steel joints in civil engineering applications: State of the art

Accepted Manuscript Environmental Durability of Adhesively Bonded FRP/Steel Joints in Civil Engineering Applications: State of the Art Mohsen Heshmati, PhD student, Reza Haghani, Assistant Professor, Mohammad AlEmrani, Assistant Professor PII:

S1359-8368(15)00424-2

DOI:

10.1016/j.compositesb.2015.07.014

Reference:

JCOMB 3674

To appear in:

Composites Part B

Received Date: 5 June 2015 Revised Date:

23 July 2015

Accepted Date: 26 July 2015

Please cite this article as: Heshmati M, Haghani R, Al-Emrani M, Environmental Durability of Adhesively Bonded FRP/Steel Joints in Civil Engineering Applications: State of the Art, Composites Part B (2015), doi: 10.1016/j.compositesb.2015.07.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Environmental Durability of Adhesively Bonded FRP/Steel Joints in Civil Engineering Applications: State of the Art

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Mohsen Heshmati1, Reza Haghani2, Mohammad Al-Emrani3

ABSTRACT

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Over the past three decades, the strengthening and repair of existing civil engineering structures using FRP laminates has attracted a great deal of attention. With the advances in polymer science, adhesive bonding has become a common joining technology in these applications. Despite numerous studies that address the short-term behaviour of adhesively bonded FRP/steel joints, uncertainty with respect to long-term performance still remains. This knowledge gap is regarded as a critical barrier, hindering the widespread application of FRPs to strengthen and retrofit steel structures. This paper presents the state of the art in terms of the durability of FRP/steel joints used in civil engineering applications. Important influential factors relating to the durability of adhesively bonded joints are reviewed and different damage mechanisms are discussed. Moreover, related investigations of the combined environmental durability of these joints are critically reviewed and the findings are presented. The paper concludes with a discussion to motivate future research topics, while it is emphasised that the generalisation of the available results is questionable.

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Keywords: A. Polymer-matrix composites (PMCs); B. Environmental degradation; B. Debonding; E. Surface treatments; B. Durability of FRP/steel joints;

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Corresponding author, PhD student, Dept. of Civil and Environmental Engineering, Division of Structural Engineering, Chalmers University of Technology, Sven Hultins gata 8, SE-412 96 Gothenburg, Sweden Tel. +46 31 772 20 21, E-mail: [email protected] 2

Assistant Professor, Dept. of Civil and Environmental Engineering, Division of Structural Engineering, Chalmers University of Technology, Sven Hultins gata 8, SE-412 96 Gothenburg, Sweden 3

Associate Professor, Dept. of Civil and Environmental Engineering, Division of Structural Engineering, Chalmers University of Technology, Sven Hultins gata 8, SE-412 96 Gothenburg, Sweden

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1. Introduction

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Fibre reinforced polymer (FRP) composites offer superior advantages compared with conventional construction materials such as steel, the most notable of which are their corrosion resistance and high strength-to-weight ratio. With the advances in polymer science, adhesive bonding has become a prominent joining technology that offers several advantages compared with mechanical fastening techniques. These advantages include lower weight, lower fabrication costs, more uniform stress distribution and the elimination of local stress concentrations. The unique properties of FRP composites, combined with the advantages of adhesive bonding, have made FRP bonding an attractive method for the strengthening, repair and refurbishment of existing structures. FRP bonds have been used increasingly in high-tech industries, such as the aerospace and automobile industries, since their advent. However, the application of FRPs, such as glass-FRP and carbon-FRP, in civil infrastructure is limited to the past three decades [1–4].

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Adhesive bonding introduces an inevitable yet crucial disadvantage by creating a weak link between composite element and steel member [5–7]. In other words, the effectiveness and success of the FRP/steel joint is dependent on the quality, integrity and durability of the adhesive bond between the adherends. Given the aggressive environments to which these adhesively bonded joints are generally subjected, durability issues take on the utmost significance. Concern about the environmental durability of adhesively bonded joints has manifested itself in recent research publications, in which the key areas of interest have been mainly to understand the underlying mechanisms of degradation [8–11] and quantify the long-term performance [12–16]. However, most of these research projects have been conducted within fields such as aerospace that have distinct differences from civil engineering applications. These differences cover a wide range of variables from material characteristics to in-service conditions. Loading, curing conditions, operating environment, material production, joint geometry and manufacturing conditions are some examples of the aforementioned dissimilarities [17,18]. In addition, the useful service life of civil infrastructures is often more than 80 years, with minimum maintenance and inspection requirements, compared with the relatively short service life and highly controlled conditions implemented in aerospace structures [19,20]. As a result, in durability assessments of adhesively bonded joints used in civil engineering applications, most of the available experimental data, testing methods and so on are not applicable and should not be directly implemented. Furthermore, this limitation highlights the need for incontext assessments of the available literature based on their intended use.

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With the increased application of FRP composites in civil infrastructure in the early 2000s, Bakis et al. [21] wrote a comprehensive state-of-the-art paper targeting the construction sector. In this paper, the authors concisely addressed the development and potential application of FRP composites, as well as the prevailing codes and standards. Concurrently, Hollaway and Cadei [22] reported on progress in the upgrading and rehabilitation of metallic structures with advanced polymer composites. Although the advantages of the investigated rehabilitation method were confirmed, the adhesive material still remained the weakest link with respect to failure. Moreover, the authors emphasised the importance of determining in-service environmental issues associated with FRP bonding. To address the identified barrier of insufficient durability data for FRP composites used in civil infrastructure applications, Karbhari et al. [17] conducted a comprehensive durability gap analysis for various environmental conditions. They concluded their paper with a list of common needs that are critical for the further implementation of FRP composites in civil infrastructure, irrespective of their intended operating conditions. Recently, Fawzia and Kabir [23] reviewed the current research on the durability of CFRP-strengthened concrete and steel structures. The

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ACCEPTED MANUSCRIPT authors remarked that, in contrast to numerous durability studies of CFRP-strengthened concrete structures, the published data on CFRP-strengthened steel structures is minimal.

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2. Factors influencing environmental durability

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To date, durability concerns remain an obstacle to the widespread application of FRPs in steel structures and they have been continuously accentuated by many other researchers [24–27]. This paper reviews the current research on the environmental durability of adhesively bonded steel/FRP joints to reflect on the durability gap and comment on the research needed in this field. Firstly, the most important factors influencing the environmental durability of adhesively bonded FRP/steel joints are presented and briefly discussed. Both material and joint degradation mechanisms are then presented. The main ageing mechanisms are identified and the key findings in pertinent publications are mentioned. Moreover, the durability test data on the materials used in civil engineering applications are collected and summarised. Lastly, the combined effect of the discussed factors on the durability of adhesively bonded joints is spotlighted, while the emphasis has been the investigations most relevant to civil engineering applications and materials. The paper concludes with a discussion to motivate future research topics.

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The bonded joints used in civil engineering applications are often exposed to different service conditions. As for any bonded assembly, the environmental durability depends both on the resistance of its constituents and on the bond between them. This means that the strength degradation of adhesively bonded FRP/steel joints is dependent on a number of factors that are substantially different in nature. A diagram of the environmental factors influencing the durability of adhesively bonded FRP/steel joints and their interaction is depicted in Figure 1. Early investigations of joints bonded with structural adhesives have demonstrated the deleterious effects of hot and wet environmental conditions on the mechanical properties of aged joints [28,29]. The effect of moisture, which can take the form of humidity, liquid water or de-icing salt solutions, as well as the effect of temperature, will therefore be discussed in detail. In addition, special attention is paid to moisture diffusion kinetics, due to its importance. Ultraviolet radiation, fire and fatigue are other factors influencing long-term performance that are not discussed in this paper.

3. Effect of moisture on adhesively bonded FRP/steel joints

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3.1. Effect of moisture on resins and adhesives Moisture primarily affects the resins and adhesives in FRP composites and bonded assemblies. Various mechanisms of degradation for different types of resin, such as epoxy, polyester and vinyl-ester, are well documented. In general, moisture can change resin through plasticisation, swelling, cracking and hydrolysis [20,30,31]. The high moisture absorption susceptibility of adhesives stems from their surface topology [32] and resin polarity [33]. Zhou and Lucas [34] studied the interaction of water molecules with epoxy resins. The authors showed that two types of bound water can be found in epoxy resins. Type I bound water is manifested by disrupting the weaker inter-chain Van der Waals forces and acts as a plasticiser. Type II bound water is a product of strong hydrogen bonds between water molecules and the resin network. The amount of Type II bound water, which is more difficult to remove and causes irreversible material changes, increases with higher temperature and longer exposure time. It is shown in [35] that Type I bound water softens the adhesive and lowers the glass transition temperature (Tg), an observation which has also been made by many other researchers (e.g. [36–41]). This is primarily important for cold-cured adhesives that generally have glass transition temperatures lower than 60°C in the dried stage [42]. As a result, the further

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ACCEPTED MANUSCRIPT depression of Tg makes it closer to normal service temperatures in some environments and, as discussed in Section 4.1, endangers the overall stability of the joint.

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In general, moisture is known to increase the ductility and reduce the elastic modulus and strength of resins/adhesives. The reported test results in terms of the effect of moisture on the mechanical properties of the most commonly used structural adhesives and resins used in civil engineering applications are summarised in Figure 2. The values are normalized with respect to the original property in the dry stage. All the tests are conducted under tensile loading at room temperature. As can be seen, a clear correlation between the elastic modulus and strength with increasing moisture content can be observed. In this context, the magnitude of the degradation of the elastic modulus is slightly larger than that of strength. The failure strain, on the other hand, does not exhibit a clear trend with increasing moisture content. This behaviour might be due to the simultaneous, and competing, deterioration of the elastic modulus and strength that does not allow the material to reach larger strains.

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By comparison, less is known about the effects of cyclic exposure conditions on the mechanical properties of adhesives. Lin and Chen [55] observed that the reduction in the elastic modulus of epoxy samples for the saturated, dried and re-saturated groups is 29%, 9% and 42% respectively, compared with the unaged group samples. Very similar results were also reported by Mubashar et al. [56]. Roy et al. [57] found that the 20% drop in the elastic modulus of epoxy adhesive samples aged in water at 40°C would be fully restored upon drying. However, this procedure would stop plasticity. They suggested that water extracts additives that plasticise the resin and, as a result, adhesive ductility is lost after cyclic moisture exposure. These observations emphasise the need for the characterisation of the material properties of adhesives and resins based on their moisture uptake history.

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3.2. Effect of moisture on FRP composites

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The effect of moisture on FRP composites has been the subject of extensive research. A comprehensive review of the durability of FRP composites for civil engineering applications in aqueous environments has been conducted by Karbhari [20]. Moreover, Böer et al. [31,58] recently reviewed the latest durability studies of FRP composites used in the construction sector. The analytical models that are relevant to the transport of moisture in structural composites have been reviewed by Bond and Smith [59]. The authors also identified different moisture sorption locations and mechanisms in FRP composites, as depicted in Figure 3. In general, moisture can potentially attack FRP composites by one or a combination of the following mechanisms: (i) altering the resin matrix; (ii) damaging the fibre/matrix interface; (iii) fibre-level degradation. It is well known that the resin-dominated properties of FRPs, such as interlaminar shear strength, are more susceptible to moisture-induced degradation than the fibre-dominated properties, such as tensile strength [60]. Karbhari [20] predicts a negligible modulus change of the order of 10% over a period of 10-15 years for FRPs used in civil engineering applications. Figure 4 shows the environmental effects on the flexural modulus, flexural strength and interlaminar shear strength of a number of FRP composites used in structural applications. All the values are normalized with respect to the unaged properties. From experiments that included tests on both flexural strength and interlaminar shear strength, the degradation of the latter property was found to be comparably larger. Moreover, a direct correlation between moisture content and the degradation of the properties cannot be identified. This is primarily due to the fundamentally different

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ACCEPTED MANUSCRIPT damaging mechanisms at the fibre/matrix interface for each FRP composite. In addition, the fibre orientation and type and the resin matrix are other influential factors.

3.3. Effect of moisture on the interfaces

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The stability of interfacial adhesion in the presence of moisture is the most important factor in the long-term durability of adhesive joints [70]. A review of the available literature reveals that the degradation of the interface is often found to be significantly larger than that of the adhesive [71–73]. Furthermore, it is well accepted that, upon moisture penetration, the failure locus almost always switches from cohesive within the adhesive to at or near the interfaces [57,72,74].

Weakening of intermolecular adhesive forces Cathodic corrosion of steel substrate Galvanic corrosion of steel substrate

3.3.1. Interfacial moisture diffusion

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All engineering metals are covered with an ultra-thin metal oxide layer that attracts water molecules due to its polar properties. The absorbed water forms substantially stronger bonds with this oxide layer than the existing interactions between oxide and adhesive [75]. As a result, the transport of moisture to the interface with the adherent may lead to irreversible changes as a result of any of the following mechanisms:

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Moisture can enter adhesive bonds through bulk adhesive diffusion, transport along the interface, capillary action and diffusion through the porous adherents such as composites [70]. This variation, plus uncertainties resulting from many different combinations of adherents, surface preparations and adhesive materials, has made interfacial diffusion a complicated phenomenon to characterise. However, it is generally accepted that the diffusion process is much faster at the interface compared with bulk adhesive [76–83]. Zanni-Deffarges and Shanahan [76] compared the diffusion rates in bulk adhesive specimens and bonded joints and found that it was greater for adhesive joints. They attributed their observation to the “capillary diffusion” phenomenon, where the higher surface energy of the dry adhesive pulls the moisture along the interface. Wahab et al. [77] also found that the initial diffusion rate was twice as rapid into laminated adhesive resin discs compared with bulk adhesive. Investigations implementing more advanced methods, such as Fourier transform infrared spectroscopy (FTIR) [78,79], nuclear reaction analysis [81] and electrochemical impedance spectroscopy (EIS) [82,83] also confirm the previous findings. This implies that employing the diffusion coefficient of bulk adhesive specimens would result in an underestimation of the actual moisture content at the interface of steel and adhesive.

3.3.2. Effect of moisture on interfacial adhesion Adhesion is typically attributed to the physical adsorption of molecules from the two different materials across the interface via secondary van der Waals forces [84]. According to this theory, the minimum amount of work that must be done to separate an adhesive/adherent interface is equal to the thermodynamic work of adhesion. As can be seen in Table 1, the work of adhesion at the epoxy/steel interface is negative in the presence of moisture, which indicates an unstable bond [85]. Unlike metals, the thermodynamic work of adhesion for the epoxy/FRP composite interface remains positive in the presence of water [86], leading to a significantly lower risk of interfacial failure upon ageing. In addition, the interfacial moisture diffusion

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at the FRP/adhesive interface can be ignored [87]. It should be noted that the thermodynamic approach describes a joint in equilibrium and not how long it takes to achieve this state. In addition, the bonding mechanism at the epoxy/metal interface is not entirely governed by intermolecular physical adsorption. The penetration of polymer into the craters and pores of a rough metal surface enhances the bond strength through mechanical interlocking as well [9,10,88]. This implies that, while the epoxy/metal interface can potentially undergo thermodynamic displacement of the adhesive, with an effective surface treatment, the apparent interfacial failure may be shifted to the adhesive or primer close to the interface. As the latter is, to a great extent, reversible on moisture removal, a good surface preparation can potentially inhibit irreversible damage caused by adhesive displacement. Table 1. Thermodynamic work of adhesion for various interfaces in air and in water [89].

Interfacial debonding after immersion in water Yes Yes Yes No

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Epoxy/steel Epoxy/aluminium Epoxy/silica Epoxy/CFRP

Work of adhesion [mJ/m2] in air in water 291 -255 232 -137 178 -57 88–90 22–44

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3.3.3. Cathodic corrosion

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Another mechanism which can contribute to interfacial degradation is the formation of a weak boundary layer through the hydration of the surface of metallic adherents [7,8,40,90]. Due to the complexity of polymer/substrate interface structures, almost no theoretical concepts that can describe and predict these corrosive delamination processes are available [91]. In general, metals with conductive oxide layers, such as iron, are prone to corrosion mechanisms, such as cathodic delamination, in humid environments [92–95]. For this reason, an adhesive layer which is in direct contact with an electrolyte can potentially continuously delaminate from the steel substrate as a result of the corrosion of the latter. Figure 5 illustrates the schematics of a continuous cathodic delamination process that begins with metal dissolution at the anode and oxygen reduction at the cathode. Hydroxide is generated as a result of oxygen reduction, which increases the interfacial pH. To compensate for this charge, cations are transported to the front of the delamination. The expansion of free volumes due to electrolyte ingress, as well as other corrosive mechanisms, then leads to the progressive de-adhesion of the coating. The electric circuit is completed by a transfer of electrons between anode and cathode [96].

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Calvez et al. [40] conducted a comprehensive experimental and analytical study to address the effect of metal surface corrosion and loss of adhesion. Their results led to an ageing model which is summarised in Figure 6. Based on this hypothesis, the degradation zone is initially located at the metal/polymer interface at the edges of the joint. During the second step, the locus of failure is essentially interfacial, but, after prolonged exposure, the failure locus shifts within a surface layer of corrosion products.

3.3.4. Galvanic corrosion Corrosion can also occur due to galvanic action when two materials with a sufficient electropotential difference are bridged together in the presence of an electrolyte [97]. Brown [98] investigated the possibility of galvanic corrosion between CFRP and mild steel and found that the electropotential difference is large enough for galvanic corrosion to occur. Bellucci [99,100] studied the effect of temperature, metal type, area ratio and environmental exposure on the galvanic corrosion rates of CFRP

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ACCEPTED MANUSCRIPT coupled to aluminium and steel substrates. He concluded that both metals exhibited significant galvanic corrosion, which would increase by raising the ambient temperature. It was also shown that the diffusion of NaCl solution into the resin matrix of CFRP could reduce its electrical resistance, leading to an increase in galvanic current.

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Galvanic coupling has also been shown to damage CFRP materials (cathode) as a result of the formation and deposition of chemical products of cathodic reaction. Tucker and Brown [101] reported blistering of vinyl-ester-based CFRPs that were coupled to steel and immersed in seawater. In another study, Sloan and Talbot [102] observed a 30 per cent reduction in the interlaminar shear strength of CFRP materials that were coupled to magnesium. In an attempt to minimise galvanic corrosion, Tavakkolizadeh and Saadatmanesh [103] found that the galvanic corrosion rate of steel in a de-icing salt solution decreased twenty-three-fold when using a 0.25 mm thick layer of epoxy adhesive between steel and carbon fibres. The successful application of a thin layer of epoxy as an electrical barrier between steel and CFRP material is also confirmed in recent work by Arronche et al. [104]. Alternatively, West [105] suggested embedding an insulating layer of glass fibres in the adhesive between steel and CFRP to prevent electrical contact between them. Dawood and Rizkalla [106] investigated the effectiveness of this method on steel/CFRP double-lap shear joints with an adhesive thickness of 0.5 mm that were exposed to accelerated environmental conditioning for up to six months. The test results indicated that, while the presence of the glass fibres helped to enhance the initial bond strength, it did not improve the durability of the bond. In the light of these results, it is anticipated that partially saturated glass fibres may adversely affect the durability of the joint by providing an enhanced route for moisture diffusion into the joint [107].

3.3.5. Effect of surface treatment in the presence of moisture

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Despite the susceptibility of steel adherents to corrosion, very few generalised surface treatment methods are reported to enhance bond durability. This is partially due to the diversity of steel products; an established treatment for one steel alloy might produce very poor results for another metallurgy. Equally importantly, structural steel bonds are usually designed with regard to cost considerations rather than performance optimisation [108]. The outcome of this kind of cost-driven mind-set is that many manufacturers prefer to use adhesives or primers that provide adequate bond durability, rather than complicated etching or surface preparations [7].

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The most common surface treatment for steel substrates is grit blasting or similar mechanical abrasion processes that produce a clean, rough and chemically active surface [4,109–111]. When grit blasting is used, extra care should be taken with the shape and hardness of grits, as it is essential that the grits cut the surface upon blasting rather than punching it [109]. Recently, Fernando et al. [111] presented a systematic experimental study to identify proper surface-adhesive combinations for strengthening of steel structures. The experimental study consisted of measurement of adhesion strength, surface chemical composition, surface roughness, and surface energy. Alumina grit was found to leave residues that are compatible with adhesives commonly used in strengthening applications. The grit size was shown to have no significant effect on the adhesion strength. Based on the results of surface characteristics, recommendations are given for minimum acceptable surface energy and fractal dimension range for surface topography. Nevertheless, in order to ensure an environmentally durable interface, it is vital to use additional methods of surface preparation in combination with grit blasting. Recently, many studies have focused on improving the durability of grit blasting by using silane coupling agents, for example [105,112–117]. Silanes improve

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ACCEPTED MANUSCRIPT the bond integrity and its environmental stability by forming primary bonds at the substrate/silane and adhesive/silane interface, instead of secondary van der Waals forces that are attributed to adhesion alone. In the case of epoxy adhesives bonded to steel substrates, Walker [112] found that applying a diluted γ-GPS silane to the surface of grit-blasted steel increased the initial bond strength by 36 per cent. The effect of solvent type, application duration and the pH of the solution on the efficacy of the silane application process was demonstrated in [113].

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Although the effectiveness of silane is well established, it does not necessarily increase the interface corrosion resistance [117]. For this purpose, chemical conversion coatings [118–122] have been widely adapted for steels. In particular, the improved durability performance of the Accomet C coating treatment compared with the silane process was shown in [118,121]. Da Silva et al. [122] also performed a detailed surface analysis of two chemical conversion coating systems and demonstrated their ability to provide optimised bonding surfaces. A detailed review of this kind of more advanced surface preparation technique is given in [123].

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In the case of adhesively bonded FRP/steel joints, the effect of surface treatment on long-term performance has been assessed in [106,124]. Dawood and Rizkalla [106] investigated the effectiveness of silane on steel/CFRP double-lap shear joints in an accelerated environmental exposure for up to six months. Two series of specimens were included, depending on the surface preparation method used for the steel substrate. In the first series, the steel surface was prepared by grit blasting alone, whereas, in the second series, an additional silane layer was applied to the grit-blasted steel surface. Linghoff and Naumes [124] investigated the effect of using a blast coating (SACO) physical-chemical surface preparation technique on the long-term performance of steel/CFRP single-lap shear joints. The steel surface in the other series was only grit blasted. Figure 7 shows the average results in terms of normalized strength versus exposure duration. The strength values are normalized with respect to the strength of grit-blasted-only specimens. As can be seen, the addition of silane or the use of a surface coating agent results in slightly higher initial strength in the joints. Furthermore, environmental ageing degrades the strength of these joints to a lesser degree compared with the grit-blasted-only series. Although the specimens with silane pre-treatment exhibited essentially no degradation in [106], the very harsh accelerated ageing scenario used in [124] still caused the corrosion of the coated steel interface and, as a result, a reduction in strength. However, both the corroded area of the bond line and the corrosion rate were reported to be significantly reduced as a result of SACO treatment. By comparison, the higher stability of the steel/adhesive interface obtained through silane treatment can be attributed to the more uniform distribution of the chemical coating on the bond line.

3.4. Effect of moisture on adhesively bonded joints In the presence of moisture, the bond between thermoset matrix composite materials, such as CFRP and GFRP, and adhesives is stable. Joint durability is thus governed to a great extent by the quality of the adhesive/steel interface. In the presence of a stable bond, moisture would primarily plasticise the adhesive in highly stressed zones, which can in turn be beneficial when it comes to reducing stress concentrations [87,125,126]. Nevertheless, due to the effects of water on the strength of adhesive and FRP, the joint strength might eventually decrease [127]. Subsequently, as matrix plasticisation is a reversible phenomenon, it has been observed in many studies that the initial joint strength was largely recovered when drying the specimens [74,128,129]. In such cases, the failure of conditioned joints usually remains cohesive in the adhesive layer but shifts to an area closer to the interface [72,74,130]. Due to the damaging effects of

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Despite numerous reports on the loss of strength in adhesively bonded joints immersed in water or exposed to highly humid conditions, some researchers have observed no joint degradation after prolonged exposure to lower humidity conditions [131–136]. These observations led Gledhill et al. [131] to specify a critical moisture content in the adhesive and a corresponding relative humidity in the air, below which no joint weakening would occur. They also proposed a model based on fracture mechanics in which the zones of adhesive with a moisture content higher than the critical moisture content were dealt with as cracked regions. This hypothesis was further investigated by testing butt joints exposed to a number of humidity levels and this led to the conclusion that the critical moisture concentration is 1.35%. A similar critical moisture content of 1.45% in the adhesive was found by Brewis et al. [134] for an epoxy/aluminium system. The authors suggested that the hydration of the metal oxide substrate in the presence of higher moisture concentrations is responsible for the sudden loss of adhesion. On the other hand, Lefebvre et al. [136] found that a completely different mechanism, which only involves the bulk adhesive properties, was responsible for the sudden loss of adhesion in an epoxy/glass system. Inspired by the importance of critical moisture content and contradictory literature regarding its underlying mechanisms, Tan et al. [137– 139] used detailed characterisation methods and proposed a combined bulk-interfacial mechanism for the loss of adhesion at or above the critical moisture content. As illustrated in Figure 8, the interface moisture content increases as the surrounding humidity level increases and this in turn reduces the contact area between adhesive and substrate. Furthermore, the increased solubility of bulk at the critical RH leads to the significant swelling of the adhesive, which, due to constraints provided by the substrate, causes the water phase to deform. This then generates normal forces that reduce the energy required for adhesive failure.

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Bowditch [140] reviewed the experimental data on the effect of water on adhesively bonded joints and proposed three possible scenarios that are shown schematically in Figure 9. As can be seen, in all cases, the joint strength varies in proportion to the water content and levels out once the solubility limit is reached. Depending on various combinations of interfacial attack and plasticisation, joints initially exhibit increasing, decreasing or steady strength up to a critical water content. However, one shortcoming of this model is that the effects of water on porous adherends such as FRP composites are not taken into account.

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The moisture ingress into the central region of bonded joints with FRP adherends occurs at significantly faster rates compared with joints with impermeable adherents [57,141]. This phenomenon was investigated by Hua et al. [139], who found that the increase in moisture concentration in the adhesive with the substrate modelled as permeable was significantly accelerated during extended exposure times. Consequently, the mechanical properties of these joints are prone to higher degradation upon prolonged exposure to humid environments. Moreover, Parker [142–145] studied the effect of various environmental factors on CFRP/epoxy-bonded assemblies, among which the effect of the pre-bond moisture content of laminates was found to have serious deleterious effects on joint performance (see also [146–148]). Nevertheless, drying out the composites prior to bonding has been reported to alleviate these problems and is suggested as a part of any standard composite bonding application. To date, the available studies focusing on the effects of moisture on the mechanical behaviour of adhesively bonded FRP/steel assemblies have considerable differences in terms of the materials used and the manufacturing techniques, ageing conditions and specimen configurations. Figure 10 depicts the schematics of test configurations used in the most relevant investigations [54,57,106,124,126,149–151]. In this figure,

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In one of the earliest investigations, Roy et al. [57] conducted a series of tests on steel/polyester GFRP specimens bonded with an epoxy adhesive that were immersed in deionised water at 25° and 40°C for up to a year, see Figure 10(a). It was concluded that the ageing mechanisms of these joints were controlled by moisture diffusion in the polyester matrix, adhesive and fibre/matrix interface. Two competing damage mechanisms were identified; the degradation of the mat layer in the composite and adhesive swelling causing the debonding of the steel/epoxy interface. Drying the specimens shifted the crack back to the laminate failure and confirmed swelling as a crucial parameter. The deleterious effects of swelling have also been outlined by other researchers and they are believed to exert additional shear stresses at the interface [152,153].

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As can be seen in Figure 10, single-lap and double-lap shear (DLS) joints are very common test configurations that are often used to study adhesively bonded joints. However, in many long-term performance investigations, the overlap length is shortened to minimise the time to saturation of the adhesive layer. McGeorge [126] studied the long-term performance of steel/composite joints and found that specimens with short overlaps tended to produce scattered and misleading results. This observation questions the applicability of many former studies designed to evaluate the performance of well-designed joints with relatively long bond lengths. He also found that, in joints with the appropriate manufacturing quality, no further degradation occurred after full saturation was reached. This implies that the wet properties of materials are appropriate values for design.

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Dawood and Rizkalla [106] investigated the effect of various bonding alternatives, as depicted in Figure 10 (d), on the environmental durability of a CFRP system for strengthening steel bridges. The experimental programme consisted of CFRP/steel DLS joints exposed to one-week-wet/one-week-dry cycles in a 5% NaCl solution at a temperature of 38°C for wet cycles and ambient temperature during the dry cycles (Tg=62°C) for a period of six months. All the unaged specimens failed due to CFRP failure, whereas various failure modes were observed for the aged specimens depending on the bonding detail. Some of the results in conjunction with those reported by Linghoff and Naumes [124] were discussed earlier in Section 3.3.5. In a recent study, Nguyen et al. [73] subjected 75 steel/CFRP DLS joints, Figure 10(f), to a number of harsh environments, including (i) simulated seawater at 20°C and 50°C for up to one year, (ii) a constant temperature at 50°C and 90% RH and (iii) a cyclic temperature between 20°C and 50°C combined with constant 90% RH up to 1,000 h. The failure mode for both unexposed and exposed joints was delamination of the CFRP. The degradation was found to be significantly larger in the case of constant immersion in seawater than the other two scenarios. In all cases, a direct analogy between the degradation of adhesive material and DLS joints was observed. This further connects the joint degradation to changes in adhesive properties and implies that, in the presence of a sound interface, a design based on wet adhesive properties would be reliable. In a different attempt, Li et al. [149] subjected steel I beams strengthened with wet lay-up CFRP sheets to constant relative humidity levels of 93% at 50°C, Figure 10 (g). The failure of all aged specimens was governed by interfacial debonding at the steel/adhesive interface. Although this configuration could result in loading similar to the case of steel bridge girders strengthened with CFRP, it requires a more complicated test set-up, which may produce additional sources of error. The effect of the stress ratio on the durability of

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bonded GFRP-to-steel assemblies was investigated by Jiang et al. [151]. As shown in Figure 10 (h), the test configuration used in this study was similar to the ARCAN test method with dissimilar adherends. Specimens were immersed in distilled water at 40°C for four months, after which they were tested at six different loading angles covering various combinations of shear and tensile loading, as well as pure shear and pure tension. Compared with unaged specimens, a sharp drop of around 60% in the strength of joints tested under pure shear and tensile loading was observed after ageing. All the other aged joints tested under combined shear and tensile loading, however, showed similar or even slightly higher strength than the unaged ones. Apart from the joints tested under pure shear conditions, whose failure took place in the adhesive layer, all the other specimens failed due to GFRP delamination or GFRP/adhesive debonding.

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A comparison of the discussed results relating to the strength variation in adhesively bonded FRP/steel joints in the presence of moisture is plotted in Figure 11. The average strength values are normalised with respect to the average strength of the same unaged joints. As is apparent from Figure 11(a), no general correlation between strength and exposure time can be found for the joints exposed to a variety of exposure scenarios. The results are therefore categorised based on exposure conditions and are plotted in Figure 11(b) to (d). As can be seen, the lower boundary of all the series is governed by the failure at the steel/adhesive interface. This observation therefore provides a new perspective on the degradation of adhesively bonded FRP/steel joints and underlines the importance of the interface zone in the durability of these joints. However, in the presence of a stable bond, the largest strength reduction (26%) is found for constant immersion in saltwater for up to a year. The failure of these series is governed by the interlaminar shear strength of the FRP adherends. In these conditions, the wet material properties can be used to predict the mechanical behaviour of the joint.

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Exposure to high relative humidity levels would generally result in lower moisture saturation content, due to the concentration-dependent nature of moisture diffusion in bonded FRP/steel joints. In these cases, as can be seen in Figure 11(c), less damage and lower strength reduction would be expected. The wet/dry ageing conditions, on the other hand, slow down moisture penetration into the joint, leading to limited damage accumulation. In addition, these conditions can provide further post-curing for the adhesive which could result in strength improvements, see Figure 11(d). A similar finding has been reported by Kim et al. [154], who used a wet/dry exposure scenario (eight hours’ submersion in a water bath followed by 16 hours drying at room temperature) to study the durability performance of adhesively bonded DLS steel/steel specimens. The test results indicated a 20% increase in the ultimate strength of the joints after 100 cycles, compared with the initial joint strength. The effect of sustained loading during environmental ageing on the strength of adhesively bonded FRP/steel joints was also studied in [106,149] by subjecting some specimens to a constant load level. In these studies, the sustained load level was chosen as a fraction of the strength of dry joints and was equivalent to 30% and 35%. A comparison of the results for loaded and unloaded series is plotted in Figure 12. As can be seen, in both studies, the sustained load did not induce any significant additional effects on the strength of the joints. Two possible mechanisms may be responsible for this observation: (i) the applied load level was not high enough to initiate micro-cracks in the joint, leading to accelerated moisture diffusion and damage development, (ii) other damage mechanisms such as interface degradation progressed at considerably faster rates than the damage caused by sustained loading and accelerated diffusion.

4. Effect of temperature on adhesively bonded FRP/steel joints

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4.1. Effect of elevated service temperatures 4.1.1. FRPs and adhesives at elevated temperatures

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Temperature variations and cycles, freezing and freeze-thaw conditions and elevated temperatures can affect the performance of FRPs and FRP-rehabilitated structures. It is generally accepted that the resin matrix, adhesive and fibre/matrix interface are the components in adhesively bonded FRP/steel joints that are most susceptible to thermal effects. Due to a wide range of produced FRPs and structural adhesives with a large number of possible fibre/matrix combinations, it is extremely difficult to make any generalisation when it comes to the effect of temperature on these joints. However, the mechanisms that may endanger the structural integrity as a result of these thermal effects are now discussed.

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Thermal exposure up to temperatures below the glass transition temperature is in fact advantageous for FRP composites and adhesives as a result of further post-curing [155,156]. Although fibres do not undergo degradation at elevated service temperatures, the softening of the resin matrix and adhesives at temperatures close to the glass transition temperature would cause an increase in the viscoelastic response. As a result, not only is the mechanical performance of the joint significantly reduced but the moisture absorption susceptibility is also increased [17]. Cao and Wang [157] tested FRP sheets at temperatures ranging from 16 to 200°C to evaluate the variation in tensile strength. It was found that the tensile strength of FRP sheets decreased as the glass transition temperature of the polymer approached and it remained constant at higher temperatures. These findings are consistent with those reported by other researchers, where the severe degradation of the mechanical properties of CFRP and GFRP composites upon exposure to elevated temperatures has been reported [158,159].

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The deleterious effects of these exposure conditions are, to a large extent, limited to the resin matrix. As a result, the degradation of matrix-dependent properties such as interlaminar shear strength is generally orders of magnitude larger than that of fibres [20,87]. In order to alleviate this problem, Di Ludovico et al. [160] replaced the conventional resin matrix with an innovative epoxy which resulted in a higher Tg and, subsequently, the superior performance of tested CFRP coupons. Fluctuating temperatures can also lead to the progressive debonding and weakening of the materials and the fibre/matrix interface. This phenomenon is mainly due to the discrepancy in the thermal expansion coefficients of fibres and resin. However, it has been shown by several researchers that, in the case of well-prepared FRP composites, this fluctuation around ambient temperature is negligible [26,87]. Karbhari et al. [17] showed that the exposure of FRP composites to elevated temperatures would result in a thermal gradient between its components. This effect, together with increased viscosity, would result in a higher probability of the premature debonding of the FRP/adhesive or the fibre/matrix interface. For the FRPs used in civil engineering applications, the glass transition temperature is in the range of 65-120°C [161–163], but it is generally above 140°C for pultruded composites [87]. Since the glass transition temperature of the common structural adhesives is within a lower range (40-65°C), the adhesive is often the governing factor.

4.1.2. FRP/steel joints at elevated temperatures At joint level, the effects of elevated temperatures on the mechanical response of adhesively bonded FRP/steel assemblies have recently been investigated in [162,164–166]. Stratford and Bisby [162] conducted experiments on CFRP-strengthened steel beams and observed increasing slip at temperatures

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close to Tg, which ultimately caused the debonding of CFRP from the beam at temperatures around the glass transition temperature of adhesive. Nguyen et al. [164] also found that the effective bond length of the tested CFRP/steel DLS joints increased with increasing temperature. The effective bond length at 40°C (near to Tg) was about twice that at room temperature and it was suggested that a larger bond length should be designed to account for this observation. Al-Shawaf [165,166] also investigated the effects of increasing temperature in service conditions on the mechanical response of CFRP/steel joints bonded using three different adhesive materials. Zhang et al. [167] conducted similar experiments on DLS samples made of prefabricated GFRP composites at temperatures ranging from -35 to 60°C. The strength and stiffness of the joints was temperature independent at temperatures below Tg. At higher temperatures, both stiffness and strength followed the thermo-mechanical behaviour of the adhesive, an observation which confirmed the greater susceptibility of adhesives compared with GFRP. The failure mode of tested specimens also changed from the fibre-tear failure of GFRP to the cohesive failure of adhesive at temperatures above Tg.

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Figure 13 summarises the effect of elevated temperatures on the mechanical response of bonded FRP joints that were tested in [164–167]. The strength and stiffness of tested joints are normalized with respect to their initial values at room temperature. It can be clearly seen that both the strength and stiffness of all the tested specimens have a direct bilinear correlation with the Tg of adhesive. As shown in Figure 13(a), the strength remained unaffected by the increase in temperature up to 5°C below the Tg of adhesive. At higher temperatures, the strength drops steadily at an average rate of 3.3%/°C of its initial value. The joints lose up to 80% of their initial load-bearing capacity at testing temperatures approximately 20°C above the Tg of their adhesive By comparison, elevated temperatures are more deleterious to the stiffness of the joint, see Figure 13(b). As can be seen, the stiffness already begins to decline at temperatures slightly above room temperature. The reduction rate is, however, considerably lower up to 5°C below the Tg of adhesive (0.5%/°C). At higher temperatures, the stiffness reduction rate increases to 4.0%/°C.

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In order to avoid undesirable performance of this kind at elevated temperatures, Hollaway [22,26] recommended using a system that has a Tg of at least 30°C above maximum service temperature. A lower margin of 5-10°C was suggested by Aiello et al. [168] and [169], which, as discussed earlier, is consistent with strength variations at elevated temperatures. However, the current design guideline recommends that an adhesive must have a Tg at least 15°C above maximum operating temperature [163].

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It should be also noted that thermal effects are more pronounced with increased exposure duration. This phenomenon, which is basically due to the increased viscosity of polymeric materials at elevated temperatures, has been recently investigated in [169–174]. For instance, Nguyen et al. [169] tested CFRP/steel DLS joints exposed to constant and cyclic temperatures below, near and above Tg. The specimens were mechanically loaded to various stress levels while placed in an environmental chamber equipped with a uniaxial testing machine. All specimens exposed to constant temperatures above Tg showed sudden failure with varying time to failure, depending on the target temperature and applied load level. On the other hand, specimens subjected to an 80% ultimate load at 35°C (5°C less than the Tg of the adhesive), did not fail after 150 min. Cooling down these specimens and pulling them until failure showed no strength reduction compared with unconditioned specimens. Furthermore, a third series of specimens were exposed to cyclic temperatures varying from room temperature to 50°C while subjected to a 20% ultimate load. No failure occurred during 400 min after which the specimens were cooled down and pulled to failure. A 30% strength reduction compared with unconditioned specimens was observed. This further indicates that the strength degradation at elevated temperatures is a function of both temperature and exposure time.

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4.2. Effect of sub-zero and freeze-thaw conditions 4.2.1. Cold-region durability of FRPs and adhesives

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Moussa et al. [174] investigated the time-related effect on a cold-curing structural adhesive and found that its Tg and curing degree continuously increased during longer exposure periods to excessive temperatures. The full recovery of mechanical properties was observed once the adhesive was cooled to temperatures below its Tg. It was therefore concluded that temporary exposure to elevated temperatures not only failed to result in any material degradation, it also significantly improved the mechanical properties. Moreover, it was found that post-curing the same adhesive at 60°C for one week resulted in an increase of 48% and 15% in tensile strength and stiffness respectively, a condition which could be obtained alternatively by ambient temperature curing for 17 years [175,176]. On the other hand, curing at low-temperatures of 5-10°C, which is very likely to happen during outdoor construction projects, decelerated the curing process to several days before a curing degree of 80% was reached [177]. As such long curing periods are not acceptable in many cases, the authors suggest designing the joints while ensuring that they are accessible to heating for accelerated curing.

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Sub-zero and extremely low temperatures can cause FRP matrix embrittlement, matrix hardening, matrix micro-cracking and fibre/matrix bond degradation [36]. This behaviour is caused by changes in the FRP constituents at low temperatures, or the incompatibility of coefficients of thermal expansion (CTE) between fibres and resins. Both carbon and glass fibres have CTEs of orders of magnitude lower than those of polymeric resins. This is more critical in the case of carbon fibres that have anisotropic properties with negative CTE in the longitudinal direction and positive CTE in the transverse direction [178]. The shrinkage of FRPs on exposure to sub-zero temperatures may form matrix micro-cracks and develop residual stresses at the fibre/matrix interface which, after repeated freeze/thaw cycling, could initiate the debonding of fibres from the surrounding matrix [179–181].

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Exposure to sub-zero temperatures is generally expected to increase matrix embrittlement. Dutta [181] investigated the cold-region durability of CFRP and GFRP composites. He found that, in spite of decreasing the tensile strength of unidirectional composites at very cold temperatures (-40°C), the transverse strength increased as a result of matrix hardening. However, it was observed that both longitudinal and transverse strengths tend to degrade on prolonged freeze/thaw cycling. Dutta and Hui [182] investigated the flexural strength of thick FRP composites at ambient and several sub-zero temperatures. They emphasised that, in general, FRP composites stiffen up at low temperatures due to increased matrix embrittlement. The composite behaviour at low temperatures therefore depends to a large degree on the type of polymer matrix and its sub-zero mechanical properties [160]. In contrast to constant sub-zero exposure, freeze/thaw cycling degraded both Young’s modulus and the shear modulus of tested composites. Dutta [183] assessed the influence of freezing temperatures on the fracture toughness of unidirectional GFRPs. It was observed that, for cracks developing in the fibre direction, fracture toughness is reduced by lowering the temperature. However, a more detrimental mechanism was identified when water fills the existing cracks and voids in the composite, which induces severe stresses on volumetric expansion caused by freezing [184,185]. Robert et al. [186] examined the effect of freeze/thaw cycles on dry and saturated GFRPs with considerable void content. After 800 freeze/thaw cycles of specimens in as-received and saturated conditions, the loss of flexural resistance was 8% and 32% and the loss of elastic modulus was 0%

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ACCEPTED MANUSCRIPT and 22% respectively. Rivera and Karbhari [180] also noted that freeze/thaw cycles in wet conditions have more pronounced effects on matrix micro-cracking and fibre/matrix debonding. As in previous studies, the authors found that constant exposure to -10°C resulted in increased matrix brittleness. Freeze-thaw cycling in combination with marine or de-icing salts is thought to be more damaging due to the potential formation and expansion of salt crystals within the FRP [161].

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Abanilla and Karbhari [36] exposed wet lay-up graphite/epoxy composites used in external strengthening to 100 weeks of freeze-thaw conditions between -10 and 23°C at the rate of one cycle a day. They observed minor degradation of 13.2% and 8.9% in the tensile modulus and flexural strength respectively, which is consistent with other published data [158,182,187,188]. In a recent study, the authors witnessed that, among GFRP, BFRP (basalt FRP) and CFRP specimens, only the latter were adversely affected by 90 freeze/thaw cycles (18% in tensile modulus and 16% in strength) [178]. Tam and Sheikh [188] investigated the higher resistance of GFRP compared with CFRP to freeze-thaw cycles by means of scanning electron microscopy (SEM) analysis. After freeze-thaw cycles, the interface of glass fibres and resin was found to be intact, whereas micro-cracks, voids and large gaps were found around carbon fibres in CFRP specimens. This highlights the importance of the compatibility of fibres and resin and its crucial role in the long-term performance of composite materials subjected to cyclic freeze/thaw loading.

4.2.2. Cold-region durability of FRP/steel joints

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To date, very few researchers have studied the effect of sub-zero and freeze/thaw environmental exposure on FRP/steel joints. On the other hand, the effect of these exposure conditions on FRP/concrete joints has been extensively researched. The results of the latter studies, however, are not directly applicable to FRP/steel joints, as the failure of the majority of FRP/concrete joints is governed by the strength of the concrete or FRP/concrete interface. In the case of FRP/steel joints, the investigation conducted by Karbahri and Shulley [1] is one of the very first. They investigated the bond durability of steel and wet lay-up unidirectional glass and carbon fibre sheets using wedge tests in various environmental conditions. In general, freeze/thaw exposure (alternating 12-hr cycles between -18°C and 25°C) was found to be among the most damaging environments, while sub-zero exposure (no cycling) enhanced bond strength.

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Al-Shawaf et al. [189] investigated the effect of sub-zero environmental exposure (no cycling) on bond characteristics between CFRP and steel by performing a series of tensile tests on DLS specimens at 20, 0, 20 and -40°C. The strain distribution along the bond length was captured by strain gauges which demonstrated higher localised bond stress values for colder environments. In another study [190], similar tests were performed on wet lay-up CFRP/steel joints which were manufactured using three rheologically different epoxy resins. In general, the joint strength was found to be extremely dependent on the adhesive/matrix rheological and thermo-mechanical properties. Consequently, the influence of sub-zero exposure on bond strength was essentially different for various adhesives; for two adhesives, the short-term sub-zero exposure led to negligible bond strength variation, whereas the average bond strength of the third series dropped by 70% at -40°C, see Figure 14. The substantial strength reduction in the latter series (adhesive c) was attributed to a fundamental mismatch between the adhesive and steel substrate, which led to an interfacial debonding failure mode. In the presence of compatible adhesive and adherends, negligible strength and stiffness variations were similarly reported by Zhang et al. [167] for GFRP/GFRP bonded joints tested at -35°C.

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In addition to the adhesive rheology, the type and stiffness of fibres used to manufacture FRP composites can affect the performance of joints at low temperatures. In this regard, Al-Shawaf et al. [191] investigated the influence of fibre stiffness on the behaviour of wet lay-up CFRP/steel DLS joints at sub-zero temperatures, cf. Figure 14. In comparison to normal modulus fibres, the higher susceptibility of joints made of high modulus fibres to strength reduction at low temperatures was observed. This was mainly due to a substantial reduction in adhesive elongation at failure for these joints which limited the shear lag deformation of the joint before failure. The strain distribution was, however, found to be more vulnerable to thermal variations for joints fabricated with normal modulus fibres in which the load transfer was adhesive controlled.

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In addition to the performance of adhesively bonded FRP/steel joints tested at sub-zero temperatures, it is also crucial fully to understand the behaviour of these joints during more severe cold-region environmental exposure of a cyclic nature. In particular, cyclic freeze/thaw exposure is a concern in many countries where it represents a typical outdoor condition. In this context, one of the earliest investigations was conducted by Colombi et al. [192], who subjected CFRP/steel double-lap shear and double-sided reinforced joints to freeze/thaw cycles (-20 and 50°C). Moreover, another test series was exposed to additional salt spray fog at the end of thermal cycles. It was reported that thermal cycles alone did not have a particular impact on joint strength, while, in combination with salt and humidity, they had a detrimental effect on joint ductility.

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The effect of adhesive type on freeze/thaw performance has recently been investigated in [193–195]. Toufigh et al. [193] studied CFRP/steel DLS joints that were manufactured using two different adhesives. The specimens were subjected to 30 cycles of temperature variation from -11 to 60°C every 24 hours and then tested afterwards. Irrespective of the adhesive type, joint failure took place at the adhesive/steel interface for both control and freeze/thawed specimens. This failure type is considered to be a result of poor manufacturing quality and/or adhesive/substrate mismatch, each of which can lead to severe bond degradation after thermal cycles. Agarwal et al. [194,195] also studied the influence of freeze/thaw cycles on the bond strength of CFRP/steel single-lap joints in relation to the adhesives used. The specimens were first put in an immersion bath at 38°C for eight hours and then put in a freezer at -18°C for 16 hours. Tests on adhesive samples after freeze/thaw exposure showed that both adhesives experienced degradation of 19% and 14% in elastic modulus, while no significant reduction in tensile strength was observed. The bond strength was more severely influenced, with reductions of 28% and 18% after 40 cycles for joints manufactured with adhesive (2a) and (2b) respectively. The larger degradation of the former series can be attributed to their failure mode, which shifted from the cohesive to the adhesive/steel interface after freeze/thaw cycling, whereas the failure of the latter series took place in CFRP. For a similar CFRP failure mode, Yoshitake et al. [196] also observed negligible variation in the ultimate strength of beams strengthened with CFRP. The latter study was conducted to investigate the effect of discrepancies in the CTE of CFRP and steel at cold temperature. CFRP laminates were bonded to the tensile flanges of steel Hbeams and were subjected to one week at -14 ± 4°C and one week at 15°C for up to 336 days. The effect of freeze/thaw cycles on the load-bearing capacity of FRP/steel joints tested in [193–196] is plotted in Figure 15. As can be seen, freeze/thaw cycles affect the strength of joints with adhesive/steel failure to a considerably larger degree than those with CFRP failure. This observation can be justified by the fact that freeze/thaw cycles mainly affect bonded steel joints through the formation of cyclic shear stresses at the adhesive/steel interface due to the discrepancy in the CTE of adhesive and steel. In joints with poor adhesive/steel interface quality, these stresses may cause micro-cracks along the interface and

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ACCEPTED MANUSCRIPT degrade the joint strength and stiffness. It should be mentioned that, even in joints with stable interfaces, freeze/thaw cycles may lead to the formation of micro-cracks at the fibre/matrix interfaces in FRP composites. However, as discussed in Section 4.2.1, the mechanical degradation of FRPs subjected to freeze/thaw cycles is generally limited (less than 20%) and can be prevented by using compatible resins and fibres.

5. Discussion and research needs

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Exposure to cold temperature can be more damaging when it is combined with a sustained load. To address this issue, Hmidan et al. [197] subjected damaged steel beams strengthened with CFRP sheets to 40% and 60% of the initial ultimate load at 25, -20 and -30°C for 7,000 hours. Afterwards, the beams were tested to failure in a three-point-bending configuration. CFRP debonding was the governing failure mode in all cases. Both strength and stiffness were found to degrade as a result of sustained loading. The degradation was, however, found to be slightly higher in colder environments. This could be attributed to the extra stresses formed at lower temperatures caused by the shrinkage of the adhesive. This behaviour was also observed by Yoshitake et al. [196] for CFRP-strengthened steel strips subjected to sustained loading at low temperatures. The additional thermal deformation was successfully predicted by the simplified flexural theory in [196]. Combined sustained loading and freeze/thaw cycling was investigated for FRP/FRP singlelap joints in [188]. For GFRP/GFRP specimens with sound fibre/matrix interfaces, no degradation was found after 300 freeze/thaw cycles combined with a sustained load equivalent to 30% of joint capacity. However, for CFRP/CFRP joints that underwent fibre/matrix cracking, the combined damage of sustained loading and freeze/thaw cycles was more than the individual damage.

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It is widely accepted that the diffusion of water in structural adhesives follows the Arrhenius equation, which implies that the rate of diffusion increases dramatically with temperature. As a result, in many studies, environmental exposure to wet conditions is performed at higher temperatures to accelerate the diffusion process into the joint and thereby increase the damage in a shorter time. However, care should be taken to avoid activating damage mechanisms at high temperatures that could not occur in service conditions. Robert et al. [198] have investigated the limitations of these accelerated testing scenarios. The results show that, although temperature is the most influential factor in long-term performance, elevated temperatures (above Tg) trigger uncontrolled degradation mechanisms at material level and this can lead to an underestimation of durability. Ashcroft et al. [199] conducted a unique study on a wide range of adhesively bonded joints to compare accelerated testing results with naturally weathered joints. It was concluded that accelerated tests are valuable tools when it comes to identifying vulnerable components and potentially hazardous environments. However, they tend to overestimate the degradation of joint strength. Using excessive temperature and high humidity levels may lead to unrealistic degradation levels far greater than those that might be experienced by a joint in service conditions. An example of these unrealistic conditions is the one used by Linghoff and Naumes [124]. In addition, as moisture absorption can lead to the depression of adhesive Tg, it is strongly recommended always to keep the maximum exposure temperature at least 10°C below the Tg of adhesive. Hygrothermal ageing, the combined effect of moisture and temperature, is known to be among the most severe exposure conditions. Exposure to constant humidity and temperature, wet/dry cycles and hot/wet exposure in saline solutions are some examples of hygrothermal ageing. Adhesively bonded joints used in outdoor applications, such as bridges, are often exposed to aggressive environments. The combined effect

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of these service conditions may be more damaging than the adverse effect of each individual condition. The review of current studies at joint level is evidence of a lack of systematic ageing conditions aimed at clearly identifying the individual and combined effect of each environmental parameter. It is therefore a future research need to conduct durability tests at both material and joint level with minimum variants each time. In addition, long-term exposure to natural weathering scenarios is very rare at material level and is not available for joints. The results of these tests, as the most realistic representative conditions, are necessary for qualitative performance evaluations of bonded FRP/steel joints and to validate accelerated testing procedures.

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The majority of the available experiments designed to address the effect of moisture on adhesively/bonded FRP steel joints are conducted with exposure durations of less than six months, cf. Figure 11. As moisture diffusion is a time-consuming phenomenon, there is a need for tests with ageing durations of at least 18 months. In this context, an analytical estimation of the moisture profile in the joint at the time of testing is also needed for the accurate evaluation and comparison of the test results. Furthermore, the adhesive layers of in-situ bonded joints in bridges are usually 1-6 mm thick. However, many of the performed studies have used adhesive layers that are orders of magnitude thinner than those used in bridges, cf. Figure 10. As adhesive layer thickness can greatly influence the interfacial damage mechanisms, such as galvanic or cathodic corrosion, it is recommended to use test configurations that are able to accommodate thicker adhesive layers.

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6. Conclusions

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From the available results relating to the effects of moisture and/or thermal cycles and variations on the mechanical performance of bonded FRP/steel joints, the stability of the adhesive/steel interface is recognised as the most influential factor. In other words, the strength degradation of joints whose failure took place at the adhesive/steel interface is found to be as high as 90%, while it is less than 25% for all other failure modes. This highlights the importance of the need for further research on interfacial moisture diffusion and damage characterisation, effective surface treatment methods and assessments of interfacial stresses caused by: (i) cyclic swelling or (ii) freeze/thaw exposure or (iii) the volumetric expansion of water trapped inside the joint at sub-zero temperatures. Motivated by other reported failure mechanisms, there is also a need to develop characterisation methods for the evaluation and improvement of fibre/matrix interfaces in FRP composites, at material level.

Long-term performance and uncertainty relating to environmental durability are a critical barrier to the wide application of FRP composites in structural applications. A comprehensive review of the available body of knowledge relating to durability issues in adhesively bonded connections and FRP/steel joints in particular is presented in this paper. Moisture and temperature are identified as the most critical environmental factors and their individual and combined effect on both constituent material level and system level are reviewed. This includes both the underlying mechanisms of environmental degradation and quantitative assessments of the available test results. The most important aspects are discussed and future research topics are identified.

Acknowledgements The authors would like to thank the Swedish Research Institute (FORMAS) for supporting this study.

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Figure 1. Diagram of environmental factors influencing the durability of adhesively bonded FRP/steel joints.

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Figure 2. Moisture-dependent mechanical properties of structural resins/adhesives, test results from [43– 54]. Figure 3. Moisture sorption locations and mechanisms in fibre reinforced polymer composites, after [59]. Figure 4. Environmental effects on the flexural and interlaminar shear strength of FRP composites; NA=not available, Mt=moisture content, DW=distilled water, SW=salt water; experimental data from [61–69]. Figure 5. Schematics of cathodic delamination in polymer coated metals, after [96]. Figure 6. Adhesion loss in lap shear joints made of steel/epoxy in humid environments, after [40].

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Figure 7. Effect of surface treatment of steel substrate on durability of bonded CFRP/steel joints. Figure 8. Schematic representation of the combined bulk-interfacial mechanism of adhesion loss at or above the adhesive critical moisture content proposed in [139]. Figure 9. Effect of water on adhesive joints, reproduced after [140].

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Figure 10. Schematic representation of test configurations used to address the long-term performance of adhesively bonded FRP/steel joints in the presence of moisture. Figure 11. Comparison of strength variation in bonded FRP/steel joints with increasing exposure duration in moist conditions.

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Figure 12. The effect of sustained loading during environmental ageing on the strength of bonded CFRP/steel joints. Figure 13. Effect of elevated temperatures on: (a) normalized strength, (b) normalized stiffness of bonded FRP joints categorised by the type of adhesive; Tt = testing temperature, Tg = glass transition temperature of adhesive. Figure 14. Effect of cold testing temperature on the load-bearing capacity of bonded FRP joints categorised by the type of adhesive and FRP; NM = normal modulus, HM = high modulus carbon fibres. Figure 15. Normalized strength versus number of freeze/thaw cycles for bonded FRP/steel joints.

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