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Punching Shear Strength of Interior and Edge Column-Slab

Connections in CFRP Reinforced Flat Plate StructuresTransferring Shear and Moment

by

Ashraf Zaghloul, B.Eng./M.A.Sc.

A thesis submitted toThe faculty of Graduate Studies and Research

in partial fulfillment of

the requirements for the degree of

Doctor of Philosophy

Department of Civil and Environmental Engineering

Carleton University

Ottawa, Ontario

The Doctor of Philosophy Program in Civil Engineering

is a joint program with University of Ottawa,

administrated by the Ottawa-Carleton Institute of Civil Engineering

Carleton University

Ottawa, Ontario, Canada

February 2007

2007, Ashraf Zaghloul

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Abstract

The purpose of this investigation is to study the punching shear strength of interior and

edge column-slab connections where the slabs are reinforced for flexure, or for flexure

and shear, with CFRP, and they are subjected to combined shear and moment transfer.

Two interior and ten edge column-slab connections are tested to study the effect of a

number of parameters, including the type and amount of slab flexural and shear

reinforcement. The flexural reinforcement is either steel or CFRP while the shear

reinforcement is either steel headed studs or a CFRP shear rail introduced in this study. It

is found that the shear reinforcement increases the punching shear strength by 20% to

26.7% in the case of the interior column connection and by only about 10% in the case of

the edge column connections. This increase is predicted with a reasonable degree of

conservatism by using the basic (vc+v5) approach of the ACI Code in conjunction with a

proposed equation for calculating vc, i.e. the concrete contribution to the punching shear

strength, which accounts for the effects of the slab flexural reinforcement rigidity and the

column size, relative to the slab thickness, on the punching shear strength.

In addition to the above simplified method, a refined and more rational model for

predicting the strength of the tested specimens is also introduced. This model uses the

compatibility and equilibrium requirements at the connection and a more realistic

punching shear perimeter for calculating the punching shear capacity of slab-column

connections, but its results are not as accurate as those of the simplified method.

Consequently, the use of the proposed simplified method is recommended for practical

applications.

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Acknowledgements

The author would like to thank God for providing him the strength, and showing him the

way during the work in this thesis. The writer wishes that this thesis be for God sake and

for the sake of useful knowledge and he will be rewarded for that.

{Behold! in the creation of the heavens and the earth; in the alternation of the Night

and the Day; in the sailing of the ships through the Ocean for the profit of mankind;

in the rain which Allah sends down from the skies, and the life which He gives

therewith to an earth that is dead; in the beasts of all kinds that He scatters through

the earth; in the change of the winds, and the clouds which they trail like their

slaves between the sky and the earth - (here) indeed are Signs for a people that are

wise} Quaran- AlBaqarah - verse 164.

The author would like to express his gratitude to his supervisor, Professor A.G.

Razaqpur, for his financial support, his keen supervision of this research and his

enormous effort during the thesis production. I am also grateful to Professors G. Hartley

and O.B. Isgor for their help and guidance. Special thanks are extended to the Carleton

University Faculty of Graduate Studies and Research and to NSERC for their financial

assistance. This research was funded by a joint grant from Materials and Manufacturing

Ontario, an Ontario Centre of Excellence and AUTOCON Composites Co. of Toronto. I

am grateful for their generosity and am particularly grateful to Mr. John Crimi, president

of AUTOCON, for providing the CFRP grids on time and per my specifications. I wish

to also acknowledge DECON Canada for donating the steel studs.

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Special thanks go out to the technical staff of the Department of Civil and Environmental

Engineering Laboratories at Carleton for their professional help and advice without

which the experimental work would not have been possible. Many thanks are due to

Messrs Ken McMartin, Pierre Trudel, Jim Whithome and Stanley Conley. The author

also likes to thank his fellow graduate students who helped in the casting of the

specimens, particularly Mr. Abd El-Zaher Mostofa, Ahmed Mostofa and Essam El-

Tahawy.

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Table of Contents

Abstract ................................................................................................................................. iii

Aknowledgments................................................................................................................... iv

Table of contents .................................................................................................................. vi

List of tab les .......................................................................................................................... xi

List of figures......................................................................................................................... xiv

List of symbols.................................................................................................................... xxvi

CHAPTER 1 Introduction .............................................................................................. 1

1.1 Introduction.......................................................................................................................... 1

1.2 Problem definition.............................................................................................................. 3

1.3 Objectives and scope........................................................................................................... 3

CHAPTER 2 Literatu re review .................................................................................. 6

Part I: Punching shear of FRP reinforced slabs...................................................................... 6

2.1 General................................................................................................................................. 6

2.2 Punching shear in slabs reinforced with FRP ................................................................... 7

(a) Concentric shear.................................................................................................................. 7

(b) Eccentric shear.................................................................................................................. 14

(c) Shear reinforcement.......................................................................................................... 17

2.3 Punching strength analysis methods for FRP reinforced slabs ..................................... 19

2.3.1 Concentric punching...................................................................................................... 19

2.3.2 Design codes and guidelines......................................................................................... 22

2.3.3 Eccentric punching......................................................................................................... 24

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Part II: Research background on steel reinforced slabs and the current design methods for

punching shear........................................................................................................................ 27

2.4 Eccentric punching shear of interior and edge column-slab connections.................... 27

2.5 Use o f shear reinforcement for slab-column connections.............................................54

2.6 Summary............................................................................................................................ 61

CHAPTER 3 Experimental program ..................................................................... 74

3.1 General.............................................................................................................................. 74

3.2 Test program.................................................................................................................... 75

3.2.1 Test materials................................................................................................................. 75

3.2.2 Interior column-slab connection test specimens ......................................................... 81

3.2.3 Edge column-slab connection test specimens............................................................. 82

3.3 Instrumentation................................................................................................................. 86

3.3.1 Electrical strain gauges................................................................................................. 87

3.3.2 Internal crack detection bar.......................................................................................... 88

3.3.3 LVDT locations............................................................................................................. 88

3.4 Loading.............................................................................................................................. 88

3.5 Test set-up .................................................................................................................... 89

3.5.1 Interior connections...................................................................................................... 89

3.5.2 Edge column slab connections..................................................................................... 90

3.6 Specimens construction ............................................................................................... 90

3.6.1 Preparation of reinforcement cages............................................................................. 90

3.6.2 Casting............................................................................................................................ 91

3.6.3 Curing............................................................................................................................. 92

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CHAPTER 4 Experimental results and discussion................................................133

4.1 General............................................................................................................................. 133

4.2 Behaviour and strength of interior column-slab specimens.........................................133

4.2.1 Crack development and propagation......................................................................... 134

4.2.2 Load Deflection Behaviour......................................................................................... 136

4.2.3 Reinforcement strain....................................................................................................138

4.3 Edge column-slab connections...................................................................................... 143

4.3.1 Specimens without shear reinforcement.................................................................... 143

4.3.1.1 Crack development and propagation...................................................................... 143

4.3.1.2 Load-deflection curves and ultimate strength.........................................................145

4.3.2 Edge column specimens with shear reinforcement.................................... 150

4.3.2.1 Crack development and propagation...................................................................... 150

4.3.2.2 Load-deflection relationship....................................................................................152

4.3.2.3 Strain in slab flexural reinforcement....................................................................... 161

(a) Specimens without shear reinforcement........................................................................ 161

(b) Specimens with shear reinforcement............................................................................. 166

(i) Slab reinforcement strain............................................................................................... 171

(ii) Shear reinforcement strain............................................................................................. 172

4.3.2.4 Slab through thickness deformations...................................................................... 177

4.3.3 Summary of results ....................................................................................................179

CHAPTER 5 Analysis of test results ...................................................................... 236

5.1 General........................................................................................................................... 236

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5.2 Punching shear analysis of interior column-slab connections without shear

reinforcement........................................................................................................................ 237

(a) Calculation of nominal shear stress, vu......................................................................... 237

(b) Calculation of punching shear resistance...................................................................... 240

5.3 Punching shear resistance of interior column-slab connections with shear

reinforcement....................................................................................................................... 242

5.4 Analysis of edge column-slab connections...................................................................243

5.5 Comparison of predicted and observed punching shear capacity of slab column

connections using the basic ACI approach......................................................................... 244

5.5.1 Interior column-slab connections subjected to concentric shear.............................245

5.5.1.1 FRP reinforced slabs................................................................................................ 245

(a) Existing methods............................................................................................................. 245

(b) Proposed method............................................................................................................. 246

5.5.1.2 Steel reinforced slabs............................................................................................... 250

5.5.2 FRP reinforced interior column-slab connections transferring shear and moment .....

................................................................................................................................................ 251

5.5.2.1 ACI method............................................................................................................... 251

5.5.2.2 Proposed refined method.........................................................................................252

(a) Description o f the refined method................................................................................. 252

(b) Comparison of test results to predicted values by the refined method ....................... 256

5.5.3 Edge column-slab connections transferring shear and unbalanced moment ..........257

5.5.4 Refined method for predicting the punching shear strength of FRP reinforced edge

column specimen using the proposed non-rectangular critical shear perimeter..............260

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5.5.5 Semi-Analytical method for calculating the punching shear capacity of edge

column-slab connections...................................................................................................... 261

5.5.5.1 Model formulation.................................................................................................. 262

5.5.4.2 Model implementation procedure........................................................................... 273

5.5.4.3 Model verification and comparison with test results............................................ 274

5.5.6 Analysis of slab-column connections by using Afhami et al.method ................... 276

5.6. Analysis of slab-column connections with shear reinforcement.............................277

5.6.1 Analysis method........................................................................................................... 277

5.6.2 Results of the analysis................................................................................................. 280

5.7 Summary....................................................................................................................... 282

CHAPTER 6 Summary, conclusions and recommendations for future study ...315

6.1 Summary........................................................................................................................ 315

6.2 Conclusions.................................................................................................................... 316

6.3 Recommendations for future work............................................................................... 320

Appendix A ................................................................................................................... 322

Appendix B ...................................................................................................................342

References..................................................................................................................... 351

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List of tables

Table 2.1 - Proportions of moment balanced by flexure, torsion and shear for interior and

edge column connections....................................................................................................... 62

Table 3.1: Concrete compressive strength at time of testing for first group of

specimens................................................................................................................................. 93

Table 3.2: Concrete compressive strength at time of testing for second group of

specimens................................................................................................................................. 93

Table 3.3: Properties of interior slab-column specimens..................................................... 94

Table 3.4: Grouping of interior slab-column specimens according to the investigated

parameters................................................................................................................................ 95

Table 3.5: Reinforcement detailing of proposed edge slab column specimens ................. 96

Table 3.6: Grouping of edge column-slab specimens according to the investigated

parameters................................................................................................................................ 97

Table 3.7: Number of strain gauges applied to each specimen........................................... 98

Table 4.1: Distribution of cracks appearance according to location and the load level for

specimens without shear reinforcement.............................................................................. 182

Table 4.2: Distribution of crack appearance according to location and the load level for

specimens with shear reinforcement....................................................................................183

Table 4.3: Loads at which diagonal tension cracks formationwas detected.....................184

Table 5.1: Comparison of observed and predicted punching shear capacity of FRP slabs

under concentric shear......................................................................................................... 284

Table 5.2: Comparison of observed and predicted punching shear capacity of steel

reinforced slabs under concentric punching....................................................................... 286

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Table 5.3: Comparison of observed and predicted shear capacity of FRP reinforced

interior column-slab connections transferring shear and moment (ACI method) 288

Table 5.4: Comparison of observed and predicted punching shear capacity of FRP

reinforced interior slabs under shear and unbalanced moment (Refined method) 289

Table 5.5: Comparison of predicted and observed punching shear capacity of FRP

reinforced edge column connections based on the ACI 318 critical shear perimeter and

different vc equations........................................................................................................... 290


reinforced edge column connections based on the proposed critical shear perimeter with

inclined sides and different vc equations........................................................................... 291


reinforced edge column-slab specimens using the refined method.................................. 292

Table 5.8: Comparison of test results and predicted values using the proposed semi-

analytical method without considering the bond efficiency of steel perpendicular to free

edge........................................................................................................................................ 293

Table 5.9: Comparison of test results and predicted values using the semi-analytical

method model considering the proposed bond efficiency of steel perpendicular to free

edge........................................................................................................................................ 295

Table 5.10: Interaction diagram principal points for Afhami et al (1998) model 297

Table 5.12: Predicted punching shear capacity of specimens reinforced with FRP shear

reinforcement based on (vcAC]+ vs ) ................................................................................ 299

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List of figures

Fig. 2.1: Critical section for diagonal tension and assumed distribution of shear stresses

according to Di Stasio and Van Buren (1960) ...................................................................... 63

Fig. 2.2: Distribution of shear stresses at ultimate, according to Moe (1961)................... 64

Fig. 2.3: Hanson and Hansons test specimens and loading arrangements....................... 65

Fig. 2.4: Dimension and reinforcement for edge and comer connections tested at Imperial

College (Stamenkovic, 1969)................................................................................................. 66

Fig. 2.5: Dimensions and reinforcement for edge connections tested by Zaghlool

(1971)....................................................................................................................................... 67

Fig. 2.6: Slab-column connection under externalactions atcritical section, (Park and

Islam 1976).............................................................................................................................. 68

Fig. 2.7: Flexural collapse mechanism, (Goli and Gesund, 1979)...................................... 69

Fig. 2.8: Test specimens of Zidan (1981)............................................................................. 70

Fig. 2.9: Free body diagram of an edge connectionaccording to strip model(Afhami,

1997)........................................................................................................................................ 71

Fig. 2.10: Moment-shear diagram for the capacity of edge connections according to strip

model (Afhami, 1997)............................................................................................................. 72

Fig. 2.11: Critical section outside shear reinforced zone for ACI and CSA Codes...........73

Fig. 3.1: Test set-up for concrete cylinders........................................................................... 99

Fig. 3.2: Typical stress strain curve of a concrete cylinder for Group 1.............................99

Fig. 3.3: Typical stress strain curve for a concrete cylinder of Group 2 ........................... 100

Fig. 3.4: CFRP ribs stress strain relationship as provided by the manufacturer.............. 100

Fig. 3.5: Cutting the carbon fibre grids for installation in the slab specimens ................ 101

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Fig. 3.6: The CFRP tension coupon held in the universal testing machine................... 101

Fig. 3.7: Typical CFRP tension coupons and its end anchors ..........................................101

Fig. 3.8: Typical stress-strainrelationship for C19 asobtained in the current testing

program.................................................................................................................................. 102

Fig. 3.9: CFRP shear reinforcement rails dimensions ...................................................... 102

Fig. 3.10: Photo of CFRP NEFMAC shear reinforcement consisting of five legs,

Zaghloul (2002)................................................................................................................... 103

Fig. 3.11: CFRP shear reinforcement disposition in the slab ............................................ 103

Fig. 3.12: CFRP shear reinforcement for slab of specimen ZJEFCS ............................... 104

Fig. 3.13: CFRP shear reinforcement in specimen ZJESCS............................................. 104

Fig. 3.14: CFRP shear reinforcementdeposition in the slabrelative to theflexural

reinforcement and column stub.......................................................................................... 105

Fig. 3.15: Layout of CFRP shear grids in specimen ZJEFCS ........................................... 105

Fig. 3.16: Specimen ZJEFSS flexural and shear reinforcement........................................106

Fig. 3.17: Layout of steel studs shear reinforcement in specimen ZJEFSS ..................... 106

Fig. 3.18: Vertical section of the slab showing the layout of steel studs..........................107

Fig. 3.19: Steel studs in position inside slab of specimen ZJEFSS.................................. 107

Fig. 3.20: Flexural and shear reinforcement for specimen ZJESSS ..................................108

Fig. 3.21: Layout of steel studs in specimen ZJESSS ........................................................ 108

Fig. 3.22: The steel studs inside slab of specimen ZJESSS.............................................. 109

Fig. 3.23: Typical interior column test specimen............................................................... 109

Fig. 3.24: Specimen ZJF8 reinforcement............................................................................ 110

Fig. 3.25: Typical test specimens o f edge column-slab connection.................................I l l

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Fig. 3.26: CFRP edge column specimen reinforcement................................................. 112

Fig. 3.27: The CFRP grid dimensions and disposition inside typical edge column-slab

specimen............................................................................................................................... 113

Fig. 3.28: Strain gaugeslocations on the bottom reinforcement of Specimen ZJEF1.... 114

Fig. 3.29: Strain gaugeslocations on the bottom reinforcement of Specimen ZJEF2....114

Fig. 3.30: Strain gaugeslocations on the bottom reinforcement of Specimen ZJEF3. ... 115

Fig. 3.31: Strain gaugeslocations on the bottom reinforcement of Specimen ZJEF5... .115

Fig. 3.32: Strain gauges locations on the bottom reinforcement of Specimen ZJEF7. ...116

Fig. 3.33: Strain gaugeslocations on the bottom reinforcement of Specimen ZJES 116

Fig. 3.34: Strain gauges locations on the bottom reinforcement of Specimen ZJEFCS..117

Fig. 3.35: Strain gauges locations on the shear reinforcement of Specimen ZJEFCS.... 117

Fig. 3.36: Strain gauges locations on the bottom reinforcement o f Specimen ZJEFSS.. 118

Fig. 3.37: Strain gauges locations on the shear reinforcement of Specimen ZJEFSS.... 118

Fig. 3.38: Strain gauges locations on the bottom reinforcement of Specimen ZJESCS..119

Fig. 3.39: Strain gauges locations on the shear reinforcement of Specimen ZJESCS... 119

Fig. 3.40: Strain gauges locations on the bottom reinforcement of Specimen ZJESSS..120

Fig. 3.41: Strain gauges locations on the shear reinforcement of Specimen ZJESSS.. ..120

Fig. 3.42: Strain gauges locations on the bottom reinforcement of Specimen ZJF8.......121

Fig. 3.43: Strain gauges locations on the bottom reinforcement (main direction) of

Specimen ZJF9...................................................................................................................... 122

Fig 3.44: Strain gauges locations on the bottom reinforcement (secondary direction) of

Specimen ZJF9....................................................................................................................... 123

Fig. 3.45: Crack detection bar details.................................................................................. 124

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Fig. 3.47: Location of the LVDTs on the bottom slab surface of Specimen ZJEF1 125

Fig. 3.48: Location of the LVDTs on the bottom surface o f Specimen ZJF9 ................. 125

Fig. 3.49: Front view of test set-up for interior column connection ................................. 126

Fig. 3.50: Side-view of test set-up for interior column connection.................................. 127

Fig. 3.51: Plan view of test set-up for interior column-slab connection ........................... 128

Fig. 3.52: The steel Z-section placed on the top edges of the slab................................... 129

Figure 3.53: Tie rods tying down the Z-section to the supporting frame .........................129

Fig. 3.54: The crack detectors hollow tubes attached to the formwork............................ 130

Fig. 3.55: Close up view of the steel reinforcement of specimen ZJESCS..................... 130

Fig. 3.56: Reinforcement in position inside the formwork for specimen ZJEFSS .......... 131

Fig. 3.57: Casting of concrete slabs using the bucket and the crane ................................ 131

Fig. 3.58: Casting the upper column stub........................................................................... 132

Fig. 3.59: Specimens after the removal of formwork........................................................ 132

Figure 4.1: Typical concentric punching failure pattern.................................................... 185

Fig. 4.2: Schematic crack patterns of interior and edge column connectionswith slab

under eccentric load.............................................................................................................. 186

Figure 4.3: Important slab parts identification and critical section location for interior

column-slab connection........................................................................................................ 187

Figure 4.4: Crack pattern on the bottom of specimen ZJF8 slab.......................................188

Figure 4.5: Crack pattern on the bottom of specimen ZJF9 slab .......................................188

Figure 4.6: Load deflection curves o f specimens ZJF4,ZJF6 and ZJF8 ........................... 189

Figure 4.7: Normalized load deflection curves of specimens ZJF4,ZJF6 and ZJF8.......189

Figure 4.8: Load deflection curves of specimens ZJF4, ZJF7 and ZJF9 .......................... 190

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Figure 4.9: Normalized load deflection curves of specimens ZJF4, ZJF7 and ZJF9 190

Figure 4.10: Strain distribution in the bottom reinforcement parallel to south face of

column stub in specimen ZJF8............................................................................................ 191

Figure 4.11: Strain distribution in the bottom reinforcement parallel to east face of

column stub in specimen ZJF8........................................................................................... 191

Figure 4.12: Strain distribution in the bottom reinforcement in specimen ZJF8...192

Figure 4.13: Strain distribution in the bottom reinforcement in specimen ZJF9...192

Figure 4.14: Distribution o f shear reinforcement strain in specimen ZJF9, (gauges 1,2 and

3) .................................................... 193

Figure 4.15: Distribution of shear reinforcement strain in specimen ZJF9, (gauges 6,7 and

8)............................................................................................................................................. 193

Figure 4.16: Distribution of shear reinforcement strain in specimen ZJF9, (gauges 10,11

and 12)................................................................................................................................... 194

Figure 4.17: Distribution of shear reinforcement strain in specimen ZJF9, (gauges

13,14,15 and 16)................................................................................................................... 194

Figure 4.18: Distribution of shear reinforcement strain in specimen ZJF9, (gauges

17,18,19,20 and 21)................................ 195

Figure 4.19: Distribution of shear reinforcement strain inspecimen ZJF9.....................195

Figure 4.20: Crack pattern o f specimen ZJEF1.................................................................. 196

Figure 4.21: Side cracks of specimen ZJEF1 ..................................................................... 196

Figure 4.22: Punching of specimen ZJEF2, fromsouth west com er................................ 197

Figure 4.23: Punching o f Specimen ZJEF1 and spalling of concrete cover.................. 197

Figure 4.24: Crack pattern at the bottom of alab in specimen ZJEF2 .............................. 198

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Figure 4.25: Crack distribution through the thickness of the slab at its connection with

column in specimen ZJEF2.................................................................................................. 198

Figure 4.26: Crack pattern o f the slab bottom in specimen ZJES .....................................199

Figure 4.27: Side cracks of specimen ZJES........................................................................ 199

Figure 4.28: Punching of specimen ZJES and spalling of concrete cover....................... 200

Figure 4.29: Normalized load-deflection curves of specimens without shear

reinforcement........................................................................................................................ 200

Figure 4.30: Load-deflection curves of specimens ZJEF1 and ZJEF3.............................201

Figure 4.31: Normalized load-deflection curves of specimens ZJEF1 and ZJEF3 201

Figure 4.32: Load-deflection curves of specimens ZJEF1, ZJEF5 and ZJEF7 ............... 202


Figure 4.34: Load-deflection curves of specimens ZJEF1 and ZJES ...............................203


Figure 4.36: Crack pattern of the bottom of the slab in specimen ZJEFCS .................... 204

Figure 4.37: Side cracks of specimen ZJEFCS ...................................................................204

Figure 4.38: Crack pattern of specimen ZJESSS............................................................... 205

Figure 4.39: Side cracks of specimen ZJESSS ...................................................................205

Figure 4.40: Crack pattern o f the bottom of the slab in specimen ZJEFSS ..................... 206

Figure 4.41: Side cracks of specimen ZJEFSS...................................................................206

Figure 4.42: Crack pattern at the bottom of the slab in specimen ZJESCS ..................... 207

Figure 4.43: Side cracks of specimen ZJESCS...................................................................207

Figure 4.44: Torsional cracks on the west side of specimen ZJEFCS.............................. 208

Figure 4.45: Opening of torsional cracks on the west side o f specimen ZJEFCS ...........208

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Figure 4.46: Load-deflection curves of shear reinforced samples.................................. 209

Figure 4.47: Load-deflection curves of specimens ZJEF3 and ZJEFCS..................... 209

Figure 4.48: Load-deflection curves of specimens ZJEF3 and ZJEFSS.......................... 210

Figure 4.49: Load-deflection curves of specimens ZJEF3 and ZJESSS .......................... 210

Figure 4.50: Load-deflection curves of specimens ZJEFCS and ZJEFSS...................... 211

Figure 4.51: Load-deflection curves of specimens ZJEFCS and ZJESCS ...................... 211

Figure 4.52: Load-deflection curves of specimens ZJEFCS and ZJESSS ....................... 212

Figure 4.53: Load-deflection curves of specimens ZJESCS and ZJESSS...................... 212

Figure 4.54: Load-deflection curves of specimens ZJEFSS and ZJESSS....................... 213

Figure 4.55: Load-deflection curves of specimens ZJES and ZJESSS.......................... 213

Figure 4.56: Load-deflection curve of specimens ZJES and ZJESSS ..............................214

Figure 4.57: Strain distribution in the bottom reinforcement, parallel to south column-

slab interface in specimen ZJEF1........................................................................................214

Figure 5.58: Strain distribution in the bottom reinforcement in specimen ZJEF5, (gauges

2,5 and 6) ............................................................................................................................. 215


4,15,18 and 19)..................................................................................................................... 215

Figure 4.60: Strain distribution of bottom reinforcement parallel to free surface, in

specimen ZJEF1.................................................................................................................... 216


column stub in specimen ZJEF2.......................................................................................... 216

Figure 4.62: Strain distribution in the bottom reinforcement parallel to free edge in

specimen ZJEF2.................................................................................................................... 217

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Figure 4.63: Strain distribution in the bottomreinforcement parallel to south face of

column stub in specimen ZJEF7..........................................................................................217


specimen ZJEF7.................................................................................................................... 218

Figure 4.65: Strain distribution in the bottomreinforcement parallel to south face of

column stub in specimen ZJES............................................................................................ 218


specimen ZJES...................................................................................................................... 219

Figure 4.67: Strain distribution in the bottomreinforcement normal to south face of

column stub in specimen ZJEFCS...................................................................................... 219


specimen ZJEFCS................................................................................................................. 220


specimen ZJEFSS................................................................................................................ .220


column stub in specimen ZJESCS...................................................................................... 221


column stub in specimen ZJESSS........................................................................................221


specimen ZJESCS................................................................................................................. 222


specimen ZJESSS................................................................................................................. 222

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4.5 and 6)............................................................................................................................... 223


8,13 and 14).......................................................................................................................... 223


2.6 and 7)............................................................................................................................... 224

Figure 4.77: Strain distribution in the bottom reinforcement in specimen ZJEFCS,

(gauges 8,11,13 and 19)....................................................................................................... 224

Figure 4.78: Strain distribution in the bottom reinforcement in specimen ZJEFSS,

(gauges 34,36,38 and 42) ..................................................................................................... 225

Figure 4.79: Distribution of shear reinforcement strain in specimen ZJEFCS,(gauges

25,26,27 and 28)................................................................................................................... 225

Figure 4.80: Distribution of shear reinforcement strain in specimen ZJESCS,(gauges

24,25,26 and 27)................................................................................................................... 226

Figure 4.81: Distribution of shear reinforcement strain in specimen ZJEFCS, (gauges

40.41.42 and 43)................................................................................................................... 226

Figure 4.82: Distribution of shear reinforcement strain in specimen ZJESCS, (gauges

37,38 and 39) ...................................................................................................................... 227

Figure 4.83: Distribution of shear reinforcement strain in specimen ZJESCS, (gauges

41.42 and 43)........................................................................................................................ 227

Figure 4.84: Distribution of shear reinforcement strain in specimen ZJESSS, (gauges 5,6

and 8)..................................................................................................................................... 228

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Figure 4.85: Distribution of shear reinforcement strain in specimen ZJEFCS, (gauges

36,37,38 and 39) ................................................................................................................. 228

Figure 4.86: Distribution of shear reinforcement strain in specimen ZJESSS, (gauges

12,13,14 and 15)................................................................................................................... 229

Figure 4.87: Distribution of shear reinforcement strain in specimen ZJEFCS at

ultimate.................................................................................................................................. 229

Figure 4.88: Distribution of shear reinforcement strain in specimen ZJEFSS at

ultimate.................................................................................................................................. 230

Figure 4.89: Distribution of shear reinforcement strain in specimen ZJESCS at

ultimate .................................................................................................................................. 230

Figure 4.90: Through-thickness slab strain in specimen ZJEF1............................. 231

Figure 4.91: Through-thickness slab strain in specimen ZJEF2 ............................. 231



Figure 4.94: Through-thickness slab strain in specimen ZJEF7............................. 233

Figure 4.95: Through-thickness slab strain in specimen ZJF9................................ 233

Figure 4.95: CFRP studs strain in specimen ZJEFCS, (for strain gauge location see Fig

3.35)....................................................................................................................................... 234

Figure 4.96: CFRP studs strain in specimen ZJESCS, (for strain gauge locations see Fig.

3.39)....................................................................................................................................... 234

Figure 4.97: Steel studs strain in specimen ZJESSS, (see Fig. 3.41 for strain gauge

locations)............................................................................................................................... 235

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Fig. 5.1: Typical distribution of shear stresses around an interior column-slab connection

transferring shear and moment (McGregor and Wight 2005) .......................................... 301

Fig. 5.2: Typical distribution of shear stresses around an edge column-slab connection

transferring shear and moment. (McGregor and Wight 2005)......................................... 302

Fig. 5.3: The idealized failure surfaces under combined shear and moments ................ 303

Fig. 5.4: Proposed critical section for shear stress distribution to be used in conjunction

with proposed refined method............................................................................................ 303

Fig. 5.5: Critical section location and its properties based on AC I318

recommendations................................................................................................................. 304

Fig. 5.6: Proposed inclined critical section for punching shear in slabs......................... 305

Fig. 5.6: Proposed inclined and rectangular critical sections and geometric properties for

edge column-slab connection............................................................................................. 306

Fig. 5.7: General layout of the critical section and shear stress distribution .................. 307

Fig. 5.8: Idealized failure mechanism for an edge column-slab connection...................308

Fig. 5.9: Assumed Mohr-Coulomb envelope according to Guralnik and Sheikh et al (

Zaghlool and de Paiva 1973 a,b)........................................................................................ 309

f vFig. 5.10: The relationships between - ^ r and kp = ~ ................................................. 309

f c f c

Fig. 5.11: Shear-moment interaction diagram for edge column connection according to

strip model prediction for specimen ZJEF1...................................................................... 310


strip model prediction for specimen ZJEF2.......................................................................310

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strip model prediction for specimen ZJEF3....................................................................... 311

Fig. 5.14:Shear-moment interaction diagram for edge column connection according to




Fig. 5.16:Shear-moment interaction diagram for edge column connection according to

strip model prediction for specimen ZJES .........................................................................312

Fig. 5.17a: Critical sections for shear in slab at d/2 from outermost peripheral o f last line

of studs.................................................................................................................................. 313

Fig. 5.17b: Critical sections for shear in slab at d/2 from outermost peripheral o f last line

of studs.................................................................................................................................. 314

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List of Symbols

a = width of column-slab interface

a' = distance from the first bar parallel to the slab edge to the slab edge

= steel perpendicular to free edge

Asp] = area o f one bar

Ast - total area o f transverse steel crossing the column-slab interface

Asv = area of shear reinforcement along the perimeter of critical section

A v = area o f set of bent bars

b = perimeter of the critical reaction taken at the periphery o f the column.

b0 = rectangular critical shear perimeter

bi = width of the critical shear section normal to the axis o f bending

b2 = width o f the critical shear section parallel to the axis of bending

c = cracked transformed section neutral axis depth

C = half width of a square column = r!2

Cl - column side perpendicular to moment vector

Ci = length of critical section parallel to the plane of the bending moment

C2 = column side parallel to moment vector

C2 = length of critical section perpendicular to the plane of the bending

moment

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d = effective depth

e = distance of the centriod of the critical section from the column centriod

E f , Efrp = modulus of elasticity of FRP reinforcement

Es, Esteei =Modulus o f elasticity of steel

fc = concrete cylinder compressive strength

f ca average compressive stress at failure

fcab = average compressive stress in the compression zone perpendicular to

the neutral axis

f cas =average concrete compressive stress in the compression zone.

f ck = concrete cube compressive strength

If = maximum tensile stress in FRP, which is obtained by strain compatibility

analysis

/f = the tensile strength of FRP reinforcement

f r = modulus of rupture of concrete

f pi = stress in the steelAspi

f y = yield stress of steel

fyv = yield stress of shear reinforcement

g = distance between column and critical section centroids

h = total thickness of slab

J c = polar moment of inertia of the assumed critical section

kt = friction coefficient

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m - modular ratio (EJEC)

M = total external moment acting on the connection

Mac = flexural moment resisted by the slab section BC

M0 = bending moment for infinite eccentricity

Ms = portion of unbalanced moment resisted by shear reinforcement

Ms = moment resisted by the slab section on one side of the column

Mtu =the total unbalanced moment acting at the centroid of the critical section

Mty = flexural capacity associated with the top reinforcement of a strip adjacent

to the spandrel strips

Mu = unbalanced bending moment strength of interior column-slab connections

Mv = bending moment produced by the eccentricity of thepart of the column

load transferred to the slab at the inside face of the column

N = number o f column-slab interfaces parallel to the applied moment

n = number of steel bars, in tension, that crosses the inside column face

77/ = modular ratio

q = reinforcement index

s = spacing of shear reinforcement, or spacing of the steel bars placed

perpendicular to the free edge of the slab

t = slab thickness

u = the peripheral length of loaded area

v = the ultimate shear strength

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V = shear force or total axial force acting on the column

Vc = punching shear strength

vc = shear stress resistance of concrete

Vc,bs =punching shear strength according to British Standards

Vc,el = punching shear strength according to ElGandour

VCie ~punching shear strength according to ElSalakawy

Vc,m&t = punching shear strength according to Matthys and Taerwe

VCio =punching shear strength according to Ospina

Vc,z&r =punching shear strength according to Zaghloul and Razaqpur

Vca - average shear stress at the moment o f failure

vCh = the average shear stress in the compression zone parallel to the neutral

axis.

Vcyb = average shear stress in the compression zone o f the failure section

perpendicular to the plane of applied bending moment and perpendicular

to the neutral axis

Vcys =average shear stress in the compression zone of the skewed section and

perpendicular to its neutral axis

Vfiex = flexural resistance of slab calculated using yield line theory approach.

Vn - nominal punching shear capacity of the slab near its connection with the

column

Vmax = the maximum allowed shear stress

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V0 =shearing force for zero eccentricity

Vs =vertical component o f force in a set of bent bars

vs = the shear resistance of the shear reinforcement

vu = ultimate shear stress

wx - the amount of shear that can be transferred by slender flexural action in

the slab strip

Xb = depth of the equivalent concrete compression block at failure

xs = depth of equivalent rectangular stress block from the top of

compressive surface of the slab

y = distance of the point of maximum shear stress from the centroid of the

critical section perimeter

y c.g. - is the distance between the centroid of the critical punching shear section

and the center o f the column

a = angle of inclination of the bent bars to horizontal

pc = ratio of longer side of column to shorter side

Ef : strain in the FRP reinforcement at failure

es = yield strain of steel reinforcement

yv = the portion of the unbalanced moment that is transferred by eccentric

shear

0o ~ Vtest! Vfiex

X = concrete density factor (1 for normal weight concrete)

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p = the average value of the slab reinforcement ratio in the two directions

pi = the ratio of the tension reinforcement placed normal to the moment vector

Pf = mean reinforcement ratio of FRP reinforcement mat

ps = the steel equivalent reinforcement ratio

ps = steel reinforcement ratio

T = the nominal shear stress associated with the tensile force in the

reinforcement

0 = reduction factor assumed by the ACI code (1983) = 0.85

c = concrete resistance factor (0 .6 )

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CHAPTER 1

INTRODUCTION

1.1 Introduction

In cold regions of the world reinforced concrete parking structures are often exposed

to de-icing salts, which cause corrosion of steel reinforcement and damage to concrete.

To solve this problem, one needs to investigate whether one can replace the steel

reinforcement by corrosion-immune carbon fibre reinforced polymer (CFRP)

reinforcement. Such a substitution would not be possible unless the CFRP reinforced

structures can economically satisfy the strength and serviceability requirements of

parking structures.

These structures often have the structural form of flat plate/slab structures that are

subjected to concentration of shear forces and moments near the slab-column

connections. Flat plate structures consist of slabs directly supported on columns while flat

slabs contain drop panels. As far as strength is concerned, the connections between the

floor slab and the columns are critically important because these regions must be able to

resist large bending moments and shear forces. Due to combined shear and moment

transfer, a connection failure can be relatively brittle and it should be prevented.

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The shear and moment to be transferred by a connection are due to different effects.

Dead and live loads acting on the slab are obvious sources, but there are other effects that

also contribute to these forces. For example, although major lateral forces caused by wind

and earthquakes are often resisted by shear walls, designers are increasingly relying on

the slab-column connections to cany an increasing portion of the lateral loads [ACI

Committee 318 (1995, 1999)]. Considerable unbalanced moment may be introduced in a

column due to uneven distribution of live loads on either side of it due to random and

unequal spacing of columns and due to volume changes caused by differences of

temperature. Differential creep between adjacent floors results in differential

displacement of the top and bottom columns, which induces moments at the slab-column

connections. In the presence of such moments, the punching shear stress distribution

becomes unsymmetrical and it reduces the strength of the slab. This phenomenon has

been observed by a number of researchers, (Hanson and Hanson, 1968, Zaghlool, 1971,

Hawkins et al, 1989, Zaghloul, 2002), and is accounted for in the modem design codes

for reinforced concrete (ACI Committee 318 (2005), CSA A23.3-94 CSA (1994), BS

8110, BSI (1997), CEB-FIP, 1993, Eurocode 2, 1992, European Committee of Concrete,

1966).

CFRP has high tensile strength, high elastic modulus and a reasonable ultimate strain

capacity. Due these characteristics and its corrosion-immunity, it has the potential to

replace the steel reinforcement in the slab, provided it can satisfy the serviceability and

strength requirements of parking structures. Limited experimental data on the punching

behaviour of FRP reinforced slabs is available in the literature, but most of the existing



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3

data are derived from tests on FRP reinforced slabs subjected to concentric punching

shear stresses only. To date no tests have been conducted on the punching behaviour of

edge column-slab connections reinforced with FRP. Since such connections are prevalent

in flat plate/slab structures, their punching shear behaviour must be investigated to derive

safe design guidelines. This need forms the motivation for the present study.

1.2 Problem definition

Carbon fibre reinforced polymer (CFRP) reinforcement is virtually immune from

corrosion. The problem that we need to investigate is whether FRP reinforced parking

structures can satisfy the serviceability and strength requirements. In flat plate/slab

parking structures a crucial structural component is the slab-column connection, which is

often subjected to large shear forces and bending moments. These forces can create

severe stresses in the connections, particularly in edge and comer column connections

with the slab. The writer is not aware of any study to date related to the strength and

behaviour of FRP reinforced edge and comer column-slab connections. Hence, before

any FRP reinforced parking structure can be constructed, it is important to investigate the

behaviour and strength o f the latter connections.

1.3 Objectives and scope

This research is aimed at the investigation of the punching shear behaviour of CFRP

reinforced slab-column connections, including edge and interior columns connections.



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4

Slab-column connections subjected to combined axial load and unbalanced bending

moments will be tested.

The study will focus on the effect of some key parameters that are known to affect

the punching shear strength and behaviour of similar steel reinforced concrete structures.

In addition, the effect of the specific characteristics of CFRP, such as its high strength to

elastic modulus ratio and its linear elastic behaviour will be studied. To increase the

punching shear strength of FRP reinforced slab-column connections, a new FRP shear

reinforcement system will be introduced and tested.

In addition, the applicability of some existing design methods or code

recommendations for punching shear to CFRP reinforced slab-column connections will

be investigated, and if necessary, modifications will be suggested to generalize the

current design recommendations to FRP reinforced structures.

More specifically, in order to study the effect of the following parameters on punching

shear behaviour of edge column-slab connections, ten specimens were constructed and

loaded to failure. The parameters of interest are:

1. MTV ratio.

2. Column side, C2, over effective depth, d, ratio, C2/d.

3. Column stub aspect ratio, C1/C2 . i.e. loading area aspect ratio.

4. Presence of shear reinforcement and its behaviour.

5. Ratio of slab flexural reinforcement,p.

6 . Type of flexural reinforcement, i.e. steel versus CFRP.



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5

In addition, two interior column-slab connections will be tested to supplement a pilot

study result previously performed by the writer. The purpose of the study is primarily to

further investigate the efficacy of the proposed new CFRP shear reinforcement.

It is expected that the present experimental data, supplemented with available data in the

literature, will provide adequate basis for the derivation of a suitable design method for

both interior and edge column connections.

It should be emphasized that the scope of this study is limited to one type of FRP

reinforcement; namely, a CFRP grid known as NEFMAC. Despite this fact, the results

may be interpreted and analyzed in the context of key characterizing parameters, such as

elastic rigidity and geometric properties o f the connection.



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CHAPTER 2

LITERATURE REVIEW__________________________

Part I: Punching shear of FRP Reinforced slabs

2.1 General

One-way shear in structural members reinforced with FRP has been investigated by a

number of researchers and is the subject of on-going research while two-way shear in

slabs reinforced with FRP reinforcement has not been widely studied. A few researchers

(Ahmad et al 1994, Banthia et al 1995, Mathys and Taerwe 1997, El-Ghandour et al

1997, Ospina et al 2003) have studied, with variable levels of detail, two-way punching

shear in flat plates and bridge decks. However, all existing studies have been concerned

with punching shear due to axial loading only. Only the work conducted by Zaghloul and

Razaqpur, (2002, 2003-A, 2003-B) included two-way punching shear of interior flat slab-

column connections reinforced with CFRP grids subjected to combined action of shear

and unbalanced moment.

This chapter begins with a review of available experimental studies concerned

with punching shear in structural slabs reinforced with different types of internal FRP

reinforcement. For the sake of comparison and to help understand those aspects of the

punching shear behaviour of FRP reinforced members that have not been fully

investigated yet, literature pertinent to the punching behaviour of steel reinforced slabs is

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subsequently discussed.7

2.2 Punching shear in slabs reinforced with FRP

(a) Concentric shear

As stated earlier, only limited experimental work is available regarding the concentric

shear resistance of FRP reinforced slabs or of slab-column connections reinforced with

FRP. Ahmad et. al. (1994), Matthys and Taerwe, (1997, 1998, 2000), ElGhandour et al.

(1996, 1997, 1998, 2000, 2003) and Opsina et al. (2000) conducted some studies on the

punching shear strength of FRP reinforced slabs. However, the behaviour of FRP slab-

column connections is still a subject that requires extensive investigation.

Ahmad et al. (1994) conducted a preliminary study on punching shear of slabs

reinforced with carbon fibre reinforced polymer (CFRP) fabric. They tested six 76 mm

thick square concrete slabs with side length of 690 mm. Four of the slabs were reinforced

with 3-D continuous carbon fibre fabric while the other specimens were reinforced with

conventional mild steel reinforcement. Two of the CFRP reinforced specimens and the

two steel reinforced control specimens were fabricated with a column stub connected to

one face of the slab; the remaining two CFRP reinforced specimens did not have column

stub.

The reinforcement ratios of the 3D grids in the three directions (x, y and z) for the

CFRP reinforcement slabs were 0.95 percent and the average effective depth of the three

layers of the 3D fabric was 41 mm. The average value for the apparent modulus of

elasticity of the CFRP reinforcement was 113 GPa with an ultimate strain varying



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8between 0.8% and 1.18%. The steel bars used for reinforcing the slabs were deformed

bars with average yield strength o f 400 MPa and the reinforcement ratios in the in the two

orthogonal directions were 1.18% and 1.35%. The average effective depth in both

directions was 61 mm and the concrete strength was 30 MPa.

The slabs were subjected to concentric punching and their crack pattern at

ultimate load indicated that they failed in punching shear. It also indicated that the total

area of the perimeter crack surrounding the loaded area was smaller for CFRP slabs

compared to steel reinforced slabs. The pre-cracking behaviour and the initial stiffness of

the two slab types were similar; however, the post cracking behaviour of CFRP

reinforced slabs drastically differed from that of the steel reinforced slabs. After initial

cracking, the stiffness of the CFRP slabs substantially diminished whereas the reduction

in the stiffness of the steel reinforced slabs was small. The CFRP reinforced slabs

exhibited significant non-linear behaviour before the maximum load, and the softening

portion of the deformation was significant. Ahmad et al. considered the post-peak load

softening behaviour as a relative measure of ductility and an indication of redistribution

of stresses after the maximum load. The experimental ultimate load values were 8 to 27

percent higher than that predicted by the ACI Code equations (ACI 318-89, 1989) and 10

to 25 percent lower than that predicted by the British Standard BS-8110-87 (BSI, 1987).

Banthia et al. (1995) studied the behaviour of slabs reinforced with 2D FRP grids

and compared it with that of slabs reinforced with a steel mesh. Their experimental

program comprised four 600 X 600 X 75 mm slabs with an effective depth of 55 mm.

Three specimens were reinforced with a CFRP grid called NEFMAC, which had 5 ribs in

each direction with centre-to-centre spacing of 100 mm in both directions. The cross



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9sectional area of each rib was 20.54 mm2. The composite tensile strength and modulus of

elasticity were 1200 MPa, and 100 GPa, respectively. The remaining slab was reinforced

with ordinary steel mesh whose centre-to-centre spacing was 1 0 2 mm with a bar cross-

sectional area of 19.62 mm ' The steel mesh was made of high-carbon steel with yield

strength of 448 MPa and ultimate strength of 917 MPa. The compressive strength of the

slabs concrete based on 100 x 200 mm concrete cylinders at 14 days were 41.0, 41.5 and

52.9 MPa for normal strength, normal strength with fibres, and high strength concrete

respectively,

The slabs were tested under concentric load, using a 100 mm diameter loading

cap and were simply supported with a clear span of 500 mm. The maximum strain

measured on the FRP ribs was 4000 micro-strains.

It was reported that slabs reinforced with FRP grids are more brittle than the slabs

reinforced with steel mesh. In all the slabs failure was due to punching and the punched

area was more pronounced in the steel reinforced slab. The FRP reinforced slabs energy

absorption and strength was improved remarkably by the use of steel fibres while the

high strength concrete slab capacity was less than that of normal concrete steel

reinforced slab but its overall energy absorption was higher.

Matthys and Taerwe, (1996, 1997, 2000a, 2000b) performed 17 concentric

punching tests on square slabs with side length of 1 0 0 0 mm and a total thickness of 1 2 0

mm or 150 mm. All test specimens, except two, were obtained by saw-cutting 1 m from

longer one-way slabs previously tested in bending. The saw-cut specimens had three to

five pre-existing cracks prior to punching shear test. The remaining two specimens were

steel reinforced slabs which were cast later and used as reference (R2 and R3).



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10These slabs were designed based on two different criteria, the first being the

flexural strength and the second the flexural stiffness to satisfy serviceability

requirements. The investigated parameters were reinforcement ratio, slab thickness and

loaded area. It is worth mentioning that these slabs did not have reinforcement near the

compression face.

The test specimens were subdivided into three series: the first series consisted of

four steel reinforced reference slabs; the second series comprised eight slabs reinforced

with different types of carbon FRP grid; and the third series contained five slabs

reinforced with a hybrid glass-carbon fibre FRP. In the first series the slabs were

reinforced with 12, (j>14, and S500 steel meshes and corresponding reinforcement

ratio of 0.58%, 1.29%, and 1.79%, and effective depth of 90 mm, 8 8 mm and 8 6 mm,

respectively. The second series included two subsets; the first one included 6 specimens

reinforced with CFRP NEFMAC grids, and the second subset consisted of two specimens

reinforced with CFRP CS mesh (carbon bars 5 mm, sanded surface). The NEFMAC

grids used were CIO, C l3 and C16 grids. The NEFMAC reinforcement ratio was 0.27%,

1.05% or 0.52%, corresponding to effective depth of 96 mm, 95 mm and 126 mm,

respectively. The CS mesh reinforced slabs were 120 mm thick and had a reinforcement

ratio of 0.19% with an effective depth of 95 mm.

The third series of slabs were reinforced with the hybrid or H type NEFMAC

designated as H10, HI 6 and HI9. The corresponding reinforcement ratios for the later

slabs were 0.62%, 1.22% and 3.76%, respectively, with effective depth of 95 mm,

122mm, or 89 mm, respectively.



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11The average concrete cylinder strength varied from 26.3 MPa to 35.1 MPa except

for one slab in the H series, which was made of high strength 96.7 MPa concrete. The

slabs were supported on 8 equidistant points arranged in a circular pattern with a 900 mm

diameter. The steel loading plate had diameter of 80 mm, 150 mm, or 230 mm and the

load was applied concentrically.

The test results showed that the stiffness of the slabs under loads up to 25% to

35% of their ultimate strength was basically independent of the reinforcement ratio or

type and for specimens designed with comparable flexural rigidity, stiffness degradation

was similar up to failure . However, the stiffness of FRP reinforced slabs was less than

that of the steel reinforced slabs designed for comparable flexural strength.

Matthys and Taerwe noted that prior to failure the damage was dominated by two

or three wide flexural cracks running parallel to the directions of the grid reinforcement

and extending over the total slab width, but ultimately the punching cone developed in all

the slabs and failure occurred due to punching shear. The average angle of inclination for

the punching cone were 30.7 for steel reinforced slabs, 29.2 for different CFRP grids

and 26.8 for H type slabs. They stated that the cracking behaviour of slabs reinforced

with steel or CS meshes were similar but significantly different from the cracking

behaviour of slabs reinforced with CFRP or H type grids. They attributed the latter to the

good bond behaviour of the steel and CS meshes, and the lack of good bond in the ribs of

CFRP and H type grids. In the case of good bond behaviour; cracks radiated outwards in

all directions, while in the case of poor bond mechanical anchorage of transverse ribs of

the grids initiated wide cracks over the total slab width parallel to the grid ribs. Although

slippage occurred in all the tested specimens, it was more pronounced in slabs reinforced



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12with CFRP and H type grids where it caused the cracks to suddenly widen. They also

noticed that whenever slippage happened, the failure was less brittle.

Matthys and Taerwe reported that for FRP reinforced slabs with similar flexural

strength as the steel reinforced slabs, the failure load was considerably smaller than the

steel reinforced slabs, except for one H type slab made of high strength concrete, which

failed in flexure. Also these FRP reinforced slabs showed low stiffness in the cracked

state, resulting in greater deflection which was twice as large as in the reference slabs.

For the FRP reinforced slabs with higher reinforcement ratio or increased slab depth with

comparable flexural stiffness in the fully cracked state as the reference steel reinforced

slabs the punching failure loads were higher or similar to those of the reference slabs. The

slab with greater slab depth exhibited the best overall behaviour, i.e. higher cracking and

ultimate load and a higher stiffness in the fully cracked state. Also, higher failure loads

were found with increasing loading plate diameter; however, this parameter was less

important than the reinforcement ratio and slab thickness.

These investigators calculated the punching failure load of their test slabs using

some well-known empirical or code equations and compared the results with their

experimental data. They found that these equations give fairly good results if an

equivalent reinforcement ratio is used. This ratio is calculated by multiplying the actual

reinforcement ratio by the ratio of the modulus of elasticity of the reinforcement to the

modulus of elasticity of steel. They found that MC90 Code (1990) equation showed the

smallest scatter. Also the modified mechanical model based on Hallgrens (1996) work

predicted the behaviour of both steel and FRP reinforced slabs rather well. Finally, they

noted that the results of the simplified model by Menetrey (1996), a model derived based



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13on finite element analysis results, were largely dependant on the assumption of the cone

angle and underestimated the punching capacity considerably.

El-Ghandour et al. (1997, 1998, 1999, 2000) reported the test results for eight

concentrically loaded slab-column connections in a flat plate structure. The 2.0 x 2.0 m2

slabs were 175 mm thick with a square column stub located centrally below the plate.

Each slab was reinforced with symmetrical top flexural reinforcement mat, four

reinforced with glass FRP bars and the other four with FRP carbon bars. Two of each

slab type contained carbon FRP shear reinforcement. Unfortunately, the first four

specimens had rather low reinforcement ratio and wide spacing between the

reinforcement bars and consequently failed due to bond slip at loads less than their

expected flexural and punching shear capacities. The shear reinforcement increased the

slab load capacity, and it retarded slip initiation, but did not eliminate it. The remaining

four slabs failed in punching, and were used to verify their theoretical analysis. The

analysis involved modification to the British Standards BS-8110 equation for calculating

punching shear strength using a modified strain approach and they reported accurate

punching shear capacity predictions for both the GFRP and the CFRP reinforced slabs.

Ospina et al. (2000) tested four specimens to simulate full-scale isolated interior

slab-column connection under a concentric load applied

momenat i proboj

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