo Structural Engineers, Honolulu, Hawaii, following design procedures presented in the literature at the time. No standard was available at that time for surface application of FRP to concrete members. The surface of the concrete was prepared by grinding to remove paint and weak surface paste. This resulted in a relatively smooth surface preparation similar to ICRI-CSP surface profile 2 (ICRI, 2002). Current FRP application specifications would generally require a slightly rougher surface profile such as ICRI-CSP 3-4. The CFRP strips were cleaned prior to installation and a layer of Sikadur 30 Hi-Mod Gel epoxy was placed on the soffit of the beam. The strips were then pressed onto the epoxy using a roller as shown in Figure 3. 150 mm wide strips of SikaWrap Hex 103C uni-direction carbon fabric were saturated with Sikadur Hex 300 epoxy and applied to the ends of the beam to restrain the ends of the flexural strips (Figure 4). Additional CFRP fabric sheets were used to wrap the epoxy mortar patches at the ledge spalls at third points along the span. Since these additional wraps are not typical of flexural strengthening, they were removed prior to testing the beam. The end anchorage wraps were however left in place as they are commonly installed as part of the flexural strengthening.

BEAM RECOVERY AND REPAIR

During demolition of the parking structure in June 2000, the precast prestressed beam with CFRP strengthening was salvaged along with two nominally identical beams (Figure 5). In the recovery process, the top slab forming the beam flange was removed to facilitate shipping. In addition, portions of the beam web suffered minor damage in the form of concrete spalls and negative bending cracks. This damage was repaired by personnel from PlasTech Inc., Hawaii, using Sika epoxy injection and epoxy mortar

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patch materials (Figure 6). It is assumed that none of these repairs affected the flexural performance of the beams during testing. The top flange of the beam was reinstated in the Structural Laboratory at UH with the same reinforcement layout as the original slab (Figure 7). The dimensions of the four-post test frame limited the flange width to 1500 mm, which corresponds to one fifth of the beam span and a slab overhang of six times the slab thickness on either side of the web. The ACI 318 building code effective flange width for this beam would be 1800 mm (ACI 2002a). Beam failures were initiated by tension reinforcement failure and not compression failure of the top flange. It is therefore assumed that the reduced flange width did not significantly affect the beam flexural performance. The cross-sectional dimensions of the beams are shown in Figure 8.

MATERIAL PROPERTIES

Table 1 lists the concrete material properties for the two T-beams tested in this study. After beam testing, concrete cores were taken from the web and anchorage blocks of each beam to determine compressive strength. Core recovery and testing were performed according to ASTM C42-99 (ASTM C42, 1999). The cores were all 100 mm diameter by 140 mm long. The compressive strength determined from each core was adjusted because the length to diameter ratio was below 1.75 (ASTM C42, 1999). The resulting concrete compressive strengths are listed in Table 1. Standard 150 mm diameter by 300 mm concrete cylinders were made while pouring the top slabs of each T-beam. These cylinders were tested in compression on the same day as the corresponding T-beam test producing the average compressive strengths listed in Table 1. Table 2 lists the material properties of the reinforcing and prestressing steel used in the test beams. Web shear reinforcing bars were recovered during beam demolition and tested in tension. The average tensile strength for shear reinforcing in each T-beam is listed in Table 2. Coupons of the 10 mm nominal diameter prestress strands were also recovered from the test beams and tested in tension producing the ultimate tensile strengths listed in Table 2. Table 3 lists the material properties for the FRP materials used in the flexural strengthening of T-beam 2. These properties are based on the manufacturer’s test information since no material coupons were available from the original repair work. Because of damage to the FRP materials during destructive testing of the T-beam, it was not possible to recover representative samples for testing. Pull-off tests on the CFRP strips would also not be representative of the installed condition because of damage caused to the epoxy bond during the beam test.

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FLEXURAL TESTING PROGRAM

Both control and strengthened T-beams were tested under four-point loading as shown schematically in Figure 9. T-beam 1, the control specimen, was tested over a span of 7.24 meters with a pinned support at one end and roller support at the other. The load was applied through two line loads each 610 mm from midspan. For T-beam 2, the strengthened beam, the support locations used for T-beam 1 would have been directly under the ends of the CFRP strips. These reactions would therefore have enhanced the restraint provided by the CFRP wrap at the ends of the strips. To avoid this additional restraint, steel support brackets were bolted to the ends of the beam so that the supports could be located beyond the ends of the beam. The span for T-beam 2 was 7.76 meters as shown in Figure 9. Figure 10 shows T-beam 2 in the test frame prior to testing. Because of the enhanced flexural capacity provided by the CFRP strengthening, the flexural capacity of T-beam 2 now exceeded the theoretical shear capacity. In order to prevent a premature shear failure, shear reinforcement was installed on both beams in the form of CFRP wet lay-up stirrups bonded to the web in the left half of the beam and CFRP wet lay-up sheets bonded to the surface of the web on the right half of the beam as seen in Figure 10. Two different configurations of shear retrofit were used so as to investigate their performance through shear testing of each half of the beam after flexural failure. Ideally this shear reinforcement would be continuous around the soffit of the beam, however, in order not to alter the CFRP flexural strips by providing additional restraint, the shear reinforcement was terminated at the bottom of the web. Anchorage of the CFRP shear reinforcement was provided at the re-entrant corners at top and bottom of the web by means of steel tubes and FRP angles secured by steel bolts through the beam web. The load was applied under displacement control in increments of 0.25 mm up to a midspan deflection of 12.5 mm, then in increments of 0.625 mm to a displacement of 50 mm, and then at increments of 1.25 mm till failure. At each displacement increment, all strain and displacement readings were recorded and the extent of beam cracking was noted.

FLEXURAL TEST RESULTS

T-beam 1 response

The bending moment at midspan of T-beam 1 (control specimen) is plotted against the midspan deflection in Figure 11. The first flexural cracks were observed at midspan at the bottom of the beam at a bending moment of 300 kN-m. As the load increased, these cracks extended up into the web and new flexural cracks formed below the load points. The theoretical moment capacity of the beam as predicted by the ACI 318-02 code, and using measured strengths of concrete and prestress steel, was 588 kN-m. The test beam was unable to reach this moment capacity. The midspan flexural crack continued to open as the load was increased, with final flexural failure occurring at a bending moment of 574 kN-m and midspan deflection of 78 mm when the ten prestress strands ruptured at this center crack (Figure 12). This failure strength was 2% below the ACI code nominal capacity.

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T-beam 2 response T-beam 2 (strengthened beam) was tested under the same loading conditions as the control T-beam 1. However, in order to prevent the support condition from providing additional restraint to the end of the Carbodur strips on the beam soffit, steel extensions were fabricated and bolted to the ends of the beam. This resulted in a longer span for T-beam 2 compared with T-beam 1 (Figure 9). During the flexural test of T-beam 2, the response was similar to that for the control specimen until flexural cracking of the beam. The post-cracking stiffness for T-beam 2 was greater than that for T-beam 1, and did not degrade as rapidly. Figure 13 shows the moment-deflection response of T-beam 2 compared with that for the control beam. ACI committee 440 recently published a report on the strengthening of concrete members using externally bonded FRP (ACI, 2002b). This ACI440R-02 report was used to predict the failure bending moment for T-beam 2. The anticipated nominal moment capacity of 846 kN-m was easily exceeded by the strengthened beam, which supported a maximum moment of 984 kN-m prior to failure, 16% greater than the predicted value. This represents a 71% increase in flexural strength compared with the control specimen, while the ACI 440R-02 suggests the increase to be 44% compared with the ACI 318 nominal capacity. The apparent conservative prediction using the ACI 440R-02 procedure is attributed to the debonding coefficient which limits the strain capacity of the CFRP strips to simulate premature debonding. Because of the unpredictable nature of a debonding failure, reasonable conservatism is warranted. The maximum midspan deflection for T-beam 2 was 100 mm compared with the 75 mm deflection for the control specimen. The addition of CFRP flexural strengthening increased the ductility of the beam. Failure occurred when the CFRP strips delaminated from the bottom of the beam. This delamination appeared to initiate at the base of a flexure-shear crack that had formed just outside the left load point. Vertical offset in the soffit of the beam on either side of this crack may have contributed to the initiation of delamination. In addition, large strain differential between the CFRP strips and the flexurally cracked concrete may also have contributed to deterioration of the bond between CFRP and concrete. If the CFRP shear reinforcement had extended around the soffit of the beam, it may have provided additional restraint to the CFRP strips and delayed the delamination, thus further increasing the flexural strength. For the first 500 mm from the delamination initiation point, the failure occurred in the surface concrete, with a thin layer of concrete remaining attached to the CFRP strips. Beyond this point, the CFRP strips separated from the epoxy, likely because of the increased angle of peeling as the CFRP stripped away from the beam soffit. The delamination occurred rapidly and extended from the shear-flexure crack to the end of the CFRP strips, which pulled part way out of the CFRP fabric wrap anchor. The anchor was not sufficient to prevent pull-out once delamination had occurred, but there was no tendency for delamination to initiate at the end of the strips as had been reported in some laboratory studies.

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SUMMARY AND CONCLUSIONS Two precast prestressed concrete T-beams were recovered from a Honolulu shopping center parking garage and tested in flexure in the University of Hawaii Structural Engineering Laboratory. One of the beams had been strengthened in 1997 using CFRP carbodur strips epoxy bonded to the soffit of the beam. The other was used as a control specimen. The following conclusions were made based on the results of these tests.

• Sika Carbodur CFRP pre-cured strips epoxy bonded to the soffit of the strengthened beam significantly increased the flexural strength over that of the control beam without reducing the beam ductility.

• There was no visually noticeable degradation of the CFRP strips, CFRP fabric wraps or epoxy bonding agents during the 5 years of field exposure between application in 1997 and testing in 2002.

• The ACI 440R-02 strain-compatibility procedure for estimating the flexural strength of concrete beams with externally bonded CFRP appears to be conservative for the condition tested here. The failure bending strength of the retrofit beam was 16% greater than that predicted by the ACI 440R-02 report procedure.

ACKNOWLEDGEMENTS

The authors are extremely grateful to Adriano “A. B.” Bortolin of Sika Products, USA, for providing valuable information concerning the original FRP application, at which he was the Sika representative. Sika Products also donated all additional FRP and epoxy materials required to repair the recovered beams and retrofit them in shear so as to avoid a premature shear failure. The authors are also indebted to Brian Ide, the structural engineer responsible for the original FRP strengthening design. Brian provided construction drawings, design calculations and photographic records of the original retrofit. Chandler Rowe and his colleagues at PlasTech Inc., Honolulu, Hawaii, are thanked for donating their labor and expertise in the repair of the recovered beams and for installation of the shear retrofit materials at the UH Structural Engineering laboratory. This project was funded through research grant No. 46507 from the Hawaii Department of Transportation Research Board. This financial support is gratefully acknowledged. The opinions and observations made in this paper are those of the authors and do not necessarily reflect the opinion of any of the project sponsors.

REFERENCES ACI 2002a, “ACI 318-02/318R-02, Building Code Requirements for Structural Concrete and Commentary”, American Concrete Institute, Farmington Hills, Michigan, 443 pp. ACI 2002b, “ ACI 440R-02, Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures”, American Concrete Institute, Farmington Hills, Michigan, 45 pp.

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ASTM C42, 1999, “Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete”, American Society for Testing and Materials, West Conshohocken, PA, 4. Fanning, P. J., and Kelly, O., 2001, “Ultimate Response of RC Beams Strengthened with CFRP Plates,” Journal of Composites for Construction, Vol. 5, No. 2, pp. 122-127. GangaRao, V. S., and Vijay, P. V., 1998, “Bending Behavior of Concrete Beams Wrapped With Carbon Fabric,” Journal of Structural Engineering, Vol. 124, No. 1, pp. 3-10. ICRI 2002, “Selecting and Specifying Concrete Surface Preparation for Coatings, Sealers, and Polymer Overlays”, International Concrete Repair Institute Technical Guideline No. 03732. Shahawy, M., Chaallal, O., Beitelman, T. E., and El-Saad, A., 2001, “Flexural Strengthening with Carbon Fiber-Reinforced Polymer Composites of Preloaded Full-Scale Girders,” ACI Structural Journal, Vol. 98, No. 5, pp. 735-742. Spadea, G., Mencardino, F., and Swamy, R. N., 2002, “Strength and Ductility of Reinforced Concrete Beams Externally Reinforced with Carbon Fiber Fabric,” ACI Structural Journal, Vol. 99, No. 2, pp. 163-171.

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Table 1: Concrete material properties

Beam Location Compressive Strength, fc’

(MPa)

Description

Precast Beam 58 (8413 psi) Average of 4 core tests T-beam 1 Control Top Slab 37 (5396 psi) Average of 3 cylinder

tests Precast Beam 58 (8400 psi) Average of 3 core tests T-beam 2

With CFRP Top Slab 62 (9025 psi) Average of 3 cylinder tests

Table 2: Steel material properties

Beam Sample

Average Tensile Yield Stress, Fy

(MPa)

Average Tensile Strength (Fpu for Prestress)

(Fu for Reinforcement) (MPa)

Prestress Strands - 1870 (272 Ksi) T-beam 1 Control Web Shear

Reinforcement 350 (50.9 Ksi) 503 (73.1 Ksi)

Prestress Strands - 1870 (272 Ksi) T-beam 2 With CFRP Web Shear

Reinforcement 350 (50.9 Ksi) 503 (73.1 Ksi)

Table 3: CFRP material properties

FRP material Description Tensile Strength, fFRP (MPa)

Tensile Modulus, EFRP

(Gpa) Carbodur Strips 102 mm x 1.2 mm 2790 (406 Ksi) 164 (23,900 Ksi) Sika Wrap Hex

103C Uni-Directional

Single Ply 1010 (147 Ksi) 73 (10,600 Ksi)

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Figure 1: Precast Prestressed T-beam repaired using CFRP strips and wrap

Figure 2: Beam cross-section showing three CFRP strips for flexural strengthening

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Figure 3: Field application of CFRP strips

Figure 4: CFRP fabric wrap at end of flexural strips

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Figure 5: Removal of T-beams during parking garage demolition

Figure 6: Repair of spalling damage to web

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Figure 7: Reconstruction of top flange in UH laboratory

420mm


3 Carbodur CFRP Strips

2-leg 10mm Ø Stirrups at 300mm o.c.

Slab Reinforcement

(10) 10mm Ø Stress-relieved prestress strands

1500mm

140mm470mm

140mm

115mm

Figure 8: T-Beam Cross-section

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TEST SETUP ELEVATION

A

SECTION "A-A"

1300 kN Actuator

Load Cell

4-post Load Frame

Spreader Beam

TestBeam

1220mm

7240mm for T-Beam 17760mm for T-Beam 2

Figure 9: T-Beam test setup

Figure 10: T-beam 2 in test frame

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Moment Capacity

0

100

200

300

400

500

600

700

800

900

1000

0 10 20 30 40 50 60 70 80 90 100

Midspan Vertical Displacement (mm)

App

lied

Mom

ent (

kN-m

)

T-beam 1, control

Mn1

Figure 11: T-beam 1, control specimen, flexural response

(T-beam 1: Shear retrofit but no flexural strengthening)

Figure 12: Flexural failure of control T-beam.

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Moment Capacity

0

200

400

600

800

1000

1200

0 20 40 60 80 100 120

Midspan Vertical Displacement (mm)

App

lied

Mom

ent (

kN-m

)

T-beam 1, controlT-beam 2, with CFRP

Mn1

MnFRP

Figure 13: Flexural performance of control and strengthened T-beams

(T-beam 2: Shear retrofit and flexural strengthening)

Figure 14: Failure of strengthened T-beam showing CFRP tension strip

delamination

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REFERENCES

1 Spadea, G., Bencardino, F., and Swamy, R. N. “Strength and Ductility of Reinforced Concrete Beams Externally Reinforced with Carbon Fiber Fabric”, ACI Structural Journal Vol.99 No.2 March-April 2002, pp.163-171 2 Spadea, G., Bencardino, F., and Swamy, R.N. “Structural Behavior of Composite RC Beams with Externally Bonded CFRP”, Journal of Composites for Construction August, 1998 Vol.2 No.3, pp. 132-137 3 GangaRao, V. S., Vijay, P. V. “Bending Behavior of Concrete Beams Wrapped With Carbon Fabric”, Journal of Structural Engineering January, 1998 Vol.124 No.1, pp. 3-10 4 Fanning, P. J. and Kelly, O. “Ultimate Response of RC Beams Strengthened with CFRP Plates”, Journal of Composites for Construction May, 2001 Vol.5 No.2, pp. 122-127 5 Shahawy, M., Chaallal, O., Beitelman, T. E., and El-Saad, A. “Flexural Strengthening with Carbon Fiber-Reinforced Polymer Composites of Preloaded Full-Scale Girders”, ACI Structural Journal September-October, 2001 Vol.98 No.5, pp. 735-742 6 White, T. W., Soudki, K. A., and Erki, M. “Response of RC Beams Strengthened with CFRP Laminates and Subjected to a High Rate of Loading”, Journal of Composites for Construction August, 2001 Vol.5 No.3, pp. 153-162 7 Masoud, S., Soudki, K., and Topper, T. “CFRP-Strengthened and Corroded RC Beams Under Monotonic and Fatigue Loads”, Journal of Composites for Construction November, 2001 Vol. 5 No. 4, pp. 228-236 8 Bonacci, J. F. and Maalej, M. “Externally Bonded Fiber-Reinforced Polymer for Rehabilitation of Corrosion Damaged Concrete Beams”, ACI Structural Journal Vol.97 No.5 September-October 2000, pp. 703-711 9 Sheikh, S. A., DeRose, D., and Mardukhi, J. “Retrofitting of Concrete Structures for Shear and Flexure with Fiber-Reinforced Polymers”, ACI Structural Journal Vol.99 No.4 July-August 2002, pp. 451-459 10 Czaderski, C. “Shear Strengthening with Prefabricated CFRP L-Shaped Plates”, IABSE Symposium Melbourne 2002

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11 Chaallal, O., Nollet, M. J., Perraton, D. “Shear Strengthening of RC Beams by Externally Bonded Side CFRP Strips”, Journal of Composites for Construction May, 1998 Vol.2 No.2, pp. 111-113 12 Triantafillou, T. C. “Shear Strengthening of Reinforced Concrete Beams Using Epoxy-Bonded FRP Composites”, ACI Structural Journal Vol.95 No 2 March-April 1998 pp. 107-115 13 Al-Sulaimani, G. J., Sharif, A., Basunbul, I. A., Baluch, M. H., and Ghaleb, B. N. “Shear Repair for Reinforced Concrete by Fiberglass Plate Bonding”, ACI Structural Journal Vol.91 No.4 July-August 1994, pp. 458-463 14 Schuman, P., and Karbhari, V. M. “A Study On Mechanical Anchorage For Shear Rehabilitation of RC Structures”, Proceedings of the fib 2003 Symposium Athens, Greece May 6-8, pp. 378-389 15 ICRI 2002, “Selecting and Specifying Concrete Surface Preparation for Coatings, Sealers, and Polymer Overlays”, International Concrete Repair Institute Technical Guideline No. 03732 16 Fung, Stephanie S. Y., and Robertson, Ian N. “Seismic Monitoring of Dynamic Bridge Deformations Using Strain Measurements”, Research Report UHM/CEE/03-02 May 2003 17 ACI (1984) “State of the Art Report on High Strength Concrete”, Report of ACI Committee 363, ACI Journal Vol. 81, No. 4 pp. 364-411 18 ACI (2002) “Guide for the Design and Construction of Externally Bonded FRP Systems For Strengthening Concrete Structures”, Reported by ACI Committee 440 pg. 44 19 ACI (2002) “Guide for the Design and Construction of Externally Bonded FRP Systems For Strengthening Concrete Structures”, Reported by ACI Committee 440 pg. 53-62 20 Nawy, Edward G. “Prestressed Concrete”, Fourth Edition 2003 pg. 56 Fig. 2.18a 21 Nakashima, Lyle “Evaluation of ACI 440 Design For FRP Repair and Retrofit Of Concrete Beams”, CEE699 Report April 2003

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