Quality Assurance of Reinforced Concrete Structures Strengthened by Externally Bonded CFRP Strips
Qualitätssicherung bei Stahlbetonsanierungen mit aufgeklebten CFK-Lamellen
©2004
Diplomarbeit
125 Seiten
Zusammenfassung
Inhaltsangabe:Zusammenfassung:
Im Dezember 2002 wurde die erste österreichische Richtlinie bezüglich Stahlbetonverstärkungen mit aufgeklebten CFK-Lamellen veröffentlicht. Das vorrangige Ziel der vorliegenden Diplomarbeit ist, die Durchführbarkeit der neuen Richtlinie - mit speziellem Augenmerk auf Qualitätssicherung - anhand Europas größter CFK-Baustelle (die Wiener Marxerbrücke) zu überprüfen.
Die Diplomarbeit enthält eine Übersicht über Eigenschaften und Anforderungen an CFK, deren gängigste Anwendungen als Verstärkung und eine Beschreibung des Hauptbezugsobjekts Marxerbrücke.
Die Methoden der Qualitätssicherung nach der neuen Richtlinie werden erklärt und Resultate aus der Qualitätssicherung auf der Marxerbrücke inklusive einer Analyse von Fehlstellen und Sanierungsmethoden werden präsentiert. Anschließend werden die Resultate aus der Qualitätssicherung auf der Marxerbrücke mit denen von vier anderen CFK-verstärkten Bezugsobjekten verglichen.
Abschließend werden Veränderungen bezüglich der Methoden der Qualitätssicherung vorgeschlagen und Empfehlungen für eine zukünftige Ausgabe der österreichischen Richtlinie werden geäußert. Inhaltsverzeichnis:Table of Contents:
AbstractI
Deutsche KurzfassungII
AcknowledgementsIII
Table of contents4
1.Content and aims of the thesis7
2.Strengthening with fibre reinforced polymers8
Fibre Reinforced Polymers9
Types of fibres9
2.1.1CFRP products and their properties10
Comparison of strengthening with carbon fibre reinforced polymers and externally bonded steel12
Differences in behaviour under tension12
Advantages of carbon fibre reinforced polymers12
2.1.2Disadvantages of carbon fibre reinforced polymers14
Guidelines and reference works in Austria concerning CFRP and design of strengthening elements14
2.1.3Verifications of the strengthening system according to the Austrian guidelines15
2.1.4Design bending moment capacity17
Preliminary measures and application of CFRP19
Examination of the state of the concrete member before surface preparation19
2.1.5Surface preparation and repair of the concrete member20
2.1.6Application of CFRP Reinforcement21
Basic techniques of strengthening with CFRP21
2.1.7Selected special techniques of strengthening with CFRP24
3.main reference object: Marxerbridge26
General description of the widening of the Marxerbridge26
Geographical position26
Arrangement of the structure of the Marxerbridge27
Division and numeration of the structure27
3.1.1Technical data […]
Im Dezember 2002 wurde die erste österreichische Richtlinie bezüglich Stahlbetonverstärkungen mit aufgeklebten CFK-Lamellen veröffentlicht. Das vorrangige Ziel der vorliegenden Diplomarbeit ist, die Durchführbarkeit der neuen Richtlinie - mit speziellem Augenmerk auf Qualitätssicherung - anhand Europas größter CFK-Baustelle (die Wiener Marxerbrücke) zu überprüfen.
Die Diplomarbeit enthält eine Übersicht über Eigenschaften und Anforderungen an CFK, deren gängigste Anwendungen als Verstärkung und eine Beschreibung des Hauptbezugsobjekts Marxerbrücke.
Die Methoden der Qualitätssicherung nach der neuen Richtlinie werden erklärt und Resultate aus der Qualitätssicherung auf der Marxerbrücke inklusive einer Analyse von Fehlstellen und Sanierungsmethoden werden präsentiert. Anschließend werden die Resultate aus der Qualitätssicherung auf der Marxerbrücke mit denen von vier anderen CFK-verstärkten Bezugsobjekten verglichen.
Abschließend werden Veränderungen bezüglich der Methoden der Qualitätssicherung vorgeschlagen und Empfehlungen für eine zukünftige Ausgabe der österreichischen Richtlinie werden geäußert. Inhaltsverzeichnis:Table of Contents:
AbstractI
Deutsche KurzfassungII
AcknowledgementsIII
Table of contents4
1.Content and aims of the thesis7
2.Strengthening with fibre reinforced polymers8
Fibre Reinforced Polymers9
Types of fibres9
2.1.1CFRP products and their properties10
Comparison of strengthening with carbon fibre reinforced polymers and externally bonded steel12
Differences in behaviour under tension12
Advantages of carbon fibre reinforced polymers12
2.1.2Disadvantages of carbon fibre reinforced polymers14
Guidelines and reference works in Austria concerning CFRP and design of strengthening elements14
2.1.3Verifications of the strengthening system according to the Austrian guidelines15
2.1.4Design bending moment capacity17
Preliminary measures and application of CFRP19
Examination of the state of the concrete member before surface preparation19
2.1.5Surface preparation and repair of the concrete member20
2.1.6Application of CFRP Reinforcement21
Basic techniques of strengthening with CFRP21
2.1.7Selected special techniques of strengthening with CFRP24
3.main reference object: Marxerbridge26
General description of the widening of the Marxerbridge26
Geographical position26
Arrangement of the structure of the Marxerbridge27
Division and numeration of the structure27
3.1.1Technical data […]
Leseprobe
Inhaltsverzeichnis
Markus Huber
Quality Assurance of Reinforced Concrete Structures Strengthened by Externally
Bonded CFRP Strips
Qualitätssicherung bei Stahlbetonsanierungen mit aufgeklebten CFK-Lamellen
ISBN: 978-3-8366-4118-0
Herstellung: Diplomica® Verlag GmbH, Hamburg, 2010
Zugl. Technische Universität Wien, Wien, Österreich, Diplomarbeit, 2004
Dieses Werk ist urheberrechtlich geschützt. Die dadurch begründeten Rechte,
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ABSRACT / DEUTSCHE KURZFASSUNG
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ABSTRACT
The first Austrian guidelines concerning the strengthening of reinforced
concrete structures with externally bonded CFRP were published in December 2002.
The main objective of the present thesis is to assess the feasibility of the new
guideline with particular reference to quality assurance on Europes largest CFRP
application (the Viennese Marxerbridge).
The thesis contains an overview of properties, requirements and common
strengthening applications of CFRP and a description of the main reference object,
the Marxerbridge.
The methods of quality assurance according to the new guideline are
described and the results of quality assurance on the Marxerbridge are presented.
These results are compared to those of four other CFRP-strengthened objects.
Furthermore encountered defects and repair methods concerning the CFRP
application on the Marxerbridge are described and analysed.
In conclusion, changes of methods of quality assurance are proposed and
recommendations for a future edition of the Austrian guideline are given.
ABSRACT / DEUTSCHE KURZFASSUNG
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DEUTSCHE KURZFASSUNG
Im Dezember 2002 wurde die erste österreichische Richtlinie bezüglich
Stahlbetonverstärkungen mit aufgeklebten CFK-Lamellen veröffentlicht. Das
vorrangige Ziel der vorliegenden Diplomarbeit ist, die Durchführbarkeit der neuen
Richtlinie - mit speziellem Augenmerk auf Qualitätssicherung - anhand Europas
größter CFK-Baustelle (die Wiener Marxerbrücke) zu überprüfen.
Die
Diplomarbeit
enthält
eine
Übersicht
über
Eigenschaften
und
Anforderungen an CFK, deren gängigste Anwendungen als Verstärkung und eine
Beschreibung des Hauptbezugsobjekts Marxerbrücke.
Die Methoden der Qualitätssicherung nach der neuen Richtlinie werden erklärt
und Resultate aus der Qualitätssicherung auf der Marxerbrücke inklusive einer
Analyse von Fehlstellen und Sanierungsmethoden werden präsentiert. Anschließend
werden die Resultate aus der Qualitätssicherung auf der Marxerbrücke mit denen
von vier anderen CFK-verstärkten Bezugsobjekten verglichen.
Abschließend werden Veränderungen bezüglich der Methoden der
Qualitätssicherung vorgeschlagen und Empfehlungen für eine zukünftige Ausgabe
der österreichischen Richtlinie werden geäußert.
ABSRACT / DEUTSCHE KURZFASSUNG
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ACKNOWLEDGEMENTS
At the institute for building construction of the Vienna University of technology
I would like to thank Professor Kolbitsch for his advice and for his willingness to
revise the thesis rapidly. I would also like to thank Mrs. Merta who introduced me to
the basics of writing a technical work.
At the OFI my thanks go to the whole department for construction. Everyone
treated me in a very friendly way from the first moment on. Dr. Rossbacher and Dr.
Fleischer had the idea of the cooperation with the university what made my
experiences on the Marxerbridge possible. Although both of them were under
permanent stress at work my questions were always answered. I would also like to
mention that the confidence of Dr Rossbacher allowed me to work in my way what
made business much more pleasant. My special thanks go to Mr. Reisner who
explained me everything concerning the construction site patiently, introduced me to
protective measures against the carbon dust and above all was willing to help me at
almost every hour of the day.
Finally I would like to thank the consultant office Mayer for drawings and Mr.
Shane Mc Manus for linguistic support.
Acknowledgements to my family and friends in German:
Ich möchte meinen Eltern, meiner Schwester, meiner Freundin Adriana und all
den Freunden und Verwandten danken, die sich nach Erfolgen auf der Uni mit mir
gefreut haben und mich nach Tiefschlägen aufgebaut haben. Abgesehen von der
moralischen Unterstützung während der Studienzeit gilt mein besonderer Dank
meinen Eltern, die es beide, nachdem sie unter schwierigsten Bedingungen nach
dem 2. Weltkrieg aufgewachsen waren, unabhängig von einander geschafft haben,
Wohlstand für sich und ihre Kinder aufzubauen. Erst der Aufstieg meiner Eltern hat
es für mich möglich gemacht, ohne erwähnenswerte Sorgen zu studieren und
nebenbei ein in allen Belangen erfüllendes Leben zu führen. Ich bewundere, wie
meine Eltern mir immer wieder neidlos Erlebnisse, wie mein Erasmus-Jahr in
Barcelona, ermöglichen, die sie selbst während ihrer Jugend nie hatten.
Ich hoffe, der gesamten Verwandtschaft, die in letzter Zeit einige unerklärbar
erscheinende Schicksalsschläge hinnehmen musste, mit dem Abschluss meines
Studiums ein paar freudige Momente bereiten zu können. Deshalb widme ich die
Diplomarbeit meiner ganzen Verwandtschaft.
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TABLE OF CONTENTS
ABSTRACT _______________________________________________________________________ I
DEUTSCHE KURZFASSUNG _______________________________________________________ II
ACKNOWLEDGEMENTS ___________________________________________________________ III
TABLE OF CONTENTS ____________________________________________________________ 4
1
CONTENT AND AIMS OF THE THESIS _________________________________________ 7
2
STRENGTHENING WITH FIBRE REINFORCED POLYMERS________________________ 8
2.1
F
IBRE
R
EINFORCED
P
OLYMERS
__________________________________________________ 9
2.1.1
Types of fibres______________________________________________________________ 9
2.1.2
CFRP products and their properties ____________________________________________ 10
2.2
C
OMPARISON OF STRENGTHENING WITH CARBON FIBRE REINFORCED POLYMERS AND EXTERNAL STEEL
REINFORCEMENTS
___________________________________________________________ 12
2.2.1
Differences in behaviour under tension _________________________________________ 12
2.2.2
Advantages of carbon fibre reinforced polymers __________________________________ 12
2.2.3
Disadvantages of carbon fibre reinforced polymers ________________________________ 14
2.3
G
UIDELINES IN
A
USTRIA CONCERNING
CFRP
AND DESIGN OF STRENGTHENING ELEMENTS
______ 14
2.3.1
Verifications of the strengthening system according to the Austrian guideline____________ 15
2.3.2
Design bending moment capacity ______________________________________________ 17
2.4
P
RELIMINARY MEASURES AND APPLICATION OF
CFRP_________________________________ 19
2.4.1
Examination of the state of the concrete member before surface preparation____________ 19
2.4.2
Surface preparation and repair of the concrete member ____________________________ 20
2.4.3
Application of CFRP Reinforcement ____________________________________________ 21
2.4.4
Basic techniques of strengthening with CFRP ____________________________________ 21
2.4.5
Selected special techniques of strengthening with CFRP ___________________________ 24
3
MAIN REFERENCE OBJECT: MARXERBRIDGE ________________________________ 26
3.1
G
ENERAL DESCRIPTION OF THE WIDENING OF THE
M
ARXERBRIDGE
_______________________ 26
3.2
G
EOGRAPHICAL POSITION
_____________________________________________________ 26
3.3
A
RRANGEMENT OF THE STRUCTURE OF THE
M
ARXERBRIDGE
____________________________ 27
3.3.1
Division and numeration of the structure ________________________________________ 27
3.3.2
Technical data and static system of the structure__________________________________ 28
3.4
W
IDENING OF THE
M
ARXERBRIDGE AND STATIC EFFECTS
_______________________________ 30
3.4.1
Involved parties in the construction_____________________________________________ 30
3.4.2
Construction phases and static effects __________________________________________ 31
3.5
S
TRENGTHENING METHODS ON THE
M
ARXERBRIDGE
__________________________________ 37
3.5.1
Reinforcement with CFRP strips _______________________________________________ 37
3.5.2
Strengthening with additional reinforced concrete _________________________________ 39
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QUALITY ASSURANCE_____________________________________________________ 42
4.1
G
ENERAL
_______________________________________________________________ 42
4.2
Q
UALITY ASSURANCE BEFORE THE APPLICATION OF
CFRP _____________________________ 43
4.2.1
Examination of the pull-off strength of the concrete member _________________________ 43
4.2.2
Control of the evenness of the concrete surface __________________________________ 44
4.3
Q
UALITY ASSURANCE AFTER THE APPLICATION OF
CFRP ______________________________ 45
4.3.1
Examination of pull-off strength after CFRP-application_____________________________ 45
4.3.2
Control of evenness after the application of CFRP_________________________________ 45
5
QUALITY ASSURANCE ON THE MARXERBRIDGE ______________________________ 48
5.1
G
ENERAL
_______________________________________________________________ 48
5.2
R
ESULTS OF PULL
-
OFF TESTS IN ALL PHASES
_______________________________________ 49
5.2.1
Statistical analysis of the results of the pull-off tests _______________________________ 49
5.2.2
Layer of rupture in pull-off tests after CFRP application _____________________________ 52
5.3
R
ESULTS OF CONTROL OF EVENNESS
_____________________________________________ 55
5.4
R
ESULTS OF SEARCH FOR HOLLOW PLACES AND AIR INCLUSIONS
_________________________ 55
6
DEFECTS AND REPAIR METHODS___________________________________________ 56
6.1
S
TARTING POINT
____________________________________________________________ 56
6.2
A
IR INCLUSIONS AND DELAMINATIONS
_____________________________________________ 57
6.2.1
Reasons for air inclusions and delaminations_____________________________________ 57
6.2.2
Development of air inclusions and delaminations__________________________________ 58
6.2.3
Repair of air inclusions and delaminations _______________________________________ 62
6.3
J
UT OUT BETWEEN
CFRP
STRIP AND CONCRETE
_____________________________________ 66
6.4
L
ONGITUDINAL CRACKS IN THE
CFRP
STRIP
________________________________________ 66
7
COMPARISON WITH OTHER OBJECTS _______________________________________ 67
8
CONCLUSIONS AND RECOMMENDATIONS ___________________________________ 70
8.1
R
ECOMMENDATIONS FOR A FUTURE EDITION OF THE
A
USTRIAN GUIDELINE
__________________ 70
8.1.1
Search for hollow places and air inclusions by knocking on CFRP strips with a nylon hammer
_____________________________________________________________ 70
8.1.2
Control of evenness ________________________________________________________ 71
8.1.3
Examination of pull-off strength _______________________________________________ 72
8.1.4
Preparation of the CFRP strips ________________________________________________ 72
8.2
A
LTERNATIVE TESTING METHODS
________________________________________________ 73
8.3
H
EALTH CONSIDERATIONS FOR WORKERS AND INSPECTORS
____________________________ 74
SUMMARY _____________________________________________________________________ 76
ZUSAMMENFASSUNG ___________________________________________________________ 78
ANNEX 1: DATA OF QUALITY CONTROL ____________________________________________ 81
ANNEX 2: PROTOCOL OF DEFECTS _______________________________________________ 95
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NNEX
2.1:
D
ATA OF DEFECTS
________________________________________________________ 97
A
NNEX
2.2:
L
OCATION OF DEFECTS
___________________________________________________ 101
ANNEX 3: PULL-OFF TEST_______________________________________________________ 106
ANNEX 4: REFERENCE OBJECTS ________________________________________________ 111
A
NNEX
4.1:
O
BJECT
A _____________________________________________________________ 111
A
NNEX
4.2:
O
BJECT
B _____________________________________________________________ 112
A
NNEX
4.3:
O
BJECT
C _____________________________________________________________ 113
A
NNEX
4.4:
O
BJECT
D _____________________________________________________________ 114
REFERENCES _________________________________________________________________ 116
LIST OF FIGURES ______________________________________________________________ 118
LIST OF TABLES _______________________________________________________________ 120
LIST OF PHOTOS_______________________________________________________________ 121
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1 CONTENT AND AIMS OF THE THESIS
In December 2002 the Austrian Association of Concrete (Österreichische
Vereinigung für Beton- und Bautechnik or abbreviated ÖVBB) published the first
national guidelines on the subject of strengthening with externally bonded
reinforcement for existing reinforced concrete structures [9]. They deal mainly with
aspects relating to the comparatively new strengthening technique with FRP and also
with externally bonded steel elements.
These new guidelines needed to be tested in terms of application and
practicability during an extensive project. The Austrian Research Institute for
Chemistry and Technology was in charge of the quality assurance on site during the
strengthening of the Marxerbridge, which is a part of the highway A 23 in Vienna, and
a project in form of this thesis was initiated in cooperation with the Vienna University
of Technology.
In April 2003 the widening of the Marxerbridge began. Changes in the static
system during the widening led to higher flexural loads in the bridges transversal
direction and made strengthening necessary. Externally bonded CFRP strips were
chosen as the main strengthening elements. The Marxerbridge was strengthened
with about 15 km of these strips, thus making this the largest application of CFRP
technology in Europe.
This leads to the practical part of the thesis. It consisted of accompanying and
monitoring the strengthening works on the Marxerbridge according to the new
guidelines. From April to October 2003 the applied CFRP-strips were inspected and
more than 700 pull-off tests were conducted by the author of the present thesis.
The main aim of the written part of the thesis is to assess the new guidelines
(concerning the quality assurance aspect in particular) and to give advice for a
possible future edition. Additionally, it is also an objective of the author to contribute
to the progress and development of practical execution methods of the CFRP
strengthening technique.
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2 STRENGTHENING WITH FIBRE REINFORCED
POLYMERS
It was necessary to develop systems of additional strengthening elements for
already existing structures, particularly reinforced concrete structures, for two main
reasons: firstly the increasing load and secondly the utilization of such structures for
different uses than originally intended. Different techniques, such as the application
of steel plates externally bonded onto existing objects or the increase of the cross
section of structural elements by additional reinforced concrete, have been used
successfully for decades.
Another significant development of the last years is the use of fibre reinforced
polymer (FRP) composites. FRP and in particular carbon fibre reinforced polymers
(CFRP) are described as a new and highly promising material in the construction
industry. FRP as a principal structural element had already been used in aeronautic
and space engineering before it was introduced to the building industry.
The method of externally bonded reinforcement (EBR) with CFRP is one of
todays state of the art techniques, which was developed in Switzerland and France
during the 1980s. The technique is mainly used as tensile- or shear-strengthening on
elements of existing structures, which are subjected to transversal loads. In addition,
FRP can also be applied to axial force-bearing elements such as columns (column
wrapping) or chimneys. CFRP is bonded onto the concrete surface with its fibres as
parallel as possible to the direction of principal tensile stresses. The important
properties of CFRP are their extremely high tensile strength and their high and
modifiable modulus of elasticity. Apart from their mechanical characteristics, CFRP
are light and therefore easy to handle and transport. They can practically be
produced in every required form and length.
The life span of structures can be increased significantly by such
strengthening and as a result partial or complete demolition can be avoided in many
cases. The main reasons, which make strengthening of existing structures
necessary, are the following:
Need for upgrading
Need for repair
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Seismic retrofit
The need for upgrading mainly results from increased service loads or is due
to stricter design requirements. The need for repair is a consequence of
deterioration, poor initial design and/or construction, ageing, environmentally induced
degradation, lack of maintenance, accidental events or a combination of various
factors.
2.1 Fibre Reinforced Polymers
2.1.1
Types of fibres
This thesis especially deals with carbon fibre reinforced polymers (CFRP)
applied on concrete structures. However all kinds of FRP can practically be bonded
onto the surface of every material. Two types of fibres, namely carbon fibres and
glass fibres are utilized for strengthening. Aramid fibres and polyester fibres are also
employed, but to a lower extent. Since none of the fibres resists heat or fire,
protection coatings are required to avoid the loss of strength.
Carbon fibres are used as active strengthening (e.g. in the case of higher or
additional loads) because they are not susceptible to stress corrosion (cracking
induced by corrosive environment and stress). Also they resist alkali and they exhibit
a high fatigue resistance. Carbon fibres can roughly be divided into two groups:
carbon fibres with a high tensile strength (Carbon HS) and carbon fibres with a high
tensile modulus of elasticity (Carbon HM) in fibre direction.
Glass fibres are cheaper and are applied as passive strengthening, for
example as a preventive measure in earthquake areas. Untreated glass fibres dont
resist alkali and they are likely to be weakened by stress corrosion. Therefore they
are not commonly used as active strengthening.
The idealized stress-strain-diagrams in Figure 2-1 illustrate the properties of
FRP for different fibre types. None of the FRP yields and the rupture is brittle for all
fibre types. Values given in the table of Figure 2-1 show the wide range of properties
of FRP. Whereas there are significant differences of about one order between the
tensile modulus of elasticity, the tensile strengths of all different fibre materials are
comparable.
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Figure 2-1: Idealized stress-strain-diagrams and properties of fibre reinforced polymers
of different fibres in fibre direction
2.1.2
CFRP products and their properties
FRP are mainly produced in two different forms thin unidirectional strips and
flexible sheets (also called fabrics). Both are bonded to the strengthened structure by
an epoxy adhesive. Normally strips are applied onto the surface of the tensile zone of
flexural members, whereas sheets are more commonly used in shear strengthening
or to wrap columns. The particular properties of CFRP products vary depending on
the source ( [4], [6], [9], [10] and [20]). The values shown in Table 2-1, Table 2-2 and
Table 2-3 are taken from [9] and [10]. The glass transition temperature is defined as
the approximate temperature above which increased molecular mobility causes a
material to become rubbery rather than brittle [4]. Therefore it is absolutely critical for
the functioning of the strengthening with CFRP that none of the components are
subjected to temperatures higher than their glass transition temperatures.
Unidirectional strips
Unidirectional strips are produced by pultrusion in flat profiles to a thickness of
1 2 mm and a width of 40 150 mm. Table 2-1 shows some typical values for the
properties of CFRP strips in fibre (longitudinal) direction.
Type of fibres
E
tensile
[GPa]
f
uk
[N/mm²]
Carbon HS
160 325
3500 5300
Carbon HM
325 640
2000 - 3500
Aramid
60-180
3100 3600
Glass
69 86
2400 3700
Steel
200
250 800
Prestressing steel
200
1000 - 1800
2000
1000
5000
4000
3000
0
5
[N/mm²]
Carbon HS
Aramid
Glass
Steel
Carbon
HM
4
3
2
1
Prestressing steel
[%]
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Characteristic value of CFRP tensile strength
f
fk
= 1000 3500 N/mm²
Characteristic value of the ultimate CFRP strain
fuk
1 %
Mean secant modulus of elasticity of CFRP
E
fm
= 100 300 kN/mm²
Volume fraction of fibres
= 60 - 70 %
Glass transition temperature
T
g
100°C
Table 2-1: Properties of CFRP strips according to the Austrian guideline [9] and [10]
Sheets
CFRP sheets are made to a thickness of between 0.1 0.3 mm and a width of
10 80 cm. If necessary, up to ten layers of sheets can be placed on top of one
another. CFRP sheets are especially suitable as reinforcement on elements with
strong curvatures. Properties of CFRP sheets are given in Table 2-2.
Characteristic value of CFRP tensile strength
f
fk
= 1000 5000 N/mm²
Characteristic value of the ultimate CFRP strain
fuk
1 %
Mean secant modulus of elasticity of CFRP
E
fm
= 100 650 kN/mm²
Volume fraction of fibres
= 25 - 35 %
Glass transition temperature
T
g
100°C
Table 2-2: Properties of CFRP sheets according to the Austrian guideline [9] and [10]
Adhesives
Most adhesives used in CFRP strengthening are epoxy-based. The function of the
adhesive is to transfer tensile stresses from the concrete surface to the FRP
strengthening element by bond (shear). A composite element under flexure explains
how tensile stresses (
tensile
) as a result of flexure in the lower part of concrete are
transferred to the CFRP element (
CFRP
) by shear stresses (
adhesive
) in the adhesive
layer (Figure 2-2).
Figure 2-2: Sketch of composite element under flexure
adhesive
reinfoced
concrete
adhesive
CFRP
tensile
CFRP
Composite element
under flexure
A
A
Cross section
A A
M
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Properties of CFRP-adhesives are presented in Table 2-3.
Characteristic value of flexural strength
f
a,fl
15N/mm²
Characteristic value of compression strength
f
a,c
60N/mm²
Mean secant modulus of elasticity
E
a,m
= 5 15 kN/mm² (strips)
E
a,m
= 2 10 kN/mm² (sheets)
Shrinkage
0,1 %
Glass transition temperature
T
g
60°C
Table 2-3: Properties of adhesives according to the Austrian guideline [9] and [10]
2.2 Comparison of strengthening with carbon fibre
reinforced polymers and externally bonded steel
The comparison of CFRP and steel as externally applied strengthening
elements is of special interest because both materials are frequently used in the
same way, namely as externally bonded reinforcement in tensile zones. Both, CFRP
and steel are bonded onto the surface by an epoxy-based adhesive. Before the
application the surface of the strengthened member must be repaired and prepared
in the same way (see chapter 2.4.2).
2.2.1
Differences in behaviour under tension
Comparing the idealized stress-strain-diagrams of CFRP and steel (Figure
2-1) it is clear that in the case of CFRP there is no yielding of the material prior to
rupture at all. Failure occurs suddenly without advance warning. In contrast to CFRP,
steel begins to yield at small strains. The tensile strength of CFRP is up to one order
higher than that of steel (Figure 2-1).
2.2.2
Advantages of carbon fibre reinforced polymers
CFRP have considerable advantages over steel due to their particular
properties. The advantages are as follows:
Immunity to corrosion
Low weight
Very high tensile strength (static and long-term)
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Negligible relaxation
In practical terms CFRP have additional advantages due to their low weights.
There is no need for temporal supports. Low weight combined with large deformation
capacity makes transport and application in limited space easier. CFRP are normally
delivered on rolls with diameters of maximal 1.5 m depending on the product [15].
Also FRP products are available in any size and geometry. Tensile strength
and stiffness can be modified to design requirements and can be chosen to fit with
other materials (i.e. the modulus of elasticity can be adapted to that of the existing
internal steel reinforcement).
As mentioned above, CFRP are significantly lighter and stronger in tension
than steel. The effects of these mechanical differences are illustrated in Figure 2-3. A
comparison of a steel with a tensile strength of 500 N/mm² and a density of 7850
kg/m³ and a CFRP strip of 2000 N/mm² and 1300 kg/m³ leads to the result (including
the division by
R,L
=1,5 according to [9]) that 1 kg/m CFRP can resist a tensile force
of 1026 kN whereas 1kg/m steel corresponds to 42 kN. It can be concluded that in
order to achieve the same increase in tensile force approximately 25 times more
steel in unit weight is required compared to CFRP. On a weight basis CFRP cost
more than steel. However CFRP are the cheaper alternative for an externally bonded
reinforcement unless the production cost of steel per unit weight is 25 times less than
that of CFRP. Taking into consideration the higher weight of steel required would add
to the transport and application costs, CFRP are more likely the cheaper option.
0
500
1000
1500
2000
2500
tensile strength [N/mm²]
force for 1kg/m [kN]
;
st eel
CFRP st rips
Figure 2-3: Comparison of the tensile strength and the resistible force for 1 kg/m of steel and CFRP strips
In addition, joints and intersections of CFRP are less problematic because
there is no need for welding or for complicated connections.
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2.2.3
Disadvantages of carbon fibre reinforced polymers
The major disadvantage of CFRP is the linear elastic behaviour whereby even
though at large strains the material ruptures without any advance warning such as
yielding and plastic deformations (according to Figure 2-1).
In contrast to steel, the thermal expansion coefficient of CFRP is different from
that of concrete. Additional shear stresses on the interfaces are caused by different
expansions when subjected to changes in temperature.
Exposure to high temperatures causes immediate degradation and collapse of
CFRP. Hence CFRP should be viewed as additional reinforcement, but not as
replacement of internal steel reinforcement on concrete structures; particularly if no
protection measures such as coatings are taken. If externally bonded CFRP is not
protected against heat, complete loss of CFRP is considered as accidental situation
according to the Austrian guidelines (refer to chapter 2.3.1).
2.3 Guidelines and reference works in Austria
concerning CFRP and design of strengthening
elements
The Austrian Association of Concrete in cooperation with the Austrian
Research Institute for Chemistry and Technology (OFI) published the first guidelines
about strengthening methods for existing reinforced concrete structures in Austria in
December 2002. The Austrian guidelines [9] refer themselves to the principles and
procedures of the Austrian regulations of reinforced concrete structures (ÖNorm B
4700), which are based on Eurocode 2. It should be mentioned that the contents of
the Austrian guidelines are concentrated on the practical execution of the CFRP
application and tendering documents. In the short section which deals with static
design and verifications, the Austrian guidelines refer to [4] without providing detailed
information. An comprehensive source of information concerning design and
verifications of CFRP-strengthening of concrete-, wood- and steel-structures was
published by Bergmeister [1] in 2003. Both [1] and [4] offer calculation procedures
including detailed explanations for design and verification of CFRP-strengthened
elements ( [4] exclusively deals with the strengthening of reinforced concrete
structures).
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The guidelines for strengthening of concrete elements by bonding of
unidirectional CFRP strips [2] published by the German Institute for Building
Technique (Deutsches Institut für Bautechnik or DIfBt) are another standard which is
applied in Austria, too. Licences or technical approvals for strengthening systems are
often given by the same DIfBt.
The Austrian guidelines for road construction (Richtlinien und Vorschriften für
den Straßenbau 13.62 or RVS 13.62) contain requirements for the direct tensile pull-
off test and the adhesion tester. The latter is used to examine the pull-off strength of
the concrete member before strengthening and to prove the composite action of
concrete and CFRP reinforcement after application.
2.3.1
Verifications of the strengthening system according to the
Austrian guidelines
The guidelines are based on Eurocode 2 and its semi probabilistic safety
concept. The verification of the strengthened structure is divided into a verification of
the ultimate limit state and a verification of the serviceability limit state. Each of these
verifications must be performed for the individual structural element and the structure
as a whole.
Ultimate limit state (ULS)
For CFRP linear elastic behaviour till rupture is assumed. The required
material safety factors are given in Table 2-4.
Material
Material safety factor
CFRP strips
R,L
=1.5
CFRP sheets
R,M
=2
(Externally bonded steel)
(
R,S
=1.5)
Table 2-4: Material safety factors of CFRP strips, CFRP sheets and externally bonded steel
according to the Austrian guideline [9]
In order to ensure the functioning of the strengthened system, the composite
action of concrete, CFRP and the adhesive must be verified. The strengthened
composite element is divided into a free section and an anchoring zone (Figure 2-1).
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In the free section the maximum possible increase in tensile stress within the CFRP
reinforcement between two subsequent flexural cracks in the concrete has to be
calculated. It must be verified that this increase in tensile stress is transferable to the
CFRP element by bond stresses. Otherwise debonding and peeling-off initiate at the
flexural cracks. The Austrian guidelines mention the envelope line of tensile force to
calculate this increase of tensile stresses (
in Figure 2-1). An approach for the
calculation of crack widths is given in [4].
In addition it has to be proven that the anchorage zone ensures the
transmission of forces from the CFRP reinforcement to the concrete element. Strips
with a higher width or special anchoring elements must be applied if this requirement
cannot be met.
Figure 2-4: Envelope line of the resisting tensile force
Unless protective measures such as special coatings are taken, an additional
verification for the unstrengthened system, in which loss of the CFRP due to fire is
assumed, has to be performed. In this case the material safety factors equal one and
the reduced partial safety coefficients as well as the combination factors for the loads
are considered with reference to Eurocode 2 (accidental situation).
Serviceability limit state (SLS)
According to the Austrian guideline verifications of the SLS may be performed
using the mean value of the modulus of elasticity of the technical approval of the
free zone
envelope line of
the tensile force
envelope line of the resisting tensile force
anchoring zone
N
S1
N
f
A
S1
A
f1
crack spacing
s
rm
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respective CFRP element. It has to be proven that under the rare load combination
(Eurocode 2) the internal steel reinforcement does not yield. The crack width
limitations do not have to be verified, unless CFRP elements are explicitly applied to
reduce or limit the width of cracks. Additionally, the Austrian guideline refers to [4],
where the following additional requirements for the SLS are sourced.
Deflections and stress limitations have to be verified according to Eurocode 2.
The tensile stresses of the CFRP element under service load are limited to 80 %
(60% according to [1]) of the characteristic value of the tensile strength (f
fk
). Apart
from that, bond interface crack initiation at the CFRP curtailment should be prevented
at service load. Otherwise the long term integrity of the anchorage zone (e.g. under
cyclic loading or freeze/thaw action) would be reduced. To meet this requirement the
maximum shear stress (
f1
) at the CFRP end must be smaller than the characteristic
value of the tensile strength of concrete (f
ctk
). If this criterion is not met, an extra
anchorage has to be provided.
2.3.2
Design bending moment capacity
The calculation of the design bending moment capacity in ULS serves as an
explanation of the composite action of a strengthened element. The Austrian
guidelines make reference to [4], which offers a possible method to determine the
design bending moment capacity (M
Rd
) of a strengthened element in flexure.
4
.
0
:
)
(
)
(
)
(
2
2
2
1
G
G
s
s
s
G
f
f
f
G
yd
s
Rd
with
d
x
E
A
x
h
E
A
x
d
f
A
M
Wherein the following assumptions should be checked:
Yielding of tensile steel reinforcement:
s
yd
cu
s
E
f
x
x
d
1
Straining of FRP is limited to the ultimate strain:
fud
cu
f
x
x
h
0
For design calculations the necessary area of the CFRP element can be
determined by the following equation [1]
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Rd
Sd
f
fud
f
s
s
s
G
cd
Sd
f
M
M
with
t
d
h
E
h
d
N
d
d
E
A
x
d
f
b
x
M
A
:
)
2
(
)
2
(
)
(
)
(
85
,
0
2
2
2
2
where A
s1
and A
s2
are the areas of the tensile and the compressive steel
reinforcements and A
f
that of the FRP reinforcement, respectively. E
f
, E
s
,
f
,
s1
and
s2
stand for the modulus of elasticity and the strain of FRP and steel (in tensile and
compressive zone), respectively. f
yd
is the design value of the steel yield strength. d,
x, h, d
2
are geometrical values, which are indicated in blue in Figure 2-5.
cu
,
o
and
fud
stand for the ultimate concrete strain, the initial strain at the extreme tensile fibre
before strengthening and the design value of the ultimate FRP strain, respectively.
M
Sd
and N are the flexural design moment and the design normal force (signs
according to Figure 2-5).
Figure 2-5: Analysis of a cross section of a rectangular reinforced concrete element strengthened by CFRP for
the ultimate limit state in bending according to [1] and [4]
Detailed explanations and calculations for shear and column strengthening
including special circumstances such as CFRP inside slits can be found in [1] and [4].
F
c
G
x
A
f
A
s1
A
s2
A
f
E
f
f
A
s1
f
y
h
M
b
f
d
2
b
t
f
d
x
s1
0
c
=
cu
=0,0035
f
s2
A
s2
E
s
s2
0.85f
cd
( =0.8 in the case of steel yielding followed by concrete crushing)
F
c
=0.85f
cd
. .x.b
N
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2.4 Preliminary measures and application of CFRP
2.4.1
Examination of the state of the concrete member before
surface preparation
Prior to surface preparation for strengthening, the structural concrete elements
should be checked visually for evidence of flaking, rust, fissures, cracks,
disintegration, etc. The sources of such features should also be investigated.
A complete evaluation of the state of the existing element prior to
strengthening should include the points listed in Table 2-5. Values differ depending
on guidelines and literature. The values cited here are taken from [1], [9] and [12].
Property
limit
Concrete tensile strength (after surface preparation)
1.5 N/mm²
Concrete compression strength (concrete class)
C 12/15 (B 15)
Concrete cover
10 mm
Steel class
not lower than S 235
Concrete cracks with a width over 0.2 mm must be explicable and need to be
documented
Examination of carbonization depth and chloride ingress
Examination of the chemical composition
Air temperature
3 K over dew point
1
Moisture content
4 %
Surface temperature
5 ( 35) °C
Table 2-5: Material requirements for a reinforced concrete element before strengthening with CFRP (from [2],
[9] and [12])
The requirements in the upper part of Table 2-5 (white) concern the condition
of the existing concrete member and ensure it can carry out the functions it was
1
According to climatic statistics relative humidity often reaches values between 80 and 90 %
in Austria. Considering this range for the relative humidity the dew point is only about 3 K higher than
the air temperature at about 6-10 °C. At lower temperatures no CFRP application should take place
without heating device. Lists and calculations of the dew point depending on the air temperature and
the relative humidity are widely available (e.g. in
[3]).
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designed for. These requirements should be tested during an appropriate period prior
to the application.
The three last requirements in Table 2-5 (grey) have to be met to guarantee
the desired performance of the CFRP element and the adhesive. These
requirements have to be examined each time immediately before work commences
and also every time the weather conditions change notably during the application.
The temperature is a particularly critical point. The ideal surface temperature
ranges from 5°C to 35°C. Heating systems can be used in the case of low
temperatures (compare to 2.4.4). Humidity has to be taken into account, too. The air
temperature has to be at least 3 K higher than the dew point during the application in
order to guarantee the functioning of the adhesive. Damage may be caused by the
intrusion of moisture into the resin-fibre interface (carbon fibres are relatively inert to
water and so the only effects of moisture on CFRP are on the resin matrix).
2.4.2
Surface preparation and repair of the concrete member
The state of the concrete member and particularly the condition of its surface
are important parameters. The surface has to provide an adequate bond with the
adhesive layer and transfer shear stresses to the inner parts of the concrete element.
Unsound areas of the concrete element such as delaminated parts, broken
pieces etc. have to be removed and voids must be filled with an appropriate repair
mortar. Uneven concrete surface irregularities (offsets) must be ground and
smoothed to avoid delaminations (delaminations can be caused by diverting forces or
stresses perpendicular to the fibre direction in the sphere of the CFRP element). If
CFRP sheets run around corners or edges, these must be rounded to a radius of
about 15 mm (dependent on the particular product). Surface materials such as
surface lubricants, broken mortar pieces, paint coatings, staining, etc. must be
removed by:
Abrasive blasting
High-pressure water jets
Disc grinding
Needle pistol
only for work of smaller extent
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Additionally, the substrate must be cleaned of any dust, debris or laitance.
Internal steel reinforcement must be identified (diameter, class, location) if no
plans are available and its condition has to be examined. In case of corrosion of
internal steel, steel and concrete must be repaired before CFRP-installation. The
static system and geometry of the building should be proven, too.
2.4.3
Application of CFRP Reinforcement
The steps in Table 2-6 are an overview of a complete orderly operational
sequence [8], [17]:
1
Primer and filler, which have cured for over 24 hours, are abraded unless the
coatings are still tacky to the touch.
2
The adhesive is prepared by mixing its components, while special attention
should be paid to the pot life.
3
The adhesive is applied to the CFRP (which should have been pre-cut to
required lengths and clearly labelled) and spread evenly until the product is fully
covered and saturated thoroughly with the adhesive. It is allowed to sit for about
a minute.
4
Saturated CFRP is applied to the concrete surface. Fibres are oriented as
detailed in project drawings and CFRP are applied taut and without any
wrinkles. Using soft plastic spreaders and suitably protected hands, the wet
CFRP product is smoothed out ensuring full contact with the surface and to
remove trapped air.
5
To join ends of CFRP products, overlaps in the longitudinal direction have to be
a minimum of 15 cm for fabrics and 30 cm for strips. No overlap is needed
between adjacent bands of fabrics. Overlaps must be staggered in the case of
multiple layers (up to ten layers of CFRP sheets can be laid over one another).
Table 2-6: Operational sequence of the application of CFRP
Applied laminates should be checked after 30-45 minutes to ensure that no
voids or delaminations are present. The installed composite must be protected from
rain, direct sunlight, dust, sand etc for 24 hours.
Optionally, coatings or paintings such as fire protection or protection against
UV light exposure can be applied.
2.4.4
Basic techniques of strengthening with CFRP
CFRP are not only applied to increase the ultimate bearing capacity of a
strengthened element but also to reduce deformations or to increase ductility.
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To minimize the dependence of CFRP systems on the climatic conditions and
to reduce the curing time of the two-component adhesive under low environmental
temperatures, heating systems have been developed. Different heating systems such
as electrical heaters, infrared heating or heating blankets can be employed. Fast
curing shortens the application time and as a consequence it increases the glass
transition temperature of the adhesive. Heating devices can be used with all of the
following strengthening methods.
Different fire protection systems, such as coatings, paintings or panels have
been developed. An example of fire protection panels for flexural members
strengthened by CFRP strips is illustrated in Figure 2-6.
Figure 2-6: Fire protection with panels (from [1])
Flexural strengthening
In the case of flexural strengthening concrete elements are reinforced by FRP
composites epoxy-bonded to their tensile zones with the direction of fibres parallel to
the member axis. Application works on flexural elements are shown in Photo 2-1 and
Photo 2-2.
Photo 2-1: CFRP strips as flexural strengthening
applied in the tensile zone of a ceiling (from [19])
Photo 2-2: CFRP strips applied in
the tensile zone of a flexural
member of a bridge (from [19])
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Strengthening in shear and torsion
The concept of shear or torsion strengthening by CFRP sheets is illustrated in
Photo 2-3 and Photo 2-4. To simplify matters, the external FRP reinforcement is
mostly applied perpendicularly to the member axis (analogue to steel stirrups)
although the principal stress trajectories in shear-critical zones are inclined at an
angle of about 45° to the member axis. The beam of a bridge (Photo 2-3) and the
joist (Photo 2-4) serve as examples: both are subjected to gravity loads and are
strengthened by CFRP sheets applied perpendicularly to their axis although the
principal stress trajectories are inclined at an angle of about 45° to the member axis.
Photo 2-3: CFRP sheets as shear strengthening
applied perpendicularly to the axis of beams of a
bridge (from [19])
Photo 2-4: CFRP sheets as shear
strengthening applied perpendicularly to the
axis of a joist (from [19])
If full wrapping around the strengthened member is not feasible (e.g. if the top
of a T-beam is not accessible), it is recommended that the CFRP elements are
anchored in the compressive zone of the concrete member (above the neutral axis).
In the case of torsion only a CFRP element which fully surrounds the strengthened
member (jacket) increases the torsion capacity.
Normal force strengthening
The manual application of CFRP sheets on columns, which is also called
wrapping, is shown in Photo 2-5, Photo 2-6 and Photo 2-7. CFRP sheets are wound
around the strengthened member to increase the compressive strength of columns or
similar objects like chimneys by activating the multi-axial compressive strength of
concrete.
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Photo 2-5: Application of CFRP
sheets on a circular column
(from [19])
Photo 2-6: Application of CFRP
sheets on a rectangular column
(from [19])
Photo 2-7: CFRP sheets applied to a
rectangular column by wrapping
(from [16])
Automated wrapping techniques have already been developed. Wet fibres are
wound continuously around columns or comparable structures under a slight angle
by a robot. Good quality control and fast application are the key advantages of the
automated technique.
2.4.5
Selected special techniques of strengthening with CFRP
Prestressed CFRP
CFRP composites are prestressed before bonding them onto the concrete
surface until the adhesive has cured. The concrete is in compression at an early
stage and existing cracks may close or alternatively crack formation may even be
avoided. Prestressing increases the applied load at which the internal steel begins to
yield, which by design often corresponds to the ultimate load. The disadvantages of
this technique are the elevated costs because of longer application times and
complicated anchorage systems.
CFRP inside slits
CFRP strips are inserted into slits, which have been cut to a depth less than
that of the cover of the internal steel reinforcement. The achieved anchoring capacity
is higher than in the case of glued CFRP strips due to the doubled area of the
interface. This is because the CFRP strips are bonded to the strengthened element
on both sides. In addition, the strips are protected against demolition. In the case of
concrete, this technique is often used to reduce the width of cracks to ensure the
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impermeability of ceilings. More frequently this technique is used on wood structures
as shown in Photo 2-8 and Photo 2-9 because of the ease of slit-cutting.
Photo 2-8: CFRP strips
applied in a slit of a
wooden joist (from [19])
Photo 2-9: Application of
CFRP-adhesive in slits of
a wooden joist (from [19])
Details
- Seiten
- Erscheinungsform
- Originalausgabe
- Jahr
- 2004
- ISBN (eBook)
- 9783836641180
- DOI
- 10.3239/9783836641180
- Dateigröße
- 9.2 MB
- Sprache
- Englisch
- Institution / Hochschule
- Technische Universität Wien – Bauingenieurwesen, Studiengang Bauingenieurwesen
- Erscheinungsdatum
- 2010 (Januar)
- Note
- 1
- Schlagworte
- statische verstärkung kohlefaserverbundwerkstoffe stahlbetonbrücken cfk-lamellen gesundheitsrisiko
