Scholarly article on topic 'Bond between TRM versus FRP composites and concrete at high temperatures'

Bond between TRM versus FRP composites and concrete at high temperatures Academic research paper on "Materials engineering"

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{Fabrics/textiles / "Carbon fibre" / Debonding / "High temperature"}

Abstract of research paper on Materials engineering, author of scientific article — Saad M. Raoof, Dionysios A. Bournas

Abstract The use of fibre reinforced polymers (FRP) as a means of external reinforcement for strengthening the existing reinforced concrete (RC) structures nowadays is the most common technique. However, the use of epoxy resins limits the effectiveness of FRP technique, and therefore, unless protective (thermal insulation) systems are provided, the bond capacity at the FRP-concrete interface will be extremely low above the glass transition temperature (T g ). To address problems associated with epoxies and to provide cost-effectiveness and durability of the strengthening intervention, a new composite cement- based material, namely textile-reinforced mortar (TRM) has been developed the last decade. This paper for the first time examines the bond performance between the TRM and concrete interfaces at high temperatures and, also compares for the first time the bond of both FRP and TRM systems to concrete at ambient and high temperatures. The key parameters investigated include: (a) the matrix used to impregnate the fibres, namely resin or mortar, resulting in two strengthening systems (TRM or FRP), (b) the level of high temperature to which the specimens are exposed (20, 50, 75, 100, and 150 °C) for FRP-reinforced specimens, and (20, 50, 75, 100, 150, 200, 300, 400, and 500 °C) for TRM-strengthened specimens, (c) the number of FRP/TRM layers (3 and 4), and (d) the loading conditions (steady state and transient conditions). A total of 68 specimens (56 specimens tested in steady state condition, and 12 specimens tested in transient condition) were constructed, strengthened and tested under double- lap direct shear. The result showed that overall TRM exhibited excellent performance at high temperature. In steady state tests, TRM specimens maintained an average of 85% of their ambient bond strength up to 400 °C, whereas the corresponding value for FRP specimens was only 17% at 150 °C. In transient test condition, TRM also outperformed over FRP in terms of both the time they maintained the applied load and the temperature reached before failure.

Academic research paper on topic "Bond between TRM versus FRP composites and concrete at high temperatures"

Accepted Manuscript

Bond between TRM versus FRP composites and concrete at high temperatures Saad M. Raoof, Dionysios A. Bournas

PII: S1359-8368(17)30417-1

DOI: 10.1016/j.compositesb.2017.05.064

Reference: JCOMB 5087

To appear in: Composites Part B

Received Date: 4 February 2017 Revised Date: 7 April 2017 Accepted Date: 21 May 2017

Please cite this article as: Raoof SM, Bournas DA, Bond between TRM versus FRP composites and concrete at high temperatures, Composites Part B (2017), doi: 10.1016/j.compositesb.2017.05.064.

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Bond between TRM versus FRP Composites and Concrete at High Temperatures

Saad M. Raoofa,b* and Dionysios A. Bournasc

a Department of Civil Engineering, University of Nottingham, NG7 2RD, Nottingham, UK b Department of Civil Engineering, University of Tikrit, Iraq- Tikrit

c European Commission, Joint Research Centre (JRC), Directorate for Space, Security and Migration, Safety and Security of Buildings Unit, via E. Fermi 2749, I-21027 Ispra, Italy.

Corresponding author: E-mails: Saad.Raoof@nottingahm.ac.uk; dionysios.bournas@ec.europa.eu

Abstract:

The use of fibre reinforced polymers (FRP) as a means of external reinforcement for strengthening the existing reinforced concrete (RC) structures nowadays is the most common technique. However, the use of epoxy resins limits the effectiveness of FRP technique, and therefore, unless protective (thermal insulation) systems are provided, the bond capacity at the FRP-concrete interface will be extremely low above the glass transition temperature (Tg). To address problems associated with epoxies and to provide cost-effectiveness and durability of the strengthening intervention, a new composite cement- based material, namely textile-reinforced mortar (TRM) has been developed the last decade. This paper for the first time examines the bond performance between the TRM and concrete interfaces at high temperatures and, also compares for the first time the bond of both FRP and TRM systems to concrete at ambient and high temperatures. The key parameters investigated include: (a) the matrix used to impregnate the fibres, namely resin or mortar, resulting in two strengthening systems (TRM or FRP), (b) the level of high temperature to which the specimens are exposed (20, 50, 75, 100, and 150 0C) for FRP-reinforced specimens, and (20, 50, 75, 100, 150, 200, 300, 400, and 500 0C) for TRM-strengthened specimens, (c) the number of FRP/TRM layers (3 and 4), and (d) the loading conditions (steady state and transient conditions). A total of 68 specimens (56 specimens tested in steady state condition, and 12 specimens tested in transient condition) were constructed, strengthened and tested under double- lap direct shear. The result showed that overall TRM exhibited excellent performance at high temperature. In steady state tests, TRM specimens maintained an average of 85% of their ambient bond strength up to 400 0C, whereas the corresponding value for FRP specimens was only 17% at 150 0C. In transient test condition, TRM also outperformed over FRP in terms of both the time they maintained the applied load and the temperature reached before failure.

Keywords: Fabrics/textiles, Carbon fibre, Debonding, High temperature

1 Introduction and background

There is a growing need for upgrading the existing reinforced concrete (RC) structures both in seismic and non-seismic areas. This is attributed to deterioration of RC structures as a result of ageing, inadequate maintenance, environmental induced degradation but also due to the increase of the applied loads and the need to comply with the modern standards (for example Eurocodes) requirements. The use of Fibre-Reinforced Polymer (FRP) as external strengthening system has gained high popularity among other techniques. This is due to the favorable properties offered by FRP such as resistance to corrosion, high strength to weight ratio, ease and speed of application and minimal change in the geometry. However, due to the epoxy resins used in these composites, the FRP systems are usually expensive, cannot be applied at low temperatures or wet surfaces, and have very poor performance at high temperature, as under loading epoxy resins normally lose their tensile capacity. Therefore, unless protective (thermal insulation) systems are not provided [1], the bond strength between the FRP and concrete substrate will be extremely low above the glass transition temperature (Tg). A review on the behaviour RC members strengthened with FRPs and subjected to fire or high temperature was recently conducted by Firmo et al. [2].

To address the problems of FRP, a novel composite material called textile-reinforced mortar (TRM) has been introduced since last decade, for structural strengthening of existing structures [3]. TRM consists of textile fibre reinforcement (with open-mesh configuration) combined with inorganic matrices (i.e. cementitious mortars). The acronym 'TRC' [4] or 'FRCM' [5] is also used in the literature for the same material. TRM is a low-cost, resistant at high temperature, compatible to masonry or concrete substrates and friendly for manual workers material, which can be applied at low temperatures or on wet surfaces. Therefore, the use of TRM is becoming more attractive for the retrofitting of existing concrete or masonry structures than FRP. A number of studies have demonstrated that TRM is an effective

technique for the flexural strengthening of beams [i.e. 6-9], one way [i.e. 10, 11], and two way slabs [12]; the shear upgrading of RC beams [i.e. 13, 14]; the seismic retrofitting of RC columns (e.g. [15- 20]); and the seismic retrofitting of infilled RC frames [21].

The effectiveness of externally bonded FRP or TRM systems depends primarily on the bond at the composite-concrete interface. At high temperatures or in case of a fire, the bond between FRP and concrete becomes negligible, reducing dramatically the performance of the FRP technique. The bond between FRP and concrete at high temperature has been addressed in Refs. [22-25]; in these studies, double-lap direct shear tests were conducted on concrete blocks externally strengthened with Carbon fibre reinforced polymers (CFRP). The specimens were exposed to a predefined temperature varied between 20 and 120 0C and then tested up to failure. It was demonstrated that the bond between FRP materials and concrete significantly deteriorated when the temperature of adhesive is equal or above Tg.

TRM could outperform FRP systems at high temperatures or fire due to the breathability, non-combustibility, and non- flammability offered by mineral-based cement mortars used as binding materials. Until now, the bond between TRM materials and concrete substrate has been addressed only at ambient temperatures [i.e. 26-28] and no single study exists at high temperatures or fire. In general, the research on the performance of TRM systems at elevated temperature or fire and the comparison between TRM and FRP systems at high temperature or fire is extremely limited [29- 33]. This is attributed to the inherent experimental difficulties applying simultaneously loading and high temperature, even for medium or small-scale specimens. For this reason, the past studies mainly evaluated the residual strength of TRM after being exposed to high temperatures and cooled down to the ambient temperature. Particularly in [29-31] uniaxial tensile tests were conducted on TRM coupons made of glass [29], carbon [30], and basalt [31] textile fibres. The test procedure

84 included the following steps: (a) exposure to elevated temperatures of 20, 200, 400, and 600

85 0C [29]; 20, 100, 150, 200, 400, and 600 0C [30]; and 20, 75, 150, 200, 300, 400, 600, and

86 1000 0C [31]; (b) exposing the specimens at these temperatures for: 2 hrs [29], 3 hrs [30], and

87 1 hr [31] (stabilizing phase); (c) cooling down the specimens to the ambient temperature; and

88 (d) conducting a uniaxial tensile test up to failure. The main conclusion of these studies was

89 that TRM coupons maintained their ambient tensile strength at high temperature up to 200 0C

90 [29, 30], and 150 0C [31]. However, above these temperatures, the residual tensile strength

91 was gradually decreased due to the deterioration of tensile strength of the textile fibres

92 themselves.

93 The only studies reported in the literature on TRM versus FRP as strengthening materials

94 at high temperature is that of Bisby et al. [32] and Tetta and Bournas [33], who did flexural

95 and shear strengthening of RC beams, respectively. In [32], a sustained load was applied on

96 medium-scale beams, and then the temperature was increased (except from anchorage zones

97 where they kept cold) up to failure. In [33], medium and full-scale beams were heated up to

98 predefined temperature (20, 100, 150 and 250 0C) and then loaded monotonically up to

99 failure. In [32], it was concluded that both strengthening systems (TRM and FRP) can have

100 same performance at high temperature if the anchorage zones of the beams kept cold.

101 Whereas, in [33] it was found that TRM exhibited superior performance over FRP at high

102 temperature where the effectiveness of the latter dropped to about zero when the temperature

103 at the concrete/adhesive reached Tg.

104 This paper investigates experimentally, for the first time the bond between TRM and

105 concrete substrates at high temperatures. Furthermore, it compares for the first time the bond

106 strength of TRM vs FRP with concrete substrates at different elevated temperatures and

107 loading conditions. The investigated parameters include: (a) the number of layers (three and

108 four layers); (b) the elevated temperatures (20, 50, 75, 100 and 150 0C for FRP strengthened

specimens and 20, 50, 75, 100, 150, 200, 300, 400 and 500 0C for TRM strengthened specimens) and; (c) the loading condition (steady-state and transient conditions).

2 Experimental programme

2.1 Test Specimens and investigated parameters

The main aim of this study was to compare the bond of two strengthening systems namely, FRP and TRM with concrete at different elevated temperatures and loading conditions. In total 68 specimens (34 twin specimens as a measure to reduce the scatter of the results) were constructed, strengthened and tested under direct tensile test. The details of the specimens are provided in Fig. 1a-f. Each specimen comprised two RC prisms with dimensions of 100x100mm cross section and 265 mm length. The two prisms were connected only by FRP/TRM layers which were bonded on two opposite sides of the prisms.

The procedure for specimen's preparation was as follows: an acrylic plate with dimensions of 100x100 mm cross sectional was fixed at the middle of a steel mould (Fig. 1a) in order to isolate the two prisms during casting stage. The acrylic plate provided with two acrylic rods with 10-mm diameter fixed at the position shown in Fig. 1b in order to create holes into concrete mass of each prism. Each prism was reinforced with a steel cage with the details shown in Fig. 1c in order to prevent the failure of prisms due to concrete splitting during the test. A 16-mm bar was fitted at the centre of each prism in order to allow for the application of the load during the test (Fig. 1 d). After 24 hour of casting, the specimen (two prisms) was removed from the mould, the acrylic plate was remove from the central zone, and the two prisms were reconnected to each other's using a 10-mm diameter aluminium rods (Fig. 1d) that were inserted into the premade holes (Fig. 1d). The purpose of these two aluminium rods was to ensure fully alignment between the two prisms and reduce the error in the measurements resulted from the possible bending of specimen due to misalignment

between the two prisms. Finally, full details about the design of test specimen including a free body diagram of the tested side of the specimen are provided in Fig. 1e and f, respectively.

A number of parameters were investigated in this study comprising: (a) the matrix used to impregnate the fibres, namely resin or mortar, resulting in two strengthening systems (TRM or FRP), (b) the temperature to which the specimens were exposed (50, 75, 100, 150 0C) for FRP and (50, 75, 100, 150, 200, 300, 400 and 500 0C) for TRM retrofitted specimens (c) the number of layers (3 and 4), and (d) the loading condition , namely steady state test and transient test conditions. In the steady state test, 28 twin specimens were heated up to a predefined temperature (see Table 1), kept at this temperature for 60 min., and then loaded monotonically to failure. In the transient test, 12 twin specimens were first loaded (at ambient temperature) up to a load fraction equal to 25%, 50%, and75% of the bond strength of the corresponding specimens tested at ambient temperature and then the specimens were heated up to failure.

The specimens' notation is BN_T, where B represents the type of bonding agent (R for epoxy resin and M for cement mortar), N refers to the number of FRP/TRM layers, whereas T denotes the exposed temperature for steady state tests, and the loading fraction of specimens tested at ambient for transient test condition. For example, M4_400 refers to a specimen strengthened with 4 TRM layers and tested monotonically (in steady state condition) at 400 0C; whereas, M4_75% denotes to a 4 layers TRM specimen, subjected to a load fraction of 75% of the bond strength measured at ambient temperature, and then exposed to high temperature up to failure. Details for each parameter of all specimens are presented in Table 1.

Note that the bond length of FRP/TRM reinforcement was the same and equal to 200 mm for all tested specimens (see Fig. 1e). This length was selected on the basis a previous study of

the authors [28], where it was found that the effective bond length (for 3-4 strengthening carbon layers) was approximately equal to 200 mm and 150 mm for the TRM and FRP systems, respectively, as illustrated in Fig. 2.

2.2 Materials and strengthening procedure

The specimens were cast in four different groups using the same mix design. The concrete compressive strength was obtained on the day of the testing. Table 1 reports the value of the concrete compressive strength (average of three 150 mm cubes).

A high strength carbon fibres textile was used as an external reinforcement which comprised equal quantity of rovings in both in the two orthogonal directions. The mesh size, the weight, and the nominal thickness is illustrated in Fig. 3. It is noted that the nominal thickness was calculated based on the equivalent smeared distribution of fibres. Uniaxial tensile tests were conducted on coupons made of bare carbon fibres textile in order to determine its tensile behaviour in the loading direction. The average calculated tensile strength, ultimate strain, and modulus of elasticity were 1518 MPa, 0.911%, and 166.8 GPa, respectively.

For the specimens received TRM as strengthening materials, an inorganic modified cement mortar was used as a bonding agent. This cement mortar was consisted of cement and polymers. The ratio of cement to polymers was 8:1 by weight. The water-cement ratio of the mortar was 0.23:1 by weight. This ratio resulted in a mortar with a good workability and plastic consistency. The compressive and flexural strength of the cement mortar both at ambient and high temperature were experimentally obtained on the day of testing. Three mortar prisms with dimensions of 40x40x160 mm were used to determine the compressive and flexural strength. The prisms were fixed in the furnace as shown in Fig. 4, heated up to the desired temperature, kept for one hour at this temperature, and then tested according to the EN 1015-22 specifications [34]. Table 1 reports the results of compressive and flexural

183 strength of the mortar prisms (average value from 3 prisms). For the specimens retrofitted

184 with FRP, a commercial epoxy resin (Sikadur® _330) comprising two-part with a mixing

185 ratio of 4:1 by weight,) was used. The tensile strength, the modulus of elasticity, and the Tg of

186 the epoxy resin were 30 MPa, 3.8 GPa, and 68 0C, respectively, according to the

187 manufacturer datasheets.

188 The strengthening procedure for both strengthening systems had the characteristics of a

189 typical wet lay-up application and comprised the following steps:

190 • Prior to the application of the strengthening materials (TRM or FRP), the concrete surface

191 was prepared as follows: (a) for FRP-strengthened specimens, a thin layer of the concrete

192 cover was removed followed by roughening the surface, and the resulted concrete surface

193 was cleaned from dust (Fig. 5a); (b) for TRM-strengthened specimens, after removing the

194 thin layer of concrete, a 50-mm mesh of grooves with a depth of approximately 2-3 mm

195 was created. Then, the resulted surface was cleaned with compressed air, followed by

196 dampening with water before applying the strengthening (Fig. 5b).

197 • Before application of strengthening materials, a 100 mm-long central zone was wrapped

198 with a foil tape (Fig. 5 a, b) in order to isolate the strengthening materials from the

199 concrete prisms at this zone and prevent any possible attachment with the concrete

200 surface.

201 • For TRM-retrofitted specimens, the first mortar layer (approximately 2 mm-thick) was

202 applied (Fig. 5c); followed by the application of the first layer of textile. (Fig. 5d). For

203 specimens received FRP, the first layer of the textile fibres was applied on a thin layer of

204 epoxy resin and impregnated using a plastic roll (Fig. 5e).

205 • The above procedure was repeated until the required number of layers (3 or 4 layers) was

206 applied.

• Finally, for the specimens retrofitted with TRM, the last textile layer was covered and levelled with an external layer of mortar (Fig. 5f).

Note that the bond width of FRP/TRM reinforcement was the same for all tested specimens and was equal to 80 mm.

2.3 Test setup, instrumentation and procedure

The specimens were positioned inside a furnace with inner chamber dimensions of 600 mm x 400 mm x 400 mm and maximum temperature capacity of 600 0C. The furnace was installed into a universal testing machine of 250 kN capacity, as shown in Fig. 6a. The instrumentations used for specimens tested in steady state condition included: (i) Two high temperature LVDTs, fixed to the specimens' un-strengthened sides to measure the relative displacement between the two prisms (Fig. 6a and b); (ii) two thermocouples type-K with diameter of 1.2 mm, fixed at the matrix- concrete interface and located at the positions shown in Fig. 1e and Fig. 6b to monitor the temperature at this interface; (iii) Five high temperature strain gages were mounted to the surface of TRM along the tested bond length to measure the strain distribution. Two steel clamps were fixed to the not-instrumented side of the specimens as shown in Fig. 6a, b. The purpose of these clamps was to prevent the failure in the un-instrumented side and ensure that the failure would occur in the instrumented side. As can also be observed in Fig. 6a, the specimen was encased in a steel box to protect the furnace in case of explosion.

For specimens tested in steady state condition the following steps were adopted: (a) positioning of the specimens inside the furnace and fixing only to the upper grip of the testing machine (Fig. 6a); (b) heating up to the predefined target temperature described in Table 1, with an average heating rate of 5.25 0C/min, and keeping the target temperature constant for

60 min. (Fig. 7); (c) fixing the lower grip of the testing machine; and (d) monotonic loading up to failure, under displacement control with a rate of 0.2 mm/min.

For specimens tested in transient condition, the following procedure was carried out: (a) positioning in the furnace (at ambient temperature) and fixing to the machine grips; (b) loading up to the targeted load fraction of 25%, 50%, and 75% of the average ultimate load recorded for the specimens tested at ambient temperature; (c) heating the specimens with the same heating rate (5.25 0C/min) up to failure. An extractor was used to remove the smoke if was released as a result of heating the specimens up.

Finally, it is worth mentioning that the FRP/TRM reinforcement was left un-bonded at 100 mm-long central zone (50 mm at each prism) of the specimen (Fig. 6b) to prevent from edge failure of the concrete prisms. Furthermore, the bond width of FRP/TRM reinforcement was the same for all tested specimens and was equal to 80 mm s (see Fig. 6b).

3 Experimental results

Fig. 1f shows the free body diagram of the tested side of the specimen. By assuming perfect symmetry of the specimen (up to peak load), the interface between the FRP/TRM strip and concrete in each side of the tested part will carry half of the measured ultimate load (Pmax). The relative displacement between the two concrete prisms measured at ultimate load will be the average of the two LVDTs' readings; Smax= (S1+S2J/2.

The main experiment results of all specimens tested in both loading conditions are presented in Tables 2 and 3. Table 2, reports the results of the steady state test including: (1) the ultimate load (Pmax) recorded for twin specimens S1 and S2; (2) the relative displacement (¿max). recorded at the ultimate load (Pmax); (3) the value of average load (Pav) of the twin specimens; (4) the average displacement (Sav) of the twin specimens; (5) the ratio of high to

H T AT

ambient temperature bond strength, expressed as Pmax ' ' IPmax ' ' to quantify the effect of high temperatures on the bond strength; (6) the average bond strength developed at the concrete-adhesive interface, calculated as (PavI2)ILb, (7) the average tensile stress (st) developed in the textile, calculated as (PavI2)Intb, and (8) the observed failure mode. Where L, and b is the bond length and width (L= 200, and b= 80 mm), respectively, n is the number of TRM layers, and t is the equivalent thickness of the textile in the longitudinal direction (t= 0.095mm).

Table 3, lists the results of the transient condition tests including: (1) the constant load (25%, 50% or 75% of the ambient temperature strength) in which specimens were subjected; (2) the time required to reach failure for both twin specimens S1 and S2; (3) the corresponding average time for the twin specimens; (4) the temperature reached at the concrete-matrix interface at failure for twin specimens S1 and S2; (5) the corresponding average temperature; and (6) the observed failure mode.

The measurements of the strain gages at high temperatures were and not reliable and therefore are not presented.

3.1 Temperature profile

Fig. 8 presents a typical temperature- time curve obtained from the two thermocouples affixed at the concrete- matrix interface, for a specimen tested in steady state condition and heated up to 400 0C. Since the readings (in all tests) were identical, the average (of the two thermocouples) temperature was used. Fig. 9 displays the actual temperature- time curves for all FRP and TRM- strengthened specimens tested in steady state condition. It can be observed that (a) the heating rate is identical between all specimens and (b) all specimens were exposed to predefined temperature for one hour before application of the load, and then tested under displacement control up to failure. Note that the consistency in the heating procedure for all tested specimens is important to reduce errors, obtain reliable, and comparable results.

3.2 Load- displacement curves

Fig. 10 presents the load-displacement curves of all FRP/TRM strengthened specimens tested in steady-state condition. For better clarity, only one of the twin specimen's curves is presented in this figure. Moreover, they were grouped on the basis of the strengthening materials used and number of layers. Starting from FRP-retrofitted specimens (Fig. 10a and c), the load vs displacement curves were characterised by a linear ascending branch with progressive decreasing in the stiffness due to softening of the (concrete-resin) interface up to failure. On the other hand, the TRM-strengthened specimens' curves were characterized by two ascending branches. The first ascending branch was linear with high axial stiffness up to mortar cracking, followed by a nonlinear one with progressively decreasing stiffness up to failure (Fig. 10.b and d).

Fig. 11a and b, depicts the increase of the crosshead displacement and the average temperature at the concrete - adhesive interface with time, for specimens strengthened with 4 FRP and TRM layers, respectively, and tested in transient condition. The initial part of the curves shows the stage of loading to reach the predefined load fractions (25%, 50%, or 75% of the ambient load); whereas the second part represents the increase of the cross-head displacement due to the heating of the specimens up to failure.

3.3 Loading condition

3.3.1 Steady state condition: ultimate load and failure mode

For the FRP retrofitted specimens, the ultimate load recorded (average of two specimens was: (a) 51.3, 30.0, 17.9, 14.7, and 9.1 kN, and (b) 62.2, 40.6, 22.6, 15.8, and 9.8 kN, for the specimens strengthened with 3 and 4 layers, at the temperatures of 20, 50, 75, 100, and 150 0C, respectively. For the TRM-retrofitted specimens the ultimate load attained was equal to: (a) 36.0, 29.3, 26.5, 29.4, 30.9, 26.2, 35.9, 35.4, and 17.9 kN; and (b) 41.5, 34.0, 34.4, 36.2,

36.5, 36.9, 38.9, 39.2, and 23.1 kN (average of two specimens) for specimens reinforced with 3 and 4 layers of TRM and testes at ambient, 50, 75, 100, 150, 200, 300, 400, and 500 0C, respectively (see Table 2).

Two types of failure modes were observed for FRP-strengthened specimens: (a) deboning of FRP from the concrete substrate including parts of the concrete cover being peeled off (Fig. 12a- d), and (b) adhesive failures at the concrete- resin interface (Fig. 12e - j). The first failure mode occurred in all FRP-strengthened specimens tested at 20 0C and 50 0C. On the other hand, when the temperature increased to 75, 100 and 150 0C, adhesive failure at the concrete-resin interface occurred for all specimens, due to the poor bond behaviour of epoxy resin at temperature above the Tg. On the contrary, for all TRM-retrofitted specimens, regardless of the number of layers, the only observed failure mode was debonding of TRM from the concrete substrate accompanied with parts of concrete cover (Fig. 13a-i, and Fig. 14a-i).

3.3.2 Transient test: time, temperature at failure, and failure mode

As reported in Table 3, the average time and temperature at failure for FRP- reinforced specimens were: 19.0 min, 17.0 min, and 12.3 min and 96.3 0C, 70.7 0C, and 48.9 0C, respectively, for specimens loaded up to 25%, 50%, and 75% of their ambient bond strength. The corresponding values of TRM- retrofitted specimens (M4_25%, M4_50%, and M4_75%) were significantly higher namely, 62.0 min, 58.8 min, and 19.5 min and 319.5 0C, 310.4 0C, and 77.3 0C.

Adhesive failure at the concrete- resin interface (Fig. 15a) was observed for FRP-strengthened specimens subjected to the low load fraction (R4_25%). Whereas debonding of FRP from the concrete substrate with including parts of concrete cover (Figs 15b, c) was noted for the moderate and high load fractions (R4_50%, and R4_75%). These failure modes

were essentially related to temperature developed at the interface at the onset of failure, namely debonding and adhesive failures for temperatures below and above the Tg, respectively. For TRM strengthened specimens, premature adhesive failure modes were prevented due to the better resistance of mortar than resin at temperatures above Tg, with all specimens failing due to debonding including part of the concrete cover (Fig. 15d-f).

4 Discussion

In terms of the various parameters investigated in this experimental programme, an examination of the results (Tables 2, 3) revealed the following information.

4.1 Matrix materials (TRM vs. FRP)

The matrix material (epoxy resin or mortar) significantly affects the bond performance of FRP and TRM composites with concrete at ambient and especially at high temperatures. At 20 0C, although both FRP and TRM-strengthening specimens failed due to debonding including part of concrete cover, the bond performance of FRP- strengthened specimens was considerably better than TRM ones. The bond strength of 3 and 4 layers FRP specimens was 1.4, and 1.5 times higher than that of counterpart TRM specimens respectively, (see Table 2). This is attributed to the excellent bond between FRP composite and concrete substrate which is confirmed by the amount of concrete being peeled off (see Fig. 12a and c for FRP specimens and Figs. 13a and Fig. 14a for TRM specimens). However, at high temperatures, the TRM system exhibited excellent bond performance with concrete, which was superior to that of FRP systems. In particular, in steady- state tests, the TRM specimens retained an average of 85% of their ambient bond strength up to 400 0C. On the contrary, the FRP systems maintained approximately 17% of their ambient bond strength at 150 0C due to the premature adhesive bond failure at the concrete-resin interface. In the next sections a

comparison between the effectiveness of FRP vs. TRM materials at high temperatures is made in terms of the exposed temperature, the number of layers, and the loading condition.

4.2 Temperature

Fig. 16a shows the variation of the ultimate load with both the temperature and the number of layers for all specimens tested in steady-state condition. The bond of the FRP strengthening system to the concrete substrate was dramatically reduced with the temperature increase. In specific, the average bond strength was decreased by 42, 65, 71, and 82%; when the temperature increased from 20 to 50, 75, 100, and 150 0C, for specimens strengthen with 3 FRP layers. The corresponding decreases in the case of 4 layers were almost identical, namely 35, 64, 75 and 84%. Similar observations were made by Firmo et al. 2015 [2], where the reductions in the bond strength were 68 and 77% when the measured temperature at the concrete-adhesive interface of FRP-strengthened specimens was 90 and 120 0C, respectively. Also, the current results, are in agreement with those of Tetta et al. 2016 [33], where the contribution of FRP U-jackets in resisting shear forces in RC strengthened beams decreased by 60 and 88% (compared to the strengthened beam tested at 20 0C) when the beams heated up to 100 and 150 0C, respectively, due an identical adhesive bond failure mode at the concrete - resin interface.

For TRM specimens, regardless the number of layers, the curves in Fig. 16a clearly demonstrate that the effectiveness of TRM in transferring the load is not significantly affected by increasing the temperature up to 400 0C. Compared to the bond strength at 20 0C, the average reduction in the bond strength was 19, 27, 18, 14, 27, 0, 2, and 50%; for the specimens subjected to temperatures of 50, 75, 100, 150, 200, 300, 400, and 500 0C, respectively, and strengthened with 3 TRM layers. The corresponding reductions for 4 TRM layers were equal to 18, 17, 13, 12, 11, 6, 6, and 44 %.

A fluctuation in the bond strength was noted at temperatures varied between 50 and 200 0C, and this could be attributed to the corresponding mechanical properties of the used cement mortar. As shown in Fig. 16b, the flexural and compressive strength of the mortar considerably deteriorated, possibly due to water vapouring process which occurred at these ranges of temperatures. However, above 200 0C, an enhancement in the TRM bond strength was observed (Fig. 16a) resulting in marginal bond reductions in comparison with the ambient strength, namely equal to 3 and 4% when the temperature attained 300 and 400 0C, respectively. The highest reduction in the bond strength was 48% for TRM specimens tested at 500 0C (Fig. 16a) seems to be attributed to the reduced tensile and compressive strength of the mortar by 87% and 68% at that temperature (Fig. 16b).

The observation that the reduction of bond strength is associated with the mortar strength is better explained if someone compares the quantity of concrete being peeled off. All TRM-strengthened specimens tested at ambient and high temperature failed due to deboning, but the concrete cover detached at high temperature was thinner than the cover detached at ambient (see Fig. 13c vs. Fig. 13a), indicating the effect of the tensile strength of the mortar on the bond strength even for failure at the concrete substrate.

Finally, an attempt was made to examine the bond performance of TRM at 600 0C; however, when the interface temperature reached 550 0C, the specimen failed due to spalling of the concrete cover in an explosive manner. It is worth noting though that the TRM was still bonded to the concrete substrate even after the specimen's failure as illustrated in Fig. 17. Such a type of failure was also observed by Chowdhury et al. 2007 [35] in column tests under fire scenario.

4.3 Influence of the number of layers

As depicted in Fig.16a, when the number of layers increased from 3 to 4, the ultimate load increased by 1.21 and 1.15 for FRP and TRM specimens tested at ambient temperatures,

respectively. However, at high temperatures, the influence of the number of layers on the bond strength was more pronounced for the TRM than FRP specimens. As shown in Fig.16a, for FRP specimens, the effect of number of layers on the bond strength was almost disappeared above the Tg, as it was controlled by the properties of the epoxy resin.

The influence of the number of TRM layers on the bond strength was not that clear, with specimens receiving 4 layers having an overall higher bond strength for all temperatures investigated. It is worth mentioning that Rambo et al. 2015 [31] observed similar results in TRM coupon tensile test, in which the tensile behaviour at high temperature of TRM coupons made of 3 and 5 fabric layers was better than the tensile performance of a TRM coupon made of one layer. Tetta and Bournas. 2016 [33] concluded that by increasing from 2 to 3 TRM layers the bond of TRM to concrete at high temperatures increases considerably.

4.4 Loading conditions

As it can be observed in Figs 19 for the transient tests, when the load fraction level was increased, the time to reach failure was decreased and consequently the temperature did, for both FRP and TRM specimens strengthened with 4 layers. Also, it is illustrated that the TRM outperformed their FRP counterparts for all load fractions. Particularly, the time required to reach failure of the TRM specimens was 3.3, 3.5 and 1.58 times higher for the low, moderate and high load fractions, respectively. Correspondingly, the attained temperature at failure was 3.3, 4,4 and 1.58 higher in the TRM-strengthened prisms.

Another interesting observation from Fig. 18a is that the bond strength attained at different temperatures was nearly identical for both loading conditions for the FRP-strengthened specimens. This confirmed that the temperature at the concrete- resin interface controlled the bond behaviour rather than the loading condition, as also reported by Firmo et al. 2015 [2]. This was not the case for the TRM system which was sensitive to the loading

conditions. In fact, the TRM specimens had increased bond strengths at higher temperatures in the steady state in respect with the transient tests. As illustrated in Fig. 18b, the measured bond strength of M4_300 which was subjected to 300 0C, was almost double and triple the predefined bond strengths of specimens M4_50% and M4_25%, respectively which failed at around 300 0C.

5 Conclusions

This paper investigates the bond between TRM vs. FRP and concrete substrates at high temperatures for the first time. The investigated parameters included the strengthening system (TRM vs FRP), the exposure temperature, the number of FRP/ TRM layers, and the loading conditions. For this purpose, 68 specimens were constructed, strengthened, and tested under double-lap direct shear at ambient and high temperatures. The main findings of the current study are summarized below:

1. The bond between the TRM strengthening system and concrete substrate remains excellent at high temperatures.

2. In steady state tests the reduction in bond strength of FRP-strengthened specimens was significantly higher than for the TRM-retrofitted specimens with the increase of the temperature. The average reduction in the bond strength of FRP-concrete interface was about 83% when the temperature reached 150 0C. Whereas the corresponding values in TRM-concrete interface was about 15% when the temperature attained 400 0C.

3. Two types of failure modes were observed in the FRP strengthened specimens tested in steady state condition. At ambient and moderate temperature (50 0C), cohesive failure was observed; in which parts of concrete cover remaining attached to the adhesive. Whereas, at elevated temperatures (i.e. 75, 100, and 150 0C), adhesive failure at the concrete-resin interface was occurred. On the other hand, for TRM specimens subjected to temperatures

446 (up to 500 0C), the failure was due to TRM debonding with parts of concrete cover peeling

447 off.

448 4. The bond strength at the FRP-concrete interface was nearly identical for the same

449 temperature regardless of the loading condition (transient or steady state). On the contrary,

450 the bond behaviour at the TRM-concrete substrate was sensitive to the loading condition,

451 and resulted to considerably higher bond strengths (for nearly the same temperature) in the

452 steady state in respect with the transient tests.

453 Further research is required to investigate the bond between TRM made of different

454 types of textile fibres materials and concrete at high temperature.

455 Acknowledgments

456 This work has been co-financed by the UK Engineering and Physical Sciences Research

457 Council (EP/L50502X/1) and the Higher Committee for Education Development in Iraq

458 (HCED).

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559 FRP-wrapped reinforced concrete columns in fire. Fire safety journal. 2007, 42 (6);

560 452-460.

561 Table 1. Specimens details, concrete compressive strength, and mortar properties on the day

562 of testing

Mortar

Specimen Temp. (0C) Number of layers compressive strength (MPa) Flexural strength (MPa)* Compressive strength (MPa)*

M3_201 Ambient 3 9.9 (0.3)* 39.9 (2.1)*

M3_50 50 3 3.93 (0.07)* 20.8(2.2)*

M3_75 75 3 3.49 (0.35)* 19.1(1.9)*

M3_100 100 3 2.35 (0.12)* 14.5(1.6)*

M3_150 150 3 33.7 (0.8)* 2.2 (0.18)* 14.1(0.9)*

M3_200 200 3 2.3 (0.19)* 15.2 (1.2)*

M3_300 300 3 3.31 (0.05)* 19.8(0.8)*

M3_400 400 3 3.73 (0.08)* 21.9(2.7)*

M3_500 500 3 1.31 (018)* 12.7(0.6)*

M4_201 Ambient 4 10.6 (1)* 40.9 (2.5)*

M4_50 50 4 3.93 (0.07)* 20.8(2.2)*

M4_75 75 4 3.49 (0.35)* 19.1(1.9)*

M4_100 100 4 2.35 (0.12)* 14.5(1.6)*

M4_150 150 4 31.4 (2.3)* 2.2 (0.18)* 14.1(0.9)*

M4_200 200 4 2.3 (0.19)* 15.2 (1.2)*

M4_300 300 4 3.31 (0.05)* 19.8(0.8)*

M4_400 400 4 3.73 (0.08)* 21.9(2.7)*

M4_500 500 4 1.31 (018)* 12.7(0.6)*

R3_20 Ambient 3 - -

R3_50 50 3 - -

R3_75 75 3 32.8 (1.6)* - -

R3_100 100 3 - -

R3_150 150 3 - -

R4_20 Ambient 4 - -

R4_50 50 4 - -

R4_75 75 4 29.7 (1.1)* - -

R4_100 100 4 - -

R4 150 150 4 - -

563 * Standard deviation in parenthesis

566 Table 2. Summary of test results

Specimen

(1) Maximum load, Pmax. (kN)

Displacement at maximum load . (mm)

(3) Average maximum load,

Pa, (kN)

(4) Average displacement at maximum load ??a, (mm)

P H.T., * max ' n A.T.

(6) Average

bond strength (MPa)

(7) Average tensile stress in the textile (MPa)

(8) Failure mode**

0.52 0.69

R3_50 R3_75 R3_100 R3 150

30.9 18.2 15.8 9.4

29 17.5 13.5 8.7

0.6 0.44 0.53 0.23

0.78 0.57 0.68 0.37

30.0 17.9 14.7 9.1

0.69 0.51 0.61 0.30

0.58 0.35 0.28 0.18

0.94 0.56 0.46 0.28

657 391 3121 198

R4_50 R4_75 R4_100 R4 150

24.3 16.7

38.8 20.8 14.8 9.1

0.76 0.53 0.5 0.37

0.88 0.42 0.67 0.51

40.6 22.6 15.8 9.8

0.82 0.48 0.59 0.44

0.65 0.36 0.25 0.16

1.27 0.70 0.49 0.30

668 371 259 160

M3_50 M3_75 M3_100 M3_150 M3_200 M3_300 M3_400 M3_500

29.0 28.9 29.8

27.2 33.8 33.2 16.6

29.6 24

25.1 38

0.99 1.1 1.04 1.33 1.56 1.46 1.55 0.78

29.3 26.5

29.4 30.9 26.2 35.9 35.4 17.9

0.87 1.20 1.17 1.22 1.46 1.63 1.70 0.74

0.81 0.73 0.82 0.86 0.73 1.00 0.98 0.50

0.92 0.83 0.92 0.97 0.82 1.12 1.11 0.56

643 580 645 678 573 787 776 393

M4_50 M4_75 M4_100 M4_150 M4_200 M4_300 M4_400 M4_500

36.7 32.3 36.2 36.9 38.5

36.4 36.2

41.2 40.7

1.14 1.02 1.28 1.17 1.44 1.46 1.72 0.75

1.39 0.85

1.26 1.05 1.18 1.43 0.87

34.4 36.2

36.5 36.9 38.9 39.2

1.27 0.94 1.27 1.22 1.25 1.32 1.58 0.81

0.82 0.83 0.87 0.88 0.89 0.94 0.94 0.56

1.06 1.07

1.15 1.21 1.22 0.72

559 565 595

600 606 639 644 379

567 * Specimen number

568 ** D: Debonding of FRP/TRM from the concrete substrate including part of the concrete cover; A: Adhesive failure at the

569 concrete-resin interface.

570 1 Specimens included in Raoof et al. 2016 [28]

575 Table 3. Results of transient condition test

Specimen (1) Load (kN) (2) Time (min.) (3) Average time (4) Temperature (0C) (5) Average temperature (6) Failure mode**

S1* S2* (min.) S1* S2* (0C)

R4 25% 15.5 19.9 18 19.0 100.8 91.8 96.3 A

R4 50% 31.1 16.3 17.7 17.0 66.4 74.9 70.7 D

R4 75% 46.6 11.9 12.7 12.3 47.5 50.2 48.9 D

M4 25% 10.4 65.6 58.3 62.0 329.8 309.2 319.5 D

M4_50% 20.8 62.3 55.2 58.8 319.6 301.1 310.4 D

M4_75% 31.1 18 21 19.5 72.4 82.2 77.3 D

576 * Specimen number

577 ** A: Adhesive failure at the concrete-resin interface (see Fig. 15a); D: Debonding of FRP/TRM from the concrete substrate

578 with peeling off part of the concrete cover (see Fig. 15 b and c for FRP specimens and Fig. 15 d-f for TRM specimens).

16-mm bar /-Internal steel cage

10-mm acrylic

rod •-100--

-16-mm bar ■i

10-mm aluminium rod for alignment

Premade holes in each prism

Thermocouple-. Unbonded

\ zone \

10-mm dia. aluminium rod

1111»1111

16-mm dia. rebar

-L,= 200mm-

TRM layers

-Lb= 200mm-

597 Fig. 1. Specimen details; (a) specimen preparation; (b) details of acrylic plate; (c) details of internal

598 reinforcement; (d) details of alignment of the prisms; (e) overall design details of the test specimen;

599 and (f) schematic diagram for the free body diagram of the tested side of the specimen (Dimensions in

600 mm)

601 602

Effective bond length-/

50 100 150 200

Bond length (mm)

Fig. 2 Variation of ultimate load with the number of layers and the bond length for both strengthening systems; the results of TRM system have already been presented in [28]

606 607

Material: Carbon fibres textile

Mesh size: 10x10 mm

Weight: 384 g/m2

Nominal thickness: 0.095 mm

Fig. 3 Carbon fibre textile used in this study

Mortar prism

Thermocouple

Fig. 4 Test setup for the mortar prisms tested at high temperature

. .■* -

Foil tape

j Foil tape k

i": »

■^■Hr

-I—I—l—l -i—1—! :

i HaSst

I Mfeg

, mt l-fTTHr irmj

'ITITI'I

'i i "! i i ; : I-.

! 1 : . r

Tn----

! I !" : : : '( i :

Fig. 5 Strengthening procedure: (a) surface preparation for TRM strengthened specimens; (b) surface preparation for FRP strengthened specimens; (c) application of the first of mortar; (d) application of the first layer of textile for TRM retrofitted specimens; (e) application of first layer of textile for FRP retrofitted specimens; and (f) application of final layer of mortar for TRM retrofitted specimens

Extractor

Machine grip

Steel box

Steel clamps

LVDTs >

Furnace

w I— ■ JJ

© ©

Thermocouple

Fig. 6: (a) Details of the test setup; (b) details of test specimen

100 150

Time (Sec.)

Fig. 7 Scheme of time- temperature curve

Time (min.)

Fig. 8 Time- temperature curve obtained from the two thermocouples for a specimen tested in steady

state and heated up to 400 0C

Time (min.)

100 150

Time (min.)

636 Fig. 9 Actual time- temperature curve of the specimens tested in steady state condition

0.5 1.0 1.5 2.0 Displacement (mm)

0.5 1.0 1.5 2.0 Displacement (mm)

Fig. 10 Load-displacement curves of the specimens strengthened with different materials and number

of layers: (a) three

--300 «

--200 u

Time (min.)

Time (min.)

644 Fig. 11 Cross-head displacement increase and average temperature at the bonded interface vs. time of

645 specimens from transient condition for specimens strengthened with (a) FRP and (b) TRM

(C) (d)

€ *J L ■ Hi ^ ? a* L

(h) (i) (j)

Fig. 12 Failure mode of specimens strengthened with three and four layers of FRP tested in steady state condition at different elevated temperature varied from 20 to 150 0C.

M3 100 ^ .

M-a 11

I ■ . -i

i! .'O *•••'»

r>w- A

"V:"' ■ «

ftr»1 Vi

: T i m

1 vt ?

M3 200 n

M3 400

M3 500

Fig. 13 Failure mode of specimens strengthened with three layers of TRM tested in steady state condition at different elevated temperature varied from 20 to 500 0C.

M4 100

M4 300

!-:. .-i-W'Jj • ' • j ^

V- I V - / — •.

-I V • '

• *J , f i I : ~ ■

' "K I'-Jr

f V ,-r-i

M4 400 rr

y L"'; p.- • ' -

M^vZ -| -; 7 3 , 'N

-•__.

It - ;•

- 1,' fll if Til' • J

- ; . ! v .

h ■ . '1-i f

M4 500

;•/■f> 'V VV iff? I • i H

P. >JU

-r } ' " J* Ct ■> r

Fig. 14 Failure mode of specimens strengthened with four layers of TRM tested in steady state condition at different elevated temperature varied from 20 to 500 0C.

M4_25%

M4 25% i r.

M4 50%

.. . . > A

* ;f t

'V VA.1 V

a «? " & ,1

: • ;

» ui*

fctfe.il

' I « "

(b) M4 50%

"•«"j-p; ;• -' ■'I

Kt^Tj 1 i J

TO! 1»

M4_75%

: £ v

Jfe, v <: *

* * if vC^i

. ...... / • 1 '

!>• * v. •. H ' '

% * •$ ' *

it jj».-^* , •• ......—1.......

M4 75%. IMBBBII

Fig. 15 Failure mode of specimens tested in transient condition

60 50 £ 40 w 30 § 20 10 0

• . - a):

R4 (

iVI L^t

, .....^K

1 ^ > '

1 > M3 ►

Temperature ( C)

..... FleYiiral strength b)

-n- Cor npressive strength (

100 200 300 400 500 Temperature (0C)

Fig. 16 (a) Variation of ultimate load with the temperature, the strengthening materials and the number of layers (steady state tests); (b) variation of mortar flexural and compressive strength with

the temperature

694 Fig. 17 Exploded specimen heated up to 550 0C

100 200

400 500 0

200 300

Temperature (C )

Temperature (C )

Fig. 18 Influence of the loading condition as a function of temperature on the bond behaviour: (a) FRP-specimens, and (b) TRM specimens