FRP Composites for Retrofitting of Existing Civil Structures in Europe: State-of-the-Art Review

Transcription

1 American Composites Manufacturers Association October 17-19, 2007 Tampa, FL USA FRP Composites for Retrofitting of Existing Civil Structures in Europe: State-of-the-Art Review Masoud Motavalli and Christoph Czaderski, Empa, Swiss Federal Laboratories for Materials Testing and Research Ueberlandstrasse 129, 8600 Dübendorf, Switzerland Abstract A substantial number of structures in Europe are more than 30 years old. Whilst they require continuous maintenance, they also require strengthening due to lack of strength, stiffness, ductility and durability. Because FRP composites are light-weight and easy to install on site, they are considered to be the most favoured material in many strengthening applications. The paper will present the state-of-the-art of the FRP composites for strengthening of existing civil structures in Europe. Existing techniques for flexural and shear strengthening, near surface mounting reinforcement as well as column confinement of Reinforced Concrete (RC) structures will be discussed. Furthermore, a few applications for FRP strengthening of historical masonry buildings will be presented. FRP pre-stressing techniques for retrofitting of existing structures will be presented as an emerging market in Europe. Introduction There are several situations in which a civil structure would require strengthening or rehabilitation due to lack of strength (flexure, shear etc.), stiffness, ductility and durability. Some of the common situations where a structure needs strengthening during its lifespan are: seismic retrofit to satisfy current code requirements; upgraded loading requirements; damage caused by accidents and environmental conditions; initial design flaws and change of usage. by 1 Because FRP composites are light-weight and easy to install on site, they are considered to be the most favoured material in many strengthening applications. The overall cost of the whole strengthening job using FRP materials can be as competitive as using conventional materials, in addition to being quick and easy to handle on site with minimum interruption to use of facility. In some situations, FRP composites are the only plausible material that could be used for strengthening, especially in places where heavy machinery cannot gain access or closure of the use is not practical. Urs Meier and his team at the Swiss Federal Laboratories for Materials Testing and Research (Empa) began research on the use of carbon FRP composites as external reinforcement for strengthening structures in the mid 1980s. This was the first worldwide research work in the field of FRP composites for strengthening. The comprehensive research work done at Empa between 1984 and 1989 (Kaiser 1989), enabled the consequent wide spread use of carbon FRP external reinforcement to strengthen structures. Based on these developments, the first application of carbon FRPs to strengthen a bridge took place in Lucerne, Switzerland in early 1990s. Ibach Bridge is a multi-span continuous box bridge, which had one of its pre-stressed tendons damaged during drilling to install new traffic signals (Meier et al. 1992). Although the material cost of carbon FRPs was several times more than that of steel plates, the fact that 6.2kg of carbon FRPs could be used in place of 175kg of steel is sufficient to explain the advantages of carbon FRPs over steel plates. The entire work was carried out in two-night shifts from a mobile platform eliminating the use of scaffolding. A substantial number of bridges on European highways and railways are more than 30 years old. Whilst they require continuous maintenance, they also require strengthening for increased loads due to heavier vehicles and traffic volume. Strengthening for upgraded loading is now common in bridge engineering and a significant portion of funding is spent on this. Since 1999, all bridges in Europe are required to carry 40 tonne vehicles, and consequently a number of old bridges needed strengthening. The traditional steel plate bonding to the decks was not viable in some cases due to various reasons including, significant weight increase, access difficulties and longer construction times. Over 30 bridges and other structures in the UK have been strengthened during 1997 alone, using about 6km of carbon FRP plates. Carbon FRP plates or sheets have been used to increase the flexural and shear capacity of decks and beams of the upgraded bridges (Loudon June 2001). In mid 1990s, the Highways Agency in the UK investigated and later implemented the use of aramid FRP composite material to increase the resistance of

2 bridge columns that were at risk due to accidental vehicular impact. Following a successful, trial site application of column wrapping with FRPs in 1997, the Highways Agency commissioned the Transport Research Laboratory (TRL) to conduct a series of tests to establish design rules and guidance, which will be published as a formal standard (TR ). Sometimes, deficiencies in the initial design have required strengthening to be carried out during the service life of the bridge. Reportedly, one of the first applications of bridge strengthening with FRP composites in Germany was carried out to correct a design flaw on a bridge [5]. A number of bridges built after the World War II in Germany consisted of pre-stressed concrete multi-span construction. These were mostly designed as continuous box girders and the joints were usually at the points of contraflexure where all the tendons were coupled. Many of these bridges now exhibit cracks at the joints. The main cause for these cracks is a temperature restraint, which was not taken into account in the initial design. In combination with other stresses, tensile stresses at the bottom increase and exceed the concrete tensile strength at the joint. This necessitated repairs on these cracked bridges for which Professor Rostasy and his colleagues from Braunschweig developed a technique to strengthen such joints with bonded steel plates. In , this method was used for the first time with glass FRP plates on the Kattenbusch Bridge (Rostasy 1987). The Kattenbusch Bridge was designed as a continuous, multi-span box girder with a total length of 478m. It consists of 9 spans of 45m and side spans of 36.5m each. There are 10 joints. The depth of the twin box girder is 2.7m. The bottom slab of the girder is 8.5m wide. One joint was strengthened with 20 glass FRP plates. Each plate is 3200mm long, 150mm wide and 30mm thick. Loading tests performed by Rostasy and colleagues showed a reduction in the crack width of 50% and a decrease in the stress amplitude of 36%, thus extending the fatigue life. A number of other European countries such as Sweden (Täljsten et al. 2003) as well as Italy, Greece, Poland and Turkey (fib April 2006) have applied FRP composites successfully for strengthening their existing structures. A selection of examples is presented in the next sections highlighting the wide range of situations where FRP composites have been implemented to improve flexural and shear capacity, ductility, and other serviceability criteria. Flexural Strengthening of RC Structures Using FRP Plates and Sheets Beams, Plates and columns may be strengthened in flexure through the use of FRP composites bonded to their tension zone using epoxy as a common adhesive for this purpose. The direction of fibers is parallel to that of high tensile stresses. Both prefabricated FRP strips, as well as sheets (wet-lay up) are applied. Figure - 1 shows the installation of the flexural strengthening of a RC girder of a building in Poland using CFRP prefabricated strips. Figure - 2 illustrates the crosswise application of a RC deck on the top and bottom side and around the columns. Well established European guidelines and codes are available for engineers for designing purposes (see section codes and guidelines). Shear Strengthening of RC Structures Using FRP Plates and Sheets Shear strengthening is usually provided by bonding the external FRP reinforcement on the sides of the webs with the principal fibre direction perpendicular or with an angle of e.g. 45 to the member axis. For this purpose prefabricated L-shaped CFRP (Czaderski et al. 2004) plates were installed for the shear strengthening of the rump of the Duttweiler bridge in Zurich Switzerland in 2001 (see Figure - 3). The L-shaped plates were installed in combination with CFRP strips for flexural strengthening. Figure - 4 shows placing of carbon fibre fabrics in the shear zone of a bridge above the railway to Laziska power plant in Poland. The strengthening was carried out in Well established European codes and guidelines are existing for shear strengthening of RC structures using FRP (see section codes and guidelines). Near Surface Mounting Reinforcement (NSMR) The externally bonded FRP to RC structures is susceptible to damage from collision, high temperature, fire and ultraviolet rays. To overcome these drawbacks, Near Surface Mounted Reinforcement (NSMR) technique has been proposed. Slits are cut into the concrete structure with a depth smaller than concrete cover. CFRP strips or bars are bonded into these slits. Tests have shown that a higher anchoring capacity compared with CFRP strips glued onto the surface of a RC structure is obtained (fib 2001; Kotynia December ). Despite the efficiency of the NSMR technique, a few applications can be found in Europe, where this technique was applied. Furthermore, codes and guidelines for this technique are missing. Figure - 5 shows the strengthening of a RC deck in Stuttgart, Germany applying the NSMR technique. 2

3 Column Confinement Confinement is generally applied to members in compression, with the aim of enhancing their load bearing capacity or, in case of seismic upgrading, to increase their ductility in the potential plastic hinge region. The confinement in seismically active regions has proven to be one of the early applications of FRP materials in infrastructure applications. Confinement may be beneficial in non-seismic zones too, where, for instance, survivability of explosive attacks is required or the axial load capacity of a column must be increased due to higher vertical loads, e.g. if new storey s have to be added to an existing building or if an existing bridge deck has to be widened. In any case, confinement with FRP may be provided by wrapping RC columns with prefabricated jackets or in situ cured sheets, in which the principal fiber direction is circumferential (Bakis et al. May 2002). Figure - 6 illustrates the confinement of RC columns applying CFRP fabric (wet-lay up technique) of the Reggio Emilia football stadium in Italy, 50 km from Bologna. Analysis of the stadium with the new Italian seismic code showed that the existing stirrups at the base of the columns were not sufficient to withstand the seismic loads. Therefore, the columns were confined in March Figure - 7 shows the seismic retrofitting of Aigaleo football stadium in Athens, Greece. The columnbeam joint is retrofitted using CFRP fabrics (wet-lay up technique). The CFRP fabric is anchored to the RC deck using steel plates. Well established European codes and guidelines are existing for designing the confinement of RC columns (see section codes and guidelines). Retrofitting of Masonry Structures Practical applications in recent years have shown the FRP s as an alternative strengthening material for masonry structures, especially those of considerable historical importance. One of the first research works worldwide was conducted at Empa, Switzerland (Schwegler 1994). FRP strips and fabrics were applied to the masonry shear walls in the laboratory using epoxy adhesives. The walls were then tested under static cyclic loading. It was shown, that the in-plane deformation capacity of the masonry shear walls after strengthening could be increased up to 300%, if the end of the FRP strips are anchored properly. A number of historical buildings especially in Italy, Greece and Portugal were retrofitted applying FRP composites. Aramid and Glass FRP was applied for restoring the Basilica of St. Francis of Assisi in Italy. The historical building was severely damaged by earthquakes and aftershocks in September and early October 1997 (Borri et al. April ). Figure - 8 and Figure - 9 shows the retrofitting of one of the masonry towers of the ancient Vercelli castle in Italy by applying CFRP rods bonded into the space between the bricks. One of the four towers showed wide vertical cracks. A reinforcement of the outer side of the masonry wall was necessary. It has been done by putting horizontal CFRP rods around the tower to prevent further opening of the cracks. Rods were bonded using epoxy resin. The strengthening was completed in May Figure - 10 shows the seismic upgrading of the masonry shear walls of a school building in Bern, Switzerland. GFRP fabrics were glued to the walls surface followed by CFRP strips, which were applied crosswise on the GFRP fabric layer. The strip ends were anchored in the RC decks using steel plates. Currently, no design codes and guidelines are available in Europe for strengthening of masonry applying FRP. RILEM has recently established a new technical committee (TC) entitled Masonry Strengthening with Composite Materials. The preliminary work of the TC will be the systematization of the current knowledge on the structural behaviour of masonry constructions and components strengthened with composite materials with the final aim of a possible proposal of international recommendations including design tools, quantitative and qualitative evaluation measures, limitation parameters of efficiency and simple experimental procedures (RILEM Technical Committee (TC) 'Masonry Strengthening with Composite Materials (MSC)', International Union of Laboratories and Experts in Construction Materials, Systems and Structures) Prestressed Systems Prestressing of composite strips prior to the bonding procedure results in a more economical use of materials but requires special clamping devices. Prestressing results in stiffer behavior; delaying the crack formation; closing cracks in structures with pre-existing cracks and therefore improving serviceability and durability. There are several prestressing methods, which are currently applied in Europe. Examples are as follows: A sport hall roof in Austria had to be retrofitted due to large deformations under dead loads and insufficient load capacity for high snow loads. The reduction of the deflection and increasing of the load bearing capacity was achieved by applying prestressed CFRP strips (Figure - 11). The details of the clamping device are illustrated in Figure Figure - 13 shows the strengthening inside the box girder of a bridge in Croatia applying prestressed CFRP strips with different clamping device than the previous example. Cracks at the coupling joints of Neckar highway bridge in Heilbronn, Germany (built in 1964) were the reason for the rehabilitation of coupling joints applying 3

4 prestressed CFRP strips with steel plates for clamping the strip ends (Figure - 14). Above-mentioned mechanical anchorages are expensive, difficult to install and subjected to corrosion. To overcome these anchorage problems, the prestressing force can be anchored using gradient method (Czaderski et al. 2007; Aram et al. accepted for publication, Feb 2007; Meier et al. November 21-23, 2005). In this method the prestressing force is reduced gradually at both ends to zero by using a special processing technique. Figure - 15 shows a RC deck, which is retrofitted applying prestressed CFRP strips with the gradient anchorage technique without any end anchorage plate. Yet, few applications are available, where this technique is applied. More experimental and analytical research work is required to optimize the gradient anchorage technique that could replace the mechanical clamping systems in near future. In Figure - 16 columns of a storage building in Portugal are shown that are seismically upgraded using prestressed wrapped aramid fibres. Only a few examples are available, where prestressed column confinement is applied. Yet, there have been no codes and guidelines available for designing prestressed strengthening systems. It can be concluded that the strengthening methods with pre-stressed FRP are not so well established yet. It will take more development work before they are suitable for practical applications since the pre-stressing methods are still complicated to use and installation techniques, both manual and automatic, have yet to be perfected. These include surface preparation, prestressing, placing and bonding, forming end anchorages and vacuum bonding. Automatic application methods will offer advantages in hazardous areas, where there is danger from traffic and will reduce traffic management and traffic delay costs. It is a need to make in the future better use of the high strength of CFRP with pre-stressed applications (Meier August 2004). Codes and Guidelines Although the technique externally bonded reinforcement is quite new, there are already several European codes and guidelines available for responsible engineers for planning a retrofitting project. However, it has to be noted that in some respects the existing design philosophies differ distinctively and various topics are still under research and development. Therefore, the strengthening techniques should be planned very carefully by experienced and educated engineers. The main focus of these codes and guidelines is the strengthening of reinforced concrete (RC). The European codes and guidelines for the strengthening of RC include following topics: - Basis of design and safety concept, 4 - Flexural strengthening, - Shear strengthening, - Confinement, - Seismic applications, - Execution and quality control. The European task group fib 9.3 FRP (Fibre Reinforced Polymer) Reinforcement for Concrete Structures (fib) was one of the first publishing a guideline in the field of externally bonded reinforcement (fib 2001). The fib (International Federation for Structural Concrete) task group comprises experts in the field of FRP as structural reinforcement for concrete structures. The work performed by fib TG 9.3 is published as fib Bulletins. Meetings are held twice a year. Started as a CEB (Comité Euro-International du Béton) Task Group in September 1996 and converted, with the merger of CEB and FIP in June 1998, into fib TG 9.3, the group forms part of Commission 9 'Reinforcing and Prestressing Materials and Systems'. The task group consists of about 50 members, representing most European universities, research institutes and industrial companies working in the field of advanced composite reinforcement for concrete structures. The work of fib TG 9.3 is organized in 2 subgroups: (1) FRP reinforcement (RC/PC) and (2) Externally bonded reinforcement (EBR). The work on an updated bulletin of (fib 2001) for EBR is under process. In Switzerland, a precode (SIA ) for externally bonded reinforcement was published in In England, already the second edition of TR55 Design guidance for strengthening concrete structures using fiber composite materials (TR ) was also published in In addition, since 2004, the Guide of the design and construction of externally bonded FRP Systems for strengthening existing structures (CNR 2004) is available in Italy. Only less information can be found in European codes and guidelines about prestressed strengthening methods using FRP materials. Furthermore, the strengthening of structures made of wood, masonry, aluminum etc. is also not very well documented. The above list of European codes and guidelines is not complete but contains important documents in the area of FRP applied as external reinforcement of concrete. Main weaknesses in these documents are lack of a unified design approach. Several topics relevant to the use and design of FRP as externally applied reinforcement are not dealt with or are poorly covered in the above documents. Research needed in these areas may be summarized as follows: (a) Better understanding and development of unified and simple design models for mechanical anchorages and mechanisms associated with debonding; (b) development and verification of systems for the protection of externally applied FRP at high temperatures; (c) derivation of

5 material safety factors; (d) better understanding of FRPstrengthened masonry, (e) development of simple design models for prestressed systems with mechanical anchorage, as well as for gradient anchorage technique without mechanical anchorage. Conclusions The use of FRP in civil and building structures is not uncommon anymore: structures have successfully been strengthened or retrofitted with FRP materials in many European countries. FRP composites are readily used for strengthening applications mainly due to the relative ease of installation. Strengthening with FRP composites have mostly been either the lowest tendered price or the only plausible solution available. The material costs of the FRP composites are several times more than that of conventional materials (e.g. steel and concrete). However, the life-cycle cost, including fabrication, application, protection and projected maintenance costs, is comparable and can be less than that of conventional materials. Many engineers believe that FRP composites must be used as a complementary material and not as a substitute for concrete and steel. FRP composites have significant advantages over conventional materials in particular situations, but composites cannot replace steel or concrete in every single application. Design guidelines and recommendations are essential for the wider use of FRP composites in strengthening of civil and structural engineering. In the last few years, European engineering institutions and societies in collaboration with researchers and practitioners in the field, either have developed or are in the process of developing codes and recommendations for professional engineers. Education of engineers is necessary to reap the full potential and the appropriate use of FRPs. Similarly, training is vital for people who fabricate and install FRP composites in the construction industry. The quality of the workmanship is a critical factor and thus specifications must address proper fabrication and installation criteria for composites. Figure - 1. Flexural strengthening of concrete girders of a cement manufacturing building in Poland using CFRP strips Acknowledgements To Mr Reto Clenin from SIKA Company AG, who provided application examples in Europe carried out by SIKA. To Mr Josef Scherer from S&P Clever Reinforcement Company AG, who provided application examples in Europe carried out by S&P. To Professor Renata Kotynia from Lodz University in Poland, who provided application examples in Poland. Figure - 2. Strengthening of a concrete deck of a building using CFRP strips on the top and underside of the deck 5

6 Figure - 3. Installation of prefabricated CFRP L- shaped plates (shear strengthening) over already installed CFRP strips for flexural strengthening; Duttweiler bridge ramp in Zurich, Switzerland Figure - 5. Flexural strengthening of a concrete deck in the region of negative bending moment using Near Surface Mounting Reinforcement (NSMR) technique by cutting a slot in the concrete deck and placing the CFRP into the slots; industry plant in Stuttgart, Germany Figure - 4. Placing of CFRP fabrics for shear strengthening of DK 81 bridge above railway to Laziska power plant in Poland Figure - 6. Application of CFRP fabrics on concrete columns for seismic retrofitting of Reggio Emilia football stadium in Italy 6

7 Figure - 7. Seismic retrofitting of column-beam joints of Aigaleo football stadium in Athens, Greece, using CFRP fabrics with steel anchorages Figure - 9. View into the castle and at the tower under strengthening and repair works, Vercelli Castle, Italy Figure - 8. Carbon rods bonded into the space between the bricks as reinforcement, Vercelli Castle, Italy Figure Seismic retrofitting of a masonry shear wall using GFRP fabric and additional CFRP strips, which are anchored in concrete using end plates; school building Zollikofen in Bern, Switzerland 7

8 Figure Reducing deflections and strengthening of a sports hall roof in Thörl, Austria by presstressed CFRP strips Figure Detail on prestressing anchorage Figure Rehabilitation of coupling joints of the Neckar highway bridge in Heilbronn, Germany using prestressed CFRP strips with steel end plates Figure Strengthening of a bridge box girder using prestressed CFRP strips with steel end anchorage; Bakar bridge, Croatia Figure Strengthening of a concrete deck using prestressed CFRP strips with recently developed gradient end anchorage technique without any additional end anchorage plates 8

9 Figure Prestressing of aramid fibers wrapped around a column for seismic retrofitting of a storage building in Portugal Authors: References Masoud Motavalli: Professor and Head of the Structural Engineering Research Laboratory at Empa, Swiss Federal Laboratories for Materials Testing and Research ( Lecturer at the Swiss Federal Institute of Technology, ETH-Zurich and at the University of Tehran, Iran Christoph Czaderski: Researcher, Project leader and PhD student at the Structural Engineering Research Laboratory at Empa, Switzerland. Member of the fib Task Group 9.3 FRP reinforcement for concrete structures Aram, M. R., C. Czaderski, et al. (accepted for publication, Feb 2007). "Effects of gradually anchored prestressed CFRP strips bonded on prestressed concrete beams." Journal of Composites for Construction, ASCE. Bakis, C. E., L. C. Bank, et al. (May 2002). "Fiber-Reinforced Polymer Composites for Construction- State-of-the- Art Review." Journal of Composites for Construction: Borri, A., M. Corradi, et al. (April ). New Materials for Strengthening and Seismic Upgrading Interventions. International Workshop Ariadne 10, Arcchip, Prague, Czech Republic. CNR (2004). Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Existing Structures, CNR-DT 200/2004. Rome, Italy, CNR - Advisory Committee on Technical Recommendations for Construction. Czaderski, C. and M. Motavalli (2004). "Fatigue Behaviour of CFRP L-Shaped Plates for Shear Strengthening of RC T-Beams." Composites Part B: Engineering 35: Czaderski, C. and M. Motavalli (2007). "40-Year-Old Full- Scale Concrete Bridge Girder Strengthened with Prestressed CFRP Plates Anchored Using Gradient Method." Composite Part B: Engineering 38: fib. Task Group 9.3 homepage: fib (2001). Externally bonded FRP reinforcement for RC structures - Bulletin 14, International Federation for Structural Concrete (fib), Switzerland. fib (April 2006). Bulletin 35, Retrofitting of Concrete Structures by Externally Bonded FRP's, with Emphasis on Seismic Applications, International Federation for Structural Concrete (fib). Kaiser, H. (1989). Bewehren von Stahlbeton mit Kohlenstoffaserverstärkten Epoxidharzen, Doctoral Thesis, ETH No. 8918, Swiss Federal Institute of Technology, ETH Zurich. Kotynia, R. (December ). Flexural Behavior of Reinforced Concrete Beams Strengthened with Near Surface Mounted CFRP Strips. CICE2006, Miami, Florida, USA. Loudon, N. (June 2001). "Strengthening Highway Structures with Fibre-Reinforced Composites." Concrete: 16pp. Meier, U. (August 2004). "External Strengthening and Rehabilitation: Where from - Where to?" IIFC FRP International, The Official Newsletter of the International Institute for FRP in Construction 1(2): 2-5. Meier, U., M. Deuring, et al. (1992). Strengthening of Structures with CFRP Laminates: Research and Applications in Switzerland. 1st Intl. Conf. on Advanced Composite Materials in Bridges and Structures, p243, Sherbrooke, Canada. Meier, U. and I. Stöcklin (November 21-23, 2005). A Novel Carbon Fiber Reinforced Polymer (CFRP) System for Post-Strengthening. International Conference on Concrete Repair, Rehabilitation and Retrofitting (ICCRRR), Cape Town, South Africa. Rostasy, F. S. (1987). Bonding of Steel and GFRP Plates in the Area of Coupling Joints, Talbrücke Kattenbusch, Research Repost No. 3126/1429. Braunschweig, Germany, Federal Institute for Materials Testing Braunschweig. Schwegler, G. (1994). Verstärken von Mauerwerk mit Faserverbundwerkstoffen in seismisch gefährdeten 9

10 Zonen. Dübendorf, Schweiz, Empa Dübendorf, Bericht Nr SIA166 (2004). Klebebewehrungen (Externally bonded reinforcement), Schweizerischer Ingenieur- und Architektenverein SIA. Täljsten, B. and A. Carolin (2003). Strengthening Two LArge Concrete Bridges in Sweden for Shear using CFRP Laminates. Structural Faults and Repair, 03, London, UK. TR55 (2004). Design guidance for strengthening concrete structures using fibre composite materials, Second Edition, Technical Report No. 55 of the Concrete Society, UK. 10