Scholarly article on topic 'Opportunities and Risks Arising from the Properties of FRP Materials Used for Structural Strengthening'

Opportunities and Risks Arising from the Properties of FRP Materials Used for Structural Strengthening Academic research paper on "Materials engineering"

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{Anchorages / "galvanic corrosion" / FRP / "material properties" / prestressining / strengthening}

Abstract of research paper on Materials engineering, author of scientific article — Wit Derkowski

Abstract Modern composite materials, made on the basis of non-metallic continuous fibers are increasingly used in civil engineering, in particular to strengthen building structures. In strengthening of RC structures the utilization of externally bonded FRP composites is only up to 35% because of the pilling-off failure mechanism. This problem can be solved by the use of pre-tensioned composite laminates. Due to more complex behavior, the strengthening of structures using prestressing technology needs a careful design approach and a full understanding of the behavior of both the materials and elements. The advantages and risks of the presented technology, which may determinate the success of the entire project, are highlighted in the article.

Academic research paper on topic "Opportunities and Risks Arising from the Properties of FRP Materials Used for Structural Strengthening"

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Procedía Engineering 108 (2015) 371 - 379

Procedía Engineering

www.elsevier.com/locate/proeedia

7th Scientific-Technical Conference Material Problems in Civil Engineering (MATBUD'2015)

Opportunities and risks arising from the properties of FRP materials used for structural strengthening

Wit Derkowskia*

aCracow University of Technology, Warszawska 24, 31-155 Krakow, Poland

Abstract

Modern composite materials, made on the basis of non-metallic continuous fibers are increasingly used in civil engineering, in particular to strengthen building structures. In strengthening of RC structures the utilization of externally bonded FRP composites is only up to 35% because of the pilling-off failure mechanism. This problem can be solved by the use of pre-tensioned composite laminates. Due to more complex behavior, the strengthening of structures using prestressing technology needs a careful design approach and a full understanding of the behavior of both the materials and elements. The advantages and risks of the presented technology, which may determinate the success of the entire project, are highlighted in the article.

©2015TheAuthors.PublishedbyElsevierLtd.Thisis anopenaccess article undertheCCBY-NC-NDlicense (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-reviewunder responsibilityof organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering Anchorages; galvanic corrosion; FRP; material properties; prestressining; strengthening

1. Introduction

The issue of the sustainability of construction is important not only at the design stage for new buildings, but also for the strengthening of existing structures. Corrosion is among the major factors affecting the long-term durability of many structures, including bridges, particularly those located in cold regions where deicing salts and other aggressive chemicals are used. Because steel is relatively weak in respect of environmental impacts, the need arose to look for other materials that will be resistant to corrosion.

* Corresponding author. Tel.: +48-502-136060; fax: +48-12-6282025. E-mail address: derkowski@pk.edu.pl

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering doi:10.1016/j.proeng.2015.06.160

In the 80s of the last century, non-metallic composites with continuous fibers (FRP - fiber reinforced plastics) began to be used in civil engineering for the first time. Even then, the application of FRPs for structural strengthening, as well as the possibility of independent use of structural elements (in whole or partly made of composite materials) was considered. It was then, that the pioneering experimental research in this area started in the Swiss Research Institute EMPA in Zurich under the direction of U. Meier [1]. Not much later, research in this field began in the Technical University of Lodz, Poland by Kaminska and Kotynia [2].

From the middle of nineties of XX century for prestressing the structures there have been started the use of composites. Initially, only cables made of carbon or aramid fibers were applied for the structural strengthening, but in the recent years pretensioned CFRP laminates have also been used. Nowadays, on the market there are several, significantly differing, CFRP strips prestressing systems. The first application in Poland of strengthening of existing building by prestressing with CFRP was carried out in 2006 by Derkowski [3].

The main advantage of composite materials could be corrosion resistance, very high strength and very low weight of strengthening systems elements. The total material mass of the FRP rehabilitation is significantly lower than performed in steel - therefore it is much easier to supply and handle it in a building.

In general, beams strengthened with FRP composites significantly contributed to enhance the flexural strength, fatigue life and the serviceability of the beams over un-strengthened beams. Prestressing the FRPs allows the material to efficiently utilize its tensile capacity which enhances its ultimate and serviceability limit capabilities.

But designers must keep in mind that FRP materials are not ideal, and - like other materials - also have its disadvantages. That is why the intended use of these materials must be preceded by a good diagnosis of their properties and behavior of the strengthened structure.

2. Characteristics of FRP materials

Composites made on the basis of non-metallic continuous fibers (FRP) are innovative construction materials and they are increasingly used in civil engineering nowadays. As a reinforcement of composites glass, aramid and carbon fibers are most often used, but still there are attempts to use also other types of fibers (e.g. basalt, bamboo, etc.). All the fibers used for FRP composites have a tensile strength substantially greater than the strength of steel. As non-metallic fibers they are characterized by high resistance to corrosion, which lacks in both mild and prestressing steel. In addition, their density is several times smaller compared to steel. Glass and carbon fibers are resistant to UV rays, while aramid fibers have some interaction of this radiation. On the other hand, most types of glass fibers are not resistant to an alkaline environment.

Polymer matrices in composites are made of epoxy resins, sometimes polyester and vinyl ester resins. Epoxy resins compared to other types of matrices are characterized by better mechanical properties and higher durability (this is, for example, due to their greater resistance to UV radiation). The resistance of resins for impact of chloride ions is very good, making that the FRP composites are resistant to de-icing salts - Dobrzanski [4]. Unfortunately, the resin matrix relatively rapidly changes its properties with increasing the temperature. Some of them have already softens at 50 °C, which limits the possibilities of their use.

Table 1. The basic properties of composite materials.

Material property GFRP AFRP CFRP

Density [kg/m3] 2 100 1 300 1 650

Tensile strength [MPa] 1 000 - 1 500 1 500 - 2 500 1 200 - 3 700

Tensile elastic modulus [GPa] 40 - 50 40 - 120 120 - 580

Thermal conductivity [W/m.K] 1 0.8 1.4

Coefficient of linear thermal expansion [1/K] 10-5 5 x 10-6 0.5 x 10-6

As a result of the embedding of a large amount of metallic thin, long fibers in polymer matrix, the FRP composite materials are formed. Matrices allow spreading evenly the tension force on all fibers and protect fibers against mechanical damage as well as the adverse effects of the environment. The matrix has practically no effect on the tensile strength of the composite, while it determines the shear and compression capacity of the material. Due to its structure composites are anisotropic materials - that significantly affect the possibility of their use. Comparison of the basic properties of composite materials based on glass fibers (FRP), aramid fibers (AFRP) and carbon fibers (CFRP) are presented in Table 1.

Laminates in the form of strips are obtained by pultrusion. For this purpose, collimated continuous fibers are pulled through the hot fluidized resin and then after removal of the excess of resin pass through the forming mold. Then, in a tunnel, under appropriate temperature and pressure conditions, the matrix is cured - Derkowski and Zych [5]. In this way long, straight portions of the strips can be produced (for the transport and storage, tapes are rolled in coils). Typically, the width of the composite strips is from 5 cm to 12 cm, and a thickness of 0.6 mm to 1.4 mm. The saturation of fiber volume is in the range between 50% and 70%.

The rheological properties of composite materials have a great influence on the durability of structural strengthening, particularly in the case of prestressing. The change in the properties of FRP composites over time is an effect of the rheological characteristics of each of the component materials, but also the changes occurring at the interface between these materials (adhesive bond). Table 2 shows the rheological properties of the most frequently used composite materials given by Borosnyoi and Balazs [6]. The amount of creep and relaxation due to load causing stress equal to 80% of the tensile strength and the time of 1000 hours are given. In the Table 2 the approximate values of the long-term tensile strength, taking into account the rheological phenomena occurring in a period of 100 years, are also shown.

Table 2. The rheological properties of composite materials.

Material property GFRP AFRP CFRP

Strain due to creep [%o] 3.0 - 10.0 1.5 - 10.0 < 0.1

Relaxation [%] 1.8 - 2.0 5.0 - 10.0 0.5 - 1.0

Long-term tensile strength (0.4 - 0.7) fLu (0.5 - 0.7) fLu > 0.9 fLu

3. Structural strengthening by prestressing with FRP laminates

According to Kotynia et al. [7] strain utilization of externally bonded carbon fiber composites for strengthening of building structures typically ranges from 30 to 35% of the tensile strength and that is why more and more effort is put into the development of an optimal technology of application of pre-tensioned FRP strips.

FRP composites have good potential of use due to their expedient properties in pre-tensioned conditions. These properties include high corrosion resistance, high strength to weight ratio, high stiffness to weight ratio, high energy absorption, and very good fatigue resistance. When used for strengthening, pre-tensioned FRPs can control the aging of construction materials and can sustain the impacts of vehicles much better than prestressing steel. Seismic upgrading and changes in the use of the structure are also more easily accommodated through the use of these polymers.

The application of pre-tensioned composite strips makes it possible to execute the strengthening practically with no change in structure dimensions or its self-weight while the bearing capacity of bent or tensile elements is significantly increased. It also improves the serviceability limit states of the strengthened structure by decreasing deflection and limiting crack width (in some cases completely closing the cracks), and increasing the structure durability thereby. It is worth noticing that in the case of pre-tensioned laminates application the force is transferred to concrete mainly by special anchorages rather than by bond constraints along the tape, thus reducing the significance of the quality of concrete surface preparation. It is even possible to use unbonded prestressing

strips. Another advantage of this technology is the fact that FRPs are active reinforcement and can carry loads immediately after their pre-tensioning (without the necessity of occurrence of the further structure strains), and the degree of their utilization is much higher than for strips glued in the passive form.

The available forms of pre-tensioned composite materials are AFRP, GFRP and CFRP. There is not a lot of information about the strengthening of structures using pre-tensioned GFRP material. Research in this area was done by Lin et al. [8]. They reported that the pre-tensioned GFRP is a good tool to produce a considerable increase in the load carrying capacity of RC beams. Glass fiber composites has a Young modulus closer to concrete but it has lower elasticity and ductility as compared to steel, AFRP and CFRP, which minimizes the usage of GFRP in the strengthening of RC structures.

A pre-tensioned AFRP performs very well with a good fatigue properties and low labor costs - Deng and Xiao [9]. Compared to prestressing steel, it also possesses a strong shield against corrosion, high temperature and adverse environmental effects. At the same time it has a lower weight than steel and a longer service life. Composites based on aramid fibers works very well under the impact loads. However, AFRP shows some non-negligible disadvantages. AFRP exhibits poorer performance in resisting acid and alkaline media which can be more efficiently resisted by CFRPs - Kurihashi et al. [10].

Both the AFRP and the GFRP show an important decrease in their tensile strength when they are subjected to a long-term constant load, while many other research showed that CFRP has very little loss in tensile strength.

Pre-tensioned CFRPs are appropriate where the strength, stiffness, lower weight and fatigue are critical issues. Moreover, carbon fiber materials are useful in applications that require chemical attack resistance and damping resistance. According to Aslam et al. [11], the main advantages of prestressing with CFRPs are its low weight, extremely high tensile strength, corrosion resistance, very good rheological behavior, electromagnetic neutrality and also quick and easy construction, as well as low labor costs. It should also be mentioned that carbon fibers has an ability to modify the elastic modulus.

When reinforced concrete structures are strengthened with externally bonded composite laminates, the crucial issue for the success of the entire project is the concentration of shear and normal stresses at the adhesive layer between the FRP and concrete in the vicinity of strip's ends. In the case of pre-tensioned laminate these stresses reach values so high that, in the case of simple bonding of FRP to the concrete it would lead to a sudden strip's end peeling-off. To prevent such a failure mechanism, several methods of strip's anchoring have been developed so far. Those ideas can be divided into systems using a mechanical anchoring of the tape in the wedges and non-mechanical anchorage systems with a gradual reduction of the prestressing force over a given length of the laminate.

Following are the basic types of structural systems which enable strengthening by prestressing with CFRP laminates:

• anchoring system using specially formed polymer heads

System "StressHead", described by Schwegler and Breset [12], allows anchoring strips tensioned with the force corresponding to the laminate deformation of 9.5 %o (up to 220 kN per strip). At the end of the laminate, a special head is made of reinforced polymer. It has a length of 110 mm and an elliptical cross section with axial dimensions 80 x 60 mm. Composite heads are fixed on massive steel blocks fastened to the reinforced concrete structure by the thick steel pin having a diameter of 100 mm. Fig. 1 a) shows a model of a passive (fixed) anchorage and Fig. 1 b) -a model of an active (moveable) anchorage of the strip. In this system one can do the structural strengthening with adhesive bonded laminates along the entire length of element or strengthening with unbonded strips.

Fig. 1. (a) StressHead passive anchorage; (b) StressHead active anchorage.

The disadvantage of this system is the need for factory implementation of polymeric heads on both ends of the strip of projected length and the need for drilling deep holes with a large diameter in strengthened structure, which is not always possible. However, it should be noted, that this technology provides the same corrosion resistance and durability for the composite laminate and the anchor head.

• anchoring system using steel clamping jaws

There are many systems of this type in the world (e.g. LEOBA, S&P and Polish systems: SIKA & IBDiM, NEOXE). Typically, these systems are for the prestressing with bonded CFRP strips - the strips are anchored in a special steel anchor blocks and additionally bonded to the reinforced concrete structure over their entire length.

For example, in LEOBA system - described by Andra et al. [13] - steel anchor block consists of two main components: an anchor plate attached to the strengthened structure (usually located in a niche in the concrete cover) and the pressure element (jaw) fixing the pre-tensioned strip to the anchor plate by means of screws. For the time of prestressing of the structure, on the strip's end (on the active anchorage side) additional tension jaws are fixed. The retaining block, located directly behind the active anchorage is tightened to the steel anchor plate which is fixed in the RC structure. The advantage of this system is ability of using the tensioning hydraulic device with a very small size.

Below, a schematic view of the LEOBA active anchorage is shown. The anchorage during the strip tensioning process is given on the Fig. 2a; immediately after anchoring the strip and before removing the tensioning jaws (Fig. 2b) and the final view, i.e. after dismantling of the temporary elements is presented on Fig 2c.

Fig. 2. LEOBA active anchorage (a) during the tensioning; (b) just after anchoring; (c) after dismantling of the temporary elements.

A certain modification of the LEOBA system is a SIKA & IBDiM system developed by Lagoda [14], in which the tape is tensioned gradually and sequentially fastened to the anchorage. The element which is fixing the pre-tensioned strip to the anchor plate is divided into four independent jaws. Thus it is possible gradation of anchored prestressing force at the anchorage length.

Another variation is the NEOXE anchorage system, in which the composite laminate is anchored in precast components - Siwowski et al. [15]. Laminate is glued between steel sheets using epoxy resin and the adhesive layer is additionally enhanced with metal nits connecting the strip to the anchor plates. Such prepared anchorages are tightened to strengthen structure.

• non-mechanical anchorage systems

The world's first system for prestressing CFRP strips without mechanical anchorages was developed in EMPA Research Laboratory in Zurich - Kotynia et al. [16].

As in the case of the conventional pretensioned structures, the prestressing force is transferred to the member only through the adhesion and friction between the strands and concrete, so in this solution, the prestressing force is transferred from the strips only by the adhesive layer made of epoxy resin. To reduce the shear stress in the adhesive in the outer parts of the strips, a gradual reduction of the prestressing force along strip's length is provided. Strengthened region of the structure is divided into areas with different prestressing force - the force varies from a maximum value in the central part of the strip to zero in the end parts of the strip. In this way a differentiated distribution of the stresses in adhesive layer is obtained and to delamination of the strip's ends is prevented.

The other system that allows application of pre-tensioned CFRP strips with non-mechanical anchors has been proposed by the Swedish company Tenroc Technologies. The system uses a specialized multi-segment tensioning device to introduce gradually variable prestressing force.

Accelerated curing of the adhesive layer can be achieved through the heating. This can be done utilizing electro-conductive properties of the carbon fibers embedded in the CFRP strip - as a result of the flow of electrical current in the strip, the temperature of the composite, and then epoxy adhesive, is significantly increased. With this technology, the curing time of the epoxy adhesive can be reduced from three days up to about two hours.

Many researchers (Kaluza and Ajdukiewicz [17], Young-Chan et al. [18], Kotynia et al. [19]) have tested the application of pre-tensioned CFRP laminates for structural strengthening and it can be stated, that obtained increase in the ultimate load-carrying capacity was even higher than 150%.

From the structural point of view, it is important to emphasize that the FRP materials has much bigger elastic elongation than traditionally used prestressing steel (~ 0.2% for steel compared to ~ 1.6% for CFRP). This may result in the fact that in the tensioned steel tendon exposed to wedges slippage or other similar effects (which reduces its extension only about few millimeters) a significant amount of pretension force will be lost, while in the composite tendon loss of prestressing force would be smaller. This is also extremely important in conditions where it is necessary to consider the accidental loading, e.g. in structures that exist in seismic areas.. In case of the carbon fibre strip with an initial elongation of 0,9% (which could be equivalent to force 220 kN, this same as in monostrand steel tendon), the prestressing system behaves like "a rubber band" and keeps its tension, because of larger elongation already obtained in the initial situation and larger capability of extension while moving of the building in accidental load situation.

4. Risk of galvanic corrosion

The use of carbon fiber composites coming into contact with steel elements (i.e. steel plates of anchorages) requires a proper understanding of the phenomenon of possible galvanic corrosion.

In theory, as long as two materials have not contacted each other, a corrosion cell will not initiate. In general, the requirements for galvanic corrosion are that the two materials, e.g. two metals or metal and non-metal (e.g. carbon in carbon fiber) must be in direct electrical contact. The materials in galvanic cell must differ in electric standard potential and they must be bridged by an electrolytic solution and a cathodic reaction must be sustained on the more noble metal or on the non-metal.

This electrolyte solution can be produced by the presence of water with a salt, fertilizer, acid or even a combustion product. If all of these conditions are met, current will flow through the electrolyte from the anodic metal to the cathodic element. The cathodic area is then protected from corrosion, but the anodic metal may suffer even greater corrosion. The reactions that occur due to galvanic corrosion, are similar to those that would take place on a single metal, however the rate of attack is increased for the anode. According to Schnerch and Stanford [20], carbon is one of possible cathodic material that can drive the corrosion of many different metals galvanically coupled to it. Steel and aluminum have similar positions in the galvanic series, and behave anodically relative to the carbon. Possible corrosion of external steel anchorages for CFRP laminates can significantly lower the durability of strengthening project.

This phenomenon is relatively new research area within the scope of structural strengthening of building structures - studies were conducted inter alia by the Tavakkolizadeh and Saadatmanesh [21], Arronche and Gordon [22], Peng and Nie [23].

The chemical reactions that cause the galvanic corrosion of the anode steel when coupled with the cathode CFRP can be explained theoretically by the following equations, as described by Tavakkolizadeh and Saadatmanesh [21]:

• Anode: location where corrosion of metal takes place. The metal M loses electrons and becomes the ion in the

solution, or it may form an insoluble product:

M ^ Mz+ + ze" (1)

• Cathode: Part that usually attracts the electrons created in the anode. The electrons are consumed by the

reactions at the cathode. Such reactions can occur as well as a reduction of oxygen (2) or reduction of hydrogen

2H2O + O2 + 4e- ^ 4OH- for pH > 7 (2)

2H+ + 2e- ^ H2 for pH < 7 (3)

where z is the number of electrons donated during the reduction, and pH is the pH of the electrolyte solution

where the galvanic corrosion is happening.

In this application, the reaction that is most likely to happen in the cathode is given by Eq. (2), where reduction of oxygen and formation of hydroxides will happen on the surface of the CFRP product.

The above information indicates that prevention of galvanic corrosion is necessary for the long-term durability of any CFRP strengthening applied in direct contact to a metallic element - in the present case, the contact of carbon fiber strips with steel anchors components exposed to environmental influences.

In strengthening applications, the potential possibility for corrosion between anode steel elements of anchorages and cathode CFRP laminates can be reduced with the use of a non-conductive, proper barrier. However, although epoxy resin can serve as a buffer for corrosion, it cannot completely prevent it - Tavakkolizadeh and Saadatmanesh [21]. The permeability of any adhesive barrier or a moisture-resistant primer may increase with time and environmental degradation. Due to exposure conditions, the introduction of moisture, when combined with salt or other components, can serve as an electrolyte, and result in the creation of a galvanic couple comprising the CFRP and steel. Should it also be noted that in most of anchoring systems (especially NEOXE system), as a result of mechanical bolting of CFRP strips, resin matrix which coats carbon fibers can be locally damaged, thus creating direct contact between the carbon and steel. According to Hollaway and Cadei [24], an intermediate layer of GFRP can also serve as buffer between the steel and CFRP layer, but such an additional layer is very difficult to use.

Monitoring the condition of an inner part of the anchorages is practically impossible during service time - the physical inspection of exterior surfaces can only be done. As the composite and anchorage plates are fixed together, the connection cannot be taken apart for more accurate inspection.

5. Summary

The poor resistance of steel elements against unfavorable environmental aspects and corrosion problems has given rise to the use an innovative technology of pre-tensioned FRPs, especially in the case of bridge structures strengthening. Moreover, when the length of the strengthening components and handling arrangements on construction sites are a significant consideration, composite materials, again, provide better performance than steel.

Prestressing of engineering structures with FRP materials is still quite rarely used, mainly because of the relatively high cost. But one needs to remember that the total cost for rehabilitation and repair in most cases is far less than the cost of replacement. Furthermore, building structure rehabilitation usually takes less time than its replacement, reducing service interruption time.

However, the technology for structure strengthening with pre-tensioned strips still needs to be improved. Some problems arise from the fact that FRP composites work in a uniaxial way, i.e. they exhibit insignificant strength in direction perpendicular to the fibers and practically do not carry any bending moments, hence even small imperfections caused by introduced force can lead to premature failure of the strip. For the same reason, it is necessary to provide protection against vandalism for the strengthened parts of structures. The issues of fire protection for external reinforcement are also often difficult to solve and needs some evaluation.

Critical issues relating to pre-tensioned FRP anchorages and the use of proper examination methods that verify their behavior have to be addressed more consistently in the future. For FRP prestressing applications, anchorage systems have to be developed to meet the same requirements as those for steel prestressing systems and reach at the same level. High prestressing levels may initiate potential slippage at end anchors. Thus, the suitability of an anchorage system sustainable to fatigue loading should be considered. The long term durability behavior of FRP prestressing systems is not yet fully defined. Furthermore, research is needed to investigate the possible galvanic action between carbon fiber laminates and steel elements of anchoring equipment.

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