The structural use of fibrous cement in partially prestressed composite concrete construction

THE _{STRUCTURAL USE} OF FIBROUS _- CEMENT IN PARTIALLY PRESTRESSED COMPOSITE

CONCRETE CONSTRUCTION

A THESIS SUBMITTED _{FOR THE DEGREE OF} DOCTOR OF _PHILOSOPHY

BY

JOHN _{SAUNDERS BSc}

DEPARTMENT OF CIVIL ENGINEERING THE UNIVERSITY OF SALFORD

SYN0PSIS

A new concept in composite construction has been deve- loped _{at the University of Salford*, involving the use of}

fibre-reinforced _{cement channels, combined structurally}

with partially prestressed composite concrete T-beams.

The British Standards Institution _{Code of Practice,}

CP110: 1972, "The _{Structural Use of Concrete", permits} the use of Class 2 and Class 3 (partially _{prestressed) concrete} members in structural design'. The limiting design _criteria

for _{such members are usually the limit'states of deflection} and cracking and therefore, _{an improvement} in their

flexural behaviour _{would be beneficial. This improvement} may be brought _about by the addition _of fibre-reinforce-

ment, in the form _of two or three dimensionally _randomly

distributed _{fibres. The Code of Practice, CP110: 1972, does} hot, however, _{give any guidance on the use of fibres in}

structural _{members and it} is _{also apl. arent} that _{the methods} outlined in the _{code for} the _{calculation of} the limit

states _are limited _{and can be improved.}

I

The flexural behaviour _{of twenty-two partially pre-}

stressed _{composite concrete} T-beams _{was investigated.}

Each beam consisted _{of a precast partially prestressed} X-joist _{web, combined with a cast-insitu lightweight}

aggregate concrete flange. Alkali-resistant _glass fibre- reinforced _{cement channels were placed at} the _{soffits of}

six beams and steel fibre-reinforced _{concrete was used} in the _{webs of two beams. The T-beams were subjected to}

short-term, long-term and fatigue loading and their struc- tural _{performance was considered} in terms _{of strength,}

cracking and deformation.

Theoretical _{relationships are derived} between the

applied moment and the depths of the _neutral axes of stress and bending, enabling a design equation relating applied

moment to the steel stress to be developed. Subsequently,

design _equations for the _{calculation of} the limit _{states of} deflection _{and cracking are} developed, _{which are} directly

applicable to both conventional and fibre-reinforced structural members.

The use of a fibre-reinforced cement channel as an integral _{structural part of a concrete member results} in many important advantages _{when compared} with conventional

concrete members and the test _results show that they con- siderably improve the structural performance of the

partially prestressed composite concrete T-beams.

University _{of Salford: Improveffients in or relating to} constructional _{elements of concrete. Patent application}

number 49179/72, British _{Patents Office, London, October} 1972.

ACKN0WLEDGEMENTS

The author is grateful to Professor T. Constantine,

BSc, PhD, CEng, FICE, FIMunE, Professor _{of Civil} Engineering and Chairman of the Department of Civil Engineering at the University _{of Salford,} for _providing the research facilities.

The author wishes to express his extreme gratitude to his _supervisor, Professor E. R. Bryan, MSc(Eng), PhD, DSc,

CEng, FICE, CIStructE, Professor _{of Structural} Engineering at the University of Salford, for his advice and guidance during the _{research work.}

The author is particularly indebted to Mr D. C. OtLeary, MSc, CEng, MICE, for his _{general supervision and constant}

advice and encouragement throughout the work.

The author would _also like to thank Dr N. J. Dave,

MEng, PhD, CEng, AMIE, _{MASCE, MICE,} for his help _{and advice} during the development _work.

Special _{and sincere} thanks _are due to the late Mr F. W. Simon, for his technical _{advice and assistance} in the

laboratories.

The author also wishes to thank Mr K Naylor and all the staff of the Civil Engineering Laboratories at the University

of Salford, for their technical assistance during the practical work.

The author would like to acknowledge the financial

support _provided by the University _{of Salford and the} Science Research _{Council. The author further acknowledges Mills}

Scaffold _{Company Limited,} for the _{supply of fibrous-cement}

channels, National Standard Company Limited, for the _supply

of steel fibres, Richard Johnson _{and Nephew Limited,} for the supply _{of prestressing-steel,} the Cement Marketing Corporation for the _{supply of cement and Fram Precast Concrete Limited}

for the initial _{supply of Xg flooring joists.}

Finally, the _{author would like to sincerely thank his}

wife for conscientiously typing the text _{and for} her _constant support, encouragement _{and, above all, patience} throughout

the work.

C0NTENTS

Page SYNOPSIS

ACKNOWLEDGEMENTS

CONTENTS v

NOTATION xii

ABBREVIATIONS xvii

LIST OF TABLES xviii

LIST OF FIGURES xix

LIST OF APPENDICES _xxiv

LIST OF PLATES _xxv

PUBLICATIONS _xxvi

CHAPTER 1- INTRODUCTION

1.1. COMPOSITE _{CONCRETE CONSTRUCTION} 1

1.2. _{PARTIAL PRESTRESSING} 2

1-3- _{LIGHTWEIGHT AGGREGATE CONCRETE} 5

1.4. _{FIBRE REINFORCEMENT} 6

1.5. _{FIBROUS-CEMENT COMPOSITE} CONSTRUCTION 7

1.5-1. _INTRODUCTION 7

1.5.2. ADVANTAGES 9

1.6. _LIMITATIONS OF THE UNIFIED CODE 11

1.7. OBJECTIVES OF THE INVESTIGATION 12

1.8. OUTLINE OF THESIS 13

CHAPTER 2- STATE OF THE ART

2.1.. COMPOSITE CONSTRUCTION 15

2.2. PARTIAL PRESTRESSING 17

2-3- FIBRE REINFORCEMENT 23

2-3-1- MECHANISM OF FIBRE REINFORCEMENT ₂₃ 2-3.2. PHYSICAL AND _-MECHANICAL PROPERTIES 25

CHAPTER _3- LIMIT STATE DESIGN Page

3-1- GENERAL 29

3.2. BASIC PRINCIPLES 30

3-3- DESIGN PRINCIPLES 31

3.4. CALCULATIONS OF THE LIMIT STATES 32

3.4.1. GENERAL 32

3.4.2. ULTIMATE LIMIT STATE 33

3.4-3- SERVICEABILITY LIMIT STATE 34

3.4-3-1. LIMIT STATE OF DEFLEC- 34

TION

3.4-3.2. LIMIT STATE OF CRACKING 37

CHAPTER 4- _{PROPOSED METHODS FOR} CALCULATING THE PRINCIPAL LIMIT STATES

4.1. _{INTRODUCTION 39}

4.2. _{ULTIMATE LIMIT STATE} 4o

4.2.1. _{CONVENTIONAL CONCRETE BEAM} 4o

4.2.2. _{FIBRE-REINFORCED CONCRETE BEAM} 43

4-3- _{SERVICEABILITY LIMIT STATE} 45

4-3-1. _{LIMIT STATE OF DEFLECTION} 45

4-3-1-1- _{UNCRACKED BEAMS} 45

4-3-1.2. _{CRACKED BEAMS} 48

4-3-1-3- _{PROPOSED RELATIONSHIP BE- 52} TWEEN THE APPLIED MOMENT

AND THE DEPTHS OF THE NEUTRAL AXES OF STRESS AND BENDING

4-3-1.4. _{RESIDUAL DEFLECTION 58}

4-3-1-5. _{SUMMARY OF PROPOSED DE- 59} FLECTION FORMULAE

CHAPTER

4-3.2.

_LIMIT

_{STATE OF CRACKING}

4-3.2.1.

_INTRODUCTION

4.3.2.2.

_{PROPOSED CRACK WIDTH}

FORMULAE

5-

_DESIGN,

_{MANUFACTURE AND TESTING}

OF

60

6o

61

THE

BEAMS

5.1.

_GENERAL

64

5.2.

_{DESIGN OF TEST BEAMS}

64

5.2-1.

_{RECTANGULAR BEAMS}

64

5.2.2.

_{COMPOSITE CONCRETE T-BEAMS}

65

5.3.

_MATERIALS

₇₀

5-3-1-

_CONCRETE

₇₀

5-3.2.

_STEEL

₇₁

5-3.2.1.

_{PRESTRESSING STEEL}

₇₁

5-3.2.2.

_{REINFORCING STEEL}

₇₂

5-3.2.3.

_{SHEAR AND BOND STEEL}

₇₃

5-3-ý-

_FIBRES

₇₄

5-3-3-1-

_{STEEL FIBRES}

₇₄

5-3-3.2.

_{GLASS FIBRES}

74

5.4.

_{MANUFACTURE OF TEST BEAMS}

₇₅

5.4.1.

_{RECTANGULAR BEAMS}

₇₅

5.4.2.

_{COMPOSITE T-BEAMS}

₇₅

5.4.2.1.

_{PRECAST WEB}

₇₅

5.4.2.2.

_INSITU

_FLANGE

76

5.4-3-

_{CONCRETE CONTROL SPECIMENS}

₇₇

5.5.

_{TEST PROCEDURE}

₇₈

5.5-1.

GENERAL

₇₈

5.5.2.

SHORT TERM TESTS

₇₈

5.5-2-1.

_{RECTANGULAR BEAMS}

₇₈

5.5.2.2.

_T-BEAMS

₇₈

5.5-3.

FATIGUE TESTS

79

5.5.4.

LONG TERM TESTS

8o

Page

5.5-5. _{INSTRUMENTATION} 8o

5.5-5-1. STRAIN MEASUREMENT 8o

5.5-5.2. DEFLECTION MEASUREMENT 81

5.5-5-3- CRACK WIDTH MEASUREMENT 81

5.5.6. _{CONTROL TESTS} 81

5.5.6.1. CONCRETE CONTROL SPECIMENS 81

5.5.6.2. STEEL CONTROL SPECIMENS 84

5.5.6-3. FIBROUS CEMENT CONTROL SPECIMENS

1 84

CHAPTER 6- ANALYSIS OF CONCRETE AND STEEL STRESSES

61

.

1.

INTRODUCTION

85

6.2.

_{STRESSES PRIOR TO TESTING}

85

6.2.1.

_{STRESSES AT TRANSFER}

₈₅

6.2.2.

_{STRESSES DUE TO SELF-WEIGHT}

₈₆

6.2-3-

_{LOSSES OF PRESTRESS}

₈₆

6.2-3-1.

_{THEORETICAL ANALYSIS}

₈₆

6.2-3.2.

_{EXPERIMENTAL ANALYSIS}

₈₈

6.2.4.

_DIFFERENTIAL

_{SHRINKAGE STRESSES}

₉₂

6.2.4.1.

_GENERAL

₉₂

6.2.4.2.

_{THEORETICAL ANALYSIS}

₉₂

6.2.4-3.

_{EXPERIMENTAL ANALYSIS}

₉₇

6-3-

_{STRESSES AFTER THE START OF THE TESTS}

₉₈

6-3-1.

_{EXPERIMENTAL ANALYSIS}

₉₈

6-3-1-1-

_{UNCRACKED BEAM}

₉₈

6-3-1.2.

_{CRACKED BEAM}

₉₉

6.4.

_{NEUTRAL AXES OF STRESS AND BENDING}

₁₀₁

6.4.1.

_GENERAL

₁₀₁

6.4.2.

_{SHORT TERM TESTS}

₁₀₂

6.4-3.

_FATIGUE

_{AND LONG TERM TESTS}

₁₀₃

Page

6.5. _{CONCRETE FORCES lo4}

6.5-1. _{GENERAL lo4}

6.5.2. _{COMPRESSIVE CONCRETE FORCE 105}

6.5-3. _{TENSILE CONCRETE FORCE 105}

CHAPTER 7- _{DISCUSSION AND CORRELATION} OF TEST RESULTS _WITH PROPOSED THEORIES

7.1. _{INTRODUCTION log}

7.2. _{GENERAL BEHAVIOUR OF TEST BEAMS log}

7.2.1. _{STRESSES PRIOR TO TESTING log}

7.2.1.1. _LOSSES OF PRESTRESS ₁₁₀

7.2.1.2. _{DIFFERENTIAL SHRINKAGE STRE SSES} 112

7.2-1-3- _{RESIDUAL PRESTRESS 118}

7.2.2. _{TEST RESULTS FOR SERIES X}

lig

7.2.2.1. _{ULTIMATE LOADS 119}

7.2.2.2. _{DEFLECTIONS 121}

7.2.2-3- _{CRACKING 124}

7.2-3- _{TEST RESULTS FOR RECTANGULAR BEAMS 125} 7.2-3-1- _{UNREINFORCED CONCRETE BEAMS 125} 7.2-3.2. _{REINFORCED CONCRETE BEAMS 126}

7-3- _{ANALYSIS OF TEST RESULTS 128}

7-3-1. _{NEUTRAL AXES OF STRESS AND BENDING 128}

7-3-1-1. _{GENERAL 128}

7-3-1.2. _{SHORT TERM TESTS 128}

7-3-1-3. _{LONG TERM TESTS 130}

7-3-1.4. _{FATIGUE TESTS 130}

7-3.2. _{STEEL STRESSES 131}

7-3.2.1. _{SHORT TERM TESTS 131}

7-3.2.2. LONG TERM _{TESTS 133}

7-3.2-3. _{FATIGUE TESTS 134}

7.4. _{ULTIMATE LIMIT STATE}

7.4.1. _{SHORT TERM TESTS} 7.4.2. _{LONG TERM TESTS} 7.4.3. _{FATIGUE TESTS}

7.5. _{SERVICEABILITY LIMIT STATE}

7.5.1. _{LIMIT STATE} OF DEFLECTION 7.5-1-1. _{SHORT TERM} TESTS 7.5-1.2. LONG TERM TESTS

7.5.1.3. FATIGUE TESTS

7.5.2. _{LIMIT STATE} OF CRACKING

7.5.2.1. _SHORT TERM TESTS 7.5.2.2. _{LONG TERM} TESTS 7-5.2.3. _FATIGUE TESTS

CHAPTER 8- CONCLUSIONS

CONCLUSIONS _{FROM PRESENT} INVESTIGATION

8.1.1. _{CONVENTIONAL CONCRETE COMPOSITE} CONSTRUCTION

8.1.1.1. _{COMPOSITE CONSTRUCTION} 8.1.1.2. _{PARTIAL PRESTRESSING}

8.1.1-3. _{LIGHTWEIGHT AGGREGATE} CONCRETE

COMPRESSION REINFORCEMENT 160

8.1.1.5. _{LOSSES OF PRESTRESS 161}

8.1.1.6. _{- DIFFERENTIAL SHRINKAGE 161}

8.1.1.7. _{ULTIMATE LIMIT STATE 162}

8.1.1.8. _{SERVICEABILITY LIMIT STATE 162} 8.1.2. _{FIBRE CONCRETE COMPOSITE CONSTRUC- 163}

TION

8.1-3- _{FIBROUS CEMENT COMPOSITE CONSTRUC- 164} TION

8.2. _{SUGGESTIONS FOR FUTURE INVESTIGATIONS 166}

Page

REFERENCES

TABLES

FIGURES

APPENDICES

PLATES

167

N0TAT10N

GENERAL

A _{- Area of transformed concrete section} Ai _{- Area of cast-insitu} flange

Ap _{- Area of precast web} A

sc - Area of compression steel A

st - Area of tensioned steel A

su - Area of untensioned steel

Asf _{- Equivalent area of reinforcing steel} b _{- Width of cast-insitu flange}

b- Width _{of precast web}

d- Effective depth _{of tension steel}

d _{- Effective} depth _{of compression steel} sc

dst _{- Effective} depth _{of tensioned steel} d

su - Effective depth of untensioned, steel

dc _{- Depth to centroid of compression} forceC d

ch - Depth to centroid of tensile force in fibrous cement channel, T

ch d

ct - Depth t o centroid of tensile force in concrete, Tc

dd _{- Depth t o centroid of compressive} force _{at decompression} c

Ec _{- Modulus of elasticity of concrete}

Ei _{- Modulus of elasticity of cast-insitu flange} EP _{- Modulus of elasticity of precast web}

Es _{- Modulus of elasticity of steel} E

sc - Modulus of elasticity of compression steel E

st - Modulus of elasticity of tensioned steel

Esu _{- Modulus of elasticity of untensioned steel}

hf- Depth _{of cast-insitu} flange

Io _{- Second moment of area of transformed} concrete section.

1- _{Clear span}

M- Esc/Ei

n- Asc/Ai

q- Sfst/sfsu

r- Ast/Asu

tf- _{Age of cast} insitu flange

tw- _{Age of web when flange cast} Vf- Volume fraction _{of fibres}

x- Depth to neutral axis

Xb _{- Depth} to neutral _{axis of bending} x

cr - Depth to neutral axis of stress at instant of first cracking

xs- Depth to neutral _{axis of stress} 1/r

b- Curvature of beam at mid point - Curvature of beam at point x

Ym _{- Partial safety} factor for _materials 0- _{Creep coefficient}

DEFORMATIONS

a- Mid point deflection a

max - Maximum deflection on previous cycle ares _{- Residual} deflection

w- Crack width

Wmax - Maximum crack width

FORCES

C _{- Total compressive} force

cc _{- Compressive} force in _concrete CS _{- Compressive} force in _steel

F _{- Differential shri nkage} force T _{- Total} tensile _{for ce}

TC _{- Tensile} force in _concrete T

ch - Tensile force in fibrous-cement channel Ts _{- Tensile} force in _steel

Tst _{- Tensile} force in tensioned _steel

Tsu _{- Tensile} force _{in untensioned steel}

MOMENTS

M

- Applied moment

SM _{- Increase in applied moment}

Mcr _{- Applied moment causing cracking} Md _{- Dead load moment}

M

dc - Applied moment causing decompression Ml _{- Live} load _moment

Mmax _{- Maximum moment on previous cycle}

Mo _{- Total external moment at decompression} M

ult - Ultimate moment

Ml _{- Maximum moment on precracking cycle}

STRAINS

e-

Strain

in concrete

c

ech

_{- Applied}

strain

in fibrous-cement

channel

ei

_{- Free shrinkage}

strain

of cast-insitu

flange

ei

_{- Apparent}

shrinkage

of cast-insitu

flange

ep-

Shrinkage

plus

creep strain,

in precast

web

esc

_{- Applied}

strain

in compression

steel

e-

Applied

strain

in tensioned

steel

st

esu

_{- Applied}

strain

in untensioned

steel

el

_{- Concrete}

strain

_{measured at bottom}

of section

e4-

Concrete

_strain

_{measured at top of section}

p-

Differential

_shrinkage

_strain

STRESSES

fc- _{Compressive stress in concrete}

.f-

cr

Tensile

stress

in concrete

at instant

of cracking

fmr

- Modulus of rupture of precast concrete fs- _{Tensile stress in steel}

f

st - Tensile stress in tensioned steel f

su - Tensile stress in untensioned steel

Sfst _{- Increase} in stress in tensioned _steel

8fsu _{- Increase} in _{stress in untensioned steel} f

stdc - Tensile stress in tensioned steel at decompression f

sudc - Tensile stress in untensioned steel at decompression f

sto - Tensile stress in tensioned steel at start of test f

suo - Tensile stress in untensioned steel at start of test ft- Hypothetical tensile _{stress in concrete}

ftmax _{- Maximum hypothetical tensile stress in concrete}

f- Differential _{shrinkage stresses at bottom of beam} f

mt - Stress in tensioned steel

f

mu - Stress in untensioned steel f

yt - Characteristic strength of tensioned steel f

yu - Characteristic strength of untensioned steel T- Nominal tensile _stress in _concrete

Ut _{- Characteristic cube strength at transfer} Uw _{- Characteristic cube strength}

ABBREV IAT IONS

A. C. I. _{- American} Concrete Institute

A. S. C. E. _{- American} Society _{of Civil} Engineers

B. S. _{- Britisli} Standard

C. and C. A. - Cement and Concrete Association

C. E. B. _{- Committee European} du Beton

C. P. _{- Code of Practice}

F. I. P. _{- Federation Internationale de la Precontrainte}

I. A. B. S. E. _{- International Association of Bridge and} Structural _Engineers

I. C. E. _{- Institution of Civil Engineers}

P. C. A. _{- Portland Cement Association}

Prestressed _{Concrete Institute}

IST0FTABLES

5.1. _{Test Programme}

5.2. _{Concrete Mix Proportions}

5-3- _{Precast Concrete} Properties

5.4. _{Insitu Concrete} Properties

5.5.

_Steel

_Properties

5.6.

Fibrous-Cement

Properties

7.1.

Prestresses(Observed)

in Soffit

_{of Beams (N/mm}

2

7.2.

_Prestresses

(Calculated)

_{in Soffit}

_{of Beams (N/mm2)}

7-3-

_Differential

_Shrinkage

_Strains

7.4.

_{Observed Loads for T-Beams (M)}

7.5.

_Calculated

_{Loads for T-Beams (M)}

7.6.

_Ratio

_{of Observed}

to Calculated

Loads

7.7.

_{Test Results}

for Rectangular

Beams

7.8.

_Test

Results

for T-Beams - Deflections

7.9.

_{Test Results}

for T-Beams - Crack Widths

LIST0FFIGURES

1.1. Composite Floor Construction

1.2. _Fibre Distributions

, 3-1- Design Assumptions for Ultimate Limit State

3.2. Design Assumptions for Limit State of Deflection

4.1. _{Strain-Compatability Curve}

4.2. _{Stress Conditions at Decompression and after} Cracking

4-3- _{Neutral Axes of Stress and Bending under Short} Term Loading

4.1 4. _{Proposed Relationships between Applied Moment} and the Depths _of the Neutral Axes of Stress

and Bending

4-5- _{Stress-Conditions at Zero Applied Load and at} Instant _{of Cracking}

4.6. _{Curvature Distribution along a Cracked Beam}

4.7. _{Proposed Relationship between Hypothetical} Tensile Stress _{and Crack} Width

5.1. Rectangular Beam Details _{- Series} P

5.2. Rectangular Beam Details _{- Series} R

5-3- T-Beam Details _{- Series} X

5.4. T-Beam. Details _{- Longitudinal} Section _{- Series} X

5.5.

_{T-Beam Details}

_{- Series}

_{S, F, And L}

5.6.

_{T-Beam-Longitudinal}

_Section

_{- Series}

_{S, F And L}

5.7.

_{Stress-Strain}

_{Curves for}

_Steel

5.8.

_Loading

_Arrangements

_for

_Short

_{Term and Fatigue}

Tests

5.9.

_Loading

_History

_{for T-Beams}

5.10.

_Loading

_Arrangement

_{for Long Term Tests}

5.11.

_Layout

_{of Demee Grid}

5.12.

_Shrinkage

_{and Creep Behaviour}

_{of Concrete}

5-13-

_{Stress-Strain}

_{Curve for Fibrous-Cemený}

6.1.

_Differential

_Shrinkage

_Stresses

6.2.

_{Nomogram for Differential}

_Shrinkage

6-3-

_Neutral

_{Axes of Stress}

_{and Bending}

_Under

Sustained

Loading

7.1.

_Distribution

_{of Differential}

_Shrinkage

_Strains

- Group XN

7.2.

_Distribution

_{of Differential}

_Shrinkage

_Strains

Group XL

7-3-

_Distribution

_{of Differential}

_Shrinkage

_Strains

- Class

2 Beams

7.4. _{Distribution of Differential Shrinkage Strains} - Class 3 Beams

7.5.

7.6.

7.7.

7.8.

7.9.

7.10.

7.11.

7.12.

7-13-

7.14.

7.15.

7.16.

7.17.

Variation. _{of Differential} Shrinkage _with Time

Differential Shrinkage Stresses

Differential Shrinkage Stresses

Load-Deflection Curves _{- Series} X

Comparison _{of Deflections - Series} X o

Crack Width-Load Curves _{- Series} X

Load-Deflection _{Curves - Series} P

Load-Deflection _{Curves - Series R}

Strain _{Profile - Beam S3}

Strain _Profile

- Beam SG3

xv sm

h _Mult-Md-Mdc

xv sm

h _Mult-Md-Mdc

xv sm

h _Mult-Md-Mdc

7.18.

_x

HV

7.19.

_Strain

7.20.

_Strain

7.21.

Strain

sm - Kult-Md-Mdc

Profile _{- Beam L2}

Profile _{- Beam LG2}

Profile _{- Beam F2}

- Class 2 Beams

- Class

3 Beams

- Beam SG2

- Beam SG3

7.22.

7.23.

7.24.

7.25.

7.26.

7.27.

7.28.

7.29.

7.30.

7-31-

7-32.

7-33-

7-34.

7-35.

7-36.

7-37.

7-38.

7-39-

7.40.

Strain Profile _{- Beam FG2}

Steel Stress _{v Load - Class} 2 Beams

Steel Stress _{v Load - Class} 3 Beams

Steel Stress _{v Load - Group SW}

Steel Stress _{v Time - Series} L

Steel Stress _{v Load Cycles - Series} F

Load-Deflection

_Curves - Groups

S2,

_S3

Load-Deflection _{Curves - Series} S

Average _{Load-Deflection Curves}

Plot _of a max _v

Mdc a

res Mmax

Deflection _{v Time Curves - Series} L

Variation _{of Deflection with} Humidity _{- Series} L

Deflection _{v Load Cycles} Series F

Crack Width _{v Load Curves} Groups S2, S3

Crack Width _{v Load Curves} Groups SG, SW

Crack Width _{v Load Curves - Series} S

Crack Width _{v Time Curves - Series} L

A. l. Eatimation _{of Shrinkage}

A. 2. Stress Profiles _{- Beam SG3}

IST0FAPPENDICES

A. l. Estimation _{of the Shrinkage of the} Precast

Concrete

A. 2. _{Determination of the Calculated Working Loads} for _{the Various Limit States}

A. 2.1. _{Limit State of Collapse}

A. 2.2. _{Limit State of Local Damage}

LIST0FPLATES

1.1. _{Normal Weight and Lightweight} Aggregates

1.2. _{Alkali-Resistant Glass} Fibres

1-3-

_Steel

_Fibres

1.4.

_{Alkali-Resistant}

_{Glass Fibre}

_Reinforced

Cement Channel

Fibre "Bridges" _across Cracks

5.1. _{Bond Failure Series X}

5.2. _{Shear Failure Series X}

5-3- _{Interface Bond Failure}

- Series X

5.4. _{Prestressing Mould}

5.5. _{Insitu Flange Mould}

5.6. _{Loading Arrangements for Short Term and} Fatigue Tests

5.7. _{Loading Arrangement} for _{Long Term Tests}

7.1. _{Typical Failure} for Conventional _Concrete Composite T-Beam

7.2. _{Typical Failure of Fibrous-Cement Channel}

7-3- _{Close-up of Fibrous-Cement Channel at Failure}

7.4. _{Concrete Web and Fibrous-Cement Channel at} Failure

PUBLICAT10NS

O'Leary, D. C., Dave, tj T. and Saunders, J. "Steel Fibres in Partially Prestressed Composite Concrete

Beams". Proceedings _of the International Symposium on Fiber Reinforced Concrete, A. C. I. Fall Convention, Ottawa, _{-SP44-27,1973, pp477-495.}

2. _{Dave, N. J.,} O'Leary, _{D. C. and Saunders,} J. "The

Structural _{Use of Fibrous} Cement in Composite Concrete

Construction". Proceedings _{of the} International Sympo- sium on Fiber Reinforced Concrete, A. C. I. Fall Conven-

tion, Ottawa, _{SP44-29,1973, PP511-532.}

3- Dave, N. J., O'Leary, _{D. C., Garwood, T. G. and Saunders,} J. "Partially _{Prestressed Concrete". Proceedings of the}

Research _{Seminar on Behaviour and Design of Structural} Concrete, Fulmer, 1973, _{C. and C. A., PP154-16o.}

4. _{Dave, N. J., O'Leary, D. C., Al-Sanjary, K. A. A. and} Saunders, J. "Asbestos _{and Glass} Fibre Reinforced

Cement Structural Concrete Composites". Proceedings

of the Conference on Fibre Reinforced Concrete, Delft, September 1973, University _{of Delft, PP115-138.}

5. _Saunders, J. "Structural _{Use of Fibrous} Cement in Partially Prestressed Composite Construction".

Proceedings _{of Rilem} Symposium _{on Fibre} Reinforced

Cement-and Concrete, London, _September 1975, _Supplemen-

II

tary Volume _{- to be publish6d.}

CHAPTER0NE

INTRODUCTION

1.1. _{COMPOSITE CONCRETE CONSTRUCTION}

A popular form _of floor _{construction consists of}

composite _concrete T-beams (figure 1.1. ). Each beam con- sists _{of a precast prestressed concrete web, combined with} a cast-insitu _{reinforced concrete} flange. Composite _con- crete _construction is _{an attempt} to economise _{over pre-}

stressed concrete design _{and yet retain} its _advantages. It leads _{to simplification of construction on site and to sav-} ings in formwork _{and labour costs} (101). _{There is also a}

substantial _saving in the _{cost of steel in a composite pre-} stressed concrete beam (102). _{The use of high strength}

concrete is _{restricted to the precast web, allowing lower} strength _{concrete to be used in the cast-insitu flange,}

resulting in a further _{saving in material costs.}

If _{properly designed and constructed, a composite beam} will exhibit _all the _{characteristics such as resilience,}

strength, _{stiffness and elasticity of conventional pre-} stressed _concrete beams (103,104).

A composite concrete T-beam may be constructed with or without the use of temporary _supports to the _{precast web.}

When. temporary _{supports are not used, the precast web} carries its own weight, the dead weight _of the _cast-insitu flange _{and the} form _{work, whilst the composite section}

carries the live load only. When temporary _{supports are} used, the composite section _carries the dead load _of the

I

cast-insitu flange as well as the live load. The use of temporary _{supports can lead to a more economic} design

(105) _{and were therefore used during the construction of} the T-beams tested in this investigation.

1.2. _{PARTIAL PRESTRESSING}

The British Standards Institution Code of Practice

CP110: 1972, "The Structural _{Use of Concrete"} (106), hence- forth _{referred to as the Unified Code, classifies}

structural concrete _members according to the degree of

flexural tensile _{stress or to the degree of cracking that} is _{permitted under design loads. The four classes of}

structural _{concrete members are: -}

Class 1: _{Fully prestressed concrete members} in which no tensile _{stresses are allowed} in the _{concrete under design loads.}

Class 2: _{Prestressed concrete members in} which limited tensile _stresses, but _no

cracks, _{are allowed under} design loads.

Class _3: Prestressed _{concrete members} in

which cracking is allowed under design loads, although the surface width of the _cracks

must not exceed O. lmm for members exposed to aggressive environments and 0.2mm for all:

other members.

Class 4: Reinforced _{concrete members. The} permissible crack width is O. lmm for _members

in _{aggressive environments hnd 0-3mm for all} other members.

Class 2 and Class 3 structural _{concrete members} are termed _{as limited or partially prestressed concrete}

members. Terms such as "Reinforced Prestressed Concrete Members"'(107) _{and "Prestressed} Reinforced Concrete

Members" (108) have _also been used. This investigation

is _{concerned with} the _structural behaviour _{of Class 2 and}

Class

_{3 members, otherwise}

_referred

to as partially

_pre-

stressed

_members.

Further _economies in _{composite concrete construction}

can be achieved by the _{use of partial} prestressing. The advantages of partial _prestressing over full prestressing

are: (109)

1. _{Lower cost of prestressing.}

2. _{Reduced camber minimising difficulties}

in construction involving _{precast mem-}

bers.

Possible _{economy resulting} from the use of smaller and therefore lighter

sections.

Increased deflection _{and crack widths} at loads in excess of the design load, giving adequate _warning before _approa- ching failure.

Increased flexibility _{and resilience} to shock.

The quantity and type _{Of steel required} in a partially

prestressed concrete member will depend upon the amount of steel required to produce the required tensile force for

the _ultimate load _{condition and the amount required} to

produce the effective prestress, ie, it will depend upon the _{magnitude of} the design load _{and the} degree _of tensile stress _{or crack width} that is permitted under that design load.

There _are basically _three different _{ways of applying} the _{prestressing force and simultaneously providing the}

total _{quantity of steel required to ensure an adequate} factor _{of safety against collapse, namely:}

- 1. _{The total quantity of steel required}

is _{prestressing steel, all of which} is tensioned _{to a stress less than} the _{maximum permitted. The degree} of prestress _will depend _upon the effective _{prestress required.}

2. _{The total quantity of steel required}

is _{prestressing steel, part of which} is fully tensioned to'the _maximum

stress permitted. The remainder _of the _steel is left _untensioned.

A proportion of the _{required quantity}

of steel is prestressing _{steel, which} is tensioned _{to the maximum stress}

permitted. The remainder _of the _steel is _{untensioned and consists of mild}

The first _method is _{seldom used, as it} is the least practical and economic. _-The second method is preferable

for _{pretensioned members where reinforcement cages are not} used. In practice, a nominal stress will be applied to

both _the "tensioned" _{and "untensioned" wires in order} to position them accurately in the mould. The stress in the

"tensioned" _{wires will then be increased to the maximum} stress permitted. The nominal stress is designed such

that _after losses, the _stress in the "untensioned" _wires

will have reduced to zero, ie prior to the application of the _external load _{the stress in the} "untensioned" _wires

will be zero. The third _method is _preferable for _post- tensioned _{members where reinforcement cages are required}

(110). _{The second}

method _{of partial} prestressing was used during _{this investigation.}

1-3 _{LIGHTWEIGHT AGGREGATE CONCRETE}

The lightweight _{aggregates that are suitable for}

use in _{concrete are} known commercially _{as Lytag, Leca. and} Aglite. _{The strength, durability and sound, insulation}

properties _of the _{concrete produced} from these lightweight aggregates _{are, at} their best, _equal to those _{of conven-}

tional _{concrete, whilst other properties make them desirable} and economic building materials. They _are: (111)

1. Lower _{unit weight.}

2. _Increased fire _resistance.

3- Increased thermal insulation. 4. _{Ease of construction.}

In addition, in areas where natural aggregates are not available or are in short supply, lightweight aggregates

provide an economic solution for the manufactureof concrete.

This investigation is _{concerned with} the _structural use of Lytag (sintered pulverised fuel ash) lightweight

aggregate (Plate 1.1. ) in composite concrete construction.

1.4. _{FIBRE REINFORCEMENT}

Recent improvements in _{cement quality and concrete mix} design _{methods, coupled with more efficient compacting}

techniques have _{made it possible to achieve concrete}

compressive strengths in excess of 10ON/sq. mm. at 28 days. The tensile _{strengths have not, however,} increased in the

same proportion. Consequently, in the design_ _{of reinforced} and partially _{prestressed concrete members,} the limiting

design _{criteria are increasingly the limit states of deflec-} tion _{and cracking. An improvement in the tensile charac-}

teristics _{of the concrete would, therefore, be beneficial}

to its flexural _{behaviour. This, improvement may be brought} about by the _addition of fibre-reinforcement to the cement or concrete matrix.

The choice of fibres available for _{use as fibre-rein-}

forcement _{in, a cementitious matrix, covers a wide range of} natural and man-made fibres. The particular fibre employed will be dependent upon the physical properties required of

the _{composite. The primary reasons} for incorporating

fibres _{in a cement or concrete matrix are} to increase the extensibility and tensile strength of the matrix before

cracking, to hold the matrix together after cracking and to

change the failure mechanism from a brittle to a ductile form. _{The fibres used} for this _{purpose are} the high

modulus types of which asbestos, glass and steel are

examples. The low modulus fibres, polypropylene and nylon, have been used to increa-se the _{energy absorption capacity}

of structural concrete subject to shock or impact loading.

However, they _{are not suitable as a means of} improving the tensile _properties.

The distribution _{of fibres} in a matrix (figure 1.2. ), may be one (1D), two (2D) _{or three-dimensional} OD). The

fibres _{may be continuous or discontinuous and in a 2D or 3D} system, the distribution _{can be random or orderly.} The

efficiency _of fibre-reinforcement _{in a preferred} direction

is 100% for _{a 1D distribution and may be as low as 33% and} 16% for _{2D and 3D random distributions, respectively} (112).

This investigation _{is concerned with the use of two-} dimensionally _{randomly distributed alkali-resistant glass}

fibres (Plate _{1.2. ) and three-dimensionally randomly distri-} buted _{steel fibres} (Plate _{1.3. ).}

FIBROUS-CEMENT COMPOSITE CONSTRUCTION

1.5-1.

_Introduction

The idea for _{a new form of construction emerged} from the desire for _{an economic form of floor construction that} would make full use of the _{new provisions} for _partially

prestressed concrete in the Unified Code (lo6). It has

already been shown in the _{prý! vious sections,} that _composite

T-beams, incorporating _{a precast partially prestressed web,}

combined with a lightweight aggregate concrete flange, can form the basis for _{an extremely economic} flooring _system

and that the addition of fibre-reinforcement to the precast web can provide for an adequate'degree of serviceability.

It _{only remains} to optimise the fibre-reinforcement.

Previous

_{work by Hannant}

(113),

has indicated

that

deflections

_{in structural}

_{members can be significantly}

reduced

by the addition

of three-dimensionally

randomly

distributed

_steel

fibres.

_{He also suggested}

that

the use of

steel

fibres

could

be of benefit,

in association

with

normal

or high

tensile

_{reinforcement,}

_{as a means of limit-}

ing crack widths

_{in concrete}

beams.

However,

the use of a

3D distribution

is far

less

_efficient

than a 2D distribution

(112).

_{In addition,}

_{the primary}

_reason

_for

_adding

_fibre-

reinforcement

to structural

_concrete

is to improve

its

tensile

_strength,

_{the inclusion}

_{in the compression}

_{zone of}

a member is,

therefore,

_neither

beneficial

_{nor economic.}

Thus,

_{the optimum solution}

_{is to have a 2D distribution,}

with

the fibres

_concentrated

in the zone of maximum tensile

stress.

This

may be achieved

by using a fibrous-cement

sheet,

covering

all

or part

of the surface

of the tensile

zone of the structural

member.

Thus,

the concept of a

fibrous-cement/structural

_concrete

_composite

_{was evolved.}

In addition,

by using fibres

in a cementitious

matrix,

a

material

is produced which will

form a natural

and complete

bond with the structural

concrete

member cast

on to it.

The fibrous-cement _composite, if _correctly designed and produced, will have an improved deflection and crack- ing _performance. This improvement _{will be a function of}

the _{properties and thickness of the} fibrous-cement _sheet. The fibrous-cement _{sheet will act as a form of surface}

reinforcement to the concrete _{member., preventing crack}

initiation _{and propagation and thereby} limiting deflections.

By profiling the _sheet into _{a channel} form (Plate 1.4. ), it is _possible to increase the _{area reinforced} by the fibrous-

cement _sheet and, therefore, surface cracks can be

completely _avoided up to and beyond the normally _accepted design load for _{the member. The structural member, which} may be reinforced or partially prestressed in the conven-

tional _manner, is _{cast in conjunction with} the _preformed

fibrous-cement _{sheet, which is either placed in the mould} or forms part of the _mould itself. Thus _{a composite} is

formed _{in which the fibrous-cement sheet forms an integral} part of the beam, with _{a natural} bond being developed _at

the interface.

Fibrous-cement _{sheets, incorporating either glass or} asbestos fibres, _{are available at present.} This investi-

gation is _{concerned with} the _{use of alkali-resistant glass} fibre-reinforced _{cement sheets, profiled into a channel}

section.

1.5.2. _Advantages

The use of a fibrous-cement sheet _{as an integral} part of a structural concrete member can result in the following

advantages: _-

1. Increased _resistance to tensile _stresses, resulting in delayed cracking.

2. Reduction in _{crack widths and the rate of} crack propagation.

Reduction in deflections due to

"increased"

_flexural

_rigidity.

Increased _{permissible working} loads _and stresses or a reduction in the overall

dimensions _{of the member.}

Reduction

_{in cover to the reinforcement.}

Increased

_{use of: -}

(a) _{lightweight aggregate concrete} (b) _{high tensile}

steel

Increase

_{in the use of partially}

_pre-

stressed _members.

Improved _surface finish.

Increased fire _resistance.

10. _Reduction in formwork _costs.

11. _Reduction in _{shear reinforcement.}

12. Potential _{use as anti-crack reinforce-} ment in liquid-retaining structures.

LIMITATIONS OF THE UNIFIED CODE

The Unified Code (106), does not give _{any guidance on} the _{use of fibre-reinforcement} in _{structural members and it} is _{also apparent} that the methods _outlined in the Code for the design _{of partially prestressed concrete members are}

limited in the following _{respects: -}

1. _{No allowance} is _{made for} the loss _of prestress in the concrete, due to the

effect of creep and shrinkage on the untensioned _steel.

2. _{No method is presented for calculat-}

ing _{the effects of differential shrin-} kage _{in composite construction.}

NO indication

_{is given as to the effects}

Of using

lightweight

_aggregate

concrete

in composite

_{construction.}

The method presented for the prediction of deflections applies to the initial

loading _{cycle only. No allowance} is

made for the residual deflections and the loss _of tensile _strength in the

concrete on subsequent loading cycles.

No method for predicting crack widths is _given.

No indication is _{given of} the _{effects of} sustained and fatigue loading.

1.7. _{OBJECTIVES OF THE INVESTIGATION}

In view of the limitations _of the Unified Code (lo6), outlined in _section 1.6., the investigation _presented in

this thesis _{was planned with} the following _{objectives: -}

To investigate the _general behaviour

of partially prestressed composite T-

I beams, with particular reference to the losses _{of prestress, the effects of}

differential _{shrinkage and the use of} lightweight _{aggregate concrete.}

2. _{To investigate the use of fibre-}

reinforcement in partially _prestressed composite _{concrete construction.}

To investigate _{the limit state beha-}

viour _{of partially prestressed composite} concrete T-beams _{with particular refere-} nce to the development _{of methods} for

calculating the _principal limit _states applicable to both _{conventional and}

fibre-reinforced _{concrete beams.}

To investigate the _{effects of sustained} and fatigue loading _{on partially pre-} stressed composite _concrete T-beams.

1.8. _{OUTLTNE OF THESIS}

Chapter _{2 reviews} the _curýent "state _of the _art" in composite _{construction, partial prestressing and fibre-}

reinforcement. A brief history _of their development is outlined, together _with the extent _{of existing research}

work.

The principles of limit state design are described in Chapter _{3, together with an outline of} the _{methods avail-}

able _{at present} for the _calculation of the principal limit states.

In Chapter 4, theoretical _{relationships are} derived between _{the applied moment and the depths of the neutral}

axes _{of stress and bending, enabling a design equation,}

relating the _{stress in the steel to the applied moment to} be developed. _{Subsequently, design equations for the cal-} culation _of the _principal limit _{states are} developed,

which _are directly _applicable to both _{conventional and} fibre-reinforced _{structural members.}

Chapter 5 covers the design, _{manufacture and testing of} the beams.

Chapter 6 deals _with the _{analysis of the stresses in} the _{concrete and steel. The methods used for calculating}

the losses _{of prestress} by both theoretical _{and experimen-} tal _{considerations are explained. Theoretical and experi-} mental methods of analysis for the differential _shrinkage

stresses are developed and the _procedures for determining the _stresses in the _{steel and the depths of the neutral}

axes of stress and bending from'the _experimental data _are described.

The limit _state behaviour _of the test beams is discus- sed in Chapter 7, _with particular reference to the use of fibre-reinforcement. _{The advantages of} fibrous-cement

composite construction, compared with conventional concrete construction a: be evaluated and the experimentally obtained

results are correlated _with the values predicted by the proposed design equations developed in Chapter 4.

Finally, in Chapter 8, conclusions _are drawn from the tests, _{and suggestions} for future _{research are} indicated.

References, tables, figures, _{plates and appendices} follow _{after Chapter}

CHAPTERTW0

STATE OF THE ART

2.1. _{COMPOSITE CONSTRUCTION}

The first _{attempt at using} lightweight _{aggregate con-}

crete in prestressed composite construction was reported by Hasnat (201) in 1965. Prior to this, investigations had

been directed _{to beams of conventional sand and gravel}

aggregate concrete (202 _{- 208).} Hasnat tested twelve pre-

stressed composite T-beams of lightweight aggregate concrete. The variables considered were the length of the shear span,

the _{strength of} the _concrete in the insitu flange _{and the}

time interval between the _{casting of} the _{web and the} flange. Hasnat _{agreed with the earlier} findings _{of Ahmed (2o8),} that

the natural bond between _{the, precast and insitu concrete}

components _{was erratic and should not} be relied _{upon for} the development _{of the ultimate flexural strength. They also}

found _{that the bond strength was directly proportional} to the _{strength of the insitu concrete. By contrast,} Evans'

and Parker (205), found that the relative qualities of the concrete did not affect the bond strength, whilst Dean and Ozell (206) found that the bond was equivalent to that bet-

ween concrete and steel. In addition, both Hasnat (201) and Ahmed (208), found that the differential _{shrinkage signifi-}

cantly reduced the cracking stress of composite concrete

T-beams. Hasnat _added that the _{actual reduction was signi-} ficantly less in _{composite beams of lightweight aggregate}

concrete than in conventional. composite concrete beams, despite the higher _{shrinkage of lightweight aggregate}