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THE USE OF FIBRE REINFORCED POLYMER (FRP) COMPOSITE MATERIALS ON STRENGTHENING OF HISTORICAL LANDMARKS

Dr. Sotiris Demis Patras Science Park

Stadiou Str. Platani, 26505, Patras, Greece

Professor Charis Apostolopoulos Department of Mechanical and Aeronautic Engineering, University of Patras

26500 Rio, Patras, Greece

Keywords: FRP, Strengthening, Historical Monuments, Durability ABSTRACT

The use of fibre reinforced polymer (FRP) composites, one of today’s most advanced material, is becoming increasingly popular as a feasible solution in strengthening and retrofitting applications. Even though the issue of upgrading our existing civil and structural engineering infrastructure has been one of great importance for over a decade, recent advances in the field of strengthening with the incorporation of FRP composite materials provide a feasible and durable alternative, than the common practices used today, for use in “sensitive” projects as the restoration of out cultural heritage. The growth of research on every aspect of FRPs and on strengthening with FRPs, have been quite substantial, providing a plethora of materials, methods and techniques, increasing in this way the choice to the practitioner engineer to enforce a robust structural and durable solution on the restoration and preservation of our historical landmarks. The aim of this study is to provide an insight and current trends on the use of FRPs in the retroffiting and strengthening of historical monuments, to the practitioner engineer, and also to raise the level of awareness on the incorporation of the technology, use and strengthening applications of these materials, on the recommendations published by the Greek Ministry of Culture regarding the restoration of our historical monuments. INTRODUCTION

It is a fact that the majority of our recently erected, 20th century, monuments are made of reinforced concrete. Given the known environmental issues of deterioration that such a type of construction faces, the need of structural interventions in the area of architectural heritage (monuments, historic buildings and bridges) for repair and strengthening has received considerable emphasis over the last years. According to the requirements set in the ‘Chapter of Venice’ guidelines, any possibly intervention should not affect the character of the monument and, if possible, should be reversible to a great extend. The majority of the practises used today, (patching, reinforced concrete jacketing, external or internal post-tensioning with steel tied etc..) present several practical difficulties in protecting the refurbished element against environmental corrosion. In addition, they offer a costly solution, not always efficient, with further implications for the future maintenance requirements and expected service life of the structure. Bearing in mind that “when traditionally used techniques prove to be inadequate, new methodologies can be implemented which their effectiveness has been proven in a scientific way (both experimental and in practice)”, according to Article no. 10 of the ‘Chapter of Venice’, the

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need for utilisation of state-of-the art materials as well as strengthening and rehabilitation techniques is imperative. On this note, fibre reinforced polymer (FRP) composites, a material used in strengthening construction projects, found their way in numerous restoration/retrofit applications on a range of concrete but also masonry and wooden monuments [1]. Utilisation of such materials and methodologies offer, as it will be discussed later on, unique properties in terms of strength, lightness, durability, ease of application and improved on-site productivity (fast execution and low labour costs). In this study the relevant properties and durability performance of these materials is summarised, and the concept of their application (with examples) is presented.

Fibre Reinforced Polymers (FRPs)

FRP are composites comprising of at least two materials, fibres which act as reinforcement and polymer binders [3, 4] (Figure 1). The purpose of the polymer is to integrally bind the fibre reinforcement together so as to distribute external loads to the reinforcement whilst protecting it from adverse environmental effects. The structural properties of the fibres largely dictate the strength and stiffness of the composite material. There are currently a number of FRP reinforcing products, mainly composed of carbon, glass, aramid or basalt fibres introduced with a certain position, volume and direction in a polyester, epoxy or vinylester binding matrix (resin), manufactured using a number of highly automated processes with pultrusion last being the most common [3].

Figure 1. FRP schematic and cross section [4] FRP materials can be designed and manufactured to meet specific requirements of a particular application. Available design variables include the choice of constituents (fibre and polymeric matrix), the volume fractions of fibre and matrix, fibre orientation and the manufacturing process. In strengthening applications the composite materials most often used are available in the form of strips (unidirectional), flexible sheets or fabrics (made by unidirectional or bi-directional fibers pre-impregnated or in-situ impregnated with resin) (Figure 1). The mechanical properties vary from one product to another, depending mainly on the nature volume and direction of the fibres and the resin. In general FRP exhibit excellent resistance to corrosion and electromagnetic neutrality. They have low weight, high tensile strength (Figure 2) and stiffness (strength to weight ratio of up to 10 times that of steel) [3, 4]. Their ease of use, but more importantly their availability in different sizes, forms and dimensions, custom made to the demands and requirements of each individual application, are briefly the reasons due to which these materials have been increasingly used in strengthening applications world wide [1].

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Figure 2. Mechanical Characteristics of FRP materials [4]

Drawbacks of using FRPs include their high cost, compared to steel reinforcement, although this is likely to change in time with increasing market demand, production volumes and competition amongst producers, the lack of ductility (which in a way is counter balanced through their increased deformation up to the point of failure) and their behaviour in extremely high temperatures (fire) [3, 4].

DURABILITY OF FRP

Each of the key elements within a composite material that influence its long term properties (matrix, fibres and fibre/matrix interface) can be susceptible to attack by various aggressive environments (as attack by chemicals, moisture uptake, temperature fluctuations and irradiation with ultra-violet light), yet all three should continue to function fully throughout the design life of the composite. A sound manufacturing process and factors as the resin wet out (how well the fibres are covered by resin), the absence of cracks or voids, the degree of resin curing (if the production process is not well controlled the resin may be insufficiently cross-linked to provide the designed protection) and overall the correct selection of an appropriate resin need to be addressed in achieving the maximum affordable durability level of the composite system. The resin should be able to resist alkali and chloride attack, be sufficiently tough to resist micro cracking, be sufficiently impermeable to resist environment penetration to the interior and compatible with the fibres to ensure a strong fibre/matrix interracial bond [3]. The performance of FRP reinforcement in certain aggressive environments varies with the materials (fibres and resins) used and with the manufacturing processes. Eventhough FRP materials perform at a more than satisfactory fashion (compared to steel reinforcement), a number of common misconceptions or “grey areas”, contribute to their poor market performance and to their wider acceptance. It has been stated that alkaline solutions, in some cases, can cause degradation of the main constituents of FRP composites (dissolution of glass fibres by an etching process, susceptibility of the resin due to hydroxyl ions attack) [3, 4]. However it is the performance of the composite system as a whole that should be the primary consideration when operating in this type of environment [4]. In general, GFRP are more susceptible to degradation from alkaline attack of the concrete environment, than carbon, or aramid fibres. CFRP are highly resistant and AFRP somewhere in between (alkalis affect AFRP bars and tendons less than GFRP, but a combination of alkali solution and high tensile stress may damage AFRP bars significantly). Factors that were found to influence

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this degradation are the alkali-diffusivity of the matrix, the quality of the fibre/resin bond and the temperature. GFRP Bars embedded in concrete at various temperatures and with good fibre-resin combinations show only limited degradation, but this increases with temperature and stress level [4]. Recent results [4, 5] investigating the durability of GFRP composites in concrete, on different levels of initial alkalinity conditions (achieved using pozzolanic cement replacement materials and accelerated carbonation) concluded that contrary to what is generally believed, alkalinity does not have a major impact on the FRP bar and its strength (moisture is thought to have some minor detrimental effect). Investigating the effect of carbonation and decoupling the influence of concrete compressive strength on the bond strength of the FRP bars immersed in concrete (Figure 3), led to the development of predictive models of deterioration (Figure 4) [4].

Figure 3. Effect of concrete alkaline environment on bond strength of GFRP bars [4] (a) raw results, (b) normalised with respect to concrete compressive strength,

(c) normalised with respect to revised concrete compressive strength (of the carbonated samples, around the position of the FRP bar)

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Figure 4. Effect of carbonation on the mechanical properties of FRP materials [4] Based on the experimental results and on the models developed, the small level of deterioration observed due to carbonation (different alkalinity levels), as a factor of the detrimental effect of CaCO3 crystals that penetrate into the FRP surface and can damage both the resin and the glass fibres, is more than compensated by the increase in strength due to time and carbonation [4]. Hence, careful selection of the resin (fully cured) is essential for optimum performance in the working environment discussed. A good manufacturing quality and a sufficient degree of curing of the resin might restrict the diffusion of moisture or alkaline ions (Conrad et al., 1998; Eurocrete, 1997). Usually vinyl ester resins, most commonly used provide better protection in alkaline exposure compared to epoxy or polyester resins, due to a better bond structure and cross-linking on a molecular level, that slow down diffusibility and absorbability. The susceptibility of FRP materials in Ultra-Violet radiation is an area where contradictory results have been reported. In general, utraviolet rays are known to affect the surface of polymeric materials [3, 4]. The main cause of degradation during weathering exposure in sunlight is a combination of the atmospheric oxygen and the UV exposure. The effects are limited to the top few microns of the exposed surface (hardening, colour change, or pigment loss). For embedded FRP reinforcement, UV-attack poses no problems, but all external FRP reinforcements (bonded strip/sheets, bars, and tendons) should be protected from sunlight using proprietary protection systems. Reduction in tensile strength (of up to 8%) has been noticed for GFRP after 2500 hours of exposure, while based on other results no degradation could be found for GFRP reinforcement after two years of exposure to sunlight at a marine test site. Tests [3, 4] on alkali resistant glass in a polyester and vinyl ester matrix suggested that the rebars were not significantly affected by UV radiation during a 6 months exposure. AFRP are generally considered to have poor resistance to ultraviolet rays (Sen et al., 1997) showing up to 13% reduction in tensile strength after 2500 hours (1250 cycle) exposure time and bad resistance to high temperature compared with glass and carbon fibres (Machida, 1993). As part of another study (Kato et al., 1997) AFRP exhibited also 13% reduction in tensile strength, after exposure to UV light for 2500 hrs, compared to 8% for GFRP and no reduction for CFRP. In general, incorporation of UV resistant coatings and utilisation of light stable resins and pigments enhances further the long term stability of FRP composites exposed to direct ultraviolet light [4].

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Overall, is is clear that while broad conclusions can be drawn about the relative performances of FRP materials, these cannot be applied strictly in all cases, due to variations in the materials and manufacturing processes used to produce FRP. One of the biggest problems is certain perceptions regarding their performance, as the perception that glass is sensitive to alkali attack and that the concrete environment is therefore intrinsically aggressive. Research has shown that the concrete environment is, however, not as aggressive as alkaline solutions and that alkali resistance can be significantly improved by the selection of appropriately treated glass fibres, suitable resins and better production techniques [4].

Considering the beneficial properties of FRPs, these materials provide a practical alternative to the commonly used techniques, in cases where a sound durable but also structurally effective solution is required [4]. Although the aim of this study is not to emphasize on the mechanisms of strengthening, a few characteristic cases are briefly presented bellow. STRENGTHENING AND RETROFITTING OF MONUMENTS USING FRP – EXAMPLE CASES

Common practices of the past, as the use of epoxy-bonded steel plates (to the external surfaces of beams or slabs), or the use of steel/concrete jacketing, although they are both simple and effective, as far as cost and mechanical performance are concerned they have certain drawbacks. Utilisation of epoxy-bonded steel plates, as a form of upgrading RC elements, is prone to corrosion of the metallic elements and hence deterioration of the bond at the steel-concrete interface. Furthermore, on-site issues as the difficulty in manipulating the heavy steel plates and the need for scaffolding, necessitate the use of FRP strips or sheets (instead of steel plates) instead. As far as concrete-steel jacketing is concerned, although it is very effective in terms of strength and ductility, it produces however undesirable weight (cross-sectional dimensions and dead loads on the structure) and stiffness increase, in addition to be labour intensive and to obstruct occupancy. On this not, wrapping of the RC element with FRP facrics/sheets results in considerable increases in strength and ductility without an excessive stiffness change, tailored to meet specific structural requirements by adjusting the placement of fibers in various directions. In this way flexural strengthening, by epoxy bonding the materials to portions of the elements in tension, with fibers parallel to the principal stress direction, shear strengthening, by epoxybonding (partial or full wrapping) FRP materials with fibers as parallel as practically possible to the principal tensile stresses [1]. Strengthening and retrofitting using FRPs have been used in a range of historical landmarks, across the globe [6-9]. From the restoration of the Archaeological Museum of Olympia [6] (Figure 5-a) to the strengthening of a elementary school in Athens, Greece [7] (listed building, constructed at 1898; Figure 5-b) and to the seismic retrofit of the Pasadena city hall in the USA [8], a landmark reinforced concrete structure, contracted at the early 1920s (Figure 5-c) using GFRP, utilisation of FRP technology produced durable (reversible if needed) retrofitting solutions.

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Figure 5. Case studies of FRP jacketing techniques [6-8]

Examples of strengthened structures demonstrate the use of a variety of materials and building techniques such as stone, clay tile or adobe masonry walls, wooden slabs and roofs, stone domes, vaults and arches. Masonry represents one of the oldest building materials used by mankind. Strengthening of masonry structures with FRPs have been applied successfully in a number of cases [6, 9-10]. The basic principle is that FRPs should be able to receive stresses that traditional materials are not able to do so. Epoxy-bond FRP strips are applied to the surface of masonry (parallel, diagonal or vertical, based on strengthening for shear or flexure) in locations and directions dictated by the principal tensile stress field, as it is illustrated in Figure 6-c.

Figure 6. Case studies of masonry strengthening applications [6, 9-10] The most important benefit of the utilisation of this technique is derived from the avoidance of total jacketing of the masonry element. In this way, the strength and ductility of the retrofitted element is enhanced without the unnecessary increases in the weight and in the dimensions of the element.

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Characteristic examples of masonry strengthening are illustrated in Figure 4, including the case of a High School in Athens, Greece (listed building, constructed at 1914) where extensive cracking during the 1999 earthquake were noticed [6] (Figure 6-a), the three-story registered building of the University of Athens Law School [9] (Figure 6-b) strengthened using unidirectional carbon strips all around its perimeter (at the top) and the restoration of Saint George’s bell tower [10] (Figure 6-e) using carbon strips. Masonry arches were often preferred to beams for covering large spans as the lack of tensile stresses allowed masonry to reveal its good behaviour in compression. Since the tensile strength of masonry is very low, under certain loading conditions, masonry arches are vulnerable to cracking and to various failure mechanisms. The use of advanced composite materials as externally bonded strengthening laminates on masonry arches, can modify their failure mode and significantly increase their load-carrying capacity, by baring the stresses occurring at the tensed edges [11-13]. The objective in this case is to activate the fibres in the locations of potential hinges during collapse and hence to prevent severe damage during earthquakes. Therefore, the brittle collapse mechanism of such structures, typically caused by the formation of hinges can be avoided. Depending on the position of the laminate the formation of the 4th hinge can be prevented. Tests on different strengthening configurations of a masonry arch using glass fibre fabric impregnated with an epoxy adhesive (Figure 7) revealed that application of GFRP on both surfaces of the arch improved the load-carrying and deformation capacity of the arch considerably [11]. Investigating the effectiveness of CFRP strips on masonry arches produced similar results [12].

Figure 7. Comparison of the behaviour of unstrengthened and strengthened masonry arch [11]

FRP composites can be applied as confinement reinforcement to masonry using unbonded strips that are color-matched with the underlying masonry structure, and can be removed if necessary at a later time (reversibility of the technique). Recent applications include strengthening of vaults in old masonry buildings either from below, using transparent glass FRP fabrics, or from above, using epoxy-bonded FRP sheets in a grid-like pattern (Figure 8) [14-15].

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Figure 8. Case studies of masonry strengthening applications [14-15] In certain Asian countries, as in China, Japan and Korea, most of their historical buildings are made of wood. The most famous examples, of such type of architectural landmarks, are the Forbidden Palace in Beijing and the Foguang Temple in Wutaishan (in China) built in the early 857 [16]. After hundreds of years, wood buildings have weathered, decayed and accumulated damaged, which results in the necessity of repair and strengthening. Strengthening techniques using steel reinforcement utilise the usual drawbacks, mentioned at the beginning of this study (difficulties on handling and installation, corrosion problems). In this way, strengthening methodologies using FRP reinforcement have been proven to be very effective in increasing the load-carrying capacity of the wooden element and its aesthetics, with minimum hazard. Strengthening of wooden beams with CFRP strips (using epoxy resin) wrapping of timber columns with GFRP sheets (as in the case of the restoration of the River House, a traditional wooden monument built in the 1880s in Singapure; Figure 9-d,e, f) and in-plane and flexural stiffening of wooden floors with positioning of FRP strips in a crossed pattern (Figure 9-a, b, c), has been proved a successful solution in certain cases (Figure 10) [17]. At the moment, efforts are being made to extend application of FRPs on wooden columns. Experimental attempts investigating CFRP confinement on short wooden columns produced increased ductile capacity and compressive stiffness.

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Figure 9. Case studies of masonry strengthening applications [16-17]

CONCLUSIONS Reinforced concrete structures consist part of our characteristic 20th century architecture. Considering the known environmental deterioration issues that such a type of construction faces, the need of structural interventions on repair and strengthening of our architectural heritage (monuments, historic buildings and bridges) has received considerable emphasis over the last years. Bearing in mind the known limitations of the effective rehabilitations techniques used today (epoxy-bonded steel plates, steel/concrete jacketing), as well as the freedom provided to the practitioner engineer by the Chapter of Venice in implementing new (proven scientifically) methodologies, the need for utilisation of state-of-the art materials, strengthening and rehabilitation techniques is imperative. On this note, fibre reinforced polymer (FRP) composites, a material used in strengthening construction projects, found their way in numerous restoration/retrofit applications on a range of concrete but also masonry and wooden monuments. Utilisation of such materials and methodologies offer, unique properties in terms of strength, lightness, durability, ease of application and improved on-site productivity (fast execution and low labour costs), among others. Strengthening and retrofitting using FRPs have been used in a range of historical landmarks, across the globe [6-9]. Most of these structures demonstrate the use of a variety of materials and building techniques such as stone, clay tile or adobe masonry walls (one of the oldest building materials used by mankind), wooden slabs and roofs, stone domes, vaults and arches. Overall, the incorporation of fibre reinforced polymers in strengthening applications of old historical structures, provides a cost-effective, attractive alternative to the commonly used techniques so far, improves the structural behaviour, stiffness and future durability of the structures, safeguarding in this way our cultural heritage. It is time to raise the level of awareness on the use of these materials on our historical landmarks, and to make the necessary provisions and arrangements for their adaptation

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as “practise” on the official guidelines of the Greek Ministry of Culture on the recommendations for maintenance and rehabilitation of our architectural heritage [2].

REFERENCES

[1] fib Technical Report. Bulletin 14. (2001). Externally Bonded FRP Reinforcement for RC Structures. [2] Hellenic Ministry of Culture (2004). Recommendations for maintenance and rehabilitation of

architectural heritage (with participation of ISCARSAH committee). [3] fib Technical Report. Bulletin 40. (2007). FRP reinforcement in RC structures. [4] Demis, S. (2007). An Investigation of the Durability of FRP in Concrete. Ph.D. Thesis. The University of

Sheffield. [5] Demis, S., Pilakoutas, K. & Apostolopoulos, C. (2009). Effect of Corrosion on Steel and Non-Metallic

Reinforcement. Materials and Corrosion. Published Online 3/08/2009 [6] Sintecno (2007) Proceedings of TEE seminar “Strengthening using composite (FRP) materials”,Athens,

2007 [7] Vachliotis, C., Petromichelakis, N., papdopoulos, C., Zaimi, M. Skari, E. and Toumpakari, E. (2008)

“Methods of analysis on rehabilitation and strengthening of list buildings – Case Studies” from the Proceedings of the TEE seminar “Seismic performance of traditional types of construction”, Athens, 2008

[8] FYFE Case studies (2007) Seismic Retrofit of Pasadena City Hall [9] Ingegneria e Architetture del Restauro - IAR (2000) The Carboniar system [10] Karantjikis M (2007) Proceedings of TEE seminar “Strengthening using composite (FRP)

materials”,Athens, 2007 [11] Taranu, N., Taranu, G., Budescu, M., Oprisan, G. & Munteanu, V. (2009) FRP Strengthened Masonry

Arches for Monumental Buildings. Proceedings of the 9th International symposium on Fiber Reinforced Polymer reinforcement for Concrete Structures

[12] Briccoli Bati, S., Rovero, L. & Tonietti, U. (2009). Masonry Arches Reinforced with CFRP. Proceedings of the 9th International symposium on Fiber Reinforced Polymer reinforcement for Concrete Structures, Sydney, Australia

[13] Borri, A., Casadei, P., Castori, G. & Ebaugh, S. (2009) Research on Composite Strengthening of Mansonry Arches. Proceedings of the 9th International symposium on Fiber Reinforced Polymer reinforcement for Concrete Structures, Sydney, Australia

[14] Spyrakos, K. (2009). Rehabilitation and Strenghening Case study. Proceedings of the Technical Chamber of Greece seminar “Pre-Earthquake Check of Structures. Strengthening of Buildings and Monuments”, Mytilini, Greece

[15] Modena, C., Casarin, F., Da Porto, F., Garbin, E., Mazzon, N., Munari, M., Panizza, M. & Valluzzi, M (2009). Structural Interventions on Historical Masonry Buildings. Proceedings of the Eurocode 8 Perspectives from the Italian Standpoint Workshop, Napoli, Italy

[16] Xu, Q., Zhu, L., Li, X. & Wang, K. (2009) Experimental Research on Wood Columns Confined with Carbon Fiber Reinforced Polymers. Proceedings of the 9th International symposium on Fiber Reinforced Polymer reinforcement for Concrete Structures, Sydney, Australia

[17] Quek, M.L., & Woo, W.S. (2009). A Case Study on the Preservation of a Timber Structure Using GFRP Composite Systems. Proceedings of the 9th International symposium on Fiber Reinforced Polymer reinforcement for Concrete Structures, Sydney, Australia