STRUCTURAL SYSTEMS
RESEARCH PROJECT
Report No.
SSRP– 06/ 12
ASSESSMENT OF FRP COMPOSITE
STRENGTHENED REINFORCED
CONCRETE STRUCTURES AT THE
COMPONENT AND SYSTEMS LEVEL
THROUGH PROGRESSIVE DAMAGE AND
NON- DESTRUCTIVE EVALUATION
by
KUMAR K. GHOSH
VISTASP M. KARBHARI
Final Report Submitted to the California Department of Transportation Under Contract No. 59A0337.
June 2006
Department of Structural Engineering
University of California, San Diego
La Jolla, California 92093- 0085
University of California, San Diego
Department of Structural Engineering
Structural Systems Research Project
Report No. SSRP– 06/ 12
Assessment of FRP Composite Strengthened Reinforced Concrete Structures at the Component and Systems Level Through Progressive Damage and Non- Destructive Evaluation
by
Kumar K. Ghosh
Graduate Student Researcher
Vistasp M. Karbhari
Professor of Structural Engineering
Final Report Submitted to the California Department of Transportation Under Contract No. 59A0337
Department of Structural Engineering
University of California, San Diego
La Jolla, California 92093- 0085
June 2006
ii
Technical Report Documentation Page
1. Report No.
SSRP 06/ 12
2. Government Accession No.
3. Recipient’s Catalog No.
4. Title and Subtitle
Assessment of FRP Composite Strengthened Reinforced Concrete Structures at Component and Systems Level Through Progressive Damage and Non- Destructive Evaluation ( NDE)
5. Report Date
05/ 23/ 2006
6. Performing Organization Code
7. Author( s)
Kumar K. Ghosh, Vistasp M. Karbhari
8. Performing Organization Report No.
UCSD / SSRP- 06/ 12
9. Performing Organization Name and Address
Department of Structural Engineering
School of Engineering
10. Work Unit No. ( TRAIS)
University of California, San Diego
La Jolla, California 92093- 0085
11. Contract or Grant No.
59A0337
12. Sponsoring Agency Name and Address
California Department of Transportation
13. Type of Report and Period Covered
Final Report -
Engineering Service Center
1801 30th St., West Building MS- 9
Sacramento, California 95807
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared in cooperation with the State of California Department of Transportation.
16. Abstract
There is growing use of FRP composite materials in the civil infrastructure for rehabilitation of deficient bridge components including deck slabs and girders. However assessment of the effectiveness of rehabilitation over time and monitoring the progression of damage or change in load paths between the structural components, caused by sequential strengthening of the components, has not been undertaken to date. Investigation was first carried out at “ component level” on both unstrengthened and field- rehabilitated slab specimens cut out from a major highway bridge. The slabs were tested to failure and the progression of damage was characterized through instrumentation and NDE. The test data on the failure modes and capacity loads were correlated to the available analytical models and design guidelines. The test capacity was also correlated to the bridge deck capacity based on local- global modeling. Research at the “ systems level” was then undertaken, in which a three- girder two- span bridge deck system was tested to simulate behavior under field loading in which the deck slabs are found to be susceptible to punching shear type failures and the longitudinal girders are usually found to be deficient in terms of shear demand. The objective of the study was to evaluate damage progression in the deck slabs and the longitudinal girders under simulated truck load and to detect changes in the overall response of structure at systems level caused by strengthening of individual components that might cause other components to reach their critical limit states under the higher load demands which can be resisted by the strengthened components. NDE techniques, including IR thermography and forced vibration based dynamic modal tests, were evaluated as means to quantify the damage localization and progression under simulated field loading as well as to quantitatively monitor changes in the response of the components, caused by subsequent modifications of the structure, at systems level. The test data on the failure modes, capacity loads and specimen behavior were correlated to the both analytical and numerical models. Based on the limitations of the available design guideline for FRP strengthening, a modified design methodology was proposed for FRP strengthening of slab- girder systems.
17. Key Words
FRP, Rehabilitation, Progressive Failure, Bridge Deck, System
18. Distribution Statement
Unlimited
19. Security Classification ( of this report)
Unclassified
20. Security Classification ( of this page)
Unclassified
21. No. of Pages
458
22. Price
Form DOT F 1700.7 ( 8- 72) Reproduction of completed page authorized
DISCLAIMER
The contents of this report reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the State of California. This report does not constitute a standard, specification, or regulation.
iii
TABLE OF CONTENTS
DISCLAIMER ......................................................................................................
iii
TABLE OF CONTENTS …………………………………………………….....
iv
LIST OF FIGURES ……………………………………………………………..
x
LIST OF TABLES ………………………………………………………………
xxii
ABSTRACT ……………………………………………………………………
xxiv
1 INTRODUCTION ………………………………………………………..
1
1.1
Problem Statement ……………………………………………………..
2
1.2
Scope of Current Research and Report Organization ……………….....
3
2 OVERVIEW OF THE STATE OF THE ART IN SYSTEMS
LEVEL STRENGTHENING WITH FRP COMPOSITES ……………
6
2.1
Introduction …………………………………………………………....
6
2.2
Strengthening of RC Components with FRP Composites ……………..
6
2.2.1
Flexural Strengthening of Beams and Slabs …………………...
6
2.2.2
Shear Strengthening of Beams ………………………………...
8
2.3
Effect of Strengthening of RC Bridge Components at Systems Level ...
8
2.3.1
Relevant Systems Level Tests Demonstrating Effect of Stress Re- Distribution and Damage Progression on Overall Structural Behavior ………………………………………………………..
9
2.3.2
Existing Systems Level Research in FRP Composite Strengthening of Bridge Components …………………………
15
2.4
Conclusions ……………………………………………………………
20
3 COMPONENT LEVEL STUDY OF FIELD SPECIMENS …………...
22
iv
3.1
Introduction ……………………………………………………………
22
3.2
Research Objectives …………………………………………………...
23
3.3
Description of Test Specimens ………………………………………...
24
3.4
Material Properties and Capacity Prediction …………………………..
28
3.5
Test Setup and Instrumentation ………………………………………..
40
3.6
Instrumentation and Data Acquisition …………………………………
41
3.7
Test Results and Discussion …………………………………………...
46
3.7.1
Ultimate Load Capacities of Slabs …………………………….
46
3.7.2
Slab Deflections and Cracking ………………………………...
47
3.7.3
Strain Development in the FRP Material ……………………...
54
3.8
Progressive Damage Characterization …………………………………
57
3.8.1
Detection and characterization of pre- existing defect/ damage ...
59
3.8.2
Damage Detection/ Progression from Thermography Data ……
67
3.8.3
Correlation of Damage Progression to Strain Gage Data ……...
76
3.9
Comparison of Predicted and Experimental Results …………………..
78
3.10
Correlating Test Specimen to Bridge Deck Capacity ………………….
81
3.11
Research Extension to the Systems Level ……………………………..
85
4 EXPERIMENTAL PROGRAM ………………………………………...
87
4.1
Introduction ……………………………………………………………
87
4.2
Research Objectives …………………………………………………...
88
4.3
Description of Test Specimen ………………………………………….
90
4.4
General Test Plan ………………………………………………………
96
4.5
Initial Analytical Modeling and Design ……………………………….
97
v
4.5.1
Design of Unstrengthened Components for Phase 1 of Test …..
101
4.5.1.1 Design capacity of deck slabs ………………………..
101
4.5.1.2 Design capacity of unstrengthened girders …………..
103
4.5.2
Design of Strengthened Deck Slabs for Phase 2 of Test ………
105
4.5.3
Design of Strengthened Girder for Phase 3 of Test ……………
116
4.6
Test Setup ……………………………………………………………...
125
4.7
CFRP Strengthening Procedures ………………………………………
129
4.7.1
Strengthening of the deck slabs ………………………………..
129
4.7.1.1 Strengthening procedure ……………………………..
129
4.7.1.2 Material properties of composite for slab strengthening …………………………………………………..
132
4.7.2
Strengthening of the girder …………………………………….
138
4.7.2.1 Strengthening procedure ……………………………..
138
4.7.2.2 Material properties of composite for girder strengthening …………………………………………………..
142
4.8
Instrumentation
145
4.8.1
Instrumentation details of linear potentiometers and load cells..
145
4.8.2
Instrumentation details of strain gages ………………………...
146
4.8.3
Instrumentation details for IR Thermography inspections ……
151
4.8.4
Instrumentation details for forced excitation based modal testing …………………………………………………………..
152
5 TEST RESULTS …………………………………………………………
156
5.1
Results from Phase 1 of Testing – Introduction ……………………….
157
5.1.1
Load Capacity and Stiffness Results …………………………..
158
5.1.2
Deflection Profiles and Crack Patterns ………………………...
168
vi
5.1.3
Strain Profiles ………………………………………………….
178
5.1.4
Comparison of Test Results with Design ……………………...
183
5.2
Results from Phase 2 of Testing – Introduction ……………………….
187
5.2.1
Load Capacity and Stiffness Results …………………………..
187
5.2.2
Deflection Profiles and Crack Patterns ………………………...
198
5.2.3
Strain Profiles ………………………………………………….
208
5.2.4
Comparison of Test Results with Design ……………………...
221
5.3
Results from Phase 3 of Testing – Introduction ……………………….
222
5.3.1
Load Capacity and Stiffness Results …………………………..
222
5.3.2
Deflection Profiles and Crack Patterns ………………………...
231
5.3.3
Strain Profiles ………………………………………………….
248
5.3.4
Comparison of Test Results with Design Basis in Phase 3 ……
259
6 NON DESTRUCTIVE EVALUATION ( NDE) 1 –
THERMOGRAPHY ……………………………………………………..
261
6.1
IR Thermography Inspection – Objectives …………………………….
261
6.2
Inspection Details and Post- Processing of Data ……………………….
262
6.3
Inspection Results from Pultruded Strips in Slab 1 ……………………
268
6.4
Comparison of Thermography Results with Crack Patterns in Slab 1 ...
286
6.5
Inspection Results from Fabric Laminates in Slab 2 …………………..
295
6.6
Comparison of Thermography Results with Crack Patterns in Slab 2 ...
302
6.7
Conclusions ……………………………………………………………
304
7 NON DESTRUCTIVE EVALUATION ( NDE) 2 – MODAL
vii
TESTING …………………………………………………………………
306
7.1
Introduction ……………………………………………………………
306
7.2
Current Modal Test Objectives ………………………………………...
307
7.3
Theoretical Basis of Modal Analysis …………………………………..
310
7.4
Test Setup and Input- Output Data ……………………………………..
313
7.5
Modal Test Results …………………………………………………….
319
7.6
Finite Element Modeling and Initial Parameter Estimation …………...
328
7.7
Model Updating Results ……………………………………………….
333
7.7.1
Segmentation of FE Model for Model Updating ………………
333
7.7.2
Model Updating ………………………………………………..
335
7.7.3
Model Updating Results ……………………………………….
338
7.8
Comparison of Model Updating With Experimental Load- Deflection Results …………………………………………………………………
350
8 FINITE ELEMENT MODELING AND ANALYSIS ………………….
359
8.1
Introduction ……………………………………………………………
359
8.2
Description of the FE Model …………………………………………..
8.2.1
Geometry and Element Types …………………………………
360
8.2.2
Material Properties …………………………………………….
360
8.2.2.1 Reinforced Concrete …………………………………
363
8.2.2.2 FRP Composite ………………………………………
368
8.2.2.3 Connectors …………………………………………...
369
8.2.3
Loading and Solution Control …………………………………
370
8.3
Analysis Results ……………………………………………………….
371
8.3.1
Phase 1 of Test …………………………………………………
371
viii
8.3.1.1 Load- Deflection Response …………………………..
371
8.3.1.2 Damage Progression and Strain Profiles …………….
374
8.3.2
Phase 2 of Test …………………………………………………
381
8.3.2.1 Load- Deflection Response …………………………..
381
8.3.2.2 Damage Progression and Strain Profiles …………….
384
8.3.3
Phase 3 of Test …………………………………………………
390
8.3.3.1 Load- Deflection Response …………………………..
391
8.3.3.2 Damage Progression and Strain Profiles …………….
394
8.4
Summary ……………………………………………………………….
402
9 DESIGN IMPLICATIONS, CONCLUSIONS AND
RECOMMENDATIONS ………………………………………………..
403
9.1
Limitations of the Current Design Methodology ……………………...
403
9.2
Proposed Design Methodology for CFRP Composite Strengthening …
406
9.3
Primary Findings ………………………………………………………
416
9.4
Conclusions ……………………………………………………………
425
9.5
Recommendations for Implementation and Future Research ………………………………..
426
APPENDIX A ……………………………………………………………………
428
REFERENCES ………………………………………………………………….
448
ix
LIST OF FIGURES
1.1
Overview of scope of research …………………………………………...
3
3.1
Onsite rehabilitation of bridge deck slabs ………………………………..
( a) Prefabricated pultruded strips
( b) Site- impregnated fabric laminates
26
3.2
Location of FRP strengthening on test slabs ……………………………..
( a) Specimen S2 with wet layup fabric
( b) Specimen S3 with adhesively bonded prefabricated strips
27
3.3
Test specimens …………………………………………………………...
( a) Specimen S1
( b) Specimen S2
( c) Specimen S3
29
3.4
FE Modeling of the test components ……………………………………..
( a) Finite element model of the bridge frame
( b) Location of test component in the bridge
( c) Finite element model of test component
29
3.5
Comparison of experimental and analytical mode shapes ……………….
( a) Experimental mode shape ( f = 205 Hz)
( b) Analytical mode shape ( f = 205 Hz) from FEM
32
3.6
Test setup …………………………………………………………………
41
3.7
Locations of linear potentiometers ……………………………………….
42
3.8
Locations of accelerometers for vibration tests …………………………..
( a) Setup 1
( b) Setup 2
44
3.9
Locations of strain gages on composite strips/ laminates ………………...
( a) Specimen S2
( b) Specimen S3
45
3.10
Load- displacement plots for test specimens ……………………………...
47
3.11
Progression of deflection in the slabs
( a) With loading at 89 kN and 267 kN ……………………………….
( b) With loading at 445 kN and 623 kN ……………………………...
( c) With loading at 791 kN and 862 kN ……………………………...
49
50
51
3.12
Crack pattern in Specimen 1 ……………………………………………..
52
x
3.13
Crack patterns in strengthened specimens 2 and 3 ……………………….
54
3.14
( a) Development of axial strains in specimen S2 along transverse laminate 3 ………………………………………………………………...
( b) Development of axial strains in specimen S2 along longitudinal laminate …………………………………………………………………..
55
56
3.15
( a) Development of axial strains in specimen S3 along transverse laminate 3 ………………………………………………………………...
( b) Development of axial strains in specimen S3 along longitudinal laminate …………………………………………………………………..
56
57
3.16
Initial thermography inspection of specimens ……………………………
( a) Data acquisition unit
( b) Setup for inspection
58
3.17
Representative detection of pre- existing defects in composite laminates ..
60
3.18
Representative thermal signatures from baseline inspection …………….
( a) Regions used for plotting thermal intensity profiles
( b) Thermal intensity profile in x- direction in Zone 1
( c) Thermal intensity profile in x- direction in Zone 2
( d) Thermal intensity profile in y- direction in Zone 2
63
3.19
Thermal signature over air voids of varying damage severity …………...
( a) Thermal intensity over minor defect
( b) Thermal intensity over major defect
63
3.20
Thermal signature over fabric undulations ……………………………….
64
3.21
Thermal signature at locations of substrate cracks ……………………….
65
3.22
Visual and baseline thermography inspection of slab 3 before testing …..
( a) Visual inspection before test
( b) Baseline thermography data before test
66
3.23
Regions of thermography inspection in slab 3 with damage areas ………
67
3.24
Progression of damage in Zone 1 of longitudinal strip 2 of Slab 3 ………
68
3.25
Appearance of debond areas in Zone 2 of longitudinal strip 2 of slab 3 …
68
3.26
Appearance of debond areas in zone 4 of longitudinal strip 1 of slab 3 …
69
3.27
Appearance of debond areas in zone 3 of longitudinal strip 1 of slab 3 …
69
3.28
Crack pattern in Slab 3 and correspondence with thermography inspection …………………………………………………………………
72
3.29
Visual observation of debonded area in composite strip …………………
73
3.30
Comparison between strain and thermography intensity profiles ………..
( a) Trend of strain profile along length of composite with increase in
75
xi
loading
( b) Damage progression detected from thermography data with increase in loading
3.31
Thermal profiles along length of fabric laminate with loading …………..
76
3.32
Theoretical punching shear failure perimeter in slab S1 …………………
82
4.1
Overall dimensions of the test specimen …………………………………
92
4.2
Reinforcement details ( Cross- section and plan) of the test specimen ……
93
4.3
Internal steel stirrup details in middle longitudinal girder ……………….
94
4.4
Construction of test specimen ……………………………………………
( a) Construction of specimen formwork
( b) Pouring of concrete and finishing of specimen surface
95
4.5
Strength development plot for concrete ………………………………….
95
4.6
Finite element model ……………………………………………………..
99
4.7
Transverse strain contours in model under simulated wheel loads ………
100
4.8
Parameters in sectional analysis for determination of flexural capacity …
101
4.9
Shear demand on middle girder for phase 2 limit load …………………..
103
4.10
Shear demand vs. capacity on middle girder for phase 2 limit load ……..
105
4.11
Manufacturing of composite panels for preliminary material tests ………
106
4.12
Pull- off test specimens …………………………………………………...
( a) Preparation of pull- off specimens
( b) Pull- off failures after tests
108
4.13
Theoretical punching shear failure perimeter …………………………….
110
4.14
Schematic of the strengthening of deck slabs ……………………………
114
4.15
Shear demand on center girder for phase 3 limit load ……………………
116
4.16
Schematic of FRP contribution parameters ………………………………
118
4.17
Schematic of shear strengthening of girder with composite stirrups …….
122
4.18
Schematic of cross- sectional view of strengthened girder ……………….
123
4.19
Shear demand vs. capacity of strengthened girder for phase 3 limit load ..
124
4.20
Cross- sectional schematic of test setup …………………………………..
125
4.21
Schematic of load cell assembly …………………………………………
126
4.22
Test setup details …………………………………………………………
128
4.23
Preparation of the concrete surface before strengthening the slabs ……...
( a) Application of primer coat
( b) Primed surface for composite installation
130
4.24
Installation of pultruded strips in slab 1 ………………………………….
131
xii
4.25
Installation of fabric laminates in slab 2 …………………………………
132
4.26
Manufacturing composite and resin test panels ………………………….
133
4.27
Surface preparation and drilling of anchor holes ………………………...
139
4.28
Installation of composite stirrup and anchor in girder …………………...
141
4.29
Comparison of shear capacities with preliminary and test batch properties …………………………………………………………………
145
4.30
Locations of vertical linear potentiometers ………………………………
146
4.31
Locations of strain gages on deck slabs ………………………………….
147
4.32
Gage locations on longitudinal and shear reinforcement of girders ……...
149
4.33
Locations of gages on composite after slab strengthening ……………….
150
4.34
Locations of gages on composite stirrup after girder strengthening ……..
150
4.35
Non- destructive inspections using IR Thermography ……………………
( a) Thermography data acquisition unit
( b) Thermography setup
152
4.36
Locations of accelerometers for the three setups ………………………...
154
4.37
Forced excitation with three different excitation sources ………………..
155
5.1
Schematic of test plan …………………………………………………….
157
5.2
Load- deflection plots for slab 1 in phase 1 ………………………………
161
5.3
Comparison of load- deflection plots between deck slabs ………………..
161
5.4
( a) Actuator load 1 vs. mid- span deflection plots for middle girder ……..
( b) Actuator load 1 vs. mid- span deflection plots for edge girder
( c) Comparison of load- deflection plots between edge girders
163
5.5
Schematic of deflection of specimen under load …………………………
166
5.6
( a) Deflection along length of middle girder ……………………………..
( b) Deflection along length of edge girder 1
( c) Deflection along length of edge girder
169
5.7
Deflection contour over deck slabs at 400 kN ( 90 kips) …………………
172
5.8
Deflection profile along Row 2 of linear potentiometers ………………...
173
5.9
Deflection profile along Row M of linear potentiometers ……………….
173
5.10
Progression of cracks in the deck slabs below load area …………………
( a) Crack pattern in Slab 1 during Phase 1 loading
( b) Crack pattern in Slab 2 during Phase 1 loading
174
5.11
Propagation of cracks from slabs into girders during Phase 1 loading …..
( a) Propagation of cracks from slabs into Center girder
( b) Propagation of cracks from slabs into Edge girder
176
xiii
5.12
Flexural cracks in the center girder at the end of Phase 1 of test ………...
177
5.13
Shear cracks in the center girder at the end of Phase 1 of test …………...
177
5.14
Strain profile in slab reinforcement at mid- span …………………………
179
5.15
Strain profile in slab reinforcement at 508 mm ( 20”) from mid- span ……
180
5.16
Strain profile in center girder stirrups ……………………………………
182
5.17
Maximum recorded strains in the instrumented steel stirrups along the length of the center girder during phase 1 loading ……………………….
183
5.18
Parameters in sectional analysis for determination of flexural capacity …
185
5.19
Deflections at support locations ………………………………………….
186
5.20
Location of maximum recorded strain in steel stirrups …………………..
188
5.21
Strain profile extrapolation for girder stirrup …………………………….
189
5.22
Load- deflection plots for slab 1 …………………………………………..
191
5.23
Effect of strengthening of slabs on load- deflection plot for slab 1 ………
192
5.24
Comparison of load- deflection plots between deck slabs in phase 2 …….
192
5.25
( a) Actuator load 1 vs. mid- span deflection plots for middle girder ……..
( b) Actuator load 2 vs. mid- span deflection plots for edge girder
( c) Comparison of load- deflection plots between edge girders
194
5.26
( a) Deflection along length of middle girder ……………………………..
( b) Deflection along length of edge girder 1
( c) Deflection along length of edge girder 2
199
5.27
contour over deck slabs at 666 kN ( 150 kips) ……………………………
202
5.28
Deflection profile along Row 2 of linear potentiometers ………………...
203
5.29
Deflection profile along Row M of linear potentiometers ……………….
203
5.30
Progression of cracks in the strengthened deck slabs …………………….
205
5.31
Crack pattern in the center girder at the end of phase 2 of test …………..
207
5.32
Crack pattern in the edge girder at the end of phase 2 of test ……………
208
5.33
Strain profile in slab reinforcement at mid- span …………………………
210
5.34
Strain profile in slab reinforcement at 508 mm ( 20”) from mid- span ……
211
5.35
Strain profile in center girder steel stirrups ………………………………
213
5.36
Maximum recorded strains in the instrumented steel stirrups along the length of the center girder during phase 2 loading ……………………….
214
5.37
( a) Strain profile in transverse composite strips ( T3 and T4) of strengthened Slab 1 for phase 2 loading ………………………………….
( b) Strain profile in transverse composite strips ( T5 and T6) of strengthened Slab 1 for phase 2 loading ………………………………….
215
216
5.38
( a) Strain profile in transverse composite laminates ( T8 and T9) of
xiv
strengthened Slab 2 for phase 2 loading ………………………………….
( b) Strain profile in transverse composite laminate T10 of strengthened Slab 2 for phase 2 loading ………………………………………………..
217
218
5.39
Strain profile in longitudinal strip/ laminate of strengthened slabs ……….
220
5.40
Strain profile extrapolation for ultimate load demand prediction ………..
223
5.41
Load- deflection plots for slab 1 …………………………………………..
226
5.42
Comparison of load- deflection plots between deck slabs in phase 3 …….
227
5.43
( a) Actuator load 1 vs. mid- span deflection plots for middle girder ……..
( b) Actuator load 2 vs. mid- span deflection plots for edge girder
( c) Comparison of mid- span load- deflection plots between edge girders
228
5.44
( a) Deflection along length of middle girder ……………………………..
( b) Deflection along length of edge girder 1
( c) Deflection along length of edge girder 1
232
5.45
Deflection contour over deck slabs at 666 kN ( 150 kips) ………………..
235
5.46
Deflection profile along Row 2 of linear potentiometers ………………...
236
5.47
Deflection profile along Row M of linear potentiometers ……………….
236
5.48
Typical visual inspection of cracks at 846 kN ( 190 kips) ………………..
238
5.49
Visual inspection of cracks at 846 kN ( 190 kips) in slab over center girder ……………………………………………………………………..
239
5.50
Visual inspection of cracks at 846 kN ( 190 kips) at slab girder joint ……
239
5.51
Schematic of punching shear failure perimeters in slabs ………………...
240
5.52
Schematic of intersection of the punching shear cracks with composite ...
241
5.53
Representative damage areas in slab 2 …………………………………...
242
5.54
Representative damage areas in slab 1 …………………………………...
243
5.55
Punch- through of the load through concrete at top of slabs ……………...
244
5.56
Failure planes along transverse cross section of specimen ………………
245
5.57
Failure planes along longitudinal cross section of specimen …………….
246
5.58
Center girder segment at the end of phase 3 loading …………………….
247
5.59
Damaged area in composite stirrup at end of test ………………………..
248
5.60
Strain profile in slab reinforcement at mid- span …………………………
249
5.61
Strain profile in slab reinforcement at 508 mm ( 20”) from mid- span ……
250
5.62
( a) Strain profile in center girder stirrups along bottom line of gages …...
( b) Strain profile in center girder stirrups along top line of gages
251
5.63
( a) Strain profile in transverse composite strips ( T3 and T4) of strengthened Slab 1 for phase 3 loading ………………………………….
( b) Strain profile in transverse composite strips ( T5 and T6) of
254
xv
strengthened Slab 1 for phase 3 loading ………………………………….
255
5.64
( a) Strain profile in transverse composite laminates ( T8 and T9) of strengthened Slab 2 for phase 3 loading ………………………………….
( b) Strain profile in transverse composite laminate T10 of strengthened Slab 2
256
257
5.65
( a) Strain profile in longitudinal composite strip of strengthened slab 1 ...
( b) Strain profile in longitudinal composite laminates of strengthened slab 2 ……………………………………………………………………...
257
258
5.66
Strain profile in composite stirrup of center girder ………………………
259
6.1
Details related to thermography data acquisition ………………………...
263
6.2
Thermal intensity profiles for pultruded strips between locations with and without defects ……………………………………………………….
( a) Thermal decay curve
( b) Thermal intensity differential
264
6.3
Representative damage area in pultruded strip …………………………...
265
6.4
Representative defect/ damage areas in fabric laminate ………………….
( a) Interlaminar air void defects
( b) Debonded area at composite- concrete interface
267
6.5
Thermal decay laminates at locations with and without defects …………
267
6.6
Thermal intensity differentials between areas with and without defects ...
268
6.7
Typical post- installation sub- surface defects detected by visual inspection …………………………………………………………………
( a) Unbonded area
( b) Defect area in strip
269
6.8
Typical sub- surface defects detected by thermography inspection ………
270
6.9
( a) Overall schematic of slab 1 strengthened with pultruded strips ……...
( b) Locations of thermography inspections in Slab 1 …………………….
271
272
6.10
Distinction between modes of debonding as observed visually and from thermography results ( 935- 0 kN stands for thermography inspection carried out on unloading the specimen after reaching load of 935 kN) ….
( a) Debonding at composite- concrete interface
( b) Interlaminar debonding occurring in the composite strip due to separation between the fibers and matrix of the strip
274
6.11
Locations of defect type 1 ( No progression of defects) ………………….
276
6.12
Locations of defect type 2 ( With pre- existing and new defects) …………
277
xvi
6.13
Damage at transverse- longitudinal strip overlap area ( Defect type 3) …...
279
6.14
Damage at concrete- longitudinal strip interface …………………………
280
6.15
Visual observation of damage at the edges of the transverse pultruded strips on unloading specimen after reaching failure load ………………...
282
6.16
Damage at edges of transverse strips ( Defect type 3) ……………………
283
6.17
Damage at concrete- transverse strip interface ( Damage after failure) …...
284
6.18
Debonded area at concrete- transverse strip interface ( Damage after failure) ……………………………………………………………………
285
6.19
Damage areas identified through thermography inspections …………….
286
6.20
Damage regions in slab 1 after removal of cracked concrete …………….
288
6.21
( a) Correspondence of thermography inspections with crack pattern ( Strip 5) …………………………………………………………………...
( b) Correspondence of thermography inspections with crack pattern ( Strip 1) …………………………………………………………………...
( c) Correspondence of thermography with crack in longitudinal direction
289
290
291
6.22
( a) Locations of thermography inspections in Slab 1 used in Figure 6.22 to show correspondence between thermography and visual observations..
( b) Comparisons between thermography and visual inspections in slab 1..
292
293
6.23
Typical post- installation sub- surface defects detected by visual inspections ………………………………………………………………..
296
6.24
Typical shallow defects detected by baseline thermography inspections ..
297
6.25
Typical debonds / deeper defects detected by baseline inspections ……...
298
6.26
Locations of thermography inspections in Slab 2 ………………………..
299
6.27
Damage area observed after failure at location L10 ……………………...
300
6.28
Comparison of inspection results at baseline and after failure …………...
301
6.29
Comparison of failure perimeter with thermography inspection ………...
302
6.30
Visual inspections of damage regions in slab 2 ………………………….
304
6.31
Visual inspection of damage at location L10 of thermography inspection.
304
7.1
Test setup with locations of accelerometers and excitation source ………
314
7.2
Typical force input by the three excitation sources ………………………
( a) Force input by shaker excitation
( b) Force input by impact hammer
( c) Force input by drop hammer
315
7.3
Typical acceleration time histories for the three excitation sources ……..
( a) Acceleration history for shaker excitation
316
xvii
( b) Acceleration history for impact hammer
( c) Acceleration history for drop hammer
7.4
Typical force spectrum for the three excitation sources ………………….
( a) Force spectrum for shaker excitation
( b) Force spectrum for impact hammer
( c) Force spectrum for drop hammer
318
7.5
FRF magnitude plots for Phase 1 of testing ……………………………...
320
7.6
FRF magnitude plots for Phase 2 of testing ……………………………...
320
7.7
FRF magnitude plots for Phase 3 of testing ……………………………...
321
7.8
Trend of frequency ratio over the load stages ……………………………
322
7.9
FRF comparisons from baseline 1 modal test for shaker and hammers ….
324
7.10
FRF comparisons from baseline 2 modal test for shaker and hammers ….
324
7.11
FRF comparisons from baseline 3 modal test for shaker and hammers ….
325
7.12
Mode shapes and complexity plots from baseline 1 modal test results …..
327
7.13
Finite element model of the test specimen ……………………………….
328
7.14
( a) Comparison of deflections below actuator load 1 between test and model ……………………………………………………………………..
( b) Comparison of deflections below actuator load 2 between test and model
( c) Comparison of deflection at center- girder midspan between test and model
332
7.15
Plan view showing regions in the specimen with parameters to be updated in the FE model ………………………………………………….
335
7.16
( a) Locations of regions in the deck slabs used for model updating ……..
( b) Ratio of updated effective modulus in the 5 regions of Slab 1 ……….
( c) Ratio of updated effective modulus in the 5 regions of Slab 2 ……….
( d) Locations of regions in the girders used for model updating ………...
( e) Ratio of updated effective modulus in the 5 regions of center girder ...
( f) Ratio of updated effective modulus in the 5 regions of edge girder 1 ..
( g) Ratio of updated effective modulus in the 5 regions of edge girder 2 ..
( h) Ratio of updated effective stiffness of the 6 springs simulating supports …………………………………………………………………..
345
346
346
347
348
348
349
349
7.17
Comparison of stiffness ratio below load area of slab 1 …………………
354
7.18
Comparison of stiffness ratio below load area of slab 2 …………………
354
7.19
Comparison of stiffness ratio at mid- span of center girder ………………
355
7.20
Comparison of stiffness ratio at mid- span of edge girder 1 ……………...
355
xviii
7.21
Comparison of stiffness ratio at mid- span of edge girder 2 ……………...
356
722
Initial load- deflection responses at S- 5 over load stages ………………...
357
7.23
Initial load- deflection responses at S- 4 over load stages ………………...
358
7.24
Initial load- deflection responses at S- 6 over load stages ………………...
358
8.1
Geometry of the baseline model ( Phase 1) ……………………………….
361
8.2
Location of multi- point constraints at locations of supports ……………..
361
8.3
Geometry of the model with strengthening of the slabs ( Phase 2) ……….
362
8.4
Geometry of model with strengthening of the center girder ( Phase 3) …..
362
8.5
Typical stress- strain curve for concrete …………………………………..
363
8.6
Compressive stress- strain curve of concrete used for model …………….
365
8.7
Tensile stress- strain curve of concrete used for model …………………..
366
8.8
( a) Stress- strain curve of # 3 rebars used for girder stirrups ……………..
( b) Stress- strain curve of # 9 rebars used for girder flexural reinforcement ……………………………………………………………..
( c) Stress- strain curve of # 5 rebars used for slab flexural reinforcement .
367
368
368
8.9
Verification of load- deflection response of slab 1 – Phase 1 …………….
372
8.10
Verification of load- deflection response of slab 2 – Phase 1 …………….
372
8.11
Verification of load- deflection response of center girder – Phase 1 ……..
373
8.12
Verification of load- deflection response of edge girder 1 – Phase 1 …….
373
8.13
Verification of load- deflection response of edge girder 2 – Phase 1 …….
373
8.14
Maximum principal strain vectors in slabs at 214 kN ( 48 kips) – Phase 1.
374
8.15
Maximum principal strain vectors in slabs at 289 kN ( 65 kips) – Phase 1.
375
8.16
Maximum principal strain vectors in slabs at 356 kN ( 80 kips) – Phase 1.
375
8.17
Maximum principal strain vectors in slabs at 400 kN ( 90 kips) – Phase 1.
375
8.18
( a) Comparison of crack patterns in slab 1 below load area from numerical model and test observations at 400 kN ( 90 kips) - End of Phase 1 ……………………………………………………………………
( a) Comparison of crack patterns in slab 2 below load area from numerical model and test observations at 400 kN ( 90 kips) - End of Phase 1 ……………………………………………………………………
376
377
8.19
( a) Comparison of strain profile in slab 1 transverse reinforcement below load ……………………………………………………………………….
( b) Comparison of strain profile in slab 2 transverse reinforcement below load ……………………………………………………………………….
378
378
8.20
Maximum principal strain vectors in center girder – Phase 1 …………....
380
xix
8.21
Comparison of crack patterns in center girder from numerical model and test observations at 400 kN ( 90 kips) - Final load cycle of Phase 1 ……...
380
8.22
Verification of load- deflection response of slab 1 – Phase 2 …………….
382
8.23
Verification of load- deflection response of slab 2 – Phase 2 …………….
382
8.24
Verification of load- deflection response of center girder – Phase 2 ……..
383
8.25
Verification of load- deflection response of edge girder 1 – Phase 2 …….
383
8.26
Verification of load- deflection response of edge girder 2 – Phase 2 …….
384
8.27
Maximum principal strain vectors ( NE > 0.0001) in center girder – Phase 2 …………………………………………………………………………..
385
8.28
Comparison of crack patterns in center girder from numerical model and test observations at 667 kN ( 150 kips) – Final load cycle of Phase 2 ……
386
8.29
Nominal strain ( N11) contours in center girder stirrups at 667 kN ………
387
8.30
Comparison of highest strain vs. load response in center girder stirrup from analytical model and strain gage data during Phase 2 ……………...
388
8.31
Strain profile comparison in slab transverse reinforcement – Phase 2 …...
389
8.32
Comparison of strain profile in slab composite reinforcement – Phase 2 ..
390
8.33
Verification of load- deflection response of slab 1 – Phase 2 …………….
392
8.34
Verification of load- deflection response of slab 2 – Phase 2 …………….
392
8.35
Verification of load- deflection response of center girder – Phase 2 ……..
393
8.36
Verification of load- deflection response of edge girder 1 – Phase 2 …….
393
8.37
Verification of load- deflection response of edge girder 2 – Phase 2 …….
394
8.38
Principal strain contours in the composite systems at 930 kN – Phase 3 ...
395
8.39
Comparison of strain profile in slab composite reinforcement – Phase 3 ..
395
8.40
Maximum principal strain contours at slab bottom at 930 kN – Phase 3 ...
397
8.41
Crack pattern at bottom of slabs below load areas from test observations.
398
8.42
Minimum principal strain contours on slab top at 930 kN – Phase 3 ……
399
8.43
Crushing of concrete on slab top below load at 930 kN – Phase 3 ………
399
8.44
Comparison of highest strain vs. load response in center girder steel stirrup from analytical model and test data for 667 kN load cycle during Phase 2 and Phase 3 ………………………………………………………
400
8.45
Comparison of highest strain vs. load response in center girder steel stirrup from analytical model and test data for 667 kN load cycle during Phase 2 and Phase 3 ………………………………………………………
401
A. 1
Mode shape and complexity plots after unloading from 214 kN – Phase 1
429
A. 2
Mode shape and complexity plots after unloading from 289 kN – Phase 1
430
xx
A. 3
Mode shape and complexity plots after unloading from 400 kN – Phase 1
431
A. 4
Mode shape and complexity plots from Baseline 2 modal test – Phase 2 ..
432
A. 5
Mode shape and complexity plots after unloading from 400 kN - Phase 2.
433
A. 6
Mode shape and complexity plots after unloading from 578 kN - Phase 2.
434
A. 7
Mode shape and complexity plots after unloading from 668 kN - Phase 2.
435
A. 8
Mode shape and complexity plots from baseline 3 modal tests – Phase 3.
436
A. 9
Mode shape and complexity plots after unloading from 668 kN - Phase 3.
437
A. 10
Mode shape and complexity plots after unloading from 756 kN - Phase 3.
438
A. 11
Mode shape and complexity plots after unloading from 846 kN - Phase 3.
439
xxi
LIST OF TABLES
3.1
Failure Loads and Mode for Test Specimens …………………………….
40
3.2
Predicted and experimental failure Loads for test specimens and bridge slabs ………………………………………………………………………
85
4.1
Performance characteristics for reinforcing steel ………………………...
94
4.2
Test phases ……………………………………………………………….
97
4.3
Preliminary material properties of FRP composite ………………………
107
4.4
Preliminary bond properties of composite ……………………………….
108
4.5
Tensile test properties of composite from slab strengthening ……………
134
4.6
test properties of pure resin from slab strengthening …………………….
134
4.7
Pull- off test results from slab strengthening ……………………………...
135
4.8
Batch ratios of properties between preliminary and test panels ………….
136
4.9
Recalculated ultimate moment capacities of the strengthened slabs ……..
138
4.10
Tensile test properties of composite ……………………………………...
143
4.11
Pull- off test results from girder strengthening …………………………...
143
4.12
Batch ratios of properties between test and preliminary design panels ….
144
4.13
Performance details of the capacitive accelerometers ……………………
153
5.1
Test phases ……………………………………………………………….
156
5.2
Loading protocol for phase 1 …………………………………………….
160
5.3
( a) Effective stiffness ratio in slabs in phase 1 from linear potentiometer data ……………………………………………………………………….
( b) Effective stiffness ratio in girders in phase 1 from potentiometer data
167
5.4
Loading protocol for phase 2 ……………………………………………
190
5.5
( a) Effective stiffness ratio in slabs in phase 2 from linear potentiometer data ……………………………………………………………………….
( b) Effective stiffness ratio in girders in phase 2 from potentiometer data.
197
198
5.6
Loading protocol for phase 3 ……………………………………………..
224
5.7
( a) Effective stiffness ratio in slabs in phase 3 from linear potentiometer data ………………………………………………………………………
( b) Effective stiffness ratio in girders in phase 3 from potentiometer data
230
6.1
Defect types and locations as identified from thermography inspections ..
275
xxii
6.2
Summary of thermal signatures for defect types …………………………
294
7.1
Load stages at which modal testing was carried out ……………………..
309
7.2
Natural frequencies of the test specimen over the load stages …………...
321
7.3
Estimated reinforced concrete properties for slab and beam elements …..
329
7.4
Initial stiffness estimates of support spring elements …………………….
331
7.5
Comparison of frequencies and MAC between the test specimen and the model ……………………………………………………………………..
340
7.6
Deflections and stiffness ratios obtained for Slabs under 2.25 kN load ….
352
7.7
Deflections and stiffness ratios for Girders under 2.25 kN load …………
353
8.1
Tensile test properties of composites …………………………………….
369
8.2
Stiffness estimates of grounded connectors at the supports ……………...
370
9.1
Proposed FRP composite strengthening methodology for slab- girder systems …………………………………………………………………...
407
9.2
Symbols and notations used in Table 9.1 ………………………………...
414
A. 1
( a) Mode 1 normalized modal amplitudes over the load stages ………….
( b) Mode 2 normalized modal amplitudes over the load stages ………….
( c) Mode 4 normalized modal amplitudes over the load stages ………….
( d) Mode 6 normalized modal amplitudes over the load stages ………….
440
441
442
443
A. 2
( a) Updated parameters for Phase 1 of testing …………………………...
( b) Updated parameters for Phase 2 of testing …………………………...
( c) Updated parameters for Phase 3 of testing …………………………...
444
445
446
xxiii
ABSTRACT
There is growing use of FRP composite materials in the civil infrastructure for rehabilitation of deficient bridge components including deck slabs and girders. However assessment of the effectiveness of rehabilitation over time and monitoring the progression of damage or change in load paths between the structural components, caused by sequential strengthening of the components, has not been undertaken to date. Investigation was first carried out at “ component level” on both unstrengthened and field- rehabilitated slab specimens cut out from a major highway bridge. The test data on the failure modes and capacity loads were correlated to the available analytical models and design guidelines. The test capacity was also correlated to the bridge deck capacity based on local- global modeling. Research at the “ systems level” was then undertaken, in which a three- girder two- span bridge deck system was tested to simulate behavior under field loading in which the deck slabs are found to be susceptible to punching shear type failures and the longitudinal girders are usually found to be deficient in terms of shear demand. The objective of the study was to evaluate damage progression in the deck slabs and the longitudinal girders under simulated truck load and to detect changes in the overall response of structure at systems level caused by strengthening of individual components that might cause other components to reach their critical limit states under the higher load demands which can be resisted by the strengthened components. NDE techniques were evaluated as means to quantify the damage localization and progression under simulated field loading as well as to quantitatively monitor changes in the response of the components, caused by subsequent modifications of the structure, at systems level. Based on the limitations of the available design guideline for FRP strengthening, a modified design methodology was proposed for FRP strengthening of slab- girder systems. xxiv
1 INTRODUCTION
A considerable number of the existing reinforced concrete bridge inventory in the United States is classified as structurally deficient or in serious need of repair and strengthening. Data from the US National Bridge Inventory indicates that in the federal aid system, which includes about 276,200 federally maintained bridges, 40% of all bridges are 15- 35 years old [ 1]. Thus most of these bridges have been subjected to significant periods of loading over their life span complemented by the increase in the number of the load bearing trucks and the weight carried by them. Moreover, over the life- span of the bridges there have been modifications in the design standards and thus components such as the deck slabs and the longitudinal girders are often found to be deficient in satisfying the truck load demands per the new design codes. The situation is made worse by the deterioration of the existing infrastructure due to environmental exposure, ( which includes extensive use of deicing salts, variations in temperature, etc.). All of these have caused extensive cracking and strength degradations to occur in a large number of these bridges. It is estimated that out of about 575,000 highway bridges in the United States, 230,000 are rated as structurally deficient or functionally obsolete and are thus in need of replacement or serious rehabilitation work [ 1].
The rapid deterioration of the infrastructure and the limited funding available for infrastructure maintenance has promoted the use of newer materials such as Fiber Reinforced Polymer ( FRP) composites as an optimized repair and strengthening technique. In this method, the FRP material ( glass or carbon) is externally bonded to the concrete surface of the deficient structural component using epoxy adhesives. The high
1
strength/ stiffness- to- weight ratio, tailorable mechanical properties, corrosion resistance and ease of installation with limited disruption of traffic are some of the well recognized advantages of the composite materials in bridge rehabilitation applications. All the above benefits have projected FRP repair and strengthening as a very promising technique for the rehabilitation/ strengthening of deficient reinforced concrete structural components in bridges. A state- of- the art survey of FRP composites for construction applications in civil engineering can be found in [ 2] and is hence not repeated herein.
1.1 Problem Statement
While a significant amount of research has been conducted on the use of externally bonded FRP composites on individual components there has been almost no laboratory based research at the systems level. There is also lack of information on the long- term ( 10+ years) in- field performance of FRP strengthened composite structures. As with all strengthening techniques which modify structural response there is a concern that changes made to a single component could cause inadvertent damage to the other components of the system either as a result of stress redistribution or through the failure of a “ weak link” in the unstrengthened portions of the structural system. This research is aimed at investigating systems level response resulting from sequential rehabilitation of components, and developing guidelines for design, which would complement the existing composite strengthening guideline [ 3], from the study relevant to slab on girder bridge systems. In addition the use of IR thermography and modal analysis, shown to be effective through prior limited field study of FRP strengthening, as non- destructive techniques to infer damage initiation and progression is also an aim of the study.
2
1.2 Scope of Current Research and Report Organization
The current research was divided into two phases. A flowchart outlining the overall scope of the research is shown in Figure 1.1. Component tests of field strengthened slab sections Analytical and FE modeling – Capacity predictions Modeling Comparison of Analytical predictions and Experimental results Capacity estimation and monitoring damage progression through instrumentation and NDE Testing Local to global modeling to predict capacity of the actual bridge deck Need to study effect of FRP strengthening of components on overall structural performance Systems level test of 3 girder- 2 span slab- on- girder bridge segment Analytical and FE modeling – Capacity predictions Modeling Testing Phase I Phase II Phase III Damage in slabs followed by strengthening of slabs with FRP Damage in girder followed by strengthening of girder with FRP Failure of strengthened slabs and damage at slab- girder joint Capacity estimation and monitoring damage progression through instrumentation and NDEComparison of analytical/ FEM predictions with test results and correlating damage progression measured through NDE with visual observations and test results Development of a Design and Monitoring Methodology
Figure 1.1 Overview of scope of research
3
Phase 1 involved component level study of field specimens through testing of slab sections cut- out from a bridge, both with externally bonded FRP, and without strengthening, after being in service for a period of time, thereby enabling assessment of the effectiveness of strengthening after being subjected to field representative loading and environmental exposures. The primary test objective was to determine the behavior and failure capacity of the deck slabs, representative of bridge deck components subjected to realistic deterioration and damage over time, to evaluate long- term performance of FRP composite strengthening. Available analytical models were used to predict the ultimate capacity of the test specimens and were compared with the test results. Local to global modeling was also used to predict the ultimate capacity of the actual bridge deck slabs from that of the test components. The failure mechanisms and progression of damage was monitored and characterized using experimental results and NDE inspections by thermography and modal testing. Further details of the component level study and the research findings have been presented in Chapter 3.
Phase 2 of the research involved assessment of FRP composite strengthening at the systems level through testing of a three- girder two bay reinforced concrete bridge deck segment under field representative loading conditions, with sequential strengthening of the slab and the girder with FRP composites. This phase of study ensued from the research review and the findings from the component level study, that there is a need for a large scale systems level testing through which the effects of progression of damage and changes in the load distribution/ failure mechanism caused by the strengthening of individual components could be assessed in terms of overall system performance. The test specimen details, preliminary design predictions and composite strengthening details
4
are presented in Chapter 4. The test protocol and the test results are presented in Chapter 5. Two NDE tools, Thermography and Modal Testing were used to quantitatively monitor the progression of damage and effects of the sequential strengthening of components on the structural performance through inspections at regular load intervals. The thermography results were used to monitor the appearance and progression of damage at the local level at the composite- concrete interface in the deck slabs with an increase in loading. The modal tests were used to determine the dynamic characteristics of the structure which were then used for subsequent model calibration and updating to predict the structural response in terms of degradation/ enhancement of the stiffness at discrete locations of the test specimen by taking into account the effect of damage progression and sequential strengthening of the specimen with FRP composites. The NDE results were correlated to the visual observations of crack patterns and failure mechanisms as well as to strain and displacement data measured during the test. The results of the thermography and modal testing inspections are presented in Chapters 6 and 7, respectively. Analytical modeling of the test specimen behavior and sequential strengthening of components over the load stages was carried out for capacity and response prediction and subsequent comparisons with the test results and this is presented in Chapter 8. A design methodology taking into account the overall structural response in the FRP strengthening design and incorporating the results of NDE methodologies for periodic structural condition monitoring are then be presented in Chapter 9.
While the specific thrust of this research is aimed at bridge structures with an emphasis on slab- on- girder systems it should be noted that the approach is general enough to allow appropriate extension to other systems, including in buildings.
5
2 OVERVIEW OF THE STATE OF THE ART IN SYSTEMS LEVEL STRENGTHENING WITH FRP COMPOSITES
2.1 Introduction
The rapid deterioration of the bridge infrastructure and the limited funding available for their maintenance has promoted the use of Fiber Reinforced Polymer ( FRP) composites as one of a range of optimized repair and strengthening techniques. Some of the benefits associated with the use of these materials in civil infrastructure renewal were discussed in the previous chapter. Considerable research has been carried out at the components level to successfully demonstrate through laboratory tests of externally bonded FRP overlays or strips, the effectiveness of these materials for flexural/ shear strengthening of beams and slabs as well as wrapping of columns for seismic retrofitting [ 2]. However research at the systems level is sparse and thus will be the primary focus of this review, as the basis of the current research. Some examples of component level applications are first summarized next in this chapter.
2.2 Strengthening of RC Components with FRP Composites
2.2.1 Flexural Strengthening of Beams and Slabs
Extensive research and field implementation has been carried out on RC beams and slabs strengthened with externally bonded- FRP reinforcement. This involves bonding the composite laminates to sections of the structural components in tension, with fibers in the composite parallel to the principal stress direction [ 2]. The early works on the strengthening of RC beams with composite plates externally bonded to the tension
6
flanges [ 4] indicated that such strengthening can result in significant enhancements in strength and stiffness of the beam. This increase is due to the resistance of the externally bonded composite laminate to the opening of flexural cracks or the formation of new cracks. Reviews on the topic have been published recently [ 5, 6, 7, 8] and will thus not be repeated herein. The gain in flexural strength of FRP strengthened beams depends on a number of factors such as the type of FRP used, the fiber volume fraction, the fiber orientation, concrete strength, proper anchorage of the composite reinforcement [ 9, 10] and mode of failure. Models [ 11, 12] and design guidelines [ 3] have also been developed to better predict the debonding strain levels at the composite- concrete interface based on the experimental data of FRP strengthened beams.
The promising applications of externally bonded composites for flexural strengthening of RC beams led to the extension of this application towards the strengthening of RC slabs. A large number of the medium and short span bridges in North America are constructed with slab- on- girder decks in which the reinforced concrete deck slabs are supported by several steel or concrete girders [ 13]. Since the distance between the girders is typically less than the span of the bridge, the slabs are subjected to one- way load transfer mechanisms in which the load on the deck slabs is transferred directly in the transverse direction to the supporting girders resulting in a positive bending moment between the beams and negative bending moment over the beams. Research on the flexural strengthening of one- way slabs with externally bonded FRP in the positive moment regions have been carried out at the component level on representative deck slabs [ 14, 15, 16, 17]. For flexural strengthening of the slabs in the negative moment regions, use of near surface mounted reinforcement in the form of 7
CFRP bars installed into grooves cut into the concrete surface have been reported [ 14]. Slabs with low or medium reinforcement ratios are usually found to fail in flexure rather than in punching shear [ 18]. Failure modes for slabs with reinforcement ratios of 1% or higher are usually found to be governed by a punching shear type mode of failure [ 19]. Thus design of the FRP composite needs to take this into account to realize the full effectiveness of the strengthening scheme.
2.2.2 Shear Strengthening of Beams
The use of externally bonded FRP at locations of high shear stresses has been demonstrated to contribute to increasing the shear capacity of RC beam components. Results of research on the shear strengthening of beams with FRP have been reported in [ 20, 21, 22, 23]. The strength gain depends on the type of FRP used, fiber volume fraction, the fiber orientation, FRP reinforcement ratio and use of mechanical anchors to prevent premature debonding of the composite [ 24, 25]. A detailed review of research on the shear strengthening of RC beams with externally bonded FRP composites can be found in [ 26] and hence is not repeated here.
2.3 Effect of Strengthening of RC Bridge Components at Systems Level
Most research on FRP composite strengthening conducted to date has been directed at the component level. Thus the focus of study has been the local response of components in terms of crack patterns, failure mechanisms and enhancement in strength and ductility. All these test results give a good understanding of the component response and help in building the confidence level in the user/ owner community to accept the use
8
of this new construction material. However, in a bridge system the strengthening of only a single component can cause significant stress re- distribution and can result in changes in overall structural performance. There is also a concern that a change to a component can result in unintended consequences to adjoining components. In addition, the failure mechanisms and structural response of a component at the systems level might be different than that if it was to be treated on its own since it will be affected by the global load distribution rather than by local load application. This necessitates further study of the effectiveness of FRP strengthening and progression of damage at the systems level through large scale systems tests and focused demonstration studies through actual in- field application and monitoring of the rehabilitated systems.
2.3.1 Relevant Systems Level Tests Demonstrating Effect of Stress Re- Distribution and Damage Progression on Overall Structural Behavior
The interaction between the different structural components of a slab- on- girder bridge towards the overall structural performance at the systems level has been reported by researchers based on systems tests. It has been recognized that in a slab- on- girder system, both the slabs and girders are equally susceptible to damage under traffic load and these damages are interlinked. The damage in a girder will increase the residual deformations which in turn can cause damage in the slabs, particularly if there are large deformation differentials between the adjacent girders [ 27]. In general the actual load demands on the deck slabs depend not only on the magnitude of the wheel load and the girder spacing but also on the stiffness of the girders and the span length of the bridge 9
[ 13]. Also in the slab- on- girder system the failure mechanism will be greatly influenced by the relative strengths of the slab and beam components.
Oh et al. [ 28] evaluated the ultimate load behavior of an existing prestressed concrete slab- I- girder bridge through an in- place failure test. The load was applied to the actual bridge at the site using hydraulic jacks and the load pattern simulated a single truck. The first occurrence of flexural cracking occurred at the bottom of the girders in the mid- span region at a load of 313 kN and therafter the girders experienced gradual stiffness degradation due to crack development. With further loading, at 1176 kN, major shear cracks were observed to develop at quarter point regions of the girders. At a load of 1960 kN, cracks were found to develop at the interface between the slab and the median strip which was cast monolithically with the slab at construction. At 2350 kN, the concrete median strip on the deck slab was detached and this resulted in redistribution of stresses due to reduction of stiffness of the slab component. This was followed by the compressive crushing failure of the slab in the loaded area at 4312 kN. The test results indicated that progression of damage can occur between the components of the bridge system depending on the relative strength/ stiffness of the components.
The behavior of multi- span slab bridges before, during and after repair was investigated by Shahrooz et al. [ 29] through study of the level of moment distribution during the various stages of repair. The study was carried out on in- service bridges in the field with varying amounts of deterioration over the pier lines in the form of spalling of concrete and loss of top steel in the negative moment regions of the deck slabs resulting in loss of continuity between the adjacent spans and thus increasing the positive moment demand and overstressing the slab bottom reinforcement. The repair method used
10
involved removal and replacement of the damaged concrete and top steel. However this repair method was seen to often result in further overstressing of the bottom steel and thus the purpose of the study was to monitor the response of the bridge both during and after repair through truckload tests. The removal of the concrete and damaged steel in the negative moment area during the repair was found to cause redistribution of moment resulting in an increase of the positive moment resisted by the slabs by about 38%. The repair method was found to enhance the participation of the top reinforcement, thereby increasing their contribution towards resisting applied negative moments by 36% as compared to before repair. However in the positive moment regions there was a permanent redistribution of live load moment and the bottom reinforcement resisted 22% more moment as compared to that before repair. The test results indicated that although the repair methodology was able to improve the stiffness and participation of the negative reinforcement it resulted in a moment redistribution increasing the moment demand on the positive reinforcement. It was suggested that shoring of the deck slabs should have been carried out during the repair to prevent such moment redistribution.
Issa et al. [ 30] studied the behavior of full- depth precast concrete panels for bridge rehabilitation. In this rehabilitation method, the damaged deck slabs are replaced by precast concrete panels that can be installed on the existing concrete or steel girders and connected by steel studs through shear pockets for composite action. The deck panels are then post- tensioned in the longitudinal ( traffic) direction of the bridge to provide continuity and secure tightness of the joints between the adjacent precast elements. The test system represented a single lane scaled down model of a two- span continuous prototype bridge with two lines of supporting beams. Three such models were tested, the
11
first being constructed without any post- tensioning in the deck system, the second being post- tensioned with a prestress level of 1.43 MPa ( 208 psi) in the deck system and the third had a larger prestress level of 2.62 MPa ( 380 psi). The presence of the pre- stressing in the second and third models were effective in delaying the initiation of cracks in the concrete deck panels as compared to the first model and increased the load capacity from 390 kN to 480 kN. The cracks in the deck panels of the first two models were found to initiate at the vicinity of the central supports and with an increase in loading gradually developed away from the supports with ultimate failure in the deck systems. However for the third specimen with the higher level of prestressing force, the initiation of cracking and damage in the deck slabs was delayed and ultimate failure at the systems level was shifted from the slabs onto the supporting steel girder with the development of a crack in the web of the girder between the top and bottom flanges. Thus even though at the component level, the deck slabs of the third specimen had higher resistance imparted by the higher level of prestressing, at the systems level no strength enhancement was achieved as compared to the second specimen since the damage progression and subsequent failure at the girder prevented the slab component from reaching full capacity.
It has also long been recognized in seismic design that by increasing the strength and ductility of critical components in a system their brittle and catastrophic failure can be prevented and the occurrence of more desirable failure mechanisms can be promoted in other components of the system [ 31]. Considerable research has been carried out in this regard on beam- column connections of RC building frame structures. However results from these tests can also be extended to bridge systems with the beam- column joints in buildings being analogous to pier column- cap beam joints in bridges. These
12
results also help to give an understanding of the progression of damage between components of a system caused by the sequential strengthening of the components. Most such existing building frame structures with non- seismic detailing are found to be dominated by weak column- strong beam behavior [ 32]. In such a structural system, the moment capacity of the beams strengthened by the participation of the slabs places high moment demands on the columns. Due to inadequate ductility of such existing columns, premature structural failure can result under lateral loads. Thus a retrofit strategy for such a structural system involves strengthening and adding ductility to the columns through the use of steel or composite jackets in the potential plastic hinge regions. However such a local strengthening of the structural system by enhancing the strength and ductility of the columns will move the failure to occur in the beam- column joint. In the presence of proper detailing of the joints, moving the failure from the column to the joint can improve the global system behavior [ 33]. However in the absence of proper detailing, shear failure in the joint can be brittle and catastrophic and thus subsequently requires the joint to be strengthened. Combined use of FRP laminates and near surface mounted FRP bars have been proposed to be used in the joint region to enhance its strength and ductility [ 34] and thus moving the failure from the beam- column joint to the beam. Thus the driving criterion behind such sequential strengthening of components is the hierarchy of strength such that by strength enhancement of those members whose failure is not desirable, it is possible to attain a ductile global performance of the system.
The above methodology of hierarchy of strength governing the seismic design of building frames has also been extended to bridge systems. However the one significant difference between the seismic retrofit of building frames and bridge systems is that for
13
the bridge systems, the desired failure mode is through formation of plastic hinges at the top of lower columns in an area that can be well confined for ductile response and can be inspected and repaired following a major earthquake [ 35]. In existing bridge systems designed before the development of stringent seismic design standards, the column- cap beam joints are usually found to have insufficient shear detailing. Thus retrofits of the joints are often suggested in the form of fiber reinforced concrete jacketing [ 36], casting of post- tensioned reinforced concrete bolsters in the joint regions [ 37], removal of existing columns and beam column joints and replacing them with well- confined circular columns and properly detailed joint region and post- tensioning of the cap beam [ 35]. Such retrofit strategies transfer the brittle failure at the joints to the columns of the bridge systems and these columns can then be retrofitted with steel, concrete or composite jackets to result in a ductile mode of failure in the well confined plastic hinge regions of the columns [ 31].
Based on the above discussion it is evident that once a component of the system is strengthened or retrofitted the failure will move or “ progress” along this hierarchy to the next weak component until ultimate failure in one of the components is achieved. A similar analogy can be applied to FRP composite strengthening of slab- on- girder system subjected to traffic loading. Based on the relative strength of the slab and girder, the initial failure will occur in the weakest component. However it is necessary to recognize that if only that component of the bridge superstructure- system ( e. g. the deck slabs or the supporting girders or other structural components such as diaphragms or slab- girder joints) is strengthened with FRP without understanding and considering the limiting capacity of the adjoining un- strengthened components, then there will be a possibility of 14
damage progression following the “ hierarchy of strength” approach to the next weak link of the system that will prevent the strengthened component from reaching its ultimate design capacity. The strengthening of a component with FRP composite also has the potential to result in changes in load- distribution and failure mechanism at the systems level as was discussed earlier since it can be treated as a structural modification. All the above emphasizes the need for an assessment of effectiveness of FRP composite strengthening of slab- girder components at the systems level to evaluate the progression of damage and change in load distribution produced by such strengthening and finally to determine the ultimate global load capacity of the system.
2.3.2 Existing Systems Level Research in FRP Composite Strengthening of Bridge Components
Of particular interest to the current research is the slab- on- girder segment of bridge superstructure which is most prone to degradation under traffic loading and environmental exposure and is also rendered strength deficient under increasing demands of truck loads and design standard requirements. Thus the deck slabs or the longitudinal supporting girders of typical existing RC bridges are often found to be in need of strength enhancement. The general trend in the field strengthening projects with externally bonded FRP composites is to strengthen only that component of the structure that shows the more imminent signs of damage and deterioration through visual inspections or non- destructive evaluation ( NDE) tests.
Some of the field applications, at the systems level, of externally bonded FRP composite in strengthening of bridge girders have been reported in [ 38, 39, 40, 41 and
15
42]. Miller et al. [ 38] reported the application of CFRP plates for the flexural strengthening of a steel girder of a slab- on- girder bridge on I- 704 in Delaware. One layer of CFRP plate was bonded to the tension flange of a steel girder. To demonstrate the effect of the retrofit on the global flexural stiffness, load tests were performed before and after application of the CFRP plate. The test measured strains in the girder as a three- axle truck was driven over the slab- on- girder bridge before and after the girder strengthening. Comparison of the load test data indicated that the addition of the CFRP plates resulted in an 11.6% increase in the global flexural stiffness of the slab- girder system.
Cardinale and Orlando [ 39] described the rehabilitation of a deficient RC bridge in Italy through application of a concrete overlay on the deck slabs, external prestressing with tendons for flexural strengthening of longitudinal girders and installation of woven CFRP composite fabric for shear strengthening of the girders. The bridge was load tested before and after the rehabilitation. Based on measurements of deflections, the static tests indicated that the stiffness of the strengthened bridge was increased by about 20% after the rehabilitation work. Vibration tests were also carried out to dynamically obtain measurement of the global stiffness. The first natural eigenfrequency was found to increase from 2.46 Hz to 2.7 Hz and based on FE modal analysis this also corresponded to about 20% increase in the global stiffness.
Hag- Elsafi et al. [ 40] conducted in- service evaluation of a FRP strengthening system in a RC T- beam bridge in New York. CFRP laminate systems were used to improve the flexural and shear capacities of the bridge system and restrain freeze- thaw cracking through strengthening of the girders with U- jackets and strengthening of the bottom soffit of the slabs between the girders with laminates in the transverse direction. 16
The bridge was instrumented and load tested before and after installation of the FRP laminates as well as after a period of 2 years after the rehabilitation to evaluate effectiveness of the strengthening systems. For a truckload of 196 kN, the strains in the girder rebars below the load area were reduced by about 5%. Also the live load distribution factors of the girders were found to improve by about 12% after installation of the composite systems indicating better load distribution between the slabs and the girders. It was commented that the benefits of the laminate systems used in the project were not fully realized because of the relatively small service loading range from the truckload application. Also the strain measurements obtained after the composite system was in service for two years showed no changes from those obtained after the rehabilitation, indicating that the composite systems did not undergo any degradation during the studied period of in- field service.
Hutchinson [ 41] reported shear strengthening of I- shaped prestressed concrete girders with CFRP sheets for the Maryland Bridge in Winnipeg, Manitoba and John Hart Bridge in British Columbia. A single CFRP layer was applied over a 4 m length near the ends of each girder of the John Hart Bridge. The shear capacity of the beams was reported to be increased by 15- 20%. However no results were presented on the effect of the strengthening on the overall stiffness of the slab- girder system.
An evaluation of the field performance of FRP bridge repairs was also reported by Stallings et al. [ 42]. The seven span bridge studied under the investigation was located in Alabama and the girders exhibited a well developed system of flexural cracks with minor spalling. These cracks extended from the bottom of the girders up to the underside of the deck slabs. One span of the bridge was repaired through installation of CFRP plates on
17
the bottom surface of the girders with the objective of mitigating the deterioration of the bridge resulting from the flexural cracks in the girders as well as to increase the load capacity of the bridge. Static load tests were performed before and after the repair using trucks with a gross weight of 346 kN and deflection and strain measurements were recorded. The application of the FRP systems was found to reduce the rebar stresses in the girders by an average of 8% and the maximum girder deflection by an average of 7%. Based on these measurements, the effective girder moment of inertia was calculated to have increased by 5% after the FRP repair.
Field applications of externally bonded FRP composite in strengthening of deck slabs in slab- on- girder bridges, though fewer than the applications on girders, have been reported by [ 27, 43, 44]. Schuman et al. [ 43] reported the strengthening of the deck slabs of a cast- in- place concrete T- girder bridge in California. The deck slabs were found to be susceptible to punching shear type failure due to the formation of evenly spaced longitudinal and transverse cracks at the slab soffit. The strengthening was designed based on the calculated internal steel reinforcement deficiency in the deck slabs. Two rehabilitation systems, namely CFRP fabric laminates and pultruded strips were bonded to the bottom soffit of the deck slabs in 5 spans of the bridge for comparative evaluation of the two systems. Forced vibration tests were performed on the bridge immediately before and after the completion of the rehabilitation of the deck slabs. The structural stiffness was computed through model updating based on the measured eigenvalues. The addition of the composite systems was found to result in stiffness enhancement of the bridge system, with the increase being a function of the amount of degradation in the
18
slabs. A maximum stiffness increase in the range of 32% was recorded in a slab span with the highest level of degradation.
Similar application of composite materials for strengthening deck slabs was reported by Lee et al [ 44]. The application was in the deck slabs of a RC T- girder bridge in which extensive longitudinal and transverse cracks in the deck slabs indicated potential for punching shear failures. Several bays of the bridge were strengthened with carbon fabric laminates and pultruded strips. The strengthening of the deck slabs resulted in stiffness enhancements with the greatest increase recorded being 29%. All the estimates of stiffness enhancements were based on global measurements through dynamic modal tests and thus reflected the effect of strengthening of the deck slabs on the slab- girder system as a whole.
Oh et al. [ 27] reported the assessment of bridge deck panels with the deck slabs strengthened with CFRP sheets. The deck slabs were found to be deficient in biaxial flexure with cracks developing in the longitudinal and transverse direction. Prototype deck panels supported on two edge girders simulating the actual bridge decks were constructed and tested. Two strengthening variables were studied in the research, namely the strengthening ratio and direction of the composite laminates. The strengthening of the deck panels by FRP was found to restrain the opening of the cracks and resulted in enhancement of flexural strength and stiffness. The deck panels strengthened in both the longitudinal and transverse directions had the best overall performance and displayed ductile failure modes in biaxial bending with the formation of numerous small cracks. A maximum flexural strength enhancement of 37% was recorded for these deck panels. 19
However no information was provided on the effect of the strengthening on the overall structural stiffness or performance.
2.4 Conclusions
Based on the review of existing research on applications of composite strengthening at the systems level it was found that no comprehensive study has been carried out on the effect of such strengthening of components on other components of the system. Even though the applications were at the systems level, the focus of study was essentially the performance of the component that showed the most degradation and damage in the system. No observations were made in any of the reported research on whether such strengthening of a component caused the other components to prematurely reach critical limit states under the higher load demands. The primary drawback seems to be that once the effectiveness of the externally bonded FRP composites in enhancing the flexural or shear strength/ stiffness of the slab or girder components was established through laboratory testing and research, the applications were extended directly to the field without exhaustive laboratory research and testing on the performance of such strengthening at the systems level. Even though valuable data on the effectiveness of FRP composite strengthening of slab or girder components at the systems level were obtained through the field applications as described previously [ 38 - 44], such assessment of global performance was limited to computations of global stiffness and changes produced by the application of composites. The true load capacity can not be obtained realistically from such condition assessments in service because the safety assessment and load tests in such field applications were conducted under service load so as not to produce any
20
substantial damage to the structure. Thus it was not possible to conclude conclusively based on the existing research on whether such strengthening of individual components had the potential to cause unintended consequences on the un- strengthened components of the system through changes in load distribution or under the higher load demands imposed by the strengthened components.
A review of existing research and the state of the art on strengthening of slab- on- girder bridge components with FRP composites emphasizes the need for a large scale systems level test of slab- on- girder segment through which the effects of progression of damage and changes in the load distribution/ failure mechanism in the components on the overall system performance could be studied under application of field- representative loading conditions and with sequential strengthening of individual components.
21
3 COMPONENT LEVEL STUDY OF FIELD SPECIMENS
3.1 Introduction
The efficacy of externally bonded composites in strength enhancement of reinforced concrete components has been well established through previous research, as was discussed in the literature review. This included evaluation of strengthening of RC slabs [ 15, 45] as well as shear strengthening of longitudinal girders [ 26, 20] with externally bonded FRP composites. However there are still unanswered questions related to time and traffic load related response especially when combined with changes in environmental conditions, including exposure to high levels of temperature and humidity. Thus there is a need to assess the response of FRP rehabilitated specimens after long- term deterioration and damage representative of field conditions. The use of analytical models to predict the behavior and ultimate capacity of such specimens also needs to be validated. In order to have a realistic prediction of the in- service response/ capacity of such bridge decks that would be representative of the in- field damage/ deterioration over time, it is necessary to first identify the “ effective” material properties of the structure to be incorporated in the appropriate parameters of the analytical model. A system identification technique had been used by Stubbs et. al. [ 46] to identify “ effective” properties based on time data obtained from dynamic modals tests on the actual structure. In addition, to increase the confidence level for widespread use of these new construction materials in field conditions, it is necessary to identify material and installation process level defects and the determination of their criticality over the service life of these materials. While some defects can be readily identified by visual means others are not as
22
easily identified, and there is a critical need for the development of methods of non- destructive testing ( NDT) that can be used effectively in the field as a means of inspection. A review of methods of NDT and their comparison in terms of potential effectiveness of use as related to FRP rehabilitated concrete was presented by Kaiser et. al. [ 47, 48]. Of the methods considered, infrared thermography was identified as being a useful tool for the detection of debonding and cracking and its use had been investigated earlier [ 49, 50, 51]. However these studies were carried out more at the materials level for characterization of defects in composites rather than evaluating the technique for monitoring damage progression in composite strengthened reinforced- concrete structures.
3.2 Research Objectives
The objectives of the component level study of field specimens were to evaluate the behavior of bridge deck slabs, with field- representative damage and deterioration, prior to and after being strengthened with externally bonded FRP composite laminates and strips. Tests were conducted on slab sections cut from a bridge, both with externally bonded FRP, and without strengthening, after being in service for a period of time, thereby enabling assessment of effectiveness. This was possible since the bridge was rehabilitated previously with externally bonded FRP [ 52] in order to both strengthen deficient regions and to provide service- life extension till a new structure could be planned. The primary objective of this test program was to evaluate the behavior and failure capacity of the RC deck slabs, both with externally bonded FRP composite and without strengthening, representative of bridge deck components subjected to realistic deterioration and damage over time to evaluate long- term performance of FRP composite
23
strengthening. Dynamic modal tests were carried out to match the analytical model parameters to the behavior of the test specimens. Cyclic load tests were used to identify the response and failure capacity of the specimens and non- destructive thermography inspections were performed at regular intervals to monitor the progression of damage in the composite rehabilitated specimens. Available analytical models were used to predict the ultimate capacity of the test specimens and were compared with the test results. A local to global modeling technique was also used to predict the ultimate capacity of the actual bridge deck slabs from that of the test components. Thus the research provided data related to both destructive, non- destructive, post- use and analytical response evaluation of field specimens after service.
3.3 Description of Test Specimens
The test specimens were cut- out from deck slab segments of the Watson Wash bridge. This was a reinforced concrete T- girder bridge, built in 1970, consisting of 18 spans each of 12.8 m ( 42 feet) length and having 5 bays with the main longitudinal girders at 2.13 m ( 7 feet) spacing. Over time the bridge had shown significant distress in the form of transverse and longitudinal cracking of the decks, efflorescence in cracks, presence of alkali silica reaction ( ASR), and local punching shear failure. Also the bridge had been designed in accordance with the 1969 California Department of Transportation ( Caltrans) Bridge Design Specification ( BDS) [ 53] and hence the reinforcement was inadequate to meet some current load requirements related to punching loads. Punching shear failure had occurred previously in a bay and although it was repaired through conventional methods of patching and filling, distress in other bays indicated the
24
potential for further occurrences. In order to strengthen the decks to meet current requirements and to avoid further occurrence of punching shear, selected bays were rehabilitated with externally bonded FRP [ 54]. The rehabilitation was also conducted to extend the service life of the structure to the point when planned demolition and replacement of the bridge could take place [ 55]. Both prefabricated carbon pultruded strips, which were adhesively bonded to the concrete substrate, and unidirectional carbon fabric laminates impregnated using wet layup were used as external rehabilitation schemes as shown in Figure 3.1.
Just prior to demolition of the bridge three test specimens of the bridge deck, each of size 3.05 m ( 10’) in length x 1.37 m ( 4.5’) in width and of 156 mm ( 6 ¼ ”) full slab thickness were cut from the bridge and removed. The sizes were determined both by logistics of removal and the desire to have sections of the slab independent of girder reinforcement. Each was cut from a midspan region, with the first being an unrehabilitated section ( from an area that did not show significant deterioration), the second being from a section rehabilitated for permit truck loading, i. e. 1.5 times the current design load, using the wet layup process and the third being from a section rehabilitated to resist punching shear, i. e. to control opening of the preexisting crack widths under current design load, through adhesive bonding of pultruded strips. Details of initial slab capacity and of strengthening were presented by Lee et. al. [ 56]. Figures 3.2( a) and ( b) show schematically the locations of the FRP on the two test sections cut from rehabilitated slabs of the bridge.
25
a) Prefabricated pultruded strips b) Site- impregnated fabric laminates
Figure 3.1 Onsite rehabilitation of bridge deck slabs
Specimen S1 was an unstrengthened control component of the bridge deck cut- off from mid- span section of the bridge. The specimens consisted of # 5 rebars ( 15.9 mm or 0.625 inch diameter) spaced at approximately 140 mm ( 5.5 inches) center- to- center in the transverse direction and # 5 rebars ( 15.9 mm or 0.625 inch diameter) spaced at 244 mm ( 9.6 inches) center- to- center in the longitudinal direction. The specimen had longitudinal and transverse cracks which had made the deck slab deficient in punching shear.
Specimen S2 was a component of the bridge deck strengthened with unidirectional carbon fiber impregnated on site with epoxy resin using the wet layup process. The 2 layer thick laminate strips were spaced at 457 mm ( 18”) center- to- center in transverse direction and single layer laminates were spaced at 380 mm ( 15”) centers in the longitudinal direction. All laminate strips were 150 mm ( 6”) wide. Specimen S3 was cut out from the segment of the bridge deck strengthened with pultruded carbon/ epoxy composite strips spaced at 610 mm ( 24”) centers in the transverse direction and at 356 mm ( 14”) center- to- center in the longitudinal direction. All the pultruded strips were 50
26
mm ( 2”) wide and consisted of only a single layer in both the longitudinal and the transverse directions. The external composite reinforcement ratio was 1.33 times higher in S2 since it was designed to carry permit truck load while specimen S3 was cut out from an area of the bridge deck where the rehabilitation scheme was designed only to prevent local punching shear failures through control of the crack width opening.
All dimensions in mm FRP fabric laminate strips All dimensions in mm FRP pultruded strips ( a) Specimen S2 with wet layup fabric ( b) Specimen S3 with adhesively bonded prefabricated strips
Figure 3.2 Location of FRP strengthening on test slabs
27
All the test specimens had a considerable number of existing cracks in both the transverse and longitudinal directions resulting from over 30 years of traffic loading as well as from the effects of severe environmental exposure. The transverse cracks were noted to be usually spaced at 140 mm ( 5.5”) and correspond to the spacing of the internal transverse steel reinforcement. Secondary cracks were also observed in both the transverse and the longitudinal directions in between the main cracks. The test specimens as obtained from the Watson Wash Bridge are presented in Figure 3.3.
3.4 Material Properties and Capacity Prediction
The boundary and loading conditions used for the component test specimens were designed to simulate one- way load transfer mechanism of the actual bridge decks where the load on the decks was transferred primarily to the longitudinal girders. In order to determine the proper boundary conditions for the test specimens, a detailed finite element model of the actual bridge was constructed and analyzed. The analysis was carried out under the application of a concentrated wheel load representative of the actual HS20 wheel load configuration acting on the bridge deck [ 53]. The finite element model used for the analysis of one of the frames ( consisting of 4 spans between hinge points) of the actual bridge deck is presented in Figure 3.4 ( a).
28
a) Specimen S1 b) Specimen S2 c) Specimen S3
Figure 3.3 Test specimens
Test specimen cut- out ( 1.37 m x 3.05 m) Longitudinal Girders @ 2.13 m c/ c spacing 12.8 m ( a) Finite element model of the bridge frame Segment of bridge for analysis ( b) Location of test component in the bridge ( c) Finite element model of test component
29
Shell elem sed al and transverse
girders
the
longitu
ents were u to model the deck slabs, longitudin
and the abutments. 3- D solid elements were used to model the footing while the footing restraints were modeled using spring elements. The location of the test component when it was cut- out from the bridge deck slab is shown in Figure 3.4 ( b). Global- local modeling was then conducted, with the FE model of the test component as shown in Figure 3.4 ( c), to develop the necessary test boundary conditions for the specimens such that they mimicked the stress conditions at global level. Under the selected test conditions ( described in the next section) contours of the transverse membrane forces from the finite element models of the full bridge deck and the component test specimens were found to match closely indicating that the test specimens would have a behavior similar to the corresponding portion of the actual bridge deck. The edges of the test specimens were simply supported on steel rollers in
dinal direction to simulate the one- way load transfer mechanism. However it was not possible to simulate the vertical deflections along the longitudinal supports representative of girder deflection in the actual bridge. Also because of size limitation, the transverse span- length of the test specimens was smaller than that of the bridge deck. The combination of these two factors resulted in higher stiffness of the test specimens and this had to be taken into account while predicting their true capacities. In order to identify the effective baseline material stiffness to be incorporated into the finite element model for capacity prediction, forced vibration based dynamic modal tests were performed on the test specimens. The time data collected from the tests was used to determine the frequencies and mode shapes. From the power spectral response, the predominant natural frequency of the control specimen ( S1) was identified to be 205 Hz 30
and from the mode shape this frequency was found to correspond to the second longitudinal bending mode. A finite element model of the test specimen was developed with 4- noded solid elements to define a baseline model, with the mass density and Poisson’s ratio of reinforced concrete assumed to be 2400 kg/ m3 ( 150 lb/ ft3) and 0.15, respectively. Since the bridge was constructed in the 1970s, no information on the concrete material property of the deck slabs was available. Thus in order to identify the concrete property in terms of its elastic modulus, an iterative process with different elastic modulus ( E) of concrete was used until the frequency corresponding to the second longitudinal bending mode from the model matched 205 Hz and the resulting “ effective” modulus was found to be 28.6 GPa ( 4150 ksi). A comparison of the mode shapes, corresponding to natural frequency of 205 Hz, obtained from the experimental vibration tests and analytical model is presented in Figure 3.5. The term “ effective” modulus will be used herein to refer to the modulus value assigned to the model for true prediction of specimen capacity. From the vibration tests of the slab components only one modal frequency could be identified to the desired level of accuracy ( because of high stiffness of the test specimen) and thus only the specimen modulus was adjusted in the model.
31
( a) Experimental mode shape ( f = 205 Hz) ( b) Analytical mode shape ( f = 205 Hz) from FEM
Figure 3.5 Comparison of experimental and analytical mode shapes
The results were compared to the “ effective” modulus identified from modal tests and system identification performed on the actual bridge decks, both prior to and after rehabilitation [ 52]. The unrehabilitated bridge decks had an effective modulus of 17.8 GPa ( 2582 ksi) and the rehabilitation of the decks with site- impregnated carbon fabric laminates and adhesively bonded prefabricated pultruded strips resulted in enhancements
32
in the effective stiffness by 20% and 13.9%, respectively [ 52]. Thus the test specimens had a higher effective stiffness, as obtained by the product of the effective modulus and the moment of inertia of the cross- section, which would result in a higher capacity as compared to the actual bridge decks, and this has to be considered while correlating the capacities of the test specimens to the actual field capacities. Corresponding to the “ effective” modulus, the “ effective” concrete strength to be used for capacity prediction was computed to be 36.56 MPa ( 5300 psi). It is to be noted that the effective concrete compressive strength might not be the true concrete strength and has been used as a parametric value to be incorporated into the capacity prediction models to take into account the model stiffness and boundary conditions.
The steel reinforcement had yield strength, fY, of 414 MPa ( 60 ksi) and a tensile modulus, ES, of 200 GPa ( 29000 ksi). The unidirectional two- layer carbon fabric was used in widths of 152 mm ( 6 in.), running in the transverse direction of the specimen, with a cured composite thickness of 1.88 mm ( 0.0739 in.) having a composite modulus and strength in the fiber direction of 78.96 GPa ( 11452 ksi) and 1.1 GPa ( 160 ksi), respectively. The unidirectional one- layer carbon fabric was also used in widths of 152 mm ( 6”), running in the longitudinal direction of the specimen, with a cured composite thickness of 1.1 mm ( 0.0434 in.) having a composite modulus and strength in the fiber direction of 76.27 GPa ( 11062 ksi) and 1.13 GPa ( 164 ksi), respectively. The prefabricated carbon/ epoxy strips were 51 mm ( 2 in.) wide and 1.3 mm ( 0.05”) thick with tensile modulus and strength in the fiber direction of 173.9 GPa ( 25222 ksi) and 2.51 GPa ( 364 ksi), respectively. The material properties of the composite strips and laminates
33
were obtained from tensile tests of the composite samples used for strengthening the bridge decks [ 52].
The general punching shear capacity of the slab, Vn, was determined from the equilibrium of forces as: tnfddbbVθθtantan2221⎟⎠ ⎞ ⎜⎝ ⎛++= ………………..( 3.1)
where, = diagonal concrete tensile strength = tf'' 33.033.017.0CCCff≤⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛ + β ( in MPa)
and is the compressive strength of concrete ( 36.56 MPa in this case), is the short side of the concentrated load area ( 203 mm or 8” in this case), is the long side of the concentrated load area ( 508 mm or 20” in this case), is the average effective depth of the section ( 122 mm or 4.8125” in this case) and ' Cf1b2bdθ is the angle between the horizontal and assumed failure plane. The theoretical formulation for punching shear ( equation 3.1) is equivalent to the AASHTO [ 57] prescribed equation for an angle of the failure plane, θ, being 450. However experimental results such as those reported by Graddy et al. [ 58] suggested a value of 380 as being more representative.
The punching shear capacities corresponding to θ values of 450 and 380, respectively, were determined as:
For θ = 450, () 56.36203/ 50833.017.012212225082032⎟⎠ ⎞ ⎜⎝ ⎛+ ×××++= nV = 425 kN
For θ = 380, 56.36203/ 50833.017.038tan12238tan12225082032⎟⎠ ⎞ ⎜⎝ ⎛+ ××⎟⎠ ⎞ ⎜⎝ ⎛ ×++= nV = 583 kN
34
The punching shear capacity of the test specimen was thus predicted to be between 425 kN ( 96 kips) and 583 kN ( 131 kips). As is evident from these formulations, the actual punching shear capacity will be governed by the angle of shear failure plane, which can be influenced by locations of the preexisting cracks. Moreover the punching shear capacity would depend on the properties of concrete in the local region of the applied concentrated wheel load as well as the amount of aggregate interlock available.
The flexural capacities of the test specimens were then computed using design oriented sectional capacity analysis using the “ effective” modulus of reinforced concrete to be 28.6 GPa ( 4150 ksi). The flexural capacity at steel yield for the unrehabilitated control specimen, corresponding to bottom steel strain of 0.002, was computed to be 63.61 kN- m/ m ( 14.3 kip- ft/ ft). From the finite element model this capacity was found to correspond to a concentrated load demand of 512 kN ( 115 kips).
For the two test specimens strengthened with FRP composite strips and laminates, it was expected that the strengthening scheme would change the mode of failure from punching shear to flexural failure, culminating in the debonding of the laminates or strips at failure of the specimens. This was based on the assumption that the strengthening of the specimens with the composite strips or laminates would limit the opening of the cracks and ensure sufficient aggregate interlock so that punching shear failure mode would be avoided. It is to be noted that the debonding strain and not the rupture strain was used as the operative limiting strain in the composite and this was estimated using an energy based procedure proposed by Niu and Wu [ 11], in which the maximum axial force in the composite at debonding is obtained as, 222max2tEGbPf= ………………..( 3.2)
35
where, E2, t2 and b2 are the elastic modulus, thickness and width of FRP, respectively. Gf is the interfacial fracture energy given by the area of the δτ− curve obtained from shear peel tests of the composite samples and were computed to be 0.976 N/ mm ( 5.6 lb/ in) for the site- impregnated 2- layer carbon composite laminates [ 59] and 1.2 N/ mm ( 6.85 lb/ in) for the 1- layer pultruded carbon composite strips [ 60].
For the site- impregnated 2- layer carbon composite laminates, with E2, t2, b2 and Gf being 78.96 GPa ( 11452 ksi), 1.88 mm ( 0.074”), 152.4 mm ( 6”) and 0.976 N/ mm ( 5.6 lb/ in), respectively, the maximum axial force in the composite at debonding was obtained using equation 3.2 as, 222max2tEGbPf= = 88.178960976.024.152××× = 82.04 kN ( 18.4 kips). …...( 3.3)
The corresponding maximum debonding strain in the composite was obtained as, 222maxmaxEbtP= ε = 789604.15288.182040×× = 3626 micro- strains ……………( 3.4)
For the site- impregnated 1- layer carbon composite laminates, with E2, t2, b2 and Gf being 76.27 GPa ( 11062 ksi), 1.1 mm ( 0.043”), 152.4 mm ( 6”) and 0.976 N/ mm ( 5.6 lb/ in), respectively, the maximum axial force in the composite at debonding was obtained as, 222max2tEGbPf= = 1.176270976.024.152××× = 61.67 kN ( 13.9 kips). ……..( 3.5)
The corresponding maximum debonding strain in the composite was obtained as, 222maxmaxEbtP= ε = 762704.1521.181670×× = 4823 micro- strains …………………( 3.6)
36
Similarly, for prefabricated 1- layer pultruded carbon composite strips, with E2, t2, b2 and Gf being 173.9 GPa ( 25222 ksi), 1.3 mm ( 0.05”), 50.8 mm ( 2”) and 1.2 N/ mm ( 6.85 lb/ in), respectively, the maximum axial force in the composite at debonding was obtained as, 222max2tEGbPf= = 3.11739002.128.50××× = 37.42 kN ( 8.4 kips) ………...( 3.7)
The corresponding maximum debonding strain in the composite was obtained as, 222maxmaxEbtP= ε = 1739008.503.137420×× = 3260 micro- strains …………( 3.8)
ACI- 440 [ 3] also gives an estimate of debonding strains in composites and was used to compare with the predicted debonding strains obtained from Niu and Wu’s [ 11] model. For the site- impregnated 2- layer carbon composite laminates, with uε, E2, and ntf being 1.4%, 78.96 MPa and 1.88 mm, respectively, the debonding strain as per ACI- 440 [ 3] was obtained as:
= fuε design rupture strain = 0.95 x 1.4% = 1.33%
For = 148,445 < 180,000, the bond co- efficient was obtained as: 88.178960×= ffntE⎟⎠ ⎞ ⎜⎝ ⎛ − ×=⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛ −= 000,360445,14810133.0601000,3601601fffumtnEεκ= 0.74 …………( 3.9)
and thus, the ultimate debonding strain = 0133.074.0×= fumεκ= 9842 micro- strains.
For the prefabricated 1- layer pultruded carbon composite strips, with uε, E2 and ntf being 1.4%, 173.9 MPa and 1.3 mm, respectively, the debonding strain as per ACI- 440 [ 3] was obtained as:
= fuε design rupture strain = 0.95 x 1.4% = 1.33% ………..( 3.10)
37
For = 226,070 > 180,000, 3.1173900×= ffntE
the bond co- efficient, ⎟⎠ ⎞ ⎜⎝ ⎛ ×=⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = 070,226000,900133.0601000,90601fffumtnEεκ= 0.5 …….( 3.11)
and thus, the ultimate debonding strain = 0133.05.0×= fumεκ= 6650 micro- strains.
………..( 3.12)
The debonding strains predicted by ACI- 440 [ 3] were thus found to be much higher as compared to those predicted by the theoretical model [ 11]. To have a conservative estimate of flexural capacity the lower debonding strain predictions were used. It is also to be noted that the strain prediction equation in ACI- 440 [ 3] only takes into account the rupture strain, elastic modulus and the thickness of the composite. The theoretical model [ 11] also takes into consideration the interfacial fracture energy, Gf, and is thus more representative of the bond characteristics of the composite. Thus in the presence of representative interfacial fracture energy information, it is expected to predict the debonding strain level more accurately than ACI- 440 [ 3].
The flexural capacities of the strengthened specimens were computed through sectional capacity analysis corresponding to the ultimate limit state at which the top concrete reached the crushing strain of 0.003 and the fabric laminate and pultruded strip at the bottom of the section reached the predicted debonding strains of 3626 and 3260 micro- strains, respectively. Also, as determined through field modal tests, there were 20% and 13.9% enhancements in the effective modulus for the decks rehabilitated with the fabric laminates and pultruded strips, respectively. Thus as compared to the effective modulus of 28.6 GPa ( 4150 ksi) for specimen S1, the effective modulus of concrete for specimens S2 and S3 were taken as 34.3 GPa ( 4975 ksi) and 32.6 GPa ( 4728 ksi),
38
respectively. The ultimate moment capacity of the specimen rehabilitated with the carbon fabric laminates, S2, was computed to be 99.42 kN- m/ m ( 22.4 kip- ft/ ft), which was found to correspond to a concentrated load demand of 827 kN ( 186 kips) from the finite element model. The ultimate moment capacity of the specimen rehabilitated with the carbon pultruded strips, S3, was computed to be 81.7 kN- m/ m ( 18.4 kip- ft/ ft), which was found to correspond to a concentrated load demand of 680 kN ( 153 kips) from the finite element model. It should be noted that the slab with the carbon fabric rehabilitation scheme was designed for permit load and thus had a higher composite reinforcement ratio as compared to the specimen strengthened with the pultruded strips which was designed only to prevent punching shear failure. Thus the specimen with the carbon fiber laminate rehabilitation scheme had a higher flexural strength. Table 3.1 summarizes the predicted capacities and failure modes of the test specimens.
39
Table 3.1 Failure Loads and Mode for Test Specimens
Specimen Description
Predicted Failure Load ( kN)
Predicted Mode of Failure
S1, unstrengthened control
425 1 to 583 2
Punching shear
S2, strengthened for permit load, using carbon fabric
827 3
Flexural failure with debonding of the FRP composite
S3, strengthened for punching shear, using prefabricated carbon/ epoxy strips
Flexural failure with debonding of pultruded strips
680 4
1 Shear failure plane angle, θ = 45o
2 Shear failure plane angle, θ = 38o
3FRPε Capacity prediction corresponding to = 0.003 and = 3626 Cεsμ
4FRPε Capacity prediction corresponding to = 0.003 and = 3660 Cεsμ
Note: The theoretical failure loads have to be reduced by a strength factor, φ = 0.9, live load factor of 1.7 and load impact factor of 1.3 to get design failure loads.
3.5 Test Setup and Instrumentation
The test specimens were placed on roller supports, simulating simply supported conditions, running continuously along the two 3.05 m ( 10’) long edges and at a distance of 152 mm ( 6”) from the outer edge of the specimens, giving a center- to- center distance between supports of 1.07 m ( 42”). The two shorter outer edges of the slab were free edges. Two 3.05 m ( 10’) long, 152 mm ( 6”) wide and 25.4 mm ( 1”) thick steel bearing plates, with 6.4 mm ( ¼ ”) thick neoprene bearing strips on top of it, were mounted between the roller supports and the underside of the slab to ensure uniform bearing of the
40
test specimens on the supports. The roller supports were welded to steel plates, which were tied down to concrete support blocks by post tensioned steel rods.
Load was applied, through a 76 mm ( 3”) thick elastomeric pad, under displacement control, over a contact area of 508 mm x 203 mm ( 20” x 8”) centered on the specimen, simulating a HS20 wheel load configuration [ 12.16]. At increments of 44.5 kN ( 10 kips) the load was cycled back to zero and then reloaded to the initial level to enable assessment of cracking and stability. The test setup is shown in Figure 3.6.
Figure 3.6 Test setup
3.6 Instrumentation and Data Acquisition
Vertical deflections in the area around load application were measured using 15, ( inch) linear potentiometers. Three of these potentiometers along the centerline of the specimens were used to measure deflections from the underside of the specimens so as not to interfere with the load actuator. The remaining 12 potentiometers were used to measure deflections from the top of the specimens. The locations of the potentiometers with respect to the specimen are presented in Figure 3.7. mm76± 3±
41
All dimensions in mm
Figure 3.7 Locations of linear potentiometers
In addition to the vertical linear potentiometers, four horizontal potentiometers were used along the long edge of the specimens two on each side of the long edges, each placed at a distance of 457 mm ( 1.5 feet) from the outside edge of the specimens. These potentiometers were used to measure any horizontal movement of the test specimens during the test. Moreover four rotation sensors were used, two on each outer unsupported shorter edge and were placed right over the supports to measure any rotations of the test specimens at the outer edges.
A data acquisition system was used to record the loads from the actuator, the deflection readings from the linear potentiometers and strain readings from the electrical resistance strain gages. Values were recorded at intervals of 4.45 kN ( 1 kip) load increments through a complete load cycle. Fatigue deterioration and crack progression as well as the appearance of any new cracks were documented at the end of each load cycle.
Forced excitation based dynamic testing was carried out for purposes of system identification to determine the “ effective” modulus of the test specimens for capacity 42
prediction of the field deteriorated specimens. A total of 15 piezoelectric accelerometers were used at two location setups in order to capture the major natural frequencies of the test specimens. The locations of the accelerometers with respect to the test specimens are presented in Figure 3.8. The dynamic tests were carried out at intervals of 178 kN ( 40 kips) at the end of the loading cycle i. e. after returning to zero load. During the dynamic tests the actuator was retracted for the purpose of placing accelerometers along the centerline of the test specimens. A small hammer with a load cell attached at the tip was used to impart the external excitation. A frequency domain transformation was used to obtain the natural frequencies of the test specimens from the accelerometer data.
Electrical resistance strain gages with a gage length of 20 mm ( 0.8”) were also used on the carbon composite laminates and strips in specimens S2 and S3 respectively, to monitor the increase in strains in the composite material with the progression of damage. A total of 27 strain gages were used in specimen S2 and 22 gages were used in specimen S3. The locations of the strain gages are presented in Figure 3.9. Since most of the deflections and load transfer in the specimens were expected to occur in the transverse direction, the laminates/ strips running in the transverse direction were more extensively instrumented as compared to the longitudinal strips/ laminates. However at least one longitudinal strip/ laminate was instrumented with strain gages in each of the specimens.
43
All dimensions in mm ( a) Setup 1 ( b) Setup 2
Figure 3.8 Locations of accelerometers for vibration tests
44
( a) Specimen S2 ( b) Specimen S3Strip 1 Strip 2 Strip 3 Strip 4 Strip 1 Strip 2 3048 mm 1067 mm 203 mm 203 mm 203 mm 203 mm 203 mm 305 mm 305 mm 508 mm 178 mm 178 mm 178 mm Strip 1 Strip 2 Strip 3 Strip 4 Strip 5 3048 mm 1067 mm 190 mm 190 mm 190 mm 228 mm 228 mm 228 mm 228 mm 228 mm 457 mm 228 mm 228 mm 228 mm 178 mm
Figure 3.9 Locations of strain gages on composite strips/ laminates
45
3.7 Test Results and Discussion
3.7.1 Ultimate Load Capacities of Slabs
Overall load- midspan displacement response envelope curves for the 3 slabs are shown in Figure 3.10. The unstrengthened slab, S1, failed in punching shear at a load of 501 kN ( 112.6 kips). Punching shear failure was followed by yielding of the internal steel reinforcement resulting in the plateau in load- deflection response as seen in Figure 3.10. The load was retained up to a maximum center deflection of approximately 12.2 mm ( 0.48) inches, beyond which there was a rapid reduction in the specimen capacity. Final failure was caused by opening of preexisting cracks at the bottom of the specimen, with the load pad punching through at the top of the slab.
Specimen S2 was cutout from the bridge deck segment strengthened with unidirectional carbon fabric field impregnated with epoxy. The specimen failed at a load of 862 kN ( 193.7 kips) with a corresponding mid- span deflection of 13.5 mm ( 0.53 inches). Slab failure was initiated in flexure by debonding of the composite strips and was followed by punching of the load pad through the concrete at the top of the slab. Thus the rehabilitation scheme resulted in enhancement in the capacity of the specimen as compared to the control specimen while at the same time changing the failure mode from punching shear to flexural failure.
Specimen S3 was cutout from the bridge deck segment strengthened with prefabricated carbon/ epoxy pultruded composite strips. The specimen failed at a load of 791 kN ( 177.76 kips) with a corresponding mid- span deflection of 11.7 mm ( 0.46 inches). The primary purpose of this rehabilitation scheme was only to prevent punching shear failure through control of the crack widths as opposed to the objective of strength
46
enhancement in specimen S2. Consequently, specimen S3 had a lower level of strength enhancement as compared to specimen S2 and also failed at a lower level of mid- span deflection. Slab failure was initiated in flexure by debonding of the prefabricated strips. This was followed by punching of the load pad through the concrete at the top of the slab, since the punching shear could not be resisted without the FRP composite.
Figure 3.10 Load- displacement plots for test specimens
3.7.2 Slab Deflections and Cracking
15 linear potentiometers were used for each specimen around the loading area in order to obtain the displacement profile of the specimens with the application of the load. The primary purpose of getting the displacement profile was to identify whether the composite strengthening schemes were able to distribute the wheel load over a larger area thus preventing localized punching shear failure. The characteristic displacement profiles of the test specimens are presented in Figure 3.11. From Figure 3.11 it is evident that specime