Quality Assurance of Reinforced Concrete Structures Strengthened by Externally Bonded CFRP Strips

Diplomarbeit von Markus Huber

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.

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
	Fibre Reinforced Polymers	9
	Types of fibres	9
2.1.1	CFRP products and their properties	10
	Comparison of strengthening with carbon fibre reinforced polymers and externally bonded steel	12
	Differences in behaviour under tension	12
	Advantages of carbon fibre reinforced polymers	12
2.1.2	Disadvantages of carbon fibre reinforced polymers	14
	Guidelines and reference works in Austria concerning CFRP and design of strengthening elements	14
2.1.3	Verifications of the strengthening system according to the Austrian guidelines	15
2.1.4	Design bending moment capacity	17
	Preliminary measures and application of CFRP	19
	Examination of the state of the concrete member before surface preparation	19
2.1.5	Surface preparation and repair of the concrete member	20
2.1.6	Application of CFRP Reinforcement	21
	Basic techniques of strengthening with CFRP	21
2.1.7	Selected special techniques of strengthening with CFRP	24
3.	main reference object: Marxerbridge	26
	General description of the widening of the Marxerbridge	26
	Geographical position	26
	Arrangement of the structure of the Marxerbridge	27
	Division and numeration of the structure	27
3.1.1	Technical data and static system of the structure	28
	Widening of the Marxerbridge and static effects	30
	Parties involved in the construction	30
3.1.2	Construction phases and static effect	31
	Strengthening methods on the Marxerbridge	37
	Reinforcement with CFRP strips	37
3.1.3	Strengthening with additional reinforced concrete	39
4.	quality assurance	42
	General	42
	Quality assurance before the application of CFRP	43
	Examination of the pull-off strength of the concrete member	43
4.1.1	Monitoring of the evenness of the concrete surface	44
	Quality assurance after the application of CFRP	45
	Examination of pull-off strength after CFRP-application	45
4.1.2	Monitoring of evenness after the application of CFRP	45
5.	quality assurance on the Marxerbridge	48
	General	48
	Results of pull-off tests in all phases	49
5.1.1	Statistical analysis of the results of the pull-off tests	49
5.1.2	Layer of rupture in pull-off tests after CFRP application	52
	Results of monitoring of evenness	55
	Results of search for hollow areas and air inclusions	55
6.	Defects and repair methods	56
	Starting point	56
	Air inclusions and delaminations	57
	Reasons for air inclusions and delaminations	57
6.1.1	Development of air inclusions and delaminations	58
6.1.2	Repair of air inclusions and delaminations	62
	Jut out between CFRP strip and concrete	66
	Longitudinal cracks in the CFRP strip	66
7.	Comparison with other objects	67
8.	Conclusions and recommendations	70
	Recommendations for a future edition of the Austrian guideline	70
8.1.1	Search for hollow areas and air inclusions by knocking on CFRP strips with a nylon hammer	70
8.1.2	Monitoring of evenness	71
8.1.3	Examination of pull-off strength	72
8.1.4	Preparation of the CFRP strips	72
	Alternative testing methods	73
	Health considerations for workers and inspectors	74
	summary	76
	Zusammenfassung	78
	Annex 1: Data of quality control	81
	Annex 2: Protocol of defects	95
	Annex 2.1: Data of defects	97
	Annex 2.2: Location of defects	101
	Annex 3: pull-off test	106
	Annex 4: Reference objects	111
	Annex 4.1: Object A	111
	Annex 4.2: Object B	112
	Annex 4.3: Object C	113
	Annex 4.4: Object D	114
	References	116
	List of Figures	118
	List of Tables	121
	List of photos	122

Text Sample:

Chapter 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 (only for work of smaller extent), -Needle pistol (only for work of smaller extent).

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 operationalsequence:

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).

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.