# Temperature dependence of the bond strength in externally bonded CFRP systems for concrete structures

E. Ferrier, C. Le Roy, A. Gagnon, Emilie Lepretre, K. Benzarti, C. Aubagnac

► To cite this version:

E. Ferrier, C. Le Roy, A. Gagnon, Emilie Lepretre, K. Benzarti, et al.. Temperature dependence of the bond strength in externally bonded CFRP systems for concrete structures. Construction and Building Materials, 2025, 486, pp.141933. 10.1016/j.conbuildmat.2025.141933. hal-05317468

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# Temperature dependence of the bond strength in externally bonded CFRP systems for concrete structures

E. Ferrier<sup>a, *</sup>, C. Le Roy<sup>b</sup>, A. Gagnon<sup>b</sup>, E. Leprete<sup>c</sup>, K. Benzarti<sup>d</sup>, C. Aubagnac<sup>b</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

CFRP strengthening system
Lap shear testing
Temperature effect
Design bond strength

## ABSTRACT

This study investigates the influence of temperature on the mechanical properties of carbon fibre-reinforced polymer (CFRP) strengthening systems externally bonded to concrete substrates. This study examined six different CFRP systems, including four pultruded CFRP plate systems and two sheet systems, which are representative of the products currently used in the French market. The CFRP concrete specimens were tested at different temperatures in two laboratories under double-lap shear or single lap-shear test configurations, representing 138 test samples. The tests were conducted at room temperature and at several other temperatures in the range $T_g$ - 20 °C to $T_g$ + 15 °C, where $T_g$ is the glass transition temperature of the epoxy adhesive or matrix. As the test temperature increases, results showed a slight increase in the maximum load capacity of the bond in the range of $T_g$ -15°C or $T_g$ -10 °C for some CFRP systems) related to polymer softening and stress redistribution along the lap joint. For most systems, the characteristic values of load capacity and bond strength remained above or near their initial levels (at 20 °C) for test temperatures up to the polymer but declined sharply beyond this limit. These experimental results were then compared with the calculation method outlined in Appendix J of Eurocode 2 focusing on externally bonded reinforcements, demonstrating that this design method can be safely applied up to $T_g$ -10 °C for all systems and even higher for certain ones. Therefore, a temperature limit of $T_g$ -10 °C is recommended for serviceability purposes. However, the testing procedure allow to assess a higher temperature limit if the system demonstrates an acceptable variation in the characteristic bond strength.

## 1. Introduction

Carbon fibre-reinforced polymer (CFRP) is a widely used strengthening technique for concrete structures. This material exhibited high mechanical performance with excellent physical and chemical properties, ensuring long-term durability. In addition, CFRP strengthening systems can be easily applied to concrete structures through a simple bonding process using epoxy adhesives that can be cured at ambient temperatures.

CFRP materials offer several vital advantages. First, they have a high strength-to-weight ratio, providing substantial strength without adding extra weight to the structure, making them ideal for applications in which minimising weight is crucial. Unlike steel, CFRPs are corrosion resistant, making them particularly suitable for environments exposed to chemicals. In addition, these materials are easy to install as they can

be applied relatively quickly and with minimal disruption, offering to be an efficient alternative to traditional strengthening methods like adding concrete or using steel reinforcements. Finally, CFRPs are versatile because they can be used for various types of structural strengthening, including flexural, shear, and axial, as well as for column confinement.

From a structural perspective, strengthening concrete structures with externally bonded CFRPs can enhance the load-bearing capacity of beams, increase the shear strength of structures, and improve the compressive strength of columns when FRP confinement is applied. CFRPs are typically bonded to tensioned areas of the structure, such as the undersides of beams or slabs. Reinforcing concrete structures with bonded composites is an effective solution for extending the lifespan of infrastructure, improving their capacity, and ensuring safety. The overall efficiency of this system is closely linked to the bond between the material and substrate and the temperature limit of the strengthening

<footnote><sup>a</sup> Composite for Construction Laboratory LMC2, University Claude Bernard Lyon 1, Campus de la Doua, 82 bd Niels Bohr, Villeurbanne 69100, France</footnote>

<footnote><sup>b</sup> Autun Agency, Cerema, Boulevard Giberstein, Autun 71400, France</footnote>

<footnote><sup>c</sup> MAST-SMC, Univ Gustave Eiffel, Bouguenais F-44344, France</footnote>

<footnote><sup>d</sup> Navier, Univ Gustave Eiffel, Ecole des Ponts, CNRS, Marne la Vallée F-77454, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: emmanuel.ferrier@univ-lyon1.fr (E. Ferrier).</footnote>
system under onsite temperature. The limit depends mainly on the polymer glass temperature value. For this purpose design code introduce a design “service temperature” depending on glass temperature value of polymer. The aim of this paper is to clarify this service temperature by mechanical testing. Thus, analysing the effectiveness of bond strength is essential and coupled the test temperature and stress during testing a good opportunity to clarify this point.

## 1.1. Literature review

Identifying the properties of CFRP bonded to concrete is crucial for evaluating the efficiency of the reinforcement systems [1]. In most cases, mechanical testing is used to measure the tensile bond strength of a CFRP system using single- or double-lap shear tests [2–4]. In these tests, a composite sample bonded to concrete was subjected to a tensile force until failure, and the maximum force and failure mode were recorded. These two testing methods have been widely adopted by researchers since 90’s [5–8].

The single-lap shear test is essential to assess the effectiveness of CFRP as a strengthening material for concrete structures. This provides valuable insights into the bond performance, which is crucial for the design and reliability of CFRP-strengthened concrete elements. During the test, a load is applied to simulate a single-lap shear condition with the concrete block remaining stationary, while the load was applied to the free end of the CFRP strip. A load cell measures the applied force while the displacement is monitored at the free end of the CFRP to track the deformation and failure progression. The load was applied at a controlled displacement rate until failure occurred, with a rate slow enough to ensure accurate measurements of the load and displacement, which were continuously recorded throughout the test. The failure modes may include cohesive failure (within the adhesive layer), adhesive failure (at the interface between the polymer adhesive and CFRP or concrete), or substrate failure (within the concrete or CFRP). Data analysis enables the calculation of the maximum shear strength based on the peak load recorded during the test, along with an examination of the load-displacement curve [8], to understand the bond behaviour and failure mechanism.

The double-lap shear test is a widely used method for assessing the bond strength at the interface between CFRP composites and concrete [9]. This test simulates the shear conditions that an adhesive interface may experience during service. The primary objective of the double-lap shear test was to measure the shear strength of the adhesive bond between the CFRP and concrete to evaluate the bond quality and performance of the strengthening system. Failure mode analysis helps identify whether the weakness originates in the bond, adhesive, or concrete substrate. The shear strength was calculated by dividing the maximum load by the surface area of the bonded joint.

In conclusion, both double-lap shear and single-lap shear tests are essential for assessing the performance of CFRP-concrete reinforcement systems, providing critical insights into the strength and durability of the adhesive interface. By conducting these two tests in accordance with established guidelines, engineers and researchers can accurately assess the shear strength of the CFRP-concrete interface, ultimately leading to safer and more efficient designs for reinforced concrete structures [10–12].

Temperature plays a significant role in the performance of epoxy adhesives, affecting their behaviour during their application to a structure during curing and in service after curing. It is essential to follow the manufacturer’s recommendations for the optimal temperatures for each stage to ensure optimal performance. FRP systems are typically bonded using bi-component epoxy adhesives comprising a polyamine hardener and an epoxy prepolymer resin. Once mixed, these adhesives can polymerise onsite at ambient temperature without requiring additional heating. Owing to the crosslinking process, the polymer network generally reaches its maximum mechanical properties after approximately one week of curing, although this duration can vary significantly

depending on the environmental conditions at the construction site. The cured polymer network is then characterized by its glass transition temperature, $T_g$, which typically falls within the range of 45–60 °C for systems cured under ambient conditions [13,14]. The glass transition temperature ($T_g$) is a phenomenon of amorphous polymers. At this temperature, polymers undergo a transition from glassy to rubbery state. $T_g$ is an important feature of polymer behaviour. It marks a region of dramatic changes in the physical and mechanical properties. DSC, DTA and DMA are by far the most dominant techniques used for glass transition temperature measurements. Differential Scanning Calorimetry (DSC) is a thermo-analytical technique using differential scanning calorimeter. It monitors the difference in heat flow between the sample and reference against time or temperature. It also programs the temperature change of the sample in a specified atmosphere. Differential Thermal Analysis (DTA) is a popular thermal analysis technique. It is often employed for measuring $T_g$ of the material. This test method is similar to differential scanning calorimetry (DSC). The technique involves an inert reference material. Dynamic Mechanical Analysis (DMA) uses a dynamic mechanical analyzer to measure the stiffness of materials as a function of temperature, humidity, dissolution media or frequency. Depending of the methods the value of the glass temperature can be slightly different.

As the service temperature approaches and exceeds the $T_g$, the molecular mobility of the polymer chains shifts from localised to large-scale motion, causing the polymer network to transition from a rigid vitreous state to a soft, rubbery state. This transition results in a significant loss of the mechanical performance of the bulk adhesive, including a sharp decline in Young’s modulus and tensile strength. In addition, the shear strength of bonded assemblies using these epoxy adhesives can deteriorate significantly beyond $T_g$ [15].

Several authors have conducted experimental studies combined with modelling to investigate the effects of temperature.

Firmo et al. [16] presented an experimental study on the bond between concrete and CFRP strengthening systems at elevated temperatures. The experimental program involved double-lap shear tests on concrete blocks strengthened with externally bonded CFRP strips using an epoxy adhesive, conducted under steady-state (series S1) and transient (series S2) conditions. In series S1, two types of specimens were tested: with and without mechanical anchorage at the CFRP strip extremities. The adhesive used to externally bond the CFRP strips had a $T_g$ of 62 °C. Specimens were heated to temperatures of 20, 55, 90, and 120 °C, before being loaded to failure. In series S2, the specimens were first loaded to 25, 50, and 75 % of their ambient temperature strength and subsequently heated to failure. The results from the specimens without mechanical anchorage indicated that increasing the temperature altered the strain distributions along the bonded length, making them more linear owing to epoxy softening, while also increasing the effective bond length. The bond-slip curves exhibited significant reductions in the stiffness and maximum shear stress. Furthermore, the failure mode changed from cohesive (at ambient temperature) to adhesive (at elevated temperatures). The overall stiffness and strength of the CFRP-concrete interface significantly decreased, with bond strength reductions in the transient tests comparable to those under steady-state conditions.

Ferrier and Agbossou [9] investigated the combined effect of temperature and mechanical loads on FRP-concrete bonds, through double-lap shear test conducted in a controlled temperature chamber, with temperature ranging from –40 to 120 °C. This study included experimental and theoretical analyses. The experimental results showed that the ultimate load decreased at temperatures above $T_g$ of the epoxy resin (40 °C) but increased at temperatures below $T_g$. However, a substantial decline in the failure load was again observed at significantly low temperatures (below –20 °C). The failure mode transitions from cohesive concrete failure between –40 °C and 40 °C to more adhesive failure modes at temperatures above 40 °C. Analytical and finite element analyses provided further insights into these experimental results,
highlighting the influence of mismatched thermal expansion coefficients and mechanical loads on the stress–strain distributions.

Leone et al. [17] conducted an experimental investigation, which led to several critical observations. The bond stress–slip curves maintained similar qualitative shapes with increasing service temperature, although significant variations were observed for some parameters. The adhesive used to externally bond the CFRP strips had a $T_g$ of 58 °C. The initial slope of the ascending branch decreased as the temperature increased, and the maximum bond stress decreased at temperatures exceeding $T_g$ of the adhesive. At 80 °C, shear stress decreased by 54 % for bonded CFRP sheets, 72 % for glass FRP (GFRP) sheets, and 25 % for bonded CFRP pultruded CFRP plates compared to their initial properties at room temperature. Bond tests performed near the $T_g$ of the matrix and adhesive exhibited significant scattering, likely owing to the chemical and physical processes occurring near the glass transition, such as post-curing and physical aging. Further investigation is required to clarify the impact of these phenomena on the bond performance of FRP-reinforced concrete elements.

Several authors have also investigated the effect of temperature on bond properties through modelling. Ferrier et al. [18] explored the response of two assembled FRP–epoxy adhesive or resin–concrete systems under thermomechanical loading. The analytical part of the study was divided into two stages. First, three empirical formulae were evaluated to assess the failure load. Subsequently, an analytical approach based on a high-order fracture mechanics model is examined. The experimental and analytical results demonstrated gradual degradation at low and high temperatures, as well as a behaviour distinct from the glass transition effect, which characterizes certain components of the layered system. Jia et al. [19] proposed a closed-form analytical solution to predict the full-range behaviour of the FRP-to-concrete bonded joints subjected to combined thermal and mechanical loadings. They compared the effects of thermal stress on the interfacial shear stress and slip distributions and observed the following: (a) the closed-form analytical solution accurately predicted the full-range behaviour of FRP-to-concrete bonded joints with limited bond lengths and varying temperature conditions; (b) this analytical solution can be used to evaluate the effects of thermal loading on the interfacial shear stress and slip distributions, as well as the global load-slip curves at the loaded and free ends of the bonded joint.

Several studies have been conducted on this structural scale. Krzywon [20] investigated the temperature effects of infrared solar radiation on beams strengthened externally with FRPs. The laboratory tests involved 15 full-scale reinforced concrete beams, seven of which were strengthened with externally bonded CFRP strips, while the other seven were strengthened with a steel-reinforced polymer (SRP) tape. The beams were tested under various temperature conditions, ranging from 20–80 °C. They were heated from the strengthened side using linear infrared radiators. Once the adhesive layer reached a predetermined temperature, the beams failed. A noticeable impact of temperature was observed, starting at around 50 °C. In all the cases, failure was followed by delamination. Differences in the behaviours of the CFRP- and SRP-strengthened beams were noted. Delamination of the CFRP strip occurred unexpectedly without any preliminary signs, whereas the SRP-strengthened beams failed with greater deflection and lower mid-span strains than the beam tested at room temperature. Significant degradation in bearing capacity was observed above 65 °C. This indicates that CFRP strengthening, which may be subjected to direct sunlight, should always be protected by thermal insulation.

## 1.2. Research objectives

This study aims to investigate the effect of temperature on the bond strength between CFRP and concrete. The primary objective is to identify the critical temperature at which the shear strength decreases significantly, thereby helping establish the serviceability temperature limit for the strengthening material. The study involves testing

specimens at different temperatures, including ambient conditions (20 °C or 23 °C), $T_g$ of the considered systems, and several temperatures in the range from ($T_g$ - 20) °C to ($T_g$ + 15) °C. For each temperature, three specimens were tested to determine the statistical characteristics of their distributions. Mechanical tests were conducted at the same temperatures to ensure consistency and enable comparisons across laboratories. The average $T_g$ values determined across the laboratories were used, provided the measurements did not differ by more than 4 °C, in accordance with NF EN 12614 standard [21]. The $T_g$ values were determined using differential scanning calorimetry (DSC) as specified in NF EN 12614.

Thirty double-shear concrete specimens were cast in the Laboratory of Composite Material for Construction from Université Claude Bernard University (LMC2 laboratory). After the surface preparation, the concrete specimens were strengthened using two CFRP pultruded CFRP plate systems equipped with adequate instrumentation. The specimens were tested at the predetermined temperatures in a thermal chamber until failure.

Additionally, 108 concrete specimens were cast at the French Center for Studies and Expertise on Risks, the Environment, Mobility and Urban Planning (CEREMA) Laboratory and strengthened with six CFRP systems: four pultruded CFRP plate systems (including two that were also investigated at LMC2) and two sheet systems. The specimens were prepared as single-lap shear test samples. After conditioning in a thermal chamber at the predetermined temperatures for an extended period, the specimens were removed from the chamber and immediately tested for failure using a custom test machine.

The obtained experimental data were first analysed to assess the influence of the test temperature on the bond performance, focusing on the characteristic load capacity, bond strength, and failure mode. The experimental dataset was then compared with a bibliographic database that included more than 78 single- or double-lap shear tests conducted at various test temperatures. Moreover, the results were compared with design strength values from Eurocode 0 and the fib Bulletin 90. Additionally, the characteristic values derived from the experiments were compared with the calculation method proposed by Eurocode 2-Appendix J [22] for externally bonded reinforcements (currently being draft). Finally, the temperature service limit was discussed based on previous findings.

## 2. Testing and analysis description

### 2.1. Double-lap adhesive joint test: geometry and preparation of the test specimens

This test corresponds to a double-overlap tensile-shear test in which two concrete blocks are connected by CFRP strengthening systems bonded to symmetrical faces.

Concrete blocks with dimensions of 140 mm × 140 mm × 260 mm were placed in a mould measuring 140 mm × 140 mm × 560 mm with a 40 mm-thick separating element. The aim was to achieve a perfect alignment of the side faces of the blocks. The concrete was mixed and cast according to the NF EN 18–422 standard [23]. Its characteristic 28-day compressive strength was 40 MPa (C40/50), in accordance with the recommendations of the NFP 18852 standard [24]. Surfaces were prepared by mechanical sanding.

The concrete blocks were positioned on a template to ensure proper alignment throughout the bonding of the composite reinforcement. The alignment tolerance on the bonding faces was 0.25 mm over a 20 cm reference length. CFRP pultruded CFRP plates, measuring 440 mm × 50 mm, were bonded onto two opposite faces while maintaining a gap of 40 mm between the two blocks. Fig. 1 and Table 1 present the specimen's geometry and dimensions. The application followed the technical specifications of the manufacturer of each system. The polymer joints were cured at room temperature (23 °C) for a minimum of 14 days before initiating the test program.
<figure><img src="image_4.png" /></figure>

<table><caption>Fig. 1. Size of the double-lap shear test specimens.</caption><tr><td></td><td>L0</td><td>L2</td><td>L1</td><td>H</td><td>B</td><td>LT</td><td>LT1</td><td>b1</td></tr><tr><td>[mm]</td><td>560</td><td>40</td><td>260</td><td>140</td><td>140</td><td>440</td><td>240</td><td>50</td></tr></table>

**Table 1**

Characteristics of the various CFRP systems tested (D refers to systems tested in the double lap shear configuration at LMC2, while S refers to those tested in the single-lap configuration at Cerema).

<table><thead><tr><th>Reference of system</th><th>Type of CFRP system</th><th>E<sub>f</sub> (GPa)</th><th>t<sub>f</sub> (mm)</th><th>T<sub>g</sub> of the epoxy resin/ adhesive (°C)</th></tr></thead><tbody><tr><td>System 1 (D or S)</td><td>Pultruded CFRP plate</td><td>196</td><td>1.40</td><td>61</td></tr><tr><td>System 2 (D or S)</td><td>Pultruded CFRP plate</td><td>160</td><td>1.20</td><td>57</td></tr><tr><td>System 3 (S)</td><td>Pultruded CFRP plate</td><td>170</td><td>1.20</td><td>55</td></tr><tr><td>System 4 (S)</td><td>Pultruded CFRP plate</td><td>170</td><td>1.20</td><td>51</td></tr><tr><td>System 5 (S)</td><td>Sheet</td><td>220</td><td>0.194</td><td>56</td></tr><tr><td>System 6 (S)</td><td>Sheet</td><td>22000</td><td>0.168</td><td>49</td></tr></tbody></table>

Two CFRP pultruded CFRP plate systems (numbered 1 and 2) were tested using a double-shear configuration. The corresponding double-shear specimens were labelled 1D and 2D, respectively. The characteristics of the two pultruded CFRP plate systems are listed in Table 1. For each system and test temperature condition, a series of three double-shear specimens were prepared to ensure statistical reliability and account for the variability in material properties and testing conditions.

## 2.2. Single overlap adhesive joint test: geometry and preparation of the test specimens

For the single lap-shear tests, the concrete blocks measure 105 mm

× 210 mm × 420 mm. The concrete mixture exhibited a characteristic strength of 40 MPa (C40/50). Before installing the CFRP reinforcement, the upper faces of the concrete blocks were prepared via sandblasting and dusted.

A CFRP plate (50 mm wide) or sheet (100 mm wide) with a length of approximately 320 mm was bonded to the concrete block over a length of 200 mm. To prevent premature failure at the concrete corner during the tests, an unbonded length of 50 mm was maintained between the edge of the block and the joint. The thickness of the adhesive layer was maintained at 2 mm using a specific roller. A thermocouple was introduced within the adhesive layer in a dedicated test zone at the back of the CFRP to measure the temperature during the testing (Fig. 2). Similar to the double shear tests, a series of three single-lap test specimens were manufactured for each system and temperature condition.

Six CFRP systems were tested using this single-lap shear test configuration, including four pultruded CFRP pultruded CFRP plates (numbered 1–4) and two CFRP sheets (numbered 5 and 6). Notably, pultruded CFRP plate systems 1 and 2 were identical to those tested in the double-shear configuration. Thus, the corresponding single shear specimens for each CFRP system will be labeled 1–6S.

CFRP sheets are installed using the wet lay-up method. First a thin resin primer coat was applied to the sandblasted concrete substrate. The fabric was pre-impregnated with the same resin and placed on the upper face of the block. A roller was used to distribute the resin evenly and ensure proper fibre impregnation. In contrast, CFRP pultruded CFRP plates are simply bonded to a concrete support. All systems are applied according to the manufacturer's specifications. All adhesive joints were cured at room temperature for at least 14 days before commencing the mechanical test program.

<figure><img src="image_5.png" /></figure>

<table><caption>Fig. 2. Geometry of the single-lap shear test specimens.</caption><tr><td></td><td>L1</td><td>L2</td><td>L3</td><td>H</td><td>B</td><td>LT1</td><td>b1</td></tr><tr><td>[mm]</td><td>420</td><td>70</td><td>50</td><td>105</td><td>210</td><td>200</td><td>50 or 100</td></tr></table>
The characteristics of the CFRP systems are listed in Table 1. These characteristics included the type of CFRP used (prefabricated pultruded CFRP plate or CFRP sheet), Young's modulus of the reinforcement ($E_f$), thickness of the composite ($t_f$), and $T_g$ of the epoxy resin or adhesive cured at room temperature (the values were averaged from at least four tests performed in different laboratories).

$T_g$ was determined using conventional DSC analyses following the NF EN 12614 standard. The adhesives were initially cured for 28 days at 23 °C, to ensure that the crosslink process was almost stabilised in this condition. Samples of the cured adhesives or resins, each weighing approximately 10 mg, were placed in dedicated aluminium crucibles and inserted into the oven of the calorimeter. The test protocol involved applying two successive heating ramps from -20 °C to 100 °C at a rate of 10 °C/min under a nitrogen flow. $T_g$ of the adhesive sample, reflecting its crosslink state after a 28-day cure at room temperature, was determined from the first heating ramp by identifying the inflection point on the thermogram (i.e., on the heat flow versus temperature curve) in the region of enthalpy variation associated with the polymer glass transition. Two measurements are done and the average value is retained. The measurement has been done in the Cerema Lab and the LMC². Results are considered to be acceptable if mean value is in a range of ± 2 °C of the obtained values between the two lab. Tables 1 and 2 gives details of the measurement.

## 2.3. Instrumentation of test specimens

In the double-shear test configuration, the distance between the two concrete blocks (ΔL1: measured by the displacement transducer) was affected by the elongation (ΔL2) of the reinforcement element (in the unbonded area) and the deformation (ΔL) at the interface between the concrete and the composite during testing. The distance between the two blocks (ΔL1) is continuously recorded using a linear variable differential transformer (LVDT)-type displacement transducers with a stroke of ± 5 mm and an accuracy of 10⁻⁴ mm (Fig. 3a).

The elongation of the composite was measured using a strain gauge located at the centre of the unbonded portion of the CFRP (gauge J1 in Fig. 3a). The applied force was monitored using a load cell. Six strain gauges were arranged along the bonded carbon-composite strip. With the origin defined at the inner end of the concrete block, all the gauges (J1 to J6) were positioned at 25, 50, 75, 100, 125 and 175 mm for double-lap shear test specimens (Fig. 3a).

For the single-lap shear test configuration, five strain gauges with a length of 10 mm were glued to the centre of the CFRP composite along the longitudinal axis. The gauges were positioned at 15, 50, 90, 135, and 180 mm from the edge of the bonded joint (Fig. 3b). Force was applied to the extremity of the composite using specific jaws and recorded

<caption>**Table 2**</caption>

Details of $T_g$ measurement Reference of system.

<table><thead><tr><th></th><th></th><th>T<sub>l</sub></th><th>T<sub>m</sub></th><th>T<sub>inflexion</sub></th><th>T<sub>e</sub></th></tr></thead><tbody><tr><td rowspan="3">System 1</td><td>Test 1</td><td>53.24</td><td>53.90</td><td>55.68</td><td>62.45</td></tr><tr><td>Test 2</td><td>53.67</td><td>54.35</td><td>56.38</td><td>62.61</td></tr><tr><td>Mean</td><td><strong>53</strong></td><td><strong>54</strong></td><td><strong>56</strong></td><td><strong>63</strong></td></tr><tr><td rowspan="3">System 2</td><td>Test 1</td><td>52.3</td><td>54.72</td><td>57.34</td><td>68.76</td></tr><tr><td>Test 2</td><td>47.22</td><td>56.46</td><td>59.68</td><td>75.2</td></tr><tr><td>Mean</td><td><strong>50</strong></td><td><strong>56</strong></td><td><strong>59</strong></td><td><strong>72</strong></td></tr><tr><td rowspan="3">System 3</td><td>Test 1</td><td>53.24</td><td>53.9</td><td>55.68</td><td>62.45</td></tr><tr><td>Test 2</td><td>53.67</td><td>54.35</td><td>56.38</td><td>62.61</td></tr><tr><td>Mean</td><td><strong>53</strong></td><td><strong>54</strong></td><td><strong>56</strong></td><td><strong>63</strong></td></tr><tr><td rowspan="3">System 4</td><td>Test 1</td><td>52.3</td><td>54.72</td><td>57.34</td><td>68.76</td></tr><tr><td>Test 2</td><td>47.22</td><td>56.46</td><td>59.68</td><td>75.2</td></tr><tr><td>Mean</td><td><strong>50</strong></td><td><strong>56</strong></td><td><strong>59</strong></td><td><strong>72</strong></td></tr><tr><td rowspan="3">System 5</td><td>Test 1</td><td></td><td></td><td></td><td></td></tr><tr><td>Test 2</td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">System 6</td><td>Test 1</td><td></td><td></td><td></td><td></td></tr><tr><td>Test 2</td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td></td><td></td><td></td><td></td></tr></tbody></table>

during the test with a load cell. Additionally, two LVDT's sensors were used to record the relative displacement between the jaws and the concrete block and to control the displacement rate throughout the test. The mean value of the two LVDT's is used.

## 2.4. Loading procedure for double shear specimens

Double-shear tests were conducted at the Claude Bernard Lyon 1 university laboratory using a ZWICK 1475 tensile press, which could operate under stress or displacement control. The control system supports various procedures, including load build-up, unloading, and limit stops, with a maximum load capacity of 65 kN.

The press was equipped with a thermal chamber, allowing the specimen to be mechanically characterised at a controlled temperature (Fig. 4). The figure shows the double-lap specimen in place without the thermal chamber (Fig. 4a) and then with (Fig. 4b). The temperature of the chamber is first set to the desired test value. Once it is stabilised (±1 °C), the specimen is placed inside the chamber. After the temperature reached equilibrium (±1 °C), an additional 60 min-period was maintained to ensure uniform temperature distribution within the sample. A thermocouple in direct contact with the concrete surface of tested specimen continuously monitored the specimen temperature throughout the test.

For each CFRP system, five test temperatures are selected (20, $T_g$ - 20, $T_g$, $T_g$ + 10 and $T_g$ + 15 °C) with three specimens tested per temperature. Thirty samples were tested.

## 2.5. Loading procedure for single-lap joint specimens

The tests were conducted at the Cerema Laboratory using a custom-built machine designed for single-lap shear tests. This machine operates under constant load or displacement rate conditions, with a maximum load capacity of 100 kN. The testing device and protocol are described in detail in [25].

Specimens are placed in a temperature-controlled chamber approximately 24 h before testing, set to 1–5 °C higher than the target test temperature. A few minutes before each test, the specimen is transferred to a heated room, where the temperature is maintained close to the test temperature, up to a limit of 60 °C. The tests were conducted in a heated room (Fig. 5).

During the test, the temperature is monitored by a thermocouple embedded in the adhesive layer (see Fig. 2), to ensure it remains within ± 1 °C of the target temperature.

For each CFRP system, single-lap shear tests are conducted at six different temperatures (23, $T_g$ - 15, $T_g$ - 10, $T_g$, $T_g$ + 10, and $T_g$ + 15 °C) with three specimens tested per temperature. In total, 108 samples were tested.

## 2.6. Methods for analysing experimental data

### 2.6.1. Characteristic values of the bond strength

To compare the experimental values with those provided by the design code (according to Appendix J of Eurocode 2 and Fib Bulletin 90 [26,27]). It is essential to calculate the characteristic values, which can be determined using Appendix F of Eurocode 0.

The characteristic strength ($F_{rk}$) was calculated according to section D 7.2 from NF EN 1990 (2002): Eurocode - Basis of Structural Design [28], as follows:

$$F_{bfr}(n) = m_{Pr} \times (1 - \text{kn}V_{Pr}), \quad (1)$$

where

$f_n$: Characteristic failure strength;

$m_{Pr}$: average value for three tested samples;

- $k_n$: Characteristic factor is 2.92 when a test includes three samples.

This value is given in section D.1 of NF EN 1990;
<figure><img src="image_6.png" /><figcaption>(a) Double shear test</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>(b) Single-lap shear tests</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>(a) Specimen in place with out thermal chamber</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>(b) with thermal chamber</figcaption></figure>

<caption>**Fig. 4.** Test setup for double-shear specimens.</caption>

$V_{Pr}$: Covariance of failure strength obtained *Pr*. $V_{Pr}$ is the ratio of the standard deviation to the average value ($\sigma_{Pr}$/ $m_{Pr}$);

$\sigma_{Pr}$: standard deviation for the three samples.

The bond strength was determined by dividing the ultimate force by the cross section of the bonded CFRP. These characteristic values could then be compared to the design values calculated according to Appendix

J of Eurocode 2 and Fib Bulletin 90 [25,26].

### 2.6.2. Anchorage length

The anchorage length $L_e$ is defined by the Yao et al. formula [1] as follows:
<figure><img src="image_10.png" /><figcaption>Fig. 5. Test setup for single-lap shear specimens.</figcaption></figure>

$$L_e = \sqrt{\frac{E_f \cdot t_f}{2 \cdot f_{ctm}}} \quad (\text{mm}) \qquad (2)$$

where $E_f$, $t_f$ and $f_{ctm}$ denote Young's modulus, thickness, and tensile strength of the CFRP composite, respectively.

For instance, in the case of the pultruded CFRP plate CFRP system 1 (with $E_f = 190,000$ MPa, $t_f = 1.4$ mm, and $f_{ctm} = 3.5$ MPa), the calculated anchorage length equals 90 mm.

### 2.6.3. Design value based of Eurocode 2 Appendix J and Fib Bulletin 90

The appendix J in the Eurocode 2 standard [26] enables the calculation of the bonded transfer length ($l_{bf,max,k}$) and tensile adhesive concrete strength in FRP ($f_{bfk,max}$) based on the characteristic concrete compressive strength properties ($f_{ck}$), tensile concrete strength ($f_{ctm}$), and FRP properties, that is, Young's modulus ($E_f$) and thickness ($t_f$). These equations are provided for design and do not consider the effect of temperature on the adhesive. The obtained values are then given for ambient temperature only.

A design example is provided below for the concrete with strengths of 40 MPa under compression and 3.5 MPa under tension.

The design bond strength of the anchorage for the externally bonded FRP system can be expressed as follows:

$$f_{bfk,max} = \sqrt{\frac{E_f \cdot \tau_{f1k} \cdot s_{f0k}}{t_f}} \qquad (3)$$

with $s_{f0k} = 0.20$ and $\tau_{f1k} = 0.37 \cdot (f_{cm} \cdot f_{ctm,surf})^{0.5}$. (4) Eurocode 2 provides a constant value for the parameter $s_{f0k}$ regardless of the CFRP strengthening system, while Fib Bulletin 90 specifies a constant value of 0.20 for CFRP pultruded CFRP plates and 0.23 for CFRP sheets. Parameter $\tau_{f1k}$ is calculated based on the concrete mean strength ($f_{cm}$) and shear concrete surface strength ($f_{ctm,surf}$). Notably, that Eurocode 2 assigns a constant value of 0.37 for both systems (pultruded CFRP plates or CFRP sheets), while Fib Bulletin 90 gives a value of 0.37 for pultruded CFRP plates and 0.44 for CFRP sheets. Then Eurocode 2 equation are used with the Fib bulletin factor for different systems.

Because the failure mode of the specimen occurs in concrete and involves shear failure, Eurocode 2 introduces a mean surface tensile strength of concrete ($f_{ctm,surf}$), which is calculated as follows for system 1, as an example:

$$f_{ctm,surf} = k_{c,surf} \cdot f_{ctm} \qquad (4)$$

$$\text{where } k_{c,surf} = 0.6 + 0.3 \cdot \left( \frac{f_{ck}}{60} - 0.20 \right)$$

In our design example, $k_{c,surf} = 0.6 + 0.3 \cdot \left( \frac{40}{60} - 0.20 \right) = 0.74$.

The transfer length for system 1 is calculated by

$$l_{bf,max,k} = 2 \cdot \sqrt{\frac{E_f \cdot t_f \cdot s_{f0k}}{\tau_{f1k}}} \qquad (5)$$

where $s_{f0k} = 0.20$ and $f_{cm} = 40$ MPa

$$\tau_{f1k} = 0.37 \cdot (f_{cm} \cdot f_{ctm,surf})^{0.5} = 4\text{MPa} \qquad (6)$$

$$l_{bf,max,k} = 2 \cdot \sqrt{\frac{E_f \cdot t_f \cdot s_{f0k}}{\tau_{f1k}}} = 230\text{mm} \qquad (7)$$

## 3. Experimental results

### 3.1. Influence of the test temperature on bond performance

Table 3 presents the maximum load capacity values recorded during the double- and single-lap shear tests at different temperatures for all CFRP systems examined, along with the mean value over three repeated tests for each set of test and the standard deviation is added to calculate latter on the characteristic bond strength. The terms D and S in Table 3 refer to double-shear and single-lap shear tests, respectively. The characteristic values of the load capacity $F_{bk}$ and the characteristic bond strength $f_{bk}$ corresponding to the maximal load capacity using the cross section of the FRP were also calculated (Table 3).

Additionally, Fig. 6 illustrates the evolution of the bond strength $f_{bk}$ as a function of $T-T_g$, where $T$ is the test temperature, and $T_g$ is the glass transition temperature of the adhesive joint for specimens strengthened with various CFRP systems. These experimental data provide insights into the dependence of the bond performance on the test temperature in relation to the $T_g$ of each system (Table 4).

First analysis of the test can compared the both set-up with system 1 and 2, for example for system, at 20°C, a value of 61.5 ± 1.1 kN is obtained with double-lap shear test and a value of 30.2 ± 1.3 kN is obtained with the single lap shear test. As expected, these values are approximately half of those obtained in the double-lap shear tests since there is only one plate instead of two in this test procedure. These two results enhance the low discrepancies between the results and confirm the reliability of the two test set-up since the same results on a same FRP system is obtained in two different laboratory. When the test is done under temperature, for example 60°C the values and slightly different, the double-lap shear test give at ultimate load of 54.7 ± 0.9 kN while the single-lap shear test gives a value of 36.6 ± 0.9 kN. This higher (an equivalent of 73.2 kN instead of 54.7 kN) value can be explained by the test set-up for double lap shear test, temperature is maintained constant during the test while with single lap shear test procedure the test specimen is preheated given higher results because a small drop in temperature can be obtained during the test. For higher temperature this difference between the two test disappear. The results outline that around $T_g$ a small change in test temperature can affect the ultimate failure load. The same observation can be done for system 2.

In double-lap tests, the highest average load capacity was approximately 60 kN for the two tested CFRP pultruded CFRP plates (Systems 1 and 2). Notably, the values obtained at ($T_g-20$) °C are slightly higher than those at ambient temperature (20 °C). As the test temperature increases beyond $T_g$, the load capacity decreases, reaching around 30 kN at $T_g + 15$ °C.

In the single-lap shear tests, the mean failure load for the test done at 23°C is around 30 kN for all CFRP pultruded CFRP plate systems (1-4) and around 25 kN for the CFRP sheets (5 and 6). Similar to the double-lap shear test, the load capacity increased below the glass transition temperature of the epoxy polymer. One result to outline in both test set-up is that around a test temperature of $T_g - 15$°C (Fig. 6) the failure load
<caption>**Table 3**</caption>

Maximum load capacities obtained for the different CFRP systems under double (D) or single-lap (S) shear tests at various temperatures. The characteristic values of the load capacity $F_{bfk}$ and bond strength $f_{bfk}$ are also provided.

<table><thead><tr><th rowspan="2">Reference of system</th><th>T</th><th rowspan="2">T-Tg [°C]</th><th colspan="5">Maximum load</th><th rowspan="2">f<sub>bfk</sub></th><th rowspan="2">f<sub>bfk</sub>/f<sub>bfk</sub> (20 °C)</th></tr><tr><th>[°C]</th><th>Sample 1 [kN]</th><th>Sample 2 [kN]</th><th>Sample 3 [kN]</th><th>Average values [kN]</th><th>Discrepancies [kN]</th><th>F<sub>bfk</sub> [kN]</th></tr><tr><th></th><th></th><th></th><th colspan="5"></th><th></th><th></th><th></th></tr><tr><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td rowspan="5">System 1 D (tf = 1.4 mm, bf = 50 mm)</td><td>20</td><td>-41</td><td>61.4</td><td>62.6</td><td>60.5</td><td>61.5</td><td>1.1</td><td>58.3</td><td>416.3</td><td>1.00</td></tr><tr><td>41</td><td>-20</td><td>64.5</td><td>65.7</td><td>66.7</td><td>65.6</td><td>1.1</td><td>62.4</td><td>445.6</td><td>1.07</td></tr><tr><td>61</td><td>0</td><td>54.7</td><td>55.7</td><td>53.9</td><td>54.7</td><td>0.9</td><td>52.1</td><td>371.9</td><td>0.89</td></tr><tr><td>71</td><td>10</td><td>52.2</td><td>53.2</td><td>51.4</td><td>52.3</td><td>0.9</td><td>49.7</td><td>354.7</td><td>0.85</td></tr><tr><td>76</td><td>15</td><td>36.8</td><td>37.6</td><td>36.3</td><td>36.9</td><td>0.6</td><td>35.1</td><td>251.0</td><td>0.60</td></tr><tr><td rowspan="5">System 2 D (tf = 1.2 mm, bf = 50 mm)</td><td>20</td><td>-37</td><td>59.0</td><td>57.0</td><td>61.0</td><td>59</td><td>2</td><td>53.1</td><td>442.8</td><td>1.00</td></tr><tr><td>37</td><td>-20</td><td>61.0</td><td>63.0</td><td>60.0</td><td>61.3</td><td>1.5</td><td>56.9</td><td>474.2</td><td>1.07</td></tr><tr><td>57</td><td>0</td><td>52.5</td><td>50.7</td><td>54.3</td><td>52.5</td><td>1.8</td><td>47.2</td><td>393.6</td><td>0.89</td></tr><tr><td>67</td><td>10</td><td>47.2</td><td>45.6</td><td>48.8</td><td>47.2</td><td>1.6</td><td>42.5</td><td>354.3</td><td>0.80</td></tr><tr><td>72</td><td>15</td><td>29.5</td><td>28.5</td><td>30.5</td><td>29.5</td><td>1</td><td>26.6</td><td>221.4</td><td>0.50</td></tr><tr><td rowspan="5">System 1 S (tf = 1.4 mm, bf = 50 mm)</td><td>23</td><td>-38</td><td>31.3</td><td>30.6</td><td>28.8</td><td>30.2</td><td>1.3</td><td>26.4</td><td>377.0</td><td>1.00</td></tr><tr><td>46</td><td>-15</td><td>55.0</td><td>52.3</td><td>47.1</td><td>51.5</td><td>4</td><td>39.8</td><td>568.3</td><td>1.51</td></tr><tr><td>51</td><td>-10</td><td>46.5</td><td>51.2</td><td></td><td>48.9</td><td>3.3</td><td>39.2</td><td>560.4</td><td>1.49</td></tr><tr><td>61</td><td>0</td><td>37.7</td><td>36.8</td><td>35.4</td><td>36.6</td><td>1.2</td><td>33.1</td><td>472.6</td><td>1.25</td></tr><tr><td>71</td><td>10</td><td>26.2</td><td>26.6</td><td>26.6</td><td>26.5</td><td>0.2</td><td>25.9</td><td>370.2</td><td>0.98</td></tr><tr><td rowspan="5">System 2 S (tf = 1.2 mm, bf = 50 mm)</td><td>76</td><td>15</td><td>18.6</td><td>21.9</td><td>21.6</td><td>20.7</td><td>1.8</td><td>15.4</td><td>220.4</td><td>0.58</td></tr><tr><td>23</td><td>-34</td><td>30.5</td><td>29.5</td><td>29.2</td><td>29.7</td><td>0.7</td><td>27.6</td><td>395.0</td><td>1.00</td></tr><tr><td>42</td><td>-15</td><td>34.2</td><td>31.9</td><td>36.0</td><td>34</td><td>2.1</td><td>27.8</td><td>397.8</td><td>1.01</td></tr><tr><td>47</td><td>-10</td><td>39.8</td><td>41.7</td><td>43.7</td><td>41.7</td><td>2</td><td>35.8</td><td>512.0</td><td>1.30</td></tr><tr><td>57</td><td>0</td><td>36.0</td><td>35.0</td><td>34.9</td><td>35.3</td><td>0.6</td><td>33.5</td><td>479.2</td><td>1.21</td></tr><tr><td rowspan="5">System 3 S (tf = 1.2 mm, bf = 50 mm)</td><td>67</td><td>10</td><td>27.2</td><td>27.8</td><td>23.6</td><td>26.2</td><td>2.3</td><td>19.5</td><td>278.0</td><td>0.70</td></tr><tr><td>72</td><td>15</td><td>11.7</td><td>20.5</td><td>21.5</td><td>17.9</td><td>5.4</td><td>-</td><td>-</td><td>-</td></tr><tr><td>23</td><td>-32</td><td>29.5</td><td>27.8</td><td>27.6</td><td>28.3</td><td>1</td><td>25.4</td><td>362.4</td><td>1.00</td></tr><tr><td>40</td><td>-15</td><td>36.4</td><td>28.2</td><td>34.6</td><td>33.1</td><td>4.3</td><td>20.5</td><td>292.9</td><td>0.81</td></tr><tr><td>45</td><td>-10</td><td>27.0</td><td>32.6</td><td>26.0</td><td>28.5</td><td>3.6</td><td>18.0</td><td>256.5</td><td>0.71</td></tr><tr><td rowspan="5">System 4 S (tf = 1.2 mm, bf = 50 mm)</td><td>55</td><td>0</td><td>18.6</td><td>33.0</td><td>24.5</td><td>25.4</td><td>7.2</td><td>4.3</td><td>61.5</td><td>0.17</td></tr><tr><td>65</td><td>10</td><td>15.3</td><td>9.9</td><td>15.3</td><td>13.5</td><td>3.1</td><td>4.4</td><td>63.1</td><td>0.17</td></tr><tr><td>70</td><td>15</td><td>9.9</td><td>20.2</td><td>11.4</td><td>13.8</td><td>5.6</td><td>-</td><td>-</td><td>-</td></tr><tr><td>23</td><td>-28</td><td>30.0</td><td>30.7</td><td>28.4</td><td>29.7</td><td>1.2</td><td>26.2</td><td>374.1</td><td>1.00</td></tr><tr><td>36</td><td>-15</td><td>38.9</td><td>43.2</td><td>40.6</td><td>40.9</td><td>2.2</td><td>34.5</td><td>492.2</td><td>1.32</td></tr><tr><td rowspan="5">System 5 S (tf = 0.194 mm, bf = 100 mm)</td><td>41</td><td>-10</td><td>42.8</td><td>48.0</td><td>49.4</td><td>47</td><td>3.7</td><td>36.2</td><td>516.6</td><td>1.38</td></tr><tr><td>51</td><td>0</td><td>31.6</td><td>39.0</td><td>45.4</td><td>38.7</td><td>6.9</td><td>18.5</td><td>264.0</td><td>0.71</td></tr><tr><td>61</td><td>10</td><td>25.0</td><td>28.5</td><td>24.4</td><td>26</td><td>2.2</td><td>19.6</td><td>279.3</td><td>0.75</td></tr><tr><td>66</td><td>15</td><td>22.7</td><td>23.9</td><td>23.3</td><td>23.3</td><td>0.6</td><td>21.5</td><td>307.7</td><td><strong>0.82</strong></td></tr><tr><td>23</td><td>-28</td><td>25.9</td><td>25.8</td><td>25.1</td><td>25.6</td><td>0.4</td><td>24.4</td><td>1265.7</td><td><strong>0.93</strong></td></tr><tr><td rowspan="5">System 6 S (tf = 0.168 mm, bf = 100 mm)</td><td>36</td><td>-15</td><td>27.9</td><td>26.7</td><td>27.3</td><td>27.3</td><td>0.6</td><td>25.5</td><td>1323.4</td><td>0.98</td></tr><tr><td>41</td><td>-10</td><td>34.1</td><td>30.8</td><td>28.7</td><td>31.2</td><td>2.7</td><td>23.3</td><td>1206.7</td><td>0.89</td></tr><tr><td>51</td><td>0</td><td>25.5</td><td>26.6</td><td>26.2</td><td>26.1</td><td>0.5</td><td>24.6</td><td>1276.4</td><td>0.94</td></tr><tr><td>61</td><td>10</td><td>14.7</td><td>17.1</td><td>15.1</td><td>15.6</td><td>1.3</td><td>11.8</td><td>610.9</td><td>0.45</td></tr><tr><td>66</td><td>15</td><td>10.9</td><td>9.9</td><td>9.5</td><td>10.1</td><td>0.7</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="5">System 7 S (tf = 0.168 mm, bf = 100 mm)</td><td>23</td><td>-26</td><td>28.9</td><td>21.5</td><td>19.8</td><td>23.4</td><td>4.9</td><td>23.4</td><td>1392.9</td><td>1.00</td></tr><tr><td>34</td><td>-15</td><td>26.8</td><td>24.0</td><td>33.7</td><td>28.2</td><td>5</td><td>13.6</td><td>806.5</td><td><strong>0.58</strong></td></tr><tr><td>39</td><td>-10</td><td>24.9</td><td>34.3</td><td>28.5</td><td>29.2</td><td>4.7</td><td>15.4</td><td>918.4</td><td><strong>0.66</strong></td></tr><tr><td>49</td><td>0</td><td>31.3</td><td>32.7</td><td>32.7</td><td>32.2</td><td>0.8</td><td>29.9</td><td>1777.1</td><td><strong>1.28</strong></td></tr><tr><td>59</td><td>10</td><td>23.7</td><td>31.8</td><td>25.1</td><td>26.9</td><td>4.3</td><td>14.3</td><td>851.3</td><td><strong>0.61</strong></td></tr><tr><td>64</td><td>15</td><td>23.9</td><td>21.1</td><td>24.1</td><td>23</td><td>1.7</td><td>18.0</td><td>1072.6</td><td><strong>0.77</strong></td></tr></tbody></table>

increase by 62 % for system 1 and by 40 % for system 2, a similar result is obtained for system 4 for the test done at a temperature of $T_g$-10 °C.

A second interesting result is that when the test temperature increase to a value higher than $T_g$ of polymers, a significant decrease in the maximum load capacity was observed. At $T_g$ + 10 °C, for instance, the load capacity becomes lower than the initial value determined at 23 °C, regardless of the system considered (for CFRP pultruded CFRP plates 1–4 and CFRP sheets 5 and 6). At $T_g$ + 15 °C, the values are 22–51 % lower than that at 23 °C, depending on the system.

The behaviour at failure for pultruded CFRP plate systems depends really on thermal polymer properties. For system 1,2 and 4 an increase of bond strength is obtained just below $T_g$ while for system 3 a decrease of bonded strength is noticed. For System using bonded FRP sheet (System 5 and 6) the behaviour is much different. For these systems, the increase in load capacity up to $T_g$ - 15 °C is less pronounced, the capacity at $T_g$ - 10 °C is close to that at 23 °C, and the load capacity at $T_g$ is lower than that at 23 °C. While the result dispersion is minimal at room temperature, it becomes larger at higher temperatures, particularly at $T_g$ - 10 °C and at $T_g$. But for system 6, a large discrepancy in the results limited the analysis and these conclusions.

Overall, a decrease limit in the bond strength relative to the value determined at ambient temperature that can be considered to be acceptable could be assessed to fix a higher service temperature limit. This criterion can determine the temperature serviceability limits in the field without deadload on the FRP system to avoid creep in the interface [3, 29]. Analysing the experimental results based on a 10 % decrease of the mean value (see the graphs in Fig. 6) allows the identification of a temperature limit for various CFRP strengthening systems that can be used in the design phase. This limit can be set to $T_g$ for Systems 1, 2, and 5. For system 3, a temperature limit of $T_g$ - 10 °C should be retained, while for system 4, a temperature limit of $T_g$ + 10 °C should be considered. For System 6, the discrepancy was too large to conduct an accurate analysis but a limit around $T_g$. seems to be acceptable. In any case, test done for temperature higher than $T_g$ + 10 °C shows a rapid loss of 45–70 % of the bond strength.

## 3.2. Analysis of the strain profiles

Fig. 7 illustrates the temperature-dependent evolution of the longitudinal strain profiles derived from the strain gauge measurements for
<figure><img src="image_11.png" /><figcaption>**Fig. 6.** Evolution of the characteristic bond strength with test temperature for various CFRP systems. The dashed orange line indicates the initial value at ambient temperature (23 °C), and the green line represents the 10 % reduction limit.</figcaption></figure>

first specimen of each set of all CFRP composites system tested in the elastic phase. Low discrepancy is obtained between a same set of specimen.

The trends were similar across different CFRP pultruded CFRP plates (Systems 1–4). Initially, the profile exhibits an exponential decay. However, as the temperature increases, it gradually shifts to a linear shape, indicating a more homogeneous stress distribution in the joint.

For the CFRP sheets (Systems 5 and 6), the behaviour was slightly different because the shift was less pronounced, and the strain profile maintained the same shape for all test temperatures.

The anchorage length at 95 % of the shear force reflected the stress

distribution within the joint, which was similar to the shear stress profile. It increases significantly for the pultruded CFRP plate CFRP systems when the test temperature exceeds $T_g$ - 15 °C (Fig. 7a–f). At higher temperatures, the anchorage length estimated directly from strain gauge measurements approached 200 mm. This length, which corresponded to the entire of the bonded length, indicated a more homogeneous stress distribution along the joint.

For the CFRP sheets (Fig. 7g and h), the anchorage length is initially much lower at 23 °C, with values around 70 mm. An increase in the anchorage length was also observed when the temperature exceeded $T_g$, with values approaching 150 mm. However, unlike the CFRP pultruded
<table><caption>**Table 4**
Experimental data from selected literature studies.</caption><thead><tr><th></th><th>T<br>[°C]</th><th>T-T<sub>g</sub><br>[°C]</th><th>Sample 1<br>[kN]</th><th>Sample 2<br>[kN]</th><th>Sample 3<br>[kN]</th><th>Average values<br>[kN]</th><th>Discrepancies<br>[kN]</th><th>F<sub>bk</sub><br>[kN]</th></tr></thead><tbody><tr><td rowspan="4">Leone et al. [17]</td><td>20</td><td>-35</td><td></td><td></td><td></td><td>14.0</td><td>0.3</td><td>13.2</td></tr><tr><td>50</td><td>-5</td><td></td><td></td><td></td><td>14.9</td><td>0.3</td><td>14.0</td></tr><tr><td>65</td><td>10</td><td></td><td></td><td></td><td>12.8</td><td>0.3</td><td>12.1</td></tr><tr><td>80</td><td>25</td><td></td><td></td><td></td><td>10.7</td><td>0.2</td><td>10.1</td></tr><tr><td rowspan="10">Ferrier et al. System 1 [18]</td><td>-40</td><td>-106</td><td>28.7</td><td>27.6</td><td>28.1</td><td>28.1</td><td>0.6</td><td>26.5</td></tr><tr><td>-20</td><td>-86</td><td>25.4</td><td>27.8</td><td>26.6</td><td>26.7</td><td>1.2</td><td>23.1</td></tr><tr><td>-10</td><td>-76</td><td>26.5</td><td>28.3</td><td>27.4</td><td>27.4</td><td>0.9</td><td>24.8</td></tr><tr><td>0</td><td>-66</td><td>30.3</td><td>28.1</td><td>29.1</td><td>29.0</td><td>1.1</td><td>25.8</td></tr><tr><td>20</td><td>-46</td><td>28.9</td><td>30.4</td><td>29.7</td><td>29.7</td><td>0.8</td><td>27.5</td></tr><tr><td>40</td><td>-26</td><td>32.4</td><td>30.6</td><td>31.5</td><td>31.5</td><td>0.9</td><td>28.8</td></tr><tr><td>70</td><td>4</td><td>18.6</td><td>17.9</td><td>18.3</td><td>18.3</td><td>0.4</td><td>17.3</td></tr><tr><td>80</td><td>14</td><td>8.4</td><td>9.7</td><td>9.1</td><td>9.1</td><td>0.7</td><td>7.2</td></tr><tr><td>-20</td><td>-110</td><td>14.4</td><td>14.2</td><td>14.0</td><td>14.2</td><td>0.2</td><td>13.6</td></tr><tr><td>20</td><td>-70</td><td>22.2</td><td>22.0</td><td>21.4</td><td>21.9</td><td>0.4</td><td>20.7</td></tr><tr><td rowspan="7">Ferrier et al. System 2 [18]</td><td>40</td><td>-50</td><td>22.0</td><td>23.2</td><td>25.8</td><td>23.7</td><td>1.9</td><td>18.0</td></tr><tr><td>50</td><td>-40</td><td>21.5</td><td>24.3</td><td>27.9</td><td>24.6</td><td>3.2</td><td>15.2</td></tr><tr><td>80</td><td>-10</td><td>21.3</td><td>21.3</td><td>17.7</td><td>20.1</td><td>2.1</td><td>14.0</td></tr><tr><td>100</td><td>10</td><td>19.5</td><td>16.5</td><td>19.4</td><td>18.5</td><td>1.7</td><td>13.5</td></tr><tr><td>110</td><td>20</td><td>17.5</td><td>16.7</td><td>17.0</td><td>17.1</td><td>0.4</td><td>15.9</td></tr><tr><td>120</td><td>30</td><td>0.8</td><td>1.0</td><td>1.0</td><td>0.9</td><td>0.1</td><td>0.6</td></tr><tr><td>15</td><td>-60</td><td>29.5</td><td>29.0</td><td>29.4</td><td>29.3</td><td>0.3</td><td>28.5</td></tr><tr><td rowspan="5">Carlos et al. [15]</td><td>40</td><td>-35</td><td>25.7</td><td>28.3</td><td>29.7</td><td>27.9</td><td>2.0</td><td>22.0</td></tr><tr><td>65</td><td>-10</td><td>17.4</td><td>17.2</td><td>17.5</td><td>17.4</td><td>0.2</td><td>16.9</td></tr><tr><td>90</td><td>15</td><td>10.5</td><td>10.1</td><td>9.4</td><td>10.0</td><td>0.6</td><td>8.4</td></tr><tr><td>115</td><td>40</td><td>6.1</td><td>6.2</td><td>6.4</td><td>6.2</td><td>0.2</td><td>5.8</td></tr><tr><td>140</td><td>65</td><td>3.5</td><td>3.2</td><td>3.8</td><td>3.5</td><td>0.3</td><td>2.6</td></tr></tbody></table>

CFRP plates, the shapes of the strain profiles remained nonlinear in this case. Overall, the strain levels near the loaded edge are significantly higher than those in the pultruded CFRP plate systems.

In summary, as the temperature increases, two opposing phenomena appear to influence the bond behaviour of CFRP systems. Initially, the increase in temperature below the value of glass transition temperature modify the internal thermal stress due to concrete and FRP thermal properties difference. This observation was confirmed Ferrier et al. [9]. This internal thermal stress in the adhesive could explain a slight increase in the ultimate failure load for some systems. of the thermal stress in the polymer adhesive promotes a more homogeneous stress distribution along the lap joint, leading to an increase in the maximum load capacity around T<sub>g</sub>, depending on the CFRP system.

However, beyond T<sub>g</sub>, the degradation of adhesive failure strength becomes dominant, reducing the joint strength. Although thermal stress temporarily improves the performance, its beneficial effect diminishes with increasing temperature, whereas the adhesive property degradation remains significant at high temperatures. Once the interfacial shear stress distribution became nearly homogeneous, further reductions in the adhesive stiffness no longer improved the bond performance.

### 3.3. Failure modes

Fig. 8 illustrates the typical appearance of fractured surfaces for specimens subjected to double- and single-lap shear tests at various temperatures, with each image showing three specimens from the same series.

Overall, the failure modes can be categorised into four types, as schematically represented in Fig. 9: cohesive failure in the concrete, adhesive failure at the concrete–glue interface, cohesive failure in the glue, and adhesive failure at the CFRP plate–glue interface. Fig. 10 shows the surface area percentages of these failure types on the fractured surfaces of the different CFRP systems and their evolution with the test temperature. These percentages are calculated over the first 10 centimeters near the loaded edge of the joint, capturing the failure mode in the region where damage initiates due to high stress concentration.

Most of failures were cohesive or partly cohesive in the concrete at test temperatures below T<sub>g</sub>. As the temperature increased, the failure

mode gradually became less cohesive in the concrete, with an increasing percentage of adhesive failure (Figs. 8 and 10). Beyond T<sub>g</sub>, the cohesive failure in concrete is minimal compared to the adhesive failure or cohesive failure within the epoxy joint. However, this trend does not clearly apply to Systems 2 and 6, in which the cohesive failure in the concrete remains significant (Fig. 10).

Design values are calculated based on Eurocode 2 Appendix J and Fib Bulletin 90.

The results obtained in this study, along with the notable findings from three studies [15,17,18] on single- or double-lap shear tests conducted at various temperatures, can be compared with the design values given by Eurocode 2 Appendix J and Fib Bulletin 90. The objective was to assess the limitations of these design equations in predicting the bond strength at the service temperature by comparing them with experimental data.

Table 3 summarises the experimental data from the selected studies. Each test series comprised three specimens, enabling the calculation of an average value along with the characteristic properties.

For example, in an experimental study by Leone et al. [17] on double-shear specimens with bonded CFRP sheets tested at different temperatures, a load capacity of 13.2 kN was obtained at the ambient temperature with an experimental anchorage length of 200 mm. This corresponds to an average bond strength of 1228 MPa, which aligns well with the values predicted by Eurocode 2, as modified according to Fib Bulletin 90.

For all tested specimens and the results from the three literature studies, the bond strength can be calculated using the Eurocode design (see Table 5). These computed values were then compared to the experimental characteristic bond strengths (see Table 6) to assess the limitations of the design predictions across the tested temperature range, particularly in relation to the difference between the tested temperature and T<sub>g</sub> of the adhesive for each CFRP system.

Fig. 11 and Table 5 compare the design values obtained with the Eurocode 2 calculation using the Fib Bulletin 90 factors. with the characteristic experimental values for the CFRP systems considered in this study, as well as for several systems investigated by Ferrier et al., Leone et al., and Carlos et al. [15,17,18]. The predictions of the calculated results demonstrated near-perfect accuracy for all tests
<figure><img src="image_12.png" /><figcaption>(c) System 2 – Double-lap shear test</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>(d) System 2 – Single-lap shear test</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>(c) System 2 – Double-lap shear test</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>(d) System 2 – Single-lap shear test</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>(e) System 3 – Single-lap shear test</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>(f) System 4 – Single-lap shear test</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>(g) System 5 – Single-lap shear test</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>(h) System 6 – Single-lap shear test</figcaption></figure>

<caption>**Fig. 7.** Influence of the test temperature on the longitudinal strain profiles of CFRP for specimen 1 of each systems.</caption>
<figure><img src="image_20.png" /><figcaption>(a) Failure at 20 °C – Double-lap shear test (system 1)</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>(b) Failure at Tg – Double-lap shear test (System 1)</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>(c) Failure at 23 °C – Single-lap shear test (system 1)</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>(d) Failure at Tg – Single-lap shear test (System 1)</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>(e) Failure at Tg – Single lap shear test (System 2)</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>(a) Cohesive failure in concrete</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>(c) Cohesive failure in the polymer</figcaption></figure>

<caption>(d) Adhesive failure at the polymer-CFRP interface</caption>

<caption>**Fig. 9.** Schematic description of the different types of failure modes.</caption>

conducted at room temperature.

Additionally, Fig. 11 shows that as long as the temperature remains below $T_g$ –10 °C, the predicted FRP bond strength closely aligns with experimental data across all system, ensuring a safe design. An exception was observed for the CFRP System 6, primarily owing to the high dispersion in its test results, as previously mentioned.

Notably, for Systems 1, 2, and 5, the design bond strength remains conservative up to $T_g$ + 10 °C. Fig. 11 also highlights the limitations of the design equation, as the bond strength decreases rapidly when the temperature exceeds $T_g$. For design and serviceability purposes, the temperature limit should be set at the polymer’s glass transition temperature minus 10 °C, provided the bonded length remains greater than the anchorage length over the temperature service range. This is consistent with the safety margin of 10 °C currently recommended by the AFGC guidelines in France [30].

Nevertheless, the methodology developed in this study could allow for higher service temperatures if the bond performance remains at an acceptable level and if there is no important dead load on the structure to avoid combined effect of creep and temperature. One proposal that could be accepted by designer is to qualify a CFRP system for a specific temperature above $T_g$ – 10 °C would typically require demonstrating that the reduction in its characteristic bond strength is less than a certain level of strength decrease (for example 10 %) relative to the value at ambient temperature (e.g., 20 °C).

## 4. Conclusions

This study investigated the effect of temperature on the bond behaviour of CFRP systems used for the external strengthening of concrete structures. Six systems were selected, four CFRP pultruded CFRP plates and two CFRP sheets, representing a range of products currently available in the French market. Mechanical characterisations were conducted by a double-shear test at the LMC2 Laboratory or by single lap-shear tests at Cerema. The tests were conducted both at room temperature, and at several temperatures in the range ($T_g$ – 20) °C to ($T_g$ + 15) °C, where $T_g$ is the glass transition temperature of the epoxy polymer used to bond or pultruded CFRP plate the CFRP reinforcements.

The results highlight two critical effects of increasing the temperature on the mechanical behaviour of bonded joints.

* Softening (favourable effect): The anchoring length increased with temperature, sometimes exceeding the bonding length of 200 mm at high temperatures. This can lead to underestimation of the maximum load capacity of the joint.
<figure><img src="image_27.png" /><figcaption>(a) System 1</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>(b) System 2</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>(c) System 3</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>(d) System 4</figcaption></figure>

<figure><img src="image_31.png" /><figcaption>(e) System 5</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>(f) System 6</figcaption></figure>

**Fig. 10.** Failure mode description for specimens tested in single lap-shear tests at various temperatures. Surface area percentages are calculated over the first 10 centimeters near the loaded edge of the joint.

* Reduction in shear stress at failure (unfavourable effect): The shear stress at which damage occurs decreases with increasing temperature.

These effects are antagonistic; while softening increases the maximum force that a bonded composite reinforcement can withstand, a reduction in the shear stress tends to decrease it.

Overall, the results show that CFRP systems exhibit an increase in the bonded joint's maximum load capacity as temperature rises within the

range of 23 °C to T<sub>g</sub> – 15 °C (or T<sub>g</sub> – 10 °C for some systems), primarily due to the softening effect. However, beyond this range, the load capacity decreased as shear stress degradation became the dominant factor. At T<sub>g</sub> + 10 °C, the maximum load capacity falls below its initial value at 23 °C for all CFRP systems, with a further decline observed at T<sub>g</sub> + 15 °C.

Beyond the observed trends, which are similar across the CFRP systems tested, significant differences may exist in the extent of variations in the mechanical behaviour of the bonded joint and the temperatures at
<table><caption>Table 5 Design values for CFRP systems from present work and selected literature studies.</caption><thead><tr><th></th><th></th><th>System 1</th><th>System 2</th><th>System 3</th><th>System 4</th><th>System 5</th><th>System 6</th><th>Leone et al. 2019</th><th>Ferrier et al. 2026 System 1</th><th>Ferrier et al. 2016 System 2</th><th>Carlos et al. 2018</th></tr></thead><tbody><tr><td>f<sub>cm</sub></td><td>[MPa]</td><td>40</td><td>40</td><td>40</td><td>40</td><td>40</td><td>40</td><td>30</td><td>30</td><td>25</td><td>25</td></tr><tr><td>f<sub>cm</sub></td><td>[MPa]</td><td>3.51</td><td>3.51</td><td>3.51</td><td>3.51</td><td>3.51</td><td>3.51</td><td>2.9</td><td>2.90</td><td>2.56</td><td>2.56</td></tr><tr><td>f<sub>ck</sub></td><td>[MPa]</td><td>40</td><td>40</td><td>40</td><td>40</td><td>40</td><td>40</td><td>30</td><td>30</td><td>25</td><td>25</td></tr><tr><td>K<sub>c, surf</sub></td><td>[MPa]</td><td>0.74</td><td>0.74</td><td>0.74</td><td>0.74</td><td>0.74</td><td>0.74</td><td>0.69</td><td>0.69</td><td>0.67</td><td>0.67</td></tr><tr><td>f<sub>mm, surf</sub></td><td>[MPa]</td><td>2.60</td><td>2.60</td><td>2.60</td><td>2.60</td><td>2.60</td><td>2.60</td><td>2.001</td><td>2.00</td><td>1.71</td><td>1.71</td></tr><tr><td>E<sub>f</sub></td><td>[MPa]</td><td>196000</td><td>160000</td><td>170000</td><td>170000</td><td>220000</td><td>220000</td><td>226000</td><td>97000</td><td>160000</td><td>170000</td></tr><tr><td>t<sub>f</sub></td><td>[mm]</td><td>1.4</td><td>1.2</td><td>1.2</td><td>1.2</td><td>1.2</td><td>0.194</td><td>0.11</td><td>1.6</td><td>1.4</td><td>1.2</td></tr><tr><td>δ<sub>ok</sub></td><td>[MPa]</td><td>0.20</td><td>0.20</td><td>0.20</td><td>0.20</td><td>0.23</td><td>0.23</td><td>0.23</td><td>0.23</td><td>0.20</td><td>0.20</td></tr><tr><td>τ<sub>ik</sub></td><td>[MPa]</td><td>3.8</td><td>3.8</td><td>3.8</td><td>3.8</td><td>3.8</td><td>4.5</td><td>3.4</td><td>3.4</td><td>2.4</td><td>2.4</td></tr><tr><td>f<sub>br,max</sub></td><td>[MPa]</td><td>325</td><td>317</td><td>327</td><td>327</td><td>327</td><td>1081</td><td>1162</td><td>1269</td><td>235</td><td>262</td></tr><tr><td>l<sub>br,max</sub></td><td>[mm]</td><td>246</td><td>206</td><td>212</td><td>212</td><td>212</td><td>97</td><td>90</td><td>82</td><td>278</td><td>265</td></tr><tr><td>L<sub>e</sub></td><td>[mm]</td><td>198</td><td>165</td><td>170</td><td>170</td><td>170</td><td>78</td><td>73</td><td>65</td><td>209</td><td>199</td></tr></tbody></table>

<caption>Table 6</caption>

Comparison between the experimental characteristic bond strength and the design values.

<table><thead><tr><th>Reference of system</th><th>T [°C]</th><th>T-Tg [°C]</th><th>f<sub>bik</sub> [MPa]</th><th>f<sub>bik,max</sub> [MPa]</th><th>f<sub>bik</sub>/f<sub>bik,max</sub> [MPa]</th></tr></thead><tbody><tr><td rowspan="5">System 1 D</td><td>20</td><td>-41</td><td>417</td><td>325</td><td>1.28</td></tr><tr><td>41</td><td>-20</td><td>446</td><td>325</td><td>1.37</td></tr><tr><td>61</td><td>0</td><td>372</td><td>325</td><td>1.14</td></tr><tr><td>71</td><td>10</td><td>355</td><td>325</td><td>1.09</td></tr><tr><td>76</td><td>15</td><td>250</td><td>325</td><td><strong>0.77</strong></td></tr><tr><td rowspan="5">System 2 D</td><td>20</td><td>-37</td><td>443</td><td>317</td><td>1.40</td></tr><tr><td>37</td><td>-20</td><td>474</td><td>317</td><td>1.49</td></tr><tr><td>57</td><td>0</td><td>394</td><td>317</td><td>1.24</td></tr><tr><td>67</td><td>10</td><td>354</td><td>317</td><td>1.12</td></tr><tr><td>72</td><td>15</td><td>222</td><td>317</td><td><strong>0.70</strong></td></tr><tr><td rowspan="5">System 1 S</td><td>23</td><td>-38</td><td>378</td><td>325</td><td>1.16</td></tr><tr><td>46</td><td>-15</td><td>568</td><td>325</td><td>1.75</td></tr><tr><td>51</td><td>-10</td><td>559</td><td>325</td><td>1.72</td></tr><tr><td>61</td><td>0</td><td>475</td><td>325</td><td>1.46</td></tr><tr><td>71</td><td>10</td><td>368</td><td>325</td><td>1.13</td></tr><tr><td rowspan="5">System 2 S</td><td>76</td><td>15</td><td>220</td><td>325</td><td><strong>0.68</strong></td></tr><tr><td>23</td><td>-34</td><td>462</td><td>317</td><td>1.46</td></tr><tr><td>42</td><td>-15</td><td>467</td><td>317</td><td>1.47</td></tr><tr><td>47</td><td>-10</td><td>601</td><td>317</td><td>1.89</td></tr><tr><td>57</td><td>0</td><td>559</td><td>317</td><td>1.76</td></tr><tr><td rowspan="5">System 3 S</td><td>67</td><td>10</td><td>326</td><td>317</td><td>1.03</td></tr><tr><td>72</td><td>15</td><td>36</td><td>317</td><td><strong>0.11</strong></td></tr><tr><td>23</td><td>-32</td><td>362</td><td>327</td><td>1.29</td></tr><tr><td>40</td><td>-15</td><td>293</td><td>327</td><td>1.04</td></tr><tr><td>45</td><td>-10</td><td>256</td><td>327</td><td><strong>0.93</strong></td></tr><tr><td rowspan="5">System 4 S</td><td>55</td><td>0</td><td>61</td><td>327</td><td><strong>0.22</strong></td></tr><tr><td>65</td><td>10</td><td>63</td><td>327</td><td><strong>0.22</strong></td></tr><tr><td>70</td><td>15</td><td>-</td><td>327</td><td>-</td></tr><tr><td>23</td><td>-28</td><td>438</td><td>327</td><td>1.34</td></tr><tr><td>36</td><td>-15</td><td>576</td><td>327</td><td>1.76</td></tr><tr><td rowspan="5">System 5 S (b<sub>f</sub> = 100 mm)</td><td>41</td><td>-10</td><td>605</td><td>327</td><td>1.85</td></tr><tr><td>51</td><td>0</td><td>308</td><td>327</td><td><strong>0.94</strong></td></tr><tr><td>61</td><td>10</td><td>325</td><td>327</td><td><strong>0.99</strong></td></tr><tr><td>66</td><td>15</td><td>359</td><td>327</td><td>1.10</td></tr><tr><td>23</td><td>-28</td><td>2509</td><td>2162</td><td>1.16</td></tr><tr><td rowspan="5">System 6 S (b<sub>f</sub> = 100 mm)</td><td>36</td><td>-15</td><td>2641</td><td>2162</td><td>1.22</td></tr><tr><td>41</td><td>-10</td><td>2403</td><td>2162</td><td>1.11</td></tr><tr><td>51</td><td>0</td><td>2526</td><td>2162</td><td>1.07</td></tr><tr><td>61</td><td>10</td><td>1232</td><td>2162</td><td>1.07</td></tr><tr><td>66</td><td>15</td><td>824</td><td>2162</td><td><strong>0.36</strong></td></tr><tr><td rowspan="5">Leone et al. [17]</td><td>23</td><td>-26</td><td>1092</td><td>1162</td><td>0.94</td></tr><tr><td>34</td><td>-15</td><td>1621</td><td>1162</td><td>1.39</td></tr><tr><td>39</td><td>-10</td><td>1833</td><td>1162</td><td>1.58</td></tr><tr><td>49</td><td>0</td><td>3549</td><td>1162</td><td>3.05</td></tr><tr><td>59</td><td>10</td><td>1708</td><td>1162</td><td>1.47</td></tr><tr><td rowspan="5">Ferrier et al. System 1 [18]</td><td>64</td><td>15</td><td>2149</td><td>1162</td><td>1.85</td></tr><tr><td>20</td><td>-35</td><td>2395</td><td>1165</td><td>2.06</td></tr><tr><td>50</td><td>-5</td><td>2541</td><td>1165</td><td>2.18</td></tr><tr><td>65</td><td>10</td><td>2193</td><td>1165</td><td>1.88</td></tr><tr><td>80</td><td>25</td><td>1841</td><td>1165</td><td>1.58</td></tr><tr><td rowspan="5">Ferrier et al. System 2 [18]</td><td>-20</td><td>-110</td><td>195</td><td>235</td><td>0.83</td></tr><tr><td>20</td><td>-70</td><td>295</td><td>235</td><td>1.26</td></tr><tr><td>40</td><td>-50</td><td>257</td><td>235</td><td>1.09</td></tr><tr><td>50</td><td>-40</td><td>217</td><td>235</td><td>0.92</td></tr><tr><td>80</td><td>-10</td><td>200</td><td>235</td><td><strong>0.85</strong></td></tr><tr><td rowspan="5">Carlos et al. [15]</td><td>100</td><td>10</td><td>193</td><td>235</td><td><strong>0.82</strong></td></tr><tr><td>110</td><td>20</td><td>227</td><td>235</td><td><strong>0.97</strong></td></tr><tr><td>120</td><td>30</td><td>9</td><td>235</td><td><strong>0.04</strong></td></tr><tr><td>15</td><td>-60</td><td>475</td><td>262</td><td>1.82</td></tr><tr><td>40</td><td>-35</td><td>366</td><td>262</td><td>1.40</td></tr><tr><td>65</td><td>-10</td><td>282</td><td>262</td><td>1.08</td></tr><tr><td>90</td><td>15</td><td>140</td><td>262</td><td><strong>0.53</strong></td></tr><tr><td>115</td><td>40</td><td>96</td><td>262</td><td><strong>0.37</strong></td></tr><tr><td>140</td><td>65</td><td>44</td><td>262</td><td><strong>0.17</strong></td></tr></tbody></table>
<figure><img src="image_33.png" /><figcaption>Fig. 11. Characteristic bond strength normalized by design value, determined using Eurocode 2 and Fib bulletin 90 recommendations, e plotted as a function of (T-Tg).</figcaption></figure>

which they occur. These differences are likely due to factors like the nature of the adhesives used, types and magnitudes of the applied loads, and other system-specific variables.

Another crucial observation is that the design equation from Eurocode 2 can be safely applied up to $T_g - 10$ °C for all tested systems. Therefore, for design and serviceability purposes, provided the bonded length is appropriate, the temperature limit should be set at $T_g - 10$ °C.

However, for certain CFRP systems, the design equation remains conservative up to $T_g$. This result suggests that a higher-temperature service limit may be considered for specific systems provided if they demonstrate acceptable bond performances and if the FRP system is not submitted to creep load to limit the viscoelastic behaviour of the adhesive at this level of temperature. In this case and under creep load, the delay behaviour of the bonded FRP should be analysed first. A proposed criterion is that the reduction in the characteristic bond strength should be less than 10 % of the value at ambient temperature (e.g. around 20 °C).

Further modelling could be explored to better interpret the results and gain a deeper understanding of the effects of temperature on CFRP-strengthened structures.

## Declaration of Competing Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Emmanuel Ferrier reports was provided by Université Claude bernard Lyon 1 laboratoire LMC2 EZUS LYON 1. Emmanuel Ferrier reports a relationship with University Claude Bernard Lyon 1 that includes: employment. Benzarti karim reports a relationship with Gustave Eiffel University that includes: employment. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

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